Effect of Pyridyl Substitution on Chemical and Photochemical Water Oxidation by [Ru(terpyridine)(bipyridine)(OH2)]2+ Scaffolds

The complexes [Ru(2-py-tpy)(bpy)(OH2)][PF6](2) (1; bpy = 2,2'-bipyridine), [Ru(3-py-tpy)(bpy)(OH2)][PF6](2) (2), and [Ru(4-py-tpy)(bpy)(OH2)][PF6](2) (3; n-py-tpy = 4'-(n-pyridyl)-2,2': 6',2 ''-terpyridine, n = 2-4), as electronic modifications of the [Ru(tpy)(bpy)(OH2)][PF6](2) scaffold, have been synthesized and characterized thoroughly by several spectroscopic methods. All three complexes were employed as catalysts for chemical and photochemical water oxidation. The free nitrogen atom of the pyridine ring in catalysts 1-3 is protonated at pH 1 and acts as an electron-withdrawing group. For 2, the meta-nitrogen atom has a larger withdrawing effect compared with those of 1 and 3, as the positive charge (d(+)) is delocalized only at the carbon atoms. However, the positive charge (d(+)) is distributed among the nitrogen and carbon atoms of the pyridine ring for the ortho-and para-nitrogen atoms in 1 and 3, respectively. Electrochemical and manometry studies revealed that 1 and 3 are better catalysts owing to the weaker electron-withdrawing effects than that for 2. All of these complexes also act as efficient catalysts towards the photo-oxidation of water in pH 6.5 phosphate buffer at 20 degrees C in the presence of [Ru(bpy)(3)](2+) as a photosensitizer and Na2S2O8 as a sacrificial electron acceptor.