Move over cyclopentadiene anion—there’s a new five-membered aromatic ring in town, and this one is made of silicon. Two research teams working independently report the first examples of pentasilacyclopentadienide—an all-silicon version of cyclopentadienides (Science 2026, DOI: 10.1126/science.aed1802 and 10.1126/science.aed0168).
Making pentasilacyclopentadienide has been a decades-long goal for both Saarland University’s David Scheschkewitz and Tohoku University’s Takeaki Iwamoto, who independently led the research efforts to make these molecules. Although they used different synthetic strategies, both research teams constructed the same pentasilacyclopentadienide, which features bulky 2,4,6-triisopropylphenyl groups on each silicon and a lithium counterion.
"This is one of my dream compounds—the idea of this was with me through my entire independent career and even earlier than that."
“At the very beginning of my career, I proposed to prepare this compound in more than one grant application,” Scheschkewitz says. Most graduate students that came through his lab over the past 20 years attempted at least one experiment to synthesize the structure. Those efforts “failed without exception,” he says.
Then Ankur, a graduate student in Scheschkewitz’s lab who doesn’t use a surname, was trying to make a different compound when he serendipitously synthesized the pentasilacyclopentadienide by reducing 2,4,6-triisopropylphenylsilyl trichloride with potassium graphite in the presence of dilithium tetrakis(trimethylsilyl)cyclobutadiendiide. The chemists hypothesize that the dilithium compound acts as a template for the formation of the pentasilacyclopentadienide.
Scheschkewitz says he almost fainted when Ankur showed him the result. “This is one of my dream compounds—the idea of this was with me through my entire independent career and even earlier than that.”
Image generated from an X-ray crystal structure shows a molecule with a five-membered silicon ring at its core. This X-ray crystal structure of the pentasilacyclopentadienide synthesized by Takeaki Iwamoto’s group shows how the anion bonds to lithium and 2,6-dimethylphenyl isocyanide. Credit: Science
Iwamoto tells C&EN that he’s been interested in making stable π-conjugated silicon compounds since he was a graduate student 30 years ago. Chemists in Iwamoto’s group successfully prepared the pentasilacyclopentadienide via a stepwise approach, in which they start with a tetrasila-1,3-diene bearing 2,4,6-triisopropylphenyl groups on each silicon and trimethylsilyl groups at both ends of the molecule. Through several steps, they coax the transformation of this molecule into the pentasilacyclopentadienide.
Both groups were able to get crystals of the compound for X-ray analysis, which Iwamoto and Scheschkewitz say was the most challenging aspect of the project. Iwamoto’s team reports that the silicon ring is nonplanar “with some pyramidalized silicon atoms and uneven silicon-silicon distances” and that it has “some degree of aromaticity.” Scheschkewitz’s team reports the silicon ring to be “essentially planar and decidedly aromatic,” although both experimental and computational data suggest that it exists in equilibrium with nonplanar isomers.
Iwamoto and Scheschkewitz say pentasilacyclopentadienides could be ligands for catalysts and materials. The cyclopentadiene anion has a long history in these areas—for example, in compounds like ferrocene—but pentasilacyclopentadienides are larger and have bulky peripheral groups that could shield a coordinating metal.
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