Novel actinide cluster fullerenes, U₂C₂@I_h(7)-C₈₀ and U₂C₂@D_3h(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 C_2n 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.

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