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).

The biological activity of two fluorescent Zn(II)-salophen derivatives has been evaluated. In vitro studies (AFM, emission and UV-vis titration with ethidium bromide and cell growth inhibition) show different mechanisms of interaction with DNA. It has been observed that these compounds enter the cells. Comet assays (with cultured fibroblast cells) have revealed that cellular uptake occurs without damaging the DNA strands. Preliminary studies carried out with living cells have shown IC50 values in a millimolar range, indicative of a non-cytotoxic behaviour. This fact could be understood by confocal microscopy co-localization studies with living cell internalization that have shown that, in fact, the compounds seem to enter the cells but not the nucleus under in vivo conditions

In the search for configurationally stable inherently chiral uranyl-salophen complexes, the newly synthesized compound 3 featuring a dodecamethylene chain was expected to be a promising candidate. Unexpectedly, dynamic HPLC on a enantioselective column showed that it still undergoes enantiomerization at high temperature. By comparison with the dynamic behavior of compounds 4 and 5, it was found that the enantiomerization rate is independent of the size of the ligand. This finding definitely rules out a jump rope-type mechanism for the enantiomerization process and points to reaction pathways involving preliminary rupture of one of the O...U coordinative bonds. This provides unprecedented evidence of the occurrence of ligand hemilability in metal-sal(oph)en complexes. Such findings inspired the synthesis of compound 6 endowed with a more rigid spacer, i.e., that derived from 4,4'-(1,4-phenylenediisopropylidene)bisphenol. DHPLC investigations showed that the new structural motif imparts a higher configurational stability, thus raising the half-life for the enantiomerization to more than 2 months at room temperature. This clearly establishes that this compound represents the first member of a new class of inherently chiral receptors, whose potential in chiral recognition and catalysis now can be feasibly explored.

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.

Abstract:The vibrational features and molecular structures of complexes formed by a series of uranyl-salophen receptors with simple anions, such as Cl À , H À , and HCOO À , have been investigated in the gas phase. Spectra of the anionic complexes were studied in the ñ = 800-1800 cm À1 range by mass-selective infrared multiple photon dissociation (IRMPD) spectroscopy with a continuously tunable free-electron laser.The gas-phase decarboxylation of the formate adducts produces uranyl-salophen monohydride anions, which have been characterized for the first time and reveal a strong UÀ H bond, the nature of which has been elucidated theoretically. The spectra are in excellent agreement with the results obtained from high-quality ab initio calculations, which provided the structure and binding features of the anion-receptor complexes.