Cooperative Binding of Cucurbit[n]urils and β-Cyclodextrin to Heteroditopic Imidazolium-Based Guests
摘要:
Imidazolium-based guests containing two distinct binding epitopes are capable of binding beta-cyclodextrin and cucurbit[6/7]uril (CB) simultaneously to form heteroternary 1:1:1 inclusion complexes. In the final the hosts occupy binding sites disfavored in the binary complexes because of the chemically induced reorganization of the intermediate 1:1 aggregate. In addition, the reported guests are capable of binding two CBs to form either 1:2 or 1:1:1 ternary assemblies despite consisting of a single cationic moiety. Whereas the adamantane site binds CB solely via hydrophobic interactions, the CB unit at the butyl site is stabilized by a combination of hydrophobic and ion-dipole interactions.
Adamantylatedbisimidazolium cations exhibit a distinct fragmentation pathway in contrast to their cucurbit[7]uril (CB7) complexes (see scheme). The observed alternative fragmentation of the guest molecule in a complex clearly correlates to the supposed sterically hindered or allowed slippage of the macrocycle over the axel molecule.
new homotritopic guests based on tris(benz)imidazolium salts with adamantane binding sites were prepared. NMR and calorimetric titration experiments revealed that each of the three sites independently binds β-cyclodextrin (β-CD) or cucurbit[7]uril (CB7) units to form binary host–guestcomplexes with 1:3 stoichiometry. The associationconstants for the single binding site for β-CD and CB7 were determined
adamantyl binding sites form water‐soluble binary or ternary aggregates with cucurbit[7]uril (CB7) and β‐cyclodextrin (β‐CD) with rotaxane and pseudorotaxane architectures. The observed arrangements result from cooperation of the supramolecular stopper binding strength and steric barriers against free slippage of the CB7 and β‐CD host molecules over the bisimidazolium guest axle.