Intermolecular Methyl Group Exchange and Reversible P–Me Bond Cleavage at Cobalt(III) Dimethyl Halide Species

The cobalt­(III) dimethyl halide complexes cis,mer-(PMe3)3Co­(CH3)2X (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(CH2CH2PMe2)2 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 PMe3 and MeP­(CH2CH2PMe2)2 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 cis,mer-(PMe3)3Co­(CH3)2I and free PMe3 was also identified during the
investigation and originates from reversible P–CH3 bond cleavage.