Equilibrium and structural studies on transition metal complexes of amino acid derivatives containing the bis(pyridin-2-yl)methyl residue

The stoichiometries, stability constants and structures of the complexes formed in the reaction of copper(ii), nickel(ii) and zinc(ii) ions and N-glycylbis(pyridin-2-yl)methylamine (Gly-BPMA) and N-histidylbis(pyridin-2-yl)methylamine (His-BPMA) ligands have been determined by potentiometric, UV-VIS and EPR spectroscopic methods. The formation of four-coordinate 1:1 copper(ii) complexes and octahedral mono and bis(ligand) nickel(ii) and zinc(ii) complexes was detected. The bis(pyridin-2-yl)methyl moiety is the main binding site in acidic media, while deprotonation and coordination of the amide nitrogen take place in the physiological pH range. The stability of the complexes of the bis(pyridin-2-yl)methyl ligands is lower than that of analogous bis(imidazol-2-yl)methyl compounds. The weaker metal binding ability of the ligands containing the bis(pyridin-2-yl)methyl group is also reflected in the fact that, in contrast with analogous bis(imidazol-2-yl)methyl ligands, the non-coordinated pyridine nitrogens cannot act as a bridge-forming polymeric species.