UB and Berkeley Lab scientists discover new heavy-metal molecule ‘berkelocene’

BUFFALO, N.Y. — Scientists from the University at Buffalo and the Department of Energy’s Lawrence Berkeley National Laboratory have discovered “berkelocene,” the first organometallic molecule to be characterized containing the heavy element berkelium.

Organometallic molecules, which consist of a metal ion surrounded by a carbon-based framework, are relatively common for early actinide elements like uranium (atomic number 92), but are scarcely known for later actinides like berkelium (atomic number 97).

“This is the first time that evidence for the formation of a chemical bond between berkelium and carbon has been obtained. The discovery provides new understanding of how berkelium and other actinides behave relative to their peers in the periodic table,” says Stefan Minasian, a scientist in Berkeley Lab’s Chemical Sciences Division, which led the research, and one of four co-corresponding authors of a new study published in the journal Science.

The electronic structure calculations of berkelocene were performed at UB by co-corresponding author Jochen Autschbach, PhD, SUNY Distinguished Professor and Larkin Professor in the UB Department of Chemistry, College of Arts and Sciences.

These calculations revealed an unexpected finding about how berkelocene, one of 15 actinides in the periodic table’s f-block, behaves in comparison to the lanthanides, which are located one row above the actinides on the periodic table.

“The electronic structure calculations, as well as the experimental observations, show that berkelocene is unlike its lanthanide analogs, which disrupts long-held assumptions about the chemical and physical properties of transplutonium elements,” Autschbach says.

The pioneering nuclear chemist Glenn Seaborg discovered berkelium at Berkeley Lab in 1949. It was one of many achievements related to transuranium elements that led to his winning the 1951 Nobel Prize in Chemistry with fellow Berkeley Lab scientist Edwin McMillan.

Organometallic compounds of actinides like berkelium typically have higher symmetries and form more covalent bonds with carbon, making them more useful for observing the unique electronic structures of the actinides.

“When scientists study higher symmetry structures, it helps them understand the underlying logic that nature is using to organize matter at the atomic level,” Minasian says.

 But berkelium is not easy to study because it is highly radioactive, and only very minute amounts of this synthetic heavy element are produced globally every year. Adding to the difficulty, organometallic molecules are extremely air-sensitive and can be pyrophoric.

Berkeley Lab is one of a few facilities in the world that can handle such materials.

The Berkeley Lab team custom-designed new gloveboxes enabling air-free syntheses with highly radioactive isotopes. Then, with just 0.3 milligram of Berkelium-249, the researchers conducted single-crystal X-ray diffraction experiments. 

The results showed a symmetrical structure with the berkelium atom sandwiched between two 8-membered carbon rings. The researchers named the molecule berkelocene because its structure is analogous to a uranium organometallic complex called “uranocene.”

The electronic structure calculations were carried out at UB by former postdoctoral researcher, Dumitru-Claudiu Sergentu, under Autschbach’s supervision, and utilizing the resources of UB’s Center for Computational Research (CCR). Sergentu is now a professor at Alexandru Ioan Cuza University in Romania. 

The calculations showed that the berkelium atom at the center of the berkelocene structure has a tetravalent oxidation state (positive charge of +4), which is stabilized by the berkelium–carbon bonds.

Traditional understanding of the periodic table suggests that berkelium would behave like the lanthanide terbium, but the berkelium ion is much happier in the +4 oxidation state than the other f-block ions that the team expected it to be most like.

The researchers say that more accurate models showing how actinide behavior changes across the periodic table are needed to solve problems related to long-term nuclear waste storage and remediation. 

This work was supported by the DOE Office of Science.