Synthesis, Characterization, and Reactivity of Iron Hydrides in Nitrogen Fixation and Proton Coupled Electron Transfer from C-H bonds Citation Schild, Dirk Jacob (2022) Synthesis, Characterization, and Reactivity of Iron Hydrides in Nitrogen Fixation and Proton Coupled Electron Transfer from C-H bonds. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1ytm-7s85. https://resolver.caltech.edu/CaltechTHESIS:10192021-161648176 Abstract Mitigating the hydrogen evolution (HER) is an outstanding challenge in small molecule reduction catalysis using protons and electrons. Nitrogen fixation is a fundamental reaction where this selectivity is of great importance. This thesis details mechanistic studies into the nitrogen fixation reaction and factors that contribute to hydrogen evolution. In addition to the mechanistic studies, the development of reagents with weak X-H bonds, with applications in N-H bond formation is presented. Chapter 1 presents a brief overview of catalytic nitrogen fixation, the role of hydride ligands, and the importance of reagents required for the formation of weak N-H bonds. Chapter 2 details the mechanism of photo-enhanced iron mediated N 2 fixation. It is shown that off-path iron complexes bearing hydride ligands play an active role in hydrogen evolution by N 2 fixation catalysts. The data presented lends further insight into the selectivity, activity, and required driving force relevant to iron (and other) N 2 RR catalysts. The third chapter describes the synthesis and characterization of a highly reactive iron(III) nitrido complex, a proposed key intermediate in nitrogen fixation mediated by [(P 3 B )Fe] + . The ability to synthesize and characterize such an intermediate provides additional support for a distal catalytic cycle for this catalyst. In Chapters 4 and 5, the reactivity of iron hydrides and their role as precursors towards weak C-H bonds is discussed. These chapters outline a valuable approach for the differentiation of a ring- versus a metal bound H-atom. Chapter 4 provides a structural, thermochemical, and mechanistic foundation for the characterization of ring protonated indene-based ligands with remarkably weak C-H bonds. Chapter 5 extends the characterization of such reactive species and presents ligand induced migration of the hydride to a Cp* ring. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: PCET; Nitrogen Fixation, Iron Catalysis; Metallocene Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Peters, Jonas C. Thesis Committee: Agapie, Theodor (chair) Miller, Thomas F. Gray, Harry B. Peters, Jonas C. Defense Date: 27 September 2021 Record Number: CaltechTHESIS:10192021-161648176 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:10192021-161648176 DOI: 10.7907/1ytm-7s85 Related URLs: URL URL Type Description https://doi.org/10.1021/acscatal.9b00523 DOI Published content for Chapter 2. https://doi.org/10.1002/anie.201909050 DOI Published content for Chapter 4. https://doi.org/10.1021/jacs.0c09363 DOI Published content for Chapter 5. ORCID: Author ORCID Schild, Dirk Jacob 0000-0003-0179-6023 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 14402 Collection: CaltechTHESIS Deposited By: Dirk Schild Deposited On: 20 Apr 2022 19:41 Last Modified: 08 Nov 2023 00:44 Thesis Files PDF - Final Version See Usage Policy. 16MB Repository Staff Only: item control page

Synthesis, Characterization, and Reactivity of Iron Hydrides in Nitrogen Fixation and Proton Coupled Electron Transfer from C-H bonds Thesis by Dirk Jacob Schild In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2022 (Defended September 27, 2021) ii © 2021 Dirk Jacob Schild ORCID 0000-0003-0179-6023 iii ACKNOWLEDGEMENTS The last 5 years at Caltech have been enjoyable, albeit challenging and frustrating at times. My time here has been incredibly formative as I developed into a better scientist. This growth is thanks to all the people I interacted with along the way. I had the opportunity to work and learn from great people and I would like to acknowledge them. First and foremost, I thank Jonas Peters for his support and mentorship during my graduate studies and as a visiting student. His feedback and approach to research have pushed me to become a better and more rigorous scientist. Jonas has given me the freedom to work on problems that interested me while providing me with advice and guidance when needed. I would also like to thank my committee, Theo Agapie, Tom Miller and Harry Gray for their advice during exams and meetings. In the Peters lab, I had the opportunity to work with incredibly talented coworkers and I would like to thank them all. Several students and postdocs worked on related projects and guided me during the first months of my time in the group. I would like to thank MarcEtienne Moret for introducing me to Jonas and starting the iron nitrido project. Trixia Buscagan paved the way for my first N2-fixation based project, and I thank her for the contributions and interest as I continued the project. Matt Chalkley and Niklas Thompson, both exceptional chemists have thaught me the basics on N2-fixation as well as EPR and Mӧssbauer spectroscopy, two techniques of paramount importance to this thesis. Besides working in the same office, I had the pleasure to collaborate in the lab with Marcus Drover on two PCET projects. Marcus kept me focused and assured that the research kept moving forward. Patricia Nance, Pablo Garrido Barros, and Mike Zott, thanks for being great glovebox coworkers! I am also grateful for the many, not always scientific, discussions in iv the office with Cooper Citek, Heejun Lee, Nina Gu, and often Christian Johansen. Recently I have been enjoing working with Lucie Nurdin in the office, who will continue the N2 fixation work. To all the people I have overlapped with during my time in the group, I wish you the best of luck in what will undoubtedly be great futures. The exceptional staff at Caltech made my research possible, and I would like to thank all of them for their contributions. Larry Henling has spent an incredible amount of time teaching me the different aspects of X-ray crystallography, while Paul Oyala has been incredible in increasing my understanding of EPR spectroscopy. I would like to thank many friends outside of the lab. Brian Sanders, Wes Kramer, Paul, and Cooper have led us to many victories at T-Boyles. I will miss the fun times with all of you. In addition to trivia, Wes kept me company during endless bike rides. Pat, Axl LeVan, Madeline Meier, Christian Goodnow, Josh Zak, and Anna Scott, thanks for the great times with you as roommates. Lastly, I would like to thank my family. My parents, brother and sister have always been there during this time. It has been hard being so far away from home, but the regular calls have made it a lot easier. I would also like to thank my grandparents who were ready to fly for the first time in their life to visit me, but then the pandemic happened. I promise I will visit Hardinxveld more often now! Finally, I would like to thank Tanvi. You have been my best friend, and your love and support may have been the most important in completing this work. v ABSTRACT Mitigating the hydrogen evolution (HER) is an outstanding challenge in small molecule reduction catalysis using protons and electrons. Nitrogen fixation is a fundamental reaction where this selectivity is of great importance. This thesis details mechanistic studies into the nitrogen fixation reaction and factors that contribute to hydrogen evolution. In addition to the mechanistic studies, the development of reagents with weak X-H bonds, with applications in N-H bond formation is presented. Chapter 1 presents a brief overview of catalytic nitrogen fixation, the role of hydride ligands, and the importance of reagents required for the formation of weak N-H bonds. Chapter 2 details the mechanism of photoenhanced iron mediated N2 fixation. It is shown that off-path iron complexes bearing hydride ligands play an active role in hydrogen evolution by N2 fixation catalysts. The data presented lends further insight into the selectivity, activity, and required driving force relevant to iron (and other) N2RR catalysts. The third chapter describes the synthesis and characterization of a highly reactive iron(III) nitrido complex, a proposed key intermediate in nitrogen fixation mediated by [(P3B)Fe]+. The ability to synthesize and characterize such an intermediate provides additional support for a distal catalytic cycle for this catalyst. In Chapters 4 and 5, the reactivity of iron hydrides and their role as precursors towards weak C-H bonds is discussed. These chapters outline a valuable approach for the differentiation of a ring- versus a metal bound H-atom. Chapter 4 provides a structural, thermochemical, and mechanistic foundation for the characterization of ring protonated indene-based ligands with remarkably weak C-H bonds. Chapter 5 extends the characterization of such reactive species and presents ligand induced migration of the hydride to a Cp* ring. vi PUBLISHED CONTENT AND CONTRIBUTIONS 1. Schild, D. J.; Peters, J. C. Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation. ACS Catal., 2019, 9, 42864295 DOI: 10.1021/acscatal.9b00523 D.J.S performed the design and execution of experimental work and calculations and participated in the preparation of the manuscript. 2. Drover, M. W.; Schild, D. J.; Oyala, P. H.; Peters, J. C. Snapshots of a Migrating HAtom: Characterization of a Reactive Iron (III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer. Angew. Chem. Int. Ed., 2019, 58, 1550415511 DOI: 10.1002/anie.201909050 DJS participated in the design and execution of the chemical reactions, performed DFT calculation, collected and analyzed spectroscopic data, and participated in the preparation of the manuscript. 3. Schild, D. J.; Drover, M. W; Oyala, P.; Peters, J. C. Generating potent C-H PCET donors: Ligand-induced Fe-to-ring proton migration from a Cp*FeIII-H complex demonstrates a promising strategy. J. Am. Chem. Soc., 2020, 142, 44, 18963–18970 DOI: 10.1021/jacs.0c09363 DJS participated in the design and execution of the chemical reactions, performed DFT calculation, collected and analyzed spectroscopic data, and participated in the preparation of the manuscript. vii TABLE OF CONTENTS ACKNOWLEDGEMENTS ............................................................................................... iii ABSTRACT........................................................................................................................ v PUBLISHED CONTENT AND CONTRIBUTIONS ....................................................... vi TABLE OF CONTENTS .................................................................................................. vii LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES .......................................................................................................... xvii Abbreviations ................................................................................................................... xix Chapter 1 : Introduction ...................................................................................................... 1 1.1 Opening Remarks ...................................................................................................... 2 1.2 Synthetic Molecular Catalysts for N2RR .................................................................. 3 1.4 Weak N-H bonds as Determinants of the Driving Force in Nitrogen Fixation......... 7 1.5 Development of Reagents with Weak X-H Bonds ................................................... 9 1.6 References ............................................................................................................... 11 Chapter 2 : Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation .................................................... 15 2.1. Introduction ............................................................................................................ 16 2.2 Results and Discussion ............................................................................................ 19 2.2.1 Synthesis and Structural Analysis .................................................................... 19 2.2.2. N2-Binding Equilibria of 5. ............................................................................. 20 2.2.3 Increased Turnover with Non-Hydride Precatalysts and Identification of OffPath Species. .............................................................................................................. 22 2.2.4 Oxidative Addition and Reductive Elimination of H2. ..................................... 26 2.2.6. Reduction of (P2PPh)Fe(N2)2. ........................................................................... 31 2.2.7 Functionalization of Formal Fe−I and Fe−II Species. ......................................... 34 2.3. Conclusions ........................................................................................................ 38 2.4. References .......................................................................................................... 40 Chapter 3 : EPR Detection of a Terminal Formal Iron(III) Nitride Stabilized by a Trisphosphine-Borane Ligand .......................................................................................... 48 3.1 Introduction ............................................................................................................. 49 3.2. Results and Discussion ........................................................................................... 52 3.3 Conclusions ............................................................................................................. 62 viii 3.4 References ............................................................................................................... 62 Chapter 4 : Snapshots of a Migrating H-atom: Characterization of a Reactive Fe(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Fe(I) Isomer .................. 68 4.1. Introduction ............................................................................................................ 69 4.2 Results and Discussion ............................................................................................ 71 4.3 Conclusions ............................................................................................................. 86 Chapter 5 : Generating potent C-H PCET donors: Ligand-induced Fe-to-ring proton migration from a Cp*FeIII-H complex demonstrates a promising strategy ...................... 92 5.1. Introduction ............................................................................................................ 93 5.2. Results and Discussion ........................................................................................... 95 5.3 Conclusions ........................................................................................................... 110 5.4. References ............................................................................................................ 112 Appendix A : Supporting Information for Chapter 2...................................................... 117 Appendix B : Supporting Information for Chapter 3 ...................................................... 211 Appendix C : Supporting Information for Chapter 4 ...................................................... 234 Appendix D : Supporting Information for Chapter 5 ..................................................... 304 ix LIST OF FIGURES Figure 1.1. Structure of FeMoco and nitrogen fixation stochiometry ............................... 3 Figure 1.2. A schematic depiction of N2 binding and reduction at an Fe site.................... 5 Figure 1.3. Molecular nitrogen fixation catalysts used in this thesis. ................................ 6 Figure 1.4. Overview of proposed HER mechanism in iron mediated nitrogen fixation. . 7 Figure 2.1. Protonation and reduction of off-path iron hydrides ..……………………...17 Figure 2.2. 1H NMR chemical shifts of [(P2PPh)Fe]2-μN2 plotted as a function of 1/T ... 21 Figure 2.3. Time profiles of the formation of H2 from HBArF4 and KC8 in Et2O at −78 °C………………………………………………………………………………………...23 Figure 2.4. Freeze Quench Mӧssbauer spectra ................................................................ 25 Figure 2.5. Schematic depiction of H2 elimination from the E4 state .............................. 27 Figure 2.6. Summary of the stoichiometric reactivity observed with [P2PPhFe(N2)x(H)y]+/0/− ...................................................................................................... 31 Figure 2.7. Cyclic voltammetry data of [(P2PPh)Fe(N2)]-. ................................................ 33 Figure 2.8. EPR Spectrum of [(P2PPh)Fe(N2)][18-crown-6)] ........................................... 37 Figure 3.1. Simplified molecular orbital diagram of nitridoiron compounds in 3 and 4fold symmetry ................................................................................................................... 50 Figure 3.2. Continous wave X-band EPR-monitored photolysis of (P3B)Fe(N3) in a frozen 1 mM 2-MeTHF solution at 77 K. ......................................................................... 54 Figure 3.3. CW X-band and psuedomodulated Q-band ESE-EPR spectra of (P3B)Fe(N) 55 Figure 4.1. Synthesis of Fe(η 3: η 2-Ind)(depe)H. ............................................................. 72 Figure 4.2. Synthesis of [FeI(η 6-IndH)(depe)][BArF4] . .................................................. 73 Figure 4.3. Freeze-quenched X-Band EPR spectrum (9.371 GHz) and corresponding Xband HYSCORE data recorded in 2-MeTHF glass at 77 K.. ........................................... 75 Figure 4.4. DFT-calculated spin-density plot for [FeIII(η3: η2-Ind)(depe)H][BArF4]+ .. 76 Figure 4.5. Freeze-quenched X-Band EPR spectrum recorded in 2-MeTHF glass at 77 K of [FeI(η 6-IndH)(depe)]+. .................................................................................................. 77 Figure 4.6. DFT optimized structures of [FeIII(η5-Cp*)(dppe)H]+. ................................. 78 Figure 4.7. Decomposition of [FeI(η 6-IndH)(depe)]+ ...................................................... 80 Figure 4.8. Square scheme for experimentally calculated thermodynamic quantities. And thermodynamic relationships that relate H• and H− transfer in MeCN. ............................ 82 x Figure 4.9. Free energy change (kcal mol-1) for PCET involving BDFEC-H . .................. 83 Figure 4.10. DFT-predicted free energy change (kcal mol-1) for metal-to-ring Hmigrations. ........................................................................................................................ 85 Figure 4.11. Free energy change (kcal mol-1) for PCET using [FeIII(η5-Cp*)(dppe)H]+ . 86 Figure 5.1. X-Band CW-EPR spectra of [FeI(endo- η 4-Cp*H)(dppe)(CO)]+]+............... 97 Figure 5.2. X-Band CW-EPR spectra and corresponding X-band HYSCORE spectra of freeze-quenched samples. ................................................................................................. 98 Figure 5.3. Solid-state structure of [FeI(exo-η4-Cp*H)(dppe)(CO)]+ ............................ 101 Figure 5.4. Frontier molecular orbitals of [Cp*(endo/exo-η4-Cp*H)CoII]+. .................. 102 Figure 5.5. X-Band CW-EPR spectra of freeze-quenched samples of [FeI(exo-η4Cp*H)(dppe)(CO)]+ ...................................................................................................... 104 Figure C.1. a) HYSCORE powder patterns for an S = 1/2, I = 1/2 spin system with an isotropic hyperfine tensor ............................................................................................... 240 Figure C.2. 1H NMR, C6D6, 400 MHz, 298 K.............................................................. 244 Figure C.3. 31P{1H} NMR, C6D6, 162 MHz, 298 K ...................................................... 244 Figure C.4. 31C{1H} NMR, THF-d8, 100 MHz, 298 K ................................................. 244 Figure C.5. UV-Visible spectrum, THF, 298 K............................................................. 245 Figure C.6. Fe(η 3: η 2-Ind)(depe)H 1H NMR, C6D6, 400 MHz, 298 K ......................... 246 Figure C.7. Fe(η 3: η 2-Ind)(depe)H 31P{1H} NMR, C6D6, 162 MHz, 298 K ................. 247 Figure C.8. Fe(η 3: η 2-Ind)(depe)H 1H NMR, C6D6, 400 MHz, 298 K ......................... 248 Figure C.9. Fe(η 3: η 2-Ind)(depe)H -57Fe, 31P{1H} NMR, C6D6, 162 MHz, 298 K ...... 248 Figure C.10. Fe(η3: η2-Ind)(depe)H 13C{1H} NMR, C6D6, 100 MHz, 298 K .............. 249 Figure C.11. Fe(η3: η2-Ind)(depe)H FT-IR ATR, thin film, 298 K .............................. 249 Figure C.12. 80 K 57Fe Mössbauer spectrum of Fe(η3:η2-Ind)(depe)H. ...................... 250 Figure C.13. Cylic Voltammogram of Fe(η3: η2-Ind)(depe)H ..................................... 251 Figure C.14. UV-Visible spectrum of Fe(η3:η 2-Ind)(depe)H, THF, 298 K ................ 252 Figure C.15. [FeI(η 6-IndH)(depe)][BArF4], 77 K X-band EPR spectrum ..................... 253 Figure C.16. 80 K 57Fe Mössbauer spectrum of [FeI(η 6-IndH)(depe)][BArF4] .......... 254 Figure C.17. [Fe(η6-toluene)(dippe)][BArF4], 1H NMR, THF-d8, 400 MHz, 298 K ..... 256 Figure C.18. [Fe(η6-toluene)(dippe)][BArF4] [BArF4], 1H NMR (expanded view), THFd8, 400 MHz, 298 K ........................................................................................................ 256 xi Figure C.19. [Fe(η6-toluene)(dippe)][BArF4] [BArF4], 77 K X-band EPR spectrum in 2MeTHF. ........................................................................................................................... 257 Figure C.20. [Fe(η6-toluene)(dippe)][BArF4], 80 K and 160 K 57Fe Mössbauer spectra258 Figure C.21. [Fe(η6-toluene)(dippe)][BArF4], Cylic Voltammogram, ........................... 259 Figure C.22. [Fe(η6-toluene)(dippe)][BArF4], Cylic Voltammogram,. .......................... 259 Figure C.23. [Fe(η6-toluene)(dippe)][BArF4], UV-Visible spectrum, THF, .................. 260 Figure C.24. [Fe(η3: η 2-Ind)(depe)N2][BArF4] [BArF4], 31P{1H} NMR, THF-d8, 162 MHz, 298 K .................................................................................................................... 261 Figure C.25. [Fe(η3: η 2-Ind)(depe)N2][BArF4], FT-IR ATR, thin film, 298 K ............ 262 Figure C.26. [Fe2(η3:η2-Ind)2(depe)2(μ-depe)][BArF4]2, 1H NMR, THF-d8, 162 MHz, 298 K...................................................................................................................................... 263 Figure C.27. [Fe2(η3:η2-Ind)2(depe)2(μ-depe)][BArF4]231P{1H} NMR, THF-d8, 162 MHz, 298 K .................................................................................................................... 264 Figure C.28. [Fe2(η3:η2-Ind)2(depe)2(μ-depe)][BArF4]2, 31C{1H} NMR, THF-d8, 100 MHz, 298 K .................................................................................................................... 264 Figure C.29. [Fe2(η3:η2-Ind)2(depe)2(μ-depe)][BArF4]2, UV-Visible spectrum, THF, 298 K...................................................................................................................................... 265 Figure C.30. [Fe(η3:η2-Ind)(depe)NCCH3][BArF4], 1H NMR, CD3CN, 162 MHz, 298 K ......................................................................................................................................... 266 Figure C.31. [Fe(η3:η2-Ind)(depe)NCCH3][BArF4], 31P{1H} NMR, CD3CN, 162 MHz, 298 K............................................................................................................................... 267 Figure C.32. [Fe(η3:η2-Ind)(depe)NCCH3][BArF4], 31C{1H} NMR, CD3CN, 100 MHz, 298 K............................................................................................................................... 267 Figure C.33. [Fe(η3:η2-Ind)(depe)NCCH3][BArF4] 80 K 57Fe Mössbauer spectrum. .... 268 Figure C.34. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of Fe(η3: η2+-Ind)(depe)H using [Fc]BArF4 at -78 oC (< 30 s). ...................................................... 269 Figure C.35. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of Fe(η3: η2-Ind)(depe)H using [Fc]BArF4 at -78 oC ...................................................................... 269 Figure C.36. A series of 77 K X-band EPR spectra in 2-MeTHF generated by oxidation of Fe(η3: η2-Ind)(depe)H using [Fc]BArF4 at -78 oC....................................................... 270 xii Figure C.37. 77 K X-band EPR spectrum of [FeI(depe)N2]+ in 2-MeTHF generated by oxidation of Fe(η3: η2-Ind)(depe)H using [Fc]BArF4 at -78 oC. ..................................... 271 Figure C.38. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of Fe(η3: η2-Ind)(depe)H using [Fc]BArF4 at -78 oC (< 30 s). ....................................................... 272 Figure C.39. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of Fe(η3: η2-Ind)(depe)Dsing [Fc]BArF4 at -78 oC . ...................................................................... 273 Figure C.40. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation using [Fc]BArF4 at -78 oC of Fe(η3:η2-Ind)(depe)H/D at -78 oC. ............................................. 274 Figure C.41. 77 K X-band EPR spectrum of [Fe(η5-Cp*)(dppe)X]+ (X = H or D) in 2MeTHF generated by oxidation of the corresponding FeII-X precursor using [Fc]BArF4 at -78 oC.. ............................................................................................................................ 275 Figure C.42. Field-dependent X-band 31P Davies ENDOR of [Fe(η3: η2-Ind)(depe)H]+ ......................................................................................................................................... 276 Figure C.43. Field-dependent X-band 31P Davies ENDOR of [[FeI(η 6-IndH)(depe)]+. 277 Figure C.44. Field-dependent X-band 31P Davies ENDOR of [Fe(η5-Cp*)(dppe)]+ . ... 278 Figure C.45. X-band HYSCORE spectra of [FeI(depe)N2]+ in 2-MeTHF ( .................. 279 Figure C.46. Field-dependent X-band HYSCORE spectra of [Fe(η3: η2Ind)(depe)H][BArF4] ...................................................................................................... 280 Figure C.47. Field-dependent X-band HYSCORE spectra of [[FeI(η 6-IndH)(depe)]+ 281 Figure C.48. Field-dependent X-band HYSCORE spectra of [FeI(η 6-IndH)(depe)]+ . 282 Figure C.49. Field-dependent X-band HYSCORE spectra of [FeIII(η5-Cp*)(dppe)]+ and [FeIII(η5-Cp*)(dppe)D]+ .................................................................................................. 283 Figure C.50. Field-dependent X-band 1H-2H difference HYSCORE spectra of [Fe(η3: η2Ind)(depe)D]BArF4 ......................................................................................................... 285 Figure C.51. Field-dependent X-band 1H-2H difference HYSCORE spectra of [FeI(η 6IndH)(depe)]+ ................................................................................................................ 286 Figure C.52. Field-dependent X-band 1H-2H difference HYSCORE spectra of [FeI(η 6IndH)(depe)]+ ................................................................................................................ 287 Figure C.53. Field-dependent X-band 1H-2H difference HYSCORE spectra of [FeIII(η5Cp*)(dppe)D]+. ............................................................................................................... 288 xiii Figure C.54. UV-VIS spectrum showing [FeI(η 6-IndH)(depe)]+ at -60 oC in 2-MeTHF. ......................................................................................................................................... 289 Figure C.55. Reaction monitoring at ε = 404 nm showing decay of [FeIII(η3: η2Ind)(depe)H][BArF4] + at -60 oC in 2-MeTHF............................................................... 289 Figure C.56. Free energy change (kcal mol-1) for PCET from the η-dienyl complex, involving BDFEC-H ......................................................................................................... 296 Figure C.57. Spin density map of gas-phase optimized hydride complexes. ................ 303 Figure D.1. a) HYSCORE powder patterns for an S = 1/2, I = 1/2 spin system with an isotropic hyperfine tensor A............................................................................................ 309 Figure D.2. [FeIII(η5-Cp*)(dppe)H]+, 1H NMR, THF-d8, 400 MHz, 298 K .................. 316 Figure D.3. [FeIII(η5-Cp*)(dppe)H]+, FT-IR ATR, thin film, 298 K (νFeH = 1874 cm-1) 317 Figure D.4. endo-[Fe(η4-Cp*H)(dppe)(CO)], 1H NMR, C6D6, 400 MHz, 298 K. ........ 318 Figure D.5. endo-[Fe(η4-Cp*H)(dppe)(CO)]-H/D stacked plot..................................... 318 Figure D.6. endo-[Fe(η4-Cp*H)(dppe)(CO)], 31P{1H} NMR, C6D6, 162 MHz, 298 K. 319 Figure D.7. endo-[Fe(η4-Cp*H)(dppe)(CO)]-H/D, FT-IR ATR, thin film, 298 K ........ 319 Figure D.8. exo-[Fe(η4-Cp*H)(dppe)(CO)], 1H NMR, C6D6, 400 MHz, 298 K............ 320 Figure D.9. exo-[Fe(η4-Cp*H)(dppe)(CO)] stacked plot, 1H NMR, C6D6, 400 MHz, 298 K...................................................................................................................................... 320 Figure D.10. exo-[Fe(η4-Cp*H)(dppe)(CO)], 31P{1H} NMR, C6D6, 162 MHz, 298 K . 321 Figure D.11. exo-[Fe(η4-Cp*H)(dppe)(CO)], 13C{1H} NMR, THF-d8, 100 MHz, ....... 321 Figure D.12. exo-[Fe(η4-Cp*H)(dppe)(CO)]-H/D, FT-IR ATR, thin film, 298 K ........ 322 Figure D.13. [FeII(η5-Cp*)(dppe)(CO)][BArF4], 1H NMR, THF-d8, 400 MHz, 298 K 322 Figure D.14. [FeII(η5-Cp*)(dppe)(CO)][BArF4], 31P{1H} NMR, THF-d8, 162 MHz, 298 K.........................................................................................Error! Bookmark not defined. Figure D.15. [FeII(η5-Cp*)(dppe)(N2)][BArF4], 1H NMR, THF-d8, 400 MHz, 298 K .. 323 Figure D.16. [FeII(η5-Cp*)(dppe)(N2)][BArF4] 31P{1H} NMR, THF-d8, 162 MHz, 298 K ......................................................................................................................................... 323 Figure D.17. [FeII(η5-Cp*)(dppe)(N2)][BArF4], 13C{1H} NMR, THF-d8, 100 MHz, 298 K ......................................................................................................................................... 324 Figure D.18 [FeII(η5-Cp*)(dppe)(CO)][BArF4], FT-IR ATR, thin film, 298 K ............. 325 Figure D.19. CNXyl, FT-IR ATR, thin film, 298 K ..................................................... 325 xiv Figure D.20. Heating [FeIII(η5-Cp*)(dppe)H]+ at 80 oC, 1H NMR, THF-d8, 400 MHz, 298 K...................................................................................................................................... 326 Figure D.21. Heating [FeIII(η5-Cp*)(dppe)H]+ at 80 oC, 31P{1H} NMR, THF-d8, 162 MHz,. .............................................................................................................................. 327 Figure D.22. [Fe(Cp*)(dppe)(N2)][BArF4], 1H NMR, THF-d8, 400 MHz ..................... 327 Figure D.23. FeII(η5-Cp*)(dppe)(CNXyl)][BArF4], 31P{1H} NMR, THF-d8, 162 MHz, 298 K............................................................................................................................... 328 Figure D.24. [FeII(η5-Cp*)(dppe)(CNXyl)][BArF4], 13C{1H} NMR, THF-d8, 100 MHz, 298 K .............................................................................................................................. 328 Figure D.25. [FeII(η5-Cp*)(dppe)(CNXyl)][BArF4] BArF4, FT-IR ATR, ...................... 329 Figure D.26. [FeII(η5-Cp*)(dppe)(NCMe)][BArF4], 1H NMR, ACN-d3, 400 MHz, ...... 330 Figure D.27. [[FeII(η5-Cp*)(dppe)(NCMe)][BArF4], 31P{1H} NMR, ACN-d3, 162 MHz, 298 K............................................................................................................................... 331 Figure D.28. 31P{1H} NMR, C6D6, 162 MHz, 298 K for the treatment of PCET reagents with CO2.......................................................................................................................... 333 Figure D.29. 1H NMR, C6D6, 162 MHz, 298 K for the treatment of PCET reagents with CO2 ................................................................................................................................. 334 Figure D.30. 1H NMR, C6D6, 400 MHz, 298 K of PCET reagents with azobenzene ... 337 Figure D.31. 31P{1H} NMR, C6D6, 162 MHz, 298 K of PCET reagents with. azobenzene ......................................................................................................................................... 338 Figure D.32. [Fe(Cp*)(dppe)(N2)][BArF4], 80 K 57Fe Mössbauer spectrum ................ 339 Figure D.33. [FeIII(η5-Cp*)(dppe)H]+, UV-Visible spectrum after 24 h, 2-MeTHF, 298 K ......................................................................................................................................... 340 Figure D.34. endo-[Fe(η4-Cp*H)(dppe)(CO)] and exo-[Fe(η4-Cp*H)(dppe)(CO)], UVVisible spectrum (2-MeTHF, 298 K).............................................................................. 340 Figure D.35. endo-[Fe(η4-Cp*H)(dppe)(CO)]+ and exo-[Fe(η4-Cp*H)(dppe)(CO)]+, UVVisible spectrum (2-MeTHF, 218 K)…………………………………………….…….341 Figure D.36. CW X-band EPR data for [FeIII(η5-Cp*)(dppe)H]+ ................................. 342 Figure D.37. CW X-band EPR data for endo-[Fe(η4-Cp*H/D)(dppe)(CO)]+ ............... 343 Figure D.38. CW X-band EPR data for endo-[[Fe(η4-Cp*H/D)(dppe)(CNXyl)]]+ ....... 344 Figure D.39. CW X-band EPR data for exo-[Fe(η4-Cp*H/D)(dppe)(CO)]]+. ............... 345 xv Figure D.40. EPR sample of [Fe(η4-Cp*H)(dppe)(CNXyl)]+ frozen at 77 K................ 346 Figure D.41. Field-dependent X-band 31P Davies ENDOR of endo-[[Fe(η4Cp*D)(dppe)(CO)]]+ ....................................................................................................... 347 Figure D.42. Field-dependent X-band 31P Davies ENDOR of exo-[[Fe(η4Cp*D)(dppe)(CO)]]+ . ..................................................................................................... 348 Figure D.43. Field-dependent X-band Davies ENDOR spectra of exo[Fe(η4Cp*H/D)(dppe)(CO)]- .................................................................................................... 349 Figure D.44. Field-dependent X-band 31P Davies ENDOR of endo-[Fe(η4Cp*H)(dppe)(CNXyl)]+ .................................................................................................. 350 Figure D.45. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*H)(dppe)(CO)]+ (top panels) .................................................................................... 351 Figure D.46. Field-dependent X-band HYSCORE spectra of endo- endo-[Fe(η4Cp*H)(dppe)(CNXyl)]+ .................................................................................................. 352 Figure D.47 Field-dependent X-band HYSCORE spectra of endo-[[Fe(η4Cp*H/D)(dppe)(CO)]]+. .................................................................................................. 353 Figure D.48. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*H/D)(dppe)(CO)]+. ................................................................................................... 354 Figure D.49. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*H/D)(dppe)(CO)]+ .................................................................................................... 355 Figure D.50. Field-dependent X-band HYSCORE spectra of exo-[Fe(η4Cp*D)(dppe)(CO)]+. ....................................................................................................... 356 Figure D.51. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*H/D)(dppe)(CXNyl)]+ ............................................................................................. 357 Figure D.52. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*H/D)(dppe)(CNXyl)]+ . ............................................................................................ 358 Figure D.53. Field-dependent X-band HYSCORE spectra of endo-[[Fe(η4Cp*H/D)(dppe)(CNXyl)]+ ............................................................................................. 359 Figure D.54. Field-dependent X-band HYSCORE spectra of endo-[Fe(η4Cp*D)(dppe)(CNXyl)]+ .................................................................................................. 360 Figure.D.55. Cyclic voltammogram of endo-[Fe(η4-Cp*H)(dppe)(CO)], . ................... 361 Figure D.56. Cyclic voltammogram of exo-[Fe(η4-Cp*H)(dppe)(CO)]. ....................... 362 xvi Figure D.57. Solid state structure of endo/exo-[Fe(η4-Cp*H)(dppe)(CO)] ................... 367 Figure D.58. Solid state structure of endo-[Fe(η4-Cp*H)(dppe)(CO)][BArF4], ............ 368 Figure D.59. Solid state structure of [Fe(Cp*)(dppe)(N2)][BArF4],............................... 368 Figure D.60. Solid state structure of [Fe(Cp*)(dppe)(CNXyl)][BArF4] ........................ 369 Figure D.61. DFT-optimized structures showing experimental and predicted A(1H) values .............................................................................................................................. 375 Figure D.62. Thermochemical scheme relating H+, H•, and H− transfers for endo-[Fe(η4Cp*H)(dppe)(CO)]+. ....................................................................................................... 407 xvii LIST OF TABLES Table 3.1. Experimental g-values, and experimental and theoretical hyperfine in MHz of (P3B)Fe(N) ......................................................................................................................... 57 Table 3.2. Calculated Lӧwdin spin populations and experimental spin density estimations of (P3B)Fe(N) ..................................................................................................................... 62 Table A1. Fit parameters for the chemicals shift of [(P2PPh)Fe]2-μ-N2.......................... 150 Table A2. Results of catalytic runs using (P2PPh)FeBr at 150 equiv. ............................. 171 Table A3. Results of individual runs using (P2PPh)Fe(N2)2 at 50 equiv. acid loading with no Hg lamp irradiation. ................................................................................................... 171 Table A4. Results of individual runs using (P2PPh)Fe(N2)2 at 150 equiv. acid loading with no Hg lamp irradiation. ........................................................................................... 171 Table A5. Results of individual runs using (P2PPh)Fe(N2)2 at 150 equiv. acid loading with Hg lamp irradiation. ................................................................................................ 172 Table A6. Results of individual runs using (P2PPh)Fe(N2)2 at 150 equiv. acid loading with one equiv. TBABr added. ....................................................................................... 172 Table A7. Results of individual runs using (P2PPh)Fe(N2)2 with 150 equiv. Ph2NH2OTf with no Hg lamp irradiation. ........................................................................................... 172 Table A8. HER Yields ................................................................................................... 174 Table A9. A comparison of gas phase optimized and experimental bond parameters of [(P2PPh)Fe(NNTMS)]−, [(P2PPh)Fe(NNH)]− and experimental values from X-ray data of [(P2PPh)Fe(NNTMS)]K. .................................................................................................. 197 Table A10. Energies of gas-phased optimized geometries of (P2PPh)Fe(N2) ................. 199 Table C1. Crystallographic data for Fe(η6-IndH(depe)][BArF4]. ................................... 292 Table C2. Crystallographic data of Fe(6-C6H5CH3)(dippe)][BArF4]. .......................... 293 Table C3. Summary of data obtained from X-ray analyses ........................................... 294 Table C4. Summary of DFT-calculated EPR parameters for [FeIIICp*(dppe)H]+ ........ 297 Table C5. Summary of DFT-calculated 57Fe Mössbauer parameters ............................ 298 Table C6. Mulliken atomic spin densities calculated for [Fe(3:2Ind)(depe)(H)][BArF4]. ................................................................................................... 299 Table C7. Mulliken atomic spin densities calculated for [Fe(6-IndH)(depe)][BArF4]. 300 xviii Table C8. Mulliken atomic spin densities calculated for [Fe(6C6H5CH3)(dippe)][BArF4] . ............................................................................................. 301 Table D1. Azobenzene reduction yields ........................................................................ 336 Table D2. Crystallographic data for endo/exo-[Fe(η4-Cp*H)(dppe)(CO)]. ................... 364 Table D3. Crystallographic data for endo-[Fe(η4-Cp*H)(dppe)(CO)]BArF4]................ 365 Table D4. Crystallographic data for [Fe(Cp*)(dppe)(CNXyl)]BArF4............................ 366 Table D5. EPR parameters of [Fe(H)(Cp*(dppe)]+ with different functionals and basis sets................................................................................................................................... 371 Table D6. EPR parameters of endo-[Fe(η4-Cp*H)(dppe)(CO)]+ with different functionals and basis sets. ................................................................................................................. 372 Table D7. Calculated energies and isotropic 1H hyperfine coupling for rotational isomers of exo-[Fe(η4-Cp*H)(dppe)(CO)]+ around the Fe-Cp*H centroid axis. ......................... 373 xix Abbreviations βe The Bohr magneton δ Isomer shift or chemical shift ΔEq Quadrupole splitting 12-c-4 12-crown-4 15-c-5 15-crown-5 18-c-6 18-crown-6 1 H Hydrogen-1 2 H Hydrogen-2 11 Boron-11 13 Carbon-13 B C 15 N Nitrogen-15 31 P Phosphorus-31 2-MeTHF 2-methyltetrahydrofuran [BArF4]− Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate [OTf] − Trifluoromethanesulfonate [TBA]+ Tetrabutylammonium nBuLi n-butyllithium A Hyperfine tensor aiso Isotropic hyperfine value BDFE Bond Dissociation Free Energy CO Carbon Monoxide Cp Cyclopentadienide Cp* Pentamethylcyclopentadienide xx CPET Concerted Proton-electron Transfer CV Cyclic Voltammogram CW Continuous Wave DFT Density Functional Theory dppe 1,2-bis(diphenylphosphino)ethane ENDOR Electron Nuclear Double Resonance EPR Electron Paramagnetic Resonance Et2O Diethyl Ether Fc Ferrocene (FeCp2) G Gauss HAT Hydrogen Atom Transfer HER Hydrogen Evolving Reaction HOMO Highest-Occupied Molecular ORbital HYSCORE Hyperfine Sublevel Correlation Hz Hertz K Kelvin KC8 Potassium Graphite kcal kilocalorie iPr isorpropyl IR Infrared MO Molecular Orbital Na/Hg Sodium–mercury amalgam N2RR Nitrogen reduction reaction xxi NMR Nuclear Magnetic Resonance P2PPh Bis(o-diisopropylphosphino-phenyl)-phenylphosphine P3B Tris(o-diisopropylphosphinophenyl)borane PCET Proton-coupled Electron Transfer Q-band Approximately 34 GHz SQUID Super-conducting Quantum Interference Device T Tesla THF Tetrahydrofuran TMS Trimethylsilyl TiPS Triisorpopylsilyl X-band Approximately 9.4 GHz 1 Chapter 1 : Introduction 2 1.1 Opening Remarks The global nitrogen cycle is a crucial biogeochemical cycle required to sustain most life on Earth.1 The proton-coupled reduction of molecular nitrogen (N2) into ammonia (NH3), a small but crucial part of this cycle, is the focus of this thesis. In the following chapters this process is referred to nitrogen fixation or the nitrogen reduction reaction (N2RR). Although almost 80% of our atmosphere consists of dinitrogen and nitrogen is a key component in many biomolecules (e.g. amino acids, DNA, and RNA), the nitrogen in our atmosphere cannot be directly incorporated into these biomolecules. Dinitrogen must first be fixed into a chemically reactive form such as nitrate or ammonia through nitrogen fixation.1 The formation of bioavailable nitrogen sources can occur via non-biological natural processes, as in the splitting of N2 by lightning, or via biological and industrial processes.1,2 The biological process of converting N2 to reduced forms, such as NH3, in essence nitrogen fixation, is performed by a class of enzymes termed Nitrogenases (Figure 1.1). These enzymes use abundantly available N2 from air, and protons derived from water to produce ammonia (Figure 1.1). In addition to the biological process, which accounts for almost half of all fixed nitrogen, an industrial counterpart exists: the Haber-Bosch process (Figure 1.1). Nowadays, biological nitrogen fixation and the Haber-Bosch process both account for nearly equivalent amounts of "fixed" nitrogen globally.3–7 Although the HaberBosch process is routinely employed today, it boasts some major drawbacks such as the requirement of high pressure and temperature. It is estimated that 3-5% of the world’s annual natural gas production is consumed in the Haber Bosch process, which translates into 1-2% of the world’s annual energy consumption.8,9 The increased access to readily 3 available ammonia in the last century resulted in a fundamental altering of the global carbon and nitrogen cycle, and the consequences of these disruptions will continue to play out for millennia.10 Figure 1.1. (Top) Biological nitrogen fixation is catalyzed by the iron-molybdenum cofactor (FeMoco) in nitrogenase enzymes. (Bottom) Stoichiometry and conditions for the conversion of dinitrogen (N2) into ammonia (NH3) by nitrogenase and the Haber-Bosch process (Bottom). 1.2 Synthetic Molecular Catalysts for N2RR Studies into the nitrogenase enzyme have revealed three major subtypes: ironmolybdenum nitrogenase, vanadium-iron nitrogenase, and all iron nitrogenase. Despite the advances made in the last decades, such as the characterization of the interstitial atom,11 an atomically-precise mechanism for N2 reduction by nitrogenase is far from established. Even basic questions, such as geometry and number of ligating sulfur atoms during catalysis remain unanswered.12 Since the discovery of the nitrogenase cofactors, biological nitrogen fixation has provided inspiration in the development of synthetic N2 fixation catalysts. 4 The binding of N2 to a synthetic transition metal center was first observed for a Ru ammine complex, synthesized by Allen and Senoff in 1965.13 Binding of N2 to a metal center results in charge-transfer from the metal to the N2 π* orbitals, polarizing the N2 ligand, towards functionalization with electrophiles. Chatt and coworkers demonstrated this activation followed by functionalization in 1972, reporting the first well-defined example of protonation of N2 bound to a transition metal center.14 Soon after, many more systems where reported, with some capable of complete reduction of N2 to NH3 and N2H4.15 On the basis of stochiometric studies, Chatt and co-workers proposed a mechanism for the catalytic reduction of N2 by a single-site transition metal catalyst (Figure 1.2).15 In the distal cycle (sometimes referred to as Chatt cycle), a metal-bound N2 moiety is sequentially reduced at Nβ prior to the key N-N bond cleavage step to produce a terminal metal nitrido (M≡N). On the other hand, a mechanism in which an N2 moiety is reduced through alternating reduction at Nβ and Nα is possible, which is referred to as the alternating mechanism (Figure 1.2). Several intermediates, such as FeNNH2+, and FeNH2NH2+ which represent intermediates along both pathways have been isolated on the ligand platforms studied in our group.16–18 Recently, our group reported the characterization of a terminal iron nitride (FeIV≡N), through stepwise protonation of Nβ from [(P3B)Fe(N2)]2, providing direct evidence for the feasibility of the distal pathway for this catalytic system. The synthesis and characterization of the proposed subsequent reduction product, a formal FeIII nitrido, is the focus of Chapter 3. 5 Figure 1.2. A schematic depiction of postulated N2 binding and reduction at an Fe site by limiting distal (top) and alternating (bottom) mechanisms. Since the initial characterization of the Ru amine complex, the search for molecular models has yielded hundreds of metal complexes that can bind dinitrogen involving transition metals far beyond those observed in nature. However, only a few systems are capable of catalytic reduction of N2 to products such as hydrazine (N2H4) and19,20 tris(trimethylsilyl)amine21–27. Even fewer examples are known of direct catalytic fixation of N2 to NH3.19,28–43 Schrock and Yandulov reported the first well-defined molecular system suitable for catalytic nitrogen fixation and observed up to eight equivalents of ammonia per molybdenum center.34 More recently, Nishibayashi and coworkers obtained higher yields with molybdenum-based systems.35,36,38,43,44 It was not until 2013 that iron-based fixation of nitrogen by a molecular model system was reported and since then the number of iron systems have been increasing steadily.20,37,40,42,45 6 Although the catalysts studied in our group are not an exact copy of the nitrogenase cofactor, design elements, including the metal identity (Fe), threefold symmetry, and the flexible anchor atom are used (Figure 1.3). Based on the success with [(P3B)Fe(N2)]-, new trisphosphine-based ligands were developed, but these have not yet matched the catalytic activity observed with the P3B ligand. Elucidating the principles behind the activity required driving force, and selectivity of these catalysts may inform the design of novel nitrogen fixation catalysts. Figure 1.3. Molecular nitrogen fixation catalysts used in this thesis. 1.3 Selectivity in Nitrogen Fixation Catalysis One of the challenges of small molecule reduction is selectivity, with the focus of recent research being the selectivity for N2 vs H+ reduction. H2 formation can occur directly via a reaction between the acid and reductant, which is strongly dependent on the reductant and acid used or mediated by the nitrogen fixation catalysts. The use of HBArF4 and KC8 provides the most complete comparison between HER catalyzed by the iron systems discussed. All P3EFe catalysts are formally N2RR catalysts, but have drastically different selectivities for N2RR and HER.41 Various mechanisms have been proposed through which 7 HER is catalyzed by the nitrogen fixation catalysts. Early intermediates along the cycle have weak bonds and are thermodynamically set up to evolve H2 (Figure 1.4). On the other hand, the selectivity for N2RR over HER is determined by the site of protonation. Protonation at the metal center leads to formation of a transition metal hydride (M-H), which can subsequently release H2 upon protonation (Figure 1.4). Figure 1.4. Overview of proposed HER mechanism in iron mediated nitrogen fixation. (Top) Direct HER via reaction of acid and reductant in solution. (Bottom) Two of the proposed pathways for HER mediated by the iron-based catalyst. Currently the factors that govern the activity and selectivity of these catalyst is not understood. Chapter 2 describes an investigation into some of the factors that govern the driving force, selectivity, and activity of (P2PPh)Fe(N2)2, and the role of hydrides in HER is discussed. 1.4 Weak N-H bonds as Determinants of the Driving Force in Nitrogen Fixation Three limiting mechanisms can be considered for the formation of N-H bonds. Two stepwise pathways, either electron transfer followed by proton transfer and vice versa 8 (Scheme 1.1). Alternatively, a concerted pathway in which a proton and electron are delivered simultaneously is possible. The stepwise pathways have long been considered in N-H bond formation, and recently the proton coupled electron transfer (PCET) has been considered based on our groups observations that acids can protonate commonly used reductants in nitrogen fixation. Scheme 1.1. Square scheme relating the stepwise ET-PT and PT-ET pathway to the PCET pathway for the first step of nitrogen fixation. The proton-coupled reduction of N2 requires the formation of a series of N-H bonds with varying bond strengths. The various intermediates encountered during the nitrogen fixation cycle (Figure 1.2) have intrinsically different N-H bond strengths, but on average, the N-H bond strengths formed during nitrogen fixation is 54.4 kcal mol-1. The strength of an X-H bond can be determined through determination of the relevant reduction potential and acidity (Eq 1.1). BDFEeff = 1.37pKa + 23.06E° + C Eq 1.1 Equation 1.1 can be extended beyond a discrete X-H bond to a reductant acid combination, as used in small molecule activation. In such a scenario, E° corresponds to 9 the reduction potential of the reductant, and the pKa corresponds to that of the acid used. During nitrogen fixation, formation of an N-H bond is only favorable if BDFEeff of the reductant and acid combination is lower than BDFEN-H (ΔG < 0). For the formation of the weakest N-H bond to be downhill, an acid/reductant combination with a weaker BDEFeff is required. For (P3B)Fe, the weakest N-H bond formed is below 30 kcal mol-1. In an ideal situation, each N-H bond formed would have the same strength i.e., 54.4 kcal mol-1, as this allows the reaction to proceed with no overpotential. 1.5 Development of Reagents with Weak X-H Bonds As the BDFE of N-H bonds formed during N2 fixation is incredibly low (< 30 kcal mol-1), reagents with even weaker BDFEeff bonds are required for a thermodynamically favorable reaction. Such low BDFE’s can be achieved by carefully choosing a reductant and acid, with a sufficiently low BDFEeff (Eq 1.1). However, these separate reagents do not necessarily go through a PCET pathway. Well defined complexes with sufficiently weak X-H are basically non-existent, as they rapidly evolve H2. Reagents with sufficiently weak X-H bonds can be generated through the coordination-induced weakening of an O-H bond, for example by paring SmI2 with a polar protic solvent. Recently, our group provided evidence for the protonation of Cp*2Co via (CW) EPR spectroscopy.46 The C-H bonds formed upon protonation of the Cp* fragment (BDFE < 29 kcal mol-1) is almost 70 kcal mol-1 weaker than a typical C-H bond and such bonds have the potential to be utilized in the formation of weak N-H bonds during the proton-coupled reduction of N2. Due to the weak C-H bond and propensity to release H2 even at temperatures below −100 °C, productive X-H bond formation through PCET could be observed. However, the developed understanding of conditions that leads to these remarkably weak C-H bonds 10 (Scheme 1.2) can be used as a design principle for other complexes with similar weak CH bonds. The development of novel reagents with such weak C-H bonds, with applications in N-H bond formation will be the focus of Chapters 4 and 5. Scheme 1.2. A comparison of the experimental BDFEC–H for a variety of related Cp*Cospecies, demonstrating the importance of aromaticity and electron count in predicting the stability of the indicated C–H bond.46 11 1.6 References (1) Smil, V. Enriching the Earth; MIT Press: Cambridge, MA, 2001. (2) Howard, J. B.; Rees, D. C. How Many Metals Does It Take to Fix N2? A Mechanistic Overview of Biological Nitrogen Fixation. Proceedings of the National Academy of Sciences 2006, 103 (46), 17088–17093. (3) Canfield, D. E.; Glazer, A. N.; Falkowski, P. G. REVIEW The Evolution and Future of Earth ’ s Nitrogen Cycle. Science 2010, 330, 192–196. (4) Gruber, N.; Galloway, J. N. An Earth-System Perspective of the Global Nitrogen Cycle. Nature 2008, 451 (7176), 293–296. (5) Thamdrup, B. New Pathways and Processes in the Global Nitrogen Cycle. 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Catalytic Dinitrogen Reduction at the Molybdenum Center Promoted by a Bulky Tetradentate Phosphine Ligand. Angewandte Chemie - International Edition 2014, 53 (51), 14206–14210. (27) Pellei, M.; Lobbia, G. G.; Santini, C.; Spagna, R.; Camalli, M.; Fedeli, D.; Falcioni, G. Synthesis, Characterization and Antioxidant Activity of New Copper(I) Complexes of Scorpionate and Water Soluble Phosphane Ligands. Dalton Trans. 2004, 17, 2822–2828. 13 (28) Fryzuk, M. D.; Johnson, S. A. The Continuing Story of Dinitrogen Activation. Coordination Chemistry Reviews 2000, 200–202, 379–409. (29) MacKay, B. a.; Fryzuk, M. D. Dinitrogen Coordination Chemistry: On the Biomimetic Borderlands. Chemical Reviews 2004, 104 (2), 385–401. (30) Gardiner, M. G.; Stringer, D. N. Dinitrogen and Related Chemistry of the Lanthanides: A Review of the Reductive Capture of Dinitrogen, as Well as Mono- and DiAza Containing Ligand Chemistry of Relevance to Known and Postulated Metal Mediated Dinitrogen Derivatives. Materials 2010, 3 (2), 841–862. (31) Shilov, A. E. Catalytic Reduction of Molecular Nitrogen in Solutions. Russian Chemical Bulletin 2003, 52 (12), 2555–2562. (32) 653. Pickett, C. J.; Talarmin, J. Electrosynthesis of Ammonia. Nature 1985, 317, 652– (33) Chatt, J.; Heath, G. A.; Richards, R. L.; Fixation, N. 2074 J.C.S. Dalton. J. Chem. Soc., Dalton Transactions 1974, 2074–2082. (34) Yandulov, D. V.; Schrock, R. R. Esearch Rticles. Science 2003, 301, 76–79. (35) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. A Molybdenum Complex Bearing PNP-Type Pincer Ligands Leads to the Catalytic Reduction of Dinitrogen into Ammonia. Nature Chemistry 2011, 3 (2), 120–125. (36) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. Catalytic Formation of Ammonia from Molecular Dinitrogen by Use of Dinitrogen-Bridged Dimolybdenum-Dinitrogen Complexes Bearing Pnp-Pincer Ligands: Remarkable Effect of Substituent at Pnp-Pincer Ligand. Journal of the American Chemical Society 2014, 136 (27), 9719–9731. (37) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501 (7465), 84–87. (38) Tanaka, H.; Arashiba, K.; Kuriyama, S.; Sasada, A.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Unique Behaviour of Dinitrogen-Bridged Dimolybdenum Complexes Bearing Pincer Ligand towards Catalytic Formation of Ammonia. Nature communications 2014, 5, 3737. (39) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock, A. S.; Davis, W. M. Molybdenum Triamidoamine Complexes That Contain Hexa-Tert-Butylterphenyl, Hexamethylterphenyl, or p-Bromohexaisopropylterphenyl Substituents. An Examination of Some Catalyst Variations for the Catalytic Reduction of Dinitrogen. Journal of the American Chemical Society 2004, 126 (19), 6150–6163. (40) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Evaluating Molecular Cobalt Complexes for the Conversion of N2 to NH3. Inorganic Chemistry 2015, 54 (19), 9256–9262. 14 (41) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench 57 Fe Mössbauer Data, and a Hydride Resting State. Journal of the American Chemical Society 2016, 138 (16), 5341–5350. (42) Creutz, S. E.; Peters, J. C. Catalytic Reduction of N2 to NH3 by an Fe-N 2 Complex Featuring a C-Atom Anchor. Journal of the American Chemical Society 2014, 136 (3), 1105–1115. (43) Arashiba, K.; Kinoshita, E.; Kuriyama, S.; Eizawa, A.; Nakajima, K.; Tanaka, H.; Yoshizawa, K.; Nishibayashi, Y. Catalytic Reduction of Dinitrogen to Ammonia by Use of Molybdenum-Nitride Complexes Bearing a Tridentate Triphosphine as Catalysts. Journal of the American Chemical Society 2015, 137 (17), 5666–5669. (44) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molybdenum-Catalysed Ammonia Production with Samarium Diiodide and Alcohols or Water. Nature 2019, 568 (7753), 536–540. (45) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. Catalytic N2-to-NH3Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET. ACS Central Science. 2017, pp 217–223. (46) 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 (11), 4721–4729. 15 Chapter 2 : Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation Reproduced in part with permission from: Schild, D. J.; Peters, J. C. Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation ACS Catal. 2019, 9, 5, 4286–4295. © 2019 American Chemical Society 16 2.1. Introduction Substantial progress has been made in the development and understanding of molecular catalysts for N2-to-NH3 conversion, commonly referred to as the nitrogen reduction reaction (N2RR).1,,3 The number of well-defined complexes capable of N2RR is expanding rapidly, and significant improvements in turnover and efficiency have been made.4,5 With the growing number of systems available, it becomes increasingly possible to uncover general design principles that will aid in further progress for the field. The selectivity of N2RR versus the competing hydrogen evolution reaction (HER) is a central selectivity issue in need of model studies.6 Competing HER not only limits the efficiency of molecular catalyst systems, but also limits nitrogenase enzymes.7,8 Additionally, and relatedly, a deeper understanding as to why seemingly related synthetic catalysts often require very different reductant and acid combinations to be competent for N2RR is needed. HER can occur by the background reaction between the reductant and acid; synthetic N2RR catalysts depend on limiting the rate of background HER relative to the catalytic N2RR rate. A catalyzed HER process, presumably accessible and competitive for many N2RR catalysts, can also limit the efficacy of N2RR selectivity. Both scenarios can be at play.4,9 For a complex that catalyzes both N2RR and HER, numerous pathways for the latter process are possible. H2 may evolve via protonation of a metal-bound hydride,3,9 a commonly proposed pathway for synthetic HER catalysts. Accordingly, the build-up of M– H species has been observed both during and after catalytic N2RR experiments.2,9,10 The accumulation of M–H species is generally thought to attenuate N2RR activity, and hydride precatalysts can give rise to diminished yields for N2RR.2,3,9 When hydride precursors serve 17 as active precatalysts for N2RR, it is presumed they react with acid and reductant to release H2, thereby generating a species that is on-path for N2RR.3,9 As an example of this, for a tris(phosphine)borane iron catalyst system studied extensively by our lab, [(P3B)Fe(N2)]− (P = o-(PiPr2)2C6H4), a dihydride intermediate was observed as an off-path resting state of the system when KC8 and HBArF4(Et2O) (BArF4 = tetrakis-(3,5- bis(trifluoromethyl)phenyl)borate) were employed (Figure 2.1).9 This dihydride species can be converted to an on path intermediate by reductive protonation.9 A conceptually similar pathway has been described by Nishibayashi and coworkers for a (PNP)Fe system (Figure 2.1).3 Figure 2.1. (Left) Protonation and reduction of off-path iron hydrides results in the formation of on-path species capable of N2RR. (Right) Previously reported iron based catalysts and the conditions under which light enhanced nitrogen fixation by mercury lamp irradiation was observed.1g Another possible competing HER pathway present within N2RR systems that has been considered by our lab involves a bimolecular, NxHy ligand-mediated step wherein two Fe(NxHy) intermediates that feature weak N–H bonds evolve H2 (Eqs. 2.1 and 2.2).6 2 Fe-NNH → 2 Fe-N2 + H2 (Eq. 2.1) 18 2 Fe-NNH2 → 2 Fe-NNH + H2 (Eq. 2.2) Protonation at the metal versus at coordinated N2 to form a metal hydride should be thermodynamically favored,6 though the kinetic site of protonation can involve the coordinated N2 ligand. Even if protonation at N2 is kinetically favored, this can be followed by intra- or intermolecular H-atom/hydride/proton transfer to form a metal hydride.11 Initial protonation at a site on the auxiliary ligand can also be kinetically favored.12 Protonation at the terminal nitrogen (Nβ) is desired for selectivity towards nitrogen fixation. For the P3BFe-system, iron is by far the thermodynamically favored site for protonation. However, the steric profile of the complex and the acids used appear to render functionalization at Nβ kinetically favorable.13 Recently, our group reported two related iron-based complexes bearing hydride ligands, (P2PPh)Fe(H)]2(µ–N2) (1) and (P2PPh)Fe(N2)(H)2 (2) (P2PPh = bis(odiisopropylphosphino-phenyl)-phenylphosphine), that are modestly active systems for N2RR (Figure 2.1, right).2 For complex 2, photo-induced H2 elimination was proposed to yield a more activated Fe–N2 species that could undergo subsequent reductive protonation steps to generate NH3. An H2-elimination step can also be observed for (P3B)(µH)Fe(N2)(H), resulting in the formation of (P3B)Fe(N2).2,14 Although precatalysts 1 and 2 are significantly less efficient for N2RR than [(P3B)Fe(N2)]− and certain other metal catalysts, they provide a fascinating model system for in-depth study because they have been shown to display substantial enhancement for N2RR under irradiation.2 Furthermore, these catalysts bear hydride ligands but are nonetheless active for N2RR, affording an opportunity to investigate the role of the hydride ligands in N2RR and competing HER. Finally, a better understanding of the electronic and 19 structural factors that influence the required redox potential for N2RR in this phosphineiron catalyst system compared to other systems can aid in the development of selective catalysts that operate at a comparatively low net driving force. 2.2 Results and Discussion 2.2.1 Synthesis and structural Analysis In our prior communication we proposed that the product of H2 elimination from well-characterized dihydride 2 might be “(P2PPh)Fe(N2)”.2 Reasoning that this or a related species might be on-path for N2RR, we targeted an independent synthesis. (P2PPh)FeBr2 (3) provided a logical starting point. Treatment of 3 with 1.05 equiv sodium mercury amalgam resulted in the formation of (P2PPh)FeBr (4) in 65% yield (Scheme 2.1). 4 exhibits Cs symmetry in solution based on its 1H NMR spectrum and a distorted tetrahedral geometry (τ4 = 0.77)15 in the solid state. Scheme 2.1. Preparation of dinitrogen adducts of (P2PPh)Fe from the bromide precursors 3 and 4. 20 Bromide 4 is a useful synthon for several complexes of present interest. For example, treatment of 4 with NaHBEt3 in toluene at −78 °C provides a more favorable route to the diiron complex 1 (Scheme 2.1), whose preparation was previously described by NaHBEt3 reduction of 3. Furthermore, 4 can be reduced with sodium mercury amalgam in either benzene or THF to provide a new, maroon red complex (P2PPh) Fe(N2)2 (5). Complex 5 can be alternatively prepared by reduction of 3 with excess sodium mercury amalgam in benzene (Scheme 2.1). 18-electron 5 exhibits two intense bands in its IR spectrum (thin film; symm = 2065 cm−1, asymm = 2005 cm−1) and its solid-state crystal structure, reveals a distorted trigonal bipyramidal geometry at iron (τ5 = 0.54)16 with Fe–P distances ~0.15 Å shorter than in 3, reflecting its singlet ground state.2 The structure and stretching frequencies of the fivecoordinate N2 complex 5 is closely related to recently reported (PRPCy2)Fe(N2)2 (PRPCy2 = RP(CH2CH2PCy2)2, R = Ph, tBu) complexes.17 In the latter N2 complexes, facile N2 dissociation hampers their isolation. Although 5 is also susceptible to N2 dissociation (vide infra) it can be readily isolated by evaporation of the solvent in vacuo followed by extraction with pentane. 2.2.2. N2-Binding Equilibria of 5. A solution equilibrium exists between 5 and a dinuclear, mono-N2-bridged complex 6 (Scheme 2.1). This is clearly gleaned from 1H and 31P NMR spectroscopies. For example, the 1H NMR signal intensities for 5 decay upon degassing the solution in a J-Young NMR tube and the signals corresponding to 6 grow in. Addition of N2 regenerates 5. The absence of an N2 stretch in the IR spectrum of 6, and the release of 1.5 equiv of N2 per Fe on 21 conversion of 5 to 6, as measured by a Toepler pump experiment, are consistent with our formulation of 6 (Scheme 2.1; Eq. 3): 2 (P2PPh)Fe(N2)2 → {(P2PPh)Fe}2(µ–N2) + 3/2 N2 (Eq. 3) Figure 2.2. The 1H NMR chemical shifts of 6 plotted as a function of 1/T display a deviation from Curie-behavior, due to the population of a low-lying exited state. A fit of the data, indicated with black lines, gives J = −940 ± 9.4 cm−1. Monitoring the 1H and 31P chemical shifts of 6 over a 130 °C range under vacuum reveals deviation from Curie-behavior (Figure 2.2). The singlet ground state of 6 likely arises from antiferromagnetic coupling of two S = 1 iron nuclei. The dramatic shifts in the NMR spectra are therefore attributed to partial population of triplet and quintet states 22 separated by 2J and 6J from the ground state, respectively (as obtained for the HeisenbergDirac-VanVleck Hamiltonian in the notation HHDVV = −2JS1S2). Fitting of the appropriate Boltzmann function to the experimental data yields J = −940 ± 9.4 cm−1.18 Antiferromagnetic coupling for an N2–bridged diiron species has been observed previously.19 2.2.3 Increased Turnover with Non-Hydride Precatalysts and Identification of OffPath Species. Previous N2RR studies using the (P2PPh)Fe-system were performed with the hydride complexes 1 and 2 as (pre)catalysts. We wondered whether increased turnover numbers might be realized with (P2PPh)Fe(N2)2, 5, devoid of hydride ligands. Indeed, catalysis under the same conditions with 5 resulted in significantly higher NH3 yields than those afforded by 1 and 2. For example, at a loading of 150 equivalents acid and 180 equivalents reductant at −78 C in Et2O, in the absence of light, complexes 1 and 2 catalyzed only 3.6 ± 0.6 and 2.6 ± 0.01 equiv NH3 per iron, respectively, whereas 5 catalyzed the generation of 6 ± 0.5 equiv. Interestingly, a comparable NH3 yield (5.1 ± 0.02 equiv per iron center) could be realized with 5 using only 1/3 as much reductant and acid (50 equiv HBArF4(Et2O) and 60 equiv KC8), which was not the case for either 1 or 2. A possible explanation for this difference is that HER catalysis from the hydrides, which are present in the highest concentration at the onset of runs with 1 and 2, outcompetes N2RR. Similar NH3 yields were obtained for the three different precatalysts 1, 2, and 5 in catalytic experiments irradiated with a mercury lamp. We presume that dihydride 2 releases H2 upon irradiation with light to yield 5, and that this transformation occurs rapidly under turn-over conditions as all (pre)catalysts give similar yields. The consumption of hydride 23 species via photolysis reduces HER catalyzed by the hydrides, thus increasing overall efficiency for NH3. To determine whether catalyzed HER contributes to the low yields of NH3 obtained with dihydride 2, hydrogen evolution was measured under catalytically relevant conditions. As shown in Figure 2.3, the initial rate of H2 evolution at −78 °C, is significantly enhanced by the presence of either 2 or 5. These data suggests both 2 and 5 are comparatively competent catalysts for HER, whereas complex 5 is a more effective (pre)catalyst for N2RR. Indeed, the fact that most of the acid is consumed within 30 minutes at −78 °C speaks to how rapidly 5 must catalyze NH3 production for N2RR to be kinetically competitive. Figure 2.3. Time profiles of the formation of H2 from HBArF4 and KC8 in Et2O at −78 °C. Data are presented for the background reaction of these reagents in the absence of catalyst (black diamonds), as well as in the presence of either 2 (red circles) or 5 (blue squares). 24 Each time course was collected continuously from a single experiment. The final data points, recorded after 16 hours, are omitted from the graph. Additional evidence for the active role of 2 in HER is obtained from Mossbauer studies of freeze-quenched samples. Freeze-quenching of a catalytic run using 57Fe-labeled 2, 50 equiv acid, and 60 equiv reductant, shows its disappearance within 5 minutes (Figure 2.4, middle trace). A new broad feature, likely due to the overlap of several species, is observed. Freeze-quenching the reaction after 30 minutes provides a similarly broad signal (Figure 2.4, bottom trace), but one that also contains 2, with its characteristic small quadrupole splitting (constituting ~ 40% of the total). Experiments using 5 as the precatalyst provide an analogously broad signal after 5 and 30 minutes (See Figure A59S60). Notably, dihydride 2 is always observed at the end of a catalytic experiment, once the sample has been warmed to room temperature. It is the major species present (typically ~ 90% by Mössbauer spectroscopy). Furthermore, IR and NMR spectra recorded after runs using 5 as the precatalyst show 2 as the only identifiable species upon warming. These data collectively suggest that the catalytic system converts to a Fe–H species (2), which is on path for HER (vide infra), as the major product.2 This finding is similar to that of [(P3B)Fe(N2)]-, which also ends tied-up in an off-path hydride-borohydride state (Figure 2.1, left).9 25 Figure 2.4. Frozen solution Mӧssbauer spectra collected at 80 K in the presence of a 50 mT parallel magnetic field. Spectra of a catalytic mixture, using 2 (blue) as precatalysts (top), quenched after 5 minutes (middle) and 30 minutes (bottom) of stirring. For parameters of individual components, see Supporting Information. 26 2.2.4 Oxidative addition and reductive elimination of H2. To investigate potential pathways by which hydride species form during catalysis, stoichiometric reactions were performed with dihydride 2, the bis–N2 complex 5, and dinuclear 6. Addition of H2 to 5 (or 6), followed by N2, resulted in the quantitative formation of 2 (Scheme 2.2). However, addition of H2 to 5 at −78 °C in a J-Young tube for one hour resulted in the appearance of a trace amount of 2. Full conversion was only observed upon warming to room temperature. The latter result strongly suggests that the formation of 2 under the catalytic conditions at −78 °C does not occur by a reaction between 5 and H2. Scheme 2.2. Light induced reductive elimination of H2 from 2 leads to a transient unobserved four coordinate species, which binds N2 to form 5. The H2 elimination is reversible as 5 reacts back to 2 in the presence of H2. Irradiating solutions of 2 with a 100 W mercury lamp at −78 °C or room temperature results in darkening of the solution and the formation of 5 (Scheme 2.2). Complete disappearance of 2 is not observed, suggesting the reaction is reversible. A possible 16- 27 electron intermediate, such as “(P2PPh)Fe(N2)”,2 could not be identified by NMR, IR, or Mössbauer spectroscopy. H2 elimination from 2 to 5 leads to a significant decrease in (NN) stretching frequencies due to increased backbonding upon H2 elimination. A similar effect was observed previously for (P3B)(µ-H)Fe(N2)(H).2 In this context, these systems crudely model a proposed N2 binding/activation via H2 elimination at the E4 state of the iron-molybdenum cofactor (Figure 2.5).20 Clearly, increased N2 activation upon H2 elimination observed for this P2PPhFe-system would be even more pronounced for the unobserved, but perhaps catalytically relevant mono-N2 adduct “P2PPhFe(N2)” (vide infra). Figure 2.5. (Top) H2 elimination from the E4 state resulting in a more electron rich center is proposed from Mo-nitrogenase, and Fe-nitrogenase. Light induced reductive elimination of H2 from 2 leads to increased back-bonding due to the formal reduction from FeII to Fe0 (Bottom). 2.2.5 Stoichiometric reactivity and hydrogen evolution. To further probe HER catalysis by the present system, HBArF4 was added to (P2PPh)Fe(N2)2 5 at −78 °C, causing a color change from maroon to dark yellow upon 28 warming. The product of protonation at iron was identified as [(P2PPh)Fe(N2)2(H)][BArF4] (7) (Scheme 2.3), featuring a diagnostic 1H NMR hydride resonance (−17 ppm) and bands at 2069, 2194 and 2264 cm−1, corresponding to the (Fe–H) and (NN) IR stretches. Its solid-state structure was also determined. This complex can also be obtained by oxidation of {(P2PPh)Fe(H)}2(µ–N2) 1 with either FcBArF4 (Fc = bis(η5-cyclopentadienyl)iron) or HBArF4 (Scheme 2.3). Of primary interest, protonation of 2 with HBArF4 likewise generates 7 with concomitant H2 release, possibly via an “[Fe(H2)(H)]+” adduct.21 29 Scheme 2.3. Pathways towards 7 and 8. Preparative and NMR scale reactions were performed at −78 °C. Monohydride 7 can be cleanly reduced to dinuclear 1 using either Cp*2Co or stoichiometric KC8. Reduction of 7 with an excess of KC8 by contrast generates a different diamagnetic species which, following addition of 18-crown-6, could be isolated in pure form as [P2PPhFe(N2)(H)][K(18-crown-6)] (8) (Scheme 2.3). Complex 8 features a diagnostic hydride resonance in its 1H NMR spectrum ( = -9.69 ppm, dt), and its solidstate structure displays a short Fe–N (1.774(1) Å) and an elongated N–N (1.139(2) Å) bond. A high degree of activation of N2 is reflected by its (NN) (1924 cm−1). 30 Stoichiometric mixing of cation 7 and anion 8 resulted in comproportionation to 1 (> 90% yield). Proton transfer (PT) from 7 to 8 might have alternatively resulted in the formation of 2 and 5 (Scheme 2.4), but this was not observed. This may be rationalized by low acidity of the hydride ligand in 7, which is not deprotonated by NaOtBu. Relatedly, 8 is weakly basic and is not protonated by MeOH at −78 °C. The absence of proton transfer between 7 and 8 makes this an unlikely step for (re)generating 2 and 5 under turnover conditions. Dihydride 2 can, however, be obtained readily by protonation of 8 with HBArF4 at low temperature (Scheme 2.4). Scheme 2.4. Electron and proton transfer from 7 and 8 Apparent differences in reactivity of the hydrides with respect to N2RR can be rationalized by the availability, or lack of, kinetically competent pathways for the hydrides to be converted to on-path Fe–N2 species at −78 °C. For the P2PPh system, no pathway has been identified via which hydrides convert back to on-path Fe–N2 species. Instead, in stoichiometric reactions, the different hydrides interconvert in an HER cycle (Figure 2.6). However, the observation of NH3 production during catalytic experiments with 1 or 2 as precatalysts indicates that there must be some pathway to a species active for N2RR, even if comparatively inefficient. 31 Figure 2.6 Summary of the stoichiometric reactivity observed with [P2PPhFe(N2)x(H)y]+/0/−. Grey arrows indicate reactions that do not occur at −78 °C. Black arrows indicate reactions that occur at −78 °C 2.2.6. Reduction of (P2PPh)Fe(N2)2. The fact that there is no reactivity between 5 and HBArF4 at −78 °C indicates that the N2 ligands are not sufficiently activated to be protonated, in accord with comparatively high N2 stretching frequencies for 5 (2065 and 2009 cm−1). To explore whether further reduction might generate a more reactive and hence on-path species, 5 was stirred with 1 equiv potassium naphthalide followed by the addition of 18-crown-6. This produced the anionic, 4-coordinate S = 1/2 (µeff = 1.80) complex [(P2PPh)Fe(N2)][K(18-crown-6)] (9) (Scheme 2.5). 9 features a single and highly activated N2 ligand (1872 cm−1). Its solid-state crystal structure shows a disordered tetrahedral iron center (τ4 = 0.75), and CV measurements 32 show a quasi-reversible Fe0/− redox event centered at −2.5 V vs Fc/Fc+ (Figure 2.7). At more negative potential, an irreversible, presumably Fe−2/−1 redox event is observed. Scheme 2.5. The reduction of 5 results in the formation of 9 or 10 depending on the equivalents of reductant used. Reduction of 5 with an excess of KC8 produced a diamagnetic species (31P NMR: δ = 113.13, doublet; 95.15 ppm, triplet) identified as [(P2PPh)Fe(N2)][K2(THF)3] (10) (Scheme 2.5). Complex 10 is an unusual iron species in that it is isoelectronic with [Fe(CO)4]2- (vide infra).22,23 A 15N-labeled analogue was synthesized by reduction of (P2PPh)FeBr2 under 15 N2; its 15N NMR spectrum (2.36 and −26.23 ppm) rules out the possibility of a dinuclear structure [{(P2PPh)Fe}2(-N2)]K2.24 Consistent with our assignment of dianion 10, its IR spectrum displays a (NN) at 1677 cm−1 (1591 cm-1 for 10–15N2) that is broadened due to ion-pairing, consistent with a very strongly activated N2 ligand. Addition of 18-crown-6 resulted in intractable decomposition, suggesting tight ion-pairing is important to its stability.25 33 The structure of 10 in the solid-state (Figure 2.7) presents two distorted tetrahedral iron centers (τ4 = 0.71) that are related by an inversion center within a dimeric unit. Tight ion pairing is evident from the close proximity of each iron center to the potassium cations (Fe–K = 3.442 and 3.567 Å); each N2 ligand interacts with three potassium ions. The Fe– N bond is remarkably short (1.728(2) Å), ~ 0.1 Å shorter than the Fe–N bonds in 5, reflective of very strong backbonding. Relatedly, significant N–N elongation is also observed (1.189(3) Å). The Fe–P bond distances are also highly contracted at 2.1494(6) Å, in line with the very strong covalency expected of a d10 tetrahedral iron center. Prior to this study, tetrahedral Fe−II species have been limited to complexes with very strong pi-acceptor ligands, such as CO,22,23 PF3,26 (C2H4),27,28 (COD) (COD = cyclooctadiene),27 and CNAr.25,29 Additional species bearing phosphorine30 and nitrosyl31 ligands have also been reported, however the assignment of their oxidation state is ambiguous. Figure 2.7. (Left) Cyclic voltammetry data of 9 scanning cathodically, (Middle) Mӧssbauer spectrum of 10, (Right) Asymmetric unit of the XRD structure of 10. In the dimeric structure, the iron centers are related by an inversion center. Hydrogen atoms and disorder in one of the i-Pr moieties are omitted for clarity. The Mössbauer spectrum of a perfectly tetrahedral d10 iron complex, such as Na2[Fe(CO)4], should show a singlet instead of a quadrupole doublet due to the spherical 34 electric field gradient at the iron nucleus;22,32 any quadrupole splitting in Na2[Fe(CO)4] is barely discernable.32 Similarly, the Mössbauer spectrum of 10 shows an apparent singlet (Figure 2.7) which can be fit by a small quadrupole splitting (δ = 0.27 mm/s, ΔEQ = 0.26 mm/s). The very small quadrupole splitting in 10, which is required to have at least a modest electric field gradient owing to the presence of three unique types of donor ligands, indicates its classification as a d10 tetrahedral structure is appropriate, at least to the extent this description is apt for Na2[Fe(CO)4] given the significant covalency in both species. 2.2.7 Functionalization of formal Fe−I and Fe−II species. Current examples of Fe-mediated N2RR are thought to proceed through Fe–N2 intermediates with ν(NN) stretching frequencies below 1970 cm−1.1d,3,33 The N2 ligand of [(P2PPh)Fe(N2)][K(18-crown-6)] (9) has a stretching frequency of 1872 cm−1 and in this context should be activated enough to be functionalized. Attempts to protonate 9 with stoichiometric HBArF4 unfortunately resulted in complex product mixtures. Silylium ions (R3Si+) have been used as surrogate electrophiles for protons to model unstable protonated Fe-NxHy species.1c,34-38Reacting 10, generated in situ, with one equivalent of Me3SiCl at −78 °C, results in an immediate color change from dark purple to dark orange. After workup, diamagnetic [(P2PPh)Fe(NNSiMe3)]K ([11-NNSiMe3]−) was isolated as a dark brown solid in 50% yield (Scheme 2.6). As for 10, a tight ion-pair seems to be important for its stability; addition of 18-crown-6 results in its decomposition. The solid-state structure of [11-NNSiMe3]− reveals a four-coordinate iron center with a distorted tetrahedral geometry (τ4 = 0.76). The Fe–N bond length of [11-NNSiMe3]− is even shorter than that in 10 (1.664(7) Å vs. 1.728(2) Å respectively), and the N–N bond length is much longer (1.270(9) Å vs. 1.189(3) Å) in 10. 35 Scheme 2.6. Synthesis of Fe–silyldiazenido complexes [11-NNSiMe3]K, [11-NNSiiPr3]K and 12-NNSiiPr3. Attempts to oxidize [11-NNSiMe3]− at −78 °C with cobaltocenium to generate the neutral diazenido species (P2PPh)Fe-NNSiMe3 resulted in a mixture of species, presumably complicated by the loss of Me3Si. Based on low temperature EPR data (Figure 2.8), we assign the major product of oxidation to be the iron-silyl complex (P2PPh)Fe(SiMe3)(N2) (its EPR signature is highly similar to that of (P2PPh)Fe(N2)(H); 2 see Figure 2.8). There is also a minor component in the EPR trace that can be tentatively assigned as the expected diazenido (P2PPh)Fe(NNSiMe3). Use of iPr3SiOTf instead leads to the analogous [11NNSiiPr3]− complex, but in this case its oxidation affords a clean EPR spectrum consistent with the diazenido species 12-NNSiiPr3 (Figure 2.8). Addition of iPr3SiOTf to [(P2PPh)Fe(N2)]− 9 generates the same species as is evident by IR and EPR spectroscopy (Figure 2.8). The IR spectrum of 12-NNSiiPr3 displays an intense band corresponding to νNN at 1660 cm−1, characteristic of iron diazenido species.34,37,39 In contrast with the - 36 SiMe3 derivative, the -SiiPr3 species is stable for days. We suspect that for the less bulky SiMe3 derivative, kinetically competitive N-to-Fe silyl migration is operative. We intuit that 9, or its further reduced state [(P2PPh)Fe(N2)]2- 10, must be reached before nitrogen functionalization occurs via protonation or silylation. The iron centers in 9 and 10 are exposed, and are therefore susceptible to direct protonation at iron, or to facile migration from N-to-Fe. An N-protonated form of 9 (or 10) can presumably react further under the catalytic conditions to produce NH3, when both excess acid and reductant are present. Such reactivity must be kinetically competitive with a step that produces an offpath hydride. The need to access an anionic state of the system (either 9 or 10) before functionalization at N2 can occur sets the requirement of a potent reductant for N2RR in the P2PPhFe-system. The Fe−I/0 couple of 9 is −2.47 V vs. Fc/Fc+, which is ~ 0.30 V more negative than the corresponding Fe−I/0 couple for [(P3B)Fe(N2)]0/−; N2RR can be driven rather efficiently with the latter system using Cp*2Co paired with anilinium acids, which are ineffective with this precatalyst.4,5 37 Figure 2.8. Collected EPR data; (Black) Spectrum of [(P2PPh)Fe(N2)][18-crown-6)] (9). (Maroon) Spectrum observed upon addition of iPr3SiOTf to 9, showing conversion to 12NNSiiPr3. (Red) Spectrum of 12-NNSiiPr3 obtained by oxidation of [11-NNSiiPr3]− with [Cp*2Co][PF6]. (Orange) Spectrum of the oxidation of [11-NNSiMe3]− with Cp*2Co][PF6] 38 showing the formation of (P2PPh)Fe(SiMe3)(N2) and (P2PPh)Fe(NNSiMe3). (Yellow) Spectrum of (P2PPh)Fe(H)(N2).2 2.3. Conclusions The present study highlights the detrimental effect of hydride ligands on an iron– catalyzed N2RR model system whose efficiency is enhanced by irradiation. Stoichiometric reactivity as well as freeze-quench Mossbauer studies reveal that off-path (P2PPh)Fe(N2)x(H)y species are formed but are not inert resting states. On the contrary, they rapidly produce hydrogen in an HER cycle operating parallel to the desired N2RR cycle. In the absence of light, these pathways compete with one another but operate along different cycles. Irradiation of (P2PPh)Fe(N2)(H)2 (2) results in photoinduced H2 elimination and the formation of (P2PPh)Fe(N2)2 (5), which is significantly more competent for N2RR. Thus, photolysis shifts the speciation from favoring an unproductive HER cycle to one where N2RR becomes kinetically competitive. A deeper understanding of the required driving force for N2 functionalization is obtained by stoichiometric reactions with Fe–N2 species. No protonation reactivity is observed with a strong acid for (P2PPh)Fe(N2)2 at −78° C; further reduction is required before functionalization can take place. Protonation experiments with the Fe−I and Fe−II species 9 and 10 provide complex mixtures, but silylation experiments are informative. The need to access an anionic or a dianionic state of the system before productive functionalization at N2 occurs sets the low reduction potential required for N2RR by this P2PPhFe-system and explains why comparatively milder reductants such as Cp*2Co, which are effective for a related (P3B)Fe-catalyst system, are ineffective in the present case. Future 39 catalyst designs for iron systems should focus on anodically shifting the needed redox couple to generate an Fe–N2 species while maintaining a strongly activated N2 ligand. 40 2.4. References 1 (a) Yandulov, D. V.; Schrock, R. R. Catalytic reduction of dinitrogen to ammonia at a single molybdenum center. Science 2003, 301, 76–79. (b) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia. Nat. Chem. 2011, 3, 120–125. (c) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic conversion of nitrogen to ammonia by an iron model complex. Nature 2013, 501, 84–87. (d) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. Selective catalytic reduction of N2 to N2H4 by a simple Fe complex. J. Am. Chem. Soc. 2016, 138, 13521–13524. (e) Sekiguchi, Y.; Arashiba, K.; Tanaka, H.; Eizawa, A.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Catalytic reduction of molecular dinitrogen to ammonia and hydrazine using vanadium complexes. Angew. Chem. Int. Ed. 2018, 57, 9064–9068. (f) Doyle, L. R.; Wooles, A. J.; Jenkins, L. C.; Tuna, F.; McInnes, E. J. L.; Liddle, S. T. Catalytic dinitrogen reduction to ammonia at a triamidoamine-titanium complex. Angew. Chem. Int. Ed. 2018, 57, 6314–6318. 2 Buscagan, T. M.; Oyala, P. H.; Peters, J. C. N2 -to-NH3 Conversion by a triphos-iron catalyst and enhanced turnover under photolysis. Angew. Chem. Int. Ed. 2017, 56, 69216926. 3 Sekiguchi, Y.; Kuriyama, S.; Eizawa, A.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Synthesis and reactivity of iron–dinitrogen complexes bearing anionic methyl- and phenylsubstituted pyrrole-based PNP-type pincer ligands toward catalytic nitrogen fixation. Chem. Commun. 2017, 53, 12040–12043. 41 4 Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. Catalytic N2- to-NH3 Ccnversion by Fe at lower driving force: A proposed role for metallocene-mediated PCET. ACS Cent. Sci. 2017, 3, 217–223. 5 Chalkley, M. J.; Castillo, T. J. Del; 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, 6122–6129. 6 Matson, B. D.; Peters, J. C. Fe-mediated HER vs N2RR: Exploring factors that contribute to selectivity in P3EFe(N2) (E = B, Si, C) catalyst model systems. ACS Catal. 2018, 8, 1448– 1455. 7 Simpson, F. B.; Burris, R. H. A nitrogen pressure of 50 atmospheres does not prevent evolution of hydrogen by nitrogenase. Science 1984, 224, 1095–1097. 8 Schubert, K. R.; Evans, H. J. Hydrogen evolution: A major factor affecting the efficiency of nitrogen fixation in nodulated symbionts. Proc. Natl. Acad. Sci. 1976, 73, 1207–1211. 9 Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. A synthetic single-site Fe nitrogenase: High turnover, freeze-quench 57Fe Mössbauer data, and a hydride resting state. J. Am. Chem. Soc. 2016, 138, 5341–5350. 10 (a) Fajardo, J.; Peters, J. C. Catalytic nitrogen-to-ammonia conversion by osmium and ruthenium complexes. J. Am. Chem. Soc. 2017, 139, 16105–16108. (b) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand. Nat. Commun. 2016, 7, 12181. (c) Kuriyama, S.; Arashiba, K.; Tanaka, H.; Matsuo, Y.; Nakajima, K.; Yoshizawa, K.; 42 Nishibayashi, Y. Direct transformation of molecular dinitrogen into ammonia catalyzed by cobalt dinitrogen complexes bearing anionic PNP pincer ligands. Angew. Chem. Int. Ed. 2016, 55, 14291–14295. (d) Creutz, S. E.; Peters, J. C. Catalytic reduction of N2 to NH3 by an Fe–N2 complex featuring a C-atom anchor. J. Am. Chem. Soc. 2014, 136, 1105–1115. 11 (a) Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.; Ceccarelli, C.; Davis, W.M. Synthesis and reactions of molybdenum triamidoamine complexes containing hexaisopropylterphenyl substituents. Inorg. Chem. 2003, 42, 796-813. (b)Yandulov, D. V.; Schrock, R. R. Studies relevant to catalytic reduction of dinitrogen to ammonia by molybdenum triamidoamine complexes. Inorg. Chem. 2005, 44, 1103-1117. 12 Kinney, R. A.; McNaughton, R. L.; Chin, J. M.; Schrock, R. R.; Hoffman, B. M. Protonation of the dinitrogen-reduction catalyst [HIPTN3N]MoIII investigated by ENDOR spectroscopy. Inorg. Chem. 2011, 50, 418-420. 13 Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W.; Zhang, L.; Hoffman, B. M.; Peters, J. C. Characterization of an Fe≡N–NH2 intermediate relevant to catalytic N2 reduction to NH3. J. Am. Chem. Soc. 2015, 137, 7803–7809. 14 Other molecular Fe(H)x complexes undergo photoinduced reductive H2 elimination with associated N2 binding. See: (a) Sacco, A.; Aresta, M. Nitrogen fixation: Hydrido- and hydrido-nitrogen-complexes of Iron(II). Chem. Commun. 1968, 1223–1224. (b) Whittlesey, M. K.; Mawby, R. J.; Osman, R.; Perutz, R. N.; Field, L. D.; Wilkinson, M. P.; George M. W. Transient and matrix photochemistry of Fe(dmpe)2H2 (dmpe = Me2PCH2CH2Me2): dynamics of C-H and H-H activation J. Am. Chem. Soc. 1993, 115, 43 8627–8637. (c) Perutz, R. N.; Procacci, B. Photochemistry of Transition Metal Hydrides, Chem. Rev. 2016, 116, 8506−8544. Okuniewski, A.; Rosiak, D.; Chojnacki, J.; Becker, B. Coordination polymers and 15 molecular structures among complexes of mercury(II) halides with selected 1benzoylthioureas. Polyhedron 2015, 90, 47–57. Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, 16 structure, and spectroscopic properties of copper(II) compounds containing nitrogensulphur donor ligands; the crystal and molecular structure of Aqua[l,7-bis(Nmethylbenzimidazol-2'-yl)-2,6-dithiaheptane]copper(II) Perchlorate. J. Chem. Soc., Dalt. Trans. 1984, 0, 1349–1356. 17 Cavaillé, A.; Joyeux, B.; Saffon-Merceron, N.; Nebra, N.; Fustier-Boutignona, M.; Mézailles, N. Triphos–Fe dinitrogen and dinitrogen–hydride complexes: relevance to catalytic N2 reductions, Chem. Commun. 2018, 54, 11953-11956. 18 Examples of deriving the coupling between metal centers by NMR can be found in a) Pfirrmann, S.; Limberg, C.; Herwig, C.; Knispel, C.; Braun, B.; Bill, E.; Stösser, R. A reduced β-diketiminato-ligated Ni3H4 unit catalyzing H/D Exchange. J. Am. Chem. Soc. 2010, 132, 13684–13691. b) Tepper, A. W. J. W.; Bubacco, L.; Canters, G. W. Paramagnetic properties of the halide-bound derivatives of oxidised tyrosinase investigated by 1H NMR spectroscopy. Chem. Eur. J. 2006, 12, 7668–7675. Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.; Münck, E.; Bominaar, 19 E. L. Mössbauer and computational study of an N2-bridged diiron diketiminate complex: 44 parallel alignment of the iron spins by direct antiferromagnetic exchange with activated dinitrogen. J. Am. Chem. Soc. 2006, 128 (31), 10181–10192. 20 (a) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L C. Mechanism of nitrogen fixation by nitrogenase: the next stage. Chem. Rev. 2014, 114, 4041–4062. (b) Lukoyanov, D.; Khadka, N.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Reversible photoinduced reductive elimination of H2 from the nitrogenase dihydride state, the E4(4H) janus intermediate. J. Am. Chem. Soc. 2016, 138, 1320–1327. (c) Lukoyanov, D.; Khadka, N.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Reductive elimination of H2 activates nitrogenase to reduce the N≡N triple bond: Characterization of the E4(4H) Janus Intermediate in wild-type enzyme. J. Am. Chem. Soc. 2016, 138, 10674– 10683. (d) Lukoyanov, D.; Yang, Z. Y.; Khadka, N.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Identification of a key catalytic intermediate semonstrates that nitrogenase is activated by the reversible exchange of N2 for H2. J. Am. Chem. Soc. 2015, 137 (10), 3610– 3615. (e) Lukoyanov, D.; Yang, Z.-Y.; Barney, B. M.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Unification of reaction pathway and kinetic scheme for N2 reduction catalyzed by nitrogenase. Proc. Natl. Acad. Sci. 2012, 109 (15), 5583–5587. 21 (a) Baker, M. V.; Field, L. D.; Young, D. J. Formation of molecular hydrogen complexes of iron by the reversible protonation of iron dihydrides with alcohols. J. Chem. Soc. Chem. Commun. 1988, 2, 546–548. Hills, A.; Hughes, D. L.; JimenezTenorio, M.; Leigh, G. J.; Rowley, A. T. Bis[1,2bis(dimethylphosphino)ethane]dihydrogenhydridoiron(II) tetraphenylborate as a model for the function of nitrogenases. J. Chem. Soc. Dalt. Trans. 1993, 3041–3049. 45 22 Erickson, N. E.; Fairhall, A. W. Mössbauer spectra of iron in Na2[Fe(CO)4] and Na[Fe3(CO)11H] and comments regarding the structure of Fe3(CO)12. Inorg. Chem. 1965, 4, 1320–1322. 23 Collman, J. P. Disodium tetracarbonylferrate, a transition metal analog of a Grignard reagent. Acc. Chem. Res. 1975, 8, 342–347. 24 Bridged M–N2–M species should give rise to only one peak in the 15N NMR spectra. See: (a) Doyle, L. R.; Hill, P. J.; Wildgoose, G. G.; Ashley, A. E. Teaching old compounds new tricks: efficient N2 fixation by simple Fe(N2)(diphosphine)2 complexes. Dalton. Trans. 2016, 45, 7550–7554. (b) Hazari, N. Homogeneous iron complexes for the conversion of dinitrogen into ammonia and hydrazine. Chem. Soc. Rev. 2010, 39, 4044–4056. (c) Eizawa, Field, L. D.; Guest, R. W.; Turner, P. Mixed-valence dinitrogen-bridged Fe(0)/Fe(II) complex. Inorg. Chem. 2010, 49, 9086–9093. (d) A.; Arashiba, K.; Tanaka, H.; Kuriyama, S.; Matsuo, Y.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation. Nat. Commun. 2017, 8, 1–12. (e) Kokubo, Y.; Yamamoto, C.; Tsuzuki, K.; Nagai, T.; Katayama, A.; Ohta, T.; Ogura, T.; Wasada-Tsutsui, Y.; Kajita, Y.; Kugimiya, S.; Masuda, H. Dinitrogen fixation by vanadium complexes with a triamidoamine ligand. Inorg. Chem. 2018, 57, 11884-11894. (f) Lindley, B. M.; van Alten, R. S.; Finger, M.; Schendzielorz, F.; Würtele, C.; Miller, A. J. M.; Siewert, I.; Schneider, S. Mechanism of chemical and electrochemical N2 splitting by a rhenium pincer complex. J. Am. Chem. Soc. 2018, 140, 7922–7935. 46 25 Other formal d10 species also suffer from decomposition upon addition of a crown ether. See: Mokhtarzadeh, C. C.; Margulieux, G. W.; Carpenter, A. E.; Weidemann, N.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. Synthesis and protonation of an encumbered iron tetraisocyanide dianion. Inorg. Chem. 2015, 54, 5579–5587. 26 Kruck, T.; Prasch, A. Synthese und Struktur der tetrakis(trifluorphosphin)‐ metalldihydride und ‐metallate(‐II) von Eisen, Ruthenium und Osmium. Z. Anorg. Allg. Chem. 1969, 371, 1–22. 27 Jonas, K.; Schieferstein, L.; Krüger, C.; Tsay, Y.-H. Tetrakis(ethylene)irondilithium and Bis(η4‐1,5‐cyclooctadiene) irondilithium. Angew. Chemie Int. Ed. 1979, 18, 550–551. 28 Martin, R.; Fürstner, A. Reversal of reactivity cross-coupling of alkyl halides with aryl Grignard reagents catalyzed by a low-valent iron complex. Angew. Chem. Int. Ed. 2004, 43, 3955–3957. 29 Brennessel, W. W.; Ellis, J. E. [Fe(CNXyl)4]2-: an isolable and structurally characterized homoleptic isocyanidemetalate dianion. Angew. Chem. Int. Ed. 2007, 46, 598–600. 30 Orsay Rosa, P.; Mézailles, N.; Ricard, L.; Mathey, F.; Floch, P. Le; Jean, Y. Dianionic iron and ruthenium(2-) bisphosphinine complexes: a formal d10 ruthenium complex with a square planar geometry. Angew. Chem. Int. Ed. 2001, 40, 1251–1253. 31 Cullen, W. R.; Crow, J. P.; Herring, F. G.; Sams, J. R.; Tapping, R. L. Mӧssbauer and electron paramagnetic resonance studies of some iron nitrosyl complexes. Inorg. Chem. 1971, 10, 1616–1623. 32 Farmery, K.; Kilner, M.; Greatrex, R.; Greenwood, N. N. Structural studies of the carbonylate and carbonyl hydride anions of iron. J. Chem. Soc. A, 1969, 2339–2345. 47 33 (a) Moret, M.-E.; Peters, J. C. Terminal iron dinitrogen and iron imide complexes supported by a tris(phosphino)borane ligand. Angew. Chem. Int. Ed. 2011, 50, 2063–2067. (b) Mankad, N. P.; Whited, M. T.; Peters, J. C. Terminal FeI-N2 and FeII...H-C interactions supported by tris(phosphino)silyl ligands. Angew. Chem., Int. Ed. 2007, 46, 5768–5771. 34 Moret, M.; Peters, J. C. N2 Functionalization at iron metallaboratranes. J. Am. Chem. Soc. 2011, 4350, 18118–18121. 35 McWilliams, S. F.; Bill, E.; Lukat-Rodgers, G.; Rodgers, K. R.; Mercado, B. Q.; Holland, P. L. Effects of N2 binding mode on iron-based functionalization of dinitrogen to form an iron(III) hydrazido complex. J. Am. Chem. Soc. 2018, 140, 8586–8598. 36 Piascik, A. D.; Li, R.; Wilkinson, H. J.; Green, J. C.; Ashley, A. E. Fe-catalyzed conversion of N2 to N(SiMe3)3 via an Fe-hydrazido resting state. J. Am. Chem. Soc. 2018, 10691-10694. 37 Piascik, A. D.; Hill, P. J.; Crawford, A. D.; Doyle, L. R.; Green, J. C.; Ashley, A. E. Cationic silyldiazenido complexes of the Fe(diphosphine)2(N2) platform: structural and electronic models for an elusive first intermediate in N2 fixation. Chem. Commun. 2017, 53, 7657–7660. 38 Alex Rudd, P.; Planas, N.; Bill, E.; Gagliardi, L.; Lu, C. C. Dinitrogen Activation at Iron and Cobalt Metallalumatranes Eur. J. Inorg. Chem. 2013, 2 (22–23), 3898–3906. 39 (a) Lee, Y.; Mankad, N.P.; Peters, J. C. Triggering N2 uptake via redox-induced expulsion of coordinated NH3 and N2 silylation at trigonal bipyramidal iron, Nat. Chem. 2010, 558–565. (b) Ung, G.; Peters, J. C. Low temperature N2 binding to 2-coordinate L2Fe0 enables reductive trapping of L2FeN2− and NH3 generation. Angew. Chem. Int. Ed. 2015, 54, 532–535. 48 Chapter 3 : EPR Detection of a Terminal Formal Iron(III) Nitride Stabilized by a Trisphosphine-Borane Ligand 49 3.1 Introduction In recent years, substantial progress has been made in the development of molecular catalysts for N2-to-NH3 conversion, with significant improvements in turnover and efficiency.1,2 With the growing number of systems available, it becomes increasingly possible to probe the mechanism(s) by which the key N-N cleavage step occurs in these functional systems. Two fundamentally different pathways are generally considered for catalysts featuring an end-on N2 moiety: (1) a Chatt-type mechanism (via an M≡N intermediate) or (2) a late state cleavage pathway (an alternating mechanism via an MN2H4 intermediate).1,3 Recent work demonstrates that with strong reductants and acids a Chatt-type mechanism is viable for a trisphosphine iron complexes [(P3B)Fe(N2)]- (P3B = tris(o-diisopropylphosphinophenyl)borane) (Scheme 3.1). Reduction of [(P3B)Fe(N2)]followed by three subsequent protonation steps yielded [(P3B)Fe(N)]+.4 Given that [(P3B)Fe≡N]+ appears stable to protonation at low temperature, the next step in the catalytic cycle is either reduction to produce the formally Fe(III) nitrido (P3B)Fe≡N, or direct N–H bond formation via PCET to produce the terminal imido [(P3B)Fe=NH]+. Scheme 3.1. Selected intermediates characterized with the P3B ligand manifold along a Chatt-type cycle. 50 Figure 3.1. (Top) Simplified molecular orbital diagram of nitridoiron compounds in 3 and 4-fold symmetry and representative three-fold symmetric d3 and d4 terminal iron nitrido complexes. The reduction of [(P3B)Fe≡N]+ during nitrogen fixation would result in a d5 electron count, akin to a highly unstable Co(IV) nitride described by Meyer and coworkers.5 The stability of compounds featuring Fe≡N triple bonds, is highly dependent on the coordination environment of the metal and the corresponding ligand-field splitting (Figure 3.1).6 In 4fold symmetry, an Fe-N bond order of 3 is only achieved in the unusual Fe(VI) state, as the Fe(V) state populates a -antibonding orbital, reducing the bond order to 2.5. Such Fe(V) and Fe(VI) nitrides have only been observed as thermally unstable compounds generated by low temperature photolysis of suitable azide precursors.7 In contrast, 3-fold 51 symmetric complexes have granted access to stable terminal nitridoiron compounds in the Fe(IV)8 and Fe(V)9 states. The pseudotetrahedral geometry affords two nonbonding d orbitals, alleviating the need to populate a σ-antibonding orbital for d3 and d4 electron counts, increasing stability in lower oxidation states (Figure 3.1). Based on the expected electronic structure, the desired Fe(III) nitride is anticipated to be highly reactive. The Co(IV) nitrodo reported by Meyer and coworkers could only be characterized by EPR spectroscopy at temperatures below 50 K (Scheme 2).5 At higher temperatures, insertion of the nitrido ligand into the M-C bond was observed. Similarly, a highly reactive Co(IV) nitrido, formed upon photolysis of an azide precursor, was proposed by Chirik and coworkers for which ligand C-H activation was observed (Scheme 3.2). These examples demonstrate that the characterization of d5 nitrides remains virtually impossible. Scheme 3.2. Examples of (proposed) CoIV nitridos and their decomposition products Here, we report the characterization of a d5 terminal Fe(III) nitride, formed by photolysis of an iron azide precursor at low temperature. Characterization by EPR, ENDOR, HYSCORE and Mӧssbauer spectroscopy in combination with DFT supports the 52 formation of a discrete but highly reactive Fe(III) nitrido stabilized by a redox active borane moiety. The observed g values and hyperfine coupling constants eliminate other potential structures formed upon photolysis of the iron azide precursor. 3.2. Results and Discussion The azide precursor (P3B)Fe(N3) (2) was synthesized by salt metathesis of NaN3 with (P3B)FeCl (1) (Scheme 3.3). Synthesis of the 15N labeled analogs was achieved using Na15NNN, yielding a 1:1 mixture of the α- and γ-15N labeled compounds. Compound 2 exhibits an intense IR absorption at 2069 cm-1, which shifts to 2058 cm-1 for the 15N labeled analog, attributed to the azide moiety. Its solid-state structure exhibits crystallographic 3fold symmetry and a geometry intermediate between trigonal-bipyramidal and pseudotetrahedral (Σ<(P-Fe-P) = 348°). Scheme 3.3. Synthesis of 2 and 3 from the chloride precursor 1 Photolysis of 2 in C6D6 solution at room temperature resulted in darkening of the brown solution accompanied by the appearance of new paramagnetic 1H NMR resonances. 53 Upon standing, these solutions deposited crystals of [(P3B)Fe]2(μ-1,3-N3) (3). XRD crystallography (See SI), revealed a symmetrical linear Fe–NNN–Fe linkage. Formation of 3 likely proceeds through photoreduction of 2 releasing one equivalent of azide radical N3• and subsequent reaction with a second equivalent of 2 (Scheme 3.3). Complex 3 could also be independently generated by mixing solutions of 2 and the previously reported complex (P3B)Fe(N2).10 Scheme 3.4. Photolysis of (P3B)Fe(N3) (2) Changing the photolysis medium from solution to frozen solvent has previously been shown to drastically affect product distributions in related reactions.7b Irradiation of a frozen 1 mM solution of 2 in 2-MeTHF with a 40 W 390 nm LED results in the formation of an axial structure-rich doublet signal centered at g = 2.02 (Figure 3.2) assigned to the S = ½ nitridoiron complex (P3B)Fe≡N (4) (Scheme 3.4). Identical signals are observed upon irradiation of 1 mM solutions in various glassing solvents such as (d8)-toluene, cis-decalin and methylcyclohexane (See SI). A similar signal, albeit broader and with less resolved hyperfine structure, is observed upon photolysis of frozen pentane solutions or thin films of 2 (See SI). After irradiating samples for 3 minutes, photolysis yields are estimated to be ca. 80 % by spin integration against an external standard (TEMPO). A decrease in signal 54 intensity is observed upon prolonged photolysis with the complete loss of signals corresponding to 4, and the concomitant formation of a new signal corresponding to an isopropyl radical in 76 % yield after 30 minutes.11 Figure 3.2. Continues wave X-band EPR-monitored photolysis of 2 in a frozen 1 mM 2MeTHF solution at 77 K. The low g-anisotropy (Δg = 0.032) and small deviation of the free electron g value (2.002) (Table 3.1) are consistent with a C3-symmetric iron nitride species having an isolated 2A ground state, where an electron resides in an orbital of dz2 parentage (Figure 1).12 The low g-anisotropy of 4 stands in stark contrast with those observed for Fe(V) nitridos (Δg > 0.175) where the electron resides in nearly degenerate dxy and dx2-y2 orbitals (Figure 3.1). The near degeneracy of the orbitals, resulting in significant spin orbit coupling and a considerable increase in Δg.5,13 The absence of such low-lying excited states in 4, due to the isolated nature of the A-symmetry orbital, results in a dramatic decrease of Δg. The 55 low g-anisotropy is a characteristic feature of 4 and is not expected for any other photolysis products (vide infra). Figure 3.3. CW X-band (a) and psuedomodulated Q-band ESE-EPR (b and c) spectra and corresponding simulations (red) of 4. The dashed dotted lines in (c) are a guide for the eye highlighting the change in hyperfine coupling with different isotopic compositions. (d) EPR spectra after 30 minutes of photolysis with a 40 W 390 nm LED. 56 The marked difference in the hyperfine patterns, especially at gz in the X and QBand EPR spectra after photolysis of 2-57Fe and 2-15N (Figure 3.3), demonstrates that the product contains iron and nitrogen. In contrast, no change is observed in the signal corresponding to liberated isopropyl radical upon isotope substitution (Figure 3.3). As an additional tool to probe the (electronic) structure of 4, Davies electronnuclear double resonance (ENDOR) spectra were acquired at X-band frequencies (ca 9.7 GHz, see SI). This in combination with hyperfine sublevel correlation (HYSCORE) spectroscopy enabled the determination of the distinct 11B, 15N, 31P and 57Fe hyperfine tensors (Table 3.1). The large axial anisotropy for 11B, 15N, and 57Fe and the presence of three identical P nuclei further corroborate the C3-symmetry present in 4. Examples of thoroughly characterized S = ½ nitrides by EPR are limited.5,14 The observed 14N hyperfine coupling ±[20.3, 20.3, 9.6] is significantly larger than those observed for [PhB(tBuIm)3FeV≡N]+, which are +[9.11, 6.48, 0.71] respectively. Just as the significant difference in Δg between the nitrides, the difference in hyperfine coupling arises from the different electronic ground states. In [PhB(tBuIm)3FeV≡N]+ the spin on nitrogen arises through spin-polarization, while in 4 direct delocalization is the origin of the spin on nitrogen (vide infra). DFT calculations (TPSS, CP(PPP) on iron and IGLO-III on all others) predict the observed hyperfine coupling constants and nuclear quadrupole moments remarkably well (Table 3.1). 57 Table 3.1. Experimental g-values, and experimental and theoretical hyperfine in MHz of [(P3B)Fe(N) (4). g⊥> g∥, g⊥ corresponds to the xy-plane (gx and gy) and g∥ to the B-Fe-N axis (gz). All experimental hyperfine values are in MHz. g g∥ g⊥ Δg 1.997 2.029 A (14N) A (11B) A (31P) A (57Fe) EPR DFT EPR DFT EPR DFT EPR DFT 20.3 21.2 81.0 79.9 39 -35.5 |46.0 42.7 49 -45.8 10.0 11.3 45 -39.8 -4.3 -7.7 21.5 20.3 0.032 Scheme 3.5. Potential products formed upon photolysis of 2. 58 It is improbable that the signal assigned to (P3B)Fe≡N (4) corresponds to potential products such as those depicted in Figure 3.4. All data indicates almost perfect C3 symmetry, where the insertion of the nitride into the Fe-L bond, as observed by Meyer and coworkers, would result in the loss of C3-symmetry. Similarly, any form of ligand activation would result in a drastic reduction of the symmetry. Either one or two hydrogen atom abstractions from e.g. solvent molecules results in the formation of (P3B)Fe≡NH and (P3B)Fe-NH2, respectively, an EPR silent singlet or previously characterized stable quartet, ruling out these products. As 4 is expected to be a strong base, one could consider protonation of the nitride. In such a scenario the expected product, [(P3B)Fe≡NH]+ would be a doublet. Based on the observed hyperfine coupling in a terminal amido by Hoffmann, hyperfine coupling to the proton is expected. For the spectrum corresponding to 4, no change in EPR, ENDOR and HYSCORE spectra is observed upon using deuterated solvents, eliminating the possibility of solvent acting as H+ source. Furthermore, the electronic structure of [(P3B)Fe≡NH]+ is expected to be comparable to that of [(P3B)Fe≡NAd]+, which has a significantly larger g anisotropy (4 Δg = 0.032) as the unpaired electron resides in a nearly degenerate e-orbital. The remarkably low g-anisotropy of 4, as well as the absence of observable proton hyperfine coupling, and the 14N quadrupole moment rule out [(P3B)Fe≡NH]+ as product. Compound 4 decays instantly at −78 °C in fluid as well as frozen solutions. The thermal instability precludes the possibility to manipulate a sample outside of an EPR tube. Furthermore, light-induced decomposition of 4 hampers the ability to perform for example Raman spectroscopy. 59 Figure 3.4 Mӧssbauer spectrum of 2 before photolysis, (b) after 5 minutes of photolysis and (c) 60 minutes of photolysis with a 40 W 390 nm LED. (d) Simulation of a fast relaxing S = ½ complex with hyperfine coupling parameters and g-values as determined by EPR spectroscopy overlaid on top of the spectrum collected after 5 minutes of photolysis. 60 Mӧssbauer spectra collected after photolysis of 2 mM 2-57Fe 2-MeTHF solutions show a decrease in signal intensity corresponding to 2, with the formation of a broad feature (See Figure 3.4). The broad feature disappears upon prolonged photolysis in line with the time dependent EPR spectra (Figure 2). Compound 4 displays remarkably slow electronic spin relaxation (τ = 265 ns at 85 K) as determined through T1 and T2 measurements by respectively an inversion recovery or a two pulse ESEEM (See SI). These relaxation times are on the same order of magnitude as the lifetime of the excited 57Fe nucleus (τM ~ 140 ns). For relaxation times τ >> τM a Mӧssbauer spectrum consist of sextets with narrow lines, while a quadrupole doublet is observed for τ << τM.15 The slow electronic relaxation, low conversion, signal intensity, and presence of 2 prevents the achievement of a reliable fit of the signal. However, simulating a spectrum using the g-values and 57Fe hyperfine, as determined by EPR spectroscopy, in combination with a quadrupole splitting of around ~ 4 mm s-1, gives a reasonable idea of what such a Mӧssbauer spectrum would look like (Figure 3.4) The thermal instability prevents the collection of spectra at higher temperatures where a quadrupole doublet with a characteristic large quadrupole splitting is expected (calculated quadrupole splitting ~ 4 mm/s). The large boron hyperfine coupling constants indicates significant contribution of boron-based orbitals to the SOMO of 4. Decomposition of 11B hyperfine tensors into its isotropic (aiso = 1/3 (Ax + Ay + Az) = ±41.6) and anisotropic components (T=[( (Ax− aiso) + (Ay− aiso) +(Az− aiso)]) can be used to estimate the spin density residing in boron 2s and 2pz orbitals.16 This analysis reveals an axial anisotropic boron hyperfine interaction with T = [-19.7 -19.7 39.33] MHz. Defining the anisotropic tensor for an electron fully localized in a B 2pz orbital as T0 = ±[-63.6 -63. 127.2], yields a spin density in the B 2pz orbital of ρ ≈ 61 0.30 (or ca. 30 % of an electron). Using aiso0 = 2547 MHz, the 2s character in the SOMO is found to be 1.7 %. Spin densities for 11B and 14N determined through this method are summarized in Table 2 and are close Lӧwdin spin populations calculated by DFT. The experimental and Lӧwdin spin populations show that the spin densities of 4 are significantly delocalized onto the nitrogen and boron atom, with the B pz orbital accommodating most of the spin. The DFT computed SOMO of 4 has a1 symmetry and nicely illustrates the proposed ground state based on EPR spectroscopy (Figure 3.5). Figure 3.5 (A) Qualitative MO diagram describing the 3-center σ bonding of (P3B)Fe≡N and (B) its SOMO and (C) resonance structure description of compound. 62 Table 3.2 Calculated Lӧwdin spin populations and experimental spin density estimations.[a] 14 11 N B EPR[b] DFT EPR[b] DFT s 0.3 0.2 1.7 2.0 pz 15.9 20.8 30.4 30.1 [a] Spin densities are given as percentages. [b]Total spin density estimated from experimental EPR hyperfine coupling is calculated assuming all spin is located in the orbitals included in the Table. 3.3 Conclusions In summary, photolysis of (P3B)Fe(N3) at low temperatures generates the transient low-spin Fe(III) nitrido complex (P3B)Fe≡N. The series of X-band and Q-band spectra in combination with ENDOR and HYSCORE spectroscopy, supported by DFT calculations, provide compelling data for the assignment of this highly reactive complex. Analysis of the 11B and 15N hyperfine coupling constants reveals significant delocalization of the unpaired electron (~ 50 %) onto these axial ligands. The degree of boron and nitrogen character in the SOMO shows the importance of these ligands in the stabilization of this unusually electron-rich iron-nitride complex. 3.4 References 63 1 Chalkley, M. J.; Drover, M. W.; Peters, J. C. Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes. Chem. Rev. 2020, 120 (12), 5582–5636 2 a) Yandulov, D. V.; Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center. Science 2003, 301 (5629), 76–78. b) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501 (7465), 84–87. c) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molybdenum-Catalysed Ammonia Production with Samarium Diiodide and Alcohols or Water. Nature 2019, 568 (7753), 536–540. d) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. Selective Catalytic Reduction of N2 to N2H4 by a Simple Fe Complex. J. Am. Chem. Soc. 2016, 138 (41), 13521–13524 e) Doyle, L. R.; Wooles, A. J.; Jenkins, L. C.; Tuna, F.; McInnes, E. J. L.; Liddle, S. T. Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine–Titanium Complex. Angew. Chem. Int. Ed. 2018, 57 (21), 6314–6318 f) 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 64 Seefeldt, L. C.; Hoffman, B. M.; Dean, D. R. Mechanism of Mo-Dependent 3 Nitrogenase. Annu. Rev. Biochem. 2009, 78, 701–722 Thompson, N. B.; Green, M. T.; Peters, J. C. Nitrogen Fixation via a Terminal 4 Fe(IV) Nitride. J. Am. Chem. Soc. 2017, 139 (43), 15312–15315 Zolnhofer, E. M.; Käß, M.; Khusniyarov, M. M.; Heinemann, F. W.; Maron, L.; van 5 Gastel, M.; Bill, E.; Meyer, K. An Intermediate Cobalt(IV) Nitrido Complex and Its N-Migratory Insertion Product. J. Am. Chem. Soc. 2014, 136 (42), 15072–15078 Mehn, M. P.; Peters, J. C. Mid- to High-Valent Imido and Nitrido Complexes of 6 Iron. J. Inorg. Biochem. 2006, 100 (4), 634–643 a) Meyer, K.; Bill, E.; Mienert, B.; Weyhermüller, T.;Wieghardt, K. Photolysis of 7 Cis- and Trans-[FeIII(Cyclam)(N3)2]+ Complexes: Spectroscopic Characterization of a Nitridoiron(V) Species. J. Am. Chem. Soc. 1999, 121 (20), 4859–4876. b) Grapperhaus, C. A.; Mienert, B.; Bill, E.; Weyhermüller, T.; Wieghardt, K. Mononuclear (Nitrido)Iron(V) and (Oxo)Iron(IV) Complexes via Photolysis of [(Cyclam-Acetato)FeIII(N3)]+ and Ozonolysis of [(Cyclam-Acetato)FeIII(O3SCF3)]+ in Water/Acetone Mixtures. Inorg. Chem. 2000, 39 (23), 5306–5317. c) AliagaAlcalde, M.; George, S. D.; Mienert, B.; Bill, E.; Wieghardt, K.; Neese, F. The Geometric and Electronic Structure of [(Cyclam-Acetato)Fe(N)+]: A Genuine 65 Iron(V) Species with a Ground-State Spin S=1/2. Angew. Chem., Int. Ed. 2005, 44 (19), 2908–2912. d) Berry, J. F.; Bill, E.; Bothe, E.; George, S. D.; Mienert, B.; Neese, F.; Wieghardt, K. An Octahedral Coordination Complex of Iron(VI). Science 2006, 312 (5782), 1937–1941. e) Wagner, W. D.; Nakamoto, K. Formation Of Nitridoiron(V) Porphyrins Detected By Resonance Raman-Spectroscopy. J. Am. Chem. Soc. 1988, 110 (12), 4044–4045 8 Betley, T. A.; Peters, J. C. A Tetrahedrally Coordinated L3Fe–Nx Platform That Accommodates Terminal Nitride (FeIV≡N) and Dinitrogen (FeI–N2–FeI) Ligands. J. Am. Chem. Soc. 2004, 126 (20), 6252–6254. b) Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.; Mehn, M. P.; Betley, T. A.; Lu, C. C.; Peters, J. C. On the Feasibility of N2 Fixation via a Single-Site FeI/FeIV Cycle: Spectroscopic Studies of FeI(N2)FeI, FeIV≡N, and Related Species. Proc. Nat. Acad. Sci. U.S.A. 2006, 103 (46), 17107–17112 c) Rohde, J.-U.; Betley, T. A.; Jackson, T. A.; Saouma, C. T.; Peters, J. C.; Que, L. XAS Characterization of a Nitridoiron(IV) Complex with a Very Short Fe-N Bond. Inorg. Chem. 2007, 46 (14), 5720–5726. d) Vogel, C.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer, K. An Iron Nitride Complex. Angew. Chem., Int. Ed. 2008, 47 (14), 2681–2684. e (20) Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler, E. N.; Kirk, M. L.; Smith, J. M. Structural and Spectroscopic 66 Characterization of an Electrophilic Iron Nitrido Complex. J. Am. Chem. Soc. 2008, 130 (32), 10515–10517 a) Scepaniak, J. J.; Vogel, C. A.; Khusniyarov, M. M.; Heinemann, F. W.; Meyer, 9 K.; Smith, J. M. Synthesis, Structure, and Reactivity of an Iron(V) Nitride. Science 2011, 331, 1049–1052. b) Keilwerth, M.; Grunwald, L.; Mao, W.; Heinemann, F. W.; Sutter, J.; Bill, E.; Meyer, K. Ligand Tailoring Toward an Air-Stable Iron(V) Nitrido Complex. J. Am. Chem. Soc. 2021, 143 (3), 1458–1465 10 Moret, M.-E.; Peters, J. C. Terminal Iron Dinitrogen and Iron Imide Complexes Supported by a Tris(Phosphino)Borane Ligand. Angew. Chem. Int. Ed. 2011, 50, 2063–2067 11 Ayscough, P. B.; Thomson, C. Electron Spin Resonance Spectra of Alkyl Radicals in γ-Irradiated Alkyl Halides. Trans. Faraday Soc. 1962, 58, 1477–1494 12 Nance, P. J.; Thompson, N. B.; Oyala, P. H.; Peters, J. C. Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center, Three-Electron Polar σ Bonds. Angewandte Chemie International Edition 2019, 58 (19), 6220–6224 13 Cutsail, G. E.; Stein, B. W.; Subedi, D.; Smith, J. M.; Kirk, M. L.; Hoffman, B. M. EPR, ENDOR, and Electronic Structure Studies of the Jahn-Teller Distortion in an Fe-V Nitride. J. Am. Chem. Soc. 2014, 136 (35), 12323–12336 67 14 Cutsail, G. E.; Stein, B. W.; Subedi, D.; Smith, J. M.; Kirk, M. L.; Hoffman, B. M. EPR, ENDOR, and Electronic Structure Studies of the Jahn-Teller Distortion in an Fe-V Nitride. J. Am. Chem. Soc. 2014, 136 (35), 12323–12336 15 See Mørup, S. Magnetic Relaxation Phenomena. In Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Applications; Gütlich, P., Bill, E., Trautwein, A. X., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2011; pp 201–234 for an in-depth discussion of lifetimes and the effect on Mӧssbauer line shapes. 16 Morton, J. R.; Preston, K. F. Atomic Parameters for Paramagnetic Resonance Data. Journal of Magnetic Resonance (1969) 1978, 30 (3), 577–582 68 Chapter 4 : Snapshots of a Migrating H-atom: Characterization of a Reactive Fe(III) Indenide Hydride and its Nearly Isoenergetic RingProtonated Fe(I) Isomer Reproduced in part with permission from: Drover, M. W.; Schild, D. J.; Oyala, P. H.; Peters, J. C. Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer Angew. Chem. Int. Ed., 2019, 58, 15504-15511 © 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim 4.1. Introduction For decades, cyclopentadienyl (Cp, C5H5-) and derivatives thereof have been used as ligands to stabilize transition metal,1 f-block,2 and main-group3 elements alike. This ligand class is ubiquitous among the synthetic community, no doubt a consequence of several alluring features, including variable hapticity,4 whereby multiple coordination modes can be accommodated, ranging from η1-to- η 5. There has been a surge of renewed interest in cooperative transformations with Cp-type ligands,5 with protonation at such “noninnocent” rings being implicated in the context of hydride or proton-coupled-electrontransfer (PCET) pathways, for instance in H2-evolving or dinitrogen reduction reactions (Chart 4.1A).5,6 Metallocene (Cp2’M) and half-sandwich complexes have been probed for decades in the context of ring protonation (or hydride attack) chemistry.7 For species that are comparatively stable, characterization data for complexes wherein a ring is “protonated” can be distinguished from those wherein the “proton” instead resides on the metal as a hydride ligand. In certain cases, the H-atom may shuttle between these two positions. For example, the [FeIII-H]+ cation, [FeIII(η 5-Cp*)(dppe)H]+ (Cp* = C5Me5-, dppe = 1,2-bis(diphenylphosphino)ethane) is resistant to ring protonation (C-H elimination), but undergoes CO binding/reduction by CoCp2 to give the diamagnetic complex Fe0(η 4Cp*H)(dppe)(CO) where H-migration has occurred onto the Cp*-ring.8 More recently, crystallography has been used to show that protonation of ferrocene (Cp2Fe) using a mixture of HF/PF5 occurs at Fe, in spite of DFT calculations that favor (by 2 kcal mol-1) a ring-protonated isomer, CpFe(η 4-CpH)+ (Chart 4.1B).7e Distinguishing such species when the respective M-H or C-H bonded isomers feature weak and hence reactive bonds (low homolytic bond dissociation free energies: BDFEM-H and BDFEC-H) is particularly 70 challenging, especially for species that are open-shell. A recent study from our lab underscores this point, where protonation of Cp*2Co is thermodynamically favored at the ring (and not at Co); multi-frequency continuous-wave (CW) and pulse-EPR spectroscopies were performed at very low temperature to assign the site(s) of protonation.6 Chart 4.1. Previous work highlighting A) Cp* non-innocence for small-molecule fixation, B) interest in assigning H-atom location by XRD analysis, and C) the present study, featuring indene as a supporting ligand. 71 As part of an effort to extend such studies to other systems, our attention turned to iron hydride precursors of the type FeII(η 3: η 2-Ind)(P2)H (Ind = indenide, C9H7-, P2 = diphosphine ligand), wherein an indenyl ligand was selected in an effort to stabilize reactive, ring-protonated complexes through [Fe]- η 6-IndH coordination, providing 6ethrough the η -system (Chart 4.1C). To our surprise, for the system described herein, the isomer of the metal-bound hydride (formally Fe(III)) is nearly isoenergetic to the isomer in which the ring is instead protonated (formally Fe(I)). This represents, what is to our knowledge, an exceptional case. In general, only one of the isomers is experimentally observed. Herein, we present the characterization of this pair of S = ½ Fe isomers, wherein Hatom migration has been validated in the solution state by CW- and pulse-EPR spectroscopy. For the Fe(I) indene complex, an X-ray crystal structure has been obtained. The data presented provide a means for facile differentiation between isomers of these types. Finally, a combination of experiment and theory provides access to relevant thermochemistry, including respective homolytic bond dissociation free energies: BDFECH and BDFEFe-H. Facile PCET from either an Fe-H or C-H bond is predicted to be feasible, and is exemplified by the propensity of the system to liberate H2 in solution. 4.2 Results and Discussion Synthetic entry into the system of present interest is as follows: trans[Fe(Br)2(depe)2]9 (depe = 1,2-bis(diethylphosphino)ethane) was reacted with lithium indenide at −78 oC with warming to room-temperature over 2 h, providing Fe(η 3: η 2Ind)(depe)Br (1) as a purple powder following work-up (δP = 92.98) (Figure 4.1A). Single crystals of 1 were grown from a saturated pentane-layered THF solution and analyzed by 72 X-ray diffraction (XRD) at 100 K (Figure 4.1B). Consistent with η 3: η 2 binding, the structure features short Fe(1)-C(1)/C(2)/C(3) bond lengths (avg. = 2.078(2) Å) and long Fe(1)-C(4)/C(5) (avg. = 2.206(4) Å) contacts, with Δ(M-C)10 = 0.134 Å [Δ(M-C) = difference between these two averages] – this is true for all complexes discussed herein. Complex 1 serves as a versatile starting material from which the FeII-hydride, Fe(η 3 : η 2-Ind)(depe)H (2) can be accessed (Figure 4.1A).11 Reaction of 1 with Li[BEt3H] gives 2 in near-quantitative yield. Most characteristically, the 31P NMR spectrum provides a doublet at δP = 92.98 ppm (2JP,H = 70.8 Hz, 1JFe,P = 60.3 Hz: for 2-57Fe) that directly couples with the FeII-H group at δH = -20.64 ppm (1JFe,H = 10.7 Hz). Figure 4.1 A) Synthesis of 2 by addition of Li[BEt3H]. B) Solid-state structure of 1 with ellipsoids shown at 50% probability. Complex 2 was also studied by cyclic voltammetry (CV), which reveals an irreversible feature centered at E1/2 = - 0.81 V vs. Fc/Fc+ (Fc = ferrocene) associated with 73 the FeII/FeIII couple; only ~75% of the signal current is maintained on the return reductive wave (SR = 50 mV/s), suggesting reactivity of the in-situ generated Fe(III) cation.12 Given our desire to generate a reactive open-shell [Fe(III)-H]+ species, we next probed the oxidative chemistry of complex 2 using [Fc]BArF4 (E1/2 = 0 V vs. Fc/Fc+; ArF4 = 3,5(CF3)2(C6H3)),13 at −78 oC (Figure 4.2). Figure 4.2 A) Synthesis of [4][BArF4] by oxidation using [Fc]BArF4 and solid-state depiction of the solid-state molecular structure of [4][BArF4] with ellipsoids shown at 50% probability). Selected bond lengths [Å] and angles (o): Fe(1)-P(1) 2.222(3), Fe(1)-P(2) 2.298(3), Fe(1)-C(centroid), 1.589,φC(1)-C(5)]-[P(1)-Fe(1)-P(2)]), 90. B) Synthesis of [5][BArF4] and solid-state depiction of the solid-state molecular structure of [5][BArF4]. Monitoring this reaction mixture for both 1H and 2H isotopologues by freezequenched X-band CW-EPR spectroscopy (77 K) shows the formation of a single S = ½ species exhibiting roughly axial symmetry (g = [2.377, 2.039, 1.993]) with couplings of similar magnitude to two distinct 31P (I = 1/2) nuclei. Significant additional couplings to 1 H are resolved in the 1H isotopologue, consistent with the presence of a strongly coupled hydride 1H nucleus (Figure 4.3A): thus we assign this spectrum to [Fe III(η3: η2Ind)(depe)H][BArF4] [3]+. Collection of a series of X-band Davies ENDOR spectra across the EPR envelope of [3-D]+ (see SI) provide additional data showing large coupling to two 74 non-equivalent phosphines (A(31Pα) = ± [100, 88, 88] MHz and A(31Pβ) = ± [82, 85, 72] MHz). To determine more accurate hyperfine parameters for the hydride ligand, than can be resolved from CW-EPR, we turned to X-band 2H-HYSCORE of this same [3-D]+ isotopologue. Simulation of field-dependent HYSCORE spectra of [3-D]+ reveal a highly anisotropic deuterium hyperfine tensor A(2H) = ± [1.84, 12.6, 10.0] MHz, with a small Euler rotation of the hyperfine tensor relative to the g-tensor of (α,β,γ) = (40, 15, 0)◦. Scaling of the 2H hyperfine tensor determined by 2H HYSCORE by the proportion of 1H/2H gyromagnetic ratios (γ1H/γ2H = 6. 514) provides a 1H hyperfine tensor A(1H) = ± [12, 82, 65] MHz which is in agreement with simulation of the X-band CW-EPR spectrum. This hyperfine coupling tensor consists of both large isotropic (aiso(1H) = ± 53 MHz) and anisotropic (T(1H) = ± [-41, 29, 12] MHz) components, consistent with those expected for a terminal metal hydride.14 For comparison, we have also undertaken study of Lapinte’s bonafide S = ½ FeIIIH species (for which an X-ray structure is available),11a FeIII(η 5-Cp*)(dppe)H+. Analogous EPR characterization reveals spectroscopic features consistent with those discussed above, namely, a large and very anisotropic 1H coupling of A(1H) = ± [4, 68, 50] MHz (aiso(1H) = ± 40.7 MHz, T(1H) = ± [-36.7, 27.3, 9.3] MHz) (Figure 4.6). 75 Figure 4.3. Freeze-quenched X-Band EPR spectrum (9.371 GHz) and corresponding Xband HYSCORE data recorded in 2-MeTHF glass at 77 K. A) [3-H][BArF4] and [3D]BArF4; B) [4-H]BArF4 and [4-D]BArF4 and C) [FeIII(η5-Cp*)(dppe)H]+ and [FeIII(η5Cp*)(dppe)D]+ HYSCORE simulations of the 2H hyperfine couplings are overlaid in red over the data, which is plotted in grey. Downward pointed arrows represent g-values at which HYSCORE has been acquired. ** = [Fe(depe)2(N2)][BArF4]18 impurity. 76 Although at early reaction periods an FeIII-H species is observed, timed freezequench experiments show that this complex is consumed at −78 oC (t1/2 = 15 min) to provide a new roughly axial S = ½ species with g = [2.332, 2.042, 1.992].15 X-band ENDOR spectroscopy provides data (see ESI) that is consistent with large coupling to two nonequivalent phosphines (A(31Pα) = ± [86, 104, 100] MHz and A(31Pβ) = ± [93, 88, 94] MHz). Unlike in the case of the terminal hydride species, FeIII-H/D [3]+ and [3-D]+, no difference in the CW X-band EPR spectrum between [4]+ and [4-D]+ (associated with a large anisotropic 1H coupling) is noted. Further analysis by 1H/2D difference HYSCORE spectroscopy reveals a very small anisotropic coupling to a single 1H nucleus, A(1H) = ± [1, -4, -4] MHz, which we assign to an indene ring proton, formed by C-H elimination to give [FeI(η 6-IndH)(depe)]+ ([4]+). Consistent with the DFT-calculated spin-density plot for [4]+, the Fe-bound indene fragment is observed to bear minimal spin c.f. the terminal hydide [3]+ (Figure 4.4). Calculated and experimental EPR parameters are summarized in Figure 4.6. Figure 4.4. DFT-calculated spin-density plot for [3]+ and [4]+ (isovalue: 0.004 e-/Å3; TPSS; def2tzvp (Fe), def2svp (all other atoms). 77 To ensure appropriate assignment of complex [4]+ by EPR spectroscopy, the S = ½ model compound, [Fe(η6-toluene)(dippe)][BArF4] ([5]+) (dippe = 1,2- bis(diisopropylphosphino)ethane) was prepared by oxidation of Fe(η6-toluene)(dippe)16 with [Fc]BArF4 at -78 oC (Figure 4.2). By EPR spectroscopy, a similar spectrum to that of [4]+ was acquired, with simulation parameters appropriate for a rhombic species with g = [2.371, 2.032, 1.990 (Figure 4.5). Figure 4.5. Freeze-quenched X-Band EPR spectrum (9.386 GHz) recorded in 2-MeTHF glass at 77 K for [4-H]BArF4 (red) and [5]BArF4 (black). Despite its propensity to lose H2 (vide infra), the assignment of [4]+ could be further cemented by storing one such oxidation mixture cold, providing orange blocks suitable for XRD analysis (Figure 4.2A). On first inspection, the Fe-P contacts for this open-shell d7 system were noted to be longer than those for the S = 0 variant (complex 1, for example) with values of 2.222(3)/2.298(3) Å compared to 2.1792(6)/2.2217(6) Å. Furthermore, metrics associated with the dearomatized five-membered indene ring are markedly 78 different, with elongation along the C(1)-C(5) and C(3)-C(4) vectors [1.437(4)/1.429(3) Å to 1.493(8)/1.503(9) Å] and contraction along the C(2)-C(3) vector [1.414(4) to 1.347(1) Å], signifying new single and double bonds, respectively.17 The presence of a “two-legged piano stool” complex is further corroborated by φ, where the plane created by P(1)-Fe(1)P(2) is perfectly perpendicular (90o) to the iron-bound indene ring. For complex [5]+, similar data were also obtained providing Fe-P contacts [2.2272(2)/2.254(2) Å] and a φ value [96.7o] close to that of [4]+ [2.222(3)/2.298(3) Å and φ = 90o] (Figure 4.2B). Figure 4.6 DFT optimized structures of [3]+, [4]+, and [FeIII( 5 -Cp*)(dppe)H]+. Experimental and theoretical A(1H) values are shown for the proton coded with a red circle. Equations: aiso = [Ax + Ay + Az]/3, T(1H) = [Ax - aiso, Ay - aiso, Az - aiso]. DFT: Structures were optimized using TPSS; def2tzvp (Fe), def2svp (all other atoms), and EPR calculations: CP(PPP) (Fe), TPSS/IGLO-III (all other atoms). 79 Complexes [3]+ and [4]+ undergo bimolecular degradation in solution, resulting in H2 loss (Figure 4.7). For example, reaction of 2 with [Fc]BArF4 in Et2O at −78 oC and warming to room-temperature results in a visible precipitation of a maroon solid and formation of H2 (~60% H2 as quantified by GC). Analysis of the crude 31P NMR spectrum (THF-d8) evidences two major by-products that are assignable to [Fe(η3: η 2Ind)(depe)N2][BArF4] ([6]+;νNN = 2151 cm-1, ca. 20%) and [Fe2(η3:η2-Ind)2(depe)2(μdepe)][BArF4]2 ([7]2+; ca. 20%), respectively.18 Consistent with formation of these two species, cooling of the reaction mixture produces yellow and violet crystals suitable for analysis by XRD (Figure 4.7B). Presumably these compounds are formed from bimolecular H2 release from one (or more) of the following pairs: {[FeIII]−H···H−[FeIII]}⧧, {[Ind]C−H···H−[FeIII]}⧧, or {[Ind]C−H···H−C[Ind]}⧧.19 To confirm bimolecular H2 release, a 1:1 ratio of 2-H and 2-D was mixed, followed by oxidation with [Fc]BArF at −78 o C, and allowing the solution to warm. The release of HD, as detected by NMR spectroscopy, substantiates a bimolecular H-H coupling process. 80 Figure 4.7. A) Decomposition of [3]+ and [4]+. B) Solid-state depiction of [6][BArF4] and [7][BArF4]2. (Displacement ellipsoids are shown at the 50% probability). Selected bond length for [6][BArF4]: N(1)-N(2)) = 1.108(6) Å. To experimentally benchmark the H• transfer propensity of [4]+, we aimed to determine an upper limit for BDFEFe-H which, based on the small energy difference between [3]+ and [4]+ (vide infra), should be similar to BDFEC-H. Figures 4.8A-C summarize our findings via experiment and theory. Reaction of an MeCN solution of 2 with 1-benzyl-3acetylpyridinium [BNAP]OTf20 results in hydride transfer and clean formation of [Fe(η3: η 2-Ind)(depe)(NCCH3)]+ [8]+ (δP = 92.8 ppm) (Figure 4.8). Transfer also proceeds in THF, providing the N2-adduct, [6]+ and μ-depe complex, [7]2+. The hydricity (defined as the heterolytic dissociation energy of M−H to give M+ and H− (ΔG(2)H−) of 2 in MeCN must therefore be less than the hydricity of 1,4-BNAPH (ΔGH− ≈ 60 kcal mol-1), i.e. more hydridic.21 In theory, loss of H− from 2 should give [Fe(η 3: η 2-Ind)(depe)]+, though this cation is not observed experimentally due to facile MeCN binding to give the adduct (ΔGo(DFT) = - 17 kcal mol-1; Fig. 8C). For comparison, hydricities of 54 kcal mol-1 and an 81 upper bound of < 44 kcal mol-1 have been established for the six-coordinate terminal iron hydrides, (SiP3)Fe(H)(H2)22 and (SiP2S)Fe(H)(H2)14b, respectively. Utilizing thermodynamic relationships that relate H• and H− in MeCN,23 the upper bound for the free energy of H− transfer from 2 and H• transfer from [3]+ can be related (Eq. 4 and 5, Figure 4.8C). The [Fe]−H bond of [3]+ is thus conservatively estimated to have an upper bound BDFE (ΔG([3]+)H•) of < 52 kcal mol-1 giving [Fe(η3: η2-Ind)(depe)(NCCH3)]+ and H•; the DFT-predicted value is 33 kcal mol-1 (TPSS; def2tzvp (Fe), def2svp (all other atoms)) – these values suggest that, on the basis of thermodynamics alone, loss of H2 should be facile (BDFEH-H = 102.3 kcal mol-1 in CH3CN).24 82 Figure 4.8. Reaction of 2 with 1-benzyl-3-acetylpyridinium triflate (BNAP)+; B) Square scheme for experimentally calculated thermodynamic quantities. C) Thermodynamic relationships that relate H• and H− transfer in MeCN. 83 Figure 4.9. Free energy change (kcal mol-1) for PCET involving BDFEC-H (TPSS; def2tzvp (Fe), def2svp (all other atoms)). To gain insight into the elementary steps associated with the reactivity of these species, we turned to DFT (Figure 4.9). First, a negligible energy difference is calculated between [3]+ (I) and [4]+ (II) (ΔGo ≈ 0 kcal mol-1), which we attribute to stabilization of [4]+ by η6indene coordination. For a related family of compounds, FeIII(η5-C5R5)(P2)H+ (R = H, P2 = dippe (1,2-bis(diisopropylphosphino)ethane or R = CH3, P2 = dppe),11 hydride-to-ring 84 migration is not observed. In this regard, a difference of + 13 kcal mol-1 is calculated between the theoretical compounds, [FeI(η4-CpH)(depe)]+ and [FeIII(η 5-Cp)(depe)H]+, and an even larger value of + 16 kcal mol-1 is calculated between [FeI(η 4-Cp*H)(depe)]+ and [FeIII(η 5-Cp*)(depe)H]+ (Figure 4.10). These values do not correlate with the higher basicity of Cp*H (pKa = 26.1) versus IndH (pKa = 20.1) versus CpH (pKa = 18.0), and are instead likely ascribable to ligand steric effects.25 Nonetheless, for this family of complexes, only the indene system manifests ring functionalization by H-migration due to a 9 kcal mol-1 stabilization resulting from ring slippage. 85 Figure 4.10. DFT-predicted free energy change (kcal mol-1) for metal-to-ring Hmigrations. In terms of BDFEC-H, the 17-electron complex III is calculated to have a BDFEC-H of 50 kcal mol-1, and the terminal hydride I is calculated to possess an equivalent energy (BDFEFe-H = 50 kcal mol-1); both pathways provide the vacant S = 1 cation, [Fe(η3: η 2Ind)(depe)]+ (IV) and H•, indicating that both species should be competent for H2 loss.22 By contrast, we calculate [FeIII(η 5-Cp*)(dppe)H]+ to possess a stronger Fe-H bond: BDFEFe-H = 56 kcal mol-1. We find experimentally this FeIII-H cation is stable in ethereal solvent (by UV-VIS and 1H NMR spectroscopy) and does not release H2 under ambient conditions (< 5% decomposition after 1 week). 86 Alternatively, from the 17-electron cation III, 19-electron complexes VI can be optimized upon associative substitution of a two-electron donor (L) with the hydrogen atom affinity ΔGoHAA being 26 kcal mol-1 for both L = NCCH3 and N2. In this transformation, the driving force for III → V is proposed to be a dual consequence of i) indene rearomization and ii) formation of a stable 18-electron product. Figure 4.11 Free energy change (kcalmol-1) for PCET using [FeIII(η5-Cp*)(dppe)H]+ (TPSS; def2tzvp (Fe), def2svp (all other atoms)). 4.3 Conclusions We have described the synthesis, electronic characterization, and thermochemistry associated with a pair of isomeric open-shell S = ½ complexes, [FeIII(η 2: η 3-Ind)(depe)H]+ and [FeI(η 6-IndH)(depe)]+. By calculation and experiment, we have shown that these species are highly reactive, having C-H and Fe-H bonds that are close in energy (BDFECH ≈ BDFEFe-H ≈ 50 kcal mol -1 ) manifesting in PCET to provide H• (identified as H2), presumably through bimolecular combination of one (or more) of the following pairs: {[FeIII]−H···H−[FeIII]}⧧, {[Ind]C−H···H−[FeIII]}⧧, or {[Ind]C−H···H−C[Ind]}⧧. With a unique opportunity to observe both species, detailed X-band (CW) and pulse EPR spectroscopic experiments were undertaken, that together, provide a reliable means to differentiate a ring- versus a metal- bound H-atom, an approach that should prove useful in other systems for which ligands might play a non-innocent role in hydride, proton, or 87 hydrogen atom transfer. Ongoing studies in our laboratory are currently focused on exploiting the PCET reactivity of weak C-H bonds in this and related systems. 88 4.4 References 1 a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry. University Science Books, CA, 1987; b) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed., John Wiley & Sons, NY, 1994. 2 Poli, R. Chem. Rev. 1991, 91, 509; b) Evans, W. J. Organometallics 2016, 35, 3088. 3 a) Jutzi, P.; Reumann, G. J. Chem. Soc. Dalton Trans. 2000, 14, 2237; b) Budzelaar, P. H.; Engelberts, J. J.; van Lenthe, J. H. Organometallics 2003, 22, 1562. 4 a) Gridnev, I. D. Coord. Chem. Rev. 2008, 252, 1798; b) Veiros, L. F. Organometallics 2000, 19, 5549; c) O’Connor, J. M.; Casey, C. P. Chem. Rev. 1987, 87, 307. 5 a) 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. PNAS 2016, 113, 6409; b) Zamorano, A.; Rendón, N.; Valpuesta, J. E. V.; Álvarez, E.; Carmona, E. Inorg. Chem. 2015, 54, 6573; c) Pal, S.; Kusumoto, S.; Nozaki, K. Organometallics 2018, 37, 906; d) Moreno, J. J.; Espada, M. F.; Campos, J.; Lopez-Serrao, J.; Macgregor, S. A.; Carmona, E. J. Am. Chem. Soc. 2019, 141, 2205; e) Sapsford, J. S.; Gates, S. J.; Doyle, L. R.; Taylor, R. A.; Diez-Gonzalez, S.; Ashley, A. E. Inorg. Chim. Acta. 2019, 488, 201; f) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Chem. Commun. 2016, 52, 9105. 6 Chalkley, M.; Del Castillo, T.; Matson, B.; Roddy, J.; Peters, J. C. ACS Cent. Sci. 2017, 3, 217; b) Chalkley, M. J.; Del Castillo, T. J.; Matson, B.; Peters, J. C. J. Am. Chem. Soc. 89 2018, 140, 6122; c) Chalkley, M. J.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2019, 141, 4721-4729. 7 a) Curphey, T. J.; Santer, J. O.; Rosenblum, M.; Richards, J. H. J. Am. Chem. Soc. 1960, 82, 5249−5250; b) Liles, D. C.; Shaver, A.; Singleton, E.; Wiege, M. B. J. Organomet. Chem. 1985, 288, c33−c36; c) Court, T. L.; Werner, H. J. Organomet. Chem. 1974, 65, 245−251; d) Koelle, U.; Khouzami, F. Angew. Chem., Int. Ed. Engl. 1980, 19, 640−641; e) Malischewski, M.; Seppelt, K.; Sutter, J.; Heinemann, F. W.; Birger, D.; Meyer, K. Angew. Chem. Int. Ed. 2017, 56, 13372. 8 Hamon, P.; Hamon, J. R.; Lapinte, C. J. Chem. Soc. Chem. Commun. 1992, 1602. 9 Mays, M. J.; Prater, B. E. Inorg. Synth. 1974, 15, 21. 10 Faller, J. W.; Crabtree, R. H.; Habib, A. Organometallics 1985, 4, 929. 11 For related Fe( 5 -Cp)(P2)H-derivatives, see: a) Hamon, P.; Toupet, L.; Hamon, J.R.; Lapinte, C. Organometallics 1992, 11, 1429; b) de la Jara Leal, A.; Tenorio, M. J.; Puerta, M. C.; Valerga, P. Organometallics 1995, 14, 3839. 12 At greater scan rate (SR), reversibility increases. 13 Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910. 14 For example: a) Chiang, K. P.; Scarborough, C. C.; Horitani, M.; Lees, N. S.; Ding, K.; Dugan, T. R.; Brennessel, W. W.; Bill, E.; Hoffman, B. M.; Holland, P. L. Angew. Chem., Int. Ed. 2012, 51, 3658; b) Gu, N. X.; Oyala, P. H.; Peters, J. C. J. Am. Chem. Soc. 2018, 140, 6374. 15 Monitoring the decay of [3]+ at – 60 C by UV/vis spectroscopy shows clean, first order kinetics (see ESI). 90 16 Hermes, A. R.; Warren. T. H.; Girolami, G. S. J. Chem. Soc. Dalton Trans. 1995, 301. 17 Similar metrics are observed for other 6 -IndH (C9H8) complexes, see: a) Bercaw, J. E.; Hazari, N.; Labinger, J. A. Organometallics 2009, 28, 5489; b) Fessenbecker, A.; Stephan, M.; Grimes, R. N.; Pritzkow, H.; Zenneck, U.; Siebert, W. J. Am. Chem. Soc. 1991, 113, 3061; c) Hung, M. Y.; Ng, M. S.; Zhou, Z.; Lau, C. P. Organometallics 2000, 19, 3692. 18 Other side products were also identified including Fe(depe)2N2+. This compound was analyzed by HYSCORE spectroscopy (see ESI), showing identical 14N couplings to those determined by: Doyle, L. R.; Scott, D. J.; Hill, P. J.; Fraser, D. A. X.; Myers, W. K.; White, A. J. P.; Green, J. C.; Ashley, A. E. Chem. Sci. 2018, 9, 7362. 19 See: a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305; b) Nocton, G.; Booth, C. H.; Maron, L.; Andersen, R. A. Organometallics 2013, 32, 1150; c) Halpern, J.; Pribanic, M. Inorg. Chem. 1970, 9, 2616. 20 a) Zhang, F.; Jia, J.; Dong, S.; Wang, W.; Tung, C. -H. Organometallics 2016, 35, 1151; b) Zhang, F.; Xu, X.; Zhao, Y.; Jia, J.; Tung, C.- H. Wang, W. Organometallics 2017, 36, 1238. 21 a) Ilic, S.; Kadel, U. P.; Basdogan, Y.; Keith, J. A.; Glusac, K. D. J. Am. Chem. Soc. 2018, 140, 4569; b) Ellis, W. W.; Raebiger, J. W.; Calvin, C. J.; Bruno, J. W.; DuBois, D. L. J. Am. Chem. Soc. 2004, 126, 2738. 22 Fong, H.; Peters, J. C. Inorg. Chem. 2015, 54, 5124. 23 a) Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R.M.; Miller, A. J. M.; Appel, A. M. Chem. Rev. 2016, 116, 8655; b) Pearson, R. G. Chem. Rev. 1985, 85, 41. 24 Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961. 91 25 Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1983, 105, 6188. 92 Chapter 5 : Generating potent C-H PCET donors: Ligand-induced Fe-toring proton migration from a Cp*FeIII-H complex demonstrates a promising strategy Reproduced in part with permission from: Schild, D. J.; Drover, M. W; Oyala, P.; Peters, Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy J. C. J. Am. Chem. Soc., 2020, 142, 44, 18963–18970. © 2020 American Chemical Society 5.1. Introduction Proton-coupled electron transfer (PCET) reactions have emerged as powerful strategies for mediating sundry reductive transformations in organic synthesis, with clever new approaches being discovered to generate in situ highly reactive H-atom surrogates as intermediates.1,2,3 Reactive fragments with weak element-hydrogen (E-H) bonds have thus been targeted to facilitate hydrogen atom delivery, revealing desirable substrate reductions that are fascinating in scope.1,2,4 For example, dissolution of SmI2 in H2O/THF mixtures confers coordination-induced bond-weakening in the resulting [Sm(H2O)n]2+ complex to provide weak and hence highly reactive O-H bonds (BDFEO-H ~ 26 kcal mol-1);1h such species can be employed towards substrate reductions such as ketones to alcohols and anthracene to dihydroanthracene,5 and also towards catalytic N2 fixation to NH3 in the presence of a Mo catalyst.6 As an outgrowth of our own mechanistic studies of Fe-mediated N2 fixation, we recently reported the rigorous characterization of a highly reactive [Cp*(endo/exo- η 4Cp*H)Co]+ (Cp* = C5Me5-) species (BDFEC-H < 29 kcal mol-1 for the exo-analogue), generated in situ via ring protonation of decamethylcobaltocene, Cp*2Co.7,8 Relatedly, we have examined an Fe(III) indenide hydride [FeIII(η5-Ind)(depe)H]+ (Ind = C8H7-, depe = 1,2-bis(diethylphosphino)ethane) and its low temperature isomerism to give a η 6-indene complex [Fe(η 6-IndH)(depe)]+, (BDFEX-H = 50 kcal mol-1; X = Fe or C).9 The highly reactive nature of these ring-protonated species has frustrated independent studies of their reactivity profiles towards exogenous substrates. We hence targeted a related system wherein a reactive ring-C-H PCET donor might be generated in solution, via association of a ligand to the metal to cause concomitant isomerization of a 94 metal-bound hydride to a Cp*-ring position. Such a strategy would, in principle, enable two unreactive partners to reside in solution, with a net PCET reaction being triggered by addition of a donor ligand L (see Chart 5.1). Chart 5.1. A) Reported generation of a 19-electron FeIII-H11; B) Introducing the concept of donor-induced PCET and C) Tunable PCET. BDFE values (calculated by DFT) are in kcal mol-1. With this goal in mind, we turned to a known iron(III) hydride half-sandwich complex, [FeIII(η5-Cp*)(dppe)H]+ [1]+, (Cp* = C5Me5-, dppe = 1,2- bis(diphenylphosphino)ethane), first prepared by Lapinte and co-workers in 1992.10 Complex [1]+ was reported to undergo associative binding of CO to provide the 19-electron 95 hydride [FeIII(η5-Cp*)(dppe)(CO)H]+ ([2]+), the latter having been characterized by Xband EPR spectroscopy. The reaction was deemed reversible with warming, returning [1]+.11 Based on our findings with [Cp*(endo- η 4-Cp*H)Co]+ and [Fe(η 6-IndH)(depe)]+, we wondered whether [2]+ would be best described as [FeI(η4-Cp*H)(dppe)(CO)]+, which would occur via proton migration to the ring upon CO binding. Herein, we use a range of methods, including crystallography and various EPR techniques, with corresponding DFT analysis, to establish that this is indeed the case. We then demonstrate that an intermolecular PCET reaction is possible, with azobenzene as a model substrate, via this ligand-induced trigger. 5.2. Results and Discussion We began our study with (re)examining the solution spectroscopy of [FeIII(η5Cp*)(dppe)H]+ [1]+ in the presence of CO. Addition of CO to the 1H and 2H isotopologues (Fe-H/D) of [1]+ results in a change in the EPR spectrum, indicating the formation of a single S = ½ species exhibiting rhombic symmetry (g = [2.085, 2.039, 2.004]), parameters that are similar to those previously reported by Lapinte (g = [2.0777, 2.0367, 2.0019], A(31P) = ± [48, 50, 50] MHz, and A(1H) = ± [14, 17, 34] MHz (aiso(1H)= ± 22 MHz)).11 Based on preliminary DFT calculations (vide infra) and the similarity of the proton hyperfine coupling to ring protonated cobaltocenes,4 we posited its structure to be [FeI(endo- η 4-Cp*H/D)(dppe)(CO)]+ (endo-isomer confirmed by XRD vide infra), endo[3-H/D]+, rather than the terminal hydride [2]+ (Scheme 5.1). 96 Scheme 5.1. Reported preparation of a 19-electron Fe(III)-H complex11 and synthesis of endo-[3]+/[4]+ by addition of CO or CNXyl with accompanying H2 release to give [5]+ and [6]+. BDFE values (calculated by DFT) are in kcal mol-1. Consumption of [1]+ is easily identified by EPR spectroscopy based on the significant shift in the value of gmax (Figure 5.1). Besides coupling to two distinct 31P (I = 1/2) nuclei with similar magnitude, significant additional coupling to 1H is evident when the 1H and 2H isotopologues are compared, consistent with the presence of a relatively strongly coupled 1H nucleus (Figure 5.2). Collection of a series of X-band Davies ENDOR 97 spectra acquired across the EPR envelope of endo-[3-D]+ (see the Supporting Information) provides additional data consistent with large couplings to two non-equivalent phosphines (A(31P1) = ± [72, 59, 58] MHz and A(31P2) = ± [49, 42, 51] MHz). Figure 5.1. X-Band CW-EPR spectra of [1]+, endo-[3-H]+, exo-[3-H]+, and endo-[4-H]+ in 2-MeTHF at 77 K. 98 Figure 5.2. X-Band CW-EPR spectra and corresponding X-band HYSCORE spectra of freeze-quenched samples in 2-MeTHF. A) endo-[3-H]BArF4 and endo-[3-D]BArF4; B) endo-[4-H]BArF4 and endo-[4-D]BArF4. HYSCORE simulations of features from 1H, 2H (red), and 14N (green, CNXyl only) hyperfine couplings are overlaid over the data, which is plotted in grey. ** = traces of [1]+. X-band HYSCORE spectroscopy of the 1H and 2H isotopologues was used to determine the hyperfine parameters for the hydride-derived hydrogen nucleus (Figure 5.2). Simulation of field-dependent HYSCORE spectra of endo-[3-H]+ reveals a relatively isotropic proton hyperfine tensor, A(1H) = ± [24, 20, 34.5] MHz (aiso(1H) = ± 26.2 MHz), with a small Euler rotation of the hyperfine tensor relative to the g-tensor of (α,β,γ) = (0, 30, 0)◦. HYSCORE spectra of endo-[3-D]+ exhibit intense features arising from deuterium which are well simulated by scaling the 1H hyperfine tensor determined from 1H HYSCORE by the proportion of 1H/2H gyromagnetic ratios (γ1H/γ2H = 6.514), with A(2H) 99 = ± [3.7, 3.1, 5.3] MHz. These parameters provide excellent agreement with simulations of the X-band CW-EPR spectra of these two isotopologues. In contrast to terminal metal-hydrides,12 the anisotropic component (T) of the 1H hyperfine of endo-[3]+, T(1H)= ± [-2.2, -6.2, 8.3] MHz), is considerably smaller, indicating greater distance between the nucleus and Fe-centered spin density. Similarly, low anisotropy is also observed for the endo-Cp*(H) protonation product of Cp*2Co. The low anisotropy suggests ring-protonation (endo-[3]+), and not a terminal hydride (i.e., [2]+) species. Hyperfine coupling constants and the predicted anisotropy as calculated by DFT further support the assignment and are independent of the method used (see see the Supporting Information). We also explored the addition of an isocyanide donor, CNXyl (Xyl = 2,6dimethylphenyl). Treatment of [1]+ with CNXyl at − 78 °C provides [FeI(endo- 4 - Cp*H)(dppe)(CNXyl)]+ (endo-[4]+) as evidenced by CW X-band EPR spectroscopy at 77 K; g = [2.132, 2.042, 2.004], A(31P1) = ± [75, 35, 54] MHz, and A(31P ) = ± [76, 64, 64] MHz. Simulations of HYSCORE spectra of endo-[4-H/D]+ give A(1H) = ± [17, 22, 32.5] MHz, aiso(1H) = ± 23.8 MHz, and T(1H) = ± [-6.8, -1.8, 8.7], which are nearly identical to those discussed above for the endo-Cp*(C-H) Fe(CO) adduct endo-[3]+. Additional features present in the (-,+) quadrant of HYSCORE of both endo-[4-H]+ and endo-[4-D]+ can be assigned to hyperfine coupling to 14N in the CNXyl ligand, with A(14N) = [7.4, 7.4, 9] MHz, a relatively small 14N nuclear quadrupole coupling constant e2qQ/h(14N) = 1.0 MHz, and a negligible electric field gradient rhombicity η(14N) ≈ 0, as expected for a triply bonded CN moiety with axial symmetry.13 100 The more stable, neutral endo/exo-species (endo/exo-3) were also prepared. Treatment of [FeII(η5-Cp*)(dppe)(CO)][BArF4] ([5]BArF4) with LiBEt3H provides exo[Fe0(η4-Cp*H)(dppe)(CO)] (exo-3) in good yield (Scheme 5.1). The alternative isomer, endo-3, was prepared via reduction of endo-[3]+ with Cp2Co at -78 oC.11 The room temperature stability allowed for isolation and growth of crystalline material suitable for single crystal X-ray diffraction. The structures of exo/endo-3 are presented in Figure 5.3. Exo- and endo-3 feature an η4-diene unit bound to a zerovalent iron center. Notably, the C(1)-C(6) bond distance in endo-3 (1.538(3) Å) is slightly longer than that in exo-3 (1.514(2) Å), possibly suggestive of Fe → C-C (σ*) donation in the former. Such an interaction is predicted based on the singularly occupied molecular orbitals (SOMOs) of endo/exo-[3]+ (Figure 5.4). Two distinct C-H stretches are observed for exo-3-H at 2711 and 2612 cm-1 that shift to 2009 and 1955 cm-1 for exo-3-D; C-H stretches for endo-3-H are not discernable from the bulk C-H stretching region i.e., > 2711 cm-1 (see the Supporting Information).14 Similar observations have been made for the pairs [Cp*(exoη4-Cp*H)CoII]/[Cp*(exo- η4-Cp*D)CoII]8 and [Cp(η4-C5H6)Co]/[Cp(exo-η4-C5H5D)Co].15 Despite its exceptionally weak C-H bond, the assignment of endo-[3]+ could be corroborated by X-ray crystallography (Figure 5.3) via growth of suitable crystals at low temperature. Gratifyingly, the solid-state structure, which was unobtainable for [Cp*(endoη4-Cp*H)Co]+, confirms the formation of an endo-Cp*(C-H) bond, with metrics associated with a dearomatized five-membered Cp*H ring that differs from that of a Cp* anion with elongation along the C(1)-C(5) and C(1)-C(2) vectors [1.51(1)/1.53(1) Å] from ca. 1.42 Å (average C-C bond distances in [7]+, vide infra), signifying new C-C single bonds. The low 101 temperature stability of endo-[3]+ likely derives from a high degree of steric shrouding of the reactive ring-bound H-atom via phenyl rings from the dppe ligand. Figure 5.3. Solid-state structure of exo-3, endo-3, and endo-[3]+ with ellipsoids shown at 50% probability. Counteranion omitted for endo-[3]+. 102 Figure 5.4. Frontier molecular orbitals (SOMOs) of endo/exo-[3]+ (staggered conformers) and [Cp*(endo/exo-η4-Cp*H)CoII]+ 7,8 optimized using TPSS; def2tzvp (Fe), def2svp (all other atoms). Inset shows aiso(1H) as obtained by 1,2H HYSCORE and/or ENDOR. Next, we sought to generate the alternative, and presumably more reactive, exoisomer wherein steric shrouding by dppe is far less prominent. Oxidation of neutral exo-3 with [Fc]BArF4 at – 78 oC provides green solutions of reactive exo-[3]+ that can be analyzed by EPR spectroscopy. From the CW data, we assign two conformational isomers of exo[3]+ (A and B), that presumably differ by rotation of the Cp*H ligand. both with rhombic symmetry (Figure 5.5): A with g = [2.116, 2.073, 1.997] and another, B with g = [2.093, 2.045, 2.013] The potential presence of multiple conformers is supported by DFT, as four distinct minima are found within 2 kcal mol-1 upon Cp*H rotation. The exact nature of the conformers present in solution could not be determined, due to the small differences in energies and predicted 1H hyperfine tensors (see the Supporting Information). Least- 103 squares optimization of simulations of the X-band CW spectra converged at relative populations of 0.6:0.4 for conformers A:B, indicating that the two conformations are indeed of similar relative energies. Simulations of 2H-1H difference ENDOR spectra of this mixture of exo-[3]+ products provided constraints on the 2H (and by proxy 1H) hyperfine couplings, with higher frequency ENDOR providing the same for 31P hyperfine couplings for the two conformers in the above CW simulations, with A(1H) = ± [85, 84, 83] MHz, aiso(1H) = ± 84 MHz, T(1H) = ± [1, 0, -1], A(31P1) = ± [96, 88, 47] MHz, and A(31P2) = ± [78, 75, 63] MHz for conformer A and A(1H) = ± [76, 74, 70] MHz, aiso(1H) = ± 73 MHz, T(1H) = ± [3, 1, -3], A(31P1) = ± [46, 44, 15] MHz, and A(31P ) = ± [70, 64, 64] MHz for conformer B. The aiso(1H) values for these isomers are much larger than that observed for the endo adduct, endo-[3]+ (aiso(1H) = ± 26.2 MHz), while the magnitude of T(1H) tensors are similar to that observed for endo-[3]+ and are consistent with a ligand C-H, rather than M-H unit. This trend, aiso(1H) (exo) > aiso(1H) (endo) is also been observed for the protonated [Cp*(endo/exo- 4 -Cp*H)CoII]+ derivative (Figure 5.4) and correlates with greater predicted spin density on the exo (versus the endo) hydrogen atom for the staggered16 isomers (0.06 versus 0.02 e-). 104 Figure 5.5. Left) X-Band CW-EPR spectra of freeze-quenched samples of exo-[3-H/D]+ in 2-MeTHF at 77 K for with simulations of conformers A and B. Right) X-band 2H-1H Difference Davies ENDOR of exo-[3-H]+ with data (black), total 2H simulation (red), conformer A (blue) ,and conformer B (green). Annealing frozen solutions of the 17-electron η4-Cp*H complexes endo-[3]+, exo[3]+, or endo-[4]+ (Scheme 5.1) provides ca. 0.5 equiv H2 and their corresponding 18electron S = 0 stable adducts, [FeII(η5-Cp*)(dppe)(CY)][BArF4] ([5]BArF4 (Y = O), [6]BArF4 (Y = NXyl)). The FeII-CO adduct [5]+ has been described previously;17 full characterization data for [6]BArF4 is in the Supporting Information. By contrast to endo[3]+, which can be isolated and characterized at temperatures below −70 oC, H2 evolution and formation of [5]+ from exo-[3]+ is observed at temperatures as low as −100 °C. As 105 previously noted, the difference in stability between endo-[3]+ and exo-[3]+ is likely due to a dramatic difference in steric hindrance between the endo- and exo-H positions. To gauge the relative energies of the species discussed herein, we turned to DFT calculations to estimate relevant BDFEs (Scheme 5.2). The starting hydride complex [1]+ was calculated to have a BDFEFe-H of 56 kcal mol-1, providing the vacant S = 1 cation, [Fe(η5-Cp*)(dppe)]+ (III). Scheme 5.2. Free energy change (kcal mol-1) for PCET involving endo-[3]+/[4]+ (TPSS; def2tzvp (Fe), def2svp (all other atoms)). 106 Associative binding of an L-type donor (prior to H• loss) was then considered. In this way, the CO (endo-[3]+) and CNXyl (endo-[4]+) adducts, IV were calculated to be - 3 and + 3 kcal mol-1 in energy relative to [1]+. The process involves adduct formation and then reductive C-H elimination (or vice-versa). H2 formation from IV to give complexes VI ([5]+ and [6]+) is calculated to be highly favorable (CO and CNXyl: BDFEC-H = 29 kcal mol-1 vs. BDFEH-H = 102.3 kcal mol-1 in CH3CN)1, correlating with a decrease in BDFEX-H of almost 30 kcal mol-1 (c.f. 56 kcal mol-1 for [1]+). The BDFEC-H of exo-[3]+ is calculated to be weaker (25 kcal mol-1), which in combination with the reduced steric crowding, manifests experimentally via facile H2 evolution at temperatures for which endo-[3]+ is stable enough to isolate. To experimentally benchmark the H• transfer propensity of [1]+, we aimed to determine an upper limit for its BDFEFe-H. Given the reduced FeII-H congener undergoes hydride transfer to 1-benzyl-3-acetamidopyridinium [BNAP]OTf (ΔGH- ≈ 59 kcal mol-1)18 and its known FeII/FeIII oxidation potential (E1/2 = - 0.71 V vs. Fc/Fc+),19 the FeIII−H bond of [1]+ is estimated to have an upper bound of BDFEFe-H < 50 kcal mol-1;20 this can be compared with a DFT-predicted value of 56 kcal mol-1. In spite of its weak BDFEFe-H, complex [1]+ is stable at room temperature in ethereal solvents (THF, Et2O; <5 % yield of H2 and [Fe(η5-Cp*)(dppe)(N2)]+ [7]+ at + 80 °C, 1 week; Scheme 5.3); details pertaining to the characterization of [7]+ are presented in the Supporting Information. Although [7]+ is thermodynamically poised to release H2, presumably a substantial kinetic barrier attenuates the rate of H2 loss (BDFEH-H = 102.3 kcal mol-1). 107 Scheme 5.3. A) Synthesis of [7]+ by H2 evolution (< 5%). Inset shows solid-state structure of [7]+ with ellipsoids shown at 50% probability; B) Non-productive H• transfer to azobenzene using [1]+. BDFE values (kcal mol-1) calculated by DFT using TPSS; def2tzvp (Fe), def2svp (all other atoms). Subsequently, we aimed to determine an upper limit for the BDFEC-H of endo-[3]+ (Scheme 5.4). Reaction of an MeCN solution of endo-3 with CO2 results in hydride transfer and formation of [Fe(η5-Cp*)(dppe)(CO)]+ [5]+. The hydricity of endo-3 in MeCN must therefore be less than the hydricity of HCO2- (ΔGH− ≈ 43 kcal mol-1). In other words, endo3 is more hydridic that HCO2-.21 Utilizing thermodynamic relationships that relate H• and H− transfer in MeCN,22 the upper bound for the free energy of H− transfer from endo-3 and H• transfer from endo-[3]+ can then be related. The endo-Cp*(C-H) bond of endo-[3]+ (Fe0/I, E1/2 = - 0.81 V vs. Fc/Fc+) is thus conservatively estimated to have an upper bound 108 BDFE (ΔG([3]+)H•) of < 36 kcal mol-1 (DFT prediction: 29 kcal mol-1), generating [5]+ and H• as products. On the basis of thermodynamics alone, H2 elimination from endo-[3]+ should be facile. The experimentally determined upper bound for the hydricity results in a pKa of 23 in acetonitrile as upper bound for endo-[3]+ (summarizing square scheme presented in the Supporting Information). Scheme 5.4. A) Experimental determination of BDFEC-H for endo-[3]+; B) Relevant thermodynamic equations relating H- and H• transfer for the endo-variant only. In line with the predicted lower BDFE of exo-[3]+, the Fe0/I redox potential is more positive (E1/2 = - 0.70 V vs. Fc/Fc+) with respect to endo-[3]+. The 110 mV difference between the endo and exo isomers corresponds to a 2.5 kcal mol-1 difference in BDFE, 109 suggesting that an additional difference in the BDFE is due to the different acidities of the isomers. We next turned our focus to exploring whether a productive H• transfer to an exogenous substrate from either endo-[3]+ or exo-[3]+ could be accomplished, as opposed to undesired H2 evolution. Azobenzene proved an interesting choice of substrate for this purpose as its conversion to diphenylhydrazine and vice versa has been studied over the years with various PCET donors and acceptors,23 with some donors in a similar BDFE range.3 Complex [1]+ does not react with a 20-fold excess of azobenzene (PhN=NPh) over a period of days (for an average BDFEN-H = 65 kcal mol-1 over two transfers), but upon addition of CO, which generates endo-[3]+ in-situ at −78 °C, a reaction is triggered to generate 1,2-diphenylhydrazine (25% yield, Scheme 5.5), H2 and [5]+. Isotopic labeling studies using the deuterated analogue, endo-[3-D]+, results in formation of the labeled isotopologue, PhDN-NDPh. In situ generation of exo-[3]+ through oxidation of exo-3 and reaction with PhN=NPh produces 1,2-diphenylhydrazine in 78% yield (Scheme 5.5). In line with endo-[3]+, only H2 and [5]+ are observed as byproducts and labeling with deuterium results in the formation of PhDN-NDPh. No other organic or iron containing products can be detected by NMR spectroscopy. A proton transfer followed by electron transfer or vice versa is unlikely as endo and exo-[3]+ are weak acids (vide supra), and the protonation step is calculated to be 27 kcal mol-1 uphill. Furthermore, endo and exo[3]+ are weak reductants (E1/2 > −0.81 V vs Fc/Fc+)24 and not capable of reducing azobenzene to the radical anion (E1/2 −1.73 V vs Fc/Fc+).25 Worth emphasizing is that the increase in reaction rate upon addition of CO to [1]+ is orders of magnitude, as no H2 formation or reduction of azobenzene is observed at room temperature in the absence of 110 added CO, while instantaneous formation of [5]+, the terminal product of H• transfer, can be observed even at −78 °C . This remarkable difference in reactivity via addition of a simple L-type ligand highlights the potential for in situ generation of a powerful PCET reagent. Scheme 5.5. Transfers to azobenzene using A) endo-[3]+ gives 25% PhNHNHPh and B) exo-[3]+ gives 78% PhNHNHPh. BDFE values (kcal mol-1) calculated by DFT using TPSS; def2tzvp (Fe), def2svp (all other atoms). 5.3 Conclusions In closing, building on recent work describing protonated metallocene and related species with very weak and hence reactive C-H bonds, we have herein described the characterization of endo-[FeI(η4-Cp*H)(dppe)(L)]+ (L = CO, CNXyl) and exo-[FeI(η4Cp*H)(dppe)(L)]+ (L = CO). Notably, one of these systems, endo-[FeI(η4Cp*H)(dppe)(CO)] was previously studied, but had been instead assigned as the 19- 111 electron hydride species [FeIII(η5-Cp*)(dppe)(CO)(H)]+. Use of pulse EPR techniques (1,2H HYSCORE, ENDOR) as well as the solid-state crystal structure of endo-[FeI(η4Cp*H)(dppe)(CO)]+, obtained at low temperature, cements this reassignment. Our interest in these particular endo-[FeI(η4-Cp*H)(dppe)(L)]+ systems is that they can be generated in situ via L addition to a far more stable iron hydride precursor, [FeIII(η5Cp*)(dppe)H]+. This affords the opportunity to transfer H• to a substrate, demonstrated for L = CO using the reduction of azobenzene to diphenylhydrazine as a model, where the H• transfer reaction is triggered at low temperature via the addition of CO to a mixture of [FeIII(η5-Cp*)(dppe)H]+ and PhN=NPh. The latter two partners do not otherwise react under the same conditions; CO triggers the isomerization that leads to the weak and hence reactive C-H bond. This approach demonstrates an attractive strategy for designing a powerful, in situ generated PCET reagent (BDFEC-H ≈ 29 kcal mol-1 for endo-[FeI(η4Cp*H)(dppe)(CO)]+), with tunability of the BDFE via the choice of L donor. 112 5.4. References 1 a) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications. Chem. Rev. 2010, 110, 6961; b) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112, 4016; c) Hammes-Schiffer, S. Theory of Proton-Coupled Electron Transfer in Energy Conversion Processes. Acc. Chem. Res. 2009, 42, 1881; d) Gentry, E. C.; Knowles, R. R. Synthetic Applications of Proton-Coupled Electron Transfer. Acc. Chem. Res. 2016, 49, 1546. e) Matos, J. L. M.; Green, S. A.; Shenvi, R. A. Markovnikov Functionalization by Hydrogen Atom Transfer. Organic Reactions 2019, 100, 383 f) Tsui, E.; Metrano, A. J.; Tsuchiya Y.; Knowles, R. R. Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton‐Coupled Electron Transfer Angew. Chem., Int. Ed. 2020, 59, 11845; g) Roos, C. B.; Demaerel, J.; Graff D. E.; Knowles, R. R. Enantioselective Hydroamination of Alkenes with Sulfonamides Enabled by Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2020, 142, 5974; h) Kolmar, S. S.; Mayer, J. M. SmI2(H2O)n Reduction of Electron Rich Enamines by Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2017, 139, 10687. 2 Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A. Mn-, Fe-, and Co- Catalyzed Radical Hydrofunctionalizations of Olefins. Chem. Rev., 2016, 116, 8912. 3 Bezdek, M. J.; Guo, S.; Chirik, P. J. Coordination-induced weakening of ammonia, water, and hydrazine X–H bonds in a molybdenum complex. Science 2016, 354, 730. 4 Chalkley, M.J.; Garrido-Barros, P.; Peters, J. C.; A molecular mediator for reductive concerted proton-electron transfers via electrocatalysis, Science 2020, 369, 850. 113 5 a) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. II. High-Affinity Proton Donors Promote Proton-Coupled Electron Transfer by Samarium Diiodide. Angew. Chem. Int. Ed. 2016, 55, 6033; b) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A. II. Proton-Coupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide-Water Complexes. J. Am. Chem. Soc. 2016, 138, 8738; c) Chciuk, T. V.; Anderson, W. R. Jr.; Flowers, R. A. II. Interplay between Substrate and Proton Donor Coordination in Reductions of Carbonyls by SmI2-Water Through proton-Coupled-Electron Transfer. J. Am. Chem. Soc. 2018, 140, 15342 6 Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molybdenum-catalyzed ammonia production with samarium diiodide and alcohols or water. Nature 2019, 568, 536. 7 a) Chalkley, M.; Del Castillo, T.; Matson, B.; Roddy, J.; Peters, J. C. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A proposed Role for Metallocene-Mediated PCET. ACS Cent. Sci. 2017, 3, 217; b) Chalkley, M. J.; Del Castillo, T. J.; Matson, B.; Peters, J. C. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis. J. Am. Chem. Soc. 2018, 140, 6122. 8 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. 9 Drover, M. W.; Schild, D. J.; Oyala, P. H. Peters, J. C. Snapshots of a Migrating H-Atom: Characterization of a reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic RingProtonated Iron(I) Isomer. Angew. Chem., Int. Ed. 2019, 58, 15504. 114 10 Hamon, P.; Toupet, L.; Hamon, J. R.; Lapinte, C. Novel diamagnetic and paramagnetic iron(II), iron(III), and iron(IV) classical and nonclassical hydrides. X-ray crystal structure of [Fe(C5Me5)(dppe)D]PF6. Organometallics 1992, 11, 1429. 11 Hamon, P.; Hamon, J. R.; Lapinte, C. Isolation and characterization of a cationic 19- electron iron(III) hydride complex; electron transfer induced hydride migration by carbon monoxide at an iron(III) center. J. Chem. Soc. Chem. Commun. 1992, 1602. 12 For example: a) Chiang, K. P.; Scarborough, C. C.; Horitani, M.; Lees, N. S.; Ding, K.; Dugan, T. R.; Brennessel, W. W.; Bill, E.; Hoffman, B. M.; Holland, P. L. Characterization of the Fe-H Bond in a Three-Coordinate Terminal Hydride Complex of Iron(I). Angew. Chem., Int. Ed. 2012, 51, 3658; b) Gu, N. X.; Oyala, P. H.; Peters, J. C. An S = ½ Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe-H Parameters. J. Am. Chem. Soc. 2018, 140, 6374. 13 Lucken, E. A. C. Nuclear Quadrupole Coupling Constants; Academic Press: London, 1969. pp 227-131. 14 For the IR spectrum of free Cp*H, see: Threlkel, R. S.; Bercaw, J. E.; Seidler, P. F.; Stryker, J. M.; Bergman, R. G. 1,2,3,4,5-pentamethylcyclopentadiene. Org. Synth., 1987, 65, 42 15 Green, M. L. H.; Pratt, L.; Wilkinson, G. 760. A new type of transition metal– cyclopentadiene compound. J. Chem. Soc. 1959, 3753-3767 16 Here, staggered refers to the puckered CH(CH3) group as being on the opposite side of the [Fe]-coordinated CO ligand. 115 17 a) Paul, F.; Toupet, L.; Roisnel, T.; Hamon, P.; Lapinte, C. Solid-state characterisation of the [(η2-dppe)(η5-C5Me5)FeCO]+ cation: an unexpected 'oxidation' product of the [(η2dppe)(η5-C5Me5)FeC=C(C6H4)NMe2]+ radical cation. C. R. Chim. 2005, 1174; b) Catheline, D.; Astruc, D. Synthesis and characterization of C5(CH3)5Fe(CO)3+PF6− and C5(CH3)5Fe(CO)2−K+. J. Organomet. Chem. 1982, 226, c52. 18 “In the reaction between [Fe(η5-Cp*)(dppe)(H)] and [BNAP]+, the solvent adduct, [Fe(η5-Cp*)(dppe)(NCMe)]+ and BNAPH are produced – in this case, acetonitrile binding will cause the experimentally determined BDFE (50 kcal mol-1) to be lower by as much as the binding strength of MeCN to the vacant Fe(II) complex, [Fe(η5-Cp*)(dppe)]+. By DFT, we estimate this value to be 12 kcal mol-1, bringing the BDFE to 62 kcal mol-1. See: Zhang, F.; Jia, J.; Dong, S.; Wang, W.; Tung, C. -H. Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)+ Analogues. Organometallics 2016, 35, 1151. 19 Tilset, M.; Fjeldah, I.; Hamon, J.R.; Haon, P.; Toupet, L.; Saillard, J.Y.; Costuas, K.; Haynes, A. Theoretical, Thermodynamic, Spectroscopic, and Structural Studies of the Consequences of One-Electron Oxidation on the Fe−X Bonds in 17- and 18-Electron Cp*Fe(dppe)X Complexes (X = F, Cl, Br, I, H, CH3). J. Am. Chem. Soc. 2001, 123, 9984. 20 This value does not take into account the free energy associated with acetonitrile binding. 21 a) Ilic, S.; Kadel, U. P.; Basdogan, Y.; Keith, J. A.; Glusac, K. D. Thermodynamic Hydricities of Biomimetic Organic Hydride Donors. J. Am. Chem. Soc. 2018, 140, 4569; b) Ellis, W. W.; Raebiger, J. W.; Calvin, C. J.; Bruno, J. W.; DuBois, D. L. Hydricities of BzNADH, C5H5Mo(PMe3)(CO)2H, and C5H5Mo(PMe3)(CO)2H in Acetonitrile. J. Am. Chem. Soc. 2004, 126, 2738. 116 22 a) Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R.M.; Miller, A. J. M.; Appel, A. M. Thermodynamic hydricity of Transition Metal Hydrides. Chem. Rev. 2016, 116, 8655; b) Pearson, R. G. The Transition-Metal-Hydrogen Bond. Chem. Rev. 1985, 85, 41. 23 a) Savéant, J.-M.; Tard, C. Proton-Coupled Electron Transfer in Azobenzene/Hydrazobenzene Couples with pendant Acid-Base Functions. HydrogenBonding and Structural Effects. J. Am. Chem. Soc. 2014, 136, 8907; b) Cattaneo, M.; Ryken S.A.; Mayer, J.M. Outer-Sphere 2e-/2H+ Transfer Reactions of Ruthenium(II)Amine and Ruthenium(IV)-Amido Complexes. Angew. Chem., Int. Ed. 2017 56, 3675. 24 Th FeI/II redox potential of endo and exo-[3]+ could not be determined experimentally but the Fe0/I couples give conservative lower limits of, respectively, −0.81 and 0.70 V vs Fc/Fc. 25 Goulet-Hanssens, A.; Utecht, M.; Mutruc, D.; Titov, E.; Schwarz, J.; Grubert, L.; Bleger, D.; Saalfrank, P.; Hecht, S. Electrocatalytic Z –> E Isomerization of Azobenzenes. J. Am. Chem. Soc. 2017, 139, 335−341 Appendix A : Supporting Information for Chapter 2 118 Experimental Section 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 argon gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. 2-MeTHF was degassed by three freeze-pump-thaw cycles, followed by drying over NaK to remove traces of water. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, filtered through an alumina plug, and dried over 3Å molecular sieves prior to use. All reagents were purchased from commercial vendors and used without further purification unless stated otherwise. P2PPhFeBr2 (3) and P2PPh57FeCl2,1 [H(OEt2)2][BArF4] (BArF4 = tetrakis(3,5‐bis(trifluoromethyl)phenyl)borate),2 Cp*2Co,3 and KC84 were synthesized following literature procedures. Physical Methods. NMR spectra were recorded at room temperature unless otherwise noted. 1H, 13C and 29Si chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent proton and 13C resonances as internal standards. 29Si NMR chemical shifts were determined from 29Si-HMBC two-dimensional spectra 15N and 31P chemical shifts are reported relative to CH3NO2 and 85 % aqueous H3PO4, respectively. Solution phase magnetic measurements were performed by the method of Evans.5 IR spectra were obtained using a Bruker Alpha Platinum ATR spectrometer with OPUS software in a glovebox under an N2 atmosphere. 119 UV-Vis measurements were collected using a Cary 50 instrument with Cary WinUV software. X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-5 mM solutions prepared as frozen glasses in 2-MeTHF. Samples were collected at powers ranging from 20 µW to 2 mW and modulation amplitudes of 1 – 5 Gauss. Spectra were simulated using the Easyspin suite of programs with Matlab 2018. Mössbauer spectra were recorded on a spectrometer from SEE Co. operating in the constant acceleration mode in a transmission geometry. Spectra were recorded with the temperature of the sample maintained at 80 K. The sample was kept in an SVT-400 Dewar from Janis. The quoted isomer shifts are relative to the centroid of the spectrum of a metallic foil of α-Fe at room temperature. Data analysis was performed using the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. Cyclic voltammetry measurements were carried out in a glovebox under an N2 atmosphere in a one-compartment cell using a CH Instruments 600B electrochemical analyzer. A glassy carbon electrode was used as the working electrode and a carbon rod was used as the auxiliary electrode. The reference electrode was AgOTf/Ag in THF isolated by a CoralPor™ frit (obtained from BASi). The ferrocenium/ferrocene couple (Fc+/Fc) was used as an external reference. THF solutions of electrolyte (0.1 M [NBu4][PF6]) and analyte were also prepared under an inert atmosphere. Hydrogen Analysis. The headspace of reaction flasks was analyzed by gas chromatography to quantify H2 evolution with an Agilent 7890A gas chromatograph (HPPLOT U, 30 m, 120 0.32 mm i.d., 30 °C isothermal, 1 mL/min flow rate, N2 carrier gas) using a thermal conductivity detector. X-Ray Crystallography. X-ray diffraction studies were carried out at the Caltech Division of Chemistry and Chemical Engineering X-ray Crystallography Facility on a Bruker threecircle SMART diffractometer with a SMART 1K CCD detector, APEX CCD detector, or Bruker D8 VENTURE Kappa Duo PHOTON 100 CMOS detector. Data were collected at 100 K using Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54178 Å). Structures were solved by direct or Patterson methods using SHELXS and refined against F2 on all data by full-matrix least squares with SHELXL-97.68 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times the Ueq of the atoms to which they are bonded. See below for any special refinement details for individual data sets. Combustion analysis measurements were collected using a PerkinElmer 2400 Series II CHN Elemental Analyzer. Computational methods Geometry optimizations were performed using the Gaussian09 package all optimizations.6 The TPSS functional7 was employed in combination with def2TZVP8 basis set on transition metals and a def2-SVP8 basis set for all remaining atoms in frequency calculations and geometry optimizations. 121 Experimental Section P2PPh: A 1.6 M n-BuLi solution (6.5 mL, 10.4 mmol) was added dropwise to a stirring solution of (2-bromophenyl)diisopropylphosphine (2.83 g, 10.3 mmol) in 40 mL diethyl ether at -78 °C. Following the addition,the light-yellow reaction mixture was stirred for 90 minutes at -78 °C. P,P-dichlorophenylphosphine (0.924 g, 5.1 mmol) in 6 mL diethyl ether was added over 30 minutes, resulting in a color change to red. The yellow suspension obtained after warming to room temperature overnight, was brought into the glovebox and filtered over celite. The flask was rinsed four times with 8 mL diethyl ether and the resulting solutionswere subsequently passed over celite and combined with the filtrate. The solvent of the combined filtrates was removed in vacuo, yielding a pale yellow powder. The powder was washed with pentane (3 x 5 mL) and dried under vacuum to give P2PPh (1) as a white solid (1.23 g, 2.5 mmol, 47 %) evident by comparing spectroscopic properties with previously reported spectra.1 1H NMR (Benzene-d6, 400 MHz) δ ppm 7.49 – 7.40 (m, 2H), 7.39 – 7.31 (m, 2H), 7.08 (t, J = 6.7 Hz, 7H), 6.97 (t, J = 7.5 Hz, 2H), 2.11 (td, J = 7.0, 2.6 Hz, 2H), 1.98 (hept, J = 7.1 Hz, 2H), 1.18 (td, J = 13.8, 7.0 Hz, 12H), 0.92 (ddd, J = 32.8, 11.5, 7.0 Hz, 12H). 13C NMR (C6D6, 101 MHz) δ ppm 148.42 (m), 142.42 (m), 139.73 (dt), 135.64 (d), 134.88 (m), 132.44 (m), 128.91 (s), 128.45 (d), 24.94 (m), 20.56 (m), 19.88 (m). 31P{1H} (C6D6, 162 MHz) δ ppm -2.17 (dd, 3JPP = 158.7, 3JPP = 147.0 Hz, 2P, P-Ar), -14.26 (dt, 3JPP = 158.7, 3JPP = 147.0 Hz, 1P, P-Ph). (P2PPh)FeBr (4): A dark purple solution of 3 (173.0 mg, 243 μmol) dissolved in 8 mL THF was stirred over sodium amalgam (5.8 mg, 252 μmol) for 2 h at room temperature during which the color changed to dark red. The red solution was filtered over celite and dried in vacuo. The resulting red solid was extracted with 4 mL Et2O and filtered over celite. 122 Washing the red solid 4 times with 1.5 mL pentane yielded P2PPhFeBr as a red solid (99.8 mg, 158 μmol 65%). Crystals suitable for XRD were obtained by vapor diffusion of pentane into a benzene solution of P2PPhFeBr. 1H NMR (400 MHz, Benzene-d6) δ 124.74 (2H), 104.79 (2H), 17.85 (2H), 7.96 (6H), 6.46,4.89, 3.84 (2H), 2.41 (2H), 1.02 (2H), -2.64 (6H), -7.19 (6H), -17.74 (2H), -20.57 (1H). UV-Vis (Benzene, nm {cm-1 M-1}): 320 {6950}, 375 {5225}, 433 {4785}, 830 {1315}. μeff (C6D6, Evans Method, 25 °C): 4.11 μB. Anal: calculated for C30H41BrFeP3: C 57.17, H 6.56 found: C 56.83, H 6.58 [(P2PPh)Fe(H)]2(μ–N2) (1): A 20 mL vial containing 4 (97.4 mg, 154.5 μmol) in 10 mL toluene was cooled down to −78 °C. 6.1 mL of an 0.25 M NaHBEt3 solution in toluene was added after which the mixture was stirred at -78 °C for 30 minutes followed by 2 h at room temperature. Upon warming to room temperaturethe color changed from dark red to forest green. The solvent was removed in vacuo, after which the residue was extracted with 6 mL pentane. Reducing the solvent to 3 mL and storing at −35 °C yielded 1 as green crystals (50.1 µmol, 61 %), as was evident by comparing the spectroscopic properties with previously reported spectra.1 1H NMR (THF-d8, 500 MHz) δ ppm 8.15 (d, J = 7.5 Hz, 2H), 7.52 (d, J = 7.5 Hz, 2H), 7.43 (t, J = 7.3 Hz, 2H), 7.26 (t, J = 7.3 Hz, 2H), 7.14 (t, J = 7.3 Hz, 2H), 6.63 (d, J = 7.4 Hz, 2H), 6.12 (t, J = 7.5 Hz, 1H), 2.99 (broad s, 2H), 2.59 (broad s, 2H), 0.72 (m, 6H), 0.42 (m, 6H). (P2PPh)Fe(N2)2 (5) from 3: 50 mL of a THF solution of 3 (555.8 mg, 782 μmol) was stirred over sodium mercury amalgam (36.7 mg, 1.596 μmol, 9.6 g Hg) for 16 h at room temperature during which the color changed to wine red. The THF solution was filtered over celite, and the solvent was subsequently removed in vacuo. The material was extracted with pentane 40 mL and filtered over celite. Cooling down the filtrate down to −35 °C 123 yields crystalline P2PPhFe(N2)2 (205.2 mg, 339 µmol 43%). Additional product can be obtained by cooling down the concentrated mother liquor. No satisfactory elemental analysis could be obtained, with C and H percentages slightly higher than expected. The nitrogen content was consistently low, which is likely due to the loss of an N2 ligand. The equilibrium discussed in the main text also hampers obtaining quantitative integrals in certain regions. 1H NMR (400 MHz, Benzene-d6) δ 7.63 (t, J = 6.3 Hz, 2H), 7.50 (dd, J = 6.0, 3.9 Hz, 2H), 7.12 – 6.75 (m, 9H), 2.86 – 2.53 (m, 4H), 1.57 – 1.40 (m, 6H), 1.35 – 0.96 (m, 18H), 13C NMR (101 MHz, THF) δ 147.69, 131.47, 130.21, 129.07, 128.66, 127.29, 32.49, 27.99, 19.29, 18.84, 31P NMR (162 MHz, Benzene-d6) δ 122.76 (t, J = 63.0 Hz, 1H), 100.26 (d, J = 63.0 Hz 2P). IR (ATR, THF C6D6 film): νN2 = 2065 cm−1, 2005 cm−1. UV-Vis (Et2O, nm {cm-1 M-1}): 251 {25000}, 322 {8816}, 405 {5480}, 490 {3820}, 830 (P2PPh)Fe(N2)2 (5) from 4: A 7 mL THF solution of 4 (81.0 mg, 128 µmol) was stirred over sodium mercury amalgam (3.6 mg, 156 µmol) for 1 hour and subsequently filtered over celite. THF was remove in vacuo and the residue was extracted with 6 mL pentane and filtered over celite. Cooling the solution to −35 °C yields P2PPhFe(N2)2 as a crystalline solid (37.8 mg, 62 µmol, 48%). Additional product can be obtained by cooling down the concentrated mother liquor. (P2PPh)57Fe(N2)2 (575): Complex 575 was prepared using the synthetic procedure for 5 with P2PPh57FeCl2 instead. The 1H NMR spectrum matched that of 5, while additional coupling with 57Fe was present in the 31P NMR spectrum. 31P NMR (162 MHz, Benzene-d6) δ 122.76 (dt, 2JPP = 64.5 Hz, 1JFeP = 46.2 Hz, 1P), 100.26 (t, J = 63.0 Hz, 2P). 124 [(P2PPh)Fe]2µ–N2 (6): An NMR tube containing 5 (20.5 mg, 33.8 µmol) in 0.6 mL toluened8 was freeze-pump thawed three times, stored for 4 hours and freeze-pump thawed three additional times. A 31P NMR spectrum was recorded to ensure full conversion to 6. During the cycles, the color changes from maroon to a dark purple. If 5 was still present, additional freeze-pump thaw cycles were performed until the signal corresponding to 5 was negligible. Attempts to isolate the product as a crystalline solid were unsuccessful. 1H NMR (500 MHz, Toluene-d8) δ 7.97 (d, J = 7.4 Hz, 2H), 7.44 (d, J = 7.3 Hz, 2H), 7.31 (dd, J = 7.3 Hz, J = 7.3 Hz, 2H), 6.94 (dd, J = 7.4 Hz, J = 7.3 Hz, 2H), 6.87 (dd, J = 7.3 Hz, J = 7.3 Hz, 2H), 6.46 (s, 2H), 5.79 (s, 1H), 5.35 (s, 2H), 5.07 (s, 2H), 1.61 (s, 6H), 1.05 (s, 6H), 1.01 (s, 6H) 0.43 (s, 6H).31P NMR (202 MHz, Toluene-d8, 298 K) δ 184.84. 31P NMR (202 MHz, Toluene-d8, 203 K ) δ 130.42, 123.63 Small amounts of an unknown species are present at 98.05 and 92.51 ppm, but they account for less than 5% (P2PPh)Fe(N2)(H)2 (2): A Schlenk tube containing 5 (60.9 mg, 100 μmol) in THF (6 mL) was freeze-pump-thawed 3 times and exposed to 1 atmosphere of H2. The reaction was stirred vigorously at room temperature for 24 hours before it was freeze‐pump‐thawed 2 times, re‐exposed to 1 atmosphere of N2, and stirred for another 24 hours, during which thesolution turned yellow. The solvent was removed in vacuo, yielding 2 quantitatively (58.0 mg, 100 µmol, 100%). The nature of the solid was determined by comparing its NMR and IR features with those reported.1 1H NMR (THF-d8, 500 MHz) δ ppm 8.09 (t, J = 6.4 Hz, 2H), 7.77 (d, J = 7.2 Hz, 2H), 7.40 (p, J = 7.2 Hz, 4H), 7.22–7.15 (m, 5H), 2.67 (h, J = 6.8 Hz, 2H), 2.56–2.50 (m, 2H), 1.43 (q, J = 7.0 Hz, 6H), 1.15–1.19 (m, 12H), 0.46 (q, J = 6.9, 6H), -9.43 (td, J = 38.2 , 15.7 Hz, 1H), -20.71 (td, J = 43.2 , 15.6 Hz, 1H), 31P NMR 125 (162 MHz, Benzene-d6) δ ppm 119.2 (2P), 110.2 (1P). IR (ATR, THF C6D6 film): 2071 cm-1 (νN2), 1796 cm-1 (νFe–H). [(P2PPh)Fe(N2)2(H)][BArF4] (7): A 20 mL vial containing 1 (16.4 mg, 14.5 µmol) in 1.5 mL diethyl ether was cooled down to −78 °C. A cooled solution of FcBArF4 (32.8 mg, 31.2 µmol) in 1.5 mL diethyl ether was added dropwise during which the color changes from green to orange. The solution was stirred at −78 °C for 1 hour and warmed to room temperature. The mixture was subsequently filtered over celite. The filtrate was layered with 10 mL pentane and stored at −35 °C overnight, which resulted in the formation of orange crystals (41.0 mg, 27.8 µmol 96 %). The obtained crystalline material was suitable for X-ray diffraction. 1H NMR (THF-d8, 400 MHz) δ ppm 8.22–8.10 (m), 7.85–7.69 (m), 7.59–7.50 (m), 7.36 (t, J = 7.9, 1H), 7.30–7.27 (m), 7.27–7.21 (m), 6.60 (t, J = 9.3 Hz, 2H), 3.14–2.92 (m, 4H), 1.56–1.41 (m), 1.35–1.14 (m), -16.85 (dt (q), J = 54.9, 1H). 11B NMR (THF-d8, 128 MHz) δ ppm -4.68 (s). 19F NMR (THF-d8, 376 MHz) δ ppm -61.51 (s). P{1H} (THF-d8, 162 MHz) δ ppm 113.41 (overlapping dt, J = 34.6, 25.0 Hz, 103 1P, 31 PPh), 95.75 (dd, J = 29.5, 7.5 Hz, 2P, PiPr2). IR (thin film from evaporation of THF-d8; cm-1): 2193 (νN–N), 2162 (νN–N), 2069 (νFe–H). UV-Vis (Et2O, nm {cm-1 M-1}): 367{2200} Anal: calculated for C62H54BF24FeN4P3: C 50.64, H 3.70 N 3.81, found: C 50.23, H 4.00, N 3.17 [(P2PPh)Fe(N2)(H)][K(18-crown-6] (8): A 4 mL vial containing 1 (28.4 mg, 25 µmol) in THF was cooled down to −78 °C. Simultaneously, a 20 mL vial containing KC 8 (7.0 mg (51 µmol) and a stir bar was cooled down. After 20 minutes, the THF solution containing 126 1 was rapidly added to the vial containing KC8. The mixture is stirred for 45 minutes at −78, after which 18-crown-6 (20.4 mg, 77 µmol) was added. The solution was stirred an additional 45 minutes at room temperature, layered with 6 mL pentane and stored at −35 °C. Over four days black crystals formed suitable for XRD, (35.4 mg, 40 µmol, 90%). 1H NMR (400 MHz, THF-d8) δ 7.75 (q, J = 4.2 Hz, 2H), 7.43 (t, J = 4.2 Hz, 2H), 7.24 (t, J = 7.6 Hz, 2H), 7.04 – 6.88 (m, 6H), 6.83 (t, J = 7.2 Hz, 1H), 2.53 (d, J = 9.2 Hz, 2H), 2.42 – 2.26 (m, 2H), 1.24 (ddt, J = 21.2, 13.8, 6.6 Hz, 6H), 0.88 (q, J = 6.8 Hz, 12H), 0.29 (q, J = 6.6 Hz, 6H), -9.69 (td, J = 68.8, 67.9, 27.3 Hz, 1H).31P NMR (162 MHz, THF-d8) δ ppm 120.62 (m, 1P) 119.89 – 118.98 (m. 2P). IR (thin film from evaporation of THF-d8; cm-1): 1924 (νN–N), 1733 (νFe–H) Anal: calculated for C62H54BF24FeN4P3: C 57.86, H 7.81 N 2.93, found: C 57.23, H 7.34, N 1.89 [(P2PPh)Fe(N2)][K(18-crown-6] (9): A 20 mL vial containing 5 (20.6 mg, 34 µmol) in 1.5 Ml THF was cooled down to −78 °C and 340 µL of a 100 µM potassium naphthalide solution (34 µmol) was added after which the species was stirred. After one hour of stirring, an excess 18-crown-6 was added (18 mg, 68 µmol). The solution was stirred for an hour at room temperature and subsequently layered with 6 mL pentane. Storing the solution in the freezer for 24 h resulted in the formation of dark crystals suitable for XRD (20.9 mg, 23.8 µmol 70%). 1H NMR (400 MHz, THF-d8) δ 63.91 26.76, 20.51, 18.28, 13.61. 9.85, 6.27, 5.22, 1.04, -2.15, -7.78, -27.16 ppm. IR (ATR, THF C6D6 film): 1872 cm-1 (νN2), Anal: calculated for C62H53BF24FeN4P3: C 57.64, H 7.61 N 2.93, found: C 57.23, H 7.34, N 1.89 127 [(P2PPh)Fe(N2)][K2(THF)3] (10): A 20 mL vial containing 5 (120.1 mg, 198 µmol) dissolved in 5 mL NaK dried THF was cooled down to −78 °C. Simultaneously, a 20 mL vial containing KC8 (80.2 mg, 600 µmol) and a stir bar was cooled down to −78 °C. The cooled solution containing 5 was added to the vial containing KC8 after which the vial was rinsed with 0.5 mL THF which was subsequently added. The solution was stirred for 15 minutes. and filtered over celite. To assure the frit and celite are sufficiently dry, 3 mL NaK dried THF was passed through the frit four times before the reaction mixture was passed over celite. The reaction vial was rinsed with 1 mL THF, and the wash was passed over celite. The filtrate was divided into two 20 mL vials and each vial was layered with 15 mL pentane. Crystals of 10 suitable for XRD formed over a period of a week. (110.1 mg, 126 µmol, 63%). 1H NMR (400 MHz, THF-d8) δ 7.87 (m, 2H), 7.51 – 7.34 (m, 2H), 6.89 (m, 8H), 6.71 (t, J = 7.2 Hz, 1H), 2.90 – 2.58 (m, 2H), 2.58 – 2.38 (m, 2H), 1.29 (m, J = 12H), 0.81 (q, J = 5.8 Hz, 6H), 0.23 (q, J = 5.9 Hz, 6H). 13C NMR (101 MHz, THF-d8) δ 157.27, 154.67, 153.04, 131.46, 129.79, 128.62, 128.05 – 122.99 (m), 35.82, 29.23, 24.12 – 19.34 (m). 15N NMR (41 MHz, THF-d8) δ 2.36, -25.91. 31P NMR (162 MHz, THF-d8) δ 113.13 (d, J = 33.8 Hz), 95.15 (t, J = 33.8 Hz). No satisfactory combustion analysis could be obtained. [(P2PPh)Fe(NNSiMe3)]K (11- NNSiMe3)K: A 20 mL vial containing 5 (55.9 mg, 92 µmol, in 5 mL NaK dried THF was cooled down to -78 °C in a glovebox Coldwell after which it was added to a vial containing KC8 (37.4mg, 276 µmol 3 equiv.) at -78 °C, Stirring the solution for 30 minutes resulted in a color change to dark purple. The cold suspension was filtered over celite, and subsequently cooled down to -78 °C followed by the addition of 128 TMSCl (11.6 µL, 92 µmol) which resulted in an immediate color change to brown. The mixture was subsequently stirred at −78 °C for 30 minutes, followed by 30 minutes at room temperature. Volatiles were removed in vacuo and the residue was dissolved in benzene and filtered over celite. Benzene was removed in vacuo, and the solid washed with pentane (3 x 2 mL) and filtered over a celite plug. The remaining solids were dissolved in diethyl ether and passed over the same celite plug. Removal of diethyl ether in vacuo yields [P2PPhFe(NNTMS)]K (35 mg, 54 µmol, 58%). Crystals suitable for XRD could be grown by layering a concentrated benzene solution with pentane. 1H NMR (C6D6, 400 MHz): δ 7.84 (m, 2H), 7.68 (d, 3JHH = 6.69 Hz, 2H), 7.10-6.96 (m, 6H), 6.66-6.42 (m, 3H), 2.992.76 (m, 2H), 2.31-2.17 (m, 2H), 1.59-1.40 (m, 6H), 1.33-0.89 (m, 18H), 0.38 (s, 9H, Si(CH3)3); 29Si NMR (79 MHz, C6D6, HMBC) δ: 4.72 (s, N-SiCH3). 13C NMR (101 MHz, C6D6) δ 130.65 (d, J = 10.4 Hz), 127.98, 127.54 (d, J = 6.0 Hz), 126.48 – 125.86 (m), 125.54 (d, J = 11.5 Hz), 32.72 (t, J = 9.4 Hz), 28.26 (d, J = 8.4 Hz), 21.36 – 19.79 (m), 0.98, -1.11. 31P NMR (162 MHz, C6D6) δ 117.48 (d, J = 16.7 Hz, 2P, PAr), 109.12 (t, J = 16.6 Hz, 1P, PPh). Anal: calculated for C33H56FeKN2P3Si: C 57.38, H 7.81 N 4.06, found: C 57.17, H 7.07, N 3.61 [(P2PPh)Fe(NNSiiPr3)]K (11-NNSiiPr3): P2PPhFe(N2)2 (50.1 mg, 83 µmol) in 2.5 mL NaK dried THF was cooled down to −78 °C in a glovebox coldwell and passed over a pipette with a thin layer of KC8 (4 mm). The mixture was passed over the pipette three times during which the color changed to dark purple. TiPSOTf (20 µL, 74 µmol was added upon which an immediate color change to orange brown was observed. The mixture was subsequently stirred at −78 °C for 30 minutes, after which it was taken out of the coldwell and stirred at room temperature for 30 minutes. Volatiles were removed in vacuo and the residue was 129 dissolved in benzene and filtered over celite. Benzene was removed in vacuo and the remaining residue washed with HMDSO (5 x 1.5 mL) and filtered over celite. The remaining brown residue in the vial and on the celite was dissolved in diethyl ether. Removal of diethyl ether in vacuo yields [P2PPhFe(NNTiPS)]K in 75 % yield (43.0 mg, 55 µmol). 1H NMR (300 MHz, Benzene-d6) δ 7.89 (s, 3H), 7.69 (d, J = 6.7 Hz, 2H), 7.09, (s, 6H), 6.62 (s, 3H), 3.55 (s, 3H) 2.88 (s, 2H), 2.62 (s, 2H), 1.50 (td, J = 14.5, 13.7, 6.4 Hz, 12H), 1.38 – 1.14 (m, 35H), 0.97 – 0.81 (m, 12H). 31P NMR (121 MHz, Benzene-d6) δ 118.22 (d, J = 17.9 Hz), 111.69 (t, J = 17.9 Hz). IR (ATR, C6D6 film): 1469 cm-1 (νN2), Stochiometric reactivity Addition of H2 to 5 at -78 °C. The headspace of a J-Young NMR tube containing 5 (5.9 mg, 9.7µmol) was degassed once by a freeze-pump-thaw cycle. The tube was warmed transferred into a dry ice acetone bath and one atmosphere of H2 was added. The cold tube was rapidly shaken for 5 seconds and inserted into the precooled NMR spectrometer. Addition of H2 to 6 at -78 °C. To a J-Young NRM tube containing 6, generated in situ according to the preparation described above, was added one atmosphere of H2. The cold tube was rapidly shaken for 5 seconds and inserted into the precooled NMR spectrometer. General procedure for the synthesis of [(P2PPh)Fe(N2)2(H)][BArF4] (7) from 1, 2 or 5 with HBArF4: A 20 mL vial containing 1, 2, or 5 (9.4 µmol for 1, 18.8 µmol for 2 and 5) in diethyl ether (1 mL) was chilled to −78 °C in the glovebox coldwell. In a separate 4 mL vial, a diethyl ether solution of HBArF4 (0.019 g, 18.4 µmol, 250 µL diethyl ether) was chilled to −78 °C. Both solutions were allowed to cool for 20 min before the HBArF4 solution was added to the vial containing 1, 2 or 5 at -78 °C in one shot. The vial containing 130 HBArF4 was subsequently rinsed with 250 µL of pre-chilled diethyl ether and the rinsing was added to the vial containing. The reaction mixture was stirred at −78 °C for 1 hour and 15 minutes before it was warmed to warm to room temperature. Analysis by NMR and IR spectroscopy confirms the nature of the product as 7. NMR analysis of addition of HBArF4 to 5 at -78 °C: 5 (5.9 mg, 9.7µmol) was dissolved in 0.4 mL THF-d8 and added to a J-Young NMR tube at −78 °C. HBArF4 (10.3 mg, 10.7 µmol) was cooled to −78 °C and added to the NMR tube. The cold tube was rapidly shaken for 5 seconds, taken out of the glovebox and inserted into the precooled NMR spectrometer at −78 °C. Oxidation of [(P2PPh)Fe(NNSiMe3)]K at room temperature. To a stirring solution of [P2PPhFe(NNTMS)]K (13.0 mg, 17 µmol) in 3 mL THF was added [Cp*2Co][PF6] (8.2 mg, 17 17 µmol) suspended in 1 mL THF. The mixture was stirred for 24 hours, followed by removal of the solvent in vacuo. Extracting the solid twice with 5 mL pentane results in an off-white residue. The pentane extracts were combined and removal of the solvents in vacuo gives a brow solid (10 mg). Analysis of the solid by NMR spectroscopy reveals the presence of 5 and cobaltocene. Oxidation of [(P2PPh)Fe(NNSiMe3)]K (11-NNSiMe3) at −78 °C. To a cooled stirring solution of [(P2PPh) Fe(NNTMS)]K (4 µmol) in 1 mL 2-MeTHF at −78 °C was added [Cp*2Co][PF6] (8.2 mg, 17 µmol) suspended in 0.5 mL 2-MeTHF. The solution was stirred for 5 minutes at −78 °C after which the sample was frozen. The frozen solution was briefly thawed and a 300 µL aliquot was transferred to a precooled EPR tube which was 131 subsequently frozen. Additional EPR spectra were recorded for the frozen samples stored at −78 °C for 24 hours. (P2PPh)Fe(NNiPr3) (12-NNiPr3): A cooled stirring solution of [(P2PPh) Fe(NNTiPS)]K (15.0 mg, 19 µmol) in 1 mL was added to [Cp*2Co][PF6] (9.1 mg, 19 µmol) at −78 °C. The solution was stirred at −78 for 10 minutes followed by 10 minutes at room temperature. The solvent was removed in vacuo and the residue extracted with pentane. Filtering over celite and removal of the solvent in vacuo gives a product characterized as 12–TiPS. 1H NMR (300 MHz, Benzene-d6) δ 1H NMR (400 MHz, THF-d8) δ 12.22 9.31 7.24 5.90, 4.92, 3.39, 2.36 1.63 – 0.48. IR (ATR, THF film): 1659 cm-1 (νN2), 132 NMR Spectra Figure A.1. 1H NMR spectrum of (P2PPh)FeBr (4) in C6D6 at room temperature Figure A.2. 1H NMR spectrum of (P2PPh)Fe(N2)2 (5) in C6D6 at room temperature. Additional broad peaks corresponding to 6 are present due to the equilibrium as described in the main text. 133 Figure A.3. 13C NMR spectrum of (P2PPh)Fe(N2)2 (5) in THF at room temperature Figure A.4. 31P NMR spectrum of (P2PPh)Fe(N2)2 (5) in C6D6 at room temperature Figure A.5. 31P NMR spectrum of (P2PPh)57Fe(N2)2 (575) in C6D6 at room temperature 134 Figure A.6. 1H NMR spectrum of [(P2PPh)Fe]2(µ– N2) (6) in Toluene-d8 at room temperature Figure A.7. 31P NMR spectrum of [(P2PPh)e]2(µ– N2) (6) in Toluene-d8 at room temperature 135 Figure A.8. 31P NMR spectrum of [(P2PPh)Fe]2(µ– N2) (6) in Toluene-d8 at 203 K Figure A.9. 1H NMR spectrum of [(P2PPh)Fe(N2)2(H)][BArF4] (7) in THF-d8 at room temperature 136 Figure A.10. 19F NMR spectrum of [(P2PPh)Fe(N2)2(H)][BArF4] (7) in THF-d8 at room temperature Figure A.11. 31P NMR spectrum of [(P2PPh)Fe(N2)2(H)][BArF4] (7) in THF-d8 at room temperature 137 Figure A.12. 1H NMR spectrum of [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (8) in THF-d8 at room temperature Figure A.13. 31P NMR spectrum of [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (8) in THF-d8 at room temperature 138 Figure A.14. 1H NMR spectrum of [(P2PPh)Fe(N2)][K(18-crown-6)] (9) in d8-THF at room temperature Figure A.15. 1H NMR spectrum of [(P2PPh)Fe(N2)][K2(THF)3] (10) in d8-THF at room temperature 139 Figure A.16. 15N NMR spectrum of [(P2PPh)Fe(N2)][K2(THF)3] in d8-THF at room temperature Figure A.17. 31P NMR spectrum of [(P2PPh)Fe(N2)][K2(THF)3] in d8-THF at room temperature 140 Figure A.18. 1H NMR spectrum of [(P2PPh)Fe(NNTMS)]K in C6D6 at room temperature. The peaks labeled with a star correspond to residual diethyl ether. Figure A.19. 13C NMR spectrum of [(P2PPh)Fe(NNSiMe3)]K (11-NNSiMe3) in THF at room temperature. The peaks labeled with a star correspond to residual diethyl ether. 141 Figure A.20. 31P NMR spectrum of [(P2PPh)Fe(NNSiMe3)]K (11-NNSiMe3) in C6D6 at room temperature. The peaks labeled with a star correspond to residual diethyl ether. Figure A.21. 1H NMR spectrum of [(P2PPh)Fe(NNSiiPr3)]K (11-NNSiiPr3) in C6D6 at room temperature. The peaks labeled with a star correspond to residual diethyl ether. 142 Figure A.22. 31P NMR spectrum of [(P2PPh)Fe(NNSiiPr3)]K (11-NNSiiPr3) in C6D6 at room temperature. The peaks labeled with a star correspond to residual diethyl ether. 143 NMR spectra of stoichiometric reactivity Figure A.23. 1H NMR spectra of 5 before and after evacuation displaying the disappearance of 5 and the increased intensity of 6. 144 Figure A.24. 31P NMR spectra of the addition of H2 to 5 before at −78 °C. The ratios between 5 and 2 (indicated by a star) remain virtually the same upon storing the sample at −78 °C 145 Figure A.25. 31P NMR spectra of the addition of H2 to 5 before at various temperatures. After the initial formation of 2 (indicated by a star), no appreciable reactivity was observed until the sample was warmed above −20 °C and a rapid change was observed at room temperature. The peaks labeled with (+) are likely due to the formation of 2P Ph )Fe(H2)(H)2 as they disappear after addition of N2. “(P- 146 Figure A.26. 31P NMR spectrum of the addition of H2 to 6 before at various temperature. Addition of H2 results in the rapid formation of 2 (indicated by a star) and an unidentified species. Figure A.27. 31P NMR spectrum of the irradiation of 2 which results in the formation of 5. 147 Figure A.28. 1H NMR spectrum of the oxidation of [(P2PPh)Fe(NNTMS)]K with [Cp*2Co][PF6] at room temperature. Figure A.29. 31P NMR spectrum of the oxidation of [(P2PPh)Fe(NNTMS)]K with [Cp*2Co][PF6] at room temperature with the characteristic features of 5. 148 Figure A.30. 1H NMR spectrum of the oxidation of [(P2PPh)Fe(NNTiPS)]K with [Cp*2Co][PF6] 149 Fitting VT NMR data The chemical shifts of the NMR active nuclei in 6 can be fit to a general equation accounting for the population of an excited state with equation S1: 𝛿 = 𝛿𝑜 + 2J 6J 2J 6J 𝐵 ∗ (𝑒 𝑘𝑇 + 5𝑒 𝑘𝑇 ) (1 + 3𝑒 𝑘𝑇 + 5𝑒 𝑘𝑇 ) ∗ 𝑇 S1 Herein, δ is the observed chemical shift for a nucleus at a given temperature, δo is the chemical shift of that atom in the ground state, B is a fitting constant, and 2J and 6J (as obtained for a Heisenberg–Dirac–VanVleck Hamiltonian in the notation H = −2JS1·S2) correspond to the energy difference between the singlet and triplet, or the singlet and quintet states respectively. The theoretical framework for the use of this equation is described by Pfirrman et al.9 Similar estimations of singlet–triplet gaps have been estimated for various systems.9–13 One should note that such a large singlet triplet splitting (corresponding to a thermal energy of E/k = 2717 K) can be hardly detected by usual static magnetic susceptibility measurements as the thermal population of the triplet at 298 K is only about 0.034 % and the quintet is 6.7·10-10 %.9 Overlap of three aromatic signals with toluene-d8 hampered their analysis and therefore only ten out of the thirteen signals, expected in Cs symmetry, could be tracked reliably over the measured temperature range. The temperature-dependent shift of these ten signals were simultaneously fit to equation S1 to give one value for J (−940 ± 9.4 cm−1), ten chemical shifts corresponding to the ground state and ten fitting constants. The obtained fitting parameters are summarized in Table A1. As expected, the chemical shifts observed at 201 K are close to those determined for the ground state. 150 Table A.1. Fit parameters for the chemicals shift of 6 δ δo (ppm) B (105) 7.73 7.76 ± 0.02 5.4 ± 0.7 7.53 7.54 ± 0.02 -2.3 ± 0.6 7.46 7.51 ± 0.02 -26.9 ± 2.3 6.95 7.84 ± 0.02 -32.5 ± 2.7 2.49 2.41 ± 0.02 68.1 ± 5.7 1.83 1.73 ± 0.03 93.3 ± 7.7 1.38 1.35 ± 0.02 7.2 ± 0.8 1.31 1.24 ± 0.02 -3.9 ± 0.6 0.95 0.92 ± 0.02 2.8 ± 0.6 0.68 0.71 ± 0.02 -7.1 ± 0.8 (ppm) at 201 K 151 Figure A.31. Selected 1H NMR spectra of 6 in d8-toluene plotted at various temperatures. Figure A.32. Selected 31P NMR spectra of 6 d8-toluene plotted at various temperatures. 152 Figure A.33. 31P NMR chemical shifts of 6 plotted as a function of 1/T. 153 IR Spectra Figure A.34. IR spectrum of (P2PPh)Fe(N2)2 (5) (thin-film from C6D6 solution). Figure A.35. IR spectrum of [(P2PPh)Fe(N2)2(H)][BArF] (7) (thin-film from d8-THF solution). 154 Figure A.36. IR spectrum of [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (8) (thin-film from d8THF solution). Figure A.37. IR spectrum of [(P2PPh)Fe(N2)][K(18-crown-6)] (9) in black (thin-film from d8-THF solution) and the spectrum of [(P2PPh)Fe(N2)][K(THF)x] recorded of a crude mixture before the addition of 18-crown-6. 155 Figure A.38. IR spectrum of [(P2PPh)Fe(N2)][K2(THF)3] (10) (thin-film from THF solution). Figure A.39. IR spectrum of [(P2PPh)Fe(15N2)][K2(THF)3] (10) (thin-film from d8-THF solution). Bands marked with a star correspond to d8-THF. 156 Figure A.40. IR spectrum recorded after the irradiation of (P2PPh)Fe(NNSiiPr3) (12NNSiiPr3) (thin-film from d8-THF solution). Figure A.41. IR spectrum recorded after the irradiation of (P2PPh)Fe(N2)(H2) (2) (thin-film from C6D6 solution). 157 Figure A.42. Thin-film IR spectra of 12-NNSiiPr3 with νNN in black and the addition of i Pr3SiOTf to 9 in blue. 158 Cyclic Voltammograms Figure A.43. Scan rate dependence of the wave observed at −2.4 V in the cyclic voltammogram of [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (8) scanning in the anodic direction. Figure A.44. Cyclic voltammogram of [(P2PPh)Fe(N2)][K(18-crown-6)] (9) scanning in the cathodic direction. 159 Figure A.45. Scan rate dependence of the r4 observed at −2.4 V in the voltammogram of [(P2PPh)Fe(N2)][K(18-crown-6)] (9). 160 EPR Spectra Figure A.46. EPR spectrum of [(P2PPh)Fe(N2)][K(18-crown-6)] in 2-MeTHF, at 77 K, microwave frequency 9.39 GHz, microwave power 6.47 mW. 161 Figure A.47. EPR spectra of the oxidation of [(P2PPh)Fe(NNSiMe3)]K with [Cp*2Co][PF6] in THF at different time points and the EPR spectrum of (P2PPh)Fe(N2)(H) . 162 Figure A.48. EPR spectrum of the oxidation of [(P2PPh)Fe(NNSiiPr3)]K (11-NNSiiPr3) and [Cp*2Co][PF6] resulting in the formation of (P2PPh)Fe(NNSiiPr3) (12-NNSiiPr3) andd fit of the spectrum in respectively black and red. Simulation parameters: g = [2.213 2.041 2.012], P = [0 40.3 57.1] P = [0 40.3 75.0] P = [0 69.4 29.0] Hstrain = [1180 50 30]. 163 Mӧssbauer Spectra Figure A.49. Mössbauer spectrum collected of (P2PPh)Fe(N2)2 (5) at 80 K Raw data shown as black points, overall simulation as a red line. The major species, 5, in purple is fit as a doublet (δ = 0.31 mm s-1, ∆EQ = 2.16 mm s-1). The minor species (~ 5%) in blue was fit as a doublet (δ = 0.10 mm s-1, ∆EQ = 0.45 mm s-1). 164 Figure A.50. Mössbauer spectrum collected of [(P2PPh)Fe(N2)2(H)][BArF4] (7) at 80 K Raw data shown as black points, overall simulation as a red line. δ = 0.21 mm s -1, ∆EQ = 1.08 mm s-1. 165 Figure A.51. Mössbauer spectrum collected of [(P2PPh)Fe(N2)]K2(THF)3] (10) at 80 K Raw data shown as black points and simulated spectrum in red (δ = 0.27 mm s -1, ∆EQ = 0.26 mm s-1). 166 UV-vis Figure A.52. UV-visible spectrum of (P2PPh)FeBr (4) THF, 293 K). Figure A.53. UV-visible spectrum of (P2PPh)Fe(N2)2 (5) (Et2O, 293 K). Spectra 167 Figure A.54. UV-visible spectrum of (P2PPh)Fe(N2)2 (5) (Et2O, 293 K). Figure A.55. UV-visible spectrum of [(P2PPh)Fe(N2)2(H)][BArF4] (7) (Et2O, 293 K). 168 Figure A.56. UV-visible spectrum of [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (8) (2-MeTHF, 293 K). Black trace corresponds to the left axis scale, while the red trace corresponds to the right axis scale. 169 Catalytic experiments Standard NH3 Generation Reaction Procedure. All solvents were stirred with Na/K for ≥1 hour and filtered through alumina prior to use. In a nitrogen-filled glovebox, a stock solution of the catalyst in THF (6.8 mM) was prepared. (Note: a fresh stock solution was prepared for each experiment and used immediately.) An aliquot of this stock solution (90 or 270 μL, 0.6 or 1.82 μmol) was added to a Schlenk tube and evaporated to dryness under vacuum to give a thin film of the precatalyst. The tube was allowed to cool to 77 K in the glovebox cold well. To the cold tube was added a solution of [H(OEt2) 2][BArF4] (93 mg, 0.092 mmol) in Et2O (0.5 mL). This solution was allowed to freeze before the vial which contained the HBArF4 was rinsed with an additional 0.5 mL of Et2O and added to the tube. After the acid layer froze, a suspension of KC8 (15 mg, 0.111 mmol) in 0.5 mL of Et2O (1.2 equiv. relative to [H(OEt2) 2][BArF4]) was added to the cold tube. A stir bar was added to the tube and the tube sealed with a Teflon screw-valve. The temperature of the system was allowed to equilibrate for 5 minutes. The Schlenk tube was passed out of the box into a liquid N2 bath and transported to a fume hood. The reaction vessel was subsequently transferred to a dry ice/acetone bath where it thawed to -78 °C and was allowed to stir for at least 1 hour. The tube was then warmed to room temperature while stirring and subsequently stirred at room temperature for 5 minutes. Standard NH3 Generation Reaction with Hg Lamp Photolysis Procedure. Preparation of the Schlenk tube containing reactants was performed as described for runs without light. The cold reaction vessel was transferred to a dry ice/isopropanol bath which was positioned 170 under a Hg lamp and turned on 1 minute prior to transfer of the Schlenk tube to the bath. The entire reaction apparatus was surrounded by aluminum foil and the reaction vessel was stirred for at least 1 hour before the Hg lamp was turned off and the Schlenk tube was allowed to warm to room temperature with stirring and stirred at room temperature for 5 minutes. Ammonia Quantification. The catalytic reaction mixture was cooled to 77 K and allowed to freeze. The reaction vessel was opened to the atmosphere and to the frozen solution was slowly added a fourfold excess (with respect to acid) solution of a NaOtBu in MeOH (0.25 M) over 1–2 minutes. The solution was allowed to freeze, then the tube was sealed, evacuated and allowed to warm to room temperature and stirred at room temperature for 10 minutes. An additional Schlenk tube was 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 were vacuum transferred into the collection flask. After completion of the vacuum transfer, the collection flask was sealed and warmed to room temperature. Solvent was removed in vacuo, and the remaining residue dissolved in H2O (1 mL) to make a stock solution that was used for ammonia quantification. An aliquot of this solution (20 μL) was then analyzed for the presence of NH3 (present as NH4Cl) by the indophenol method. Quantification was performed with UV−Vis spectroscopy by analyzing the absorbance at 635 nm. 171 Yields of Independent Catalytic Runs Table A.2. Results of individual runs using 4 at 150 equiv. acid loading with no Hg lamp irradiation. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.017 0.86 1.7 B 0.022 0.63 1.3 Table A.3. Results of individual runs using 5 at 50 equiv. acid loading with no Hg lamp irradiation. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.401 5.09 30.6 B 0.399 5.06 30.4 Table A.4. Results of individual runs using 5 at 150 equiv. acid loading with no Hg lamp irradiation. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.171 6.40 12.8 B 0.152 5.66 11.3 172 Table A.5. Results of individual runs using 5 at 150 equiv. acid loading with Hg lamp irradiation. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.240 8.83 17.7 B 0.315 12.01 24.0 C 0.207 7.78 15.6 Table A.6. Results of individual runs using 5 at 150 equiv. acid loading with one equiv. TBABr added. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.055 1.88 3.8 B 0.072 2.55 5.1 Table A.7. Results of individual runs using 5 with 150 equiv. Ph2NH2OTf with no Hg lamp irradiation. Run Absorbance Equiv. NH3/Fe % Yield (Based of H+) A 0.019 0.51 1.0 B 0.018 0.46 0.9 173 General Procedure for Time-resolved H2 Quantification: Inside of a nitrogen filled glovebox, the Fe precursor (2 or 5, 1.5 μmol) was added to a 300 mL Schlenk flask as a solution in THF, and subsequently deposited as a thin film in a Schlenk flask by removing the solvent in vacuo. To this flask was added solid HBArF4 (0.23 mmol), KC8 (0.28 mmol), and a stir bar. The flask was sealed with a septum at room temperature and subsequently chilled to -196 °C in the cold well of a nitrogen filled glovebox. Et2O (3 mL) was added via syringe into the flask and completely frozen. The flask was passed out of the glovebox into a liquid N2 bath, and subsequently thawed in a dry ice/acetone bath. The timer was set to zero as soon as the flask 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 immediately loaded into the GC, and analyzed for the presence of H2(g). From these data, the percent H2 evolved (relative to HBArF4) 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. 174 Table A.8. HER Yields Entry Fe precursor Time % Yield (Based of H+) A 2 0 0 B 5 25.9 C 25 70.7 .7D 60 74.4 E 120 72.6 F 1040 72.8 0 0 H 5 22.0 I 25 71.6 J 60 78.4 K 120 78.9 L 1040 79.5 G 5 175 General Procedure for the Preperation of Rapid-freeze-quenched Mӧssbauer Samples. The precatalyst was weighed into a vial (3 µmol) and dissolved in 0.5 mL Et2O. The solvent was evaporated in a Schlenk tube to form a thin film of the precatalyst to which a stir bar was added. HBArF4 (150 mg, 0.148 mmol) and KC8 (24 mg, 0.178 mmol) are added as solids. The Schlenk tube was cooled to 77 K. To the cooled tube, 1 mL of Et 2O was added. The tube was then sealed with a Teflon screw tap and transferred to a prechilled cold well at − 78°C. The timer was set to zero as soon as the stir bar was freed from the thawing solvent. At the desired time, the tube was opened and the suspension was transferred to a Delrin cup pre-chilled to −78 °C using a pre-chilled pipette. The sample in the Delrin Cup was then rapidly frozen in liquid nitrogen. At this point, the frozen sample was taken outside the glovebox and mounted in the cryostat. General Procedure for Fitting of Rapid-freeze-quench Mӧssbauer Samples. 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. It is known that the exact linewidths are sensitive to the particular sample, but the relative line breadth should be fairly constant. Using the nonlinear 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 the included components 176 are necessary to generate satisfactory fits of the data and therefore are believed to be present in the reaction mixtures. Details of Individual of Rapid-freeze-quench Mӧssbauer Spectra Figure A.57. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 5 minutes using (P2PPh)Fe(N2)(H)2 (2) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Fitting details for Figure A.57: Various fits were attempted for the Mӧssbauer spectrum. The species could be fit with one quadruple doublet with a large linewidth. A better fit was obtained by fitting the spectrum with two quadrupole doublets with slightly different parameters. One of the species has parameters similar to [(P2PPh)Fe(N2)2(H)][BArF4] (7) The broad feature at 2 mm/s, present in various freeze quenched spectra, probably due to a magnetically split species, could not be fit. 177 Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Γ (mm s−1) Relative area A 0.22 1.01 0.54 0.63 B 0.24 1.34 0.41 0.37 Figure A.58. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 30 minutes using (P2PPh)Fe(N2)(H)2 (2) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Fitting details for Figure A.58: Three quadrupole doublet were found to be necessary to obtain an adequate simulation. Two of the species could be fit with the same parameters as 178 required for the freeze-quenched sample measured after 5 minutes. The remaining quadrupole doublet can be well simulated as (P2PPh)Fe(N2)(H2). Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Γ (mm s−1) Relative area A 0.22 1.01 0.50 0.15 B 0.25 1.16 0.46 0.42 C 0.06 0.43 0.48 0.43 Figure A.59. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 30 minutes using (P2PPh)Fe(N2)2 (5) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. 179 Fitting details for Figure A.59. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 30 minutes using (P2PPh)Fe(N2)2 (5) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Various fits were attempted for the Mӧssbauer spectrum. The species could be fit with one quadruple doublet with a large linewidth. A better fit was obtained by fitting the spectrum with two quadrupole doublets with slightly different parameters. Of the several options, one gave a species with a similar isomer shift (0.26 mm/s) and quadrupole splitting (1.12 mm/s) as observed in the reactions with 2. Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Γ (mm s−1) Relative area A 0.16 1.00 0.41 0.20 B 0.26 1.12 0.52 0.80 180 Figure A.60. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 30 minutes using (P2PPh)Fe(N2)2 (5) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Fitting details for Figure A.59. Mössbauer spectrum collected after rapid-freeze-quenching a catalytic reaction after 30 minutes using (P2PPh)Fe(N2)2 (5) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. Various fits were attempted for the Mӧssbauer spectrum. The species could be fit with one quadruple doublet with a large linewidth. A better fit was obtained by fitting the spectrum with two quadrupole doublets with slightly different 181 parameters. Of the several options, one gave a species with a similar isomer shift (0.26 mm/s) and quadrupole splitting (1.12 mm/s) as observed in the reactions with 2. Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Γ (mm s−1) Relative area A 0.16 1.00 0.41 0.20 B 0.26 1.12 0.52 0.80 Figure A.61. Mössbauer spectrum collected from a catalytic reaction warmed to room temperature using (P2PPh)Fe(N2)2 (5) as precatalyst. Raw data shown as black points, overall simulation as a grey line, with components in green, blue and red (see Table for parameters). The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT. 182 Fitting details for Figure A.61: Three quadrupole doublet were found to be necessary to obtain an adequate simulation. The major species in the spectrum can be well simulated as (P2PPh)Fe(N2)(H2). The residual signal exhibits two quadrupole doublets and a broad feature. The broad feature, potentially due to a magnetically split species, could not be fit. The remaining quadrupole doublets could be fit as two species with respective areas of 0.05 and 0.13. 183 Component δ (mm s−1) ΔEQ (mm s−1) Linewidths, Γ (mm s−1) Relative area A 0.04 0.48 0.37 0.82 B 0.20 2.08 0.50 0.05 C 0.45 1.02 0.50 0.13 184 Crystallographic Details and Tables [(P2PPh)Fe(N2)2(H)][BArF4] (7) This structure contains disorder in the CF3 moieties of the BArF4 anion which could only be poorly modeled. B level alerts persist due to this disorder. [(P2PPh)Fe(N2)(H)][K(18-crown-6)] (9) This structure contains residual electron density assignable to disordered solvent molecules. OLEX2 was used to identify voids and a solvent mask was applied. This application gave a good improvement of data statistics. In addition, A level alerts persist [PLAT360, PLAT 410] in the checkcif file related to treated disorder about an 18-crown-6 molecule that is positioned on a mirror plane CCDC 1824471-1824478 contains 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.uk/data_request/cif 185 Compound 3 5 Identification code p17260 p17278 Empirical formula C30H41Br1.03FeP3 C30H41FeN4P3 Formula weight 632.69 606.43 Temperature/K 99.97 99.99 Crystal system triclinic monoclinic Space group P-1 P21/c a/Å 9.4966(10) 17.626(2) b/Å 9.5930(8) 10.7764(8) c/Å 17.630(3) 16.1774(19) α/° 101.894(6) 90 β/° 96.481(6) 98.630(3) γ/° 102.236(4) 90 Volume/Å3 1515.2(3) 3038.0(5) Z 2 4 ρcalcg/cm3 1.387 1.326 μ/mm-1 2.032 0.681 F(000) 656 1280 Crystal size/mm3 0.187 × 0.163 × 0.02 0.16 × 0.15 × 0.076 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) 4.448 to 55.118 4.444 to 64.162 2Θ range collection/° for data 186 -12 ≤ h ≤ 12, -12 ≤ k ≤ 12, -22 -26 ≤ h ≤ 26, -16 ≤ k ≤ 16, -24 ≤ Index ranges ≤ l ≤ 22 l ≤ 24 Reflections collected 87122 112007 7004 [Rint = 0.0784, Rsigma = 10593 [Rint = 0.0944, Rsigma = Independent reflections 0.0244] 0.0420] Data/restraints/parameters 7004/0/345 10593/3/381 Goodness-of-fit on F2 1.061 1.114 Final R indexes [I>=2σ (I)] R1 = 0.0232, wR2 = 0.0557 R1 = 0.0385, wR2 = 0.1013 Final R indexes [all data] R1 = 0.0272, wR2 = 0.0573 R1 = 0.0583, wR2 = 0.1167 0.53/-0.37 1.39/-0.95 Largest diff. peak/hole / e Å-3 187 Compound 7 8 Identification code p17303 p17651 Empirical formula C129H106B2F48Fe2N8P6 C42H65FeKN2O6P3 Formula weight 2999.35 877.47 Temperature/K 100 100 Crystal system monoclinic monoclinic Space group P21/n P21/n a/Å 22.4451(18) 10.4864(6) b/Å 12.1053(6) 23.9154(17) c/Å 25.482(2) 17.8619(10) α/° 90 90 β/° 101.904(4) 93.645(3) γ/° 90 90 Volume/Å3 6774.6(9) 4470.5(5) Z 2 4 ρcalcg/cm3 1.47 1.304 μ/mm-1 3.434 0.584 F(000) 3040 1859 Crystal size/mm3 0.284 × 0.142 × 0.06 0.26 × 0.26 × 0.23 Radiation CuKα (λ = 1.54178) MoKα (λ = 0.71073) 5.886 to 159.312 4.248 to 79.494 2Θ range collection/° for data 188 -28 ≤ h ≤ 28, -14 ≤ k ≤ 15, -32 -18 ≤ h ≤ 18, -42 ≤ k ≤ 42, -31 ≤ Index ranges ≤ l ≤ 31 l ≤ 32 Reflections collected 86754 220099 14524 [Rint = 0.0840, Rsigma = 26751 [Rint = 0.0480, Rsigma = Independent reflections 0.0509] 0.0334] Data/restraints/parameters 14524/800/907 26751/0/544 Goodness-of-fit on F2 1.022 1.061 Final R indexes [I>=2σ (I)] R1 = 0.0967, wR2 = 0.2547 R1 = 0.0460, wR2 = 0.0998 Final R indexes [all data] R1 = 0.1178, wR2 = 0.2737 R1 = 0.0682, wR2 = 0.1080 2.19/-0.84 2.07/-0.65 Largest diff. peak/hole / e Å-3 189 Compound 9 10 Identification code p17634Pnma p17581 Empirical formula C40H61FeKN2O4P3 C42H65FeK2N2O3P3 Formula weight 816.72 872.92 Temperature/K 100(2) 100 Crystal system orthorhombic monoclinic Space group Pnma P21/n a/Å 25.9569(13) 14.6713(13) b/Å 17.0296(9) 16.6256(9) c/Å 12.1851(6) 18.7129(12) α/° 90 90 β/° 90 102.506(3) γ/° 90 90 Volume/Å3 5386.2(5) 4456.1(5) Z 4 4 ρcalcg/cm3 1.007 1.301 μ/mm-1 0.478 0.671 F(000) 1728 1856 Crystal size/mm3 0.22 × 0.16 × 0.12 0.22 × 0.199 × 0.055 Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2Θ range for data 4.586 to 55.012 collection/° 5.088 to 66.06 190 -33 ≤ h ≤ 33, -20 ≤ k ≤ 22, -15 -22 ≤ h ≤ 22, -24 ≤ k ≤ 25, -28 ≤ Index ranges ≤ l ≤ 15 l ≤ 25 Reflections collected 73356 64947 6368 [Rint = 0.0545, Rsigma = 15164 [Rint = Independent reflections 0.0235] Rsigma = 0.0787] Data/restraints/parameters 6368/303/287 15164/0/486 Goodness-of-fit on F2 1.123 1.014 0.0574, Final R indexes [I>=2σ (I)] R1 = 0.1026, wR2 = 0.3059 R1 = 0.0569, wR2 = 0.1094 Final R indexes [all data] R1 = 0.1074, wR2 = 0.1263 R1 = 0.1162, wR2 = 0.3201 Largest diff. peak/hole / e 3.98/-1.50 Å-3 1.56/-0.61 191 Compound 11 Identification code p17555 Empirical formula C33H50FeKN2P3Si Formula weight 690.7 Temperature/K 124.99 Crystal system orthorhombic Space group P212121 a/Å 10.5863(9) b/Å 16.3326(12) c/Å 20.8095(17) α/° 90 β/° 90 γ/° 90 Volume/Å3 3598.0(5) Z 4 ρcalcg/cm3 1.275 μ/mm-1 6.154 F(000) 1464 Crystal size/mm3 0.164 × 0.077 × 0.059 Radiation CuKα (λ = 1.54178) 2Θ range for data 6.88 to 158.208 collection/° 192 -12 ≤ h ≤ 13, -19 ≤ k ≤ 20, -26 Index ranges ≤ l ≤ 24 Reflections collected 38615 7481 [Rint = 0.0728, Rsigma = Independent reflections 0.0653] Data/restraints/parameters 7481/0/382 Goodness-of-fit on F2 1.062 Final R indexes [I>=2σ (I)] R1 = 0.0670, wR2 = 0.1490 Final R indexes [all data] R1 = 0.0873, wR2 = 0.1606 Largest diff. peak/hole / e 1.31/-0.81 Å-3 193 Figure A.62. ORTEP depiction of the solid-state molecular structure of 4 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms and disorder from cocrystalized 3 are omitted for clarity). Figure A.63. ORTEP depiction of the solid-state molecular structure of 5 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms are omitted for clarity). 194 Figure A.64. ORTEP depiction of the solid-state molecular structure of 7 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms, BArF4 and disordered solvent molecules are omitted for clarity). Figure A.65. ORTEP depiction of the solid-state molecular structure of 8 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms are omitted for clarity). 195 Figure A.66. ORTEP depiction of the solid-state molecular structure of 9 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms are omitted for clarity). Figure A.67. ORTEP depiction of the solid-state molecular structure of 11 (displacement ellipsoids are shown at the 50% probability; hydrogens atoms are omitted for clarity). 196 DFT Calculations Figure A.68. Density function theory calculated molecular orbitals. (Left) HOMO of [(P2PPh)Fe(NNH)]− and (Right) HOMO of [(P2PPh)Fe(NNTMS)]− (isovalue = 0.06). 197 Table A.9. A comparison of gas phase optimized and experimental bond parameters demonstrating the good agreement between optimized gas-phase structures of [(P2PPh)Fe(NNTMS)]−, [(P2PPh)Fe(NNH)]− and experimental values from X-ray data of [(P2PPh)Fe(NNTMS)]K. Species Fe-N (Å) Fe-PPh (Å) N-N (Å) [(P2PPh)Fe(NNTMS)]K (XRD) 1.664(7) 2.115(2) 1.270(9) [(P2PPh)Fe(NNTMS)]− 1.662 2.112 1.244 [(P2PPh)Fe(NNH)]− 1.669 2.111 1.250 198 Figure A.69. The gas-phase optimized geometry of (P2PPh)Fe(N2) with different spin states and geometries. (Left) S = 0, (Middle) S = 1 and (Right) S = 1. 199 Table A.10. Energies of gas-phased optimized geometries of (P2PPh)Fe(N2). The energies are given relative to the lowest energy triplet state Species Spin Energy N–N PPh–Fe–N νN2 (P2PPh)Fe(N State S=0 −1 (kcal 7.4 mol ) (Å) 1.145 (°) 173.8 −1 (cm 2076.7) S=1 1.3 1.141 159.1 2093.4 S=1 0 1.142 106.5 2093.2 2) (P2PPh)Fe(N 2) (P2PPh)Fe(N 2) DFT Coordinates H 0.812900 3.509600 77 C -3.086900 0.451800 -2.264000 P2PPhFeN2_singlet H -3.188400 1.315700 -1.584200 N 0.356700 -2.595000 -1.420600 H -3.798100 0.596100 -3.098900 Fe 0.127600 -0.946000 -0.809200 H -2.062200 0.469000 -2.674400 P 2.229500 -0.844000 -0.184700 C -3.378100 -0.870700 -1.539700 P -1.996900 -1.247100 -0.269500 H -3.234700 -1.691300 -2.269800 P -0.087800 0.996300 0.152500 C 3.576600 0.467000 -2.306200 C 3.827000 1.059300 1.334400 H 2.642600 0.466200 -2.897900 H 4.696900 0.409900 1.193200 H 4.424800 0.503800 -3.015400 C 4.219400 -2.517400 1.235600 H 3.600500 1.389100 -1.700200 H 4.798900 -2.668100 0.309500 C 1.474800 1.566400 H 4.342100 -3.428800 1.851300 C -1.916500 4.514600 -1.230400 H 4.674800 -1.684900 1.800300 H -2.763400 5.153200 -0.954700 C 2.722100 -2.278600 0.977200 C 0.243600 2.871300 -1.923600 H 2.317200 -3.153800 0.433300 H 1.088200 2.229400 -2.198100 C 3.983000 2.272600 2.024400 C -1.622900 3.379600 -0.457500 H 4.963400 2.545300 2.430700 H -2.247200 3.141600 0.409700 C 3.668500 -0.797300 -1.435600 C -2.366300 -0.137900 1.189800 H 4.607400 -0.754400 -0.850600 C -4.822700 -0.922300 -1.017900 C 2.894800 3.144800 2.170400 H -5.066500 -1.885100 -0.535600 H 3.022800 4.102600 2.686700 H -5.532000 -0.785300 -1.856500 C 2.571800 0.692800 0.808500 H -5.012600 -0.114100 -0.289500 C 1.948700 -2.137100 2.300000 C 3.681600 -2.063700 -2.309600 H 2.354800 -1.307800 2.907300 H 3.751200 -2.990900 -1.715800 H 2.032700 -3.066200 2.894100 H 4.548100 -2.038500 -2.996900 H 0.881600 -1.932800 2.109300 H 2.766100 -2.129800 -2.922300 C 1.645100 1.633100 C -1.634900 -3.449200 2.800300 1.700400 0.975900 1.481800 201 H -0.641000 -3.680100 1.063500 P 2.180500 -0.815200 -0.202600 H -2.044800 -4.373800 1.930700 P -1.674200 -1.669700 -0.419200 H -1.502600 -2.708500 2.289200 P -0.252000 1.011700 0.115700 C -0.051300 4.004000 -2.693800 C 3.663300 1.113200 1.397600 H 0.565000 4.241800 -3.568100 H 4.556600 0.496900 1.249600 C -1.133300 4.830100 -2.349300 C 3.909800 -2.497300 1.527100 H -1.364500 5.714800 -2.952700 H 4.549100 -2.803400 0.682100 C -2.596100 1.508000 3.479200 H 3.910100 -3.329300 2.257100 H -2.672100 2.133800 4.375200 H 4.380600 -1.631200 2.025100 C -1.537800 1.704100 2.575600 C 2.464100 -2.191300 1.103200 H -0.798300 2.485200 2.773900 H 2.060300 -3.082200 0.584100 C -3.437800 -0.301800 2.089700 C 3.768100 2.298200 2.142000 H -4.186300 -1.082700 1.922200 H 4.732800 2.587100 2.574100 C -0.537000 2.546800 -0.792700 C 3.771100 -0.900400 -1.235100 C -1.417900 0.887200 1.437100 H 4.618400 -1.001500 -0.530600 C -3.548100 0.512100 3.231400 C 2.644000 3.118800 2.316300 H -4.376600 0.355900 3.931200 H 2.726100 4.051800 2.885000 C -2.582600 -2.934900 0.387000 C 2.434700 0.729800 0.821800 H -3.569700 -2.739200 0.848000 C 1.585300 -1.909500 2.333800 C -2.782700 -3.982600 -0.723700 H 2.009700 -1.088200 2.938000 H -3.545500 -3.675600 -1.459300 H 1.522400 -2.807100 2.976600 H -3.126900 -4.931600 -0.271300 H 0.560700 -1.620800 2.044700 H -1.844600 -4.192000 -1.263000 C 1.417500 2.753000 1.743300 N 0.524900 -3.640300 -1.857500 H 0.551400 3.416100 1.847900 77 C -3.161300 0.146600 -2.041100 P2PPhFeN2_triplet_acute H -3.277000 0.881500 -1.225500 N 0.587800 0.092700 -2.853300 H -3.991300 0.305200 -2.754900 Fe 0.188300 -0.648800 -1.244300 H -2.211500 0.362800 -2.557900 202 C -3.202400 -1.292700 -1.507100 C -1.975800 5.136400 -1.290900 H -3.049500 -1.976600 -2.363900 H -2.364300 6.092600 -1.658300 C 3.983200 0.375500 -2.067500 C -2.444600 0.494400 3.671300 H 3.170100 0.521600 -2.796400 H -2.581100 1.003900 4.631600 H 4.932500 0.298100 -2.630600 C -1.594400 1.045400 2.700400 H 4.040500 1.275300 -1.431600 H -1.074000 1.985200 2.912600 C 1.302200 1.559800 1.003100 C -2.923700 -1.364600 2.183600 C -2.700100 4.396800 -0.346000 H -3.449300 -2.306800 1.995900 H -3.658100 4.774200 0.029700 C -0.973100 2.666800 -0.339400 C -0.259600 3.414200 -1.304700 C -1.394100 0.396800 1.466900 H 0.686100 3.023700 -1.698300 C -3.113200 -0.709900 3.410100 C -2.204200 3.171100 0.128100 H -3.779600 -1.145000 4.163300 H -2.782700 2.607300 0.867100 C -1.948900 -3.519200 -0.069500 C -2.058300 -0.831900 1.206300 H -2.987600 -3.643300 C -4.560700 -1.606300 -0.860000 C -1.789900 -4.312300 -1.380600 H -4.643200 -2.652900 -0.518700 H -2.554400 -4.047500 -2.130700 H -5.369200 -1.437100 -1.595900 H -1.882800 -5.395600 -1.179600 H -4.754600 -0.945000 0.002900 H -0.795900 -4.132200 -1.830000 C 3.719400 -2.152600 -2.129600 N 0.708800 H 3.620600 -3.083300 -1.542700 77 H 4.643000 -2.232300 -2.732700 P2PPhFeN2_triplet H 2.861500 -2.098200 -2.823700 N 0.966900 -1.482800 -2.669000 C -0.990000 -4.051600 1.005800 Fe 0.388700 -0.566600 -1.210800 H 0.059100 -3.958400 0.676000 P 2.271400 H -1.192300 -5.123100 1.190900 P -0.920300 -2.072700 -0.183200 H -1.096300 -3.515700 1.963300 P -0.645300 0.974800 -0.026000 C -0.751900 4.639600 -1.769300 C 2.961100 2.611300 1.078900 H -0.182300 5.205900 -2.514700 H 4.016500 2.335500 0.985200 0.292100 0.540900 -3.896600 0.036900 -0.109800 203 C 4.417900 -0.494700 1.863000 H 3.036000 2.257300 -1.983100 H 5.170900 -0.580600 1.061900 C 0.602800 2.138500 H 4.733000 -1.166100 2.684700 C -4.139200 3.161700 -0.856400 H 4.446000 0.535500 2.259300 H -5.177200 3.210600 -0.508200 C 3.003300 -0.882800 1.402000 C -1.489100 3.013000 -1.744700 H 3.027600 -1.927400 1.036700 H -0.456700 2.942500 -2.108800 C 2.630700 3.856300 1.637000 C -3.244100 2.269700 -0.242600 H 3.425900 4.527500 1.980500 H -3.596200 1.635100 0.576800 C 3.800500 0.355700 -1.185400 C -1.659200 -1.252800 1.326000 H 4.555600 0.857400 -0.550300 C -3.608800 -3.210300 -0.552400 C 1.287400 4.244400 1.742400 H -3.289100 -4.174500 -0.120400 H 1.026800 5.219100 2.169700 H -4.413000 -3.429200 -1.280200 C 1.956900 1.733300 0.623100 H -4.050300 -2.600100 C 2.009500 -0.797500 2.571600 C 4.391600 -0.970900 -1.697200 H 2.019500 0.211800 3.020300 H 4.699000 -1.639100 -0.873800 H 2.279800 -1.522800 3.361200 H 5.287800 -0.771000 -2.314200 H 0.979500 -1.012400 2.242700 H 3.666500 -1.515800 -2.324200 C 0.276800 3.390600 1.277600 C 0.543000 -3.858800 1.532300 H -0.769600 3.713300 1.320000 H 1.504000 -3.437200 1.192000 C -2.971000 -1.184800 -1.938100 H 0.726800 -4.902500 1.849700 H -3.370700 -0.464700 -1.202400 H 0.212500 -3.289500 2.416800 H -3.785600 -1.428200 -2.645700 C -2.379100 3.909500 -2.347300 H -2.165100 -0.679000 -2.499400 H -2.035800 4.545000 -3.171300 C -2.463300 -2.463300 -1.253900 C -3.710800 3.986200 -1.905400 H -2.041800 -3.118300 -2.040900 H -4.410500 4.681400 -2.382000 C 3.436700 1.295500 -2.347700 C -2.589900 0.147700 3.601300 H 2.676600 0.835200 -3.003300 H -2.935700 0.690200 4.488100 H 4.332400 1.508300 -2.960900 C -1.983100 0.845100 2.545100 0.720400 0.255600 204 H -1.862400 1.931400 2.615500 C 4.215200 3.258500 -0.470800 C -2.290500 -1.935600 2.385500 H 4.858200 4.101100 -0.754600 H -2.418800 -3.022300 2.339300 C 1.516400 -0.381400 C -1.904800 2.188100 -0.674600 C 1.418200 -2.481200 -0.919300 C -1.517800 0.157500 1.408500 C 2.839900 3.297500 -0.759600 C -2.746300 -1.242800 3.518400 H 2.426200 4.182200 -1.258400 H -3.219300 -1.793000 4.339600 C 4.762900 2.141600 0.175700 C -0.496300 -3.836300 0.402700 H 5.836700 2.103100 0.397400 H -1.433300 -4.286700 0.783000 C -1.302800 3.324500 -2.960400 C -0.008900 -4.666800 -0.800100 H -1.826900 2.353800 -3.041100 H -0.781500 -4.774700 -1.580000 H -1.294900 3.795900 -3.963600 H 0.270600 -5.683400 -0.466400 H -1.896900 3.972400 -2.294500 H 0.878900 -4.204200 -1.266500 C 2.104000 -1.826300 N 1.297800 -2.032700 -3.612800 C -5.309500 0.663000 -0.579300 90 H -4.778600 1.593200 -0.852700 P2PPhFeNNTMS_anion_singlet H -6.397600 0.869900 -0.593000 Fe -0.575400 H -5.092500 -0.081700 -1.366800 P 1.279600 -0.219700 0.674600 C 1.569200 -0.655600 P 0.148500 2.094500 -0.800500 C 3.073900 -4.280800 -0.857200 P -0.168700 -1.634100 -1.508400 H 3.447400 -5.238300 -1.241800 C 3.928400 1.073100 0.546400 C -1.360400 -3.125600 -1.261800 H 4.357200 0.213000 1.072000 H -0.959700 -3.970300 -1.859500 C 1.914100 -3.707200 -1.405300 C 2.096100 0.613500 3.347500 H 1.389200 -4.229600 -2.215300 H 2.533700 1.505500 2.887400 C 1.999600 2.217000 -0.423300 C -5.629600 -1.591000 1.533100 C 2.551500 1.104300 0.253800 H -5.434000 -2.358800 0.761700 C 0.132600 3.125400 -2.446300 H -6.725900 -1.450600 1.607800 H 0.549300 4.121000 -2.191100 H -5.270500 -1.992600 2.499100 0.101100 -0.281700 2.530800 0.126400 5.359400 205 C 2.121500 0.478100 4.745200 H -2.405800 4.202700 1.193700 C 3.257800 -2.411800 0.683600 H -2.296900 2.425000 0.948800 H 3.768300 -1.925800 1.523700 C 1.533800 -1.271700 -3.778300 C -2.784900 -2.801300 -1.741700 H 1.736400 -0.308300 -3.276600 H -3.133200 -1.861400 -1.279200 H 1.630500 -1.114500 -4.871200 H -3.482500 -3.611800 -1.449800 H 2.318400 -1.979000 -3.463300 H -2.847600 -2.691200 -2.837600 C 0.151800 3.457000 1.736400 C -0.513100 3.509500 0.352000 H 0.002300 2.463000 2.196000 H -0.254900 4.472500 -0.135400 H -0.301500 4.213700 2.407400 C -1.385500 -3.527700 0.222400 H 1.236400 3.652600 1.685400 H -0.381800 -3.810700 0.584600 C -5.227500 1.306000 2.452800 H -2.058200 -4.396400 0.370400 H -6.326100 1.438300 2.509200 H -1.755800 -2.690600 0.839500 H -4.774000 2.293100 2.246300 C 1.006700 2.494600 -3.538700 H -4.871100 0.979100 3.447400 H 2.038200 2.309500 -3.193200 C 0.125800 -1.778700 -3.420900 H 1.056200 3.161400 -4.423100 H 0.043000 -2.849900 -3.697300 H 0.586200 1.531800 -3.871100 C -0.949600 -0.981800 -4.177500 Si -4.734800 0.036300 1.123200 H -1.953300 -1.424500 -4.069300 N -2.038400 -0.033100 0.496400 H -0.714900 -0.933300 -5.259700 N -3.022200 -0.258100 1.224100 H -1.007300 C 0.950300 -1.510600 3.166700 H 1.590400 -0.761500 6.450600 H 0.472600 -2.282800 2.553700 H 2.579400 5.356500 C 0.981300 -1.650500 4.559600 78 H 0.538200 -2.539200 5.025600 P2PPhFeNNH_anion_singlet C 3.744400 -3.634200 0.192100 Fe -0.083400 -0.757600 -0.968900 H 4.642300 -4.085400 0.632700 P 0.045200 C -2.038000 3.406100 0.515900 P 1.871000 -1.284600 -0.176100 H -2.577900 3.500100 -0.440800 P -1.987300 -1.128200 -0.020400 0.047400 -3.780400 1.265800 1.025000 0.154600 206 C 1.670100 1.901400 2.460700 C 1.610100 4.686400 -1.237000 H 0.984400 2.741900 2.613900 C -1.802200 2.536800 1.890200 C -3.705500 0.485900 1.732900 H -1.076200 3.357900 1.922400 H -4.463000 -0.305700 1.667900 C -3.536800 -2.323700 -2.133400 C 2.335200 -0.093500 1.216700 H -2.592000 -2.350200 -2.704000 C 1.445200 0.989300 1.411700 H -4.371700 -2.230500 -2.856900 C 2.515900 -2.930500 0.629300 H -3.657100 -3.289300 -1.613400 H 3.599100 -2.789700 0.821600 C 3.376800 -0.995200 -1.363800 C 3.675900 0.694600 3.107600 H 4.264400 -1.471800 -0.897400 H 4.546500 0.575500 3.764900 C -3.587600 0.186200 -1.960300 C 0.281000 2.724100 -0.616000 H -3.617700 1.064400 -1.291900 C -2.472000 0.337600 1.068500 H -4.494100 0.212600 -2.598300 C 3.460500 -0.214200 2.055900 H -2.699500 0.273500 -2.610300 H 4.177700 -1.029100 1.899200 C 1.814500 -3.226200 1.962700 C 2.775300 1.748000 3.316700 H 1.894900 -2.385500 2.672800 H 2.934300 2.455100 4.140400 H 2.260200 -4.122300 2.440100 C 2.343700 -4.117400 -0.333500 H 0.743100 -3.430700 1.803800 H 1.308500 -4.152400 -0.722400 H -0.449000 -1.852100 -4.253000 H 2.549600 -5.074500 0.186500 N -0.228400 -0.920700 -2.623300 H 3.025700 -4.056800 -1.198100 N -0.377900 -0.884000 -3.863400 C -1.512000 1.368500 1.158600 C -0.755300 3.208900 -1.446600 C 0.567000 5.162100 -2.044800 H -1.672800 2.618800 -1.550900 C -3.985700 1.646300 2.475600 C -0.618500 4.414000 -2.146500 H -4.952000 1.752000 2.985000 H -1.439100 4.769000 -2.781900 C -3.536800 -1.128800 -1.165400 C -3.034500 2.674600 2.551500 H -4.431500 -1.194200 -0.512200 H -3.255200 3.588400 3.117400 C 1.468400 3.481200 -0.529500 C 3.116900 -1.628500 -2.742000 H 2.293600 3.125600 H 2.945000 -2.716300 -2.687800 0.096300 207 H 3.986700 -1.457000 -3.408400 H 2.225400 -1.175300 -3.207700 C -1.937700 -2.318600 2.588100 H -0.870100 -2.038000 2.534800 H -2.019800 -3.230600 3.212900 H -2.469000 -1.501200 3.102800 C 3.648900 0.506800 -1.541500 H 2.732900 1.019900 -1.887100 H 4.438100 0.667200 -2.303000 H 3.978100 0.985500 -0.603600 C -2.495200 -2.572700 1.177100 H -3.603200 -2.609500 1.222400 C -1.965400 -3.906400 0.624800 H -2.464600 -4.203800 -0.312100 H -2.108700 -4.723200 1.360500 H -0.887100 -3.814000 0.401000 H 0.677700 6.103300 -2.596700 H 2.544000 5.256500 -1.153600 208 Supplementary References (1) Buscagan, T. M.; Oyala, P. H.; Peters, J. C. N2-to-NH3 Conversion by a triphos–iron catalyst and enhanced turnover under photolysis. Angew. Chem. Int. Ed. 2017, 56, 6921–6926. (2) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. A synthetic single-site Fe nitrogenase: high turnover, freeze-quench 57Fe Mössbauer data, and a hydride resting state. J. Am. Chem. Soc. 2016, 138, 5341–5350. (3) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. Syntheses and Electronic Structures of Decamethylmetallocenes. J. Am. Chem. Soc. 1982, 104 (7), 1882– 1893. (4) Weitz, I. S.; Rabinovitz, M. The application of C8K for organic synthesis: reduction of substituted naphthalenes. J. Chem. Soc. Perkin Trans. 1993, 1, 117. (5) Evans, D. F. The determination of the pararnagnetic susceptibility of substances in solution by nuclear magnetic resonance. J. Chem. Soc. 1959, 2003–2005. (6) Gaussian 09, Revision A.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R. ; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, 209 T.; Throssell, K.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M. ; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2016 (7) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta–generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91 (14), 3–6. (8) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (9) Pfirrmann, S.; Limberg, C.; Herwig, C.; Knispel, C.; Braun, B.; Bill, E.; Stösser, R. A reduced β-diketiminato-ligated Ni3H4 unit catalyzing H/D Exchange. J. Am. Chem. Soc. 2010, 132, 13684–13691. (10) Creutz, S. E.; Peters, J. C. Spin-state tuning at pseudo-tetrahedral d6 ions: spin crossover in [BP3]FeII−X complexes. Inorg. Chem. 2016, 55, 3894–3906. (11) Rittle, J.; Mccrory, C. C. L.; Peters, J. C. A 106 fold enhancement in N2 binding affinity of an Fe2(μ-H)2 core upon reduction to a mixed-valence FeIIFeI state. J. Am. Chem. Soc. 2014, 136, 13853–13862. (12) Tepper, A. W. J. W.; Bubacco, L.; Canters, G. W. Paramagnetic properties of the halide-bound derivatives of oxidised tyrosinase investigated by spectroscopy. Chem. - A Eur. J. 2006, 12, 7668–7675. 1 H NMR 210 (13) Zaballa, M. E.; Ziegler, L.; Kosman, D. J.; Vila, A. J. NMR study of the exchange coupling in the trinuclear cluster of the multicopperoxidase Fet3p. J. Am. Chem. Soc. 2010, 132, 11191–11196. 211 Appendix B : Supporting Information for Chapter 3 212 Experimental Section General considerations. All manipulations were carried out using standard Schlenk or glovebox techniques under an N2 atmosphere, except for the synthesis of trimethyltin azide. Unless otherwise noted, solvents were deoxygenated and dried by thoroughly sparging with Ar gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC. 2-MeTHF was degassed by three freezepump-thaw cycles, followed by drying over NaK to remove traces of water. All reagents were purchased from commercial vendors and used without further purification unless stated otherwise. P3B,1 [(P3B)Fe(N2)],2 [(P3B)Fe][BArF],3 and were synthesized following literature procedures. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, and dried over activated 3-Å molecular sieves prior to use. 1 H and 13C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent proton and 13C resonances as internal standards. 31P chemical shifts are reported relative to 85 % aqueous H3PO4. Solution phase magnetic measurement were performed by the method of Evans. IR measurements were performed on a Bruker Alpha Platinum ATR spectrometer. X-band EPR spectra were obtained on a Bruker EMX spectrometer and simulated using Easyspin. ESEEM and inversion recovery experiments were aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker D2 resonator. Temperature control was achieved using an ER 4118HV‐CF5‐L Flexline Cryogen‐Free VT cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC. 213 Mössbauer spectra were recorded on a spectrometer from SEE Co. operating in the constant acceleration mode in a transmission geometry. Spectra were recorded with the temperature of the sample maintained at 80 K. The sample was kept in an SVT-400 Dewar from Janis. The quoted isomer shifts are relative to the centroid of the spectrum of a metallic foil of αFe at room temperature. Data analysis was performed using the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes. X-Ray Crystallography. X-ray diffraction studies were carried out at the Caltech Division of Chemistry and Chemical Engineering X-ray Crystallography Facility on a Bruker three-circle SMART diffractometer with a SMART 1K CCD detector, APEX CCD detector, or Bruker D8 VENTURE Kappa Duo PHOTON 100 CMOS detector. Data were collected at 100 K using Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54178 Å). Structures were solved by direct or Patterson methods using SHELXS and refined against F2 on all data by full-matrix least squares with SHELXL-97.68. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times the Ueq of the atoms to which they are bonded. [(P3B))Fe(Cl)] (1). A mixture of FeCl2 (87 mg, 0.69 mmol), P3B (400 mg, 0.69 μmol), iron powder (415 mg, 7.4 mmol), and THF (20mL) was heated to 90 °C in a sealed bomb under vigorous stirring for 62 h, during which time the color of the liquid phase turned from pale yellow to brown. The remaining iron powder was removed by filtration, and the solvent was removed in vacuo. The brown residue was taken in toluene (10 mL) and dried in vacuo. The brown residue was extracted with pentane (200 mL) to give a brown solution. Solvent 214 evaporation in vacuo afforded the product as a greenish-brown powder (422 mg, 90%). An analytically pure sample was obtained by slow concentration of a saturated pentane solution. Crystals suitable for XRD were obtained upon cooling a saturated solution of (P3B)FeCl in pentane to −35 °C . 1H NMR (C6D6, 300 MHz): δ 96.9 (1H), 35.0 (1H), 23.6 (1H), 9.8(1H), 5.8 (1H), 1.9 (3H), -0.3 (3H), -2.3 (3H), −22.4 (1H). UV-vis (THF, nm {cm– 1 M–1}): 280 {2.0·104}, 320 {sh}, 560 {sh}, 790 {150}, 960 {190}. μeff (C6D6, Evans method, 20 °C): 4.0 μB. Anal: calcd for C36H54BClFeP3: C 63.41, H 7.98; found: C 63.16, H 7.72. [(P3B)Fe(N3)] (2. A mixture of 1 (100 mg, 0.15 mmol) and sodium azide (15 mg, 0.23 mmol) in THF (3 mL) was stirred for 10 days at room temperature. The solvent was removed in vacuo and the solid residue was extracted with benzene. Lyophilization afforded the product as a brown powder (100 mg, 99%). 1H NMR (C6D6, 300 MHz): δ 72.5 (3H), 34.3 (3H), 23.0 (3H), 12.3 (3H), 5.9 (3H), 1.7 (9H), 1.1 (9H), 0.5 (9H), −1.8 (9H), −21.5 (3H). UV-vis (DEE, nm {cm–1 M–1}): 280 {sh}, 340 {sh}, 370 {1.1·104}, 600 {sh}, 820 {160}, 1000{190}. μeff (C6D6, Evans method, 20 °C): 3.8 μB. IR (ATR, THF film): νNNN = 2070 cm-1. Anal: calcd for C36H54BFeN3P3: C 62.81, H 7.91, N 6.10; found: C 63.26, H 7.65, N 6.09. [(P3B)Fe(N*NN*) (N*: 50% 15N (2-15N)] was prepared as 2 using Na15NNN. νNNN = 2058 cm-1. [(P3B)57Fe(N3)] (2-57Fe) was prepared as 2 using (P3B)57FeCl. [(P3B)]2(μ-1,3-N3) (3) A brown solution of (TPBiPr)Fe(N2) (20 mg, 30 μmol) in benzene (1 mL) was layered on top of a brown solution of 2 (20 mg, 29 μmol) in benzene (1 mL) 215 and left standing for 2 days. Filtration, washing with benzene (2 × 1 mL) and pentane (2 × 1 mL) followed by drying in vacuo afforded the product as a dark brown powder (29 mg, 76%). IR (ATR): νNN= 2073 cm-1. NIR (KBr): 5400 cm-1 (IVCT). Anal: calcd for C72H108B2Fe2N3P6: C 64.79, H 8.16, N 3.15; found: C 64.39, H 7.81, N 3.18. Additional characterization was obtained on 3 generated in solution from equimolar amounts of 2 and (TPBiPr)Fe(N2). 1H NMR (C6D6, 300 MHz): δ 47.0, 46.0 (sh), 39.3, 19.6, 12.0, 11.0 (sh), 4.3, 3.3, 2.1, 0.2, −1.8, −3.4, −7.2, −8.0. UV-vis (THF, nm {cm−1M−1}): 640 {sh}, 750 {sh}, 1000{sh}. NIR (THF, cm−1{cm−1M−1}): 5250 {2.2 •103} (IVCT). IR (ATR, THF film): νNNN = 2090 cm−1. [(P3B)Fe(NCO)] (6). A mixture of [(P3B)Fe][BArF4] (15 mg, 0.01 mmol) and KNCO (8.1 mg, 0.1 mmol) in THF (3 mL) was stirred for 48 hours at room temperature. The solvent was removed in vacuo and the solid residue extracted with pentane (2 x 5 mL). Removal of the solid in vacuo afforded the product as a yellow powder. 1H NMR (400 MHz, Benzene-d6) δ 84.42 (3H), 33.96 (3H), 23.31 (3H), 10.17 (3H), 1.97 (9H), 1.89 (3H), 1.40 (9H), 0.60 (9H), -1.35 (s, 9H), -22.29 (s, 3H). 216 2. NMR Spectra Figure B..1. 70 1H NMR spectrum of (P3B)FeCl 1 in C6D6. Figure B..2 1H NMR spectrum of (P3B)Fe(N3) 2 in C6D6. 217 Figure B..3 1H NMR spectra monitoring the photolysis of (P3B)Fe(N3) (2) in C6D6 at room temperature and 1H NMR spectrum of [(P3B)]2(μ-1,3-N3) (3) generated in situ by reaction of 2 and (P3B)Fe(N2) recorded before complete precipitation of insoluble 3. 218 3. IR Spectra Figure B..4 IR spectrum of (P3B)Fe(N3) (2) (thin-film from C6D6 solution). 219 Figure B.5 IR spectrum of (P3B)Fe(15NNN) (2-15N) (thin-film from C6D6 solution). 220 4. EPR Spectra Figure B.6. EPR spectra recorded after irradiating a frozen 1 mM solution of 2, 2-15N, and 2-57Fe, showcasing the absence of isotope effects on the signal and the absence of a signal corresponding to (P3B)Fe≡N (4). The observed signal is identical to i-Pr radical.4 221 Figure B.6. EPR spectra recorded after irradiating a frozen 1 mM solution of 2 in methylcyclohexane, toluene, d8-toluene, 2-MeTHF, pentane, and as thin films deposited from pentane solutions. 222 Figure B.7. Field-Dependent Q-band Davies ENDOR spectra (black) of natural abundance TPB-FeN with spectral simulations of contributions from the three 31P nuclei (blue), 10/11B (green) overlaid using parameters in main text. Figure B.8. Field-Dependent Q-band Davies ENDOR 57Fe-minus-Natural abundance difference spectra (black) of TPB-FeN with spectral simulations of contributions from the 57 Fe overlaid using parameters in main text. 223 Figure B.9. (Top) Q-band HYSCORE spectrum of natural abundance TPB-FeN at acquired at 1201.5 mT (g = 2.028). (Bottom) Simulation of 14N HYSCORE spectrum (red) overlaid over experimental data (grey) using parameters in main text. Figure B.10. (Top) Q-band HYSCORE spectrum of 15N enriched (~ 50%) TPB-FeN at acquired at 1201.5 mT (g = 2.028). (Bottom) Simulation of 15N HYSCORE spectrum (red) overlaid over experimental data (grey) using parameters in main text. 224 Figure B.11 (Top) Q-band HYSCORE spectrum of 15N enriched (~ 50%) TPB-FeN at acquired at 1201.5 mT (g = 2.028). (Bottom) Simulation of both 14N and 15N contributions to HYSCORE spectrum (red) overlaid over experimental data (grey) using parameters in main text. 225 Figure B.12 Figure B.13 226 Figure B.14. T1 relaxation times of 4. Temperature T1 T2 τ (K) (μs) (ns) (ns) 15 1128 993.3 992.4 30 33.25 980.3 952.2 50 3.879 909 736.4 85 0.623 450.4 261.5 227 5. UV-Vis Spectra Figure B.15. UV-visible spectrum of (P3B)Fe(Cl) (1) (THF, 293 K). Black trace corresponds to the left axis scale, while the red trace corresponds to the right axis scale. Figure B.16. UV-visible spectrum of (P3B)Fe(N3) (2) (THF, 293 K). Black trace corresponds to the left axis scale, while the red trace corresponds to the right axis scale. 228 Figure B.17. UV-visible spectrum [(P3B)]2(μ-1,3-N3) (3) generated in-situ by mixing equimolar solution of 2 and (P3B)Fe(N2) (THF, 293 K). 229 6. Crystallographic Details and Tables Table B.1. Crystallographic Data for Compounds 1-3 chem formula fw cryst syst space group a [Å] b [Å] c [Å] α [°] β [°] γ [°] V [Å3] Z Dcalcd [g cm-3] F(000) μ [mm-1] temp. [K] wavelength [Å] measd rflns unique rflns data/restraints/param R(F) (I>2σ(I)) wR(F2) (all) GOF 1 2 3 C35H54BClFeP3 C38.5H60BFeN3P3 C84H120B2Fe2N3P6 681.81 724.46 1490.97 Triclinic Cubic Monoclinic P-1 Pa-3 P21/c 10.8929(7) 19.953(3) 11.0614(7) 11.5482(7) 19.953(3) 17.7504(11) 15.8266(9) 19.953(3) 20.8069(12) 91.335(2) 90 90 97.048(2) 90 102.344(3) 117.325(2) 90 90 1748.20(19) 7944(4) 3990.9(4) 2 8 2 1.295 1.213 1.241 726 3107.4 1594 0.67 0.53 0.529 100 100 100 0.71073 0.71073 0.71073 104461 156002 41848 21944 8697 8148 21944/6/417 8697/68/182 8148/0/451 0.0363 0.0379 0.0537 0.0942 0.1101 0.113 1.021 1.085 1.018 230 DFT Calculations General All calculations were performed using the ORCA 4.05,6 program. In cases where crystal structures were available these coordinates were used as the input. The calculations were performed using the TPSS (meta-GGA)7 functional with the def2-SVP basis set on C and H and the def2-TZVP basis set on Fe. To assure that optimized structures represented true stationary points was checked by doing a single-point frequency calculations on the optimized structure. EPR parameters were calculated using TPPSh, TPSS, B3LYP, M06L, and BP86. The TPSS-optimized structures were calculated by doing a single point calculation on the optimized structures using CP(PPP)8 on Fe and IGLO-III9 on everything else grid 7. These basis sets and functionals have previously shown to be good predictors of phosphine and hydrogen hyperfine coupling constants in iron phosphine complexes.10,11 See below for a summary of the results. 231 7.2 EPR predictions ≈0 ± [-4.3, 4.3, 20.3] ± 3.9 iso) iso) [11.3 11.3 42.7] [9.2 9.3 39.7] [10.5 10.6 44.6] [6.5 6.5 37.23] [10.1 10.2 41.1] 21.8 19.4 21.9 16.75 20.5 [-35.5 -39.8 45.8] [-28.7 -33.5 38.3] [-33.6 38.2 43.6] [-30.2 -35.56 -40.51] [-52.4 -59.4 65.1] -40.4 -33.5 -38.5 -35.4 -59.0 [20.3 20.3 79.7] [18.0 18.0 81.0] [13.8 13.80 75.1] [2.3 2.8 69.8] 40.1 39.0 34.23 25.0 [-8.0 -8.0 20.87] [-4.8 -4.8 20.5] [-9.0 -9.0 17.3] [-11.6 11.7 33.9] 1.62 3.6 -0.26 3.52 IGLOIII IGLOIII IGLOIII IGLOIII TPSS B3LYP BP86 M06l ± [10, 10, 46] ± 22 A(57Fe) (MHz) ± [49, 45, 39] (MHz) 57Fe (a ± 44.3 A(31P) (MHz) [20.3 20.3 79.65] ≈0 iso) (MHz) 31P (a 40.1 ≈± 2.8 e2Qq/h η (11B) (11B) [-7.7 -7.8 21.2] ±[21.5, 21.5, 81] A(11B) (MHz) ± 41.3 (MHz) 11B (a 1.9 ≈± 3.1 e2Qq/h η (14N) (14N) A(14N) (MHz) iso) (MHz) 14N (a IGLOIII Basis set TPSSh experimental Functional 232 Table B.2. Experimental and DFT-calculated EPR parameters. Experimental g = [2.0293, 2.0293, 1.9975]. 233 References: (1) Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. QuasiThermoneutral P -> B Interactions within Di- and Tri-Phosphine Boranes. Inorg. Chem. 2007, 46 (13), 5149–5151. (2) Moret, M.-E.; Peters, J. C. Terminal Iron Dinitrogen and Iron Imide Complexes Supported by a Tris(Phosphino)Borane Ligand. Angew. Chem. Int. Ed. 2011, 50, 2063– 2067. (3) Anderson, J. S.; Moret, M. E.; Peters, J. C. Conversion of Fe–NH2 to Fe–N2 with Release of NH3. J. Am. Chem. Soc. 2013, 135 (2), 534–537.. (4) Ayscough, P. B.; Thomson, C. Electron Spin Resonance Spectra of Alkyl Radicals in γ-Irradiated Alkyl Halides. Trans. Faraday Soc. 1962, 58 (0), 1477–1494. (5) Neese, F. The ORCA Program System. WIREs Computational Molecular Science 2012, 2 (1), 73–78. (6) Neese, F. Software Update: The ORCA Program System, Version 4.0. WIREs Computational Molecular Science 2018, 8 (1), e1327. (7) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91 (14), 146401–146404. (8) Neese, F. Prediction and Interpretation of the 57Fe Isomer Shift in Mössbauer Spectra by Density Functional Theory. Inorganica Chimica Acta 2002, 337, 181–192. (9) Kutzelnigg, W.; Fleischer, U.; Schindler, M. The IGLO-Method: Ab-Initio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities. In Deuterium and Shift Calculation; Fleischer, U., Kutzelnigg, W., Limbach, H.-H., Martin, G. J., Martin, M. L., Schindler, M., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 1991; pp 165–262. (10) Drover, M. W.; Schild, D. J.; Oyala, P. H.; Peters, J. C. Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and Its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer. Angewandte Chemie International Edition 2019, 58 (43), 15504–15511. (11) Schild, D. J.; Drover, M. W.; Oyala, P. H.; Peters, J. C. Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy. J. Am. Chem. Soc. 2020, 142 (44), 18963–18970. 234 Appendix C : Supporting Information for Chapter 4 235 General Considerations All experiments were carried out employing standard Schlenk techniques under an atmosphere of dry nitrogen employing degassed, dried solvents in a solvent purification system supplied by SG Water, LLC. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective moisture removal. trans-FeBr2(depe)2cvi, FeBr2(dippe)cvii, Fe(η6- toluene)(dippe), Fe(Cp*)(dppe)H were prepared according to a literature procedure. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Hydrogen Analysis: The headspace of reaction flasks was analyzed by gas chromatography to quantify H2 evolution with an Agilent 7890A gas chromatograph (HPPLOT U, 30 m, 0.32 mm i.d., 30 °C isothermal, 1 mL/min flow rate, N2 carrier gas) using a thermal conductivity detector. Nuclear Magnetic Resonance Spectroscopy 1 H and 13C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. 31P chemical shifts are reported in ppm and referenced externally to 85% aqueous H3PO4 at 0 ppm. 236 III. 57Fe Mössbauer Spectroscopy Mössbauer spectra were recorded on a spectrometer from SEE Co. (Edina, MN) operating in the constant acceleration mode in transmission geometry. The sample was kept in an SVT-400 cryostat form Janis (Wilmington, MA), using liquid N2 as a cryogen for 80 K measurements. 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 solid material into a fine powder and then mounted in to a Delrin cup fitted with a screw cap as a boron nitride pellet. Solution samples were transferred to a sample cup and chilled to 77 K inside of the glovebox, and quickly removed from the glovebox and immersed in liquid N2 until mounted in the cryostat. Data analysis was performed using WMOSS version 4 (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes.cviii IV. Infrared Spectroscopy: Solid and thin film IR measurements were obtained on a Bruker Alpha spectrometer equipped with a diamond ATR probe. 237 UV-VIS Spectroscopy UV-Visible spectroscopy measurements were collected with a Cary 50 UV-Vis spectrophotometer using a 1 cm two-window quartz cell. EPR Spectroscopy: Continuous wave X-band EPR spectra were obtained on a Bruker EMX spectrometer using solutions prepared as frozen glasses in 2-MeTHF. Pulse EPR spectroscopy: All pulse Xband (9.4-9.7) EPR, electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments were acquired using a Bruker ELEXSYS E580 pulse EPR spectrometer. X-band ENDOR and HYSCORE experiments were performed using a Bruker MD-4 X-band ENDOR 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. All pulse X-band (ν ≈ 9.4-9.7 GHz) EPR and electron nuclear double resonance (ENDOR) experiments were aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker MD-4 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC. Pulse X-band ENDOR was acquired using the Davies pulse sequence (π − TRF − πRF − TRF − π/2 – τ – π – echo), where TRF is the delay between mw pulses and RF pulses, πRF is the length of the RF pulse and the RF frequency is randomly sampled during each pulse sequence. 238 X-band HYSCORE spectra were acquired using the 4-pulse sequence (π/2 − τ − π/2 − t1 − π –t 2 – π/2 – echo), where τ is a fixed delay, while t1 and t 2 are independently incremented by Δt1 and Δt 2 , respectively. The time domain data was baseline-corrected (third-order polynomial) to eliminate the exponential decay in the echo intensity, apodized with a Hamming window function, zero-filled to eight-fold points, and fast Fouriertransformed to yield the 2-dimensional frequency domain. For 2H-1H difference spectra, the time domain of the HYSCORE spectrum of the 1H sample was subtracted from that of the 2H sample, and the same data processing procedure detailed above was used to generate the frequency spectrum. In general, the ENDOR spectrum for a given nucleus with spin I= ½ (1H, 31P) coupled to the S = ½ electron spin exhibits a doublet at frequencies A ν± = | ± νN | 2 (1) Where νN is the nuclear Larmor frequency and A is the hyperfine coupling. For nuclei with I ≥ 1 (2H), an additonal splitting of the ν± manifolds is produced by the nuclear quadrupole interaction (P) ν±,mI = | νN ± 3P(2mI − 1) | 2 (2) In HYSCORE spectra, these signals manifest as cross-peaks or ridges in the 2-D frequency spectrum which are generally symmetric about the diagonal of a given quadrant. This technique allows hyperfine levels corresponding to the same electron-nuclear submanifold to be differentiated, as well as separating features from hyperfine couplings in the weakcoupling regime (|A| < 2|νI | ) in the (+,+) quadrant from those in the strong coupling 239 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 within the excitation bandwidth of the MW pulses at the magnetic field position at which the spectrum is collected. The length and curvature of these correlation ridges can allow for the separation and estimation of the magnitude of the isotropic and dipolar components of the hyperfine tensor, as shown in Fig. C1. 240 Figure C.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 (aiso ) and dipolar (T) contributions. Blue correlation ridges represent the strong coupling case; red correlation ridges represent the weak coupling case. 241 EPR Simulations. Simulations of all CW and pulse EPR data were achieved using the EasySpincix simulation toolbox (release 5.2.21) with Matlab 2018b using the following Hamiltonian: ̂ = μB ⃑B0 gŜ + μN g N ⃑B0 Î + hŜ ∙ 𝐀 ∙ Î + hÎ ∙ 𝐏 ∙ Î H (3) In this expression, the first term corresponds to the electron Zeeman interaction term where μB is the Bohr magneton, g is the electron spin g-value matrix with principle components g = [gxx gyy gzz], and Ŝ is the electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where μN is the nuclear magneton, g N is the characteristic nuclear g-value for each nucleus (e.g. 1H, 2H ,31P) and Î 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 principal axis system (PAS), 𝐏 is traceless and parametrized by the quadrupole coupling constant e2 Qq/h and the asymmetry parameter η such that: Pxx 𝐏= (0 0 where e2 Qq h 0 Pyy 0 0 e2 Qq/h −(1 − η) 0)= ( 0 4I(2I − 1) Pzz 0 = 2I(2I − 1)Pzz and η = Pxx −Pyy Pzz 0 0 −(1 + η) 0) 0 2 (4) . The asymmetry parameter may have values between 0 and 1, with 0 corresponding to an electric field gradient with axial symmetry and 1 corresponding to a fully rhombic efg. 242 The orientations between the hyperfine and NQI tensor principal axis systems and the gmatrix reference frame are defined by the Euler angles (α, β, γ), with rotations performed within the zyz convention where α rotates xyz counterclockwise about z-axis to give x'y'z', β rotates x'y'z counterclockwise about y'-axis to give x",y",z", γ rotates xyz counterclockwise about z"-axis to give final frame orientation. Electrochemistry Electrochemical measurements were carried out using a CD instruments 600B electrochemical analyzer. A freshly polished glassy carbon electrode was used as the working electrode and a graphite rod was used as the auxiliary electrode. Solutions (THF) of electrolyte (0.4 M tetra-n-butylammonium hexafluorophosphate) contained ferrocene (0.1 mM), to serve as an internal reference, and analyte (0.2 mM). All reported potentials are referenced to the ferrocene/ferrocenium couple, [Cp2Fe]+ /Cp2Fe. Synthetic Procedures Fe(η3:η2-Ind)(depe)(Br) (1): To a solution of trans-FeBr2(depe)2 (402 mg, 0.64 mmol, 1 equiv.) in THF at -78 °C was added drop-wise a chilled (-78 °C) solution of lithium indenide (78 mg, 0.64 mmol, 1 equiv.). Following addition, the resulting mixture was stirred for an additional 2 h at room temperature, giving a clear purple solution. Subsequently, all volatiles were removed in-vacuo and the residue was washed with pentane (2 x 20 mL) and Et2O (2 x 20 mL). The resulting purple solid was dissolved in THF and filtered through a pad of Celite®. Cooling a pentane-layered THF solution at –35 ºC afforded 1 as dark purple crystals (132 243 mg, 45%). N.B. Allowing this reaction mixture to stir for longer than 2 h results in appreciable formation of Fe(Ind)2 and free ligand. 1H NMR (C D , 400 MHz, 298 K): δ = 7.54 (m, 2H), 7.10 (m, 2H), 4.56 (br s, 1H), 4.01 6 6 (br s, 2H), 2.13 (m, 2H), 1.79 (m, 2H), 1.64 (m, 2H), 1.28 (m, 2H), 1.12 (m, 4H), 1.01 (m, 6H), 0.77 (m, 6H). 31P{1H} NMR (C6D6, 162 MHz, 298 K): δ = 92.98. 13C NMR (THFd8, 100 MHz, 298 K): δ = 127.99, 125.39, 83.13 (η3:η2-C9H7), 58.07 (η3:η2-C9H7), 23.60, 21.92, 20.40, 10.13, 8.99. CV data (1 mM, vs. Fc/Fc+): - 0.53 V (FeII/FeIII). UV-VIS (THF, 1 cm cell, 298 K): ε = 529 {313 M-1cm-1}, 687 {208 M-1cm-1}. Anal. Calcd. for C19H31BrFeP2 (456.04): C, 49.92; H, 6.84. Found: C, 50.40; H, 6.96. 244 Figure C.2 1 H NMR, C6D6, Figure C.3. 31P{1H} NMR, C6D6, 162 MHz, 298 K Figure C.4. 31C{1H} NMR, THF-d8, 100 MHz, 298 K 400 MHz, 298 K 245 Figure C.5. UV-Visible spectrum, THF, 298 K ( = 529, 687 nm) Fe(η3:η2-Ind)(depe)(H) (2): To a solution of 1 (21.5 mg, 0.047 mmol, 1 equiv.) in THF at -78 °C was added drop-wise a chilled (-78 °C) 1.0 M solution of Li[BEt3H] (47.1 L, 0.047 mmol, 1 equiv.). Following addition, the resulting mixture was stirred for an additional 10 min at 78 °C and then at room temperature for 2 h, giving a clear red solution. Subsequently, all volatiles were removed in-vacuo and the residue was dissolved in pentane (5 mL) and filtered through a pad of Celite®. This was repeated three times to give 2 as a red oil (17.5 mg, 98%). Efforts to recrystallize 2 were unsuccessful. 1H NMR (C D , 400 MHz, 298 K): δ = 7.39 (m, 2H), 6.81 (m, 2H), 4.73 (br s, 2H), 4.71 6 6 (m, 1H), 1.57-1.34 (m, 8H), 1.25 (m, 4H), 0.99 (m, 6H), 0.75 (m, 6H), -20.64 (t, 2JH,P = 70.8 Hz, 1JFe,H = 10.3 Hz). 31P{1H} NMR (C6D6, 162 MHz, 298 K) δ = 106.38 (2JH,P = 70.8 Hz, 1JFe,P = 60.3 Hz). 13C NMR (C6D6, 100 MHz, 298 K): δ = 126.69, 121.00, 97.73, 80.09 246 (η3:η2-C9H7), 62.22 (η3:η2-C9H7), 27.01 (m), 25.89 (dd, JC,P = 21.4 Hz, JC,P = 19.5 Hz), 25.24 (dd, JC,P = 6.9 Hz, JC,P = 4.89 Hz), 9.27, 9.14. IR (thin film, 298 K, cm–1): 1851 cm1 ( FeH). 57Fe Mössbauer (80 K, Et O solution, mm/s): δ = 0.28, ΔE = 1.61. CV data 2 Q (1 mM, vs. Fc/Fc+): - 0.81 V (FeII/FeIII). UV-VIS (THF, 1 cm cell, 298 K): ε = 396 {498 M-1cm-1}, 506 {912 M-1cm-1}. N.B. Given the physical nature of 2, elemental analysis was not acquired. Probing bimolecular H2 loss: To a J. Young NMR tube cooled to -78 oC containing a THF-d8 solution (300 μL) of 4.5 mg (12 μmol) 2-H and 4.5 mg (12 μmol) 2-D (500 μL) was added 25.6 mg (25 μmol) Figure C.6. 1H NMR, C6D6, 400 MHz, 298 K [Fc]BArF4 in 200 μL THF-d8. The tube was shaken and warmed to room temperature. NMR spectroscopy confirms the presence of both H2 and HD. 1H NMR (THF-d8, 400 MHz, 298 K):δ= 4.50 ppm (t, 1JH,D = 42 Hz; HD), 4.55 (s, H2). 247 Figure C.7. 31P{1H} NMR, C6D6, 162 MHz, 298 K 248 Figure C.8. 1H NMR, C6D6, 400 MHz, 298 K Figure C.9. 2-57Fe, 31P{1H} NMR, C6D6, 162 MHz, 298 K 249 Figure C.10. 13C{1H} NMR, C6D6, 100 MHz, 298 K Figure C.11. 2, FT-IR ATR, thin film, 298 K (μFeH = 1851 cm-1) 250 Figure C.12. 80 K 57Fe Mössbauer spectrum collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution in Et2O).δ= 0.28 mm/s, ΔEQ = 1.61 mm/s.ΓL = ΓR = 0.50 mm/s. 251 , Figure C.13. Cylic Voltammogram, THF, 298 K showing a reversible feature at - 0.81 V for the Fe(II)/Fe(III) couple (vs. Fc/Fc+) in 4 mL THF, 0.4 M [NBu4][PF6] and analyte (1 mM). Data was collected at x mV/s with Pt counter, glassy carbon working, and Ag/AgOTf reference electrodes (◼ 100 mV/s, ◼ 200 mV/s, ◼ 300 mV/s, and ◼ 400 mV/s). 252 Figure C.14. UV-Visible spectrum, THF, 298 K [Fe(η6-IndH)(depe)][BArF4] [4][BArF4]: To a solution of 2 (12.9 mg, 0.034 mmol, 1 equiv.) in Et2O at -78 °C was added drop-wise a chilled (-78 °C) solution of Fc[BArF4] (35.9 mg, 0.034 mmol, 1 equiv.). Following addition, the resulting dark orange mixture was stirred for an additional 20 min at -78 °C and then all volatiles were removed in-vacuo at -78 °C. Next, the residue was washed with 20 mL cold (-78 °C) pentane, dissolved in cold (-78 °C) Et2O, layered with pentane and placed in the freezer at -35 °C causing deposition of yellow ([4][BArF4]) and purple ([7][BArF4]2) crystals. N.B. Considerably fewer (~ 1:10) orange crystals were observed. Yield: According to a 57Fe Mössbauer experiment, oxidation of 2-57Fe at -78 oC produces ca. 30% of [4]+ after 5 min. 253 57Fe Mössbauer (80 K, Et O solution, mm/s): δ= 0.59 mm/s, ΔE = 1.80 mm/s. X-Band 2 Q EPR (77 K, 2-MeTHF): Sys.g = [2.332, 2.042, 1.992], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [86, 104, 100; 93, 88, 94], Sys.HStrain = [319 67 11], Exp.mwFreq = 9.389. Figure C.15. [4][BArF4], 77 K X-band EPR spectrum in 2-MeTHF with simulations; blue trace: experiment; red trace: simulation (Sys.g = [2.332, 2.042, 1.992], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [86, 104, 100; 93, 88, 94], Sys.HStrain = [319 67 11], Exp.mwFreq = 9.389]. 254 Figure C.16. 80 K 57Fe Mössbauer spectrum collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution in 2-MeTHF). Addition of [Fc]BArF4 to 2-57Fe at -78 oC. Top spectrum shows freeze-quenched sample after 5 min. Bottom spectrum shows freeze-quench after stirring at room temperature for 15 min. Parameters: δ = 0.23 mm/s, ΔEQ = 1.73 mm/s (green, 42%); δ = 0.59 mm/s, ΔEQ = 1.80 mm/s (blue, 28%), δ = 0.40 mm/s, ΔEQ = 0.46 mm/s (violet, 16%); ΓL = ΓR = 0.50 mm/s. N.B. These fits are not strictly required by the data. 255 [Fe(η6-toluene)(dippe)][BArF4] [5][BArF4]: To a solution of Fe0(η6toluene)(dippe) (16.7 mg, 0.041 mmol, 1 equiv.) in Et2O at -78 °C was added drop-wise a chilled (-78 °C) solution of Fc[BArF4] (38.4 mg, 0.037 mmol, 0.9 equiv.). Following addition, the resulting dark orange mixture was stirred for an additional 10 min at -78 °C and then at room temperature for 10 min. Next, all volatiles were removed in-vacuo and the orange residue was washed with 20 mL pentane. Cooling a pentane-layered THF solution at –35 ºC afforded [7][BArF4] as orange crystals (40 mg, 77%). 1H NMR (THF-d , 400 MHz, 298 K):δ = 35.37, 19.83, 7.78 (BArF ), 7.56 (BArF ), 6.60, 8 4 4 3.04, -0.17, -7.04. X-Band EPR (77 K, 2-MeTHF): Sys.g = [2.371, 2.032, 1.990], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [--, 90, 105; --, 99, 89], Sys.HStrain = [256 71 10], Exp.mwFreq = 9.389 mT. 57Fe Mössbauer (160 K, frozen THF solution, mm/s):δ= 0.50 ΔEQ = 1.71. CV data (1 mM, vs. Fc/Fc+): - 1.51 V (FeI/Fe0). UV-VIS (THF, 1 cm cell, 298 K):ε = 267 {3231 M-1cm-1}, 280 {2615 M-1cm-1}, 352 {849 M-1cm-1}, 428 {282 M1 cm-1}. Anal. Calcd. for C53H52BF24FeP2 (1273.3): C, 49.98; H, 4.12. Found: C, 51.20; H, 4.66. 256 Figure C.17. [5][BArF4], 1H NMR, THF-d8, 400 MHz, 298 K Figure C.18. [5][BArF4], 1H NMR (expanded view), THF-d8, 400 MHz, 298 K 257 Figure C.19. [5][BArF4], 77 K X-band EPR spectrum in 2-MeTHF with simulations; blue trace: experiment; red trace: simulation. Sys.g = [2.371, 2.032, 1.990], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [--, 90, 105; --, 99, 89], Sys.HStrain = [256 81 40], Exp.mwFreq = 9.389]. 258 Figure C.20. [5][BArF4], 80 K (bottom) and 160 K (top) 57Fe Mössbauer spectrum collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution in Et2O). ):δ~ 0.50 mm/s, ΔEQ ~ 1.71 mm/s. 259 Figure C.21. [5][BArF4], Cylic Voltammogram, THF, 298 K showing a reversible feature at - 1.51 V for the Fe(I)/Fe(0) couple (vs. Fc/Fc+) in 4 mL THF, 0.4 M [NBu4][PF6] and analyte (1 mM). Data was collected at 100 mV/s with Pt counter, glassy carbon working, and Ag/AgOTf reference electrodes Figure C.22. [5][BArF4], Cylic Voltammogram, THF, 298 K showing a reversible feature at - 1.51 V for the Fe(I)/Fe(0) couple (vs. Fc/Fc+) as a function of scan rate (50 mV/s to 500 mV/s at 50 mV/s increments). 260 Figure C.23. [5][BArF4], UV-Visible spectrum, THF, 298 K ( [Fe(η3:η2-Ind)(depe)N2][BArF4]2 [6][BArF4]: N.B. = 267, 280, 352, 428 nm) This compound can be prepared via treatment of 2 with Fc[BArF4] or H(OEt2)2[BArF4]. To a solution of 2 (22.1 mg, 0.058 mmol, 1 equiv.) in Et2O (2 mL) at -78 °C was added drop-wise a chilled (78 °C) solution of H(OEt2)2[BArF4] (53.4 mg, 0.053 mmol, 0.9 equiv.) in Et2O (2 mL). Following addition, the resulting mixture was stirred at -78 °C and warmed to 25 °C over 2 h. During this time, a purple solid ([7][BArF4]2) precipitated and was removed by filtration. The supernatant was dried in-vacuo and washed with pentane. Cooling a pentane-layered THF solution at –35 ºC afforded [6][BArF4] as orange crystals that were mechanically separated from [7][BArF4]2 for analysis by X-ray diffraction. Complex [6][BArF4] was not obtained in pure form, however, by 31P{1H} NMR spectroscopy, the yield is estimated to 261 be ~30%. The N2 ligand in [6][BArF4] was also observed to be labile; exposure to vacuum caused a change in color from orange to maroon. 1H NMR (THF-d , 400 MHz, 298 K, select signals):δ= 7.79 (s, 8H; BArF ), 7.70 (m, 2H), 8 4 7.58 (br s, 4H, BArF4), 7.47 (m, 2H), 5.23 (s, 1H), 5.01 (s, 2H). 31P{1H} NMR (THF-d8, 162 MHz, 298 K):δ= 89.7. IR (thin film, 298 K, cm–1): 2151 cm-1 (νNN). Figure C.24. [6][BArF4], 31P{1H} NMR, THF-d8, 162 MHz, 298 K [signals at = 87.51 and 41.16 are due to [7][BArF4]2, those at = 56.0, 81.1, 87.3, and 92.9 ppm are due to impurities]. 262 Figure C.25. [6][BArF4], FT-IR ATR, thin film, 298 K ( NN = 2151 cm -1 ) [Fe2(η3:η2-Ind)2(depe)2(μ-depe)][BArF4]2 [7][BArF4]2: N.B. This compound can be prepared via treatment of 2 with Fc[BArF4] or H(OEt)2[BArF4]. To a solution of 2 (22.1 mg, 0.058 mmol, 1 equiv.) in Et2O (2 mL) at -78 °C was added dropwise a chilled (-78 °C) solution of H(OEt2)2[BArF4] (53.4 mg, 0.053 mmol, 0.9 equiv.) in Et2O (2 mL). Following addition, the resulting mixture was stirred at -78 °C and warmed to 25 °C over 2 h. During this time, a purple solid precipitated that was isolated via filtration, washed with pentane, and dried in-vacuo. Cooling a pentane-layered THF solution at –35 ºC afforded [7][BArF4]2 as a dark purple crystals (11.4 mg, 22%). 1H NMR (THF-d , 400 MHz, 298 K):δ= 7.79 (s, 16H; BArF ), 7.76 (m, 4H), 7.58 (br s, 8 4 8H, BArF4), 7.41 (m, 2H), 5.39 (2H), 4.83 (m, 4H), 2.58 (m, 4H), 2.28-2.04 (m, 8H), 1.97 (m, 4H), 1.67 (m, 8H), 1.43 (m, 4H), 1.37-1.07 (m, 26H), 1.01 (m, 18H). 31P{1H} NMR 263 (THF-d8, 162 MHz, 298 K):δ= 87.51 (d, 2JP,P = 37.8 Hz), 41.16 (t, 2JP,P = 37.8 Hz). 13C NMR (THF-d8, 100 MHz, 298 K):𝛿= 162.63 (q, 1JC,B = 37 Hz, BArF4, ipso quaternary C), 135.41 (BArF4, ortho C), 129.81 (q, 2JC,F = 31 Hz, BArF4, meta quaternary C), 128.76, 127.38, 124.93 (q, 1JC,F = 273 Hz, BArF4, CF3), 117.97 (m, BArF4, para C), 104.28, 86.62 (η3:η2-C9H7), 64.62 (η3:η2-C9H7), 23.67 (br), 21.41 (br), 21.18 (br), 9.78 (br), 9.17 (br), 7.85 (br). UV-VIS (THF, 1 cm cell, 298 K): ε = 329 {1657 M-1cm-1}, 491 {637 M-1cm-1}. Anal. Calcd. for C112H110B2F48Fe2P6 (2685.5): C, 50.06; H, 4.13. Found: C, 49.86; H, 4.21. Figure C.26. [7][BArF4]2, 1H NMR, THF-d8, 162 MHz, 298 K 264 Figure C.27. [7][BArF4]2, 31P{1H} NMR, THF-d8, 162 MHz, 298 K Figure C.28. [7][BArF4]2, 31C{1H} NMR, THF-d8, 100 MHz, 298 K 265 Figure C.29. [7][BArF4]2, UV-Visible spectrum, THF, 298 K [Fe(η3:η2-Ind)(depe)NCCH3][BArF4] [8][BArF4]: Route A): To a J. Young tube containing CD3CN (500 μL) and 2 was added 1-benzyl-3-acetylpyridinium triflate [BNAP]OTf as a solid; the color changed from red to orange. Analysis by NMR spectroscopy evidenced formation of [Fe(η3:η2-Ind)(depe)(NCCH3)]OTf and 1,4-BNAPH (~65% by 31P NMR). Route B): To a solution of 2 (4.7 mg, 0.012 mmol, 1 equiv.) in CH3CN (2 mL) at -35 °C was added drop-wise a chilled (-35 °C) solution of H(OEt2)2[BArF4] (12.6 mg, 0.012 mmol, 1 equiv.) in CH3CN (1 mL). Following addition, the resulting mixture was stirred at 25 °C over 10 min giving a clear purple solution. Removal of volatiles in-vacuo and washing with pentane gave [8][BArF4] as a purple solid (15.1 mg, 95%). 266 1H NMR (CD CN, 400 MHz, 298 K, 1,4-BNAPH):δ= 5.82 (dq, 3 J = 8.1 Hz, J = 1.5 Hz, 1H), 4.86 (dt, J = 8.1 Hz, J = 3.4 Hz, 1H), 4.38 (s, 2H), 2.94 (br s, 2H). 1H NMR (CD CN, 400 MHz, 298 K):δ = 7.69 (s, 8H; BArF ), 7.67 (s, 4H; BArF ), 7.56 3 4 4 (dd, 3JH,H = 6.6 Hz, 3JH,H = 3.1 Hz), 7.27 (dd, 3JH,H = 6.6 Hz, 3JH,H = 3.1 Hz), 4.66 (m, 1H), 4.49 (2H), 2.27 (m, 2H), 1.88 (m, 2H), 1.73 (m, 2H), 1.44 (m, 2H), 1.24 (m, 2H), 1.14 (m, 6H), 0.90 (m, 6H). 31P{1H} NMR (CD3CN, 162 MHz, 298 K):δ= 92.90. 13C NMR (CD3CN, 100 MHz, 298 K):δ = 162.52 (q, 1JC,B = 37 Hz, BArF4, ipso quaternary C), 135.61 (BArF4, ortho C), 129.86 (q, 2JC,F = 31 Hz, BArF4, meta quaternary C), 127.74, 127.25, 125.42 (q, 1JC,F = 273 Hz, BArF4, CF3), 117.97 (m, BArF4, para C), 84.98 ( 3 : 2 -C9H7), 62.17 (η3:η2-C9H7) 23.29 (m), 21.24 (m), 18.63 (m). Anal. Calcd. for C53H46BF24FeNP2 (1281.2): C, 49.67; H, 3.62; N, 1.09. Found: C, 49.30; H, 3.54; N, 1.02. Figure C.30. [8][BArF4], 1H NMR, CD3CN, 162 MHz, 298 K 267 Figure C.31. [8][BArF4], 31P{1H} NMR, CD3CN, 162 MHz, 298 K Figure C.32. [8][BArF4], 31C{1H} NMR, CD3CN, 100 MHz, 298 K 268 Figure C.33. 80 K 57Fe Mössbauer spectrum collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution in 2-MeTHF). Addition of [H(OEt2)2]BArF4 to 2-57Fe at -78 oC. Top spectrum shows freeze-quenched sample after 5 min. Bottom spectrum shows freeze-quench after stirring at room temperature for 15 min. Parameters: δ= 0.33 mm/s, ΔEQ = 1.69 mm/s (blue, 82%), δ= 0.14 mm/s, ΔEQ = 0.29 mm/s (green, 18%); ΓL = ΓR = - 0.50 mm/s. 269 IX. EPR Spectroscopy data: Figure C.34. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of 2-H using [Fc]BArF4 at -78 oC (< 30 s). Figure C.35. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of 2-H using [Fc]BArF4 at -78 oC, red (< 30 s), blue (20 min). 270 Figure C.36. A series of 77 K X-band EPR spectra in 2-MeTHF generated by oxidation of 2-H using [Fc]BArF4 at -78 oC showing red ([3]+) giving green ([4]+), which decomposes to give [FeI(depe)N2]+ (purple). 271 N P Et2 N Et2 Et2 P Fe P BArF4 P Et2 Figure C.37. 77 K X-band EPR spectrum of [FeI(depe)N2]+ in 2-MeTHF generated by oxidation of 2 using [Fc]BArF4 at -78 oC (after 24 h). The fit is consistent with that provided by Ashleycx and co-workers. Microwave frequency 9.369 GHz; red trace: experiment; blue trace: simulation. Simulation parameters: Sys.g = [2.125 2.093 2.0016] with four 31P hyperfine interactions, two of type 1, A(31P) = [65.6 61.4 60.9] MHz and two of type 2, A(31P) = [72 61.7 64.3] MHz. 272 Figure C.38. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of 2-H (red) and 2-D (blue) using [Fc]BArF4 at -78 oC (< 30 s). 273 Figure C.39. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation of 2-D using [Fc]BArF4 at -78 oC blue (< 30 s), red (20 min). 274 Figure C.40. 77 K X-band EPR spectrum in 2-MeTHF generated by oxidation using [Fc]BArF4 at -78 oC of 2-D: blue (< 30 s), red (20 min) or 2-H: green (< 30 s), orange (20 min) or 2-D at -78 oC. 275 Gauss [G] Figure C.41. 77 K X-band EPR spectrum of [Fe(η5-Cp*)(dppe)X]+ (X = H or D) in 2-MeTHF generated by oxidation of the corresponding FeII-X precursor using [Fc]BArF4 at -78 oC. These spectra show changes at high field, due to coupling of the unpaired electron to a hydride nucleus. For X = D (blue), the following fit paramaters were obtained: Sys.g = [2.352, 2.041, 1.992], Sys.lw = 1.2, Sys.Nucs = ‘31P, 31P’, Sys.A = [88, 82, 79; 82, 71, 76], Sys.HStrain = [180 32 31], Exp.mwFreq = 9.370]. 276 Figure C.42. Field-dependent X-band 31P Davies ENDOR of [3-D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31Pα, (blue), 31Pβ (green)). Simulation parameters: g = [2.377, 2.039, 1.993]; A(31Pα) = [100, 88, 88] MHz, Euler rotation angles [α, β, γ] = [35, 0, 0]◦; A(31Pβ) = [82, 85, 72] MHz, Euler rotation angles [α,β,γ] = [10, 0, 0]◦. Acquisition parameters: temperature = 10 K; MW frequency = 9.718 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; RF pulse length = 15 µs; shot repetition time = 2 ms. 277 Figure C.43. Field-dependent X-band 31P Davies ENDOR of [4-D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31Pα, (blue), 31Pβ (green)). Simulation parameters: g = [2.332, 2.042, 1.992]; A(31Pα) = [86, 104, 100] MHz, Euler rotation angles [α, β, γ] = [40, 0, 0]◦; A(31Pβ) = [93, 88, 94] MHz, Euler rotation angles [α,β,γ] = (0, 0, 0)◦. Acquisition parameters: temperature = 10 K; MW frequency = 9.712 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; RF pulse length = 15 µs; shot repetition time = 2 ms. 278 Figure C.44. Field-dependent X-band 31P Davies ENDOR of [Fe(η5-Cp*)(dppe)D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31 Pα, (blue), 31Pβ (green)). Simulation parameters: g = [2.352, 2.041, 1.992]; A(31Pα) = [88, 82, 79] MHz, Euler rotation angles (α, β, γ) = [0, 0, 0]◦; A(31Pβ) = [82, 71, 76] MHz, Euler rotation angles (α,β,γ) = [0, 0, 0]◦. Acquisition parameters: temperature = 10 K; MW frequency = 9.719 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; RF pulse length = 15 µs; shot repetition time = 2 ms. 279 Figure C.45. X-band HYSCORE spectra of [FeI(depe)N2]+ in 2-MeTHF (top panels) with simulations of features arising from hyperfine couplings to proximal 14N (14Np, red) and distal 14N (14Nd, blue) which are consistent with those previously reported by Ashley and co-workers.7 Simulation parameters: g = [2.125 2.093 2.0016]; A(14Np) = [13.6, 13.6, 15.4] MHz, e2qQ/h(14Np) = 3.2 MHz, η(14Np) ≈ 0; A(14Nd) = [2.7, 2.7, 6.9] MHz, e2qQ/h(14Nd) = 3.2 MHz, η(14Nd) ≈ 0. Acquisition parameters: temperature = 20 K; microwave frequency = 9.711 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 144 ns (g = 2.114), 138 ns (g = 2.038); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 280 Figure C.46. Field-dependent X-band HYSCORE spectra of [3-H][BArF4] (top panels) [3D]BArF4 (middle panels) and 1H-2H difference HYSCORE spectra (bottom panels). Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.314), 148 ns (g = 2.197); 144 ns (g = 2.116); 138 ns (g = 2.042); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 281 Figure C.47. Field-dependent X-band HYSCORE spectra of [4-H][BArF4] (top panels) [4D]BArF4 (middle panels) and 1H-2H difference HYSCORE spectra (bottom panels). Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.312), 148 ns (g = 2.195); 144 ns (g = 2.111); 138 ns (g = 2.002); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 282 Figure C.48. Field-dependent X-band HYSCORE spectra of [4-H][BArF4] (top panels) [4D]BArF4 (middle panels) and 1H-2H difference HYSCORE spectra (bottom panels) focused on the region in which 1H features manifest. Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.312), 148 ns (g = 2.195); 144 ns (g = 2.111); 138 ns (g = 2.002); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 283 Figure C.49. Field-dependent X-band HYSCORE spectra of [FeIII(η5-Cp*)(dppe)H]+ (top panels), [FeIII(η5-Cp*)(dppe)D]+ (middle panels), and 1H-2H difference HYSCORE spectra (bottom panels). Acquisition parameters: temperature = 12 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 158 ns (g = 2.349), 152 ns (g = 2.237); 144 ns (g = 2.135); 138 ns (g = 2.042); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 284 cvi Mays, M. J.; Prater, B. E. Inorg. Synth. 1974, 15, 21. cvii Hermes, A.R.; Girolami, G. S. Organometallics 1987, 6, 763. cviii Prisecaru, I. WMOSS4 Mössbauer Spectral Analysis Software, www.wmoss.org, 2009-2016. cix Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42. cx Doyle, L. R.; Scott, D. J.; Hill, P. J.; Fraser, D. A. X.; Myers, W. K.; White, A. J. P.; Green, J. C.; Ashley, A. E. Chem. Sci. 2018, 9, 7362. 285 Figure C.50 Field-dependent X-band 1H-2H difference HYSCORE spectra of [3-D]BArF4 (top panels) and simulations of deuteride 2H features overlaid in red over experimental data displayed in grey (bottom panels). Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.314), 148 ns (g = 2.197); 144 ns (g = 2.116); 138 ns (g = 2.042); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 286 Figure C.51 Field-dependent X-band 1H-2H difference HYSCORE spectra of [4-D]BArF4 (top panels) and simulations of Indene 2H features overlaid in red over experimental data displayed in grey (bottom panels). Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.312), 148 ns (g = 2.195); 144 ns (g = 2.111); 138 ns (g = 2.002); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 287 Figure C.52 Field-dependent X-band 1H-2H difference HYSCORE spectra of [4-D]BArF4 (top panels) and simulations of Indene 1H features overlaid in red over experimental data displayed in grey (bottom panels). Acquisition parameters: temperature = 10 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 156 ns (g = 2.312), 148 ns (g = 2.195); 144 ns (g = 2.111); 138 ns (g = 2.002); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 288 Figure C.53 Field-dependent X-band 1H-2H difference HYSCORE spectra of [FeIII(η5Cp*)(dppe)D]+ (top panels) and simulations of deuteride 2H features overlaid in red over experimental data displayed in grey (bottom panels). Acquisition parameters: temperature = 12 K; microwave frequency = 9.718 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 158 ns (g = 2.349), 152 ns (g = 2.237); 144 ns (g = 2.135); 138 ns (g = 2.042); t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 289 Rate Data Figure C.54 UV-VIS spectrum showing decay of [3]+ to give [4]+ at -60 oC in 2-MeTHF. Figure C.55 Reaction monitoring at ε = 404 nm showing decay of [3]+ at -60 oC in 2MeTHF. Crystallographic details 290 All crystals were mounted on a glass fiber loop. All measurements were made using graphite-monochromated Mo or Cu Kα radiation (λ = 0.71073 or 1.54178 Å) on a Bruker AXS D8 VENTURE KAPPA diffractometer coupled to a PHOTON 100 CMOS detector. The structures were solved by direct methods111 and refined by full-matrix least-squares procedures on F2 (SHELXL-2013)6 using the OLEX2 interface.112 All hydrogen atoms were placed in calculated positions. Non-hydrogen atoms were refined anisotropically. 1: This crystal is a twin. The following twin law was obtained: TWIN LAW (-1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, -1.0, 2.0), BASF [0.562(6)] using OLEX2; this led to improved data statistics. 4: This crystal features disorder about an entire depe ligand that was modeled as a 41:59 split; this led to improved data statistics. [5][BArF4]: A q-peak near Fe was assigned as a partially occupied Fe site (PLAT307). Qpeaks for the remainder of the Fe-containing molecule were not observed, due to the degree of disorder being relatively small. PLAT971 suggested that there was residual electron density, however, this is not attributable to an atom. The electron density appears near a phenyl ring and Fe. [6][BArF4]: One side of the depe ligand was modeled in two orientations in a 38/62 split C16/C17/C20/C21 [38] and C18/C19/C22/C23 [62]; this led to improved data statistics. [7][BArF4]2: This crystal contains a disordered pentane molecule located on an inversion center (this was not modeled). 291 CCDC 1896047-1896051 contains 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.uk/data_request/cif 111 Sheldrick, G. M.; IUCr. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. 112 Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339. 292 Table C.1. Crystallographic data for 1, 4, [5][BArF4], and [6][BArF4]. Compound Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° 𝛽/° γ/° V/Å3 Z ρ/ g/cm-3 μ/ mm-1 F(000) Crystal size/ mm3 Radiation 2θ range for collection/° data Index ranges Independent reflections Data/restraints/parameters Goodness-of-fit on F2 R [I>=2θ (I)] (R1, wR2) R (all data) (R1, wR2) Largest diff. peak/hole / (e Å-3) 1 C19H31BrFeP2 457.14 100(2) Orthorhombic Pna21 15.4701(4) 17.1275(5) 7.5971(2) 90 90 90 2012.96(9) 4 1.508 2.891 944.0 0.18 × 0.18 × 0.16 MoKα (λ = 0.71073) 4 C51H44BF24FeP2 1241.46 100(2) Monoclinic P21/c 11.9688(6) 17.5103(11) 24.9005(16) 90 95.858(2) 90 5191.3(5) 4 1.588 0.475 2508.0 0.23 × 0.17 × 0.17 MoKα (λ = 0.71073) [5][BArF4] C53H52BF24FeP2 1273.54 100(2) Monoclinic P21/c 17.9424(6) 17.3777(5) 19.0513(6) 90 111.447(2) 90 5528.8(3) 4 1.530 3.802 2588.0 0.38 × 0.29 × 0.23 CuKα (λ = 1.54178) 4.756 to 54.994 4.498 to 52.878 5.292 to 147.104 -20 ≤ h ≤ 20, -22 ≤ k ≤ 22, -9 ≤ l ≤ 9 4612 [Rint = 0.0752, Rsigma = 0.0242] 4612/1/213 1.046 R1 = 0.0175, wR2 = 0.0432 R1 = 0.0189, wR2 = 0.0439 -14 ≤ h ≤ 14, -21 ≤ k ≤ 21, -31 ≤ l ≤ 31 10453 [Rint = 0.1878, Rsigma = 0.1618] 10453/912/866 0.995 R1 = 0.0828, wR2 = 0.2121 R1 = 0.1053, wR2 = 0.2400 -21 ≤ h ≤ 22, -20 ≤ k ≤ 21, -23 ≤ l ≤ 23 11079 [Rint = 0.0572, Rsigma = 0.0199] 11079/729/744 1.051 R1 = 0.0995, wR2 = 0.2894 R1 = 0.1122, wR2 = 0.3046 0.27/-0.32 1.01/-0.48 4.25/-1.24 293 Table C2. Crystallographic data for [7][BArF4]2. Compound Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° 𝛽/° γ/° V/Å3 Z ρ/ g/cm-3 μ/ mm-1 F(000) Crystal size/ mm3 Radiation 2θ range for collection/° data Index ranges Independent reflections Data/restraints/parameters Goodness-of-fit on F2 R [I>=2θ (I)] (R1, wR2) [6][BArF4] C56H52BF24FeN2P2 1337.59 100(2) Monoclinic P21/n 16.4173(6) 16.3342(5) 22.0278(8) 90 91.9240(10) 90 5903.7(4) 4 1.505 0.425 2716.0 0.28 × 0.2 × 0.17 MoKα (λ = 0.71073) [7][BArF4]2 C60H55BF24FeP3 1391.61 100(2) Triclinic P-1 12.5836(7) 13.0412(7) 19.6731(11) 103.588(2) 106.523(2) 90.595(2) 2998.2(3) 2 1.541 0.446 1414.0 0.71 × 0.22 × 0.16 MoKα (λ = 0.71073) 4.462 to 55.826 4.352 to 70.178 -21 ≤ h ≤ 21, -21 ≤ k ≤ 21, -29 ≤ l ≤ 29 14128 [Rint = 0.0849, Rsigma = 0.0379] 14128/786/821 1.014 R1 = 0.0744, wR2 = 0.1872 R1 = 0.1060, wR2 = 0.2106 -20 ≤ h ≤ 20, -20 ≤ k ≤ 18, -30 ≤ l ≤ 28 19818 [Rint = 0.0368, Rsigma = 0.0568] 19818/0/808 1.051 R1 = 0.0546, wR2 = 0.1311 R1 = 0.0853, wR2 = 0.1533 R (all data) (R1, wR2) Largest diff. peak/hole / (e 1.26/-0.97 Å-3) 0.78/-0.77 R1 = Σ ||Fo|-|Fc|| / Σ |Fo|; wR2 = [Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]1/2 294 Table C.3. Summary of data obtained from X-ray analyses 1 [4][BArF4] [5][BArF4] [6][BArF4] [7][BArF4]2 Spin-state S=0 S=½ S= ½ S=0 S=0 o.s. 2+ 1+ 1+ 2+ 2+ d(Fe-P1) (Å) 2.1792(6) 2.222(3) 2.254(2) 2.208(1) 2.1960(7) d(Fe-P2) (Å) 2.2217(6) 2.298(3) 2.272(2) 2.206(1) 2.2413(7) Ω 4.58 - - 4.30 6.02 0.134 - - 0.089 0.134 90 96.7 127.5 135.1 M-C) 127.5 295 DFT Calculations General: Geometry optimizations were performed using Gaussian 09 [Rev. A.02] at the following level of theory: TPSS functional, a def2-TZVP basis set on iron and a def2-SVP basis set on all other atoms. Frequency calculations were used to confirm true minima and to determine gas phase free energy values (Ggas). BDFEX-H Calculations: Consistent with a previous report, a calibration curve of ΔG vs. BDFElit was employed. The free-energy 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. 57Fe Mӧssbauer Calculations: The 57Fe Mӧssbauer isomer shifts in Table C4 have been calculated using the extended calibration parameters according to Neese. Isomer shifts were calculated according to the equation δcalc = α(ρ(0)-C+β, with α = -0.17683, β = 0.35964 and C = 23600.9 Taking the systematic overestimation of this method into account, this provides a good indication of the isomer shifts. In contrast, the older set of calibration parameters (not included in Table C4) underestimate the observed isomer shifts. 296 Figure C.56 Free energy change (kcal mol-1) for PCET from the η-dienyl complex, involving BDFEC-H (TPSS; def2tzvp (Fe), def2svp (all other atoms)). ΔGoHAA = hydrogen atom affinity. 297 Functional Basis set 1H (a 1H A (MHz) 1H T (MHz) 36.7 [-10, 60, 60] [-46.7, 23.3 23.3] -66.34 [-19.33 -83.31 -85.15] [43.23, -22.55, -20.67] iso) (MHz) experimental TPSSh EPR-III (C & H) IGLO-III (P) TPSSh IGLO-III -62.59 [-23.11, -88.89, 87.01] [43.26, -20.72, -22.54] TPSS EPR-III (C & H) -42.33 [-3.85, -62.55, -60.59] [38.48, -20.22, -18.26] -39.72 [-1.21, -59.92, -58.02] [38.51, -20.20, -18.30] IGLO-III (P) TPSS IGLO-III Table C.0.4. Summary of DFT-calculated EPR parameters for [FeIIICp*(dppe)H]+ 298 Table C.0.5. Summary of DFT-calculated 57Fe Mössbauer parameters i 0.47 ii 16 Ref. 0 1.86 iii,iv Compound 23598.94994 1.91 1.10 this work i 0.59 23600.32909 0.84 1.62 this work 2.6 0.57 0.33 23600.16782 1.61 0.90 this work |∆EQcalc| (mms−1) 0.26 0.33 23600.02363 -- 0.75 this work 2.5 Ferrocenium 0.26 0.35 23600.13298 1.80 1.72 this work |∆EQexp| (mms−1) Fe(5-Cp)(dppe)(H) 0.28 0.33 23598.90502 -- 1.69 this work 23598.91995 [Fe(5-Cp*)(dppe)(H)]+ -0.55 23599.42829 -- 1.65 this work ρ(0) TPSSh+DKH Fe(3:2-Ind)(depe)(H) (2) 0.59 0.36 23599.72057 -- 1.66 this work 0.54 [Fe(3:2-Ind)(depe)(H)]+ -- 0.40 23599.78043 -- 1.35 this work δcalc (mms−1) [Fe(3:2-Ind)(depe)(PEt3)]+ -- 0.40 23601.04992 -- 0.70 0.51 [Fe(3:2-Ind)(depe)(N2)]+ -- 0.17 23598.94974 1.71 δexp (mms−1) [Fe(3:2-Ind)(depe)(H2)]+ -- 0.55 23598.82654 Ferrocene [Fe(3:2-Ind)(depe)(H)2]+ -- 0.56 [Fe(6-IndH)(depe)]+ ([4][BArF4]) [Fe(3:2-Ind)(depe)]+ (S = 1) 0.50 [Fe(6-C6H5CH3)(dippe)]+([5][BArF4]) Malischewski, M.; Seppelt, K.; Sutter, J.; Munz, D.; Meyer, K. Angew. Chem. Int. Ed. 2018, 57, Patel, D.; Wooles, A.; Cornish, A.D.; Steven, L.; Davies, S.E.; Evans, D.J.; McMaster, J.; Lewis, 14597. ii iii Hamon, P.; Hamon, J.R.; Lapinte, C. J. Chem. Soc., Chem. Commun. 1992, 1602. Hamon, P.; Toupet, L.; Hamon, J.R.; Lapinte, C. Organometallics 1992, 11, 1429. W.; Blake, A.J.; Liddle, S.T. Dalton Trans. 2015, 44, 14159. iv 299 Table C.0.6. Mulliken atomic spin densities calculated for [Fe(3:2Ind)(depe)(H)][BArF4] ([3][BArF4]). The red italicized atom corresponds to the hydride ligand with the value of aiso(1H) provided to its right. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Fe P P C C C C H C H C C H C H C H C C H H C H C H H C C H C H H H C H H H C H 1.004222 -0.023195 -0.029529 0.049194 -0.041834 0.003854 0.075785 -0.003139 -0.047046 0.001708 0.002200 0.020057 -0.001148 0.004944 -0.000302 -0.037762 0.001462 0.003755 0.004541 0.002211 -0.000232 -0.042396 0.001438 0.000510 -0.000910 0.000278 0.002046 0.081118 -0.003645 0.000412 0.000098 0.000298 0.000009 0.000508 0.000364 -0.000094 0.000008 0.000560 -0.000002 300 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 H H C H H H H H H H H H H H H -0.000152 -0.000484 -0.000034 0.000031 0.000339 0.000035 -0.028443 -0.001568 0.000283 0.000351 -0.000554 -0.000013 -0.000248 0.000445 -0.000331 aiso(1H) = 40.4 MHz Table C.0.7. Mulliken atomic spin densities calculated for [Fe(6-IndH)(depe)][BArF4] ([4][BArF4]). The red italicized atoms correspond to the CH2 moiety of the indene ligand with the value of aiso(1H) provided to its right. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Fe C C C H C C H C H C H C H C H P P C H H H C 1.107057 -0.020525 -0.007300 0.030112 0.002934 0.005009 -0.013393 0.000614 0.008598 -0.000140 -0.014147 0.002396 -0.034156 0.002322 -0.002793 0.001041 -0.045210 -0.043973 0.000078 -0.000075 0.000442 0.000062 0.000278 301 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 H H H C H H C H H C H H C H H H C H H H C H H C H H C H H H H 0.000043 0.000802 0.000188 -0.002335 0.000578 0.000021 -0.001156 -0.000991 -0.000082 0.010649 0.003180 -0.000727 0.001213 -0.000028 0.000390 -0.000062 0.000236 0.000004 0.000174 -0.000123 0.004419 0.000103 -0.001413 0.008216 0.000470 -0.000183 -0.000183 -0.001390 0.000085 -0.000061 -0.001270 aiso(1H) = 0.087 MHz aiso(1H) = 1.80 MHz Table C.0.8. Mulliken atomic spin densities calculated for [Fe(6C6H5CH3)(dippe)][BArF4] ([5][BArF4]). The red italicized atoms correspond to the toluene hydrogen atoms with the value of aiso(1H) provided to its right. 1 2 3 4 5 6 7 Fe P P C H C H 1.109224 -0.051012 -0.045018 0.008734 0.003451 -0.000236 0.000277 aiso(1H) = 4.90 MHz 302 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 H H C C H C H C H H H C H C H C H C H H H C H C H H H C H C H H H C H H C H H H C H H 0.000530 0.000063 0.001060 0.012531 0.000427 -0.027437 0.001502 0.000183 -0.000005 0.000301 0.000008 0.009207 -0.000350 -0.003332 0.001605 -0.022314 0.001202 0.000830 0.000004 -0.000220 -0.000595 -0.009873 0.001819 0.000214 -0.000003 -0.000140 0.000312 0.005435 -0.001251 -0.000346 0.000101 0.000873 0.000020 -0.001815 0.000681 -0.000093 0.002110 -0.000682 -0.000168 0.000017 0.002660 -0.000541 0.001438 aiso(1H) = 2.13 MHz aiso(1H) = 2.28 MHz aiso(1H) = 1.71 MHz aiso(1H) = 2.58 MHz aiso(1H) = -0.77 MHz aiso(1H) = 2.04 MHz 303 51 52 53 54 55 56 57 58 59 60 61 62 63 H C H C H H C H H H C H H -0.000513 0.002485 0.000163 -0.002381 -0.000963 0.000033 0.000474 0.000526 -0.000122 -0.000004 -0.000895 0.000069 -0.000299 64 H 0.000040 aiso(1H) = 0.73 MHz [4]+ [3]+ H P C Figure C57 Spin density map of gas-phase optimized structures (isovalue: 0.004 e-/Å3; TPSS functional: def2tzvp (Fe), def2svp (all other atoms). 304 Appendix D : Supporting Information for Chapter 5 305 Experimental Section General Considerations All experiments were carried out employing standard Schlenk techniques under an atmosphere of dry nitrogen or argon employing degassed, dried solvents in a solvent purification system supplied by SG Water, LLC. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran to confirm effective moisture removal. FeII(η5-Cp*)(dppe)X (X = Cl, H, CH3, OTf) were prepared according to a literature procedure. All other reagents were purchased from commercial vendors and used without further purification unless otherwise stated. Nuclear Magnetic Resonance Spectroscopy 1 H and 13C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. 31P chemical shifts are reported in ppm and referenced externally to 85% aqueous H3PO4 at 0 ppm. 57 Fe Mössbauer Spectroscopy Mössbauer spectra were recorded on a spectrometer from SEE Co. (Edina, MN) operating in the constant acceleration mode in transmission geometry. The sample was kept in an SVT-400 cryostat form Janis (Wilmington, MA), using liquid N2 as a cryogen for 80 K measurements. 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 solid material into a fine powder and then mounted in to a Delrin cup fitted with a screw cap as a boron nitride pellet. Solution samples were transferred to a sample cup and chilled to 77 K inside of the glovebox, and quickly removed from the glovebox and immersed in liquid N2 until mounted in the cryostat. Data analysis was performed using WMOSS version 4 (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes.1 Infrared Spectroscopy Solid and thin film IR measurements were obtained on a Bruker Alpha spectrometer equipped with a diamond ATR probe. UV-VIS Spectroscopy UV-Visible spectroscopy measurements were collected with a Cary 50 UV-Vis spectrophotometer using a 1 cm two-window quartz cell. 307 EPR Spectroscopy Continuous wave X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-9 mM solutions prepared as frozen glasses in 2-MeTHF. Pulse EPR spectroscopy: All pulse X-band (9.4-9.7 GHz) EPR, electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments were acquired using a Bruker ELEXSYS E580 pulse EPR spectrometer. X-band ENDOR experiments were performed using a Bruker MD-4 X-band ENDOR resonator, and X-band HYSCORE experiments were performed using a Bruker MS-5 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. All pulse X-band (ν ≈ 9.4-9.7 GHz) EPR and electron nuclear double resonance (ENDOR) experiments were aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker MD-4 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC. Pulse X-band ENDOR was acquired using the Davies pulse sequence (π − TRF − πRF − TRF − π/2 – τ – π – echo), where TRF is the delay between mw pulses and RF pulses, πRF is the length of the RF pulse and the RF frequency is randomly sampled during each pulse sequence. X-band HYSCORE spectra were acquired using the 4-pulse sequence (π/2 − τ − π/2 − t1 − π –t 2 – π/2 – echo), where τ is a fixed delay, while t1 and t 2 are independently incremented by Δt1 and Δt 2 , respectively. The time domain data was baseline-corrected 308 (third-order polynomial) to eliminate the exponential decay in the echo intensity, apodized with a Hamming window function, zero-filled to eight-fold points, and fast Fouriertransformed to yield the 2-dimensional frequency domain. For 2H-1H difference spectra, the time domain of the HYSCORE spectrum of the 1H sample was subtracted from that of the 2H sample, and the same data processing procedure detailed above was used to generate the frequency spectrum. In general, the ENDOR spectrum for a given nucleus with spin I= ½ (1H, 31P) coupled to the S = ½ electron spin exhibits a doublet at frequencies A ν± = | ± νN | 2 (1) Where νN is the nuclear Larmor frequency and A is the hyperfine coupling. For nuclei with I ≥ 1 (2H), an additonal splitting of the ν± manifolds is produced by the nuclear quadrupole interaction (P) ν±,mI = | νN ± 3P(2mI − 1) | 2 (2) In HYSCORE spectra, these signals manifest as cross-peaks or ridges in the 2-D frequency spectrum which are generally symmetric about the diagonal of a given quadrant. This technique allows hyperfine levels corresponding to the same electron-nuclear submanifold to be differentiated, as well as separating features from hyperfine couplings in the weakcoupling 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 displayed in literature. 309 For systems with appreciable hyperfine anisotropy in frozen solutions or solids, HYSCORE spectra typically do not exhibit sharp cross peaks, but show ridges that represent the sum of cross peaks from selected orientations within the excitation bandwidth of the MW pulses at the magnetic field position at which the spectrum is collected. The length and curvature of these correlation ridges can allow for the separation and estimation of the magnitude of the isotropic and dipolar components of the hyperfine tensor, as shown in Fig. S1. 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 (aiso ) and dipolar (T) contributions. Blue correlation ridges represent the strong coupling case; red correlation ridges represent the weak coupling case. 310 EPR Simulations. Simulations of all CW and pulse EPR data were achieved using the EasySpin2 simulation toolbox (release 5.2.21) with Matlab 2018b using the following Hamiltonian: ̂ = μB ⃑B0 gŜ + μN g N ⃑B0 Î + hŜ ∙ 𝐀 ∙ Î + hÎ ∙ 𝐏 ∙ Î H (3) In this expression, the first term corresponds to the electron Zeeman interaction term where μB is the Bohr magneton, g is the electron spin g-value matrix with principle components g = [gxx gyy gzz], and Ŝ is the electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where μN is the nuclear magneton, g N is the characteristic nuclear g-value for each nucleus (e.g. 1H, 2H ,31P) and Î 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 parameterized by the quadrupole coupling constant e2 Qq/h and the asymmetry parameter η such that: Pxx 𝐏= (0 0 where e2 Qq h 0 Pyy 0 0 e2 Qq/h −(1 − η) 0)= ( 0 4I(2I − 1) Pzz 0 = 2I(2I − 1)Pzz and η = Pxx −Pyy Pzz 0 0 −(1 + η) 0) 0 2 (4) . The asymmetry parameter may have values between 0 and 1, with 0 corresponding to an electric field gradient with axial symmetry and 1 corresponding to a fully rhombic efg. 311 The orientations between the hyperfine and NQI tensor principle axis systems and the gmatrix reference frame are defined by the Euler angles (α, β, γ). Electrochemistry Electrochemical measurements were carried out using a CD instruments 600B electrochemical analyzer. A freshly polished glassy carbon electrode was used as the working electrode and a graphite rod was used as the auxiliary electrode. Solutions (THF) of electrolyte (0.4 M tetra-n-butylammonium hexafluorophosphate) contained ferrocene (0.1 mM), to serve as an internal reference, and analyte (0.2 mM). All reported potentials are referenced to the ferrocene/ferrocenium couple, [Cp2Fe]+ /Cp2Fe. 312 Synthetic Procedures [FeIII(η5-Cp*)(dppe)H]BArF4 ([1]BArF4: C68H52BF24FeP2; 1453.3 gmol-1): To a solution of FeII(η5-Cp*)(dppe)H (40 mg, 0.07 mmol, 1 equiv.) in Et2O (2 mL) at -78 °C was added an Et2O (2 mL) solution of [Fc]BArF4 (60 mg, 0.06 mmol, 0.85 equiv.). Following addition, the resulting dark orange mixture was stirred at 25 °C over 10 min giving a dark red-orange solution. Removal of volatiles in-vacuo and washing with pentane gave [1]BArF4 as a red solid (72 mg, 74%). N.B. The PF6- salt has been prepared previously, though 1H NMR data was not provided.3 1H NMR (THF-d8, 400 MHz, 298 K):δ = 54.12 (br), 26.46 (br), 9.88 (br), 8.71 (br), 7.80 (BArF4), 7.58 (BArF4), 7.40 (br), 5.61 (br), -0.15 (br), -2.51 (br), -9.00 (br). Fe(endo η4-Cp*H)(dppe)(CO) (endo-3): Prepared as previously reported.4 The solidstate molecular structure was not reported and is shown in the crystallography section. UVVIS (THF, nm {cm-1 M-1): 450 {1130}. IR (ATR, C6D6 film): 1864 cm-1 (νCO). [Fe(endo η4-Cp*H)(dppe)(CO)]BArF4 (endo-[3]BArF4): To a solution of [1]BArF4 (10 mg, 0.007 mmol) in 2-MeTHF (2 mL) at -78 °C was added CO (~1 atm) in a J. Young EPR tube, giving a green solution. CW X-band EPR spectroscopy evidenced complete consumption of the FeIII-H starting material. UV-VIS (THF, nm {cm-1 M-1): 891 {252}, 712 {425}, 459 {870}, 383 {1530}. EPR Parameters (30 K, 2-MeTHF, 9.717 GHz): g = [2.085, 2.039, 2.004]; A(31P1) = [72, 59, 58] MHz; A(31P2) = [49, 42, 51] MHz; A(1H) = ± [24, 20, 34.5] MHz. Fe(exo η4-Cp*H)(dppe)(CO) (exo-3): To a solution of [Fe(η5-Cp*)(dppe)CO]OTf (229.9 mg, 0.3 mmol, 1 equiv.) in Et2O (20 mL) at -78 °C was added drop-wise a solution of 1 M LiBEt3H in Et2O (0.3 mL, 0.3 mmol, 1 equiv. Following addition, the resulting mixture 313 was stirred at 25 °C over 10 min giving a clear yellow solution. Volatiles were removed in-vacuo and the sample was extracted with 200 mL pentane, and filtered over celite. Removal of pentane in-vacuo yields exo-3 as a yellow solid (158 mg, 85%). X-ray quality crystals are formed by cooling down a concentrated pentane solution of exo-3 to -35 °C. 3 1H NMR (400 MHz, C D ): δ= 7.91 (t, 3J 6 6 H,H = 8.2 Hz, 4H), 7.33 (h, JH,H = 3.8, 2.9 Hz, 4H), 7.27 – 7.18 (m, 4H), 7.14 – 7.03 (m, 8H), 3.04 (q, 3JH,H = 6.8 Hz, 1H), 2.23 – 2.01 (m, 2H), 1.83 (q, 3JH,H = 2.2 Hz, 9H), 1.65 (td, 3JH,H = 15.7, 12.7, 6.8 Hz, 2H), 0.97 (s, 6H), 13C NMR (101 MHz, THF-d8): δ = 139.7 – 138.8 (m), 138.3 (d, J = 16.8 Hz), 133.8 (t, J = 6.0 Hz), 133.1 (d, J = 5.3 Hz), 128.6 (d, J = 27.6 Hz), 127.3 (dt, J = 11.6, 4.1 Hz), 92.3, 60.8, 58.1, 31.0 – 29.9 (m), 13.1, 11.3. 31P NMR (162 MHz, C6D6): δ= 85.3. UV-VIS (THF, nm {cm-1 M-1): 441 {1850}. IR (ATR, C6D6 film): 2711 cm-1 (νC–H), 2612 cm-1 (νC–H), 1854 cm-1 (νCO). [Fe(exo-η4-Cp*H)(dppe)(CO)]BArF4 (exo-[3]BArF4): In an 4 mm EPR tube, a frozen solution of exo-3 (1.3 mg, 0.002 mmol) in 2-MeTHF (0.25 mL) was layered with a frozen solution of [Fc]BArF4 (2.2 mg, 0.002 mmol, 1 equiv) in 0.25 mL 2-MeTHF. The two frozen solutions where slow thawed and stirred with a needle by taking the tube out of a cold well cooled with liquid nitrogen. Upon mixing, a color change to green can be observed. CW X-band EPR spectroscopy evidenced complete consumption of the FeIII-H starting material. UV-VIS (THF, nm {cm-1 M-1): 923 {130}, 767 {180}, 441 {1645}. EPR Parameters (30 K, 2-MeTHF, 9.717 GHz): g = [2.116, 2.073, 1.997]; A(31P1) = [96, 88, 47] MHz; A(31P2) = [78, 75, 63] MHz; A(1H) = ± [85, 84, 83] MHz, HStrain = [70, 22, 22] MHz for conformer A (0.6 weight) and g = [2.093, 2.045, 2.013]; A(31P1) = [46, 44, 15] 314 MHz; A(31P2) = [70, 64, 64] MHz; A(1H) = ± [76, 74, 70] MHz, HStrain = [70, 22, 22] MHz for conformer B (0.4 weight). [Fe(endo-η4-Cp*H)(dppe)(CNXyl)]BArF4 (endo-[4]BArF4): To a solution of [1]BArF4 (10 mg, 0.007 mmol) in 2-MeTHF (2 mL) at -78 °C was added CNXyl (~1 mg, 0.008 mmol, ~1.1 equiv.), giving a green solution. CW X-band EPR spectroscopy evidenced complete consumption of the FeIII-H starting material. UV-VIS (THF, nm): 828. EPR Parameters (20 K, 2-MeTHF, 9.716 GHz): g = [2.132, 2.042, 2.004]; A(31P1) = [75, 35, 54] MHz; A(31P2) = [76, 64, 64] MHz; A(1H) = ± [17.0, 22.0, 32.5] MHz; A(14N) = [7.4, 7.4, 9] MHz. [Fe(η5-Cp*)(dppe)CO]BArF4 ([5]BArF4): This molecule and H2 cleanly result (>99%) by annealing of solutions of exo- or endo-[3]+ to room temperature. Characterization data is consistent with that reported in ref. 6. [Fe(η5-Cp*)(dppe)CNXyl]BArF4 ([6]BArF4): To a solution of FeII(η5-Cp*)(dppe)H (9.5 mg, 0.016 mmol, 1 equiv.) and CNXyl (2.1 mg, 0.016 mmol, 1 equiv.) in Et2O (2 mL) at 78 °C was added drop-wise a chilled (-78 °C) solution of [H(OEt)2]BArF4 (16.3 mg, 0.016 mmol, 1 equiv.) in Et2O (1 mL). Following addition, the resulting mixture was stirred at 25 °C over 10 min giving a clear yellow solution. Removal of volatiles in-vacuo and washing with pentane gave [6]BArF4 as a yellow solid (20 mg, 80%). 1H NMR (THF-d8, 400 MHz, 298 K):δ = 7.79 (s, 8H; BArF4), 7.57 (s, 4H; BArF4), 7.55 (m, 10H; Ph), 7.39 (m, 10H; Ph), 7.08 (t, 3JH,H = 7.2 Hz, 1H; CNXyl), 7.00 (d, 3JH,H = 7.2 Hz, 2H; CNXyl), 2.67 (m, 2H; CH2), 2.45 (m, 2H; CH2), 1.63 (s, 6H; CNXyl), 1.55 (s, 15H; Cp*). 31P{1H} NMR (THF-d8, 162 MHz, 298 K):δ = 94.0. 13C NMR (THF-d8, 100 MHz, 298 K): δ = 162.5 (q, 1JC,B = 37 Hz, BArF4, ipso quaternary C), 135.6 (BArF4, ortho C), 135.0, 134.6, 133.7, 131.9, 129.9, 129.9 (q, 2JC,F = 31 Hz, BArF4, meta quaternary C), 129.6, 129.4, 129.2, 315 129.1, 125.4 (q, 1JC,F = 273 Hz, BArF4, CF3), 117.9 (m, BArF4, para C), 93.6 (Cp*), 30.5 (CH2; dppe), 18.8, 10.1 (Cp*). IR (THF film): 2050 cm-1 (νC≡N). 57Fe Mössbauer (80 K, Et2O solution, mm/s): δ = 0.16, ΔEQ = 1.75. [Fe(η5-Cp*)(dppe)N2]BArF4 ([7]BArF4): To a solution of Fe(η5-Cp*)(dppe)CH3Error! Bookmark not defined. (21.8 mg, 0.036 mmol, 1 equiv.) in Et O (2 mL) at -78 °C was added drop2 wise a chilled (-78 °C) solution of [H(OEt)2]BArF4 (36.5 mg, 0.036 mmol, 1 equiv.) in Et2O (1 mL). Following addition, the resulting mixture was stirred at 25 °C over 10 min giving a clear yellow solution. Removal of volatiles in-vacuo and washing with pentane gave [7]BArF4 as a yellow solid (53 mg, 92%). [7]BArF4 is also the product of H2 release by [1]BArF4 in THF (< 5% yield after 1 week, + 80 oC, THF-d8). 1H NMR (THF-d8, 400 MHz, 298 K): δ= 7.79 (s, 8H; BArF4), 7.57 (s, 4H; BArF4), 7.75-7.54 (m, 16H; Ph), 7.44 (m, 4H; Ph), 2.54 (m, 2H; CH2), 2.38 (m, 2H; CH2), 1.43 (s, 15H; Cp*). 31P{1H} NMR (THF-d8, 162 MHz, 298 K): δ= 86.6. 13C NMR (THF-d8, 100 MHz, 298 K): δ= 162.5 (q, 1JC,B = 37 Hz, BArF4, ipso quaternary C), 135.6 (BArF4, ortho C), 135.4, 134.3, 133.2, 132.4, 132.2, 129.9 (q, 2JC,F = 31 Hz, BArF4, meta quaternary C), 125.4 (q, 1JC,F = 273 Hz, BArF4, CF3), 118.0 (m, BArF4, para C), 117.0, 92.7 (Cp*), 28.8 (CH2; dppe), 9.2 (Cp*). IR (THF film): 2119 cm-1 (νNN). 7.79 7.57 7.39 7.30 7.27 3.63 3.61 3.58 3.41 3.39 3.38 3.36 1.79 1.77 1.73 1.34 1.32 1.31 1.29 1.28 1.13 1.11 1.10 0.90 0.89 0.87 27.44 24.87 9.68 8.58 7.79 7.57 7.39 7.30 7.27 7.20 7.19 7.13 5.69 5.64 4.11 3.63 3.61 3.58 3.41 3.39 3.38 3.36 2.51 2.31 1.79 1.77 1.73 1.34 1.32 1.31 1.29 1.28 1.25 1.13 1.11 1.10 0.90 0.89 0.87 -1.91 -8.07 -8.90 316 Spectroscopic Data 13 60 55 12 11 50 10 9 45 8 7 40 6 5 f1 (ppm) 4 35 30 3 2 1 25 20 f1 (ppm) 0 -1 -2 15 10 Figure D.2 [1]BArF4, 1H NMR, THF-d8, 400 MHz, 298 K 5 0 -5 -10 -15 317 Figure D.3 [1]BArF4, FT-IR ATR, thin film, 298 K (νFeH = 1874 cm-1) 318 * Figure D.4 endo-3, 1H NMR, C6D6, 400 MHz, 298 K. The data match that previously reported.5 (* = C6D6) Figure D.5 endo-3-H/D stacked plot, 1H NMR, C6D6, 400 MHz, 298 K showing disappearance of a signal at δH = 2.65. 319 Figure D.6 endo-3, 31P{1H} NMR, C6D6, 162 MHz, 298 K. The data match that previously reported.5 Figure D.7. endo-3-H/D, FT-IR ATR, thin film, 298 K 320 * Figure D9. exo-3-H/D stacked plot, 1H NMR, C6D6, 400 MHz, 298 K showing disappearance of a signal at H = 3.04. Figure D.8. exo-3, 1H NMR, C6D6, 400 MHz, 298 K (* = C6D6) Figure D.9. exo-3-H/D stacked plot, 1H NMR, C6D6, 400 MHz, 298 K showing disappearance of a signal at δH = 2.65. 321 Figure D.10. exo-3, 31P{1H} NMR, C6D6, 162 MHz, 298 K * * Figure D.11. exo-3, 13C{1H} NMR, THF-d8, 100 MHz, 298 K (* = THF-d8) 322 Figure D.13. exo-3-H/D, FT-IR ATR, thin film, 298 K (exo-3-H: 2711 and 2612 cm-1 and exo-3-D: 2009 and 1955 cm-1). Figure D.12. [5]BArF4, 1H NMR, THF-d8, 400 MHz, 298 K (this compound is formed from H2 evolution from 0.5 equiv. exo-/endo-[3]+ - signals match those previously provided in ref. 1). (* = THF-d8) 7.80 7.79 7.79 7.63 7.61 7.59 7.58 7.56 7.54 7.52 7.51 7.43 7.41 7.40 7.38 7.36 7.31 7.11 7.09 7.07 7.02 7.00 3.62 3.58 3.40 3.38 2.69 2.67 2.65 2.45 2.41 2.19 2.06 1.79 1.78 1.73 1.64 1.56 1.42 1.40 1.33 1.30 1.14 1.12 1.10 0.91 0.89 0.87 323 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.0 5.5 5.0 f1 (ppm) 4.5 4.0 3.5 3.0 3.06 2.96 6.5 5.53 13.17 * 8.00 14.98 7.54 1.41 2.26 * 2.5 2.0 1.5 93.99 Figure D.14. [6]BArF4, 1H NMR, THF-d8, 400 MHz, 298 K (* = THF-d8) F 31 190 180 170 160 150 140 4 130 90 80 THF-d 70 60 8,50162 40 MHz, 30 20 298 10 0K -10 -20 -30 -40 Figure D.15. [6]BAr , 120 P{1110H}100NMR, f1 (ppm) 1.0 190 (ppm) Figure D.16. [6]BArF4, 13C{1H} NMR, f1THF-d 8, 100 MHz, 298 K (* = THF-d8) 180 170 160 150 140 130 120 110 100 90 80 70 60 30.54 25.66 thf 25.53 thf 25.46 thf 25.33 thf 25.26 thf 25.13 thf 24.93 thf 24.73 18.75 15.49 14.18 10.10 135.40 135.00 134.62 134.57 133.77 133.72 131.98 131.86 129.98 129.56 129.39 129.28 129.23 129.11 128.12 126.68 123.97 93.58 67.78 thf 67.65 thf 67.43 thf 67.21 thf 66.99 66.77 162.88 161.88 324 * * 50 40 30 20 10 0 325 Figure D.17 [6]BArF4, FT-IR ATR, thin film, 298 K (νCN = 2050 cm-1) Figure D.18. CNXyl, FT-IR ATR, thin film, 298 K (νCN = 2151 cm-1) 326 H2 Evolution by [1]BArF4 in THF: To a J-Young NMR tube containing THF-d8 (606 mg) was added [1]BArF4 (4.9 mg, 0.003 mmol). The reaction mixture was monitored by NMR spectroscopy, showing no observable reaction. However, heating the reaction mixture at 80 oC for 24 h results in minimal (<5 % deterioration) to give [FeII( 5 -Cp*)(dppe)(N2)]BArF4 ([7]BArF4) and H2. [2]BArF4 is cleanly accessible on preparative scale by protonation of FeII( 5 - Cp*)(dppe)CH3 using [H(OEt)2]BArF4 (as outlined above). 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 f1 (ppm) Figure D.19. Heating [1]BArF4 at 80 oC, 1H NMR, THF-d8, 400 MHz, 298 K (Red; 1 h, green; 5 h, blue; 23 h); dppe(CH2) and Cp*(CH3) groups for [7]BArF4 highlighted in pink. 327 95 90 85 80 75 70 65 60 55 50 45 40 f1 (ppm) 35 30 25 20 15 10 5 0 -5 -10 7.79 7.78 7.78 7.74 7.71 7.70 7.69 7.68 7.67 7.67 7.65 7.63 7.61 7.59 7.57 7.46 7.45 7.44 7.42 7.30 3.63 3.62 3.58 3.41 3.39 3.38 3.36 2.39 1.79 1.78 1.78 1.73 1.50 1.43 1.35 1.34 1.33 1.31 1.31 1.29 1.28 1.25 1.13 1.12 1.10 0.91 0.89 0.87 Figure D.20. Heating [1]BArF4 at 80 oC, 31P{1H} NMR, THF-d8, 162 MHz, 298 K (Red; 1 h, green; 5 h, blue; 23 h) (free ligand at = -12.8 ppm). 9.0 8.5 8.0 7.5 4.82 15.00 * 9.03 20.95 4.77 * 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Figure D.21. [7]BArF4, 1H NMR, THF-df18(ppm) , 400 MHz, 298 K (* = THF-d8) 0.5 0.0 86.63 328 180 170 160 150 140 130 120 110 100 90 f1 (ppm) 80 70 60 50 40 30 20 67.65 thf 67.43 thf 67.21 thf 66.99 66.77 92.66 135.41 134.26 133.22 132.41 132.17 129.93 129.67 129.38 126.68 123.96 118.01 163.37 162.88 162.38 161.88 Figure D.22. [7]BArF4, 31P{1H} NMR, THF-d8, 162 MHz, 298 K 10 * 200 190 180 170 160 150 140 130 120 110 100 f1 (ppm) 90 80 70 0 -10 29.05 28.84 28.63 25.52 thf 25.33 thf 25.12 thf 24.93 thf 24.73 9.22 190 * 60 50 40 30 20 Figure D.23. [7]BArF4, 13C{1H} NMR, THF-d8, 100 MHz, 298 K (* = THF-d8) 10 0 329 Figure D.24. [7]BArF4, FT-IR ATR, thin film, 298 K ( NN = 2119 cm -1 ) 330 H2 evolution by [1]BArF4 in MeCN: To a J-Young NMR tube containing MeCN-d3 (646 mg) was added [1]BArF4 (5.0 mg, 0.003 mmol). The reaction mixture was monitored by NMR spectroscopy, showing consumption of [1]BArF4 to cleanly give [FeII(η5-Cp*)(dppe)(NCMe)]BArF4 ([8]BArF4) 7.70 7.70 7.69 7.68 7.68 7.67 7.61 7.60 7.60 7.59 7.58 7.57 7.57 7.56 7.56 7.55 7.55 7.54 7.54 7.53 7.52 7.46 7.46 7.45 7.45 7.44 7.44 7.43 7.43 7.42 7.37 4.16 3.64 3.43 3.41 1.95 1.95 1.94 1.93 1.93 1.80 1.79 1.30 1.14 1.12 1.10 0.89 0.88 and H2.5 4.57 * 4.65 4.60 4.55 f1 (ppm) 4.50 4.45 7.0 6.5 δ = 4.57 ppm).8.0 (* =7.5ACD-d 3) 15.00 4.24 12.68 14.71 3.90 Figure D.25. [8]BArF4, 1H NMR, ACN-d3, 400 MHz, 298 K (inset shows signal for generated H2 at 6.0 5.5 5.0 4.5 4.0 f1 (ppm) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 91.22 331 160 150 140 130 120 110 100 90 80 70 f1 (ppm) 60 50 40 Figure D.26. [8]BArF4, 31P{1H} NMR, ACN-d3, 162 MHz, 298 K 30 20 10 0 -10 332 3.3 Hydricity measurement: Hydride transfer to CO2: To an NMR J-Young tube containing a solution of endo-3 (3.9 mg, 0.006 mmol, 1 equiv.) in d3-acetonitrile (ca. 0.6 mL) was added CO2. The reaction mixture was monitored by NMR spectroscopy, showing consumption of endo-3 to give [5]+ and HCO2-. 333 Figure D.27. 31P{1H} NMR, C6D6, 162 MHz, 298 K for the treatment of endo-3 with CO2 before addition, 5 days, 1 week, and 3 weeks post addition showing consumption of endo-3 (δP = 84.5 ppm) and formation of [5]+ (δP = 89.8 ppm) 334 Figure D.28. 1H NMR, C6D6, 162 MHz, 298 K for the treatment of endo-3 with CO2 before addition, 5 days, 1 week, and 3 weeks post addition showing formation of HCO2- at δH = 8.6 ppm) 335 3.4 Azobenzene reduction: Using [1]+: To a J-Young NMR tube containing a solution of [1]+ (14.5 mg, 1 mmol) in d8-THF (ca. 500 μL) was added PhNNPh (36 mg, 20 mmol). The reaction mixture was monitored by NMR spectroscopy, showing no reaction. Azobenzene reduction by addition of CO: On top of a 0.5 mL of a frozen diethyl ether solution containing 2 μmol azobenzene and 0.1 μmol of endo/exo-[3]+ was layered 0.25 mL of a 0.4 mM FcBArF solution in diethyl ether. The reaction mixture was stirred at −78 °C for 30 minutes followed by 10 minutes at room temperature. The solvent was removed in vacuo and the residue was dissolved in 0.6 mL C6D6 with trimethylbenzene as internal standard and analyzed by NMR. General procedure for azobenzene reduction by oxidation: On top of a 0.5 mL of a frozen diethyl ether solution containing 2 μmol azobenzene and 0.1 μmol of endo/exo-[3]+ was layered 0.25 mL of a 0.4 mM FcBArF solution in diethyl ether. The reaction mixture was stirred at −78 °C for 30 minutes followed by 10 minutes at room temperature. The solvent was removed in vacuo and the residue was dissolved in 0.6 mL C6D6 with trimethylbenzene as internal standard and analyzed by NMR. 336 Table D.1. Summary of azobenzene reduction yields based on transferred H-atom equivalents Fe complex Yield [1]+ no reaction endo-[3]+ 25 % exo-[3]+ 78 % 337 endo-[3]+ + PhNNPh + spiked PhNHNHPh endo-[3]+ + PhNNPh PhNHNHPh Figure D.29. 1H NMR, C6D6, 400 MHz, 298 K showing the aromatic region following treatment of endo-[3]+ with azobenzene to give [5]+ and PhNHNHPh. Bottom trace: PhNNPh; Middle trace: oxidation of endo-3 using [Fc]BArF4 to give endo-[3]+ in the presence of azobenzene, giving PhNHNHPh, and Top trace, the reaction mixture spiked with authentic PhNHNHPh. 338 Figure D.30. 31P{1H} NMR, C6D6, 162 MHz, 298 K following treatment of endo-[3]+ with azobenzene to give [5]+. 339 3.5 57Fe Mössbauer Spectroscopy: Figure D.31. [6]BArF4, 80 K 57Fe Mössbauer spectrum collected in the presence of a 50 mT magnetic field oriented parallel to the propagation of the γ-beam (frozen solution in Et2O). δ = 0.16 mm/s, ΔEQ = 1.75 mm/s for major species. ΓL = ΓR = 0.50 mm/s. δ= 0.16 and ΔEQ = 1.75 (81%) δ = 0.33 and ΔEQ = 0.84 (17%) δ = 0.24 and ΔEQ = 0.23 (3%) 340 UV-Visible Spectroscopy: Figure D.32. [1]BArF4, UV-Visible spectrum showing stability over 24 h, 2-MeTHF, 298 K Figure D.33. endo-3 and exo-3, UV-Visible spectrum (2-MeTHF, 298 K, 1 cm cell). endo-3:δ = 450 {1130}, exo-3: δ = 441 {1850}. 341 Figure D.34. endo-[3]+ and exo-[3]+, UV-Visible spectrum (2-MeTHF, 218 K, 1 cm cell). endo-[3]+: ε = 891 {252}, 712 {425}, 459 {870}, 383 {1530}, exo-[3]+: ε = 923 {130}, 767, {180}, 441 {1645}. 342 EPR Spectroscopy CW-EPR Figure D.35. CW X-band EPR data for [1]+ (2-MeTHF, 77 K; MW frequency = 9.36 GHz; MW power = 20 μW; modulation amplitude = 10 mT; conversion time = 10.24 ms). Simulation parameters: g = [2.352, 2.041, 1.992]; A(31P1) = [88, 82, 79] MHz; A(31P2) = [82, 71, 76] MHz. For details, see: Drover, M. W.; Schild, D. J.; Oyala, P. H.; Peters, J. C. Angew. Chem. Int. Ed. 2019, 58, 15504. 343 [3-H]+ H Ph Ph P + FeI P Ph CO Ph endo-[3]+ [3-D]+ simulation data Figure D.36. CW X-band EPR data for endo-[3-H/D]+ (2-MeTHF, 77 K; MW frequency = 9.37 GHz; MW power = 2 mW; modulation amplitude = 10.0 mT; conversion time = 5.12 ms). Parameters: g = [2.085, 2.039, 2.004]; A(31P1) = [72, 59, 58] MHz, A(31P2) = [49, 42, 51] MHz. 344 H Ph Ph P ** + [4-H]+ FeI P Ph CNXyl Ph endo-[4]+ ** [4-D]+ simulation data Figure D.37. CW X-band EPR data for endo-[4-H/D]+ (2-MeTHF, 77 K; MW frequency = 9.37 GHz; MW power = 2 mW; modulation amplitude = 10 mT; conversion time = 10.24 ms). Parameters: g = [2.132, 2.042, 2.004]; A(31P1) = [75, 35, 54] MHz, A(31P2) = [76, 64, 64] MHz. ** = traces of [1]+. 345 H + Ph Ph P Fe I P Ph CO Ph exo-[3]+ Figure D.38. CW X-band EPR data for exo-[3-H]+ and exo-[3-D]+ (2-MeTHF, 77 K, 9.33 GHz; MW power = 6.44 mW; modulation amplitude = 2.0 mT; conversion time = 20.48 ms). Parameters: g = [2.116, 2.073, 1.997]; A(31P1) = [96, 88, 47] MHz; A(31P2) = [78, 75, 63] MHz; A(1H) = ± [85, 84, 83] MHz, HStrain = [70, 22, 22] MHz for conformer A (0.6 weight) and g = [2.093, 2.045, 2.013]; A(31P1) = [46, 44, 15] MHz; A(31P2) = [70, 64, 64] MHz; A(1H) = ± [76, 74, 70] MHz, HStrain = [70, 22, 22] MHz for conformer B (0.4 weight). 346 Figure D.39. EPR sample of [4-H]+ frozen at 77 K. 347 3.7.2 X-band Davies ENDOR: Figure D.40. Field-dependent X-band 31P Davies ENDOR of endo-[3-D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31P1, (green), 31 P2 (blue)). Simulation parameters: g = [2.085, 2.039, 2.004]; A(31P1) = [72, 59, 58] MHz, A(31P2) = [49, 42, 51] MHz. Acquisition parameters: temperature = 30 K; MW frequency = 9.717 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; tau = 200 ns; RF pulse length = 15 µs; shot repetition time = 4 ms. 348 Figure D.41. Field-dependent X-band 31P Davies ENDOR of exo-[3-D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31P1 conformer A, (dark blue), 31P2 conformer A (turquoise), 31P1 conformer B, (forest green), 31P2 conformer B (lime green). Simulation parameters for conformer A: g = [2.116, 2.073, 1.997]; A(31P1) = [96, 88, 47] MHz, A(31P2) = [78, 75, 63] MHz. Simulation parameters for conformer B: g = [2.093, 2.045, 2.013]; A(31P1) = [46, 44, 15] MHz, A(31P2) = [70, 64, 64] MHz. Acquisition parameters: temperature = 20 K; MW frequency = 9.734 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; tau = 200 ns; RF pulse length = 15 µs; shot repetition time = 4 ms. 349 Figure D.42. Field-dependent X-band Davies ENDOR spectra of exo-[3-H]+ (black) and exo-[3-D]+ (black). Acquisition parameters: temperature = 20 K; microwave frequency = 9.734 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; τ = 200; RF pulse length πRF = 15 µs; TRF = 2 µs; shot repetition time (srt) = 4 ms. Asterisk at ~ 4 MHz indicates an RF artifact. 350 Figure D.43. Field-dependent X-band 31P Davies ENDOR of endo-[4-D]+ (black), with simulations of 31P hyperfine couplings overlaid (total 31P simulation (red), 31P1, (green), 31 P2 (blue)). Simulation parameters: g = [2.132, 2.042, 2.004]; A(31P1) = [75, 35, 54] MHz, A(31P2) = [76, 64, 64] MHz. Acquisition parameters: temperature = 20 K; MW frequency = 9.716 GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; tau = 200 ns; RF pulse length = 15 µs; shot repetition time = 4 ms. 351 X-band HYSCORE Figure D.44. Field-dependent X-band HYSCORE spectra of endo-[3-H]+ (top panels) endo-[3-D]+ (middle panels) and 1H-2H difference HYSCORE spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 140 ns (g = 2.079), 138 ns (g = 2.037); 138 ns (g = 2.004); t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 352 Figure D.45. Field-dependent X-band HYSCORE spectra of endo-[4-H]+ (top panels) endo-[4-D]+ (middle panels) and 1H-2H difference HYSCORE spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 144 ns (g = 2.130), 142 ns (g = 2.088); 138 ns (g = 2.048); 138 ns (g = 2.004); t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 353 Figure D.46 Field-dependent X-band HYSCORE spectra of endo-[3-H]+ and endo-[3-D]+ (top panels) and 1H/2H HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; Magnetic field = 334 mT; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 140 ns, t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 354 Figure D.47. Field-dependent X-band HYSCORE spectra of endo-[3-H]+ and endo-[3-D]+ (top panels) and 1H/2H HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; Magnetic field = 340.8 mT; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns, t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 355 Figure D.48. Field-dependent X-band HYSCORE spectra of endo-[3-H]+ and endo-[3-D]+ (top panels) and 1H/2H HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; Magnetic field = 346.5 mT; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns, t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 356 Figure D.49. Field-dependent X-band HYSCORE spectra of exo-[3-D]+ (top panels) and 2 H HYSCORE simulations (bottom panels) of conformations A (blue) and B (green) (bottom panels). Acquisition parameters: temperature = 25 K; microwave frequency = 9.734 GHz; Magnetic field = 330.8 mT (g = 2.102), 340.0 mT (g = 2.045), 346.5 mT (g = 2.007); MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 142 ns (g = 2.102), τ = 138 ns (g = 2.045), τ = 136 ns (g = 2.007); t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 357 Figure D.50. Field-dependent X-band HYSCORE spectra of endo-[4-H]+ and endo-[4-D]+ (top panels) and 1H,14N/2H,14N HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 144 ns; Magnetic Field = 326.0 mT; t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 358 Figure D.51. Field-dependent X-band HYSCORE spectra of endo-[4-H]+ and endo-[4-D]+ (top panels) and 1H,14N/2H,14N HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 142 ns; Magnetic Field = 332.5 mT; t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 359 Figure D.52. Field-dependent X-band HYSCORE spectra of endo-[4-H]+ and endo-[4-D]+ (top panels) and 1H,14N/2H,14N HYSCORE simulation spectra (bottom panels). Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns; Magnetic Field = 339.0 mT; t1 = t2 = 100 ns; Δt1 = Δt2 = 12 ns; shot repetition time (srt) = 2 ms. 360 Figure D.53. Field-dependent X-band HYSCORE spectra of endo-[4-H]+ and endo-[4-D]+ (top panels) and 1H,14N/2H,14N HYSCORE simulation spectra (bottom panels). Intense features in the (+,-) quadrant of the HYSCORE spectrum of endo-[4-D]+ are due to contribution of a small amount of residual starting [1-D]+. Acquisition parameters: temperature = 30 K; microwave frequency = 9.717 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138 ns; Magnetic Field = 346.5 mT; t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 2 ms. 361 3.8 Electrochemistry Figure. 0.54 Cyclic voltammogram of endo-3, showing a reversible Fe0/Fe1+ feature at Eox = -0.81 V (versus Fc/Fc+). 362 Figure D.55. Cyclic voltammogram of exo-3, showing the Fe0/Fe1+ feature become more reversable upon increasing the scan rate from 1 V/s to 100 V/s at Eox = -0.70 V (versus Fc/Fc+). 363 Crystallographic details All crystals were mounted on a glass fiber loop. All measurements were made using graphite-monochromated Mo or Cu Kα radiation (λ = 0.71073 or 1.54178 Å) radiation on a Bruker AXS D8 VENTURE KAPPA diffractometer coupled to a PHOTON 100 CMOS detector. The structures were solved by direct methods6 and refined by full-matrix leastsquares procedures on F2 (SHELXL-2013)6 using the OLEX2 interface.7 All hydrogen atoms were placed in calculated positions. Non-hydrogen atoms were refined anisotropically. endo-[3]BArF4 (L = CO): ISOR (0.001 0.001) was applied to a 2methylcyclohexane moiety (C70-C76). Applications of these constraints improved data statistics. An A-level alert also persists for this molecule: PLAT029_ALERT_3_A _diffrn_measured_fraction_theta_full value Low . 0.938 Why? Author Response: This crystal has many weak reflections at higher values of 2theta (it is weakly diffracting and small). That said, it more than corroborates the connectivity for this species. [6]BArF4 (L = CNXyl): Rotational disorder about two of the -CF3 groups of the BArF4counterion was modeled F1/F2/F3 [46/54] and F7/F8/F9 [51/49]. RIGU was applied to two pentane solvent molecules [C77-C87] and ISOR (0.01 0.01) was applied to F1/F2/F3 and F7/F8/F9. Applications of these constraints improved data statistics. [7]BArF4 (L = N2): Rotational disorder about one of the -CF3 groups of the BArF4counterion was modeled [50/50]. Application of these constraints improved data statistics. CCDC 1943147-1943149 and 2021150-2021151 contains the supplementary 364 crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif Table D.2. Crystallographic data for endo-3 and exo-3. Compound Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z ρ/ g/cm-3 μ mm-1 F(000) Crystal size/ mm3 Radiation 2θ range for data collection/° Index ranges Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 R [I>=2θ (I)] (R1, wR2) R (all data) (R1, wR2) Largest diff. peak/hole / (e Å-3) endo-3 C37H40FeOP2 618.48 100(2) Monoclinic P21/n 12.071(4) 19.165(3) 14.020(4) 90 102.72(3) 90 3164.0(16) 4 1.298 4.983 1304.0 0.12 × 0.11 × 0.09 CuKα (λ = 1.54178) exo-3 C37H40FeOP2 618.48 100(2) Monoclinic P21/n 12.161(5) 19.178(6) 13.920(5) 90 104.70(2) 90 3140.2(19) 4 1.308 5.021 1304.0 0.32 × 0.19 × 0.14 CuKα (λ = 1.54178) 7.942 to 163.416 8.024 to 149.76 -15 ≤ h ≤ 15, -24 ≤ k ≤ 23, -17 ≤ l ≤ 17 58389 6809 [Rint = 0.0727, Rsigma = 0.0358] 6809/0/375 1.049 R1 = 0.0364, wR2 = 0.0859 R1 = 0.0409, wR2 = 0.0880 -13 ≤ h ≤ 15, -23 ≤ k ≤ 23, -17 ≤ l ≤ 17 102570 6420 [Rint = 0.0536, Rsigma = 0.0209] 6420/0/375 1.052 R1 = 0.0272, wR2 = 0.0754 R1 = 0.0276, wR2 = 0.0757 0.55/-0.42 0.28/-0.39 R1 = Σ ||Fo|-|Fc|| / Σ |Fo|; wR2 = [Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]1/2 365 Table D.3. Crystallographic data for endo-[3]BArF4 and [6]BArF4. Compound Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z ρ/ g/cm-3 μ mm-1 F(000) Crystal size/ mm3 Radiation 2θ range for data collection/° Index ranges Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 R [I>=2θ (I)] (R1, wR2) R (all data) (R1, wR2) Largest diff. peak/hole / (e Å-3) endo-[3]BArF4 C76H66BF24FeOP2 1579.88 100(2) Triclinic P-1 14.148(6) 16.393(4) 17.640(7) 113.13(2) 106.52(2) 90.961(16) 3568(2) 2 1.470 3.079 1614.0 0.23 × 0.13 × 0.11 CuKα (λ = 1.54178) [6]BArF4 (L = CNXyl) C87H84BF24FeNP2 1728.15 100(2) Triclinic P-1 13.7115(6) 16.4188(8) 19.4299(9) 86.338(2) 69.671(2) 88.504(2) 4093.3(3) 2 1.402 2.727 1780.0 0.21 × 0.16 × 0.14 CuKα (λ = 1.54178) 5.744 to 161.046 4.858 to 161.348 -18 ≤ h ≤ 16, -20 ≤ k ≤ 14, -20 ≤ l ≤ 21 20853 13512 [Rint = 0.1012, Rsigma = 0.1579] 13512/42/940 1.086 R1 = 0.1289, wR2 = 0.2951 R1 = 0.1857, wR2 = 0.3381 -17 ≤ h ≤ 17, -19 ≤ k ≤ 20, -24 ≤ l ≤ 24 204590 17660 [Rint = 0.0836, Rsigma = 0.0278] 17660/114/1112 1.049 R1 = 0.0693, wR2 = 0.2042 R1 = 0.0710, wR2 = 0.2061 2.13/-0.92 2.12/-0.94 R1 = Σ ||Fo|-|Fc|| / Σ |Fo|; wR2 = [Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]1/2 366 Table D.4. Crystallographic data for [7]BArF4. Compound Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z ρ/ g/cm-3 μ mm-1 F(000) Crystal size/ mm3 Radiation 2θ range for data collection/° Index ranges Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 R [I>=2θ (I)] (R1, wR2) R (all data) (R1, wR2) Largest diff. peak/hole / (e Å-3) [7]BArF4 (L = N2) C68H51BF24FeN2P2 1480.70 100(2) Triclinic P-1 12.139(5) 14.282(3) 19.549(4) 91.970(16) 93.347(19) 106.142(18) 3245.4(15) 2 1.515 3.343 1500.0 0.20 × 0.16 × 0.14 CuKα (λ = 1.54178) 6.452 to 162.704 -15 ≤ h ≤ 15, -18 ≤ k ≤ 18, -24 ≤ l ≤ 24 175771 14063 [Rint = 0.0447, Rsigma = 0.0204] 14063/0/916 1.066 R1 = 0.0465, wR2 = 0.1288 R1 = 0.0510, wR2 = 0.1331 1.12/-0.55 R1 = Σ ||Fo|-|Fc|| / Σ |Fo|; wR2 = [Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]1/2 367 A. B. Figure D.56 . Thermal ellipsoid plot (50%) of A) endo-3 and B) exo-3 (endo-3 had been reported previously5, though the crystal structure was not obtained). 368 Figure D.57. endo-[3]BArF4, Thermal ellipsoid plot (50%). Figure D.58. [6]BArF4, Thermal ellipsoid plot (50%, hydrogens omitted for clarity). 369 Figure D.59. [7]BArF4, Thermal ellipsoid plot (50%, hydrogens omitted for clarity). 370 DFT Calculations General All calculations were performed using the ORCA 4.08,9 program. In cases where crystal structures were available these coordinates were used as the input. The calculations were performed using the TPSS (meta-GGA)10 functional with the def2-SVP basis set was on C and H and the def2-TZVP basis set on Fe.11 To assure that optimized structures represented true stationary points was checked by doing a single-point frequency calculations on the optimized structure. EPR parameters were calculated using, TPPSh, TPSS, B3LYP, M06L and BP86 with for the TPSS-optimized structure were calculated by doing a single point calculation on the optimized structures using CP(PPP)12 on Fe and IGLO-III13 on everything else 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 Fe and def2-TZVP on C and H with Grid7. See below for a summary of the results. BDFEX-H Calculations: Consistent with a previous report, a calibration curve of ΔG vs. BDFElit was employed.14 The free-energy 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. 371 EPR predictions Table D.5. EPR parameters of [1]+ with different functionals and basis sets. Although aiso varies significantly between the methods, T(1H) remains constant and is characteristic for iron hydrides. Functional Basis set 1H A(1H) (MHz) T(1H) MHz) 36.7 [-10, 60, 60] [-46.7, 23.3 23.3] -66.34 [-19.33 -83.31 -85.15] [43.23, -22.55, -20.67] (aiso) (MHz) experimental TPSSh EPR-III (C & H) IGLO-III (P) TPSSh IGLO-III -62.59 [-23.11, -88.89, 87.01] [43.26, -20.72, -22.54] TPSS EPR-III (C & H) -42.33 [-3.85, -62.55, -60.59] [38.48, -20.22, -18.26] IGLO-III (P) TPSS IGLO-III -39.72 [-1.21, -59.92, -58.02] [38.51, -20.20, -18.30] B3LYP IGLO-III -81.68 [-36.01, -105.51, 103.51] [45.67, -23.83, -21.83] BP86 IGLO-III -34.39 [2.68, -54.06, -51.78] [37.07, -19.67, -17.39] M06l IGLO-III -49.08 [-4.69, -72.89, -69.67] [44.39, -23.81, -20.59] 372 Table D.6. EPR parameters of endo-[3]+ with different functionals and basis sets. Although aiso varies significantly between the methods, T(1H) remains constant and the small anisotropy is characteristic for ring functionalized (and not metal-hydride) species. Functional Basis set 1H A(1H) (MHz) T(1H) (MHz) [34, 16, 20] [10.67, -7.33, - (aiso) (MHz) experimental 23.33 3.33] TPSSh IGLO-III 25.16 [33.11, 19.93, 22.43] [7.95, -5.23, -2.73] TPSS IGLO-III 26.52 [34.11, 21.34, 24.1] [7.59, -5.17, -2.42] B3LYP IGLO-III 24.24 [32.39, 18.89, 21.42] [8.16, -5.34, -2.81] BP86 IGLO-III 23.96 [33.6, 20.81, 23.96] [7.48, -5.31, -2.16] M06l IGLO-III 20.85 [29.43, 15.27, 17.86] [8.58, -5.58, -2.99] 373 Table D.7. Calculated energies and isotropic 1H hyperfine coupling for rotational isomers of exo-[3]+ around the Fe-Cp*H centroid axis. C-Fe-Centroid-C ΔG Dihedral Angle (kcal/mol) (MHz) -53.25 0 92.27 [95.44, 91.12, 90.25] [3.17, -1.15, -2.02] 19 0.446322 43.24 [45.81, 41.67, 42.24] [2.57, -1.57, -1.00] 89.36 1.098098 63.07 [66.46, 61.83, 60.92] [3.39.-1.24, -2.15] -170.03 2.108162 86.32 [81.02, 86.88, 91.05] [-5,30, 0.56, 4.73] 1H (a iso) T(1H) (MHz) T(1H) (MHz) (°) DFT A(1H) = ± [85, 84, 83] MHz aiso(1H) = ± 84.0 MHz T(1H) = ± [1, 0, -1] MHz A(1H) = [81.0, 86.9, 91.0] MHz aiso(1H) = 86.3 MHz T(1H) = [4.7, 0.5, -5.3] MHz aiso(1H) = ± 23.3 MHz T(1H) = ± [-3.3, -7.3, 10.7] MHz A(1H) = ± [20, 16, 34] MHz A(1H) = [24.1, 21.4, 34.1] MHz aiso(1H) = 26.5 MHz T(1H) = [-2.4, -5.2, 7.6] MHz ---- A(1H) = [31.9, 29.2, -70.2] MHz aiso(1H) = 43.8 MHz T(1H) = [-11.9, -14.9, 26.4] MHz aiso(1H) = ± 40.7 MHz T(1H) = ± [-36.7, 27.3, 9.3] MHz A(1H) = [-1.2, -59.9, -58.0] MHz aiso(1H) = - 39.7 MHz T(1H) = [38.5, -20.2, -18.3] MHz A(1H) = ± [4, 68, 50] MHz --- g = [2.116 2.073 1.997] A(31P1) = ± [86, 84, 87] MHz A(31P2) = ± [78, 75, 63] MHz g = [2.083, 2.037, 2.003] A(31P1) = ± [73, 60, 49] MHz A(31P2) = ± [49, 51, 49] MHz [FeI(exo-h 4-Cp*H)(dppe)]+ A Fe -- [FeI(endo-h4-Cp*H)(dppe)]+ Fe A(31P1) = ± [88, 82, 79] MHz A(31P2) = ± [82, 71, 76] MHz O g = [2.352, 2.041, 1.992] C [FeIII(h 5-Cp*)(dppe)(CO)(H)]+ P Fe [FeIII(h5-Cp*)(dppe)(H)]+ Fe A(1H) = [66.4, 61.8 60.9] MHz aiso(1H) = 63.1 MHz T(1H) = [3.3, -1.3, -2.2] MHz A(1H) = ± [76 74 70] MHz aiso(1H) = ± 73 MHz T(1H) = ± [2 1 3] MHz A(31P1) = ± [46, 44, 15] MHz A(31P2) = ± [70, 64, 64] MHz g = [2.093 2.035, 2.0012] [FeI(exo-h 4-Cp*H)(dppe)]+ B Fe A(1H) = [30.8 27.9 41.4] MHz aiso(1H) = 33.4 MHz T(1H) =[-2.6, -5.5, 8.0] MHz A(1H) = ± [19.0, 15.0, 19.5] MHz aiso(1H) = ± 17.8 MHz T(1H) = ± [1.2, -2.8, 1.7] MHz --- g = [2.626, 2.349, 1.984] [Cp*(endo-h4-Cp*H)Co]+ Co A(1H) = [116.6, 122,5, 117.7] MHz aiso(1H) = 118.9 MHz T(1H) = [-2.3, 3.6, -1.2] MHz A(1H) = ± [106.5 112.5, 108.2] MHz aiso(1H) = ± 109.1 MHz T(1H) = ± [-2.6, 3.4, -0.9] MHz --- g = [2.170, 2.085, 2.005] [Cp*(exo-h4-Cp*H)Co]+ Co 374 375 Figure D.60. DFT-optimized structures showing experimental and predicted A(1H) values 376 DFT Coordinates 79 dppeFe Cp* hydride cation doublet Fe -0.056500 -0.514900 0.761900 P 1.568200 0.112700 -0.666800 P -1.575600 0.278700 -0.684900 C -1.221500 -0.778900 2.516800 C -0.940300 -2.042600 1.869500 C 0.498000 -2.216400 1.843200 C 1.094700 -1.067300 2.484200 C 0.033700 -0.192400 2.908100 C -2.586500 -0.258100 2.876500 C -1.961300 -3.068100 1.464200 C 1.237000 -3.440000 1.376200 C 2.556000 -0.909400 2.802200 C 0.192500 1.054500 3.734700 C 0.703200 0.770300 -2.214800 C -0.729900 0.246300 -2.357500 C 2.730400 1.505000 -0.317400 C 2.523300 2.319200 0.811800 C 3.333500 3.441900 1.040000 C 4.357600 3.760700 0.137300 C 4.569200 2.957000 -0.994400 C 3.761500 1.835700 -1.224600 C 2.665700 -1.243300 -1.269600 C 3.974300 -1.403900 -0.768200 C 4.775000 -2.470200 -1.204300 C 4.282800 -3.385200 -2.145300 C 2.982400 -3.232900 -2.649100 C 2.176900 -2.173900 -2.210300 C -2.092100 2.045800 -0.532700 C -2.933800 2.653300 -1.491400 C -3.264100 4.009600 -1.379600 C -2.759800 4.775200 -0.315700 C -1.921700 4.181700 0.637900 C -1.590800 2.822600 0.528900 C -3.144800 -0.645000 -0.975200 C -3.117900 -1.879500 -1.659900 C -4.300000 -2.601100 -1.871500 C -5.522300 -2.109200 -1.389800 C -5.556900 -0.892600 -0.694000 C -4.377600 -0.161700 -0.487400 H -2.620200 0.843800 2.880700 H -3.357100 -0.623300 2.179800 H -2.864400 -0.604200 3.889800 377 H -2.900100 -2.603700 1.126800 H -1.590300 -3.714600 0.653100 H -2.198800 -3.718000 2.327700 H 0.659700 -3.995800 0.620900 H 2.213700 -3.184700 0.935000 H 1.420500 -4.119300 2.230100 H 3.190800 -1.413800 2.056900 H 2.859500 0.147900 2.856700 H 2.773900 -1.370000 3.784500 H 1.185600 1.514600 3.610600 H -0.577300 1.809800 3.506100 H 0.086300 0.800600 4.807600 H 1.316200 0.543300 -3.104400 H 0.693300 1.867100 -2.088100 H -1.305500 0.846400 -3.082900 H -0.743200 -0.801100 -2.704600 H 1.717500 2.072100 1.511000 H 3.164800 4.068000 1.922600 H 4.991100 4.636500 0.312100 H 5.366400 3.204600 -1.703400 H 3.936800 1.216300 -2.111200 H 4.380700 -0.685200 -0.048900 H 5.790900 -2.578800 -0.810900 H 4.912100 -4.212200 -2.489900 H 2.593600 -3.940000 -3.389700 H 1.162300 -2.075300 -2.610800 H -3.343100 2.066500 -2.321600 H -3.919100 4.471700 -2.126100 H -3.022600 5.834700 -0.231800 H -1.524500 4.776100 1.467300 H -0.934000 2.354200 1.271300 H -2.171800 -2.290300 -2.027900 H -4.264700 -3.551900 -2.412800 H -6.445400 -2.673200 -1.557400 H -6.507200 -0.503100 -0.314900 H -4.422700 0.794500 0.043000 H -0.106300 -1.484400 -0.400800 81 dppeFe H Cp* CO cation doublet Fe 2.143600 1.077600 1.582100 P 4.107000 0.498600 2.517100 P 2.727200 0.138600 -0.368600 C 1.635800 2.818000 2.900200 C 2.401600 3.409100 4.052000 H 3.376700 3.808100 3.737700 H 2.579400 2.674600 4.854100 378 H C C H H H C C H H H C C H H H C C H H H C C H H C C H C H C H C H C H C C H C H C H C H C 1.822300 4.244300 4.490700 0.464700 1.967900 3.038800 -0.081700 1.406000 4.319900 -0.367800 0.344000 4.220200 -0.995500 1.956700 4.612300 0.634400 1.499600 5.151300 -0.142800 1.833700 1.763700 -1.429500 1.107900 1.495400 -1.406900 0.064400 1.858700 -1.684800 1.097600 0.424400 -2.260700 1.609100 2.025500 0.667300 2.563700 0.804600 0.279500 2.847900 -0.621400 -0.216200 1.986100 -1.097200 1.151500 3.119500 -1.235900 -0.429800 3.697700 -0.653300 1.689300 3.269300 1.536300 2.537100 4.388600 0.998700 3.452100 4.529100 1.593900 1.967100 5.336900 1.033400 2.828300 4.219100 -0.050600 5.131600 -0.199500 1.107600 4.254100 -0.876300 0.050800 3.857700 -1.834200 0.431400 4.833500 -1.088300 -0.863800 5.234700 1.799900 3.201100 5.705500 2.830600 2.359900 5.383300 2.878400 1.313500 6.582200 3.808100 2.847600 6.948100 4.593300 2.177600 6.988600 3.780600 4.190900 7.672500 4.545600 4.573500 6.517100 2.768900 5.037800 6.831400 2.739000 6.086100 5.647800 1.781600 4.548700 5.298800 0.992000 5.221100 4.135700 -0.789400 3.842900 5.230900 -1.662900 4.015400 6.090100 -1.629000 3.337200 5.246100 -2.580000 5.074600 6.100900 -3.254700 5.192800 4.177900 -2.629200 5.982400 4.194000 -3.345900 6.810400 3.091700 -1.756900 5.827400 2.254700 -1.788200 6.533300 3.069900 -0.845700 4.762600 379 H 2.216000 -0.172300 4.641800 C 3.233900 1.226000 -1.781100 C 2.532000 1.203900 -3.004700 H 1.669800 0.542500 -3.132100 C 2.933100 2.024500 -4.070000 H 2.375700 1.995300 -5.012000 C 4.041500 2.871000 -3.934500 H 4.357300 3.503200 -4.770700 C 4.744900 2.904300 -2.720500 H 5.614000 3.560500 -2.605400 C 4.337200 2.097400 -1.650100 H 4.889000 2.148600 -0.706300 C 1.659000 -1.119900 -1.194800 C 0.279200 -1.184200 -0.930200 H -0.165900 -0.491100 -0.212000 C -0.526600 -2.136200 -1.572000 H -1.598500 -2.177400 -1.352300 C 0.041200 -3.035600 -2.484100 H -0.585600 -3.782100 -2.982800 C 1.416700 -2.977600 -2.757900 H 1.865900 -3.676200 -3.471900 C 2.222500 -2.025100 -2.122400 H 3.289600 -1.984900 -2.363900 O 1.088000 -1.634000 2.293300 C 1.416700 -0.543700 2.058400 H 5.678000 0.657000 0.679200 H 5.891800 -0.898100 1.493500 H 3.329500 1.846300 0.982200 81 endo dppeFe Cp*H CO cation doublet Fe 1.706100 0.849800 1.780200 P 3.772200 0.554300 2.668900 P 2.714500 0.147400 -0.141100 C 1.223800 2.679600 2.842800 C 1.974200 3.476200 3.873800 H 2.898100 3.917800 3.469500 H 2.233900 2.867200 4.753200 H 1.339000 4.310800 4.230600 C 0.289700 1.650800 3.141700 C -0.035600 1.116300 4.509100 H -0.407100 0.080300 4.469700 H -0.821200 1.739100 4.977300 H 0.846600 1.138600 5.168500 C -0.370700 1.265600 1.896800 C -1.523900 0.307000 1.779900 H -1.504300 -0.475000 2.555500 380 H H C C H H H C C H H H C H H C H H C C H C H C H C H C H C C H C H C H C H C H C C H C H C -1.544700 -0.182700 0.794200 -2.471500 0.867100 1.892800 0.178000 2.042900 0.839300 -0.365700 2.051500 -0.564700 -0.623000 1.039400 -0.914500 0.344500 2.487300 -1.286100 -1.287800 2.663300 -0.607700 0.912900 3.239100 1.450300 0.046700 4.534400 1.492500 0.600600 5.352000 1.983600 -0.890100 4.358600 2.048200 -0.204900 4.859600 0.469300 4.621000 -0.836400 1.692700 5.610000 -0.444700 1.406500 4.792300 -1.693000 2.365200 3.807700 -1.250400 0.453800 3.100000 -2.060700 0.696500 4.451000 -1.626500 -0.359800 5.059300 1.882600 2.591600 4.845000 3.041500 1.824100 3.887900 3.180800 1.313100 5.855200 4.005700 1.689900 5.674600 4.902200 1.087900 7.092600 3.817200 2.319700 7.882000 4.569500 2.216800 7.320600 2.659200 3.080000 8.287400 2.505500 3.570400 6.313700 1.694800 3.215500 6.505900 0.796200 3.811900 3.827600 -0.010400 4.428500 3.500400 -1.344300 4.755400 3.248800 -2.066500 3.972900 3.489200 -1.766300 6.091700 3.240500 -2.806200 6.327200 3.795400 -0.864300 7.121000 3.789000 -1.197300 8.163900 4.116900 0.463800 6.807500 4.366300 1.172900 7.603600 4.133100 0.890900 5.471800 4.408800 1.925000 5.243800 3.858600 1.286500 -1.046900 3.380500 2.540800 -1.485300 2.343300 2.827400 -1.289000 4.212900 3.422500 -2.186900 3.819900 4.385600 -2.528200 5.545200 3.070900 -2.452000 381 H 6.199100 3.760900 -2.995500 C 6.033300 1.830100 -2.020700 H 7.069600 1.544800 -2.227600 C 5.197300 0.941900 -1.327900 H 5.603400 -0.024600 -1.017300 C 1.729300 -0.651100 -1.484500 C 0.745300 -1.602400 -1.135900 H 0.551700 -1.840100 -0.084900 C 0.013300 -2.261800 -2.131600 H -0.739100 -3.005400 -1.847400 C 0.242300 -1.970500 -3.485300 H -0.333300 -2.483700 -4.262200 C 1.211900 -1.022200 -3.838200 H 1.398000 -0.791900 -4.892600 C 1.955600 -0.365900 -2.846400 H 2.716300 0.364900 -3.137900 O 0.952300 -1.900400 2.583200 C 1.283100 -0.825800 2.263400 H 1.835800 3.488900 0.889600 81 exo dppeFe Cp*H CO D 89 cation doublet Fe 1.659500 1.152600 1.279000 P 3.165700 -0.005700 2.656400 P 3.288500 0.931300 -0.413900 C -0.326000 1.800300 1.582200 C -1.586900 0.981700 1.546200 H -1.601700 0.211800 2.336300 H -1.735000 0.482000 0.576600 H -2.458700 1.642300 1.721100 C 0.254300 2.469200 0.452400 C -0.279300 2.472900 -0.953100 H 0.517900 2.624300 -1.697500 H -1.015800 3.290700 -1.074400 H -0.791700 1.527800 -1.193200 C 1.351600 3.277900 0.947300 C 2.077400 4.294100 0.109000 H 2.319000 3.919300 -0.897600 H 3.003100 4.653100 0.579500 H 1.410400 5.168000 -0.019400 C 1.478300 3.070900 2.348300 C 2.398700 3.823400 3.268100 H 3.288700 4.213700 2.749700 H 2.741000 3.201200 4.110700 H 1.862400 4.692000 3.698800 C 0.147200 2.497700 2.859300 C 4.550500 -0.594600 1.519800 382 H H C H H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C 5.375800 0.133400 1.595000 4.924700 -1.562300 1.892900 4.081800 -0.700500 0.063100 3.336100 -1.504900 -0.056200 4.929200 -0.914800 -0.610800 4.067800 0.756700 4.086000 5.269300 1.478200 3.921300 5.729800 1.584000 2.934400 5.905800 2.066800 5.023600 6.846900 2.606400 4.877200 5.348600 1.956100 6.305000 5.849200 2.412400 7.164800 4.150400 1.248200 6.479600 3.711000 1.148500 7.477700 3.515200 0.651800 5.382300 2.592400 0.085700 5.541900 2.557600 -1.584700 3.415100 3.485300 -2.446600 4.043400 4.544800 -2.171700 4.101300 3.060700 -3.653300 4.611100 3.791000 -4.314600 5.088700 1.703600 -4.012200 4.571100 1.372900 -4.956900 5.014500 0.774300 -3.157100 3.965500 -0.286000 -3.428700 3.935000 1.199000 -1.949300 3.390200 0.464400 -1.289700 2.920100 4.806300 1.992100 -0.501900 5.102200 2.879000 0.550000 4.388100 2.991500 1.372500 6.302300 3.607300 0.558400 6.516600 4.295400 1.383300 7.221700 3.453400 -0.488300 8.157600 4.021500 -0.486300 6.940300 2.565300 -1.538800 7.658100 2.436900 -2.356300 5.743700 1.836300 -1.548100 5.539400 1.146700 -2.373900 2.797400 0.725800 -2.179100 2.273300 -0.497600 -2.649600 2.188800 -1.363600 -1.986600 1.850200 -0.623200 -3.980500 1.454100 -1.581800 -4.331100 1.936100 0.466800 -4.858100 1.606700 0.364000 -5.896900 2.451300 1.688200 -4.399500 383 H 2.530600 2.542900 -5.079500 C 2.877200 1.819400 -3.070900 H 3.297100 2.774100 -2.735900 O 0.520500 -1.296600 0.142800 C 1.020900 -0.351600 0.613000 C 0.151800 1.681600 4.158900 H 0.556800 2.283900 4.989400 H 0.757200 0.764500 4.072900 H -0.875200 1.388900 4.434200 H -0.531500 3.369200 3.037300 81 exo dppeFe Cp*H CO D -53 cation doublet Opt Freq Fe 1.717900 1.116100 1.485500 P 3.594100 0.356900 2.688800 P 2.876000 0.493900 -0.376400 C 0.992900 3.094700 0.790000 C 1.287100 3.907400 -0.437800 H 1.108900 3.331000 -1.360100 H 2.317200 4.290300 -0.462200 H 0.602400 4.777700 -0.467100 C 1.624000 3.215700 2.052400 C 2.702600 4.191100 2.437200 H 3.490600 3.731500 3.054300 H 2.250400 5.007000 3.032900 H 3.170700 4.656300 1.556200 C 0.927600 2.351900 3.000800 C 1.158700 2.358600 4.486500 H 2.213600 2.546100 4.737800 H 0.848800 1.414700 4.960700 H 0.559300 3.168100 4.945900 C -0.150400 1.720200 2.310100 C -1.232700 0.902100 2.960200 H -0.871500 0.369300 3.854000 H -1.665100 0.160200 2.269600 H -2.053700 1.572000 3.282000 C -0.388800 2.453900 0.989000 C 4.705300 -0.500900 1.421100 H 5.476300 0.224800 1.112300 H 5.221300 -1.348200 1.902000 C 3.901900 -0.959400 0.199000 H 3.218300 -1.785300 0.459900 H 4.564200 -1.302200 -0.614800 C 4.807700 1.437500 3.596400 C 5.805700 2.165900 2.912000 H 5.906100 2.088900 1.824600 C 6.696100 2.993200 3.611700 384 H C H C H C H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H O C C H H H 7.473500 3.535300 3.062700 6.598800 3.119500 5.004900 7.297100 3.762700 5.550500 5.607800 2.407200 5.695500 5.529300 2.490400 6.785200 4.721600 1.572000 5.000600 3.973500 1.002400 5.561100 3.328800 -0.983200 3.946300 4.437900 -1.697700 4.454100 5.453600 -1.450900 4.124500 4.257600 -2.715300 5.398000 5.126300 -3.262500 5.778200 2.968800 -3.029200 5.858500 2.828500 -3.826000 6.596500 1.864400 -2.318100 5.373300 0.857200 -2.554900 5.730900 2.044300 -1.301700 4.422100 1.176100 -0.755600 4.044700 4.177600 1.560000 -1.144700 4.604000 2.737600 -0.504500 4.111400 3.052600 0.419300 5.656100 3.499200 -1.036700 5.975500 4.416400 -0.531200 6.293400 3.086200 -2.214200 7.113100 3.679400 -2.632400 5.879200 1.909200 -2.857400 6.375100 1.582300 -3.777000 4.829200 1.147400 -2.328700 4.513700 0.233600 -2.843400 1.905400 -0.120000 -1.819000 1.378500 -1.429800 -1.833500 1.566900 -2.118600 -1.005200 0.602800 -1.868300 -2.916200 0.209600 -2.889900 -2.918500 0.335000 -1.007300 -3.990300 -0.269500 -1.353900 -4.834900 0.848600 0.297800 -3.981200 0.648200 0.974400 -4.818600 1.628300 0.741000 -2.904500 2.040400 1.754900 -2.920900 0.508400 -1.560800 1.463300 1.025500 -0.513300 1.491000 -1.012000 1.669900 -0.177200 -2.009000 1.294500 0.107100 -0.393400 0.812600 -0.486900 -1.148700 2.328400 -1.051300 385 H -1.086000 3.301800 1.210500 81 exo dppeFe Cp*H CO D -170 cation doublet Fe 1.766800 0.941300 1.709800 P 3.826100 0.583500 2.630400 P 2.707700 0.218500 -0.240400 C 1.304600 2.776900 2.810400 C 2.063400 3.584200 3.825400 H 3.003400 3.995000 3.427000 H 2.295200 3.000500 4.728900 H 1.438200 4.444400 4.138300 C 0.404900 1.722100 3.131300 C 0.131400 1.165300 4.500900 H -0.220400 0.122600 4.458500 H -0.653300 1.766500 4.998300 H 1.030300 1.196300 5.136000 C -0.312200 1.355500 1.913300 C -1.484400 0.414400 1.856300 H -1.379000 -0.443300 2.538700 H -1.655000 0.032700 0.838300 H -2.396000 0.966700 2.154000 C 0.172700 2.164800 0.849600 C -0.417000 2.221000 -0.532100 H -0.760300 1.237000 -0.886700 H 0.303400 2.618700 -1.264100 H -1.290100 2.903800 -0.533500 C 0.872500 3.381500 1.465600 C 4.714300 -0.726500 1.582800 H 5.706200 -0.304500 1.353300 H 4.890700 -1.618500 2.206400 C 3.947300 -1.088800 0.301000 H 3.316700 -1.979600 0.464900 H 4.629800 -1.322400 -0.533700 C 5.163400 1.863100 2.763800 C 5.084700 3.069200 2.045200 H 4.192600 3.288400 1.454800 C 6.147000 3.986600 2.069300 H 6.068400 4.922600 1.506600 C 7.302500 3.702800 2.808600 H 8.131400 4.418600 2.831600 C 7.397200 2.495300 3.518500 H 8.300400 2.264800 4.093000 C 6.338300 1.578400 3.497400 H 6.427800 0.641400 4.056900 C 3.781400 -0.129600 4.337800 C 3.522500 -1.501100 4.544500 386 H 3.378700 -2.178400 3.697500 C 3.440800 -2.021900 5.843500 H 3.245500 -3.090100 5.984700 C 3.608900 -1.183000 6.953800 H 3.547600 -1.593200 7.967100 C 3.863700 0.182200 6.760000 H 4.008100 0.843000 7.621000 C 3.948800 0.707400 5.463100 H 4.174700 1.770000 5.329500 C 3.662500 1.382800 -1.315900 C 3.007500 2.084000 -2.353900 H 1.948800 1.896200 -2.558700 C 3.710500 2.993400 -3.154500 H 3.189400 3.519000 -3.961600 C 5.077200 3.221200 -2.932300 H 5.626600 3.929400 -3.561200 C 5.738300 2.528600 -1.908600 H 6.807100 2.691600 -1.733800 C 5.037800 1.616900 -1.104800 H 5.579900 1.088800 -0.315600 C 1.722700 -0.756000 -1.465700 C 0.466600 -1.286600 -1.114300 H 0.040700 -1.066100 -0.132000 C -0.236900 -2.105200 -2.009800 H -1.213800 -2.507800 -1.724400 C 0.309600 -2.404900 -3.265100 H -0.240500 -3.040900 -3.966400 C 1.564700 -1.889000 -3.620900 H 1.998800 -2.124600 -4.597900 C 2.270400 -1.070300 -2.729600 H 3.248300 -0.675000 -3.022600 O 1.037200 -1.837600 2.432000 C 1.352100 -0.748600 2.152100 C 1.895900 4.125200 0.599700 H 1.388000 4.620800 -0.244400 H 2.656300 3.448400 0.177800 H 2.399600 4.914900 1.182700 H 0.073700 4.126900 1.708500 81 exo dppeFe Cp*H CO D 19 cation doublet Fe 1.930200 11.829200 2.651800 P 1.089800 12.865800 4.495200 P 3.287000 13.655100 2.407400 O 3.807200 10.589400 4.561400 C 2.646100 16.459700 -1.284000 H 2.494800 17.105800 -2.155200 387 C H C C C H C C H C C H H H C H C H C H H C H H H C H C H C H C H H H C H C H C H H C H C H 2.098800 9.375100 1.435100 1.537700 8.681800 0.758200 2.199300 10.706200 0.687200 0.954900 11.376400 0.816200 1.490900 11.336400 6.855100 2.561400 11.512000 6.717400 3.045700 14.802000 0.970800 -0.826300 11.629100 6.173300 -1.577900 12.060900 5.504100 1.106900 9.798100 2.519400 -1.190800 11.034500 2.331800 -1.374800 10.836300 3.399100 -1.821700 10.333000 1.753900 -1.535800 12.054000 2.105100 -2.127000 16.222900 4.041300 -2.865700 17.024300 3.934100 3.683900 15.516700 -1.277200 4.349900 15.425900 -2.142200 2.519500 13.915600 5.104300 2.198300 14.556200 5.943400 3.311700 13.239800 5.471200 0.402400 12.394600 -0.144500 -0.405300 12.996200 0.301600 -0.024500 11.874500 -1.023400 1.181900 13.079800 -0.513700 7.063500 11.941600 2.120200 7.481100 10.943300 1.952900 7.914800 13.040500 2.292800 9.001100 12.903900 2.264100 7.372200 14.319200 2.497500 8.032200 15.183600 2.624400 3.177700 10.926700 -0.429300 3.172000 11.968400 -0.785700 2.913100 10.277100 -1.288100 4.207800 10.662800 -0.138300 -1.242400 10.841700 7.255600 -2.310600 10.656800 7.411000 -1.656100 15.867100 5.315200 -2.025200 16.391100 6.202700 3.019000 14.751800 3.919500 3.956500 15.278600 4.162700 2.268800 15.514800 3.650600 3.885700 14.694700 -0.159800 4.715900 13.980500 -0.161800 0.738800 8.896800 3.663800 1.632500 8.523900 4.190900 388 H 0.192900 8.012100 3.280100 H 0.092500 9.401700 4.399100 C 5.984000 14.496400 2.534900 H 5.578900 15.505300 2.672400 C 3.106500 11.040000 3.741000 C -0.297300 10.301400 8.139900 H -0.624700 9.691800 8.988500 C -1.651400 15.549900 2.908100 H -2.017000 15.822000 1.912500 C 0.546900 11.888000 5.961600 C 3.384000 8.636200 1.810700 H 3.923800 8.333000 0.897300 H 3.151400 7.714100 2.370000 H 4.065800 9.242900 2.426600 C 2.002300 15.752000 0.952300 H 1.333400 15.864900 1.811400 C -0.705900 14.522000 3.049700 H -0.326700 14.001800 2.164700 C 5.117700 13.390900 2.377900 C -0.714500 14.839900 5.458200 H -0.363200 14.566300 6.459100 C 1.809700 16.577200 -0.164800 H 1.003700 17.319200 -0.155300 C 1.067300 10.551800 7.937400 H 1.810700 10.138300 8.626500 C 0.254300 10.813100 1.970700 C 5.671800 12.115900 2.162200 H 5.010600 11.256300 2.023400 C -0.230000 14.154400 4.321600 80 dppeFe Cp* CO cation singlet Fe 1.819800 0.784200 1.741600 P 3.826000 0.582200 2.757600 P 2.927200 0.150100 -0.118400 C 1.253500 2.578600 2.805500 C 1.944800 3.445100 3.819900 H 2.860400 3.910700 3.427400 H 2.202400 2.882400 4.729600 H 1.256600 4.257600 4.120000 C 0.382600 1.476800 3.123500 C 0.072500 0.962100 4.501900 H -0.249200 -0.091100 4.479300 H -0.746400 1.553500 4.953100 H 0.946400 1.035600 5.168500 C -0.276500 1.069900 1.894800 C -1.417200 0.094600 1.797200 389 H H H C C H H H C C H H H C H H C H H C C H C H C H C H C H C C H C H C H C H C H C C H C H -1.314900 -0.746700 2.500700 -1.522500 -0.314300 0.780600 -2.361000 0.617800 2.040500 0.232600 1.882300 0.826200 -0.307800 1.931500 -0.576100 -0.572000 0.935100 -0.962500 0.409300 2.393000 -1.271600 -1.225000 2.550000 -0.591300 1.181600 2.821000 1.387500 1.735600 4.012200 0.649700 2.398800 4.614400 1.289000 0.902500 4.668500 0.335700 2.291500 3.729000 -0.259200 4.738100 -0.832500 1.892600 5.801900 -0.547500 1.851000 4.671300 -1.725500 2.535200 4.169100 -1.127100 0.489500 3.586000 -2.063200 0.502900 4.962700 -1.256600 -0.265500 5.118100 1.910500 2.730000 4.947500 3.068800 1.953500 4.021500 3.201900 1.390300 5.963000 4.035300 1.878000 5.814200 4.933200 1.269500 7.165600 3.844100 2.570300 7.959600 4.596700 2.512800 7.354400 2.680900 3.334400 8.295200 2.523000 3.871900 6.340300 1.718400 3.415700 6.500900 0.819200 4.020700 3.788900 0.075300 4.538700 3.410900 -1.237300 4.896800 3.173500 -1.981300 4.130600 3.330200 -1.613100 6.244400 3.044800 -2.638200 6.502000 3.613900 -0.684700 7.256500 3.552700 -0.981800 8.308500 3.982200 0.623500 6.913400 4.213100 1.354200 7.695500 4.069400 1.003000 5.566000 4.379900 2.022700 5.318600 3.920100 1.339100 -1.131200 3.288200 2.094000 -2.145200 2.222900 1.949100 -2.352100 4.021600 2.994900 -2.928800 3.517000 3.561500 -3.718500 390 C 5.398900 3.156700 -2.714300 H 5.973900 3.854300 -3.332300 C 6.037400 2.410500 -1.714400 H 7.113800 2.522800 -1.545800 C 5.305000 1.509600 -0.926700 H 5.831300 0.940500 -0.154500 C 2.059000 -0.837500 -1.424000 C 0.820800 -1.451200 -1.152300 H 0.331800 -1.286200 -0.188000 C 0.212600 -2.282300 -2.104200 H -0.752000 -2.748900 -1.880200 C 0.837100 -2.512800 -3.337600 H 0.361600 -3.158900 -4.082800 C 2.074800 -1.914000 -3.614300 H 2.571500 -2.095400 -4.573100 C 2.684800 -1.081400 -2.666700 H 3.651200 -0.622700 -2.898400 C 1.520800 -0.901200 2.111100 O 1.243900 -2.003300 2.387700 78 dppeFe Cp* cation singlet Fe 0.187400 -0.217000 0.934900 P -1.398000 0.108800 -0.658300 P 1.645700 -0.165200 -0.768500 C 0.977200 -1.849400 1.952400 C 1.275000 -0.630700 2.658100 C 0.006300 -0.017700 3.013700 C -1.060000 -0.834900 2.519200 C -0.458000 -1.966800 1.830700 C 1.966100 -2.896600 1.527900 C 2.635100 -0.153200 3.090400 C -0.149000 1.230300 3.840600 C -2.522900 -0.666200 2.821700 C -1.184700 -3.178200 1.321200 C -0.554600 0.344200 -2.344800 C 0.788400 -0.392100 -2.416600 C -2.659300 -1.192800 -1.043400 C -2.306800 -2.315200 -1.822700 C -3.254000 -3.303800 -2.121300 C -4.566300 -3.192000 -1.637900 C -4.926900 -2.082100 -0.860600 C -3.982800 -1.086300 -0.567200 C -2.436900 1.638900 -0.564200 C -2.418700 2.435700 0.595200 C -3.185200 3.609100 0.667400 C -3.972600 3.999700 -0.424400 391 C C C C C C C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H -3.997700 3.211600 -1.586500 -3.239300 2.036200 -1.657100 3.308600 -0.912900 -0.991000 3.461200 -2.138100 -1.675800 4.731000 -2.712500 -1.824300 5.860000 -2.075700 -1.289100 5.716800 -0.860600 -0.603300 4.449600 -0.280900 -0.451100 1.886400 1.652000 -0.701500 1.224200 2.241200 0.395000 1.181500 3.632600 0.554300 1.827600 4.443700 -0.391400 2.494700 3.868600 -1.486000 2.518400 2.475700 -1.651100 1.644900 -3.418300 0.611600 2.969100 -2.479200 1.352600 2.054600 -3.658700 2.326300 3.427800 -0.540500 2.431100 2.704500 0.947900 3.093700 2.857700 -0.500300 4.117300 0.675400 1.944600 3.677300 -1.103300 1.744900 3.641800 -0.139600 0.966300 4.915900 -2.804600 0.392100 2.939100 -3.157200 -1.106500 2.036900 -2.768700 -1.182600 3.769300 -2.219900 -2.950200 1.027000 -0.675200 -3.628800 0.454500 -1.224700 -3.945400 2.119600 -0.394300 1.431600 -2.444700 -1.237700 0.026200 -3.150100 0.650800 -1.476300 -2.575400 1.424000 -0.012700 -3.236500 -1.287600 -2.424600 -2.207200 -2.965900 -4.162900 -2.736400 -5.306800 -3.963200 -1.874100 -5.951800 -1.981000 -0.488800 -4.288400 -0.211100 0.015300 -1.796400 2.129000 1.441900 -3.165900 4.219400 1.576800 -4.567900 4.917100 -0.371900 -4.613000 3.512000 -2.441200 -3.281300 1.424200 -2.564900 2.592900 -2.652100 -2.101500 4.837400 -3.659800 -2.362200 6.851400 -2.524000 -1.410300 392 H 6.596100 -0.355600 -0.190500 H 4.351900 0.675400 0.073400 H 0.776400 1.604700 1.202300 H 0.662000 4.082700 1.406100 H 1.812200 5.532600 -0.273500 H 2.997200 4.511100 -2.216700 H 3.034200 2.034400 -2.511200 78 dppeFe Cp* cation triplet Fe 0.056500 -0.478000 0.834500 P -1.539400 0.085200 -0.717600 P 1.617000 0.114700 -0.700000 C 0.882900 -2.009800 2.012600 C 1.176400 -0.718100 2.627500 C -0.067600 -0.115000 3.014000 C -1.130300 -0.981900 2.586100 C -0.544200 -2.167500 1.991200 C 1.902700 -3.056100 1.651700 C 2.549800 -0.209700 2.973200 C -0.223200 1.161800 3.796300 C -2.594600 -0.789300 2.867800 C -1.301500 -3.386700 1.537900 C -0.622200 0.495000 -2.320300 C 0.773800 -0.141100 -2.358700 C -2.719700 -1.250600 -1.202000 C -2.294400 -2.300900 -2.043500 C -3.168100 -3.345200 -2.375900 C -4.474700 -3.360700 -1.865700 C -4.904200 -2.325200 -1.023000 C -4.034400 -1.276000 -0.690400 C -2.602600 1.571100 -0.487000 C -2.426900 2.391200 0.643400 C -3.178000 3.567200 0.791000 C -4.108400 3.932000 -0.192200 C -4.290200 3.118700 -1.322800 C -3.543500 1.943300 -1.472800 C 3.249900 -0.716500 -0.904200 C 3.325300 -1.998200 -1.492300 C 4.560000 -2.647000 -1.625300 C 5.732800 -2.032800 -1.161500 C 5.666000 -0.766500 -0.563300 C 4.433900 -0.109200 -0.434000 C 2.006200 1.917000 -0.714100 C 1.533300 2.731600 0.332300 C 1.783200 4.111900 0.328600 C 2.510100 4.687500 -0.722600 393 C 2.988400 3.882400 -1.769400 C 2.740700 2.503900 -1.767900 H 1.530300 -3.738000 0.869600 H 2.843600 -2.610400 1.294100 H 2.142400 -3.673300 2.538900 H 3.308100 -0.561200 2.255200 H 2.586800 0.892100 3.000900 H 2.847900 -0.576300 3.973800 H 0.564100 1.896600 3.557700 H -1.204100 1.635900 3.627900 H -0.149700 0.951600 4.881200 H -2.876800 0.275200 2.898600 H -3.224200 -1.294300 2.118300 H -2.845500 -1.226700 3.853200 H -2.288300 -3.127700 1.121200 H -0.745800 -3.947100 0.768400 H -1.469900 -4.072600 2.390200 H -0.536400 1.595700 -2.332700 H -1.233100 0.195700 -3.189500 H 0.715200 -1.230100 -2.532500 H 1.391600 0.288700 -3.166600 H -1.278300 -2.309800 -2.452300 H -2.827200 -4.147300 -3.039200 H -5.158000 -4.174500 -2.129400 H -5.925100 -2.325900 -0.627600 H -4.389900 -0.463700 -0.047800 H -1.696100 2.103500 1.406700 H -3.035700 4.198900 1.674300 H -4.693700 4.850400 -0.080000 H -5.016600 3.401200 -2.092100 H -3.697900 1.313400 -2.356000 H 2.420800 -2.501900 -1.849800 H 4.604600 -3.635900 -2.092200 H 6.697000 -2.540100 -1.267700 H 6.577900 -0.281300 -0.200600 H 4.398900 0.885600 0.021300 H 0.968800 2.273500 1.153500 H 1.410100 4.736800 1.146600 H 2.708800 5.764400 -0.727000 H 3.559500 4.330500 -2.589600 H 3.132400 1.885200 -2.583300 97 dppeFe Cp* CNXyl cation singlet Fe 1.850700 0.959100 1.750600 P 3.872200 0.572800 2.691100 P 2.740300 0.230300 -0.173300 394 C C H H H C C H H H C C H H H C C H H H C C H H H C C H H C C H C H C H C H C H C C H C H C 1.532500 2.849700 2.757300 2.322500 3.685800 3.724900 3.255100 4.076900 3.294300 2.572100 3.131900 4.642700 1.703200 4.552000 4.026600 0.539800 1.883200 3.144300 0.169000 1.546500 4.561400 -0.499600 0.675700 4.607800 -0.361400 2.400400 5.024800 1.052800 1.327900 5.182000 -0.194200 1.506900 1.953500 -1.448500 0.677300 1.927200 -1.391700 -0.189900 2.604500 -1.675100 0.307100 0.915500 -2.307200 1.296000 2.248800 0.373600 2.211600 0.840400 -0.214600 2.312300 -0.540100 -0.682200 1.374600 -0.875700 0.541200 2.607100 -1.282800 -0.998900 3.093900 -0.540700 1.457100 3.038200 1.333900 2.143800 4.109900 0.529300 2.946200 4.598900 1.102200 1.412200 4.894400 0.258700 2.571900 3.724000 -0.410100 4.874300 -0.552300 1.537400 4.075300 -1.003100 0.311800 3.512100 -1.924600 0.529900 4.730500 -1.209300 -0.551300 5.164400 1.876500 2.986000 5.241000 2.962800 2.091700 4.481000 3.071500 1.312300 6.283000 3.896700 2.184800 6.324600 4.736500 1.482400 7.266100 3.755300 3.174900 8.079200 4.485100 3.252600 7.206500 2.670900 4.061800 7.976000 2.547300 4.831600 6.167200 1.733100 3.968400 6.147600 0.886000 4.661000 3.834600 -0.334100 4.307500 3.893000 -1.741300 4.369800 3.999900 -2.336900 3.457900 3.817300 -2.408600 5.601400 3.871900 -3.502200 5.626500 3.676500 -1.681700 6.791300 395 H C H C H C C H C H C H C H C H C C H C H C H C H C H N C C C C H C H C H C C H H H C H H H 3.624400 -2.203300 7.752700 3.609900 -0.281300 6.743400 3.508000 0.296800 7.668000 3.685300 0.386100 5.513300 3.658900 1.480900 5.497700 3.586700 1.359100 -1.382900 2.897600 1.852200 -2.513500 1.869500 1.534400 -2.711700 3.529100 2.720700 -3.415200 2.978900 3.086000 -4.288700 4.860600 3.108600 -3.209500 5.354700 3.781200 -3.918400 5.559300 2.618300 -2.097500 6.603900 2.902700 -1.933300 4.927800 1.754200 -1.191300 5.499800 1.385800 -0.335000 1.723900 -0.817600 -1.318700 0.397400 -1.160800 -0.999200 -0.043900 -0.780200 -0.074100 -0.349300 -1.995700 -1.845200 -1.382100 -2.250100 -1.586100 0.228200 -2.505200 -3.015900 -0.352900 -3.156000 -3.678000 1.554800 -2.178200 -3.339300 2.011500 -2.575100 -4.251700 2.298100 -1.337800 -2.500600 3.326600 -1.079600 -2.774300 1.171700 -1.893000 2.474400 1.444800 -0.763400 2.179500 0.847400 -3.170600 2.914600 1.158900 -4.295800 2.100300 0.811700 -5.569900 2.578800 1.040700 -6.443600 1.959000 0.185200 -5.738700 3.818400 -0.078200 -6.741800 4.168200 -0.103900 -4.620500 4.609200 -0.589900 -4.749100 5.582000 0.216600 -3.322600 4.182300 1.865500 -4.152400 0.774700 1.478100 -3.303300 0.189500 1.751900 -5.068600 0.173800 2.951200 -3.993100 0.921600 -0.106700 -2.128300 5.042100 -0.907600 -1.517400 4.584500 0.773900 -1.476000 5.175200 -0.457500 -2.446700 6.035600 396 H 5.747100 0.052800 1.243000 H 5.271700 -1.410000 2.104400 98 dppeFe endo Cp*H CNXyl cation singlet Fe 1.792800 0.952800 1.781300 P 3.860800 0.601200 2.672700 P 2.675000 0.223900 -0.180100 C 1.445100 2.806100 2.832100 C 2.169300 3.569000 3.908500 H 3.130900 3.980400 3.567500 H 2.352700 2.950500 4.800600 H 1.542300 4.422900 4.233600 C 0.410100 1.866900 3.100000 C -0.038700 1.427100 4.464500 H -0.658400 0.521400 4.417100 H -0.643100 2.227300 4.933900 H 0.815000 1.223100 5.130700 C -0.249200 1.545500 1.844600 C -1.497600 0.715000 1.720500 H -1.447400 -0.212800 2.312800 H -1.711300 0.448900 0.673900 H -2.361700 1.299300 2.090000 C 0.390200 2.279300 0.806600 C -0.111000 2.394100 -0.608200 H -0.469000 1.434200 -1.012400 H 0.670200 2.784700 -1.279000 H -0.960200 3.104800 -0.646100 C 1.233800 3.390800 1.431700 C 0.520700 4.776800 1.441000 H 1.152300 5.530800 1.940000 H -0.443500 4.714600 1.973700 H 0.334100 5.118200 0.409000 C 4.877100 -0.474200 1.484200 H 5.714200 0.165200 1.159600 H 5.327000 -1.312600 2.041600 C 4.058100 -0.964000 0.287000 H 3.523700 -1.895400 0.535800 H 4.698400 -1.168300 -0.588000 C 5.110200 1.943700 2.972200 C 5.064000 3.117700 2.193700 H 4.235500 3.272700 1.494400 C 6.069500 4.089800 2.302300 H 6.016500 4.997600 1.691900 C 7.135200 3.900600 3.193400 H 7.919000 4.660500 3.283800 C 7.194900 2.732700 3.967700 397 H C H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H N C C C C H C H C H 8.028100 2.575000 4.661100 6.193500 1.757400 3.857800 6.261600 0.848000 4.463500 3.869600 -0.332400 4.271400 3.923600 -1.740800 4.300400 4.001500 -2.315800 3.372300 3.878400 -2.434000 5.519000 3.928600 -3.528000 5.519500 3.771500 -1.731800 6.727300 3.741600 -2.273600 7.678400 3.711300 -0.330300 6.711300 3.637700 0.228200 7.650600 3.758200 0.363900 5.494700 3.739200 1.459000 5.502500 3.466200 1.407100 -1.375000 2.802600 1.805700 -2.556800 1.823700 1.386300 -2.806600 3.400200 2.716200 -3.439800 2.872200 3.005500 -4.354400 4.669600 3.242800 -3.164000 5.137100 3.947300 -3.859800 5.340300 2.853100 -1.996400 6.336600 3.248500 -1.773900 4.742400 1.948400 -1.108000 5.288400 1.665000 -0.204000 1.678800 -0.853800 -1.308300 0.356700 -1.209500 -0.983800 -0.089900 -0.821000 -0.064800 -0.377300 -2.067600 -1.817700 -1.406900 -2.332100 -1.555900 0.209200 -2.588000 -2.979200 -0.361900 -3.257400 -3.631300 1.532000 -2.249100 -3.305900 1.995400 -2.654900 -4.211000 2.263200 -1.386100 -2.479600 3.288700 -1.118700 -2.755500 1.085400 -1.923600 2.604700 1.345100 -0.798600 2.283100 0.789100 -3.212600 3.031800 1.125200 -4.320900 2.204600 0.800600 -5.607400 2.665500 1.047500 -6.468700 2.035500 0.174400 -5.803200 3.901400 -0.071800 -6.815300 4.237600 -0.135600 -4.701000 4.706700 -0.619600 -4.851600 5.677300 398 C 0.161000 -3.391800 4.296900 C 1.834000 -4.143500 0.884800 H 1.412400 -3.312300 0.297300 H 1.769000 -5.063400 0.282600 H 2.909100 -3.931100 1.041000 C -0.183300 -2.214600 5.172600 H -0.996200 -1.612800 4.724400 H 0.686100 -1.548300 5.312800 H -0.526200 -2.552600 6.162400 H 2.188300 3.524700 0.882900 98 dppeFe hydride Cp* CNXyl cation doublet Fe 1.634700 1.309700 1.476700 P 4.208700 0.554000 3.036900 P 2.609900 0.323200 -0.237000 C 1.323700 3.223000 2.300100 C 2.254400 4.035300 3.156800 H 3.101400 4.431600 2.576900 H 2.664500 3.442200 3.990200 H 1.714000 4.897600 3.590800 C 0.318300 2.327400 2.781700 C 0.001700 2.088100 4.230900 H -0.618600 1.192500 4.372200 H -0.561100 2.950000 4.637700 H 0.916900 1.977600 4.835000 C -0.457300 1.862900 1.635600 C -1.698800 1.019100 1.713900 H -1.606200 0.213500 2.460000 H -1.945500 0.560400 0.743600 H -2.561300 1.646400 2.008000 C 0.067500 2.482900 0.460000 C -0.512900 2.418300 -0.926300 H -1.069400 1.484200 -1.102600 H 0.266800 2.505800 -1.700000 H -1.219800 3.256900 -1.074800 C 1.186800 3.301900 0.857100 C 1.935000 4.245500 -0.041700 H 2.951000 4.449000 0.330400 H 1.396000 5.210800 -0.087800 H 2.018700 3.860000 -1.068800 C 5.013900 0.057900 1.411000 C 4.112800 -0.631600 0.371000 H 3.699100 -1.567500 0.781300 H 4.716700 -0.904400 -0.512600 C 5.553600 1.634700 3.716600 C 5.836500 2.847900 3.049100 399 H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H N C C C 5.278300 3.121500 2.145200 6.829300 3.712400 3.525900 7.042500 4.643100 2.989500 7.542200 3.391000 4.690500 8.315800 4.068700 5.066300 7.256900 2.199100 5.370200 7.809600 1.940500 6.279600 6.271500 1.324100 4.890000 6.071400 0.391700 5.427000 4.304000 -0.961000 4.088700 5.003000 -2.136900 3.745100 5.559000 -2.200500 2.804700 5.012700 -3.240000 4.612700 5.563200 -4.144300 4.332000 4.333200 -3.183200 5.837500 4.347800 -4.044000 6.514300 3.634700 -2.018800 6.192000 3.103700 -1.966100 7.148100 3.610100 -0.922900 5.319300 3.055400 -0.018800 5.599000 3.256400 1.381300 -1.615300 2.601000 1.415600 -2.865300 1.714300 0.797900 -3.037800 3.084500 2.225800 -3.903800 2.562900 2.237100 -4.866600 4.233700 3.006000 -3.716200 4.616500 3.627700 -4.532000 4.891700 2.983100 -2.477400 5.791700 3.586700 -2.321400 4.401000 2.187800 -1.432900 4.920600 2.202700 -0.470300 1.713300 -1.006300 -1.172200 0.352300 -1.253200 -0.914800 -0.163500 -0.652700 -0.159500 -0.332800 -2.266300 -1.603900 -1.392100 -2.448600 -1.394300 0.340500 -3.046400 -2.554200 -0.191400 -3.839000 -3.090800 1.698600 -2.807800 -2.819400 2.227700 -3.411800 -3.564200 2.381900 -1.792400 -2.137500 3.436100 -1.607800 -2.371100 1.108400 -1.485200 2.621500 1.346300 -0.398700 2.187300 0.805900 -2.730600 3.159500 1.276100 -3.907700 2.512500 400 C 0.932400 -5.146400 3.076800 H 1.282300 -6.060900 2.586500 C 0.160500 -5.230100 4.240700 H -0.095200 -6.208100 4.660000 C -0.281900 -4.060200 4.868400 H -0.880100 -4.122600 5.783500 C 0.026300 -2.793700 4.349000 C 2.139800 -3.849200 1.278400 H 1.740700 -3.147800 0.528400 H 2.221100 -4.845200 0.816200 H 3.162600 -3.518500 1.537100 C -0.460300 -1.544500 5.037100 H -1.192400 -1.003300 4.409400 H 0.373100 -0.851700 5.245800 H -0.953400 -1.792300 5.989400 H 5.431500 0.995300 1.008700 H 5.888500 -0.583800 1.621700 H 3.058000 1.784800 1.195900 52 Cp* endo Cp*H Co cation doublet Co 0.057800 0.005100 -0.066800 C 1.739600 1.231500 -0.241100 C -1.636200 -1.155500 -0.197500 C -1.401300 -0.785700 1.161600 C 1.506200 -2.052800 -2.116500 H 2.515600 -2.260700 -2.518500 H 0.853500 -1.821600 -2.973800 H 1.145600 -2.981100 -1.647500 C 1.856100 -1.034100 0.285500 C 1.573300 -0.915400 -1.136500 C 2.097700 -2.321900 1.019200 H 3.157900 -2.619900 0.909000 H 1.484900 -3.146300 0.620200 H 1.895500 -2.227900 2.098000 C 1.854800 2.724400 -0.129100 H 2.913400 3.023200 -0.249700 H 1.522900 3.092100 0.854800 H 1.277700 3.244900 -0.909000 C 1.344900 1.072600 -2.835400 H 2.326900 1.190000 -3.331400 H 0.876100 2.068800 -2.798800 H 0.729800 0.423400 -3.480200 C -1.389800 0.670500 1.245600 C -1.616600 1.196400 -0.062700 C 1.967100 0.287600 0.834000 C 1.528800 0.488500 -1.463000 401 C -1.248000 -1.721500 2.326000 H -2.233700 -1.879300 2.802900 H -0.570600 -1.318500 3.095900 H -0.875700 -2.708200 2.011200 C -1.228100 1.466300 2.509200 H -2.218400 1.614700 2.979300 H -0.808700 2.465500 2.311600 H -0.587500 0.954400 3.244500 C -2.137700 0.072400 -0.962300 C 2.349400 0.635700 2.244900 H 3.450400 0.706200 2.331800 H 2.014400 -0.129300 2.964900 H 1.935800 1.607700 2.556100 C -3.692500 0.094200 -1.128800 H -4.017200 -0.762300 -1.742000 H -4.004400 1.020600 -1.638300 H -4.185600 0.041000 -0.144600 C -1.762700 2.644500 -0.427200 H -2.821200 2.955900 -0.328600 H -1.467700 2.829800 -1.473800 H -1.167500 3.301100 0.226300 C -1.803700 -2.550800 -0.723200 H -2.859800 -2.871200 -0.625200 H -1.189500 -3.278700 -0.169500 H -1.543000 -2.614200 -1.792600 H -1.705400 0.127300 -1.982800 52 Cp* exo Cp*H Co cation doublet Co 0.012400 -0.001600 -0.037400 C -1.714900 -1.164000 -0.249300 C 1.753300 1.118800 0.007900 C 1.397000 0.693500 1.323600 C -1.308500 2.108700 -2.119500 H -2.301300 2.375800 -2.527700 H -0.662200 1.845700 -2.972200 H -0.899100 3.012600 -1.642500 C -1.754100 1.104400 0.269800 C -1.444600 0.972700 -1.144400 C -1.970700 2.402600 0.993000 H -3.011700 2.744000 0.834900 H -1.307500 3.200000 0.620800 H -1.821900 2.302200 2.079700 C -1.884400 -2.652200 -0.143100 H -2.945700 -2.917100 -0.310900 H -1.608800 -3.029800 0.854500 H -1.290300 -3.191100 -0.897400 402 C -1.323400 -1.038500 -2.837600 H -2.329600 -1.255900 -3.242900 H -0.766800 -1.989400 -2.820700 H -0.827200 -0.357600 -3.546900 C 1.344500 -0.763400 1.337100 C 1.665100 -1.235700 0.027600 C -1.932200 -0.211400 0.817700 C -1.453600 -0.432900 -1.468100 C 1.186600 1.577100 2.519700 H 2.139500 1.671300 3.074600 H 0.440100 1.164900 3.217800 H 0.875100 2.592000 2.230200 C 1.077700 -1.611700 2.547400 H 2.034000 -1.823800 3.061900 H 0.632600 -2.581800 2.276800 H 0.418500 -1.109600 3.272500 C 2.340300 -0.087500 -0.743400 C -2.376300 -0.540400 2.214700 H -3.482000 -0.550200 2.263400 H -2.025500 0.203300 2.949000 H -2.028000 -1.534600 2.534200 H 3.422800 -0.128600 -0.464500 C 1.803400 -2.671100 -0.385400 H 2.854400 -3.000100 -0.260600 H 1.545400 -2.817300 -1.447600 H 1.173900 -3.339900 0.221600 C 1.993500 2.532900 -0.430400 H 3.061700 2.793500 -0.292800 H 1.399200 3.253800 0.152500 H 1.761700 2.674400 -1.498700 C 2.274300 -0.090600 -2.276600 H 2.727100 -1.011400 -2.680200 H 2.845200 0.759100 -2.685900 H 1.240400 -0.023600 -2.648300 2 CO C 0.000000 0.000000 -0.651200 O 0.000000 0.000000 0.488400 19 CNXyl N 1.118400 -1.963900 2.628900 C 1.404500 -0.868500 2.268400 C 0.800400 -3.246900 3.053500 C 1.127600 -4.340200 2.212400 C 0.794900 -5.626600 2.666800 H 1.035600 -6.490000 2.037000 403 C 0.164500 -5.815400 3.903800 H -0.087700 -6.827200 4.238000 C -0.146400 -4.715700 4.714500 H -0.638300 -4.869300 5.680900 C 0.163000 -3.407200 4.309700 C 1.806700 -4.110800 0.884100 H 1.192400 -3.467300 0.228200 H 1.988800 -5.065900 0.367200 H 2.775200 -3.594900 1.016100 C -0.162300 -2.206200 5.164100 H -0.824500 -1.502600 4.627200 H 0.752400 -1.642900 5.424100 H -0.659100 -2.511800 6.098200 81 dppeFeI exo CpstarH CO D 89 cation doublet Fe 1.659500 1.152600 1.279000 P 3.165700 -0.005700 2.656400 P 3.288500 0.931300 -0.413900 C -0.326000 1.800300 1.582200 C -1.586900 0.981700 1.546200 H -1.601700 0.211800 2.336300 H -1.735000 0.482000 0.576600 H -2.458700 1.642300 1.721100 C 0.254300 2.469200 0.452400 C -0.279300 2.472900 -0.953100 H 0.517900 2.624300 -1.697500 H -1.015800 3.290700 -1.074400 H -0.791700 1.527800 -1.193200 C 1.351600 3.277900 0.947300 C 2.077400 4.294100 0.109000 H 2.319000 3.919300 -0.897600 H 3.003100 4.653100 0.579500 H 1.410400 5.168000 -0.019400 C 1.478300 3.070900 2.348300 C 2.398700 3.823400 3.268100 H 3.288700 4.213700 2.749700 H 2.741000 3.201200 4.110700 H 1.862400 4.692000 3.698800 C 0.147200 2.497700 2.859300 C 4.550500 -0.594600 1.519800 H 5.375800 0.133400 1.595000 H 4.924700 -1.562300 1.892900 C 4.081800 -0.700500 0.063100 H 3.336100 -1.504900 -0.056200 H 4.929200 -0.914800 -0.610800 C 4.067800 0.756700 4.086000 404 C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H O C C 5.269300 1.478200 3.921300 5.729800 1.584000 2.934400 5.905800 2.066800 5.023600 6.846900 2.606400 4.877200 5.348600 1.956100 6.305000 5.849200 2.412400 7.164800 4.150400 1.248200 6.479600 3.711000 1.148500 7.477700 3.515200 0.651800 5.382300 2.592400 0.085700 5.541900 2.557600 -1.584700 3.415100 3.485300 -2.446600 4.043400 4.544800 -2.171700 4.101300 3.060700 -3.653300 4.611100 3.791000 -4.314600 5.088700 1.703600 -4.012200 4.571100 1.372900 -4.956900 5.014500 0.774300 -3.157100 3.965500 -0.286000 -3.428700 3.935000 1.199000 -1.949300 3.390200 0.464400 -1.289700 2.920100 4.806300 1.992100 -0.501900 5.102200 2.879000 0.550000 4.388100 2.991500 1.372500 6.302300 3.607300 0.558400 6.516600 4.295400 1.383300 7.221700 3.453400 -0.488300 8.157600 4.021500 -0.486300 6.940300 2.565300 -1.538800 7.658100 2.436900 -2.356300 5.743700 1.836300 -1.548100 5.539400 1.146700 -2.373900 2.797400 0.725800 -2.179100 2.273300 -0.497600 -2.649600 2.188800 -1.363600 -1.986600 1.850200 -0.623200 -3.980500 1.454100 -1.581800 -4.331100 1.936100 0.466800 -4.858100 1.606700 0.364000 -5.896900 2.451300 1.688200 -4.399500 2.530600 2.542900 -5.079500 2.877200 1.819400 -3.070900 3.297100 2.774100 -2.735900 0.520500 -1.296600 0.142800 1.020900 -0.351600 0.613000 0.151800 1.681600 4.158900 405 H 0.556800 2.283900 4.989400 H 0.757200 0.764500 4.072900 H -0.875200 1.388900 4.434200 H -0.531500 3.369200 3.037300 81 dppeFeI exo CpstarH CO D -53 cation doublet Opt Freq Fe 1.717900 1.116100 1.485500 P 3.594100 0.356900 2.688800 P 2.876000 0.493900 -0.376400 C 0.992900 3.094700 0.790000 C 1.287100 3.907400 -0.437800 H 1.108900 3.331000 -1.360100 H 2.317200 4.290300 -0.462200 H 0.602400 4.777700 -0.467100 C 1.624000 3.215700 2.052400 C 2.702600 4.191100 2.437200 H 3.490600 3.731500 3.054300 H 2.250400 5.007000 3.032900 H 3.170700 4.656300 1.556200 C 0.927600 2.351900 3.000800 C 1.158700 2.358600 4.486500 H 2.213600 2.546100 4.737800 H 0.848800 1.414700 4.960700 H 0.559300 3.168100 4.945900 C -0.150400 1.720200 2.310100 C -1.232700 0.902100 2.960200 H -0.871500 0.369300 3.854000 H -1.665100 0.160200 2.269600 H -2.053700 1.572000 3.282000 C -0.388800 2.453900 0.989000 C 4.705300 -0.500900 1.421100 H 5.476300 0.224800 1.112300 H 5.221300 -1.348200 1.902000 C 3.901900 -0.959400 0.199000 H 3.218300 -1.785300 0.459900 H 4.564200 -1.302200 -0.614800 C 4.807700 1.437500 3.596400 C 5.805700 2.165900 2.912000 H 5.906100 2.088900 1.824600 C 6.696100 2.993200 3.611700 H 7.473500 3.535300 3.062700 C 6.598800 3.119500 5.004900 H 7.297100 3.762700 5.550500 C 5.607800 2.407200 5.695500 H 5.529300 2.490400 6.785200 C 4.721600 1.572000 5.000600 406 H C C H C H C H C H C H C C H C H C H C H C H C C H C H C H C H C H O C C H H H H 3.973500 1.002400 5.561100 3.328800 -0.983200 3.946300 4.437900 -1.697700 4.454100 5.453600 -1.450900 4.124500 4.257600 -2.715300 5.398000 5.126300 -3.262500 5.778200 2.968800 -3.029200 5.858500 2.828500 -3.826000 6.596500 1.864400 -2.318100 5.373300 0.857200 -2.554900 5.730900 2.044300 -1.301700 4.422100 1.176100 -0.755600 4.044700 4.177600 1.560000 -1.144700 4.604000 2.737600 -0.504500 4.111400 3.052600 0.419300 5.656100 3.499200 -1.036700 5.975500 4.416400 -0.531200 6.293400 3.086200 -2.214200 7.113100 3.679400 -2.632400 5.879200 1.909200 -2.857400 6.375100 1.582300 -3.777000 4.829200 1.147400 -2.328700 4.513700 0.233600 -2.843400 1.905400 -0.120000 -1.819000 1.378500 -1.429800 -1.833500 1.566900 -2.118600 -1.005200 0.602800 -1.868300 -2.916200 0.209600 -2.889900 -2.918500 0.335000 -1.007300 -3.990300 -0.269500 -1.353900 -4.834900 0.848600 0.297800 -3.981200 0.648200 0.974400 -4.818600 1.628300 0.741000 -2.904500 2.040400 1.754900 -2.920900 0.508400 -1.560800 1.463300 1.025500 -0.513300 1.491000 -1.012000 1.669900 -0.177200 -2.009000 1.294500 0.107100 -0.393400 0.812600 -0.486900 -1.148700 2.328400 -1.051300 -1.086000 3.301800 1.210500 407 Square Scheme: Figure D.61. Thermochemical scheme relating H+, H•, and H− transfers for endo-[3]+. The oxidation potential of B and the pKa of C could not be determined due to the low stability of the species involved. The upper limit for the pKa of B was estimated using the experimentally determined upper limit of the BDFE of B and reduction potential of E 408 References: 1 Prisecaru, I. WMOSS4 Mössbauer Spectral Analysis Software, www.wmoss.org, 20092016. 2 Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 2006, 178, 42. 2 Hamon, P.; Toupet, L.; Hamon, J. R.; Lapinte, C. Novel diamagnetic and paramagnetic iron(II), iron(III), and iron(IV) classical and nonclassical hyrides. X-ray crystal structure of [Fe(C5Me5)(dppe)D]PF6. Organometallics 1992, 11, 1429. 4 Hamon, P.; Hamon, J. R.; Lapinte, C. Isolation and characterization of a cationic 19electron iron(III) hydride complex; electron transfer induced hydride migration by carbon monoxide at an iron(III) centre. J. Chem. Soc. Chem. Commun. 1992, 1602. 4 Similar behavior has been noted: a) Zhang, F.; Xu, X.; Zhao, Y.; Jia, J.; Tung, C. –H.; Wang, W. Solvent Effects on Hydride Transfer from Cp*(P-P)FeH to BNA+ Cation. Organometallics 2017, 36, 1238; b) Zhang, F.; Jia, J.; Dong, S.; Wang, W.; Tung, C. –H. Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)+ Analogues. Organometallics 2016, 35 1151. 409 6 Sheldrick, G. M.; A short history of SHELX. IUCr. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. 7 Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339. 8 Neese, F. The ORCA program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73. 9 Neese, F. Software update: the ORCA program system, version 4.0. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8, 1327. 10 Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the density functional ladder: nonempirical meta-generalized gradient approximation designed for molecules and solids. Phys. Rev. Lett. 2003, 91, 146401. 11Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297. 12 Neese, F. Prediction and interpretation of the 57Fe isomer shift in Mössbauer spectra by density functional theory. Inorg. Chim. Acta 2002, 337, 181. 13 Kutzelnigg, Werner, Fleischer, Ulrich, Schindler, Michael. The IGLO-Method: Abinitio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities in 'Deuterium and Shift Calculation' ed. Maryvonne L. Martin, Gérard J. Martin, NMR Basic Principles and Progress, 16, 165-262 14 Matson, B. D.; Peters, J. C. ACS Catal. 2018, 8, 1448.