Thermodynamics of formation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) complexes with calcium, strontium and barium ions in water and dimethylformamide

Abstract

The formation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) complexes with calcium, strontium and barium ions has been studied by titration calorimetry in water and dimethylformamide (dmf) containing 0.1 mol dm^–3 NEt₄Cl and NEt₄ClO₄, respectively, as constant ionic media at 25 °C. The formation of the [M(18-crown-6)]²⁺(M = Ca²⁺, Sr²⁺ or Ba²⁺) complexes was revealed and their formation constants, reaction enthalpies and entropies were determined. The formation constant increases in the order Ca < Sr < Ba, showing the barium complex to be the most stable in water and dmf. Despite the large difference in the electron-pair donating and accepting abilities of water and dmf, 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 exothermic, 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 compensation leads to the similar stability constant of [M(18-crown-6)]²⁺ in water and dmf. The differences between enthalpies in dmf and in water; ΔH°₁(dmf)–ΔH°₁(water), are 10.9, –6.7 and –11.3 kJ mol^–1 for the calcium, strontium and barium complexes, respectively. Since dmf has a stronger electron-pair donating ability than water, the solvation of the bivalent cations is expected to be more pronounced in dmf than in water. On the other hand, water has a much stronger electron-pair accepting ability than dmf and the solvation of 18-crown-6 is more enhanced in water than in dmf, i.e. 18-crown-6 is much stabilized in water by forming hydrogen bonds with water molecules. Thus, these two factors compensate and the enthalpic differences change from positive to negative for M²⁺, i.e., Ca > O > Sr > Ba. The decreasing order suggests that the difference between the desolvation energies upon complexation of the metal ions in dmf and in water is more significant for the ion of smaller size.