Pentasilacyclopentadienide: A Hückel aromatic species at the border of resonance and equilibrium.

Aromatic rings such as the ubiquitous π-ligand cyclopentadienide (C5H5−) 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 (Si3R3+; R, silyl) has been experimentally realized. We report a stable persilacyclopentadienide (Si5R5−; R, aryl), which is essentially planar and decidedly aromatic, although experimental and computational data suggest an ultrarapid equilibrium between nonplanar isomers. With trimethylchlorostannane, Si5R5− rearranges the Si5 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. 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]