Conductometric Studies of Thermodynamics of Complexation of Co~(2+), Ni~(2+), Cu~(2+), and Zn~(2+) Cations with Aza-18-crown-6 in Binary Acetonitrile-Methanol Mixtures

摘要

The complexation reactions between aza-18-crown-6 (A18C6) and Co~(2+), Ni~(2+), Cu~(2+), and Zn~(2+) ions were studied conductometri-cally in different acetonitrile-methanol 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. Selectivity of A18C6 for Co~(2+), Ni~(2+), Cu~(2+), and Zn~(2+) cations is sensitive to the solvent composition. At 20℃ and in acetonitrile solvent, the stability of the resulting complexes varied in the order Zn~(2+) > Cu~(2+) > Co~(2+) ～Ni~(2+) but the order was reversed by adding 20% methanol. 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 decreased with increasing methanol in the solvent mixture. The T△S° versus △H° plot of thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy-entropy compensation in the complexation reactions. In addition, binding energies of Ni~(2+), Cu~(2+), and Zn~(2+) complexes with A18C6 were calculated at B3LYP/6-31G level of theory.