Bifunctional rare earth complexes have been developed for the cycloaddition reactions of epoxides and CO₂, under atmospheric pressure, without co-catalyst to produce value-added cyclic carbonates. Comparative and kinetic experiments confirm the existence of intramolecular synergies between the central metal and the nucleophilic reagent within the catalytic molecule.

Comprehensive Summary

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 °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.