Room-Temperature-Observable Interconversion Between Si(IV) and Si(II) via Reversible Intramolecular Insertion Into an Aromatic C−C Bond

An easily isolable silacycloheptatriene (silepin) 1 b was synthesized from the reaction of a N-heterocyclic imino (IPrN) substituted tribromosilane IPrNSiBr₃ with the sterically congested bis(trimethylsilyl)triisopropylsilyl silanide KSi(TMS)₂Si(ⁱPr)₃ (BTTPS). In solution, the Si(IV) silepin 1 b is in a thermodynamic equilibrium with the acyclic Si(II) silylene 1 a. The relative concentration of the Si(II) or Si(IV) isomers can be controlled by temperature variation and observed by variable temperature NMR and UV/Vis spectroscopy. DFT calculations show a small reaction barrier for the Si(II)⇌Si(IV) interconversion and a small energy gap between the Si(II) and Si(IV) species. The reactivity of 1 a/b is demonstrated on a variety of small molecules.