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 ((NN)-N-boolean AND) 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-(RuCl2)-Cl-II((NN)-N-boolean AND)(CO)(2) are prepared by reacting the new proligands and three known ones with the polymeric precursor ](RuCl2)-Cl-II(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-(RuCl2)-Cl-II( (NN)-N-boolean AND)(CO)(2) complexes and their mono-aquo derivatives [RuII(H2O)(CO)(2)Cl((NN)-N-boolean AND)](+) with 4-methoxy-benzyl 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 [Co-III(NH3)(5)Cl](2+) as a sacrificial oxidant.