Crystal structure of 4-({5-[(E)-(3,5-difluorophenyl)diazenyl]-2-hydroxybenzylidene}amino)-2,2,6,6-tetramethylpiperidin-1-oxyl

The asymmetric unit of the title compound contains two crystallographically independent molecules with the similar conformation, the piperidine rings in both molecules adopt a similar distorted chair conformation and have pseudo mirror planes passing through the N—O bond.

The asymmetric unit of the title compound, C 22 H 25 F 2 N 4 O 2 , contains two crystallographically independent molecules. In one molecule, the two benzene rings are oriented at a dihedral angle of 1.93 (10) and in the other molecule the corresponding dihedral angle is 7.19 (9) . The piperidine rings in the two molecules adopt a similar distorted chair conformation, and both have pseudomirror planes passing through the N-O bonds. An intramolecular O-HÁ Á ÁN hydrogen bond between the hydroxy group and the imine N atom is observed in both molecules. In the crystal, weak C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds, enclosing R 2 2 (6) ring motifs, and weakstacking interactions link the molecules into a three-dimensional supramolecular network, with centroid-tocentroid distances between the nearly parallel phenyl and benzene rings of adjacent molecules of 3.975 (2) and 3.782 (2) Å .

Chemical context
It is well known that the 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) free nitroxyl radical has been attached to various organic compounds (such as aldehydes, ketons, azo compounds and carboxylic and amino acids) and biomolecules (such as lipids, proteins, steroids and metalloenzymes) (Gallez et al. 1992;Berliner, 1976) to yield a wide variety of TEMPO-bearing molecules named as spinlabeled compounds (Rosen et al., 1999;Gnewuch & Sosnovsky, 1986). These types of nitroxide free radicals have different applications such as magnetic resonance imaging (Likhtenstein et al., 2008), protection from oxidative stress and irradiative damage (Hahn et al., 1994), controlled 'living' freeradical polymerization (Hawker, 1997), spin trapping and spin-labeling in various fields of chemistry, biology and material sciences (Tretyakov & Ovcharenko, 2009). Our literature searches revealed that while a verity of TEMPOlabeled radicals with various imines, alcohol amines, carboxylic acids, salicylaldehydes, azo compounds, ketone derivatives have been designed, no TEMPO-labeled compound on the basis of phenylazo-salicylaldehyde compounds has been reported. We report herein the synthesis and structure of the new class title spin-labeled compound.

Structural commentary
The asymmetric unit of the title compound contains two crystallographically independent molecules (Fig. 1). The molecules include short intramolecular O-H Á Á Á N hydrogen bonds (Table 1), which mean that the ligand is in the phenol-

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Intramolecular O-HÁ Á ÁN hydrogen bonds are shown as dashed lines. C-bound H atoms have been omitted for clarity.

Figure 2
Part of the crystal structure, viewed down [001]. Intramolecular O-HÁ Á ÁN and intermolecular C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds, which enclose R 2 2 (6) ring motifs, are shown as dashed lines. H atoms not involved in these hydrogen bonds have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Atoms H1 and H3 (for OH) and H13 and H35 (for CH) were located in a difference Fourier map and were refined freely. The other C-bound H atoms were positioned geometrically with C-H = 0.93 Å (for aromatic CH), 0.96 Å (for CH 3 ), 0.97 Å (for CH 2 ) and 0.98 Å (for CH), and constrained to ride on their parent atoms, with U iso (H) = xU eq (C), where x = 1.5 for methyl H atoms and x = 1.2 for other H atoms.
Acta Cryst. (2015). E71, 864-866 research communications   program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).  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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.