Structural Isomers and Superatomic Character of the Smallest Endohedral Azafullerene U@C27N

Abstract

Endohedral metalloazafullerenes (EMAFs) and endohedral metalloborafullerenes (EMBFs) are nitrogen- and boron-doped carbon cages, respectively, with metallic species inside. Recent experiments have successfully synthesized U@C₂₇B as the smallest EMBF. Herein, density functional theory (DFT) calculations were conducted to systematically investigate the possible structure of its corresponding EMAF analogue, U@C₂₇N. By fully considering the relative energies at 0 K and entropy effect at high temperatures for all potential U@C₂₇N isomers, we predicted U@T_d-C₂₇N-[5,5,5] and U@T_d-C₂₇N-[5,5,6] with different nitrogen doping positions as its two major isomers. The encapsulated U atom formally adopts the highest VI oxidation state, and is effectively stabilized by the strong covalent interactions with the cage. Holding an unpaired electron on the cage, U@C₂₇N could easily dimerize. Interestingly, it maintains a superatomic state with a perfect closed-shell 1S²1P⁶1D¹⁰1F¹⁴ electronic configuration by using 32 out of 35 valence electrons in both the monomers and dimers. The novel structures and good stability promise the possible synthesis of this smallest EMAF in the experiments.