エネルギー変換を目指した錯体触媒による無機および有機小分子の酸化還元反応の開発

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  • 田中 晃二
    分子科学研究所生命·錯体分子科学研究領域

書誌事項

タイトル別名
  • Metal Catalyzed Redox Reactions of Small Inorganic and Organic Molecules Aimed at Energy Conversion
  • エネルギー ヘンカン オ メザシタ サクタイ ショクバイ ニ ヨル ムキ オヨビ ユウキ ショウブンシ ノ サンカ カンゲン ハンノウ ノ カイハツ

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An aqua-Ru-quinone complex, [Ru(trpy)(Q)(OH2)]2+ (trpy=2,2’: 6’,2”-terpyridine, Q=3,5-di-tert-butyl-1,2-benzoquinone, or 3,6-di-tert-butyl-1,2-benzoquinone) dissociates two protons in a stepwise fashion. The pKavalues of [Ru(trpy)(Q)(OH2)]+ and [Ru(trpy)(Q)(OH)]+ are 5.5 and 10.5, respectively. Proton dissociation of the latter is coupled with electron transfer from the resultant O2− to Q to produce an unusual O-radical complex, [Ru(trpy)(Sq)(O−·)]0 (Sq=3,5-di-tert-butyl-1,2-benzosemiquinone). Dissociation of an amino proton of analogous [Ru(trpy)(Q)(NH3)]2+ (pKa6.2) results in similar intramolecular electron transfer from NH2− to Q with generating an N-radical complex, [Ru(trpy)(Sq)(NH2·)]+. One-electron oxidation of [Ru(trpy)(Sq)(NH2·)]2+ produces [Ru(trpy)(Q)(NH2·)]2+, which reacts with alcohols to give the correspondent aldehyde or ketone with regeneration of [Ru(trpy)(Sq)(NH3)]+. On the other hand, two electron oxidation of [Ru(trpy)(Sq)(O−·)]0 generates a catalytic ability to oxidize alcohols. The catalytic activity of the O-radical complex is less than that of N-radical one. Dinuclear Ru complexes bridged with an anthracene framework, [Ru2(btpyan)(Q)2(OH)2]2+ and [Ru2(btpyan)(bpy)2(OH)2]2+ (btpyan=1,8-bis(2,2’: 6’,2”-terpyrid-4’-yl)anthracene) catalyze four-electron oxidation of water. The catalytic activity for O2 production by [Ru2(btpyan)(Q)2(OH)2]2+ is much higher than [Ru2(btpyan)(bpy)2(OH)2]2+. Proton dissociation of [Ru2(btpyan)(Q)2(OH)2]2+ spontaneously produces [Ru2(btpyan)(Sq)2(O−·)2]0, and the subsequent oxidation of the two Ru(Sq)(O−·) groups induces O-O bond formation through the coupling reaction of two O-radicals. On the other hand, deprotonation of [Ru2(btpyan)(bpy)2(OH)2]2+ is caused by oxidation of [Ru2(btpyan)(bpy)2(OH)2]2+, and four electron oxidation affords [RuIV2(btpyan)(bpy)2(=O)2]4+ without passing through an O-radical complex. High catalytic activity of [Ru2(btpyan)(Q)2(OH)2]2+ compared with [Ru2(btpyan)(bpy)2(OH)2]2+ as to the O2 evolution is associated with the easiness of the O-O bond formation by the coupling reaction of two Ru-O· radicals compared with that of two RuIV=O groups. A new pbn (pbn=2-pyridylbenzo[b]-1,5-naphthyridine) ligand was designed to simulate the NAD+/NADH redox reaction as a possible renewable hydride donor. A series of [Ru(bpy)3−n(pbn)n]2+ (n=1, 2, 3) complexes undergoes photo-induced two-(n=1), four-(n=2), and six-electron reductions (n=3) under irradiation of visible light in the presence of N(CH2CH2OH)3.

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