3-Aminopyridinium trans-diaquadioxalatochromate(III)

In the structure of the title compound, (C5H7N2)[Cr(C2O4)2(H2O)2], two crystallographically independent formula units are present. Both chromium atoms are six-coordinated in a distorted octahedral geometry by two chelating equatorial oxalato ligands and two axial water molecules. The [Cr(C2O4)2(H2O)2]− anions and C5H7N2 + cations are linked through a complex three-dimensional hydrogen-bonding network consisting of N—H⋯O and O—H⋯O interactions.


Ichraf Chérif, Mohamed Faouzi Zid, Malika El-Ghozzi and Daniel Avignant Comment
In recent years, much work has been focused on the great versatility of the oxalato ligand which affords a wide number of homo and heteropolynuclear complexes with various dimensionalities and network architectures (Marinescu et al., 2011;Martak et al., 2009;Hernández-Molina et al., 2001). This exceptional versatility of the oxalato ligands offers possibilities for the tuning of magnetic properties and thus makes them very useful for obtaining materials with potential applications in magnetism (Chen et al., 2005;Marinescu et al., 2011). The present investigation deals with the new oxalato chromium (III) salt: (C 5 The structure is made up of two crystallographically independent entities of formula (C 5 Nenwa et al., 2010;Chérif et al., 2011). The charge balance of the mononuclear anion is provided by an uncoordinated 3-aminopyridinium cation.

Experimental
To 10 cm 3 of a solution of chromium chloride CrCl 3 .6H 2 O (1 mmol) was added under stirring at room temperature previously prepared solutions of 3-aminopyridine C 5 H 6 N 2 (1 mmol, 10 cm 3 ) and oxalic acid H 2 C 2 O 4 .2H 2 O (2 mmol, 10 cm 3 ) in water. The obtained solution was stirred for 3 h at 323 K and prismatic violet single crystals were grown by slow evaporation at room temperature.

Refinement
All non hydrogen atoms were refined anisotropically. The hydrogen atoms of the water molecules were located in a at 0.97 Å from the C4 atom and the deepest hole was located at 0.62 Å from the Cr2 atom.

Figure 1
A view of the title compound with the atom-numbering scheme. Thermal ellipsoids are drawn at the 50% probability level for non-H atoms.  View of the hydrogen bonding system (dashed lines) and π-π stacking interactions between pairs of 3-aminopyridinium cations showing the layered structure.

Figure 3
Structure projection along a axis showing connection between layers via hydrogen bonds (dashed lines).

3-Aminopyridinium trans-diaquadioxalatochromate(III)
Crystal data (C 5   Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 R-factors(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.