Characterization of the ionic liquid obtained by chlorosulfonation of 1-methylimidazole: 1-methyl-3-sulfonic acid imidazolium chloride, 1-methylimidazolium chlorosulfate or a zwitterionic salt?

A great number of organic reactions catalyzed by the ionic liquid product of chlorosulfonation of 1-methylimidazole have been reported recently. At the same time controversial assumptions have appeared on the real structure of the catalyst. In the present report the primarily formed chlorosulfonation product is proved to be 1-methylimidazolium chlorosulfate ([HMim]+[SO3Cl]−) instead of 1-methyl-3-sulfonic acid imidazolium chloride, reported previously. The former structure is confirmed by X-ray crystallography and NMR spectroscopy, including 1H-, 13C-, 17O- and 15N–1H HSQC measurements. 1H and 17O NMR experiments support fast hydrolysis of [HMim] [SO3Cl] resulting in the formation of [HMim][HSO4] in the presence of traces of water.