Calixarene Receptors in Ion Recognition and Sensing

Cations and anions are ubiquitous in biological and chemical systems and their efficient and selective recognition is one of the main goals of Supramolecular Chemistry. As a matter of fact, research in this field started with Pressman’s 1964 discovery that valinomycin and other natural antibiotics increase the permeability of lipid bilayer membranes through the selective binding of potassium ion, and with Pedersen’s finding that macrocyclic polyethers (crowns) are able to complex salts of alkali metal ions and dissolve them in organic media. Since then, the topic of ion recognition by synthetic receptors has developed tremendously and is still quite fertile as testified to by the recent review articles and books concerning cation and anion complexation and sensing. More recently, a special role in ion recognition has been played by calixarenes, which are phenolic macrocycles derived from the base-catalyzed condensation of p-substituted phenols and formaldehyde. Calixarenes have been extensively used as building blocks for the synthesis of very efficient and selective ionophores. It is almost impossible to cover all fields of ion complexation by calixarene ligands and therefore we will confine ourselves to spherical cation recognition and anion binding by calixarenes, which are subjects close to our own work. We will not discuss other important topics such as i) cation coordination by calixarene phenoxides, ii) ammonium and transition metal ion binding iii) anion recognition by calixpyrroles, which have been authoritatively covered in the recent review articles quoted for each case.