Thermodynamics of complexation of a lower rim calix(4)arene ketone derivative and the sodium cation in N, N-dimethylformamide/acetonitrile mixtures

Stability constants and derived standard Gibbs energies, enthalpies, and entropies of complexation of a lower rim calix(4)arene ketone derivative, namely 5,11,17,23-tetra p-tert-butyl-25,26,27,28-tetra(benzoyl)methoxycalix-(4)arene and the sodium cation in N, N-dimethylformamide/acetonitrile mixtures at 298.15 K are first reported. Stability constants were determined potentiometrically by the use of a sodium selective electrode with the silver/silver (I) reference electrode. A linear relationship is found between the stability constant and the solvent composition. The strength of complexation decreases from acetonitrile through the mixtures. The minimum stability for this system was found in N, N-dimethylformamide. The striking feature of these data are the small variations in enthalpy observed in the solvent mixtures relative to their pure solvents. In all cases, the complexation process in these solvent mixtures is enthalpically controlled. However, the higher stability found by increasing the content of acetonitrile in the mixture is accompanied by an increase of entropy. Standard enthalpies of solution of the reactants (NaClO₄ and ligand) and the product (metal-ion complex) are reported. These data are used to calculate the standard coordination enthalpies as a means of checking the reliability of the data. Transfer enthalpy data from acetonitrile are calculated in an attempt to establish the factors which contribute to the enthalpies of complexation in the mixed solvents relative to corresponding data in the pure solvents. The exothermic character of the transfer process involving the sodium complex electrolyte increases as the content of DMF in the mixture increases but Δ_tH° from acetonitrile to neat DMF became less negative. The ¹H NMR study carried out in CD₃CN, d₇ - DMF and mixed solvents shows that the interaction between these two solvents enhances the interaction of the molecule of acetonitrile through its methyl group with the hydrophobic cavity of the complex. This explains the gain in the enthalpic stability observed in the transfer of the sodium complex salt from acetonitrile to the mixtures.