Mechanism of photocatalytic CO₂ reduction to HCO₂H by a robust multifunctional iridium complex

The tetradentate PNNP-type Ir^III complex Mes-IrPCY2 ([Cl-Ir^III-H]⁺) is reported to be an efficient catalyst for the reduction of CO₂ to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H (¹[Ir^I-H]⁰) demonstrates higher activity than the single-reduced complex 2-H (²[Ir^III(L˙^-)-H]⁺), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO₂ reduction reaction. In the HCOO^- formation pathway, the hydride of 1-H performs a nucleophilic attack on CO₂via an outer-sphere fashion to generate species 1-OCHO (¹[Ir^I-OCHO]⁰), which then releases HCOO^- to produce an Ir^I intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst ¹[Cl-Ir^III-H]⁺. For the CO production, a nucleophilic attack on CO₂ takes place by the Ir atom of 1-Hvia an inner-sphere manner to afford complex O2C-3-H (¹[O₂C-Ir^III-H]⁰), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex (²[OC-Ir^III(L˙^-)-H]⁺) after liberating a H₂O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H (¹[Ir^I-H]⁰) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO₂ reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.