Polyoxometalates (POMs) are a kind of inorganic clusters, which can be combined with organic ligands and metal ions to form supramolecular compounds. These groups have been extensively studied due to their potential applications in various fields such as catalysis  and materials science  . The size and the structure are two important criteria among others that determine the properties and hence the applications of these species.
Among the various types of POMs, decavanadate represents an important part of the group which has been studied extensively in the past decades because of their excellent catalysis properties   and versatile bioactivities     . In addition, the decavanadate anion [V10O28]6− has been characterized by numerous cations for antibacterial, antidiabetes and catalytic activities  -  . Recently, organic decavanadates have been synthesized with the aim to understand their biological  , magnetic  , and optical properties  .
Several decavanadate compounds containing organic cations have been synthesized such us: (C5H12N)9[HV10O28](NO3)4  , (C7H10N)4[H2V10O28]∙2H2O  , (C12H12N2)3(V10O28)∙2H2O  , [C8NH20]9[H2V10O28][HV10O28]∙2∙13H2O  .
In a continuation of these studies, a new decavanadate compound containing organic cations has been prepared.
In this paper, we report the synthesis, crystal structure, IR and UV-Vis spectroscopies of the novel decavanadate (C4N2H7)4(C6N2H10)V10O28∙2H2O.
2.1. Synthesis and Crystallization of 2-Amino-4-Picolinium, 2-Methylimidazolium Decavanadate Hydrate: (C4N2H7)4(C6N2H10)V10O28∙2H2O (1)
The compound (C4N2H7)4(C6N2H10)V10O28∙2H2O was prepared from a mixture of 0.321 g of vanadium oxide V2O5, 0.091 g of 2-methylimidazole (C4H6N2) and 0.057 g of 2-amino-4-picoline (C6H8N2) in water. The mixture obtained was stirred and heated for about 3 hours. Finally, the solution was allowed to stand at room temperature and after a week, single orange crystals suitable for ray diffraction analysis were obtained.
2.2. X-Ray Diffraction Study
The collect of diffracted intensities was performed using an Enraf-Nonius CAD4 four-circle diffractometer with Kα radiation of molybdenum (λ = 0.71067 Å). The structure was solved by direct method using the SHELXS-97 program  and refined by the full-matrix least squares method on all F2 data using the program SHELXL-2014  .
Anisotropic thermal parameters were used to refine all the non-hydrogen.
B atoms and the positions of the hydrogen atoms were calculated using the HFIX instruction.
Crystal data and details on data collection and refinement are summarized in Table 1.
DIAMOND  package was used for molecular graphics. CIF file containing complete information about the structure of (1) was deposited with the Cambridge Crystallographic Data Center (CCDC 1908881). The file is freely available upon request from the following web site: https://www.ccdc.cam.ac.uk/data_request/cif.
2.3. X-Ray Powder Diffraction Study of Compound (1)
The purity of phase (I) were checked by comparing, using the Origin software  , the experimental diffractogram (λCu (Kα1) = 1.5406 Å) with the calculated diffractogram from single-crystal X-ray diffraction data, obtained using the VESTA program  . The superposition of the two diffractograms (Figure 1) confirms the purity of the sythesized phase.
Table 2 groups the different peaks, their indications and their intensities.
2.4. IR Spectra of Compound (1)
In Figure 2, we present the IR spectrum of the compound (1) (Pellets with KBr, mg: analyte 2, KBr 250; Perkin-Elmer spectrometer) cm–1: 585, 608, 745, 830 ν(VO6), 970 ν(V=O), 1310 ν(C=C) and cycle vibrations, 1430 ν(C=N), 1610 δ(O-H), 1830, 1930, 2690 ν(C-H), ν(C-N), ν(N-H) and ν(C-C), 2905 hydrogen bonds, 3095, 3145, 3365 ν(O-H).
2.5. UV-Vis Spectra of Compound (1)
The UV-Vis spectra were recorded on a T70/T70 + UV-V is spectrophotometer in the range 200 - 700 nm (Figure 3). The UV-Vis spectrum of 1 displays two intense absorption bands. The band observed at 300 nm can be assigned to charge-transfer (CT) transitions of the type π(O) → d(V) and the band at 494.64 nm can be explained the color (orange) of crystals of the title compound. The band gap is estimated by extrapolation of the linear part. The absorbance spectrum measurement shows an optical band gap of 3.27 eV which allows us to conclude that this compound is a semiconductor material (the gap energy is less than 4 eV).
3. Results and Discussion
The formula unit is composed of one decavanadate [V10O28]6− anion, four 2-methylimidazolium (C4N2H7)+ cations, one 2-amino-4-picolinium (C6N2H10)2+ cation and two water molecules (Figure 4).
Each decavanadate cluster is composed by ten distorted edge-sharing VO6 octahedra. In fact, the distortion indexes vary between 7.6% and 9.9%  (Table 3). The vanadium is in the +5 oxidation. This result was confirmed by the bond valence sums calculations (Table 3) according to Brown   (S = Σsi = Σexp[(R0 − Ri)/B]. The V-Oterminal bond distances range between 1.593(4) and 1.621(4) Å. The V-O2b bonds are in the range of 1.678(4) and 2.071(4) Å. The
Table 1. Crystallographic characteristics, the X-ray data collection and structure-refinement parameters for (C4N2H7)4(C6N2H10)V10O28∙2H2O.
Table 2. Indexation of DRX diffractogram of compound (1).
*Only the most significant peaks are considered.
Figure 1. Experimental and calculated diffractogram DRX of compound (1).
Figure 2. Infrared spectra of compound (1).
V-O3b bond distances range from 1.929(4) and 2.059(4) Å. The V-O6b bond lengths are within 2.061(4) and 2.374(4) Å. The V-V distances are in the range 3.051(1) to 3.120(1) Å.
The 2-methylimidazolium (C4N2H7)+ and 2-amino-4-picolinium (C6N2H10)2+ cations forming two different alternating layers. The first layer is composed by 2-amino-4-picolinium (C6N2H10)2+ and 2-methylimidazolium (C4N2H7)+ cations. The second layer is formed by 2-methylimidazolium (C4N2H7)+ cations (Figure 5). The bond lengths of C-C and C-N are from 1.353(1) to 1.502(1) Å and from
Figure 3. UV-Vis spectra of compound (1).
Figure 4. Projection of the (C4N2H7)4(C6N2H10)V10O28∙2H2O compound along the a-axis. Hydrogen atoms are omitted for clarity.
Table 3. Distortion, indexes ID and BVS calculations for (C4N2H7)4(C6N2H10)V10O28∙2H2O.
1.292(1) to 1.511(1) Å, respectively. These bond lengths are in agreement with those reported in literature   .
Decavanadate groups, organic cations and water molecules form infinite zigzag chains viewed along the c axis as shown in Figure 6. The cohesion of these layers by N-H···O, O-H···O hydrogen bonds and Van der Waals interactions leads to a three-dimensional structure.
The decavanadate groups form hydrogen bonds on the one hand with the water molecules and on the other hand with the organic molecules. Similarly organic cations form hydrogen bonds with oxygen atoms of groups and
Figure 5. Projection along the b-axis of the organic cations in the (C4N2H7)4(C6N2H10)V10O28∙2H2O compound.
Figure 6. (a) Structure view of compound (1) along the a-axis. (b) Disposition of the layers parallel to the plane (011).
water molecules (Figure 7). In this structure, five hydrogen bonds are strong with a range of D---A bond lengths from 2.677 to 2.876 Å and the other five are weak with the D---A bond lengths vary from 3.112 to 3.350 Å according to Brown  (Table 4).
Figure 7. Representation of hydrogen bonds in the (C4N2H7)4(C6N2H10)V10O28∙2H2O compound.
Table 4. Hydrogen bonds in crystal of (1).
Symmetry codes: (i) –x + 1, −y + 1, −z; (ii) −x, −y + 1, −z + 1; (iii) x − 1, y, z; (ivi) –x + 1, −y + 1, −z + 1; (vi) x−1, y, z.
4. Conclusion and Perspectives
A novel decavanadate has been synthesized and characterized by single crystal X-ray diffraction, IR and UV-Vis spectroscopies. The decavanadates anions, organic cations and the water molecules are connected through N-H···O, O-H···O hydrogen bonds and Van der Waals interactions to form a three-dimensional structure. In perspective, biological analyzes will be carried out, as part of a project with the Institute of Pasteur of Tunis, to better understand the mode of action of vanadium crystals developed and therefore evaluate their effect on the germination of spores. Germination tests are also very useful for evaluating the mechanism of action of the antifungal compounds.
Acknowledgements and Funding Information
Financial support from the Ministry of Higher Education and Scientific Research of Tunisia is gratefully acknowledged.
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