Carbon's unique chemical properties allow it to be an essential building block for life on Earth and many other molecules we rely on for day-to-day life - but what about carbon's neighbour? Silicon is located one row below carbon in the periodic table of elements, and similarly has many possible uses, and is a key component of semiconductors, silicon carbide fibers, and silicones. However, silicon has some key weaknesses compared to carbon.

For example, carbon forms very stable π-electron compounds (compounds linked by pi bonds, or π-bonds, which affect a molecule's reactivity) called benzene and fullerene. In comparison, silicon cannot readily form these compounds, as the π-bonds forming π-electron compounds are not strong in this element. Synthesizing such silicon-based π-electron compounds consequently becomes increasingly difficult as the number of silicon atoms increases. However, researchers at Tohoku University found a way to overcome these limitations.

A research group led by Professor Takeaki Iwamoto, Graduate Student Tomoki Ishikawa, and Associate Professor Shintaro Ishida at the Graduate School of Science, Tohoku University, has successfully synthesized π-electron compounds with a pentagonal silicon framework, 'pentasilacyclopentadienide', and elucidated their molecular structures.

'Pentasilacyclopentadienide' is a compound characterized by a nonplanar pentagonal skeleton. Despite its nonplanarity, it has delocalized π-electrons and aromaticity similar to a carbon-based planar molecule called cyclopentadienide. Given the utility of cyclopentadienide, this resemblance is reassuring.

"Cyclopentadienide is a highly useful molecule used widely in catalysts that speed up reactions and materials science research," remarks Iwamoto, "We are very excited to have created a similar, stable molecule using silicon."

Quantum chemical calculations were used to determine the underlying mechanism behind how this compound remains stable. The research team found that the delocalization of π-electrons on the silicon skeleton and the protecting groups on the silicon skeleton are the secret to stabilizing this compound.

The new silicon compounds emerging from this research may have the potential to unlock the latent physical properties, functions, and novel applications inherent in silicon - an abundant element with no risk of depletion.

And in a neat twist of fate, the very same compound was discovered almost simultaneously in Professor David Scheschkewitz's laboratory at Saarland University in Saarbruecken, Germany. By mutual agreement, the research teams in Japan and in Germany have published their findings side by side in the same issue of Science on February 6, 2026.

Publication Details:

Title: Silicon Cyclopentadienides Featuring a Nonplanar 6π Aromatic Si₅ Ring

Authors: Takeaki Iwamoto, Tomoki Ishikawa, Shintaro Ishida

Journal: Science

DOI: 10.1126/science.aed0168

Press release in Japanese