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

Abstract

Low-valent (^Me BDI)Al and (^Me BDI)Ga and highly Lewis acidic cations in [(^tBu BDI)M⁺ ⋅C₆ H₆ ][(B(C₆ F₅ )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 )→Mg²⁺ 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.