Thermodynamics of complexation of sodium, potassium, rubidium, caesium and ammonium ions with 18-crown-6 in acetonitrile and propylene carbonate

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 acetonitrile (MeCN) and propylene carbonate (PC) containing 0.05 mol dm^–3(C₂H₅)₄NClO₄ as a constant ionic medium at 25 °C. The formation of [M(18-crown-6)]⁺(M = Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺) ions was revealed, and their formation constants, reaction enthalpies and entropies were determined. Consistent with the similar donor and acceptor properties of MeCN and PC, the stability of the 18-crown-6–cation complexes is similar in the two solvents. However, the enthalpic and entropic behaviour are very different. The formation enthalpy of the sodium complex is positive in MeCN, while negative in PC. The reactions for the potassium, rubidium, caesium and ammonium ions are less exothermic in MeCN than in PC. On the other hand, the formation of the sodium complex in MeCN is accompanied by a more positive entropy change than that in PC. The reaction entropies for the potassium, rubidium, caesium and ammonium systems are positive in MeCN, whereas they are negative in PC. Thus, enthalpy–entropy compensation results in a similar stability of [M(18-crown-6)]⁺(M = Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺) in MeCN and PC.