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 m acrocyclic ligands cucurbituril and 18-crown-6, was studied by calorim etric titration in aqueous formic acid solution (50 vol.%). The solubi lity of cucurbituril strongly depends on the concentration of formic a cid. At low and very high formic acid concentrations, the solubility o f cucurbituril is low. For both ligands the formation of proton comple xes influence the complexation reactions with cations. Thus, the compl ex formation in most cases is favored by entropic contributions. Only in the case of the Ba2+ ion stability constants and reaction enthalpie s could be measured at lower formic acid concentrations. Using a highl y sensitive calorimeter, the stability constants and reaction enthalpi es for the reaction of Na+ and Ba2+ with cucurbituril in aqueous solut ion could be determined.