Strong ion-pairing effects in a room temperature ionic liquid

1,4- and 1,2-Dinitrobenzenes exhibit two one-electron waves in acetonitrile but a single two-electron wave in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. The latter effect is ascribed to strong ion pairing between the imidazolium cation and the dinitrobenzene dianion. The strength of this ion-pairing effect is further demonstrated by the fact that complete wave coalescence is observed in acetonitrile containing as little as 10% of the ionic liquid. On the other hand, no wave coalescence is observed in the ‘hydrocarbon electrolyte’ (toluene)3–tetrabutylammonium hexafluorophosphate, demonstrating the unusual ion pairing tendency of the 1-butyl-3-methylimidazolium cation compared to tetraalkylammonium ions. This is tentatively ascribed to the planarity of the imidazolium moiety.