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(PPh3)(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 [Ru2Cl6(btpyan)](0) with the corresponding catechol. The electronic structure of [1](0) is approximated by [Ru-2(II)(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-2(III)(OH)(2) (cat)(2)(btpyan)](0) (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes [Ru-2(III)(OH)(2)(cat)(2)(btpyan)](0) rather than [Ru-2(II)(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 HClO4. Furthermore, controlled potential electrolysis of [1](0) deposited on an ITO (indium-tin oxide) electrode catalyzed the four-electron oxidation of H2O 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-2(II)(OH)(2)(sq)(2)(btpyan)](0) and [Ru-2(III)(OH)(2)(cat)(2)(btpyan)](0).