Synthesis and properties of new calixarene-based ditopic receptors for the simultaneous complexation of cations and carboxylate anions

Abstract

Calix[4]arene tetramide mono(thio)urea derivatives 5a,b and 9 are able to solubilize solid sodium salts of different anions in CDCl₃ thanks to their ability to complex simultaneously the cation at the lower rim and the anion at the upper rim. Therefore they behave as ditopic receptors. Binding studies in [²H₆]DMSO performed only on the thiourea derivatives allowed us to establish that these receptors complex carboxylates better than spherical anions and that receptor 9 having the thiourea group directly connected to the aromatic nucleus is more efficient than 5a, which has a CH₂ spacer between the two units; in the latter case good selectivity for the acetate anion is observed. Complexation of sodium ion by the amide groups at the lower rim causes an increase in the anion binding properties of receptor 9 and has little effect on receptor 5a. This is explained by the effects of the cation encapsulation on the hydrogen bonding ability of the thiourea units in the two receptors.