The Medium Effect on the Thermodynamics of Complexation of 5,11,17,23-Tetrakis-(1,1-dimethylethyl)-25,27-bis(methylthioethoxy)-26,28-bis[(diethylamine)ethoxy]calix(4)arene and the Silver Cation

摘要

The interaction of 5,11,17,23-tetrakis-(1,1-dimethylethyl)-25,24-bis(methylthioethoxy)-26,28-bis[(diethylamine)ethoxy]calix(4)arene and the silver cation was investigated by ~1H NMR in CD_3OD and CD_3CN at 298 K. The results suggest that the hydrophilic cavity of the ligand hosts the metal cation through interaction with the different donor atoms (O, N, S). Conductometric titrations of Ag~+ and this ligand in methanol and in ethanol indicate that the stoichiometry of the metal ion-ligand complex is 1:1. The thermodynamics of complexation of 1a and silver in six different solvents (methanol, ethanol, propan-1-ol, N,N-dimethylformamide, acetonitrile, and benzonitrile) at 298.15 K was derived from titration calorimetry. Stability constants were also checked by direct potentiometry using silver electrodes. Standard Gibbs energies of solution of 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis(methylthioethoxy)-26,28-bis[(diethylamine)ethoxy]calix(4)arene derived from solubility measurements in different solvents are used to calculate transfer Gibbs energies from acetonitrile to various solvents. The medium effect on the complexation process is assessed in terms of Gibbs energies taking into account the differences in solvation of the free and the complex metal cation and the ligand in the appropriate solvents. Representative examples are given to demonstrate that the medium alters the stability of the complex in a selective manner which is dependent upon the solvation changes that the reactants and the product undergo upon complexation in the various solvents.