Hydrogen bonding in the crystal structure of the molecular salt of pyrazole–pyrazolium picrate

A 2:1 organic salt formed from pyrazole and picric acid was obtained from methanol solution. In the crystal, N—H⋯O hydrogen bonds give rise to a hydrogen-bonded chain along [100]. Adjacent [100] chains are linked by a weak C—H⋯O interaction.


Chemical context
Research interest on co-crystals or organic complex salts in recent years has been prompted by their potential utilization in the pharmaceutical industry (Blagden et al., 2014;Duggirala et al., 2016). Imidazole and pyrazole derivatives are often used as co-crystallized pharmaceutical ingredients (Shimpi et al., 2014). Our investigations involve studies of weak intermolecular interactions in co-crystallized compounds. As part of our continuing study on organic salts formed by imidazole derivatives and picric acid (Song et al., 2016;Su et al., 2008), we report herein the crystal structure of the title compound (I).

Structural commentary
The asymmetric unit of the title compound is shown in Fig. 1. It consists of one picrate anion and two pyrazole molecules, which are connected by an N-HÁ Á ÁN hydrogen bond (Table 1), forming a dimeric pyrazolium monocation. The H atom of the hydrogen bond is disordered over both pyrazole molecules. In the dimeric monocation, the two pyrazole rings form a dihedral angle of 74.6 (1) . In the anion, the C-O phenol bond [1.257 (3)Å ] is shorter by ca 0.05Å than an average C-O single bond in a neutral picric acid molecule [1.308 (2)Å ] calculated statistically by analysis of a CSD search (Groom et al., 2016;Allen, 2002). The C1-C2 [1.438 (4)Å ] and C1-C6 [1.449 (4)Å ] bonds are significantly longer than the other four benzene C-C bonds [1.367 (4)-1.380 (4)Å ]. The C2-C1-C6 [111.9 (2) ] angle is smaller than the ideal value of 120 for a regular hexagon and the other five benzene inner angles of 119.0 (3)-124.4 (3). All variations of bond lengths and angles demonstrate that the negative charge on the phenol oxygen atom is delocalized over the aromatic ring, giving double-bond character for the C1-O1 bond due to the electron-withdrawing effect of the three nitro groups. This is similar to what is observed in some picrate-containing analogs (Zakharov et al., 2015;Gomathi & Kalaivani, 2015). The mean planes of the nitro groups in the anion, are twisted from the benzene ring by dihedral angles of 30.8 (2), 4.8 (3) and 27.2 (4) for N1/O2/ O3, N2/O4/O5 and N3/O6/O7, respectively. The two orthonitro groups are twisted out of the benzene ring to a greater extent than the para-nitro group. This is most likely due to the steric hindrance between the ortho-nitro groups and the phenolic oxygen atom.

Database survey
A search of the Cambridge Structural Database (CSD Version 5.37 plus one update; Groom et al., 2016) indicates there are some analogs prepared from picric acid and pyrazole derivatives, viz. SASKII, SASLAB, SASKUU, SASLUB (Singh et al., 2012) and SASKII01 (Dhanabal et al., 2013). A similar solvated organic adduct, C 5 H 9 N 2 + ÁC 6 H 2 N 3 O 7 À (SASKII; Singh et al., 2012) indicates that the solvent used for the crystallization process can affect the final product in which the ratio of component ions are different.

Synthesis and crystallization
Pyrazole (20.0 mmol, 136.0 mg) and picric acid (10. 0 mmol, 230.0mg) were dissolved in a 2:1 molar ratio in 95% methanol (50.0 ml). The mixture was stirred for an hour at 323 K and then cooled to room temperature and filtered. The resulting yellow solution was kept in air for two weeks. Needle-like yellow crystals of (I) suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of the solution. The crystals were separated by filtration (yield, 60%, ca 0.22 g).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were positioned geometrically with C-H = 0.93 Å (aromatic) and refined in a riding-model approximation with U iso (H) = 1.2U eq (C). H atoms bonded to N atoms were refined with a Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Part of the crystal structure of (I), showing the formation of hydrogenbonded columns along [100]. For clarity, H atoms not involved in the motif have been omitted. Green and red dashed lines indicate the N-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁO hydrogen bonds, respectively.

Figure 1
The molecular structure of (I), showing the atom-numbering scheme.  Computer programs: SMART and SAINT (Bruker, 2001), SHELXS and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 2006 Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXTL (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.