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

Pentamethylcyclopentadienyl iridium (Cp*Ir) complexes with bidentate ligands consisting of a pyridine ring and an electron-rich diazole ring were prepared. Their catalytic activity towards 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 TOF values of 1300 h^-1, 1550 h^-1 and 2000 h^-1, respectively. Here, 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 the high catalytic performance in basic media.