Catalytic Water Oxidation by Mononuclear Ru Complexes with an Anionic Ancillary Ligand

Abstract

Mononuclear Ru-based water oxidation catalysts containing anionic ancillary ligands have shown promising catalytic efficiency and intriguing properties. However, their insolubility in water restricts a detailed mechanism investigation. In order to overcome this disadvantage, complexes [Ru^II(bpc)­(bpy)­OH₂]⁺ (<b>1</b>⁺, bpc = 2,2′-bipyridine-6-carboxylate, bpy = 2,2′-bipyridine) and [Ru^II(bpc)­(pic)₃]⁺ (<b>2</b>⁺, pic = 4-picoline) were prepared and fully characterized, which features an anionic tridentate ligand and has enough solubility for spectroscopic study in water. Using Ce^IV as an electron acceptor, both complexes are able to catalyze O₂-evolving reaction with an impressive rate constant. On the basis of the electrochemical and kinetic studies, a water nucleophilic attack pathway was proposed as the dominant catalytic cycle of the catalytic water oxidation by <b>1</b>⁺, within which several intermediates were detected by MS. Meanwhile, an auxiliary pathway that is related to the concentration of Ce^IV was also revealed. The effect of anionic ligand regarding catalytic water oxidation was discussed explicitly in comparison with previously reported mononuclear Ru catalysts carrying neutral tridentate ligands, for example, 2,2′:6′,2″-terpyridine (tpy). When <b>2</b>⁺ was oxidized to the trivalent state, one of its picoline ligands dissociated from the Ru center. The rate constant of picoline dissociation was evaluated from time-resolved UV–vis spectra