The Unanticipated Dimerization of Ce@C (9)-C upon Co-crystallization with Ni(octaethylporphyrin) and Comparison with Monomeric M@C (9)-C (M = La, Sc, and Y).

We report that Ce@C (9)-C forms a centrosymmetric dimer when co-crystallized with Ni(OEP) (OEP = octaethylporphyrin dianion). The crystal structure of {Ce@C (9)-C } ⋅2[Ni(OEP)]⋅4 C H shows that a new C-C bond with a bond length of 1.605(5) Å connects the two cages. The high spin density of the singly occupied molecular orbital (SOMO) on the cage and the pyramidalization of the cage are factors that favor dimerization. In contrast, the treatment of Ni(OEP) with M@C (9)-C (M = La, Sc, and Y) results in crystallization of monomeric endohedral fullerenes. A systematic comparison of the X-ray structures of M@C (9)-C (M = Sc, Y, La, Ce, Gd, Yb, and Sm) reveals that the major metal site in each case is located at an off-center position adjacent to a hexagonal ring along the C  axis of the C (9)-C cage. DFT calculations at the M06-2X level revealed that the positions of the metal centers in these metallofullerenes M@C (9)-C (M = Sc, Y, and Ce), as determined by single-crystal X-ray structure studies, correspond to an energy minimum for each compound.