N-[(E)-Quinolin-2-ylmethylidene]-1,2,4-triazol-4-amine hemihydrate

In the title hemihydrate, the Schiff base molecule adopts an E configuration about the C=N bond and is approximately planar, with a dihedral angle between the quinoline ring system and the 1,2,4-triazole ring of 12.2 (1)°. In the crystal, one water molecule bridges two Schiff base molecules via O—H⋯N hydrogen bonds.


Synthesis and crystallization
A solution of quinoline-2-carbaldehyde (1.00 g, 6.0 mmol) was mixed with an equimolar solution of 4-amino-4H-1,2,4-triazole (0.54 g, 6.0 mmol) in a mixture of absolute ethanol and chloroform (1:1) (20 ml). Glacial acetic acid (2 drops) was added into the reaction mixture, followed by heating at 351 K for 6 h for complete conversion to the product (as confirmed by TLC analysis). The mixture was then kept in an ambient environment for two weeks. The crude product obtained was recrystallized from ethyl acetate solution, giving clear brown crystals (yield 55%) suitable for X-ray analysis. Presumably the water molecule of crystallization was absorbed from the atmosphere or as a by product during the synthesis.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A partial packing diagram of the title compound, showing two Schiff base molecules linked by two identical intermolecular O-HÁ Á ÁN hydrogen bonds.

Figure 3
A partial packing diagram of the title compound, showing a molecular column formed bystacking interactions.

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme and 50% probability displacement ellipsoids.  The title compound was also synthesized using a green synthesis method. A solution of 4-amino-4H-1,2,4-triazole (0.11 g, 1.3 mmol) in 5 ml of distilled water was added to quinoline-2-carbaldehyde (0.20 g, 1.3 mmol) in 5 ml of distilled water. The resulting mixture was then stirred at room temperature for 1 h while the reaction progress was monitored by TLC. The clear light-brown crude product formed quantitatively after 1 h and was vacuum filtered, dried and recrystallized from ethyl acetate solution to give clear dark-brown crystals with 60% yield. The recrystallized compound from the conventional synthesis method and that from the green method were confirmed to be identical as given by the similar data from FTIR, TLC, MS, and melting-point measurements.

Refinement
Crystal data, data collection and structure refinement details of the title compound are summarized in Table 2.

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.