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

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)]^<i>m</i>+ [X, Y = H₂O, OH, O, O₂; 3,6-tBu₂Q = 3,6-di-<i>tert</i>-butyl-1,2-benzoquinone; btpyan =1,8-bis­(2,2′:6′,2″-terpyrid-4′-yl)­anthracene] (<i>m</i> = 2, 3, 4) (<b>1</b>). 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 <b>1</b> work together to drive the catalytic cycle for water oxidation. Two substrate water molecules can bind to <b>1</b> 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 <b>1