Co^I‐ and Co⁰‐Bipyridine Complexes Obtained by Reduction of CoBr₂bpy: Electrochemical Behaviour and Investigation of Their Reactions with Aromatic Halides and Vinylic Acetates

Abstract

The electrochemical behaviour of CoBr(2)bpy (bpy=2,2'-bipyridine) catalyst precursor in acetonitrile has been studied, revealing its possible reduction into the corresponding Co(I) and Co(0) complexes. These low-valent cobalt species appear to be stable on the time scale of cyclic voltammetry. In the presence of aromatic halides, both complexes undergo oxidative addition, the latter Co(0) species allowing the activation of poorly reactive aromatic chlorides. The arylcobalt(III) and arylcobalt(II) obtained are reduced at the same potential as the original Co(II) and Co(I) complexes, respectively, resulting in the observation of overall ECE mechanisms in both cases. The electrochemical study shows that vinylic acetates competitively react with electrogenerated Co(0) species, leading to a labile complex. Preparative scale electrolyses carried out from solutions containing aromatic halides (ArX), vinyl acetate (vinylOAc) and a catalytic amount of CoBr(2)bpy lead to a mixture of biaryl (Ar-Ar) and arene (ArH) as long as the potential is set on the plateau of the Co(II) right arrow over left arrow Co(I) reduction wave. The coupling product (Ar-vinyl) is formed only if the electrolysis is performed on the plateau of the Co(I)/Co(0) reduction wave. A mechanism is proposed for the overall cobalt-catalyzed coupling reaction between aromatic halides and allylic acetates.

Fundamental Input of Analytical Electrochemistry in the Determination of Intermediates and Reaction Mechanisms in Electrosynthetic Processes

“…This competitive situation was also encountered in the CoBr 2 bpy catalyzed vinylation of aromatic halides, where the vinyl acetate both stabilized the Co(I) complex and reacted on the arylcobalt(II) intermediate. These examples displaying competitive reactions illustrate the unique capacity of cyclic voltammetry in providing fast, quantitative and unambiguous information on the nature and kinetics of follow‐up reactions, thus allowing to establish mechanisms from consolidated data.…”

“…Interestingly, the cobalt(I) species was also found to be stabilized by more classical ligands such as bipyridine, but through an unusual 1/1 metal/ligand stoichiometry. The electrochemical behavior of CoBr 2 bpy revealed the possibility to generate both Co I and Co 0 redox states in acetonitrile, the latter being stable only over few seconds . Note that such Co 0 ‐bipyridine complexes cannot be obtained from the stable Co(bpy) 3 2+ precursor which is generally preferred to carry out academic electroanalytical studies.…”

“…Generally, the weaker the coordination of cobalt, the higher its capacity to break (or insert in) carbon‐halogen bonds, namely C(sp 2 )−X bonds. The CoBr 2 bpy complex was particularly interesting in this context, since oxidative addition on aromatic carbon‐halogen bonds depended on the oxidation state of cobalt . As shown in Figure , the reaction of electrogenerated Co(I) on iodo‐, bromo‐ and chlorobenzoates can be monitored by cyclic voltammetry.…”

Synthesis of Symmetrical Diaryl Ketones by Cobalt‐Catalyzed Reaction of Arylzinc Reagents with Ethyl Chloroformate

“…The filtration of arylzinc‐containing solutions was carried with Interchim PTFE syringe filters UptiDisc 22 mm, 0.45 µm. CoBr 2 (bpy) and CoBr 2 (phen) were prepared according to Labbé and Buriez . CoBr 2 (PPh 3 ) 2 was prepared according to Cotton and Holm .…”

A new bipyridyl cobalt complex for reductive dechlorination of pesticides

“…More recently, it was reported that CoBr2 in the presence of a single equivalent of 2,2'-bipyridine or pyridine exhibits a high catalytic activity toward the reduction of aromatic halides [41,42]. In an aqueous solution containing a surfactant, the reduced tris-complex was stabilized in the micelle and a mild kinetic enhancement was reported in the catalytic reduction of allyl halides by the Co(bpy)3 + complex.…”