Isolation and crystallographic characterization of Sm@C2v(3)-C80 through cocrystal formation with Ni(II)(octaethylporphyrin) or bis(ethylenedithio)tetrathiafulvalene.

Sm@C(2v)(3)-C(80) has been separated from the carbon soot produced by electrical arc vaporization of graphite rods doped with Sm(2)O(3) and purified. Its structure has been determined by single crystal X-ray diffraction using cocrystals obtained from either Ni(II)(octaethylporphyrin) (Ni(II)(OEP)) to form Sm@C(2v)(3)-C(80)·Ni(II)(OEP)·1.68(toluene)·0.32(benzene) or bis(ethylenedithio)-tetrathiafulvalene (ET) to produce Sm@C(2v)(3)-C(80)·ET·0.5(toluene). Thus, this study offers the first opportunity to compare a common endohedral fullerene in two different cocrystals. Both cocrystals provide consistent information on the basic structure of Sm@C(2v)(3)-C(80) but show that the distribution of samarium ion sites inside the carbon cage depends upon whether Ni(II)(OEP) or ET is present. The samarium ion is disordered in both structures, but the prominent sites lie slightly off the 2-fold symmetry axis of the cage. Computational studies at the B3LYP level indicate that Sm@C(2v)(3)-C(80) is more stable than any of the other six isomers of Sm@C(80) that obey the isolated pentagon rule (IPR). The surface electrostatic potential of the interacting components in the cocrystals has been examined to identify factors responsible for the ordering of the fullerene cages. The regions of the Ni(II)(OEP) or ET molecules that are closest to the fullerene display negative potential, while the corresponding regions of the endohedral fullerene show positive potential in a consistent fashion in both cocrystals.