Role of pK a in establishing the crystal structures of six hydrogen-bonded compounds of 4-methylquinoline with different isomers of chloro- and nitro-substituted benzoic acids

The structures of the six hydrogen-bonded 1:1 compounds of 4-methylquinoline with 2-chloro-4-nitrobenzoic acid, 2-chloro-5-nitrobenzoic acid, 2-chloro-6-nitrobenzoic acid, 3-chloro-2-nitrobenzoic acid, 4-chloro-2-nitrobenzoic acid and 5-chloro-2-nitrobenzoic acid have been determined at 185–190 K. In each crystal, the acid and base molecules are linked by a short hydrogen bond between a carboxy/carboxylate O atom and an N atom of the base.


Structural commentary
The molecular structures of compounds (I)-(VI) are shown in Fig. 1. In each compound, the acid and base molecules are linked by a short hydrogen bond between the O atom of the carboxy (or carboxylate) group and the N atom of the base with OÁ Á ÁN distances of 2.5652 (14), 2.556 (3), 2.5485 (13), 2.5364 (13), 2.5568 (13) and 2.5252 (11) Å , respectively, for compounds (1)-(VI) (Tables 2-7). In (III) and (IV), the H atoms in these hydrogen bonds are each disordered over two sites with O site:N site occupancies of 0.37 (3):0.63 (3) and 0.46 (3):0.54 (3), respectively, for (III) and (IV). In (I), (V) and (VI), the H atoms in the hydrogen bonds are located at the N site, while in (II) they are located at the O-atom site. In addition, a weak C-HÁ Á ÁO hydrogen bond is observed in each of the acid-base units of (I) and (VI) (C15-H15Á Á ÁO2; Tables 2 and 7). The nitro group in (III) is disordered over two orientations around the N1-C6 bond with occupancies of 0.46 (3) and 0.54 (3).
The dihedral angles made by the benzene C1-C6 ring, the carboxy/carboxylate O1/C7/O2 plane and the nitro O3/N1/O4 plane of the acid, and the quinoline N2/C8-C16 ring system of the base in each hydrogen-bonded acid-base unit of (I)-(VI) are summarized in Table 1, together with those in compounds of other quinoline derivatives with chloro-and nitro-substituted benzoic acids, which contain similar hydrogen-bonded acid-base units (Gotoh & Ishida, 2009, 2011, 2019a,b, 2020. The H-atom position in the short hydrogen bond and the ÁpK a value of each compound are also given in Table 1. In each acid-base unit of compounds of (I) and (III)-(VI), the acid C1-C6 ring and the quinoline N2/C8-C16 ring system are considerably twisted with respect to each other with dihedral angles of 58.90 (4)-69.15 (5) , which are much larger than those of other compounds. In the acid-base unit of (II), the acid ring and the quinoline ring system are slightly twisted by 13.18 (10) , which is still larger compared with those of quinoline-2-chloro-5-nitrobenzoic acid [1.92 (4) ] and 6-methylquinoline-2-chloro-5-nitrobenzoic acid [2.15 (4) ]. These results suggest that the methyl group substituted to the quinoline ring system at the 4-position has an effect on the molecular packing, which prevents the aromatic rings of the acid and base lying in the same plane in the crystal.
In all the compounds of 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitrobenzoic acid, the nitro O3/N1/O4 group is approximately perpendicular to the benzene C1-C6 ring with dihedral angles of 74.4 (3)-88.54 (13) , while in the 2-chloro-6nitrobenzoic acid molecule of compound (III), where the nitro group and the Cl atom are adjacent to the carboxy group, the carboxy O1/C7/O2 group is almost perpendicular to the benzene ring with a dihedral angle of 84.53 (16) . In the compounds of 5-chloro-2-nitrobenzoic acid, the nitro and carboxy/carboxylate groups are both twisted by 33.31 (13)-57.13 (11) out of the benzene ring plane. These large twists are mainly ascribable to intramolecular steric repulsion between the nitro group and the carboxy/carboxylate group. Table 1 Dihedral angles in the acid-base unit ( ), hydrogen position and ÁpK a .

Figure 4
A packing diagram of (II) viewed along the c axis, showing the tape structure formed via the C-HÁ Á ÁO hydrogen bonds (green dashed lines).
Hirshfeld surfaces for compounds (I)-(VI) mapped over d norm and shape index (Turner et al., 2017;McKinnon et al., 2004McKinnon et al., , 2007 are shown in Fig. 12. Theinteractions are indicated by blue and red triangles on the shape-index surfaces (white circles in Fig. 12). On all the surfaces of the quinoline ring systems except one of the back view of (II), the interactions between the quinoline ring systems are observed. On the surfaces of both acid and base molecules of the back view of (II), theinteractions between the acid ring and the quinoline ring system are shown, while the interactions between the acid rings are observed on the acid ring surfaces of (IV)-(VI). The C-HÁ Á ÁO interactions in (I)-(VI) are indicated by faint-red spots on the d norm surfaces (black arrows). In addition, the short ClÁ Á ÁCl contact and the N-OÁ Á Á interaction in (I), and the C-HÁ Á ÁCl interaction in (II) are shown as faint-red spots on the d norm surfaces (green, magenta and cyan arrows, respectively). On the shape-index surfaces of (I) and (III), large red areas corresponding to the N-OÁ Á Á and C-HÁ Á Á interactions (magenta and violet arrows, respectively) are observed.

Synthesis and crystallization
Single crystals of the title compounds (I)-(VI) were obtained by slow evaporation from acetonitrile solutions of 4-methylquinoline with the appropriate chloro-nitrobenzoic acid in a 1:1 molar ratio at room temperature [120 ml of an acetonitrile solution of 4-methylquinoline (0.20 g) and chloro-nitrobenzoic acid (0.28 g for each acid)].

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 )  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.91 e Å −3 Δρ min = −0.58 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.

4-Methylquinolinium 2-chloro-6-nitrobenzoate (III)
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. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (<1) Cl1 0.71027 (7) 0.10339 (2) 1.02427 (4) 0.05678 (15) (17)  Hydrogen-bond geometry (Å, º) Cg1 is the centroid of the C1-C6 ring.  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.

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

4-Methylquinolinium 5-chloro-2-nitrobenzoate (VI)
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.

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