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

The regulation of ancillary ligands is critical to improve catalysisof Cp*Ir complexes for CO2 hydrogenation. Herein, a seriesof Cp*Ir complexes with N<^>N or N<^>O ancillary ligands were designedand synthesized. These N<^>N and N<^>O donors were derived from the pyridylpyrroleligand. The solid-state structures of Cp*Ir complexes featured apendant 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 employedas catalysts for CO2 hydrogenation to formate in the presenceof alkali under a pressure range of 0.1-8 MPa and temperaturerange of 25-120 & DEG;C. The catalytic activity of 2-SO ( 4 ) with a pyridyloxy pendant group dramaticallyoutperformed that of 1-SO ( 4 ) and 3-SO ( 4 ). The TOF of conversion of CO2 into formate reached 263 h(-1) at 25 & DEG;Cunder a total pressure of 8 MPa (CO2/H-2 = 1:1).The experiments and density functional theory calculations revealedthat a pendant base in metal complexes plays a key role in the rate-determiningheterolytic H-2 splitting and enhancing the proton transferby forming a hydrogen bonding bridge thereby improving the catalyticactivity. This paper systemically examined themultifunctional pyridylpyrroleligand effect for CO2 hydrogenation. A water molecule hasa remarkable influence on the heterolytic cleavage of H-2 in the case of 1-SO ( 4 ) with apyridine pendant base, most likely through participation in the catalyticcycle, while in the case of 2-SO ( 4 ), a proton-responsive pyridyloxy group directly assists cooperativeintramolecular H-H bond cleavage in CO2 hydrogenation.