Five-membered metal ring pushes the boundaries of aromaticity

Professor Dr. Robert Kretschmer, professor of inorganic chemistry at the Chemnitz University of Technology, examines crystals in front of a diffractometer. Credit: Jacob Müller | Chemnitz University of Technology

Researchers from Chemnitz University of Technology, Humboldt Universität Berlin and Philipps-Universität Marburg have succeeded in synthesizing a molecule that should not exist.

How many atoms can an aromatic ring have if it only contains two electrons? The answer has recently changed: five! For the first time, a team of chemists has succeeded in synthesizing and describing a five-membered aromatic compound – an important class of substances in chemistry – by combining experience and theory. They thus achieved what had been attempted for decades and which had been considered impossible in the meantime. The research groups led by Professor of Chemistry Dr. Robert Kretschmer of the Chemnitz University of Technology, Dr. Oliver Dumele of Humboldt University Berlin and Professor of Chemistry Dr. Florian Weigend of the Philipps University of Marburg report their findings in the journal International Edition of Applied Chemistry.

Aromatic compounds do not have to be fragrant

“The term aromatic is usually associated with the smell of a compound. In chemistry, however, it also refers to compounds that form a ring in which electrons can be distributed. This makes rings more stable, which is why we estimates that about two-thirds of the chemical compounds known today are wholly or partly aromatic,” says Kretschmer.

Aromaticity is one of the most fundamental concepts in chemistry and dates back to the German chemist August Kekulé. Since its seminal work in 1865, researchers have been trying to find a general definition and continue to discover new types of aromaticity. For a molecule to be considered aromatic, it must distribute at least two electrons through the ring. In most cases, however, there are many more, varying between six and, so far, a maximum of 162 electrons. While the record for the maximum number of electrons has been steadily set, it was previously assumed that the minimum required two electrons could only stabilize rings with a maximum of four atoms. Indeed, the larger a ring, the fewer additional electrons available per atom, which is why the stabilizing effect becomes smaller and smaller.

Surprising conclusion: two electrons can stabilize rings with five atoms

The team led by Professor Robert Kretschmer, who recently took over the chair of inorganic chemistry at the Chemnitz University of Technology, has now succeeded in shifting this limit. They synthesized a compound that contains a planar ring of five gallium atoms.

“When we saw the crystal structure, we were quite impressed,” says Kretschmer, “because the fact that the ring is flat and the bonds inside the ring have similar spacing are already clear indications of aromatic character. of the molecule.”

Spectroscopic studies then provided further evidence of its aromatic nature. In cooperation with the research groups of Oliver Dumele in Berlin and Florian Weigend in Marburg, the new molecule was also studied by computer. The result: “The compound is aromatic, albeit weakly, but that’s to be expected if two electrons are to be spread over five ring atoms,” Kretschmer explains.

Then the researchers from Berlin, Chemnitz and Marburg want to study the reactivity of the new compound in more detail and use it for the synthesis of new complexes.

Largest aromatic ring size record broken with 162 π electrons

More information:
Oleksandr Kysliak et al, A five-membered planar aromatic ring stabilized by only two π electrons, International Edition of Applied Chemistry (2022). DOI: 10.1002/anie.202206963

Provided by Chemnitz University of Technology

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