As part of an international team, researchers from the Karlsruhe Institute of Technology (KIT) have synthesized an extraordinary aromatic cycle. Consisting of three metal atoms, it is the heaviest example of this type of molecule: A triangle made from three bismuth atoms is held between two metal complexes in a structure known as "inverse‑sandwich" complex. This discovery contributes to a better understanding of aromaticity in heavy elements. In addition, the study might pave the way for new functional materials. The results have been published in Nature Chemistry. (DOI: 10.1038/s41557-026-02123-8 )
Aromatic modules are characterized by a ring shape and a special bonding situation that allows electrons to move freely around the ring if positioned in a magnetic field. The increased stability brought about by the resulting ring currents opens up interesting prospects for various applications of aromatic molecules. "For a long time, aromatic molecules were known only in organic chemistry, in the form of ring-shaped carbon compounds. Benzene, present in gasoline, is the prototypic aromatic molecule and a key component in the production of synthetics," said Professor Stefanie Dehnen, Director of KIT's Institute of Nanotechnology (INT) and head of the Cluster-Based Materials research group established there. "But aromatic molecules can also occur in combination with other nonmetal atoms or as metal-organic compounds. Recently, researchers have also been able to synthesize aromatic molecules composed of only metal atoms."
Now, a collaboration of Dehnen's research group at KIT with a team from the University of Manchester and several Chinese research organizations has led to a breakthrough in the field of all-metal aromaticity: The international team was able to achieve a stable integration of the heaviest aromatic three-membered bismuth ring to date into a complex molecule.
Stabilizing Ring Currents Found in the Bismuth Ring
The researchers trapped a ring consisting of three bismuth atoms between two other heavy metal atoms - uranium or thorium - in a so-called inverse-sandwich complex. X-ray crystallography confirmed the shape and the symmetric structure of the triangular bismuth-atom cycle. Magnetic measurements, spectroscopy, and advanced computations revealed magnetically induced ring currents that stabilize the bismuth ring. The study thus provides groundbreaking new insights on aromaticity beyond organic chemistry and shows that aromatic three-membered rings consisting solely of metal atoms can occur in unusual clusters of heavy-metal atoms.
Discovery Might Pave the Way for New Functional Materials
The researchers led by Dehnen and Professor Florian Weigend, Head of the Molecular Quantum Systems research group at KIT's Institute for Quantum Materials and Technologies (IQMT), have been collaborating for several years in the field of all-metal aromaticity. Within the scope of this project, they have partnered with a research team led by Professor Stephen T. Liddle from the University of Manchester. For the recent study, Dehnen's research group provided bismuth chemicals, while Weigand's team contributed quantum chemistry calculations on the bonding conditions and ring currents. "Our work bridges the gap between organic chemistry and the chemistry of metallic semiconductors," said Dehnen. "It helps us to understand aromaticity in heavy metal clusters and might pave the way for new functional materials, for example intermetallic compounds, nano-scale semiconductor components, or catalysts."
The study is closely related to the HEiKA STAR - DEUsAroMet project (short for All-Metal Aromaticity: Definition, Evidence, Use) coordinated by KIT and the University of Heidelberg, an interdisciplinary collaboration of scientists investigating all-metal compounds with aromatic properties. The goal is to establish a theoretical and experimental basis for the research of potential applications in nanotechnology, energy technology, or quantum computing.
Original publication
Junru Ding, John A. Seed, Katrin Beuthert, Benjamin Peerless, Julia Rienmüller, Andreas Schmidt, Ashley J. Wooles, Louise S. Natrajan, Chuan-Ling Chen, Zhong-Ming Sun, Florian Weigend, Stefanie Dehnen, Jingzhen Du & Stephen T. Liddle: All-metal aromaticity of cyclo-Bi33− in diuranium and dithorium inverse-sandwich-type complexes. Nature Chemistry, 2026. DOI: 10.1038/s41557-026-02123-8
