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The Lewis superacid Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> and its higher homolog Ga[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> – structural features, theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF<sub>5</sub>
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Citations
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References
2017
Year
Herein we present the synthesis of the two Lewis acids Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> (ALTA) and Ga[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> (GATA) <i>via</i> 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<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> and Ga[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol)<sub>3</sub>]<sup>+</sup>[FAl(N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)<sub>3</sub>]<sup>-</sup> and [AsPh<sub>4</sub>]<sup>+</sup>[ClGa(N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)<sub>3</sub>]<sup>-</sup> contain interesting weakly coordinating anions. The reaction of Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> with trityl fluoride yielded [CPh<sub>3</sub>]<sup>+</sup>[FAl(N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>)<sub>3</sub>]<sup>-</sup> which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> and Ga[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> 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<sup>-1</sup> for Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> and 472 kJ mol<sup>-1</sup> for Ga[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub>. Thus, Al[N(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>]<sub>3</sub> can be considered a Lewis superacid with a fluoride affinity higher than SbF<sub>5</sub> (493 kJ mol<sup>-1</sup>) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.
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