Precisely Controlling Ancillary Ligands to Improve Catalysis of Cp*Ir Complexes for CO2 Hydrogenation

The regulation of ancillary ligands is critical to improve catalysis of Cp*Ir complexes for CO₂ 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-SO₄ and a pyridyloxy group in 2-Cl, 3-Cl, 2-SO₄, and 3-SO₄. These complexes were employed as catalysts for CO₂ 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-SO₄ with a pyridyloxy pendant group dramatically outperformed that of 1-SO₄ and 3-SO₄. The TOF of conversion of CO₂ into formate reached 263 h^-1 at 25 °C under a total pressure of 8 MPa (CO₂/H₂ = 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 H₂ splitting and enhancing the proton transfer by forming a hydrogen bonding bridge thereby improving the catalytic activity.