This paper from my lab has been getting a lot of press lately and so I figured it would be timely to discuss it here today. This paper represents the accomplishment of one of the holy grails of organofluorine chemistry – namely, the characterization of the CF3 anion as a long-lived species in the condensed phase. This paper effectively dispels one of the long-standing myths in organofluorine chemistry, that the anion is too kinetically unstable to be observed as a distinct species due to fast α-defluorination to generate difluorocarbene.
The project was initially conceived by my friend Dr. Fang Wang, who is now a postdoc working for Stephen Lippard at MIT. Prof. Ralf Haiges carefully prepared the NMR samples on his vacuum line, and Fang and Zhe Zhang conducted the NMR experiments and synthetic reactions. Fang and Dr. Martin Rahm also carried out quantum calculations at all stages of the project. In fact, the idea that CF3– could be observed as a distinct species first came from the results of initial quantum calculations, which showed that the barrier to defluorination was rather high (approx 17 kcal/mol).
Thus, by carefully preparing the NMR samples with rigorously anhydrous reagents (TMSCF3, 18-crown-6, KOtBu) and solvents (THF) it is possible to generate solutions containing appreciable amounts of “free” CF3–; these can be characterized by 13C and 19F NMR at low temperature (-80 ºC).
Prior evidence for the intermediacy of “free” CF3– in synthetic reactions came from work in our lab conducted towards another publication. It was found that KHMDS (potassium bis(trimethylsilyl)amide) was the base that worked best for deprotonating fluoroform. Other alkali metal bases were not as effective. Thus, it was also proposed that under the right conditions, K+ CF3– could be trapped and observed in the condensed phase. Unfortunately, we have been unable to get a crystal structure of the CF3– salt so far.
C&EN recently highlighted this work, and Angewandte Chemie made it a cover issue!
EDIT: Highlight by Prof. Gilmour in Angewandte Chemie