5-Amino-3-(4H-1,2,4-triazol-4-yl)-1H-1,2,4-triazole

The asymmetric unit of the title compound, C4H5N7, comprises two independent but virtually superimposable molecules. Each molecule is planar with the dihedral angles between the five-membered rings being 2.8 (3) and 2.1 (3)°. The crystal structure is formed by an extensive network of relatively strong N—H⋯N hydrogen-bond interactions. Individual molecules are arranged into supramolecular zigzag chains running parallel to [001] by way of the strongest N—H⋯N interactions. Adjacent chains are interconnected by rather long (D⋯A distances range from ca 3.00 to 3.03 Å) but highly directional (interaction angles above ca 173°) hydrogen bonds forming a supramolecular layer in the bc plane.

The asymmetric unit of the title compound, C 4 H 5 N 7 , comprises two independent but virtually superimposable molecules. Each molecule is planar with the dihedral angles between the five-membered rings being 2.8 (3) and 2.1 (3) . The crystal structure is formed by an extensive network of relatively strong N-HÁ Á ÁN hydrogen-bond interactions. Individual molecules are arranged into supramolecular zigzag chains running parallel to [001] by way of the strongest N-HÁ Á ÁN interactions. Adjacent chains are interconnected by rather long (DÁ Á ÁA distances range from ca 3.00 to 3.03 Å ) but highly directional (interaction angles above ca 173 ) hydrogen bonds forming a supramolecular layer in the bc plane.

Comment
In the context of our interest in Crystal Engineering approaches (Wang et al. 2012;Silva et al., 2011;Amarante, Gonçalves et al., 2009;Amarante et al., 2009;Paz & Klinowski, 2007), we are currently designing new triazole molecules which could be simultaneously employed in the construction of novel Metal-Organic Frameworks (MOFs) or organic crystals. We note that this type of molecule has received considerable interest in the synthesis of polynuclear complexes and in the preparation of MOFs (Zhang et al., 2009). A survey in the Cambridge structural Database (Allen, 2002) and in the literature revealed that the use of the asymmetrical bridging bitriazole molecule 5amino-3-(1,2,4-triazol-4-yl)-1H-1,2,4-triazole (HAtrtr) has not been reported to date, either in MOFs nor in organic crystals. Furthermore, to the best of our knowledge, its crystal structure has not been reported. Following our recent efforts we were able to isolate good-quality single-crystals of the title compound as a minor secondary phase and here we wish to report its crystal structure at ambient temperature.
The asymmetric unit of the title compound (I) comprises two whole molecules (i.e., Z′=2) of HAtrtr as depicted in Fig.   1. We note that these two individual moieties are almost perfectly overlaid by assuming a combination of both inversion and molecular flexibility (maximum distance of ca 0.021 Å with RMS of ca 0.014 Å). Nevertheless, this feature is not described by crystal symmetry as the "head-to-tail" orientation of the molecules can not be described by the screw-axis parallel to the b-axis of the unit cell. Both molecules are almost planar with the dihedral angles between the 1,2,4-triazole rings being only of ca 2.8 and 2.1°. In addition, the medium planes of all non-hydrogen atoms of each molecule subtend an angle of just ca 7.1°, indicating that the molecules can also be envisaged as coplanar.
HAtrtr is rich in groups capable of forming strong N-H···N hydrogen bonding interactions (see Table 1 for further geometrical details), which direct the crystal packing features of the title compound. The most striking intermolecular interactions concern the double donation of hydrogen atoms from each 5-amino-3-(1,2,4-triazole moiety to the 1,2,4triazole group of an adjacent molecule (N1-H1A···N14, N2-H2···N13, N8-H8A···N7 and N10-H10···N6), forming a R 2 2 (7) graph set motif (dashed yellow lines in Fig. 2) (Grell et al., 1999). This supramolecular motif constitutes the basis of the formation of a supramolecular zigzag tape parallel to the c axis, for which the repeating motif are the two molecules composing the asymmetric unit. Tapes are interconnected by additional N-H···N interactions (N1-H1B···N4 and N8-H8B···N11) which, despite being slightly long (d D···A ranging from 3.002 (6) to 3.034 (6) Å), are nevertheless highly directional with the interaction angles approaching linearity (of ca 173 and 176°). Noteworthy, these inter-chain connections are further strengthened by the presence of two weak C-H···N interactions (not represented): C3-H3···N3 i with d C···N =3.389 (6) Å and <(CHN)=174°; C8-H8···N9 ii with d C···N =3.435 (6) Å and <(CHN)=167° (symmetry codes: (i) x, 1 + y, z; (ii) x, -1 + y, z). The combination of these N-H···N and C-H···N contacts can be described by the R 2 2 (8) The hydrogen bonding interactions connecting adjacent molecular units and summarized in the previous paragraph lead to the formation of a two-dimensional supramolecular layer placed in the bc plane as shown in Fig. 2. Individual layers close pack parallel to the a-axis of the unit cell mediated by offset π-π contacts between HAtrtr molecules ( Fig. 3 and Table 2 for intercentroid distances). Noteworthy, the inter-layer distance along the a axis alternates: layers are disposed into pairs so to promote the aforementioned strong offset π-π contacts within one pair; connections between adjacent pairs of supramolecular layers are weaker (i.e. longer inter-centroid distances -not shown).
HAtrtr (0.2 mmol, 0.0407 g) and Mn(OAc) 2 . 4H 2 O (1.0 mmol, 0.2451 g) were mixed with ca 0.50 g of [BMI]Br in a 25 mL teflon-lined stainless-steel reaction vessel. The resulting mixture was heated to 110 °C for 7 days. The vessel was then allowed to cool to ambient temperature at a rate of ca 1 °C/h. Small colourless platelets of HAtrtr were formed as a minor secondary product, whose crystal structure is reported in this manuscript.

Refinement
Hydrogen atoms bound to carbon were placed at their idealized positions with C-H = 0.93 Å and included in the final structural model in the riding-motion approximation with isotropic displacement parameters fixed at 1.2×U eq (C).
Hydrogen atoms associated with nitrogen have been directly located from difference Fourier maps and were included in the structural model with the N-H and H···H (only for the -NH 2 moieties) distances restrained to 0.900 (5) and 1.560 (5) Å, respectively, in order to ensure a chemically reasonable environment. These hydrogen atoms were refined using a riding-motion approximation with isotropic displacement parameters fixed at 1.5×U eq (N).      -3-(4H-1,2,4-triazol-4-yl)-1H-1,2,4 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 R-factors(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.  (5)