Iridium Complexes with Proton-Responsive Azole-Type Ligands as Effective Catalysts for CO2 Hydrogenation

Abstract

Pentamethylcyclopentadienyl iridium (Cp*Ir) complexes with bidentate ligands consisting of a pyridine ring and an electron-rich diazole ring were prepared. Their catalytic activity toward CO2 hydrogenation in 2.0 m KHCO3 aqueous solutions (pH 8.5) at 50 °C, under 1.0 MPa CO2/H2 (1:1) have been reported as an alternative to photo- and electrochemical CO2 reduction. Bidentate ligands incorporating an electron-rich diazole ring improved the catalytic performance of the Ir complexes compared to the bipyridine ligand. Complexes 2, 4, and 6, possessing both a hydroxy group and an uncoordinated NH group, which are proton-responsive and capable of generating pendent bases in basic media, recorded high initial turnover frequency values of 1300, 1550, and 2000 h−1, respectively. Spectroscopic and computational investigations revealed that the reversible deprotonation changes the electronic properties of the complexes and causes interactions between pendent base and substrate and/or solvent water molecules, resulting in high catalytic performance in basic media.