Cycloaddition Reactions of Epoxides and CO2 Catalyzed by Bifunctional Rare‐Earth Metal Complexes Bearing Amino‐Bridged Tris(phenolato) Ligands

Abstract

Comprehensive SummaryEight zwitterionic rare earth metal complexes stabilized by amino‐bridged tris(phenolato) ligands bearing quaternary ammonium side‐arms were synthesized and characterized. These complexes were used as single‐component catalysts for the cycloaddition of CO2 and epoxides, and their catalytic activities are obviously higher than those of their binary analogues. Further studies revealed that the halide anions (Cl–, Br–, I–) and the metal complexes influenced the catalytic activity, and the lanthanum complex bearing iodide anion showed the highest catalytic activity for this addition reaction. A variety of mono‐substituted epoxides were converted to cyclic carbonates in good to excellent yields (55%—99%) with high selectivity (> 99%) at 30 °C and 1 bar CO2, whereas internal epoxides required higher both reaction temperatures (60—120 °C) and catalyst loading (2 mol%) for high yields. The catalyst was recyclable for four times without noticeable loss of catalytic activity. Based on the results of kinetic studies and in situ IR reactions, a plausible reaction mechanism was proposed.