Water oxidation by a ruthenium complex with noninnocent quinone ligands: Possible formation of an O-O bond at a low oxidation state of the metal

Tanaka and co-workers reported a novel dinuclear Ru complex, [Ru ₂(OH)₂(3,6-Bu₂Q)₂(btpyan)](SbF ₆)₂ (3,6-Bu₂Q = 3,6-ditert-butyl-1,2- benzoquinone, btpyan = 1,8-bis(2,2′:6′,2″-terpyrid-4′- yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for water oxidation when immobilized on an indium-tin-oxide electrode (Inorg. Chem., 2001, 40, 329-337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogues with the bpy ligand (bpy = 2,2′-bipyridine) replacing quinone, are summarized here together with new theoretical and experimental results that show striking features for both the dinuclear and mononuclear species. The identity and oxidation state of key mononuclear species, including the previously reported oxyl radical, have been reassigned. Our gas-phase theoretical calculations indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and water moiety are involved in redox reactions throughout the entire catalytic cycle for water oxidation. Our theoretical study identifies [Ru₂(O₂ ^-)(Q^-1.5)₂(btpyan) ]⁰ as a key intermediate and the most reduced catalyst species that is formed by removal of all four protons before four-electron oxidation takes place. While our study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic investigations, and key issues and challenges in water oxidation with the Tanaka catalyst (and its analogues with Cl- or NO₂-substituted quinones and a species with a xanthene bridge instead an antheracene) are discussed.