Stabilities of five-coordinate ternary copper(II) complexes containing 2,2′-dipicolylamine and α-amino acids and X-ray crystal structure of [Cu(dpa)(l-phe)]ClO4 (dpa=2,2′-dipicolylamine, l-phe=l-phenylalaninate)

Stabilities of the five-coordinate ternary copper(II) complexes [Cu(dpa)(am)]+ containing 2,2′-dipicolylamine (dpa) and 11 different α-amino acid anions (am) have been studied by potentiometric and spectrophotometric titrations. The effect of the side chains of the amino acid ligand on stability is examined. The ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids, indicating the presence of a hydrophobic intramolecular interaction between the hydrophobic moiety of dpa and the aromatic side chains. The X-ray crystal structure of [Cu(dpa)(l-phe)]ClO4, where l-phe is l-phenylalaninate, is also described. The copper ion takes on a slightly distorted square pyramidal geometry with facially-coordinated dpa. There is no intramolecular ring-stacking between the pyridine ring of dpa and the phenyl ring of l-phe, whereas intermolecular stacking is observed between the pyridine rings of adjacent complex molecules.