Isolation of R6Si6 Dianion: A Bridged Tricyclic Isomer of Dianionic Hexasilabenzene

A new strategy for the highly selective synthesis of tricyclo­[2,2,0,02,5]­hexasilanes R6Si6X2 (R = 2,4,6-Me3C6H2; X = H, Cl) and a bridged tricyclic R6Si6 dianion starting from the tetrachlorotrisilane RCl2SiSi­(H)­RSiCl2R (1) was described. Reduction of 1 with lithium naphthalene afforded tricyclohexasilane R6Si6H2 (2), which was halogenated to give the dichloride R6Si6Cl2 (3). Reduction of 3 with four equivalents of potassium graphite in the presence of 18-crown-6 afforded the first R6Si6 dianion (5) paired with [K­(18-crown-6)]+2 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.