U₂@ I _h(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 _h(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 _h-C₈₀)^6-. Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U₂@ I _h(7)-C₈₀ is strengthened upon reduction and weakened upon oxidation. The comprehensive characterization of U₂@ I _h(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.