Discovery of a Superatom inside the Fullerene Cage

The stability of endohedral clusterfullerenes is generally understood in terms of the metal cluster shape, cage structure, and metal-cage interactions, with the electronic state of the internal cluster mostly neglected. Herein, theoretical calculations reveal that the (Ti3C3)6+ unit of recently synthesized Ti3C3@Ih(7)-C80 exhibits a superatomic state with a perfect closed-shell 1S21P61D10 electronic configuration in accordance with the famous jellium model. This "trapped superatom" features considerable aromaticity and hyperconjugation interactions never reported for other clusterfullerenes. Besides the localized two-center two-electron (2c-2e) Ti-C/C-C bonds, it also has two 3c-2e Ti-C-Ti bonds. Furthermore, the ring strain of the cyclopropane-like C3 core is effectively released upon the metal coordination. All these factors greatly stabilize the (Ti3C3)6+ cluster, showing the critical role of metal-to-cage charge transfer and cage encapsulation in enhancing the stability of this exotic metal cluster.