Discovery of Non-Isolated-Pentagon-Rule Fullerenes from Computational Characterization of U2O@C72

The reported actinide-based endohedral clusterfullerenes (ECFs) are rather scarce thus far. Though several members have been detected in mass spectra, their exact structures and properties mostly remain unclear. Herein, density functional theory calculations revealed that the U2O@C72 observed in recent experiments should be U2O@D2(10611)-C72, U2O@C1(10610)-C72, or U2O@Cs(10616)-C72. Featuring two pairs of fused pentagons, their outer cages all break the well-known isolated pentagon rule. U2O@D2(10611)-C72 is the first clusterfullerene based on the D2(10611)-C72 cage, which only encapsulated dimetals (Sc2, La2, Ce2, Pr2) before. It is also the first time to reveal that C1(10610)-C72 and Cs(10616)-C72 can serve as the parent cage of an endohedral fullerene. Interestingly, the three isomers could interconvert with each other via Stone-Wales transformation with one internal U atom dynamically changing its orientation according to the position of pentagon adjacencies. A common electronic structure of (U4+)2(O)2-@C726- can be formally assigned to the three ECFs but with obvious covalent character for both U-O and U-C bonds. Their spatially extended U-5f orbitals substantially enhance the metal-cage interactions. Their various spectra were also simulated to assist future experiments. Moreover, our work shows that the careful choice of exchange-correlation functionals is rather critical for the structural characterization of ECFs.