2-Amino-5-chloropyridine–fumaric acid (1/2)

The asymmetric unit of the title compound, C5H5ClN2·0.5C4H4O4, comprises a neutral 2-amino-5-chloropyridine molecule and one half of a fumaric acid molecule which lies on an inversion center. The dihedral angle between the pyridine ring and the plane formed by the fumaric acid molecule is 3.22 (8)°. The 2-amino-5-chloropyridine molecule is planar, with a maximum deviation of 0.004 (1) Å for the pyridine N atom. In the crystal, the 2-amino-5-chloropyridine molecules interact with the carboxyl groups of fumaric acid molecules through N—H⋯O and O—H⋯N hydrogen bonds, forming centrosymmetric R 2 2(8) ring motifs and another N—H⋯O hydrogen bond links these motifs into a two-dimensional network parallel to (100).

The asymmetric unit of the title compound, C 5 H 5 ClN 2 Á-0.5C 4 H 4 O 4 , comprises a neutral 2-amino-5-chloropyridine molecule and one half of a fumaric acid molecule which lies on an inversion center. The dihedral angle between the pyridine ring and the plane formed by the fumaric acid molecule is 3.22 (8) . The 2-amino-5-chloropyridine molecule is planar, with a maximum deviation of 0.004 (1) Å for the pyridine N atom. In the crystal, the 2-amino-5-chloropyridine molecules interact with the carboxyl groups of fumaric acid molecules through N-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, forming centrosymmetric R 2 2 (8) ring motifs and another N-HÁ Á ÁO hydrogen bond links these motifs into a twodimensional network parallel to (100).
They are often involved in hydrogen-bond interactions (Jeffrey & Saenger, 1991;Jeffrey, 1997;Scheiner, 1997). Fumaric acid is among the organic compounds widely found in nature, and is 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, (I), is presented here.
The asymmetric unit of the title compond consists of a 2-amino-5-chloropyridine molecule and a half of the fumaric acid molecule (Fig. 1). The planar fumaric acid molecule is centrosymmetric with the mid-point of the C═C double bond located at an inversion center. The C6-O1 bond distance of 1.2375 (17) Å is much shorter than the C6-O2 bond distance of 13061 (16) Å, suggesting that the carboxyl group is not deprotonated in the crystal structure. The 2-amino-5-chloropyridine molecule is planar, with a maximum deviation of 0.004 (1) Å for atom N1. The bond lengths (Allen et al., 1987) and angles are normal.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-chloropyridine (64 mg, Aldrich) and fumaric acid (58 mg, Merck) was mixed and warmed over a 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.

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 cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq