Theoretical investigation on conversion of CO2 with epoxides to cyclic carbonates by bifunctional metal-salen complexes bearing ionic liquid substsituents

Abstract

• The influence on CO 2 conversion is: metal center > ionic liquids > epoxide substituent. • Amino functionalized ILs exhibit higher activity to CO 2 conversion than the non functionalized ILs or hydroxyl functionalized ones. • The electronic effect of the substituent in epoxide exceeds its steric effect in most cases. • The length from the metal to epoxide oxygen is a descriptor for catalyst screening. A series of bifunctional metal-salen complexes bearing ionic liquid substituents were investigated as catalysts for the synthesis of cyclic carbonates from carbon dioxide (CO 2 ) and epoxides using density functional theory (DFT) and energy span model. Detailed studies reveal that the three-step mechanism involving the ring-opening of propylene oxide (PO), CO 2 insertion, and ring-closure to the cyclic carbonate is kinetically more favorable than the two-step mechanism. The effect of ionic liquids, epoxide substituents and metal center of the metal-salen complexes were explored. Amino functionalized ionic liquids substituents tethered to the salen ligand of the metal-salen complexes performed well. Epoxide substituents affect the cycloaddition mainly through the electronic effect and the substituent that lowers the negative charges of the ring carbon atom to be attacked favors the cycloaddition. When the salen ligand is fixed, the catalytic activity metal-salen complexes can be measured by the interaction between the oxygen atom (O) of epoxides and the central metal (M) and there is a linear relationship between the energy span and the M-O bond length, which can be used as a descriptor to screen CO 2 conversion catalysts.