What determines the Th/U atom positions inside fullerenes?

Abstract

Understanding the exact position of metal atom(s) within a cage is critical for elucidating the structural characteristics, metal–cage interplay and stability mechanism of endohedral metallofullerenes (EMFs). But it becomes rather challenging for the emerging actinide EMFs due to the variable oxidation state and complex valence orbitals of the encapsulated metals. Herein, density functional theory calculations were carried out for all 25 reported Th/U-based mono-metallofullerenes (mono-EMFs) with well-defined single crystal structures. The electronic ground states of Th- and U-contained mono-EMFs are singlet and triplet/quintet, respectively. Th always formally assumes the IV oxidation state inside the cage, whereas U has the III or IV valence state highly depending on the cage structure. Due to the considerable intramolecular charge transfer (Th: 4e; U: 3e or 4e), their metal–cage interactions are mainly ionic, but with non-negligible covalent components largely contributed by the spatially extended Th/U-5f orbitals. Importantly, all the internal Th⁴⁺/U^3+/4+ cations can be located by only considering the frontier molecular orbital distributions of the corresponding empty cages rather than the electrostatic potentials of cage anions commonly used for the classic lanthanide mono-EMFs. Such an obvious deviation from the simple ionic model (M^q+@C_2n^q−) is ascribed to the strong actinide–cage covalency, which leads to much lower actual metal charge than the formal one. Our work unprecedently finds the overlooked important role of cage orbitals in determining the internal metal positions of all actinide mono-EMFs. It not only demonstrates the uniqueness of actinide EMFs, but also could help understand the structural characteristics of more EMFs with multiple possible internal metal locations.