Enlarging the size of calix[4]arene-crowns-6 to improve Cs/K selectivity: a theoretical and experimental study

Ab initio calculations in the gas-phase indicate that the substitution of an ethylene with a propylene moiety in the polyether bridge of 1,3-di-iso-propoxycalix4]arene-crowns-6 could result in an enhanced Cs⁺/K⁺ selectivity which is of particular interest in nuclear waste treatment. We therefore synthesised two novel calix4]arene-crown-6 compounds (1 and 2) having a propylene moiety in their structure and for this named calix4]arene-propylene-crown-6. The structures of compounds 1 and 2 were elucidated by NMR in solution and for 1 also by X-ray diffraction studies in the solid state. Association constants (K_a) in CHCl₃ of the two novel calix-crowns were measured and pointed out a plateau selectivity towards alkali metal ions which was not predicted by molecular modelling calculations. These results indicate the important role played by the solvent molecules and counter-anions in binding for this class of ionophores.