Crystal structures are reported for two molecular salts containing the 4-(4-nitrophenyl)piperazin-1-ium cation. Co-crystallization from methanol/ethyl acetate solution of N-(4-nitrophenyl)piperazine with benzoic acid gives the benzoate salt, which crystallizes as a monohydrate, C10H14N3O2·C7H5O2·H2O, (I), and similar co-crystallization with 3,5-dinitrosalicylic acid yields the 2-carboxy-4,6-dinitrophenolate salt, C10H14N3O2·C7H3N2O7, (II). In the structure of (I), a combination of O—H...O, N—H...O and C—H...O hydrogen bonds links the components into sheets, while in the structure of (II), the supramolecular assembly, generated by hydrogen bonds of the same types as in (I), is three dimensional. Comparisons are made with the structures of some related compounds.

Co-crystallization of N-(4-methoxyphenyl)piperazine with 4-methylbenzoic acid and with benzene-1,2-dicarboxylic acid yields the salts 4-(4-methoxyphenyl)piperazin-1-ium 4-methylbenzoate monohydrate, C11H17N2O+·C8H7O2 −·H2O (I), and bis[4-(4-methoxyphenyl)piperazin-1-ium] benzene-1,2-dicarboxylate, 2C11H17N2O+·C8H4O4 2− (II). These salts both crystallize with Z′ = 2, in space groups P\overline{1} and Pna21, respectively. In compound (I), a combination of four O—H...O, four N—H...O, one C—H...O and one C—H...π(arene) hydrogen bonds link the six independent components into complex sheets, within which the two piperazine rings, the two anions, and the two water molecules are related by an approximate, non-crystallographic translation along the b-axis direction. In compound (II), sheets containing R 4 4(18) and R 10 12(38) rings are formed by the combined action of eight independent N—H...O hydrogen bonds. Comparisons are made with the structures of some related compounds.

The synthesis and crystal structures of the molecular salts of 4-(4-nitrophenyl)piperazine with trifluoroacetate, namely, 4-(4-nitrophenyl)piperazin-1-ium trifluoroacetate, C10H14N3O2 +·C2F3O2 − (I), and with trichloroacetate, namely, 4-(4-nitrophenyl)piperazin-1-ium trichloroacetate, C10H14N3O2 +·C2Cl3O2 −, (II), are reported and compared. A partial positional disorder of the anions was found. In both structures, the piperazine rings adopt a chair conformation, whereas the positions of the nitrophenyl group on the piperazine ring differ from bisectional in (I) to equatorial in (II). In both structures, the supramolecular assemblies are mono-periodic on the basis of the chain-of-rings motifs supported by aromatic π–π interactions. Hirshfeld surface analysis was used to explore the intermolecular close contacts in both crystals. The most dominant contacts of the Hirshfeld surface of the cation–anion pairs of the asymmetric units are O...H/H...O, and those with a contribution of halogen atoms: F...H/H...F in (I) and Cl...H/H...Cl in (II), respectively.

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Three organic salts of sparfloxacin, a difluorinated third-generation fluoroquinolone antibiotic, have been synthesized and their crystal structures determined. The salts, sparfloxacinium 4-nitrobenzoate dihydrate, C19H23F2N4O3 +·C7H4NO4 −·2H2O (I), sparfloxacinium 2-phenylacetate, C19H23F2N4O3 +·C8H7O2 − (II), and sparfloxacinium 4-methylbenzoate trihydrate, C19H23F2N4O3 +·C8H7O2 −·3H2O (III), exhibit similar inter-species packing interactions. The overall crystal structures each, however, have their own distinct characteristics, which are described here along with a Hirshfeld surface analysis of the various atom–atom contacts involving the sparfloxacinium cations. In the crystal structure of III, an extended supramolecular tape of edge-fused hydrogen-bonded water pentagons was found. These pentagonal water and tape motifs are compared to related constructs in a broad selection of structure types, ranging from macromolecules to small molecules, clathrates, and exotic `ice' formations on clean metal surfaces.