3-Phenyl-1H-1,2,4-triazol-5-amine–5-phenyl-1H-1,2,4-triazol-3-amine (1/1)

In the title compound, C8H8N4·C8H8N4, two tautomers, viz. 3-phenyl-1,2,4-triazol-5-amine and 5-phenyl-1,2,4-triazol-3-amine, are crystallized together in equal amounts. The 3-phenyl-1,2,4-triazol-5-amine molecule is essentially planar; the phenyl ring makes a dihedral angle of 2.3 (2)° with the mean plane of the 1,2,4-triazole ring. In the 5-phenyl-1,2,4-triazol-3-amine tautomer, the mean planes of the phenyl and 1,2,4-triazole rings form a dihedral angle of 30.8 (2)°. The π-electron delocalization of the amino group with the 1,2,4-triazole nucleus in the 3-phenyl-1,2,4-triazol-5-amine molecule is more extensive than that in the 5-phenyl-1,2,4-triazol-3-amine tautomer. The molecules are linked into a two-dimensional network parallel to (100) by N—H⋯N hydrogen bonds.

Usually, tautomerizable 1,2,4-triazoles with nonequivalent substituents at positions 3 and 5 crystallize as a tautomer bearing at position 5 substituent with relatively more pronounced electronodonor properties (Buzykin et al., 2006). Considering significant difference in electronic properties of phenyl and amino group, the crystal would be assembled from the molecules of tautomer I analogously to the reported 3-pyridin-2-yl-1,2,4-triazol-5-amine (Dolzhenko et al., 2009). Surprisingly, two tautomeric forms I and II were found crystallized together in the crystal. To the best of our knowledge, this is the first example of existence in crystal of unequally 3,5-disubstituted tautomerizable 1,2,4-triazole tautomeric form with electronodonor group located at position 3.
The geometry of the tautomer I molecule is essentially planar (Fig.2). The amino group is involved in π-electron delocalization with the 1,2,4-triazole nucleus. It is almost planar with small deviation 0.06 (2) Å of the nitrogen atom from the C8/H4A/H4B plane. The length of the C8-N4 bond is 1.337 (3) Å. The π-electron delocalization of the amino group of II with the 1,2,4-triazole nucleus is significantly lower. The nitrogen atom (N8) of the amino group adopts a pyramidal configuration with 0.21 (2) Å deviation of the nitrogen atom from the C16/H8A/H8B plane. The C16-N8 bond [1.372 (3) Å] is also longer. The phenyl ring of I makes a small dihedral angle of 2.3 (2)° with the mean plane of the 1,2,4-triazole ring. The molecule of tautomer II loses this planarity. The mean planes of the phenyl and 1,2,4-triazole rings of II form a dihedral angle of 30.8 (2)°.
The molecules are linked into a two-dimensional network parallel to the (100) by N-H···N hydrogen bonds (Table 1 and Fig.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.
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 > σ(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.