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Crystallography reveals two polymorphs for the salt [4-(4-acetylphenyl)piperazin-1-ium][2-amino-4-nitrobenzoate], a monoclinic form ( 2 ; modelled as  P 2 1 / n  with Z′=4) formed directly from the reaction mixture, and a triclinic form ( 1 ; Z′=1) isolated from recrystallisation. Relatively minor differences are noted in the conformations of the anions and of the cations, mainly relating to the twist of, respectively, the carboxylate groups and piperazin-1-ium rings with respect to the phenyl rings they are connected to. The key feature of the packing of both forms is the formation of charge-assisted ammonium-N–H···O (carboxylate) hydrogen bonds which lead to cyclic 12-membered {···HNH ···OCO} 2  synthons in the case of  1  but, supramolecular chains in  2 . The three-dimensional architecture in the crystal of  1  is further stabilised by amine-N–H···O (nitro) and amine-N–H···O (acetyl) hydrogen bonds, leading to double-layers in the bc-plane, which are linked along the  a -axis by methylene-C–H···O (carboxylate) and π-stacking interactions. The combination of ammonium-N–H···O (carboxylate) and amine-N–H···O (carboxylate, acetyl) hydrogen bonds consolidate the three-dimensional packing in the crystal of  2 . The greater crystal density, packing efficiency and calculated lattice energy for  1  compared with  2 , suggest the former to be the thermodynamically most stable crystal. An analysis of the Hirshfeld surfaces for  1  and  2  reveal distinctive features that differentiate between the constituents of the two forms and between the ions comprising the asymmetric unit of  2 .

Supramolecular association in the triclinic (Z′=1) and monoclinic (Z′=4) polymorphs of 4-(4-acetylphenyl)piperazin-1-ium 2-amino-4-nitrobenzoate