Evaluation of the effect of the dicationic ionic liquid structure on the cycloaddition of CO2 to epoxides

The cycloaddition of CO2 to epoxides has attracted a great deal of interest in recent years as an atom efficient way to employ this greenhouse gas as a useful C1 building block. While several ionic liquids have been tested as catalysts for the reaction, little attention has been devoted to dicationic ionic liquids (DILs). Herein, the catalytic activity of a panel of structurally-related DILs has been evaluated in the cycloaddition of CO2 to epichlorohydrin, in order to study the influence of the DILs' structure on their activity. The solubilities of a series of bromide DILs in epichlorohydrin, featuring various linker lengths and imidazolium substituents, were investigated. The bromide Cn(MIM)2 series displayed a limited solubility in this medium, which particularly affected the performances of C3(MIM)2 2Br. However, the addition of 20※mol% of reaction product to the reactants was found to be an effective way to overcome this issue. This way, it was possible to compare the catalytic activity of both soluble and only partially soluble systems in epichlorohydrin. C6(MIM)2 2Br and C5(HIM)2 2Br were found to be the best performing DILs and have been tested, together with monocation C4MIM Br, also on less reactive epoxides. Mono and dicarboxylate (D)ILs instead proved to be less active than the bromide DILs and, when considering their synthesis, thermal stability and moisture sensitivity, ultimately less attractive for the development of future catalysts. Finally, a convenient and operationally simple way of recovering and reusing the bromide DILs at the end of the reaction was also evaluated.