Synthesis and characterization of chlorinated alkenylcobaloximes to probe the mechanism of vitamin B(12)-catalyzed dechlorination of priority pollutants.

Vitamin B(12) catalyzes the reductive dechlorination of several ubiquitous pollutants such as perchloroethylene (PCE) and trichloroethylene (TCE). Several mechanisms have been proposed for these transformations, some of which involve the intermediacy of chlorinated vinylcobalamins. To evaluate the currently unknown chemical and physical properties of such species, various chlorinated vinylcobaloxime complexes [cobaloxime = bis(dimethylglyoximato)(pyridine)cobalt(III)] were prepared and characterized. X-ray structures are reported for (cis-1,2-dichloroethenyl)cobaloxime (4), (cis-monochloroethenyl)cobaloxime (5), (alpha-chloroethenyl)cobaloxime (6), and vinylcobaloxime (7), and the reactivities of these isolated complexes were investigated. They were stable in the presence of ethanolic NaBH(4) unless external cobaloximes were added. The cob(I)aloxime formed under the latter conditions promoted the conversion of 4 to 5 and 6, and of 5 and 6 into 7. Mechanistic studies of these transformations are consistent with a pathway in which the conversion of 4 into 5 and 6 takes place via chloroacetylene as an intermediate, and the conversion of 6 to 7 involves vinyl chloride as an intermediate. Cyclic voltammetry on the chlorinated vinylcobaloximes resulted in irreversible reduction waves, with 4 displaying the least negative and 7 the most negative peak potential. These results are discussed in the context of the B(12)-catalyzed reductive dechlorination of PCE and TCE.