Ditopic receptors based on lower- and upper-rim substituted hexahomotrioxacalix[3]arenes: Cation-controlled hydrogen bonding of anion

Heteroditopic hexahomotrioxacalix[3]arene receptors that are capable of binding an anion and a cation simultaneously in a cooperative fashion were synthesized. The structure of one of the triamide derivatives was confirmed by single-crystal X-ray diffraction. The binding of alkali metals at the lower rim, and the binding of anions (chloride, bromide) at the upper rim, has been investigated by using 1H NMR titration experiments. Alkali metal binding at the lower rim controls the calix cavity. Li+-ion binding to the lower rim can improve the binding ability of anions at the upper rim amide moiety by a factor of 15, thus suggesting a strong positive allosteric effect for anion recognition. However, when a Na+ cation is bound to the ionophoric site on the lower rim, the calix cavity is changed from a “flattened cone” to a more-upright form, which is favored for intramolecular hydrogen bonding between the neighboring NH and CO groups; this change can block the inclusion of anions onto the amide moiety at the upper rim, which strongly suggests a negative allosteric effect of Na+-ion binding, which controls the cooperative recognition system.