There is a promethium complex in the solution

Effects of 4f electron orbitals on the Ln-PyDGA contraction and the dynamics of PmIII complexation in Aqueous media

In order to get a good fit to the Ln-PyDGA data, the model used for promethium was used. Figure 3c represents the most comprehensive view of the Ln contraction phenomenon obtained from experiments and shows how the inner-sphere Ln–O bond distances change depending on the number of 4f electrons in the electronic structure of LnIII. A correlation was found between the decrease of Ln–O bonds and the number of Lu bonds. Filling the 4f orbitals apparently influences shielding of the nuclear charge and according to our data this effect was most pronounced early in the series from La to Pm, accounting for as much as roughly 36% of the overall Ln contraction. After Pm, there was a steadier shortening of the Ln–O bonds. The difference between this and the Shannon’s effective ion radii decrease is that this one is larger at the start of the series than at the end. It is also worth mentioning that the observed accelerated contraction parallels well with the Ln extraction performance of lipophilic diglycolamides in a liquid–liquid extraction process, where better separation between adjacent lanthanides was achieved for the light (La–Nd) than for the heavy (Er–Lu) members of the series9,10. The shielding constant for f electrons (s > 0.74) we derived from adapting the modified model36 to our experimental dataset is in good agreement with the reported value of 0.69 obtained from the Ln ionization energies42. We note, however, that accurate fully relativistic quantum mechanical calculations using a new generation of supercomputers will be important to further investigate the observed Ln contraction behaviour in future studies.

We performed simulations of the dynamics of PmIII complexation in an Aqueous environment. The creation of a Pm(PyDGA)3+ complex was found to have very good agreement with the AIMD trajectory simulations and the theoretical EXAFS spectrum and its Fourier transform. Key structural parameters can be determined by the analyses of radial distribution functions (RDFs) and AIMD predicted Pm–O bond length. Beyond the inner-sphere Pm–O correlations, the AIMD results also indicated some water structuring around the complex at 4.43 Å through transient hydrogen bond interactions with the O donor groups of the PyDGA ligands. It is also worth noting that, like in the experimental EXAFS data, the amide carbonyl and etheric Pm–O bonds could not be resolved in the AIMD and thus appeared as a single peak in the corresponding RDF, pointing to the dynamic nature of the first-sphere ligand-metal interactions in aqueous solution (Supplementary Video 1).

Exploiting coordination complexes to separate lanthanide ion-oxide ions using three electron-rich oxygen atoms

“It’s a tour de force,” says Polly Arnold, a chemist at Lawrence Berkeley National Laboratory in Berkeley, California, who was not involved in the research.

Together with a couple of other metals, the lanthanides are collectively known as rare-earth elements, and many are prized for their uses in technology, including lasers and powerful magnets. Many rare-earth elements are abundant in the Earth’s crust but are hard to find. It is difficult to extract just one lanthanide element and separate it from the rest because of their similar chemistry.

Current separation methods often use molecules known as ligands to bind to positively charged lanthanide ions in solution, forming coordination complexes. Chemists can then exploit subtle differences between these complexes to separate them: for example, by selectively washing the complexes out of water using organic solvents. Oak Ridge chemist Ilja Popovs co-led the research and said that it takes a lot of separations to get to pure material.

Promethium has been something of a closed book for researchers working on improved separation methods. Chemists have succeeded at making only a handful of promethium compounds, all of them simple solids such an oxide3 — but never a complex that shows how promethium might bond to separation ligands in solution.

The team combined these ions with a ligand called bispyrrolidine diglycolamide, which contains three electron-rich oxygen atoms. Three of these ligands hugged each promethium ion, generating complexes with nine promethium–oxygen bonds.

Using X-ray absorption spectroscopy and theoretical simulations, the researchers measured the average length of these bonds. Oxygen gives pairs of electrons that fit around the empty energy levels around promethium to form bonds.

“It’s just incredibly difficult, skilful work, and it’s really impressive that they’ve been able to do it,” says Arnold, who studies lanthanides and their heavier cousins, the actinide elements.

With every step along the lanthanide series, from lanthanum to lutetium, each element gains one proton and one electron. There are two components to an atom, a nucleus and electrons. When using lanthanides, the electrons gradually fill up a set of 4f that are diffuse and therefore can not shield the negatively charged electrons in an atom from the growing positive charge of its nucleus. The nucleus can exert a stronger pull on some orbitals and contract the atom more than expected.

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