Oxonium picrate

The title compound, H3O+·C6H2N3O7 −, consists of one picrate anion and one oxonium cation. The oxonium cation is located on a crystallographic twofold axis and both its H atoms are disordered, each over two symmetry-equivalent positions with occupancy ratios of 0.75. The picrate anions are also located on twofold axes bisecting the phenolate and p-nitro groups. π–π interactions between the rings of the picrates [centroid-to-centroid distances of 3.324 (2) Å] connect the anions to form stacks along the a-axis direction. The stacks are further joined together by the protonated water molecules through hydrogen bonds to form two-dimensional sheets extending parallel to the ab plane. The sheets are stacked on top of each other along the c-axis direction and connected through C—H⋯O interactions between the CH groups of the benzene rings and the picrate nitro groups, with C⋯O distances of 3.450 (2) Å.

The title compound, H 3 O + ÁC 6 H 2 N 3 O 7 À , consists of one picrate anion and one oxonium cation. The oxonium cation is located on a crystallographic twofold axis and both its H atoms are disordered, each over two symmetry-equivalent positions with occupancy ratios of 0.75. The picrate anions are also located on twofold axes bisecting the phenolate and p-nitro groups.
interactions between the rings of the picrates [centroidto-centroid distances of 3.324 (2) Å ] connect the anions to form stacks along the a-axis direction. The stacks are further joined together by the protonated water molecules through hydrogen bonds to form two-dimensional sheets extending parallel to the ab plane. The sheets are stacked on top of each other along the c-axis direction and connected through C-HÁ Á ÁO interactions between the CH groups of the benzene rings and the picrate nitro groups, with CÁ Á ÁO distances of 3.450 (2) Å .   Table 1 Hydrogen-bond geometry (Å , ).

Comment
It is well known that picric acid is used primarily to prepare explosives, and as an intermediate to manufacture dyes. As a strong organic acid, picric acid forms salts with many N-containing organic bases (Smith et al., 2004;Harrison et al., 2007;Muthamizhchelvan et al., 2005). As an extension of our study concerning organic salts based on picric acid (Jin et al., 2010), we herein report the crystal structure of oxonium picrate.
The single crystal of the title compound ( Fig. 1) with the formula C 6 H 5 N 3 O 8 was obtained by recrystallization of picric acid and 2-chloropyridine from a methanol solution. However the 2-chloropyridine molecules do not appear in the title compound. X-ray diffraction analysis indicated that in the title compound there are one protonated water molecule, and one picrate. The OH group of the picric acid is ionized and the proton is transferred to the water molecule. In the title compound all of the bond distances and angles are in the normal range. The oxonium cation is located on a crystallographic two-fold axis and both its H atoms are disordered over each two symmetry equivalent positions with occupancy rates of 0.75 each.
The benzene ring of the picrate is almost planar. The ortho-nitro groups (N1-O2-O3, and N1A-O2A-O3A) deviate from the benzene ring plane and have a dihedral angle of 25.6 (2)° with the benzene plane, whereas the para-nitro group lies almost in the benzene plane [with a dihedral angle of 2.0 (1)° between the N2-O4-O4A group and the benzene ring].
These structural data are similar to those in other structurally described picrates (Muthamizhchelvan et al., 2005). π-π Interactions between the phenyl rings of the picrates (with Cg-Cg distances of 3.324 (2) Å) connect the picrate anions to form stacks along the a axis direction. Within one stack molecules alternate and are arranged in an antiparallel fashion. The one-dimensional picrate stacks are further linked together by the oxonium ions to form a two-dimensional sheet structure when it is viewed from the c axis direction (Fig. 2). The sheets are further stacked along the c axis direction through CH-O interactions between CH of the benzene rings and the nitro groups of the picrates with C-O distances of 3.450 (2) Å to form a three-dimensional structure.

Experimental
Crystals of oxonium picrate were formed by slow evaporation of its methanol solution at room temperature. Picric acid (23 mg, 0.10 mmol) was dissolved in 4 ml of methanol, and 2-chloropyridine (11 mg, 0.10 mmol) was added to the methanol solution. The solution was then filtered into a test tube and left standing at room temperature. After about one week yellow block crystals were obtained.

Refinement
H atoms H5A and H5B bonded to the oxonium O atom were located in a difference Fourier map and refined isotropically.
The oxonium cation is located on a crystallographic two-fold axis and both H5A and H5B are disordered over each two symmetry equivalent positions, and both have an occupancy of 0.75. Other H atoms were positioned geometrically with C-H = 0.93 Å for aromatic H atoms, and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C).  Fig. 1. The structure of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Symmetry code: (i) -x + 1/2, y, -z.  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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (