Intermolecular Methyl
Group Exchange and Reversible
P–Me Bond Cleavage at Cobalt(III) Dimethyl Halide Species
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Abstract
The cobalt(III) dimethyl halide complexes <i>cis,mer</i>-(PMe<sub>3</sub>)<sub>3</sub>Co(CH<sub>3</sub>)<sub>2</sub>X (X
= Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer
of the methyl ligands during isotopic labeling experiments. Extensive
mechanistic studies exclude radical, methyl iodide elimination, and
disproportionation/comproportionation pathways for exchange of the
methyl groups between metals. A related cobalt(III) dimethyl complex
supported by the tridentate phosphine ligand MeP(CH<sub>2</sub>CH<sub>2</sub>PMe<sub>2</sub>)<sub>2</sub> showed dramatically slower methyl
ligand transfer, indicative of a mechanism for intermetallic exchange
with a requisite phosphine dissociation. Crossover experiments between
cobalt(III) dimethyl halide complexes supported by PMe<sub>3</sub> and MeP(CH<sub>2</sub>CH<sub>2</sub>PMe<sub>2</sub>)<sub>2</sub> are consistent with a dicobalt transition structure in which only
one cobalt center requires phosphine dissociation prior to methyl
transfer. An additional methyl group scrambling process between <i>cis,mer</i>-(PMe<sub>3</sub>)<sub>3</sub>Co(CH<sub>3</sub>)<sub>2</sub>I and free PMe<sub>3</sub> was also identified during the
investigation and originates from reversible P–CH<sub>3</sub> bond cleavage
