Complexation of calixarene derivatives and lanthanide cations in nonaqueous media

The complexing properties of two lower rim calix(4)arene derivatives, namely, 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28-bis[2-(diethylamine)ethoxy ]calix(4)arene (1a) and p-tert-butylcalix(4)arene tetradiisopropylethanoamide (1b) toward lanthanide(III), scandium, and yttrium cations in acetonitrile and in NN-dimethylformamide at 298.15 K were investigated. H-1 NMR complexation experiments established the presence of interactions between the hydrophilic cavity of these macrocycles and these metal cations and revealed the active sites of complexation of these ligands. Conductance measurements were used to (i) establish the concentrations at which the lanthanide trifluoromethanesulfonate salts are fully dissociated 3:1 electrolytes in these solvents and (ii) determine the composition of the metal-ion complex in these solvents. Titration microcalorimetry was used to derive the thermodynamics of complexation of these macrocycles and lanthanide(III) cations in acetonitrile and NN-dimethylformamide at 298.15 K. No reliable thermodynamic data could be obtained from classical calorimetry due to the slow kinetics observed in the complexation of these calixarene derivatives and these cations in these solvents. Stability constants of la were also determined by the competitive potentiometric method using the silver electrodes. Excellent agreement was found between the data derived from calorimetry and those derived by potentiometry. For all the systems investigated, the complexation process between these cations and these ligands was enthalpically controlled. Enthalpy-entropy compensation effects were observed in the complexation of la and the different lanthanide(III) cations in acetonitrile and in N,N-dimethylformamide, as suggested by the absence of significant variations in the Gibbs energies of complexation in each case. As far Ib is concerned, a selective behavior was observed for this ligand and the various cations in acetonitrile with the highest stabilities found for gadolinium and europium. The enthalpic and entropic contributions to the Gibbs energy associated with these processes are analyzed. Final conclusions are given.