ACS Meeting News: Cesium atom coordinates with 16 fluorine atoms in record-setting complex

Among the elements, cesium, located in the lower left corner of the periodic table, and fluorine, in the upper right corner, are among the largest electropositive and smallest electronegative elements, respectively. When chemists look at possible ways to get the two elements together, something interesting is bound to happen. And it has.

Klaus-Richard Pörschke, David Pollak, and Richard Goddard of the Max Planck Institute for Coal Research have prepared a molecule in which a central cesium atom is coordinated by 16 fluorine atoms—achieving a perfect score for the maximum number of bonds possible and establishing a new precedent for bonding in the process. Pörschke announced the team’s discovery in a Division of Inorganic Chemistry symposium at the American Chemical Society national meeting yesterday in San Diego.

Going beyond 12 bonds is rare because of the limited space available around the central atom of a molecule and electrostatic repulsion between the ligands. Chemists have flirted with 16-coordinate compounds for years, reporting isolated 15- and 16-coordinate Th-H molecules and a gas-phase 16-coordinate Co-B species.

For Pörschke and coworkers, pairing the large singly charged Cs⁺ cation with the weakly coordinating [H₂NB₂(C₆F₅)₆]^– anion allowed them to go beyond 12 bonds in a complex for the first time without using hydrogen as a ligand. The team prepared Cs[H₂NB₂(C₆F₅)₆] by using ultrasound to agitate a solution of [Na(OCH₂CH₃)₄][H₂NB₂(C₆F₅)₆] and CsF in dichloromethane. The researchers concentrated the solution and isolated crystals to study by X-ray crystallography.

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The poor aqueous solubility of the new compound suggested to the researchers that [H₂NB₂(C₆F₅)₆]^– could be a good scavenger of Cs⁺ in water, which they demonstrated with a set of experiments. Pörschke says that the anion might therefore be useful to pull ¹³⁴Cs and ¹³⁷Cs from nuclear waste solutions, as a treatment for ¹³⁴Cs and ¹³⁷Cs radiation poisoning, or to prepare implantable ¹³¹Cs and ¹³⁷Cs seeds for radiation therapy.

“Pörschke and coworkers have plumbed the limits of coordination chemistry by a careful matching of cation and anion properties,” commented Warren E. Piers, of the University of Calgary, an expert in coordination chemistry. “In addition to the sheer beauty of the molecule, they demonstrate exciting possibilities for radioactive cesium ion sequestration.”

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