Binding studies on CB[6] with a series of 1-alkyl-3-methylimidazolium ionic liquids in an aqueous system

The host-guest chemistry between a series of 1-alkyl-3-methyl-imidazolium bromide ([C(n)mim]Br) guests and the macrocyclic host molecule cucurbit[6]uril (CB[6]) in an aqueous system is systematically studied in neutral aqueous media. Both 1D and 2D NMR experiments in conjunction with isothermal titration calorimetry (ITC) unveil the binding characteristics of the host-guest interaction. Solution binding constants (K(a)) up to 10(5) M(-1) are measured directly. Additionally, this [C(n)mim]Br-CB[6] interaction was found to significantly increase the solubility of CB[6] in neutral water, in some cases by at least four orders of magnitude. From these studies, a detailed host-guest binding model has been constructed and is fully discussed. In this model, the delocalized positive charge on the imidazolium ring becomes partially localized on either one of the nitrogen atoms upon complexation with CB[6]. Localization of the positive charge is directly related to the length of the "1-alkyl" chain on the imidazolium ring, which causes an induced local dipole subsequently allowing for an ion-dipole interaction with the carbonyl portal of CB[6].