A very short O—H⋯O hydrogen bond in the structure of clozapinium hydrogen bis(3,5-dinitrobenzoate)

The title compound features a very short, but asymmetric, O—H⋯O hydrogen bond having an O⋯O distance of 2.452 (3) Å within the anion.


Structural commentary
Compound (I) is an acid salt, i.e., the asymmetric unit contains one C 18 H 20 ClN 4 + clozapinium cation and one C 14 H 7 N 4 O 12 À hydrogen bis(3,5-dinotrobenzoate) anion ( Fig. 1). An alternative description is one cation, one 3,5-dinitrobenzote anion and one neutral molecule of 3,5-dinitrobenzoic acid, i.e., C 18 H 20 ClN 4 + Á(C 7 H 3 N 2 O 6 ) À Á(C 7 H 4 N 2 O 6 ). The -CO 2 H and -CO 2 À groups in the anion are linked by a very short O22-H22AÁ Á ÁO32 hydrogen bond (Table 1) (Speakman, 1972;Emsley, 1980;Gerlt et al., 1997) but, although it is nearly linear [169 (3) ], it is not symmetric as the two independent O-H distances are significantly different [O22-H22A = 1.11 (4); H22AÁ Á ÁO32 = 1.35 (4) Å ]. There is a similarly short O-HÁ Á ÁO hydrogen bond in the corresponding species of the chlorprothixene salt (III) (Shaibah et al., 2019), where the OÁ Á ÁO distance, 2.4197 (15) Å , is slightly shorter than that found here for (I), while the difference between the two independent O-H distances is about 50% higher in (III) as compared to (I). For (I) it is possible to select a compact asymmetric unit in which the components are linked by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds (Table 1, Fig. 1). Within this asymmetric unit, there are also two fairly short C-HÁ Á ÁO contacts. That involving atom C4 has a small C-HÁ Á ÁO angle, and so it probably not structurally significant (Wood et al., 2009), while for that involving atom C13, the HÁ Á ÁO distance is not significantly shorter that the sum of the van der Waals radii (Rowland & Taylor, 1996). These are both probably better regarded as adventitious contacts rather than as structurally significant hydrogen bonds.
One of the nitro groups of the anion, that attached to C35, is disordered over two sets of atomic sites, with occupancies of 0.56 (3) and 0.44 (3) for the oxygen atoms. The major and minor disorder components make dihedral angles with the adjacent aryl ring of 17.2 (8) and 19.4 (8) , with a dihedral angle between their own planes of 36.5 (14) , so that these components are rotated out of the plane of the aryl ring in opposite senses.
In the C 18 H 20 ClN 4 + cation of (I), the fused tricyclic component adopts a butterfly conformation with a dihedral angle of 45.59 (6) between the planes of the two outer aryl rings. The piperazine ring adopts a chair conformation, as indicated by the value of the ring-puckering angle = 176.0 (3) , as calculated for the atom sequence N11/C12/C13/ N14/C15/C16: for an idealized chair form this angle takes a value of either zero or 180 (Boeyens, 1978). The site of protonation is the methylated atom N14 where the methyl substituent occupies the equatorial site (Fig. 1). The geometry at the other N atom in this ring, atom N11, is nearly planar: the sum of the C-N-C angles at N11 is 351.9 , as compared with 344.1 at N14, while the displacements of these N atoms from the planes of the adjacent three C atoms are 0.449 (3) Å for N14 and 0.236 (2) Å for N11. Symmetry codes: (i) Àx þ 3 2 ; y À 1 2 ; Àz þ 1 2 ; (ii) x À 1 2 ; Ày þ 1 2 ; z À 1 2 ; (iii) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 1
The molecular structure of (I), showing displacement ellipsoids drawn at the 30% probability level and hydrogen bonds (dashed lines) within the asymmetric unit.

Supramolecular features
Aggregates of the type defining the selected asymmetric unit ( Fig. 1) are linked by a combination of one N-HÁ Á ÁO, one O-HÁ Á ÁO and two C-HÁ Á ÁO hydrogen bonds (Table 1) to form a three-dimensional network: since both disorder components participate in similar hydrogen bonds, it is necessary to consider only the interactions involving the major component. The formation of the hydrogen-bonded network is readily analysed in terms of three simple sub-structures (Ferguson et al., 1998a,b;Gregson et al., 2000), in which the asymmetric unit aggregates are linked in different ways, each utilizing just one of the three inter-aggregate hydrogen bonds. The N14-H14Á Á ÁO31 i (see Table 1 for symmetry codes) hydrogen bond links the aggregates into a C 3 3 (17) (Etter, 1990;Etter et al., 1990;Bernstein et al., 1995) chain running parallel to the [010] direction (Fig. 2). In the second sub-structure, the C1-H1Á Á ÁO35 ii hydrogen bond links the aggregates into another C 3 3 (17) chain, this time running parallel to the [101] direction (Fig. 3). In the final sub-structure, the C7-H7Á Á ÁO36 iii hydrogen bond links inversion-related pairs of aggregates into a cyclic centrosymmetric system characterized by an R 6 6 (34) motif (Fig. 4). The combination of the chains along [010] and [101] generates a complex sheet lying parallel to (101), and adjacent sheets are linked by the R 6 6 (34) motif, thereby generating a three-dimensional array.

Database survey
Here we briefly compare the salient features of the structure of compound (I), with those of some related structures. As Part of the crystal structure of (I) showing the formation of a hydrogenbonded C 3 3 (17) chain running parallel to [010]. Hydrogen bonds are drawn as dashed lines. For the sake of clarity, the H atoms bonded to C atoms have all been omitted.

Figure 3
Part of the crystal structure of (I) showing the formation of a hydrogenbonded C 3 noted above (Section 2), the OÁ Á ÁO distances in the anion of the chlorothixene salt (III) (Shaibah et al., 2019), is slightly shorter than that found here for compound (I). Although the OÁ Á ÁO distances in (I) and (III) are very short, some even shorter distances have been reported, some below 2.40 Å . One of the simplest organic compounds to display such a short distance is the enol form, Me 3 C(OH) C(CN)COCMe 3 , of the 1,3 diketone 4-cyano 2,2,6,6-tetramethyl3,5-heptanedione, where the intramolecular O-HÁ Á ÁO hydrogen bond has an OÁ Á ÁO distance of 2.3936 (15) Å (Belot et al., 2004), while the corresponding distances in some cyclic phosphate derivatives are reported to be as low as 2.368 (4) Å (Kumara Swamy et al., 2001).
The dihedral angles between the planes of the pendent aryl rings in the fused tricyclic portion of various clozapine derivatives show some curious variations. In the free base (Fillers & Hawkinson, 1982) this angle is 67.3 [unfortunately, the atomic coordinates retrieved from the CSD (Groom et al., 2016) have no s.u. values] and in the monohydrate (CSD refcode DEHBUP; the publication cited in the CSD could not be traced) and the methanol solvate (Verma et al., 2018), the corresponding angles are 63.4 and 56.1 , respectively. In the 1:1 salt formed with 3,5-dinitrobenzoic acid (II), this angle is 62.21 (11) (Kaur et al., 2015), fairly similar to the values of 60.97 (9) and 59.07 (16) in the 1:1 salts formed with maleic and 2-hydroxybenzoic acids, respectively (Kaur et al., 2015). In the di(hydrobomide) salt, the angle is 52.3 (Fillers & Hawkinson, 1982), while in the ethanol solvate of clozapine N-oxide, the corresponding angle is 56.2 (van der Peet et al., 2018). There are, at present, too few data for any pattern to be discernible in the variation of this dihedral angle.
The hydrogen-bonded supramolecular assembly of compound (I) is three dimensional (Section 3, above), but in the solvated 1:1 salt (II), the hydrogen-bonded ion pairs are linked into chains by astacking interaction (Kaur et al., 2015). There are no hydrogen bonds in the structure of clozapine itself (Fillers & Hawkinson, 1982), but in the monohydrate (DEHBUP), a combination of one N-HÁ Á ÁO hydrogen bond and two O-HÁ Á ÁN hydrogen bonds links the components into a chain of rings. In the methanol solvate of clozapine (Verma et al., 2018), the components are linked by an O-HÁ Á ÁN hydrogen bond, but with no further aggregation. In the hydrogenmaleate and 2-hydroxybenzoate salts, multiple hydrogen bonds generate sheets and a three-dimensional supramolecular network, respectively (Kaur et al., 2015), while in the di(hydrobromide) salt, the ions are linked into chains by N-HÁ Á ÁBr hydrogen bonds (Fillers & Hawkinson, 1982).

Synthesis and crystallization
Clozapine (100 mg, 0.31 mmol) and 3,5-dinitrobenzoic acid (66 mg, 0.31mmol) were dissolved in methanol (10 ml), and this mixture was then stirred at 333 K for a few minutes. The solution was permitted to cool to room temperature and the resulting crystals were then collected by filtration and dried over P 2 O 5 . Crystals of (I) suitable for single-crystal X-ray diffraction were obtained by slow evaporation, at room temperature and in the presence of air, of a solution in the mixed solvents of chloroform and methanol (initial composition 1:1, v/v); m.p. 494-497 K.

Refinement
Crystal data, data collection and refinement details are summarized in Table 2. All H atoms 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), 0.96 Å (CH 3 ) or 0.97 Å (CH 2 ), and with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C); the CH 3 group was permitted to rotate but not to tilt. For the H atoms bonded to N or O atoms, the atomic coordinates were refined with U iso (H) = 1.2U eq (N) or 1.5U eq (O). For the minor disorder component, the N-O distances and the 1,3-non-bonded OÁ Á ÁO distances were restrained to be the same of the corresponding distances in the major component, subject to s.u. values of 0.01 and 0.02 Å , respectively. In addition, a similarity restraint was applied to the disordered O-atom sites and for each of the disorder components, the C-NO 2 fragment was restrained to be planar. Subject to these conditions, the refined disorder occupancies are 0.56 (3)  Part of the crystal structure of (I) showing the formation of a hydrogenbonded R 6 6 (34) ring. Hydrogen bonds are drawn as dashed lines. For the sake of clarity, the H atoms bonded to those C atoms that are not involved in the motif shown have been omitted.   (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.

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