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

Abstract

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, [Ru₂(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^II₂(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^III₂(OH)₂ (cat)₂(btpyan)]⁰ (cat = catecholato), indicating that a dioxolene having electron-withdrawing groups stabilizes [Ru^III₂(OH)₂(cat)₂(btpyan)]⁰ rather than [Ru^II₂(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 O₂ 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^II₂(OH)₂(sq)₂(btpyan)]⁰ and [Ru^III₂(OH)₂(cat)₂(btpyan)]⁰.