Molecular Structure and Chemical Property of a Divalent Metallofullerene Yb@<i>C</i>₂(13)‑C₈₄

Abstract

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/Ni^II(OEP) cocrystals (OEP is the dianion of octaethylporphyrin) unambiguously established the chiral <i>C</i>₂(13)-C₈₄ cage structure and revealed multiple sites for Yb²⁺, indicating a moving metal ion inside the cage. The chemical property of Yb@<i>C</i>₂(13)-C₈₄ was probed with the electrophillic adamantylidene carbene (<b>1</b>). Three monoadduct isomers were isolated and characterized. Crystallographic results of the major isomer (<b>2b</b>) 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@<i>C</i>₂(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 <b>1</b> to Yb@<i>C</i>₂(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@<i>C</i>_2<i>v</i>(9)-C₈₂ (M = Sc, Y, La, Ce, Gd) toward <b>1</b>. 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