Strategies for the structure determination of endohedral fullerenes applied to the example of Ba@C74•Co(octaethylporphyrin)•2C6H6

The structure determination of endohedral fullerenes is complicated because of the high degree of disorder which may affect the different structural components, e.g. the endohedral atom/molecule, the fullerene cage and solvent molecules. Even worse, the data-to-parameter ratio and, in particular, the scattering power is low. Different strategies are employed to deal with these problems. The observed fraction of the reciprocal lattice may be increased by means of synchrotron diffraction studies. The number of parameters may be reduced by applying rigid-body refinement strategies and by restricting the displacement parameters in the framework of the TLS approach. In the first section, we will give a short overview of the structure determinations of endohedral fullerenes from single-crystal data and discuss the severe problems one may encounter. Then we will give an example of a successful structure analysis of a metal-containing endohedral fullerene, i.e. Ba@C-74.Co(OEP).2C(6)H(6) (OEP = octaethylporphyrin). Several strategies of refinement, beyond the black box level, have been checked and compared, e.g. TLS, split-atom models, twin and single crystal models, anharmonic displacement parameters, and rigid-body models.