Isolation and Structural Characterization of Er@C2v(9)-C82 and Er@Cs(6)-C82: Regioselective Dimerization of a Pristine Endohedral Metallofullerene Induced by Cage Symmetry

Two Er@C82 isomers have been isolated and unambiguously characterized as Er@C2v(9)-C82 and Er@Cs(6)-C82, respectively, by single-crystal X-ray diffraction. Er@Cs(6)-C82 is identified as a dimeric structure in the crystalline state, but dimerization does not occur for Er@C2v(9)-C82 under identical crystallization conditions, indicating a cage-symmetry-induced dimerization process. Density functional theory calculations reveal that the major unpaired spin resides on a special C atom of Er@Cs(6)-C82, which leads to regioselective dimerization. Calculations also found that the dimeric structure of Er@Cs(6)-C82·Ni(OEP) is much more stable than the two monomers, suggesting a thermodynamically favorable dimerization process. Vis–near-IR spectrometric and electrochemical results demonstrate that the electronic structure of Er@C82 isomers is Er3+@C823–, instead of the theoretically proposed Er2+@C822–.