Thermodynamics of calix(4)arene esters. 1. Complexation of alkyl p-tert-butylcalix(4)arenetetraethanoates and alkali-metal cations in acetonitrile and in benzonitrile

Using silver electrodes a double competition reaction involving cryptands and calixarene esters has been for the first time used for the potentiometric determination of stability constants of highly stable complexes of calixarene esters and metal cations (lithium and sodium) in acetonitrile and in benzonitrile at 298.15 K. Corresponding data for less stable complexes were derived from thermochemical measurements using titration calorimetry (macro and micro). Thus, Gibbs energies, enthalpies, and entropies of complexation of alkyl (methyl, ethyl, and n-butyl) p-tert-butylcalix(4)tetraethanoates and alkali-metal cations in acetonitrile and in benzonitrile at 298.15 K are reported. The results show that as the electron-donating effect of the ligand increases in moving from the methyl to the ethyl and to a lesser extent to the tert-butylcalix(4)arene ester, making the carbonyl oxygen more electronegative, their interaction with metal cations increases. The implication of these finding on the selective recognition of alkali-metal cations by calix(4)arene esters in these solvents is discussed. It is concluded that in both solvents (i) metal ion-ligand stability (hence Gibbs energy) is enthalpically controlled and (ii) entropies of calix(4)arenate formation reflect marked differences between the solvation of the ligands relative to their metal-ion complexes.