Diuranium(IV) Carbide Cluster U2C2 Stabilized Inside Fullerene Cages

Novel actinide cluster fullerenes, U2C2@Ih(7)-C80 and U2C2@D3h(5)-C78, were synthesized and fully characterized by mass spectrometry, single-crystal X-ray crystallography, UV–vis–NIR, nuclear magnetic resonance spectroscopy (NMR), X-ray absorption spectroscopy (XAS), Raman spectroscopy, IR spectroscopy, as well as density functional and multireference wave function calculations. The encapsulated U2C2 is the first example of a uranium carbide cluster featuring two U centers bridged by a CC unit. The U–C bond distances in these U2C2 clusters are in the range between 2.130 and 2.421 Å. While the U2C2 cluster in U2C2@C80 adopts a butterfly-shaped geometry with a U–C2–U dihedral angle of 112.7° and a U–U distance of 3.855 Å, the U–U distance in U2C2@C78 is 4.164 Å and the resulting U–C2–U dihedral angle is increased to 149.1°. The combined experimental and quantum-chemical results suggest that the formal U oxidation state is +4 in the U2C2 cluster, and each U center transfers three electrons to the C2n cage and one electron to C2. Different from the strong UC covalent bonding reported for U2C@C80, the U–C bonds in U2C2 are less covalent and predominantly ionic. The C–C triple bond is somewhat weaker than in HCCH, and the C–C π bonds undergo donation bonding with the U centers. This work demonstrates that the combination of the unique encapsulation effect of fullerene cages and the variable oxidation states of actinide elements can lead to the stabilization of novel actinide clusters, which are not accessible by conventional synthetic methods.