Use of imidazolium transition metal halides in the synthesis of cyclic carbonates from the coupling reaction of epoxide and CO₂

The synthesis of cyclic carbonates by the coupling reactions of epoxides with CO₂ has attracted much attention with regard to the utilization of CO₂, which is responsible for global warming. The synthesis and reactivity of a series of ionic liquid-derived imidazolium metal halide complexes consisting of 1-alkyl-3-methylimidazolium cations and metal halide anion was studied. The catalytic activities were evaluated for the coupling reactions of CO₂ and ethylene oxide or propylene oxide at 100°C for 1 hr. Imidazolium zinc tetrahalide showed the highest activities. Such enhanced activities were most likely to be due to the in situ formation of new active complexes, bis(1-butyl-3-methylimidazolium) zinc tetrahalides from 1-butyl-3-methylimidazolium halide and ZnBr₂. The dissociation of a halide ion and the following attack of the dissociated halide ion on the carbon atom of the coordinated ethylene oxide would occur more easily for more nucleophilic bromide ion.