Thermodynamic Data for Complex Formation Between Cucurbituril and Alkali and Alkaline Earth Cations in Aqueous Formic Acid Solution

Complex formation, between alkali and alkaline earth cations and the macrocyclic ligands cucurbituril and 18-crown-6, was studied by calorimetric titration in aqueous formic acid solution (50 vol.%). The solubility of cucurbituril strongly depends on the concentration of formic acid. At low and very high formic acid concentrations, the solubility of cucurbituril is low. For both ligands the formation of proton complexes influence the complexation reactions with cations. Thus, the complex formation in most cases is favored by entropic contributions. Only in the case of the Ba²⁺ ion stability constants and reaction enthalpies could be measured at lower formic acid concentrations. Using a highly sensitive calorimeter, the stability constants and reaction enthalpies for the reaction of Na⁺ and Ba²⁺ with cucurbituril in aqueous solution could be determined.