Crystallographic X-ray Analyses of Yb@<i>C</i>_2<i>v</i>(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@C2v(3)-C80·NiII(OEP)·CS2·1.5C6H6 (OEP = octaethylporphinate) reveal that a relatively flat region of the fullerene interacts with the NiII(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 C2v(3)-C80 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 C2 axis of C2v(3)-C80. 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 C80 and C82.