Anion selectivity of Zn–salophen receptors: influence of ligand substituents

Four Zn–salophen complexes that differ for the electrodonating/electrowithdrawing character of the substituents located in the positions para to the phenolic oxygens have been used for the molecular recognition of several anions. Spectrophotometric and spectrofluorimetric titrations carried out in ethanol show for three of them strong binding affinity and also marked selectivity in the recognition of ATP, when methoxy electrodonating substituents are present. The finding that a Zn–salophen derivative with overall electrodonating substituents is a more selective receptor for this anion than those decorated with electron withdrawing groups, support the occurrence in this case of supramolecular pi–pi stacking interactions between the adenosine residue and the aromatic ligand surface that counterbalance the decrease in electrophilicity of the metal center.