Facile Light-Induced Transformation of [RuII(bpy)2(bpyNO)]2+ to [RuII(bpy)3]2.

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [RuII(bpy)2(bpyNO)]2+ (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine-N-oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [RuII(bpy)2(bpyNO)]2+ using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic analysis was augmented with the DFT calculations. We concluded that upon 530-650 nm light excitation, 3[RuIII(bpyNO-•)(bpy)2]2+ is formed similarly to the 3[RuIII(bpy-•)(bpy)2]2+ light-induced state of the well-known photosensitizer [RuII(bpy)3]2+. An electron localization on the bpyNO ligand was confirmed by obtaining a unique EPR signal of reduced [RuII(bpy)2(bpyNO-•)]+ (gxx = 2.02, gyy = 1.99, and gzz = 1.87 and 14N hfs Axx = 12 G, Ayy = 34 G, and Azz = 11 G). 3[RuIII(bpyNO-•)(bpy)2]2+ may evolve via breaking of the Ru-O-N fragment at two different positions resulting in [RuIV═O(bpy)2(bpyout)]2+ for breakage at the O-|-N bond and [RuII(H2O)(bpy)2(bpyNOout)]2+ for breakage at the Ru-|-O bond. These pathways were found to have comparable ΔG. A reduction of [RuIV═O(bpy)2(bpyout)]2+ may result in water elimination and formation of [RuII(bpy)3]2+. The expected intermediates, [RuIII(bpy)2(bpyNO)]3+ and [RuIII(bpy)3]3+, were detected by EPR. In addition, a new signal with gxx = 2.38, gyy = 2.10, and gzz = 1.85 was observed and tentatively assigned to a complex with the dissociated ligand, such as [RuIII(H2O)(bpy)2(bpyNOout)]3+. The spectroscopic signatures of [RuIV═O(bpy)2(bpyout)]2+ were not observed, although DFT analysis and [RuII(bpy)3]2+ formation suggest this intermediate. Thus, [RuII(bpy)2(bpyNO)]2+ has potential as a light-induced oxidizer.