Crystal structures of 3,3′-bis(hydroxydimethylsilanyl)azobenzene and 4,4′-bis(hydroxydimethylsilane)azobenzene

In each of the crystal structures of the two title compounds, two molecules are found in the asymmetric unit. Individual molecules are linked by intermolecular O—H⋯O hydrogen bonding and show significant differences in the torsions about the N=N bond and the dihedral angle between the benzene rings.

The title compounds {systematic names (E)-[diazene-1,2-diylbis(3,1-phenylene)]bis(dimethylsilanol) and (E)-[diazene-1,2-diylbis(4,1-phenylene)]bis(dimethylsilanol)}, both of the sum formula C 16 H 22 N 2 O 2 Si 2 , were obtained by transmetallation of the respective bis-stannylated azobenzenes with dichlorodimethylsilane and esterification followed by hydrolysis. The asymmetric unit of 3,3 0 -diazenediylbis[dimethyl(phenyl)silanol] (with the silanol functional group in a meta position) consists of two molecules, of which one occupies a general position, whereas the second is located on a centre of inversion. In 4,4 0 -diazenediylbis[dimethyl(phenyl)silanol] (with the silanol functional group in a para position) likewise two molecules are present in the asymmetric unit, but in this case both occupy general positions. Differences between all molecules can be found in the torsions about the N N bond, as well as in the dihedral angles between the benzene rings. In both structures, intermolecular O-HÁ Á ÁO hydrogen bonding is observed, leading to the formation of layers parallel to (011) for (I) and to chains parallel to the a axis for (II).

Chemical context
Azobenzenes have been widely investigated as photoswitches due to their photochemically induced trans/cis-isomerization. Furthermore, they are common motifs in dyes due to their high thermal and photochemical stability (Yesodha et al., 2004;Lagrasta et al., 1997). Their application as molecular switches is sometimes limited by their synthetical accessability. For ortho, meta and para-substituted azobenzenes, a novel functionalization has been presented recently (Strü ben et al., 2014(Strü ben et al., , 2015. This opens access to new synthetic pathways and hence new dyes and materials, for example light-responsive polymers (Yu et al., 2003;Kizilkan et al., 2016).
In the above context, we report here on the synthesis and crystal structures of two regioisomers with composition ISSN 2056-9890 C 16 H 22 N 2 O 2 Si 2 , obtained by transmetalation of the respective bis-stannylated azobenzenes.

Supramolecular features
In the crystal structure of isomer (I), neighboring molecules are linked by intermolecular O-HÁ Á ÁO hydrogen bonding between the silylhydroxyl hydrogen atoms of the first independent molecules, forming chains that elongate in the a-axis direction (Fig. 3 top). These chains are further linked via O-HÁ Á ÁO hydrogen bonds to the second crystallographically independent molecules, forming layers that are parallel to (011) (Fig. 3, bottom, Table 1). The O-HÁ Á ÁO angles and OÁ Á ÁO contacts indicate that these are rather strong hydrogen bonds (Table 1). Between the layers, slippedinteractions [centroid-to-centroid distances 3.767 (2) and 3.811 (2) Å ] are present, consolidating the crystal packing. In isomer (II), the molecules are likewise linked by intermolecular O-HÁ Á ÁO hydrogen bonding into tetrameric units, which are further linked into chains that elongate in the a-axis direction (Fig. 4, top, Table 2). By this arrangement, 16-membered cyclic hydrogen-bonded motifs are formed that consist of eight alternating hydroxysilyl groups and that can be described as R 8 8 (16) according to the graph-set notation (Etter et al., 1990;Bernstein et al., 1995). As in isomer (I), the values of the O-HÁ Á ÁO angles and OÁ Á ÁO distances indicate rather strong hydrogen bonding (Table 2). These tetrameric chains are packed along the a axis in a pseudo-hexagonal arrangement (Fig. 4, bottom). The molecular structures of the two crystallographically independent molecules in the crystal structure of isomer (I) (a top and b bottom) with labelling and displacement ellipsoids drawn at the 50% probability level. Symmetry code for the generation of equivalent atoms: Àx + 2, Ày + 2, Àz + 2.

Database Survey
Hundreds of azobenze-based structures are found in the Cambridge Structural Database (Groom et al., 2016) but compounds with silanol groups are unknown (ConQuest Version 1.18, CSD Version 5.37). There are also no compounds reported with silyl groups in a meta or a para position but some compounds have been deposited in which both benzene rings are substituted in the ortho position by, e.g., trimethylsilyl, fluoro-dimethylsilyl, difluoro-methylsilyl or trifluorosilyl groups (Kano et al., 2001). It is noted that two structures are reported in which two azobenenzene molecules are bridged by

Synthesis and crystallization
The syntheses of 3,3 0 -bis(trimethylstannyl)azobenzene and 4,4 0 -bis(trimethylstannyl)azobenzene were described in the literature (Strü ben et al., 2014). For further details of the transmetallation, see: Strü ben et al. (2015). Dimethyldichlorosilane (99%) was purchased from ABCR Inc., degassed and distilled from calcium hydride. Methyl lithium (1.6 M in diethyl ether) was purchased from Acros Organics, monopotassium phosphate (99.7%) was purchased from Sigma-Aldrich, sodium methoxide (99%) from TCI Inc. and used without further purification. THF was purchased from Merck-Polaro and was dried and degassed with a PS-MD-5 by Innovation Technology. Methanol as obtained from BCD was distilled from sodium and was stored over molecular sieves (3 Å ). 3,3 0 0 0 -Bis(Hydroxydimethylsilane)azobenzene 3,3 0 -Bis(trimethylstannyl)azobenzene (3.80 g, 7.48 mmol) was dissolved in dry THF (100 ml). Then, at 195 K, methyl lithium (12.0 ml, 19.0 mmol, 1.6 M solution in diethyl ether) in THF (18.0 ml) was added and the mixture was stirred for 10 min at 195 K. Then the reaction was quenched with dichlorodimethylsilane (30.0 ml, 32.1 g, 249 mmol) and the reaction mixture allowed to warm to 298 K in a cooling bath. Subsequently the solvent and the excess of dichlorodimethylsilane were evaporated in inert conditions under reduced pressure. The residual orange solid was dissolved in diethyl ether (25 ml) and added dropwise over the course of 15 min to a solution of sodium methoxide (4.00 g, 74.0 mmol) in methanol (50 ml). Both of the latter steps were performed under inert conditions. To this mixture, a solution of sodium    Crystal structure of isomer (II) showing a view of the hydrogen-bonded hydroxide (17.5 g, 438 mmol) in methanol (105 ml) and water (10.0 ml) was added. The resulting solution was stirred for 15 minutes and then a further portion of sodium hydroxide (17.5 g, 438 mmol) in water (105 ml) was added. The reaction mixture was stirred for 1 h. This mixture was finally poured into a vigorously stirred solution of monopotassium phosphate (155 g, 1.14 mol) in water (200 ml). The orange precipitate was filtered and purified by three recrystallization cycles from diethyl ether/n-hexane (v/v 1:1). The final product was isolated as an orange solid in a yield of 500 mg (20%). Crystals were obtained by dissolving the product in chlroroform, adding a layer of n-hexane and allowing the n-hexane to diffuse into the chloroform, leading to crystal formation at the phase boundary. 4,4 0 0 0 -Bis(hydroxydimethylsilane)azobenzene 4,4 0 -Bis(trimethylstannyl)azobenzene (3.80 g, 7.48 mmol) was dissolved in dry THF (100 ml). A solution of methyl lithium (12.0 ml, 19.0 mmol, 1.6 M solution in diethyl ether) in THF (18.0 ml) was added at 195 K. The orange solution turned dark and was stirred for 10 min. Then dichlorodimethylsilane (30.0 ml, 32.1 g, 249 mmol) was added to quench the reaction and the reaction mixture allowed to warm to 298 K in a cooling bath. Then the solvent and the excess of dichlorodimethylsilane were evaporated in inert conditions under reduced pressure. The residual orange solid was dissolved in diethyl ether (25 ml) and added dropwise over the course of 15 min to a solution of sodium methoxide (4.00 g, 74.0 mmol) in methanol (50 ml). Both of the latter steps were performed under inert conditions. To this mixture, a solution of sodium hydroxide (17.5 g, 435 mmol) in methanol (105 ml) and water (10 ml) was added. The resulting mixture was stirred 15 minutes and then a further portion of sodium hydroxide (17.5 g) in water (105 ml) was added. The reaction mixture was stirred for 1 h. This mixture was then poured into a vigorously stirred solution of monopotassium phosphate (155 g, 1.14 mol) in water (200 ml). The orange precipitate was filtered and purified by three recrystallization cycles from diethyl ether/n-hexane (v/v, 1:1). The product was isolated as a bright-orange solid in a yield of 864 mg (35%). Crystals were obtained by dissolving the product in chlroroform, adding a layer of n-hexane and allowing the n-hexane to diffuse into the

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-and O-bound H atoms were located in difference maps but were positioned with idealized geometry (methyl and hydroxyl H atoms allowed to rotate but not to tip) and refined with U iso (H) = 1.2U eq (C) (1.5 for methyl and hydroxyl H atoms) using a riding model. For both compounds, data collection: X-AREA (Stoe, 2008); cell refinement: X-AREA (Stoe, 2008); data reduction: X-AREA (Stoe, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

(I) (E)-[Diazene-1,2-diylbis(3,1-phenylene)]bis(dimethylsilanol)
Crystal data  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.