Metallic five-membered ring pushes the boundaries of aromaticity

The university of technology in Chemnitz.

A molecule that should not exist has been synthesised by researchers.

If there are only two electrons in the aromatic ring, how many atoms can it hold? The answer has been changed. A chemistry team has synthesised and described a five-membered aromatic compound for the first time. They achieved what had been attempted for a long time and had been considered impossible. The research groups were led by professors from the Chemnitz University of Technology.

It is possible that aromatic compounds do not need to be fragrant.

The term aromatic is used to describe the smell of a compound. It stands for compounds that form a ring in chemistry. It is estimated that around two-thirds of the chemical compounds are fully or partially aromatic.

One of the most fundamental concepts in chemistry is aromaticity. Researchers have been trying to find a general definition since 1865 when his seminal work was published. At least two electrons must be distributed across the ring for a molecule to be considered aromatic. There are between six and a maximum of 161 electrons in most cases. The record for the maximum number of electrons has been raised many times, but it was previously thought that only two electrons could stable rings with a maximum of four atoms. The stabilizing effect becomes smaller and less effective when the ring gets larger.

The conclusion is surprising: two electrons can hold the rings together.

This boundary has been shifted by the team led by Prof. Robert Kretschmer, who recently took over the professorship of Inorganic Chemistry at Chemnitz University of Technology. There is a ring of five gallium atoms in the compound they created.

The ring is flat and the bonds within it have similar spacing, which are clear indications of the aromatic character of the molecule.

Further studies showed that it was aromatic. The new molecule was investigated using a computer. If two electrons have to be distributed over five ring atoms, the compound is aromatic.

Researchers in Berlin, Chemnitz and Marburg want to study the reactivity of the new compound and use it for the synthesis of novel complexes.

More information: Oleksandr Kysliak et al, A Planar Five‐Membered Aromatic Ring Stabilized by Only Two π‐Electrons, Angewandte Chemie International Edition (2022). DOI: 10.1002/anie.202206963 Journal information: Angewandte Chemie International Edition Provided by Chemnitz University of Technology