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Polyhedron 19 (2000) 1917–1923

Spectroscopic and electrochemical investigations of theK8[P2VMoW16O62]·31H2O heteropolyoxometalate

L. David a,*, Cora Craciun a, Mariana Rusu b, O. Cozar a, P. Ilea b, D. Rusu c

a Department of Physics, ‘Babes-Bolyai’ Uni6ersity, 1, M. Kogalniceanu, 3400 Cluj-Napoca, Romaniab Department of Chemistry, ‘Babes-Bolyai’ Uni6ersity, 3400 Cluj-Napoca, Romania

c Department of Chemical-Physics, Medical and Pharmacy Uni6ersity, 3400 Cluj-Napoca, Romania

Received 25 February 2000; accepted 7 June 2000

Abstract

The K8[P2VMoW16O62]·31H2O heteropolyoxometalate complex was synthesized and investigated by electrochemistry measure-ments and IR, UV–Vis and EPR spectroscopy. The substitution of one W�Od unit by the VO2+ group was inferred from the shiftof the nas(M�Oa,d) and nas(M�Ob,c�M) vibrational bands (M=W, Mo) and the appearance of nas(V�Od):960 cm−1 vibration inthe IR spectrum of the complex and from the electrochemistry studies. The three transitions in the electronic spectra were assignedto 2B2�2E (13 700 cm−1), 2B2�2B1 (16 490 cm−1) and 2B2�2A1 (23 140 cm−1), indicating a six coordination of thevanadium(IV) ion in an axial C4v environment. EPR spectrum of the complex obtained at room temperature shows resolvedhyperfine features in the parallel and perpendicular regions typical for monomeric V4+ ion. EPR parameters (g��=1.940,gÞ=1.982, A��=178.5, AÞ=62.9 G) and the values of LCAO–MO coefficients (b1

2=0.863, b22=0.875 and b3

2=0.935)correspond to a dominant ionic character of the s and p V�O bondings and to an important delocalization of the electronstowards the Ob,c atoms. © 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Vanadium; Molybdenum; Heteropolyoxometalate; IR spectroscopy; UV–Vis spectroscopy; EPR spectroscopy

1. Introduction

Heteropolyoxometalates (HPOM) are intensely stud-ied for their use in catalysis, biology and materialsscience. When these polyanions contain vanadium andmolybdenum atoms near the tungsten and oxygenatoms, they exhibit important catalytic activities forredox reactions [1]. In this paper we investigate theK8[P2VMoW16O62]·31H2O complex by electrochemicaland spectroscopic methods. This complex has an a-Dawson–Wells structure [1,2] with one tungsten atomsubstituted by one easier molybdenum atom and an-other tungsten atom replaced by one vanadium ion(Fig. 1). The two phosphorus atoms are tetrahedrallycoordinated in the center of the investigated hetero-polyoxometalate complex.

Usually, the oxygen atoms of a-Dawson–Wellsstructure coordinate octahedrally the vanadium and

molybdenum ions, similarly to the fundamental WO6

units [3,4]. The MoO6 and VO6 substitute WO6 octahe-dra in a 6W belt or in a 3W cap regions of thea-Dawson–Wells structure [3]. On the other hand,these could be in the vicinity, sharing a corner oxygenatom or an edge, or could be nonadjacent. Previousstudies revealed the affinity of the molybdenum ion tocoordinate in the ‘cap’ region [3,5] and the affinity ofvanadyl ions for the ‘belt’ region of the complex [5].Also, the vanadium ion is more easily reducible (toV4+) compared to molybdenum ion (to Mo5+) [6]. Theelectron trapped on the vanadium ion is slightly delo-calized, more into the V�O�Mo bond than into theV�O�W bond [7].

2. Experimental

2.1. Synthesis of K8[P2VMoW16O62]·31H2O complex

A solution of 0.163 g (1 mmol) of VOSO4 in 20 mlacetate buffer was added to 4.81 g (1 mmol) of

* Corresponding author. Fax: +40-64-191906.E-mail address: leodavid@phys.ubbcluj.ro (L. David).

0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.PII: S 0277 -5387 (00 )00448 -4

L. Da6id et al. / Polyhedron 19 (2000) 1917–19231918

K10[P2MoW16O61]·19H2O (obtained as described in [8])in 50 ml acetate buffer solution (pH 4.7) at 60–70°C,under vigorous stirring for 1 h. After cooling at room

temperature (r.t.), KCl saturated solution was added toblue polyoxometalate complex solution.

The dark blue precipitate was collected on a mediumfrit, dried for 30 min under vacuum, washed withsaturated KCl solution and dried again under vacuum.The chemical analysis was carried out by atomic ab-sorption methods after the complex decomposition byboiling with 6 M NaOH solution. The chemical analysisdata are: Anal. Calc.: H2O, 11.14; K, 6.21; Mo, 1.95;P,1.24; V, 1.02; W, 58.68%. Found: H2O, 11.05; K, 6.16;Mo, 1.87; P, 1.26; V, 1.00; W, 58.42%.

2.2. Physical measurements

UV and visible electronic spectra were performed inl=200–800 nm range in aqueous solutions, using astandard Specord UV–Vis spectrometer. IR spectrawere obtained in KBr pellets in the 4000–400 cm−1

range with a Carl Zeiss Jena model UR-20 spectropho-tometer. Cyclic voltammetry experiments were per-formed using a computer-controlled potentiostatAUTOLAB 10. A conventional three-electrode cell wasused. The working electrode (WE) was a vitreous car-bon disk (f=3 mm), the reference electrode was asaturated calomel electrode (SCE, o0=0.244 V NHE)and the counter electrode was a platinum plate (S=1cm2). The WE was polished before use on abrasivepaper 1200, followed with a paste of Al2O3 (Struers,Denmark). The electrolyte solutions were preparedfrom distilled water and heteropolyoxometalates. Allmeasurements were performed in phosphates buffer atpH 3. EPR measurements were performed at 9.4 GHz(X band) using a standard JEOL-JES-3B equipment atr.t.

3. Results and discussion

3.1. IR spectra

By compairing the IR frequencies of the monolacu-nary K10[P2MoW16O61]·19H2O ligand with those of thevanadium–polyoxometalate complex, we have obtainedinformation concerning the coordination of the vana-dium ion at the polyoxometalate. Fig. 2 presents themain part of the IR spectra and the most important IRabsorption bands and their assignments are shown inTable 1.

The shoulder at :960 cm−1 in the IR spectrum ofthe complex is due to the stretching vibration of avanadyl group, whose oxygen is a terminal atom [7,9].

The bands of the nas(W�Od) and nas(Mo�Od) strech-ing vibrations [10] are supperposed in the IR spectrumof the ligand. One of the two shoulders (at 910 cm−1)

Fig. 1. The structure of the K8[P2VMoW16O62]·31H2O heteropolyoxo-metalate (Shaded octahedron in the ‘belt’ position represents VO6

site. Pointed octahedron in the ‘cap’ position represents the MoO6

site).

Fig. 2. IR spectra of the ligand (a) and vanadium–HPOM complex(b).

L. Da6id et al. / Polyhedron 19 (2000) 1917–1923 1919

Table 1Some IR bands (cm−1) of the ligand and vanadium–HPOM com-plex a

Band V(IV)–HPOM (cm−1)Ligand (cm−1)

3470 (s,b)n(H2O) 3500 (s,b)3570 (s,b)

1643 (m,sp)d(H�O�H) 1620 (m,sp)1080 (s,sp)nas(P�Oa) 1082 (s,sp)1060 (m,sh)1064 (m,sp)

1021 (m,sh) 1015 (m,sp)1054 (m,sh)

960 (m,sh)nas(V�Od)945 (s,sp)nas(W�Od) 945 (s,sp)

+nas(Mo�Od)923 (m,sp) 910 (s,sp)

890 (m,sh)895 (m,sp)nas(W�Oc�W)nas(W�Ob�W) 795 (s,b)807 (s,b)

840 (s,sh)839 (m,sh)730 (s,sp)nas(W�Oa) 745 (m,sh)677 (m,sh)nas(O�W�O) 685 (m,b)

603 (w,sp)603 (w,b)nas(P�O�W)565 (w,sp)586 (w,sp)

a w, weak; m, medium; s, strong; sh, shoulder; b, broad; sp, sharp;vs, very strong; vw, very weak.

The frequency of the nas(W�Oa) vibration is shiftedwith 15 cm−1 to smaller frequencies in the IR spectrumof the complex. This is due to the coordination of thevanadyl group at the ligand which also affects theneighboring octahedra of the structure [14].

The WO6 octahedra from the equatorial regions havethree Ob atoms and one Oc, Oa and Od atom, while theWO6 octahedra from the polar regions have two Ob,two Oc, one Oa and one Od atoms. The value of thenas(W�Ob�W) vibration and the shape of its band aremore affected by the vanadium coordination than thoseof nas(W�Oc�W) vibration. We conclude that the vana-dium ion coordinates in the region with more Ob atoms(the equatorial region) of the a-Dawson–Wellsstructure.

The three absorption bands assigned to nas(P�Oa)vibrations [7,15] correspond to distorted PO4 tetrahedrawhose symmetry decreases to Cs. These frequencies arered-shifted with 2–6 cm−1 comparatively to the ligandspectrum. The shoulder at :1054 cm−1 arrises fromthe nonequivalent character of the two PO4 distortedtetrahedra [16]. The shifts of the nas(P�Oa) frequenciesand the lowering of the local symmetry around thephosphorus atoms are due to the vanadium coordina-tion at Oa type atoms.

The frequency of the deformation band of the watermolecules changes because of the raising of basic char-acter of the Od atom bounded to the vanadium ion [17].The terminal Od oxygen could be involved in hydrogenbonds with the neighbouring water molecules [18]. Thisfact is confirmed by the shape of the nas(OH) vibrationband, which spreads in the 3100–3700 cm−1 range andpresents two separate shoulders, at 3460 and 3570cm−1, respectively. The first shoulder, which is also lessintense, corresponds to the coordination water involvedin hydrogen bonds, while the other is due to thecrystalization water.

The IR spectrum of vanadium–HPOM complex indi-cates the coordination of the vanadyl ion at Oa, Ob andOc type oxygen atoms, in the equatorial region ofDawson–Wells structure.

3.2. Electronic spectra

The visible electronic spectrum of the complex inaqueous solution (Fig. 3) presents typical absorptionsfor vanadium(IV) ion in C46 local symmetry [19,20]. Interms of the Ballhausen and Gray molecular orbitaltheory [21], these bands can be assigned to the transi-tions indicated in Table 2 and are consistent with thedxyBdxz,yzBdx2–y2Bdz2 energetical orbital ordering[22].

The absorption band centered at :13 700 cm−1 isdue to the transition into V�Od double bond. Thesecond absorption band (:16 490 cm−1) is a measureof the nonaxial ligand field strength. The smaller value

Fig. 3. Visible electronic spectra of the vanadium–HPOM complex in10−2 mol l−1 aqueous solution.

is red-shifted with �13 cm−1 due to the coordinationof the vanadyl group at the polyoxometalate [11].

In the IR spectrum of the complex the strechingvibration of the tricentric W�Oc�W bonds of the edge-sharing WO6 octahedra [12] appears as shoulder and isshifted with 5 cm−1 to lower frequencies than thecorresponding value in the IR spectrum of the ligand.

Due to the coordination of the vanadium ion at Oa

and Ob atoms, the shape of the spectra is modified inthe 700–850 cm−1 region [13]. IR spectrum of themonolacunary ligand presents two distinct bands, oneat 807 cm−1 (with a shoulder at 839 cm−1) assigned tonas(W�Ob�W) vibration and another at 730 cm−1 corre-sponding to nas(W�Oa) vibration, while in the IR spec-trum of the complex there is only a broad band withsome features at 840, 795 and 745 cm−1.

L. Da6id et al. / Polyhedron 19 (2000) 1917–19231920

comparative to the other vanadium–HPOM complexes[19] indicates the presence of one important ligand fieldin the oxy plane (ortogonal to V�Od direction).

For the assignment of the band centered at :23 140cm−1, there are two posibilities: this can arise from adxy�dz2 transition [22] or can be a heteronuclearcharge-transfer transition (V4+�W6+, Mo6+) [23].For some vanadium–heteropolyoxometalates, M.T.Pope [24] and E. Cadot [19] have assigned the bands at:20 000 and :25 000 cm−1 to the charge-transfertransitions, owing to their important intensity compara-tive to d–d transitions. Because in the present case theabsorption at :23 140 cm−1 is less intense than theother two bands and also than the charge-transfer bandappearing up to 25 000 cm−1 (responsible for the in-tense colour of the anion), we attribute this band to ad–d transition [25].

The UV electronic spectra of the vanadium complexand of the ligand are similar in shape and number ofbands (Fig. 4). The band centered at 47 040 cm−1 inthe ligand spectrum is assigned to pp–dp transitionsinto the W�O and Mo�O bonds [26]. The shift of themaximum of this band for the complex is due to thedistortions introduced by the weaker vanadium(IV) ioninto its neighbouring MO6 octahedra (M=W, Mo).

The broad bands centered at :34 040 cm−1 for theligand and at :34 000 cm−1 for the complex (Fig. 4)are due to electron transition between the energeticlevels of the M�O�M% tricentric bonds (M, M%=Mo,W) [27]. The two shoulders arrising from differentM�O�M% bonds are more resolved in the vanadiumcomplex spectrum.

3.3. Electrochemistry

To establish the electrochemical behaviour of theK8[P2VMoW16O62]·31H2O heteropolyoxometalate, thecyclic voltammograms of the heteropolyanions: a2-[P2W17O61]10− (abbreviated P2W17), a2-[P2MoW16O61]10− (abbreviated P2MoW16) anda2-[P2VMoW16O62]8− (abbreviated P2VMoW16) wereperformed at different potential scan rates (Fig. 5).

The four pairs of peaks in the case of voltam-mograms obtained for P2W17 (Fig. 5a) showed clearlythree reduction reversible processes (peaks I, II, andIII) confirmed by a linear dependence between the peakcurrent (Ipc) logarithm and the scan rate logarithm, andby the fact that the peak potentials are independent ofscan rates. In the case of peak IV, the behaviour is lessclear due to the fact that the maximum correspondingto anodic peak is in cathodic current range. For thefour pairs of peaks the charge transfer and protonationreactions are the same with those confirmed by litera-ture [28–31]:

a2-[P2W17O61]10− +1e−

�a2-[P2W16W5+O61]11− (peak I) (1)

a2-[P2W6+16 W5+O61]11− +1e−

�a2-[P2W6+15 W5+

2 O61]12− (peak II) (2)

a2-[P2W6+15 W5+

2 O61]12− +2e− +2H+

�a2-[H2P2W6+15 W5+

2 O61]12− (peak III) (3)

a2-[H2P2W6+15 W5+

2 O61]12− +2e− +2H+

�a2-[H4P2W6+15 W5+

2 O61]12− (peak IV) (4)

The voltammetric behaviour of the compoundP2MoW16 (Fig. 5b) is similar to that of the P2W17 anionbut the peak for Mo6+ –Mo5+ appears at a potentialabout +0.2 V SCE and the corresponding reaction is:

a2-[P2Mo6+W6+16 O61]10− +1e−

�a2-[P2Mo5+W6+16 O61]11− (5)

For the P2VMoW16 compound (Fig. 5c) a new peak forthe V5+ –V4+ appears at +0.36 V SCE and the corre-sponding reaction is:

a2-[P2V5+Mo6+W6+16 O62]7− +1e−

�a2-[P2V4+Mo6+W6+16 O62]8− (6)

Table 2UV–Vis spectral features (cm−1) of the vanadium–HPOM complex a

n (cm−1)Electronic transition

2B2(dxy)�2E(dxz,yz) 13700 (sh)16490 (s)2B2(dxy)�2B1(dx2−y2)

2B2(dxy)�2A1(dz2) 23140 (m)34000 (m)dp�pp�dp (M�O�M%)47440 (s)pp�dp(M�O)

a sh, shoulder; s, strong; m, medium.

Fig. 4. UV spectra of the ligand (a) and vanadium–HPOM complex(b) obtained in 5×10−5 mol l−1 aqueous solution.

L. Da6id et al. / Polyhedron 19 (2000) 1917–1923 1921

Fig. 5. Experimental cyclic voltammograms of 10−3 M K10P2W17O61 (a), 2×10−3 M K10P2MoW16O61 (b) and 2×10−3 M K8P2VMoW16O62

(c) complexes at different scan rates at pH 3.

Our results are in good agreement with those pro-posed by Pope [32]. Both iso and heteropolyoxometa-lates containing vanadium, molybdenum or tungstenshows the ability to form reduced species only when the

metal atom is coordinated by the oxygen terminalatom. The presence of the three cations (vanadium,molybdenum and tungsten) in the same polyoxometa-late structure leads first to electrochemical reductions of

L. Da6id et al. / Polyhedron 19 (2000) 1917–19231922

vanadium (V5+), this being more easy reducible, thento that of molybdenum (Mo6+) and finally to that oftungsten (W6+) [33,34].

Voltammetric study of P2VMoW16 in acid mediashowed the modification of the electrochemical be-haviour compared with that of the P2W17 anion becauseof the vanadium and molybdenum addenda presence.

The electrochemical signal of vanadium is very dis-tinct and creates favorable views for using it as a redoxmediator for electrochemical polyoxometalate modifiedelectrodes.

3.4. EPR spectra

The powder EPR spectrum of the complex recordedat r.t. (Fig. 6, normal line) is typical for mononuclearoxovanadium species in an axial environment. Thisspectrum can be described by an axial spin Hamilto-nian for a d1 system Eq. (7) [27]:

H=b [g��BzSz+gÞ(BxSx+BySy)]+A��SzIz

+AÞ(SxIx+SyIy) (7)

The spectrum exhibits eight components, both in theperpendicular and in the parallel bands, due to themetallic hyperfine coupling of the electron spin with thenuclear spin (I=7/2) of the 99.75% natural abundant51V isotope. The absence of Mo5+ and W5+ EPRsignals confirms that all the molybdenum and tungstenatoms are in the +6 oxidation state [35]. The bestfitting simulated EPR parameters are: g��=1.940, gÞ=1.982, A��=178.5, AÞ=62.9 G, with the line widthsDBpp(��)=52 and DBpp(Þ)=38 G in the parallel andperpendicular bands, respectively. The principal axis ofg and A tensors have been presumed coincident, withthe g�� and A�� directions parallel to the V�Od bond.

Differences between the experimental and simulatedspectra indicate a small rhombic distortion of the VO6

octahedron.The electronic ground state for V4+ ion in a C46 local

symmetry is the antibonding B2 molecular orbital,formed by the dxy orbital of the vanadium ion. TheLCAO–MO approach gives the spin Hamiltonianparameters (Eq. (8)) [21,37]:

g =g0−8b1

2b22l

DE(dxy�dx2−y2)

gÞ=g0−2b2

2b32l

DE(dxy�dxz,yz)

A = −P�

b22�K+

47�

+ (g0−g )+37

(g0−gÞ)n

AÞ= −P�

b22�K−

27�

+1114

(g0−gÞ)n

(8)

where l is the spin-orbit coupling constant for thevanadium(IV) ion, P=2.0023begNbN�r−3� is the dipo-lar interaction term, K is the isotropic Fermi contactterm, b1

2, b22 and b3

2 are the fractional contributions ofdx2–y2, dxy, dxz,yz vanadium orbitals to the molecularorbitals. Using P=0.0128 [38] and l=170 cm−1 [39],we obtained b1

2=0.863, b22=0.875 and b3

2=0.935 forthe in-plane s-bonding, in-plane p-bonding and out-of-plane p bonding, respectively.

The out-of-plane p bonding has a dominant ioniccharacter, with the unpaired electron spending moretime into the dxz,yz orbitals of the vanadium ion. Thenot too high values of b1

2 and b22 coefficients suggest

that the VO6 and MoO6 octahedra could share a cor-ner, as previous studies of polyoxometalates withvanadyl and molybdenum ions have indicated [19]. Thecalculation was made by considering all the possibilitiesfor the signs of A�� and AÞ, but only the case A��B0,

Fig. 6. Experimental (normal line) and simulated (dashed line) EPR spectra of the powder vanadium–HPOM complex, at room temperature.

L. Da6id et al. / Polyhedron 19 (2000) 1917–1923 1923

AÞB0 gives rise to acceptable values for the molecularorbital coefficients (for A��\0, AÞ\0 and A��\0,AÞB0 we obtained the negative b2

2= −1.003 andb2

2= −2.060 values respectively and for A��B0, AÞB0, the b2

2=1.932 exceed the b22=1 superior limit). This

fact together with the low K=0.785 value are in goodagreement with the presence of a negative charge on theOb,c oxygen atoms, resulted from the delocalization ofthe unpaired electron of the vanadium(IV) ion[24,36,38].

4. Conclusions

The spectroscopic and electrochemical investigationsof K8[P2VMoW16O62]·31H2O complex indicate the co-ordination of the vanadyl group at the monolacunarya-Dawson–Wells fragment. The vanadium(IV) ion issix coordinated by oxygen atoms in a C46 local environ-ment, with a 2B2(dxy) ground state. EPR parametersobtained from the simulated spectrum confirm the axialsymmetry. The values of the molecular orbital coeffi-cients for the in-plane s and p bondings show theelectron delocalization towards the oxygen atoms fromV�Ob,c bonds.

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