Crystal structures of the solvent-free and ethanol disolvate forms of 4,4′-(diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid) exemplifying self-stabilized azobenzene cis-configurations

The synthesis and the crystal structure of cis-2,3,5,6,2′,3′,5′,6′-octafluoro-4,4′-azinodibenzoic acid with and without residual ethanol are reported.


Chemical context
The parent structure of azobenzene and its numerous differently substituted derivatives is comprised of two aromatic benzene rings separated by an azo group. One of the most intriguing properties of these artificial molecules is their capability to shape reversibly the configuration of the azo group from the linear trans form, usually more stable, to the bent cis form, in the presence of an appropriate light irradiation, e.g. lasers or LEDs. Such controlled trans-cis interconversions at the molecular scale, typically performed on the microsecond time interval or faster, have been amplified successfully to a macroscopic material photomechanical response, suggesting a highly promising route toward creating and applying diverse photoresponsive systems (Mahimwalla et al., 2012;Bushuyev et al., 2018). In this context, azobenzenes, capable of adopting long-term stabilized cis-forms, represent an important tool for studying the trans-cis isomerization mechanisms, as well as for tuning the photomechanical properties. Particular attention has therefore been paid to polyfluorinated azobenzene derivatives employed as components of various photoresponsive homo-and heteromolecular crystals (Bushuyev et al., 2013(Bushuyev et al., , 2014(Bushuyev et al., , 2016a. ISSN 2056-9890 In the present study, we report the crystal structures of 4,4 0 -(diazenediyl)bis(2,3,5,6-tetrafluorobenzoic acid) with (I) and without residual ethanol (II), both adopting the cis configuration during a common crystallization procedure from the same solution in ethanol at room temperature under normal laboratory lighting conditions.

Structural commentary
The molecular structure of the title compound with (I) and without residual ethanol solvent molecules (II), Figs. 1 and 2, respectively, is constituted of two 2,3,5,6-tetrafluorobenzoic acid residues linked to each other by a cis-configured azo group. In the solvent-free form (II), the molecule is characterized by rotational symmetry around a twofold rotation axis bisecting its central N N bond while this symmetry is not present in the solvated form (I).

Supramolecular features
The inclusion of ethanol molecules in the crystal composition renders different the patterns of interactions through hydrogen bonds for the forms (I) and (II) (Tables 1 and 2, respectively). For the solvated structure (I), the hydrogen bonds between the alternating hydroxy groups of residual ethanol and the carboxyl groups of the title molecule are arranged in two different ways, by forming either 12membered rings involving four molecules (two molecules of each component), according to graph-set descriptor R 4 4 (12) (Etter et al., 1990), or an open-chain pattern extending parallel The molecular structure of (II) showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. H atoms are drawn as spheres of arbitrary radius. [Symmetry code: (i) Àx + 1, y, Àz + 1 2 ].

Figure 1
The molecular structure of (I) showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. H atoms are drawn as spheres of arbitrary radius, and hydrogen bonds are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ) for (I). Symmetry codes: (i) Àx; Ày þ 1; Àz þ 1; (ii) x À 1; y; z.

Synthesis and crystallization
The title compound was synthesized according to a modified general protocol for obtaining symmetrically substituted azobenzenes from the corresponding initial anilines (Clarke, 1971). Briefly, 3 g (0.014 mol) of 4-amino-2,3,5,6-tetrafluorobenzoic acid was neutralized in 60 ml of water by NaOH solution and adjusted to pH ' 8.5-9.0, and added dropwise to 100 ml of the commercial bleach solution Clorox TM (The Clorox Company of Canada Ltd., ON, Canada), preliminary cooled to 273-278 K in an ice bath. The mixture was allowed to reach room temperature with overnight stirring. The resulting red-coloured solution was first treated with 80 ml of acetone and stirred for 1 h, to neutralize the excess of NaOCl, and then with aqueous HCl to pH 1.0 to give a pink sediment. After filtering and drying overnight at room temperature, the solid crude product was purified by extraction with ethanol followed by filtering. The final removal of solvent under reduced pressure gave 1.2 g of the target product with the yield of 40.4%. The structure and purity of the desired product were confirmed by LC-MS analysis performed on an Agilent Technologies 1260 Infinity LC-MS spectrometer (Santa Clara, CA, US) in ESI positive and negative modes. Separation was performed with an Agilent Poroshell 120 EC-C18 2.7 mm column, using as eluent the 0-100% gradient of solvent mixtures A and B [where A: water-acetonitrile (95% vol -5% vol ) and acetic acid ( Partial view of the packing showing two hydrogen-bonded chains in (II). Hydrogen bonds are shown as dotted lines and hanging hydrogen bonds were omitted for clarity.

Figure 3
Partial view of the packing of (I), showing the hydrogen-bonding interactions (dotted lines). Hanging hydrogen bonds were omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms of the hydroxy and carboxyl groups in (I) were first positioned from Fourier synthesis and refined leveraging a riding model with U iso (H) set to 1.5 times U eq (O). All other H atoms of (I) were treated by using appropriate constraints. For (II), all the H atoms, including those belonging to the carboxyl group, were positioned from the difference synthesis and fully refined. For (I), non-merohedral twinning was found using the TwinRotMat Routine in PLATON (Spek, 2009). The twin law matrix was found to be (1 0 0, À0.621 À 1 0, À0.951 0 À 1). Processing the data as a two-component specimen with SAINT (Bruker, 2013) and TWINABS (Bruker, 2013) did not lead to an improvement in the refinement. Therefore, the initial data set was kept with the refinement performed using the HKLF5 file as generated with PLATON. The final BASF parameter indication the ratio of the two crystal domains was 0.646 (10).

Funding information
Funding for this research was provided by: Natural Sciences and Engineering Research Council of Canada; Fonds de Recherche du Qué bec -Nature et Technologies.

Special details
Experimental. X-ray crystallographic data for I were collected from a single crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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. Refined as a 2-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.9889 (11) 0.7393 (6) (7) Special details Experimental. X-ray crystallographic data for I were collected from a single crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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.