Allosteric effects in a tetrapodal imidazolium-derived calix[4]arene anion receptor

A ditopic tetrapodal imidazolium-based 1,3-alternate calix[4]arene host binds anions as 1:2 complexes via a range of CH center dot center dot center dot anion hydrogen bonds. Allosteric enhancement of the affinity for the second chloride anion is observed. X-Ray crystal structures of the chloride, bromide and nitrate complexes suggest that this behaviour is linked to inter-binding site communication mediated by the calixarene framework. The imidazolium groups exist as either in-out or out-out conformers with respect to the calixarene, with the chloride complex exhibiting conformational isomorphism such that both forms are present in the same crystal structure.