THERMODYNAMICS OF FORMATION OF 1,4,7,10,13,16-HEXAOXACYCLOOCTADECANE (18-CROWN-6) COMPLEXES WITH CALCIUM, STRONTIUM AND BARIUM IONS IN WAFER AND DIMETHYLFORMAMIDE

The formation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) co mplexes with calcium, strontium and barium ions has been studied by ti tration calorimetry in water and dimethylformamide (dmf) containing 0. 1 mol dm(-3) NEt(4)Cl and NEt(4)ClO(4), respectively, as constant ioni c media at 25 degrees C. The formation of the [M(18-crown-6)](2+) (M = Ca2+, Sr2+ or Ba2+) complexes was revealed and their formation consta nts, reaction enthalpies and entropies were determined. The formation constant increases in the order Ca < Sr < Ba, showing the barium compl ex to be the most stable in water and dmf. Despite the large differenc e in the electron-pair donating and accepting abilities of water and d mf, the stability of the [M(18-crown-6)](2+) complexes is very similar in the two solvents. The formation of [Ca(18-Crown-6)](2+) is less ex othermic, while that of [Sr(18-crown-6)](2+) and [Ba(18-crown-6)](2+) is more exothermic in dmf than in water. The formation entropy is less negative for the calcium complex but more negative for the strontium and barium complexes in dmf than in water. Thus, enthalpy-entropy comp ensation leads to the similar stability constant of [M(18-crown-6)](2) in water and dmf. The differences between enthalpies in dmf and in w ater, Delta H degrees(1)(dmf) - Delta H degrees(1)(water). are 10.9. - 6.7 and -11.3 kJ mol(-1) for the calcium. strontium and barium complex es, respectively. Since dmf has a stronger electron-pair donating abil ity than water, the solvation of the bivalent cations is expected to b e more pronounced in dmf than in water. On the other hand, water has a much stronger electron-pair accepting ability than dmf and the solvat ion of 18-crown-6 is more enhanced in water than in dmf, i.e., 18-crow n-6 is much stabilized in water by forming hydrogen bonds with water m olecules, Thus, these two factors compensate and the enthalpic differe nces change from positive to negative for M(2+), i.e., Ca > O > Sr > B a. The decreasing order suggests that the difference between the desol vation energies upon complexation of the metal ions in dmf and in wate r is more significant for the ion of smaller size.