PREPARATION AND STRUCTURE OF CRYSTALS OF THE METALLOFULLERENE SC-2-AT-C-84

IT was first proposed in 1985(1) that fullerenes can confine atoms in their interior because of their closed-cage structure. Attempts to verify this conjecture following the mass production of fullerenes(2,3) have yielded metallofullerenes in bulk, and there is now good evidence that these compounds are endohedral(4)-that is, that the metal atoms are inside. But direct confirmation in the form of structural data has been lacking, in part because of the difficulty of separating different metallofullerenes and obtaining pure crystals. Here we report the preparation of pure crystalline Sc-2@C-84 and analyses of its structure by electron diffraction and high-resolution transmission electron microscopy. At room temperature the Sc-2@C-84 molecules pack in a hexagonal-close-packed structure with a ratio of lattice constants c/a = 1.63, the value expected for ideal-sphere packing. The molecular spacing of 11.2 Angstrom is the same as that found earlier in crystalline C-84 (refs 5,6). The match between our microscopic images and simulations is markedly better for endohedral models than for those in which the metal atoms reside in the lattice outside the C-84 cages. We believe that this combination of observations points inevitably to the conclusion that the metal atoms are inside the fullerenes.