A Ru-Hbpp-based water-oxidation catalyst anchored on rutile TiO2

A water oxidation catalyst based on ruthenium bis-(2-pyridyl)pyrazole anchored on rutile TiO₂ was prepared. The performance of this new material with regard to its capacity to catalytically oxidize water to molecular oxygen in a heterogeneous phase was evaluated. Two organic ligands based on bis-(2-pyridyl)pyrazole (Hbpp) functionalized with a para-methylenebenzoic acid (Hbpp-Ra) or its ester derivative (Hbpp-Re) were prepared and characterized. The ester-functionalized ligand was then used to prepare a series of related dinuclear ruthenium complexes of general formula [Ru^II₂(L-L)(bpp-Rn)(trpy)₂]^m+ (L-L= -Cl, -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 [Ru^II₂(μ-Cl)(bpp-Re)(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^II₂(H₂O)₂(bpp-Ra)(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.