Crystal structure of 5-amino-4H-1,2,4-triazol-1-ium pyrazine-2-carboxylate: an unexpected salt arising from the decarboxylation of both precursors

The salt 3-amino-2H,4H-1,2,4-triazolium pyrazine-2-carboxylate was isolated by reacting 5-amino-1H-1,2,4-triazole-3-carboxylic acid and pyrazine-2,3-dicarboxylic acid in the presence of silver nitrate and 1-butyl-3-methylimidazolium bromide in ionothermal conditions.


Chemical context
A remarkable feature of ionothermal synthesis is the fact that ionic liquids (ILs) can act simultaneously as sustainable solvents and structure-directing agents (also known as templates). This has been widely demonstrated by their potential in the discovery of unprecedented crystalline materials . Following our interest in the design and preparation of new types of metal-organic frameworks (MOFs), we have been exploring the use of 5-amino-1H-1,2,4triazole-3-carboxylic acid (H 2 atrc) and pyrazine-2,3-dicarboxylic acid (H 2 Pzdc) as a double-ligand system in the presence of transition metal centers using ionothermal synthetic conditions. In the presence of AgNO 3 the obtained product revealed, however, to be an unexpected organic salt (Bond, 2007) composed of the 3-amino-2H,4H(+)-1,2,4-triazolium cation and the pyrazine-2-carboxylato anion.

Structural commentary
The title compound is a product of decomposition of the H 2 atrc and H 2 Pzdc organic molecules by way of decarboxylation leading to, respectively, 3-amino-2H,4H-1,2,4-triazolium ISSN 2056-9890 [(C 2 H 5 N 4 ) + ] and pyrazine-2-carboxylate [(C 5 H 3 N 2 O 2 ) À ]. The asymmetric unit is composed of one of each of these moieties, as depicted in both the chemical diagram and in Fig. 1.

Supramolecular features
The cation present in the title compound is rich in groups capable of forming strong N-HÁ Á ÁN,O hydrogen-bonding interactions (see Table 1 for further geometrical details), many highly directional with the observed <(D-HÁ Á ÁA) interaction angles being above 165 . These supramolecular contacts are the main driving force which mediate the crystal packing features of the title compound. Indeed, the donation of hydrogen atoms from the cation to the carboxylate group of an adjacent anion (N6-H6BÁ Á ÁO2 and N5-H5Á Á ÁO1) forms the known structurally robust R 2 2 (8) graph-set motif (dashed pink lines in Fig. 2) (Grell et al., 1999). This graph-set motif has already been found in salts containing the title compound cation and carboxylic acids (see Database survey below). Two other interactions, N6-H6AÁ Á ÁN1 (dashed aqua lines) and N4-H4AÁ Á ÁO2, describe a second R 2 2 (9) hydrogen-bond motif. In contrast to the previous graph-set motif, the R 2 2 (9) ring has not been observed in structures containing the titlecompound cation. The zigzag alternation of these two graphset motifs leads to the formation of a highly coplanar supramolecular tape running parallel to the [010] direction of the unit cell (Fig. 2). Adjacent tapes interact by way of weakstacking contacts between triazole and pyrazine rings, with the inter-centroid distance being 3.75 (3) Å (dashed orange lines in Fig. 2).

Database survey
Triazole molecules have been extensively used in the preparation of organic co-crystals (Kastelic et al., 2011;Remenar et al., 2003), and a survey of the Cambridge Structural Database (Groom & Allen, 2014) revealed the existence of about a dozen of crystallographic reports of co-crystals of the title compound cation Essid et al., 2013; research communications The asymmetric unit of the title salt. Non-H atoms are represented as displacement ellipsoids drawn at the 50% probability level, while H atoms are depicted as small spheres with arbitrary radii. The atomic labelling scheme for all non-H atoms is provided. Hydrogen bonds are represented as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). (1) 3.099 (3) 169 (3) Symmetry code: (i) Àx þ 1; y À 1 2 ; Àz þ 3 2 .

Figure 2
Supramolecular tape running parallel to the [010] direction of the unit cell. N-HÁ Á ÁN and N-HÁ Á ÁN hydrogen bonds are depicted as dashed aqua and pink lines, respectively. Graph-set motifs present in the structure are highlighted. For geometric details of the represented supramolecular contacts, see Table 1.stacking interactions between two adjacent supramolecular tapes are shown as orange dashed lines. Joo et al., 2013;Luo et al., 2013;Lynch et al., 1992Lynch et al., , 1998Lynch et al., , 1999Matulková et al., 2007;Smith et al., 1996). The only compounds known with both of the title compound entities present is a bimetallic complex also containing Cd 2+ and NO 3À ions (Chen et al., 2009).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms bound to carbon were placed at idealized positions with C-H = 0.95 Å , and included in the final structural model in a riding-motion approximation with the isotropic thermal displacement parameters fixed at 1.2U eq of the carbon atom to which they are attached. Hydrogen atoms associated with nitrogen atoms were located directly from difference Fourier maps and were included in the model with the N-H and HÁ Á ÁH (only for the -NH 2 groups) distances restrained to 0.90 (1) and 1.55 (1) Å , respectively, in order to ensure a chemically reasonable environment for these groups. These hydrogen atoms were modelled with the isotropic thermal displacement parameters fixed at 1.5U eq (N).  Computer programs: APEX2 (Bruker, 2006), SAINT-Plus (Bruker, 2005), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 2009 Data collection: APEX2 (Bruker, 2006); cell refinement: SAINT-Plus (Bruker, 2005); data reduction: SAINT-Plus (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry.  (3) 169 (3)