anti-2,2,3,3,6,6,7,7,10,10,11,11,14,14,15,15-Hexadecamethyl-2,3,6,7,10,11,14,15-octasilapentacyclo[10.4.2.24,9.05,8.013,16]icosa-1(17),4,8,12(18),13(16),19-hexaene

The title compound, C28H52Si8, was synthesized by condensation of two molecules of 1,2,3,4-tetrakis(chlorodimethylsilyl)benzene with lithium. The 3,4-disila-1,2-benzocyclobutene rings in the centrosymmetric molecule are bridged by 1,1,2,2-tetramethyldisilanylene chains with an anti conformation. The benzene rings are deformed by fusion with a 3,4-disilacyclobutene ring resulting in a slight boat conformation. Two Si—C bonds are bent to reduce the steric repulsion between the methyl groups on the two Si atoms and the methyl groups on another two Si atoms.

The title compound, C 28 H 52 Si 8 , was synthesized by condensation of two molecules of 1,2,3,4-tetrakis(chlorodimethylsilyl)benzene with lithium. The 3,4-disila-1,2-benzocyclobutene rings in the centrosymmetric molecule are bridged by 1,1,2,2-tetramethyldisilanylene chains with an anti conformation. The benzene rings are deformed by fusion with a 3,4disilacyclobutene ring resulting in a slight boat conformation. Two Si-C bonds are bent to reduce the steric repulsion between the methyl groups on the two Si atoms and the methyl groups on another two Si atoms.

Experimental
The condensation of two molecules of 1,2,3,4-tetrakis(chlorodimethylsilyl)benzene with lithium in THF gave 1 in 2% yield (Fig. 1). The structure of 1 was determined by X-ray crystallography (Fig. 2). The molecule lies on an inversion center, and one half of the molecule corresponds to the asymmetric unit. Two 3,4-disila-1,2-benzocyclobutene rings are bridged by 1,1,2,2-tetramethyldisilanylene chains with an anti structure. The anti structure is favorable to avoid the steric hindrance among methyl groups on the 3,4-disilacyclobutene rings.

Experimental
All operations except for Kugelrohr distillation were carried out in a glovebox. A mixture of 1,2,3,4-tetrakis(chlorodimethylsilyl)benzene (0.200 g, 0.446 mmol) and lithium (13.0 mg, 1.87 mmol) in THF (25 ml) was stirred at room temperature for 14 h. After removal of the solvent, the residue was dissolved in toluene, and insoluble materials were filtered off. The solvent was removed under reduced pressure. Kugelrohr distillation (300 °C/0.9 mm H g) of the residue gave a colorless solid. The solid was recrystallized from hexane to give 1 (3 mg, 2%) as colorless crystals. Single crystals were obtained from hexane by slow evaporation.

Refinement
All hydrogen atoms were generated at calculated positions and refined as riding atoms with C-H = 0.95 (phenyl) or 0.98 (methyl) Å and U iso (H) = 1.2U eq (phenyl C) or 1.5U eq (methyl C).       -2,2,3,3,6,6,7,7,10,10,11,11,14,14,15,15-Hexadecamethyl-2,3,6,7,10,11,14 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq