Pyridinium 3-nitrobenzoate–3-nitrobenzoic acid (1/1)

The structure of the neutralization product of 3-nitrobenzoic acid and pyridine is reported. A combination of hydrogen bonding and π–π interactions link the molecules, forming long-range order.

The crystal structure of the product of the neutralization reaction between 3nitrobenzoic acid and pyridine is reported. The entities that crystallized are a pyridinium cation, a 3-nitrobenzoate anion and a 3-nitrobenzoic acid molecule in a 1:1:1 molar ratio, C 5 H 6 N + ÁC 7 H 4 NO 4 À ÁC 7 H 5 NO 4 . Distinct sets of hydrogen bonds link the pyridinium and benzoate ions (N-HÁ Á ÁO) and the acid and benzoate moieties (O-HÁ Á ÁO). The hydrogen bonding along withstacking between the acid and benzoate moieties accounts for the long-range ordering of the crystal.

Structure description
The sample crystallizes in the monoclinic crystal system in the Pc space group. Three discrete entities in a 1:1:1 molar ratio comprise the asymmetric unit of this structure: 3nitrobenzoic acid, 3-nitrobenzoate, and a pyridinium cation (Fig. 1). The structure is the result of a neutralization reaction of the carboxylic acid and pyridine (see Synthesis and crystallization section for details). The benzoic acid molecule and benzoate anion in the asymmetric unit are nearly coplanar with a 1.16 (14) dihedral angle. The dihedral angle between the pyridinium and the acid is 99.99 (10) and the dihedral angle between the benzoate anion and pyridinium cation is 99.58 (10) .
Parallel, offset interactions between benzoate anions and between benzoic acid molecules account, in part, for the long-range ordering of the structure. The interactions data reports range in distance from approximately 3.3 to 3.5 Å . Given the offset interactions of the aromatic rings, it appears that interactions are between the nitro groups and the aromatic rings in a manner similar to previously reported structures (Sá nchez-Moreno et al., 2003). A depiction of these interactions is shown in Fig. 3. No interactions are observed from the pyridinium moiety.
Both distinct nitro groups, that is the nitro group on the acid molecule and the nitro group on the benzoate anion, interact with hydrogen atoms on the pyridinium ring. The shortest HÁ Á ÁO NO2 interactions are between O7 and O8 with H19 in one of the positions of the pyridinium ring. Both O atoms of the nitro moiety participate in a nearly symmetric, bifurcated interaction with the H19 atom at distances of 2.695 (3) and 2.714 (3) Å , respectively. The other nitro oxygen atoms (O3 and O4) also display a nearly symmetric bifurcated set of interactions with H16 in the position of the pyridinium ring, at HÁ Á ÁO NO2 distances of 2.882 (3) and 2.820 (3) Å , respectively. The HÁ Á ÁO NO2 interactions observed herein are similar to those observed in some previously reported compounds (Allen et al., 1997;Gu et al., 1999;Vijayvergiya et al., 1995).

Synthesis and crystallization
The reported crystal is an impurity from residual water from an esterification reaction. A sample of 3-nitrobenzoyl chloride (1 eq.) was dissolved in dichloromethane (30 ml) with stirring. Pyridine (2 eq.) and ethanol (5 eq.) were added to the solution, the flask sealed, and the entire mixture allowed to stir overnight at room temperature. A white crystalline solid Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Depiction of the interactions in the title structure. Lines in green display multiple close points of contact between the aromatic rings. Distances in Å .

Figure 1
The asymmetric unit of the structure with 50% probability ellipsoids and hydrogen bonds indicated by red dotted lines.

Special details
Refinement. H atoms on the aromatic (sp 2 ) carbons were included in calculated positions and treated as riding atoms: C -H = 0.95 Å with U iso (H) = 1.2 × U eq (carrier atom). H atoms H1 and H3A were located as residual electron density and allowed to refine freely with U iso (H) = 1.5 × U eq (O or N).