Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO by Re(I)-Complex-Anchored TiO Hybrid Catalyst.

A series of cationic Ir(III) complexes ([Ir(btp)(bpy-X)] (Ir-X: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X = 4,4'-X-2,2'-bipyridine (X = OMe, Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO reduction to CO using a hybrid catalyst (TiO/ReP) prepared by anchoring of Re(4,4'-Y-bpy)(CO)Cl (ReP; Y = CHPO(OH)) on TiO particles. Irradiation of a solution containing Ir-X, TiO/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO to CO with efficiencies in the order X = OMe > Bu ≈ Me > H; Ir-CN has no photosensitization effect. A notable observation is that Ir-Bu and Ir-Me are less efficient than Ir-OMe at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X sensitizers are commonly superior compared to Ru(bpy), a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe and TiO/ReP is much more efficient than a homogeneous-solution system using Ir-OMe and Re(4,4'-Y'-bpy)(CO)Cl (Y' = CHPO(OEt)). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.