Crystal structure of a new mol­ecular salt: 4-amino­benzenaminium 5-carb­oxy­penta­noate

The asymmetric unit of the title mol­ecular salt (systematic name: 4-amino­anilinium 5-carb­oxy­penta­noate), C(6)H(9)N(2) (+)·C(6)H(9)O(4) (−), consists of half a 4-amino­benzenaminium cation (4-ABA) and half a 5-carb­oxy­penta­noate anion (5-CP); the other half of each ion is generated by inversion symmetry. Protonation of p-phenyl­enedi­amine (PPDA) leads to the formation of a 1:1 salt, but scrutiny of the crystal structure reveals that both of the amine groups of PPDA are partially protonated, with a half-occupied H atom. For the 5-CP anion, an H atom is positioned on an inversion center midway between two O atoms of inversion-related 5-CP ions. In the crystal, the 5-CP anions are linked by the O—H⋯O hydrogen bond to form chains propagating along the [1-10] direction. The chains are linked via N—H⋯O and N—H⋯N hydrogen bonds involving the 4-ABA cations, forming a three-dimensional supra­molecular framework. The title salt was also prepared by mechanochemical synthesis using liquid-assisted grinding (LAG). Its PXRD pattern matches that of the simulated pattern of the crystal structure of the title mol­ecular salt.