Visible-Light-Induced Photooxidation of Ruthenium(II) Complex with 2,2′-Biimidazole-like Ligand by Singlet Oxygen

Four new ruthenium­(II) complexes [Ru­(bpy)2(TMBiimH2)]­(ClO4)2 (Ru-5; bpy is 2,2′-bipyridine and TMBiimH2 is 4,5,4′,5′-tetramethyl-2,2′-biimidazole), [Ru­(bpy)2(L1H2)]­(ClO4)2·H2O (Ru-6; L1H2 is 4,5-dimethyl-2-(N,N-diacetyl)­carboximidamide-1H-imidazole), [Ru­(bpy)2(L2H2)]­(ClO4)2 (Ru-7; L2H2 is N1,N1,N2,N2-tetrakis­(acetyl)­ethanediimidamide), and [Ru­(phen)2(TMBiimH2)]­(ClO4)2 (Ru-8; phen is 1,10′-phenanthroline) have been synthesized and characterized. Their photophysical and electrochemical properties have been studied and compared to the previously reported [Ru­(bpy)2(BiimH2)]­(PF6)2 (Ru-1), [Ru­(bpy)2(BbimH2)]­(PF6)2 (Ru-2), [Ru­(bpy)2(DMBbimH2)]­(PF6)2 (Ru-3), and [Ru­(bpy)2(TMBbimH2)]­(PF6)2 (Ru-4). Under irradiation with either sunlight or household light in atmosphere, Ru-5 reacts with molecular oxygen to produce Ru-6 in an acetonitrile solution with a relatively high concentration and Ru-7 in a methanol or dilute acetonitrile solution, respectively. The mechanism studies show that singlet oxygen is the reactive oxygen species in the ring-opening reaction and the photooxidation reaction is solvent- and concentration-dependent. The photoreaction product Ru-6 is an intermediate, which has been isolated and structurally characterized by single-crystal X-ray diffraction. Ru-6 is stable in the solid state and an acetonitrile solution with a high concentration, but can be further oxidized to Ru-7 in a methanol or dilute acetonitrile solution.