Synthesis, crystal structure and water oxidation activity of [Ru(terpy)(bipy)Cl]+ complexes: influence of ancillary ligands on O2 generation

Four new Ru(II) complexes, [Ru-II(MeMPTP)(bpy)Cl]PF6 (1), [Ru-II(MeMPTP)(dmbpy)Cl]PF6 (2), [Ru-II(MeMPTP)-(dmcbpy)Cl]PF6 (3) and [Ru-II(MeMPTP)(Pic)(2)Cl]PF6 (4) [where, MeMPTP = 4 '-(4-methylmercaptophenyl)-2,2 ': 6 ' 2 ''-terpyridine, bpy = 2,2 '-bipyridine, dmbpy = 4,4 '-dimethyl-2,2 '-bipyridine, dmcbpy = 4,4 '-dimethoxycarbonyl-2,2 '-bipyridine and pic = 4-picoline] were synthesized and characterized via various spectroscopic techniques. The molecular structures of the complexes 1 and 2 were determined by single crystal X-ray diffraction analysis. Catalytic activity for chemical oxidation of water of the complexes 1-4 reveals that the rate of O-2 evolution follows the trend 1 > 4 > 2 > 3. Except the unsubstituted complex 1, the catalytic rate for O-2 generation of 2 and 4, containing electron-donating (-CH3) groups, is higher than that of 3, bearing an electron-withdrawing (-COOMe) group on the bpy, while the turn over number (TON) of the complexes follows an opposite trend. The difference in the water oxidation activity of the complexes has been correlated to the effect of the substituents on the ancillary ligands in facilitating the electron density on the Ru(II) center to achieve the higher oxidation states required for the water oxidation catalysis. Interestingly, water oxidation study of the complexes 1-4 fills the missing gap between the well-studied mononuclear ruthenium complexes based on terpy/bpy and the MeMPTP/phen ligands.