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

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)°].

The title compound,  ) 2 OSi(CH 3 ) 2 O-] 2 , was obtained unintentionally as the product of an attempted crystallization of caesium bis(biphenyl-2,2 0 -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(C 12 H 9 ) 2 unit, the orientation of one terminal phenyl ring relative to the phenylene ring of the other biphenyl moiety suggests a parallel displacedstacking interaction [centroid distance = 4.2377 (11) Å and dihedral angle = 15.40 (9) ].
Financial assistance for this project was provided by the Dutch Organization for Scientific Research, Chemical Sciences (NWO-CW).

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(C 12 H 9 ) 2 OSi(CH 3 ) 2 O-] 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 eightmembered 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 2,2,6,6tetraphenyl-4,4,8,8-tetramethyltetrasiloxane (Malinovskii et al., 2007). Each Si(C 12 H 9 ) 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)°.
Anhydrous DMF (3 ml) was added and the mixture was stirred in the glovebox for 2 days. 19 F and 1 H 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 supplementary materials sup-2 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.