Crystal structure and hydrogen-bonding patterns in 5-fluorocytosinium picrate

In the crystal, the 5FC+ cation interacts with the PA− anion through three-centre N—H⋯O hydrogen bonds, forming two conjoined rings having (6) and (6) motifs, and is extended by N—H⋯O hydrogen bonds and C—H⋯O interactions into a two-dimensional sheet structure lying parallel to (001). Also present in the crystal structure are weak C—F⋯π interactions.

In the crystal structure of the title compound, 5-fluorocytosinium picrate, C 4 H 5 FN 3 O + ÁC 6 H 2 N 3 O 7 À , one N heteroatom of the 5-fluorocytosine (5FC) ring is protonated. The 5FC ring forms a dihedral angle of 19.97 (11) with the ring of the picrate (PA À ) anion. In the crystal, the 5FC + cation interacts with the PA À anion through three-centre N-HÁ Á ÁO hydrogen bonds, forming two conjoined rings having R 2 1 (6) and R 1 2 (6) motifs, and is extended by N-HÁ Á ÁO hydrogen bonds and C-HÁ Á ÁO interactions into a two-dimensional sheet structure lying parallel to (001). Also present in the crystal structure are weak C-FÁ Á Á interactions.

Chemical context
Crystal engineering is defined as the rational design of crystalline solids through control of intermolecular interactions (hydrogen bonding, hydrophobic forces, van der Waals forces, interactions and electrostatic forces). New solid forms of pharmaceuticals are designed using the crystal engineering approach. These engineered solids have technological and legal importance. Among the intermolecular interactions, hydrogen bonding is the master key for molecular recognition in biological systems because of its strength and directionality (Almarsson & Zaworoko, 2004;Desiraju, 1995). It plays a dominant role in molecular aggregates (Samuel, 1997;Tutughamiarso & Egert, 2012) and three-dimensional structure, stability and function of biomacromolecules (Gould, 1986). In particular, pyrimidine derivatives are used in the treatment of antiviral, antifungal, antitumor and cardiovascular diseases. 5-fluorocytosine (5FC) is a synthetic antimycotic compound, first synthesized in 1957 and widely used as an antitumor agent it is also active against fungal infection (Heidelberger et al., 1957;Portalone & Colapietro, 2007;Vermes et al., 2000). It becomes active by deamination of 5FC into 5-fluorouracil by the enzyme cytosine deaminase (CD) and inhibits RNA and DNA synthesis (Morschhä user, 2003). Picric acid forms charge-transfer complexes with many organic compounds. It functions not only as an acceptor to form -stacking complexes with aromatic biomolecules, but also as an acidic ligand to form salts with polar biomolecules through specific electrostatic hydrogen-bonding interactions . The present work is focused on the understanding of supramolecular hydrogen-bonding patterns exhibited by the interaction of 5FC and picric acid, giving the (1:1) title salt, C 4 H 5 FN 3 O + ÁC 6 H 2 N 3 O 7 À whose structure and hydrogenbonding patterns are reported on herein.

Supramolecular features
In this crystal structure, the N4-amino group and protonated N3 atom of the 5FC + cation interact with atoms O3 and O9 of the picrate anion through three-centre N-HÁ Á ÁO hydrogen bonds, forming two fused-ring motifs with graph-sets R 2 1 (6) and R 1 2 (6) (Fig. 1). One of the N4-amino hydrogen atoms of the 5FC + cation acts as a three-centre donor and the O3 atom of the PA À anion acts as a three-centre acceptor. This type of interaction has also been reported in the crystal structures of 2-amino-4,6-dimethylpyrimidinium picrate (Subashini et al., 2006) and 2-amino-4,6-dimethoxypyrimidinium picrate, pyrimethaminium picrate dimethyl sulfoxide (Thanigaimani et al., 2009). Similarly, the other hetero nitrogen atom (N1) of the cation and both the phenolate O3 i and a nitro O4 i atom of a PA À anion form an R 2 1 (6) ring motif through N-HÁ Á ÁO hydrogen bonds with a second C-HÁ Á ÁO4 i interaction, closing an R 1 2 (5) ring (Table 1). A similar type of interaction has also been observed in the crystal structure of cytosinium hydrogen chloroanilate monohydrate (Gotoh et al., 2006).

Figure 2
A view of the supramolecular network formed via N-HÁ Á ÁO and C-HÁ Á ÁO interactions. Dashed lines represent hydrogen bonds. For symmetry codes, see Table 1.

Figure 1
The naming scheme for the 5FC + cation and the PA À anion in the title compound, showing 30% probability level displacement ellipsoids. Dashed lines represent hydrogen bonds.

Synthesis and crystallization
A hot aqueous solution of 5-fluorocytosine (32 mg) and picric acid (57 mg) were mixed in a 1:1 molar ratio. The resulting solution was warmed to 353 K wrong symmetry descriptioninversion centre in central benzene ring over a water bath for half an hour and kept for slow evaporation. After a week, colourless prismatic crystals were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were positioned geometrically (C-H = 0.95 Å and N-H = 0.88 Å ) and were refined using a riding model with U iso (H) = 1.2U eq (parent atom).

5-Fluorocytosinium picrate
Crystal data 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.