Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology Citation Reed, Christopher John (2019) Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QWMZ-HA45. https://resolver.caltech.edu/CaltechTHESIS:06072019-140931627 Abstract This dissertation discusses the synthesis, characterization, and reactivity of site-differentiated tetranuclear clusters containing Fe and Mn with NO and H 2 O-derived ligands. The motivation of this work was to conduct a detailed examination of structure-property relationships in well-defined molecular systems focused on unique features of multinuclear systems, such as bridging ligands, neighboring metal identity, and cluster oxidation state. Reactivity towards NO and H 2 O-derived ligands was targeted due to their relevance to biological multinuclear transition metal active sites that promote multi-electron small molecule transformations. Chapter 2 discusses the synthesis of Fe-nitrosyl clusters bearing an interstitial μ4-F atom. These clusters were prepared to compare their reactivity to previously synthesized [Fe3 3 OFeNO] clusters with an analogous structure. A redox series of the [Fe 3 FFe] and [Fe 3 FFeNO] clusters were accessed, with the nitrosyl clusters displaying five cluster oxidation states, from Fe II 3 {FeNO} 8 to Fe III 3 {FeNO} 7 . Overall, the weaker bonding of the F - ligand resulted in attenuation of the activation and reactivity of the {FeNO} 7 , relative to the corresponding μ4-O clusters. Furthermore, the ability of distal Fe oxidation state changes to influence the activation of NO was decreased, demonstrating lower cooperativity between metals in clusters linked by a weaker μ4-atom This represents a rare case where the effects of bridging atom ligands could be compared in isostructural multinuclear complexes and decoupled from changes in metal ion coordination number, oxidation states, or geometry. Chapter 3 describes the synthesis of site-differentiated heterometallic clusters of [Fe 3 OMn], displaying facile ligand substitution at the five-coordinate Mn. This system was able to coordinate H 2 O and thermodynamic parameters of the proton and electron transfer processes from the Mn II –OH 2 to form a Mn III –OH moiety were studied. The oxidation state distribution of the neighboring Fe centers had a significant influence on these thermodynamic parameters, which was similar to the analogous parameters for mononuclear Mn systems, demonstrating that oxidation state changes in neighboring metals of a cluster can perturb the reactivity of a Mn–OH x unit nearly as much as an oxidation state change at the Mn–OH x . Subsequent experiments attempted to find spectroscopic or electrochemical evidence for formation of a terminal Mn-oxo in this system; however, that was not obtained, even in relatively extreme conditions. This established a lower limit for the bond dissociation enthalpy of the Mn III –OH of ca. 93 kcal/mol, which makes formation of a terminal Mn-oxo cluster unfavorable in most organic solvents, due to expected facile hydrogen atom abstraction of a solvent C–H bond. The insights obtained on the reactivity of these tetranuclear metal-hydroxide clusters was applied towards stabilizing a terminal metal-oxo in a multinuclear complex, as outlined in Chapter 4. Through the use of pendant hydrogen bond donors with tert-butyl-aminopyrazolate ligands, tetranuclear Fe clusters bearing terminal-hydroxide and -oxo ligands could be stabilized and structurally characterized. A similar thermodynamic analysis of the Fe III –OH bond dissociation enthalpy was conducted, which demonstrated Fe III -oxo clusters could be accessed with a range of reactivity at the terminal-oxo ligand, based on the redox distribution of the neighboring Fe centers. The kinetics of C–H activation for the [Fe II 2 Fe III 2 ]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pK a on the rate of proton coupled electron transfer. Lastly, Chapter 5 describes the synthesis and reactivity of tetranuclear Fe clusters bearing unsubstituted pyrazolate ligands, focusing on attempts to observe evidence for a terminal Fe-oxo or Fe-imido motif. Clusters bearing a labile trifluoromethanesulfonate ligand at the five-coordinate Fe center could be prepared, and would react with oxygen atom transfer reagents to produce a terminal Fe-hydroxide cluster, which, upon dehydration, led to isolation of an octanuclear μ2-O cluster. The pathway for Fe-hydroxide formation was investigated, but could not conclusively determine whether reactivity occurred from a transient terminal Fe-oxo. Similarly, the reduced tetra-iron cluster, in the [Fe II 3 Fe III ], redox state was prepared, and demonstrated reactivity towards electron deficient aryl azides. Isolation of aryl amide clusters (Fe-NHAr) was observed, suggesting, again, formation of a reactive Fe-imido which decomposes through formal hydrogen atom abstraction. Efforts to stabilize either of these Fe=O/NR multiply-bonded species through a more acidic Fe were investigated by synthesizing the corresponding pyrazolate bridged μ4-F clusters. The [Fe II 4 ] cluster also displayed reactivity towards oxygen atom transfer reagents, and produced a similar octanuclear μ2-O cluster, but the observation of μ4-F substitution with oxygen to produce μ4-O clusters with a terminal F ligand likely precluded formation of a reactive terminal-oxo cluster. Instead, thermodynamically favorable cluster rearrangement to the [Fe 3 OFe] structure dominates. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: transition metal, cluster, Fe-oxo Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Agapie, Theodor Group: Resnick Sustainability Institute Thesis Committee: Peters, Jonas C. (chair) Dougherty, Dennis A. Gray, Harry B. Agapie, Theodor Defense Date: 30 May 2019 Funders: Funding Agency Grant Number Resnick Sustainability Institute Graduate Research Fellowship UNSPECIFIED Record Number: CaltechTHESIS:06072019-140931627 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:06072019-140931627 DOI: 10.7907/QWMZ-HA45 Related URLs: URL URL Type Description https://doi.org/10.1021/acs.inorgchem.7b02114 DOI Article adapted for chapter 2 https://doi.org/10.1021/jacs.8b06426 DOI Article adapted for chapter 3 https://doi.org/10.1021/jacs.9b03157 DOI Article adapted for chapter 4 ORCID: Author ORCID Reed, Christopher John 0000-0002-8774-5106 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 11714 Collection: CaltechTHESIS Deposited By: Christopher Reed Deposited On: 11 Jun 2019 18:36 Last Modified: 12 Dec 2019 21:44 Thesis Files Preview PDF (Full Thesis) - Final Version See Usage Policy. 21MB Preview PDF (Title) - Final Version See Usage Policy. 59kB Preview PDF (Acknowledgements - Abstract - Published Content and Contributions - Table of Contents) - Final Version See Usage Policy. 214kB Preview PDF (Chapter 1 - General Introduction) - Final Version See Usage Policy. 1MB Preview PDF (Chapter 2 - Tetranuclear Iron Clusters with a Varied Interstitial Ligand: Effects On Structure, Redox Properties, and Nitric Oxide Activation) - Final Version See Usage Policy. 2MB Preview PDF (Chapter 3 - Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II) - Final Version See Usage Policy. 2MB Preview PDF (Chapter 4 - A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity) - Final Version See Usage Policy. 3MB Preview PDF (Chapter 5 - Intermolecular Reactivity of Tetranuclear Fe Clusters Via Putative Fe–Oxo and –Imido Intermediates) - Final Version See Usage Policy. 2MB Preview PDF (Appendix A - NMR Data) - Final Version See Usage Policy. 2MB Preview PDF (Appendix B - 57Fe Mossbauer Data) - Final Version See Usage Policy. 4MB Preview PDF (Appendix C - Miscellaneous Crystal Structures) - Final Version See Usage Policy. 2MB Preview PDF (About the Author) - Final Version See Usage Policy. 46kB Repository Staff Only: item control page

Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology Thesis by Christopher John Reed In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Division of Chemistry and Chemical Engineering Pasadena, California 2019 (Defended on May 30", 2019) 248 ABOUT THE AUTHOR Christopher John Reed was born in Boise, ID on July 2nd, 1991. He, and his brother Alexander Reed, were raised in Idaho by their mother, Freda Reed. He attended Mountain View High School and graduated in 2009. Then, he moved to Pocatello, ID to attend Idaho State University for undergraduate. During his time at ISU, he worked in the ‘extreme biochemistry’ laboratory of Prof. Caryn Evilia where he studied protein structure and stability of halophilic microorganisms using circular dichroism spectroscopy. In the summer of 2012, he completed a research internship at Utah State University, in the protein crystallography lab of Prof. Sean Johnson. Chris graduated from ISU in 2014 with a B.S. in Chemistry and Biochemistry; he received the ISU Outstanding Student Achievement Award from the College of Science and Engineering (Science Division). He completed his Ph. D. studies in the lab of Prof. Theodor Agapie at the California Institute of Technology in 2019. In 2017, he received a Resnick Sustainability Institute Graduate Fellowship to support the remainder of his graduate studies. He is currently planning to start his post-doctoral studies with Prof. Yi Lu at University of Illinois, Urbana-Champaign.  CHAPTER 1 GENERAL INTRODUCTION 2 INTRODUCTION Multi-Electron Transformations Relevant to Global Biogeochemical Cycles of Oxygen and Nitrogen. The chemical basis of every living organism is centralized around a handful of elements C, H, N, O, P, and S; the transformation of molecules containing these elements occurs ubiquitously and on massive scales, directly affecting life around Earth. The global chemistry of oxygen and nitrogen are particularly relevant to human life. For example, the beginning of production of molecular oxygen eons ago, and its build up in the planet’s atmosphere, has been linked to the development of complex eukaryotic organisms.1 A constant production of O2 is required to support aerobic life, and is intimately involved in biochemical processes related to the oxidation of other biological elements. Similarly, ammonium (NH4+) serves as the crucial building block to all biological nitrogen-containing molecules, which includes amino acids and nucleotides, and the combination of manmade and natural ammonium synthesis is directly tied to the production of sufficient supplies of food for humans and other animals.2 The global cycles of oxygen and nitrogen revolve around electron transfer; the formal oxidation state of oxygen commonly varies from -2 (i.e. H2O) to 0 (O2), and nitrogen can vary from -3 (NH4+) to +5 (NO3-).1 The four-electron reduction of dioxygen by cellular respiration constitutes a key component of the global oxygen cycle. The consumed dioxygen is replaced by photosynthesis through the analogous four-electron oxidation of water (Scheme 1). In a similar way, multi-electron redox processes of nitrogen provide access to its various biologically relevant forms (i.e. N2, NH4+, NO, N2O) and processes to regenerate them. The global redox transformations of oxygen and nitrogen typically occur through multi-electron pathways. This is because, often, the partially reduced/oxidized molecule is less thermodynamically stable (Figure 1). 3 Scheme 1. Multi-Electron Redox Processes of Oxygen and Nitrogen Relevant to Their Global Biogeochemical Cycles. Figure 1. Equilibrium standard potentials of forms of oxygen (red) and nitrogen (blue).3 Biological Catalysts for Multi-Electron Redox Transformations of Oxygen and Nitrogen. Nature contains a variety of catalysts which constitute major steps in the global cycles of oxygen and nitrogen. For example, yearly, 300 tetragrams of N2 are reduced to NH4+ by nitrogen-fixing microbes through a metalloenzyme called nitrogenase; this process is considered to be responsible for more than half the amount of bio-available nitrogen in the environment.2 These enzymes display remarkable efficiency towards their native reactions and are capable of effecting complex, challenging multi-electron transformations under relatively mild, and sustainable, conditions. For this reason, researchers have examined these biological systems to gain insight into the features of these enzymes that promote efficient catalysis. 4 Biological enzymes responsible for redox transformations of oxygen and nitrogen are all metalloenzymes, containing either Fe, Mn, and/or Cu ions in their active site.4 Within this set of enzymes, many of the active sites contain complex transition metal clusters (Figure 2). The structural diversity of these multinuclear active sites is notable, displaying various transition metals, nuclearities, bridging ligands, and geometric arrangements. Detailed mechanistic study of these globally relevant metalloenzymes has been a culmination of efforts from biochemists, crystallographers, spectroscopists, and theoreticians, which remain at the frontier of bioinorganic research, leading to developments in enzymology, metal ion spectroscopy, and computational techniques. Figure 2. Active site structures of metalloenzymes competent for multi-electron redox transformations of oxygen and nitrogen: nitric oxide reductase (PDB code: 3O0R), 5 heme/Cu oxygen reductase (1V54), nitrous oxide reductase (1FWX), photosystem II (3WU2), and nitrogenase (1M1N). Fe (light brown), Cu (cyan), Mn (light purple), Mo (purple), Ca (green), S (yellow), O (red), N (blue). Case Study: Mechanistic Insight to the Oxygen-Ovolving Complex (OEC) in Photosystem II. Photosynthetic organisms use the energy in sunlight to drive the reduction of plastoquinol and produce energy in the form of adenosine triphosphate (ATP). The reducing equivalents obtained are derived from water, one of the most abundant sources of electrons in the environment.4b, 5 In a separate process, the energy and reducing equivalents are used for reduction of CO2 and production of carbohydrates in the Calvin cycle. Water oxidation occurs in a protein assembly known as photosystem II (PSII), at a multinuclear active site composed of a heterometallic [Mn4CaO5] cluster, called the OEC.6 Incoming photons induce a charge separated state at by a nearby heme center (P680), which transfers a single electron through a series of mediators until reaching plastoquinone. P680 is regenerated by reduction from a nearby tyrosine residue (TyrZ). This resulting organic radical oxidizes the OEC, which translates four separate electron transfer events by P680 to one catalytic turnover, producing dioxygen from two molecules of water.4b The molecular model of OEC turnover is considered within the framework of the Kok cycle, which describes five distinct so-called Sstates (S0, S1, …, S4) of the OEC (Figure 3).4c, 7 Electrons, and protons from coordinated HxO ligands, are removed from the OEC during each S-state transition; states S0 through S3 have been observed spectroscopically (principally through advanced EPR and X-ray absorption techniques), and in some cases structurally characterized by X-ray diffraction. These investigations have established a number of important characteristics of the OEC catalytic cycle, including (i) the lowest oxidation state of the OEC during turnover is [MnIII3MnIV] and 6 this cluster undergoes four subsequent oxidations to reach a formal [MnVI3MnV] redox state, responsible for O–O bond formation (S4);4b, 8 (ii) EPR spectroscopy of the OEC has shown a dynamic structure within the cluster, where at least one of the bridging oxygen atoms is exchangeable (and implicated as one of the substrate oxygen atoms);9 (iii) recent X-ray techniques have captured structural snapshots of the OEC in the S3 state, which displays coordination of the second substrate HxO molecule to one of the cubane Mn centers;10 and (iv) extensive computational studies based on experimental structural and spectroscopic parameters for the OEC suggest a high-valent terminal Mn-oxo is the key O–O bond forming intermediate in the unobserved S4 state.11 Figure 3. Contemporary proposed structures of the OEC in each stage of the Kok cycle. These studies provide a remarkably detailed picture of water oxidation by the OEC; however, due to the inherent complexity of studying this massive protein (700 kDa), with its 7 multiple subunits and cofactors, there are a number of challenges to realizing a complete mechanistic understanding of the OEC. For example, the precise protonation state of the OEC, and its neighboring protein environment, is not well-understood for any S-state.11b Also, since PSII is necessarily studied in aqueous conditions, the OEC is always present in a large excess of substrate, which complicates isolation and characterization of the oxidized states of the cluster. Our mechanistic understanding of the OEC exemplifies a number of possible functional roles for multiple transition metals arranged within a multinuclear active site. The presence of many redox active transition metals allows for the storage of multiple oxidizing equivalents, without requiring a large buildup of charge at a single site. This has been implicated as a cause for the high selectivity of the OEC, which produces little to no partially oxidized forms of oxygen, i.e. H2O2 or O2-, which would be detrimental to the organism. One can also envision how the relatively unique dangling cubane geometry of the metal centers in the OEC promotes reactivity between specific metal-bound oxygen atoms. Furthermore, the coupling of unpaired spins in the Mn centers may be crucial for efficient release of dioxygen, avoiding the production of reactive singlet oxygen. In general, a number of functions for neighboring metal centers in various catalytic systems can be proposed: (i) storage of redox equivalents, (ii) structurally directing reactive moieties, and (iii) tuning the electronic characteristics of a reactive metal or coordinated ligand. Biologically Inspired Transition Metal Complexes Relevant to Small Molecule Chemistry of Oxygen and Nitrogen. Due to the potential complexity and constraints of studying metalloenzymes directly, the complementary development of well-defined small molecule transition metal complexes has provided significant chemical insight related to these biological processes. The synthetic inorganic chemistry of Fe, Mn, and Cu complexes has been 8 studied extensively, including chemistry related to the global oxygen and nitrogen cycles through reactions involving relevant O- and N-containing small molecules such as H2O, O2, N2, and NO. A majority of these studies are performed with mononuclear, or binuclear, transition metal complexes; the development of multinuclear systems with greater complexity that bear closer resemblance to biological active sites remains a challenge for synthetic chemists. The following survey of relevant synthetic metal complexes is by no means exhaustive, but its discussion will place this work within the wider context of previously reported synthetic transition metal complexes that are relevant to biological transformations of oxygen and nitrogen. Synthetic Inorganic Chemistry Related to Water Oxidation by the OEC. Efforts towards a full structural model of the OEC were undertaken with the goal of elucidating the structureproperty relationships of a well-defined [Mn4CaO5] cluster with spectroscopic and/or functional relevance to the native metalloenzyme. A variety of di- and tetramanganese oxo clusters have been reported with relevance to the OEC; notable early achievements within this field include the isolation of high-valent [Mn4O4] cubane clusters from the groups of Dismukes (1) and Christou, and the incorporation of Ca into high nuclearity Mn-oxo clusters by Christou and co-workers.12 In 2011, Agapie and co-workers reported a [Mn3CaO4] cluster analogous to the cubane subunit of the OEC, 3.13 Since then, a more complete structural model of the OEC, with the dangler Mn has been reported by Zhang, Dong, Dau, and co-workers (4).14 With these complexes, a great deal of insight has been obtained, towards understanding the electronic interactions between Mn centers in cubane clusters, and the influence of the redoxinactive Ca ion on redox and reactivity properties.15 Most structural models of the OEC display coordinatively saturated Mn centers (Scheme 2), precluding extensive reactivity studies with 9 exogenous H2O; however, these and related systems have been used to study the reactivity of the bridging oxo ligands of high valent Mn complexes with relevance to the OEC.16 Scheme 2. Selected Structural Models of the OEC.12b, 13-14, 15b Numerous lower nuclearity metal complexes have been examined for their relevance to the OEC (Scheme 3). For example, Borovik and co-workers have studied the reactivity and electronic structure of mononuclear Mn-OH, Mn-oxo, and Mn–(OH)–Ca motifs, which are related to possible intermediates of the OEC; a unique ligand capable of hydrogen bonding to the Mn–OHx moiety facilitated characterization of a series of MnIII-, MnIV-, and MnV-oxo complexes (5 – 7).17 Particularly relevant to the biological system are understanding aspects that affect the homolytic bond dissociation enthalpy (BDE) of Mn–OHx motifs; proton coupled electron transfer (PCET) has been implicated as a crucial aspect of OEC catalysis, as it avoids charge built up at the active site, promoting progression to the fully oxidized state of the cluster.18 Along these lines, other groups have examined the PCET reactivity of mononuclear Mn–OH and -oxo complexes to understand the influence of Mn oxidation state, ligand, field, and protonation state on reactivity.19 Examples of these types of studies with multinuclear Mn complexes are less common;16b, 16c, 20 a binuclear Mn system has been reported by Pecoraro and co-workers (9), which is able to support –aquo and –hydroxide ligands in multiple Mn oxidation states, and access a reactive terminal Mn-oxo.21 10 O–O bond formation of synthetic Mn-oxo complexes has also been examined, predominantly with porphyrin ligands. Nucleophilic attack of MnV-oxo (10) by hydroxide produces peroxo- intermediates in Mn-corrole systems.22 In some cases, subsequent oxidation of this intermediate releases dioxygen. The reactivity of related corrole complexes in the presence of a redox-inactive metal has shown significant perturbations to the Mn–O bonding, which could be relevant to Mn–O–Ca motifs in the OEC.23 Scheme 3. Selected Examples of PCET and O–O Bond Formation with Synthetic Complexes Relevant to the OEC. Synthetic Complexes Relevant to Heme/Cu Oxygen Reductase (HCO). Dioxygen reduction by the bimetallic active site of HCO is a key step in cellular respiration that drives transmembrane proton pumping to ultimately obtain ATP from cellular reducing equivalents.4e HCO enzymes are part of a wider class of metalloenzymes that reduce dioxygen; other enzymes of this class employ mono- and binuclear active sites of Fe and Cu with O2 to oxidize organic molecules for a variety of metabolic pathways. A common element between HCO (and other Fecontaining O2 reducing enzymes) and the OEC is the key role of a putative high-valent terminal metal-oxo intermediate. In HCO, O–O bond cleavage of a Fe–(O2)–Cu intermediate is proposed to produce CuII–OH and FeIV-oxo, which is further reduced and protonated to afford two molecules of H2O. 11 The synthetic chemistry of heme and non-heme Fe-oxo complexes has been extensively investigated, due to their relevance to members of O2-reducing metalloenzymes.24 Groves and co-workers have recently reviewed this topic for heme Fe-oxo complexes.25 Close structural mimics of the bimetallic HCO active site have also been prepared (i.e. 11); a survey of these, and related synthetic Fe/Cu systems, has been reviewed recently by Karlin and co-workers.26 Biosynthetic approaches to study the mechanism of O2-reduction by binuclear heme/nonheme active sites has provided great insight to structure-property relationships in HCO; Lu and co-workers have used protein scaffolds to produce close structural models of the HCO active site, through mutagenesis studies of a simpler heme protein. With a single protein scaffold, they were able to introduce binding sites for various non-heme metals (Zn, Fe, Cu) and investigate their effect on the activity and selectivity for HCO-like activity.27 Scheme 4. Synthetic HCO Model Complex26b Synthetic Complexes Related to the Fe-Mo Cofactor (FeMoCo) of Nitrogenase. The FeMoco cluster of nitrogenase is a [MFe7S9C] cluster with a fused-cubane geometry (M = Mo);28 versions of nitrogenase where the eighth metal is V, Mo, or Fe have been observed, but the Fe-Mo cofactor is the most well-studied. One of the unique structural features of this cluster is the interstitial μ6-C ligand, which is not observed in any other biological cluster, and a rare motif in reported synthetic complexes. Extensive mechanistic investigations of nitrogenase by EPR 12 spectroscopy has characterized a number of reduced oxidation states of the cluster;29 however the precise binding mode of the FeMoco substrate, N2, has not been established. Peters and co-workers have examined a series of mononuclear Fe–N2 complexes, bearing different transligand donors (12 – 14), including an anionic carbon donor;30 notably, 13 is the first example of an Fe-based molecular N2-reduction catalyst.31 The identity of the trans-donor had a strong influence on the Fe–N2 bonding and reactivity. More recently, crystal structures of FeMoco with a displace μ2-S between to Fe centers have been obtained, suggesting a possible substrate binding site.32 This has led to the investigation of binuclear Fe complexes as models of FeMoco.33 The N2-activation chemistry of higher nuclearity Fe complexes have also been investigated, although well-characterized high-spin Fe clusters (of more than two Fe centers) with a bound N2 ligand have yet to be reported in the literature. Despite this, reduction of multinuclear Fe complexes in the presence of N2 has led to the isolation of a number of Fenitride and –imido clusters (15 and 16), with relevance to putative intermediates in FeMoco.34 Scheme 5. Selected Functional Models of FeMoco.30, 34 Like the OEC, efforts to make rigorous structural mimics of FeMoco are motivated by a desire to prepare well-defined molecular models for spectroscopic and structure-property investigations. Many fused-cubane clusters reminiscent of FeMoco have been reported by the 13 research groups of Holm and Ohki, including a [Fe8S10] cluster bearing a μ6-S (17).35 Although the small molecule chemistry of these clusters has not been reported, related [Fe6S9] clusters have been combined with apo-nitrogenase proteins to produce artificial metalloproteins competent for reductive coupling of CO, -CN, and C2H4.36 Scheme 6. Structural Models of FeMoco and Related Nitrogenase Clusters.37 Synthetic Metal Complexes Related to Biological Denitrification. The process of denitrification is an important part of the global nitrogen cycle, nitrate (NO3-) is reduced to N2 over four steps, via nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O) intermediates, as a terminal electron acceptor for an anaerobic analogue to cellular respiration.4a The NO and N2O reducing steps are accomplished by multinuclear active sites of Fe and Cu, respectively. Nitric oxide reductase (NOR) contains an active site structure similar to HCO, with a nonheme Fe center instead of Cu; similar molecular and biosynthetic systems that have been used to understand HCO have been applied to NOR, as well.38 NO-reducing metalloenzymes with a binuclear non-heme Fe active site are also present in pathogenic bacteria for NO 14 detoxification. A faithful structural and functional model of this active site has recently been reported by Lehnert and co-workers (20).39 Nitrous oxide reductase (N2O) is composed of a tetranuclear Cu active site, containing a bridging S ligand.40 Mankad and co-workers have reported a tetranuclear Cu complex bearing a μ4-S with a square pyramidal geometry (21); this complex is capable of reducing N2O to N2, structurally and functionally mimicking the native enzyme.41 The proposed mechanism of N2OR suggests N2O is activated across two Cu centers; further mechanistic investigations of 21 or the native enzyme are required to establish the precise role of the four Cu centers in the cluster’s ability to drive this transformation. Scheme 8. Multinuclear Model Complexes of NOR and N2OR39, 41 CONSPECTUS The oxidation and reduction of small molecules of oxygen and nitrogen occurs on a global scale and has relevance to many of the chemical processes that encompass life. In organisms across all domains of life, the metalloenzymes that catalyze the multi-electron transformations of molecules such as H2O, O2, N2, and NO3- contain active sites with diverse, complex multinuclear transition metal structures. Mechanistic investigations of these metalloenzymes have sought to elucidate the functional purpose of these unique multinuclear arrangements; and come with a number of inherent challenges, due to difficulties in preparation and 15 manipulation of large protein assemblies, or the limitations of aqueous conditions. For this reason, complementary studies of synthetic transition metal complexes and their chemistry towards O- and N-based small molecules is useful for understanding mechanistic details of native biological systems. While a majority of the reported literature has focused on the synthetic chemistry of mononuclear and binuclear transition metal complexes, the synthesis and study of higher nuclearity clusters comprises an important development towards a more complete understanding of the multi-electron redox transformations of globally relevant molecules. The work detailed in this dissertation addresses the development of tetranuclear clusters of biologically relevant transition metals Fe and Mn, in particular their chemistry towards NO and H2O, with relevance to biological multinuclear active sites responsible for multi-electron transformations of small molecules. 16 References 1. Falkowski Paul, G.; Godfrey Linda, V. Philosophical Transactions of the Royal Society B: Biological Sciences 2008, 363, 2705-2716. 2. Kuypers, M. M. M.; Marchant, H. K.; Kartal, B. Nature Reviews Microbiology 2018, 16, 263. 3. van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem. Soc. 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Soc. 2016, 138, 13107-13110.  CHAPTER 2 TETRANUCLEAR IRON CLUSTERS WITH A VARIED INTERSTITIAL LIGAND: EFFECTS ON STRUCTURE, REDOX PROPERTIES, AND NITRIC OXIDE ACTIVATION The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. Inorg. Chem., 2017, 56, 13360-13367 20 ABSTRACT A new series of tetranuclear Fe clusters displaying an interstitial μ4-F ligand was prepared for a comparison to previously reported μ4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with NO stretching frequencies ranging from 1680 to 1855 cm−1, respectively. Despite accessing more reduced states with an F− bridge, a two-electron reduction of the distal Fe centers is necessary for the μ4-F clusters to activate NO to the same degree as the μ4-O system; consequently, NO reactivity is observed at more positive potentials with μ4-O than μ4-F. Moreover, the μ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation. 21 INTRODUCTION Transition metal clusters perform diverse functions in proteins, including metal storage, sensing, electron transfer, and multi-electron small molecule conversions (such as H2O oxidation, CO2 fixation, and N2 reduction).1 A common element of these multinuclear sites is the presence of highly bridged (≥ µ3-binding) single atom ligands, such as sulfide,2 oxide,3 or carbide.4 Quantitative measures of the effects these ligands play in small molecule activation remain rare. This is particularly relevant to understanding the role the interstitial µ6-C ligand in the FeMo cofactor (FeMoco) of nitrogenase (Figure 1A). Synthetic clusters suitable for structure-function studies of bridging ligands with respect to the activation of a small molecule are rare, likely because of design constraints that are difficult to overcome by self-assembly, which is the route typically employed in cluster synthesis. Maintaining the exact same structure while changing the bridging ligands and redox states while limiting ligand binding to a single small molecule, desirable for quantification of the effect and for mimicking substrate activation by protein active sites, are two major challenges. A host of iron carbonyl clusters have been synthesized with a variety of bridging (≥ µ3) single-atom ligands, including µ6-C clusters, such as [(µ6-C)Fe6(CO)16]2-, with arrangements reminiscent of the FeMoco structure (Figure 1B).5 While a related cluster has been reported displaying a µ6-N ligand, [(µ6-N)Fe6(CO)15]3- has been reported, with potential for structure-function studies of the effect of the interstitial ligand, changes in the structure and number of carbon monoxide (CO) ligands complicate interpretations. In the cases where completely isostructural clusters can be prepared with bridging elements of the second row of the periodic table, the large number (≥ 9) of diatomic ligands limits interpretations regarding the activation of a single small molecule substrate, which is most relevant to biological systems. Recently, in an elegant demonstration of the effect of the µ4-ligand (N vs C) on reactivity, the hydride ligands in [HFe4C(CO)12]2- and [HFe4N(CO)12]- 22 Figure 1 (A) Depiction of FeMoco cluster of nitrogenase with putative binding of nitrogenous ligand and design elements of the clusters reported herein (B) Reported Fe clusters with different interstitial (or psedo-interstitial, X) and diatomic (CO) ligands; right, limitations of these clusters for determining the effect of interstitial ligand on small molecule activation (C) Synthesis of tetranuclear iron clusters. 23 have been shown to have distinct behavior for H2 and formate generation.6 Other synthetic clusters have been studied to address effects of a bridging ligand on reduction potentials or to model FeMoco, but small molecule binding by the clusters with different bridging ligands has not been reported.7 Toward directly interrogating the effect of a cluster’s interstitial ligand on reactivity, we have developed synthetic methodologies to access site-differentiated multinuclear complexes that allow variation of the bridging ligands. Initial studies by Dr. Graham de Ruiter established a synthetic route to tetranuclear Fe clusters arranged in a pseudo-tetrahedral geometry around a µ4-O ligand. One of these Fe centers displays a trigonal pyramidal coordination geometry, with an open coordination site trans to the µ4-O and the three neighboring Fe centers; this open coordination site is facilitated by steric protection of phenyl-pyrazolate ligands, which preclude binding of most ligands except for small molecules. These clusters, LFe3O(PhPz)3Fen+, were competent to bind and activate nitric oxide (NO; Scheme 1), where redox changes of the distal Fe centers were able to modulate the degree of NO activation and reactivity.8 In summary, reduction of a distal FeIII to FeII leads to an average decrease of N–O stretching frequency of ~ 50 – 30 cm-1, where the lowest oxidation state NO cluster observed (FeII2FeIII{FeNO}7) displayed reactivity towards NO disproportionation. Scheme 1. Related Tetranuclear Clusters Previously Reported by the Agapie Group8 24 Herein, we present investigations of a series of tetranuclear iron clusters containing a µ4-F motif, isostructural with the previously reported µ4-O clusters (Figure 1C).8-9 These compounds allow for the evaluation of the effects of the nature of the interstitial atom on cluster properties related to the activation of a single diatomic ligand (NO). RESULTS AND DISCUSSION We have recently reported the synthesis of site differentiated tetranuclear clusters, where three (basal) metal centers are coordinated by a hexapyridyl trialkoxide framework (L3-, Figure 1C) and bridged to a fourth (apical) metal site through three pyrazolate ligands and a µ4-O ligand.8-9 The all-ferrous fluoride-bridged cluster, 1, was synthesized via addition of a 2:1 ratio of phenylpyrazole and potassium phenylpyrazolate along with 1 equiv of anhydrous tetrabutylammonium fluoride to a previously reported trinuclear iron precursor (LFe3(OAc)(OTf)2; Figure 1C).8, 10 The fourth Fe equivalent was delivered as Fe(N(SiMe3)2)2 to complete the tetranuclear cluster (1). This redox-neutral route of installing the interstitial F proved to be the most reliable way to avoid the generation of mixtures of clusters, with some µ4-O clusters likely having formed due to trace moisture. Subsequent chemical oxidations afford two additional redox states, [FeII3FeIII] (2) and [FeII2FeIII2] (3); cyclic voltammetry of 1 displays two quasi-reversible features for these oxidations at potentials of -0.51 V (all potentials vs Fc/Fc+) and +0.18 V (Figure 2). Characterization by Mössbauer spectroscopy is consistent with charge localization on each Fe center and with oxidations occurring exclusively in the basal triiron core, the apical Fe remaining FeII (Figure 3), as was observed for the µ4-O analogs.9a Structural characterization by single crystal X-ray diffraction (XRD) reveals that the most oxidized cluster, 3, displays a five-coordinate apical FeII, due to acetonitrile (MeCN) binding (Figure 4). Removal of this ligand under vacuum results in decomposition. This behavior is in contrast to the analogous µ4-O clusters, which have been isolated in the 25 [FeII2FeIII2] and [FeIIFeIII3] oxidation states, both displaying a four-coordinate apical FeII. This difference suggests that that the µ4-F clusters are more Lewis acidic than their µ4-O analogues. Consistent with this interpretation, µ4-O clusters with electron withdrawing substituents show increased coordination numbers at the apical metal.9a II II Fe 3Fe III II III Fe 2Fe 2 Current Fe 4 20 μA -1.5 -1 -0.5 0 0.5 1 Potential vs. Fc/Fc+ (V) Figure 2. Cyclic voltammogram (black trace) of 1 (3 mM) in MeCN with 85 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Square wave voltammograms (gray dashed trace) overlaid with 0.1 V amplitude, 1.0 s period, and 0.01 V increment. 26 Figure 3. 57Fe Mössbauer spectra at 80K of (A) 1, (B) 2, and (C) 3. Black dots represent the data, gray traces are the sum of the simulated fits, and colored traces represent the individual fits for the Fe centers (See Table 1 for parameters). Blue traces represent assignments made to basal FeII, orange traces represent basal FeIII assignments, green traces represent apical FeII. Table 1. 57Fe Mössbauer Parameters for Complexes 1 – 3 with Oxidation State Assignments δ (mm/s) % assignment 1.16 |ΔEq| (mm/s) 1 3.42 25 h.s. FeII 1.15 3.18 25 h.s. FeII 1.18 3.07 25 1.10 25 h.s. FeII h.s. FeII 3.30 2.90 1.21 1.50 25 25 29 21 h.s. FeII h.s. FeII h.s. FeIII h.s. FeII 3.07 0.89 1.45 2.49 25 25 25 25 h.s. FeII h.s. FeIII h.s. FeIII h.s. FeII 0.95 2 1.14 1.12 0.48 0.99 3 1.12 0.49 0.49 0.98 NO provides a diagnostic vibrational spectroscopic signature for comparing different complexes to address the effects of the multinuclear supporting platform and the interstitial ligand on small-molecule binding.11 Studies of the chemistry of Fe clusters with NO have been 27 Figure 4. Crystal structure of 3. Ellipsoids are shown at the 50% probability level. Hydrogen atoms, solvent molecules, and outersphere counterions omitted for clarity. principally focused on understanding the biologically relevant conversion of Fe–S clusters to nitrosylated products.12 However, there are few examples of multinuclear mononitrosyl complexes containing nearby redox-active metal centers.8, 13 The clusters targeted here provide insight into the influence of neighboring metal centers on the chemistry of the metal nitrosyl moiety. The addition of NO to compound 1 leads to the formation of the corresponding nitrosyl adduct. Cyclic voltammetry of the monocationic nitrosyl cluster, 1-NO, displays three electrochemically quasi-reversible oxidations and one quasi-reversible reduction (Figure 5). Each of the five redox states of the nitrosyl clusters observed electrochemically was accessed synthetically (Figure 1C). Stepwise treatment of 1-NO with AgOTf (2-NO and 3-NO) and [(2,4-Br-C6H4)3N][SbCl6] (4-NO) provides access to the oxidized NO adducts. 4-NO decomposes in solution and as a solid on the time scale of attempted crystallizations, preventing structural characterization. Reduction of 1-NO with decamethylcobaltocene in 28 MeCN precipitates a purple solid assigned as 5-NO. Dissolution of 5-NO in tetrahydrofuran, pyridine, or dichloromethane (CH2Cl2), leads to rapid decomposition, preventing structural characterization of this complex as well (Figure 6). Reoxidation of a MeCN suspension of 5NO with silver triflate (AgOTf) leads to isolation of the one electron oxidized cluster, 1-NO, in good yield (Figure 7). Nitrous oxide (N2O) is detected upon decomposition of 5-NO, albeit in low yield (~0.1 equiv, gas chromatography-mass spectrometry). Figure 5. Cyclic voltammogram of monocationic nitrosyl cluster, 1-NO (2mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt wire, and Ag wire as working, reference, and counter electrodes, respectively. The measured open-circuit potential was -0.7 V. Mössbauer spectroscopy was performed on 1-NO – 5-NO. As observed in the µ4-O system, Mössbauer parameters are consistent with oxidations being localized at the basal triiron core as characterized previously.8-9, 14 In the Mössbauer spectrum of 1-NO, the Fe–NO signal is readily distinguished from the basal iron centers in the cluster, and was fit with an isomer shift (δ) of 0.62 mm/s and a quadrupole splitting value (|ΔEq|) of 1.16 mm/s (Figure 29 Figure 6. (Top) 1H NMR (300 MHz; CD3CN) of reaction mixture of 5-NO in THF over 24 hours. The spectrum of the major species is identical to [LFe3F(PhPz)3Fe][OTf] (1) in CD3CN. (Bottom) 1H NMR (300 MHz; CD2Cl2) of reaction mixture of 5-NO in THF over 24 hours. The spectrum of the new species is identical to [LFe3O(PhPz)3Fe][OTf]. We have previously observed decomposition of LFe3O(PhPz)3Fe in dichloromethane to the monocationic cluster. This is consistent with formation of a mixture of [LFe3F(PhPz)3Fe]+ and LFe3O(PhPz)3Fe from the decomposition of 5-NO in THF. Figure 7. 1H NMR (300 MHz; CD3CN) of reaction mixture of AgOTf addition to LFe3F(PhPz)3Fe(NO) (5-NO) in thawing THF. The spectrum is identical to [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO) in CD3CN. 30 8B; Table 2). The exact Mössbauer parameters for the Fe–NO centers in 2-NO – 4-NO are more difficult to assign due to spectral overlap with signals from the FeIII centers of the triiron core. The overlap is consistent, however, with only small changes in the Mössbauer parameters for the Fe-NO sites in 1-NO – 4-NO (Figures 8C-D and Table 2). These parameters are also similar to the previously reported µ4-O NO clusters, which have δ values ranging from 0.55 to0.62 mm/s, and |ΔEq| values of 1.94 to 2.38 mm/s.8 Overall, these data, along with the IR spectroscopy data (vide infra), are consistent with the {FeNO}7 formulation, according to Figure 8. Zero applied field 57Fe Mössbauer spectra at 80K of (A) 5-NO,(B) 1-NO, (C) 2NO, (D) 3-NO, and (E) 4-NO. Black dots represent the data, gray traces are the sum of the simulated fits, and colored traces represent the individual fits for the Fe centers (see Table 2 for parameters). Blue traces represent assignments made to basal FeII, orange traces represent basal FeIII assignments, green and purple traces represent {FeNO}7 and {FeNO}8 units, respectively. 31 Table 2. Fe–µ4-F Distances and 57Fe Mössbauer Parameters for Complexes 1-NO – 5NO with Oxidation State Assignments Fe Center Fe–µ4-F distance (Ǻ) δ (mm/s) |Eq| (mm/s) assignment 1.15; 1.15; 1.16 3.59; 3.40;3.23 h.s. FeII 1.67 {FeNO}7 1.17 3.31; 3.03 1.39 h.s. FeIII h.s. FeII {FeNO}7 3.10 0.87; 1.47 1.51 h.s. FeII h.s. FeIII {FeNO}7 1.42 h.s. FeIII and {FeNO}7 1-NO Fe1, Fe2, Fe3 Fe4 2.129(7); 2.205(6); 2.169(5) 2.065(7) Fe1 Fe2, Fe3 Fe4 2.030(4) 2.237(4); 2.101(4) 2.093(4) Fe1 Fe2, Fe3 Fe4 2.207(3) 2.080(3); 2.091(3) 2.155(3) Fe1-Fe4 - 0.63 2-NO 0.44 1.12; 1.15 0.62 3-NO 1.09 0.48; 0.40 0.62 4-NO a 0.47 - 5-NO 1.15; 1.15; 1.15 3.56; 3.17; 3.75 h.s. FeII - 0.95 1.63 {FeNO}8 Fe1, Fe2, Fe3 Fe4 aIn this case, the signals for the Fe centers overlap preventing reliable parameter determination for the unique apical {FeNO}7 center. The presence of an {FeNO}7 moiety is supported via the IR spectroscopy data. Enemark-Feltham notation.15 The Mössbauer spectrum of 5-NO was fit with three FeII in the triiron core and an apical Fe–NO signal distinct from the ones observed for 1-NO – 4NO; this is assigned as {FeNO}8 (δ = 0.94 mm/s and |ΔEq| = 1.63 mm/s; Figure 8A), consistent with reduction of the Fe–NO moiety rather than a remote metal site. Compounds 1-NO, 2-NO, and 3-NO were structurally characterized by XRD. In all cases, binding of NO to the apical Fe occurs in a linear fashion (∠Fe4–N40–O40 > 175°, Figure 9A). As observed in the µ4-O system and from Mössbauer spectra (Figure8B-D), bond metrics are consistent 32 with oxidations being localized at the basal triiron core of these three clusters (Table 2). The Fe– µ4-F bonds, which range from 2.07 to 2.24 Å, are longer than the Fe–µ4-O bonds (1.93 to 2.18 Å) despite the shorter ionic radius of F- which suggests a significantly weaker interaction with the fluoride resulting in more electron deficient metal centers.16 IR spectroscopy reveals a large range of νN–O for complexes 1-NO – 5-NO, from 1680 cm-1 to 1855 cm-1 (Figure 10). Comparison of νN–O for 1-NO – 4-NO (1799 – 1855 cm-1) provides insight into the effect of remote redox changes on NO activation. Oxidation of the Fe centers not bound to NO leads to an average of 19 cm-1 per redox change, with redox changes of more reduced clusters having a larger effect. The shift in νN–O to higher energy upon oxidation is matched by an increase in Fe4-µ4-F distance, and likely results from a more electron deficient Fe4 center due to this elongation. The nature and type of interaction with axial ligand has been previously demonstrated to effect the level of NO activation in mononuclear Fe complexes.17 Analogous shifts in the distance between Fe and axial ligands trans to coordinated N2 have been reported for monoiron models of nitrogenase.18 The correlation between the increase in the Fe4-µ4-ligand distance and the increase in the νN–O frequency observed previously for µ4-O and now with µ4-F interstitial ligands suggests that this structural parameter generally serves to relay the effect of remote redox changes to the metal that binds the small molecule. However, the magnitude of the change in NO activation as a result of these distal redox changes varies with the nature of the interstitial atom. For µ4-O clusters, the νN–O changes from 1715/1759 to 1823 cm-1 over two redox events with an average change of 54/33 cm-1 per electron transfer, in contrast to only 19 cm-1 for μ4F. The stronger O2- ligand roughly doubles the effect of the remote redox changes on the activation of NO compared to F-. This is a unique observation, which relies on the ability to access multiple oxidation states of these clusters, and demonstrates that an interstitial ligand 33 Figure 9. (A) Crystal structure of tetranuclear iron nitrosyl cluster 2-NO with ellipsoids shown at the 50% probability level. Solvents molecules, outer-sphere counterions, and H atoms are omitted for clarity. (B) Simplified depiction of the tetranuclear iron clusters discussed. Measured redox potentials, NO stretching frequencies, and apical Fe-µ4-ligand distances are included for comparison. Data for the µ4-O clusters were previously reported.8 Rel. Transmission [LFe3F(PhPz)3Fe][OTf] LFe3F(PhPz)3Fe(NO), 5-NO [LFe3F(PhPz)3Fe(NO)][OTf], 1-NO [LFe3F(PhPz)3Fe(NO)][OTf]2, 2-NO [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6], 4-NO [LFe3F(PhPz)3Fe(NO)][OTf]3, 3-NO 3400 2900 2400 1900 1400 Frequency (ν, cm-1) 900 400 Figure 10. Solid state IR spectra of complexes 1, 1-NO - 5-NO. can influence small molecule activation in two ways: first, by its direct interaction with the small molecule-binding metal center, and second, by modulation of the degree to which other 34 metals in the cluster can perturb this meta-interstitial ligand interaction. A structural comparison of the Fe4-μ4-ligand distances over two oxidation states shows that redox changes at the remote Fe centers shift the Fe4-μ4-F distance by 0.09 Å and the Fe4-μ4-O bond by 0.12 Å (Figure 9B). The more donating interstitial ligand is able to more efficiently translate remote redox changes in the cluster into NO activation. A consequence of varying the μ4-ligand in these clusters is that the weaker F− donor increases the overall cluster charge of a particular redox state by 1 compared to the O2− version. In reported mononuclear complexes, related modifications of a ligand‘s charge at a distal site (i.e. R3BH- vs R3CH) leads to observable shifts of bound carbonyl stretching frequencies by ~10 – 40 cm-1.19 For these clusters, separating the effect of higher positive charge from the effect of the donating abilities of the interstitial ligand on NO activation can be addressed by comparing clusters 2-NO−4-NO and the μ4-O analogues. For the same cluster redox state, significantly higher νN−O are observed for the μ4-F ligand compared to μ4-O, as expected. The overall cluster charge, which is higher by 1 compared to μ4-O clusters of the same Fe redox states, is not sufficient to explain the higher NO activation. A comparison of clusters of the same charge for μ4-O and μ4-F, but higher overall Fe redox state for μ4-O (for example, (μ4F)FeIIFeIII2{FeNO}7 (3-NO) with νN−O = 1842 cm−1 vs (μ4-O)FeIII2{FeNO}7 with νN−O = 1823 cm−1) still shows a higher degree of NO activation with O2−. This difference suggests that the higher-charge interstitial ligand leads to a more electron-rich cluster and a lower νN−O due to its direct interaction with the metal centers rather than solely due to the reduced cluster charge. IR spectroscopy of 5-NO corroborates the Mössbauer data and is consistent with the formation of a {FeNO}8 motif; the νN–O at 1680 cm-1, is ~120 cm-1 lower than νN–O for the {FeNO}7 moiety of 1-NO. A similarly large shift was observed upon reduction for a structurally related mononuclear trigonal bipyramidal Fe-NO complex,20 and more generally 35 for nonheme {FeNO}7/{FeNO}8 complexes.21 An analogous species is not observable for the µ4-O clusters. A comparison of the redox potentials of the µ4-F and the µ4-O systems (Figure 9B)8 reveals that the F- ligand shifts the redox potentials positively by approximately 1 V for the same cluster oxidation states compared to the O2- ligand because of the lower negative charge and weaker electron donating ability of F-. An analogous effect is observed for other clusters upon changing the bridging ligand to alter the charge of the cluster.6, 7e The shift in redox potentials allows access to more reduced states of the µ4-F clusters within the electrochemical solvent window, which could be beneficial for storing additional reducing equivalents at more positive potentials. However, this is counterbalanced by weaker activation of the diatomic ligand, as reflected by IR spectroscopy (vide supra). In fact, to achieve the same level of NO activation, the µ4-F clusters need to have Fe oxidation states lower by two levels compared to the µ4-O clusters. This is in contrast to the behavior observed for certain iron-multicarbonyl clusters, where data is available for isostructural motifs. For example, [Fe4C(CO)12]2- shows lower average CO activation than the one electron more reduced, but same-charge cluster, [Fe4N(CO)12]2-.6, 22 The difference is likely a result of distribution of charge and small molecule activation over many (12) CO ligands. In the present system, which displays a more biomimetic, single ligand binding, the NO activation is relayed remotely through the interstitial atom and provides a test for the ability of the µ4-ligand to communicate the redox change at metals not bound to NO. Furthermore, differences in chemical reactivity of the diatomic ligand are observed. The addition of NO to (µ4-O)FeII2FeIII{FeNO}7 leads to NO disproportionation to generate N2O and the one electron oxidized nitrosyl cluster.8 In contrast, addition of NO to 1-NO, which is one electron more reduced (µ4-F)FeII3{FeNO}7, does not result in a reaction. This difference in reactivity as a function of interstitial ligand is likely due to a more activated NO and a 250 mV lower redox potential for the µ4-O cluster. 36 Only 5-NO, with an electronically different, {FeNO}8 moiety, undergoes conversion to N2O with a fluoride interstitial ligand, albeit not cleanly. Overall, despite more negative potentials compared to µ4-F analogs of the same redox state, reactivity of NO is observed at milder potentials with the µ4-O cluster. CONCLUSIONS In this report, we have demonstrated the significant effects that the change of interstitial ligands (µ4-O vs µ4-F) has on the small molecule activation properties of tetranuclear Fe clusters. The more positive redox potentials of µ4-F clusters allow access to more reduced Fe states. However, this does not result in more efficient activation of small molecule ligands, as inferred from IR spectroscopy and reactivity of NO complexes. The higher νN–O values of the µ4-F species for the same Fe oxidation states compared to the µ4-O analogues are not due to the difference in cluster charge but rather the nature of the interactions with the bridging ligand. To achieve similar NO activation, the cluster needs to be two electrons more reduced with the µ4-F compared to the µ4-O ligand. Consequently, NO disproportionation is observed with a µ4-O ligand at higher Fe oxidation states and more positive potentials than with a µ4-F ligand. Furthermore, the µ4-O ligand is a better relay of remote redox changes. The structurefunction studies described here suggest that a higher charge interstitial ligand, such as the carbide in FeMoco of nitrogenase, is more efficient at tuning cluster properties in a variety of ways toward the activation of small molecule. Cluster analogs with interstitial C and N moieties are currently being pursued for comparison. 37 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. LFe3(OAc)(OTf)2,8 Fe(N(SiMe3)2)2,23 benzyl potassium,24 1-H-3-phenyl pyrazole (HPhPz),25 anhydrous [NBu4][F]26, and [(2,4-Br-C6H3)3N][SbCl6]27 were prepared according to literature procedures. [(4-Br-C6H4)3N][OTf] was prepared according to a modified literature procedure.28 Tetrahydrofuran was dried using sodium/benzophenone ketyl, degassed with three freeze-pump-thaw cycles, vacuum transferred, and stored over 3Å molecular sieves prior to use. CH2Cl2, diethyl ether, benzene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H and 19F NMR spectra were recorded on a Varian 300 MHz spectrometer. 13C NMR spectra were recorded on a Varian 500 MHz spectrometer. CD3CN and CD2Cl2 was purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior to use. Infrared (ATR-IR) spectra were recorded on a Bruker ALPHA ATR-IR spectrometer at 4 cm1 resolution. Headspace analysis was conducted on a HP 5972 GC-MS. Physical Methods. Mössbauer measurements. Zero applied field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated acetonitrile solution in the cup. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). 38 Mössbauer simulation details for compounds 1 – 3 and 1-NO – 5-NO. All spectra were simulated as four pairs of symmetric quadrupole doubles with equal populations and Lorentzian lineshipes (the parameter defining the width, Г, is reported). They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.8-9, 14 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.29 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter), and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Dry acetonitrile or CH2Cl2 that contained ~85 mM [Bu4N][PF6] was used as the electrolyte solution. The ferrocene/ferrocinium (Fc/Fc+) redox wave was used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of 39 the compounds (1 - 3 and 1-NO – 3-NO), most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Synthetic Procedures. Synthesis of Potassium 3-phenyl-pyrazolate (KPhPz). In the glovebox, a solution of 1-H-3-phenyl-pyrazole (1.54 g, 11.8 mmol) in THF (5 mL) was stirred while a solution of benzyl potassium (1.70 g, 11.8 mmol) in THF (10 mL) was added drop-wise. Addition caused the solution to change from colorless to pale yellow. After 30 minutes, the solvent was removed under reduced pressure to obtain 1.83 g off-white powder (85% yield). 1 H NMR (300 MHz, CD3CN) δ 7.83 (d, 2H), 7.44 (s, 1H), 7.28 (t, 2H), 7.07 (t, 1H), 6.39 (s, 1H). 13C NMR (500 MHz, CD3CN) δ 100.01 (Pz NCCH), 125.02 (p-Ar CH), 125.37 (m-Ar CH), 128.98 (o-Ar CH), 139.34 (Pz CHCHN), 150.27 (Pz NCCH). An expected signal ~ 138 ppm (i-Ar C)8 could not be observed, likely due to the low solubility of KPhPz. Synthesis of tris-4-bromo-phenylamininum trifluoromethanesulfonate ([(4-Br-C6H4)3N][OTf]). This was prepared through a modification of a literature procedure for [(4-Br-C6H4)3N][BF4].28 Tris4-bromo-phenylamine (1.5 g, 3.11 mmol) was dissolved in 30 mL diethyl ether with silver trifluoromethanesulfonate (AgOTf; 1.2 g, 4.67 mmol). This light green solution was added to a 100 mL Schlenk tube and cooled to -40 ºC under N2 atmosphere. Iodine powder (0.75 g, 2.96 mmol) was added with a counter-flow of N2 while stirring; addition caused the solution to turn dark blue. The Schlenk tube was warmed to room temperature and filtered over a course porosity frit. The collected precipitate was filtered with 30 mL CH2Cl2 in the glovebox. To the resulting dark blue solution, 40 mL diethyl ether was added and the flask was cooled to -40 ºC. [(4-Br-C6H4)3N][OTf] was collected as a dark purple solid upon filtration (1.36 g, 69% yield). Anal. Calc. (%) for C19H12Br3F3NO3S: C, 36.16; H, 1.92; N, 2.22. Found: C, 36.70; H, 1.94; N, 2.27. 40 Synthesis of [LFe3F(PhPz)3Fe][OTf] (1). In the glovebox, a suspension of LFe3(OAc)(OTf)2 (1047 mg, 0.76 mmol) in THF (3 mL) was frozen in the cold well. To the thawing suspension, solutions of potassium 3-phenyl-pyrazolate (190 mg, 1.04 mmol) in THF (3 mL) and 1-H-3phenyl-pyrzole (220 mg, 1.52 mmol) in THF (3 mL) were added. The suspension changed color from yellow to orange upon addition of the potassium 3-phenyl-pyrazolate. [Bu4N][F] (208 mg, 0.79 mmol) was added as a suspension in THF (3 mL), causing the solution to become dark red. A solution of Fe(N(SiMe3)2)2 (288 mg, 0.76 mmol) in THF (2 mL) was added. The reaction was stirred for 20 h, after which an orange precipitate was observed. The suspension was filtered over a bed of celite on a fine porosity glass frit and washed with 5 mL THF. The orange solid was collected with 60 mL MeCN. The solvent was removed under reduced pressure to obtain [LFe3F(PhPz)3Fe][OTf] as an orange solid (950 mg, 75% yield). 1H NMR (300 MHz, CD2Cl2) δ 104.77, 78.57, 75.13, 48.82, 37.46, 30.48, 27.17, 26.44, 25.63, 19.69, 18.42, 11.60, 10.53, 4.54, 4.22, 3.44, 1.99, 1.27, 1.16, -1.13, -2.80, -46.96. 19F NMR (300 MHz, CD2Cl2) δ -78.45. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 251 nm (9.2 ×104), 463 nm (3.9 ×103). Anal. Calcd. (%) for C85H60F4Fe4N12O6S: C, 60.88; H, 3.61; N, 10.02. Found: C, 61.16; H, 3.75; N, 9.74. Synthesis of [LFe3F(PhPz)3Fe][OTf]2 (2). To a suspension of [LFe3F(PhPz)3Fe][OTf] (1; 94 mg, 0.06 mmol) in THF (2 mL), a solution of AgOTf (14 mg, 0.06 mmol) in THF (2 mL) was added. The color of the suspension changed from orange to brown and, after 2 hours, the solvent was removed under reduced pressure. The brown residue was dissolved in CH2Cl2 and filtered over a bed of celite on glass filter paper. The solvent was removed under reduced pressure to obtain [LFe3F(PhPz)3Fe][OTf]2 as a brown solid (100 mg, 98% yield). 1H NMR (300 MHz, CD2Cl2) δ 101.33, 87.83, 79.33, 47.73, 46.79, 35.24, 34.14, 28.86, 26.35, 18.15, 16.58, 16.33, 12.10, 8.55, 7.28, 6.79, 6.25, 5.25, 4.63, -42.36. 19F NMR (300 MHz, CD2Cl2) - 41 78.19. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 250 nm (10.9 ×104), 432 nm (4.8 ×103). Anal. Calcd. (%) for C86H60F7Fe4N12O9S2: C, 56.57; H, 3.31; N, 9.21. Found: C, 56.47; H, 3.13; N, 8.88. Synthesis of [LFe3F(PhPz)3Fe(MeCN)][OTf]3 (3). To a stirring solution of [LFe3F(PhPz)3Fe][OTf]2 (2; 78.5 mg, 0.04 mmol) in acetonitrile (2 mL), [(p-Br-C6H4)3N][OTf] (27.1 mg, 0.04 mmol) was added as an MeCN solution (2 mL). The brown solution became purple upon addition. After 30 minutes, the solution was filtered. 5 mL of CH2Cl2 was added to the filtrate, then 10 mL pentane, to obtain a purple precipitate. The supernatant was decanted and the remaining solid was briefly dried under reduced pressure to obtain [LFe3F(PhPz)3Fe(MeCN)][OTf]3 as a purple solid (42.3 mg, 50% yield). 1H NMR (300 MHz, CD3CN) δ 125.15, 91.53, 82.45, 80.10, 61.48, 51.98, 43.99, 15.30, 13.93, 12.33, 8.44, 6.48, 5.67, 5.30, 0.46, -5.74, -18.78. 19F NMR (300 MHz, CD3CN) -75.66. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 250 nm (10.3 ×104), 465 nm (3.6 ×103). Anal. Calcd. (%) for C88H62Cl2F10Fe4N12O12S3 (3 with CH2Cl2 instead of MeCN; compound recrystallized in CH2Cl2): C, 51.31; H, 3.03; N, 8.16. Found: C, 51.26; H, 3.04; N, 8.43. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). Method A. In the glovebox, a 100 mL Schlenk tube was charged with a solution of [LFe3F(PhPz)3Fe][OTf] (1; 179 mg, 0.11 mmol) in CH2Cl2 (5 mL). The solution was degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 59 mmHg, 0.11 mmol) was condensed in the tube. The reaction was stirred at room temperature for 2 h and changed color from orange to brown. The solvent was removed under reduced pressure to yield [LFe3F(PhPz)3Fe(NO)][OTf] as a brown solid (181 mg, 99% yield). 1H NMR (300 MHz, CD2Cl2) δ 98.43, 76.64, 74.24, 42.59, 40.12, 35.92, 32.51, 27.06, 20.05, 15.27, 14.16, 11.24, 10.79, 4.27, 2.46, 1.13, 0.58, 0.46, -10.77, -23.61. 19F NMR (300 MHz, CD2Cl2) δ -78.71. Anal. Calcd. (%) for C86H62Cl2F4Fe4N13O7S (1-NO ∙ 42 CH2Cl2; compound recrystallized from CH2Cl2/pentane): C, 57.66; H, 3.49; N, 10.16. Found: C, 57.40; H, 3.46; N, 10.01. Method B. In the glovebox, solid LFe3F(PhPz)3Fe(NO) (5-NO; 22 mg, 0.014 mmol) was cooled to -196 °C in a cold well in a 20 mL vial with a stir bar. AgOTf (3.7 mg, 0.014 mmol) in 0.5 mL thawing tetrahydrofuran was added to the cooled powder. This reaction was stirred at room temperature for 30 minutes then pumped down. The purple suspension became a brown solution. 1H NMR analysis of the crude reaction showed mostly (>90%) [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). The brown solid was filtered in CH2Cl2 to obtain 16.8 mg of [LFe3F(PhPz)3Fe(NO)][OTf] after recrystallization (69% yield). Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO). Method A. In the glovebox, a 100 mL Schlenk tube was charged with a solution of [LFe3F(PhPz)3Fe][OTf]2 (2; 163 mg, 0.09 mmol) in CH2Cl2 (5 mL). The solution was degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 50 mmHg, 0.09 mmol) was condensed in the tube. The reaction was stirred at room temperature for 2 h, changing color from brown to yellow-green. The solvent was removed under reduced pressure to yield [LFe3F(PhPz)3Fe(NO)][OTf]2 as a dark green solid (162 mg, 98% yield). 1H NMR (300 MHz, CD2Cl2) δ 100.10, 83.22, 80.63, 66.68, 50.74, 46.79, 41.32, 17.25, 14.62, 14.38, 12.35, 11.71, 3.31, 0.30, -3.31, -17.33. 19F (300 MHz, CD2Cl2) δ -77.52. Anal. Calcd. (%) for C86H60F7Fe4N13O10S2: C, 55.65; H, 3.26; N, 9.81. Found: C, 55.59; H, 3.25; N, 9.53. Method B. In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 160 mg, 0.10 mmol) in MeCN (3 mL) was added to a solution of AgOTf (25 mg, 0.10 mmol) in MeCN (2 mL). The solution changed color from brown to yellow-green. After 1 h, the solvent was removed under reduced pressure. The green residue was dissolved in CH2Cl2 and filtered over a bed of celite. The solvent was removed under reduced pressure to obtain 43 [LFe3F(PhPz)3Fe(NO)][OTf]2 as a dark green solid (164 mg, 95% yield). 1H NMR is identical to that observed for method A. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO). In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO; 27.6 mg, 0.015 mmol) in CH2Cl2 (1 mL) was stirred as a solution of [(4-Br-C6H4)3N][OTf] (10.0 mg, 0.016 mmol) in CH2Cl2 (1 mL) was added. The addition caused the yellow-green solution to turn purple. After 30 minutes, the reaction was filtered and layered under pentane to afford purple crystals of [LFe3F(PhPz)3Fe(NO)][OTf]3 (20.3 mg, 68% yield). 1H NMR (300 MHz, CD2Cl2) δ 123.58, 98.80, 89.32, 60.89, 41.42, 14.25, 13.41, 10.34, 5.32, 4.35, 3.93, 3.71, 3.47, 2.07, 1.85, 1.18, -2.45, -8.26. Anal. Calcd. (%) for C87H60F10Fe4N13O13S3: C, 52.12; H, 3.02; N, 9.08. Found: C, 51.88; H, 2.94; N, 8.74. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] (4-NO). In the glovebox, a thawing solution of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO; 25.7 mg, 0.013 mmol) in CH2Cl2 (1 mL) was stirred as a solution of [(2,4-Br-C6H3)3N][SbCl6] (13.9 mg, 0.013 mmol) in MeCN (1 mL) was added. The addition caused the purple solution to turn blue. Cold toluene was added until a precipitate was observed. This was kept in a liquid nitrogen-cooled cold well for 2 minutes. The supernatant was decanted and the resulting solid was dried under vacuum. This afforded [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] as a blue solid (15 mg, 49% yield). This compound decomposes over time in solution and the solid state, even at reduced temperatures. Characterization of this compound was conducted with freshly prepared samples to minimize decomposition. 1H NMR (300 MHz, CD3CN) δ 124.54, 97.65, 80.33, 77.50, 74.55, 37.57, 18.30, 15.25, 13.39, 9.04, 0.01, -1.66, -5.71, -6.88. Anal. Calcd. (%) for C101H76Cl6F10Fe4N13O13S3Sb (NO4 ∙ 2 C7H8; compound precipitated with toluene): C, 48.07; H, 3.04; N, 7.21. Found: C, 47.83; H, 2.97; N, 7.88. 44 Synthesis of LFe3F(PhPz)3Fe(NO) (5-NO). In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 82.9 mg, 0.049 mmol) in MeCN was stirred as a solution of CoCp*2 (16.8 mg, 0.051 mmol) in MeCN was added. The addition caused the brown solution to become a purple suspension. After 2 hours, the solids were collected, washed with minimal MeCN, and dried under vacuum to afford LFe3F(PhPz)3Fe(NO) as a purple solid (44.3 mg, 59% yield). This species decomposes upon dissolution in tetrahydrofuran, pyridine, or CH2Cl2 and is mostly insoluble in acetonitrile, benzene, and toluene. Therefore, NMR and UV-Vis Absorbance data could not be collected for this complex. Anal. Calcd. (%) for C84H60FFe4N13O4: C, 64.76; H, 3.88; N, 11.69. Found: C, 64.21; H, 3.86; N, 11.51. Decomposition of LFe3F(PhPz)3Fe(NO) (5-NO). In the glovebox, solid LFe3F(PhPz)3Fe(NO) (26 mg, 0.02 mmol) was added to a 20 mL vial with septum cap and stir bar. 10 mL tetrahydrofuran was added and the vial was quickly sealed. Upon dissolving, the solution appeared brown. After stirring for 24 hr, the headspace was analyzed via GC-MS. A blue precipitate was observed in a brown-orange solution. 45 ELECTROCHEMICAL DETAILS 100mV/s 400mV/s 200mV/s 500mV/s Current 50mV/s 300 mV/s 50 μA -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V vs. Fc/Fc+) Figure 11. Cyclic voltammogram (solid traces) of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 2 mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Agwire as working, reference, and counter electrode, respectively. Square wave voltammograms (gray dashed trace) overlaid with 0.1 V amplitude, 1.0 s period, and 0.01 V increment. The open circuit potential (OCP) was measured to be -0.7 V. 3500 2500 jp (µA/cm2) 1500 500 -500 -1500 ○ ● FeII3{FeNO}8/FeII3{FeNO}7 □ ■ FeII3{FeNO}7/FeIIIFeII2{FeNO}7  ▲ FeIIIFeII2{FeNO}7/FeIII2FeII{FeNO}7 -2500 -3500 -4500 5 10 15 ν1/2 ((V/s)1/2) 20 Figure 12. Current density (jp) dependence of the square root of the scan rate ν1/2 for the electrochemical events observed in the CV of [LFe3F(PhPz)3Fe(NO)][OTf], 1-NO. 46 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3 and 1-NO – 3-NO. 1 2 3 1-NO 2-NO 3-NO CCDC Number 1554599 1554601 1554596 1554600 1554598 1554597 Empirical formula C86.75H62Cl4F 4Fe4N12O6S C96H71Cl2F7F e4N12O9S2 C101.5H75F10F e4N14O12S3 C93H76Cl16F4 Fe4N13O7S C93H74Cl14F7 Fe4N13O10S2 Formula weight (g/mol) C89.92H64.86Cl5 .85F10Fe4N13 O13S3 1837.80 2028.06 2235.21 2386.32 2450.16 2252.38 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) a (Å) 12.4213(18) 40.6924(19) 16.1006(6) 18.7000(7) 20.1692(15) 17.8680(9) b (Å) 16.108(2) 17.6138(8) 16.1006(6) 16.8237(7) 17.4343(13) 20.4024(11) c (Å) 20.502(3) 25.6670(13) 67.515(3) 32.4277(11) 28.440(2) 26.3282(15) α (°) 78.323(6) 90 90 90 90 78.386(4) β (°) 78.274(7) 114.015(2) 90 103.821(2) 99.091(2) 72.564(3) γ (°) 85.537(6) 90 120 90 90 82.836(3) V (Å3) 3930.6(10) 16804.3(14) 15157.0(13) 9906.5(7) 9874.8(13) 8948.2(9) 2 8 6 4 4 4 Cryst. syst. triclinic monoclinic trigonal monoclinic monoclinic triclinic Space group P-1 C2/c R-3 P21/n P21/n P-1 ρcalcg (cm3) 1.553 1.603 1.469 1.600 1.648 1.672 2Θ range (°) 2.584 to 61.034 4.626 to 55.754 5.060 to 61.146 5.004 to 130.168 4.464 to 51.356 9.042 to 179.202 μ (mm-1) 0.960 0.877 0.712 9.351 1.076 8.167 GOF 0.998 1.026 1.051 1.052 1.060 1.033 R1, wR2 (I>2σ (I)) 0.0400, 0.1003 0.0458, 0.0959 0.0835, 0.2216 0.1232, 0.2937 0.0944, 0.2594 0.0786, 0.2045 Z 47 Special refinement details for [LFe3F(PhPz)3Fe][OTf] (1). The structure contains several co-crystallized solvent molecules, many of which are on special positions. The only complete solvent molecule in the asymmetric unit that could be refined was disordered over two positions refined as 34.1% (C14 through C102) and 65.9% (Cl2 through C101). The two remaining solvent molecules were also disordered over two positions, but on a symmetry element. One disordered dichloromethane was refined as a partially occupied carbon 25% (C103) with ~50% occupied chlorine groups (Cl6 and Cl7). The other disordered dichloromethane was refined as a half occupied molecule, disordered over a symmetry element. Special refinement details for [LFe3F(PhPz)3Fe][OTf]2 (2). This structure contains two triflate counterions, one of which is positionally disordered over two positions with refined occupancies of 78.5% (S201 through C201) and 21.5% (S202 through C202). The structure also contains a co-crystallized dichloromethane (C0AA through Cl20), and two benzene molceules; one is complete (C101 through C106) the other (C107 through C109) is on a special position. Rigid bond restraints were used on the triflate counterions. Special refinement details for [LFe3F(PhPz)3Fe(MeCN)][OTf]3 (3). This structure contains the cluster on a C3 rotation axis, and therefore the three irons in the tri-iron core (Fe1 through Fe1’’) are indistinct. One triflate counterion is observed in the asymmetric unit along with four solvent molecules, only two of which could be modeled satisfactorily. A toluene molecule (C104 through C107) was disordered over a special position. There was another disordered toluene and acetonitrile that were disordered near special positions, based on residual electron density peaks; however, they could not be satisfactorily modeled. A solvent mask was used to account for the electron density of these molecules. 48 Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). This structure contained numerous co-crystallized solvent molecules (eight dichloromethane molecules). One solvent molecule was disordered and modeled with occupancies of 69% (Cl111 through C105) and 31% (Cl8 through C104). Another dichloromethane contained a disordered chlorine atom that was modeled with occupancies of 63% (Cl15) and 37% (Cl14). The triflate counterion was positionally disordered, whereby the sulfur would point either towards or away from the cluster. It was refined as two molecules with occupancies of 61% (S200 through C200) and 39% (S201 through C201). The standard deviations of some atoms in the phenyl ring of the trinucleating ligand (C34 – C36) were restrained to be the same. Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO). The structure contains two triflate counterions. One counterion is disordered over two positions with occupancies of 79.3% (S202 through C202) and 20.7% (S201 through C201). Three of the seven co-crystallized dichloromethane molecules are disordered over two positions. The first has occupancies of 59% (Cl11 through C105) and 41% (C103 through C108). Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO). There are two molecules in the asymmetric unit of the crystal structure. One of the clusters (Fe0A through C199) has a disordered phenyl pyrazolate ligand. Because of this disorder, the bond lengths and angles were not considered with in this molecule and only the other cluster, for which there was no evidence of disorder, (Fe1 through C99) was used for reporting bond metrics. All but two triflate counterions were disordered. Three triflates were positionally disordered; the first two had occupancies of 78% (S303 through C303 and S304 through C304) and 22% (S302 through C302 and S305 through C305). The third triflate had occupancies of 76% (S306 through C306) and 24% (S307 thgouh C307). The two remaining triflates were disordered as a pair. They could be modeled as being adjacent to one another with occupancies of 51% 49 (S308 through C308) and 62% (S130 through C130). This would be disordered with two triflates, one occupying the same space as the first pair with an occupancy of 37% (S309 through C309), and the other, itself disordered, next to a symmetry element with an occupancy of 50%. There were seven dichloromethane solvent molecules modeled in the structure. Two had positionally disordered chlorine atoms with occupancies of 78% (Cl9) and 22% (Cl16), and 51% (Cl5) and 49% (Cl6). Another dichloromethane was only partially occupied (taking up the same space as the disordered pair of triflates, as discussed above); it had an occupancy of 36% (Cl10 through C205). 50 Selected bond angles and distances for complexes 1-3, 1-NO – 3-NO. Bond Distance (Å) Fe1–F1 Fe2–F1 Fe3–F1 Fe4–F1 Fe1–N13 Fe2–N23 Fe3–N33 Fe4–N14 Fe4–N24 Fe4–N34 N13–N14 N23–N24 N33–N34 Fe4–N40 N40–O40 Bond Angles (º) N14–Fe4–N24 N24–Fe4–N34 N34–Fe4–N14 Fe4–N40–O40 Torsion Angles (º) Fe1–N13– N14–Fe4 Fe2–N23– N24–Fe4 Fe3–N33– N34–Fe4 Centroid Distances (Å) Fe1|Fe2|Fe3N 14|N24|N34 Fe1|Fe2Fe3– O11|O21|O31 Fe1|Fe2|Fe3– F1 N14|N24|N34 –Fe4 Complex 1-NO 1 2 3 2-NO 3-NO 2.167(1) 2.154(1) 2.174(1) 1.997(1) 2.170(2) 2.187(2) 2.157(2) 2.034(2) 2.056(2) 2.046(2) 1.390(2) 1.383(2) 1.387(2) – – 2.024(1) 2.204(1) 2.216(1) 2.011(1) 2.076(2) 2.125(2) 2.108(2) 2.063(2) 2.056(2) 2.044(2) 1.386(3) 1.389(3) 1.388(3) – – 2.132(2) – – 2.172(4) 2.061(3) – – – – 1.387(5) – – 2.112(8) – 2.129(7) 2.205(6) 2.169(5) 2.065(7) 2.118(10) 2.122(8) 2.116(11) 2.049(10) 2.060(9) 2.044(10) 1.401(15) 1.398(13) 1.384(13) 1.757(10) 1.163(13) 2.030(4) 2.237(4) 2.101(4) 2.093(4) 2.076(5) 2.102(5) 2.083(6) 2.064(6) 2.057(6) 2.064(5) 1.389(7) 1.380(8) 1.379(8) 1.773(6) 1.147(7) 2.207(3) 2.080(3) 2.091(3) 2.155(3) 2.057(4) 2.046(4) 2.025(4) 2.020(4) 2.049(4) 2.025(4) 1.369(6) 1.371(6) 1.389(5) 1.754(4) 1.133(6) 119.24(7) 117.34(7) 123.35(7) – 119.34(8) 120.91(8) 119.26(8) – 117.323 – – – 118.5(4) 117.1(4) 119.1(4) 175.7(9) 114.6(2) 119.8(2) 119.6(2) 177.0(6) 119.99(2) 116.37(2) 114.83(2) 179.2(4) 3.640 3.40 27.095 23.02 29.86 23.320 1.317 4.02 - 31.89 30.26 23.035 4.968 2.14 - 30.32 22.64 25.985 3.090 2.972 2.832 2.889 2.856 2.828 0.974 0.945 1.008 0.959 0.987 0.926 1.093 1.051 0.975 1.100 1.063 1.029 0.053 0.084 0.344 0.276 0.296 0.353 51 References 1. 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D. J. Chem. Soc., Perkin Trans. 1 1975, 2055-2065. 29. (a) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58; (b) Singh, A. K.; Jacob, W.; Boudalis, A. K.; Tuchagues, J.-P.; Mukherjee, R. Eur. J. Inorg. Chem. 2008, 2008, 2820-2829; (c) Sutradhar, M.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem. 2012, 2012, 4273-4278; (d) Schünemann, V.; Hauke, P. Mössbauer Spectroscopy. In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269.  CHAPTER 3 THERMODYNAMICS OF PROTON AND ELECTRON TRANSFER IN TETRANUCLEAR CLUSTERS WITH MN–OH2/OH MOTIFS RELEVANT TO H2O ACTIVATION BY THE OXYGEN EVOLVING COMPLEX IN PHOTOSYSTEM II The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. J. Am. Chem. Soc., 2018, 140, 10900 – 10908 54 ABSTRACT We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of [FeIII/II3MnII–OH2] clusters leads to internal redox reorganization resulting in formal oxidation at Mn to generate [FeIII/II3MnIII–OH]. 57Fe Mössbauer spectroscopy reveals that oxidation state changes (three for [FeIII/II3MnII–OH2] and four for [FeIII/II3MnIII–OH] clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined pKa (17.4) of the [FeIII2FeIIMnII–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn– OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69, to 78, and 85 kcal/mol for the [FeIIIFeII2MnII-OH2], [FeIII2FeIIMnII-OH2], and [FeIII3MnII-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn–OHx] system was obtained by constructing a potential–pKa diagram; the shift in reduction potentials of the [Fe3Mn–OHx] clusters in the presence of different bases supports the BDEO–H values reported for the [Fe3Mn–OH2] clusters. A lower limit of the pKa for the hydroxide ligand of the [Fe3Mn– OH] clusters was estimated for two oxidation states. These data suggest BDEO–H values for the [FeIII2FeIIMnIII–OH] and [FeIII3MnIII–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn=O] in this and related multinuclear systems. 55 INTRODUCTION During photosynthesis, water oxidation is catalyzed at the active site of Photosystem II (PSII) by a [CaMn4O5] cluster known as the oxygen evolving complex (OEC).1 The catalytic mechanism is outlined by the Kok cycle, with the cluster transitioning through five distinct so-called S-states (S0, S1, …, S4).2 Four sequential oxidations of the cluster occur (S0 → → S4), followed by the O–O bond forming step, with concomitant loss of O2 and binding of H2O to complete the cycle (S4 →S0). Protons are sequentially released from the active site during the S-state cycle; deprotonation of bound H2O in this stepwise manner prevents the buildup of significant charge at the active site, facilitating the further oxidation of the [CaMn4O5] cluster.3 PSII utilizes a nearby tyrosine radical (Yz•) as a mediator to transfer electrons/protons away from the OEC during turnover; because of the nature of the tyrosine radical, proton-coupled electron transfer (PCET) of the H2O-derived ligands bound to the OEC is considered to play an important role in the Kok cycle.3-4 Due to the wealth of information available in X-ray crystallographic1b-d, 5, EPR6, and X-ray absorption2b, 6d, 7 spectroscopic techniques, much is known about the Mn oxidation states and electronic environment of the OEC in the S0 through S3 states of the Kok cycle. More challenging has been understanding the precise protonation state of H2O ligands and relevant neighboring amino acid residues of any S-state; computational studies of the OEC have considered a variety of possible protonation states.4c, 8 Experimentally, time-resolved IR spectroscopy has been helpful in gaining insight to the dynamics of protons at the active site during turnover.6j, 9 Furthermore, multiple computational models of the OEC mechanism invoke a terminal Mn-oxo as a crucial part of the O–O bond forming S4 intermediate2c, 10. Therefore, there is significant interest in understanding the chemistry of a Mn–OH2 species undergoing multiple proton and electron transfers to reach a reactive terminal Mn-oxo. 56 The chemistry of synthetic Mn-aquo, -hydroxo, and -oxo motifs has been a subject of interest for inorganic chemists, particularly within the context of gaining insight into the thermodynamic basis of Mn–OHx PCET reactivity and how it relates to the mechanism of the OEC.11, 12, 13 Reported mononuclear systems have been able to probe the roles of Mn oxidation state12c, 12d, 12i, ligand field11a, 11f, 13b, and oxygen ligand protonation state11j, 12j on PCET reactions and the intrinsic O–H bond dissociation enthalpies (BDEO–H) of Mn–OHx moieties. There are fewer examples of such studies with multinuclear Mn complexes, with most of the reports examining the PCET chemistry of bridging oxo moieties,14 as opposed to terminally bound OHx ligands.15 Most of these reports are limited to binuclear Mn complexes or systems where only a single redox couple could be examined. The PCET reactivity of a synthetic MnIII3MnIVO3(OH) cubane cluster has been examined, where the BDEO–H of the μ3-OH could be estimated to be >94 kcal/mol; however, precise determination of the thermodynamic bond strength was complicated by subsequent decomposition of the protonated cubane.14d A report of proton and electron transfer at a terminal Mn-OHx moiety with an adjacent Mn center over three oxidation states (MnIII2, MnIIIMnIV, and MnIV2) represents a very rare example of thermodynamic studies of a terminal Mn-OHx in a multinuclear system.15a Access to a suitable synthetic platform to interrogate the effects of multiple neighboring redox-active metal centers on the chemistry of a terminal Mn–OHx motif may facilitate a more complete picture of the dynamics of proton and electron transfer of the OEC leading up to its reactive S4 state, and more generally lead to a better understanding of the behavior of metal clusters in reactions involving water, dioxygen, and multi-electron transformations. Our group has demonstrated the utility of rationally-designed, well-defined molecular clusters for probing structure-function relationships in multinuclear first-row transition metal complexes, acting as models of complex active sites found in biology.16, 17 Recently, we have 57 studied a family of tetranuclear Fe and Mn complexes composed of three coordinativelysaturated metal ions bridged to a fourth (apical) metal center through substituted pyrazolate (or imidazolate) ligands and a µ4-single atom ligand.18 The apical metal has a coordination site available for exogenous ligands, allowing for the study of substrate binding and reactivity by a molecular cluster. With bulky and nonpolar phenyl substituents in the 3 position of the pyrazolate ligands coordination of bulkier ligands remains inhibited, and intramolecular ligand activation had been observed.18d-f In contrast, previous group members, Drs. Zhiji Han and Kyle Horak, have established that amino-phenylpyrazolate ligands, which are more open and facilitate hydrogen bonding interactions, support oxo-bridged tetramanganese clusters bearing a MnIII–OH moiety (Scheme 1; [LMn3O(PzNHPh)3Mn(OH)][OTf]), which are competent for catalyzing electrochemical water oxidation to H2O2.19 Detailed examination of the PCET reactivity of this cluster was challenging, however, which may have been due, in part, to the acidity and possible redox non-innocence of the amino-phenylpyrazolate ligands. Other previous efforts within the Agapie group to examine PCET reactivity of clusters includes studies of related tri-iron-oxo/-hydroxo clusters bearing a pendant redox inactive metal, [LFe3O2M(OAc)2(DME)(OTf)]n+ (Scheme 1), by Dr. Davide Lionetti.20 Scheme 1. Previous Multinuclear Complexes Studied by the Agapie Group for PCET Reactivity19-20 58 Here, clusters with unsubstituted bridging pyrazolate ligands (Pz-) were synthesized to further promote intermolecular reactivity between apical Mn–OHx groups and external acids, bases, or hydrogen atom donors/acceptors. A heterometallic cluster composition, in this case [Fe3Mn], was targeted to provide a spectroscopic handle of metal oxidation states within the cluster, via 57Fe Mössbauer spectroscopy. The thermodynamic aspects of the PCET reactivity of these LFe3O(Pz)3Mn(OHx)n+ clusters were investigated through examination of the discrete electron and proton transfers taking place over multiple redox states. The results herein establish the significant influence redox changes at distal metal sites in a cluster have on a Mn– OHx motif and, conversely, how this motif’s protonation state can modulate the electron distribution between metals in the cluster. RESULTS AND DISCUSSION Synthesis and Characterization of Pyrazolate Bridged [Fe3Mn] Clusters. The [FeIII2FeIIMnII] cluster (2-[OTf]) can be prepared via one-pot synthesis, starting from previously reported [LFe3(OAc)(OTf)][OTf] complex.18a Sequential addition of Ca(OTf)2 (which serves to sequester the equivalent of acetate in the starting material, to avoid mixtures of counterions), potassium pyrazolate, iodosobenzene (PhIO), and manganese (II) trifluoromethanesulfonate bis-acetonitrile solvate (Mn(OTf)2 • 2 MeCN) allows for isolation of the desired complex (Figure 1C). 1H NMR and Mössbauer spectra of 2-[OTf] are similar to our previously reported [LFe3O(PhPz)3Mn][OTf]2 cluster which was synthesized using sodium phenyl pyrazolate, supporting the assignment that the apical metal is Mn (Figure 2).18e The structure of 2-[OTf] was confirmed by single crystal X-ray diffraction (see Figure 1A for isostructural 1-[OTf]); the cluster geometry is analogous to the substituted pyrazolate and imidizolate tetranuclear clusters, with a single µ4-interstitital ligand and pyrazolates bridging 59 Figure 1. (A) Crystal structure of tetranuclear Fe3Mn cluster 1-[OTf] with ellipsoids shown at the 50% probability level. Solvent molecules, outer-sphere counterions, and H atoms are omitted for clarity. (B) 1,3,5-triarylbenzene ligand platform (L3-). (C) Synthetic scheme of Fe3Mn clusters with triflate and -[BArF4] counterions. Conditions: (i) One-pot synthesis in THF with (1) Ca(OTf)2 (1 equiv., 60 min), (2) potassium pyrazolate (3.1 equiv., 20 min), (3) PhIO (1 equiv., 90 min), and (4) Mn(OTf)2 • 2 MeCN (1.3 equiv., 18 hr); (ii) CoCp2 (1 equiv.), THF, 60 min; (iii) Na/Hg (2.6 equiv. Na), THF, 4 hr; (iv) AgOTf (1 equiv.), THF, 30 min; (v) Ag[BArF4] • 2 MeCN (2 equiv.), Et2O, 15 min; (vi) [AcFc][BArF4] (1 equiv.), THF, 10 min; (vii) Ag[BArF4] • 2 MeCN (1 equiv.), Et2O, 15 min. each Fe center of the tri-nuclear core to the apical Mn.18 In the case of the previously reported clusters, the apical metal typically adopts a four-coordinate, trigonal pyramidal geometry since the sterics of the substituted pyrazolate ligands disfavor binding of one of the triflate counterions to the apical metal. Here, the apical Mn is ligated by one triflate counterion with 60 a trigonal bipyramidal geometry, indicative of the increased steric accessibility of the apical metal with the unsubstituted pyrazolates. Figure 2. (A) 1H NMR (300 MHz) of 2-[OTf] in CD3CN. (B) Zero applied field 57Fe Mössbauer spectrum of 2-[OTf] (black dots) fit with three equal quadrupole doublets (gray line) with parameters: (i) δ = 1.12 mm/s, |ΔEq| = 2.93 mm/s (blue trace), (ii) δ = 0.47 mm/s, |ΔEq| = 0.58 mm/s (solid orange trace), and (iii) δ = 0.42 mm/s, |ΔEq| = 0.91 mm/s (dashed orange trace). Cyclic voltammetry (CV) data of 2-[OTf] in MeCN show a quasi-reversible oxidation at 0.11 V, a quasi-reversible reduction wave at -0.84 V, and an irreversible reductive process below -1.50 V (Figure 3; all potentials vs Fc/Fc+). The one electron reduced (1-[OTf]), and one electron oxidized (3-[OTf]) clusters were prepared via chemical reduction/oxidation of 2-[OTf] with cobaltocene (CoCp2) and silver trifluoromethanesulfonate (AgOTf), respectively. The X-ray crystal structures of these three compounds all have identical coordination modes for the metal centers (Figure 1A). Bond distances between the metals and the µ4-oxo are consistent with the redox processes taking place at the Fe centers, with the apical Mn maintaining a +2 oxidation state across the series 1-[OTf] – 3-[OTf] (Table 1). Mössbauer data corroborate these oxidation state assignments, and are similar to our previously characterized clusters.18a-e 61 FeII2FeIIIMnII FeIIFeIII2MnII FeIII3MnII Current FeII3MnII 100 µA -2.5 -1.5 -0.5 0.5 Potential (V, vs Fc/Fc+) 1.5 Figure 3. Cyclic voltammogram (green trace) of 2-[OTf] (2.8 mM) in MeCN and 100 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.5 V. (Red trace) Partial CV of 2-[OTf] of the quasi-reversible electrochemical features. 62 Table 1. Selected Bond Distances, 57Fe Mössbauer Parameters, and Oxidation State Assignments for Structurally Characterized Compounds Metal Center Fe1 Fe2, Fe3 Mn1 Fe1, Fe2 Fe3 Mn1 M–µ4-O1; (Mn1–O2) distance (Å) 1.912(2) 2.054(2), 2.112(2) δ (mm/s) 1-[OTf] 0.56 1.14, 1.13 |ΔEq| (mm/s) Assignment 1.32 3.51, 3.02 h.s. FeIII h.s. FeII 1.997(2); (2.249(2)) 1.951(2), 1.966(2) 2.097(2) 2.053(2); (2.167(3)) MnII 2-[OTf] 0.47, 0.42 0.58, 0.91 h.s. FeIII 1.12 2.93 h.s. FeII MnII 2-[OTf] (H2O) Fe1, Fe2 Fe3 Mn1 Fe1, Fe2, Fe3 Mn1 Fe1 Fe2, Fe3 Mn1 1.923(5), 1.984(5) h.s. FeIII 2.092(5) 2.064(5); (2.163(6)) h.s. FeII MnII 1.980(4), 1.982(4), 1.989(4) 3-[OTf] 0.44 0.80 2.107(4); (2.162(5)) 2.003(7) 2.126(7), 2.051(7) 1.838(8); (1.843(9)) h.s. FeIII MnII 6-[OTf] 0.53 1.09, 1.08 0.76 3.09, 2.58 h.s. FeIII h.s. FeII MnIII Preparation of Mn–OH2 and Mn–OH Clusters. Binding of water to these clusters was investigated; however, the coordination of triflate to the apical Mn complicates direct access to the Mn–OH2 moiety for all oxidation states of the cluster. The triflate ligand in 2-[OTf] is sufficiently labile to allow for isolation of the Mn–OH2 cluster as single crystals by slow diffusion of Et2O into a MeCN/5% H2O solution of the cluster, and its structure was confirmed via X-ray crystallography (2-[OTf] (H2O); Figure 4B). Attempts to obtain crystals of the analogous reduced Mn-OH2 cluster (1-[OTf] (H2O)) were unsuccessful; we postulate that the difficulty lies in poor crystallinity of the complex, as opposed to an inability to 63 coordinate H2O over triflate. Crystallization attempts of 3-[OTf] in MeCN/5% H2O solutions produced crystals of triflate coordinated clusters, demonstrating the complication of preparing Mn–OH2 clusters across these oxidation states with the triflate counterions. The structure of 2-[OTf] (H2O) displays H2O coordinated to the apical Mn, with a long Mn–O distance of 2.163(6) Ǻ, consistent with a MnII–OH2 assignment;13e, 21 furthermore, both triflate counterions are hydrogen bonding to each proton of the Mn–OH2 moiety through one of the sulfonate oxygen atoms (Oaquo–OOTf distances of 2.787 and 2.695 Ǻ). Figure 4. Truncated crystal structures of (A) 1-[OTf], (B) 2-[OTf] (H2O), and (C) 6-[OTf]. Ellipsoids are shown at the 50% probability level with solvent molecules, outersphere counterions, and hydrogen atoms (except for hydrogen atoms on O2) are omitted for clarity. To ensure that H2O remained coordinated to the cluster in solution, experiments were performed in THF, a less coordinating solvent than MeCN, and triflate counterions were replaced with the non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([BArF4]) anion. This was accomplished by reducing the dicationic cluster, 2-[OTf], with Na/Hg amalgam in THF to obtain the neutral all MII cluster, 4, as a blue solid (Figure 1C). Similar to the related neutral phenyl pyrazolate clusters,18a 4 is either insoluble or unstable in most organic 64 solvents, so its chemistry towards H2O was not pursued. Oxidation of 4 with 1 and 2 equiv of Ag[BArF4] • 2 MeCN affords 1-[BArF4] and 2-[BArF4], respectively (Figure 1C). The [FeIII3MnII] cluster, 3-[BArF4], was prepared by oxidation of 2-[BArF4] with acetyl-ferrocenium ([AcFc][BArF4]). All these clusters are highly soluble in THF and bind H2O under conditions where it is present in ~100 molar equivalents (Figures 5 - 7). Significant decomposition is observed when H2O concentrations above ~1000 equivalents were used; therefore, all the studies described herein were performed on ca. 2 mM of a cluster with -[BArF4] counterions in THF solution with 250 mM H2O, as these conditions displayed 1H NMR spectra consistent with complete or near complete binding of H2O to the apical Mn. 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O 500 equiv. H2O + ca. 150 equiv. DBU Figure 5. 1H NMR spectra (500 MHz) of 2mM [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]) in THF/C6D6 with various equivalents of H2O. Splitting of the peak at ~35 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. Addition of excess 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) leads to no significant change in the 1H NMR spectrum. 65 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O Figure 6. 1H NMR spectra (500 MHz) of 2mM [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) in THF/C6D6 with various equivalents of H2O. Coalescence of the two peaks at ~45 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O Figure 7. 1H NMR spectra (500 MHz) of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) in THF/C6D6 with various equivalents of H2O. The upfield shift of the peak at ~50 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. Deprotonation of the Mn–OH2 moiety in the [FeIII2FeIIMnII] cluster, 2-[BArF4], was accomplished by addition of 1 equivalent of a relatively strong organic base, 1,8diazabicyclo(5.4.0)undec-7-ene (DBU; pKa(THF) = 19.1)22, or by stirring a THF solution of 2[BArF4] over solid KOH for 30 minutes. Both reactions lead to the same species based on the 66 Figure 8. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]; top) and [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; bottom) in CD3CN. 1 H NMR and UV-Vis absorbance features, assigned to the Mn–OH cluster, 6-[BArF4]. Structural confirmation of this species was obtained by performing analogous reactions on 2[OTf] to prepare the triflate salt, 6-[OTf], which displays the same 1H NMR features (Figure 8). This species could be crystallized from MeCN solution by slow diffusion of Et2O, and was characterized via X-ray diffraction (Figure 4C). The structure of 6-[OTf] is similar to 2-[OTf] (H2O), displaying Mn binding to the hydroxide ligand with a trigonal bipyramidal geometry. Notably, the Mn–O distance to the hydroxide ligand is contracted by approximately 0.3 Å relative to 2-[OTf] (H2O) (1.843(9) vs 2.163(6) Å). Furthermore, the distance of the apical Mn to the interstitial µ4-O in the cluster is also shortened significantly (1.838(8) vs 2.064(5) Å in 2-[OTf] (H2O); Table 1); both of these observations are consistent with a MnIII–OH assignment.23 The Mn–OH and Mn–µ4-O distances of ~1.8 Ǻ are similar to the bond metrics observed in our previously reported hydroxide-bound tetramanganese cluster in the [MnIII2MnII2] oxidation state.19 There, the Mn–OH bond is slightly longer (1.872(2) Å) due to hydrogen bonding to the pendant tert-butyl-phenyl-aminopyrazolate ligands. The structural data for 6-[OTf] are consistent to an oxidation state assignment of [FeII2FeIIIMnIII] for the 67 cluster; corroborated by the Mössbauer spectrum of 6-[BArF4] (Figure 9B). Deprotonation of the MnII–OH2 in 2 to form 6 leads to rearrangement of the redox states of the metals in the cluster to produce a MnIII–OH site. Similar ‘valence tautomerizations’ have been observed in MnV(O)-corrole systems, where protonation or binding a Lewis acid to the oxo moiety leads to reversible formation of a MnIV(O–X)-(corrole-radical cation) complexes.24 Figure 9. (A) Mössbauer spectrum of 5 (black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.12 mm/s; ΔEq = 3.40 mm/s (solid blue trace), (ii) δ = 1.12 mm/s; ΔEq = 2.95 mm/s (dashed blue trace), and (iii) δ = 1.08 mm/s; ΔEq = 2.42 mm/s (dotted blue trace). (B) Mössbauer spectrum of 6-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.09 mm/s; ΔEq = 3.09 mm/s (solid blue trace), (ii) δ = 1.08 mm/s; ΔEq = 2.58 mm/s (dashed blue trace), and (iii) δ = 0.53 mm/s; ΔEq = 0.76 mm/s (solid orange trace). (C) Mössbauer spectrum of 7-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.10 mm/s; ΔEq = 3.03 mm/s (solid blue trace), and (ii) δ = 0.43 mm/s; ΔEq = 0.55 mm/s (solid orange trace), (iii) δ = 0.46 mm/s; ΔEq = 1.02 mm/s (dashed orange trace). 68 Electrochemistry. The electrochemistry of the [Fe3OMn–OH2] and [Fe3OMn–OH] clusters was investigated by cyclic voltammetry of 2-[BArF4] and 6-[BArF4]. Two quasireversible redox events were observed in 2-[BArF4]: an oxidation at -0.02 V and a reduction at -0.90 V (all potentials vs Fc/Fc+; Figure 10, red trace). These redox events were assigned to the [FeIII2FeIIMnII]→[FeIII3MnII] and [FeIIIFeII2MnII]→[FeIII2FeIIMnII] couples. Mössbauer spectra collected on 1-[BArF4] – 3-[BArF4] in THF with 250 mM H2O show that both oxidation state changes occur at the Fe centers (Figure 11), leading to the conclusion that the apical Mn remains divalent when bound to H2O across all the oxidation states observed in the CV experiment, and only the distal Fe centers change oxidation states. The hydroxide-bound cluster, 6-[BArF4], displays two oxidations: a quasi-reversible couple at -0.49 V ([FeIIIFeII2MnIII]→[FeIII2FeIIMnIII]), and an ([FeIII2FeIIMnIII]→[FeIII3MnIII]). A quasi-reversible irreversible event at reduction +0.26 V for the [FeIIIFeII2MnIII]→[FeII3MnIII] couple is also observed at -1.34 V. The Mössbauer spectra of 5 7-[BArF4] (Figure 9) are again consistent with redox changes occurring exclusively at Fe. Notably, no catalytic oxidation wave is observed at high potentials for 6-[BArF4], in contrast to our previously reported tetramanganese cluster bridged with tert-butyl- phenylaminopyrazolates.19 Reasons for this difference may be the ~100 mV negative shift in reduction potentials for the [Fe3Mn–OH] clusters, along with the lower concentration of H2O. The lack of electrocatalytic oxidation by 6-[BArF4] could also suggest the importance of pendant basic groups near the MnIII–OH moiety for water oxidation catalysis. 69 Figure 10. Cyclic voltammograms of 2-[BArF4] (red trace) and 6-[BArF4] (green trace); 2 mM compound in THF with 250 mM H2O and 100 mM [nPr4N][BArF4] at a scan rate of 50 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrodes, respectively. The open circuit potentials were -0.2 V for 2-[BArF4] and -1.0 V for 6-[BArF4]. Figure 11. (A) Mössbauer spectrum of 1-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.12 mm/s; ΔEq = 3.46 mm/s (solid blue trace), (ii) δ = 1.10 mm/s; ΔEq = 2.86 mm/s (dashed blue trace), and (iii) δ = 0.57 mm/s; ΔEq = 1.27 mm/s (solid orange trace). (B) Mössbauer spectrum of 2-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: 70 (i) δ = 1.13 mm/s; ΔEq = 2.86 mm/s (solid blue trace), (ii) δ = 0.49 mm/s; ΔEq = 1.00 mm/s (solid orange trace), and (iii) δ = 0.48 mm/s; ΔEq = 0.546 mm/s (dashed orange trace). (C) Mössbauer spectrum of 3-[BArF4] (THF solution [250 mM H2O]; black dots) fit with a single quadrupole doublet (orange trace) with parameters: δ = 0.44 mm/s and ΔEq = 0.80 mm/s Determination of pKa of H2O Ligand in [Fe3MnII–OH2] Clusters. The pKa of the aquo-ligand bound to 2-[BArF4] was measured by mixing this cluster with various concentrations of 1,1,3,3-tetramethyl-2-phenylguanidine (PhTMG; pKa(THF) = 16.5).22 The ratio of 2-[BArF4] and 6-[BArF4] was examined by UV-Vis absorbance spectroscopy as a function of PhTMG concentration, and a pKa value of 17.5 for 2-[BArF4] was obtained (Figure 12). Analogous experiments were attempted on the oxidized aquo-cluster, 3-[BArF4]; however, the changes in UV-Vis spectral features upon deprotonation are minor. The pKa of 3-[BArF4] could be obtained by examining its 1H NMR resonances in the presence of a suitable exogenous base, 2,6-Me2-pyridine (Figure 13; pKa(THF) = 9.5).22 As expected, the acidity of the [Fe3Mn–OH2] cluster increases upon oxidation; a pKa value of 9.2 was obtained for 3[BArF4]. While a titration on the reduced [FeIIIFeII2MnII] cluster, 1-[BArF4], was not conducted, we determined that its pKa is significantly higher than the other clusters investigated, since it does not react with excess DBU (Figure 5; pKa(THF) = 19.1).22 71 A) 0.8 B) 0.7 Absorbance (a.u.) 0.6 0.5 0.4 4.5 [6-[BArF4]][[PhTMGH][BArF4]]/ [2-[BArF4]] (M, 10-5) 0.5 equiv. 1 equiv. 2 equiv. 5 equiv. 10 equiv. 15 equiv. 0.3 0.2 0.1 4 y = 0.0899x R² = 0.8892 3.5 3 2.5 2 1.5 1 0.5 0 0 300 400 500 Wavelength (nm) 600 0 0.0002 0.0004 0.0006 [PhTMG] (M) Figure 12. (A) UV-Vis absorbance spectra of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; 1 cm cuvette; 100μM) in THF [250 mM H2O] after addition of various equivalents of 1,1,3,3tetramethyl-2-phenylguanidine (PhTMG; pKa(THF) = 16.5).22 (B) Titration plot for deprotonation of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) to [LFe3O(Pz)3Mn(OH)][BArF4] (6[BArF4]) based on multiple titrations; the slope of the line represents an equilibrium constant value of K = 0.09, where: 𝐾= [𝟔– [𝐁𝐀𝐫𝟒𝐅 ]][(PhTMGH)([BAr4F ])] [𝟐– [𝐁𝐀𝐫𝟒𝐅 ]][PhTMG] 3-[BArF4] 3-[BArF4]+ 0.5 2,6-Me2-pyridine 3-[BArF4]+ 1 2,6-Me2-pyridine 3-[BArF4]+ 2 2,6-Me2-pyridine 3-[BArF4]+ 3 2,6-Me2-pyridine 7-[BArF4] Figure 13. 1H NMR spectrum (500 MHz) of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) with various equivalents of 2,6-dimethyl-pyridine in THF/C6D6 [250 mM H2O]. 72 Table 2. pKa titration of 3-[BArF4] via 1H NMR spectroscopy with 2,6-dimethylpyridine. 1H NMR Χ3-[BArF4]b Kc 0.632 1.625 0.343 3.661 0.319 1.100 0.296 0.730 Average K 1.779 (±0.654) a A sharp resonance ~ 11 ppm was selected to measure the mole fraction of 3-[BArF4] (Χ3-[BArF4]), as 3-[BArF4] and 7-[BArF4] undergo fast exchange on the NMR time-scale. b The concentrations of 3-[BArF4], 2,6-Me2-Py, and [2,6-Me2-PyH][BArF4] were determined from Χ3-[BArF4] via mass balance. cAccording to the equation below: [𝟕– [𝐁𝑨𝒓𝑭𝟒 ]][(2,6– Me2 – PyH)([B𝐴𝑟4𝐹 ])] 𝐾= [𝟑– [𝐁𝑨𝒓𝑭𝟒 ]][2,6– Me2 – Py] 3-[BArF4] 3-[BArF4] + 0.5 2,6-Me2-Py 3-[BArF4] + 1 2,6-Me2-Py 3-[BArF4] + 2 2,6-Me2-Py 3-[BArF4] + 3 2,6-Me2-Py 7-[BArF4] δa (ppm) 11.12 10.90 10.72 10.70 10.69 10.51 BDEO–H and PCET Reactivity of the Different Redox States. The homolytic bond dissociation enthalpy of the aquo O–H (BDEO–H) were determined for the three cluster oxidation states observed (1-[BArF4] – 3-[BArF4]) by analyzing the pKa and reduction potentials of the aquo- and hydroxide-bound clusters (Figure 14). This calculation combines the energies of the discrete proton and electron transfers involved, along with the energy of formation of the hydrogen atom in THF (C; 66 kcal/mol25):26 BDEO–H (kcal/mol) = 1.37 pKa + 23.06 E° + C (1) Therefore, summing the energy of the oxidation of 1-[BArF4] (-0.90 V; -20.6 kcal/mol) and the energy of deprotonation of 2-[BArF4] (17.5; 24.0 kcal/mol) with C establishes an energy of 69 kcal/mol for the formal H-atom transfer from 1-[BArF4] to 6-[BArF4]. Likewise, the oxidation of 2-[BArF4] (-0.02 V; -0.5 kcal/mol) followed by the deprotonation of 3-[BArF4] (9.2; 12.6 kcal/mol) leads to a BDEO–H of 78 kcal/mol for the aquo-ligand of 2-[BArF4] (formal HAT to form 7-[BArF4]). An alternate way to determine the BDEO–H of 2-[BArF4] is from the pKa of 2-[BArF4] and the reduction potential of 6-[BArF4], leading to a similar bond enthalpy of 78.7 kcal/mol. The same square scheme analysis can be done to obtain a BDEO-H 73 Figure 14. Thermodynamic cycles to evaluate the BDEO–H of Mn–OH2 clusters 1-[BArF4] 3-[BArF4]. Reduction potentials (horizontal lines) are referenced to Fc/Fc+. pKa values (vertical lines) are based on relative pKa values of acids in THF. Diagonal lines are the BDEO– H values calculated from the reduction potential and pKa according to the Bordwell equation (equation 1). Approximate values (~) are extrapolated from the Bordwell equation. of 85 kcal/mol for 3-[BArF4]. With these measured values, the pKa of 1-[BArF4] could be estimated; a BDEO–H of 69 kcal/mol for 1-[BArF4] means the enthalpy of deprotonation for this cluster is expected to be ~34 kcal/mol (pKa = 24.9), by combining this energy with the oxidation of 5 (-1.34 V; -30.9 kcal/mol). The BDEO–H values of these clusters were evaluated by investigating their proton-coupled electron transfer (PCET) reactivity towards different organic radicals. The PCET reagents employed were (2,2,6,6,-tetramethylpiperidin-1yl)oxyl (TEMPO; BDEO–H = 70 kcal/mol) and 2,4,6-tri-tert-butylphenoxy radical (2,4,6-TBPR; BDEO–H = 82 kcal/mol).26c Formal hydrogen atom transfer from 1-[BArF4] to form 6-[BArF4] could be accomplished using either one equivalent of TEMPO or 2,4,6-TBPR, consistent with a BDEO–H less than 70 kcal/mol 74 (Scheme 2). 2-[BArF4] reacts with 1 equivalent 2,4,6-TBPR to form 7-[BArF4], but no reaction is observed between this cluster and TEMPO, indicative of a BDEO–H that is between 70 and 82 kcal/mol. 3-[BArF4] does not react with either PCET reagent, which supports the assignment of a BDEO–H greater than 82 kcal/mol). Scheme 2. Proton Coupled Electron Transfer (PCET) Reactions of Mn–OH2 clusters, 1-[BArF4] - 3-[BArF4]. 75 Potential–pKa Diagram of [Fe3Mn–OHx] Clusters. Further insight into the basis of PCET reactivity of these clusters was obtained by investigating the effect of external bases on the reduction potentials of the aquo- and hydroxide-bound clusters. Typically, this type of analysis is conducted under aqueous conditions, where the reduction potentials can be measured as a function of solution pH; data are presented as a potential–pH plot, known as a Pourbaix diagram.27 Aqueous Pourbaix diagrams have been helpful in understanding the speciation of a number of molecular Ru/Mn water oxidation catalysts and related complexes.15b, 28 Recently, Pourbaix theory has been applied to nonaqueous solvents, where the reduction potential of PCET will depend on the pKa of an external acid/base (and the concentration of this acid/base relative to its conjugate base/acid at Nernstian equilibrium).29 For the system reported here, a potential– pKa plot was constructed as a means of providing experimental support for the aquo-ligand pKa and BDEO–H values of 1-[BArF4] - 3-[BArF4] and to gain information about the thermochemistry of PCET with the Mn–OH clusters to form a terminal Mn-oxo moiety. Measuring the CV of 2-[BArF4] with one equivalent of various organic bases, with pKa values of their conjugate acids in THF ranging from 7.5 to 28.1, produced the potential–pKa plot depicted in Figure 15 (blue points; see Electrochemical Details section for individual CVs). For relatively weak bases (pKa < 10), the reduction potentials of 2-[BArF4] do not significantly deviate from their potentials in the absence of any base. As the strength of the base increases, the reduction potential for the oxidation of 2-[BArF4] is lowered as a function of the conjugate acid pKa, consistent with PCET occurring between the pKa range 10-17. The data points in this pKa range follow the diagonal line calculated for the 2-[BArF4]→7-[BArF4] PCET process, based on the reduction potentials of 2-[BArF4] and 6-[BArF4] and the pKa values of 2-[BArF4] and 3-[BArF4]. Observing the predicted linear decrease in reduction 76 potential for 3-[BArF4] in the pKa range 10 - 17 supports the pKa values reported for 2-[BArF4] and 3-[BArF4] in Figure 14. Similar support is given to the pKa of 1-[BArF4] (24.9) when using strong bases (pKa > 17.5). Evidence for the 1-[BArF4]→6-[BArF4] PCET process was observed under these conditions, again with the data roughly matching the diagonal line with an intercept at -1.34 V and a pKa of 24.9. Furthermore, when a base was employed with a pKa > 24.9, the reduction potentials observed were nearly identical to the potentials reported for 6-[BArF4] in the absence of any acid or base. Figure 15. Potential–pKa diagram of 2-[BArF4] (blue squares) and 6-[BArF4] (green squares). Data points are the observed reduction potentials of the compounds (y-axis) in the presence of one equivalent base with various pKa values in THF (x-axis). The horizontal lines correspond to the measured reduction potentials of 2-[BArF4] and 6-[BArF4] in the absence of any external base. Vertical lines correspond to the pKa values of Mn–OH2 clusters 1-[BArF4] - 3-[BArF4]. The horizontal line for the 7-[BArF4]→8-[BArF4] redox couple is dashed at high pKa values due to the possibility of PCET from the Mn–OH with a strong enough base. 77 As expected, deviations of the data from the calculated diagonal line occur as the base pKa approaches the pKa of the cluster (see 2-[BArF4]→7-[BArF4] around pKa of 10, for example), based on predicted potential–pKa relationships for ET-PT or PT-ET reaction mechanisms.29 Further deviations from the predicted solid lines in Figure 15 could be due to incompatibility of the chosen base with this system (too coordinating, electrochemically unstable, etc.). Ultimately, inconsistencies between the potential–pKa data of 2-[BArF4] and the BDEO–H values reported in Figure 14 only amount to a difference of ~3 kcal/mol, which is a reasonable uncertainty for these bond energydeterminations.26c Based on the PCET reactivity of these complexes towards TEMPO and TBPR (vide supra), it is unlikely that these BDEO–H values could deviate more than a few kcal/mol. Potential–pKa data were also obtained for 6-[BArF4] in the presence of relatively strong organic bases in attempts to observe a PCET process accessing Mn=O clusters, since this technique has been previously useful for gaining insight into the proton and electron transfer thermodynamics for unstable species.29 Based on the potential–pKa plot constructed in Figure 15, PCET to form a Mn-oxo cluster could be possible at high potentials with a strong base (top right area of the diagram). The CV of 6-[BArF4] with one equivalent tert-butyliminotri(pyrrolidino)phosphorene (t-BuP1(pyrr); pKa(THF) = 22.8)22 shows no shift in the Mn–OH cluster reduction potentials. Similarly, no change is observed with the 5→6-[BArF4] and 6[BArF4]→7-[BArF4] reduction potentials with 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (EtP2(dma); pKa(THF) = 28.1).22, 30 These experimental observations provide a lower limit to the pKa values of 8-[BArF4] and 7-[BArF4], respectively. With these values, the BDEO–H of 7-[BArF4] and 8-[BArF4] are predicted to be greater than 93 and 103 kcal/mol, respectively (Figure 16).31 These BDEO–H estimates are higher than Mn complexes, where these bond strengths have been reported.11c, 12d, 12i, 12k The large BDEO–H 78 values for these hydroxide clusters suggest that if these terminal oxo moieties could be accessed, they would be highly reactive. Indeed, previous attempts to generate a terminal oxo species on related phenyl-pyrazolate bridged multimetallic clusters results in activation of strong bonds, although the identity of the reactive intermediate in these reactions could not be established (terminal metal-oxo or iodosylbenzene adduct).18d-f Figure 16. Estimated BDEO–H values for MnIII–OH clusters 6-[BArF4] and 7-[BArF4] based on their oxidation potentials and lower bound of their pKa (where no PCET occurs in their CV with an external base). The metal oxidation states of the resulting putative Mn-oxo clusters are left ambiguous since multiple oxidation state distributions are possible. CONCLUSIONS We have reported the synthesis of tetranuclear [Fe3Mn–OHx] clusters bearing bridging unsubstituted pyrazolate ligands, leading to a sterically open coordination environment around the apical Mn center. These clusters were used to investigate the implications of distal metal redox changes on the activation of water by Mn in terms of the aquo-cluster pKa, reduction potential, and homolytic O–H bond strength. Increasing the oxidation state of a distal Fe 79 center by one increases the acidity of the aquo ligand by ~7 pKa units (in THF), while raising the BDEO–H 8 kcal/mol, on average. By only changing the redox states of the distal Fe centers, a wide range of BDEO–H values could be measured for the Mn–OH2 moiety (69 – 85 kcal/mol), which nearly spans the range of the reported BDEO–H measured in reported in mononuclear Mn–OH2 complexes.11c, 12e, 12f, 12i The three different oxidation states of the aquo-cluster (1[BArF4] – 3-[BArF4]) underwent PCET reactions with TEMPO and 2,4,6-TBPR consistent with their measured BDEO–H values. The increase in BDEO–H of ~8 kcal/mol by increasing distal Fe oxidation state is similar to the increases that have been observed in mononuclear Mn systems where BDEO–H studies could be accomplished over multiple Mn oxidation states.12d, 12i This is in contrast to the previous example of a binuclear Mn system, where Mn– OH2 BDEO–H values could be measured over three oxidation states, where small changes in the bond strength of ~ 4 kcal/mol for the MnIIIMnIV–OH2 complex versus MnIII2–OH2 were observed.15a Importantly, the large effect of the remote metals on the BDEO–H demonstrates that the cluster as a whole has a significant impact in the activation of substrate water molecules. The range of BDEO–H reported here is achieved without change in the redox state of Mn; therefore, consideration of the effect of metal centers not directly supporting the substrate must be taken into account for multimetallic biological active sites as well as synthetic clusters. Additionally, the present findings demonstrate that, in comparison to monometallic complexes, transition metal clusters not only provide the possibility of increased storage of redox equivalents, but also can serve to dynamically tune reactivity through remote oxidation state changes. 80 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified; reactions of compounds in THF/H2O mixtures were performed in an N2-filled VAC wetbox. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. [LFe3(OAc)(OTf)][OTf]18a, Mn(OTf)2 • 2 MeCN32, benzyl potassium33, iodosobenzene34, silver (Ag[BArF4] MeCN)35, • 2 tetrakis[3,5-bis(trifluoromethyl)phenyl]borate 2,4,6-tri-tert-butylphenoxy radical bis-acetonitrile (2,4,6-TBPR)36, and tetrapropylammnoium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([nPr4N][BArF4])37 were prepared according to literature procedures. All organic solvents were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H and 19F NMR spectra were recorded on a Varian 300 MHz spectrometer. 1H NMR spectra in THF/C6D6 were recorded on a Varian 500 MHz spectrometer using solvent suppression protocols. CD3CN, CD2Cl2, and C6D6 were purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freezepump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated solution in MeCN or THF. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). 81 Mössbauer simulation details for all compounds. All spectra were simulated by three pairs of symmetric quadrupole doublets with equal populations and Lorentzian lineshapes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts of ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.17a, 18a, 18d, 18e The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.38 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter) and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Either the ferrocene/ferrocinium (Fc/Fc+) or decamethylferrocene/ decamethylferrocinium (Fc*/Fc*+; -0.524 V vs Fc/Fc+ in THF/250 mM H2O, under our experimental conditions) redox waves were used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of the compounds most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. 82 Synthetic Procedures. Synthesis of Potassium pyrazolate (KPz). 1.09 g (16.0 mmol) pyrazole was dissolved in 2 mL THF. To this stirring solution, a 10 mL THF solution of benzyl potassium, 2.03 g (15.6 mmol), was added dropwise; an off-white precipitate formed. After stirring for 20 minutes, the reaction was concentrated to 10 mL; the solids were collected on a glass frit and washed with 2 mL THF. The white solid was dried completely under vacuum to obtain 1.37 g (83% yield) potassium pyrazolate. Anal. calcd. (%) for C3H3KN2: C. 33.94; H, 2.85; N, 26.39. Found: C, 34.12; H, 2.89; N, 25.38. Synthesis of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). A suspension of 387 mg (0.28 mmol) [LFe3(OAc)(OTf)][OTf] in 7 mL THF was stirred with 98.4 mg (0.29 mmol) Ca(OTf)2 for an hour before being frozen with LN2. To this mixture, 93.2 mg (0.88 mmol) KPz was added in thawing THF (4 mL) and stirred for 20 minutes at room temperature to obtain a dark redorange solution. Iodosylbenzene, 63.6 mg (0.29 mmol), was added with 1 mL THF and the reaction was stirred for 90 minutes. 160 mg (0.37 mmol) Mn(OTf)2 • 2 MeCN solution in 2 mL THF was then added to the reaction. After 18 hours, the reaction was concentrated to 10 mL and filtered over a bed of celite; the precipitate was dried under vacuum, extracted with 8 mL DCM, and recrystallized via vapor diffusion of Et2O into the filtrate. Dark green crystals of 2-[OTf] were collected on a glass frit and dried (147 mg, 33% yield). Another 69 mg of 2[OTf] can be obtained by drying the crude reaction filtrate, extracting with 6 mL DCM and recrystallizing via Et2O vapor diffusion (46% overall yield). X-ray diffraction quality crystals were obtained via oxidation of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) with 1 equivalent of AgBPh4; Et2O vapor diffusion into a DCM/THF solution of the resulting [LFe3O(Pz)3Mn][OTf][BPh4] produced crystals of suitable quality. 1H NMR (300 MHz, CD3CN): δ 120.8 (br), 80.8 (br), 71.0, 70.1, 52.9, 52.3, 42.2, 28.0 (br), 15.5, 13.0, 10.4, 8.1 (br), 4.38, 3.01, -2.51 (br). UV-Vis 83 (MeCN) [ε (M-1 cm-1)] 241 nm (6.53 104), 368 nm (6.49  103). Anal. calcd. (%) for C68H48F6Fe3MnN12O10S2: C. 51.25; H, 3.04; N, 10.55. Found: C, 50.81; H, 3.12; N, 10.18. Synthesis of [LFe3O(Pz)3Mn][OTf] (1-[OTf]). A suspension of [LFe3O(Pz)3Mn][OTf]2 (2[OTf]; 91.5 mg, 0.057 mmol) in 2 mL THF was stirred as a THF solution of 10.9 mg CoCp2 (0.058 mmol) was added. After 1 hour, the reaction was dried under vacuum. 4 mL DME was added to the purple solid and stirred for 12 hours. The resulting purple precipitate was collected on a bed of celite, washed with 2 mL DME, dried, and eluted with 2:1 THF/MeCN; crystals of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) were obtained by vapor diffusion of Et2O into this solution (46.3 mg, 56% yield). 1H NMR (300 MHz, CD3CN): δ 96.4 (br), 57.8, 55.5, 37.8 (br), 36.4, 34.3, 34.0, 25.2, 13.4, 13.0, 12.0, 11.4, 3.4, 2.6, -6.4 (br). UV-Vis (MeCN) [ε (M-1 cm1 )] 250 nm (6.08  104), 517 nm (3.72  103). Anal. calcd. (%) for C67H48F3Fe3MnN12O7S: C, 55.70; H, 3.35; N, 11.63. Found: C, 55.36; H, 3.58; N, 11.20. Synthesis of [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]). 9.2 mg (0.036 mmol) of AgOTf in THF was added to a stirring suspension of 56.8 mg (0.036 mmol) [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]) in THF. The resulting brown suspension was pumped down after 30 minutes. The reaction was filtered over a celite pad using DCM and the solvent was removed under reduced pressure. Crystals of [LFe3O(Pz)3Mn][OTf]3 were obtained via vapor diffusion of Et2O into a concentrated DCM/MeCN solution of the crude product, 57.4 mg (92% yield). 1H NMR (300 MHz, CD2Cl2): δ 162.2 (br), 118.9 (br), 81.2, 76.9, 74.4, 73.1, 45.7, 18.8 (br), 16.3, 9.5, 3.34, 1., -6.5 (br). UV-Vis (MeCN) [ε (M-1 cm-1)] 241 nm (7.84  104), 411 nm (9.22  103). Anal. calcd. (%) for C69H48F9Fe3MnN12O13S3: C, 47.55; H, 2.78; N, 9.64. Found: C, 47.57; H, 3.07; N, 9.21. Synthesis of [LFe3O(Pz)3Mn] (4). 4.1 mg (0.18 mmol) sodium metal was mixed ~6 g elemental mercury with a pre-reduced stirbar. After 12 hours, a 5 mL THF suspension of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; 114 mg, 0.07 mmol) was added to the Na/Hg amalgam. 84 Over 4 hours, a blue precipitate formed; this resulting suspension was decanted from the amalgam and filtered over a fine porosity glass frit. The solids were washed with 5 mL THF and dried under vacuum. The resulting blue material, [LFe3O(Pz)3Mn] (78.1 mg; 84% yield), is insoluble or unstable in most typical organic solvents. Anal. calcd. (%) for C66H48Fe3MnN12O4: C. 61.18; H, 3.73; N, 12.94. Found: C, 60.44; H, 3.82; N, 12.87 Synthesis of [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]). 14.0 mg (0.013 mmol) Ag[BArF4] • 2 MeCN in 2 mL Et2O was added to a stirring suspension of [LFe3O(Pz)3Mn] (4; 17.2 mg, 0.013 mmol); the blue suspension changed to a purple solution. After 15 minutes, the solvent was removed under reduced pressure. 3 mL Et2O was added to the purple residue and filtered over a pad of celite. The filtrate was dried to afford [LFe3O(Pz)3Mn][BArF4] as a purple solid, 26.5 mg (92% yield). 1H NMR (300 MHz, CD3CN) is identical to [LFe3O(Pz)3Mn][OTf] (1-[OTf]). Anal. calcd. (%) for C98H60BF24Fe3MnN12O4: C. 54.52; H, 2.80; N, 7.79. Found: C, 54.06; H, 2.84; N, 7.33. Synthesis of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]). 45.0 mg (0.043 mmol) Ag[BArF4] • 2 MeCN in 2 mL Et2O was added to a stirring suspension of [LFe3O(Pz)3Mn] (4; 27.6 mg, 0.021 mmol); the blue suspension changed to a brown-green solution. After 15 minutes, the solvent was removed under reduced pressure. 3 mL Et2O was added to the brown residue and filtered over a pad of celite. 6 mL benzene was added to the filtrate to produce an oily precipitate; after 30 minutes, the supernatant was removed and the remaining brown-green residue was dried under reduced pressure. 36.6 mg (57% yield) of the brown-green solid, 2-[BArF4], was obtained; the 1H NMR (300 MHz, CD3CN) is identical to [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 368 nm (5.11 103). Anal. calcd. (%) for C130H72B2F48Fe3MnN12O4: C. 51.67; H, 2.40; N, 5.56. Found: C, 51.38; H, 2.56; N, 5.46. 85 Synthesis of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]). 6.4 mg (0.006 mmol) [AcFc][BArF4] in 0.5 mL THF was added to [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; 18.6 mg, 0.006 mmol). After 10 minutes, 5 mL benzene was added to the solution to produce an oily brown precipitate; after 30 minutes, the yellow supernatant was removed and the remaining brown residue was dried under reduced pressure. 18.4 mg of a brown solid was obtained (77% yield). 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 83.8, 78.2, 75.9, 50.2, 24.9 (br), 16.6, 9.8 (br), 0.1. UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 405 nm (7.64 103). Anal. calcd. (%) for C162H84B3F72Fe3MnN12O4: C. 50.08; H, 2.18; N, 4.33. Found: C, 50.34; H, 2.38; N, 4.29. Synthesis of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]). Addition of 100 µL of a 50 mM solution of DBU in THF/250 mM H2O to 2 mL 2 mM solution of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) in THF/250 mM H2O leads to a color change of the solution from green to red. Crystals for X-ray diffraction (6-[OTf]) were obtained by conducting the analogous reaction with [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]) and DBU in 95:5 MeCN/H2O and crystalizing via vapor diffusion of Et2O into this solution; considerable decomposition occurs on the timescale of crystallization, making crystallization unsuitable for preparing analytically pure solid samples of [LFe3O(Pz)3Mn(OH)][OTf]. Solutions of [LFe3O(Pz)3Mn(OH)][BArF4] were prepared for electrochemistry experiments by stirring 4 mL of 2.5 mM [LFe3O(Pz)3Mn][[BArF4]]2 (2-[BArF4]) and 0.1 M [nPr4N][BArF4] solution in THF/250 mM H2O with ~2 mg of solid KOH pellet for 1 hour; the resulting red solution was decanted off the remaining KOH before electrochemical measurements were conducted. 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 153.1 (br), 102.7 (br), 85.9, 80.0, 64.8, 60.8, 58.1, 57.3, 23.0, 15.7. 12.5, 10.9 (br). UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 467 nm (3.29  103). Synthesis of [LFe3O(Pz)3Mn(OH)] (5). Addition of 11 mg (0.03 mmol) decamethylcobaltocene in THF/250 mM H2O to 4 mL 7 mM solution of 86 [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; 0.03 mmol) in THF/250 mM H2O leads to a color change of the solution from red to blue. The reaction was pumped down after 30 minutes. 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 126.0 (br), 76.1 (br), 59.6, 49.0, 46.9, 42.7, 37.0, 23.9 (br), 17.2, 15.6, 12.8, -14.9. Synthesis of [LFe3O(Pz)3Mn(OH)][BArF4]2 (7-[BArF4]). Method A. Addition of 160 µL of a 50 mM solution of Et3N in THF/250 mM H2O to 2 mL 2 mM solution of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) in THF/250 mM H2O leads to a color change of the solution from brown to brown-green. 1H NMR spectroscopy confirms complete conversion to [LFe3O(Pz)3Mn(OH)][BArF4]2. Method B. Addition of 200 μL of 6 mM solution of Ag[BArF4] • 2 MeCN in THF/250 mM H2O to 400 μL of a 3mM solution of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]) in THF/250 mM H2O leads to formation of a grey precipitate. Filtration of this solution yields a brown-green solution with an identical 1H NMR obtained from Method A. 1H NMR (500 MHz, THF/C6D6 [250 mM H2O]: δ 110.2 (br), 89.1, 85.1, 70.0, 67.2, 62.0, 19.1 (br), 15.7, 13.1, 9.8 (br), 8.6 (br), 6.2 (br), 1.1, 0.7, 0. UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 389 nm (5.29  103). 87 ELECTROCHEMICAL DETAILS Current 50 µA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2 -1.5 -1 -0.5 0 0.5 Potential (V, vs Fc/Fc+) 1 1.5 Figure 17. Cyclic voltammograms of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf], 2.8 mM) in MeCN and 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as Current working, counter, and reference electrode, respectively. 20 µA -2 -1.5 -1 -0.5 0 0.5 + Potential (V, vs Fc/Fc ) 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 1 1.5 88 Figure 18. Cyclic voltammograms of [LFe3O(Pz)3Mn(OH2)][BArF4]2 (2-[BArF4], 2.1 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] at various scan rates with glassy carbon, Pt- Current wire, and Ag-wire as working, counter, and reference electrode, respectively. 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 20 μA -2 -1.5 -1 -0.5 0 0.5 Potential (V, vs Fc/Fc+) 1 1.5 Figure 19. Cyclic voltammograms of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Table 3. Peak-to-peak separation (ΔEp; mV) and peak area ratio (Aa/c) for the redox couples in 2-[OTf], 2-[BArF4], and 6-[BArF4]. Redox Couple Assignment [FeII2FeIIIMnII]→ [FeIIFeIII2MnII] [FeIIFeIII2MnII]→ [FeIII3MnII] [FeII2FeIIIMnII]→ [FeIIFeIII2MnII] [FeIIFeIII2MnII]→ [FeIII3MnII] [FeII3MnIII] → [FeII2FeIIIMnIII] [FeII2FeIIIMnIII] → [FeIIFeIII2MnIII] [FeIIFeIII2MnIII] → [FeIII3MnIII] Epa (mV) Epc(mV) 2-[OTf] -799 -66 2-[BArF4] -958 -831 -93 44 6-[BArF4] -1,406 -1,274 -548 -426 172 354 -882 -154 ΔEp(mV) Aa (μW) Ac (μW) Aa/Ap 83 88 22.0 11.4 18.5 12.2 1.2 0.9 127 137 7.9 2.6 2.8 2.5 2.8 1.0 132 122 182 6.6 4.2 1.6 2.6 3.6 3.0 2.5 1.2 0.5 89 Constructing the Potential – pKa Diagram of [LFe3O(Pz)3Mn(OHx)] Clusters. Cyclic voltammetry was performed on ~2 mM solutions of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]), or [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]) with glassy carbon working, Pt wire counter, and Ag wire reference electrodes in THF [250 mM H2O] and ca. 100 mM [nPr4N][BArF4]. After collecting a blank CV, and CV of the cluster, one equivalent of a base was added by injecting a concentrated solution of it to the cluster solution and mixing via pipette. It was observed that quasi-reversible waves corresponding to PCET could be observed best at slow scan rates (< 200 mV/s) for all bases tested; faster scan rates led to loss of a return wave for the PCET. We postulate that proton transfer in these experiments is slow relative to the time scale of electrochemistry. With some bases, redox events for the PCET and fully protonated/deprotonated cluster could be observed simultaneously; we propose that this is due to a lower local concentration of base at the electrode surface, or slow proton transfer kinetics. For all measurements reported, it is assumed that half an equivalent of available base is consumed at the electrode at the PCET E½ potential; making the observed potential based only on the redox potential of the Mn–OHx cluster, and the pKa of the added base.29 All THF pKa values used here were obtained from a report by Rosés and co-workers.22 90 Base with 2-[BArF4] pKa (THF) E½(1) (V) 2-methyl-aniline 7.5 -0.885 2-methyl-pyridine 2,6-dimethyl-pyridine 2,4,6-trimethyl-pyridine triethylamine 2-phenyl-1,1,3,3-tetramethylguanidine 1,1,3,3-tetramethylguanidine 1,8-diazabicyclo[5.4.0]undec-7-ene 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene 1,5,7-triazabicyclo[4.4.0]dec-5-ene tert-butylimino-tri(pyrrolidino)phosphorane 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) Base with 6-[BArF4] tert-butylimino-tri(pyrrolidino)phosphorane 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) 8.6 9.5 10.4 14.9 16.5 17.8 19.1 20.5 22.0 22.8 28.1 22.8 28.1 E½(3) (V) -0.891 -0.894 -0.907 -0.900 -0.956 -1.034 -1.066 -1.148 -1.180 -1.271 -1.317 E½(2) (V) -0.002 (-0.266) -0.028 -0.082 -0.390 -0.373 -0.468 -0.454 -0.468 -0.460 -0.451 -0.461 -0.453 -1.307 -1.333 -0.517 -0.449 0.245 0.403 0.327 All reported potentials referenced to Fc/Fc+. 25 1 equiv. 2-Me-aniline Current (μA) 15 5 * -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs Fc/Fc+) Figure 20. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-methyl-aniline (pKa(THF) = 7.5) at a scan rate of 50 mV/s. Asterisk (*) denotes redox couple of the decamethylferrocene internal standard. The open circuit potential was -0.4 V. 91 25 1 equiv. 2-Me-pyridine Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs Fc/Fc+) Figure 21. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-methyl-pyridine (pKa(THF) = 8.6) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. 25 1 equiv. 2,6-Me-pyridine Current (μA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 22. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2,6-dimethyl-pyridine 92 (pKa(THF) = 9.5) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. 25 1 equiv. 2,4,6-Me-pyridine Current (μA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 23. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2,4,6-trimethyl-pyridine (pKa(THF) = 10.4) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. The E½ of middle peak was determined via square wave voltammetry since its return wave was low in current, and overlapping with another peak. 93 60 1 equiv. Et3N Current (µA) 40 20 0 -20 -40 -60 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 24. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent triethylamine (pKa(THF) = 14.9) at a scan rate of 200 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.6 V. 25 1 equiv. PhTMG Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 25. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-phenyl-1,1,3,3tetramethylguanidine (pKa(THF) = 16.5) at a scan rate of 50 mV/s. An independent scan in 94 the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 1 equiv. TMG 25 Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 26. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,1,3,3tetramethylguanidine (pKa(THF) = 17.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.5 V. 25 1 equiv. DBU Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 95 Figure 27. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,8diazabicyclo[5.4.0]undec-7-ene (pKa(THF) = 19.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 30 1 equiv. MTBD Current (µA) 20 10 0 -10 -20 -30 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 28. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (pKa(THF) = 20.5) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 96 20 1 equiv. TBD Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 29. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,5,7triazabicyclo[4.4.0]dec-5-ene (pKa(THF) = 22.0) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 20 1 equiv. tBuP1(pyrr) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 30. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent tert-butylimino- 97 tri(pyrrolidino)phosphorane (pKa(THF) = 22.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 20 1 equiv. EtP2(dma) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 31. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1-ethyl-2,2,4,4,4pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (pKa(THF) = 28.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 98 20 1 equiv. tBuP1(pyrr) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 Figure 32. Cyclic voltammogram of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent tert-butyliminotri(pyrrolidino)phosphorane (pKa(THF) = 22.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -1.3 V. 1 equiv. EtP2(dma) 25 Current (µA) 15 5 -5 -15 -25 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 33. Cyclic voltammogram of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1-ethyl-2,2,4,4,4- 99 pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (pKa(THF) = 28.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -1.3 V. 100 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1-[OTf] – 3-[OTf], 6-[OTf] and 2-[OTf] (H2O). 1-[OTf] 2-[OTf] 3-[OTf] 6-[OTf] 2-[OTf] (H2O) CCDC Number 1848679 1848681 1848680 1848678 1848677 Empirical formula C77H62F3Fe3Mn N12O8S C105.2H81.2BCl0.5 F3Fe3MnN12O1 0.5S C76H53.5F9Fe3M nN13.4O14.1S3 C67H49F3Fe3M nN12O8S C72H62F6Fe3Mn N12O13S2 Formula weight (g/mol) 1594.9 2019.8 1869.7 1461.7 1703.9 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) a (Å) 12.2741(5) 14.2908(12) 44.125(2) 14.7283(7) 12.2685(6) b (Å) 19.4126(8) 15.9691(13) 14.3106(7) 19.3808(10) 29.896(2) c (Å) 15.5112(6) 24.3709(17) 24.8034(10) 45.518(2) 19.6152(17) α (°) 90 71.236(4) 90 90 90 β (°) 108.397(2) 75.366(2) 90.402(3) 92.474(3) 92.393(5) γ (°) 90 70.262(4) 90 90 90 V (Å3) 3507.0(2) 4891.6(7) 15661.9(13) 12980.9(11) 7188.2(9) 2 2 8 8 4 Cryst. syst. monoclinic triclinic monoclinic monoclinic monoclinic Space group P21 P-1 C2/c C2/c P21/c ρcalcg (cm3) 1.510 1.371 1.586 1.496 1.575 2Θ range (°) 5.028 to 56.648 5.076 to 60.444 6.492 to 145.272 7.544 to 132.498 5.392 to 149.51 μ (mm-1) 0.899 0.668 7.226 7.742 7.461 GOF 1.031 1.029 1.037 1.160 1.140 0.0244, 0.0583 0.0635, 0.1712 0.0840, 0.2131 0.1305, 0.2771 0.1109, 0.2060 Z R1, wR2 (I>2σ (I)) 101 Special refinement details for [LFe3O(Pz)3Mn][OTf] (1-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 12% (S200 through C200) and 88% (S201 through C201). Special refinement details for [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 51% (S200 through C200) and 49% (S201 through C201). A disordered THF molecule was modeled over two positions with occupancies of 81% (O102 through C107) and 19% (O101 through C111). A different THF molecule was modeled to be only partially occupied (56%; O103 through C115). A co-crystallized solvent site was modeled to contain a mixture of three different molecules: a THF (27% O105 through C123), a DCM (22%; Cl10 through C124), and Et2O (64%; O104 through C119). Special refinement details for [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 30% (S200 through C200) and 70% (S201 through C201). An outersphere triflate was modeled in two different positions with occupancies of 38% (S203 through C203) and 62% (S204 through C204). For the S203 through C203 triflate, a nearby Et2O molecule was modeled as partially occupied at 62%. For the S204 through C204, a nearby MeCN molecule was modeled as partially occupied at 38%. Special refinement details for [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]).. The outersphere triflate is disordered over two positions, modeled at an occupancy of 50% each. Both triflates are on symmetry elements and positionally disordered. For the S200 through C200 triflate, this was modeled with EXYZ/EADP constraints. For the S201 throughC201 triflate, the C and S atoms were constrained with EXYZ/EACDP, and the O203 through F205 atoms were modeled in alternating positions, at 50% occupancy each. 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The redox couple that would be assigned to the 7-[BArF] to 8-[BArF] reduction potential unexpectedly increases, inconsistent with PCET to Mn=O and may suggest an undesirable side reaction takes place at these potentials with the base employed. 31. Attempts to obtain supporting data via stoichiometric deprotonation of 7-[BArF4] with various bases led to decomposition via reduction, with no evidence of forming a transient Mn=O moiety. Investigations into the characterization and reactivity of these putative M=O species (M = Fe, Mn) in these and related clusters is ongoing. 32. Riedel, P. J.; Arulsamy, N.; Mehn, M. P. Inorg. Chem. Commun. 2011, 14, 734-737. 33. Izod, K.; Rayner, D. G.; El-Hamruni, S. M.; Harrington, R. W.; Baisch, U. Angew. Chem. Int. Ed. 2014, 53, 3636-3640. 106 34. Huang, C.-Y.; Doyle, A. G. J. Am. Chem. Soc. 2012, 134, 9541-9544. 35. Zhang, Y.; Santos, A. M.; Herdtweck, E.; Mink, J.; Kuhn, F. E. New J. Chem. 2005, 29, 366-370. 36. Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.; Mayer, J. M. Chem. Commun. 2008, 256-258. 37. Thomson, R. K.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Comptes Rendus Chimie 2010, 13, 790802. 38. (a) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58; (b) Singh, A. K.; Jacob, W.; Boudalis, A. K.; Tuchagues, J.-P.; Mukherjee, R. Eur. J. Inorg. Chem. 2008, 2008, 2820-2829; (c) Sutradhar, M.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem. 2012, 2012, 4273-4278; (d) Schünemann, V.; Hauke, P. Mössbauer Spectroscopy. In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269.  CHAPTER 4 A TERMINAL FEIII-OXO IN A TETRANUCLEAR CLUSTER: EFFECTS OF DISTAL METAL CENTERS ON STRUCTURE AND REACTIVITY The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. J. Am. Chem. Soc.., 2019, Accepted Manuscript. doi:10.1021/jacs.9b03157 108 ABSTRACT Tetranuclear Fe clusters have been synthesized bearing a terminal FeIII-oxo center stabilized by hydrogen bonding interactions from pendant tert-butyl amino pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [FeII2FeIII2] to [FeIII4]; two oxidation states were structurally characterized by single crystal X-ray diffraction. The reactivity of the FeIII-oxo center in proton coupled electron transfer (PCET) with X–H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role adjacent metal centers have on modulating the reactivity of a terminal metal-oxo. 109 INTRODUCTION Terminal metal-oxo moieties are invoked as key intermediates in both natural and synthetic catalysts of mid-first-row transition metal ions (Mn, Fe, and Co).1 For example in photosynthesis, water is oxidized in photosystem II by a CaMn4O5 cluster known as the oxygen evolving complex (OEC);2 numerous computational studies of the catalytic mechanism have proposed a high-valent Mn-oxo playing a key role in O–O bond formation.3 Similarly, a number of synthetic water oxidation catalysts employing various multinuclear scaffolds have been reported, where a terminal metal-oxo is implicated as a key intermediate (Figure 1).1e-g, 4 Figure 1. Multinuclear catalysts with proposed terminal metal-oxo intermediates (top), and structurally characterized terminal FeIII-oxo complexes (bottom). Studies of synthetic transition metal-oxo complexes have been integral for understanding these reactive moieties in catalytic systems.1a, 5 However, there is a paucity of literature concerning multinuclear complexes bearing well-characterized terminal metal-oxo motifs.6 In a rare example where the effects of a neighboring metal oxidation state on a terminal metaloxo could be interrogated, Que and coworkers reported that the spin state of an FeIV-oxo 110 center would change depending on the oxidation state of a neighboring Fe in a μ2-O bridged bimetallic complex (L’2OFe2(OH)(O)2+/3+).6c The authors demonstrated that structural and spin-state changes due to reduction of this secondary Fe leads to a thousand-fold activation of the [Fe2] complex towards C–H oxidation. To gain further insight into these multimetallic effects, previous group members, Dr. Graham de Ruiter and Kurtis Carsch, studied the reactivity of Fe4, Fe3Mn, and Mn4 clusters, bearing aryl-substituted pyrazolate ligands, towards oxygen atom transfer reagents; in all cases, intramolecular C–H (or C–F) activation occurs, forming a five-coordinate apical metal with a phenoxypyrazolate donor.7 Analysis of the reaction mechanism were consistent with ratelimiting iodosylarene activation step, producing a transient reactive moiety (either iodosylarene adduct or terminal-oxo) that could not be directly observed. Inspired by reports of mononuclear terminal metal-oxo motifs stabilized by second coordination sphere hydrogen bonding interactions,8 our group has previously used this strategy to access a terminal MnIII– OH moiety as part of a [Mn4] cluster.9 Dr. Kyle Horak was able to serendipitously isolate the analogous Fe cluster, [LFe3O(PzNHPh)3Fe(OH)][OTf], however, there were challenges with accessing this cluster in reasonable purity and yield.10 Due to the observed difficulties in supporting clusters with the amino-phenylpyrazolate ligand, new amino-pyrazolate donors were investigated. Herein, we describe the synthesis, structural characterization, and reactivity studies of clusters bearing a terminal FeIII-oxo motif, stabilized by tert-butyl-amino-pyrazolates, to probe the significance of a multinuclear scaffold on structural and reactivity aspects of a terminal metal-oxo. 111 Scheme 1. Previous Efforts Towards Isolation of a Terminal Metal-Oxo in a Multinuclear System by the Agapie Group RESULTS AND DISCUSSION Synthesis of Fe4-Hyroxide and Fe4-Oxo Clusters. Treatment of the reported LFe3(OAc)(OTf)2 cluster (-OTf, triflate = trifluoromethane sulfonate)11 with three equivalents of potassium tert-butyl-amino-pyrazolate (KPzNHtBu) and iodosylbenzene (PhIO), followed by addition of iron (II) triflate bis-acetonitrile (Fe(OTf)2 • 2 MeCN) and excess potassium hydroxide in tetrahydrofuran (THF) produces the neutral [FeII3FeIII] cluster, 1 (Scheme 2). Single crystal X-ray diffraction (XRD) studies of 1 reveal a structure similar to our previously reported [Mn4] cluster bearing a terminal hydroxide ligand (Figure 2);9 the apical metal displays a trigonal bipyramidal geometry, with the terminal hydroxide ligand hydrogen bonded to each amino-pyrazolate (N–O distances of 2.826(1), 2.765(1), 2.789(1) Å for 1). The relatively short distance between the apical Fe and the interstitial μ4-O (Fe4–O1), 1.837(1) Å, is consistent with an FeIII in the apical position of the cluster, with the remaining Fe centers being FeII.7a, 12 112 Scheme 2. Synthesis of [Fe4] clusters. (Inset) 1,3,5-triarylbenzene ligand (L3-) and tertbutyl amino pyrazolate ligand (PzNHtBu-). Figure 2. Crystal structure of 1. Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules removed for clarity. 113 The electrochemistry of the [Fe4] hydroxide clusters in THF features three quasi-reversible events assigned to the [FeII3FeIII]→[FeII2FeIII2] (-1.53 V; all potentials vs. Fc/Fc+), [FeII2FeIII2]→[FeIIFeIII3] (-0.68 V), and [FeIIFeIII3]→[FeIII4] (-0.10 V) redox couples (Figure 3). Each of the corresponding oxidation states of the cluster could be isolated (Scheme 1). Mössbauer spectra of the oxidized clusters 2, 3, and 4 are consistent with oxidations occurring at the FeII centers in the tri-iron core and the Fe–OH moiety remaining FeIII (Figure 4 and Table 1). FeII2FeIII2 FeIIFeIII3 FeIII4 Current FeII3FeIII 10 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 Figure 3. Cyclic voltammetry of 2, (2.5 mM) at 50 mV/s in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 200 mM [Bu4N][PF6]. Figure 4. Mössbauer spectra of (A) 1, (B) 2, (C) 3, and (D) 4; see parameters in Table 1. 114 Table 1. 57Fe Mössbauer Parameters for Complexes 1 – 4. δ (mm/s) |ΔEq| (mm/s) assignment δ (mm/s) 1 (FeII3FeIII) Fe1, Fe2, Fe3 Fe4 assignment 3 (FeIIFeIII3) 1.10, 1.11, 1.13 3.52, 3.03, 2.64 h.s. FeII Fe1 1.15 2.83 h.s. FeII 0.41 2.71 apical FeIII Fe2, Fe3 0.47, 0.46 0.61, 1.06 h.s. FeIII Fe4 0.38 1.69 apical FeIII 2 (FeII2FeIII2) Fe1, Fe2 |ΔEq| (mm/s) 1.12, 1.10 3.20, 2.76 h.s. FeII 4 (FeIII4) Fe3 0.52 0.81 h.s. FeIII Fe1, Fe2, Fe3 0.45, 0.53, 0.36 0.64, 1.15, 1.17 h.s. FeIII Fe4 0.41 2.17 apical FeIII Fe4 0.41 1.71 apical FeIII Access to a terminal FeIII-oxo moiety was achieved by deprotonation of the [FeII2FeIII2] hydroxide cluster, 2, with potassium tert-butoxide (KOtBu; Scheme 1). The resulting compound, 5, was crystallographically characterized (Figure 5); deprotonation of the hydroxide ligand leads to structural changes to the apical Fe in 5. The Fe4–O2 distance contracts to 1.817(2) Å, compared to the distances in 1 (1.937(1) Å) and the precursor 2 (1.907(3) Å); this bond length matches closely with the structurally characterized FeIII-oxo complexes reported by Borovik and Fout.8e, 8h, 8i Compound 6, prepared by deprotonating 3, also displays a short Fe4–O2 distance (1.795(8) Å). Furthermore, the apical Fe-μ4-O distance (Fe4–O1) elongates to 1.965(2) Å in 5 and 2.049(7) Å in 6, from 1.890(3) Å in 2 and 1.948(2) Å in 3, which is consistent with a greater trans influence exerted by the terminal oxo ligand. Terminal FeIII-oxo complexes are rare, and typically stabilized through hydrogen bonding interactions.8e, 8h, 8i, 13 The structures of 5 and 6 display comparable hydrogen bonding distances 115 Figure 5. Truncated crystal structure of FeIII-oxo cluster, 5. Hydrogen atoms and solvent molecules removed for clarity. Table 2. Selected Bond Distances and Angles, Structural Index Parameter, and Mössbauer Parameters of Reported FeIII-Oxo Complexes a 5 6 [(H3beau)Fe(O)]2-8e [N(afaCy)3Fe(O)]+8h Fe–O (Å) 1.817(2) 1.795(8) 1.813(3) 1.806(1) Fe–Nequatorial (Å) 2.104(2), 2.098(2), 2.093(2) 2.100(8), 2.085(9), 2.087(9) 2.030(4), 2.060(4), 2.082(4) 2.049(1), 2.049(1), 2.052(1) Fe–Ltrans (Å) 1.965(2) (L=O2-) 2.049(7) (L=O2-) 2.271(4) (L=NR3) 2.276(1) (L=NR3) N–O (H-bond; Å) 2.647, 2.717, 2.685 2.718, 2.790, 2.750 2.732, 2.702, 2.686 2.641, 2.645, 2.673 ∠Nequatorial–Fe–O (°) 96.3, 92.8, 92.0 93.6, 97.5, 96.3 103.3, 99.7, 100.8 102.6, 103.1, 103.1 Fe–N|N’|N’’equatorial (Å) 0.14 0.22 0.42 0.45 Structural Index Parameter (τ)a 0.9 0.8 0.5 0.4 Mossbauer parameters (mm/s) δ = 0.43, |ΔEq| = 3.04 δ = 0.47, |ΔEq| = 2.53 δ = 0.30, |ΔEq| = 0.91 - τ = [Σ (∠Nequit.–Fe–N’equit.) - Σ (∠Nequit.–Fe–O)]/90 to other structurally characterized FeIII-oxo complexes, [(H3beau)Fe(O)]2- and [N(afaCy)3Fe(O)]+, along with similar equatorial Fe–N distances (Table 2). However, the μ4O distances in 5 (1.965(2) Å) and 6 (2.049(7) Å) are significantly shorter than the Fe–N 116 distances for the amine trans to the oxo in the mononuclear systems (~2.27 Å). This is likely a result of greater ligand flexibility in the mononuclear systems; the geometry of these FeIIIoxo complexes display greater deviations from ideal trigonal bipyramidal geometry compared to the apical Fe in 5 and 6, based on a structural index parameter (τ; ideal trigonal bipyramidal geometry = 1.0). For the clusters reported here, the rigid geometry of the pyrazolate ligands prevents significant distortion of the apical Fe out of the equatorial plane. Electronic Structure Investigations of Tetranuclear Fe Clusters. The 57Fe Mössbauer spectra of the FeIII-oxo clusters 5 – 7 display relatively unique parameters for the apical Fe, relative to the structurally related FeIII-oxo, where Mössbauer parameters have been reported, [(H3beau)Fe(O)]2- (Table 2 and Figure 4).8e, 14 For example, the Mössbauer parameters assigned to the apical Fe of 5, δ = 0.43 mm/s and |ΔEq| = 3.04 mm/s, are atypical for highspin (S= 5/2) FeIII centers, which typically display low quadrupole splitting values. A detailed examination of the electronic structure of the Fe4 clusters was conducted through magnetic susceptibility measurements and EPR spectroscopy. Variable temperature, and vaeriable temperature variable field, magnetic susceptibility measusurements were conducted on a series of the thermally stable Fe4-hydroxide clusters (2 Figure 4. Zero applied-field Mössbauer spectra of (A) 5, (B) 6, and (C) 7. The parameters of each doublet are listed in Table 3. 117 Table 3. 57Fe Mössbauer Parameters for Complexes 5 – 7. δ (mm/s) |ΔEq| (mm/s) assignment 5 (FeII3FeIII) Fe1, Fe2 1.12, 1.10 3.14, 2.87 h.s. FeII Fe3 0.52 1.13 h.s. FeIII Fe4 0.43 3.04 apical FeIII Fe1 Fe2, Fe3 Fe4 1.09 2.87 h.s. FeII 0.51, 0.49 1.09, 0.72 h.s. FeIII 0.47 2.53 apical FeIII assignment 0.43, 0.44, 0.41 1.44, 0.95, 0.38 h.s. FeIII 0.43 2.03 apical FeIII 7 (FeIII4) Fe1, Fe2, Fe3 6 (FeII2FeIII2) |ΔEq| (mm/s) δ (mm/s) Fe4 – 4) to establish their electronic ground states (Figure 5) The variable temperature magnetic susceptibility data for these compounds is consistent with high-spin Fe centers composing all cluster redox states for the series; the best fit for each complex is obtained by using exclusively S = 2 FeII and S = 5/2 FeIII. The spin coupling model for these clusters is similar to that of structurally related high-spin Fe clusters bearing bridging imidazolate ligands, where strong antiferromagnetic coupling between the apical FeIII and the tri-iron core promotes in ferromagnetic alignment of the spins with the remaining three Fe centers (Figure 5B).12 The variable temperature variable field magnetization data for these compounds can be fit adequately using this coupling scheme, supporting assignments for spin ground states of S = 4, 9/2, and 5 for 2, 3, and 4, respectively (Figure 6). The presence of increasing zero field splitting as 4 is reduced to 3 and 2 is observed in the magnetization data, causing saturation below the expected 2S limit (10 for 4, 9 for 3, and 8 for 2). 118 Figure 5. (A) Variable temperature direct current magnetic susceptibility data for Fe4hydroxide clusters 2 (yellow), 3 (green), and 4 (blue) at 0.1 T. The spin coupling model has strong antiferromagnetic alignment of the apical Fe and the tri-iron core. (B) Simulation of coupling scheme for 2 – 4, with all metal centers locally high spin. Figure 6. Variable temperature-variable field magnetization data for 2 (A), 3 (B), and 4 (D) from 2.5 K to 8 K. The spin and zero field splitting parameters (S, D, and |E/D|) were used to simulate the data. 119 Continuous wave EPR spectra of these paramagnetic clusters at low temperature (< 20 K) in frozen solutions display a number of features in regions expected for these high-spin systems (g >16). Figure 7 summarizes the parallel (for integer spin clusters 2, 4, 5, and 7) and perpendicular (for 3 and 6) mode data collected. Attempts to simulate these spectra were challenging, even for the complexes where magnetization data was obtained. For example, 2 displays a sharp peak at ca. g = 17 at low temperatures (<15 K), which can be tentatively assigned to the Ms = ± 4 doublet transition (Figure 7A).12 In contrast, the corresponding [FeIII2FeII2] Fe-oxo cluster, 5, displays one major transition near g ~ 19, which is consistent with an S = 4 or 5 system (Figure 7B). Further studies will examine the field-dependent Mössbauer spectra of these clusters to identify their spin ground states. Even without simulating the EPR data, there are noticable differences for each cluster oxidation state between Fe4-hydroxide and –oxo, demonstrating the strong influence of the apical ligand on the electronics of the cluster. Figure 7. Parallel-mode EPR spectrum in 2-MeTHF of(A) 2 collected at 5 K (red), 10 K (orange), 15 K (green), and 25 K (blue), and (B) 5 at 5 K (red), 10 K (orange), and 15 K (green). 120 Figure 7 cont. (C) Perpendicular-mode CW-EPR spectrum of 3 collected in EtCN/PrCN at 5 K (red), 7.5 K (orange), 10 K (green), 15 K (blue), 20 K (purple), and 30 K (black). (D) Perpendicular-mode spectrum of 6 collected in EtCN/PrCN at 5 K (red), 7.5 K (orange), 10 K (green), 15 K (blue), and 20 K (black). (E) Parallel-mode spectrum of 4 in EtCN/PrCN at 5 K (red), 10 K (green), and 15 K (purple). (F) Parallel-mode spectrum of 7 in EtCN/PrCN at 5 K (red), 10 K (orange), 15 K (green), and 20 K (black). pKa and BDEO–H Determination for Fe4-Hydroxide Clusters. The hydroxide ligand in 2 was determined to be very basic in THF (pKa = 30.1; Table 4). Analogous equilibrium studies were performed on 3 and, as expected, oxidation of the cluster reduces the basicity of the 121 FeIII-oxo moiety (pKa = 23.0 for 3; Table 5). Attempts to deprotonate 4 with various bases, even at low temperatures, only resulted in decomposition, so a pKa value for this oxidation state was not measured. These data were combined with electrochemical information for clusters 1 (vide supra) and 5 (Figure 8), to produce thermodynamic square schemes according to equation 1 (Figure 9): 15 BDEO–H = 23.06 E° + 1.37 pKa + C (1) Table 4. pKa determination of 2 via 31P NMR spectroscopy.a NMR 2 + 1.3 Ph3PCH2 2 + 5 Ph3PCH2 2 + 7 Ph3PCH2 2 + 10 Ph3PCH2 2 + 18 Ph3PCH2 Equiv. Ph3PCH2 ~ 1.3 4.54 6.38 9.24 16.97 Equiv. [Ph3PCH2]+ 0.46 0.62 0.76 1.03 [5]/[2] n.d. 0.85 1.63 3.17 n.d. Average K Kb n.d. 0.09 0.16 0.26 n.d. 0.17 (±0.09) aThe ratio of 2 to 5 was estimated from the relative integrals of the 31P NMR peaks for Ph PCH (~15 ppm) and 3 2 Ph3PCH3+ (~18 ppm); it was assumed that the equivalents of [Ph3PCH3]+ produced in the NMR were due to partial deprotonation of 2, and corresponded to equivalents of 5 ([[Ph3PCH3][OTf]] = [5]). bAn equilibrium constant was determined according to the equation below: [𝟓][[Ph3 PCH3 ][OTf]] [𝟐][Ph3 PCH2 ] where this value, along with the reported pKa of Ph3PCH2 in THF, 29.3, was used to obtain a pKa value of 30.1 (±1.0) for 2.16 𝐾= Table 5. pKa determination of 3 via 1H NMR spectroscopy NMR 3 + 1 tBuP1(pyrr) 3 + 5 tBuP1(pyrr) 3 + 10 tBuP1(pyrr) Equiv. 3a 0.55 0.27 0.12 Equiv. 6a 0.45 0.73 0.88 Average K Kb 0.70 0.46 0.71 0.62 (±0.14) aThe ratio of 3 to 6 was based on the relative integrals of the 1H NMR peaks at 17.0 (for 3) and 14.5 ppm (for 6); the relative amounts of base and conjugate acid of tert-butylimino-tri(pyrrolidino)phosphorene (tBuP1(pyrr)) were assumed based on mass balance. bAn equilibrium constant was determined according to the equation below: [𝟔][[H′tBuP1 (pyrr)′][OTf]] [𝟑][′tBuP1 (pyrr)′] where this value, along with the reported pKa of tBuP1(pyrr) in THF, 22.8, was used to obtain a pKa value of 23.0 (±1.0) for 3.16 𝐾= 122 FeII2FeIII2 FeIIFeIII3 FeIII4 Current * * * 15 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V vs. Fc/Fc+) 0.5 1 Figure 8. Cyclic voltammetry of 5 (2.3 mM at 200 mV/s scan rate) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 100 mM [Bu4N][PF6]. Electrochemical events marked with an asterisk (*) are assigned to a small amount of [LFe3O(PzNHtBu)3Fe(OH)]n+ that formed due to decomposition. Figure 9. Thermodynamic cycles to evaluate the BDEO–H values of the hydroxide clusters 1 – 3. Reduction potentials (horizontal lines) are references to Fc/Fc+. pKa values (vertical lines) are based on relative pKa values of cationic acids in THF. Diagonal lines are the BDEO–H values calculated from these parameters according to the Bordwell equation (eq 1). Approximate values (~) have been extrapolated from the Bordwell equation. 123 Similar to our previously reported studies on [Fe3Mn] hydroxide and aquo clusters, the bond dissociation enthalpy of the O–H bond (BDEO–H) increases upon oxidation of the distal Fe centers, ranging from 72 kcal/mol in 1 to 84 kcal/mol in 3.17 Reactivity Studies of Fe4-Oxo Clusters. The three distal Fe oxidation states have a dramatic effect on the reactivity of the FeIII-oxo center through modifying the pKa and BDEO– H values. For example, 5 is incapable of performing proton coupled electron transfer (PCET) reactions18,19 with substituted phenols over a range of phenol BDEO–H values (79 – 85 kcal/mol); only proton transfer to generate 2 is observed as expected from the combination of low BDEO–H for 1 and high pKa of 2. Oxidation of the remote Fe centers in 6 and 7 enables PCET reactivity with these phenols, resulting in the formation of 2 and 3, respectively (Figure 10). Figure 10. 1H NMR spectra (400 MHz) in THF/C6D6 of reaction products with 5 - 7 and 2,4,6-tri-tert-butyl phenol (2,4,6-tBu3-PhOH; BDE = 82 kcal/mol). The major species in the maroon and green spectra corresponds to 2. The major species in the blue spectrum corresponds to 3. 31 P NMR and GC/MS analyses suggest that 7 is capable of transferring an oxygen atom to trimethylphoshine (PMe3), where the other FeIII-oxo clusters display no reaction towards the phosphine on a similar timescale (Figure 11). The difference in reactivity is likely due to 124 the low reduction potentials of 5 and 6 precluding efficient oxygen atom transfer reactivity. A more oxidizing cluster, through oxidations of the distal Fe centers, 7 can undergo OAT. Figure 11. (A) 1H NMR spectrum (400 MHz) in THF/C6D6 of 5 with 40 equivalents trimethylphosphine (PMe3); the resonances are consistent with no reaction occurring. (B) 31P NMR spectrum of 6 and 8 equivalents PMe3; the only resonances observed on those assigned to unreacted PMe3 and the conjugate acide of the based used to prepare 6 (Ph3PCH3+). (C) 31P NMR spectrum of 7 and 20 equivalents PMe3, where a resonance assigned to trimethylphosphine oxide (OPMe3) is observed ~ 40 ppm. (D) GC/MS analysis of reaction between 18O-7 and PMe3 contains mass fragments of 18OPMe3 (94 m/z), consistent with oxygen atom transfer from 7 to form the phoshpine oxide. The kinetics of C–H activation by these clusters was investigated. The reaction between 5 and 9,10-dihydroanthracene (DHA; BDEC–H = 78 kcal/mol)15c displays an expected first order dependence on substrate concentration, with an overall second order rate constant of 87 M-1 s-1, and a considerable kinetic isotope effect (KIE) of 7 with d4-DHA. These data are consistent with a rate-limiting C–H bond activation for the PCET process to form 1 and anthracene. The 125 second-order rate constants between 5 and C–H bonds of varying BDEC–H and pKa values were measured and display a linear dependence of the PCET reaction rate on the pKa of the organic substrate (Figure 12), suggesting either a concerted or stepwise pKa-driven process.20 Reactions between DHA and 6 or 7 produce the corresponding hydroxide-clusters and anthracene in yields comparable to 5 (Table 6) indicating PCET processes, but complex kinetics precluded the determination of rate constants and further insights into the mechanism of these reactions. Figure 12. Plots of log (k2) (normalized to number of reactive C-H bonds) versus BDE (left) and pKa (in DMSO; right) for 5 with various organic substrates. 126 Table 6. Product Analysis of PCET and OAT Reactivity of 5 – 7. 5 organic product anthracene (53%)a 6 7 organic cluster organic cluster Substrate product product product product anthracene anthracene 3 9,10 -dihydroanthracene 1 (67%)b 2 (66%)b (43%)a (44%)a (110%)b 9,9’-bifluorenyl 3 fluorene n.d.c 1 (83%)b 2 (81%) n.d.c (1%)a (107%)b 2,4,6-tri-tertbutylphenol phenoxyl 2 2 3 radicald e e e e trimethylphosphine N.R. N.R. N.R. N.R. OPMe3 2 aQuanitified by GC/MS versus authentic samples, with triphenylphosphine as an internal standard; percent yield based on a 2:1 cluster/product stoichiometry. bQuantified by 1H NMR spectroscopy with 1,3-trimethylsilylbenzene as an internal standard. cNot detected by GC/MS. dX-band EPR signal detected at 77 K that matches authentic sample of the corresponding phenoxyl radical.e No reaction observed. cluster product CONCLUSIONS Overall, this report offers a rare systematic study of the effects of neighboring redox active metals on structural and reactivity aspects of a terminal metal-oxo. Because it is part of a cluster, the reactivity of the terminal metal-oxo motif can be tuned without changing the formal redox state of the metal supporting it; however, redox events at distal centers have significant effect on the acidity and BDE of the corresponding O-H bond. Clearly, the cluster as an assembly is essential for reactivity beyond the structural aspects of the isolated metaloxo motif. Further development of multinuclear model systems is necessary to fully understand the nature and amplitude of these effects. 127 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified; reactions with KOH were performed in an N2-filled VAC wetbox. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. [LFe3(OAc)(OTf)][OTf],11 iodosylbenzene,21 benzyl potassium,22 Fe(OTf)2 • 2 MeCN,23 ferrocenium trifluoromethane sulfonate ([Fc][OTf]),24 and Ph3PCH225 were prepared according to literature procedures. N-tert-butyl-1H-pyrazol-5-amine (HPzNHtBu) was prepared according to a modified literature procedure.26 18-oxygen labeled potassium hydroxide (K18OH) was prepared by quenching a tetrahydrofuran solution of benzyl potassium (less than 1 mmol) with H218O, and drying the resulting white suspension under vacuum. Tetrahydrofuran, CH2Cl2, diethyl ether, benzene, toluene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H spectra were recorded on a Varian 300 MHz spectrometer; 13C NMR spectra were recorded on a Varian 500 MHz spectrometer. 1H and 31P NMR spectra in THF/C6D6 were recorded on a Varian 500 MHz spectrometer using solvent suppression protocols. NMR spectra collected at low temperature were recorded on a Bruker 500 MHz spectrometer. CD3CN, C6D6, and CD2Cl2 was purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freezepump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero applied field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to 128 the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated acetonitrile solution in the cup. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). Mössbauer simulation details for all compounds. All spectra were simulated by four pairs of symmetric quadrupole doublets with equal populations and Lorentzian lineshapes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts of ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported Fe clusters.7a, 7b, 11, 27 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.28 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter) and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Dry acetonitrile or tetrahydrofuran that contained ~100 mM [Bu4N][PF6] was used as the electrolyte solution. The ferrocene/ferrocinium (Fc/Fc+) redox wave was used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All 129 non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of the compounds (1 – 3, 5, and 6), most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Magnetic measurements. Magnetic susceptibility measurements were collected on a Quantum Design DynaCool 14T PPMS instrument at the University of Southern California, Los Angeles. Polycrystalline samples (10 – 20 mg) of 2 – 4 were packed in VSM sample holders. Magnetization data at 100 K from 0 to 4 T were collected to confirm the absence of ferromagnetic impurities. Magnetic susceptibility measurements were collected between 2 and 300 K with a 0.1 T field. Reduced magnetization data was collected between 2 K and 8 K at fields between 1 T and 12 T. Magnetic data was simulated with PHI.29 Synthetic Procedures. Synthesis of 2-tert-butyl-isoxazolium tetrafluoroborate. 25 mL isoxazole (0.4 mol) was combined with 37 mL tert-butanol (0.4 mol) in a 500 mL roundbottom flask. This was cooled to -20 °C with an ice/sodium chloride bath while 160 mL tetrafluoroboric acid diethyl ether complex (1.2 mol) was added dropwise over 1 hour. After the addition was complete, the reaction was warmed to room temperature and stirred for 4 hours. Then, 100 mL Et2O and 50 mL THF was added to the reaction and cooled to -20 °C; the resulting precipitate was collected on a glass frit, washed three times with 200 mL Et2O and dried under reduced pressure. 60 g of 2-tert-butyl-isoxazolium tetrafluoroborate (72% yield) can be obtained this way; another 6 g can be obtained by cooling the filtrate and Et2O washings to 20 °C overnight and collecting the resulting crystals (79% overall yield). 1H NMR (300 MHz, (CD3)2CO): δ 1.90 (s, 9H), 7.46 (s, 1H), 9.55 (s, 1H), 9.77 (s, 1H) ppm. Synthesis of N-tert-butyl-1H-pyrazol-3-amine (HPzNHtBu). This procedure was adapted from a report describing the synthesis of tert-butyl substituted 3-aminopyrazoles.26 10.0 g of 2-tert- 130 butyl-isoxazolium tetrafluoroborate (47 mmol) was suspended in 100 mL EtOH in a 250 mL roundbottom flask and cooled with an ice bath to 0 °C. A solution of 4.56 mL hydrazine monohydrate (94 mmol) in 20 mL EtOH was added dropwise to the cooled flask. After complete addition, the reaction was warmed to room temperature and stirred for 30 minutes. EtOH was removed via rotary evaporation and an aqueous work up was performed with 100 mL H2O and 3 x 100mL CH2Cl2, collecting the organic layers. The combined organic fractions were dried with Na2SO4, filtered, and dried to yield an orange oil. The crude product was purified via Kugelrohr distillation under dynamic vacuum at 90 °C. The distillate was recrystallized with Et2O and the resulting white solid was sublimed under vacuum at 60 °C to yield 1.6 g of HPzNHtBu (24% yield). 1H NMR (400 MHz, CD2Cl2): δ 1.27 (s, 9H), 3.61 (br), 5.71 (d, 1H), 7.34 (d, 1H), 9.75 (br) ppm. 13C{1H} NMR (100 MHz, CD2Cl2): 53.28, 75.11, 118.60, 154.26 ppm (a signal for the tert-butyl quaternary carbon was likely not observed). Anal. calcd. (%) for C7H13N3: C, 60.40; H, 9.41; N, 30.19. Found: C, 60.75; H, 9.37; N, 30.20. Synthesis of potassium N-tert-butyl-1H-pyrazol-3-amine-ate (KPzNHtBu). 1.25 g N-tert-butyl-1Hpyrazol-3-amine (9 mmol) was dissolved in 5 mL THF. A THF solution of 1.17 g benzyl potassium (9 mmol) was added dropwise, while stirring. After 30 minutes, the reaction was concentrated to 5 mL, and the precipitate was collected via filtration. The precipitate was washed with Et2O and dried under vacuum to yield 1.2 g KPzNHtBu as a white solid (75% yield). Anal. calcd. (%) for C7H12KN3: C, 47.42; H, 6.82; N, 23.70. Found: C, 47.50; H, 6.83; N, 23.61. Synthesis of LFe3O(PzNHtBu)3Fe(OH) (1). 1.287 g (0.93 mmol) LFe3(OAc)(OTf)2 was suspended in THF and froze in a liquid nitrogen cooled cold well. 502.6 mg (2.83 mmol) KPzNHtBu was added with THF while the suspension was thawing. After stirring at room temperature for 1 hour, 207.0 mg (0.94 mmol) iodosylbenzene was added with THF. After 4 131 hours, the solvent was removed under reduced pressure. The brown solid was transferred to a coarse porosity glass frit with celite using 50 mL pentane. The desired compound was extracted using toluene until the filtrate appeared colorless. This red-brown solution was dried completely under reduced pressure; the resulting solid (1.207 g obtained) is used in the following steps assuming a molecular formula of LFe3O(PzNHtBu)3, however this could not be confirmed via X-ray crystallography due to its poor crystallinity. 110.7 mg (0.076 mmol) of the LFe3O(PzNHtBu)3 solid was dissolved in 5 mL THF. 33.0 mg (0.076 mmol) Fe(OTf)2 • 2 MeCN was added with 1 mL THF. After 45 minutes, 26 mg (0.464 mmol) KOH was added as a THF suspension. After 18 hours, the reaction appeared dark blue; this solution was transferred to a Schlenk tube and dried under vacuum at 100 °C for 1 hour. The reaction mixture is suspended in MeCN and the blue precipitate was collected over a coarse porosity frit with celite. The precipitate was washed with MeCN until the filtrate was colorless, and then dried under vacuum. The dry blue precipitate was extracted with toluene and dried under reduced pressure. This residue was recrystallized via benzene/HMDSO vapor diffusion to yield 25.7 mg (0.017 mmol; 22% yield) of 1 as a blue solid. 1H NMR (300 MHz, C6D6): δ 123.0 (br), 64.6 (br), 56.4, 50.1, 44.1, 41.0, 24.6, 19.6, 14.2, 12.2, 4.4, 3.2, 1.7, -40.6 (br) ppm. UV-Vis (THF) [ε (M-1 cm-1)] 253 nm (5.19 x 104), 494 nm (3.26 x 103), 609 nm (3.81 x 103). Anal. calcd. (%) for C78H76Fe4N15O5: C, 61.36; H, 5.02; N, 13.76. Found: C, 61.27; H, 5.40; N, 13.12. The 18-O labeled cluster could be prepared through the analogous protocol, substituting K18OH for KOH. The resulting product has identical spectroscopic features to that of 1, and was used to prepare the remaining 18-O labeled clusters (via oxidations and/or deprotonation). ESI-MS analysis was consistent with 18-O incorporation of the cluster (Figure S18). 132 Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2). 265.2 mg (0.17 mmol) LFe3O(PzNHtBu)3Fe(OH) was dissolved in 5 mL THF. This was transferred to a stirring suspension of 52.3 mg (0.16 mmol) [Fc][OTf] in 3 mL THF. After 1 hour, the reaction was concentrated under vacuum to 1 mL and 15 mL toluene was added. The reaction was stirred for 15 minutes and the resulting red-purple precipitate was collected on a coarse frit with celite and dried completely under vacuum. The red-purple solid was extracted by washing with MeCN until the filtrate appeared colorless; this solution was dried under reduced pressure. The resulting residue was recrystallized via THF/Et2O vapor diffusion to yield 211 mg of redpurple crystals of 2 (0.13 mmol; 82% yield). 1H NMR (300 MHz, CD3CN): δ 127.2 (br), 82.1 (br), 54.4, 49.0, 22.1, 16.5 (br), 14.1, 13.8, 13.3, 10.3 (br), 8.4, 7.8, 7.3, 1.0, -4.9, -5.1, -22.8 (br) ppm. UV-Vis (ACN) [ε (M-1 cm-1)] 243 nm (5.96 x 104), 328 nm (8.83 x 103), 503 nm (4.88 x 103). Anal. calcd. (%) for C79H76F3Fe4N15O8S: C, 56.61; H, 4.57; N, 12.54. Found: C, 56.72; H, 4.70; N, 12.03. Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3). 102.3 mg (0.06 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf] was dissolved in 3 mL DCM and a solution of 20.3 mg (0.06 mmol) [Fc][OTf] in 2 mL DCM was transferred to this stirring solution. After 2 hours, 10 mL pentane was added to the reaction and the blue precipitate was collected on a coarse porosity glass frit with celite. The blue powder was dried under vacuum and extracted with DCM until colorless, and recrystallized from DCM/Et2O to obtain 76.8 mg of 3 as blue crystals (69% yield) 1H NMR (300 MHz, CD2Cl2): δ 144.3 (br), 103.7 (br), 82.0, 79.7, 66.0, 63.1, 15.5, 12.8, 9.9, 3.5, 1.2, -0.5, -2.3 (br), -11.6 (br) ppm. [ε (M-1 cm-1)] 238 nm (5.76 x 104), 345 nm (7.74 x 103), 634 nm (4.80 x 103). Anal. calcd. (%) for C80H76F6Fe4N15O11S2: C, 52.65; H, 4.20; N, 11.51. Found: C, 51.70; H, 4.37; N, 11.11. 133 Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4). 42.9 mg (0.024 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 was dissolved in 2 mL DCM and 8.1 mg (0.024 mmol) [Fc][OTf] was added with 2 mL DCM. After 30 minutes, the reaction was concentrated and 10 mL Et2O was added to produce a green precipitate. This was collected on a frit over celite and rinsed with Et2O. The precipitate was collected with DCM and recrystallized via vapor diffusion of Et2O to obtain 39.0 mg (0.020 mmol; 82% yield) 4 as green crystals. 1H NMR (300 MHz, CD2Cl2): δ 95.2, 85.0, 14.5, -1.8, -44.9. -48.3 ppm. UV-Vis (ACN) [ε (M-1 cm-1)] 242 nm (7.11 x 104), 355 nm (8.85 x 103), 748 nm (7.39 x 103). Anal. calcd. (%) for C81H76F9Fe4N15O14S3: C, 49.28; H, 3.88; N, 10.64. Found: C, 49.19; H, 4.09; N, 10.02. Synthesis of LFe3O(PzNHtBu)3Fe(O) (5). 102.6 mg (0.06 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf] was dissolved in 15mL THF and froze in a liquid nitrogen cooled cold well. 7.2 mg (0.06 mmol) KOtBu was added to the thawing solution, and the reaction was stirred at room temperature for 1 hour. The solvent was removed under reduced pressure and the crude product was recrystallized via benzene/HMDSO vapor diffusion to obtain 26.2 mg of 5 as purple crystals (0.02 mmol; 28% yield). 1H NMR (300 MHz, C6D6): δ 105.9 (br), 58.5 (br), 55.3, 53.0, 40.9, 38.9, 33.9, 21.8, 14.4, 11.7, 2.4, 1.1, -21.5 (br) ppm. UVVis (THF) [ε (M-1 cm-1)] 248 nm (4.40 x 104), 342 nm (6.73 x 103), 539 nm (3.41 x 103). Anal. calcd. (%) for C78H75Fe4N15O5: C, 61.40; H, 4.95; N, 13.77. Found: C, 60.04; H, 5.01; N, 13.06 (Calcd. (%) for C78H75Fe4N15O5 • 0.5 (C6H18OSi2): C, 60.05; H, 5.27; N, 13.08; compound recrystallized from benzene/HMDSO). Synthesis of [LFe3O(PzNHtBu)3Fe(O)][OTf] (6). 50.3 mg (0.03 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 was dissolved in 2 mL THF/DCM (1:1) and froze in a liquid nitrogen cooled cold well. 8 mg (0.03 mmol) Ph3PCH2 was added to the thawing solution as a THF solution. The reaction turned a deep blue, and at this point care was taken to avoid 134 warming the mixture to room temperature. The compound was precipitated by addition of cold Et2O, and the precipitate was dried under vacuum to yield 6 as a blue powder. NMR analysis of this powder revealed the presence of residual [Ph3PCH3][OTf], which were difficult to remove with Et2O washes. This mixture could be recrystallized in THF/Et2O at -35 °C to obtain X-ray quality crystals of 6; however, due to the decomposition of this compound, obtaining 6 cleanly as a bulk solid for elemental analysis was unsuccessful. 1H NMR (300 MHz, CD3CN): δ 122.2 (br), 90.2 (br), 68.5, 66.1, 55.0, 53.2, 14.5, 13.9, 13.0, 10.7, -31.0 (br) ppm. Synthesis of [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7). 43.0 mg [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3; 0.02 mmol) was dissolved in 1:1 DCM/THF and froze in a liquid nitrogen cooled cold well. A THF solution of 6.8 mg Ph3PCH2 (0.02 mmol) was added to the thawing solution. The reaction was then combined, while thawing, with a DCM solution of 7.8 mg [Fc][OTf] (0.02 mmol). Keeping this mixture as cold as possible, thawing Et2O was added to precipitate the oxidized cluster; The blue-green solid was collected on a fine porosity glass frit, and dried under vacuum. The 1H NMR of this solid always contained minor amounts of impurities (<20%, mostly ascribed to 3 and 4), which 7 could not be isolated from due to its thermal instability. For any subsequent reactions performed on this material, the moles of initial cluster 3 were used to approximate the amount of 7 present. 1H NMR (300 MHz, 1:1 CD3CN/CD2Cl2): δ 145.6 (br), 105.4 (br), 85.2, 81.4, 71.0, 67.2, 19.0, 13.8, 11.1, 8.8, -61.1, 67.3 (br). Experimental Protocols. Reactions for product analysis. 2mM solutions of Fe-oxo clusters 5 – 7 were stirred for 12-24 hours with one equivalent of 9,10-dihydroanthracene (DHA) or fluorene. The solvent was removed under vacuum and the organic products were extracted with Et2O containing triphenylphosphine as an internal standard. The suspensions were filtered over celite and analyzed via GC-MS. The oxidized products (anthracene and 9,9’- 135 bifluorene) were quantified based on a calibration curve of authentic samples. Other possible oxidation products, such as fluorenone, or anthraquinone, were not observed. Oxygen atom transfer studies with 18O-7 and PMe3.18O-7 cluster was prepared in situ by combining a thawing 1:1 THF/DCM solution of 50.3 mg 18O-3 (0.03 mmol) with 7.5 mg Ph3PCH2 (0.03 mmol) to prepare a solution of 6-18O. This solution was combined, while thawing, to a DCM solution of 9.5 mg [Fc][OTf] (0.03 mmol). Keeping this mixture as cold as possible, thawing Et2O was added to precipitate the oxidized cluster; boron nitride (BN) was added to ease separation of precipitate from the solution. This suspension was filtered to obtain solid 18O-7, which was eluted from BN with cold 1:1 THF/DCM. 50 μL PMe3 (0.50 mmol) was added to solution as it thawed, and was gradually warmed to room temperature. 31P NMR analysis of the reaction mixture at this stage showed a peak consistent with trimethylphosphine oxide (OPMe3) formation at ~35 ppm. After 30 minutes, the reaction was pumped down. On the bench, the crude reaction mixture was separated via silica plug; 10% MeOH in DCM was used to elute a dark solution, at which point MeOH was washed through the plug to collect a fraction containing OPMe3. The MeOH fraction was pumped down and analyzed via GC/MS, which displayed a GC peak characteristic of OPMe3, with both 16OPMe3 and 18OPMe3 based on its mass spectrum. 136 ELECTROCHEMICAL DETAILS Current (μA) 20 μA -2.5 50 mV/s 100 mV/s 150 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2 -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 1 Figure 13. Cyclic voltammetry of [LFe3O(PzNHtBu)3Fe(OH)][OTf], 2, (2.5 mM) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 200 mM [Bu4N][PF6] at various scan rates. * Current 15 μA * * 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2.5 -2 -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 1 Figure 14. Cyclic voltammetry of LFe3O(PzNHtBu)3Fe(O), 5, (2.3 mM) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 100 mM [Bu4N][PF6] at various scan rates. Electrochemical events marked with an asterisk (*) are assigned to a small amount of [LFe3O(PzNHtBu)3Fe(OH)]n+ that formed due to decomposition. 137 KINETICS DETAILS Substrate xanthene 1,4-cyclohexadiene 9,10-dihydroanthracene triphenylmethane fluorene BDE (kcal/mol)30 75.2 76.0 76.3 81.0 82.2 pKa (DMSO) 30.031 ~34a 30.131 30.632 22.632 k2 (M-1 s-1) with 5 40 ~0.3 87 ~0.7 ~3 x 106 Reported bond dissociation enthalpies (in kcal mol-1) and pKa values (in DMSO) of various organic substrates investigated for PCET reactivity with LFe3O(PzNHtBu)3Fe(O) (5), with their measured second order rate constants. 1H NMR analysis of the reaction mixtures after kinetics measurements show formation of 1 in all cases, consistent with a PCET process. aA reported pK a value for 1,4-cyclohexadiene could not be obtained, but is approximated based on the reported pKa value of 1,3-cyclohexadiene in DMSO (pKa(1,3) = 35.0) and the energy of isomerization between 1,3cyclohexadiene and 1,4-cyclohexadiene (-1.6 kcal/mol):33 1 Absorbance (a.u.) 0.8 0.6 0.4 0.2 0 350 450 550 650 750 850 950 Wavelength (nm) Figure 15. UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and xanthene (10 mM) at ambient temperature. 138 0 0.03 y = -0.0067x + 0.0285 R² = 0.9945 0.02 -1 -1.5 -3 0.015 0.01 -2 -2.5 y = 0.6676x + 0.0054 R² = 0.9436 0.025 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf]) -0.5 y = -0.014x + 0.0657 R² = 0.9917 0.005 y = -0.0242x - 0.0016 y = -0.0199x + 0.0936 R² = 0.9996 R² = 0.9884 0 50 100 150 200 0 0 250 0.01 0.02 0.03 [Xylene] (M) Time (min) Figure 16. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and xanthene (4.8, 10.1, 19.9, and 29.8 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. ln([A]-[Ainf]/[A0]-[Ainf]) Absorbance (a.u.) 0 0.5 -0.5 -1 -1.5 y = -0.0041x - 0.1039 R² = 0.9984 -2 -2.5 0 400 600 Wavelength (nm) 800 0 200 400 Time (min) Figure 17. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 1,4-cyclohexadiene (1 M) at ambient temperature. (Right) Pseudo-first order kinetics plot of the reaction by following the decay of the signal at 516 nm; this wavelength was used since the background decomposition of the compound did not affect this wavelength. 139 1 Absorbance (a.u.) 0.8 0.6 0.4 0.2 0 350 450 550 650 750 850 950 Wavelength (nm) Figure 18. UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 9,10dihydroanthracene (10 mM) at ambient temperature. 0.05 y = 1.4433x + 0.0007 R² = 0.9275 y = -0.0049x - 0.0073 R² = 0.9997 -1 -2 0.04 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf]) 0 0.03 y = -0.021x - 0.0193 R² = 0.999 -3 0.02 y = -0.0255x - 0.042 R² = 0.9983 -4 0.01 y = -0.0452x - 0.1816 R² = 0.9958 -5 0 0 100 200 300 Time (min) 400 500 0 0.01 0.02 0.03 [9,10-DHA] (M) Figure 19. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 9,10-dihydroanthracene (9,10-DHA; 5, 10, 20, and 30 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. 140 0 0.04 y = -0.0106x - 0.1387 R² = 0.9923 y = 0.217x R² = 0.9494 0.03 -2 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf] -1 0.02 y = -0.0146x - 0.2006 R² = 0.993 -3 y = -0.0282x - 0.2029 R² = 0.9935 -4 y = -0.0209x - 0.2058 R² = 0.9936 0.01 -5 0 50 100 150 200 0 250 0 Time (min) 0.05 0.1 0.15 0.2 [d4-9,10-DHA] (M) Figure 20. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and d4-9,10-dihydroanthracene (d4-9,10-DHA; 40, 60, 100, and 130 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. 0 1 -0.5 Absorbance (a.u.) ln([A]-[Ainf]/[A0]-[Ainf]) -1 -1.5 -2 0.5 -2.5 -3 -3.5 y = -0.0111x + 0.1612 R² = 0.9918 -4 0 -4.5 400 500 600 Wavelength (nm) 700 800 0 100 200 300 400 Time (min) Figure 21. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and triphenylmethane (1 M) at ambient temperature. (Right) Pseudo-first order kinetics plot of the reaction by following the decay of the signal at 516 nm; this wavelength was used since the background decomposition of the compound did not affect this wavelength. 141 x103 0.6 1/([5][fluorene]) (M-1) Absorbance (a.u.) 0.5 0.4 0.3 0.2 0.1 7000 y = 35845x + 1E+06 R² = 0.9721 y = 51310x + 522253 R² = 0.9819 y = 45347x + 463112 R² = 0.9862 6000 5000 4000 3000 2000 1000 0 350 550 750 0 0 20 Wavelength (nm) 40 60 80 100 Time (min) Figure 22. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 1 mM) and fluorene (2 mM) at ambient temperature in a 1 mm cuvette. (Right) Second order kinetics plot of the reaction by following the growth of the signal at 645 nm. 142 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3, 5, and 6. 1 1903350 2-PF6 1903348 3 1903352 5 1903351 6 1903349 Empirical formula C90H88Fe4N15 O5 C83H76F6Fe4 N15O5.5P C80H76F6Fe4 N15O11S2 C90H87Fe4N15 O5 C91H99F3Fe4N 15O11S Formula weight (g/mol) 1683.15 1793.95 1825.07 1682.14 1673.00 Radiation MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ=1.54178) CuKα (λ=1.54178) 14.1115(11) 11.9919(11) 12.150(2) 12.3162(13) 19.122(9) 15.0509(11) 13.7630(9) 14.975(5) 15.5743(15) 18.204(5) 21.1556(16) 25.905(2) 23.386(6) 21.6599(15) 24.698(6) 70.794(3) 89.286(4) 95.271(14) 102.390(6) 90 86.911(3) 87.757(4) 90.124(12) 94.445(4) 90 70.570(3) 79.589(4) 104.172(19) 102.897(9) 90 3993.7(5) 4201.8(6) 4106.5(18) 3921.5(7) 8597(5) 2 2 2 2 4 triclinic triclinic triclinic triclinic orthorhombic P-1 P-1 P-1 P-1 Pna21 1.400 1.375 1.476 1.425 1.463 4.74 to 77.068 6.53 to 149.628 3.796 to 148.742 6.454 to 160.188 6.032 to 130.72 0.777 6.219 6.726 6.338 6.172 1.019 1.044 1.063 1.012 1.064 0.0412, 0.0980 0.0710, 0.1919 0.0375, 0.0940 0.0400, 0.0954 0.0732, 0.1352 CCDC Number a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Cryst. syst. Space group ρcalcg (cm3) 2 Θ range (°) μ (mm-1) GOF R1, wR2 (I>2σ (I)) 143 Special refinement details for [LFe3O(PzNHtBu)3Fe(OH)][PF6]. A tert-butyl group of one of the pyrazolate ligands is partially disordered over two positions with occupancies of 39% (C93 and C94) and 61% (C97 and C98). There is significant solvent disorder that could not be fully refined, however electron density for a tetrahydrofuran (O100 and C100-C103), and two partial diethyl ether molecules (refined as C104-C109) were refined isotropically with only partial occupancy. Special refinement details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]2. One of the outersphere triflates was modeled as disordered over two nearly identical positions, with occupancies of 75% (S101 through C101) and 25% (S102 through C102). A ‘SAME’ constraint was used to favor distances and angles of these disordered triflates to the non-disordered one. A partially occupied solvent molecule (likely Et2O) was present; however, due to its position near a symmetry element, its residual electron density was removed via a solvent mask, as opposed to modeling this disordered solvent. Special refinement details for [LFe3O(PzNHtBu)3Fe(O)]. A benzene solvent is positionally disordered over two positions with occupancies of 29% (C106-C111) and 71% (C206-C211). Special refinement details for [LFe3O(PzNHtBu)3Fe(O)][OTf]. Generally, these crystals were of relatively poor quality compared to the other structures obtained; the crystal was twinned with a 13% twinned crystal component. While no disorder had to be modeled in the molecule, the outerphere triflate, or the three additional molecules of THF, the low intensity of high angle diffraction data led to low C–C bond precision. Initially, some carbon atoms in the ligand backbone had highly skewed ellipsoids, which were addressed with SIMU/DELU restraints (on O11, C11, C12, C26, C27, and C42 – C45) or, in one case, an ISOR restraint (C44). 144 Selected bond parameters for structurally characterized compounds 1-3, 5, and 6. Bond Distance (Å) Fe1–O1 Fe2–O1 Fe3–O1 Fe4–O1 Fe4–O2 Fe1–N13 Fe2–N23 Fe3–N33 Fe4–N14 Fe4–N24 Fe4–N34 N13–N14 N23–N24 N33–N34 N15–C72 N25–C82 (N26–C82) N35–C92 Bond Angles (º) N14–Fe4–N24 N24–Fe4–N34 N34–Fe4–N14 N14–Fe4–O2 N24–Fe4–O2 N34–Fe4–O2 O1–Fe4–O2 Torsion Angles (º) Fe1–N13–N14–Fe4 Fe2–N23–N24–Fe4 Fe3–N33–N34–Fe4 Centroid Distances (Å) Fe1|Fe2|Fe3– N14|N24|N34 Fe1|Fe2|Fe3– O11|O21|O31 Fe1|Fe2|Fe3–O1 N14|N24|N34–Fe4 1 2.102(1) 2.109(1) 2.089(1) 1.837(1) 1.937(1) 2.129(1) 2.126(1) 2.120(1) 2.097(1) 2.168(1) 2.111(1) 1.382(1) 1.368(1) 1.386(1) 1.397(2) 1.422(2) 1.393(2) 2-PF6 2.142(3) 2.101(3) 1.952(3) 1.890(3) 1.907(3) 2.107(4) 2.106(3) 2.071(4) 2.091(4) 2.056(4) 2.105(4) 1.373(5) 1.387(5) 1.389(5) 1.417(7) 1.354(7) 1.383(7) 3 2.148(1) 2.002(2) 1.971(1) 1.948(2) 1.879(2) 2.074(2) 2.057(2) 2.017(2) 2.083(2) 2.047(2) 2.059(2) 1.377(2) 1.394(2) 1.397(2) 1.422(3) 1.356(3) 1.350(3) 5 2.139(1) 2.050(2) 1.967(2) 1.965(2) 1.817(2) 2.124(2) 2.084(2) 2.090(2) 2.093(2) 2.104(2) 2.098(2) 1.388(2) 1.387(3) 1.378(3) 1.400(3) 1.366(5) 1.41(2) 1.402(3) 6 2.154(7) 1.927(7) 1.948(7) 2.049(7) 1.795(8) 2.091(9) 2.039(9) 2.015(8) 2.085(9) 2.087(9) 2.100(8) 1.396(12) 1.384(11) 1.387(12) 1.379(14) 1.391(14) 1.341(14) 120.1 122.1 117.8 88.4 91.3 92.2 177.1 119.1 118.8 121.7 91.7 93.5 91.1 177.3 116.6 119.8 122.7 92.6 95.0 92.1 178.7 118.3 123.0 117.5 96.3 92.0 92.8 177.1 113.4 119.6 124.0 97.6 96.3 93.6 176.0 13.9 11.6 13.4 2.7 9.9 12.5 6.2 8.2 11.1 19.4 4.5 13.7 21.9 17.4 2.7 2.050 1.701 1.643 1.685 1.627 1.121 1.110 1.078 1.120 1.122 1.153 0.025 1.105 0.075 1.036 0.120 1.076 0.138 1.012 0.218 145 References 1. (a) Que Jr, L.; Tolman, W. B. Nature 2008, 455, 333; (b) Hohenberger, J.; Ray, K.; Meyer, K. Nature Communications 2012, 3, 720; (c) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L. Chem. Rev. 2004, 104, 939-986; (d) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Chem. Rev. 2015, 115, 12974-13005; (e) Nguyen, A. I.; Ziegler, M. S.; Oña-Burgos, P.; Sturzbecher-Hohne, M.; Kim, W.; Bellone, D. E.; Tilley, T. D. J. Am. Chem. Soc. 2015, 137, 12865-12872; (f) Okamura, M.; Kondo, M.; Kuga, R.; Kurashige, Y.; Yanai, T.; Hayami, S.; Praneeth, V. K. K.; Yoshida, M.; Yoneda, K.; Kawata, S.; Masaoka, S. Nature 2016, 530, 465-468; (g) Hunter, B. M.; Thompson, N. B.; Müller, A. M.; Rossman, G. R.; Hill, M. G.; Winkler, J. R.; Gray, H. B. Joule 2018, 2, 747-763. 2. Yano, J.; Yachandra, V. Chem. Rev. 2014, 114, 4175-4205. 3. (a) Ichino, T.; Yoshioka, Y. Chem. Phys. 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B.; Takase, M. K.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 14861489; (b) de Ruiter, G.; Carsch, K. M.; Gul, S.; Chatterjee, R.; Thompson, N. B.; Takase, M. K.; Yano, J.; Agapie, T. Angew. Chem. Int. Ed. 2017, 56, 4772-4776; (c) Carsch, K. M.; de Ruiter, G.; Agapie, T. Inorg. Chem. 2017, 56, 9044-9054. 8. (a) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188-12190; (b) Gupta, R.; Taguchi, T.; Lassalle-Kaiser, B.; Bominaar, E. L.; Yano, J.; Hendrich, M. P.; Borovik, A. S. Proc. Natl. Acad. Sci. 2015, 112, 5319-5324; (c) Gupta, R.; MacBeth, C. E.; Young, V. G.; Borovik, A. S. J. Am. Chem. Soc. 2002, 124, 1136-1137; (d) Gupta, R.; Borovik, A. S. J. Am. Chem. Soc. 2003, 125, 13234-13242; (e) MacBeth, C. E.; Golombek, A. P.; Young, V. G.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938-941; (f) Ford, C. L.; Park, Y. J.; Matson, E. M.; Gordon, Z.; Fout, A. R. Science 2016, 354, 741; (g) Park, Y. J.; Matson, E. M.; Nilges, M. J.; Fout, A. R. Chem. Commun. 2015, 51, 5310-5313; (h) Matson, E. M.; Park, Y. J.; Fout, A. R. J. Am. Chem. Soc. 2014, 136, 17398-17401; (i) Gordon, Z.; Drummond, M. J.; Matson, E. M.; Bogart, J. A.; Schelter, E. J.; Lord, R. L.; Fout, A. R. Inorg. Chem. 2017, 56, 4852-4863. 9. Han, Z.; Horak, K. T.; Lee, H. B.; Agapie, T. J. Am. Chem. Soc. 2017, 139, 9108-9111. 10. Horak, K. T. The Design and Synthesis of Transition Metal Complexes Supported by Noninnocent Ligand Scaffolds for Small Molecule Activation. PhD dissertation, California Institute of Technology, Pasadena, California, 2016. 11. de Ruiter, G.; Thompson, N. B.; Lionetti, D.; Agapie, T. J. Am. Chem. Soc. 2015, 137, 14094-14106. 146 12. Arnett, C. H.; Chalkley, M. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 5569-5578. 13. Andris, E.; Navrátil, R.; Jašík, J.; Puri, M.; Costas, M.; Que, L.; Roithová, J. J. Am. Chem. Soc. 2018, 140, 14391-14400. 14. McDonald, A. R.; Que Jr, L. Coord. Chem. Rev. 2013, 257, 414-428. 15. (a) Bordwell, F. G.; Satish, A. V.; Zhang, S.; Zhang, X. M. Using thermodynamic cycles to study reactive intermediates. In Pure Appl. Chem., 1995; Vol. 67, p 735; (b) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441-450; (c) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. 16. (a) Saame, J.; Rodima, T.; Tshepelevitsh, S.; Kütt, A.; Kaljurand, I.; Haljasorg, T.; Koppel, I. A.; Leito, I. The Journal of Organic Chemistry 2016, 81, 7349-7361; (b) Garrido, G.; Koort, E.; Ràfols, C.; Bosch, E.; Rodima, T.; Leito, I.; Rosés, M. The Journal of Organic Chemistry 2006, 71, 9062-9067. 17. Reed, C. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 10900-10908. 18. PCET is broadly referred to here as the transfer of a proton and an electron to different parts of a complex (see ref. 17); the precise mechanism, whether concerted (CPET or EPT) or stepwise (either PTET or ETPT), is left ambiguous, as the present experiments cannot differentiate them. 19. Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Chem. Rev. 2012, 112, 4016-4093. 20. Goetz, M. K.; Anderson, J. S. J. Am. Chem. Soc. 2019, 141, 4051-4062. 21. Huang, C.-Y.; Doyle, A. G. J. Am. Chem. Soc. 2012, 134, 9541-9544. 22. Izod, K.; Rayner, D. G.; El-Hamruni, S. M.; Harrington, R. W.; Baisch, U. Angew. Chem. Int. Ed. 2014, 53, 3636-3640. 23. Hagadorn, J. R.; Que, L.; Tolman, W. B. Inorg. Chem. 2000, 39, 6086-6090. 24. Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676-3682. 25. Ludwiczak, M.; Majchrzak, M.; Marciniec, B.; Kubicki, M. J. Organomet. Chem. 2011, 696, 1456-1464. 26. Alberola, A.; Antolín, L. F.; Cuadrado, P.; González, A. M.; Laguna, M. A.; Pulido, F. J. Synthesis 1988, 1988, 203-207. 27. Herbert, D. E.; Lionetti, D.; Rittle, J.; Agapie, T. J. Am. Chem. Soc. 2013, 135, 19075-19078. 28. (a) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58; (b) Singh, A. K.; Jacob, W.; Boudalis, A. K.; Tuchagues, J.-P.; Mukherjee, R. Eur. J. Inorg. Chem. 2008, 2008, 2820-2829; (c) Sutradhar, M.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem. 2012, 2012, 4273-4278; (d) Schünemann, V.; Hauke, P. Mössbauer Spectroscopy. In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269. 29. Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. J. Comput. Chem. 2013, 34, 1164-1175. 30. Xue, X.-S.; Ji, P.; Zhou, B.; Cheng, J.-P. Chem. Rev. 2017, 117, 8622-8648. 31. Bordwell, F. G.; Bares, J. E.; Bartmess, J. E.; McCollum, G. J.; Van der Puy, M.; Vanier, N. R.; Matthews, W. S. The Journal of Organic Chemistry 1977, 42, 321-325. 32. Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.; Cornforth, F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCollum, G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006-7014. 33. Taskinen, E.; Nummelin, K. The Journal of Organic Chemistry 1985, 50, 4844-4847.  CHAPTER 5 INTERMOLECULAR REACTIVITY OF TETRANUCLEAR FE CLUSTERS VIA PUTATIVE FE–OXO AND –IMIDO INTERMEDIATES 148 ABSTRACT Sterically open pyrazolate-bridged tetranuclear Fe clusters were examined for their reactivity towards oxygen and nitrogen transfer reagents. Addition of iodosylarene to a FeII2FeIII2 cluster produces a one electron oxidized terminal-hydroxide cluster, which ultimately forms an octanuclear μ2-O cluster, upon dehydration. Formation of the terminal hydroxide cluster is considered to occur due to formal hydrogen atom abstraction from a reactive intermediate that could not be extensively characterized (either terminal Fe-oxo or iodosylarene adduct). The one electron reduced pyrazolate cluster is capable of activating electron deficient aryl azides, leading to isolation of clusters bearing an NHAr amide ligand, via a putative Fe-imido moiety. Reactivity studies were also performed with an interstitial fluoride containing Fe cluster. Oxygen atom transfer to a μ4-F containing FeII4 cluster leads to formation of analogous octanuclear μ2-O cluster, or cluster rearrangement to afford a fluoride bound μ4-O cluster. The intermolecular reactivity of these putative Fe-oxo and –imido moieties were limited to decomposition by formal hydrogen atom transfer in solution, which highlights the high reactivity of these complexes, likely due, in part, to the open coordination environment of the unsubstituted bridging pyrazolates. 149 INTRODUCTION Nature utilizes a variety of multinuclear transition metal arrangements to accomplish many catalytic transformations.1 Three general cases can be considered for the roles these clusters possibly have in metalloenzymes: (i) the transfer of electrons, with no direct substrate-cluster binding or interaction (i.e. [4Fe-4S] clusters); (ii) binding and activation of substrate at a single metal site with the auxiliary metals providing a specific structural or electronic environment for the substrate-binding metal; or (iii) binding and activation of substrate across multiple metal sites within the cluster. Often, the precise role of each metal center within an active site cluster is ambiguous, based on the available biochemical data. Therefore, developing our understanding of the reactivity of transition metal clusters related to these metalloenzymes can help establish the functional purposes of their unique multinuclear architectures. A number of synthetic mononuclear transition metal complexes have been studied with the goal of providing insight into biological multinuclear active sites that are thought to activate substrates at a single metal site.2 Due to a number of strategies to tune the reactivity of transition metal complexes through ligand modifications, and the relative ease of structural and spectroscopic characterization of these small molecules, the transition metal chemistry of these systems, which are relevant to biological processes, can be probed in fine detail. However, these studies are not able to establish the possible role of auxiliary metal centers within multinuclear active sites. Examples of multinuclear systems where the reactivity of one metal site can be probed as a function of an auxiliary metal are relatively rare,3 with most examples limited to binuclear complexes.4 A challenge to developing better models of multinuclear active sites, where the level of detail in study can match that of mononuclear systems, lies in the difficulty of producing welldefined multinuclear structures which bear distinct metal coordination environments, 150 specifically ones where substrate binding and activation is limited to a single metal site within the cluster. These ‘site-differentiated’ clusters require a fine balance of stability and reactivity to be suitable for detailed studies of their property-reactivity relationships. Along these lines, our group has developed routes to synthesize a family of transition metal clusters supporting various first row transition metal centers (Mn, Fe, Co, Ni, Cu, Zn) in a robust scaffold that arranges the metal in a site-differentiated fashion capable of supporting these propertyreactivity studies.5 These clusters are all based on a common symmetric trinuclear metal precursor, where a fourth (apical) metal site is introduced through the use of bridging pyrazolates (or imidazolates), and anchored into a tetrahedral metal arrangement through a central interstitial μ4-atom ligand (either O or F).6 The resulting apical metal center is four coordinate, with a trigonal pyramidal geometry, suitable to study its reactivity as a function of the auxiliary coordinatively saturated metal ions. The reactivity of these clusters towards accessing M=E (E = O, N) moieties is appealing to study, due to the implication of terminal Mn=O species in the OEC of photosystem II7 and the possibility of Fe=NR intermediates in nitrogenase FeMo cofactor.8 Understanding the ways in which neighboring metal centers can affect the nature of these reactive intermediates, therefore, has relevance towards our understanding of multinuclear active sites. Furthermore, interest in the chemistry of these types of reactive intermediates has led to many examples of mononuclear systems capable of supporting these moieties;9 significantly less developed is the chemistry of these intermediates in complexes with more than two redox active metals.10 Previous attempts to examine oxygen- or nitrogen-atom transfer reactivity through terminal M=E intermediates were performed by a number of group members on various pyrazolate-bridged clusters of Fe and Mn. Dr. Graham de Ruiter and Kurtis Carsch studied intramolecular oxygen atom transfer reactions between Fe4, Mn4, and Fe3Mn clusters bearing 151 arylpyrazolate ligands (Scheme 1);6b, 11 fast C–H (or C–F) activation of the pendant arene moiety was observed, precluding any reactivity with external substrates, even in large excesses. Attempts to observe intermolecular reactivity led to the development of clusters supported by unsubstituted pyrazolate ligands by Dr. Kyle Horak.12 Early synthetic routes to these clusters relied on FeCl2 as the source of the apical metal, leading to isolation of [LFe3O(Pz)3FeCl][OTf]; removal of the Cl ligand proved challenging, precluding extensive reactivity studies. Exchange of the Cl- ligand for N3- was accomplished, serving as a possible precursor to a reactive Fe-nitride cluster; however, photolysis or thermolysis of this cluster did not lead to activation of the azide ligand. Scheme 1. Related Studies of Tetranuclear Clusters By Previous Members of the Agapie Group6b, 11-12 Herein is an extension of the chemistry of the tetranuclear Fe clusters supported by unsubstituted pyrazolate ligands, towards examining intermolecular reactivity via Fe=O or Fe=NR intermediates. Characterization of reaction products demonstrates decomposition by formal hydrogen atom transfer of these putative intermediates to produce the corresponding Fe–OH and Fe–NHR (R = -aryl or –tosyl) species. 152 RESULTS AND DISCUSSION Synthesis and Characterization of Fe4 Clusters Bearing Unsubstituted Pyrazolate Ligands with a Labile Apical Fe Ligand. Tetranuclear Fe clusters with unsubstituted pyrazolates and a labile trifluoromethanesulfonate (triflate, -OTf) ligand bound to the apical Fe can be prepared in two steps starting from the reported tri-iron cluster LFe3(OAc)(OTf)2 by stirring this cluster with three equivalents potassium pyrazolate (KPz), iodosylbenzene (PhIO), and Fe(II) acetate (Figure 1A). This produces a tetranuclear Fe cluster bearing unsubstituted pyrazolate ligands and an acetate ligand bound to the apical Fe, 1. A partial Xray crystal diffraction dataset was collected to confirm the identity of this cluster; the bond metrics of the Fe–μ4-O distances were similar to the previously synthesized Fe4-chloride Figure 1. (A) Synthesis of tetranuclear Fe clusters with unsubstituted pyrazolate ligands and labile ligands bound to the apical Fe; inset, 1,3,5-triarylbenzene ligand platform (L3-). (B) Preliminary crystal structure of 1 and crystal structure of 2 (C); hydrogen atoms, outersphere anions, and solvent molecules omitted for clarity. 153 cluster of the same oxidation state, consistent with a redox distribution where the apical Fe is trivalent (Table 1). Addition of calcium triflate to this cluster (to precipitate the less soluble calcium acetate) affords isolation of the tetra-iron dication bis-triflate, 2. Structural characterization by XRD displays binding of the a triflate counterion to the apical Fe, with changes in the Fe–μ4-O distances consistent with reduction of the apical Fe to the 2+ oxidation state, and concomitant oxidation of an Fe in the tri-iron core to FeIII. This is supported by the Mössbauer spectrum of 2, which contains parameters for the apical Fe consistent with a five-coordinate high-spin FeII (δ = 0.95 mm/s; |ΔEq| = 2.22 mm/s; Figure 2 and Table 2). Table 1. Selected Bond Distances for Structurally Characterized Pz-Fe4 Clusters Metric (Å) b [LFe3O(Pz)3Fe(Cl)][OTf]12 1a 3-MeCN 2 4 Fe1-O1 2.071(4) 2.17 2.086(9) 2.055(6) 2.143(7) Fe2-O1 2.145(3) 2.15 2.049(8) 2.018(5) 1.977(7) Fe3-O1 2.024(4) 1.91 1.926(9) 1.949(5) 1.989(6) Fe4-O1 1.864(4) 1.87 1.977(8) 1.999(5) 1.959(6) 2.04 2.169(11) 2.155(6) 2.087(8) (Fe4-O2) (Fe4-N2) (Fe4-O2) (Fe4-O2) aPreliminary structure bBold bond distances denote bonds with FeIII centers, the rest are assigned to FeII. Fe4-L 2.339(2) (Fe4-Cl) Cyclic voltammetry of 2 in acetonitrile (MeCN) displays two quasi-reversible peaks, corresponding to the oxidation and reduction of 2, with reduction potentials of -0.89 V (all potentials vs. Fc/Fc+) and -0.13 V (Figure 3). A second quasi-reversible oxidation is observed at 0.57 V, however the return reductive scan produces new electrochemical events, suggesting a putative FeIII4 cluster is accessible, but unstable under the electrochemical conditions. The reduced and oxidized clusters 3 and 4 were accessed through treatment of 2 with cobaltocene (CoCp2) and silver triflate (AgOTf), respectively (Figure 2). Mössbauer spectra of these clusters was consistent with reduction of 2 occurring at an Fe within the tri-iron core, but oxidation 154 occurring at the apical Fe, as opposed to the FeII in the core. This is an unusual observation for these types of tetranuclear clusters, where redox changes are typically restricted to the sixcoordinate metal centers (only when there are changes to are ligand bound to the apical metal are redox changes for that metal observed). The redox distribution of 4 can be reversibly perturbed by displacement of triflate bound to the apical Fe; the solution state Mossbauer spectrum of 4 in MeCN (4-MeCN) displays notable changes to the parameters for the Fe centers, consistent with the loss of high-spin FeII in the tri-iron core and reduction of the apical Fe. Figure 2. Chemical reduction and oxidation of 2 to afford 3 and 4, respectively. Zero appliedfield 57Fe Mössbauer spectra of these clusters in the solid state, and 4 in a solution of MeCN (4-MeCN). 155 Table 2. 57Fe Mössbauer Parameters for Pz-Fe4 Clusters 2 – 4, and 4-MeCN δ (mm/s) |ΔEq| (mm/s) assignment |ΔEq| (mm/s) δ (mm/s) 3 (FeII3FeIII) assignment 4 (FeIIFeIII3) Fe1, Fe2 1.12, 1.11 3.49, 3.10 h.s. FeII Fe1 1.12 2.81 h.s. FeII Fe3 0.53 1.43 h.s. FeIII Fe2, Fe3 0.46, 0.48 0.62, 0.91 h.s. FeIII Fe4 0.99 1.37 h.s. apical FeII Fe4 0.37 0.34 h.s. apical FeIII 2 (FeII2FeIII2) 4-MeCN (FeIIFeIII3) Fe1 1.10 3.15 h.s. FeII Fe1, Fe2, Fe3 0.45, 0.46, 0.46 1.03, 0.72, 0.42 h.s. FeIII Fe2, Fe3 0.47, 0.48 0.58, 0.96 h.s. FeIII Fe4 0.89 1.74 h.s. apical FeII Fe4 0.95 2.22 h.s. apical FeII FeII2FeIII2 FeIIFeIII3 Current FeII3FeIII 50 μA -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 Figure 3. Cyclic voltammetry of 2 in MeCN (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.4 V. 156 Investigations of Pz-Fe4 Clusters with Oxygen Atom Transfer Reagents. With access to these pyrazolate-bridged Fe clusters with a relatively labile ligand (OTf) bound to the site-differentiated Fe, their reactivity towards oxygen atom transfer (OAT) agents were investigated towards performing reactivity studies through a terminal Fe-oxo moiety. Treatment of the [FeII2FeIII2] cluster, 2, with 2-tert-butyl-sulfonyl iodosylbenzene (sPhIO) produces a [FeIIFeIII3] cluster with a terminal hydroxide ligand bound to the apical Fe, 5, based on TOF-MS, 1H NMR spectroscopy, and Mössbauer spectroscopy (Figure 4). This cluster decomposes upon standing, or if concentrated under vacuum, to produce an octanuclear Fe Figure 4. (A) Reactivity of 2 towards OAT reagent sPhIO. (B) Zero applied-field Mössbauer spectrum of 5 in MeCN with parameters for each unique quadrupole doublet. (C) Structure of 6, with hydrogen atoms, outersphere counterions, and solvent molecules omitted for clarity. (D) Zero applied-field Mössbauer spectrum of 6 in MeCN with parameters for each unique quadrupole doublet. 157 cluster with a μ2-O ligand bound to two pyrazolate-bridged [FeIIFeIII3] cluster subunits, 6; this compound was structurally characterized via XRD. Similar results are obtained with different OAT reagents, such as tert-butyl hydroperoxide and tetrabutylammonium meta-periodate. Observation of 5 in solution, likely followed by dehydration to form 6, supports the conclusion that, under the experimental conditions employed here, a 1:1 cluster to sPhIO stoichiometry is dominant, as opposed to consuming half an equivalent of sPhIO to afford 6 directly. Producing 5 from 2 and sPhIO is consistent with formation of a reactive species which undergoes formal hydrogen atom transfer; both terminal metal-oxo and iodosylarene adduct complexes are known to undergo this type of chemistry.13 In an attempt to distinguish between these two possible reactive intermediates, variable temperature 1H NMR experiments were performed. In deuterated solvents (CD2Cl2 or CD3CN) at low temperatures (below -20 °C), mixtures of 5 and a putative intermediate were Figure 5. (A) 1H NMR (400 MHz, CD2Cl2, -40 °C) spectra of 2 (blue), intermediate of 2 and sPhIO before formation of 5 (green), and 5 formed by addition of excess toluene to this intermediate (red). (B) Mössbauer spectrum of the putative intermediate, collected by cold pentane precipitation, with parameters: (i) δ = 1.06 mm/s, |ΔEq| = 3.13 mm/s (blue trace), (ii) δ = 0.40 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.46 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.49 mm/s, |ΔEq| = 1.44 mm/s (green trace). 158 observed (Figure 5). A Mössbauer spectrum of this mixture was obtained, and it ruled out the presence of an FeIV-oxo moiety, with parameters that were best fit with 5 and another [FeIIFeIII3] cluster. However, assigning this species to either possible oxidizing intermediate (FeIII-oxo or -iodosylarene adduct) was inconclusive. Analogous experiments were performed on the other oxidation states of the Pz-Fe4 clusters, 3 and 4. In neither case could a reactive intermediate be observed by NMR at an appreciable concentration; 3 would react completely, even at -40 °C, while 4 would remain mostly unreacted towards sPhIO at low temperatures. The electrochemical and spectroscopic properties of 6 were investigated; however, it was later determined that isolation and recrystallization of samples of 6 contained two species by 1 H NMR with nearly identical resonances (Figure 6). These species could be separated from each other through extraction of 6 (as confirmed by XRD) in MeCN. Attempts to structurally characterize the remaining MeCN precipitate were unsuccessful. Figure 6. 1H NMR (300 MHz, CD2Cl2) of as-isolated and recrystallized 6 (red) and the species separated by trituration in MeCN (green and blue). (Inset) Paramagnetic regions highlighting the differences in peak positions of these two species. Cyclic voltammetry of 6 in CH2Cl2 displays five quasi-reversible redox events separated into closely spaced pairs, attributable to redox changes occurring within each of the tetra-iron 159 subunits (Figure 7). The reduction potentials of these processes, where the charge of the cluster is used to abbreviate oxidation state (0 = [FeII3FeIII]2O, 1+ = [FeII3FeIII]O[FeIII2FeII2], etc.): 0 → 1+, -1.98 V (all potentials vs Fc/Fc+); 1+ → 2+, -1.64 V; 2+ → 3+, -0.75 V; 3+ → 4+, -0.56 V; 4+ → 5+, 0.10 V. The small separation between the pairs of redox events is consistent with relatively little electronic interaction between the individual Pz-Fe4 subunits. The redox event to form the 6+ cluster ([FeIII4]2O) was not observed in the CV, or via squarewave voltammetry. 1+ 2+ 3+ 4+ 5+ Current 0 20 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 Figure 7. Cyclic voltammetry of 6 in CH2Cl2 (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was 0 V. Reactivity of Pz-Fe4 Clusters with Organic Azides and N-Tosylimino-Transfer Reagent. Due to the challenges in observing evidence for formation of, and reactivity from, a terminal Fe-oxo with 2 and OAT reagents, reactions targeting an Fe-imido species were attempted; it was hypothesized that substituents on the nitrogen atom could be selected to stabilize the Fe-bound intermediate, i.e. having a bulky, electron withdrawing group would 160 slow down decomposition by formal hydrogen atom transfer, and disfavor formation of μ2NR clusters (analogous to 6). 2 proved to be unreactive towards aryl- or alkyl-azides, but the one electron reduced cluster, 3, was competent for activation of relatively electron poor aryl azides. These reactions were typically complete upon warming to room temperature; preliminary structural characterization of reactions between 3 and 2-azideobiphenyl and 3,5trifluoromethyl-phenylazide displayed apical Fe centers with a coordinated aryl amide (– NHAr), as evidenced by the relatively long apical Fe–N distance of ~2.0 Å and small Fe–N– C angle of ~130°, consistent with reported structures of FeIII–NHAr complexes.14 Figure 8. (A) Synthesis of aryl- and tosyl-amide clusters 7 – 9 from 3; (inset) possible Febound intermediates include a terminal Fe-imido or iodo-tosylimino adduct. (B) Truncated preliminary crystal structure of 7. (C) Truncated crystal structure of 8; hydrogen atoms, counterions, and solvent molecules omitted for clarity. 161 Table 3. Selected Bond Distances and Angles for Complexes 7 – 9. Metric (Å)b 7a 8a 9 Fe1-O1 2.10 2.16 2.047(8) Fe2-O1 2.08 2.09 2.044(9) Fe3-O1 2.04 1.99 1.993(7) Fe4-O1 1.87 1.90 2.035(8) Fe4-N1 2.01 1.99 1.856(10) ∠Fe4–N1–C1 131 131 146.8 (∠Fe4–N1–S1) a Preliminary structure. bBold bond distances denote bonds with FeIII centers, the rest are assigned to FeII. Similarly, treatment of 3 with an iodoimino transfer agent, 2-tert-butyl-sulfonyl-N-paratoluenesulfonyl-iminoiodobenzene (sPhNTs), leads to isolation of the tosylamide-bound cluster, 9. Overall, attempts to stabilize Fe-imido clusters with electron withdrawing or sterically bulky aryl groups were unable to lead to observation of this intermediate before formal hydrogen atom transfer to produce the corresponding amide. For reactions with aryl azides, it is possible that azide activation leads to a short-lived aryl-imido cluster, although off-metal reactive species cannot be ruled out, i.e. outersphere electron transfer producing reactive nitrene. Application of these reactions towards amination of cyclohexene were attempted, however no N-transfer product was observed. Reactivity of μ4-F Pz-Fe4 Clusters Towards Oxygen Atom Transfer Reagents and Azide. Due to the challenge of observing evidence of a terminal Fe-oxo or –imido moiety within these clusters, investigations on related clusters bearing a different electronic environment were pursued. Previous investigations on μ4-F bridged clusters demonstrated a significant effect the interstitial ligand has on the properties and reactivity of the Fe4 cluster (see Chapter 2).6d A μ4-F ligand could make the apical Fe more electron deficient, and any resulting terminal Fe-oxo less basic; furthermore, weaker the bonding to the μ4-F may promote 162 multiple bonding to a terminal-oxo by facilitating pseudo-tetrahedral geometry at the apical Fe. Unsubstituted pyrazolate-bridged clusters bearing an interstitial F- ligand were prepared through addition of tetrabutylammonium fluoride to LFe3(OAc)(OTf)2 with a combination of potassium pyrazolate and two equivalents of pyrazole (HPz). The apical Fe is introduced with Fe(N(SiMe3)2)2 and calcium triflate was added to sequester acetate from the initial trinuclear cluster, to afford isolation of a fluoride-bridged FeII4 cluster, 10 (Scheme 2). Scheme 2. Synthesis and Reactivity of Pz-Fe4 Clusters Bearing μ4-F Ligand. Structural characterization of 10 confirms a cluster geometry analogous to the μ4-O versions (Figure 9A). The electrochemistry of this cluster is more complex than 2. In MeCN, only one quasi-reversible feature is observed, corresponding to a reduction potential for the oxidation of 10 at -0.52 V (vs. Fc/Fc+), followed by two irreversible oxidations, which change 163 over multiple CV scans (Figure 10), likely due to decomposition of the FeIIFeIII3 cluster oxidation state. Figure 9. Truncated crystal structures of 10 (A), 11 (B), and 13 (C; preliminary). Current 40 μA -2.5 -1.5 -0.5 0.5 Potential (V, vs. Fc/Fc+) 1.5 2.5 Figure 10. Cyclic voltammetry of 10 in MeCN (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.7 V. The full CV scan (blue) displays two irreversible oxidations, which shift over multiple CV scans. Treatment of 10 with different OAT reagents results in multiple reaction products, with product distributions that depends on the nature of the OAT reagent. Reactions with 164 iodosylbenzene (PhIO), which is polymeric and insoluble, leads to isolation of a μ2-O cluster (11), with a structure analogous to 6 (Figure 9B). Analysis of bond metrics of 6 and 11 display shorter bonds to the μ2-O in 11 (1.7853(6) vs. 1.8079(7) Å), with a shift of the apical Fe out of the equatorial plane (Table 4). Table 4. Selected Bond Distances in Complexes 6 and 11. Metric (Å) 6 ([FeIIFeIII3]2O) 11 ([FeII3FeIII]2O) Fe1-O1/F1 1.942(3) 2.191(2) Fe2-O1/F1 2.096(3) 2.205(2) Fe3-O1/F1 2.176(3) 2.235(2) Fe4-O1/F1 1.925(3) 1.991(2) Fe4-O2 1.8079(7) 1.7853(6) 0.099 0.174 Fe4-N14|N24|N34 Alternatively, tert-butyl hydroperoxide (tBuOOH) or tetrabutylammonium periodate ([Bu4N][IO4]) react very quickly with 10 to produce a species which is assigned to an interstitial O cluster with a terminal F ligand, 12. This is supported by independent synthesis of 12 from 1 and tetrabutylammonium fluoride, as evidenced by 1H NMR spectroscopy. A possible intermediate to 12 could be a terminal Fe-oxo, followed by internal ligand rearrangement to form the more stable interstitial oxo cluster, although direct substitution of the μ4-F by the OAT reagent is also a possible route to this complex. Observation of substitution of the μ4-F in 10 suggested a potential route to tetranuclear clusters with a bridging nitride ligand, via formation of a terminal nitride that undergoes cluster rearrangement. The azide-bound Pz-Fe4 cluster, 13, can be prepared through addition of tetrabutylammonium azide to 10 (Figure 9C). Attempts to thermalyze or photolyze this moiety, however, led to no reaction being observed (Scheme 1). 165 CONCLUSIONS A series of tetranuclear Fe clusters bearing a sterically open coordination environment at the apical Fe could be prepared with a coordinated labile triflate ligand, suitable for reactivity studies towards oxygen and nitrogen atom transfer reagents. For 2, and its one electron reduced form, 3, fast intermolecular decomposition of putative Fe-oxo and Fe-imido moieties (or iodosylarene adducts) occur by formal hydrogen atom transfer, likely from organic solvent. A cluster bearing an interstitial F ligand, 10, demonstrated similar reactivity, forming an octanuclear μ2-O cluster containing fluoride, but was also capable undergoing substitution of this interstitial fluoride ligand with oxygen. Overall, it is challenging to reach a suitable balance between stability and reactivity for these Pz-Fe4 clusters to prepare terminal-oxo or -imido moieties. Subsequent studies demonstrated the necessity of pendant hydrogen bonding donors on the pyrazolate ligands to stabilize an Fe-oxo (Chapter 4). Further investigations could examine strategies to tune these secondary coordination sphere interactions to access a suitably reactive, but still well-defined, Fe-oxo cluster. 166 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. LFe3(OAc)(OTf)2,6a potassium pyrazolate (KPz),6e iodosobenzene (PhIO),15, tetrabutylammonium fluoride ([Bu4N][F]),16, iron(II) bis-hexamethyldisilylamide (Fe(N(SiMe3)2)2),17 2-tert-butyl-sulfonyl-iodosylbenzene (sPhIO),18 2-tert-butyl-sulfonyl-Npara-toluenesulfonyl-iminoiodobenzene (sPhINTs),18 and 3.5-trifluoromethyl-phenylazide,19 were prepared according to literature procedures. All organic solvents were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H spectra were recorded on a Varian 300 MHz spectrometer; variable temperature spectra were recorded on a Varian 400 MHz spectrometer. CD3CN and CD2Cl2 were purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated solution in MeCN. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). Mössbauer simulation details for all compounds. All spectra were simulated as four pairs of symmetric quadrupole doubles with equal populations and Lorentzian lineshipes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). 167 Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.6a, 11b, 20 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.21 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (reference) and bare platinum wire (counter), in a N2 filled M. Braun glovebox at RT. The ferrocene/ferrocinium (Fc/Fc+) redox wave were used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and were refined using a riding model. Due to the size of the compounds most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Synthetic Procedures. Synthesis of [LFe3O(Pz)3Fe(OAc)][OTf] (1). 627 mg (0.45 mmol) LFe3(OAc)(OTf)2 was suspended in 8 mL THF in a 20 mL scintillation vial. This suspension was frozen in a liquid N2-cooled cold-well. In a separate vial, a suspension of 152 mg (1.43 mmol) potassium pyrazolate in 2 mL was also frozen. The suspensions were combined, while 168 thawing, and stirred for 10 minutes. Then, 2 mL THF was used to transfer 101 mg (0.46 mmol) iodosylbenzene to the stirring suspension; after 60 minutes, a brown suspension forms. 80 mg (0.465 mmol) Fe(OAc)2 was added to the reaction with 3 mL THF and stirred for 20 hours. The precipitate was collected over a bed of celite on a glass frit, washed with 3 mL THF, and dried under vacuum. A brown solution was collected by washing this solid with CH2Cl2 and filtering it through the frit. Recrystallization via CH2Cl2/Et2O vapor diffusion affords 375 mg (55% yield) 1 as brown crystals. 1H NMR (300 MHz, CD2Cl2) δ 131.9 (br), 83.9 (br), 75.9, 56.6, 50.9, 37.6, 26.2, 22.9, 14.4, 13.6, 11.0, 2.6), 1.3, -2.3 (br), -12.3, -17.8 (br) ppm. Synthesis of [LFe3O(Pz)3Fe][OTf]2 (2). 124 mg Ca(OTf)2 (0.37 mmol) was dissolved in 10 mL MeCN and 290 mg (0.19 mmol) 1 was added as a 10 mL DCM solution. This was stirred for 24 – 48 hours, until the 1H NMR showed complete conversion to [LFe3O(Pz)3Fe][OTf]2. The reaction was dried completely under vacuum, and a green-brown compound was dissolved in 15 mL DCM, filtered over celite, and recrystallized via vapor diffusion of Et2O. 293 mg (0.184 mmol; 96% yield) of 2 were obtained after drying the resulting crystals under vacuum. X-ray quality crystals were obtained by oxidizing 3 with 1 equivalent AgBPh4 in THF. Crystallization via diffusion of pentane into a CH2Cl2 solution of this compound affords crystals of [LFe3O(Pz)3Fe][OTf][BPh4] (1H NMR identical to [LFe3O(Pz)3Fe][OTf]2). 1H NMR (300 MHz, CD3CN) δ 122.0 (br), 71.1, 69.0, 53.3, 50.9, 42.4, 38.4, 16.7, 14.2, 13.9 (br), 12.5, 8.4 (br), 7.2, 3.9, 3.4, -5.5 (br) ppm. 19F NMR (300 MHz, CD3CN) δ -76 ppm. UV-Vis (CH3CN) [ε (M-1 cm-1)]: 246 nm (6.4 x 104), 369 nm (6.7 x 103). Anal. Calcd. (%) for C68H48F6Fe4N12O10S2: C, 51.77; H, 3.03; N, 10.54. Found: C, 51.17; H, 3.11; N, 10.46. Synthesis of [LFe3O(Pz)3Fe][OTf] (3). A suspension of [LFe3O(Pz)3Fe][OTf]2 (2; 43.8 mg, 0.027 mmol) in 2 mL THF was stirred as a THF solution of 5.9 mg CoCp2 (0.031 mmol) was added. After 1 hour, the reaction was dried under vacuum. 4 mL DME was added to the purple 169 solid and stirred for 12 hours. The resulting purple precipitate was collected on a bed of celite, washed with 2 mL DME, dried, and eluted with 2:1 THF/MeCN; crystals of [LFe3O(Pz)3Fe][OTf] (3) were obtained by vapor diffusion of Et2O into this solution. 1H NMR (300 MHz, CD3CN): 101.8 (br), 59.4, 57.4, 35.8, 30.0, 29.5, 19.5, 15.9, 13.3, 7.2, 4.5, 3.9, -1.6 (br), -5.9 (br) ppm. Synthesis of [LFe3O(Pz)3Fe][OTf]3 (4). 45 mg (0.03 mmol) of [LFe3O(Pz)3Fe][OTf]2 (2) was stirred as a suspension in THF. A THF solution of AgOTf (7 mg; 0.03 mmol) was added. After 30 minutes the reaction was dried under vacuum. The resulting solid was filtered over celite with DCM and the filtrate was dried under vacuum. This brown solid was crystallized via vapor diffusion of Et2O into a DCM/MeCN solution to obtain 47 mg [LFe3O(Pz)3Fe][OTf]3 (95% yield). 1H NMR (300 MHz, CD3CN) δ 169.0 (br), 94.7 (br), 81.8, 78.9, 72.9, 64.5, 45.0, 17.3, 10.4, 8.5, 7.9, 5.2, -0.8 (br) ppm. 19F NMR (300 MHz, CD3CN) δ 76 ppm. Synthesis of [LFe3O(Pz)3Fe(OH)][OTf]2 (5). Method A. 10.8 mg (0.007 mmol) 1 was layered in an NMR tube in MeCN with 100 μL of a 0.11 M MeCN solution of tBuOOH (0.011 mmol; prepared by diluting a 3.3 M stock in toluene) in a liquid N2-cooled cold well. The layers were mixed upon thawing. 5 decomposes upon pumping down, or standing for long periods of time. In some experiments, excess (~2 equiv.) DABCO was added, but this had no noticeable effect on the production 5. 1H NMR (300 MHz, CD3CN) δ 145.0 (br), 103.7 (br), 85.6, 66.6, 63.9, 45.5, 30.0 (br), 15.3, 13.6, 11.1, 7.2, -3.0 (br) ppm. Method B. 54 mg (0.034 mmol) 2 was dissolved in 10 mL MeCN and froze in a liquid N2cooled cold well. This was combined with 12 mg (0.035 mmol) sPhIO upon thawing and mixed for 30 minutes. This produces an NMR identical to Method A, with a minor amount of 6, due to decomposition. 170 Synthesis of [(LFe3O(Pz)3Fe)2O][OTf]4 (6) Method A. Concentration of a solution of 5 under vacuum produces a change in the NMR consistent with formation of 6. 1H NMR (300 MHz, CD2Cl2) δ 137.4 (br), 98.9 (br), 80.3, 63.7, 60.2, 50.3, 25.3 (br), 14.6, 13.1, 10.8, 9.9, 2.7, 1.7, 2.6 (br) ppm. Method B. A MeCN solution of 2 was mixed with 0.6 equivalents PhIO for 5 hours at room temperature. The reaction was dried under vacuum and filtered with CH2Cl2. This crude product was recrystallized via CH2Cl2/Et2O vapor diffusion to obtain mostly 6 (with an unidentified impurity, vide infra), with an NMR identical to Method A. Regardless of preparation method, a side product (ca. 30% relative to 6) would be present with nearly identical NMR shifts: 1H NMR (300 MHz, CD2Cl2) δ 137.5 (br), 98.7 (br), 80.6, 63.6, 60.1, 50.9, 25.7 (br), 14.6, 13.2, 9.9, 2.7, 0.84, -2.6 (br) ppm. This could be separated from 6 by stirring a solution of the mixture for 18 hours in MeCN and collecting the filtrate, or via MeCN/Et2O vapor diffusion, where the mother liquor is collected upon precipitation of the side product. Attempts to identify this side product by XRD were unsuccessful. Synthesis of [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7). 22.3 mg (0.014 mmol) 2 was combined with 2.6 mg (0.013 mmol) 2-azidobiphenyl in thawing THF. After 24 hours, no reaction had been observed and 3.8 mg (0.02 mmol) CoCp2 was added to the reaction; the solution turned purple, signifying the formation of 3, and over multiple hours became blue. After stirring 20 hours, the reaction was recrystallized via THF/pentane vapor diffusion to afford crystals of 7. 1H NMR (500 MHz, THF/C6D6) δ 122.2 (br), 78.5 (br), 71.7, 71.1, 53.2, 47.5, 37.7, 33.4, 21.2, 14.0, 13.5, 11.4, 9.4, -16.8 ppm. Synthesis of [LFe3O(Pz)3Fe(3,5-trifluoromethyl-anilide)][OTf] (8). 11.2 mg (0.008 mmol) 3 was dissolved in 3 mL THF and froze in a liquid N2-cooled cold well. This was combined with a 1 mL solution of 2 mg (0.008 mmol) 3,5-trifluoromethyl phenylazide in THF, while thawing. 171 The solution turned blue as it warmed to room temperature, producing a mixture of 8 and side products. This was recrystallized via THF/pentane vapor diffusion. 1H NMR (500 MHz, THF/C6D6) δ 123.4 (br), 78.9 (br), 70.4, 69.5, 50.7, 44.8, 25.7, 17.1, 8.9, 8.5, -6.8, -25.3 (br) ppm. Synthesis of [LFe3O(Pz)3Fe(p-toluenesulfonimide)][OTf] (9). 11.3 mg (0.008 mmol) 3 was dissolved in THF and froze in a liquid N2-cooled cold well. This was combined with 4.5 mg (0.009 mmol) sPhINTs in THF while thawing. The solution turns orange-red upon warming to room temperature to produce 9. This was recrystallized via MeCN/Et2O. 1H NMR (300 MHz, CD3CN) δ 125.7 (br), 91.6 (br), 73.7, 58.0, 55.1, 51.1, 14.0, 13.4, 534, 3.4, -1.1 (br) ppm. Synthesis of [LFe3F(Pz)3Fe][OTf] (10). 763 mg (0.55 mmol) LFe3(OAc)(OTf)2 was suspended in 5 mL THF in a 20 mL scintillation vial. This suspension was frozen in a liquid N2-cooled cold-well. In a separate vial, a suspension of 64 mg (0.60 mmol) potassium pyrazolate and 88 mg (1.30 mmol) pyrazole in 2 mL THF was also frozen. These were combined while thawing, followed by addition of a THF suspension of [Bu4N][F], 156 mg (0.60 mmol). After stirring for 30 minutes, a solution of 210 mg (0.56 mmol) Fe(N(SiMe3)2)2 was added. The solvent was removed under vacuum upon stirring for 20 hours. The solid was washed with Et2O and toluene on a course porosity glass frit with celite; The remaining precipitate was eluted with a 1:1 mixture of THF/MeCN, until the washings from the frit were colorless. The filtrate was dried completely under vacuum, then suspended in 10 mL MeCN. 120 mg (0.34 mmol) Ca(OTf)2 was added to the suspension and stirred for 24 hours. The red orange precipitate was collected over celite, dried under vacuum, then eluted with CH2Cl2 and recrystallized via CH2Cl2/Et2O vapor diffusion to yield 450 mg of 10 as red-orange crystals (56% yield). 1H NMR (300 MHz, CD2Cl2) δ 110.3 (br), 77.2, 72.5, 40.4, 30.0, 27.1, 23.3, 16.7, 11.9, 11.4, 4.5, 4.1, 1.2, -1.3 (br), -30.1 (br) ppm 172 Synthesis of [(LFe3F(Pz)3Fe)2O][OTf]2 (11). Method A. 102 mg (0.07 mmol) 10 was dissolved in CH2Cl2 a connected to the Schlenk line in a Schlenk tube. The reaction was degassed with three freeze-pump-thaw cycles and froze with liquid N2. On the Schlenk line, 10.5 cmHg of O2 was collected in a volumetric gas bulb (36.7 mL; 0.21 mmol O2), which was connected between the Schlenk line and the reaction vessel. This was introduced to the frozen solution for 5 minutes, then thawed -78 °C and stirred briefly before warming to -30 °C. The reaction was left open to the partial atmosphere of O2 for 30 minutes and stirred. Then, the solvent was removed under vacuum and the solid was recrystallized via CH2Cl2/Et2O vapor diffusion to afford crude 11 (with an unidentified side product, vide infra). 1H NMR (300 MHz, CD2Cl2) δ 92.1 (br), 77.3, 76.3, 43.2, 42.7 (br), 38.3, 26.7, 25.2, 24.3, 22.1,16.0, 9.2, 0.9, -1.1, -10.5 (br), -11.9 (br) ppm Method B. 48 mg (0.033 mmol) 10 was dissolved in MeCN and combined with 4.6 mg (0.021 mmol) PhIO; 11 with its side product was produced as the major species after 5 minutes, with a 1H NMR identical to Method A. Similar to 6, regardless of preparation method, a side product would be present with nearly identical NMR shifts: 1H NMR (300 MHz, CD2Cl2) δ 77.9, 77.1, 43.1, 38.6, 27.4, 23.7, 22.7, 16.1 (br), 9.2, 0.89, -1.3, -10.8 (br) ppm. This could be separated from 11 by stirring a solution of the mixture for 18 hours in MeCN and collecting the filtrate, or via MeCN/Et2O vapor diffusion, where the mother liquor is collected upon precipitation of the side product. Attempts to identify this side product by XRD were unsuccessful. Synthesis of [LFe3O(Pz)3Fe(F)][OTf] (12). Method A. 20.2 mg (0.014 mmol) 10 was dissolved in MeCN and froze in a liquid N2 -cooled cold well. This was combined with 6.8 mg (0.016 mmol) [Bu4N][IO4] in MeCN while thawing. The reaction became a brown suspension. After 16 hours, the solvents were removed under vacuum and the residue was washed with Et2O, 173 toluene, THF, and eluted with MeCN to obtain 14.2 mg of a brown solid, corresponding to mostly clean (>90%) 12 (~70% yield). 1H NMR (300 MHz, CD2Cl2) δ 127.7 (br), 77.3 (br), 74.5, 73.9, 59.9, 49.0, 31.1, 28.6, 23.4, 14.4,13.3, 11.4, 3.6 (br), -10.7 (br) ppm Method B. 83.3 mg (0.058 mmol) 10 was dissolved in MeCN and froze in a liquid N2 -cooled cold well. This was combined with 370 μL of a tBuOOH solution (3.3 M in toluene diluted to 5% in MeCN; 0.061 mmol) and 7.1 mg (0.063 mmol) DABCO while thawing. The reaction became a brown suspension. After 12 hours, reaction was filtered. Method C. 36.4 mg (0.024 mmol) of a suspension of 1 in MeCN was combined with 8.7 mg (0.033 mmol) [Bu4N][F] in MeCN to produce 12. Synthesis of LFe3F(Pz)3Fe(N3) (13). 20.9 mg (0.014 mmol) 10 in MeCN was mixed with 4.3 mg (0.015 mmol) [Bu4N][N3] to produce an orange precipitate. After 2 hours, the precipitate was collected over a frit with celite, washed with minimal MeCN, eluted with CH2Cl2, and dried under vacuum to obtain 14.1 mg 13 as a red orange solid (75% yield). 1H NMR (300 MHz, CD2Cl2) δ 111.2 (br), 75.9, 71.5, 41.1, 31.5, 30.8, 29.3, 15.0, 14.2, 11.1, 3.6, -2.5 (br), 18.9 (br) ppm 174 ELECTROCHEMICAL DETAILS Current 50 μA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 Figure 11. Cyclic voltammograms of [LFe3O(Pz)3Fe][OTf]2 (2, 2 mM) in MeCN with 100 mM [Bu 4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Current 40 μA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2.5 -1.5 -0.5 Potential (V, vs. Fc/Fc+) 0.5 1.5 Figure 12. Cyclic voltammograms of [(LFe3O(Pz)3Fe)2O][OTf]4 (6, 2 mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. 175 Current 30 μA -1.2 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -1 -0.8 -0.6 -0.4 -0.2 + Potential (V, vs. Fc/Fc ) 0 0.2 Figure 13. Cyclic voltammograms of [LFe3F(Pz)3Fe][OTf] (10, 2 mM) in MeCN with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. 176 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3-MeCN, 4, 6 – 11, and 13 1a 2 3-MeCN 4 6 Empirical formula C78F2Fe4N13O8 C95.28H74.56BCl8.56 F3Fe4N12O7S C73H52F3Fe4N15 O7S C71.75H49.5ClF8.34 Fe4N10.5O12.61S3 C141.57H102.25Cl1.7 9F12Fe8N24O28.11 S4 Formula weight (g/mol) 1594.6 2124.57 1445.63 1717.62 3273.34 Radiation CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) a (Å) 14.5023(10) 13.5663(9) 14.6522(6) 15.0846(9) 27.057(2) b (Å) 19.6777(15) 17.3765(12) 19.7336(8) 15.0846(9) 14.2472(9) c (Å) 47.912(4) 20.9073(14) 47.0533(17) 57.491(4) 40.302(3) α (°) 90 84.592(3) 90 90 90 β (°) 93.256(3) 71.407(3) 91.934(2) 90 105.371(3) γ (°) 90 80.890(2) 90 120 90 V (Å3) 13650.8(18) 4607.4(5) 13597.3(9) 11329.2(15) 14980.0(18) 8 2 8 6 4 Cryst. syst. monoclinic triclinic monoclinic trigonal monoclinic Space group C2/c P-1 C2/c P31 C2/c ρcalcg (cm3) 1.552 1.531 1.412 1.511 1.451 2Θ range (°) 7.392 to 69.236 4.656 to 56.496 7.518 to 158.694 4.61 to 158.116 6.776 to 149.24 μ (mm-1) 4.997 0.937 7.573 7.400 7.521 GOF 1.673 1.066 1.151 1.010 1.050 R1 = 0.1227, wR2 = 0.3416 R1 = 0.0694, wR2 = 0.1905 R1 = 0.1808, wR2 = 0.3527 R1 = 0.0674, wR2 = 0.1620 R1 = 0.1030, wR2 = 0.2767 Z R1, wR2 (I>2σ (I)) a Preliminary structure 177 7a 8a 9 10 11 13a Empirical formula C91F3Fe4N13 O9S C80Cl0.14F7Fe4 N12.36O7 C81H55Fe4N13 O9S2 C68H51Cl1.91F 4Fe4N12O6S C134H94F8Fe8 N24O13S2 C74.5H47FFe4 N18.39O3 Formula weight (g/mol) 1873.11 1598.15 1641.90 1531.3 2884.12 1315.48 Radiation MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) a (Å) 18.1272(9) 19.0607(9) 55.590(10) 12.1433(15) 23.1614(12) 45.024(3) b (Å) 16.5232(7) 15.6115(10) 12.4282(12) 23.562(4) 16.6931(8) 12.4123(7) c (Å) 30.3724(15) 47.591(3) 22.632(2) 23.005(3) 33.0662(17) 51.116(3) α (°) 90 90 90 90 90 90 β (°) 103.749(2) 91.952(2) 98.040(7) 99.807(5) 94.436(3) 109.157(5) γ (°) 90 90 90 90 90 90 V (Å3) 8836.5(7) 14153.3(14) 15482(3) 6485.8(16) 12746.3(11) 26984(3) 4 8 8 4 4 16 Cryst. syst. monoclinic monoclinic monoclinic monoclinic monoclinic monoclinic Space group P21/n C2/c C2/c P21/C C2/c C2/c ρcalcg (cm3) 1.408 1.500 1.409 1.568 1.503 1.295 2Θ range (°) 4.7 to 60.842 7.32 to 127.554 7.292 to 116.164 5.41 to 130.452 3.306 to 60.97 4.156 to 129.078 μ (mm-1) 0.739 7.504 6.937 8.690 1.003 7.231 GOF 1.517 1.620 1.115 1.042 1.119 1.032 R1 = 0.1346, wR2 = 0.3671 R1 = 0.1532, wR2 = 0.4118 R1 = 0.1353, wR2 = 0.3364 R1 = 0.0801, wR2 = 0.163 R1 = 0.0891, wR2 = 0.2464 R1 = 0.0945, wR2 = 0.2274 Z R1, wR2 (I>2σ (I)) a Preliminary structure 178 Special refinement details for [LFe3O(Pz)3Fe(OAc)][OTf] (1). This is a preliminary structure; 80% complete dataset. The counterions and solvent molecules were not modeled beyond the XT structure solution. The cluster was kept isotropic. Special refinement details for [LFe3O(Pz)3Fe][OTf][BPh4] (2). This structure contains five co-crystalized CH2Cl2 molecules. One was modeled as partially occupied (28%; C014 and Cl1 and Cl12). Another was positionally disordered, with a common carbon atom (Cl4 and Cl5, 21%, and Cl2 and Cl3, 79%). Relatively high residual Q peak density near the bound triflate, corresponding to a small amount of disorder for this ligand, was present, but could not be modeled adequately. Special refinement details for [LFe3O(Pz)3Fe(MeCN)][OTf] (3-MeCN). The outersphere triflate is disordered over two positions, which is on a symmetry element. Each half triflate was modeled as partially occupied (fixed to 50%) with the CF3 and SO3 groups refined via EXYZ constraints. There were 1 or 2 co-crystalized solvent molecules, which could not be adequately modeled, and were left as isotropic C atoms. Special refinement details for [LFe3O(Pz)3Fe][OTf]3 (4). There are two clusters in the asymmetric unit. All but two of the six triflates are disordered; the disordered triflates bound to the clusters could not be completely modeled, but the sulfur and carbon of the minor component were modeled isotropically. The two outersphere triflates are positionally disordered with occupancies of 64% (S11 through O141) and 36% (S3CA through O142), and 60% (S0AA through O2AA) and 40% (S7CA through O21). Special refinement details for [(LFe3O(Pz)3Fe)2O][OTf]4 (6). There was a significant amount of solvent disorder in the crystal. It could be adequately modeled with CH2Cl2, Et2O, and H2O molecules. They were all modeled as partially occupied: Et2O, 45%; CH2Cl2, 45%; H2O, 55% (O18, O3AA) and 87% (O15). 179 Special refinement details for [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7). This is a preliminary structure with a 37% complete dataset. The outersphere triflate was modeled, along with two co-crystalized THF molecules. There was some remaining solvent that could not be adequately modeled. Everything was modeled isotropically. Special refinement details for [LFe3O(Pz)3Fe(3,5-trifluoromethyl-anilide)][OTf] (8). This is a preliminary structure with a 77% complete dataset. Everything was modeled isotropically. A complete triflate counterion and the trifluoromethyl groups could not be modeled adequately. Special refinement details for [LFe3O(Pz)3Fe(para-toluenesulfonamide)][OTf] (9). Everything but the cluster was modeled isotropically, where no complete solvent molecule or counterion could be modeled. Special refinement details for [LFe3F(Pz)3Fe][OTf] (10). The crystal contained a disordered co-crystalized CH2Cl2 molecule, with partial occupancies of 48% (C3, Cl2, and Cl6) and 52% (C3A, Cl1, and Cl3). 89% complete dataset. Special refinement details for [(LFe3F(Pz)3Fe)2O][OTf]2 (11). The outersphere triflate was disordered over two positions in a ~50/50 ratio; however, each molecule could not be modeled completely. The presence of co-crystalized solvent molecules was suggested by large residual Q peaks, but nothing could be modeled adequately. Special refinement details for LFe3F(Pz)3Fe(N3) (13). This is a preliminary structure with a 71% complete dataset. Everything was left isotropic. A number of co-crystallized MeCN molecules could be modeled. 180 References 1. Bertini, I.; Gray, H. B.; Stiefel, E. I.; Valentine, J. S. Biological Inorganic Chemistry: Structure and Reactivity. 1st ed.; University Science Books: Sausolito, California, 2007. 2. (a) Rittle, J.; Peters, J. C. Proc. Natl. Acad. 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Soc. 2015, 137, 1409414106; (b) Carsch, K. M.; de Ruiter, G.; Agapie, T. Inorg. Chem. 2017, 56, 9044-9054; (c) Arnett, C. H.; Chalkley, M. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 5569-5578; (d) Reed, C. J.; Agapie, T. Inorg. Chem. 2017, 56, 13360-13367; (e) Reed, C. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 10900-10908. 7. (a) Siegbahn, P. E. M. BBA - Bioenergetics 2013, 1827, 1003-1019; (b) Vinyard, D. J.; Brudvig, G. W. Annu. Rev. Phys. Chem. 2017, 68, 101-116; (c) Sproviero, E. M.; Gascón, J. A.; McEvoy, J. P.; Brudvig, G. W.; Batista, V. S. J. Am. Chem. Soc. 2008, 130, 3428-3442. 8. Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041-4062. 9. (a) McDonald, A. R.; Que Jr, L. Coord. Chem. Rev. 2013, 257, 414-428; (b) Berry, J. F. Comments Inorg. Chem. 2009, 30, 28-66; (c) Lu, X.; Li, X.-X.; Seo, M. S.; Lee, Y.-M.; Clémancey, M.; Maldivi, P.; Latour, J.-M.; Sarangi, R.; Fukuzumi, S.; Nam, W. J. Am. Chem. Soc. 2019, 141, 80-83; (d) Wilding, M. J. T.; Iovan, D. A.; Wrobel, A. T.; Lukens, J. T.; MacMillan, S. N.; Lancaster, K. M.; Betley, T. A. J. Am. Chem. Soc. 2017, 139, 14757-14766. 10. (a) Vaddypally, S.; Kondaveeti, S. K.; Karki, S.; Van Vliet, M. M.; Levis, R. J.; Zdilla, M. J. J. Am. Chem. Soc. 2017, 139, 4675-4681; (b) de Visser, S. P.; Kumar, D.; Neumann, R.; Shaik, S. Angew. Chem. Int. Ed. 2004, 43, 5661-5665; (c) Khenkin, A. M.; Kumar, D.; Shaik, S.; Neumann, R. J. Am. Chem. Soc. 2006, 128, 15451-15460. 11. (a) Ruiter, G. d.; Carsch, K. M.; Takase, M. K.; Agapie, T. Chemistry – A European Journal 2017, 23, 10744-10748; (b) de Ruiter, G.; Thompson, N. B.; Takase, M. K.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 1486-1489; (c) de Ruiter, G.; Carsch, K. M.; Gul, S.; Chatterjee, R.; Thompson, N. B.; Takase, M. K.; Yano, J.; Agapie, T. Angew. Chem. Int. Ed. 2017, 56, 4772-4776. 12. Horak, K. T. The Design and Synthesis of Transition Metal Complexes Supported by Noninnocent Ligand Scaffolds for Small Molecule Activation. PhD dissertation, California Institute of Technology, Pasadena, California, 2016. 13. (a) Nam, W.; Choi, S. K.; Lim, M. H.; Rohde, J.-U.; Kim, I.; Kim, J.; Kim, C.; Que, J. L. Angew. Chem. Int. Ed. 2003, 42, 109-111; (b) Wang, B.; Lee, Y.-M.; Seo, M. S.; Nam, W. Angew. Chem. Int. Ed. 2015, 54, 11740-11744; (c) MacBeth, C. E.; Golombek, A. P.; Young, V. G.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938-941; (d) Wijeratne, G. B.; Corzine, B.; Day, V. W.; Jackson, T. A. Inorg. Chem. 2014, 53, 7622-7634. 14. (a) Mankad, N. P.; Müller, P.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 4083-4085; (b) Iovan, D. A.; Betley, T. A. J. Am. Chem. Soc. 2016, 138, 1983-1993; (c) Spasyuk, D. M.; Carpenter, S. H.; Kefalidis, C. E.; Piers, W. E.; Neidig, M. L.; Maron, L. Chemical Science 2016, 7, 5939-5944; (d) Sazama, G. T.; Betley, T. A. Inorg. 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In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269.  NMR DATA 183 CHAPTER 2 Figure 1. 1H NMR spectrum (300 MHz) of potassium 3-phenyl pyrazolate in CD3CN. Figure 2. 13C {1H} NMR spectrum (300 MHz) of potassium 3-phenyl pyrazolate in CD3CN. Figure 3. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf] (1) in CD2Cl2. 184 Figure 4. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf] (1) in CD2Cl2. Figure 5. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf]2 (2) in CD2Cl2. Figure 6. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf]2 (2) in CD2Cl2. 185 Figure 7. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3) in CD3CN. Figure 8. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3) in CD3CN. Figure 9. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO) in CD2Cl2. 186 CHAPTER 3 Figure 10. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) (top) and [LFe3O(Pz)3Mn][[BArF4]] (1-[BArF4]; bottom) in CD3CN. Figure 11. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf] (1-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]; bottom) in CD3CN. Figure 12. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; bottom) in CD3CN. 187 Figure 13. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; bottom) in CD3CN. Figure 14. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]3 in CD2Cl2 (3-[OTf]; top), [LFe3O(Pz)3Mn][BArF4]3 in THF/C6D6 with three equivalents tetrabutylammonium trifluoromethanesulfonate (400 MHz, middle), and [LFe3O(Pz)3Mn][BArF4]3 in THF/C6D6 (500 MHz) (3-[BArF4]; bottom). Figure 15. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]) in CD2Cl2 (top) and [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]; bottom) in THF/C6D6 (400 MHz). 188 Figure 16. 1H NMR spectra (400 MHz) of [LFe3O(Pz)3Mn(OH)] (5) in THF/C6D6 [250 mM H2O]. Figure 17. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]; top) and [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; middle) in CD3CN. 1H NMR spectrum (400 MHz) of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; bottom) in THF/C6D6 [250 mM H2O]. Figure 18. 1H NMR spectrum (400 MHz) of [LFe3O(Pz)3Mn(OH)][BArF4]2 (7-[BArF4]) in THF/C6D6 [250 mM H2O]. 189 CHAPTER 4 Figure 19. 1H NMR spectrum (300 MHz) of 2-tert-butyl-isoxazolium tetrafluoroborate in (CD3)2CO. Figure 20. 1H NMR spectrum (400 MHz) of N-tert-butyl-1H-pyrazol-3-amine in CD2Cl2. Figure 21. 13C{1H} NMR spectrum (100 MHz) of N-tert-butyl-1H-pyrazol-3-amine in CD2Cl2. 190 Figure 22. 1H NMR spectrum (300 MHz) of LFe3O(PzNHtBu)3Fe(OH) (1) in C6D6. The sharp signal ~ 95 ppm is a spectral artifact. Figure 23. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2) in CD3CN. The sharp signal ~ 90 ppm is a spectral artifact. Figure 24. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3) in CD2Cl2. 191 Figure 25. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4) in CD2Cl2. Figure 26. 1H NMR spectrum (300 MHz) of LFe3O(PzNHtBu)3Fe(O) (5) in C6D6. Figure 27. 1H NMR spectrum (400 MHz) of [LFe3O(PzNHtBu)3Fe(O)][OTf] (6) in THF/C6D6. 192 Figure 28. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7) in 1:1 CD3CN/CD2Cl2. 193 CHAPTER 5 Figure 29. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(OAc)][OTf] (1) in CD2Cl2. Figure 30. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf]2 (2) in CD3CN. Figure 31. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf] (3) in CD3CN. 194 Figure 32. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf]3 (4) in CD2Cl2. Figure 33. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN) in CD3CN. Figure 34. 19F NMR spectra (300 MHz) of [LFe3O(Pz)3Fe][OTf]3 (4) in CD2Cl2 (left) and [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN) in CD3CN (right). 195 Figure 35. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(OH)][OTf]2 (5) in CD3CN. Figure 36. 1H NMR spectrum (300 MHz) of [(LFe3O(Pz)3Fe)2O][OTf]4 (6) in CD2Cl2. Figure 37. 1H NMR spectrum (500 MHz) of [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7) in THF/C6D6. 196 Figure 38. 1H NMR spectrum (500 MHz) of reaction mixture containing [LFe3O(Pz)3Fe(3,5trifluoromethyl-anilide)][OTf] (8) in THF/C6D6. Figure 39. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(para-tolylsulfonamide)][OTf] (9) in CD3CN. Figure 40. 1H NMR spectrum (300 MHz) of [LFe3F(Pz)3Fe][OTf] (10) in CD2Cl2. 197 Figure 41. 1H NMR spectrum (300 MHz) of [(LFe3F(Pz)3Fe)2O][OTf]2 (11) in CD2Cl2. Figure 42. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(F)][OTf] (12) in CD2Cl2. Figure 43. 1H NMR spectrum (300 MHz) of LFe3N(Pz)3Fe(N3) (13) in CD2Cl2.  57 FE MÖSSBAUER DATA 199 CHAPTER 2 Simulation details for [LFe3(PhPz)3FFe][OTf] (1): The Mössbauer spectrum of 1 features an intense signal around 3 mm/s characteristic of a high-spin Fe(II) center. Fitting a quadrupole doublet for this peak led to a model that accounted for ~75% of the overall signal, with a second doublet satisfactorily accounting for the rest of the signal (Figure 1). The final refinement spli t the 75%-abundant high-spin Fe(II) signal into three equally populated doublets (Figure 2). Figure 1. Mössbauer spectrum of 1 (black dots) fit with two doublets in ~3:1 ratio refined with parameters: δ = 1.156 mm/s; ΔEq = 3.260 mm/s (blue trace) and δ = 0.925 mm/s; ΔEq = 1.171 mm/s (green trace). The overall fit is the gray trace. δ (mm/s) 1.156 |ΔEq| (mm/s) 3.424 Г 0.25 Rel. % 25 1.152 3.176 0.23 25 1.178 3.065 0.50 25 0.951 1.099 0.70 25 Figure 2. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe][OTf] (1). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent signals assigned to the high-spin FeII of the tri-iron core and the green trace is assigned to the apical high-spin FeII. 200 Simulation details for [LFe3(PhPz)3FFe][OTf]2 (2): The Mössbauer spectrum of 2 was fit by first fitting the signal at 3 mm/s to high-spin Fe(II) (Figure 3A); this fit accounted for ~50% the total spectrum. The remaining spectrum was fit with two nearly equal doublets (Figure 3B); other combinations of this signal to arrive at alternative parameters for these two signals could not be satisfactorily modeled, so these are not included. The final fit split the 50%-abundant Fe(II) signal into two equal doublets (Figure 4). A) B) Figure 3. Mössbauer spectrum of 2 (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.132 mm/s; ΔEq = 3.101 mm/s (blue trace). (B) Mössbauer spectrum of 2 with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.462 mm/s; ΔEq = 1.213 mm/s (orange trace), and δ = 0.928 mm/s; ΔEq = 1.522 mm/s (green trace). δ (mm/s) 1.138 |ΔEq| (mm/s) 3.297 Г 0.35 Rel. % 25 1.122 2.899 0.39 25 0.479 1.211 0.50 29 0.987 1.504 0.70 21 Figure 4. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe][OTf]2 (2). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent signals assigned to the high-spin FeII of the tri-iron core, the orange trace is assigned to high- 201 spin FeIII in the tri-iron core, and the green trace is assigned to the apical high-spin FeII. The best fit was obtained by having a slight deviation from four equally abundant Fe centers. Simulation details for [LFe3(PhPz)3FFe(CH3CN)][OTf]3 (3): The Mössbauer spectrum of 3 contains three easily observable features, which was first approximated by fitting two doublets (Figure 5A). This fit shows inadequate modeling for the signal around 2 mm/s, so a third doubled was modeled in (Figure 5B-D). This fit led to three signals in an approximately 1:1:2 ratios. Of the possible distribution of these three signals, the fit in Figure 5D is favored, because it has isomer shifts self-consistent for the two Fe(III) centers (0.4 – 0.5 mm/s) and the Fe(II) center (1.00 – 1.15 mm/s) of the ‘tri-iron’ core of our related iron clusters presented here and previously reported. Furthermore, the third signal is most consistent with a high-spin five-coordinate Fe(II) (~0.1 mm/s lower than the corresponding six-coordinate Fe(II) signal), which is in agreement of the coordination of the apical Fe(II) in the crystal structure of 3. The final fit was refined by splitting the 50%-abundant Fe(III) signal into two equal signals (Figure 6). A) B) C) D) Figure 5. Mössbauer spectrum of 3 (black dots) (A) fit with two signals (gray trace) refined with parameters: δ = 1.091 mm/s; ΔEq = 2.841 mm/s (blue trace), and δ = 0.471 mm/s; ΔEq = 1.253 mm/s (orange trace). (B) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 202 ratio (gray trace) with the parameters: δ = 1.261 mm/s; ΔEq = 2.797 mm/s (blue trace), δ = 1.137 mm/s; ΔEq = 2.216 mm/s (green trace), and δ = 0.374 mm/s; ΔEq = 1.433 mm/s (orange trace). (C) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 ratio (gray trace) with the parameters: δ = 1.106 mm/s; ΔEq = 3.050mm/s (blue trace), δ = 1.083 mm/s; ΔEq = 2.098 mm/s (green trace), and δ = 0.447 mm/s; ΔEq = 1.269 mm/s (orange trace). (D) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 ratio (gray trace) with the parameters: δ = 1.120 mm/s; ΔEq = 3.071 mm/s (blue trace), δ = 0.981 mm/s; ΔEq = 2.486 mm/s (green trace), and δ = 0.498 mm/s; ΔEq = 1.186 mm/s (orange trace). δ (mm/s) 1.120 |ΔEq| (mm/s) 3.071 Г 0.51 Rel. % 25 0.492 0.889 0.46 25 0.487 1.449 0.42 25 0.981 2.486 0.72 25 Figure 6. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue trace represents the signal assigned to the high-spin FeII of the tri-iron core, the orange traces are assigned to high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical high-spin FeII. 203 Simulation details for LFe3(PhPz)3FFe(NO) (5-NO): The Mössbauer spectrum of 5-NO displays three readily distinguished signals. The major two were initially fit with a single doublet, which had parameters consistent with high-spin Fe(II) (Figure 7); the remaining signal was fit in a 3:1 ratio relative to the first signal. The final fit split the high-spin Fe(II) doublet into three equal doublets (Figure 8). Figure 7. Mössbauer spectrum of 5-NO (black dots) fit with two doublets in a ~3:1 ratio (gray trace) with the parameters: δ = 1.149 mm/s; ΔEq = 3.177 mm/s (blue trace), and δ = 0.906 mm/s; ΔEq = 1.651 mm/s (purple trace). δ (mm/s) 1.148 |ΔEq| (mm/s) 3.561 Г 0.37 Rel. % 25 1.150 3.172 0.33 25 1.153 2.753 0.39 25 0.947 1.629 0.63 25 Figure 8. Zero applied field 57Fe Mössbauer spectrum of LFe3F(PhPz)3Fe(NO) (5-NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core and the purple trace is assigned to the apical {FeNO}8. 204 Simulation details for [LFe3(PhPz)3FFe(NO)][OTf] (1-NO): The Mössbauer spectrum of 1-NO contains four distinguishable signals (one being a shoulder on the peak below 0 mm/s). These four signals were fit to two doublets based on their relative intensities and had an approximate 3:1 ratio (Figure 9). The final fit refined the intense Fe(II) doublet into three equal signals (Figure 10). Figure 9. Mössbauer spectrum of 1-NO (black dots) fit with two doublets in a ~3:1 ratio (gray trace) with the parameters: δ = 1.139 mm/s; ΔEq = 3.308 mm/s (blue trace), and δ = 0.609 mm/s; ΔEq = 1.631 mm/s (green trace). δ (mm/s) 1.152 |ΔEq| (mm/s) 3.592 Г 0.26 Rel. % 25 1.150 3.403 0.22 25 1.162 3.229 0.31 25 0.630 1.669 0.32 25 Figure 10. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf] (1NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core and the green trace is assigned to the apical {FeNO}7. 205 Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]2 (2-NO): The Mössbauer spectrum of 2-NO displays four readily-distinguished signals. The signal above 3 mm/s was initially fit as high-spin Fe(II), and this doublet accounts for 50% of the overall spectrum (Figure 11A). The remaining signal was fit as two equally abundant doublets in three different ways (Figures 11B-D). The fit in Figure S39B gives unreasonable isomer shifts of -0.1 and 0.2 mm/s (orange and green traces, respectively). The fits in Figures 11C and 11D are only slightly different, but the fit in Figure 11D gives parameters that are more self-consistent with isomer shift values for the Fe(III) center of the ‘tri-iron core’ (0.4 mm/s; orange trace) and the {FeNO}7 center of the fluoride-bridged tetranuclear iron clusters (0.6 mm/s; green trace; see parameter refinement for 1-NO). The final fit split the 50%-abundant Fe(II) signal into two doublets (Figure 12). A) B) C) D) Figure 11. Mössbauer spectrum of 2-NO (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.132 mm/s; ΔEq = 3.211 mm/s (blue trace). (B) Mössbauer spectrum of 2-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = -0.111 mm/s; ΔEq = 0.165 mm/s (orange trace), and δ = 1.158 mm/s; ΔEq = 0.270 mm/s (green trace). (C) Mössbauer spectrum of 2NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.577 mm/s; ΔEq = 1.456 mm/s (green trace), and δ = 0.469 mm/s; ΔEq = 1.094 206 mm/s (orange trace). (D) Mössbauer spectrum of 2-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.616 mm/s; ΔEq = 1.387 mm/s (green trace), and δ = 0.435 mm/s; ΔEq = 1.171 mm/s (orange trace). δ (mm/s) 1.121 |ΔEq| (mm/s) 3.309 Г 0.30 Rel. % 25 1.150 3.026 0.46 25 0.435 1.171 0.41 25 0.616 1.387 0.42 25 Figure 12. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core, the orange trace is assigned to the high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical {FeNO}7. Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]3 (3-NO): The Mössbauer spectrum of 3-NO displays three major features (instead of the expected eight for four Fe centers). The peak around 3 mm/s was modeled as a high-spin Fe(II) with parameters consistent with other Fe(II) centers in the ‘tri-iron core’ of the tetranuclear clusters; this fit accounted for 25% of the total signal (Figure 13A). Subtracting this doublet from the data, the remaining signal could be fit reasonably well with either one or two equally abundant quadrupole doublets (Figures 13B and 13C), however these fits were ruled out since they were inconsistent with our crystallographic analysis of 3-NO. There are many possible ways to fit 3 doublets in this residual signal; below are shown three fits that give reasonable fit parameters for two highspin Fe(III) centers and a high-spin five-coordinate {FeNO}7 (Figure 13D-F). The fit in Figure 13F gave parameters the most self-consistent within this series of clusters, and was used for the final refinement (Figure 14). 207 A) B) C) D) E) F) Figure 13. Mössbauer spectrum of 3-NO (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.087 mm/s; ΔEq = 3.100 mm/s (blue trace). (B) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and 208 fit with a single quadrupole doublet (gray trace): δ = 0.501 mm/s; ΔEq = 1.319 mm/s. (C) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.354 mm/s; ΔEq = 1.291 mm/s (orange trace), and δ = 0.643 mm/s; ΔEq = 1.314 mm/s (green trace). (D) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.488 mm/s; ΔEq = 0.819 mm/s (solid orange trace), and δ = 0.499 mm/s; ΔEq = 1.292 mm/s (dashed orange trace), and δ = 0.512 mm/s; ΔEq = 1.750 mm/s (green trace). (E) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.387 mm/s; ΔEq = 1.058 mm/s (solid orange trace), and δ = 0.409 mm/s; ΔEq = 1.503 mm/s (dashed orange trace), and δ = 0.719 mm/s; ΔEq = 1.321 mm/s (green trace). (F) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.486 mm/s; ΔEq = 0.840 mm/s (solid orange trace), and δ = 0.395 mm/s; ΔEq = 1.486 mm/s (dashed orange trace), and δ = 0.620 mm/s; ΔEq = 1.512 mm/s (green trace). δ (mm/s) 1.087 |ΔEq| (mm/s) 3.100 Г 0.59 Rel. % 25 0.484 0.868 0.49 25 0.402 1.471 0.41 25 0.615 1.507 0.44 25 Figure 14. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue trace represents the signal assigned to the high-spin FeII of the tri-iron core, the orange traces are assigned to the high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical {FeNO}7. Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]3[SbCl6] (4-NO): The Mössbauer spectrum of 4-NO displays two broad features, which were adequately fit as a single quadrupole doublet (Figure 15). Since it is likely that the Fe(III) signals are overlapping with the {FeNO}7 signal (see Mössbauer spectra of 2-NO and 3-NO), modeling the four separate Fe centers is not included. This spectrum is consistent, however, with our assignment of 4NO as a cluster containing no Fe(II) centers, since there the characteristic signal around 3 mm/s is absent. 209 δ (mm/s) 0.471 |ΔEq| (mm/s) 1.420 Г 0.61 Rel. % 100 57 Figure 15. Zero applied field Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] (4-NO). The data (black dots) was fit to a single Fe quadrupole doublet (gray trace). We postulate that the signal for the apical {FeNO}7 is overlapping with the doublet(s) for the high-spin Fe(III) in the ‘tri-iron core’. 210 CHAPTER 3 Simulation details for 1-[OTf]: The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 16). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 17). Figure 16. Mössbauer spectrum of 1-[OTf] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.543 mm/s; ΔEq = 1.366 mm/s (orange trace) and δ = 1.126 mm/s; ΔEq = 3.294 mm/s (blue trace). δ (mm/s) 1.135 |ΔEq| (mm/s) 3.513 Rel. % 1.127 3.016 33 0.563 1.318 33 33 Figure 17. Zero applied field Mössbauer spectrum of 1-[OTf] (black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned as high-spin Fe(III). 211 Simulation details for 2-[OTf]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 18). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(II). The final fit split the large doublet into two equal signals (Figure 19). Figure 18. Mössbauer spectrum of 2-[OTf] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.469 mm/s; ΔEq = 0.740 mm/s (orange trace) and δ = 1.124 mm/s; ΔEq = 2.926 mm/s (blue trace). δ (mm/s) 1.119 |ΔEq| (mm/s) 2.931 Rel. % 0.468 0.578 33 0.423 0.913 33 33 Figure 19. Zero applied field Mössbauer spectrum of 2-[OTf] (black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). Simulation details for 3-[OTf]: The spectrum displays a single quadrupole doublet signal. Although three Fe(III) signals are expected, the best fit was obtained with only one set of parameters (Figure 20). 212 δ (mm/s) 0.443 |ΔEq| (mm/s) 0.804 Rel. % 100 Figure 20. Zero applied field Mössbauer spectrum of 3-[OTf] (black dots) fit to a single quadrupole doublet (orange trace); this signal is assigned to the high-spin Fe(III) centers in the cluster. Simulation details for 1-[BArF4]: The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 21). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 22). Figure 21. Mössbauer spectrum of 1-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.556 mm/s; ΔEq = 1.267 mm/s (orange trace) and δ = 1.115 mm/s; ΔEq = 3.153 mm/s (blue trace). 213 δ (mm/s) 1.119 |ΔEq| (mm/s) 3.460 Rel. % 33 1.099 2.860 33 0.570 1.266 33 Figure 22. Zero applied field Mössbauer spectrum of 1-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned to high-spin Fe(III). Simulation details for 2-[BArF4]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 23). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(II). The final fit split the large doublet into two equal signals (Figure 24). Figure 23. Mössbauer spectrum of 2-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.485 mm/s; ΔEq = 0.746 mm/s (orange trace) and δ = 1.132 mm/s; ΔEq = 2.858 mm/s (blue trace). 214 δ (mm/s) 1.128 |ΔEq| (mm/s) 2.858 Rel. % 0.487 0.999 33 0.478 0.540 33 33 Figure 24. Zero applied field Mössbauer spectrum of 2-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). Simulation details for 3-[BArF4]: The spectrum displays a single quadrupole doublet signal. Although three Fe(III) signals are expected, the best fit was obtained with only one set of parameters (Figure 25). δ (mm/s) 0.444 |ΔEq| (mm/s) 0.800 Rel. % 100 Figure 25. Zero applied field Mössbauer spectrum of 3-[BArF4] (THF solution [250 mM H2O]; black dots) fit with a single quadrupole doublet assigned to high-spin Fe(III) centers. 215 Simulation details for 5: The spectrum displays an apparently asymmetric quadrupole doublet signal. The data could be fit to a single quadrupole doublet (Figure 26). The final fit split this signal into three equally abundant Fe(II) centers, to account for the asymmetry of the doublet (Figure 27). Figure 26. Mössbauer spectrum of 5 (black dots) fit with a single quadrupole doublet with parameters δ = 1.113 mm/s; ΔEq = 2.985 mm/s (blue trace). δ (mm/s) 1.115 |ΔEq| (mm/s) 3.399 Rel. % 33 1.115 2.947 33 1.078 2.417 33 Figure 27. Zero applied field Mössbauer spectrum of 5 (black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II). Simulation details for 6-[BArF4]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 28). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 29). 216 Figure 28. Mössbauer spectrum of 6-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.525 mm/s; ΔEq = 0.769 mm/s (orange trace) and δ = 1.075 mm/s; ΔEq = 2.893 mm/s (blue trace). δ (mm/s) 1.093 |ΔEq| (mm/s) 3.091 Rel. % 33 1.083 2.576 33 0.534 0.756 33 Figure 29. Zero applied field Mössbauer spectrum of 6-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned to high-spin Fe(III). Simulation details for 7-[BArF4]: The spectrum displays three discernable peaks, with a shoulder on the lowest peak, corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 30). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 31). 217 Figure 30. Mössbauer spectrum of 7-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.447 mm/s; ΔEq = 0.790 mm/s (orange trace) and δ = 1.109 mm/s; ΔEq = 3.018 mm/s (blue trace). δ (mm/s) 1.096 |ΔEq| (mm/s) 3.033 Rel. % 33 0.434 0.547 33 0.462 1.019 33 Figure 31. Zero applied field Mössbauer spectrum of 7-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). 218 CHAPTER 4 Simulation details for LFe3O(PzNHtBu)3Fe(OH) (1): The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet could be fit a number of ways, two of which are shown here (Figure 32). Figure 32B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into three equal signals (Figure 33). A) B) Figure 32. Mössbauer spectrum of 1 (black dots) fit with two doublets in a ~1:3 ratio (gray trace) with parameters (A) δ = 1.035 mm/s; ΔEq = 3.255 mm/s (blue trace) and δ = 0.756 mm/s; ΔEq = 1.993 mm/s (green trace) and (B) δ = 1.107 mm/s; ΔEq = 3.126 mm/s (blue trace) and δ = 0.436 mm/s; ΔEq = 2.663 mm/s (green trace). δ (mm/s) 1.099 |ΔEq| (mm/s) 3.519 Rel. % 25 1.109 3.029 25 1.134 2.640 25 0.407 2.706 25 Figure 33. Zero applied field Mössbauer spectrum of 1 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the green trace is assigned as Fe(III). 219 Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways (Figures 34). Figure 34B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into two equal signals (Figure 35). A) B) Figure 34. Mössbauer spectrum of 2 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.020 mm/s; ΔEq = 3.197 mm/s (blue trace), δ = 0.502 mm/s; ΔEq = 0.871 mm/s (orange trace) and δ = 0.612 mm/s; ΔEq = 1.832 mm/s (green trace) and (B) δ = 1.107 mm/s; ΔEq = 3.126 mm/s (blue trace), δ = 0.504 mm/s; ΔEq = 0.857 mm/s (orange trace) and δ = 0.425 mm/s; ΔEq = 2.172 mm/s (green trace). δ (mm/s) 1.123 |ΔEq| (mm/s) 3.197 Rel. % 25 1.100 2.757 25 0.515 0.809 25 0.406 2.171 25 Figure 35. Zero applied field Mössbauer spectrum of 2 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned as Fe(III). 220 Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3): The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the less intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways. Based on the Mossbauer parameters for the other cluster oxidation states (Figures 33 and 35), a fit was performed to obtain selfconsistent parameters (i.e. making apical Fe(III) doublet with second largest quadrupole splitting). This fit led to parameters that are in good agreement for a high-spin Fe(II), two high-spin Fe(III), and the unique apical Fe(III) environment (Figure 36). δ (mm/s) 1.153 |ΔEq| (mm/s) 2.829 Rel. % 25 0.469 0.613 25 0.459 1.056 25 0.381 1.693 25 Figure 36. Zero applied field Mössbauer spectrum of 3 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned as Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4): The spectrum displays two peaks of different intensities and shapes, consistent with multiple overlapping quadrupole doublets. In order to obtain self-consistent parameters, the spectrum was fit to four quadrupole doublets, with the doublet with the highest quadrupole splitting assigned to the apical Fe(III) (Figure 37). 221 δ (mm/s) 0.449 |ΔEq| (mm/s) 0.641 Rel. % 25 0.543 1.145 25 0.363 1.166 25 0.411 1.712 25 Figure 37. Zero applied field Mössbauer spectrum of 4 (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III) and the green trace is assigned as Fe(III). Simulation details for LFe3O(PzNHtBu)3Fe(O) (5): The spectrum displays six discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways (Figure 38). Figure 38C gives parameters most consistent with an apical Fe(III) and high-spin Fe(III) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 39). 222 A) B) C) Figure 38. Mössbauer spectrum of 5 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.106 mm/s; ΔEq = 3.023 mm/s (blue trace), δ = 1.522 mm/s; ΔEq = 0.868 mm/s (orange trace) and δ = -0.569 mm/s; ΔEq = 1.042 mm/s (green trace)., (B) δ = 1.106 mm/s; ΔEq = 3.029 mm/s (blue trace), δ = 0.952 mm/s; ΔEq = 2.010 mm/s (orange trace) and δ = -0.003 mm/s; ΔEq = 2.178 mm/s (green trace), and (C) δ = 1.103 mm/s; ΔEq = 3.027 mm/s (blue trace), δ = 0.518 mm/s; ΔEq = 1.146 mm/s (orange trace) and δ = 0.432 mm/s; ΔEq = 3.051 mm/s (green trace). 223 δ (mm/s) 1.115 |ΔEq| (mm/s) 3.141 Rel. % 25 1.097 2.869 25 0.524 1.126 25 0.433 3.042 25 Figure 39. Zero applied field Mössbauer spectrum of 5 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned to Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(O)][OTf] (6): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figures 40). Figure 40B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into two equal signals (Figure 41). A) B) Figure 40. Mössbauer spectrum of 6 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 0.864 mm/s; ΔEq = 3.311 mm/s (blue trace), δ = 0.495 mm/s; ΔEq = 0.912 mm/s (orange trace) and δ = 0.706 mm/s; ΔEq = 2.089 mm/s (green trace), and (B) δ = 1.077 mm/s; ΔEq = 2.870 mm/s (blue trace), δ = 0.496 mm/s; ΔEq = 0.916 mm/s (orange trace) and δ = 0.465 mm/s; ΔEq = 2.565 mm/s (green trace). 224 δ (mm/s) 1.085 |ΔEq| (mm/s) 2.867 Rel. % 25 0.507 1.093 25 0.485 0.717 25 0.473 2.532 25 Figure 41. Zero applied field Mössbauer spectrum of 6 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7): The small peak at >2 mm/s indicated the presence of a FeII-containing impurity; NMRs of this cluster typically contained [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3) as an impurity (Figures 36 and 42). Using the parameters for 3 above, the FeII doublet was used to estimate the amount of this impurity (30 – 40% of the spectrum). This was subtracted from the data, and the remaining data appeared to be one broad doublet, similar to the [FeIII4] spectrum of 4. It was fit as four equal doublets, with the apical Fe(III) tentatively assigned to the doublet with the highest quadrupole splitting, to represent the highest value possible for this parameter (Figure 43). Figure 42. Mössbauer spectrum of reaction mixture containing 7 and 3 (black dots) fit with parameters to account for 3 (gray trace). The remaining signal (red dots) is assigned to the Mossbauer signal of 7. 225 δ (mm/s) 0.430 |ΔEq| (mm/s) 1.435 Rel. % 25 0.440 0.953 25 0.407 0.379 25 0.426 2.033 25 Figure 43. Zero applied field Mössbauer spectrum of 7 (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III), and the green trace is assigned to the unique Fe(III). 226 CHAPTER 5 Simulation details for [LFe3O(Pz)3Fe][OTf]2 (2): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 44). Figure 44B gives parameters most consistent with an apical Fe(II). The final fit split the large doublet into two equal signals (Figure 45). A) B) Figure 44. Mössbauer spectrum of 2 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.215 mm/s; ΔEq = 2.905 mm/s (blue trace), δ = 0.487 mm/s; ΔEq = 0.750 mm/s (orange trace) and δ = 0.773 mm/s; ΔEq = 2.615 mm/s (green trace) and (B) δ = 1.101 mm/s; ΔEq = 3.158 mm/s (blue trace), δ = 0.481 mm/s; ΔEq = 0.757 mm/s (orange trace) and δ = 0.975 mm/s; ΔEq = 2.239 mm/s (green trace). δ (mm/s) 1.101 |ΔEq| (mm/s) 3.148 Rel. % 25 0.469 0.580 25 0.485 0.959 25 0.950 2.218 25 Figure 45. Zero applied field Mössbauer spectrum of 2 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to Fe(II). 227 Simulation details for [LFe3O(Pz)3Fe][OTf] (3): The spectrum displays ca. four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment (Figure 46A), and the remainder of the signal can be fit a number of ways (Figures 45B - D). Figure 45D gives parameters most consistent with an apical Fe(II) and high-spin Fe(III) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 46). A) B) C) D) Figure 45. (A) Mössbauer spectrum of 3 (black dots) fit with a single doublet (blue trace) with parameters δ = 1.116 mm/s; ΔEq = 3.304 mm/s, consistent with high-spin FeII. (B) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with a single doublet (gray trace) with parameters δ = 0.689 mm/s; ΔEq = 1.467 mm/s. (C) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with two doublets (gray trace) with parameters δ = 0.742 mm/s; ΔEq = 0.950 mm/s (orange trace), and δ = 0.700 mm/s; ΔEq = 1.837 mm/s (green trace).(D) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with two doublets (gray trace) with parameters δ = 0.517 mm/s; ΔEq = 1.455 mm/s (orange trace), and δ = 0.970 mm/s; ΔEq = 1.373 mm/s (green trace). 228 δ (mm/s) 1.119 |ΔEq| (mm/s) 3.489 Rel. % 25 1.110 3.099 25 0.534 1.425 25 0.985 1.366 25 Figure 46. Zero applied field Mössbauer spectrum of 3 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned to Fe(II). Simulation details for [LFe3O(Pz)3Fe][OTf]3 (4): The spectrum displays three discernable peaks, corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal is consistent with high-spin Fe(II) (Figure 47). The final fit split the large doublet into three equal signals, one of which was assigned to the apical Fe(III) (Figure 48). Figure 47. Mössbauer spectrum of 4 (black dots) fit with two doublets in ~ 1:3 ratio (gray trace) with parameters δ = 1.211 mm/s; ΔEq = 2.664 mm/s (blue trace), and δ = 0.448 mm/s; ΔEq = 0.662 mm/s (orange trace). 229 δ (mm/s) 1.120 |ΔEq| (mm/s) 2.807 Rel. % 25 0.458 0.621 25 0.457 0.910 25 0.368 0.343 25 Figure 48. Zero applied field Mössbauer spectrum of 4 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN): The spectrum displays three discernable peaks, corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal is consistent with five-coordinate high-spin Fe(II) (Figure 49). The final fit split the large doublet into three equal signals, one of which was assigned to the apical Fe(II) (Figure 50). Figure 49. Mössbauer spectrum of 4-MeCN (black dots) fit with two doublets in ~ 1:3 ratio (gray trace) with parameters δ = 0.863 mm/s; ΔEq = 1.830 mm/s (green trace), and δ = 0.470 mm/s; ΔEq = 0.722 mm/s (orange trace). 230 δ (mm/s) 0.457 |ΔEq| (mm/s) 0.420 Rel. % 25 0.464 0.716 25 0.452 1.033 25 0.886 1.738 25 Figure 50. Zero applied field Mössbauer spectrum of 4-MeCN (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III), and the green trace is assigned to the apical Fe(II). Simulation details for [LFe3O(Pz)3Fe(OH)][OTf]2 (5): The spectrum displays four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 51). Figure 51C gives parameters most consistent with an apical Fe(III) and high-spin Fe(II) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 52). 231 A) B) C) Figure 51. Mössbauer spectrum of 5 (black dots) fit with three doublets in ~ 1:1:2 ratio (gray trace) with parameters (A) δ = 0.463 mm/s; ΔEq = 0.722 mm/s (green trace), δ = 0.401 mm/s; ΔEq = 1.343 mm/s (orange trace), and δ = 1.062 mm/s; ΔEq = 2.969 mm/s (blue trace), (B) δ = 0.171 mm/s; ΔEq = 1.154 mm/s (green trace), δ = 0.549 mm/s; ΔEq = 0.910 mm/s (orange trace), and δ = 1.147 mm/s; ΔEq = 2.800 mm/s (blue trace), and (C) δ = 0.331 mm/s; ΔEq = 1.478 mm/s (green trace), δ = 0.456 mm/s; ΔEq = 0.723 mm/s (orange trace), and δ = 1.161 mm/s; ΔEq = 2.766 mm/s (blue trace). 232 δ (mm/s) 1.145 |ΔEq| (mm/s) 2.778 Rel. % 25 0.452 0.893 25 0.454 0.519 25 0.339 1.493 25 Figure 52. Zero applied field Mössbauer spectrum of 5 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for [(LFe3O(Pz)3Fe)2O][OTf]4 (6): The spectrum displays four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 53). Figure 53C gives parameters most consistent with an apical Fe(III) and high-spin Fe(II) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 54). 233 A) B) C) Figure 53. Mössbauer spectrum of 6 (black dots) fit with three doublets in ~ 1:1:2 ratio (gray trace) with parameters (A) δ = 0.156 mm/s; ΔEq = 1.128 mm/s (green trace), δ = 0.560 mm/s; ΔEq = 1.057 mm/s (orange trace), and δ = 0.999 mm/s; ΔEq = 3.188 mm/s (blue trace), (B) δ = 0.762 mm/s; ΔEq = 1.176 mm/s (green trace), δ = 0.350 mm/s; ΔEq = 1.049 mm/s (orange trace), and δ = 0.995 mm/s; ΔEq = 3.180 mm/s (blue trace), and (C) δ = 0.382 mm/s; ΔEq = 1.835 mm/s (green trace), δ = 0.463 mm/s; ΔEq = 0.711 mm/s (orange trace), and δ = 1.160 mm/s; ΔEq = 2.827 mm/s (blue trace). 234 δ (mm/s) 1.099 |ΔEq| (mm/s) 2.909 Rel. % 25 0.645 1.005 25 0.464 0.530 25 0.398 1.875 25 Figure 54. Zero applied field Mössbauer spectrum of 6 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for putative intermediate of 2 and sPhIO: Based on variable temperature 1H NMR data, the putative intermediate is the major species at low temperature, with a small (~20%) impurity of 5. The parameters for 5 in Figure 63 were used to model this amount of impurity, and was subtracted out (Figure 55). The remaining spectrum displays three discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. It was modeled with parameters similar to 6, based on its similar match to the data (Figure 56). Figure 55. Mössbauer spectrum of putative intermediate between 2 and sPhIO, with ~ 20% of an impurity, 5 (black dots). The signal attributed to the impurity was subtracted from the data (gray trace). 235 A) B) Figure 56. Mössbauer spectrum of putative intermediate between 2 and sPhIO (black dots) fit with four equally abundant quadrupole doublets (gray trace). Two potential fits with the following parameters: (A) (i) δ = 1.21 mm/s, |ΔEq| = 2.83 mm/s (blue trace), (ii) δ = 0.41 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.45 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.35 mm/s, |ΔEq| = 1.70 mm/s (green trace), and (B) (i) δ = 1.06 mm/s, |ΔEq| = 3.13 mm/s (blue trace), (ii) δ = 0.40 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.46 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.49 mm/s, |ΔEq| = 1.44 mm/s (green trace).  MISCELLANEOUS CRYSTAL STRUCTURES 237 Figure 1. Structure of [LFe3F(PhPz)3Fe(MeCN)][OTf], obtained via recrystallization of [LFe3F(PhPz)3Fe][OTf] in MeCN/Et2O vapor diffusion. Figure 2. Structure of [LFe3F(PhPz)3Fe(MeCN)][OTf]2, obtained via recrystallization of [LFe3F(PhPz)3Fe][OTf]2 in MeCN/Et2O vapor diffusion. 238 Figure 3. Structure of [LM3F(PhPz)3M][OTf], from LZn3(OTf)3 and Fe(N(SiMe3)2)2. The occupancies of the metals are similar at each site: ~130% Fe and ~105% Zn. This is rationalized as Fe scrambling to each position in the cluster. Figure 4. Structure of LLi3Fe(PhPz)2 (tentative assignment), obtained as a decomposition product of [NEt4][Fe4N2Cl10] (Bennett, et al. J. Am. Chem. Soc. 127, 12378), KPhPz, and LLi3 in THF. The Et2O soluble product is pink, and gives this structure. 239 Figure 5. Structure of [(LFe3O(Pz)3Mn)2O][OTf]2, obtained from decomposition of [LFe3O(Pz)3Mn(OH)][OTf]. Figure 6. Structure of [LFe3O(Pz)3Mn(OMe)][OTf], obtained from addition of methanol solution of sodium methoxide to . [LFe3O(Pz)3Mn][OTf]2. 240 Figure 7. Structure of LMn3O(PzNHtBu)3Mn(OH), obtained from the same synthetic route as LFe3O(PzNHtBu)3Fe(OH), starting from LMn3OTf3 and Mn(OTf)2 • 2 MeCN. Figure 8. Structure of [LMn3O(PzNHtBu)3Mn(OH)][OTf], obtained from oxidation of LMn3O(PzNHtBu)3Mn(OH) with [Fc][OTf]. 241 Figure 9. Structure of [LFe3O(Pz)3Fe(MeCN)][OTf], obtained from recrystallization of [LFe3O(Pz)3Fe][OTf] by MeCN/Et2O vapor diffusion. Figure 10. Structure of [LFe3O(Pz)3Fe(NO)][OTf]2, obtained from addition of one equivalent NO gas to [LFe3O(Pz)3Fe][OTf]2. 242 Figure 11. Structure of [(LFe3O(Pz)3FeO)2FeCl2] (tentative assignment), obtained from addition of one equivalent [NBu4][IO4] to [LFe3O(Pz)3FeCl][OTf] in THF. Figure 12. Structure of [LFe3O(Pz)3Fe(NHAr/Cl)][OTf] (Ar = 3,5-CF3-Ph; ~6:4 amide to chloride), obtained from partial decomposition of the reaction product between [LFe3O(Pz)3Fe][OTf] and ArN3. 243 Figure 13. Structure of [LFe3F(Pz)3Fe(MeCN)][OTf], obtained by recrystallization of [LFe3F(Pz)3Fe][OTf] in MeCN/Et2O vapor diffusion. Figure 14. Structure of LMn3(OTf)3, obtained by an analogous synthesis to that of LFe3(OTf)3 (LFe3(OAc)3 and 10 equivalents Me3SiOTf; Arnett, et al. J. Am. Chem. Soc. 140, 5569). 244 Figure 15. Structure of [LMn3O(Pz)3Mn(OAc)][OTf], obtained by an analogous synthesis to that of [LFe3O(Pz)3Fe(OAc)][OTf]. Figure 16. Structure of [LMn3O(Pz)3Mn][OTf]2, obtained by an analogous synthesis to that of [LFe3O(Pz)3Fe][OTf]2. 245 Figure 17. Structure of [LFe3O(iPrPz)3Mn][OTf]2, obtained by addition of KiPrPz, PhIO, and Mn(OTf)2 • 2 MeCN to LFe3(OTf)3. Figure 18. Structure of [LFe3O(iPrPz)3Mn][OTf], [LFe3O(iPrPz)3Mn][OTf]2 with CoCp2. obtained by reduction of 246 Figure 19. Structure of [LFe3O(iPrPz)3K][OTf], obtained by addition of KiPrPz, PhIO, and Fe(OTf)2 to LFe3(OTf)3. Full metalation of Fe was not observed by NMR, and this byproduct was crystalized out instead of the desired compound. Figure 20. Structure of [LFe3O(iPrPz)3Fe][OTf]2, obtained by addition of KiPrPz, PhIO, and Fe(OTf)2 • 2 MeCN to LFe3(OTf)3. Full metalation of Fe was observed with this reagent by NMR. 247 Figure 21. Structures of a mixture of [LFe3O(iPrPz)3Mn(OH)][OTf] and [LFe3O(iPrPz)2(OiPrPz)Mn][OTf] (roughly 50:50; tentative assignments), obtained by addition of (2-tert-butyl-sulfonyl)-iodosylbenzene to [LFe3O(iPrPz)3Mn][OTf].  ii  2019 Christopher John Reed ORCID: 0000-0002-8774-5106 iii ACKNOWLEDGEMENTS I would like to thank Theo for his mentorship over the past five years. I would not be the scientist that I am without your advice, patience, and motivation. I also thank the rest of my PhD committee, Profs. Jonas Peters, Dennis Dougherty, and Harry Gray, who were an excellent source of advice and insight during my studies, and pushed me to think critically of my research. It has been a great opportunity to study chemistry at Caltech. I would like to thank all the members of the Agapie group, particularly the ones who shared their expertise and resources with me over the years, which was crucial for my development as a chemist; I thank Dr. Graham de Ruiter for training me, and instilling a levelheaded approach to working in a synthetic lab. I want to acknowledge Dr. Kyle Horak for beginning the work on the unsubstituted pyrazolate and hydrogen-bonding pyrazolate clusters in the last year of his PhD. This served as a footing for my later graduate research. Special thanks to the members of the Peters and Gray groups who had allowed me to use their reagents and instruments, especially the Mӧssbauer spectrometer, these were invaluable aids in my studies. I would also like to thank the people who ran the facilities in the division, including the NMR (David VanderVelde), Mass spectrometry (Mona Shagoli), X-Ray Crystallography (Mike Takase and Larry Henling), and EPR facilities (Paul Oyala), along with the BILRC (Prof. Harry Gray and Jay Winkler). There were countless times I turned to them for advice on my experiments. I am grateful for the Resnick Sustainability Institute at Caltech for supporting the last two years of my graduate education, and giving me the opportunity to share my chemistry at the Resnick Fellow Day Seminar. iv ABSTRACT This dissertation discusses the synthesis, characterization, and reactivity of sitedifferentiated tetranuclear clusters containing Fe and Mn with NO and H2O-derived ligands. The motivation of this work was to conduct a detailed examination of structure-property relationships in well-defined molecular systems focused on unique features of multinuclear systems, such as bridging ligands, neighboring metal identity, and cluster oxidation state. Reactivity towards NO and H2O-derived ligands was targeted due to their relevance to biological multinuclear transition metal active sites that promote multi-electron small molecule transformations. Chapter 2 discusses the synthesis of Fe-nitrosyl clusters bearing an interstitial μ4-F atom. These clusters were prepared to compare their reactivity to previously synthesized [Fe3OFeNO] clusters with an analogous structure. A redox series of the [Fe3FFe] and [Fe3FFeNO] clusters were accessed, with the nitrosyl clusters displaying five cluster oxidation states, from FeII3{FeNO}8 to FeIII3{FeNO}7. Overall, the weaker bonding of the F- ligand resulted in attenuation of the activation and reactivity of the {FeNO}7, relative to the corresponding μ4-O clusters. Furthermore, the ability of distal Fe oxidation state changes to influence the activation of NO was decreased, demonstrating lower cooperativity between metals in clusters linked by a weaker μ4-atom This represents a rare case where the effects of bridging atom ligands could be compared in isostructural multinuclear complexes and decoupled from changes in metal ion coordination number, oxidation states, or geometry. Chapter 3 describes the synthesis of site-differentiated heterometallic clusters of [Fe3OMn], displaying facile ligand substitution at the five-coordinate Mn. This system was able to coordinate H2O and thermodynamic parameters of the proton and electron transfer processes from the MnII–OH2 to form a MnIII–OH moiety were studied. The oxidation state v distribution of the neighboring Fe centers had a significant influence on these thermodynamic parameters, which was similar to the analogous parameters for mononuclear Mn systems, demonstrating that oxidation state changes in neighboring metals of a cluster can perturb the reactivity of a Mn–OHx unit nearly as much as an oxidation state change at the Mn–OHx. Subsequent experiments attempted to find spectroscopic or electrochemical evidence for formation of a terminal Mn-oxo in this system; however, that was not obtained, even in relatively extreme conditions. This established a lower limit for the bond dissociation enthalpy of the MnIII–OH of ca. 93 kcal/mol, which makes formation of a terminal Mn-oxo cluster unfavorable in most organic solvents, due to expected facile hydrogen atom abstraction of a solvent C–H bond. The insights obtained on the reactivity of these tetranuclear metal-hydroxide clusters was applied towards stabilizing a terminal metal-oxo in a multinuclear complex, as outlined in Chapter 4. Through the use of pendant hydrogen bond donors with tert-butyl-aminopyrazolate ligands, tetranuclear Fe clusters bearing terminal-hydroxide and -oxo ligands could be stabilized and structurally characterized. A similar thermodynamic analysis of the Fe III–OH bond dissociation enthalpy was conducted, which demonstrated FeIII-oxo clusters could be accessed with a range of reactivity at the terminal-oxo ligand, based on the redox distribution of the neighboring Fe centers. The kinetics of C–H activation for the [FeII2FeIII2]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pKa on the rate of proton coupled electron transfer. Lastly, Chapter 5 describes the synthesis and reactivity of tetranuclear Fe clusters bearing unsubstituted pyrazolate ligands, focusing on attempts to observe evidence for a terminal Feoxo or Fe-imido motif. Clusters bearing a labile trifluoromethanesulfonate ligand at the fivecoordinate Fe center could be prepared, and would react with oxygen atom transfer reagents vi to produce a terminal Fe-hydroxide cluster, which, upon dehydration, led to isolation of an octanuclear μ2-O cluster. The pathway for Fe-hydroxide formation was investigated, but could not conclusively determine whether reactivity occurred from a transient terminal Fe-oxo. Similarly, the reduced tetra-iron cluster, in the [FeII3FeIII], redox state was prepared, and demonstrated reactivity towards electron deficient aryl azides. Isolation of aryl amide clusters (Fe-NHAr) was observed, suggesting, again, formation of a reactive Fe-imido which decomposes through formal hydrogen atom abstraction. Efforts to stabilize either of these Fe=O/NR multiply-bonded species through a more acidic Fe were investigated by synthesizing the corresponding pyrazolate bridged μ4-F clusters. The [FeII4] cluster also displayed reactivity towards oxygen atom transfer reagents, and produced a similar octanuclear μ2-O cluster, but the observation of μ4-F substitution with oxygen to produce μ4-O clusters with a terminal F ligand likely precluded formation of a reactive terminal-oxo cluster. Instead, thermodynamically favorable cluster rearrangement to the [Fe3OFe] structure dominates. vii PUBLISHED CONTENT AND CONTRIBUTIONS Parts of this thesis have been adapted from published articles co-written by the author. The following articles were reproduced in part with permission from the American Chemical Society: “Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation” Reed, C. J.; Agapie, T. Inorganic Chemistry, 2017, 56, 13360-13367. “Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn– OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II” Reed, C. J.; Agapie, T. Journal of the American Chemical Society, 2018, 140, 1090010908. “A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity” Reed, C. J.; Agapie, T. Journal of the American Chemical Society, 2019, Just Accepted Manuscript. doi: 10.1021/jacs.9b03157. viii TABLE OF CONTENTS Acknowledgments Abstract Published Content and Contributions Table of Contents iii iv vii viii Chapter 1 General Introduction 1 Chapter 2 Tetranuclear Iron Clusters with a Varied Interstitial Ligand: Effects On Structure, Redox Properties, and Nitric Oxide Activation Abstract Introduction Results and Discussion Conclusions Supporting Data References 19 Chapter 3 Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II Abstract Introduction Results and Discussion Conclusions Supporting Data References Chapter 4 A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity Abstract Introduction Results and Discussion Conclusions Supporting Data References Chapter 5 Intermolecular Reactivity of Tetranuclear Fe Clusters Via Putative Fe–Oxo and –Imido Intermediates Abstract 20 21 24 36 37 51 53 54 55 58 78 80 103 107 108 109 111 126 127 145 147 148 ix Introduction Results and Discussion Conclusions Supporting Data References Appendix A NMR Data Chapter 2 Chapter 3 Chapter 4 Chapter 5 Appendix B 57 Fe Mössbauer Data Chapter 2 Chapter 3 Chapter 4 Chapter 5 149 152 165 166 180 182 183 186 189 193 198 199 210 218 226 Appendix C Miscellaneous Crystal Structures 236 About the Author 248  Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology Thesis by Christopher John Reed In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Division of Chemistry and Chemical Engineering Pasadena, California 2019 (Defended on May 30th, 2019) ii  2019 Christopher John Reed ORCID: 0000-0002-8774-5106 iii ACKNOWLEDGEMENTS I would like to thank Theo for his mentorship over the past five years. I would not be the scientist that I am without your advice, patience, and motivation. I also thank the rest of my PhD committee, Profs. Jonas Peters, Dennis Dougherty, and Harry Gray, who were an excellent source of advice and insight during my studies, and pushed me to think critically of my research. It has been a great opportunity to study chemistry at Caltech. I would like to thank all the members of the Agapie group, particularly the ones who shared their expertise and resources with me over the years, which was crucial for my development as a chemist; I thank Dr. Graham de Ruiter for training me, and instilling a levelheaded approach to working in a synthetic lab. I want to acknowledge Dr. Kyle Horak for beginning the work on the unsubstituted pyrazolate and hydrogen-bonding pyrazolate clusters in the last year of his PhD. This served as a footing for my later graduate research. Special thanks to the members of the Peters and Gray groups who had allowed me to use their reagents and instruments, especially the Mӧssbauer spectrometer, these were invaluable aids in my studies. I would also like to thank the people who ran the facilities in the division, including the NMR (David VanderVelde), Mass spectrometry (Mona Shagoli), X-Ray Crystallography (Mike Takase and Larry Henling), and EPR facilities (Paul Oyala), along with the BILRC (Prof. Harry Gray and Jay Winkler). There were countless times I turned to them for advice on my experiments. I am grateful for the Resnick Sustainability Institute at Caltech for supporting the last two years of my graduate education, and giving me the opportunity to share my chemistry at the Resnick Fellow Day Seminar. iv ABSTRACT This dissertation discusses the synthesis, characterization, and reactivity of sitedifferentiated tetranuclear clusters containing Fe and Mn with NO and H2O-derived ligands. The motivation of this work was to conduct a detailed examination of structure-property relationships in well-defined molecular systems focused on unique features of multinuclear systems, such as bridging ligands, neighboring metal identity, and cluster oxidation state. Reactivity towards NO and H2O-derived ligands was targeted due to their relevance to biological multinuclear transition metal active sites that promote multi-electron small molecule transformations. Chapter 2 discusses the synthesis of Fe-nitrosyl clusters bearing an interstitial μ4-F atom. These clusters were prepared to compare their reactivity to previously synthesized [Fe3OFeNO] clusters with an analogous structure. A redox series of the [Fe3FFe] and [Fe3FFeNO] clusters were accessed, with the nitrosyl clusters displaying five cluster oxidation states, from FeII3{FeNO}8 to FeIII3{FeNO}7. Overall, the weaker bonding of the F- ligand resulted in attenuation of the activation and reactivity of the {FeNO}7, relative to the corresponding μ4-O clusters. Furthermore, the ability of distal Fe oxidation state changes to influence the activation of NO was decreased, demonstrating lower cooperativity between metals in clusters linked by a weaker μ4-atom This represents a rare case where the effects of bridging atom ligands could be compared in isostructural multinuclear complexes and decoupled from changes in metal ion coordination number, oxidation states, or geometry. Chapter 3 describes the synthesis of site-differentiated heterometallic clusters of [Fe3OMn], displaying facile ligand substitution at the five-coordinate Mn. This system was able to coordinate H2O and thermodynamic parameters of the proton and electron transfer processes from the MnII–OH2 to form a MnIII–OH moiety were studied. The oxidation state v distribution of the neighboring Fe centers had a significant influence on these thermodynamic parameters, which was similar to the analogous parameters for mononuclear Mn systems, demonstrating that oxidation state changes in neighboring metals of a cluster can perturb the reactivity of a Mn–OHx unit nearly as much as an oxidation state change at the Mn–OHx. Subsequent experiments attempted to find spectroscopic or electrochemical evidence for formation of a terminal Mn-oxo in this system; however, that was not obtained, even in relatively extreme conditions. This established a lower limit for the bond dissociation enthalpy of the MnIII–OH of ca. 93 kcal/mol, which makes formation of a terminal Mn-oxo cluster unfavorable in most organic solvents, due to expected facile hydrogen atom abstraction of a solvent C–H bond. The insights obtained on the reactivity of these tetranuclear metal-hydroxide clusters was applied towards stabilizing a terminal metal-oxo in a multinuclear complex, as outlined in Chapter 4. Through the use of pendant hydrogen bond donors with tert-butyl-aminopyrazolate ligands, tetranuclear Fe clusters bearing terminal-hydroxide and -oxo ligands could be stabilized and structurally characterized. A similar thermodynamic analysis of the Fe III–OH bond dissociation enthalpy was conducted, which demonstrated FeIII-oxo clusters could be accessed with a range of reactivity at the terminal-oxo ligand, based on the redox distribution of the neighboring Fe centers. The kinetics of C–H activation for the [FeII2FeIII2]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pKa on the rate of proton coupled electron transfer. Lastly, Chapter 5 describes the synthesis and reactivity of tetranuclear Fe clusters bearing unsubstituted pyrazolate ligands, focusing on attempts to observe evidence for a terminal Feoxo or Fe-imido motif. Clusters bearing a labile trifluoromethanesulfonate ligand at the fivecoordinate Fe center could be prepared, and would react with oxygen atom transfer reagents vi to produce a terminal Fe-hydroxide cluster, which, upon dehydration, led to isolation of an octanuclear μ2-O cluster. The pathway for Fe-hydroxide formation was investigated, but could not conclusively determine whether reactivity occurred from a transient terminal Fe-oxo. Similarly, the reduced tetra-iron cluster, in the [FeII3FeIII], redox state was prepared, and demonstrated reactivity towards electron deficient aryl azides. Isolation of aryl amide clusters (Fe-NHAr) was observed, suggesting, again, formation of a reactive Fe-imido which decomposes through formal hydrogen atom abstraction. Efforts to stabilize either of these Fe=O/NR multiply-bonded species through a more acidic Fe were investigated by synthesizing the corresponding pyrazolate bridged μ4-F clusters. The [FeII4] cluster also displayed reactivity towards oxygen atom transfer reagents, and produced a similar octanuclear μ2-O cluster, but the observation of μ4-F substitution with oxygen to produce μ4-O clusters with a terminal F ligand likely precluded formation of a reactive terminal-oxo cluster. Instead, thermodynamically favorable cluster rearrangement to the [Fe3OFe] structure dominates. vii PUBLISHED CONTENT AND CONTRIBUTIONS Parts of this thesis have been adapted from published articles co-written by the author. The following articles were reproduced in part with permission from the American Chemical Society: “Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation” Reed, C. J.; Agapie, T. Inorganic Chemistry, 2017, 56, 13360-13367. “Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn– OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II” Reed, C. J.; Agapie, T. Journal of the American Chemical Society, 2018, 140, 1090010908. “A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity” Reed, C. J.; Agapie, T. Journal of the American Chemical Society, 2019, Just Accepted Manuscript. doi: 10.1021/jacs.9b03157. viii TABLE OF CONTENTS Acknowledgments Abstract Published Content and Contributions Table of Contents iii iv vii viii Chapter 1 General Introduction 1 Chapter 2 Tetranuclear Iron Clusters with a Varied Interstitial Ligand: Effects On Structure, Redox Properties, and Nitric Oxide Activation Abstract Introduction Results and Discussion Conclusions Supporting Data References 19 Chapter 3 Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II Abstract Introduction Results and Discussion Conclusions Supporting Data References Chapter 4 A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity Abstract Introduction Results and Discussion Conclusions Supporting Data References Chapter 5 Intermolecular Reactivity of Tetranuclear Fe Clusters Via Putative Fe–Oxo and –Imido Intermediates Abstract 20 21 24 36 37 51 53 54 55 58 78 80 103 107 108 109 111 126 127 145 147 148 ix Introduction Results and Discussion Conclusions Supporting Data References Appendix A NMR Data Chapter 2 Chapter 3 Chapter 4 Chapter 5 Appendix B 57 Fe Mössbauer Data Chapter 2 Chapter 3 Chapter 4 Chapter 5 149 152 165 166 180 182 183 186 189 193 198 199 210 218 226 Appendix C Miscellaneous Crystal Structures 236 About the Author 248 CHAPTER 1 GENERAL INTRODUCTION 2 INTRODUCTION Multi-Electron Transformations Relevant to Global Biogeochemical Cycles of Oxygen and Nitrogen. The chemical basis of every living organism is centralized around a handful of elements C, H, N, O, P, and S; the transformation of molecules containing these elements occurs ubiquitously and on massive scales, directly affecting life around Earth. The global chemistry of oxygen and nitrogen are particularly relevant to human life. For example, the beginning of production of molecular oxygen eons ago, and its build up in the planet’s atmosphere, has been linked to the development of complex eukaryotic organisms.1 A constant production of O2 is required to support aerobic life, and is intimately involved in biochemical processes related to the oxidation of other biological elements. Similarly, ammonium (NH4+) serves as the crucial building block to all biological nitrogen-containing molecules, which includes amino acids and nucleotides, and the combination of manmade and natural ammonium synthesis is directly tied to the production of sufficient supplies of food for humans and other animals.2 The global cycles of oxygen and nitrogen revolve around electron transfer; the formal oxidation state of oxygen commonly varies from -2 (i.e. H2O) to 0 (O2), and nitrogen can vary from -3 (NH4+) to +5 (NO3-).1 The four-electron reduction of dioxygen by cellular respiration constitutes a key component of the global oxygen cycle. The consumed dioxygen is replaced by photosynthesis through the analogous four-electron oxidation of water (Scheme 1). In a similar way, multi-electron redox processes of nitrogen provide access to its various biologically relevant forms (i.e. N2, NH4+, NO, N2O) and processes to regenerate them. The global redox transformations of oxygen and nitrogen typically occur through multi-electron pathways. This is because, often, the partially reduced/oxidized molecule is less thermodynamically stable (Figure 1). 3 Scheme 1. Multi-Electron Redox Processes of Oxygen and Nitrogen Relevant to Their Global Biogeochemical Cycles. Figure 1. Equilibrium standard potentials of forms of oxygen (red) and nitrogen (blue).3 Biological Catalysts for Multi-Electron Redox Transformations of Oxygen and Nitrogen. Nature contains a variety of catalysts which constitute major steps in the global cycles of oxygen and nitrogen. For example, yearly, 300 tetragrams of N2 are reduced to NH4+ by nitrogen-fixing microbes through a metalloenzyme called nitrogenase; this process is considered to be responsible for more than half the amount of bio-available nitrogen in the environment.2 These enzymes display remarkable efficiency towards their native reactions and are capable of effecting complex, challenging multi-electron transformations under relatively mild, and sustainable, conditions. For this reason, researchers have examined these biological systems to gain insight into the features of these enzymes that promote efficient catalysis. 4 Biological enzymes responsible for redox transformations of oxygen and nitrogen are all metalloenzymes, containing either Fe, Mn, and/or Cu ions in their active site.4 Within this set of enzymes, many of the active sites contain complex transition metal clusters (Figure 2). The structural diversity of these multinuclear active sites is notable, displaying various transition metals, nuclearities, bridging ligands, and geometric arrangements. Detailed mechanistic study of these globally relevant metalloenzymes has been a culmination of efforts from biochemists, crystallographers, spectroscopists, and theoreticians, which remain at the frontier of bioinorganic research, leading to developments in enzymology, metal ion spectroscopy, and computational techniques. Figure 2. Active site structures of metalloenzymes competent for multi-electron redox transformations of oxygen and nitrogen: nitric oxide reductase (PDB code: 3O0R), 5 heme/Cu oxygen reductase (1V54), nitrous oxide reductase (1FWX), photosystem II (3WU2), and nitrogenase (1M1N). Fe (light brown), Cu (cyan), Mn (light purple), Mo (purple), Ca (green), S (yellow), O (red), N (blue). Case Study: Mechanistic Insight to the Oxygen-Ovolving Complex (OEC) in Photosystem II. Photosynthetic organisms use the energy in sunlight to drive the reduction of plastoquinol and produce energy in the form of adenosine triphosphate (ATP). The reducing equivalents obtained are derived from water, one of the most abundant sources of electrons in the environment.4b, 5 In a separate process, the energy and reducing equivalents are used for reduction of CO2 and production of carbohydrates in the Calvin cycle. Water oxidation occurs in a protein assembly known as photosystem II (PSII), at a multinuclear active site composed of a heterometallic [Mn4CaO5] cluster, called the OEC.6 Incoming photons induce a charge separated state at by a nearby heme center (P680), which transfers a single electron through a series of mediators until reaching plastoquinone. P680 is regenerated by reduction from a nearby tyrosine residue (TyrZ). This resulting organic radical oxidizes the OEC, which translates four separate electron transfer events by P680 to one catalytic turnover, producing dioxygen from two molecules of water.4b The molecular model of OEC turnover is considered within the framework of the Kok cycle, which describes five distinct so-called Sstates (S0, S1, …, S4) of the OEC (Figure 3).4c, 7 Electrons, and protons from coordinated HxO ligands, are removed from the OEC during each S-state transition; states S0 through S3 have been observed spectroscopically (principally through advanced EPR and X-ray absorption techniques), and in some cases structurally characterized by X-ray diffraction. These investigations have established a number of important characteristics of the OEC catalytic cycle, including (i) the lowest oxidation state of the OEC during turnover is [MnIII3MnIV] and 6 this cluster undergoes four subsequent oxidations to reach a formal [MnVI3MnV] redox state, responsible for O–O bond formation (S4);4b, 8 (ii) EPR spectroscopy of the OEC has shown a dynamic structure within the cluster, where at least one of the bridging oxygen atoms is exchangeable (and implicated as one of the substrate oxygen atoms);9 (iii) recent X-ray techniques have captured structural snapshots of the OEC in the S3 state, which displays coordination of the second substrate HxO molecule to one of the cubane Mn centers;10 and (iv) extensive computational studies based on experimental structural and spectroscopic parameters for the OEC suggest a high-valent terminal Mn-oxo is the key O–O bond forming intermediate in the unobserved S4 state.11 Figure 3. Contemporary proposed structures of the OEC in each stage of the Kok cycle. These studies provide a remarkably detailed picture of water oxidation by the OEC; however, due to the inherent complexity of studying this massive protein (700 kDa), with its 7 multiple subunits and cofactors, there are a number of challenges to realizing a complete mechanistic understanding of the OEC. For example, the precise protonation state of the OEC, and its neighboring protein environment, is not well-understood for any S-state.11b Also, since PSII is necessarily studied in aqueous conditions, the OEC is always present in a large excess of substrate, which complicates isolation and characterization of the oxidized states of the cluster. Our mechanistic understanding of the OEC exemplifies a number of possible functional roles for multiple transition metals arranged within a multinuclear active site. The presence of many redox active transition metals allows for the storage of multiple oxidizing equivalents, without requiring a large buildup of charge at a single site. This has been implicated as a cause for the high selectivity of the OEC, which produces little to no partially oxidized forms of oxygen, i.e. H2O2 or O2-, which would be detrimental to the organism. One can also envision how the relatively unique dangling cubane geometry of the metal centers in the OEC promotes reactivity between specific metal-bound oxygen atoms. Furthermore, the coupling of unpaired spins in the Mn centers may be crucial for efficient release of dioxygen, avoiding the production of reactive singlet oxygen. In general, a number of functions for neighboring metal centers in various catalytic systems can be proposed: (i) storage of redox equivalents, (ii) structurally directing reactive moieties, and (iii) tuning the electronic characteristics of a reactive metal or coordinated ligand. Biologically Inspired Transition Metal Complexes Relevant to Small Molecule Chemistry of Oxygen and Nitrogen. Due to the potential complexity and constraints of studying metalloenzymes directly, the complementary development of well-defined small molecule transition metal complexes has provided significant chemical insight related to these biological processes. The synthetic inorganic chemistry of Fe, Mn, and Cu complexes has been 8 studied extensively, including chemistry related to the global oxygen and nitrogen cycles through reactions involving relevant O- and N-containing small molecules such as H2O, O2, N2, and NO. A majority of these studies are performed with mononuclear, or binuclear, transition metal complexes; the development of multinuclear systems with greater complexity that bear closer resemblance to biological active sites remains a challenge for synthetic chemists. The following survey of relevant synthetic metal complexes is by no means exhaustive, but its discussion will place this work within the wider context of previously reported synthetic transition metal complexes that are relevant to biological transformations of oxygen and nitrogen. Synthetic Inorganic Chemistry Related to Water Oxidation by the OEC. Efforts towards a full structural model of the OEC were undertaken with the goal of elucidating the structureproperty relationships of a well-defined [Mn4CaO5] cluster with spectroscopic and/or functional relevance to the native metalloenzyme. A variety of di- and tetramanganese oxo clusters have been reported with relevance to the OEC; notable early achievements within this field include the isolation of high-valent [Mn4O4] cubane clusters from the groups of Dismukes (1) and Christou, and the incorporation of Ca into high nuclearity Mn-oxo clusters by Christou and co-workers.12 In 2011, Agapie and co-workers reported a [Mn3CaO4] cluster analogous to the cubane subunit of the OEC, 3.13 Since then, a more complete structural model of the OEC, with the dangler Mn has been reported by Zhang, Dong, Dau, and co-workers (4).14 With these complexes, a great deal of insight has been obtained, towards understanding the electronic interactions between Mn centers in cubane clusters, and the influence of the redoxinactive Ca ion on redox and reactivity properties.15 Most structural models of the OEC display coordinatively saturated Mn centers (Scheme 2), precluding extensive reactivity studies with 9 exogenous H2O; however, these and related systems have been used to study the reactivity of the bridging oxo ligands of high valent Mn complexes with relevance to the OEC.16 Scheme 2. Selected Structural Models of the OEC.12b, 13-14, 15b Numerous lower nuclearity metal complexes have been examined for their relevance to the OEC (Scheme 3). For example, Borovik and co-workers have studied the reactivity and electronic structure of mononuclear Mn-OH, Mn-oxo, and Mn–(OH)–Ca motifs, which are related to possible intermediates of the OEC; a unique ligand capable of hydrogen bonding to the Mn–OHx moiety facilitated characterization of a series of MnIII-, MnIV-, and MnV-oxo complexes (5 – 7).17 Particularly relevant to the biological system are understanding aspects that affect the homolytic bond dissociation enthalpy (BDE) of Mn–OHx motifs; proton coupled electron transfer (PCET) has been implicated as a crucial aspect of OEC catalysis, as it avoids charge built up at the active site, promoting progression to the fully oxidized state of the cluster.18 Along these lines, other groups have examined the PCET reactivity of mononuclear Mn–OH and -oxo complexes to understand the influence of Mn oxidation state, ligand, field, and protonation state on reactivity.19 Examples of these types of studies with multinuclear Mn complexes are less common;16b, 16c, 20 a binuclear Mn system has been reported by Pecoraro and co-workers (9), which is able to support –aquo and –hydroxide ligands in multiple Mn oxidation states, and access a reactive terminal Mn-oxo.21 10 O–O bond formation of synthetic Mn-oxo complexes has also been examined, predominantly with porphyrin ligands. Nucleophilic attack of MnV-oxo (10) by hydroxide produces peroxo- intermediates in Mn-corrole systems.22 In some cases, subsequent oxidation of this intermediate releases dioxygen. The reactivity of related corrole complexes in the presence of a redox-inactive metal has shown significant perturbations to the Mn–O bonding, which could be relevant to Mn–O–Ca motifs in the OEC.23 Scheme 3. Selected Examples of PCET and O–O Bond Formation with Synthetic Complexes Relevant to the OEC. Synthetic Complexes Relevant to Heme/Cu Oxygen Reductase (HCO). Dioxygen reduction by the bimetallic active site of HCO is a key step in cellular respiration that drives transmembrane proton pumping to ultimately obtain ATP from cellular reducing equivalents.4e HCO enzymes are part of a wider class of metalloenzymes that reduce dioxygen; other enzymes of this class employ mono- and binuclear active sites of Fe and Cu with O2 to oxidize organic molecules for a variety of metabolic pathways. A common element between HCO (and other Fecontaining O2 reducing enzymes) and the OEC is the key role of a putative high-valent terminal metal-oxo intermediate. In HCO, O–O bond cleavage of a Fe–(O2)–Cu intermediate is proposed to produce CuII–OH and FeIV-oxo, which is further reduced and protonated to afford two molecules of H2O. 11 The synthetic chemistry of heme and non-heme Fe-oxo complexes has been extensively investigated, due to their relevance to members of O2-reducing metalloenzymes.24 Groves and co-workers have recently reviewed this topic for heme Fe-oxo complexes.25 Close structural mimics of the bimetallic HCO active site have also been prepared (i.e. 11); a survey of these, and related synthetic Fe/Cu systems, has been reviewed recently by Karlin and co-workers.26 Biosynthetic approaches to study the mechanism of O2-reduction by binuclear heme/nonheme active sites has provided great insight to structure-property relationships in HCO; Lu and co-workers have used protein scaffolds to produce close structural models of the HCO active site, through mutagenesis studies of a simpler heme protein. With a single protein scaffold, they were able to introduce binding sites for various non-heme metals (Zn, Fe, Cu) and investigate their effect on the activity and selectivity for HCO-like activity.27 Scheme 4. Synthetic HCO Model Complex26b Synthetic Complexes Related to the Fe-Mo Cofactor (FeMoCo) of Nitrogenase. The FeMoco cluster of nitrogenase is a [MFe7S9C] cluster with a fused-cubane geometry (M = Mo);28 versions of nitrogenase where the eighth metal is V, Mo, or Fe have been observed, but the Fe-Mo cofactor is the most well-studied. One of the unique structural features of this cluster is the interstitial μ6-C ligand, which is not observed in any other biological cluster, and a rare motif in reported synthetic complexes. Extensive mechanistic investigations of nitrogenase by EPR 12 spectroscopy has characterized a number of reduced oxidation states of the cluster;29 however the precise binding mode of the FeMoco substrate, N2, has not been established. Peters and co-workers have examined a series of mononuclear Fe–N2 complexes, bearing different transligand donors (12 – 14), including an anionic carbon donor;30 notably, 13 is the first example of an Fe-based molecular N2-reduction catalyst.31 The identity of the trans-donor had a strong influence on the Fe–N2 bonding and reactivity. More recently, crystal structures of FeMoco with a displace μ2-S between to Fe centers have been obtained, suggesting a possible substrate binding site.32 This has led to the investigation of binuclear Fe complexes as models of FeMoco.33 The N2-activation chemistry of higher nuclearity Fe complexes have also been investigated, although well-characterized high-spin Fe clusters (of more than two Fe centers) with a bound N2 ligand have yet to be reported in the literature. Despite this, reduction of multinuclear Fe complexes in the presence of N2 has led to the isolation of a number of Fenitride and –imido clusters (15 and 16), with relevance to putative intermediates in FeMoco.34 Scheme 5. Selected Functional Models of FeMoco.30, 34 Like the OEC, efforts to make rigorous structural mimics of FeMoco are motivated by a desire to prepare well-defined molecular models for spectroscopic and structure-property investigations. Many fused-cubane clusters reminiscent of FeMoco have been reported by the 13 research groups of Holm and Ohki, including a [Fe8S10] cluster bearing a μ6-S (17).35 Although the small molecule chemistry of these clusters has not been reported, related [Fe6S9] clusters have been combined with apo-nitrogenase proteins to produce artificial metalloproteins competent for reductive coupling of CO, -CN, and C2H4.36 Scheme 6. Structural Models of FeMoco and Related Nitrogenase Clusters.37 Synthetic Metal Complexes Related to Biological Denitrification. The process of denitrification is an important part of the global nitrogen cycle, nitrate (NO3-) is reduced to N2 over four steps, via nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O) intermediates, as a terminal electron acceptor for an anaerobic analogue to cellular respiration.4a The NO and N2O reducing steps are accomplished by multinuclear active sites of Fe and Cu, respectively. Nitric oxide reductase (NOR) contains an active site structure similar to HCO, with a nonheme Fe center instead of Cu; similar molecular and biosynthetic systems that have been used to understand HCO have been applied to NOR, as well.38 NO-reducing metalloenzymes with a binuclear non-heme Fe active site are also present in pathogenic bacteria for NO 14 detoxification. A faithful structural and functional model of this active site has recently been reported by Lehnert and co-workers (20).39 Nitrous oxide reductase (N2O) is composed of a tetranuclear Cu active site, containing a bridging S ligand.40 Mankad and co-workers have reported a tetranuclear Cu complex bearing a μ4-S with a square pyramidal geometry (21); this complex is capable of reducing N2O to N2, structurally and functionally mimicking the native enzyme.41 The proposed mechanism of N2OR suggests N2O is activated across two Cu centers; further mechanistic investigations of 21 or the native enzyme are required to establish the precise role of the four Cu centers in the cluster’s ability to drive this transformation. Scheme 8. Multinuclear Model Complexes of NOR and N2OR39, 41 CONSPECTUS The oxidation and reduction of small molecules of oxygen and nitrogen occurs on a global scale and has relevance to many of the chemical processes that encompass life. In organisms across all domains of life, the metalloenzymes that catalyze the multi-electron transformations of molecules such as H2O, O2, N2, and NO3- contain active sites with diverse, complex multinuclear transition metal structures. Mechanistic investigations of these metalloenzymes have sought to elucidate the functional purpose of these unique multinuclear arrangements; and come with a number of inherent challenges, due to difficulties in preparation and 15 manipulation of large protein assemblies, or the limitations of aqueous conditions. For this reason, complementary studies of synthetic transition metal complexes and their chemistry towards O- and N-based small molecules is useful for understanding mechanistic details of native biological systems. While a majority of the reported literature has focused on the synthetic chemistry of mononuclear and binuclear transition metal complexes, the synthesis and study of higher nuclearity clusters comprises an important development towards a more complete understanding of the multi-electron redox transformations of globally relevant molecules. The work detailed in this dissertation addresses the development of tetranuclear clusters of biologically relevant transition metals Fe and Mn, in particular their chemistry towards NO and H2O, with relevance to biological multinuclear active sites responsible for multi-electron transformations of small molecules. 16 References 1. Falkowski Paul, G.; Godfrey Linda, V. Philosophical Transactions of the Royal Society B: Biological Sciences 2008, 363, 2705-2716. 2. Kuypers, M. M. M.; Marchant, H. K.; Kartal, B. Nature Reviews Microbiology 2018, 16, 263. 3. van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem. Soc. 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Soc. 2016, 138, 13107-13110. CHAPTER 2 TETRANUCLEAR IRON CLUSTERS WITH A VARIED INTERSTITIAL LIGAND: EFFECTS ON STRUCTURE, REDOX PROPERTIES, AND NITRIC OXIDE ACTIVATION The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. Inorg. Chem., 2017, 56, 13360-13367 20 ABSTRACT A new series of tetranuclear Fe clusters displaying an interstitial μ4-F ligand was prepared for a comparison to previously reported μ4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with NO stretching frequencies ranging from 1680 to 1855 cm−1, respectively. Despite accessing more reduced states with an F− bridge, a two-electron reduction of the distal Fe centers is necessary for the μ4-F clusters to activate NO to the same degree as the μ4-O system; consequently, NO reactivity is observed at more positive potentials with μ4-O than μ4-F. Moreover, the μ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation. 21 INTRODUCTION Transition metal clusters perform diverse functions in proteins, including metal storage, sensing, electron transfer, and multi-electron small molecule conversions (such as H2O oxidation, CO2 fixation, and N2 reduction).1 A common element of these multinuclear sites is the presence of highly bridged (≥ µ3-binding) single atom ligands, such as sulfide,2 oxide,3 or carbide.4 Quantitative measures of the effects these ligands play in small molecule activation remain rare. This is particularly relevant to understanding the role the interstitial µ6-C ligand in the FeMo cofactor (FeMoco) of nitrogenase (Figure 1A). Synthetic clusters suitable for structure-function studies of bridging ligands with respect to the activation of a small molecule are rare, likely because of design constraints that are difficult to overcome by self-assembly, which is the route typically employed in cluster synthesis. Maintaining the exact same structure while changing the bridging ligands and redox states while limiting ligand binding to a single small molecule, desirable for quantification of the effect and for mimicking substrate activation by protein active sites, are two major challenges. A host of iron carbonyl clusters have been synthesized with a variety of bridging (≥ µ3) single-atom ligands, including µ6-C clusters, such as [(µ6-C)Fe6(CO)16]2-, with arrangements reminiscent of the FeMoco structure (Figure 1B).5 While a related cluster has been reported displaying a µ6-N ligand, [(µ6-N)Fe6(CO)15]3- has been reported, with potential for structure-function studies of the effect of the interstitial ligand, changes in the structure and number of carbon monoxide (CO) ligands complicate interpretations. In the cases where completely isostructural clusters can be prepared with bridging elements of the second row of the periodic table, the large number (≥ 9) of diatomic ligands limits interpretations regarding the activation of a single small molecule substrate, which is most relevant to biological systems. Recently, in an elegant demonstration of the effect of the µ4-ligand (N vs C) on reactivity, the hydride ligands in [HFe4C(CO)12]2- and [HFe4N(CO)12]- 22 Figure 1 (A) Depiction of FeMoco cluster of nitrogenase with putative binding of nitrogenous ligand and design elements of the clusters reported herein (B) Reported Fe clusters with different interstitial (or psedo-interstitial, X) and diatomic (CO) ligands; right, limitations of these clusters for determining the effect of interstitial ligand on small molecule activation (C) Synthesis of tetranuclear iron clusters. 23 have been shown to have distinct behavior for H2 and formate generation.6 Other synthetic clusters have been studied to address effects of a bridging ligand on reduction potentials or to model FeMoco, but small molecule binding by the clusters with different bridging ligands has not been reported.7 Toward directly interrogating the effect of a cluster’s interstitial ligand on reactivity, we have developed synthetic methodologies to access site-differentiated multinuclear complexes that allow variation of the bridging ligands. Initial studies by Dr. Graham de Ruiter established a synthetic route to tetranuclear Fe clusters arranged in a pseudo-tetrahedral geometry around a µ4-O ligand. One of these Fe centers displays a trigonal pyramidal coordination geometry, with an open coordination site trans to the µ4-O and the three neighboring Fe centers; this open coordination site is facilitated by steric protection of phenyl-pyrazolate ligands, which preclude binding of most ligands except for small molecules. These clusters, LFe3O(PhPz)3Fen+, were competent to bind and activate nitric oxide (NO; Scheme 1), where redox changes of the distal Fe centers were able to modulate the degree of NO activation and reactivity.8 In summary, reduction of a distal FeIII to FeII leads to an average decrease of N–O stretching frequency of ~ 50 – 30 cm-1, where the lowest oxidation state NO cluster observed (FeII2FeIII{FeNO}7) displayed reactivity towards NO disproportionation. Scheme 1. Related Tetranuclear Clusters Previously Reported by the Agapie Group8 24 Herein, we present investigations of a series of tetranuclear iron clusters containing a µ4-F motif, isostructural with the previously reported µ4-O clusters (Figure 1C).8-9 These compounds allow for the evaluation of the effects of the nature of the interstitial atom on cluster properties related to the activation of a single diatomic ligand (NO). RESULTS AND DISCUSSION We have recently reported the synthesis of site differentiated tetranuclear clusters, where three (basal) metal centers are coordinated by a hexapyridyl trialkoxide framework (L3-, Figure 1C) and bridged to a fourth (apical) metal site through three pyrazolate ligands and a µ4-O ligand.8-9 The all-ferrous fluoride-bridged cluster, 1, was synthesized via addition of a 2:1 ratio of phenylpyrazole and potassium phenylpyrazolate along with 1 equiv of anhydrous tetrabutylammonium fluoride to a previously reported trinuclear iron precursor (LFe3(OAc)(OTf)2; Figure 1C).8, 10 The fourth Fe equivalent was delivered as Fe(N(SiMe3)2)2 to complete the tetranuclear cluster (1). This redox-neutral route of installing the interstitial F proved to be the most reliable way to avoid the generation of mixtures of clusters, with some µ4-O clusters likely having formed due to trace moisture. Subsequent chemical oxidations afford two additional redox states, [FeII3FeIII] (2) and [FeII2FeIII2] (3); cyclic voltammetry of 1 displays two quasi-reversible features for these oxidations at potentials of -0.51 V (all potentials vs Fc/Fc+) and +0.18 V (Figure 2). Characterization by Mössbauer spectroscopy is consistent with charge localization on each Fe center and with oxidations occurring exclusively in the basal triiron core, the apical Fe remaining FeII (Figure 3), as was observed for the µ4-O analogs.9a Structural characterization by single crystal X-ray diffraction (XRD) reveals that the most oxidized cluster, 3, displays a five-coordinate apical FeII, due to acetonitrile (MeCN) binding (Figure 4). Removal of this ligand under vacuum results in decomposition. This behavior is in contrast to the analogous µ4-O clusters, which have been isolated in the 25 [FeII2FeIII2] and [FeIIFeIII3] oxidation states, both displaying a four-coordinate apical FeII. This difference suggests that that the µ4-F clusters are more Lewis acidic than their µ4-O analogues. Consistent with this interpretation, µ4-O clusters with electron withdrawing substituents show increased coordination numbers at the apical metal.9a II II Fe 3Fe III II III Fe 2Fe 2 Current Fe 4 20 μA -1.5 -1 -0.5 0 0.5 1 Potential vs. Fc/Fc+ (V) Figure 2. Cyclic voltammogram (black trace) of 1 (3 mM) in MeCN with 85 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Square wave voltammograms (gray dashed trace) overlaid with 0.1 V amplitude, 1.0 s period, and 0.01 V increment. 26 Figure 3. 57Fe Mössbauer spectra at 80K of (A) 1, (B) 2, and (C) 3. Black dots represent the data, gray traces are the sum of the simulated fits, and colored traces represent the individual fits for the Fe centers (See Table 1 for parameters). Blue traces represent assignments made to basal FeII, orange traces represent basal FeIII assignments, green traces represent apical FeII. Table 1. 57Fe Mössbauer Parameters for Complexes 1 – 3 with Oxidation State Assignments δ (mm/s) % assignment 1.16 |ΔEq| (mm/s) 1 3.42 25 h.s. FeII 1.15 3.18 25 h.s. FeII 1.18 3.07 25 1.10 25 h.s. FeII h.s. FeII 3.30 2.90 1.21 1.50 25 25 29 21 h.s. FeII h.s. FeII h.s. FeIII h.s. FeII 3.07 0.89 1.45 2.49 25 25 25 25 h.s. FeII h.s. FeIII h.s. FeIII h.s. FeII 0.95 2 1.14 1.12 0.48 0.99 3 1.12 0.49 0.49 0.98 NO provides a diagnostic vibrational spectroscopic signature for comparing different complexes to address the effects of the multinuclear supporting platform and the interstitial ligand on small-molecule binding.11 Studies of the chemistry of Fe clusters with NO have been 27 Figure 4. Crystal structure of 3. Ellipsoids are shown at the 50% probability level. Hydrogen atoms, solvent molecules, and outersphere counterions omitted for clarity. principally focused on understanding the biologically relevant conversion of Fe–S clusters to nitrosylated products.12 However, there are few examples of multinuclear mononitrosyl complexes containing nearby redox-active metal centers.8, 13 The clusters targeted here provide insight into the influence of neighboring metal centers on the chemistry of the metal nitrosyl moiety. The addition of NO to compound 1 leads to the formation of the corresponding nitrosyl adduct. Cyclic voltammetry of the monocationic nitrosyl cluster, 1-NO, displays three electrochemically quasi-reversible oxidations and one quasi-reversible reduction (Figure 5). Each of the five redox states of the nitrosyl clusters observed electrochemically was accessed synthetically (Figure 1C). Stepwise treatment of 1-NO with AgOTf (2-NO and 3-NO) and [(2,4-Br-C6H4)3N][SbCl6] (4-NO) provides access to the oxidized NO adducts. 4-NO decomposes in solution and as a solid on the time scale of attempted crystallizations, preventing structural characterization. Reduction of 1-NO with decamethylcobaltocene in 28 MeCN precipitates a purple solid assigned as 5-NO. Dissolution of 5-NO in tetrahydrofuran, pyridine, or dichloromethane (CH2Cl2), leads to rapid decomposition, preventing structural characterization of this complex as well (Figure 6). Reoxidation of a MeCN suspension of 5NO with silver triflate (AgOTf) leads to isolation of the one electron oxidized cluster, 1-NO, in good yield (Figure 7). Nitrous oxide (N2O) is detected upon decomposition of 5-NO, albeit in low yield (~0.1 equiv, gas chromatography-mass spectrometry). Figure 5. Cyclic voltammogram of monocationic nitrosyl cluster, 1-NO (2mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt wire, and Ag wire as working, reference, and counter electrodes, respectively. The measured open-circuit potential was -0.7 V. Mössbauer spectroscopy was performed on 1-NO – 5-NO. As observed in the µ4-O system, Mössbauer parameters are consistent with oxidations being localized at the basal triiron core as characterized previously.8-9, 14 In the Mössbauer spectrum of 1-NO, the Fe–NO signal is readily distinguished from the basal iron centers in the cluster, and was fit with an isomer shift (δ) of 0.62 mm/s and a quadrupole splitting value (|ΔEq|) of 1.16 mm/s (Figure 29 Figure 6. (Top) 1H NMR (300 MHz; CD3CN) of reaction mixture of 5-NO in THF over 24 hours. The spectrum of the major species is identical to [LFe3F(PhPz)3Fe][OTf] (1) in CD3CN. (Bottom) 1H NMR (300 MHz; CD2Cl2) of reaction mixture of 5-NO in THF over 24 hours. The spectrum of the new species is identical to [LFe3O(PhPz)3Fe][OTf]. We have previously observed decomposition of LFe3O(PhPz)3Fe in dichloromethane to the monocationic cluster. This is consistent with formation of a mixture of [LFe3F(PhPz)3Fe]+ and LFe3O(PhPz)3Fe from the decomposition of 5-NO in THF. Figure 7. 1H NMR (300 MHz; CD3CN) of reaction mixture of AgOTf addition to LFe3F(PhPz)3Fe(NO) (5-NO) in thawing THF. The spectrum is identical to [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO) in CD3CN. 30 8B; Table 2). The exact Mössbauer parameters for the Fe–NO centers in 2-NO – 4-NO are more difficult to assign due to spectral overlap with signals from the FeIII centers of the triiron core. The overlap is consistent, however, with only small changes in the Mössbauer parameters for the Fe-NO sites in 1-NO – 4-NO (Figures 8C-D and Table 2). These parameters are also similar to the previously reported µ4-O NO clusters, which have δ values ranging from 0.55 to0.62 mm/s, and |ΔEq| values of 1.94 to 2.38 mm/s.8 Overall, these data, along with the IR spectroscopy data (vide infra), are consistent with the {FeNO}7 formulation, according to Figure 8. Zero applied field 57Fe Mössbauer spectra at 80K of (A) 5-NO,(B) 1-NO, (C) 2NO, (D) 3-NO, and (E) 4-NO. Black dots represent the data, gray traces are the sum of the simulated fits, and colored traces represent the individual fits for the Fe centers (see Table 2 for parameters). Blue traces represent assignments made to basal FeII, orange traces represent basal FeIII assignments, green and purple traces represent {FeNO}7 and {FeNO}8 units, respectively. 31 Table 2. Fe–µ4-F Distances and 57Fe Mössbauer Parameters for Complexes 1-NO – 5NO with Oxidation State Assignments Fe Center Fe–µ4-F distance (Ǻ) δ (mm/s) |Eq| (mm/s) assignment 1.15; 1.15; 1.16 3.59; 3.40;3.23 h.s. FeII 1.67 {FeNO}7 1.17 3.31; 3.03 1.39 h.s. FeIII h.s. FeII {FeNO}7 3.10 0.87; 1.47 1.51 h.s. FeII h.s. FeIII {FeNO}7 1.42 h.s. FeIII and {FeNO}7 1-NO Fe1, Fe2, Fe3 Fe4 2.129(7); 2.205(6); 2.169(5) 2.065(7) Fe1 Fe2, Fe3 Fe4 2.030(4) 2.237(4); 2.101(4) 2.093(4) Fe1 Fe2, Fe3 Fe4 2.207(3) 2.080(3); 2.091(3) 2.155(3) Fe1-Fe4 - 0.63 2-NO 0.44 1.12; 1.15 0.62 3-NO 1.09 0.48; 0.40 0.62 4-NO a 0.47 - 5-NO 1.15; 1.15; 1.15 3.56; 3.17; 3.75 h.s. FeII - 0.95 1.63 {FeNO}8 Fe1, Fe2, Fe3 Fe4 aIn this case, the signals for the Fe centers overlap preventing reliable parameter determination for the unique apical {FeNO}7 center. The presence of an {FeNO}7 moiety is supported via the IR spectroscopy data. Enemark-Feltham notation.15 The Mössbauer spectrum of 5-NO was fit with three FeII in the triiron core and an apical Fe–NO signal distinct from the ones observed for 1-NO – 4NO; this is assigned as {FeNO}8 (δ = 0.94 mm/s and |ΔEq| = 1.63 mm/s; Figure 8A), consistent with reduction of the Fe–NO moiety rather than a remote metal site. Compounds 1-NO, 2-NO, and 3-NO were structurally characterized by XRD. In all cases, binding of NO to the apical Fe occurs in a linear fashion (∠Fe4–N40–O40 > 175°, Figure 9A). As observed in the µ4-O system and from Mössbauer spectra (Figure8B-D), bond metrics are consistent 32 with oxidations being localized at the basal triiron core of these three clusters (Table 2). The Fe– µ4-F bonds, which range from 2.07 to 2.24 Å, are longer than the Fe–µ4-O bonds (1.93 to 2.18 Å) despite the shorter ionic radius of F- which suggests a significantly weaker interaction with the fluoride resulting in more electron deficient metal centers.16 IR spectroscopy reveals a large range of νN–O for complexes 1-NO – 5-NO, from 1680 cm-1 to 1855 cm-1 (Figure 10). Comparison of νN–O for 1-NO – 4-NO (1799 – 1855 cm-1) provides insight into the effect of remote redox changes on NO activation. Oxidation of the Fe centers not bound to NO leads to an average of 19 cm-1 per redox change, with redox changes of more reduced clusters having a larger effect. The shift in νN–O to higher energy upon oxidation is matched by an increase in Fe4-µ4-F distance, and likely results from a more electron deficient Fe4 center due to this elongation. The nature and type of interaction with axial ligand has been previously demonstrated to effect the level of NO activation in mononuclear Fe complexes.17 Analogous shifts in the distance between Fe and axial ligands trans to coordinated N2 have been reported for monoiron models of nitrogenase.18 The correlation between the increase in the Fe4-µ4-ligand distance and the increase in the νN–O frequency observed previously for µ4-O and now with µ4-F interstitial ligands suggests that this structural parameter generally serves to relay the effect of remote redox changes to the metal that binds the small molecule. However, the magnitude of the change in NO activation as a result of these distal redox changes varies with the nature of the interstitial atom. For µ4-O clusters, the νN–O changes from 1715/1759 to 1823 cm-1 over two redox events with an average change of 54/33 cm-1 per electron transfer, in contrast to only 19 cm-1 for μ4F. The stronger O2- ligand roughly doubles the effect of the remote redox changes on the activation of NO compared to F-. This is a unique observation, which relies on the ability to access multiple oxidation states of these clusters, and demonstrates that an interstitial ligand 33 Figure 9. (A) Crystal structure of tetranuclear iron nitrosyl cluster 2-NO with ellipsoids shown at the 50% probability level. Solvents molecules, outer-sphere counterions, and H atoms are omitted for clarity. (B) Simplified depiction of the tetranuclear iron clusters discussed. Measured redox potentials, NO stretching frequencies, and apical Fe-µ4-ligand distances are included for comparison. Data for the µ4-O clusters were previously reported.8 Rel. Transmission [LFe3F(PhPz)3Fe][OTf] LFe3F(PhPz)3Fe(NO), 5-NO [LFe3F(PhPz)3Fe(NO)][OTf], 1-NO [LFe3F(PhPz)3Fe(NO)][OTf]2, 2-NO [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6], 4-NO [LFe3F(PhPz)3Fe(NO)][OTf]3, 3-NO 3400 2900 2400 1900 1400 Frequency (ν, cm-1) 900 400 Figure 10. Solid state IR spectra of complexes 1, 1-NO - 5-NO. can influence small molecule activation in two ways: first, by its direct interaction with the small molecule-binding metal center, and second, by modulation of the degree to which other 34 metals in the cluster can perturb this meta-interstitial ligand interaction. A structural comparison of the Fe4-μ4-ligand distances over two oxidation states shows that redox changes at the remote Fe centers shift the Fe4-μ4-F distance by 0.09 Å and the Fe4-μ4-O bond by 0.12 Å (Figure 9B). The more donating interstitial ligand is able to more efficiently translate remote redox changes in the cluster into NO activation. A consequence of varying the μ4-ligand in these clusters is that the weaker F− donor increases the overall cluster charge of a particular redox state by 1 compared to the O2− version. In reported mononuclear complexes, related modifications of a ligand‘s charge at a distal site (i.e. R3BH- vs R3CH) leads to observable shifts of bound carbonyl stretching frequencies by ~10 – 40 cm-1.19 For these clusters, separating the effect of higher positive charge from the effect of the donating abilities of the interstitial ligand on NO activation can be addressed by comparing clusters 2-NO−4-NO and the μ4-O analogues. For the same cluster redox state, significantly higher νN−O are observed for the μ4-F ligand compared to μ4-O, as expected. The overall cluster charge, which is higher by 1 compared to μ4-O clusters of the same Fe redox states, is not sufficient to explain the higher NO activation. A comparison of clusters of the same charge for μ4-O and μ4-F, but higher overall Fe redox state for μ4-O (for example, (μ4F)FeIIFeIII2{FeNO}7 (3-NO) with νN−O = 1842 cm−1 vs (μ4-O)FeIII2{FeNO}7 with νN−O = 1823 cm−1) still shows a higher degree of NO activation with O2−. This difference suggests that the higher-charge interstitial ligand leads to a more electron-rich cluster and a lower νN−O due to its direct interaction with the metal centers rather than solely due to the reduced cluster charge. IR spectroscopy of 5-NO corroborates the Mössbauer data and is consistent with the formation of a {FeNO}8 motif; the νN–O at 1680 cm-1, is ~120 cm-1 lower than νN–O for the {FeNO}7 moiety of 1-NO. A similarly large shift was observed upon reduction for a structurally related mononuclear trigonal bipyramidal Fe-NO complex,20 and more generally 35 for nonheme {FeNO}7/{FeNO}8 complexes.21 An analogous species is not observable for the µ4-O clusters. A comparison of the redox potentials of the µ4-F and the µ4-O systems (Figure 9B)8 reveals that the F- ligand shifts the redox potentials positively by approximately 1 V for the same cluster oxidation states compared to the O2- ligand because of the lower negative charge and weaker electron donating ability of F-. An analogous effect is observed for other clusters upon changing the bridging ligand to alter the charge of the cluster.6, 7e The shift in redox potentials allows access to more reduced states of the µ4-F clusters within the electrochemical solvent window, which could be beneficial for storing additional reducing equivalents at more positive potentials. However, this is counterbalanced by weaker activation of the diatomic ligand, as reflected by IR spectroscopy (vide supra). In fact, to achieve the same level of NO activation, the µ4-F clusters need to have Fe oxidation states lower by two levels compared to the µ4-O clusters. This is in contrast to the behavior observed for certain iron-multicarbonyl clusters, where data is available for isostructural motifs. For example, [Fe4C(CO)12]2- shows lower average CO activation than the one electron more reduced, but same-charge cluster, [Fe4N(CO)12]2-.6, 22 The difference is likely a result of distribution of charge and small molecule activation over many (12) CO ligands. In the present system, which displays a more biomimetic, single ligand binding, the NO activation is relayed remotely through the interstitial atom and provides a test for the ability of the µ4-ligand to communicate the redox change at metals not bound to NO. Furthermore, differences in chemical reactivity of the diatomic ligand are observed. The addition of NO to (µ4-O)FeII2FeIII{FeNO}7 leads to NO disproportionation to generate N2O and the one electron oxidized nitrosyl cluster.8 In contrast, addition of NO to 1-NO, which is one electron more reduced (µ4-F)FeII3{FeNO}7, does not result in a reaction. This difference in reactivity as a function of interstitial ligand is likely due to a more activated NO and a 250 mV lower redox potential for the µ4-O cluster. 36 Only 5-NO, with an electronically different, {FeNO}8 moiety, undergoes conversion to N2O with a fluoride interstitial ligand, albeit not cleanly. Overall, despite more negative potentials compared to µ4-F analogs of the same redox state, reactivity of NO is observed at milder potentials with the µ4-O cluster. CONCLUSIONS In this report, we have demonstrated the significant effects that the change of interstitial ligands (µ4-O vs µ4-F) has on the small molecule activation properties of tetranuclear Fe clusters. The more positive redox potentials of µ4-F clusters allow access to more reduced Fe states. However, this does not result in more efficient activation of small molecule ligands, as inferred from IR spectroscopy and reactivity of NO complexes. The higher νN–O values of the µ4-F species for the same Fe oxidation states compared to the µ4-O analogues are not due to the difference in cluster charge but rather the nature of the interactions with the bridging ligand. To achieve similar NO activation, the cluster needs to be two electrons more reduced with the µ4-F compared to the µ4-O ligand. Consequently, NO disproportionation is observed with a µ4-O ligand at higher Fe oxidation states and more positive potentials than with a µ4-F ligand. Furthermore, the µ4-O ligand is a better relay of remote redox changes. The structurefunction studies described here suggest that a higher charge interstitial ligand, such as the carbide in FeMoco of nitrogenase, is more efficient at tuning cluster properties in a variety of ways toward the activation of small molecule. Cluster analogs with interstitial C and N moieties are currently being pursued for comparison. 37 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. LFe3(OAc)(OTf)2,8 Fe(N(SiMe3)2)2,23 benzyl potassium,24 1-H-3-phenyl pyrazole (HPhPz),25 anhydrous [NBu4][F]26, and [(2,4-Br-C6H3)3N][SbCl6]27 were prepared according to literature procedures. [(4-Br-C6H4)3N][OTf] was prepared according to a modified literature procedure.28 Tetrahydrofuran was dried using sodium/benzophenone ketyl, degassed with three freeze-pump-thaw cycles, vacuum transferred, and stored over 3Å molecular sieves prior to use. CH2Cl2, diethyl ether, benzene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H and 19F NMR spectra were recorded on a Varian 300 MHz spectrometer. 13C NMR spectra were recorded on a Varian 500 MHz spectrometer. CD3CN and CD2Cl2 was purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior to use. Infrared (ATR-IR) spectra were recorded on a Bruker ALPHA ATR-IR spectrometer at 4 cm1 resolution. Headspace analysis was conducted on a HP 5972 GC-MS. Physical Methods. Mössbauer measurements. Zero applied field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated acetonitrile solution in the cup. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). 38 Mössbauer simulation details for compounds 1 – 3 and 1-NO – 5-NO. All spectra were simulated as four pairs of symmetric quadrupole doubles with equal populations and Lorentzian lineshipes (the parameter defining the width, Г, is reported). They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.8-9, 14 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.29 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter), and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Dry acetonitrile or CH2Cl2 that contained ~85 mM [Bu4N][PF6] was used as the electrolyte solution. The ferrocene/ferrocinium (Fc/Fc+) redox wave was used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of 39 the compounds (1 - 3 and 1-NO – 3-NO), most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Synthetic Procedures. Synthesis of Potassium 3-phenyl-pyrazolate (KPhPz). In the glovebox, a solution of 1-H-3-phenyl-pyrazole (1.54 g, 11.8 mmol) in THF (5 mL) was stirred while a solution of benzyl potassium (1.70 g, 11.8 mmol) in THF (10 mL) was added drop-wise. Addition caused the solution to change from colorless to pale yellow. After 30 minutes, the solvent was removed under reduced pressure to obtain 1.83 g off-white powder (85% yield). 1 H NMR (300 MHz, CD3CN) δ 7.83 (d, 2H), 7.44 (s, 1H), 7.28 (t, 2H), 7.07 (t, 1H), 6.39 (s, 1H). 13C NMR (500 MHz, CD3CN) δ 100.01 (Pz NCCH), 125.02 (p-Ar CH), 125.37 (m-Ar CH), 128.98 (o-Ar CH), 139.34 (Pz CHCHN), 150.27 (Pz NCCH). An expected signal ~ 138 ppm (i-Ar C)8 could not be observed, likely due to the low solubility of KPhPz. Synthesis of tris-4-bromo-phenylamininum trifluoromethanesulfonate ([(4-Br-C6H4)3N][OTf]). This was prepared through a modification of a literature procedure for [(4-Br-C6H4)3N][BF4].28 Tris4-bromo-phenylamine (1.5 g, 3.11 mmol) was dissolved in 30 mL diethyl ether with silver trifluoromethanesulfonate (AgOTf; 1.2 g, 4.67 mmol). This light green solution was added to a 100 mL Schlenk tube and cooled to -40 ºC under N2 atmosphere. Iodine powder (0.75 g, 2.96 mmol) was added with a counter-flow of N2 while stirring; addition caused the solution to turn dark blue. The Schlenk tube was warmed to room temperature and filtered over a course porosity frit. The collected precipitate was filtered with 30 mL CH2Cl2 in the glovebox. To the resulting dark blue solution, 40 mL diethyl ether was added and the flask was cooled to -40 ºC. [(4-Br-C6H4)3N][OTf] was collected as a dark purple solid upon filtration (1.36 g, 69% yield). Anal. Calc. (%) for C19H12Br3F3NO3S: C, 36.16; H, 1.92; N, 2.22. Found: C, 36.70; H, 1.94; N, 2.27. 40 Synthesis of [LFe3F(PhPz)3Fe][OTf] (1). In the glovebox, a suspension of LFe3(OAc)(OTf)2 (1047 mg, 0.76 mmol) in THF (3 mL) was frozen in the cold well. To the thawing suspension, solutions of potassium 3-phenyl-pyrazolate (190 mg, 1.04 mmol) in THF (3 mL) and 1-H-3phenyl-pyrzole (220 mg, 1.52 mmol) in THF (3 mL) were added. The suspension changed color from yellow to orange upon addition of the potassium 3-phenyl-pyrazolate. [Bu4N][F] (208 mg, 0.79 mmol) was added as a suspension in THF (3 mL), causing the solution to become dark red. A solution of Fe(N(SiMe3)2)2 (288 mg, 0.76 mmol) in THF (2 mL) was added. The reaction was stirred for 20 h, after which an orange precipitate was observed. The suspension was filtered over a bed of celite on a fine porosity glass frit and washed with 5 mL THF. The orange solid was collected with 60 mL MeCN. The solvent was removed under reduced pressure to obtain [LFe3F(PhPz)3Fe][OTf] as an orange solid (950 mg, 75% yield). 1H NMR (300 MHz, CD2Cl2) δ 104.77, 78.57, 75.13, 48.82, 37.46, 30.48, 27.17, 26.44, 25.63, 19.69, 18.42, 11.60, 10.53, 4.54, 4.22, 3.44, 1.99, 1.27, 1.16, -1.13, -2.80, -46.96. 19F NMR (300 MHz, CD2Cl2) δ -78.45. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 251 nm (9.2 ×104), 463 nm (3.9 ×103). Anal. Calcd. (%) for C85H60F4Fe4N12O6S: C, 60.88; H, 3.61; N, 10.02. Found: C, 61.16; H, 3.75; N, 9.74. Synthesis of [LFe3F(PhPz)3Fe][OTf]2 (2). To a suspension of [LFe3F(PhPz)3Fe][OTf] (1; 94 mg, 0.06 mmol) in THF (2 mL), a solution of AgOTf (14 mg, 0.06 mmol) in THF (2 mL) was added. The color of the suspension changed from orange to brown and, after 2 hours, the solvent was removed under reduced pressure. The brown residue was dissolved in CH2Cl2 and filtered over a bed of celite on glass filter paper. The solvent was removed under reduced pressure to obtain [LFe3F(PhPz)3Fe][OTf]2 as a brown solid (100 mg, 98% yield). 1H NMR (300 MHz, CD2Cl2) δ 101.33, 87.83, 79.33, 47.73, 46.79, 35.24, 34.14, 28.86, 26.35, 18.15, 16.58, 16.33, 12.10, 8.55, 7.28, 6.79, 6.25, 5.25, 4.63, -42.36. 19F NMR (300 MHz, CD2Cl2) - 41 78.19. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 250 nm (10.9 ×104), 432 nm (4.8 ×103). Anal. Calcd. (%) for C86H60F7Fe4N12O9S2: C, 56.57; H, 3.31; N, 9.21. Found: C, 56.47; H, 3.13; N, 8.88. Synthesis of [LFe3F(PhPz)3Fe(MeCN)][OTf]3 (3). To a stirring solution of [LFe3F(PhPz)3Fe][OTf]2 (2; 78.5 mg, 0.04 mmol) in acetonitrile (2 mL), [(p-Br-C6H4)3N][OTf] (27.1 mg, 0.04 mmol) was added as an MeCN solution (2 mL). The brown solution became purple upon addition. After 30 minutes, the solution was filtered. 5 mL of CH2Cl2 was added to the filtrate, then 10 mL pentane, to obtain a purple precipitate. The supernatant was decanted and the remaining solid was briefly dried under reduced pressure to obtain [LFe3F(PhPz)3Fe(MeCN)][OTf]3 as a purple solid (42.3 mg, 50% yield). 1H NMR (300 MHz, CD3CN) δ 125.15, 91.53, 82.45, 80.10, 61.48, 51.98, 43.99, 15.30, 13.93, 12.33, 8.44, 6.48, 5.67, 5.30, 0.46, -5.74, -18.78. 19F NMR (300 MHz, CD3CN) -75.66. UV-vis (CH2Cl2) [ε (M-1 cm-1)]: 250 nm (10.3 ×104), 465 nm (3.6 ×103). Anal. Calcd. (%) for C88H62Cl2F10Fe4N12O12S3 (3 with CH2Cl2 instead of MeCN; compound recrystallized in CH2Cl2): C, 51.31; H, 3.03; N, 8.16. Found: C, 51.26; H, 3.04; N, 8.43. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). Method A. In the glovebox, a 100 mL Schlenk tube was charged with a solution of [LFe3F(PhPz)3Fe][OTf] (1; 179 mg, 0.11 mmol) in CH2Cl2 (5 mL). The solution was degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 59 mmHg, 0.11 mmol) was condensed in the tube. The reaction was stirred at room temperature for 2 h and changed color from orange to brown. The solvent was removed under reduced pressure to yield [LFe3F(PhPz)3Fe(NO)][OTf] as a brown solid (181 mg, 99% yield). 1H NMR (300 MHz, CD2Cl2) δ 98.43, 76.64, 74.24, 42.59, 40.12, 35.92, 32.51, 27.06, 20.05, 15.27, 14.16, 11.24, 10.79, 4.27, 2.46, 1.13, 0.58, 0.46, -10.77, -23.61. 19F NMR (300 MHz, CD2Cl2) δ -78.71. Anal. Calcd. (%) for C86H62Cl2F4Fe4N13O7S (1-NO ∙ 42 CH2Cl2; compound recrystallized from CH2Cl2/pentane): C, 57.66; H, 3.49; N, 10.16. Found: C, 57.40; H, 3.46; N, 10.01. Method B. In the glovebox, solid LFe3F(PhPz)3Fe(NO) (5-NO; 22 mg, 0.014 mmol) was cooled to -196 °C in a cold well in a 20 mL vial with a stir bar. AgOTf (3.7 mg, 0.014 mmol) in 0.5 mL thawing tetrahydrofuran was added to the cooled powder. This reaction was stirred at room temperature for 30 minutes then pumped down. The purple suspension became a brown solution. 1H NMR analysis of the crude reaction showed mostly (>90%) [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). The brown solid was filtered in CH2Cl2 to obtain 16.8 mg of [LFe3F(PhPz)3Fe(NO)][OTf] after recrystallization (69% yield). Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO). Method A. In the glovebox, a 100 mL Schlenk tube was charged with a solution of [LFe3F(PhPz)3Fe][OTf]2 (2; 163 mg, 0.09 mmol) in CH2Cl2 (5 mL). The solution was degassed by three freeze-pump-thaw cycles. While frozen, gaseous nitric oxide (33 mL, 50 mmHg, 0.09 mmol) was condensed in the tube. The reaction was stirred at room temperature for 2 h, changing color from brown to yellow-green. The solvent was removed under reduced pressure to yield [LFe3F(PhPz)3Fe(NO)][OTf]2 as a dark green solid (162 mg, 98% yield). 1H NMR (300 MHz, CD2Cl2) δ 100.10, 83.22, 80.63, 66.68, 50.74, 46.79, 41.32, 17.25, 14.62, 14.38, 12.35, 11.71, 3.31, 0.30, -3.31, -17.33. 19F (300 MHz, CD2Cl2) δ -77.52. Anal. Calcd. (%) for C86H60F7Fe4N13O10S2: C, 55.65; H, 3.26; N, 9.81. Found: C, 55.59; H, 3.25; N, 9.53. Method B. In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 160 mg, 0.10 mmol) in MeCN (3 mL) was added to a solution of AgOTf (25 mg, 0.10 mmol) in MeCN (2 mL). The solution changed color from brown to yellow-green. After 1 h, the solvent was removed under reduced pressure. The green residue was dissolved in CH2Cl2 and filtered over a bed of celite. The solvent was removed under reduced pressure to obtain 43 [LFe3F(PhPz)3Fe(NO)][OTf]2 as a dark green solid (164 mg, 95% yield). 1H NMR is identical to that observed for method A. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO). In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO; 27.6 mg, 0.015 mmol) in CH2Cl2 (1 mL) was stirred as a solution of [(4-Br-C6H4)3N][OTf] (10.0 mg, 0.016 mmol) in CH2Cl2 (1 mL) was added. The addition caused the yellow-green solution to turn purple. After 30 minutes, the reaction was filtered and layered under pentane to afford purple crystals of [LFe3F(PhPz)3Fe(NO)][OTf]3 (20.3 mg, 68% yield). 1H NMR (300 MHz, CD2Cl2) δ 123.58, 98.80, 89.32, 60.89, 41.42, 14.25, 13.41, 10.34, 5.32, 4.35, 3.93, 3.71, 3.47, 2.07, 1.85, 1.18, -2.45, -8.26. Anal. Calcd. (%) for C87H60F10Fe4N13O13S3: C, 52.12; H, 3.02; N, 9.08. Found: C, 51.88; H, 2.94; N, 8.74. Synthesis of [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] (4-NO). In the glovebox, a thawing solution of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO; 25.7 mg, 0.013 mmol) in CH2Cl2 (1 mL) was stirred as a solution of [(2,4-Br-C6H3)3N][SbCl6] (13.9 mg, 0.013 mmol) in MeCN (1 mL) was added. The addition caused the purple solution to turn blue. Cold toluene was added until a precipitate was observed. This was kept in a liquid nitrogen-cooled cold well for 2 minutes. The supernatant was decanted and the resulting solid was dried under vacuum. This afforded [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] as a blue solid (15 mg, 49% yield). This compound decomposes over time in solution and the solid state, even at reduced temperatures. Characterization of this compound was conducted with freshly prepared samples to minimize decomposition. 1H NMR (300 MHz, CD3CN) δ 124.54, 97.65, 80.33, 77.50, 74.55, 37.57, 18.30, 15.25, 13.39, 9.04, 0.01, -1.66, -5.71, -6.88. Anal. Calcd. (%) for C101H76Cl6F10Fe4N13O13S3Sb (NO4 ∙ 2 C7H8; compound precipitated with toluene): C, 48.07; H, 3.04; N, 7.21. Found: C, 47.83; H, 2.97; N, 7.88. 44 Synthesis of LFe3F(PhPz)3Fe(NO) (5-NO). In the glovebox, a solution of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 82.9 mg, 0.049 mmol) in MeCN was stirred as a solution of CoCp*2 (16.8 mg, 0.051 mmol) in MeCN was added. The addition caused the brown solution to become a purple suspension. After 2 hours, the solids were collected, washed with minimal MeCN, and dried under vacuum to afford LFe3F(PhPz)3Fe(NO) as a purple solid (44.3 mg, 59% yield). This species decomposes upon dissolution in tetrahydrofuran, pyridine, or CH2Cl2 and is mostly insoluble in acetonitrile, benzene, and toluene. Therefore, NMR and UV-Vis Absorbance data could not be collected for this complex. Anal. Calcd. (%) for C84H60FFe4N13O4: C, 64.76; H, 3.88; N, 11.69. Found: C, 64.21; H, 3.86; N, 11.51. Decomposition of LFe3F(PhPz)3Fe(NO) (5-NO). In the glovebox, solid LFe3F(PhPz)3Fe(NO) (26 mg, 0.02 mmol) was added to a 20 mL vial with septum cap and stir bar. 10 mL tetrahydrofuran was added and the vial was quickly sealed. Upon dissolving, the solution appeared brown. After stirring for 24 hr, the headspace was analyzed via GC-MS. A blue precipitate was observed in a brown-orange solution. 45 ELECTROCHEMICAL DETAILS 100mV/s 400mV/s 200mV/s 500mV/s Current 50mV/s 300 mV/s 50 μA -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V vs. Fc/Fc+) Figure 11. Cyclic voltammogram (solid traces) of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO; 2 mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Agwire as working, reference, and counter electrode, respectively. Square wave voltammograms (gray dashed trace) overlaid with 0.1 V amplitude, 1.0 s period, and 0.01 V increment. The open circuit potential (OCP) was measured to be -0.7 V. 3500 2500 jp (µA/cm2) 1500 500 -500 -1500 ○ ● FeII3{FeNO}8/FeII3{FeNO}7 □ ■ FeII3{FeNO}7/FeIIIFeII2{FeNO}7  ▲ FeIIIFeII2{FeNO}7/FeIII2FeII{FeNO}7 -2500 -3500 -4500 5 10 15 ν1/2 ((V/s)1/2) 20 Figure 12. Current density (jp) dependence of the square root of the scan rate ν1/2 for the electrochemical events observed in the CV of [LFe3F(PhPz)3Fe(NO)][OTf], 1-NO. 46 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3 and 1-NO – 3-NO. 1 2 3 1-NO 2-NO 3-NO CCDC Number 1554599 1554601 1554596 1554600 1554598 1554597 Empirical formula C86.75H62Cl4F 4Fe4N12O6S C96H71Cl2F7F e4N12O9S2 C101.5H75F10F e4N14O12S3 C93H76Cl16F4 Fe4N13O7S C93H74Cl14F7 Fe4N13O10S2 Formula weight (g/mol) C89.92H64.86Cl5 .85F10Fe4N13 O13S3 1837.80 2028.06 2235.21 2386.32 2450.16 2252.38 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) MoKα (λ = 0.71073) CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) a (Å) 12.4213(18) 40.6924(19) 16.1006(6) 18.7000(7) 20.1692(15) 17.8680(9) b (Å) 16.108(2) 17.6138(8) 16.1006(6) 16.8237(7) 17.4343(13) 20.4024(11) c (Å) 20.502(3) 25.6670(13) 67.515(3) 32.4277(11) 28.440(2) 26.3282(15) α (°) 78.323(6) 90 90 90 90 78.386(4) β (°) 78.274(7) 114.015(2) 90 103.821(2) 99.091(2) 72.564(3) γ (°) 85.537(6) 90 120 90 90 82.836(3) V (Å3) 3930.6(10) 16804.3(14) 15157.0(13) 9906.5(7) 9874.8(13) 8948.2(9) 2 8 6 4 4 4 Cryst. syst. triclinic monoclinic trigonal monoclinic monoclinic triclinic Space group P-1 C2/c R-3 P21/n P21/n P-1 ρcalcg (cm3) 1.553 1.603 1.469 1.600 1.648 1.672 2Θ range (°) 2.584 to 61.034 4.626 to 55.754 5.060 to 61.146 5.004 to 130.168 4.464 to 51.356 9.042 to 179.202 μ (mm-1) 0.960 0.877 0.712 9.351 1.076 8.167 GOF 0.998 1.026 1.051 1.052 1.060 1.033 R1, wR2 (I>2σ (I)) 0.0400, 0.1003 0.0458, 0.0959 0.0835, 0.2216 0.1232, 0.2937 0.0944, 0.2594 0.0786, 0.2045 Z 47 Special refinement details for [LFe3F(PhPz)3Fe][OTf] (1). The structure contains several co-crystallized solvent molecules, many of which are on special positions. The only complete solvent molecule in the asymmetric unit that could be refined was disordered over two positions refined as 34.1% (C14 through C102) and 65.9% (Cl2 through C101). The two remaining solvent molecules were also disordered over two positions, but on a symmetry element. One disordered dichloromethane was refined as a partially occupied carbon 25% (C103) with ~50% occupied chlorine groups (Cl6 and Cl7). The other disordered dichloromethane was refined as a half occupied molecule, disordered over a symmetry element. Special refinement details for [LFe3F(PhPz)3Fe][OTf]2 (2). This structure contains two triflate counterions, one of which is positionally disordered over two positions with refined occupancies of 78.5% (S201 through C201) and 21.5% (S202 through C202). The structure also contains a co-crystallized dichloromethane (C0AA through Cl20), and two benzene molceules; one is complete (C101 through C106) the other (C107 through C109) is on a special position. Rigid bond restraints were used on the triflate counterions. Special refinement details for [LFe3F(PhPz)3Fe(MeCN)][OTf]3 (3). This structure contains the cluster on a C3 rotation axis, and therefore the three irons in the tri-iron core (Fe1 through Fe1’’) are indistinct. One triflate counterion is observed in the asymmetric unit along with four solvent molecules, only two of which could be modeled satisfactorily. A toluene molecule (C104 through C107) was disordered over a special position. There was another disordered toluene and acetonitrile that were disordered near special positions, based on residual electron density peaks; however, they could not be satisfactorily modeled. A solvent mask was used to account for the electron density of these molecules. 48 Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO). This structure contained numerous co-crystallized solvent molecules (eight dichloromethane molecules). One solvent molecule was disordered and modeled with occupancies of 69% (Cl111 through C105) and 31% (Cl8 through C104). Another dichloromethane contained a disordered chlorine atom that was modeled with occupancies of 63% (Cl15) and 37% (Cl14). The triflate counterion was positionally disordered, whereby the sulfur would point either towards or away from the cluster. It was refined as two molecules with occupancies of 61% (S200 through C200) and 39% (S201 through C201). The standard deviations of some atoms in the phenyl ring of the trinucleating ligand (C34 – C36) were restrained to be the same. Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf]2 (2-NO). The structure contains two triflate counterions. One counterion is disordered over two positions with occupancies of 79.3% (S202 through C202) and 20.7% (S201 through C201). Three of the seven co-crystallized dichloromethane molecules are disordered over two positions. The first has occupancies of 59% (Cl11 through C105) and 41% (C103 through C108). Special refinement details for [LFe3F(PhPz)3Fe(NO)][OTf]3 (3-NO). There are two molecules in the asymmetric unit of the crystal structure. One of the clusters (Fe0A through C199) has a disordered phenyl pyrazolate ligand. Because of this disorder, the bond lengths and angles were not considered with in this molecule and only the other cluster, for which there was no evidence of disorder, (Fe1 through C99) was used for reporting bond metrics. All but two triflate counterions were disordered. Three triflates were positionally disordered; the first two had occupancies of 78% (S303 through C303 and S304 through C304) and 22% (S302 through C302 and S305 through C305). The third triflate had occupancies of 76% (S306 through C306) and 24% (S307 thgouh C307). The two remaining triflates were disordered as a pair. They could be modeled as being adjacent to one another with occupancies of 51% 49 (S308 through C308) and 62% (S130 through C130). This would be disordered with two triflates, one occupying the same space as the first pair with an occupancy of 37% (S309 through C309), and the other, itself disordered, next to a symmetry element with an occupancy of 50%. There were seven dichloromethane solvent molecules modeled in the structure. Two had positionally disordered chlorine atoms with occupancies of 78% (Cl9) and 22% (Cl16), and 51% (Cl5) and 49% (Cl6). Another dichloromethane was only partially occupied (taking up the same space as the disordered pair of triflates, as discussed above); it had an occupancy of 36% (Cl10 through C205). 50 Selected bond angles and distances for complexes 1-3, 1-NO – 3-NO. Bond Distance (Å) Fe1–F1 Fe2–F1 Fe3–F1 Fe4–F1 Fe1–N13 Fe2–N23 Fe3–N33 Fe4–N14 Fe4–N24 Fe4–N34 N13–N14 N23–N24 N33–N34 Fe4–N40 N40–O40 Bond Angles (º) N14–Fe4–N24 N24–Fe4–N34 N34–Fe4–N14 Fe4–N40–O40 Torsion Angles (º) Fe1–N13– N14–Fe4 Fe2–N23– N24–Fe4 Fe3–N33– N34–Fe4 Centroid Distances (Å) Fe1|Fe2|Fe3N 14|N24|N34 Fe1|Fe2Fe3– O11|O21|O31 Fe1|Fe2|Fe3– F1 N14|N24|N34 –Fe4 Complex 1-NO 1 2 3 2-NO 3-NO 2.167(1) 2.154(1) 2.174(1) 1.997(1) 2.170(2) 2.187(2) 2.157(2) 2.034(2) 2.056(2) 2.046(2) 1.390(2) 1.383(2) 1.387(2) – – 2.024(1) 2.204(1) 2.216(1) 2.011(1) 2.076(2) 2.125(2) 2.108(2) 2.063(2) 2.056(2) 2.044(2) 1.386(3) 1.389(3) 1.388(3) – – 2.132(2) – – 2.172(4) 2.061(3) – – – – 1.387(5) – – 2.112(8) – 2.129(7) 2.205(6) 2.169(5) 2.065(7) 2.118(10) 2.122(8) 2.116(11) 2.049(10) 2.060(9) 2.044(10) 1.401(15) 1.398(13) 1.384(13) 1.757(10) 1.163(13) 2.030(4) 2.237(4) 2.101(4) 2.093(4) 2.076(5) 2.102(5) 2.083(6) 2.064(6) 2.057(6) 2.064(5) 1.389(7) 1.380(8) 1.379(8) 1.773(6) 1.147(7) 2.207(3) 2.080(3) 2.091(3) 2.155(3) 2.057(4) 2.046(4) 2.025(4) 2.020(4) 2.049(4) 2.025(4) 1.369(6) 1.371(6) 1.389(5) 1.754(4) 1.133(6) 119.24(7) 117.34(7) 123.35(7) – 119.34(8) 120.91(8) 119.26(8) – 117.323 – – – 118.5(4) 117.1(4) 119.1(4) 175.7(9) 114.6(2) 119.8(2) 119.6(2) 177.0(6) 119.99(2) 116.37(2) 114.83(2) 179.2(4) 3.640 3.40 27.095 23.02 29.86 23.320 1.317 4.02 - 31.89 30.26 23.035 4.968 2.14 - 30.32 22.64 25.985 3.090 2.972 2.832 2.889 2.856 2.828 0.974 0.945 1.008 0.959 0.987 0.926 1.093 1.051 0.975 1.100 1.063 1.029 0.053 0.084 0.344 0.276 0.296 0.353 51 References 1. 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E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105-1115; (b) Rittle, J.; Peters, J. C. Proc. Natl. Acad. Sci. 2013, 110, 15898-15903; (c) Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Coord. Chem. Rev. 2017, 334, 100-111. 19. (a) Schoenberg, A. R.; Anderson, W. P. Inorg. Chem. 1972, 11, 85-87; (b) Hallett, A. J.; Angharad Baber, R.; Guy Orpen, A.; Ward, B. D. Dalton Transactions 2011, 40, 9276-9283; (c) Imai, S.; Fujisawa, K.; Kobayashi, T.; Shirasawa, N.; Fujii, H.; Yoshimura, T.; Kitajima, N.; Moro-oka, Y. Inorg. Chem. 1998, 37, 3066-3070; (d) Kujime, M.; Kurahashi, T.; Tomura, M.; Fujii, H. Inorg. Chem. 2007, 46, 541551; (e) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870-8888; (f) Thomas, J. C.; Peters, J. C. Inorg. Chem. 2003, 42, 5055-5073. 20. (a) Speelman, A. L.; Lehnert, N. Angew. Chem. Int. Ed. 2013, 52, 12283-12287; (b) Speelman, A. L.; Zhang, B.; Krebs, C.; Lehnert, N. Angew. Chem. Int. Ed. 2016, 55, 6685-6688. 21. (a) Sanders, B. C.; Patra, A. K.; Harrop, T. C. J. Inorg. Biochem. 2013, 118, 115-127; (b) Kupper, C.; Rees, J. A.; Dechert, S.; DeBeer, S.; Meyer, F. J. Am. Chem. Soc. 2016, 138, 7888-7898. 22. Taheri, A.; Thompson, E. J.; Fettinger, J. C.; Berben, L. A. ACS Catalysis 2015, 5, 7140-7151. 23. Rauchfuss, T. B. Bio-Inspired Iron and Nickel Complexes. In Inorg. Synth., John Wiley & Sons, Inc.: 2010; pp 129-147. 24. Izod, K.; Rayner, D. G.; El-Hamruni, S. M.; Harrington, R. W.; Baisch, U. Angew. Chem. Int. Ed. 2014, 53, 3636-3640. 25. (a) Larina, N. A.; Lokshin, V.; Berthet, J.; Delbaere, S.; Vermeersch, G.; Khodorkovsky, V. Tetrahedron 2010, 66, 8291-8299; (b) Kiss, L.; David, L.; Da Costa Pereira Rosa, C. P.; De Nornha, R. G.; Palma, P. N. L.; Da Silva, A. S.; Beliaev, A. US Patent 2012/65191, Mar 15, 2012. 26. Sun, H.; DiMagno, S. G. J. Am. Chem. Soc. 2005, 127, 2050-2051. 27. Yueh, W.; Bauld, N. L. J. Am. Chem. Soc. 1995, 117, 5671-5676. 28. Barton, D. H. R.; Haynes, R. K.; Leclerc, G.; Magnus, P. D.; Menzies, I. D. J. Chem. Soc., Perkin Trans. 1 1975, 2055-2065. 29. (a) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58; (b) Singh, A. K.; Jacob, W.; Boudalis, A. K.; Tuchagues, J.-P.; Mukherjee, R. Eur. J. Inorg. Chem. 2008, 2008, 2820-2829; (c) Sutradhar, M.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem. 2012, 2012, 4273-4278; (d) Schünemann, V.; Hauke, P. Mössbauer Spectroscopy. In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269. CHAPTER 3 THERMODYNAMICS OF PROTON AND ELECTRON TRANSFER IN TETRANUCLEAR CLUSTERS WITH MN–OH2/OH MOTIFS RELEVANT TO H2O ACTIVATION BY THE OXYGEN EVOLVING COMPLEX IN PHOTOSYSTEM II The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. J. Am. Chem. Soc., 2018, 140, 10900 – 10908 54 ABSTRACT We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of [FeIII/II3MnII–OH2] clusters leads to internal redox reorganization resulting in formal oxidation at Mn to generate [FeIII/II3MnIII–OH]. 57Fe Mössbauer spectroscopy reveals that oxidation state changes (three for [FeIII/II3MnII–OH2] and four for [FeIII/II3MnIII–OH] clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined pKa (17.4) of the [FeIII2FeIIMnII–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn– OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69, to 78, and 85 kcal/mol for the [FeIIIFeII2MnII-OH2], [FeIII2FeIIMnII-OH2], and [FeIII3MnII-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn–OHx] system was obtained by constructing a potential–pKa diagram; the shift in reduction potentials of the [Fe3Mn–OHx] clusters in the presence of different bases supports the BDEO–H values reported for the [Fe3Mn–OH2] clusters. A lower limit of the pKa for the hydroxide ligand of the [Fe3Mn– OH] clusters was estimated for two oxidation states. These data suggest BDEO–H values for the [FeIII2FeIIMnIII–OH] and [FeIII3MnIII–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn=O] in this and related multinuclear systems. 55 INTRODUCTION During photosynthesis, water oxidation is catalyzed at the active site of Photosystem II (PSII) by a [CaMn4O5] cluster known as the oxygen evolving complex (OEC).1 The catalytic mechanism is outlined by the Kok cycle, with the cluster transitioning through five distinct so-called S-states (S0, S1, …, S4).2 Four sequential oxidations of the cluster occur (S0 → → S4), followed by the O–O bond forming step, with concomitant loss of O2 and binding of H2O to complete the cycle (S4 →S0). Protons are sequentially released from the active site during the S-state cycle; deprotonation of bound H2O in this stepwise manner prevents the buildup of significant charge at the active site, facilitating the further oxidation of the [CaMn4O5] cluster.3 PSII utilizes a nearby tyrosine radical (Yz•) as a mediator to transfer electrons/protons away from the OEC during turnover; because of the nature of the tyrosine radical, proton-coupled electron transfer (PCET) of the H2O-derived ligands bound to the OEC is considered to play an important role in the Kok cycle.3-4 Due to the wealth of information available in X-ray crystallographic1b-d, 5, EPR6, and X-ray absorption2b, 6d, 7 spectroscopic techniques, much is known about the Mn oxidation states and electronic environment of the OEC in the S0 through S3 states of the Kok cycle. More challenging has been understanding the precise protonation state of H2O ligands and relevant neighboring amino acid residues of any S-state; computational studies of the OEC have considered a variety of possible protonation states.4c, 8 Experimentally, time-resolved IR spectroscopy has been helpful in gaining insight to the dynamics of protons at the active site during turnover.6j, 9 Furthermore, multiple computational models of the OEC mechanism invoke a terminal Mn-oxo as a crucial part of the O–O bond forming S4 intermediate2c, 10. Therefore, there is significant interest in understanding the chemistry of a Mn–OH2 species undergoing multiple proton and electron transfers to reach a reactive terminal Mn-oxo. 56 The chemistry of synthetic Mn-aquo, -hydroxo, and -oxo motifs has been a subject of interest for inorganic chemists, particularly within the context of gaining insight into the thermodynamic basis of Mn–OHx PCET reactivity and how it relates to the mechanism of the OEC.11, 12, 13 Reported mononuclear systems have been able to probe the roles of Mn oxidation state12c, 12d, 12i, ligand field11a, 11f, 13b, and oxygen ligand protonation state11j, 12j on PCET reactions and the intrinsic O–H bond dissociation enthalpies (BDEO–H) of Mn–OHx moieties. There are fewer examples of such studies with multinuclear Mn complexes, with most of the reports examining the PCET chemistry of bridging oxo moieties,14 as opposed to terminally bound OHx ligands.15 Most of these reports are limited to binuclear Mn complexes or systems where only a single redox couple could be examined. The PCET reactivity of a synthetic MnIII3MnIVO3(OH) cubane cluster has been examined, where the BDEO–H of the μ3-OH could be estimated to be >94 kcal/mol; however, precise determination of the thermodynamic bond strength was complicated by subsequent decomposition of the protonated cubane.14d A report of proton and electron transfer at a terminal Mn-OHx moiety with an adjacent Mn center over three oxidation states (MnIII2, MnIIIMnIV, and MnIV2) represents a very rare example of thermodynamic studies of a terminal Mn-OHx in a multinuclear system.15a Access to a suitable synthetic platform to interrogate the effects of multiple neighboring redox-active metal centers on the chemistry of a terminal Mn–OHx motif may facilitate a more complete picture of the dynamics of proton and electron transfer of the OEC leading up to its reactive S4 state, and more generally lead to a better understanding of the behavior of metal clusters in reactions involving water, dioxygen, and multi-electron transformations. Our group has demonstrated the utility of rationally-designed, well-defined molecular clusters for probing structure-function relationships in multinuclear first-row transition metal complexes, acting as models of complex active sites found in biology.16, 17 Recently, we have 57 studied a family of tetranuclear Fe and Mn complexes composed of three coordinativelysaturated metal ions bridged to a fourth (apical) metal center through substituted pyrazolate (or imidazolate) ligands and a µ4-single atom ligand.18 The apical metal has a coordination site available for exogenous ligands, allowing for the study of substrate binding and reactivity by a molecular cluster. With bulky and nonpolar phenyl substituents in the 3 position of the pyrazolate ligands coordination of bulkier ligands remains inhibited, and intramolecular ligand activation had been observed.18d-f In contrast, previous group members, Drs. Zhiji Han and Kyle Horak, have established that amino-phenylpyrazolate ligands, which are more open and facilitate hydrogen bonding interactions, support oxo-bridged tetramanganese clusters bearing a MnIII–OH moiety (Scheme 1; [LMn3O(PzNHPh)3Mn(OH)][OTf]), which are competent for catalyzing electrochemical water oxidation to H2O2.19 Detailed examination of the PCET reactivity of this cluster was challenging, however, which may have been due, in part, to the acidity and possible redox non-innocence of the amino-phenylpyrazolate ligands. Other previous efforts within the Agapie group to examine PCET reactivity of clusters includes studies of related tri-iron-oxo/-hydroxo clusters bearing a pendant redox inactive metal, [LFe3O2M(OAc)2(DME)(OTf)]n+ (Scheme 1), by Dr. Davide Lionetti.20 Scheme 1. Previous Multinuclear Complexes Studied by the Agapie Group for PCET Reactivity19-20 58 Here, clusters with unsubstituted bridging pyrazolate ligands (Pz-) were synthesized to further promote intermolecular reactivity between apical Mn–OHx groups and external acids, bases, or hydrogen atom donors/acceptors. A heterometallic cluster composition, in this case [Fe3Mn], was targeted to provide a spectroscopic handle of metal oxidation states within the cluster, via 57Fe Mössbauer spectroscopy. The thermodynamic aspects of the PCET reactivity of these LFe3O(Pz)3Mn(OHx)n+ clusters were investigated through examination of the discrete electron and proton transfers taking place over multiple redox states. The results herein establish the significant influence redox changes at distal metal sites in a cluster have on a Mn– OHx motif and, conversely, how this motif’s protonation state can modulate the electron distribution between metals in the cluster. RESULTS AND DISCUSSION Synthesis and Characterization of Pyrazolate Bridged [Fe3Mn] Clusters. The [FeIII2FeIIMnII] cluster (2-[OTf]) can be prepared via one-pot synthesis, starting from previously reported [LFe3(OAc)(OTf)][OTf] complex.18a Sequential addition of Ca(OTf)2 (which serves to sequester the equivalent of acetate in the starting material, to avoid mixtures of counterions), potassium pyrazolate, iodosobenzene (PhIO), and manganese (II) trifluoromethanesulfonate bis-acetonitrile solvate (Mn(OTf)2 • 2 MeCN) allows for isolation of the desired complex (Figure 1C). 1H NMR and Mössbauer spectra of 2-[OTf] are similar to our previously reported [LFe3O(PhPz)3Mn][OTf]2 cluster which was synthesized using sodium phenyl pyrazolate, supporting the assignment that the apical metal is Mn (Figure 2).18e The structure of 2-[OTf] was confirmed by single crystal X-ray diffraction (see Figure 1A for isostructural 1-[OTf]); the cluster geometry is analogous to the substituted pyrazolate and imidizolate tetranuclear clusters, with a single µ4-interstitital ligand and pyrazolates bridging 59 Figure 1. (A) Crystal structure of tetranuclear Fe3Mn cluster 1-[OTf] with ellipsoids shown at the 50% probability level. Solvent molecules, outer-sphere counterions, and H atoms are omitted for clarity. (B) 1,3,5-triarylbenzene ligand platform (L3-). (C) Synthetic scheme of Fe3Mn clusters with triflate and -[BArF4] counterions. Conditions: (i) One-pot synthesis in THF with (1) Ca(OTf)2 (1 equiv., 60 min), (2) potassium pyrazolate (3.1 equiv., 20 min), (3) PhIO (1 equiv., 90 min), and (4) Mn(OTf)2 • 2 MeCN (1.3 equiv., 18 hr); (ii) CoCp2 (1 equiv.), THF, 60 min; (iii) Na/Hg (2.6 equiv. Na), THF, 4 hr; (iv) AgOTf (1 equiv.), THF, 30 min; (v) Ag[BArF4] • 2 MeCN (2 equiv.), Et2O, 15 min; (vi) [AcFc][BArF4] (1 equiv.), THF, 10 min; (vii) Ag[BArF4] • 2 MeCN (1 equiv.), Et2O, 15 min. each Fe center of the tri-nuclear core to the apical Mn.18 In the case of the previously reported clusters, the apical metal typically adopts a four-coordinate, trigonal pyramidal geometry since the sterics of the substituted pyrazolate ligands disfavor binding of one of the triflate counterions to the apical metal. Here, the apical Mn is ligated by one triflate counterion with 60 a trigonal bipyramidal geometry, indicative of the increased steric accessibility of the apical metal with the unsubstituted pyrazolates. Figure 2. (A) 1H NMR (300 MHz) of 2-[OTf] in CD3CN. (B) Zero applied field 57Fe Mössbauer spectrum of 2-[OTf] (black dots) fit with three equal quadrupole doublets (gray line) with parameters: (i) δ = 1.12 mm/s, |ΔEq| = 2.93 mm/s (blue trace), (ii) δ = 0.47 mm/s, |ΔEq| = 0.58 mm/s (solid orange trace), and (iii) δ = 0.42 mm/s, |ΔEq| = 0.91 mm/s (dashed orange trace). Cyclic voltammetry (CV) data of 2-[OTf] in MeCN show a quasi-reversible oxidation at 0.11 V, a quasi-reversible reduction wave at -0.84 V, and an irreversible reductive process below -1.50 V (Figure 3; all potentials vs Fc/Fc+). The one electron reduced (1-[OTf]), and one electron oxidized (3-[OTf]) clusters were prepared via chemical reduction/oxidation of 2-[OTf] with cobaltocene (CoCp2) and silver trifluoromethanesulfonate (AgOTf), respectively. The X-ray crystal structures of these three compounds all have identical coordination modes for the metal centers (Figure 1A). Bond distances between the metals and the µ4-oxo are consistent with the redox processes taking place at the Fe centers, with the apical Mn maintaining a +2 oxidation state across the series 1-[OTf] – 3-[OTf] (Table 1). Mössbauer data corroborate these oxidation state assignments, and are similar to our previously characterized clusters.18a-e 61 FeII2FeIIIMnII FeIIFeIII2MnII FeIII3MnII Current FeII3MnII 100 µA -2.5 -1.5 -0.5 0.5 Potential (V, vs Fc/Fc+) 1.5 Figure 3. Cyclic voltammogram (green trace) of 2-[OTf] (2.8 mM) in MeCN and 100 mM [Bu4N][PF6] at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.5 V. (Red trace) Partial CV of 2-[OTf] of the quasi-reversible electrochemical features. 62 Table 1. Selected Bond Distances, 57Fe Mössbauer Parameters, and Oxidation State Assignments for Structurally Characterized Compounds Metal Center Fe1 Fe2, Fe3 Mn1 Fe1, Fe2 Fe3 Mn1 M–µ4-O1; (Mn1–O2) distance (Å) 1.912(2) 2.054(2), 2.112(2) δ (mm/s) 1-[OTf] 0.56 1.14, 1.13 |ΔEq| (mm/s) Assignment 1.32 3.51, 3.02 h.s. FeIII h.s. FeII 1.997(2); (2.249(2)) 1.951(2), 1.966(2) 2.097(2) 2.053(2); (2.167(3)) MnII 2-[OTf] 0.47, 0.42 0.58, 0.91 h.s. FeIII 1.12 2.93 h.s. FeII MnII 2-[OTf] (H2O) Fe1, Fe2 Fe3 Mn1 Fe1, Fe2, Fe3 Mn1 Fe1 Fe2, Fe3 Mn1 1.923(5), 1.984(5) h.s. FeIII 2.092(5) 2.064(5); (2.163(6)) h.s. FeII MnII 1.980(4), 1.982(4), 1.989(4) 3-[OTf] 0.44 0.80 2.107(4); (2.162(5)) 2.003(7) 2.126(7), 2.051(7) 1.838(8); (1.843(9)) h.s. FeIII MnII 6-[OTf] 0.53 1.09, 1.08 0.76 3.09, 2.58 h.s. FeIII h.s. FeII MnIII Preparation of Mn–OH2 and Mn–OH Clusters. Binding of water to these clusters was investigated; however, the coordination of triflate to the apical Mn complicates direct access to the Mn–OH2 moiety for all oxidation states of the cluster. The triflate ligand in 2-[OTf] is sufficiently labile to allow for isolation of the Mn–OH2 cluster as single crystals by slow diffusion of Et2O into a MeCN/5% H2O solution of the cluster, and its structure was confirmed via X-ray crystallography (2-[OTf] (H2O); Figure 4B). Attempts to obtain crystals of the analogous reduced Mn-OH2 cluster (1-[OTf] (H2O)) were unsuccessful; we postulate that the difficulty lies in poor crystallinity of the complex, as opposed to an inability to 63 coordinate H2O over triflate. Crystallization attempts of 3-[OTf] in MeCN/5% H2O solutions produced crystals of triflate coordinated clusters, demonstrating the complication of preparing Mn–OH2 clusters across these oxidation states with the triflate counterions. The structure of 2-[OTf] (H2O) displays H2O coordinated to the apical Mn, with a long Mn–O distance of 2.163(6) Ǻ, consistent with a MnII–OH2 assignment;13e, 21 furthermore, both triflate counterions are hydrogen bonding to each proton of the Mn–OH2 moiety through one of the sulfonate oxygen atoms (Oaquo–OOTf distances of 2.787 and 2.695 Ǻ). Figure 4. Truncated crystal structures of (A) 1-[OTf], (B) 2-[OTf] (H2O), and (C) 6-[OTf]. Ellipsoids are shown at the 50% probability level with solvent molecules, outersphere counterions, and hydrogen atoms (except for hydrogen atoms on O2) are omitted for clarity. To ensure that H2O remained coordinated to the cluster in solution, experiments were performed in THF, a less coordinating solvent than MeCN, and triflate counterions were replaced with the non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([BArF4]) anion. This was accomplished by reducing the dicationic cluster, 2-[OTf], with Na/Hg amalgam in THF to obtain the neutral all MII cluster, 4, as a blue solid (Figure 1C). Similar to the related neutral phenyl pyrazolate clusters,18a 4 is either insoluble or unstable in most organic 64 solvents, so its chemistry towards H2O was not pursued. Oxidation of 4 with 1 and 2 equiv of Ag[BArF4] • 2 MeCN affords 1-[BArF4] and 2-[BArF4], respectively (Figure 1C). The [FeIII3MnII] cluster, 3-[BArF4], was prepared by oxidation of 2-[BArF4] with acetyl-ferrocenium ([AcFc][BArF4]). All these clusters are highly soluble in THF and bind H2O under conditions where it is present in ~100 molar equivalents (Figures 5 - 7). Significant decomposition is observed when H2O concentrations above ~1000 equivalents were used; therefore, all the studies described herein were performed on ca. 2 mM of a cluster with -[BArF4] counterions in THF solution with 250 mM H2O, as these conditions displayed 1H NMR spectra consistent with complete or near complete binding of H2O to the apical Mn. 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O 500 equiv. H2O + ca. 150 equiv. DBU Figure 5. 1H NMR spectra (500 MHz) of 2mM [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]) in THF/C6D6 with various equivalents of H2O. Splitting of the peak at ~35 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. Addition of excess 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) leads to no significant change in the 1H NMR spectrum. 65 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O Figure 6. 1H NMR spectra (500 MHz) of 2mM [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) in THF/C6D6 with various equivalents of H2O. Coalescence of the two peaks at ~45 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. 0 equiv. H2O 5 equiv. H2O 20 equiv. H2O 100 equiv. H2O 300 equiv. H2O 1000 equiv. H2O Figure 7. 1H NMR spectra (500 MHz) of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) in THF/C6D6 with various equivalents of H2O. The upfield shift of the peak at ~50 ppm was used to judge the amount of H2O coordination, which appeared complete at > 20 equivalents H2O. Deprotonation of the Mn–OH2 moiety in the [FeIII2FeIIMnII] cluster, 2-[BArF4], was accomplished by addition of 1 equivalent of a relatively strong organic base, 1,8diazabicyclo(5.4.0)undec-7-ene (DBU; pKa(THF) = 19.1)22, or by stirring a THF solution of 2[BArF4] over solid KOH for 30 minutes. Both reactions lead to the same species based on the 66 Figure 8. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]; top) and [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; bottom) in CD3CN. 1 H NMR and UV-Vis absorbance features, assigned to the Mn–OH cluster, 6-[BArF4]. Structural confirmation of this species was obtained by performing analogous reactions on 2[OTf] to prepare the triflate salt, 6-[OTf], which displays the same 1H NMR features (Figure 8). This species could be crystallized from MeCN solution by slow diffusion of Et2O, and was characterized via X-ray diffraction (Figure 4C). The structure of 6-[OTf] is similar to 2-[OTf] (H2O), displaying Mn binding to the hydroxide ligand with a trigonal bipyramidal geometry. Notably, the Mn–O distance to the hydroxide ligand is contracted by approximately 0.3 Å relative to 2-[OTf] (H2O) (1.843(9) vs 2.163(6) Å). Furthermore, the distance of the apical Mn to the interstitial µ4-O in the cluster is also shortened significantly (1.838(8) vs 2.064(5) Å in 2-[OTf] (H2O); Table 1); both of these observations are consistent with a MnIII–OH assignment.23 The Mn–OH and Mn–µ4-O distances of ~1.8 Ǻ are similar to the bond metrics observed in our previously reported hydroxide-bound tetramanganese cluster in the [MnIII2MnII2] oxidation state.19 There, the Mn–OH bond is slightly longer (1.872(2) Å) due to hydrogen bonding to the pendant tert-butyl-phenyl-aminopyrazolate ligands. The structural data for 6-[OTf] are consistent to an oxidation state assignment of [FeII2FeIIIMnIII] for the 67 cluster; corroborated by the Mössbauer spectrum of 6-[BArF4] (Figure 9B). Deprotonation of the MnII–OH2 in 2 to form 6 leads to rearrangement of the redox states of the metals in the cluster to produce a MnIII–OH site. Similar ‘valence tautomerizations’ have been observed in MnV(O)-corrole systems, where protonation or binding a Lewis acid to the oxo moiety leads to reversible formation of a MnIV(O–X)-(corrole-radical cation) complexes.24 Figure 9. (A) Mössbauer spectrum of 5 (black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.12 mm/s; ΔEq = 3.40 mm/s (solid blue trace), (ii) δ = 1.12 mm/s; ΔEq = 2.95 mm/s (dashed blue trace), and (iii) δ = 1.08 mm/s; ΔEq = 2.42 mm/s (dotted blue trace). (B) Mössbauer spectrum of 6-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.09 mm/s; ΔEq = 3.09 mm/s (solid blue trace), (ii) δ = 1.08 mm/s; ΔEq = 2.58 mm/s (dashed blue trace), and (iii) δ = 0.53 mm/s; ΔEq = 0.76 mm/s (solid orange trace). (C) Mössbauer spectrum of 7-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.10 mm/s; ΔEq = 3.03 mm/s (solid blue trace), and (ii) δ = 0.43 mm/s; ΔEq = 0.55 mm/s (solid orange trace), (iii) δ = 0.46 mm/s; ΔEq = 1.02 mm/s (dashed orange trace). 68 Electrochemistry. The electrochemistry of the [Fe3OMn–OH2] and [Fe3OMn–OH] clusters was investigated by cyclic voltammetry of 2-[BArF4] and 6-[BArF4]. Two quasireversible redox events were observed in 2-[BArF4]: an oxidation at -0.02 V and a reduction at -0.90 V (all potentials vs Fc/Fc+; Figure 10, red trace). These redox events were assigned to the [FeIII2FeIIMnII]→[FeIII3MnII] and [FeIIIFeII2MnII]→[FeIII2FeIIMnII] couples. Mössbauer spectra collected on 1-[BArF4] – 3-[BArF4] in THF with 250 mM H2O show that both oxidation state changes occur at the Fe centers (Figure 11), leading to the conclusion that the apical Mn remains divalent when bound to H2O across all the oxidation states observed in the CV experiment, and only the distal Fe centers change oxidation states. The hydroxide-bound cluster, 6-[BArF4], displays two oxidations: a quasi-reversible couple at -0.49 V ([FeIIIFeII2MnIII]→[FeIII2FeIIMnIII]), and an ([FeIII2FeIIMnIII]→[FeIII3MnIII]). A quasi-reversible irreversible event at reduction +0.26 V for the [FeIIIFeII2MnIII]→[FeII3MnIII] couple is also observed at -1.34 V. The Mössbauer spectra of 5 7-[BArF4] (Figure 9) are again consistent with redox changes occurring exclusively at Fe. Notably, no catalytic oxidation wave is observed at high potentials for 6-[BArF4], in contrast to our previously reported tetramanganese cluster bridged with tert-butyl- phenylaminopyrazolates.19 Reasons for this difference may be the ~100 mV negative shift in reduction potentials for the [Fe3Mn–OH] clusters, along with the lower concentration of H2O. The lack of electrocatalytic oxidation by 6-[BArF4] could also suggest the importance of pendant basic groups near the MnIII–OH moiety for water oxidation catalysis. 69 Figure 10. Cyclic voltammograms of 2-[BArF4] (red trace) and 6-[BArF4] (green trace); 2 mM compound in THF with 250 mM H2O and 100 mM [nPr4N][BArF4] at a scan rate of 50 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrodes, respectively. The open circuit potentials were -0.2 V for 2-[BArF4] and -1.0 V for 6-[BArF4]. Figure 11. (A) Mössbauer spectrum of 1-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: (i) δ = 1.12 mm/s; ΔEq = 3.46 mm/s (solid blue trace), (ii) δ = 1.10 mm/s; ΔEq = 2.86 mm/s (dashed blue trace), and (iii) δ = 0.57 mm/s; ΔEq = 1.27 mm/s (solid orange trace). (B) Mössbauer spectrum of 2-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three equal doublets (gray trace) with parameters: 70 (i) δ = 1.13 mm/s; ΔEq = 2.86 mm/s (solid blue trace), (ii) δ = 0.49 mm/s; ΔEq = 1.00 mm/s (solid orange trace), and (iii) δ = 0.48 mm/s; ΔEq = 0.546 mm/s (dashed orange trace). (C) Mössbauer spectrum of 3-[BArF4] (THF solution [250 mM H2O]; black dots) fit with a single quadrupole doublet (orange trace) with parameters: δ = 0.44 mm/s and ΔEq = 0.80 mm/s Determination of pKa of H2O Ligand in [Fe3MnII–OH2] Clusters. The pKa of the aquo-ligand bound to 2-[BArF4] was measured by mixing this cluster with various concentrations of 1,1,3,3-tetramethyl-2-phenylguanidine (PhTMG; pKa(THF) = 16.5).22 The ratio of 2-[BArF4] and 6-[BArF4] was examined by UV-Vis absorbance spectroscopy as a function of PhTMG concentration, and a pKa value of 17.5 for 2-[BArF4] was obtained (Figure 12). Analogous experiments were attempted on the oxidized aquo-cluster, 3-[BArF4]; however, the changes in UV-Vis spectral features upon deprotonation are minor. The pKa of 3-[BArF4] could be obtained by examining its 1H NMR resonances in the presence of a suitable exogenous base, 2,6-Me2-pyridine (Figure 13; pKa(THF) = 9.5).22 As expected, the acidity of the [Fe3Mn–OH2] cluster increases upon oxidation; a pKa value of 9.2 was obtained for 3[BArF4]. While a titration on the reduced [FeIIIFeII2MnII] cluster, 1-[BArF4], was not conducted, we determined that its pKa is significantly higher than the other clusters investigated, since it does not react with excess DBU (Figure 5; pKa(THF) = 19.1).22 71 A) 0.8 B) 0.7 Absorbance (a.u.) 0.6 0.5 0.4 4.5 [6-[BArF4]][[PhTMGH][BArF4]]/ [2-[BArF4]] (M, 10-5) 0.5 equiv. 1 equiv. 2 equiv. 5 equiv. 10 equiv. 15 equiv. 0.3 0.2 0.1 4 y = 0.0899x R² = 0.8892 3.5 3 2.5 2 1.5 1 0.5 0 0 300 400 500 Wavelength (nm) 600 0 0.0002 0.0004 0.0006 [PhTMG] (M) Figure 12. (A) UV-Vis absorbance spectra of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; 1 cm cuvette; 100μM) in THF [250 mM H2O] after addition of various equivalents of 1,1,3,3tetramethyl-2-phenylguanidine (PhTMG; pKa(THF) = 16.5).22 (B) Titration plot for deprotonation of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) to [LFe3O(Pz)3Mn(OH)][BArF4] (6[BArF4]) based on multiple titrations; the slope of the line represents an equilibrium constant value of K = 0.09, where: 𝐾= [𝟔– [𝐁𝐀𝐫𝟒𝐅 ]][(PhTMGH)([BAr4F ])] [𝟐– [𝐁𝐀𝐫𝟒𝐅 ]][PhTMG] 3-[BArF4] 3-[BArF4]+ 0.5 2,6-Me2-pyridine 3-[BArF4]+ 1 2,6-Me2-pyridine 3-[BArF4]+ 2 2,6-Me2-pyridine 3-[BArF4]+ 3 2,6-Me2-pyridine 7-[BArF4] Figure 13. 1H NMR spectrum (500 MHz) of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) with various equivalents of 2,6-dimethyl-pyridine in THF/C6D6 [250 mM H2O]. 72 Table 2. pKa titration of 3-[BArF4] via 1H NMR spectroscopy with 2,6-dimethylpyridine. 1H NMR Χ3-[BArF4]b Kc 0.632 1.625 0.343 3.661 0.319 1.100 0.296 0.730 Average K 1.779 (±0.654) a A sharp resonance ~ 11 ppm was selected to measure the mole fraction of 3-[BArF4] (Χ3-[BArF4]), as 3-[BArF4] and 7-[BArF4] undergo fast exchange on the NMR time-scale. b The concentrations of 3-[BArF4], 2,6-Me2-Py, and [2,6-Me2-PyH][BArF4] were determined from Χ3-[BArF4] via mass balance. cAccording to the equation below: [𝟕– [𝐁𝑨𝒓𝑭𝟒 ]][(2,6– Me2 – PyH)([B𝐴𝑟4𝐹 ])] 𝐾= [𝟑– [𝐁𝑨𝒓𝑭𝟒 ]][2,6– Me2 – Py] 3-[BArF4] 3-[BArF4] + 0.5 2,6-Me2-Py 3-[BArF4] + 1 2,6-Me2-Py 3-[BArF4] + 2 2,6-Me2-Py 3-[BArF4] + 3 2,6-Me2-Py 7-[BArF4] δa (ppm) 11.12 10.90 10.72 10.70 10.69 10.51 BDEO–H and PCET Reactivity of the Different Redox States. The homolytic bond dissociation enthalpy of the aquo O–H (BDEO–H) were determined for the three cluster oxidation states observed (1-[BArF4] – 3-[BArF4]) by analyzing the pKa and reduction potentials of the aquo- and hydroxide-bound clusters (Figure 14). This calculation combines the energies of the discrete proton and electron transfers involved, along with the energy of formation of the hydrogen atom in THF (C; 66 kcal/mol25):26 BDEO–H (kcal/mol) = 1.37 pKa + 23.06 E° + C (1) Therefore, summing the energy of the oxidation of 1-[BArF4] (-0.90 V; -20.6 kcal/mol) and the energy of deprotonation of 2-[BArF4] (17.5; 24.0 kcal/mol) with C establishes an energy of 69 kcal/mol for the formal H-atom transfer from 1-[BArF4] to 6-[BArF4]. Likewise, the oxidation of 2-[BArF4] (-0.02 V; -0.5 kcal/mol) followed by the deprotonation of 3-[BArF4] (9.2; 12.6 kcal/mol) leads to a BDEO–H of 78 kcal/mol for the aquo-ligand of 2-[BArF4] (formal HAT to form 7-[BArF4]). An alternate way to determine the BDEO–H of 2-[BArF4] is from the pKa of 2-[BArF4] and the reduction potential of 6-[BArF4], leading to a similar bond enthalpy of 78.7 kcal/mol. The same square scheme analysis can be done to obtain a BDEO-H 73 Figure 14. Thermodynamic cycles to evaluate the BDEO–H of Mn–OH2 clusters 1-[BArF4] 3-[BArF4]. Reduction potentials (horizontal lines) are referenced to Fc/Fc+. pKa values (vertical lines) are based on relative pKa values of acids in THF. Diagonal lines are the BDEO– H values calculated from the reduction potential and pKa according to the Bordwell equation (equation 1). Approximate values (~) are extrapolated from the Bordwell equation. of 85 kcal/mol for 3-[BArF4]. With these measured values, the pKa of 1-[BArF4] could be estimated; a BDEO–H of 69 kcal/mol for 1-[BArF4] means the enthalpy of deprotonation for this cluster is expected to be ~34 kcal/mol (pKa = 24.9), by combining this energy with the oxidation of 5 (-1.34 V; -30.9 kcal/mol). The BDEO–H values of these clusters were evaluated by investigating their proton-coupled electron transfer (PCET) reactivity towards different organic radicals. The PCET reagents employed were (2,2,6,6,-tetramethylpiperidin-1yl)oxyl (TEMPO; BDEO–H = 70 kcal/mol) and 2,4,6-tri-tert-butylphenoxy radical (2,4,6-TBPR; BDEO–H = 82 kcal/mol).26c Formal hydrogen atom transfer from 1-[BArF4] to form 6-[BArF4] could be accomplished using either one equivalent of TEMPO or 2,4,6-TBPR, consistent with a BDEO–H less than 70 kcal/mol 74 (Scheme 2). 2-[BArF4] reacts with 1 equivalent 2,4,6-TBPR to form 7-[BArF4], but no reaction is observed between this cluster and TEMPO, indicative of a BDEO–H that is between 70 and 82 kcal/mol. 3-[BArF4] does not react with either PCET reagent, which supports the assignment of a BDEO–H greater than 82 kcal/mol). Scheme 2. Proton Coupled Electron Transfer (PCET) Reactions of Mn–OH2 clusters, 1-[BArF4] - 3-[BArF4]. 75 Potential–pKa Diagram of [Fe3Mn–OHx] Clusters. Further insight into the basis of PCET reactivity of these clusters was obtained by investigating the effect of external bases on the reduction potentials of the aquo- and hydroxide-bound clusters. Typically, this type of analysis is conducted under aqueous conditions, where the reduction potentials can be measured as a function of solution pH; data are presented as a potential–pH plot, known as a Pourbaix diagram.27 Aqueous Pourbaix diagrams have been helpful in understanding the speciation of a number of molecular Ru/Mn water oxidation catalysts and related complexes.15b, 28 Recently, Pourbaix theory has been applied to nonaqueous solvents, where the reduction potential of PCET will depend on the pKa of an external acid/base (and the concentration of this acid/base relative to its conjugate base/acid at Nernstian equilibrium).29 For the system reported here, a potential– pKa plot was constructed as a means of providing experimental support for the aquo-ligand pKa and BDEO–H values of 1-[BArF4] - 3-[BArF4] and to gain information about the thermochemistry of PCET with the Mn–OH clusters to form a terminal Mn-oxo moiety. Measuring the CV of 2-[BArF4] with one equivalent of various organic bases, with pKa values of their conjugate acids in THF ranging from 7.5 to 28.1, produced the potential–pKa plot depicted in Figure 15 (blue points; see Electrochemical Details section for individual CVs). For relatively weak bases (pKa < 10), the reduction potentials of 2-[BArF4] do not significantly deviate from their potentials in the absence of any base. As the strength of the base increases, the reduction potential for the oxidation of 2-[BArF4] is lowered as a function of the conjugate acid pKa, consistent with PCET occurring between the pKa range 10-17. The data points in this pKa range follow the diagonal line calculated for the 2-[BArF4]→7-[BArF4] PCET process, based on the reduction potentials of 2-[BArF4] and 6-[BArF4] and the pKa values of 2-[BArF4] and 3-[BArF4]. Observing the predicted linear decrease in reduction 76 potential for 3-[BArF4] in the pKa range 10 - 17 supports the pKa values reported for 2-[BArF4] and 3-[BArF4] in Figure 14. Similar support is given to the pKa of 1-[BArF4] (24.9) when using strong bases (pKa > 17.5). Evidence for the 1-[BArF4]→6-[BArF4] PCET process was observed under these conditions, again with the data roughly matching the diagonal line with an intercept at -1.34 V and a pKa of 24.9. Furthermore, when a base was employed with a pKa > 24.9, the reduction potentials observed were nearly identical to the potentials reported for 6-[BArF4] in the absence of any acid or base. Figure 15. Potential–pKa diagram of 2-[BArF4] (blue squares) and 6-[BArF4] (green squares). Data points are the observed reduction potentials of the compounds (y-axis) in the presence of one equivalent base with various pKa values in THF (x-axis). The horizontal lines correspond to the measured reduction potentials of 2-[BArF4] and 6-[BArF4] in the absence of any external base. Vertical lines correspond to the pKa values of Mn–OH2 clusters 1-[BArF4] - 3-[BArF4]. The horizontal line for the 7-[BArF4]→8-[BArF4] redox couple is dashed at high pKa values due to the possibility of PCET from the Mn–OH with a strong enough base. 77 As expected, deviations of the data from the calculated diagonal line occur as the base pKa approaches the pKa of the cluster (see 2-[BArF4]→7-[BArF4] around pKa of 10, for example), based on predicted potential–pKa relationships for ET-PT or PT-ET reaction mechanisms.29 Further deviations from the predicted solid lines in Figure 15 could be due to incompatibility of the chosen base with this system (too coordinating, electrochemically unstable, etc.). Ultimately, inconsistencies between the potential–pKa data of 2-[BArF4] and the BDEO–H values reported in Figure 14 only amount to a difference of ~3 kcal/mol, which is a reasonable uncertainty for these bond energydeterminations.26c Based on the PCET reactivity of these complexes towards TEMPO and TBPR (vide supra), it is unlikely that these BDEO–H values could deviate more than a few kcal/mol. Potential–pKa data were also obtained for 6-[BArF4] in the presence of relatively strong organic bases in attempts to observe a PCET process accessing Mn=O clusters, since this technique has been previously useful for gaining insight into the proton and electron transfer thermodynamics for unstable species.29 Based on the potential–pKa plot constructed in Figure 15, PCET to form a Mn-oxo cluster could be possible at high potentials with a strong base (top right area of the diagram). The CV of 6-[BArF4] with one equivalent tert-butyliminotri(pyrrolidino)phosphorene (t-BuP1(pyrr); pKa(THF) = 22.8)22 shows no shift in the Mn–OH cluster reduction potentials. Similarly, no change is observed with the 5→6-[BArF4] and 6[BArF4]→7-[BArF4] reduction potentials with 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (EtP2(dma); pKa(THF) = 28.1).22, 30 These experimental observations provide a lower limit to the pKa values of 8-[BArF4] and 7-[BArF4], respectively. With these values, the BDEO–H of 7-[BArF4] and 8-[BArF4] are predicted to be greater than 93 and 103 kcal/mol, respectively (Figure 16).31 These BDEO–H estimates are higher than Mn complexes, where these bond strengths have been reported.11c, 12d, 12i, 12k The large BDEO–H 78 values for these hydroxide clusters suggest that if these terminal oxo moieties could be accessed, they would be highly reactive. Indeed, previous attempts to generate a terminal oxo species on related phenyl-pyrazolate bridged multimetallic clusters results in activation of strong bonds, although the identity of the reactive intermediate in these reactions could not be established (terminal metal-oxo or iodosylbenzene adduct).18d-f Figure 16. Estimated BDEO–H values for MnIII–OH clusters 6-[BArF4] and 7-[BArF4] based on their oxidation potentials and lower bound of their pKa (where no PCET occurs in their CV with an external base). The metal oxidation states of the resulting putative Mn-oxo clusters are left ambiguous since multiple oxidation state distributions are possible. CONCLUSIONS We have reported the synthesis of tetranuclear [Fe3Mn–OHx] clusters bearing bridging unsubstituted pyrazolate ligands, leading to a sterically open coordination environment around the apical Mn center. These clusters were used to investigate the implications of distal metal redox changes on the activation of water by Mn in terms of the aquo-cluster pKa, reduction potential, and homolytic O–H bond strength. Increasing the oxidation state of a distal Fe 79 center by one increases the acidity of the aquo ligand by ~7 pKa units (in THF), while raising the BDEO–H 8 kcal/mol, on average. By only changing the redox states of the distal Fe centers, a wide range of BDEO–H values could be measured for the Mn–OH2 moiety (69 – 85 kcal/mol), which nearly spans the range of the reported BDEO–H measured in reported in mononuclear Mn–OH2 complexes.11c, 12e, 12f, 12i The three different oxidation states of the aquo-cluster (1[BArF4] – 3-[BArF4]) underwent PCET reactions with TEMPO and 2,4,6-TBPR consistent with their measured BDEO–H values. The increase in BDEO–H of ~8 kcal/mol by increasing distal Fe oxidation state is similar to the increases that have been observed in mononuclear Mn systems where BDEO–H studies could be accomplished over multiple Mn oxidation states.12d, 12i This is in contrast to the previous example of a binuclear Mn system, where Mn– OH2 BDEO–H values could be measured over three oxidation states, where small changes in the bond strength of ~ 4 kcal/mol for the MnIIIMnIV–OH2 complex versus MnIII2–OH2 were observed.15a Importantly, the large effect of the remote metals on the BDEO–H demonstrates that the cluster as a whole has a significant impact in the activation of substrate water molecules. The range of BDEO–H reported here is achieved without change in the redox state of Mn; therefore, consideration of the effect of metal centers not directly supporting the substrate must be taken into account for multimetallic biological active sites as well as synthetic clusters. Additionally, the present findings demonstrate that, in comparison to monometallic complexes, transition metal clusters not only provide the possibility of increased storage of redox equivalents, but also can serve to dynamically tune reactivity through remote oxidation state changes. 80 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified; reactions of compounds in THF/H2O mixtures were performed in an N2-filled VAC wetbox. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. [LFe3(OAc)(OTf)][OTf]18a, Mn(OTf)2 • 2 MeCN32, benzyl potassium33, iodosobenzene34, silver (Ag[BArF4] MeCN)35, • 2 tetrakis[3,5-bis(trifluoromethyl)phenyl]borate 2,4,6-tri-tert-butylphenoxy radical bis-acetonitrile (2,4,6-TBPR)36, and tetrapropylammnoium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([nPr4N][BArF4])37 were prepared according to literature procedures. All organic solvents were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H and 19F NMR spectra were recorded on a Varian 300 MHz spectrometer. 1H NMR spectra in THF/C6D6 were recorded on a Varian 500 MHz spectrometer using solvent suppression protocols. CD3CN, CD2Cl2, and C6D6 were purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freezepump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated solution in MeCN or THF. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). 81 Mössbauer simulation details for all compounds. All spectra were simulated by three pairs of symmetric quadrupole doublets with equal populations and Lorentzian lineshapes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts of ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.17a, 18a, 18d, 18e The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.38 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter) and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Either the ferrocene/ferrocinium (Fc/Fc+) or decamethylferrocene/ decamethylferrocinium (Fc*/Fc*+; -0.524 V vs Fc/Fc+ in THF/250 mM H2O, under our experimental conditions) redox waves were used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of the compounds most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. 82 Synthetic Procedures. Synthesis of Potassium pyrazolate (KPz). 1.09 g (16.0 mmol) pyrazole was dissolved in 2 mL THF. To this stirring solution, a 10 mL THF solution of benzyl potassium, 2.03 g (15.6 mmol), was added dropwise; an off-white precipitate formed. After stirring for 20 minutes, the reaction was concentrated to 10 mL; the solids were collected on a glass frit and washed with 2 mL THF. The white solid was dried completely under vacuum to obtain 1.37 g (83% yield) potassium pyrazolate. Anal. calcd. (%) for C3H3KN2: C. 33.94; H, 2.85; N, 26.39. Found: C, 34.12; H, 2.89; N, 25.38. Synthesis of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). A suspension of 387 mg (0.28 mmol) [LFe3(OAc)(OTf)][OTf] in 7 mL THF was stirred with 98.4 mg (0.29 mmol) Ca(OTf)2 for an hour before being frozen with LN2. To this mixture, 93.2 mg (0.88 mmol) KPz was added in thawing THF (4 mL) and stirred for 20 minutes at room temperature to obtain a dark redorange solution. Iodosylbenzene, 63.6 mg (0.29 mmol), was added with 1 mL THF and the reaction was stirred for 90 minutes. 160 mg (0.37 mmol) Mn(OTf)2 • 2 MeCN solution in 2 mL THF was then added to the reaction. After 18 hours, the reaction was concentrated to 10 mL and filtered over a bed of celite; the precipitate was dried under vacuum, extracted with 8 mL DCM, and recrystallized via vapor diffusion of Et2O into the filtrate. Dark green crystals of 2-[OTf] were collected on a glass frit and dried (147 mg, 33% yield). Another 69 mg of 2[OTf] can be obtained by drying the crude reaction filtrate, extracting with 6 mL DCM and recrystallizing via Et2O vapor diffusion (46% overall yield). X-ray diffraction quality crystals were obtained via oxidation of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) with 1 equivalent of AgBPh4; Et2O vapor diffusion into a DCM/THF solution of the resulting [LFe3O(Pz)3Mn][OTf][BPh4] produced crystals of suitable quality. 1H NMR (300 MHz, CD3CN): δ 120.8 (br), 80.8 (br), 71.0, 70.1, 52.9, 52.3, 42.2, 28.0 (br), 15.5, 13.0, 10.4, 8.1 (br), 4.38, 3.01, -2.51 (br). UV-Vis 83 (MeCN) [ε (M-1 cm-1)] 241 nm (6.53 104), 368 nm (6.49  103). Anal. calcd. (%) for C68H48F6Fe3MnN12O10S2: C. 51.25; H, 3.04; N, 10.55. Found: C, 50.81; H, 3.12; N, 10.18. Synthesis of [LFe3O(Pz)3Mn][OTf] (1-[OTf]). A suspension of [LFe3O(Pz)3Mn][OTf]2 (2[OTf]; 91.5 mg, 0.057 mmol) in 2 mL THF was stirred as a THF solution of 10.9 mg CoCp2 (0.058 mmol) was added. After 1 hour, the reaction was dried under vacuum. 4 mL DME was added to the purple solid and stirred for 12 hours. The resulting purple precipitate was collected on a bed of celite, washed with 2 mL DME, dried, and eluted with 2:1 THF/MeCN; crystals of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) were obtained by vapor diffusion of Et2O into this solution (46.3 mg, 56% yield). 1H NMR (300 MHz, CD3CN): δ 96.4 (br), 57.8, 55.5, 37.8 (br), 36.4, 34.3, 34.0, 25.2, 13.4, 13.0, 12.0, 11.4, 3.4, 2.6, -6.4 (br). UV-Vis (MeCN) [ε (M-1 cm1 )] 250 nm (6.08  104), 517 nm (3.72  103). Anal. calcd. (%) for C67H48F3Fe3MnN12O7S: C, 55.70; H, 3.35; N, 11.63. Found: C, 55.36; H, 3.58; N, 11.20. Synthesis of [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]). 9.2 mg (0.036 mmol) of AgOTf in THF was added to a stirring suspension of 56.8 mg (0.036 mmol) [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]) in THF. The resulting brown suspension was pumped down after 30 minutes. The reaction was filtered over a celite pad using DCM and the solvent was removed under reduced pressure. Crystals of [LFe3O(Pz)3Mn][OTf]3 were obtained via vapor diffusion of Et2O into a concentrated DCM/MeCN solution of the crude product, 57.4 mg (92% yield). 1H NMR (300 MHz, CD2Cl2): δ 162.2 (br), 118.9 (br), 81.2, 76.9, 74.4, 73.1, 45.7, 18.8 (br), 16.3, 9.5, 3.34, 1., -6.5 (br). UV-Vis (MeCN) [ε (M-1 cm-1)] 241 nm (7.84  104), 411 nm (9.22  103). Anal. calcd. (%) for C69H48F9Fe3MnN12O13S3: C, 47.55; H, 2.78; N, 9.64. Found: C, 47.57; H, 3.07; N, 9.21. Synthesis of [LFe3O(Pz)3Mn] (4). 4.1 mg (0.18 mmol) sodium metal was mixed ~6 g elemental mercury with a pre-reduced stirbar. After 12 hours, a 5 mL THF suspension of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; 114 mg, 0.07 mmol) was added to the Na/Hg amalgam. 84 Over 4 hours, a blue precipitate formed; this resulting suspension was decanted from the amalgam and filtered over a fine porosity glass frit. The solids were washed with 5 mL THF and dried under vacuum. The resulting blue material, [LFe3O(Pz)3Mn] (78.1 mg; 84% yield), is insoluble or unstable in most typical organic solvents. Anal. calcd. (%) for C66H48Fe3MnN12O4: C. 61.18; H, 3.73; N, 12.94. Found: C, 60.44; H, 3.82; N, 12.87 Synthesis of [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]). 14.0 mg (0.013 mmol) Ag[BArF4] • 2 MeCN in 2 mL Et2O was added to a stirring suspension of [LFe3O(Pz)3Mn] (4; 17.2 mg, 0.013 mmol); the blue suspension changed to a purple solution. After 15 minutes, the solvent was removed under reduced pressure. 3 mL Et2O was added to the purple residue and filtered over a pad of celite. The filtrate was dried to afford [LFe3O(Pz)3Mn][BArF4] as a purple solid, 26.5 mg (92% yield). 1H NMR (300 MHz, CD3CN) is identical to [LFe3O(Pz)3Mn][OTf] (1-[OTf]). Anal. calcd. (%) for C98H60BF24Fe3MnN12O4: C. 54.52; H, 2.80; N, 7.79. Found: C, 54.06; H, 2.84; N, 7.33. Synthesis of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]). 45.0 mg (0.043 mmol) Ag[BArF4] • 2 MeCN in 2 mL Et2O was added to a stirring suspension of [LFe3O(Pz)3Mn] (4; 27.6 mg, 0.021 mmol); the blue suspension changed to a brown-green solution. After 15 minutes, the solvent was removed under reduced pressure. 3 mL Et2O was added to the brown residue and filtered over a pad of celite. 6 mL benzene was added to the filtrate to produce an oily precipitate; after 30 minutes, the supernatant was removed and the remaining brown-green residue was dried under reduced pressure. 36.6 mg (57% yield) of the brown-green solid, 2-[BArF4], was obtained; the 1H NMR (300 MHz, CD3CN) is identical to [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 368 nm (5.11 103). Anal. calcd. (%) for C130H72B2F48Fe3MnN12O4: C. 51.67; H, 2.40; N, 5.56. Found: C, 51.38; H, 2.56; N, 5.46. 85 Synthesis of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]). 6.4 mg (0.006 mmol) [AcFc][BArF4] in 0.5 mL THF was added to [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; 18.6 mg, 0.006 mmol). After 10 minutes, 5 mL benzene was added to the solution to produce an oily brown precipitate; after 30 minutes, the yellow supernatant was removed and the remaining brown residue was dried under reduced pressure. 18.4 mg of a brown solid was obtained (77% yield). 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 83.8, 78.2, 75.9, 50.2, 24.9 (br), 16.6, 9.8 (br), 0.1. UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 405 nm (7.64 103). Anal. calcd. (%) for C162H84B3F72Fe3MnN12O4: C. 50.08; H, 2.18; N, 4.33. Found: C, 50.34; H, 2.38; N, 4.29. Synthesis of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]). Addition of 100 µL of a 50 mM solution of DBU in THF/250 mM H2O to 2 mL 2 mM solution of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]) in THF/250 mM H2O leads to a color change of the solution from green to red. Crystals for X-ray diffraction (6-[OTf]) were obtained by conducting the analogous reaction with [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]) and DBU in 95:5 MeCN/H2O and crystalizing via vapor diffusion of Et2O into this solution; considerable decomposition occurs on the timescale of crystallization, making crystallization unsuitable for preparing analytically pure solid samples of [LFe3O(Pz)3Mn(OH)][OTf]. Solutions of [LFe3O(Pz)3Mn(OH)][BArF4] were prepared for electrochemistry experiments by stirring 4 mL of 2.5 mM [LFe3O(Pz)3Mn][[BArF4]]2 (2-[BArF4]) and 0.1 M [nPr4N][BArF4] solution in THF/250 mM H2O with ~2 mg of solid KOH pellet for 1 hour; the resulting red solution was decanted off the remaining KOH before electrochemical measurements were conducted. 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 153.1 (br), 102.7 (br), 85.9, 80.0, 64.8, 60.8, 58.1, 57.3, 23.0, 15.7. 12.5, 10.9 (br). UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 467 nm (3.29  103). Synthesis of [LFe3O(Pz)3Mn(OH)] (5). Addition of 11 mg (0.03 mmol) decamethylcobaltocene in THF/250 mM H2O to 4 mL 7 mM solution of 86 [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; 0.03 mmol) in THF/250 mM H2O leads to a color change of the solution from red to blue. The reaction was pumped down after 30 minutes. 1H NMR (500 MHz, THF/C6D6 [250mM H2O]): δ 126.0 (br), 76.1 (br), 59.6, 49.0, 46.9, 42.7, 37.0, 23.9 (br), 17.2, 15.6, 12.8, -14.9. Synthesis of [LFe3O(Pz)3Mn(OH)][BArF4]2 (7-[BArF4]). Method A. Addition of 160 µL of a 50 mM solution of Et3N in THF/250 mM H2O to 2 mL 2 mM solution of [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]) in THF/250 mM H2O leads to a color change of the solution from brown to brown-green. 1H NMR spectroscopy confirms complete conversion to [LFe3O(Pz)3Mn(OH)][BArF4]2. Method B. Addition of 200 μL of 6 mM solution of Ag[BArF4] • 2 MeCN in THF/250 mM H2O to 400 μL of a 3mM solution of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]) in THF/250 mM H2O leads to formation of a grey precipitate. Filtration of this solution yields a brown-green solution with an identical 1H NMR obtained from Method A. 1H NMR (500 MHz, THF/C6D6 [250 mM H2O]: δ 110.2 (br), 89.1, 85.1, 70.0, 67.2, 62.0, 19.1 (br), 15.7, 13.1, 9.8 (br), 8.6 (br), 6.2 (br), 1.1, 0.7, 0. UV-Vis (THF/250 mM H2O) [ε (M-1 cm-1)] 389 nm (5.29  103). 87 ELECTROCHEMICAL DETAILS Current 50 µA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2 -1.5 -1 -0.5 0 0.5 Potential (V, vs Fc/Fc+) 1 1.5 Figure 17. Cyclic voltammograms of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf], 2.8 mM) in MeCN and 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as Current working, counter, and reference electrode, respectively. 20 µA -2 -1.5 -1 -0.5 0 0.5 + Potential (V, vs Fc/Fc ) 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 1 1.5 88 Figure 18. Cyclic voltammograms of [LFe3O(Pz)3Mn(OH2)][BArF4]2 (2-[BArF4], 2.1 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] at various scan rates with glassy carbon, Pt- Current wire, and Ag-wire as working, counter, and reference electrode, respectively. 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s 20 μA -2 -1.5 -1 -0.5 0 0.5 Potential (V, vs Fc/Fc+) 1 1.5 Figure 19. Cyclic voltammograms of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Table 3. Peak-to-peak separation (ΔEp; mV) and peak area ratio (Aa/c) for the redox couples in 2-[OTf], 2-[BArF4], and 6-[BArF4]. Redox Couple Assignment [FeII2FeIIIMnII]→ [FeIIFeIII2MnII] [FeIIFeIII2MnII]→ [FeIII3MnII] [FeII2FeIIIMnII]→ [FeIIFeIII2MnII] [FeIIFeIII2MnII]→ [FeIII3MnII] [FeII3MnIII] → [FeII2FeIIIMnIII] [FeII2FeIIIMnIII] → [FeIIFeIII2MnIII] [FeIIFeIII2MnIII] → [FeIII3MnIII] Epa (mV) Epc(mV) 2-[OTf] -799 -66 2-[BArF4] -958 -831 -93 44 6-[BArF4] -1,406 -1,274 -548 -426 172 354 -882 -154 ΔEp(mV) Aa (μW) Ac (μW) Aa/Ap 83 88 22.0 11.4 18.5 12.2 1.2 0.9 127 137 7.9 2.6 2.8 2.5 2.8 1.0 132 122 182 6.6 4.2 1.6 2.6 3.6 3.0 2.5 1.2 0.5 89 Constructing the Potential – pKa Diagram of [LFe3O(Pz)3Mn(OHx)] Clusters. Cyclic voltammetry was performed on ~2 mM solutions of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]), or [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]) with glassy carbon working, Pt wire counter, and Ag wire reference electrodes in THF [250 mM H2O] and ca. 100 mM [nPr4N][BArF4]. After collecting a blank CV, and CV of the cluster, one equivalent of a base was added by injecting a concentrated solution of it to the cluster solution and mixing via pipette. It was observed that quasi-reversible waves corresponding to PCET could be observed best at slow scan rates (< 200 mV/s) for all bases tested; faster scan rates led to loss of a return wave for the PCET. We postulate that proton transfer in these experiments is slow relative to the time scale of electrochemistry. With some bases, redox events for the PCET and fully protonated/deprotonated cluster could be observed simultaneously; we propose that this is due to a lower local concentration of base at the electrode surface, or slow proton transfer kinetics. For all measurements reported, it is assumed that half an equivalent of available base is consumed at the electrode at the PCET E½ potential; making the observed potential based only on the redox potential of the Mn–OHx cluster, and the pKa of the added base.29 All THF pKa values used here were obtained from a report by Rosés and co-workers.22 90 Base with 2-[BArF4] pKa (THF) E½(1) (V) 2-methyl-aniline 7.5 -0.885 2-methyl-pyridine 2,6-dimethyl-pyridine 2,4,6-trimethyl-pyridine triethylamine 2-phenyl-1,1,3,3-tetramethylguanidine 1,1,3,3-tetramethylguanidine 1,8-diazabicyclo[5.4.0]undec-7-ene 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene 1,5,7-triazabicyclo[4.4.0]dec-5-ene tert-butylimino-tri(pyrrolidino)phosphorane 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) Base with 6-[BArF4] tert-butylimino-tri(pyrrolidino)phosphorane 1-ethyl-2,2,4,4,4-pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) 8.6 9.5 10.4 14.9 16.5 17.8 19.1 20.5 22.0 22.8 28.1 22.8 28.1 E½(3) (V) -0.891 -0.894 -0.907 -0.900 -0.956 -1.034 -1.066 -1.148 -1.180 -1.271 -1.317 E½(2) (V) -0.002 (-0.266) -0.028 -0.082 -0.390 -0.373 -0.468 -0.454 -0.468 -0.460 -0.451 -0.461 -0.453 -1.307 -1.333 -0.517 -0.449 0.245 0.403 0.327 All reported potentials referenced to Fc/Fc+. 25 1 equiv. 2-Me-aniline Current (μA) 15 5 * -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs Fc/Fc+) Figure 20. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-methyl-aniline (pKa(THF) = 7.5) at a scan rate of 50 mV/s. Asterisk (*) denotes redox couple of the decamethylferrocene internal standard. The open circuit potential was -0.4 V. 91 25 1 equiv. 2-Me-pyridine Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs Fc/Fc+) Figure 21. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-methyl-pyridine (pKa(THF) = 8.6) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. 25 1 equiv. 2,6-Me-pyridine Current (μA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 22. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2,6-dimethyl-pyridine 92 (pKa(THF) = 9.5) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. 25 1 equiv. 2,4,6-Me-pyridine Current (μA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 23. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2,4,6-trimethyl-pyridine (pKa(THF) = 10.4) at a scan rate of 50 mV/s. An independent scan in the presence of a decamethylferrocene internal standard was used as a reference. The open circuit potential was -0.3 V. The E½ of middle peak was determined via square wave voltammetry since its return wave was low in current, and overlapping with another peak. 93 60 1 equiv. Et3N Current (µA) 40 20 0 -20 -40 -60 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 24. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent triethylamine (pKa(THF) = 14.9) at a scan rate of 200 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.6 V. 25 1 equiv. PhTMG Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 25. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 2-phenyl-1,1,3,3tetramethylguanidine (pKa(THF) = 16.5) at a scan rate of 50 mV/s. An independent scan in 94 the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 1 equiv. TMG 25 Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 26. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,1,3,3tetramethylguanidine (pKa(THF) = 17.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.5 V. 25 1 equiv. DBU Current (µA) 15 5 -5 -15 -25 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 95 Figure 27. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,8diazabicyclo[5.4.0]undec-7-ene (pKa(THF) = 19.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 30 1 equiv. MTBD Current (µA) 20 10 0 -10 -20 -30 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 28. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (pKa(THF) = 20.5) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 96 20 1 equiv. TBD Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 29. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1,5,7triazabicyclo[4.4.0]dec-5-ene (pKa(THF) = 22.0) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 20 1 equiv. tBuP1(pyrr) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 30. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent tert-butylimino- 97 tri(pyrrolidino)phosphorane (pKa(THF) = 22.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 20 1 equiv. EtP2(dma) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 31. Cyclic voltammogram of [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1-ethyl-2,2,4,4,4pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (pKa(THF) = 28.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -0.7 V. 98 20 1 equiv. tBuP1(pyrr) Current (µA) 10 0 -10 -20 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 Figure 32. Cyclic voltammogram of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent tert-butyliminotri(pyrrolidino)phosphorane (pKa(THF) = 22.8) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -1.3 V. 1 equiv. EtP2(dma) 25 Current (µA) 15 5 -5 -15 -25 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Potential (V, vs. Fc/Fc+) Figure 33. Cyclic voltammogram of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4], 2 mM) in THF [250 mM H2O] and 100 mM [nPr4N][BArF4] upon addition of 1 equivalent 1-ethyl-2,2,4,4,4- 99 pentakis(dimethylamino)2λ5,4λ5-catenadi(phosphazene) (pKa(THF) = 28.1) at a scan rate of 50 mV/s. An independent scan in the presence of a ferrocene internal standard was used as a reference. The open circuit potential was -1.3 V. 100 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1-[OTf] – 3-[OTf], 6-[OTf] and 2-[OTf] (H2O). 1-[OTf] 2-[OTf] 3-[OTf] 6-[OTf] 2-[OTf] (H2O) CCDC Number 1848679 1848681 1848680 1848678 1848677 Empirical formula C77H62F3Fe3Mn N12O8S C105.2H81.2BCl0.5 F3Fe3MnN12O1 0.5S C76H53.5F9Fe3M nN13.4O14.1S3 C67H49F3Fe3M nN12O8S C72H62F6Fe3Mn N12O13S2 Formula weight (g/mol) 1594.9 2019.8 1869.7 1461.7 1703.9 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) a (Å) 12.2741(5) 14.2908(12) 44.125(2) 14.7283(7) 12.2685(6) b (Å) 19.4126(8) 15.9691(13) 14.3106(7) 19.3808(10) 29.896(2) c (Å) 15.5112(6) 24.3709(17) 24.8034(10) 45.518(2) 19.6152(17) α (°) 90 71.236(4) 90 90 90 β (°) 108.397(2) 75.366(2) 90.402(3) 92.474(3) 92.393(5) γ (°) 90 70.262(4) 90 90 90 V (Å3) 3507.0(2) 4891.6(7) 15661.9(13) 12980.9(11) 7188.2(9) 2 2 8 8 4 Cryst. syst. monoclinic triclinic monoclinic monoclinic monoclinic Space group P21 P-1 C2/c C2/c P21/c ρcalcg (cm3) 1.510 1.371 1.586 1.496 1.575 2Θ range (°) 5.028 to 56.648 5.076 to 60.444 6.492 to 145.272 7.544 to 132.498 5.392 to 149.51 μ (mm-1) 0.899 0.668 7.226 7.742 7.461 GOF 1.031 1.029 1.037 1.160 1.140 0.0244, 0.0583 0.0635, 0.1712 0.0840, 0.2131 0.1305, 0.2771 0.1109, 0.2060 Z R1, wR2 (I>2σ (I)) 101 Special refinement details for [LFe3O(Pz)3Mn][OTf] (1-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 12% (S200 through C200) and 88% (S201 through C201). Special refinement details for [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 51% (S200 through C200) and 49% (S201 through C201). A disordered THF molecule was modeled over two positions with occupancies of 81% (O102 through C107) and 19% (O101 through C111). A different THF molecule was modeled to be only partially occupied (56%; O103 through C115). A co-crystallized solvent site was modeled to contain a mixture of three different molecules: a THF (27% O105 through C123), a DCM (22%; Cl10 through C124), and Et2O (64%; O104 through C119). Special refinement details for [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]). The triflate counterion bound to Mn1 is disordered over two positions with refined occupancies of 30% (S200 through C200) and 70% (S201 through C201). An outersphere triflate was modeled in two different positions with occupancies of 38% (S203 through C203) and 62% (S204 through C204). For the S203 through C203 triflate, a nearby Et2O molecule was modeled as partially occupied at 62%. For the S204 through C204, a nearby MeCN molecule was modeled as partially occupied at 38%. Special refinement details for [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]).. The outersphere triflate is disordered over two positions, modeled at an occupancy of 50% each. Both triflates are on symmetry elements and positionally disordered. For the S200 through C200 triflate, this was modeled with EXYZ/EADP constraints. For the S201 throughC201 triflate, the C and S atoms were constrained with EXYZ/EACDP, and the O203 through F205 atoms were modeled in alternating positions, at 50% occupancy each. 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The redox couple that would be assigned to the 7-[BArF] to 8-[BArF] reduction potential unexpectedly increases, inconsistent with PCET to Mn=O and may suggest an undesirable side reaction takes place at these potentials with the base employed. 31. Attempts to obtain supporting data via stoichiometric deprotonation of 7-[BArF4] with various bases led to decomposition via reduction, with no evidence of forming a transient Mn=O moiety. Investigations into the characterization and reactivity of these putative M=O species (M = Fe, Mn) in these and related clusters is ongoing. 32. Riedel, P. J.; Arulsamy, N.; Mehn, M. P. Inorg. Chem. Commun. 2011, 14, 734-737. 33. Izod, K.; Rayner, D. G.; El-Hamruni, S. M.; Harrington, R. W.; Baisch, U. Angew. Chem. Int. Ed. 2014, 53, 3636-3640. 106 34. Huang, C.-Y.; Doyle, A. G. J. Am. Chem. Soc. 2012, 134, 9541-9544. 35. Zhang, Y.; Santos, A. M.; Herdtweck, E.; Mink, J.; Kuhn, F. E. New J. Chem. 2005, 29, 366-370. 36. Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.; Mayer, J. M. Chem. Commun. 2008, 256-258. 37. Thomson, R. K.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Comptes Rendus Chimie 2010, 13, 790802. 38. (a) Herold, S.; Lippard, S. J. Inorg. Chem. 1997, 36, 50-58; (b) Singh, A. K.; Jacob, W.; Boudalis, A. K.; Tuchagues, J.-P.; Mukherjee, R. Eur. J. Inorg. Chem. 2008, 2008, 2820-2829; (c) Sutradhar, M.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem. 2012, 2012, 4273-4278; (d) Schünemann, V.; Hauke, P. Mössbauer Spectroscopy. In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269. CHAPTER 4 A TERMINAL FEIII-OXO IN A TETRANUCLEAR CLUSTER: EFFECTS OF DISTAL METAL CENTERS ON STRUCTURE AND REACTIVITY The text for this chapter was reproduced in part from: Reed, C. J.; Agapie, T. J. Am. Chem. Soc.., 2019, Accepted Manuscript. doi:10.1021/jacs.9b03157 108 ABSTRACT Tetranuclear Fe clusters have been synthesized bearing a terminal FeIII-oxo center stabilized by hydrogen bonding interactions from pendant tert-butyl amino pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [FeII2FeIII2] to [FeIII4]; two oxidation states were structurally characterized by single crystal X-ray diffraction. The reactivity of the FeIII-oxo center in proton coupled electron transfer (PCET) with X–H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role adjacent metal centers have on modulating the reactivity of a terminal metal-oxo. 109 INTRODUCTION Terminal metal-oxo moieties are invoked as key intermediates in both natural and synthetic catalysts of mid-first-row transition metal ions (Mn, Fe, and Co).1 For example in photosynthesis, water is oxidized in photosystem II by a CaMn4O5 cluster known as the oxygen evolving complex (OEC);2 numerous computational studies of the catalytic mechanism have proposed a high-valent Mn-oxo playing a key role in O–O bond formation.3 Similarly, a number of synthetic water oxidation catalysts employing various multinuclear scaffolds have been reported, where a terminal metal-oxo is implicated as a key intermediate (Figure 1).1e-g, 4 Figure 1. Multinuclear catalysts with proposed terminal metal-oxo intermediates (top), and structurally characterized terminal FeIII-oxo complexes (bottom). Studies of synthetic transition metal-oxo complexes have been integral for understanding these reactive moieties in catalytic systems.1a, 5 However, there is a paucity of literature concerning multinuclear complexes bearing well-characterized terminal metal-oxo motifs.6 In a rare example where the effects of a neighboring metal oxidation state on a terminal metaloxo could be interrogated, Que and coworkers reported that the spin state of an FeIV-oxo 110 center would change depending on the oxidation state of a neighboring Fe in a μ2-O bridged bimetallic complex (L’2OFe2(OH)(O)2+/3+).6c The authors demonstrated that structural and spin-state changes due to reduction of this secondary Fe leads to a thousand-fold activation of the [Fe2] complex towards C–H oxidation. To gain further insight into these multimetallic effects, previous group members, Dr. Graham de Ruiter and Kurtis Carsch, studied the reactivity of Fe4, Fe3Mn, and Mn4 clusters, bearing aryl-substituted pyrazolate ligands, towards oxygen atom transfer reagents; in all cases, intramolecular C–H (or C–F) activation occurs, forming a five-coordinate apical metal with a phenoxypyrazolate donor.7 Analysis of the reaction mechanism were consistent with ratelimiting iodosylarene activation step, producing a transient reactive moiety (either iodosylarene adduct or terminal-oxo) that could not be directly observed. Inspired by reports of mononuclear terminal metal-oxo motifs stabilized by second coordination sphere hydrogen bonding interactions,8 our group has previously used this strategy to access a terminal MnIII– OH moiety as part of a [Mn4] cluster.9 Dr. Kyle Horak was able to serendipitously isolate the analogous Fe cluster, [LFe3O(PzNHPh)3Fe(OH)][OTf], however, there were challenges with accessing this cluster in reasonable purity and yield.10 Due to the observed difficulties in supporting clusters with the amino-phenylpyrazolate ligand, new amino-pyrazolate donors were investigated. Herein, we describe the synthesis, structural characterization, and reactivity studies of clusters bearing a terminal FeIII-oxo motif, stabilized by tert-butyl-amino-pyrazolates, to probe the significance of a multinuclear scaffold on structural and reactivity aspects of a terminal metal-oxo. 111 Scheme 1. Previous Efforts Towards Isolation of a Terminal Metal-Oxo in a Multinuclear System by the Agapie Group RESULTS AND DISCUSSION Synthesis of Fe4-Hyroxide and Fe4-Oxo Clusters. Treatment of the reported LFe3(OAc)(OTf)2 cluster (-OTf, triflate = trifluoromethane sulfonate)11 with three equivalents of potassium tert-butyl-amino-pyrazolate (KPzNHtBu) and iodosylbenzene (PhIO), followed by addition of iron (II) triflate bis-acetonitrile (Fe(OTf)2 • 2 MeCN) and excess potassium hydroxide in tetrahydrofuran (THF) produces the neutral [FeII3FeIII] cluster, 1 (Scheme 2). Single crystal X-ray diffraction (XRD) studies of 1 reveal a structure similar to our previously reported [Mn4] cluster bearing a terminal hydroxide ligand (Figure 2);9 the apical metal displays a trigonal bipyramidal geometry, with the terminal hydroxide ligand hydrogen bonded to each amino-pyrazolate (N–O distances of 2.826(1), 2.765(1), 2.789(1) Å for 1). The relatively short distance between the apical Fe and the interstitial μ4-O (Fe4–O1), 1.837(1) Å, is consistent with an FeIII in the apical position of the cluster, with the remaining Fe centers being FeII.7a, 12 112 Scheme 2. Synthesis of [Fe4] clusters. (Inset) 1,3,5-triarylbenzene ligand (L3-) and tertbutyl amino pyrazolate ligand (PzNHtBu-). Figure 2. Crystal structure of 1. Ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules removed for clarity. 113 The electrochemistry of the [Fe4] hydroxide clusters in THF features three quasi-reversible events assigned to the [FeII3FeIII]→[FeII2FeIII2] (-1.53 V; all potentials vs. Fc/Fc+), [FeII2FeIII2]→[FeIIFeIII3] (-0.68 V), and [FeIIFeIII3]→[FeIII4] (-0.10 V) redox couples (Figure 3). Each of the corresponding oxidation states of the cluster could be isolated (Scheme 1). Mössbauer spectra of the oxidized clusters 2, 3, and 4 are consistent with oxidations occurring at the FeII centers in the tri-iron core and the Fe–OH moiety remaining FeIII (Figure 4 and Table 1). FeII2FeIII2 FeIIFeIII3 FeIII4 Current FeII3FeIII 10 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 Figure 3. Cyclic voltammetry of 2, (2.5 mM) at 50 mV/s in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 200 mM [Bu4N][PF6]. Figure 4. Mössbauer spectra of (A) 1, (B) 2, (C) 3, and (D) 4; see parameters in Table 1. 114 Table 1. 57Fe Mössbauer Parameters for Complexes 1 – 4. δ (mm/s) |ΔEq| (mm/s) assignment δ (mm/s) 1 (FeII3FeIII) Fe1, Fe2, Fe3 Fe4 assignment 3 (FeIIFeIII3) 1.10, 1.11, 1.13 3.52, 3.03, 2.64 h.s. FeII Fe1 1.15 2.83 h.s. FeII 0.41 2.71 apical FeIII Fe2, Fe3 0.47, 0.46 0.61, 1.06 h.s. FeIII Fe4 0.38 1.69 apical FeIII 2 (FeII2FeIII2) Fe1, Fe2 |ΔEq| (mm/s) 1.12, 1.10 3.20, 2.76 h.s. FeII 4 (FeIII4) Fe3 0.52 0.81 h.s. FeIII Fe1, Fe2, Fe3 0.45, 0.53, 0.36 0.64, 1.15, 1.17 h.s. FeIII Fe4 0.41 2.17 apical FeIII Fe4 0.41 1.71 apical FeIII Access to a terminal FeIII-oxo moiety was achieved by deprotonation of the [FeII2FeIII2] hydroxide cluster, 2, with potassium tert-butoxide (KOtBu; Scheme 1). The resulting compound, 5, was crystallographically characterized (Figure 5); deprotonation of the hydroxide ligand leads to structural changes to the apical Fe in 5. The Fe4–O2 distance contracts to 1.817(2) Å, compared to the distances in 1 (1.937(1) Å) and the precursor 2 (1.907(3) Å); this bond length matches closely with the structurally characterized FeIII-oxo complexes reported by Borovik and Fout.8e, 8h, 8i Compound 6, prepared by deprotonating 3, also displays a short Fe4–O2 distance (1.795(8) Å). Furthermore, the apical Fe-μ4-O distance (Fe4–O1) elongates to 1.965(2) Å in 5 and 2.049(7) Å in 6, from 1.890(3) Å in 2 and 1.948(2) Å in 3, which is consistent with a greater trans influence exerted by the terminal oxo ligand. Terminal FeIII-oxo complexes are rare, and typically stabilized through hydrogen bonding interactions.8e, 8h, 8i, 13 The structures of 5 and 6 display comparable hydrogen bonding distances 115 Figure 5. Truncated crystal structure of FeIII-oxo cluster, 5. Hydrogen atoms and solvent molecules removed for clarity. Table 2. Selected Bond Distances and Angles, Structural Index Parameter, and Mössbauer Parameters of Reported FeIII-Oxo Complexes a 5 6 [(H3beau)Fe(O)]2-8e [N(afaCy)3Fe(O)]+8h Fe–O (Å) 1.817(2) 1.795(8) 1.813(3) 1.806(1) Fe–Nequatorial (Å) 2.104(2), 2.098(2), 2.093(2) 2.100(8), 2.085(9), 2.087(9) 2.030(4), 2.060(4), 2.082(4) 2.049(1), 2.049(1), 2.052(1) Fe–Ltrans (Å) 1.965(2) (L=O2-) 2.049(7) (L=O2-) 2.271(4) (L=NR3) 2.276(1) (L=NR3) N–O (H-bond; Å) 2.647, 2.717, 2.685 2.718, 2.790, 2.750 2.732, 2.702, 2.686 2.641, 2.645, 2.673 ∠Nequatorial–Fe–O (°) 96.3, 92.8, 92.0 93.6, 97.5, 96.3 103.3, 99.7, 100.8 102.6, 103.1, 103.1 Fe–N|N’|N’’equatorial (Å) 0.14 0.22 0.42 0.45 Structural Index Parameter (τ)a 0.9 0.8 0.5 0.4 Mossbauer parameters (mm/s) δ = 0.43, |ΔEq| = 3.04 δ = 0.47, |ΔEq| = 2.53 δ = 0.30, |ΔEq| = 0.91 - τ = [Σ (∠Nequit.–Fe–N’equit.) - Σ (∠Nequit.–Fe–O)]/90 to other structurally characterized FeIII-oxo complexes, [(H3beau)Fe(O)]2- and [N(afaCy)3Fe(O)]+, along with similar equatorial Fe–N distances (Table 2). However, the μ4O distances in 5 (1.965(2) Å) and 6 (2.049(7) Å) are significantly shorter than the Fe–N 116 distances for the amine trans to the oxo in the mononuclear systems (~2.27 Å). This is likely a result of greater ligand flexibility in the mononuclear systems; the geometry of these FeIIIoxo complexes display greater deviations from ideal trigonal bipyramidal geometry compared to the apical Fe in 5 and 6, based on a structural index parameter (τ; ideal trigonal bipyramidal geometry = 1.0). For the clusters reported here, the rigid geometry of the pyrazolate ligands prevents significant distortion of the apical Fe out of the equatorial plane. Electronic Structure Investigations of Tetranuclear Fe Clusters. The 57Fe Mössbauer spectra of the FeIII-oxo clusters 5 – 7 display relatively unique parameters for the apical Fe, relative to the structurally related FeIII-oxo, where Mössbauer parameters have been reported, [(H3beau)Fe(O)]2- (Table 2 and Figure 4).8e, 14 For example, the Mössbauer parameters assigned to the apical Fe of 5, δ = 0.43 mm/s and |ΔEq| = 3.04 mm/s, are atypical for highspin (S= 5/2) FeIII centers, which typically display low quadrupole splitting values. A detailed examination of the electronic structure of the Fe4 clusters was conducted through magnetic susceptibility measurements and EPR spectroscopy. Variable temperature, and vaeriable temperature variable field, magnetic susceptibility measusurements were conducted on a series of the thermally stable Fe4-hydroxide clusters (2 Figure 4. Zero applied-field Mössbauer spectra of (A) 5, (B) 6, and (C) 7. The parameters of each doublet are listed in Table 3. 117 Table 3. 57Fe Mössbauer Parameters for Complexes 5 – 7. δ (mm/s) |ΔEq| (mm/s) assignment 5 (FeII3FeIII) Fe1, Fe2 1.12, 1.10 3.14, 2.87 h.s. FeII Fe3 0.52 1.13 h.s. FeIII Fe4 0.43 3.04 apical FeIII Fe1 Fe2, Fe3 Fe4 1.09 2.87 h.s. FeII 0.51, 0.49 1.09, 0.72 h.s. FeIII 0.47 2.53 apical FeIII assignment 0.43, 0.44, 0.41 1.44, 0.95, 0.38 h.s. FeIII 0.43 2.03 apical FeIII 7 (FeIII4) Fe1, Fe2, Fe3 6 (FeII2FeIII2) |ΔEq| (mm/s) δ (mm/s) Fe4 – 4) to establish their electronic ground states (Figure 5) The variable temperature magnetic susceptibility data for these compounds is consistent with high-spin Fe centers composing all cluster redox states for the series; the best fit for each complex is obtained by using exclusively S = 2 FeII and S = 5/2 FeIII. The spin coupling model for these clusters is similar to that of structurally related high-spin Fe clusters bearing bridging imidazolate ligands, where strong antiferromagnetic coupling between the apical FeIII and the tri-iron core promotes in ferromagnetic alignment of the spins with the remaining three Fe centers (Figure 5B).12 The variable temperature variable field magnetization data for these compounds can be fit adequately using this coupling scheme, supporting assignments for spin ground states of S = 4, 9/2, and 5 for 2, 3, and 4, respectively (Figure 6). The presence of increasing zero field splitting as 4 is reduced to 3 and 2 is observed in the magnetization data, causing saturation below the expected 2S limit (10 for 4, 9 for 3, and 8 for 2). 118 Figure 5. (A) Variable temperature direct current magnetic susceptibility data for Fe4hydroxide clusters 2 (yellow), 3 (green), and 4 (blue) at 0.1 T. The spin coupling model has strong antiferromagnetic alignment of the apical Fe and the tri-iron core. (B) Simulation of coupling scheme for 2 – 4, with all metal centers locally high spin. Figure 6. Variable temperature-variable field magnetization data for 2 (A), 3 (B), and 4 (D) from 2.5 K to 8 K. The spin and zero field splitting parameters (S, D, and |E/D|) were used to simulate the data. 119 Continuous wave EPR spectra of these paramagnetic clusters at low temperature (< 20 K) in frozen solutions display a number of features in regions expected for these high-spin systems (g >16). Figure 7 summarizes the parallel (for integer spin clusters 2, 4, 5, and 7) and perpendicular (for 3 and 6) mode data collected. Attempts to simulate these spectra were challenging, even for the complexes where magnetization data was obtained. For example, 2 displays a sharp peak at ca. g = 17 at low temperatures (<15 K), which can be tentatively assigned to the Ms = ± 4 doublet transition (Figure 7A).12 In contrast, the corresponding [FeIII2FeII2] Fe-oxo cluster, 5, displays one major transition near g ~ 19, which is consistent with an S = 4 or 5 system (Figure 7B). Further studies will examine the field-dependent Mössbauer spectra of these clusters to identify their spin ground states. Even without simulating the EPR data, there are noticable differences for each cluster oxidation state between Fe4-hydroxide and –oxo, demonstrating the strong influence of the apical ligand on the electronics of the cluster. Figure 7. Parallel-mode EPR spectrum in 2-MeTHF of(A) 2 collected at 5 K (red), 10 K (orange), 15 K (green), and 25 K (blue), and (B) 5 at 5 K (red), 10 K (orange), and 15 K (green). 120 Figure 7 cont. (C) Perpendicular-mode CW-EPR spectrum of 3 collected in EtCN/PrCN at 5 K (red), 7.5 K (orange), 10 K (green), 15 K (blue), 20 K (purple), and 30 K (black). (D) Perpendicular-mode spectrum of 6 collected in EtCN/PrCN at 5 K (red), 7.5 K (orange), 10 K (green), 15 K (blue), and 20 K (black). (E) Parallel-mode spectrum of 4 in EtCN/PrCN at 5 K (red), 10 K (green), and 15 K (purple). (F) Parallel-mode spectrum of 7 in EtCN/PrCN at 5 K (red), 10 K (orange), 15 K (green), and 20 K (black). pKa and BDEO–H Determination for Fe4-Hydroxide Clusters. The hydroxide ligand in 2 was determined to be very basic in THF (pKa = 30.1; Table 4). Analogous equilibrium studies were performed on 3 and, as expected, oxidation of the cluster reduces the basicity of the 121 FeIII-oxo moiety (pKa = 23.0 for 3; Table 5). Attempts to deprotonate 4 with various bases, even at low temperatures, only resulted in decomposition, so a pKa value for this oxidation state was not measured. These data were combined with electrochemical information for clusters 1 (vide supra) and 5 (Figure 8), to produce thermodynamic square schemes according to equation 1 (Figure 9): 15 BDEO–H = 23.06 E° + 1.37 pKa + C (1) Table 4. pKa determination of 2 via 31P NMR spectroscopy.a NMR 2 + 1.3 Ph3PCH2 2 + 5 Ph3PCH2 2 + 7 Ph3PCH2 2 + 10 Ph3PCH2 2 + 18 Ph3PCH2 Equiv. Ph3PCH2 ~ 1.3 4.54 6.38 9.24 16.97 Equiv. [Ph3PCH2]+ 0.46 0.62 0.76 1.03 [5]/[2] n.d. 0.85 1.63 3.17 n.d. Average K Kb n.d. 0.09 0.16 0.26 n.d. 0.17 (±0.09) aThe ratio of 2 to 5 was estimated from the relative integrals of the 31P NMR peaks for Ph PCH (~15 ppm) and 3 2 Ph3PCH3+ (~18 ppm); it was assumed that the equivalents of [Ph3PCH3]+ produced in the NMR were due to partial deprotonation of 2, and corresponded to equivalents of 5 ([[Ph3PCH3][OTf]] = [5]). bAn equilibrium constant was determined according to the equation below: [𝟓][[Ph3 PCH3 ][OTf]] [𝟐][Ph3 PCH2 ] where this value, along with the reported pKa of Ph3PCH2 in THF, 29.3, was used to obtain a pKa value of 30.1 (±1.0) for 2.16 𝐾= Table 5. pKa determination of 3 via 1H NMR spectroscopy NMR 3 + 1 tBuP1(pyrr) 3 + 5 tBuP1(pyrr) 3 + 10 tBuP1(pyrr) Equiv. 3a 0.55 0.27 0.12 Equiv. 6a 0.45 0.73 0.88 Average K Kb 0.70 0.46 0.71 0.62 (±0.14) aThe ratio of 3 to 6 was based on the relative integrals of the 1H NMR peaks at 17.0 (for 3) and 14.5 ppm (for 6); the relative amounts of base and conjugate acid of tert-butylimino-tri(pyrrolidino)phosphorene (tBuP1(pyrr)) were assumed based on mass balance. bAn equilibrium constant was determined according to the equation below: [𝟔][[H′tBuP1 (pyrr)′][OTf]] [𝟑][′tBuP1 (pyrr)′] where this value, along with the reported pKa of tBuP1(pyrr) in THF, 22.8, was used to obtain a pKa value of 23.0 (±1.0) for 3.16 𝐾= 122 FeII2FeIII2 FeIIFeIII3 FeIII4 Current * * * 15 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V vs. Fc/Fc+) 0.5 1 Figure 8. Cyclic voltammetry of 5 (2.3 mM at 200 mV/s scan rate) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 100 mM [Bu4N][PF6]. Electrochemical events marked with an asterisk (*) are assigned to a small amount of [LFe3O(PzNHtBu)3Fe(OH)]n+ that formed due to decomposition. Figure 9. Thermodynamic cycles to evaluate the BDEO–H values of the hydroxide clusters 1 – 3. Reduction potentials (horizontal lines) are references to Fc/Fc+. pKa values (vertical lines) are based on relative pKa values of cationic acids in THF. Diagonal lines are the BDEO–H values calculated from these parameters according to the Bordwell equation (eq 1). Approximate values (~) have been extrapolated from the Bordwell equation. 123 Similar to our previously reported studies on [Fe3Mn] hydroxide and aquo clusters, the bond dissociation enthalpy of the O–H bond (BDEO–H) increases upon oxidation of the distal Fe centers, ranging from 72 kcal/mol in 1 to 84 kcal/mol in 3.17 Reactivity Studies of Fe4-Oxo Clusters. The three distal Fe oxidation states have a dramatic effect on the reactivity of the FeIII-oxo center through modifying the pKa and BDEO– H values. For example, 5 is incapable of performing proton coupled electron transfer (PCET) reactions18,19 with substituted phenols over a range of phenol BDEO–H values (79 – 85 kcal/mol); only proton transfer to generate 2 is observed as expected from the combination of low BDEO–H for 1 and high pKa of 2. Oxidation of the remote Fe centers in 6 and 7 enables PCET reactivity with these phenols, resulting in the formation of 2 and 3, respectively (Figure 10). Figure 10. 1H NMR spectra (400 MHz) in THF/C6D6 of reaction products with 5 - 7 and 2,4,6-tri-tert-butyl phenol (2,4,6-tBu3-PhOH; BDE = 82 kcal/mol). The major species in the maroon and green spectra corresponds to 2. The major species in the blue spectrum corresponds to 3. 31 P NMR and GC/MS analyses suggest that 7 is capable of transferring an oxygen atom to trimethylphoshine (PMe3), where the other FeIII-oxo clusters display no reaction towards the phosphine on a similar timescale (Figure 11). The difference in reactivity is likely due to 124 the low reduction potentials of 5 and 6 precluding efficient oxygen atom transfer reactivity. A more oxidizing cluster, through oxidations of the distal Fe centers, 7 can undergo OAT. Figure 11. (A) 1H NMR spectrum (400 MHz) in THF/C6D6 of 5 with 40 equivalents trimethylphosphine (PMe3); the resonances are consistent with no reaction occurring. (B) 31P NMR spectrum of 6 and 8 equivalents PMe3; the only resonances observed on those assigned to unreacted PMe3 and the conjugate acide of the based used to prepare 6 (Ph3PCH3+). (C) 31P NMR spectrum of 7 and 20 equivalents PMe3, where a resonance assigned to trimethylphosphine oxide (OPMe3) is observed ~ 40 ppm. (D) GC/MS analysis of reaction between 18O-7 and PMe3 contains mass fragments of 18OPMe3 (94 m/z), consistent with oxygen atom transfer from 7 to form the phoshpine oxide. The kinetics of C–H activation by these clusters was investigated. The reaction between 5 and 9,10-dihydroanthracene (DHA; BDEC–H = 78 kcal/mol)15c displays an expected first order dependence on substrate concentration, with an overall second order rate constant of 87 M-1 s-1, and a considerable kinetic isotope effect (KIE) of 7 with d4-DHA. These data are consistent with a rate-limiting C–H bond activation for the PCET process to form 1 and anthracene. The 125 second-order rate constants between 5 and C–H bonds of varying BDEC–H and pKa values were measured and display a linear dependence of the PCET reaction rate on the pKa of the organic substrate (Figure 12), suggesting either a concerted or stepwise pKa-driven process.20 Reactions between DHA and 6 or 7 produce the corresponding hydroxide-clusters and anthracene in yields comparable to 5 (Table 6) indicating PCET processes, but complex kinetics precluded the determination of rate constants and further insights into the mechanism of these reactions. Figure 12. Plots of log (k2) (normalized to number of reactive C-H bonds) versus BDE (left) and pKa (in DMSO; right) for 5 with various organic substrates. 126 Table 6. Product Analysis of PCET and OAT Reactivity of 5 – 7. 5 organic product anthracene (53%)a 6 7 organic cluster organic cluster Substrate product product product product anthracene anthracene 3 9,10 -dihydroanthracene 1 (67%)b 2 (66%)b (43%)a (44%)a (110%)b 9,9’-bifluorenyl 3 fluorene n.d.c 1 (83%)b 2 (81%) n.d.c (1%)a (107%)b 2,4,6-tri-tertbutylphenol phenoxyl 2 2 3 radicald e e e e trimethylphosphine N.R. N.R. N.R. N.R. OPMe3 2 aQuanitified by GC/MS versus authentic samples, with triphenylphosphine as an internal standard; percent yield based on a 2:1 cluster/product stoichiometry. bQuantified by 1H NMR spectroscopy with 1,3-trimethylsilylbenzene as an internal standard. cNot detected by GC/MS. dX-band EPR signal detected at 77 K that matches authentic sample of the corresponding phenoxyl radical.e No reaction observed. cluster product CONCLUSIONS Overall, this report offers a rare systematic study of the effects of neighboring redox active metals on structural and reactivity aspects of a terminal metal-oxo. Because it is part of a cluster, the reactivity of the terminal metal-oxo motif can be tuned without changing the formal redox state of the metal supporting it; however, redox events at distal centers have significant effect on the acidity and BDE of the corresponding O-H bond. Clearly, the cluster as an assembly is essential for reactivity beyond the structural aspects of the isolated metaloxo motif. Further development of multinuclear model systems is necessary to fully understand the nature and amplitude of these effects. 127 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified; reactions with KOH were performed in an N2-filled VAC wetbox. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. [LFe3(OAc)(OTf)][OTf],11 iodosylbenzene,21 benzyl potassium,22 Fe(OTf)2 • 2 MeCN,23 ferrocenium trifluoromethane sulfonate ([Fc][OTf]),24 and Ph3PCH225 were prepared according to literature procedures. N-tert-butyl-1H-pyrazol-5-amine (HPzNHtBu) was prepared according to a modified literature procedure.26 18-oxygen labeled potassium hydroxide (K18OH) was prepared by quenching a tetrahydrofuran solution of benzyl potassium (less than 1 mmol) with H218O, and drying the resulting white suspension under vacuum. Tetrahydrofuran, CH2Cl2, diethyl ether, benzene, toluene, acetonitrile, hexanes, and pentane were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H spectra were recorded on a Varian 300 MHz spectrometer; 13C NMR spectra were recorded on a Varian 500 MHz spectrometer. 1H and 31P NMR spectra in THF/C6D6 were recorded on a Varian 500 MHz spectrometer using solvent suppression protocols. NMR spectra collected at low temperature were recorded on a Bruker 500 MHz spectrometer. CD3CN, C6D6, and CD2Cl2 was purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freezepump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero applied field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to 128 the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated acetonitrile solution in the cup. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). Mössbauer simulation details for all compounds. All spectra were simulated by four pairs of symmetric quadrupole doublets with equal populations and Lorentzian lineshapes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts of ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported Fe clusters.7a, 7b, 11, 27 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.28 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (counter) and bare platinum wire (reference), in a N2 filled M. Braun glovebox at RT. Dry acetonitrile or tetrahydrofuran that contained ~100 mM [Bu4N][PF6] was used as the electrolyte solution. The ferrocene/ferrocinium (Fc/Fc+) redox wave was used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All 129 non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and refined using a riding model. Due to the size of the compounds (1 – 3, 5, and 6), most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Magnetic measurements. Magnetic susceptibility measurements were collected on a Quantum Design DynaCool 14T PPMS instrument at the University of Southern California, Los Angeles. Polycrystalline samples (10 – 20 mg) of 2 – 4 were packed in VSM sample holders. Magnetization data at 100 K from 0 to 4 T were collected to confirm the absence of ferromagnetic impurities. Magnetic susceptibility measurements were collected between 2 and 300 K with a 0.1 T field. Reduced magnetization data was collected between 2 K and 8 K at fields between 1 T and 12 T. Magnetic data was simulated with PHI.29 Synthetic Procedures. Synthesis of 2-tert-butyl-isoxazolium tetrafluoroborate. 25 mL isoxazole (0.4 mol) was combined with 37 mL tert-butanol (0.4 mol) in a 500 mL roundbottom flask. This was cooled to -20 °C with an ice/sodium chloride bath while 160 mL tetrafluoroboric acid diethyl ether complex (1.2 mol) was added dropwise over 1 hour. After the addition was complete, the reaction was warmed to room temperature and stirred for 4 hours. Then, 100 mL Et2O and 50 mL THF was added to the reaction and cooled to -20 °C; the resulting precipitate was collected on a glass frit, washed three times with 200 mL Et2O and dried under reduced pressure. 60 g of 2-tert-butyl-isoxazolium tetrafluoroborate (72% yield) can be obtained this way; another 6 g can be obtained by cooling the filtrate and Et2O washings to 20 °C overnight and collecting the resulting crystals (79% overall yield). 1H NMR (300 MHz, (CD3)2CO): δ 1.90 (s, 9H), 7.46 (s, 1H), 9.55 (s, 1H), 9.77 (s, 1H) ppm. Synthesis of N-tert-butyl-1H-pyrazol-3-amine (HPzNHtBu). This procedure was adapted from a report describing the synthesis of tert-butyl substituted 3-aminopyrazoles.26 10.0 g of 2-tert- 130 butyl-isoxazolium tetrafluoroborate (47 mmol) was suspended in 100 mL EtOH in a 250 mL roundbottom flask and cooled with an ice bath to 0 °C. A solution of 4.56 mL hydrazine monohydrate (94 mmol) in 20 mL EtOH was added dropwise to the cooled flask. After complete addition, the reaction was warmed to room temperature and stirred for 30 minutes. EtOH was removed via rotary evaporation and an aqueous work up was performed with 100 mL H2O and 3 x 100mL CH2Cl2, collecting the organic layers. The combined organic fractions were dried with Na2SO4, filtered, and dried to yield an orange oil. The crude product was purified via Kugelrohr distillation under dynamic vacuum at 90 °C. The distillate was recrystallized with Et2O and the resulting white solid was sublimed under vacuum at 60 °C to yield 1.6 g of HPzNHtBu (24% yield). 1H NMR (400 MHz, CD2Cl2): δ 1.27 (s, 9H), 3.61 (br), 5.71 (d, 1H), 7.34 (d, 1H), 9.75 (br) ppm. 13C{1H} NMR (100 MHz, CD2Cl2): 53.28, 75.11, 118.60, 154.26 ppm (a signal for the tert-butyl quaternary carbon was likely not observed). Anal. calcd. (%) for C7H13N3: C, 60.40; H, 9.41; N, 30.19. Found: C, 60.75; H, 9.37; N, 30.20. Synthesis of potassium N-tert-butyl-1H-pyrazol-3-amine-ate (KPzNHtBu). 1.25 g N-tert-butyl-1Hpyrazol-3-amine (9 mmol) was dissolved in 5 mL THF. A THF solution of 1.17 g benzyl potassium (9 mmol) was added dropwise, while stirring. After 30 minutes, the reaction was concentrated to 5 mL, and the precipitate was collected via filtration. The precipitate was washed with Et2O and dried under vacuum to yield 1.2 g KPzNHtBu as a white solid (75% yield). Anal. calcd. (%) for C7H12KN3: C, 47.42; H, 6.82; N, 23.70. Found: C, 47.50; H, 6.83; N, 23.61. Synthesis of LFe3O(PzNHtBu)3Fe(OH) (1). 1.287 g (0.93 mmol) LFe3(OAc)(OTf)2 was suspended in THF and froze in a liquid nitrogen cooled cold well. 502.6 mg (2.83 mmol) KPzNHtBu was added with THF while the suspension was thawing. After stirring at room temperature for 1 hour, 207.0 mg (0.94 mmol) iodosylbenzene was added with THF. After 4 131 hours, the solvent was removed under reduced pressure. The brown solid was transferred to a coarse porosity glass frit with celite using 50 mL pentane. The desired compound was extracted using toluene until the filtrate appeared colorless. This red-brown solution was dried completely under reduced pressure; the resulting solid (1.207 g obtained) is used in the following steps assuming a molecular formula of LFe3O(PzNHtBu)3, however this could not be confirmed via X-ray crystallography due to its poor crystallinity. 110.7 mg (0.076 mmol) of the LFe3O(PzNHtBu)3 solid was dissolved in 5 mL THF. 33.0 mg (0.076 mmol) Fe(OTf)2 • 2 MeCN was added with 1 mL THF. After 45 minutes, 26 mg (0.464 mmol) KOH was added as a THF suspension. After 18 hours, the reaction appeared dark blue; this solution was transferred to a Schlenk tube and dried under vacuum at 100 °C for 1 hour. The reaction mixture is suspended in MeCN and the blue precipitate was collected over a coarse porosity frit with celite. The precipitate was washed with MeCN until the filtrate was colorless, and then dried under vacuum. The dry blue precipitate was extracted with toluene and dried under reduced pressure. This residue was recrystallized via benzene/HMDSO vapor diffusion to yield 25.7 mg (0.017 mmol; 22% yield) of 1 as a blue solid. 1H NMR (300 MHz, C6D6): δ 123.0 (br), 64.6 (br), 56.4, 50.1, 44.1, 41.0, 24.6, 19.6, 14.2, 12.2, 4.4, 3.2, 1.7, -40.6 (br) ppm. UV-Vis (THF) [ε (M-1 cm-1)] 253 nm (5.19 x 104), 494 nm (3.26 x 103), 609 nm (3.81 x 103). Anal. calcd. (%) for C78H76Fe4N15O5: C, 61.36; H, 5.02; N, 13.76. Found: C, 61.27; H, 5.40; N, 13.12. The 18-O labeled cluster could be prepared through the analogous protocol, substituting K18OH for KOH. The resulting product has identical spectroscopic features to that of 1, and was used to prepare the remaining 18-O labeled clusters (via oxidations and/or deprotonation). ESI-MS analysis was consistent with 18-O incorporation of the cluster (Figure S18). 132 Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2). 265.2 mg (0.17 mmol) LFe3O(PzNHtBu)3Fe(OH) was dissolved in 5 mL THF. This was transferred to a stirring suspension of 52.3 mg (0.16 mmol) [Fc][OTf] in 3 mL THF. After 1 hour, the reaction was concentrated under vacuum to 1 mL and 15 mL toluene was added. The reaction was stirred for 15 minutes and the resulting red-purple precipitate was collected on a coarse frit with celite and dried completely under vacuum. The red-purple solid was extracted by washing with MeCN until the filtrate appeared colorless; this solution was dried under reduced pressure. The resulting residue was recrystallized via THF/Et2O vapor diffusion to yield 211 mg of redpurple crystals of 2 (0.13 mmol; 82% yield). 1H NMR (300 MHz, CD3CN): δ 127.2 (br), 82.1 (br), 54.4, 49.0, 22.1, 16.5 (br), 14.1, 13.8, 13.3, 10.3 (br), 8.4, 7.8, 7.3, 1.0, -4.9, -5.1, -22.8 (br) ppm. UV-Vis (ACN) [ε (M-1 cm-1)] 243 nm (5.96 x 104), 328 nm (8.83 x 103), 503 nm (4.88 x 103). Anal. calcd. (%) for C79H76F3Fe4N15O8S: C, 56.61; H, 4.57; N, 12.54. Found: C, 56.72; H, 4.70; N, 12.03. Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3). 102.3 mg (0.06 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf] was dissolved in 3 mL DCM and a solution of 20.3 mg (0.06 mmol) [Fc][OTf] in 2 mL DCM was transferred to this stirring solution. After 2 hours, 10 mL pentane was added to the reaction and the blue precipitate was collected on a coarse porosity glass frit with celite. The blue powder was dried under vacuum and extracted with DCM until colorless, and recrystallized from DCM/Et2O to obtain 76.8 mg of 3 as blue crystals (69% yield) 1H NMR (300 MHz, CD2Cl2): δ 144.3 (br), 103.7 (br), 82.0, 79.7, 66.0, 63.1, 15.5, 12.8, 9.9, 3.5, 1.2, -0.5, -2.3 (br), -11.6 (br) ppm. [ε (M-1 cm-1)] 238 nm (5.76 x 104), 345 nm (7.74 x 103), 634 nm (4.80 x 103). Anal. calcd. (%) for C80H76F6Fe4N15O11S2: C, 52.65; H, 4.20; N, 11.51. Found: C, 51.70; H, 4.37; N, 11.11. 133 Synthesis of [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4). 42.9 mg (0.024 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 was dissolved in 2 mL DCM and 8.1 mg (0.024 mmol) [Fc][OTf] was added with 2 mL DCM. After 30 minutes, the reaction was concentrated and 10 mL Et2O was added to produce a green precipitate. This was collected on a frit over celite and rinsed with Et2O. The precipitate was collected with DCM and recrystallized via vapor diffusion of Et2O to obtain 39.0 mg (0.020 mmol; 82% yield) 4 as green crystals. 1H NMR (300 MHz, CD2Cl2): δ 95.2, 85.0, 14.5, -1.8, -44.9. -48.3 ppm. UV-Vis (ACN) [ε (M-1 cm-1)] 242 nm (7.11 x 104), 355 nm (8.85 x 103), 748 nm (7.39 x 103). Anal. calcd. (%) for C81H76F9Fe4N15O14S3: C, 49.28; H, 3.88; N, 10.64. Found: C, 49.19; H, 4.09; N, 10.02. Synthesis of LFe3O(PzNHtBu)3Fe(O) (5). 102.6 mg (0.06 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf] was dissolved in 15mL THF and froze in a liquid nitrogen cooled cold well. 7.2 mg (0.06 mmol) KOtBu was added to the thawing solution, and the reaction was stirred at room temperature for 1 hour. The solvent was removed under reduced pressure and the crude product was recrystallized via benzene/HMDSO vapor diffusion to obtain 26.2 mg of 5 as purple crystals (0.02 mmol; 28% yield). 1H NMR (300 MHz, C6D6): δ 105.9 (br), 58.5 (br), 55.3, 53.0, 40.9, 38.9, 33.9, 21.8, 14.4, 11.7, 2.4, 1.1, -21.5 (br) ppm. UVVis (THF) [ε (M-1 cm-1)] 248 nm (4.40 x 104), 342 nm (6.73 x 103), 539 nm (3.41 x 103). Anal. calcd. (%) for C78H75Fe4N15O5: C, 61.40; H, 4.95; N, 13.77. Found: C, 60.04; H, 5.01; N, 13.06 (Calcd. (%) for C78H75Fe4N15O5 • 0.5 (C6H18OSi2): C, 60.05; H, 5.27; N, 13.08; compound recrystallized from benzene/HMDSO). Synthesis of [LFe3O(PzNHtBu)3Fe(O)][OTf] (6). 50.3 mg (0.03 mmol) [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 was dissolved in 2 mL THF/DCM (1:1) and froze in a liquid nitrogen cooled cold well. 8 mg (0.03 mmol) Ph3PCH2 was added to the thawing solution as a THF solution. The reaction turned a deep blue, and at this point care was taken to avoid 134 warming the mixture to room temperature. The compound was precipitated by addition of cold Et2O, and the precipitate was dried under vacuum to yield 6 as a blue powder. NMR analysis of this powder revealed the presence of residual [Ph3PCH3][OTf], which were difficult to remove with Et2O washes. This mixture could be recrystallized in THF/Et2O at -35 °C to obtain X-ray quality crystals of 6; however, due to the decomposition of this compound, obtaining 6 cleanly as a bulk solid for elemental analysis was unsuccessful. 1H NMR (300 MHz, CD3CN): δ 122.2 (br), 90.2 (br), 68.5, 66.1, 55.0, 53.2, 14.5, 13.9, 13.0, 10.7, -31.0 (br) ppm. Synthesis of [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7). 43.0 mg [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3; 0.02 mmol) was dissolved in 1:1 DCM/THF and froze in a liquid nitrogen cooled cold well. A THF solution of 6.8 mg Ph3PCH2 (0.02 mmol) was added to the thawing solution. The reaction was then combined, while thawing, with a DCM solution of 7.8 mg [Fc][OTf] (0.02 mmol). Keeping this mixture as cold as possible, thawing Et2O was added to precipitate the oxidized cluster; The blue-green solid was collected on a fine porosity glass frit, and dried under vacuum. The 1H NMR of this solid always contained minor amounts of impurities (<20%, mostly ascribed to 3 and 4), which 7 could not be isolated from due to its thermal instability. For any subsequent reactions performed on this material, the moles of initial cluster 3 were used to approximate the amount of 7 present. 1H NMR (300 MHz, 1:1 CD3CN/CD2Cl2): δ 145.6 (br), 105.4 (br), 85.2, 81.4, 71.0, 67.2, 19.0, 13.8, 11.1, 8.8, -61.1, 67.3 (br). Experimental Protocols. Reactions for product analysis. 2mM solutions of Fe-oxo clusters 5 – 7 were stirred for 12-24 hours with one equivalent of 9,10-dihydroanthracene (DHA) or fluorene. The solvent was removed under vacuum and the organic products were extracted with Et2O containing triphenylphosphine as an internal standard. The suspensions were filtered over celite and analyzed via GC-MS. The oxidized products (anthracene and 9,9’- 135 bifluorene) were quantified based on a calibration curve of authentic samples. Other possible oxidation products, such as fluorenone, or anthraquinone, were not observed. Oxygen atom transfer studies with 18O-7 and PMe3.18O-7 cluster was prepared in situ by combining a thawing 1:1 THF/DCM solution of 50.3 mg 18O-3 (0.03 mmol) with 7.5 mg Ph3PCH2 (0.03 mmol) to prepare a solution of 6-18O. This solution was combined, while thawing, to a DCM solution of 9.5 mg [Fc][OTf] (0.03 mmol). Keeping this mixture as cold as possible, thawing Et2O was added to precipitate the oxidized cluster; boron nitride (BN) was added to ease separation of precipitate from the solution. This suspension was filtered to obtain solid 18O-7, which was eluted from BN with cold 1:1 THF/DCM. 50 μL PMe3 (0.50 mmol) was added to solution as it thawed, and was gradually warmed to room temperature. 31P NMR analysis of the reaction mixture at this stage showed a peak consistent with trimethylphosphine oxide (OPMe3) formation at ~35 ppm. After 30 minutes, the reaction was pumped down. On the bench, the crude reaction mixture was separated via silica plug; 10% MeOH in DCM was used to elute a dark solution, at which point MeOH was washed through the plug to collect a fraction containing OPMe3. The MeOH fraction was pumped down and analyzed via GC/MS, which displayed a GC peak characteristic of OPMe3, with both 16OPMe3 and 18OPMe3 based on its mass spectrum. 136 ELECTROCHEMICAL DETAILS Current (μA) 20 μA -2.5 50 mV/s 100 mV/s 150 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2 -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 1 Figure 13. Cyclic voltammetry of [LFe3O(PzNHtBu)3Fe(OH)][OTf], 2, (2.5 mM) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 200 mM [Bu4N][PF6] at various scan rates. * Current 15 μA * * 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2.5 -2 -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 1 Figure 14. Cyclic voltammetry of LFe3O(PzNHtBu)3Fe(O), 5, (2.3 mM) in THF with a glassy carbon working, platinum counter, and silver wire reference electrodes and ca. 100 mM [Bu4N][PF6] at various scan rates. Electrochemical events marked with an asterisk (*) are assigned to a small amount of [LFe3O(PzNHtBu)3Fe(OH)]n+ that formed due to decomposition. 137 KINETICS DETAILS Substrate xanthene 1,4-cyclohexadiene 9,10-dihydroanthracene triphenylmethane fluorene BDE (kcal/mol)30 75.2 76.0 76.3 81.0 82.2 pKa (DMSO) 30.031 ~34a 30.131 30.632 22.632 k2 (M-1 s-1) with 5 40 ~0.3 87 ~0.7 ~3 x 106 Reported bond dissociation enthalpies (in kcal mol-1) and pKa values (in DMSO) of various organic substrates investigated for PCET reactivity with LFe3O(PzNHtBu)3Fe(O) (5), with their measured second order rate constants. 1H NMR analysis of the reaction mixtures after kinetics measurements show formation of 1 in all cases, consistent with a PCET process. aA reported pK a value for 1,4-cyclohexadiene could not be obtained, but is approximated based on the reported pKa value of 1,3-cyclohexadiene in DMSO (pKa(1,3) = 35.0) and the energy of isomerization between 1,3cyclohexadiene and 1,4-cyclohexadiene (-1.6 kcal/mol):33 1 Absorbance (a.u.) 0.8 0.6 0.4 0.2 0 350 450 550 650 750 850 950 Wavelength (nm) Figure 15. UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and xanthene (10 mM) at ambient temperature. 138 0 0.03 y = -0.0067x + 0.0285 R² = 0.9945 0.02 -1 -1.5 -3 0.015 0.01 -2 -2.5 y = 0.6676x + 0.0054 R² = 0.9436 0.025 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf]) -0.5 y = -0.014x + 0.0657 R² = 0.9917 0.005 y = -0.0242x - 0.0016 y = -0.0199x + 0.0936 R² = 0.9996 R² = 0.9884 0 50 100 150 200 0 0 250 0.01 0.02 0.03 [Xylene] (M) Time (min) Figure 16. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and xanthene (4.8, 10.1, 19.9, and 29.8 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. ln([A]-[Ainf]/[A0]-[Ainf]) Absorbance (a.u.) 0 0.5 -0.5 -1 -1.5 y = -0.0041x - 0.1039 R² = 0.9984 -2 -2.5 0 400 600 Wavelength (nm) 800 0 200 400 Time (min) Figure 17. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 1,4-cyclohexadiene (1 M) at ambient temperature. (Right) Pseudo-first order kinetics plot of the reaction by following the decay of the signal at 516 nm; this wavelength was used since the background decomposition of the compound did not affect this wavelength. 139 1 Absorbance (a.u.) 0.8 0.6 0.4 0.2 0 350 450 550 650 750 850 950 Wavelength (nm) Figure 18. UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 9,10dihydroanthracene (10 mM) at ambient temperature. 0.05 y = 1.4433x + 0.0007 R² = 0.9275 y = -0.0049x - 0.0073 R² = 0.9997 -1 -2 0.04 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf]) 0 0.03 y = -0.021x - 0.0193 R² = 0.999 -3 0.02 y = -0.0255x - 0.042 R² = 0.9983 -4 0.01 y = -0.0452x - 0.1816 R² = 0.9958 -5 0 0 100 200 300 Time (min) 400 500 0 0.01 0.02 0.03 [9,10-DHA] (M) Figure 19. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and 9,10-dihydroanthracene (9,10-DHA; 5, 10, 20, and 30 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. 140 0 0.04 y = -0.0106x - 0.1387 R² = 0.9923 y = 0.217x R² = 0.9494 0.03 -2 kobs (min-1) ln([A]-[Ainf]/[A0]-[Ainf] -1 0.02 y = -0.0146x - 0.2006 R² = 0.993 -3 y = -0.0282x - 0.2029 R² = 0.9935 -4 y = -0.0209x - 0.2058 R² = 0.9936 0.01 -5 0 50 100 150 200 0 250 0 Time (min) 0.05 0.1 0.15 0.2 [d4-9,10-DHA] (M) Figure 20. Kinetics data for the reaction between LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and d4-9,10-dihydroanthracene (d4-9,10-DHA; 40, 60, 100, and 130 mM) at ambient temperature. The decay of the UV-Vis absorbance feature at 540 nm was used to follow the reaction. 0 1 -0.5 Absorbance (a.u.) ln([A]-[Ainf]/[A0]-[Ainf]) -1 -1.5 -2 0.5 -2.5 -3 -3.5 y = -0.0111x + 0.1612 R² = 0.9918 -4 0 -4.5 400 500 600 Wavelength (nm) 700 800 0 100 200 300 400 Time (min) Figure 21. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 200 μM) and triphenylmethane (1 M) at ambient temperature. (Right) Pseudo-first order kinetics plot of the reaction by following the decay of the signal at 516 nm; this wavelength was used since the background decomposition of the compound did not affect this wavelength. 141 x103 0.6 1/([5][fluorene]) (M-1) Absorbance (a.u.) 0.5 0.4 0.3 0.2 0.1 7000 y = 35845x + 1E+06 R² = 0.9721 y = 51310x + 522253 R² = 0.9819 y = 45347x + 463112 R² = 0.9862 6000 5000 4000 3000 2000 1000 0 350 550 750 0 0 20 Wavelength (nm) 40 60 80 100 Time (min) Figure 22. (Left) UV-Vis absorbance spectra of LFe3O(PzNHtBu)3Fe(O) (5; 1 mM) and fluorene (2 mM) at ambient temperature in a 1 mm cuvette. (Right) Second order kinetics plot of the reaction by following the growth of the signal at 645 nm. 142 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3, 5, and 6. 1 1903350 2-PF6 1903348 3 1903352 5 1903351 6 1903349 Empirical formula C90H88Fe4N15 O5 C83H76F6Fe4 N15O5.5P C80H76F6Fe4 N15O11S2 C90H87Fe4N15 O5 C91H99F3Fe4N 15O11S Formula weight (g/mol) 1683.15 1793.95 1825.07 1682.14 1673.00 Radiation MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ=1.54178) CuKα (λ=1.54178) 14.1115(11) 11.9919(11) 12.150(2) 12.3162(13) 19.122(9) 15.0509(11) 13.7630(9) 14.975(5) 15.5743(15) 18.204(5) 21.1556(16) 25.905(2) 23.386(6) 21.6599(15) 24.698(6) 70.794(3) 89.286(4) 95.271(14) 102.390(6) 90 86.911(3) 87.757(4) 90.124(12) 94.445(4) 90 70.570(3) 79.589(4) 104.172(19) 102.897(9) 90 3993.7(5) 4201.8(6) 4106.5(18) 3921.5(7) 8597(5) 2 2 2 2 4 triclinic triclinic triclinic triclinic orthorhombic P-1 P-1 P-1 P-1 Pna21 1.400 1.375 1.476 1.425 1.463 4.74 to 77.068 6.53 to 149.628 3.796 to 148.742 6.454 to 160.188 6.032 to 130.72 0.777 6.219 6.726 6.338 6.172 1.019 1.044 1.063 1.012 1.064 0.0412, 0.0980 0.0710, 0.1919 0.0375, 0.0940 0.0400, 0.0954 0.0732, 0.1352 CCDC Number a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Cryst. syst. Space group ρcalcg (cm3) 2 Θ range (°) μ (mm-1) GOF R1, wR2 (I>2σ (I)) 143 Special refinement details for [LFe3O(PzNHtBu)3Fe(OH)][PF6]. A tert-butyl group of one of the pyrazolate ligands is partially disordered over two positions with occupancies of 39% (C93 and C94) and 61% (C97 and C98). There is significant solvent disorder that could not be fully refined, however electron density for a tetrahydrofuran (O100 and C100-C103), and two partial diethyl ether molecules (refined as C104-C109) were refined isotropically with only partial occupancy. Special refinement details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]2. One of the outersphere triflates was modeled as disordered over two nearly identical positions, with occupancies of 75% (S101 through C101) and 25% (S102 through C102). A ‘SAME’ constraint was used to favor distances and angles of these disordered triflates to the non-disordered one. A partially occupied solvent molecule (likely Et2O) was present; however, due to its position near a symmetry element, its residual electron density was removed via a solvent mask, as opposed to modeling this disordered solvent. Special refinement details for [LFe3O(PzNHtBu)3Fe(O)]. A benzene solvent is positionally disordered over two positions with occupancies of 29% (C106-C111) and 71% (C206-C211). Special refinement details for [LFe3O(PzNHtBu)3Fe(O)][OTf]. Generally, these crystals were of relatively poor quality compared to the other structures obtained; the crystal was twinned with a 13% twinned crystal component. While no disorder had to be modeled in the molecule, the outerphere triflate, or the three additional molecules of THF, the low intensity of high angle diffraction data led to low C–C bond precision. Initially, some carbon atoms in the ligand backbone had highly skewed ellipsoids, which were addressed with SIMU/DELU restraints (on O11, C11, C12, C26, C27, and C42 – C45) or, in one case, an ISOR restraint (C44). 144 Selected bond parameters for structurally characterized compounds 1-3, 5, and 6. Bond Distance (Å) Fe1–O1 Fe2–O1 Fe3–O1 Fe4–O1 Fe4–O2 Fe1–N13 Fe2–N23 Fe3–N33 Fe4–N14 Fe4–N24 Fe4–N34 N13–N14 N23–N24 N33–N34 N15–C72 N25–C82 (N26–C82) N35–C92 Bond Angles (º) N14–Fe4–N24 N24–Fe4–N34 N34–Fe4–N14 N14–Fe4–O2 N24–Fe4–O2 N34–Fe4–O2 O1–Fe4–O2 Torsion Angles (º) Fe1–N13–N14–Fe4 Fe2–N23–N24–Fe4 Fe3–N33–N34–Fe4 Centroid Distances (Å) Fe1|Fe2|Fe3– N14|N24|N34 Fe1|Fe2|Fe3– O11|O21|O31 Fe1|Fe2|Fe3–O1 N14|N24|N34–Fe4 1 2.102(1) 2.109(1) 2.089(1) 1.837(1) 1.937(1) 2.129(1) 2.126(1) 2.120(1) 2.097(1) 2.168(1) 2.111(1) 1.382(1) 1.368(1) 1.386(1) 1.397(2) 1.422(2) 1.393(2) 2-PF6 2.142(3) 2.101(3) 1.952(3) 1.890(3) 1.907(3) 2.107(4) 2.106(3) 2.071(4) 2.091(4) 2.056(4) 2.105(4) 1.373(5) 1.387(5) 1.389(5) 1.417(7) 1.354(7) 1.383(7) 3 2.148(1) 2.002(2) 1.971(1) 1.948(2) 1.879(2) 2.074(2) 2.057(2) 2.017(2) 2.083(2) 2.047(2) 2.059(2) 1.377(2) 1.394(2) 1.397(2) 1.422(3) 1.356(3) 1.350(3) 5 2.139(1) 2.050(2) 1.967(2) 1.965(2) 1.817(2) 2.124(2) 2.084(2) 2.090(2) 2.093(2) 2.104(2) 2.098(2) 1.388(2) 1.387(3) 1.378(3) 1.400(3) 1.366(5) 1.41(2) 1.402(3) 6 2.154(7) 1.927(7) 1.948(7) 2.049(7) 1.795(8) 2.091(9) 2.039(9) 2.015(8) 2.085(9) 2.087(9) 2.100(8) 1.396(12) 1.384(11) 1.387(12) 1.379(14) 1.391(14) 1.341(14) 120.1 122.1 117.8 88.4 91.3 92.2 177.1 119.1 118.8 121.7 91.7 93.5 91.1 177.3 116.6 119.8 122.7 92.6 95.0 92.1 178.7 118.3 123.0 117.5 96.3 92.0 92.8 177.1 113.4 119.6 124.0 97.6 96.3 93.6 176.0 13.9 11.6 13.4 2.7 9.9 12.5 6.2 8.2 11.1 19.4 4.5 13.7 21.9 17.4 2.7 2.050 1.701 1.643 1.685 1.627 1.121 1.110 1.078 1.120 1.122 1.153 0.025 1.105 0.075 1.036 0.120 1.076 0.138 1.012 0.218 145 References 1. 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B.; Takase, M. K.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 14861489; (b) de Ruiter, G.; Carsch, K. M.; Gul, S.; Chatterjee, R.; Thompson, N. B.; Takase, M. K.; Yano, J.; Agapie, T. Angew. Chem. Int. Ed. 2017, 56, 4772-4776; (c) Carsch, K. M.; de Ruiter, G.; Agapie, T. Inorg. Chem. 2017, 56, 9044-9054. 8. (a) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188-12190; (b) Gupta, R.; Taguchi, T.; Lassalle-Kaiser, B.; Bominaar, E. L.; Yano, J.; Hendrich, M. P.; Borovik, A. S. Proc. Natl. Acad. Sci. 2015, 112, 5319-5324; (c) Gupta, R.; MacBeth, C. E.; Young, V. G.; Borovik, A. S. J. Am. Chem. Soc. 2002, 124, 1136-1137; (d) Gupta, R.; Borovik, A. S. J. Am. Chem. Soc. 2003, 125, 13234-13242; (e) MacBeth, C. E.; Golombek, A. P.; Young, V. G.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938-941; (f) Ford, C. L.; Park, Y. J.; Matson, E. M.; Gordon, Z.; Fout, A. R. Science 2016, 354, 741; (g) Park, Y. J.; Matson, E. M.; Nilges, M. J.; Fout, A. R. Chem. Commun. 2015, 51, 5310-5313; (h) Matson, E. M.; Park, Y. J.; Fout, A. R. J. Am. Chem. Soc. 2014, 136, 17398-17401; (i) Gordon, Z.; Drummond, M. J.; Matson, E. M.; Bogart, J. A.; Schelter, E. J.; Lord, R. L.; Fout, A. R. Inorg. Chem. 2017, 56, 4852-4863. 9. Han, Z.; Horak, K. T.; Lee, H. B.; Agapie, T. J. Am. Chem. Soc. 2017, 139, 9108-9111. 10. Horak, K. T. The Design and Synthesis of Transition Metal Complexes Supported by Noninnocent Ligand Scaffolds for Small Molecule Activation. PhD dissertation, California Institute of Technology, Pasadena, California, 2016. 11. de Ruiter, G.; Thompson, N. B.; Lionetti, D.; Agapie, T. J. Am. Chem. Soc. 2015, 137, 14094-14106. 146 12. Arnett, C. H.; Chalkley, M. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 5569-5578. 13. Andris, E.; Navrátil, R.; Jašík, J.; Puri, M.; Costas, M.; Que, L.; Roithová, J. J. Am. Chem. Soc. 2018, 140, 14391-14400. 14. McDonald, A. R.; Que Jr, L. Coord. Chem. Rev. 2013, 257, 414-428. 15. (a) Bordwell, F. G.; Satish, A. V.; Zhang, S.; Zhang, X. M. Using thermodynamic cycles to study reactive intermediates. In Pure Appl. Chem., 1995; Vol. 67, p 735; (b) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441-450; (c) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. 16. (a) Saame, J.; Rodima, T.; Tshepelevitsh, S.; Kütt, A.; Kaljurand, I.; Haljasorg, T.; Koppel, I. A.; Leito, I. The Journal of Organic Chemistry 2016, 81, 7349-7361; (b) Garrido, G.; Koort, E.; Ràfols, C.; Bosch, E.; Rodima, T.; Leito, I.; Rosés, M. The Journal of Organic Chemistry 2006, 71, 9062-9067. 17. Reed, C. J.; Agapie, T. J. Am. Chem. Soc. 2018, 140, 10900-10908. 18. PCET is broadly referred to here as the transfer of a proton and an electron to different parts of a complex (see ref. 17); the precise mechanism, whether concerted (CPET or EPT) or stepwise (either PTET or ETPT), is left ambiguous, as the present experiments cannot differentiate them. 19. Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Chem. Rev. 2012, 112, 4016-4093. 20. Goetz, M. K.; Anderson, J. S. J. Am. Chem. Soc. 2019, 141, 4051-4062. 21. Huang, C.-Y.; Doyle, A. G. J. Am. Chem. Soc. 2012, 134, 9541-9544. 22. Izod, K.; Rayner, D. G.; El-Hamruni, S. M.; Harrington, R. W.; Baisch, U. Angew. Chem. Int. Ed. 2014, 53, 3636-3640. 23. Hagadorn, J. R.; Que, L.; Tolman, W. B. Inorg. Chem. 2000, 39, 6086-6090. 24. Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676-3682. 25. 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Chem. Rev. 2017, 117, 8622-8648. 31. Bordwell, F. G.; Bares, J. E.; Bartmess, J. E.; McCollum, G. J.; Van der Puy, M.; Vanier, N. R.; Matthews, W. S. The Journal of Organic Chemistry 1977, 42, 321-325. 32. Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.; Cornforth, F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCollum, G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006-7014. 33. Taskinen, E.; Nummelin, K. The Journal of Organic Chemistry 1985, 50, 4844-4847. CHAPTER 5 INTERMOLECULAR REACTIVITY OF TETRANUCLEAR FE CLUSTERS VIA PUTATIVE FE–OXO AND –IMIDO INTERMEDIATES 148 ABSTRACT Sterically open pyrazolate-bridged tetranuclear Fe clusters were examined for their reactivity towards oxygen and nitrogen transfer reagents. Addition of iodosylarene to a FeII2FeIII2 cluster produces a one electron oxidized terminal-hydroxide cluster, which ultimately forms an octanuclear μ2-O cluster, upon dehydration. Formation of the terminal hydroxide cluster is considered to occur due to formal hydrogen atom abstraction from a reactive intermediate that could not be extensively characterized (either terminal Fe-oxo or iodosylarene adduct). The one electron reduced pyrazolate cluster is capable of activating electron deficient aryl azides, leading to isolation of clusters bearing an NHAr amide ligand, via a putative Fe-imido moiety. Reactivity studies were also performed with an interstitial fluoride containing Fe cluster. Oxygen atom transfer to a μ4-F containing FeII4 cluster leads to formation of analogous octanuclear μ2-O cluster, or cluster rearrangement to afford a fluoride bound μ4-O cluster. The intermolecular reactivity of these putative Fe-oxo and –imido moieties were limited to decomposition by formal hydrogen atom transfer in solution, which highlights the high reactivity of these complexes, likely due, in part, to the open coordination environment of the unsubstituted bridging pyrazolates. 149 INTRODUCTION Nature utilizes a variety of multinuclear transition metal arrangements to accomplish many catalytic transformations.1 Three general cases can be considered for the roles these clusters possibly have in metalloenzymes: (i) the transfer of electrons, with no direct substrate-cluster binding or interaction (i.e. [4Fe-4S] clusters); (ii) binding and activation of substrate at a single metal site with the auxiliary metals providing a specific structural or electronic environment for the substrate-binding metal; or (iii) binding and activation of substrate across multiple metal sites within the cluster. Often, the precise role of each metal center within an active site cluster is ambiguous, based on the available biochemical data. Therefore, developing our understanding of the reactivity of transition metal clusters related to these metalloenzymes can help establish the functional purposes of their unique multinuclear architectures. A number of synthetic mononuclear transition metal complexes have been studied with the goal of providing insight into biological multinuclear active sites that are thought to activate substrates at a single metal site.2 Due to a number of strategies to tune the reactivity of transition metal complexes through ligand modifications, and the relative ease of structural and spectroscopic characterization of these small molecules, the transition metal chemistry of these systems, which are relevant to biological processes, can be probed in fine detail. However, these studies are not able to establish the possible role of auxiliary metal centers within multinuclear active sites. Examples of multinuclear systems where the reactivity of one metal site can be probed as a function of an auxiliary metal are relatively rare,3 with most examples limited to binuclear complexes.4 A challenge to developing better models of multinuclear active sites, where the level of detail in study can match that of mononuclear systems, lies in the difficulty of producing welldefined multinuclear structures which bear distinct metal coordination environments, 150 specifically ones where substrate binding and activation is limited to a single metal site within the cluster. These ‘site-differentiated’ clusters require a fine balance of stability and reactivity to be suitable for detailed studies of their property-reactivity relationships. Along these lines, our group has developed routes to synthesize a family of transition metal clusters supporting various first row transition metal centers (Mn, Fe, Co, Ni, Cu, Zn) in a robust scaffold that arranges the metal in a site-differentiated fashion capable of supporting these propertyreactivity studies.5 These clusters are all based on a common symmetric trinuclear metal precursor, where a fourth (apical) metal site is introduced through the use of bridging pyrazolates (or imidazolates), and anchored into a tetrahedral metal arrangement through a central interstitial μ4-atom ligand (either O or F).6 The resulting apical metal center is four coordinate, with a trigonal pyramidal geometry, suitable to study its reactivity as a function of the auxiliary coordinatively saturated metal ions. The reactivity of these clusters towards accessing M=E (E = O, N) moieties is appealing to study, due to the implication of terminal Mn=O species in the OEC of photosystem II7 and the possibility of Fe=NR intermediates in nitrogenase FeMo cofactor.8 Understanding the ways in which neighboring metal centers can affect the nature of these reactive intermediates, therefore, has relevance towards our understanding of multinuclear active sites. Furthermore, interest in the chemistry of these types of reactive intermediates has led to many examples of mononuclear systems capable of supporting these moieties;9 significantly less developed is the chemistry of these intermediates in complexes with more than two redox active metals.10 Previous attempts to examine oxygen- or nitrogen-atom transfer reactivity through terminal M=E intermediates were performed by a number of group members on various pyrazolate-bridged clusters of Fe and Mn. Dr. Graham de Ruiter and Kurtis Carsch studied intramolecular oxygen atom transfer reactions between Fe4, Mn4, and Fe3Mn clusters bearing 151 arylpyrazolate ligands (Scheme 1);6b, 11 fast C–H (or C–F) activation of the pendant arene moiety was observed, precluding any reactivity with external substrates, even in large excesses. Attempts to observe intermolecular reactivity led to the development of clusters supported by unsubstituted pyrazolate ligands by Dr. Kyle Horak.12 Early synthetic routes to these clusters relied on FeCl2 as the source of the apical metal, leading to isolation of [LFe3O(Pz)3FeCl][OTf]; removal of the Cl ligand proved challenging, precluding extensive reactivity studies. Exchange of the Cl- ligand for N3- was accomplished, serving as a possible precursor to a reactive Fe-nitride cluster; however, photolysis or thermolysis of this cluster did not lead to activation of the azide ligand. Scheme 1. Related Studies of Tetranuclear Clusters By Previous Members of the Agapie Group6b, 11-12 Herein is an extension of the chemistry of the tetranuclear Fe clusters supported by unsubstituted pyrazolate ligands, towards examining intermolecular reactivity via Fe=O or Fe=NR intermediates. Characterization of reaction products demonstrates decomposition by formal hydrogen atom transfer of these putative intermediates to produce the corresponding Fe–OH and Fe–NHR (R = -aryl or –tosyl) species. 152 RESULTS AND DISCUSSION Synthesis and Characterization of Fe4 Clusters Bearing Unsubstituted Pyrazolate Ligands with a Labile Apical Fe Ligand. Tetranuclear Fe clusters with unsubstituted pyrazolates and a labile trifluoromethanesulfonate (triflate, -OTf) ligand bound to the apical Fe can be prepared in two steps starting from the reported tri-iron cluster LFe3(OAc)(OTf)2 by stirring this cluster with three equivalents potassium pyrazolate (KPz), iodosylbenzene (PhIO), and Fe(II) acetate (Figure 1A). This produces a tetranuclear Fe cluster bearing unsubstituted pyrazolate ligands and an acetate ligand bound to the apical Fe, 1. A partial Xray crystal diffraction dataset was collected to confirm the identity of this cluster; the bond metrics of the Fe–μ4-O distances were similar to the previously synthesized Fe4-chloride Figure 1. (A) Synthesis of tetranuclear Fe clusters with unsubstituted pyrazolate ligands and labile ligands bound to the apical Fe; inset, 1,3,5-triarylbenzene ligand platform (L3-). (B) Preliminary crystal structure of 1 and crystal structure of 2 (C); hydrogen atoms, outersphere anions, and solvent molecules omitted for clarity. 153 cluster of the same oxidation state, consistent with a redox distribution where the apical Fe is trivalent (Table 1). Addition of calcium triflate to this cluster (to precipitate the less soluble calcium acetate) affords isolation of the tetra-iron dication bis-triflate, 2. Structural characterization by XRD displays binding of the a triflate counterion to the apical Fe, with changes in the Fe–μ4-O distances consistent with reduction of the apical Fe to the 2+ oxidation state, and concomitant oxidation of an Fe in the tri-iron core to FeIII. This is supported by the Mössbauer spectrum of 2, which contains parameters for the apical Fe consistent with a five-coordinate high-spin FeII (δ = 0.95 mm/s; |ΔEq| = 2.22 mm/s; Figure 2 and Table 2). Table 1. Selected Bond Distances for Structurally Characterized Pz-Fe4 Clusters Metric (Å) b [LFe3O(Pz)3Fe(Cl)][OTf]12 1a 3-MeCN 2 4 Fe1-O1 2.071(4) 2.17 2.086(9) 2.055(6) 2.143(7) Fe2-O1 2.145(3) 2.15 2.049(8) 2.018(5) 1.977(7) Fe3-O1 2.024(4) 1.91 1.926(9) 1.949(5) 1.989(6) Fe4-O1 1.864(4) 1.87 1.977(8) 1.999(5) 1.959(6) 2.04 2.169(11) 2.155(6) 2.087(8) (Fe4-O2) (Fe4-N2) (Fe4-O2) (Fe4-O2) aPreliminary structure bBold bond distances denote bonds with FeIII centers, the rest are assigned to FeII. Fe4-L 2.339(2) (Fe4-Cl) Cyclic voltammetry of 2 in acetonitrile (MeCN) displays two quasi-reversible peaks, corresponding to the oxidation and reduction of 2, with reduction potentials of -0.89 V (all potentials vs. Fc/Fc+) and -0.13 V (Figure 3). A second quasi-reversible oxidation is observed at 0.57 V, however the return reductive scan produces new electrochemical events, suggesting a putative FeIII4 cluster is accessible, but unstable under the electrochemical conditions. The reduced and oxidized clusters 3 and 4 were accessed through treatment of 2 with cobaltocene (CoCp2) and silver triflate (AgOTf), respectively (Figure 2). Mössbauer spectra of these clusters was consistent with reduction of 2 occurring at an Fe within the tri-iron core, but oxidation 154 occurring at the apical Fe, as opposed to the FeII in the core. This is an unusual observation for these types of tetranuclear clusters, where redox changes are typically restricted to the sixcoordinate metal centers (only when there are changes to are ligand bound to the apical metal are redox changes for that metal observed). The redox distribution of 4 can be reversibly perturbed by displacement of triflate bound to the apical Fe; the solution state Mossbauer spectrum of 4 in MeCN (4-MeCN) displays notable changes to the parameters for the Fe centers, consistent with the loss of high-spin FeII in the tri-iron core and reduction of the apical Fe. Figure 2. Chemical reduction and oxidation of 2 to afford 3 and 4, respectively. Zero appliedfield 57Fe Mössbauer spectra of these clusters in the solid state, and 4 in a solution of MeCN (4-MeCN). 155 Table 2. 57Fe Mössbauer Parameters for Pz-Fe4 Clusters 2 – 4, and 4-MeCN δ (mm/s) |ΔEq| (mm/s) assignment |ΔEq| (mm/s) δ (mm/s) 3 (FeII3FeIII) assignment 4 (FeIIFeIII3) Fe1, Fe2 1.12, 1.11 3.49, 3.10 h.s. FeII Fe1 1.12 2.81 h.s. FeII Fe3 0.53 1.43 h.s. FeIII Fe2, Fe3 0.46, 0.48 0.62, 0.91 h.s. FeIII Fe4 0.99 1.37 h.s. apical FeII Fe4 0.37 0.34 h.s. apical FeIII 2 (FeII2FeIII2) 4-MeCN (FeIIFeIII3) Fe1 1.10 3.15 h.s. FeII Fe1, Fe2, Fe3 0.45, 0.46, 0.46 1.03, 0.72, 0.42 h.s. FeIII Fe2, Fe3 0.47, 0.48 0.58, 0.96 h.s. FeIII Fe4 0.89 1.74 h.s. apical FeII Fe4 0.95 2.22 h.s. apical FeII FeII2FeIII2 FeIIFeIII3 Current FeII3FeIII 50 μA -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 Figure 3. Cyclic voltammetry of 2 in MeCN (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.4 V. 156 Investigations of Pz-Fe4 Clusters with Oxygen Atom Transfer Reagents. With access to these pyrazolate-bridged Fe clusters with a relatively labile ligand (OTf) bound to the site-differentiated Fe, their reactivity towards oxygen atom transfer (OAT) agents were investigated towards performing reactivity studies through a terminal Fe-oxo moiety. Treatment of the [FeII2FeIII2] cluster, 2, with 2-tert-butyl-sulfonyl iodosylbenzene (sPhIO) produces a [FeIIFeIII3] cluster with a terminal hydroxide ligand bound to the apical Fe, 5, based on TOF-MS, 1H NMR spectroscopy, and Mössbauer spectroscopy (Figure 4). This cluster decomposes upon standing, or if concentrated under vacuum, to produce an octanuclear Fe Figure 4. (A) Reactivity of 2 towards OAT reagent sPhIO. (B) Zero applied-field Mössbauer spectrum of 5 in MeCN with parameters for each unique quadrupole doublet. (C) Structure of 6, with hydrogen atoms, outersphere counterions, and solvent molecules omitted for clarity. (D) Zero applied-field Mössbauer spectrum of 6 in MeCN with parameters for each unique quadrupole doublet. 157 cluster with a μ2-O ligand bound to two pyrazolate-bridged [FeIIFeIII3] cluster subunits, 6; this compound was structurally characterized via XRD. Similar results are obtained with different OAT reagents, such as tert-butyl hydroperoxide and tetrabutylammonium meta-periodate. Observation of 5 in solution, likely followed by dehydration to form 6, supports the conclusion that, under the experimental conditions employed here, a 1:1 cluster to sPhIO stoichiometry is dominant, as opposed to consuming half an equivalent of sPhIO to afford 6 directly. Producing 5 from 2 and sPhIO is consistent with formation of a reactive species which undergoes formal hydrogen atom transfer; both terminal metal-oxo and iodosylarene adduct complexes are known to undergo this type of chemistry.13 In an attempt to distinguish between these two possible reactive intermediates, variable temperature 1H NMR experiments were performed. In deuterated solvents (CD2Cl2 or CD3CN) at low temperatures (below -20 °C), mixtures of 5 and a putative intermediate were Figure 5. (A) 1H NMR (400 MHz, CD2Cl2, -40 °C) spectra of 2 (blue), intermediate of 2 and sPhIO before formation of 5 (green), and 5 formed by addition of excess toluene to this intermediate (red). (B) Mössbauer spectrum of the putative intermediate, collected by cold pentane precipitation, with parameters: (i) δ = 1.06 mm/s, |ΔEq| = 3.13 mm/s (blue trace), (ii) δ = 0.40 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.46 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.49 mm/s, |ΔEq| = 1.44 mm/s (green trace). 158 observed (Figure 5). A Mössbauer spectrum of this mixture was obtained, and it ruled out the presence of an FeIV-oxo moiety, with parameters that were best fit with 5 and another [FeIIFeIII3] cluster. However, assigning this species to either possible oxidizing intermediate (FeIII-oxo or -iodosylarene adduct) was inconclusive. Analogous experiments were performed on the other oxidation states of the Pz-Fe4 clusters, 3 and 4. In neither case could a reactive intermediate be observed by NMR at an appreciable concentration; 3 would react completely, even at -40 °C, while 4 would remain mostly unreacted towards sPhIO at low temperatures. The electrochemical and spectroscopic properties of 6 were investigated; however, it was later determined that isolation and recrystallization of samples of 6 contained two species by 1 H NMR with nearly identical resonances (Figure 6). These species could be separated from each other through extraction of 6 (as confirmed by XRD) in MeCN. Attempts to structurally characterize the remaining MeCN precipitate were unsuccessful. Figure 6. 1H NMR (300 MHz, CD2Cl2) of as-isolated and recrystallized 6 (red) and the species separated by trituration in MeCN (green and blue). (Inset) Paramagnetic regions highlighting the differences in peak positions of these two species. Cyclic voltammetry of 6 in CH2Cl2 displays five quasi-reversible redox events separated into closely spaced pairs, attributable to redox changes occurring within each of the tetra-iron 159 subunits (Figure 7). The reduction potentials of these processes, where the charge of the cluster is used to abbreviate oxidation state (0 = [FeII3FeIII]2O, 1+ = [FeII3FeIII]O[FeIII2FeII2], etc.): 0 → 1+, -1.98 V (all potentials vs Fc/Fc+); 1+ → 2+, -1.64 V; 2+ → 3+, -0.75 V; 3+ → 4+, -0.56 V; 4+ → 5+, 0.10 V. The small separation between the pairs of redox events is consistent with relatively little electronic interaction between the individual Pz-Fe4 subunits. The redox event to form the 6+ cluster ([FeIII4]2O) was not observed in the CV, or via squarewave voltammetry. 1+ 2+ 3+ 4+ 5+ Current 0 20 μA -2.5 -2 -1.5 -1 -0.5 0 Potential (V, vs. Fc/Fc+) 0.5 1 1.5 Figure 7. Cyclic voltammetry of 6 in CH2Cl2 (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was 0 V. Reactivity of Pz-Fe4 Clusters with Organic Azides and N-Tosylimino-Transfer Reagent. Due to the challenges in observing evidence for formation of, and reactivity from, a terminal Fe-oxo with 2 and OAT reagents, reactions targeting an Fe-imido species were attempted; it was hypothesized that substituents on the nitrogen atom could be selected to stabilize the Fe-bound intermediate, i.e. having a bulky, electron withdrawing group would 160 slow down decomposition by formal hydrogen atom transfer, and disfavor formation of μ2NR clusters (analogous to 6). 2 proved to be unreactive towards aryl- or alkyl-azides, but the one electron reduced cluster, 3, was competent for activation of relatively electron poor aryl azides. These reactions were typically complete upon warming to room temperature; preliminary structural characterization of reactions between 3 and 2-azideobiphenyl and 3,5trifluoromethyl-phenylazide displayed apical Fe centers with a coordinated aryl amide (– NHAr), as evidenced by the relatively long apical Fe–N distance of ~2.0 Å and small Fe–N– C angle of ~130°, consistent with reported structures of FeIII–NHAr complexes.14 Figure 8. (A) Synthesis of aryl- and tosyl-amide clusters 7 – 9 from 3; (inset) possible Febound intermediates include a terminal Fe-imido or iodo-tosylimino adduct. (B) Truncated preliminary crystal structure of 7. (C) Truncated crystal structure of 8; hydrogen atoms, counterions, and solvent molecules omitted for clarity. 161 Table 3. Selected Bond Distances and Angles for Complexes 7 – 9. Metric (Å)b 7a 8a 9 Fe1-O1 2.10 2.16 2.047(8) Fe2-O1 2.08 2.09 2.044(9) Fe3-O1 2.04 1.99 1.993(7) Fe4-O1 1.87 1.90 2.035(8) Fe4-N1 2.01 1.99 1.856(10) ∠Fe4–N1–C1 131 131 146.8 (∠Fe4–N1–S1) a Preliminary structure. bBold bond distances denote bonds with FeIII centers, the rest are assigned to FeII. Similarly, treatment of 3 with an iodoimino transfer agent, 2-tert-butyl-sulfonyl-N-paratoluenesulfonyl-iminoiodobenzene (sPhNTs), leads to isolation of the tosylamide-bound cluster, 9. Overall, attempts to stabilize Fe-imido clusters with electron withdrawing or sterically bulky aryl groups were unable to lead to observation of this intermediate before formal hydrogen atom transfer to produce the corresponding amide. For reactions with aryl azides, it is possible that azide activation leads to a short-lived aryl-imido cluster, although off-metal reactive species cannot be ruled out, i.e. outersphere electron transfer producing reactive nitrene. Application of these reactions towards amination of cyclohexene were attempted, however no N-transfer product was observed. Reactivity of μ4-F Pz-Fe4 Clusters Towards Oxygen Atom Transfer Reagents and Azide. Due to the challenge of observing evidence of a terminal Fe-oxo or –imido moiety within these clusters, investigations on related clusters bearing a different electronic environment were pursued. Previous investigations on μ4-F bridged clusters demonstrated a significant effect the interstitial ligand has on the properties and reactivity of the Fe4 cluster (see Chapter 2).6d A μ4-F ligand could make the apical Fe more electron deficient, and any resulting terminal Fe-oxo less basic; furthermore, weaker the bonding to the μ4-F may promote 162 multiple bonding to a terminal-oxo by facilitating pseudo-tetrahedral geometry at the apical Fe. Unsubstituted pyrazolate-bridged clusters bearing an interstitial F- ligand were prepared through addition of tetrabutylammonium fluoride to LFe3(OAc)(OTf)2 with a combination of potassium pyrazolate and two equivalents of pyrazole (HPz). The apical Fe is introduced with Fe(N(SiMe3)2)2 and calcium triflate was added to sequester acetate from the initial trinuclear cluster, to afford isolation of a fluoride-bridged FeII4 cluster, 10 (Scheme 2). Scheme 2. Synthesis and Reactivity of Pz-Fe4 Clusters Bearing μ4-F Ligand. Structural characterization of 10 confirms a cluster geometry analogous to the μ4-O versions (Figure 9A). The electrochemistry of this cluster is more complex than 2. In MeCN, only one quasi-reversible feature is observed, corresponding to a reduction potential for the oxidation of 10 at -0.52 V (vs. Fc/Fc+), followed by two irreversible oxidations, which change 163 over multiple CV scans (Figure 10), likely due to decomposition of the FeIIFeIII3 cluster oxidation state. Figure 9. Truncated crystal structures of 10 (A), 11 (B), and 13 (C; preliminary). Current 40 μA -2.5 -1.5 -0.5 0.5 Potential (V, vs. Fc/Fc+) 1.5 2.5 Figure 10. Cyclic voltammetry of 10 in MeCN (2 mM) with [Bu4N][PF6] electrolyte (100 mM) at a scan rate of 200 mV/s with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. The open circuit potential was -0.7 V. The full CV scan (blue) displays two irreversible oxidations, which shift over multiple CV scans. Treatment of 10 with different OAT reagents results in multiple reaction products, with product distributions that depends on the nature of the OAT reagent. Reactions with 164 iodosylbenzene (PhIO), which is polymeric and insoluble, leads to isolation of a μ2-O cluster (11), with a structure analogous to 6 (Figure 9B). Analysis of bond metrics of 6 and 11 display shorter bonds to the μ2-O in 11 (1.7853(6) vs. 1.8079(7) Å), with a shift of the apical Fe out of the equatorial plane (Table 4). Table 4. Selected Bond Distances in Complexes 6 and 11. Metric (Å) 6 ([FeIIFeIII3]2O) 11 ([FeII3FeIII]2O) Fe1-O1/F1 1.942(3) 2.191(2) Fe2-O1/F1 2.096(3) 2.205(2) Fe3-O1/F1 2.176(3) 2.235(2) Fe4-O1/F1 1.925(3) 1.991(2) Fe4-O2 1.8079(7) 1.7853(6) 0.099 0.174 Fe4-N14|N24|N34 Alternatively, tert-butyl hydroperoxide (tBuOOH) or tetrabutylammonium periodate ([Bu4N][IO4]) react very quickly with 10 to produce a species which is assigned to an interstitial O cluster with a terminal F ligand, 12. This is supported by independent synthesis of 12 from 1 and tetrabutylammonium fluoride, as evidenced by 1H NMR spectroscopy. A possible intermediate to 12 could be a terminal Fe-oxo, followed by internal ligand rearrangement to form the more stable interstitial oxo cluster, although direct substitution of the μ4-F by the OAT reagent is also a possible route to this complex. Observation of substitution of the μ4-F in 10 suggested a potential route to tetranuclear clusters with a bridging nitride ligand, via formation of a terminal nitride that undergoes cluster rearrangement. The azide-bound Pz-Fe4 cluster, 13, can be prepared through addition of tetrabutylammonium azide to 10 (Figure 9C). Attempts to thermalyze or photolyze this moiety, however, led to no reaction being observed (Scheme 1). 165 CONCLUSIONS A series of tetranuclear Fe clusters bearing a sterically open coordination environment at the apical Fe could be prepared with a coordinated labile triflate ligand, suitable for reactivity studies towards oxygen and nitrogen atom transfer reagents. For 2, and its one electron reduced form, 3, fast intermolecular decomposition of putative Fe-oxo and Fe-imido moieties (or iodosylarene adducts) occur by formal hydrogen atom transfer, likely from organic solvent. A cluster bearing an interstitial F ligand, 10, demonstrated similar reactivity, forming an octanuclear μ2-O cluster containing fluoride, but was also capable undergoing substitution of this interstitial fluoride ligand with oxygen. Overall, it is challenging to reach a suitable balance between stability and reactivity for these Pz-Fe4 clusters to prepare terminal-oxo or -imido moieties. Subsequent studies demonstrated the necessity of pendant hydrogen bonding donors on the pyrazolate ligands to stabilize an Fe-oxo (Chapter 4). Further investigations could examine strategies to tune these secondary coordination sphere interactions to access a suitably reactive, but still well-defined, Fe-oxo cluster. 166 EXPERIMENTAL DETAILS General Considerations. All reactions were performed at room temperature in an N2filled M. Braun glovebox or using standard Schlenk techniques unless otherwise specified. Glassware was oven dried at 140 ºC for at least 2 h prior to use, and allowed to cool under vacuum. LFe3(OAc)(OTf)2,6a potassium pyrazolate (KPz),6e iodosobenzene (PhIO),15, tetrabutylammonium fluoride ([Bu4N][F]),16, iron(II) bis-hexamethyldisilylamide (Fe(N(SiMe3)2)2),17 2-tert-butyl-sulfonyl-iodosylbenzene (sPhIO),18 2-tert-butyl-sulfonyl-Npara-toluenesulfonyl-iminoiodobenzene (sPhINTs),18 and 3.5-trifluoromethyl-phenylazide,19 were prepared according to literature procedures. All organic solvents were dried by sparging with nitrogen for at least 15 minutes, then passing through a column of activated A2 alumina under positive N2 pressure. 1H spectra were recorded on a Varian 300 MHz spectrometer; variable temperature spectra were recorded on a Varian 400 MHz spectrometer. CD3CN and CD2Cl2 were purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw cycles, and vacuum transferred prior to use. Physical Methods. Mössbauer measurements. Zero field 57Fe Mossbauer spectra were recorded at 80 K in constant acceleration mode on a spectrometer from See Co (Edina, MN) equipped with an SVT-400 cryostat (Janis, Wilmington, WA). The isomer shifts are relative to the centroid of an α-Fe foil signal at room temperature. Samples were prepared by mixing polycrystalline material (20 mg) with boron nitride in a cup fitted with screw cap or freezing a concentrated solution in MeCN. The data were fit to Lorentzian lineshapes using WMOSS (www.wmoss.org). Mössbauer simulation details for all compounds. All spectra were simulated as four pairs of symmetric quadrupole doubles with equal populations and Lorentzian lineshipes. They were refined to a minimum via least squares optimization (13 fitting parameters per spectrum). 167 Signals appearing above 2 mm/s were indicative with the presence of high-spin FeII centers and correspond to species with isomer shifts ~ 1 mm/s. The Mössbauer data were fit to be consistent with our previously reported iron clusters.6a, 11b, 20 The observed Mossbauer parameters are in agreement with related six-coordinate high-spin FeII/FeIII centers.21 Electrochemical measurements. CVs and SWVs were recorded with a Pine Instrument Company AFCBP1 biopotentiostat with the AfterMath software package. All measurements were performed in a three electrode cell, which consisted of glassy carbon (working; ø = 3.0 mm), silver wire (reference) and bare platinum wire (counter), in a N2 filled M. Braun glovebox at RT. The ferrocene/ferrocinium (Fc/Fc+) redox wave were used as an internal standard for all measurements. X-ray crystallography. X-ray diffraction data was collected at 100 K on a Bruker PHOTON100 CMOS based diffractometer (microfocus sealed X-ray tube, Mo Kα (λ) = 0.71073 Å or Cu Kα (λ) = 1.54178 Å). All manipulations, including data collection, integration, and scaling, were carried out using the Bruker APEXII software. Absorption corrections were applied using SADABS. Structures were solved by direct methods using XS (incorporated into SHELXTL) and refined by using ShelXL least squares on Olex2-1.2.7 to convergence. All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and were refined using a riding model. Due to the size of the compounds most crystals included solvent-accessible voids that contained disordered solvent. In most cases the solvent could be modeled satisfactorily. Synthetic Procedures. Synthesis of [LFe3O(Pz)3Fe(OAc)][OTf] (1). 627 mg (0.45 mmol) LFe3(OAc)(OTf)2 was suspended in 8 mL THF in a 20 mL scintillation vial. This suspension was frozen in a liquid N2-cooled cold-well. In a separate vial, a suspension of 152 mg (1.43 mmol) potassium pyrazolate in 2 mL was also frozen. The suspensions were combined, while 168 thawing, and stirred for 10 minutes. Then, 2 mL THF was used to transfer 101 mg (0.46 mmol) iodosylbenzene to the stirring suspension; after 60 minutes, a brown suspension forms. 80 mg (0.465 mmol) Fe(OAc)2 was added to the reaction with 3 mL THF and stirred for 20 hours. The precipitate was collected over a bed of celite on a glass frit, washed with 3 mL THF, and dried under vacuum. A brown solution was collected by washing this solid with CH2Cl2 and filtering it through the frit. Recrystallization via CH2Cl2/Et2O vapor diffusion affords 375 mg (55% yield) 1 as brown crystals. 1H NMR (300 MHz, CD2Cl2) δ 131.9 (br), 83.9 (br), 75.9, 56.6, 50.9, 37.6, 26.2, 22.9, 14.4, 13.6, 11.0, 2.6), 1.3, -2.3 (br), -12.3, -17.8 (br) ppm. Synthesis of [LFe3O(Pz)3Fe][OTf]2 (2). 124 mg Ca(OTf)2 (0.37 mmol) was dissolved in 10 mL MeCN and 290 mg (0.19 mmol) 1 was added as a 10 mL DCM solution. This was stirred for 24 – 48 hours, until the 1H NMR showed complete conversion to [LFe3O(Pz)3Fe][OTf]2. The reaction was dried completely under vacuum, and a green-brown compound was dissolved in 15 mL DCM, filtered over celite, and recrystallized via vapor diffusion of Et2O. 293 mg (0.184 mmol; 96% yield) of 2 were obtained after drying the resulting crystals under vacuum. X-ray quality crystals were obtained by oxidizing 3 with 1 equivalent AgBPh4 in THF. Crystallization via diffusion of pentane into a CH2Cl2 solution of this compound affords crystals of [LFe3O(Pz)3Fe][OTf][BPh4] (1H NMR identical to [LFe3O(Pz)3Fe][OTf]2). 1H NMR (300 MHz, CD3CN) δ 122.0 (br), 71.1, 69.0, 53.3, 50.9, 42.4, 38.4, 16.7, 14.2, 13.9 (br), 12.5, 8.4 (br), 7.2, 3.9, 3.4, -5.5 (br) ppm. 19F NMR (300 MHz, CD3CN) δ -76 ppm. UV-Vis (CH3CN) [ε (M-1 cm-1)]: 246 nm (6.4 x 104), 369 nm (6.7 x 103). Anal. Calcd. (%) for C68H48F6Fe4N12O10S2: C, 51.77; H, 3.03; N, 10.54. Found: C, 51.17; H, 3.11; N, 10.46. Synthesis of [LFe3O(Pz)3Fe][OTf] (3). A suspension of [LFe3O(Pz)3Fe][OTf]2 (2; 43.8 mg, 0.027 mmol) in 2 mL THF was stirred as a THF solution of 5.9 mg CoCp2 (0.031 mmol) was added. After 1 hour, the reaction was dried under vacuum. 4 mL DME was added to the purple 169 solid and stirred for 12 hours. The resulting purple precipitate was collected on a bed of celite, washed with 2 mL DME, dried, and eluted with 2:1 THF/MeCN; crystals of [LFe3O(Pz)3Fe][OTf] (3) were obtained by vapor diffusion of Et2O into this solution. 1H NMR (300 MHz, CD3CN): 101.8 (br), 59.4, 57.4, 35.8, 30.0, 29.5, 19.5, 15.9, 13.3, 7.2, 4.5, 3.9, -1.6 (br), -5.9 (br) ppm. Synthesis of [LFe3O(Pz)3Fe][OTf]3 (4). 45 mg (0.03 mmol) of [LFe3O(Pz)3Fe][OTf]2 (2) was stirred as a suspension in THF. A THF solution of AgOTf (7 mg; 0.03 mmol) was added. After 30 minutes the reaction was dried under vacuum. The resulting solid was filtered over celite with DCM and the filtrate was dried under vacuum. This brown solid was crystallized via vapor diffusion of Et2O into a DCM/MeCN solution to obtain 47 mg [LFe3O(Pz)3Fe][OTf]3 (95% yield). 1H NMR (300 MHz, CD3CN) δ 169.0 (br), 94.7 (br), 81.8, 78.9, 72.9, 64.5, 45.0, 17.3, 10.4, 8.5, 7.9, 5.2, -0.8 (br) ppm. 19F NMR (300 MHz, CD3CN) δ 76 ppm. Synthesis of [LFe3O(Pz)3Fe(OH)][OTf]2 (5). Method A. 10.8 mg (0.007 mmol) 1 was layered in an NMR tube in MeCN with 100 μL of a 0.11 M MeCN solution of tBuOOH (0.011 mmol; prepared by diluting a 3.3 M stock in toluene) in a liquid N2-cooled cold well. The layers were mixed upon thawing. 5 decomposes upon pumping down, or standing for long periods of time. In some experiments, excess (~2 equiv.) DABCO was added, but this had no noticeable effect on the production 5. 1H NMR (300 MHz, CD3CN) δ 145.0 (br), 103.7 (br), 85.6, 66.6, 63.9, 45.5, 30.0 (br), 15.3, 13.6, 11.1, 7.2, -3.0 (br) ppm. Method B. 54 mg (0.034 mmol) 2 was dissolved in 10 mL MeCN and froze in a liquid N2cooled cold well. This was combined with 12 mg (0.035 mmol) sPhIO upon thawing and mixed for 30 minutes. This produces an NMR identical to Method A, with a minor amount of 6, due to decomposition. 170 Synthesis of [(LFe3O(Pz)3Fe)2O][OTf]4 (6) Method A. Concentration of a solution of 5 under vacuum produces a change in the NMR consistent with formation of 6. 1H NMR (300 MHz, CD2Cl2) δ 137.4 (br), 98.9 (br), 80.3, 63.7, 60.2, 50.3, 25.3 (br), 14.6, 13.1, 10.8, 9.9, 2.7, 1.7, 2.6 (br) ppm. Method B. A MeCN solution of 2 was mixed with 0.6 equivalents PhIO for 5 hours at room temperature. The reaction was dried under vacuum and filtered with CH2Cl2. This crude product was recrystallized via CH2Cl2/Et2O vapor diffusion to obtain mostly 6 (with an unidentified impurity, vide infra), with an NMR identical to Method A. Regardless of preparation method, a side product (ca. 30% relative to 6) would be present with nearly identical NMR shifts: 1H NMR (300 MHz, CD2Cl2) δ 137.5 (br), 98.7 (br), 80.6, 63.6, 60.1, 50.9, 25.7 (br), 14.6, 13.2, 9.9, 2.7, 0.84, -2.6 (br) ppm. This could be separated from 6 by stirring a solution of the mixture for 18 hours in MeCN and collecting the filtrate, or via MeCN/Et2O vapor diffusion, where the mother liquor is collected upon precipitation of the side product. Attempts to identify this side product by XRD were unsuccessful. Synthesis of [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7). 22.3 mg (0.014 mmol) 2 was combined with 2.6 mg (0.013 mmol) 2-azidobiphenyl in thawing THF. After 24 hours, no reaction had been observed and 3.8 mg (0.02 mmol) CoCp2 was added to the reaction; the solution turned purple, signifying the formation of 3, and over multiple hours became blue. After stirring 20 hours, the reaction was recrystallized via THF/pentane vapor diffusion to afford crystals of 7. 1H NMR (500 MHz, THF/C6D6) δ 122.2 (br), 78.5 (br), 71.7, 71.1, 53.2, 47.5, 37.7, 33.4, 21.2, 14.0, 13.5, 11.4, 9.4, -16.8 ppm. Synthesis of [LFe3O(Pz)3Fe(3,5-trifluoromethyl-anilide)][OTf] (8). 11.2 mg (0.008 mmol) 3 was dissolved in 3 mL THF and froze in a liquid N2-cooled cold well. This was combined with a 1 mL solution of 2 mg (0.008 mmol) 3,5-trifluoromethyl phenylazide in THF, while thawing. 171 The solution turned blue as it warmed to room temperature, producing a mixture of 8 and side products. This was recrystallized via THF/pentane vapor diffusion. 1H NMR (500 MHz, THF/C6D6) δ 123.4 (br), 78.9 (br), 70.4, 69.5, 50.7, 44.8, 25.7, 17.1, 8.9, 8.5, -6.8, -25.3 (br) ppm. Synthesis of [LFe3O(Pz)3Fe(p-toluenesulfonimide)][OTf] (9). 11.3 mg (0.008 mmol) 3 was dissolved in THF and froze in a liquid N2-cooled cold well. This was combined with 4.5 mg (0.009 mmol) sPhINTs in THF while thawing. The solution turns orange-red upon warming to room temperature to produce 9. This was recrystallized via MeCN/Et2O. 1H NMR (300 MHz, CD3CN) δ 125.7 (br), 91.6 (br), 73.7, 58.0, 55.1, 51.1, 14.0, 13.4, 534, 3.4, -1.1 (br) ppm. Synthesis of [LFe3F(Pz)3Fe][OTf] (10). 763 mg (0.55 mmol) LFe3(OAc)(OTf)2 was suspended in 5 mL THF in a 20 mL scintillation vial. This suspension was frozen in a liquid N2-cooled cold-well. In a separate vial, a suspension of 64 mg (0.60 mmol) potassium pyrazolate and 88 mg (1.30 mmol) pyrazole in 2 mL THF was also frozen. These were combined while thawing, followed by addition of a THF suspension of [Bu4N][F], 156 mg (0.60 mmol). After stirring for 30 minutes, a solution of 210 mg (0.56 mmol) Fe(N(SiMe3)2)2 was added. The solvent was removed under vacuum upon stirring for 20 hours. The solid was washed with Et2O and toluene on a course porosity glass frit with celite; The remaining precipitate was eluted with a 1:1 mixture of THF/MeCN, until the washings from the frit were colorless. The filtrate was dried completely under vacuum, then suspended in 10 mL MeCN. 120 mg (0.34 mmol) Ca(OTf)2 was added to the suspension and stirred for 24 hours. The red orange precipitate was collected over celite, dried under vacuum, then eluted with CH2Cl2 and recrystallized via CH2Cl2/Et2O vapor diffusion to yield 450 mg of 10 as red-orange crystals (56% yield). 1H NMR (300 MHz, CD2Cl2) δ 110.3 (br), 77.2, 72.5, 40.4, 30.0, 27.1, 23.3, 16.7, 11.9, 11.4, 4.5, 4.1, 1.2, -1.3 (br), -30.1 (br) ppm 172 Synthesis of [(LFe3F(Pz)3Fe)2O][OTf]2 (11). Method A. 102 mg (0.07 mmol) 10 was dissolved in CH2Cl2 a connected to the Schlenk line in a Schlenk tube. The reaction was degassed with three freeze-pump-thaw cycles and froze with liquid N2. On the Schlenk line, 10.5 cmHg of O2 was collected in a volumetric gas bulb (36.7 mL; 0.21 mmol O2), which was connected between the Schlenk line and the reaction vessel. This was introduced to the frozen solution for 5 minutes, then thawed -78 °C and stirred briefly before warming to -30 °C. The reaction was left open to the partial atmosphere of O2 for 30 minutes and stirred. Then, the solvent was removed under vacuum and the solid was recrystallized via CH2Cl2/Et2O vapor diffusion to afford crude 11 (with an unidentified side product, vide infra). 1H NMR (300 MHz, CD2Cl2) δ 92.1 (br), 77.3, 76.3, 43.2, 42.7 (br), 38.3, 26.7, 25.2, 24.3, 22.1,16.0, 9.2, 0.9, -1.1, -10.5 (br), -11.9 (br) ppm Method B. 48 mg (0.033 mmol) 10 was dissolved in MeCN and combined with 4.6 mg (0.021 mmol) PhIO; 11 with its side product was produced as the major species after 5 minutes, with a 1H NMR identical to Method A. Similar to 6, regardless of preparation method, a side product would be present with nearly identical NMR shifts: 1H NMR (300 MHz, CD2Cl2) δ 77.9, 77.1, 43.1, 38.6, 27.4, 23.7, 22.7, 16.1 (br), 9.2, 0.89, -1.3, -10.8 (br) ppm. This could be separated from 11 by stirring a solution of the mixture for 18 hours in MeCN and collecting the filtrate, or via MeCN/Et2O vapor diffusion, where the mother liquor is collected upon precipitation of the side product. Attempts to identify this side product by XRD were unsuccessful. Synthesis of [LFe3O(Pz)3Fe(F)][OTf] (12). Method A. 20.2 mg (0.014 mmol) 10 was dissolved in MeCN and froze in a liquid N2 -cooled cold well. This was combined with 6.8 mg (0.016 mmol) [Bu4N][IO4] in MeCN while thawing. The reaction became a brown suspension. After 16 hours, the solvents were removed under vacuum and the residue was washed with Et2O, 173 toluene, THF, and eluted with MeCN to obtain 14.2 mg of a brown solid, corresponding to mostly clean (>90%) 12 (~70% yield). 1H NMR (300 MHz, CD2Cl2) δ 127.7 (br), 77.3 (br), 74.5, 73.9, 59.9, 49.0, 31.1, 28.6, 23.4, 14.4,13.3, 11.4, 3.6 (br), -10.7 (br) ppm Method B. 83.3 mg (0.058 mmol) 10 was dissolved in MeCN and froze in a liquid N2 -cooled cold well. This was combined with 370 μL of a tBuOOH solution (3.3 M in toluene diluted to 5% in MeCN; 0.061 mmol) and 7.1 mg (0.063 mmol) DABCO while thawing. The reaction became a brown suspension. After 12 hours, reaction was filtered. Method C. 36.4 mg (0.024 mmol) of a suspension of 1 in MeCN was combined with 8.7 mg (0.033 mmol) [Bu4N][F] in MeCN to produce 12. Synthesis of LFe3F(Pz)3Fe(N3) (13). 20.9 mg (0.014 mmol) 10 in MeCN was mixed with 4.3 mg (0.015 mmol) [Bu4N][N3] to produce an orange precipitate. After 2 hours, the precipitate was collected over a frit with celite, washed with minimal MeCN, eluted with CH2Cl2, and dried under vacuum to obtain 14.1 mg 13 as a red orange solid (75% yield). 1H NMR (300 MHz, CD2Cl2) δ 111.2 (br), 75.9, 71.5, 41.1, 31.5, 30.8, 29.3, 15.0, 14.2, 11.1, 3.6, -2.5 (br), 18.9 (br) ppm 174 ELECTROCHEMICAL DETAILS Current 50 μA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -1.5 -1 -0.5 Potential (V, vs. Fc/Fc+) 0 0.5 Figure 11. Cyclic voltammograms of [LFe3O(Pz)3Fe][OTf]2 (2, 2 mM) in MeCN with 100 mM [Bu 4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. Current 40 μA 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -2.5 -1.5 -0.5 Potential (V, vs. Fc/Fc+) 0.5 1.5 Figure 12. Cyclic voltammograms of [(LFe3O(Pz)3Fe)2O][OTf]4 (6, 2 mM) in CH2Cl2 with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. 175 Current 30 μA -1.2 50 mV/s 100 mV/s 200 mV/s 300 mV/s 400 mV/s 500 mV/s -1 -0.8 -0.6 -0.4 -0.2 + Potential (V, vs. Fc/Fc ) 0 0.2 Figure 13. Cyclic voltammograms of [LFe3F(Pz)3Fe][OTf] (10, 2 mM) in MeCN with 100 mM [Bu4N][PF6] at various scan rates with glassy carbon, Pt-wire, and Ag-wire as working, counter, and reference electrode, respectively. 176 CRYSTALOGRAPHIC DETAILS Crystal and refinement data for complexes 1 – 3-MeCN, 4, 6 – 11, and 13 1a 2 3-MeCN 4 6 Empirical formula C78F2Fe4N13O8 C95.28H74.56BCl8.56 F3Fe4N12O7S C73H52F3Fe4N15 O7S C71.75H49.5ClF8.34 Fe4N10.5O12.61S3 C141.57H102.25Cl1.7 9F12Fe8N24O28.11 S4 Formula weight (g/mol) 1594.6 2124.57 1445.63 1717.62 3273.34 Radiation CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) a (Å) 14.5023(10) 13.5663(9) 14.6522(6) 15.0846(9) 27.057(2) b (Å) 19.6777(15) 17.3765(12) 19.7336(8) 15.0846(9) 14.2472(9) c (Å) 47.912(4) 20.9073(14) 47.0533(17) 57.491(4) 40.302(3) α (°) 90 84.592(3) 90 90 90 β (°) 93.256(3) 71.407(3) 91.934(2) 90 105.371(3) γ (°) 90 80.890(2) 90 120 90 V (Å3) 13650.8(18) 4607.4(5) 13597.3(9) 11329.2(15) 14980.0(18) 8 2 8 6 4 Cryst. syst. monoclinic triclinic monoclinic trigonal monoclinic Space group C2/c P-1 C2/c P31 C2/c ρcalcg (cm3) 1.552 1.531 1.412 1.511 1.451 2Θ range (°) 7.392 to 69.236 4.656 to 56.496 7.518 to 158.694 4.61 to 158.116 6.776 to 149.24 μ (mm-1) 4.997 0.937 7.573 7.400 7.521 GOF 1.673 1.066 1.151 1.010 1.050 R1 = 0.1227, wR2 = 0.3416 R1 = 0.0694, wR2 = 0.1905 R1 = 0.1808, wR2 = 0.3527 R1 = 0.0674, wR2 = 0.1620 R1 = 0.1030, wR2 = 0.2767 Z R1, wR2 (I>2σ (I)) a Preliminary structure 177 7a 8a 9 10 11 13a Empirical formula C91F3Fe4N13 O9S C80Cl0.14F7Fe4 N12.36O7 C81H55Fe4N13 O9S2 C68H51Cl1.91F 4Fe4N12O6S C134H94F8Fe8 N24O13S2 C74.5H47FFe4 N18.39O3 Formula weight (g/mol) 1873.11 1598.15 1641.90 1531.3 2884.12 1315.48 Radiation MoKα (λ = 0.71073) CuKα (λ = 1.54178) CuKα (λ = 1.54178) CuKα (λ = 1.54178) MoKα (λ = 0.71073) CuKα (λ = 1.54178) a (Å) 18.1272(9) 19.0607(9) 55.590(10) 12.1433(15) 23.1614(12) 45.024(3) b (Å) 16.5232(7) 15.6115(10) 12.4282(12) 23.562(4) 16.6931(8) 12.4123(7) c (Å) 30.3724(15) 47.591(3) 22.632(2) 23.005(3) 33.0662(17) 51.116(3) α (°) 90 90 90 90 90 90 β (°) 103.749(2) 91.952(2) 98.040(7) 99.807(5) 94.436(3) 109.157(5) γ (°) 90 90 90 90 90 90 V (Å3) 8836.5(7) 14153.3(14) 15482(3) 6485.8(16) 12746.3(11) 26984(3) 4 8 8 4 4 16 Cryst. syst. monoclinic monoclinic monoclinic monoclinic monoclinic monoclinic Space group P21/n C2/c C2/c P21/C C2/c C2/c ρcalcg (cm3) 1.408 1.500 1.409 1.568 1.503 1.295 2Θ range (°) 4.7 to 60.842 7.32 to 127.554 7.292 to 116.164 5.41 to 130.452 3.306 to 60.97 4.156 to 129.078 μ (mm-1) 0.739 7.504 6.937 8.690 1.003 7.231 GOF 1.517 1.620 1.115 1.042 1.119 1.032 R1 = 0.1346, wR2 = 0.3671 R1 = 0.1532, wR2 = 0.4118 R1 = 0.1353, wR2 = 0.3364 R1 = 0.0801, wR2 = 0.163 R1 = 0.0891, wR2 = 0.2464 R1 = 0.0945, wR2 = 0.2274 Z R1, wR2 (I>2σ (I)) a Preliminary structure 178 Special refinement details for [LFe3O(Pz)3Fe(OAc)][OTf] (1). This is a preliminary structure; 80% complete dataset. The counterions and solvent molecules were not modeled beyond the XT structure solution. The cluster was kept isotropic. Special refinement details for [LFe3O(Pz)3Fe][OTf][BPh4] (2). This structure contains five co-crystalized CH2Cl2 molecules. One was modeled as partially occupied (28%; C014 and Cl1 and Cl12). Another was positionally disordered, with a common carbon atom (Cl4 and Cl5, 21%, and Cl2 and Cl3, 79%). Relatively high residual Q peak density near the bound triflate, corresponding to a small amount of disorder for this ligand, was present, but could not be modeled adequately. Special refinement details for [LFe3O(Pz)3Fe(MeCN)][OTf] (3-MeCN). The outersphere triflate is disordered over two positions, which is on a symmetry element. Each half triflate was modeled as partially occupied (fixed to 50%) with the CF3 and SO3 groups refined via EXYZ constraints. There were 1 or 2 co-crystalized solvent molecules, which could not be adequately modeled, and were left as isotropic C atoms. Special refinement details for [LFe3O(Pz)3Fe][OTf]3 (4). There are two clusters in the asymmetric unit. All but two of the six triflates are disordered; the disordered triflates bound to the clusters could not be completely modeled, but the sulfur and carbon of the minor component were modeled isotropically. The two outersphere triflates are positionally disordered with occupancies of 64% (S11 through O141) and 36% (S3CA through O142), and 60% (S0AA through O2AA) and 40% (S7CA through O21). Special refinement details for [(LFe3O(Pz)3Fe)2O][OTf]4 (6). There was a significant amount of solvent disorder in the crystal. It could be adequately modeled with CH2Cl2, Et2O, and H2O molecules. They were all modeled as partially occupied: Et2O, 45%; CH2Cl2, 45%; H2O, 55% (O18, O3AA) and 87% (O15). 179 Special refinement details for [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7). This is a preliminary structure with a 37% complete dataset. The outersphere triflate was modeled, along with two co-crystalized THF molecules. There was some remaining solvent that could not be adequately modeled. Everything was modeled isotropically. Special refinement details for [LFe3O(Pz)3Fe(3,5-trifluoromethyl-anilide)][OTf] (8). This is a preliminary structure with a 77% complete dataset. Everything was modeled isotropically. A complete triflate counterion and the trifluoromethyl groups could not be modeled adequately. Special refinement details for [LFe3O(Pz)3Fe(para-toluenesulfonamide)][OTf] (9). Everything but the cluster was modeled isotropically, where no complete solvent molecule or counterion could be modeled. Special refinement details for [LFe3F(Pz)3Fe][OTf] (10). The crystal contained a disordered co-crystalized CH2Cl2 molecule, with partial occupancies of 48% (C3, Cl2, and Cl6) and 52% (C3A, Cl1, and Cl3). 89% complete dataset. Special refinement details for [(LFe3F(Pz)3Fe)2O][OTf]2 (11). The outersphere triflate was disordered over two positions in a ~50/50 ratio; however, each molecule could not be modeled completely. The presence of co-crystalized solvent molecules was suggested by large residual Q peaks, but nothing could be modeled adequately. Special refinement details for LFe3F(Pz)3Fe(N3) (13). This is a preliminary structure with a 71% complete dataset. Everything was left isotropic. A number of co-crystallized MeCN molecules could be modeled. 180 References 1. Bertini, I.; Gray, H. B.; Stiefel, E. I.; Valentine, J. S. Biological Inorganic Chemistry: Structure and Reactivity. 1st ed.; University Science Books: Sausolito, California, 2007. 2. (a) Rittle, J.; Peters, J. C. Proc. Natl. Acad. 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The Design and Synthesis of Transition Metal Complexes Supported by Noninnocent Ligand Scaffolds for Small Molecule Activation. PhD dissertation, California Institute of Technology, Pasadena, California, 2016. 13. (a) Nam, W.; Choi, S. K.; Lim, M. H.; Rohde, J.-U.; Kim, I.; Kim, J.; Kim, C.; Que, J. L. Angew. Chem. Int. Ed. 2003, 42, 109-111; (b) Wang, B.; Lee, Y.-M.; Seo, M. S.; Nam, W. Angew. Chem. Int. Ed. 2015, 54, 11740-11744; (c) MacBeth, C. E.; Golombek, A. P.; Young, V. G.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938-941; (d) Wijeratne, G. B.; Corzine, B.; Day, V. W.; Jackson, T. A. Inorg. Chem. 2014, 53, 7622-7634. 14. (a) Mankad, N. P.; Müller, P.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 4083-4085; (b) Iovan, D. A.; Betley, T. A. J. Am. Chem. Soc. 2016, 138, 1983-1993; (c) Spasyuk, D. M.; Carpenter, S. H.; Kefalidis, C. E.; Piers, W. E.; Neidig, M. L.; Maron, L. Chemical Science 2016, 7, 5939-5944; (d) Sazama, G. T.; Betley, T. A. Inorg. 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In Applications of Physical Methods to Inorganic and Bioinorganic Chemistry, Scott, R. A.; Lukehart, C. M., Eds. John Wiley & Sons: West Sussex, England, 2007; pp 243-269. NMR DATA 183 CHAPTER 2 Figure 1. 1H NMR spectrum (300 MHz) of potassium 3-phenyl pyrazolate in CD3CN. Figure 2. 13C {1H} NMR spectrum (300 MHz) of potassium 3-phenyl pyrazolate in CD3CN. Figure 3. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf] (1) in CD2Cl2. 184 Figure 4. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf] (1) in CD2Cl2. Figure 5. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf]2 (2) in CD2Cl2. Figure 6. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe][OTf]2 (2) in CD2Cl2. 185 Figure 7. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3) in CD3CN. Figure 8. 19F NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3) in CD3CN. Figure 9. 1H NMR spectrum (300 MHz) of [LFe3F(PhPz)3Fe(NO)][OTf] (1-NO) in CD2Cl2. 186 CHAPTER 3 Figure 10. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf] (1-[OTf]) (top) and [LFe3O(Pz)3Mn][[BArF4]] (1-[BArF4]; bottom) in CD3CN. Figure 11. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf] (1-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4] (1-[BArF4]; bottom) in CD3CN. Figure 12. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; bottom) in CD3CN. 187 Figure 13. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]2 (2-[OTf]; top) and [LFe3O(Pz)3Mn][BArF4]2 (2-[BArF4]; bottom) in CD3CN. Figure 14. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]3 in CD2Cl2 (3-[OTf]; top), [LFe3O(Pz)3Mn][BArF4]3 in THF/C6D6 with three equivalents tetrabutylammonium trifluoromethanesulfonate (400 MHz, middle), and [LFe3O(Pz)3Mn][BArF4]3 in THF/C6D6 (500 MHz) (3-[BArF4]; bottom). Figure 15. 19F NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn][OTf]3 (3-[OTf]) in CD2Cl2 (top) and [LFe3O(Pz)3Mn][BArF4]3 (3-[BArF4]; bottom) in THF/C6D6 (400 MHz). 188 Figure 16. 1H NMR spectra (400 MHz) of [LFe3O(Pz)3Mn(OH)] (5) in THF/C6D6 [250 mM H2O]. Figure 17. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Mn(OH)][OTf] (6-[OTf]; top) and [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; middle) in CD3CN. 1H NMR spectrum (400 MHz) of [LFe3O(Pz)3Mn(OH)][BArF4] (6-[BArF4]; bottom) in THF/C6D6 [250 mM H2O]. Figure 18. 1H NMR spectrum (400 MHz) of [LFe3O(Pz)3Mn(OH)][BArF4]2 (7-[BArF4]) in THF/C6D6 [250 mM H2O]. 189 CHAPTER 4 Figure 19. 1H NMR spectrum (300 MHz) of 2-tert-butyl-isoxazolium tetrafluoroborate in (CD3)2CO. Figure 20. 1H NMR spectrum (400 MHz) of N-tert-butyl-1H-pyrazol-3-amine in CD2Cl2. Figure 21. 13C{1H} NMR spectrum (100 MHz) of N-tert-butyl-1H-pyrazol-3-amine in CD2Cl2. 190 Figure 22. 1H NMR spectrum (300 MHz) of LFe3O(PzNHtBu)3Fe(OH) (1) in C6D6. The sharp signal ~ 95 ppm is a spectral artifact. Figure 23. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2) in CD3CN. The sharp signal ~ 90 ppm is a spectral artifact. Figure 24. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3) in CD2Cl2. 191 Figure 25. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4) in CD2Cl2. Figure 26. 1H NMR spectrum (300 MHz) of LFe3O(PzNHtBu)3Fe(O) (5) in C6D6. Figure 27. 1H NMR spectrum (400 MHz) of [LFe3O(PzNHtBu)3Fe(O)][OTf] (6) in THF/C6D6. 192 Figure 28. 1H NMR spectrum (300 MHz) of [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7) in 1:1 CD3CN/CD2Cl2. 193 CHAPTER 5 Figure 29. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(OAc)][OTf] (1) in CD2Cl2. Figure 30. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf]2 (2) in CD3CN. Figure 31. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf] (3) in CD3CN. 194 Figure 32. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe][OTf]3 (4) in CD2Cl2. Figure 33. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN) in CD3CN. Figure 34. 19F NMR spectra (300 MHz) of [LFe3O(Pz)3Fe][OTf]3 (4) in CD2Cl2 (left) and [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN) in CD3CN (right). 195 Figure 35. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(OH)][OTf]2 (5) in CD3CN. Figure 36. 1H NMR spectrum (300 MHz) of [(LFe3O(Pz)3Fe)2O][OTf]4 (6) in CD2Cl2. Figure 37. 1H NMR spectrum (500 MHz) of [LFe3O(Pz)3Fe(2-phenyl-anilide)][OTf] (7) in THF/C6D6. 196 Figure 38. 1H NMR spectrum (500 MHz) of reaction mixture containing [LFe3O(Pz)3Fe(3,5trifluoromethyl-anilide)][OTf] (8) in THF/C6D6. Figure 39. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(para-tolylsulfonamide)][OTf] (9) in CD3CN. Figure 40. 1H NMR spectrum (300 MHz) of [LFe3F(Pz)3Fe][OTf] (10) in CD2Cl2. 197 Figure 41. 1H NMR spectrum (300 MHz) of [(LFe3F(Pz)3Fe)2O][OTf]2 (11) in CD2Cl2. Figure 42. 1H NMR spectrum (300 MHz) of [LFe3O(Pz)3Fe(F)][OTf] (12) in CD2Cl2. Figure 43. 1H NMR spectrum (300 MHz) of LFe3N(Pz)3Fe(N3) (13) in CD2Cl2. 57 FE MÖSSBAUER DATA 199 CHAPTER 2 Simulation details for [LFe3(PhPz)3FFe][OTf] (1): The Mössbauer spectrum of 1 features an intense signal around 3 mm/s characteristic of a high-spin Fe(II) center. Fitting a quadrupole doublet for this peak led to a model that accounted for ~75% of the overall signal, with a second doublet satisfactorily accounting for the rest of the signal (Figure 1). The final refinement spli t the 75%-abundant high-spin Fe(II) signal into three equally populated doublets (Figure 2). Figure 1. Mössbauer spectrum of 1 (black dots) fit with two doublets in ~3:1 ratio refined with parameters: δ = 1.156 mm/s; ΔEq = 3.260 mm/s (blue trace) and δ = 0.925 mm/s; ΔEq = 1.171 mm/s (green trace). The overall fit is the gray trace. δ (mm/s) 1.156 |ΔEq| (mm/s) 3.424 Г 0.25 Rel. % 25 1.152 3.176 0.23 25 1.178 3.065 0.50 25 0.951 1.099 0.70 25 Figure 2. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe][OTf] (1). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent signals assigned to the high-spin FeII of the tri-iron core and the green trace is assigned to the apical high-spin FeII. 200 Simulation details for [LFe3(PhPz)3FFe][OTf]2 (2): The Mössbauer spectrum of 2 was fit by first fitting the signal at 3 mm/s to high-spin Fe(II) (Figure 3A); this fit accounted for ~50% the total spectrum. The remaining spectrum was fit with two nearly equal doublets (Figure 3B); other combinations of this signal to arrive at alternative parameters for these two signals could not be satisfactorily modeled, so these are not included. The final fit split the 50%-abundant Fe(II) signal into two equal doublets (Figure 4). A) B) Figure 3. Mössbauer spectrum of 2 (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.132 mm/s; ΔEq = 3.101 mm/s (blue trace). (B) Mössbauer spectrum of 2 with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.462 mm/s; ΔEq = 1.213 mm/s (orange trace), and δ = 0.928 mm/s; ΔEq = 1.522 mm/s (green trace). δ (mm/s) 1.138 |ΔEq| (mm/s) 3.297 Г 0.35 Rel. % 25 1.122 2.899 0.39 25 0.479 1.211 0.50 29 0.987 1.504 0.70 21 Figure 4. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe][OTf]2 (2). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent signals assigned to the high-spin FeII of the tri-iron core, the orange trace is assigned to high- 201 spin FeIII in the tri-iron core, and the green trace is assigned to the apical high-spin FeII. The best fit was obtained by having a slight deviation from four equally abundant Fe centers. Simulation details for [LFe3(PhPz)3FFe(CH3CN)][OTf]3 (3): The Mössbauer spectrum of 3 contains three easily observable features, which was first approximated by fitting two doublets (Figure 5A). This fit shows inadequate modeling for the signal around 2 mm/s, so a third doubled was modeled in (Figure 5B-D). This fit led to three signals in an approximately 1:1:2 ratios. Of the possible distribution of these three signals, the fit in Figure 5D is favored, because it has isomer shifts self-consistent for the two Fe(III) centers (0.4 – 0.5 mm/s) and the Fe(II) center (1.00 – 1.15 mm/s) of the ‘tri-iron’ core of our related iron clusters presented here and previously reported. Furthermore, the third signal is most consistent with a high-spin five-coordinate Fe(II) (~0.1 mm/s lower than the corresponding six-coordinate Fe(II) signal), which is in agreement of the coordination of the apical Fe(II) in the crystal structure of 3. The final fit was refined by splitting the 50%-abundant Fe(III) signal into two equal signals (Figure 6). A) B) C) D) Figure 5. Mössbauer spectrum of 3 (black dots) (A) fit with two signals (gray trace) refined with parameters: δ = 1.091 mm/s; ΔEq = 2.841 mm/s (blue trace), and δ = 0.471 mm/s; ΔEq = 1.253 mm/s (orange trace). (B) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 202 ratio (gray trace) with the parameters: δ = 1.261 mm/s; ΔEq = 2.797 mm/s (blue trace), δ = 1.137 mm/s; ΔEq = 2.216 mm/s (green trace), and δ = 0.374 mm/s; ΔEq = 1.433 mm/s (orange trace). (C) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 ratio (gray trace) with the parameters: δ = 1.106 mm/s; ΔEq = 3.050mm/s (blue trace), δ = 1.083 mm/s; ΔEq = 2.098 mm/s (green trace), and δ = 0.447 mm/s; ΔEq = 1.269 mm/s (orange trace). (D) Mössbauer spectrum of 3 fit with three signals in a 1:1:2 ratio (gray trace) with the parameters: δ = 1.120 mm/s; ΔEq = 3.071 mm/s (blue trace), δ = 0.981 mm/s; ΔEq = 2.486 mm/s (green trace), and δ = 0.498 mm/s; ΔEq = 1.186 mm/s (orange trace). δ (mm/s) 1.120 |ΔEq| (mm/s) 3.071 Г 0.51 Rel. % 25 0.492 0.889 0.46 25 0.487 1.449 0.42 25 0.981 2.486 0.72 25 Figure 6. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(CH3CN)][OTf]3 (3). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue trace represents the signal assigned to the high-spin FeII of the tri-iron core, the orange traces are assigned to high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical high-spin FeII. 203 Simulation details for LFe3(PhPz)3FFe(NO) (5-NO): The Mössbauer spectrum of 5-NO displays three readily distinguished signals. The major two were initially fit with a single doublet, which had parameters consistent with high-spin Fe(II) (Figure 7); the remaining signal was fit in a 3:1 ratio relative to the first signal. The final fit split the high-spin Fe(II) doublet into three equal doublets (Figure 8). Figure 7. Mössbauer spectrum of 5-NO (black dots) fit with two doublets in a ~3:1 ratio (gray trace) with the parameters: δ = 1.149 mm/s; ΔEq = 3.177 mm/s (blue trace), and δ = 0.906 mm/s; ΔEq = 1.651 mm/s (purple trace). δ (mm/s) 1.148 |ΔEq| (mm/s) 3.561 Г 0.37 Rel. % 25 1.150 3.172 0.33 25 1.153 2.753 0.39 25 0.947 1.629 0.63 25 Figure 8. Zero applied field 57Fe Mössbauer spectrum of LFe3F(PhPz)3Fe(NO) (5-NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core and the purple trace is assigned to the apical {FeNO}8. 204 Simulation details for [LFe3(PhPz)3FFe(NO)][OTf] (1-NO): The Mössbauer spectrum of 1-NO contains four distinguishable signals (one being a shoulder on the peak below 0 mm/s). These four signals were fit to two doublets based on their relative intensities and had an approximate 3:1 ratio (Figure 9). The final fit refined the intense Fe(II) doublet into three equal signals (Figure 10). Figure 9. Mössbauer spectrum of 1-NO (black dots) fit with two doublets in a ~3:1 ratio (gray trace) with the parameters: δ = 1.139 mm/s; ΔEq = 3.308 mm/s (blue trace), and δ = 0.609 mm/s; ΔEq = 1.631 mm/s (green trace). δ (mm/s) 1.152 |ΔEq| (mm/s) 3.592 Г 0.26 Rel. % 25 1.150 3.403 0.22 25 1.162 3.229 0.31 25 0.630 1.669 0.32 25 Figure 10. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf] (1NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core and the green trace is assigned to the apical {FeNO}7. 205 Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]2 (2-NO): The Mössbauer spectrum of 2-NO displays four readily-distinguished signals. The signal above 3 mm/s was initially fit as high-spin Fe(II), and this doublet accounts for 50% of the overall spectrum (Figure 11A). The remaining signal was fit as two equally abundant doublets in three different ways (Figures 11B-D). The fit in Figure S39B gives unreasonable isomer shifts of -0.1 and 0.2 mm/s (orange and green traces, respectively). The fits in Figures 11C and 11D are only slightly different, but the fit in Figure 11D gives parameters that are more self-consistent with isomer shift values for the Fe(III) center of the ‘tri-iron core’ (0.4 mm/s; orange trace) and the {FeNO}7 center of the fluoride-bridged tetranuclear iron clusters (0.6 mm/s; green trace; see parameter refinement for 1-NO). The final fit split the 50%-abundant Fe(II) signal into two doublets (Figure 12). A) B) C) D) Figure 11. Mössbauer spectrum of 2-NO (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.132 mm/s; ΔEq = 3.211 mm/s (blue trace). (B) Mössbauer spectrum of 2-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = -0.111 mm/s; ΔEq = 0.165 mm/s (orange trace), and δ = 1.158 mm/s; ΔEq = 0.270 mm/s (green trace). (C) Mössbauer spectrum of 2NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.577 mm/s; ΔEq = 1.456 mm/s (green trace), and δ = 0.469 mm/s; ΔEq = 1.094 206 mm/s (orange trace). (D) Mössbauer spectrum of 2-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.616 mm/s; ΔEq = 1.387 mm/s (green trace), and δ = 0.435 mm/s; ΔEq = 1.171 mm/s (orange trace). δ (mm/s) 1.121 |ΔEq| (mm/s) 3.309 Г 0.30 Rel. % 25 1.150 3.026 0.46 25 0.435 1.171 0.41 25 0.616 1.387 0.42 25 Figure 12. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]2 (2NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue traces represent the signal assigned to the high-spin FeII of the tri-iron core, the orange trace is assigned to the high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical {FeNO}7. Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]3 (3-NO): The Mössbauer spectrum of 3-NO displays three major features (instead of the expected eight for four Fe centers). The peak around 3 mm/s was modeled as a high-spin Fe(II) with parameters consistent with other Fe(II) centers in the ‘tri-iron core’ of the tetranuclear clusters; this fit accounted for 25% of the total signal (Figure 13A). Subtracting this doublet from the data, the remaining signal could be fit reasonably well with either one or two equally abundant quadrupole doublets (Figures 13B and 13C), however these fits were ruled out since they were inconsistent with our crystallographic analysis of 3-NO. There are many possible ways to fit 3 doublets in this residual signal; below are shown three fits that give reasonable fit parameters for two highspin Fe(III) centers and a high-spin five-coordinate {FeNO}7 (Figure 13D-F). The fit in Figure 13F gave parameters the most self-consistent within this series of clusters, and was used for the final refinement (Figure 14). 207 A) B) C) D) E) F) Figure 13. Mössbauer spectrum of 3-NO (black dots) (A) fit with a single doublet refined with parameters consistent with high-spin Fe(II): δ = 1.087 mm/s; ΔEq = 3.100 mm/s (blue trace). (B) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and 208 fit with a single quadrupole doublet (gray trace): δ = 0.501 mm/s; ΔEq = 1.319 mm/s. (C) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and two equally abundant signals fit (gray trace): δ = 0.354 mm/s; ΔEq = 1.291 mm/s (orange trace), and δ = 0.643 mm/s; ΔEq = 1.314 mm/s (green trace). (D) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.488 mm/s; ΔEq = 0.819 mm/s (solid orange trace), and δ = 0.499 mm/s; ΔEq = 1.292 mm/s (dashed orange trace), and δ = 0.512 mm/s; ΔEq = 1.750 mm/s (green trace). (E) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.387 mm/s; ΔEq = 1.058 mm/s (solid orange trace), and δ = 0.409 mm/s; ΔEq = 1.503 mm/s (dashed orange trace), and δ = 0.719 mm/s; ΔEq = 1.321 mm/s (green trace). (F) Mössbauer spectrum of 3-NO with this Fe(II) doublet subtracted (black dots) and three equally abundant signals fit (gray trace): δ = 0.486 mm/s; ΔEq = 0.840 mm/s (solid orange trace), and δ = 0.395 mm/s; ΔEq = 1.486 mm/s (dashed orange trace), and δ = 0.620 mm/s; ΔEq = 1.512 mm/s (green trace). δ (mm/s) 1.087 |ΔEq| (mm/s) 3.100 Г 0.59 Rel. % 25 0.484 0.868 0.49 25 0.402 1.471 0.41 25 0.615 1.507 0.44 25 Figure 14. Zero applied field 57Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]3 (3NO). The data (black dots) was fit to four Fe quadrupole doublets (gray trace). The blue trace represents the signal assigned to the high-spin FeII of the tri-iron core, the orange traces are assigned to the high-spin FeIII in the tri-iron core, and the green trace is assigned to the apical {FeNO}7. Simulation details for [LFe3(PhPz)3FFe(NO)][OTf]3[SbCl6] (4-NO): The Mössbauer spectrum of 4-NO displays two broad features, which were adequately fit as a single quadrupole doublet (Figure 15). Since it is likely that the Fe(III) signals are overlapping with the {FeNO}7 signal (see Mössbauer spectra of 2-NO and 3-NO), modeling the four separate Fe centers is not included. This spectrum is consistent, however, with our assignment of 4NO as a cluster containing no Fe(II) centers, since there the characteristic signal around 3 mm/s is absent. 209 δ (mm/s) 0.471 |ΔEq| (mm/s) 1.420 Г 0.61 Rel. % 100 57 Figure 15. Zero applied field Fe Mössbauer spectrum of [LFe3F(PhPz)3Fe(NO)][OTf]3[SbCl6] (4-NO). The data (black dots) was fit to a single Fe quadrupole doublet (gray trace). We postulate that the signal for the apical {FeNO}7 is overlapping with the doublet(s) for the high-spin Fe(III) in the ‘tri-iron core’. 210 CHAPTER 3 Simulation details for 1-[OTf]: The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 16). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 17). Figure 16. Mössbauer spectrum of 1-[OTf] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.543 mm/s; ΔEq = 1.366 mm/s (orange trace) and δ = 1.126 mm/s; ΔEq = 3.294 mm/s (blue trace). δ (mm/s) 1.135 |ΔEq| (mm/s) 3.513 Rel. % 1.127 3.016 33 0.563 1.318 33 33 Figure 17. Zero applied field Mössbauer spectrum of 1-[OTf] (black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned as high-spin Fe(III). 211 Simulation details for 2-[OTf]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 18). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(II). The final fit split the large doublet into two equal signals (Figure 19). Figure 18. Mössbauer spectrum of 2-[OTf] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.469 mm/s; ΔEq = 0.740 mm/s (orange trace) and δ = 1.124 mm/s; ΔEq = 2.926 mm/s (blue trace). δ (mm/s) 1.119 |ΔEq| (mm/s) 2.931 Rel. % 0.468 0.578 33 0.423 0.913 33 33 Figure 19. Zero applied field Mössbauer spectrum of 2-[OTf] (black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). Simulation details for 3-[OTf]: The spectrum displays a single quadrupole doublet signal. Although three Fe(III) signals are expected, the best fit was obtained with only one set of parameters (Figure 20). 212 δ (mm/s) 0.443 |ΔEq| (mm/s) 0.804 Rel. % 100 Figure 20. Zero applied field Mössbauer spectrum of 3-[OTf] (black dots) fit to a single quadrupole doublet (orange trace); this signal is assigned to the high-spin Fe(III) centers in the cluster. Simulation details for 1-[BArF4]: The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 21). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 22). Figure 21. Mössbauer spectrum of 1-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.556 mm/s; ΔEq = 1.267 mm/s (orange trace) and δ = 1.115 mm/s; ΔEq = 3.153 mm/s (blue trace). 213 δ (mm/s) 1.119 |ΔEq| (mm/s) 3.460 Rel. % 33 1.099 2.860 33 0.570 1.266 33 Figure 22. Zero applied field Mössbauer spectrum of 1-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned to high-spin Fe(III). Simulation details for 2-[BArF4]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 23). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(II). The final fit split the large doublet into two equal signals (Figure 24). Figure 23. Mössbauer spectrum of 2-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.485 mm/s; ΔEq = 0.746 mm/s (orange trace) and δ = 1.132 mm/s; ΔEq = 2.858 mm/s (blue trace). 214 δ (mm/s) 1.128 |ΔEq| (mm/s) 2.858 Rel. % 0.487 0.999 33 0.478 0.540 33 33 Figure 24. Zero applied field Mössbauer spectrum of 2-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). Simulation details for 3-[BArF4]: The spectrum displays a single quadrupole doublet signal. Although three Fe(III) signals are expected, the best fit was obtained with only one set of parameters (Figure 25). δ (mm/s) 0.444 |ΔEq| (mm/s) 0.800 Rel. % 100 Figure 25. Zero applied field Mössbauer spectrum of 3-[BArF4] (THF solution [250 mM H2O]; black dots) fit with a single quadrupole doublet assigned to high-spin Fe(III) centers. 215 Simulation details for 5: The spectrum displays an apparently asymmetric quadrupole doublet signal. The data could be fit to a single quadrupole doublet (Figure 26). The final fit split this signal into three equally abundant Fe(II) centers, to account for the asymmetry of the doublet (Figure 27). Figure 26. Mössbauer spectrum of 5 (black dots) fit with a single quadrupole doublet with parameters δ = 1.113 mm/s; ΔEq = 2.985 mm/s (blue trace). δ (mm/s) 1.115 |ΔEq| (mm/s) 3.399 Rel. % 33 1.115 2.947 33 1.078 2.417 33 Figure 27. Zero applied field Mössbauer spectrum of 5 (black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II). Simulation details for 6-[BArF4]: The spectrum displays four discernable peaks corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 28). The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 29). 216 Figure 28. Mössbauer spectrum of 6-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.525 mm/s; ΔEq = 0.769 mm/s (orange trace) and δ = 1.075 mm/s; ΔEq = 2.893 mm/s (blue trace). δ (mm/s) 1.093 |ΔEq| (mm/s) 3.091 Rel. % 33 1.083 2.576 33 0.534 0.756 33 Figure 29. Zero applied field Mössbauer spectrum of 6-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the orange trace is assigned to high-spin Fe(III). Simulation details for 7-[BArF4]: The spectrum displays three discernable peaks, with a shoulder on the lowest peak, corresponding to two quadrupole doublets in ~ 1:2 ratio (Figure 30). The parameters of the more intense peak are consistent with a high-spin Fe(III) assignment, while the smaller doublet displays parameters consistent with high-spin Fe(III). The final fit split the large doublet into two equal signals (Figure 31). 217 Figure 30. Mössbauer spectrum of 7-[BArF4] (black dots) fit with two doublets in a ~1:2 ratio (gray trace) with parameters δ = 0.447 mm/s; ΔEq = 0.790 mm/s (orange trace) and δ = 1.109 mm/s; ΔEq = 3.018 mm/s (blue trace). δ (mm/s) 1.096 |ΔEq| (mm/s) 3.033 Rel. % 33 0.434 0.547 33 0.462 1.019 33 Figure 31. Zero applied field Mössbauer spectrum of 7-[BArF4] (THF solution [250 mM H2O]; black dots) fit with three quadrupole doublets. The blue trace is assigned to high-spin Fe(II) and the orange traces are assigned to high-spin Fe(III). 218 CHAPTER 4 Simulation details for LFe3O(PzNHtBu)3Fe(OH) (1): The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the more intense peak are consistent with a high-spin Fe(II) assignment, while the smaller doublet could be fit a number of ways, two of which are shown here (Figure 32). Figure 32B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into three equal signals (Figure 33). A) B) Figure 32. Mössbauer spectrum of 1 (black dots) fit with two doublets in a ~1:3 ratio (gray trace) with parameters (A) δ = 1.035 mm/s; ΔEq = 3.255 mm/s (blue trace) and δ = 0.756 mm/s; ΔEq = 1.993 mm/s (green trace) and (B) δ = 1.107 mm/s; ΔEq = 3.126 mm/s (blue trace) and δ = 0.436 mm/s; ΔEq = 2.663 mm/s (green trace). δ (mm/s) 1.099 |ΔEq| (mm/s) 3.519 Rel. % 25 1.109 3.029 25 1.134 2.640 25 0.407 2.706 25 Figure 33. Zero applied field Mössbauer spectrum of 1 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II) and the green trace is assigned as Fe(III). 219 Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf] (2): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways (Figures 34). Figure 34B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into two equal signals (Figure 35). A) B) Figure 34. Mössbauer spectrum of 2 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.020 mm/s; ΔEq = 3.197 mm/s (blue trace), δ = 0.502 mm/s; ΔEq = 0.871 mm/s (orange trace) and δ = 0.612 mm/s; ΔEq = 1.832 mm/s (green trace) and (B) δ = 1.107 mm/s; ΔEq = 3.126 mm/s (blue trace), δ = 0.504 mm/s; ΔEq = 0.857 mm/s (orange trace) and δ = 0.425 mm/s; ΔEq = 2.172 mm/s (green trace). δ (mm/s) 1.123 |ΔEq| (mm/s) 3.197 Rel. % 25 1.100 2.757 25 0.515 0.809 25 0.406 2.171 25 Figure 35. Zero applied field Mössbauer spectrum of 2 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned as Fe(III). 220 Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3): The spectrum displays three discernable peaks corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the less intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways. Based on the Mossbauer parameters for the other cluster oxidation states (Figures 33 and 35), a fit was performed to obtain selfconsistent parameters (i.e. making apical Fe(III) doublet with second largest quadrupole splitting). This fit led to parameters that are in good agreement for a high-spin Fe(II), two high-spin Fe(III), and the unique apical Fe(III) environment (Figure 36). δ (mm/s) 1.153 |ΔEq| (mm/s) 2.829 Rel. % 25 0.469 0.613 25 0.459 1.056 25 0.381 1.693 25 Figure 36. Zero applied field Mössbauer spectrum of 3 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned as Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(OH)][OTf]3 (4): The spectrum displays two peaks of different intensities and shapes, consistent with multiple overlapping quadrupole doublets. In order to obtain self-consistent parameters, the spectrum was fit to four quadrupole doublets, with the doublet with the highest quadrupole splitting assigned to the apical Fe(III) (Figure 37). 221 δ (mm/s) 0.449 |ΔEq| (mm/s) 0.641 Rel. % 25 0.543 1.145 25 0.363 1.166 25 0.411 1.712 25 Figure 37. Zero applied field Mössbauer spectrum of 4 (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III) and the green trace is assigned as Fe(III). Simulation details for LFe3O(PzNHtBu)3Fe(O) (5): The spectrum displays six discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment, and the remainder of the signal can be fit a number of ways (Figure 38). Figure 38C gives parameters most consistent with an apical Fe(III) and high-spin Fe(III) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 39). 222 A) B) C) Figure 38. Mössbauer spectrum of 5 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.106 mm/s; ΔEq = 3.023 mm/s (blue trace), δ = 1.522 mm/s; ΔEq = 0.868 mm/s (orange trace) and δ = -0.569 mm/s; ΔEq = 1.042 mm/s (green trace)., (B) δ = 1.106 mm/s; ΔEq = 3.029 mm/s (blue trace), δ = 0.952 mm/s; ΔEq = 2.010 mm/s (orange trace) and δ = -0.003 mm/s; ΔEq = 2.178 mm/s (green trace), and (C) δ = 1.103 mm/s; ΔEq = 3.027 mm/s (blue trace), δ = 0.518 mm/s; ΔEq = 1.146 mm/s (orange trace) and δ = 0.432 mm/s; ΔEq = 3.051 mm/s (green trace). 223 δ (mm/s) 1.115 |ΔEq| (mm/s) 3.141 Rel. % 25 1.097 2.869 25 0.524 1.126 25 0.433 3.042 25 Figure 39. Zero applied field Mössbauer spectrum of 5 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned to Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(O)][OTf] (6): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figures 40). Figure 40B gives parameters most consistent with an apical Fe(III). The final fit split the large doublet into two equal signals (Figure 41). A) B) Figure 40. Mössbauer spectrum of 6 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 0.864 mm/s; ΔEq = 3.311 mm/s (blue trace), δ = 0.495 mm/s; ΔEq = 0.912 mm/s (orange trace) and δ = 0.706 mm/s; ΔEq = 2.089 mm/s (green trace), and (B) δ = 1.077 mm/s; ΔEq = 2.870 mm/s (blue trace), δ = 0.496 mm/s; ΔEq = 0.916 mm/s (orange trace) and δ = 0.465 mm/s; ΔEq = 2.565 mm/s (green trace). 224 δ (mm/s) 1.085 |ΔEq| (mm/s) 2.867 Rel. % 25 0.507 1.093 25 0.485 0.717 25 0.473 2.532 25 Figure 41. Zero applied field Mössbauer spectrum of 6 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to Fe(III). Simulation details for [LFe3O(PzNHtBu)3Fe(O)][OTf]2 (7): The small peak at >2 mm/s indicated the presence of a FeII-containing impurity; NMRs of this cluster typically contained [LFe3O(PzNHtBu)3Fe(OH)][OTf]2 (3) as an impurity (Figures 36 and 42). Using the parameters for 3 above, the FeII doublet was used to estimate the amount of this impurity (30 – 40% of the spectrum). This was subtracted from the data, and the remaining data appeared to be one broad doublet, similar to the [FeIII4] spectrum of 4. It was fit as four equal doublets, with the apical Fe(III) tentatively assigned to the doublet with the highest quadrupole splitting, to represent the highest value possible for this parameter (Figure 43). Figure 42. Mössbauer spectrum of reaction mixture containing 7 and 3 (black dots) fit with parameters to account for 3 (gray trace). The remaining signal (red dots) is assigned to the Mossbauer signal of 7. 225 δ (mm/s) 0.430 |ΔEq| (mm/s) 1.435 Rel. % 25 0.440 0.953 25 0.407 0.379 25 0.426 2.033 25 Figure 43. Zero applied field Mössbauer spectrum of 7 (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III), and the green trace is assigned to the unique Fe(III). 226 CHAPTER 5 Simulation details for [LFe3O(Pz)3Fe][OTf]2 (2): The spectrum displays five discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 44). Figure 44B gives parameters most consistent with an apical Fe(II). The final fit split the large doublet into two equal signals (Figure 45). A) B) Figure 44. Mössbauer spectrum of 2 (black dots) fit with three doublets in a ~1:1:2 ratio (gray trace) with parameters (A) δ = 1.215 mm/s; ΔEq = 2.905 mm/s (blue trace), δ = 0.487 mm/s; ΔEq = 0.750 mm/s (orange trace) and δ = 0.773 mm/s; ΔEq = 2.615 mm/s (green trace) and (B) δ = 1.101 mm/s; ΔEq = 3.158 mm/s (blue trace), δ = 0.481 mm/s; ΔEq = 0.757 mm/s (orange trace) and δ = 0.975 mm/s; ΔEq = 2.239 mm/s (green trace). δ (mm/s) 1.101 |ΔEq| (mm/s) 3.148 Rel. % 25 0.469 0.580 25 0.485 0.959 25 0.950 2.218 25 Figure 45. Zero applied field Mössbauer spectrum of 2 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to Fe(II). 227 Simulation details for [LFe3O(Pz)3Fe][OTf] (3): The spectrum displays ca. four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(II) assignment (Figure 46A), and the remainder of the signal can be fit a number of ways (Figures 45B - D). Figure 45D gives parameters most consistent with an apical Fe(II) and high-spin Fe(III) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 46). A) B) C) D) Figure 45. (A) Mössbauer spectrum of 3 (black dots) fit with a single doublet (blue trace) with parameters δ = 1.116 mm/s; ΔEq = 3.304 mm/s, consistent with high-spin FeII. (B) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with a single doublet (gray trace) with parameters δ = 0.689 mm/s; ΔEq = 1.467 mm/s. (C) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with two doublets (gray trace) with parameters δ = 0.742 mm/s; ΔEq = 0.950 mm/s (orange trace), and δ = 0.700 mm/s; ΔEq = 1.837 mm/s (green trace).(D) Mössbauer spectrum of 3 minus the quadrupole doublet for the high-spin Fe(II) (black dots) fit with two doublets (gray trace) with parameters δ = 0.517 mm/s; ΔEq = 1.455 mm/s (orange trace), and δ = 0.970 mm/s; ΔEq = 1.373 mm/s (green trace). 228 δ (mm/s) 1.119 |ΔEq| (mm/s) 3.489 Rel. % 25 1.110 3.099 25 0.534 1.425 25 0.985 1.366 25 Figure 46. Zero applied field Mössbauer spectrum of 3 (black dots) fit with four quadrupole doublets. The blue traces are assigned to high-spin Fe(II), the orange trace is assigned to highspin Fe(III), and the green trace is assigned to Fe(II). Simulation details for [LFe3O(Pz)3Fe][OTf]3 (4): The spectrum displays three discernable peaks, corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal is consistent with high-spin Fe(II) (Figure 47). The final fit split the large doublet into three equal signals, one of which was assigned to the apical Fe(III) (Figure 48). Figure 47. Mössbauer spectrum of 4 (black dots) fit with two doublets in ~ 1:3 ratio (gray trace) with parameters δ = 1.211 mm/s; ΔEq = 2.664 mm/s (blue trace), and δ = 0.448 mm/s; ΔEq = 0.662 mm/s (orange trace). 229 δ (mm/s) 1.120 |ΔEq| (mm/s) 2.807 Rel. % 25 0.458 0.621 25 0.457 0.910 25 0.368 0.343 25 Figure 48. Zero applied field Mössbauer spectrum of 4 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for [LFe3O(Pz)3Fe(MeCN)][OTf]3 (4-MeCN): The spectrum displays three discernable peaks, corresponding to two quadrupole doublets in ~ 1:3 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal is consistent with five-coordinate high-spin Fe(II) (Figure 49). The final fit split the large doublet into three equal signals, one of which was assigned to the apical Fe(II) (Figure 50). Figure 49. Mössbauer spectrum of 4-MeCN (black dots) fit with two doublets in ~ 1:3 ratio (gray trace) with parameters δ = 0.863 mm/s; ΔEq = 1.830 mm/s (green trace), and δ = 0.470 mm/s; ΔEq = 0.722 mm/s (orange trace). 230 δ (mm/s) 0.457 |ΔEq| (mm/s) 0.420 Rel. % 25 0.464 0.716 25 0.452 1.033 25 0.886 1.738 25 Figure 50. Zero applied field Mössbauer spectrum of 4-MeCN (black dots) fit with four quadrupole doublets. The orange traces are assigned to high-spin Fe(III), and the green trace is assigned to the apical Fe(II). Simulation details for [LFe3O(Pz)3Fe(OH)][OTf]2 (5): The spectrum displays four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 51). Figure 51C gives parameters most consistent with an apical Fe(III) and high-spin Fe(II) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 52). 231 A) B) C) Figure 51. Mössbauer spectrum of 5 (black dots) fit with three doublets in ~ 1:1:2 ratio (gray trace) with parameters (A) δ = 0.463 mm/s; ΔEq = 0.722 mm/s (green trace), δ = 0.401 mm/s; ΔEq = 1.343 mm/s (orange trace), and δ = 1.062 mm/s; ΔEq = 2.969 mm/s (blue trace), (B) δ = 0.171 mm/s; ΔEq = 1.154 mm/s (green trace), δ = 0.549 mm/s; ΔEq = 0.910 mm/s (orange trace), and δ = 1.147 mm/s; ΔEq = 2.800 mm/s (blue trace), and (C) δ = 0.331 mm/s; ΔEq = 1.478 mm/s (green trace), δ = 0.456 mm/s; ΔEq = 0.723 mm/s (orange trace), and δ = 1.161 mm/s; ΔEq = 2.766 mm/s (blue trace). 232 δ (mm/s) 1.145 |ΔEq| (mm/s) 2.778 Rel. % 25 0.452 0.893 25 0.454 0.519 25 0.339 1.493 25 Figure 52. Zero applied field Mössbauer spectrum of 5 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for [(LFe3O(Pz)3Fe)2O][OTf]4 (6): The spectrum displays four discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. The parameters of the most intense peak are consistent with a high-spin Fe(III) assignment, and the remainder of the signal can be fit a number of ways (Figure 53). Figure 53C gives parameters most consistent with an apical Fe(III) and high-spin Fe(II) in the tri-iron core. The final fit split the large doublet into two equal signals (Figure 54). 233 A) B) C) Figure 53. Mössbauer spectrum of 6 (black dots) fit with three doublets in ~ 1:1:2 ratio (gray trace) with parameters (A) δ = 0.156 mm/s; ΔEq = 1.128 mm/s (green trace), δ = 0.560 mm/s; ΔEq = 1.057 mm/s (orange trace), and δ = 0.999 mm/s; ΔEq = 3.188 mm/s (blue trace), (B) δ = 0.762 mm/s; ΔEq = 1.176 mm/s (green trace), δ = 0.350 mm/s; ΔEq = 1.049 mm/s (orange trace), and δ = 0.995 mm/s; ΔEq = 3.180 mm/s (blue trace), and (C) δ = 0.382 mm/s; ΔEq = 1.835 mm/s (green trace), δ = 0.463 mm/s; ΔEq = 0.711 mm/s (orange trace), and δ = 1.160 mm/s; ΔEq = 2.827 mm/s (blue trace). 234 δ (mm/s) 1.099 |ΔEq| (mm/s) 2.909 Rel. % 25 0.645 1.005 25 0.464 0.530 25 0.398 1.875 25 Figure 54. Zero applied field Mössbauer spectrum of 6 (black dots) fit with four quadrupole doublets. The blue trace is assigned to high-spin Fe(II), the orange traces are assigned to highspin Fe(III), and the green trace is assigned to the apical Fe(III). Simulation details for putative intermediate of 2 and sPhIO: Based on variable temperature 1H NMR data, the putative intermediate is the major species at low temperature, with a small (~20%) impurity of 5. The parameters for 5 in Figure 63 were used to model this amount of impurity, and was subtracted out (Figure 55). The remaining spectrum displays three discernable peaks corresponding to three quadrupole doublets in ~ 1:1:2 ratio. It was modeled with parameters similar to 6, based on its similar match to the data (Figure 56). Figure 55. Mössbauer spectrum of putative intermediate between 2 and sPhIO, with ~ 20% of an impurity, 5 (black dots). The signal attributed to the impurity was subtracted from the data (gray trace). 235 A) B) Figure 56. Mössbauer spectrum of putative intermediate between 2 and sPhIO (black dots) fit with four equally abundant quadrupole doublets (gray trace). Two potential fits with the following parameters: (A) (i) δ = 1.21 mm/s, |ΔEq| = 2.83 mm/s (blue trace), (ii) δ = 0.41 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.45 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.35 mm/s, |ΔEq| = 1.70 mm/s (green trace), and (B) (i) δ = 1.06 mm/s, |ΔEq| = 3.13 mm/s (blue trace), (ii) δ = 0.40 mm/s, |ΔEq| = 0.36 mm/s (solid orange trace), and (iii) δ = 0.46 mm/s, |ΔEq| = 0.87 mm/s (dashed orange trace), (iv) δ = 0.49 mm/s, |ΔEq| = 1.44 mm/s (green trace). MISCELLANEOUS CRYSTAL STRUCTURES 237 Figure 1. Structure of [LFe3F(PhPz)3Fe(MeCN)][OTf], obtained via recrystallization of [LFe3F(PhPz)3Fe][OTf] in MeCN/Et2O vapor diffusion. Figure 2. Structure of [LFe3F(PhPz)3Fe(MeCN)][OTf]2, obtained via recrystallization of [LFe3F(PhPz)3Fe][OTf]2 in MeCN/Et2O vapor diffusion. 238 Figure 3. Structure of [LM3F(PhPz)3M][OTf], from LZn3(OTf)3 and Fe(N(SiMe3)2)2. The occupancies of the metals are similar at each site: ~130% Fe and ~105% Zn. This is rationalized as Fe scrambling to each position in the cluster. Figure 4. Structure of LLi3Fe(PhPz)2 (tentative assignment), obtained as a decomposition product of [NEt4][Fe4N2Cl10] (Bennett, et al. J. Am. Chem. Soc. 127, 12378), KPhPz, and LLi3 in THF. The Et2O soluble product is pink, and gives this structure. 239 Figure 5. Structure of [(LFe3O(Pz)3Mn)2O][OTf]2, obtained from decomposition of [LFe3O(Pz)3Mn(OH)][OTf]. Figure 6. Structure of [LFe3O(Pz)3Mn(OMe)][OTf], obtained from addition of methanol solution of sodium methoxide to . [LFe3O(Pz)3Mn][OTf]2. 240 Figure 7. Structure of LMn3O(PzNHtBu)3Mn(OH), obtained from the same synthetic route as LFe3O(PzNHtBu)3Fe(OH), starting from LMn3OTf3 and Mn(OTf)2 • 2 MeCN. Figure 8. Structure of [LMn3O(PzNHtBu)3Mn(OH)][OTf], obtained from oxidation of LMn3O(PzNHtBu)3Mn(OH) with [Fc][OTf]. 241 Figure 9. Structure of [LFe3O(Pz)3Fe(MeCN)][OTf], obtained from recrystallization of [LFe3O(Pz)3Fe][OTf] by MeCN/Et2O vapor diffusion. Figure 10. Structure of [LFe3O(Pz)3Fe(NO)][OTf]2, obtained from addition of one equivalent NO gas to [LFe3O(Pz)3Fe][OTf]2. 242 Figure 11. Structure of [(LFe3O(Pz)3FeO)2FeCl2] (tentative assignment), obtained from addition of one equivalent [NBu4][IO4] to [LFe3O(Pz)3FeCl][OTf] in THF. Figure 12. Structure of [LFe3O(Pz)3Fe(NHAr/Cl)][OTf] (Ar = 3,5-CF3-Ph; ~6:4 amide to chloride), obtained from partial decomposition of the reaction product between [LFe3O(Pz)3Fe][OTf] and ArN3. 243 Figure 13. Structure of [LFe3F(Pz)3Fe(MeCN)][OTf], obtained by recrystallization of [LFe3F(Pz)3Fe][OTf] in MeCN/Et2O vapor diffusion. Figure 14. Structure of LMn3(OTf)3, obtained by an analogous synthesis to that of LFe3(OTf)3 (LFe3(OAc)3 and 10 equivalents Me3SiOTf; Arnett, et al. J. Am. Chem. Soc. 140, 5569). 244 Figure 15. Structure of [LMn3O(Pz)3Mn(OAc)][OTf], obtained by an analogous synthesis to that of [LFe3O(Pz)3Fe(OAc)][OTf]. Figure 16. Structure of [LMn3O(Pz)3Mn][OTf]2, obtained by an analogous synthesis to that of [LFe3O(Pz)3Fe][OTf]2. 245 Figure 17. Structure of [LFe3O(iPrPz)3Mn][OTf]2, obtained by addition of KiPrPz, PhIO, and Mn(OTf)2 • 2 MeCN to LFe3(OTf)3. Figure 18. Structure of [LFe3O(iPrPz)3Mn][OTf], [LFe3O(iPrPz)3Mn][OTf]2 with CoCp2. obtained by reduction of 246 Figure 19. Structure of [LFe3O(iPrPz)3K][OTf], obtained by addition of KiPrPz, PhIO, and Fe(OTf)2 to LFe3(OTf)3. Full metalation of Fe was not observed by NMR, and this byproduct was crystalized out instead of the desired compound. Figure 20. Structure of [LFe3O(iPrPz)3Fe][OTf]2, obtained by addition of KiPrPz, PhIO, and Fe(OTf)2 • 2 MeCN to LFe3(OTf)3. Full metalation of Fe was observed with this reagent by NMR. 247 Figure 21. Structures of a mixture of [LFe3O(iPrPz)3Mn(OH)][OTf] and [LFe3O(iPrPz)2(OiPrPz)Mn][OTf] (roughly 50:50; tentative assignments), obtained by addition of (2-tert-butyl-sulfonyl)-iodosylbenzene to [LFe3O(iPrPz)3Mn][OTf]. 248 ABOUT THE AUTHOR Christopher John Reed was born in Boise, ID on July 2nd, 1991. He, and his brother Alexander Reed, were raised in Idaho by their mother, Freda Reed. He attended Mountain View High School and graduated in 2009. Then, he moved to Pocatello, ID to attend Idaho State University for undergraduate. During his time at ISU, he worked in the ‘extreme biochemistry’ laboratory of Prof. Caryn Evilia where he studied protein structure and stability of halophilic microorganisms using circular dichroism spectroscopy. In the summer of 2012, he completed a research internship at Utah State University, in the protein crystallography lab of Prof. Sean Johnson. Chris graduated from ISU in 2014 with a B.S. in Chemistry and Biochemistry; he received the ISU Outstanding Student Achievement Award from the College of Science and Engineering (Science Division). He completed his Ph. D. studies in the lab of Prof. Theodor Agapie at the California Institute of Technology in 2019. In 2017, he received a Resnick Sustainability Institute Graduate Fellowship to support the remainder of his graduate studies. He is currently planning to start his post-doctoral studies with Prof. Yi Lu at University of Illinois, Urbana-Champaign.