Cycloaddition reactivity of Yb@D3h-C74: the carbon cage size matters†

Abstract

Divalent endohedral metallofullerenes (Di-EMFs) with a closed-shell electronic configuration and characterized by a two-electron transfer from the inner cluster to the outer cage exhibit a very even distribution of charge over the whole cage, resulting in low chemical reactivity and thus limited chemical modification approaches. To date, the reported chemical modifications of Di-EMFs are limited to those with large carbon cages, such as C₈₀, C₈₂, and C₈₄, whereas the chemical reactivity of Di-EMFs bearing cages smaller than C₈₀ remains unknown. Herein, we synthesized a medium-sized Di-EMF, Yb@D_3h-C₇₄, and we investigated its 1,3-dipolar cycloaddition reaction and found that it has unusually high chemical reactivity at room temperature. This finding is significantly different from previously 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 using single-crystal X-ray crystallography. The addition sites were found to be far from the carbon cage region adjacent to the inner Yb cation, and this addition pattern is different to those of previously reported Di-EMFs with larger carbon cage sizes. Theoretical calculations rationally explained the high reactivity of Yb@D_3h-C₇₄ as well as its addition pattern.