Synthesis, Electrochemical Characterization, and Water Oxidation Chemistry of Ru Complexes Containing the 2,6-Pyridinedicarboxylato Ligand

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc2–) has been used to prepare complexes [RuII(pdc-κ3-N1O2)(DMSO)2Cl]- (1II), [RuII(pdc-κ3-N1O2)(bpy)(DMSO)] (2II), and {[RuIII(pdc-κ3-N1O2)(bpy)]2(μ-O)} (5III,III), where bpy = 2,2′-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical, and single-crystal X-ray diffraction techniques. Compounds 1II and 2II show S → O linkage isomerization of the DMSO ligand upon oxidation from RuII to RuIII, and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments. Dimeric complex 5III,III is a precursor of the monomeric complex [RuII(pdc-κ3-N1O2)(bpy)(H2O)] (4II) which is a water oxidation catalyst. The electrochemistry and catalytic activity of 4II has been ascertained for the first time and compared with related Ru-aquo complexes that are also active for the water oxidation reaction. It shows a TOFmax = 0.2 s–1 and overpotential of 240 mV in pH = 1. The overpotential shown by 4II is one of the lowest reported in the literature and is associated with the role of the two carboxylato groups of the pdc ligand, providing high electron density to the ruthenium complex.