A diuranium carbide cluster stabilized inside a C(80) fullerene cage

Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing I(h)(7)-C(80) fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@I(h)(7)-C(80). This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (Ni(II)-OEP) to produce UCU@I(h)(7)-C(80)·[Ni(II)-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@I(h)(7)-C(80) and the covalent nature of the U(f(1))=C double bonds were further affirmed through various spectroscopic and theoretical analyses.