Photochemical CO2-to-Formate/CO Conversion Catalyzed by Half-Metallocene Ir(III) Catalyst and Its Mechanistic Investigation

The catalytic efficacy of photochemical CO2 reduction by the bipyridyl (bpy) half-metallocene Ir(III) complex, [Cp*Ir(bpy)Cl]+, was evaluated in both homogeneous and heterogeneous manners. The catalyst and photosensitizer were modified in order to be commonly engaged in each system, [Cp*Ir(4,4′-Y2-bpy)Cl]+ (Cp*IrPE, Y = CH2PO(OEt)2; Cp*IrP, Y = CH2PO(OH)2) and [Ir(C∧N)2(4,4′-Y2-bpy)]+ (IrPE, C∧N = 1-phenylisoquinoline, Y = CH2PO(OEt)2; IrP, Y = CH2PO(OH)2), respectively. This modification rendered the mixed homogeneous or heterogeneous ternary hybrid system, IrPE + Cp*IrPE or IrP/TiO2/Cp*IrP, respectively, from which the catalytic performance of the half-metallocene Ir(III) was assessed. The mixed homogeneous system (IrPE + Cp*IrPE) produced formate as a major CO2 reduction product with a maximal turnover number (TON) of ∼800 for 48 h. In contrast, the heterogeneous ternary hybrid (IrP/TiO2/Cp*IrP) yielded both CO and formate with 16.7 vol % TEOA additive (TONCO/formate > 560 for 100 h), reflecting the idea that two different catalytic routes for CO2 reduction exist. The mechanistic investigations along with electrochemical and photophysical studies suggest that the homogeneous catalysis involves Cp*IrIII–H intermediate for formate production, while the heterogeneous catalysis undergoes multiple electron transfer pathways involving the energy lowering of the bipyridine ligand as it is anchored onto the electron-withdrawing n-type TiO2 support.