Photochemical hydrogen production catalyzed by polypyridyl ruthenium–cobaloxime heterobinuclear complexes with different bridges

Two heterobinuclear complexes [(bpy)2Ru(bpy-4-CH3,4′-CONH(4-py)Co(dmgBF2)2(OH2)](PF6)2 (1, dmgBF2 = (difluoroboryl)dimethylglyoximato) and [(bpy)2Ru(bpy-4-CH3,4′-CONHCH2(4-py)Co(dmgBF2)2(OH2)](PF6)2 (2) were prepared, in which the polypyridyl ruthenium photosensitizer and the cobaloxime catalyst are connected either by a conjugated bridge (1) or by an unconjugated one (2). Complexes 1 and 2 were used as photocatalysts for hydrogen generation. Under optimal conditions, the turnover numbers (ton) for hydrogen evolution were 38 for 1 and 48 for 2 in the presence of 300 equiv of both Et3N and [Et3NH][BF4] in the acetone solution during an 8-h irradiation of visible light (λ > ca. 400 nm). The complex 2 with an unconjugated bridge proved to be more efficient for photochemical hydrogen generation than the complex 1 with a conjugated bridge under the same reaction condition.