In this critical review the use of metal-salophen complexes as anion receptors is discussed with an emphasis on the supramolecular control of selectivity derived by the presence of additional interaction sites on the ligand skeleton. Some examples of application in sensing are also reported (112 references).

A new water soluble zinc-salophen complex with appended D-glucose moieties was synthesized and characterized. Its binding properties toward six aminoacids were investigated by UV-vis spectrophotometric titrations. Association constants showed to be unfavorably affected by increasing steric hindrance of the side chain. Quite surprisingly, different association constants were measured for the L and D enantiomers. These data suggest that aminoacids are bound via two interactions, zinc-carboxylate coordination and two hydrogen bonds between the ammonium group of the aminoacid and two oxygen atoms of one D-glucose moiety. Such conclusion is supported by the result of semiempirical (PM3) calculations. Notably, the K(L)/K(D) value of 9.6 observed for the association of phenylalanine rivals with the highest values found for the chiral recognition of aminoacids in water.

Fluoride recognition by molecular receptors is attracting much interest in the scientific community and a variety of approaches are used to achieve recognition with high affinity and selectivity. Most of the synthetic systems reported have been extensively studied in organic solvents by using tetrabutylammonium fluoride (TBAF) as the source of fluoride. In many cases, titration behaviors are observed that cannot be ascribed to a classical 1:1 binding isotherm. By using UV/Vis, 19F NMR, and 1H NMR spectroscopy we have been able to highlight that the equilibrium between the fluoride and the corresponding bihalide ion, HF2, which is inevitably generated in situ due to trace amounts of water, can be at the origin of this singular behavior. Our results highlight that when undertaking titrations with fluoride in DMSO, the data can be affected by the fluoridebihalide equilibrium and that the latter species can even be the dominating species at low TBAF concentrations

The development of heteroditopic receptor systems that can simultaneously bind cationic and anionic species is one of the most challenging research topics in supramolecular chemistry, attracting the attention of a large number of research groups worldwide. Such an interest is due especially to the fact that the overall receptor–ion-pair complex is neutral and this can be advantageous in many situations, such as salt solubilization and extraction, and membrane-transport applications. Receptors designed for ion-pair complexation are molecules comprising well-known anion-binding motifs and familiar cation-binding sites. An important family of compounds that can use metal Lewis-acidic centres for anion recognition and that can be easily derivatized to introduce an additional binding site for the cation is metal–salophen and metal–salen complexes. This short review shows that the high versatility of salen and salophen ligands and of the corresponding metal complexes allows, through simple modifications of the basic skeleton, the obtention of highly efficient receptors for ion pairs.