Published: June 24, 2011 r2011 American Chemical Society 11605 dx.doi.org/10.1021/ja2024965 | J. Am. Chem. Soc. 2011, 133, 11605–11613 ARTICLE pubs.acs.org/JACS Catalytic Mechanism of Water Oxidation with Single-Site RutheniumHeteropolytungstate Complexes Masato Murakami, † Dachao Hong, † Tomoyoshi Suenobu, † Satoru Yamaguchi, ‡ Takashi Ogura, ‡ and Shunichi Fukuzumi* ,†,§ † Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Picobiology Institute, Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan § Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea b S Supporting Information ’ INTRODUCTION Effective and sustained catalysts for water oxidation are absolutely imperative to achieve the conversion of sunlight into clean and accumulable chemical energy by artificial photosynth- esis as typified by sunlight-driven water splitting into hydrogen and oxygen (eqs 1, 2). 1 Thus, extensive efforts have so far been devoted to the development of water oxidation catalysts (WOCs) over the past three decades. 24 2H 2 O f O 2 þ 4H þ þ 4e  ð1Þ 4H þ þ 4e  f 2H 2 ð2Þ WOCs can be classified into heterogeneous 2 and homoge- neous 3,4 types. Heterogeneous WOCs are generally robust and easy to fabricate, providing potential applications. However, the difficulty to identify the catalytically active species of hetero- geneous catalysts has precluded clarifying the catalytic mechan- ism of water oxidation. 2,5 In contrast to heterogeneous catalysts, homogeneous catalysts provide the better opportunity to study the catalytic mechanism of water oxidation, which has yet to be clarified. The first homogeneous WOC reported by Meyer et al. 68 is cis,cis-[(bpy) 2 (H 2 O)Ru III ORu III (H 2 O)(bpy) 2 ] 4+ (bpy = 2,2 0 - bipyridine), so-called “blue dimer”. To date, several bimetallic Ru 919 and Mn 20,21 WOCs have been documented, and the multinuclear structure similar to that in the oxygen evolving complex (OEC) in Photosystem II 22 was regarded to be essential for mediating four-electron process of water oxidation. However, a number of mononuclear Ru 2334 and Ir 35,36 complexes have also been reported as efficient WOCs. Although the catalytic mechanisms of multinuclear WOCs as the functional model of OEC have attracted considerable attention, there are a few detailed mechanistic studies on binuclear Ru complexes due presumably to the complexity of multinuclear catalysis. 7,11,16,37 Mononuclear WOCs may be more suitable to elucidate the catalytic mechanism of water oxidation because of their simple structure. However, all of the single-site Ru complexes reported so far contain organic ligands, 2334 and oxidation of the organic ligands under the water oxidation conditions would make it difficult to obtain mechanistic insight into the catalytic water oxidation. To avoid the oxidative damage of WOCs, the adoption Received: April 6, 2011 ABSTRACT: Catalytic water oxidation to generate oxygen was achieved using all-inorganic mononuclear ruthenium com- plexes bearing Keggin-type lacunary heteropolytungstate, [Ru III (H 2 O)SiW 11 O 39 ] 5 (1) and [Ru III (H 2 O)GeW 11 O 39 ] 5 (2), as catalysts with (NH 4 ) 2 [Ce IV (NO 3 ) 6 ] (CAN) as a one- electron oxidant in water. The oxygen atoms of evolved oxygen come from water as confirmed by isotope-labeled experiments. Cyclic voltammetric measurements of 1 and 2 at various pH’s indicate that both complexes 1 and 2 exhibit three one-electron redox couples based on ruthenium center. The Pourbaix dia- grams (plots of E 1/2 vs pH) support that the Ru(III) complexes are oxidized to the Ru(V)oxo complexes with CAN. The Ru(V)oxo complex derived from 1 was detected by UVvisible absorption, EPR, and resonance Raman measurements in situ as an active species during the water oxidation reaction. This indicates that the Ru(V)oxo complex is involved in the rate-determining step of the catalytic cycle of water oxidation. The overall catalytic mechanism of water oxidation was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates. Complex 2 exhibited a higher catalytic reactivity for the water oxidation with CAN than did complex 1.