Structural Dependence of Aromatic Ring Stacking and Related Weak Interactions in Ternary Amino Acid−Copper(II) Complexes and Its Biological Implication

Structures and stabilization due to stacking of ternary copper(II) complexes containing an aromatic amino acid (AA) and an aromatic diamine (DA), Cu(AA)(DA), have been investigated by potentiometric, spectroscopic, and X-ray diffraction methods. For the systems with AA = para-X-substituted l-phenylalanine (l-XPhe; X = H, NO2, OH, NH2) and DA = 2,2‘-bipyridine (bpy) or 1,10-phenanthroline (phen), the difference absorption spectra in the region 320−400 nm exhibited a peak assignable to the charge transfer interaction between the aromatic rings of DA and l-XPhe, the intensity being in the order NH2 > OH > H ≫ NO2 with respect to X. The stability constants of Cu(AA)(DA) determined for AA = dl-XPhe (X = F, Cl, Br) and l-XPhe (X = NH2, NO2, I) at 25 °C and I = 0.1 M (KNO3) indicated that stabilization of Cu(l-XPhe)(DA) relative to Cu(l-Ala)(en) (Ala = alanine; en = ethylenediamine) is in the order Br > OH > Cl ≈ NH2 > NO2 ≥ H ≥ F. The structures of [Cu(l-NH2Phe)(bpy)]NO3·H2O (1), [Cu(l-Tyr)(phen)]ClO4·2.5H2O (2), [Cu(l-Phe)(phen)]Cl·3H2O (3), and [Cu(l-Phe)(bpy)]ClO4·H2O (4), isolated as crystals, were determined by the X-ray diffraction method: 1, orthorhombic, P212121, a = 10.292(1) Å, b = 13.576(4) Å, c = 14.407(1) Å, V = 2012.9 Å3, Z = 4, R = 0.037, R w = 0.038; 2, orthorhombic, P212121, a = 18.20(2) Å, b = 32.63(1) Å, c = 8.14(1) Å, V = 4833 Å3, Z = 4, R = 0.111, R w = 0.087; 3, monoclinic, P21, a = 11.738(2) Å, b = 16.301(1) Å, c = 11.795(1) Å, β = 102.01(1)°, V = 2207.4 Å3, Z = 2, R = 0.045, R w = 0.035; 4, monoclinic, P21, a = 9.954(2) Å, b = 24.179(3) Å, c = 9.780(2) Å, β = 107.32(1)°, V = 2257.1 Å3, Z = 4, R = 0.065, R w = 0.060. All of the complexes have a similar distorted square-pyramidal structure around the central Cu(II) ion. While 1 and 2 have a structure involving aromatic ring stacking in the solid state, 4 has a structure without it and 3 has both types of structures in the unit cell.