Disodium dihydrogen pyridine-2,3,5,6-tetracarboxylate trihydrate

In the title compound, 2Na+·C9H3NO8 2−·3H2O, the asymmetric unit consists of two Na+ cations, one dihydrogen pyridine-2,3,5,6-tetracarboxylate dianion (H2pdtc2−) and three water molecules coordinated to the Na+ cations. The configuration of the anion is stabilized by intramolecular O—H⋯O hydrogen bonding between vicinal carboxylate/carboxy groups. The Na+ cations are bridged by the H2pdtc2− dianions, generating layers extending infinitely in sheets parallel to (001), and further pillared by the water molecule linkers to build up a three-dimensional framework.

In the title compound, 2Na + ÁC 9 H 3 NO 8 2À Á3H 2 O, the asymmetric unit consists of two Na + cations, one dihydrogen pyridine-2,3,5,6-tetracarboxylate dianion (H 2 pdtc 2À ) and three water molecules coordinated to the Na + cations. The configuration of the anion is stabilized by intramolecular O-HÁ Á ÁO hydrogen bonding between vicinal carboxylate/ carboxy groups. The Na + cations are bridged by the H 2 pdtc 2À dianions, generating layers extending infinitely in sheets parallel to (001), and further pillared by the water molecule linkers to build up a three-dimensional framework.
Experimental 0.0766 g (0.3 mmol) pyridine-2,3,5,6-tetracarboxylic acid and 0.024 g (0.6 mmol) NaOH were successively added to 10.0 ml H 2 O and stirred at room temperature for 2 h, and the resulting colorless solution (pH = 3.48) was then transferred to a 50 ml beaker for slow evaporation at room temperature for several months, affording colorless block crystals (yield: 0.05 g).

Refinement
H atoms bonded to C atoms were placed in geometrically calculated positions and were refined using a riding model, with U iso (H) = 1.2U eq (C

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 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.