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The 2021 study provided the first single-crystal X-ray structure of an organic [C–F–C]+ fluoronium cation. In the crystal, one fluorine atom bridges two carbon centers at nearly equal distances, supporting a symmetrical, bridged structure rather than a rapidly equilibrating classical carbocation model. Earlier inorganic fluoronium salts had already been crystallographically characterized, so the milestone is specifically the first organic example.
What the 2021 crystal structure showed
The researchers isolated a double-norbornyl-type organic cation as an [Sb2F11]− salt and determined its solid-state structure using single-crystal X-ray diffraction. The bridging fluorine lies between two carbon atoms. The study reports C–F distances of 156.6(3) pm and 158.5(3) pm and a C–F–C angle of 115.78(15)°.
The close bond lengths and measured angle show a nearly symmetrical C–F–C arrangement in the crystal. The authors described their result as the single-crystal structure of a symmetrical [C–F–C]+ fluoronium cation in their Nature Communications paper, published 6 September 2021.
Which structural controversy did it address?
Earlier work had provided evidence for the organic species in solution, including observations of transient formation and NMR evidence, but had not settled its solid-state structure with a crystal structure. The competing pictures were a fluorine atom bridging two carbon centers in a divalent fluoronium cation, or a classical carbocation that rapidly equilibrates between arrangements.
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| Question | Bridged fluoronium model | Rapidly equilibrating carbocation model |
|---|---|---|
| Connectivity | One fluorine bridges two carbon centers. | The structure is interpreted as a rapidly shifting classical carbocation, not a fixed symmetrical bridge. |
| What the crystal revealed | Two close C–F distances, 156.6(3) pm and 158.5(3) pm, and a C–F–C angle of 115.78(15)° support a nearly symmetrical bridge. | The observed crystal geometry does not match the alternative as well as the bridged structural description. |
| What the geometry says about bonding | The measurements establish the arrangement of atoms in the crystal. | The measurements alone do not establish a routine shared-electron bonding picture; the authors’ electronic analysis is a separate question. |
Why symmetry does not mean two ordinary covalent bonds
The similar C–F distances describe the cation’s geometry, but they should not be taken as proof that the bridging fluorine forms two ordinary covalent bonds. The authors’ electronic analysis points to strong repulsion between fluorine lone pairs and the C–F sigma bonds, making the bonds barely covalent in the conventional shared-electron sense. They discuss charge-shift bonding, where electron fluctuations give rise to a bonding interaction. Chemistry World also explains this distinction in its coverage of the crystallized fluoronium ion.
Why “first fluoronium crystal” needs qualification
The achievement was not the first crystal structure of any fluoronium salt. In 1988, Dietrich Mootz and Klemens Bartmann reported crystal structures of H2F+ and H3F2+ associates from the HF–SbF5 superacid system. Their 1988 study described those as the first crystal structure determinations on fluoronium salts. The 2021 result’s precise historical distinction is that it was the first crystallographically characterized organic [C–F–C]+ fluoronium cation; the authors also place their work in the context of later inorganic and silylated fluoronium structures.
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How the researchers grew crystals
For the 2021 synthesis, the team used the crystalline antimony pentafluoride–sulfur dioxide adduct, SbF5·SO2, as a fluoride abstractor. They added the precursor to a cooled mixture in SO2ClF, partially evaporated the solvent, and slowly cooled the reaction mixture to grow crystals suitable for X-ray diffraction.
Chemistry World’s account of earlier preparation conditions specifies fluoride abstraction with antimony pentafluoride at −50 °C, followed by holding the mixture at −80 °C for two weeks until yellow crystals formed. Those details describe the procedure reported in that coverage, not a general preparation recipe.
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