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 H-1 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 H-1 NMR titration experiments in CDCl3 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-Bu4NX (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-BuNH3+ through intermolecular hydrogen bonding with the amide NH hydrogen atoms in the receptors in CDCl3 solution. Association constants were calculated from the chemical shift changes of the amide protons.