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

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Ternary Cu(ii) complexes containing an aromatic diimine (DA = di(2-pyridylmethyl)amine (dpa), 4,4′-disubstituted 2,2′-bipyridine (Y2bpy; 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)]ClO4·2H 2O and [Cu((CONH2)2bpy)(Phe)]ClO 4·H2O in the solid state were revealed to have intramolecular π-π interactions between the Cu(ii)-coordinated aromatic ring moiety, Cu(DA) (Mπ), and the side chain aromatic ring of the AA (Lπ). The intensities of Mπ-Lπ interactions were evaluated by the stability constants of the ternary Cu(ii) complexes determined at 25 °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 (CONH 2)2bpy < bpy < Me2bpy < (Et 2N)2bpy 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 Mπ-Lπ interactions are also influenced by the substituents, X, of Lπ and are in linear relationship with their Hammett σp values with the exception of X = Cl and Br. © The Royal Society of Chemistry

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