Bis(2,3-diaminopyridinium) succinate trihydrate

In the title salt, 2C5H8N3 +·C4H4O4 2−·3H2O, the asymmetric unit contains a protonated 2,3-diaminopyridinium cation, half of a succinate dianion (disposed about a centre of inversion), and one and a half water molecules. One of the water molecules is disordered over two sites with occupancies of 0.670 (17) and 0.330 (17). The other water molecule has an occupancy of 0.5 (from refinement). The pyridine N atom of the 2,3-diaminopyridine molecule is protonated. The protonated N atom and one of the 2-amino H atoms are hydrogen bonded to the succinate anion through a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In the crystal, molecules are consolidated into a three-dimensional network by N—H⋯O and O—H⋯O interactions.

In the title salt, 2C 5 H 8 N 3 + ÁC 4 H 4 O 4 2À Á3H 2 O, the asymmetric unit contains a protonated 2,3-diaminopyridinium cation, half of a succinate dianion (disposed about a centre of inversion), and one and a half water molecules. One of the water molecules is disordered over two sites with occupancies of 0.670 (17) and 0.330 (17). The other water molecule has an occupancy of 0.5 (from refinement). The pyridine N atom of the 2,3-diaminopyridine molecule is protonated. The protonated N atom and one of the 2-amino H atoms are hydrogen bonded to the succinate anion through a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. In the crystal, molecules are consolidated into a three-dimensional network by N-HÁ Á ÁO and O-HÁ Á ÁO interactions.  Balasubramani & Fun (2009a,b). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986 Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx; y À 1 2 ; Àz þ 1 2 ; (ii) x; Ày þ 1 2 ; z À 1 2 .
In the crystal packing (Fig. 2), the protonated N1 atom and a nitrogen atom of the 2-amino group (N2) are hydrogenbonded to the succinate oxygen atoms (O2 and O1) via a pair of N-H···O hydrogen bonds forming a ring motif R 2 2 (8) (Bernstein et al., 1995). The 2-amino groups (N2 and N3) are involved in N-H···O hydrogen-bonding interactions to form a R 2 1 (7) ring motif. The crystal structure is further stabilized by water molecules via O(water)-H···O and N-H···O(water) hydrogen bonding (Table 1 and Fig. 2).

S2. Experimental
An aqueous solution of hot methanol (10 ml/ 10 ml) of 2,3-diaminopyridine (27 mg, Aldrich) and succinic acid (29 mg, Merck) were mixed and warmed over a heating magnetic stirrer for 5 minutes. The resulting solution was allowed to cool slowly at room temperature. Crystals of (I) appeared from the mother liquor after a few days.

S3. Refinement
All the H atoms (other than the water H-atoms) were located from the difference Fourier map and allowed to refine freely  The molecular structure of (I), showing 50% probability displacement ellipsoids and the atom numbering scheme.
Dashed lines indicate the hydrogen bonding. The O1 water molecule is disordered over two positions. Symmetry operation A:-x, 2-y, 1-z.    (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 > σ(F 2 ) is used only for calculating R-factors(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 Occ. (