화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.141, No.51, 20249-20260, 2019
Diuranium(IV) Carbide Cluster U2C2 Stabilized Inside Fullerene Cages
Novel actinide cluster fullerenes, U2C2@I-h(7)-C-80 and U2C2@D-3h(5)-C-78, 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 angstrom. While the U2C2 cluster in U2C2@C-80 adopts a butterfly-shaped geometry with a U-C-2-U dihedral angle of 112.7 degrees and a U-U distance of 3.855 angstrom, the U U distance in U2C2@C-78 is 4.164 angstrom and the resulting U-C-2-U dihedral angle is increased to 149.1 degrees. 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 C-2. Different from the strong U=C bonding reported for U2C@C-80, 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 pi 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.