Ion Pair Formation of Alkylimidazolium Ionic Liquids in Dichloromethane

The 1:1 ion pair formation constants (KIP0) of 1-alkyl-3-methylimidazolium ([RMeIm]+; R = butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe2Im]+) ions with tetrafluoroborate ([BF4]−), hexafluorophosphate ([PF6]−), bis(trifluoromethanesulfonyl)amide ([NTf2]−), and 2,4,6-trinitrophenolate (picrate, [Pic]−) ions have been determined conductometrically in dichloromethane at 25 °C. The KIP0 determinations have also been made for symmetric tetraalkylammonium ions ([R4N]+; R = methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the KIP0 value of the [RMeIm]+ salt is almost independent of the length of the alkyl chain (R), whereas that of the [R4N]+ salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The KIP0 values of [BuMeIm]+, [BuMe2Im]+, and [Et4N]+, which are similar in the van der Waals volume, are in the order of [BuMeIm]+ ≫ [BuMe2Im]+ ≈ [Et4N]+, showing that the C2−H atom on the imidazolium ring makes an important contribution to the strong ion pair formation ability of [RMeIm]+. For a given cation, the KIP0 value is generally smaller for the larger anion, i.e., [BF4]− ≥ [PF6]− ≥ [NTf2]− > [Pic]− for [Et4N]+ and [BuMe2Im]+, and [BF4]− > [PF6]− ≥ [Pic]− ≥ [NTf2]− for [RMeIm]+.