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 (C(10)H(9)N) with chloro- and nitro-substituted benzoic acids (C(7)H(4)ClNO(4)), namely, 4-methyl­quinolinium 2-chloro-4-nitro­benzoate, C(10)H(10)N(+)·C(7)H(3)ClNO(4) (−), (I), 4-methyl­quinoline–2-chloro-5-nitro­benzoic acid (1/1), C(10)H(9)N·C(7)H(4)ClNO(4), (II), 4-methyl­quinolinium 2-chloro-6-nitro­benzoate, C(10)H(9.63)N(0.63+)·C(7)H(3.37)ClNO(4) (0.63−), (III), 4-methyl­quinolinium 3-chloro-2-nitro­benzoate, C(10)H(9.54)N(0.54+)·C(7)H(3.46)ClNO(4) (0.54−), (IV), 4-methyl­quinolinium 4-chloro-2-nitro­benzoate, C(10)H(10)N(+)·C(7)H(3)ClNO(4) (−), (V), and 4-methyl­quinolinium 5-chloro-2-nitro­benzoate, C(10)H(10)N(+)·C(7)H(3)ClNO(4) (−), have been determined at 185–190 K. In each compound, the acid and base mol­ecules are linked by a short hydrogen bond between a carb­oxy (or carboxyl­ate) 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 inter­actions between the quinoline ring systems and C—H⋯O hydrogen bonds are observed. Similar layer structures are constructed in (IV)–(VI) through these inter­actions together with π–π inter­actions between the benzene rings of the adjacent acid mol­ecules. A short Cl⋯Cl contact and an N—O⋯π inter­action are present in (I), while a C—H⋯Cl hydrogen bond and a π–π inter­action between the benzene ring of the acid mol­ecule and the quinoline ring system in (II), and a C—H⋯π inter­action in (III) are observed. Hirshfeld surfaces for the title compounds mapped over d (norm) and shape index were generated to visualize the weak inter­molecular inter­actions.