Structural Elucidation and Regioselective Functionalization of An Unexplored Carbide Cluster Metallofullerene Sc2C2@Cs(6)-C82

A Sc2C84 isomer, previously assumed to be Sc2@C84, is unambiguously identified as a new carbide cluster metallofullerene Sc2C2@Cs(6)-C82 using both NMR spectroscopy and X-ray crystallography. The 13C-nuclei signal of the internal C2-unit was observed at 244.4 ppm with a 15% 13C-enriched sample. Temperature-dependent dynamic motion of the internal Sc2C2 cluster is also revealed with NMR spectrometry. Moreover, the chemical property of Sc2C2@Cs(6)-C82 is investigated for the first time using 3-triphenylmethyl-5-oxazolidinone (1) which provides a 1,3-dipolar reagent under heating. Regarding the low cage symmetry of this endohedral which contains 44 types of nonequivalent cage carbons, it is surprising to find that only one monoadduct isomer is formed in the reaction. Single-crystal X-ray results of the isolated pyrrolidino derivative Sc2C2@Cs(6)-C82N(CH2)2Trt (2) reveal that the addition takes place at a [6,6]-bond junction, which is far from either of the two Sc atoms. Such a highly regioselective addition pattern can be reasonably interpreted by analyzing the frontier molecular orbitals of the endohedral. Electronic and electrochemical investigations reveal that adduct 2 has a larger highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap than pristine Sc2C2@Cs(6)-C82; accordingly, it is more stable.