Electron Transfer Mediator Effects in Water Oxidation Catalysis by Solution and Surface-Bound Ruthenium Bpy-Dicarboxylate Complexes

Abstract

Electrocatalytic water oxidation by the catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru(bda)(isoq)2], 1, was investigated at metal oxide electrodes surface-derivatized with electron transfer (ET) mediators. At indium-doped tin oxide (ITO) in pH 7.2 in H2PO4–/HPO42– buffers in 0.5 M NaClO4 with added acetonitrile (MeCN), the catalytic activity of 1 is enhanced by the surface-bound redox mediators [Ru (4,4′-PO3H2-bpy)(4,4′-R-bpy)2]2+ (RuPbpyR22+, R = Br, H, Me, or OMe, bpy = 2,2′-bipyridine). Rate-limiting ET between the Ru3+ form of the mediator and the RuIV(O) form in the [RuV/IV(O)]+/0 couple of 1 is observed at relatively high concentrations of HPO42– buffer base under conditions where O···O bond formation is facilitated by atom-proton transfer (APT). For the solution [Ru(bpy)3]3+/2+ mediator couple and 1 as the catalyst, catalytic currents vary systematically with the concentration of mediator and the HPO42– buffer base concentration. Electron transfer mediation of water oxidation catalysis was also investigated on nanoparticle TiO2 electrodes co-loaded with catalyst [Ru(bda)(py-4-O(CH2)3-PO3H2)2], 2, (py = pyridine) and RuPbpyR22+ (R = H, Me, or OMe) with an interplay between rate-limiting catalyst oxidation and rate-limiting O···O bond formation by APT. Lastly, the co-loaded assembly RuPbpyR22+ + 2 has been investigated in a dye-sensitized photoelectrosynthesis cell for water splitting.