Base Metal Complexes of Ketone and Imine Ligands: Metal-Ligand Cooperativity and Catalytic Hydrosilylation

Abstract

The incorporation of cooperative ligands in metal complexes is gaining interest for the development of new homogeneous catalysts. Productive elementary reactions in catalysis, such as bond-making and bond-breaking processes, are often two-electron transformations which are generally well mediated by precious metals, i.e. late second and third row transition metals. However, economic and environmental considerations warrant the investigation of catalysts based on base metals such as Fe, Co, Ni and Cu, which tend to undergo one-electron reactions. Employing cooperative ligands – ligands that directly participate in chemical reactions – in base metal catalysis can lead to enhanced reactivity and selectivity, overcoming often undesired one-electron pathways. The use of π-ligands, for example ketone and imine ligands, for such processes is a promising strategy, as these ligands can adapt their bonding in versatile ways and therewith stabilize the metal center during reactivity. In this thesis, the synthesis, complexation, and reactivity of two phosphine tethered π-ligand frameworks, containing a ketone or an imine backbone, are described. The investigated complexes show good to excellent reactivity in the catalytic hydrosilylation of alkene and carbonyl substrates with either phenyl- or diphenylsilane under mild conditions. Interestingly, the use of a nickel-based complex with the imine ligand lead to highly selective stoichiometric reactivity of the complex with silanes, allowing for in-depth characterization of possible reactive intermediates, which were investigated both experimentally and computationally. In the long run, metal-ligand cooperativity is envisioned to become a key-feature in the design of earth abundant metal complexes. The performed research on the coordination and reactivity of π-ligands offers a better understanding of these ligands and their reactions, and will hopefully inspire future research regarding new avenues in metal-ligand cooperativity.