Crystal structure of ammonium bis(pyridine-2,6-dicarboxylato-κ3 O,N,O′)chromate(III) from synchrotron data

In the title complex, the CrIII ion is coordinated by two nearly perpendicular pyridine-2,6-dicarboxylate (pydc) dianions acting as tridentate ligands through the pyridine N atoms and two of the O atoms of each carboxylate group, in a distorted octahedral geometry. The ammonium cation is linked to non-coordinating carbonyl O atoms from neighboring pydc groups through N—H⋯O hydrogen-bonding interactions.


Chemical context
Pyridine-2,6-dicarboxylic acid (also known as dipicolinic acid and abbreviated here as H 2 pydc) can coordinate a metal center as a neutral molecule (H 2 pydc), the univalent anion (Hpydc À ), or the divalent anion (pydc 2À ). In particular, the pyridine-2,6-dicarboxylate ligand frequently acts as a meridional tridentate ligand and sometimes also as a bidentate or bridging ligand (Park et al., 2007). The first [Cr(pydc) 2 ] À complex was prepared as the Na + salt according to the literature (Hoggard & Schmidtke, 1973) and its crystal structure determined using synchrotron data. Structural analysis showed the compound to be a dihydrate (Dai et al., 2006;Gonzá lez-Baró et al., 2008) rather than the 1.5 or 2.5 hydrates that had been suggested previously (Hoggard & Schmidtke, 1973;Fü rst et al., 1979). The crystal structures of K[Cr(pydc) 2 ] (Hakimi et al., 2012) and Rb[Cr(pydc) 2 ] (Fü rst et al., 1979) have also been reported previously but the structure of the ammonium salt is not known.
Here we report the crystal structure of (NH 4 )[Cr(pydc) 2 ] in order to clarify unambiguously the bonding mode of the two ISSN 2056-9890 pyridine-2,6-dicarboxylato ligands and the structural arrangement of this ammonium salt.

Structural commentary
Counter-ionic species play a very important role in coordination chemistry. The structure reported here is another example of a [Cr(pydc) 2 ] À salt but with a different cation. The structural analysis shows that the two tridentate pyridine-2,6dicarboxylate (pydc) dianions octahedrally coordinate to the Cr III metal center through one N atom and two carboxylate O atoms in a meridional arrangement. The Cr III ion is located on a crystallographic fourfold rotoinversion axis (4). An ellipsoid plot of title complex together with the atomic numbering is illustrated in Fig. 1.

Supramolecular features
The pattern of hydrogen bonding around the cation is very similar to the coordination environment in the related potassium salt (Hakimi et al., 2012). The non-coordinating carbonyl O atom forms weak C-HÁ Á ÁO hydrogen bonds that contribute to the crystal packing. The ammonium cation is also linked to the carbonyl O atoms from four neighboring pydc 2À ligands through classical N-HÁ Á ÁO hydrogen bonds (Table 1). An extensive array of these contacts generate a threedimensional network of molecules stacked along the a-axis direction (Fig. 2).interactions involving adjacent pyridine rings further link the components of the structure into a threedimensional network. The centroid-centroid distance between thestacked rings ( The molecular structure of (NH 4 )[Cr(pydc) 2 ], showing the atomnumbering scheme. Non-H atoms are shown as displacement ellipsoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).  (11) 158 (2) Symmetry codes: (i) Àx þ 3 2 ; Ày þ 3 2 ; Àz þ 3 2 ; (ii) Ày þ 5 4 ; x þ 1 4 ; Àz þ 5 4 .

Synthesis and crystallization
All chemicals were reagent-grade materials and were used without further purification. The starting material, Na[Cr(pydc) 2 ]Á2H 2 O was prepared as described previously (Hoggard & Schmidtke, 1973). The sodium salt (0.20 g) was dissolved in 15 mL of water at 323 K and added to 3 mL of water containing 0.5 g of NH 4 Cl. The resulting solution was filtered and allowed to stand at room temperature for several days to give brown block-like crystals of the ammonium salt NH 4 [Cr(pydc) 2 ] suitable for X-ray structural analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.95 (ring H atoms) and with U iso (H) = 1.2 U eq (parent atom). The H atoms of the ammonium cation were located from difference Fourier maps and refined with restraints and a fixed N-H distance of 0.87 Å , with U iso (H) = 1.2U eq (N). One reflection with F o <<<F c was omitted from the final refinement cycles. The slightly low fraction of measured reflections results from the geometry of the 2D-SMC beamline goniostat.  Computer programs: PAL ADSC Quantum-210 ADX (Arvai & Nielsen, 1983), HKL3000sm (Otwinowski & Minor, 1997), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2014/7 (Sheldrick, 2008(Sheldrick, , 2015b, DIAMOND (Brandenburg, 2007) and publCIF (Westrip 2010).

Bis(pyridine-2,6-dicarboxylato-κ 3 O,N,O′)chromate(III)
Crystal data (NH 4  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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )