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. Editor's summary: Unlike carbon, silicon naturally tends to produce networks and clusters held together exclusively with single bonds. That hasn't stopped chemists from artificially coaxing the heavier element into constrained geometries that induce the formation of double or triple bonds. Iwamoto et al. and Ankur et al. independently reported syntheses of negatively charged five-membered silicon rings directly analogous to the well-known cyclopentadienide ligands common in organometallic chemistry. The silicon rings were characterized in solid state and solution and showed signs of some degree of aromaticity in their electronic structure. —Jake S. Yeston [ABSTRACT FROM AUTHOR]