Cycloaddition Reactivity of Yb@D3h-C74: the Carbon Cage Size Matters

Divalent endohedral metallofullerenes (Di-EMFs) featuring a closed-shell electronic configuration and two-electron transfer from the inner cluster to outer cage exhibit a more evenly distributed charge over the whole cage, resulting in a low chemical reactivity and thus limited chemical modification approaches. Up to date, the reported chemical modifications of Di-EMFs are limited to those with large carbon cages based on C₈₀, C₈₂, and C₈₄, whereas the chemical reactivity of Di-EMFs bearing cage smaller than C₈₀ remains unknown. Herein, on the basis of synthesis of a medium-sized Di-EMF Yb@D_3h-C₇₄, we investigated its 1,3-dipolar cycloaddition reaction, and found its unusually high chemical reactivity at room temperature, significantly different from the reported Di-EMFs with larger carbon cages for which cycloaddition reactions generally require light irradiation or heating conditions. As a result, four monoadducts of Yb@D_3h-C₇₄, labelled as 2a-2d, were obtained, among which the molecular structure of 2a was unambiguously determined by single-crystal X-ray crystallography. The addition sites were far from the carbon cage region adjacent to the inner Yb cation, and this addition pattern is different to those of the reported Di-EMFs with larger carbon cage size. Theoretical calculations rationally explained the high reactivity of Yb@D_3hC₇₄ as well as its addition pattern.