Metal-catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society

Proton dissociation of an aqua-Ru-quinone complex, [Ru(trpy)(q)(OH2)]2+ (trpy = 2,2' : 6',2''-terpyridine, q = 3,5-di-t-butylquinone) proceeded in two steps (pK(a) = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(q)(OH)]+, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O-*)]0 (sq = 3,5-di-t-butylsemiquinone), owing to an intramolecular electron transfer from the resultant O2- to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru2(btpyan)(q)2(OH)2]2+ (btpyan = 1,8-bis(2,2'-terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru2(btpyan)(q)2(OH)2]2+ generated two oxyl radical Ru-O-* groups, which worked as a precursor for O2 evolution in the oxidation of water. The [Ru2(btpyan)(q)2(OH)2](SbF6)2 modified ITO electrode effectively catalyzed four-electron oxidation of water to evolve O2 (TON = 33500) under 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 = 3,5-di-t-butylcathechol) framework with a mu-superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru2(btpyan)(cat)(sq)(mu-O2-)]0 assisted with an attack of two water (or OH-) to Ru centers, which causes smooth O2 evolution with regeneration of [Ru2(btpyan)(q)2(OH)2]2+. Deprotonation of an Ru-quinone-ammonia complex also gave the corresponding 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. Three complexes, [Ru(bpy)2(CO)2]2+, [Ru(bpy)2(CO)(C(O)OH)]+, and [Ru(bpy)2(CO)(CO2)]0 exist as an equilibrium mixture in water. Treatment of [Ru(bpy)2(CO)2]2+ with BH4- gave [Ru(bpy)2(CO)(C(O)H)]+, [Ru(bpy)2(CO)(CH2OH)]+, and [Ru(bpy)2(CO)(OH2)]2+ with generation of CH3OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO2 to CH3OH in electro- and photochemical CO2 reduction is proposed. A new pbn (pbn = 2-pyridylbenzo[b]-1,5-naphthyridine) ligand was designed as a renewable hydride donor for the six-electron reduction of CO2. A series of [Ru(bpy)(3-n)(pbn)n]2+ (n = 1, 2, 3) complexes undergoes photochemical two- (n = 1), four- (n = 2), and six-electron reductions (n = 3) under irradiation of visible light in the presence of N(CH2CH2OH)3.