Crystal structure and thermal behaviour of pyridinium styphnate

In the crystal of the title molecular salt, the pyridinium cation and the 3-hydroxy-2,4,6-trinitrophenolate anion are linked through bifurcated N—H⋯(O,O) hydrogen bonds forming an (6) ring motif. Impact, friction sensitivity tests and TGA/DTA studies on this compound imply that it is an insensitive high-energy-density material.

In the crystal structure of the title molecular salt, C 5 H 6 N + ÁC 6 H 2 N 3 O 8 À (systematic name: pyridinium 3-hydroxy-2,4,6-trinitrophenolate), the pyridinium cation and the 3-hydroxy-2,4,6-trinitrophenolate anion are linked through bifurcated N-HÁ Á Á(O,O) hydrogen bonds, forming an R 1 2 (6) ring motif. The nitro group para with respect to phenolate ion forms an intramolecular hydrogen bond with the adjacent phenolic -OH group, which results in an S(6) ring motif. The nitro group flanked by the phenolate ion and the phenolic -OH group deviates noticeably from the benzene ring, subtending a dihedral angle of 89.2 (4) . The other two nitro groups deviate only slightly from the plane of the benzene ring, making dihedral angles of 2.8 (4) and 3.4 (3) . In the crystal, the 3-hydroxy-2,4,6-trinitrophenolate anions are linked through O-HÁ Á ÁO hydrogen bonds, forming chains along [100]. These anionic chains, to which the cations are attached, are linked via C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional structure. Impact friction sensitivity tests and TGA/DTA studies on the title molecular salt imply that it is an insensitive high-energydensity material.

Structural commentary
The molecular structure of the title molecular salt is depicted in Fig. 1. The asymmetric unit is comprised of one phenolate anion and a pyridinium cation. The loss of a single proton of the styphnate anion is confirmed by the increase in the bond lengths of the C-C bonds adjacent to the phenolate ion (C1-C2 and C2-C3), which are 1.439 (4) and 1.420 (4) Å , respectively. There is an increase of the value of the bond angles C2-C1-C6 and C2-C3-C4 in the benzene ring to 122.4 (3) and 126.3 (3) , respectively, and a decrease of the C4-C5-C6 bond angle to 120.5 (2) compared to the values observed for free styphic acid (Pierce-Butler, 1982). The nitro group (N3/O5/O6) flanked by the phenolate ion and the phenolic -OH group deviates noticeably from the benzene ring plane, subtending a dihedral angle of 89.2 (4) . The other two nitro groups, O1/N1/O2 and O3/N2/O4, lie close to the plane of the attached benzene ring, making dihedral angles of 2.8 (4) and 3.4 (3) , respectively. The nitro group (N2/O3/O4) para with respect to the phenolate O atom, O7, forms an intramolecular hydrogen bond with the adjacent phenolic -OH group (O8-H8), which results in an S(6) ring motif ( Fig. 1 and Table 1).

Supramolecular features
In the crystal, the cation and anion are linked via bifurcated N-HÁ Á Á(O,O) hydrogen bonds forming an R 2 1 (6) ring motif (Table 1 and Figs. 1 and 2). Inversion-related anions are connected through pairs of C-HÁ Á ÁO hydrogen bonds, forming dimers enclosing an R 2 2 (10) ring motif. The phenolate oxygen, O7, is also bifurcated and forms hydrogen bonds with the protonated nitrogen atom, N4, of the pyridinium moiety and the C-H H atom adjacent to the protonated nitrogen atom, forming an R 1 2 (5) ring motif. The combination of these various N-HÁ Á ÁO, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds leads to the formation of a three-dimensional structure (

Figure 2
A view along the a axis of the crystal packing of the title molecular salt. Hydrogen bonds are shown as dashed lines (see Table 1 for details).

Thermal behaviour and friction sensitivity
As styphnic acid derivatives are energetic salts, the thermal behaviour of the title molecular salt has also been examined. The exothermic decomposition has been observed at four different heating rates (5 K/min, 10 K/min, 20 K/min and 40 K/min). The title molecular salt decomposes (70-80%) in two stages. For each stage, the energy of activation was determined employing Kissinger (1957) [stage I: 27.2 kcal/mol; stage II: 50.5 kcal/mol] and Ozawa (1965) methods [stage I: 28.5 kcal/mol; stage II: 51.8 kcal/mol]. The title molecular salt was observed to be insensitive towards the impact of a 2 kg mass hammer up to the height limit (160 cm) of the instrument, even at the maximum energy level of 31.392 J (Meyer & Kohler, 1993a). The friction sensitivity was determined under defined conditions according to the BAM method (Meyer & Kohler, 1993b). The title molecular salt was insensitive at the maximum force of 360 Newton. The title molecular salt is an insensitive high-energy-density material, confirmed through the impact, friction-sensitivity test, and the energy of activation from TGA/DTA curves.

Synthesis and crystallization
Styphnic acid (2.45 g, 0.01 mol) dissolved in 25 mL of absolute alcohol was mixed with pyridine (0.79 g, 0.01 mol) and stirred continuously for 6 hrs and then kept aside for 2 h. The yellowcoloured amorphous solid obtained was filtered, washed with 30 ml of dry ether and recrystallized from ethylene glycol. Yellow crystals formed in an ethylene glycol solution after slow evaporation at 298 K over a period of 2 weeks (m.p: 455 K; yield: 80%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The NH H atom was located from a difference Fourier map and freely refined. The OH and C-bound H atoms were included in calculated positions and treated as riding atoms: O-H = 0.82, C-H = 0.93 Å , with U iso (H) = 1.5U eq (O) for the hydroxyl H atom and = 1.2U eq (C) for the other H atoms. Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 and SAINT (Bruker, 2004); data reduction: SAINT and XPREP (Bruker, 2004); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

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. 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 > 2sigma(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.