2-Amino-5-chloropyridinium (Z)-3-carboxyprop-2-enoate 0.25-hydrate

In the title hydrated salt, C5H6ClN2 +·C4H3O4 −·0.25H2O, the water O atom lies on a twofold axis with 0.25 occupancy. The 2-amino-5-chloropyridinium cation is almost planar, with a maximum deviation of 0.015 (3) Å. In the hydrogen malate anion, an intramolecular O—H⋯O hydrogen bond generates an S(7) ring and results in a folded conformation. 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. The ion pairs are further connected via O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds, forming layers parallel to the ab plane which stack down the c axis.

In the title hydrated salt, C 5 H 6 ClN 2 + ÁC 4 H 3 O 4 À Á0.25H 2 O, the water O atom lies on a twofold axis with 0.25 occupancy. The 2-amino-5-chloropyridinium cation is almost planar, with a maximum deviation of 0.015 (3) Å . In the hydrogen malate anion, an intramolecular O-HÁ Á ÁO hydrogen bond generates an S(7) ring and results in a folded conformation. 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-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. The ion pairs are further connected via O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming layers parallel to the ab plane which stack down the c axis.

Comment
There is growing interest in the construction of supramolecular assemblies with hydrogen bonds as the building blocks (Aakeröy & Seddon, 1993;Fredericks & Hamilton, 1996). The maleic acid anion can exist in the fully deprotonated form or as hydrogen maleate, with one of the carboxylic acid groups protonated. The crystal structures of glycinium maleate (Rajagopal et al., 2001a), β-alaninium maleate (Rajagopal et al., 2001b), sarcosinium maleate (Rajagopal et al., 2002) and L-alaninium maleate (Alagar et al., 2001) have been reported in the literature. The present study reports the crystal structure of 2-amino-5-chloropyridinium hydrogen maleate 0.25-hydrate, (I), a complex of 2-amino-5-chloropyridinium with maleic acid.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-chloropyridine (64 mg, Aldrich) and maleic 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 colourless needles of (I) 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 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.