Tanaka and co-workers reported a novel dinuclear Ru complex, [Ru2(OH)2(3,6-Bu2Q)2(btpyan)](SbF6)2 (3,6-Bu2Q = 3,6-di tert-butyl-1,2-benzoquinone, btpyan = 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for water oxidation when immobilized on an indium-tin-oxide electrode (Inorg. Chem., 2001, 40, 329-337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogues with the bpy ligand (bpy = 2,2'-bipyridine) replacing quinone, are summarized here together with new theoretical and experimental results that show striking features for both the dinuclear and mononuclear species. The identity and oxidation state of key mononuclear species, including the previously reported oxyl radical, have been reassigned. Our gas-phase theoretical calculations indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and water moiety are involved in redox reactions throughout the entire catalytic cycle for water oxidation. Our theoretical study identifies [Ru2(O2(-))(Q(-1.5))2(btpyan)](0) as a key intermediate and the most reduced catalyst species that is formed by removal of all four protons before four-electron oxidation takes place. While our study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic investigations, and key issues and challenges in water oxidation with the Tanaka catalyst (and its analogues with Cl(-) or NO(2-)substituted quinones and a species with a xanthene bridge instead an antheracene) are discussed.

The novel bridging ligand 1,8-bis(2,2':6',2"-terpyridyl)anthracene (btpyan) is synthesized by three reactions from 1,8-diformylanthracene to connect two [Ru(L)(OH)]+ units (L = 3,6-di-tert-butyl-1,2-benzoquinone (3,6-tBu2qui) and 2,2'-bipyridine (bpy)). An addition of tBuOK (2.0 equiv) to a methanolic solution of [RuII2(OH)2(3,6-tBu2qui)2(btpyan)](SbF6)2 ([1](SbF6)2) results in the generation of [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 (3,6-tBu2sq = 3,6-di-tert-butyl-1,2-semiquinone) due to the reduction of quinone coupled with the dissociation of the hydroxo protons. The resultant complex [RuII2(O)2(3,6-tBu2sq)2(btpyan)]0 undergoes ligand-localized oxidation at E1/2 = +0.40 V (vs Ag/AgCl) to give [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ in MeOH solution. Furthermore, metal-localized oxidation of [RuII2(O)2(3,6-tBu2qui)2(btpyan)]2+ at Ep = +1.2 V in CF3CH2OH/ether or water gives [RuIII2(O)2(3,6-tBu2qui)2(btpyan)]4+, which catalyzes water oxidation. Controlled-potential electrolysis of [1](SbF6)2 at +1.70 V in the presence of H2O in CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of O2 evolution increases to 33,500 when the electrolysis is conducted in water (pH 4.0) by using a [1](SbF6)2-modified ITO electrode. On the other hand, the analogous complex [RuII2(OH)2(bpy)2(btpyan)](SbF6)2 ([2](SbF6)2) shows neither dissociation of the hydroxo protons, even in the presence of a large excess of tBuOK, nor activity for the oxidation of H2O under similar conditions.

Much attention has been paid to the oxidation of water to dioxygen by homogeneous catalysts. Of particular interest are di-and tetranuclear transition metal complexes, since extended X-ray absorption fine structure studies have indicated that the O 2 -evolving center (OEC) in photosystem II is composed of a tetranuclear Mn cluster with dimeric di-moxo Mn units. [1] A variety of di-and tetranuclear metal complexes have been prepared as structural models of the OEC, and four-electron oxidation of water has been attained with only a few dinuclear metal (Mn, Ru) complexes. [2±4] The key intermediates for O 2 evolution in the OEC is thought to be high-valent metal ± oxo complexes derived from aqua signals range from 0.1 ± 0.3 and increase as the temperature lowers. Finally, an empirical model based on PM3 conformational analysis permits a tentative assignment of configurations at the carbon atom a to the metal, relative ligand on the basis of both anisotropic effects and free energy criteria. Structural studies aimed at finding definitive proof for the assignment of the absolute configuration as well as studies of other organometallic species are underway. Experimental SectionOrganozinc bromides were obtained by reaction of active zinc with the corresponding bromides, previously distilled over P 2 O 5 , in dry THF at RT in an Ar atmosphere. [21] . (R)-1 and (S)-1 were purified by precipitation with ZnCl 2 (THF), extraction (0.2 m aqueous NaCN in CHCl 3 ), and vacuum distillation. (RS)-1 ([a] 25 D 0.0) was prepared by mixing equal amounts of the former reagents. The concentration in all the NMR experiments was 0.5 m in THF/C 6 D 6 (1/1), except for those performed at 150 8C in which it was 1.5 m in THF/[D 8 ]THF (4/1). TMS was added as an internal standard.[1] a) H. M. Walborsky, Acc. Chem.