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

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc^2–) has been used to prepare complexes [Ru^II(pdc-κ³-N¹O²)(DMSO)₂Cl]^– (1^II), [Ru^II(pdc-κ³-N¹O²)(bpy)(DMSO)] (2^II), and {[Ru^III(pdc-κ³-N¹O²)(bpy)]₂(μ-O)} (5^III,III), where bpy = 2,2′-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical, and single-crystal X-ray diffraction techniques. Compounds 1^II and 2^II show S → O linkage isomerization of the DMSO ligand upon oxidation from Ru^II to Ru^III, and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments. Dimeric complex 5^III,III is a precursor of the monomeric complex [Ru^II(pdc-κ³-N¹O²)(bpy)(H₂O)] (4^II) which is a water oxidation catalyst. The electrochemistry and catalytic activity of 4^II 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 TOF_max = 0.2 s^–1 and overpotential of 240 mV in pH = 1. The overpotential shown by 4^II 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.