Novel ion-pair receptors based on hexahomotrioxacalix[3]arene derivatives.

A series of novel heteroditopic hexahomotrioxacalix[3]arene triamide receptors capable of binding an anion and cation simultaneously in a cooperative fashion has been prepared. The lower rim functionalized cone-hexahomotrioxacalix[3]arene derivatives cone-5a-5d bearing three amide groups were synthesized from cone-3 by a stepwise reaction. The crystal structures of 5c and 5d and (1)H NMR studies in nonpolar solvents strongly indicate that a number of interesting intramolecular hydrogen bonding interactions exist in these receptors. The binding abilities of these compounds towards n-butylammonium chloride and bromide salts have been investigated using (1)H NMR titration experiments in CDCl(3) solvent. Owing to the 'flattened cone' conformations and intramolecular hydrogen bonding involving the amide NH and neighbouring O atoms in cone-5a-5d, the affinities toward n-Bu(4)NX (X = Cl(-) and Br(-)) were weakened. However, it should be noted that triamides cone-5a-5d show a single selectivity for halide anions in the presence of n-BuNH(3)(+) through intermolecular hydrogen bonding with the amide NH hydrogen atoms in the receptors in CDCl(3) solution. Association constants were calculated from the chemical shift changes of the amide protons.