Computational understanding of the structural characteristics of Sm-based endohedral metallofullerenes

Abstract

Endohedral metallofullerenes (EMFs) containing samarium (Sm) are rich and cover all kinds of conventional endofullerene types. Herein, an exhaustive density functional theory study was conducted to understand the structural characteristics of all 21 Sm-based EMFs characterized by single-crystal X-ray diffraction. The mono-, di- and tri-EMFs have open-shell septet, 13-et and 19-et electronic states, respectively. The internal Sm atoms hold all the unpaired spins (six for each), which are completely shielded by carbon cages. A formal divalent state can be assigned to each metal but with obvious electron backdonation from cage orbitals, rendering the metal-metal and metal-cage interactions mainly ionic. Thus, their Sm positions can be easily located by the electrostatic potentials inside negatively charged hollow cages and also affected by intramolecular covalent interactions. All Sm-based EMFs are stable with large encapsulation energies and SOMO-LUMO gap energies. Interestingly, although the two Sm2+ cations are repulsive, the limited cage cavity of Sm2@D2(35)-C88 may push them to have obvious covalent interactions. While for Sm3@Ih(7)-C80, the three Sm2+ cations exhibit markedly reduced positive charges to effectively alleviate the intermetallic repulsions. Our findings rationalize the structural characteristics and favorable formation of all Sm-based EMFs, and could deepen the understanding for more types of EMFs.