Isolation and Structure Determination of a Missing Endohedral Fullerene La@C70 through In Situ Trifluoromethylation

D-5h-symmetric fullerene C-70 (D-5h-C-70) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D-5h-C-70 is capable of accommodating a rare-earth metal atom to form an endohedral metallofullerene M@ D-5h-C-70, which would be expected to show novel electronic properties. The molecular structure of La@C-70 remains unresolved since its discovery three decades ago because of its extremely high instability under ambient conditions and insolubility in organic solvents. Herein, we report the single-crystal X-ray structure of La@C-70(CF3)(3), which was obtained through in situ exohedral functionalization by means of trifluoromethylation. The X-ray crystallographic study reveals that La@C-70(CF3)(3) is the first example of an endohedral rare-earth fullerene based on D5hC70. The dramatically enhanced stability of La@C-70(CF3)(3) compared to La@C-70 can be ascribed to trifluoromethylationinduced bandgap enlargement.