The macrocyclic and cryptate effects. 5. complexation of alkali ions by monocyclic and bicyclic ligands in methanol

Abstract

The complexation of alkali ions by the ligand 18- crown 6 and by different azacrown ethers and cryptands was studied in methanol by calorimetric and potentiometric titrations. The macrocyclic effect found for the reactions of 18-crown-6 is caused only by favourable entropic changes. Due to the different conformational forms of the uncomplexed azacrown ethers, their complexes with alkali ions are not as stable as the complexes of unsubstituted crown ethers. Cations too big to be encapsulated by the bicyclic cryptands form exclusive complexes. However, 2:1 complexes could not be observed in these cases. During the complex formation the cryptands have to change their conformation. Therefore, the observed reaction enthalpies are smaller than expected. On the other hand, the stability constants measured for the reactions of bicyclic ligands with alkali ions are several orders of magnitude higher in most cases in comparison with monocyclic ligands. It is concluded that the macrocyclic and cryptate effects are caused only by favourable entropic changes.