Synthesis and structural study of tris(2,6-diaminopyridinium) bis(oxalato)dioxidovanadate(V) 2.5-hydrate

The synthesis of complex compounds based on vanadium oxalates has grown considerably during the last decades, because of there biological and catalytic applications. This paper describes the synthesis and characterization of a new dioxovanadate(V) complex, (C5H8N3)3[VO2(C2O4)2]·2.5H2O.


Structural commentary
The asymmetric unit of (I) is composed of a complex [VO 2 (C 2 O 4 ) 2 ] 3À ion, three protonated 2,6-diaminopyridinium ISSN 2056-9890 cations (C 5 H 8 N 3 ) + and two and a half uncoordinated water molecules (Fig. 1). The anionic complex has an overall charge of À3, requiring a vanadium atom with an oxidation state +5. This formal value is in good agreement with the bond-valencesum calculation (Brown & Altermatt, 1985), which gives a value of 4.99 valence units.
In the coordination polyhedron of V V , the central vanadium has distorted octahedral geometry with two terminal oxygen atoms and four oxygen atoms from two oxalate groups. The two terminal oxygen atoms O1 and O2 are located at shortened V-O distances of 1.6433 (8) and 1.6317 (8) Å , respectively, which is typical for a double-bonded vanadyl group, and form a cis-vanadyl grouping in the usual monodentate fashion. Substantially elongated complexing bonds [2.1644 (8) and 2.2248 (8) Å ] extend from the vanadium to the two carboxylate oxygen atoms O4 and O7, while two other carboxylate oxygen atoms O3 and O8 are at 2.0020 (8) and 2.0026 (8) Å respectively.
The geometric parameters for the 2,6-diaminopyridinium cations do not show any unusual features and are in agreement with those previously reported for bis(2,6-diaminopyridinium) oxalate dihydrate, 2C 5 H 8 N 3 Odabaşog lu et al., 2006).

Supramolecular features
The charged components are connected by an extensive hydrogen-bonding network. The amine and pyridine nitrogen atoms of the 2,6-diaminopyridinuim cations act as hydrogenbond donors and coordinate the complex ions [VO 2 (C 2 O 4 ) 2 ] 3À to each other or to water molecules via N-HÁ Á ÁO hydrogen bonds as shown in Fig. 2, with bond lengths between 1.87 (2) and 2.61 (2) Å (Table 1).

Figure 1
The asymmetric unit of (I) showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms.
The water molecules act as hydrogen-bond donors via five O-HÁ Á ÁO hydrogen bonds involving their oxygen atoms, (Table 2) and coordinate the complex ions to the water molecules, generating R 5 5 (13) and R 10 10 (36) hydrogen-bonded rings, as shown in Fig. 3.

Synthesis and crystallization
All reagents and solvents were commercially available and used without further purification. Elemental analyses for carbon, nitrogen and hydrogen were performed on a Flash2000 Organic Elemental Analyser, CHNS-O analyser by Thermo Scientific (Centre of Scientific Instrumentation of the University of Granada). An ICP-OES Perkin-Elmer Optima 8300 Spectrometer (Centre of Scientific Instrumentation of the University of Granada) was used to determine the metal content in the complex.   (13) and R 10 10 (36) motifs.
Under continuous stirring at 373 K, a solution of oxalic acid dihydrate (0.126 g, 1 mmol) dissolved in 10 cm 3 of distilled water was added dropwise to a stirring solution of vanadium pentoxide (0.181 g, 1 mmol) dissolved in 20 cm 3 of distilled water. After 15 minutes of mixture stirring, 2,6-diaminopyridine (0.218 g, 2 mmol) was added to the mixture without prior dissolution. The final solution was kept under continuous stirring and heated for a further hour. After filtration, the filtrate was placed in a petri dish and kept at room temperature. After a week to ten days, orange-brown crystals, stable at room temperature and of suitable size for a structural study, appeared.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms of the 2,6-diaminopyridinium cations and water molecules were located in difference-Fourier maps and refined freely with isotropic displacement parameters.

Acknowledgements
HS thanks Dr Elisa Barea (Department of Inorganic Chemistry, University of Granada) for support and advice during her short-term stay in the University of Granada where the single-crystal X-ray diffraction, elemental analysis and ICP-MS studies were carried out (Centre of Scientific Instrumentation of the University of Granada).

Funding information
Financial support from the Ministry of Higher Education and Scientific Research of Tunisia is gratefully acknowledged. View of the packing of the title compound. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Tris(2,6-diaminopyridinium) bis(oxalato)dioxidovanadate(V) 2.5-hydrate
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.