MECHANISM OF BASE-CATALYZED METHANE FORMATION FROM METHYL(AQUO)COBALOXIME

The formation of methane in aqueous base from methylcobaloximes and some analogues has been studied by GLC, mass spectral, and manometric techniques. Decomposition of methyl(aquo)cobaloxime in D2O (1.0 N NaOD) yielded mono-deuteriomethane, while trideuteriomethyl(aquo)cobaloxime gave trideuteriomethane in H2O (1.0 N KOH) and tetradeuteriomethane in D2O (1.0 N NaOD). In all cases (in 1.0 N base) the yield was less than stoichiometric (ca. 70%) but additional methane was obtained by photolysis indicating formation of a methylcobalt side product. Studies of the dependence of the rate and yield of methane formation on [OH-] (50°C, ionic strength 1.0 M) showed that only the hydroxo complex of methylcobaloxime is reactive for methane formation, and both methane and side product formation are first order in [OH-]. The second-order rate constants for methane formation were determined to be zero and 1.36 X 10-4 M-1 s-1 while those for side-product formation were 7.47 X 10-5 and 5.52 X 10-5 M-1 s-1 for the aquo and hydroxo complexes, respectively. Studies of the dependence of the rate constant for methane formation on the concentration of four different Lewis base axial Iigands (including sulfur and nitrogen donors, with affinity for the cobalt center varying from 6.06 to 3500 M-1) showed that the N-and S-liganded species are at least four orders of magnitude less reactive than the hydroxo complex. The cobalt-containing dealkylated product of this cleavage reaction was found, after neutralization of reaction mixtures, to be a cobaloxime analogue in which one Schiffs base linkage has become hydrated. This material was reconverted to a cobaloxime upon stirring in aqueous acid. The methylcobalt side product proved to be extremely labile and all attempts to isolate it led to its reversion to starting material. These observations are discussed in the framework of several ionic mechanisms, although a homolytic mechanisms cannot be ruled out. © 1979, American Chemical Society. All rights reserved.