Crystal structure of 2,4-diamino-6-oxo-3,6-dihydropyrimidin-1-ium p-toluenesulfonate

In the crystal of the title salt, the 2,6-diamino-4-oxo-1,3-dihydropyrimidin-1-ium cations and the p-toluenesulfonate anions are linked by a series of N—H⋯O hydrogen bonds, forming tunnel-like polymer chains propagating along [010].

In the title salt, C 4 H 7 N 4 O + ÁC 7 H 7 O 3 S À , the 2,6-diamino-4-oxo-1,3-dihydropyrimidin-1-ium cation interacts with the sulfonate group of the p-toluenesulfonate anion via a pair of N-HÁ Á ÁO hydrogen bonds, forming a heterosynthon R 2 2 (8) that mimics the role of a carboxylate. The self-assembled cations form a homo-synthon R 2 1 (6) motif which is further linked with the sulfonate anion via N-HÁ Á ÁO hydrogen bonds to generate an R 3 2 (10) ring motif. The three motifs are fused together and extended as supramolecular ribbons along the baxis direction. Adjacent ribbons are further linked via N-HÁ Á ÁO hydrogen bonds to form an annulus, with an R 4 4 (20) ring motif, resulting in a tunnel-like arrangement propagating along [010]. There are slipped parallelstacking interactions [inter-centroid distance = 3.6539 (7) Å ], between the tunnel-like polymer chains, forming slabs parallel to (100).

Chemical context
Di-and tri-aminopyrimidines show various biological and pharmacological properties like tyrosine kinase (Thomas, 1995a,b), dihydrofolate reductase inhibitors (Ayer, 1991) and are used as antiviral and antiprotozoan agents. 2,6-Diamino-4hydroxy pyrimidine (DAHP), an inhibitor of guanosine triphosphate cyclohydrolase I, blocks the synthesis of tetrahydrobiopterin which is a known cofactor of inducible nitric oxide synthesis (iNOS) (Bogdan et al., 1995). The study of hydrogen-bonding patterns involving sulfonate groups in biological systems and metal complexes are of current interest (Gomathi & Muthiah, 2011;Wang, 2006). The present report deals with the supramolecular interactions exhibited by the title salt.

Structural commentary
The asymmetric unit of the title salt contains one 2,6-diamino-4-oxo-1,3-dihydropyrimidin-1-ium cation and one p-toluene sulfonate anion (Fig. 1). The cation is protonated at the N3 position, which is reflected by the slight increase in the C2-N3-C4 bond angle to 123.2 (1) . The dihedral angle between the cation and anion ring mean planes is 54.04 (6) .

Supramolecular features
The primary interaction between the cation and anion takes place via a pair of N-HÁ Á ÁO hydrogen bonds, forming a robust six-membered hetero-synthon, R 2 2 (8), and here the sulfonate group mimics the role of a carboxylate. This motif links the protonated ring N atom, N3, and the 2-amino N atom, N2, of the cation with the sulfonate atoms O2 and O3 of the anion. Adjacent R 2 2 (8) ring motifs are connected via an N-HÁ Á ÁO hydrogen bond by linking the 2-amino N atom, N2 with atom O3 i [symmetry code: x, y + 1, z]. The cation undergoes self-association via a pair of bifurcated N-HÁ Á Á(O,O) hydrogen bonds, forming a homo-synthon, R 1 2 (6). This motif involves ring N1 and the 6-amino N atoms and carbonyl atom O1 i of the cation ( Table 1). The self-assembled cations extend as a supramolecular chain propagating along [010]. The homo-and hetero-synthons [R 2 2 (8) and R 1 2 (6)] are linked by an R 2 3 (10) ring motif. The three motifs are fused together continuously, forming supramolecular ribbons along [010]. Two such ribbons in adjacent planes are connected via N-HÁ Á ÁO hydrogen bonds by linking the 6-amino N of the cation and the sulfonate atom O4 ii [symmetry code: Àx + 1, Ày + 1, Àz + 1] of the anion, generating an annulus (Su et al., 2007) with an R 4 4 (20) graph-set motif (Fig. 2). This motif extends in the direction of the supramolecular ribbons and generates a tunnel-like architecture along the b-axis direction (Figs. 2 and 3).

Figure 3
A view along the b axis of the crystal packing of the title molecular salt. Hydrogen bonds (see Table 1 for details) andinteractions are shown as dashed lines.

Database survey
A search of the Cambridge Structural Database (Version 5.36; Groom & Allen, 2014) revealed the presence of over 700 compounds involving p-toluene sulfonate but only three hits for the 2,6-diamino-4-oxo-1,3-dihydropyrimidin-1-ium cation. These include the sulfate monohydrate (ACEYUD; Muthiah et al., 2004), the di(methanesulfanyl)amide (ESAQOE; Wijaya et al., 2004) and the chloride dihydrate (SUZFOJ; Suleiman Gwaram et al., 2010). In ACEYUD the cation is protonated at the N atom adjacent to the carbonyl group, as in the title compound, while in compounds ESAQOE and SUZFOJ it is the N atom para to the carbonyl group that is protonated. Otherwise, the bond distances in these three compounds are very similar and close to those observed for the title compound.

Synthesis and crystallization
A hot methanolic solution (20 ml) of 2,6-diamino-4-hydroxy pyrimidine (31.5 mg, Aldrich) and p-toluene sulfonic acid (43 mg, Loba chemie) was warmed at 323 K for 30 min over a water bath. The mixture was cooled slowly and kept at room temperature and after three weeks light-yellow needle-shaped crystals were obtained.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses 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 observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs 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.