Crystal structure and Hirshfeld surface analysis of 1,2,4-triazolium hydrogen oxalate

Charge-assisted N—H⋯O and O—H⋯O hydrogen bonds along with π–π interactions stabilize the crystalline state. Intermolecular interactions are quantified by Hirshfeld surface analysis.


Chemical context
The oxalate anion (C 2 O 4 2-), i.e. the complete deprotonation product of oxalic acid (C 2 H 2 O 4 ), is a small, rigid, planar species and has been widely used as a ligand in the formation of coordination polymers (Gruselle et al., 2006;Abraham et al., 2014). This ligand possesses four electron-donating O atoms and can display versatile coordination modes upon metal complexation. As a result, a large number of compounds with multi-dimensional coordination networks with short intermetallic distances have been synthesized along with the investigation of interesting properties (Clemente-Leó n et al., 2011). During our synthetic efforts to develop novel lanthanide coordination polymers with rigid, short, organic ligands including the oxalate anion, the title salt C 2 H 4 N 3 + ÁC 2 HO 4 À (I) was obtained unexpectedly from the reaction of terbium(III) chloride hexahydrate, oxalic acid, and 1,2,4-triazole in water at room temperature.
Herein, we describe the crystal structure and Hirshfeld surface analysis of the title salt (I).

Supramolecular features
Extensive hydrogen-bonding interactions in the crystal of the title salt (I) are observed, the numerical values of which are collated in Table 1. As shown in Fig. 2, each hydrogen oxalate anion is linked with another anion by O-HÁ Á ÁO hydrogen bonds into an infinite chain running parallel to [100]. The anionic chains are linked by charge-assisted + N-HÁ Á ÁO À hydrogen bonds involving the 1,2,4-triazolium cations into sheets extending parallel to (011). Additionally, intrasheet C-HÁ Á ÁO hydrogen and C-HÁ Á ÁN hydrogen bonds involving the cationic molecules are also observed. The sheets are further stacked throughinteractions between the 1,2,4-triazolium rings [centroid-to-centroid distance = 3.642 (3) Å , normal distance = 3.225 (3) Å , slippage 1.691 Å ], Fig. 3, resulting in the formation of a three-dimensional supramolecular network.

Figure 3
A view of thestacking interactions along with the C-HÁ Á ÁN hydrogen bonds in the title salt (I).

Figure 1
The structures of the molecular entities in the title salt (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen bonds are shown as dotted lines.
bution of only 7.7% to the entire Hirshfeld surfaces of the title salt.

Synthesis and crystallization
An aqueous solution (5 ml) of oxalic acid (0.09 g, 0.01 mol) and 1,2,4-triazole (0.07 g, 0.01 mmol) was added dropwise to an aqueous solution (5 ml) of TbCl 3 Á6H 2 O (0.37 g, 0.01 mol) under constant stirring for one h. The resulting solution was filtered to remove any undissolved solid. The filtrate was allowed to slowly evaporate at room temperature. After two weeks, colourless block-shaped crystals of the title salt (I) suitable for X-ray analysis were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The carboxyl and triazolium H atoms were located in difference-Fourier maps and were freely refined. Carbon-bound H atoms were placed in calculated positions and refined using a riding-model approximation with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). Computer programs: APEX3 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

1,2,4-Triazolium hydrogen oxalate
Crystal data 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.