Efficient Solvent-Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by New Imidazolinium Functionalized Metal Complexes Under 0.1 MPa

A series of imidazolinium functionalized metal complexes were prepared as efficient and recyclable homogeneous catalysts for the synthesis of cyclic carbonates via the coupling reaction of carbon dioxide and epoxides in the absence of co-catalysts and solvent. Due to the synergistic effect between the metal center as Lewis acid and the imidazolinium groups with halide anions as Lewis base, these catalysts exhibited excellent catalytic capability. Particularly, high turnover frequency (TOF) values of (1174, 1240 and 1262 h−1) for bifunctional catalysts (2c, 2i and 2j, respectively) were achieved via adjusting reaction parameters. These catalysts also showed excellent catalytic performances under atmospheric pressure. Recyclability test showed that this kind of bifunctional catalyst can be easily recovered and reused for at least five times without dramatic loss of activity and selectivity. The optimum reaction conditions were also screened as (120 °C, 1.0 MPa CO2 and 2 h). The catalyst could also be applied to a variety of substrates and be used for a long time with high catalytic activities. We also performed the Kinetic studies, which suggested that the activity of catalysts following the order I− > Br− > Cl−. Moreover, a plausible reaction mechanism by imidazolinium functionalized metal complexes was proposed.