The crystal structures of salts of N-(4-fluorophenyl)piperazine with four aromatic carboxylic acids and with picric acid

The structures of five 4-(4-fluorophenyl)piperazin-1-ium salts of closely related aromatic acids all show different patterns of supramolecular assembly.


Figure 3
The independent components of compound (III) showing the atomlabeling scheme and the hydrogen bonds, drawn as dashed lines, within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level.

Figure 4
The independent components of compound (IV) showing the atomlabeling scheme and the hydrogen bonds, drawn as dashed lines, within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level.

Figure 5
The independent components of compound (V) showing the atomlabeling scheme and the hydrogen bonds, drawn as dashed lines, within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level.

Figure 1
The independent components of compound (I) showing the atomlabeling scheme and the hydrogen bonds, drawn as dashed lines, within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level. as a partial hydrate: the refined occupancy of the water molecule is 0.353 (8) and the O atom of this component, O41 (Fig. 2), lies close to an inversion centre, such that the OÁ Á ÁO distance across this centre is only 0.962 (16) Å . Hence, if either of this pair of sites is occupied, the other must be vacant. By contrast, the 2-iodobenzoate (III) crystallizes in solvent-free form with Z 0 = 2 in space group P1 (Fig. 3). The refined structure of the 3,5-dinitrobenzoate salt (V) contains four void spaces per unit cell, each of volume 59 Å 3 , which lie on the twofold rotation axes, and which are connected into narrow channels lying along these axes. Examination of the refined structure using the SQUEEZE procedure (Spek, 2015) showed the the presence of 48 electrons per unit cell that were not accounted for by the ionic components, i.e. an average of 12 electrons per void, or rather less than the equivalent of one water molecule. There were two significant peaks in the difference maps, but no plausible solvent model could be developed from these. Hence the SQUEEZE procedure was used prior to the final refinement, and the nature of the solvent component remains unknown: it seem likely that the included molecules are disordered and/or mobile within the channels.
In each of the cations in compounds (I)-(V), the piperazine ring adopts an almost perfect chair conformation: in every case the reference cation was selected as one having the ringpuckering angle (Cremer & Pople, 1975), as calculated for the atom sequence (N1,C2,C3,N4,C5,C6), or the equivalent sequences in compound (III), close to the ideal value (Boeyens, 1978) of zero, rather than close to 180 as expected for the enantiomeric form of the cation.
The dihedral angles between the aryl ring and the carboxylate group in compounds (I)-(III) and (V) vary from 5.94 (11) in (V) to 75.9 (2) in the anion of (III), which contains atom I132 (Fig. 3). The corresponding angles involving the nitro groups in compounds (IV) and (V) span a much smaller range, from 5.24 (11) for the group containing atom N35 in (V) to 29.09 (6) for the group containing atom N36 in (IV). This contrasting behaviour may be associated with the differences in the hydrogen-bonding participation of the carboxylate and nitro groups, as discussed below (x 3).
Within the anion of compound (IV), the distances C31-C32 and C31-C36 of 1.451 (2) and 1.449 (2) Å , respectively are very much longer than the other C-C distances in this ring, which range from 1.365 (2) Å to 1.383 (2) Å ; in addition the distance C31-O31 [1.2398 (19) Å ] is much close to the values typically found in ketones than to those in phenols (Allen et al., 1987). These values indicate significant delocalization of the negative charge from the atom O31 into the adjacent ring, as shown in the Scheme.

Supramolecular features
It is possible to select a compact asymmetric unit for compound (I) (Fig. 1) in which the three independent components are linked by two N-HÁ Á ÁO hydrogen bonds ( Table 1). The supramolecular assembly of (I) is determined by a combination of two N-HÁ Á ÁO hydrogen bonds and two O-HÁ Á ÁO hydrogen bonds (Table 1). Together these link the three independent components into a chain of centrosymmetric rings running parallel to the [100] direction in which rings of R 4 6 (12) type (Bernstein et al., 1995) centred at (n, 0.5, 0.5) alternate with R 6 6 (16) rings centred at (n + 0.5, 0.5, 0.5), where n represents an integer in each case (Fig. 6). A weak C-HÁ Á ÁO hydrogen bond (Table 1), having a fairly small C-HÁ Á ÁO angle (Wood et al., 2009), links these chains into complex sheets lying parallel to (001).
The ionic components of compound (II) are linked by two independent N-HÁ Á ÁO hydrogen bonds (Table 1) to form a centrosymmetric four-ion aggregate, characterized by an Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded chain of alternating R 4 6 (12) and R 6 6 (16) rings along [100]. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have been omitted.

Figure 7
Part of the crystal structure of compound (II) showing the formation of a centrosymmetric four-ion R 4 4 (12) aggregate. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the partial-occupancy water molecules and the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 1 À z).

Figure 8
Part of the crystal structure of compound (II) showing the formation of a molecular ribbon of centrosymmetric rings running parallel to the [100] direction. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the partial-occupancy water molecules and the H atoms bonded to those C atoms which are not involved in the motifs shown have been omitted. Table 1 Hydrogen bonds and short intermolecular contacts (Å , ) for compounds (I)-(V).

Figure 9
Part of the crystal structure of compound (III) showing the formation of a molecular ribbon of edge-fused rings running parallel to the [010] direction. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to those C atoms which are not involved in the motifs shown have been omitted. symmetry with its centroid close to (0.25, 0.25, 0.5): a search for possible additional crystallographic symmetry found none. A combination of one C-HÁ Á ÁO and one C-HÁ Á Á(arene) hydrogen bonds (Table 1) links the four-ion aggregates into a complex molecular ribbon running parallel to the [010] direction (Fig. 9). The component ions in compound (IV) are linked by a three-centre (bifurcated) N-HÁ Á Á(O) 2 hydrogen bond within the selected asymmetric unit (Fig. 4, Table 1), while a two centre N-HÁ Á ÁO hydrogen bond links the ions to form a centrosymmetric four-ion aggregate, in which rings of R 2 1 (6), R 4 4 (12) and R 4 4 (16) types can be identified ( Fig. 10): it is interesting to note the occurrence of the R 4 4 (12) ring type, exactly as in the structures of compounds (II) and (III). The structure of compound (IV) contains several short C-HÁ Á ÁO contacts, but in all of these contacts the C-HÁ Á ÁO angle is close to 140 (Table 1), so that their structural significance is likely to be minimal, at best (Wood et al., 2009).
Inversion-related ion pairs in compound (V) form the same type of R 4 4 (12) motif ( Fig. 11) as previously seen in each of research communications Figure 10 Part of the crystal structure of compound (IV) showing the formation of a centrosymmetric four-ion aggregate containing R 2 1 (6), R 4 4 (12) and R 4 4 (16) ring types. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 1 À z).

Figure 11
Part of the crystal structure of compound (V) showing the formation of a centrosymmetric four-ion R 4 4 (12) aggregate. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 1 À z).

Figure 12
Part of the crystal structure of compound (V) showing the formation of a sheet lying parallel to (101) and formed from N-HÁ Á ÁO and C-HÁ Á Á(arene) hydrogen bonds, drawn as dashed lines. For the sake of clarity, the H atoms not involved in the motifs shown have been omitted. compounds (II)-(IV). In addition, an almost linear C-HÁ Á Á(arene) hydrogen bond links these four-ion aggregates into a sheet lying parallel to (101) (Fig. 12). A second sheet of this type, related to the first by the twofold rotation axes, also passes through each unit cell, but there are no directionspecific interactions between adjacent sheets. However, the stacking of the sheets leaves void space in the form of narrow channels lying along the twofold axes (Fig. 13). As noted above (x 2), the channels appear to contain solvent molecules, which are disordered and/or mobile.
Thus the cyclic R 4 4 (12) motif can be identified in some form in each of compounds (II)-(V), although such rings are centrosymmetric in each of (II), (IV) and (V), but noncentrosymmetric in (III), and they are explicit in (II), (III) and (V) (Figs. 3,7,11), but masked within a more complex four-ion aggregate in (IV) (Fig. 10).

Database survey
It is of interest briefly to compare the structures reported here with those of some closely related compounds. One obvious comparison is between the monohydrated compound (I) reported here and a series of isostructural salts (benzoate, 4fluorobenzoate, 4-chlorobenzoate and 4-bromobenzoate) of MeOPP [compounds (VI)-(IX)], all of which crystallize as monohydrates (Kiran Kumar et al., 2019). Compounds (VI)-(IX) all form a chain of centrosymmetric R 4 6 (12) and R 6 6 (16) rings comparable to that found here in compound (I). It is important to emphasize, however, that (I) is not isostructural with (VI)-(IX); thus, although the repeat vectors for the unit cell of (I) are somewhat similar to those in (VI)-(IX), the inter-axial angles in (I) are all greater than 90 , whereas in (VI)-(IX) they are consistently less than 90 .
The picrate salt of MeOPP (X) (Kiran Kumar et al., 2020) is analogous to the picrate salt (IV) formed by 4-FPP, and both show the same pattern of electronic delocalization within the anion. While (IV) and (X) both crystallize in space group P2 1 / n, they are by no means isomorphous, and in (X) two of the nitro groups exhibit disorder. Whereas the N-HÁ Á ÁO hydrogen bonds in (IV) generate a dimeric structure (Fig. 10), in (X) they generate a chain of rings, and adjacent chains are linked by a C-HÁ Á Á(arene) hydrogen bond to form a sheet parallel to (001) (Kiran Kumar et al., 2020). Similar delocalization is apparent in the anion of the 5-hydroxy-3,5-dinitrobenzoate salts of both 4-FPP (XI)  and MeOPP (XII) (Kiran Kumar et al., 2019). However, in (XI) the ions are linked into a chain of R 2 1 (4) and R 2 1 (6) rings by two independent three-centre N-HÁ Á Á(O) 2 hydrogen bonds, while in (XII) a combination of N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds generates a chain of alternating R 2 2 (10) and R 4 6 (16) rings, with the chains further linked into a threedimensional framework structure by a single C-HÁ Á Á(arene) hydrogen bond.

Synthesis and crystallization
All starting materials were obtained commercially, and all were used as received: solutions of N-(4-fluorophenyl)piperazine (100 mg, 0.55 mol) in methanol (10 ml) were mixed with solutions of the appropriate acids (0.55 mol) in methanol (10 ml), viz. 2-fluorobenzoic acid (77.1 mg) for (I), 2-bromobenzoic acid (110.6 mg) for (II), 2-iodobenzoic acid (136.4 mg) for (III), picric acid (126 mg) for (IV) and 3,5-dinitrobenzoic acid (116.7 mg) for (V). The corresponding pairs of solution were mixed and then briefly held at 323 K, before being set aside to crystallize. After two days at room temperature, the resulting solid products were collected by filtration, dried in air and then crystallized, at ambient temperature and in the presence of air from mixture of ethyl acetate and acetone (initial composition 9:1, v/v) for (I), or ethyl acetate and methanol [initial composition 9:1, v/v for (II)-(IV), and 1:1 v/v for (V)]. M.p. (I) 349-351 K, (II) 411-414 K, (III) 409-411 K, (IV) 405-410 K, and (V) 426-430 K. Despite repeated efforts, we have been unable to obtain satisfactory crystals of the 2chlorobenzoate salt, using solvents such as acetonitrile, acetone, ethyl acetate or methanol, and a number of mixtures of such solvents.

Refinement
Crystal data, data collection and refinement details are summarized in Table 2. All H atoms, apart from those of the partial-occupancy water molecule in compound (II), were located in difference maps. The H atoms bonded to C atoms were then treated as riding atoms in geometrically idealized positions with C-H distances of 0.93 Å (aromatic) or 0.97 Å (CH 2 ), and with U iso (H) = 1.2U eq (C). For the H atoms bonded to N atoms, the atomic coordinates were refined with U iso (H) = 1.2U eq (N) giving the N-H distances shown in Table 1  reflection, (110), which had been attenuated by the beam stop, and one bad outlier reflection, (004), were removed from the data set. It was not possible to reliably locate in difference maps the H atoms of the partial-occupancy water molecule in (II); hence they were included in calculated positions, riding at 0.90 Å from the atom O41, at positions calculated by interpolation along the relevant OÁ Á ÁO vectors, with U iso (H) = 1.5U eq (O): the water atom O41 was refined isotropically and the refined occupancy of the water molecule was 0.353 (8). The largest maximum and minimum in the final difference map for (III), +2.92 and À2.00 e Å -3 , respectively, were both close to atom I232, at distances of 0.92 and 0.77 Å , respectively. Examination of the difference map for compound (V) after conventional refinement showed the presence of two significant peaks, one of 2.98 e Å À3 lying on a twofold rotation axis, at (0.5, À0.0533, 0.75), and the other, 1.94 e Å À3 , lying in a general position at (0.534, 0.882, 0.760). No plausible solvent model could be developed based upon these two maxima and, accordingly, the data at this stage were subjected to the SQUEEZE procedure (Spek, 2015) prior to the final refinements, and it is the results of that refinement which are reported here for compound (V).  (2) 9.8892 (3), 11.6831 (7), 16.4547 (9) 16.658 (2), 6.6734 (6), 17.553 (3) 19.871 (1), 7.3420 (7)  program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2020); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b) and PLATON (Spek, 2020).  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.

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.    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 2.92 e Å −3 Δρ min = −2.00 e Å −3 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.