SYNTHESIS OF NEW CALIX[4]ARENE-BASED IONOPHORES

A synthesis of new calix[4]arene-based ionophores is reported. These ligands are characterized by the presence of two types of coordination sites on their lower rim: one crowning unit of different length, linking two proximal calix[4]arene phenolic oxygens, and two esters or amide groups acting as additional hard binding sites. The choice of the proper reaction conditions, such as the base counterion and the solvent polarity, during the functionalization of the macrocycle lower rim, allows to modulate the stereochemistry of the final ligands. In this way the ligands in cone and 1,2-alternate conformations were obtained in good yields. Complexation properties of these new compounds towards alkali and alkaline-earth cations were studied by means of liquid-liquid extraction experiments. The X-ray solid state structure of the p-tert-butylcalix[4]arene-N,N-diethylacetamide-crown-5 (6a) complex with strontium picrate shows the effective cooperativity among the two type of coordination site in the binding event.