Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage.

Low-valent ( ^Me BDI)Al and ( ^Me BDI)Ga and highly Lewis acidic cations in [( ^tBu BDI)M ⁺ ⋅C ₆ H ₆ ][(B(C ₆ F ₅ ) ₄ ^- ] (M=Mg or Zn, ^Me BDI=HC[C(Me)N-DIPP] ₂ , ^tBu BDI=HC[C(tBu)N-DIPP] ₂ , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [( ^tBu BDI)Mg-Al( ^Me BDI) ⁺ ], [( ^tBu BDI)Mg-Ga( ^Me BDI) ⁺ ] and [( ^tBu BDI)Zn-Ga( ^Me BDI) ⁺ ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [( ^tBu BDI)Zn-Al( ^Me BDI) ⁺ ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak Al ^I (or Ga ^I )→Mg ²⁺ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give ( ^tBu BDI)ZnPh and ( ^Me BDI)AlF ⁺ which is extremely Lewis acidic and was not observed, but ( ^Me BDI)Al(F)-(μ-F)-(F)Al( ^Me BDI) ⁺ was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route.
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