Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals Citation Gu, Nina Xiao (2020) Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8zwz-gh20. https://resolver.caltech.edu/CaltechTHESIS:04102020-135244808 Abstract Reactive metalloradical intermediates have been implicated in both biological and synthetic catalyst systems for small molecule activation processes, including proton reduction and ammonia oxidation. Towards a greater mechanistic understanding of such transformations on well-defined model complexes, this thesis explores relevant H–H and N–N bond-forming reactions mediated by trivalent Fe and Ni species, as well as catalytic N–N bond cleavage mediated by an open-shell VFe bimetallic complex. First, a pair of thiolate-supported, S = ½ iron and nickel hydrides are synthesized and spectroscopically characterized at low temperatures (Chapters 2, 3). Paramagnetic iron and nickel hydrides have been proposed as catalytic intermediates of [NiFe] hydrogenase and nitrogenase, but characterization of such molecular species are limited. For both the Fe III and Ni III hydride complexes described herein, spin delocalization onto the thiolate ligand is proposed to stabilize the formal 3+ metal oxidation state. Furthermore, both the Fe III –H and Ni III –H species are demonstrated to undergo the bimolecular reductive elimination of dihydrogen upon warming, albeit with distinct activation parameters consistent with different proposed pathways for H–H bond formation. Chapter 4 expands upon the H–H bond forming chemistry demonstrated on the Ni system to demonstrate related N–N bond formation from an analogous Ni III –NH 2 species, resulting in the formation of a Ni II 2 (N 2 H 4 ) complex. Given the diverse mechanistic possibilities for the overall 6e - /6H + transformation to oxidize ammonia to dinitrogen, identification of the active M(NH x ) intermediate and pathway for N–N bond formation is a central mechanistic question. While the homocoupling of M–NH 2 species to form hydrazine has been hypothesized as the key N–N bond forming step in ammonia oxidation systems, stoichiometric examples of this transformation from M–NH 2 complexes are rare. Lastly, Chapter 5 details the synthesis of a heterobimetallic VFe complex featuring a bridging thiolate, inspired by the structure of the VFe nitrogenase cofactor. This VFe species is demonstrated to be an active catalyst for the disproportionation of hydrazine to dinitrogen and ammonia. Notably, the heterobimetallic complex is appreciably more active than monometallic analogues of the individual V and Fe sites, suggesting that bimetallic cooperativity may facilitate the observed catalysis. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Iron; nickel; vanadium; metal hydrides; dihydrogen; ammonia; hydrazine Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Peters, Jonas C. Thesis Committee: Agapie, Theodor (chair) Hsieh-Wilson, Linda C. Fu, Gregory C. Peters, Jonas C. Defense Date: 9 March 2020 Funders: Funding Agency Grant Number Department of Energy (DOE) DOE-0235032 NIH GM-070757 National Science Foundation Graduate Research Fellowship UNSPECIFIED NSF NSF-1531940 Dow Next Generation Fund UNSPECIFIED Record Number: CaltechTHESIS:04102020-135244808 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:04102020-135244808 DOI: 10.7907/8zwz-gh20 Related URLs: URL URL Type Description https://doi.org/10.1021/jacs.8b02603 DOI Published content for Chapter 2. https://doi.org/10.1021/jacs.0c00712 DOI Published content for Chapter 3. https://doi.org/10.1039/C9CC00345B DOI Published content for Chapter 5. ORCID: Author ORCID Gu, Nina Xiao 0000-0002-4637-8418 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 13672 Collection: CaltechTHESIS Deposited By: Nina Gu Deposited On: 21 May 2020 16:55 Last Modified: 08 Nov 2023 00:44 Thesis Files Preview PDF - Final Version See Usage Policy. 25MB Repository Staff Only: item control page

Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals Thesis by Nina Xiao Gu In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2020 (Defended March 9, 2020) ii ã 2020 Nina Xiao Gu ORCID: 0000-0002-4637-8418 iii ACKNOWLEDGEMENTS My time at Caltech has been a challenging yet enriching experience, and I am lucky to have had many fantastic mentors and co-workers who have helped me reach the finish line. First and foremost, I thank Jonas for his mentorship and support during my graduate studies. His feedback has challenged me to grow as a scientist and to think more critically about my science, for which I am grateful. I’m also appreciative of the freedom that Jonas has allowed me to pursue chemistry that sparked my interest. Although at times it can feel like nothing is working, it has also been a lot of fun to see if reactions that I dream up actually pan out in the lab. I would additionally like to thank the rest of my committee, Theo Agapie, Linda Hsieh-Wilson, and Greg Fu, for their constructive feedback and advice during our meetings. I am fortunate to have had wonderful undergraduate research experiences, which sowed my interest in chemistry and motivated me to pursue graduate studies in the first place. Ted Betley is thanked for his encouragement and his dedication to mentoring undergraduate students. During my time in his lab, I also had the privilege to learn from two talented graduate students, Tamara Powers and Benji Lin, who each taught me a great deal about synthesis. Additionally, I would like to thank Cliff Kubiak for his mentorship during my first stint in a research lab, and my postdoc mentor, Kyle Grice, for his guidance and patience with a very green undergraduate. The staff who run the facilities at Caltech are truly outstanding, and I owe them a big thank you for their help over these years. I’ve been fortunate to work on several projects with Paul Oyala, who has been a great person to talk to for all things EPR-related. Mike Takase and Larry Henling are thanked for their help with my crystals, as well as their patience and good humor with the many, many amorphous samples that I’ve brought them. Dave VanderVelde, Nathan Dalleska, Mona Shahgholi are additionally acknowledged for sharing their expertise in NMR, GC, and mass spectrometry, respectively. I’m grateful to all the individuals that I’ve overlapped with during my time in Jonas’ lab. In particular, I’d like to thank Gaël Ung for helping me get on my feet during my first year and for being a great role model. Meaghan Deegan is an exceptionally hard-working and rigorous scientist, and I am grateful for the helpful suggestions and friendship over these years. In addition to their wise words and support, I also thank Kumiko Yamamoto and Gerri Roberts for tasty food outings and Trixia Buscagan for much-needed coffee breaks! I am also appreciative of the people that I’ve shared an iv office bay with, most recently Cooper Citek, Heejun Lee, Dirk Schild, and Christian Johansen (honorary member), for the interesting scientific discussions and entertaining, not-quite-so-scientific ones as well. Lastly, I’d like to thank Kareem, Olivia, and my parents for their unwavering support (I’m the luckiest!). None of this would have been possible without the hard work and sacrifices of my parents – this thesis is dedicated to them. v ABSTRACT Reactive metalloradical intermediates have been implicated in both biological and synthetic catalyst systems for small molecule activation processes, including proton reduction and ammonia oxidation. Towards a greater mechanistic understanding of such transformations on well-defined model complexes, this thesis explores relevant H–H and N– N bond-forming reactions mediated by trivalent Fe, Ni species, as well as catalytic N–N bond cleavage mediated by an open-shell VFe bimetallic complex. First, a pair of thiolatesupported, S = ½ iron and nickel hydrides are synthesized and spectroscopically characterized at low temperatures (Chapters 2, 3). Paramagnetic iron and nickel hydrides have been proposed as catalytic intermediates of [NiFe] hydrogenase and nitrogenase, but characterization of such molecular species are limited. For both the Fe and Ni hydride complexes described herein, spin delocalization onto the thiolate ligand is proposed to stabilize the formal 3+ metal oxidation state. Furthermore, both the FeIII–H and NiIII–H species are demonstrated to undergo the bimolecular reductive elimination of dihydrogen upon warming, albeit with distinct activation parameters consistent with different proposed pathways for H–H bond formation. Chapter 4 expands upon the H–H bond forming chemistry demonstrated on the Ni system to demonstrate related N–N bond formation proposed from an analogous NiIII–NH2 species, resulting in the formation of a NiII2(N2H4) complex. Given the diverse mechanistic possibilities for the overall 6e–/6H+ transformation to oxidize ammonia to dinitrogen, identification of the active M(NHx) intermediate and pathway for N–N bond formation is a central mechanistic question. While the homocoupling of M–NH2 species to form hydrazine has been hypothesized as the key N–N bond forming step in ammonia oxidation systems, stoichiometric examples of this transformation from M– NH2 complexes are rare. Lastly, Chapter 5 details the synthesis of a heterobimetallic VFe complex featuring a bridging thiolate, inspired by the structure of the VFe nitrogenase cofactor. This VFe species is demonstrated to be an active catalyst for the disproportionation of hydrazine to dinitrogen and ammonia. Notably, the heterobimetallic complex is appreciably more active than monometallic analogues of the individual V and Fe sites, suggesting that bimetallic cooperativity may facilitate the observed catalysis. vi PUBLISHED CONTENT AND CONTRIBUTIONS (1) Gu, N. X.; Oyala, P. H.; Peters, J. C. An S = ½ Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe– H Parameters. J. Am. Chem. Soc. 2018, 140, 6374-6382. DOI: 10.1021/jacs.8b02603. N.X.G. synthesized all of the compounds, collected characterization data, performed computational studies, and prepared the manuscript. (2) Gu, N. X.; Ung, G.; Peters, J. C. Catalytic hydrazine disproportionation mediated by a thiolate-bridged VFe complex. Chem. Commun. 2019, 55, 5363-5366. DOI: 10.1039/C9CC00345B. N.X.G. synthesized the metallated complexes, performed all catalytic reactions and kinetic studies, and prepared the manuscript. (3) Gu, N. X.; Oyala, P. H.; Peters, J. C. H2 Evolution from a Thiolate-Bound Ni(III) Hydride. J. Am. Chem. Soc. 2020, accepted. DOI: 10.1021/jacs.0c00712. N.X.G. synthesized all of the compounds, collected characterization data, performed computational studies, and prepared the manuscript. vii TABLE OF CONTENTS Acknowledgments .......................................................................................................... iii Abstract ........................................................................................................................... v Published Content and Contributions ............................................................................... vi Table of Contents ........................................................................................................... vii List of Illustrations and/or Tables .................................................................................... ix Nomenclature ................................................................................................................. xi Chapter I: Introduction ..................................................................................................... 1 1.1 Motivation............................................................................................................ 1 1.2 Proton Reduction to Dihydrogen ........................................................................... 1 1.3 Ammonia Oxidation to Dinitrogen ........................................................................ 7 1.4 Redox Non-Innocence of Thiolate Ligands............................................................ 9 1.5 Chapter Summaries ............................................................................................ 11 1.6 References.......................................................................................................... 12 Chapter II: An S = ½ Iron Complex Featuring N2, Thiolate and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe–H Parameters ............... 15 2.1 Introduction ........................................................................................................ 15 2.2 Results and Discussion ....................................................................................... 17 2.3 Conclusion ......................................................................................................... 31 2.4 Experimental Section .......................................................................................... 32 2.5 References.......................................................................................................... 42 Chapter III: H2 Evolution from a Thiolate-Bound Ni(III) Hydride ................................... 49 3.1 Introduction ........................................................................................................ 49 3.2 Results and Discussion ....................................................................................... 50 3.3 Conclusion ......................................................................................................... 62 3.4 Experimental Section .......................................................................................... 63 3.5 References.......................................................................................................... 74 viii Chapter IV: H2N–NH2 Bond Formation from a NiIII–NH2 Species: Mechanistic Insights into Ni-Mediated Ammonia Oxidation ........................................... 81 4.1 Introduction ........................................................................................................ 81 4.2 Results and Discussion ....................................................................................... 84 4.3 Conclusion ......................................................................................................... 87 4.4 Experimental Section .......................................................................................... 88 4.5 References.......................................................................................................... 96 Chapter V: Catalytic Hydrazine Disproportionation Mediated by a Thiolate-Bridged VFe Complex ..................................................................................... 99 5.1 Introduction ........................................................................................................ 99 5.2 Results and Discussion ..................................................................................... 100 5.3 Conclusion ....................................................................................................... 106 5.4 Experimental Section ........................................................................................ 106 5.5 References........................................................................................................ 112 Appendix A: Supplementary Information for Chapter 2 ................................................ 116 Appendix B: Supplementary Information for Chapter 3................................................. 156 Appendix C: Supplementary Information for Chapter 4................................................. 215 Appendix D: Supplementary Information for Chapter 5 ................................................ 240 ix LIST OF ILLUSTRATIONS AND TABLES Chapter 1 Page Figure 1.1. Hydride-bound hydrogenase intermediates and proposed mechanism of [NiFe] hydrogenase ................................................................................................ 3 Figure 1.2. Proposed mechanism for biological nitrogen fixation ................................. 4 Figure 1.3. Homolytic and heterolytic pathways for molecular HER ............................ 5 Figure 1.4. Examples of paramagnetic Fe and Ni hydrides........................................... 6 Figure 1.5. Examples of molecular catalysts for ammonia oxidation ............................ 8 Figure 1.6. Pathways for O–O and N–N bond formation.............................................. 9 Figure 1.7. Resonance structures of thiolate-bound metal complexes ........................ 10 Chapter 2 Scheme 2.1. Synthesis of HSiP2SiPr and metallated Fe complexes .............................. 18 Figure 2.1. X-ray structures of (SiP2S)Fe complexes ................................................. 19 Table 2.1. Select bond metrics and angles of (SiP2S)Fe complexes ............................ 19 Figure 2.2. Characterization data on (SiP2S)FeHN2 ................................................... 21 Figure 2.3. Calculated spin density map for (SiP2S)FeHN2 ........................................ 23 Figure 2.4. ENDOR and CW EPR data on (SiP2S)FeHN2 and (SiP2S)FeDN2 ............. 24 Figure 2.5. Spectroscopic and kinetic data on H2 evolution from (SiP2S)FeHN2 ........ 25 Figure 2.6. Proposed mechanism for H2 evolution from (SiP2S)FeHN2 ..................... 27 Scheme 2.2. Synthesis of (SiP2S)FeN2–/2–and hydride transfer from (SiP2S)FeHN2– ... 28 Figure 2.7. Thermochemical scheme relating H+, H•, and H– transfers from (SiP2S)FeHN20/– ....................................................................................................... 30 Table 2.2. Oxidation potentials and thermochemical parameters of pertaining to Fe–H cleavage from (SiP2S)FeHN20/– ....................................................................... 31 Chapter 3 Figure 3.1. Proposed biological NiIII hydride intermediates and pathways for H2 evolution from (SiP2S)NiH .................................................................................. 50 x Scheme 3.1. Synthesis of (SiP2S)Ni complexes ......................................................... 51 Figure 3.2. X-ray structures of (SiP2S)Ni complexes ................................................. 52 Table 3.1. Select bond lengths of (SiP2S)Ni complexes ............................................. 53 Figure 3.3. CW EPR and ENDOR data on (SiP2S)NiH .............................................. 55 Figure 3.4. Synthesis, X-ray structure, and CW EPR data of (SiP2S)NiMe ................. 57 Table 3.2. Bond lengths of (SiP2S)NiH (DFT) and (SiP2S)NiMe (XRD) .................... 57 Figure 3.5. Calculated Mulliken spin densities of the Ni-C hydrogenase state and (SiP2S)NiH ............................................................................................... 59 Figure 3.6. UV-vis spectra monitoring the decay of (SiP2S)NiH ................................ 60 Scheme 3.2. Comparison of H2 evolution mechanisms from (SiP2S)NiH and (SiP2S)FeHN2 ................................................................................................... 61 Chapter 4 Figure 4.1. Pathways for O–O bond formation in water oxidation and Ni-mediated amide coupling ..................................................................................... 81 Scheme 4.1. Synthesis of (SiP2S)Nix(NyHz) complexes.............................................. 83 Figure 4.2. X-ray structure of (SiP2S)Nix(NyHz) complexes ....................................... 84 Figure 4.3. NMR spectra of [(SiP2S)Ni]2(N2H4) and isotopologues ............................ 85 Scheme 4.2. Comparison of H–H and N–N bond formation from (SiP2S)NiIIIX ......... 86 Scheme 4.3. Summary of (SiP2S)Ni-mediated ammonia oxidation ............................ 87 Chapter 5 Figure 5.1. Thiolate-bridged FeFe and VFe bimetallic scaffolds .............................. 100 Scheme 5.1. Synthesis of thiolate-bridged VFe complex ......................................... 101 Figure 5.2. X-ray structure of thiolate-bridged VFe complex ................................... 102 Figure 5.3. Monometallic Fe and V complexes........................................................ 103 Table 5.1. Catalytic hydrazine disproportionation mediated by the VFe catalyst ....... 104 Table 5.2. Hydrazine disproportionation mediated by V and Fe complexes .............. 105 Figure 5.4. Proposed general pathway for catalyzed hydrazine disproportionation .................................................................................................. 106 xi NOMENCLATURE Å. Angstrom µB. Bohr Magneton µeff. Effective magnetic moment 12-c-4. 12-crown-4 2-MeTHF. 2-methyltetrahydrofuran [BArF4]−. Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ca. Circa Cat. Catalyst Cp. Cyclopentadienyl CV. Cyclic voltammetry CW. Continuous wave DFT. Density functional theory DME. 1,2-dimethoxyethane EPR. Electron paramagnetic resonance ENDOR. Electron nuclear double resonance Equiv. Equivalents Fc. Ferrocene g. Electron g-factor. G. Gibbs free energy GC. Gas chromatography Gt. Gigatonne H. Enthalpy HER. Hydrogen evolution reaction HYSCORE. Hyperfine sublevel correlation. IR. Infrared NMR. Nuclear magnetic resonance MeCN. Acetonitrile S. Entropy S. Spin quantum number TEMPO. 2,2,6,6-Tetramethyl-1-piperidinyloxy UV-vis. Ultraviolet-visible PPN. Bis(triphenylphosphine)iminium 1 Chapter 1 INTRODUCTION 1.1 Motivation Given the environmental impacts of carbon-based fuels and finite fossil fuel reserves, largescale adoption of a carbon-neutral energy economy is central to global sustainability.1 Thus, the development of active, efficient, and Earth-abundant catalysts for relevant transformations are critical. A unifying theme of this thesis is the study of first-row transition metal complexes that mediate small molecule activation chemistry pertinent to renewable energy processes, serving as model systems to understand these bond-breaking and -making reactions on a molecular level. Model systems are uniquely well-suited for spectroscopic studies and probing structure-function relationships, which may in turn aid the understanding of more complex catalyst systems, as well as uncover new reactivity modes for a ground-up approach to catalyst design. Specifically, the characterization of model species allow for the identification of spectroscopic handles to support reliable characterization of transient catalytic intermediates, and the reactivity of these model species can inform mechanistic hypotheses involving such motifs in complex enzymatic or synthetic systems. Furthermore, the molecular understanding of these elementary transformations may aid the rational design of new catalysts for processes relevant to sustainable energy. These general principles motivate the study of the model complexes described in this thesis. 1.2 Proton Reduction to Dihydrogen 1.2.1 Dihydrogen as a Fuel Molecular hydrogen is a promising alternative fuel given its high gravimetric density and because it is clean burning, generating water as a byproduct of combustion.2 However, one challenge to adopting a largescale hydrogen fuel economy is the carbon footprint associated with current hydrogen production methods; industrial steam reforming processes 2 convert methane and water into carbon dioxide and dihydrogen and account for greater than 90% of H2 produced in the United States today.3 An attractive alternative method to generate hydrogen is through direct proton reduction (H+ + e– → ½ H2), as such a process would circumvent the reliance on, and generation of, carbon-based compounds. Ideally, water would serve the source of protons, and solar photovoltaic cells would generate the necessary energy.1 Platinum electrodes are highly efficient catalysts for the hydrogen evolution reaction (HER), but the high cost and limit supply of platinum are prohibitive toward large-scale use. It would thus be highly desirable to replicate this efficient activity with base metal catalysts, which is currently an unmet challenge and an active area of research.4 As a demonstration of the viability of such a goal, Nature employs hydrogenase enzymes containing active sites comprised of Ni and Fe centers to mediate proton reduction with high turnover frequency and low overpotentials, exhibiting efficiencies comparable to that of platinum electrodes.5 1.2.2 Biological Proton Reduction 1.2.2.1 Hydrogenase Enzymes Hydrogenase enzymes catalyze the reversible reduction of protons to H2, which allow for a broad range of microorganisms to employ H2 as fuel and regulate intracellular redox properties.6 Exhibiting turnover frequencies of up to ca. 104 s-1, hydrogenases catalyze waterderived proton reduction under ambient temperatures and pressures at low overpotentials.7 It is estimated that hydrogenase enzymes consume and produce a total of ca. 0.3 Gt H2 per year,8 making them a key player in the global hydrogen cycle.9 These enzymes can be categorized as [NiFe], [FeFe], or [Fe] hydrogenases based on the metal composition in the active sites.7,10 [NiFe] hydrogenase is the most abundant variant, and the proposed pathway for reversible H2 evolution catalyzed by [NiFe] hydrogenase is depicted in Fig. 1.1. Considering the case of proton reduction, introduction of 1e–/1H+ to the diamagnetic Ni-SIa state is believed to protonate a cysteine ligand to yield the S = ½ Ni-L state. Isomerization of the Ni-L state results in the S = ½ Ni-C intermediate, which is assigned as bearing a hydride bridging the NiIII and FeII centers through pulse EPR and computational studies.11 Introduction of another electron and proton results in cysteine protonation (Ni-R 3 state), and finally a heterolytic H–H bond formation yields dihydrogen to regenerate the NiSIa state. Catalysis is hypothesized to be Ni-centered rather than Fe-centered; under turnover conditions, the iron center persists in the divalent state, whereas the Ni center CO accesses formal NH H Cys-S [4Fe-4S] 1+, 2+, and states.12 Accordingly, Ni-substituted rubredoxin CN 3+ oxidation Feis studied as a H O S-Cys NC CO S S S H Ni Fe N functional model enzyme for [NiFe] hydrogenase and bears an active site comprised Cys OH of single Fe Fe OC S-Cys S S CN OC C Ni center bound four cysteines.13,14 Proton CystoCys COcatalysis mediated by Ni-substituted NC reduction O R rubredoxin is believed to proceed via an analogous pathway as that of [NiFe] hydrogenase, [NiFe] hydrogenase [FeFe] hydrogenase [Fe] hydrogenase including accessing a terminally bound NiIII-H species akin to the Ni-C intermediate. Ni-SIa H2 NC CN S-Cys NiII FeII OC Ni-R S S Cys Cys e -, H + S-Cys Ni-L H NC CN H FeII OC H S-Cys NC NiII S S Cys Cys CN OC S-Cys S-Cys FeII NiI S S Cys Cys S-Cys Ni-C e -, H + NC CN OC S-Cys H FeII NiIII S S Cys Cys S-Cys Figure 1.1. Proposed mechanism for reversible proton reduction mediated by [NiFe] hydrogenase 1.2.2.2 Nitrogenase Enzymes Nitrogenase enzymes catalyze the conversion of dinitrogen to ammonia and play a crucial role in the global nitrogen cycle, generating around half of the bioavailable nitrogen on the planet.15 Three nitrogenase variants are known, MoFe, VFe and Fe-only nitrogenases, which are categorized by the metal composition of the active sites.16 Research on nitrogenase is generally motivated by its ability to fix dinitrogen, but a curious aspect of this reductive 4 process is the concomitant generation of at least one equivalent of H2 per fixed N2.17 Given the tandem occurrence of biological N2 and H+ reduction, H2 evolution has been proposed as an obligate step in the pathway for nitrogenase-catalyzed N2 reduction. H2 H2 H2 N2 E0 e-/H+ E1(H) e-/H+ E2(2H) e-/H+ E4(3H) e-/H+ H2 E4(4H) E4(2N2H) H2 N2 2 NH3 4e-/4H+ H S S S S Fe S Cys Fe Fe S OR Fe C Fe S Mo OR’ Fe S Fe S S N(His) S E0 Fe 4 H+ Fe S 4 e- Fe H H S Fe reductive elimination H2 + N2 + E4(2N2H) S H E4(4H) Figure 1.2. (top) Simplified Lowe-Thorneley model for biological nitrogen fixation; (bottom) Structure of the resting E0 state of the FeMo cofactor and a depiction of bridging hydrides proposed in the E4(4H) state The Lowe-Thorneley kinetic model describes the biological nitrogen fixation cycle as a function of electron loading, and a simplified model is illustrated in Figure 1.2.18 Generated after introduction of 4 e–/4H+ to the resting E0 state, the S = ½ E4(4H) state is a key intermediate where N2 binding can occur and nitrogen fixation can proceed, or alternatively, evolution of two equivalents of dihydrogen can regenerate the resting E0 state.19 Pulse EPR studies on the purported E4(4H) state reveal two D2O-exchangeable hydrogen nuclei assigned as bridging hydride (Fe–H–Fe) motifs.19 In the productive pathway for ammonia generation, it is hypothesized that reductive elimination of dihydrogen from the two bridging hydrides precedes dinitrogen binding at E4 state, and the formal 2 e– reduction 5 upon dihydrogen evolution promotes dinitrogen activation at iron (Fig. 1.2).20 Such a mechanism has been suggested to be universally operative for the MoFe, VFe, and Fe-only nitrogenases at different efficiencies.21 1.2.3 Molecular Proton Reduction Catalysis 1.2.3.1 Pathways for Molecular H−H Bond Formation H2 [Mn] ½ H2 H+ Bimolecular Reductive Elimination Homolytic M-H Cleavage e- H+ Heterolytic M-H Cleavage [Mn] H [Mn+1] H e- Figure 1.3. General pathways for proton reduction featuring the intermediacy of metal hydrides that may proceed through a homolytic and/or heterolytic pathway for H2 evolution. Molecular HER electrocatalysts containing Earth-abundant transition metals such as Mo, Fe, Co, and Ni have been developed,21,22 and metal hydride intermediates are often implicated in HER schemes, akin to the biological HER pathways described above. The general mechanism for molecular HER catalysts involves protonation of the metal center to yield a metal hydride intermediate, and subsequent H–H bond formation may proceed either through a heterolytic or homolytic pathway for M–H bond cleavage (Fig. 1.3). The heterolytic pathway invokes metal hydride protonation to release H2, whereas the homolytic pathway proceeds via bimolecular reductive elimination of H2 from a metal hydride intermediate. Multiple pathways may be operative under turnover conditions. The stoichiometric protonation of metal hydrides to release H2 is a commonly demonstrated transformation.22 Considering the case of a solvated proton in MeCN, the hydricity (ΔGH-) of the metal hydride needs to be < 76 kcal/mol for protonation to be thermodynamically 6 favorable.23 However, examples of bimolecular reductive elimination of dihydrogen from a well-defined metal hydride are limited. For metal hydrides with a M–H bond dissociation free energy less than ca. 52 kcal/mol, release of H2 is thermodynamically favorable.24 However, the demonstration of such reactivity from an observable metal hydride species is limited given the comparatively weak M–H bond and propensity for self-reaction. 1.2.3.2 Paramagnetic Metal Hydride Complexes A F2 B O O H N N FeIII N N O O H C6F5 C6F5 B tBu tBu iPr iPr N Fe N I 2+ C6F5 C6F5 N C6F5 N NiIII N H NH N NiIII N N C6F5 C6F5 N N iPr2P FeI H iPr2P P H O C O iPr H iPr aiso(1H) = -18.5 MHz (S = 3/2) C6F5 H N |aiso(1H)| = 45 MHz C NiI C O |aiso(1H)| = 293 MHz [H-Ni(CN)n]x |aiso(1H)| = ca. 427 MHz Figure 1.4. (A) Proposed paramagnetic iron and nickel hydride intermediates in electrochemical proton reduction. (B) Examples of terminally bound, paramagnetic hydrides with measured aiso(1H) values. Spin states for these complexes are S = ½ unless otherwise noted. In addition to being implicated in biological systems, the intermediacy of paramagnetic metal hydrides has been implicated in proton reduction catalyzed by synthetic electrocatalysts (Fig. 1.4A),25–27 but such species are difficult to observe and study due to general instability, especially under turnover conditions.28 Thermochemical studies reported by Tilset demonstrate considerable M–H bond weakening upon oxidation of several diamagnetic metal hydrides to the radical cation analogues.29 Accordingly, spectroscopic data on such paramagnetic species in general are limited, and the study of model 7 paramagnetic metal hydride species can serve to canvass accessible pathways for H2 evolution and be utilized to obtain relevant spectroscopic handles. Particularly given that pulse EPR techniques have been employed to characterize enzymatic hydride-bound intermediates, the study of molecular metal hydrides through such techniques may aid the reliable assignment of these motifs in both biological and synthetic systems. Figure 1.4B depicts several examples of paramagnetic, terminal M–H species with measured hyperfine coupling values determined through EPR spectroscopy.30–33 The hyperfine coupling tensor (A = [Ax, Ay, Az]) to an EPR-active nucleus can be parsed into the isotropic hyperfine constant (aiso = (Ax + Ay + Az)/3) and the dipolar tensor (T; A = aiso + T).34 These parameters can be utilized to estimate spin density and probe local geometry; the isotropic hyperfine constant is a measure of the Fermi contact interaction, and the dipolar tensor reports on dipolar interactions. In general, spin density in atomic s orbitals contribute to the isotropic hyperfine constant for that nucleus, and spin density in p or d orbitals contribute to the dipolar tensor due to orbital nodes at the nucleus. Since hydrogen comprises of only the 1s orbital, the aiso(1H) value can be directly used to estimate spin density on the hydride by scaling by the aiso(1H) of a free hydrogen atom (1420 MHz; aiso(1H)/1420 = ρ(1H)).35 Furthermore, point-dipole models for metal hydrides suggest that terminally bound hydrides (M–H) are anticipated to exhibit approximately axial dipolar tensors (T ≈ [t, t, −2t]), whereas approximately symmetric bridging paramagnetic hydrides (M–H–M) are predicted to exhibit rhombic dipolar tensors (T ≈ [t, −t, 0]).36 1.3 Ammonia Oxidation to Dinitrogen 1.3.1 Ammonia as a Hydrogen Carrier and Fuel In addition to the aforementioned carbon footprint associated with hydrogen production, another challenge to adopting a hydrogen fuel economy is the low volumetric energy density of hydrogen, which makes it challenging to transport and store.37 The volumetric energy density of hydrogen at 700 bar is 5.6 MJ/L, compared to 32 MJ/L for gasoline at atmospheric pressure.2 One solution is to use hydrogen carriers for more efficient transportation, which can then be converted to H2 on site when power is required. Ammonia 8 is a promising candidate given its higher volumetric energy density (13.6 MJ/L at 10 bar), its relative ease of condensation, and the existing infrastructure and technology to distribute and store large quantities of ammonia.38 Furthermore, research is also ongoing to utilize ammonia directly in fuel cells.39 For example, the anodic reaction would be the oxidation of ammonia to dinitrogen (2 NH3 → N2 + 6 H+ + 6 e–), which could be coupled to dioxygen reduction to water at the cathode (O2 + 4 H+ + 4 e– → 2 H2O). Thus, for both hydrogen carrier and fuel cell applications, the development of efficient catalysts for the overall 6e–/6H+ oxidation of ammonia to dinitrogen is essential. 1.3.2 Ammonia Oxidation Catalysts and Mechanistic Analogy to Water Oxidation + N Ph N P N RuII P N RuII N NH3 NH3 Mes N N N RuII Mes NH3 Mes N NMe2 Ph N N Mes NMe2 L O NH3 N N N O 2+ NH3 RuII O O N FeII 2+ NH3 N FeII N L Figure 1.5. Reported homogenous catalysts for ammonia oxidation to dinitrogen. Complexes highlighted in gray are also demonstrated to catalyze water oxidation to dioxygen. P = PtBu, Mes = mesityl Early examples of stoichiometric ammonia oxidation include reports from the Taube, Meyer, and Collman groups using Os40–43 and Ru44–47 complexes, and there has been a recent renewed interest in this transformation. The first demonstration of catalytic ammonia oxidation was demonstrated last year from a polypyridyl Ru electrocatalyst,48 and since then, several Ru49–51 and Fe52,53 catalysts for ammonia oxidation to dinitrogen have been reported, either under electrochemical or chemical reaction conditions (Fig. 1.5). Notably, several of 9 these systems for ammonia oxidation are also known to be water oxidation catalysts.54–57 For the catalysts can mediate both water and ammonia oxidation, do these two transformations operate through analogous pathways? If so, is this mechanistic analogy more generally true for all complexes that mediate ammonia oxidation? For water oxidation catalysts, metal oxo species are widely proposed intermediates, and the key O–O bond forming step is generally believed to be either nucleophilic attack by an exogenous equivalent of water/hydroxide or bimolecular coupling of two oxo fragments (Fig. 1.6A).58 In the related case of ammonia oxidation to dinitrogen, analogous intermediates featuring metal-ligand multiple bonding have been proposed, namely metal imide and metal nitride species, which have been suggested to form N–N bonds either through nucleophilic attack by exogenous NH3 or bimolecular coupling (Fig. 1.6B).48,51 In contrast, while the N–N coupling of metal amides to yield hydrazine has been proposed as the N–N coupling step in ammonia oxidation catalyzed by ferrocene53 and (tetramesitylporphyrin)Ru(NH3),50 there are no reported examples of such reactivity from a characterized M–NH2 species. Bimolecular coupling A Nucleophilic attack O + [M] [M] O O OH + OH2, - H+ [M] [M] B O O [M] NHy Hx [M] N + [M] N [M] Hy NHx [M] + NH3 N2Hx+3 [M] Figure 1.6. Possible pathways for (A) O−O bond formation in water oxidation and (B) N−N bond formation in ammonia oxidation 1.4 Redox Non-Innocence of Thiolate Ligands Cysteine ligation is commonly found in metalloenzyme active sites, and the redox noninnocence of such motifs has been suggested to be a stabilizing factor in accessing formal 10 high-valent metal states under biological conditions.59,60 The redox noninnocence of thiolate ligands is also observed in synthetic complexes, and in fact, dithiolene ligands were the first recognized example of redox non-innocent ligands in coordination compounds.61 More generally, thiolate ligands have been spectroscopically demonstrated to bear significant spin density upon oxidation, which arises from the high degree of covalency in the M–S bond. For example, oxidation state formalism assigns the complexes depicted in Fig. 1.7 as Ni3+/V5+, whereas a purely ligand-based oxidation to generate a thiyl radical would yield no change in metal oxidation state. However, in both cases, the authors suggest that the “true oxidation state” of the metal center is somewhere in between the two resonance structures based on spectroscopic and computational data. In other words, the one-electron oxidation of the starting Ni2+/V4+ complexes is borne out to a significant degree on both the metal center and supporting thiolate ligand(s), which stabilizes the oxidized species of high formal oxidation state. Thiolate-supported transition metal complexes are thus a suitable platform to pursue the spectroscopic characterization and reactivity of oxidized, formally high-valent complexes. Metal-centered oxidation N R S NiIII O N R R S S S V S V S S P N N R R R R S NiII O S P R Ligand-centered oxidation S P R R R S S S IV S V S S P R R R Figure 1.7. Resonance structures for thiolate-supported Ni and V complexes, illustrating the limiting metal-centered and ligand-centered oxidation descriptors. 11 1.5 Chapter Summaries Chapter 2 describes the synthesis and characterization of a thiolate-supported FeIII(H)(N2) species. Based on DFT calculations, there is significant spin delocalization onto the thiolate ligand, which likely stabilizes this S = ½ species at low temperatures to allow for characterization by methods including pulse EPR spectroscopy and X-ray diffraction. Upon warming, this ferric hydride is demonstrated to release H2, and thermochemical and kinetic data support bimolecular reductive elimination as the mechanism for H2 evolution. Notably, a positive entropic activation term is measured for this transformation, which is postulated to arise from dinitrogen dissociation prior to the rate-determining H–H bond forming step. Chapter 3 details the synthesis of an S = ½ NiIII–H species supported by the tetradentate ligand introduced in Chapter 2. Akin to the ferric hydride analogue, this NiIII–H is observed to undergo bimolecular reductive elimination of dihydrogen upon warming, albeit with a measured negative entropic activation parameter. Spectroscopic characterization of the NiIII–H species was carried out at low temperatures, and an isotropic hyperfine coupling value to the hydride is measured (|aiso(1H)| = 11.7 MHz). This measured aiso(1H) is comparable to that measured for the bridging hydride in the Ni-C hydrogenase state, demonstrating that hydrides covalently bound to a paramagnetic nickel center can bear aiso(1H) values on this order of magnitude; previous work on matrix-trapped Ni–H species exhibited aiso(1H) values on an order of magnitude larger. Chapter 4 demonstrates stoichiometric ammonia oxidation mediated by the Ni platform introduced in Chapter 3. This work demonstrates that the H–H homocoupling chemistry observed from the NiIII–H species can be extended to other X-type ligands; the key N–N bond forming step is believed to be the coupling of two NiIII–NH2 species to yield a Ni2(N2H4) complex. Subsequent oxidation of the hydrazine fragment can be carried out to ultimately generate ammonia-derived dinitrogen, and several Ni-bound NxHy intermediates are synthesized and characterized. In Chapter 5, a bimetallic VFe complex featuring a thiolate bridge is prepared, inspired by the V–S–Fe linkage of VFe nitrogenase. This VFe complex is demonstrated to be a highly active catalyst for the disproportionation of hydrazine to ammonia and dinitrogen. 12 The catalytic activity of the heterobimetallic VFe complex is greater than the sum of the activity of representative monometallic V and Fe analogues, suggesting a possible role for bimetallic cooperativity in the hydrazine disproportionation activity. 1.6 References (1) Gray, H. B. Nat. 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Chem. 2011, 50, 9741-9751. 15 Chapter 2 AN S = ½ IRON COMPLEX FEATURING N2, THIOLATE, AND HYDRIDE LIGANDS: REDUCTIVE ELIMINATION OF H2 AND RELEVANT THERMOCHEMICAL FE–H PARAMETERS 2.1 Introduction Despite a wealth of recent progress towards functional models of biological N2-toNH3 conversion,1,2 many questions remain as to how the active site cofactors of nitrogenase enzymes, as in the iron-molybdenum cofactor (FeMoco) of MoFe-nitrogenase,3 manage N2 binding and the subsequent bond-breaking and making steps en route to NH3 formation. While the community has primarily focused on iron as the most likely site/s of N2 binding in recent years,4 a particular conundrum concerns the presence of relatively weak-field sulfide (S2–) ligands around the candidate iron sites,5 as yet unknown ligands in Fe–N2 model chemistry, and the related requirement that N2 binding must occur at a biologically accessible redox potential.6 One plausible scenario to help account for N2 binding at iron at the FeMoco is that redox leveling of successive electron transfer steps may be achieved by concomitant proton transfers7 to generate, for example, protonated sulfides (e.g., SH–)8 and/or iron hydrides (e.g. Fe–H, Fe–(µ-H)–Fe).9 Such a scenario would not only alleviate local charge build-up, thereby buffering redox potential, but would also install comparatively strong-field hydrides that are compatible with, and might even facilitate, Fe–N2 bound states.4,10 Hence, there is substantial motivation to prepare synthetic model complexes featuring Fe–N2 with a combination of thioether11,12/thiolate13,14 and hydride ligands within the immediate iron coordination sphere.15 Relatedly, spin-active model complexes of these types, and S = ½ systems especially, may provide needed spectroscopic parameters to help guide reliable assignments of 16 intermediate states within the biological systems. Accordingly, various EPR techniques have proven effective for observing S = ½ intermediate states during nitrogenase turnover (e.g., hydride- and NxHy-bound states),4,9,16 and an EPR-active, hydride-bound nitrogenase state, where the hydride ligands have been assigned as iron-ligated, has been proposed to undergo H2 elimination concurrent with nitrogen uptake and subsequent reduction.9,17 H2 elimination steps from metal-bound hydride states are also presumed to be relevant to iron-rich hydrogenase enzymes,18,19 and often in synthetic molecular catalysts for the hydrogen evolution reaction (HER),19,20 including examples featuring Fe, Co, and Ni.20bc,21,22 When considering the key H–H bond formation step from metal hydride complexes, there are a number of pathways one might consider, including bond formation via direct protonation of the hydride ligand,23 ligand-facilitated protonation of metal hydrides,20b,24 and reductive elimination from a metal polyhydride species.25 In particular, although bimolecular reductive elimination of H2 from metal hydride species has been demonstrated to play a role in electrocatalytic systems for proton reduction,22 examples of such reactivity from well-defined terminal metal hydride complexes are limited.26 There is thus substantial motivation to map the reactivity patterns and fundamental thermochemical parameters of Fe–H (and other M–H) species in the presence of thiolate and N2 ligands, particularly in systems where H2 evolution is viable. In this study, we report the synthesis and characterization of structurally unusual Fe(H)(N2)(thiolate) complexes in two redox states, S = 0 FeII(H)(N2)(thiolate) and S = ½ FeIII(H)(N2)(thiolate), which have each been characterized by numerous techniques, including by XRD analysis and pulse electron-nuclear double resonance (ENDOR) spectroscopy for the S = ½ state. EPR data for terminally bound and open-shell Fe–H species, regardless of the other ligands in the coordination sphere, is highly limited.2,27,28 Furthermore, prior to this report, Fe–N2 complexes have, to our knowledge, only been characterized in oxidation states of 2+ and lower;29 our finding that Fe(III) binds a weakly activated N2 ligand in the presence of thiolate and hydride ligands is hence noteworthy. The FeIII(H)(N2)(thiolate) species persists at low temperatures in solution but undergoes bimolecular conversion to a diiron FeII–N2–FeII product upon warming, along 17 with associated loss of H2. Such reactivity has strong precedence for second- and third-row transition metal hydrides,26 but is still unusual among first-row metal hydrides,30,31,32,33,34 and among paramagnetic hydrides in general.26ab,27 Thus, the present FeII/FeIII hydride system offers an opportunity to map kinetic parameters for H2 evolution and fundamental Fe–H thermochemical (H+, H•, H−) parameters of broad current interest in the context of HER and small molecule reduction catalysis.35 2.2 Results and Discussion 2.2.1 Synthesis and Characterization of FeII(H)(N2)(thiolate), 2.7-H and FeIII(H)(N2)(thiolate), 2.8-H To template a thiolate-supported iron(N2) complex, we envisioned the incorporation of a thiolate group within a polyphosphino silyl framework, a motif our lab has previously utilized to gain entry into Fe(N2) chemistry (Scheme 2.1).36 Alkylation of 2phenylbenzenethiol (2.1)37 followed by directed ortho lithiation provides the aryl lithium salt (2.3) as a TMEDA adduct (88%). Treatment of bis(o-diisopropylphosphino-phenyl)chlorosilane (2.4)12 with 2.3 affords the ligand HSiP2S (2.5) (Scheme 2.1a; 66%). A singlet corresponding to the two phosphines is observed by 31P NMR spectroscopy at 1.10 ppm, and IR spectroscopy reveals an Si–H stretch at 2228 cm-1. Complexation of 2.5 using FeCl2 and subsequent addition of excess MeMgCl promotes cleavage of the Si–H and S–iPr bonds to afford the thiolate-bound, yellow-brown complex [(SiP2S)Fe]2(µ-N2) (2.6) in moderate yield (Scheme 2.1b). The two previous reports of thiolate-coordinated Fe(N2) species exhibit terminally bound N2 ligands.13,14 Compound 2.6 exhibits a bridging N2 ligand coordinated end-on to each iron center. This is confirmed by X-ray crystallography, which elucidates two similar but crystallographically distinct iron centers in the solid state with a bridging N≡N bond length of 1.138(2) Å (Fig. 2.1a, Table 2.1). A weak N2 stretch at 1888 cm-1 is observed in the solidstate by IR spectroscopy, consistent with the absence of a rigorous inversion center. The 80 K 57Fe Mössbauer spectrum of 2.6 can be fit to one unique iron center (δ = 0.447 mm/s, ΔEQ = 1.776 mm/s) with an isomer shift similar to other five-coordinate, S = 1 iron species ligated 18 A SH SiPr iPrBr Ph nBuLi Ph K2CO3 TMEDA 2.1 2.2 iPr2P iPr2P iPr2P iPr2P H Si Cl SiPr H 2.4 Ph Si SiPr Ph Li · 0.5 TMEDA 2.5 2.3 B Ph 2.5 + FeCl2 iPr2 P PiPr2 MeMgCl 0.5 S Si Fe N N Fe Si S Ph iPr2P P iPr2 2.6 - 0.5 H2 - 0.5 N2 LiEt3BH N [Cp2Co][PF6] or [Cp*2Fe][PF6] N iPr2P H Fe iPr2P Si S Ph iPr2P Si Li(THF)2 S Ph 2.7-H Li(12-c-4)(THF) N N iPr2P H Fe N iPr2P H Fe iPr2P Si 12-crown-4 2.8-H N S Ph 2.7-H(crown) Scheme 2.1. (A) Synthesis of ligand precursor 2.5. (B) Metallation procedure of 2.5 to yield diiron 2.6, and the synthesis of species 2.7-H, 2.7-H(crown) and 2.8-H. 19 Figure 2.1. X-ray structures of (A) 2.6 (B) 2.7-H(crown) (C) 2.8-H. C-H hydrogen atoms, solvent molecules, and the counterion of 2.7-H(crown) are omitted for clarity. Ellipsoids are depicted at 50% occupancy. Table 2.1. Select bond metrics and angles of complexes 2.6, 2.7-H(crown), and 2.8aa d(Fe–N) d(N–N) d(Fe–P) d(Fe–S) ∠(P–Fe–P) ∠(P–Fe–S) 1.871(1) 1.138(2) 2.3059(6) 2.3132(7) 2.2314(5) 130.72(2) 110.53(2) 112.59(2) 1.865(1) - 2.3444(6) 2.2323(6) 2.3189(5) 2.71.810(4) 1.117(6) 2.16(1) 2.339(2) H(crown) 2.180(2) 1.822(3) 1.077(4) 2.2541(8) 2.2185(7) 2.8-H 2.2422(8) a Bond distances listed in Å and bond angles listed in degrees 131.34(2) 110.75(2) 111.98(2) 104.21(6) 106.3(3) 106.98(3) 111.19(3) 2.6 146.5(3) 138.70(3) by related polyphosphine ligands.38 The room temperature solution-state magnetic moment of compound 2.6 (µeff = 4.8µB) suggests an overall S = 2 species, and magnetic susceptibility data collected between 25 ˚C and −75 ˚C obey the Curie-Weiss law. These data are consistent 20 with a description for 2.6 featuring two S = 1 centers with strong ferromagnetic exchange. Strong ferromagnetic coupling has been observed in an S = 3/2, dinitrogen-bridged Fe(I)/Fe(II) species,12 and weak ferromagnetic exchange in an S = 3 Fe(I)/Fe(I) species,39 both previously described by our lab. Alternatively, Fe–NºN–Fe species are known to exhibit antiferromagnetic coupling between iron centers,2 or bear iron centers antiferromagnetically coupled to an S = 1 dinitrogen (N22-) unit.40 To install the desired hydride ligand on iron, we found that treatment of diiron 2.6 with lithium triethylborohydride affords red, diamagnetic [(SiP2S)FeII(H)(N2)]Li(THF)2 (2.7-H). X-ray diffraction data confirms the assignment of 2.7-H and reveals a coordinated dinitrogen with an N–N bond length of 1.128(9) Å. Although the hydride ligand could not be resolved from the XRD analysis of 2.7-H, a wide P–Fe–P angle of 145.24(7)° is consistent with the presence of a hydride located between the two phosphine groups. Furthermore, a hydridic resonance coupled to the two 31P nuclei is observed by 1H NMR spectroscopy at −19.32 ppm (t, 3 JH, P = 71.3 Hz), absent in the 1 H NMR spectrum of [(SiP2S)FeII(D)(N2)]Li(THF)2 (2.7-D); the latter species is obtained in an analogous fashion to 2.7-H with lithium triethylborodeuteride. IR spectroscopy of compound 2.7-H reveals three N2 stretches (2020, 1976, 1935 cm-1) that presumably arise from distinct coordination modes of the lithium cation, and an Fe–H stretch at 1864 cm-1 that is absent in the IR spectrum of 2.7-D. Treatment of 2.7-H with 12-crown-4 affords [(SiP2S)FeII(H)(N2)][Li(12crown-4)(THF)] (2.7-H(crown)), which is isolated as a red solid. IR spectroscopy of 2.7H(crown) shows a single N2 stretch at 1971 cm-1 (n(Fe–H) at 1886 cm-1), and XRD analysis of 2.7-H(crown) confirms its relation to 2.7-H (Fig. 2.1b, Table 1). Room temperature cyclic voltammetry measurements of 2.7-H in THF reveal a reversible oxidation event at −1.63 V vs. Fc/Fc+, corresponding to the FeIII/II couple (Fig. 2.2a). Chemical oxidation of the Fe(II) precursor 2.7-H with [Cp2Co][PF6] affords a dark blue solution that persists for several hours at room temperature, enabling the characterization of the title complex (SiP2S)FeIII(H)(N2) (2.8-H). Neutral 2.8-H cocrystallizes with crystals of the cobaltocene byproduct; despite repeated attempts, 2.8-H and 21 Cp2Co could not be separated, owing to very similar solubility. Nevertheless, high quality X-ray data confirms the solid-state structure of 2.8-H (Fig. 2.1, Table 2.1). Figure 2.2. (A) Cyclic voltammetry of 2.7-H at various scan rates depicting the FeIII/II couple at −1.63 V vs. Fc/Fc+ (0.4 M [NBu4][PF6] in THF). (B) Thin-film IR spectra of isotopologues 2.8-H and 2.8-D. (C) 57Fe Mössbauer spectrum of 2.8-H collected at 80 K with a 50 mT applied parallel field. (D) 77 K CW X-band EPR spectra of isotopologues 2.8-H and 2.8-D in 2-MeTHF. Compared to the crystallographic data for 2.7-H(crown), the structure of 2.8-H reveals elongation of the Fe–N≡N bond (from 1.810(4) Å to 1.882(3) Å) and contraction of the N≡N bond (1.117(6) Å to 1.077(4) Å) upon oxidation, consistent with poorer π-donicity from the more oxidized iron center. The crystallographically determined N≡N bond length of 2.8-H of 1.077(4) Å suggests that the dinitrogen fragment is only weakly activated (1.0975 Å in free N2).41 Furthermore, the Fe–P bond lengths elongate upon conversion of diamagnetic 2.7-H(crown) to doublet 2.8-H, consistent with the observed correlation between spin state and Fe–P bond length noted on related polyphosphino Fe complexes.42 The hydride ligand can be located in the difference map of 2.8-H and freely refined to an Fe–H bond length of 1.54(4) Å, comparable to the crystallographically determined Fe–H bond lengths of other paramagnetic, terminal iron hydride species.28a,b The isotopologue (SiP2S)FeIII(D)(N2) (2.8D) is prepared in an analogous fashion to 2.8-H via oxidation of 2.7-D. Spectroscopic 22 characterization of charge-neutral 2.8-H and 2.8-D is carried out on samples generated in situ at −78 °C. Furthermore, manipulations with 2.8-H and 2.8-D are carried out at low temperature due to their thermal instability (vide infra). The IR spectrum of 2.8-H reveals N2 and Fe–H stretches at 2123 cm−1 and 1852 cm−1, respectively (Fig. 2.2b). Compared to 2.7-H(crown), there is a 152 cm−1 shift in the N2 stretching frequency upon oxidation, consistent with a metal-centered oxidation. The observed Fe–D IR stretch of 2.8-D is in good agreement with the values predicted by the simple harmonic oscillator model (calculated: 1321 cm-1; observed: 1333 cm-1). Additionally, the 57Fe Mössbauer spectrum of 2.8-H is consistent with a single ironcontaining species (δ = 0.227 mm/s, ΔEQ = 1.734 mm/s); these parameters correspond to an approximate fit due to asymmetric line broadening of the spectrum (Fig. 2.2c).43 Broad, paramagnetically shifted peaks are observed by 1H NMR spectroscopy of 2.8-H at −78 °C, and the S = ½ spin state is corroborated by continuous-wave (CW) EPR spectroscopy (Fig. 2.2d, vide infra).44 In addition to the coordination of a strong field hydride ligand, the ferric Fe–N2 species 2.8-H is additionally stabilized by partial spin delocalization onto the aryl thiolate ligand;45 this attenuates the perturbation to Fe–N2 and Fe–H bonding upon oxidation. There is significant contraction in the Fe–S bond of the more oxidized 2.8-H (2.2185(7) Å) compared to 2.7-H(crown) (2.339(2) Å), which contrasts with the observed elongation of the Fe–P bonds in the ferric analogue (Fe–Pavg: 2.17 Å in 2.7-H(crown), 2.25 Å in 2.8-H). The spin density map of the gas-phase optimized 2.8-H structure (M06-L: def2tzvp (Fe), def2svp (all other atoms)) indeed reveals partial spin leakage onto the sulfur atom, with lesser delocalization onto the other coordinated fragments (Fig. 2.3). However, the calculated distribution of unpaired spin suggests that the oxidation is predominantly metal based, which is consistent with the significant shift in N2 stretching frequency observed between 2.7H(crown) and 2.8-H. There is a calculated spin density of 0.92 e– (58%) localized at Fe and 0.18 e– (11%) at sulfur. Additionally, the calculated −0.029 e– (2%) localized on the hydride fragment is consistent with the experimentally observed hyperfine coupling obtained via EPR and ENDOR spectroscopies (vide infra). 23 Figure 2.3. Spin density map of gas-phase optimized structure of compound 2.8-H (Isovalue: 0.005 e–/Å3; M06-L functional: def2tzvp (Fe), def2svp (all other atoms)). Atom colors: Fe (purple), S (yellow), H (white), P (orange), C (gray), N (blue), Si (light blue). 2.2.2 EPR and ENDOR Characterization of 2.8-H The 77 K X-band CW EPR spectrum of 2.8-H (Fig. 2.2d) shows a rhombic signal, which is simulated with slight g anisotropy (g = [2.07, 2.0475, 2.02]). Q-band Davies ENDOR spectra collected at 18.5 K on 2.8-H across the EPR envelope are simulated well (Fig. 2.4a) with coupling to the hydridic 1H nucleus (A(1H) = ±[15, 56, 58] MHz) with a small Euler rotation β = 25° of the 1H A tensor relative to the g-tensor, and two similar but inequivalent 31P nuclei (A(31Pα) = ±[31, 36, 27] MHz; A(31Pβ) = ±[28, 25, 23] MHz). Additional coupling to a 1H nucleus is also observable (A(1H’) = ±[6.8, 10, 6.8] MHz), likely arising from coupling to hydrogen atom(s) of the ligand; this coupling is also present in the spectra of 2.8-D. ENDOR data on 2.8-D are additionally simulated (Fig. 2.4b) with coupling to two 31P nuclei with identical parameters as that of 2.8-H and a 2H nucleus; almost complete disappearance of the 1H hydride signal is also evident. The 2H signal can be suitably simulated by scaling A(1H) by the ratio of the 2H/1H gyromagnetic ratios, (A(2H) = ±[2.3, 8.6, 8.9] MHz), and the X-band CW EPR spectra of both 2.8-H and 2.8-D are simulated well by using the hyperfine coupling constants obtained via ENDOR spectroscopy (Fig. 2.4c,d). Pulse EPR data on an S = ½ freeze-trapped state of MoFe nitrogenase, which has been observed during FeMoco-catalyzed proton reduction9 and nitrogen fixation,46 are consistent with the accumulation of hydride intermediates at the cofactor under turnover conditions. Two 1H nuclei (H1: aiso = 24.3 MHz, T = [−13.3, 0.7, 12.7] MHz; H2: aiso = 22.3 24 MHz, T = [10.7, −12.3, 1.7] MHz) are assigned as Fe–(µ-H)–Fe moieties.9a The 1H signals are believed to arise from bridging hydrides, as opposed to terminal, due to the approximate rhombic symmetry of the dipolar tensor; a point-dipole model predicts a dipolar tensor of approximate axial symmetry for a terminally bound hydride.47 Figure 2.4. Field-dependent Q-band Davies ENDOR of (a) 2.8-H and (b) 2.8-D in 2-MeTHF with simulations. Simulation parameters: g = [2.07, 2.0475, 2.02]; A(31Pα) = ±[31, 36, 27] MHz; A(31Pβ) = ±[28, 25, 23] MHz; A(1H’) = ±[6.8, 10, 6.8] MHz. Fig. 2.4a was simulated with additional coupling to Fe–H (A(1H) = ±[15, 56, 58] MHz), whereas Fig. 2.4b was simulated with additional coupling to Fe–D (A(2H) = ±[2.3, 8.6, 8.9] MHz). Summation of individual component ENDOR simulations is displayed in red. Experimental conditions: microwave frequency = 33.674 GHz; MW π pulse length = 40 ns; interpulse delay τ = 300 ns; "#$ pulse length = 15 µs; TRF delay = 1 µs; shot repetition time (srt) = 5 ms; temperature = 18.5 K; RF frequency randomly sampled. 77 K X-band CW EPR spectra of (c) 2.8-H and (d) 2.8-D in 2-MeTHF with simulations. The CW EPR spectra were simulated with the same parameters as the corresponding ENDOR spectra. 25 Decomposition of the hydridic 1H coupling constants of 2.8-H to the isotropic and dipolar contributions yields an isotropic value of aiso = ± 43 MHz and an approximately axial dipolar tensor of T = ± [−28, 13, 15] MHz, in good agreement with the predicted tensor symmetry for a terminal hydride. Notably, this experimental aiso value indicates that ± 0.030 e– is localized at the hydride ligand,48 which is consistent with the DFT-calculated value of –0.029 e–. The greater aiso value observed for 2.8-H compared to that of the hydride-bound form of FeMoco correlates with greater spin density localized at the hydrogen atom of 2.8H, presumably due to a greater degree of spin delocalization within the cofactor. 2.2.3 Bimolecular H2 Elimination from 2.8-H. Figure 2.5. (A) 1H NMR spectra of 2.8-H in THF-d8 collected at −78 °C (top) and after stirring at 25 °C overnight (middle), with 1H NMR spectrum of 2.6 in THF-d8 (bottom). (B) UV-vis spectra showing the decay of 2.8-H and the growth of 2.6 at 50 °C. Spectra were collected every 30 min. (C) Plot of [2.8-H]-1 (M-1) versus time, using the UV-vis data shown in Fig. 2.5b. (D) Eyring plot of the conversion of 2.8-H to 2.6. (T in K, k in M-1·min-1). Although FeIII(H)(N2)(thiolate) 2.8-H could be spectroscopically characterized, it is thermally unstable in solution and liberates H2. Monitoring a THF-d8 solution of 2.8-H at room temperature by 1H NMR spectroscopy reveals the near quantitative conversion of dark blue 2.8-H to yellow-brown 2.6 overnight, which corresponds to formal loss of an H• and 26 half an N2 molecule per Fe (Fig. 2.5a). H2 liberation was confirmed by GC analysis of the headspace. Monitoring the decay of 2.8-H at 50 °C by UV-vis spectroscopy shows that 2.8-H, with an absorption at 607 nm, decays in concert with the growth of 2.6 (Fig. 2.5b). The decay of 2.8-H follows second-order kinetics (Fig. 2.5c), consistent with a bimolecular H2 reductive elimination pathway to all-ferrous diiron 2.6. The UV-vis timecourse data display isosbestic behavior, and the absence of observable intermediates indicates that transformations prior to the rate determining step are both endergonic and reversible. At 25 °C, the conversion of 2.8H to 2.6 proceeds with a second-order rate constant of k = 0.068 M-1·min-1. Monitoring the decay of 2.8-D reveals a kinetic isotope effect of 1.7 at 25 °C, suggesting a role for the hydride ligand in the transition state of the rate determining step. An early transition state featuring substantial Fe–H character in a bimolecular reductive elimination step is consistent with these data.49 Eyring plot analysis of the conversion of 2.8-H to diiron 2.6 provides the following activation parameters: ∆H‡ = 31(4) kcal/mol; ∆S‡ = 39(13) cal/(mol·K); ∆G‡ = 19 kcal/mol (25 °C) (Fig. 2.5d). The large and positive ∆S‡ value is surprising as the transition state of two iron centers interacting in an ordered manner is likely to incur high entropic cost. For comparison, Wayland’s classic study of Rh(II)-porphyrin metalloradical M–H/M–R species shows that bimolecular release of R–H provides negative entropies of activation correlated with highly ordered, tertiary transition states (e.g., {[Rh]–H×××R–[Rh]}‡).50 For the present iron system, we suggest that the large and positive ∆S‡ value may be rationalized if a requisite N2-dissociation step precedes the rate determining step via preequilibration of an N2-bound and an N2-dissociated state. A plausible mechanism consistent with this scenario and the data in hand is depicted in Fig. 2.6. Accordingly, an N2 ligand of 2.8-H first dissociates, giving rise to free N2 and an unobserved, reactive hydride intermediate, (SiP2S)FeIIIH. This Fe–H intermediate is intercepted by 2.8-H, present at much higher concentration, via a transition state containing one N2 ligand and two Fe–H subunits, then decaying to H2 and the final product, diiron 2.6. While in principle it may also be 27 possible for two molecules of (SiP2S)FeH to react directly to form H2 without a coordinated N2 ligand in the transition state, we favor the scenario shown in Fig. 2.6. N N - H2 , N2 [Fe] H [Fe] N N 2.8-H - N2 2.6 - H2 + N2 H [Fe] [Fe] THF N2 H [Fe] [Fe] H Figure 2.6. Proposed mechanism for the conversion of 2.8-H to 2.6, where [Fe] = (SiP2S)Fe. 2.2.4 Thermochemical Parameters for the Fe–H Subunit in 2.7-H(crown) and 2.8-H The bimolecular elimination of H2 from 2.8-H is indicative of an Fe–H bond with a relatively low homolytic bond strength, consistent with spin density localized on the hydride ligand as evidenced by the large coupling constants to the hydridic 1H nucleus.27,28 Hence, our use of the term “hydride” masks radical H• character present in the Fe–H subunit. We therefore sought to explore this idea in greater detail. The immediate product of H• loss from 2.8-H is the iron(II) species (SiP2S)Fe(N2), but this species is not experimentally observed, presumably because the formation of diiron 2.6 is too facile. However, we have been able to generate and spectroscopically characterize the one- and two-electron reduced relatives of this species, (SiP2S)Fe(N2)n– (n = 1, 2, Scheme 2.2a). Under an N2 atmosphere, reduction of 2.6 with excess Na(Hg) in THF followed by addition of 12-crown-4 affords the S = ½ iron(I) species [(SiP2S)Fe(N2)][Na(12-crown-4)2] (2.9) as a dark red solid (n(N2) = 1963 cm-1). The gas-phase optimized structure of 2.9 (M06L: def2tzvp (Fe), def2svp (all other atoms)) indicates less unpaired spin density localized at the sulfur atom (0.02 e–, 1% overall spin density) than for 2.8-H (0.18 e–, 11% overall spin density). Alternatively, treatment of 2.6 with excess potassium metal in THF under N2 28 provides the dianionic and diamagnetic iron(0) complex [(SiP2S)Fe(N2)][K(THF)x]2 (10) as a dark brown species (n(N2) = 1805 cm-1). The availability of 2.9 and 2.10 provides access to additional data needed to assess the thermochemical properties of the hydride ligand in 2.8-H. A Na(12-c-4)2 N Na(Hg) 12-crown-4 N iPr2P Fe iPr2P Si S Ph 2.9 2.6 K2(THF)x N N K (s) iPr2P Fe iPr2P Si S Ph 2.10 B Me C 2.7-H(crown) 1 atm CO2 MeCN-d3 N iPr2P Fe iPr2P Si S Ph - [HCO2][Li(12-c-4)(THF)] 2.11-d3 Scheme 2.2. (A) Synthesis of 2.9 and 2.10 via reduction of 2.6. (B) Hydride transfer upon treatment of 2.7-H(crown) with CO2. Complex 2.9 shows a reversible FeII/I redox couple in THF at −1.75 V vs. Fc/Fc+, but data collection in MeCN reveals instead an irreversible oxidation event at −1.71 V vs. Fc/Fc+, presumably due to rapid solvent substitution for the N2 ligand upon oxidation to produce (SiP2S)Fe(MeCN), 2.11 (see below). Additionally, a reversible reductive couple (FeI/0) is observed at −2.94 V vs. Fc/Fc+ in MeCN, corresponding to the conversion of [(SiP2S)Fe(N2)]– to [(SiP2S)Fe(N2)]2–. 29 Cyclic voltammograms of the FeII(H)(N2)(thiolate) 2.7-H(crown) derivative in MeCN shows an irreversible oxidative feature at −0.58 V vs. Fc/Fc+. This feature is significantly shifted from the reversible oxidative FeIII/II couple that is recorded in THF (−1.63 V vs. Fc/Fc+). The irreversibility and extreme solvent dependence of the oxidation potential of 2.7-H(crown) indicates that a process that is more complex than outer-sphere electron transfer may be occurring in MeCN. Thus, for the thermochemical calculations presented below, we have utilized −1.63 V vs. Fc/Fc+ as the value for the 2.7-H(crown)/ 2.8H couple, because it is a well-defined, reversible feature as obtained in THF. Exposing a degassed MeCN-d3 solution of 2.7-H(crown) to an atmosphere of CO2 results in complete hydride transfer to yield the formate salt [Li(12-crown-4)][HCO2] and the solvent complex (SiP2S)Fe(CD3CN) (2.11-d3, Scheme 2.2b); the hydricity of 2.7H(crown) in MeCN must therefore be close to, or less than, the hydricity of formate (∆GH= 44 kcal/mol in MeCN).35b Loss of H– from 2.7-H(crown) should generate (SiP2S)Fe(N2), which is not observed due to facile ligand substitution by MeCN to produce the MeCN adduct 2.11-d3 instead. Thus, the hydricity of 2.7-H(crown) can be estimated to have an upper bound of ~44 kcal/mol in MeCN.51 There are two previous reports of the hydricity of terminally-bound Fe–H species.52,53 A study by our lab of a related five-coordinate iron(II) hydride complex, (SiP3)Fe(H)(H2), estimated a hydricity of 54.3 ± 0.9 kcal/mol in MeCN,52 significantly less hydridic than 2.7-H(crown), likely in part reflective of the different charges. Utilizing available thermodynamic values that relate H+, H•, and H– in MeCN (eq. 2.1, 2.2),35b the upper bounds for the free energies of H+/H•/H– transfer from 2.7-H(crown) and H+/H• transfer from 2.8-H can be related (Fig. 2.7, eq. 2.3-2.6). The approximated upper bounds for homolytic and heterolytic values of Fe–H bond cleavage from 2.7-H(crown) and 2.8-H are shown in Table 2.2. Although absolute free energy values for the H+/H•/H– transfers (∆G) are not established, the difference between any two such free energies (∆∆G) can be determined from eq. 2.1-2.6. The Fe–H bond of 2.7-H(crown) is estimated to have a bond dissociation free energy (BDFE) of < 57 kcal/mol and a pKa of < 53. Compared to the 30 Fe–H bond of 2.7-H(crown), the Fe–H bond of 2.8-H is considerably more acidic, by more than 22 pKa units, with an estimated upper bound of pKa < 30. + -H +H + + H (SiP2S)Fe(H)N2 + e- e- - H. + H. (SiP2S)FeN22- - + -H- - eH+ + H. - e- + - H. + e- (SiP2S)FeN2- -H + e- (SiP2S)FeN2 (SiP2S)Fe(H)N2- Figure 2.7. Thermochemical scheme relating H+, H•, and H– transfers from 2.7-H(crown) and 2.8-H. H• → H+ + e- (ΔG = - 53.6 kcal/mol) (2.1) H- → H • + e- (ΔG = - 26.0 kcal/mol) (2.2) ΔG(2.7-H(crown))H+ = ΔG(2.7-H(crown))H. - 23.06(Eox(2.10)) - 53.6 (2.3) ΔG(2.7-H(crown))H- = 23.06(Eox(2.7-H(crown)) + ΔG(2.8-H)H. + 26.0 (2.4) ΔG(2.8-H)H+ = ΔG(2.8-H)H. - 23.06(Eox(2.9)) - 53.6 (2.5) ΔG(2.7-H(crown))H. = ΔG(2.8-H)H. + 23.06(Eox(2.7-H(crown) - Eox(2.9)) (2.6) The estimated upper bound for the BDFE of the Fe–H bond in FeIII(H)(N2)(thiolate) 2.8-H of 56 kcal/mol is in close agreement with the DFT-predicted value of 55.6 kcal/mol (M06-L; def2tzvp (Fe), def2svp (all other atoms) in MeCN solvation). It has been previously suggested that M–H bonds with bond dissociation enthalpies of < 56 kcal/mol54 are competent to release H2 at room temperature (BDFE(H2) in MeCN = 102.3 kcal/mol).55 Interestingly, eq. 2.6 indicates that the Fe–H BDFE of Fe(II) 2.7-H(crown) is only 2.8 kcal/mol greater than that of Fe(III) 2.8-H in THF, but 2.7-H(crown) is observed to be stable in solution at room temperature, whereas 2.8-H undergoes clean conversion to 2.6 with release of H2. 31 Table 2.2. Oxidation potentials and thermochemical parameters pertaining to Fe–H bond cleavage of 2.7-H(crown) and 2.8-H a Compound Hydricitya BDFEa pKa Eoxb 2.7-H(crown) < 44 < 57 < 53 −1.63 2.8-H - < 56 < 30 - 2.9 - - - −1.71 2.10 - - - −2.94 values in kcal/mol b potentials reported in V vs. Fc/Fc+ Although there is only a small difference in BDFE between the ferrous and ferric hydrides, the Keq for homolytic Fe–H bond cleavage is thus about 100 times greater at room temperature for 2.8-H than 2.7-H(crown); this is further biased by the thermodynamic stability afforded by the formation of 2.6 rather than (SiP2S)FeN2, the direct product of Fe– H bond homolysis from 2.8-H. Additionally, electrostatic penalties associated with a biomolecular reaction between two anionic species may be a kinetic impediment to H2 release. Finally, assuming N2 dissociation would need to precede an H2 evolution in 2.7H(crown), as we have suggested is likely for 2.8-H (Fig. 2.6), the stability of 2.7-H(crown) may be correlated to a greater kinetic barrier to N2 dissociation from 2.7-H(crown), which features a more strongly coordinated N2 ligand. 2.3 Conclusion The synthesis of a trivalent, thiolate-FeIII(H)(N2) complex has been accomplished and serves as an unusual Fe–N2 species from the perspective of both oxidation state and the presence of both hydride and thiolate ligands. This is the first example of a ferric dinitrogen complex, and it is sufficiently persistent at low temperature to be amenable to characterization by X-ray crystallography and various spectroscopic techniques, including CW EPR and ENDOR spectroscopies. Additionally, we have demonstrated, via kinetic measurements, that this ferric Fe–H complex undergoes bimolecular reductive elimination to liberate H2 with a primary kinetic deuterium isotope effect. To the best of our knowledge, 32 this is the first example of a well-defined, bimolecular H2 elimination process from a terminal Fe–H species. Surprisingly, the entropy of activation for H2 elimination is positive (∆S‡ = 39(13) cal/(mol·K)), whereas a large and negative value would be anticipated based solely on the prediction of a highly ordered transition state ([Fe–H×××H–Fe]‡). Pre-dissociation of N2 from thiolate–FeIII(H)(N2) to afford thiolate–FeIII(H), which is then captured by thiolateFeIII(H)(N2) to proceed to an H–H bond-forming transition state, is invoked to accommodate the collective data. Finally, synthetic access to thiolate–FeII(H)(N2) and thiolate–FeIII(H)(N2) species, in addition to the reduced derivatives thiolate–FeI(N2)– and thiolate–Fe0(N2)2–, has provided access to the physical detail needed to estimate important thermochemical H+, H•, and H– parameters of broad current interest and within an S = ½ Fe system that evolves H2. 2.4 Experimental Section 2.4.1 Experimental Details 2.4.1.1 General Considerations All syntheses and measurements, unless otherwise stated, were carried out under an inert atmosphere (N2) in a glovebox or using standard Schlenk techniques, and solvents were dried and degassed by thoroughly sparging with N2 and then passing through an activated alumina column in a solvent purification system supplied by SG Water, LLC. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3 Å molecular sieves before use. 2-phenylbenzenethiol,56 bis(odiisopropylphosphino-phenyl)-chlorosilane (2.4),57 lithium triethylborodeuteride58 were prepared according to literature procedures. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. 2.4.1.2 Physical Methods Electrochemical measurements were carried out in a glovebox under an N2 atmosphere in a one-compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and a platinum wire was used as the auxiliary electrode. A silver pseudoreference electrode was used with the 33 ferrocene couple (Fc/Fc+) as an internal reference, unless otherwise noted. Solutions of electrolyte (0.4 M [NBu4][PF6] in THF) and analyte were also prepared under an N2 atmosphere. NMR spectra (1H, 13C, 31P) were collected on Varian 300, 400, or 500 MHz spectrometers (25 °C unless otherwise specified). 1H and 13C chemical shifts are reported in ppm, relative to tetramethylsilane using residual proton and 13C resonances from solvent as internal standards. 31P chemical shifts are reported in ppm relative to 85% aqueous H3PO4. Thin film IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. Optical spectroscopy measurements were taken on a Cary 50 UV-Vis spectrophotometer using a 1-cm quartz cell, unless otherwise noted. Temperature regulation for UV-Vis measurements was carried out with a Unisoku cryostat. Time-course UV-Vis spectra were collected with the Scanning Kinetics application of the Cary WinUV software and fit to Gaussian lineshapes with the Curve Fitting tool of Matlab. H2 was analyzed on an Agilent 7890A gas chromatograph (HP-PLOT U, 30 m, 0.32 mm ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. Combustion analyses were carried out by either Midwest Microlabs (Indianapolis) or the Beckman Institute Crystallography Facility (Caltech). 2.4.1.3 X-ray Crystallography X-ray diffraction measurements were carried out in the Beckman Institute Crystallography Facility. XRD measurements were collected using a dual source Bruker D8 Venture, four-circle diffractometer with a PHOTON CMOS detector. Structures were solved using SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL. The crystals were mounted on a glass fiber under Paratone N oil. 2.4.1.4 DFT Calculations Optimization and frequency calculations were performed using the Gaussian 09 program.59 Structures of 2.8-H and 2.9 were optimized using the crystal structure coordinates as the input. Structures utilized for thermochemical estimations were optimized with MeCN solvation. The M06-L functional60 with the def2-TZVP61 basis set was used on Fe, and the 34 def2-SVP basis set was used on all other atoms. DFT-estimated free energies of H• transfer in MeCN are referenced to H• dissociation from TEMPO in MeCN (BDFE of TEMPO-H = 66.5 kcal/mol).62 2.4.1.5 57Fe Mössbauer Spectroscopy Mössbauer spectra were recorded on a spectrometer from SEE Co. (Edina, MN) operating in the constant acceleration mode in a transmission geometry. The sample was kept in an SVT-400 cryostat from Janis (Wilmington, MA). The quoted isomer shifts are relative to the centroid of the spectrum of a metallic foil of α-Fe at room temperature. Solid samples were prepared by grinding polycrystalline material into a fine powder and then mounted in a Delrin cup fitted with a screwcap as a boron nitride pellet. Frozen solution samples were prepared by freezing the sample in a Delrin cup. All samples were prepared within a glovebox with rapid transfer to a liquid nitrogen bath before mounting in the cryostat. Data analysis was performed using the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. 2.4.1.6 CW EPR Spectroscopy 77 K X-band EPR spectra were obtained on a Bruker EMX spectrometer on solutions prepared as frozen glasses in 2-MeTHF. Spectra were simulated using the EasySpin63 suite of programs with Matlab. 2.4.1.7 Pulse EPR Spectroscopy All pulse Q-band (≈ 33.7 GHz) EPR and electron nuclear double resonance (ENDOR) experiments were aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker D2 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC. Pulse Q-band ENDOR was acquired using the Davies pulse sequence (" − &#$ − "#$ − &#$ − "/2 – * – " – echo), where &#$ is the delay between mw pulses and RF pulses, 35 "#$ is the length of the RF pulse and the RF frequency is randomly sampled during each pulse sequence. In general, the ENDOR spectrum for a given nucleus with spin += ½ (1H, 31P) coupled to the S = ½ electron spin exhibits a doublet at frequencies 0 ,± = / ± ,1 / 2 (2.7) where ,1 is the nuclear Larmor frequency and 0 is the hyperfine coupling. For nuclei with + ≥ 1 (2H), an additonal splitting of the ,± manifolds is produced by the nuclear quadrupole interaction (P) ,±,56 = / ,1 ± 38(2:; − 1) / 2 (2.8) Simulations of all pulse EPR data were achieved using the EasySpin63 simulation toolbox (release 5.1.8) with Matlab 2016 using the following Hamiltonian: > = ?@ A B⃑D EFG + ?1 E1 A B⃑D +G + ℎFG ∙ K ∙ +G + ℎ+G ∙ L ∙ +G = (2.9) In this expression, the first term corresponds to the electron Zeeman interaction term where ?@ is the Bohr magneton, g is the electron spin g-value matrix with principle components g = [gxx gyy gzz], and FG is the electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where ?1 is the nuclear magneton, E1 is the characteristic nuclear g-value for each nucleus (e.g. 1H, 2H, 31P) and +G is the nuclear spin operator; the third term corresponds to the electron-nuclear hyperfine term, where K is the hyperfine coupling tensor with principle components K = [Axx Ayy Azz]; and for nuclei with + ≥ 1, the final term corresponds to the nuclear quadrupole (NQI) term which arises from the interaction of the nuclear quadrupole moment with the local electric field gradient (efg) at the nucleus, where L is the quadrupole coupling tensor. In the principle axis system (PAS), L is traceless and 36 parametrized by the quadrupole coupling constant M N OP/ℎ and the asymmetry parameter Q such that: 8SS L= R 0 0 where [ \ ]^ _ 0 8UU 0 0 M N OP/ℎ −(1 − Q) 0W= Y 0 4+(2+ − 1) 8VV 0 = 2+ (2+ − 1)8VV and Q = `aa b`cc `dd 0 −(1 + Q) 0 0 0Z 2 (2.10) . The asymmetry parameter may have values between 0 and 1, with 0 corresponding to an electric field gradient with axial symmetry and 1 corresponding to a fully rhombic efg. The orientations between the hyperfine and NQI tensor principle axis systems and the g-matrix reference frame are defined by the Euler angles (α, β, γ). 2.4.2 Synthetic Details 2.4.2.1 Synthesis of 2-(isopropylthio)biphenyl (2.2) A mixture of 2-phenylbenzenethiol (26.5 g, 0.142 mol), 2-bromopropane (17.5 g, 0.142 mol) and K2CO3 (29.4 g, 0.213 mol) in acetone (500 mL) was allowed to reflux for 5 h. The reaction was cooled to room temperature and filtered through Celite. The filtrate was concentrated to an oil and purified via vacuum distillation to yield the title compound as a pale yellow oil (26.0 g, 80%). 1H NMR (CDCl3, 400 MHz, 298 K, δ): 7.49 – 7.22 (m, 9H), 3.20 (hept, J = 6.7 Hz, 1H), 1.19 (d, J = 6.9 Hz, 6H). 13C NMR (CDCl3, 101 MHz, 298 K) δ 143.5 (s), 141.1 (s), 134.7 (s), 130.7 (s), 130.5 (s), 129.6 (s), 127.9 (s), 127.7 (s), 127.3 (s), 126.1 (s), 37.3 (s), 23.0 (s). 2.4.2.2 Synthesis of (2-(isopropylthio)-[1,1'-biphenyl]-3-yl)lithium · 0.5 TMEDA (2.3) Neat tetramethylethylenediamine (4.0 mL, 26.7 mmol) and nBuLi (16.4 mL, 1.6 M in hexanes) were sequentially added dropwise to a stirring solution of compound 2.2 (6.0 g, 26.3 mmol) in pentane (100 mL) at 0 °C. The yellow reaction mixture was stirred at 23 °C overnight, which yielded ample white precipitate. The solids were collected by filtration and 37 washed with pentane (3 x 10 mL) to yield the title compound as an off-white solid. (6.75 g, 88%) 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.05 (bs, 1H), 7.79 (d, J = 6.0 Hz, 2H), 7.41 – 7.31 (m, 3H), 7.21 – 7.17 (m, 2H), 2.98 (bs, 1H), 1.91 (s, 6H, TMEDA), 1.75 (s, 2H, TMEDA), 1.06 (bs, 6H). 2.4.2.3 Synthesis of HSiP2S (2.5) A suspension of compound 2.3 (4.54 g, 15.5 mmol) in toluene (20 mL) was added to a stirring solution of bis(o-diisopropylphosphino-phenyl)-chlorosilane (2.4) (5.84 g, 12.9 mmol) in toluene (30 mL) at –78 °C. The reaction mixture was stirred for 12 h, with gradual warming from –78 °C to 23 °C. The resulting orange reaction mixture was filtered through Celite and concentrated. The resulting orange oil was triturated with pentane (20 mL) to yield the title compound as an off-white solid, which was isolated by filtration and washed with cold pentane (–78 °C, 40 mL). The filtrate was concentrated to 10 mL and stored at –33 °C overnight, which yielded additional product. (5.47 g, 66%) 1H NMR (C6D6, 400 MHz, 298 K, δ): 7.55 (d, J = 7.3 Hz, 2H), 7.44 (d, J = 8.0 Hz, 2H), 7.40 (d, J = 7.5 Hz, 2H), 7.29 – 7.26 (m, 3H), 7.23 – 7.16 (m, 3H), 7.11 – 7.01 (m, 4H), 2.99 (hept, J = 6.3 Hz, 1H), 2.14 – 1.98 (m, 4H), 1.15 (m, 12H), 1.04 – 0.97 (m, 18H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 1.10 (s). IR (solid, cm-1): 2228 (Si–H). ESI-MS (positive ion, amu): Calcd. 643.3 ([M+H]+); Found. 643.2. 2.4.2.4 Synthesis of [(SiP2S)Fe]2(µ-N2) (2.6) A solution of 2.5 (1.0 g, 1.6 mmol) in THF (15 mL) was added to solid FeCl2 (0.28 g, 1.6 mmol) and stirred for 9 h. The reaction mixture was cooled to −78 °C, and MeMgCl (3.0 M in THF, 1.6 mL) was added dropwise to the stirring solution. The reaction mixture was stirred overnight with gradual warming from –78 °C to 23 °C and stirred at 23 °C for an additional day. The reaction mixture was filtered through Celite and concentrated to dryness. The resulting brown solids were washed with pentane (20 mL), extracted with C6H6 (50 mL) and lyophilized. The brown material was redissolved in C6H6 (20 mL) and filtered through Celite. The filtrate was lyophilized to yield the title compound as a dark brown solid. (crude yield: 1.09 g, 52%) Smaller portions of the crude product were recrystallized prior to use: Pentane 38 (10 mL) was layered over a benzene (5 mL) solution of 2.4 (160.0 mg, 0.1 mmol) and allowed to stand at 23 °C, which yielded the product as dark brown blocks (62 mg, 39% yield from recrystallization). Dark brown crystals suitable for XRD were grown from slow evaporation of a Et2O solution at 23 °C. 1H NMR (C6D6, 300 MHz, 298 K, δ): 83.0, 52.6, 27.0, 13.7, 9.9, 8.7, 7.2, 6.4, 6.0, 4.9, 4.0, 3.6, 3.3, −2.5, −7.7, −70.3. µeff (C6D5CD3, Evans method, 298 K): 4.8µB. IR (solid, cm-1): 1888 (N≡N). UV-Visible (THF, 298 K, nm {M-1cm1 }): 495 {4478}, 678 {1273}, 773 {1250}, 1019 {1210}. Anal. Calcd. for [(SiP2S)Fe]2(N2)·(C6H6)0.5 (C75H141Fe2N2P4S2Si2): C, 65.45; H, 6.66; N, 2.04. Found: C, 65.51; H, 7.03; N, 2.16. 2.4.2.5 Synthesis of [(SiP2S)Fe(H)(N2)]Li(THF)2 (2.7-H) To a stirring solution of 2.6 (0.100 g, 0.074 mmol) in THF (5 mL), LiEt3BH (1.0 M in THF, 0.16 mL) was added dropwise at –78 °C. The reaction mixture was stirred at 23 °C for 30 min. The reaction mixture was subsequently chilled to –78 °C, and a second portion of LiEt3BH (1.0 M in THF, 0.16 mL) was added. The dark red reaction mixture was stirred at 23 °C for 30 min, and the volatiles were then removed in vacuo. The resulting red residue was washed with 2:1 pentane/Et2O (3 mL) and pentane (10 mL) then extracted with THF (10 mL) to yield compound 2.7-H as a red solid. (97 mg, 78%) Dark red crystals suitable for XRD were grown from slow evaporation of a Et2O solution into HMDSO at 23 °C. 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.58 (d, J = 7.4 Hz, 2H), 8.10 (d, J = 7.4 Hz, 1H), 7.71 (d, J = 7.5 Hz, 2H), 7.52 (d, J = 7.3 Hz, 2H), 7.39 (t, J = 7.3 Hz, 2H), 7.22 (t, J = 7.4 Hz, 2H), 7.10 (t, J = 7.6 Hz, 2H), 6.97 (q, J = 10.3, 9.0 Hz, 2H), 6.88 (d, J = 7.2 Hz, 1H), 3.30 (bs, 8H, THF), 2.78 (bs, 2H), 2.51 (bs, 2H), 1.64 (q, J = 6.9 Hz, 8H, THF), 1.36 – 1.28 (m, 12H), 1.10 – 1.00 (m 12H), −19.32 (t, J = 71.3 Hz, 1H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 96.8 (s), 96.4 (s). IR (solid, cm-1): 2020 (N≡N), 1976 (N≡N), 1935 (N≡N), 1864 (Fe–H); the inequivalent N2 stretches arise from distinct coordination modes of the Li cation. UV-Visible (THF, 298 K, nm {M-1cm-1}): 336 {6903}, 379 {4263}, 437 {3194}, 535 {1471}. Anal. Calcd. for C44H61FeLiN2O2P2SSi: C, 63.30; H, 7.37; N, 3.36. Found: C, 63.32; H, 7.59; N, 3.23. 39 2.4.2.6 Synthesis of [(SiP2S)Fe(D)(N2)]Li(THF)2 (2.7-D) Prepared in an analogous fashion to 2.7-H but employing LiEt3BD (1.0 M in THF). Except for the absence of the hydridic proton resonance at −19.32 ppm, spectroscopic features in the 1 H NMR spectrum of 2.7-D were identical to that of 2.7-H. 31P NMR (C6D6, 162 MHz, 298 K, δ): 96.85 (t, J = 9.3 Hz). 31P{2H} NMR (C6D6, 162 MHz, 298 K, δ): 96.85 (s). IR (solid, cm-1): 2016 (N≡N), 1975 (N≡N), 1924 (N≡N); the inequivalent N2 stretches arise from distinct coordination modes of the Li cation. 2.4.2.7 Synthesis of [(SiP2S)Fe(H)(N2)][Li(12-crown-4)(THF)] (2.7-H(crown)) Neat 12-crown-4 (5.8 µL, 36 µmol) was added in one portion to a homogeneous stirring solution of 2.7-H (30 mg, 36 µmol) in Et2O (10 mL) at 23 °C. Red solids immediately precipitated from solution upon addition of 12-crown-4. The reaction mixture was stirred at 23 °C for 5 min. The resulting solids were isolated by filtration, washed with Et2O (5 mL) and pentane (5 mL), then extracted with THF (5 mL) to yield the title compound as a red solid. (26 mg, 76%) Dark red crystals suitable for XRD were grown from slow diffusion of pentane into a concentrated THF solution at 23 °C. 1H NMR (THF-d8, 400 MHz, 298 K, δ): 8.25 (d, J = 6.9 Hz, 2H), 7.60 (d, J = 7.5 Hz, 2H), 7.36 (bs, 3H), 7.19 – 7.11 (m, 4H), 7.01 (t, J = 7.2 Hz, 3H), 3.62 (bs, 16H, 12-crown-4), 2.60 (bs, 2H), 2.40 (bs, 2H), 1.41 (q, J = 6.8 Hz, 6H), 1.32 (q, J = 6.9 Hz, 6H), 0.78 (q, J = 6.3 Hz, 6H), 0.71 (q, J = 6.4 Hz, 6H), −19.39 (t, J = 69.6 Hz, 1H). 31P NMR (THF-d8, 162 MHz, 298 K, δ): 101.20 (s), 100.87 (s). IR (solid, cm-1): 1971 (N≡N), 1886 (Fe–H). UV-Visible (THF, 298 K, nm {M-1cm-1}): 333 {8713}, 421 {4674}, 532 {1906}, 885 {280}. Anal. Calcd. for C48H69FeLiN2O5P2SSi: C, 61.40; H, 7.41; N, 2.98. Found: C, 61.26; H, 7.71; N, 2.67. 2.4.2.8 Synthesis of (SiP2S)Fe(H)(N2) (8-H) 1 H NMR (THF-d8, 500 MHz, 195 K, δ): 11.0, 5.8, 5.4, 4.7, 3.6, 1.3, 0.9, -7.5. IR (solid, cm- 1): 2123 (N≡N), 1852 (Fe–H). UV-Visible (THF, 298 K, nm {M-1cm-1}): 607 {2830}. *Note: The thermal instability of 2.8-H precluded characterization by combustion analysis. 40 2.4.2.8.1 EPR and ENDOR Spectroscopies Solid [Cp*2Fe][PF6] (1.4 mg, 3.0 µmol) was added in one portion to a solution of 2.7-H (4.8 mg, 5.7 µmol) in 2-MeTHF (0.5 mL, –78 °C). The reaction was allowed to stir at –78 °C for 1 h and the resulting dark blue solution was filtered through a pre-chilled pipette filter (pipette/glass fiber) to remove excess [Cp*2Fe][PF6]. The filtered solution was analyzed directly. *Note: 77 K X-band EPR spectra of 2.7-H generated via oxidation with [Cp2Co][PF6] and [Cp*2Fe][PF6] were identical, see Figure S41. 2.4.2.8.2 Representative Sample Preparation for all other Spectroscopies Solid [Cp2Co][PF6] (2.2 mg, 6.6 µmol) was added in one portion to a solution of 2.7-H (5.0 mg) in THF (0.5 mL, −78 °C). The reaction was allowed to stir at –78 °C for 1 h and the resulting dark blue solution was filtered through a pre-chilled pipette filter (pipette/glass fiber) to remove excess [Cp2Co][PF6]. This filtered solution was analyzed via the procedures detailed below. 2.4.2.9 Synthesis of (SiP2S)Fe(D)(N2) (2.8-D) Prepared in situ in an analogous fashion to 8-H but employing 7-D. IR (solid, cm-1): 2121 (N≡N), 1333 (Fe–D). 2.4.2.10 Synthesis of [(SiP2S)Fe(N2)][Na(12-crown-4)2] (2.9) A solution of 2.6 (50 mg, 0.037 mmol) in THF (2 mL) was added to Na(Hg) (12 mg Na, 0.52 mmol; 2 g Hg) and stirred vigorously for 5 h at 23 °C. The resulting red solution was decanted, and neat 12-crown-4 (24 µL) was added in one portion to the reaction mixture. The reaction was stirred at 23 °C for 1 h and subsequently layered with pentane (2 mL). The mixture was allowed to stand at –33 °C overnight, resulting in the precipitation of dark red solids. The solids were collected by vacuum filtration, washed with Et2O (2 mL x 2) and pentane (2 mL x 2), then extracted with THF (5 mL) to yield the title compound as a red solid. (61.0 mg, 77%). Dark red crystals suitable for XRD were grown from slow diffusion of pentane into a concentrated THF solution at 23 °C. 1H NMR (THF-d8, 400 MHz, 298 K, 41 δ) 11.6, 8.90, 7.6, 6.6, 6.2, 5.7, 3.6, 0.1, −3.3. µeff (THF-d8, Evans method, 298 K): 1.8µB. IR (solid, cm-1): 1963 (N≡N). UV-Visible (THF, 298 K, nm {M-1cm-1}): 547 {2020}, 826 {499}. Anal. Calcd. for [(SiP2S)Fe(N2)][Na(12-crown-4)2]·THF0.5 (C54H80FeN2NaO8.5P2SSi): C, 59.28; H, 7.37; N, 2.56. Found: C, 58.98; H, 7.66; N, 2.40. 2.4.2.11 Synthesis of [(SiP2S)Fe(N2)][K(THF)x] (2.10) A solution of 2.6 (30 mg, 0.022 mmol) in THF (5 mL) was stirred with potassium metal (25 mg, 0.64 mmol) at 23 °C for 15 min. The resulting dark brown solution was filtered and concentrated to dryness to yield the title complex as a dark brown solid. (37 mg) *Note: The broad signals in the room temperature 1H NMR spectrum hindered reliable peak integration. 1 H NMR (THF-d8, 300 MHz, 298 K, δ) 7.92 (bs), 7.63 (bs), 7.23 (bs), 7.02 (t, J = 7.0, 2.2 Hz), 6.79 (bs), 6.43 (bs), 2.41 (bs), 1.09 (bs), 0.78 (bs). 31P NMR (THF-d8, 121 MHz, 298 K, δ): 93.6 (bs). IR (solid, cm-1): 1805 cm-1. 2.4.2.12 Synthesis of (SiP2S)Fe(NCMe) (2.11) MeCN (0.5 mL) was added to a solution of 2.6 (50 mg, 0.037 mmol) in C6H6 (3 mL) at 23 °C, and the resulting dark red solution was stirred at 23 °C for 15 min. The reaction mixture was concentrated to dryness, and the red solids were washed with pentane (3 mL x 2) and extracted with C6H6 (5 mL). The volatiles were removed in vacuo, and the title complex was isolated as a red solid. (48 mg, 92%) Dark red crystals suitable for XRD were grown from slow diffusion of pentane into a concentrated THF solution 23 °C. 1H NMR (400 MHz, C6D6) δ 96.1, 80.5, 51.3, 15.9, 8.9, 8.5, 7.3, 7.2, 6.1, 5.8, 4.2, 2.4, −1.0, −2.8, −4.1, −43.6. µeff (C6D6, Evans method, 298 K): 2.9µB. UV-Visible (THF, 298 K, nm {M-1cm-1}): 485 {2405}, 680 {325}, 1029 {639}. Anal. Calcd. for C38H47FeNP2SSi: C, 65.60; H, 6.81; N, 2.01. Found: C, 65.72; H, 6.85; N, 1.91 2.4.3 Quantification of H2 from the Conversion of 2.8-H to 2.6 A representative sample preparation: A filtered solution of 2.8-H, which was generated in situ in THF at −78 °C from 2.7-H and [Cp2Co][PF6], (800 µL, 35.5 mM) was transferred into a Schlenk tube and sealed. 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In Recent Advances in Hydride Chemistry; Poli, R., Peruzzini, M., Eds. Elsevier B. V. 2001; pp 139188. (28) (a) Chiang, K. P.; Scarborough, C. C.; Horitani, M.; Lees, N. S.; Ding, K.; Dugan, T. R.; Brennessel, W. W.; Bill, E.; Hoffman, B. M.; Holland, P. L. Angew. Chemie. Int. Ed. 2012, 51, 3658-3662. (b) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Organometallics. 1992, 11, 1429-1431. (c) Hamon, P.; Hamon, J.-R.; Lapinte, C. J. Chem. Soc., Chem. Commun. 1992, 1602-1603 (29) Crossland, J. L.; Tyler, D. R. Chem. Rev. 2010, 254, 1883-1894. (30) For studies of heterobimetallic bimolecular HER, see: (a) Mazzacano, T. J.; Mankad, N. P. J. Am. Chem. Soc. 2013, 135, 17258-17261. (b) Parmelee, S. R.; Mazzacano, T. J.; Zhu, Y.; Mankad, N. P.; Keith, J. A. ACS Catal. 2015, 5, 3689-3699. (31) Koelle, U.; Ohst, S. Inorg. Chem. 1986, 25, 2689-2694. (32) For related examples of unimolecular reductive elimination from multimetallic, firstrow transition metal hydride complexes, see: (a) Vollhardt, K. P. C.; Cammack, J. K.; Matzger, A. J.; Bauer, A.; Capps, K. B.; Hoff, C. D. Inorg. Chem. 1999, 38, 2624-2631. (b) Lee, Y.; Anderton, K. J.; Sloane, F. T.; Ermert, D. M.; Abboud, K. A.; García-Serres, R.; Murray, L. J. J. Am. Chem. Soc. 2015, 137, 10610-10617. (c) Manz, D.-H.; Duan, P.-C.; 45 Dechert, S.; Demeshko, S.; Oswald, R.; John, M.; Mata, R. A.; Meyer, F. J. Am. Chem. Soc. 2017, 139, 16720-16731. (d) Bellows, S. M.; Arnet, N. A.; Gurubasavaraj, P. M.; Brennessel, W. W.; Eckhard, B.; Cundari, T. R.; Holland, P. L. J. Am. Chem. Soc. 2016, 138, 1211212123. (e) Yu, Y.; Sadique, A. R.; Smith, J. R.; Dugan, T. R.; Cowley, R. E.; Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6624-6638. (f) Ding, K.; Brennessel, W. W.; Holland, P. L. J. Am. Chem. Soc. 2009, 131(31), 10804-10805. (33) The elimination of H2 from two M–H species (2 M–H → H2 + M2) may also occur through a M–M bonded species, whereby the intermediate prior to reductive elimination is a dinuclear M2H2 species, see: (a) Halpern, J.; Pribaníc, M.; Inorg. Chem. 1970, 9, 2616-2618. (b) Ungváry, F.; Markó, L. J. Organomet. Chem. 1969, 20, 205-209. (c) Trinquier, G.; Hoffmann, R. Organometallics 1984, 3, 370-380. (34) It has been reported that CpFe(CO)2H (FpH) undergoes reductive elimination of H2 to yield {CpFe(CO)2}2. See: Green, M. L. H.; Street, C. N.; Wilkinson, G. Z. Naturforschg. 1959, 14, 738. However, subsequent studies provided evidence to suggest that the H2 evolution was catalyzed by the presence of a trace oxidant. See: Shackleton, T. A.; Mackie, S. C.; Fergusson, S. B.; Johnston, L. J.; Baird, M. C. Organometallics 1990, 9, 2248-2253. The reverse bimolecular oxidative addition of H2 (Fp2 + H2 ® 2 FpH) is known to occur at high pressures of H2 (Fp2 + H2 ® 2 FpH). See: Chang, B.-H.; Coil, P. C.; Brown, M. J.; Barnett, K. W. J. Organomet. Chem. 1984, 270, C23-C25. (35) (a) Waldie, K. M.; Ostericher, A. L.; Reineke, M. H.; Sasayama, A. F.; Kubiak, C. P. ACS Catal. 2018, 8, 1313-1324. (b) Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M. Chem. Rev. 2016, 116, 8655-8692. (c) Pearson, R. G. Chem. Rev. 1985, 85, 41-49. (36) (a) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507-2517. (b) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558-565. (37) McGinley, P. L.; Koh, J. T. J. Am. Chem. Soc. 2007, 129, 3822-3823. (38) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 53415350. 46 (39) Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.; Mehn, M. P.; Betley, T. A.; Lu, C. C.; Peters, J. C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17107-17112. (40) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.; Münck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181-10192. (41) M. D. Fryzuk and S. A. Johnson Coord. Chem. Rev. 2000, 200, 379. (42) (a) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 7148-7165. (b) Creutz, S. E.; Peters, J. C. Inorg. Chem. 2016, 55, 3894-3906. (43) An asymmetric 57Fe Mössbauer spectrum has been similarly observed for (a) [Fe(Cp*)(dppe)(CO)(H)]PF6, an S = ½ Fe(III) hydride. See Ref. 28c; (b) polyphosphinebound iron complexes studied by previously by our lab. See, for example: Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 4243-4248. (44) Unless otherwise noted, [Cp*2Fe][PF6] was utilized as the oxidant to prepare EPR and ENDOR samples of 2.8-H and 2.8-D. Decamethylferrocenium was favored over cobaltocenium because the resultant Cp*2Fe byproduct is diamagnetic. However, the 77 K X-band EPR spectrum obtained via oxidation with [Cp2Co][PF6] was indiscernible from that obtained utilizing [Cp*2Fe][PF6] as the oxidant (see Appendix A for complete details). (45) a) Chang, Y.-H.; Su, C.-L.; Wu, R.-R.; Liao, J.-H.; Liu, Y.-H.; Hsu, H.-F. J. Am. Chem. Soc. 2011, 133, 5708-5711. (b) Broering, E. P.; Dillon, S.; Gale, E. M.; Steiner, R. A.; Telser, J.; Brunold, T. C.; Harrop, T. C. Inorg. Chem. 2015, 54, 3815-3828. (46) Lukoyanov, D.; Khadka, N.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2016, 138, 10674-10683. (47) Kinney, R. A.; Saouma, C. T.; Peters, J. C.; Hoffman, B. M. J. Am. Chem. Soc. 2012, 134, 12637-12647. (48) The spin density at the hydride ligand is calculated using the aiso value for a free hydrogen atom of 1420 MHz, see: Wittke, J. P.; Dicke, R. H. Dicke Phys. Rev. 1956, 103, 620-631. (49) Gómez-Gallego, M.; Sierra, M. A. Chem. Rev. 2011, 111, 4857-4963. (50) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305-5311. 47 (51) Following the reaction of 2.7-H(crown) with CO2 by 1H NMR reveals direct conversion of 2.7-H(crown) to 2.11; a N2-bound intermediate is not observed in the reaction mixture. It may be possible that the thermodynamic stability afforded by MeCN coordination drives the hydride transfer reaction forward, even if the hydricity of 2.7-H(crown) is slightly greater than 44 kcal/mol. Thus, 44 kcal/mol is determined to be an approximate upper bound for the hydricity of 2.7-H(crown). (52) Fong, H.; Peters, J. C. Inorg. Chem. 2015, 54, 5124-5135. (53) Estes, D. P.; Vannucci, A. K.; Hall, A. R.; Lichtenberger, D. L.; Norton, J. R. Organometallics 2011, 30, 3444-3447. (54) Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D. J. Am. Chem. Soc. 1990, 112, 56575658. (55) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. (56) McGinley, P. L.; Koh, J. T. J. Am. Chem. Soc. 2007, 129, 3822-3823. (57) Takaoka, A.; Mankad, N. P.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 8440-8443. (58) Bhattacharyya, K. X.; Dreyfuss, S.; Saffon-Merceron, N.; Mézailles, N. Chem. Commun. 2016, 52, 5179-5182. (59) Gaussian 09, Revision B.01, M.J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. (60) Zhao, Y.; Truhlar, D. G. J. Chem, Phys. 2006, 125, 1-18. 48 (61) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305. (62) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. (63) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42-55. 49 Chapter 3 H2 ELIMINATION FROM A THIOLATE-BOUND NI(III) HYDRIDE 3.1 Introduction Hydrogen is a promising alternative to carbon-based fuel, and homogenous electrocatalysts for the H2 evolution reaction (HER) have been scrutinized for possible practical applications and especially as well-defined systems for mechanistic studies.1 Although metal hydrides may not be required intermediates of HER,2 for HER catalysts featuring nickel, terminally bound NiIII hydride intermediates have been implicated in both stoichiometric3 and catalytic4 proton reduction.5 Additionally, paramagnetic NiIII hydride intermediates have been proposed in enzymatic H2 evolution (Fig. 3.1, top). A bridging hydride has been identified in the S = ½ [NiFe] hydrogenase intermediate (NiIII–H–FeII) assigned as the Ni-C state,6 and computational data suggest that the hydride is bound more tightly to Ni than to Fe (Ni–H: 1.61 Å, Fe–H: 1.72 Å).7,8 Additionally, EPR data support an estimated Ni–H bond length of ~1.6 Å.6a The possible role of a [NiFe] hydrogenase state featuring a terminal NiIII–H has also been computationally investigated for the related case of H2 oxidation.9 Furthermore, studies on Ni-substituted rubredoxin, a model enzyme for [NiFe] hydrogenase bearing a single Ni center in the active site, support the intermediacy of a terminal NiIII–H species in proton reduction catalysis.10 Owing to their posited role as intermediates in HER catalysis, well-characterized paramagnetic nickel hydride model complexes are needed for detailed study, but examples are lacking, whereas examples of related terminally bound NiI/III–Me species are available.11 For hydride cases, such species largely feature bridging hydrides bound to two metal centers (Ni–H–Ni),12 three metal centers (Ni3–(H)),12d,13 or as borohydride adducts (Ni–H–BR3).14 Characterization data for terminally bound NiI/III hydride complexes are scant. In previous work, irradiation of a matrix-isolated sample containing Ni(CO)4 and HI yielded several EPR-active compounds, including a species assigned as H–NiI(CO)4.15 Related solid-state 50 Figure 3.1. (top) Proposed hydrogenase intermediates featuring a NiIII hydride motif (bottom) Accessible pathways for H2 evolution from the Ni hydride complexes described in this work. experiments employing Ni(CN)42– generated a [H–Ni(CN)n]x species as one of the EPRactive products.16 Additionally, the NiIII hydride [(PS3)NiIIIH][PPN] has been reported in solution by treatment of the analogous NiIII–OPh complex with pinacolborane.17,18 Characterization data for this species was limited; in particular, the inferred hydride ligand was not confirmed by spectroscopic analysis (vide infra). Herein, we generate and spectroscopically characterize a thiolate-supported, terminally-bound NiIII–H species at low temperature. Direct identification of the terminal hydride ligand is confirmed by both vibrational and pulse EPR data. Of note, bimolecular reductive elimination of H2 proceeds upon warming this NiIII–H species, followed with N2 binding to quantitatively generate a NiII–N2 product. Stoichiometric reactivity studies from the NiII–H– and NiIII–H species featured herein demonstrate the viability of both a heterolytic and homolytic pathway for H2 evolution. 3.2 Results and Discussion 3.2.1 Synthesis and Characterization of Ni precursors 51 Scheme 3.1. Synthesis and numbering scheme of compounds discussed herein. Following a recent study of a ferric Fe(H)(N2) species featuring a tetradentate bis(phosphine)(silyl)(thiolate) ligand (Scheme 3.1),19 we targeted the generation of a trivalent Ni–H species supported by this ligand framework. Treatment of HSiP2SiPr (3.1)19 with Ni(COD)2 (COD = 1,5-cyclooctadiene) yields a thioether-bound Ni(II) hydride, (SiP2SiPr)NiIIH (3.2, Scheme 3.1a; see Fig. 3.2A for solid-state structure) with a Ni–H stretch 52 at 1737 cm-1 and a 1H NMR hydride signal at −6.90 ppm (t, 3JH, P = 46.3 Hz, in C6D6). Addition of KC8 to 3.2 results in the reductive cleavage of the S–iPr bond to furnish a thiolate-bound Ni(II) hydride, [(SiP2S)NiIIH]K (3.3, see Fig. 3.2B for solid-state structure; n(Ni–H) at 1677 cm-1; hydridic resonance at −8.10 ppm (t, 3JH, P = 43.0 Hz) in C6D6). Figure 3.2. X-ray structures of 3.2 (A), 3.3 (B), 3.4’ (C), 3.7 (D), and the residual positive electron density plot of 3.7 (isovalue: 0.78) (E). C–H hydrogen atoms and solvent molecules are omitted for clarity. Ellipsoids are depicted at 50% probability. Protonation of hydride 3.3 with [H(OEt2)2][BArF4] under an N2 atmosphere affords a nickel(II) complex, (SiP2S)NiII(N2) (3.4), which crystallizes as the dimeric, dinitrogenbridged species, [(SiP2S)NiII]2(N2) (3.4’) (Scheme 3.1B). XRD data confirms the structure 53 of 3.4’ (Fig. 3.2C; Table 3.1) and reveals an N–N bond length of 1.115(2) Å. Solid-state IR data of crystals of 3.4’ do not show an N2 stretch, consistent with the inversion center gleaned in the solid-state structure. However, an intense N2 stretch at 2200 cm-1 is observed in solution under an N2 atmosphere, indicating that 3.4’ dissociates to monomeric 3.4 in solution. Notably, a solid thin-film produced by concentration of a solution of 3.4 under an N2 stream does not exhibit the N2 stretch at 2200 cm-1, demonstrating that 3.4’ predominates upon concentration and is consistent with an equilibrium between 3.4 and 3.4’. Table 3.1. Bond lengths of 3.4’ and 3.7. 3.4 d(Ni–S) d(Ni–P) ∠(P–Ni–P) ∠(P–Ni–S) 2.3320(6) 2.2405(5) 129.82(3) 109.69(2) 2.2627(6) 3.7 2.3089(9) 2.208(1) 116.49(2) 129.12(4) 2.2246(6) 108.62(4) 119.77(4) A degassed solution of diamagnetic 3.4 bears NMR features distinct from those acquired under an N2 atmosphere, intimating the loss of N2 to generate a diamagnetic fourcoordinate Ni species, (SiP2S)NiII (3.6), under vacuum (Scheme 3.1C). Exposure of 3.6 to an atmosphere of H2 yields the five-coordinate NiII–H2 complex, (SiP2S)NiII(H2) (3.7). XRD data obtained on crystals grown under an atmosphere of H2 are consistent with the assignment of 3.7; while the H2 unit could not be reliably identified from the XRD data, positive residual electron density located trans to the silyl group is consistent with a bound H2 (Fig. 3.2d,e).20 Notably, the structural similarity between 3.7 and 3.4’ suggests that 3.7 bears an intact H–H fragment in the solid state, as opposed to a Ni(H)2 or NiH(SH) species (Table 3.1). Additionally, the HD analogue, (SiP2S)NiII(HD), exhibits a 1JHD of 35 Hz (toluene-d8, −80 °C), indicative of an intact dihydrogen unit in solution.21 Examples of Ni(H2) complexes are rare,22,23 and 3.7 is distinct by virtue of the thiolate donor ligand; H2 activation across a Ni–thiolate bonds has been invoked elsewhere.24 54 3.2.2 Generation and Characterization of a NiIII–H The cyclic voltammogram of 3.4 in THF reveals a reduction event at −2.33 V vs. Cp2Fe/Cp2Fe+, and 3.4 slowly catalyzes HER in the presence of PhOH as a weak acid source at a strongly cathodic potential (see Appendix B for details).25 Treatment of the NiII–H– 3.3 with PhOH in THF results in quantitative generation of H2 and (SiP2S)NiOPh-, demonstrating a heterolytic pathway for H–H bond formation within the system. For comparison, we also explored the viability of a homolytic pathway for H2 generation from the corresponding NiIII–H species. Examples of Mn/n+1 hydride pairs that can access H2 evolution via a homolytic pathway from one oxidation state and a heterolytic pathway from another are rare.27e,28a Whereas protonation of a metal hydride complex to release H2 is commonly observed,1,26 examples of bimolecular reductive elimination of H2 from two welldefined M–H units is less precedented,19,27,28,29 although such a pathway has been proposed in a number of HER electrocatalysts.1,4f,27e,30 The cyclic voltammogram of the NiII–H– species 3.3 exhibits a reversible feature at −1.26 V vs. Cp2Fe/Cp2Fe+ in THF that corresponds to the formal NiII/III couple, and chemical oxidation using [Cp*2Fe][PF6] at −78 °C in THF yields a dark blue-green solution of the desired (SiP2S)NiIIIH (3.5-H) species. Solutions of 3.5-H are thermally sensitive (vide infra) and were therefore handled at low temperatures to obtain spectroscopic data. As expected for a Kramer’s doublet species, 3.5-H exhibits paramagnetically shifted peaks in its 1H NMR spectrum (THF-d8, −40 °C). A characteristic terminal Ni–H stretch is observed in the IR spectrum at 1728 cm-1 (THF solution, −78 °C) that vanishes upon deuteration via the analogue (SiP2S)NiIIID (3.5-D).31 This Ni–H stretch is in good agreement with the DFTpredicted gas-phase value of 1720 cm-1.32 The 77 K X-band CW EPR spectrum of 3.5-H confirms the expected S = ½ spin state (Fig. 3.3) and is simulated with a rhombic g tensor of g = [2.166, 2.056, 2.039] and hyperfine coupling to two equivalent 31P nuclei (A(31Pα) = A(31Pβ) = ±[200, 210, 260] MHz) and a Ni– H nucleus (A(1H) = ±[1.63, 1.63, 31.9] MHz; for 3.5-D, A(2H) = ±[0.25, 0.25, 4.9] MHz). These simulation parameters are consistent with Q-band 1H, 31P Davies ENDOR and 2H HYSCORE data for 3.5-H and 3.5-D (see Appendix B). Notably, the 1H dipolar tensor (T) 55 56 Figure 3.3. (A) 77 K X-band CW EPR spectrum of 3.5-H in 2-MeTHF with simulation. Simulation parameters: g = [2.166, 2.056, 2.039]; A(31Pα) = A(31Pβ) = [200, 210, 260] MHz); A(1H) = [1.6, 1.6, 31.9] MHz. (B) Field-dependent 31P Q-band Davies ENDOR spectra of 3.5-D. (C) Field-dependent Q-band Davies ENDOR 1H minus 2H difference spectra of 3.5H and 3.5-D; difference spectra were smoothed using a 5-point Savitzky-Golay filter. ENDOR samples were prepared in 2-MeTHF, and spectra are simulated with the same gvalues and hyperfine coupling as Fig. 3.3A. Experimental conditions for Fig. 3.3B, 3.3C: microwave frequency = 34.040 GHz; MW π pulse length = 80 ns; interpulse delay τ = 400 ns; πRF pulse length = 15 µs; TRF delay = 2 µs; shot repetition time (srt) = 2 ms; temperature = 12 K. of 3.5-H is axially symmetric (A(1H) = aiso + T; T = ±[−10.1, −10.1, 20.2] MHz), as can be anticipated for a terminally bound metal hydride (M−H), whereas a rhombic dipolar tensor would instead be expected for an approximately symmetric bridging hydride (M−H−M).33 By scaling the isotropic component of the 1H hyperfine coupling tensor (|aiso(1H)| = 11.7 MHz) by the aiso value for a hydrogen atom (1420 MHz),34 spin density localized at the hydride is estimated as ±0.008 e–. The DFT-optimized structure in the gas phase for 3.5-H (M06l) predicts a Mulliken spin density of −0.005 e– on the hydride, consistent with the experimental data. Complex 3.5-H could not be obtained in solid-state form due to its highly reactive nature. However, an analogous and more stable NiIII-Me species could be prepared, isolated, and crystallographically characterized. Accordingly, treatment of 3.4 with methyl lithium yields diamagnetic [(SiP2S)NiIIMe]Li (3.8), and its oxidation by [Cp*2Fe][PF6] at −78 °C affords a dark blue-green solution of (SiP2S)NiIIIMe (3.9, Fig. 3.4).35 Structural parameters for 3.9 compare favorably to the gas-phase, DFT-optimized parameters for 3.5-H (Table 3.2). The 77 K X-band CW EPR spectrum exhibits a rhombic g tensor (g = [2.255, 2.073, 2.037]) and hyperfine coupling to two similar but distinct 31P nuclei (A(31Pα) = ±[170, 133, 330] MHz; A(31Pβ) = ±[260, 257, 130] MHz). Compared to that of 3.5-H, the EPR spectrum of 9 exhibits slightly greater g-anisotropy and comparable |aiso(31P)| values (3.5-H: |aiso(31Pα)| = |aiso(31Pβ)| = 223 MHz; 3.9: |aiso(31Pα)| = 211 MHz, |aiso(31Pβ)| = 216 MHz). 57 Figure 3.4. (A) Synthetic route to access compound 3.9. (B) X-ray structure of 3.9. Hydrogen atoms and disordered components are omitted for clarity. Ellipsoids are depicted at 50% probability. (C) 77 K X-band CW EPR spectrum of 3.9 in 2-MeTHF with simulation. Simulation parameters: g = [2.255, 2.073, 2.037]; A(31Pα) = [170, 133, 330] MHz; A(31Pβ) = [260, 257, 130] MHz. Table 3.2. Bond lengths of 3.5-H (DFT) and 3.9 (XRD) d(Ni–X) d(Ni–S) d(Ni–P)avg d(Ni–Si) 3.5-H 1.55 2.22 2.25 2.25 3.9 2.047(5) 2.1972(6) 2.2428(6) 2.2541(6) X = H (3.5-H), CH3 (3.9) 58 3.2.3 EPR characterization of [(PS3)NiH][PPN] Given the potential value for EPR data of model NiIII–H species to guide reliable assignments of such intermediates in Ni-containing hydrogenases, we also undertook the generation of the previously reported [(PS3)NiH][PPN] species17 and the analogous nickel deuteride for related characterization by EPR techniques. As noted above, previously reported vibrational and EPR data did not locate the presence of the terminal hydride moiety, though its chemical reactivity was consistent with such a formulation. Through X-band HYSCORE experiments, the 2 H hyperfine coupling for [(PS3)NiD][PPN] could be detected: A(2H) = ±[1.6, 9.3, 9.3] MHz (See Appendix B for more details). Scaling A(2H) by the ratio of the 1H/2H gyromagnetic ratios (1He/2Hγ = 6.514) approximates a 1H hyperfine coupling of A(1H) = ±[10.4, 60.6, 60.6] MHz in [(PS3)NiH][PPN], with |aiso(1H)| = 43.9 MHz and T = ±[−33.5, 16.7, 16.7] MHz. The |aiso(1H)| value corresponds to approximately ±0.03 e– of spin density localized on the hydride of [(PS3)NiH][PPN], in agreement with a DFT-estimated value of −0.05 e– (gas phase, M06l; see Appendix B). There hence appears to be greater spin delocalization onto the hydride ligand of [(PS3)NiH][PPN] compared to 3.5-H. This difference presumably arises from an increased metal-ligand covalency in 3.5-H; a Mulliken spin density of 0.73 e– is calculated on Ni in [(PS3)NiH][PPN], compared to 0.60 e– in 3.5-H. 3.2.4 Comparison of NiIII–H Species with the Ni-C Hydrogenase State In both systems, spin delocalization onto the ligand framework likely stabilizes the NiIII–H species. Consistent with this idea, DFT-calculated Mulliken spin densities (M06-L functional: def2tzvp [Ni] and def2svp [all other atoms] basis sets) suggests that there is considerable spin leakage onto the supporting thiolate ligands of 3.5-H (0.22 e–) and [(PS3)NiH][PPN] (0.07, 0.07, 0.16 e–). For comparison, in the case of the Ni-C hydrogenase state, DFT calculations suggest the presence of significant spin delocalization onto one of the bridging cysteine groups (Fig. 3.5, BP86 functional),36 with the majority of spin localized on Ni (0.72 e–) rather than Fe (0.01 e–). Additionally, a spin density of −0.01 e– is calculated on the hydride bridging the Ni and Fe centers; experimental data support |aiso(1H)| = ca. 11 59 MHz for the hydride ligand in D. gigas hydrogenase,6c and a value of |aiso(1H)| = 3.5 MHz is measured for both Ralstonia eutropha6a and D. vulgaris Miyazaki F hydrogenases.6d [NiFe] hydrogenase NC -0.01 eH CN FeII OC 0.01 e- 0.72 eS-Cys NiIII S-Cys S S Cys Cys 0.26 e|⍴| < 0.1 e- 3.5-H -0.005 eH 0.60 e- iPr2P NiIII S 0.22 e iPr2P Ph Si Figure 3.5. Calculated Mulliken spin densities of the Ni-C hydrogenase state and 3.5-H. It has been noted that the |aiso(1H)| value for the hydride of the Ni-C hydrogenase state is significantly smaller than that anticipiated for a hydride covalently bound to Ni,6a,b,c given that |aiso(1H)| for H–NiI(CO)3 was reported to be 293 MHz,15,37 and that of [H–Ni(CN)n]x was estimated to be ca. 427 MHz.16 Assuming these assignments are reliable, one suggested explanation for the discrepancy is the presence of a proximal iron center in the Ni-C state, which may in turn perturb the spin distribution.6a However, it has been observed that the spin density on Fe is very low in the Ni-C state,38 corroborated by DFT calculations.36 In this context, it is therefore significant that 3.5-H and [(PS3)NiH][PPN] are measured to have |aiso(1H)| values quite comparable to that of the Ni-C hydride. The data presented here demonstrates that hydrides covalently bound to a paramagnetic Ni center can exhibit comparatively small |aiso(1H)| values. We attribute the small |aiso(1H)| of these complexes in part to the significant spin delocalization onto the thiolate ligands of 3.5-H and [(PS3)NiH][PPN], and by extension suggest that the bridging cysteine in the Ni-C state mitigates the magnitude of |aiso(1H)| in the Ni-C state hydride, and likely more so than the proximal Fe center.39 Furthermore, although DFT-calculated EPR parameters for the hydrogenase structure featuring a bridging hydride between NiIII and FeII provide a satisfactory fit to experimental data for the Ni-C state,6f,g the present study demonstrates that 60 terminally bound NiIII hydrides can bear |aiso(1H)| values comparable to that measured for the Ni-C state. 3.2.2 Bimolecular H2 Release from Thiolate–NiIII–H and Comparison with H2 Release from Thiolate–FeIII–H Our ability to generate and reliably characterize the NiIII–H species 3.5-H enabled us to evaluate its propensity to undergo a bimolecular homocoupling process to generate H2.40 Accordingly, dark blue-green solutions of 3.5-H prepared in situ at −78 °C quantitatively convert to orange solutions of 3.4 upon warming to 25 °C under N2. H2 production was confirmed by GC analysis (98%). The decay of 3.5-H to 3.4 monitored by UV-Vis spectroscopy exhibits isosbestic behavior and is second-order with respect to 3.5-H (Fig. 3.6, k = 20 M−1·min−1 at 25 °C). A kinetic isotope effect is observed (kH/kD = 1.6 at 25 °C), suggesting that the hydride ligand is present in the rate-determining transition state, and Eyring analysis reveals activation parameters of ΔS‡ = −30(7) cal/(mol·K) and ΔH‡ = 9(2) kcal/mol. The large and negative entropic activation term is consistent with an ordered ratedetermining transition state, and these data support the bimolecular reductive elimination of H2 between two NiIII–H fragments. Figure 3.6. UV-vis spectra depicting the decay of 3.5-H to 3.4 at 25 °C in THF. Spectra collected in 20 min intervals. Metal hydrides with BDFE(M–H) less than half the BDFE of H2 (BDFE(M–H) < ca. 52 kcal/mol in MeCN)41 are thermodynamically favored to undergo bimolecular reductive elimination of H2 (2 M–H → 2 M + H2). Under vacuum, a THF solution of 3.5-H reacts to 61 form the vacant species 3.6, suggesting that the Ni–H BDFE is < ca. 52 kcal/mol. This is in approximate agreement with the DFT-estimated Ni–H gas-phase BDFE of 57 kcal/mol (3.5H → H• + 3.6). Scheme 3.2. Comparison of 3.5-H and (SiP2S)Fe(H)(N2), which both undergo bimolecular reductive elimination of H2 upon warming but with different rates and activation parameters in accord with the presented pathways. Gas-phase BDFEs were calculated with the M06-L functional (def2tzvp [Fe or Ni], def2svp [all other atoms]). A related S = ½ ferric hydride, (SiP2S)FeIII(H)(N2), that we have previously reported,19 has a DFT-estimated gas phase Fe–H BDFE of 56 kcal/mol ((SiP2S)Fe(H)(N2) → H• + (SiP2S)Fe(N2)) and also undergoes bimolecular reductive elimination of H2 upon warming to yield the N2-bridged species [(SiP2S)FeII]2(N2). Compared to the Ni system, there is a greater degree of spin delocalization onto the hydride ligand in the Fe system, evidenced 62 by a greater |aiso(1H)| value of 43 MHz. However, HER from the ferric hydride proceeds with a significantly smaller second-order rate constant at 25 °C compared to the Ni system ([Fe]: k = 0.068 M−1·min−1, Scheme 3.2). Additionally, the bimolecular transformation of the ferric hydride bears a large and positive ΔS‡ of 39(13) cal/(mol·K) that we hypothesized to arise from N2 dissociation prior to H−H bond formation (e.g. {N2 (g) + [FeIII]−H···H−[FeIII(N2)]}⧧). Such a scenario contrasts with the negative ΔS‡ determined for the present Ni system, which is consistent with an ordered rate-determining transition state (e.g. {[NiIII]−H···H−[NiIII]}⧧). The unobserved, N2-dissociated ferric hydride (SiP2S)FeIII(H) would be an electrondeficient 15 e– species, favoring N2 binding to yield a 17 e– species (SiP2S)FeIII(H)(N2). In contrast, the Ni(III) hydride 3.5-H is a 17 e– species, and N2 binding to form the 19 e– species (SiP2S)NiIII(H)(N2) is thus disfavored. Pre-dissociation of N2 hence does not play a role in the H2 evolution chemistry of the Ni system, whereas it does in the Fe system. This is likely a significant factor in the dramatically enhanced rate of H2 formation in the Ni system compared to Fe. Additionally, because the terminal product is an N2-bound species, thermodynamic stabilization afforded by N2 binding may also contribute to the relative rates of H2 elimination. 3.3 Conclusion In closing, we have reported the synthesis and spectroscopic characterization of an unusual S = ½, terminal NiIII–H species, 3.5-H, and its propensity to undergo homolytic bimolecular H–H coupling to release H2. Heterolytic H2 evolution via protonation of its 1electron reduced state, NiII–H–, has also been demonstrated. For the NiIII–H of most interest, the sulfur donor bears a considerable amount of spin density (0.22 e–) based on DFT calculations, stabilizing the system and allowing, for the first time, direct measurement of salient spectroscopic parameters, including a terminal Ni–H vibration (1728 cm-1 in THF), and hyperfine coupling to the terminal hydride ligand (|aiso(1H)| = 11.7 MHz) as detected via pulse EPR studies. Importantly, this isotropic hyperfine coupling value is similar to that of the hydride ligand in the Ni-C hydrogenase state,6a,c,d whereas significantly larger |aiso(1H)| 63 values, on the order of 102 MHz, were reported for H–NiI(CO)415 and [H–Ni(CN)n]x 16 (via generation and detection in a solid matrix), the only previous examples of Ni–H species with reported |aiso(1H)| values. The discrepancy between the significantly smaller |aiso(1H)| measured for the assigned Ni–H–Fe moiety of the Ni-C hydrogenase state, compared to the previously reported paramagnetic nickel hydride species, has been highlighted previously.6a,b,c The EPR data for the well-defined NiIII–H species featured herein demonstrates that smaller |aiso(1H)| values are in fact compatible with a nickel hydride ligand assignment, especially when covalently bound to a spin active nickel center with one (or more) sulfur donors, as is suggested for the Ni-C hydrogenase state. Thus, the NiIII–H species reported here, and related structures in future studies, can provide valuable platforms for constraining spectroscopic assignments in enzymatic systems, and specific reactivity patterns including H–H bond formation to generation H2. 3.4 Experimental Section 3.4.1 Experimental Details 3.4.1.1 General Considerations All syntheses and measurements, unless otherwise stated, were carried out under an inert atmosphere (N2) in a glovebox or using standard Schlenk techniques, and solvents were dried and degassed by thoroughly sparging with N2 and then passing through an activated alumina column in a solvent purification system supplied by SG Water, LLC. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3 Å molecular sieves before use. 3.1,19 [Cp*2Fe][PF6],42 KC8,43 [H(OEt2)2][BArF4],44 PPh3PCH2,45 lithium triethylborodeuteride,46 [(PS3)NiOPh][PPN],47 [(PS3)NiH][PPN],17 and DBPin48 were prepared according to literature procedures. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. 64 3.4.1.2 Physical Methods Electrochemical measurements were carried out in a glovebox under an N2 atmosphere in a one compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and a platinum wire was used as the auxiliary electrode. A Ag/AgOTf reference electrode was used with the ferrocene couple (Fc/Fc+) as an external reference, unless otherwise noted. Solutions of electrolyte (0.4 M [NBu4][PF6] in THF) and analyte were also prepared under an N2 atmosphere. NMR spectra (1H, 31P) were collected on Varian 300, 400, or 500 MHz spectrometers (25 °C unless otherwise specified). 1H chemical shifts are reported in ppm, relative to tetramethylsilane using residual proton resonances from solvent as internal standards. 31P chemical shifts are reported in ppm relative to 85% aqueous H3PO4. IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. Optical spectroscopy measurements were taken on a Cary 50 UV−Vis spectrophotometer using a 1-cm quartz cell, unless otherwise noted. Temperature regulation for UV-Vis measurements was carried out with a Unisoku cryostat. Time-course UV-Vis spectra were collected with the Scanning Kinetics application of the Cary WinUV software. H2 was analyzed on an Agilent 7890A gas chromatograph (HPPLOT U, 30 m, 0.32 mm ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. Combustion analyses were carried out by the Beckman Institute Crystallography Facility (Caltech). 3.4.1.3 X-Ray Crystallography X-ray diffraction measurements were carried out in the Beckman Institute Crystallography Facility. XRD measurements were collected using a dual source Bruker D8 Venture, four-circle diffractometer with a PHOTON CMOS detector. Structures were solved using SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL. The crystals were mounted on a glass fiber under Paratone N oil. 65 3.4.1.4 DFT Calculations Optimization and frequency calculations were performed using the Gaussian 09 program.49 Structures utilized for thermochemical estimations were optimized in the gas phase. The M06-L functional50 with the def2-TZVP51 basis set was used on Ni, and the def2SVP basis set was used on all other atoms. DFT-estimated free energies of H• transfer are referenced to H• dissociation from TEMPOH.52 3.4.1.5 CW EPR Spectroscopy 77 K X-band EPR spectra were obtained on a Bruker EMX spectrometer on solutions prepared as frozen glasses in 2-MeTHF, unless otherwise noted. 3.4.1.6 Pulse EPR Spectroscopy All pulse Q-band (ν ≈ 34 GHz) EPR and electron nuclear double resonance (ENDOR) experiments were aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker D2 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC. Pulse Q-band ENDOR was acquired using the Davies pulse sequence (" − &#$ − "#$ − &#$ − "/2 – * – " – echo), where &#$ is the delay between mw pulses and RF pulses, "#$ is the length of the RF pulse, and the RF frequency is randomly sampled during each pulse sequence. Q-band HYSCORE spectra were acquired using the 4-pulse sequence ("/2 − * − "/2 − gh − " –gN – "/2 – echo), where * is a fixed delay, while gh and gN are independently incremented by Δgh and ΔgN , respectively. The time domain data was baseline-corrected (third-order polynomial) to eliminate the exponential decay in the echo intensity, apodized with a Hamming window function, zero-filled to eight-fold points, and fast Fouriertransformed to yield the 2-dimensional frequency domain. For 2H-1H difference spectra, the time domain of the HYSCORE spectrum of the 1H sample was subtracted from that of the 66 2 H sample, and the same data processing procedure detailed above was used to generate the frequency spectrum. In general, the ENDOR spectrum for a given nucleus with spin += ½ (1H, 31P) coupled to the S = ½ electron spin exhibits a doublet at frequencies 0 ,± = / ± ,1 / 2 (3.1) where ,1 is the nuclear Larmor frequency and 0 is the hyperfine coupling. For nuclei with + ≥ 1 (2H), an additonal splitting of the ,± manifolds is produced by the nuclear quadrupole interaction (P) ,±,56 = / ,1 ± 38(2:; − 1) / 2 (3.2) In HYSCORE spectra, these signals manifest as cross-peaks or ridges in the 2-D frequency spectrum which are generally symmetric about the diagonal of a given quadrant. This technique allows hyperfine levels corresponding to the same electron-nuclear submanifold to be differentiated, as well as separating features from hyperfine couplings in the weak-coupling regime (|0| < 2|,; | ) in the (+,+) quadrant from those in the strong coupling regime (|0| > 2|,; | ) in the (−,+) quadrant. The (−,−) and (+,−) quadrants of these frequency spectra are symmetric to the (+,+) and (−,+) quadrants, thus typically only two of the quadrants are typically displayed in literature. For systems with appreciable hyperfine anisotropy in frozen solutions or solids, HYSCORE spectra typically do not exhibit sharp cross peaks, but show ridges that represent the sum of cross peaks from selected orientations within the excitation bandwidth of the MW pulses at the magnetic field position at which the spectrum is collected. The length and curvature of these correlation ridges can allow for the separation and estimation of the magnitude of the isotropic and dipolar components of the hyperfine tensor, as shown in Fig. 3.7. 67 Figure 3.7. a) HYSCORE powder patterns for an S = ½, I = ½ spin system with an isotropic hyperfine tensor A. b) HYSCORE powder patterns for an S = ½, I = ½ spin system with an isotropic hyperfine tensor which contains isotropic (lmno ) and dipolar (&) contributions. Blue correlation ridges represent the strong coupling case; red correlation ridges represent the weak coupling case. 3.4.1.7 EPR Simulations Simulations of all CW and pulse EPR data were achieved using the EasySpin53 simulation toolbox (release 5.2.21) with Matlab 2018b using the following Hamiltonian: > = ?@ A B⃑D EFG + ?1 E1 A B⃑D +G + ℎFG ∙ K ∙ +G + ℎ+G ∙ L ∙ +G = (3.3) In this expression, the first term corresponds to the electron Zeeman interaction term where ?@ is the Bohr magneton, g is the electron spin g-value matrix with principle components g = [gxx gyy gzz], and FG is the electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where ?1 is the nuclear magneton, E1 is the characteristic nuclear g-value for each nucleus (e.g. 1H, 2H, 31P), and +G is the nuclear spin operator; the third term 68 corresponds to the electron-nuclear hyperfine term, where K is the hyperfine coupling tensor with principle components K = [Axx, Ayy, Azz]; and for nuclei with + ≥ 1, the final term corresponds to the nuclear quadrupole (NQI) term which arises from the interaction of the nuclear quadrupole moment with the local electric field gradient (efg) at the nucleus, where L is the quadrupole coupling tensor. In the principle axis system (PAS), L is traceless and parametrized by the quadrupole coupling constant M N OP/ℎ and the asymmetry parameter Q such that: 8SS L= R0 0 where [ \ ]^ _ 0 8UU 0 0 M N OP/ℎ −(1 − Q) 0W= Y 0 4+(2+ − 1) 8VV 0 = 2+ (2+ − 1)8VV and Q = `aa b`cc `dd 0 −(1 + Q) 0 0 0Z 2 (3.4) . The asymmetry parameter may have values between 0 and 1, with 0 corresponding to an electric field gradient with axial symmetry and 1 corresponding to a fully rhombic efg. The orientations between the hyperfine and NQI tensor principle axis systems and the g-matrix reference frame are defined by the Euler angles (α, β, γ). 3.4.2 Synthetic Details 3.4.2.1 Synthesis of (SiP2SiPr)NiH (3.2) A solution of 3.1 (1.83 g, 2.8 mmol) in Et2O (20 mL) was added to solid Ni(COD)2 (0.82 g, 3.0 mmol), and the mixture was stirred at 23 °C for 1 d. The yellow-brown reaction was concentrated to dryness. The resulting yellow-brown material was extracted with C6H6 (10 mL), and the solution was filtered through Celite. The filtrate was lyophilized and dissolved in Et2O (25 mL). The solution was filtered through Celite, and the product was crystallized at 23 °C via slow concentration of the filtrate. The yellow crystals were isolated by filtration and washed with pentane (10 mL) to afford the title complex (1.14 g, 57%). Yellow crystals suitable for XRD were grown from slow concentration of an Et2O solution at 23 °C. 1H NMR (C6D6, 300 MHz, 298 K, δ): 8.22 (d, J = 7.2 Hz, 2H), 8.13 (dd, J = 7.1, 1.4 Hz, 1H), 7.39 – 69 6.98 (m, 13H), 2.64 (hept, J = 6.6 Hz, 1H), 2.29 (m, 4H), 1.13 – 0.85 (m, 30H), −6.90 (t, J = 46.3 Hz, 1H). 31P NMR (C6D6, 121 MHz, 298 K, δ): 73.8. IR (solid, cm-1): 1737 (Ni–H). Anal. Calcd. For C39H52NiP2SSi : C, 66.76; H, 7.47; N, 0.00. Found: C, 67.19; H, 7.48; N, 0.03. 3.4.2.2 Synthesis of [(SiP2S)NiH]K (3.3-H) A suspension of KC8 (0.26 g, 1.9 mmol) in THF (3 mL) was added dropwise to a stirring solution of 3.2 (0.64 g, 0.9 mmol) in THF (3 mL) at 78 °C. The reaction mixture was stirred at −78 °C for 30 min and then stirred at 23 °C for 1 h. The dark yellow reaction was filtered through Celite, and the filtrate was concentrated to dryness. The resulting solids were washed with pentane (3 x 5 mL) to afford the title complex as a yellow solid. (0.58 g, 91%). Yellow crystals suitable for XRD were grown from slow concentration of an Et2O solution at 23 °C. 1 H NMR (THF-d8, 300 MHz, 298 K, δ): 8.18 (dd, J = 7.3, 1.3 Hz, 2H), 7.57 (dd, J = 5.3, 3.2 Hz, 1H), 7.55 – 7.49 (m, 2H), 7.44 (d, J = 7.4 Hz, 2H), 7.35 – 7.22 (m, 2H), 7.21 – 6.96 (m, 5H), 6.61 (d, J = 2.1 Hz, 1H), 6.59 (s, 1H), 2.33 – 2.07 (m, 4H), 1.05 (q, J = 7.0 Hz, 6H), 1.00 – 0.89 (m, 12H), 0.74 (q, J = 6.8 Hz, 6H), −7.83 (t, J = 44.3 Hz, 1H). 31P NMR (THFd8, 121 MHz, 298 K, δ): 77.47. IR (solid, cm-1): 1677 (Ni–H). Anal. Calcd. For C36H45KNiP2SSi: C, 61.98; H, 6.50; N, 0.00. Found: C, 61.73; H, 6.73; N, 0.03. 3.4.2.3 Synthesis of [(SiP2S)Ni]2(N2) (3.4’) A solution of [H(OEt2)2][BArF4] (0.73 g, 0.72 mmol) in Et2O (3 mL) was added dropwise to a stirring solution of 3.3-H (0.50 g, 0.72 mmol) in Et2O (3 mL) at –78 °C. The stirring reaction mixture was slowly warmed from –78 °C to 23 °C overnight, and the resulting dark red solution was concentrated to dryness. The crude material was extracted with 2:1 C6H6/pentane (10 mL) and filtered through Celite. The volatiles were removed in vacuo, and the resulting solid was recrystallized from slow evaporation of an Et2O solution at 23 °C to yield the title complex as a red solid (0.24 g, 50%). Red crystals suitable for XRD were grown from slow concentration of an Et2O solution at 23 °C. Anal. Calcd. For C72H88N2Ni2P4S2Si2: C, 64.39; H, 6.60; N, 2.09. Found: C, 63.83; H, 6.61; N, 1.84. In 70 solution under an N2 atmosphere, 3.4’ is in equilibrium with the monomeric form (SiP2S)NiN2 (3.4) and has the following spectroscopic parameters: 1H NMR (THF-d8, 400 MHz, 298 K, δ): 7.75 – 7.66 (m, 8H), 7.64-7.60 (m, 16H), 7.33 – 7.27 (m, 16H), 7.22 (t, J = 7.6 Hz, 8H), 7.17 – 7.08 (m, 4H), 6.90 (ddd, J = 19.3, 7.2, 1.4 Hz, 8H), 6.73 (t, J = 7.2 Hz, 4H), 2.60 (m, 4H), 2.38 (m, 4H), 1.22 (q, J = 7.1 Hz, 12H), 1.16 (q, J = 7.0 Hz, 12H), 1.07 (q, J = 7.4 Hz, H), 0.99 (q, J = 6.9 Hz, 6H). 31P NMR (THF-d8, 162 MHz, 298 K, δ): 45.50. IR (C6H6 solution): 2200 (N2). UV-Visible (THF, 298 K, nm {M-1mm-1}): 354 {1400}, 446 {560}, 531 {320}. 3.4.2.4 Synthesis of [(SiP2S)NiD]Li (3.3-D) A solution of LiBEt3D (66 µL, 1.0 M in THF) was added in one portion to a stirring THF solution of 3.4 (0.040 g, 0.040 mmol) at –78 °C. The reaction mixture was stirred at 23 °C for 30 min, and the resulting yellow solution was concentrated to dryness. The resulting yellow solids were washed with 3:1 pentane/Et2O (5 mL) and extracted with THF (5 mL). The filtrate was filtered through Celite and concentrated to afford the title complex as a yellow solid (0.036 g, 90%). 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.16 (d, J = 7.2 Hz, 2H), 7.73 (dd, J = 6.9, 1.6 Hz, 1H), 7.67 – 7.60 (m, 2H), 7.44 (d, J = 7.6 Hz, 3H), 7.27 (t, J = 7.3 Hz, 2H), 7.19 (s, 1H), 7.08 (td, J = 7.3, 4.3 Hz, 3H), 7.00 (dd, J = 8.3, 6.5 Hz, 1H), 6.90 (d, J = 7.6 Hz, 1H), 2.30 – 2.07 (m, 2H), 1.15 (t, J = 7.1 Hz, H), 1.14 (q, J = 7.1 Hz, 4H), 0.94 (q, J = 6.6 Hz, 3H), 0.79 (q, J = 7.1 Hz, 3H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 69.68. 2 H NMR (C6H6, 61 MHz, 298 K, δ): −8.38. 3.4.2.5 Generation of (SiP2S)NiH (3.5-H) in situ A representative sample preparation: Solid [Cp*2Fe][PF6] (7 mg, 16 µmol) was added in one portion to a THF solution of 3.3-H (10 mg, 14 µmol, 1 mL) at –78 °C. The resulting dark blue-green solution was stirred at –78 °C and then filtered through a pre-chilled glass fiber pipette filter. The dark blue-green filtrate was analyzed directly. 1H NMR (THF-d8, 500 MHz, −40 °C, δ) δ 16.0, 13.7, 10.7, 6.3, 4.5, −1.2. IR (THF solution, −78 °C): 1728 (Ni–H). UV-Visible (THF, 298 K, nm {M-1cm-1}): 395 {3900}, 632 {1400}. 71 3.4.2.6 Generation of (SiP2S)NiD (3.5-D) in situ The title compound was generated in situ in an analogous manner as 3.5-H employing 3.3D. 1H NMR (THF-d8, 500 MHz, −40 °C, δ) 16.3, 13.8, 10.8, 6.2, 4.4, −1.3. *Note: Based on the simple harmonic oscillator model, the estimated Ni–D stretch for 3.5-D is 1232 cm-1, given that 3.5-H exhibits a Ni–H stretch at 1728 cm-1. However, this falls in a region that is dominated by intense IR stretches from THF, so the Ni–D stretch could not be resolved in the solution IR spectrum collected in THF at −78 °C. Nonetheless, the broad, intense stretch at 1728 cm-1 observed in 3.5-H was not observed in the IR spectrum of 3.5-D, which supports the assignment as a Ni–H stretch (See Figs. B.2.8, B.2.9) 3.4.2.7 Generation of (SiP2S)Ni (3.6) in situ A solution of 3.4 in THF was degassed via freeze-pump-thaw cycles (x 3) to generate a red solution of the title complex, which is quantitative by NMR spectroscopy. 1H NMR (THFd8, 400 MHz, 298 K, δ): 7.72 – 7.63 (m, 4H), 7.61 (dt, J = 6.0, 2.1 Hz, 2H), 7.30 – 7.24 (m, 4H), 7.22 (t, J = 7.7 Hz, 2H), 7.14 – 7.04 (m, 1H), 6.93 – 6.85 (m, 2H), 6.72 (t, J = 7.2 Hz, 1H), 2.62 (m, 1H), 2.35 (m, 1H), 1.24 (q, J = 7.2 Hz, 3H), 1.22 (q, J = 7.2 Hz, 3H), 1.10 (q, J = 7.1 Hz, 3H), 0.98 (q, J = 6.8 Hz, 3H). 31P NMR (THF-d8, 162 MHz, 298 K, δ): 41.27. UV-Visible (THF, 298 K, nm {M-1mm-1}): 340 {700}, 535 {130}. Combustion analysis was not obtained on compound 3.6 due to conversion to 3.4 in the presence of N2. 3.4.2.8 Generation of (SiP2S)Ni(H2) (3.7) in situ A solution of 3.4 in THF-d8 was degassed via freeze-pump-thaw cycles (x 3), and the solution was exposed to 1 atm of H2 at 23 °C, which immediately yielded an orange solution of the title complex and was quantitative by NMR spectroscopy. Orange crystals were grown from a concentrated Et2O solution of 3.7 at −35 °C under an atmosphere of H2. 1H NMR (THFd8, 400 MHz, 298 K, δ): 7.88 – 7.71 (m, 2H), 7.74 – 7.50 (m, 4H), 7.31 (dd, J = 6.8, 3.3 Hz, 4H), 7.20 (t, J = 7.5 Hz, 2H), 7.09 (t, J = 7.4 Hz, 1H), 6.99 (d, J = 7.1 Hz, 1H), 6.86 (d, J = 7.3 Hz, 1H), 6.73 (t, J = 7.2 Hz, 1H), 2.61 – 2.42 (m, 1H), 2.50 (m, 1H), 2.34 (m, 1H), 1.18 (dq, J = 18.1, 7.2 Hz, 6H), 0.95 (dq, J = 14.7, 6.8 Hz, 6H), −1.68 (s, 2H). (Note: At 23 °C, 72 bound H2 and free H2 are in exchange, so the peak at −1.68 ppm may shift depending on sample conditions. At −80 °C in toluene-d8, a distinct peak for free H2 is observed, and the 1 H NMR shift corresponding to bound H2 is observed at −2.89 ppm). 31P NMR (THF-d8, 162 MHz, 298 K, δ): 50.93. UV-Visible (THF, 298 K, nm {M-1mm-1}): 360 {600}, 535 {200}. Combustion analysis was not obtained on compound 3.7 due to conversion to 3.4 in the presence of N2. 3.4.2.9 Synthesis of [(SiP2S)Ni(CH3)]Li(solv)x (3.8) A solution of methyl lithium (3.6 mg, 0.16 mmol) in Et2O (3 mL) was added to a stirring solution of 3.4 (0.100 g, 0.075 mmol) in Et2O (3 mL) at −78 °C. The stirring reaction mixture was stirred at −78 °C for 1 h and then stirred at 23 °C for 30 min, resulting in a yellow-orange solution. The reaction mixture was concentrated to dryness, and the resulting solid was washed with pentane (5 mL, −78 °C) to afford the title compound as a yellow-orange solid (0.073 g, 72%). Yellow-orange crystals suitable for XRD were grown from slow concentration of a pentane solution at −35 °C. 1H NMR (C6D6, 300 MHz, 298 K, δ): 8.08 (dd, J = 7.1, 1.4 Hz, 2H), 7.72 (d, J = 7.5 Hz, 2H), 7.63 (dd, J = 6.9, 1.5 Hz, 1H), 7.41 (d, J = 7.6 Hz, 2H), 7.30 – 7.17 (m, 5H), 7.08 (t, J = 7.2 Hz, 3H), 7.03 – 6.90 (m, 1H), 2.61 – 2.25 (m, 4H), 1.19 (q, J = 6.9, 6.6 Hz, 6H), 1.13 (q, J = 7.0 Hz, 6H), 1.00 (q, J = 6.7, 6.2 Hz, 6H), 0.89 (q, J = 6.9 Hz, 6H), −0.57 (t, J = 7.1 Hz, 3H). 31P NMR (C6D6, 121 MHz, 298 K, δ): 47.22. UV-Visible (C6H6, 298 K, nm {M-1cm-1}): 360 {4100}, 432 {2600}. Anal. Calcd. For C37H47LiNiP2SSi: C, 65.40; H, 6.97; N, 0.00. Found: C, 64.25 ; H, 6.87 ; N, 0.05. 3.4.2.10 Synthesis of (SiP2S)Ni(CH3) (3.9) Solid [Cp*2Fe][PF6] (0.035 g, 0.074 mmol) was added in one portion to a stirring Et2O solution of 3.8 (0.046 g, 0.068 mmol, 1 mL) at −78 °C. The resulting dark green solution was stirred at −78 °C for 30 min then concentrated to dryness at 25 °C. The resulting solid was washed with chilled pentane (5 mL, −78 °C) and Et2O (1 mL, −78 °C), then extracted with THF (0.5 mL, −78 °C) and filtered. The dark blue-green filtrate was layered with chilled pentane (3 mL, −78 °C) and stored at −78 °C, which yielded a dark blue-green precipitate. 73 The solids were isolated by vacuum filtration, then washed with pentane (3 mL, −78 °C) and dried under vacuum to yield the title complex as a dark blue-green solid (0.014 g, 31%). 1H NMR (400 MHz, THF-d8) δ 73.4, 59.3, 25.2, 22.2, 13.9, 10.7, 9.2, 8.0, 7.3, 5.8, 5.4, −0.1, −0.8, −50.7. µeff (C6D6, Evans method, 298 K): 1.70µB. UV-Visible (THF, 298 K, nm {M1 cm-1}): 641 {2300}, 937 {620}. Satisfactory combustion analysis was not obtained on compound 3.9, likely due to relatively facile Ni–CH3 bond homolysis. 3.4.3. H2 Quantification from the Decay of 3.5-H to 3.4 A solution of 3.3-H (0.030 g, 0.045 mmol) in THF (1 mL) was added to a Schlenk tube and frozen. Subsequently, a thawing solution of [Cp*2Fe][PF6] (0.022 g, 0.047 mmol) in THF (1 mL) was added to the tube and frozen. The tube was stirred at −78 °C for 30 min to generate 3.5-H as a dark blue-green solution, then warmed to 23 °C and stirred for 1 h. During this period, the solution changed from blue-green to red. An aliquot of the headspace was sampled and analyzed for H2 by GC. Yield of H2: 98% 3.4.4. H2 quantification from the protonation of 3.3-H with [H(OEt2)2][BArF4] Solid 3.3-H (0.020 g, 0.029 mmol) and [H(OEt2)2][BArF4] (0.029 g, 0.029) were added to a round bottom, and the flask was sealed with a septum. Chilled Et2O (1 mL, −78 °C) was added to the round bottom via syringe, and the flask was stirred at 23 °C for 10 min. An aliquot of the headspace was sampled and analyzed for H2 by GC. Yield of H2: 99% 3.4.5. H2 Quantification from the Protonation of 3.3-H with PhOH A THF solution of 3.3-H (0.030 g, 0.045 mmol, 1 mL) was added to a round bottom and frozen. To the chilled round bottom, a THF solution of PhOH (4.2 mg, 0.045 mmol, 1 mL) was added and frozen. The flask was sealed, and the sample was warmed to 23 °C and stirred at 23 °C for 30 min. During this period, the solution changed color from yellow-orange to dark red. An aliquot of the headspace was sampled and analyzed for H2 by GC. Yield of H2: 97%. 74 NMR analysis of the protonation reaction of 3.3-H with PhOH in THF-d8 reveals quantitative conversion to [(SiP2S)NiOPh]K (See Figs. B.1.30, B.1.31) : 1H NMR (THF-d8, 400 MHz, 298 K, δ): 7.76 (d, J = 7.0 Hz, 2H), 7.60 (d, J = 7.6 Hz, 2H), 7.47 (d, J = 7.3 Hz, 2H), 7.21 – 7.01 (m, 8H), 6.75 (t, J = 7.6 Hz, 2H), 6.69 (d, J = 7.3 Hz, 1H), 6.60 (d, J = 7.5 Hz, 3H), 5.99 (t, J = 7.0 Hz, 1H), 2.55 (bs, 4H), 1.12 (dd, J = 15.6, 8.4 Hz, 6H), 1.06 (q, J = 6.5 Hz, 12H), 0.74 (q, J = 7.1 Hz, 6H). 31P NMR (THF-d8, 162 MHz, 298 K, δ): 36.74 3.4.6. Bulk Electrolysis Experiments for Proton Reduction Catalyzed by 3.4 Controlled-potential electrolysis experiments were conducted in a sealed two-chambered H cell equipped with a glassy carbon plate working electrode (5.5 cm × 1 cm × 2 mm) and a Ag/AgOTf reference electrode in the working chamber and a glassy carbon plate counter electrode in the auxiliary chamber. The two chambers were separated by a fine-porosity glass frit, and the cell was sealed under a N2 atmosphere. Bulk electrolysis runs were performed at 23 °C in THF containing 0.4 M [NBu4][PF6] as the electrolyte, with a controlled potential at −2.7 V vs. Cp2Fe/Cp2Fe+. In a standard run, the working chamber contained 0.5 mM complex 3.4 and 50 mM PhOH. Following the electrolysis period of 4 h, H2 was quantified by sampling the headspace volume and analyzing by gas chromatography. 3.5 References (1) (a) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5, 865-878. (b) Bullock, R. 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Dalton Trans. 2019, 48, 16322-16329. (e) Luca, O. R.; Blakemore, J. D.; Konezny, S. J.; Praetorius, J. M.; Schmeier, T. J.; Hunsinger, G. B.; Batista, V. S.; Brudvig, G. W.; Hazari, N.; Crabtree, R. H. Inorg. Chem. 2012, 51, 8704-8709. (f) Han, Y.; Fang, H.; Jing, H.; Sun, H.; Lei, H.; Lai, W.; Cao, R. Angew. Chem. Int. Ed. 2016, 55, 5457-5462. (5) Additionally, paramagnetic NiI/III hydrides have been proposed as intermediates in catalytic organometallic transformations, see: (a) Kuang, Y.; Anthony, D.; Katigbak, J.; Marrucci, F.; Humagain, S.; Diao, T. Chem 2017, 3, 268-280. (b) Zarate, C.; Yang, H.; Bezdek, M. J.; Hesk, D.; Chirik, P. J. J. Am. Chem. Soc. 2019, 141, 5034-5044. (6) (a) Brecht, M.; van Gastel, M.; Buhrke, T.; Friedrich, B.; Lubitz, W. J. Am. Chem. Soc. 2003, 125, 13075-13083. (b) Whitehead, J. P.; Gurbiel, R. J.; Bagyinka, C.; Hoffman, B. M.; Maroney, M. J. J. Am. Chem. Soc. 1993, 115, 5629-5635. (c) Fan, C.; Teixeira, M.; Moura, J.; Moura, I.; Hanh, H. 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(b) Journaux, Y.; Lozan, V.; Klingele, J.; Kersting, B. Chem. Commun. 2006, 83-84. (c) MacMillan, S. N. Ph.D. Dissertation, Massachusetts Institute of Technology, Cambridge, MA, 2013. 77 (15) The authors favor the assignment as a trigonal pyramidal H–Ni(CO)3 species, but also note that H–Ni(CO)x (x ≥ 2) could not be experimentally ruled out as possible assignments, see: Morton, J. R.; Preston, K. F. J. Chem. Phys. 1984, 81, 5775-5778. (16) Irradiation of Ni(CN)42– in the presence of water results in the generation of several EPR-active species. One of the species exhibits a large |aiso(1H)| value of ca. 427 MHz and an axial g tensor (g = [2.00, 2.05, 2.05]), which was initially attributed to H–NiIII(CN)42– (See: Symons, M. C. R.; Aly, M. M.; West, D. X. J. Chem. Soc., Chem. Commun. 1979, 5152). However, the subsequent study of H–NiI(CO)3 revealed similar EPR parameters to that of the putative H–NiIII(CN)42– complex, and the authors suggest that H–NiI(CN)33– is also a plausible assignment given the available data (See Ref. 15). (17) Lai, K.-T.; Ho, W.-C.; Chiou, T.-W.; Liaw, W.-F. Inorg. Chem. 2013, 52, 4151-4153. (18) In Ref. 17, it is noted that H2 and [Ni2II(P(C6H3-3-SiMe3-2-S)3)2]2– are detected upon warming a solution of [(PS3)NiH][PPN]. However, because the yields of either product are not included, it is not clear whether H2 loss is a major reaction pathway. (19) Gu, N. X.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6374-6382. (20) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 14158-14164. (21) Crabtree, R. H. Chem. Rev. 2016, 116, 8750-8769. (22) (a) Tsay, C.; Peters, J. C. Chem. Sci. 2012, 3, 1313-1318. (b) Connelly, S. J.; Zimmerman, A. C.; Kaminsky, W.; Heinekey, D. M. Chem. Eur. J. 2012, 18, 15932-15934. (c) He, T.; Tsvetkov, N. P.; Andino, J. G.; Gao, X.; Fullmer, B. C.; Caulton, K. G. J. Am. Chem. Soc. 2010, 132, 910-911. (d) Sweany, R. L.; Polito, M. A.; Moroz, A. Organometallics 1989, 8, 2305-2308. (23) For examples of d10 Ni(H2) complexes supported by group 13 Z-type ligands, see: (a) Cammarota, R. C.; Lu, C. C. J. Am. Chem. Soc. 2015, 137, 12486-12489. (b) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chemie. Int. Ed. 2014, 53, 1081-1086. (24) (a) Olechnowicz, F.; Hillhouse, G. L.; Cundari, T. R.; Jordan, R. F. Inorg. Chem. 2017, 56, 9922-9930. (b) Sellmann, D.; Geipel, F.; Moll, M. Angew. Chem. Int. Ed. 2000, 39, 561563. 78 (25) Bulk electrolysis experiments confirmed the generation of 5.8 ± 0.6 equivalents of H2 per Ni center (106 ± 5% FE) over 4 hours in THF at a controlled potential of −2.7 V vs. Cp2Fe/Cp2Fe+. The yield of H2 is the average of two independent runs. Only trace H2 is detected in the absence of PhOH. In the absence of 3.4, there is a small but measurable background HER process accounting for 0.6 equiv H2, corrected for in the yield of the Nicatalyzed process. Rinse test experiments were consistent with a molecular catalyst but do not reliably rule out a heterogeneous component. (26) Besora, M.; Lledos, A.; Maseras, F. Chem. Soc. Rev. 2009, 38, 957−966. (27) For examples of bimolecular HER from first-row transition metal hydrides, see: (a) Halpern, J.; Pribanić Inorg. Chem. 1970, 9, 2616-2618. (b) Ungváry, F.; Markó, L. J. Organomet. Chem. 1969, 20, 205-209. (c) Prokopchuk, D. E.; Chambers, G. M.; Walter, E. D.; Mock, M. T.; Bullock, R. M. J. Am. Chem. Soc. 2019, 141, 1871-1876. (d) Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Chem. Eur. J. 2014, 20, 9657-9665. (e) Marinescu, S. C.; Winkler, J. R.; Gray, H. B. Proc. Natl. Acad. Sci. 2012, 109, 15127-15131. (28) For examples of bimolecular HER from 2nd and 3rd row transition metal hydrides, see: (a) Collman, J. P.; Wagenknecht, P. S.; Lewis, N. S. J. Am. Chem. Soc. 1992, 114, 5665−5673. (b) Collman, J. P.; Hutchison, J. E.; Wagenknecht, P. S.; Lewis, N. S.; Lopez, M. A.; Guilard, R. J. Am. Chem. Soc. 1990, 112, 8206−8208. (c) Norton, J. R. Acc. Chem. Res. 1979, 12, 139−145. (d) Evans, J.; Norton, J. R. J. Am. Chem. Soc. 1974, 96, 7577−7578. (e) Inoki, D.; Matsumoto, T.; Nakai, H.; Ogo, S. Organometallics 2012, 31, 2996−3001. (29) For an example of unimolecular H2 evolution from a dinickel(II) dihydride species, see: Manz, D.-H.; Duan, P.-C.; Dechert, S.; Demeshko, S.; Oswald, R.; John, M.; Mata, R. A.; Meyer, F. J. Am. Chem. Soc. 2017, 139, 16720-16731. (30) (a) Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988-8998. (b) Rose, M. J.; Gray, H. B.; Winkler, J. R. J. Am. Chem. Soc. 2012, 134, 8310-8313. (31) Based on the harmonic oscillator model, the Ni–D stretch of 3.5-D is estimated to be 1232 cm-1. This falls in a region that is dominated by intense IR stretches from THF, and the Ni–D stretch could not be resolved in the solution IR spectrum collected in THF at −78 °C. (See Figs. B.2.8, B.2.9). 79 (32) The gas-phase DFT calculated Ni–H stretches for 3.2 (calc: 1742 cm-1, expt: 1737 cm−1) and 3.3 (calc: 1689 cm-1, expt: 1677 cm-1) are also in good agreement with the experimental solid-state IR stretches. (M06-L functional: def2tzvp [Ni], def2svp [all other atoms]) (33) (a) Kinney, R. A.; Saouma, C. T.; Peters, J. C.; Hoffman, B. M. J. Am. Chem. Soc. 2012, 134, 12637-12647. (b) Hoeke, V.; Tociu, L.; Case, D. A.; Seefeldt, L. C.; Raugei, S.; Hoffman, B. M. J. Am. Chem. Soc. 2019, 141, 11984-11996. (34) Wittke, J. P.; Dicke, R. H. Phys. Rev. 1956, 103, 620-631. (35) The apical ligand of compound 3.9 is modeled with 87% occupancy as Ni–CH3 and 13% occupancy as Ni–N2. The presence of the disordered component results from the slow decomposition of 3.9 in solution at −35 °C. (36) Kampa, M.; Lubitz, W.; van Gastel, M.; Neese, F. J. Biol. Inorg. Chem. 2012, 17, 12691281. (37) Stein, M.; van Lenthe, E.; Baerends, E. J.; Lubitz, W. J. Phys. Chem. A. 2001, 105, 416425. (38) Huyett, J. E.; Carepo, M.; Pamplona, A.; Franco, R.; Moura, I.; Moura, J. J. G.; Hoffman, B. M. J. Am. Chem. Soc. 1997, 119, 9291-9292. (39) Another proposed cause for the small 1H hyperfine coupling of the Ni-C state is that the hydride resides close to the equatorial plane (near the nodal surface) of the singly-populated Ni(dz2) orbital of a distorted square pyramidal Ni center, where three cysteines and the hydride lie approximately along the xy-plane (See Refs. 6a,d,e and 37). For comparison, the DFT-optimized structure of 3.5-H is best described as a distorted trigonal bipyramid. Nonetheless, a S = ½ , d7 configuration for a trigonal bipyramidal complex would place the unpaired electron largely in xy-plane (defining the z-axis along the C3-axis). The combination of significant spin delocalization onto the thiolate and limited spatial overlap with the singly-occupied Ni d orbital in the xy-plane likely both contribute to the small aiso(1H) value for 3.5-H. These are analogous factors to what is hypothesized for the Ni-C hydrogenase state, which is measured to have a hydride |aiso(1H)| value of comparable magnitude as 3.5-H. 80 (40) Related H• transfer from a Ni(II)–H species has been previously demonstrated, see: (a) Ref. 27d (b) Yao, C.; Wang, S.; Norton, J. R.; Hammond, M. DOI: 10.1021/jacs.9b13757 (41) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. (42) Hess, C. R.; Weyhermüller, T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2010, 49, 56865700. (43) Weitz, I. S.; Rabinovitz, M. J. Chem. Soc. Perkin Trans. 1993, 1, 117-120. (44) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920-3922. (45) Vedejs, E.; Meier, G. P.; Snoble, K. A. J. J. Am. Chem. Soc. 1981, 103, 2823-2831. (46) Bhattacharyya, K. X.; Dreyfuss, S.; Saffon-Merceron, N.; Mézailles, N. Chem. Commun. 2016, 52, 5179-5182. (47) Chiou, T.-W.; Liaw, W.-F. Inorg. Chem. 2008, 47, 7908-7913. (48) Smith, S. M.; Takacs, J. M. Org. Lett. 2010, 12, 4612-4615. (49) Gaussian 09, Revision B.01, M.J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. (50) Zhao, Y.; Truhlar, D. G. J. Chem, Phys. 2006, 125, 194101: 1-18. (51) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (52) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. (53) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42-55. 81 Chapter 4 H2N–NH2 BOND FORMATION FROM A NI–NH2 COMPLEX: MECHANISTIC INSIGHTS INTO NI-MEDIATED AMMONIA OXIDATION TO DINITROGEN 4.1 Introduction Intermediates featuring metal-ligand multiple bonding are often implicated in the mechanism of water oxidation to O2, as well as in the related oxidation of ammonia to N2. For water oxidation catalysts, O–O bond formation is generally proposed to occur either via bimolecular oxo coupling or nucleophilic attack of a metal oxo intermediate by water (Fig. 4.1),1 and the ability to stabilize oxo ligands has been suggested to be a design requirement for water oxidation catalysts.1a The related 6e–/6H+ process of ammonia oxidation to N2 has been studied stoichiometrically on Mo,2 Mn,3 Os,4 and Ru5,6 complexes, and molecular Ru7 and Fe8 species have been recently demonstrated to mediate the transformation catalytically. In analogy to the mechanisms of water oxidation catalysts, several systems have invoked the role of metal nitride and imido intermediates in N–N bond formation.2,3abd,4,5ab,7ac Furthermore, bimolecular nitride3ab,4b,9 or imide10 coupling and the nucleophilic attack of amines on M=NR4d,11 species has been implicated in N–N bond forming reactions. Water oxidation: Pathways for O-O bond formation O M OH + H2O, - H+ O M metal oxo intermediate ½ M O O M This work: Bimolecular coupling of M-NH2 NH2 [NiII] - e- NH2 [NiIII] ½ H2 [NiII] N N [NiII] H2 Figure 4.1 General mechanisms for O−O coupling proposed for water oxidation catalysts, and the N−N coupling mechanism described in this work 82 In contrast, although N–N bond formation from a M–NH2 intermediate has been hypothesized as the N–N coupling step in the catalytic ammonia oxidation mediated by ferrocene8b and (tetramesitylporphyrin) Ru(NH3)2,7d stoichiometric examples of such reactivity from a M–NH2 complex are rare.12 Collman and coworkers reported that treatment of a cofacial diporphyrin Ru2(NH3)2 complex with either tBuOOH or a combination of oxidant and base yields a mixture of Ru2(N2H4), Ru2(N2H2), and Ru2(N2) species, and it has been hypothesized that the hydrazine formation occurs through the coupling of two Ru–NH2 fragments.5c,d Relatedly, Sellmann and coworkers detected trace amounts of CpMn(CO)2N2H4 in electrochemical bulk oxidation experiments of CpMn(CO)2NH3.3c Generation of N–N coupled product was hypothesized to proceed through a similar pathway as the Ru diporphyrin system.5d Stoichiometric examples of H2N–NH2 reductive elimination are limited likely due to the relatively weak bond that is generated. The N–N bond dissociation enthalpy of free hydrazine is 66 kcal/mol,13 although the coordination of hydrazine to a metal center influences the N–N bond strength. In this work, we demonstrate the stepwise oxidation of NH3 facilitated by a Ni complex, the first example of ammonia oxidation mediated by a molecular late transition metal system. We propose that the key N– N bond formation step occurs via the bimolecular H2N–NH2 reductive elimination from a transient NiIII–NH2 species. 4.2 Results and Discussion Our group has previously reported the synthesis of the tetradentate ligand framework HSiP2S14 and metalation procedures to yield [(SiP2S)Ni]2(N2) (4.1).15,16 Treatment of 4.1 with a THF solution of ammonia yields (SiP2S)NiII(NH3) (4.2), and deprotonation of the coordinated NH3 with nBuLi gives quantitative conversion to [(SiP2S)NiII(NH2)]Li (4.3), which can also be prepared by treatment of 4.1 with LiNH2 in THF (Scheme 4.1A, Fig. 4.2A). 1H NMR data of 4.3 in C6D6 support the coordination of three THF molecules to the lithium cation,17 and recrystallization in benzene allows structural characterization of the solvent-free dimer, [(SiP2S)NiIINH2]2Li2 (Scheme 4.1A, Fig. 4.2B). The cyclic voltammogram of 4.3 in 0.4 M [NBu4][PF6] in THF reveals an oxidation event at −0.72 V 83 vs. Cp2Fe/Cp2Fe+, corresponding to the formal NiII/NiIII couple. Our group has previously demonstrated that oxidation of a related FeII–NH2 complex leads to the generation of the FeII–NH3 species, likely due to hydrogen atom abstraction from solvent by the transient FeIII– NH2 species.18 In contrast, the chemical oxidation reaction of 4.3 with one equivalent of [Cp2Fe][BArF4] (BArF4 = B(3,5-(CF3)2C6H3)4) at −78 ºC in 1,2-dimethoxyethane (DME) generates both 4.2 (60(4)%) and the N–N coupled product, [(SiP2S)NiII]2(N2H4) (4.4) (41(3)%). Complex 4.4 can be independently generated via treatment of 4.1 with half an equivalent of hydrazine. XRD data of compound 4.4 reveal an N–N bond length of 1.488(4) Å, which is slightly elongated compared to free hydrazine (1.449 Å, Scheme 4.1A, Fig. 4.2C).19 A Ph iPr2 P PiPr2 NH3 S Si Ni N N Ni Si S iPr2P P iPr2 NH3 iPr2P iPr2P THF 4.1 Ph Ni S Si 4.2 N 2H 4 THF THF LiNH2 nBuLi THF Ph iPr2 P PiPr2 H2 Si Ni N S Ph Li NH2 S iPr2P iPr2P N Ni Si H2 iPr2P P iPr2 4.4 4.4 O tBu THF Ni S Si Ph 4.3 B tBu Ph Ph tBu ½ iPr2 P PiPr2 H Si Ni N S Ph S N Ni Si H iPr2P P iPr2 4.5 Scheme 4.1 (A) Synthesis of compounds 4.2, 4.3, and 4.4. (B) Hydrogen atom abstraction from 4.4 to generate 4.5 84 Figure 4.2. Crystal structures of compounds (A) 4.2, (B) 4.3, (C) 4.4, and (D) 4.6. Solvent molecules, disordered components, and C–H hydrogens omitted for clarity. Ellipsoids shown at 50% probability. 85 Figure 4.3. Partial 1H NMR spectra of (A) compound 4.4 generated by treatment of 4.1 with N2H4 and (B-E) reaction mixtures generated by oxidation of isotopologues of 4.3 in DME, showing the N–H resonance at 4.93 ppm (C6D6, 400 MHz, 298 K) Oxidation of the 15N-labeled amide, [(SiP2S)NiII(15NH2)]Li, and deuterated amide, [(SiP2S)NiII(ND2)]Li, yields [(SiP2S)NiII]2(15N2H4) and [(SiP2S)NiII]2(N2D4) respectively, as the N–N coupled products, confirming that the coordinated hydrazine fragment is exclusively amide-derived (Fig. 4.3).20 Of note, H2 is not detected in the headspace, and NH3 is absent in the volatiles of the oxidation reaction of 4.3. Considering that N–N coupling follows addition of two equivalents of oxidant, it is possible that N–N bond formation occurs between two molecules of (SiP2S)NiIII(NH2) or alternatively between (SiP2S)NiIII(NH2) and 4.3, followed by one-electron oxidation. However, treatment of 4.3 with 0.5 equivalents of [Cp2Fe][BArF4] in DME at −78 ºC generated no EPR-active species, yielding no evidence to support the intermediacy of [(SiP2S)Ni]2(N2H4)–. This one-electron reduced congener of 4.4 would be a mixed-valent NiI/NiII species with the NiI center bearing 19 e–, and thus likely to be a high energy intermediate. Thus, we favor the mechanism of N−N coupling between two NiIII-NH2 units to directly generate 4.4. The concurrent generation of 4.2 in the oxidation reaction of 4.3 likely arises from H-atom abstraction from solvent by (SiP2S)NiIII(NH2). Previous work from our lab has demonstrated that the SiP2S ligand framework can support a formal NiIII–H species competent for the bimolecular release of H2. H2 evolution is posited to occur via a transition state featuring the direct H–H coupling between two NiIII–H 86 fragments, which is analogous to the proposed N–N coupling pathway discussed in this work (Scheme 4.2). H iPr2P NiIII S iPr2P Si [Ni] H Ph ½ H2 H [Ni] proposed transition state for H-H coupling (SiP2S)NiIII(H) NH2 iPr2P NiIII S iPr2P Si [Ni] NH2 Ph ½ 4.4 H2N [Ni] proposed transition state for N-N coupling (SiP2S)NiIII(NH2) Scheme 4.2. Comparison of H–H and N–N bond formation by (SiP2S)NiIIIX species (X = H, NH2) The coordinated hydrazine in complex 4.4 can be further oxidized to the diazenebridged dinickel species, [(SiP2S)Ni]2(trans-N2H2) (4.5), by treatment with two equivalents of 2,4,6-tri-tert-butylphenoxyl radical and generating two equivalents of 2,4,6-tri-tertbutylphenol as byproducts (Scheme 4.1B; Fig. 4.2D). Additionally, in the absence of a radical abstractor, complex 4.4 slowly undergoes disproportionation in solution at 25 ºC to yield 4.2 and 4.5 in a 1:1 molar ratio, where a formal H2 equivalence is transferred from between two molecules of 4.4.21 To the best of our knowledge, there is only one example of a nickel complex featuring a coordinated N2H2 unit, which is bound side-on to a single Ni center.22 The reported crystal structure features an N–N bond length of 1.351(3) Å, and the authors suggest that it is best described as a (HN–NH)2– ligand. XRD data confirm the assignment of 4.5 and reveal a contracted N–N bond length of 1.277(2) Å compared to that of 4.4 (1.488(4) Å). Given that free diazene bears an N–N bond length of 1.252(2) Å,23 we believe that compound 4.5 is best described as a N=N doubly bonded species. A related CuI2(trans-N2H2) species has been structurally characterized, but the determination of reliable bond metrics of the N2H2 fragment was hindered by crystal disorder.24 However, the 87 methylated CuI2(trans-N2(CH3)2) species exhibited an N=N bond length of 1.27(2) Å, which is comparable to that of 4.5. Treatment of the diazene-bridged species 4.5 with excess NH3 in THF results in the quantitative displacement of the diazene ligand with NH3 to yield the ammonia-bound species 4.2, as determined by 31P NMR spectroscopy. Free diazene is unstable toward disproportionation to N2 and N2H4,25 thereby liberating ammonia-derived N2 upon displacement (Scheme 4.3). Liberation of N2 from 4.5 closes the ammonia oxidation cycle, and regenerates compound 4.2. In this stoichiometric cycle, the conversion of two NH3 molecules to ½ N2 and ½ N2H4 is an overall 4e–/4H+ process, but under potential turnover conditions, the N2H4 generated upon diazene disproportionation could undergo further iterative oxidation via compounds 4.4 and 4.5 to achieve the full 6e–/6H+ conversion of 2 NH3 to N2. NH3 NH2 - H+ [NiII] 4.3 4.2 N 2H 2 + NH3 H [NiII] N - e- [NiII] N H 4.5 [NiIII] (SiP2S)NiIII(NH2) ½ N 2 + ½ N 2H 4 - H• [NiII] NH2 H2 [NiII] N N [NiII] H2 4.4 [Ni] NH2 H2N [Ni] proposed transition state for N-N coupling Scheme 4.3. Oxidation of NH3 to N2 mediated by (SiP2S)Ni complexes 4.3 Conclusion In this work, we report the first example of Ni-mediated oxidation of ammonia, where the key N–N bond formation step is believed to be the homocoupling of a NiIII–NH2 species to generate a hydrazine-bridged dinickel complex. The reductive elimination of an N–N single bond has been proposed in several ammonia oxidation systems, but stoichiometric observation of such reactivity is limited. This homocoupling pathway from a M–NH2 species is in contrast to mechanisms for water oxidation catalysts, which commonly invoke high 88 valent metal oxo intermediates prior to O–O bond formation. Compared to the oxidation of water to O2, catalysts for the analogous oxidative conversion of ammonia to dinitrogen may traverse an even greater oxidation state change during turnover (e.g. Mn + NH3 →→ Mn+3≡N). It is thus notable that the ammonia oxidation cycle described herein is carried out exclusively in the NiII/NiIII oxidation states. This is attributable to the fact that the N−N bond forming step is from a M–NH2 species (rather than a metal-ligand multiply bonded intermediate), and the redox load is distributed between two metal centers (e.g. H2N–NH2 reductive elimination from a mononuclear M(NH2)2 species would be a 2e– process). Carrying out multielectron oxidation processes at a single redox couple (Mn/Mn+1) could potentially bypass the need to access high valent intermediates at highly oxidizing potentials, and incorporating these two components in the design of novel ammonia or water oxidation systems may facilitate catalysis at lower overpotentials. 4.4 Experimental Section 4.4.1 Experimental Details 4.4.1.1 General Considerations All syntheses and measurements, unless otherwise stated, were carried out under an inert atmosphere (N2) in a glovebox or using standard Schlenk techniques, and solvents were dried and degassed by thoroughly sparging with N2 and then passing through an activated alumina column in a solvent purification system supplied by SG Water, LLC. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3 Å molecular sieves before use. Complex 4.1,15 ferrocenium tetrakis[3,5bis(trifluoromethyl)phenyl]borate ([Cp2Fe][BArF4]),26 LiNH2,27 and 2,4,6-tri-tert- butylphenoxyl radical28 were prepared according to literature procedures. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. 89 4.4.1.2 Physical Methods Electrochemical measurements were carried out in a glovebox under an N2 atmosphere in a one-compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and a platinum wire was used as the auxiliary electrode. A Ag/AgOTf reference electrode was used with the ferrocene couple (Cp2Fe/Cp2Fe+) as an external reference, unless otherwise noted. Solutions of electrolyte (0.4 M [NBu4][PF6] in THF) and analyte were also prepared under an N2 atmosphere. NMR spectra (1H, 31P) were collected on Varian 300, 400, or 500 MHz spectrometers (25 °C unless otherwise specified). 1H chemical shifts are reported in ppm, relative to tetramethylsilane using residual proton resonances from solvent as internal standards. 31P chemical shifts are reported in ppm relative to 85% aqueous H3PO4. IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. Optical spectroscopy measurements were taken on a Cary 50 UV-Vis spectrophotometer using a 1-cm quartz cell, unless otherwise noted. H2 was analyzed on an Agilent 7890A gas chromatograph (HP-PLOT U, 30 m, 0.32 mm ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. Combustion analyses were carried out by the Beckman Institute Crystallography Facility (Caltech). 4.4.1.3 X-Ray Crystallography X-ray diffraction measurements were carried out in the Beckman Institute Crystallography Facility. XRD measurements were collected using a dual source Bruker D8 Venture, four-circle diffractometer with a PHOTON CMOS detector. Structures were solved using SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL. The crystals were mounted on a glass fiber under Paratone N oil. 4.4.2 Synthetic Details 4.4.2.1 Synthesis of (SiP2S)Ni(NH3) (4.2) A THF solution of NH3 (0.4 M, 1.5 mL) was added at 25 ºC to a stirring red solution of 4.1 (0.20 g, 0.15 mmol) in 10 mL THF, resulting in a yellow-orange solution. The reaction 90 mixture was stirred at 25 ºC for 1 h and then concentrated to dryness. The orange solids were washed with pentane (5 mL), then extracted with C6H6 (10 mL) and filtered through Celite. The filtrate was lyophilized to yield the title complex as an orange solid (0.18 g, 89%). Orange crystals suitable for XRD were grown at −35 ºC by layering pentane over a concentrated THF solution. 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.02 (d, J = 7.8 Hz, 2H), 7.94 (d, J = 7.2 Hz, 2H), 7.42 (d, J = 7.2 Hz, 1H), 7.33 (t, J = 7.5 Hz, 2H), 7.31 – 7.23 (m, 3H), 7.20 – 7.17 (m, 3H), 7.11 (t, J = 7.4 Hz, 2H), 7.04 (t, J = 7.2 Hz, 1H), 2.14 (m, 4H), 1.13 (q, J = 6.9 Hz, 6H), 1.01 (q, J = 7.0 Hz, 6H), 0.82 (q, J = 6.7 Hz, 6H), 0.77 (s, 3H), 0.67 (q, J = 7.0 Hz, 6H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 42.18. UV-Visible (THF, 298 K, nm {M-1cm-1}): 363 {6200}, 469 {3000}. Anal. Calcd. For C36H47NNiP2SSi: C, 64.10; H, 7.02; N, 2.08. Found: C, 64.46; H, 6.74; N, 1.70. 4.4.2.1 Synthesis of (SiP2S)Ni(15NH3) (4.2-15N) A solution of nBuLi (1.6 M in hexanes, 0.45 mmol) was added to a suspension of 15NH4Cl (0.024 g, 0.44 mmol) in THF (5 mL) at 25 ºC, and the reaction mixture was stirred at 50 ºC for 2 h. The volatiles were subsequently vacuum transferred to a frozen solution of 4.1 (0.100 g, 0.07 mmol) in THF (5 mL). The frozen reaction mixture was thawed and stirred at 25 ºC for 3 h, then the orange reaction mixture was concentrated to dryness. The resulting orange solids were washed with pentane (10 mL), then extracted with C6H6 (5 mL) and filtered through Celite. The filtrate was lyophilized to yield the title complex as an orange solid (0.078 g, 77%). 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.01 (d, J = 7.6 Hz, 2H), 7.93 (d, J = 7.2 Hz, 2H), 7.42 (d, J = 7.1 Hz, 1H), 7.32 (t, J = 7.5 Hz, 2H), 7.29 – 7.26 (m, 3H), 7.19 – 7.17 (m, 3H), 7.11 (t, J = 7.3 Hz, 2H), 7.04 (t, J = 7.2 Hz, 1H), 2.14 (m, 4H), 1.13 (q, J = 6.9 Hz, 6H), 1.01 (q, J = 7.0 Hz, 6H), 0.82 (q, J = 6.8 Hz, 6H), 0.69 (s, 3H), 0.66 (q, J = 7.0 Hz, 6H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 42.17. 15N NMR (C6D6, 41 MHz, 298 K, δ): −24.41 91 4.4.2.1 Synthesis of (SiP2S)Ni(ND3) (4.2-2H) Prepared in an analogous manner as 4.2-15N but employing ND4Cl. 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.02 (d, J = 7.9 Hz, 2H), 7.94 (d, J = 7.2 Hz, 2H), 7.42 (d, J = 7.1 Hz, 1H), 7.33 (t, J = 7.7 Hz, 2H), 7.30 – 7.24 (m, 3H), 7.19 – 7.09 (m, 5H), 7.04 (t, J = 7.3 Hz, 1H), 2.13 (m, 4H), 1.13 (q, J = 6.9 Hz, 6H), 1.01 (q, J = 7.0 Hz, 6H), 0.81 (q, J = 6.7 Hz, 6H), 0.66 (q, J = 7.0 Hz, 6H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 42.27. 2H NMR (C6H6, 61 MHz, 298 K, δ): 0.70. 4.4.2.1 Synthesis of [(SiP2S)Ni(NH2)]Li (4.3) Method A: A solution of 4.1 (0.029 g, 0.022 mmol) in THF (1 mL) was added to a suspension of LiNH2 (0.002 g, 0.087) in THF (1 mL) at −78 ºC. The reaction mixture was stirred at −78 ºC for 1 h and then stirred at 25 ºC for 1 h. The red-orange reaction mixture was filtered through Celite and concentrated to dryness to yield the title complex as a red-orange solid (0.035 g, 90%). Method B: A solution of nBuLi (1.6 M in hexanes, 0.071 mmol) was added in one portion at −78 ºC to a stirring solution of 4.2 (0.040 g, 0.059 mmol) in THF (3 mL). The reaction mixture was stirred at −78 ºC for 30 min and then stirred at 25 ºC for 30 min. The red-orange reaction mixture was filtered and concentrated to dryness. The resulting red-orange solid was dissolved in minimal benzene (~0.5 mL) and allowed to stand at 25 ºC, resulting in the formation of red-orange crystals. The red-orange crystals were isolated by filtration and washed with 1:1 C6H6/pentane (3 mL) and dried under vacuum (0.035 g, 87%). Note: Unlike with Method A, material prepared via Method B does not contain THF molecules coordinated the lithium cation. Instead, [(SiP2S)Ni(NH2)]Li is crystallized in benzene as a dimer with no solvent molecules bound to lithium. Prior to recrystallization, the crude material generated by deprotonation of 4.2 in THF also bears three coordinated THF molecules, as determined by 1H NMR spectroscopy in C6D6 (See Figs. C.1.30, C.1.31). 92 Red-orange crystals suitable for XRD were grown from a benzene solution at 25 ºC. 1H NMR (C6D6, 300 MHz, 298 K, δ): 7.92 – 7.85 (m, 2H), 7.67 (d, J = 7.5 Hz, 2H), 7.42 (dd, J = 7.1, 1.6 Hz, 1H), 7.34 (dt, J = 6.0, 2.1 Hz, 2H), 7.24 – 6.95 (m, 9H), 2.40 (m, 2H), 3.51 (m, THF, 12H), 2.27 (bs, 2H), 1.38 (m, THF, 12H), 1.19 (dd, J = 13.7, 6.9 Hz, 6H), 1.15 (dd, J = 13.4, 6.9 Hz, 6H), 1.04 (q, J = 6.7 Hz, 4H), 0.83 (q, J = 7.2 Hz, 6H), −1.98 (t, J = 4.9 Hz, 2H). 31P NMR (C6D6, 121 MHz, 298 K, δ): 40.94. 1H NMR (1:5 THF-d8/C6D6, 400 MHz, 298 K, δ): 7.97 (d, J = 7.0 Hz, 2H), 7.71 (d, J = 7.4 Hz, 2H), 7.46 – 7.42 (m, 3H), 7.21 – 7.08 (m, 7H), 7.03 – 6.94 (m, 2H), 2.50 (bs, 2H), 2.42 (bs, 2H), 1.24 (q, J = 6.5 Hz, 6H), 1.16 (q, J = 6.9 Hz, 6H), 1.05 (q, J = 6.4 Hz, 6H), 1.00 (q, J = 6.7 Hz, 6H), −2.31 (t, J = 6.2 Hz, 2H). 1H NMR (1:5 THF-d8/C6D6, 400 MHz, 298 K, δ): 40.69. UV-Visible (THF, 298 K, nm {M-1cm1 }): 365 {5000}, 471 {2300}. Anal. Calcd. For C36H46LiNNiP2SSi·(C6H6): C, 66.49; H, 6.91; N, 1.85; Found: C, 66.81; H, 7.00; N, 1.91. 4.4.2.1 Synthesis of [(SiP2S)Ni(15NH2)]Li (4.3-15N) A solution of nBuLi (1.6 M in hexanes, 0.048 mmol) was added in one portion to a stirring solution of 4.2-15N (0.030 g, 0.044 mmol) in THF at −78 ºC. The reaction mixture was stirred at −78 ºC for 30 min and then stirred at 25 ºC for 30 min. The red-orange reaction mixture was filtered and concentrated to dryness. The resulting red-orange solid was dissolved in minimal benzene (~ 0.5 mL) and allowed to stand at 25 ºC, resulting in the formation of redorange crystals. The crystals were isolated by filtration and washed with 1:1 C6H6/pentane (3 mL) and dried under vacuum (0.020 g, 65%). 1H NMR (1:5 THF-d8/C6D6, 400 MHz, 298 K, δ): 7.96 (d, J = 7.1 Hz, 2H), 7.71 (d, J = 7.5 Hz, 2H), 7.46 – 7.42 (m, 3H), 7.21 – 7.08 (m, 7H), 7.02 – 6.93 (m, 2H), 2.50 (bs, 2H), 2.42 (bs, 2H), 1.24 (q, J = 6.6 Hz, 6H), 1.16 (q, J = 6.9 Hz, 6H), 1.05 (q, J = 6.4 Hz, 6H), 1.00 (q, J = 7.0 Hz, 6H), −2.31 (dt, J = 55.6, 5.9 Hz, 2H). 31P NMR (1:5 THF-d8/C6D6, 162 MHz, 298 K, δ): 40.69. 15N NMR (1:5 THF-d8/C6D6, 41 MHz, 298 K, δ): −62.07 93 4.4.2.1 Synthesis of [(SiP2S)Ni(ND2)]Li (4.3-2H) Prepared in an analogous manner as 4.3-15N but employing 4.2-2H. 1H NMR (1:5 THFd8/C6D6, 400 MHz, 298 K, δ): 7.97 (d, J = 6.6 Hz, 2H), 7.71 (d, J = 8.1 Hz, 2H), 7.47 – 7.42 (m, 3H), 7.21 – 7.08 (m, 7H), 7.03 – 6.94 (m, 2H), 2.50 (bs, 2H), 2.42 (bs, 2H), 1.24 (q, J = 6.6 Hz, 6H), 1.16 (q, J = 7.0 Hz, 6H), 1.05 (q, J = 6.3 Hz, 6H), 1.00 (q, J = 7.0 Hz, 6H). 31P NMR (1:5 THF-d8/C6D6, 162 MHz, 298 K, δ): 40.86. 2H NMR (1:5 THF-H8/C6H6, 61 MHz, 298 K, δ): −2.3. 4.4.2.1 Synthesis of (SiP2S)Ni(NH2)Na(THF)3 A mixture of NaNH2 (0.004 g, 0.10 mmol) in THF (1 mL) was added to a stirring solution of 4.1 (0.060 g, 0.045 mmol) in THF (1 mL) at −78 ºC. The reaction mixture was stirred at −78 ºC for 1 h and then stirred at 25 ºC for 1 h. The orange mixture was filtered and concentrated to dryness to yield the title complex as an orange solid (0.045 g, 55%) Red crystals suitable for XRD were grown by layering pentane onto a concentrated THF solution at −35 ºC. 1H NMR (C6D6, 400 MHz, 298 K, δ): 8.04 (d, J = 7.2 Hz, 2H), 7.77 – 7.52 (m, 1H), 7.42 (dd, J = 22.5, 7.3 Hz, 4H), 7.21 (t, J = 7.3 Hz, 2H), 7.15 – 6.94 (m, 6H), 6.79 (t, J = 6.8 Hz, 1H), 3.57 (m, THF, 12H), 2.56 (bs, 4H), 1.41 (m, THF, 12H), 1.29 (q, J = 6.4 Hz, 6H), 1.18 (bs, 12H), 1.05 (q, J = 6.8 Hz, 6H), −2.22 (bs, 2H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 42.76. 1H NMR (1:5 THF-d8/C6D6, 400 MHz, 298 K, δ): 8.02 (d, J = 7.2 Hz, 2H), 7.81 (d, J = 7.5 Hz, 2H), 7.50 (d, J = 7.0 Hz, 1H), 7.42 (d, J = 7.8 Hz, 2H), 7.27 – 7.09 (m, 7H), 7.04 (t, J = 7.3 Hz, 1H), 6.95 (t, J = 7.2 Hz, 1H), 2.48 (bs, 2H), 2.35 (bs, 2H), 1.25-1.05 (m, 18H), 1.01 (q, J = 6.6 Hz, 6H), −2.21 (bs, 2H).31P NMR (1:5 THF-d8/C6D6, 162 MHz, 298 K, δ): 41.40 4.4.2.1 Synthesis of [(SiP2S)Ni]2(N2H4) (4.4) Neat hydrazine (0.0012 g, 0.037 mmol) was added in one portion to a stirring solution of 4.1 in THF (1 mL) at −78 ºC. The dark orange solution was stirred at 25 ºC for 10 min, then concentrated to dryness. The resulting orange material was washed with pentane (1 mL) and Et2O (2 mL), then extracted with THF (5 mL) and filtered through Celite. The filtrate was 94 then layered with pentane (5 mL) and stored at −35 ºC. The title complex crystallized as dark red crystals suitable for XRD (0.020 g, 39%). 1H NMR (C6D6, 400 MHz, 298 K, δ): 7.83 (d, J = 7.1 Hz, 4H), 7.63 (d, J = 7.7 Hz, 4H), 7.34 (d, J = 7.3 Hz, 2H), 7.32 – 7.17 (m, 8H), 7.17 – 7.10 (m, 14H), 4.86 (s, 4H), 2.33 (s, 4H), 2.24 (hept, J = 7.2 Hz, 4H), 1.16 (q, J = 6.3 Hz, 12H), 1.00 (q, J = 7.3 Hz, 12H), 0.87 (t, J = 6.6 Hz, 12H), 0.72 (q, J = 7.5 Hz, 12H). 31P NMR (C7D8, 162 MHz, 298 K, δ): 40.79. UV-Visible (THF, 298 K, nm {M-1cm-1}): 355 {9600}, 472 {3500}. Anal. Calcd. For C72H92N2Ni2P4S2Si2: C, 64.20; H, 6.88; N, 2.08. Found: C, 64.55; H, 7.19; N, 1.80. 4.4.2.1 Synthesis of [(SiP2S)Ni]2(N2H2) (4.5) A solution of 2,4,6-tri-tert-butylphenoxyl radical (0.047 g, 0.18 mmol) in THF (3 mL) was added dropwise to a stirring solution of 4.4 (0.117 g, 0.085 mmol) in THF (3 mL) at −78 ºC, which resulted in a green solution. The reaction mixture was stirred at 25 ºC for 1 h and then concentrated to dryness. The resulting material was washed with pentane (20 mL) and Et2O (10 mL), then extracted with THF (20 mL) and filtered through Celite. The green filtrate was concentrated to dryness to yield the title complex as a green solid (0.088 g, 75%). Dark greenbrown crystals suitable for XRD were grown at −35 ºC by layering pentane over a concentrated THF solution. 1H NMR (C7D8, 400 MHz, 298 K, δ): 15.62 (s, 2H), 7.87 (d, J = 7.2 Hz, 4H), 7.67 (d, J = 7.6 Hz, 4H), 7.36 (d, J = 7.1 Hz, 2H), 7.29 – 7.23 (m, 11 H), 7.17 – 6.97 (m, 11H), 2.36 (bs, 4H), 2.20 (hept, J = 7.2 Hz, 4H), 1.02 – 0.93 (m, 24H), 0.86 (q, J = 7.2, 6.8 Hz, 12H), 0.66 (q, J = 7.3, 6.5 Hz, 12H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 49.54. UV-Visible (THF, 298 K, nm {M-1cm-1}): 349 {13000}, 444 {13000}, 789 {6000}. Anal. Calcd. For C72H90N2Ni2P4S2Si2: C, 64.29; H, 6.74; N, 2.08. Found: C, 64.42; H, 6.84; N, 1.97. 4.4.3 Deprotonation of 4.2 with nBuLi to generate 4.3 A solution of nBuLi (0.025 mL, 1.6 M in hexanes) was added to a stirring solution of 4.2 (0.025 g, 0.019 mmol) in THF (1 mL) at −78 ºC. The orange reaction mixture was stirred at −78 ºC for 30 min, then stirred at 25 ºC for 30 min and concentrated to dryness. 1H, 31P NMR 95 spectra of the crude reaction mixture reveal quantitative conversion of 4.2 to 4.3 (See Figs. C.1.30, C.1.31). 4.4.2.1 Hydrogen Atom Abstraction from 4.4 to Generate 4.5 A solution of 2,4,6-tri-tert-butylphenoxyl radical (2.4 mg, 9.2 µmol) in THF (1 mL) was added to a stirring solution of 4.4 (5.0 mg, 3.6 µmol) in THF (1 mL) at −78 ºC, resulting in a green solution. The reaction mixture was stirred at −78 ºC for 1 h, then stirred at 25 ºC for 30 min and concentrated to dryness. The crude material was analyzed by 1H, 31P NMR spectroscopy to reveal quantitative conversion of 4.4 to 4.5 (See Figs. C.1.37, C.1.38). 4.4.2.1 Conversion of 4.5 to 4.2 in 0.4 M NH3 in THF A solution of NH3 in THF (0.4 M, 1 mL) was added to solid 4.5 (0.005 g, 0.0036 mmol) at 25 ºC. The reaction mixture was transferred to a J. Young NMR tube, and ca. 50 µL C6D6 was added to the tube and sealed. The dark green solution was stirred at 25 ºC for 3 h, which resulted in a color change to an orange solution. 31P NMR data of the reaction mixture reveals the quantitative formation of 4.2 (See Figs. C.1.41, C.1.42). 4.4.2.1 Detection of H2 and free NH3 in the oxidation of 4.3 Solid 4.3 (0.008 g, 0.010 mmol) and solid [Cp2Fe][BArF4] (0.010 g, 0.010 mmol) were added to a septum-sealed Schlenk tube. Cold dimethoxyethane (0.5 mL, −78 ºC) was added by syringe, and the reaction mixture stirred at −78 ºC for 30 min. An aliquot of the headspace was sampled and analyzed for H2. Subsequently, the reaction volatiles were vacuum transferred into a vessel containing 2M HCl in Et2O (3 mL) and analyzed for ammonia (in the form of [NH4][Cl]) by the indophenol method.29 No H2 or ammonia was detected in the reaction mixture. 4.4.2.1 Oxidation of 4.3 to yield 4.2 and 4.4 Cold solutions of 4.3 (0.008 g, 0.01 mmol) in DME (0.25 mL) and [Cp2Fe][BArF4] (0.010 g, 0.01 mmol) in DME (0.25 mL) were mixed in a glovebox coldwell cooled with an external dry ice/acetone bath (−78 ºC). The reaction mixture was green immediately upon addition, 96 and then changed color within a few seconds to orange, which persisted for the duration of the reaction. The reaction mixture was stirred at −78 ºC for 30 min, then the volatiles were removed under vacuum. The remaining orange solids were dissolved in C6D6 (0.5 mL), then filtered through a glass fiber filter pipette and analyzed by NMR spectroscopy. Yield of 4.4: 41 ± 3%; yield of 4.2: 60 ± 4% 4.5 References (1) (a) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Chem. Rev. 2014, 114, 11863-12001. (b) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Chem. Rev. 2015, 23, 12974-13005. (2) Johnson, S. I.; Heins, S. P.; Klug, C. M.; Wiedner, E. S.; Bullock, R. M.; Raugei, S. Chem. Commun. 2019, 55, 5083-5086. (3) (a) Clarke, R. M.; Storr, T. J. Am. Chem. Soc. 2016, 138, 15299-15302. (b) Keener, M.; Peterson, M.; Hernández Sánchez, R.; Oswald, V. F.; Wu, G.; Ménard, G. Chem. Eur. J. 2017, 23, 11479-11484. (c) Würminghausen, T.; Sellmann, D. J. Organomet. Chem. 1980, 1999, 77-85. (d) Chantarojsiri, T.; Reath, A. H.; Yang, J. Y. Angew. Chem. Int. Ed. 2018, 57, 14037-14042. (4) (a) Pipes, D. W.; Bakir, M.; Vitols, S. E.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 5507-5514. (b) Demadis, K. D.; Meyer, T. J.; White, P. S. Inorg. Chem. 1997, 36, 5678-5679. (c) Buhr, J. D.; Taube, H. Inorg. Chem. 1979, 18, 2208-2212. (d) Coia, G. M.; Devenney, M.; White, P. S.; Meyer, T. J.; Wink, D. A. Inorg. Chem. 1997, 36, 23412351. (5) (a) Ishitani, O.; White, P. S.; Meyer, T. J. Inorg. Chem. 1996, 35, 2167-2168. (b) Ishitani, O.; Ando, E.; Meyer, T. J. Inorg. Chem. 2003, 42, 1707-1710. (c) Collman, J. P.; Hutchison, J. E.; Lopez, M. A.; Guilard, R.; Reed, R. A. J. Am. Chem. Soc. 1991, 113, 2794-2796. (d) Collman, J. P.; Hutchison, J. E.; Ennis, M. S.; Lopez, M. A.; Guilard, R. J. Am. Chem. Soc. 1992, 114, 8074-8080. 97 (6) For a related example of Ru-catalyzed ammonia oxidation to nitrite/nitrate, see: Thompson, M. S.; Meyer, T. J. J. Am. Chem. Soc. 1981, 103, 5577-5579. (7) (a) Habibzadeh, F.; Miller, S. L.; Hamann, T. W.; Smith III, M. R. Proc. Natl. Acad. Sci. 2019, 116, 2849-2853. (b) Bhattacharya, P.; Heiden, Z. M.; Chambers, G. M.; Johnson, S. I.; Bullock, R. M.; Mock, M. T. Angew. Chemie. Int. Ed. 2019, 58, 11618-11624. (c) Nakajima, K.; Toda, H.; Sakata, K.; Nishibayashi, Y. Nat. Chem. 2019, 11, 702-709. (d) Dunn, P.; Johnson, S. I.; Kaminsky, W.; Bullock, R. M. J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b13706. (8) (a) Zott, M. D.; Garrido-Barros, P.; Peters, J. C. ACS Catal. 2019, 9, 10101-10108. (b) Raghibi Boroujeni, M.; Greene, C.; Bertke, J. A.; Warren, T. H. ChemRxiv 2019, DOI: 10.26434/chemrxiv. 9729635.v1. (9) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252-6254. (10) (a) Mankad, N. P.; Müller, P.; Peters, J. C. J. Am. Chem. Soc. 2010, 132, 4083-4085. (b) Powers, I. G.; Andjaba, J. M.; Luo, X.; Mei, J.; Uyeda, C. J. Am. Chem. Soc. 2018, 140, 4110-4118. (c) Yiu, S.-M.; Lam, W. W. Y.; Ho, C.-M.; Lau, T.-C. J. Am. Chem. Soc. 2007, 129, 803-809. (11) Coia, G. M.; White, P. S.; Meyer, T. J.; Wink, D. A.; Keefer, L. K.; Davis, W. M. J. Am. Chem. Soc. 1994, 116, 3649-3650. (12) The related reductive elimination of an N−N single bond between nitrogen-based heterocyclic ligands has been demonstrated on a dinuclear Ni system, see: Diccianni, J. B.; Hu, C.; Diao, T. Angew. Chemie. Int. Ed. 2016, 55, 7534-7538. Additionally, homocoupling reactions have been proposed between putative CuII(NR2) species to yield N−N coupled products, see: (a) Gephart III, R. T.; Huang, D. L; Aguila, M. J. B.; Schmidt, G.; Shahu, A.; Warren, T. H. Angew. Chem. Int. Ed. 2012, 51, 6488-6492. (b) Kinoshita, K. Bull. Chem. Soc. Jpn. 1959, 32, 780-783. (c) Ryan, M. C.; Kim, Y. J.; Gerken, J. B.; Wang, F.; Aristov, M. M.; Martinelli, J. R.; Stahl, S. S. Chem. Sci. 2020, 11, 1170-1175. (13) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961-7001. (14) Gu, N. X.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6374-6382. 98 (15) Gu, N. X.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2020, DOI: 10.1021/ jacs.0c00712. (16) Complex 4.1 is isolated as a dinickel species featuring a bridging N2 unit, but is in equilibrium with the monomeric (SiP2S)NiN2 in solution under an N2 atmosphere. (17) Crystals of the sodium analogue, [(SiP2S)NiII(NH2)]Na(THF)3, can be grown from THF/pentane mixtures, and XRD data reveal the coordination of three THF molecules to the sodium ion. 1H NMR data of [(SiP2S)NiII(NH2)]Na(THF)3 in C6D6 also corroborate the presence of three THF molecules. See Appendix C for more information. (18) Creutz, S. E.; Peters, J. C. Chem. Sci. 2017, 8, 2321-2328. (19) Kohata, K.; Fukuyama, T.; Kuchitsu, K. J. Phys. Chem. 1982, 86, 602-606. (20) The 1H NMR spectrum of the oxidation reaction of [(SiP2S)NiII(ND2)]Li bears a minor component of [(SiP2S)Ni]2(N2H4) due to incomplete deuteration of the starting 2H-labeled amide. (21) For an example of this disproportionation reaction mediated by a Ru system, see: Sellmann, D.; Hille, A.; Rösler, A.; Heinemann, F. W.; Moll, M.; Brehm, G.; Schneider, S.; Reiher, M.; Hess, B. A.; Bauer, W. Chem. Eur. J. 2004, 10, 819-830. (22) Köthe, C.; Braun, B.; Herwig, C.; Limberg, C. Eur. J. Inorg. Chem. 2014, 5296-5303. (23) Carlotti, M.; Johns, J. W. C.; Trombetti, A. Can. J. Phys. 1974, 52, 340-344. (24) Fujisawa, K.; Lehnert, H.; Ishikawa, Y.; Okamoto, K. Angew. Chemie. Int. Ed. 2004, 43, 4944-4947. (25) Stanbury, D. M. Inorg. Chem. 1991, 30, 1293-1296. (26) Chávez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniukiewicz, W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manrı́quez, J. M. J. Organomet. Chem. 2000, 601, 126-132. (27) Baldissin, G.; Boag, N. M.; Tang, C. C.; Bull, D. J. Eur. J. Inorg. Chem. 2013, 19931996. (28) Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.; Mayer, J. M. Chem. Commun. 2008, 256-258. (29) Weatherburn, M. W. Anal. Chem. 1967, 39, 971-974. 99 Chapter 5 CATALYTIC HYDRAZINE DISPORPORTIONATION MEDIATED BY A THIOLATE-BRIDGED VFE COMPLEX 5.1 Introduction Despite structural similarities between the MoFe and VFe nitrogenase cofactors,1 these variants demonstrate variable efficiencies for nitrogen fixation,2 and it is unknown how the distinct metal compositions of the cofactors influence catalytic performance, and the mechanistic pathways that are viable. Similarly, questions remain regarding which metal site(s) are active toward nitrogen fixation at the VFe cofactor,3 and whether bimetallic cooperativity between V and Fe sites may play a role. Synthetic V4 and Fe5 complexes are now known that catalyse molecular N2 reduction to NH3 (or hydrazine). It remains of interest to study the reactivity of bimetallic MFe (M= Mo, V, Fe) complexes toward N2 and other nitrogenous substrates to canvas accessible chemistry on such systems. N2 binding to heterobimetallic VFe complexes has not been previously demonstrated, and relatedly, there are as yet no examples of heterobimetallic VFe-based molecular N2 fixation catalysts. However, VFe complexes are known to generate ammonia via reduction of hydrazine (N2H4 + 2 H+ + 2 e– ® 2 NH3),6 which is both a detectable product of biological nitrogen fixation,7 as well as a substrate for some nitrogenases.8 Alternatively, the 2 H+/2 e– employed in the reduction of hydrazine may be derived from hydrazine itself via a disproportionation pathway, coupling the reduction of N2H4 to NH3 to the oxidation of N2H4 to N2 (3 N2H4 ® 4 NH3 + N2).9,10 Whether N2H4 is an essential intermediate of biological nitrogen fixation is not known. Nonetheless, one possible pathway to catalyse nitrogen fixation to ammonia is the selective reduction N2 to N2H4, which has been demonstrated both stoichiometrically11 and catalytically in molecular Fe systems,5d followed by subsequent hydrazine disproportionation. 100 Herein, we synthesize a thiolate-supported vanadium-iron complex, with N2 and hydride ligands bound to the Fe centre, and demonstrate that it is an active catalyst for the disproportionation of hydrazine under ambient conditions, achieving yields of up to 1073 equivalents of NH3 per VFe complex. In comparison to the bimetallic complex, monometallic analogues of the individual V and Fe centres display attenuated catalytic activity, which suggests a cooperative bimetallic mechanism may be operative in the disproportionation reaction. 5.2 Results and Discussion To template the target VFe complex, we modified the binucleating ligand used to support the thiolate-bridged diiron complex N2Fe-(µ-SAr)-FeN2 (5.1), previously reported by our lab (Fig. 5.1),10 to incorporate a (polythiolate)phosphine fragment to coordinate V.12 Treatment of the thioether-substituted aryllithium 5.213 with dichloro(diethylamino) phosphine (0.5 equivalent), followed by addition of HCl, yields the thioether-substituted diaryl(chloro)phosphine (5.3, Scheme 5.1a). Lithium-halogen exchange of (diphosphino)thioether-functionalized triarylsilane 5.4 and subsequent treatment with 5.3 yields the thioether-protected ligand precursor 5.5. Reductive cleavage of the S-iPr groups with excess KC8 and subsequent protonation yields the thiol-functionalized ligand 5.6. Figure 5.1. Thiolate-bridged bimetallic complexes N2Fe–(µ-SAr)–Fe(N2) (5.1) and V–(µSAr)–Fe(N2)(H) (5.8) 101 Scheme 5.1. (A) Synthesis of ligand 5.6 and (B) metalation of complexes 5.7 and 5.8. Treatment of 5.6 with VMes3(py) (Mes = 2,4,6-Me3C6H2, py = NC5H5) generates V(SAr) (5.7, Scheme 5.1b). Compound 5.7 is a yellow-orange solid and has a spin state of S = 1 at 25 °C. Stirring 5.7 with iron dibromide, followed by reduction with Na(Hg) amalgam 102 (2.1 equiv. Na) under an N2 atmosphere yields V–(µ-SAr)–Fe(N2)(H) (5.8) in moderate yields, whereby reduction promotes the Fe-centred Si–H bond cleavage and N2 coordination. Compound 5.8 has a solution magnetic moment of 3.1µB (25 °C), consistent with an overall S = 1 species. Figure 5.2. X-ray structure of 5.8. Hydrogen atoms and solvent molecules are omitted for clarity. Ellipsoids are depicted at 50% probability. Despite repeated attempts, only moderate-quality crystallographic data could be obtained on compound 5.8. Nonetheless, the XRD data support the structural assignment of 5.8 (Fig. 5.2). The Fe-bound hydride could not be located in the XRD data of 5.8, but the large
P–Fe–P angle (153.0(6)°) suggests it is positioned trans to the thiolate. The IR spectrum of 5.8 reveals an Fe–H stretch at 1898 cm-1, which corroborates the presence of the hydride ligand. Additionally, treatment of 5.8 with one equivalent of [H(OEt2)][BArF4] results in the protonation of the hydride ligand to form H2. The N2 stretch is observed at 2054 cm-1, similar to that of a structurally related diamagnetic ferrous species, (SiP2SAd)Fe(H)(N2) (5.9, 2055 cm-1, Fig. 5.3).14 This comparison is consistent with a description for 5.8 as a V(III)/Fe(II) species. In an alternate oxidation state assignment, if the bridging thiolate is described as an L-type ligand to V and an X-type ligand to Fe, compound 5.8 can instead be considered a V(II)/Fe(III) species. However, the IR spectrum of a related ferric hydride, (SiP2S)Fe(H)(N2) (5.10),13 exhibits a significantly less activated N2 stretch of 2123 cm-1.15 Thus, we favour the description of 5.8 as bearing an S = 1 V(III) centre and an S = 0 Fe(II) 103 centre. Of note, a related thiolate-bound V(III) species (5.11) has a spin state of S = 1 at 25 °C. Figure 5.3. Monometallic complexes 5.9-5.11. Ad = adamantyl Complex 5.8 is an active catalyst for the disproportionation of hydrazine (3 N2H4 ® 4 NH3 + N2).16 Treatment of 5.8 with 50 equivalents of hydrazine results in the detection of NH3 after 2 hours (Table 5.1), with higher yields of ammonia using Et2O (10 equiv. NH3/5.8, 16%, entry a) as a solvent as compared to THF (5 equiv. NH3/5.8, 7%, entry b). Employing Et2O as the solvent, 46 equivalents of NH3 per VFe complex were detected after 12 hours of reaction time (69%, entry c), and 60 equivalents of NH3 were produced after 24 hours of reaction time (89%, entry d). At higher substrate loading (100 equiv. N2H4/5.8), 89 equivalents of NH3 were produced after 24 h (67%, entry e), and 113 equivalents after 48 h (85%, entry f). Addition of elemental Hg or PPh3 did not affect catalysis.17 There are no previously reported examples of vanadium complexes that facilitate the catalytic disproportionation of N2H4, although V complexes have been demonstrated to catalyse hydrazine reduction12d and the disproportionation of substituted hydrazines.18 When considering Fe-only catalysts, diiron 5.1 bears the highest previously reported turnover number; in the presence of one equivalent of co-acid, treatment of 5.1 with 50 equivalents N2H4 yields 29 equivalents of NH3 (44% yield).10 To the best of our knowledge, the most active molecular hydrazine disproportionation catalyst with respect to turnover number is a dimolybdenum complex, which generated 1232 equivalents of NH3 per complex (1440 equivalents of N2H4, 64% yield) over 17 days.9g,h For direct comparison, treatment of 5.8 with 1440 equivalents of N2H4 yielded 1073 equivalents of ammonia (56%) over the same time period (Table 5.1, entry g), demonstrating their similar activities. 104 Table 5.1. N2H4 disproportionation catalysed by compound 5.8 Entry Time N2H4 NH3 NH3 (equiv./5.8) (equiv./5.8) (%) a 2h 50 10(1) 16 b1 2h 50 5(1) 7 c 12 h 50 46(7) 69 d 24 h 50 60(4) 89 e 24 h 100 89(12) 67 f 48 h 100 113(16) 85 g 17 d 1440 1073(84) 56 Catalytic runs are performed at catalyst concentrations of 1 mM unless otherwise noted. Yields are an average of 2 runs; the spread between the two runs is shown in parentheses as a std deviation for N = 2, and values for all individual runs are presented in Appendix D. 1 Entry b was carried out with THF as the solvent. 2Entry g was carried out with a catalyst concentration of 0.1 mM. The catalytic activities of the monometallic Fe and V analogues (5.9 and 5.11) were assayed to compare their respective activities with that of bimetallic 5.8. Upon treatment of 50 equiv of hydrazine in THF (Table 5.2),19 complexes 5.9 (0.4 equiv/5.9, entry h) and 5.11 (0.2 equiv/5.11, entry i) yielded far less NH3 than 5.8 (5 equiv/5.8, Table 5.1, entry b) after 2 hours. Additionally, an equimolar mixture of 5.9 and 5.11 yielded 1.0 equiv of NH3/5.9 after 2 hours (Table 5.2, entry j), which is comparable to the sum of their independent yields in entries h and i. While we do not know all the factors at play, the poorer activity of the equimolar mixture of 5.9 and 5.11 compared to that of 5.8 suggests that arrangement of the Fe and V centres in a binucleating scaffold may enhance the activity for ammonia formation. 105 Employing the method of initial rates, a pseudo-zero order dependence on N2H4 is observed at high substrate loadings (ca. 50 equiv. N2H4) in THF,20 which is due to saturation kinetics and is relevant to the catalysis at early timepoints. Probing kinetics at lower substrate Table 5.2. N2H4 disproportionation catalysed by compounds 5.9 and 5.11 Entry Catalyst NH3 (equiv./cat.) h 5.9 0.4(0.2) i 5.11 0.2(0.1) j1 5.9 + 5.11 1.0(0.2) Catalytic runs are performed at catalyst concentrations of 1 mM unless otherwise noted. Yields are an average of 2 runs; the spread between the two runs is shown in parentheses as a std deviation for N = 2, and values for all individual runs are presented in the Appendix D. 1 [5.9] = [5.11] = 1 mM. loadings (ca. 10 equiv. of N2H4), a first order dependence on N2H4 is determined, which is the regime that the catalyst operates under at late timepoints when most of the substrate has been consumed. Additionally, a first order dependence on 5.8 is measured, which is consistent with a unimolecular pathway with respect to the VFe catalyst, as depicted in Figure 5.4. However, an on-path step involving two VFe species cannot be ruled out. Here, a formal dihydrogen transfer between two hydrazine species yields two equivalents of ammonia and one equivalent of diazene, which can undergo thermal decomposition upon release,21 followed by N2H4 binding to close the catalytic cycle. The dominant decomposition pathway for diazene yields half an equivalent of dinitrogen and half an equivalent hydrazine. Additionally, it has been proposed as an intermediate in other hydrazine disproportionation systems.9e,i 106 Figure 5.4. Proposed catalytic cycle for hydrazine disproportionation catalysed by 5.8. 5.3 Conclusion Drawing inspiration from the VFe cofactor, we have synthesized an N2-bound VFe complex with supporting thiolate, phosphine, and silyl donors, and demonstrated that it is a highly active catalyst for hydrazine disproportionation with respect to turnover number, as active as any known. Whereas there are no known V catalysts for N2H4 disproportionation, there are several less active Fe complexes that also catalyse this transformation.9 dfi,10 It is unclear whether both metal centres are synergistically participating in the key bond-breaking and -forming steps, but assuming that the monometallic complexes 5.9 and 5.11 are representative models for the catalytic activities of the Fe and V centres of 5.8, the much poorer activity of the vanadium and iron monometallic species compared to the activity of 5.8 indicates that arranging the two metal centres within the bimetallic scaffold described promotes hydrazine disproportionation catalysis. 5.4 Experimental Section 5.4.1 Experimental Details 5.4.1.1 General Considerations All syntheses and measurements, unless otherwise stated, were carried out under an inert atmosphere (N2) in a glovebox or using standard Schlenk techniques, and solvents were dried and degassed by thoroughly sparging with N2 and then passing through an activated alumina column in a solvent purification system supplied by SG Water, LLC. 2-phenylbenzenethiol,22 VMes3(py),23 VMes3(THF),24 PhPS3H325 , 5.2,13 5.4,10 and 5.914 were prepared according to 107 literature procedures. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. 5.4.1.2 Physical Methods NMR spectra (1H, 31P) were collected on Varian 300 or 400 MHz spectrometers (25 °C unless otherwise specified). 1 H chemical shifts are reported in ppm, relative to tetramethylsilane using residual proton and 31P chemical shifts are reported in ppm relative to 85% aqueous H3PO4. Thin film IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. Optical spectroscopy measurements were taken on a Cary 50 UV-Vis spectrophotometer using a 1 cm quartz cell, unless otherwise noted. Combustion analysis was carried out by either Midwest Microlabs (Indianapolis) or the Beckman Institute Crystallography Facility (Caltech). H2 was analyzed on an Agilent 7890A gas chromatograph (HP-PLOT U, 30 m, 0.32 mm ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. Mass spectroscopy data for 5.5 and 5.6 were obtained using a Thermo LCQ ion trap mass spectrometer. High-resolution mass spectroscopy data for 5.7 was collected by direct infusion electrospray ionization in the positive ion mode using an LCT Premier XE time-offlight mass spectrometer (Waters). 5.4.1.3 X-Ray Crystallography X-ray diffraction (XRD) measurements were carried out in the Beckman Institute Crystallography Facility. XRD measurements were collected using a dual source Bruker D8 Venture, four-circle diffractometer with a PHOTON CMOS detector. Structures were solved using SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL. The crystals were mounted on a glass fiber under Paratone N oil. 108 5.4.2 Synthetic Details 5.4.2.1 Synthesis of Chlorobis(2-(isopropylthio)-[1,1'-biphenyl]-3-yl)phosphine (5.3) Neat P(NEt2)Cl2 (1.3 g, 7.1 mmol) was added dropwise to a stirring suspension of 5.2 (5.0 g, 14.3 mmol) in Et2O (100 mL) at −78 °C. The reaction was stirred overnight with slow warming to 25 °C. The volatiles were subsequently removed under reduced pressure, and the resulting solids were extracted with pentane (3 x 25 mL), then dried and redissolved in Et2O (100 mL). HCl (2 M in Et2O, 28.5 mL) was added dropwise to the stirring reaction mixture at 0 °C, and the reaction was allowed to stir at 25 °C for 1 h. The reaction volatiles were subsequently removed under reduced pressure, and the crude product was extracted from the mixture with pentane (3 x 50 mL). The pentane solution was concentrated to 15 mL and chilled to −78 °C to precipitate the product. The title compound was isolated by filtration as a white solid. The filtrate was collected, concentrated, and cooled to −78 °C to yield additional material (1.94 g, 26%). 1H NMR (C6D6, 400 MHz, 298 K, δ): 7.81 (d, J = 7.5 Hz, 2H), 7.40 (d, J = 7.2 Hz 2H), 7.12 – 7.03 (m, 16H), 2.84 (hept, J = 7.0 Hz, 1H), 1.00 (d, J = 6.7 Hz, 6H), 0.84 (d, J = 6.8 Hz, 6H). 31P NMR (C6D6, 300 MHz, 298 K, δ): 49.81 (s). 5.4.2.2 Synthesis of (((3-(bis(2-(isopropylthio)-[1,1'-biphenyl]-3-yl) phosphino)-2(isopropylthio)phenyl)silanediyl)bis(2,1-phenylene))bis(diisopropylphosphine) (5.5) N-BuLi (1.6 M in hexane, 0.9 mL, 1.4 mmol) was added dropwise to a stirring solution of 5.4 (0.82 g, 1.3 mmol) in Et2O (20 mL) at −78 °C. The reaction was allowed to stir at −78 °C for 1 h and subsequently allowed to stir at 25 °C for 1 h. The volatiles were removed under reduced pressure, and the resulting off-white solid was redissolved in toluene (20 mL). A cold solution of 5.3 (0.66 g, 1.3 mmol) in toluene (20 mL) was added dropwise to the stirring reaction, and the reaction mixture was stirred overnight with slow warming to 25 °C. The volatiles were subsequently removed in vacuo. The resulting residue was redissolved in Et2O (20 mL) and filtered through Celite. The filtrate was concentrated to yield the title compound as an off-white solid (1.10 g, 82%). The title compound can be recrystallized from a concentrated pentane solution at 25 °C. 1H NMR (C6D6, 400 MHz, 298 K, δ): 7.52 (dd, J = 17.5, 7.4 Hz, 4H), 7.41 (t, J = 8.6 Hz, 2H), 7.36 – 7.02 (m, 20H), 6.96 (t, J = 7.4 Hz, 1H), 109 4.59 (bs, 1H), 3.31 (hept, 1H), 3.10 (hept, 1H), 2.20 – 1.89 (m, 4H), 1.52 (d, J = 6.8 Hz, 6H), 1.36 – 1.25 (m, 3H), 1.25 – 0.94 (m, 21H), 0.90 – 0.79 (m, 12H). 31P NMR (C6D6, 300 MHz, 298 K, δ): 2.12 (s), 1.84 (s), −17.00 (s). ESI-MS (positive ion, amu): Calcd. 1089.4 ([MNa]+); Found. 1089.2. 5.4.2.3 Synthesis of 3,3''-((3-(bis(2-(diisopropylphosphino)phenyl)silyl)-2mercaptophenyl)phosphinediyl)bis(([1,1'-biphenyl]-2-thiol)) (5.6) A suspension of KC8 (0.470 g, 3.5 mmol) in THF (15 mL) was added dropwise to a stirring solution of 5.5 (1.108 g, 1.05 mmol) in THF (50 mL) at −78 °C. The reaction was stirred at 25 °C for 1 h to yield a deep red solution. The reaction mixture was filtered to remove graphite, and solid pyridinium chloride (0.670 g, 5.8 mmol) was added at 25 °C to the stirring solution. The color of the reaction mixture changed from deep red to pale yellow during the addition of acid. The reaction was stirred at 25 °C for 1 h, and the volatiles were subsequently removed in vacuo. The product was extracted from the remaining white solid with pentane (3 x 25 mL), and the volatiles were removed under reduced pressure to afford the title compound as an off-white solid and used as is (0.803 g, 82%). 1H NMR (C6D6, 300 MHz, 298 K, δ): 7.43 – 7.01 (m, 19 H), 6.91 (t, J = 7.6 Hz, 5H), 6.83 (t, J = 7.4 Hz, 3H), 4.72 (d, J = 3.7 Hz, 2H), 4.65 (bs, 1H), 2.06-1.88 (m, 4H), 1.09 (ddd, J = 14.4, 9.4, 6.9 Hz, 12H), 0.93 (dd, J = 12.0, 7.1 Hz, 6H), 0.86 (dd, J = 11.7, 7.2 Hz, 6H). 31P NMR (C6D6, 162 MHz, 298 K, δ): 0.95 (s), −24.15 (s). ESI-MS (positive ion, amu): Calcd. 925.3 ([M-H]+); Found. 925.0. 5.4.2.4 Synthesis of V-(SAr) (5.7) A solution of VMes3(py) (0.218 g, 0.45 mmol) and pyridine (0.1 mL) in THF (10 mL) was added dropwise to a stirring solution of 5.6 (0.376 g, 0.41 mmol) in THF at 25 °C. The reaction mixture was stirred for 5 h at 25 °C, and the volatiles were subsequently removed in vacuo to yield a dark yellow-brown residue. The resulting solids were washed with THF (3 x 5 mL) to yield the title compound as a yellow-orange solid. The filtrate was concentrated and pentane was allowed to slowly diffuse onto the solution to precipitate additional material (0.125 g, 29%). Yellow-orange crystals suitable for XRD were grown from slow diffusion of pentane into a concentrated THF solution at 25 °C. 1H and 31P NMR in C6D6 at 25 °C were 110 silent. µeff (THF, Evans method, 298 K): 3.3µB. Anal. Calcd. for C59H61NP3S3SiV: C, 67.34; H, 5.84; N, 1.33. Found: C, 66.52, H, 6.00; N, 1.23. UV-Vis (THF, 298 K, nm {cm-1 M-1}): 462 {3600}. ESI-MS (positive ion, amu): Calc (M+): 1051.2388, Found: 1051.2361. 5.4.2.5 Synthesis of V-(µ-SAr)-Fe(N2)(H) (5.8) A solution of 5.7 (44.9 mg, 0.043 mmol) in THF (10 mL) was added to solid FeBr2 (10.1 mg, 0.047 mmol) and stirred at 25 °C for 1 h to yield an orange solution. The volatiles were subsequently removed under reduced pressure, and the remaining orange solid was suspended in Et2O (5 mL) and allowed to stir for 10 min at 23 °C. The volatiles were removed in vacuo, and the resulting dark orange solid was dissolved in 10 mL of benzene and added to Na(Hg) amalgam (2.1 mg of Na, 0.091 mmol). The reaction was allowed to vigorously stir at 25 °C for 2 h, which yielded a dark orange-brown solution. The solution was filtered through Celite and lyophilized. The resulting material was washed with pentane (3 x 5 mL) and extracted with diethyl ether (10 mL) to afford the title complex as a dark orange-brown solid (26.7 mg, 55% yield). Dark orange-brown crystals were grown from slow concentration of a solution of 5.8 (3 mL Et2O with ca. 100 µL pyridine) at -35 °C. 1H and 31P NMR in C6D6 at 25 °C were silent. µeff (C6D6, Evans method, 298 K): 3.1µB. µeff (toluene-d8, Evans method, 223K): 3.3µB. Anal. Calcd. for C59H61FeN3P3S3SiV: C, 62.37; H, 5.41; N, 3.70. Found: C, 62.58; H, 5.45; N, 1.73 (Note: low nitrogen content may be due to lability of the coordinated N2). UV-Vis (THF, 298 K, nm {cm-1 M-1}): 446 {5100}, 613 {2700}, 726 {2600}. IR (solid, cm-1): 2054 (N2), 1898 (Fe–H). 5.4.2.6 Synthesis of PhPS3V(THF) A solution of VMes3(THF) (30.1 mg, 0.063 mmol) in THF (5 mL) was added dropwise to a stirring solution of PhPS3H3 (36.8 mg, 0.063 mmol) in THF (5 mL) at 25 °C. The reaction was stirred for 2 h, and the volatiles were subsequently removed in vacuo. The resulting yellow-brown solid was washed with pentane (3 x 5 mL) and Et2O (5 mL). The solids were dissolved in THF and filtered through Celite. The solution was concentrated to yield the product as a yellow-orange solid (35.0 mg, 79% yield). The product was recrystallized in a 111 THF/pentane vapor diffusion cell at 25 °C, which yielded yellow-orange crystals suitable for XRD. 1H and 31P NMR in C6D6 at 25 °C were silent. µeff (C6D6, Evans method, 298 K): 2.6µB. Anal. Calcd. for C40H32OPS3V: C, 67.97; H, 4.56; N, 0.00. Found: C, 68.04; H, 4.88; N, −0.08. UV-Vis (THF, 298 K, nm {cm-1 M-1}): 462 {3800}. 5.4.2.7 Synthesis of PhPS3V(py) (5.11) Pyridine (ca. 100 µL) was added to a solution of PhPS3V(THF) (0.040 g, 0.056 mmol) in benzene (10 mL) and stirred at 25 °C overnight. The reaction mixture was lyophilized to yield the title compound as a yellow-orange solid. The product was recrystallized in a C6H6/pentane vapor diffusion cell at 25 °C, which yielded yellow-orange crystals suitable for XRD. 1H and 31P NMR in C6D6 at 25 °C were silent. µeff (C6D6, Evans method, 298 K): 2.4µB. Anal. Calcd. for C41H29NPS3V: C, 68.99; H, 4.10; N, 1.96. Found: C, 68.89; H, 4.26; N, 1.94. UV-vis (THF, 298 K, nm {cm-1 M-1}): 464 {4800}. 5.4.3 Quantification of H2 from the protonation of compound 5.8 A solution of [H(OEt2)][BArF4] in Et2O (2.4 mM, 1.0 mL) was added dropwise to a stirring solution of 5.8 in Et2O (1.2 mM, 2.0 mL) in a septum-sealed round bottom, yielding a dark red solution. The solution was stirred at 25 °C for 10 min, then an aliquot of the headspace was sampled and analyzed for H2 by GC. Yield of H2: 91% 5.4.4 General procedure for catalytic hydrazine disproportionation *Caution: All catalytic runs were performed on small scales to avoid a significant buildup of pressure from gas generated during catalysis* A solution of catalyst (1 mM catalyst, 2.5 mL solvent, unless otherwise noted) is transferred into a Schlenk tube and frozen. Neat N2H4 is added in one portion to the vessel, which is then immediately sealed and allowed to stir at 25 °C. After the appropriate amount of time elapsed, the volatiles are vacuum transferred into a Schlenk tube containing HCl (2M in Et2O, 5 mL), which is analyzed for ammonia content (in the form of [NH4][Cl]) by the indophenol method.26 (*Note: Upon thawing of the evacuated frozen vessels, care is taken to allow the 112 vacuum transfer to proceed at 25 °C. Hydrazine is a thermally-sensitive compound, and heating a vessel containing solely N2H4 in THF during the work-up results in positive ammonia detection) D.5.6 Toepler pump quantification of gaseous products Neat N2H4 (0.1 mmol) was added in one portion at 25 °C to a stirring Et2O solution of 5.8 (2.4 mg, 0.002 mmol) in a 100 mL Kontes Schlenk tube equipped with a ground-glass side arm. The tube was degassed via three freeze-pump-thaw cycles and sealed. The reaction mixture stirred at 25 °C under static vacuum. The volatiles in the tube were passed through a cold trap (77 K), and the non-condensed gaseous products were quantified with a Toepler pump. The frozen volatiles condensed in the traps were vacuum transferred into a Schlenk tube containing an ethereal solution of HCl (5 mL, 2 M) and concentrated to dryness. The resulting material was quantified for ammonia (in the form of [NH4][Cl]) via the indophenol method.26 Omitting the equivalent of N2 that is introduced from 5.8, 4.0 molecules of NH3 per gas molecule was generated. 5.5 References (1) (a) Sippel, D.; Einsle, O. Nat. Chem. Bio. 2017, 13, 956-960. (b) Spatzal, T.; Aksoyoglu, M.; Zhang, L.; Andrade, S. L. A.; Schleicher, E.; Weber, S.; Rees, D. C.; Einsle, O. Science 2011, 334, 940. (2) (a) Eady, R. R. Chem. Rev. 1996, 96, 3013-3030. (b) Hu, Y.; Ribbe, M. W. J. Biol. Inorg. Chem. 2015, 20, 435-445. (3) (a) Sippel, D; Rohde, M.; Netzer, J.; Trncik, C.; Gies, J.; Grunau, K.; Djurdjevic, I.; Decamps, L.; Andrade, S. L. A.; Einsle, O. Science 2018, 359, 1484-1489. (b) Benediktsson, B.; Thorhallsson, A. T.; Bjornsson, R. Chem. Commun. 2018, 54, 7310-7313. (4) Sekiguchi, Y.; Arashiba, K.; Tanaka, H.; Eizawa, A.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chemie. Int. Ed. 2018, 57, 9064-9068. (5) (a) For representative examples see: Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84-87. (b) Ung, G; Peters, J. C. Angew. Chemie. Int. Ed. 2015, 54, 532-535. (c) 113 Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181. (d) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. J. Am. Chem. Soc. 2016, 138, 13521-13524. (e) Buscagan, T. M.; Oyala, P. H.; Peters, J. C. Angew. Chem. Int. Ed. 2017, 56, 6921-6926. (f) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105-1115. (g) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217-223. (h) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 61226129. (6) (a) Malinak, S. M.; Demadis, K. D.; Coucouvanis, D. J. Am. Chem. Soc. 1995, 117, 31263133. (b) Coucouvanis, D.; Demadis, K. D.; Malinak, S. M.; Mosier, P. E.; Tyson, M. A.; Laughlin, L. J. J. Mol. Catal. A: Chem. 1996, 107, 123-135. (7) Dilworth, M. J.; Eady, R. R. Biochem. J. 1991, 277, 465-468. (8) Davis, L. C. Arch. Biochem. Biophys. 1980, 204, 270-276. (9) (a) Kuwata, S.; Mizobe, Y.; Hidai, M. Inorg. Chem. 1994, 33, 3619-3620. (b) Takei, I.; Dohki, K.; Kobayashi, K.; Suzuki, T.; Hidai, M. Inorg. Chem. 2005, 44, 3768-3770. (c) Umehara, K.; Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2013, 135, 6754-6757. (d) Hitchcock, P. B.; Hughes, D. L.; Maguire, M. J.; Marjani, K.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1997, 4747-4752. (e) Wu, B.; Gramigna, K. M.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Inorg. Chem. 2015, 54, 10909-10917. (f) Malinak, S. M.; Coucouvanis, D. Prog. Inorg. Chem. 2001, 49, 599-662. (g) Block, E.; Ofori-Okai, G.; Kang, H.; Zubieta, J. J. Am. Chem. Soc. 1992, 114, 758-759. (h) Under photolysis, [Mo(CN)8]4was demonstrated to catalyze N2H4 disproportionation with yields of >103 equivalents of NH3. See: Szkiarzewicz, J.; Matoga, D.; Klyś, A.; Łasocha, W. Inorg. Chem. 2008, 47, 54645472. (i) Saouma, C. T.; Moore, C. E.; Rheingold, A. L.; Peters, J. C. Inorg. Chem. 2011, 50, 11285-11287. (10) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7310-7313. (11) Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem. Int. Ed. 2007, 46, 5768-5771. (12) (a) Chang, Y.-H.; Su, C.-L.; Wu, R.-R.; Liao, J.-H.; Liu, Y.-H.; Hsu, H.-F. J. Am. Chem. Soc. 2011, 133, 5708-5711. (b) Hsu, H.-F.; Chu, W.-C.; Hung, C.-H.; Liao, J.-H. Inorg. 114 Chem. 2003, 42, 7369-7371. (c) Ye, S.; Neese, F.; Ozarowski, A.; Smirnov, D.; Krzystek, J.; Telser, J.; Liao, J.-H.; Hung, C.-H.; Chu, W.-C.; Tsai, Y.-F.; Wang, R.-C.; Chen, K.-Y.; Hsu, H.-F. Inorg. Chem. 2010, 49, 977-988. (d) Chu, W.-C.; Wu, C.-C.; Hsu, H.-F. Inorg. Chem. 2006, 45, 3164-3166. (13) Gu, N. X.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6374-6382. (14) Takaoka, A; Mankad, N. P.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 8440-8443. (15) For reference, the Fe–H stretches of 5.9 and 5.10 are 1910 cm-1 and 1852 cm-1, respectively. (16) Volatiles from a catalytic run performed in Et2O were passed through a 77 K cold trap, and the remaining gas was quantified with a Toepler pump. The ammonia content from the same run was also determined. Representative IR data of catalyst mixtures after turnover do not reveal any Fe–N2 stretches, indicating that the bound N2 of 5.8 dissociates during catalysis (see Fig. D.2.4). Omitting the equivalent of N2 that is introduced into the headspace from 5.8, a ratio of 4.0 NH3 molecules per gas molecule is generated, consistent with the anticipated ratios for hydrazine disproportion (3 N2H4 ® 4 NH3 + N2). (17) Carrying out entry C in the presence of elemental Hg or PPh3 (0.3 equiv./ 5.8) demonstrated no significant attenuation of catalytic activity. (Hg: 46 equiv. NH3/5.8; PPh3: 42 equiv. NH3/5.8) (18) Milsmann, C.; Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 12099-12107. (19) The catalytic runs in Table 5.2 were performed in THF rather than Et2O because of the poor solubility of 5.9 in Et2O. (20) Although complex 5.8 is a more active catalyst in Et2O, the reaction order on N2H4 and 5.8 were determined in THF because it is miscible with hydrazine. (21) (a) Tang, H. R.; Stanbury, D. M. Inorg. Chem. 1994, 33, 1388-1391. (b) Wiberg, E.; Wiberg, N. Inorganic Chemistry, Academic Press: San Diego, CA, 2001. (c) Wiberg, N.; Bachhuber, H.; Fischer, G. Angew. Chem. Int. Ed. 1972, 11, 829-830. (22) McGinley, P. L.; Koh, J. T. J. Am. Chem. Soc. 2007, 129, 3822-3823. (23) Seidel, V. W.; Kreisel, G. Z. anorg. allg. Chem. 1977, 435, 146-152. 115 (24) Hoang, T. K. A.; Hamaed, A.; Moula, G.; Aroca, R.; Trudeau, M.; Antonelli, D. M. J. Am. Chem. Soc. 2011, 133, 4955-4964. (25) Tsai, J.-C. Masters thesis, Stony Brook University, 2011. (26) Weatherburn, M. W. Anal. Chem. 1967, 39, 971-974. 116 Appendix A SUPPLEMENTARY INFORMATION FOR CHAPTER 2 A.1 NMR Spectra Figure A.1.1. 1H NMR spectrum (400 MHz) of 2.2 in CDCl3 Figure A.1.2. 13C NMR spectrum (101 MHz) of 2.2 in CDCl3 117 Figure A.1.3. 1H NMR spectrum (400 MHz) of 2.3 in C6D6 Figure A.1.4. 1H NMR spectrum (400 MHz) of 2.5 in C6D6 Figure A.1.5. 31P NMR spectrum (162 MHz) of 2.5 in C6D6 118 Figure A.1.6. 1H NMR spectrum (300 MHz) of 2.6 in C6D6 Figure A.1.7. 1H NMR spectrum (300 MHz) of 2.7-H in C6D6 Figure A.1.8. 31P NMR spectrum (162 MHz) of 2.7-H in C6D6 119 Figure A.1.9. 1H NMR spectrum (300 MHz) of 2.7-D in C6D6 Figure A.1.10. 31P NMR spectrum (162 MHz) of 2.7-D in C6D6 Figure A.1.11. 31P{2H} NMR spectrum (162 MHz) of 2.7-D in C6D6 120 * * Figure A.1.12. 1H NMR spectrum (400 MHz) of 2.7-H(crown) in THF-d8 (*Et2O) Figure A.1.13. 31P NMR spectrum (162 MHz) of 2.7-H(crown) in THF-d8 Figure A.1.14. 1H NMR spectrum (500 MHz) of 2.8-H in THF-d8 (−78 ºC; generated in the presence of one equivalent of Cp2Co) 121 Figure A.1.15. 1H NMR spectrum (400 MHz) of 2.9 in THF-d8 Figure A.1.16. 1H NMR spectrum (300 MHz) of 2.10 in THF-d8 (*C6H6, #Et2O, &pentane) Figure A.1.17. 31H NMR spectrum (121 MHz) of 2.10 in THF-d8 122 Figure A.1.18. 1H NMR spectrum (400 MHz) of 2.11 in C6D6 Figure A.1.19. 1H spectra (300 MHz) of (a) complex 2.7-H(crown) (b) reaction mixture of 2.7-H(crown) under 1 atm of CO2, collected 1 h after addition of CO2 (c) complex 2.11 prepared from diiron 2.6. All spectra collected in MeCN-d3 123 Figure A.1.20. Partial 1H spectra (300 MHz) of (a) complex 2.7-H(crown) (b) reaction mixture of 2.7-H(crown) under 1 atm of CO2, collected 1 h after addition of CO2 (c) complex 2.11. All spectra collected in MeCN-d3 (* = C6H6) A.2 IR Spectra Figure A.2.1. IR spectrum of 2.5 (solid powder sample) Figure A.2.2. IR spectrum of 2.6 (solid crystalline sample) Figure A.2.3. IR spectrum of 2.7-H (thin-film from THF solution) 124 Figure A.2.4. IR spectrum of 2.7-D (thin-film from THF solution) Figure A.2.5. IR spectrum of 2.7-H(crown) (thin-film from THF solution) Figure A.2.6. IR spectrum of 2.8-H and 2.8-D (thin-film from THF solution; generated in the presence of one equivalent of Cp2Co) 125 Figure A.2.7. IR difference spectrum of 2.8-H and 2.8-D exhibiting an Fe–D stretch at 1333 cm-1 and Fe–H stretch at 1852 cm-1 Figure A.2.8. IR spectrum of 2.9 (solid crystalline sample) Figure A.2.9. IR spectrum of 2.10 (solid powder sample) 126 Figure A.2.10. IR spectrum of 2.11 (solid powder sample) Figure A.2.11. IR spectrum of the reaction of 2.7-H(crown) with CO2 (solid powder sample) A.3 X-Ray Diffraction Figure A.3.1. X-ray structure of 2.7-H. Hydrogen and disordered atoms omitted for clarity. Ellipsoids shown at 50% probability. Partial substitution of diethyl ether in place of THF was observed with regards to the solvent coordination at lithium, due to the crystals being grown from a diethyl ether solution. 127 Figure A.3.2. Structure of 2.9 with omission of countercation, disordered components, hydrogen atoms and solvent molecules. Thermal ellipsoids at 50% probability. Figure A.3.3. Full asymmetric unit of 2.9 including solvent and disordered components. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure A.3.4. Structure of 2.11 with omission of disordered components and hydrogen atoms. Thermal ellipsoids at 50% probability. 128 Figure A.3.5. Full asymmetric unit of 2.11 including disordered components. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. A.4 57Fe Mössbauer Spectra Figure A.4.1. 80K 57Fe Mössbauer spectrum of 2.6 collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (solid sample suspended in boron nitride) Figure A.4.2. 80 K 57Fe Mössbauer spectrum of 2.7-H collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution sample in THF) 129 Figure A.4.3. 80K 57Fe Mössbauer spectrum of 2.7-H(crown) collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (solid sample suspended in boron nitride) Figure A.4.4. 80K 57Fe Mössbauer spectrum of 2.8-H collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (solid sample suspended in boron nitride; generated in the presence of one equivalent of Cp2Co) A.5 EPR Spectra Figure A.5.1. 77 K X-band EPR spectra of 2.8-H in 2-MeTHF, generated with [Cp2Co][PF6] and [Cp*2Fe][PF6] as oxidants 130 Figure A.5.2. 77K K X-band EPR spectra of 2.9 in 2-MeTHF with simulation (Sys.g = [2.1440 2.0350 2.0300], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [0 50 45; 0 50 45], Sys.HStrain = [120 0 60], Exp.mwFreq = 9.389) A.6 ENDOR Spectroscopy Figure A.6.1. Field-dependent Q-band Davies ENDOR of (a) 2.8-H and (b) 2.8-D in 2MeTHF with simulations (partial spectrum of Fig. 5ab). Summation of individual component ENDOR simulations is displayed in red. Experimental conditions: microwave frequency = 33.674 GHz; MW π pulse length = 40 ns; interpulse delay τ = 300 ns; "#$ pulse length = 15 µs; TRF delay = 1 µs; shot repetition time (srt) = 5 ms; temperature = 18.5 K; RF frequency randomly sampled. The simulated 31P signals at lower frequencies have very low intensity in the experimental data, possibly as a product of the hyperfine enhancement effect and lower RF magnetic field intensity generated by the ENDOR coils at very low frequencies. 131 A.7 UV-Visible Spectra Figure A.7.1. UV-Vis spectrum of 2.6 in THF Figure A.7.2. UV-Vis spectrum of 2.7-H in THF Figure A.7.3. UV-Vis spectrum of 2.7-H(crown) in THF 132 Figure A.7.4. UV-Vis spectrum of 2.8-H in THF (1 mm cuvette, generated in the presence of one equivalent of Cp2Co) Figure A.7.5 UV-Vis spectrum of 2.9 in THF Figure A.7.6. UV-Vis spectrum of 2.11 in THF 133 A.8 Cyclic Voltammograms Figure A.8.1. Cyclic voltammogram of 2.7-H(crown) in THF (0.4 M [NBu4][PF6]) showing the reversible Fe(II)/Fe(III) couple centered at −1.63 V vs Fc/Fc+. Data was collected at 400 mV/s with Pt counter, glassy carbon working and Ag/AgOTf reference electrodes Figure A.8.2. Cyclic voltammogram of 2.9 in THF (0.4 M [NBu4][PF6]) showing the reversible Fe(I)/Fe(II) couple centered at −1.75 V vs Fc/Fc+. Data was collected at 400 mV/s with Pt counter, glassy carbon working and Ag/AgOTf reference electrodes 134 Figure A.8.3. Cyclic voltammogram (blue) and square wave voltammogram (red) of 2.9 in MeCN (0.4 M [NBu4][PF6]) showing the irreversible Fe(I)/Fe(II) couple centered at −1.71 V vs. Fc/Fc+. Data was collected at 100 mV/s with Pt counter, glassy carbon working and Ag/AgOTf reference electrodes Figure A.8.4. Cyclic voltammogram of 2.9 in MeCN (0.4 M [NBu4][PF6]) showing the reversible Fe(I)/Fe(0) couple centered at −2.94 V vs. Fc/Fc+. Data was collected at 100 mV/s with Pt counter, glassy carbon working and Ag/AgOTf reference electrodes 135 Figure A.8.5. Cyclic voltammogram of 2.7-H(crown) in MeCN (0.4 M [NBu4][PF6]) showing the reversible Fe(II)/Fe(III) couple with an onset at −0.58 V vs. Fc/Fc+. Data was collected with Pt counter, glassy carbon working and Ag/AgOTf reference electrodes A.9 Variable Temperature Evans Method Measurements Figure A.9.1.Variable temperature 1H NMR spectra (500 MHz) of 2.6 in C6D5CD3 136 Temperature (K) µeff/µB 298 4.79 288 4.82 278 4.83 268 4.86 258 4.88 248 4.90 238 4.93 228 4.95 218 4.98 208 5.00 198 5.02 Table A.9.1. Solution state magnetic susceptibility of 2.6 in C6D5CD3 at variable temperatures A.10 Time-Course UV-Visible Spectra * Note: For Tables A.10.1-A.10.7, concentrations of 2.8-H and 2.8-D over time were obtained by fitting the 640 nm – 900 nm region of the corresponding time-course UV-vis traces following the conversion of 2.8-H or 2.8-D to 2.6. The absorbance data (Abs) were fit to Gaussian lineshapes with the following equation: Figure A.10.1. UV-visible spectra showing the conversion of 2.8-H to 2.6 at 15 °C in a 1 mm cuvette; traces collected every 30 min for 270 min 137 Figure A.10.2. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.1) Time (min) [2.8-H] (mol/L) 0 0.006212 30 0.006191 60 0.006163 90 0.006148 120 0.006113 150 0.006081 180 0.006095 210 0.006078 240 0.006039 270 0.006049 Table A.10.1. Fitted concentration of 2.8-H vs. time (at 15 °C) Figure A.10.3. Plot of [2.8-H]-1 vs. time (at 15 °C) 138 Figure A.10.4. UV-visible spectra showing the conversion of 2.8-H to 2.6 at 25 °C in a 1 mm cuvette; traces collected every 30 min for 270 min Figure A.10.5. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.4) Time (min) [2.8-H] (mol/L) 0 0.005435 30 0.005346 60 0.005276 90 0.005261 120 0.005184 150 0.005144 180 0.005095 210 0.005032 240 0.004993 270 0.004908 Table A.10.2. Fitted concentration of 2.8-H vs. time (at 25 °C) 139 Figure A.10.6. Plot of [2.8-H]-1 vs time (at 25 °C) Figure A.10.7. UV-visible spectra showing the conversion of 2.8-H to 2.6 at 30 °C in a 1 mm cuvette; traces collected every 30 min for 270 min Figure A.10.8. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.7) Time (min) [2.8-H] (mol/L) 0 0.005067 30 0.004989 140 60 0.004936 90 0.004869 120 0.004756 150 0.004721 180 0.004625 210 0.004551 240 0.004449 270 0.004389 Table A.10.3. Fitted concentration of 2.8-H vs. time (at 30 °C) Figure A.10.9. Plot of [2.8-H]-1 vs. time (at 30 °C) Figure A.10.10. UV-visible spectra showing the conversion of 2.8-H to 2.6 at 40 °C in a 1 mm cuvette; traces collected every 30 min for 270 min 141 Figure A.10.11. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.10) Time (min) [2.8-H] (mol/L) 0 0.003282 30 0.002946 60 0.002723 90 0.002510 120 0.002307 150 0.002124 180 0.001949 210 0.001806 240 0.001668 270 0.001547 Table A.10.4. Fitted concentration of 2.8-H vs. time (at 40 °C) Figure A.10.12. Plot of [2.8-H]-1 vs. time (at 40 °C) 142 Figure A.10.13. UV-visible spectra showing the conversion of 2.8-H to 2.6 at 50 °C in a 1 mm cuvette; traces collected every 30 min for 270 min Figure A.10.14. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.13) Time (min) [2.8-H] (mol/L) 0 0.001873 30 0.001438 60 0.001155 90 0.0009611 120 0.0008223 150 0.0007216 180 0.0006399 210 0.0005678 240 0.0005187 270 0.0004784 Table A.10.5. Fitted concentration of 2.8-H vs. time (at 50 °C) 143 Figure A.10.15. (a) Plot of [2.8-H]-1 vs. time (at 50 °C) (b) Plot of ln([2.8-H]) vs. time (at 50 °C) T (K) k (M-1min -1 ) 288 0.01693 298 0.06793 303 0.1141 313 1.254 323 5.828 Table A.10.6. Temperature dependence of the second order rate constant (k) for the conversion of 2.8-H to 2.6 Figure A.10.16. UV-visible spectra showing the conversion of 2.8-D to 2.6 at 25 °C in a 1 mm cuvette; traces collected every 30 min for 270 min 144 Figure A.10.17. Partial UV-vis spectra and fits (640 nm to 900 nm region of Fig. A.10.16) Time (min) [2.8-D] (mol/L) 0 0.005650 30 0.005604 60 0.005534 90 0.005523 120 0.005466 150 0.005452 180 0.005420 210 0.005371 240 0.005378 270 0.005304 Table A.10.7. Fitted concentration of 2.8-D vs. time (at 25 °C) Figure A.10.18. Plot of [2.8-D]-1 vs. time (at 25 °C) 145 A.11 DFT-Optimized Structures Fe P S P Si N C C H C C C C H C H C H C H C H C H C H C C H C H H H C C H C H H H C C 0.51351100 0.05922200 -1.03017400 1.38765300 -1.99402600 -0.80215900 -1.71508100 -0.14263000 -0.79525400 1.05446900 2.21884800 -0.77401200 0.60659500 0.05068200 1.24023200 0.23385300 0.04219400 -2.87573300 2.05136100 -2.32109200 0.87337700 2.78554100 -2.48360700 -1.93306000 2.97617200 -3.54564700 -1.69634900 1.55693400 2.65748400 0.93360400 -3.53858000 -0.17437400 1.30077700 1.36786000 1.66557100 1.90902300 -3.84306400 -0.21410100 2.66995700 -4.89541600 -0.26191500 2.96501000 0.11806900 -3.36551600 -1.02362700 -0.51582500 -2.98556600 -1.84531000 2.77406700 -2.81946800 3.51871600 3.05124800 -3.01623200 4.55755400 -0.30566400 3.47812400 -1.07459500 0.22290800 4.45108700 -1.08557200 2.84490100 -3.43330100 1.18651200 3.19218400 -4.11215100 0.40128600 -2.85275700 -0.16608500 3.64651400 -3.12464200 -0.18637200 4.70452400 2.48741800 2.85356900 -1.80672600 2.36561000 3.95292700 -1.82998500 1.62901900 -1.42952600 1.87731700 1.99659600 -1.70670700 3.20315600 1.67318400 -1.04463400 4.01268300 4.05687000 -1.70046500 -1.65359400 3.93062700 -0.63702500 -1.90748400 4.88497600 -2.08449300 -2.26822500 4.37011600 -1.75606000 -0.60123600 -1.16237000 -0.05528300 1.90623700 3.20822400 -3.67884100 2.50817600 3.82949400 -4.54395500 2.75247100 -0.73885000 -3.47953500 0.22907700 -0.18654400 -3.97226800 1.04407300 -1.63524300 -4.08295800 0.02412500 -1.08282200 -2.50551900 0.60168200 -4.65929900 -0.23083700 0.33392800 -2.17354400 -0.10392800 0.92144800 146 C H H H C H C H C H H H C H C H C H H H N C H C H H H C H C H C H C H C H H H C H C H 2.45502100 2.32318900 -3.23196300 2.65142000 1.23972900 -3.24509100 3.24098200 2.80395300 -3.83320700 1.50057700 2.48751600 -3.74801800 2.15245900 3.24455600 3.58925200 2.38630000 3.47308900 4.63223100 -1.51664900 -0.08320000 3.25849300 -0.73717300 -0.04194900 4.02778900 -1.00032000 3.27047600 -2.41066500 -0.31240400 3.27797500 -3.26697400 -1.73923200 4.06768700 -2.58186300 -1.54409600 2.31322400 -2.42192500 2.02809900 3.93142100 1.27978800 2.16433100 4.70249600 0.51341700 2.32828200 4.22286400 2.60674500 2.69796200 5.21439500 2.87960500 3.82638600 2.51682000 -1.16533800 3.98919000 3.02134800 -0.20500400 4.65018200 2.80212500 -1.83666100 3.91600300 1.43393900 -0.98370400 -0.02147200 0.03228300 -3.97141000 1.67558800 1.98339200 3.24210000 1.53661500 1.23760000 4.03101000 -1.31272000 3.47638800 0.06507100 -1.79419400 2.49251900 0.16744000 -2.10912700 4.20922700 -0.13456200 -0.86441800 3.73111400 1.03483900 -5.64232600 -1.22308900 0.46891100 -5.53639300 -1.96953000 1.26156600 -6.72900300 -1.28274300 -0.40071800 -7.47562300 -2.07215600 -0.28312600 -4.80826900 0.71348600 -0.69423700 -4.06361800 1.50530400 -0.80583700 -6.86083300 -0.34068300 -1.41911400 -7.71225400 -0.38357300 -2.10267500 2.38522300 -2.37470400 -3.39645400 1.40738300 -2.82686100 -3.61844500 3.12964400 -2.86749000 -4.03945200 2.33389700 -1.32286600 -3.71089300 -5.89834300 0.65998000 -1.55803600 -5.99937200 1.40955400 -2.34681400 0.68229100 -4.72295200 -1.41064000 1.21633300 -4.72330000 -2.37056600 147 H -0.13729300 -5.45202600 -1.50030600 H 1.36501100 -5.11500800 -0.64046200 H 2.05682000 0.17816000 -1.08009400 Table A.11.1. Gas-phase optimized coordinates for 2.8-H (m06l, S = 1/2) Fe P P S Si C H N C H N C C C C C C C H C C H H H C H C H C H C H C C H H H C -0.40651200 -0.02742200 -1.05973600 -1.43431500 -1.97339400 -0.67790500 -1.47887300 1.90612200 -0.82623600 1.88980900 0.04304100 -0.87360400 -0.38392000 0.06386500 1.20164500 -0.20460800 -3.40847500 -0.91118300 -0.82326500 -4.32563600 -0.98560600 -0.37449700 -0.10237400 -2.88106500 -0.46129900 3.39985700 -1.40816200 -1.15610800 4.26289800 -1.40683700 -0.34305500 -0.15563500 -4.01698500 -1.77546700 2.46941900 0.90585200 -1.20236000 1.65625700 1.89688700 1.40432300 0.02232900 1.84428300 2.39150100 0.05601300 0.82501400 -1.92292100 -2.27864500 1.08060000 3.76805300 0.03633800 1.18984800 -2.41834300 3.66788700 1.24311000 -2.85458700 4.30070700 0.46096600 4.88003000 0.02388600 0.21155200 0.68241700 3.68080300 -0.44340800 0.33745000 3.98733100 0.55378800 1.32541600 4.48606000 -0.83464100 1.31482700 2.78660600 -0.31997200 -1.63166400 -1.57200000 3.36991500 -1.22545300 -0.87423400 4.11080000 -1.28145900 2.08441000 3.23205100 -0.82541600 1.48435300 4.02775900 -1.92284300 3.27472800 3.57152700 -1.97466300 3.59124400 4.61779100 4.97186900 0.93932000 -0.85099900 4.17406400 1.67164500 -0.98880900 -1.38294800 -1.36453300 2.00478000 0.61512300 -3.23876400 -2.18322000 0.01057600 -3.05712400 -3.08128200 1.22831200 -4.13588800 -2.36818700 1.30000400 -2.38292900 -2.08216900 -2.93231700 -3.54707000 2.88429900 148 H C H C H H H C H C H C H C H C H C H C H C H H H C H C H C H C H H H C H C H H H C H -3.53643500 -4.39388600 3.22351500 3.13831800 -0.05071400 3.55221800 3.43334400 -0.08706100 4.60456300 0.72764900 -3.53665100 0.28488800 1.27143300 -2.59352600 0.46132100 1.48800000 -4.31183900 0.09728100 0.20237800 -3.80037900 1.21349600 -2.68935900 -3.36522500 1.52363200 -3.09876900 -4.08675400 0.80871000 1.79201600 -0.03185500 3.18508200 1.02628800 -0.06885400 3.96959100 6.06222700 0.92220400 -1.71662100 6.11239700 1.64892400 -2.53235600 -2.39968400 -2.64898900 3.81117000 -2.58801600 -2.78979100 4.87994400 7.08467900 -0.01488500 -1.55471700 7.93295100 -0.03152000 -2.24468100 -2.92928000 -2.71066000 -1.57385100 -3.03334800 -3.74105200 -1.18475600 -3.15962400 2.26070500 -1.62529700 -3.25088500 3.35734400 -1.74795600 0.08380600 3.19219700 -2.81132100 0.79248600 2.34982400 -2.82368600 0.62894100 4.08749900 -3.15232600 -0.69455800 2.97953000 -3.55771400 -2.49820000 4.06776000 2.57505600 -3.00208800 5.00238600 2.83929300 4.10482600 -0.02007600 2.55165500 5.16589100 -0.02370000 2.82258100 7.00905700 -0.93065300 -0.50730100 7.79729600 -1.67679100 -0.37138200 -4.19967100 -1.94277800 -1.25194400 -4.35214000 -1.80022500 -0.17176300 -5.08474400 -2.46755200 -1.64753200 -4.18235400 -0.94588400 -1.71620800 5.92196100 -0.90558100 0.36415000 5.85817200 -1.63705400 1.17522900 -4.28078800 1.76207300 -0.72221400 -4.05218400 0.75977200 -0.32437700 -5.23272600 1.68697100 -1.27330600 -4.44543700 2.41369700 0.14567300 -3.23584400 1.60036100 -2.99251800 -2.46586400 1.95801000 -3.68904100 149 H -4.21850600 1.77078400 -3.46235600 H -3.08691400 0.51254000 -2.90119400 C -2.72991600 -2.79457000 -3.07860200 H -2.47058600 -1.81481600 -3.50887900 H -3.65538100 -3.13391500 -3.57273400 H -1.93743900 -3.49890300 -3.36360100 Table A.11.2. Gas-phase optimized coordinates for 2.9 (m06l, S = 1/2) Fe P S P Si N C C H C C C C H C H C H C H C H C H C H C C H C H H H C C 0.51937400 0.06404600 -1.02451700 1.40648300 -1.99850900 -0.80591400 -1.72736800 -0.12756600 -0.82249300 1.08641900 2.23212000 -0.75962900 0.58462200 0.04165800 1.24653400 0.29356300 0.06672600 -2.86412100 2.03901100 -2.32546600 0.88245900 2.82252300 -2.46267100 -1.91736700 3.01923300 -3.52234200 -1.67811100 1.52205600 2.65825300 0.96963700 -3.57213700 -0.18872400 1.26456400 1.31530100 1.66053700 1.93700800 -3.88555500 -0.25027300 2.63248700 -4.93888300 -0.30721500 2.92313200 0.14401300 -3.36863200 -1.05790500 -0.47076300 -2.98954900 -1.89417600 2.73318700 -2.83190900 3.53558600 3.00009600 -3.03224400 4.57668900 -0.24681600 3.49911300 -1.11844400 0.29569400 4.46262000 -1.10649700 2.83365400 -3.43662700 1.19908800 3.19028000 -4.11042500 0.41413600 -2.90099600 -0.21684300 3.61627900 -3.17904500 -0.25538800 4.67249800 2.57523000 2.82990800 -1.72661700 2.47838900 3.93079900 -1.74358400 1.60093100 -1.44006000 1.88677400 1.95465900 -1.71916300 3.21748700 1.62092200 -1.06005700 4.02519000 4.07968000 -1.66666400 -1.61764300 3.95737200 -0.60699400 -1.88982100 4.92328700 -2.05551400 -2.20797800 4.37437400 -1.70802000 -0.55865200 -1.19936900 -0.07745100 1.88507800 3.18135400 -3.68579000 2.52527300 150 H C H H H C C C H H H C H C H C H H H C H C H C H H H N C H C H H H C H C H C H C H C 3.80243300 -4.55041600 2.77281500 -0.73588000 -3.49058900 0.17665000 -0.20223300 -3.99560700 0.99710300 -1.63053500 -4.09102400 -0.04786300 -1.08167200 -2.51733100 0.55303500 -4.69242100 -0.23425200 0.29448400 -2.20417500 -0.11158200 0.89060800 2.59326300 2.31065100 -3.15567900 2.76570900 1.22278000 -3.17784000 3.41942500 2.77862700 -3.71195300 1.67049100 2.51326300 -3.71498400 2.05300700 3.23545100 3.64266000 2.26200400 3.46068000 4.69192200 -1.56173800 -0.12764000 3.23581800 -0.78558200 -0.10146000 4.00924900 -0.88357100 3.30405000 -2.48387200 -0.15554900 3.30972200 -3.30618500 -1.60285300 4.11297100 -2.68327400 -1.44064300 2.35496700 -2.53042600 1.98000600 3.93341700 1.33176400 2.12955400 4.70631300 0.57032800 2.24688100 4.21911800 2.66774000 2.60539100 5.21131000 2.95386800 3.87436300 2.45749300 -1.02778000 4.01796300 2.97444000 -0.07024500 4.73106400 2.71241000 -1.66974400 3.93091500 1.37414100 -0.83189000 0.06687200 0.07061500 -3.96813400 1.58959100 1.97219100 3.28048900 1.43580400 1.22214200 4.06266300 -1.30227100 3.51381600 -0.02473900 -1.79760900 2.53495900 0.06795500 -2.08393000 4.25010100 -0.26763600 -0.89661800 3.78160600 0.96097000 -5.67738000 -1.22778400 0.41965400 -5.57603600 -1.98302700 1.20472000 -6.76654900 -1.27592200 -0.44934300 -7.51416800 -2.06563900 -0.33842300 -4.84408400 0.72410900 -0.72217700 -4.10233400 1.52046500 -0.82396300 -6.90059200 -0.32083500 -1.45656700 -7.75396600 -0.35434700 -2.13861300 2.44256900 -2.35217600 -3.38579700 151 H 1.46284900 -2.79400500 -3.62077500 H 3.19053800 -2.85981500 -4.01316200 H 2.41634900 -1.30052600 -3.70627700 C -5.93717700 0.68209700 -1.58495700 H -6.04095700 1.44369000 -2.36224400 C 0.72193800 -4.72021600 -1.44186400 H 1.26952600 -4.71017700 -2.39385200 H -0.09510900 -5.45022600 -1.55000600 H 1.39668600 -5.11421100 -0.66602300 H 2.05996600 0.17104800 -1.02231200 Table A.11.3. Optimized coordinates for 2.8-H with MeCN solvation (m06l, S = 1/2) Fe P S P Si N C C H C C C C H C H C H C H C H C H C H C C H C H 0.48346400 0.07403100 -1.09971000 1.50080400 -2.00381700 -0.79711900 -1.81017900 -0.07945800 -0.87777700 1.16000500 2.27375100 -0.74716800 0.52214500 0.04646900 1.20664600 0.27529700 0.06980100 -2.94778000 2.06720800 -2.29739500 0.91979100 2.95866200 -2.45705200 -1.85947200 3.18043600 -3.51200200 -1.61962000 1.45179800 2.69435600 1.01100000 -3.64523100 -0.22657600 1.21944000 1.18688400 1.67952500 1.94703300 -3.94651600 -0.33448500 2.58734100 -4.99691800 -0.40996600 2.88474300 0.25167300 -3.37583200 -1.07887400 -0.33053100 -3.01741500 -1.94718600 2.67802600 -2.73017400 3.60592800 2.91382800 -2.89942900 4.66005100 -0.04946100 3.59810000 -1.26887100 0.53401300 4.53670200 -1.23251000 2.87663000 -3.38144100 1.28915000 3.27516700 -4.06241400 0.53088800 -2.95478600 -0.32682000 3.56428600 -3.22352600 -0.40213200 4.62101200 2.78016000 2.70850000 -1.57882500 2.78941000 3.81008400 -1.66826300 1.56697000 -1.40345500 1.88830500 1.88085700 -1.64706300 3.23616300 1.50300900 -0.98076000 4.01812300 4.17462100 -1.61439900 -1.51940800 3.98913600 -0.54959500 -1.72879700 152 H H C C H C H H H C C C H H H C H C H C H H H C H C H C H H H N C H C H H H C H C H C 5.03617300 -1.91910100 -2.13272400 4.47578700 -1.70043300 -0.46521500 -1.26903100 -0.11779300 1.82338700 3.18387700 -3.59370500 2.63155400 3.81777500 -4.43617200 2.91971200 -0.67855000 -3.47213500 0.12065700 -0.17597700 -3.94628100 0.97838800 -1.55705500 -4.08840000 -0.12344200 -1.04756800 -2.48970100 0.44962600 -4.77451500 -0.26309700 0.25892200 -2.28072700 -0.12316700 0.83355600 2.87587300 2.09178700 -2.96573500 2.90845300 0.99182100 -2.90767700 3.80095800 2.41956500 -3.46361200 2.03811500 2.35809300 -3.62369200 1.82175800 3.22868200 3.71868600 1.96571100 3.43685200 4.78239700 -1.61961700 -0.21650200 3.17482000 -0.83707000 -0.21167900 3.94262600 -0.56482900 3.38388100 -2.68204800 0.23546900 3.31512700 -3.43153200 -1.21386900 4.22206600 -2.97749700 -1.16955200 2.46556400 -2.74624500 1.89111600 3.96188000 1.41975900 2.08951800 4.74421300 0.67932000 2.07859500 4.22651400 2.77317600 2.42403800 5.21205100 3.09608300 3.95284400 2.26561400 -0.71449100 4.05041700 2.85575200 0.20603500 4.89655300 2.36561100 -1.27198600 3.86369000 1.20759300 -0.41674300 0.08540600 0.06395900 -4.06118800 1.37758700 1.97265700 3.30955300 1.17325500 1.21293400 4.07094000 -1.20540700 3.69250900 -0.28412000 -1.75678200 2.74031400 -0.22444200 -1.91830200 4.46311900 -0.61477100 -0.88369900 3.95977300 0.73210400 -5.75228600 -1.26508900 0.37795200 -5.63957400 -2.03081100 1.15127200 -6.84820800 -1.31042800 -0.48288700 -7.58871200 -2.10747100 -0.37619400 -4.94277600 0.70638000 -0.74476400 153 H -4.20855000 1.50909200 -0.84422700 C -6.99733200 -0.34429300 -1.47736500 H -7.85517700 -0.37615400 -2.15391300 C 2.60635300 -2.34482400 -3.33475100 H 1.64297600 -2.80990600 -3.59200700 H 3.37957300 -2.82602400 -3.95226800 H 2.55625400 -1.29085600 -3.64653200 C -6.04218600 0.66719000 -1.59965900 H -6.15682000 1.43761400 -2.36674000 C 0.85507800 -4.72947000 -1.41049200 H 1.44047400 -4.73154000 -2.33989700 H 0.05113000 -5.47093500 -1.53640400 H 1.50340900 -5.09841000 -0.60043600 Table A.11.4. Optimized coordinates for (SiP2S)FeN2 with MeCN solvation (m06l, S = 1) Atom 1 Fe 2 P 3 S 4 P 5 Si 6 N 7 C 8 C 9 H 10 C 11 C 12 C 13 C 14 H 15 C 16 H 17 C 18 H 19 C 20 H 21 C 22 H 23 C 24 H 25 C Mulliken atomic spin density (e–) 0.918617 0.017703 0.176109 0.00877 -0.086852 -0.007824 -0.004151 -0.004923 -0.000571 -0.003273 0.039217 0.002189 -0.021828 0.000714 0.008125 0.00059 -0.000679 -0.000058 0.00837 0.002665 -0.000748 0.000084 0.042411 -0.002586 -0.004668 Percentage of total spin density (%) 57.54 1.11 11.03 0.55 5.44 0.49 0.26 0.31 0.04 0.21 2.46 0.14 1.37 0.04 0.51 0.04 0.04 0 0.52 0.17 0.05 0.01 2.66 0.16 0.29 154 26 H 27 C 28 C 29 H 30 C 31 H 32 H 33 H 34 C 35 C 36 H 37 C 38 H 39 H 40 H 41 C 42 C 43 C 44 H 45 H 46 H 47 C 48 H 49 C 50 H 51 C 52 H 53 H 54 H 55 C 56 H 57 C 58 H 59 C 60 H 61 H 62 H 63 N 64 C 65 H 66 C 67 H -0.000517 0.003183 0.000803 -0.000086 0.000523 -0.000615 -0.000202 0.000013 0.047127 0.00096 -0.000086 -0.000735 0.000194 0.000108 0.001205 -0.00438 -0.03821 0.000202 0.000416 -0.000009 0.000274 -0.000714 -0.000067 -0.020507 0.00072 0.000177 0.000025 0.000139 0.001091 -0.000763 0.000027 0.000792 -0.000076 -0.000393 -0.000019 -0.000194 0.000553 -0.061058 0.000775 -0.00012 0.000773 0.000602 0.03 0.2 0.05 0.01 0.03 0.04 0.01 0 2.95 0.06 0.01 0.05 0.01 0.01 0.08 0.27 2.39 0.01 0.03 0 0.02 0.04 0 1.28 0.05 0.01 0 0.01 0.07 0.05 0 0.05 0 0.02 0 0.01 0.03 3.82 0.05 0.01 0.05 0.04 155 68 H 0.000277 0.02 69 H -0.000104 0.01 70 C 0.003106 0.19 71 H -0.000147 0.01 72 C -0.000981 0.06 73 H 0.000074 0 74 C 0.002351 0.15 75 H 0.000382 0.02 76 C 0.002736 0.17 77 H -0.000133 0.01 78 C 0.000223 0.01 79 H 0.000135 0.01 80 H 0.000069 0 81 H 0.000715 0.04 82 C -0.00105 0.07 83 H 0.000133 0.01 84 C 0.00053 0.03 85 H -0.000059 0 86 H 0.001241 0.08 87 H 0.000052 0 88 H -0.028886 1.81 Table A.11.5. Mulliken atomic spin densities calculated for 2.8-H (gas-phase, m06l, S = 1/2) 156 Appendix B SUPPLEMENTARY INFORMATION FOR CHAPTER 3 B.1 NMR Spectra -5 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 -6 -7 ppm 5.5 5.0 -8 -9 4.5 4.0 3.5 ppm Figure B.1.1. 1H NMR spectrum of 3.2 in C6D6 (300 MHz) Figure B.1.2. 31P NMR of 3.2 in C6D6 (121 MHz) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 157 * Et2O -7 -8 ppm -9 * Et2O C5H12 & 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -20 -30 -40 Figure B.1.3. 1H NMR spectrum of 3.3-H in THF-d8 (300 MHz) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 ppm Figure B.1.4. 31P NMR spectrum of 3.3-H in THF-d8 (121 MHz) -5 -10 ppm -15 THF THF Et2O Et2O 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 ppm 3.5 3.0 2.5 Figure B.1.5. 1H NMR spectrum of 3.3-D in C6H6 (400 MHz) 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 158 69.8 190 180 170 69.7 ppm 160 150 69.6 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 20 10 0 -10 -20 -30 -40 -11 -12 ppm Figure B.1.6. 31P NMR spectrum of 3.3-D in C6H6 (162 MHz) 70.5 190 180 170 70.0 160 69.5 ppm 150 69.0 140 68.5 130 120 110 100 90 80 70 60 50 40 30 ppm 7.16 -8.38 Figure B.1.7. 31P{2H} NMR spectrum of 3.3-D in C6H6 (162 MHz) 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 ppm 1 0 -1 -2 -3 Figure B.1.8. 2H NMR spectrum of 3.3-D in C6H6 (61 MHz) -4 -5 -6 -7 -8 -9 -10 -13 -14 159 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 ppm Figure B.1.9. 1H NMR spectrum of 3.4 in THF-d8 (400 MHz) 140 130 120 110 100 90 80 70 60 50 40 30 ppm 20 10 0 -10 -20 -30 -40 -50 -1 -2 -60 -70 -80 -90 -6 -7 Figure B.1.10. 31P NMR spectrum of 3.4 in THF-d8 (162 MHz) 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -3 -4 -5 ppm Figure B.1.11. 1H NMR spectrum of 3.5-H in THF-d8 (500 MHz, −40 °C) *Note: Due to facile decay of 3.5-H to 3.4, observable amounts of 3.4 are also present in the sample 160 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 ppm Figure B.1.12. 1H NMR spectrum of 3.5-D in THF-d8 (500 MHz, −40 °C) *Note: Due to facile decay of 3.5-D to 3.4, observable amounts of 3.4 are also present in the sample 3 1 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 Figure B.1.13. (top) 1H NMR spectrum of 3.4 generated upon warming a solution of 3.5-H in THF-d8 (500 MHz, 298 K) (bottom) 1H NMR spectrum of an authentic sample of 3.4 in THF-d8 (400 MHz) 50 45 40 35 30 25 20 15 10 5 0 ppm -5 -10 -15 -20 -25 -30 -35 -40 -45 -50 Figure B.1.14. 1H NMR spectrum of 3.4 generated upon warming a solution of 3.5-H in THF-d8 (500 MHz, 298 K) 161 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 ppm 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 Figure B.1.15. 31P NMR spectrum of 3.4 generated upon warming a solution of 3.5-H in THF-d8 (202 MHz, 298 K) 2 1 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Figure B.1.16. (top) 1H NMR spectrum of 3.6 generated upon warming a solution of 3.5-H in THF-d8 under vacuum (400 MHz, 298 K) (bottom) 1H NMR spectrum of an authentic sample of 3.6 in THF-d8 (400 MHz) *Et2O *Et2O 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 Figure B.1.17. 1H NMR spectrum of 3.6 in THF-d8 (400 MHz) 2.5 2.0 1.5 1.0 0.5 162 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 -140 -150 -160 -170 -180 -190 -200 -210 -220 -230 -240 ppm Figure B.1.18. 31P NMR spectrum of 3.6 in THF-d8 (162 MHz) *Et2O *Et2O 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 ppm 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Figure B.1.19. 1H NMR spectrum of 3.7 in THF-d8 (400 MHz, 23 °C) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 -140 -150 -160 -170 -180 -190 -200 -210 -220 -230 -240 ppm Figure B.1.20. 31P NMR spectrum of 3.7 in THF-d8 (162 MHz, 23 °C) 163 3 N2 atmosphere 2 vacuum 1 H2 atmosphere 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 ppm 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure B.1.21. 1H NMR spectra of 3.4 (top), 3.6 (middle), and 3.7 (bottom) in THF-d8 (400 MHz) 3 N2 atmosphere 2 vacuum 1 H2 atmosphere 76 74 72 70 68 66 64 62 60 58 56 54 52 50 48 46 44 42 ppm 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 -2.89 Figure B.1.22. 31P NMR spectra of 3.4 (top), 3.6 (middle), and 3.7 (bottom) in THF-d8 (162 MHz, 23 °C) * -1 -2 -3 -4 -5 ppm 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 ppm Figure B.1.23. 1H NMR spectrum of 3.7 in toluene-d8. Inset shows the peak corresponding to the bound H2. (400 MHz, −80°C, *free H2) 164 150 140 130 120 110 100 90 80 70 60 50 40 30 ppm 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 Figure B.1.24. 31P NMR spectrum of 3.7 in toluene-d8 (162 MHz, −80°C) -2.6 -2.7 -2.8 -2.9 -3.0 ppm -1.0 -1.5 -2.0 -2.5 -3.1 -3.2 -3.3 * 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 ppm 1.5 1.0 0.5 0.0 -0.5 -3.0 -3.5 -4.0 -4.5 Figure B.1.25. 1H NMR spectrum of (SiP2S)Ni(HD) in toluene-d8. Inset shows the peak corresponding to the bound HD. (400 MHz, −70°C, *free H2 and HD, generated from CaH2 and D2O) 130 125 120 115 110 105 100 95 90 85 80 75 70 65 60 55 50 ppm 45 40 35 30 25 20 15 10 5 0 -5 -10 -15 Figure B.1.26. 31P NMR spectrum of (SiP2S)Ni(HD) in toluene-d8 (162 MHz, −70°C) -20 165 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 ppm Figure B.1.27. 1H NMR spectrum of 3.8 (C6D6, 300 MHz) 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 ppm Figure B.1.28. 31P NMR spectrum of 3.8 (C6D6, 121 MHz) 90 80 70 60 50 40 30 20 10 0 ppm -10 -20 -30 Figure B.1.29. 1H NMR spectrum of 3.9 (THF-d8, 400 MHz) -40 166 Figure B.1.30. 1H NMR spectrum of the crude protonation reaction of 3.3-H with PhOH in THF-d8 (400 MHz) Figure B.1.31. 31P NMR spectrum of the crude protonation reaction of 3.3-H with PhOH in THF-d8 (162 MHz) B.2 IR Spectra Figure B.2.1. IR spectrum of 3.2 (thin film from THF solution) 167 Figure B.2.2. IR spectrum of 3.3-H (thin film from THF solution) Figure B.2.3. IR spectrum of 3.3-D (solid powder sample) Figure B.2.4. IR spectrum of 3.4’ (solid crystalline sample) 168 Figure B.2.5. IR spectrum of 3.4 (THF solution sample) Figure B.2.6. IR spectrum of 3.4 (C6H6 solution sample) Figure B.2.7. IR spectrum of 3.4’ (thin film from THF solution dried under N2 stream, collected in N2 glovebox) 169 Figure B.2.8. IR spectrum of 3.5-H (THF solution sample, −78 °C) Figure B.2.9. IR spectrum of 3.5-D (THF solution sample, −78 °C) B.3 X-Ray Diffraction Data Figure B.3.1. Full asymmetric unit of 3.2. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. 170 Figure B.3.2. Structure of 3.3 with omission of disordered components, C–H hydrogen atoms, and solvent molecules. Thermal ellipsoids at 50% probability. Figure B.3.3. Full asymmetric unit of 3.3 including disordered components. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. 171 Figure B.3.4. Structure of 3.4’ with omission of C–H hydrogen atoms and solvent molecules. Thermal ellipsoids at 50% probability. Figure B.3.5. Full asymmetric unit of 3.4’ including solvent. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. 172 Figure B.3.6. Full asymmetric unit of 3.7. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure B.3.7. Structure of 3.8 with omission of disordered components and hydrogen atoms. Thermal ellipsoids at 50% probability. 173 Figure B.3.8. Full asymmetric unit of 3.8 including disordered components. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure B.3.9. Structure of 3.9 with omission of disordered components, hydrogen atoms, and solvent molecules. Thermal ellipsoids at 50% probability. 174 Figure B.3.10. Full asymmetric unit of 3.9 including disordered components. The apical ligand is modeled with 87% occupancy as Ni–CH3 and 13% occupancy as Ni–N2. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. B.4 UV-Visible Data Figure B.4.1. UV-vis spectrum of 3.2 in THF Figure B.4.2. UV-vis spectrum of 3.3-H in THF 175 Figure B.4.3. UV-vis spectrum of 3.4 in THF (1 mm cuvette) Figure B.4.4. UV-vis spectrum of 3.5-H in THF Figure B.4.5. UV-vis spectrum of 3.6 in THF (1 mm cuvette) 176 Figure B.4.6. UV-vis spectrum of 3.7 in THF (1 mm cuvette) Figure B.4.7. UV-vis spectrum of 3.8 in C6H6 Figure B.4.8. UV-vis spectrum of 3.9 in THF 177 B.5 Cyclic Voltammetry Data Figure B.5.1. Cyclic voltammograms of 3.3-H at various scan rates, depicting the NiIII/II couple at −1.26 V vs Fc/Fc+ (0.4 M [NBu4][PF6] in THF). A glassy carbon working, Pt counter, and Ag/Ag+ reference electrodes were utilized. Figure B.5.2. Cyclic voltammograms of 3.4 in the presence of PhOH in 0.4 M [NBu4][PF6] in THF (scan rate = 100 mV/s). A glassy carbon working electrode, Ag/AgOTf reference electrode and Pt counter electrodes were utilized. 178 B.6 Bulk Electrolysis Data Table B.6.1. H2 generation from catalyzed reduction of PhOH Entry Complex 3.4 (mM) PhOH (mM) Equiv. H2/Nia Faradaic eff. (%) a 0.5 50 6.4±0.6 106±5 b 0 50 0.6±0.2 96±6 c 0.5 0 0.0±0.0 - db 0 50 0.9 103 Yields are an average of 2 runs; the spread between the two runs is shown as a standard deviation for N = 2. aEquivalents of H2 are listed per Ni center. bA working electrode from Entry A was rinsed with THF after electrolysis and then used directly as the working electrode in Entry D. Yield of H2 from a single run. B.7 CW EPR Data Figure B.7.1. X-band CW EPR spectrum of 3.5-D in 2-MeTHF with simulations. Simulation parameters: g = [2.166, 2.056, 2.039]; A(31Pα) = A(31Pβ) = [200, 210, 260] MHz); A(2H) = [0.25, 0.25, 4.9] MHz.; Acquisition parameters: MW frequency = 9.371 GHz; temperature = 77 K; MW power = 2 mW; modulation amplitude = 2 G; conversion time = 82 ms. 179 Figure B.7.2. X-band CW EPR spectra of 3.5-H (blue) and 3.5-D (orange) in 2-MeTHF. Acquisition parameters: MW frequency = 9.371 GHz; temperature = 77 K; MW power = 2 mW; modulation amplitude = 2 G; conversion time = 82 ms. B.8 ESE-EPR Data Figure B.8.1. Q-band ESE-EPR spectrum of 3.5-D in 2Me-THF with simulations. Simulation parameters: g = [2.166, 2.056, 2.039]; A(31Pα) = A(31Pβ) = [200, 210, 260] MHz; A(2H) = [0.25, 0.25, 4.9] MHz. Acquisition parameters: MW frequency = 34.032 GHz, temperature = 12 K; π pulse length = 160 ns, τ = 400 ns, shot rep time (srt) = 1 ms. 180 B.9 HYSCORE Data Figure B.9.1. Q-band HYSCORE spectra of 3.5-D (top) and 3.5-H (middle) in 2Me-THF measured at magnetic fields corresponding to g = 2.038. The corresponding 2H – 1H difference spectrum is depicted on the bottom. Experimental conditions: microwave frequency = 34.055 GHz; temperature = 14 K; B0 = 1194 mT; τ = 100 µs t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 181 Figure B.9.2. Q-band HYSCORE spectra of 3.5-D (top) and 3.5-H (middle) in 2Me-THF measured at magnetic fields corresponding to g = 2.058. The corresponding 2H – 1H difference spectrum is depicted on the bottom. Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1182.4 mT G; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 182 Figure B.9.3. Q-band HYSCORE spectra of 3.5-D (top) and 3.5-H (middle) in 2Me-THF measured at magnetic fields corresponding to g = 2.092. The corresponding 2H – 1H difference spectrum is depicted on the bottom. Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1163 mT; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 183 Figure B.9.4. Q-band HYSCORE spectra of 3.5-D (top) and 3.5-H (middle) in 2Me-THF measured at magnetic fields corresponding to g = 2.128. The corresponding 2H – 1H difference spectrum is depicted on the bottom. Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1143.6 mT; τ = 100 µs t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 184 Figure B.9.5. Q-band HYSCORE spectra of 3.5-D (top) and 3.5-H (middle) in 2Me-THF measured at magnetic fields corresponding to g = 2.164. The corresponding 2H – 1H difference spectrum is depicted on the bottom. Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1124.2 mT; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 185 Figure B.9.6. Q-band HYSCORE 2H – 1H difference spectrum of 3.5-D and 3.5-H (top) in 2Me-THF measured at magnetic fields corresponding to g = 2.038. Overlay of 2H simulation contours (red) with experimental difference contours (gray). Experimental conditions: microwave frequency = 34.055 GHz; temperature = 14 K; B0 = 1194 mT; τ = 100 µs t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms). Simulation parameters: g = [2.166, 2.056, 2.039]; A(2H) = [0.25, 0.25, 4.9] MHz. 186 Figure B.9.7. Q-band HYSCORE 2H – 1H difference spectrum of 3.5-D and 3.5-H (top) in 2Me-THF measured at magnetic fields corresponding to g = 2.058. Overlay of 2H simulation contours (red) with experimental difference contours (gray). Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1182.4 mT G; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms). Simulation parameters: g = [2.166, 2.056, 2.039]; A(2H) = [0.25, 0.25, 4.9] MHz. 187 Figure B.9.8. Q-band HYSCORE 2H – 1H difference spectrum of 3.5-D and 3.5-H (top) in 2Me-THF measured at magnetic fields corresponding to g = 2.092. Overlay of 2H simulation contours (red) with experimental difference contours (gray). Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1163 mT; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms). Simulation parameters: g = [2.166, 2.056, 2.039]; A(2H) = [0.25, 0.25, 4.9] MHz. 188 Figure B.9.9. Q-band HYSCORE 2H – 1H difference spectrum of 3.5-D and 3.5-H (top) in 2Me-THF measured at magnetic fields corresponding to g = 2.128. Overlay of 2H simulation contours (red) with experimental difference contours (gray). Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1143.6 mT; τ = 100 µs t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms). Simulation parameters: g = [2.166, 2.056, 2.039]; A(2H) = [0.25, 0.25, 4.9] MHz. 189 Figure B.9.10. Q-band HYSCORE 2H – 1H difference spectrum of 3.5-D and 3.5-H (top) in 2Me-THF measured at magnetic fields corresponding to g = 2.164. Overlay of 2H simulation contours (red) with experimental difference contours (gray). Experimental conditions: microwave frequency = 34.05514 GHz; temperature = 14 K; B0 = 1124.2 mT; τ = 100 µs; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms). Simulation parameters: g = [2.166, 2.056, 2.039]; A(2H) = [0.25, 0.25, 4.9] MHz. 190 B.10 UV-Visible Data for the Conversion of 3.5-H to 3.4 Table B.10.1. Rate of the conversion of 3.5-H to 3.4 T (K) K (M-1 S-1) 228 0.0015 243 0.0103 258 0.0145 273 0.0411 298 0.3293 Figure B.10.1. Eyring plot for the conversion of 3.5-H to 3.4. T in K, k in M-1s-1 Figure B.10.2. Plot of time vs. ln(Abs638nm) (left) and time vs. 1/Abs638nm (right) for the conversion of 3.5-H to 3.4 at 25 °C 191 Figure B.10.3. UV-visible spectra showing the conversion of 3.5-H to 3.4 at 25 °C in a 1 cm cuvette; traces collected every 10 min Figure B.10.4. Plot of Aλ vs. (Aλ- Aλ,0)/t at 25 °C ([3.5-H]0 = 0.00014 M, Δt = 10 min, k = 0.3293 M-1s-1 Figure B.10.5. UV-visible spectra showing the conversion of 3.5-H to 3.4 at 0 °C in a 1 mm cuvette; traces collected every 10 min 192 Figure B.10.6. Plot of Aλ vs. (Aλ- Aλ,0)/t at 0 °C ([3.5-H]0 = 0.0074 M, Δt = 10 min, k = 0.0411 M-1s-1) Figure B.10.7. UV-visible spectra showing the conversion of 3.5-H to 3.4 at -15 °C in a 1 mm cuvette; traces collected every 5 min Figure B.10.8. Plot of Aλ vs. (Aλ- Aλ,0)/t at -15 °C ( [3.5-H]0 = 0.0072 M, Δt = 5 min, k = 0.0145 M-1s-1 193 Figure B.10.9. UV-visible spectra showing the conversion of 3.5-H to 3.4 at -30 °C in a 1 mm cuvette; traces collected every 10 min Figure B.10.10. Plot of Aλ vs. (Aλ- Aλ,0)/t at -30 °C ([3.5-H]0 = 0.0072 M, Δt = 10 min, k = 0.0103 M-1s-1) Figure B.10.11. UV-visible spectra showing the conversion of 3.5-H to 3.4 at -45 °C in a 1 mm cuvette; traces collected every 10 min. 194 Figure B.10.12. Plot of Aλ vs. (Aλ- Aλ,0)/t at -45 °C ([3.5-H]0 = 0.0078 M, Δt = 10 min, k = 0.0015 M-1s-1 B.11 DFT Calculations Figure B.11.1. Gas-phase optimized DFT structure of 3.5-H. C-H hydrogens not depicted for clarity. (M06L: def2tzvp (Ni), def2svp (all other atoms)) Figure B.11.2. Gas-phase optimized DFT spin density plot of 3.5-H. C-H hydrogens not depicted for clarity. (isovalue: 0.005; M06L: def2tzvp (Ni), def2svp (all other atoms)) 195 Table B.11.1. Gas-phase optimized coordinates for 3.5-H (M06l, S = 1/2) Ni S P Si P C C C C H C H C C H C C C H C C H C H C H C H C H C H C H C H C H C H H H 0.33404900 0.03766300 -1.11190900 -1.88209400 0.11591200 -0.97158800 1.43418100 1.97516600 -0.71237300 0.27831300 -0.16112300 1.13234400 1.53628800 -1.83406700 -1.00297300 -1.46400000 -0.64669100 1.64807200 -2.43065300 -0.36194300 0.65530400 -3.80863500 -0.55941700 0.93389800 -1.85691600 -1.16698200 2.88162000 -1.10388800 -1.40070500 3.64307100 -4.15713200 -1.09389700 2.18568300 -5.21757400 -1.25060900 2.40345900 0.51588000 1.59053600 1.84817300 -3.20518400 -1.40492000 3.15083500 -3.51650700 -1.81850600 4.11313100 1.65401100 -1.35386300 1.70034500 -4.89521200 -0.25794900 -0.02445400 3.29972400 2.13079000 -0.59838000 3.66140500 1.69211400 -1.54314400 0.91947600 2.58888100 0.93867700 2.14753000 -1.49511300 3.00420900 1.68062100 -0.94476900 3.82816300 0.19042400 1.97186700 3.15836500 -0.15364200 1.21942400 3.87634600 0.26044700 3.30521200 3.55739600 -0.00231300 3.58490900 4.58085500 -4.96329100 0.96545600 -0.71050500 -4.17661000 1.70719100 -0.55743100 0.48819300 -3.37299700 -1.20069700 0.03678900 -3.24093800 -2.20029300 -5.92954600 -1.18579800 -0.22400400 -5.88765900 -2.14907000 0.29246600 -6.02547400 1.24551700 -1.56497800 -6.06157700 2.20708400 -2.08312500 0.96475800 3.93237800 1.33451700 1.24245600 4.71392100 0.62103100 0.95474800 3.31758000 -1.90506700 1.35692200 4.25704700 -1.48418900 -0.55727100 3.43435800 -2.00236400 -0.99336700 2.52696600 -2.44549400 -0.83441100 4.28847700 -2.63805500 -1.03090700 3.57785700 -1.02002300 196 C H C H C C H H H C H H H C H C H C H H H C H H H C H H H C H C H C H C H H H C H H H 0.63713800 4.28818200 2.64136700 0.66656700 5.33784300 2.94366300 -6.98895100 -0.90908000 -1.08493300 -7.77581100 -1.65297800 -1.23282100 2.29074900 -2.06755600 0.65852000 3.81881900 1.28739200 0.55508800 3.52450100 1.71188300 1.52815500 4.91824300 1.23862700 0.53864300 3.43857600 0.25701700 0.52463600 -0.63455100 -3.40589400 -0.17946000 -0.25051100 -3.44010600 0.85311500 -1.25350900 -4.30369600 -0.32380700 -1.29130600 -2.53279300 -0.27079900 -7.04115100 0.30902600 -1.75973100 -7.87104200 0.52951100 -2.43559400 3.25435000 -2.30111200 3.27151800 3.63803800 -2.38395900 4.29162900 3.81762600 3.55693800 -0.50299300 3.53245000 4.18412500 -1.35854400 4.91776700 3.55203300 -0.46443100 3.47369400 4.05553400 0.41580300 1.58857500 3.07333600 -3.26538400 2.68815000 3.07613900 -3.23470700 1.28086000 3.85156900 -3.97946000 1.26518600 2.10171400 -3.67068700 1.29295400 -4.66289800 -1.17440800 2.07013800 -4.71584500 -1.94931900 0.62562000 -5.52360000 -1.33285400 1.77772500 -4.81437700 -0.19754400 2.88971000 -2.11773800 -2.25450700 3.43772600 -3.02094200 -1.93162800 3.88385000 -2.98779500 2.23356200 4.75872700 -3.60943800 2.43919000 3.39832200 -2.87963200 0.93055900 3.90201500 -3.42435400 0.12596500 3.85143000 -0.94497000 -2.29032200 3.32738300 -0.04024600 -2.63706400 4.67455200 -1.13863800 -2.99440300 4.30027900 -0.73098600 -1.30937500 2.29023800 -2.36466400 -3.63153400 1.71353300 -3.29772700 -3.69060300 3.08664500 -2.42735800 -4.38783200 1.62149200 -1.53814700 -3.92009800 197 H 0.40336600 0.21962900 -2.65366100 Table B.11.2. Gas-phase optimized coordinates for 3.6 (M06l, S = 0) Ni 0.40716700 0.13111300 -0.96350100 S -1.69248300 0.73762800 -0.52163600 P 1.32445200 -1.82612600 -1.16952800 P 1.58059800 2.01010900 -0.58325500 Si 0.25899200 -0.33035500 1.17029600 C -2.38934100 -0.32867600 0.71566900 C -1.46773100 -0.87697500 1.63659800 C -4.80522700 -0.03009900 -0.04590000 C -3.77518800 -0.57371100 0.86553900 C 0.70169400 1.33105500 1.98026100 C -1.90702200 -1.70074000 2.67326700 H -1.18273600 -2.14719700 3.36435800 C -4.67053700 -0.09852200 -1.44218100 H -3.77120200 -0.54660300 -1.87081600 C -4.17702400 -1.39295500 1.93336100 H -5.24302400 -1.61676700 2.03847400 C -3.26881500 -1.95986400 2.82603200 H -3.62612900 -2.60640500 3.63097600 C 0.46491200 1.57192900 3.34112200 H 0.05636200 0.77353000 3.97067000 C 3.91578700 0.95720900 0.51285500 H 3.75914400 1.35809800 1.52647400 H 4.98901700 0.73431100 0.41151600 H 3.37139500 0.00244400 0.45377100 C 0.70575300 2.82286900 3.90767700 H 0.50699200 2.99167700 4.96907500 C 1.20043700 2.38565500 1.18067400 C 0.19893700 -3.24491400 -1.66774600 H 0.84118900 -4.14636200 -1.63780100 C 3.46315700 1.95029400 -0.54702700 H 3.72330200 1.54278700 -1.54273200 C 1.51240900 -1.71106900 1.55666400 C -5.67666200 0.37463300 -2.28010300 H -5.55477300 0.30233300 -3.36380200 C 1.96353200 -2.43739100 0.43571600 C 1.92820600 -2.11946500 2.83162800 H 1.60036100 -1.56800700 3.71952800 C 1.41987700 3.64527000 1.75420300 H 1.77293700 4.47828700 1.13878000 C 1.17576700 3.86385800 3.10947200 198 H C H C H H H C H H H C H C H C H C H H H C H H H C H C H H H C H C H C H C H H H C H 1.34596500 4.85360300 3.54051900 1.22515100 3.59936000 -1.49119300 1.80668500 4.40569900 -1.00912900 -0.92787100 -3.39858600 -0.66137900 -0.56656100 -3.51101500 0.37134900 -1.53541300 -4.28535300 -0.89660500 -1.59670100 -2.52405000 -0.68239000 -0.25250700 3.94366700 -1.40574500 -0.87283500 3.17935100 -1.89772100 -0.45605700 4.90469400 -1.90222100 -0.60522700 4.02029600 -0.36751800 2.70817900 -2.00211900 -2.43563500 2.67860000 -3.05365600 -2.77986800 -5.98317900 0.52685900 0.47713000 -6.09996500 0.59874000 1.56222500 2.78019500 -3.56237000 0.59740100 3.10434000 -4.14135300 -0.27392300 4.16904700 3.28459400 -0.37372600 3.95285500 4.00000300 -1.17917100 5.26051400 3.13953900 -0.36064200 3.90372800 3.76072300 0.58257600 1.68735600 3.46165600 -2.93479700 2.76187300 3.24057600 -3.02137900 1.49896800 4.38774400 -3.49875900 1.14322700 2.65288500 -3.44883600 -6.83746000 0.93292100 -1.74496000 -7.62319800 1.30849600 -2.40502000 -0.35060800 -3.07439000 -3.07454000 -0.88075800 -2.11331700 -3.18008500 -1.07957500 -3.86787000 -3.29815800 0.42488000 -3.12669400 -3.85093700 3.18415200 -3.95103100 1.87369900 3.82886800 -4.82423400 1.99996700 -6.98666600 1.00685400 -0.36086400 -7.88937700 1.44648000 0.07099300 2.76564900 -3.22356800 2.98921500 3.09080300 -3.52485600 3.98835900 4.09341000 -1.69880500 -1.89005500 4.16774400 -0.64847100 -1.57517000 4.85049200 -1.85069600 -2.67445500 4.37822700 -2.31858200 -1.03007000 2.41325100 -1.07540400 -3.61068800 1.41421800 -1.21746900 -4.04433800 199 H H 3.14785200 -1.21719000 -4.41777800 2.47427300 -0.02232800 -3.28835100 Table B.11.3. Optimized coordinates for 3.5-H (M06l, S = ½, MeCN solvation) Ni 0.33898600 0.04737800 -1.13646800 S -1.90037100 0.15979100 -0.99695000 P 1.49897500 1.98333800 -0.68193600 Si 0.25389000 -0.16391400 1.11422700 P 1.56424800 -1.84312500 -1.00635700 C -1.49842900 -0.65401400 1.60241400 C -2.46450000 -0.35211300 0.61570400 C -3.84238600 -0.55456900 0.89055100 C -1.89061000 -1.19264000 2.82964000 H -1.13614700 -1.43237400 3.58724200 C -4.19219100 -1.11142500 2.13380400 H -5.25186900 -1.27836200 2.34870400 C 0.46844600 1.58555700 1.83382900 C -3.23970900 -1.43601900 3.09490900 H -3.55124600 -1.86500600 4.05055400 C 1.59681500 -1.37134300 1.70257600 C -4.93107800 -0.23690700 -0.06207700 C 3.35563900 2.10499900 -0.47894400 H 3.75306100 1.66723200 -1.40903800 C 0.90814300 2.58772800 0.94521200 C 2.03523700 -1.53170100 3.02470800 H 1.54871100 -0.97938000 3.83576200 C 0.10577600 1.95504200 3.13866800 H -0.25334900 1.19701700 3.84334300 C 0.17058500 3.28603900 3.54950200 H -0.11824500 3.55886100 4.56794000 C -5.01156100 1.00651900 -0.71237200 H -4.23604700 1.75518000 -0.53484000 C 0.51106800 -3.36550900 -1.26697200 H 0.10170200 -3.21878900 -2.28277400 C -5.95833100 -1.16848100 -0.28717500 H -5.91337400 -2.14406800 0.20552100 C -6.07788700 1.30276500 -1.55844200 H -6.12520500 2.28038800 -2.04530900 C 0.95232800 3.92822400 1.35381100 H 1.26876400 4.71278300 0.66077400 C 1.09578900 3.33056800 -1.89459500 H 1.48060300 4.25890600 -1.43787200 C -0.40437200 3.47091200 -2.08260600 200 H H H C H C H C C H H H C H H H C H C H C H H H C H H H C H H H C H C H C H C H H H C -0.82969500 2.58212300 -2.57341400 -0.62770300 4.34006200 -2.71940400 -0.93980500 3.60969100 -1.13119800 0.58257500 4.27386800 2.65286300 0.61255600 5.32065800 2.96533200 -7.02165300 -0.87524600 -1.13972300 -7.80374000 -1.62063000 -1.30539600 2.25173000 -2.09080200 0.67794800 3.82013300 1.25611400 0.69257900 3.48380400 1.67375800 1.65568700 4.91968500 1.21869900 0.72301400 3.45820700 0.22046000 0.63622700 -0.64814700 -3.40631800 -0.28838400 -0.29956500 -3.47825900 0.75509200 -1.27442300 -4.29033300 -0.48062500 -1.29017900 -2.52032100 -0.37595900 -7.08575800 0.36214100 -1.77974900 -7.91979100 0.59560700 -2.44638000 3.11127500 -2.36861000 3.32309500 3.45490800 -2.46992600 4.35585500 3.87921600 3.52648900 -0.35684800 3.63591400 4.16020000 -1.22019900 4.97699300 3.50442000 -0.27781200 3.50781700 4.02789000 0.54982900 1.81046400 3.10660600 -3.21741000 2.90529300 3.07715700 -3.11907600 1.57002300 3.91933200 -3.91912000 1.49083200 2.16406300 -3.68979400 1.30210900 -4.66272200 -1.22537700 2.13697300 -4.69846500 -1.93849300 0.63840100 -5.50742900 -1.46529200 1.70904800 -4.85425600 -0.22039400 2.96002800 -2.09873800 -2.21020300 3.49646100 -2.99689400 -1.85798000 3.75907900 -3.06501600 2.30160600 4.60834400 -3.71248700 2.53382200 3.32680200 -2.93557100 0.98136300 3.84157900 -3.49060500 0.19180200 3.92095100 -0.92535700 -2.20448200 3.43199100 -0.01815900 -2.59431600 4.78091400 -1.13516300 -2.85805200 4.31742800 -0.70337800 -1.20324600 2.43086900 -2.36059400 -3.61141900 201 H H H H 1.84663700 3.27093000 1.79559400 0.40807300 -3.28727500 -3.69147300 -2.44565500 -4.31670800 -1.53178000 -3.96318100 0.22362300 -2.69964000 Table B.11.4. Optimized coordinates for 3.6 (M06l, S = 0, MeCN solvation) Ni 0.42104400 0.11667700 -0.99020100 S -1.72916800 0.75619400 -0.60660800 P 1.26989600 -1.89115800 -1.12923200 P 1.65056400 1.98061800 -0.61483100 Si 0.29284000 -0.28873400 1.14907200 C -2.39531800 -0.25230000 0.68901300 C -1.46012300 -0.78029400 1.61270800 C -4.83504700 0.03017500 -0.02216400 C -3.78208500 -0.48215200 0.88444000 C 0.73553800 1.38030400 1.94742500 C -1.88812900 -1.55869900 2.69005600 H -1.15363500 -1.98271900 3.38468100 C -4.75271200 -0.11837000 -1.41788400 H -3.87537300 -0.60197600 -1.85446500 C -4.17173500 -1.25285500 1.99383000 H -5.23809000 -1.45552700 2.13527100 C -3.24957800 -1.79663400 2.88726500 H -3.59424900 -2.40544100 3.72687000 C 0.49451800 1.64837900 3.30405600 H 0.08128500 0.86587300 3.95050500 C 3.94953800 0.88064100 0.51526600 H 3.80057800 1.29617500 1.52528400 H 5.01870600 0.63482300 0.42338200 H 3.38867200 -0.06537100 0.46175300 C 0.75175800 2.90774900 3.84685200 H 0.55253000 3.09961500 4.90452800 C 1.25829700 2.41098400 1.13209800 C 0.12971500 -3.30375100 -1.59924900 H 0.77257000 -4.20343500 -1.57098700 C 3.52738200 1.86840600 -0.56071400 H 3.77954000 1.43666500 -1.54702000 C 1.49497800 -1.69308700 1.58939100 C -5.78439200 0.32009200 -2.24531000 H -5.70468500 0.18235000 -3.32690800 C 1.91142600 -2.46806800 0.48712500 C 1.91189400 -2.06883500 2.87471400 H 1.60729800 -1.48114100 3.74740500 202 C H C H C H C H H H C H H H C H C H C H C H H H C H H H C H C H H H C H C H C H C H H 1.49685200 3.67874400 1.68146800 1.87674000 4.49079100 1.05419700 1.24455000 3.92627400 3.03115200 1.43075200 4.92035600 3.44593600 1.34557500 3.55070600 -1.56932000 1.94402100 4.34893700 -1.09658500 -0.98702600 -3.45624300 -0.58282900 -0.61503700 -3.57530600 0.44632800 -1.59752600 -4.34272000 -0.81413000 -1.65890600 -2.58301400 -0.59600800 -0.11927200 3.94684200 -1.51257900 -0.75986400 3.20001700 -2.00657200 -0.27855500 4.90618100 -2.02895600 -0.48563800 4.06379400 -0.48185300 2.62961800 -2.08177400 -2.41417000 2.55751100 -3.12681200 -2.76819700 -5.98823800 0.63120700 0.51020100 -6.07071400 0.75978900 1.59341700 2.69931100 -3.60983800 0.67575800 2.99729900 -4.22366600 -0.18018700 4.26475400 3.18682500 -0.40825900 4.07513500 3.88854700 -1.23213300 5.35173800 3.01047900 -0.38507200 4.00628600 3.69470900 0.53402400 1.81711900 3.36845800 -3.00428100 2.88298400 3.10522200 -3.07595700 1.67204300 4.29466100 -3.58127700 1.24607100 2.57571200 -3.51457500 -6.91863600 0.92565900 -1.70097400 -7.72366000 1.27403800 -2.35307400 -0.42735700 -3.13987600 -3.00261100 -0.97756700 -2.18995100 -3.10963200 -1.13828500 -3.95004500 -3.22657300 0.34868900 -3.17319200 -3.77931200 3.10650300 -3.96587200 1.96156700 3.72890500 -4.85193700 2.11020000 -7.01636700 1.07896400 -0.31807400 -7.89832100 1.55253000 0.12130700 2.72025300 -3.19162900 3.05835500 3.04711200 -3.47064900 4.06363500 4.03197000 -1.83598800 -1.88855300 4.15492000 -0.79848400 -1.54565000 4.76427300 -1.99151700 -2.69580400 203 H C H H H 4.31541000 -2.49808200 2.33973100 -1.13142300 1.31648300 -1.21305300 3.02953100 -1.31475300 2.48224700 -0.08446000 -1.05961200 -3.57040300 -3.96370900 -4.40833900 -3.253052003 Table B.11.5. DFT-calculated spin density of 3.5-H (gas phase) ATOM LABEL 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 ATOM Ni S P Si P C C C C H C H C C H C C C H C C H C H C H C H C H C H C H SPIN DENSITY (E-) 0.598 0.224 0.083 -0.043 0.069 0.056 -0.042 0.052 -0.028 0.001 -0.029 0.001 0.009 0.056 -0.003 0.000 -0.006 -0.005 0.000 -0.001 0.000 0.000 -0.001 0.000 0.000 0.000 0.004 0.000 0.002 0.000 0.004 0.000 -0.002 0.000 SPIN DENSITY (%) 44.0 16.5 6.1 3.1 5.1 4.1 3.1 3.8 2.0 0.1 2.1 0.1 0.7 4.1 0.2 0.0 0.4 0.3 0.0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.3 0.0 0.2 0.0 0.3 0.0 0.2 0.0 204 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 C H C H C H H H C H C H C C H H H C H H H C H C H C H H H C H H H C H H H C H C H C H 0.000 0.000 0.000 0.003 0.000 0.001 0.000 0.000 0.000 0.000 -0.002 0.000 -0.004 0.000 0.000 0.000 -0.001 0.000 0.000 0.000 -0.001 0.004 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.002 0.000 0.004 0.001 0.000 0.000 0.000 0.000 0.0 0.0 0.0 0.2 0.0 0.1 0.0 0.0 0.0 0.0 0.1 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.1 0.0 0.3 0.1 0.0 0.0 0.0 0.0 205 78 79 80 81 82 83 84 85 86 C H H H C H H H H 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.005 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.4 B.12 Discussion on the Characterization of [(PS3)NiH][PPN] Prepared according to the published synthesis,17 treatment of [(PS3)NiOPh][PPN] in THF with 1.3 equivalents of HBPin at −78 °C resulted generation of a dark red solution containing the terminal NiIII hydride [(PS3)NiH][PPN]. The UV-Visible data that we obtained of this dark red sample are in good agreement with the reported spectrum with respect to spectral features, although we determined larger extinction coefficients for the transitions. In our hands, EPR samples of this dark red sample contained the approximate axial signal assigned as [(PS3)NiH][PPN], with additional S = ½ signals present. However, although impurities were present in the sample, we were able to obtain X-band HYSCORE data on the signal corresponding to [(PS3)NiH][PPN] at g = 2.42 and g = 1.99, with minimal overlap with the additional S = ½ signals. The NiIII deuteride, [(PS3)NiD][PPN], was analogously generated by treatment of [(PS3)NiOPh][PPN] with 1.3 equivalents of DBPin at −78 °C in THF. Xband HYSCORE data of the NiIII deuteride revealed 2H hyperfine coupling of A(2H) = [1.6, 9.3, 9.3] MHz. Scaling A(2H) by the ratio of 1H/2H gyromagnetic ratios approximates a 1H hyperfine coupling of A(1H) = [10.4, 60.6, 60.6] MHz in [(PS3)NiH][PPN] (aiso = 43.9 MHz; T = [-33.5, 16.7, 16.7] MHz). Additional hyperfine coupling to 31P and 13C (natural abundance) was resolved in the HYSCORE data: A(31P) = [16, 22, 16] MHz; A(13C) = [−0.5, 4, −0.5] MHz. The |aiso(1H)| of 43.9 MHz corresponds to approximately ±0.03 e– of spin density localized on the hydride of [(PS3)NiH][PPN], in agreement with the DFT-estimated value of −0.05 e– (gas phase, M06l). 206 PPN R tBu PPN tBu Cl R HS Cl O S NiII tBu P THF [(PS2SH)NiCl][PPN] 3 NaOPh S S S R R NiIII S P R R [(PS3)NiCl][PPN] 3:1 THF/MeCN PPN PPN H OPh S S R R NiIII S P R 1.3 HBPin THF, -78 °C [(PS3)NiOPh][PPN] S S R R NiIII S P R [(PS3)NiH][PPN] Scheme B.12.1. Synthesis of [(PS3)NiH][PPN] (R = SiMe3). B.12.1 Modified Preparation of [(PS3)NiCl][PPN] A THF solution of 2,4,6-tri-tert-butylphenoxy radical (12.5 mg, 0.048 mmol, 2 mL) was added to a THF solution of [(PS2SH)NiCl][PPN] (58 mg, 0.048 mmol, 2 mL) at 23 °C, resulting in the formation of a dark green solution. The reaction was stirred at 23 °C for 15 min and then concentrated to dryness. The resulting dark green material was washed with pentane (5 mL), then extracted with THF (10 mL) and filtered through Celite. Pentane (3 mL) was layered over the THF solution and stored at −35 °C overnight, which yielded dark green crystals of the title compound (0.040 g, 69%). 1H NMR data on the material obtained through this route is in agreement with the published characterization of [(PS3)NiCl][PPN]. 207 -6.50 2.32 9.59 16.23 B.12.2 NMR Data 8 40 35 6 30 4 ppm 25 2 0 20 15 10 5 0 -5 -10 -15 -20 -25 ppm 10 -5.02 10.32 15 9.15 15.27 Figure B.12.2.1. 1H NMR spectra of [(PS3)NiCl][PPN] in CD3CN (300 MHz) THF-d8 20 5 0 -5 0 -5 -10 -15 -10 -15 -5.40 10.41 9.43 15.87 ppm CD3CN 25 20 15 10 5 ppm Figure B.12.2.2. 1H NMR spectra of [(PS3)NiOPh][PPN] (300 MHz) B.12.3 UV-Visible data Figure B.12.3.1 UV-vis spectrum of [(PS3)NiOPh][PPN] in THF (1 cm cuvette) -20 208 Figure B.12.3.2 UV-vis spectrum of [(PS3)NiH][PPN] in THF (1 mm cuvette, −80 °C) B.12.4 CW EPR Data Figure B.12.4.1. X-band CW EPR spectrum of [(PS3)NiOPh][PPN] in 3:1 THF/MeCN. Acquisition parameters: MW frequency = 9.371 GHz; temperature = 77 K; MW power = 2 mW; modulation amplitude = 2 G; conversion time = 82 ms. Figure B.12.4.2. X-band CW EPR spectrum of [(PS3)NiH][PPN] generated in situ in THF (black trace) and simulation in red. Arrows indicate field positions at which X-band HYSCORE spectra were acquired. Simulation parameters: g = [2.46, 1.974, 1.93]; gStrain = [0.13 0.002 0.07]. Acquisition parameters: MW frequency = 9.370 GHz; temperature = 5 K; MW power = 8 mW; modulation amplitude = 8 G; conversion time = 41 ms. 209 B.12.5 HYSCORE Data Figure B.12.5.1. X-band HYSCORE spectra of [(PS3)NiH][PPN] (top) and [(PS3)NiD][PPN] (middle) in THF measured at magnetic fields corresponding to g = 2.42. The 2H spectrum is depicted on the bottom in grey, with simulations of 2H (red) 31P (green), natural abundance 13C (blue) features overlaid. Simulation Parameters: g = [2.43, 1.99, 1.99]; A(2H) = [1.6, 9.3, 9.3] MHz; A(31P) = [16, 22, 16] MHz; A(13C) = [−0.5, 4, −0.5] MHz. Acquisition Parameters: microwave frequency = 9.399 GHz; temperature = 5.6 K; B0 = 277.5 mT; τ = 170 ns; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 210 Figure B.12.5.2. X-band HYSCORE spectra of [(PS3)NiH][PPN] (top) and [(PS3)NiD][PPN] (middle) in THF measured at magnetic fields corresponding to g = 1.975. The 2H spectrum is depicted on the bottom in grey, with simulations of 2H (red) 31P (green), natural abundance 13C (blue) features overlaid. Simulation Parameters: g = [2.43, 1.99, 1.99]; A(2H) = [1.6, 9.3, 9.3] MHz; A(31P) = [16, 22, 16] MHz; A(13C) = [−0.5, 4, −0.5] MHz. Acquisition Parameters: microwave frequency = 9.399 GHz; temperature = 5.6 K; B0 = 277.5 mT; τ = 138 ns; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms. 211 B.12.6 DFT Data Figure B.12.6.1. Gas-phase optimized DFT spin density plot of [(PS3)NiH][PPN]. C-H hydrogens not depicted for clarity. (isovalue: 0.005; M06L: def2tzvp (Ni), def2svp (all other atoms)) Table B.12.6.1. Gas-phase optimized coordinates for [(PS3)NiH]- (M06l, S = 1/2) Ni S S S P Si Si Si C C H C H C H C C C H H H C 0.07752900 -0.14827000 -1.51955100 -1.97980500 -1.16344700 -1.41979400 2.26573300 -0.78157300 -1.44866800 -0.17820000 2.15969400 -1.38328400 0.06789100 -0.18113800 0.65366900 -4.55175400 -3.11755500 -0.61602200 5.33256100 -1.74904000 -0.60684500 -0.91998600 5.27291800 -0.44991900 -1.17900500 -1.36872000 1.23127100 -1.26259800 -1.87031600 2.53078400 -0.49478600 -1.60834400 3.26576000 -2.30714700 -2.71764500 2.89098400 -2.37015600 -3.12017600 3.90491100 -3.26896000 -3.04584600 1.93398100 -4.09021100 -3.70708500 2.23105500 -3.21749300 -2.57726800 0.61368700 -2.13414000 -1.72757000 0.24452300 -5.78456600 -4.21241000 0.30995000 -5.31598100 -5.11868400 0.72381600 -6.57938700 -4.54471100 -0.37608000 -6.27563700 -3.68281200 1.14099600 -3.82653300 -4.15171600 -2.00915300 212 H H H C H H H C C H C H C H C C C H H H C H H H C H H H C C H C H C H C C C H H H C H -3.06626400 -3.58744600 -2.56705300 -4.60888100 -4.47125000 -2.71546400 -3.34400600 -5.05850700 -1.61382300 -5.51552100 -1.65331400 -1.29438000 -6.00290600 -1.09318000 -0.48212300 -6.30322100 -1.98879000 -1.98720900 -4.85843900 -0.95529500 -1.83069300 1.72179100 -0.60531900 1.27252200 2.08188100 -0.61499900 2.62222100 1.34987900 -0.32642700 3.38340300 3.37114100 -0.97104300 3.00513800 3.65352200 -0.97942300 4.06058000 4.29768200 -1.31318200 2.01776700 5.30698000 -1.60010200 2.33198600 3.99254700 -1.28669400 0.65049400 2.67385100 -0.90673900 0.26356700 5.79977500 -0.30804400 -1.71978700 6.10837600 0.56615100 -1.12723300 4.95581200 0.00140200 -2.35030700 6.64132300 -0.58002800 -2.37637500 6.88604200 -2.22643200 0.36046800 7.68642100 -2.52140200 -0.33621100 6.71516900 -3.07844700 1.03659000 7.27649000 -1.39440300 0.96679700 4.83596600 -3.24591400 -1.63093400 3.92555300 -3.04542500 -2.21239900 4.63541100 -4.11639100 -0.98825000 5.63851400 -3.52590800 -2.33128400 -0.40803900 1.45421900 1.27945400 -0.70039900 1.70102700 2.62297100 -0.68464900 0.87416600 3.34165200 -1.03173300 2.98283700 3.04741600 -1.26539300 3.17861700 4.09664700 -1.06895000 4.01403000 2.10371400 -1.33019900 5.02080400 2.44803700 -0.80501000 3.81073700 0.74419500 -0.47399400 2.48956300 0.30969300 0.71614500 5.60877000 -1.31450000 0.63714600 6.48574400 -1.97616500 1.03565500 4.74859700 -1.91833700 1.51225500 5.81384300 -0.58296000 -1.33438000 6.81813400 0.55988100 -0.57190500 7.03805000 1.32320700 213 H H C H H H H -2.30524600 6.73847800 1.07313100 -1.39086300 7.69583400 -0.10306100 -2.30260500 5.04932300 -1.70429100 -2.36473500 5.91742100 -2.37932800 -3.27627500 4.94552200 -1.20225800 -2.14710100 4.14910700 -2.31386600 0.06892100 -0.07698900 -3.01855200 Table B.12.6.2. DFT-calculated spin density of [(PS3)NiH]- (gas phase) ATOM LABEL 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 ATOM Ni S S S P Si Si Si C C H C H C H C C C H H H C H H H C H H H C C H SPIN DENSITY (E–) 0.734 0.067 0.067 0.156 -0.009 0.001 0.000 0.007 0.009 -0.004 0.000 0.008 -0.001 -0.003 0.000 0.007 -0.005 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.001 0.015 -0.007 0.000 SPIN DENSITY (%) 60.6 5.5 5.5 12.9 0.8 0.1 0.0 0.6 0.7 0.3 0.0 0.7 0.0 0.3 0.0 0.6 0.4 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 1.2 0.5 0.0 214 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 C H C H C C C H H H C H H H C H H H C C H C H C H C C C H H H C H H H C H H H H 0.014 -0.001 -0.006 0.000 0.011 -0.008 0.000 0.000 -0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.002 0.001 0.000 -0.003 0.000 0.000 0.000 0.000 -0.007 -0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 -0.001 0.000 0.000 0.000 -0.049 1.1 0.1 0.5 0.0 0.9 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.2 0.1 0.0 0.2 0.0 0.0 0.0 0.0 0.6 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 4.0 215 Appendix C SUPPLEMENTARY INFORMATION FOR CHAPTER 4 C.1 NMR Spectra 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure C.1.1. 1H NMR spectrum of 4.2 in C6D6 (400 MHz) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 ppm Figure C.1.2. 31P NMR spectrum of 4.2 in C6D6 (162 MHz) 30 20 10 0 -10 -20 -30 -40 216 * * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm Figure C.1.3. 1H NMR spectrum of 4.2-15N in C6D6 (400 MHz, *H grease) 130 120 110 100 90 80 70 60 50 40 ppm 30 20 10 0 -10 -20 -30 -40 Figure C.1.4. 31P NMR spectrum of 4.2-15N in C6D6 (162 MHz) 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 ppm Figure C.1.5. 15N NMR spectrum of 4.2-15N in C6D6 (41 MHz) -140 -160 -180 -200 -220 -240 -260 -280 217 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Figure C.1.6. 1H NMR spectrum of 4.2-2H in C6D6 (400 MHz) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 ppm Figure C.1.7. 31P NMR spectrum of 4.2-2H in C6D6 (162 MHz) 12 11 10 9 8 7 6 5 4 3 2 1 ppm Figure C.1.8. 2H NMR spectrum of 4.2-2H in C6H6 (61 MHz) 0 -1 -2 -3 -4 218 * * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 ppm 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -15 -20 Figure C.1.9. 1H NMR spectrum of 4.3 in C6D6 (300 MHz , *Et2O) 110 105 100 95 90 85 80 75 70 65 60 55 50 45 ppm 40 35 30 25 20 15 10 5 0 -5 -10 Figure C.1.10. 31P NMR spectrum of 4.3 in C6D6 (121 MHz) -2.2 -2.3 ppm -2.4 # * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 ppm 3.5 3.0 2.5 2.0 1.5 1.0 0.5 -2.0 -2.5 Figure C.1.11. 1H NMR spectrum of 4.3 in 1:5 THF-d8/C6D6 (400 MHz, *Et2O, #pentane) 219 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 ppm Figure C.1.12. 31P NMR spectrum of 4.3 in 1:5 THF-d8/C6D6 (162 MHz) # 4 * 3 2 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 ppm 3.5 3.0 2.5 2.0 1.5 1.0 -2.0 -2.5 Figure C.1.13. {top} 1H NMR spectrum of 4.3-15N in 1:5 THF-d8/C6D6 (400 MHz, *Et2O, # pentane), {middle} 1H{15N} NMR spectrum of 4.3-15N in 1:5 THF-d8/C6D6 (400 MHz), {bottom} 1H NMR spectrum of 4.3 in 1:5 THF-d8/C6D6 (400 MHz) 4 3 2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2.0 -2.1 -2.2 -2.3 ppm -2.4 -2.5 -2.6 -2.7 -2.8 -2.9 -3.0 -3.1 -3.2 -3.3 Figure C.1.14. {top} Partial 1H NMR spectrum of 4.3-15N in 1:5 THF-d8/C6D6 (400 MHz), {middle} Partial 1H{15N} NMR spectrum of 4.3-15N in 1:5 THF-d8/C6D6 (400 MHz), {bottom} Partial 1H NMR spectrum of 4.3 in 1:5 THF-d8/C6D6 (400 MHz) 220 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 ppm Figure C.1.15. 31P NMR spectrum of 4.3-15N in 1:5 THF-d8/C6D6 (162 MHz) 0 f1 (ppm) -100 100 200 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 ppm Figure C.1.16. 1H-15N HSQC of 4.3-15N in 1:5 THF-d8/C6D6 (1H{15N} spectrum on top; 400 MHz, 41 MHz) -90 -70 -60 f1 (ppm) -80 -50 -40 -30 -1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2.0 -2.1 -2.2 -2.3 ppm -2.4 -2.5 -2.6 -2.7 -2.8 -2.9 -3.0 -3.1 -3.2 Figure C.1.17. Partial 1H-15N HSQC of 4.3-15N in 1:5 THF-d8/C6D6 (1H{15N} spectrum on top; 400 MHz, 41 MHz) 221 * 0 # * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 -2.0 -2.5 ppm Figure C.1.18. 1H NMR spectrum of 4.3-2H in 1:5 THF-d8/C6D6 (400 MHz, *Et2O, # pentane) 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 ppm Figure C.1.19. 31P NMR spectrum of 4.3-2H in 1:5 THF-d8/C6D6 (162 MHz) – 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 ppm 4 3 2 1 0 -1 -2 -3 -4 Figure C.1.20. 2H NMR spectrum of 4.3-2H in 1:5 THF-h8/C6H6 (61 MHz) -5 -6 -7 -8 -9 -10 222 0 8.0 7.5 7.0 6.5 -1 -2 ppm 6.0 5.5 -3 -4 5.0 -5 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 ppm 1.98 4.00 Figure C.1.21. 1H NMR spectrum of (SiP2S)NiNH2Na(THF)3 in C6D6 (400 MHz) 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 Figure C.1.22. 1H NMR spectrum of (SiP2S)NiNH2Na(THF)3 in C6D6 (400 MHz) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 ppm Figure C.1.23. 31P NMR spectrum (SiP2S)NiNH2Na(THF)3 in C6D6 (162 MHz) -20 -30 -40 -3.5 223 -1 -2 -3 -4 ppm # * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure C.1.24. 1H NMR spectrum of (SiP2S)NiNH2Na(THF)3 in 1:5 THF-d8/C6D6 (400 MHz) 120 110 100 90 80 70 60 50 40 ppm 30 20 10 0 -10 -20 -30 -40 Figure C.1.25. 31P NMR spectrum of (SiP2S)NiNH2Na(THF)3 in 1:5 THF-d8/C6D6 (162 MHz) * * 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 Figure C.1.26. 1H NMR spectrum of 4.4 in toluene-d8 (400 MHz, *pentane) 1.5 1.0 0.5 224 100 95 90 85 80 75 70 65 60 55 50 ppm 45 40 35 30 25 20 15 10 5 Figure C.1.27. 31P NMR spectrum of 4.4 in toluene-d8 (162 MHz). Minor 31P shifts at 42.1 ppm and 49.4 ppm correspond to 4.2 and 4.5, respectively, due to the disproportionation of 4.4 in solution. * 15.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure C.1.28. 1H NMR spectrum of 4.5 in toluene-d8 (400 MHz, *C6H6) 140 130 120 110 100 90 80 70 60 50 40 30 ppm Figure C.1.29. 31P NMR spectrum of 4.5 in C6D6 (162 MHz) 20 10 0 -10 -20 -30 225 2 1 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 ppm 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 Figure C.1.30. {top} 1H NMR spectrum of 4.3 in C6D6, generated from LiNH2 and 4.1 (300 MHz) {bottom} 1H NMR spectrum in C6D6 of the crude reaction mixture upon treatment of 4.2 with nBuLi (300MHz) 2 1 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 ppm Figure C.1.31. {top} 31P NMR spectrum of 4.3 in C6D6, generated from LiNH2 and 4.1 (121MHz) {bottom} 31P NMR spectrum in C6D6 of the crude reaction mixture upon treatment of 4.2 with nBuLi (121 MHz) 226 3 2 1 15.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm Figure C.1.32. {top} 1H NMR spectrum of 4.4 in toluene-d8 (400 MHz) {middle} 1H NMR spectrum of sample after 1 h at 25 ºC in toluene-d8 (400 MHz) {bottom} 1H NMR spectrum of sample after 1 d at 25 ºC in toluene-d8 (400 MHz) * 3 * 2 1 16.0 15.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 ppm 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure C.1.33. {top} 1H NMR spectrum of 4.2 and 4.5 (generated via disproportionation of 4.4 after 1 d at 25 ºC) in toluene-d8 (400 MHz,*pentane) {middle} 1H NMR spectrum of 4.2 in toluene-d8 (400 MHz) {bottom} 1H NMR spectrum of 4.5 in toluene-d8 (400 MHz) 227 3 2 1 130 120 110 100 90 80 70 60 ppm 50 40 30 20 10 0 -10 Figure C.1.34. {top} 31P NMR spectrum of 4.4 in toluene-d8 (162 MHz) {middle} 31P NMR spectrum of sample after 1 h at 25 ºC in toluene-d8 (162 MHz) {bottom} 31P NMR spectrum of sample after 1 d at 25 ºC in toluene-d8 (162 MHz) 3 2 1 100 95 90 85 80 75 70 65 60 55 50 ppm 45 40 35 30 25 20 15 10 5 0 Figure C.1.35. {top} 31P NMR spectrum of 4.2 and 4.5 (generated via disproportionation of 4.4 after 1 d at 25 ºC) in toluene-d8 (162 MHz) {middle} 31P NMR spectrum of 4.2 in toluened8 (162 MHz) {bottom} 31P NMR spectrum of 4.5 in toluene-d8 (162 MHz) 120 115 110 105 100 95 90 85 80 75 70 65 60 55 ppm 50 1.00 1.00 228 45 40 35 30 25 20 15 10 5 0 -5 -10 Figure C.1.36. 31P NMR spectrum of 4.2 and 4.5 (generated via disproportionation of 4.4 after 1 d at 25 ºC) in toluene-d8 with integrations (162 MHz) 2 * ** * 1 # 16.0 15.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm Figure C.1.37. {top} 1H NMR spectrum of 4.5 in toluene-d8 (400 MHz) {bottom} 1H NMR spectrum in toluene-d8 of the crude reaction mixture upon treatment of 4.4 with 2,4,6-tri-tertbutylphenoxyl radical (300 MHz, *2,4,6-tri-tert-butylphenol, #THF) 2 1 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 ppm Figure C.1.38. {top} 31P NMR spectrum of 4.5 in toluene-d8 (162 MHz) {bottom} 31P NMR spectrum in toluene-d8 of the crude reaction mixture upon treatment of 4.4 with 2,4,6-tri-tertbutylphenoxyl radical (121 MHz) 229 * 6 * 4 1 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure C.1.39. {top} 1H NMR spectrum of the crude mixture upon oxidation of 4.3 in DME, in C6D6 (400 MHz, *1,3,5-trimethoxybenzene) {middle} 1H NMR spectrum of 4.4 in C6D6 {bottom}1H NMR spectrum of 4.2 in C6D6 8 4 2 54 53 52 51 50 49 48 47 46 45 44 ppm 43 42 41 40 39 38 37 36 35 34 Figure C.1.40. {top} 31P NMR spectrum of the crude mixture upon oxidation of 4.3 in DME, in C6D6 (162 MHz) {middle} 31P NMR spectrum of 4.4 in C6D6 {bottom} 31P NMR spectrum of 4.2 in C6D6 230 A 5 B 4 C 3 D 2 E 1 170 160 150 140 130 120 110 100 90 80 70 ppm 60 50 40 30 20 10 0 -10 -20 Figure C.1.41. 31P NMR spectra monitoring the conversion of 4.5 to 4.2 in 0.4 M NH3 in THF (with ca. 5% C6D6), (A) 30 min after addition (B) 1 h after addition (C) 3 h after addition (121 MHz). Authentic 31P NMR spectra of (D) 4.5 and (E) 4.2 in THF with ca. 5% C6D6 (162 MHz) A 5 B 4 C 3 D 2 E 1 54 53 52 51 50 49 48 47 46 ppm 45 44 43 42 41 40 39 38 Figure C.1.42. 31P NMR spectra monitoring the conversion of 4.5 to 4.2 in 0.4 M NH3 in THF (with ca. 5% C6D6), (A) 30 min after addition (B) 1 h after addition (C) 3 h after addition (121 MHz). Authentic 31P NMR spectra of (D) 4.5 and (E) 4.2 in THF with ca. 5% C6D6 (162 MHz) 231 C.2 UV-Visible Spectra Figure C.2.1. UV-vis spectrum of 4.2 in THF Figure C.2.2. UV-vis spectrum of 4.3 in THF Figure C.2.3. UV-vis spectrum of 4.4 in THF (1 mm cuvette) 232 Figure C.2.4. UV-vis spectrum of 4.5 in THF Figure C.2.5. UV-vis spectra monitoring the disproportionation of 4.4 → ½ 4.2 + ½ 4.5 in THF at 25 ºC. Red trace represents the initial spectrum of 4.4, and the blue trace represents spectrum of the mixture overnight. (1 mm cuvette) Figure C.2.6. UV-vis spectra monitoring the disproportionation of 4.4 → ½ 4.2 + ½ 4.5 in THF at 25 ºC. Red trace represents the initial spectrum of 4.4, and the blue trace represents spectrum of the mixture overnight. Dashed spectra are included for reference (4.2 in dashed red, 4.5 in dashed blue) and are shown at arbitrary absorbance scale relative to the solid spectra). 233 C.3 IR Spectra Figure C.3.1. IR spectrum of 4.2 (solid powder sample) Figure C.3.2. IR spectrum of 4.3 (solid powder sample) Figure C.3.3. IR spectrum of (SiP2S)Ni(NH2)Na(THF)3 (solid powder sample) 234 Figure C.3.4. IR spectrum of 4.4 (solid powder sample) Figure C.3.5. IR spectrum of 4.5 (solid powder sample) C.4 Cyclic Voltammogram Figure C.4.1. Cyclic voltammograms of 4.3 at various scan rates, depicting the NiIII/II couple at −0.72 V V vs. Fc/Fc+ (0.4 M [NBu4][PF6] in THF). Data was collected with glassy carbon working, Pt counter, and Ag/Ag+ reference electrodes. 235 C.5 X-ray Diffraction Data Figure C.5.1. Full asymmetric unit of 4.2. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure C.5.2. Structure of 4.3 with omission of C–H hydrogen atoms, solvent molecules, and disordered components. Thermal ellipsoids at 50% probability. 236 Figure C.5.3. Full asymmetric unit of 4.3 including solvent. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure C.5.4. Full asymmetric unit of 4.4 including solvent. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. 237 Figure C.5.5. Structure of 4.5 with omission of C–H hydrogen atoms, solvent molecules, and disordered components. Thermal ellipsoids at 50% probability. Figure C.5.6. Full asymmetric unit of 4.5 including solvent. C–H hydrogen atoms omitted; thermal ellipsoids at 50% probability. 238 Figure C.5.7. Structure of (SiP2S)NiNH2Na(THF)3 with omission of hydrogen atoms and solvent molecules. Thermal ellipsoids at 50% probability. Figure C.5.8. Full asymmetric unit of (SiP2S)NiNH2Na(THF)3 including solvent. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. 239 C.6 Oxidative N−N Coupling from 4.3 Table C.6.1. Single run yields for the generation of 4.4 and 4.2 by oxidation of 4.3 4.3 [Cp2Fe][BArF4] (mg) (mg) a 8 10 b 8 c 8 Entry Solvent Yield of 4.4 Yield of 4.2 (%) (%) DME 42 60 10 DME 36 65 10 DME 44 54 240 Appendix D SUPPLEMENTARY INFORMATION FOR CHAPTER 5 D.1 NMR Spectra 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Figure D.1.1. 1H NMR spectrum of 5.3 in C6D6 (400 MHz) 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 ppm Figure D.1.2. 31P NMR spectrum of 5.3 in C6D6 (162 MHz) 30 20 10 0 -10 -20 -30 -40 241 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 Figure D.1.3. 1H NMR spectrum of 5.4 in C6D6 (300 MHz) 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 ppm 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -45 -50 Figure D.1.4. 31P NMR spectrum of 5.4 in C6D6 (162 MHz) * 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 ppm 4.0 3.5 3.0 2.5 Figure D.1.5. 1H NMR spectrum of 5.6 in C6D6 (400 MHz). *impurity 2.0 1.5 1.0 0.5 242 100 90 80 70 60 50 40 30 20 10 ppm 0 -10 Figure D.1.6. 31P NMR spectrum of 5.6 in C6D6 (162 MHz) D.2 IR Spectra Figure D.2.1. IR spectrum of 5.7 (solid sample) Figure D.2.2. IR spectrum of 5.8 (solid sample) -20 -30 -40 -50 -60 -70 243 Figure D.2.3. IR spectrum of 5.9 (solid sample) Figure D.2.4. Representative IR spectrum of solids remaining after vacuum transfer of volatiles in a N2H4 disproportion catalytic run with 5.8 (solid sample) D.3 UV-Visible Spectra Figure D.3.1. UV-vis spectrum of 5.7 in THF 244 Figure D.3.2. UV-vis spectrum of 5.8 in THF Figure D.3.3. UV-vis spectrum of 5.11 in THF D.4 X-Ray Diffraction Data Figure D.4.1. Structure of 5.7 with omission of hydrogen atoms and solvent molecules. Thermal ellipsoids at 50% probability. 245 Figure D.4.2. Full asymmetric unit of 5.7 including solvent and disordered components. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. Figure D.4.3. Full asymmetric unit of 5.8 including solvent. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. 246 Figure D.4.4. Structure of PhPS3V(THF) with omission of hydrogen atoms and solvent molecules. Thermal ellipsoids at 50% probability. Figure D.4.5. Full asymmetric unit of PhPS3V(THF) including solvent. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. 247 Figure D.4.6. Structure of 5.11 with omission of disordered components and hydrogen atoms. Thermal ellipsoids at 50% probability. Figure D.4.7. Full asymmetric unit of 5.11 including solvent and disordered components. Hydrogen atoms omitted; thermal ellipsoids at 50% probability. 248 D.5. Hydrazine Disproportionation Catalysis Table D.5.1 Single runs from Table 5.1. Entry Solvent Time N2H4 (equiv./5.8) NH3 (equiv./5.8) a THF 2h 50 4.0, 5.5 b Et2O 2h 50 9.7, 11 c Et2O 12 h 50 41, 50 d Et2O 24 h 50 57, 62 e Et2O 24 h 100 80, 98 f Et2O 48 h 100 102, 124 g1 Et2O 17 d 1440 1014, 1132 S1* Et2O 12 h 50 46 S2‡ Et2O 12 h 50 42 S3# Et2O 12 h 50 39 Runs performed at catalyst concentrations of 1 mM in 2.5 mL solvent. 1Run performed with at 0.1 mM catalyst concentration in 2.5 mL solvent.*Performed in the presence of elemental Hg. ‡Performed in the presence of 0.3 equivalents of PPh3 per equivalent of 5.8 (1 mM in 2 mL Et2O). # Performed in the presence of 10 equivalents of pyridine per equivalent of 5.8 (1 mM in 2 mL Et2O). Table D.5.2 Single runs from Table 5.2. Entry Catalyst NH3 (equiv./catalyst) h 5.9 0.26, 0.51 i 5.11 0.27, 0.13 j* 5.9 + 5.11 0.88, 1.19 Runs performed at catalyst concentrations of 1 mM in 2.5 mL solvent. * [5.9] = [5.11] = 1 mM 249 Table D.5.3 Determination of the reaction order of compound 5.8 via method of initial rates 5.8 (mg) NH3 (µmol) ln([5.8]0)1 ln([NH3]/t)1 1 3.5, 3.6 −8.0 −7.3 2 5.7, 8.1 −7.3 −6.6 3 12.2, 13.1 −6.9 −6.0 4 16.0, 16.1 −6.6 −5.7 Runs performed with 0.13 mmol N2H4 in 2.5 mL THF. Yield of NH3 determined after 2 h of reaction time. 1concentrations in M, time in h Figure D.5.1. Plot of ln([5.8]0) vs. ln(NH3]/t) Table D.5.4. Determination of the reaction order on substrate via method of initial rates [N2H4] NH3 (µmol) ln([N2H4]0)1 ln([NH3]/t)1 0.002 0.23, 0.24 −6.2 −7.3 0.005 0.39, 0.46 −5.3 −6.7 0.010 0.97, 1.4 −4.6 −5.7 Runs performed in 2 mL THF with 1 mM catalyst (a THF solution of substrate is added at 25 °C to a Schlenk tube containing a stirring THF solution of catalyst and immediately sealed).Yield of NH3 determined after 10 min of reaction time. 1concentrations in M, time in h 250 Figure D.5.2. Plot of ln([N2H4]0) vs. ln(NH3]/t) Table D.5.5. Determination of the reaction order on substrate via method of initial rates (saturation kinetics) [N2H4] NH3 (µmol) ln([N2H4]0)1 ln([NH3]/t)1 0.025 9.0, 9.6 −3.6 −6.3 0.070 7.9, 8.8 −2.7 −6.4 0.083 8.3, 8.5 −2.5 −6.4 Runs performed in 2.5 mL THF with 1 mM catalyst. Yield of NH3 determined after 2 h of reaction time. 1concentrations in M, time in h Figure D.5.2. Plot of ln([N2H4]0) vs. ln(NH3]/t) under saturation kinetics