O-O bond formation and the transition metal chemistry of [beta]-diketiminate, siloxide, and triamide ligands

Abstract

The electrochemical splitting of water into hydrogen and oxygen has been proposed as an alternative means to store electrical energy. The limiting aspect of this reaction is the oxygen forming reaction, which can be catalyzed by transition metal species with varying degrees of efficiency. This thesis examines the characteristics of oxygen bonding that complicate the 0-0 bond formation reaction, and examines ligand platforms that can stabilize high valent metal oxo intermediates. Siloxide ligands were used to generate a series of 4-coordinate Cr, Mn, Fe, and Co complexes, but these could not support the corresponding high valent oxo species. Instead, a remarkably stable 4-coordinate Crv tetrasiloxide complex was isolated. Chelating triamide ligands were explored, which might generate pseudotetrahedral metal oxo species, but complexes of various metal ions could not be reliably isolated. Planar bis-pdiketiminate (NacNac) complexes were synthesized by reaction of acetonitrile with Fe and Co mesityl species. The Co complex undergoes a ligand centered oxidation event to yield the first structurally characterized NacNac radical cation. In contrast to known redox noninnocent ligand platforms, no significant changes in C-C or C-N bond lengths are observed by X-ray crystallography. DFT calculations and the electronic and structural characterization of the oxidized and reduced Co complex confirm these conclusions.