Precisely Controlling Ancillary Ligands to Improve Catalysis of Cp*Ir Complexes for CO 2 Hydrogenation.

The regulation of ancillary ligands is critical to improve catalysis of Cp*Ir complexes for CO 2 hydrogenation. Herein, a series of Cp*Ir complexes with N^N or N^O ancillary ligands were designed and synthesized. These N^N and N^O donors were derived from the pyridylpyrrole ligand. The solid-state structures of Cp*Ir complexes featured a pendant pyridyl group in 1-Cl and 1-SO 4 and a pyridyloxy group in 2-Cl , 3-Cl , 2-SO 4 , and 3-SO 4 . These complexes were employed as catalysts for CO 2 hydrogenation to formate in the presence of alkali under a pressure range of 0.1-8 MPa and temperature range of 25-120 °C. The catalytic activity of 2-SO 4 with a pyridyloxy pendant group dramatically outperformed that of 1-SO 4 and 3-SO 4 . The TOF of conversion of CO 2 into formate reached 263 h -1 at 25 °C under a total pressure of 8 MPa (CO 2 /H 2 = 1:1). The experiments and density functional theory calculations revealed that a pendant base in metal complexes plays a key role in the rate-determining heterolytic H 2 splitting and enhancing the proton transfer by forming a hydrogen bonding bridge thereby improving the catalytic activity.