Efficient Cp*Ir Catalysts with Imidazoline Ligands for CO2 Hydrogenation

We report newly developed iridium catalysts with electron-donating imidazoline moieties as ligands for the hydrogenation of CO2 to formate in aqueous solution. Interestingly, these new complexes promote CO2 hydrogenation much more effectively than their imidazole analogues and exhibit a turnover frequency (TOF) of 1290 h(-1) for the bisimidazoline complex compared to that of 20 h(-1) for the bisimidazole complex at 1 MPa and 50 degrees C. In addition, the hydrogenation proceeds smoothly even under atmospheric pressure at room temperature. The TOF of 43 h(-1) for the bisimidazoline complex is comparable to that of a dinuclear complex (70 h(-1), highest TOF reported) [Nat. Chem. 2012, 4, 383], which in-corporates proton-responsive ligands with pendent-OH groups in the second coordination sphere. The catalytic activity of the complex with an N-methylated imidazoline moiety is much the same as that of the corresponding pyridyl-imidazoline analogue. This result and the UV/Vis titrations of the imidazoline complexes indicate that the high activity is not attributable to the deprotonation of NH on the imidazoline under the reaction conditions.