Conversion of CO₂ into Cyclic Carbonates under Ambient Conditions Catalyzed by Rare-Earth Metal Complexes Bearing Poly(phenolato) Ligand

Abstract

A series of rare-earth (RE) metal complexes (Y, Sm, Nd, and La) stabilized by polydentate N-methylethylenediamine-bridged tris(phenolato) ligands was synthesized and characterized. Lanthanum complexes showed good activity in catalyzing the cycloaddition reaction of terminal epoxides with CO2 under ambient conditions (i.e., room temperature, 1 bar CO2), giving rise to cyclic carbonates in 49–99% yields. More importantly, generally challenging internal epoxides were also transformed into cyclic carbonates in 70–99% yields in the presence of 1 bar CO2 at 60 °C. This is the first RE-based catalyst for efficient cycloaddition of CO2 and epoxides under ambient conditions and is among the most active catalysts for this important transformation. The lanthanum complex was also recycled six times. Kinetic study of the cycloaddition of cyclohexene oxide and CO2 was conducted, and the kinetic equation was determined as follows: rate = k[epoxide]1[CO2]0[cat.]1.26[TBAI]0.4. The Gibbs activation energy (333 K) was determined to be 29.8 kcal/mol.