Variations in the coordination environment of Co, Cu and Zn complexes prepared from a tridentate (imino)pyridine ligand and their structural comparisons

The variations in the coordination environment of Co(II), Cu(II) and Zn(II) complexes with the neutral, tridentate ligand bis1-(cyclohexylimino)ethyl]pyridine (BCIP) are reported. Analogous syntheses were carried out utilizing either the M(BF₄)₂ · xH₂O or MCl₂ · xH₂O metal salts (where M = Co(II), Cu(II) or Zn(II)) with one equivalent of BCIP. When the hydrated metal starting material was used, cationic, octahedral complexes of the type M(BCIP)₂]²⁺ were isolated as the tetrafluoroborate salt (4, 5). Conversely, when the hydrated chloride metal salt was used as the starting material, only neutral, pentacoordinate M(BCIP)Cl₂] complexes (1-3) formed. All complexes were characterized by X-ray diffraction studies. The three complexes that are five coordinate have distortions due mainly to the pyridine di-imine bite angle. The Cu(BCIP)Cl₂] (2) also exhibits deviations in the Cu(II)-Cl bond distances with values of 2.4242(9) and 2.2505(9) Å, which are not seen in the analogous Zn(II) and Co(II) structures. Similarly, the two six coordinate complexes (5, 6) are also altered by the ligand frame bite angle giving rise to distorted octahedral geometries in each complex. The Cu(BCIP)₂](BF₄)₂ (6) also exhibits Cu(II)-N_imine bond lengths that are on average 0.14 Å longer than those found in the analogous 5 coordinate complex, Cu(BCIP)Cl₂]. In addition to X-ray analysis, all complexes were also characterized by UV/Vis and IR spectroscopy with ¹H NMR spectroscopy being used for the analysis of the Zn(II) analogue (3).