Methyl-to-double bond migration in methylene arenium rhodium complexes

The novel methylene arenium Rh(I)-Me complexes 5 and 10 were prepared by selective protonation of the corresponding xylylene complexes with HBF₄ in cold pentane. These complexes are stable both in the solid state and in solution. Upon reaction with carbon monoxide, the corresponding CO adducts 11 are obtained, which undergo under ambient conditions selective methyl migration to the coordinated double bond of the methylene arenium moiety. This provides an uncommon example of a migratory insertion process in a saturated complex that is promoted by the incoming ligand. No evidence for a concurrent methyl migration to the CO ligand was observed, indicating that the methyl-to-olefin migration is thermodynamically preferred over the carbonylation process. The observed activation entropy (ΔS^‡ = -36.1 eu) is in good agreement with an organized transition state for the migration process.