Are U–U Bonds Inside Fullerenes Really Unwilling Bonds?

[Image: see text] Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th(2)@C(80) and U(2)@C(80) have shown that although the two Th(3+) ions form a strong covalent bond within the carbon cage, the interaction between the U(3+) 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 U(2)@C(2n) 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(5f(3))-U(5f(3)) triple bonds, two U(3+) 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 U(2)@C(80). Smaller cages like C(60) 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 Å, overlap between 7s6d orbitals is still detected above 4 Å. 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.