Two dialkylammonium salts of 2-amino-4-nitrobenzoic acid: crystal structures and Hirshfeld surface analysis

Abstract

The crystal structures of two ammonium salts of 2-amino-4-nitrobenzoic acid are described, namely dimethylazanium 2-amino-4-nitrobenzoate, C2H8N+·C7H5N2O4−, (I), and dibutylazanium 2-amino-4-nitrobenzoate, C8H20N+·C7H5N2O4−, (II). The asymmetric unit of (I) comprises a single cation and a single anion. In the anion, small twists are noted for the carboxylate and nitro groups from the ring to which they are connected, as indicated by the dihedral angles of 11.45 (13) and 3.71 (15)°, respectively; the dihedral angle between the substituents is 7.9 (2)°. The asymmetric unit of (II) comprises two independent pairs of cations and anions. In the cations, different conformations are noted in the side chains in that three chains have an all-trans[(+)-antiperiplanar] conformation, while one has a distinctive kink resulting in a (+)-synclinal conformation. The anions, again, exhibit twists with the dihedral angles between the carboxylate and nitro groups and the ring being 12.73 (6) and 4.30 (10)°, respectively, for the first anion and 8.1 (4) and 12.6 (3)°, respectively, for the second. The difference between anions in (I) and (II) is that in the anions of (II), the terminal groups are conrotatory, forming dihedral angles of 17.02 (8) and 19.0 (5)°, respectively. In each independent anion of (I) and (II), an intramolecular amino-N—H...O(carboxylate) hydrogen bond is formed. In the crystal of (I), anions are linked into a jagged supramolecular chain by charge-assisted amine-N—H...O(carboxylate) hydrogen bonds and these are connected into layersviacharge-assisted ammonium-N—H...O(carboxylate) hydrogen bonds. The resulting layers stack along theaaxis, being connected by nitro-N—O...π(arene) and methyl-C—H...O(nitro) interactions. In the crystal of (II), the anions are connected into four-ion aggregates by charge-assisted amino-N—H...O(carboxylate) hydrogen bonding. The formation of ammonium-N—H...O(carboxylate) hydrogen bonds, involving all ammonium-N—H and carboxylate O atoms leads to a three-dimensional architecture; additional C—H...O(nitro) interactions contribute to the packing. The Hirshfeld surface analysis confirms the importance of the hydrogen bonding in both crystal structures. Indeed, O...H/H...O interactions contribute nearly 50% to the entire Hirshfeld surface in (I).