Are U-U Bonds Inside Fullerenes Really Unwilling Bonds?

Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th2@C80 and U2@C80 have shown that although the two Th3+ ions form a strong covalent bond within the carbon cage, the interaction between the U3+ ions is weaker and described as an unwilling bond. To evaluate the feasibility of covalent U-U bonds, which are neglected in classical actinide chemistry, we have first investigated the formation of smaller diuranium EMFs by laser ablation using mass spectrometric detection of dimetallic U2@C2n species with 2n >= 50. DFT, CASPT2 calculations, and MD simulations for several fullerenes of different sizes and symmetries showed that thanks to the formation of strong U(5f3)-U(5f3) triple bonds, two U3+ ions can be incarcerated inside the fullerene. The formation of U-U bonds competes with U-cage interactions that tend to separate the U ions, hindering the observation of short U-U distances in the crystalline structures of diuranium endofullerenes as in U2@C80. Smaller cages like C60 exhibit the two interactions, and a strong triple U-U bond with an effective bond order higher than 2 is observed. Although 5f-5f interactions are responsible for the covalent interactions at distances close to 2.5 angstrom, overlap between 7s6d orbitals is still detected above 4 angstrom. In general, metal ions within fullerenes should be regarded as templates in cage formation, not as statistically confined units that have little chance of being observed.

Automated summary

Previous studies on diactinide endohedral metallofullerenes (EMFs) Th2@C80 and U2@C80 have shown that the formation of covalent bonds between U3+ ions is weaker compared to Th3+ ions. This study investigates the feasibility of covalent U-U bonds in smaller diuranium EMFs and finds that the formation of strong U(5f3)-U(5f3) triple bonds allows for incarceration of two U3+ ions inside the fullerene. It is observed that U-cage interactions hinder the observation of short U-U distances in larger diuranium endofullerenes.