Catalytic reduction of hexachlorobenzene and pentachlorobenzene by cobalt(I) salen electrogenerated at vitreous carbon cathodes in dimethylformamide

Cyclic voltammetry and controlled-potential electrolysis have been used to investigate the catalytic reduction of hexachlorobenzene and pentachlorobenzene by cobalt(I) salen that is electrogenerated at a carbon cathode in dimethylformamide containing 0.050 M tetramethylammonium tetrafluoroborate. Cyclic voltammograms for the reduction of cobalt(II) salen in the presence of either hexachlorobenzene or pentachlorobenzene show an enhanced cathodic peak for the catalyst–substrate reaction as well as loss of the anodic peak for reoxidation of cobalt(I) salen, revealing the effectiveness of the catalyst. However, cobalt(I) salen shows no catalytic activity toward tetrachlorobenzene or any less-chlorinated species. Controlled-potential electrolyses of cobalt(II) salen–hexachlorobenzene mixtures give both pentachlorobenzene and three isomers of tetrachlorobenzene, whereas bulk catalytic reduction of pentachlorobenzene affords three different tetrachlorobenzenes; for both substrates, substantial starting material remains unreduced. To account for the formation of the various products, a mechanistic scheme is proposed and discussed.