Complexation of Sodium, Potassium, Rubidium, Cesium and Ammonium Ions with 12-Crown-4 in N,N-Dimethylformamide and Water

Abstract

The complexation of 1, 4, 7, 10-tetraoxacyclododecane (12-crown-4) with sodium, potassium, rubidium, cesium and ammonium ions has been studied by titration calorimetry in N, N-dimethylformamide (DMF) and water containing 0.1mol dm-3 (C2H5)4NClO4 and (C2H5)4NCl, respectively, as constant ionic media at 25°C. A very small heat of complexation other than the heat of dilution of titrant was evolved in the course of the titration of sodium, potassium, rubidium, cesium and ammonium ions with 12-crown-4 in water; thus, the complexation of 12-crown-4 with these cations is expected to be very weak. The formation of [M(12-crown-4)]+ and [M(12-crown-4)2]+ (M=Na+, K+, Rb+, Cs+) complexes was revealed in DMF, and their formation constants, reaction enthalpies and entropies were determined. On the other hand, no significant heat ascribable to complexation was liberated for the ammonium 12-crown-4 DMF solutions, and the formation of the ammonium 12-crown-4 complex was very weak, practically negligible. The cavity size of 12-crown-4 is 57pm in radius, while the alkali-metal ion has an ionic radius of 116-181pm. Since 12-crown-4 is too small to fully accommodate any alkali-metal ion, the formation of the [M(12-crown-4)]+ and [M(12-crown-4)2]+ (M=Na+, K+ Rb+, Cs+) complexes results. The formation constants, enthalpies and entropies for the [M(12-crown-4)]+ (M=Na+, K+, Rb+, Cs+) complexes do not differ significantly by metal ions. Thus, in DMF, 12-crown-4 is not able to distinguish metal ions having different sizes.