Metal‐catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society |
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| Authors: | Koji Tanaka |
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| Institution: | 1. Institute for Molecular Science Higashiyama 5‐1, Myodaiji, Okazaki 444‐8787, Japan. Telephone: +81‐564‐59‐5580;2. Fax: +81‐564‐59‐5582 |
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| Abstract: | 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 (pKa = 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 μ‐superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of Ru2(btpyan)(cat)(sq)(μ‐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‐pyridylbenzob]‐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. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200800039 |
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| Keywords: | energy conversion green chemistry photochemistry redox chemistry ruthenium |
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