Poly[diaquatris(μ6-4,6-dioxo-1,4,5,6-tetrahydro-1,3,5-triazine-2-carboxylato)tripotassium]

The asymmetric unit of the title compound, [K3(C4H2N3O4)3(H2O)2]n, contains two potassium cations (one in general position, one located on a twofold rotation axis), one and a half oxonate anions (the other half generated by twofold symmetry) and one water molecule. As a result of the twofold symmetry, one H atom of the symmetric anion is statistically occupied. Both potassium cations are surrounded by eight oxygen atoms in the form of distorted polyhedra. Adjacent cations are interconnected by oxygen bridges, generating layers parallel to (100). The aromatic ring system of the oxonate anions link these layers into a network structure. The crystal packing is stabilized by N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds, three of which are bifurcated. In addition, intermolecular π–π stacking interactions exist between neighboring aromatic rings with a centroid–centroid distance of 3.241 (2) Å.

The asymmetric unit of the title compound, [K 3 (C 4 H 2 N 3 O 4 ) 3 -(H 2 O) 2 ] n , contains two potassium cations (one in general position, one located on a twofold rotation axis), one and a half oxonate anions (the other half generated by twofold symmetry) and one water molecule. As a result of the twofold symmetry, one H atom of the symmetric anion is statistically occupied. Both potassium cations are surrounded by eight oxygen atoms in the form of distorted polyhedra. Adjacent cations are interconnected by oxygen bridges, generating layers parallel to (100). The aromatic ring system of the oxonate anions link these layers into a network structure. The crystal packing is stabilized by N-HÁ Á ÁO, O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, three of which are bifurcated. In addition, intermolecularstacking interactions exist between neighboring aromatic rings with a centroid-centroid distance of 3.241 (2) Å .
Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT; program(s) used to solve structure: MoPro (Jelsch et al., 2005); program(s) used to refine structure: MoPro; molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: MoPro.  Oxonic acid has antibacterial and antiviral properties (Pancheva, 1977); it is a competitive inhibitor of pyrimidine biosynthesis (Cihak et al., 1968) and occupies an unique biologic position by being the only effective precursor in the biosynthesis. Besides being biologically important, oxonic acid has also been of interest in coordination and supramolecular chemistry. Despite its importance in biochemistry, physical chemistry studies of oxonic acid are rare, probably due to its low solubility and, particularly, to the instability of oxonic acid solutions which easily decarboxylate into 5-azauracil. The study of the kinetics of metal-oxonic acid decarboxylation has been conducted some time ago (Lalart et al., 1981).
In recent years, much attention has been paid for crystal engineering of metal-organic coordination compounds (Yaghi et al., 2003). This arises not only from fundamental properties of these materials, such as their intriguing topological frameworks, but also from their unexpected potential applications in various fields such as engineering, device manufacturing or materials science (Janiak, 2003;Mori et al., 2005Mori et al., , 2006Dybtsev et al., 2004).
As a contribution to the investigation of the above materials, we report here the crystal structure of the hydrated potassium salt of oxonic acid, K 3 (C 4 H 2 N 3 O 4 ) 3 . 2H 2 O, (I).
The asymmetric unit of the structure of (I) contains two potassium cations (one in general position, one located on a twofold rotation axis), one water molecule and one and a half molecules of the oxonic acid anion (1,4,5,6-tetrahydro-4,6dioxo-1,3,5-triazine-2-carboxylate), the second half completed by a twofold rotation axis ( Fig. 1). Due to symmetry, one hydrogen atom (H12_4) of this anion is equally disordered between two equivalent sites. The two potassium cations are octa-coordinated to oxygen atoms in the form of distorted cubic antiprisms. The coordination environment of K1_1 is defined by three oxygen atoms of carboxylate groups, four oxygen atoms of carbonyl groups and one oxygen atom of the water molecule. K2_2 is surrounded by four oxygen atoms of carbonyl groups, two oxygen atoms of carboxyl groups and two oxygen atoms of water molecules. Each K1_1 potassium atom shares four bridging oxygen atoms (O9_3 v , O9_3 vi , O10_3 iii and O10_3 iv ) with a symmetry-related cation K1_1 i , and two bridging oxygen atoms (O11_3 and O17_4) with the potassium cation K2_2 (for symmetry codes, see Table). The K-O distances, ranging from 2.6893 (6) to 3.1649 (6) Å are similar than in related potassium complexes (Sheldrick & Poonia, 1986).

Experimental
Potassium oxonate (4,6-dihydroxy-1,3,5-triazine-2-carboxylic acid potassium salt) was obtained as a commercially available salt (Aldrich, 97%) and was dissolved in a minimum amount of water at 323 K. The solution was slowly cooled in two days in an incubator from 323 K to 277 K. Crystals of the title compound could then be isolated after two days and were subjected to X-ray diffraction analysis.

Refinement
After initial refinement with SHELXL97, the structure was further refined with the program MoPro (Jelsch et al., 2005) using a multipolar atom model transfered from the ELMAM2 electron density database (Domagała et al., 2012). The R(F) factor improved from 4.3 to 3.4%. The residual difference electron density showed a positive/negative peak when the nitrogen atom N12_4 was modeled as deprotonated or fully protonated, respectively. Due to the twofold symmetry of this anion the hydrogen atom H12_4 was modelled with half-occupancy on the two crystallographically equivalent sites.
The other H atom positions were refined using distance restraints; the target values were 1.01 (2) and 0.97 (2) Å for N-H and O-H bond lengths, respectively. In the oxonate moieties, angle similarity restraints (σ = 0.2°) were also applied to the C-N-H triplets. The H atoms were restrained to remain close to the planes of the oxonate moieties (σ = 0.03). The H atoms of the water molecule were refined using two O-H distance and one distance similarity restraints, and the target of the H-O-H angle was set to 105.0 (2)°. The fractal analysis of the residual electron density (Meindl & Henn, 2008) in Fig. 4 shows a more symmetric curve for the multipolar model, with notably a reduced shoulder on the positive side.  The basic structure units in the structure of (I), showing 50% probability displacement ellipsoids and spheres of arbitrary radius for the H atoms. [Symmetry code: (xii) -x + 1, y, -z + 1/2.]

,5-triazine-2-carboxylato)tripotassium]
Crystal data [K 3 (C 4  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.005 Δρ max = 0.50 e Å −3 Δρ min = −0.36 e Å −3 Special details Refinement. Refinement of F 2 against reflections. The threshold expression of F 2 > σ(F 2 ) is used for calculating Rfactors(gt) 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. (