Azacryptands as molecular cages for anions and metal ions

This is a short overview on azacryptands, as molecular receptors for cations and anions. A particular attention was devoted to the results obtained by women researchers working in the field. The terms ‘cryptand’ and ‘cryptate’ were coined by Lehn. Since then, much work has been done to improve the knowledge on this class of receptors. Small azacryptands, as free bases, were found to bind a single metal ion within their cavities. When fully protonated, the same systems could also behave as selective hosts for anions, through the cooperation of H-bonding and electrostatic interactions. Proceeding to systems with larger cavities, the inclusion of two metal ions and a bridging anion was possible, forming the so-called ‘cascade’ complexes. Azacryptates carrying fluorescent spacers or exploiting the indicator displacement paradigm allowed the sensing of anionic species in water at micromolar concentrations. Moreover, immobilisation on solid matrices and surfaces yielded new materials for the solid-phase extraction of anionic pollutants and the construction of selective electrodes for analytes in water.