化学エネルギーと電気エネルギーの相互変換を目指した錯体触媒の設計と合成

概要

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Proton dissociation of an aqua-Ru-quinone complex, [Ru(trpy)(3,5-t-Bu2q)(OH2)]2+ (trpy = 2,2’:6’,2”-terpyridine, 3,5-t-Bu2q = 3,5-di-tert-butyl-1,2-banzoquinone) proceeded in two steps (pKa = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(3,5-t-Bu2q)(OH)]+, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(3,5-t-Bu2sq)(O-.)]0 (3,5-t-Bu2sq = 3,5-di-tert-butyl-1,2-banzosemiquinone), due to an intramolecular electron transfer from the resultant O2- to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru2(btpyan)(3,6-t-Bu2q)2(OH)2]2+ (btpyan = 1,8-bis(2,2’:6’,2”-terpyrid-4-yl)anthracene, 3,6-t-Bu2q = 3,6-di-tert-butyl-1,2-banzoquinone), was prepared to place two Ru(trpy)(q)(OH) groups in a close distance. The [Ru2(btpyan)(3,6-t-Bu2q)2(OH)2](SbF6)2 modified ITO electrode effectively catalyzed four-electron oxidation of water to evolve O2 (TON = 33,500) under the electrolysis at +1.70 V in H2O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O-.) groups forms a (cat)Ru-O-O-Ru(sq) (cat = cathecholato) framework with a μ-superoxo bond. Deprotonation of an Ru-quinone-ammonia complex also gave the corresponding an Ru-semiquinone-aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base.