Propane-1,2-diammonium bis(6-carboxypyridine-2-carboxylate) dihydrate

The reaction of propane-1,2-diamine (pn) and pyridine-2,6-dicarboxylic acid (pydcH2) in a 1:2 molar ratio in aqueous solution resulted in the formation of the title compound, C3H12N2 2+·2C7H4NO4·2H2O or (pnH2)(pydcH)2·2H2O. The structure contains two monoanionic deprotonated forms of pyridine-2,6-dicarboxylic acid molecules (pydcH)−, a diprotonated propane-1,2-diamine (pnH2)2+, and two water molecules. A significant π–π stacking interaction is observed between the pyridyl rings of the (pydcH)− fragments, with a face-to-face distance of 3.6194 (9) Å. In the crystal structure, a wide range of non-covalent interactions consisting of ion pairing, hydrogen bonding [of the types of O—H⋯O, N—H⋯O, N—H⋯N and C—H⋯O, with D⋯A distances in the range 2.454 (2)–3.222 (2)Å] and π–π stacking interactions [centroid–centroid distance = 3.6194 (9) Å] connect the components into a supramolecular structure.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: OM2231).
The molecular structure of the title compound is shown in Fig. 1. The crystal structure shows that a single proton from each of the carboxyl groups was transferred to the propane-1,2-diamine molecule (pn), rendering it a dication. Thus, the negative charges of two monoanionic 6-carboxypyridine-2-carboxylate groups, (pydcH) -, are neutralized by a doubly protonated propane-1,2-diammonium, (pnH 2 ) 2+ , fragment.
The C-O distances for this compound support the existence of both ionic and non-ionic acid moieties.  (2) to 3.222 (2) Å, are observed in the crystal structure of the title compound (Table 1). The shortest hydrogen bond is O5-H5···O3 viii (x -1, y -1, z) with D···A = 2.454 (2) Å, a strong interaction. Water molecules in this structure increase the number of hydrogen bonding interactions. Ion pairing, π-π stacking and van der Waals interactions are also effective in the packing of the crystal structure. These interactions result in the formation of a supramolecular structure (Fig. 3).
Yellow crystals of the title compound were obtained from the solution after three weeks at room temperature.

S3. Refinement
The hydrogen atoms of NH 3 and OH groups, and also H atoms of water molecules were found in difference Fourier synthesis. The H(C) atom positions were calculated. All H(N) and H(O) atoms were refined in isotropic approximation in supporting information sup-2 Acta Cryst. (2008). E64, o1045-o1046 rigid model, the H(C) atoms were refined in isotropic approximatiom in riding model with with the U iso (H) parameters equal to 1.2 U eq (Xi) for OH, CH and CH 2 gropus and 1.5 U eq (Xii) for NH 3 and CH 3 group, where U(Xi) and U(Ni) are respectively the equivalent thermal parameters of the atoms to which corresponding H atoms are bonded.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level.

Figure 2
A view of π-π stacking interactions between the two aromatic rings of (pydcH)fragments with centroid-centroid distance  The crystal packing of the title compound with hydrogen bonds shown as dashed lines as viewed approximately down a. 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.