Isolation of RSi Dianion: A Bridged Tricyclic Isomer of Dianionic Hexasilabenzene.

A new strategy for the highly selective synthesis of tricyclo[2,2,0,0]hexasilanes RSiX (R = 2,4,6-MeCH; X = H, Cl) and a bridged tricyclic RSi dianion starting from the tetrachlorotrisilane RClSiSi(H)RSiClR (1) was described. Reduction of 1 with lithium naphthalene afforded tricyclohexasilane RSiH (2), which was halogenated to give the dichloride RSiCl (3). Reduction of 3 with four equivalents of potassium graphite in the presence of 18-crown-6 afforded the first RSi dianion (5) paired with [K(18-crown-6)] counterions. The dianion 5 could act as a two-electron reductant toward transition metal halides and a nucleophile toward chlorosilanes. These reactions allowed the efficient and selective access to three types of silicon cages. The structures of the representative cages were confirmed by X-ray diffraction studies. Density functional theory calculations on 5 indicate that the negative charges are localized mainly on the anionic silicon atoms.