Halogen-elimination photochemistry and oxygen-activation chemistry of late transition-metal complexes

Abstract

Multi-electron reaction chemistry, from both ground- and excited-state species, is at the heart of many topics in renewable energy and catalysis. In this thesis, two classes of reactions central to the themes of energy conversion and multi-electron chemistry are studied on mono- and bimetallic late transition-metal platforms. In the early chapters, studies of photochemical halogen elimination, the key energy-storing step in photocatalytic hydrogen production from HX (X = Cl, Br), are described. In the latter sections of the thesis, the oxygen-activation and reduction chemistries of rhodium and iridium hydride complexes are highlighted. In Chapters 1 and 2, photochemical halogen elimination from a variety of late transition-metal complexes is described. Studies of phosphine-terminated gold(III) halide complexes demonstrated that efficient halogen photoelimination can be promoted by ligand-to-metal charge-transfer (LMCT) excitation, in complexes devoid of a formal metal-metal interaction. In addition, gold was partnered with rhodium and iridium in a series of heterobimetallic complexes, and these complexes were also shown to cleanly eliminate halogen when illuminated, with additional electronic structural insights and reactivity trends emerging from this latter suite of compounds. In Chapters 3-6, small-molecule reactivity studies of rhodium and iridium complexes, with a particular slant towards oxygen reduction, are disclosed. A new class of two-electron mixedvalent dirhodium and diiridium complexes is described. Featuring a coordinatively unsaturated M0 center, these complexes display an expansive reactivity with numerous small-molecule substrates. A dirhodium hydride complex, prepared by HCl addition to the mixed-valent precursor, mediates the reduction of oxygen to water. Studies on iridium model complexes, coupled with detailed kinetic studies, produced a clear mechanistic understanding of this chemistry. In particular, the preparation and reactivity of a diiridium hydroperoxo complex gave many key insights into the activation of O2 and the subsequent release of water. Analogous oxygen-reduction chemistry was also demonstrated to occur on a monorhodium platform, which will facilitate detailed mechanistic studies enabled by systematic ligand alteration.