Silicon cyclopentadienides featuring a nonplanar 6π aromatic Si₅ ring

Compared with the wide variety of reports on carbon π-electron compounds, the silicon counterparts remain scarce because of the intrinsic preference of silicon to form σ-bonded compounds. In particular, silicon compounds in which π-electrons are delocalized over five or more silicon atoms have been elusive. We report the synthesis of pentasilicon analogs of cyclopentadienides (pentasilacyclopentadienides). These compounds contain nonplanar five-membered silicon rings with some pyramidalized silicon atoms and uneven silicon-silicon distances. The highly shielded ⁷Li nuclear magnetic resonance signal of a lithium pentasilacyclopentadienide corroborates the presence of a diamagnetic ring current in the five-membered ring, which is indicative of at least some degree of aromaticity. Furthermore, a computational study revealed that bulky substituents and the delocalization of π-electrons stabilize the pentasilacyclopentadienide structure.