The syntheses and crystal structures of three molecular salts of protonated 3,4-diaminobenzoic acid, it viz. 2-amino-5-carboxyanilinium chloride, C$sb 7$H$sb 9$N$sb 2$O$sb 2$$sp +$$Cl$sp $-$$, (I), 2-amino-5-carboxyanilinium bromide, C$sb 7$H$sb 9$N$sb 2$O$sb 2$$sp +$$Br$sp $-$$, (II), and 2-amino-5-carboxyanilinium nitrate monohydrate, C$sb 7$H$sb 9$N$sb 2$O$sb 2$$sp +$$NO$sb 3$$sp $-$$$H$sb 2$O, (III), are described. The cation is protonated at the it meta-N atom (with respect to the carboxy group) in each case. In the crystal of (I), carboxylic acid inversion dimers linked by pairwise O---H$O hydrogen bonds are seen and each N---H group forms a hydrogen bond to a chloride ion to result in (100) undulating layers of chloride ions bridged by the inversion dimers into a three-dimensional network. The extended structure of (II) features O---H$Br, N---H$Br and N---H$O hydrogen bonds: the last of these generates it C(7) chains of cations. Overall, the packing in (II) features undulating (100) sheets of bromide ions alternating with the organic cations. Intermolecular interactions in the crystal of (III) include O---H$O, O---H$(O,O), N---H$O, N---H$N and O---H$N links. The cations are linked into (001) sheets, and the nitrate ions and water molecules form undulating chains. Taken together, alternating (001) slabs of organic cations plus anions/water molecules result. Hirshfeld surfaces and fingerprint plots were generated to give further insight into the intermolecular interactions in these structures. The crystal used for the data collection of (II) was twinned by rotation about  in reciprocal space in a 0.4896(15):0.5104(15) ratio.
Bibliographical noteWe thank the EPSRC National Crystallography Service (University of Southampton) for the intensity data collections.
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- benzoic acid
- molecular salt
- hydrogen bonds
- Hirshfeld surface
- crystal structure