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

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+) (RuPbpyR(2)(2+), R = Br, H, Me, or OMe, bpy = 2,2'-bipyridine). Rate-limiting ET between the Ru3+ form of the mediator and the Ru-IV(O) form in the [Ru-V/IV(O)](+/0) couple of 1 is observed at relatively high concentrations of HPO42- buffer base under conditions where O center dot center dot center dot 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 nanopartide TiO2 electrodes co-loaded with catalyst [Ru(bda)(py-4-O(CH2)(3)-PO3H2)(2)], 2, (py = pyridine) and RuPbpyR(2)(2+) (R = H, Me, or OMe) with an interplay between rate-limiting catalyst oxidation and rate-limiting O center dot center dot center dot O bond formation by APT. Lastly, the co-loaded assembly RuPbpyR(2)(2+) + 2 has been investigated in a dye-sensitized photoelectrosynthesis cell for water splitting.