A microcalorimetric study of the macrocyclic effect. Enthalpies of formation of copper(II) and zinc(II) complexes with some tetra-aza macrocyclic ligands in aqueous solution

Enthalpies of formation of copper(II) and zinc(II) complexes with three tetra-aza macrocyclic ligands of differing ring size have been determined directly by microcalorimetry. The values for the copper(II) complexes, which reach a maximum with the ligand 1,4,8,11 -tetra-azacyclotetradecane, have been correlated with the calculated ideal size of the aperture in the macrocyclic ligand required to maintain the ligand in a minimum-strain conformation. The macrocyclic effect has been interpreted as being due to a favourable entropy term and to a normally favourable enthalpy term, the magnitude of which is critically dependent on matching the size of the metal ion to that of the aperture in the macrocyclic ligand.