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

Maik Finze; Guido J Reiss; Hermann-Josef Frohn

doi:10.1107/S1600536812010677

. 2012 Mar 17;68(Pt 4):o1082. doi: 10.1107/S1600536812010677

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

Maik Finze ^a,^*, Guido J Reiss ^b, Hermann-Josef Frohn ^c

PMCID: PMC3344037 PMID: 22589946

Abstract

The asymmetric unit of the title compound, C₁₂H₁₈F₄Si₂, 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.

Related literature

For the synthesis and chemistry of 1,4-(Me₃Si)₂—C₆F₄, see: Fearon & Gilman (1967 ▶); Tamborski & Soloski (1969 ▶); Fields et al. (1970 ▶); Sartori & Frohn (1974 ▶); Bardin et al. (1991 ▶); Frohn et al. (1998 ▶); Kashiwabara & Tanaka (2006 ▶). For related structures see: Rehm et al. (1999 ▶); Sekiguchi et al. (2000 ▶); Haberecht et al. (2002 ▶, 2004 ▶); Krumm et al. (2005 ▶); Hanamoto et al. (2006 ▶).

Experimental

Crystal data

C₁₂H₁₈F₄Si₂
M _r = 294.44
Monoclinic,
a = 19.8389 (4) Å
b = 6.35013 (10) Å
c = 12.3827 (2) Å
β = 107.407 (2)°
V = 1488.53 (5) Å³
Z = 4
Mo Kα radiation
μ = 0.26 mm⁻¹
T = 199 K
0.25 × 0.22 × 0.20 mm

Data collection

Oxford Xcalibur Eos diffractometer
8888 measured reflections
2626 independent reflections
2496 reflections with I > 2σ(I)
R _int = 0.015

Refinement

R[F ² > 2σ(F ²)] = 0.027
wR(F ²) = 0.068
S = 1.09
2626 reflections
187 parameters
H-atom parameters constrained
Δρ_max = 0.39 e Å⁻³
Δρ_min = −0.22 e Å⁻³

Data collection: CrysAlis PRO (Oxford Diffraction, 2009 ▶); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 2011) ▶; software used to prepare material for publication: publCIF (Westrip, 2010 ▶).

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536812010677/mw2055sup1.cif

e-68-o1082-sup1.cif^{(17.5KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536812010677/mw2055Isup2.hkl

e-68-o1082-Isup2.hkl^{(129KB, hkl)}

Supplementary material file. DOI: 10.1107/S1600536812010677/mw2055Isup3.cml

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected geometric parameters (Å, °).

Si1—C2	1.9101 (15)
Si2—C8	1.9077 (15)

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Acknowledgments

This publication was funded by the German Research Foundation (DFG) and the University of Würzburg under the funding programme Open Access Publishing.

supplementary crystallographic information

Comment

The first synthesis of the title compound 1,4-bis(trimethylsilyl)tetrafluorobenzene, 1,4-(Me₃Si)₂—C₆F₄, was reported in 1967 starting from 1,2,4,5-tetrafluorobenzene, n-butyl lithium, and trimethylsilyl chloride (Fearon & Gilman 1967). Later, the compound was observed as a by-product in related reactions, improved methods for its selective synthesis were published and some reactions of 1,4-(Me₃Si)₂—C₆F₄ were described (Tamborski & Soloski, 1969; Fields et al. 1970; Sartori & Frohn, 1974; Bardin et al. 1991; Frohn et al. 1998; Kashiwabara & Tanaka, 2006). The 1,4-(Me₃Si)₂—C₆F₄ employed in the present study was prepared by a different route using poly(cadmium-2,3,5,6-tetrafluorobenzene), [1,4-Cd—C₆F₄]_n, and trimethylsilyl chloride as starting materials.

The title compound 1,4-bis(trimethylsilyl)tetrafluorobenzene (Figure 1) crystallizes in the monoclinic space group P2₁/c with two independent molecules each of which is located on a center of symmetry. Both crystallographically independent molecules display very similar geometric parameters. The C–C, C_methyl–Si and C–F bond lengths are in the expected range. Both C_arene–Si distances are slightly longer than those found for the non-fluorinated analogue 1,4-(Me₃Si)₂—C₆H₄ [d(C_arene–Si) = 1.8817 (12) Å] (Haberecht et al. 2004) and other non-fluorinated Me₃Si–C_arene compounds (Haberecht et al. 2004; Rehm et al. 1999). In contrast, for related 1-trimethylsilyl-2,3,5,6-tetrafluoro benzene fragments similar values were reported (Sekiguchi et al. 2000, Krumm et al. 2005). In the closely related compound 1,2,4-(iPr₃Si)₃—C₆F₃ (Hanamoto et al. 2006) the d(C_arene–Si) of the iPr₃Si groups in ortho positions [d(C–Si) = 1.937 (2), 1.934 (2)] are significantly longer than those in the title compound whereas the third C_arene–Si distance [d(C–Si) = 1.914 (2) Å], which corresponds to the iPr₃Si group that has two F atoms in the ortho positions, is close to the values determined for 1,4-(Me₃Si)₂—C₆F₄.

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—C₆F₄]_n, was synthesized by thermolysis of Cd(1,4-O₂C—C₆F₄) 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—C₆F₄]_n, was charged into a Duran-glass Carius tube inside a glove box. 6.55 g (60.3 mmol) freshly distilled (CH₃)₃SiCl 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 (CH₃)₃SiCl were recovered by condensation under high-vacuum. The dark grey solid residue, which contained the co-product CdCl₂, 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. Yield ca 80%; mp 46°C [31–32°C isomeric mixture (Fields et al., 1970)]; NMR (20% CCl₄ solution): ¹H -0.39 p.p.m., ¹⁹F -124.2 p.p.m.; MS (EI, 73 eV) M^+. 294, the fragment ions 81 (CH₃SiF₂), 77 ((CH₃)₂SiF), and 73 ((CH₃)₃Si) possessed higher intensities than the parent ion; IR (neat): 2946 (m), 2889 (m), 1575 (w), 1487 (w), 1400 (st, b), 1343 (w), 1334 (w), 1286 (w), 1240 (st), 1214 (st), 1183 (m), 1035 (w), 921 (st), 830 (st, b), 748 (st), 684 (m), 613 (m), 566 (w), 432 (w).