Thermodynamic and kinetic effects of the organic group in the electrochemical reduction of organocobaloximes

Methyl Transfer from CH3CoIIIPc to Thiophenoxides Revisited: Remote Substituent Effect on the Rates

A two-step mechanism of the reaction of CH(3)Co(III)Pc (Pc = dianion of phthalocyanine) with thiophenoxides in DMA has been confirmed, and the visible spectrum of the inactive transient, CH(3)Co(III)Pc(SAr)(-), has been determined. Rapid rates for ligation of CH(3)Co(III)Pc, yielding CH(3)Co(III)Pc(S-C(6)H(4)-X)(-), are virtually independent of X; this step proceeds probably by an I(d) mechanism. Kinetic data for the follow-up methyl-transfer step yield second-order rate constants and stability constants for CH(3)Co(III)Pc(S-C(6)H(4)-X)(-) consistent with those estimated from concentration dependence of the amplitude of the ligand-exchange step. Cyclic voltammetry provides first reduction potential for CH(3)Co(III)Pc(DMA) of -1.42 V vs Fc(+)/Fc, which makes an OSET mechanism unlikely. Homolytic decay of CH(3)Co(III)Pc(SAr)(-) has also been ruled out. All of the kinetic data, including Hammett's rho = -2.3 +/- 0.1, N-donor inhibition, and alkyl group effect, Me > Et, indicate that the reaction is a normal S(N)2 methyl transfer, only very fast. Methyl transfer to aliphatic thiolates is also rapid and follows the same S(N)2 mechanism. Exceptional methyl-transfer reactivity of the phthalocyanine model sharply contrasting with the inertness of methylcobaloxime is explained.

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.