Highly efficient synthesis of cyclic carbonates from epoxides catalyzed by salen aluminum complexes with built-in “CO2 capture” capability under mild conditions
A series of monometallic salen aluminum complexes were prepared by covalent linkage of the imidazolium-based ionic liquid moieties containing various polyether chains with the salen ligand at the two sides of the 5,5′-position. The salen aluminum complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst. The catalysts presented excellent “CO2 capture” capability due to the molecules containing polyether chains and the metal aluminum center, in which >90% yield of cyclic carbonate could be obtained under mild conditions. The catalysts can be easily recovered and six times reused without significant loss of activity and selectivity. Moreover, based on experimental and previous work, the “CO2 capture and activation” cycloaddition reaction mechanisms by monometallic or bimetallic salen aluminum complexes were both proposed.