Imidazolium-based ionic liquids containing multipoint hydrogen bond donors as bifunctional organocatalysts for efficient cooperative conversion of CO2 to cyclic carbonates

Abstract

A series of imidazolium-based ionic liquids (ILs) possessing multipoint hydrogen bond donors (HBDs) were developed as efficient catalysts at a significantly low catalyst loading (0.2 mol%) for the synthesis of cyclic carbonates by the cycloaddition of CO₂ with a variety of epoxides under metal and solvent-free conditions. The effect of the structure of imidazolium-based ILs with different spatial positions, substituents and nucleophilic anions on the catalytic activity was systematically studied. Single crystal X-ray diffraction, NMR spectroscopy and DFT calculations were used to reveal that the presence of multipoint HBDs within the imidazolium-based ILs is considered to be beneficial for synergistic effects that can both activate epoxide substrates and stabilize intermediates and transition states during catalysis. Additionally, the most active catalyst could be reused at least five times without a decrease in selectivity and conversion. This study provides fruitful information to deeply understand the catalytic role of multipoint HBDs within imidazolium-based ILs, which would contribute to the design of novel and efficient bifunctional organocatalysts for CO₂ utilization.