(Propane-1,3-diammonium) bis(4-hydroxypyridine-2,6-dicarboxylato-κ3 O 2,N,O 6)zinc(II) 3.5-hydrate

The asymmetric unit of the title compound, (C3H12N2)[Zn(C7H3NO5)2]·3.5H2O, contains two formula units. The compound consists of an anionic complex, a doubly protonated propane-1,3-diamine as a counter-ion and 3.5 uncoordinated water molecules. The coordination polyhedron around the ZnII atom is distorted octahedral, defined by four O atoms and two N atoms from two Hchel (H3chel = 4-hydroxypyridine-2,6-dicarboxylic acid) ligands. In the crystal structure, O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds along with π–π stacking interactions [centroid–centroid distance = 3.473 (2) Å] are observed to reinforce the crystal cohesion.

Data collection: SMART (Bruker, 2007); cell refinement: SAINT-Plus (Bruker, 2007); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL. The title compound has two molecules in the asymmetric unit ( Fig. 1). All Zn-O and Zn-N bond lengths and angles are in normal range (Table 1). In the asymmetric unit, the two complexes slightly differ in bond lengths and bond angles.
Both complexes have a six-coordinated octahedral geometry around the Zn II atom. The N1-Zn1-N2 and N3-Zn2-N4 angles, 157.03 (13) and 160.29 (12)°, respectively, are the nearest angles to linearity, so it can be concluded that the N atoms of the two Hchel ligands locate at the axial positions, and the other atoms at the equatorial positions of the distorted octahedral geometry.
The crystal structure contains many O-H···O, N-H···O and C-H···O hydrogen bonds, shown in Table 2. The π-π stacking with centroid-centroid distance of 3.473 (2) Å is illustrated in Fig. 2. The crystal packing is shown in Fig. 3.

S2. Experimental
From a solution of propane-1,3-diamine (0.029 g, 0.4 mmol) and 4-hydroxypyridine-2,6-dicarboxylic acid (0.073 g, 0.4 mmol) in THF (30 ml), a white precipitate was obtained. By mixing the precipitate with Zn(NO 3 ) 2 .6H 2 O (0.052 g, 0.2 mmol) in water (25 ml) and heating for 2 h at 333 K, colourless crystals of the title compound were obtained after allowing the mixture to stand for four weeks at room temperature.

S3. Refinement
H atoms of OH and NH 3 groups and water molecules were found in difference Fourier maps. The H atoms bound to O5, O15, O1W and O2W were refined with U iso (H) = 1.2U eq (O), and the others were refined as riding atoms, with U iso (H) = 1.2U eq (O,N). H atoms on C atoms were positioned geometrically and refined as riding, with C-H = 0.93 (aromatic) and 0.97 (CH 2 ) Å and with U iso (H) = 1.2U eq (C).  Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines denote hydrogen bonds.

Figure 3
The crystal packing of the title compound.