Plutonium (Pu) exhibits one of the most diverse and complex chemistries of any element in the periodic table. Since its discovery in 1940, scientists have synthesized and studied many different types of plutonium-containing compounds using tools that reveal both their atomic structures and how they interact with light.
Not only does plutonium have numerous alloys and metallic phases, but it can also exist in coordination compounds - molecules in which a central metal atom is surrounded by and bound to other molecules or ions, known as ligands. In these compounds, plutonium appears as plutonium cations, meaning positively charged plutonium ions that form chemical bonds with their surroundings. While hundreds of such plutonium coordination structures are known today, only a tiny fraction of these compounds involve polyoxometalates (POMs), a special class of large, metal-oxygen molecular clusters that can act as rigid, inorganic "molecular cages" for metal ions.
Until a recent study by researchers at Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories and Oregon State University, only five plutonium-POM compounds had ever been isolated, representing less than one percent of all known plutonium compounds and highlighting how unexplored this area of plutonium chemistry remains.
The team's latest research, published in Inorganic Chemistry, is the third in a series of papers, building on earlier work in Chemical Communications and Inorganic Chemistry that laid the foundation for understanding how POMs interact with some of the most chemically challenging actinide elements.
In this new study, the researchers used a POM shape called a Keggin ion - a hollow, negatively charged cluster built mostly from tungsten and oxygen, with a small phosphorus atom at its center. Before this study, chemists had used Keggin POMs to bind many metals, but never plutonium.
Using a carefully prepared chemical solution and only six micrograms of plutonium (six millionths of a gram), the team was able to successfully bind a plutonium ion between two Keggin cages. The researchers then applied a suite of advanced tools (X-ray crystallography, optical spectroscopy techniques, nuclear magnetic resonance and X-ray scattering experiments) to confirm the stability and structural makeup of their new plutonium-POM complex.
After comparing the plutonium to similar metals such as cerium, hafnium, thorium and zirconium, the researchers noticed something unexpected - while the immediate bonding around the plutonium atom looked familiar, the plutonium complexes had actually arranged themselves at right angles to one another (perpendicularly), unlike the parallel arrangements seen in other metals. This unusual behavior highlights plutonium's reputation as a chemical "wild card" and helps explain why it has long defied simple models.
These findings indicate that the team's microscale POM approach offers a viable pathway to exploring some of the most challenging elements in the periodic table - one molecule at a time.
LLNL authors of this study include Ian Colliard, Derrick Kaseman, Christopher Colla and Gauthier Deblonde.
-- Shelby Conn
