Water Oxidation Catalyzed by Ruthenium Complexes with 4-Hydroxypyridine-2,6-dicarboxylate as a Negatively Charged Tridentate Ligand

In order to reduce the overpotential for water oxidation, 4-hydroxypyridine-2,6-dicarboxylate (hdc(2-)), was used as a ligand to prepare Ru-based water oxidation catalysts (WOCs) where the hydroxyl group of hdc(2-) endows strong electron-donating ability to the pyridine ring due to the p-pi conjugation. A family of Ru-hdc complexes, [Ru(hdc)(py-R)(3)] (py-R = 4-pyridyl ligands; R = OMe, 1; Me, 2; H, 3; COOMe, 4) with electron-donating and -withdrawing groups were prepared and fully characterized. Electrochemical water oxidation experiments demonstrated that (i) the redox potentials of Ru-III/II are sensitive to the substituents, and stronger electron-donating groups give lower oxidation potentials and (ii) the catalytic current for water oxidation correlates well with their electron-donating ability of pyridyl ligands, 1 > 2 > 3 > 4. A similar trend for the performance of Ce-IV-driven water oxidation was also observed. Mechanistic studies revealed that the equatorial pyridyl ligands are prone to ligand exchange with water under acidic conditions, especially upon oxidation, leading to the formation of Ru-aqua species, for instance [Ru-III(hdc)(py)(2)(OH2)](+). Further oxidation of the Ru-aqua species gives dimeric Ru species, [(hdc)(py)(2)Ru-III-(mu-O)-Ru-IV(hdc)(py)(2)](+), which is proposed to be closely related to the real water oxidation catalyst toward water oxidation.