New crown ether complex cation ionic liquids with N-heterocycle anions: preparation and application in CO2 fixation

Abstract

A series of crown ether complex cation ionic liquids (CECILs) have been devised, synthesized and characterized by NMR, IR, elemental analysis and TGA-DSC. These new CECILs ([18-C-6K]⁺Y⁻ and [15-C-5Na]⁺Y⁻, Y⁻ = imidazolide, pyrazolide, 1,2,4-triazolide, benzo[d]imidazolide, benzo[d]triazolide) were applied as efficient catalysts to the cycloaddition of epoxides and carbon dioxide without a co-catalyst under atmospheric pressure and solvent-free conditions. The results showed that the activity of catalysts deeply relies on their structural features. The CECILs of [18-C-6K]⁺Y⁻ are superior to [15-C-5Na]⁺Y⁻ under optimal reaction conditions. A preliminary kinetic study of this reaction was carried out using [18-C-6K][Im] and [15-C-5Na][Im] catalysts. The obtained activation energies are consistent with their reaction rates. An anion–cation synergistic catalytic mechanism has been carefully proposed in light of the features of the new catalysts and experimental results, which was confirmed as well by DFT calculations.