Abstract
This thesis investigates the complexing properties of synthetic macrocyclic ligands such as calix[4]arenes (L1-L6) and cryptand 222 (L7) towards lanthanide(III) cations in dipolar aprotic media (acetonitrile and N,N-dimethylformamide) at 298.15 K. Molecular modelling studies performed on calix[4]arene derivatives suggested that solvent molecules such as acetonitrile and N,N-dimethylformamide interact with the hydrophobic cavity, thus inducing conformational changes on these ligands, 'preorganising' them for complexation. 1H NMR complexation experiments established the presence of interactions between the hydrophilic cavity of calix[4]arene derivatives and the metal cations and revealed the sites of complexation of the ligands. Conductance measurements clearly demonstrated that 1:1 and 2:1 metal cation : ligand stoichiometries are found with these cations in acetonitrile. The thermodynamics of complexation of macrocyclic ligands and lanthanide(III) cations in acetonitrile and in N,N-dimethylformamide at 298.15 K was derived from titration microcalorimetry. Stability constants were also determined by the competitive potentiometric method using silver electrodes. Excellent agreement was found between the data derived from calorimetry and those derived by potentiometry. The complexation process between these cations and these ligands was enthalpically controlled for all systems studied. Enthalpy-entropy compensation effects were observed in the complexation of 5,11,17,23-tetrakis-(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28-bis[2(diethylamine)ethoxy]calix[4]arene, L1 and the different lanthanide(III) cations in acetonitrile, as well as L1, L7 and [tetrakis(N,N-diethylaminoethyl)oxyl]p-tert-butylcalix[4]arene, L2, in N,N-dimethyl formamide, as suggested by the absence of significant variations in the free energies of complexation in each case. As far as p-tert-butylcalix[4]arene tetradiisopropyl acetamide, L3, is concerned, a selective behaviour was observed for these cations in acetonitrile with the highest stabilities found for gadolinium and europium. Metal-ion complexes were isolated based on their stability and the solution thermodynamics of the free and complexed salts was investigated. Lastly, the complexation process in the two solvents was discussed, taking into account the differences in solvation of the reactants and the product in these solvents.