5-Amino-1H-1,2,4-triazol-4-ium-3-carboxylate hemihydrate

The asymmetric unit of the title compound, C3H4N4O2·0.5H2O, comprises two whole molecules of 5-amino-1H-1,2,4-triazole-3-carboxylic acid in its zwitterionic form (proton transfer occurs from the carboxylic acid group to the N heteroatom at position 1), plus one water molecule of crystallization. The organic moieties are disposed into supramolecular layers linked by N—H⋯O and N—H⋯N hydrogen bonds parallel to the bc plane. Additional O—H⋯O and N—H⋯O hydrogen bonds involving the water molecules and the organic molecules lead to the formation of double-deck supramolecular arrangements which are interconnected along the a axis via π–π stacking [centroid–centroid distance = 3.507 (3) Å].

We are grateful to the Fundaçã o para a Ciê ncia e a Tecnologia (FCT, Portugal) for their general financial support under the R&D projects PTDC/QUI-QUI/098098/2008 and PTDC/CTM/100357/2008, and for the post-doctoral research grants, Nos. SFRH/BPD/63736/2009 (to JAF) and SFRH/ BPD/47566/2008 (to BL). Thanks are also due to FCT for specific funding toward the purchase of the single-crystal diffractometer.
5-Amino-1H-1,2,4-triazol-4-ium-3-carboxylate hemihydrate J. A. Fernandes, B. Liu, J. C. Tomé, L. Cunha-Silva and F. A. Almeida Paz Comment 5-Amino-1H-1,2,4-triazole-3-carboxylic acid (H 2 Atrc) arises as a promising ligand which can be employed in the preparation of coordination compounds as a consequence of its multiple available sites to establish direct connections with metallic centres. However, surveying the Cambridge Structural Database (Allen, 2002) showed only a handful of crystalline compounds reported to date, namely organic derivatives (Masiukiewicz et al., 2007;Wawrzycka-Gorczyca et al., 2003), the sulfate salt (Ouakkaf et al., 2011) and a three-dimensional metal-organic framework (MOF) with Dy 3+ , very recently reported by Sun et al. (2011). Following our on-going interest on crystal engineering approaches of both organic crystals Amarante, Figueiredo et al., 2009) and metal-organic frameworks (Shi et al., 2008;Paz & Klinowski, 2007;Paz et al., 2005;Paz & Klinowski, 2004), we are currently interested in exploring the coordination capabilities of H 2 Atrc and its residues. The title compound was isolated as a secondary minor product for which we wish to report its crystal structure at the low temperature of 150 K.
The asymmetric unit of title compound comprises two whole molecules of H 2 Atrc in its zwitterionic form (proton transference occurs from the carboxylic acid group to the N heteroatom at position 1) and a water molecule of crystallization as depicted in Fig. 1. The two individual molecular units are almost planar, with the observed deviations being smaller than 0.037 Å. The two organic moieties are also mutually located in the same average plane, with the average planes subtending an angle of ca 7.2 °. This planarity is extended throughout the entire crystal structure with the organic moieties being disposed in layers placed in the bc plane of the unit cell.
Due to the presence of a considerable number of proton donors and acceptors, the crystal structure is rich in hydrogen bonding interactions (see Table 1 for further details). In this context, the structural function of the two non-equivalent organic molecules composing the asymmetric unit is not the same since the hydrogen bonding interactions in which each moiety is involved differ considerably. While the moiety coined as A interacts with other symmetry-related moieties and also with B, the residue coined as B only interacts with A and with water molecules of crystallization. Given the coplanarity of the two non-equivalent H 2 Atrc molecules, mutual interactions occur solely along the aforementioned layers, forming several fused hydrogen-bonded rings (green dashed lines in Figs 2 and 3), best described by the graph set motifs R 2 2 (8), R 3 3 (9) and R 3 2 (11) (Grell et al., 1999). Because of the hydrogen bonds directly involving the crystallographically independent water molecule of crystallization (pink dashed lines in Fig. 3), individual moieties are arranged into double decker layers as depicted in Fig.   3. These supramolecular arrays interact between each other along the a-axis of the unit cell via weak interactions such as π-π stacking. The most structurally relevant of such interactions occurs between two symmetry-equivalent A moieties with an inter-centroid distance of 3.507 (3) Å (orange dashed lines in Figure 3).
supplementary materials sup-2 H 2 Atrc (0.1 mmol, 12.8 mg) was dissolved in ca 15 ml of hot water (ca 358 K). The solution was then cooled to ambient temperature. A second aqueous solution of MnSO 4 (0.1 mmol, 11.7 mg in ca 2 ml) was added drop wise to that containing the dissolved H 2 Atrc ligand. The resulting mixture solution was allowed to stand still over a period of one week and small colourless blocks were formed as a secondary product.

Refinement
All hydrogen atoms bound to nitrogen (organic molecules) and to oxygen (water molecule of crystallization) were directly located from difference Fourier maps and included in the structural model with the O-H and N-H distances restrained to 0.95 (1) Å. The H···H distances in the water molecule and in the -NH 2 groups were further restrained to 1.55 (1) Å in order to ensure a chemically reasonable geometry for these moieties. The U iso of these hydrogen atoms were fixed at 1.5×U eq of the parent nitrogen or oxygen atoms. Fig. 1. Molecular structures of the units composing the asymmetric unit of the title compound, showing the atomic labelling for all atoms. Non-hydrogen atoms are represented as displacement ellipsoids drawn at the 70% probability level. Hydrogen atoms are depicted as small spheres with arbitrary radii.

Fig. 2. N-H···N and N-H···O interactions (dashed green lines)
forming the supramolecular layer placed in the bc plane of the unit cell. The two distinct H 2 Atrc residues are depicted as molecules A and B. Supramolecular arrangements are described by their graph set notation following Grell et al. (1999). For geometric details on the represented supramolecular contacts see Table 1. Symmetry transformations used to generate equivalent atoms have been omitted for clarity. 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.