Crystal structure of (RS)-4-(3-carboxy-1-ethyl-6,8-difluoro-4-oxo-1,4-dihydroquinolin-7-yl)-2-methylpiperazin-1-ium 3-carboxy-5-fluorobenzoate

Reaction of lomefloxacin [(RS)-4-(3-carboxy-1-ethyl-6,8-difluoro-4-oxo-1,4-dihydroquinolin-7-yl)-2-methylpiperazine, Lf] with 5-fluoroisophthalic acid leads to a charge-assistant hydrogen-bonding network between HLf+ cations and 3-carboxy-5-fluorobenzoate anions.

In the title organic salt, C 17 H 20 F 2 N 3 O 3 + ÁC 8 H 4 FO 4 À , proton transfer leads to one protonated lomefloxacin molecule (HLf + ) and one 3-carboxy-5-fluorobenzoate (5-F-Hip À ) anion in the asymmetric unit. The HLf + cation is bent, with a dihedral angle of 38.3 (1) between the quinoline ring and the piperazinium moiety. In the crystal, two kinds of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen-bonded chains cross-link each other to produce a three-dimensional network structure that is additionally stabilized by weak C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds, as well asinteractions. The methyl group attached to the piperazinium ring is disordered over two sets of sites [refined ratio: 0.645 (5):0.335 (5)], indicating the presence of both enantiomers of the cation in the structure.

Structural commentary
The structures of the molecular entities of (I) are displayed in Fig. 1. Unlike other lomefloxacin salts (Zhang et al., 2015), the title compound reveals no guest solvents residing in the crystal structure. In the asymmetric unit, there is one HLf + cation and one 5-F-Hip À anion, i.e. only one proton has been transferred from the free acid. Within the HLf + moiety, a non-planar conformation of the molecule is formed with a dihedral angle of 38.3 (1) between the aromatic ring plane and the piperazinium ring (the latter exhibits a chair conformation). An intramolecular S(6) hydrogen-bonding pattern (Etter et al., 1990) is found between the carboxylic group and the carbonyl O atom (O2-H2Á Á ÁO1; Table 1). The 5-F-Hip À anion is nearly planar (r.m.s. deviation = 0.132 Å ), with the highest deviation of 0.2645 (13) Å for the carboxylate O6 atom.

Figure 2
A perspective view of (I) showing the N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen-bonding interactions (dotted lines) between the two kinds of chains. 'Acidic' chains, i.e. chains involving only the anion, are shown in red for clarity.

Figure 1
Molecular structures of the cation and anion in the title salt. Displacement ellipsoids are drawn at the 30% probability level.
phenyl rings of the anions with a centroid-to-centroid separation of 3.7895 (12) Å .

Synthesis and crystallization
A methanol solution (6 ml) of 5-fluoroisophthalic acid (5-F-H 2 ip; 20 mg, 0.1 mmol) was mixed with a slurry of lome-floxacin (Lf) (35 mg, 0.1 mmol) in 5 ml water under stirring. The mixture was exposed to ultrasound for ca 20 min, and was then filtered and left to slowly evaporate. Colourless blocklike single crystals suitable for X-ray analysis were obtained after several weeks. Yield: 65% (35 mg, based on Lf). Analysis calculated for C 25 H 24 F 3 N 3 O 7 : C, 56.08; H, 4.52; N, 7.85%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C were placed geometrically and refined in a riding model: C-H = 0.96-0.98 Å ; U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). All O-bound and N-bound H atoms were initially found in difference electron-density maps, and then refined using a riding model [O-H = 0.82 Å and N-H = 0.89 Å ; U iso (H) = 1.2U eq (N) and 1.5U eq (O)]. The methyl group bound to the piperazinium ring is disordered over two positions with occupancies of 0.645 (5) and 0.355 (5).

sup-1
Acta Cryst. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010). 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.