Crystal engineering is defined as the rational design of crystalline solids through control of inter­molecular inter­actions (hydrogen bonding, hydro­phobic forces, van der Waals forces, π–π 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 (Morschhäuser, 2003 ▸). Picric acid forms charge-transfer complexes with many organic compounds. It functions not only as an acceptor to form π-stacking complexes with aromatic biomolecules, but also as an acidic ligand to form salts with polar biomolecules through specific electrostatic hydrogen-bonding inter­actions (In et al., 1997 ▸). 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, C₄H₅FN₃O⁺·C₆H₂N₃O₇ ⁻ whose structure and hydrogen-bonding patterns are reported on herein.

The asymmetric unit contains one 5-fluoro­cytosinium cation (5FC⁺) and one picrate anion (PA⁻) (Fig. 1 ▸). The 5-fluoro­cytosine cation is protonated at the N3 atom, as is evident from the widening of the corresponding inter­nal angle from 120.8 (5)° to 125.37 (17)° compared to neutral 5FC (Louis et al., 1982 ▸). The dihedral angle between the planes of the rigs in the cation and anion is 19.97 (11)°. In the picrate (PA⁻) anion, the nitro groups lie variously out of the parent benzene ring, with torsion angles C9—C8—N5—O4, C9—C10—N6—O7 and C11—C12—N7—O9 of 166.2 (2), −171.7 (2) and 147.2 (2)°, respectively.

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—H⋯O hydrogen bonds, forming two fused-ring motifs with graph-sets (6) and (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⁻ 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 O3ⁱ and a nitro O4ⁱ atom of a PA⁻ anion form an (6) ring motif through N—H⋯O hydrogen bonds with a second C—H⋯O4ⁱ inter­action, closing an (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 ▸).

Further, the symmetry-related O2ⁱⁱ atom and the amino group of the 5FC⁺ cation are connected through an N—H⋯O hydrogen bond, forming a two-dimensional supra­molecular network lying parallel to (001) (Fig. 2 ▸). Also present in the crystal structure is a weak C5—F5⋯π inter­action (Fig. 3 ▸) between 5FC⁺ cations [C5⋯Cg ^iv = 3.4002 (19) Å; C—F⋯Cg = 88.34 (12)°, where Cg is the centroid of the N1–C6 ring; symmetry code: (iv) −x, −y, −z + 1]. A similar angle [90.5 (2)°] has been reported for a C—F⋯Cg inter­action in an acridinium tri­fluoro­methane sulfonate compound (Sikorski et al., 2005 ▸).

The crystal structures of 5-fluoro­cytosine monohydrates (Louis et al., 1982 ▸; Portalone & Colapietro, 2006 ▸; Portalone & Colapietro, 2007 ▸; Portalone, 2011 ▸), polymorphs (Hulme & Tocher, 2006 ▸; Tutughamiarso & Egert, 2012 ▸), salts (Perumalla et al., 2013 ▸) and co-crystals (Tutughamiarso et al., 2012 ▸; da Silva et al., 2014 ▸) have been reported in the literature. From our laboratory, 5-fluoro­cytosinium salicylate (Prabakaran et al., 2001 ▸), 5-fluoro­cytosinium 3-hy­droxy­picolinate (Karthikeyan et al., 2014 ▸) and 5-fluoro­cytosine melamine (Mohana et al., 2016 ▸) have been reported. Various salts and co-crystals of picric acid have also been reported in the literature (Subashini et al., 2006 ▸; Thanigaimani et al., 2009 ▸; Nagata et al., 1995 ▸; Smith et al., 2004 ▸; Gotoh et al., 2004 ▸).

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 description - inversion 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.

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).