Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO<sub>2</sub> by Re(I)-Complex-Anchored TiO<sub>2</sub> Hybrid Catalyst

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