Insight into Water Oxidation by Mononuclear Polypyridyl Ru Catalysts

A family of compounds based on the mononuclear coordination complex [Ru(tpy)(bpy)(OH2)](2+) (1b; tpy=2,2':6',2 ''-terpyridine, bpy=2,2'-bipyridine) are shown to be competent catalysts in the Ce(IV)-driven oxidation of water in acidic media. The systematic installation of electron-withdrawing (e.g., -Cl, -COOH) and -donating (e.g., -OMe) groups at various positions about the periphery of the polypyridyl framework offers insight into how electronic parameters affect the properties of water oxidation catalysts, It is observed, in general, that electron-withdrawing groups (EWGs) on the bpy ligands suppress catalytic activity (k(obs)) and enhance catalytic turnover numbers (TONs); conversely, the presence of electron-donating groups (EDGs) accelerate catalytic rates while decreasing catalyst stability. We found that 2,2'-bipyridine N,N'-dioxide is produced when 1b is subject to excess Ce(IV) in acidic media, which suggests that dissociation of the bpy ligand is a source of catalyst deactivation and/or decomposition. Density functional theory (DFT) calculations corroborate these findings by showing that the Ru-N-bpy bond trans to the O atom is weakened at higher oxidation levels while the other Ru-N bonds are affected to a lesser extent. We also show that the Ru-Cl bond is not robust in aqueous media, which has implications in studying the catalytic behavior of systems of this type.