Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute, Wako, Saitama, Japan.
Science. 2011 Mar 11;331(6022):1306-9. doi: 10.1126/science.1199906.
The cyclobutadiene (CBD) molecule C(4)H(4) deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si(4)(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)SiBr(3) with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si(4) four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large (29)Si 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.
环丁二烯 (CBD) 分子 C(4)H(4) 偏离高度对称的正方形几何形状，以补偿其反芳香电子结构。在这里，我们报告了一种 CBD 的硅类似物，Si(4)(EMind)(4)（1），由体积庞大的 1,1,7,7-四乙基-3,3,5,5-四甲基-s-氢茚-4-基 (EMind) 基团稳定，通过三当量的锂萘基化物还原 (EMind)SiBr(3) 得到，得到橙色晶体。X 射线晶体学揭示了 Si(4) 四元环的平面和菱形结构，硅原子的交替为金字塔形和平面构型。固态核磁共振谱中较大的 (29)Si 化学位移差（Δδ>350 ppm）表明存在交替电荷分离结构的贡献。因此，化合物 1 的菱形电荷分离单重态以一种与键长交替形成鲜明对比的方式稳定其环状 4π 电子反芳香性，这与基于碳的 CBD 的矩形扭曲形成对比。
