Bis(2,6-diamino-4-chloropyrimidin-1-ium) fumarate

In the title salt, 2C4H6ClN4 +·C4H2O4 2−, the complete fumarate dianion is generated by crystallographic inversion symmetry. The cation is essentially planar, with a maximum deviation of 0.018 (1) Å. In the anion, the carboxylate group is twisted slightly away from the attached plane, the dihedral angle between the carboxylate and (E)-but-2-ene planes being 12.78 (13)°. In the crystal, the protonated N atom and the 2-amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In addition, another type of R 2 2(8) motif is formed by centrosymmetrically related pyrimidinium cations via N—H⋯N hydrogen bonds. These two combined motifs form a heterotetramer. The crystal structure is further stabilized by stong N—H⋯O, N—H⋯Cl and weak C—H⋯O hydrogen bonds, resulting a three-dimensional network.

In the title salt, 2C 4 H 6 ClN 4 + ÁC 4 H 2 O 4 2À , the complete fumarate dianion is generated by crystallographic inversion symmetry. The cation is essentially planar, with a maximum deviation of 0.018 (1) Å . In the anion, the carboxylate group is twisted slightly away from the attached plane, the dihedral angle between the carboxylate and (E)-but-2-ene planes being 12.78 (13) . In the crystal, the protonated N atom and the 2amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. In addition, another type of R 2 2 (8) motif is formed by centrosymmetrically related pyrimidinium cations via N-HÁ Á ÁN hydrogen bonds. These two combined motifs form a heterotetramer. The crystal structure is further stabilized by stong N-HÁ Á ÁO, N-HÁ Á ÁCl and weak C-HÁ Á ÁO hydrogen bonds, resulting a threedimensional network.
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 Pyrimidine derivatives are very important molecules in biology and have many application in the areas of pesticide and pharmaceutical agents (Condon et al., 1993). For example, imazosulfuron, ethirmol and mepanipyrim have been commercialized as agrochemicals (Maeno et al., 1990). Pyrimidine derivatives have also been developed as antiviral agents, such as AZT, which is the most widely-used anti-AIDS drug (Gilchrist, 1997). Fumaric acid is among the organic compounds widely found in nature, and is a key intermediate in the biosynthesis of organic acids. Fumaric acid is of interest since it is known to form supramolecular assemblies with N-aromatic complexes (Batchelor et al., 2000). In order to study some interesting hydrogen bonding interactions, the synthesis and structure of the title compound is presented here.
In the crystal structure (Fig. 2), the protonated N atom and the 2-amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N-H···O hydrogen bonds, forming R 2 2 (8) (Bernstein et al., 1995) ring motifs. In addition, another type of R 2 2 (8) motif is formed by centrosymmetrically related pyrimidinium cation through a pair of N3-H3···N1 iii hydrogen bonds (symmetry codes in Table 1). These two different motifs generate a linear heterotetrameric unit known to be one of the most stable synthons (Thakur & Desiraju, 2008). One of the O atoms of the carboxylate group acts as an acceptors of bifurcated N2-H1···O2 ii and N4-H5···O2 ii hydrogen bonds (symmetry codes in Table 1). The crystal structure is further stabilized by strong N4-H4···O1 iv , N4-H5···Cl1 V and weak C3-H3A···O2 i hydrogen bonds (symmetry codes in Table 1), resulting in a three-dimensional network.

Experimental
Hot methanol solutions (20 ml) of 2,6-diamino-4-chloropyrimidine (36 mg, Aldrich) and fumaric acid (29 mg, Merck) were mixed and warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title compound appeared after a few days.

Refinement
N-bound H Atoms were located in a difference Fourier maps and refined isotropically. The N2-H1 bond length was constrained to 0.85 (1) Å. The remaining hydrogen atoms were positioned geometrically [C-H= 0.95 Å] and were refined using a riding model, with U iso (H) = 1.2 U eq (C).

Figure 1
The molecular structure of the title compound with 50% probability displacement ellipsoids.  The crystal packing of the title compound viewed down the a axis. Hydrogen atoms not involved in the intermolecular interactions (dashed lines) have been omitted for clarity.  (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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.