New Synthetic Receptors Based on Calix[4]Arenes for the Selective Recognition of Ions and Neutral Molecules

The conformational properties of calix[4]arene derivatives have been exploited to synthesize new receptors for the selective encapsulation of metal ions and neutral molecules. 1,3-Dialkoxycalix[4]arene-crown-5 conformers fixed in the 1,3-alternate structure are the most selective potassium ionophores known sofar which have a K+/Na+ selectivity higher than valinomycin. The corresponding crown-6 derivatives show a very high Cs+/Na+ selectivity which allows their use in the removal of Cs-137 from highly salted radioactive waste. Highly preorganized and rigid cone calix[4]arene derivatives, in which the C-2v double left right arrow C-2v interconversion is inhibited after the introduction of bridges at the lower and upper rim, are able to complex small organic molecules having acidic CH groups, both in the gas phase and in CDCl3 solution.