Crystal structure of 3-[2-(1,3-thiazol-2-yl)diazen-1-yl]pyridine-2,6-diamine monohydrate

The organic molecule in the title hydrate shows an E configuration with respect to the azo functionality.


Chemical context
Azo compounds are one of the most important organic dyes used in industrial applications to colour various consumer goods such as leather, plastics and cosmetics (Kaur et al., 2018). The main characteristic of these compounds is the chromophore of the azo group (-N N-), which is responsible for the color of the dyes. Compounds with an aromatic thiazolylazo moiety are a subclass of azo dyes, which contain the thiazole group on one side of the azo linkage and are important ligands in coordination chemistry (Kaim, 2001). In this regard, zinc complexes with polydentate chelating thiazolylazo ligands have been prepared as luminescence probes for selectively sensing phosphates (Hens et al., 2015). Recently, Piyasaengthong et al. (2015) reported the synthesis of a gold(III) complex of 3-(2 0 -thiazolylazo)-2,6-diaminopyridine and investigated its pepsin inhibition.

Figure 2
The unit-cell packing in (I) , viewed approximately down [100]. The classical O-HÁ Á ÁN, and N-HÁ Á ÁO hydrogen bonds are shown as green dashed lines (see Table 1 for numerical details).

Figure 1
The molecular structure of the organic entity and the water molecule in compound (I), with the atom labelling and displacement ellipsoids drawn at the 50% probability level.

Synthesis and crystallization
2-Aminothiazole (1.0 g, 0.009 mol) was dissolved in 6 M hydrochloric acid (16 ml) with sodium nitrite (0.7 g, 0.01 mol). The mixture was stirred at a temperature between 268 and 273 K while a solution of 2,6-diaminopyridine (1.0 g, 0.009 mol) in 40 ml of 4 M hydrochloric acid was added. The reaction mixture was stirred for 1 h and then adjusted to pH 6.0 by 0.001 M sodium hydroxide. The red precipitate formed was filtered through suction and washed with water. Suitable crystals for X-ray analysis were grown by recrystallization using the vapor diffusion technique in a methanol-hexane mixture at 253 K [yield 1.12 g, 51%]. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to O and N atoms were located in difference-Fourier maps and refined with distance restraints of 0.84AE0.02 Å with U iso (H) = 1.5U eq (O) and 0.86AE0.02 Å with U iso (H) = 1.2U eq (N), respectively. The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93 Å with U iso (H) = 1.2U eq (C).

Funding information
RC and BB thank the Faculty of Science, Naresuan University for financial support. AS thanks the Development and Promotion of Science and Technology Talents Project (DPST) for a scholarship. The authors thank the Faculty of Science and Technology, Thammasat University, for funds to purchase the X-ray diffractometer.  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); 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.