Binding Studies on CB[6] with a Series of 1‐Alkyl‐3‐methylimidazolium Ionic Liquids in an Aqueous System

Abstract

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].

Capture of Carbon Dioxide by Cucurbit[7]uril Complex

“…However for component of NH 2 (CH 2 ) 6 NH 2 CB [7] complex, still no carbamate intermediate could be detected under the same concentration. The proton signals of bicarbonate product corresponding to bound amine shifted from 0.67, 0.86, 2.37 ppm to 0.61, 0.89, 2.51 ppm, whereas the signal shifts appeared and the conversion stayed complete since 45 s based on integration of the resonance signal at 0.61, 0.89 or 2.51 ppm (Figure 3b) The absence of carbamate product for bound amine whether or not in a mixture with free amine could be mostly ascribed to the deshielding effect [13,14] in that nucleophilicity of bound amine to attack CO 2 decreases as influenced by the electronegative portal oxygens, while the steric effect also inhibits the formation of carbamate to some extent. On the other hand, increased electrophilicity of bound amine caused by the deshielding effect favors the acid-base neutralization towards bicarbonate product, thus resulting in higher selectivity and faster reaction rate for bound amine compared to free amine.…”

pH-controlled release of auxin plant hormones from cucurbit[7]uril macrocycle

“…Conversely, the considerably smaller Δδ lim value for the benzyl protons f next to the IAA carboxyl group suggested a proximity to the portal (Table 1). These CB [7]-induced shifts are highly suggestive of IAA-CB [7] complex formation, and that association/dissociation of the host-guest complex was fast on the NMR time scale since the observed chemical shifts were a time-weighted average of free and bound proton signals [38].…”

“…An upfield shift of a guest proton resonance (Δδ lim = δ free − δ bound ) indicates its average position within the shielding hydrophobic cavity, whereas a downfield shift suggests that the guest proton is near one of the deshielding carbonyl-laced portals [38].…”

Reversible Capture of CO2 by Cucurbit[7]uril Absorbent

“…They can accommodate cationic and neutral guests through ion-dipole interactions, hydrophobic effect or hydrogen bonds [6,7]. Mock firstly reported that 1,3-dipolar cycloaddition reaction could be accelerated by a factor of 5.5×10 4 under the influence of CB [6] [8].…”

“…In this work, without resort to the chemical modification step of amino group, a supramolecular approach to CO 2 capture catalyzed by CB [7] was developed.…”