pi-pi Stacking assisted binding of aromatic amino acids by copper(II)-aromatic diimine complexes. Effects of ring substituents on ternary complex stability

Ternary Cu(II) complexes containing an aromatic diimine (DA = di(2-pyridylmethyl)amine (dpa), 4,4'-disubstituted 2,2'-bipyridine (Y(2)bpy; Y = H (bpy), Me, Cl, N(Et)(2), CONH2 or COOEt) or 2,2'-bipyrimidine) and an aromatic amino acid (AA = L-phenylalanine (Phe), p-substituted phenylalanine (XPhe; X = NH2, NO2, F, Cl or Br), L-tyrosine (Tyr), L-tryptophan (Trp) or L-alanine (Ala)) were characterized by X-ray diffraction, spectroscopic and potentiometric measurements. The structures of [Cu(dpa)(Trp)]ClO4 center dot 2H(2)O and [Cu((CONH2)(2)bpy)(Phe)]ClO4 center dot H2O in the solid state were revealed to have intramolecular pi-pi interactions between the Cu(II)-coordinated aromatic ring moiety, Cu(DA) (M pi), and the side chain aromatic ring of the AA (L pi). The intensities of M pi-L pi interactions were evaluated by the stability constants of the ternary Cu(II) complexes determined at 25 degrees C and I = 0.1 M (KNO3), which revealed that the stability enhancement of the Cu(DA)(AA) systems due to the interactions is in the order (CONH2)(2)bpy < bpy < Me(2)bpy < (Et2N)(2)bpy with respect to DA. The results indicate that the electron density of coordinated aromatic diimines influences the intensities of the stacking interactions in the Cu(DA)(AA) systems. The M pi-L pi interactions are also influenced by the substituents, X, of L pi and are in linear relationship with their Hammett sigma(p) values with the exception of X = Cl and Br.