A study of new catalyst systems for the homogeneous oxidation of water.

摘要

The ability to drive energetically uphill homogeneous and microheterogeneous chemistry using radiative energy, particularly the visible wavelengths prevalent in sunlight, has long been pursued in chemical research both for industrial as well as purely scientific goals. One of the most fundamental challenges in the progression of this field is the conversion of water into dihydrogen and dioxygen gas. Advancement in water splitting is currently hindered by the discrepancy in progress between the catalysis of its two half reactions. While water reduction to form hydrogen has been achieved photocatalytically with large quantum yield and respectable longevity, the complementary water oxidation to form oxygen has never been driven strictly by visible light. Furthermore, homogeneous catalysis of the reaction suffers both from slow turnover frequencies and short-lived catalyst systems, and are only reported under the most extreme of oxidative environments, precluding simultaneous hydrogen generation. In short, this entire subfield awaits a revolution in water oxidation catalysis.;Within this thesis are described two new classes of molecular catalysts for oxygen evolution. The first of these contains an iridium(III) center, chelated by two cyclometalating moieties, typically substituted phenylpyridine. The remaining two octahedral coordination sites are occupied either by acetonitrile or water, and are believed to be the active site for oxidation of water. Their investigation is the first report of an iridium-based homogeneous catalyst for water oxidation, and the second report of oxygen evolution from a single metal center. The second catalyst studied bears resemblance to one proposed structure of the tetramanganese core in Photosystem II's WOC enzyme. While the natural, theorized structure consists of four manganese atoms attached by oxygen in either an oxo-cubane or oxo-adamantane structure, its molecular mimic herein described instead possesses four ruthenium atoms connected as an oxo-adamantane, sandwiched by two decatungstosilicate stabilizing units. This immense structure falls into a class of species known as polyoxometalates. Its synthesis was perfected by the Bonchio group at the University of Padova, and its catalysis was studied here in the Bernhard lab. Devoid of oxidizable organic substrates, this catalyst can achieve water oxidation with turnover frequencies of 450 hr-1 and lifetimes approaching 500 turnovers.