2,3,5,6-Tetrafluoro-1,4-bis(trimethylsilyl)benzene

The asymmetric unit of the title compound, C12H18F4Si2, contains two independent molecules, both lying on inversion centers. The Carene—Si distances are significantly longer than in the analogous non-fluorinated compound. The packing of the molecules results in a herringbone motif in the ac plane.

The asymmetric unit of the title compound, C 12 H 18 F 4 Si 2 , contains two independent molecules, both lying on inversion centers. The C arene -Si distances are significantly longer than in the analogous non-fluorinated compound. The packing of the molecules results in a herringbone motif in the ac plane.
The 1,4-bis(trimethylsilyl)tetrafluorobenzene molecules are arranged in the ac plane to form a herringbone structure ( Figure 2). The trimethylsilyl groups are interlinked by van der Waals interactions with methyl groups of neighboring molecules.

Experimental
The starting material poly(cadmium-2,3,5,6-tetrafluorobenzene), [1,4-Cd-C 6 F 4 ] n , was synthesized by thermolysis of Cd(1,4-O 2 C-C 6 F 4 ) at 270 °C under vacuum according to a literature procedure (Sartori & Frohn, 1974). 7.1 g (27.3 mmol) poly(cadmium-2,3,5,6-tetrafluorobenzene), [1,4-Cd-C 6 F 4 ] n , was charged into a Duran-glass Carius tube inside a glove box. 6.55 g (60.3 mmol) freshly distilled (CH 3 ) 3 SiCl was added under protection of dry nitrogen. The tube was sealed and shaken and heated inside an oven. The temperature was increased stepwise over 30 h to 222°C without visual change of the reaction components. Further heating from 230 to 250°C over 26 h was accompanied by a reduction of the liquid phase and a change of the color to light grey. The Carius tube was cooled stepwise to -78°C before opening under nitrogen protection. CAUTION: Handling of the sealed Carius tube should proceed behind a large protection screen with long-sleeve leather gloves. At 0°C 3.4 g (31.3 mmol) of (CH 3 ) 3 SiCl were recovered by supplementary materials condensation under high-vacuum. The dark grey solid residue, which contained the co-product CdCl 2 , was extracted with boiling petrol ether (60-70°C fraction). After removing the solvent from the extract a slightly brownish oil remained which was sublimed under high vacuum. The colorless crystals were collected on a water cooled sublimation finger.

Refinement
Methyl H atoms were identified in a difference map, idealized and refined using rigid groups allowed to rotate about the Si-C bond (AFIX 137 option of the SHELXL97 program). All U iso (H) values were refined unrestrictedly.

Figure 1
The two crystallographically independent molecules of the title compound (H-atoms are drawn with arbitrary radii; ′ = -x, 1 -y, -z; ′′ = 1 -x, 2 -y, 1 -z).   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.39 e Å −3 Δρ min = −0.22 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. 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 > σ(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.