Isoxazoline-Ring Fused Derivatives of Sc3n@C80 and C60: The Internal Effect on the Exohedral Configuration

Fullerenes are unique among all carbon allotropes as their molecular structures can be fully confirmed by single-crystal X-ray diffraction. With metal atoms or metal complexes encapsulated, endohedral metallofullerenes (EMFs) redistribute electrons between the internal unit and the cage, and thus afford different electronic structures and different chemical properties compared to empty fullerenes. However, because of the relatively low yield of endohedral metallofullerenes, only a limited number of chemical modifications have been reported for them. In this work, an isoxazoline ring was added to Sc₃N@C₈₀ for the first time to form only one mono-adduct with high regio-selectivity (2b). An analogous product was obtained for empty cage C₆₀ (2a).

X-ray crystallography has revealed their structures, as shown in Fig. 1. The addition site of exohedral moieties of C₆₀ was the [6,6]-bond while it is a [5,6]-bond for Sc₃N@C₈₀. Inside the Sc₃N@C₈₀, disorder of the Sc atoms represents the dynamic motion of the cluster, and the sum of the three Sc–N–Sc angles (342°) suggests that the trimetallic nitride cluster slightly derives from a coplanar structure, confirming the motion control effect of the exohedral modification. Moreover, it is very interesting to find that the configuration of exohedral moieties differed in 2a and 2b. In detail, the methoxy indicated by the arrow shown in Fig. 1 occurs at opposite sides of the benzene plane. These results reveal the significant internal effect on the exohedral configuration.

Figure 1. Drawings of 2a and 2b showing thermal ellipsoids at the 30% probability level. Arrows indicate the special methoxy group that changes conformations when added to different fullerenes. Solvent molecules are omitted for clarity.