The Anion [Mg₆Cl₈Cp*₅]^-: A Final Intermediate on the Way to the Molecular Donor-Free Grignard Compound MgClCp*?

The reaction at −70 °C of decamethylmagnesocene MgCp*2 (Cp* = pentamethylcyclopentadienyl) with monovalent aluminum chloride, AlCl, stabilized with diethyl ether and dissolved in toluene, results in tetrameric tetrahedrotetrakis(pentamethylcyclopentadienylaluminum), [AlCp*]4, and the dimeric bis(pentamethylcyclopentadienylmagnesium chloride diethyl etherate), [MgCp*Cl·Et2O]2. At a slightly higher temperature and by stepwise decrease of the diethyl ether concentration before reaction, a preferred formation of metal clusters such as dodekakispentamethylcyclopentadienylpentaconta-aluminum, Al50Cp*12, was frequently observed. Interestingly, after working up the residue of this reaction, a new kind of Grignard compound could be isolated. We report here on the synthetic details and X-ray structure of a donor-free Grignard compound with an inverse magnesocene motif, μ-Cp*Mg2, in the anionic part of the compound, [Mg6Cl8Cp*5][AlCp*2]·C6H6 (1).