Caged actinides form unusual bonds in simulations

Computational chemists predict that actinide elements trapped in fullerene cages will form some unusual metal-metal bonds, including very long covalent bonds and π bonds without accompanying σ bonds (Inorg. Chem. 2020, DOI: 10.1021/acs.inorgchem.0c01713).

Scientists know of just a few actinide-actinide bonds, including gas-phase U₂ and Th₂, and U₂ inside an 80-carbon fullerene cage. Cina Foroutan-Nejad of Masaryk University,Adam Jaroš of Charles University and Michal Straka of the Czech Academy of Sciences used density functional theory calculations to predict other actinide bonds that could form in fullerene. They modeled bonds between the elements from Ac to Cm inside C₇₀, C₈₀, and C₉₀ fullerenes. The researchers found the cages encourage bonding not just by bringing the atoms close together, but also because bonds between the cage and the actinides improve the electronic conditions for actinide-actinide bonding. Plutonium proved the most interesting system, forming banana-shaped σ bonds in C₇₀ fullerenes, two π bonds without a σ bond in C₈₀, and a 5.93 Å bond in C₉₀, which would be a record-long metal-metal covalent bond.

These bonds remain theoretical for now, and experts say it’s hard to assess the group’s predictions without experimental evidence. The researchers say they don’t have the ability to synthesize these complexes, although other groups are working on making similar systems.

Computational chemists want to see more sophisticated modeling. Laura Gagliardi of the University of Chicago says the calculations should include accounting for electron correlation and relativistic effects in a more comprehensive way. Straka doesn’t think those effects are likely to change their results. And he says the group found more advanced computations were impractical on systems with so many atoms.

Straka hopes they can model similar two-element actinide bonds soon. But the researchers say that, in addition to their interesting bonds, actinide-fullerene systems might prove useful as switches, spin filters, or magnets in advanced electronics. Foroutan-Nejad says the group is working on a hypothetical “Swiss Army knife” molecular device.