Lubomír Rulíšek Group
CS

Exotic chemical bonds

Actinide-actinide bonds in fullerenes

Actinide–actinide bonds are very rare. So far the An–An bonds have been only described in a gas-phase U2 and (indirectly) Th2 diatomics, and in the interior of endohedral fullerenes, where a strong metal-cage interaction keeps the two actinide atoms at distances where An–An bonds are formed. In 2015, we predicted actinide-actinide bonding in U2@Ih(7)-C80, coined unwilling bonding (Foroutan-Nejad, 2015). This molecule was later confirmed experimentally (Zhang, 2018). In 2020, we searched for actinide-actinide bonding in An2@D5h(1)-C70, An2@Ih(7)-C80, and An2@D5h(1)-C90 (An=Ac-Cm) series of endohedral metallofullerenes (EMFs). We showed that most of the studied An2@C70 and An2@C80 systems feature one or more one-electron two-center actinide-actinide bonds, in particular in Th and Pa EMFs (Jaroš, 2020). Notably, Th2@C80 predicted therein was later characterized experimentally (Zhuang, 2021). However, our benchmark study for theoretical predictions of actinide-actinide bonding revealed, that the DFT BP86-predicted bonding is overestimated (Jaroš, 2023). High-level ab initio calculations confirmed bonding in Th and Pa systems and in U2@C60, but only minor interaction was found in U2@C80 and heavy-actinide (Np, Pu, Am, Cm) compounds.

Comparison of bonding picture calculated by DFT and CASPT2

Fullerene molecular crystal

An interesting example of exotic bonding in the endohedral fullerenes is molecular crystal F2-@C60+. In collaboration with G. Frenking (University of Marburg), C. Foroutan-Nejad (Polish Academy of Sciences), and I. Fernandez (Universidad Complutense of Madrid) we have predicted that F2 molecule enclosed in C60 oxidizes the fullerene cage and stabilizes itself as F2- ion (Foroutan-Nejad, 2018). Notably, the interaction between F2- and C60+ is predicted to be of purely electrostatic nature, making this molecule a single-molecule crystal. Additionally, F2-@C60+ is a rare example of an endohedral fullerene, where the enclosed moiety is negatively charged while the fullerene is oxidized.


Electron density in the fullerene molecular crystal

Charge-depletion bonding inside fullerenes

What happens when neutral molecular systems are enclosed in a fullerene, for example HF···H2O system, a prototype of Lewis acid-base pair? Experiments have shown that the HF···OH2 hydrogen bond strongly shortens while the H–F bond elongates upon encapsulation of the cluster in C70 (Zhang, 2017). Our calculations revealed that it is not a mere compression of the HF·H2O pair inside the cavity of C70 but it is related to a strong LP–𝞹 bonding interaction (LP = lone pair of electrons) with the fullerene cage (Jaroš, 2018). A bonding analysis revealed unique LPF–𝞹 cage interactions with a charge-depletion character in the bonding region, unlike usual LP–𝞹 bonds. The LPF–𝞹 cage interactions are responsible for the elongation of the H–F bond and HF thus appears to be more acidic inside the cage. The shortening of the FH···OH2 contact in (HF·H2O)@C70 originates from increased acidity of the HF. Investigation L1L2@Cn (L1/2 = HF, H2O, and NH3; n = 70, 80, 90) systems showed more examples of the charge-depletion bonding. The existence of charge depletion bonding is important for understanding of bonding in endohedral fullerenes.

Charge-depleted bond in (HF·H2O)@C70