Synthesis and X-ray structure of dumb-bell-shaped C120

The discovery and large-scale synthesis of fullerenes have aroused interdisciplinary interest in these closed-cage molecules1,2,3,4,5,6. C60 can be photopolymerized into a form in which the cages are thought to be linked by cyclic C4 units in a [2 + 2] cycloaddition7, provoking theoretical studies of the C60 dimer8,9,10,11,12,13,14,15, the smallest subunit of such a polymer. The C60 dimers C120O (refs 16, 17), C121H2( ref. 17) and C120O2(ref. 18) have been reported, in which the two C60molecules are linked by, respectively, a furan group, a cyclopentane ring and a cyclobutane ring plus two oxygen bridges; but the simplest dimer, C120linked by a cyclobutane ring alone, has not so far been observed. We now report that this dumb-bell-shaped molecule can be synthesized by a solid-state mechanochemical reaction of C60 with potassium cyanide. Our X-ray structural analysis shows that the C4 ring connecting the cages is square rather than rectangular—the latter is predicted theoretically8,9,13,14,15. The dimer dissociates cleanly into two C60 molecules on heating or one-electron reduction, but in the gas phase during mass-spectrometric measurements it undergoes successive loss of C2 units, shrinking to even-numbered fullerenes such as C118 and C116 in a sequence similar to that seen for other large fullerenes19,20.