Conductometric studies of thermodynamics of complexation of Li+, Na+ and K+ ions with 4′,4"(5")-di-tert-butyldibenzo-18-crown-6 in binary acetonitrile-nitromethane mixturess

The complexation reactions between 4',4aEuro(3)(5aEuro(3))-di-tert-butyldibenzo-18-crown-6 (DTBDB18C6) and Li+, Na+ and K+ ions were studied conductometrically in different acetonitrile-nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance-mole ratio data at different temperatures. At 20 A degrees C and in nitromethane solvent, the stability of the resulting complexes varied in the order K+ > Na+ > Li+. The enthalpy and entropy changes of the complexation reactions were evaluated from the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture. The T Delta SA degrees versus Delta HA degrees plot of thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy-entropy compensation in the complexation reactions. The ab initio studies calculated at B3LYP/6-31G level of theory, indicate the binding energy of complexes decreases with increasing cation size in the gas phase. In the solution phase, DTBDB18C6 preferentially forms complexes with the larger ions rather than the smaller ions because the solvation energies of the smaller ions are large enough to overcome and reverse the trends in gas phase complexation. The findings of this study suggest that the current understanding of the factors influencing the selectivity of metal ion complexation by crown ethers may be in need of revision.