Molecular Structure and Chemical Property of a Divalent Metallofullerene Yb@C2(13)‑C84

Endohedral metallofullerenes (EMFs) encapsulating divalent metal ions have received limited attention because of their low production yields. Here, we report the results of structural determination and chemical functionalization of a typical divalent metallofullerene, Yb@C84(II). Single-crystal X-ray crystallographic studies of Yb@C84/NiII(OEP) cocrystals (OEP is the dianion of octaethylporphyrin) unambiguously established the chiral C2(13)-C84 cage structure and revealed multiple sites for Yb2+, indicating a moving metal ion inside the cage. The chemical property of Yb@C2(13)-C84 was probed with the electrophillic adamantylidene carbene (1). Three monoadduct isomers were isolated and characterized. Crystallographic results of the major isomer (2b) revealed that, although the cycloaddition breaks a [5,6]-bond on the cage, Yb2+ is localized under a hexagonal ring distant from the sites of addition. Thus, it is proved that the dynamic motion of the divalent metal ion in Yb@C84 has been effectively halted by exohedral functionalization. Spectroscopic results show that the electronic property of Yb@C2(13)-C84 is pertained in the derivatives, although the addend exerts a mild reduction effect on the electrochemical behavior of the EMF. Computational works demonstrated that addition of 1 to Yb@C2(13)-C84 is mainly driven by releasing the local strains of cage carbons rather than charge recombination, which is always prominent to the affinity of typical trivalent EMFs such as M@C2v(9)-C82 (M = Sc, Y, La, Ce, Gd) toward 1. Accordingly, it is speculated that the chemical behaviors of divalent EMFs more likely resemble those of empty fullerenes because both are closed-shell compounds, but they differ from those of trivalent EMFs, which have open-shell electronic configurations instead.