Syntheses, crystal structures and Hirshfeld surface analysis of 4-(4-nitrophenyl)piperazin-1-ium trifluoroacetate and 4-(4-nitrophenyl)piperazin-1-ium trichloroacetate

The supramolecular assemblies of the two title structures are one-dimensional: the chain-of-rings motifs are supported by aromatic π–π interactions.

The synthesis and crystal structures of the molecular salts of 4-(4-nitrophenyl)piperazine with trifluoroacetate, namely, 4-(4-nitrophenyl)piperazin-1-ium trifluoroacetate, C 10 H 14 N 3 O 2 + ÁC 2 F 3 O 2 À (I), and with trichloroacetate, namely, 4-(4-nitrophenyl)piperazin-1-ium trichloroacetate, C 10 H 14 N 3 O 2 + ÁC 2 Cl 3 O 2 À , (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 aromaticinteractions. 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 OÁ Á ÁH/HÁ Á ÁO, and those with a contribution of halogen atoms: FÁ Á ÁH/HÁ Á ÁF in (I) and ClÁ Á ÁH/HÁ Á ÁCl in (II), respectively.

Structural commentary
The title compounds are shown in Figs. 1 and 2. The piperazine rings adopt a chair conformation with puckering parameters (Cremer & Pople, 1975) in (I) of Q = 0.576 (2) Å , = 177.8 (2) , ' = 182 (4) , and in (II) of Q = 0.571 (2) Å , = 177.1 (2) , ' = 189 (4) , respectively. The position of the nitrophenyl group on the piperazine ring differs in the two structures, from bisectional in (I) to occupying an equatorial site in (II) (Fig. 3). The angle between the N1-C1 bond and the normal to the Cremer & Pople mean plane is 39.57 (11) in (I) and 60.87 (14) in (II) (Spek, 2020; see Database survey section for further comparisons). In addition, the delocalization effect within the benzene ring is slightly disturbed due to the presence of the electron-donating piperazinyl [-C 4 H 8 N 2 ; for the structurally similar piperidino substituent the Hammett p constant is À0.12 (Perrin et al., 1981)] and the electron-withdrawing nitro [-NO 2 , p = 0.78 (Hansch et al., 1991)] groups located in the para-position: the lengthening of the C1-C2 and C1-C6 bonds is accompanied by the shortening of the remaining C-C bonds within the ring and C-N distances to the substituents.
In the anions, the C-O bond lengths in the carboxylate group are more similar in compound (II) than in compound (I), although in both cases these distances are shorter than the mean value for its type (Allen et al., 1987). The geometries of the COO À groups can be affected by the positional disorder of the CF 3 group in (I) and the chlorine atoms in (II). In (I), the CF 3 group is found to be disordered over two orientations, with a refined occupancy ratio of 0.779 (4):0.221 (4), while in (II), the disordered chlorine atoms in the CCl 3 group show an almost equivalent contribution of components A and B

Supramolecular features
In (I), the 4-(4-nitrophenyl)piperazin-1-ium cation interacts with two trifluoroacetate anions, which are related by translation, by two N-HÁ Á ÁO hydrogen bonds: N2-H21Á Á ÁO3 and N2-H21Á Á ÁO4(x + 1, y, z). Additionally, if one considers the C7-H7AÁ Á ÁO3(x + 1, y, z) interaction the hydrogen-bonded Independent components of compound (I) showing the atom-labelling scheme and the hydrogen bond (drawn as dashed line) within the selected asymmetric unit. The major disorder component is drawn using unbroken lines (A) and the minor disorder component is drawn using dashed lines (B). Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
Independent components of compound (II) showing the atom-labelling scheme and the hydrogen bonds (drawn as dashed lines) within the selected asymmetric unit. The disorder components A and B of chlorine atoms have equal site-occupancies (1/2) within s.u. Displacement ellipsoids are drawn at the 30% probability level.

Figure 6
A part of the crystal structure of compound (I) showing the aromaticinteractions between adjacent chains of rings. Red balls represent the centroids of the phenyl rings (Cg1). Table 1 Hydrogen-bond geometry (Å , ) for (I).

Figure 4
Part of the crystal structure of compound (I) showing the formation of a chain of rings parallel to the [100] direction. Hydrogen bonds are drawn as dashed lines, and for the sake of clarity, the H atoms bonded to C atoms have been omitted. Symmetry code: (i) x + 1, y, z.

Figure 5
Part of the crystal structure of compound (II) showing the formation of a chain of rings parallel to the [010] direction. Hydrogen bonds are drawn as dashed lines, and for the sake of clarity, the H atoms bonded to C atoms have been omitted. Symmetry code: (i) x, y + 1, z.
distances from the centroid to the plane of the opposite ring are 3.333 (1) and 3.253 (1) Å in (I) and 3.303 (1) Å in (II).
Although in (I) the slippage distance (2.764 Å ) between the centroids spaced by 4.27 Å is markedly far from a value of 1.8 Å (suggesting an overlap of rings), one can still consider molecular stacks along the [100] direction to be comparable to those undoubtedly observed in structure (II) in the [010] direction. Finally, both supramolecular structures can be described as mono-periodic; no other specific close contacts or interactions can be found in addition to those mentioned above. Despite the similarities in the formation of 1D-chains of rings and their stacking assemblies, the packing of these motifs in the analysed crystals is fundamentally different. In (I), the packing fashion can be described as herringbone-type (Fig. 8), whereas in (II) a linear mode is seen (Fig. 9). It seems that the halogen atoms [F in (I) and Cl in (II)] in the anions influence the crystal-packing modes because of the difference in their van der Waals radii.
A part of the crystal structure of compound (II) showing the aromaticinteractions between adjacent chains of rings. Red balls represent the centroids of the phenyl rings (Cg1).

Figure 8
Crystal packing of (I) in a view along the crystallographic a axis (herringbone type).

Figure 9
Crystal packing of (II) in a view along the crystallographic b axis (linear type).

Hirshfeld surface analysis
The Hirshfeld surface analysis is a valuable tool for understanding crystal packing. It offers both identification and visualization of intermolecular interactions, as well as reflecting the interplay between atoms in the structure. The Hirshfeld surfaces of ionic pairs in the asymmetric units of (I) and (II), are shown in Fig. 10. In addition, in Fig. 10, the corresponding 2D fingerprint plots of the most dominant contacts are also presented and combined with the information about their percentage contributions to the Hirshfeld surface. For both structures, the most significant contacts percentages are attributed to OÁ Á ÁH/HÁ Á ÁO interactions, 34.3% in (I) and 31.7% in (II). The closest contacts of this type appear as two sharp symmetric spikes in the 2D maps, and the intermolecular contacts as representatives are visualized between the Hirshfeld surface of the ionic components and neighbouring molecules. Competing close contacts are those with halogen atom, ClÁ Á ÁH/HÁ Á ÁCl type in (I) (32.1%) and FÁ Á ÁH/HÁ Á ÁF in (II) (28.8%). The former contacts in the fingerprint plot of (II) can be seen as wings, whereas the latter contacts dominate in the structure of (I) are spread over the central part of plot; their distances are essentially comparable or longer than the sum of the van der Waals radii of the atoms involved. The much lower contributions of the HÁ Á ÁH contacts are consistent with the moderate number of H atoms per two molecules in the asymmetric units. The contributions of the remaining contact types constitute about 20%, among which 6-8% of the Hirshfeld surface area of (I) and (II) is covered by CÁ Á ÁH/HÁ Á ÁC contacts.  Lu, 2007). In addition, two neutral NPP molecules have been reported in an inclusion material (Kö nig et al., 1997) or co-crystal (Wang et al., 2014). We have compared the molecular conformation of thirteen independent 4-(4-nitrophenyl)piperazin-1-ium cations: nine published structures (2 with Z 0 > 1) and the two reported in this article. As shown in Fig. 11, the molecular structures of the NPP cations differ from each other with respect to the position of the nitrophenyl group on the piperazine ring: the equatorial site is preferred (9/13), whereas the axial position (3/13) is rare, and bisectional is uncommon (1/13). All compared piperazine rings adopt a chair conformation.

Synthesis and crystallization
A solution of commercially available (from Sigma-Aldrich) 4-nitrophenylpiperazine (100 mg, 0.483 mol) in methanol (10 ml) was mixed with equimolar solutions of the appropriate acids in methanol (10 ml) viz., trifluoroacetic acid (55 mg, 0.483 mol) for (I) and trichloroacetic acid (79 mg, 0.483 mol) for (II). The corresponding solutions were stirred for 30 minutes at 323 K and allowed to stand at room temperature. X-ray quality crystals were formed on slow evaporation for a week for both of the compounds, where ethanol ethyl acetate (1:1) was used for crystallization. The corresponding melting points were 425-427 K (I) and 388-390 K (II).

Refinement
Crystal data, data collection and structure refinement details for both compounds are summarized in Table 3. In both structures, an extinction parameter was refined.
The CF 3 group of (I) was found to be disordered over two orientations, with a refined occupancy ratio of 0.779 (4):0.221 (4). The disorder was restrained using SIMU, ISOR and DELU commands in SHELXL for the six resulting fluorine atoms. Anisotropic displacement parameters for pairs of the disordered carbon atom (C12A and C12B) were constrained to be the same. The three C-F bonds of the minor disorder component (B) and two C11-C12 bonds were restrained to be similar in length.
In (II), the refined occupancies of disordered chlorine atoms in the CCl 3 group of 0.494 (15) and 0.506 (15), show the equivalent contribution of the components A and B. The ellipsoids of three chlorine atoms of the B disorder component were modelled using SIMU, ISOR and DELU commands in SHELXL. All six C-Cl distances were restrained to be similar in length.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (