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

Article

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

Hiroshi Isobe *†‡, Koji Tanaka §, Jian-Ren Shen †, and Kizashi Yamaguchi ‡||

† Division of Bioscience, Graduate School of Natural Science and Technology/Faculty of Science, Okayama University, Okayama 700-8530, Japan

‡ Department of Regulatory Bioorganic Chemistry, The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan

§ Institute for Integrated Cell-Material Sciences, Kyoto University, Kyoto 615-8530, Japan

|| Institute for NanoScience Design, Osaka University, Toyonaka, Osaka 560-0043, Japan

Inorg. Chem., Article ASAP

DOI: 10.1021/ic402340d

Publication Date (Web): April 2, 2014

*E-mail: h-isobe@cc.okayama-u.ac.jp, isobe@chem.sci.osaka-u.ac.jp.

Synopsis

Water oxidation by a dinuclear ruthenium complex has been investigated theoretically. The ability of redox-active quinone ligands to store redox equivalents leads to an electronic flexibility of the catalyst that causes water activation to be thermodynamically and kinetically favorable. On the other hand, the catalyst binds a μ-peroxo ligand too strongly to release O2, unless a large hinge-like bending motion permits the catalyst to open its reaction site for incorporating a substrate water molecule.

Section:

Catalysis, Reaction Kinetics, and Inorganic Reaction Mechanisms

Abstract

We investigated theoretically the catalytic mechanism of electrochemical water oxidation in aqueous solution by a dinuclear ruthenium complex containing redox-active quinone ligands, [Ru2(X)(Y)(3,6-tBu2Q)2(btpyan)]m+ [X, Y = H2O, OH, O, O2; 3,6-tBu2Q = 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• units in an anti-coplanar conformation leading to bridged μ-peroxo or μ-superoxo intermediates. The μ-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 μ-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.

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