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

Heats of formation of the macrocyclic complexes ML (M = Cu, Zn; L = I, n = m = k = l = 2 (II); n = l = 2, m = k = 3 (III); n = m =k = 2, l = 3 (IV)) were detd. by microcalorimetry. The values for the Cu complexes were correlated with the calcd. ideal size of aperture in the ligand required to keep it in a min. strain configuration; CuL (L = IV) had the max. value. The macrocyclic effect was studied by reaction of [(CH2)x[NH(CH2)2NH2]2] (x = 2,3) with II and III, and was due to favorable entropy and enthalpy terms. The latter is dependent in magnitude on matching the size of M2+ to that of the ligand aperture.