Iridium Complexes with Proton‐Responsive Azole‐Type Ligands as Effective Catalysts for CO 2 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 toward CO 2 hydrogenation in 2.0  m KHCO 3 aqueous solutions (pH 8.5) at 50 °C, under 1.0 MPa CO 2 /H 2 (1:1) have been reported as an alternative to photo‐ and electrochemical CO 2 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.