2,2,6,6-Tetra­kis(biphenyl-2-yl)-4,4,8,8-tetra­methyl­cyclo­tetra­siloxane

Erik P A Couzijn; Martin Lutz; Anthony L Spek; Koop Lammertsma

doi:10.1107/S1600536809031961

. 2009 Aug 19;65(Pt 9):o2182–o2183. doi: 10.1107/S1600536809031961

2,2,6,6-Tetrakis(biphenyl-2-yl)-4,4,8,8-tetramethylcyclotetrasiloxane

Erik P A Couzijn ^a, Martin Lutz ^b, Anthony L Spek ^b,^*, Koop Lammertsma ^a

PMCID: PMC2969922 PMID: 21577587

Abstract

The title compound, [–Si(C₁₂H₉)₂OSi(CH₃)₂O–]₂, was obtained unintentionally as the product of an attempted crystallization of caesium bis(biphenyl-2,2′-diyl)fluorosilicate from dimethylformamide. In the crystal, the molecule is located on an inversion center and the siloxane ring adopts a twist-chair conformation with the two dimethyl-substituted Si atoms lying 0.7081 (5) Å out of the plane defined by the two bis(biphenyl-2-yl)-substituted Si atoms and the four O atoms. In each Si(C₁₂H₉)₂ unit, the orientation of one terminal phenyl ring relative to the phenylene ring of the other biphenyl moiety suggests a parallel displaced π–π stacking interaction [centroid distance = 4.2377 (11) Å and dihedral angle = 15.40 (9)°].

Related literature

For general background to stable compounds of pentavalent, anionic silicon bearing five organic substituents, see: Couzijn et al. (2004 ▶, 2006 ▶, 2009 ▶); Deerenberg et al. (2002 ▶); de Keijzer et al. (1997 ▶). For related structures, see: Malinovskii et al. (2007 ▶); Steinfink et al. (1955 ▶); Hensen et al. (1997 ▶). For puckering analysis,, see: Evans & Boeyens (1989 ▶). Bis(biphenyl-2,2′-diyl)silane was synthesized using a slight modification of a literature procedure (Gilman & Gorsich, 1958 ▶).

Experimental

Crystal data

C₅₂H₄₈O₄Si₄
M _r = 849.26
Orthorhombic,
a = 17.3418 (2) Å
b = 14.6488 (2) Å
c = 17.9584 (2) Å
V = 4562.09 (10) Å³
Z = 4
Mo Kα radiation
μ = 0.18 mm⁻¹
T = 110 K
0.30 × 0.12 × 0.03 mm

Data collection

Nonius KappaCCD diffractometer
Absorption correction: none
68467 measured reflections
4317 independent reflections
3247 reflections with I > 2σ(I)
R _int = 0.081

Refinement

R[F ² > 2σ(F ²)] = 0.036
wR(F ²) = 0.097
S = 1.07
4317 reflections
273 parameters
H-atom parameters constrained
Δρ_max = 0.26 e Å⁻³
Δρ_min = −0.33 e Å⁻³

Data collection: COLLECT (Nonius, 1999 ▶); cell refinement: HKL-2000 (Otwinowski & Minor, 1997 ▶); data reduction: HKL-2000 and SORTAV (Blessing, 1997 ▶); program(s) used to solve structure: SIR97 (Altomare et al., 1999 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: PLATON (Spek, 2009 ▶); software used to prepare material for publication: manual editing of SHELXL output.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809031961/vm2002sup1.cif

e-65-o2182-sup1.cif^{(23.2KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536809031961/vm2002Isup2.hkl

e-65-o2182-Isup2.hkl^{(211.6KB, hkl)}

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

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

Si1—O1	1.6287 (12)
Si1—O2	1.6290 (13)
Si1—C12	1.8684 (19)
Si1—C11	1.8746 (17)
Si2—O2ⁱ	1.6342 (13)
Si2—O1	1.6347 (12)
Si2—C2	1.8452 (19)
Si2—C1	1.8494 (19)

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O1—Si1—O2	109.98 (7)
O2ⁱ—Si2—O1	107.88 (7)
Si1—O1—Si2	141.85 (8)
Si1—O2—Si2ⁱ	142.12 (8)

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O1—Si2—O2ⁱ—Si1ⁱ	−74.36 (14)
O2—Si1—O1—Si2	−52.51 (15)
O2ⁱ—Si2—O1—Si1	84.84 (14)
O1—Si1—O2—Si2ⁱ	−37.42 (15)
C11—C61—C71—C81	81.5 (2)
C12—C62—C72—C82	56.1 (3)

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Symmetry code: (i) Inline graphic .

Acknowledgments

Financial assistance for this project was provided by the Dutch Organization for Scientific Research, Chemical Sciences (NWO-CW).

supplementary crystallographic information

Comment

Our research is focused on stable compounds of pentavalent, anionic silicon bearing five organic substituents (Couzijn et al., 2009; Couzijn et al., 2006; Couzijn et al., 2004). Such pentaorganosilicates are commonly proposed as intermediates for nucleophilic substitution reactions on silanes, but were only recently characterized in the condensed phase (e.g., de Keijzer et al., 1997; Deerenberg et al., 2002). In solution, these five-coordinate species undergo intramolecular substituent interchange via Berry pseudorotation and related processes. For a better understanding of the influence of the substituents on the stereomutational barrier, we synthesized bis(biphenyl-2,2'-diyl)fluorosilicate as the caesium and tetramethylammonium salts (Couzijn et al., 2009). While attempting to crystallize these salts, we obtained crystals of the title compound, [–Si(C₁₂H₉)₂OSi(CH₃)₂O–]₂, instead. This siloxane was most probably formed by reaction with silicone grease and adventitious water.

The title compound crystallizes with C_i point group symmetry, adopting a twist-chair conformation of the eight-membered siloxane ring (Fig. 1, Table 1). A ring puckering analysis (Evans & Boeyens, 1989) shows that the out-of-plane displacements in the ring can be described as a linear combination of the E_3g (sin form) and E_3g (cos form) normal modes, respectively, in a ratio of 0.891:0.109. The two bis(biphenyl)-substituted silicon atoms and the four oxygen atoms lie in a plane (RMS deviation 0.025 Å), whereas the two dimethyl-substituted silicon atoms are situated at 0.7081 (5)Å above and below this plane, respectively. Similar arrangements have been reported for octamethyl- (Steinfink et al., 1955) and 2,2,6,6-tetraphenyl-4,4,8,8-tetramethyltetrasiloxane (Malinovskii et al., 2007). Each Si(C₁₂H₉)₂ unit features a parallel displaced π-π stacking interaction between one terminal phenyl ring and the phenylene ring of the other biphenyl moiety. The ring centroids are 4.2377 (11)Å apart and their vector makes an angle of 38.65° with the phenylene plane, while the ring planes make a dihedral angle of 15.40 (9)°.

Experimental

General procedures: dimethylformamide (DMF) was distilled from phenylzinc iodide and stored in a glovebox on 3Å molecular sieves. Commercial caesium fluoride was dried in vacuo at > 373 K. Bis(biphenyl-2,2'-diyl)silane was synthesized from 1,1'-dibromobiphenyl and tetrachlorosilane using a slight modification of a literature procedure (Gilman & Gorsich, 1958).

The title compound was obtained as follows. In the purified nitrogen atmosphere of a glovebox, a flame-dried Schlenk tube was charged with caesium fluoride (71.45 mg, 470 µmol) and bis(biphenyl-2,2'-diyl)silane (73.95 mg, 222 µmol). Anhydrous DMF (3 ml) was added and the mixture was stirred in the glovebox for 2 days. ¹⁹F and ¹H NMR indicated almost quantitative conversion to the desired caesium bis(biphenyl-2,2'-diyl)fluorosilicate (Couzijn et al., 2009). The clear colorless supernate was filtered through glass wool in the glovebox and evaporated on a Schlenk line to afford the crude fluorosilicate as a white solid. Recrystallization of the fluorosilicate from DMF was initially performed by cooling the Schlenk tube in a freezer outside the glovebox. After repeated attempts, colorless blocks were obtained that were shown by X-ray analysis to be the title compound. Recrystallizations of the fluorosilicate using Young-type glassware (closed with a greaseless Teflon tap) in the freezer of a glovebox invariably afforded an amorphous solid.