Abstract
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.
Highlights
The structures of the six hydrogen-bonded 1:1 compounds of 4-methylquinoline (C10H9N) with chloro- and nitro-substituted benzoic acids (C7H4ClNO4), namely, 4-methylquinolinium 2-chloro-4-nitrobenzoate, C10H10N+ÁC7H3ClNO4À, (I), 4-methylquinoline–2-chloro-5-nitrobenzoic acid (1/1), C10H9NÁC7H4ClNO4, (II), 4-methylquinolinium 2-chloro-6-nitrobenzoate, C10H9.63N0.63+ÁC7H3.37ClNO40.63À, (III), 4-methylquinolinium 3-chloro-2-nitrobenzoate, C10H9.54N0.54+ÁC7H3.46ClNO40.54À, (IV), 4-methylquinolinium 4-chloro-2-nitrobenzoate, C10H10N+ÁC7H3ClNO4À, (V), and 4-methylquinolinium 5-chloro-2-nitrobenzoate, C10H10N+ÁC7H3ClNO4À, have been determined at 185–190 K
In our ongoing studies of crystal structures for the system of quinoline derivatives–chloro- and nitro-substituted benzoic acids, we have shown that three compounds of quinoline with 3-chloro-2-nitrobenzoic acid, 4-chloro-2-nitrobenzoic acid and 5-chloro-2-nitrobenzoic acid (Gotoh & Ishida, 2009), and three compounds of 6-methylquinoline with 2-chloro-4-nitrobenzoic acid, 3-chloro-2-nitrobenzoic acid and 4-chloro-2-nitrobenzoic acid (Gotoh & Ishida, 2020) have a short double-well O—HÁ Á ÁN/OÁ Á ÁH—N hydrogen bond
The pKa value of 4-methylquinoline is 5.66, which is slight larger than quinoline and 6-methylquinoline, the system of 4-methylquinoline–chloro- and nitro-substituted benzoic acids is an attractive candidate for studying short hydrogen bonds and weak intermolecular interactions
Summary
The properties of hydrogen bonds formed between organic acids and organic bases depend on the pKa values of the acids and bases as well as the intermolecular interactions in the crystals. The pKa value of 4-methylquinoline is 5.66, which is slight larger than quinoline (pKa = 4.90) and 6-methylquinoline (pKa = 5.20), the system of 4-methylquinoline–chloro- and nitro-substituted benzoic acids is an attractive candidate for studying short hydrogen bonds and weak intermolecular interactions. 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. These large twists are mainly ascribable to intramolecular steric repulsion between the nitro group and the carboxy/carboxylate group
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