STRUCTURES AND PROPERTIES OF TERNARY COPPER(II) COMPLEXES INVOLVING A HALOGEN-SUBSTITUTED PHENYLALANINE AND 2,2'-BIPYRIDINE - INTERMOLECULAR INTERACTIONS BETWEEN HALO AND ORGANIC GROUPS

Structural and spectroscopic studies have been performed on the ternary Cu(II) complexes containing 4-halophenylalanines (XPhe) (X=I or Br) and 2,2'-bipyridine (bpy). The absorption spectra of the 1:1:1 Cu(bpy)(XPhe) systems exhibited a single d-d peak at 605-608 nm (pK 6-7), and the band centered at ca. 320 nm due to the charge transfer interaction between the substituted phenyl ring and bpy was observed in the difference spectra in the near ultraviolet region. Three complexes with XPhe, [Cu(bpy)(DL-IPhe) (NO3)] (1), [Cu(bpy)(DL-BrPhe)(NO3)] (2), and [Cu(bpy)(L-IPhe)]ClO4 (3) were isolated as crystals, and the structures were determined by the X-ray diffraction method. Complex 1 crystallizes in the monoclinic space group, P2(1)/n, with four molecules in a unit cell of dimensions a=5.292(1), b=27.315(8), c=13.848(2) Angstrom, beta=90.05(2)degrees. Complex 2 also crystallizes in the monoclinic space group, P2(1)/n, with four molecules in a unit cell of dimensions a=5.293(5), b=27.18(1), c=13.540(6) Angstrom, beta=90.03(6)degrees. The Cu(II) ion in each complex is in a five-coordinate distorted square-pyramidal geometry. Complex 3 crystallizes in triclinic space group P1 with two independent molecules in the asymmetric unit in a unit cell of dimensions a=18.329(4), b=10.701(1), c=5.9287(9) Angstrom, alpha=73.91(1), beta=90.02(2), gamma=105.93(2)degrees. The Cu(II) ion is in a four-coordinate, slightly distorted square-planar geometry. The phenyl ring of XPhe in 1 and 2 is located perpendicular to the bpy plane, and no intramolecular stacking was observed. The phenyl ring of L-IPhe in 3 is tilted over the coordination plane to be stacked with bpy with the average spacing of 3.23 and 3.24 Angstrom. The crystal structures revealed close contacts between the halogen atoms and the carbon atoms of the bpy ring in complexes 1 and 2, with the distances of I...C=3.54 Angstrom and Br...C=3.47 Angstrom for 1 and 2, respectively. A close contact between the iodine and carbonyl oxygen was also detected in the crystal structure of 3 with the I...O distance of 3.1 Angstrom. Possible significance of these interactions has been discussed in relation to thyroid hormone-receptor bonding.