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@I_h(7)-C₈₀, coined unwilling bonding.¹ This molecule was later confirmed experimentally.² In 2020, we searched for actinide-actinide bonding in An₂@D_5h(1)-C₇₀, An₂@I_h(7)-C₈₀, and An₂@D_5h(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.³ Notably, Th₂@C₈₀ predicted therein was later characterized experimentally.⁴ However, our benchmark study for theoretical predictions of actinide-actinide bonding revealed, that the DFT BP86-predicted bonding is overestimated.⁵ 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.⁶ 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.

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 C₇₀.⁷ 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.⁸ 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@C_n (L_1/2 = 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.

References:

1. Foroutan-Nejad, C.; Vícha, J.; Marek, R.; Patzschke, M.; Straka, M. Unwilling U–U Bonding in U₂@C₈₀: Cage-Driven Metal–Metal Bonds in Di-Uranium Fullerenes. Phys. Chem. Chem. Phys. 2015, 17, 24182–24192. https://doi.org/10.1039/C5CP04280A

2. Zhang, X.; Wang, Y.; Zhang, J.; Wang, C.; Wang, Y.; Wang, S.; Wang, G.; Wang, B. U₂@I_h(7)-C₈₀: Crystallographic Characterization of a Long-Sought Fullerene Cage with a U–U Bond. J. Am. Chem. Soc. 2017, 139, 16447–16450. https://doi.org/10.1021/jacs.7b10865

3. Jaroš, A.; Foroutan-Nejad, C.; Straka, M. From π Bonds without σ Bonds to the Longest Metal–Metal Bond Ever: A Survey on Actinide–Actinide Bonding in Fullerenes. Inorg. Chem. 2020, 59, 12608–12615. https://doi.org/10.1021/acs.inorgchem.0c01713

4. Zhuang, J.; Morales-Martínez, R.; Zhang, J.; Wang, Y.; Yao, Y.-R.; Pei, C.; Rodríguez-Fortea, A.; Wang, S.; Echegoyen, L.; de Graaf, C.; Poblet, J. M.; Chen, N. Characterization of a Strong Covalent Th³⁺–Th³⁺ Bond Inside an I_h(7)-C₈₀ Fullerene Cage. Nat. Commun. 2021, 12, 2372. https://doi.org/10.1038/s41467-021-22659-2

5. Jaroš, A.; Straka, M. Unraveling Actinide–Actinide Bonding in Fullerene Cages: A DFT versus ab Initio Methodological Study. Phys. Chem. Chem. Phys. 2023, 25, 31500–31513. https://doi.org/10.1039/D3CP03606E

6. Zhang, J.; Wang, Y.; Wang, S.; Wang, G.; Wang, B. Buckyball Difluoride F₂^-@C₆₀⁺—A Single-Molecule Crystal. Angew. Chem. Int. Ed. 2018, 57, 13931–13934. https://doi.org/10.1002/anie.201809699

7. Zhou, X.; Zhang, J.; Wang, Y.; Wang, S.; Wang, G.; Wang, B. Isolation of the Simplest Hydrated Acid. Sci. Adv. 2017, 3, e1602833. https://doi.org/10.1126/sciadv.1602833

8. Jaroš, A.; Badri, Z.; Bora, P. L.; Bonab, E.; Marek, R.; Straka, M; Foroutan-Nejad, C. How Does a Container Affect Acidity of Its Content: Charge Distribution and Acidity of H₂O@C₆₀. Chem. Eur. J. 2018, 24, 4245–4249. https://doi.org/10.1002/chem.201706017

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Exotic chemical bonds