Electrogenerated homogeneous-phase cobalt(I) salen or nickel(I) salen can serve as a catalyst for the reductive cleavage of carbon-halogen bonds in a variety of organic compounds. Alternatively, electrodes coated with anodically formed polymeric cobalt(II) salen or nickel(II) salen can be used for similar catalytic reductions; the potential of a polymer-coated electrode is controlled to convert the cobalt(II) or nickel(II) sites into cobalt(I) or nickel(I) sites that catalytically reduce the organohalogen compounds. In this review, a summary of recent research from our laboratory will be presented that deals with nickel(I) salen- and cobalt(I) salen-catalyzed reductions of some organohalogen compounds. After the electrochemical behavior of homogeneous-phase cobalt(II) salen and nickel(II) salen is described, the anodic polymerization of nickel(II) salen onto electrode surfaces as well as the electrochemical properties of the resulting chemically modified electrodes will be discussed, with emphasis on the electrogeneration of catalytically active, polymer-bound nickel(I) salen. Thereafter, we will consider the catalytic reduction of a number of classes of halogenated organic compounds. For these different species, there will be discussions of (a) the products obtained from the catalytic reductions, (b) comparisons between catalytic and noncatalytic (direct) reduction of the organohalogen compounds, (c) differences arising from the use of cobalt(I) salen and nickel(I) salen, and (d) mechanistic aspects of the catalytic processes. Mention will be made of unpublished research from our laboratory that offers additional insight into the nature of these catalytic reactions. Throughout this review, wherever it is appropriate, work done by other investigators on closely related topics will be cited.
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