Crystal structure of poly[μ2-aqua-aqua(μ2-4-nitro-2,5,6-trioxo-1,2,5,6-tetrahydropyridin-3-olato)hemi-μ4-oxalato-barium(II)]

In a Ba coordination polymer incorporating 4-nitro-2,5,6-trioxo-1,2,5,6-tetrahydropyridin-3-olate, oxalate and aqua ligands, the complex monomers are connected into polymeric two-dimensional layers parallel to the bc plane. Intermolecular O—H⋯O hydrogen bonds link these layers into a three-dimensional supramolecular framework.

In the title coordination polymer, [Ba(C 5 HN 2 O 6 )(C 2 O 4 ) 0.5 (H 2 O) 2 ] n , the tenfold coordination of the Ba centre consists of four O atoms from the two 4-nitro-2,5,6-trioxo-1,2,5,6-tetrahydropyridin-3-olate (L) anions, three O atoms of two oxalate anions and three water molecules. The Ba-O bond lengths fall in the range 2.698 (3)-2.978 (3) Å . The L ligand chelates two Ba atoms related by a screw axis, leading to formation of fused five-and six-membered chelate rings. Due to the bridging function of the ligands and water molecules, the complex monomers are connected into polymeric two-dimensional layers parallel to the bc plane. Intermolecular O-HÁ Á ÁO hydrogen bonds link these layers into a three-dimensional supramolecular framework.

Chemical context
Mixed ligand coordination polymers containing bridging oxalate anions and 1,2-dicarbonyl carbocyclic or heterocyclic compounds exhibit high reactivity and different types of magnetism (Aldoshin, 2008;Coronado et al., 2007;Kitagawa & Kawata, 2002;Ohba & Okawa, 2000). Such compounds are of chemical interest, since a large number of potential donors available in the ligands predetermine a variety of coordination modes, which afford different geometries and dimensionalities of coordination polymers. Recently, we reported the synthesis, crystal structure and some properties of metal complexes of the 2,3,5,6tetraoxo-4-nitro-4-idene anion Dinh Do et al., 2013). The above-mentioned anion does not replace the water molecules from the inner sphere of the hydrated metal cations [M(H 2 O) 6 ] n+ , but can coordinate metal centres like sodium and silver(I). In the present paper, we report the molecular and crystal structure of a mixed-ligand barium complex containing 4-nitro-2,5,6-trioxo-1,2,5,6-tetrahydropyridin-3-olate (L) and oxalate anions as ligands.

Supramolecular features
Due to the bridging function of the L ligand and the O9 water molecule, the hydrated complex cations [Ba(L)(H 2 O) 2 ] + form wide zigzag bands running along the screw axes in the b-axis direction (Fig. 2). Oxalate anions connect the bands into thick two-dimensional networks parallel to bc. The networks have corrugated surfaces with terminal O3, O4 and O6 atoms of the L ligand on the 'hills' and water molecules in the 'hollows'. In the packing (Fig. 3), the 'hills' enter the 'hollows' of adjacent networks. Two-centre hydrogen bonds O9-H2Á Á ÁO3 and Acta Cryst. (2015). E71, 459-462 research communications Table 1 Selected bond lengths (Å ).

Figure 2
One-dimensional polymeric chain in (I). Oxalato ligands omitted for clarity.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms of water molecules were localized in a difference map; O-H distances were normalized. The position of the amino H atom was calculated. All H atoms were refined within the riding model, with U iso (H) = 1.2U eq of the parent atom. The crystal studied was a twin without superposition of reciprocal lattices. Apparently, an accidental overlapping of reflections of two domains is responsible for increased displacement ellipsoids of some atoms. The U ij components of atom O5 were restrained to approximate the isotropic behaviour.  Computer programs: APEX2 and SAINT (Bruker, 2004), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2013 (Sheldrick, 2015), ORTEP (Johnson & Burnett, 1996) and Mercury (Macrae et al., 2008). Crystal structure of poly[µ 2 -aqua-aqua(µ 2 -4-nitro-2,5,6-trioxo-1,2,5,6-tetra-