4-(5-Amino-1H-1,2,4-triazol-3-yl)pyridinium chloride monohydrate

In the cation of the title compound, C7H8N5 +·Cl−·H2O, the mean planes of the pyridine and 1,2,4-triazole rings form a dihedral angle of 2.3 (1)°. The N atom of the amino group adopts a trigonal–pyramidal configuration. The N atom of the pyridine ring is protonated, forming a chloride salt. In the crystal, intermolecular N—H⋯O, N—H⋯N, N—H⋯Cl and O—H⋯Cl hydrogen bonds link the cations, anions and water molecules into layers parallel to the (1, 0, ) plane.

In the cation of the title compound, C 7 H 8 N 5 + ÁCl À ÁH 2 O, the mean planes of the pyridine and 1,2,4-triazole rings form a dihedral angle of 2.3 (1) . The N atom of the amino group adopts a trigonal-pyramidal configuration. The N atom of the pyridine ring is protonated, forming a chloride salt. In the crystal, intermolecular N-HÁ Á ÁO, N-HÁ Á ÁN, N-HÁ Á ÁCl and O-HÁ Á ÁCl hydrogen bonds link the cations, anions and water molecules into layers parallel to the (1, 0, 1 2 ) plane.
By potentiometric titration with 0.1 M hydrochloric acid we established that the pK α of the 5-amino-3-(pyridin-4-yl)-1,2,4-triazole in water is 4.68 (5) at 293 K. A model compound, 5-amino-3-phenyl-1H-1,2,4-triazole, which is protonated at the N 4 of triazole cycle in acid solutions (Voronkov et al., 1976), has the pK α = 3.80 (3) at 293 K. Since the basicity of the 5-amino-3-(pyridin-4-yl)-1,2,4-triazole is almost eight times higher than the model compound, it is possible to assume that in water solution the pyridine rather than triazole cycle is protonated. In the 13 C NMR spectrum of the hydrochloride of 5-amino-3-(pyridin-4-yl)-1,2,4-triazole in dimethyl sulfoxide (DMSO-d 6 ), the signals of the triazole carbons C 3' and C 5' are observed at 153.71 and 157.83 ppm, correspondingly (for the chemical numbering scheme, see Fig. 1). These values are very close to the same signals of the unprotonated 5-amino-3-(pyridin-4-yl)-1H-1,2,4-triazole (Dolzhenko et al., 2009a) and are typical for 5-amino-1H-1,2,4-triazoles (Chernyshev et al., 2010). The chemical shift of the carbon connected to amino group is most representative. Thus, in the 5-amino-and 3-amino-4H-1,2,4-triazolium salts the signals of the same atoms are high field shifted to 149.3-154.7 ppm (Chernyshev et al., 2010). Therefore, it could be concluded that the triazole cycle is unprotonated in DMSO solution of the studied salt. However, the signals of the carbons of the pyridine cycle of the hydrochloride (especially the carbons, connected with nitrogen atom) differ sufficiently from the ones of unprotonated 5-amino-3-(pyridin-4-yl)-1,2,4-triazole. In the unprotonated compound the signals of C 2 and C 6 are detected at 149.9 ppm (Dolzhenko et al., 2009a), while in the hydrochloride they are observed at 142.9 ppm. Therefore, we can conclude that the pyridine cycle is protonated and the tautomeric form A is predominant in DMSO (Fig. 1). For unambiguous confirmation of the proposed structure, we performed an X-ray investigation of the title compound. In the ensuing discussion of the structure, the crystallographic numbering system will be used (Fig. 3). In accordance with the X-ray diffraction data, the studied compound in the crystal exists as the tautomer A (Fig. 3). The pyridine and triazole rings are almost coplanar, the dihedral angle between the planes of the rings is 2.3 (1)°. Bond lengths and angles in the triazole cycle are within the normal ranges and are comparable with those found in the other 3-substituted 5-amino-1H-1,2,4-triazoles (Rusinov et al., 1991;Daro et supplementary materials sup-2 al., 2000;Boechat et al., 2004;Dolzhenko et al., 2009b,c). The nitrogen atom of the amino group is in a trigonal pyramidal configuration (sum of valence angles is 356.0° and deviates from the triazole plane by only 0.020 (3)Å. Conjugation between the unshared electron pair of N21 and the π-system of the triazole fragment leads to a shortening of the N21-C2 bond (1.330 (2)Å) relative to the standard length of a purely single Nsp 2 -Csp 2 bond (1.43-1.45Å) (Burke-Laing & Laing, 1976;Allen et al., 1987). Bond lengths and angles in the pyridine cycle are analogous to the ones in the pyridyl-substituted 1,2,4-triazoles, protonated at the pyridine cycle (Ren & Jian, 2008;Xie et al., 2009;Du et al., 2009).

Experimental
The title compound was prepared by the following procedure. A mixture of aminoguanidine hydrogen carbonate (5.53 g, 40.6 mmol), isonicotinic acid (5.01 g, 40.7 mmol) and 33.5% hydrochloric acid (5.0 ml) was heated to reflux for 15 min, then water was distilled off until the temperature of the reaction mixture raised to 448-453 K. The reaction mixture was heated at the same temperature for 6 h, cooled to ~373 K and dissolved in water (5 ml). The resulted solution was cooled to 276-278 K, the precipitate formed was isolated by filtration, recrystallized from 50% ethanol and dried at 403 K to give Calcd for C 7 H 8 ClN 5 : C,42.54;H,4.08;N,35.44. Found: C 42.35;H 4.19;N 35.18. The crystals of title compound suitable for X-ray analysis were grown by slow evaporation from water at room temperature.

Refinement
C-bound H atoms were placed in calculated positions C-H 0.93Å and refined as riding, with U iso (H) = 1.2U eq (C). H-atoms forming hydrogen (N-and O-bound H atoms) bonds were found from difference Fourier map and refined independently.
The initial experimental data were obtained for independent area of reciprocal space, but at the final stage of refinement procedure 'MERG 2' instruction was used and 'DIFABS CAD4' (Walker & Stuart, 1983) was applied. As a result, we have FVAR = 1, R int = 0. Fig. 1. Possible tautomeric forms for hydrochloride of 5-amino-3-(pyridin-4-yl)-1,2,4triazole. Fig. 2. Synthesis of the title compound.   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 > 2σ(F 2 ) is used only for calculating Rfactors(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.