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Abstract

This thesis examines the use of bidentate N,N and P,N-chelating ligands in nickel(0) and nickel(II) mediated activation of inert carbon-hydrogen and carbon-halogen bonds in organic substrates. Fundamental mechanistic investigations, coordination chemistry and implications in catalysis are explored and provide insights into factors that can both improve and impede reactivity. Chapter 1 discusses the need for developing earth abundant first-row transition-metal catalysts for progressing organic synthesis through reaction discovery. Contrasting trends and reactivity differences between nickel and palladium catalysts, and their roles in commonly encountered C−C bond forming reactions are explored. Lastly, the importance of ligand design towards the development of new catalysts is explained, and commonly encountered nitrogen and phosphorus bidentate ligands for established nickel catalysts are provided. Chapter 2 investigates the electronic influence of 8-aminoquinoline, a directing group used to promote C−H activation. We monitored the C(sp³)−H activation of tertiary ureas through kinetic experiments and a Hammett analysis, revealing unproductive nickel speciation resulting from electronic modifications. Synthetic experiments to identify the unwanted nickel complexes were conducted, and the implications of the electronic influence of the directing group were used in the development of a catalytic C−C bond forming reaction to create allylic ureas. In Chapter 3 a library of neutral bidentate P,N ligands was investigated for their coordination behavior with nickel(II). Phosphorus donor atoms with sterically bulky, electron-rich alkyl groups demonstrate a highly strained κ²⁻P,N-nickel(II) complexes that demonstrate fluxional geometries. Spin-crossover events, where the metal center switches from a low-spin (S = 0) square planar state to a high-spin (S = 1) tetrahedral state, are investigated experimentally with the support of computational calculations from a collaborator. Additionally, the ambient stability and unexpected reactivity of the κ²⁻P,N-nickel(II) complexes are presented. Chapter 4 attempts to leverage κ²⁻P,N-nickel(II) complexes to promote cross-electrophile coupling of commercially available N-containing heterocycles. Selective homo-coupling of chloropyridine substrates is observed under mild conditions and is hypothesized to be the result of a bridging coordination mode with hemilabile P,N-ligands. Mechanistic insights, intermediates, and catalyst resting states are explored and a catalytic cycle resulting in the exclusive homo-coupled products is proposed.