Hydrothermal Synthesis , Characterization and Photoluminescent Properties of a New Vanadium Substituted Keggin Assembly Supported by Ni Complex

a Laboratoire de Chimie des Matériaux, Département Chimie, Faculté des Sciences de Bizerte, 7021 Zarzouna, Tunisie b Laboratoire CEISAM, UMR CNRS 6230, UFR des sciences et des techniques, 2 rue de la Houssinière BP 92208 44322 Nantes cedex 03 ABSTRACT A new monosubstituted vanadium Keggin-type tungstophosphate derivative, [Ni(phen)3]2[PVW11O40].4H2O (phen = 1,10’phenanthroline), has been hydrothermally synthesized and characterized by powder and single-crystal XRD methods, scanning electron microscopy (SEM), IR, UV-Vis and photoluminescence spectroscopy measurements. X-ray diffraction analysis reveals that the compound crystallizes in the monoclinic system with space group C2/c with a = 19.659 (2) Å, b = 18.054 (3) Å, c = 25.227 (4) Å and β = 100.63 (2)° and its crystal packing displays alternating [PV1W11O40(H2O)4]n 4-

Intrigued by these results, we attempted to explore our research on the preparation of vanadium-substituted Keggin-type tungstophosphate materials using multidentate N-donor ligands and with the aim of achieving luminescent properties; we selected the well-known bidentate heterocyclic N-donor ligand, the phenanthroline (phen).In the course of these investigations and by varying the metal chelating agent, we succeeds to prepare new two Keggin-type tungstovanado phosphate cluster decorated by metal-organic mononuclear complex.The first with Co ion was recently published [Co(phen)3]2[PVW11O40].2H2O [25] and the second with Ni metal; [Ni(phen)3]2[PVW11O40].4H2O is the subject of this paper.Its preparation and its structure and physicochemical characterization are discussed.

1. Synthesis of [Ni(phen) 3 ] 2 [PVW 11 O 40 ].4H 2 O
All reagents used were purchased and used without further purification.A reaction mixture of Na2WO4.2H2O(2 g, 6 mmol), NaH2PO4.2H2O(0.156 g, 1 mmol), NiCl2.6H2O(0.24 g, 1 mmol), V2O5 (0,045 g; 0.25 mmol) and phen.H2O (0.6 g, 3 mmol) were added to water (10 mL).The mixture was adjusted to pH = 5.5 by the addition of 4 mol L -1 HCl aqueous solution; then stirred for 30 min in air.The solution was transferred into a 23 mL Teflon-lined autoclave and crystallized at 180°C for 4 days.Then the autoclave was cooled at 10°C h -1 to room temperature.The resulting dark block crystals of 1 were filtered off, washed with water, and dried at ambient temperature to give yields of 68% based on W. Anal.Calc

X-Ray crystal structure determination
A single crystal of the title compound, [Ni(C12H8N2)3]2[PVW11O40].4H2O, was glued to fiber glass for data collection at room temperature on a Nonius KappaCCD diffractometer with graphite-monochromated MoKα (λ = 071073 Å) radiation at room temperature The structure was solved by Shelxs97 [26] in the Wingx package [27] and refined by fullmatrix least-squares techniques on F 2 with the help of the Shelxl97 [27] crystallographic software package All the nonhydrogen atoms were refined anisotropically Hydrogen atoms of organic ligands were located in the calculated positions and refined in a riding mode where the hydrogen atoms of water molecule were located by finding difference Fourier maps.A summary of the crystallographic data and structural determination for 1 is listed in Table 1.The hydrogen bonding scheme and selected bond lengths and angles are given respectively in Tables 2 and 3.The molecular graphics are drawn using Diamond [28].

II. 3. Materials and physical measurements
Elemental analysis (C, H, and N) were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer.P, Ni, V and W were determined on a ICP-AES Inductively Coupled Plasma spectrometer.
Morphology and EDAX measurements were performed on Philips XL30 Scanning electronic microscope along with an integrated EDAX X-ray system.
The infrared spectra were recorded with a Perkin Spectrum 1000 spectrometer with a KBr pellet in the 4000 -400cm -1 region.
UV-Vis spectra were recorded on a Perkin Elmer Lambda 19 spectrophotometer in the 200 -800 nm range.
Emission spectra were obtained on a Perkin-Elmer LS55 spectrofluorometer equipped with a 450 W xenon lamp as the excitation source using solid samples at room temperature.

1. Crystal structure
The title Compound was prepared by solvothermal reaction conditions in good yields as dark green blocks.Its morphology is detected by scanning electron microscopy SEM and the qualitative analysis of these crystals by electron microscope probe revealed the presence of the W, V, Ni, O, N, and C atoms (Figure 1) as well as confirmed by elemental analysis.

Figure 1. The SEM micrograph and EDAX pattern of [Ni(C12H8N2)3]2[PVW11O40].4H2O
The XRD pattern depicted in Figure 2, shows the purity of the as synthesized sample.Moreover the presence of the typical reflexions at 8°<2<10° well prove that the sample possesses well-defined Keggin structure [29][30][31].n− with asymmetric unit made up of one Ni 2+ ion, three phenanthroline ligands, a half -Keggin-type polyanion and two uncoordinated water molecules.As central phosphorus atom is located on inversion centre site, the whole formula unit is generated by this element symmetry (Figure 3).The elemental analysis and EDAX patterns supported by IR spectra (Figure 6) measurements as well as charge balance calculations, indicate that there is a monosubstituted vanadium Keggin-type [PVW11O40] cluster.The assignment of oxidation states for the tungsten and vanadium atoms is confirmed by bond valence sum calculations using an empirical formula of bond valence, S = exp[-(R -Ro)/0.37](S = bond valence, R bond length) [32].The BVS results show that vanadium atom has +V oxidation state (average 5.0485 valence units for the V atom), while tungsten atoms have +VI oxidation state (average 6.3309 valence units for the W atoms).These oxidation states are identical with the charge balance considerations and so consistent with the expected [PV  .Attempts to definitely locate the V atom were unsuccessful due to its low occupancy.As a result, each X site in the polyanion structure was assigned as 11/12W and 1/12 V while the V was constrained in the final refinement.This statistically disorder is very common for this kind of heteropolyoxotungstate as regarded in bibliography [23][24][25].Commonly, the X-O bond distances are grouped into three sets: X-Ot, X-Ob and X-Oc (with Ot: terminal oxygen atoms of type X-Ot, Ob: bridging oxygen atoms of type X-Ob-X and Oc: central oxygen atoms of type X-Oc-P) which are respectively ranged between 1.648 (15)-1.693(17), 1.83 (2)-2.08 (3)Å and 2.37 (2) -2.51 (2)Å as shown in Table 3.The X-O-X bond angles are in the range from 91.1 (9)° to 142.2 ( 14)° (Table 3).These geometrical features are comparable to those observed in related structure of substituted keggin type clusters [23][24][25]33].The Ni 2+ ion metal is also coordinated by six nitrogen atoms from three chelating 1,10-phenanthroline ligands to form a rather regular MN6 octahedron with bond lenghts around Ni are 2.11 (2) -2.16 (2) Å (Ni-N), 77.3 (7) -172.8 ( 8) ° (N-Ni-N) (Table 3).The distortion index [34]   H…O ( mean C…O = 3.22 Å ) (Table 4) (Figure 5) and electrostatic interactions so as to build 3D-supramolecular networks generating vacant 1D-channels along c-axis as can be seen in Figure 4.

III. 3. UV/Vis properties and optical band gaps
The UV-Vis electronic spectra of [Ni(C12H8N2)3]2[PVW11O40].4H2O and pure phenanthroline in DMSO solution at room temperature are depicted in Figure 7A.The UV-Vis electronic spectrum of the complex exhibits broad and strong bands spreading between 200 nm and 350 nm.The strong band with a maximum centered at 295 nm is relatively close to this observed for the free phen ligand (max = 300 nm) and corresponds to intraligand and ligand to-metal charge transfer (LMCT) of Ni metal [40][41].In addition, the broad band centered at about 230 nm may be due to Ot→X and Ob/c→X (X = W and V) charge transfer (LMCT), characteristic of Keggin anions [42][43].Moreover the electronic spectra of the compound provided by using the Tauc model [44], optical band gaps of ca.3.9 eV as reported in Figure 7B, suggesting that the materials may possess semiconducting properties [45].

III. 4. Fluorescence properties
The photoluminescent properties of free phen and the compound in the solid state at room temperature have been investigated.The emission spectrum of the compound (Figure 8(b)) shows broad emission bands red-shifted in comparison to the free phen ligand (Figure 8(a)) with maxima at 422 nm, 481 nm, 530 nm upon excitation at ca. 295 nm.The reported material exhibits blue photoluminescence which can be assigned to the emission to intraligand and ligand tometal charge transfer (LMCT) [46][47][48][49] as evidenced by UV-visible analysis.2+ subunits thanks to C-H…O and electrostatic interactions so as to build 3D-hydrogen bondsupported supramolecular networks generating vacant 1D-channels along c-axis.Meanwhile, the electronic investigations provide on the one hand, the optic band gaps describing the semiconductor behavior owing to the electron delocalization and on the other hand, the fluorescent property of the reported material.

Figure 5 .
Figure 5.View of O-H…O and C-H…O intermolecular interactions in [Ni(C12H8N2)3]2[PVW11O40].4H2O.Hydrogen atoms of phenanthroline ligands not included in H-bond scheme are omitted for clarity.