U2@D3h(5)-C78: Experimental Validation of a Theoretically Predicted Structure and Insights into U-U Bonding in a Fullerene Cage.

The nature of U-U bonding remains one of the most challenging issues in actinide chemistry. Recent theoretical studies have predicted that the fullerene C78 cage can stabilize a U2 dimer in two distinct bonding configurations, namely, a long-bonding model with a weak bond and a short-bonding model featuring a long-sought triple bond. Herein, we report the synthesis, isolation, and molecular characterization of U2@D3h(5)-C78, providing experimental validation of this theoretically proposed system. Single-crystal X-ray diffraction analysis reveals that the U2 dimer is located on the 3-fold axis of the D3h(5)-C78 cage, with a U-U distance of 4.073 Å. This geometry is in good agreement with the predicted long-bonding configuration, indicating a weak bond between the two U atoms. Further analysis indicates a plausible strategy for achieving shorter U-U distances through modulation of cage size. These findings highlight the tunability of U-U bonding in confined carbon cages and provide valuable guidance for the future exploration of higher-order U-U multiple bonding within fullerene cages.