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Proton-Coupled Reduction of N2 Facilitated by Molecular Fe Complexes

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Proton-Coupled Reduction of N2 Facilitated by Molecular Fe Complexes Proton-Coupled Reduction of N2 Facilitated by Molecular Fe Complexes Thesis by Jonathan Rittle In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2016 (Defended November 9, 2015) ii 2016 Jonathan Rittle All Rights Reserved iii ACKNOWLEDGEMENTS My epochal experience in graduate school has been marked by hard work, intellectual growth, and no shortage of deadlines. These facts leave little opportunity for an acknowledgement of those whose efforts were indispensable to my progress and perseverance, and are therefore disclosed here. First and foremost, I would like to thank my advisor, Jonas Peters, for his tireless efforts in molding me into the scientist that I am today. His scientific rigor has constantly challenged me to strive for excellence and I am grateful for the knowledge and guidance he has provided. Jonas has given me the freedom to pursue scientific research of my choosing and shown a remarkable degree of patience while dealing with my belligerent tendencies. Perhaps equally important, Jonas has built a research group that is perpetually filled with the best students and postdoctoral scholars. Their contributions to my graduate experience are immeasurable and I wish them all the best of luck in their future activities. In particular, John Anderson, Dan Suess, and Ayumi Takaoka were senior graduate students who took me under their wings when I joined the lab as a naïve first-year graduate student. I am grateful for the time and effort that they collectively spent in teaching me the art of chemical synthesis and for our unforgettable experiences outside of lab. Hill Harman and Marc-Etienne Moret were excellent postdocs that taught me a great deal about all aspects of science while providing comprehensive guidance. I joined the lab concurrent with Sid Creutz, who has been a truly great friend and one of the most brilliant people I have met. I have had the distinct pleasure of eating lunch with Charles McCrory over much of the past five years and I will sorely miss his humor and ‘positive’ attitude. My thesis committee has additionally consisted of Theo Agapie, Harry Gray, and Doug Rees, who have provided me with a wealth of feedback, criticism, and insight towards my research objectives that will undoubtedly shape many aspects of my future in science. The exceptional staff at Caltech have 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 aspects of X-ray crystallography and I am grateful for these efforts and his friendship. Michael T. Green has been a dedicated mentor and friend of mine during the past 8 years and I am grateful for everything he has done for me. I would like to thank my family and friends that I have met at Caltech or otherwise who have provided unwavering support during the past five years. Finally, I would like to thank my best friend, Liz Smith, whose sincerity and compassion during the past four years may have played the single largest role in my completion of these endeavors. iv ABSTRACT The activation of Fe-coordinated N2 via the formal addition of hydrogen atom equivalents is explored in this thesis. These reactions may occur in nitrogenase enzymes during the biological conversion of N2 to NH3. To understand these reactions, the N2 reactivity of a series of molecular I Fe(N2) platforms is investigated. A trigonal pyramidal, carbon-ligated Fe complex was prepared that displays a similar geometry to that of the resting state ‘belt’ Fe atoms of nitrogenase. Upon reduction, this species was shown to coordinate N2, concomitant with significant weakening of the C-Fe interaction. This hemilability of the axial ligand may play a critical role in mediating the interconversion of Fe(NxHy) species during N2 conversion to NH3. In fact, a trigonal pyramidal borane-ligated Fe complex was shown to catalyze this transformation, generating up to 8.49 equivalents of NH3. To shed light on the mechanistic details of this reaction, protonation of a borane-ligated Fe(N2) complex was investigated and found to give rise to a mixture of species that contains an iron hydrazido(2-) [Fe(NNH2)] complex. The identification of this species is suggestive of an early N-N bond cleavage event en route to NH3 production, but the highly-reactive nature of this complex frustrated direct attempts to probe this possibility. A structurally-analogous silyl- ligated Fe(N2) complex was found to react productively with hydrogen atom equivalents, giving rise to an isolable Fe(NNH2) species. Spectroscopic and crystallographic studies benefitted from the enhanced stability of this complex relative to the borane analogue. One-electron reduction of this species initiates a spontaneous disproportionation reaction with an iron hydrazine [Fe(NH2NH2)] complex as the predominant reaction product. This transformation provides support for an Fe- mediated N2 activation mechanism that proceeds via a late N-N bond cleavage. In hopes of gaining more fundamental insight into these reactions, a series of Fe(CN) complexes were prepared and reacted with hydrogen-atom equivalents. Significant quantities of CH4 and NH3 are generated in these reactions as a result of complete C-N bond activation. A series of Fe(CNHx) were found to be exceptionally stable and may be intermediates in these reactions. The stability of these compounds permitted collection of thermodynamic parameters pertinent to the unique N-H bonds. This data is comparatively discussed with the theoretically-predicted data of the N2-derived Fe(NNHx) species. Exceptionally-weak N-H bond enthalpies are found for many of these compounds, and sheds light on their short-lived nature and tendency to evolve H2. As a whole, these works both establish and provide a means to understand Fe-mediated N2 activation via the addition of hydrogen atom equivalents. v TABLE OF CONTENTS Acknowledgements ........................................................................................................ iii Abstract ........................................................................................................................... iv Table of Contents ............................................................................................................ v Detailed Table of Contents ............................................................................................. vi List of Figures ............................................................................................................... viii List of Schemes ............................................................................................................. xv List of Tables ................................................................................................................ xvi List of Abbreviations ................................................................................................... xvii Published Content and Contributions .......................................................................... xxi Chapter 1: Introduction ................................................................................................... 1 Chapter 2: Catalytic Conversion of N2 to NH3 by an Fe Model Complex .................. 14 Chapter 3: Fe-N2/CO Complexes that Model a Possible Role for the Interstitial C Atom of FeMoco ..................................................................................... 40 Chapter 4: Stepwise Conversion of Fe-N≡N to Fe-NH2-NH2 via a Redox-Active Fe=N=NH2 Complex ................................................................................... 66 Chapter 5: Proton-Coupled Reduction of Fe-CN to CH4 and NH3 ............................. 90 6 Chapter 6: A 10 -Fold Enhancement in N2-Binding Affinity of an Fe2(μ-H)2 Core Upon Reduction to a Mixed-Valence FeIIFeI State .................................. 121 Appendix 1: Supplementary Data for Chapter 4 ........................................................ 158 Appendix 2: Supplementary Data for Chapter 5 ........................................................ 207 vi DETAILED TABLE OF CONTENTS Chapter 1: Introduction ................................................................................................... 1 1.1. Opening Remarks .................................................................................................. 2 1.2. Transition Metal-mediated Dinitrogen Activation ............................................... 2 1.3. Biological N2 activation ........................................................................................ 6 1.4. N2 Activation at Well-defined Fe Sites ................................................................. 7 1.5. Overview of Individual Chapters .......................................................................... 9 1.6. Cited References.................................................................................................. 12 Chapter 2: Catalytic Conversion of N2 to NH3 by an Fe Model Complex .................. 14 2.1. Introduction ......................................................................................................... 15 2.2. Results ................................................................................................................. 17 2.2.1. Stoichiometric NH3 Generation from (TPB)Fe(N2) Complexes .............. 17 2.2.2. Catalytic NH3 Generation from (TPB)Fe(N2) Complexes ........................ 20 2.2.3. NH3 Production from Alternative
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