Proton-Coupled Electron Transfer in Nitrogen Fixation Citation Chalkley, Matthew J. (2020) Proton-Coupled Electron Transfer in Nitrogen Fixation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FE9D-9K14. https://resolver.caltech.edu/CaltechTHESIS:02052020-203503014 Abstract This thesis focuses on the management of protons and electrons in the formation of X−H bonds. In our pursuit of better understanding this process, we have been particularly interested in the nitrogen fixation reaction (N 2 -to-NH 3 ) because of the high number of protons and electrons involved in this conversion (6) and the significant difficulty of functionalizing N 2 . The first chapter introduces the important themes of this thesis: (i) multiple bonding, (ii) proton-coupled electron transfer, (iii) overpotential in N 2 fixation, and (iv) selectivity in N 2 fixation. The second chapter discusses the bonding of an iron complex with a small molecule (NO) and how this bonding is key to activating the small molecule for reactivity. The third chapter looks at how employing a new proton and electron source allows an Fe catalyst to achieve improved selectivity and turnover number for the reduction of N 2 to NH 3 despite a lowered overpotential relative to previous reactions. It also raises the hypothesis that this is possible due to proton-coupled electron transfer mediated by a metallocene. The fourth chapter studies the effect of acid strength on N 2 fixation selectivity and demonstrates circumstantial evidence for the involvement of a decamethylcobaltocene (Cp* 2 Co) in the formation of N−H bonds via proton-coupled electron transfer. It also highlights how the addition of co-catalytic [Cp* 2 Co] + to electrochemical experiments with our Fe catalyst enabled truly electrocatalytic N 2 fixation for the first time. The fifth chapter provides both atomistic detail on the protonation reactivity of Cp* 2 Co and experimentally verifies the prediction that this species would be an extremely strong hydrogen-atom donor. It also develops a conceptual framework to explain the uniquely weak C−H bonds both homolytic and heterolytic that result from metallocene protonation and discusses their potential to play a role in not only the hydrogen evolution reaction (HER), but also the N 2 fixation reaction. In the final chapter, we develop a synthetic route to a base appended cobaltocene. We demonstrate that this second-generation cobaltocene can, unlike the first generation, serve as a net hydrogen-atom donor under electrocatalytic conditions. As a demonstration of the utility of this, we use the base-appended cobaltocene for the selective, proton-coupled reduction of ketones to pinacols via a rate-determing concerted proton-electron transfer. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Proton-coupled electron transfer, nitrogen fixation, electrocatalysis Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Awards: The Herbert Newby McCoy Award, 2020. Thesis Availability: Public (worldwide access) Research Advisor(s): Peters, Jonas C. Group: Resnick Sustainability Institute Thesis Committee: Agapie, Theodor (chair) Gray, Harry B. Miller, Thomas F. Peters, Jonas C. Defense Date: 17 January 2020 Funders: Funding Agency Grant Number NIH GM070757 Department of Energy (DOE) 0235302 Record Number: CaltechTHESIS:02052020-203503014 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:02052020-203503014 DOI: 10.7907/FE9D-9K14 Related URLs: URL URL Type Description https://doi.org/10.1002/anie.201605403 DOI Published content for Chapter 2. https://doi.org/10.1021/acscentsci.7b00014 DOI Published content for Chapter 3. https://doi.org/10.1021/jacs.8b02335 DOI Published content for Chapter 4. https://doi.org/10.1021/jacs.9b00193 DOI Published content for Chapter 5. ORCID: Author ORCID Chalkley, Matthew J. 0000-0002-0484-7335 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 13634 Collection: CaltechTHESIS Deposited By: Matthew Chalkley Deposited On: 25 Feb 2020 00:37 Last Modified: 08 Nov 2023 00:44 Thesis Files Preview PDF - Final Version See Usage Policy. 17MB Repository Staff Only: item control page

Proton-Coupled Electron Transfer in Nitrogen Fixation Thesis by Matthew J. Chalkley In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2020 Defended: January 17, 2020 ii  2020 Matthew J. Chalkley ORCID: 0000-0002-0484-7335 All rights reserved iii ACKNOWLEDGEMENTS At this critical waypoint in my chemistry journey, I want to reach back in time and thank some of the important people who influenced me to start on this path. Firstly, a thank you to two high school science teachers, Dr. Sohn and Mr. Smith, who with their excellent teaching helped to arouse my interest in science. Also, I want to thank the New Jersey Governor’s School in the Sciences, which was an amazing experience and gave me a lot of confidence with respect to pursuing chemistry in college. At Yale, I joined the lab of Professor Nilay Hazari after my freshman year. This is one of those fortuitous life decisions that one can only credit chance for. Nilay is an incredibly generous mentor and friend. He taught me so much and allowed me a remarkable amount of freedom to explore. I continue to benefit tremendously from his advice and imagine that this will always be the case. Nilay also attracted a remarkable group of people to the lab, who I also want to thank. These include Professor Eddie Luzik and Dr. Damian Hruszkewycz, who did much of my hands-on training and welcomed me into the group. Members who joined the lab later including Louise Guard, Heewon Suh, and Amymarie Bartholomew, also became friends. The larger chemistry community at Yale was also an incredibly welcoming environment and I have been lucky to cross paths multiple times with a number of chemists I first met at Yale: Patrick Melvin, Irene Cai, Graham Dobereiner, Nathan Schley, Alissa Hare, and James Blakemore. I was also lucky enough to make non-chemistry friends at Yale. Despite the fact that I have not seen most of them as often as I have hoped in the past five years, through skype and phone calls I have been able to enjoy their company. So thank you to all of these friends and others who have stayed in touch over the years via Skype and phone calls. After Yale, I spent a year in Germany in the lab of Professor Karsten Meyer. I’m grateful for that opportunity to experience a different culture, both in the laboratory and in general, during my time there. I also met a number of students (Dominik and Mario), postdocs (Pete) and visitors (Donald), who helped me to integrate into the group and enjoy my year there. iv Arriving at Caltech, I was fortunate to be able to join the lab of Professor Jonas Peters. Jonas has pushed me throughout my Ph.D. to choose scientific problems more carefully, to consider them more deeply, and to communicate them more coherently; for this, I am very grateful. I also believe that we have a relatively unique rapport that has allowed us to work effectively together. Most importantly though, I have always trusted that Jonas was there to support me, and I believe that if you can find that in a Ph.D. advisor you are very lucky. Furthermore, Jonas has also built a tremendous environment to do science in. The people and the resources of the Peters lab and Caltech at large are hard to match. At the beginning of my Ph.D., I received invaluable mentorship from Jon Rittle and Gaël Ung. In addition to these two, I landed in an office with Tanvi Ratani and Mark Nesbit, who I am sure answered more than their fair share of questions. I hope that I have been able to pay back this help to newly joined group members, who have become my officemates in the years since (Kumiko, Jian, Patricia, Teddy, Pablo, Emily, Tucker, and Nick). The support staff who run the facilities/operations at Caltech are consistently excellent. I want to thank in particular Julianne Just, who does so much to keep the Peters’ group running, and Mike Takase, who has been a friendly face ever since I first met him at Yale. I did a lot of collaborative work at Caltech. This started with two papers that I coauthored with Trevor del Castillo and Ben Matson. These papers really re-oriented the way I was thinking about science and formed a springboard for the work that I would do going forward. That I am still excited by the hypothesis that we formulated several years ago now is a testament to its strength (or maybe my myopic nature). Subsequently, I worked on a paper with Paul Oyala. In hindsight, this project looks very straightforward, but it took a long time for us to gain the first foothold of understanding and so I want to thank him for his persistence in working to solve this problem with me. Thank you to Marcus Drover, who I hope has forgiven me for accidentally committing us both to writing a Chemical Reviews article with Jonas. Most recently, I have been incredibly lucky to get to work with Pablo Garrido-Barros. Pablo has already taught me so much and our current project would not have been possible without his expertise and eternal optimism. I also want to thank Dirk Schild, with whom I was a teaching assistant for Ch 153b, and Nik Thompson, with whom I worked closely on both Mӧssbauer spectroscopy and nuclear resonance vibrational spectroscopy v over the years. Both were also always eager to chat about chemistry and have given formative input over the years. I have also gotten valuable input from my committee members: Professor Theodor Agapie, Professor Tom Miller, and Professor Harry Gray. They always came ready with comments and questions to my meetings which helped to push me to do better. They also provided important support and guidance towards building the next phase of my chemistry career. I have also gotten support and guidance from the Resnick Institute, most typically in the form of advice or help offered by Heidi Rusina and Neil Fromer. The Resnick Institute is already a great organization that brings together many scientists on campus, and I am excited to see how it will grow in the future. I also received generous financial support from both the NSF Graduate Research Fellowship and the Caltech Center for Environmental Microbial Interactions (CEMI). The latter of these was particularly patient in putting up with my completely non-microbial work. I have not had many hobbies over the years at Caltech, but one of them was playing soccer. In my earlier years this was with Gaël, Trevor, Javier Fajardo, Josh Buss, Kyle Horak, Davide Lionetti, and others less familiar to the Caltech community. At the end of my Caltech experience, I have been grateful to again play soccer this time with Pablo, Nick Watkins, Heejun Lee, Alonso Rosas, Mike Zott, and others. I was also involved with volunteering with the RISE program at Caltech Y and want to thank all those who made that possible. This leaves my family to thank. As always, my parents have been an incredible support to me over the past years. Despite the increased distance, they have remained an ever-present part of my life sharing in my frustrations and joys. I think of how lucky I am to have them as my parents regularly and love them very much. During my Ph.D., we have added Grace to our family. In the time that I have been in graduate school, Grace has completed medical school, (almost) completed residency, and is now interviewing to become an attending physician. I’m so proud of her. Alongside all her own accomplishments, she has spent endless time listening to me talk about my chemistry and experience at Caltech. She and my parents know far more about PCET, metallocenes, nitrogen fixation, Mӧssbauer, and vi all the other topics that you will read about in the coming chapters than they ever could have expected or wanted. So thank you so much to them and to all of the other people mentioned above, without whom this thesis would never have happened. vii ABSTRACT This thesis focuses on the management of protons and electrons in the formation of X−H bonds. In our pursuit of better understanding this process, we have been particularly interested in the nitrogen fixation reaction (N2-to-NH3) because of the high number of protons and electrons involved in this conversion (6) and the significant difficulty of functionalizing N2. The first chapter introduces the important themes of this thesis: (i) multiple bonding, (ii) proton-coupled electron transfer, (iii) overpotential in N2 fixation, and (iv) selectivity in N2 fixation. The second chapter discusses the bonding of an iron complex with a small molecule (NO) and how this bonding is key to activating the small molecule for reactivity. The third chapter looks at how employing a new proton and electron source allows an Fe catalyst to achieve improved selectivity and turnover number for the reduction of N2 to NH3 despite a lowered overpotential relative to previous reactions. It also raises the hypothesis that this is possible due to proton-coupled electron transfer mediated by a metallocene. The fourth chapter studies the effect of acid strength on N2 fixation selectivity and demonstrates circumstantial evidence for the involvement of a decamethylcobaltocene (Cp*2Co) in the formation of N−H bonds via proton-coupled electron transfer. It also highlights how the addition of co-catalytic [Cp*2Co]+ to electrochemical experiments with our Fe catalyst enabled truly electrocatalytic N2 fixation for the first time. The fifth chapter provides both atomistic detail on the protonation reactivity of Cp*2Co and experimentally verifies the prediction that this species would be an extremely strong hydrogen-atom donor. It also develops a conceptual framework to explain the uniquely weak C−H bonds both homolytic and heterolytic that result from metallocene protonation and discusses their potential to play a role in not only the hydrogen evolution reaction (HER), but also the N2 fixation reaction. In the final chapter, we develop a synthetic route to a base appended cobaltocene. We demonstrate that this second-generation cobaltocene can, unlike the first generation, serve as a net hydrogen-atom donor under electrocatalytic conditions. As a demonstration of the utility of this, we use the base-appended cobaltocene for the selective, proton-coupled reduction of ketones to pinacols via a rate-determing concerted protonelectron transfer. viii PUBLISHED CONTENT AND CONTRIBUTIONS Chalkley, M. J.; Peters, J. C. A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}8-10. Angew. Chem. Int. Ed. 2016, 128, 12174. doi:10.1002/anie.201605403. M.J.C. designed and performed all experiments. M.J.C. and J.C.P. wrote the manuscript. Chalkley, M. J.;† Del Castillo, T. J.;† Matson, B. D.;† Roddy, J. P.; Peters, J. C. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for MetalloceneMediated PCET. ACS Cent. Sci. 2017, 3, 217. doi:10.1021/acscentsci.7b00014. † These authors contributed equally. M.J.C. performed catalytic runs and freeze-quench spectroscopic measurements. M.J.C. helped to develop the PCET hypothesis and write the manuscript. Chalkley, M. J.;† Del Castillo, T. J.;† Matson, B. D.;† Peters, J. C. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis. J. Am. Chem. Soc. 2018, 140, 6122. doi:10.1021/jacs.8b02335. † These authors contributed equally. M.J.C. performed the pKa study, the freeze-quench spectroscopic measurements, and the stoichiometric reactivity. M.J.C. helped to write the manuscript. Chalkley, M. J.; Oyala, P. H.; Peters, J. C. Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation. J. Am. Chem. Soc. 2019, 141, 4721. doi:10.1021/jacs.9b00193. M.J.C. performed all synthesis, quantum chemical calculations, and spectroscopic measurements, other than pulse EPR data. M.J.C. helped to interpret pulse EPR data. M.J.C. wrote the manuscript. ix TABLE OF CONTENTS Introduction .................................................................................................... 1 1.1 Opening Remarks ......................................................................................................... 2 1.2 Multiple Bonding ......................................................................................................... 6 1.3 Proton-Coupled Electron Transfer: .............................................................................. 9 1.4 Overpotential in Nitrogen Fixation ............................................................................ 11 1.5 Selectivity in Nitrogen Fixation Catalysis ................................................................. 13 1.6 Chapter Summaries .................................................................................................... 15 1.7 References .................................................................................................................. 17 Chapter 2. A Triad of Highly-Reduced, Linear Iron Nitrosyls: {Fe(NO)}8-10 ......... 21 2.1 Introduction ................................................................................................................ 22 2.2 Results ......................................................................................................................... 23 2.2.1 Synthesis of {Fe(NO)}8-10................................................................................. 23 2.2.2 Characterization and Electronic Structure of {Fe(NO)}8-10 ............................. 24 2.3 Conclusion .................................................................................................................. 32 2.4 References .................................................................................................................. 33 Chapter 3. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET ............................................................. 36 3.1 Introduction ................................................................................................................ 37 3.2 Results ......................................................................................................................... 40 3.2.1 Catalytic N2RR at Lowered Overpotential ....................................................... 40 3.2.2 Mechanism of N2RR Catalysis with (P3B)Fe ................................................... 42 3.2.3 PCET from a Protonated Metallocene.............................................................. 48 3.3 Discussion ................................................................................................................... 52 3.4 Conclusion .................................................................................................................. 53 3.5 References .................................................................................................................. 54 Chapter 4. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis .............................................................. 58 4.1 Introduction ................................................................................................................ 59 x 4.2 Results and Discussion ............................................................................................. 62 4.2.1 Effect of pKa on the Selectivity of N2 Fixation by [(P3B)Fe]+ ......................... 62 4.2.2 Computational Studies on Cp*2Co Protonation and N–H Bond Formation ... 67 4.2.3 Electrocatalytic N2 Fixation with [(P3B)Fe]+ .................................................... 73 4.3 Conclusion .................................................................................................................. 81 4.4 References .................................................................................................................. 82 Cp* Non-innocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation ...................................................................................................... 86 5.1 Introduction ................................................................................................................ 87 5.2 Results ......................................................................................................................... 90 5.2.1 Pulse Electron Paramagnetic Resonance Spectroscopy on Protonated Cp*2Co........................................................................................................................ 90 5.2.2 Stereochemical Assignment of Cp*2Co Protonation ....................................... 95 5.2.3 Thermochemical Measurements Relevant to Cp*2Co Protonation ................. 99 5.3 Discussion ................................................................................................................. 103 5.4 Conclusion ................................................................................................................ 107 5.5 References ................................................................................................................ 108 Transfer Metallocenes: Beyond Electron Transfer to Proton-Coupled Electron ...................................................................................................................... 113 6.1 Introduction .............................................................................................................. 114 6.2 Results ....................................................................................................................... 117 6.2A Mechanism of Electrocatalytic HER by [Cp2Co]+ ......................................... 117 6.2B Synthesis of a Brønsted Base-Appended Cobaltocene .................................. 119 6.2C Thermochemical Properties of a Brønsted Base-Appended Cobaltocene ..... 122 6.2D Electrochemical behavior of (Cp)Co(CpN) with Excess Acid ....................... 124 6.2E Electrochemical Behavior in the Presence of a Hydrogen Atom Acceptor ... 125 6.3 Discussion ................................................................................................................. 131 6.4 Conclusion: ............................................................................................................... 135 6.5 References: ............................................................................................................... 136 Appendix A. Supplementary Information for Chapter 2 .......................................... 141 xi Appendix B. Supplementary Information for Chapter 3 ....................................... 178 Appendix C. Supplementary Information for Chapter 4 .......................................... 213 Appendix D. Supplementary Information for Chapter 5 .......................................... 251 Appendix E. Supplementary Information for Chapter 6 .......................................... 304 xii LIST OF FIGURES Figure 1.1: Depiction of the active site of the FeMo nitrogenase .......................................... 3 Figure 1.2: Molecular nitrogen fixation catalysts used in this thesis. .................................... 4 Figure 1.3: Cooperative binding of N2 via lone-pair donation from N2 and πbackdonation. ......................................................................................................... 6 Figure 1.4: (top) Mo and W complexes promote the functionalization of N2 and its conversion to NH3.15 (bottom) Mo complex that binds N2 and then cleaves it to form to Mo(N) complexes.16 ................................................................................. 7 Figure 1.5: Comparison of the electronic structure of Fe complexes in C3 and C4 symmetry. ................................................................................................................................ 8 Figure 1.6: Canonical examples of hydrogen atom transfer (HAT), concerted-proton electron transfer (CPET), and multi-site proton-coupled electron transfer (MSPCET). ................................................................................................................. 10 Figure 1.7: (left) The square scheme that relates the stepwise ET-PT and PT-ET pathway to the PCET pathway. (right) Definition of CG ...................................................... 11 Figure 1.8: Calculated gas-phase bond dissociation enthalpies (BDEs) for intermediates of potential relevance to nitrogen fixation.32........................................................... 12 Figure 1.9: The possible pathways for background HER and hypothesized N-H bond forming reactions via H• or H− transfer .............................................................. 15 Figure 2.1: Synthetic route for {(P3B)Fe(NO)}8-10. .............................................................. 23 Figure 2.2: Variable scan rate cyclic voltammograms in 0.1 M [TBA][BArF4] THF of the {(P3B)Fe(NO)}8/9 and {(P3B)Fe(NO)}9/10 redox couples ................................... 24 Figure 2.3: IR spectra for {(P3B)Fe(NO)}8-10 highlighting the NO stretch. ......................... 25 Figure 2.4: X-ray crystal structures of {(P3B)Fe(NO)}8-10 ................................................... 26 Figure 2.5: Experimental bond lengths from X-ray crystallography and Wiberg bond indices from DFT ............................................................................................................. 26 Figure 2.6: Zero-field Mӧssbauer spectra of {(P3B)Fe(NO)}8-10 ......................................... 28 Figure 2.7: Density-corrected room temperature (red) and cryogenic (black) UV-Vis spectra of {(P3B)Fe(NO)}9 in 2-MeTHF highlighting the region that demonstrates vibronic coupling. ................................................................................................ 30 xiii Figure 2.8: CW X-band EPR spectrum of {(P3B)Fe(NO)}9 ............................................ 31 Figure 2.9: A molecular orbital diagram for {(P3B)Fe(NO)}9. ............................................ 32 Figure 3.1: Summary of conditions used for catalytic N2-to-NH3 conversion by [(P3B)Fe]+ highlighting the estimated driving force (ΔΔHf). ............................................... 39 Figure 3.2: Mössbauer spectrum at 80 K with 50 mT applied parallel field of a freezequenched catalytic reaction (54 equiv Cp*2Co, 108 equiv [Ph2NH2][OTf], 1 equiv [(P3B)57Fe]+) after five minutes of reaction time. ..................................... 44 Figure 3.3: DFT calculations regarding the interaction of metallocenes and acids relevant to nitrogen fixation catalysis. .................................................................................. 49 Figure 3.4: X-band 77 K powder EPR spectrum (red) and simulation (blue) of the isolated purple precipitate (assigned as endo- and [(Cp*)Co(exo-η4-C5Me5H)]+) from reaction between Cp*2Co and HOTf at −78 °C. ................................................ 51 Figure 4.1: Electrosynthetic cycle for the formation of NH3 from N2 by a Chatt-type Wphosphine complex.19 .......................................................................................... 60 Figure 4.2. Effect of acid strength on the percentage of electrons being used to form NH3 and H2................................................................................................................... 64 Figure 4.3: The kinetics and thermodynamics of protonation of Cp*2Co for three acids from different catalytic efficiency regimes .................................................................. 70 Figure 4.4: The calculated thermodynamics and kinetics of synchronous PCET and fully asynchronous PCET (PT-ET), between (P3B)Fe(NNH) and [(Cp*)Co(exo-η4C5Me5H)] [OTf] to generate (P3B)Fe(NNH2). .................................................... 72 Figure 4.5: Cyclic voltammograms of relevance to electrocatalytic N2 fixation in the with co-catalytic [(P3B)Fe]+ and [Cp*2Co]+ ................................................................ 75 Figure 5.1: Reaction pathways to consider for protonated Cp*2Co species, illustrating both undesired HER and possible N–H bond forming steps relevant to N2RR......... 89 Figure 5.2: Pseudomodulated24 Q-band ESE-detected EPR spectra of the reaction of Cp*2Co with HOTf (black traces), and DOTf (blue traces) measured at 10 K (top traces) and 6 K (bottom traces). ...................................................................................... 91 Figure 5.3: a) Q-band Davies ENDOR spectra at 10 K and b) narrow Q-band Davies ENDOR spectra at 10 K. c) Q-band Davies ENDOR spectra at 6 K and d) narrow Q-band Davies ENDOR spectra at 6 K. Data from the reaction with HOTf (black) and DOTf (blue) are shown................................................................................. 93 xiv Figure 5.4: Comparison of Q-band HYSCORE spectra acquired at the center of the EPR spectrum of the endo- (left) and exo- (right) protonated Cp*2Co species generated via reaction with DOTF (top panels) and HOTF (2nd panels from top). .......... 94 Figure 5.5: (top) X-Band CW EPR spectrum of the reaction of Cp*2Co with HOTf (black) and its simulation (red). (bottom) X-Band CW EPR spectrum of the reaction of Cp*2Co with HNTf2 (black) and its simulation (red). ........................................ 97 Figure 5.6: a) DFT optimized structures for the isomers of protonated Cp*2Co. The predicted A(1H) values for the protons circled in red. b) X-ray crystal structure of (Cp*)Co(exo-η4-C5Me5H). Thermal ellipsoids are shown at 50% probability. c) Experimentally derived 1H hyperfine parameters for the endo- and exo-isomers of protonated Cp*2Co. ......................................................................................... 98 Figure 5.7: Proposed mechanism for the oxidation of (Cp*)Co(exo-η4-C5Me5D) with HBArF4. ................................................................................................................ 99 Figure 5.8: Cyclic voltammograms of the [(Cp*)Co(exo-η4-C5Me5H)]+/0 ........................ 100 Figure 5.9: Thermochemistry of neutral and cationic (Cp*)Co(exo-η4-C5Me5H). ........... 102 Figure 5.10: (left) A comparison of the experimental BDFEC–H for a variety of related Cp*Co-species, demonstrating the importance of aromaticity and electron count in predicting the stability of the indicated C–H bond. (right) A comparison of computational BDFEC–H values for a redox series of [(Cp*)Ni(exo-η4C5Me5H)]n+. ....................................................................................................... 106 Figure 6.1: Overview of PCET for the functionalization of unsaturated substrates.......... 117 Figure 6.2: Cyclic voltammetry relevant to HER by [Cp2Co]+ ......................................... 118 Figure 6.3: Scheme describing the synthesis of the Brønsted base appended cobaltocenes. ...................................................................................................... 120 Figure 6.4: X-ray structure of (Cp)Co(CpN), [(Cp)Co(CpN)]+, and [(Cp)Co(CpNH)]2+ with counteranions and solvent omitted for clarity. Ellipsoids shown at 50% probability. ......................................................................................................... 120 Figure 6.5: Spectroscopic data for [(Cp)Co(CpNH)]+. ........................................................ 121 Figure 6.6: Variable scan rate CVs of 1 mM [(Cp)Co(CpNH)]2+ and 1 mM Fc in 100 mM [TBA][PF6] solution of acetonitrile at a boron-doped diamond working electrode. ............................................................................................................ 123 Figure 6.7: Thermochemistry of [(Cp)Co(CpN(H))]n+ in acetonitrile.................................. 124 N xv 4- Figure 6.8: (left) Cyclic voltammogram of (Cp)Co(Cp ) in the presence of 100 mM of [ CN PhNH3][OTf] in 200 mM [TBA][PF6] DME at 10 mV/s with the acid background subtracted. (right) Spin density plot of [(Cp)Co(CpNH)][OTf] at an isovalue of 0.04 demonstrating that the N–H bond is entirely decoupled from the spin density. ....................................................................................................... 125 Figure 6.9: DFT calculated thermodynamics of PT, ET, and PCET for the reaction of acetophenone with the [(Cp)Co(CpNH)]+. ......................................................... 126 Figure 6.10: Time course of the current (I) and charge (Q) in a CPE experiment at −1.45 V ......................................................................................................................... 128 Figure 6.11: (left) Cyclic voltammograms of 1 mM [(Cp)Co(CpN)][OTf] at 10 mV/s in the presence of 100 mM [4-CNPhNH3][OTf] and the presence (red) and absence (blue) of 50 mM acetophenone. (right) Plot of the dependence of the plateau current on the ketone concentration with a line of best fit. ................................................ 130 Figure 6.12: (left) Plateau current dependence on the concentration of [(Cp)Co(CpN)][OTf] in the reaction with 100 mM [4-CNPhNH3][OTf] and 50 mM acetophenone in a 200 mM [TBA][PF6] solution of DME at 100 mV/s. (right) Repeated cyclic voltammograms in 200 mM [TBA][PF6] DME solution of [(Cp)Co(CpNH)][OTf]2 at 100 mV/s with increasing equivalents of acetophenone (0 to 50 mM). ..................................................................................................... 131 Figure A.1: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. .............................................................................................................. 149 Figure A.2: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}8 at -78 °C in THFd8. ....................................................................................................................... 149 Figure A.3: The 31P NMR spectrum (161.9 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. .......................................................................................................... 150 Figure A.4: The 31P NMR spectrum (202.4 MHz) of {(P3B)Fe(NO)}8 at -78 °C in THFd8. ....................................................................................................................... 150 Figure A.5: The 11B NMR spectrum (128.3 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. .......................................................................................................... 151 Figure A.6: The 19F NMR spectrum (376.3 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. .......................................................................................................... 151 Figure A.7: The 13C NMR spectrum (100.6 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. .......................................................................................................... 152 Figure A.8: The 1H NMR spectrum (400 MHz) of {(P3B)Fe(NO)}9 at room temperature in C6D6. .................................................................................................................. 152 1 xvi H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}10 at room temperature Figure A.9: The in THF-d8. .......................................................................................................... 153 Figure A.10: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THFd8. ....................................................................................................................... 153 Figure A.11: The 31P NMR spectrum (202.4 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THFd8. ....................................................................................................................... 154 Figure A.12: The 11B NMR spectrum (128.3 MHz) of {(P3B)Fe(NO)}10 at room temperature in THF-d8. .......................................................................................................... 154 Figure A.13: The 13C NMR spectrum (125.7 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THFd8. ....................................................................................................................... 155 Figure A.14: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}8. ............................... 155 Figure A.15: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}9. ............................... 156 Figure A.16: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}10. ............................. 156 Figure A.17: UV/Visible spectra of {(P3B)Fe(NO)}8 in 2-MeTHF .................................. 157 Figure A.18: UV/Visible spectra of {(P3B)Fe(NO)}10 in 2-MeTHF.................................. 157 Figure A.19: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}8 as a microcrystalline solid suspended in a boron nitride matrix. ........................................................ 160 Figure A.20: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}8 as a powder suspended in a boron nitride matrix. ................................................................................... 161 Figure A.21: The 80 K, zero field 57Fe Mössbauer of a 2mM solution of {(P3B)57Fe(NO)}8 in 2-MeTHF. ...................................................................................................... 161 Figure A.22: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}9 as a microcrystalline solid suspended in a boron nitride matrix. ........................................................ 161 Figure A.23: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}10 as a microcrystalline solid suspende in a boron nitride matrix. .......................................................... 162 Figure A.24: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}10 as a powder suspended in a boron nitride matrix. ................................................................. 162 Figure A.25: The 80 K, zero field 57Fe Mössbauer of a 2 mM solution of {(P3B)57Fe(NO)}10 in THF. ............................................................................................................... 163 Figure A.26: The 80 K, 50 mT parallel field 57Fe Mössbauer of a 2mM solution of {(P3B)57Fe(NO)}10 in THF. ............................................................................... 163 57 xvii Figure A.27:The 80 K, 50 mT perpendicular field Fe Mössbauer of a 2mM solution of {(P3B)57Fe(NO)}10 in THF. ............................................................................... 164 Figure A.28: The 80 K, 50 mT parallel field Mössbauer of [(P3B)Fe(N2)]− [(P3B)Fe(N2)][Na(12-crown-4)2]) as a powder suspended in a boron-nitride matrix. ................................................................................................................ 164 Figure A.29: Scan rate dependence of the {(P3B)Fe(NO)}8/9 couple in a THF solution of 100 mM [TBA][PF6] under argon. ........................................................................... 165 Figure A.30: Scan rate dependence of the {(P3B)Fe(NO)}9/10 couple in a THF solution of 100 mM [TBA][PF6] under argon..................................................................... 165 Figure A.31: Cyclic voltammetry data at 100 mV s-1 for {(P3B)Fe(NO)}8 in a THF solution of 100 mM [TBA][BArF] under N2. ................................................................. 166 Figure A.32: X-Band EPR spectrum of {(P3B)Fe(NO)}9 in 2-MeTHF at 7 K and its simulation........................................................................................................... 169 Figure A.33: A molecular orbital diagram for {(P3B)Fe(NO)}8 ........................................ 174 Figure A.34: A molecular orbital diagram for {(P3B)Fe(NO)}10 ....................................... 175 Figure A.35: A molecular orbital diagram for {(P3B)Fe(NO)}9 ........................................ 176 Figure B.1: 1H NMR spectrum (300 MHz, DMSO-d6) of [14NH4][Cl] produced from catalytic N2-to-NH3 conversion conducted with [(P3B)Fe]+ catalyst, 54 equiv Cp*2Co, and 108 equiv [Ph215NH2][OTf] under an atmosphere of 14N2. ........ 186 Figure B.2: Mössbauer spectrum collected on [(P3B)57Fe]+ that was used for the Mössbauer experiments conducted in this paper. ................................................................ 192 Figure B.3: Mössbauer spectrum collected from a reaction freeze quenched after 5 minutes between P3BFe+ and excess Cp*2Co (20 equiv)................................................ 193 Figure B.4: Mössbauer spectrum collected from a reaction freeze quenched after 5 minutes between P3BFe+ and excess Cp*2Cr (20 equiv). ............................................... 194 Figure B.5: Mossbauer spectrum collected from a catalytic reaction freeze quenched after 5 minutes. .............................................................................................................. 195 Figure B.6: Mössbauer spectrum collected from a catalytic reaction freeze quenched after 30 minutes. ......................................................................................................... 196 Figure B.7: The X-band EPR spectrum in a 2-MeTHF glass of 2.3 mM [(P3B)Fe(N2)][Na(12-crown-4)2] at 77K. .......................................................... 199 Figure B.8: The X-band EPR spectrum in Et2O of 2.3 mM [(P3B)Fe][BArF4] at 77 K..... 200 xviii Figure B.9: The X-band EPR spectrum in Et2O of 46 mM Cp*2Co at 77K. ............. 200 Figure B.10: The X-band EPR spectrum in Et2O (1 mL) of the reaction between [(P3B)Fe]+ (3.5 mg, 0.0023 mmol) and Cp*2Co (15.2 mg, 0.046 mmol) stirred for 5 minutes at −78 °C then rapidly frozen to 77 K. .............................................................. 201 Figure B.11: The X-band EPR spectrum in Et2O (1 mL) of the reaction between [(P3B)Fe]+ (3.5 mg, 0.0023 mmol) and Cp*2Co (40.3 mg, 0.124 mmol) and [Ph2NH2][OTf] (79.4 mg, 0.248 mmol) stirred for 5 minutes at −78 °C then rapidly frozen to 77 K. ........................................................................................................................ 201 Figure B.12: 77 K X-band EPR spectrum of a toluene solution of Cp*2Co (green), 77 K powder X-Band EPR spectrum of the purple solid isolated from the reaction of Cp*2Co and HOTf as described in Section B2.2 (red), and 77 K powder X-Band EPR spectrum after annealing the EPR tube RT for two hours (blue). ........... 203 Figure B.13: Powder EPR spectrum at 77 K for the reaction of HOTf and Cp*2Co and its simulation........................................................................................................... 204 Figure B.14: Comparison of the EPR spectra obtained using HOTf and DOTf in the reaction with Cp*2Co. ..................................................................................................... 205 Figure B.15: EPR spectrum obtained when reacting DOTf with Cp*2Co and its simulation........................................................................................................... 205 Figure B.16: Calculated BDE vs literature BDE for the species shown in Table B.13.... 209 Figure B.17: Activation barrier for PCET reactions between selected (P3B)Fe(NxHy) species (as labeled, total spin-state of the surface in parenthesis) with [(Cp*)Co(η4C5Me5H)]+ as a function of outer-sphere reorganization energy. .................... 210 Figure C.1: Comparison of catalyzed and uncatalyzed H2 evolution from 2,6dichloroanlinium triflate and Cp*2Co at early time points. ............................. 222 Figure C.2: Mössbauer spectrum collected from a reaction freeze quenched after stirring for 5 minutes at −78 °C in 1 mL of Et2O between [P3B(57Fe)N2][Na(Et2O)3] and excess 2,6-[2,6-ClPhNH3][OTf] (50 equiv). ........................................................ 223 Figure C.3: The continuous wave, X-band EPR at 77K in Et2O of reaction mixtures freezequenched after five minutes. In red is the reaction of [(P3B)Fe(N2)][Na(Et2O)3] with 50 equiv of [2,6-ClPhNH3][BArF4] clearly demonstrating the formation of [(P3B)Fe(NNH2)][BArF4].18 In green is reaction of [(P3B)Fe(N2)][Na(Et2O)3] with 50 equiv of [2,6-ClPhNH3][OTf] in which the small residual species is neither the starting material ([(P3B)Fe(N2)][Na(Et2O)3]) or the desired product ([(P3B)Fe(NNH2)][OTf]). .................................................................................. 226 xix Figure C.4: In blue is the continuous wave, X-band EPR spectrum at 77K of a reaction mixture of 50 equiv [2,6-ClPhNH3][BArF4] with [(P3B)Fe(N2)][Na(12-crown-4)2] quenched with liquid nitrogen after 5 minutes. ................................................ 227 Figure C.5: Cyclic voltammograms of 0.5 mM [(P3B)Fe][BArF4] at varied scan rates (left) and plot of peak current versus square root of scan rate for each feature (right) showing linear dependence in all cases. ........................................................... 235 Figure C.6: Cyclic voltammograms of 0.5 mM [(P3B)Fe][BArF4] (red trace) and [(P3B)Fe][BArF4] with 10 equiv of tetrabutylammonium trifluoromethanesulfonate ([TBA][OTf]) (blue and gray traces). .................... 235 Figure C.7: BDFEcalc and BDFElit plotted for species of known BDFEE-H. Line of best fit is shown. ................................................................................................................ 239 Figure C.8: Cyclic voltammograms of 5 mM [Ph2NH2][OTf] (gray trace) and 5 mM [perCl PhNH3][OTf] (red trace) ................................................................................. 242 Figure C.9: XPS survey scan of a section of a glassy carbon plate which was not exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0) ................................................................................................................... 245 Figure C.10: XPS survey scan of a section of a glassy carbon plate which was exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0) ................................................................................................................... 247 Figure D.1: a) HYSCORE powder patterns ....................................................................... 256 Figure D.2: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5H).. 262 Figure D.3: 1H-COSY NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4C5Me5H). ........................................................................................................... 263 Figure D.4: 13C NMR spectrum (100 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5H). 263 Figure D.5: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5D).. 264 Figure D.6: 2H{1H} NMR spectrum (61.42 MHz, 90% C6H6:10% C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5D). ................................................................................ 265 Figure D.7: 13C{1H} NMR spectrum (100 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5D). ............................................................................................................................ 265 Figure D.8: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4C5Me513Me) ....................................................................................................... 266 xx Figure D.9: C NMR spectrum (100 MHz, C6D6, 25 °C) (Cp*)Co(exo-η C5Me513Me) ....................................................................................................... 266 13 4 Figure D.10: Reaction of (Cp*)Co(exo-η4-C5Me5H) with 4MeOTEMPO. .......................... 267 Figure D.11: 1H NMR spectrum (400 MHz, MeCN-d3, RT) of the reaction between (Cp*)Co(exo-η4-C5Me5H) and 4MeOTEMPO. ................................................... 268 Figure D.12: Zoomed in view of the 1H NMR (500 MHz, C7D8, −78 °C) of the reaction between (Cp*)Co(exo-η4-C5Me5H) and 2 equivalents of 4Me0TEMPO ........... 269 Figure D.13: 1H NMR spectrum (500 MHz, C7D8, RT) of the reaction between Cp*2Co and 4MeO TEMPO. ...................................................................................................... 270 Figure D.14: 1H NMR spectrum (500 MHz, C7D8, −78 °C) of the reaction between Cp*2Co and 4MeOTEMPO. ............................................................................................... 271 Figure D.15: Reaction scheme of Cp*2Co with 4-MeOTEMPO ........................................... 271 Figure D.16:13C NMR spectrum (125 MHz, C7D8, −78 °C) of between Cp*2Co and 4Me0 TEMPO ....................................................................................................... 272 Figure D.17: Zoomed in data from the 1H-ROESY NMR (500 MHz, C7D8, −78 °C) of the reaction between Cp*2Co and 4Me0TEMPO. ..................................................... 273 Figure D.18: 1H NMR spectrum (400 MHz, C7D8, RT) of the reaction between octamethylcobaltocene (4 mg, 0.013 mmol, 1 eq) and 4MeOTEMPO (2.5 mg, 0.013 mmol, 1 eq). ............................................................................................. 273 Figure D.19: 1H NMR (400 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exo-η4C5Me5H) with one atmosphere of CO2 ............................................................. 275 Figure D.20: 1H NMR (400 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exo-η4C5Me5H) with 4 equivalents of [Pt(dmpe)2][PF6]2 ........................................... 276 Figure D.21: 31P{1H} NMR (162 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exo-η4C5Me5H) with 4 equivalents of [Pt(dmpe)2][PF6]2 ........................................... 276 Figure D.22: Q-band inversion recovery traces of endo- (black) and exo-isomers (red) of Cp*2Co protonation ........................................................................................... 277 Figure D.23: Pseudomodulated Q-band ESE-EPR spectrum at 10 K of a coal standard sample ................................................................................................................ 278 Figure D.24: X-Band CW EPR spectrum (blue) at 77 K in the solid state of the precipitate from the reaction of bis(trifluoromethanesulfon)imide) with Cp*2Co at −78 °C in toluene............................................................................................................ 281 xxi Figure D.25: A comparison of the 77 K, X-band EPR spectra of solid samples of [(Cp*)Co(exo-η4-C5Me5H)][BArF] (blue) and [(Cp*)Co(exo-η4C5Me5D)][BArF] (orange). ............................................................................... 282 Figure D.26: A depiction of our mechanistic proposal to account for observations upon the reaction of (Cp*)Co(exo-C5Me5D) with HBArF4. ............................................ 283 Figure D.27: A comparison of the data to the simulation (green) for the 77 K, X-band EPR spectra of solid samples of [(Cp*)Co(exo-η4-C5Me5H)][BArF] (blue) and [(Cp*)Co(exo-η4-C5Me5D)][BArF] (orange). .................................................. 284 Figure D.28: X-Band CW EPR spectrum (blue) at 77 K in the solid state of the precipitate from the reaction of trifluoromethanesulfonic acid with Cp*2Co at −78 °C in toluene. ............................................................................................................... 285 Figure D.29: Cyclic voltammogram of (Cp*)Co(η4-C5Me6) at room temperature ........... 285 Figure D.30: Scan rate dependence of the [(Cp*)Co(η4-C5Me6)]+/0 at room temperature 286 Figure D.31: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at room temperature ........................................................................................................ 286 Figure D.32: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at room temperature at different scan rates in a butyronitrile solution of 0.4 M [TBA][PF6]. ............. 287 Figure D.33: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at −78 °C ................ 288 Figure D.34: Comparison of cyclic voltammograms taken at room temperature (~22 °C) and at −78 °C. ........................................................................................................... 288 Figure D.35: Cyclic voltammograms of Cp*2Co at room temperature ............................. 289 Figure D.36: Thin film IR spectra of (Cp*)Co(exo-η4-C5Me5H) (blue) and (Cp*)Co(exo-η4C5Me5D) (orange).............................................................................................. 291 Figure D.37: Solution state IR spectra of (Cp*)Co(exo-η4-C5Me5H) (blue) and (Cp*)Co(exo-η4-C5Me5D) (orange) in C6D6. ................................................... 292 Figure D.38: Thin film IR spectrum of the reaction between (Cp*)Co(exo-η4-C5Me5H) and CO2 demonstrating the formation of [Cp*2Co][HCO2]. .................................. 292 Figure D.39: Measured thermochemical parameters in acetonitrile. Potentials in V vs Fc+/0. ................................................................................................................... 293 Figure D.40: Describes the process of determining the C–H BDFE in [(Cp*)Co(exo-η4C5Me5H)]+ using the measured hydricity of (Cp*)Co(exo-η4-C5Me5H) and the redox potential of (Cp*)Co(exo-η4-C5Me5H). .................................................. 293 4 xxii Figure D.41: Describes the process of determining the C–H BDFE in [(Cp*)Co(exo-η C5Me5H)]+ using the measured BDFE C–H of (Cp*)Co(exo-η4-C5Me5H) and the redox potential of (Cp*)Co(exo-η4-C5Me5H) and Cp*2Co. ............................. 294 Figure E.1: 1H NMR spectrum of Cp(η4-C5H5PhNMe2)Co in C6D6, 400 MHz, 298 K. .. 313 Figure E.2:13C{1H} NMR spectrum of Cp(η4-C5H5PhNMe2)Co in C6D6, 100 MHz, 298 K. ........................................................................................................................ 314 Figure E.3: 1H NMR spectrum of (Cp)Co(CpN) in CD3CN, 400 MHz, 298 K. ............... 314 Figure E.4: 1H NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 400 MHz, 298 K. .. 315 Figure E.5: 19F NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 376 MHz, 298 K. .. 315 Figure E.6: 13C{1H} NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 100 MHz, 298 K. ........................................................................................................................ 316 Figure E.7: 1H NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 400 MHz, 298 K. 316 Figure E.8: 19F NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 376 MHz, 298 K. ........................................................................................................................ 317 Figure E.9: 13C{1H} NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 100 MHz, 298 K. ........................................................................................................................ 317 Figure E.10: UV-Vis spectrum at room temperature for [(Cp)Co(CpNH)][OTf]2 in 2MeTHF:THF solution (4:1)............................................................................... 318 Figure E.11: UV-Vis spectra of the reaction of trifluoromethanesulfonimide and (Cp)Co(CpN) at −130 °C in 2 UV-Vis spectrum in 2-MeTHF:THF solution (4:1) ............................................................................................................................ 318 Figure E.12: UV-Vis spectrum at room temperature for [(Cp)Co(CpNH)][OTf]2 in acetonitrile. ........................................................................................................ 319 Figure E.13:(blue) X-band EPR spectrum of a 1 mM solution of (Cp)Co(CpN) in a 2methyltetrahydrofuran glass at 77 K. ................................................................ 319 Figure E.14: (blue) X-band EPR spectrum of a 1 mM solution of [(Cp)Co(CpNH)][OTf] in a 2-methyltetrahydrofuran glass at 77 K generated via protonation of (Cp)Co(CpN). (red) X-band EPR spectrum of a 1 mM solution of [(Cp)Co(CpNH)][OTf] in frozen acetonitrile at 77 K generated via reduction of [(Cp)Co(CpNH)][OTf]2. ..................................................................................... 321 Figure E.15: IR spectrum of [(Cp)Co(CpNH)][OTf]2 in a KBr pellet. Weak N−H vibration is evident at 3430 cm−1. ..................................................................................... 322 xxiii Figure E.16: Variable scan rate cyclic voltammograms with a glassy carbon working electrode of a 1 mM solution of the (Cp)Co(CpN)+/0 in acetonitrile with 100 mM [TBA][PF6]. ....................................................................................................... 324 Figure E.17: Variable scan rate cyclic voltammograms with a glassy carbon working electrode of a 1 mM solution of the [(Cp)Co(CpN)]2+/+ in acetonitrile with 100 mM [TBA][PF6]. ............................................................................................... 324 Figure E.18: Variable scan rate cyclic voltammograms with a boron doped diamond working electrode of a 1 mM solution of [(Cp)Co(CpNH)][OTf]2 and 1 mM solution of ferrocene in acetonitrile with 100 mM [TBA][PF6]. ..................... 325 Figure E.19: Cyclic voltammogram of a glassy carbon working electrode of a 1 mM solution of the (Cp)Co(CpN)+/0 in DME with 100 mM [TBA][PF6].............................. 325 Figure E.20: Cathodic scan of CV’s at 100 mV/s of different concentrations of [(Cp)Co(CpN)][OTf] on a BDD working electrode of a DME solution of 100 mM [4-CNPhNH3][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6]. ............ 326 Figure E.21: Plot of plateau current (−1.314 V vs Fc+/0) against concentration of [(Cp)Co(CpN)][OTf] for the CV’s shown in Figure E.20 with a line of best fit. ....................................................................................................................... 326 Figure E.22: Cathodic scan of CV’s at 10 mV/s of different concentrations of acetophenone on a BDD working electrode of a DME solution of 100 mM [4-CNPhNH3][OTf], 1 mM [(Cp)Co(CpN)][OTf], and 200 mM [TBA][PF6]. .................................. 327 Figure E.23: Plot of plateau current (−1.314 V vs Fc+/0) against concentration of acetophenone for the CV’s shown in Figure E.22 with a line of best fit. ....... 327 Figure E.24: Comparison of CV’s on BDD working electrode at 10 mV/s with 1 mM [(Cp)Co(CpN)][OTf], 100 mM [4-CNPhNH3][OTf], and 200 mM [TBA][PF6] with variable amount of acetophenone (0 mM, blue) vs (50 mM, red). .......... 328 Figure E.25: Differential pulse voltammogram of a DME solution containing 1 mM ferrocene, 1 mM [(Cp)Co(CpN)][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6]. ....................................................................................................... 328 Figure E.26: CV at 100 mV/s of a DME solution containing 1 mM ferrocene, 1 mM [(Cp)Co(CpN)][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6]. ....... 329 Figure E.27: 400 MHz 1H NMR spectrum of 4-chloroaniline (4-ClPhNH2) in MeCN-d3 at room temperature............................................................................................... 334 Figure E.28: 400 MHz 1H NMR spectrum of 4-chloroanilinium triflate ([4-ClPhNH3][OTf]) in MeCN-d3 at room temperature. .................................................................... 335 1 2-Cl xxiv Figure E.29: 400 MHz H NMR spectrum of 2-chloroaniline ( PhNH2) in MeCN-d3 at room temperature. .......................................................................................... 335 Figure E.30: 400 MHz 1H NMR spectrum of 2-chloroanilinium triflate ([2-ClPhNH3][OTf]) in MeCN-d3 at room temperature. .................................................................... 336 Figure E.31: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline in MeCN-d3 at room temperature.................................... 337 Figure E.32: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline in MeCN-d3 at room temperature.................................... 338 Figure E.33: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 2-chloroaniline in MeCN-d3 at room temperature.................................... 339 Figure E.34: Thermochemical relationships in acetonitrile for cobaltocene species in this study. .................................................................................................................. 341 Figure E.35: Calibration curve used for GC-TCD detection of H2. .................................. 343 Figure E.36: Calibration curve used for GC-FID quantification of pinacol (2,3-diphenylbutanediol). ........................................................................................................ 343 Figure E.37: Calibration curve used for GC-FID quantification of ketone (acetophenone)................................................................................................... 344 Figure E.38: CV’s relevant to the set-up of Entry 1 in Table E.14. .................................. 345 Figure E.39: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment ........................................................................................... 346 Figure E.40: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 1). ............................................................................................................. 346 Figure E.41: First derivative of DPV taken after the CPE experiment (Table E.14, Entry 1). ............................................................................................................................ 347 Figure E.42: XPS of the BDD electrode in the Co 2p region ............................................ 347 Figure E.43: XPS of the BDD electrode in the S 2p region, showing no evidence of S deposition after the CPE experiment (Table E.14, Entry 1). Sulfur is found in both the triflate counteranion and the tosic acid. .............................................. 348 Figure E.44: XPS of the BDD electrode in the N 1s region .............................................. 348 Figure E.45: CV’s relevant to the set-up of Entry 2 in Table E.14. .................................. 349 xxv Figure E.46: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 2). ..................................................... 349 Figure E.47: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 2). ............................................................................................................. 350 Figure E.48: First derivative of DPV taken after the CPE experiment (Table E.14, Entry 2). ............................................................................................................................ 350 Figure E.49: CV’s relevant to the set-up of Entry 3 in Table E.14. .................................. 351 Figure E.50: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 3). ..................................................... 351 Figure E.51: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 4) .............................................................................................................. 352 Figure E.52: CV’s relevant to the set-up of Entry 4 in Table E.14. .................................. 352 Figure E.53: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 4). ..................................................... 353 xxvi LIST OF EQUATIONS Eq 1.1) η = Eapp – Emin ............................................................................................................ 12 Eq 1.2) BDFEeff = 1.37×pKa + 23.06×E° + CG ..................................................................... 13 Eq 1.3) ΔΔGf = 3×(BDFE(H2)/2−BDFEeff) .......................................................................... 13 Eq 1.4) BDEeff = 1.37×pKa + 23.06×E° + CH........................................................................ 13 Eq 1.5) ΔΔHf = 3×(BDE(H2)/2−BDEeff) ............................................................................... 13 Eq 3.1) BDEeff = 1.37×pKa + 23.06×E° + Ch ........................................................................ 39 Eq 3.2) ΔΔHf = 3×(BDEeff – BDE(H2)/2) ............................................................................. 39 Eq 3.3) (P3B)Fe(N2) + e−  [(P3B)Fe(N2)]− ....................................................................... 46 Eq 3.4) [(P3B)Fe(N2)]− + H+  (P3B)Fe(NNH) ................................................................. 46 Eq 3.5) (P3B)Fe(N2) + H+  [(P3B)Fe(NNH)]+ ................................................................. 46 Eq 3.6) [(P3B)Fe(NNH)]+ + e-  (P3B)Fe(NNH) ............................................................... 46 Eq 3.7) (P3B)Fe(N2) + H•  (P3B)Fe(NNH) ..................................................................... 46 Eq 3.8) (P3B)Fe(N2) + [(Cp*)Co(η4-C5Me5H)]+  (P3B)Fe(NNH) + [Cp*2Co]+................. 52 Eq 4.1) [(P3B)Fe(N2)]− + H+  (P3B)Fe(NNH) ................................................................. 66 Eq 4.2) (P3B)Fe(NNH) + H+  [(P3B)Fe(NNH2)]+ ........................................................... 66 Eq 4.3) (P3B)Fe(NNH)  (P3B)Fe(N2) + 1/2 H2 .............................................................. 66 Eq 4.4) PhNH2 + [2,6-ClPhNH3]+  [PhNH3]+ + 2,6-ClPhNH2 .......................................... 68 Eq 4.5) pKa([PhNH3+]) = −ΔGa/(2.303×RT) + pKa([2,6-ClPhNH3]+) ................................. 68 Eq 4.6) PhNH2 + [2,6-ClPhNH3][OTf]  [PhNH3][OTf] + 2,6-ClPhNH2 ........................... 68 Eq 4.7) pKd([PhNH3][OTf]) = −ΔGd/(2.303×RT) + pKa([2,6-ClPhNH3]+) ......................... 68 Eq 4.8) Keq = ([[(RPhNH2-(Cp*)Co(exo-η4C5Me5H)][OTf]])/ ............................................. 69 Eq 4.9) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  ........................................... 71 Eq 4.10) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf] ......................................... 71 xxvii Eq 4.11) [(P3B)Fe(NNH2)][OTf] + Cp*2Co  (P3B)Fe(NNH2) + [Cp*2Co][OTf] ..... 71 Eq 5.1) (Cp*)Co(exo-η4-C5Me5H) + 4MeOTEMPO•  Cp*2Co + 4MeOTEMPO–H .......... 102 Eq 5.2) Cp*2Co + 4MeOTEMPO•  (Cp*)Co(η4-C5Me4CH2) + 4MeOTEMPO–H ............. 102 Eq 6.1) BDFE = 1.37×pKa + 23.06×E0 + CG ..................................................................... 123 Eq 6.2) BDFEeff = 1.37×pKa(acid) + 23.06×E0(reductant) + CG ........................................ 132 Eq 6.3) Eonset (vs NHE) = −0.99 + 0.60×pH ........................................................................ 134 Eq B.1) ∆Gsolv ≈ Esoln – Egas .................................................................................................. 182 Eq B.2) Gsoln = Ggas + ∆Gsolv ................................................................................................ 182 Eq B.3) ∆G = −nFE0 ............................................................................................................ 182 Eq B.4) ΔG = ΔH − TΔS ...................................................................................................... 207 Eq B.5) ΔH = BDE(H2) − 2×BDE([(Cp*)Co(η4-C5Me5H)]+) ............................................ 207 Eq B.6) TΔS = TΔSH − 2×TΔSH = −TΔSH = −4.6 kcal mol−1 ............................................. 207 Eq B.7) 105.8 kcal mol−1 − 2×BDE([(Cp*)Co(η4-C5Me5H)]+) + 4.6 kcal mol−1............... 207 Eq B.8) ⸫ BDE(([(Cp*)Co(η4-C5Me5H)]+) < 50.6 kcal mol−1 ........................................... 207 Eq C.1) ∆Gsolv ≈ Esoln – Egas.................................................................................................. 216 Eq C.2) Gsoln = Ggas + ∆Gsolv ................................................................................................ 216 Eq C.3) ∆G = −nFE0 ............................................................................................................ 216 Eq C.4) λis,PT/PCET = Σj (fjrfjp)/(fjr+fjp)×Δqj2 .......................................................................... 236 Eq C.5) λos,PCET = λos,PT + λos,ET – (λos,PT* λos,ET)cos(θ) ................................................... 237 Eq C.6) kbi = KA*kuni ........................................................................................................... 237 Eq C.7) [2,6-ClPhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + .................... 237 Eq C.8) [2,6-MePhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + ................... 238 Eq C.9) [4-OMePhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + ................... 238 Eq C.10) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  ...................................... 238 Eq C.11) [(P3B)Fe(NNH2)][OTf] + Cp*2Co  (P3B)Fe(NNH2) + [Cp2*Co][OTf] .......... 238 xxviii Eq C.12) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  ............................. 238 Eq C.13) pKa(THF) = 0.78×pKa(MeCN) − 0.52 ................................................................. 247 Eq C.14) pKa(THF) = 1.19×pKa(H2O) + 2.13 ..................................................................... 247 Eq D.1) ν± = |A/2 ± νN| .......................................................................................................... 254 Eq D.2) ν±,mI = |νN ± 3P(2mI−1)/2| ....................................................................................... 254 Eq D.3) H = μB B0 gS +μNgNB0I +hSAI+hIPI..................................................................... 256 Eq D.4) 𝑷 = 𝑃𝑥𝑥000𝑃𝑦𝑦000𝑃𝑧𝑧 = 𝑒2𝑄𝑞/ℎ4𝐼(2𝐼 − 1) − (1 − 𝜂)000 − (1 + 𝜂)0002............................................................................................................... 257 Eq D.5) e2qQ/h = 2I(2I−1)Pzz .............................................................................................. 257 Eq D.6) η = (Pxx−Pyy)/Pzz ..................................................................................................... 257 Eq D.7) Mz = 0, T ≈ ln(2)×T1’ ............................................................................................. 278 Eq D.8) ΔG = (G(X•) + G(H•)) – G(X–H) .......................................................................... 296 Eq D.9) BDFE = 0.996 × ΔG + 4.376 ................................................................................. 296 Eq E.1) CSCo = CSNH×y+ CSN×(1−y) ................................................................................. 330 Eq E.2) CSAcid/Base = CSAcid×y+ CSBase×(1−y)..................................................................... 330 Eq E.3) [Acid] = y×Integral ................................................................................................. 330 Eq E.4) Keq = ([Acid][(Cp)Co(CpN)]+])/([Base][[(Cp)Co(CpNH)]2+)................................. 330 Eq E.5) ΔG = RT×ln(Keq) .................................................................................................... 330 Eq E.6) pKa([(Cp)Co(CpNH)]2+) = Reference + ΔG/1.37 ................................................... 330 Eq E.7) BDFE = 1.37×pKa + 23.06×E°1 + CG .................................................................... 340 Eq E.8) BDFE1 = 1.37×8.6 + 23.06×−1.21 + 54.9 ............................................................. 340 Eq E.9) BDFE1 = 38.8 kcal∙mol−1 ....................................................................................... 340 Eq E.10) BDFE = 1.37×pKa + 23.06×E°2 + CG .................................................................. 340 Eq E.11) BDFE2 = 1.37×8.6 + 23.06×0.54 + 54.9.............................................................. 340 Eq E.12) BDFE2 = 79.1 kcal∙mol−1...................................................................................... 340 xxix Eq E.13) ΔG(H ) = 1.37×pKa + 23.06×E°1 + 23.06×E°2 + CHyd ................................. 340 − Eq E.14) ΔG(H−) = 1.37×8.6 + 23.06×0.54 + 23.06×−1.21 + 79.6 ................................... 340 Eq E.15) ΔG(H−) = 75.9 kcal∙mol−1..................................................................................... 340 Eq E.16) BDFE = 1.37×pKa + 23.06×E°3 + CG .................................................................. 340 Eq E.17) pKa = (BDFE − 23.06×E° − CG)/1.37 .................................................................. 340 Eq E.18) pKa = (38.8 – 23.06×−1.35 – 54.9)/1.37 .............................................................. 340 Eq E.19) pKa = 11.0 ............................................................................................................. 340 xxx LIST OF TABLES Table 3.1: N2-to-NH3 Conversion with (P3E)M Complexes (M = Fe, Co) .......................... 41 Table 3.2: Calculated pKa Values and BDEs of Selected Species ....................................... 46 Table 4.1. Literature and calculated pKa values and efficiencies observed in catalytic N2-toNH3 conversion.................................................................................................... 64 Table 4.2. Yields and Faradaic Efficiencies of NH3 from CPE Experiments with [(P3B)Fe]+ ............................................................................................................. 76 Table 6.1. Results from CPE experiments on a DME solution of 200 mM [TBA][PF6], 100 mM tosic acid, 50 mM acetophenone at −1.45 V vs Ag+/0 .............................. 127 Table A.1: Comparison of selected data for {(P3B)Fe(NO)}9 and [Cr(CN)5(NO)]3-.18 ..... 158 Table A.2: X-ray parameters for {(P3B)Fe(NO)}9, and {(P3B)Fe(NO)}10. ........................ 166 Table A.3: X-ray parameters for {(P3B)Fe(NO)}8. ............................................................. 168 Table A.4: A comparison of calculated (BP86, low-spin) and experimental bond parameters ............................................................................................................................ 171 Table A.5: Spin ladder energies for different functionals for {(P3B)Fe(NO)}8-10. Broken symmetry solutions in which an S = 1 NO- unit is antiferromagnetically coupled to the metal are indicated by BS. ...................................................................... 171 Table A.6: Calculated Mössbauer parameters for the spin ladder of {(P3B)Fe(NO)}8-10. . 173 Table A.7: Comparison of crystallographically determined bond lengths and Wiberg Bond Indices for {(P3B)Fe(NO)}8-10. .......................................................................... 175 Table B.1: UV-vis quantification results for standard NH3 generation experiments with [(P3B)Fe]+ ........................................................................................................... 184 Table B.2: UV-vis quantification results for standard NH3 generation experiments with (P3Si)Fe(N2) ........................................................................................................ 185 Table B.3: UV-vis quantification results for standard NH3 generation experiments with [(P3B)Co(N2)]− ................................................................................................... 185 Table B.4: UV-vis quantification results for standard NH3 generation experiments with (P3Si)Co(N2) ....................................................................................................... 186 xxxi Table B.5: UV-vis quantification results for NH3 generation experiments with [(P3B)Fe]+, with reloading.................................................................................. 187 Table B.6:Time-resolved H2 quantification for the reaction of Cp*2Co and acid in Et2O at 78 °C in the absence of an Fe precatalyst ......................................................... 188 Table B.7: Fit parameters for [(P3B)57Fe]+ .......................................................................... 192 Table B.8: Simulation parameters for Mossbauer spectrum in Figure B.3....................... 193 Table B.9: Simulation parameters for Mossbauer spectrum in Figure B.4....................... 195 Table B.10: Simulation parameters for Mossbauer spectrum in Figure B.5..................... 196 Table B.11: Simulation parameters for Mossbauer spectrum in Figure B.6..................... 197 Table B.12: A comparison of the g-tensors for the authentic sample of [(P3B)Fe(N2)]− (Figure B.7), the freeze quench of the reaction with the reductant (Figure B.10), the freeze quench of the catalytic reaction mixture (Figure B.11) .................. 202 Table B.13: Calculated ΔH values and literature BDE values used for BDE calibration . 208 Table B.14: Calculated entropy (S) for selected XH and X• species ................................. 209 Table B.15: Calculated Reorganization Energies, Free-Energies of Reaction and Activation Barriers for Selected PCET Reactions .............................................................. 210 Table C.1: NMR quantification results for standard NH3 generation experiments with [(P3B)Fe][BArF4] ................................................................................................ 219 Table C.24: Data for Background H2 Quantification Experiments ................................... 220 Table C.25: Time points for catalyzed H2 evolution from 2,6-dichloroanlinium triflate and Cp*2Co ............................................................................................................... 221 Table C.26: Time points for uncatalyzed H2 evolution from 2,6-dichloroanlinium triflate and Cp*2Co ............................................................................................................... 221 Table C.5: Simulation parameters for Mossbauer spectrum in Figure C.2. ..................... 224 Table C.6: Comparative NH3 and H2 Yields for [2,6-ClPhNH3][OTf] and [2,6-ClPhNH3][BArF4] ............................................................................................................................ 228 Table C.7: Controlled Potential Electrolysis Data .............................................................. 232 Table C.8: Overview of Parameters Used to Calculate Kinetic Barriers ........................... 238 xxxii Table C.9: Data used to generate the plot and line of best fit shown in Figure C.7 . 239 Table C.10: Calculated Reduction Potentials of Selected Species .................................... 240 Table C.11: pKa values of anilinium acids in different solvents ........................................ 247 Table D.1: Simulation parameters derived from the ENDOR, HYSCORE, and CW-EPR spectroscopy. ..................................................................................................... 279 Table D.2: Crystallographic details for (Cp*)Co(exo-η4-C5Me5H) and (Cp*)Co(η4-C5Me6). ............................................................................................................................ 289 Table D.3: Thermochemistry for the structures optimized with different functionals. ..... 294 Table D.4: Thermochemistry for the different potential isomers of neutral, H-functionalized Co species. ......................................................................................................... 295 Table D.5: Thermochemistry for the different potential isomers resulting from the protonation of (Cp*)Co(exo-η4-C5Me5H). ....................................................... 295 Table D.6: Gas phase Gibbs Free Energies for species necessary for determining the thermochemical parameters. ............................................................................. 297 Table D.7: Particular thermochemical properties of interest. ............................................. 297 Table D.8: Effect of functional on structure optimization. ................................................. 298 Table D.9: Effect of functional on the calculation of EPR parameters. ............................. 298 Table D.10: Higher level EPR calculations. ....................................................................... 300 Table E.1: X-ray crystallography details for (Cp)Co(CpN), [(Cp)Co(CpNH)][OTf], and [(Cp)Co(CpN)][OTf].......................................................................................... 322 Table E.2: NMR shifts of pure compounds used in the pKa determination experiments. . 330 Table E.3: NMR data for the cobalt center during titration of [(Cp)Co(CpNH)][OTf]2 with 4chloroaniline ...................................................................................................... 330 Table E.4: NMR data for the aniline/anilinium during titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline........................................................................................... 331 Table E.5: Equilibrium Constants derived from data in Table E.3 and Table E.4. ......... 331 Table E.6: NMR data for the cobalt center during second titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline........................................................................................... 332 xxxiii Table E.7: NMR data for the aniline/anilinium during second titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline .................................................... 332 Table E.8: Equilibrium Constants derived from data in Table E.6 and Table E.7. ......... 332 Table E.9: NMR data for the cobalt center during titration of [(Cp)Co(CpNH)][OTf]2 with 2chloroaniline ...................................................................................................... 333 Table E.10: NMR data for the aniline/anilinium during titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline........................................................................................... 333 Table E.11: Equilibrium Constants derived from data in Table E.9 and Table E.10. ..... 333 Table E.12: Final equilibrium constants from data in Table E.5, Table E.8, and Table E.11 ............................................................................................................................ 334 Table E.13: DFT calculated thermochemical parameters for parent cobaltocene and baseappended cobaltocenes. ..................................................................................... 341 Table E.14: Results from CPE experiments under the standard conditions described using the procedures described in Sections E.1.5 and E.1.6 ..................................... 344 xxxiv NOMENCLATURE A. Hyperfine tensor aiso. Isotropic hyperfine value ADP. Adenosine diphosphate ATP. Adenosine triphosphate. Avg. Average [BArF4]−. Tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate BDD. Boron-doped diamond BDE. Bond dissociation enthalpy BDFE. Bond dissociation free energy B0. Magnetic Field BS. Broken symmetry CG. Thermodynamic constant accounting for the free energy of forming H• from H+ and e− CH. Thermodynamic constant accounting for the enthalpy of forming H• from H+ and e− cm−1. Wavenumbers Cp. Cyclopentadienide Cp2Co. Cobaltocene Cp2Fe. Ferrocene Cp2Ni. Nickelocene Cp*. Pentamethylcyclopentadienide Cp*2Co. Decamethylcobaltocene Cp*2Cr. Decamethylchromocene xxxv Cp*2Ni. Decamethylnickelocene CPE. Controlled potential electrolysis CPET. Concerted proton electron transfer CW. Continuous wave CV. Cyclic voltammogram °C. Degrees Celsius δ. Isomer shift Δeq. Quadrupole splitting ΔΔGf. Excess energy relative to the free energy of formation ΔG. Gibbs free energy ΔG‡. Gibbs free energy of the transition state ΔGf. Gibbs free energy of formation ΔG(H−). Gibbs free energy of hydricity ∆Gsolv. Free energy of solvation ∆S. Change in entropy ΔΔHf. Change in enthalpy of formation DFT. Density functional theory dmpe. Dimethylphosphinoethane DMSO. Dimethylsulfoxide depe. Diethylphosphinoethane DME. Dimethoxyethane DNA. Deoxyribose nucleic acid xxxvi DOTf. Deuterium-labeled trifluoromethanesulfonic acid e. Electron charge E. Reduction potential e−. Electron EC. Electrochemical-chemical efg. Electric field gradient Eg. Gas phase internal energy ENDOR. Electron nuclear double resonance Eq. Equation equiv. Equivalent EPR. Electron paramagnetic resonance ESE. Electron spin echo Esoln. Solution phase internal energy ET. Electron transfer Et2O. Diethyl ether eV. Electron volt η. Overpotential η. Rhombicity F. Faraday’s constant Fc. Ferrocene FE. Faradaic efficiency 4-MeOTEMPO⦁. 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy 4-MeOTEMPO−H. 1-hydroxy,4-methoxy-2,2,6,6-tetramethyl-1-piperidine GC. Glassy carbon gn. Nuclear g-value Ggas. Gas phase free energy Gsoln. Solution phase free energy h. Planck’s constant H•. Hydrogen atom H−. Hydride H+. Proton 1H. Hydrogen 2H. Deuteron HAT. Hydrogen atom transfer HER. Hydrogen evolution reaction HIPTN3N. [(3,5-(2,4,6- iPr3C6H2)2C6H3NCH2CH2)3N]3− HNTf2. Bistrifluoromethanesulfonimide HOTf. Trifluoromethanesulfonic acid HYSCORE. Hyperfine sub-level correlation I. Nuclear spin IR. Infrared IRC. Internal reaction coordinate K. Kelvin kcal. Kilocalorie xxxvii xxxviii Keq. Equilibrium constant krel. Relative rate constant KC8. Potassium Graphite λ. Wavelength λOS. Outer sphere reorganization energy Me. Methyl MeCN. Acetonitrile MeCy. Methylcyclohexane MHz. Megahertz μB. Bohr magneton μN. Nuclear magneton μL. Microliters μmol. Micromoles mL. Milliliters mm. Millimeters mol. Moles ms. Milliseconds MSE. Mean signed error mT. Milliteslas MUE. Mean unsigned error mV. Millivolts MS-PCET. Multi-site proton-coupled electron transfer xxxix NADH. Nicotinamide adenine dinucleotide NADPH. Nicotinamide adenine dinucleotide phosphate n. Number of moles nm. Nanometers NMR. Nuclear magnetic resonance NQI. Nuclear quadrupole interaction ns. Nanosecond N2RR. Nitrogen reduction reaction ν. Frequency νN. Larmor frequency [OTf]−. Trifluromethanesulfonate P. Nuclear quadrupole interaction PCET. Proton-coupled electron transfer [PF6]−. Hexafluorophosphate Ph. Phenyl [Ph2NH2]+. Diphenylammonium Pi. Phosphate PT. Proton transfer P3B. Tris(o-diisopropylphosphinophenyl)-borane P3Si. Tris(o-diisopropylphosphinophenyl)-silyl q. Electric field gradient Q. Electric quadrupolar moment xl R. Gas constant RF. Radio frequency RNA. Ribose nucleic acid [RPhNH3]+. Anilinium substituted on the phenyl ring by an R group rpm. Rotations per minute s. Seconds S. Electron spin SOMO. Singly occupied molecular orbital srt. Shot repetition time T. Temperature T. Anisotropic hyperfine tensor τ. Fixed delay time T1’. Apparent relaxation rate [TBA]+. Tetrabutylammonium tBu. Tert-butyl THF. Tetrahydrofuran 2-MeTHF. 2-methyltetrahydrofuran UV-Vis. Ultraviolet-visible V. Volt XRD. X-ray diffraction XPS. X-ray photoelectron spectroscopy XSEDE. Xtreme Science and Engineering Discovery Environment 1 Introduction 2 1.1 Opening Remarks This thesis concerns itself with the proton-coupled reduction of dinitrogen (N2) to ammonia (NH3). Throughout the following chapters you will see this process referred to as nitrogen fixation, N2-to-NH3 conversion, and the nitrogen reduction reaction (N2RR). This reaction has motivated this work because of its critical importance to human and environmental health. Although dinitrogen is the largest component of our atmosphere (~80%) and is a key component of many biomolecules (i.e., DNA, RNA, amino acids, etc.), it cannot be directly incorporated into these biomolecules but must first be fixed into a chemically reactive form, NH3.1 Biologically this fixation is achieved by a highly conserved family of nitrogenase enzymes which feature, I would argue, the most complicated transition metal architectures known in biology (Figure 1.1). Metalloenzymes typically operate at low overpotential and with high selectivity, and thus the significant excess energetic demand (fixation of 1 N2 requires the hydrolysis of 14 ATP) and the poor selectivity (under typical conditions ~2 equivalents of H2 are produced for every NH3) demonstrates both the significant challenge presented by this reaction and its critical importance to the organism(s).2 Energy demand and selectivity of N2RR will form key threads through this work. 3 Figure 1.1: Depiction of the active site of the FeMo nitrogenase (PDB: 1M1N)3 with the idealized stoichiometry for biological nitrogen fixation. 2 Industrially, nitrogen fixation is (almost) exclusively achieved through the HaberBosch process in which NH3 is formed via the high-temperature and high-pressure combination of H2 with N2 on an Fe surface in the presence of promoters. The increased access to fixed nitrogen provided by the discovery of Haber-Bosch in the early 20th century truly transformed the world.4 It resulted not only in us fundamentally altering the global biogeochemical nitrogen cycle, but also, via the population boom it enabled, the carbon cycle. The environmental consequences of the disruption of these key biogeochemical cycles will continue to play out for millennia.5 The importance of nitrogen fixation would easily motivate study of either of these two globally-important NH3-producing systems, biological or industrial. The nitrogen fixing system that we have chosen to study is far removed from both of these in both its atomistic structure and its practicality, although certain design elements are drawn from the active site 4 cluster in biological nitrogenases. These design elements include the metal identity (Fe), the symmetry (threefold), and the flexible anchoring atom (Figure 1.2). Nonetheless, the catalyst is clearly bio-inspired rather than bio-mimetic. One of the key analogies to the biological system is our choice to use protons and electrons (rather than H2) to reduce N2. Indeed, the work presented here is largely focused on the mechanisms by which protons and electrons can be combined to form N–H (or O–H) bonds. Figure 1.2: Molecular nitrogen fixation catalysts used in this thesis. They feature three inplane phosphine donors and a variable anchoring atom, E (E = B, C, Si). The ratio of NH 3 to H2 depends on the exact conditions (i.e., temperature, solvent, acid, reductant). We have chosen to study the mechanism of X–H bond formation, because we anticipate that combining protons with electrons for X–H bond formation will be one of the defining chemical challenges of the coming decades.6 Humans have long used X–H bonds to store, transport, and release energy. Indeed, it is the abundance of energy-rich C–H bonds in biomass such as wood, coal, or oil that makes them such an excellent source of energy: dense and easily-controlled. As energy is increasingly derived from renewable sources such as solar, wind, and hydro, the relevant energy currency will increasingly be electrons. Only by pairing these electrons with protons to form new energy-rich X–H bonds will we be able 5 to match the benefits in terms of storage, transportation, and controlled release that characterize our current carbon-based fuel cycle.7 The conversion of N2 to NH3 is thus a fertile training ground for the chemist, as it requires the coordination of 6 protons and 6 electrons to form the energy-rich N–H bonds. For reactions of biogeochemical relevance, this is matched only by sulfite (SO32−) reduction to sulfide (S2−)8 and nitrite (NO2−) reduction to ammonia.9 Furthermore, no substrate is as resistant to initial functionalization as N2. N2 has an electron affinity lower than that of the noble gases. It has a proton affinity less than methane.10 N2 is so unreactive that the addition of a free hydrogen radical to form NNH, the first intermediate of nitrogen fixation, is unfavorable.11 Thus understanding the mechanism by which we can use transition metals in the presence of protons and electrons to functionalize N2 is a useful whetstone for the chemist interested in the problem of X–H bond formation. Furthermore, NH3 is a promising fuel candidate with high energy-density, well-developed infrastructure for transportation, and the potential to be carbon-free.12 Rather than reviewing a history of molecular species for nitrogen fixation, I will now attempt to sketch out the ideas that have animated my thesis work. In doing so, I hope to pull together some threads that might otherwise seem unrelated to the reader. Naturally, this logic is being applied with the benefit of hindsight and I will inevitably engage in some degree of ex post facto rationalization, in order to improve coherence. However, I hope that this exercise will help to inform anyone who would endeavor to read this thesis in its entirety. 6 1.2 Multiple Bonding The challenge of functionalizing N2 can be restated in terms of the high strength of the N≡N triple bond. The perfect energy match and overlap of the two nitrogen atoms make their bonding interactions very strong and consequently the anti-bonding interactions very high in energy.10 Thus, efforts to activate N2 typically take advantage of transition metals. Considering only the most common case of N2 binding (terminal and end-on), the transition metal accepts electron density from the poorly basic N-lone pair. This interaction only minimally perturbs the N≡N bonding, as can be surmised from the small change in stretching frequency and bond length observed in formally “σ-only” N2-adducts. Rather, activation of the N≡N bond is primarily achieved via π-backdonation from the metal center. This backdonation into the π* orbitals of the N2 moiety serves to reduce the N–N bond order with a concomitant increase in the M–N bond order. Nonetheless, most M–N2 complexes are best described as containing a M–N single bond and an N≡N triple bond. However, this backdonation increases the negative charge density at the distal nitrogen atom enhancing its basicity and pre-arranges the M–N multiple bonding interactions that will emerge upon functionalization (Figure 1.3).13 Figure 1.3: Cooperative binding of N2 via lone-pair donation from N2 and π-backdonation. This polarizes the N2 molecule and primes it for protonation at Nβ. Upon protonation the N≡N bond order is reduced to 2. This reduced bond order is compensated for (at least in part) by enhanced M–N bonding. Typically the M–N bonding 7 does not fully compensate for the reduction in N–N bond order and thus the resultant N–H bonds have bond dissociation free energies (BDFEs) that are far weaker than typical N–H bonds (≤50 kcal mol−1 compared to ~100 kcal mol−1) and thus these species are prone to loss of ½ H2 to reform the N2 complex.14 Considering the significance of M–N bond order for facilitating N≡N bond disruption, it becomes clear why most N2-cleavage and functionalization reactions have been performed with heavier, earlier metals, such as Mo, which due to their more extensive d-orbitals better activate N2 via backdonation. Indeed, examples of N2 protonation reactions and N2 cleavage reactions were both first discovered with Mo (Figure 1.5).15,16 Figure 1.4: (top) Mo and W complexes promote the functionalization of N 2 and its conversion to NH3.15 (bottom) Mo complex that binds N2 and then cleaves it to form to Mo(N) complexes.16 8 For our metal of interest, Fe, the breakthrough in terms of productive N2functionalization reactions came upon lowering the symmetry from fourfold to threefold (Figure 1.5).17,18 In doing so, an additional d-orbital becomes formally non-bonding. This facilitates the formation of lower-valent Fe centers (FeI and Fe0), thereby enhancing the πbackbonding and, consequently, the nucleophilicity of the distal nitrogen atom (Nβ).19 Figure 1.5: Comparison of the electronic structure of Fe complexes in C3 and C4 symmetry. (left) In C4 symmetry only FeII is accessible. By going to the C3 symmetry (right), an extra non-bonding orbital becomes available and so the Fe 0 oxidation state is accessible and more activated N2 complexes are accessible. Bonding interactions described here are ignoring π-interactions. The unusual stability of the complexes presented in Chapter 2 and their electronic structure is a direct consequence of this reduction from fourfold to threefold symmetry. Although these complexes are iron nitrosyls (Fe–NO) rather than Fe–N2 complexes, the same electronic structure arguments apply. Indeed, NO+, the canonical resonance form assigned to linear Fe–NO complexes such as these, is isoelectronic with N2. Given the importance of porphyrinic iron nitrosyl complexes, Fe–NO complexes have traditionally been studied in fourfold symmetry.20 In that symmetry, reduction of Fe– NO complexes to lower valent states leads to occupation of dz2 resulting in the bending of the Fe–NO ligand and in turn promoting reactivity at the proximal nitrogen (Nα). In threefold symmetry, we see sequential occupation of a dxy-type orbital (defining the xy plane to be that 9 formed by the three equatorial donors). As we would expect, upon addition of electrons into this orbital the Fe becomes more reduced and consequently the π-backbonding (with the nitrosyl) increases as measured via vibrational spectroscopy and corroborated via density functional theory (DFT). The minimal perturbation of isomer shift in the Mӧssbauer spectrum further highlights the high covalency of the system that allows electrons to be distributed onto the ligand platform. Similar results have been observed for other redox series of NO complexes in threefold symmetry.21,22 In this way, the electronic structure promoted by three-fold symmetry provides a means of achieving low-valent Fe. In turn the Fe redistributes the electron density to the ligand platform via backdonation. In the case of both nitrosyl and dinitrogen complexes this leads to a build-up of metal-ligand multiple bonding character with Nα, the attenuation of the ligand-ligand multiple bonding (N–N or N–O), and the enhanced electron density at the distal-atom all prime the small molecule for the formation of X–H bonds via protonation. 1.3 Proton-Coupled Electron Transfer: Considering the mechanism of formation for an X–H bond one can consider three limiting mechanisms. Two stepwise pathways, either electron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET), are possible. Alternatively, a concerted step, namely a proton-coupled electron transfer (PCET), is possible. While PT-ET and ET-PT pathways have been long discussed in the context of nitrogen fixation and discussion of PCET mechanisms for N–H bond formation have taken place in the context of late nitrogen fixation intermediates,23–25 such pathways were essentially overlooked in the discussion of forming early N2 fixation intermediates during catalysis. 10 PCET comes in many flavors, which I will define briefly here as a reference for the reader (Figure 1.6).26,27 The simplest is the hydrogen atom transfer (HAT) in which both the proton and electron derive from the same orbital. Abstracting by one degree, one arrives at concerted-proton electron transfer (CPET), in which the proton and electron are derived from different orbitals in the same molecule or derived from the same orbital but travel to two different receptor orbitals in the same molecule. Taking the PCET concept to its limit, one arrives at multi-site proton-coupled electron transfer (MS-PCET). In this extreme, the proton and electron originate from different molecules; nonetheless their movement remains concerted. Throughout this thesis, we will conservatively refer to all reactions involving the concerted movement of protons and electrons as PCET, although deeper study would reveal the particular flavor of each. Figure 1.6: Canonical examples of hydrogen atom transfer (HAT), concerted-proton electron transfer (CPET), and multi-site proton-coupled electron transfer (MS-PCET). Physical organic chemists long ago realized that the homolytic X–H bond strengths of relevance to PCET reactions could be more readily evaluated via measurement of pKa and E° values and then an application of Hess’s law (Figure 1.7).28 This provided a technically more straightforward approach to measuring homolytic bond strengths given the frequent solution instability of radicals formed via H• loss. These concepts, which have now been employed far beyond their initial home in physical organic chemistry, 29–31 11 will prove to be an invaluable tool in much of the work in this thesis, particularly Chapters 5 and 6. Figure 1.7: (left) The square scheme that relates the stepwise ET-PT and PT-ET pathway to the PCET pathway. (right) Definition of CG 1.4 Overpotential in Nitrogen Fixation Zooming out, the proton-coupled reduction of any small molecule (e.g., N2, O2, CO2, etc.) requires the formation of a series of X–H bonds. In our case, we use a proton (H+) source and an electron (e−) source, which together are able to form new X–H bonds. The strength of the acid is defined by its pKa; the strength of the reductant is defined by its E°. The various intermediates of a small molecule reduction pathway (i.e., N2-to-NH3) have different intrinsic X–H bond strengths. In the case of nitrogen fixation, the N–H bond strengths generally increase throughout the pathway until the final, thermodynamic product, NH3, is formed (Figure 1.8).32 An ideal catalyst would thus, from a thermodynamic perspective, (de)stabilize the various intermediates so that each of the N–H bond strength are equal. 12 Figure 1.8: Calculated gas-phase bond dissociation enthalpies (BDEs) for intermediates of potential relevance to nitrogen fixation.32 Naturally, no ideal nitrogen fixing catalyst has been uncovered, so being able to define the (thermodynamic) shortcomings of the various catalysts becomes important in terms of measuring progress within the field. In the proton-coupled reduction of most small molecules, overpotentials are calculated by defining an operating pKa for the system which in turn defines a thermodynamic potential Emin. The overpotential is then the difference between Eapplied and Emin and thus is defined as a voltage (Eq 1.1). This approach is popular in the electrocatalytic nitrogen fixation field, particularly in papers discussing heterogeneous electrocatalysts.33 Eq 1.1) η = Eapp – Emin Within the field of homogeneous nitrogen fixation catalysis, a complementary approach to defining the overpotential has become popular. This approach uses as a point of comparison not Emin but rather the ΔGf.24,34,35 For NH3, the ΔGf is defined as the combination of 1/2 N2 with 3/2 H2. Thus, formally each N–H bond in the NH3 can be thought of as being formed from H• derived from H2. One can therefore compare the energy of forming H• from H2 (i.e., the homolytic bond strength of H2) with that supplied by the reaction, defined as the effective bond dissociation free energy (BDFEeff) of the hypothetical H• formed from a combination of the H+ and e− source.11 This value can be evaluated from the pKa and the Eapp via Error! Reference source not found. or Error! Reference source not found., where CG 13 and CH are thermodynamic constants that account for the energy or enthalpy of formation of H• from H+ and e− in a particular solvent, respectively. This difference between these energies is then multiplied by three to give the ΔΔGf in terms of energy per mol ( ). The same approach can be used in enthalpic terms with bond dissociation enthalpies (BDEs, Error! Reference source not found.) to generate the ΔΔHf (Error! Reference source not found.). Eq 1.2) BDFEeff = 1.37×pKa + 23.06×E° + CG Eq 1.3) ΔΔGf = 3×(BDFE(H2)/2−BDFEeff) Eq 1.4) BDEeff = 1.37×pKa + 23.06×E° + CH Eq 1.5) ΔΔHf = 3×(BDE(H2)/2−BDEeff) One of the key insights of this thesis was recognizing that the hypothetical H• represented by the BD(F)Eeff could potentially represent a chemical species that was forming in solution and actually reacting via PCET. This idea is hinted at in Chapters 3 and 4 and then fully realized in Chapter 5. 1.5 Selectivity in Nitrogen Fixation Catalysis One of the reasons that the direct reaction of proton and electron sources to form a PCET donor had been overlooked for so long was that this reaction was believed to solely contribute to the undesirable background hydrogen evolution reaction (HER). One of the challenges of any proton-coupled reduction of a small molecule is selectivity. Selectivity problems can come in several varieties: product selectivity (i.e., fixation of N2 to NH3 or to 14 N2H4), small molecule selectivity (i.e., in atmosphere there is N2, O2, and CO2 which could all be reduced), and small molecule vs H+ reduction. This thesis will not touch on either of the first two problems although both pose interesting challenges in the context of developing nitrogen fixation catalysis. However, the issue of selectivity for fixed nitrogen products vs H2 plays is a focus of the Chapters 3 and 4. H2 formation can either be mediated by the catalyst via a variety of mechanisms14,36 or formed directly via a reaction between the acid and reductant. Indeed, we and others have observed H2 formation from direct reaction of the acid and reductant in the absence of the catalyst on the same time scale as the N2 fixation reaction, suggesting the kinetic competence of the background HER reaction. This background HER must necessarily proceed via multiple steps given it necessitates the combination of two protons and two electrons. Thus, the chemical identity of the acid and reductant is going to, in turn, impact the identity and reactivity of the intermediates on this background HER pathway. Reaction of the acid and reductant will typically generate a species that is a powerful PCET reagent (BDFEX-H ≤ 50 kcal mol−1). This species can potentially transfer H• to form a desired X–H bond or react with itself in a bimolecular fashion to release H2. One-electron reduction of this species would result in an intermediate with hydridic characteristics. Again this species could react in a productive fashion (i.e., to form N-H bonds) or it could release H2 upon protonation. In theory the intermediates on the background HER pathway provide access to reactive modalities that have traditionally not been considered in nitrogen fixation (H• or H− transfer). Productive N–H bond formation from these intermediates can thus both enhance NH3 formation and suppress H2 formation. These concepts animate the work in Chapter 6 15 where we use a synthetic strategy to prevent reduction of a protonated metallocene to a hydridic species, thereby allowing us to electrochemically generate a strong PCET donor. Figure 1.9: The possible pathways for background HER and hypothesized N-H bond forming reactions via H• or H− transfer shown with solid and dotted arrows respectively. 1.6 Chapter Summaries The intellectual motivation of each of the chapters is alluded to in the above paragraphs. Here, I will provide a succinct summary of the means by which we interrogated these concepts. In Chapter 2, we synthesize a redox series of trisphosphine borane (P3B) ironsupported nitrosyl complexes. These complexes are unusual in their highly reduced state and their retention of a linear Fe-N-O unit. We use spectroscopic methods (Mӧssbauer, infrared, ultraviolet-visible, and electron paramagnetic resonance) to interrogate the electronic structure of these complexes, which are further verified with density functional theory (DFT).37 In Chapter 3, we develop new conditions for the reduction of N2-to-NH3 using (P3B)Fe. These new conditions utilize a metallocene reductant (Cp*2Co, Cp* = pentamethylcyclopentadienide) and an anilinium acid ([RPhNH3]+, R = substitution on the phenyl ring). With this acid and reductant, we can use the same catalyst, in the same solvent, 16 at the same temperature and pressure to achieve higher turnover numbers and improved selectivity compared to previous results.38,39 The milder nature of the reagents mean that the BDFEeff is higher, indirectly suggesting that PCET mechanisms might be accessible. This hypothesis is further supported by DFT calculations demonstrating that metallocene protonation leads to very reactive PCET reagents.34 We believe that this mechanism may be operative in a number of N2-fixing systems that operate at high-efficiency with metallocenes.40,41 In Chapter 4, we studied the effect of varying the acid pKa on the selectivity for NH3 formation vs H2 formation with (P3B)Fe. Circumstantial evidence in support of PCET mechanisms under catalytic conditions was derived from stoichiometric reactions and calculations. Furthermore, we found that although Cp*2Co on its own cannot reduce N2-toNH3, using it as a co-catalyst in the presence of (P3B)Fe boosted the yield of NH3 in electrodemediated nitrogen fixation reactions into the clearly catalytic regime. This provided the first bona fide example of electrocatalytic N2 fixation and provided further circumstantial evidence of a role for Cp*2Co in mediating productive N–H bond formation.42 In Chapter 5, we provide definitive experimental evidence for the protonation of Cp*2Co via continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy and pulse-EPR spectroscopy. We also use thermochemical techniques to validate the very weak C–H bond formed upon protonation of Cp*2Co that we had predicted computationally. We provide a framework for understanding why these bonds are ~70 kcal mol−1 weaker than typical C–H bonds and thus very strong H• donors. We also demonstrate that this H• donor could be readily converted into an H− donor by one-electron reduction. We believe that this work has significant implications for reconsidering the interplay between the background 17 HER reaction and productive X–H bond forming reactions during the proton-coupled reduction of small molecules generally.43 In Chapter 6, we confirm that the mechanism of electrocatalytic proton reduction by [Cp2Co]+ (Cp = cyclopentadienide) proceeds via an ECEC-mechanism in which it is initially reduced to Cp2Co, followed by protonation (presumably on the Cp ring), then reduction leads to a hydridic species from which H2 is released via protonation.. In order to develop a metallocene based system that would access H• reactivity under electrocatalytic conditions, we synthetically append a Brønsted basic moiety to the Cp2Co. By directing protonation to this remote Brønsted basic site rather than the Cp ring, we prevent further reduction. Thus, the molecule must either bimolecularly eliminate H2, which is slow, or transfer H• to an acceptor. We take advantage of this to achieve the electrocatalytic reduction of ketones to pinacols. Mechanistic evidence from cyclic voltammetry indicates that this reaction proceeds via a rate-limiting PCET step and generates a synthetically useful neutral ketyl radical. 1.7 References (1) Stein, L. Y.; Klotz, M. G. Curr. Biol. 2016, 26, R94–R98. (2) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041–4062. (3) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696–1700. (4) Leigh, G. J. Catalysts for Nitrogen Fixation: Nitrogenases, Relevant Chemical Models and Commercial Processes; Smith, B. E., Richards, R. L., Newton, W. E., Eds.; 18 Nitrogen Fixation: Origins, Applications, and Research Progress; Springer Netherlands: Dordrecht, 2004; pp 33–54. (5) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. Nat. Geosci. 2008, 1, 636–639. (6) Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; et al. Science 2018, 360, 873–879. (7) Detz, R. J.; Reek, J. N. H.; Zwaan, B. C. C. van der. Energy Environ. Sci. 2018, 11, 1653–1669. (8) Parey, K.; Schiffer, A.; Steuber, J.; Fritz, G.; Ermler, U.; Kroneck, P. M. Handbook of Metalloproteins; American Cancer Society, 2010. (9) Maia, L. B.; Moura, J. J. G. Chem. Rev. 2014, 114, 5273–5357. (10) Bazhenova, T. A.; Shilov, A. E. Coord. Chem. Rev. 1995, 144, 69–145. (11) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. (12) Valera-Medina, A.; Xiao, H.; Owen-Jones, M.; David, W. I. F.; Bowen, P. J. Progress in Energy and Combustion Science 2018, 69, 63–102. (13) Burford, R. J.; Fryzuk, M. D. Nat. Rev. Chem. 2017, 1, 1–13. (14) Matson, B. D.; Peters, J. C. ACS Cat. 2018, 8, 1448–1455. (15) Chatt, J.; Heath, G. A.; Richards, R. L. J. Chem. Soc., Chem. Commun. 1972, No. 18, 1010–1011. (16) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861–863. (17) Moret, M. E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118–18121. (18) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558–565. 19 (19) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Inorg. Chem. 2015, 54, 9256–9262. (20) Lehnert, N.; Scheidt, W. R.; Wolf, M. W. Nitrosyl Complexes in Inorganic Chemistry, Biochemistry and Medicine II; Mingos, D. M. P., Ed.; Structure and Bonding; Springer: Berlin, Heidelberg, 2014; pp 155–223. (21) Speelman, A. L.; White, C. J.; Zhang, B.; Alp, E. E.; Zhao, J.; Hu, M.; Krebs, C.; Penner-Hahn, J.; Lehnert, N. J. Am. Chem. Soc. 2018, 140, 11341–11359. (22) Keilwerth, M.; Hohenberger, J.; Heinemann, F. W.; Sutter, J.; Scheurer, A.; Fang, H.; Bill, E.; Neese, F.; Ye, S.; Meyer, K. J. Am. Chem. Soc. 2019, 141, 17217–17235. (23) Scepaniak, J. J.; Vogel, C. A.; Khusniyarov, M. M.; Heinemann, F. W.; Meyer, K.; Smith, J. M. Science 2011, 331, 1049–1052. (24) Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2016, 138, 13379–13389. (25) MacLeod, K. C.; McWilliams, S. F.; Mercado, B. Q.; Holland, P. L. Chem. Sci. 2016, 7, 5736–5746. (26) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004–5064. (27) Darcy, J. W.; Koronkiewicz, B.; Parada, G. A.; Mayer, J. M. Acc. Chem. Res. 2018, 51, 2391–2399. (28) Friedrich, L. E. J. Org. Chem. 1983, 48, 3851–3852. (29) Miller, D. C.; Tarantino, K. T.; Knowles, R. R. Top. Curr. Chem. 2016, 30, 374–432. (30) Saouma, C. T.; Richard, S.; Smolders, S.; Delley, M. F.; Ameloot, R.; Vermoortele, F.; De Vos, D. E.; Mayer, J. M. J. Am. Chem. Soc. 2018, 140, 16184–16189. (31) Valdez, C. N.; Schimpf, A. M.; Gamelin, D. R.; Mayer, J. M. J. Am. Chem. Soc. 2016, 138, 1377–1385. 20 (32) Siegbahn, P. E. M.; Westerberg, J.; Svensson, M.; Crabtree, R. H. J. Phys. Chem. B 1998, 102, 1615–1623. (33) Lindley, B. M.; Appel, A. M.; Krogh-Jespersen, K.; Mayer, J. M.; Miller, A. J. M. ACS Energy Lett. 2016, 1, 698–704. (34) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. (35) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Nature 2019, 568, 536–540. (36) Benedek, Z.; Papp, M.; Oláh, J.; Szilvási, T. Inorg. Chem. 2019. (37) Chalkley, M. J.; Peters, J. C. Angew. Chem. Int. Ed. 2016, 55, 11995–11998. (38) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84–87. (39) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (40) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76–78. (41) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120–125. (42) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. (43) Chalkley, M. J.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2019, 141, 4721–4729. 21 Chapter 2. A Triad of Highly-Reduced, Linear Iron Nitrosyls: {Fe(NO)}8-10 Adapted from: Chalkley, M. J.; Peters, J. C. Angew. Chem. Int. Ed. 2016, 55, 11995–11998. DOI: 10.1002/anie.201605403 22 2.1 Introduction Due to the prevalence of both heme and non-heme iron nitrosyls in biology,1,2 iron model complexes bearing nitrosyl ligands have been the subject of study for decades.3 Iron nitrosyl complexes are frequently characterized by their Enemark-Feltham number, which is the sum of the Fe d-electrons and NO π* electrons. Many examples of {Fe(NO)}6 and {Fe(NO)}7 complexes are known4,5 and, more recently, several {Fe(NO)}8 complexes have been thoroughly characterized.6–11 However, {Fe(NO)}9 complexes are unknown, and the unique properties of the only known example of an {Fe(NO)}10 complex, [Fe(CO)3(NO)]−, prompted its reinvestigation in 2014 by Plietker and coworkers.12,13 The activated NO stretching frequency (1647 cm-1) and long N–O bond (1.212 Å) but linear Fe–N–O angle (180°) observed in this complex stand in contrast to the iron nitrosyl literature, where high Enemark-Feltham numbers are associated with a bent geometry.6–11 Therefore, we sought to investigate the Fe–NO unit under local three-fold symmetry on a ligand platform that would engender strong Fe–N multiple bonding while also providing redox flexibility. With this in mind, we pursued the synthesis of a series of Fe–NO complexes supported by a tris(phosphine)borane (P3B) ligand ().14 Herein we report the synthesis of a monoironmononitrosyl complex that has been crystallographically characterized across three oxidation states, including highly unusual examples of {Fe(NO)}9 and {Fe(NO)}10 complexes. These nitrosyl complexes are distinct in their retention of a strictly linear Fe–N–O unit across the series; a high degree of covalency factilitates this atypical structural behavior. Similar results have been observed recently for a Fe-NO unit supported by four N-heterocyclic carbene ligands.15 23 2.2 Results 2.2.1 Synthesis of {Fe(NO)}8-10 In our exploration of Fe–N2 chemistry, the P3B ligand has demonstrated the ability to support strong Fe–N π-bonding, while geometric (and electronic) flexibility of the Fe– B interaction allows for stabilization of highly reduced Fe species. Given that (P 3B)Fe(N2) has an irreversible oxidation event that causes dissociation of the N 2 ligand to generate [(P3B)Fe]+, we hypothesized that nitrosonium hexafluorophosphate ([NO][PF 6]) could serve both to oxidize (P3B)Fe(N2) and to act as a source of in situ nitric oxide to bind Fe.16 Successful isolation of the {(P3B)Fe(NO)}8 cation as its [BArF4]− salt (after salt metathesis with [Na][BArF4]; BArF4 = B(3,5-(CF3)2-C6H3)4) proved viable (). The cyclic Figure 2.1: Synthetic route for {(P3B)Fe(NO)}8-10. B 8 24 voltammetry of {(P3 )Fe(NO)} (Figure 2.2) demonstrated both a quasi-reversible reduction at −0.56 V vs. Fc+/0 and a second, fully reversible feature at −1.97 V vs. Fc +/0. We attribute these redox features to the {Fe(NO)}8/9 and {Fe(NO)}9/10 couples, respectively. Synthetic generation of neutral {(P 3B)Fe(NO)}9 via cobaltocene reduction of {(P3B)Fe(NO)}8, and anionic {(P3B)Fe(NO)}10 by Na/Hg reduction of {(P3B)Fe(NO)}9, provided the desired {(P3B)Fe(NO)}8-10 series (). In contrast to most other reports of highly reduced Fe–NO complexes,6–11,17 these three species are stable both in solution and in the solid state at room temperature under an inert atmosphere. Figure 2.2: Variable scan rate cyclic voltammograms in 0.1 M [TBA][BArF4] THF of the {(P3B)Fe(NO)}8/9 and {(P3B)Fe(NO)}9/10 redox couples on the left and right respectively. 2.2.2 Characterization and Electronic Structure of {Fe(NO)} 8-10 The infrared spectra of {(P3B)Fe(NO)}8-10 demonstrate an approximately 100 cm-1 decrease in the stretching frequency of the NO bond upon each successive reduction (Figure 2.3). This behaviour is reminiscent of that seen in transition metal complexes of π- accepting ligands such as N2 and CO.16,18 However, this behaviour stands in contrast to that of most previously characterized iron nitrosyls, which more typically show much larger changes (between 200-350 cm-1) in the NO stretching frequency per unit change in their of Enemark-Feltham number.6,11 This observation suggested to us that the Fe–NO 25 linkage remains linear throughout the redox series described here, behaviour that is rare in redox series of metal nitrosyls.4,5 The crystal structures of {{(P3B)Fe(NO)}8-10 confirm that the Fe–N–O angle is highly linear in each complex: 175.8(3)° in {(P 3B)Fe(NO)}8, 176.18(6)° in {(P3B)Fe(NO)}9, and 179.05(12)° in {(P3B)Fe(NO)}10. Figure 2.3: IR spectra for {(P3B)Fe(NO)}8-10 highlighting the NO stretch. The crystal structures19 (Figure 2.4) of {(P3B)Fe(NO)}8-10 further reveal that the only significant ligand rearrangement across the series is the presence of an intramolecular η4-BCCP interaction in {(P3B)Fe(NO)}8. Variable temperature 1 H and 31 P NMR experiments indicate that this interaction is maintained in solution. In contrast, both {(P3B)Fe(NO)}9 and {(P3B)Fe(NO)}10 demonstrate approximate three-fold symmetry both in solution and in the solid state. Although Fe–N–O linearity is maintained, the N–O bond does lengthen about 0.03 Å upon each reduction, in agreement with the activation observed by IR spectroscopy. Although the Fe–N bond distance remains fairly constant (Figure 2.5), the Wiberg bond indices (Figure 2.5) find that the Fe–N bond order increases slightly from {(P3B)Fe(NO)}8 to {(P3B)Fe(NO)}10 with a concomitant decrease in the N–O bond order.20,21 In addition to Fe–N bonding, the Wiberg Bond Index finds significant Fe–O 26 bonding (bond order of ~0.5). This through bonding interaction has been previously interpreted as indicative of a highly covalent interaction.20 Although the Fe–B bond Figure 2.4: X-ray crystal structures of {(P3B)Fe(NO)}8-10 with hydrogens and counteranions omitted for clarity. Fe is orange, phosphorus is purple, nitrogen is pink, oxygen is red, boron is brown, and carbon is blue. distance increases with reduction, suggestive of a weakening Fe–B interaction, the boron does become more pyramidalized, which agrees with both the DFT calculations (vide infra) and the 11B NMR spectra, and suggests increased Fe–B bonding upon reduction. Finally, the Fe–P distances are significantly shorter in {(P3B)Fe(NO)}10 than in both {(P3B)Fe(NO)}8, and {(P3B)Fe(NO)}9, potentially suggestive of increased Fe–P backbonding in the most reduced species. Figure 2.5: Experimental bond lengths from X-ray crystallography and Wiberg bond indices from DFT, along with a comparison of experimental and computational NO stretching frequencies. 27 Mössbauer spectroscopy has been frequently employed in the study of iron nitrosyl complexes as an experimental probe of the relative state of oxidation of the iron center.6,11,13,15,17,22,23 Typically octahedral {FeNO}6 complexes have isomer shifts between 0.0 and 0.05 mm s-1, {FeNO}7 complexes have isomer shifts between 0.25 and 0.33 mm s-1 and {FeNO}8 complexes have isomer shifts between 0.4 and 0.5 mm s -1. These significant changes in the isomer shift occur despite NO-centered reduction, as the decreasing ability of the NO ligand to accept electron density through backbonding leads the Fe-center to become more electron rich.6 For the high-spin, pseudo-C3 {FeNO}6-8 recently reported by Lehnert and coworkers, even larger changes in isomer shift (~0.4 mm s-1) per unit reduction are observed. These authors suggest that such a large shift is indicative of metal-centered reduction.22 The zero-field, 80 K Mössbauer spectra of {(P3B)Fe(NO)}8-10 (Figure 2.6) show only very minimal changes in the isomer shift as a function of the overall redox state. The linear and therefore strongly π-bonded NO ligand enforces low-spin configurations for all three redox states, leading to short metal–ligand bonds and correspondingly low isomer shifts.24 These low isomer shifts are consistent with the behavior we have observed in other highly-reduced (P3B)Fe complexes, related tris(phosphine)silyl supported Fe complexes, and a very recently reported series of linear {FeNO}6-8 with four N-heterocyclic carbene ligands.15,18,25,26 Even within this context, the {(P3B)Fe(NO)} system is remarkable for the minimal change in isomer shift observed across the redox series; this suggests a high degree of metal–ligand covalency that buffers against any buildup of electron density on the iron center upon successive reductions. The nitrosyl ligand enhances the already strong covalency between the Fe and the P3B ligand, which we have previously noted leads to a 28 non-classical relationship between isomer shift and redox state in these species. Lowspin complexes of these types feature low isomer shifts regardless of formal oxidation state.26 Figure 2.6: Zero-field Mӧssbauer spectra of {(P3B)Fe(NO)}8-10 obtained as microcrystalline material suspended in a boron nitride matrix at 80 K. Data is shown as black dots and the simulation with the isomer shift (δ) and quadrupole splitting (Δ eq) given below shown as a red line. The electronic structures of metal nitrosyls are still debated, 27 but linear NO complexes are most commonly described by a π-accepting NO+ resonance form. In Fe– (NO+) complexes, the (NO) stretching frequency is typically found between 1900 and 2000 cm-1.[19] There are also cases where a linear nitrosyl is considered to be a π-donating NO− ligand.[4e,6b] These two limiting resonance forms indicate formal charge transfer either from the NO to the metal or from the metal to the NO. To help determine which, if either, of these limiting cases more accurately describes the complexes featured herein, we draw comparisons to the known dinitrogen (N2 is isolobal to NO+) and imido (NR is isolobal to NO−) complexes of the (P3B)Fe scaffold.15,18,25,26 Relative to the (P3B)Fe(NO) complexes, the (P3B)Fe(N2) complexes have longer Fe–N (~1.78 Å) distances and shorter Fe–B distances (~1.3 Å), suggesting a weaker π-interaction between the Fe and the N 2 ligand and 29 more σ-backdonation into the borane. For comparison, the Wiberg bond index calculation for [(P3B)Fe(N2)]− (isoelectronic to {(P3B)Fe(NO)}9) provides an Fe–B bond order of 0.5526 compared to 0.4402 and an Fe–N bond order of 0.9796 compared to 1.5958. In contrast, the (P3B)Fe(NR) (R = adamantyl or 4-methoxyphenyl) complexes have similarly short Fe–N (1.66 Å) distances but much longer Fe–B (2.6-2.8 Å) distances, arising from a distortion to a more tetrahedral symmetry at iron that further enhances Fe– N π-bonding, and a more electron poor Fe center with minimal backdonation into the borane. For comparison, the Wiberg bond index calculation for (P 3B)Fe(NAd) (Ad = adamantyl) (isoelectronic to {(P3B)Fe(NO)}9) provides an Fe–B bond order of 0.2718 compared to 0.4402 and an Fe–N bond order of 1.7980 compared to 1.5958. It therefore seems apparent that neither limiting scenario (Fe–(NO+) vs Fe–(NO−)) reliably describes the bonding situation observed in the {(P3B)Fe(NO)}8-10 series. A description of the entire ligand sphere about the iron center as covalent seems more appropriate than descriptions that imply significant charge transfer. This covalent description is further supported by the cryogenic-temperature (−180 °C) UV-vis spectrum of {(P3B)Fe(NO)}9 (Figure 2.7). Upon cooling, a vibronic progression with spacing of 452, 457, 476, 509, 499 cm -1 emerges on the electronic transition centered at 521 nm. To our knowledge, a related vibronic progression has been observed in only one other M–NO system in the cryogenic electronic spectrum of [Cr(CN)5(NO)]3−, characterized by Gray and coworkers in 1966. In that case, the electronic transition featuring the vibronic progression was centered at 470 nm and was attributed to a transition from a metal-based dxy or dx2-y2 orbital into a Cr–N π* orbital. Although this 30 previous study investigated a series of isoelectronic, pentacyano metal (M = V, Cr, Mn, Fe) nitrosyl complexes, only [Cr(CN)5(NO)]3− revealed this vibronic progression upon cooling. The orbital contribution from Cr and NO π* to the Cr–N π* orbital was deduced to be nearly equal, leading the authors to posit that this might be requisite for the observation of vibronic coupling.28 Likewise, cooling of {(P3B)Fe(NO)}8 and {(P3B)Fe(NO)}10, which exhibit absorption features at a similar wavelength, does not lead to the emergence of any vibronic coupling. Due to the similarities in M–N–O angle, N–O distance, and ν(NO) of [Cr(CN)5(NO)]3− and {(P3B)Fe(NO)}9, and the observation that the calculated dxy-Fe–N π* gap (SOMO-LUMO gap) in {(P3B)Fe(NO)}9 is 520 nm, we also assign this absorption feature to a dxy-M–N π* transition.29 Based on our own observations, those of Gray and coworkers, and the paucity of metal–NO complexes demonstrating such vibronic coupling, it appears that vibronic coupling of this type is only (albeit not necessarily) observed in highly covalent, linear M–NO units. Figure 2.7: Density-corrected room temperature (red) and cryogenic (black) UV-Vis spectra of {(P3B)Fe(NO)}9 in 2-MeTHF highlighting the region that demonstrates vibronic coupling. The solution magnetic susceptibility of {(P3B)Fe(NO)}9 is 1.7μB (C6D6, RT), 31 consistent with an S = ½ species. The X-band EPR spectrum of {(P3B)Fe(NO)}9 (Figure 2.8) shows a nearly axial signal with significant g anisotropy and broad features. We favor an electronic structure consistent with a description in which the SOMO consists of an iron dorbital that is primarily of dxy or dx2-y2 parentage, akin to ferrocenium.30 Unrestricted DFT Figure 2.8: CW X-band EPR spectrum of {(P3B)Fe(NO)}9 at 77 K in a 2-MeTHF glass (black) and its simulation (red). Simulation parameters: g = [2.50048, 1.99439, 1.96918] and HStrain = [450.420, 159.384, 205.277]. calculations using BP86/def2-TZVPP (Fe, B, P, N, O) and 631-G(d) (C, H)31–35 reproduce the structure of {(P3B)Fe(NO)}9 well and support this description (Figure 2.9). Broken symmetry calculations, in which an S = 1 NO− is antiferromagnetically coupled to a metal center, are often used in the case of linear M–NO complexes.27 Attempts to optimize such wavefunctions with BP86 led to their collapse back to the low-spin wavefunction. Calculations using B3LYP were observed to converge broken symmetry wavefunctions, but 32 this functional predicted the ground states to be intermediate spin, rather than the observed low-spin. 2.3 Conclusion The collective data presented here lend support to an electronic description of {(P3B)Fe(NO)}8-10 in which reduction of the system across the Enemark-Feltham numbers from 8 to 9 and 9 to 10 occurs in primarily Fe-based orbitals that are orthogonal to both the Figure 2.9: A molecular orbital diagram for {(P3B)Fe(NO)}9. The energies given are relative to the SOMO which was set to be 0 eV. The molecular orbitals depicted (Isovalue = 0.05) are from the α-spin manifold as are the energies. The energy and appearance of the β-spin orbitals are similar. Fe–NO π-bonds and the Fe–B σ-bond. However, as the Fe becomes more electron rich, backbonding into the NO, σ-backbonding into the borane, and presumably π-backbonding into the phosphines all become stronger, leading to very little overall change in the electron density at the Fe-center (i.e., its relative state of oxidation.) This high degree of covalency, particularly in the Fe–NO bond but also between the Fe and the P3B ligand, leads to the atypical maintenance of a linear Fe–N–O geometry upon 33 successive reductions, and appears to be key to the ability of the P3B ligand to support an Fe–NO unit across three redox states. Finally, trigonal symmetry, rather than tetragonal symmetry, gives rise to an additional non-bonding orbital, providing access to unusually high Enemark-Feltham numbers (8, 9, and 10). 2.4 References (1) Pacher, P.; Beckman, J. S.; Liaudet, L. Physiol. Rev. 2007, 87, 315–424. (2) Tennyson, A. G.; Lippard, S. J. Chem. Biol. (Oxford, U.K.) 2011, 18, 1211–1220. (3) Enemark, J. H.; Feltham, R. D. Coord. Chem. Rev. 1974, 13, 339–406. (4) Hayton, T. W.; Legzdins, P.; Sharp, W. B. Chem. Rev. 2002, 102, 935–992. (5) Wright, A. M.; Hayton, T. W. Comments Inorg. Chem. 2012, 33, 207–248. (6) Serres, R. G.; Grapperhaus, C. A.; Bothe, E.; Bill, E.; Weyhermüller, T.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2004, 126, 5138–5153. (7) Pellegrino, J.; Bari, S. E.; Bikiel, D. E.; Doctorovich, F. J. Am. Chem. Soc. 2010, 132, 989–995. (8) Sanders, B. C.; Patra, A. K.; Harrop, T. C. J. Inorg. Biochem. 2013, 118, 115–127. (9) Goodrich, L. E.; Roy, S.; Alp, E. E.; Zhao, J.; Hu, M. Y.; Lehnert, N. Inorg. Chem. 2013, 52, 7766–7780. (10) Speelman, A. L.; Lehnert, N. Angew. Chem. Int. Ed. 2013, 52, 12283–12287. (11) Hu, B.; Li, J. Angew. Chem. Int. Ed. 2015, 54, 10579–10582. (12) Hieber, W.; Beutner, K. Z. Naturforsch., B: Anorg. Chem., Org. Chem., Biochem., Biophys., Biol. 1960, 15, 323–324. 34 (13) Klein, J. E. M. N.; Miehlich, B.; Holzwarth, M. S.; Bauer, M.; Milek, M.; Khusniyarov, M. M.; Knizia, G.; Werner, H.-J.; Plietker, B. Angew. Chem. Int. Ed. 2014, 53, 1790–1794. (14) Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149–5151. (15) Kupper, C.; Rees, J. A.; Dechert, S.; DeBeer, S.; Meyer, F. J. Am. Chem. Soc. 2016, 138, 7888–7898. (16) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (17) Patra, A. K.; Dube, K. S.; Sanders, B. C.; Papaefthymiou, G. C.; Conradie, J.; Ghosh, A.; Harrop, T. C. Chem. Sci. 2012, 3, 364–369. (18) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558–565. (19) CCDC 1481636 ({(P3B)Fe(NO)}8), 1481637 ({(P3B)Fe(NO)}9), and 1481638 ({(P3B)Fe(NO)}10) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.ukj/data_request/cif. (20) Sizova, O. V.; Sokolov, A. Yu.; Skripnikov, L. V.; Baranovski, V. I. Polyhedron 2007, 26, 4680–4690. (21) Moret, M. E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118–18121. (22) Speelman, A. L.; Zhang, B.; Krebs, C.; Lehnert, N. Angew. Chem. Int. Ed. 2016, 55, 6685–6688. (23) Zhang, Y.; Oldfield, E. J. Am. Chem. Soc. 2004, 126, 9494–9495. 35 (24) Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Applications; Springer-Verlag: Berlin Heidelberg, 2011. (25) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (26) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (27) Tomson, N. C.; Crimmin, M. R.; Petrenko, T.; Rosebrugh, L. E.; Sproules, S.; Boyd, W. C.; Bergman, R. G.; DeBeer, S.; Toste, F. D.; Wieghardt, K. J. Am. Chem. Soc. 2011, 133, 18785–18801. (28) Manoharan, P. T.; Gray, H. B. Inorg. Chem. 1966, 5, 823–839. (29) Enemark, J. H.; Quinby, M. S.; Reed, L. L.; Steuck, M. J.; Walthers, K. K. Inorg. Chem. 1970, 9, 2397–2403. (30) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1970, 92, 3233–3234. (31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257–2261. (32) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (33) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (34) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571–2577. (35) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. 36 Chapter 3. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET Adapted from: Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. DOI: 10.1021/acscentsci.7b00014 37 3.1 Introduction The reduction of N2 to NH3 is critical for life and is performed on a massive scale both industrially and biologically.1 The high stability of the N≡N triple bond necessitates catalysts and high-energy reagents/conditions to achieve the desired transformation. 2 Synthetic studies of catalytic N2-to-NH3 conversion by model complexes are of interest to constrain hypotheses concerning the mechanism/s of biological (or industrial) N 2-fixation and to map fundamental catalyst design principles for multi-electron reductive transformations.3,4 Interest in Fe model systems that catalyze N2-to-NH3 conversion has grown in part due to the postulate that one or more Fe centers in the FeMo-cofactor of FeMo-nitrogenase may serve as the site of N2 binding and activation during key bondbreaking and -making steps.5 Previous examples of synthetic molecular Fe catalysts that mediate N2-to-NH3 conversion operate with high driving force, relying on a very strong acid (pKa ca. 0) and reductant (E° < −3.0 V vs Fc+/0).6–9 In contrast, several Mo catalysts have been shown to facilitate N2-to-NH3 conversion with significantly lower driving force10–13 and one Fe catalyst has been recently reported to facilitate N2-to-N2H4 conversion at lower driving force.14 There is thus interest in exploring the viability of Fe-mediated catalytic N2-to-NH3 conversion under less forcing conditions from a practical perspective and to continue assessing these systems as functional models of biological nitrogenases, in which 8 ATP are consumed per NH3 formed leading to a driving force of 58 kcal/mol.2 Herein we demonstrate that catalytic conversion of N 2 to NH3 by [(P3B)Fe]+ (P3B = tris(odiisopropylphosphinophenyl)borane) can be achieved with a significantly lower driving force by coupling Cp*2Co with [Ph2NH2]+ or [PhNH3]+ (Figure 3.1). Such conditions 38 additionally afford unusually high selectivity and catalytic turnover for NH3. It was similarly observed that this reagent combination with (depe)2Fe(N2) likewise provides high selectivity for N2H4.14 We note that the use of milder reagents as reductant (E°) and acid (pKa), as compared to previous Fe-mediated N2-to-NH3 conversion, engenders a higher effective bond dissociation enthalpy (BDEeff, Error! Reference source not found.),15 which in turn lowers the enthalpic driving force (ΔΔHf, Error! Reference source not found.), as discussed in Chapter 1. Furthermore, this higher BDEeff suggests access to proton-coupled electron transfer (PCET) pathways (e.g., FeN2 + H·  FeN2H) in addition to electron transfer (ET)/proton transfer (PT) pathways, thus enhancing overall catalytic efficiency. Theoretical considerations, including DFT calculations, and experimental details are discussed that suggest the viability of a decamethylcobaltocene-mediated PCET pathway in this system; by extension we suggest metallocene-mediated (e.g., Cp*2Cr) PCET pathways may be operative in previously studied Mo and Fe N 2-fixing systems that use metallocene reductants.10–14 39 Figure 3.1: Summary of conditions used for catalytic N2-to-NH3 conversion by [(P3B)Fe]+ highlighting the estimated driving force (ΔΔHf).6,16 Eq 3.1) BDEeff = 1.37×pKa + 23.06×E° + Ch Eq 3.2) ΔΔHf = 3×(BDEeff – BDE(H2)/2) Various observations of (P3B)Fe complexes in the presence of acids and reductants suggested that this system might be capable of N 2-to-NH3 conversion with lower driving force than that originally reported. Accordingly, we had observed that the treatment of [(P3B)Fe(N2)]− with KC8 and weaker acids (pKa > 0) led to greater than stoichiometric NH3 formation (e.g., under unoptimized conditions [2,6-dimethylanilinium][OTf] afforded 2.1 equiv NH3 per Fe).17 Similarly, the treatment of [(P3B)Fe(N2)]− with [H(OEt2)2][BArF4] (HBArF4, BArF4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) and weaker reductants led to modest yields of NH3. For example, under unoptimized conditions we had observed that decamethylcobaltocene (Cp*2Co) and HBArF4 afforded 0.6 equiv NH3 per Fe.16,17 Most recently, an apparent catalytic response was observed during a cyclic voltammetry experiment at the [(P3B)Fe(N2)]0/− couple (−2.1 V vs Fc+/0) upon addition of excess HBArF4 B + 40 under an N2 atmosphere. Electrolytic NH3 generation by [(P3 )Fe] was observed at −2.4 V vs Fc+/0 in Et2O,23 and Na/Hg (−2.4 V vs Fc+/0 in THF)16 could instead be used for N2to-NH3 conversion catalysis (albeit less selectively and with low turnover). Finally, mixing [(P3B)Fe]+ with Cp*2Co in Et2O at −78 °C under N2 generates some [(P3B)Fe(N2)]− as observed by freeze-quench X-band EPR and Mössbauer spectroscopy, suggesting that Cp*2Co is, in principle, a sufficiently strong reductant to trigger N 2RR catalysis by [(P3B)Fe]+. 3.2 Results 3.2.1 Catalytic N2RR at Lowered Overpotential Treatment of [(P3B)Fe]+ with Cp*2Co and [Ph2NH2][OTf], [Ph2NH2][BArF4], or [PhNH3][OTf] in Et2O at −78 °C under an N2 atmosphere affords catalytic yields of NH3 (Table 3.1). Notably, the highest selectivity for NH3 obtained among this series (72% at standard substrate loading; Entry 1) is significantly improved compared to all previously described (molecular) Fe catalysts for N2-to-NH3 conversion.6–9,14,16 Tripling the initial substrate loading (Entry 2) nearly triples the NH3 production with only modest loss in efficiency for NH3 (63 ± 2%). Preliminary attempts to further increase the initial substrate loading have led to substantially decreased efficiency (Entry 3). However, substrate reloading experiments (Entries 4 and 5) maintain greater than 50% efficiency for NH 3 overall; a turnover number for NH3 generation via two reloadings has been achieved as high as 89 in a single run (84 ± 8 equiv NH3 per Fe; Entry 5). This is an unusually high turnover number for a molecular N2-to-NH3 conversion catalyst under any conditions.14 In catalytic runs performed with labeled [Ph215NH2][OTf] under an atmosphere of natural 14 41 14 abundance N2 the production of exclusively NH3 is observed, demonstrating that the NH3 formed during catalysis is derived from N2 and not degradation of the acid. The use of the more soluble acid [Ph2NH2][BArF4] (Entry 6) provides significantly lower, but still catalytic, yields of NH3. This more soluble acid presumably increases background reactivity with Cp*2Co. Perhaps more significantly, [PhNH3][OTf] is a considerably weaker acid than [Ph2NH2][OTf] (Figure 3.1), but still provides substantial catalytic yields of NH3 (Entries 7 and 8), at efficiencies that compare well with those obtained previously using HBArF4 and KC8 despite a difference in driving force of nearly 100 kcal/mol. 16 Table 3.1: N2-to-NH3 Conversion with (P3E)M Complexes (M = Fe, Co) Cp*2Co Acid Acid Equiv Catalyst (equiv) (equiv) NH3/Fe % Yield NH3/e- 1 [(P3B)Fe]+ 54 108 [Ph2NH2][OTf] 12.8 ± 0.5 72 ± 3 2 [(P3B)Fe]+ 162 322 [Ph2NH2][OTf] 3 [(P3B)Fe]+ 322 638 [Ph2NH2][OTf] 26.7 ± 0.9 25 ± 1 4a [(P3B)Fe]+ [162]x2 [322]x2 [Ph2NH2][OTf] 56 ± 9 52 ± 9 5a [(P3B)Fe]+ [162]x3 [322]x3 [Ph2NH2][OTf] 84 ± 8 52 ± 5 6 [(P3B)Fe]+ 54 108 [Ph2NH2][BArF4] 8±1 42 ± 6 7 [(P3B)Fe]+ 54 108 [PhNH3][OTf] 7±1 38 ± 7 8 [(P3B)Fe]+ 162 322 [PhNH3][OTf] 16 ± 3 29 ± 4 9 (P3Si)Fe(N2) 54 108 [Ph2NH2][OTf] 1.2 ± 0.1 6±1 10 [(P3B)Co(N2)]− 54 108 [Ph2NH2][OTf] 1.1 ± 0.4 6±2 34 ± 1 63 ± 2 42 11 Si (P3 )Co(N2) 54 108 [Ph2NH2][OTf] 0±0 0±0 The catalyst, acid, Cp*2Co, and Et2O were sealed in a vessel at −196 °C under an N2 atmosphere followed by warming to −78 °C and stirring. Yields are reported as an average of at least 2 runs. aFor these experiments, the reaction was allowed to proceed for 3 hours at −78 °C before cooling to −196 °C and furnished with additional substrate and solvent We also screened several related phosphine-ligated, (P3E)M(N2) (M = Fe, Co; E = B, Si) complexes18,19 under the optimized reaction conditions with [Ph2NH2][OTf] and Cp*2Co (Entries 9–11) but found that none of these other systems were competent catalysts at these loadings. While we anticipate other catalyst systems for N 2-to-NH3 conversion may yet be found that function under the conditions described herein, 14 certain features of the (P3B)Fe system correlate with unusually productive catalysis.19 3.2.2 Mechanism of N2RR Catalysis with (P3B)Fe Also significant is that when [(P3B)Fe]+ is loaded with 322 equiv [Ph2NH2][OTf] and 162 equiv Cp*2Co in Et2O at −78 °C, modest levels of N2H4 are detected (< 1 equiv per Fe).9,14 We had previously reported that catalytic N2 reduction with KC8 and HBArF4 yielded no detectable hydrazine, but observed that if hydrazine was added at the outset of a catalytic run, it was consumed.6 When 5 equiv of N2H4 were added at the beginning of a catalytic run (again with 322 equiv [Ph2NH2][OTf] and 162 equiv Cp*2Co), only 0.22 equiv of N2H4 (4.4% recovery) remained after workup. This result indicates that liberated hydrazine can also be reduced or disproportionated under the present conditions or that the work-up 43 conditions convert N2H4 to NH3. That N2H4 is detected to any extent in the absence of initially added N2H4 under these conditions indicates that a late N–N cleavage mechanism to produce NH3 (e.g., alternating or hybrid cross-over) is accessible.4,20,21 A recent report by Ashley and coworkers describes a phosphine-supported Fe system for which catalytic hydrazine formation is kinetically dominant.14 Whether such a pathway is kinetically dominant in this system is as yet unclear.16,22 The (P3B)Fe speciation under turnover conditions was probed via freeze-quench Mössbauer spectroscopy.16 The Mössbauer spectrum of a catalytic reaction mixture after five minutes of reaction time ( Figure 3.2) reveals the presence of multiple species featuring well-resolved sets of quadrupole doublets. The spectrum is satisfactorily simulated with (P 3B)Fe(N2) (δ = 0.55 mm/s, ΔEQ = 3.24 mm/s, 32%; Figure 3.2 green), [(P3B)Fe(N2)]− (δ = 0.40 mm/s, ΔEQ = 0.98 mm/s, 26%; Figure 3.2 blue),16 an unknown, likely P3B-metallated Fe species (δ = 0.42 mm/s, ΔEQ = 1.84 mm/s, 18%; Figure 3.2 yellow), and a final species that is modeled with δ = 0.96 mm/s and ΔE Q = 3.10 mm/s (24%; Figure 3.2 orange). The presence of [(P3B)Fe(N2)]− was confirmed by freeze-quench EPR spectroscopy experiments. The broad nature of this last signal ( Figure 3.2 orange) and its overlap with other features in the spectrum prevents its precise assignment, but its high isomer shift and large quadrupole splitting are suggestive of a tetrahedral, S = 2 Fe(II) complex, which have distinct Mӧssbauer properties. 23,24 The 44 Mössbauer spectrum of a catalytic reaction mixture after 30 minutes was also analyzed. The spectrum still shows (P3B)Fe(N2) (53%), the same unknown (P3B)Fe species (18%), and again a tetrahedral, high-spin Fe(II) component (22%). However, [(P 3B)Fe]+ is now present (δ = 0.75 mm/s, ΔEQ = 2.55 mm/s, 8%) and [(P3B)Fe(N2)]− is no longer observed. The reloading experiments described above provide strong evidence that “(P 3B)Fe” species represent an “active catalyst” population; interpretation of the relative speciation via spectroscopy should hence bear on the mechanism of the overall catalysis. Figure 3.2: Mössbauer spectrum at 80 K with 50 mT applied parallel field of a freezequenched catalytic reaction (54 equiv Cp*2Co, 108 equiv [Ph2NH2][OTf], 1 equiv [(P3B)57Fe]+) after five minutes of reaction time. The appearance of a presumed high-spin (S = 2), tetrahedral Fe(II) species during catalysis (ca. 25%) might arise via dechelation of a phosphine arm. This species could represent an off-path state, or a downstream deactivation product. Interestingly, under the present catalytic conditions we do not observe the borohydrido-hydrido species (P 3B)(μ- 45 H)Fe(H)(L) (L = N2 or H2); this species was postulated to be an off-path resting state during N2-to-NH3 conversion catalysis using HBArF4 and KC8 and was the major component observed at early times (ca. 60% at 5 min).16 It therefore appears that a larger fraction of the “(P3B)Fe” species are in a catalytically on-path state at early reaction times under these new catalytic conditions. Additionally, the presence of a significant degree of [(P3B)Fe(N2)]− ( Figure 3.2) at an early time point is distinct from conditions with HBArF4 and KC8.16 This observation is consistent with the notion that protonation of [(P 3B)Fe(N2)]− is slowed under the present conditions, likely as a result of the insolubility of the triflate salt [Ph 2NH2][OTf] and its attenuated acidity relative to HBArF4,25–27 Clearly, differences in the rates of key elementary steps under the new conditions described here may lead to new mechanistic scenarios for N2-to-NH3 conversion. The improved catalytic efficiency at significantly lower driving force warrants additional consideration. When using HBArF4 and KC8 we have previously suggested that protonation of [(P3B)Fe(N2)]−, which itself can be generated by reduction of (P3B)Fe(N2), to produce (P3B)Fe(NNH) is a critical first step; (P3B)Fe(NNH) can then be trapped by acid to produce spectroscopically observable [(P3B)Fe(NNH2)]+.22 These steps, shown in Error! Reference source not found. and Error! Reference source not found. represent an ET-PT pathway. A PT-ET pathway, where (P3B)Fe(N2) is sufficiently basic to be protonated to generate [(P3B)Fe(NNH)]+ as a first step, followed by ET, is also worth considering (Error! Reference source not found., Error! Reference source not found.) A direct PCET pathway (Error! Reference source not found.) where H-atom delivery to (P3B)Fe(N2) occurs, thus 46 obviating the need to access either [(P3B)Fe(N2)]− or [(P3B)Fe(NNH)]+, needs also to be considered. Eq 3.3) (P3B)Fe(N2) + e−  [(P3B)Fe(N2)]− Eq 3.4) [(P3B)Fe(N2)]− + H+  (P3B)Fe(NNH) Eq 3.5) (P3B)Fe(N2) + H+  [(P3B)Fe(NNH)]+ Eq 3.6) [(P3B)Fe(NNH)]+ + e-  (P3B)Fe(NNH) Eq 3.7) (P3B)Fe(N2) + H•  (P3B)Fe(NNH) Initial PT to (P3B)Fe(N2) to generate [(P3B)Fe(NNH)]+ (Error! Reference source not found.) is unlikely under the present conditions due to the high predicted acidity of [(P3B)Fe(NNH)]+ (pKa = −3.7; estimated via DFT); efficient generation of such a species seems implausible for acids whose pKa’s are calculated at 1.4 (Ph2NH2+) and 6.8 (PhNH3+) in Et2O (Table 3.2). We note that [Ph2NH2][OTf] does not react productively with (P3B)Fe(N2) at −78 °C in Et2O, as analyzed by Mössbauer spectroscopy. Table 3.2: Calculated pKa Values and BDEs of Selected Species BDE a Species pKa + 1.4 (kcal/mol) - [PhNH3]+ 6.8 - Lutidinium 14.5 - [(Cp*)Co(endo-η4-C5Me5H)]+ 16.8 31 [(Cp*)Co(exo-η4-C5Me5H)]+ 16.8 31 [Ph2NH2] 47 4 + 17.3 37 [(Cp*)Cr(exo-η4-C5Me5H)]+ 12.1 30 [(P3B)Fe(NNH)]+ −3.7 - (P3B)Fe(NNH) 38.7 35 [(P3B)Fe(NNH2)]+ 14.4 51 (P3B)Fe(NNH2) - 47 (HIPTN3N)Mo(NNH) - 51 [(Cp*)Cr(endo-η -C5Me5H)] a pKa values were calculated in Et2O and reported relative to (Et2O)2H+. To analyze these potential pathways DFT calculations were performed using the M06-L28 functional with a def2-TZVP basis set on Fe and a def2-SVP basis set on all other atoms. 29 This computational method was selected on the basis that it reproduced the experimentallyderived BDE’s of related N-H bonds on the (P3Si)Fe platform.30 We were particularly interested in accurate calculation of the BDE’s because the DFT-calculated BDE N-H for (P3B)Fe(NNH) (35 kcal/mol; Table 3.2) is larger than the effective BDE (Error! Reference source not found.) of either Cp*2Co/Ph2NH2+ or Cp*2Co/PhNH3+ (25 and 31 kcal/mol, respectively). This suggests that PCET (Error! Reference source not found.) is plausible on thermodynamic grounds. Given that we have employed Cp* 2Co in this study, and that metallocenes generally (i.e., Cp2Co, Cp*2Co and Cp*2Cr) have been effective in other N2fixing molecular catalyst systems,10–14 we have explored via DFT several putative metallocene-derived PCET reagents. Independent studies of H 2 evolution from cobaltocene have invoked a protonated cobaltocene intermediate. 31–33 48 The observation of a background H2 evolution reaction (HER) when employing metallocene reductants, but in the absence of an N2-to-NH3 conversion catalyst, suggests that metallocene protonation is kinetically competent.10,34,35 Based on the analysis we describe below, we propose that protonated metallocenes may serve as discrete and highly active H• sources for PCET. 3.2.3 PCET from a Protonated Metallocene We find that the formation of endo- and [(Cp*)Co(exo-η4-C5Me5H)]+ are predicted to be thermodynamically favorable via protonation of Cp*2Co by either [Ph2NH2]+ or [PhNH3]+ (−21 and −13 kcal/mol, respectively; Figure 3.3A). Efforts to optimize a Co–H species led to migration of the hydrogen to the ring; however, transition metal hydride radical cations are also known to display PCET behavior.36 We have calculated the BDEC-H’s for both endo- and [(Cp*)Co(exo-η4-C5Me5H)]+ as 31 kcal/mol (Figure 3.3B; Table 3.2), 49 indicating that they must be among the strongest PCET reagents accessible in this catalyst cocktail. Indeed, they would be among the strongest PCET reagents known. 15 Figure 3.3: DFT calculations regarding the interaction of metallocenes and acids relevant to nitrogen fixation catalysis. (A) Calculated free-energy changes for the protonation of Cp*2Co. (B) DFT optimized structure of [(Cp*)Co(endo-η4-C5Me5H)]+ (methyl protons omitted for clarity). (C) The unfavorable reduction of 2,6-lutidinium by Cp* 2Cr with the calculated free energy change. (D) The favorable protonation of Cp*2Cr by lutidinium with the calculated free energy change. We anticipate these species would be extremely unstable in solution and hence difficult to detect in situ, but via trapping in the solid state by rapid precipitation from toluene we have isolated a species whose EPR data and chemical behavior are consistent with [(Cp*)Co(η4C5Me5H)][OTf]. Accordingly, slow addition of a toluene solution of Cp* 2Co at −78 °C to triflic acid (HOTf) leads to the instantaneous precipitation of a purple solid that can be isolated at low temperature. The purple solid can be characterized at 77 K by powder EPR 50 spectroscopy via its highly structured signal. By contrast, at this temperature S = ½ Cp*2Co does not display a discernable EPR signal. The new signal shows strong Co hyperfine coupling and significant g-anisotropy, consistent with a new S = ½ cobalt species (Figure 3.4). Furthermore, the resulting EPR signal is slightly perturbed when this purple solid is instead generated from the reaction between deuterated triflic acid (DOTf) and Cp*2Co, suggesting that the acidic proton is directly associated with the new Co species and consistent with its assignment as a protonated decamethylcobaltocene species. Close inspection of these spectra indicate they likely represent a mixture of two signals arising from similar Co-containing complexes. This observation is fully consistent with the presence of both endo- and [(Cp*)Co(exo-η4-C5Me5H)]+, as is to be expected given they are predicted to be nearly isoenergetic. Allowing the purple precipitate to warm to room temperature either as a solid or a stirred suspension in toluene leads to the formation of H 2 and [Cp*2Co]+. 51 Figure 3.4: X-band 77 K powder EPR spectrum (red) and simulation (blue) of the isolated purple precipitate (assigned as endo- and [(Cp*)Co(exo-η4-C5Me5H)]+) from reaction between Cp*2Co and HOTf at −78 °C. Simulation parameters are g1 = [2.63 2.345 1.984], A1,Co = [248 160 187] MHz, lw1 = 1 MHz, HStrain1 = [60 50 60] MHz, Weight1 = 1; g2 = [2.347 2.1 1.982], A2,Co = [200 50 110] MHz, lw2 = 1, HStrain2 = [40 40 40] MHz, Weight2 = 0.2. To better understand the potential role of PCET in N2-to-NH3 conversion catalysis by (P3B)Fe, we have additionally calculated the N–H bond strengths (Table 2) of several early stage candidate intermediates, including the aforementioned (P3B)Fe(NNH) (35 kcal/mol), [(P3B)Fe(NNH2)]+ (51 kcal/mol), and (P3B)Fe(NNH2) (47 kcal/mol). We conclude that PCET from [(Cp*)Co(exo-η4-C5Me5H)]+ to generate intermediates of these types is thermodynamically favorable in each case. Although the dissolution equilibria and kinetics of the insoluble reagents used complicate analysis of the kinetics of individual ET, PT, and PCET steps, the low activation barriers (ΔG‡ < 9 kcal/mol) calculated for all proposed PCET reactions are consistent with these reactions being kinetically accessible. To generate the first 52 and most challenging intermediate (Error! Reference source not found.), the enthalpic driving force for PCET is estimated at ~4 kcal/mol (ΔGcalc = −9 kcal/mol). This driving force of PCET steps increases sharply as further downstream Fe-NxHy intermediates are considered. Given that the Marcus cross-relation between driving force and reaction kinetics has been shown to hold quite well for many PCET reactions this suggests that these reactions would also be kinetically accessible. 39–41 Eq 3.8) (P3B)Fe(N2) + [(Cp*)Co(η4-C5Me5H)]+  (P3B)Fe(NNH) + [Cp*2Co]+ 3.3 Discussion Given the prevalence of metallocene reductants in N 2-to-NH3 (or -N2H4) conversion,10–14 especially for the well-studied Mo catalyst systems, it is worth considering metallocenemediated PCET more generally. For instance, a role for ET/PT steps (or conversely PT/ET) in N2-to-NH3 conversion catalyzed by (HIPTN3N)Mo (HIPTN3N = [(3,5-(2,4,6i Pr3C6H2)2C6H3NCH2CH2)3N]3-, a bulky triamidoamine ligand) has been frequently posited.40–44 But PCET steps may play a critical role, too. In the latter context, we note reports from Schrock and coworkers that have shown both acid and reductant are required to observe productive reactivity with (HIPTN3N)Mo(N2). These observations are consistent with PCET to generate (HIPTN3N)Mo(NNH).44 A PCET scenario has been discussed in this general context of N2-to-NH3 conversion, where a lutidinyl radical intermediate formed via ET from Cp*2Cr has been suggested as a PCET reagent that can be generated in situ.34,45 However, our own calculations predict that the lutidinyl radical should be uphill with Cp*2Cr as the reductant (∆Gcalc = +10 kcal/mol; Figure 3C). Although estimates of the reduction potential of pyridinium radicals has been controversial, this calculation is consistent with previous findings. 46,47 53 We instead propose protonation of Cp*2Cr by the lutidinium acid as more plausible (∆Gcalc = −5.3 kcal/mol; Figure 3D) to generate a highly reactive decamethylchromocene-derived PCET reagent. While N–H bond strengths have not been experimentally determined for the (HIPTN3N)Mo-system, using published data we deduce the N–H bond of (HIPTN3N)Mo(NNH) to be ca. 49 kcal/mol, and we calculate it via DFT with a truncated HIPTN3N ligand to be 51 kcal/mol.34,44 The BDEN-H for this Mo diazenido species is hence much larger than we predict for (P3B)Fe(NNH) (35 kcal/mol), perhaps accounting for its higher stability.44 A PCET reaction between [(Cp*)Cr(endo-η4-C5Me5H)]+ (BDEcalc = 37 kcal/mol) and (HIPTN3N)Mo(N2) to generate (HIPTN3N)Mo(NNH) and [Cp*2Cr]+ would be highly exergonic. Furthermore, we predict a similarly weak BDEC-H for Cp-protonated cobaltocene, [(Cp)Co(η4-C5H6)]+ (BDEcalc = 35 kcal/mol). These considerations are consistent with the reported rapid formation of (HIPTN 3N)Mo(NNH) using either Cp*2Cr or Cp2Co in the presence of lutidinium acid. 3.4 Conclusion To close, we have demonstrated catalytic N2-to-NH3 conversion by [(P3B)Fe]+ at a much lower driving force (nearly 100 kcal/mol) than originally reported via combination of a weaker reductant (Cp*2Co) and acid ([Ph2NH2][OTf] or [Ph3NH][OTf]). Significantly improved efficiency for NH3 formation is observed (up to 72% at standard substrate loading), and by reloading additional substrate at low temperature a turnover number that is unusually high for a synthetic molecular catalyst (84 ± 8 equiv NH3 per Fe) has been achieved. Freeze-quench Mössbauer spectroscopy under turnover conditions reveals B differences in the speciation of (P3 )Fe compared to previous studies with HBArF4 and 54 KC8, suggesting changes in the rates of key elementary steps. Using DFT calculations we have considered the viability of a decamethylcobaltocene-mediated PCET pathway as an additional contributor to the previously formulated ET-PT and PT-ET pathways. Based on our calculations, we propose that protonated metallocenes should serve as discrete, very reactive PCET reagents in N2-to-NH3 conversion catalysis. Furthermore, we present preliminary experimental data that suggest protonated decamethylcobaltocene can be accessed synthetically and that such a species may be a potent PCET reagent. Indeed, the achievement of high efficiency for N2-to-NH3 conversion by both (P3B)Fe and various Mo catalysts that benefit from metallocene reductants raises the intriguing possibility that metallocene-based PCET reactivity is a potentially widespread and overlooked mechanism. Efforts are underway to experimentally probe such pathways. 3.5 References (1) Smil, V. MIT Press: Cambridge, MA, 2001. (2) van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem. Soc. Rev. 2014, 43, 5183–5191. (3) Shaver, M. P.; Fryzuk, M. D. Adv. Synth. Catal. 2003, 345, 1061–1076. (4) MacLeod, K. C.; Holland, P. L. Nat. Chem. 2013, 5, 559–565. (5) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041–4062. (6) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84–87. (7) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105–1115. 55 (8) Ung, G.; Peters, J. C. Angew. Chem. Int. Ed. 2015, 54, 532–535. (9) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181–12189. (10) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76–78. (11) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120–125. (12) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2014, 136, 9719–9731. (13) Arashiba, K.; Kinoshita, E.; Kuriyama, S.; Eizawa, A.; Nakajima, K.; Tanaka, H.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2015, 137, 5666–5669. (14) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. J. Am. Chem. Soc. 2016, 138, 13521–13524. (15) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. (16) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (17) Anderson, J. S. phd, California Institute of Technology, 2014. (18) Whited, M. T.; Mankad, N. P.; Lee, Y. H.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507–2517. (19) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Inorg. Chem. 2015, 54, 9256–9262. (20) Dilworth, M. J.; Eady, R. R. Biochem. J. 1991, 277, 465–468. (21) Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 4243–4248. (22) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. 56 (23) Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Applications; Springer-Verlag: Berlin Heidelberg, 2011. (24) Daifuku, S. L.; Kneebone, J. L.; Snyder, B. E. R.; Neidig, M. L. J. Am. Chem. Soc. 2015, 137, 11432–11444. (25) Garrido, G.; Rosés, M.; Ràfols, C.; Bosch, E. J. Solution Chem. 2008, 37, 689–700. (26) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (27) Hamashima, Y.; Somei, H.; Shimura, Y.; Tamura, T.; Sodeoka, M. Org. Lett. 2004, 6, 1861–1864. (28) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (29) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (30) Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 3161–3170. (31) Koelle, U.; Infelta, P. P.; Graetzel, M. Inorg. Chem. 1988, 27, 879–883. (32) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Chem. Commun. 2016, 52, 9105– 9108. (33) Quintana, L. M. A.; Johnson, S. I.; Corona, S. L.; Villatoro, W.; Goddard, W. A.; Takase, M. K.; VanderVelde, D. G.; Winkler, J. R.; Gray, H. B.; Blakemore, J. D. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 6409–6414. (34) Munisamy, T.; Schrock, R. R. Dalton Trans. 2011, 41, 130–137. (35) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Schrock, R. R.; Müller, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17099–17106. (36) Hu, Y.; Shaw, A. P.; Estes, D. P.; Norton, J. R. Chem. Rev. 2016, 116, 8427–8462. 57 (37) Roth, J. P.; Yoder, J. C.; Won, T.-J.; Mayer, J. M. Science 2001, 294, 2524–2526. (38) Mayer, J. M.; Rhile, I. J. Biochim. Biophys. Acta-Bioenerg. 2004, 1655, 51–58. (39) Hammes-Schiffer, S. Acc. Chem. Res. 2001, 34, 273–281. (40) Studt, F.; Tuczek, F. Angew. Chem. Int. Ed. 2005, 44, 5639–5642. (41) Reiher, M.; Le Guennic, B.; Kirchner, B. Inorg. Chem. 2005, 44, 9640–9642. (42) Studt, F.; Tuczek, F. J. Comput. Chem. 2006, 27, 1278–1291. (43) Thimm, W.; Gradert, C.; Broda, H.; Wennmohs, F.; Neese, F.; Tuczek, F. Inorg. Chem. 2015, 54, 9248–9255. (44) Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103–1117. (45) Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2016, 138, 13379–13389. (46) Yan, Y.; Zeitler, E. L.; Gu, J.; Hu, Y.; Bocarsly, A. B. J. Am. Chem. Soc. 2013, 135, 14020–14023. (47) Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 7580–7583. 58 Chapter 4. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis Adapted from: Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. DOI: 10.1021/jacs.8b02335 59 4.1 Introduction There has been substantial recent progress in the development of soluble, welldefined molecular catalysts for N2-to-NH3 conversion, commonly referred to as the nitrogen reduction reaction (N2RR).1–10 Nevertheless, a significant and unmet challenge is to develop molecular catalysts, and conditions, compatible with electrocatalytic N2RR. Progress in this area could have both fundamental and practical benefits, including access to informative in situ mechanistic studies via electrochemical techniques, and an electrochemical means to translate solar or otherwise derived chemical currency (H+/e−) into NH3. The latter goal, which has been the subject of numerous studies using heterogeneous catalysts, is key to the long-term delivery of new ammonia synthesis technologies for fertilizer and/or fuel.11,12 Many soluble coordination complexes are now known that electrocatalytically mediate the hydrogen evolution reaction (HER),13,14 the carbon dioxide reduction reaction (CO2RR),15–17 and the oxygen reduction reaction (O2RR).18 The study of such systems has matured at a rapid pace in recent years, coinciding with expanded research efforts towards solar-derived fuel systems. In this context, it is noteworthy how little corresponding progress has been made towards the discovery of soluble molecular catalysts that mediate electrocatalytic N2RR. To our knowledge, only two prior systems address this topic directly.19–22 More than three decades ago, Pickett and coworkers reported that a Chatt-type tungsten-hydrazido(2−) complex could be electrochemically reduced to release ammonia (and trace hydrazine), along with some amount of a reduced tungsten-dinitrogen product (; the latter species serves as the source of the tungsten-hydrazido(2−) complex (via its protonation by acid).19 By cycling through such a process, an electrochemical, but not an 60 electrocatalytic, synthesis of ammonia was demonstrated. Indeed, efforts to demonstrate electrocatalysis with this and related systems instead led to substoichiometric NH3 yields.20,21 Figure 4.1: Electrosynthetic cycle for the formation of NH3 from N2 by a Chatt-type Wphosphine complex.19 An obvious limitation to progress in electrochemical N2RR by molecular systems concerns the small number of synthetic N2RR catalysts that have been available for study; it is only in the past five years that sufficiently robust catalyst systems have been identified to motivate such studies. In addition, the conditions that have to date been employed to mediate N2RR have typically included non-polar solvents, such as heptane, toluene, and diethyl ether (Et2O),1–8,10 that are not particularly well-suited to electrochemical studies owing to the lack of compatible electrolytes. Nevertheless, several recent developments, including ones from our lab, point to the likelihood that iron (and perhaps other) molecular coordination complexes may be able to mediate electrocatalytic N2RR in organic solvent. Specifically, our lab recently reported that a tris(phosphine)borane iron complex, [(P3B)Fe]+, that is competent for catalytic N2RR with chemical reductants, can also mediate electrolytic N2-to-NH3 conversion, 22 61 with the available data (including that presented in this study) pointing to bona fide electrocatalysis in Et2O. Focusing on the [(P3B)Fe]+ catalytic N2RR system, a development germane to the present study was its recently discovered catalysis with reagents milder than those that had been originally employed.3 Decamethylcobaltocene (Cp*2Co) and diphenylammonium acid are effective for N2RR catalysis; these reagents give rise to fast, and also quite selective (> 70% NH3), N2RR catalysis at low temperature and ambient pressure in ethereal solvent. In addition, based on preliminary spectroscopic evidence and density functional theory (DFT) predictions, it appears that a protonated metallocene species, [(Cp*)Co(η4-C5Me5H)]+, may be an important intermediate of N2RR catalysis under such conditions. Indeed, we have suggested that [(Cp*)Co(η4-C5Me5H)]+ may serve as a proton-coupled-electron-transfer (PCET) donor (BDEC–H(calc) = 31 kcal mol−1), thereby mediating net H-atom transfers to generate N–H bonds during N2RR.23 The presence of a metallocene mediator might, we therefore reasoned, enhance N2RR during electrocatalysis. We present here a study of the effect of pKa on the selectivity of [(P3B)Fe]+ for N2RR vs HER. By using substituted anilinium acids, we are able to vary the acid pKa over 9 orders of magnitude and find that the selectivity is highly correlated with the pKa. In our efforts to investigate the origin of the observed pKa effect, we found, to our surprise, that in stoichiometric reactions, the catalytically competent anilinium triflate acids are unable to facilitate productive N–H bond formation with early-stage N2-fixation intermediates. We therefore hypothesize that the formation of a protonated metallocene species, [(Cp*)Co(η4C5Me5H)]+, plays a critical role in N–H bond-forming reactions, either via PCET, PT, or a 62 combination of both during N2RR catalysis. DFT studies support this hypothesis and also establish that the observed pKa correlation with N2RR selectivity can be explained by the varying ability of the acids to protonate Cp*2Co. The suggested role of this protonated metallocene intermediate in N–H bond forming reactions led us to test the effect of [Cp*2Co]+ as an additive in the electrolytic synthesis of NH3 mediated by [(P3B)Fe]+. We find that the addition of co-catalytic [Cp*2Co]+ enhances the yield of NH3 without decreasing the Faradaic efficiency (FE), and furnishes what is to our knowledge the first unambiguous demonstration of electrocatalytic N2RR mediated by a soluble, molecular coordination complex. 4.2 Results and Discussion 4.2.1 Effect of pKa on the Selectivity of N2 Fixation by [(P3B)Fe]+ In our recent study on the ability of [(P3B)Fe]+ to perform N2RR with Cp*2Co as the chemical reductant,23 we found that there was a marked difference in efficiency for NH3 generation with diphenylammonium triflate ([Ph2NH2][OTf]) versus anilinium triflate ([PhNH3][OTf]). In that study, we posited that this difference could arise from several possibilities, including the differential solubility, sterics, or pKa’s of these acids.23 To investigate the last possibility, we have studied the efficiency of the catalysis by quantifying the NH3 and H2 produced when using substituted anilinium acids with different pKa values (Table 4.1). The table is organized in increasing acid strength, from [ 4OMe PhNH3][OTf] as the weakest acid to the perchlorinated derivative ([ per-ClPhNH3][OTf]) as the strongest. Importantly, good total electron yields (85.8 ± 3.3) were obtained in all cases. As can be seen from the table, the NH3 efficiencies are found to be strongly correlated with pKa. In some cases, the pKa of a particular anilinium acid was already 63 known in THF, in which case this value was used. In cases where the pKa has not been reported in THF a literature procedure was used to appropriately convert the pKa from the solvent in which it was measured into a value for THF. 24 In particular, a comparison of the efficiency for NH3 with the pKa of the anilinium acid used gives rise to four distinct activity regimes (Table 4.1, Figure 4.2). One regime that is completely inactive for N2RR, but active for HER, is defined by the weakest acid, [4OMe PhNH3][OTf] (pKa = 8.8). Consistent with this observation is that efforts to use other weak, non anilinium acids such as benzylammonium triflate (pKa in THF of 13.2) and collidinium triflate (pKa in THF of 11.2) also led to no observed NH3 formation. With stronger anilinium acids, a gradual increase in observed NH3 yields, coupled with a decrease in H2 yield comprises the second regime ([PhNH3][OTf], pKa = 7.8; [2,6-MePhNH3][OTf], pKa = 6.8; [2-ClPhNH3][OTf], pKa = 5.6). Yet stronger acids, [2,5-ClPhNH3][OTf] (pKa = 4.3), [2,6Cl PhNH3][OTf] (pKa = 3.4), and [2,4,6-ClPhNH3][OTf] (pKa = 2.1), constitute another, most active N2RR regime, one in which the H2 yields are nearly invariant. These results are consistent with our previous observation with [Ph2NH2][OTf] (pKa of 3.2 in THF) yielding 72 ± 3% NH3; H2 yields were not measured in that study.23 The highest selectivity for N2RR (77.5 ± 3.8%) was observed using [2,5-ClPhNH3][OTf] as the acid. A final regime of very low N2RR activity is encountered with [per-ClPhNH3][OTf] (pKa = 1.3) as the acid. We suspect this last acid undergoes unproductive reduction via ET, thereby short-circuiting N2RR. The only other N2RR system for which this type of acid-dependent correlation has been systematically studied is the enzyme MoFe-nitrogenase.25 In some other reports on N2RR by 64 molecular catalysts, efficiencies for NH3 have been reported for several acids, but typically these acids span only a small pKa range, electron yields are inconsistent, and variations are not explained.2,26 As shown in Figure 4.2, the N2RR vs HER activity of [(P3B)Fe]+ as a function of acid strength, is, in broad terms, similar to the behavior of the enzyme25 across a wide pH range. Figure 4.2. Effect of acid strength on the percentage of electrons being used to form NH 3 and H2. (left) Selectivity of the FeMo-nitrogenase in A. vinelandi at different pH values. Reprinted with permission from Pham, D. N.; Burgess, B. K. Biochemistry 1993, 32, 13725-13731. Copyright 1993 American Chemical Society. 25 (right) Selectivity of [(P3B)Fe]+ at different pKa values. Table 4.1. Literature and calculated pKa values and efficiencies observed in catalytic N2to-NH3 conversion pKaexp pKacalc pKdcalc (THF) (298 K)a (195 K)b % yield Equiv of NH3/Fe % yield of NH3/e- Total of -c Yield/e- H2/e [4-OMePhNH3]+ 8.8 9.6 15.7 0.04 ± 0.01 0.2 ± 0.1 89.1 ± 0.2 89.3 [PhNH3]+ 7.8 7.7 13.8 7.3 ± 0.1 40.4 ± 0.5 48.6 ± 0.7 87.5 [2,6-MePhNH3]+ 6.8 7.3 13.2 8.6 ± 0.7 47.5 ± 4.0 37.8 ± 0.2 85.6 65 [(Cp*)Co(exoN/A 9.2 11.8 [2-ClPhNH3]+ 5.6 5.6 6.0 10.7 ± 0.1 53.9 ± 0.4 26.1 ± 1.9 80.0 [2,5-ClPhNH3]+ 4.3 4.0 5.0 13.9 ± 0.7 77.5 ± 3.8 10.5 ± 1.1 87.7 [2,6-ClPhNH3]+ 3.4 3.4 3.4 13.8 ± 0 .9 76.7 ± 4.9 12.6 ± 2.5 89.3 [2,4,6-ClPhNH3]+ 2.1 2.7 1.8 12.8 ± 0.4 70.9 ± 2.2 12.0 ± 0.8 83.1 [per-ClPhNH3]+ 1.3 0.8 0.4 3.6 ± 0.1 83.5 η4-C5Me5H)]+ ― ― ― 19.9 ± 0.5 63.5 ± 1.1 ― [4-OMePhNH3]+= 4-methoxyanilinium; [PhNH3]+ = anilinium; [2,6-MePhNH3]+ = 2,6dimethylanilinium; [2-ClPhNH3]+ = 2-chloroanilinium; [2,5-ClPhNH3]+ = 2,5dichloroanilinium; [2,6-ClPhNH3]+ = 2,6-dichloroanilinium; [2,4,6-ClPhNH3]+ = 2,4,6trichloroanilinium; [per-ClPhNH3]+ = 2,3,4,5,6-pentachloroanilinium. aAcidities calculated at 298 K in THF and referenced to the known literature value for [2,6-ClPhNH3]+. b All species calculated as the ion-paired [OTf]− species in Et2O at 195 K and referenced to the known literature value for [2,6-ClPhNH3][OTf] in THF. In a previous study of Cp*2Co-mediated N2RR by [(P3B)Fe]+,23 we identified that [(P3B)Fe(N2)]− forms under the catalytic conditions. Earlier studies on the reactivity of [(P3B)Fe(N2)]− with an excess of soluble acids, including HOTf and [H(OEt2)2][BArF4] (HBArF4, BArF4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)), at −78 °C in Et2O, established rapid formation of the doubly protonated species, [(P3B)Fe(NNH2)]+.27 Recent computational work from our group suggests that, under catalytic conditions with a soluble acid, different efficiencies for N2RR (versus HER) by (P3E)Fe catalysts (E = B, C, Si) are likely correlated to the rate of formation and consumption of early N2RR intermediates (i.e., (P3E)Fe(NNH) and [(P3E)Fe(NNH2)]+/0).28 Thus, we were interested in the reactivity of these anilinium triflate acids with [(P3B)Fe(N2)]−, reasoning they may show differential efficiency in the formation of [(P3B)Fe(NNH2)]+. 66 To our surprise, a freeze-quench EPR spectrum of the reaction of excess [ 2,6- Cl PhNH3][OTf] (high N2RR efficiency regime) at −78 °C in Et2O with [(P3B)Fe(N2)]− does not show any [(P3B)Fe(NNH2)]+. Freeze-quench Mössbauer analysis shows the formation of the oxidized products (P3B)Fe(N2) and [(P3B)Fe]+, but nothing assignable to [(P3B)Fe(NNH2)]+. Analysis of such a reaction for NH3 or N2H4 after warming to room temperature shows no fixed-N products. The observation of exclusive oxidation, rather than productive N–H bond formation, is analogous to what is observed upon addition of 1 equiv of a soluble acid (HBArF4 or HOTf) to [(P3B)Fe(N2)]−. We have previously suggested that if unstable (P3B)Fe(NNH) is formed (Error! Reference source not found.) without excess acid to trap it as the more stable [(P3B)Fe(NNH2)]+ (Error! Reference source not found.), then it can decay bimolecularly with the loss of 1/2 H2 to form (P3B)Fe(N2), a net oxidation (Error! Reference source not found.). Eq 4.1) [(P3B)Fe(N2)]− + H+  (P3B)Fe(NNH) Eq 4.2) (P3B)Fe(NNH) + H+  [(P3B)Fe(NNH2)]+ Eq 4.3) (P3B)Fe(NNH)  (P3B)Fe(N2) + 1/2 H2 The low solubility of the anilinium triflate acids studied herein, under the catalytically relevant conditions (1 mL Et2O, −78 °C), likely leads to a similar scenario, despite the excess employed; consequently, (P3B)Fe(NNH) that is generated is not efficiently captured by acid, leading instead to bimolecular H2 loss. In accord with this idea, a freeze-quench EPR spectrum of the addition of 50 equiv of [2,6-ClPhNH3][BArF4], a far more ether-soluble derivative of the same anilinium, to [(P3B)Fe(N2)]− shows [(P3B)Fe(NNH2)]+ formation, and the detection of fixed-N products upon warming (0.20 ± 0.04 equiv NH3 per Fe). 67 These observations must be reconciled with the seemingly contradictory observation that comparatively efficient N2RR catalysis is observed when [2,6Cl PhNH3][OTf], and other anilinium triflate acids, are employed under catalytic conditions. For example, [Ph2NH2][OTf] leads to better efficiency for NH3 formation versus [Ph2NH2][BArF4] (72 ± 3% and 42 ± 6%, respectively). A key difference between the stoichiometric reactions described above, and the catalytic reaction, is the presence of Cp*2Co in the latter. We have suggested that Cp*2Co can be protonated under the catalytic reaction conditions, to form [(Cp*)Co(η4-C5Me5H)]+,23 which may then play a role in N–H bond forming steps. Recent results on a Cr–N2 species also support the role of PCET in the formation of early-stage N2 fixation intermediates in the presence of collidinium triflate and a cobaltocene .29 The results presented here (and below) suggest that such a mechanism is not only plausible, but is likely necessary, to explain the observed catalytic results with anilinium triflate acids. Given the effect of pKa on the efficiency for N2RR, we now hypothesize that this effect can arise from the relative energetics and rates of Cp*2Co protonation by the different anilinium triflate acids. 4.2.2 Computational Studies on Cp*2Co Protonation and N–H Bond Formation To investigate the kinetics and thermodynamics of Cp*2Co protonation by anilinium triflate acids we turned to a computational study. DFT-D330 calculations were undertaken at the TPSS/def2-TZVP(Fe); def2-SVP31,32 level of theory, as has been previously demonstrated is capable of not only reproducing crystallographic details but also experimentally measured singlet-triplet gaps, reduction potentials, and N–H BDFE’s on the (P3E)Fe platforms.28 The free energy of H+ exchange (ΔGa) was calculated for all of the 68 anilinium acids used (representative example shown in Error! Reference source not found.), and also for [(Cp*)Co(exo-η4-C5Me5H)]+, in Et2O at 298 K. These free energies were then used to determine the acid pKa’s, with inclusion of a term to reference them to the literature pKa value for [2,6-ClPhNH3][OTf] at 298 K in THF (Error! Reference source not found.). Eq 4.4) PhNH2 + [2,6-ClPhNH3]+  [PhNH3]+ + 2,6-ClPhNH2 Eq 4.5) pKa([PhNH3+]) = −ΔGa/(2.303×RT) + pKa([2,6-ClPhNH3]+) Because we presume that variable triflate hydrogen bonding effects (0.5-10 kcal mol−1) are likely to be important under the catalytic conditions (low temperature and low polarity solvent), we additionally calculated the free energy for net HOTf exchange reactions (ΔGd) at 195 K in Et2O (representative example shown in Error! Reference source not found.). The free energies of these reactions can then be used to determine a pKd, referenced to the pKa value for [2,6-ClPhNH3][OTf] at 298 K in THF, for ease of comparison (Error! Reference source not found.). Hereafter, we use these pKd values for discussion, but we note that use of the pKa values instead does not substantively alter the conclusions drawn. Eq 4.6) PhNH2 + [2,6-ClPhNH3][OTf]  [PhNH3][OTf] + 2,6-ClPhNH2 Eq 4.7) pKd([PhNH3][OTf]) = −ΔGd/(2.303×RT) + pKa([2,6-ClPhNH3]+) Calculations of the pKd of all of the relevant species (Table 4.1) shows that the pKd of [(Cp*)Co(exo-η4-C5Me5H)][OTf] (pKdcalc = 11.8; Table 4.1) falls within the range of the anilinium acids studied (0.4 ≤ pKdcalc ≤ 15.7), suggesting there should be a significant acid 69 dependence on the kinetics and thermodynamics of Cp*2Co protonation. To better elucidate the differences in Cp*2Co protonation between the acids studied, we investigated the kinetics of protonation for three acids, [2,6-ClPhNH3][OTf] (high selectivity; pKdcalc = 3.4), [2,6-MePhNH3][OTf] (modest selectivity; pKdcalc = 13.2), and [4-OMePhNH3][OTf] (poor selectivity; pKdcalc = 15.8). Transition states for Cp*2Co protonation were located for all three acids. To confirm that these transition states accurately reflect proton transfer, internal reaction coordinates (IRC) were followed to determine the reactant (IRC-A) and product (IRC-B) minima (Figure 4.3). These minima represent hydrogen bonded arrangements of the reactants and products. Protonation is found to have only a moderate barrier (ΔG‡ in kcal mol−1) in all three cases: ([4-OMePhNH3][OTf], +4.5; [2,6- MePhNH3][OTf], +3.8; [2,6-ClPhNH3][OTf], +1.3). This suggests that Cp*2Co protonation is kinetically accessible in all cases, in agreement with the experimental observation of background HER with each of these acids. The small differences in rate, and the large variance in the equilibrium constant Keq defined in Eq 4.8, points to a significant difference in the population of protonated metallocene, [(Cp*)Co(exo-η4-C5Me5H)][OTf], for these anilinium acids during catalysis. Eq 4.8) Keq = ([[(RPhNH2-(Cp*)Co(exo-η4C5Me5H)][OTf]])/ ([[(RPhNH3–Cp*2Co][OTf]]) We reason that the low solubility of the anilinium triflate acids, and the low catalyst concentration (2.3 mM (P3B)Fe), leads to a scenario in which the interaction between the acid and the Cp*2Co, the latter being present in excess relative to the iron catalyst (measured solubility of Cp*2Co at −78 °C in Et2O is ~ 6 mM), significantly affects the overall kinetics 70 Figure 4.3: The kinetics and thermodynamics of protonation of Cp*2Co for three acids from different catalytic efficiency regimes ([4-OMePhNH3][OTf] = poor selectivity]; [2,6Me PhNH3][OTf] = modest selectivity; [2,6-ClPhNH3][OTf] = high selectivity). of productive N–H bond formation. As such, the difference in [(Cp*)Co(exo-η4C5Me5H)][OTf] concentration and formation rate should relate to, and likely dominate, the origin of the observed pKa effect. This explanation, rather than one that involves differences in rates for the direct interaction of a given (P3B)Fe(NxHy) species with an anilinium acid, better captures the collected data available. [(Cp*)Co(exo-η4-C5Me5H)][OTf], characterized by a very weak C–H bond, should be a strong PCET donor, and we presume it serves such a role under the catalytic conditions being discussed herein. Its reactions with (P3B)Fe(NxHy) intermediates may occur in a synchronous fashion, akin to HAT, as DFT calculations suggest that in almost all cases the (P3B)Fe(NxHy) intermediates have stronger N–H bonds than the C–H bond in [(Cp*)Co(exoη4-C5Me5H)]+. Asynchronous reactions more akin to a PT-ET reaction are likely also possible because in cases where the basicity of (P3B)Fe(NxHy) has been evaluated, they are 4 + 71 predicted to be readily protonated by [(Cp*)Co(exo-η -C5Me5H)] . The reactivity of ringfunctionalized Cp* rings has been discussed previously in the context of electrocatalytic HER by 4d and 5d metals but via a mechanism of hydride transfer rather than PCET.33–35 So while multiple N–H bond forming mechanisms must be considered, the evidence suggests that a PCET reaction with [(Cp*)Co(exo-η4-C5Me5H)][OTf], plays a critical role in productively trapping the highly reactive first fixed intermediate, (P3B)Fe(NNH) (Figure 4.4), before it can bimolecularly release H2 (Eq 4.9). Productive N–H bond formation via PCET with models of late-stage N2 fixation intermediates (i.e., M≡N or M–NH2) has been observed previously,36–39 but examples with early-stage N2 fixation intermediates are lacking, so we investigated this reaction in more detail via DFT. Both a synchronous PCET (ΔGPCET = −17.3 kcal mol−1; Eq 4.9) and a fully asynchronous net-PCET path (ΔGPT = −5.7 kcal mol−1, ΔGET = −11.6 kcal mol−1; Eq 4.10 and Eq 4.11), are predicted to be thermodynamically favorable. Eq 4.9) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  (P3B)Fe(NNH2) + [Cp*2Co][OTf] Eq 4.10) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  [(P3B)Fe(NNH2)][OTf] + Cp*2Co Eq 4.11) [(P3B)Fe(NNH2)][OTf] + Cp*2Co  (P3B)Fe(NNH2) + [Cp*2Co][OTf] To evaluate the kinetics of these reactions the Marcus theory expressions40 and the Hammes-Schiffer method41 were used to approximate relative rates of bimolecular ET and 72 PCET. We find that there is a slight kinetic preference for the fully synchronous PCET reaction (krelPCET ~ 3 × 103 M−1s−1) compared to the fully asynchronous PT-ET reaction (krelPT-ET ≈ krelET ≡ 1 M−1 s−1; Figure 4.4) assuming in that case that ET is rate limiting based on lower reorganization energies and barriers for PT than ET. Figure 4.4: The calculated thermodynamics and kinetics of synchronous PCET and fully asynchronous PCET (PT-ET), between (P3B)Fe(NNH) and [(Cp*)Co(exo-η4-C5Me5H)] [OTf] to generate (P3B)Fe(NNH2). The above discussion leads to the conclusion that the efficiency for NH3 formation in this system is coupled to the kinetics and/or thermodynamics of the reaction between the anilinium triflate acid and the Cp*2Co reductant. This conclusion is counterintuitive, as the protonation of Cp*2Co is also the requisite first step for background HER.42 The fact that a key HER intermediate can be intercepted and used for productive N2RR steps is an important 73 design principle for such catalysis. Similar design strategies are currently being used to repurpose molecular cobalt HER catalysts for the reduction of unsaturated substrates.43 Efforts are often undertaken to suppress background reactivity between acid and reductant in catalytic N2RR systems.1,2 We were hence particularly interested to explore whether the inclusion of a metallocene co-catalyst, in this case Cp*2Co, might improve the yield, and/or the Faradaic efficiency (FE), for N2RR versus HER, in controlled potential electrolysis (CPE) experiments with [(P3B)Fe]+ under N2. 4.2.3 Electrocatalytic N2 Fixation with [(P3B)Fe]+ To set the context for this section of the present study, we had shown previously that ~ 2.2 equiv NH3 per Fe could be generated via controlled potential electrolysis (CPE; −2.3 V vs Fc+/0) at a reticulated vitreous carbon working electrode, using [(P3B)Fe]+ as the (pre)catalyst in the presence of HBArF4 (50 equiv) at −45 °C under an atmosphere of N2. This yield of NH3 corresponded to a ~ 25% FE22 which, while modest in terms of overall chemoselectivity, compares very favorably to FE’s most typically reported for heterogeneous electrocatalysts for N2RR that operate below 100 °C (< 2%), although some recently reported studies have reported heterogeneous systems with significantly improved yields (> 10% FE).11,12 To further explore the possibility of using [(P3B)Fe]+ as an electrocatalyst for N2RR, various conditions were surveyed to determine whether enhanced yields of NH3 could be obtained from CPE experiments. For example, various applied potentials were studied (ranging from −2.1 to −3.0 V vs Fc+/0), the concentrations of [(P3B)Fe]+ and HBArF4 were varied, the ratio of acid to catalyst was varied, and the rate at which acid was delivered to the system was varied (e.g., initial full loadings, batch-wise additions, reloadings, or continuous 74 slow additions). None of these studies led to substantial improvement in N2RR; in all cases, < 2.5 equiv of NH3 was obtained per Fe. Attempts to vary the ratio of the electrode surface area to the working compartment solution volume, either by employing smaller cell geometries or by using different morphologies of glassy carbon as the working electrode (e.g., reticulated porous materials of different pore density or plates of different dimensions), also failed to provide substantial improvement in NH3 yield. The replacement of HBArF4, the original acid used in our electrolysis studies,22 by 50 equiv of [Ph2NH2][OTf] led to similar yields of NH3 (Table 4.2, entry 1). We next investigated the effect of [Cp*2Co]+ as an additive on the electrolysis/electrocatalysis. Traces of relevant cyclic voltammograms (Figure 4.5) collected with glassy carbon as the working electrode in Et2O under glovebox atmosphere N2 at −35 °C are provided. Background traces including only [Ph2NH2][OTf] are present in both panels (gray traces, Figure 4.5). [Cp*2Co]+ (yellow trace, Figure 4.5A), [Cp*2Co]+ with the addition of ten equiv of [Ph2NH2][OTf] (green trace, Figure 4.5A), [(P3B)Fe]+ (dark blue trace, Figure 4.5B), [(P3B)Fe]+ with the addition of ten equiv of [Ph2NH2][OTf] (light blue trace, Figure 4.5B), and [(P3B)Fe]+ with the addition of one equiv of [Cp*2Co]+ and ten equiv of [Ph2NH2][OTf] (red traces, Figure 4.5). 75 Figure 4.5: Cyclic voltammograms of relevance to electrocatalytic N2 fixation in the with co-catalytic [(P3B)Fe]+ and [Cp*2Co]+. A) 10 equiv [Ph2NH2][OTf] (gray trace), 1 equiv [Cp*2Co][BArF4] ([Cp*2Co]+) (yellow trace), 1 equiv [Cp*2Co]+ with 10 equiv [Ph2NH2][OTf] (green trace), and [(P3B)Fe]+ with 1 equiv of [Cp*2Co]+ and 10 equiv [Ph2NH2][OTf] (red trace). B) 10 equiv [Ph2NH2][OTf] (gray trace), [(P3B)Fe]+ (dark blue trace), [(P3B)Fe]+ with 10 equiv [Ph2NH2][OTf] (light blue trace), and [(P3B)Fe]+ with 1 equiv of [Cp*2Co]+ and 10 equiv [Ph2NH2][OTf] (red trace). All voltammograms are collected in 0.1 M NaBArF4 solution in Et2O at −35 °C using a glassy carbon working electrode, and externally referenced to the Fc+/0 couple. Scan rate is 100 mV/s. The cyclic voltammogram of [Cp*2Co]+ is shown in Figure 4.5A (yellow trace), displaying the reversible [Cp*2Co]+/0 couple at −2.0 V. The addition of [Ph2NH2][OTf] to [Cp*2Co]+ causes an increase in current at this potential, consistent with HER catalyzed by [Cp*2Co]+ (green trace, Figure 4.5A). Figure 4.5B provides the cyclic voltammogram of [(P3B)Fe]+ in the absence (dark blue trace, showing previously assigned and quasi-reversible [(P3B)Fe(N2)]0/− couple at ~ −2.1 V) and in the presence (light blue trace) of [Ph2NH2][OTf].44 The latter is indicative of modest HER and N2RR. Also evident upon the addition of acid is the disappearance of a wave corresponding to the [(P3B)Fe]+/0 couple at ~ −1.6 V. This wave, in the absence of acid, is broad and shows a large peak-to-peak separation, likely due to the presence of both [(P3B)Fe]+ and [(P3B)Fe(N2)]+ in solution at −35 °C. The addition of a large excess of [Ph2NH2][OTf] presumably results in triflate binding (to generate (P3B)Fe(OTf), thereby attenuating the waves associated with the reduction of 76 [(P3B)Fe]+ and [(P3B)Fe(N2)]+). The red trace of Figure 4.5A is reproduced in Figure 4.5B to illustrate the marked increase in current observed when Cp*2Co is added. Table 4.2. Yields and Faradaic Efficiencies of NH3 from CPE Experiments with [(P3B)Fe]+ Equiv Equiv NH3 Equiv NH3 NH3 FE Entry [Cp*2Co]+ (per Fe) (per Co) (%) 1 0 2.6 ± 0.3d ― 24 ± 5 2a 0 2.6 ± 0.6 ― 18 ± 1 4.0 ± 0.6 4.0 ± 0.6 28 ± 5 3 4 5 4.0 ± 0.6d 0.8 ± 0.1 25 ± 3 5a 5 5.5 ± 0.9e 1.1 ± 0.2 19 ± 1 6 10 4±1 0.4 ± 0.1 24 ± 7 7b 5 1.9 ± 0.2 0.4 ± 0.1 10 ± 1 8c 5 0.9 ± 0.4 0.2 ± 0.1 6±3 All CPE experiments conducted at −2.1 V vs Fc+/0 with 0.1 M NaBArF4 in Et2O as solvent at −35 °C under an N2 atmosphere, featuring a glassy carbon plate working electrode, Ag+/0 reference couple isolated by a CoralPorTM frit referenced externally to Fc+/0, and a solid sodium auxiliary electrode. Working and auxiliary chambers separated by a sintered glass frit. Averages represent two runs unless noted. aAfter initial electrolysis with 50 equiv [Ph2NH2][OTf], an additional 50 equiv [Ph2NH2][OTf] in 0.1 M NaBArF4 Et2O solution was 77 added to the working chamber, via syringe through a rubber septum, followed by additional CPE at −2.1 V vs Fc+/0. The listed Equiv NH3 (per Fe or Co) for these runs is the total yield from both electrolysis experiments. b[2,6-ClPhNH3][OTf] employed as the acid. c [PhNH3][OTf] employed as the acid. dAverage of three runs. eAverage of five runs. CPE studies were undertaken to characterize the reduction products associated with the red trace at ~ −2.1 V vs Fc+/0. These studies employed a glassy carbon plate electrode, a Ag+/0 reference electrode that was isolated by a CoralPorTM frit and referenced externally Fc+/0 redox couple, and a solid sodium auxiliary electrode. The latter was used to avoid excessive, non-productive redox cycling between the working and auxiliary chambers. Unless otherwise noted, CPE experiments were performed at −2.1 V versus Fc+/0, with 0.1 M NaBArF4 as the ether-soluble electrolyte, under a glovebox N2 atmosphere at −35 °C. The electrolysis was continued until the current had dropped to 1% of the initial current measured, or until 21.5 hours had passed. CPE experiments were conducted with the inclusion of 0, 1, 5, and 10 equiv of [Cp*2Co]+ with respect to [(P3B)Fe]+, using excess [Ph2NH2][OTf] as the acid. In the absence of added [Cp*2Co]+, a significant amount of NH3 was generated (2.6 ± 0.3 equiv per Fe, Table 4.2, entry 1), consistent with the previous finding that, in the presence of a strong acid, [(P3B)Fe]+ can electrolytically mediate N2-to-NH3 conversion.22 Notably, when a CPE experiment that did not include [Cp*2Co]+ was reloaded with additional acid after electrolysis and electrolyzed again, the total yield of NH3 (2.6 ± 0.6 equiv NH3 per Fe, Table 4.2, entry 2) did not improve. We found that inclusion of 1.0 equiv of [Cp*2Co]+ enhanced the NH3 yield, by a factor of ~ 1.5 (Table 4.2, entry 3) without decreasing the FE. The data provide a total yield, 78 with respect to both Fe and Co, that confirms modest but unequivocal N2RR electrocatalysis. In single run experiments, the highest NH3 yield in the absence of [Cp*2Co]+ was 2.8 equiv, compared with 4.4 equiv in the presence of 1 equiv of [Cp*2Co]+. Conversely, the lowest single run NH3 yield in the absence of [Cp*2Co]+ was 2.3 equiv, compared with 3.5 equiv in the presence of 1 equiv of [Cp*2Co]+. Increasing the amount of added [Cp*2Co]+ did not affect the NH3 yield (Table 4.2, entry 4). However, the addition of a second loading of [Ph2NH2][OTf] following the first electrolysis (Table 4.2, entry 5), followed by additional electrolysis, led to an improved yield of NH3, suggesting that some active catalyst is still present after the first run.22,23 Even higher [Cp*2Co]+ loading did not lead to higher NH3 yields (Table 4.2, entry 6). CPE of [(P3B)Fe]+ in the presence of [Cp*2Co]+ was also explored with other acids. Replacing [Ph2NH2][OTf] in these experiments with [2,6-ClPhNH3][OTf] led to lower yields of NH3, and with [PhNH3][OTf] even lower yields of NH3 were observed (Table 4.2, entries 7 and 8 respectively). The lower, but nonzero, yield of NH3 provided by [PhNH3][OTf] in these CPE experiments is consistent with chemical trials employing various acids (vide supra) and can be rationalized by the relative pKa of the acids (Table 4.1). The intermediate yield of NH3 provided by [2,6-ClPhNH3][OTf] in these CPE experiments is less consistent with simple pKa considerations, suggesting that additional factors are at play, perhaps including the relative stability of the acid or conjugate base to electrolysis. To probe whether electrode-immobilized iron might contribute to the N2RR electrocatalysis, X-ray photoelectron spectroscopy (XPS) was used to study the electrode. After a standard CPE experiment with [(P3B)Fe]+, 5 equiv of [Cp*2Co]+, and 50 equiv 79 [Ph2NH2][OTf], the electrode was removed, washed with fresh 0.1 M NaBArF4 Et2O solution, then fresh Et2O, and probed by XPS. A very low coverage of Fe (< 0.3 atom % Fe) was detected in the post-electrolysis material; no Fe was detected on a segment of the electrode which was not exposed to the electrolytic solution. This observation implies a detectable but likely small degree of degradation of [(P3B)Fe]+ over the course of a 15 hour CPE experiment. Worth noting is that no Co was detected on the post-electrolysis electrode. This may be consistent with the known stability of metallocenes and the recently discovered stability of protonated-Cp* ligands on Rh.45 To test whether the small amount of deposited Fe material might be catalytically active for N2RR, following a standard CPE experiment the electrode was removed from the cold electrolysis solution, washed with fresh 0.1 M NaBArF4 Et2O at −35 °C (the electrode itself was maintained at −35 °C at all times), and then used for an additional CPE experiment, under identical conditions except that [(P3B)Fe]+ was excluded. This CPE experiment yielded no detectable NH3. The charge passed, and H2 yield, were very similar to a “no [(P3B)Fe]+” control experiment conducted with a freshly cleaned electrode. Accordingly, a CPE experiment in the absence of [(P3B)Fe]+ demonstrated that [Cp*2Co]+ serves as an effective electrocatalyst for HER with [Ph2NH2][OTf] as the acid source, but does not catalyze the N2RR reaction (0% FE for NH3, 75% FE for H2). This background HER, and the observed catalytic response to the addition of [Ph2NH2][OTf] at the [Cp*2Co]+/0 couple, provides circumstantial evidence for the formation of a protonated decamethylcobaltocene intermediate, [(Cp*)Co(η4-C5Me5H)]+, on a timescale similar to that of the N2RR mediated by [(P3B)Fe]+. 80 To probe whether the sodium auxiliary electrode used in the CPE experiments might play a non-innocent role as a chemical reductant, a standard CPE experiment with [(P3B)Fe]+, 5 equiv [Cp*2Co]+, and 50 equiv [Ph2NH2][OTf] was assembled, but was left to stir at −35 °C for 43 hours without an applied potential bias. This experiment yielded 0.3 equiv NH3 (relative to Fe), suggesting that background N2RR due to the sodium auxiliary electrode is very minor. To ensure the NH3 produced was derived from the N2 atmosphere during these electrolysis experiments, as opposed to degradation of the anilinium acid used, a standard CPE experiment using [(P3B)Fe]+, 5 equiv [Cp*2Co]+, and 50 equiv of [Ph215NH2][OTf] was performed. Only 14NH3 product was detected. We also sought to compare the chemical N2RR catalysis efficiency of the [(P3B)Fe]+ catalyst under conditions similar to those used for electrocatalysis. Hence, chemical catalysis with [(P3B)Fe]+, employing Cp*2Co as a reductant and [Ph2NH2][OTf] as the acid at −35 °C instead of the more typical temperature of −78 °C, in a 0.1 M NaBArF4 Et2O solution, afforded lower yields of NH3 (1.8 ± 0.7 equiv of NH3 per Fe) than the yields observed via electrolysis with [Cp*2Co]+ as an additive. The lower yields of NH3 in these chemical trials, compared with our previously reported conditions (12.8 ± 0.5 equiv of NH3 per Fe at −78 °C),23 may be attributable to increased competitive HER resulting from a more solubilizing medium (0.1 M NaBArF4 Et2O vs pure Et2O) and a higher temperature (−35 °C vs −78 °C).23 These results suggest that an electrochemical approach to NH3 formation can improve performance, based on selectivity for N2RR, of a molecular catalyst under comparable conditions. 81 4.3 Conclusion Herein we described the first systematic pKa studies on a synthetic nitrogen fixation catalyst and find a strong correlation between pKa and N2RR vs HER efficiency. Chemical studies reveal that, on their own, the anilinium triflate acids employed in the catalysis are unable to generate the N–H bonds of early-stage N2RR intermediates such as [(P3B)Fe(NNH2)]+. We propose that the insolubility of these triflate acids prevents the sufficiently rapid proton transfer necessary to capture the critical but unstable first fixed intermediate, (P3B)Fe(NNH). Under catalytic conditions, we believe that the presence of the metallocene reductant (Cp*2Co) is essential, as this species can be protonated in situ to form [(Cp*)Co(exo-η4-C5Me5H)]+, which in turn is effective in N–H bond formation with early intermediates. This leads to the intriguing conclusion that an intermediate of the background HER pathway is redirected for productive N2RR chemistry during catalysis. DFT studies illustrate that the pKa effect on the N2RR efficiency may be explained by the variation in the kinetics and thermodynamics of Cp*2Co protonation by the different acids. Investigation of the reactivity of [(Cp*)Co(exo-η4-C5Me5H)]+ with the (P3B)Fe(NNH) intermediate revealed that PCET reactivity, both synchronous or fully asynchronous, is favorable and may proceed with only a small barrier, suggesting that (P3B)Fe(NNH) can be rapidly trapped by [(Cp*)Co(exo-η4-C5Me5H)]+. We suspect [(Cp*)Co(exo-η4-C5Me5H)]+ is likely involved in a variety of N–H bond forming reactions during the overall catalysis, including reactions with late-stage nitrogen fixation intermediates. Despite the fact that [Cp*2Co]+ itself catalyzes HER under the conditions employed for electrocatalytic N2RR, we found that its inclusion in CPE experiments containing [(P3B)Fe]+ and acid under an N2 atmosphere led to modest improvements in the overall 82 catalytic yield of NH3. This system represents the first unambiguous example of electrocatalytic N2RR mediated by a soluble, molecular coordination complex. 4.4 References (1) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76–78. (2) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120–125. (3) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84–87. (4) Ung, G.; Peters, J. C. Angew. Chem. Int. Ed. 2015, 54, 532–535. (5) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181–12189. (6) Kuriyama, S.; Arashiba, K.; Tanaka, H.; Matsuo, Y.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chem. Int. Ed. 2016, 55, 14291–14295. (7) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. J. Am. Chem. Soc. 2016, 138, 13521–13524. (8) Buscagan, T. M.; Oyala, P. H.; Peters, J. C. Angew. Chem. Int. Ed. 2017, 56, 6921– 6926. (9) Wickramasinghe, L. A.; Ogawa, T.; Schrock, R. R.; Müller, P. J. Am. Chem. Soc. 2017, 139, 9132–9135. (10) Fajardo Jr., J.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 16105–16108. (11) Shipman, M. A.; Symes, M. D. Catal. Today 2017, 286, 57–68. (12) Kyriakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Catal. Today 2017, 286, 2–13. (13) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5, 865–878. 83 (14) Bullock, R. M.; Appel, A. M.; Helm, M. L. Chem. Commun. 2014, 50, 3125–3143. (15) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem. Soc. Rev. 2008, 38, 89–99. (16) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; et al. Chem. Rev. 2013, 113, 6621– 6658. (17) Francke, R.; Schille, B.; Roemelt, M. Chem. Rev. 2018, 118, 4631–4701. (18) Zhang, W.; Lai, W.; Cao, R. Chem. Rev. 2017, 117, 3717–3797. (19) Pickett, C. J.; Talarmin, J. Nature 1985, 317, 652–653. (20) Pickett, C. J.; Ryder, K. S.; Talarmin, J. J. Chem. Soc., Dalton Trans. 1986, 7, 1453– 1457. (21) Becker, J. Y.; Avraham, S. J. Electroanal. Chem. 1990, 280, 119–127. (22) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (23) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. (24) Garrido, G.; Rosés, M.; Ràfols, C.; Bosch, E. J. Solution Chem. 2008, 37, 689–700. (25) Pham, D. N.; Burgess, B. K. Biochemistry 1993, 32, 13725–13731. (26) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Schrock, R. R.; Müller, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17099–17106. (27) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (28) Matson, B. D.; Peters, J. C. ACS Cat. 2018, 8, 1448–1455. 84 (29) Kendall, A. J.; Johnson, S. I.; Bullock, R. M.; Mock, M. T. J. Am. Chem. Soc. 2018, 140, 2528–2536. (30) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104– 154119. (31) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (32) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (33) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Chem. Commun. 2016, 52, 9105– 9108. (34) Quintana, L. M. A.; Johnson, S. I.; Corona, S. L.; Villatoro, W.; Goddard, W. A.; Takase, M. K.; VanderVelde, D. G.; Winkler, J. R.; Gray, H. B.; Blakemore, J. D. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 6409–6414. (35) Peng, Y.; Ramos-Garces, M. V.; Lionetti, D.; Blakemore, J. D. Inorg. Chem. 2017, 56, 10824–10831. (36) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M. Angew. Chem. Int. Ed. 2009, 48, 3158–3160. (37) Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2015, 137, 3498–3501. (38) MacLeod, K. C.; McWilliams, S. F.; Mercado, B. Q.; Holland, P. L. Chem. Sci. 2016, 7, 5736–5746. (39) Lindley, B. M.; Bruch, Q. J.; White, P. S.; Hasanayn, F.; Miller, A. J. M. J. Am. Chem. Soc. 2017, 139, 5305–5308. (40) Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978. 85 (41) Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2001, 123, 3723–3733. (42) Koelle, U.; Infelta, P. P.; Graetzel, M. Inorg. Chem. 1988, 27, 879–883. (43) Call, A.; Casadevall, C.; Acuña-Parés, F.; Casitas, A.; Lloret-Fillol, J. Chem. Sci. 2017, 8, 4739–4749. (44) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (45) Henke, W. C.; Lionetti, D.; Moore, W. N. G.; Hopkins, J. A.; Day, V. W.; Blakemore, J. D. ChemSusChem 2017, 10, 4589–4598. 86 Cp* Non-innocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation Adapted from: Chalkley, M. J.; Oyala, P. H..; Peters, J. C. J. Am. Chem. Soc. 2019, 141, 4721–4729. DOI: 10.1021/jacs.9b00193 87 5.1 Introduction Metallocenes such as ferrocene, chromocene, and cobaltocene have enjoyed a privileged role in the development of organometallic chemistry and serve as useful reagents owing to their high compositional stabilities and accessible redox couples. 1,2 Indeed, many chemists first encounter metallocenes in the context of their one-electron redox chemistry, with the [Cp2Fe]+/0, [Cp2Cr]+/0, and [Cp2Co]+/0 couples, and those of their related permethylated variants, being some of the most commonly exploited in all of synthetic chemistry.3 An area where divalent metallocene reductants (e.g., Cp* 2Cr, Cp*2Co) have been proven particularly effective is catalytic N2-to-NH3 conversion (N2RR).4–6 Schrock first identified their utility in this context via the discovery of a molybdenum tris(amido)amine ((HIPTN3N)Mo, HIPTN3N = [(3,5-(2,4,6- iPr3C6H2)2C6H3NCH2CH2)3N]3−) N2RR catalyst system employing lutidinium as the acid and Cp*2Cr as the reductant.4 Since that discovery, other labs have exploited related cocktails that pair a metallocene reductant with an acid to drive N2RR using a range of metal catalysts, with selectivities and turnover numbers that continue to improve.5–9 The protonation chemistry of metallocenes is well studied, especially among Group 810,11 and 1012,13 metallocenes. Related studies on more reducing Group 6 and 9 metallocenes (e.g., Cp*2Cr, Cp2Co, Cp*2Co), which are relevant to the aforementioned proton-coupled reduction of N2, have been much more limited. While studies have shown that the release of H2 is highly favorable on both thermodynamic and kinetic grounds,14 protonated Group 6 and 9 metallocene intermediates have not been reliably identified and 88 characterized. It has been presumed that the direct reactions of acids with reducing metallocenes are deleterious to selectivity for N2RR versus H2 generation.4,15,16 Our lab became interested in metallocenes following the observation that Cp* 2Co could serve as the electron source for Fe-mediated N2RR in the presence of anilinium acids and an iron catalyst, (P3B)Fe (P3B = tris(o-diisopropylphosphinophenyl)-borane).8,9 Indeed, the selectivity for N2RR under these conditions proved far more efficient for NH3 formation (up to 78%) than our originally published conditions using KC 8 and [H(OEt2)2][BArF4] (HBArF4, BArF4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)).17,18 However, contrary to our mechanistic experiments with HBArF4, reaction of [(P3B)Fe(N2)]− with anilinium triflate acids led neither to the observation of relevant intermediates (e.g., [(P3B)Fe(NNH2)]+) in freeze-quench spectroscopic methods, nor to the observation of fixed-N products upon annealing.9,19,20 The apparent need for both acid and reductant to be present to achieve productive N–H bond formation is reminiscent of Schrock and coworkers’ observations when attempting to functionalize (HIPTN3N)Mo(N2) with catalytically relevant reagents.21 In both cases, we have hypothesized that metallocene-mediated proton-coupled electron transfer reactions may play a key role in N–H bond-forming steps.8,9,22 Furthermore, given the ubiquity of these metallocene reagents in N2RR, we wondered whether metallocenemediated N–H bond forming steps might provide a contributing, or even dominant, mechanistic pathway. Density functional theory (DFT) studies by our group supported the notion that protonation of metallocenes such as Cp*2Co or Cp*2Cr by catalytically relevant acids is 89 thermodynamically favorable. To our surprise, these DFT studies also predicted that ring protonation is thermodyamically favored versus protonation at the metal (to form a metal hydride).8 Such selectivity would contrast with the classic case of ferrocene, where protonation at Fe has been firmly established.10,23 The Cp*2Co and Cp*2Cr ring-protonated species are predicted to have remarkably weak C–H bond dissociation enthalpies (BDE <37 kcal mol−1), which should in turn make them excellent PCET donors.8,9 These observations intimate that protonated metallocene intermediates might thereby offer viable pathways for N–H bond formation via PCET (or even hydride transfer; vide infra), in addition to the more commonly presumed pathway for deleterious H 2 evolution (Figure 5.1). Figure 5.1: Reaction pathways to consider for protonated Cp*2Co species, illustrating both undesired HER and possible N–H bond forming steps relevant to N 2RR. We previously reported preliminary investigations into the protonation of Cp* 2Co. In brief, slow addition of a toluene solution of Cp* 2Co to a vigorously stirred suspension of trifluoromethanesulfonic acid (HOTf) in toluene at −78 °C led to precipitation of a purple solid that could be isolated via filtration. On the basis of the X-band continuous wave (CW) electron paramagnetic resonance (EPR) spectrum (77 K, Figure 4) of the solid, we speculated that it was a protonated Cp*2Co species.8 Herein, we undertake the rigorous characterization of the protonation products of Cp* 2Co using pulse EPR techniques, and provide unequivocal evidence for the assignment of the ring protonated isomers [(Cp*)Co(exo/endo-η4-C5Me5H)]+. Variable temperature Q-Band (34 GHz) pulse EPR spectroscopy, in conjunction with DFT predictions, are key to enabling the assignment. We 90 also demonstrate that exo-protonation can be favored when using a bulkier acid. Of further interest, we provide physical data to experimentally assess the C−H bond dissociation free energy (BDFEC−H) for [(Cp*)Co(exo-η4-C5Me5H)]+, which support our earlier DFT predictions that it has an exceptionally weak C–H bond (Figure 5.1). This behavior should not be limited to the present protonated metallocene, and we thus outline a general approach to understanding the reactivity of arene-protonated metal species. 5.2 Results 5.2.1 Pulse Electron Paramagnetic Resonance Spectroscopy on Protonated Cp* 2Co To interrogate the reaction of Cp*2Co with HOTf, we employed Q-band pulse EPR experiments at very low temperatures. Electron spin-echo (ESE) detected, field-swept spectra at Q-band, performed at 6 K and 10 K, clearly identify the presence of two different species with dramatically different g-anisotropy in the precipitated solid (Figure 5.2). Fortuitously, measurement of the approximate spin-lattice relaxation rates via inversion recovery reveals that the two species have significantly different T1’ times. The species with higher g-anisotropy (g = [2.625, 2.349, 1.984]) exhibits a much shorter T1’ than the species exhibiting a narrower spectrum (g = [2.170, 2.085, 2.005]), even at 6 K. This difference in relaxation rates becomes more dramatic upon warming the sample to 10 K; at this temperature, T1’ for the species with high g-anisotropy is short enough to greatly diminish its electron-nuclear double resonance (ENDOR) response relative to the other species, even at magnetic fields with significant spectral overlap. Thus, the signals arising 91 from these two species in pulse EPR experiments can be largely isolated by recording spectra at these two different temperatures (6 K and 10 K; Figure 5.2). Figure 5.2: Pseudomodulated24 Q-band ESE-detected EPR spectra of the reaction of Cp*2Co with HOTf (black traces), and DOTf (blue traces) measured at 10 K (top traces) and 6 K (bottom traces). Simulations for each species are displayed in red. The g-values and 1H hyperfine coupling is in Figure 5.6c. Furthermore, for the species with larger ganisotropy a 59Co coupling used was [245, 155, 187] MHz, e2qQ/h = 170 MHz, η < 0.1. For the species with smaller g-anisotropy, a 59Co coupling of [15, 15, 225] MHz was used. The sharp signals with asterisks above them are due to a background signal arising from a component of the EPR cavity. For the narrower, more slowly relaxing species, a comparison of the Q-band ESEdetected EPR spectra from the reaction of Cp*2Co with HOTf and DOTf shows a clear change, from 16 resolved splittings centered at 1270 mT, to the 8 lines expected for a large hyperfine coupling to an I = 7/2 59Co nucleus (A(59Co)10 K = [15, 15, 225] MHz). This observation indicates that, at least at the orientation corresponding to g3, there is a single 1 59 92 H nucleus with a hyperfine coupling of approximately ½ of the magnitude of the Co hyperfine coupling. Analysis of the field-dependent 1H ENDOR (Figure 5.3a-b) at 10 K, with corroboration from 2H hyperfine sub-level correlation (HYSCORE) spectra (Figure 5.4) of each respective species, allows determination of the full hyperfine tensor of this acidderived proton: A(1H)10 K = [106.5, 112.5, 108.2] MHz, with aiso = 109.1 MHz. Comparing this value to that known for the hydrogen atom (1420 MHz) indicates that approximately 0.08 e− are localized on this proton.25 The amount of spin density localized on this proton is unusually large, even when compared with highly reactive, paramagnetic transition metal hydrides.26 Characterization of the species with greater g-anisotropy was targeted by performing analogous experiments at 6 K. In this case, the Q-band ESE-detected EPR spectra for this species in samples generated with HOTf and DOTf are indistinguishable, indicating that the 1H hyperfine coupling to the acid-derived proton is small in comparison to the 59Co hyperfine and the inhomogeneous line broadening. This was confirmed via field-dependent ENDOR (Figure 5.3c-d) and HYSCORE (Figure 5.4) spectra acquired at 1 93 6 K, which reveal a single acid-derived proton coupling of A( H)6 K = [19.0, 15.0, 19.5] MHz, aiso = 17.8 MHz. Figure 5.3: a) Q-band Davies ENDOR spectra at 10 K and b) narrow Q-band Davies ENDOR spectra at 10 K. c) Q-band Davies ENDOR spectra at 6 K and d) narrow Q-band Davies ENDOR spectra at 6 K. Data from the reaction with HOTf (black) and DOTf (blue) are shown. A simulation (red) is given and in the spectra at 6 K the simulation is further decomposed into the components for the 1H simulation (green) and the 59Co simulation (light blue). 94 Figure 5.4: Comparison of Q-band HYSCORE spectra acquired at the center of the EPR spectrum of the endo- (left) and exo- (right) protonated Cp*2Co species generated via reaction with DOTF (top panels) and HOTF (2nd panels from top). To minimize the intensity of the features centered at 5 MHz in the (−,+) quadrant common to both isotopologues, the 2H−1H difference spectrum is shown third from the top. Simulations of the 2H hyperfine couplings derived from the 1H couplings determined via ENDOR for the endo- and exo-species are overlaid in red over the difference spectra, which are plotted in grey. Additionally, features from 59 59 95 Co (A( Co)6 K = [245, 160, 187] MHz) are observable in the ENDOR spectra at all fields, which is likely due to the more isotropic nature of the coupling symmetry of this species in comparison to the species with smaller ganisotropy. In the ENDOR acquired at 1230 mT, additional splittings due to the 59Co nuclear quadrupole interaction are resolved. Simulations of these ENDOR spectra indicate a quadrupole coupling constant of e2qQ/h = 170 MHz, with negligible electric field gradient rhombicity (η < 0.1). These values are very similar to those reported for Cp2Co+ (e2qQ/h = 171.5 MHz and η = 0), as previously determined by nuclear quadrupole resonance.27 This suggests that protonation results in only a relatively minor perturbation of the environment around Co. 5.2.2 Stereochemical Assignment of Cp*2Co Protonation Notably, both of the proton hyperfine coupling tensors are highly isotropic in nature, with only small anisotropic components (T(1H)10 K = [−2.6, 3.4, −0.9] MHz; T(1H)6 K = [1.2, −2.8, 1.7] MHz). To evaluate possible chemical assignments for these observations, DFT calculations were performed to optimize the structure of the three plausible protonation isomers (i.e., Co–H, exo-C–H, and endo-C–H) and then single point calculations were performed to predict the relevant hyperfine tensors (Figure 5.6a). Different functionals (TPSS,28 TPSSH,28 BP86,29,30 B3LYP29,31–33) were used for both the optimization and also the single point calculation to determine the hyperfine coupling. In all cases, similar T-tensors and the highly disparate magnitude of the aiso for the endo- and exo-functionalized species were observed (vide supra). Consistent with previous experimental observations for paramagnetic transition metal hydrides, DFT predicts the 1 96 Co–H isomer to have a large, roughly axial dipolar coupling tensor (T( H)Co–H = [34.1, −20.7, −12.9]).34,35 Furthermore, the predicted aiso value for the Co–H of −50 MHz is inconsistent with our experimental EPR data for the protonated species. In contrast, the hyperfine coupling tensor for both exo- and endo-isomers are predicted to be far more isotropic (T(1H)exo-C−H = [−2.4, 3.8, −1.4] MHz and T(1H)endo-C−H = [−3.1, 8.7, −5.6] MHz), consistent with the available EPR data. Importantly, our DFT calculations also predict that the two ring-protonated isomers have very different aiso values, with the exo-isomer predicted to have aiso = 119 MHz and the endo-isomer predicted to have aiso = 31 MHz. Thus, we assign the species with small g-anisotropy to be the exo-isomer and that with large g-anisotropy to be the endo-isomer. Assuming this assignment is correct then, on the basis of the recorded CW EPR spectra, the endo-isomer is formed in higher yield with HOTf (Figure 5.5, top). We wondered whether it might be possible to achieve preferential exo-functionalization by using a bulkier acid. Indeed, we have found that slow addition of a toluene solution of Cp*2Co at −78 °C to a rapidly stirred suspension of the more sterically encumbered bis(trifluoromethane)sulfonimide (HNTf2) in toluene also precipitates a purple solid (Figure 5.5, bottom). Analysis of this solid by CW X-Band EPR at 77 K reveals a near complete inversion of the protonation-site selectivity suggesting that with the smaller HOTf, endoprotonation is preferred (Figure 5.6a), but with the bulkier HNTf2, steric clash with the opposite Cp* ring leads to exo-protonation being favored (Figure 5.6b). We believe that both of the protonation reactions are under kinetic control due to the rapid precipitation upon proton transfer. 97 Figure 5.5: (top) X-Band CW EPR spectrum of the reaction of Cp*2Co with HOTf (black) and its simulation (red). (bottom) X-Band CW EPR spectrum of the reaction of Cp* 2Co with HNTf2 (black) and its simulation (red). The simulations are generated using the same parameters except for the weighting of the two species. For the top simulation it is 10:1 endo:exo and in the bottom simulation it is 3:10 endo:exo. Further chemical confirmation of the protonation site was obtained by prefunctionalization of the Cp* ring. Taking a cue from classic literature, we noted that Wilkinson and coworkers previously characterized a far more stable, neutral ringfunctionalized species, (Cp)Co(η4-C5H6).36 By analogy to their approach, we generated (Cp*)Co(exo-η4-C5Me5H) in moderate yield via the reaction of [Cp*2Co]+ with excess tetrabutylammonium borohydride in refluxing THF. The stereospecificity of exofunctionalization could be confirmed in the solid state by XRD analysis (Figure 5.6b) and in solution via NMR spectroscopy. 98 Figure 5.6: a) DFT optimized structures for the isomers of protonated Cp*2Co. The predicted A(1H) values for the protons circled in red. b) X-ray crystal structure of (Cp*)Co(exo-η4-C5Me5H). Thermal ellipsoids are shown at 50% probability. c) Experimentally derived 1H hyperfine parameters for the endo- and exo-isomers of protonated Cp*2Co. Although our efforts to use common oxidants (i.e., Fc+, Ag+) to affect the electron transfer were unsuccessful, we found that (Cp*)Co(exo-η4-C5Me5H) could be oxidized by reaction with HBArF4 at −78 °C in pentane. The purple precipitate was analyzed by X-Band CW EPR and, as expected, demonstrated only the signal that we had assigned to the exoisomer. To confirm that the strongly coupled proton observed derived from our prefunctionalized ring and not the acid, (Cp*)Co(exo-η4-C5Me5D)was reacted with HBArF4. Only the formation of [(Cp*)Co(exo-η4-C5Me5D)]+ was detected by EPR (Figure 5.7). 99 Figure 5.7: Proposed mechanism for the oxidation of (Cp*)Co(exo-η4-C5Me5D) with HBArF4. 5.2.3 Thermochemical Measurements Relevant to Cp*2Co Protonation We were also interested in experimentally validating the DFT-predicted thermochemical properties of these species. The high kinetic instability of [(Cp*)Co(endo/exo-η4-C5Me5H)]+ in solution precludes direct measurement of the thermochemical properties that we have predicted by DFT. Of particular interest is experimental validation of a remarkably weak BDFEC-H. We therefore turned to the neutral species (Cp*)Co(exo-η4-C5Me5H) as a means to indirectly measure pertinent thermochemical properties for [(Cp*)Co(exo-η4-C5Me5H)]+. One important parameter in this regard is the [(Cp*)Co(exo-η4-C5Me5H)]+/0 redox couple. In cyclic voltammograms (CVs) of (Cp*)Co(exo-η4-C5Me5H), obtained at typical scan rates (0.01 to 1.0 V s−1) at room temperature in butyronitrile, only an irreversible oxidation is observed. Continuing to scan these voltammograms further in the cathodic direction leads to the observation of the fully reversible [Cp*2Co]+/0 couple (Figure 5.8, top), 100 consistent with the loss of 0.5 equivalents of H2, as expected from [(Cp*)Co(exo-η 4 C5Me5H)]+ in solution. By scanning rapidly (>10 V s−1) at room temperature (Figure 5.8, bottom), or alternatively by cooling the reaction mixture to −78 °C, voltammograms with appreciable reversibility could be obtained, from which E0 could be determined to be −0.62 V vs Fc+/0 (Figure 5.9). Butyronitrile was used in this study, because potentials in this solvent have been shown to be a similar to those in acetonitrile, and it enables low-temperature electrochemistry, which was useful for verifying the oxidation potential.37,38 Figure 5.8: Cyclic voltammograms of the [(Cp*)Co(exo-η4-C5Me5H)]+/0 couple at room temperature in a 0.4 M [TBA][PF6] solution of butyronitrile. (top) Scan showing that the oxidation of (Cp*)Co(exo-η4-C5Me5H)0 leads to the emergence of the [Cp*2Co]+/0 couple. (bottom) Variable scan rate measurements on the [(Cp*)Co(exo-η4-C5Me5H)]+/0 feature. To further confirm this value, the methylated derivative, (Cp*)Co(η4-C5Me6), was prepared.13 The oxidation event for this species is reversible.39 This result is consistent with the significantly higher kinetic barrier anticipated for Me• loss/transfer compared to H• 0 +/0 loss/transfer. In acetonitrile, the E we measure for [(Cp*)Co(C5Me6)] 101 is −0.61 V vs Fc+/0, in good agreement with our experimental data for the [(Cp*)Co(exo-η4-C5Me5H)]+/0 couple. An estimate of the hydricity (ΔG(H−)) of (Cp*)Co(exo-η4-C5Me5H) provides another useful parameter. Dissolution of (Cp*)Co(exo-η4-C5Me5H)in MeCN-d3 and reaction with 1 atm of CO2 or with excess [Pt(dmpe)2]2+ (dmpe = 1,2-dimethylphosphinoethane), leads in both cases to quantitative hydride transfer. From this observation we can determine a lower bound for the hydricity of (Cp*)Co(exo-η4-C5Me5H) (ΔG(H−) <41 kcal mol−1; Figure 5.9).40 This is in good agreement with our DFT prediction of ∆G(H−) = 37 kcal mol−1 for this species. The C–H bond of (Cp*)Co(exo-η4-C5Me5H) is thus about 15 kcal mol−1 more hydridic than the C–H bonds in the common biological hydride donors NADH and NADPH.41 These observations hint at the possibility that, at least in principle, species such as (Cp*)Co(exo-η4-C5Me5H) could mediate hydride transfer steps relevant to N2RR, such as that shown in Figure 5.1. In an attempt to estimate the homolytic C−H bond strength (BDFE) of (Cp*)Co(exo-η4-C5Me5H), it was reacted with excess 4-methoxy-2,2,6,6-tetramethyl-1piperidinyloxy (4-MeOTEMPO⦁). To our surprise, two equivalents of 4-MeOTEMPO−H were formed. The first equivalent derives from the expected H-atom abstraction to form Cp* 2Co, providing an upper limit to the BDFEexo-C–H: <65 kcal mol−1 (Error! Reference source not found., Figure 5.9). The second 4-MeOTEMPO−H equiv is derived from a second H-atom abstraction step between 4-MeOTEMPO⦁ and Cp*2Co (Error! Reference source not found.). This generates the known fulvene complex, (Cp*)Co(exo-η4-C5Me4CH2).42 These 102 observations are consistent with our BDFEC–H predictions for the C–H bond in both (Cp*)Co(exo-η4-C5Me5H) and Cp*2Co (53 and 62 kcal/mol, respectively). Eq 5.1) (Cp*)Co(exo-η4-C5Me5H) + 4MeOTEMPO•  Cp*2Co + 4MeOTEMPO–H Eq 5.2) Cp*2Co + 4MeOTEMPO•  (Cp*)Co(η4-C5Me4CH2) + 4MeOTEMPO–H With this thermochemical data for (Cp*)Co(exo-η4-C5Me5H) in hand, it is possible to constrain the BDFEC−H for [(Cp*)Co(exo-η4-C5Me5H)]+. Using the bound established for the BDFEC−H for neutral (Cp*)Co(exo-η4-C5Me5H), we can establish an upper limit for the BDFEC–H of [(Cp*)Co(exo-η4-C5Me5H)]+ of 34 kcal mol−1. But using the upper limit determined for the ΔG(H−) of (Cp*)Co(exo-η4-C5Me5H) allows us to place an even lower upper limit for BDFE([(Cp*)Co(exo-η4-C5Me5H)]+) of <29 kcal mol−1. This experimental upper limit predicted from the solution phase data is in reasonable agreement with our gasphase DFT prediction of 23 kcal mol−1, thus confirming that the C–H bond of [(Cp*)Co(exo-η4-C5Me5H)]+ is extremely weak. Figure 5.9: Thermochemistry of neutral and cationic (Cp*)Co(exo-η4-C5Me5H). Computational values are shown in parentheses. Thermodynamic quantities are in kcal mol−1 and potentials are against Fc+/0. 103 5.3 Discussion It is well established that group 8 metallocenes form metal hydrides upon protonation.10,11,23 In the case of ferrocene, computational trajectories have been used to argue that there is fast exchange between a terminal hydride and a hydride that forms an agostic interaction with the Cp ring.43,44 In contrast, neutral group 10 metallocenes (Cp2Ni and Cp*2Ni), and the isoelectronic [Cp2Co]−, are known to undergo exo-protonation of the ring.12,13,45 Here we find that, consistent with the near isoenergetic protonation predicted by DFT, Cp*2Co undergoes both exo- and endo-protonation on the ring. This result provides a distinct example of a metallocene that undergoes non-specific protonation. Furthermore, the protonation selectivity can be altered by changing the steric profile of the acid. We suspect that formation of the exo-isomer is likely critical to observing productive PCET reactions in N2RR mediated by the (P3B)Fe-system, as this isomer provides significantly less steric shielding for the reactive H•. Given the steric profile of the catalytically relevant acids that we and others have used (e.g., anilinium and pyridinium),4–9 we expect that exo-protonation is far more likely under N2RR conditions. Indeed, we have calculated only small barriers (∆G‡ <5 kcal mol−1) for the exo-protonation of Cp*2Co by substituted anilinium triflate acids.9 Facile endo-protonation by these acids is inconsistent with simple space-filling models. Although Cp* is most typically considered to be an innocent ligand, evidence continues to emerge that it can be involved in the management of protons. In addition to the well-established protonation of Cp*2Ni,13,46 several half-sandwich Rh complexes have recently been reported to form Cp*–H linkages following reductive elimination of a Rh–H. – + 104 In these cases, the Cp*–H bond has been directly implicated in H transfer, either to H or to NAD+.47–49 The present study illustrates that Cp*–H linkages are not limited to H– transfer pathways. Rather, the type of reactivity can be predicted by the tendency of the metal center to achieve a closed-shell, 18e− d6 configuration. Thus, the d7 Co center in [(Cp*)Co(exo-η4C5Me5H)]+ should favor a one electron process (H• transfer), while the d8 Co center in (Cp*)Co(exo-η4-C5Me5H) should favor a two electron process (H− transfer), akin to those observed for the aforementioned d8 Rh centers. In this work, we have derived a BDFE for three different C–H bonds relevant to decamethylcobaltocene [(Cp*)Co(exo-η4-C5Me5H)]+, (Cp*)Co(exo-η4-C5Me5H), and (Cp*)Co(η5-C5Me4CH2H) (Figure 5.10). All of these bonds are weak, but that in [(Cp*)Co(exo-η4-C5Me5H)]+ is significantly weaker than the other two. This can be readily explained in terms of the two primary factors affecting the stability of the starting and product complexes: aromaticity and electron count. In the case of H-atom abstraction from (Cp*)Co(exo-η4-C5Me5H) or from a methyl substituent in Cp*2Co, these factors offset one another to provide a weak, but not an exceptionally weak, BDFEC–H. For (Cp*)Co(exo-η4C5Me5H), H-atom abstraction aromatizes the Cp* ring, offset by the formation of a 19e− center (Figure 5.10). On the other hand, in Cp*2Co, H-atom abstraction transforms the 19e− center to an 18e− center, but the Cp* ring is dearomatized (Figure 5.10). Only in the case of [(Cp*)Co(exo-η4-C5Me5H)]+ are both stabilizing factors driving formation of the product. Hatom abstraction affords aromatic, 18e− Cp*2Co+, and correspondingly the C–H bond is remarkably weak (BDFE <29 kcal mol−1, Figure 5.10). 105 Reagents with such weak X−H bonds have been sought due to their utility in organic synthesis for the stepwise reduction of unsaturated substrates, such as olefins, ketones, aldehydes, esters, and enamines via H• transfer.50,51 Traditional strategies for developing such reagents have focused on reactive metal hydrides, for which the M• product of an overall hydrogen atom transfer is stabilized by dimerization via M–M bond formation, and/or the formation of bridging carbonyl products.52 Another strategy has involved the coordination of substrates that contain otherwise strong X–H bonds to a highly reducing, but nonetheless oxophilic/azaphilic, metal centers, resulting in remarkable weakening of the X–H bond.53 One system where this phenomenon has proven particularly effective for engendering synthetically useful PCET reactions is SmI2-H2O, in which coordination of H2O to SmII has been estimated to result in an O–H bond weakening of almost 100 kcal mol–1.54–57 The present study presents the protonation of Cp*2Co to form [(Cp*)Co(exo-η4C5Me5H)]+ as a distinct and promising strategy for developing extremely strong PCET donors. In general, this strategy involves coupling a d7 (or d4) metal ion to a dearomatized arene ligand. Given the prevalence of sandwich and half- sandwich complexes in organometallic chemistry, it is likely that as yet unrecognized examples of such PCET reagents already exist or are readily accessible. [(Cp*)Ni(exo-η4-C5Me5H)]2+ provides one such example. Electrochemical oxidation of the stable [(Cp*)Ni(exo-η4-C5Me5H)]+ to [(Cp*)Ni(exo-η4-C5Me5H)]2+ leads to rapid generation of [Cp*2Ni]2+ on the CV time scale. This transformation was originally proposed to occur via H+ loss followed by e− loss.58 Alternatively, we suspect that, in analogy to 4 + 106 [(Cp*)Co(exo-η -C5Me5H)] , this transformation may occur via rapid H• loss. In contrast, electrochemical reduction of the cation [(Cp*)Ni(exo-η4-C5Me5H)]+ to (Cp*)Ni(exo-η4-C5Me5H) is fully reversible on the CV time scale.58 Figure 5.10: (left) A comparison of the experimental BDFEC–H for a variety of related Cp*Co-species, demonstrating the importance of aromaticity and electron count in predicting the stability of the indicated C–H bond. (right) A comparison of computational BDFEC–H values for a redox series of [(Cp*)Ni(exo-η4-C5Me5H)]n+. These results emphasize that the electron count of the metal center, instead of its reducing power, can be a good predictor of the reactivity. Indeed, our DFT calculations and the relative experimental stability of (Cp*)Ni(exo-η4-C5Me5H) and [(Cp*)Ni(exo-η4C5Me5H)]+ suggest that even though H• loss involves formal oxidation of the metal center the d9 NiI and d8 NiII species are less prone to PCET reactivity. However, upon oxidation to 7 III −1 107 the d Ni species, the C–H bond is weakened by approximately 20 kcal mol . This weakening is due to the high stability of the 18e−, d6 [Cp*2Ni]2+ product resulting from net hydrogen atom transfer. 5.4 Conclusion We have demonstrated using pulse EPR spectroscopy, supported by DFT calculations, that for Cp*2Co the Cp* ring is the site of protonation. Both ring-protonated isomers (endo and exo) can be formed and observed, with the selectivity being determined by the bulk of the acid. For the exo-species, we were able to use the one-electron reduced, neutral congener, (Cp*)Co(exo-η4-C5Me5H), to verify our DFT prediction that the protonated species, [(Cp*)Co(exo-η4-C5Me5H)]+, has a remarkably weak C−H bond (<29 kcal mol−1). This is consistent with the suggestion that it may serve as a PCET donor in catalytic N2RR in which it is generated in situ. The facile protonation of Cp*2Co to generate [(Cp*)Co(exo-η4-C5Me5H)]+ points to a more general strategy for developing conceptually related, strong PCET donors. Such strategies would complement current approaches for the development of PCET reagents, which rely on creating weak M–H bonds or coordination-induced weakening of O–H or N–H bonds. We have also shown that [(Cp*)Co(exo-η4-C5Me5H)]+ can be converted from a PCET donor to a strong hydride donor by one electron reduction, as demonstrated by the capacity of (Cp*)Co(exo-η4-C5Me5H) to convert CO2 to formate. This observation highlights the dual potential for metallocenes to mediate both hydride transfer and PCET steps during the proton-coupled reduction of small molecule substrates (Figure 5.1). Both 108 types of reactivity differ from the canonical role associated with metallocenes as electron transfer reagents. 5.5 References (1) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J. Am. Chem. Soc. 1952, 74, 2125–2126. (2) Stepnicka, P. John Wiley & Sons, Ltd: Chichester, England, 2008. (3) Geiger, W. E. Organometallics 2007, 26, 5738–5765. (4) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76–78. (5) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120–125. (6) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. J. Am. Chem. Soc. 2016, 138, 13521–13524. (7) Arashiba, K.; Eizawa, A.; Tanaka, H.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Bull. Chem. Soc. Jpn. 2017, 90, 1111–1118. (8) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. (9) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. (10) Curphey, T. J.; Santer, J. O.; Rosenblum, M.; Richards, J. H. J. Am. Chem. Soc. 1960, 82, 5249–5250. (11) Liles, D. C.; Shaver, A.; Singleton, E.; Wiege, M. B. J. Organomet. Chem. 1985, 288, c33–c36. (12) Court, T. L.; Werner, H. J. Organomet. Chem. 1974, 65, 245–251. 109 (13) Koelle, U.; Khouzami, F. Angew. Chem. Int. Ed. 1980, 19, 640–641. (14) Koelle, U.; Infelta, P. P.; Graetzel, M. Inorg. Chem. 1988, 27, 879–883. (15) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Schrock, R. R.; Müller, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17099–17106. (16) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2014, 136, 9719–9731. (17) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84–87. (18) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (19) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (20) Thompson, N. B.; Green, M. T.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 15312– 15315. (21) Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103–1117. (22) Wickramasinghe, L. A.; Schrock, R. R.; Tsay, C.; Müller, P. Inorg. Chem. 2018, 57, 15566–15574. (23) Malischewski, M.; Seppelt, K.; Sutter, J.; Heinemann, F. W.; Dittrich, B.; Meyer, K. Angew. Chem. Int. Ed. 2017, 56, 13372–13376. (24) Hyde, J. S.; Pasenkiewicz-Gierula, M.; Jesmanowicz, A.; Antholine, W. E. Appl. Magn. Reson. 1990, 1, 483–496. (25) Wittke, J. P.; Dicke, R. H. Phys. Rev. 1956, 103, 620–631. (26) Hu, Y.; Shaw, A. P.; Estes, D. P.; Norton, J. R. Chem. Rev. 2016, 116, 8427–8462. (27) Brown, T. L. Acc. Chem. Res. 1974, 7, 408–415. 110 (28) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (29) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (30) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (31) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (32) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chemical Physics Letters 1989, 157, 200–206. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (34) Harmer, J.; Finazzo, C.; Piskorski, R.; Ebner, S.; Duin, E. C.; Goenrich, M.; Thauer, R. K.; Reiher, M.; Schweiger, A.; Hinderberger, D.; et al. J. Am. Chem. Soc. 2008, 130, 10907–10920. (35) Kinney, R. A.; Saouma, C. T.; Peters, J. C.; Hoffman, B. M. J. Am. Chem. Soc. 2012, 134, 12637–12647. (36) Green, M. L. H.; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1959, 0, 3753–3767. (37) Izutsu, K. Handbook of Reference Electrodes; Inzelt, G., Lewenstam, A., Scholz, F., Eds.; Springer: Berlin, Heidelberg, 2013; pp 145–187. (38) Van Duyne, R. P.; Reilley, C. N. Anal. Chem. 1972, 44, 142–152. (39) Koelle, U. Inorg. Chim. Acta 1981, 47, 13–18. (40) 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. (41) Ilic, S.; Alherz, A.; Musgrave, C. B.; Glusac, K. D. Chem. Soc. Rev. 2018, 47, 2809– 2836. (42) Ohki, Y.; Murata, A.; Imada, M.; Tatsumi, K. Inorg. Chem. 2009, 48, 4271–4273. 111 (43) Bühl, M.; Grigoleit, S. Organometallics 2005, 24, 1516–1527. (44) Sharma, N.; Ajay, J. K.; Venkatasubbaiah, K.; Lourderaj, U. Phys. Chem. Chem. Phys. 2015, 17, 22204–22209. (45) Geiger, W. E.; Bowden, W. L.; El Murr, N. Inorg. Chem. 1979, 18, 2358–2361. (46) Werner, H.; Dernberger, Th. J. Organomet. Chem. 1980, 198, 97–103. (47) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Chem. Commun. 2016, 52, 9105– 9108. (48) Quintana, L. M. A.; Johnson, S. I.; Corona, S. L.; Villatoro, W.; Goddard, W. A.; Takase, M. K.; VanderVelde, D. G.; Winkler, J. R.; Gray, H. B.; Blakemore, J. D. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 6409–6414. (49) Johnson, S. I.; Gray, H. B.; Blakemore, J. D.; Goddard, W. A. Inorg. Chem. 2017, 56, 11375–11386. (50) Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A. Chem. Rev. 2016, 116, 8912–9000. (51) Gentry, E. C.; Knowles, R. R. Acc. Chem. Res. 2016, 49, 1546–1556. (52) Hartung, J.; Norton, J. R. Catalysis without Precious Metals; Bullock, R. M., Ed.; WILEY-VCH Verlag GmbH & Co., 2010; pp 1–21. (53) Cuerva, J. M.; Campaña, A. G.; Justicia, J.; Rosales, A.; Oller‐López, J. L.; Robles, R.; Cárdenas, D. J.; Buñuel, E.; Oltra, J. E. Angew. Chem. Int. Ed. 2006, 45, 5522– 5526. (54) Chciuk, T. V.; Flowers, R. A. J. Am. Chem. Soc. 2015, 137, 11526–11531. (55) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. J. Am. Chem. Soc. 2016, 138, 8738– 8741. 112 (56) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. J. Am. Chem. Soc. 2018, 140, 15342–15352. (57) Kolmar, S. S.; Mayer, J. M. J. Am. Chem. Soc. 2017, 139, 10687–10692. (58) Kölle, U.; Khouzami, F.; Lueken, H. Chem. Ber. 1982, 115, 1178–1196. 113 Metallocenes: Beyond Electron Transfer to Proton-Coupled Electron Transfer 114 6.1 Introduction The high stability of the bis-cyclopentadienyl (Cp) ligand framework has allowed a remarkable number of metallocenes (Cp2M) to be isolated, often in multiple redox states [Cp2M]n+ making many of these reagents useful as electron transfer reagents. Indeed, the [Cp2Fe]0/+ redox couple is the canonical example of a reversible electron transfer in organometallic chemistry. The bis-cyclopentadienyl scaffold also stabilizes formally electron excessive (>18e−) complexes such as Cp2Co that are strong reductants. Indeed, our group first became interested in metallocenes, when we discovered that permethylated cobaltocene, Cp*2Co, is an efficient electron donor for the reduction of N2-to-NH3 in the presence of a trisphosphine borane Fe species, (P3B)Fe, and excess diphenylammonium or anilinium acids. Under such conditions high selectivity for NH3 was attainable (up to 80%),1,2 akin to results from other groups with Mo3,4 and Fe.5 We hypothesized that given the ubiquity of metallocenes in high-efficiency N2-fixation catalysis that Cp*2Co or related reducing metallocenes (e.g., Cp2Co or Cp*2Cr) were not only serving as electron transfer reagents, but could also be protonated to generate species with weak C–H bonds that would therefore be strong PCET reagents.1 Significant circumstantial evidence from stoichiometric experiments and theory suggested that such species had a role in N–H bond forming reactions during nitrogen fixation both chemical and electrochemical.2 We ultimately found conditions under which, we could characterize the highly reactive protonated Cp*2Co via its precipitation at low temperature and the employ of solid-state pulse electron paramagnetic resonance (EPR) spectroscopy. Consistent with our calculations, we found that Cp*2Co underwent protonation on the Cp* ring to form [(Cp*)Co(endo/exo-η4-C5Me5H)]+ with the endo- vs exo-selectivity determined by the steric bulk of the acid. This allowed us to use the much more stable 115 (Cp*)Co(exo-η -C5Me5H) to indirectly verify our hypothesis that the protonated Cp*2Co 4 is a strong hydrogen atom donor (BDFEC–H < 29 kcal mol−1).6 In our efforts to study electrocatalytic nitrogen fixation, we had observed that the addition of co-catalytic [Cp*2Co]+ to controlled potential electrolyses (CPEs) containing [(P3B)Fe]+, N2, and excess diphenylammonium boosted the yield of NH3 from the electrosynthetic (2.6 ± 0.3 equiv per Fe) to the electrocatalytic regime (4.0 ± 0.6 equiv per Fe and Co). This was despite the fact that CPE experiments of [Cp*2Co]+ under an N2 atmosphere in the presence of excess acid do not reveal the formation of any NH3 only H2. Probing the [Cp*2Co]+/0 couple by cyclic voltammetry (CV) reveals that it becomes irreversible and shows moderate current enhancement upon the addition of diphenylammonium, consistent with catalytic HER.2 Although these data suggest that under electrocatalytic conditions in the presence of excess acid, the strong hydrogen atom donors [(Cp*)Co(endo/exo-η4-C5Me5H)]+ could be formed, it remained unclear whether such a species built up in such a way that might allow it to react via H-atom donation. Instead, it might be that Cp*2Co, an electron donor (E0 = −1.91 V vs Fc+/0 in MeCN), or that (Cp*)Co(endo/exo-η4-C5Me5H), a hydride donor (ΔG(H−) < 41 kcal mol−1 in MeCN), was the kinetically relevant species under these conditions. We were interested in the active species, because there are a paucity of examples of species that provide access to freely diffusing strong H-atom donors (BDFEX–H < 50 kcal mol−1) without rapidly evolving H2 under electrocatalytic conditions. This contrasts with the many organic or organometallic species that under reductive conditions in the presence of acid provide access to freely diffusing electron donors 7,8 116 or freely diffusing hydride donors.9–11 Indeed, freely diffusing electron donors are frequently employed as redox mediators7,8 while freely diffusing hydride donors are involved in many proton-coupled reductions.9–11 This led us to believe that freely diffusing strong PCET reagents would be similarly useful. Weak PCET reagents (BDFEX–H > 50 kcal mol−1) have already found utility as co-catalysts in the electrocatalytic reduction of O2 to H2O.12,13 An electrochemical means of generating a strong net H-atom donors would be complementary to photochemical strategies using acids and Rh or Ir photoreductants,14–16 and chemical strategies that use SmIIaquo complexes17–19 or M–H species derived from the reaction of MII salts (MII = Mn, Fe, Co) with silanes and alcohols20,21 that have been exploited for organic synthesis. Strong PCET donors are particularly useful for the functionalization of unsaturated substrates, because hydrogen-atom donation cleaves a π-bond; the resultant X–H bond is consequently weakened by the α-radical. Although challenging to generate, this radical intermediate has proven highly synthetically versatile allowing for hydrofunctionalization reactions, non-syn hydrogenations, and three-component couplings.20 Although, we ultimately uncovered that parent cobaltocenes are not suitable for the electrocatalytic generation of strong PCET donors, we demonstrate a simple synthetic strategy for modifying them to achieve this goal. We characterize this novel PCET donor via chemical and electrochemical means and demonstrate the utility of this new, base-appended cobaltocene in the electrocatalytic proton-coupled reduction of ketones to pinacols. 117 Figure 6.1: Overview of PCET for the functionalization of unsaturated substrates. (top left) The combination of phosphoric acids and iridium photoreductants have been shown to be strong PCET donors.15,16 (bottom left) The combination of silanes and FeIII alkoxides20,21 and SmII with water have been shown to generate strong PCET donors.17–19,22 (top right) PCET to an unsaturated organic substrate results in a new X–H bond that is unusually weak due to the α-radical. (bottom right) A novel, strong PCET donor synthesized and characterized herein suitable for the electrocatalytic reduction of ketones to pinacols. 6.2 Results 6.2A Mechanism of Electrocatalytic HER by [Cp 2Co]+ DFT calculations suggest that much like Cp*2Co, Cp2Co should undergo ringprotonation to form a species that is a strong PCET reagent (BDFEcalc = 33.8 kcal mol−1). Cyclic voltammograms of 1 mM [Cp2Co]+ at 10 mV/s with a boron-doped diamond (BDD) working electrode of in the presence of 100 equiv of 4-cyanoanilinium ([4-CNPhNH3]+) in a 200 mM [TBA][PF6] solution of DME demonstrate an irreversible wave at the [Cp2Co]+/0 couple. This feature is indicative of electrocatalytic HER, albeit the shallow slope and minimal current enhancement suggest slow catalysis. Measurement of the HER current at different concentrations of [Cp2Co]+ are consistent with a reaction that is first order in Co (Figure 6.2). Increasing the scan rate to 100 mV/s leads to enhanced reversibility at the +/0 [Cp2Co] 4 +/0 couple (Figure 6.2), rather than at the [(Cp)Co(η -C5H6)] 118 couple (E = 0 −0.33 V vs Fc+/0) indicating that the rate-determining step (RDS) in the HER cycle is the initial protonation of Cp2Co. These data are consistent with previous pulse radiolytic measurements of HER by Cp2Co.23 Given the facile reduction of the cobaltocene protonation product, [(Cp)Co(η4-C5H6)]+, at −1.33 V (ΔG(ET) = −23 kcal mol−1), it seems likely that the electrocatalytic HER follows a traditional ECEC mechanism, but the current data are not definitive on this issue. Regardless, although protonation of cobaltocene (and Cp*2Co) generates a strong PCET donor, they are not suitable as electrocatalytic PCET mediators under our envisioned CPE conditions (negative applied bias, excess acid, vide infra). Figure 6.2: Cyclic voltammetry relevant to HER by [Cp2Co]+ (left) Line of best fit for relationship between the plateau current and [Cp2Co]+ concentration in the HER reaction at a BDD electrode in DME with 100 mM [4-CNPhNH3][OTf] and 200 mM [TBA][PF6] at 10 mV/s. (right) CV of [Cp2Co]+ at a BDD electrode in DME with 100 mM [ 4CN PhNH3][OTf] and 200 mM [TBA][PF6] at 100 mV/s demonstrating significant reversibility at the [Cp2Co]+/0 couple. This lack of suitability emerges from two problems. The first is that protonation of the Cp ligand is slow. This is consistent with a large body of physical organic literature 119 demonstrating that (de)protonations at carbon are slow relative to those at nitrogen or oxygen.24 Secondly, protonation of the Cp (or Cp*) ligand results in the formation of an electron-accepting diolefinic ligand which makes subsequent reduction more favorable. This effect has been discussed in the context of half-sandwich (Cp*)Rh complexes that undergo ring-protonation.25 We anticipated that by synthetically appending a Brønsted base to the cobaltocene, we would both accelerate the rate of protonation and retain the bis-Cp ligand framework thereby disfavoring a second reduction by avoiding formation of an electronaccepting diolefin ligand. 6.2B Synthesis of a Brønsted Base-Appended Cobaltocene Unlike with ferrocene,26 there are relatively few synthetic strategies for elaborating cobaltocenes. One strategy that has been successfully used is the addition of aryllithium reagents to [Cp2Co]+ to generate an exo-substituted cyclopentadienyl cyclopentadiene Co complex from which a hydride can often be extracted to yield the desired, mono-substituted cobaltocenium in reasonable yields.27,28 Addition of a THF solution of 4-lithio-N,Ndimethylaniline to a stirred suspension of [Cp2Co][PF6] in THF at −78 °C led to the desired nucleophilic addition product (Figure 6.3). Although hydride abstraction from this species was unsuccessful, reflux of this species in 5:1 methylcyclohexane:benzene led to the formal loss of 1/2 H2 and rearomatization of the Cp ring yielding (Cp)Co(CpN) (CpN = 4cyclopentadienyl-N,N-dimethylaniline, Figure 6.3). This species was readily oxidized to [(Cp)Co(CpN)][OTf] by reaction with AgOTf at room temperature (Figure 6.3). This oxidized species is readily protonated by HOTf in DME to form [(Cp)Co(CpNH)][OTf]2 (CpNH = 4-cyclopentadienyl-N,N-dimethylanilinium, Figure 6.3). X-ray structures of N N + NH 2+ (Cp)Co(Cp ), [(Cp)Co(Cp )] , [(Cp)Co(Cp )] 120 confirm that, as hoped, the primary coordination sphere is minimally perturbed by redox or protonation (Figure 6.4). Figure 6.3: Scheme describing the synthesis of the Brønsted base appended cobaltocenes. Figure 6.4: X-ray structure of (Cp)Co(CpN), [(Cp)Co(CpN)]+, and [(Cp)Co(CpNH)]2+ with counteranions and solvent omitted for clarity. Ellipsoids shown at 50% probability. 121 With these complexes in hand, we assessed our ability to chemically access our desired PCET donor, [(Cp)Co(CpNH)]+. Continuous-wave X-band electron paramagnetic resonance spectroscopy at 77 K of rapidly freeze quenched reaction mixtures (Figure 6.5) of either [(Cp)Co(CpNH)]2+ with SmI2 or (Cp)Co(CpN) with trifluoromethanesulfonimide led to the observation of the same EPR signal. The signal is best fit with a roughly axial g-tensor with g-values at or below the free-electron value (g = [2.056, 1.994, 1.791]) consistent with other d9 metallocenes29 and distinct from the g-tensors we observed for endo- and exoprotonation of the Cp* ring in decamethylcobaltocene.6 We thus assign this species to be [(Cp)Co(CpNH)]+. UV-Vis spectroscopy of the reaction of (Cp)Co(CpN) with trifluoromethanesulfonimide at −130 °C in 4:1 2-MeTHF:THF demonstrates isobestic behavior, consistent with the formation of a single product (Figure 6.5). Warming of these reactions to room temperature ultimately led to decomposition to form [(Cp)Co(CpN)]+ consistent with the high thermodynamic driving force for formal H2 loss. Figure 6.5: Spectroscopic data for [(Cp)Co(CpNH)]+. (left) Freeze-quench EPR data for [(Cp)Co(CpNH)]+ formed either via the reduction of [(Cp)Co(CpNH)]2+ with SmI2 (blue) or the protonation of (Cp)Co(CpN) with bistrifluoromethanesulfonimide (red) and their simulation (orange). (right) UV-Vis spectroscopy for the reaction of (Cp)Co(CpN) with 1.5 equiv of bistrifluoromethanesulfonimide in 4:1 2-MeTHF:THF at −130 °C. 122 6.2C Thermochemical Properties of a Brønsted Base-Appended Cobaltocene We next set out to perform thermochemical experiments in order to confirm that [(Cp)Co(CpNH)]+ is indeed a strong PCET donor. The electrochemical properties of [(Cp)Co(CpN)]+ were evaluated in a 100 mM [TBA][PF6] acetonitrile solution on a glassy carbon working electrode. A cyclic voltammogram revealed a reversible reduction to (Cp)Co(CpN) at −1.35 V vs Fc+/0, a minimal perturbation from the [Cp2Co]+/0 couple (E0 = −1.33 V vs Fc+/0). Also apparent is a pseudo-reversible oxidation to [(Cp)Co(CpN)]2+ at 0.52 V vs Fc+/0, which we assign to an N-centered oxidation on the basis of the similar oxidation potential for N,N-dimethylaniline (0.38 V vs Fc+/0 in acetonitrile).30 The enhanced reversibility of this peak relative to that of parent N,N-dimethylaniline suggests that the cobaltocenium substituent helps to stabilize the resultant aminium radical. We also observe a pseudo-reversible reduction at ~ −2.25 V vs Fc+/0 to form [(Cp)Co(CpN)]−; this redox feature is fully reversible in DME (−2.42 V vs Fc+/0) consistent with observations of the [Cp2Co]0/− couple.31 To ascertain the pKa of [(Cp)Co(CpNH)]2+, 1H NMR spectra in MeCN-d3 were recorded of titration experiments with aniline bases. In particular, titration experiments with 4-Cl PhNH2 (pKa([4-ClPhNH3]+) = 9.60)32 and 2-ClPhNH2 (pKa([2-ClPhNH3]+) = 7.86)33 reveal that the pKa of [(Cp)Co(CpNH)]2+ is 8.86 ± 0.02. The enhanced acidity relative to N,Ndimethylanilinum (pKa = 11.4)33 is consistent with the introduction of the electronwithdrawing cobaltocenium at the para-position. NH 2+ 123 Oxidative scans of [(Cp)Co(Cp )] in 100 mM [TBA][PF6] do not lead to the observation of any waves before the solvent window, consistent with our above assignment of the [(Cp)Co(CpN)]2+/+ couple to an N-centered oxidation. Cathodic scans of [(Cp)Co(CpNH)]2+ lead to the observation of a fully reversible wave (Ered = −1.20 V vs Fc+/0) at moderate scan rates 100-1000 mV/s ( Figure 6.6). With the collected data, we can determine the N–H BDFE of [(Cp)Co(CpNH)]+, to be 38.8 kcal mol−1 (Figure 6.7, Eq 6.1) satisfying one of our criteria for a strong PCET donor (BDFE < 50 kcal mol−1). Eq 6.1) BDFE = 1.37×pKa + 23.06×E0 + CG Figure 6.6: Variable scan rate CVs of 1 mM [(Cp)Co(CpNH)]2+ and 1 mM Fc in 100 mM [TBA][PF6] solution of acetonitrile at a boron-doped diamond working electrode. 124 Figure 6.7: Thermochemistry of [(Cp)Co(CpN(H))]n+ in acetonitrile. Potentials are in V vs Fc+/0. 6.2D Electrochemical behavior of (Cp)Co(CpN) with Excess Acid Addition of 1 mmol of [(Cp)Co(CpN)]+ to a DME solution containing 100 mM [4CN PhNH3][OTf] leads to instantaneous formation of [(Cp)Co(CpNH)]2+ consistent with their respective pKa’s in acetonitrile (7.0 vs 8.6). Notably protonation is the slow step in Cp2Comediated HER. Here the Co species is protonated even before reduction, nonetheless these CV’s demonstrate appreciable reversibility even at 10 mV/s (Figure 6.8). Thus, [(Cp)Co(CpNH)]+ is an even worse HER catalyst than Cp2Co presumably because further protonation or reduction is unfavorable under these conditions and thus the only mechanism for HER is bimolecular. Potentially, the slow bimolecular H2 evolution is due to the electronically decoupled nature of the Co center and the N–H bond (Figure 6.8). These data suggest that we have developed a species that met all of the criteria we had established for an electrocatalytic, strong PCET donor: 1) slow HER catalyst, 2) X–H bond strength of less −1 125 than 50 kcal mol , and 3) H• transfer is the thermodynamically preferred mode of reactivity as opposed to H+ or H− transfer (Figure 6.7). Figure 6.8: (left) Cyclic voltammogram of (Cp)Co(CpN) in the presence of 100 mM of [4CN PhNH3][OTf] in 200 mM [TBA][PF6] DME at 10 mV/s with the acid background subtracted. (right) Spin density plot of [(Cp)Co(CpNH)][OTf] at an isovalue of 0.04 demonstrating that the N–H bond is entirely decoupled from the spin density. 6.2E Electrochemical Behavior in the Presence of a Hydrogen Atom Acceptor To explore the possibility of electrocatalytic PCET, we sought out a model substrate for which thermodynamics favored a PCET pathway. DFT calculations suggest that acetophenone is an ideal substrate. The calculated O–H bond strength of the radical intermediate is well-matched to that of our catalyst (BDFEO–H = 38.7 kcal mol−1, ΔG(PCET) = 0 kcal mol−1), while both the ET to form the ketyl anion (ΔG(ET) = +26 kcal mol−1) and PT to form the oxonium (ΔG(PT) = +19 kcal mol−1) are significantly uphill ( Figure 6.9). These calculated energies for electron transfer and proton transfer are consistent with the known experimental reduction potential and pKa of acetophenone.34,35 We were particularly interested in this substrate because recently it has been demonstrated that photochemical PCET strategies are a means of selectively forming pinacol products at potentials much more mild than traditional, ET routes.36,37 126 Figure 6.9: DFT calculated thermodynamics of PT, ET, and PCET for the reaction of acetophenone with the [(Cp)Co(CpNH)]+. To test the suitability of acetophenone as a hydrogen-atom acceptor, we added 12 equiv of the ketone to the aforedescribed solution containing 1 mM [(Cp)Co(CpNH)][OTf], 100 mM [4-CNPhNH3][OTf], and 200 mM [TBA][PF6]. Cyclic voltammograms at 100 mV/s after the addition of the ketone lead to the observation of enhanced current (180% increase) and the complete loss of reversibility indicating a catalytic reaction. Control CV’s without [(Cp)Co(CpN)][OTf] do reveal slightly enhanced current densities relative to the background HER reaction but significantly less than the catalytic reaction at the relevant potential. To interrogate the product of these CV experiments, we performed CPE experiments. Optimization of the CPE conditions ultimately led us to the use of a one-compartment cell with tosic acid (pKa of 8.45 in MeCN),38 and a glassy-carbon counter electrode. The onecompartment cell was necessary to avoid diffusion of the acetophenone out of the working compartment. The tosic acid was necessary due to undesirable anodic reactivity of the conjugate base of anilinium acids. Lastly, although reasonable results could be obtained with a Pt mesh counter electrode (35.7% FE for pinacol, 38.4% FE for H2, Table 4.1, Entry 1) 127 substantially higher turnover number (10.5 vs 41.6) could be achieved with a glassycarbon counter electrode (Table 6.1). This improved performance with a glassy carbon counter electrode could be due to favorable adsorption of the ketone on the Pt electrode, as no ketone was recovered in this experiment despite the lower yield of pinacol. Table 6.1. Results from CPE experiments on a DME solution of 200 mM [TBA][PF 6], 100 mM tosic acid, 50 mM acetophenone at −1.45 V vs Ag+/0 Counter Pinacol Ketone Pinacol H2 Yield Catalyst Q (C) Electrode Yield Pt 22% Recovery TON (FE) 1 mM N 11 N [(Cp)Co(Cp )][OTf] Glassy 83% Glassy 6% Glassy 10% Carbon (36%) (38%) 42 48% 72 (39%) (45%) 3 66% 0% Carbon none 21 11% Carbon 1 mM [Cp2Co][PF6] 12% 5% [(Cp)Co(Cp )][OTf] 1 mM (FE) 72 (3%) (62%) N/A 1% (47%) (8%) 0% 8 Under our optimized conditions (200 mM [TBA][PF6], 100 mM tosic acid, 50 mM acetophenone, 1 mM [(Cp)Co(CpN)][OTf], BDD working electrode, and glassy carbon counter electrode) a CPE experiment at −1.45 V for ~53 hours with stirring results in the passage of 71.6 Coulombs (Figure 6.10). Analysis of the headspace by gas chromatography revealed the formation of H2 (45.0% FE). Analysis of the solution phase products by comparison of their GC-MS and GC-FID traces to authentic standards after work-up led to 128 the identification of 83.2% yield for pinacol (39.0% FE) and 10.8% recovery of the ketone (Table 6.1, Entry 2). No 1-phenylethanol or other reduction products were identified. Differential pulse voltammograms (DPV’s) taken of the bulk solution with a fresh electrode after the CPE experiment reveals redox features assignable to both the reduction and oxidation of [(Cp)Co(CpN)][OTf]. Furthermore, x-ray photoelectron spectroscopy measurements of the BDD electrode surface after the CPE experiment does not reveal the presence of any cobalt. These observations are consistent with catalyst stability under the CPE conditions. Figure 6.10: Time course of the current (I) and charge (Q) in a CPE experiment at −1.45 Vvs Ag+/0 of a 200 mM [TBA][PF6], 100 mM tosic acid, 1 mM [(Cp)Co(CpN)][OTf] DME solution at a BDD working electrode with a glassy carbon counter electrode, and Ag wire reference electrode. A CPE experiment for the same amount of time in the absence of our catalyst (Table 4.1, Entry 3) reveals minimal pinacol formation (10.4% yield) and H2 formation (1.0% yield). The low conversion is consistent with the minimal currents observed in the CV experiments. Intriguingly, no ketone is observed suggesting that the ketone may become adsorbed or degraded over long-times in the absence of a catalytic reaction promoting its conversion. A 129 CPE experiment performed with 1 mM [Cp2Co][PF6] (Table 6.1, Entry 4) instead of our base-appended cobalt catalyst, revealed slightly enhanced rates for HER (65.8% yield and 62.0% Fe for H2) and a slight deterioration relative to the background for pinacol formation (5.6%). This is consistent with our hypothesis that the protonated Cp2Co species is not the predominant reactive species under these conditions. To mechanistically interrogate the [(Cp)Co(CpN)]2+ mediated catalysis, we returned to performing cyclic voltammograms. For mechanistic experiments, we continued to use [4CN PhNH3][OTf], as the similar pKa of [(Cp)Co(CpNH)][OTf]2 and tosic acid (8.6 and 8.45 in MeCN respectively) complicates the analysis due to incomplete protonation of the Co catalyst. Titrating acetophenone into a DME solution of 200 mM [TBA][PF6], 100 mM [4CN PhNH3][OTf], and 1 mM [(Cp)Co(CpN)][OTf] results in S-shaped CV’s with increasing current density (representative CV shown in Figure 6.11).39 Plotting the plateau current observed after subtraction of the background current against the concentration of acetophenone reveals a first-order dependence on ketone (Figure 6.11). 130 Figure 6.11: (left) Cyclic voltammograms of 1 mM [(Cp)Co(CpN)][OTf] at 10 mV/s in the presence of 100 mM [4-CNPhNH3][OTf] and the presence (red) and absence (blue) of 50 mM acetophenone. (right) Plot of the dependence of the plateau current on the ketone concentration with a line of best fit. Titration of [(Cp)Co(CpN)][OTf] into a DME solution of 100 mM [4-CnPhNH3][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6] led to observation of a first order dependence on Co concentration. Although these data indicate a rate-limiting reaction between Co and acetophenone, we find that CV’s of a DME solution of 1 mM [(Cp)Co(CpN)][OTf] and 50 mM acetophenone in the absence of acid demonstrates no evidence of an interaction between these two species. In contrast, CV’s of [(Cp)Co(CpNH)][OTf]2 at 100 mV/s in the presence of acetophenone (increasing from 0 to 50 equiv) leads to observation of an increasing amount of the product of H-atom transfer, [(Cp)Co(CpN)][OTf] (Figure 6.12), behavior which is not observed in the absence of ketone. Thus, we conclude that the cumulative evidence support a mechanism involving a rate limiting reaction between [(Cp)Co(CpNH)][OTf] and acetophenone. We suggest that this reaction is a concerted proton-electron transfer (CPET). 131 Figure 6.12: (left) Plateau current dependence on the concentration of [(Cp)Co(CpN)][OTf] in the reaction with 100 mM [4-CNPhNH3][OTf] and 50 mM acetophenone in a 200 mM [TBA][PF6] solution of DME at 100 mV/s. (right) Repeated cyclic voltammograms in 200 mM [TBA][PF6] DME solution of [(Cp)Co(CpNH)][OTf]2 at 100 mV/s with increasing equivalents of acetophenone (0 to 50 mM). 6.3 Discussion We were interested in the thermochemical consequences of synthetic integration of the cobaltocene moiety with N,N-dimethylanilinium. We find that the BDFEN–H of [(Cp)Co(CpNH)]2+ (79.1 kcal mol−1) is in excellent agreement with that of N,Ndimethylanilinium (BDFEN–H = 79 kcal mol−1).30,40 While the introduction of the [Cp2Co]+ group at the para-position of the anilinium makes it more acidic (pKa of 8.6 vs 11.4), this effect is balanced by the increased oxidation potential (0.52 V vs Fc+/0 vs 0.38 V vs Fc+/0). These data are consistent with the [Cp2Co]+ acting as an innocent, electron-withdrawing substituent in this redox state, although the enhanced stability of [(Cp)Co(CpN)]2+ relative to [N,N-dimethylanilinium]+ suggests some electronic interaction between these species. It also points to the potential utility of [(Cp)Co(CpN)]2+ as a hydrogen-atom abstracting reagent 132 given the similarity of its hydrogen-atom abstracting power to that of 2,4,6-tri-tertbutylphenoxy radical (BDFEO–H(2,4,6-tri-tert-butylphenol) = 77.1 kcal mol−1 in MeCN).40 One electron reduction of [(Cp)Co(CpNH)]2+ attenuates the BDFEN–H by 40 kcal mol−1. The BDFEN–H of [(Cp)Co(CpNH)]+ is thus not well-predicted by that of the parent anilinium but rather by the effective BDFE (BDFEeff = 37.9 kcal mol−1, Eq 6.2) of its constituent elements, namely cobaltocene (E0(reductant) = −1.33 V vs Fc+/0) and N,Ndimethylanilinium (pKa(acid) = 11.4).40 This suggests that although the loci of electron donation has changed from being the aniline to the Co, the thermochemical properties of the constituent elements are again only minimally perturbed by synthetic integration. Density functional theory calculations with the TPSS functional,41 the def2-tzvpp basis set,42,43 Grimme-d3 dispersion correction,44 and SMD45 solvation in acetonitrile also predict the homolytic bond strength of both species well, BDFEcalc = 75.8 and 38.5 kcal mol−1 respectively. The ready prediction of the BDFEN–H both by effective BDFE calculations and by DFT is promising for the development of future catalytic systems with targeted H-atom donor strengths. Eq 6.2) BDFEeff = 1.37×pKa(acid) + 23.06×E0(reductant) + CG The importance of the BDFE in determining the reactivity is demonstrated by the selective proton-coupled reduction of acetophenone to its pinacol in CPE experiments. Thermodynamic considerations and mechanistic data derived from CV experiments strongly support the active mechanism to be a rate-limiting CPET from [(Cp)Co(CpNH]+ to acetophenone to generate the neutral ketyl radical (BDFEcalc = 38.7 kcal mol−1). This 133 demonstrates that the base-appended cobaltocene is able to take advantage of the inherent reversible electron-transfer properties and the kinectically facile protonation of anilines and harness them to achieve a thermodynamically challenging CPET reaction. The proton-coupled reduction of acetophenone directly on metal electrodes has been extensively studied.46–50 In general, metals with a low-overpotential for hydrogen evolution (i.e., Pt) preferentially hydrogenate the substrate to the alcohol.48,51 For metals with highoverpotential for HER, such as Hg, more complicated and sometimes contradictory results have been observed, but at least some pinacol product is always observed.46,47,50,52,53 Summarized here are the clear results from the literature and how they compare to our own observations of Co-mediated pinacolization. Firstly, with Hg, at low-pH values the reduction of the acetophenone is proton-coupled and thus pH-dependent (Eq 6.3).47 In contrast, the onset potential in our system is neither pH- (or pKa-) dependent but is rather fixed by the [(Cp)Co(CpNH)]2+/+ redox couple. This is highlighted by the fact that we observe catalysis upon the addition of cobalt even though the relevant cobalt species, [(Cp)Co(CpNH)]+ is four orders of magnitude less acidic than the acid that was already present, [4-CNPhNH3]+ (pKa = 11.0 and 7.0 respectively). Secondly, the pinacolization reaction at a Hg electrode is secondorder in acetophenone indicating a rate-determining step coupling step. In our case, the reaction is first order in [(Cp)Co(CpN)][OTf] and acetophenone, which we interpret as resulting from a rate-limiting proton-coupled electron transfer step. Lastly, it has been established that at the potentials relevant to Hg-mediated electropinacolization reactions that aromatic ketones are strongly adsorbed to Hg electrodes.54–56 This is consistent with results on the proton-coupled electrode-mediated reduction of ketones with other metals,50,51 and the 134 strongly sterically-dependent onset potential observed for differently substituted phenyl ketones. In our case, we believe that the reaction is happening in the homogeneous phase, consistent with the 1:1 ratio of diasteromers that we are observing, which is typically not the case in surface-mediated pinacolizations.52,54 Eq 6.3) Eonset (vs NHE) = −0.99 + 0.60×pH While the overall transformation mediated by our catalyst and a Hg pool electrode are similar, we believe that the comparison is illustrative. Firstly, the mechanistic differences observed support our contention that we are performing a Co-mediated CPET reaction in the homogeneous phase. Secondly, we anticipate that the mechanistic differences identified in this first study will position us to develop reactions that are not accessible directly on a mercury electrode. Adsorption on Hg is exquisitely sensitive to both sterics and electronics,53,55 so it is likely that many ketones would not be suitable for that reaction, although studies on the scope of the electropinacolization reaction are very limited. Furthermore, the necessity of adsorbing a substrate to the electrode may prevent hydrofunctionalization or three-component coupling reactions that are potentially accessible to the freely diffusing radical, such observations have been made previously in related electrohydrodimerization reactions.56 Lastly, with a Hg electrode, the proton-coupled reduction of a substrate relies on pre-association of the acid with the substrate, and, thus, proton-coupled electron transfer reactivity to substrates lacking polar motifs, such as olefins, is not observed.57 In our case, the acid and electron are co-localized by the cobalt catalyst 135 suggesting that we may also be able to mediate the proton-coupled reduction of non-polar unsaturated moieties. 6.4 Conclusion: Herein, we expanded upon our interest in metallocenes as mediators of electrocatalytic reactions. In particular, we uncovered that although protonated cobaltocenes are strong proton-coupled electron transfer donors upon protonation,6 which we1,2 and others58 have suggested could be relevant to N−H bond formation during chemical nitrogen fixation reactions, they are likely not relevant under electrocatalytic conditions due to slow protonation of the Cp(*) ring. Furthermore, protonation to form the PCET donor (i.e., [(Cp)Co(η4-C5H6)]+) results in a species that is rapidly reduced by the electron to a species whose preferred mechanism of reaction is as a hydride donor. This is not a unique mechanism; indeed, many organometallic species undergo ECEC mechanisms for HER. In contrast, relatively few are able to access EC (or CE) mechanisms that produce stable, strong hydrogen-atom donors (BDFEX–H < 50 kcal mol−1). As such, we became interested in synthetic modifications that would both kinetically accelerate protonation of the cobaltocene and prevent its further reduction to a hydridic species. To this end, we synthetically append an aniline base to cobaltocenium. Now, protonation no longer requires reduction instead readily occuring in the CoIII state. Furthermore, by effectively decoupling protonation from the primary coordination sphere, a second reduction to form a hydridic species (i.e., an ECE mechanism) is no longer favorable. Instead, as desired, the aniline-protonated, CoII species, [(Cp)Co(CpNH)]+, is a strong H-atom −1 136 donor (BDFEN–H = 38.8 kcal mol ) that is fairly stable under reductive conditions despite the high thermodynamic driving force for HER and the presence of excess protons and electrons. CPE experiments in the presence of excess acid and acetophenone form the pinacolization product selectively demonstrating the potential of this catalyst as a PCET mediator. Mechanistic information gleaned from cyclic voltammetry indicate a first-order reaction in cobalt and acetophenone, which is dependent on formation of [(Cp)Co(CpNH)]+. These data point at a rate-limiting CPET to acetophenone. On the basis of these data and known reactions,20,56 we believe that this catalyst will now provide access to electrocatalytic hydrofunctionalization or three-component cross coupling reactions preceding via carbon-centered radicals derived from the CPET to unsaturated substrates. Furthermore, given the growing acknowledgment of the role of strong PCET donors in nitrogen fixation catalysis1,2,6,59,60 and the demonstrated utility of PCET mediators as co-catalysts in electrocatalytic O2-reduction,12,13 we believe that similar cobalt catalysts may have a role as co-catalysts in the proton-coupled reduction of small molecules. 6.5 References: (1) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. (2) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. (3) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76–78. (4) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2010, 3, 120–125. 137 (5) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. J. Am. Chem. Soc. 2016, 138, 13521–13524. (6) Chalkley, M. J.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2019, 141, 4721–4729. (7) Francke, R.; Little, R. D. Chem. Soc. Rev. 2014, 43, 2492–2521. (8) Oh, Y.; Hu, X. Chem. Soc. Rev. 2013, 42, 2253–2261. (9) Taheri, A.; Berben, L. A. Chem. Commun. 2016, 52, 1768–1777. (10) Bullock, R. M.; Appel, A. M.; Helm, M. L. Chem. Commun. 2014, 50, 3125–3143. (11) Kang, P.; Chen, Z.; Brookhart, M.; Meyer, T. J. Top. Catal. 2015, 58, 30–45. (12) Gerken, J. B.; Stahl, S. S. ACS Cent. Sci. 2015, 1, 234–243. (13) Anson, C. W.; Stahl, S. S. J. Am. Chem. Soc. 2017, 139, 18472–18475. (14) Tarantino, K. T.; Liu, P.; Knowles, R. R. J. Am. Chem. Soc. 2013, 135, 10022–10025. (15) Rono, L. J.; Yayla, H. G.; Wang, D. Y.; Armstrong, M. F.; Knowles, R. R. J. Am. Chem. Soc. 2013, 135, 17735–17738. (16) Qiu, G.; Knowles, R. R. J. Am. Chem. Soc. 2019, 141, 2721–2730. (17) Chciuk, T. V.; Flowers, R. A. J. Am. Chem. Soc. 2015, 137, 11526–11531. (18) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. J. Am. Chem. Soc. 2018, 140, 15342– 15352. (19) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. J. Am. Chem. Soc. 2016, 138, 8738– 8741. (20) Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A. Chem. Rev. 2016, 116, 8912–9000. (21) Kim, D.; Rahaman, S. M. W.; Mercado, B. Q.; Poli, R.; Holland, P. L. J. Am. Chem. Soc. 2019, 141, 7473–7485. 138 (22) Kolmar, S. S.; Mayer, J. M. J. Am. Chem. Soc. 2017, 139, 10687–10692. (23) Koelle, U.; Infelta, P. P.; Graetzel, M. Inorg. Chem. 1988, 27, 879–883. (24) Henderson, R. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 946–967. (25) Peng, Y.; Ramos-Garces, M. V.; Lionetti, D.; Blakemore, J. D. Inorg. Chem. 2017, 56, 10824–10831. (26) Stepnicka, P. John Wiley & Sons, Ltd: Chichester, England, 2008. (27) El Murr, N. J. Organomet. Chem. 1981, 208, C9–C11. (28) Pauson, P. L. Encyclopedia of Reagents for Organic Synthesis; American Cancer Society, 2001. (29) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882–1893. (30) Liu, W.-Z.; Bordwell, F. G. J. Org. Chem. 1996, 61, 4778–4783. (31) Geiger, W. E. J. Am. Chem. Soc. 1974, 96, 2632–2634. (32) Garrido, G.; Rosés, M.; Ràfols, C.; Bosch, E. J. Solution Chem. 2008, 37, 689–700. (33) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (34) Loutfy, R. O.; Loutfy, R. O. Tetrahedron 1973, 29, 2251–2252. (35) Arnett, E. M.; Quirk, R. P.; Larsen, J. W. J. Am. Chem. Soc. 1970, 92, 3977–3984. (36) Nakajima, M.; Fava, E.; Loescher, S.; Jiang, Z.; Rueping, M. Angew. Chem. Int. Ed. 2015, 54, 8828–8832. (37) Caron, A.; Morin, É.; Collins, S. K. ACS Catal. 2019, 9, 9458–9464. (38) Eckert, F.; Leito, I.; Kaljurand, I.; Kütt, A.; Klamt, A.; Diedenhofen, M. J. Comput. Chem. 2009, 30, 799–810. 139 (39) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M. J. Am. Chem. Soc. 2012, 134, 11235–11242. (40) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. (41) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (42) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (43) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065. (44) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104– 154119. (45) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378– 6396. (46) Brennan, M. P. J.; Brown, O. R. J. Chem. Soc., Faraday Trans. 1 1973, 69, 132–142. (47) Elving, P. J.; Leone, J. T. J. Am. Chem. Soc. 1958, 80, 1021–1029. (48) Bondue, C. J.; Koper, M. T. M. J. Am. Chem. Soc. 2019, 141, 12071–12078. (49) Lopez-Ruiz, J. A.; Andrews, E.; Akhade, S. A.; Lee, M.-S.; Koh, K.; Sanyal, U.; Yuk, S. F.; Karkamkar, A. J.; Derewinski, M. A.; Holladay, J.; et al. ACS Catal. 2019, 9, 9964–9972. (50) Rudd, E. J.; Conway, B. E. Trans. Faraday Soc. 1971, 67, 440–457. (51) Yue, Y.-N.; Wu, D.; Zeng, S.; Yang, M.-P.; Wang, H.; Lu, J.-X. New J. Chem. 2017, 41, 7853–7856. (52) Bewick, A.; P. Cleghorn, H. Journal of the Chemical Society, Perkin Transactions 2 1973, 0, 1410–1413. 140 (53) Tilborg, W. J. M. van; Dekker, L. J. G.; Smit, C. J. Recl. Trav. Chim. Pays-Bas 1978, 97, 321–326. (54) Stocker, J. H.; Jenevein, R. M. J. Org. Chem. 1968, 33, 294–297. (55) Blomgren, E.; Bockris, J. O.; Jesch, C. J. Phys. Chem. 1961, 65, 2000–2010. (56) Nielsen, M. F.; Utley, J. H. P. Organic Electrochemistry; Marcel Dekker: New York, NY, 1999; pp 795–882. (57) Laitinen, H. A.; Wawzonek, S. J. Am. Chem. Soc. 1942, 64, 1765–1768. (58) Bruch, Q. J.; Connor, G. P.; Chen, C.-H.; Holland, P. L.; Mayer, J. M.; Hasanayn, F.; Miller, A. J. M. J. Am. Chem. Soc. 2019, 141, 20198–20208. (59) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Nature 2019, 568, 536–540. (60) Munisamy, T.; Schrock, R. R. Dalton Trans. 2011, 41, 130–137. 141 APPENDIX A. Supplementary Information for Chapter 2 Adapted from: Chalkley, M. J.; Peters, J. C. Angew. Chem. Int. Ed. 2016, 55, 11995–11998. DOI: 10.1002/anie.201605403 142 A.1 Experimental Details A.1.1 General Considerations All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective oxygen and moisture removal. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. (P3B)Fe(N2),1 and [TBA][BArF4]2 were synthesized via known literature procedures. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. C 6D6 was degassed and stored over activated 3 Å molecular sieves prior to use, and THF-d8 was dried with NaK and vac-transferred into a dry vessel. Elemental analysis for {(P3B)Fe(NO)}8 was performed by Midwest Microlab, LLC, Indianapolis, IN, and elemental analysis for {(P3B)Fe(NO)}9-10 was performed by the Beckman Institute Elemental Analysis facility at California Institute of Technology. 1H and 13C NMR chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. 31P NMR chemical shifts are reported in ppm relative to 85% aqueous H 3PO4. 11B NMR chemical shifts are reported in ppm relative to a 15% solution of BF3∙OEt2 in CDCl3 Solution phase magnetic measurements were performed by the method of Evans. 3 Solid IR measurements were obtained on a Bruker Alpha spectrometer equipped with a diamond ATR probe. 143 A.1.2 EPR Spectroscopy X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-5 mM solutions prepared as frozen glasses in 2-methyltetrahydrofuran (2-MeTHF). Samples were collected at powers ranging from 20 μW to 15 mW and modulation amplitudes of 1-5 Gauss. Spectra were simulated using the EasySpin4 suite of programs with Matlab 2013. A.1.3 Optical Spectroscopy Measurements were taken on a Cary 50 UV/Visible spectrophotometer using a 1 cm quartz cell connected to a round-bottom flask and sealed with a Teflon stopcock. Variable temperature measurements were collected with a Unisoku CoolSpek cryostat mounted within the Cary spectrophotometer. Density corrections were applied using literature temperature vs. density data available for 2-MeTHF.5 A.1.4 X-ray Crystallography XRD studies were carried out at the Beckman Institute Crystallography Facility on a Brüker Kappa Apex II diffractometer (Mo Kα radiation). Structures were solved using SHELXS or SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL.6 The crystals were mounted on a glass fiber under Paratone N oil. A.1.5 Electrochemistry Electrochemical measurements were carried out in a thick-walled one-component electrochemical cell fitted with a Teflon stopcock and tungsten leads protruding from the top of apparatus. A CD instruments 600B electrochemical analyzer was used for data collection. A freshly-polished glassy carbon electrode was used as the working electrode, a platinum wire was used as the auxiliary electrode, and a silver wire as a reference 144 electrode. The analyte was used in 1 mM concentration. After the desired scans were completed, ferrocene (1 mM) was added to serve as an internal reference. All reported potentials are referenced to the ferrocene couple, Fc/Fc +. A.1.6 57Fe Mössbauer Spectroscopy 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 SVT400 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 crystalline material and then mounted in a Delrin cup fitted with a screw-cap as a boron nitride pellet. Data analysis was performed using the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes except where noted. A.1.7 Density Functional Theory Calculations Optimizations were performed using the ORCA program.7 Frequency calculations were performed using the Gaussian 09 program. Gas-phase structures were optimized using the crystal structure coordinates as the input. The BP86 8,9 and B3LYP10 functional with the 631G(d) basis set was used on C and H,11 and the def2-TZVPP basis set was used on P, B, Fe, N, and O.12,13 That optimized structures represented true stationary points was checked by doing a single-point frequency calculations on the optimized structure, which in all cases revealed no negative frequencies or one negative frequency that was small (≥ -30 cm 1 ) and weak. Bond indices were calculated using the built-in Wiberg Bond Index algorithm in Gaussian 09. Broken symmetry solutions were found by first optimizing the high spin 145 wavefunction and then using the Flip Spin keyword in ORCA to find and optimize a broken symmetry solution. Mössbauer parameters were calculated by doing a single point calculation on the BP86 optimized structure using TPSSH14 and CP(PPP) on Fe;15 def2TZVP on P, B, and N;12,13 and def2-SVP on C and H.16 The obtained density was converted to an isomer shift using the calibration constant obtained by Neese and coworkers for this functional.17 A.2 Synthetic Procedures: {(P3B)Fe(NO)}8-A vial containing [NO][PF6] (11.4 mg, 0.065 mmol, 1.0 eq.) and a stir bar was chilled to -78 °C. A suspension of (P3B)Fe(N2) (45.0 mg, 0.067 mmol, 1.02 eq.) in DME (3 mL) was chilled to -78 °C. Using a syringe the suspension of (P3B)Fe(N2) was added in one portion to the stirring [NO][PF6]. This mixture was then immediately capped and allowed to stir at -78 °C for a half hour before being warmed to room temperature. It was then stirred at room temperature for an additional half hour to yield a black solution. The solvent was removed under reduced pressure, and the oily solid was washed with pentane (3 x 1 mL), ether (3 x 1 mL) and benzene (3 x 1 mL). The residual solid was dissolved in THF and filtered through celite. The THF solution was then layered with benzene, and precipitated by slow diffusion of pentane overnight. This yielded crystalline purple material of [(P3B)Fe(NO)][PF6] (not characterized due to its very poor solubility after recrystallization) which was then suspended in Et2O (2 mL) and stirred with NaBArF (0.95 eq.) overnight. The ether was then filtered to remove salts and the solvent was removed in vacuo. The purple solid is then dissolved in minimal THF and pentane was allowed to diffuse in yielding purple crystals of {(P3B)Fe(NO)}8 overnight (yield = 41.1 mg, 41%). 146 H NMR (RT, THF-d8, 400 MHz): δ = 7.80 (11 H, br s, Ar-H), 7.68-7.48 (10 H, 1 m, Ar-H), 7.27 (3 H, d, 3J(H-H) = 7.2 Hz, Ar-H), 2.86 (3 H, m, PCH), 1.73 (9 H, m), 1.58 (9 H, ‘dd’ 4J(P-H) = 14.5 Hz and 3J(H-H) = 7.0 Hz, PCHCH3), 1.02 (9 H, ‘dd’ 4J(P-H) = 14.5 Hz and 3J(H-H) = 6.3 Hz, PCHCH3), 0.81 (3 H, br s, PCH), 0.71 (9 H, ‘dd’ 4J(P-H) = 12.0 Hz and 3J(H-H) = 6.4 Hz, PCHCH3). 1H NMR (-78 °C, THF-d8, 500 MHz): δ = 8.17 (2 H, ‘t’ 3J(H-H) = 8.9 Hz, Ar-H), 7.99 (2 H, br s, Ar-H), 7.90 (8 H, s, BArF-H), 7.74 (4 H, s, BArF-H), 7.54 (2 H, br s, Ar-H), 7.49 (2 H, br s, Ar-H), 7.34 (2 H, br s, Ar-H), 7.30 (1 H, br s, Ar-H), 6.88 (1 H, br s, Ar-H), 3.62 (1 H, br s, PCH), 3.41 (1 H, br s, PCH), 3.06 (1 H, br s, PCH), 2.33 (1 H, br s, PCH), 1.94 (4 H, br s, PCHCH3), 1.89-1.77 (4 H, m, PCHCH3), 1.60 (3 H, br s, PCHCH3), 1.51 (3 H, br s, PCHCH3), 1.42 (3 H, d 3J(H-H) = 15.7 Hz, PCHCH3), 1.24 (4 H, br s, PCHCH3), 1.11 (6 H, br s, PCHCH3), 0.86 (3 H, d 3J(H-H) = 12.7 Hz, PCHCH3), 0.55 (3 H, br s, PCHCH3), .47 (3 H, br s, PCHCH3), -0.07 (1 H, br s, PCH), -0.17 (1 H, br s, PCH). 31P NMR (RT, THF-d8, 162 MHz): δ = 64.0 (br). 31P NMR (-78 °C, THFd8, 202 MHz): δ = 84.4 (br), 80.0 (br), 19.6 (br). 11B NMR (RT, THF-d8, 128 MHz): δ = 36.7 (br, P3B), -2.0 (sh, BArF). 13C NMR (RT, THF-d8, 100 MHz): δ = 164.8 (q, 2J(C-B) = 29.7 Hz), 139.1 (br), 137.5 (s), 134.3 (d, 2J(C-P) = 17.3 Hz), 133.2 (br), 131.9 (m), 127.4 (q, 2J(CF) = 72.2 Hz), 120.1 (m), 35.0 (d, 2J(C-P) = 19.0 Hz), 30.6 (d, 2J(C-P) = 8.9 Hz), 24.8 (d, 3 J(C-P) = 4.2 Hz), 24.0 (s), 23.7 (s), 21.7 (s). 19F NMR (RT, THF-d8, 376 MHz): δ = -63.4 (s). IR (Thin Film): 1745 cm-1 (νNO). UV/Vis (2-MeTHF, nm {cm-1 M-1}): 463 {220}, 565 {350}. Elemental Analysis: theory [C 53.05, H 4.32, N 0.91]; found [C 53.11, H 4.17, N 1.08] 147 {(P )Fe(NO)} -The microcrystalline material, [(P )Fe(NO)][PF6], (38.0 mg, B 3 9 B 3 0.046 mmol, 1.0 eq) was suspended in THF (5 mL) and added to a vial containing Cp2Co (8.7 mg, 0.046 mmol, 1.0 eq.). The suspension was stirred for three hours during which the color of the solution lightened to an orange-pink color. The solvent was then removed under reduced pressure, and the residue was extracted with pentane and filtered through celite. The pentane was then reduced to a minimal volume under reduced pressure and the solvent was allowed to concentrate slowly overnight by slow evaporation into an HMDSO antisolvent at room temperature. The mother liquor was decanted and the solids were washed with HMDSO (3 x 1 mL). This yielded red-brown crystalline material of {(P3B)Fe(NO)}9 (yield = 21.3 mg, 68%). 1 H NMR (C6D6, 400 MHz, RT): 17.48 (3 H), 11.27 (3 H), 8.58 (3 H), 6.11 (3 H), 2.11 (9H), 1.87 (9H), 0.57 (3 H), -1.23 (9H), -2.10 (9 H). μeff (RT, Evans Method, C6D6): 1.7μB. IR (Thin Film): 1664 cm-1 (νNO). UV/Vis (2-MeTHF, nm {cm-1 M-1}): 535 {420}, 916 {180}. Elemental Analysis: theory [C 63.93, H 8.05, N 2.07]; found [63.59, 8.11, 2.18] {(P3B)Fe(NO)}10-[(P3B)Fe(NO)][PF]6 (66.3 mg, 0.081 mmol, 1.0 eq.) was suspended in THF (10 mL) and added to a vial containing 1% Na/Hg amalgam (3.8 mg of Na, 0.162 mmol, 2.0 eq.). The suspension was stirred at room temperature for four hours yielding a dark red solution. The solvent was removed under reduced pressure, and the residue was extracted with ether (3 x 3 mL) and filtered through celite. The solvent was then evaporated to dryness under reduced pressure. The residue was dissolved in minimal THF (~2 mL) and layered with a 2 mL benzene solution of 12-crown-4 (47 mg, 0.267 mmol, 3.3 eq.). After standing at room temperature overnight, red needles of the desired compound were obtained. 10 148 Decanting and washing with minimal ether and then drying yielded {(P )Fe(NO)} (48 B 3 mg, 57 %). The best yields could be obtained using this procedure but reduction of {(P3B)Fe(NO)}9 with 1 eq. of Na/Hg and the above reported isolation procedure also led reproducibly to pure material. 1 H NMR (-78 °C, THF-d8, 500 MHz): δ = 7.15 (3 H, br s, Ar-H), 7.01 (3 H, br s, Ar- H), 6.73 (3 H, br s, Ar-H), 6.59 (3 H, br s, Ar-H), 3.74 (32 H, s, 12-c-4), 2.79 (3 H, br s, PCH), 2.00 (3 H, br s, PCH), 1.52-0.80 (27 H, m, PCHCH3), -0.43 (9H, s, PCHCH3). 31P NMR (-78 °C, THF-d8, 202 MHz): δ = 88.7 (br). 11B NMR (RT, THF-d8, 128 MHz): δ = 19.9 (br). 13C NMR (-78 °C, 126 MHz, THF-d8, 100 MHz): δ = 175.9 (br), 146.0, 130.7, 125.2, 124.9, 119.9, 70.4, 65.1, 30.7, 27.6, 20.3, 18.9, 17.2. At room temperature there is no detectable 31P NMR signal and the 1H NMR spectrum is broadened presumably by the presence of undetectably small amounts of {(P3B)Fe(NO)}9 that undergoes fast electron transfer with {(P3B)Fe(NO)}10 in solution. Cooling the solution to -78 °C makes this process slower than the NMR time scale allowing 1 H and 31P NMR spectra to be obtained. IR (Thin Film): 1568 cm-1 (νNO). UV/Vis (2-MeTHF, nm {cm-1 M-1}): 314 {11700}, 389 {4000}, 499 {1800}. Elemental Analysis: {(P3B)Fe(NO)}10∙.5C6H6 theory [C 60.56, H 8.22, N 1.28]; found [C 60.32, H 8.10, N 1.41] Taking crystalline {(P3B)FeNO}10 and then pulling vacuum on it before dissolution in THF-d8 leads to the presence of 0.5 equivalents of C6H6 as seen in Figure S9. The crystal structure of {(P3B)Fe(NO)}10 has two half-occupied benzene molecules present confirming the presence of benzene in crystalline {(P3B)Fe(NO)}10. 149 A.3 NMR Spectra: Figure A.1: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. Figure A.2: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}8 at -78 °C in THF-d8. 150 Figure A.3: The 31P NMR spectrum (161.9 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. Figure A.4: The 31P NMR spectrum (202.4 MHz) of {(P3B)Fe(NO)}8 at -78 °C in THF-d8. 151 Figure A.5: The 11B NMR spectrum (128.3 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. Figure A.6: The 19F NMR spectrum (376.3 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. 152 Figure A.7: The 13C NMR spectrum (100.6 MHz) of {(P3B)Fe(NO)}8 at room temperature in THF-d8. Figure A.8: The 1H NMR spectrum (400 MHz) of {(P3B)Fe(NO)}9 at room temperature in C6D6. 153 Figure A.9: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}10 at room temperature in THF-d8. Figure A.10: The 1H NMR spectrum (500 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THF-d8. 154 Figure A.11: The 31P NMR spectrum (202.4 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THF-d8. Figure A.12: The 11B NMR spectrum (128.3 MHz) of {(P3B)Fe(NO)}10 at room temperature in THF-d8. 155 Figure A.13: The 13C NMR spectrum (125.7 MHz) of {(P3B)Fe(NO)}10 at -78 °C in THFd8 . A.4 IR Spectra Trel {(P3B)Fe(NO)}8 3400 2900 2400 1900 1400 900 -1 ν / cm Figure A.14: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}8. 400 156 Trel {(P3B)Fe(NO)}9 3400 2900 2400 1900 1400 900 400 ν / cm-1 Figure A.15: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}9. Trel {(P3B)Fe(NO)}10 3400 2900 2400 1900 1400 900 ν / cm-1 Figure A.16: Thin-film IR Absorption spectrum of {(P3B)Fe(NO)}10. 400 157 Absorption Coefficient / M-1cm-1 A.5 UV/Visible Spectra {(P3B)Fe(NO)}8 3000 -190 C 2500 20 C 2000 1500 1000 500 0 375 575 775 Wavelength / nm 975 Figure A.17: UV/Visible spectra of {(P3B)Fe(NO)}8 in 2-MeTHF accounting for changes in density with temperature.[5] Absorption Coefficient / M-1cm-1 {(P3B)Fe(NO)}10 14000 ° -190 C 12000 20 C ° 10000 8000 6000 4000 2000 0 280 480 680 880 Wavelength / nm Figure A.18: UV/Visible spectra of {(P3B)Fe(NO)}10 in 2-MeTHF accounting for changes in density with temperature.[5] 158 ° ° Table A.1: Comparison of selected data for {(P3B)Fe(NO)}9 and [Cr(CN)5(NO)]3-.18 A.6 57Fe Mössbauer Spectroscopy A.6.1 Mössbauer Discussion There are a variety of mechanisms that can lead to an asymmetric quadrupole doublet arising. The most common cause is the presence of a local magnetic field (usually from the measured iron complex being paramagnetic) that causes a difference in the relaxation rate of the nuclear transitions.19 However, other mechanisms can cause broad and asymmetric line shapes even in diamagnetic samples particularly when they are measured as crystalline or microcrystalline solids. Such phenomena include the Goldanskii-Kargaryin effect,20 “texture,”21 or cosine smearing.22 Despite the appearance of asymmetric line shapes in the crystalline, 80 K, zero field Mössbauer of {(P3B)Fe(NO)}8 and {(P3B)Fe(NO)}10 we believe that these species are diamagnetic in the solid state. The X-ray structures manifest short metal-ligand bond lengths that are well predicted by DFT calculations with an S = 0 ground state. In contrast optimizations from using the crystal structures as input in an S = 1 or 2 ground state lead to considerably different structures and higher energies. To further verify that the bulk material was diamagnetic we performed 80 K, zero field Mössbauer on 159 powdered samples obtained by dissolving crystalline material in THF, evaporating the solvent and then triturating it with pentane. This procedure led to the obtainment of signals with symmetric line shapes and in the case of {(P3B)Fe(NO)}10 significantly sharper line widths. Furthermore, the spin state of {(P3B)Fe(NO)}8 and {(P3B)Fe(NO)}10 is known to be S = 0 in solution due to the multinuclear NMR data. As such 2 mM THF solutions of 57Fe enriched samples of {(P3B)Fe(NO)}8 and {(P3B)Fe(NO)}10 were measured in the presence (50 mT) and absence of a magnetic field. These samples also manifested the same isomer shift and quadrupole splitting as the solid samples confirming that the spin state is the same in solution and solid state. In the {(P3B)57Fe(NO)}10 there is the presence of a ~20 % contamination of [(P3B)Fe(N2)]−. This contaminant was verified by the measurement of an independently prepared sample of [(P3B)Fe(N2)]−. As can be seen in the electrochemistry under N2 (Figure S32), addition of excess strong reductant leads to the formation of [(P3B)Fe(N2)]− from samples of {(P3B)Fe(NO)}8. Such an issue can be readily avoided on large scale syntheses of {(P3B)Fe(NO)}10 by the use of stoichiometric reducing agent but in the preparation of small amounts of 57Fe enriched material (~ 2 mg) consistent over reduction was observed. However the presence of this impurity does not prevent the observation of symmetric and narrow linewidths for solution state measurements of {(P3B)Fe(NO)}10. The calculated Mössbauer parameters (Table S5) consistently underestimate the isomer shift by ~ 0.08 mm s-1 an error within the standard found by Neese and coworkers.[16] The predictions of the quadrupole splitting are remarkably accurate within 0.04 mm s-1 in all 160 cases. These calculations also illustrate the trend discussed in the main text in which lower spin states are associated with low isomer shifts. A.6.2 Mössbauer Spectra Trel {(P3B)Fe(NO)}8 Data Fit -4 -2 0 2 4 Velocity / mm s-1 Figure A.19: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}8 as a microcrystalline solid suspended in a boron nitride matrix. Fit with δ = 0.24 mm s -1, ΔEq = 1.50 mm s-1, ΓR = 0.34 mm s-1, and ΓL = 0.29 mm s-1. {(P3B)Fe(NO)}8 Trel Data Fit -4 -2 0 Velocity / mm s-1 2 4 161 Figure A.20: The 80 K, zero field Fe Mössbauer of {(P3 )Fe(NO)} as a powder suspended in a boron nitride matrix. Fit with δ = 0.24 mms -1, ΔEq = 1.50 mm s-1, ΓR = 0.34 mm s-1, and ΓL = 0.34 mm s-1. 57 B 8 Figure A.21: The 80 K, zero field 57Fe Mössbauer of a 2mM solution of {(P 3B)57Fe(NO)}8 in 2-MeTHF. Fit with δ = 0.25 mm s-1, ΔEq = 1.47 mm s-1, ΓR = 0.27 mm s-1, and ΓL = 0.27 mm s-1. Trel {(P3B)Fe(NO)}9 Data Fit -4 -2 0 Velocity / mm s-1 2 4 Figure A.22: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}9 as a microcrystalline solid suspended in a boron nitride matrix. Fit with δ = 0.26 mm s -1, ΔEq = 0.92 mm s-1, ΓR = 0.34 mm s-1, and ΓL = 0.31 mm s-1. 162 Trel {(P3B)Fe(NO)}10 -4 Data Fit -2 0 2 4 Velocity / mm s-1 Figure A.23: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}10 as a microcrystalline solid suspende in a boron nitride matrix. Fit with δ = 0.17 mm s -1, ΔEq = 1.62 mm s-1, ΓR = -0.61 mm s-1, and ΓL = -0.64 mm s-1. Given the microcrystalline nature of the material a Voigtian distribution was considered to be an appropriate model, [13,17] and the purity of the material was confirmed by NMR and IR spectroscopy. The most likely impurity would be {(P3B)Fe(NO)}9 due to the handling procedure needed to mount the Mössbauer sample, but including a second species with the parameters for {(P 3B)Fe(NO)}9 leads to that component only representing 1% of the total intensity and no improvement of the fit as judged by the χ2-value. Figure A.24: The 80 K, zero field 57Fe Mössbauer of {(P3B)Fe(NO)}10 as a powder suspended in a boron nitride matrix. Fit with δ = 0.16 mm s -1, ΔEq = 1.62 mm s-1, ΓR = 0.44 mm s-1, and ΓL = 0.44 mm s-1. 163 Trel {(P3B)Fe(NO)}10 Data Fit {TPBFeNO}10 [TPBFeN2]- -4 -2 0 2 4 -1 Velocity / mm s Figure A.25: The 80 K, zero field 57Fe Mössbauer of a 2 mM solution of {(P3B)57Fe(NO)}10 in THF. Fit for {(P3B)Fe(NO)}10 δ = 0.17 mm s-1, ΔEq = 1.64 mm s-1, ΓR = 0.26 mm s-1, and ΓL = 0.26 mm s-1 Weight = 0.81. Fit for [(P3B)Fe(N2)]− δ = 0.36 mm s-1, ΔEq = 0.95 mm s-1, ΓR = 0.67 mm s-1, and ΓL = 0.67 mm s-1 Weight = 0.19. {(P3B)Fe(NO)}10 Trel Data Fit {TPBFeNO}10 [TPBFeN2]- -4 -2 0 2 4 Velocity / mm s-1 Figure A.26: The 80 K, 50 mT parallel field 57Fe Mössbauer of a 2mM solution of {(P3B)57Fe(NO)}10 in THF. Fit for {(P3B)Fe(NO)}10 δ = 0.17 mm s-1, ΔEq = 1.64 mm s-1, ΓR = 0.36 mm s-1, and ΓL = 0.36 mm s-1 Weight = 0.81. Fit for [(P3B)Fe(N2)]− δ = 0.36 mm s1 , ΔEq = 0.95 mm s-1, ΓR = 0.54 mm s-1, and ΓL = 0.54 mm s-1 Weight = 0.19. 164 {(P3B)Fe(NO)}10 Trel Data Fit {TPBFeNO}10 [TPBFeN2]- -4 -2 0 2 4 Velocity / mm s-1 Figure A.27:The 80 K, 50 mT perpendicular field 57Fe Mössbauer of a 2mM solution of {(P3B)57Fe(NO)}10 in THF. Fit for {(P3B)Fe(NO)}10 δ = 0.17 mm s-1, ΔEq = 1.63 mm s-1, ΓR = 0.26 mm s-1, and ΓL = 0.26 mm s-1 Weight = 0.78. Fit for [(P3B)Fe(N2)]− δ = 0.36 mm s1 , ΔEq = 0.95 mm s-1, ΓR = 0.54 mm s-1, and ΓL = 0.54 mm s-1 Weight = 0.22. Trel [(P3B)Fe(N2)]− Data Fit -4 -2 0 2 4 Velocity / mm s-1 Figure A.28: The 80 K, 50 mT parallel field Mössbauer of [(P3B)Fe(N2)]− [(P3B)Fe(N2)][Na(12-crown-4)2]) as a powder suspended in a boron-nitride matrix. Fit with δ = 0.36 mm s-1, ΔEq = 0.95 mm s-1, ΓR = 0.52 mm s-1, and ΓL = 0.52 mm s-1. 165 A.7 Electrochemistry Figure A.29: Scan rate dependence of the {(P3B)Fe(NO)}8/9 couple in a THF solution of 100 mM [TBA][PF6] under argon. Figure A.30: Scan rate dependence of the {(P3B)Fe(NO)}9/10 couple in a THF solution of 100 mM [TBA][PF6] under argon. 166 {(P3B)Fe(NO)}9/10 3.5E-05 2.5E-05 I/A 1.5E-05 5.0E-06 -5.0E-06 -1.5E-05 -2.5E-05 -0.5 -1 -1.5 -2 -2.5 -3 E / V vs. Fc/Fc+ Figure A.31: Cyclic voltammetry data at 100 mV s-1 for {(P3B)Fe(NO)}8 in a THF solution of 100 mM [TBA][BArF] under N2. A.8 X-ray Crystallography Table A.2: X-ray parameters for {(P3B)Fe(NO)}9, and {(P3B)Fe(NO)}10. Compound {(P3B)Fe(NO)}9 {(P3B)Fe(NO)}10 ** Chemical Formula C36H54FeNOBP3 C58H92FeNO9NaP3 Formula Weight 676.41 1129.89 Crystal System Triclinic Monoclinc Space Group P-1 C2/c a [Å] 10.8566(3) 34.611(2) b [Å] 11.3808(4) 10.8164(7) c [Å] 15.9526(5) 32.356(2) 167 α [°] 91.345(2) 90 β [°] 95.217(1) 98.5813 (18) γ [°] 118.023(1) 90 V [Å3] 1727.84(10) 11977.5(13) Z 2 8 Dcalcd [g cm-1] 1.300 1.253 F000 722.0 4848.0 μ [mm-1] 0.605 0.393 Temperature [K] 100 100 Wavelength [Å] 0.71073 0.71073 Measured Reflections 29705 22866 Unique Reflections 22786 14652 Data/Restraints/Parameters 22786/0/593 14652/1316/928 R(F) (I>2σ(I)) 0.0353 0.0499 wR(F2) (all) 0.0790 0.1073 GOOF 1.030 1.013 ** Both 12-crown-4 units that encapsulate the Na counteraction are rotationally disordered over two positions. As such, restraints (EADP and SAME) have been applied to the smaller component. No changes to the structural parameters of the {(P 3B)Fe(NO)}10 anion of interest occurred due to this process. B 8 Table A.3: X-ray parameters for {(P3 )Fe(NO)} . Compound {(P3B)Fe(NO)}8 * Chemical Formula C68H66FeNOB2P3F24 Formula Weight 1539.63 Crystal System Monoclinic Space Group P21/c a [Å] 20.1940(2) b [Å] 13.9917(8) c [Å] 25.7210(14) α [°] 90 β [°] 109.837(3) γ [°] 90 V [Å3] 6836.2(7) Z 4 Dcalcd [g cm-1] 1.496 F000 3144.0 μ [mm-1] 0.400 Temperature [K] 100 Wavelength [Å] 0.71073 168 169 Measured Reflections 19288 Unique Reflections 127279 Data/Restraints/Parameters 12729/54/941 R(F) (I>2σ(I)) 0.0622 wR(F2) (all) 0.1854 GOOF 1.037 * There is one highly disordered CF3 group on the BArF24 counteranion. Modeling this disorder led to no improvement in the {(P3B)Fe(NO)}8 cation of interest, hence such disorder was not modeled. A.9 EPR Spectroscopy Figure A.32: X-Band EPR spectrum of {(P3B)Fe(NO)}9 in 2-MeTHF at 7 K and its simulation. The simulation was done using the least-squares fitting program of EasySpin [4] and produced the following parameters: g = [2.50048, 1.99439, 1.96918] and HStrain = [450.420, 159.384, 205.277]. 170 A.10 DFT Calculations A.10.1 DFT Discussion The calculated structures using BP86/def2-TZVVP (Fe, B, P, N, O); 631-G(d) (C, H) led to very good agreement with the experimental values as can be seen in Table S4. Higher spin states were also calculated and revealed poor matches to the experimental data (long Fe–P, Fe–N, and N–O bond distances) and bent Fe–N–O angles. They were also notably higher in energy. Attempts to generate broken symmetry solutions using BP86 with S = 1 NOantiferromagnetically coupled to an intermediate spin metal center led to collapse of the wavefunction to the low-spin solution. This is unsurprising because BP86 is a pure functional and as such will favor electron pairing. To find broken symmetry solutions we used B3LYP, which as a hybrid functional should favor such solutions. In these cases broken-symmetry solutions are found, which in all cases are of a similar energy to the standard low-spin calculation. However, these calculations despite using the same basis sets provide both a poorer estimate of the solid state structure and more importantly predict that the triplet ground state for {(P3B)Fe(NO)}8 is more stable than the singlet ground state by 23.1 kcal mol-1 (Table S4). This triplet structure fails to capture the η4-BCCP interaction that is characteristic of this molecule in the solid state and solution. Therefore we believe that these calculations do not provide as accurate a picture of the electronic structure of these molecules. As such only the BP86 calculations are discussed in Chapter 2. 171 Table A.4: A comparison of calculated (BP86, low-spin) and experimental bond parameters demonstrating good agreement between optimized gas-phase structures and experimental values from X-ray data for {(P3B)Fe(NO)}8, {(P3B)Fe(NO)}9, and {(P3B)Fe(NO)}10. A.10.2 DFT Tables Table A.5: Spin ladder energies for different functionals for {(P3B)Fe(NO)}8-10. Broken symmetry solutions in which an S = 1 NO- unit is antiferromagnetically coupled to the metal are indicated by BS. In all cases attempts to find broken symmetry solutions with BP86 led to collapse of the wavefunction to the electron-paired, low-spin solution. The energies given are relative to the low-spin wavefunction for that functional and species. Functional Spin State Species Energy (kcal mol-1) BP86 S=0 {(P3B)Fe(NO)}8 0 BP86 S=1 {(P3B)Fe(NO)}8 3.6 BP86 S=2 {(P3B)Fe(NO)}8 27.5 B3LYP S=0 {(P3B)Fe(NO)}8 0 B 8 172 B3LYP S=1 {(P3 )Fe(NO)} -23.1 B3LYP S=2 {(P3B)Fe(NO)}8 -7.4 B3LYP BS S = 0 {(P3B)Fe(NO)}8 1.5 BP86 S = 1/2 {(P3B)Fe(NO)}9 0 BP86 S = 3/2 {(P3B)Fe(NO)}9 28.9 BP86 S = 5/2 {(P3B)Fe(NO)}9 53.1 B3LYP S = 1/2 {(P3B)Fe(NO)}9 0 B3LYP S = 3/2 {(P3B)Fe(NO)}9 17.5 B3LYP S = 5/2 {(P3B)Fe(NO)}9 34.5 B3LYP BS S = 1/2 {(P3B)Fe(NO)}9 0.1 BP86 S=0 {(P3B)Fe(NO)}10 0 BP86 S=1 {(P3B)Fe(NO)}10 25.6 BP86 S=2 {(P3B)Fe(NO)}10 53.4 B3LYP S=0 {(P3B)Fe(NO)}10 0 B3LYP S=1 {(P3B)Fe(NO)}10 4.6 B3LYP S=2 {(P3B)Fe(NO)}10 17.5 B3LYP BS S = 0 {(P3B)Fe(NO)}10 1.8 B Table A.6: Calculated Mössbauer parameters for the spin ladder of {(P 3 )Fe(NO)} 8-10 173 . Isomer Shift (mm s-1) Quadrupole Splitting (mm s-1) {(P3B)Fe(NO)}8 S = 0 0.19 -1.48 {(P3B)Fe(NO)}8 S = 1 0.23 1.84 {(P3B)Fe(NO)}8 S = 2 0.35 -2.27 {(P3B)Fe(NO)}9 S = 0 0.17 0.87 {(P3B)Fe(NO)}9 S = 1 0.33 1.53 {(P3B)Fe(NO)}9 S = 2 0.47 -2.04 {(P3B)Fe(NO)}10 S = 0 0.10 1.68 {(P3B)Fe(NO)}10 S = 1 0.33 1.05 {(P3B)Fe(NO)}10 S = 2 0.51 -0.44 Species Spin State 174 A.10.3 DFT Orbitals Figure A.33: A molecular orbital diagram for {(P3B)Fe(NO)}8 picturing select valence orbitals. The orbitals drawn in red correspond to those depicted (isovalue = 0.05) on the right. The orbitals are denoted by their d-orbital parentage. Energies are relative to the HOMO (dz2) which was set to be 0 eV. 175 Figure A.34: A molecular orbital diagram for {(P3B)Fe(NO)}10 picturing select valence orbitals. The orbitals drawn in red correspond to those depicted (isovalue = 0.05) on the right. The orbitals are denoted by their d-orbital parentage. Energies are relative to the HOMO (dxy) which was set to be 0 eV. Table A.7: Comparison of crystallographically determined bond lengths and Wiberg Bond Indices for {(P3B)Fe(NO)}8-10. 176 Figure A.35: A molecular orbital diagram for {(P3B)Fe(NO)}9 picturing selected valence orbitals from the β-spin manifold. The orbitals are denoted with their d-orbital parentage. Energies given are relative to the HOMO (dx2-y2) for the β-spin set which was set to be 0 eV. The depicted orbitals are shown at an isovalue of 0.05. A.11 References (1) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (2) Ono, T.; Ohta, M.; Sada, K. ACS Macro Lett. 2012, 1, 1270–1273. (3) Evans, D. F. J. Chem. Soc. 1959, 0, 2003–2005. (4) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42–55. (5) 177 Shinsaka, K.; Gee, N.; Freeman, G. R. J. Chem. Thermodyn. 1985, 17, 1111–1119. (6) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3–8. (7) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73–78. (8) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (9) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (10) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (11) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257–2261. (12) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571–2577. (13) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (14) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (15) Neese, F. Inorg. Chim. Acta 2002, 337, 181–192. (16) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829–5835. (17) Römelt, M.; Ye, S.; Neese, F. Inorg. Chem. 2009, 48, 784–785. (18) Enemark, J. H.; Quinby, M. S.; Reed, L. L.; Steuck, M. J.; Walthers, K. K. Inorg. Chem. 1970, 9, 2397–2403. (19) Münck, E. Physical Methods in Bioinorganic Spectroscopy; University Science Books: Sausalito, CA, 2000. (20) Goldanskii, V. I.; Makarov, E. F.; Khrapov, V. V. Phys. Lett. 1963, 3, 344–346. (21) Greneche, J. M.; Varret, F. J. Phys. C: Solid State Phys. 1982, 15, 5333–5344. (22) Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Applications; Springer-Verlag: Berlin Heidelberg, 2011. 178 APPENDIX B. Supplementary Information for Chapter 3 Adapted from: Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. DOI: 10.1021/acscentsci.7b00014 179 B.1 Experimental Details B.1.1 General Considerations All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with N 2 followed by passage through an activated alumina column in a solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with sodium benzophenone ketyl in tetrahydrofuran (THF) in order to confirm the absence of oxygen and water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3Å molecular sieves prior to use. Cp*2Co,1 [(P3B)Fe][BArF4],2 (P3Si)Fe(N2),3 [(P3B)Co(N2)][Na(12-crown-4)2],4 (P3Si)Co(N2),5 [(P3B)Fe(N2)][Na(12-crown-4)2],6 and [Ph215NH2][OTf]7,8 were prepared according to literature procedures. Ph15NH2 was obtained from Sigma-Aldrich, Inc. degassed, and dried over activated 3-Å molecular sieves prior to use. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Diethyl ether (Et2O) used in the experiments herein was stirred over Na/K (≥ 2 hours) and filtered or vacuum-transferred before use unless otherwise stated. B.1.2 Physical Methods 1 H chemical shifts are reported in ppm relative to tetramethylsilane, using 1H resonances from residual solvent as internal standards. IR measurements were obtained as solutions or thin films formed by evaporation of solutions using a Bruker Alpha Platinum ATR spectrometer with OPUS software (solution IR collected in a cell with KBr windows and a 1 mm pathlength). H2 was quantified on an Agilent 7890A gas chromatograph (HP- 180 PLOT U, 30 m, 0.32 mm ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. B.1.3 Mӧssbauer 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 form 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 (RT). Solution samples were transferred to a sample cup and chilled to 77 K inside of the glovebox, and unless noted otherwise, quickly removed from the glovebox and immersed in liquid N2 until mounted in the cryostat. Data analysis was performed using version 4 of the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. See discussion below for detailed notes on the fitting procedure. B.1.4 Ammonia and Hydrazine Quantification Reaction mixtures are cooled to 77 K and allowed to freeze. The reaction vessel is then opened to atmosphere and to the frozen solution is slowly added an excess (with respect to acid) solution of a NaOtBu solution in MeOH (0.25 mM) over 1-2 minutes. This solution is allowed to freeze, then the headspace of the tube is evacuated and the tube is sealed. The tube is then allowed to warm to RT and stirred at RT for at least 10 minutes. An additional Schlenk tube is charged with HCl (3 mL of a 2.0 M solution in Et2O, 6 mmol) to serve as a collection flask. The volatiles of the reaction mixture are vacuum transferred at RT into this collection flask. After completion of the vacuum transfer, the collection flask is sealed and warmed to RT. Solvent is removed in vacuo, and the remaining residue is dissolved in H2O (1 mL). An aliquot of this solution (10–100 μL) is then analyzed for the presence of NH3 9 181 (present as NH4Cl) by the indophenol method. A further aliquot of this solution is analyzed for the presence of N2H4 (present as N2H5Cl) by a standard colorimetric method.10 Quantification is performed with UV−vis spectroscopy by analyzing absorbance at 635 nm. In this case of runs with [PhNH3][OTf] we found that aniline in the form of anilinium chloride was present in the receiving vessels. The anilinium chloride interfered with the indophenol and hydrazine detection method. Therefore, quantification for NH3 was performed by extracting the solid residue into 1 mL of DMSO-d6 that has 20 mmol of trimethoxybenzene as an internal standard. Integration of the 1H NMR peak observed for NH4 was then integrated against the two peaks of trimethoxybenzene to quantify the ammonium present. This 1H NMR detection method was also used to differentiate [14NH4][Cl] and [15NH4][Cl] produced in the control reactions conducted with [15NPh2H2][OTf]. B.1.5 EPR Spectroscopy X-band EPR spectra were obtained on a Bruker EMX spectrometer. Samples were collected at powers ranging from 6-7 mW with modulation amplitudes of 2.00 G, modulation frequencies of 100.00 kHz, over a range of 1800 to 4500 Gauss. Spectra were baseline corrected using the algorithm in SpinCount. EPR spectra were modeled using the easyspin program.11 B.1.6 Density Functional Theory All stationary point geometries were calculated using DFT with an M06-L functional,12 a def2-TZVP13 basis set on transition metals (Stuttgart ECP14 was used on Mo atoms) and a def2-SVP13 basis set on all other atoms. Calculations were performed, in part, using Xtreme Science and Engineering Discovery Environment (XSEDE) resources.15 Calculations were performed on the full (P3E)Fe 182 scaffolds. Calculations on the (HIPTN3N)Mo system were performed on a truncated scaffold in which the isopropyl groups were removed (i.e. [{3,5-(C6H4)2C6H3NCH2CH2}3N]3–). Geometries were optimized using the NWChem 6.5 package.16 All single point energy, frequency and solvation energy calculations were performed with the Gaussian09 package. Frequency calculations were used to confirm true minima and to determine gas phase free energy values (Ggas). Single point solvation calculations were done using an SMD solvation model with diethyl ether solvent and were used to determine solvated internal energy (Esoln). Free energies of solvation were approximated using the difference in gas phase internal energy (Egas) and solvated internal energy (Eq B.1) and the free energy of a species in solution was then calculated using the gas phase free energy (Ggas) and the free energy of solvation (Eq B.2).17–19 All reduction potentials were calculated referenced to Fc+/0 using the standard Nernst relation (Eq B.3). Eq B.1) ∆Gsolv ≈ Esoln – Egas Eq B.2) Gsoln = Ggas + ∆Gsolv Eq B.3) ∆G = −nFE0 B.1.7 Gas Chromatography H2 was quantified on an HP 5890 Series II Plus Gas Chromatograph (nitrogen carrier gas) using a thermal conductivity detector. All measurements were obtained using a 100 μL manual injection and the final value was obtained as an average of two runs. 183 B.2 Synthetic Details B.2.1 General Procedure for the Synthesis of the Acids Prior to use the amine was purified (aniline by distillation and diphenylamine by recrystallization). To a 250 mL round bottom flask in the glovebox was added the amine which was subsequently dissolved in 100 mL of Et2O (no additional drying with NaK). To this was added dropwise (1 equiv) of HOTf with stirring over five minutes. Immediate precipitation of white solid was observed and the reaction mixture was allowed to stir for one hour at RT. The reaction mixture was then filtered and the resulting white powder was washed with Et2O (50 mL), pentane (50 mL) and Et2O again (50 mL). The resulting white microcrystalline material was then dried under vacuum. Yields of greater than 90% of microcrystalline material was obtained in this manner in all cases. B.2.2 Procedure for Reaction of Cp*2Co with Acid A 1 mL solution of HOTf or DOTf (23 μL, 3.0 equiv) in toluene and a 2 mL solution of Cp*2Co (40 mg, 1.0 equiv) was chilled to −78 °C for ten minutes in a cold well. With strong stirring the Cp*2Co solution was added dropwise over ten minutes to the HOTf solution. Purple precipitate could be observed upon the addition of each drop. After the completion of the addition the reaction mixture was allowed to stir for 5 more minutes. The reaction was then vacuum filtered in the cold well through a medium porosity frit to yield a purple solid. This solid was then washed with toluene that had been prechilled to −78 °C (5 mL) and then likewise prechilled pentane (15 mL). After drying on the frit for ten minutes the solid was then transferred to a prechilled vial. The solid was then dried under vacuum for several hours at −78 °C. Exact yields were not obtained due to the solid retaining some solvent even after extended drying at these temperatures; however, the material isolated 184 reproducibly represents > 75% of the expected chemical yield. All further spectroscopic and reactivity characterization of this material was carried out immediately after its synthesis and with the maintenance of the material at ≤ −78 °C except where specifically noted. B.3. Ammonia Production and Quantification Studies B.3.1 Standard NH3 Generation Reaction Procedure All solvents are stirred with Na/K for ≥2 hours and filtered prior to use. In a nitrogenfilled glovebox, the precatalyst (2.3 μmol) was weighed into a vial.* The precatalyst was then transferred quantitatively into a Schlenk tube using THF. The THF was then evaporated to provide a thin film of precatalyst at the bottom of the Schlenk tube. The tube is then charged with a stir bar and the acid and reductant are added as solids. The tube is then cooled to 77 K in a cold well. To the cold tube is added Et2O to produce a concentration of precatalyst of 2.3 mM. The temperature of the system is allowed to equilibrate for 5 minutes and then the tube is sealed with a Teflon screw-valve. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is allowed to stir at −78 °C for three hours. At this point the tube is allowed to warm to RT with stirring, and stirred at RT for 5 minutes. To ensure reproducibility, all experiments were conducted in 200 mL Schlenk tubes (51 mm OD) using 25 mm stir bars, and stirring was conducted at ~900 rpm. * In cases where less than 2.3 μmol of precatalyst was used stock solutions were used to avoid having to weigh very small amounts. Table B.1: UV-vis quantification results for standard NH3 generation experiments with [(P3B)Fe]+ Entry [(P3B)Fe]+ Acid Cp*2Co NH4Cl N2H5Cl Equiv % Yield NH3 (μmol) equiv equiv (μmol) (μmol) NH3/Fe Based on e- A B C Avg. 2.3 2.3 2.3 a 108 108a 108a 185 54 54 54 31.4 28.5 29.2 0.0 0.0 0.0 13.5 75.6 12.3 68.6 12.6 70.4 12.8 ± 0.5 72 ± 3 D 2.3 322a 162 76.4 2.0 33.0 61.4 a E 2.3 322 162 80.0 0.7 34.5 64.2 Avg. 34 ± 1 63 ± 2 F 2.3 638a 322 60.4 0.5 26.0 24.3 a G 2.3 638 322 63.2 0.3 27.3 25.4 Avg. 26.7 ± 0.9 25 ± 1 b H 1.1 108 54 7.8 0.0 6.9 37.6 I 2.3 108b 54 19.2 0.0 8.3 46.3 Avg. 8±1 42 ± 6 J 2.3 108c 54 17.7 N.D. 7.7 43.1 c K 2.3 108 54 13.8 N.D. 6.0 33.6 Avg. 7±1 38 ± 7 L 2.3 322c 162 39.8 N.D. 17.3 32.0 c M 2.3 322 162 31.9 N.D. 13.9 25.7 Avg. 16 ± 3 29 ± 4 N.D. indicates the value was not determined aAcid used is [Ph2NH2][OTf] bAcid used is [Ph2NH2][BArF4] cAcid used is [PhNH3][OTf] Table B.2: UV-vis quantification results for standard NH3 generation experiments with (P3Si)Fe(N2) Entry P3SiFeN2 Acid Cp*2Co NH4Cl N2H5Cl Equiv % Yield NH3 (μmol) equiv equiv (μmol) (μmol) NH3/Fe Based on ea A 2.3 108 54 6.6 0.0 1.7 9.3 B 2.3 108a 54 2.7 0.0 0.7 3.8 Avg. 1.2 ± 0.2 6.5 ± 0.3 a Acid used is [Ph2NH2][OTf] Table B.3: UV-vis quantification results for standard NH3 generation experiments with [(P3B)Co(N2)]− Entry [(P3B)Co(N2)]− Acid Cp*2Co NH4Cl N2H5Cl Equiv % Yield (μmol) equiv equiv (μmol) (μmol) NH3/Co NH3 Based on ea A 2.3 108 54 3.0 0.0 1.3 7.2 B 2.3 108a 54 1.8 0.0 0.8 4.4 Avg. 1.1 ± 0.4 6±2 186 a Acid used is [Ph2NH2][OTf] Table B.4: UV-vis quantification results for standard NH3 generation experiments with (P3Si)Co(N2) Entry (P3Si)Co(N2) Acid Cp*2Co NH4Cl N2H5Cl Equiv % Yield (μmol) equiv equiv (μmol) (μmol) NH3/Fe NH3 Based on eA 2.3 108a 54 0.0 0.0 0.0 0.0 B 2.3 108a 54 0.0 0.0 0.0 0.0 Avg. 0.0 0.0 a Acid used is [Ph2NH2][OTf] B.3.2 Ammonia production studies with [Ph215NH2][OTf] The procedure was the same as the general procedure presented in section 3.1 with 2.3 μmol of P3BFe+ catalyst, 54 equiv Cp*2Co, and 108 equiv [Ph215NH2][OTf]. Product analyzed by 1 H NMR as described in section 1.4 and only the diagnostic triplet of [14NH4][Cl] is observed. Figure B.1: 1H NMR spectrum (300 MHz, DMSO-d6) of [14NH4][Cl] produced from catalytic N2-to-NH3 conversion conducted with [(P3B)Fe]+ catalyst, 54 equiv Cp*2Co, and 108 equiv [Ph215NH2][OTf] under an atmosphere of 14N2. B.3.3 NH3 Generation Reaction with Periodic Substrate Reloading – Procedure with [(P3B)Fe]+ All solvents are stirred with Na/K for ≥2 hours and filtered prior to use. In a nitrogenfilled glovebox, the precatalyst (2.3 μmol) was weighed into a vial. The precatalyst was then transferred quantitatively into a Schlenk tube using THF. The THF was then evaporated to provide a thin film of precatalyst at the bottom of the Schlenk tube. The tube is then charged 187 with a stir bar and the acid and reductant are added as solids. The tube is then cooled to 77 K in a cold well. To the cold tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes and then the tube is sealed with a Teflon screw-valve. The cold well cooling bath is switched from a N2(l) bath to a dry ice/acetone bath. In the cold well the mixture in the sealed tube thaws with stirring and is allowed to stir at −78 °C for 3 hours. Then, without allowing the tube to warm above −78 °C, the cold well bath is switched from dry ice/acetone to N2(l). After fifteen minutes the reaction mixture is observed to have frozen, at this time the tube is opened. To the cold tube is added acid (324 equiv) and reductant (162 equiv) as solids. To the tube then 1 additional mL of Na/K-dried Et2O is added. The cold well cooling bath is switched from a N2(l) bath to a dry ice/acetone bath. In the cold well the mixture in the sealed tube thaws with stirring and is allowed to stir at −78 °C for 3 hours. These reloading steps are repeated the desired number of times. Then the tube is allowed to warm to RT with stirring and stirred at RT for 5 minutes. Table B.5: UV-vis quantification results for NH3 generation experiments with [(P3B)Fe]+, with reloading Entry Load [(P3B)Fe]+ Acid Cp*2Co NH4Cl N2H5 Equiv % Yield # (μmol) equiv equiv (μmol) Cl NH3/ Based on (μmol) Fe H+ A [322] [162]x2 0.1 2 2.3 x2a 115.0 49.6 46.2 B [322] [162]x2 0.0 2 2.3 x2a 145.6 62.8 58.5 Avg. 56 ± 9 52 ± 9 C [322] [162]x3 0.3 3 2.3 x3a 182.4 78.7 48.9 D [322] [162]x3 0.1 3 2.3 x3a 207.3 89.5 55.5 Avg. 84 ± 8 52 ± 5 a Acid used is [Ph2NH2][OTf] 188 B.4. Time Resolved H2 Quantification Inside of a nitrogen filled glovebox, solid acid (0.248 mmol) and Cp*2Co (0.124 mmol) are added to a 260 mL glass tube charged with a stir bar. The vessel is sealed with a septum at RT and subsequently chilled to −196 °C in a cold well in the nitrogen filled glovebox. Et2O (1 mL) is added via syringe into the vessel and completely frozen. The vessel is passed out of the glovebox into a liquid N2 bath, and subsequently thawed in a dry ice/acetone bath with stirring at ~900 rpm. The timer was started as soon as the vessel was transferred to the dry ice/acetone bath. The headspace of the reaction vessel was periodically sampled with a sealable gas sampling syringe (10 mL), which was loaded into a gas chromatograph, and analyzed for the presence of H2(g). From these data, the percent H2 evolved (relative to Cp*2Co) was calculated, correcting for the vapor pressure of Et2O and the removed H2 from previous samplings. Each time course was measured from a single reaction maintained at −78 °C. Table B.6:Time-resolved H2 quantification for the reaction of Cp*2Co and acid in Et2O at 78 °C in the absence of an Fe precatalyst Acid Time (min) H2(g) % H2 Based (μmol) on Cp*2Co 10 1.0 ± 0.4 1.6 ± 0.6 [Ph2NH2][OTf]a 60 2.1 ± 0.6 3±1 10 3.7 ± 0.1 6.0 ± 0.2 [Ph2NH2][BArF4]b 60 12.7 ± 0.8 21 ± 1 a b Average of two experiments Average of three experiments B.5. Time Resolved NH3 Quantification All solvents are stirred with Na/K for ≥2 hours and filtered prior to use. In a nitrogenfilled glovebox, the precatalyst (2.3 μmol) was weighed into a vial. The precatalyst was then transferred quantitatively into a Schlenk tube using THF. The THF was then evaporated to 189 provide a thin film of precatalyst at the bottom of the Schlenk tube. The tube is then charged with a stir bar and diphenylammonium triflate (108 eq) and decamethylcobaltocene (54 eq) are added as solids. The tube is then cooled to 77 K in a cold well. To the cold tube is added Et2O to produce a concentration of precatalyst of 2.3 mM. The temperature of the system is allowed to equilibrate for 5 minutes and then the tube is sealed with a Teflon screwvalve. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. For the control reaction at this point a 2.6 M heptane solution of tBuLi (2 eq with respect to the acid) was added to the tube under N2 backflow and the headspace was evacuated. The tube was then allowed to warm to room temperature with stirring and then stirred for a further ten minutes at room temperature. At this point the normal procedure was used to quantify NH3 and N2H4. No NH3 or N2H4 was observed. To test catalytic activity at five minutes, a tube prepared as described above was allowed to stir for five minutes at −78 °C. At five minutes the tube was frozen in a liquid N2 bath and allowed to equilibrate for five minutes. Under N2 backflow a 2.6 M heptane solution of tBuLi (2 eq with respect to the acid) was added to the tube. The tube was then sealed and the headspace was evacuated. The reaction mixture was then allowed to warm to room temperature with stirring and then stirred for a further ten minutes at room temperature. At this point the normal procedure was used to quantify NH3 and N2H4. Ammonia (1.2 ± 0.5 eq) was detected. No hydrazine was detected. 190 B.6. Mössbauer Spectroscopy B.6.1 General procedure for preparation of rapid-freeze-quench Mössbauer samples of catalytic reaction mixtures using [(P3B)Fe]+ All manipulations are carried out inside of a nitrogen filled glovebox. The precatalyst, [(P3B)57Fe][BArF4], is weighed into a vial (3.5 mg, 2.3 μmol) and transferred using THF into a 150 mL Schlenk tube. The solvent is evaporated to form a thin film of the precatalyst and a stir bar is added. The [Ph2NH2][OTf] (79.4 mg, 0.248 mmol) and Cp*2Co (40.3 mg, 0.124 mmol) are added to the Schlenk tube as solids. The Schlenk tube is then placed in N2(l) and the temperature is allowed to equilibrate. To the tube 1 mL of Et2O is added. The tube is then sealed with a Teflon screw tap and transferred to a pre-chilled cold well at −78 °C. The timer is set to zero as soon as the stir bar is freed from the thawing solvent. At the desired time, the tube is opened and the well-stirred suspension is transferred to a Delrin cup pre-chilled to −78 °C using a similarly pre-chilled pipette. The sample in the Delrin cup is then rapidly frozen in N2(l). At this point the sample, immersed in N2(l), is taken outside of the glovebox and mounted in the cryostat. B.6.2 General Procedure for Preparation of Rapid-freeze-quench Mössbauer Samples of the Reaction of [(P3B)Fe]+ with Reductants All manipulations are carried out inside of a nitrogen filled glovebox. The precatalyst, [(P3B)57Fe][BArF4], is weighed into a vial (3.5 mg, 2.3 μmol) and .5 mL of THF is added. The solvent is then evaporated to provide a thin film of [(P3B)57Fe][BArF4]. To this is added the desired reductant as a solid (46.0 μmol, 20 equiv). This vial is then placed in N2(l) and the temperature is allowed to equilibrate. To this is added 1 mL of NaK-dried Et2O. The vial is then sealed with a cap and transferred to a pre-chilled cold well at −78 °C. The timer is set 191 to zero as soon as the stir bar is freed from the thawing solvent. After five minutes using a pre-chilled pipette the well-stirred reaction mixture is transferred to a Delrin cup that has been pre-chilled to −78 °C. The sample in the Delrin cup is then rapidly frozen in N2(l). At this point the sample, immersed in N2(l), is taken outside of the glovebox and mounted in the cryostat. B.6.3 General Procedure for Fitting of Rapid-freeze-quench Mössbauer Samples Data analysis was performed using version 4 of the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. Simulations were constructed from the minimum number of quadrupole doublets required to attain a quality fit to the data (convergence of χR2). Quadrupole doublets were constrained to be symmetric, unless [(P3B)Fe(N2)][Na(12-crown-4)2] was included in the model. With [(P3B)Fe(N2)][Na(12-crown-4)2] since it is known to have characteristic asymmetry we started with the observed linewidths in the authentic sample and allowed them to then relax. It is known that the exact linewidths are sensitive to the particular sample but the relative line breadth should be fairly constant. Using the non-linear error analysis algorithm provided by WMOSS, the errors in the computed parameters are estimated to be 0.02 mm s-1 for δ and 2% for ΔEq. We additionally note that in these spectra the exact percentage contributions given do not represent exact percentages. Particularly for components that represent less than 10% of the overall spectrum, these values are subject to a high degree of uncertainty; however, all of the included components are necessary to generate satisfactory fits of the data and therefore are believed to be present in the reaction mixtures. 192 B.6.4 Details of Individual RFQ Mossbauer spectra Figure B.2: Mössbauer spectrum collected on [(P3B)57Fe]+ that was used for the Mössbauer experiments conducted in this paper. The parameters used to model this species are well within the experimental error of those used previously to model this species (δ = 0.75 mm/sec, ΔEq = 2.55 mm/sec, Γr = Γl = 0.52 mm/sec).20 Table B.7: Fit parameters for [(P3B)57Fe]+ Component δ (mm s-1) ΔEQ (mm s-1) Linewidths, ΓL/ ΓR (mm s-1) A 0.75 ± 0.02 2.50 ± 0.05 0.54/0.58 193 Figure B.3: Mössbauer spectrum collected from a reaction freeze quenched after 5 minutes between P3BFe+ and excess Cp*2Co (20 equiv). Raw data shown as black points, simulation as a solid red line, with components in green, blue, yellow, and purple (see Table B.8 for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT as a suspension in Et2O. Fitting details for Figure B.3 are as follows: four quadrupole doublets were found to be necessary to obtain an adequate simulation. The simulation parameters are given in Table B.8. The two major species in this spectrum are well simulated as (P3B)Fe(N2) and [(P3B)Fe(N2)]−. The residual signal exhibits only two well resolved absorbances but to obtain a good fit with symmetric lineshapes two additional quadrupole doublets were necessary. One of these can be identified as [(P3B)Fe]+ based on the asymmetry in the lineshape of the right feature of [(P3B)Fe(N2)]−. The similarity of the other two quadrupole doublets to those identified in the five-minute freeze quench make this a logically consistent fit but one that is not strictly required by the data. Table B.8: Simulation parameters for Mossbauer spectrum in Figure B.3 Component δ (mm s-1) ΔEQ (mm s- Linewidths, ΓL/ ΓR (mm s- Relative 1) 1) area A (green) 0.57 ± 0.02 3.26 ± 0.06 0.29/0.29 0.33 194 B (purple) C (yellow) D (blue) 0.75 ± 0.02 0.45 ± 0.02 0.40 ± 0.02 2.55 ± 0.05 1.76 ± 0.04 0.98 ± 0.02 0.27/0.27 0.45/0.45 0.48/0.45 0.06 0.23 0.39 Figure B.4: Mössbauer spectrum collected from a reaction freeze quenched after 5 minutes between P3BFe+ and excess Cp*2Cr (20 equiv). Raw data shown as black points, simulation as a solid red line, with components in green and brown (see Table S9 for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT as a suspension in Et2O. Fitting details for Figure B.4 are as follows: the two well-resolved quadrupole doublets can be simulated. The simulation parameters are given in Table B.9. One of the two major species in this spectrum is well simulated as (P3B)Fe(N2). The other feature has a very similar isomer shift but a significantly narrower quadrupole splitting. Given the labile nature of the N2 ligand this other species may represent a vacant neutral species or a dimeric N2 bridged species. 195 Table B.9: Simulation parameters for Mossbauer spectrum in Figure B.4 Component δ (mm s-1) A (green) B (brown) 0.57 ± 0.02 0.58 ± 0.02 ΔEQ (mm s-1) Linewidths, ΓL/ ΓR (mm s- Relative area 1) 3.22 ± 0.06 0.29/0.29 0.46 1.60 ± 0.05 0.71/0.71 0.54 Figure B.5: Mossbauer spectrum collected from a catalytic reaction freeze quenched after 5 minutes. Conditions: [(P3B)57Fe][BArF] = 0.23 mM, [Ph2NH2][OTf] = 24.8 mM (108 equiv), and Cp*2Co 12.4 mM (54 equiv). Raw data shown as black points, simulation as a solid red line, with components in green, blue, yellow, and orange (see Table B.10 for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Fitting details for Figure B.5 are as follows four pairs of quadrupole doublets were found to be necessary to obtain an adequate simulation of these data. The simulation parameters are given in Table S10. The outer pair of sharp features clearly belong to (P3B)Fe(N2). The inner feature is highly suggestive of [(P3B)Fe(N2)]− the presence of which was confirmed by freeze-quench EPR. The residual then consists of two sharp features which were simulated with the quadrupole doublet in yellow and a broader residual feature that is 196 captured by the quadrupole doublet in orange. The exact isomer shift and quadrupole splitting of orange is not determined by this model but the one here is representative. Table B.10: Simulation parameters for Mossbauer spectrum in Figure B.5 Component δ (mm s-1) ΔEQ (mm s- Linewidths, ΓL/ ΓR (mm s- Relative 1) 1) area A (green) 0.55 ± 0.02 3.24 ± 0.06 0.25/0.25 0.32 B (blue) 0.40 ± 0.02 0.98 ± 0.02 0.49/0.34 0.26 C (yellow) 0.42 ± 0.02 1.82 ± 0.04 0.31/0.31 0.18 D (orange) 0.93 ± 0.02 2.99 ± 0.06 0.87/0.87 0.24 Figure B.6: Mössbauer spectrum collected from a catalytic reaction freeze quenched after 30 minutes. Conditions: [(P3B)57Fe][BArF] = 0.23 mM, [Ph2NH2][OTf] = 24.8 mM (108 equiv), and Cp*2Co 12.4 mM (54 equiv). Raw data shown as black points, simulation as a solid red line, with components in green, purple, yellow, and orange (see Table B.11 for parameters). The spectrum was collected at 80 K with a parallel-applied magnetic field of 50 mT. Fitting details for Figure B.6 are as follows: four quadrupole doublets were found to be necessary to obtain an adequate simulation. The simulation parameters are given in Table B.11. The major species in this spectrum is again well simulated as (P3B)Fe(N2). The residual signal exhibits only three well resolved absorbances. To obtain a good fit with symmetric lineshapes three additional quadrupole doublets were necessary. One of these can be identified as [(P3B)Fe]+ based on the asymmetry in the lineshape of the right feature of 197 (P3B)Fe(N2). The similarity of the other two quadrupole doublets to those identified in the five-minute freeze quench make this a logically consistent fit but one that is not strictly required by the data. Table B.11: Simulation parameters for Mossbauer spectrum in Figure B.6 Component δ (mm s-1) ΔEQ (mm s- Linewidths, ΓL/ ΓR (mm s- Relative 1) 1) area A (green) 0.55 ± 0.02 3.24 ± 0.06 0.29/0.29 0.53 B (purple) 0.75 ± 0.02 2.55 ± 0.05 0.27/0.27 0.08 C (yellow) 0.44 ± 0.02 1.74 ± 0.04 0.48/0.48 0.18 D (orange) 1.35 ± 0.02 3.00 ± 0.06 0.67/0.67 0.22 B.7. EPR Spectroscopy B.7.1 General Procedure for Preparation of Rapid-freeze-quench EPR Samples of Catalytic Reaction Mixtures using [(P3B)Fe]+ All manipulations are carried out inside of a nitrogen filled glovebox. The precatalyst, [(P3B)Fe][ BArF4], is weighed into a vial (3.5 mg, 2.3 μmol) and transferred using THF into a 150 mL Schlenk tube. The solvent is evaporated to form a thin film of the precatalyst and a stir bar is added. The [Ph2NH2][OTf] (79.4 mg, 0.248 mmol) and Cp*2Co (40.3 mg, 0.124 mmol) are added to the Schlenk tube as solids. The Schlenk tube is then placed in N2(l) and the temperature is allowed to equilibrate. To the tube 1 mL of Et2O is added. The tube is then sealed with a Teflon screw tap and transferred to a pre-chilled cold well at −78 °C. The timer is set to zero as soon as the stir bar is freed from the thawing solvent. At the desired time, the tube is opened and the well-stirred suspension is transferred to an EPR tube that is prechilled to −78 °C using a pipette that has similarly been pre-chilled to −78 °C. The EPR sample is 198 then rapidly frozen in N2(l). At this point the sample is quickly transferred out of the glovebox and put into N2(l) before it can warm. B.7.2 General Procedure for Preparation of Rapid-freeze-quench EPR Samples of the Reaction of [(P3B)Fe]+ with Reductants All manipulations are carried out inside of a nitrogen filled glovebox. The precatalyst, [(P3B)Fe][BArF4], is weighed into a vial (3.5 mg, 2.3 μmol) and .5 mL of THF is added. The solvent is then evaporated to provide a thin film of [(P3B)Fe][BArF4]. To this is added (46.0 μmol, 20 equiv) of the desired reductant as a solid. This vial is then placed in N2(l) and the temperature is allowed to equilibrate. To this is added 1 mL of NaK-dried Et2O. The vial is then sealed with a cap and transferred to a pre-chilled cold well at −78 °C. The timer is set to zero as soon as the stir bar is freed from the thawing solvent. At the desired time, the tube is opened and the well-stirred suspension is transferred to an EPR tube that is prechilled to −78 °C using a pipette that has similarly been pre-chilled to −78 °C. The EPR sample is then rapidly frozen in N2(l). At this point the sample is quickly transferred out of the glovebox and put into N2(l) before it can warm. B.7.3 General Procedure for Preparation of EPR Samples of Cp*2Co, [(P3B)Fe][BArF4], and [(P3B)Fe(N2)][Na(12-crown-4)2] The desired species was dissolved in 1 mL of Et2O at RT and transferred to an EPR tube. The EPR tube was then chilled to -78 °C for five minutes. It was then rapidly frozen by transfer to a bath of N2(l). 199 B.7.3 Procedure for EPR Characterization of the Reaction of Cp*2Co with Acid The as isolated solid was added to a J-Young or septum-sealed X-Band EPR tube after prechilling both in the cold well to 77 K. Specific experimental details are listed with the accompanying spectra. Figure B.7: The X-band EPR spectrum in a 2-MeTHF glass of 2.3 mM B [(P3 )Fe(N2)][Na(12-crown-4)2] at 77K. Note that the exceeding insolubility of these species when encapsulated in a crown salt prevented its measurement in ether. We note that this species has significantly different parameters than the species in which the Na is not encapsulated with a crown ether and is therefore interacting with the N2 ligand. We think this species is more representative of what a hypothetical [(P3B)Fe(N2)][Cp*2Co] species would look like if isolated. 200 Figure B.8: The X-band EPR spectrum in Et2O of 2.3 mM [(P3B)Fe][BArF4] at 77 K. Note this species is S = 3/2. We attribute the extremely weak signal observed here to background signal from the cavity. Figure B.9: The X-band EPR spectrum in Et2O of 46 mM Cp*2Co at 77K. Decamethylcobaltocene is known to be EPR silent at 77 K24 but at these high concentrations it becomes apparent that there is a small S = ½ impurity present in this spectrum. This persistent impurity is observable in both freeze quenched reactions of this reductant with [(P3B)Fe][BArF4] and in spectra of the freeze quenched catalytic reaction mixtures. 201 Figure B.10: The X-band EPR spectrum in Et2O (1 mL) of the reaction between [(P3B)Fe]+ (3.5 mg, 0.0023 mmol) and Cp*2Co (15.2 mg, 0.046 mmol) stirred for 5 minutes at −78 °C then rapidly frozen to 77 K. Figure B.11: The X-band EPR spectrum in Et2O (1 mL) of the reaction between [(P3B)Fe]+ (3.5 mg, 0.0023 mmol) and Cp*2Co (40.3 mg, 0.124 mmol) and [Ph2NH2][OTf] (79.4 mg, 0.248 mmol) stirred for 5 minutes at −78 °C then rapidly frozen to 77 K. Table B.12: A comparison of the g-tensors for the authentic sample of [(P3B)Fe(N2)]− 202 (Figure B.7), the freeze quench of the reaction with the reductant (Figure B.10), the freeze quench of the catalytic reaction mixture (Figure B.11) Reaction gx gy gz Figure B.7 2.304 2.048 2.032 2.295 2.048 2.032 Figure B.10 2.298 2.048 2.032 Figure B.11 B.7.5 Discussion of the EPR spectra obtained by reacting Cp*2Co with Acid Previous studies have demonstrated that the EPR signal for Cp*2Co is only apparent at temperatures below the 77 K used in this study. In line with this expectation the solution spectra of Cp*2Co at 77 K do not show any signal ( Figure B.12, green). The 77 K powder, X-band EPR spectrum obtained of the material isolated as described in SI 2.2 ( Figure B.12, red) demonstrates a signal with significant g-anisotropy and Cohyperfine coupling. Although 1H-hyperfine coupling is not resolved in this spectrum we believe that this is due to the large Co-hyperfine coupling and the significant linewidths observed (Figure B.13). Attempts to obtain narrower linewidths by diluting the solid in KBr did not lead to any observable improvement. However, comparison of the spectrum obtained from reaction of Cp*2Co with HOTf and DOTf (Figure B.14) strongly supports the hypothesis that this material represents a protonated Cp*2Co. In particular, the narrower lineshapes observed in the reaction with DOTf evidence both that the metallocene has been protonated but also that this proton is strongly coupled to the spin. The narrower line shapes manifest because of the lesser gyromagnetic ratio of 2H compared to 1H confirming that the linewidths in the reaction with HOTf are in part broadened by coupling to the added proton. That the appearance of the extra lines observed in the reaction with DOTf can be well- 203 simulated simply by dividing the anisotropic strain parameter (HStrain in EasySpin) by the ratio of the 1H gyromagnetic ratio:2H gyromagnetic ratio (~6.5) strongly supports this hypothesis (Figure B.15). That two species are present (more obvious in the DOTf reaction due to the sharper lineshapes) is further evidence that these spectra represent endo- and exoprotonated decamethylcobaltocene [(Cp*)Co(η4-C5Me5H)]+, as we would expect both the endo- and exo-protonated isomers to be kinetically and thermodynamically accessible under these conditions, and they should manifest distinct EPR signatures. Although preliminary in nature the observed reactivity of these species (discussed in SI 9.1-9.3) is further evidence that this material is competent for PCET reactivity as predicted by DFT. Figure B.12: 77 K X-band EPR spectrum of a toluene solution of Cp*2Co (green), 77 K powder X-Band EPR spectrum of the purple solid isolated from the reaction of Cp* 2Co 204 and HOTf as described in Section B2.2 (red), and 77 K powder X-Band EPR spectrum after annealing the EPR tube RT for two hours (blue). Figure B.13: Powder EPR spectrum at 77 K for the reaction of HOTf and Cp* 2Co and its simulation. Simulation parameters are g1 = [2.63 2.345 1.984], A1,Co = [248 160 187], lw1 205 = 1 MHz, HStrain1 = [60 50 60], Weight1 = 1; g2 = [2.347 2.1 1.982], A2,Co = [200 50 110], lw2 = 1, HStrain2 = [40 40 40], Weight2 = 0.2. Figure B.14: Comparison of the EPR spectra obtained using HOTf and DOTf in the reaction with Cp*2Co. The zoomed in spectrum highlights the middle g-value where the differences are most apparent between the two reactions. Figure B.15: EPR spectrum obtained when reacting DOTf with Cp*2Co and its simulation. Simulation parameters are g1 = [2.63 2.345 1.984], A1,Co = [248 160 187], lw1 = 1 MHz, HStrain1 = [9.2 7.7 9.2], Weight1 = 1; g2 = [2.347 2.1 1.982], A2,Co = [200 50 110], lw2 = 1, HStrain2 = [6.2 6.2 6.2], Weight2 = 0.2. 206 B.8. Reactivity of [(Cp*)Co(η4-C5Me5H)]+ B.8.1 Annealing the Purple Solid Purple solid isolated as described in Section B2.2 was placed in an X-band EPR tube at 77 K and sealed with a rubber septum. A 77 K, powder X-band EPR spectrum was then taken to confirm the presence of [(Cp*)Co(η4-C5Me5H)]+. After two hours at room temperature a second 77 K, powder EPR spectrum was taken to confirm the quenching of [(Cp*)Co(η4-C5Me5H)]+. At this point the headspace was analyzed for H2 via GC (14% yield). B.8.2 Annealing a Suspension of the Purple Solid in Toluene Into a 26 mL vial with a septum seal was loaded 2 mL of toluene which was frozen at 77 K. To this purple solid isolated as described in Section B2.2 was added along with a stir bar. The suspension was stirred for 1 hour at −78 °C and then warmed to room temperature and stirred for an additional 15 minutes. At this point the headspace was sampled for H2 via GC (35% yield). The solvent was then removed and to the yellow residue was added 1,3,5-trimethoxybenzene (1 eq.). The solid was then extracted with d6-acetone and 1H NMR was obtained. [Cp*2Co]+ was observed (>95 % yield). B.8.3 Upper Bound on Protonated Metallocene BDE An upper bound for the BDE of the putative protonated metallocene was estimated using the literature BDE value for H2 (105.8 kcal/mol) in MeCN, as well as the approximation for TSH• (4.6 kcal/mol) in MeCN.24 The maximum BDE was then approximated as follows: 207 Eq B.4) ΔG = ΔH − TΔS Eq B.5) ΔH = BDE(H2) − 2×BDE([(Cp*)Co(η4-C5Me5H)]+) = 105.8 kcal mol−1 −2 BDE([(Cp*)Co(η4-C5Me5H)]+) Eq B.6) TΔS = TΔSH − 2×TΔSH = −TΔSH = −4.6 kcal mol−1 Eq B.7) 105.8 kcal mol−1 − 2×BDE([(Cp*)Co(η4-C5Me5H)]+) + 4.6 kcal mol−1 < 0 kcal mol−1 Eq B.8) ⸫ BDE(([(Cp*)Co(η4-C5Me5H)]+) < 50.6 kcal mol−1 B.9. Details on DFT Estimates of pKa and BDE B.9.1 Computational Estimates of pKa in Et2O The pKa values in diethyl ether were calculated referenced to [H(OEt2)2]+ and were predicted on the basis of the free-energy change of the exchange reaction with [H(OEt2)2]+ and application of Hess’ law on the closed chemical cycle. The pKa of [H(OEt2)2]+ was defined as 0.0. B.9.2 Computational Estimates of BDEs Bond dissociation enthalpies (BDE) of X–H bonds were calculated in the gas-phase using a series of known reference compounds containing M–OH, M–H and M–NH bonds (Table B.13).20 The enthalpy difference between the H-atom donor/acceptor pair was calculated based on the thermochemical information provided by frequency calculations after structure optimizations using the procedure described in the general computational section. A linear plot of ΔH vs BDElit was generated to form a calibration curve (Figure B.16). BDE predictions were generated by application of the line of best fit to the calculated ΔH of the unknown species. Error were calculated by application of the trend line to the calculated enthalpies of known species and comparison to their literature BDE value.21 Errors 208 are reported as the average of BDEcalc-BDElit (mean signed error, MSE) and the average of the absolute values of BDEcalc-BDElit (mean unsigned error, MUE). The use of the Bordwell equation for bond dissociation enthalpies is well supported by small Scalc = S(X●) – S(XH), as shown in Table S14. Table B.13: Calculated ΔH values and literature BDE values used for BDE calibration Species [Cr(H2O)5(OH)]2+ [Fe(H2O)6]2+ [Cr(H2O)5(OOH)]2+ [(bim)Fe(N2)]2+ [(P3Si)Fe(CNH)]+ [(bip)Fe(H)]2+ [(Tren)Fe(OH)]2(Cp)Fe(CO)2(H) (HIPTN3N)Mo(NNH) [(P3Si)Fe(NNMeH)]+ (P3Si)Fe(CNH) (P3Si)Fe(CNMeH) [(P3Si)Fe(CNMeH)]+ Hcalc (kcal mol−1)  97.735 77.985 77.175 69.255 65.905 65.475 64.105 57.455 47.715 43.915 38.915 34.375 32.955 BDElit (kcal mol−1) BDEcalc (kcal mol−1) Notes 89 77 79 67 65 62 66 56 49 48 44 45 44 90 75 75 68 66 65 64 59 51 48 44 40 39 MSE MUE ref 21 ref 21 ref 21 ref 21 ref 21 ref 21 ref 21 ref 21 Truncated; ref 22 ref 23 ref 23 ref 23 ref 23 −0.9 2.1 209 Figure B.16: Calculated BDE vs literature BDE for the species shown in Table B.13. Table B.14: Calculated entropy (S) for selected XH and X• species Species (P3B)Fe(NNH) [(P3B)Fe(NNH2)]+ (P3B)Fe(NNH2) (Cp*)Co(η4-C5Me5H)]+ [(Cp*)Cr(η4-C5Me5H)]+ S(XH) S(X•) (cal/mol*K) (cal/mol*K) 271.6 268.9 266.3 273.1 268.9 281.3 168.8 162.0 159.5 163.4 S (kcal/mol*K) 2.7x10−3 −6.8x10−3 −1.2x10−2 6.6x10−3 −3.9x10−3 B.9.3 Estimation of PCET Activation Barriers Activation barriers were bracketed using the methods develop by the HammesSchiffer group. The inner sphere reorganization energy was estimated using force constants for normal modes in the Fe- and Co-coordination sphere.24 The outer sphere reorganization energy (λOS) was estimated by calculating the outer sphere reorganization energy for a single ET in diethyl ether using the continuum solvation model25 and assuming λOS,PCET ≤ λOS,ET.24 210 The activation barrier was plotted as a function of λOS (Figure B.17) to determine a maximum and minimum barrier for each PCET reaction (Table B.15). Figure B.17: Activation barrier for PCET reactions between selected (P3B)Fe(NxHy) species (as labeled, total spin-state of the surface in parenthesis) with [(Cp*)Co(η4-C5Me5H)]+ as a function of outer-sphere reorganization energy. Table B.15: Calculated Reorganization Energies, Free-Energies of Reaction and Activation Barriers for Selected PCET Reactions Acceptor Spin State Donor λtot Range (kcal/mol) Grxn (kcal/mol) G‡ Range (kcal/mol) (P3B)Fe(N2) (P3B)Fe(NNH) (P3B)Fe(NNH) S=½ S=0 S=1 [(Cp*)Co(η4-C5Me5H)]+ [(Cp*)Co(η4-C5Me5H)]+ [(Cp*)Co(η4-C5Me5H)]+ 26.1 - 53.5 17.7 – 45.1 10.2 – 37.6 –9.1 –24.6 –24.6 3-9 0-2 0-5 B.10. References (1) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882–1893. (2) Anderson, J. S.; Moret, M. E.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534–537. 211 (3) Mankad, N. P.; Whited, M. T.; Peters, J. C. Angew. Chem. Int. Ed. 2007, 46, 5768– 5771. (4) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Inorg. Chem. 2015, 54, 9256–9262. (5) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 14158–14164. (6) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (7) Melzer, M. M.; Mossin, S.; Dai, X.; Bartell, A. M.; Kapoor, P.; Meyer, K.; Warren, T. H. Angew. Chem. Int. Ed. 2010, 49, 904–907. (8) Vicente, J.; Chicote, M.-T.; Guerrero, R.; Jones, P. G. J. Chem. Soc., Dalton Trans. 1995, 8, 1251–1254. (9) Weatherburn, M. W. Anal. Chem. 1967, 39, 971–974. (10) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006–2008. (11) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42–55. (12) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (13) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (14) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theoret. Chim. Acta 1990, 77, 123–141. (15) Towns, J.; Cockerill, T.; Dahan, M.; Foster, I.; Gaither, K.; Grimshaw, A.; Hazlewood, V.; Lathrop, S.; Lifka, D.; Peterson, G. D.; et al. Comput. Sci. Eng. 2014, 16, 62–74. (16) Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; et al. Comput. Phys. Commun. 2010, 181, 1477–1489. 212 (17) Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2011, 115, 14556–14562. (18) Wang, T.; Brudvig, G.; Batista, V. S. J. Chem. Theory Comput. 2010, 6, 755–760. (19) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. (20) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (21) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. (22) Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103–1117. (23) Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 3161–3170. (24) Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2001, 123, 3723–3733. (25) Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978. 213 APPENDIX C. Supplementary Information for Chapter 4 Adapted from: Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. DOI: 10.1021/jacs.8b02335 214 C.1 Experimental Details C.1.1 General Considerations All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with N 2 followed by passage through an activated alumina column in a solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with sodium benzophenone ketyl in tetrahydrofuran (THF) in order to confirm the absence of oxygen and water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3Å molecular sieves prior to use. Cp*2Co,1 crown-4)2],3 [(P3B)Fe][BArF4],2 [H(OEt2)][BArF4] [(P3B)Fe(N2)][Na(Et2O)3],3 (HBArF4; BArF4 = [(P3B)Fe(N2)][Na(12tetrakis- (3,5- bis(trifluoromethyl)phenyl)borate),4 sodium BArF4 (NaBArF4),4 and 15N-diphenylammonium triflate ([Ph215NH2][OTf]).5,6 Ph15NH2 was obtained from Sigma-Aldrich, Inc. degassed, and dried over activated 3-Å molecular sieves prior to use. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Diethyl ether (Et2O) used in the experiments herein was stirred over Na/K (≥ 2 hours) and filtered or vacuum-transferred before use unless otherwise stated. C.1.2 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 form 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 (RT). Solution 215 samples were transferred to a sample cup and chilled to 77 K inside of the glovebox, and unless noted otherwise, quickly removed from the glovebox and immersed in liquid N2 until mounted in the cryostat. Data analysis was performed using version 4 of the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. See discussion below for detailed notes on the fitting procedure. C.1.3 Ammonia and Hydrazine Quantification Reaction mixtures are cooled to 77 K and allowed to freeze. The reaction vessel is then opened to atmosphere and to the frozen solution is slowly added a twofold excess (with respect to acid) solution of a NaOtBu solution in MeOH (0.25 mM) over 1-2 minutes. This solution is allowed to freeze and a Schlenk tube adapter is added and the headspace of the tube is evacuated. After sealing the tube is then allowed to warm to RT and stirred at RT for at least 10 minutes. An additional Schlenk tube is charged with HCl (3 mL of a 2.0 M solution in Et2O, 6 mmol) to serve as a collection flask. The volatiles of the reaction mixture are vacuum transferred at RT into this collection flask. After completion of the vacuum transfer, the collection flask is sealed and warmed to RT and stirred vigorously for 10 minutes. Solvent is removed in vacuo, and the remaining residue is dissolved in DMSO-d6 containing 20 mM 1,3,5-trimethoxybenzene as an internal standard. The ammonium chloride is quantified by integration relative to the 1,3,5-trimethoxybenzene internal standard. C.1.4 EPR Spectroscopy X-band EPR spectra were obtained on a Bruker EMX spectrometer. Samples were collected at powers ranging from 6-7 mW with modulation amplitudes of 2.00 G, modulation frequencies of 100.00 kHz, over a range of 1800 to 4500 Gauss. Spectra were baseline 216 corrected using the algorithm in SpinCount. EPR spectra were modeled using the easyspin program.7 C.1.5 Density Functional Theory All stationary point geometries were calculated using DFT-D3 (Grimmes D3 dispersion correction)8 with an TPSS functional,9 a def2-TZVP10 basis set on transition metals and a def2-SVP7 basis set on all other atoms. Calculations were performed, in part, using Xtreme Science and Engineering Discovery Environment (XSEDE) resources.11 Calculations were performed on the full (P3B)Fe scaffold. Geometries were optimized using the NWChem 6.5 package.12 All single point energy, frequency and solvation energy calculations were performed with the ORCA package.13 Frequency calculations were used to confirm true minima and to determine gas phase free energy values (Ggas). Single point solvation calculations were done using an SMD solvation model14,15 with diethyl ether solvent and were used to determine solvated internal energy (Esoln). Free energies of solvation were approximated (Eq C.1) using the difference in gas phase internal energy (Egas) and solvated internal energy (Esoln) and the free energy of a species in solution (Gsoln) was then calculated (Eq C.2) using the gas phase free energy (Ggas) and the free energy of solvation (∆Gsolv).16,17 All reduction potentials were calculated referenced to Fc+/0 and using the standard Nernst relationEq C.3. Eq C.1) ∆Gsolv ≈ Esoln – Egas Eq C.2) Gsoln = Ggas + ∆Gsolv Eq C.3) ∆G = −nFE0 217 C.1.7 Gas Chromatography H2 was quantified 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. A 10 mL manual injection was used and integration area was converted to percent H2 composition by use of a calibration obtained from injection of H2 solutions in N2 of known concentration. C.2 Synthetic Details C.2.1 Synthesis of Anilinium Triflates Prior to use the amine was purified (aniline and 2,6-dimethylaniline by distillation and the remaining substituted anilines by sublimation). To a 100 mL round bottom flask in the glovebox was added the desired aniline which was subsequently dissolved in 50 mL of Et2O (no additional drying with NaK). To this was added dropwise (1 equiv) of HOTf with stirring over five minutes. Immediate precipitation of white solid was observed and the reaction mixture was allowed to stir for thirty minutes. The reaction mixture was then filtered and the resulting white powder was washed with Et2O (50 mL) and pentane (50 mL). The resulting white microcrystalline material was then dried under vacuum. Yields of greater than 90% of microcrystalline material was obtained in this manner in all cases. 4-methoxyanilinium triflate ([4-OMePhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.52 (m, 2 H), 7.28 (m, 2H), 4.30 (br, 3H). anilinium triflate ([PhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.50 (m, 2 H), 7.41 (m, 1H), 7.34 (m, 2H). 2,6-dimethylanilinium triflate ([2,6-MePhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.14 (m, 3H), 2.32 (br, 6H). 2-chloroanilinium triflate ([ 2-Cl 1 218 PhNH3][OTf]): H NMR (DMSO-d6, 400 MHz): 7.32 (m, 1H), 7.15 (m, 1H), 7.02 (m, 1H), 6.82 (m, 1H). 2,5-chloroanilinium triflate ([2,5-ClPhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.19 (apparent d, 1H, 3J(H-H) = 8.5 Hz), 6.83 (apparent dd, 1H, 3J(H-H) = 2.5 Hz, 0.9 Hz), 6.56 (m, 1H). 2,6-chloroanilinium triflate ([2,6-ClPhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.22 (d, 2H, 3J(H-H) = 8.0 Hz), 6.57 (t, 1H, 3J(H-H) = 8.0 Hz). 2,4,6-chloroanilinium triflate ([2,4,6-ClPhNH3][OTf]): 1H NMR (DMSO-d6, 400 MHz): 7.37 (s, 2H). C.2.2 Preparation of [Cp*2Co][BArF4] A RT solution of HBArF4 (96.1 mg, 0.095 mmol) in Et2O (6 mL) is added dropwise to a stirred, RT solution of Cp*2Co (32.9 mg, 0.1 mmol) in Et2O (6 mL). This mixture is allowed to stir 30 min and then reduced to dryness in vacuo. The resulting solid residue is washed with pentane (3 x 2 mL) to yield [Cp*2Co][BArF4] as a bright yellow solid (104 mg isolated, 92% yield). 1 H NMR (THF-d8, 300 MHz): δ 7.79 (8H, s, BArF4), δ 7.58 (4H, s, BArF4), δ 1.75 (30H, s, Cp*2Co). C.3 Ammonia Generation Details All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogenfilled glovebox, the precatalyst (2.3 μmol) was weighed into a vial. The precatalyst was then transferred quantitatively into a long tube with a female 24-40 joint at the top using THF. The THF was then evaporated to provide a thin film of precatalyst at the bottom of tube. The tube is then charged with a stir bar, the acid (108 equiv), and Cp*2Co (41.2 mg, 54 equiv) as 219 solids. The tube is then sealed at RT with a septum that is secured with copper wire (this ensures a known volume of N2 in the reaction vessel, which is important for H2 detection). The tube is then chilled to 77 K and allowed to equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is allowed to stir at −78 °C for four hours. At this point the headspace of the tube is sampled with a 10 mL sealable gas syringe which is used to analyze for H2 by GC. The tube is then allowed to warm to RT with stirring and then stirred at RT for a further ten minutes. At this point the previously described procedure for quantifying ammonia was employed. To ensure reproducibility, all experiments were conducted in 395 mL tubes (51 mm OD) using 25 mm stir bars and stirring was conducted at ~ 650 rpm. Table C.1: NMR quantification results for standard NH3 generation experiments with [(P3B)Fe][BArF4] Acid Integration Relative % Yield NH3 % Yield H2 to Internal Standard (error) (error) [4-OMePhNH3][OTf] 0.01, 0.02 0.2 ± 0.1 89.1 ± 0.2 [PhNH3][OTf] 3.42, 3.33 40.4 ± 0.5 48.6 ± 0.7 2,6-Me [ PhNH3][OTf] 4.30, 3.63 47.5 ± 4.0 37.8 ± 0.2 [2-ClPhNH3][OTf] 4.98, 4.92 59.3 ± 0.4 26.1 ± 1.9 [2,5-ClPhNH3][OTf] 6.78, 6.15 77.5 ± 3.8 10.5 ± 1.1 [2,6-ClPhNH3][OTf] 6.81, 6.00 76.7 ± 4.9 12.6 ± 2.5 2,6-Cl * [ PhNH3][OTf] 6.60, 5.81 74.4 ± 4.7 14.2 ± 3.4 [2,6-ClPhNH3][BArF4] 4.12, 3.0 42.7 ± 6.7 18.8 ± 0.8 2,4,6-Cl [ PhNH3][OTf] 5.73, 6.10 70.9 ± 2.2 12.0 ± 0.8 pentachloroanilinium triflate 1.62, 1.70 19.9 ± 0.5 63.5 ± 1.1 ([per-ClPhNH3][OTf]) *Run performed with [(P3B)Fe(N2)][Na(Et2O)3] as the precatalyst. 220 C.4 H2 Monitoring Details C.4.1 Standard Background Generation Reaction All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogenfilled glovebox, a long tube with a female 24-40 joint is charged with a stir bar, the acid (108 equiv) and Cp*2Co (41.2 mg, 54 equiv). The tube is then sealed at RT with a septum that is secured with copper wire. The tube is then chilled to 77 K and allowed to equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is allowed to stir at −78 °C for four hours. At this point the headspace of the tube is sampled with a 10 mL sealable gas syringe which is used to analyze for H2. Table C.2: Data for Background H2 Quantification Experiments Acid GC Integration for H2 % Yield H2 4-OMe [ PhNH3][OTf] 49.8 31.5 [PhNH3][OTf] 24.0 15.2 2,6-Me [ PhNH3][OTf] 8.2 5.2 [2-ClPhNH3][OTf] 47.2 29.9 2,5-Cl [ PhNH3][OTf] 37.1 23.5 [2,6-ClPhNH3][OTf] 77.8 49.2 2,4,6-Cl [ PhNH3][OTf] 34.8 22.0 per-Cl [ PhNH3][OTf] 98.3 62.3 C.4.2 H2 Evolution Kinetics All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. For the catalyzed run, the precatalyst was then transferred quantitatively into a Schlenk tube using THF. The THF was then evaporated to provide a thin film of precatalyst at the bottom of the long tube with a female 24-40 joint. The tube is then charged with a stir bar and the [2,6- Cl 221 PhNH3][OTf] (77.9 mg, 108 equiv) and Cp*2Co (41.2 mg, 54 equiv) are added as solids. The tube is then sealed at RT with a septum that is secured with copper wire. The tube is then chilled to 77 K and allowed to equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is allowed to stir at −78 °C. As soon as the stir bar is freed from the frozen solution and stirring begins the timing is started. At the time points noted below the headspace was sampled for H2 with a 10 mL sealable gas syringe. Table C.3: Time points for catalyzed H2 evolution from 2,6-dichloroanlinium triflate and Cp*2Co Time (min) GC Integration for H2 % Yield H2 (error) 5 3.8, 6.4 3.3 ± 0.9 15 11.6, 16.9 9.3 ± 1.8 25 14.7, 26.2 13.4 ± 3.8 35 22.5, 20.8 13.9 ± 0.5 Table C.4: Time points for uncatalyzed H2 evolution from 2,6-dichloroanlinium triflate and Cp*2Co Time (min) GC Integration for H2 % Yield H2 (error) 5 3.3, 2.9 2.0 ± 0.1 15 7.0, 6.2 4.3 ± 0.3 25 8.8, 11.1 6.3 ± 0.8 65 20.7, 27.0 14.5 ± 1.7 222 18 16 14 %H2 Evolution 12 10 8 6 4 2 0 0 10 20 30 40 Time (min) Catalyzed H2 Formation 50 60 Background H2 Evolution Figure C.1: Comparison of catalyzed and uncatalyzed H2 evolution from 2,6dichloroanlinium triflate and Cp*2Co at early time points. C.5 Mössbauer Spectroscopy All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogenfilled glovebox, the desired 57Fe species (0.0023 mmol) is quantitatively transferred using THF to a vial and then evaporated to yield a thin film. That vial is charged with a small stir bar and the other reagents as solids. The vial is then chilled to 77 K in a liquid nitrogen bath and allowed to equilibrate for five minutes. To the chilled tube is added 1 mL of Et2O and this allowed to equilibrate for another five minutes. The vial is then transferred to a cold well 70 223 that has been pre-cooled for at least fifteen minutes to −78 °C with a dry ice/acetone bath. When the stir bar is freed from the frozen solvent and begins to stir the time is started. At the time noted the stirring is stopped and using a prechilled pipette the reaction mixture is transferred in one portion to a pre-chilled Mössbauer cup sitting in a vial. The vial is then placed in a liquid nitrogen bath causing the reaction mixture to freeze in approximately twenty seconds. The Mössbauer cup is then submerged in the liquid nitrogen and then removed from the glovebox and standard procedure is used to mount the sample on the Mössbauer spectrometer. Figure C.2: Mössbauer spectrum collected from a reaction freeze quenched after stirring for 5 minutes at −78 °C in 1 mL of Et2O between [P3B(57Fe)N2][Na(Et2O)3] and excess 2,6[2,6-ClPhNH3][OTf] (50 equiv). Raw data shown as black points, simulation as a solid red line, with components in green, purple, and yellow (see Table S3 for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT in Et2O. Fitting details for Figure C.2 are as follows: three quadrupole doublets were found to be necessary to obtain an adequate simulation. Although a variety of parameters could potentially simulate the relatively broad absorptions observed here, previous reactivity of [(P3B)Fe(N2)]− with acid18 suggested that (P3B)Fe(N2) and [(P3B)Fe]+ were likely products. Satisfyingly if the known isomer shift and quadrupole splitting for one of those species was fixed during the fitting process and the other components were allowed to refine freely the 2 224 other major component was found to be the complementary species. The third species was always unchanged in these simulations and represents an unknown species. Its presence in the fit is demanded by the inflection point on the more negative side of the right-hand absorbance. Modeling this feature also helps to capture the asymmetry of the left-hand absorbance while using the symmetric line-shapes we expect for (P3B)Fe(N2) (green) and [(P3B)Fe]+ (purple). The broad linewidths for [(P3B)Fe]+ have been observed previously and may be explained by the existence of unbound and bound varieties of the species with the reaction mixture providing potential ligands such as [OTf]−, 2,6-ClPhNH2, and N2. Table C.5: Simulation parameters for Mossbauer spectrum in Figure C.2. Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Relative area ΓL/ ΓR (mm s−1) A (green) 0.58 ± 0.02 3.28 ± 0.07 0.52/0.52 0.26 B (purple) 0.76 ± 0.02 2.57 ± 0.05 1.10/1.10 0.63 C (yellow) 0.13 ± 0.02 2.24 ± 0.04 0.50/0.50 0.11 C.6 EPR Spectroscopy C.6.1 General Procedure for EPR Spectroscopy All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogenfilled glovebox, the desired Fe species (0.0023 mmol) is quantitatively transferred using THF to a vial and then evaporated to yield a thin film. That vial is charged with a small stir bar and the acid (0.116 mmol, 50 equiv) as solids ([2,6-ClPhNH3][OTf] or [2,6-ClPhNH3][BArF4]). The vial is then chilled to 77 K in a liquid nitrogen bath and allowed to equilibrate for five minutes. To the chilled tube is added 1 mL of Et2O (for HOTf 50 equiv have been dissolved in this 1 mL of Et2O at RT) and this allowed to equilibrate for another five minutes. The vial is then transferred to a cold well that has been pre-cooled for at least fifteen minutes to −78 225 °C with a dry ice/acetone bath. When the stir bar is freed from the frozen solvent and begins to stir the time is started. The reaction mixture is stirred for five minutes and then stirring is stopped. Using a pre-chilled pipette approximately 0.5 mL of the reaction mixture is rapidly transferred to a pre-chilled X-band EPR tube. The X-band EPR tube is then placed in a liquid nitrogen bath causing the reaction mixture to freeze in approximately twenty seconds. The EPR tube is then sealed and removed from the glovebox in liquid nitrogen. C.6.2 Comment on Stoichiometric Reactivity In our attempt to model the catalytic reaction mixture we were interested in the reactivity of [(P3B)Fe(N2)]− (observed previously both from mixing [(P3B)Fe][BArF4] with excess Cp*2Co and under the catalytic reaction conditions) with acid. In order to achieve this we wanted to prepare independently known [(P3B)Fe(N2)]− species to model the proposed catalytic intermediate [(P3B)Fe(N2)][Cp*2Co]. We chose [(P3B)Fe(N2)][Na(Et2O)3] because we believed that its solubility in Et2O likely modeled that of [(P3B)Fe(N2)][Cp*2Co]. 226 Figure C.3: The continuous wave, X-band EPR at 77K in Et2O of reaction mixtures freezequenched after five minutes. In red is the reaction of [(P 3B)Fe(N2)][Na(Et2O)3] with 50 equiv of [2,6-ClPhNH3][BArF4] clearly demonstrating the formation of [(P3B)Fe(NNH2)][BArF4].18 In green is reaction of [(P3B)Fe(N2)][Na(Et2O)3] with 50 equiv of [2,6-ClPhNH3][OTf] in which the small residual species is neither the starting material ([(P3B)Fe(N2)][Na(Et2O)3]) or the desired product ([(P3B)Fe(NNH2)][OTf]). Although we do not know the chemical identity of this species we note that it is very similar to the EPR observed in the reaction of [(P3B)Fe(N2)][Na(12-crown-4)2] with 1 equiv of HBArF4.19 We hypothesize therefore that it may represent a Fe–H side product. 227 Figure C.4: In blue is the continuous wave, X-band EPR spectrum at 77K of a reaction mixture of 50 equiv [2,6-ClPhNH3][BArF4] with [(P3B)Fe(N2)][Na(12-crown-4)2] quenched with liquid nitrogen after 5 minutes. In orange is the simulation of this spectrum (fitting details below) Fitting details for Figure C.4 are as follows: the parameters used to fit the spectrum were obtained using the esfit application in the easyspin program.20 The fitting program obtains the best fit by minimizing the root mean square deviation from the data. The data was fit with the following parameters: g1 = 2.23899, g2 = 2.09189, g3 = 2.00664, and a line broadening of 323.8530, 71.2309, and 38.7902 MHz respectively. These parameters represent only a very small perturbation from those used previously to model [(P3B)Fe(NNH2)][BArF4]: g1 = 2.222, g2 = 2.091, g3 = 2.006 and a line broadening of 256, 113, and 41 MHz respectively.19 The slightly broader spectrum observed here precludes resolution of the small phosphorus coupling on g3. We believe that this broadening arises 228 from either the use of a non-glassing solvent (Et2O vs 2-MeTHF) or via small differences in hydrogen-bonding that arise from the presence of 2,6-dichloroaniline. C.7 Acid Quench of [(P3B)Fe(N2)]All solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogenfilled glovebox, the desired Fe species (2.3 μmol) was weighed into a vial. The Fe species was then transferred quantitatively into a Schlenk tube using THF. The THF was then evaporated to provide a thin film of Fe species at the bottom of the Schlenk tube. The tube is then charged with a stir bar and acid (0.116 mmol, 50 equiv) as solids 9[2,6-ClPhNH3][OTf] or [2,6-ClPhNH3][BArF4]) is added as a solid. The tube is then sealed at RT with a septum and a Konte’s valve that is left partially open. The tube is then chilled to 77 K and allowed to equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et2O through the septum. The temperature of the system is allowed to equilibrate for 5 minutes and then the Konte’s valve is sealed. This tube is passed out of the box into a liquid N2 bath and transported to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is allowed to stir at −78 °C for three hours. At the end of the reaction the Konte’s valve is opened and the reaction headspace is allowed to equilibrate. At this point the headspace of the tube is sampled with a 10 mL sealable gas syringe which is used to analyze for H2. The tube is then allowed to warm to RT with stirring and then stirred at RT for a further ten minutes. At this point the previously described procedure for quantifying ammonia was employed. Table C.6: Comparative NH3 and H2 Yields for [2,6-ClPhNH3][OTf] and [2,6-ClPhNH3][BArF4] Acid Yield of NH3 (equiv) % Yield H2 2,6-Cl [ PhNH3][OTf] 0.0 ± 0.0 43.7 ± 4.6 [2,6-ClPhNH3][BArF4] 0.20 ± 0.03 37.8 ± 7.6 229 C.8 Procedure for Measuring Solubility To measure the solubility of Cp*2Co, all solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogen-filled glovebox, a Schlenk tube is charged with a stir bar and the Cp*2Co (41.2 mg, 0.125 mmol) is added to the tube. The tube is then chilled to 77 K in a liquid nitrogen bath and allowed to equilibrate for 5 minutes. To the chilled tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes and then the Schlenk tube is transferred to the cold well which has been prechilled to −78 °C for fifteen minutes. After five minutes of stirring at ~ 620 rpm, the stirring is stopped. With a prechilled pipette the entirety of the reaction mixture is transferred to a similarly prechilled celite pad for filtration. Filtration yielded a pale green solution that was then warmed to RT and the solvent was removed under reduced pressure. The vial was then extracted with a 20 mM solution of 1,3,5-trimethoxybenzene in C6D6. The NMR was then measured and the Cp*2Co signal was integrated relative to the 1,3,5-trimethoxybenzene standard. The accuracy of this integration procedure was confirmed by performing this procedure on a sample of Cp*2Co that had simply been weighed into a vial. Repetition of this experiment resulted in Cp*2Co concentrations between 5-6 mM. To measure the solubility of [2,6-ClPhNH3][OTf], all solvents are stirred with Na/K for ≥ 2 hours and filtered prior to use. In a nitrogen-filled glovebox, a Schlenk tube is charged with a stir bar and the [2,6-ClPhNH3][OTf] (77.9 mg, 0.250 mmol) is added to the tube. The tube is then chilled to 77 K in a liquid nitrogen bath and allowed to equilibrate for 5 minutes. To the chilled tube is added 1 mL of Et2O. The temperature of the system is allowed to equilibrate for 5 minutes and then the Schlenk tube is transferred to the cold well which has been prechilled to −78 °C for fifteen minutes. After five minutes of stirring at ~ 620 rpm, the 230 stirring is stopped. With a prechilled pipette the entirety of the reaction mixture is transferred to a similarly prechilled celite pad for filtration. Filtration yielded a colorless solution that was then warmed to RT and the solvent was removed under reduced pressure. The vial was then extracted with a 20 mM solution of 1,3,5-trimethoxybenzene in THF-d8. The NMR was then measured and the two signals for [2,6-ClPhNH3][OTf] were integrated relative to the 1,3,5-trimethoxybenzene standard. The result was a [2,6-ClPhNH3][OTf] concentration of 0.4 mM. C.9 Controlled Potential Electrolysis and Cyclic Voltammetry Details C.9.1 General Considerations All manipulations are carried out in an N2-filled glove box. For CPE experiments a sealable H-cell consisting of two compartments separated by a fine porosity sintered glass frit is cooled to −35 °C in a cold well and charged with 4 mL (working chamber) and 4 mL (auxiliary chamber) of 0.1 M NaBArF4 solution in Et2O, the solutions are also cooled to −35 °C and the solution for the working chamber may contain additional chemical components as described below. The working chamber is outfitted with a glassy carbon working electrode, rectangular prismatic in shape with dimensions of 10 mm × 2 mm and submerged in the working chamber solution to a depth of ~ 10 mm. The working chamber is also equipped with a Ag/AgPF6 in 0.1 M NaBArF4 Et2O reference electrode isolated by a CoralPor™ frit (obtained from BASi) and referenced externally to Fc+/0. The auxiliary chamber is outfitted with a solid sodium auxiliary electrode (~ 5 mm × ~ 1 mm rectangular prism, submerged to ~ 5 mm). The cell is sealed before electrolysis. The cell is connected to a CH Instruments 600B electrochemical analyzer and controlled potential bulk electrolysis 231 experiments were performed at −35 °C with stirring, cold well external bath temperature maintained by a SP Scientific FTS Systems FC100 immersion cooler. CV experiments are conducted in a single compartment cell cooled to −35 °C in a cold well in 0.1 M NaBArF4 Et2O solution, again cold well external bath temperature maintained by a SP Scientific FTS Systems FC100 immersion cooler. The working electrode is a glassy carbon disk, the reference electrode is a Ag/AgPF6 in 0.1 M NaBArF4 Et2O reference electrode isolated by a CoralPor™ frit (obtained from BASi) and referenced externally to Fc+/0, the auxiliary electrode is a platinum wire. Measurements conducted with a CH Instruments 600B electrochemical analyzer C.9.2 General Methodology for Controlled Potential Electrolysis Experiments To the working chamber is added 3 mg of [(P3B)Fe][BArF4] (2 μmol), 100 μmol of acid (e.g. [Ph2NH2][OTf]), 0-23.8 mg of [Cp*2Co][BArF4] (0-20 μmol), and a magnetic stir bar. The cell is held at a working potential of −2.1 V vs Fc+/0 until the current passed in the cell falls to 1% of the initial current pass or until 21.5 hours have passed. After that time the potential bias is removed, the headspace of the cell is sampled with a sealable gas syringe (10 mL), which is immediately analyzed by GC for the presence of H2. Then an additional 100 μmol of acid in 2 mL 0.1 M NaBArF4 solution in Et2O is injected through rubber septa into both chambers to sequester NH3 as [NH4][OTf]. The cell is allowed to stir at −35 °C for 10 minutes and then warmed to RT. The contents of both chambers are then transferred to a Schlenk tube (cell washed with additional Et2O) and this material is analyzed for NH3 by base digestion, vacuum transfer of volatiles, and NMR integration as described in section S1.4 232 C.9.3 Methodology for Controlled Potential Electrolysis Experiments with Reloading of Substrate To the working chamber is added 3 mg of [(P3B)Fe][BArF4] (2 μmol), 100 μmol of acid (e.g. [Ph2NH2][OTf]), 0-23.8 mg of [Cp*2Co][BArF4] (0-20 μmol), and a magnetic stir bar. The cell is held at a working potential of −2.1 V vs Fc+/0 until the current passed in the cell falls to 1% of the initial current pass or until 21.5 hours have passed. After that time the potential bias is removed. An additional 100 μmol of acid in 2 mL 0.1 M NaBArF4 solution in Et2O is then added to the working chamber of the cell via injection through a rubber septum. The cell is then held at a working potential of −2.1 V vs Fc+/0 until the current passed in the cell falls to 1% of the initial current pass or until 21.5 hours have passed. After that time the potential bias is removed, the headspace of the cell is sampled with a sealable gas syringe (10 mL), which is immediately analyzed by GC for the presence of H2. Then an additional 100 μmol of acid in 2 mL 0.1 M NaBArF4 solution in Et2O is injected through rubber septa into both chambers of the cell to sequester NH3 as [NH4][OTf]. The cell is allowed to stir at −35 °C for 10 minutes and then warmed to RT. The contents of both chambers are then transferred to a Schlenk tube (cell washed with additional Et2O) and this material is analyzed for NH3 by base digestion, vacuum transfer of volatiles, and NMR integration as described in section S1.4 Table C.7: Controlled Potential Electrolysis Data Entry Acid Equiv Time [Cp*2Co] (h) [BArF4] Charge Passed (C) 1 2 3 7.5 6.2 5.4 [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] 0 0 0 42 63 43 Yield of NH3 (equiv per Fe) 2.3 2.8 2.6 FE NH3 FE H2a (%) (%) 18 26 28 80 25 53 233 Avg 4b 5b Avg 6 7 Avg 8 9 10 Avg 11 12 Avg 13 14 Avg 15 16 Avg 17b 18b 19b 20b 21b Avg 22Ac 22Bc 23d 24e 25f 26f 9.5 9.2 9.2 0.0 N/A 2.6 ± 0.3 2.2 3.0 2.6 ± 0.6 4.4 3.5 4.0 ± 0.6 3.9 3.5 4.6 4.0 ± 0.6 3.0 5.1 4±1 1.2 0.6 0.9 ± 0.4 2.0 1.7 1.9 ± 0.2 6.1 6.7 4.7 4.8 5.4 5.5 ± 0.9 4.6 0.0 0.0 0.3 1.3 24 ± 5 17 19 18 ± 1 31 24 28 ± 5 26 22 28 25 ± 3 19 29 24 ± 7 8 4 6±3 11 9 10 ± 1 20 21 20 18 18 19 ± 1 28 0 0 N/A 7.8 e- N/A 2.3 13.8 e- [Ph2NH2][OTf] [Ph2NH2][OTf] 0 0 43 43 7.5 9.0 [Ph2NH2][OTf] [Ph2NH2][OTf] 1 1 17 22 8.1 8.3 [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] 5 5 5 17 21 22 8.5 9.1 9.5 [Ph2NH2][OTf] [Ph2NH2][OTf] 10 10 21 10 9.4 10.2 [PhNH3][OTf] [PhNH3][OTf] 5 5 15 22 9.0 7.8 [2,6-ClPhNH3][OTf] [2,6-ClPhNH3][OTf] 5 5 17 17 10.6 10.7 [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] 5 5 5 5 5 32 22 37 41 43 17.3 18.7 13.7 15.3 17.8 [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] [Ph2NH2][OTf] 5 5 5 5 Chemical runs Chemical runs 21.5 11.5 16 43 21.5 21.5 [Ph2NH2][OTf] 67 22 56 47 61 57 27 64 47 48 35 44 41 43 32 38 52 31 27 88 75 N/A 50 e31 e- Avg 1.8 ± 0.7 11 ± 4 Some ports of the cell are sealed with septa and one of these is pierced before the electrolysis begins to pressure equilibrate the cell as it cools to −35 °C, we note therefore that H2 gas may escape from the cell particular during long experiments, indeed a test of H2 retention in the cell under equivalent conditions revealed leakage of H2 (60% recovery), thus the detected % yield of H2 reported here should be considered a lower limit. bThese experiments were conducted using the reloading protocol as described above. cElectrode rinse test as described in main text. dControl experiment with no [(P3B)Fe][BArF4] included but including a typical loading of 11.9 mg (10 μmol) of [Cp*2Co][BArF4]. eControl experiment in which the cell a 234 with all components, including the sodium auxiliary electrode, was assembled and stirred at −35 °C for 43 hours but neither a potential bias was applied, nor the working and auxiliary electrodes externally connected. This experiment thus interrogates the ability of the sodium electrode to function as a chemical reductant. fChemical catalysis runs at −35 °C in 0.1 M NaBArF4 Et2O solution with 50 equiv (100 μmol) of Cp*2Co included as a chemical reductant as well as [(P3B)Fe][BArF4] (2 μmol) and 100 μmol of acid ([Ph2NH2][OTf]). C.9.4 Control experiment for the possibility of NH3 being generated in a chemical rather than electrochemical process during acidic workup As per the general CPE methodologies described in S9.2 and S9.3, after electrolysis additional acid is added to the cell to sequester generated NH3 as an ammonium salt to facilitate transfer of these materials to a Schlenk tube, ultimately allowing NH3 quantitation via base digestion and vaccum transfer as described in S1.4. This presents the possibility that electrochemically reduced species formed during electrolysis (e.g., [(P3B)Fe(N2)]− and Cp*2Co) could react with this additional acid after the electrolysis was complete to generate NH3 in a chemical reaction. A control experiment to determine the extent to which this type of reactivity might contribute to the total NH3 yield observed from the CPE experiments was conducted. The H-cell is assembled via the standard methodology and charged with a typical loading of [(P3B)Fe][BArF4] (2 μmol) and [Cp*2Co][BArF4] (10 μmol, 5 equiv) but without initial acid (i.e., no [Ph2NH2][OTf]). Electrolysis is then carried out at −2.1 V vs Fc+/0 until the current pass in the cell falls to 1% of the initial current passed (in this case 1.12 C of charge were passed, corresponding to 11.6 μmol of reducing equivalents stored in the system, which approaches the theoretical limit of reducing equivalents that the loading of [(P3B)Fe][BArF4] (2 μmol) and [Cp*2Co][BArF4] (10 μmol) could store) at this potential. This post electrolysis mixture is then treated with acid and analyzed for NH3 via the standard methodology. This experiment yielded 0.2 equiv NH3 (relative to Fe) indicating that 235 chemical N2RR between electrochemically reduced species during the acidic workup is very minor. C.9.5 Additional CV Data Figure C.5: Cyclic voltammograms of 0.5 mM [(P3B)Fe][BArF4] at varied scan rates (left) and plot of peak current versus square root of scan rate for each feature (right) showing linear dependence in all cases. All spectra are collected in 0.1 M NaBArF4 solution in Et2O at −35 °C using a glassy carbon working electrode, and externally referenced to the Fc+/0 couple. Figure C.6: Cyclic voltammograms of 0.5 mM [(P3B)Fe][BArF4] (red trace) and [(P3B)Fe][BArF4] with 10 equiv of tetrabutylammonium trifluoromethanesulfonate ([TBA][OTf]) (blue and gray traces). The traces with [TBA][OTf] show disappearance of a wave corresponding to the [(P3B)Fe]+/0 couple at ~ −1.58 V (present in the red trace). This phenomenon, as in the case with acidic triflate sources as described in the main text, is likely due to triflate binding (to generate (P3B)Fe(OTf), thereby attenuating the wave associated with the reduction of [(P3B)Fe]+ and [(P3B)Fe(N2)]+). If the scan is stopped and reversed at −2.0 (before the [(P3B)Fe(N2)]0/− couple) no reversibility is observed, consistent with a chemical step (dissociation of triflate) being coupled with this redox event. We note that in 236 the presence of OTf it appears that the second reductive feature is also slightly anodically shifted. We believe this to be due to slow N2 binding kinetics and thus this wave would represent a convolution of the 0/1− reduction processes for both a vacant and an N2 bound (P3B)Fe. All spectra are collected in 0.1 M NaBArF4 solution in Et2O at −35 °C using a glassy carbon working electrode, and externally referenced to the Fc+/0 couple. C.10 Computational Details − C.10.1 Calculation of Acid Dissociation Constants Acid dissociation constants (pKa and pKd) were performed were optimized and solvated as discussed in the general methods section. For pKa values, the G for the exchange of a proton (H+) between the acid of interest and 2,6-ClPhNH2/[2,6-ClPhNH3]+. For pKd values, the same approach was used except that the net exchange of a HOTf unit was calculated. In all cases the dissociation constant was reference to the literature value for the pKa of [2,6Cl PhNH3]+ in THF. C.10.2 Determination of PT, ET and PCET Kinetics Kinetic barriers for reported for PT, ET and PCET were performed in one of two ways. Internal consistency between the methods was determined where possible. Values are summarized in Table C.8. Method A. Marcus Theory. Standard Marcus theory expressions19 were used in Method A. Inner sphere reorganization energies for PT or PCET were calculated using the method developed by the group of Hammes-Schiffer (Error! Reference source not found.) utilizing the force constants for the reactant (𝑓 ) and product (𝑓 ) species and the change in equilibrium bond length (qj).20 Eq C.4) λis,PT/PCET = Σj (fjrfjp)/(fjr+fjp)×Δqj2 Outer sphere reorganization energies were calculated using a continuum solvation model for the solvation of a point charge (λos,ET)19 or a dipole (λos,PT).20–22 The λos,PCET was 20 237 approximated using Eq C.5, where θ is the angle between the ET and PT vectors. It was determined via analysis of the structure of a constrained optimization (in which the Fe−H– Co distance was kept constant) that θ is between 0 and 45o, a range which corresponds to an insignificant variation (less than 0.2 kcal mol−1) in λos,PCET. Eq C.5) λos,PCET = λos,PT + λos,ET – (λos,PT* λos,ET)cos(θ) Relative rates for a bimolecular PT/ET vs PCET (kbi) pathway for reaction shown in Table C.8, Eq C.12 were determined via the method outline by the group of HammesSchiffer in which the bimolecular rate constant for PT, ET or PCET is approximated by Eq C.6. Eq C.6) kbi = KA*kuni KA represents the pre-arrangement equilibrium constant and kuni represents the unimolecular rate constant for PCET or ET.2325 Along an PT/ET pathway, the barriers calculated suggest that kPT > kET. In approximating kun for PCET and ET, we made extensive use of the webPCET portal.24 The electronic coupling for PCET and ET was assumed to be equal. In order to approximate a lower bound for kPCET/kET, the pre-arrangement equilibrium (KA) was also assumed to be equal for PCET and ET. We believe this represents a lower bound as the approximation for KA does not include any hydrogen bonding interactions for a PCET pathway. Method B. Optimization of a 1st Order Saddle Point. PT barriers for the protonation of Cp*2Co were also found by optimization of a 1st order saddle point. That the optimized structure represented a 1st order saddle point was confirmed with a frequency calculation, which showed only one imaginary frequency. Eq C.7) [2,6-ClPhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + 238 2,6-Cl PhNH2 Eq C.8) [2,6-MePhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + 2,6-Me PhNH2 Eq C.9) [4-OMePhNH3][OTf] + Cp*2Co  (Cp*)Co(exo-η4-C5Me5H)-OTf + 4-OMe PhNH2 Eq C.10) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  [(P3B)Fe(NNH2)][OTf] + Cp*2Co Eq C.11) [(P3B)Fe(NNH2)][OTf] + Cp*2Co  (P3B)Fe(NNH2) + [Cp2*Co][OTf] Eq C.12) (P3B)Fe(NNH) + [(Cp*)Co(exo-η4-C5Me5H)][OTf]  (P3B)Fe(NNH2)+ [Cp*2Co][OTf] Table C.8: Overview of Parameters Used to Calculate Kinetic Barriers Reaction λis λos Barrier Method {krel} 1.3 kcal mol−1 A 1.3 kcal mol−1 B −1 3.8 kcal mol A 3.6 kcal mol−1 B −1 4.5 kcal mol A 4.8 kcal mol−1 B −1 1.5 kcal mol A 4.1 kcal mol−1 Aa {krel ≡ 1} 13.7 kcal mol−1 0-10 kcal mol−1 0.2 – 0.6 kcal mol−1 A Eq C.12 {2000 – 4500} a The barrier for [(P3B)Fe(NNH2)][OTf] reduction was calculated assuming that ratedetermining reduction to [(P3B)Fe(NNH2)][OTf]− precedes OTf– release. Eq C.7 Eq C.7 Eq C.8 Eq C.8 Eq C.9 Eq C.9 Eq C.10 Eq C.11 N/A 7.5 kcal mol−1 N/A 7.5 kcal mol−1 N/A 7.5 kcal mol−1 8.9 kcal mol−1 8.9 kcal mol−1 N/A 6.3 kcal mol−1 N/A 6.3 kcal mol−1 N/A 6.3 kcal mol−1 6.3 kcal mol−1 25.0 kcal mol−1 C.10.3 BDFE Calculations Bond dissociation free energies (BDFE) of X–H bonds were calculated in the gasphase using a series of known reference compounds.25 The free-energy difference between 239 the H-atom donor/acceptor pair was calculated based on the thermochemical information provided by frequency calculations after structure optimizations using the procedure described in the general computational section. A linear plot of ΔG vs BDFElit was generated to form a calibration curve (Figure C.7). BDFE predictions were generated by application of the line of best fit to the calculated ΔG of the unknown species. Figure C.7: BDFEcalc and BDFElit plotted for species of known BDFEE-H. Line of best fit is shown. Table C.9: Data used to generate the plot and line of best fit shown in Figure C.7 G (E-H) G (E●) Gcalc BDFEE-H Species (kcal mol−1) (kcal mol−1) (kcal mol−1) (kcal mol−1) HOOH −151.4 −150.8 69.8 79.7 MeOH −115.6 −115.0 88.3 96.4 EtOOH −230.0 −229.4 68.7 76.6 H2O −76.4 −75.7 104.2 111.0 NH3 −56.5 −55.8 94.0 99.4 Me3CH −158.3 −157.6 82.7 88.3 240 PhOH −307.2 −306.6 74.0 79.8 Et2NH −213.6 −212.9 81.0 86.4 NH2NH2 −111.8 −111.1 67.3 72.6 − [OH] −75.7 −75.0 98.6 103.1 PhSH −630.2 −629.5 70.3 75.3 [NH4]+ −56.8 −56.1 113.0 116.9 Me2CH2 −119.0 −118.4 85.9 90.4 HC(O)OOH −264.7 −264.1 82.2 86.8 OOH −150.8 −150.2 37.5 42.7 C6H6 −232.1 −231.4 101.6 104.7 C2H4 −78.5 −77.8 99.7 102.5 C2H6 −79.7 −79.1 90.0 92.9 PhCH3 −271.3 −270.7 79.0 81.6 CH4 −40.5 −39.8 95.1 96.8 CpH −193.9 −193.3 71.0 73.2 EtSH −477.8 −477.2 77.2 79.1 MeSH −438.6 −437.9 77.3 79.2 PhNH2 −287.4 −286.7 79.8 81.5 NHNH −110.6 −110.0 51.0 52.6 H2S −399.3 −398.7 83.1 83.0 H2 −1.2 −0.5 98.8 97.2 C.10.4 Calculated Reduction Potentials for Selected [ArnNH(4-n)][OTf] and ArnNH(3-n) Table C.10: Calculated Reduction Potentials of Selected Species Species Eo (V vs Fc+/0) [4-OMePhNH3][OTf] −3.8 V 4-OMe PhNH2 −3.4 V [2,6-MePhNH3][OTf] −3.8 V [2,6-ClPhNH3][OTf] −2.4 V 241 a [2,6-ClPhNH3]+ −2.0 V [Ph2NH2]+ < −2.5 Va Ph2NH −3.1 V [per-ClPhNH3][OTf] −2.0 V Cp*2Co −2.2 V Potential for the formation of Ph2NH● + H● is reported. No ‘reversible’ minima was found. C.10.5 Calculated Reduction Potentials for Selected [ArnNH(4-n)][OTf] and ArnNH(3-n) Determining the reduction potential of the acids using electrochemical techniques is challenging due to the significant, electrode catalyzed HER observed upon scanning anodically (see Figure C.8). However, as expected due to the lower pKa of [perCl PhNH3][OTf] compared to [Ph2NH2][OTf] we see an earlier onset of the reduction potential and a higher current density. These processes are likely electrode-mediated and thus do not reflect a pure reduction potential, so to better estimate the outer-sphere reduction potential of the acids employed we have used DFT (Table C.10). In many cases, attempts to optimize the one electron reduced species results in a chemical step (i.e., loss of Cl– or H⦁) precluding determination of the reversible redox potential for the system. We are, however, able to find (in silico) a well-behaved reduction for [per-ClPhNH3][OTf] of −2.0 V. As a comparison, the high efficiency acid, [2,6-ClPhNH3][OTf], has a reduction potential of −2.4 V. This leads us to believe for [per-ClPhNH3][OTf] rather than engaging in an inner-sphere proton transfer with Cp*2Co (E0calc(Cp*2Co0/+) = −2.18 V) it is likely that an outer sphere electron transfer occurs first. This change in mechanism would explain the increased H2 yields and the decreased N2RR efficiency. In contrast, [2,6-ClPhNH3][OTf] should be resistant to reduction and thus able to protonate the metallocene and engage in the mechanism discussed in Chapter 4. 242 Figure C.8: Cyclic voltammograms of 5 mM [Ph2NH2][OTf] (gray trace) and 5 mM [perCl PhNH3][OTf] (red trace). All spectra are collected in 0.1 M NaBArF4 solution in Et2O at −35 °C using a glassy carbon working electrode and externally referenced to the Fc+/0 couple. Scan rate is 100 mV/s. C.11 X-ray Photoelectron Spectroscopy Details The surface composition of the carbon electrode surface after a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 was determined via XPS on a Kratos Axis Nova spectrometer with DLD (Kratos Analytical; Manchester, UK). The excitation source for all analysis was monochromatic Al Kα1,2 (hv = 1486.6 eV) operating at 10 mA and 15 kV. The X-ray source was directed at 54° with respect to the sample normal. A base pressure of 1 × 10−9 Torr is maintained in the analytical chamber, which rises to 5 × 10−9 Torr during spectral acquisition. All spectra were acquired using the hybrid lens magnification mode and slot aperture, resulting in an analyzed area of 700 μm × 400 μm. Survey scans were collected using 160 eV pass energy, while narrow region scans used 10 eV; charge compensation via the attached e−-flood source was not necessary in this study. Subsequent peak fitting and composition analysis was performed using CasaXPS version 2.3.16 (Casa Software Ltd.; Teignmouth, UK). Energy scale correction for the survey 243 and narrow energy regions was accomplished by setting the large component in the C 1s spectrum, corresponding to a C 1s C(=C) transition, to 284.8 eV. All components were fit using a Gaussian 30% Lorentzian convolution function. For quantification, Shirley baselines were employed where there was a noticeable change in CPS before and after the peak in the survey spectrum; otherwise, linear was chosen. Atomic percentages were calculated using the CasaXPS packages for regions and/or components and are reported herein. Calculations were performed using region or component areas normalized to relative sensitivity factors specific to the instrument conditions with deconvolution from the spectrometer transmission function. 244 245 Figure C.9: XPS survey scan of a section of a glassy carbon plate which was not exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0). XPS and Auger peaks are assigned as labeled in the legend, which also includes atomic percentages calculated from component fits from scans of individual XPS regions. This material represents a baseline of the electrode surface composition resulting from cleaning, polishing, and handling prior to CPE experiments and is provided for comparison to a XPS survey scan of a section of the same glassy carbon plate which was exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0) presented in figure S7. 246 247 Figure C.10: XPS survey scan of a section of a glassy carbon plate which was exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0) XPS and Auger peaks are assigned as labeled in the legend, which also includes atomic percentages calculated from component fits from scans of individual XPS regions. This material represents a post-electrolysis state of the electrode surface composition for comparison to a XPS survey scan of a section of the same glassy carbon plate which was not exposed to the working chamber solution during a 15 hour bulk electrolysis in the presence of [(P3B)Fe][BArF4], [Cp*2Co][BArF4], [Ph2NH2][OTf] and N2 at −2.1 V (vs Fc+/0) presented in figure S1. Notably this active surface scan reveals a small Fe signal, likely resulting from some degree of decomposition of the [(P3B)Fe][BArF4] catalyst over the course of the 15 hour electrolysis; however it is also possible that this small Fe signal is the result of contamination during the handling of the sample. This Fe 2p signal occurs at 710.4 eV, but due to the weak signal intensity it is not possible to confidently assign it’s oxidation state. Although, the signal does not appear consistent with Fe(0) whose signal is typically around 707 eV; we would also note though that the handling process involves transferring the electrode quickly in air which could result in oxidation of Fe(0) that was present.Also notable is that no new Co signal is observed in the post-electrolysis scan suggesting that [Cp*2Co][BArF4] does not decompose to a surface bound Co species in detectable amounts during the electrolysis. Phosphorus is also detectable in the survey scan but the signal is too small for quantitation. C.12 pKa Determination Strategy Bosch et al. published a procedure for converting a pKa in THF into the equivalent pKa in different solvents.26 Although not all of the pKa values have been experimentally determined in THF the values obtained from converting from MeCN or H2O into a THF value is quite accurate. So we have used these converted values in the text. Where available a number measured in THF has been used, if not the MeCN derived value is used. If neither is available then the H2O derived value is used. Solvent conversion equations: Eq C.13) pKa(THF) = 0.78×pKa(MeCN) − 0.52 Eq C.14) pKa(THF) = 1.19×pKa(H2O) + 2.13 Table C.11: pKa values of anilinium acids in different solvents 248 pKa in pKa in Converted Experimental MeCN H2O pKa a pKa in THF 4-OMe 27 28 [ PhNH3][OTf] 11.86 5.29 8.8 (8.4) 8.826 27 28 [PhNH3][OTf] 10.62 4.58 7.8 (7.6) 8.026 [2,6-MePhNH3][OTf] -3.8928 -- (6.8) 2-Cl 27 [ PhNH3][OTf] 7.86 2.6428 5.6 (5.3) 6.026 [2,5-ClPhNH3][OTf] 6.2129 1.5327 4.3 (4.0) 4.526 2,6-Cl 27 30 [ PhNH3][OTf] 5.06 0.42 3.4 (2.6) [2,4,6-ClPhNH3][OTf] -−0.0330 -- (2.1) per-Cl 29 [ PhNH3][OTf] 2.35 -1.3 (--) collidinium triflate 14.9827 -11.2 (--) benzylammonium triflate -9.3431 -- (13.2) a First is listed the value converted from THF and then in parentheses is the value converted from H2O. Acid C.13 References (1) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882–1893. (2) Anderson, J. S.; Moret, M. E.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 534–537. (3) Moret, M.-E.; Peters, J. C. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (4) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (5) Melzer, M. M.; Mossin, S.; Dai, X.; Bartell, A. M.; Kapoor, P.; Meyer, K.; Warren, T. H. Angew. Chem. Int. Ed. 2010, 49, 904–907. (6) Vicente, J.; Chicote, M.-T.; Guerrero, R.; Jones, P. G. J. Chem. Soc., Dalton Trans. 1995, 8, 1251–1254. (7) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42–55. (8) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104– 154119. 249 (9) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (10) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (11) Towns, J.; Cockerill, T.; Dahan, M.; Foster, I.; Gaither, K.; Grimshaw, A.; Hazlewood, V.; Lathrop, S.; Lifka, D.; Peterson, G. D.; et al. Comput. Sci. Eng. 2014, 16, 62–74. (12) Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; et al. Comput. Phys. Commun. 2010, 181, 1477–1489. (13) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73–78. (14) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 5, 799–805. (15) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. (16) Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2011, 115, 14556–14562. (17) Wang, T.; Brudvig, G.; Batista, V. S. J. Chem. Theory Comput. 2010, 6, 755–760. (18) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (19) Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978. (20) Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2001, 123, 3723–3733. (21) Koper, M. T. M. Chem. Sci. 2013, 4, 2710–2723. (22) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486–1493. 250 (23) Thoss, M.; Evers, F. J. Chem. Phys. 2018, 148, 030901. (24) Hammes-Schiffer, S.; Soudackov, A. V. J. Phys. Chem. B 2008, 112, 14108–14123. (25) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. (26) Garrido, G.; Rosés, M.; Ràfols, C.; Bosch, E. J. Solution Chem. 2008, 37, 689–700. (27) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (28) Pankratov, A. N.; Uchaeva, I. M.; Doronin, S. Yu.; Chernova, R. K. J. Struct. Chem. 2001, 42, 739–746. (29) Haav, K.; Saame, J.; Kütt, A.; Leito, I. Eur. J. Org. Chem. 2012, 2012, 2167–2172. (30) Liao, K.; Pack, B. W.; Toltl, N. P. J. Pharm. Biomed. Anal. 2007, 44, 118–126. (31) Hall, H. K. J. Am. Chem. Soc. 1957, 79, 5441–5444. 251 Appendix D. Supplementary Information for Chapter 5 252 D.1 Experimental Details D.1.1 General Considerations All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective oxygen and moisture removal. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Cp*2Co,1 (Cp*)Co(η4-C5Me6),2 [H(OEt2)2][BArF4] (HBArF4, BArF4 = tetrakis-(3,5- bis(trifluoromethyl)phenyl)borate),3 [Fc][BArF4] (Fc = ferrocenium),4 and [TBA][BD4] (TBA = tetrabutylammonium, BD4 = borodeuteride)5 were synthesized according to a literature procedure. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. C6D6 and MeCN-d3 were degassed and stored over activated 3 Å molecular sieves prior to use. Elemental analysis was performed by the Beckman Institute Elemental Analysis facility at California Institute of Technology. 1H and 13C NMR chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. Solid IR measurements were obtained on a Bruker Alpha spectrometer equipped with a diamond ATR probe. D.1.2 Continuous Wave EPR Spectroscopy X-band (9.4 GHz) CW EPR spectra were acquired using a Bruker EMX spectrometer equipped with a Super High-Q (SHQE) resonator using Bruker Win-EPR software (ver. 3.0). 253 Spectra were acquired at 77 K using a vacuum-insulated quartz liquid nitrogen immersion dewar inserted into the EPR resonator. D.1.3 Pulse EPR Spectroscopy All pulse Q-band (34 GHz) electron nuclear double resonance (ENDOR), hyperfine sublevel correlation (HYSCORE), and electron spin echo detected field-swept spectra were acquired using a Bruker 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 equipped with an Oxford Instruments Mercury ITC temperature controller. Q-band electron spin-echo detected EPR (ESE-EPR) field-swept spectra were acquired using the 2-pulse “Hahn-echo” sequence (𝜋/2 – 𝜏 – 𝜋 – 𝜏 – echo) where 𝜏 was held constant. Subsequently, each field swept echo-detected EPR absorption spectrum was modified using a pseudo-modulation function to approximate the effect of field modulation and produce the CW-like 1st derivative spectrum.6 Specific acquisition parameters: 𝜋/2 = 12 ns; 𝜋 = 24 ns; 𝜏 = 160 ns (6 K spectra), 600 ns (10 K spectra); shot repetition time (srt) = 5 ms (6 K spectra), 2 ms (10 K spectra). Q-band inversion recovery experiments were conducted using the pulse sequence 𝜋 – 𝑇 – 𝜋/2 – 𝜏 – 𝜋 – 𝜏 – echo, where 𝑇 is a variable delay and 𝜏 is a fixed delay. Specific acquisition parameters: 𝜋/2 = 12 ns; 𝜋 = 24 ns; 𝜏 = 160 ns Q-band HYSCORE spectra were acquired using the 4-pulse sequence (𝜋/2 − 𝜏 − 𝜋/2 − 𝑡 − 𝜋 − 𝑡 − 𝜋/2 – 𝜏 – echo), where 𝜏 is a fixed delay, and 𝑡 and 𝑡 are variable delays independently incremented by ∆𝑡 and ∆𝑡 , respectively. Sixteen step phase cycling 254 was utilized. The time domain spectra were baseline-corrected (third-order polynomial), apodized with a Hamming window function, zero-filled to eight-fold points, and fast Fourier-transformed to yield the frequency domain. For 2H-1H difference spectra, the time domain of the HYSCORE spectrum of the 1H sample was subtracted from that of the 2H sample, and the same data processing procedure detailed above was used to generate the frequency spectrum. Q-band ENDOR spectra were acquired using the Davies pulse sequence (𝜋 − 𝑡 𝜋 −𝑡 pulses, 𝜋 − 𝜋/2 – 𝜏 – 𝜋 – echo), where 𝑡 − is the delay between MW pulses and RF is the length of the RF pulse. The RF frequency was randomly sampled during each pulse sequence. Specific acquisition parameters: 𝜋/2 = 40 ns; 𝜋 = 15 µs; 𝑡 =2 µs; 𝜋 = 80 ns; 𝜏 = 300 ; shot repetition time (srt) = 5 ms In general, the ENDOR spectrum for a given nucleus with spin 𝐼 = ½ (1H) coupled to the S = ½ electron spin exhibits a doublet at frequencies (Eq D.1) Eq D.1) ν± = |A/2 ± νN| Where 𝜈 is the nuclear Larmor frequency and 𝐴 is the hyperfine coupling. For nuclei with 𝐼 ≥ 1 (14N, 2H), an additional splitting of the 𝜈± (Eq D.2) manifolds is produced by the nuclear quadrupole interaction (P) Eq D.2) ν±,mI = |νN ± 3P(2mI−1)/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 255 in the weak-coupling regime (|A| < 2|νI|) in the (+,+) quadrant from those in the strong coupling regime (|A| > 2|νI|) 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 at the magnetic field position at which the spectrum is collected. The length and curvature of these correlation ridges allow for the separation and estimation of the magnitude of the isotropic and dipolar components of the hyperfine tensor, as shown in Figure D.1. 256 Figure D.1: a) HYSCORE powder patterns for an S = 1/2, I = 1/2 spin system with an isotropic hyperfine tensor A. b) HYSCORE powder patterns for an S = 1/2, I = 1/2 spin system with an isotropic hyperfine tensor which contains isotropic (𝑎 ) and dipolar (𝑇) contributions. Blue correlation ridges represent the strong coupling case; red correlation ridges represent the weak coupling case. D.1.4 EPR Simulations All CW and pulse EPR spectra were simulated using the EasySpin 7 suite of programs with Matlab 2017 using the following Hamiltonian (Eq D.3): Eq D.3) H = μB B0 gS +μNgNB0I +hSAI+hIPI In this expression, the first term corresponds to the electron Zeeman interaction term where 𝜇 is the Bohr magneton, g is the electron spin g-value matrix with principle components g = [gxx, gyy, gzz], and 𝑆 is the electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where 𝜇 257 is the is the nuclear magneton, 𝑔 characteristic nuclear g-value for each nucleus (e.g. 1H, 2H, 31P) and 𝐼 is the nuclear spin operator; the third term corresponds to the electron-nuclear hyperfine term, where 𝐴 is the hyperfine coupling tensor with principle components 𝐴 = [Axx, Ayy, Azz]; and for nuclei with 𝐼 ≥ 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 𝑷 is the quadrupole coupling tensor. In the principle axis system (PAS), 𝑷 is traceless and parametrized by the quadrupole coupling constant 𝑒 𝑞𝑄/ℎ and the asymmetry parameter 𝜂 such that (Eq D.4-D.6): Eq D.4) 𝑷 = 𝑃 0 0 0 𝑃 0 0 0 𝑃 = / ( ) −(1 − 𝜂) 0 0 −(1 + 𝜂) 0 0 0 0 2 Eq D.5) e2qQ/h = 2I(2I−1)Pzz Eq D.6) η = (Pxx−Pyy)/Pzz 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). D.1.5 X-ray Crystallography XRD studies were carried out at the Beckman Institute Crystallography Facility on a Bruker Kappa Apex II diffractometer (Mo Kα radiation). Structures were solved using SHELXS or SHELXT and refined against F2 on all data by full-matrix least squares with 8 9 258 SHELXL. All of the solutions were performed in the Olex2 program. The crystals were mounted on a glass fiber under Paratone N oil or fluorolube. D.1.6 Electrochemistry Electrochemical measurements were carried out in a thick-walled one-component electrochemical cell fitted with a Teflon stopcock and tungsten leads protruding from the top of apparatus. A CH instruments 600B electrochemical analyzer was used for data collection. A freshly-polished glassy carbon electrode was used as the working electrode, a platinum wire was used as the auxiliary electrode, and a silver wire as a reference electrode. The analyte was used in 1 mM concentration. The solvent and the concentration of the electrolyte are noted with the individual voltammograms. After the desired scans were completed, ferrocene (1 mM) was added to serve as an internal reference or the known decamethylcobaltocenium/decamethylcobaltocene couple was used as the internal reference. All reported potentials are referenced to the ferrocenium/ferrocene couple (Fc+/0). D.1.7 Density Functional Theory Calculations All calculations were performed using the ORCA 4.0 program. 10,11 In cases where crystal structures were available these coordinates were used as the input. The calculations were performed using the TPSS (meta-GGA)12 functional with the def2-SVP basis set was on C and H and the def2-TZVP basis set on Co13 and Grimme-d3 dispersion correction.14 That optimized structures represented true stationary points was checked by doing a singlepoint frequency calculations on the optimized structure. For species where calculating the EPR parameters was of interest further calculations were performed in order to verify the robustness of those results. Using the 259 structures optimized as described above additional optimizations were performed with other functionals: BP86 (GGA),15,16 B3LYP (hybrid+GGA),15,17,18 and TPSSH (hybrid+meta-GGA).12,19 In all cases the ring-functionalized structures were found to be minima by means of a frequency calculation. For the Co–H structures in all cases (except with TPSS) there was a small negative frequency that did involve motion of the Co–H between the top and bottom ring suggesting that with these other functionals that this structure may only be a transition state. This is consistent with our previous observations with M06-L20 where a Co–H structure could not be optimized.21 In all cases the thermochemistry was very similar. EPR parameters for the TPSS-optimized structure were calculated by doing a single point calculation on the optimized structures using CP(PPP) 22 on Co and def2-TZVP on C and H with Grid6 and TPSSH as the functional. To check the robustness of this basis set a higher level calculation was also done using CP(PPP) on Co and EPR-III 23 on C and H with grid 7. The results were very similar. Thus the EPR parameters were also calculated using CP(PPP) on Co and def2-TZVP on C and H with TPSS, BP86, and B3LYP. Lastly, the EPR parameters for the structures optimized using TPSSH, BP86, and B3LYP were all calculated via a single point calculation using TPSSH with CP(PPP) on Co and def2-TZVP on C and H with Grid6. See below for a discussion of the results. For the calculation of thermochemical properties (reduction potential, pKa, and hydricity) an additional solvation calculation was done using the CPCM solvation model with acetonitrile solvent to determine the solvated internal energy (E soln).24–26 Free energies of solvation were approximated using the difference in gas phase internal energy (E gas) and solvated internal energy (∆Gsolv ≈ Esoln – Egas) and the free energy of a species in solution 260 was then calculated using the gas phase free energy (Ggas) and the free energy of solvation (Gsoln = Ggas + ∆Gsolv).27,28 The calculations of BDFE were done without the additional solvation correction as there is no change in charge for either reactants or products and it was found to introduce additional error in previous studies. D.2 Synthetic Procedures (Cp*)Co(exo-η4-C5Me5H)-[Cp*2Co][PF6] (100.0 mg, 0.21 mmol) was added as a solid to a THF solution (15 mL) of [TBA][BH4] (271.2 mg, 1.05 mmol, 5.0 equiv) in a Schlenk tube. This reaction was then heated to reflux and allowed to stir overnight. The reaction mixture was cooled to room temperature and the solvent was evaporated. The orange solid was extracted with pentane (8 x 5 mL) and filtered through an alumina plug. The solvent was then removed under reduced pressure to yield an orange solid. X-ray diffraction quality crystals were obtained by slow evaporation of a concentrated pentane solution. (Yield: 40.2 mg, 57.8%). 1 H NMR (RT, C6D6, 400 MHz): δ = 2.11 (1H, m, C(H)-CH3), 1.87 (6H, s, β-CH3), 1.65 (15H, s, Cp*), 1.42 (3H, d, 4J(H-H) = 6.7 Hz, C(H)-CH3) 0.76 (6H, d, 5J(H-H) = 1.2 Hz, α-CH3).13C{1H} NMR (RT, C6D6, 100 MHz): δ = 87.74, 86.14, 53.21, 51.66, 17.08, 11.39, 10.21, 9.55. IR (Thin Film): 2708 and 2612 cm-1 (νC–H for the ring-bound C–H). UV/Vis (2MeTHF): nm [cm-1 M-1]): 463 [220], 565 [350]. Elemental Analysis: theory [C: 72.71 H: 9.46]; found [C: 72.82, H: 9.64]. See the IR spectra for a further discussion of why two stretches are observed for the C−H mode. Cp*(exo-η4-C5Me5D)Co-[Cp*2Co][PF6] (100 mg, 0.21 mmol) and [TBA][BD4] (275.4 mg, 1.05 mmol, 5 equiv) were added to a Schlenk tube as solids. A small stir bar and 261 20 mL of THF were then added. The reaction was then heated to reflux overnight. At this point the solvent was removed in vacuo and the solid extracted with (8 x 5 mL) pentane. This material was then filtered through alumina and the solvent was removed under reduced pressure. (Yield: 36.5 mg, 52.0%) 1 H NMR (RT, C6D6, 400 MHz): δ = 1.87 (6H, s, β-CH3), 1.65 (15H, s, Cp*), 1.42 (3H, s, C(D)-CH3) 0.76 (6H, s, α-CH3). A small residual peak from the protio-incorporated material can be observed at 2.11 ppm. Integration of this peak suggests that there has been ~95% deuterium incorporation. 13C{1H} NMR (RT, C6D6, 100 MHz): δ = 51.21 (t, 2J(C-D) = 18.0 Hz). 2H{1H} NMR (RT, 90% C6H6:10% C6D6, 61.42 MHz): δ = 2.05 (s). IR (Thin Film): 2001 and 1957 cm-1 (νC–D for the ring-bound C–H). (Cp*)Co(exo-η4-C5Me513Me)-Cp*2Co (50 mg, 0.15 mmol) was dissolved in toluene and chilled to −78 °C with stirring. To this was added dropwise 13C-MeOTf (75 mg, 0.45 mmol, 3 eq). The reaction was stirred for one hour at −78 °C followed by warming to room temperature for five hours. The reaction was then filtered to remove [Cp*2Co][OTf]. The desired product could then be obtained by evaporation. (Yield: 23.1 mg, 87%) 1 H NMR (RT, C6D6, 400 MHz): δ = 1.84 (6H, s), 1.65 (15H, s, Cp*), 1.42 (3H, d, 3 J(C-H) = 4.5 Hz) 0.76 (6H, s), 0.47 (3H, d, 1J(C-H) = 124.4 Hz). 13C{1H} NMR (RT, C6D6, 100 MHz): δ = 26.38. 262 D.3 NMR Characterization of New Species Figure D.2: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5H). 263 Figure D.3: 1H-COSY NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4C5Me5H). This data was used to establish the chemical shift of the Me groups that are α and β to the quaternary carbon. Figure D.4: 13C NMR spectrum (100 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5H). 264 Figure D.5: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5D). Integration of the residual 1H signal at 2.11 indicates approximately 95% deuterium incorporation. 265 Figure D.6: 2H{1H} NMR spectrum (61.42 MHz, 90% C6H6:10% C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me5D). In addition to the expected deuterium incorporation into the exoposition it appears that there is also a small amount of deuterium incorporation into the methyl group that is attached to the quaternary carbon. Figure D.7: 13C{1H} NMR spectrum (100 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4C5Me5D). Here we can also see a small amount of contamination with the protio species at 51.66. The triplet is centered at 51.21 demonstrating the expected change in chemical shift upon deuteration. 266 Figure D.8: 1H NMR spectrum (400 MHz, C6D6, 25 °C) of (Cp*)Co(exo-η4-C5Me513Me) demonstrating the 13C-1H coupling evident in the splitting of the peak for the α-protons at 0.46 ppm and the smaller coupling to the γ-protons at 1.41 ppm. Figure D.9: 13C NMR spectrum (100 MHz, C6D6, 25 °C) (Cp*)Co(exo-η4-C5Me513Me) highlighting the selective 13C incorporation. 267 D.4 Reactivity of (Cp*)Co(exo-η4-C5Me5H) Figure D.10: Reaction of (Cp*)Co(exo-η4-C5Me5H) with 4MeOTEMPO. (Cp*)Co(exo-η4-C5Me5H) (1.5 mg,0.005 mmol, 1 eq) and 4MeOTEMPO (1.9 mg, 0.010, 2.2 eq) were dissolved in MeCN-d3. These were then allowed to react at room temperature for thirty minutes with shaking. A 1H NMR was then taken. In this the formation of 2 equivalents of 4MeOTEMPOH is observed (Figure D.11)29 confirmed by integration relative to a benzene internal standard containing 2 equivalents. However, the Co product is unclear. On the basis of reactions performed in toluene, we believe that this is because of the high instability of the fulvene product, (Cp*)Co(η4-C5Me4CH2), in coordinating solvents. 268 Figure D.11: 1H NMR spectrum (400 MHz, MeCN-d3, RT) of the reaction between (Cp*)Co(exo-η4-C5Me5H) and 4MeOTEMPO. Due to the poor solubility of the reagents in acetonitrile, the limited ability to cool that solvent, and that the NMR chemical shifts of the fulvene complex have only been previously reported in aromatic solvents the reaction was performed again in toluene-d8.30,31 (Cp*)Co(exo-η4-C5Me5H) (4 mg, 0.012 mmol, 1 eq) and 4MeOTEMPO (4.5 mg, 0.024 mmol, 1 eq) were each dissolved in minimal d8-toluene and both added to a J-Young NMR tube (Figure S10). Due to broadening at room temperature the reaction mixture was cooled to −78 °C and a 1H (Figure D.12). 269 Figure D.12: Zoomed in view of the 1H NMR (500 MHz, C7D8, −78 °C) of the reaction between (Cp*)Co(exo-η4-C5Me5H) and 2 equivalents of 4Me0TEMPO demonstrating that two equivalents of 4-MeOTEMPOH are formed for each equivalent of Cp*(η4-C5Me4CH2)Co. As further evidence for the proposed scheme, decamethylcobaltocene (8 mg, 0.024 mmol, 1 eq) and 4MeOTEMPO (4.5 mg, 0.024 mmol, 1 eq) were each dissolved in minimal d8-toluene and both added to a J-Young NMR tube (Figure S13). Due to broadening at room temperature (Figure D.13) the reaction mixture was cooled to −78 °C and a 1H (Figure D.14) and 13C NMR (Figure D.16) were taken. These data are in good agreement with the previously reported spectroscopic data.30,31 270 Figure D.13: 1H NMR spectrum (500 MHz, C7D8, RT) of the reaction between Cp*2Co and 4MeO TEMPO. 271 Figure D.14: 1H NMR spectrum (500 MHz, C7D8, −78 °C) of the reaction between Cp*2Co and 4MeOTEMPO. b Co + 4MeO TEMPO PCET a c i d Co O + e f j g/h N O H k electron transfer + Co proton transfer + 4MeOTEMPO Figure D.15: Reaction scheme of Cp*2Co with 4-MeOTEMPO and the left side is labeled with the assignments made in the 1H NMR based on known spectroscopic properties of these molecules. Both the concerted and separated PCET reaction pathways are possible. The deprotonation of decamethylcobaltocenium by TEMPO− has been observed previously as part of a more complicated reaction scheme.31 1 3 272 H NMR (−78 °C, C7D8, 500 MHz): δ = 5.10 (1H, s, k), 3.39 (1H, app t, J(H-H) = 11.3 MHz, f), 3.26 (2H, s, d), 3.13 (3H, s, e), 1.98 (2H, d, 3J(H-H) = 11.3 Hz, g/h), 1.64 (2H, t, 2J(H-H) = 11.2 MHz, g/h), 1.53 (6H, s, b/c), 1.40 (15 H, s, a), 1.29 (6H, s, i/j), 1.21 (6H, s, i/j), 1.09 (6H, s, b/c). Figure D.16:13C NMR spectrum (125 MHz, C7D8, −78 °C) of between Cp*2Co and 4Me0 TEMPO which demonstrates the formation of Cp*(η4-C5Me4CH2)Co and 4MeOTEMPOH. Residual solvents are indicated with symbols (# = THF, & = diethylether, $ = pentane). 273 Figure D.17: Zoomed in data from the 1H-ROESY NMR (500 MHz, C7D8, −78 °C) of the reaction between Cp*2Co and 4Me0TEMPO. Figure D.18: 1H NMR spectrum (400 MHz, C7D8, RT) of the reaction between octamethylcobaltocene (4 mg, 0.013 mmol, 1 eq) and 4MeOTEMPO (2.5 mg, 0.013 mmol, 1 eq). 274 In the ROESY NMR spectrum at −78 °C (Figure S17), there is clear chemical exchange between the different positions of the fulvene complex. The ROESY experiment is phase-sensitive, so you can observe both the through-space interaction between the axial and equatorial protons on the piperidin-1-oxyl ring, and the exchange coupling between all the positions that originate from the Cp*2Co species. The lack of through space interaction between the resonances from the 4MeOTEMPO-H and those from the (Cp*)Co-derived fragment suggest that they are not coupled. However, the exchange between the resonances attributed to (Cp*)Co(η4-C5Me4CH2) along with the broadening at room temperature indicates that the reaction between these two is reversible with two possibilities shown in Figure S15. The observation of similar reactivity between 4MeO TEMPO and octamethylcobaltocene (Figure S18) biases us towards the latter explanation but does not provide definitive evidence. This reversibility is not evident in acetonitrile. This may be because the O–H bond is stronger in this more polar solvents due to preferential hydrogen bonding interactions relative to the C–H bond of the methyl in Cp*2Co. It has been observed that the O–H bond of TEMPO–H is a little more than 1 kcal mol−1 stronger in acetonitrile than in benzene. Attempts to isolate the fulvene product from this reaction were precluded by the similar solubility of the two products and as had been previously observed, the relative instability of this complex at room temperature. Reaction of (Cp*)Co(exo-η4-C5Me5H) with CO2 is done as follows: (Cp*)Co(exoη4-C5Me5H) (2.0 mg, 0.006 mmol) is dissolved in MeCN-d3 and added to a J-Young NMR 275 tube. The J-Young tube is freeze-pump-thawed 3x and then backfilled with CO2 that is passed through a dry-ice/acetonitrile bath to one atmosphere of pressure. The reactions is then rotated for three days and a pale yellow color is obtained. A new 1H NMR reveals the formation of [Cp*2Co][HCO2] (Figure D.19). This product can be further confirmed by a thin film IR spectrum of the reaction (Figure D.38). Figure D.19: 1H NMR (400 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exo-η4C5Me5H) with one atmosphere of CO2 illustrating the formation of [Cp*2Co][HCO2] with the characteristic peak at 8.5 ppm for the formate proton. Reaction of (Cp*)Co(exo-η4-C5Me5H) with [Pt(dmpe)2][PF6]2 is done as follows: (Cp*)Co(exo-η4-C5Me5H) (1.8 mg, 0.0055 mmol) and [Pt(dmpe)2][PF6]2 (9.0 mg, 0.014 mmol, 4 equiv) are added as solids to a vial. They are then dissolved in MeCN-d3 and added to a J-Young NMR tube. They are then allowed to react for one day after which 1H NMR (Figure D.20) and 31P NMR (Figure D.21) revealed complete transfer of the hydride to the Pt to form [HPt(dmpe)2][PF6] and [Cp*2Co][PF6]. 276 Figure D.20: 1H NMR (400 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exo-η4C5Me5H) with 4 equivalents of [Pt(dmpe)2][PF6]2 illustrating the quantitative hydride transfer to form [HPt(dmpe)2][PF6]2. The signal at 1.82 consists of three different peaks, the methyl protons for the hydride product, the methylene protons for the Pt starting material, and also a residual peak from THF. The inset highlights the formation of the hydride peak. Figure D.21: 31P{1H} NMR (162 MHz, MeCN-d3, RT) of the reaction of (Cp*)Co(exoη4-C5Me5H) with 4 equivalents of [Pt(dmpe)2][PF6]2 illustrating the quantitative hydride transfer to form [HPt(dmpe)2][PF6]2. 277 D.5 Pulse EPR Spectroscopy Samples for pulse EPR spectroscopy were prepared as previously described.21 Figure D.22: Q-band inversion recovery traces of endo- (black) and exo-isomers (red) of Cp*2Co protonation acquired at 6 K (dashed lines) and 10 K (solid lines). Data for each species was acquired at the field corresponding to peak spectral intensity for that species. This illustrates the significantly faster relaxation properties of the endo-isomer. Accurate measurement of the T1’ in these species is technically challenging due to their significant g-anisotropy. This is because there are contributions to the relaxation from spectral diffusion in addition to explicit spin-lattice relaxation, as only a very small portion of the EPR spectral envelope is acted upon by the initial inversion pulse. Furthermore, the EPR signals of the exo-product, overlap with those of the endo-product. Thus a multiexponential function would be necessary to model its inversion recovery data and would thus not be well constrained. Nonetheless, a rough estimate of the T1’ can be obtained without fitting of exponential functions by using Eq D.8 for the longitudinal magnetization as a function of time in the inversion recovery experiment. 278 Eq D.7) Mz = 0, T ≈ ln(2)×T1’ This allows for the following estimates to be made. For the exo-species the T1’ is ~80 μs at 6 K and ~25 μs at 10 K. For the endo-species the T1’ is ~20 μs at 6 K and ~6 μs at 10 K. This clearly illustrates the significant difference observed in the longitudinal relaxation rates of the two species, which enables relatively clean pulse EPR data (particularly in the case of ENDOR) to be collected. Figure D.23: Pseudomodulated Q-band ESE-EPR spectrum at 10 K of a coal standard sample (seen as an intense, isotropic signal at 1200 mT) using the same Bruker Q-band D2 resonator as in this study. There are additionally three sharp signals present in the backround (1025 mT, 1205 mT, and 1418 mT) that arise from a trace paramagnetic species present in a component of the resonator, as well as a broad axial feature from a small amount of Cu(II) at 1184 mT. Two of these sharp background signals, marked with asterisks, are observed in the ESE-EPR spectra acquired at 10 K presented in the main text (Figure 2). The other signal is outside of the measured spectral window in this work. These features are much less intense relative to the signals derived from Cp*2Co-protonation at 6 K and thus are not observed in the spectra acquired at that temperature. 279 Table D.1: Simulation parameters derived from the ENDOR, HYSCORE, and CW-EPR spectroscopy. g-tensor A(1H) (MHz) A(59Co) (MHz) e2qQ/h for Co η (MHz) [(Cp*)Co(endo- [2.626, C5Me5H) ]+ 2.349, 1.984] [19.0, 15.0, 19.5] [245, 155, 187] 170 <0.1 [(Cp*)Co(exoC5Me5H)]+ [106.5, 112.5, 108.2] [15, 15, N.D. 225] N.D. Species [2.170, 2.085, 2.005] D.6 CW EPR Spectroscopy Experimental details for the reaction of Cp*2Co with HOTf have been reported previously21 but are repeated here for convenience. A 1 mL solution of HOTf (23 µL, 3.0 eq) in toluene and a 2 mL solution of Cp*2Co (40 mg, 1.0 eq) were chilled to −78 °C for ten minutes in a cold well. With strong stirring of the HOTf solution, the Cp*2Co solution was added dropwise over ten minutes. Purple precipitate could be observed upon the addition of each drop. After the completion of the addition, the reaction mixture was allowed to stir for 5 more minutes. The reaction was then vacuum filtered in the cold well through a medium porosity frit to yield a purple solid. This solid was then washed with toluene that had been prechilled to −78 °C (5 mL) and then likewise with prechilled pentane (15 mL). After drying on the frit for ten minutes the solid was then transferred to a prechilled vial. The solid was then dried under vacuum for several hours at −78 °C. This solid was then transferred to an EPR tube and passed out of the glovebox into a dewar filled with liquid nitrogen. 280 For reactions with DOTf, the same experimental protocol as above was used with the exception that DOTf was employed and in all cases where toluene had previously been used it was replaced with toluene-d8. For the reaction of Cp*2Co with HNTf2, decamethylcobaltocene (20 mg, 0.061 mmol) was weighed into a 20 mL scintillation vial and dissolved in 5 mL of toluene. Bis(trifluoromethanesulfon)imide (HNTf2) is weighed into a 20 mL scintillation vial and 5 mL of toluene is added and the vial is charged with a stir bar. Both solutions are cooled to −78 °c in the cold well for ten minutes. Then the Cp*2Co solution is added dropwise to the HNTf2 solution while it was being vigorously stirred over five minutes. The reaction mixture is allowed to stir for ten minutes during which it turns purple. Prechilled pentane (10 mL) is added to the toluene solution and the reaction is stirred for another five minutes. The reaction mixture is then filtered through a pre-chilled frit. The solid is then washed with pentane (5 x 2 mL). The solid is then transferred to a pre-chilled vial. The solid is then placed under vacuum for three hours to yield a pale purple solid. This solid is then transferred into a prechilled EPR tube and the cold well is then switched to a liquid nitrogen bath. The tube is allowed to cool for ten minutes before it is then quickly transferred out of the box and into a dewar with liquid nitrogen (Figure D.24). 281 Figure D.24: X-Band CW EPR spectrum (blue) at 77 K in the solid state of the precipitate from the reaction of bis(trifluoromethanesulfon)imide) with Cp* 2Co at −78 °C in toluene. Simulation (green) of this data using the parameters from Table D.1 with a weight of 0.3 on the on the endo-isomer and a weight of 1.0 on the exo-isomer. Both of the protonation reactions are under kinetic control due to the rapid precipitation upon proton transfer. Thus the relative energies of either the transition state or a pre-equilibrium must be affected by the steric bulk of the acid. We believe that this likely due to a hydrogen bonding interaction between the acid and the π-electrons of the Cp* ring. Such hydrogen bonds have previously been observed experimentally by IR and NMR for ferrocene,32,33 and we have observed computationally the formation of hydrogen bonded interactions between anilinium triflates and Cp*2Co preceding proton transfer.34 In this model, the thermodynamics of hydrogen bonding for the smaller HOTf reflect those of the isolated products and thus endo-protonation is preferred. With the bulkier HNTf2, steric clash with the opposite Cp* ring leads to destabilization of this endo-hydrogen bond and thus exohydrogen bonding and consequently exo-protonation is favored. 4 For oxidation of (Cp*)Co(exo-η -C5Me5H) with HBArF4, 282 (Cp*)Co(exo-η 4 C5Me5H) (20 mg, 0.061 mmol) was dissolved in pentane and cooled to −78 °C. It was then added to a 50 mL round bottom flask with a large stir bar that was also chilled to −78 °C after it was charged with HBArF4 (122.6 mg, 0.12 mmol, 2 equiv.). This was then stirred for several hours during which a pale purple solid was seen to form in the reaction. This can then be filtered through a pre-chilled frit and washed with pre-chilled pentane (5 x 2 mL). The solid was then transferred to a pre-chilled EPR tube. The cold well was then swapped for one that contained liquid nitrogen and the solid was allowed to equilibrate to 77 K. The tube was then quickly passed out of the glovebox into a dewar containing liquid nitrogen (Figure D.25). For oxidation of (Cp*)Co(exo-η4-C5Me5D) with HBArF4, the same procedure as above was used except now (Cp*)Co(exo-η4-C5Me5D) was used (Figure D.25). Figure D.25: A comparison of the 77 K, X-band EPR spectra of solid samples of [(Cp*)Co(exo-η4-C5Me5H)][BArF] (blue) and [(Cp*)Co(exo-η4-C5Me5D)][BArF] (orange). 283 Figure D.26: A depiction of our mechanistic proposal to account for observations upon the reaction of (Cp*)Co(exo-C5Me5D) with HBArF4. Efforts to identify the bracketed CoIII– H intermediate at low temperature using NMR spectroscopy always resulted in the sole observation of the terminal oxidation product Cp*2Co+. However, we believe that formation of this species is reasonable on the basis of DFT which suggests that the most favorable site of protonation (endo, exo, or Co) is the Co (see Table S5). This proposal is supported by the known protonation of 18e− Cp-CoI species to form hydrides.35,36 DFT calculations suggest that this Co–H bond is homolytically weak (BDFECo–H = 51 kcal mol−1). Thus it should be able to react with itself or with the starting material to release H2 or HD bimolecularly and form the experimentally observed product, [(Cp*)Co(exo-C5Me5D)Co]+. It is quite possible that the precipitated solid is contaminated with the CoIII–H species, which we would also expect to be fairly insoluble under the reaction conditions, but due to the expected diamagnetic nature of this species we would not observe it by EPR techniques. 284 Figure D.27: A comparison of the data to the simulation (green) for the 77 K, X-band EPR spectra of solid samples of [(Cp*)Co(exo-η4-C5Me5H)][BArF] (blue) and [(Cp*)Co(exoη4-C5Me5D)][BArF] (orange). Simulation parameters are the same in both cases (Table D.1 Table ) except for A(1H) being scaled down by the ratio of the gyromagnetic ratio for proton:deuterium (~6.5) in the simulation for [(Cp*)Co(exo-η4-C5Me5D)][BArF] (bottom). We believe that the small deviation of the observed EPR spectra from the simulations can be accounted for by the fact that there is likely a significant difference in hydrogen bonding with the [BArF4]− counteranion as compared to the [OTf]− counteranion for which the simulation parameters were derived using pulse EPR. 285 Figure D.28: X-Band CW EPR spectrum (blue) at 77 K in the solid state of the precipitate from the reaction of trifluoromethanesulfonic acid with Cp* 2Co at −78 °C in toluene.21 Simulation (green) of this data using the parameters from Table D.1 Table with a weight of 1.0 on the species measured at 6 K (endo) and a weight of 0.1 on the species measured at 10 K (exo). D.7 Electrochemistry 3.00E-05 2.00E-05 1.00E-05 I (A) 0.00E+00 -1.00E-05 -2.00E-05 -3.00E-05 -4.00E-05 -5.00E-05 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 Potential (V vs Fc+/0) Figure D.29: Cyclic voltammogram of (Cp*)Co(η4-C5Me6) at room temperature in an acetonitrile solution of 200 mM [TBA][PF6] with ferrocene added as an internal reference. From this a potential of −0.61 V is determined for the [(Cp*)Co(η4-C5Me6)]0/+ couple. This potential is shifted by ~0.05 V from that previously reported in DCM (−0.21 V vs SCE in 286 37 DCM and −0.16 V vs SCE in DME. This small shift is attributed to going to the more coordinating acetonitrile solvent. 1.70E-04 10 mV/sec 50 mV/sec 250 mV/sec 1000 mV/sec 5000 mV/sec 1.20E-04 7.00E-05 25 mV/sec 100 mV/sec 500 mV/sec 2000 mV/sec 10000 mV/sec I (A) 2.00E-05 -3.00E-05 -8.00E-05 -1.30E-04 -1.80E-04 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 +/0 Potential (V vs Fc ) Figure D.30: Scan rate dependence of the [(Cp*)Co(η4-C5Me6)]+/0 at room temperature in an acetonitrile solution of 200 mM [TBA][PF6]. This demonstrates the high reversibility of this couple in contrast to what is observed with (Cp*)Co(exo-η4-C5Me5H) 2.00E-04 1.50E-04 0.1 V/s 1 V/s I (A) 1.00E-04 5.00E-05 0.00E+00 -5.00E-05 -1.00E-04 -1.50E-04 0 -0.5 -1 -1.5 -2 -2.5 -3 +/0 Potential (V vs Fc ) Figure D.31: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at room temperature at different scan rates in a butyronitrile solution of 0.4 M [TBA][PF6]. The quasi-reversible 287 oxidation event at ~−0.6 V corresponds to the (Cp*)Co(exo-η -C5Me5H) couple while the highly reversible redox couple at ~−1.9 V corresponds to the known Cp*2Co0/+ couple observed due to decomposition of [(Cp*)Co(exo-η4-C5Me5H)]+ to form [Cp*2Co]+. There is 4 0/+ 1.80E-04 8.00E-05 I (A) -2.00E-05 -1.20E-04 1 V/s 2 V/s -2.20E-04 5 V/s -3.20E-04 0 -0.5 -1 -1.5 -2 Potential (V vs Fc+/0) the onset of another apparently irreversible reduction event apparent at even more negative potentials. Although this was not explored thoroughly it is in agreement with reports on Cp(η4-C5H5R)Co complexes (R = alkyl or H) which were found to undergo an irreversible reduction at negative potentials.38 Figure D.32: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at room temperature at different scan rates in a butyronitrile solution of 0.4 M [TBA][PF6]. These demonstrate the increasing reversibility of the couple at extremely high scan rates (maximum ic/ia ~0.62). From the highest scan rate we are able to determine an oxidation potential for (Cp*)Co(exoη4-C5Me5H) of −0.62 V in good agreement with that observed for (Cp*)Co(η4-C5Me6). 288 3.00E-06 1.00E-06 I (A) -1.00E-06 -3.00E-06 100 mV/s -5.00E-06 250 mV/s -7.00E-06 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 -1.4 Potential (V vs Fc+/0) Figure D.33: Cyclic voltammograms of (Cp*)Co(exo-η4-C5Me5H) at −78 °C at different scan rates in a butyronitrile solution of 0.4 M [TBA][PF6]. We can see that a degree of reversibility is achieved at lower scan rates than are necessary at room temperature. A comparison is shown in Figure S15. Attempts to go to the higher scan rates that were used at room temperature were not successful likely due to the increased resistivity at low temperature. Scaled Current 0.4 -0.1 -0.6 RT -1.1 0.4 -0.1 -0.6 -1.1 -1.6 Potential (V vs Fc+/0) Figure D.34: Comparison of cyclic voltammograms taken at room temperature (~22 °C) and at −78 °C. Both voltammograms are taken at a scan rate of 1 V/s with a concentration of analyte of 1 mM and 0.4 M [TBA][PF6] in butyronitrile. This demonstrates that cooling 289 the cell leads to a significant enhancement of reversibility at a particular scan rate. In order to facilitate the comparison each of the currents have been scaled by dividing them by the absolute value of the peak cathodic current. It is apparent here that the redox couple appears to drift to a slightly more positive potential of −0.56 V when measured at −78 °C. This drift in potential is not uncommon and only represents a ΔBDFE = 23.06 × 0.05 = 1.4 kcal mol−1. We have used the potential measured at room temperature in our thermodynamic calculations in the main text as that represents the standard state. 2.50E-04 I (A) 1.50E-04 5.00E-05 -5.00E-05 -1.50E-04 0 -1 -2 -3 -4 Potential (V vs Fc+/0) Figure D.35: Cyclic voltammograms of Cp*2Co at room temperature in 0.1 M [TBA][PF6] dissolved in THF. The spectrum is referenced to the Cp*2Co reduction potential in MeCN of −1.91 V as a reference. After scanning through the reduction of Cp*2Co you can then observe the irreversible feature at ~−0.6 V associated with Cp*(η4-C5Me5H)Co0/+. This suggests that there is a small amount of available proton source, likely water. D.8 X-ray Crystallography Table D.2: Crystallographic details for (Cp*)Co(exo-η4-C5Me5H) and (Cp*)Co(η4-C5Me6). Compound (Cp*)Co(exo-η4-C5Me5H) Cp*(C5Me6)Co Chemical Formula C20H31Co C21H33Co Formula Weight 330.38 344.43 Crystal System Monoclinic Monoclinic Space Group P21/n P21/c a [Å] 8.0700(6) 12.8973(5) 290 b [Å] 8.4605(6) 9.6767(4) c [Å] 12.8684(10) 15.1146(6) α [°] 90 90 β [°] 93.611(2) 92.956(2) γ [°] 90 90 V [Å3] 876.86(11) 1883.84(13) Z 2 4 Dcalcd [g cm-1] 1.251 1.214 F000 356.0 984.0 μ [mm-1] 0.971 0.93 Temperature [K] 100(2) 100(2) Wavelength [Å] 0.71073 0.71073 Measured Reflections 38760 117581 Unique Reflections 5620 8739 Data/Restraints/Parameter 5620/0/115 8739/0/210 R(F) (I>2σ(I)) 0.0382 0.0471 wR(F2) (all) 0.0838 0.0897 GOOF 1.016 1.045 s 291 D.9 IR Spectroscopy Figure D.36: Thin film IR spectra of (Cp*)Co(exo-η4-C5Me5H) (blue) and (Cp*)Co(exoη4-C5Me5D) (orange). C–H vibrations at 2708 cm−1 and 2612 cm−1 shift to 2005 cm−1 and 1962 cm−1. The predicted shift for deuterium substitution based on a simple harmonic oscillator model would be to 1987 cm−1 and 1917 cm−1. It is unclear why there are two stretches visible for this C–H/D mode. It is clear from solution NMR studies that these molecules consist of only one regioisomer (exo with no endo- contamination) but this phenomenon persists in the solution IR (Figure S37). Similar behavior was observed for (Cp)Co(η4-C5H6) and (Cp)Co(exo-η4-C5H5D) by Wilkinson and no regioisomerism is possible in that molecule.39 We have not found other examples of Cp−H or Cp*−H species in the literature that have been characterized by IR spectroscopy. We suggest that the different stretches arise from conformational isomers relating to the eclipsed or staggered nature of the ring that are causing the different stretching frequencies. This has been observed before for Cp* species of M−H.40 292 Figure D.37: Solution state IR spectra of (Cp*)Co(exo-η4-C5Me5H) (blue) and (Cp*)Co(exo-η4-C5Me5D) (orange) in C6D6. This spectra demonstrates the same behavior as the thin film IR does demonstrating that the observed behavior is not only a solid state phenomenon. Figure D.38: Thin film IR spectrum of the reaction between (Cp*)Co(exo-η4-C5Me5H) and CO2 demonstrating the formation of [Cp*2Co][HCO2]. The features demonstrated here are similar to those previously characterized for tetramethylammonium formate.41 In particular the unusually low C–H stretch (2666 cm−1) and the C=O (1626 cm−1) stretch are diagnostic for this species. 293 D.10 Thermochemistry [Cp2*Co]+ H ( G(H )) <41 kcal mol 1 e (E0) 1.96 V [Cp*( 4-C5Me5H)Co]+ Cp*2Co 0 e (E ) 3.5 V [Cp2*Co]- <6 H 5k (B ca l DF E mo ) l 1 e (E0) 0.62 V Cp*( 4-C5Me5H)Co Figure D.39: Measured thermochemical parameters in acetonitrile. Potentials in V vs Fc+/0. Figure D.40: Describes the process of determining the C–H BDFE in [(Cp*)Co(exo-η4C5Me5H)]+ using the measured hydricity of (Cp*)Co(exo-η4-C5Me5H) and the redox potential of (Cp*)Co(exo-η4-C5Me5H). There is some disagreement over the hydricity of [HPt(dmpe)2][PF6] in the literature so we have chosen to use the more conservative value of ΔG(H−) = 41 kcal mol−1.42,43 294 Figure D.41: Describes the process of determining the C–H BDFE in [(Cp*)Co(exo-η4C5Me5H)]+ using the measured BDFE C–H of (Cp*)Co(exo-η4-C5Me5H) and the redox potential of (Cp*)Co(exo-η4-C5Me5H) and Cp*2Co. D.11 DFT Calculations D.11.1 Comparison of Isomer Stability Table D.3: Thermochemistry for the structures optimized with different functionals. In the B3LYP, TPSSH, and BP86 structures the Co–H has a small negative frequency which could affect the relative energies but given the magnitude likely would not affect the relative ordering. Functional for Optimization TPSSH TPSSH TPSSH BP86 BP86 BP86 B3LYP B3LYP B3LYP TPSS TPSS Species Spin State Co–H endo-C–H exo-C–H Co–H endo-C–H exo-C–H Co–H endo-C–H exo-C–H Co–H Co–H S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 1/2 S = 3/2 Absolute Energy (Hartrees) −2162.647 −2162.676 −2162.673 −2162.773 −2162.800 −2162.796 −2161.879 −2161.913 −2161.911 −2162.785 −2162.725 Relative Energy (kcal mol−1) 18.42 0 1.82 16.93 0 2.10 20.91 0 1.02 15.72 46.73 295 TPSS TPSS TPSS TPSS endo-C–H endo-C–H exo-C–H exo-C–H S = 1/2 S = 3/2 S = 1/2 S = 3/2 −2162.810 −2162.746 −2162.809 −2162.744 0 33.61 0.51 34.92 Table D.4: Thermochemistry for the different potential isomers of neutral, Hfunctionalized Co species. Note that both of the Co–H structures have small imaginary frequencies (~−80 cm−1) indicating that they may be transition states representing transfer of the hydride between the two rings but they have been included here for completeness. Species (Cp*)Co(exo-η4C5Me5H) (Cp*)Co(exo-η4C5Me5H) (Cp*)Co(endo-η4C5Me5H) (Cp*)Co(endo-η4C5Me5H) Cp*2Co–H Cp*2Co–H Spin State S=0 Absolute Energy (Hartrees) −2162.995 Relative Energy (kcal mol−1) 0 S=1 −2162.954 26.18 S=0 −2162.995 0.22 S=1 −2162.955 25.29 S=0 S=1 −2162.940 −2162.940 34.44 34.46 Table D.5: Thermochemistry for the different potential isomers resulting from the protonation of (Cp*)Co(exo-η4-C5Me5H). Species Spin State [(Cp*)Co(exo-η4-C5Me5H)–H]+ [(Cp*)Co(exo-η4-C5Me5H)–H]+ [(exo-η4-C5Me5H)Co(endo-η4-C5Me5H)]+ [(exo-η4-C5Me5H)Co(endo-η4-C5Me5H)]+ [(exo-η4-C5Me5H)Co(exo-η4-C5Me5H)]+ [(exo-η4-C5Me5H)Co(exo-η4-C5Me5H)]+ S=0 S=1 S=0 S=1 S=0 S=1 Absolute Relative Energy Energy (kcal mol−1) (Hartrees) −2163.381 0 −2163.354 17.34 −2163.358 14.38 −2163.342 24.60 −2163.357 15.18 −2163.340 26.12 D.11.2 Determination of Thermochemical Parameters via DFT To determine the DFT-predicted gas-phase BDFE’s (Main Text Figure 1) we followed a protocol that has been published previously.44 In this protocol, the ΔG for a series 296 of small molecules with known gas-phase BDFE’s are calculated (Eq D.8). A correlation between ΔG and the BDFE is then established (Eq D.9). This allows for accurate gas-phase BDFE’s to be calculated. Because of the large differences between how acetonitrile solvation effects X–H bond strengths45 no efforts have been made to calculate solution state BDFE values but it can be anticipated that they would be stronger (5-10 kcal mol−1) than those in the gas phase with larger errors being encountered for more acidic species in which Hbonding plays a more noticeable role. Eq D.8) ΔG = (G(X•) + G(H•)) – G(X–H) Eq D.9) BDFE = 0.996 × ΔG + 4.376 To determine redox potentials, Ferrocene and ferrocenium were optimized using the same input parameters used for all of the thermochemical calculations. By creating a square scheme and taking the known Fc+/0 couple as 0 V we were then able to determine the DFTpredicted redox potential. The energies used in this case were after the solvation correction as described in the general methods. To determine the gas-phase pKa we again used a square scheme by calculating the ΔG for a conjugate acid/base pair with a known pKa in acetonitrile. All calculated ΔG values could then be referenced using this to generate an absolute pKa value. We chose aniline and anilinium as our pair due to its catalytic relevance. The energies used in this case were after the solvation correction as described in the general methods. To determine the solution-phase hydricity we again used a square scheme by calculating the ΔG for a hydride donor/acceptor pair with a known hydricity in acetonitrile. 297 In this case we chose CO2 and formate given their relevance to this report. Based on the ΔG of this reaction, we then determined the hydricity of the desired compound. The energies used in this case were after the solvation correction as described in the general methods. Table D.6: Gas phase Gibbs Free Energies for species necessary for determining the thermochemical parameters. Species Cp*2Co− (S = 0) Cp*2Co− (S = 1) Cp*2Co Cp*2Co+ Cp*(η4-C5Me4CH2)Co Cp2Fe Cp2Fe+ Aniline Anilinium CO2 Formate H• Cp*2Ni+ Cp*2Ni2+ [(Cp*)Ni(exo-η4-C5Me5H)]+ [(Cp*)Ni(exo-η4-C5Me5H)]2+ Energy (Hartrees) −2162.408 −2162.384 −2162.409 −2162.258 −2161.81 −1650.623 −1650.37969 −287.363 −287.714 −188.484 −189.069 −319.972 −2287.758 −2287.426 −2288.343 −2287.977 Table D.7: Particular thermochemical properties of interest. Species [Cp*2Co]− Cp*2Co (Cp*)Co(exo-η4C5Me5H) (Cp*)Co(endo-η4C5Me5H) (Cp*)2Co(H) [(Cp*)Co(exo-η4C5Me5H)]+ BDFE (kcal mol−1) N/A 62.4 52.6 Oxidation Potential (V vs Fc+/0) −3.28 −2.16 −0.88 Hydricity (kcal mol−1) pKa N/A N/A 37 N/A N/A 49.8 52.3 −0.98 37 49.6 7.3 22.1 −1.63 N/A 4 N/A 25.2 9.3 4 [(Cp*)Co(endo-η C5Me5H)]+ [(Cp*)2Co(H)]+ [(Cp*)Ni(exo-η4C5Me5H)]+ [(Cp*)Ni(exo-η4C5Me5H)]2+ 298 24.2 N/A N/A 10.8 18.3 51.4 N/A N/A N/A N/A −2.6 N/A 30.1 N/A N/A N/A D.11.3 EPR Predictions by DFT Table D.8: Effect of functional on structure optimization. EPR parameters calculated for structures optimized with different functionals but the same basis sets (def2-SVP(C and H)/def2-TZVP(Co)). Each structure was then used as the input for the calculation of EPR parameters using the same exact conditions (TPSSH/def2-TZVP (C and H)/CP(PPP) (Co)). More detail provided in the general methods section. Functional Species Ax Ay Az aiso Tx Ty Tz ΔT for Optimization TPSSH Co–H −62.3 −70. −15.8 −49.7 −12.7 −21.2 33.9 55.1 9 TPSSH endo29.4 41.4 26.7 32.5 −3.1 8.9 −5.8 14.7 C–H TPSSH exo-C– 110.5 117.0 111.6 113.1 −2.5 3.9 −1.41 6.5 H BP86 Co–H −62.5 −70.4 −15.8 −49.6 −13.0 −20.8 33.78 54.6 BP86 endo27.7 39.3 25.2 30.8 −3.0 8.6 −5.59 14.2 C–H BP86 exo-C– 121.9 116.9 115.8 118.2 3.8 −1.3 −2.45 6.2 H B3LYP Co–H −64.2 −72.0 −15.4 −50.5 −13.7 −21.5 35.16 56.5 B3LYP endo25.4 28.3 40.4 31.4 −5.9 −3.1 9.04 15.0 C–H B3LYP exo-C– 100.8 95.5 94.6 97.0 3.9 −1.4 −2.42 6.3 H TPSS Co–H −62.6 −70.5 −16.3 −49.8 −12.8 −20.7 33.50 54.2 TPSS endo28.6 40.3 26.0 31.6 −3.0 8.7 −5.67 14.4 C–H TPSS exo-C– 120.1 114. 113.8 116.3 3.9 −2.5 −1.37 6.4 H 9 Table D.9: Effect of functional on the calculation of EPR parameters. Structures were optimized using (TPSS/def2-SVP(C and H)/def2-TZVP(Co)). Each structure was then used 299 as the input for the calculation of EPR parameters using different functionals but otherwise the same conditions (def2-TZVP (C and H)/CP(PPP) (Co)). Functional Species Ax Ay Az aiso Tx Ty Tz ΔT for EPR Calculation TPSSH Co–H −62.6 −70.5 −16.3 −49.8 −12.8 −20.7 33.5 54.2 TPSSH endo28.6 40.3 26.0 31.6 −3.0 8.7 −5.7 14.4 C–H TPSSH exo-C– 120.1 114.9 113.8 116.3 3.9 −2.5 −1.4 6.4 H BP86 Co–H −49.2 −57.5 −8.8 −38.5 −10.7 −19.0 29.7 48.7 BP86 endo30.3 40.1 26.9 32.4 −2.1 7.7 −5.5 13.2 C–H BP86 exo-C– 118.9 112.8 111.6 114.5 4.5 −1.6 −2.8 7.3 H B3LYP Co–H −91.9 −99.9 −40.5 −77.4 −14.4 −22.5 36.9 59.4 B3LYP endo26.9 38.9 24.1 29.9 −3.1 8.9 −5.4 14.8 C–H B3LYP exo-C– 99.9 107.3 101.6 102.8 −2.8 4.5 −1.7 7.4 H TPSS Co–H −45.0 −53.0 −4.2 −34.1 −10.9 −18.9 29.9 48.8 TPSS endo30.8 41.2 27.8 33.3 −2.5 7.9 −5.5 13.5 C–H TPSS exo-C– 128.2 123.0 121.8 124.4 3.9 −1.4 −2.5 6.4 H We can see when we optimize the isomers with different functionals and then do a single point EPR prediction under the same conditions that the results are nearly identical (Table D.8). When we use different functionals for the single point calculation we see larger differences (Table D.9). However, the ΔT and the large disparity in the aiso (aiso = (Ax + Ay + Az)/3) for the endo- and exo-isomer persist regardless. The performance of TPSSH in predicting the magnitude of aiso was the best so it was selected for use at the higher level of theory (Table D.10). In Table D.10, it can be seen that aiso and ΔT are again similar. However, at this level of theory we now also capture that |Ty| is larger than |Tx| or |Tz| in accordance with 300 experiment. The absolute signs of the individual T components are not relevant because we have not determined the absolute sign of the 1H hyperfine tensors experimentally. Switching the sign of the hyperfine tensor would switch all of the signs of the components of T as well. Table D.10: Higher level EPR calculations. Structures were optimized using (TPSS/def2SVP(C and H)/def2-TZVP(Co). Then a single point calculation was done with CP(PPP) on Co and EPR-III on C and H with grid 7. Functional Species Ax Ay Az aiso Tx Ty Tz ΔT for EPR Calculation TPSSH Co–H −64.2 −72.0 −17.8 −51.3 −12.9 −17.7 33.6 51.3 TPSSH endo28.3 40.1 25.8 31.4 −3.1 8.7 −5.6 14.3 C–H TPSSH exo-C– 116.3 122.6 117.4 118.8 −2.4 3.8 −1.3 5.1 H D.12 References (1) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882–1893. (2) Koelle, U.; Khouzami, F. Angew. Chem. Int. Ed. 1980, 19, 640–641. (3) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341– 5350. (4) Manson, J. L.; Buschmann, W. E.; Miller, J. S. Inorg. Chem. 2001, 40, 1926–1935. (5) Cazelles, J.; Robert, A.; Meunier, B. J. Org. Chem. 2002, 67, 609–619. (6) Hyde, J. S.; Pasenkiewicz-Gierula, M.; Jesmanowicz, A.; Antholine, W. E. Appl. Magn. Reson. 1990, 1, 483–496. (7) Stoll, S.; Britt, R. D. Phys. Chem. Chem. Phys. 2009, 11, 6614–6625. (8) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3–8. 301 (9) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. a. K.; Puschmann, H. J. Appl. Cryst. 2009, 42, 339–341. (10) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73–78. (11) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8, e1327. (12) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401–146404. (13) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (14) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104– 154119. (15) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (16) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (17) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (18) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (19) Staroverov, V. N.; Scuseria, G. E.; Tao, J.; Perdew, J. P. J. Chem. Phys. 2003, 119, 12129–12137. (20) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (21) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217–223. (22) Neese, F. Inorg. Chim. Acta 2002, 337, 181–192. (23) Barone, V. Recent Advances in Density Functional Methods; Recent Advances in Computational Chemistry; WORLD SCIENTIFIC, 1995; Vol. Volume 1, pp 287– 334. (24) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 5, 799–805. 302 (25) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995–2001. (26) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669–681. (27) Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2011, 115, 14556–14562. (28) Wang, T.; Brudvig, G.; Batista, V. S. J. Chem. Theory Comput. 2010, 6, 755–760. (29) Wu, A.; Mader, E. A.; Datta, A.; Hrovat, D. A.; Borden, W. T.; Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 11985–11997. (30) Ohki, Y.; Murata, A.; Imada, M.; Tatsumi, K. Inorg. Chem. 2009, 48, 4271–4273. (31) Bauer, J.; Braunschweig, H.; Hörl, C.; Radacki, K.; Wahler, J. Chem. - Eur. J. 2013, 19, 13396–13401. (32) Yoshida, Z.; [UNK]Osawa, E. J. Am. Chem. Soc. 1966, 88, 4019–4026. (33) Cerichelli, G.; Illuminati, G.; Ortaggi, G.; Maria Giuliani, A. Journal of Organometallic Chemistry 1977, 127, 357–370. (34) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6122–6129. (35) Werner, H.; Lippert, F.; Peters, K.; Schnering, H. G. V. Chem. Ber. 1992, 125, 347– 352. (36) Fang, M.; Wiedner, E. S.; Dougherty, W. G.; Kassel, W. S.; Liu, T.; DuBois, D. L.; Bullock, R. M. Organometallics 2014, 33, 5820–5833. (37) Koelle, U. Inorg. Chim. Acta 1981, 47, 13–18. (38) Murr, N. E.; Laviron, E. Can. J. Chem. 1976, 54, 3350–3356. (39) Green, M. L. H.; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1959, 0, 3753–3767. 303 (40) Belkova, N. V.; Epstein, L. M.; Filippov, O. A.; Shubina, E. S. Spectroscopic Properties of Inorganic and Organometallic Compounds; 2012; pp 1–28. (41) Albert, B.; Jansen, M. Z. Anorg. Allg. Chem. 1995, 621, 1735–1740. (42) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999, 121, 11432–11447. (43) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem. Soc. 2002, 124, 1918–1925. (44) Matson, B. D.; Peters, J. C. ACS Cat. 2018, 8, 1448–1455. (45) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961–7001. 304 Appendix E. Supplementary Information for Chapter 6 305 E.1 Experimental Details E.1.1 General Considerations All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with N2 followed by passage through an activated alumina column in a solvent purification system by SG Water, USA LLC. Non-halogenated solvents were tested with sodium benzophenone ketyl in tetrahydrofuran (THF) in order to confirm the absence of oxygen and water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3-Å molecular sieves prior to use. Cobaltocenium hexafluorophosphate, trifluoromethanesulfonimide, triflic acid and tetrabutylammonium hexafluorophosphate were all used as purchased. Ferrocenium triflate1 and 4-lithiodimethylaniline2 were synthesized as described previously. E.1.2 Computational Details All calculations were performed using the ORCA 4.0 program. 3,4 In cases where crystal structures were available these coordinates were used as the input. The calculations were performed using the TPSS (meta-GGA)5 functional with def2-TZVPP6,7 on all atoms, Grimme-d3 dispersion correction,8 and SMD9 solvent modelling acetonitrile. That optimized structures represented true stationary points was checked by doing a single-point frequency calculations on the optimized structure and ensuring that there were no negative frequencies (≤ −50 cm−1). Reduction potentials were determined via exchange reactions with ferrocene/ferrocenium. The pKa values were determined via exchange reactions with 2- 10 306 chloroanilinium/2-chloroaniline. The hydricity values were determined via exchange reactions with CO2/HCO2− (formate).11 The bond dissociation free energies (BDFEs) were calculated directly using the free energy of H∙. E.1.3 EPR Spectroscopy X-band (9.4 GHz) CW EPR spectra were acquired using a Bruker EMX spectrometer equipped with a Super High-Q (SHQE) resonator using Bruker Win-EPR software (ver. 3.0). Spectra were acquired at 77 K using a vacuum-insulated quartz liquid nitrogen immersion dewar inserted into the EPR resonator. E.1.4 X-ray Crystallography XRD studies were carried out at the Beckman Institute Crystallography Facility on a Bruker Kappa Apex II diffractometer (Mo Kα radiation). Structures were solved using SHELXS or SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL.12 All of the solutions were performed in the Olex2 program. 13 The crystals were mounted on a glass fiber under Paratone N oil. E.1.5 Electrochemistry A CHI instruments 600B electrochemical analyzer was used for data collection. Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV) and Differential Pulse Voltammetry (DPV) experiments were carried out in a one-compartment three-electrode cell using a glassy carbon (GC) disk or a boron doped diamond (BDD) disk as the working electrode (3 mm diameter), a Pt wire as the counter electrode, and a Ag/AgOTf (5 mM) 307 reference electrode. Details for the CVs and LSVs are noted as they appear. DPVs were obtained with the following parameters: amplitude= 50 mV, step height=4 mV, pulse width= 0.05 s, pulse period= 0.5 s and sampling width= 0.0167 s. E 1/2 values for the reversible waves were obtained from the half potential between the oxidative and reductive peaks, and the one for irreversible processes are estimated according to the potential at the Imax in DPV measurements. All the measurements were done applying IR compensation at 85% of the total resistance. All the reported potentials are referenced to the ferrocenium/ferrocene couple (Fc+/0) used as an internal standard. Controlled potential electrolyses (CPEs) were carried out in a gas-tight single compartment cell using a BDD plate (dimensions 1x2 cm) working electrode, a Ag/AgOTf (5 mM) reference electrode, and a GC plate (dimensions 1x5 cm) counter electrode. GC electrodes were pre-treated by polishing with 1, 0.3 and 0.05 μm alumina paste followed by rinsing with water and acetone. The BDD electrode was pre-treated according to a literature procedure.14 A fresh DME solution of 5 mM AgOTf was used in the reference electrode for each experiment. In a typical CPE experiment, 7 ml of a 200 mM of [TBA][PF 6] DME solution was added to the gas-tight single compartment electrochemical cell. Then 120.5 mg of toluenesulfonic acid (100 mM) was added and a CV’s were performed to ensure a stable HER background. Then, 40 μL of acetophenone (50 mM) was added. For the experiments using a catalyst, either 3.2 mg of [CpCpNCo][OTf] or 2.3 mg of [CpCpCo][PF6] were then 308 added to generate a 1 mM solution of the catalyst. The solution was then electrolyzed at −1.45 V vs Ag+/0 with stirring throughout the CPE. E.1.6 Analysis of Products from CPE Upon completion of the CPE, the headspace was analyzed for H 2 via gas chromatography with a thermal conductivity detector (GC-TCD). A 100 μL Hamilton syringe was used to sample the headspace and for the injection into the GC-TCD. GC-TCD was performed in the Environmental Analysis Center (Caltech) using a HP 5890 Series II instrument with N2 as carrier gas. Calibration curve was obtained by direct injection of hydrogen (Figure E.35). The solution was then analyzed via electrochemical methods (CV and DPV) with the BDD plate working electrode and in some cases additionally with a glassy carbon disk electrode (3 mm diameter) prior to working up. The work up procedure consisted first of quenching the solution with an ethereal 2 M HCl solution. This step was found to be necessary to help remove the acid, not to pronate the pinacol product. After 5 min of stirring, the solvent was then removed under reduced pressure with a rotavap until dryness. Then, the organics products were extracted from the resulting solid by extraction with ether (3x10 ml) and filtration through a medium frit. The resulting filtrate was washed with an aqueous solution containing 100 mM Na2CO3 (20 ml). This aqueous phase was then further washed with ether (5x20 ml) and the organic phases were combined and evaporated until dryness. The resulting solid was extracted with 4 ml of ethyl acetate and filtered through filter paper. The resulting filtrate was used as analysis solution for detection and 309 quantification of products and remaining substrate. We verified that work-up via this procedure of a DME solution containing tosic acid, pinacol, ketone, and [TBA][PF 6] led to >95% recovery of the pinacol and ketone. To the ethyl acetate solution was then added dodecane as an internal standard and the solution was then analyzed via gas chromatography coupled to either mass spectrometry (GC-MS, for products identification) or flame ionization detector (GC-FID, for product identification and quantification). The products were verified by analysis of the compounds (acetophenone, 1-phenylethanol, and 2,3-diphenyl-2,3-butanediol) obtained from commercial sources. Calibration curves (GC-FID, Figure E.36-Figure E.37) were obtained by comparing integration areas of known concentrations of the pure compounds with dodecane which was added as an internal standard. GC-MS was performed in an Agilent 5975C instrument in the Center for Catalysis and Chemical Synthesis (Caltech). GC-FID was performed using an Agilent 6850 instrument equipped with an Agilent HP-5 5% phenyl methyl siloxane capillary column (J&W Scientific). E.1.7 IR Spectroscopy Spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. E.1.8 UV-Vis Spectroscopy Measurements were taken on a Cary 50 UV/Visible spectrophotometer using a 1 cm quartz cell connected to a round-bottom flask and sealed with a Teflon stopcock. 310 Variable temperature measurements were collected with a Unisoku CoolSpek cryostat mounted within the Cary spectrophotometer. The 2-MeTHF:THF mixture was used to achieve low temperature solutions that are mixable as has been noted previously. 15 E.1.9 NMR Spectroscopy 1 H and 13C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. Solution phase magnetic measurements were performed by the method of Evans.16 E.1.10 X-ray Photoelectron Spectroscopy The BDD working electrode used in the CPE experiment was analyzed by X-ray photoelectron spectroscopy using a Kratos AXIS Ultra XPS instrument with a x-ray source consisting in monochromatic Al K alfa line at 1486.6 eV, with 0.2 eV resolution at full width half maximum. XPS results were analyzed using CasaXPS, Casa Spftware Ltd. The XPS instrument was calibrated to the Au 4f7/2 peak at 84 eV. Samples were calibrated to the adventitious carbon peak at 284.5 eV. E.2 Synthetic Details To synthesize (Cp)Co(η4-C5H5PhNMe2) a suspension of [Cp2Co][PF6] (399 mg, 1.2 mmol) was stirred in THF (6 mL) at −78 °C. To this was added dropwise a room temperature, THF (10 mL) solution of 4-lithiodimethylaniline (152 mg, 1.2 mmol, 1 eq.) causing the reaction mixture to go orange. The suspension was allowed to stir at −78 °C for three hours and then allowed to warm slowly to room temperature at which point the 311 reaction became homogeneous. The reaction was then evaporated to dryness and the solid was extracted with pentane (3x 100 mL). The pentane was then filtered through an alumina plug and the solution was evaporated. (Yield: 148 mg, 40%) 1 H NMR (C6D6, 400 MHz, 298 K): δ 6.90 (2H, “d”, J = 8.5 Hz), 6.58 (2H, “d”, J = 8.5 Hz), 5.13 (2 H, t, 3JH-H = 2 Hz), 4.63 (5H, s, Cp), 3.94 (1H, t, 3JH-H = 2.4 Hz), 2.93 (2H, dt 3JH-H = 2.4, 2.0 Hz), 2.51 (6H, s, NMe2). 13C{1H} NMR (CD3CN 100 MHz, 298 K): δ 149.57, 135.96, 126.35, 113.05, 79.42, 75.01, 56.35, 45.99, 40.67. Elemental Analysis: (calculated) C: 69.90%, H: 6.52%, N: 4.53%; (observed) C: 70.33%, H: 6.85%, N: 4.51%. To synthesize (Cp)Co(CpN), (Cp)Co(η4-C5H5PhNMe2) (50 mg, 0.16 mmol) was dissolved in 5:1 methylcyclohexane:benzene. The solution was brought to reflux and went from orange to dark red. The solution was refluxed for 18 hours at which point the solution was cooled to room temperature. The solvent was removed under reduced pressure. The solid was extracted with pentane and filtered through celite. The solution was then concentrated by slow evaporation of pentane to yield crystalline (Cp)Co(CpN). (Yield: 29.4 mg, 59%). 1 H NMR (C6D6, 400 MHz, 298 K): δ 11.32 (2H), 4.99 (6H), −2.82 (2H), −41.49 (2H), −47.45 (2H), −50.12 (5H). μeff (C6D6, 298 K): 1.75. Elemental Analysis: (calculated) C: 70.13%, H: 6.21%, N: 4.54%; (observed) C: 70.01%, H: 6.30%, N: 4.62% To synthesize [(Cp)Co(CpN)][OTf], (Cp)Co(CpN) (43.7 mg, 0.147 mmol) was dissolved in THF (3 mL) and added dropwise to a stirring a THF suspension (3 mL) of ferrocenium triflate (50 mg, 0.149 mmol) was added dropwise. The solution was allowed 312 to stir for thirty minutes and then the solvent was evaporated. The solid was washed with pentane to remove ferrocene and then the solid was redissolved in THF. The solution was then layered with diethyl ether and cooled to −35 °C. Overnight crystalline material of [(Cp)Co(CpN)][OTf] was deposited. (Yield: 48 mg, 74 %) 1 H NMR (CD3CN, 400 MHz, 298 K): δ 7.61 (2H, d, 3JH,H = 8.5 Hz), 6.78 (2H, d, 3 JH,H = 8.5 Hz), 6.07 (2 H, s), 5.69 (2H, s), 5.39 (5H, s, Cp), 3.01 (6H, s, NMe 2). 19F NMR (CD3CN, 376 MHz, 298 K): δ 76.6. 13C{1H} NMR (CD3CN 100 MHz, 298 K): δ 153.10, 129.91, 113.21, 86.11, 84.52, 78.99, 68.25, 40.30, 26.22. [(Cp)Co(CpNH)][OTf]2: A dimethoxyethane (5 mL) solution of [(Cp)Co(CpN)][OTf] (37 mg, 0.08 mmol) was stirred. To this a dimethoxyethane (1 mL) solution of HOTf (60.8 mg, 39 μL, 0.40 mmol) was added dropwise. This caused the reaction to turn yellow and then over thirty minutes a yellow solid precipitated. The solvent was removed and the solid residue was extracted with acetonitrile. The acetonitrile solution was filtered through celite and then concentrated. Layering with toluene at −35 °C led to the precipitation of crystalline material of [(Cp)Co(CpNH)][OTf]2 overnight. (Yield: 25 mg, 50%) 1 H NMR (C6D6, 400 MHz): δ 10.08 (1H, br s, NHMe2) 7.96 (2H, “d”, 3JH,H = 8.6 Hz), 7.73 (2H, “d”, 3JH,H = 8.6 Hz), 6.26 (2 H, tt, J = 2.3, 2.3 Hz), 5.83 (2H, tt, J = 2.3, 2.3 Hz), 5.54 (5H, s, Cp), 3.29 (6H, s, NHMe2). 19F NMR (CD3CN, 376 MHz, 298 K): δ 76.5. 13 C NMR (C6D6, 100 MHz): δ 144.31, 130.72, 122.69, 102.76, 86.93, 86.12, 82.57, 66.16, 313 47.73. Elemental Analysis: (calculated) C: 39.55%, H: 3.32%, N: 2.31%; (observed) C: 39.23%, H: 3.44%, N: 2.35% E.3 NMR Spectroscopy of New Complexes Figure E.1: 1H NMR spectrum of Cp(η4-C5H5PhNMe2)Co in C6D6, 400 MHz, 298 K. 314 Figure E.2:13C{1H} NMR spectrum of Cp(η4-C5H5PhNMe2)Co in C6D6, 100 MHz, 298 K. Figure E.3: 1H NMR spectrum of (Cp)Co(CpN) in CD3CN, 400 MHz, 298 K. 315 Figure E.4: 1H NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 400 MHz, 298 K. Figure E.5: 19F NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 376 MHz, 298 K. 316 Figure E.6: 13C{1H} NMR spectrum of [(Cp)Co(CpN)][OTf] in CD3CN, 100 MHz, 298 K. Figure E.7: 1H NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 400 MHz, 298 K. 317 Figure E.8: 19F NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 376 MHz, 298 K. Figure E.9: 13C{1H} NMR spectrum of [(Cp)Co(CpNH)][OTf]2 in CD3CN, 100 MHz, 298 K. 318 E.4 UV-Vis Spectroscopy (Cp)Co(CpN) Epsilon (M-1 cm-1) 20000 15000 10000 5000 0 350 550 750 950 Wavelength (nm) Figure E.10: UV-Vis spectrum at room temperature for [(Cp)Co(CpNH)][OTf]2 in 2MeTHF:THF solution (4:1). (Cp)Co(CpN) + HNTf2 2 0 minutes 1.6 2 minutes 1.4 4 minutes Absorbance 1.8 1.2 6 minutes 1 10 minutes 0.8 16 minutes 0.6 0.4 0.2 0 360 560 760 960 Wavelength (nm) Figure E.11: UV-Vis spectra of the reaction of trifluoromethanesulfonimide and (Cp)Co(CpN) at −130 °C in 2 UV-Vis spectrum in 2-MeTHF:THF solution (4:1) showing conversion to [(Cp)Co(CpNH)][NTf2]. 319 [(Cp)Co(CpNH)][OTf]2 6000 Epsilon (M-1 cm-1) 5000 4000 3000 2000 1000 0 310 410 510 610 710 Wavelength (nm) 810 910 Figure E.12: UV-Vis spectrum at room temperature for [(Cp)Co(CpNH)][OTf]2 in acetonitrile. E.5 EPR Spectroscopy Figure E.13:(blue) X-band EPR spectrum of a 1 mM solution of (Cp)Co(CpN) in a 2methyltetrahydrofuran glass at 77 K. (orange) Spectral simulation with the following parameters: g = [1.8669 1.8654 1.5172], lw = 61.42 Hz, Hstrain = [0.0965 19.3818 510.669] Hz. 320 Sample Preparation for [(Cp)Co(CpNH)][OTf]: Protonation: In a glovebox cold well, a 2 mM solution of (Cp)Co(CpN) in 2methyltetrahydrofuran (0.5 mL) was layered on a frozen solution of 2-chloroanilinium triflate in 2-methyltetrahydrofuran (2 mM, 0.5 mL). The layers were thawed were briefly allowed to thaw and agitated with a pre-chilled spatula. The solution turned a dark blue color. It was then rapidly transferred with a prechilled pipette to an X-band EPR tube. The solution was then frozen, brought out of the glovebox, and analyzed by continuous wave X-band EPR. Reduction: In a glovebox cold well, a 2 mM solution of [(Cp)Co(CpNH)][OTf]2 in acetonitrile (0.5 mL) was added to a vial. This was frozen and on this was layered a 2 mM solution of SmI2 in acetonitrile (0.5 mL). The layers were thawed and quickly mechanically mixed with a pre-chilled spatula. The solution turned a dark blue color and then transferred with a pre-chilled pipette to an X-band EPR tube. The solution was then frozen, brought out of the glovebox, and analyzed by continuous wave X-band EPR. 321 Figure E.14: (blue) X-band EPR spectrum of a 1 mM solution of [(Cp)Co(CpNH)][OTf] in a 2-methyltetrahydrofuran glass at 77 K generated via protonation of (Cp)Co(CpN). (red) X-band EPR spectrum of a 1 mM solution of [(Cp)Co(CpNH)][OTf] in frozen acetonitrile at 77 K generated via reduction of [(Cp)Co(CpNH)][OTf]2. (green) Simulation of these data. g = [2.0555 1.9932 1.7959], A(59Co) = [401.1050 27.8926 41.6297], HStrain = [220.0820 656.3320 1885.5], gStrain=[0.0307 0.0305 0.0342] The high degree of strain in the simulation is suggestive of a variety of conformations or hydrogen bonding interactions, or poor homogeneity of the solution. The broadness of (Cp)Co(CpN) in 2Me-THF glass supports the possibility of the variety of conformations. Indeed the g-values of many cobaltocenes display high sensitivity to their local environment.17–19 The increased broadness of the acetonitrile sample relative to the 2-MeTHF sample provides support for poor solution homogeneity. 322 E.6 IR Spectroscopy [(Cp)Co(CpNH)][OTf]2 1 0.95 0.9 0.8 0.75 Absorbance 0.85 0.7 0.65 0.6 0.55 3500 2500 1500 500 Wavelength (nm) Figure E.15: IR spectrum of [(Cp)Co(CpNH)][OTf]2 in a KBr pellet. Weak N−H vibration is evident at 3430 cm−1. E.7 X-ray Crystallography Table E.1: X-ray crystallography details for (Cp)Co(CpN), [(Cp)Co(CpNH)][OTf], and [(Cp)Co(CpN)][OTf] Compound (Cp)Co(CpN) [(Cp)Co(CpNH)][OTf]2 [(Cp)Co(CpN)][OTf] Chemical Formula C18H19CoN C20H20CoNF6O6S2 C19H19CoNF3O3S Formula Weight 308.27 607.42 457.34 Crystal System Triclinic Triclinic Triclinic Space Group P −1 P −1 P −1 a (Å) 13.3390(15) 8.2326(5) 10.5419(7) 323 b (Å) 10.8922(13) 12.5729(7) 11.6519(8) c (Å) 9.6039(11) 12.9491(8) 16.4972(10) α (°) 90 67.614(3) 89.708(2) β (°) 90 76.729(3) 79.981(2) γ (°) 90 81.730(3) 67.013(2) V (Å3) 1395.4(3) 1203.85(13) 1832.6(2) Z 4 2 4 Dcalcd (g∙cm−1) 9.518 1.676 1.658 F000 644.0 616.0 936.0 μ (mm−1) 9.518 7.997 1.101 Temperature (K) 100(2) 100(2) 100(2) Wavelength (Å) 0.71073 1.54178 Measured Reflections 1457 4716 12299 Unique Reflections 1314 4491 10976 Data/Restraints/ 1457/0/509 4716/0/327 10976/0/509 R(F) (I>2σ(I)) 0.0543 0.0411 0.0297 wR(F2) (all) 0.1400 0.0970 0.0793 GOOF 1.068 1.046 1.033 Parameters 324 E.8 Cyclic Voltammetry [(Cp)Co(CpN)]0/+ 1.20E-04 I /(Amps) 6.00E-05 100 mV/s 250 mV/s 500 mV/s 1000 mV/s 0.00E+00 -6.00E-05 -1.20E-04 -1 -1.2 -1.4 -1.6 -1.8 Potential (V vs Fc+/0) Figure E.16: Variable scan rate cyclic voltammograms with a glassy carbon working electrode of a 1 mM solution of the (Cp)Co(CpN)+/0 in acetonitrile with 100 mM [TBA][PF6]. [(Cp)Co(CpN)]2+/+ 8.00E-05 100 mV/s 250 mV/s 500 mV/s 1000 mV/s 6.00E-05 4.00E-05 I (Amps) 2.00E-05 0.00E+00 -2.00E-05 -4.00E-05 -6.00E-05 -8.00E-05 1.1 0.9 0.7 0.5 0.3 0.1 Potential (V vs Fc+/0) Figure E.17: Variable scan rate cyclic voltammograms with a glassy carbon working electrode of a 1 mM solution of the [(Cp)Co(CpN)]2+/+ in acetonitrile with 100 mM [TBA][PF6]. 325 [(Cp)Co(CpNH)]2+/+ 6.00E-05 I (Amps) 3.00E-05 0.00E+00 -3.00E-05 -6.00E-05 100 mV/s 250 mV/s 500 mV/s 1000 mV/s -9.00E-05 0.3 -0.2 -0.7 -1.2 -1.7 +/0 Potential (V vs Fc ) Figure E.18: Variable scan rate cyclic voltammograms with a boron doped diamond working electrode of a 1 mM solution of [(Cp)Co(CpNH)][OTf]2 and 1 mM solution of ferrocene in acetonitrile with 100 mM [TBA][PF6]. [(Cp)Co(CpN)][OTf] 5.00E-05 I (Amps) 3.00E-05 1.00E-05 -1.00E-05 -3.00E-05 -5.00E-05 -7.00E-05 1 0 -1 -2 Potential (V vs Fc+/0) -3 Figure E.19: Cyclic voltammogram of a glassy carbon working electrode of a 1 mM solution of the (Cp)Co(CpN)+/0 in DME with 100 mM [TBA][PF6]. 326 I (Amps) 4.00E-05 3.50E-05 0.33 mM Co 3.00E-05 0.66 mM Co 2.50E-05 0.99 mM Co 2.00E-05 1.32 mM Co 1.50E-05 1.65 mM Co 1.00E-05 1.98 mM Co 5.00E-06 0.00E+00 -5.00E-06 -0.5 -1 -1.5 +/0 Potential (V vs Fc ) Figure E.20: Cathodic scan of CV’s at 100 mV/s of different concentrations of [(Cp)Co(CpN)][OTf] on a BDD working electrode of a DME solution of 100 mM [ 4CN PhNH3][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6]. I (Amps) 0.5 0 0.000045 0.00004 0.000035 0.00003 0.000025 0.00002 0.000015 0.00001 0.000005 0 y = 1.67E-05x + 5.20E-06 R² = 9.83E-01 0 0.5 1 1.5 2 2.5 Concentration of [(Cp)Co(CpNH)][OTf] (mM) Figure E.21: Plot of plateau current (−1.314 V vs Fc+/0) against concentration of [(Cp)Co(CpN)][OTf] for the CV’s shown in Figure E.20 with a line of best fit. 327 2.00E-05 I (Amps) 1.50E-05 1.00E-05 5.00E-06 0.00E+00 0 -0.5 -1 -1.5 Potential (V vs Fc+/0) Figure E.22: Cathodic scan of CV’s at 10 mV/s of different concentrations of acetophenone on a BDD working electrode of a DME solution of 100 mM [ 4CN PhNH3][OTf], 1 mM [(Cp)Co(CpN)][OTf], and 200 mM [TBA][PF6]. 0.00002 0.000019 y = 7.93E-08x + 1.23E-05 R² = 9.91E-01 I (Amps) 0.000018 0.000017 0.000016 0.000015 0.000014 0.000013 0.000012 0 20 40 60 80 Concentration of Acetophenone (mM) 100 Figure E.23: Plot of plateau current (−1.314 V vs Fc+/0) against concentration of acetophenone for the CV’s shown in Figure E.22 with a line of best fit. I (Amps) 328 1.90E-05 Co + Acid + Ketone 1.40E-05 Co + Acid 9.00E-06 4.00E-06 -1.00E-06 -0.6 -0.8 -1 -1.2 Potential (V vs Fc+/0) -1.4 -1.6 Figure E.24: Comparison of CV’s on BDD working electrode at 10 mV/s with 1 mM [(Cp)Co(CpN)][OTf], 100 mM [4-CNPhNH3][OTf], and 200 mM [TBA][PF6] with variable amount of acetophenone (0 mM, blue) vs (50 mM, red). Figure E.25: Differential pulse voltammogram of a DME solution containing 1 mM ferrocene, 1 mM [(Cp)Co(CpN)][OTf], 50 mM acetophenone, and 200 mM [TBA][PF 6]. 329 Figure E.26: CV at 100 mV/s of a DME solution containing 1 mM ferrocene, 1 mM [(Cp)Co(CpN)][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6]. E.9 pKa Determination Experiments for pKa determination were performed in MeCN-d3 using equilibria constants determined using 1H NMR spectroscopy on a 400 MHz spectrometer. In a standard experiment a cobalt containing starting material was dissolved in MeCN-d3 and added to a J-Young NMR tube. An initial NMR was taken to verify purity and then aliquots of acid (or base) were added to the tube from a stock solution. Although care was taken to add exact amounts of acid (or base) the relative integrals of the species was used to evaluate the amounts of the species rather than an absolute amount as determined by the concentrations/volumes. In all cases, the counterion used was triflate to simplify the analysis. The relative concentration of the protonated/deprotonated species was determined by the chemical shift (CS) of each peak (Eq B.1-Eq E.3). For each component (i.e., acid/base and [(Cp)Co(CpN)][OTf]/ [(Cp)Co(CpNH)][OTf]2) the average of each of these 330 equilibria were taken. These averages were then used to determine K eq for a given loading of acid/base (Eq E.4). These Keq values from each loading were then averaged to give a Keq from each run, which could then be converted to a pKa based on the known pKa of the anilinium acids in acetonitrile (Eq E.5-Eq E.6). Eq E.1) CSCo = CSNH×y+ CSN×(1−y) Eq E.2) CSAcid/Base = CSAcid×y+ CSBase×(1−y) Eq E.3) [Acid] = y×Integral Eq E.4) Keq = ([Acid][(Cp)Co(CpN)]+])/([Base][[(Cp)Co(CpNH)]2+) Eq E.5) ΔG = RT×ln(Keq) Eq E.6) pKa([(Cp)Co(CpNH)]2+) = Reference + ΔG/1.37 Table E.2: NMR shifts of pure compounds used in the pKa determination experiments. Compoun d [(Cp)Co(CpN )] [(Cp)Co(CpNH )] [OTf] [OTf]2 4Cl PhNH 2 [4Cl PhNH3 ] 2Cl PhNH 2 [OTf] [2Cl PhNH3 ] [OTf] Shift 1 7.61 7.96 7.04 7.53 7.20 7.63 Shift 2 6.78 7.73 6.62 7.42 7.06 7.51 Shift 3 6.08 6.26 6.81 7.49 Shift 4 5.69 5.83 6.63 7.49 Shift 5 5.4 5.54 Shift 6 3.01 3.29 Table E.3: NMR data for the cobalt center during titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline. All chemical shifts are in ppm. In parentheses is the fraction of [(Cp)Co(CpNH)][OTf]2 indicated by that chemical shift. Volume Shift 1 Shift 2 Shift 3 Shift 4 Shift 5 Shift 6 Average Std. Dev. 331 25 μL 7.84 (0.66) 7.42 (0.67 6.19 (0.61) 5.78 (0.64) 5.49 (0.64) 3.2 (0.68) 0.65 0.02 50 μL 7.75 (0.40) 7.17 (0.41) 6.14 (0.33) 5.74 (0.36) 5.45 (0.36) 3.12 (0.39) 0.38 0.03 75 μL 7.70 (0.26) 7.04 (0.27) 6.12 (0.22) 5.72 (0.21) 5.43 (0.21) 3.08 (0.25) 0.24 0.02 100 μL 7.67 (0.17) 6.97 (0.11) 6.10 (0.11) 5.71 (0.14) 5.42 (0.14 3.06 (0.18) 0.16 0.03 Table E.4: NMR data for the aniline/anilinium during titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline. Relative integral is determined by setting the most downfield aryl peaks in the cobalt center to an integral of 1 and integrating the acid peaks in comparison and taking the average of those integrals. Volume Shift 1 Shift 2 Average Std. Dev. Relative Integral 25 μL 7.49 (0.92) 7.35 (0.91) 0.92 0.003 0.35 50 μL 7.42 (0.78) 7.23 (0.76) 0.77 0.007 0.77 75 μL 7.34 (0.61) 7.11 (0.61) 0.61 0.0001 1.09 100 μL 7.28 (0.49) 7.01 (0.49) 0.49 0.001 1.54 Table E.5: Equilibrium Constants derived from data in Table E.3 and Table E.4. Volume Keq 25 μL 0.17 50 μL 0.18 75 μL 0.20 100 μL 0.19 332 Table E.6: NMR data for the cobalt center during second titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline. All chemical shifts are in ppm. In parentheses is the fraction of [(Cp)Co(CpNH)][OTf]2 indicated by that chemical shift. Volume Shift 1 Shift 2 Shift 3 Shift 4 Shift 5 Shift 6 Average Std. Dev. 25 μL 7.85 (0.69) 7.43 (0.68) 6.19 (0.61) 5.78 (0.64) 5.49 (0.64) 3.20 (0.68) 0.66 0.03 100 μL 7.68 (0.2) 6.11 (0.17) 5.72 (0.21) 5.72 (0.21) 5.43 (0.21 3.07 (0.21) 0.21 0.02 125 μL 7.67 (0.17) 6.94 (0.17) 6.10 (0.11) 5.71 (0.14) 5.42 (0.14) 3.06 (0.18) 0.15 0.02 Table E.7: NMR data for the aniline/anilinium during second titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline. Relative integral is determined by setting the most downfield aryl peaks in the cobalt center to an integral of 1 and integrating the acid peaks in comparison and taking the average of those integrals. Volume Shift 1 Shift 2 Average Std. Dev. Relative Integral 25 μL 7.50 (0.94) 7.35 (0.91) 0.93 0.01 0.37 100 μL 7.33 (0.59) 7.08 (0.58) 0.58 0.01 1.54 125 μL 7.29 (0.51) 7.02 (0.50) 0.51 0.01 1.85 Table E.8: Equilibrium Constants derived from data in Table E.6 and Table E.7. Volume Keq 25 μL 0.15 100 μL 0.18 125 μL 0.18 333 Table E.9: NMR data for the cobalt center during titration of [(Cp)Co(CpNH)][OTf]2 with 2-chloroaniline. All chemical shifts are in ppm. In parentheses is the fraction of [(Cp)Co(CpNH)][OTf]2 indicated by that chemical shift. Volume Shift 1 Shift 2 Shift 3 Shift 4 Shift 5 Shift 6 Average Std. Dev. 10 μL 7.90 (0.83) 7.57 (0.83) 6.22 (0.78) 5.81 (0.86) 5.52 (0.86) 3.25 (0.86) 0.83 0.03 20 μL 7.88 (0.77) 7.52 (0.78) 6.21 (0.72) 5.80 (0.79) 5.51 (0.79) 3.23 (0.79) 0.78 0.02 40 μL 7.85 (0.69) 7.44 (0.69) 6.19 (0.61) 5.79 (0.71) 5.49 (0.64) 3.21 (0.71) 0.68 0.04 80 μL 7.81 (0.57) 7.35 (0.60) 6.17 (0.50) 5.77 (0.57) 5.48 (0.57) 3.18 (0.60) 0.57 0.03 Table E.10: NMR data for the aniline/anilinium during titration of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline. Relative integral is determined by setting the most downfield aryl peak of the Co complex to 1 and then taking the average of the integrals from the acid/base system. Volume Shift 1 Shift 2 Shift 3 Shift 4 Average Std. Dev. Relative Integral 10 μL 7.33 (0.30) 7.19 (0.29) 7.05 (0.35) 6.91 (0.33) 0.32 0.02 0.28 20 μL 7.30 (0.23) 7.16 (0.22) 7.00 (0.28) 6.85 (0.26) 0.25 0.02 0.51 40 μL 7.27 (0.16) 7.13 (0.16) 6.94 (0.17) 6.78 (0.17) 0.17 0.01 0.90 80 μL 7.25 (0.12) 7.11 (0.11) 6.90 (0.13) 6.73 (0.11) 0.12 0.01 1.62 Table E.11: Equilibrium Constants derived from data in Table E.9 and Table E.10. Volume Keq 334 10 μL 10.9 20 μL 10.3 40 μL 10.2 80 μL 10.3 Table E.12: Final equilibrium constants from data in Table E.5, Table E.8, and Table E.11 Experiment # Keq Std. Dev. pKa 1 (4-ClPhNH2) 0.19 0.01 (6%) 8.83 2 (4-ClPhNH2) 0.17 0.01 (7%) 8.88 3 (2-ClPhNH2) 10.3 0.4 (3%) 8.87 Final pKa is 8.86 ± 0.02 Figure E.27: 400 MHz 1H NMR spectrum of 4-chloroaniline (4-ClPhNH2) in MeCN-d3 at room temperature. 335 Figure E.28: 400 MHz 1H NMR spectrum of 4-chloroanilinium triflate ([4-ClPhNH3][OTf]) in MeCN-d3 at room temperature. Figure E.29: 400 MHz 1H NMR spectrum of 2-chloroaniline (2-ClPhNH2) in MeCN-d3 at room temperature. 336 Figure E.30: 400 MHz 1H NMR spectrum of 2-chloroanilinium triflate ([2-ClPhNH3][OTf]) in MeCN-d3 at room temperature. 337 Figure E.31: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline in MeCN-d3 at room temperature. Only one integral and peak pick for both Co and the aniline are shown for clarity, but all the data shown in the above tables was used to determine the equilibrium constants (Experiment #1). 338 Figure E.32: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 4-chloroaniline in MeCN-d3 at room temperature. Only one integral and peak pick for both Co and the aniline are shown for clarity, but all the data shown in the above tables was used to determine the equilibrium constants (Experiment #2). 339 Figure E.33: 400 MHz 1H NMR spectra of titration experiments of [(Cp)Co(CpNH)][OTf]2 with 2-chloroaniline in MeCN-d3 at room temperature. Only one integral and peak pick for both Co and the aniline are shown for clarity, but all the data shown in the above tables was used to determine the equilibrium constants (Experiment #3). E.10 Application of Hess’s Law to [(Cp)Co(CpNH)]n+ BDFE = bond dissociation free energy 340 CG = solvent-dependent thermodynamic constant accounting for the energy of formation of H∙ from one proton and one electron ΔG(H−) = hydricity CHyd = solvent-dependent thermodynamic constant accounting for the energy of formation of H− from one proton and two electrons Calculation of the Homolytic N–H Bond Strength for [(Cp)Co(CpNH)][OTf]: Eq E.7) BDFE = 1.37×pKa + 23.06×E°1 + CG Eq E.8) BDFE1 = 1.37×8.6 + 23.06×−1.21 + 54.9 Eq E.9) BDFE1 = 38.8 kcal∙mol−1 Calculation of the Homolytic N–H Bond Strength for [(Cp)Co(CpNH)][OTf]2: Eq E.10) BDFE = 1.37×pKa + 23.06×E°2 + CG Eq E.11) BDFE2 = 1.37×8.6 + 23.06×0.54 + 54.9 Eq E.12) BDFE2 = 79.1 kcal∙mol−1 Calculation of the Heterolytic N–H Bond Strength for [(Cp)Co(CpNH)][OTf]: Eq E.13) ΔG(H−) = 1.37×pKa + 23.06×E°1 + 23.06×E°2 + CHyd Eq E.14) ΔG(H−) = 1.37×8.6 + 23.06×0.54 + 23.06×−1.21 + 79.6 Eq E.15) ΔG(H−) = 75.9 kcal∙mol−1 Calculation of the pKa of [(Cp)Co(CpNH)][OTf] Eq E.16) BDFE = 1.37×pKa + 23.06×E°3 + CG Eq E.17) pKa = (BDFE − 23.06×E° − CG)/1.37 Eq E.18) pKa = (38.8 – 23.06×−1.35 – 54.9)/1.37 Eq E.19) pKa = 11.0 341 Figure E.34: Thermochemical relationships in acetonitrile for cobaltocene species in this study. Potentials are reported vs Fc+/0. E.11 Thermochemistry from DFT In cases, where an experimental spin state was known this was used for the calculation. In cases, where the experimental spin state was unknown the singlet, triplet and quintet or doublet and quartet states were calculated. The lowest energy state was used for the thermochemical calculations and is indicated. Both Co I species, [(Cp)Co(CpN)]− and CpCpNHCo, were calculated to be triplets consistent with the known spin state of the isoelectronic nickelocene.20 Table E.13: DFT calculated thermochemical parameters for parent cobaltocene and baseappended cobaltocenes. Species Reduction Potential pKa (V vs Fc+/0) BDFE Hydricity −1 (kcal∙mol ) (kcal∙mol−1) [(Cp)Co(CpN)]2+ (S 0.42 = ½) N/A N/A N/A [(Cp)Co(CpN)] + N/A N/A N/A −1.58 N 342 (Cp)Co(Cp ) −2.28 N/A N/A N/A [(Cp)Co(CpNH)]2+ −1.19 7.9 75.6 N/A 14.5 38.5 70.8 [(Cp)Co(CpNH)]+ [Cp2Co]+ −1.45 N/A N/A N/A Cp2Co −2.20 N/A N/A N/A [(Cp)Co(η4-C5H6)]+ −0.42 (S = ½) 9.0 33.8 N/A 57.8 46.8 (Cp)Co(η4-C5H6) N/A E.13 Controlled Potential Electrolyses To set-up the bulk electrolyses a single-compartent cell was used. The reference electrode contained a silver wire in a DME solution of 5 mM AgOTf and a silver. The counter electrode was a glassy carbon plate. The working electrode was a BDD plate that had been treated as previously described in the literature to generate a clean, Hterminated surface.14 Additionally, repeated CV’s at 100 mV’s were taken with the BDD plate in a 100 mM DME solution of tosic acid and [TBA][PF 6] were taken until a stable HER background was observed. The electrodes were then placed into a fresh solution of DME containing 100 mM tosic acid and 200 mM [TBA][PF6]. A CV at 100 mV/s was taken to confirm the stable HER background. To this was then added via syringe a sufficient amount of acetophenone to generate a 0.05 M solution. Another CV was taken to evaluate the current. Finally, enough catalyst was added to generate a 1 mM solution and a final CV was taken. Controlled potential electrolyses at −1.45 V with stirring until sufficient charge had passed to consume all of the acid or in the case of the control experiment without catalyst until the same amount of time had passed. 343 Figure E.35: Calibration curve used for GC-TCD detection of H2. Figure E.36: Calibration curve used for GC-FID quantification of pinacol (2,3-diphenylbutanediol). 344 Figure E.37: Calibration curve used for GC-FID quantification of ketone (acetophenone). Table E.14: Results from CPE experiments under the standard conditions described using the procedures described in Sections E.1.5 and E.1.6 Counter Catalyst Ketone Pinacol H2 Yield Recovery TON (FE) (FE) 10.5 11.8% Pinacol Yield Electrode Q (C) 1 mM Pt 21.9% 5.4% [(Cp)Co(CpN)][OTf] 1 mM Glassy [(Cp)Co(CpN)][OTf] Carbon 83.2% 20.7 (35.7%) (38.4%) 41.5 47.7% 10.8% 71.6 (39.0%) Glassy 1 mM [Cp2Co][PF6] 65.8% 5.6% Carbon (44.9%) 0% 2.8 71.6 345 Glassy none 10.4% Carbon (2.6%) (62.0%) N/A 1.0% 0% 7.5 (46.6%) (7.5%) Figure E.38: CV’s relevant to the set-up of Entry 1 in Table E.14. In blue is tosic acid only. In orange is tosic acid with acetophenone (50 mM) added. In gray is tosic acid and acetophenone with [(Cp)Co(CpN)][OTf] (1 mM) added. Similar CV’s are consistently observed during the set-up. 346 Figure E.39: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 1). Figure E.40: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 1). Peak at −1.33 V corresponds to the reduction of [(Cp)Co(CpN)][OTf] 347 Figure E.41: First derivative of DPV taken after the CPE experiment (Table E.14, Entry 1). Peak at 0.45 V corresponds to the reduction of [(Cp)Co(CpN)][OTf] Figure E.42: XPS of the BDD electrode in the Co 2p region, showing no evidence of Co deposition after the CPE experiment with [(Cp)Co(CpN)][OTf] (Table E.14, Entry 1). 348 Figure E.43: XPS of the BDD electrode in the S 2p region, showing no evidence of S deposition after the CPE experiment (Table E.14, Entry 1). Sulfur is found in both the triflate counteranion and the tosic acid. Figure E.44: XPS of the BDD electrode in the N 1s region, showing no minimal evidence of N deposition after the CPE experiment (Table E.14, Entry 1). The likeliest source of nitrogen given the lack of adsorbed Co is either environmental contamination or the [TBA]+. 349 Figure E.45: CV’s relevant to the set-up of Entry 2 in Table E.14. In blue is tosic acid only. In orange is tosic acid with acetophenone (50 mM) added. In gray is tosic acid and acetophenone with [(Cp)Co(CpN)][OTf] (1 mM) added. Similar CV’s are consistently observed during the set-up. Figure E.46: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 2). 350 Figure E.47: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 2). Peak at −1.33 V corresponds to the reduction of [(Cp)Co(CpN)][OTf] Figure E.48: First derivative of DPV taken after the CPE experiment (Table E.14, Entry 2). Peak at 0.45 V corresponds to the reduction of [(Cp)Co(CpN)][OTf]. 351 Figure E.49: CV’s relevant to the set-up of Entry 3 in Table E.14. In blue is tosic acid only. In orange is tosic acid with [Cp2Co][PF6] (1 mM) added. In gray is tosic acid and [Cp2Co][PF6] with acetophenone (50 mM) added. Figure E.50: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 3). 352 Figure E.51: Negative first derivative of DPV taken after the CPE experiment (Table E.14, Entry 4) with a fresh glassy carbon disk electrode. Peak at −1.31 V corresponds to the reduction of [Cp2Co][PF6] Figure E.52: CV’s relevant to the set-up of Entry 4 in Table E.14. In blue is tosic acid only. In orange is tosic acid with acetophenone (50 mM) added. 353 Figure E.53: Current (left axis) and charge passed (right axis) with respect to time during the CPE experiment (Table E.14, Entry 4). E.14 References (1) Tanabe, Y.; Nakajima, K.; Nishibayashi, Y. Chem. – Eur. J. 2018, 24, 18618– 18622. (2) Kato, K.; Kim, W.; Kim, D.; Yorimitsu, H.; Osuka, A. Chem. – Eur. J. 2016, 22, 7041–7045. (3) Neese, F. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73–78. (4) Neese, F. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2018, 8, e1327. (5) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. (6) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (7) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065. 354 (8) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104–154119. (9) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378– 6396. (10) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (11) 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. (12) Sheldrick, G. M. Acta Crystallogr. Sect. C Struct. Chem. 2015, 71, 3–8. (13) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. a. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339–341. (14) Macpherson, J. V. Phys. Chem. Chem. Phys. 2015, 17, 2935–2949. (15) Chiang, L.; Keown, W.; Citek, C.; Wasinger, E. C.; Stack, T. D. P. Angew. Chem. Int. Ed. 2016, 55, 10453–10457. (16) Evans, D. F. J. Chem. Soc. Resumed 1959, No. 0, 2003–2005. (17) Ammeter, J. H. J. Magn. Reson. 1969 1978, 30, 299–325. (18) Ammeter, J. H.; Swalen, J. D. J. Chem. Phys. 1972, 57, 678–698. (19) Hulliger, J.; Zoller, L.; Ammeter, J. H. J. Magn. Reson. 1969 1982, 48, 512–518. (20) Wilkinson, G.; Pauson, P. L.; Cotton, F. A. J. Am. Chem. Soc. 1954, 76, 1970– 1974.