The complexing properties of two lower rim calix(4)arene derivatives, namely, 5,11,17,23-tetrakis(1,1dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28-bis[2-(diethylamine)ethoxy]calix(4)arene (1a) and p-tertbutylcalix(4)arene tetradiisopropylethanoamide (1b) toward lanthanide(III), scandium, and yttrium cations in acetonitrile and in N,N-dimethylformamide at 298.15 K were investigated. 1 H 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 N,N-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 1a 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 1a 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 1b 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.

The complexation of p-tert-butylcalix[4]arene tetraethanoate, 1a, p-tert-butylcalix[4]arene tetramethyl ketone, 1b, and p-tert-butylcalix[4]arene tetraacetamide, 1c, and trivalent cations was investigated in acetonitrile and N,N-dimethylformamide at 298.15 K using several techniques. 1H NMR measurements in CD3CN at 298 K were carried out for the systems involving 1a, 1b, and 1c as ligands and Sc3+, Y3+, Eu3+, and Yb3+ as cations. For the latter ligand, 1H NMR titration with La3+ was also carried out to assess the sites of interaction of these ligands and the appropriate cation in this solvent. Conductance measurements were performed in acetonitrile and N,N-dimethylformamide with the aim of determining the composition of the metal ion complexes. Stability constants and derived standard Gibbs energies, enthalpies, and entropies reveal that, as far as 1b is concerned, this ligand is not able to distinguish among the trivalent cations as a result of a remarkable enthalpy−entropy compensation effect. This is not the case for 1c and these cations in acetonitrile and to a lesser extent in N,N-dimethylformamide. The selective behavior of this ligand for these metal cations is reflected in the stability constants, which are higher in acetonitrile than in N,N-dimethylformamide. A plot of log K s values against the cation radius shows a “selectivity” peak. In acetonitrile, the complex stability is greater than that previously observed for an analogous derivative and these cations in this solvent. The medium effect on the complexation process is discussed.

The interaction of 5,11,17,23-tetra-tert-butyl [25,26,27,28-tetrakis(2-pyridylmethyl)oxy]calix(4)arene 1a with alkali-metal cations in dipolar aprotic media (acetonitrile and benzonitrile) has been investigated. 1H NMR experiments were carried out by adding to the ligand an excess amount of the appropriate metal cation (Li`, Na`, K`) in at 298 K. Measurements were taken at di †erent time intervals ranging from 30 min to 10 CD 3 CN days. It was found that the kinetics of the process in this solvent is fast. Conductance measurements demonstrated that 1 : 1 metal cation : ligand stoichiometries are found with these cations in these solvents. Thermodynamic parameters of complexation for this ligand and alkali-metal cations in acetonitrile and in benzonitrile at 298.15 K were derived from titration microcalorimetry. Stability constants were also determined by the competitive potentiometric method using silver electrodes. Excellent agreement is found between the data derived from calorimetry and those derived by potentiometry. The highest stability is found for lithium with this ligand. The results are compared with data previously reported for systems involving a calix(4)arene ester derivative and these cations in the same solvents. The implications of stability constant data on the selective behaviour of this ligand for metal cations in these solvents are discussed. It is shown that this ligand in these solvents is able to discriminate between the smaller cations (Li`and Na`) and the larger ones (K`and Rb`). The sodium perchlorate complex of 1a was synthesised and the molecular structure of this complex has been determined from X-ray di †raction data. The substance crystallises in the tetragonal group P4cc with a \ 21.791(2), c \ 26.958(3) and z \ 8. There are three di †erent complexes in the lattice, two sited Ó on fourfold axes and a third one on a twofold axis. All ligands exhibit a " cone Ï conformation and the Na`ion is encapsulated in their hydrophilic pockets with an acetonitrile molecule Ðlling their hydrophobic cavities. Based on 1H NMR, conductimetric, microcalorimetric and X-ray di †raction studies Ðnal conclusions are given.

The thermodynamics of trivalent cations (Y 3þ , La 3þ , Pr 3þ , Nd 3þ , Eu 3þ , Gd 3þ , Tb 3þ , Ho 3þ , Er 3þ , Yb 3þ ) and cryptand 222 in acetonitrile at 298.15 K is discussed. Recent reports regarding the behavior of lanthanide(III) trifluoromethane sulfonate salts in acetonitrile are considered. Thus, the experimental work was carried out under conditions in which ions (M 3þ ) are predominantly in solution. Therefore, conductiometric titrations were carried out to establish the composition of the cation-cryptand 222 complexes and their ionic behavior in solution. Stability constants and derived standard Gibbs energies, enthalpies and entropies were determined by competitive titration microcalorimetry. Previously reported thermodynamic data for the complexation of cryptand 222 and a few lanthanide cations (La 3þ , Pr 3þ and Nd 3þ ) in acetonitrile are revisited. The medium effect on the stability of complex formation in acetonitrile relative to N,N-dimethylformamide is demonstrated. Thus, a drop in stability by a factor of 8 Â 10 10 is observed for the latter relative to the former solvent. The selectivity of cryptand 222 for these cations relative to La 3þ in acetonitrile is discussed.