2-Amino-4-methylpyridinium (E)-3-carboxyprop-2-enoate

In the title salt, C6H9N2 +·C4H3O4 −, the dihedral angle between the pyridine ring and the plane formed by the hydrogen fumarate anion is 85.67 (6)°. Excluding the amino and methyl groups, the atoms of the cation are coplanar, with a maximum deviation of 0.005 (1) Å. In the crystal structure, 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. These motifs are further connected through N—H⋯O and C—H⋯O hydrogen bonds, leading to a supramolecular chain along the c axis. These chains are further cross-linked via a pair of O—H⋯O hydrogen bonds involving centrosymmetrically related hydrogen fumarate anions, forming a two-dimensional network parallel to (101). These planes are further interconnected by O—H⋯O interactions into a three-dimensional network.

In the title salt, C 6 H 9 N 2 + ÁC 4 H 3 O 4 À , the dihedral angle between the pyridine ring and the plane formed by the hydrogen fumarate anion is 85.67 (6) . Excluding the amino and methyl groups, the atoms of the cation are coplanar, with a maximum deviation of 0.005 (1) Å . In the crystal structure, 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-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. These motifs are further connected through N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, leading to a supramolecular chain along the c axis. These chains are further cross-linked via a pair of O-HÁ Á ÁO hydrogen bonds involving centrosymmetrically related hydrogen fumarate anions, forming a twodimensional network parallel to (101). These planes are further interconnected by O-HÁ Á ÁO interactions into a threedimensional network.
The asymmetric unit of the title compound consists of a 2-amino-4-methyl pyridinium cation and a hydrogen fumarate anion (Fig. 1). In the 2-amino-4-methylpyridinium cation, a wider than normal angle [C1-N1-C5 121.94 (11)°] is subtended at the protonated N1 atom. The C10-O3 bond distance of 1.2259 (16) Å is much shorter than the C10-O4 bond distance of 1.3224 (15) Å, suggesting that the carboxyl group is not deprotonated in the crystal structure. The dihedral angle between the pyridine ring and the plane formed by the hydrogen fumarate anion is 85.67 (6)°. Excluding amino and methyl groups, the atoms of the cation are coplanar, with a maximum deviation of 0.005 (1) Å for atom C2. The bond lengths (Allen et al., 1987) and angles are normal.
In the crystal packing (Fig. 2), the protonated N1 atom and the 2-amino group (N2) are hydrogen-bonded to the carboxylate oxygen atoms (O1 and O2) via a pair of N-H···O hydrogen bonds, forming a R 2 2 (8) ring motif (Bernstein et al., 1995). Furthermore, these motifs are connected through N-H···O and C-H···O hydrogen bonds (Table 1), leading to a one-dimensional supramolecular chain along the c-axis. These chains are further connected via a pair of O-H···O hydrogen bonds involving centrosymmetric hydrogen fumarate anions, forming a two-dimensional network parallel to (010). These planes are further interconnected by O4-H1O4···O3 hydrogen bonds into a 3D network.

Experimental
A hot methanol solution (20 ml) of 2-amino-4-methylpyridine (54 mg, Aldrich) and fumaric acid (58 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
All the H atoms were located from a difference Fourier map and refined freely [C-H = 0.953 (19) Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are shown as spheres of arbitrary radius.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.