N-Phosphinoamidinate Nickel Complexes
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Society demands cheaper and more abundant metal catalysts to accomplish the tasks previously performed by precious metal catalysts. In turn, chemists have developed ancillary ligands designed to harness the power of first-row metals, such as Mn, Fe, Co, and Ni. In Chapter 1, these design principles are rationalized and summarized, and applied to a variety of important ligand classes. One such class includes N-phosphinoamidinate (PN) ligands. (PN) complexes of first-row metals have been applied as precatalysts for hydrosilative carbonyl reductions (in the case of Fe) and terminally selective alkene isomerization/hydroboration (I-H; in the case of Co). Given a recent surge of advances in homogeneous catalysis using Ni, I began my thesis research with synthetic attempts towards (PN) complexes of Ni. Over the course of my research, I developed a family of related complexes. Ultimately, I was able to harness these complexes to generate the reactive, transient, three-coordinate Ni hydride (PN)NiH, which unlocked three thesis chapters. In Chapter 2, I describe a mechanistic study of alkene I-H via isostructural (PN) Mn, Fe, Co, and Ni precatalysts. The Ni precatalyst performed poorly, and isolable Ni hydride complexes proved unsuitable for the reaction, but the diamagnetism of the Ni complexes afforded in situ monitoring of catalytic intermediates to help determine the origin of terminal selectivity. Chapter 3 contains an analogous study of hydrosilative amide reduction, wherein (PN)NiH instead proved remarkably competent to reduce challenging substrates via optimization of reaction conditions. Iterative optimization of the precatalyst proved the utility of (PN)Ni(OtBu), itself a rare example of a three-coordinate nickel alkoxo complex. Chapter 4 reports the spectroscopic and X-ray characterization of a unique compound conceived by (PN)NiH inserting benzene. It also includes unusual β- or γ- agostic (PN)Ni(alkyl) complexes synthesized in attempts to access (PN)NiH in a controlled fashion via β-hydride elimination.