Cd(II) complexation in aqueous solution with dipyridine-and phenanthroline-containing polyamine macrocycles

The coordination features toward Cd(II) of four phenanthroline-containing and three dipyridine-containing macrocyclic polyamines have been studied by means of potentiometric and spectrophotometric UV-Vis measurements in aqueous solutions. All ligands form stable 1:1 metal complexes. The stability constants of the Cd(II) are generally lower than those reported for Cd(II) complexes with aliphatic macrocyclic polyamines containing the same number of nitrogen donors. This effect is mainly related to the stiffening of the ligands, due to the insertion of a large and rigid heteroaromatic moiety within the aliphatic polyamine framework. This structural feature does not allow the nitrogen donor to achieve an optimal arrangement around the metal ion. The decreased stability is often accompanied by facile protonation of the complexes, suggesting that some amine groups are not involved in metal binding.