The X-ray crystal structure and packing of a hexakis-adduct of C-60: Temperature dependence of weak C-H O interactions

The crystal structure and packing of the C-60 hexakis-adduct 1 has been determined from accurate X-ray diffraction measurements. The largest deformations on the C-60 surface occur within the bridged pyracylene subunits; the bridgehead 6-6 bond lengths of the four cyclopropane and the two cyclohexene rings, attached in pseudo-octahedral positions, are close to 1.6 Angstrom. Significant deformations also occur in the remaining part of the C-60 skeleton, not bridged to any of the functional groups. The crystal packing, analyzed at 270, 230, and 180 K is characterized by a three-dimensional network of 14 weak C-H ... O interactions. For most of these, H ... O and C ... O distances decrease on cooling the crystal specimen, indicating that they are attractive in nature. On the other hand, the shortest, nearly linear C(sp(3))-H ... O contact seems to be repulsive, since cooling of the crystal specimen leads to significantly longer H ... O and C ... O distances and to a reduction of the C-H ... O linearity. Probably as a result of this intermolecular expansion, strain is produced in the crystal. Depending on the crystal-mounting procedure, this can lead either to a disordered structure (below ca. 200 K), or to destruction of the crystal specimen.