U₂@<i>I</i>_<i>h</i>(7)‑C₈₀: Crystallographic Characterization of a Long-Sought Dimetallic Actinide Endohedral Fullerene

The nature of actinide–actinide bonds has attracted considerable attention for a long time, especially since recent theoretical studies suggest that triple and up to quintuple bonds should be possible, but little is known experimentally. Actinide–actinide bonds inside fullerene cages have also been proposed, but their existence has been debated intensively by theoreticians. Despite all the theoretical arguments, critical experimental data for a dimetallic actinide endohedral fullerene have never been obtained. Herein, we report the synthesis and isolation of a dimetallic actinide endohedral metallofullerene (EMF), U₂@C₈₀. This compound was fully characterized by mass spectrometry, single crystal X-ray crystallography, UV–vis–NIR spectroscopy, Raman spectroscopy, cyclic voltammetry, and X-ray absorption spectroscopy (XAS). The single crystal X-ray crystallographic analysis unambiguously assigned the molecular structure to U₂@<i>I</i>_<i>h</i>(7)-C₈₀. In particular, the crystallographic data revealed that the U–U distance is within the range of 3.46–3.79 Å, which is shorter than the 3.9 Å previously predicted for an elongated weak U–U bond inside the C₈₀ cage. The XAS results reveal that the formal charge of the U atoms trapped inside the fullerene cage is +3, which agrees with the computational and crystallographic studies that assign a hexaanionic carbon cage, (<i>I</i>_<i>h</i>-C₈₀)^6–. Theoretical studies confirm the presence of a U–U bonding interaction and suggest that the weak U–U bond in U₂@<i>I</i>_<i>h</i>(7)-C₈₀ is strengthened upon reduction and weakened upon oxidation. The comprehensive characterization of U₂@<i>I</i>_<i>h</i>(7)-C₈₀ and the overall agreement between the experimental data and theoretical investigations provide experimental proof and deeper understanding for actinide metal–metal bonding interactions inside a fullerene cage