Crystal structures of isoquinoline–3-chloro-2-nitrobenzoic acid (1/1) and isoquinolinium 4-chloro-2-nitrobenzoate

The structures of two isomeric compounds of isoquinoline with 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitrobenzoic acid have been determined at 190 K. In each compound, the acid and base molecules are held together by a short hydrogen bond between a carboxy O atom and a base N atom. In the hydrogen-bonded unit of the former, the H atom is disordered over two positions, while in the latter, an acid–base interaction involving H-atom transfer occurs and the H atom is located at the N site.


Structural commentary
The molecular structure of (I) is shown in Fig. 1. The base and acid molecules are held together by a short hydrogen bond between the N atom of the base and the carboxy O atom. The H atom in the hydrogen bond is disordered over two positions with the N and O sites occupancies refined to 0.30 (3) and 0.70 (3), respectively. In addition, a C-HÁ Á ÁO hydrogen bond (C8-H8Á Á ÁO2; Table 1) is observed in the hydrogen-bonded acid-base unit. In the unit, the isoquinoline ring system, the carboxy group and the benzene ring of the acid molecule are almost coplanar with each other; the carboxy group makes dihedral angles of 5.35 (15) and 5.91 (15) , respectively, with the isoquinoline ring system and the benzene ring, and the dihedral angle between the isoquinoline ring system and the benzene ring is 1.21 (4) . On the other hand, the nitro group and the benzene ring are almost perpendicular with a dihedral angle of 83.71 (13) .
The molecular structure of (II) is shown in Fig. 2. An acidbase interaction involving H-atom transfer occurs and the base and acid molecules are linked by an N + -HÁ Á ÁO À hydrogen bond. In the hydrogen-bonded unit, the isoquinoline ring system make dihedral angles of 54.12 (15) and 71.89 (5) , respectively, with the carboxy group and the benzene ring of the acid. In the acid molecule, the benzene ring makes dihedral angles of 26.59 (15) and 67.69 (15) , respectively, with the carboxy and nitro groups.

Supramolecular features
In the crystal of (I), the hydrogen-bonded acid-base units are linked by a C-HÁ Á ÁO hydrogen bond (C5-H5Á Á ÁO2 i ; Table 1), forming a tape structure along the b-axis direction (Fig. 3). Adjacent tapes, which are related by an inversion center, are further linked throughinteractions between the benzene ring of the acid and the isoquinoline ring system 32 Gotoh and Ishida C 9 H 7.3 NÁC 7 H 3.7 ClNO 4 and C 9 H 8 N + ÁC 7 H 3 ClNO 4 À Acta Cryst. A molecular view of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. The disordered O-HÁ Á ÁN/N-HÁ Á ÁO hydrogen bond and the C-HÁ Á ÁO interaction are indicated by dashed lines.

Synthesis and crystallization
Crystals of compounds (I) and (II) were obtained by slow evaporation from acetonitrile solutions of isoquinoline with the corresponding chloro-and nitro-substituted benzoic acid in a 1:1 molar ratio at room temperature [50 ml acetonitrile solution of isoquinoline (0.202 g) and 3-chloro-2nitrobenzoic acid (0.315 g) for (I), and 150 ml solution of isoquinoline (0.204 g) and 4-chloro-2-nitrobenzoic acid (0.318 g) for (II)].

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms in compounds (I) and (II) were found in difference Fourier maps. The H atom in (I), which is involved in the NÁ Á ÁHÁ Á ÁO hydrogen bonds, was found to be disordered over two positions in a difference Fourier map. Since the site-occupancy factors and isotropic displacement parameters were strongly correlated, the occupancy factors were refined, with U iso (H) = 1.5U eq (N or O). The positional parameters were refined with bond restraints of O-H = 0.84 (2) Å and N-H = 0.88 (2) Å . Atom H2 in (II) was refined freely [refined distance N2-H2 = 0.91 (2) Å ]. Other H atoms of compounds (I) and (II) were positioned geometrically (C-H = 0.95 Å ) and treated as riding, with U iso (H) = 1.2U eq (C).