Pentasilacyclopentadienide: A Hückel aromatic species at the border of resonance and equilibrium.
basic_science · Level V
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- Record sourced from PubMed, PMID 41643004.
- Also identified by DOI 10.1126/science.aed1802.
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Abstract
Aromatic rings such as the ubiquitous π-ligand cyclopentadienide (C<sub>5</sub>H<sub>5</sub><sup>-</sup>) adhere to the famous Hückel rule with their 4n+2 π cyclically delocalized electrons. Since the advent of stable Si-Si π-bonds in 1981, all-silicon aromatics have been targeted, yet only a three-membered cyclopropenium analog (Si<sub>3</sub>R<sub>3</sub><sup>+</sup>; R, silyl) has been experimentally realized. We report a stable persilacyclopentadienide (Si<sub>5</sub>R<sub>5</sub><sup>-</sup>; R, aryl), which is essentially planar and decidedly aromatic, although experimental and computational data suggest an ultrarapid equilibrium between nonplanar isomers. With trimethylchlorostannane, Si<sub>5</sub>R<sub>5</sub><sup>-</sup> rearranges the Si<sub>5</sub> core to a tricyclic isomer confirming the pivotal role of the lithium cation for stability. The persilacyclopentadienide promises a rich chemistry akin to carbon-based cyclopentadienides. Its structural flexibility questions the paradigm of a clear-cut distinction between resonance and equilibrium.