Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst

A series of cationic Ir­(III) complexes ([Ir­(btp)2(bpy-X2)]+ (Ir-X + : btp = (2-pyridyl)­benzo­[b]­thiophen-3-yl; bpy-X2 = 4,4′-X2-2,2′-bipyridine (X = OMe, t Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO2 reduction to CO using a hybrid catalyst (TiO2/ReP) prepared by anchoring of Re­(4,4′-Y2-bpy)­(CO)3Cl (ReP; Y = CH2PO­(OH)2) on TiO2 particles. Irradiation of a solution containing Ir-X + , TiO2/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 CO2 to CO with efficiencies in the order X = OMe > t Bu ≈ Me > H; Ir-CN + has no photosensitization effect. A notable observation is that Ir- t 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)3 2+, a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe + and TiO2/ReP is much more efficient than a homogeneous-solution system using Ir-OMe + and Re­(4,4′-Y′2-bpy)­(CO)3Cl (Y′ = CH2PO­(OEt)2). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.