Crystal structures of four isomeric hydrogen-bonded co-crystals of 6-methylquinoline with 2-chloro-4-nitrobenzoic acid, 2-chloro-5-nitrobenzoic acid, 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitrobenzoic acid

The structures of the four isomeric hydrogen-bonded 1:1 co-crystals of 6-methylquinoline with 2-chloro-4-nitrobenzoic acid, 2-chloro-5-nitrobenzoic acid, 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitrobenzoic acid have been determined at 185–190 K. In each crystal, the acid and base molecules are linked by a short O—H⋯N hydrogen bond.

The structures of the four isomeric compounds of 6-methylquinoline with chloro-and nitro-substituted benzoic acids, C 7 H 4 ClNO 4 ÁC 10 H 9 N, namely, 2-chloro-4-nitrobenzoic acid-6-methylquinoline (1/1), (I), 2-chloro-5-nitrobenzoic acid-6-methylquinoline (1/1), (II), 3-chloro-2-nitrobenzoic acid-6methylquinoline (1/1), (III), and 4-chloro-2-nitrobenzoic acid-6-methylquinoline (1/1), (IV), have been determined at 185-190 K. In each compound, the acid and base molecules are linked by a short hydrogen bond between a carboxyl O atom and an N atom of the base. The OÁ Á ÁN distances are 2.5452 (12), 2.6569 (13), 2.5640 (17) and 2.514 (2) Å , respectively, for compounds (I)-(IV). In the hydrogen-bonded acid-base units of (I), (III) and (IV), the H atoms are each disordered over two positions with O site:N site occupancies of 0.65 (3):0.35 (3), 0.59 (4):0.41 (4) and 0.48 (5):0.52 (5), respectively, for (I), (III) and (IV). The H atom in the hydrogen-bonded unit of (II) is located at the O-atom site. In all of the crystals of (I)-(IV),interactions between the quinoline ring system and the benzene ring of the acid molecule are observed. In addition, ainteraction between the benzene rings of adjacent acid molecules and a C-HÁ Á ÁO hydrogen bond are observed in the crystal of (I), and C-HÁ Á ÁO hydrogen bonds and OÁ Á ÁCl contacts occur in the crystals of (III) and (IV). These intermolecular interactions connect the acid and base molecules, forming a layer structure parallel to the bc plane in (I), a column along the a-axis direction in (II), a layer parallel to the ab plane in (III) and a three-dimensional network in (IV). Hirshfeld surfaces for the title compounds mapped over d norm and shape index were generated to visualize the weak intermolecular interactions.

Structural commentary
The molecular structures of compounds (I)-(IV) are shown in Fig. 1. In each compound, the acid and base molecules are linked by a hydrogen bond between the carboxy group and the N atom of the base. In (I), (III) and (IV), short hydrogen bonds are observed with NÁ Á ÁO distances of 2.5452 (12), 2.5640 (17) and 2.515 (2) Å , respectively. (Tables 1, 3
In the hydrogen-bonded acid-base unit of compound (I), the quinoline ring system (N2/C8-C16) and the benzene ring (C1-C6) are almost coplanar with a dihedral angle of 1.11 (4) , while the quinoline ring system and the carboxy group (O1/ C7/O2) of the acid are twisted to each other with a dihedral angle of 28.59 (12) . In the acid molecule, the benzene ring makes dihedral angles of 29.36 (12) and 8.24 (11) , respectively, with the carboxy group and the nitro group (O3/N1/ O4).
Similar to (I), the quinoline ring system (N2/C8-C16) in the hydrogen-bonded acid-base unit of (II) makes dihedral angles of 2.15 (4) and 24.51 (15) , respectively, with the benzene ring and the carboxy group. The benzene ring makes dihedral angles of 22.63 (15) and 0.77 (14) , respectively, with the carboxy group and the nitro group.     Table 3 Hydrogen-bond geometry (Å , ) for (III).  Hydrogen-bond geometry (Å , ) for (IV). Compound (III) crystallizes in the non-centrosymmetric space group P2 1 2 1 2 1 . In the acid-base unit, the quinoline ring system and the benzene ring of the acid are slightly twisted to each other with a dihedral angle of 14.50 (5) . The quinoline ring system and the carboxy group are also slightly twisted with a dihedral angle of 12.55 (18) . The benzene ring makes dihedral angles of 3.14 (18) and 85.04 (11) , respectively, with the carboxy group and the nitro group.
Compound (IV) crystallizes in the non-centrosymmetric space group Cc. In the acid-base unit, the quinoline ring system and the benzene ring of the acid are twisted to each other with a dihedral angle of 30.39 (9) . The quinoline ring system and the carboxy group are also twisted with a dihedral angle of 21.7 (3) . The benzene ring makes dihedral angles of 16.4 (3) and 74.4 (3) , respectively, with the carboxy group and the nitro group.

Figure 7
A packing diagram of (IV), showing the zigzag chain structure along the c axis via O-HÁ Á ÁN/OÁ Á ÁÁH-N and C-HÁ Á ÁO hydrogen bonds. H atoms not involved in the hydrogen bonds are omitted for clarity. Symmetry code: (i) x, Ày + 1, z + 1 2 .

Synthesis and crystallization
Single crystals of the title compounds (I)-(IV) were obtained by slow evaporation from acetonitrile solutions of 6-methylquinoline with chloro-nitrobenzoic acids in a 1:1 molar ratio at room temperature [80 ml acetonitrile solution of 6-methylquinoline (0.20 g) and chloro-nitrobenzoic acid (0.28 g for each acid)].

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All H atoms in compounds (I)-(IV) were found in difference-Fourier maps. The O-bound H atom in (II) was refined freely; the refined distance is given in Table 2. For (I), (III) and (IV), H atoms in the NÁ Á ÁHÁ Á ÁO hydrogen bonds were found to be disordered over two positions in difference-Fourier maps. Since the site-occupancy factors and isotropic displacement parameters are strongly collated, the positional parameters and occupancy factors were refined, with bond length restraints of N-H = 0.88 (1) Å and O-H = 0.84 (1) Å , and with U iso (H) = 1.5U eq (N or O); the refined distances are given in Tables 1, 3 and 4. Other H atoms were positioned geometrically (C-H = 0.95 Å ) and treated as riding, with U iso (H) = 1.2 or 1.5U eq (C).

2-Chloro-4-nitrobenzoic acid-6-methylquinoline (1/1) (I)
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.48 e Å −3 Δρ min = −0.26 e Å −3 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.

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

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (<1) Cl1 0.53139 (7) 0.88067 (5) (9) 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.