Halogen Exchange and Scrambling between C−X and M−X‘ Bonds in Copper, Nickel, and Cobalt Complexes of 6,6‘-bis(bromo/ chloromethyl)-2,2‘-bipyridine. Structural, Electrochemical, and Photochemical Studies

The synthesis, reactivities, spectroscopic, electrochemical, and structural studies of copper(I), copper(II), nickel(II),
and cobalt(II) complexes of 6,6‘-bis(bromomethyl)-2,2‘-bipyridine (bpy-Br2) and 6,6‘-bis(chloromethyl)-2,2‘-bipyridine
(bpy-Cl2) have been reported. The copper(I) complex [CuI(bpy-Br2)2](ClO4) (1) has been obtained in two crystallographic
modifications, in which the coordination geometry of the metal center has the D2d symmetry. The reaction between
CuCl2·2H2O and bpy-Br2 has been followed spectrophotometrically at 45 °C over a period of 7 h, and a mechanism
for the intramolecular halogen exchange and scrambling in the initially formed compound [CuII(bpy-Br2)Cl2] (5) has
been proposed. Depending upon the reaction conditions, several halogen-exchanged products, namely [CuII(bpy-Br1.86Cl0.14)(Cl1.89Br0.11)] (2), [CuII(bpy-Br1.81Cl0.19)(Cl1.70Br0.30)(H2O)] (3), and [CuII(bpy-Br0.63Cl1.37)(Cl0.54Br1.46)] (4), have
been isolated in crystalline form. The reaction between bpy-Cl2 and CuCl2·2H2O provides [CuII(bpy-Cl2)Cl2] (7) and
[CuII(bpy-Cl2)Cl2(H2O)] (8), whereas CoCl2·6H2O and NiCl2·6H20 on reaction with bpy-Br2 under boiling condition
produce [CoII(bpy-Br0.5Cl1.5)(ClBr)] (11) and [NiII(bpy-Br0.46Cl1.54)(Cl0.73Br1.27)(H2O)] (12), respectively. The X-ray
structures determined for the 4-coordinate compounds 2, 4, and 7 show flattened tetrahedral geometry for the
metal center with the D2 symmetry. Both 5-coordinate compounds 3 and 12 have square pyramidal geometry, and
whereas the nickel(II) complex 12 has near-perfect geometry (τ = 0.015), considerable distortion is observed for
the copper(II) complex 3 (τ = 0.25). Complexes [CuII(bpy-Cl2)Br2] (6) and [CuII(bpy-Br2)Br2] under boiling condition
undergo photoreduction to produce the dimeric copper(I) complexes [{CuI(bpy-Cl1.30Br0.70)(μ-Br)}2](9) and [{CuI(bpy-Br2)(μ-Br)}2] (10), respectively. The fact that the photoreduction of [CuII(bpy-Cl2)Br2] (6) and [CuII(bpy-Br2)Br2]
do not take place in absence of light has been established by spectrophotometric measurements. The crystal
structures of 9 and 10 have been determined. The electrochemical behavior of all the copper complexes 1−10 has
been studied in acetonitrile and dichloromethane. The E1/2 values for the CuI/CuII redox couples show strong solvent
dependence and for a given system the E1/2 value is more positive in dichloromethane relative to that in acetonitrile.
For the compounds [CuII(bpy-Br2-xClx)(Cl2-yBry)] (x = 0−2, y = 0−2), the E1/2 values become more positive with
the increase of y value.