Bis(2-amino-5-bromopyridinium) fumarate dihydrate

In the title compound, 2C5H6BrN2 +·C4H2O4 2−·2H2O, the complete fumarate dianion is generated by crystallographic inversion symmetry. The cation is approximately planar, with a maximum deviation of 0.036 (1) Å. In the anion, the carboxylate group is twisted slightly away from the attached plane; the dihedral angle between carboxylate and (E)-but-2-ene planes is 6.11 (14)°. In the crystal, the carboxylate O atoms form bifurcated (N—H⋯O and C—H⋯O) and N—H⋯O hydrogen bonds with the cations. The crystal packing is stabilized by R 2 2(8) ring motifs which are generated by pairs of N—H⋯O hydrogen bonds. The crystal structure is further consolidated by water molecules via O(water)—H⋯O and N—H⋯O(water) hydrogen bonds. The components are linked by these interactions into three-dimensional network.

In the title compound, 2C 5 H 6 BrN 2 + ÁC 4 H 2 O 4 2À Á2H 2 O, the complete fumarate dianion is generated by crystallographic inversion symmetry. The cation is approximately planar, with a maximum deviation of 0.036 (1) Å . In the anion, the carboxylate group is twisted slightly away from the attached plane; the dihedral angle between carboxylate and (E)-but-2-ene planes is 6.11 (14) . In the crystal, the carboxylate O atoms form bifurcated (N-HÁ Á ÁO and C-HÁ Á ÁO) and N-HÁ Á ÁO hydrogen bonds with the cations. The crystal packing is stabilized by R 2 2 (8) ring motifs which are generated by pairs of N-HÁ Á ÁO hydrogen bonds. The crystal structure is further consolidated by water molecules via O(water)-HÁ Á ÁO and N-HÁ Á ÁO(water) hydrogen bonds. The components are linked by these interactions into three-dimensional network.

Comment
Hydrogen bonding plays a key role in molecular recognition (Goswami & Ghosh, 1997) and crystal engineering research (Goswami et al., 1998). Fumaric acid, the E isomer of butenedioic acid, is of interest since it is known to form supramolecular assemblies with N-aromatic compounds (Batchelor et al., 2000). It tends to form infinite chains arranged in a nearly coplanar manner via pairs of strong O-H···O hydrogen bonds. The crystal structures of 2-aminopyridinium-fumarate-fumaric acid (2/1/1) (Büyükgüngör et al., 2004) and 2,6-diamino pyridinium hydrogen fumarate (Büyükgüngör & Odabąsoǧlu, 2006) have been reported in the literature. We have recently reported the crystal structures of 2-amino-5-chloropyridine-fumaric acid (1/2) ) and 2-amino-4-methylpyridinium (E)-3-carboxyprop-2-enoate ) from our laboratory. In order to study some interesting hydrogen bonding interactions, the synthesis and structure of the title compound, (I), is presented here.
The anions, cations and water molecules are linked by these interactions into three-dimensional network.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-bromopyridine (86 mg, Aldrich) and fumaric acid (58 mg, Merck) was mixed and warmed over a 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
O-and N-bound H atoms were located in a difference Fourier map and allowed to refined freely. The rest of the hydrogen atoms were positioned geometrically and refined using a riding model with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C). The highest residual electron density peak is located at 0.66 Å from C6 and the deepest hole is located at 1.19 Å from Br1.  Fig. 1. The molecular structure of the title compound showing 50% probability displacement ellipsoids for non-H atoms and the atom-numbering scheme. Symmetry code: ($) -x + 1, -y + 1, -z. Intramolecular interactions are shown as dashed lines.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (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 mat- Geometric parameters (Å, °)