Electronic Tuning of Iridium(III)-Terpyridine Photocatalysts via the Caryl^CNHC-Chelate Ligands for Highly Selective CO2 Reduction.

There is a significant challenge to achieving high selectivity and efficiency in photocatalytic CO2 reduction. Multifunctional photocatalysts are hoped to solve present problems through improving electron transfer, visible light absorption, and electron structure adjustment. Herein, two new [Ir(N^N^N)(C^C)X]+-type Ir(III)-terpyridine complexes (Ir-pmi and Ir-pbmi) were designed as multifunctional photocatalysts for CO2 reduction, featuring Caryl^CNHC-chelate ligands with strong electron-donating N-heterocyclic carbene (NHC) groups. Crucially, through the conjugation and weak electron-withdrawing effects of the benzene rings in the Caryl^CNHC-chelate ligand, the electronic structure of the Ir metal center is precisely regulated to suppress the competitive H2 evolution reaction. Ir-pbmi exhibits excellent performance in photocatalytic CO2 reduction to CO, with a turnover number (TON) of 190, a maximum turnover frequency (TOFmax) of 20 h-1, and a selectivity as high as 98%. Experiments combined with theoretical calculations confirm the regulatory effect of the ligand on the electronic structure of the active center and reveal the intrinsic mechanism and dynamic process of its photocatalytic CO2 reduction. This work provides new insight for the design and catalytic mechanism research of Ir(III)-terpyridine photocatalysts.