Bis(2,6-diaminopyridinium) bis(hydrogen oxalate) monohydrate

The asymmetric unit of the title compound, 2C5H8N3 +·2C2HO4 −·H2O, contains two crystallographically independent 2,6-diaminopyridinium cations, a pair of hydrogen oxalate anions and a water molecule. Both 2,6-diaminopyridinium cations are planar, with maximum deviations of 0.011 (2) and 0.015 (1) Å, and are protonated at the pyridine N atoms. The hydrogen oxalate anions adopt twisted conformations and the dihedral angles between the planes of their carboxyl groups are 31.01 (11) and 63.48 (11)°. In the crystal, the cations, anions and water molecules are linked via O—H⋯O and N—H⋯O hydrogen bonds, forming a three-dimensional network.

The asymmetric unit of the title compound, 2C 5 H 8 N 3 + Á-2C 2 HO 4 À ÁH 2 O, contains two crystallographically independent 2,6-diaminopyridinium cations, a pair of hydrogen oxalate anions and a water molecule. Both 2,6-diaminopyridinium cations are planar, with maximum deviations of 0.011 (2) and 0.015 (1) Å , and are protonated at the pyridine N atoms. The hydrogen oxalate anions adopt twisted conformations and the dihedral angles between the planes of their carboxyl groups are 31.01 (11) and 63.48 (11) . In the crystal, the cations, anions and water molecules are linked via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, forming a three-dimensional network.
The asymmetric unit of the title compound consists of two crystallographically independent 2,6-diaminopyridinium and C2B-N1B-C6B [123.62 (15)°] angles compared to those observed in an unprotonated structure (Schwalbe et al., 1987). The oxalic acid molecule exists in a mono-ionized state in the crystals. Similar observations were also found in the crystal structure of glycinium hydrogen oxalate (Subha Nandhini et al., 2001), creatininium hydrogen oxalate monohydrate (Bahadur et al., 2007) and 3-carboxypyridinium hydrogen oxalate (Athimoolam & Natarajan, 2007). Here, the hydrogen oxalate anions adopt twisted conformations and the dihedral angles between planes of their carboxylic groups are 31.01 (11)°a nd 63.48 (11)° for anions A and B, respectively.
In the crystal structure, the carboxylate groups of each hydrogen oxalate anion interact with the corresponding 2,6diaminopyridinium cations via a pair of N-H···O hydrogen bonds forming an R 2 2 (8) ring motif ( Fig. 1) (Bernstein et al., 1995). The ionic units and water molecules are linked by O-H···O and N-H···O (Table 1) hydrogen bonds to form a three-dimensional network (Fig. 2).

Experimental
Oxalic acid dihydrate (0.01 mol, 1.3 g) was dissolved in 50 ml of methanol in a round bottom flask. 2,6-diaminopyridine (0.01mol, 1.1 g) was dissolved in 50 ml of methanol in a flask and then added in small portions to the oxalic acid with stirring. The reaction mixture was left stirring for 3 hours at room temperature. Brown precipitate was formed, filtered, and washed with methanol. Recrystallization of the brown precipitate with water has yielded after 48 hours brown crystals which was washed with methanol and dried at 353 K.

Special details
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.