Thermodynamics of complexation of 18-crown-6 with sodium, potassium, rubidium, caesium and ammonium ions in N,N-dimethylformamide

Abstract

The Complexation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with sodium, potassium, rubidium, caesium and ammonium ions has been studied by titration calorimetry in N,N-dimethylformamide (DMF) containing 0.1 mol dm^–3(C₂H₅)₄NCIO₄ as a constant ionic medium at 25 °C. The calorimetric titration data were well explained in terms of the formation of the [M(18-crown-6)]⁺(M = Na⁺, K⁺, Rb⁺, Cs⁺, NH⁺₄) and [Cs(18-crown-6)₂]⁺ complexes, and their formation constants, enthalpies and entropies were determined. The stability of the [M(18-crown-6)]⁺(M = Na⁺, K⁺, Rb⁺, Cs⁺, NH⁺₄) complexes is much more pronounced in DMF than in water. The formation entropies are similar for the sodium and potassium complexes in DMF and water, but are smaller for the rubidium, caesium and ammonium complexes in DMF than for the corresponding complexes in water. The stabilization of the [M(18-crown-6)]⁺ complexes is thus due to the more exothermic complexation in DMF than in water. Because the solvation of the univalent cations is expected to be enhanced to a greater extent in DMF than in water owing to the relatively large electron-pair donating ability of DMF, the more exothermic complexation in DMF than in water is ascribed to the different solvation of 18-crown-6 in the two solvents, i.e. 18-crown-6 is much more stabilized in water by forming hydrogen bonds with water molecules.