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Six piperazinium salts, namely 4-(4-nitro­phenyl)­piperazin-1-ium 4-bromo­ben­zo­ate dihydrate, C10H14N3O2+·C7H4BrO2¿·2H2O, (I), 4-(4-nitro­phenyl)­pi­per­a­zin-1-ium 4-iodo­benzoate dihydrate, C10H14N3O2+·C7H4IO2¿·2H2O, (II), 4-(4-nitro­phenyl)­piperazin-1-ium 4-hy­droxy­benzoate monohydrate, C10H14N3O2+·C7H5O3¿·H2O, (III), 4-(4-nitro­phenyl)­piperazin-1-ium 4-methyl­benzoate monohydrate, C10H14N3O2+·C8H7O2¿·H2O, (IV), 4-(4-nitro­phenyl)­piperazin-1-ium 4-meth­oxy­benzoate hemihydrate, 2C10H14N3O2+·2C8H7O3¿·H2O, (V), and 4-(4-nitro­phenyl)­piperazin-1-ium 4-eth­oxy­benzoate, 2C10H14N3O2+·2C9H9O3¿, (VI), have been synthesized and their crystal structures solved by single-crystal X-ray diffraction, revealing that all of them crystallize in the triclinic space group P except for (V), which crystallizes in the monoclinic space group P21/c and has a disordered nitro group. Compounds (I) and (II) are isostructural. The crystal packing of (I)¿(V) is constructed from organic chains formed by a combination of hydrogen bonds of type N¿H¿O and/or O¿H¿O and other weak inter­actions of type C¿H¿O and/or C¿H¿¿, forming sheets, whereas (VI) shows a cationic and anionic-based layer structure.

In this study, four new piperazinium salts, namely, 4-phenylpiperazin-1-ium 4ethoxybenzoate monohydrate, C9H9O3.C10H15N2.H2O(I); 4-phenylpiperazin-1ium 4-methoxybenzoate monohydrate, C10H15N2.C8H7O3.H2O(II); 4-phenylpiperazin-1-ium 4-methylbenzoate monohydrate, C10H15N2.C8H7O2.H2O(III); and 4-phenylpiperazin-1-ium trifluoroacetate 0.12 hydrate, C10H15N2.C2F3O20.12H2O(IV), have been synthesized. The single-crystal structures of these compounds reveal that all of them crystallize in the triclinic P1space group and the crystal packing of (I)–(III) is built up of ribbons formed by a combination of hydrogen bonds of type N—HO, O—HO and other weak interactions of type C—HOand C—H,leading to a three-dimensional network. In the crystal of (IV), the cations and the anions are connected by C—HO, N— HOandC—HFhydrogen bonds and by C—Hinteractions, forming sheets which in turn interact to maintain the crystal structure by linking through the oxygen atoms of water molecules and van der Waals interactions, giving the whole structure.

As part of our ongoing investigation on the chemical and biological properties of piperazinium salts, we synthesized three novel compounds: 1-ethyl­piperazinium 3,5-di­nitro­benzoate (I), 1-methyl­piperazinium 3,5-di­nitro­benzoate (II) and 1-methyl­piperazinium 4-iodo­benzoate (III). The crystal structures of these compounds are built up of organic layers formed by the strong connection between the mol­ecules by hydrogen bonds of type N—H...O. These layers are linked through N—H...O hydrogen bonds and C—H...O inter­actions or C—I...N halogen bonding, leading to the formation of a three-dimensional network.

The geometric parameters of the title compound, C4H6N3+·C6H2N3O7-, are in the usual ranges. While two nitro groups are almost coplanar with the aromatic picrate ring [dihedral angles 3.0 (2) and 4.4 (3)°], the third is significantly twisted out of this plane [dihedral angle 46.47 (8)°]. Anions and cations are connected via N-H...O hydrogen bonds. The molecules crystallize in planes parallel to (1\overline{2}1). Key indicators: single-crystal X-ray study; T = 173 K; mean σ(C–C) = 0.002 Å; R factor = 0.036; wR factor = 0.099; data-to-parameter ratio = 10.9.

The geometric parameters of the title compound, C8H6N2O·C6H3N3O7, are in the usual ranges. The three nitro groups are almost coplanar with the aromatic picrate ring [dihedral angles 10.2 (2)°, 7.62 (16) and 8.08 (17)°]. The molecular conformation of the picric acid is stabilized by an intramolecular O-H...O hydrogen bond. The phthalazin-1(2H)-one molecules are connected via N-H...O hydrogen bonds, forming centrosymmetric dimers. Key indicators: single-crystal X-ray study; T = 173 K; mean σ(C–C) = 0.002 Å; R factor = 0.034; wR factor = 0.091; data-to-parameter ratio = 11.1.