The Lewis superacid Al[N(C 6 F 5 ) 2 ] 3 and its higher homolog Ga[N(C 6 F 5 ) 2 ] 3 - structural features, theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF 5 .

Herein we present the synthesis of the two Lewis acids Al[N(C ₆ F ₅ ) ₂ ] ₃ (ALTA) and Ga[N(C ₆ F ₅ ) ₂ ] ₃ (GATA) via salt elimination reactions. The metal complexes were characterized by NMR-spectroscopic methods and X-ray diffraction analysis revealing the stabilization of the highly Lewis acidic metal centers by secondary metal-fluorine contacts. The Lewis acidic properties of Al[N(C ₆ F ₅ ) ₂ ] ₃ and Ga[N(C ₆ F ₅ ) ₂ ] ₃ are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol) ₃ ] ⁺ [FAl(N(C ₆ F ₅ ) ₂ ) ₃ ] ^- and [AsPh ₄ ] ⁺ [ClGa(N(C ₆ F ₅ ) ₂ ) ₃ ] ^- contain interesting weakly coordinating anions. The reaction of Al[N(C ₆ F ₅ ) ₂ ] ₃ with trityl fluoride yielded [CPh ₃ ] ⁺ [FAl(N(C ₆ F ₅ ) ₂ ) ₃ ] ^- which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C ₆ F ₅ ) ₂ ] ₃ and Ga[N(C ₆ F ₅ ) ₂ ] ₃ was investigated by means of competition experiments for chloride ions in solution. DFT calculations yielded fluoride ion affinities in the gas phase (FIA) of 555 kJ mol ^-1 for Al[N(C ₆ F ₅ ) ₂ ] ₃ and 472 kJ mol ^-1 for Ga[N(C ₆ F ₅ ) ₂ ] ₃ . Thus, Al[N(C ₆ F ₅ ) ₂ ] ₃ can be considered a Lewis superacid with a fluoride affinity higher than SbF ₅ (493 kJ mol ^-1 ) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.