Stability constants and derived Gibbs energies and enthalpies and entropies of complexation of alkali−metal cations and macrocycles (18-crown-6 and ethyl p-tert-butylcalix(6)arenehexanoate) in benzonitrile at 298.15 K derived from titration calorimetry (macro and micro) are first reported. These data are compared with those previously reported for cryptand 222 and these cations in this solvent. A “peak” selectivity is observed for the complexation of the calix(6)arene ester and alkali metal cations with a monotonic increase in stability from lithium to potassium followed by a decrease from the latter to rubidium. This behavior is analogous to that shown for cation binding involving 18-crown-6 and cryptand 222. Similar to the process of complex formation involving these ligands, that involving the calix(6)arene ester and these metal cations is enthalpy controlled with an exothermic maximum for the potassium cation. Among the ligands considered, thermodynamic data involving cation−calix(6)arene ester interactions are characterized by a lower enthalpic stability (less exothermic) and a more favorable entropy (except for K+) than corresponding data involving cryptand 222 or indeed 18-crown-6. These results are analysed in terms of solute−solvent interactions reflected on the solution thermodynamic data of the free and the complexed electrolytes and the ligand in benzonitrile. Standard enthalpies of solution of macrocycles and their sodium and potassium salts in benzonitrile measured calorimetrically are discussed in relation to (i) corresponding data for the uncomplexed salts and (ii) previously reported data for sodium and potassium cryptate salts in various solvents. Enthalpies of coordination referred to the process in the solid state for eighteen systems involving sodium and potassium coronates, cryptates, and calix(6)arenates are derived and whenever possible these are discussed on the basis of (i) available X-ray crystallographic data and (ii) the anion effect on the coordination process.

The synthesis and characterization ( 1 H NMR) of two new lower rim calix(4)arene derivatives containing mixed pendent arms, namely, 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28bis[(pyrid-2-ylmethyl)oxy]calix(4)arene (1) and 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis[2-(methylthio)ethoxy]-26,28-bis[(pyrid-3-ylmethyl)oxy]calix(4)arene (2), are reported. Transfer thermodynamic parameters of these ligands to various solvents show that these macrocycles undergo selective solvation in nonaqueous media. As far as the alkali-metal cations are concerned, 1 H NMR measurements reflect that 1 in CD 3 CN discriminates against the largest cations (K + , Rb + , and Cs + ) but interacts with Li + and Na + . These interactions appears to be more pronounced with the pendent arms containing the pyridyl ring (through the ethereal oxygens and nitrogen donor atoms) than with those containing the (methylthio)ethoxy arms (only ethereal oxygens are involved). No interaction takes place between its isomer 2 and alkali-metal cations in CD 3 CN. The complexation of the lithium and the sodium complexes was established through conductometric titrations. In all cases investigated 1:1 (ligand: metal cation) complexes are formed in acetonitrile and in benzonitrile. On the basis of the semiquantitative information provided by conductance measurements regarding the strength of complexation of 1 and the sodium cation, the sodium monoacetonitrile complex of 1 was isolated and its structure determined by X-ray diffraction methods. Thus, the cation was found in the hydrophilic cavity while the hydrophobic cavity hosts a molecule of acetonitrile. The crystallographic results confirm the key role played in the chelating process by the ethereal oxygens and the pyridyl nitrogens at the 2-position in the terminal pyridyl groups, as found in solution. The thermodynamics of complexation of 1 and alkali-metal cations (Li + and Na + ) in two dipolar aprotic solvents is interpreted in terms of the solution thermodynamics of the reactants and the product. It is the availability of these data that allows interpreting the different enthalpic and entropic contributions resulting from cation, solvent, and ligand effects. Final conclusions are given.