pubs.acs.org/IC Published on Web 12/08/2009 r 2009 American Chemical Society Inorg. Chem. 2010, 49, 209–215 209 DOI: 10.1021/ic9017486 Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System Lele Duan, † Yunhua Xu, † Pan Zhang, ‡ Mei Wang, ‡ and Licheng Sun* ,†,‡ † Department of Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden and ‡ DUT-KTH Joint Education and Research Center on Molecular Devices, State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), 116012, Dalian, China Received September 2, 2009 Discovery of an efficient catalyst bearing low overpotential toward water oxidation is a key step for light-driven water splitting into dioxygen and dihydrogen. A mononuclear ruthenium complex, Ru(II)L(pic) 2 (1)(H 2 L = 2,2 0 -bipyridine- 6,6 0 -dicarboxylic acid; pic = 4-picoline), was found capable of oxidizing water eletrochemically at a relatively low potential and promoting light-driven water oxidation using a three-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochemical properties of 1 were studied, and the onset potentials of the electrochemically catalytic curves in pH 7.0 and pH 1.0 solutions are 1.0 and 1.5 V, respectively. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy) 3 ] 2þ and even [Ru(dmbpy) 3 ] 2þ as a photosensitizer for photochemical water oxidation. Two different sacrificial electron acceptors, [Co(NH 3 ) 5 Cl]Cl 2 and Na 2 S 2 O 8 , were used to generate the oxidized state of ruthenium tris(2,2 0 -bipyridyl) photosensitizers. In addition, a two- hour photolysis of 1 in a pH 7.0 phosphate buffer did not lead to obvious degradation, indicating the good photostability of our catalyst. However, under conditions of light-driven water oxidation, the catalyst deactivates quickly. In both solution and the solid state under aerobic conditions, complex 1 gradually decomposed via oxidative degradation of its ligands, and two of the decomposed products, sp 3 C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp 3 C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degradation of the catalyst. Introduction In nature, water is oxidized by the oxygen-evolving com- plex (OEC) in photosystem II (PSII) driven by light, provid- ing electrons and protons for the sustainability of all life forms on earth. Inspired by the function of OEC, tremendous efforts have been made on artificial photosynthesis systems aiming at light-driven water splitting into molecular hydro- gen and oxygen. 1-5 This approach is extremely important for solar energy conversion into a fuel, and the ultimate challenge in this approach is the catalytic water oxidation driven by visible light. 5 In heterogeneous systems, light-driven water oxidation or water splitting has been demonstrated in several ways, such as electrolysis using photovoltaic cells, 6 semicon- ductor-based photoelectrodes via the photoinduced electron and hole separation, 7-10 and catalytic systems composed of transition-metal photosensitizers and metal (Ir and Ru) oxide water oxidation catalysts. 11,12 On the other hand, light- driven water oxidation in homogeneous systems is rarely reported. A few molecular complexes have been reported to catalyze the oxidation of water. 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