Stabilization of High-Energy Metallic Clusters via Superatom Formation Inside Fullerenes

Labile cluster isomers are elusive owing to their energies far above the global minimum. Herein, density functional theory and coupled cluster calculations reveal that certain isomers of 24-valence-electron Ti3C3, Sc4C3 and Ti3B4 clusters, although spanning a wide relative energy range of 0-62 kcal/mol in the isolated state, can be captured by carbon cages to form corresponding endohedral clusterfullerenes (ECFs). After donating six electrons to cages, their cations all become superatoms with 18 valence electrons on nine superatom molecular orbitals (SAMOs), which endow the corresponding endohedrals with the lowest energy. The stabilizing role of internal SAMOs on ECFs is further confirmed by the fact that all those ECF isomers with fewer SAMOs exhibit higher relative energies. Noteworthily, three of these ECFs, namely Sc4C3@Ih(7)-C80, Ti3C3@Ih(7)-C80 and Ti3C3@C2v(9)-C82, have been experimentally achieved, suggesting that such a radical cage-encapsulation-induced structural change of metallic clusters does not occur by coincidence. Furthermore, Ti3B4@Ih(7)-C80 is predicted as the first boride ECF with high synthesis potential. This work confirms that fullerene confinement enables the stabilization of high-energy metastable clusters and offers guidelines for the rational design of new ECFs encapsulating superatoms. It also cautions theoretical investigations of metallofullerene catalysts that adopt the most stable free metal clusters as the core in the models.