Crystallographic X-ray Analyses of Yb@C2v(3)-C80 Reveal a Feasible Rule That Governs the Location of a Rare Earth Metal inside a Medium-Sized Fullerene

Single crystal X-ray diffraction studies of Yb@C-2v(3)-C-80 center dot Ni-II(OEP) center dot CS2 center dot 1.5C(6)H(6) (OEP = octa-ethylporphinate) reveal that a relatively flat region of the fullerene interacts with the Ni-II(OEP) molecule, featuring shape-matching interactions. Surprisingly, it is found that the internal metal is located under a hexagonal carbon ring apart from the 2-fold axis of the C-2v(3)-C-80 cage, presenting the first example of metallofullerenes with an asymmetrically positioned metal. Such an anomalous location of Yb2+ is associated with its strong ability to pursue a large coordination number and the lack of hexagon along the C-2 axis of C-2v(3)-C-80. It is accordingly assumed that a suitable cage hexagon is most likely to be preferred by the single rare earth metal to stay behind inside a medium-sized fullerene, such as C-80 and C-82.