The tris(pentafluorophenyl)methyl cation

The tris(pentafluorophenyl)methyl cation
18 April 2022 - Chemical Zoo

The laboratory of Sebastian Hasenstab-Riedel from the Institut für Chemie und Biochemie in Berlin has reported the synthesis and isolation of the tris(pentafluorophenyl)methyl cation (Hoffmann et al. DOI). The parent trityl cation (positively charged carbon atom with three phenyl substituents) is well known and can for example be found in the compound triphenylmethyl hexafluorophosphate. The perfluorined trityl cation is isoelectronic with another well-known compound tris(pentafluorophenyl)borane which is extensively used as a very stable Lewis acid.

In an article in 2014 titled “Is the Perfluorinated Trityl Cation Worth a Revisit?” (DOI) Jason Dutton of La Trobe University in Melbourne predicted that the first could very well surpass the latter in terms of hydride affinity and that got some research groups exited. His own attempt at synthesis and isolation failed (counterion ate the phenyls) and in 2018 he could only report observation of the cation in a magic acid solution (DOI).

In the 2022 Hasenstab-Riedel work triethylaluminum was mixed with treflic acid in ortho-difluorobenzene forming a superacid with a protonated aryl cation and an aluminum tetraoxo anion (Al(OTeF5)4) and with ethane eliminated. The trityl compound was formed by reaction of this acid with tri(pentafluorophenyl)chloromethane with removal of hydrochloric acid. It was possible to form crystals and x-ray diffraction showed the methyl cation and the aluminium anion observing a safe distance, leaving close contact to one of the phenyl fluorine atoms and one of the fluorine orthotellurate units.

High proton affinity of the new compound was demonstrated in two ways. In one not by design, the compound was found to slowly degrade as the triarylmethane compound with solvent as proton source. In the other, added isobutane was found to replace the trityl cation. The perfluorinated trityl compound was also found to be an oxidizing reagent, converting ferrocene to a green ferrocenium cation and converting tris(4-bromophenyl)amine to deep-blue magic blue. In both reactions the tris(pentafluorophenyl)methyl radical is formed.

By happy coincidence, this science month saw a second fluorinated trityl cation article published. In it (DOI) the laboratory of Oleg Ozerov at Texas A&M University described the synthesis and isolation of trityl compound with just the phenyl meta positions fluorinated and having a carborane counter ion. Its proton abstraction capabilities were tested on mesitylene and tetraethylsilane.