From π Bonds without σ Bonds to the Longest Metal-Metal Bond Ever: A Survey on Actinide-Actinide Bonding in Fullerenes.

Actinide-actinide bonds are rare. Only a few experimental systems with An-An bonds have been described so far. Recent experimental characterization of the U2@Ih(7)-C80 (J. Am. Chem. Soc. 2018, 140, 3907) system with one-electron two-center (OETC) U-U bonds as was predicted by some of us (Phys. Chem. Chem. Phys. 2015, 17, 24182) encourages the search for more examples of actinide-actinide bonding in fullerene cages. Here, we investigate actinide-actinide bonding in An2@D5h(1)-C70, An2@Ih(7)-C80, and An2@D5h(1)-C90 (An = Ac-Cm) endohedral metallofullerenes (EMFs). Using different methods of the chemical bonding analysis, we show that most of the studied An2@C70 and An2@C80 systems feature one or more one-electron two-center actinide-actinide bonds. Unique bonding patterns are revealed in plutonium EMFs. The Pu2@Ih(7)-C80 features two OETC Pu-Pu π bonds without any evidence of a corresponding σ bond. In the Pu2@D5h(1)-C90 with rPu-Pu = 5.9 Å, theory predicts the longest metal-metal bond ever described. Predicted systems are thermodynamically stable and should be, in principle, experimentally accessible, though radioactivity of studied metals may be a serious obstacle.