Diuranium(IV) Carbide Cluster UC Stabilized Inside Fullerene Cages.

Novel actinide cluster fullerenes, UC@(7)-C and UC@(5)-C, 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 UC 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 UC clusters are in the range between 2.130 and 2.421 Å. While the UC cluster in UC@C adopts a butterfly-shaped geometry with a U-C-U dihedral angle of 112.7° and a U-U distance of 3.855 Å, the U-U distance in UC@C is 4.164 Å and the resulting U-C-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 UC cluster, and each U center transfers three electrons to the C cage and one electron to C. Different from the strong U═C covalent bonding reported for UC@C, the U-C bonds in UC 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.