Smallest Endohedral Metallofullerenes [Mg@C₂₀] (n = 4, 2, 0, -2, and -4): Endo-Ionic Interaction in Superatoms

This work reports a series of endohedral metallofullerene superatoms [Mg@C₂₀], where n = 4, 2, 0, -2, and -4. It was found that Mg transfers virtually all of its 3s electrons to the C₂₀ shell, resulting in the ionic states of Mg²⁺@[C₂₀]. Detailed calculations revealed that the superatomic electronic configuration of these clusters is 1S²1P⁶1D¹⁰1F^4-. The first nine superatomic molecular orbitals (SAMOs), 1S²1P⁶1D¹⁰, housed with 18 electrons, are largely based on [C₂₀] with small contributions from magnesium, while the outmost SAMOs, 1F^4-, with 4 - n extra electrons, reside solely on [C₂₀]. The interaction between the Mg²⁺ ion and [C₂₀] was found to be predominately ionic in character. Furthermore, ultraviolet-visible spectra provide a theoretical basis for fingerprinting these clusters. It is hoped that this work will encourage the synthetic pursuit of these smallest superatomic systems.