Preparation and characterization of C60Br6 and C60Br8

BUCKMINSTERFULLERENE (C 60 ) is much more reactive than was originally anticipated, because resonance structures that place double bonds in the pentagonal rings are energetically unfavourable 1 and lead to restricted electron delocalization. C 60 therefore behaves as an electron-deficient 'super-alkene' rather than as a 'super-aromatic', as demonstrated by the addition of osmium tetroxide 2 , platinum 3 , dipolar molecules 4, alkyl radicals 5 and amines 6 . Reaction of anions derived from C 60 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 C 60 Br 6 (which forms magenta plates) and C 60 Br 8 (dark brown prisms) by mixing solutions of C 60 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 C 60 with bromine in the absence of solvent gives C 60 Br 24 (which also contains occluded bromine). We propose a configuration for C 60 Br 24 (and sterically hindered C 60 X 24 compounds) based on that for C 60 Br 8 . On being warmed in solution, C 60 Br 6 rearranges and disproportionates to C 60 Br 8 , and all three bromo derivatives revert to C 60 on strong heating. These results provide an insight into the pattern of addition of bulky reagents to C 60 .