The macrocyclic and cryptate effect. 3. Complexation of barium(II) by monocyclic and bicyclic ligands in methanol

Abstract

The complexation of Ba 2+ by different crown ethers, azacrown ethers and cryptands wa studied in methanol by calorimetric and potentiometric titrations. A macrocyclic effect was found for the reactions of crown ethers, caused by favourable entropic changes. Azacrown ethers exist in different conformational forms. The energy for the nitrogen inversion which takes place when the conformation changes was calculated using the estimated Ba 2+O bond strength. The bicyclic ligand (211) is too small to form a real inclusion compound. This is possible with the larger bicyclic ligands (221) and (222). A big rise in complex stability with these ligands is observed. The cryptate effect is mainly due to an increase in reaction enthalpies.