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The structures and Hirshfeld surface analysis of three salts of 1-(4-nitrophenyl)piperazine are discussed. In 4-(4-nitrophenyl)piperazin-1-ium salicylate (C10H14N3O2+·C7H5O3−), there are strong hydrogen bonds between cation and anion and the 4-nitrophenyl substituent occupies an equatorial position in the piperazinium ring. The cation and anion are linked together by supramolecular interactions [graph-set notation of hydrogen bonding C_{2}^{2}(6) propagating in the a-axis direction]. Additionally, there is π–π stacking involving the salicylate anion and the piperazinium cation in adjacent asymmetric units as well as a C—H...π interaction between a hydrogen atom on the piperazine ring and the phenyl ring within the salicyclate anion. In bis[4-(4-nitrophenyl)piperazin-1-ium] bis(4-fluorobenzoate) trihydrate (2C10H14N3O2+·2C7H4FO2−·3H2O), there are two cations, two anions, and three water molecules of solvation in the asymmetric unit, all linked by hydrogen bonds [graph-set notation of hydrogen bonding R22(20) between adjacent cations and R33(9) between a cation and its adjacent anion]. In the anion, the 4-nitrophenyl ring occupies an axial substitution position in the piperazinium ring, which is relatively rare. Within the asymmetric unit, the phenyl groups in the cations show an offset π–π interaction. Additionally, there is a C—H...π interaction between a hydrogen atom on the phenyl ring within a cation and the phenyl ring within an anion. In 4-(4-nitrophenyl)piperazin-1-ium 3,5-dinitrobenzoate (C10H14N3O2+·C7H4N2O6−), there is a strong N—H...O hydrogen bond linking the cation and anion and the 4-nitrophenyl ring occupies an axial substitution position in the piperazinium ring, as seen in the previous structure. In the crystal, the cation and the anion form a complex three-dimensional hydrogen-bonded array involving R22(8), R44(12) and R44(20) rings propogating in the a-axis direction. The nitrophenyl group is disordered with occupancies of 0.806 (10) and 0.194 (10).

Eleven (4-phenyl)piperazinium salts containing organic anions have been prepared and structurally characterized, namely, 4-phenylpiperazin-1-ium 4-fluorobenzoate monohydrate, C10H15N2+·C7H4FO2−·H2O, 1; 4-phenylpiperazin-1-ium 4-bromobenzoate monohydrate, C10H15N2+·C7H4BrO2−·H2O, 3; 4-phenylpiperazin-1-ium 4-iodobenzoate, C10H15N2+·C7H4IO2−, 4; 4-phenylpiperazin-1-ium 4-nitrobenzoate, C10H15N2+·C7H4NO4−, 5; 4-phenylpiperazin-1-ium 3,5-dinitrosalicylate, C10H15N2+·C7H3N2O7−, 6; 4-phenylpiperazin-1-ium 3,5-dinitrobenzoate, C10H15N2+·C7H3N2O6−, 7; 4-phenylpiperazin-1-ium picrate, C10H15N2+·C6H2N3O7−, 8; 4-phenylpiperazin-1-ium benzoate monohydrate, C10H15N2+·C7H5O2−·H2O, 9; 4-phenylpiperazin-1-ium p-toluenesulfonate, C10H15N2+·C7H7O3S−, 10; 4-phenylpiperazin-1-ium tartarate monohydrate, C10H15N2+·C4H5O6−·H2O, 11; and 4-phenylpiperazin-1-ium fumarate, C10H15N2+·C4H3O4−, 12. Compounds 1 and 3–12 are all 1:1 salts with the acid proton transferred to the phenylpiperaizine basic N atom (the secondary amine) with the exception of 3 where there is disorder in the proton position with it being 68% attached to the base and 32% attached to the acid. Of the structures with similar stoichiometries only 3 and 9 are isomorphous. The 4-phenyl substituent in all cases occupies an equatorial position except for 12 where it is in an axial position. The crystal chosen for structure 7 was refined as a non-merohedral twin. There is disorder in 5, 6, 10 and 11. For both 5 and 6, a nitro group is disordered and was modeled with two equivalent orientations with occupancies of 0.62 (3)/0.38 (3) and 0.690 (11)/0.310 (11), respectively. For 6, 10 and 11, this disorder is associated with the phenyl ring of the phenylpiperazinium cation with occupancies of 0.687 (10)/0.313 (10), 0.51 (7)/0.49 (7) and 0.611 (13)/389 (13), respectively. For all salts, the packing is dominated by the N—H...O hydrogen bonds formed by the cation and anion. In addition, several structures contain C—H...π (1, 3, 4, 8, 9, 10, and 12) and aromatic π–π stacking interactions (6 and 8) and one structure (5) contains a –NO2...π interaction. For all structures, the Hirshfeld surface fingerprint plots show the expected prominent spikes as a result of the N—H...O and O—H...O hydrogen bonds.

Synthesis and crystal structures of 1-benzoyl-4-(4-nitrophenyl)piperazine, C17H17N3O3, (I) and 1-(4-bromobenzoyl)-4-phenylpiperazine, C17H17BrN2O, (II) are described. Compounds I and II crystallize in the orthorhombic and monoclinic crystal systems with space groups Pna21 (Z′ = 2, I) and P21 (Z′ = 1, II), respectively. The crystal of II was a two-component aggregate, treated as a `twin' for data-acquisition purposes. There are no conventional hydrogen bonds in either I or II, but there are weaker C—H...O contacts. Each molecule consists of a central piperazine ring in a chair conformation, with either benzoyl and nitrophenyl (I) or 4-bromobenzoyl and phenyl (II) groups attached to different nitrogen atoms of the piperazine. The various atom–atom contact coverages as quantified by Hirshfeld surface analysis fingerprint plots are given.

The title compound, C29H25N3O3S, crystallizes in the monoclinic space group P21/n. The molecule contains a central four-ring system in which all of the rings are almost coplanar. Both the 4-methoxyphenyl ring and the prop-2-ynyloxy substituent are disordered over two equivalent conformations with occupancy ratios of 0.903 (2):0.097 (2) and 0.776 (5):0.224 (5), respectively. In the crystal, π–π interactions [centroid–centroid distance = 3.7327 (11) Å] between the dihydropyrazole ring and the 4-methoxyphenyl ring link the molecules into centrosymmetric dimers. In addition, there are weak C—H...S, C—H...N and C—H...O interactions, which link the molecules into a complex three-dimensional array.

In the title compound, C28H22FN3OS, four rings are almost coplanar, with the fluorophenyl ring substantially twisted. In the extended structure, aromatic π–π stacking interactions between the pyrazole ring and the tolyl ring link the molecules into centrosymmetric dimers.

In the title compound, C18H16N2O2, the phenyl and pyrazole rings subtend a dihedral angle of 22.68 (8)°. The packing of the title compound features aromatic π–π stacking and weak C—H...π interactions.

Three salts containing the 4-(4-fluorophenyl)piperazin-1-ium cation have been prepared and structurally characterized. In 4-(4-fluorophenyl)piperazin-1-ium 2-hydroxy-3,5-dinitrobenzoate, C10H14FN2+·C7H3N2O7−, (I), the anion contains an intramolecular O—H...O hydrogen bond, and it has a structure similar to that of the picrate ion. The cations and anions are linked into [001] chains of rings by a combination of two three-centre N—H...(O)2 hydrogen bonds. The anion in 4-(4-fluorophenyl)piperazin-1-ium hydrogen oxalate, C10H14FN2+·C2HO4−, (II), is planar, and the cations and anions are linked into (100) sheets by multiple hydrogen bonds including two-centre N—H...O, three-centre N—H...(O)2, O—H...O, C—H...O and C—H...π(arene) types. In 4-(4-fluorophenyl)piperazin-1-ium hydrogen (2R,3R)-tartrate monohydrate, C10H14FN2+·C4H5O6−·H2O, (III), the anion exhibits an approximate non-crystallographic twofold rotation symmetry with antiperiplanar carboxyl groups. A combination of eight hydrogen bonds, encompassing two- and three-centre N—H...O systems, O—H...O and C—H...π(arene) types, link the independent components into a three-dimensional framework. Comparisons are made with some related structures.