Ciprofloxacin salt and salt co-crystal with dihydroxybenzoic acids

The crystal structure of a ciprofloxacin salt with 2,6-dihydroxybenzoic acid and a ciprofloxacin hydrochloride salt co-crystal with 3,5-dihydroxybenzoic acid are reported.


Chemical context
The design and exploration of multi-component crystals of active pharmaceutical ingredients (APIs) have gained increasing interest over recent decades. The formation of multi-component crystals, i.e. salts and co-crystals through a crystal-engineering approach has been continuously demonstrated as a versatile tool to improve the physicochemical properties of APIs (Kavanagh et al., 2019;Putra & Uekusa, 2020;Thakur & Thakuria, 2020). Recently, the co-crystallization of salt APIs or salt co-crystal formation has been increasingly studied. Salt co-crystallization has been utilized to suppress hydrate formation of salt APIs (Nugraha & Uekusa, 2018;Fujito et al., 2021). As a part of our study of salt cocrystals of APIs, we investigated multi-component crystals of ciprofloxacin. Ciprofloxacin is a Biopharmaceutics Classification System (BCS) class IV fluoroquinolone antibiotic that is widely used therapeutically as the free base and the hydrochloride salt (Olivera et al., 2011).

Structural commentary
Compound (I) was obtained as an anion-exchange product between ciprofloxacin hydrochloride and 2,6-dihydrobenzoic acid in solution. 2,6-Dihydroxybenzoic acid (2,6HBA) is a ISSN 2056-9890 relatively strong carboxylic acid with a pK a of 1.30 (Gdaniec et al., 1994;Habibi-yangjeh et al., 2005). Compound (I) crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit consists of one ciprofloxacin cation and one 2,6HBA anion ( Fig. 1). The C-O distances of the ciprofloxacin carboxylic group i.e., 1.218 (3) and 1.325 (3) Å indicate that it exists as the neutral carboxylic form. However, in 2,6HBA, the C-O distances are very similar i.e., 1.263 (4) and 1.267 (3) Å due to resonance stabilization in the carboxylate anion (Childs et al., 2007;Aakerö y et al., 2006). As a result, the piperazinyl group of ciprofloxacin is protonated. Therefore, compound (I) is a salt. The formation of a salt is well-predicted by the pK a rule (Cruz-Cabeza, 2012). The pK a of ciprofloxacin are 6.18 and 8.73 for the carboxylic acid and the piperazinyl ring, respectively (Sun et al., 2002). Therefore, salt formation is expected because the ÁpK a between the piperazinyl ring of ciprofloxacin and the carboxylic acid of 2,6HBA is greater than 4. Similar behaviour is observed in the salicylate salt of ciprofloxacin (Surov et al., 2019;Nugrahani et al., 2020).
Compound (II) crystallizes in the non-centrosymmetric P1 space group despite the lack of a chiral centre. The asymmetric unit comprises one ciprofloxacin cation, one chloride anion and one 3,5HBA molecule, as shown in Fig. 2. In addition, one water molecule is incorporated into the crystal lattice. An anion-exchange reaction during crystallization did not occur in this system. Compared to 2,6HBA, the coformer is a weaker acid with a pK a of 4.04 (Habibi-yangjeh et al., 2005). Contrary to the previous structures, the coformer exists as a neutral molecule in the crystal. The carboxylic C18-O4 and C18-O5 distances of 2,6HBA are 1.320 (4) and 1.216 (4) Å , respec-tively, confirming its neutral state. Additionally, the carboxylic C1-O1 and C1-O2 distances of ciprofloxacin, i.e. 1.227 (4) and 1.314 (4) Å , respectively, also confirm the neutral state of this moiety. On the other hand, the piperazinyl group is protonated. Hence, compound (II) is a salt co-crystal monohydrate of ciprofloxacin.

Supramolecular features
In compound (I), the carboxylate anion of 2,6HBA acts as a hydrogen-bond donor for intramolecular hydrogen bonds involving two hydroxyl groups, namely O6-H6Á Á ÁO5 and O7-H7Á Á ÁO4. The protonated nitrogen atom N3 of the piperazinyl ring is involved in the formation of trifurcated hydrogen bonds with O4, O5, and O6 of the coformer. These charge-assisted hydrogen bonds, i.e. N3-H3BÁ Á ÁO4, N3-      Molecular overlay of ciprofloxacin cation in compounds (I) (red) and (II) (blue). Hydrogen atoms are omitted for clarity.

Figure 4
Intermolecular hydrogen-bonding motifs in (I) showing infinite chains along the a-axis direction formed by ciprofloxacin and 2,6HBA (red). Hydrogen atoms are omitted for clarity. H3BÁ Á ÁO5, and N3-H3AÁ Á ÁO6, form an infinite chain structure along the a-axis direction (Table 1, Fig. 4). The chains are connected to the adjacent ciprofloxacin molecule through head-to-tail N3-H3AÁ Á ÁO1 hydrogen bonding. The crystal packing of (I) is shown in Fig. 5. Along the a-axis, centrosymmetric pairs of ciprofloxacin molecules are stacked byinteractions. The distance between the centroids of symmetryrelated C4-C9 rings is 3.4986 (11) Å . This arrangement leads to the formation of a columnar packing arrangement. Interestingly, a similar packing feature was observed in the 1.75 hydrate of ciprofloxacin salicylate (Nugrahani et al., 2020). In addition, compound (I) shows a layered structure with alternating ciprofloxacin and 2,6HBA layers along the b axis.

Figure 5
Packing motifs of (I) viewed along (a) the a axis and (b) the c axis highlighting the alternating layers of ciprofloxacin and the coformer.

Figure 6
Intermolecular hydrogen-bonding motifs in (II) highlighting the role of the chloride ion and water molecule in bridging ciprofloxacin and 3,5HBA (blue). Hydrogen atoms are omitted for clarity. (b) The crystal packing of (II) showing the alternating layered arrangement.

Synthesis and crystallization
Single crystals of (I) and (II) were obtained by preparing a saturated solution of equimolar ciprofloxacin hydrochloride and the respective coformer in methanol/water (1:1) at room temperature. The saturated solution was allowed to slowly evaporate at room temperature. A suitable single crystal was selected and measured for structure determination.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were refined using a riding model and their displacement parameters (U iso ) were fixed to 1.2U eq of the parent carbon or nitrogen atom and 1.5U eq for hydroxyl groups.    For both structures, data collection: CrysAlis PRO (Rigaku OD, 2020); cell refinement: CrysAlis PRO (Rigaku OD, 2020); data reduction: CrysAlis PRO (Rigaku OD, 2020); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: Mercury (Macrae et al., 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.