A Ru-Hbpp-Based Water-Oxidation Catalyst Anchored on Rutile TiO2

Two organic ligands based on bis-(2-pyridyl)pyrazole (Hbpp) functionalized with a para-methylenebenzoic acid (Hbpp-R-a) or its ester derivative (Hbpp-R-e) were prepared and characterized. The ester-functionalized ligand was then used to prepare a series of related dinuclear ruthenium complexes of general formula [RU2II(L-L)(bpp-R-n)(trpy)(2)](m+) (L-L = mu-Cl, mu-acetato, or (H2O)(2); n=e or a; trpy=2,2':6',2 ''-terpyridine; m=2 or 3). The complexes were characterized in solution by 1D and 2D NMR spectroscopy, UV/Vis spectroscopy, and electrochemical techniques. The [RU2II(mu-Cl)(bpp-R-e)(trpy)(2)](PF6)(2) complex was further characterized in the solid state by X-ray diffraction. The complexes containing the free carboxylic acid ligand were anchored onto rutile TiO2 and treated with 0.1 M triflic acid solution to generate the homologous water-oxidation catalysts TiO2- (Ru-2(II)(H2O)(2)(bpp-R-a)(trpy)(2)](2+). This new hybrid material catalytically oxidizes water to molecular oxygen in a heterogeneous manner using Ce-IV as chemical oxidant. The generation of molecular oxygen is accompanied by the formation of carbon dioxide as well as some leaching of the Ru catalyst.