Rare-earth metals are indispensable for many technical products, from
smartphones, laptops, batteries, electromotors, and wind turbines, to
catalysts. In the journal Angewandte Chemie, a Japanese team has
now introduced a molecular “cage” with “caps” that can be used to
selectively “confine” certain rare-earth-metal ions for isolation or
recycling.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
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 the rare-earth metals are not actually rare.
They are everywhere in the environment but are highly dispersed and
bound in minerals (“earths”); large deposits are rare. Reclaiming these
elements from electronic waste is becoming more important. Some
microorganisms have been discovered that contain enzymes with rare-earth
metals. These could be useful in extraction and reclamation and provide
inspiration for the use of rare-earth metals as catalysts.
Rare-earth-metal ions are also found in bodies of water and in
effluent. However, they are hard to separate individually from aqueous
solutions. One reason for this is that they are usually hydrated,
meaning that they are bound to water molecules. Their states of
hydration are different and may change. This makes identification and
isolation of the ions through binding to ligands more difficult.
A team led by Makoto Fujita at the University of Tokyo and the
Institute for Molecular Science has now managed to “confine” the
hydrated forms of trivalent ions of a series of rare-earth metals in
closed “cages”. Each cage molecule consists of four organic ligands
shaped like triangular “plates” that are connected by their tips to six
palladium ions to make an octahedral cage with two large openings. The
rare-earth-metal ion fits into the cage with its nine bound water
molecules. The critical feature of the cage are its two “caps” that
cover the openings. These are planar molecules with three negatively
charged binding arms that bind to the rare-earth-metal ion’s water
molecules through hydrogen bridges. In addition, they are held tight by
electrostatic interactions with the positively charged palladium ions in
the cage.
Not all rare-earth-metal ions are captured equally well by 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, are bound significantly more
strongly than the later lanthanides, like ytterbium. Scandium, for
example, only has six water molecules bound to it and cannot find a
stable position within the cage. It is thus barely held in place.
Confinement of hydrophilic metal species in a closed cavity could be
an approach for the isolation of rare-earth metals, as well as for the
development of novel catalysts analogous to metal-containing enzymes
(metallozymes) in microorganisms.
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About the Author
Dr. Makoto Fujita is
the University Distinguished Professor at the University of Tokyo, and
has been actively working in the field of molecular self-assembly (using
metals) for over 30 years. He is the recipient of the Wolf prize in
Chemistry (2018) and Clarivate Citation Laureates (2020).
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