Water Oxidation Chemistry of a Synthetic Dinuclear Ruthenium Complex Containing Redox-Active Quinone Ligands

作者：Isobe Hiroshi^*; Tanaka Koji; Shen Jian Ren; Yamaguchi Kizashi

来源：Inorganic Chemistry, 2014, 53(8): 3973-3984.

DOI：10.1021/ic402340d

We investigated theoretically the catalytic mechanism of electrochemical water oxidation in aqueous solution by a dinuclear ruthenium complex containing redox-active quinone ligands, [Ru-2(X)(Y)(3,6-tBu(2)Q)(2)(btpyan)](m+) [X, Y = H2O, OH, O, O-2; 3,6-tBu(2)Q = 3,6-di-tert-butyl-1,2-benzoquinone; btpyan = 1,8-bis(2,2':6',2"-terpyrid-4'-yl)anthracene] (m = 2, 3, 4) (1). The reaction involves a series of electron and proton transfers to achieve redox leveling, with intervening chemical transformations in a mesh scheme, and the entire molecular structure and motion of the catalyst 1 work together to drive the catalytic cycle for water oxidation. Two substrate water molecules can bind to 1 with simultaneous loss of one or two proton(s), which allows pH-dependent variability in the proportion of substrate-bound structures and following pathways for oxidative activation of the aqua/hydroxo ligands at low thermodynamic and kinetic costs. The resulting bis-oxo intermediates then undergo endothermic O-O radical coupling between two Ru(III)-O-center dot units in an anti-coplanar conformation leading to bridged mu-peroxo or mu-superoxo intermediates. The mu-superoxo species can liberate oxygen with the necessity for the preceding binding of a water molecule, which is possible only after four-electron oxidation is completed. The magnitude of catalytic current would be limited by the inherent sluggishness of the hinge-like bending motion of the bridged mu-superoxo complex that opens up the compact, hydrophobic active site of the catalyst and thereby allows water entry under dynamic conditions. On the basis of a newly proposed mechanism, we rationalize the experimentally observed behavior of electrode kinetics with respect to potential and discuss what causes a high overpotential for water oxidation by 1.

• 出版日期2014-4-21