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

Abstract

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 [Cu^I(bpy-Br₂)₂](ClO₄) (1) has been obtained in two crystallographic modifications, in which the coordination geometry of the metal center has the D_2d 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 [Cu^II(bpy-Br₂)Cl₂] (5) has been proposed. Depending upon the reaction conditions, several halogen-exchanged products, namely [Cu^II(bpy-Br_1.86Cl_0.14)(Cl_1.89Br_0.11)] (2), [Cu^II(bpy-Br_1.81Cl_0.19)(Cl_1.70Br_0.30)(H₂O)] (3), and [Cu^II(bpy-Br_0.63Cl_1.37)(Cl_0.54Br_1.46)] (4), have been isolated in crystalline form. The reaction between bpy-Cl₂ and CuCl₂·2H₂O provides [Cu^II(bpy-Cl₂)Cl₂] (7) and [Cu^II(bpy-Cl₂)Cl₂(H₂O)] (8), whereas CoCl₂·6H₂O and NiCl₂·6H₂0 on reaction with bpy-Br₂ under boiling condition produce [Co^II(bpy-Br_0.5Cl_1.5)(ClBr)] (11) and [Ni^II(bpy-Br_0.46Cl_1.54)(Cl_0.73Br_1.27)(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 [Cu^II(bpy-Cl₂)Br₂] (6) and [Cu^II(bpy-Br₂)Br₂] under boiling condition undergo photoreduction to produce the dimeric copper(I) complexes [{Cu^I(bpy-Cl_1.30Br_0.70)(μ-Br)}₂](9) and [{Cu^I(bpy-Br₂)(μ-Br)}₂] (10), respectively. The fact that the photoreduction of [Cu^II(bpy-Cl₂)Br₂] (6) and [Cu^II(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 E_1/2 values for the Cu^I/Cu^II redox couples show strong solvent dependence and for a given system the E_1/2 value is more positive in dichloromethane relative to that in acetonitrile. For the compounds [Cu^II(bpy-Br_2-xCl_x)(Cl_2-yBr_y)] (x = 0−2, y = 0−2), the E_1/2 values become more positive with the increase of y value.