![]() ![]() Since that seminal work, several theoretical articles have been devoted to the analysis and rationalization of the U–U interaction in different environments. (15) The computed U–U bond distance of 2.43 Å was interpreted in terms of the presence of a quintuple bond. (13) The complexity of the electronic structure of the naked U 2 was already pointed out in 1990, (1,14) but it was not until 2005 when Gagliardi and Roos predicted using CASPT2 calculations that U 2 is a stable molecule in the gas phase with a dissociation energy of about 30 kcal (11) Mass spectrometric evidence has been reported on the formation of Th 2 and U 2 in gas phase (12) as well as the isolation and characterization of uranium hydride molecules. Since the initial studies, it became clear that the propensity to form U-ligand bonds was greater than that for forming U–U bonds. (8−10) Nevertheless, much less experimental information is available about the formation of diactinides. (4) Huge advances have been made in the chemistry of the f-elements over the last decades, (5,6) including the synthesis of complexes containing actinide–ligand multiple bonds, (7) or uranyl peroxide capsules with high oxidation states. (1−3) In nature, uranium is generally found as an oxide, showing several oxidation states. Understanding how atoms stay together to form stable structures is at the core of chemistry. 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. 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. 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. 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 80. 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. 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 2n species with 2 n ≥ 50. Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th 80 and U 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. ![]()
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