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Lewis Superacidity?

The definition may be seen in analogy to Brønsted acids: Brønsted superacids are stronger than the strongest conventional Brønsted acid, 100% H2SO4.

"Molecular Lewis acids, which are stronger than monomeric SbF5 in the gas phase, are Lewis Superacids."

We propose using the FIA as a quantitative measure for Lewis acidity (Table 1). So Lewis acids with a FIA higher than that of monomeric SbF5 (489 kJ/mol) are termed Lewis superacids.[2] 

Because of the tendency of monomeric AlBr3 and AlI3 towards aggregation the halogenides were substituted for the bulky perfluorinated alkoxy ligand ORF (ORF = OC(CF3)3). According to quantum chemical calculations the FIA of Al(ORF)3 is 537 kJ/mol (Table 1). The isolation of the new superacid Al(ORF)3 from different solvents under ambient conditions was very difficult, because of self-decomposition with C-F activation. After having changed the solvent to fluorobenzene the adduct PhF→Al(ORF)3 was formed in 98% yield and can be easiliy handled as a room temperature sensitive powder.



figure 1

Figure 1. Molecular structure of [PhF→Al(ORF)3] (1; RF=C(CF3)3) at 103.1 K with thermal ellipsoids set at 50% probability. Selected bond lengths [Å] and angles [°]: Al1-O3 1.685(2), Al1-O2 1.693(2), Al1-O1 1.706(2), Al1-F1 1.864(2), Al1-F2 2.770(8), O1-C7 1.369(3), O2-C11 1.365(3), O3-C15 1.364(3), F1-C1 1.447(3); O3-Al1-O2 115.83 (10), O3-Al1-O1 121.43(10), O2-Al1-O1 113.22(10), O3-Al1-F1 102.83(9), O2-Al1-F1 103.01, O1-Al1-F1 95.30(9), C7-O1-Al1 138.08(18), C11-O2-Al1 150.16(19), C15-O3-Al1 151.56(19), C1-F1-Al1 129.99(15).



[2] Lutz O. Müller, Daniel Himmel, Julia Stauffer, Gunther Steinfeld, John Slattery, Gustavo Santiso-Quiñones, Volker Brecht, Ingo Krossing* (2008): „Simple Access to the Non-Oxidizing Lewis Superacid Ph F→Al(ORF)3 (RF = C(CF3)3).”, Angew. Chem. 120, 7772-7776.


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