Factors enhancing the interactions of lower rim calix[4]arene derivatives towards toxic metal cations in solution : versatile behaviour in the solid state

The synthesis of calix[4]arene derivatives, 5,11,17,23-tetra-tert-butyl[25,27-bis(hydroxy)-26,28-bis(ethylthioethoxy)]-calix[4]arene, L3, and 5,11,17,23-tetra-tert-butyl[25,27-bis(ethylethanoate)oxy-26,28-bis(ethylthioethoxy)]-calix[4]arene, L4, are reported. These ligands are structurally (1H, 13C NMR and X-ray crystallography) and thermodynamically (solubility, calorimetry) characterised. In addition to L3 and L4, 5,11,17,23-tetra-tert-butyl[25,26,27,28-tetrakis(ethylethanoate)- calix[4]arene, L1 and 5,11,17,23-tetra-tert-butyl[25,26,27,28-tetrakis(methylketonemethoxy)-calix[4]arene, L2, are included in this study. Therefore, ligands L1, L2, L3 and L4 were investigated by 1H NMR technique to obtain information regarding their conformation in solution and the active sites of interaction with metal cations in different deuterated solvents at 298 K. Conductance measurements were performed to establish the composition of the metal-ion complexes with L1, L3 and L4 in acetonitrile, methanol, N,N-dimethylformamide and propylene carbonate at 298.15 K. Weak or non-existing interaction was observed in the latter solvent. However, complexes of 1:1 stoichiometry are formed in all cases. Potentiometric and calorimetric measurements were carried out to establish the thermodynamic parameters of complexation of these ligands in acetonitrile, methanol, and N,N-dimethylformamide at 298.15 K. L1 and L2 behaved in a parallel fashion in acetonitrile, with a peak selectivity for Ca2+ relative to other bivalent metal cations investigated. Complexation was absent in other media. On the other hand, L4 showed higher discrimination ability towards metal cations relative to L1 in acetonitrile, methanol and N,N-dunethylformamide. In moving to L3, the selectivity of the ligand was improved with respect to L1 and L4 to recognize only Ag+ and Hg2+. The ligand and the medium effects on the complexation process are quantitatively assessed. Solution and transfer thermodynamic parameters for the reactants and the product involved in the complexation of Ag+, Hg2+ with L3 and L4 in acetonitrile (as a reference solvent), methanol and N,N-dimethylformamide (as receiving solvents) are discussed. The complexation and solution data for the reactants and the product were used to calculate the standard enthalpies of coordination as a means of checking the reliability of the data. Extraction affinity of L3 for Ag+ was investigated in the water-dichloromethane saturated system. Crystals of several ligands and their metal-ion complexes were isolated and characterized by X-ray diffraction studies. Conclusions and suggestions for further work in this area are given.