Chemists Synthesize Elusive Aromatic 5-Silicon Rings

Two independent research teams report the first examples of pentasilacyclopentadienide, a long-sought silicon-based aromatic compound.

Published on Feb. 9, 2026

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Researchers at Saarland University and Tohoku University have independently synthesized the first examples of pentasilacyclopentadienide, an all-silicon version of cyclopentadienides that has been a decades-long goal for both teams. The chemists used different synthetic strategies but constructed the same pentasilacyclopentadienide molecule, which features bulky 2,4,6-triisopropylphenyl groups on each silicon and a lithium counterion. Both teams were able to obtain crystals of the compound for X-ray analysis, which revealed the silicon ring is either essentially planar and aromatic or has some pyramidalized silicon atoms and uneven silicon-silicon distances with some degree of aromaticity.

Why it matters

Pentasilacyclopentadienides could serve as ligands for catalysts and materials, similar to how the cyclopentadiene anion has been used. The larger size and bulky peripheral groups of the silicon-based compounds could provide new opportunities to shield coordinating metals. The differences between these silicon compounds and their carbon counterparts also provide new insights into the relationship between structure, bonding, and physical properties beyond just carbon.

The details

The two research teams used different synthetic strategies to construct the pentasilacyclopentadienide. Saarland University's David Scheschkewitz's graduate student Ankur serendipitously synthesized the compound by reducing 2,4,6-triisopropylphenylsilyl trichloride with potassium graphite in the presence of dilithium tetrakis(trimethylsilyl)cyclobutadiendiide, which may have acted as a template. Tohoku University's Takeaki Iwamoto's group took a stepwise approach, starting with a tetrasila-1,3-diene bearing 2,4,6-triisopropylphenyl groups on each silicon and trimethylsilyl groups at both ends, and then transforming this molecule into the pentasilacyclopentadienide.

  • Scheschkewitz has been interested in making this compound since the beginning of his independent career, over 20 years ago.
  • Iwamoto has been interested in stable π-conjugated silicon compounds since he was a graduate student 30 years ago.

The players

David Scheschkewitz

A professor at Saarland University who has been trying to synthesize pentasilacyclopentadienide for over 20 years.

Takeaki Iwamoto

A professor at Tohoku University who has been interested in stable π-conjugated silicon compounds since he was a graduate student 30 years ago.

Ankur

A graduate student in Scheschkewitz's lab who serendipitously synthesized the pentasilacyclopentadienide while trying to make a different compound.

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What they’re saying

“This is one of my dream compounds—the idea of this was with me through my entire independent career and even earlier than that.”

— David Scheschkewitz, Professor (C&EN)

“The true beauty of this work lies in their exploration of how and why these carbon and silicon ring structures are different, as these nuances create new understanding for how structure, bonding, and physical properties are related for elements beyond carbon.”

— Timothy A. Su, Chemistry Professor, University of California, Riverside (Email)

“The pentasilacyclopentadienides 'are long sought 'dream' molecules that were theoretically predicted more than three decades ago.' Their synthesis 'challenges the limits of bonding theory to explain their existence and nonclassical bonding nature.'”

— Vladimir Ya. Lee (Email)

What’s next

The researchers plan to further study the properties and potential applications of pentasilacyclopentadienides as ligands for catalysts and materials.

The takeaway

The successful synthesis of pentasilacyclopentadienide, a long-sought aromatic silicon compound, represents a major breakthrough in the field of silicon chemistry and opens up new possibilities for the development of silicon-based materials and catalysts.