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

Herein we present the synthesis of the two Lewis acids Al[N(CF)] (ALTA) and Ga[N(CF)] (GATA) 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(CF)] and Ga[N(CF)] are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol)][FAl(N(CF))] and [AsPh][ClGa(N(CF))] contain interesting weakly coordinating anions. The reaction of Al[N(CF)] with trityl fluoride yielded [CPh][FAl(N(CF))] which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(CF)] and Ga[N(CF)] 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 for Al[N(CF)] and 472 kJ mol for Ga[N(CF)]. Thus, Al[N(CF)] can be considered a Lewis superacid with a fluoride affinity higher than SbF (493 kJ mol) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.