The conversion of solar energy into a useful chemical fuel represents a major scientific goal in the drive towards a society powered by renewable energy. Several potential solar fuels seem possible, including hydrogen from proton reduction, or hydrocarbons from the reduction of carbon dioxide. Useful rates of solar fuel production must rely on an efficient oxidation that generates electrons and protons. In nature's photosynthesis, this is accomplished by a tetra-manganese oxo cluster (the oxygen-evolving complex, OEC), which yields 4 protons and 4 electrons by the photo-oxidation of water (the oxygen evolution reaction, OER). For solar fuel applications this water-splitting half reaction must be catalyzed to make it energetically feasible, and transition-metal oxo cubane clusters related to the OEC represent intriguing model systems and design motifs for new water-splitting catalysts based on abundant metals. Molecularly derived catalysts of this type offer potential advantages, including the synthetic tunability of catalytic and chemical properties. In addition, the study of high-valent molecular species can provide key insights into the mechanism of water oxidation and help bridge the gap between solid-state and molecular systems to allow for more rational design of catalysts. This presentation will describe high-valent metal complexes and clusters, and a detailed mechanism for the evolution of oxygen via water oxidation at a tetranuclear cobalt oxo cubane. An important aspect to this catalysis relates to the manner in which metals cooperate to mediate multi-electron, multi-proton oxidations. In further pursuit of these concepts, the synthesis and study of related heterometallic oxo clusters have been targeted.

Don Tilley (University of Caifornia, Berkeley)