Crystal structure of 2-[2-(pyridin-3-yl)diazen-1-yl]aniline

In the crystal structure of the title compound the molecules are coplanar and are linked into helical chains via N—H⋯N hydrogen bonding between one of the amino H atoms and the pyridine N atoms.


Chemical context
Azobenzenes are among the most frequently used photochromic compounds with numerous applications in different fields being reported (Szymań ski et al., 2013;Merino & Ribagorda, 2012;Kay et al., 2007). Moreover, azobenzenes are easily accessible, and their photochromic functions are quite reliable. The stretched trans isomer is usually the thermodynamically stable conformation. Upon irradiation with UV light, the bent cis isomer is formed. This cis conformation switches back to the trans isomer either upon irradiation with visible light or thermochemically (Hartley, 1937). A highly important variation of azobenzenes are azopyridines, as pyridines coordinate to various metals, e.g. nickel (Thies et al., 2010;Dommaschk et al., 2015c). Thus, azopyridines can be used as switchable ligands. In this context, we have reported an approach to switch the spin state of azopyridine-functionalized Ni-porphyrins (Thies et al., 2011(Thies et al., , 2012Venkataramani et al., 2011;Dommaschk et al., 2015a,b). Aiming at further functionalization of azopyridines and in view of applications as molecular spin switches, we have synthesized 2-[2-(pyridin-3-yl)diazen-1-yl]aniline and report here its molecular and crystal structure. ISSN 2056-9890

Structural commentary
The crystal structure of the title compound comprises 2-[2-(pyridin-3-yl)diazen-1-yl]aniline molecules, located on general positions, adopting a trans-conformation with a C1-N2-N3-C6 torsion angle of À179.80 (8) , which corresponds to the energetically favored arrangement (Fig. 1). The two sixmembered rings are coplanar [the maximum deviation from the least-squares plane for all non-H atoms is 0.0569 (9) Å for N4] and the dihedral angle between the ring planes is 0.11 (8) . The amino H atoms are also in the plane of the adjacent benzene ring. There is an intramolecular N-HÁ Á ÁN hydrogen bond between one of the amino H atoms and one nitrogen atom of the azo group with an NÁ Á ÁH distance of 2.066 (15) Å and an N-HÁ Á ÁN angle of 127.30 (12) ( Table 1). Even if this corresponds to a weak interaction, it might stabilize the planar arrangement.

Supramolecular features
In the crystal structure of the title compound, the molecules are linked into chains along the b-axis direction via N-HÁ Á ÁN hydrogen bonds between the amino hydrogen atom that is not involved in intramolecular hydrogen bonding and one of the nitrogen atoms of the azo group (Fig. 2, top). The NÁ Á ÁH distance amounts to 2.163 (16) Å and the N-HÁ Á ÁN angle of 159.7.14 (12) is slightly bent, indicating that this is a relatively strong interaction (Table 1). The dihedral angle between the pyridine ring that carries the acceptor N atom and the aminophenyl moiety of a neighbouring molecule that carries the donor group is 66.12 (8) . Therefore, the molecules exhibit a helical arrangement along the chain (Fig. 2, top). The chains are closely packed in such a way that each chain is surrounded by eight neighboring chains (Fig. 2, bottom). The molecules exhibit a herringbone-like pattern along the a axis ( Fig. 3) in which the pyridine and benzene rings of adjacent molecules are perfectly coplanar. The distance between the ring planes is 3.462 Å and the centroid-centroid distance is 3.8040 (7) Å , indicatinginteractions between the chains.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx þ 1 2 ; y À 1; Àz þ 3 2 .

Figure 2
Crystal structure of the title compound showing a chain (top) and a view along the b axis (bottom). Intra-and intermolecular hydrogen bonds are indicated by dashed lines.

Figure 3
Crystal structure of the title compound in a view along the a axis.

Figure 1
Molecular structure of the title compound with labeling and displacement elliposids drawn at the 50% probability level. The intramolecular N-HÁ Á ÁN hydrogen bond is shown with dashed lines.

Synthesis and crystallization
The synthesis of the title compound can be performed in two steps.
After cooling to room temperature, zinc dust (4.93 g, 75.3 mmol) was added and the mixture was stirred for 1 h at 333 K. After filtration, the filtrate was poured into an aqueous ice-cooled iron(III) chloride solution (hexaaqua complex, 5.40 g, 20.3 mmol in 150 ml) whereby a green solid precipitated. After 15 min of stirring, the solid was filtered off and washed with water. The crude product was a mixture of 2-nitrosoacetanilide and starting material, which was used for azo condensation without further purification. 3-Aminopyridine (800 mg, 8.51 mmol) was dissolved in a mixture of pyridine (25 ml) and aqueous sodium hydroxide (5 ml, 25%). The crude product of 2-nitrosoacetanilide dissolved in pyridine (30 ml) was added to the solution containing the 3aminopyridine. The reaction mixture was stirred for 1 h at 353 K and overnight at room temperature. After addition of dichloromethane (200 ml) the phases were separated. The organic layer was washed with water twice and dried over sodium sulfate. The solvent was removed under reduced pressure. The crude product was purified by column chromatography (ethyl ester/n-hexane, R f = 0.16). The product was obtained as an orange solid. Yield: 200 mg (0.83 mmol, 10%).  147.7, 146.3, 138.9, 136.4, 133.8, 127.3, 124.1, 123.4, 120.6, 120.4 (ii) Synthesis of 2-[2-(pyridin-3-yl)diazen-1-yl]aniline: 3-(2acetanilide)azopyridine (640 mg, 2.66 mmol) was dissolved in methanol (50 ml). A sodium hydroxide solution (5 ml, 30%) was added and stirred for 6 h at 343 K. 2-[2-(Pyridin-3yl)diazen-1-yl]aniline precipitated and was filtered off. The solid was washed with water. Recrystallization from acetone gave orange single crystals. Yield: 520 mg (2.63 mmol, 99%). Comparison of the experimental X-ray powder diffraction pattern with that calculated from single crystal data proves that the title compound was obtained as a pure phase (see Fig.  S1 in the supporting information). The UV-Vis spectrum shows the strong !* band of the trans conformation (Fig.  S2 in the supporting information). If the sample is exposed to light of 365 nm, no isomerization into the cis conformer is observed, and the sample starts to decompose. However, conversion of the amino to an amide group will probably restore the photochromic properties.  Computer programs: X-AREA (Stoe, 2008), SHELXS97 and XP (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 2014) and publCIF (Westrip, 2010).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-H hydrogen atoms were located in a difference map but were positioned with idealized geometry and refined with U iso (H) = 1.2U eq (C,N) using a riding model with C aromatic -H = 0.95 Å . The N-H hydrogen atoms were located in a difference map and were freely refined.