The cooperative effect in ion-pair recognition by a ditopic hemicryptophane host.

The heteroditopic hemicryptophane 1, which bears a tripodal anion binding site and a cation recognition site in the molecular cavity, proved to be an efficient ion-pair receptor. The hemicryptophane host binds anions selectively depending on shape and hydrogen-bond-accepting ability. It forms an inclusion complex with the Me4N+ ion, which can simultaneously bind anionic species to provide anion@[1⋅Me4N+] complexes. The increased affinity of [1⋅Me4N+] for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the hemicryptophane cavity, thus allowing the formation of the contact ion pair. Density functional theory calculations were performed to analyze the Coulomb interactions of the ion pairs, which compete with the ion-dipole ones, that originate in the ion–hemicryptophane contacts.