Putting the Squeeze on REEs

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"Rare earth elements represent one of the final frontiers of chemical exploration."

Rare earth elements (REEs) are a series of elements that represent one of the final frontiers of chemical exploration. The vigorous reactivity of these substances, however, has made it difficult for researchers to transform them into stable materials with well-defined structures. But when they succeed, the payoff can be enormous—rare earth compounds have important applications in areas ranging from catalysis to clean energy.

PNP ligands

Now, Zhaomin Hou and colleagues from the RIKEN Advanced Science Institute have discovered a new way to isolate REEs as hydrogen-infused crystals by using wedge-shaped bis(phosphinophenyl)amido (PNP) ligands to 'pinch' them in place. These ligands squeeze rare earth yttrium atoms together tighter than any previous material, and can even stabilize highly volatile charged complexes.

According to Hou, the trick to producing rare earth polyhydrides is to surround them with large, cumbersome molecules that easily pack together to form crystals. The distinct structure of PNP ligands—two phosphorus atoms, linked together by a rigid aromatic–amino core that can bind to elements with a pincer-like grip—made this ligand a promising candidate for the researchers’ investigation.

By first substituting extra methyl units onto the aromatic backbone of PNP to increase its bulkiness, and then mixing the ligand with an yttrium alkyl precursor and hydrogen gas, the team synthesized pale yellow crystals of a new yttrium polyhydride complex. X-ray structural analysis revealed that three yttrium atoms, held in place by PNP ‘pincers’, were interlinked by a set of double- and triple-bridged hydrogen ligands (see figure).

The researchers found that an ammonium proton could remove a hydride from the complex without disrupting crystallization, yielding the first-ever cationic tri- and di-yttrium polyhydrides. The charged nature of these materials should impart potent chemical activity, attributes Hou and his team are currently investigating.

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