Rare earth metals are essential for many technical products, from smartphones, laptops, batteries, electromotors, and wind turbines, to catalysts. A Japanese team has now introduced a molecular “cage” with “caps” that can be used to prevent certain rare earth metal ions from being selected for isolation or recycling.
The rare earth elements include 17 metals: scandium, yttrium, lanthanum, and the lanthanides, the 14 elements that follow after lanthanum in the periodic table, including neodymium and europium. The name is misleading because rare earth metals are not actually rare. They are everywhere in the environment but very dispersed and bound in minerals (“soils”); rare deposits.
Reclaiming these elements from electronic waste is becoming increasingly important. Some microorganisms have been discovered to have enzymes that contain rare earth metals. It can be used in extraction and reclamation and will inspire the use of rare earth metals as catalysts.
Rare earth metal ions are also found in water bodies and effluents. However, they are difficult to separate individually from aqueous solutions. One reason for this is that they are usually hydrated, which means they are bound to water molecules. Their hydration states are different and can change. This makes the identification and isolation of ions by binding ligands more difficult.
A team led by Makoto Fujita of the University of Tokyo and the Institute for Molecular Science has now managed to confine the hydrated forms of trivalent ions to a series of rare earth metals in closed cages. Each molecule of the cage consists of four organic ligands shaped like triangular plates connected at their tips to six palladium ions to form an octahedral cage with two large openings. . The rare-earth-metal ion fits into the cage with its nine bonded water molecules.
Tabuchi and so on.
The critical part of the cage is the two “caps” that cover the openings. These are planar molecules with three negatively charged binding arms that bind water molecules to rare earth-metal ions through hydrogen bridges. In addition, it is held tightly by electrostatic interactions with the positively charged palladium ions in the cage.
Not all rare-earth-metal ions are captured equally well in this system. Subtle differences in their radii and preferred modes of hydration determine how well they fit into the cages: lanthanum and the early lanthanides, such as europium, bind more strongly than the later lanthanides, like ytterbium. Scandium, for example, has only six water molecules bound to it and cannot find a stable position within the cage. Thus it is hardly held in place.
Confinement of hydrophilic metal species in a closed cavity can be a method for the isolation of rare earth metals, as well as for the development of new catalysts similar to enzymes with metals (metallozymes) in microorganisms.
Resources
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R. Tabuchi, H. Takezawa, M. Fujita (2022) “Selective Confinement of Rare-Earth-Metal Hydrates in a Capped Metallo-Cage under Aqueous Conditions”, Angew. Chem. Int. Ed. two: 10.1002/anie.202208866
