Bis(4-aminopyridinium) bis(hydrogen oxalate) monohydrate

In the title compound, 2C5H7N2 +·2C2HO4 −·H2O, the asymmetric unit consists of an aminopyridinium cation, an oxalic actetate anion and a half-molecule of water, which lies on a two-fold rotation axis. The crystal packing is consolidated by intermolecular O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds. The molecules are linked into an infinite one dimensional chain along [010].

In the title compound, 2C 5 H 7 N 2 + Á2C 2 HO 4 À ÁH 2 O, the asymmetric unit consists of an aminopyridinium cation, an oxalic actetate anion and a half-molecule of water, which lies on a two-fold rotation axis. The crystal packing is consolidated by intermolecular O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. The molecules are linked into an infinite one dimensional chain along [010].
The asymmetric unit of (I) (Fig. 1) contains one molecule of 4-aminopyridine cation, one molecule of oxalate anion and half-a-molecule of water. A proton transfer from the carboxyl group of oxalic acid to atom N1 of 4-aminopyridine resulted in the formation of salts. This protonation lead to the widening of C1-N1-C5 angle of the pyridine ring to 121.0 (8)°, compared to 115.25 (13)° in the unprotonated 4-aminopyridine (Anderson et al., 2005). This type of protonation is observed in various 4-aminopyridine acid complexes (Bhattacharya et al., 1994;Karle et al., 2003). The bond lengths and bond angles of the 4-aminopyridine are comparable to the values reported earlier for 4-aminopyridine (Chao & Schempp, 1977;Anderson et al., 2005). The 4-aminopyridine ring is essentially planar with the maximum deviation from planarity being 0.0075 (9)Å for atom N1. The bond lengths and bond angles of the oxalate are comparable to the values reported for oxalic acid (Derissen & Smith, 1974).
The solution was refluxed at 323 K for 12 h. Colourless crystals were harvested after two months of solvent evaporation.

Refinement
All the hydrogen atoms were located from the Fourier map and were allowed to refine freely. Fig. 1. The molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atom numbering scheme.

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. 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 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 )