Substituents dependent capability of bis(ruthenium-dioxolene-terpyridine) complexes toward water oxidation

The bridging ligand, 1,8-bis(2,2':6',2'-terpyrid-4'-yl)anthracene (btpyan) was synthesized by the Miyaura-Suzuki cross coupling reaction of anthracenyl-1,8-diboronic acid and 4'-triflyl-2,2':6'-2'-terpyridine in the presence of Pd(PPh(3))(4) (5 mol%) with 68% in yield. Three ruthenium-dioxolene dimers, Ru(2)(OH)(2)(dioxolene)(2)(btpyan)](0) (dioxolene = 3,6-di-tert-butyl-1,2-benzosemiquinone (1](0)), 3,5-dichloro-1,2-benzosemiquinone (2](0)) and 4-nitro-1,2-benzosemiquinone (3](0))) were prepared by the reaction of Ru(2)Cl(6)(btpyan)](0) with the corresponding catechol. The electronic structure of 1](0) is approximated by Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) (sq = semiquinonato). On the other hand, the electronic states of 2](0) and 3](0) are close to Ru(III)(2)(OH)(2) (cat)(2)(btpyan)](0) (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes Ru(III)(2)(OH)(2)(cat)(2)(btpyan)](0) rather than Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) as resonance isomers. No sign was found of deprotonation of the hydroxo groups of 1](0), whereas 2](0) and 3](0) showed an acid-base equilibrium in treatments with t-BuOLi followed by HClO(4). Furthermore, controlled potential electrolysis of 1](0) deposited on an ITO (indium-tin oxide) electrode catalyzed the four-electron oxidation of H(2)O to evolve O(2) at potentials more positive than +1.6 V (vs. SCE) at pH 4.0. On the other hand, the electrolysis of 2](0) and 3](0) deposited on ITO electrodes did not show catalytic activity for water oxidation under similar conditions. Such a difference in the reactivity among 1](0), 2](0) and 3](0) is ascribed to the shift of the resonance equilibrium between Ru(II)(2)(OH)(2)(sq)(2)(btpyan)](0) and Ru(III)(2)(OH)(2)(cat)(2)(btpyan)](0).