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.

四种新型二价钌配合物已被合成并表征，具体包括：[Ru(bpy)₂(TMBiimH₂)](ClO₄)₂（记为Ru-5；其中bpy为2,2′-联吡啶（2,2′-bipyridine），TMBiimH₂为4,5,4′,5′-四甲基-2,2′-联咪唑）、[Ru(bpy)₂(L1H₂)](ClO₄)₂·H₂O（记为Ru-6；L1H₂为4,5-二甲基-2-(N,N-二乙酰基甲脒基)-1H-咪唑）、[Ru(bpy)₂(L2H₂)](ClO₄)₂（记为Ru-7；L2H₂为N1,N1,N2,N2-四乙酰基乙二脒）以及[Ru(phen)₂(TMBiimH₂)](ClO₄)₂（记为Ru-8；其中phen为1,10′-邻菲啰啉（1,10′-phenanthroline））。研究了该系列配合物的光物理与电化学性质，并与此前已报道的[Ru(bpy)₂(BiimH₂)](PF₆)₂（Ru-1）、[Ru(bpy)₂(BbimH₂)](PF₆)₂（Ru-2）、[Ru(bpy)₂(DMBbimH₂)](PF₆)₂（Ru-3）以及[Ru(bpy)₂(TMBbimH₂)](PF₆)₂（Ru-4）进行了对比。在大气环境下经日光或家用灯光辐照时，Ru-5可与分子氧发生反应：在较高浓度的乙腈溶液中生成Ru-6，而在甲醇或低浓度乙腈溶液中则生成Ru-7。机理研究表明，开环反应中的活性氧物种为单线态氧，且该光氧化反应受溶剂与浓度共同调控。光反应产物Ru-6为反应中间体，已通过单晶X射线衍射分离并完成结构表征。Ru-6在固态与高浓度乙腈溶液中均较为稳定，但在甲醇或低浓度乙腈溶液中可被进一步氧化为Ru-7。