FT-IR, UV-VIS AND ESR INVESTIGATION OF THREE VANADIUM(IV) SUBSTITUTED KEGGIN POLIOXOMETALATE

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1 FT-IR, UV-VIS AND ESR INVESTIGATION OF THREE VANADIUM(IV) SUBSTITUTED KEGGIN POLIOXOMETALATE Cora Crăciun, L. David, O. Cozar, Sanda Filip*, Ioana Panic "Babes-Bolyai" Univ., Dept. of Physics, Cluj-Napoca, 3400 Romania * University of Oradea, Dept. of Physics, 3700 Oradea, Romania ABSTRACT The K 3 [PV 3 W 9 O 40 ] 19H O substituted polyoxometalate was investigated by spectroscopic methods. The coordination of the vanadium(iv) ions at the trilacunary Keggin unit is indicated by the appearance in the FT-IR spectrum of the complex of ν as (V=O d )=963 cm 1 vibration. In the visible electronic spectrum the B (d xy ) E(d xz,yz ) (14184 cm 1 ), B (d xy ) B 1 (d x y ) (19169 cm 1 ), B (d xy ) A 1 (d z ) (4390 cm 1 ) transitions for V(IV) ions in C 4v local symmetry are obtain. The isotropic ESR spectrum of the complex was simulated by considering a Gaussian lineshape and g 0 =.009, B pp = 65 G parameters. This indicates the presence of dipolar interactions between the V(IV) ions. 1. INTRODUCTION

2 Heteropolyanions have won particular attention mainly because of their use in heterogeneous catalysis as well as in oxidation and acidic processes. Heteropolyoxometalates (HPOM) with mixed addend atoms like vanadium and tungsten are very interesting from structural point of view [1]. The unpaired electron delocalization into the HPOM or contrary the presence of trapped electrons on different ions are some structural features leading to different spectroscopic properties and applications of these compounds. Owing to the nonequivalent positions of the addend atoms into the HPOM, the sample could contain different geometrical isomers []. Figure 1. The structure of K 3 [PV 3 W 9 O 40 ] 19H O heteropolyoxometalate (Shaded octahedra in the cap position represent VO 6 sites. Empty polyhedra are WO 6 units and the full central tetrahedron in PO 4 ) In this paper we report some spectroscopic investigation on K 3 [PV 3 W 9 O 40 ] 19H O heteropolyoxometalate (Figure 1). This complex presents one α-keggin structure and it derives from the trivacant α-[pw 9 O 34 ] 9- anion to which three vanadium ions are added. Previous X-ray and multinuclear NMR studies on isostructural complexes showed that vanadium are usually octahedrally coordinated by the oxygen atoms. The VO 6 octahedra are adjacent and share a corner oxygen atom O b. 166

3 FT-IR SPECTRA The coordination of the vanadyl groups at the tricunary ligand arrises from the appearance in the FT-IR spectrum of the complex comparative to that of the ligand of the ν as (V=O d ) stretching vibration (Figure ) and from the shift of ν as (W-O a ), ν as (W-O b -W) and ν as (W=O d ) bands (Table 1). Figure. FT-IR spectra of the V(IV)-HPOM complex (a) and the ligand (b) Table 1. Some IR bands (cm -1 ) of the ligand and V(IV)-HPOM complex Band Ligand (cm -1 ) Complex (cm -1 ) δ(o a -P-O a )+ ν s (V-O b,c -W) 599 w,b 618 s,sp ν as (W-O a ) + ν as (W-O b -W) 754 s,b 83 s,b w,b 794 s,vb ν as (W-O c -W) 884 m,b 888 w,b ν as (W=O d ) 933 s,sp 936 w,sh

4 ν as (V=O d ) m,sh ν as (P-O a ) 1010 w,sp 105 m,sp 991 m,sh 109 w,b 1066 m,sp δ(hoh) 1637 m,sp 160 w,sp ν as (OH) 360 w,sh 3381 m,b 3513 m,sp 3178 w,vb 345 m,vb 355 m,vb w-weak, m-medium, s-strong, sh-shoulder, b-broad, sp-sharp, vs-very strong, vw-very weak UV-VIS SPECTRA The UV electronic spectrum of the vanadium complex contains two bands for the d π* -p π transition into the W=O bonds (47040 cm -1 ) and for d π* -p π -d π transition into the tricentric W-O-W bonds (38800 cm -1 ) respectively. The ligand spectrum contains only a broad centered at cm -1. The shift of the maxima of the complex bands is due to the distorsions introduced by the weaker vanadiu (IV) ions into its neighbouring WO 6 octahedra. The electronic spectrum of the complex in the visible region (Figure 3) contains three bands for V(IV) ions in C 4v local symmetry. In terms of Ballhausen and Gray molecular orbital theory, these bands are cocnsistent with the d xy <d xz,yz <d x y <d z energetical orbital ordering and can be assigned to the following transitions: B (d xy ) E(d ) : cm -1 xz, yz B (d ) B (d ) : cm -1 xy 1 x y B (d ) A (d ) : 4390 cm -1 xy 1 z 168

5 Figure 3. Electronic spectrum of the K 3 [PV 3 W 9 O 40 ] 19H O complex in the visible region ESR SPECTRUM The ESR spectrum of the K 3 [PV 3 W 9 O 40 ] 19H O complex obtained at room temperature is isotropic, symmetric without any hyperfine structure (Figure 4). The ESR parameters have been obtained form the simulation of the experimental spectrum: g 0 =.009 and B pp = 65 G. The best results (as the figure indicates) were obtained by considering one Gaussian lineshape. This fact shows the presence of dipolar interactions, if they are present, are very small can t be evaluated from the ESR spectrum. 169

6 Figure 4. Experimental ESR spectrum of the K 3 [PV 3 W 9 O 40 ] 19H O complex obtained in the X band at room temperature (normal line) and the corresponding simulated spectrum (dot line). CONCLUSIONS IR spectra indicate the coordination of the vanadyl ions at the trilacunary α-[pw 9 O 34 ] 9- anion. The symmetry of the PO 4 central group decreases after the vanadium ions coordination from T d to C s. The vavadium (IV) ions are six coordinated by oxigen atoms in C 4v local environments and have B (d xy ) ground state as the VIS spectrum of the complex shows. The three V(IV) ions are dipolar coupled. The broad ESR line obtained both at room and nitrogen liquid temperatures can arise also from the delocalization of the unpaired electrons over the three VO 6 octahedra. REFERENCES 1. I.V.Kozhevnikov Chem.Rev., 1998, 98, 171. E.Cadot, M.Fournier, A.Teze, G.Herve Inorg.Chem., 1996, 35, 8 170

7 3. C.J.Gomez-Garcia, C.Gimenez-Saiz, S.Triki, E.Coronado, P.Le Magueres, L.Ouahab, L.Ducasse, C.Sourisseau, P.Delhaes Inorg.Chem., 1995, 34,