The Trifluoromethyl Anion

Abstract

First evidence for the existence of free trifluoromethyl anion CF3 (-) has been obtained. The 3D-caged potassium cation in [K(crypt-222)](+) is inaccessible to CF3 (-) , thus rendering it uncoordinated ("naked"). Ionic [K(crypt-222)](+) CF3 (-) has been characterized by single-crystal X-ray diffraction, solution NMR spectroscopy, DFT calculations, and reactivity toward electrophiles.

A Reinterpretation of the Crystal Structure Analysis of [K(crypt‐222)]+CF3−: No Proof for the Trifluoromethanide Ion

“…All these warning signs prompted us to have ac loser look at the submitted diffraction data and subsequently to undertake ac omplete re-determination of the structure based on Grushin's data set. À ,asp ublished recently by Grushin et al, [5] (CCDC 1417273).…”

“…The description of the X-ray structure determination of [K(crypt-222)] + CF 3 À in the paper under discussion already revealed several inconsistencies. [5] It starts with the incorrectly reported crystal system ("hexagonal" for at rigonal space group) and continuest ob eing overly descriptivea nd unnecessarily verbose about how exactly the crystals were harvested from the crystallization vials, while omitting vital information about data reduction such as absorption correction, the scaling procedure, and resolution ranges. The authors mention technical difficulties in mounting as ingle crystal:" multiple crystals rather than as ingle one were mounted on the goniometer head"; [5] however,adescription of how they proceeded with data reduction is not provided.…”

“…Yet, Grushin et al interpreted these maxima as lone pairs, generating aC F 3 À anion and thus professing that "the existence of the free trifluoromethyl anion CF 3 À has been established for the first time". [5] To determine the natureo ft he CF 3 unit, Grushin et al carried out DFT calculations and compared bond distances and angles obtained from their crystal structure to computed geometry parameters for CF 3 À and CHF 3 .H owever,aconclusion "cannot not be drawn because of the limited accuracy of the X-ray data as ar esult of the above-described disorder in the crystal". [5] Indeed, the bond distances and angles obtained from Grushin's own structural model fall in between the computed geometry parameters of CF 3 À and CHF 3 ( Table 1).…”

“…These challenges notwithstanding, X-ray crystallography is of vital importance in many research areas and, in the interest of science in general, structures of potentially groundbreaking molecules deserve acertainamount of scrutiny by experienced crystallographers.Agood example is the X-ray structure of the proposed trifluoromethyl anion that has been published recently by Grushin et al (see Figure1). [5] Structural characterization of nucleophilic trifluoromethanide-related species is of great interest, [5,6] considering that they are key intermediates in trifluoromethylationr eactions. [7][8][9][10][11][12] The most criticalp oint in the determination of the structureo ft his compound is the distinction between CF 3 À and CHF 3 ,t hat is, the assignment of residual electron density in closep roximity to the carbon atom as either al one pair or ah ydrogen atom.…”

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“…Using DFT to calculate the CF 3 binding affinity of over 40 Lewis acids, the authors of this work were able to narrow in on an ideal regime where the Lewis acid additive provides just enough stabilization to keep CF 3 -from decom posing, yet reactive enough to be used as a reagent. "The best Lewis acid, hexamethylborazine [B 3 N 3 (CH 3 ) 6 ], appears in many chemistry textbooks as an 'inorganic benzene' yet has few applications as a reagent in organic synthesis," said Pro fessor Szymczak, whose lab had previous experience working with borazines for energystorage related projects: "The very challenge we had been facing in our previous investigations with borazines -delivering anionic nucleophiles -suggested they might be perfect candidates for CF 3 -generation/capture/ release experiments because they are very weak Lewis acids," he added. Professor Szymczak and coworkers found that using the borazine Lewis acid facilitated the preparation of highly nucleophilic CF 3 -transfer reagents from HCF 3 in quan titative yield at room temperature.…”