Scientific paper
Synthesis and Physical-Chemical Study of Two Sandwich-Type Heteropolyoxometalates With Dinuclear Vanadium Clusters
Nicoleta Jooa, Mihaela Hossub, Dan Rusuc, Anca Marcub, Mariana Rusua, Cristina Pascad and Leontin Davidb*
a Deptment of Chemistry, "Babes-Bolyai" University, 11 Arany Janos Str., 400028, Cluj-Napoca, Romania
b Deptment of Physics, "Babes-Bolyai" University, 1 Kogalniceanu Str., 400084, Cluj-Napoca, Romania
c Deptment of Chemistry - Physics, "Iuliu Hatieganu" Medical and Pharmaceutical University, 6 Pasteur Str.,
400349, Cluj-Napoca, Romania
d Departmant of Biology, "Babes-Bolyai" University, 5-7 Clinicilor Str., 400006, Cluj-Napoca, Romania
* Corresponding author: Tel.: + 004 0264 405300 5185; E-mail: leodavid@ohys.ubbcluj.ro
Received: 15-06-2006
Abstract
The sandwich-type K12[(VO)2Sb2W20O70] • 31H2O (1) and K11[(VO)2Bi2W20O70] • 21H2O (2) heteropolyoxotungstates were investigated by means of elemental analyses, thermogravimetric and spectroscopic (FT-IR, UV-VIS and EPR) methods.
The analysis of vanadium ions coordination mode was made comparing the FT-IR spectra of the complexes 1 and 2 with the K12[Sb2W22O74(OH)2] • 38H2O (Lx) and K12[Bi2W22O74(OH)2] • 40H2O (L2) ligands. FT-IR spectra of the complexes show the presence of the V=O bonds characterized by vas(V=O) vibrations. In both complexes, the coordination of the vanadium shifts the v (W-O -W) vibration bands.
sv	c,e '
In UV spectra, the charge transfer pn(Oce) ^ dn*(W) band is shifted in complexes compared to the ligands spectra with = 245 nm for 1 towards higher wavelength and with = 250 nmfor 2 towards lower wave numbers. Visible spectra contain 2B2(dxy) ^ 2E(dxzyz) and 2B2(dxy) ^ 2B1(dx2-y2) transition bands for vanadyl ions in C4v local symmetry, at 12020 cm-1 respectively 14710 cm-1 for the complex 1, and at 12405 cm-1 and 15905 cm-1 for the complex 2. The powder EPR spectra at room temperature exhibit eight components both in the perpendicular and in the parallel bands and are typical for mononuclear oxovanadium species in axial environment.
Keywords: Heteropolyoxotungstates, VO complexes, FT-IR spectroscopy, UV-VIS spectroscopy, EPR spectroscopy
1. Introduction
During the last years, interest for heteropolyoxometalates (HPOM) substituted by early transition metals (3d) has been continuously growing.1-3 These complexes have the capacity to include more transition metals, which interact by means of dipolar or exchange coupling.4,5 This aspect recommends heteropolyoxometalates as potential hosts of high dimensional clusters.3, 4 6 7
A special class of heteropolyoxometalates is the unsaturated trilacunary Keggin-type [Xn+W9O33](12-n)- structure, where the heteroatom X is one of the Bim, As111 or Sb111 ions.8-10 The main characteristic of these ions is the presence of one pair of electrons, which prevents further
condensation to a saturated Keggin structure.8 However, transition metal ions could link the lacunary units, resulting a sandwich-type structure. During the last decade syntheses and structural characterization of a series of di-meric polyoxotungstates containing Sb111 and Bi111 as subvalent heteroatom have been reported, the chemistry of heteropolyoxotungstates being a new, but expanding field of research. Were synthesized compounds containing transition-metals: [M2Sb2W20O70 (H2O)2]10-, where Mn+=Fe3+, Co2+, Mn2+, Ni2+, [M2Bi2W20O70 (H2O)2]10-, where Mn+=Fe3+, Co2+, Zn2+, Mn2+, Cu2+, and also a series of Sn11 compounds.3
In this work we investigate the new K10[(VO)2Sb2 W20O70] ■ 20H2O (1) and K10[(VO)2Bi2W20O70] ■ 24H2O
Fig. 1 The structure of the complexes [X2(VO)2W20O70(H2O)4]10-X=Sb, Bi.11
(2) sandwich-type complexes by spectroscopic (FT-IR, UV-Vis, EPR) methods.
The main goal was to obtain information about the vanadium ions coordination to the trilacunary ligand, the local symmetry around the vanadium ions and the presence of possible vanadium-vanadium couplings.
The investigated compounds contain two identical P-B-[XW9O33]9- heteropolyanion fragments, X=Sbm, Bi111 related by a center of inversion and facing each other with their open sites (Fig. 1).11-14
A belt of two vanadium ions connects the trilacunary anions.3, 15Formally, the fac-WO3 groups have been exchanged for transition-metal ions with three aqua molecules as ligand. This unusual formation leads to three free coordination sites at the transition-metal atoms that are completed by water molecules.
2. Experimental Section
All chemicals were of reagent grade and used without further purification. The [SbW9O33]9- unit have been synthesized as previously described.1
Synthesis of K12[Sb2W22O74(OH)2] • 38H2O (Lj)
The sodium salt of [SbW^J9- 1(10 g, 3.49 mmol) and NajWO4 ■ 2H2O (2.3 g, 6.99 mmol) were dissolved in distilled water (10 mL) while gently heated. By drop wise addition of 1 M HCl (23.5 mL) the pH of the reaction mixture was set to 4-5; the mixture was then evaporated to one third of its volume. After cooling, the sodium salt of [Sb2W22O74(OH)2]12- was formed with a yield of 6.7 g (63%). Crystals of K12[Sb2W22O74(OH)2] ■ 38H2O were obtained after recristallization of this compound with KCl (2.68 g, 35.9 mmol) in water (10 mL) by diffusion techniques. The compound was characterized by IR spectrum.
Synthesis of K10[(VO)2Sb2W20O70] • 20H2O (1)
The salt of [(VO)2Sb2W20O70]10~ was prepared by the reaction of stoichiometric amounts of K12[Sb2 W22O74(OH)2] ■ with the transition-metal salt (VO)SO4 ■ 2Hp. The potassium salt of [Sb2W22O74(OH)2]12- (2 g, 0.3 mmol) prepared above was suspended in 40 mL Na-OAc/HOAc buffer solution (pH = 5.0) and heated to 70 °C while stirring. Then (VO)SO4 ■ 2H2O (0.23 g, 1.27 mmol) was slowly added in portions to the slightly yellowish solution of K12[Sb2W22O74(OH)2]. The dark-brown reaction mixture resulted was stirred for 1 h at 70 °C, and then was allowed to cool at room temperature. The resulting pH value of the mixture was 4.5. After three days, crystals of K10[(VO)2Sb2W20O70] ■ 20H2O were obtained.
Synthesis of K12[Bi2W22O74(OH)2] • 40H2O (L2)
A 30 g (90.95 mmol) amount of Na2W04 ■ 2H2O was dissolved in 40 mL of 4 M NaOAc/HOAc buffer solution. The mixture was heated to 100 °C and 2.52g (8.26 mmol) of BiONO3 ■ H2O was dissolved in 10 mL of concentrated HNO3 (65%). After addition of 20 mL distilled water the bismuth-containing solution was added drop wise to the tungstate solution. The resulting mixture was heated for 2 h (95 °C). The potassium salt was precipitated by adding of grinded KCl solid (85 g, 114.09 mmol) with stirring. The desired product crystallized within 48 h as colorless plates.16
Synthesis of K10[(VO)2Bi2W20O70] • 24H2O ( 2)
The salt of [(vO)2Bi2W20O70]10- was prepared by the reaction of stoichiometric amounts of K12[Bi2W22 O74(OH)2] with the transition-metal salt (VO)SO4 ■ 2H2O. The potassium salt of [Bi2W22O74(OH)2]12- (2 g, 0.29 mmol) prepared above was dissolved in 40 mL NaOA-c/HOAc buffer solution (pH = 5.0) and heated to 70 °C while stirring. To this pale yellow solution of K12[Bi2 W22O74(OH)2] was given slowly (VO)SO4 ■ 2H2O (0.215 g, 1.18 mmol) in portions, leading to a deep-brown reaction mixture, with the final pH 4.3. After heating and stirring for 1 h at 70 °C, the mixture was allowed to cool to ambient temperature and then was filtered. After one week, the green-brown crystals of K10[(VO)2Bi2W20O70] ■ 20H2O complex were obtained. The translucent crystals were recrystallized from distilled water (pH = 4.5).
Physical-Chemical Measurements
The composition in vanadium, potassium and bismuth of each complex was determined by Atomic absorption.
The water content was estimated on the difference between the initial weight of the complex samples and their weight after they were heated at 120 °C for 30 minutes (Table 1). FT-IR spectra were recorded on a Jasco FT/IR 610 spectrometer in the 4000-400 cm-1 range, using KBr pellets.
Electronic spectra were performed in aqueous solutions having 10-5 -10-3 M concentrations, within a range
Table 1. Analytical data of the synthesized compounds.
Complex Yield (g/%)	Color		Found (calc.) (%)			
		K	Sb/Bi	W	V	H2O
1 2.56 / 70	yellow-brown	6.57 (6.48)	4.08 (4.12)	(62.24) 62.10	1.71 (1.60)	6.07 (6.23)
2 2.40 / 74	green-brown	6.31 (6.24)	6.77 (6.84)	59.60 (59.81)	1.65 (1.60)	(7.10)
of X = 190-1000 nm on an ATI UNICAM-UV-Visible spectrophotometer with Vision Software V 3.20.
EPR spectra on powdered solids were recorded at room temperature at ca. 9.6 GHz (X band) using a Bruker ESP 380 spectrometer.
3. Results and Discussion
3. 1. FT-IR spectra
Some information about the coordination of the vanadium ions to the trilacunary POM units and the bonds strength were obtained by comparing the FT-IR spectra of the metallic complexes 1 and 2 and the corresponding ligand. The characteristic bands of the ligand and complexes are summarized in Table 2 and the main regions of the FT-IR spectra (400-1000 cm-1) are given in Fig. 2 and Fig. 3.
The stretching vibration of the terminal W=Ot bonds is shifted (with 8 cm-1 for 1 and 22 cm-1 for 2) towards higher wave numbers in the FT-IR spectra of the complexes (Table 2), which indicates the involving of the terminal oxygen atoms in the coordination to the vanadium ions. The vas(W=Ot) vibration band is broader in the complexes spectra than the corresponding band in the ligand spectra because of its superposition with the stretching vibration vas(V=O).17 The equivalence of the V=O groups in
Table 2. Some FT-IR bands (cm-1) of the ligands (L1; L2) and (VO)II-POM compounds (1, 2).a b
Band	L1	L2	1	2
Vas(OH)	3332 s, b	3423 m, b	3427 s, b	3420 m, b
			3235 m, sh	3242 m, sh
			3050 w, sh	
S(HOH)	1619 w	1620 w, b	1617 w	1623 w, b
S(OH)	1374	1384		
Vas(W=°t)	939 s	948 s	947 s	970 s
Vas(X_Oi)	850 s	848 s	849 s	849 s
Vas(W-°c,e"W)	798 vs	794 vs	797 vs	792 vs
	761 s	726 s, b	750 s	756 s, b
Vs(W-Ob-W)	618 m, b	613 m, b	681 m, b	602 m, b
a Oj is the oxygen which links the As and W atoms, Oc e connect corner and edge-sharing octahedral, respectively, Ot is a terminal oxygen b w, weak; m, medium; s, strong; vs, very strong; sh, shoulder; b, broad.
1000	800	600	400
Waveriumber (em"1)
Fig. 2 FT-IR spectra of the L1 ligand (a) and complex 1 (b).
100G	800	6G0	400
Waveriumber (cm-1) Fig. 3 FT-IR spectra of the L2 ligand (a) and complex 2 (b).
the both complexes makes the corresponding vibration bands to be broad and unsplitted.
The bicentric X-Oi bond is not shifted in complexes spectra compared to the ligands spectra due to their non-involving into the coordination of VIV ions by the ligand. The vibration bands for the tricentric W-Oc-W bonds of the corner-sharing WO6 octahedra observed in the FT-IR spectra of the complexes are non shifted comparing with the li-gands. This is due to their non-involving into the coordination of the VIV ions by Oi atoms. The tricentric W-Oe-W bonds of the edge-sharing WO6 octahedra have different stretching vibrations in both complexes. The vas(W-Oe-W) vibration is blue shifted with 30 cm-1 in complex 2 FT-IR spectrum comparing to the ligand spectrum, but red shifted with 11 cm-1 in complex 1 FT-IR spectrum. This behavior arrises from different deformations induced by the vanadium ions coordination in the frame of the trilacunary li-gands. The decrease of the vas(W-Oe-W) frequency in complex 1 indicates the stretching of these bonds after the metallic ion complexation.18 The increase of this frequency in complex 2 is in agreement with the shortening of these bonds after the complexation of the VIV ions by the ligand.17 The vs(W-Oe-W) vibration is blue shifted with 63 cm-1 in complex 1 FT-IR spectrum comparing to the ligand spectrum, but red shifted with 11 cm-1 and blue shifted with 33 cm-1, respectively, in complex 2 FT-IR spectrum. In addition, the FT-IR spectrum of complex 2 contains two W-Oe-W tricentric bands while the ligand spectrum contains a single band. This suggests the presence in the complexes of two nonequivalent W-Oc-W bonds.19
The W-Oj bonds, where Oi connects the tungsten with the heteroatom, present a single vibration in both li-gands and complexes spectra (Table 2).
There is no evidence about the involving of these bonds in coordination process at the vanadium ions.20 SRTF bookmark start: cOLE_LINK 1 The shift of vas(Bi-Ob,c-W), vs(W-Ob-W), vas(W-Oc-W) bands in the complex 1 comparative to the ligand L1 is due to the substitution of the lateral WO6 octahedra by the (VO)O4 square pyramid and the coordination of (VO)II ions at Ob,c type oxygens.7 The FT-IR bands are broader in the complex, a part of them being overlapped. The vas(W=Od) frequency is unchanged by the substitution of two tungsten ions by two vanadyl ions, which is a sign of the structural stability of the P-B-BiW9O33 units.
The local symmetries around the (VO)11 ions in both (VO)II-POM complexes are distorted C4v ((VO)O4 local unit).
3. 2. Electronic Spectra
The UV electronic spectra of the (VO)II-POM complexes and the two ligands L1 and L2 are similar (Fig. 4, Table 3). Each spectrum presents two bands assigned to ligand to metal charge transfer pn ^ dn transitions in the W=Ot bonds (at high wavenumbers) and the electron transition dn ^ pn ^ dn between the energetic levels of the tricentric bonds W-Ob-W (at low wave numbers).12
a) 1 i y\ 1	(1)
b)\.	
\ \ V ■Si \	
150 200 250 300 Wavelenght (nm)
350
400
a)	(ID
i \ \ b) \\	
	
150 200 250 300 Wavelenght (nm)
350 400
Fig. 4 UV spectra of synthetized ligands and complexes obtained in 5 • 10-5 mol l-1 aqueous solutions: ligand L1 (a) and complex 1 (b) (I); ligand L2 (a) and complex 2 (b), (II).
Table 3. UV spectral features (cm 1 /nm) of the(V O)II-POM compounds and corresponding li-gands.
	52631/190	51280/195	54054/185	52631/190
(W=Ot)				
Band	Li	L2	1	2
cm-1/nm				
dn ^ pn ^ dn	37735/265	37037/270	37037/270	36363/275
(W ObW)	(40000/250 sh -	(39215/255 sh -	(40816/245 sh -	(40000/250 sh -
	(35714/280 sh)	35087/285 sh)	33898/295 sh)	33333/300 sh)
The shifts of the maximum of the bands for the complex comparative to the ligand are due to the distortions introduced by the (VO)11 ions coordination to their neighboring WO6 octahedra. The bicentric W=Ot band is weakly shifted for complexes comparative to ligands. The band at lower wavelength for the pn(Ot) ^ dn„(W) transitions21 appears at approximate the same wavelength (~ 200 nm) in ligands spectra as well as in complexes spectra. The charge transfer transition is situated at « 190 nm in L1 ligand spectrum, shifted towards higher energies in complex 1 (at « 185 nm), and is situated at « 195 nm in L2 li-gand spectrum, shifted towards higher energies in complex 2 (at = 190 nm) (Fig. 4).
The tricentric charge transfer band dn ^ pn ^ dn presents two shoulders for the (VO)n-POM complexes and for the ligands. These bands are shifted in complexes towards lower energies comparative to the ligands because of the weakness of W-Ob-W bonds after the (VO)11 complexation.
The visible electronic spectra of both complexes (Fig. 5) show a relative stronger absorption above 16000 cm1 and a band with a shoulder at lower wave numbers. The strong absorptions correspond to the VIV ^ WVI charge transfer transitions.22
The Gaussian analyses of the spectra lead to obtaining the position of the bands for VIV ions d-d transitions. The two bands appear at 12040 cm1 and 14705 cm1 for complex 1 and at 12410 cm1 and 15915 cm1 for complex 2. The bands of each complex are related to the 2B2(dxy) ^ 2E(dXZyZ) (I) and 2B2(dXy) ^ ^(d^- y2) (II) transitions in the Ballhausen and Gray molecular orbital theory for vanadyl ions in C4v local symmetry.23 The higher energies for complex 2 are related to different degrees of delocali-Zation of the unpaired electrons from the parent vanadium ions towards the neighboring oxygens, by means of out-of plane n bondings and in-plane -bondings, respectively.
3. 3. EPR spectra
The axial powder EPR spectra of both complexes were simulated by considering (VO)II ions noninteracting (Fig. 6).
Fig. 5. Visible spectra of the complex 1 (a) and 2 (b), performed in 5 • 10-3 mol l-1 aqueous solutions. The Gaussian components are represented with dashed lines.
Fig. 6. Experimental (normal line) and simulated (dashed line) EPR spectra of the powder 1 (a) and 2 (b) complexes, at room temperature.
Powder EPR spectra of the K10[(VivO)2X2W20O70] ■ xH2O complexes (X=Sbm, Bim), obtained in the X band at room temperature, correspond to the Viv ions from the vanadyl groups of each molecule. The obtained spectra contain eight components, both in the perpendicular and in the parallel bands due to the hyperfine coupling of the spin of one unpaired electron with the nuclear spin of the 51V isotope (1=7/2). The spectra can be described by an axial spin Hamiltonian characteristic for 5=1/2 system
with C4v local symmetry:
.24
H = ^[ftBA + gL(BxSx + BySy)] + AjSA + A^ + S/p
where gn, g^t and An, A^t are the axial principal values of the g and hyperfine tensors respectively, is the Bohr magneton, Bx, By, Bz are the components of the applied. magnetic field in direction of the principal g axes, Sx, Sy, Sz and /x, /y, /z are the components of the electronic and nuclear spin angular momentum operators, respectively.
The best fit of the EPR spectra was made considering the parameters: g^t = 1.973, gII = 1.912, A^t = 69.5 G, AII = 201.6 G for complex 1 and gn = 1.908, git = 1.974, An = 202.1 G, A it = 71.6 G for complex 2. These values suggest the equivalence of the two paramagnetic VIV ions in each K10[(VivO)2X2W20O70] ■ xH2O units.
4.	Conclusions
Two new polyoxometalate complexes of [(VO)2 X2W20070]10-, X=Sbm, BiIII, were synthesized and investigated by means of elemental analysis, thermogravimetric, and spectroscopic methods (FT-IR, UV-VIS, EPR).
FT-IR data indicate the coordination of each vanadyl ion to oxygen atoms from corner-sharing and edge sharing octahedra. The UV spectra show that in the studied compounds, trilacunary Keggin anion plays the ligand role, as well as the secondary heteroatoms are the vanadyl cations. Visible electronic spectra indicate the penta-coor-dination in square-pyramidal environment of the vanadyl ions (C4v symmetry with a dxy orbital as ground state) in the investigated complexes. EPR parameters confirm the axial symmetry and noninteracting (VO)n ions.
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Povzetek
Pripravili smo kompleksa s sendvično strukturo K12[(VO)2Sb2W20O70] 31H2O (1) in K11[(VO)2Bi2W20O70] 21H2O (2) in jih preiskali s elementno analizo, termogravimetrijo in spektroskopijo (FT-IR, UV-VIS in EPR). Vrsto koordinacijo na vanadijev ion smo študirali s primerjavo FT-IR spektrov 1 in 2 s spektri ligandov Na9[BiW9O33] 14H2O in K12[B-i2W22O74(OH)2] 40H2O. UV spektri kažejo premik trakov pn(Oce) ^ dn*(W) v kompleksih = 245 nm v 1 in = 250 nm v 2 proti proti nižjim vrednostim. Vidni spektri imajo trakove 2B2(dxy) ^ 2E(dxzyz) in 2B2(dxy) ^ 2B1(dx2 _y2) vanadilnega iona s C4v točkovno simetrijo. EPR spektri pri sobni temperature kažejo na enojedrne oksovanadijeve spojine.