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 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 [(P₃^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.

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