Crystal structure of tris[4-(dimethylamino)pyridinium] tris(oxalato-κ2 O,O′)chromate(III) tetrahydrate

In the crystal structure of the title compound, the molecular components, viz. 4-(dimethylamino)pyridinium cations, tris[oxalatochromate(III)] anions and lattice water molecules, are linked through O—H⋯O and N—H⋯O hydrogen bonds into a three-dimensional network. Additional π–π interactions between pyridinium rings stabilize this arrangement.

In the title hybrid salt, (C 7 H 11 N 2 ) 3 [Cr(C 2 O 4 ) 3 ]Á4H 2 O, the central Cr III ion of the complex anion (point group symmetry 2) is coordinated by six O atoms from three chelating oxalate(2À) ligands in a slightly distorted octahedral coordination sphere. The Cr-O bond lengths vary from 1.9577 (11) to 1.9804 (11) Å , while the chelate O-Cr-O angles range from 82.11 (6) to 93.41 (5) . The 4-(dimethylamino)pyridinium cations (one situated in a general position and one on a twofold rotation axis) are protonated at the pyridine N atoms. In the crystal, N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds link the cations and anions into a three-dimensional network.interactions between the pyridine rings of adjacent cations provide additional stabilization of the crystal packing, with the closest centroid-to-centroid distances being 3.541 (1) and 3.575 (1) Å .

Chemical context
The coordination chemistry of oxalate (C 2 O 4 2À ) continues to receive considerable attention because of the ability of this ion to act as a remarkably flexible ligand system in complexations with a wide range of metal ions (Martin et al., 2007). Over the last decade, Bé lombé and coworkers (Bé lombé et al., 2003) prepared a novel barium-oxalatochromate(III), {Ba 6 (H 2 O) 17 -[Cr(ox) 3 ] 4 }Á7H 2 O, and demonstrated the use of this complex as a suitable precursor for the synthesis of multi-functional crystalline materials (Bé lombé et al., 2009a,b;Mbiangué et al., 2012). Moreover, this complex has received much attention in the field of materials science for its use as a convenient route for the preparation of technologically important metallic composite oxides (Neo et al., 2006). As part of our ongoing research program, we have now combined this versatile barium-oxalatochromate(III) complex with 4-(dimethylamino)pyridinium oxalate to isolate the organic-inorganic hybrid salt, (C 7 H 11 N 2 ) 3 [Cr(C 2 O 4 ) 3 ]Á4H 2 O.

Structural commentary
The molecular components of the title compound are shown in Fig. 1. The asymmetric unit contains one and a half 4-(dimethylamino)pyridinium cations, one half of the tris-(oxalato)chromate(III) complex anion and two lattice water molecules. The entities are completed by application of twofold rotation symmetry. The central Cr III ion of the complex anion is coordinated by six O atoms from three chelating oxalato(2À) ligands in a slightly distorted (2 + 2 + 2) octahedral coordination sphere. The chelate O-Cr-O angles range from 82.11 (6) to 93.41 (5) . The Cr-O bond lengths vary from 1.9577 (11) to 1.9804 (11) Å and are similar to those found in the guanidinium tris(oxalato)chromate(III) salt (Golič & Bulc, 1988). Bond lengths and angles in the organic cations, [C 7 H 11 N 2 ] + , are in agreement with those found in salts with the same cationic entity (Nenwa et al., 2010;Ghouili et al., 2010;Benslimane et al., 2012;Ben Nasr et al., 2015).

Figure 2
Projection on the ab plane of the crystal structure of the title compound. Hydrogen bonds are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
The title compound was obtained by reaction of an aqueous solution of the freshly prepared barium-oxalatochromate(III) salt {Ba 6 (H 2 O) 17 [Cr(C 2 O 4 ) 3 ] 4 }Á7H 2 O (1 mmol, 2.536 g), with an aqueous solution of 4-(dimethylamino)pyridine (12 mmol, 1.464 g) and oxalic acid (6 mmol, 0.756 g). The mixture was stirred at 333 K for about 30 minutes and then cooled to room temperature and filtered. The title compound crystallized by slow evaporation of the solvent at room temperature in form of light-violet crystals with dimensions up to 3 mm within a few weeks.