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

The structures of the six hydrogen-bonded 1:1 compounds of 4-methyl-quinoline (C10H9N) with chloro- and nitro-substituted benzoic acids (C7H4ClNO4), namely, 4-methyl-quinolinium 2-chloro-4-nitro-benzoate, C10H10N+·C7H3ClNO4 -, (I), 4-methyl-quinoline-2-chloro-5-nitro-benzoic acid (1/1), C10H9N·C7H4ClNO4, (II), 4-methyl-quinolinium 2-chloro-6-nitro-benzoate, C10H9.63N0.63+·C7H3.37ClNO4 0.63-, (III), 4-methyl-quinolinium 3-chloro-2-nitro-benzoate, C10H9.54N0.54+·C7H3.46ClNO4 0.54-, (IV), 4-methyl-quinolinium 4-chloro-2-nitro-benzoate, C10H10N+·C7H3ClNO4 -, (V), and 4-methyl-quinolinium 5-chloro-2-nitro-benzoate, C10H10N+·C7H3ClNO4 -, have been determined at 185-190 K. In each compound, the acid and base mol-ecules are linked by a short hydrogen bond between a carb-oxy (or carboxyl-ate) O atom and an N atom of the base. The O⋯N distances are 2.5652 (14), 2.556 (3), 2.5485 (13), 2.5364 (13), 2.5568 (13) and 2.5252 (11) Å, respectively, for compounds (I)-(VI). In the hydrogen-bonded acid-base units of (III) and (IV), the H atoms are each disordered over two positions with O site:N site occupancies of 0.37 (3):0.63 (3) and 0.46 (3):0.54 (4), respectively, for (III) and (IV). The H atoms in the hydrogen-bonded units of (I), (V) and (VI) are located at the N-atom site, while the H atom in (II) is located at the O-atom site. In all the crystals of (I)-(VI), π-π stacking inter-actions between the quinoline ring systems and C-H⋯O hydrogen bonds are observed. Similar layer structures are constructed in (IV)-(VI) through these inter-actions together with π-π inter-actions between the benzene rings of the adjacent acid mol-ecules. A short Cl⋯Cl contact and an N-O⋯π inter-action are present in (I), while a C-H⋯Cl hydrogen bond and a π-π inter-action between the benzene ring of the acid mol-ecule and the quinoline ring system in (II), and a C-H⋯π inter-action in (III) are observed. Hirshfeld surfaces for the title compounds mapped over d norm and shape index were generated to visualize the weak inter-molecular inter-actions.