A co-crystal of nonahydrated disodium(II) with mixed anions from m-chlorobenzoic acid and furosemide

In view of its potential for developing useful pharamaceutical formulations of furosemide, a widely-used loop diuretic, the crystal structure of the furosemide anion with m-chlorobenzoate has been investigated. In these co-crystals, the monoanions of furosemide and m-chlorobenzoate are balanced by two independent Na+ ions, both of which are hexacoordinated by three monodentate water molecules, two double-water bridge molecules and one single-water bridge molecules, thus yielding centrosymmetric Na2(OH2)8 units linked by single water bridges to form chains in the [10] direction.

In the title compound, [Na 2 (H 2 O) 9 ](C 7 H 4 ClO 2 )(C 12 H 10 ClN 2 O 5 S) {systematic name: catena-poly [[[triaquasodium(I)]-di--aqua-[triaquasodium(I)]--aqua] 3-chlorobenzoate 4-chloro-2-[(furan-2-ylmethyl)amino]-5-sulfamoylbenzoate]}, both the original m-chlorobenzoic acid and furosemide exist with deprotonated carboxylates, and the sodium cations and water molecules exist in chains with stoichiometry [Na 2 (OH 2 ) 9 ] 2+ that propagate in the [110] direction. Each of the two independent Na + ions is coordinated by three monodentate water molecules, two double-water bridges, and one single-water bridge. There is considerable cross-linking between the [Na 2 (OH 2 ) 9 ] 2+ chains and to furosemide sulfonamide and carboxylate by intermolecular O-HÁ Á ÁO hydrogen bonds. All hydrogen-bond donors participate in a complex two-dimensional array parallel to the ab plane. The furosemide NH group donates an intramolecular hydrogen bond to the carboxylate group, and the furosemide NH 2 group donates an intramolecular hydrogen bond to the Cl atom and an intermolecular one to the m-chlorobenzoate O atom. The plethora of hydrogen-bond donors on the cation/water chain leads to many large rings, up to graph set R 4 4 (24), involving two chains and two furosemide anions. The chlorobenzoate is involved in only one R 2

Chemical context
Furosemide is a widely used diuretic for the treatment of hypertension and edema (Krumlovsky & del Greco, 1976;Musini et al., 2015), and to a lesser extent, hypercalcemia (Belen et al., 2014;Carvalhana et al., 2006). While this furancontaining compound is of interest, the toxicity elicited by these core compounds is not well understood. The free furan itself is a known hepato-carcinogen and toxicant, as studied in rats (Gill et al., 2010) and mice (Terrell et al., 2014). The epoxide metabolite of furans, formed in CYP450mediated oxidations, can isomerize to highly reactive electrophilic intermediates such as cis-2-butene-1,4-dial (Chen et al., 1995;Peterson 2015;Vargas et al., 1998).
We have performed the oxidation of furosemide with m-chloroperbenzoic acid (m-CPBA), and isolated various epoxide and isomerized products in support of our efforts to understand this type of toxicity mechanism, and to also identify potential biomarkers for furosemide in humans. During the separation and drying of the products of the furosemide-m-CPBA reaction, we observed the formation of crystals in the mother liquor (the organic layer). Analysis of these crystals by X-ray crystallography revealed a disodium nonahydrate co-crystal with furosemide (starting material) and m-chlorobenzoic acid (an inadvertent contaminant or the reduced product of m-CPBA). Analogous to the known properties of co-crystals of furosemide with nicotinamide and their pharmaceutical importance (Aitipamula et al., 2012;Chadha et al., 2012;Goud et al., 2012;Stepanovs & Mishnev, 2012;Ueto et al., 2012), we believe that the co-crystals of furosemide with m-chlorobenzoic acid could have useful applications in drug development and may lead to formulations with improved potency, solubility, and stability. Therefore, this serendipitous finding may have important applications for improving furosemide bioavailability.

Structural commentary
The asymmetric unit is illustrated in Fig. 1. The furosemide moiety is present as the monoanion, with the COOH group deprotonated, N2 as NH and the primary amine nitrogen N1 as NH 2 . The m-chlorobenzoic acid moiety is also deprotonated. Balancing the charge of the two types of anions are two independent sodium cations, both of which are hexacoordinate, with NaÁ Á ÁO(water) distances in the range 2.3558 (13)-2.4500 (13) Å . Each Na + cation is coordinated by three monodentate water molecules, two double-water bridge molecules, and one single-water bridge molecule, as shown in Fig. 2. Thus, centrosymmetric Na 2 (OH 2 ) 8 units are linked by single water bridges, forming chains in the [110] direction.

Figure 1
The asymmetric unit with 50% ellipsoids.

Figure 2
A portion of the Na-water chain, showing the centrosymmetric Na 2 (OH 2 ) 2 bridges and the single water bridges. anion (carboxylate), and water O-HÁ Á ÁO hydrogen bonds to the sulfonamide O atom, to both types of carboxylates, and to other water molecules. The direction of the normal to the hydrogen-bonding network is [001]. The furan oxygen atom O5 is not involved in the hydrogen bonding. A supramolecular layer in the ab plane is shown in Fig. 3.

Synthesis and crystallization
Furosemide (8.2 mmol; 2.71 g), dissolved in 3 ml of dichloromethane (DCM), was added dropwise over 5 min to a solution of 8.2 mmol of m-CPBA (1.84 g) and 10.5 mmol NaHCO 3 (0.88 g) in 20 ml of DCM on ice with rapid stirring (Fig. 4). After 2 h, an additional 4 mmol of m-CPBA in 10 ml of DCM was added to the reaction mixture. Upon removal from the ice bath, 4 ml of aqueous sodium sulfite solution (10%) was added with stirring for an additional 15 min. After partitioning the layers with deionized water (resistance 18.2 M cm À1 ), the organic layer was collected and the aqueous layer was extracted with another 10 ml of DCM. The combined mixture of the organic layer was washed with 10 ml of aqueous solution of NaHCO 3 (5%, w/v), dried over anhydrous Na 2 SO 4 , and then subjected to partial evaporation under low pressure (ca 4 psi) at 308 K. The partially evaporated sample was left at ambient pressure and temperature overnight. Crystals were formed with slow evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms on C were idealized with C-H distances of 0.95 Å for sp 2 C and 0.99 Å for CH 2 . Those on N and O were assigned from difference maps, and their positions refined, with O-H distances restrained to be equal. U iso (H) were assigned as 1.2 times U eq of the attached atoms (1.5 for water). Six reflections with Fo<<Fc were omitted from the calculations.   Computer programs: COLLECT (Nonius, 1999), HKL SCALEPACK and DENZO and SCALEPACK (Otwinowski & Minor 1997), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008) and ORTEP-3 for Windows (Farrugia, 2012).

Figure 4
Proposed scheme of reactions of furosemide with m-chloroperoxybenzoic acid.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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