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A Planar Rhombic Charge-Separated Tetrasilacyclobutadiene

A stable silicon analog of a reactive, antiaromatic hydrocarbon has been synthesized and characterized. The cyclobutadiene (CBD) molecule C4H4 deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si4(EMind)4 (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtained as air- and moisture-sensitive orange crystals by the reduction of (EMind)SiBr3 with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si4 four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large 29Si chemical shift differences (Δδ > 350 parts per million) in the solid-state nuclear magnetic resonance spectra suggest a contribution of an alternately charge-separated structure. The rhombic-shaped charge-separated singlet state of compound 1 thus stabilizes its cyclic 4π-electron antiaromaticity in a manner that contrasts sharply with the bond-length alternation, characterizing the rectangular distortion of carbon-based CBD.