Syntheses, spectroscopy, and crystal structures of 3-(4-bromophenyl)-1,5-diphenylformazan and the 3-(4-bromophenyl)-1,5-diphenylverdazyl radical and the crystal structure of the by-product 5-anilino-3-(4-bromophenyl)-1-phenyl-1H-1,2,4-triazole

The syntheses of a formazan and a verdazyl radical are reported along with their crystal structures, UV–Vis spectra, and the EPR spectrum of the radical. In addition, the isolation of a possible by-product was achieved.


Chemical context
Verdazyl radicals are a family of organic radicals first reported by Kuhn & Trischmann (1963) who emphasized their intense green color and their stability. These Kuhn-verdazyls require formazan precursors, which are intensely red in color and interesting in their own respect (Nineham, 1955;Scudiero et al. 1988). A few years after Kuhn's discovery, syntheses leading to the orange 6-oxo-and 6-thioxoverdazyls were developed (Neugebauer & Fischer, 1980;Neugebauer et al., 1988). As of late, verdazyls experience renewed interest, partially as a result of the improvements concerning their syntheses, enabling the introduction of a large variety of substitution patterns (Paré et al., 2005;Bancerz et al., 2012;Matuschek et al., 2015;Le et al., 2017). Such tailor-made radicals have possible applications as fundamental building blocks in molecular magnets or in spintronic materials (Koivisto & Hicks, 2005;Train et al., 2009;Ratera & Veciana, 2012). Verdazyls often avoid stacking, preventing the occurrence of strong magnetic interactions. However, some exceptions to this rule have been reported, where strong antiferromagnetic coupling occurs as a consequence (Koivisto et al., 2006;Eusterwiemann et al., 2017). With respect to applications in spintronics, tetrathiafulvalene-substituted verdazyl compounds represent interesting examples (Chahma et al., 2006;Venneri et al., 2015). Herein, the preparation and crystal structures of three molecules involved in verdazyl synthesis are reported.

Structural commentary
The molecular structures of 1 and 3 are shown in Fig. 1a and b, respectively. Compound 2 has a structure typical for verdazyls, for details see Iwase et al. (2013). For 1, interesting structural features are the bond lengths in the central NNCNN atomic chain. Taking into account the 3 criterion, the bond lengths N1-N2 and N3-N4 are identical [1.309 (5) and 1.300 (5) Å , respectively] and the same is true for N1-C7 and N3-C7 [1.350 (5) and 1.364 (5) Å , respectively]. These bond lengths lie between values typical for single and double bonds. The pairwisely identical bond lengths are in agreement with rapid intramolecular H-atom exchange (Nineham, 1955;Otting & Neugebauer, 1969;Buemi et al., 1998). Correspondingly, the H atom was considered to be split between the two possible positions at N2 and N4. In both positions, an intramolecular hydrogen bond is formed with HÁ Á ÁA distances amounting to 1.93 (10) Å for N2-H2Á Á ÁN4 and 1.86 (12) Å for N4-H4Á Á ÁN2 (Table 1). Finally, it is noted that the molecule is essentially planar with angles between the normal vectors of the NNCNN mean plane A and the three rings B, C, and D amounting to 9.71 (16) (A/B), 5.28 (3) (A/C), and 12.18 (13) (A/D).
Compound 3 was isolated in later fractions of the column that was used to purify 2. Such triazole compounds have been identified as products of thermal verdazyl decomposition at 473 K or after four days of refluxing at 353 K in benzene (Neugebauer et al., 1972). Here, the formation of 3 was observed under much less harsh conditions. The bond lengths within ring A suggest bond orders between single and double bonds, in accordance with the aromatic character of 1,2,4-

Figure 2
(a) Unit cell of 1 viewed parallel to the (100) plane. (b) Stacks of dimers formed along the a-axis direction. Two nitrogen atoms of two molecules are labelled.  Compound 3 has a similar structure to 1 in space group Pbca and with pairs of molecules stacked along the a-axis direction (Fig. 3). Here, the centroid-to-centroid distances of the A rings are 3.564 (3) and 4.661 (3) Å within and between the dimers, respectively. However, the shortest intra-dimer contact is a C-HÁ Á Á interaction (Table 2) between rings C and D (C10-H10Á Á ÁC20, 2.75 Å ). A similar contact is found between H17 and C19 (C17-H17Á Á ÁC19, 2.84 Å ), forming a contact between different stacks. -Stacking is observed between rings A and B, connecting pairs of dimers, with the shortest contacts being 3.229 (6) (C8Á Á ÁN3), 3.318 (6) (C8Á Á ÁC2), and 3.378 (6) Å (C7Á Á ÁC2). As with 1 and 2, no halogen bonding is observed, but the Br atom is involved in a very weak hydrogen bond (C14-H14Á Á ÁBr1, 2.99 Å ; Table 2). represents the EPR spectrum of 2 and its simulation (black and red lines, respectively). The UV-Vis spectra of 1 and 2 are typical for formazans and verdazyls, respectively, with the peaks in the visible range at 490 nm (1) as well as at 425 and 720 nm (2) being responsible for their intense red (1) or green colors (2). The EPR spectrum of 2 was simulated by assuming a g value of 2.00354 and hyperfine coupling constants (HFCC) of 16.77 and 16.48 MHz for the two pairs of nitrogen nuclei. In addition, the approximate values for the HFCC of the phenyl ring protons could be obtained, amounting to 0.01 (CH 2 ), 3.04 (H, rings B and D, ortho), 1.14 (H, rings B and D, meta), 3.34 (H, rings B and D, para), 1.14, (H, ring C, ortho), and 0.52 MHz (H, ring C, meta). The assignment of the protons is in accordance with that of Kopf et al. (1971).

Database survey
The Cambridge Structural Database (CSD, Version 5.36; Groom et al., 2016) was queried for other formazans, verdazyls, and 1,2,4-triazoles. The search revealed 21 examples of formazans if the only restriction was to have carbon substituents in the 1,3,5-positions. This number reduced to nine if all of these substituents were required to be phenyl-based, one of these nine examples being a metal complex of a formazan. The remaining eight structures include examples in which the bond lengths in the NNCNN unit alternate, as well as examples in which they are pairwisely equal in a similar manner to that described herein. Interestingly, 3,5-diphenyl-1-(4-bromophenyl)formazan (regioisomer of 1, CCDC code EMEVUO; Tunç & Yıldırım, 2010) shows alternating bond lengths, which reflects the fact that the two nitrogen atoms are chemically inequivalent in this molecule. An example with split hydrogen positions is 1,5-diphenyl-3-(p-nitrophenyl)formazan (GUHCIW; Iqbal et al., 2009), which shows a similar stacking to that observed in 1 and can be formally derived from 1 by replacing the bromine with a nitro group. 33 examples for 1,3,5-aryl-substituted verdazyls were found in the CSD, only 14 of them Kuhn-verdazyls. The largest hitlist was obtained for 1,3,5-substituted 1,2,4-triazoles (1001 entries). This number reduced drastically if purely organic compounds were considered exclusively (42 hits) and even further if the substitutent at C5 was required to be a nitrogen atom (four hits, no further restriction).

Synthesis and crystallization
The syntheses were performed following Berry et al., 2009 (Fig. 5). The hydrazone 4 required for the synthesis of 1 was synthesized by refluxing a solution of p-bromobenzaldehyde with phenylhydrazine in ethanol and collecting the slightly yellow precipitate that formed after cooling the solution down to room temperature (rt).

Figure 5
Synthesis of 1, 2, and 3. of sodium carbonate in 11 mL of water to form a biphasic system, which was stirred at 273 K for 30 min. During this time, 1.8 mL (186 mg, 2 mmol) of aniline were dissolved in 4.5 mL of dilute hydrochloric acid (ca 12%) and stirred at 273 K. To this solution, 55 mg (3.3 mmol) of sodium nitrite in 3 mL of water were added dropwise over the course of ten minutes, leading to the occurrence of a slight yellow color. This yellow solution was added carefully to the biphasic solution of 4 and an intense red color evolved within minutes. After one h, 20 mL of water were added and the temperature was allowed to increase to rt. After stirring for another 30 minutes at rt, the phases were separated. The organic phase was washed with water and dried over Na 2 SO 4 before removing the solvent under reduced pressure. The raw product was subjected to column chromatography using aluminum oxide (AlOx, water content 5%) as stationary phase and DCM/cyclohexane (1:4). The red fractions were collected, yielding 1 as red solid in 66% yield (307 mg). Crystals of 1 were obtained by dissolving the solid in a mixture of DCM and hexane which was left to evaporate.
To obtain 2, 119 mg (0.31 mmol) of 1 were dissolved in 10 mL of dimethylformamide and mixed with 0.7 mL 2 M aqueous sodium hydroxide solution and 0.65 mL of 37% formaldehyde solution. The mixture was stirred at rt in an open vessel with contact to air, leading to a change of color from red to green over the course of an hour. 20 mL of water and diethyl ether were then added to the solution and the phases were separated from each other. After drying the organic phase over Na 2 SO 4 , the raw product was subjected to column chromatography using AlOx (water content 5%) and diethylether/cyclohexane (1:5) as eluent. The green fractions were collected and the solvent was removed under reduced pressure (yield: 37 mg, 30%). Crystals of 2 were obtained by dissolving the product in a mixture of DCM and hexane and leaving the green solution to evaporate.
Compound 3 was obtained by collecting the slightly yellow fractions that eluted from the column after 2 and removing the solvent. Dissolving the resulting brownish solid in a mixture of DCM and hexane and leaving the solution to evaporate afforded crystals suitable for X-ray crystallography.
Additional analytical data for 1 and 2.

Computing details
Data collection: APEX2 (Bruker, 2015) for (1) (2). Program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) for (2), (3). For all structures, program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.72 e Å −3 Δρ min = −0.65 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.46 e Å −3 Δρ min = −0.84 e Å −3 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.