Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals

Citation

Gu, Nina Xiao (2020) Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8zwz-gh20. https://resolver.caltech.edu/CaltechTHESIS:04102020-135244808

Abstract

Reactive metalloradical intermediates have been implicated in both biological and synthetic catalyst systems for small molecule activation processes, including proton reduction and ammonia oxidation. Towards a greater mechanistic understanding of such transformations on well-defined model complexes, this thesis explores relevant H–H and N–N bond-forming reactions mediated by trivalent Fe and Ni species, as well as catalytic N–N bond cleavage mediated by an open-shell VFe bimetallic complex. First, a pair of thiolate-supported, S = ½ iron and nickel hydrides are synthesized and spectroscopically characterized at low temperatures (Chapters 2, 3). Paramagnetic iron and nickel hydrides have been proposed as catalytic intermediates of [NiFe] hydrogenase and nitrogenase, but characterization of such molecular species are limited. For both the Fe^III and Ni^III hydride complexes described herein, spin delocalization onto the thiolate ligand is proposed to stabilize the formal 3+ metal oxidation state. Furthermore, both the Fe^III–H and Ni^III–H species are demonstrated to undergo the bimolecular reductive elimination of dihydrogen upon warming, albeit with distinct activation parameters consistent with different proposed pathways for H–H bond formation. Chapter 4 expands upon the H–H bond forming chemistry demonstrated on the Ni system to demonstrate related N–N bond formation from an analogous Ni^III–NH₂ species, resulting in the formation of a Ni^II₂(N₂H₄) complex. Given the diverse mechanistic possibilities for the overall 6e^-/6H⁺ transformation to oxidize ammonia to dinitrogen, identification of the active M(NH_x) intermediate and pathway for N–N bond formation is a central mechanistic question. While the homocoupling of M–NH₂ species to form hydrazine has been hypothesized as the key N–N bond forming step in ammonia oxidation systems, stoichiometric examples of this transformation from M–NH₂ complexes are rare. Lastly, Chapter 5 details the synthesis of a heterobimetallic VFe complex featuring a bridging thiolate, inspired by the structure of the VFe nitrogenase cofactor. This VFe species is demonstrated to be an active catalyst for the disproportionation of hydrazine to dinitrogen and ammonia. Notably, the heterobimetallic complex is appreciably more active than monometallic analogues of the individual V and Fe sites, suggesting that bimetallic cooperativity may facilitate the observed catalysis.

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