Hydrogen-bonded network in the salt 4-methyl-1H-imidazol-3-ium picrate

A salt formed from 4-methylimidazole and picric acid was obtained in methanol solution. A three-dimensional hydrogen-bonded network is observed which can be topologically simplified into a uninodal 5-connected {42.85} net.

In the title molecular salt, C 4 H 7 N 2 + ÁC 6 H 2 N 3 O 7 À , the phenolic proton of the starting picric acid has been transferred to the imidazole N atom. The nitro groups are twisted away from the benzene ring plane, making dihedral angles of 12.8 (2), 9.2 (4) and 29.3 (2) . In the crystal, the component ions are linked into chains along [010] via N-HÁ Á ÁO and bifurcated N-HÁ Á Á(O,O) hydrogen bonds. These chains are further linked by weak C-HÁ Á ÁO hydrogen bonds into a threedimensional network. The complex three-dimensional network can be topologically simplified into a 4-connected uninodal net with the point symbol {4.8 5 }.

Chemical context
Co-crystallization, the crystallization of more than one solid component into a new compound, forming a new co-crystal or molecular salt, is a well known research field involving, for example, active pharmaceutical ingredients (Aitipamula et al., 2015;Weyna et al., 2012;Robinson, 2010;Arenas-García et al., 2010) and crystal engineering (Manoj et al., 2014). 4-Methylimidazole is an often used pharmaceutical intermediate (Shimpi et al., 2014). The study of its crystallization can facilitate its related organic synthesis and theoretical optimization calculations. Picric acid, as a strong organic protondonating reagent, is often adopted 2as an organic acid in the synthesis of co-crystallized complexes. Herein, we report the crystal structure of the molecular salt, 4-methylimidazolium picrate, (I). Future work will concentrate on how the crystallization behavior is affected by the solvent and temperature.

Structural commentary
The asymmetric unit of (I) consists of one 4-methylimidazolium cation and one picrate anion (Fig. 1). The phenolic proton in the original picric acid starting material was ISSN 2056-9890 transferred from the picric acid OH group to the imidazole nitrogen atom, forming a molecular salt. In the picrate anion, the C-O phenol bond distance is shorter than in an earlier reported un-deprotonated compound [1.33 (2) Å ; Bertolasi et al., 2011] with a value of 1.244 (2) Å in (I). The adjacent C1-C2 [1.453 (2) Å ] and C1-C6 [1.457 (3) Å ] bonds are also lengthened from the values expected in a completely delocalized benzene ring. The C2-C1-C6 angle [111.0 (2) ] is smaller by ca 10 than the average value of the other five phenyl inner angles [121.8 (1) ]. This is mainly due to the electron-withdrawing effect of the three nitro groups attached to the aromatic system, delocalizing electron density on the phenolate oxygen atom over the system. The three nitro groups, N1/O2/O3, N2/O4/O5 and N3/O6/O6, are twisted away from the benzene ring plane, making dihedral angles of 12.8 (2), 9.2 (4) and 29.3 (2) , respectively. In the 4-methylimidazolium cation, the C9-N4

Supramolecular features
In the crystal structure of (I), the component ions are linked into chains along [010] by N-HÁ Á ÁO hydrogen bonds (

Figure 2
Part of the crystal structure of (I), showing the formation of the three-dimensional network. N-HÁ Á ÁO Hydrogen bonds and C-HÁ Á ÁO interactions are shown as green dashed lines. For the sake of clarity, H atoms not involved in the motif have been omitted.

Figure 1
Molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines.
dimensional framework. In the cation, all H atoms except for the methyl group H atoms act as hydrogen-bond donors. Each cation is bonded to four adjacent picrate anions. In turn, each picrate anion utilizes the one phenolic and four nitro oxygen atoms, acting as hydrogen-bond acceptors, linked to four 4methylimidazolium cations. No other interactions such asand C-HÁ Á Á are observed (Spek, 2009). In order to better understand the three-dimensional structure, we can regard both the cation and anion as 4-connected nodes (Fig. 3), i.e. each one 4-methylimidazolium ion links with four other picrate ions, and vice versa. Thus, the whole network is simplified into a uninodal 4-connected net with the point symbol {4.8 5 } (Baburin & Blatov, 2007;Blatov et al., 2014) (Fig. 4).

Database survey
A CSD search (CSD Version 5.37 plus one update; Groom et al., 2016) found some analogs of the title compound, viz.

Synthesis and crystallization
Equivalent molar amounts of 4-methyl imidazole (1.0 mmol, 80.0 mg) and picric acid (1 mmol, 230.0mg) were dissolved in 95% methanol (40.0 ml). The mixture was stirred for half an hour at room temperature and then filtered. The resulting yellow solution was kept in air for two weeks. Needle-shaped yellow crystals of (I) suitable for single-crystal X-ray diffraction analysis were grown at the bottom of the vessel by slow evaporation of the solution. The crystals were separated by filtration (yield, 75%, ca 0.23 g).

Figure 3
Part of the crystal structure of (I), showing the topologically connected relationship between 4-methylimidazolium and picrate ions (shown as gray and pink balls, respectively).
atoms were found in Fourier difference maps; N-H distances were refined freely with U iso (H) = 1.2U eq (N).

Computing details
Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 2001); data reduction: SAINT-Plus (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.