Crystal structure of bis[4-(dimethylamino)pyridinium] aquabis(oxalato)oxidovanadate(IV) dihydrate

The vanadium(IV) atom in the title compound is located on a twofold rotation axis and has a distorted octahedral coordination sphere made up from two symmetry-related oxalate ligands, one vanadyl O atom and a water molecule.

The title organic-inorganic hybrid salt, (C 7 H 11 N 2 ) 2 [V(C 2 O 4 ) 2 O(H 2 O)]Á2H 2 O, shows a distorted octahedral coordination environment for the vanadium(IV) atom in the anion (point group symmetry 2), with four O atoms from two symmetry-related chelating oxalate dianions and two O atoms in trans configuration from a coordinating water molecule and a terminal vanadyl O atom. In the crystal, (001) layers of cations and anions alternate along [001]. The anionic layers are built up by intermolecular O-HÁ Á ÁO hydrogen bonds involving the coordinating and solvent water molecules. The cationic layers are linked to the anionic layers via N-HÁ Á ÁO hydrogen bonds between the pyridinium group and the non-coordinating O atoms of the oxalate group. The 4-(dimethylamino)pyridinium cations are also engaged instacking with their antiparallel neighbours [centroid-to-centroid distance = 3.686 (2) Å ]. Considering all supramolecular features, a three-dimensional network structure is accomplished.

Chemical context
Because of the great importance of vanadium as an effective metal antitumor agent (Evangelou, 2002) and the vanadyl antidiabetic factor via its manifested insulin-mimetic activity (Goc, 2006), the coordination chemistry of this element has received much attention over the past years through the design and synthesis of organic-inorganic hybrid salts and the investigation of their solution chemistry. In addition to that, the use of pyridine and its derivatives in those hybrid materials may also provide biological activity as reported by Markees et al. (1968). Many compounds containing the vanadyl V O group combined with oxalate ligands have been isolated as mononuclear (Lin et al., 2004;Aghabozorg et al., 2007;Oughtred et al., 1976) or dinuclear (Zheng et al., 1998) compounds.
In this context, we report on the synthesis and crystal structure of the title organic-inorganic hybrid salt,

Structural commentary
The vanadium atom V1, the double-bonded oxygen atom O3 of the vanadyl group and the oxygen atom of the coordinating water molecule OW1 lie on a twofold rotation axis. Thus, the asymmetric unit of the title compound corresponds to half of the molecular formula which consequently contains one halfanionic complex [V 1/2 (C 2 O 4 )O 1/2 (HO 1/2 )] À , one 4-(dimethylamino)pyridinium cation (C 7 H 11 N 2 ) + protonated at the N2 atom of the heterocyclic ring, and one solvent water molecule (Fig. 1). The anionic complex has an overall charge of 2À, requiring a vanadium atom with an oxidation state of +IV. This formal value is in good agreement with the bond-valencesum calculation (Brown & Altermatt, 1985), resulting in a value of 4.20 (3) valence units.
The V IV ion is coordinated by four oxygen atoms of two symmetry-related chelating oxalate dianions, defining the equatorial plane, and two axial oxygen atoms from a water molecule and the vanadyl oxygen atom. The resulting octahedral coordination sphere is considerably distorted. The V-O oxalate bond lengths (  (Oughtred et al., 1976) or piperazinium (Lin et al., 2004) as counter-cations. The short V1 O3 distance of 1.600 (3) Å is typical for a double-bonded vanadyl group and the longest V-O bond involves the aqua ligand, again in agreement with the structures of the related compounds with different cations. The shortest distances between vanadium atoms in the isolated complexes are equal to 7.689 (4) Å along [010] (corresponding to the length of the b axis) and 8.287 (1) Å along [010], while a shorter distance equal to 5.176 (5) Å along the [001] direction is reported by Aghabozorg et al. (2007) for the related piperazinium compound. The oxalate anion is planar (root-mean-deviation of fitted atoms = 0.0343 Å ); the two symmetry-related oxalate ligands subtend a dihedral angle of 32.59 (4) between the least-squares planes. The slightly elongated C-C bond length of 1.552 (3) Å in the oxalate anion is in agreement with the value of 1.539 (2) Å research communications Acta Cryst. (2016). E72, 1002-1005 The asymmetric unit of (I) showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms. Table 1 Selected bond lengths (Å ).

Figure 2
View of O-HÁ Á ÁO hydrogen bonds (orange dashed lines) developed by both coordinating and non-coordinating water molecules in (a) the title compound [symmetry codes: reported for other oxalate complexes (Belaj et al., 2000). Bond lengths and angles of the 4-(dimethylamino)pyridinium cation are consistent with those found in salts with the same cationic entity (

Supramolecular features
Within the crystal packing, all components are connected by an extensive hydrogen-bonding network ( Table 2). The cations and anions are aligned into layers parallel to (001). O-HÁ Á ÁO hydrogen bonds involving the coordinating OW1 water molecule as donor group and the solvent OW2 molecule as both acceptor and donor groups consolidate the anionic layers parallel to (001), as shown in Fig. 2a. In the structure of the related piperazinium compound (Aghabozorg et al., 2007), a more complex three-dimensional arrangement of the O-HÁ Á ÁO hydrogen bonds is realized (Fig. 2b). Along the [001] direction, N-HÁ Á ÁO hydrogen bonds involving the protonated N2 atom of the 4-(dimethylamino)pyridinium cation as double-donor group and non-coordinating O atoms of the oxalate dianion as acceptors ensure the connection between the anionic and cationic layers in the title structure, as shown in Fig. 3. Furthermore,stacking interactions between antiparallel-arranged pyridinium rings [centroid-to-centroid distance = 3.686 (2) Å ; Fig. 4] are present and consolidate the three-dimensional network (Fig. 5).

Synthesis and crystallization
A solution of 0.5 mmol of vanadium(V) pentoxide dissolved in 10 cm 3 of distilled water was added to a solution of 1 mmol of oxalic acid dissolved in 10 cm 3 of distilled water. Then, a solution of 1 mmol of 4-(dimethylamino)pyridine dissolved in 10 cm 3 of distilled water was poured slowly until pH ' 4. The obtained blue solution was placed in a petri dish at room temperature for almost one month until purple crystals suitable for a structural study appeared.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms bonded to C and N atoms were placed at geometrically calculated positions using a riding model. C-H distances were fixed at 0.93 Å for aromatic H atoms and 0.96 Å for methyl H atoms, with U iso (H) = 1.2U eq (C aromatic ) or 1.5U eq (C methyl  View of the N-HÁ Á ÁO hydrogen bonds (blue dashed lines) developed between anionic and cationic entities.

Figure 4
stacking interactions (green dashed lines) between adjacent anti-parallel organic cations, forming zigzag chains.

Bis[4-(dimethylamino)pyridinium] aquabis(oxalato)oxidovanadate(IV) dihydrate
Crystal data 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. 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 > 2sigma(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.