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, ^tBu, 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 > ^tBu ≈ Me > H; Ir-CN⁺ has no photosensitization effect. A notable observation is that Ir-^tBu⁺ 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)₃²⁺, 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.