Synthesis and properties of new mononuclear Ru(II)-based photocatalysts containing 4,4′-diphenyl-2,2′-bipyridyl ligands

The synthesis and characterisation of a series of eleven new 4,4′-diphenyl-2,2′-bipyridyl derivatives (N^N) with varying 4- or 3,5-substituents on the phenyl rings are reported. The molecular structures of two of these compounds, 4,4′-bis(3,5-diheptyloxyphenyl)-2,2′-bipyridyl and 4,4′-bis(3,5-di-4,4,5,5,5-pentafluoropentyloxyphenyl)-2,2′-bipyridyl are confirmed by X-ray crystallography. Fourteen neutral complexes trans-RuIICl2(N^N)(CO)2 are prepared by reacting the new proligands and three known ones with the polymeric precursor [RuIICl2(CO)2]n, and their optical and electrochemical properties are studied. Density functional theory (DFT) and time-dependent DFT calculations have been carried out on selected complexes in order to rationalise their electronic structures and absorption properties. The low energy electronic absorption bands have metal-to-ligand charge-transfer and ligand-to-ligand charge-transfer (LLCT) character, but these dominate only in the 4-substituted species. In the 3,5-disubstituted ones, intraligand charge-transfer (ILCT) within the bpy ligands becomes more important. The complexes show only irreversible reductive electrochemistry and no signs of polymerisation. The photocatalytic oxidation behaviour of selected trans-RuIICl2(N^N)(CO)2 complexes and their mono-aquo derivatives [RuII(H2O)(CO)2Cl(N^N)]+ with 4-methoxybenzyl alcohol is studied. Turnover numbers of up to ca. 130 are achieved when using [RuII(bpy)3]2+ (bpy = 2,2′-bipyridyl) as photosensitizer and [CoIII(NH3)5Cl]2+ as a sacrificial oxidant.