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) (dioxolene = 3,6-di-tert-butyl-1,2-benzosemiquinone (1), 3,5-dichloro-1,2-benzosemiquinone (2) and 4-nitro-1,2-benzosemiquinone (3)) were prepared by the reaction of Ru(2)Cl(6)(btpyan) with the corresponding catechol. The electronic structure of 1 is approximated by Ru(II)(2)(OH)(2)(sq)(2)(btpyan) (sq = semiquinonato). On the other hand, the electronic states of 2 and 3 are close to Ru(III)(2)(OH)(2) (cat)(2)(btpyan) (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes Ru(III)(2)(OH)(2)(cat)(2)(btpyan) rather than Ru(II)(2)(OH)(2)(sq)(2)(btpyan) as resonance isomers. No sign was found of deprotonation of the hydroxo groups of 1, whereas 2 and 3 showed an acid-base equilibrium in treatments with t-BuOLi followed by HClO(4). Furthermore, controlled potential electrolysis of 1 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 and 3 deposited on ITO electrodes did not show catalytic activity for water oxidation under similar conditions. Such a difference in the reactivity among 1, 2 and 3 is ascribed to the shift of the resonance equilibrium between Ru(II)(2)(OH)(2)(sq)(2)(btpyan) and Ru(III)(2)(OH)(2)(cat)(2)(btpyan).