Squaramide functionalized ionic liquids with well-designed structures: Highly-active and recyclable catalyst platform for promoting cycloaddition of CO2 to epoxides

Novel squaramide-based ionic liquids (SAILs) were first presented through properly molecular structure design. They were successfully applied to the cycloaddition of CO2 and epoxide to form cyclic carbonate as single-component catalyst. The catalytic behaviors of SAILs with different structures were thoroughly studied for the cycloaddition reaction. The squaramide groups were easy to form strong hydrogen bonds with oxygen atoms of epoxide, leading to an intensive activation of the epoxide. Under the optimum reaction conditions, good to excellent product yields with satisfactory selectivities were obtained. The structure-designed catalyst was easily recovered and showed excellent reusability with no significant loss of catalytic activity after six cycles. An insight into the synergistic catalytic mechanism deriving from the multiple hydrogen bond donors and bromine anions was proposed. The new strategy helps to solve the problems of cocatalyst/nucleophile leaching and complex separation/purification existed in binary organocatalysts. The multiple hydrogen bond catalysis shows a green and promising alternative to Lewis acid catalysis in the relevant CO2 conversion applications.