The overarching goal of the studies presented herein was to investigate the mechanisms of catalytic homogeneous water oxidation in order to develop more active and robust catalysts.
In Chapter 2, a series of polypyridyl Ru complexes related to [RuII(tpy)(bpy)(OH2)]2+ were synthesized and characterized. The complexes were investigated for catalytic water oxidation capabilities when driven by the sacrificial, one-electron oxidant, (NH¬4)2[CeIV(NO3)6]. Chemical modification of the bidentate ligand that resides trans to the active Ru-O vector has the most pronounced effects on catalytic performance in terms of catalytic turnovers and rates. Decomposition pathways were also identified. Chapter 3 provides spectroscopic, electrochemical and kinetic investigations of complexes related to [RuII(tpy)(bpy)(OH2)]2+, and expose the role that electronic modification of the ligand framework has on the kinetics of the catalytic water oxidation mechanism. Experiments using in situ electrospray-ionization mass spectrometry provided structural verification of proposed catalytic intermediates. Additional experiments suggested an unprecedented O-O bond formation pathway involving a Ce species.
The effects of reaction medium on the Ce(IV) oxidant, and the catalytic mechanism were investigated in Chapter 4. It was found that the Ce(IV) reduction potential is only mildly perturbed by the nature of the acid anion (e.g. HNO3, HClO4, CF3SO3H and H2SO4), and that certain conditions promote the spontaneous decomposition of Ce(IV). The basicity of NO3- relative to the other acids anions facilitates certain reaction steps, however, at the cost of undesired sources of O-atoms.
Chapter 5 discusses the development of [CoII(PY5)(OH2)]2+ as a possible water oxidation electrocatalyst. Unprecedented and well-defined proton-coupled electron transfer chemistry conceivably enables access to a high-valent and catalytically active [CoIV-OH]3+ species that undergoes O-O bond formation with OH-. Detection of dioxygen using electrolytic methods confirmed catalysis, and thus established [CoII(PY5)(OH2)]2+ as the first single-site Co water oxidation electrocatalyst. Kinetic and mechanistic investigations using cyclic voltammetry provided insights into the proposed catalytic mechanism. Issues associated with potential decomposition of [CoII(PY5)(OH2)]2+ into catalytically active Co-oxides were investigated and suggest the possibility of genuine molecular origins to catalysis, but with significant encumbrances brought on by potential decomposition species.