Preparation and characterization of C₆₀Br₆ and C₆₀Br₈

Abstract

BUCKMINSTERFULLERENE (C₆₀) is much more reactive than was originally anticipated, because resonance structures that place double bonds in the pentagonal rings are energetically unfavourable¹ and lead to restricted electron delocalization. C₆₀ therefore behaves as an electron-deficient 'super-alkene' rather than as a 'super-aromatic', as demonstrated by the addition of osmium tetroxide², platinum³, dipolar molecules^4, alkyl radicals⁵and amines⁶. Reaction of anions derived from C₆₀ with iodomethane results in up to 24 methyl groups becoming attached to the cage, although compounds containing either eight or six methyl groups are dominant in the mass spectrum^7. Here we report the synthesis of crystalline C₆₀Br₆ (which forms magenta plates) and C₆₀Br₈ (dark brown prisms) by mixing solutions of C₆₀ with bromine. The structures of these compounds, which contain occluded bromine when precipitated from the brominating medium, have been determined by single-crystal X-ray diffraction. Reaction of C₆₀ with bromine in the absence of solvent gives C₆₀Br₂₄ (which also contains occluded bromine). We propose a configuration for C₆₀Br₂₄ (and sterically hindered C₆₀X₂₄ compounds) based on that for C₆₀Br₈. On being warmed in solution, C₆₀Br₆ rearranges and disproportionates to C₆₀Br₈, and all three bromo derivatives revert to C₆₀ on strong heating. These results provide an insight into the pattern of addition of bulky reagents to C₆₀.