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

Eight 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 degrees C and 1 bar CO2, whereas internal epoxides required higher both reaction temperatures (60-120 degrees 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.