Crystal structure of 4-chloro-2-nitrobenzoic acid with 4-hydroxyquinoline: a disordered structure over two states of 4-chloro-2-nitrobenzoic acid–quinolin-4(1H)-one (1/1) and 4-hydroxyquinolinium 4-chloro-2-nitrobenzoate

The title compound was analysed as a disordered structure over two states, viz. co-crystal and salt, accompanied by a keto–enol tautomerization in the base molecule. In the compound, the acid and base molecules are linked by a short hydrogen bond [O⋯O = 2.4393 (15) Å], in which the H atom is disordered over two positions with equal occupancies.


Chemical context
In our previous study on D-HÁ Á ÁA hydrogen bonding (D = N, O, or C, A = N, O or Cl) in chloro-and nitro-substituted benzoic acid-pyridine derivative systems, we have shown that several compounds, namely, three compounds of quinoline with 3-chloro-2-nitrobenzoic acid, 4-chloro-2-nitrobenzoic acid and 5-chloro-2-nitrobenzoic acid (Gotoh & Ishida, 2009), two compounds of phthalazine with 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitorbenzoic acid (Gotoh & Ishida, 2011), and 3-chloro-2-nitrobenzoic acid-isoquinoline (Gotoh & Ishida, 2015), have a short double-well OÁ Á ÁHÁ Á ÁN hydrogen bond between the carboxy O atom and the aromatic N atom. Hydroxyquinolines, which have hydrogen-bond acceptor as well as donor groups, appear attractive as a base molecule in the above systems for investigating the hydrogen bonds (Babu & Chandrasekaran, 2014;Gotoh & Ishida, 2019). We report here the crystal structure of the title compound, in which there exists another type of short double-well hydrogen bond, namely, an OÁ Á ÁHÁ Á ÁO hydrogen bond between the acid and base molecules, accompanied by a keto-enol tautomerization of the base molecule.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The acid and base molecules are held together by a short hydrogen bond between atom O1 of the acid molecule and atom O5 of the base [O1Á Á ÁO5 = 2.4393 (15) Å ; Table 1]. In the hydrogen bond, the H atom is disordered as indicated in the difference-Fourier map (Fig. 2), in which a broad peak along the line connecting the two O atoms is observed. Although two distinct peaks were not clearly observed in the map, the H atom was successfully analysed as being disordered over two positions of the O1 and O5 sites with equal occupancies. The title compound is, thus, interpreted as a disordered structure over two states, viz. the co-crystal, 4-chloro-2nitrobenzoic acid-4(1H)-quinolinone (1/1), and the salt, 4hydroxyquinolinium 4-chloro-2-nitrobenzoate, accompanied by a keto-enol tautomerization in the base molecule. The C10-O5 bond length [1.2956 (18) (Allen et al., 1987), supporting that hypothesis that the base molecule has an intermediate state between the keto and enol forms.

Figure 2
A difference-Fourier map of the title compound associated with the OÁ Á ÁHÁ Á ÁO hydrogen bond between the acid and the base. The map was calculated on the plane of atoms O1, C7 and O5 from a model containing all atoms apart from the H atom in the hydrogen bond.

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.

Synthesis and crystallization
Single crystals of the title compound suitable for X-ray diffraction analysis were obtained by slow evaporation from     (2) and (3)] and -[arrows (4) and (5)] interactions.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms except one H atom between the two O atoms (O1 and O5) of the acid and base molecules were found in a difference-Fourier map. A broad peak in a difference-Fourier map between atoms O1 and O5 was observed (Fig. 2). Although two distinct peaks were not observed in the map, the H atom between the O atoms was analysed using a model of an H atom disordered over two positions. The occupancies of the two sites were refined to 0.47 (4) and 0.53 (4) for H1A (O1 site) and H1B (O5 site), respectively, with bond restraints of O-H = 0.84 (1) Å and with U iso (H) = 1.5U eq (O). In the final refinement, the occupancies were fixed at 0.5, and one outlier (6,8,13) was omitted. The N-bound H atom was refined freely [refined distance: N2-H2 = 0.89 (2)    Data collection: PROCESS-AUTO (Rigaku, 2006); cell refinement: PROCESS-AUTO (Rigaku, 2006); data reduction: PROCESS-AUTO (Rigaku, 2006); program(s) used to solve structure: SHELXT2018 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows and WinGX (Farrugia, 2012); Mercury (Macrae et al., 2008); software used to prepare material for publication: CrystalStructure (Rigaku, 2018) and PLATON (Spek, 2015).

4-Chloro-2-nitrobenzoic acid-quinolin-4(1H)-one (1/1)-4-hydroxyquinolinium 4-chloro-2-nitrobenzoate
Crystal data 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.