Aromatic compounds: A ring made up solely of metal atoms
Heidelberg University chemists led by Prof. Dr. Lutz Greb discovered a new aromatic metal ring made of bismuth atoms, challenging traditional carbon-based aromatic compounds. Their innovative stabilization method offers insights for future research.
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Heidelberg University chemists, led by Prof. Dr. Lutz Greb, have characterized a new basic structure in the field of aromaticity by isolating a metal ring composed solely of elemental bismuth atoms. This groundbreaking discovery challenges the conventional understanding of aromatic compounds, typically associated with carbon rings. The research team achieved this by employing a novel supramolecular stabilization approach, encapsulating the positively charged metal ring with a negatively charged molecular shell to prevent decomposition reactions. This innovative method not only expands the fundamental knowledge of aromatic compounds but also hints at potential applications in charge transport within metals. The study, published in Nature Chemistry, sheds light on a new concept in aromaticity and opens up possibilities for further research in stabilizing positively charged rings and cages. Prof. Greb emphasizes the significance of this work in advancing the understanding of aromatic compounds and its potential implications in various scientific fields.
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4 commentsAromatic metal compounds might be usable as highly efficient catalysts for organic reactions. These compounds could be used to create novel materials with interesting electronic, optical, or magnetic properties, so we could see better organic LEDs, better organic solar cells, conductive polymers, and improvements to magnetic materials for data storage. The unique structures of aromatic metal compounds could be exploited in drug design, potentially leading to new classes of therapeutic agents with improved efficacy or novel mechanisms of action. These compounds might be also used to create new types of nanostructures or improve existing ones, with potential applications in areas like sensing, drug delivery, or nanoelectronics. The electronic properties of aromatic metal compounds could potentially be harnessed to develop new types of batteries or supercapacitors with improved performance. The specific interactions between aromatic metal compounds and various analytes could be used to develop highly sensitive and selective chemical sensors. These compounds might exhibit interesting photochemical properties, leading to advancements in areas such as photocatalysis or light-harvesting materials for solar energy conversion.
We will know in ten years!
This weirdness, that anything weird can happen at any time, leading to misery or a nature paper, is what makes chemistry so interesting and frustrating.
Relevant passage from the Nature article (https://www.nature.com/articles/s41557-024-01530-z)
During our investigation of 2 as a transmetallation agent, we observed an unusual reactivity with InBr3. The reaction of 2 with InBr3 in dichloromethane resulted in a rapid colour change from light orange to dark red. After standing for 3 days at room temperature, dark-coloured crystals of 3 developed (Fig. 3f). The synthesis is reproducible, yielding up to 28% of isolated 3. scXRD analysis showed a planar rhomboid {Bi4} ring encapsulated by two indiumbromide-calix[4]pyrrolates (...)Elemental bismuth is not a metal, but a semi-metal.
Solid metals have 2 characteristics that distinguish them from other kinds of substances. The first is the existence of free electrons, which makes them electrically conductive and optically reflective. The second is a crystal structure where each atom has many neighbors, whose consequence is that metals are ductile and malleable.
Already in the 18th century, semi-metals have been distinguished from metals. Semi-metals have only the first property. They have free electrons, but they have a crystal structure where each atom has only few neighbors, whose consequence is that semi-metals are fragile, even when pure (most metals can be made fragile by impurities that disrupt the crystal structure, but they are not fragile when pure).
A typical example of a semi-metal is carbon as graphite. Graphite has free electrons and it is a good electric conductor, but it is easily broken by any attempt to deform it.
Elemental bismuth is similar to graphite. It has free electrons, but like graphite, its crystal structure is made of planes of atoms between which there are relatively large distances, so it is fragile like graphite.
The fact that neither bismuth nor graphite are metals, but only semi-metals, is determined by the fact that they have a much higher electro-negativity than metals. The electro-negativity of bismuth is similar to silicon, germanium and antimony (which are also not metals, Si and Ge are semiconductors, while antimony is a semi-metal), and higher than for any true metals. (Semiconductors differ from semi-metals by not having free electrons when pure and at low temperatures; free electrons are produced in semiconductors by various means, e.g. high temperatures, impurities, light, injection etc.)
So the existence of aromatic rings made of bismuth atoms is much less surprising than the existence of aromatic rings made of truly metallic atoms would be.
Despite the fact that the distinction between metals and semi-metals has already been done for almost 300 years, many modern chemistry manuals fail to distinguish them in a consistent way (or worse, some confuse semi-metals with semiconductors). This is a serious mistake, because in practical applications the distinctions between metals, semi-metals and semiconductors are very important. Many technically important materials are semi-metals, for instance many of the carbides or nitrides of the transitional metals.
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