Synthesis and characterization of trifluoromethylcarboxonium salts

Abstract

The low-temperature protonation of trifluoroacetic acid with the superacids HF/SbF₅ and HF/AsF₅ resulted in the syntheses of the first examples of trifluoromethylcarboxonium salts. The less acidic trifluoromethylacetamide was also protonated in the same fashion, resulting in exclusive protonation of the carbonyl function. Their [SbF₆]⁻ and [AsF₆]⁻ salts were characterized by crystal structures, vibrational and multinuclear NMR spectra, and by electronic structure calculations. These salts are thermally unstable, colorless solids, stabilized by very strong hydrogen bonding. The proton NMR resonances of the [CF₃C(OH)₂]⁺[SbF₆]⁻ and [CF₃C(OH)₂]⁺[AsF₆]⁻ salts occur at an unprecedented 16.0 and 15.7 ppm, respectively, thus extending the upper limit of the range of observed proton NMR chemical shifts from 14 to 16 ppm.