Efficient Cp*Ir Catalysts with Imidazoline Ligands for CO₂ Hydrogenation

Abstract

Abstract We report newly developed iridium catalysts with electron‐donating imidazoline moieties as ligands for the hydrogenation of CO 2 to formate in aqueous solution. Interestingly, these new complexes promote CO 2 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 °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 incorporates 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 pyridylimidazoline 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.