5-Amino-1H-1,2,4-triazol-4-ium hydrogen oxalate

In the title salt, C2H5N4 +·C2HO4 −, the hydrogen oxalate anions form corrugated chains parallel to the c axis, linked by intermolecular O—H⋯O hydrogen bonds. The 5-amino-1H-1,2,4-triazol-4-ium cations are connected into centrosymmetric clusters via weak C—H⋯N hydrogen bonds forming nine-membered rings with an R 3 3(9) motif. These clusters are interconnected via anions through N—H⋯O hydrogen bonds, building a three-dimensional network.

The crystal structure is based on a three dimensional network of hydrogen oxalic acid anions interconnected by O-H···O hydrogen bonds with lengths of 2.585 Å.
Planar 5-amino-1,2,4-triazolinium cations are located in the cavities of the hydrogen oxalic acid network and connected with anions via linear and bifurcated N-H···O hydrogen bonds. The donor-acceptor distances in these hydrogen bonds attain values from 2.765 to 3.087 Å (Tab. 1 and Fig. 2).
The oxalate ion is maintained by moderate hydrogen bonds that link the oxygen atoms of oxalate ion and the hydrogen of the other oxalate into corrugated chains parallel to the c axis. In addition, there are weak C-H···N hydrogen bonds in the crystal structure between 5-amino-1,2,4-triazilium cations forming an R 3 3 (9) motif ( Fig. 2) (Bernstein et al., 1995).
These cations are interconnected via anions through N-H···O hydrogen bonds, building a three dimensional network.

Experimental
An aqueous solution of H 2 C 2 O 4 (2 mmol in 10 ml water) was added to an aqueous solution of 5-amino-1H-1,2,4-triazole (2 mmol in 10 ml of water). The obtained solution was stirred at 333 K for 30 min and then left to stand at room temperature. Colorless single crystals of the title compound were obtained after some days.

Refinement
The hydrogen atoms bonded to O1 and N1 were located from a difference map and were allowed to refine. The rest of the H atoms were treated as riding, with C-H = 0.93 Å and N-H = 0.86 Å and with U iso (H) = 1.2U eq (C or N).

Computing details
Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf-Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS86 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).   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.