Electrogeneration of cobalt(I) species stabilized by pyridine and its reaction with aryl halides

Cyclic voltammetry and controlled-potential electrolysis have been used to investigate the electrochemical reduction of CoBr2 in dimethylformamide (DMF) and DMF + pyridine mixtures. The presence of excess pyridine allows the stabilization of the electrogenerated Co(I) on the time scale of cyclic voltammetry. However, on the time scale of slow cyclic voltammetry or electrolysis, even in a DMF + pyridine (9:1) mixture, the cobalt(I) species undergoes a disproportionation reaction leading to solid Co and regeneration of Co(II). It is shown, for the first time, that this electrogenerated Co(I) species reacts with aryl halides, ArX, leading to the corresponding arylcobalt(III) complex which is reducible into arylcobalt(II) species at the same potential as for the starting compound CoBr2. The rate constant for this oxidative addition has been determined and discussed according to the nature of ArX. The competition between both chemical reactions (oxidative, addition, and disproportionation) is also discussed. Finally, the catalytic electroreduction of aryl halides (C6H5Br and p-EtO2C-C6H4Br) by cobalt(I) leading to biaryl and arene has been performed. (C) 2000 Elsevier Science B.V. All rights reserved.