Teflic acid, or pentafluoroorthotelluric acid, is a strong acid with the formula HOTeF₅. It is a fluorinated derivative of orthotelluric acid (Te(OH)₆). The teflate, or pentafluoroorthotellurate, group (OTeF₅) is stable against oxidization and electrophilic attack and has strong electron-withdrawing properties. It can, thus, be used, e.g., in weakly coordinating anions or strong Lewis acids. However, the group is sensitive to hydrolysis and can form HF as a degradation product. This means compounds with a teflate group usually require inert conditions, which hampers their use.

Sebastian Riedel, Free University Berlin, Germany, and colleagues have synthesized air-stable derivatives of pentafluoroorthotellurate substituted with fluorinated or nonfluorinated aryl groups. The team first prepared the acid cis-PhTeF₄OH (pictured) from PhTeF₅ bis dissolving it in a MeCN/water mixture. According to the researchers, this product is the first crystallized species with a TeF₄OH moiety connected to an aryl group. It was further transformed to Ag[cis-PhTeF₄O] in two steps: a reaction with Me₃SiCl to give cis-PhTeF₄OSiMe₃, followed by a reaction with AgF. The silver salt was then used as a cis-PhTeF₄O transfer reagent, e.g., in a reaction with [PPh₄]Cl that gives the salt [PPh₄][cis-PhTeF₄O].

The team also prepared derivatives with fluorinated aryl groups. They obtained trans-(C₆F₅)₂TeF₄ via a fluorination of Te(C₆F₅)₂ using trichloroisocyanuric acid, potassium fluoride, and trifluoroacetic acid. Further transformations gave K[trans-(C₆F₅)₂TeF₃O] and trans-(C₆F₅)₂TeF₃OH. K[trans-(C₆F₅)₂TeF₃O] is both moisture- and air-stable, as well as soluble in organic solvents. Overall, the work could be useful for the synthesis of new weakly coordinating anions or Lewis acids.

• Air-stable Aryl Derivatives of the Pentafluoroorthotellurate,
  Daniel Wegener, Kurt F. Hoffmann, Alberto Pérez-Bitrián, Ilayda Bayindir, Amiera Nadien Hadi, Anja Wiesner, Sebastian Riedel,
  Chem. Commun. 2022.
  https://doi.org/10.1039/d2cc03936b