After 50 Years, Chemists Isolate Uranocenium, the Missing Uranium 'Sandwich' Molecule
Manchester and Lancaster researchers found the long-sought cation was never unstable. The salt used to study it was destroying it.
Chemists at the University of Manchester and Lancaster University have isolated uranocenium, a positively charged uranium molecule that researchers have sought for more than half a century. They report it is stable at room temperature and behaves in a way that challenges how chemists usually think about bonding in heavy elements. The study appears in Nature Communications.
Uranocenium is the oxidized form of uranocene, a landmark uranium compound. Uranocene is a "sandwich" molecule: one uranium atom held between two flat rings, each made of eight carbon atoms. It is the uranium counterpart of ferrocene, an iron sandwich molecule whose discovery contributed to the 1973 Nobel Prize in Chemistry. Ferrocene readily gives up an electron to form the ferrocenium cation, a switch so clean and reliable that labs worldwide use it as a reference standard. Until now no one had isolated the uranium equivalent.
When chemists first tried to remove an electron from uranocene in the late 1970s and early 1980s, the molecule seemed to fall apart into poorly defined products. The Manchester team found that the problem was not uranocene. It was a fluorine-containing salt routinely added to help solutions conduct electricity. The uranocenium cation strips fluorine from that salt and decomposes in the process. By choosing a different chemical to remove the electron and a less reactive solvent, the team isolated uranocenium as crystals. The breakthrough began partly by chance: the reaction was designed for another purpose, and uranocenium turned up instead.
The team, which included Cameron Deakin, Iulia Turcas, Benjamin Réant, Liam Griffin, John Seed, Adam Brookfield, professors David Collison and Eric McInnes and Dr. Conrad Goodwin of Manchester, and Dr. Andrew Kerridge of Lancaster, then studied it in detail. They used single-crystal X-ray diffraction to map atom positions, magnetic and electron paramagnetic resonance measurements to probe unpaired electrons, and computer modeling. The results confirmed that uranium has given up one more electron than in uranocene, raising its oxidation state from +4 to +5. Electrochemical tests showed uranocene and uranocenium switch back and forth as cleanly as ferrocene and ferrocenium do.
The surprise was how little the molecule changed. Normally, when a metal loses an electron, its greater positive charge pulls neighboring atoms closer, and uranium bonds are usually held mainly by that electrostatic attraction. The team expected the uranium-carbon distances to shrink noticeably. X-ray measurements showed they shortened by only about 0.02 angstroms, several times less than expected from the change in uranium-ion size alone.
Calculations explained why. When uranium loses an electron, it shares electrons with its carbon neighbors much more, a type of bonding called covalent. The uranium share of the key bonding electrons rises from around 30% to over 40%. That builds up electron density between uranium and carbon, and because electrons repel one another, the crowding resists further squeezing. "What surprised us most was that the molecule hardly changes shape when it loses an electron, and that turns out to be a direct consequence of uranium sharing its electrons with carbon," said Goodwin, a Royal Society university research fellow.
Uranocene was one of the first molecules to show that uranium's outermost electrons, in so-called f-orbitals, take part in chemical bonding, and it remains a benchmark for heavy-element chemistry. With uranocenium, researchers now have a matching set of uranium sandwich molecules in three oxidation states (+3, +4 and +5), letting them track changes in bonding while the structure stays almost the same. Next, the team plans to explore uranocenium's reactivity and test whether heavier elements, such as neptunium, can form similar compounds.
