Water Oxidation Catalysis: Influence of Anionic Ligands upon the Redox Properties and Catalytic Performance of Mononuclear Ruthenium Complexes

Abstract

Aiming at highly efficient molecular catalysts for water oxidation, a mononuclear ruthenium complex Ru^II(hqc)­(pic)₃ (<b>1</b>; H₂hqc = 8-hydroxyquinoline-2-carboxylic acid and pic = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru^II(pdc)­(pic)₃ (<b>2</b>; H₂pdc = 2,6-pyridine-dicarboxylic acid) and Ru^II(tpy)­(pic)₃ (<b>3</b>; tpy = 2,2′:6′,2″-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex <b>1</b> showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce^IV in a pH 1 solution) with a initial turnover number of 0.32 s^–1, which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three-component system with [Ru­(bpy)₃]²⁺ types of photosensitizers. Electrospray ionization MS results revealed that at the Ru^III state complex <b>1</b> undergoes ligand exchange of 4-picoline with water, forming the authentic water oxidation catalyst in situ. Density functional theory (DFT) was employed to explain how anionic ligands (hqc and pdc) facilitate the 4-picoline dissociation compared with a neutral ligand (tpy). Electrochemical measurements show that complex <b>1</b> has a much lower <i>E</i>(Ru^III/Ru^II) than that of reference complex <b>2</b> because of the introduction of a phenolate ligand. DFT was further used to study the influence of anionic ligands upon the redox properties of mononuclear aquaruthenium species, which are postulated to be involved in the catalysis cycle of water oxidation