The Existence of Hexafluoroarsenic(V) Acid

Axhausen, J.; Lux, K.; Kornath, A.Angew. Chem. Int. Ed. 2014. 53,  Early View.

DOI:10.1002/anie.201308023

The homogeneous mixture of anhydrous hydrogen fluoride aHF and antimony pentafluoride AsF₅ is known as a superacidic system. The high acidity is derived from the formation of [H₂F]⁺ [AsF₆]⁻. No experimental evidence exists for the existence of the free acid molecule HAsF₆. The reaction of trimethylsilyl N,N-dimethylcarbamate in the binary system aHF/AsF₅ led to decomposition of trimethylsilyl N,N-dimethylcarbamate at −50 °C to dimethylammonium hexafluoridoarsenate and cocrystallization of HAsF₆. The single-crystal X-ray structure displays an HAsF₆ molecule involved in an asymmetric hydrogen bridge to the hexafluoridoarsenate anion. As a result of the incalculable situation in the crystal lattice, the molecular structure of HAsF₆ is calculated by quantum chemical structure optimization of the extreme cases of [FHF-AsF₅]⁻ (strong hydrogen bond) and HAsF₆ (no hydrogen bond) at the PBE1PBE/6-311G(3df,3pd) level of theory.

 

Superacids are defined as stronger acids than 100% sulfuric acid. As you might know the story on the discovery of FSO₃H/SbF₅, when a member of Olah group put a candle into the acid during a Christmas party, the candle dissolved immediately.After that, FSO₃H/SbF₅ has been called Magic acid, known as one of the superacids. In this report, Kornath group show the protonation of trimethylsilyl-N,N-dimethylcarbamate in the superacid media HF/AsF₅. This reaction gave not only dimethyl ammoniumhexafluoridoarsenate [Me₂NH₂]⁺[AsF₆]^–, but the hexafluoroarsenic(V) acid HAsF₆ which was structurally characterized by X-ray diffraction by chance in co-crystalline.

• References

[1] “Superacids”,Olah, G. A. ; Prakash, G. K.; Sommer, J. 2nd ed., Wiley, Hoboken, 2009. ISBN: 9780470421543

The chemistry of superacids has developed in the last two decades into a field of growing interest and importance. Now available in a new expanded second edition, this definitive work on superacids offers a comprehensive review of superacids and discusses the development of new superacid systems and applications of superacids in the promotion of unusual reactions.

 

• Related Books

[amazonjs asin=”047159668X” locale=”US” title=”Superacid Chemistry”]

 

• Related Links

Kornath Lab home Page 

Olah; University of Southern California web site 

Magic acid, Superacids