Structure and Electronic Configurations of the Intermediates of Water Oxidation in a Highly Active and Robust Molecular Ruthenium Catalyst

Abstract

Photosynthesis is one of the most important chemical processes on our planet. Splitting water into O2 and H2 using sunlight is a clean and sustainable proposition for addressing energy and environmental problems encountered in our society. Mimicking this reaction in a manmade device will allow for sunlight-to-energy conversion with water providing electrons and protons for production of chemical fuels. About 30 years ago Meyer and coworkers reported the first ruthenium-based catalyst for water oxidation, known as the “blue dimer”. This catalyst may be considered as an artificial analog of the oxygen-evolving complex (OEC) in Photosystem II (PS II). Recently it has been demonstrated that single-site Ru and Ir catalysts are also active in water oxidation. These single-site catalysts are attractive model compounds for both experimental and theoretical studies of mechanism of water oxidation and show improved catalytic activity compared to “blue dimer”. A better understanding of this mechanism and identification of the rate-limiting steps could pave the way to light-driven generation of molecular hydrogen by water splitting.A mononuclear ruthenium complex [Ru(bda)(pic)2] (pic=4-picoline) was found to show high catalytic activity as well as high chemical stability. EPR, Raman and XAS characterization of the electronic structure and molecular geometry of Ru(III)-OH2,Ru(IV)=O and Ru(IV)-OO-Ru(IV) are reported in the single site water oxidizing complex. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts. However, such intermediates were never isolated and their structural and electronic characterization has not been reported. We believe that the intermediates described here are direct products of the O-O bond formation step in the studied catalyst.