In contrast to the well-known 2-norbornyl cation, the structure of which was a matter of long debate until its pentacoordinated nature was recently proven by an X-ray structure, the pentagonal-pyramidal dication of hexamethylbenzene has received considerably less attention. This species was first prepared by Hogeveen in 1973 at low temperatures in magic acid (HSO3 F/SbF5 ), for which he proposed a non-classical structure (containing a hexacoordinated carbon) based on NMR spectroscopy and reactivity studies, but no X-ray crystal structure has been reported. C6 (CH3 )62+ can be obtained through the dissolution of hexamethyl Dewar benzene epoxide in HSO3 F/SbF5 and crystallized as the SbF6- salt upon addition of excess anhydrous hydrogen fluoride. The crystal structure of C6 (CH3 )62+ (SbF6- )2 ⋅HSO3 F confirms the pentagonal pyramidal structure of the dication. The apical carbon is bound to one methyl group (distance 1.479(3) Å) and to the five basal carbon atoms (distances 1.694(2)-1.715(3) Å).
Keywords: carbocations; electron-deficient compounds; hypercoordination; oxidation; superacids.
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