Interplay between conformational flexibility, intermolecular H-bonding and 3d-metal cation extraction ability in a series of thiacalix[4]arene lower rim disubstituted Schiff base derivatives

The rational design of organic ligands with the aim to control their binding abilities towards different metal ions can be considered as one of the key concepts in supramolecular coordination chemistry. Regarding the macrocyclic compounds of thiacalix[4]arene family, this can be achieved <i>via</i> the targeted modulation of macrocyclic platform rigidity as well as the proper choice of appended binding sites. Four macrocyclic salen-type ligands based on lower rim disubstituted thiacalix[4]arene derivatives, adopted in a cone conformation, bearing highly coordinating iminophenolic or catecholic groups and two -CH₂- moieties as spacers but presenting different abilities to form H-bonds, were chosen to elucidate the interplay between the conformational flexibility of the macrocyclic ligands, propensity to participate in the intermolecular H-bonding and the extraction ability of 3d-metal cations. X-ray diffraction analysis, theoretical DFT calculations, IR and Raman spectroscopies, and dynamic light scattering (DLS) studies performed in combination with liquid-liquid metal extraction study revealed that compounds 4, and 6, based on a thiacalix[4]arene macrocyclic platform, display a higher extraction ability towards all studied 3d-metal ions, caused by enhanced conformational flexibility. This is in good accordance with the ability of 6 to form H-bonded supramolecular assemblies in solution and crystalline phases due to recognition between the catecholic moieties.