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Abstract

This thesis describes the chemistry of Ni complexes supported by an N-Heterocyclic Carbene (NHC) ligand that contain sulfur atoms in different binding modes. The unique steric and electronic properties of NHCs allow for the preparation and interconversion of four different sulfur-based groups. These complexes are markedly different than related complexes in the literature, and can be used as a model to study the H2 activation step of the hydrodesulfurization process. Chapter One describes the prevalence of nickel complexes containing bridging thiolate and sulfide groups. These complexes serve key roles in the small molecule activation and electron transport in enzyme active sites, and perform the key activation step of H2 in the hydrodesulfurization process. Model used to study this activation step exhibit fluxional behavior in solution, which limits the conclusions about the exact mechanistic details. These limitations can be overcome by using an NHC ligand. The core imidazole ring imparts NHC ligands with net electron donor ability that is greater than that of phosphine ligands. Additionally NHC ligands possess lateral steric bulk that can shelter a metal center and support low-coordinate environments. Chapter Two describes the synthesis and reactivity of the dinuclear bridging disulfide complex {(IPr)ClNi}2(μ2 η2,η2-S2) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) . This complex can be converted to a mononuclear (IPr)(AdNC)Ni(η2-S2) complex with a terminal disulfide, a dinuclear {(IPr)Ni(μ-S)}2 complex with two bridging sulfides, and a dinuclear {(IPr)Ni(μ-SH)}2 complex two bridging hydrosulfides. Most interestingly, the bridging sulfide and bridging hydrosulfide complexes are interconverted through reaction with H2 and H atom abstractor 2,4,6-tBu3-phenoxy radical. In Chapter Three, the hydrogenation of {(IPr)Ni(μ-S)}2 to {(IPr)Ni(μ-SH)}2 is probed in depth through a combination of kinetic studies and DFT calculations. These results show that H2 adds across a Ni-S bond in a heterolytic manner to generate a Ni(H)(μ-S)(μ-SH)Ni intermediate that rearranges to the product by H atom migration from Ni to the remaining μ-S ligand. Chapter Four describes the reaction of {(IPr)Ni(μ-S)}2 with H-BPin to produce {(IPr)Ni}2(μ-SH)(μ-SBPin). This reaction suggests that heterolytic E-H bond activation by the Ni2(μ-S)2 unit may be a general reaction.

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