U2O@C76: Non-Isolated-Pentagon-Rule Cages Prevail with the U2O Configuration Determined by Cage Shape and Dominated by Multicenter Bonds

Endohedral clusterfullerenes (ECFs) are fullerene cages with various metallic clusters trapped inside. So far, the actinide-based ECFs are rather scarce with their possible structures and chemistry remaining largely unexplored. Herein, density functional theory calculations characterized that the recently synthesized U2O@C-76 could be U2O@C-s(17 490)-C-76 or U2O@C-2v(19 138)-C-76, whose cages have two or one pentagon adjacencies (PAs) and thus both violate the isolated pentagon rule (IPR). It is noteworthy that they are the first actinide-based ECFs bearing non-IPR outer cages. They are also the first C-s(17 490)- and C-2v (19 138)-C-76-based oxide ECFs. Moreover, U2O@C-2v(19 138)-C-76 is the first example of a hexavalent metal cluster within the C-2v(19 138)-C-76 cage. Interestingly, although trapped by the two same-sized cages, the U2O unit exhibits a bent and a perfect linear configuration, respectively, indicative of the crucial role of cage shape in steering the internal cluster configuration. Their electronic structures can be formally described as (U2O)(6+)@C-76(6-) with primary electrostatic attractions and secondary covalent interactions between cluster and cage. Significantly, bonding analyses reveal that the encaged U2O moiety may only features two three-center, two-electron (3c-2e) U-O-U bonds with completely absent common two-center bonds.