The Lewis superacid AlN(C6F5)23 and its higher homolog GaN(C6F5)23 – structural features theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF5

摘要

Herein we present the synthesis of the two Lewis acids Al[N(C6F5)2]3 (ALTA) and Ga[N(C6F5)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(C6F5)2]3 and Ga[N(C6F5)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(C₆F₅)₂)₃]^– and [AsPh₄]⁺[ClGa(N(C₆F₅)₂)₃]^– contain interesting weakly coordinating anions. The reaction of Al[N(C₆F₅)₂]₃ with trityl fluoride yielded [CPh₃]⁺[FAl(N(C₆F₅)₂)₃]^– which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C₆F₅)₂]₃ and Ga[N(C₆F₅)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(C₆F₅)₂]₃ and 472 kJ mol^–1 for Ga[N(C₆F₅)₂]₃. Thus, Al[N(C₆F₅)₂]₃ can be considered a Lewis superacid with a fluoride affinity higher than SbF₅ (493 kJ mol^–1) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.