First Unsymmetrical Bisfullerene, C121:  Evidence for the Presence of Both Homofullerene and Methanofullerene Cages in One Molecule

We report the isolation and characterization of the bisfullerene C121, the first all-carbon molecule to contain a homofullerene (also called a fulleroid) cage. This unsymmetrical isomer of C121 ,along with a symmetrical isomer of C121 and C122, were obtained by thermolysis of a mixture of C60CBr2 and C60 and separated by high performance liquid chromatography. The predominant isomer of C121 has a spiro carbon atom bridge that connects to one of the cages through an open [5.6] ring junction (i.e., between a pentagon and a hexagon) and to the other through a closed [6.6] ring junction. Ab initio calculations indicate that the unsymmetrical structure is more stable than either of the symmetrical alternatives, with the bridging carbon atom attached to both C60 cages through closed [6.6] ring junctions or attached to each cage through open [5.6] ring junctions. Experimental evidence for the unsymmetrical structure comes from the 13C NMR and UV/vis spectra. Electrochemical reduction of this bisfullerene shows three pairs of distinct, reversible peaks that correspond to each of the first three reductions of the [60]fullerene cages. This is consistent with the presence of a homofullerene unit and shows the similarity of the redox behavior to that of C60. The first reduction potential of C121 is slightly shifted toward more positive values than that of [60]fullerene.