Spectroscopic and thermal behavior of Na6[(VO)3(P2O7)3] ·7H2O

Article abstract of DOI:10.1023/A:1011588904771

The electronic (absorption and reflectance) as well as the infrared and Raman spectra of Na₆[(VO)₃(P₂O₇)₃] ·7H₂O were recorded and discussed. Its thermal behavior was investigated by means of TG and DTA measurements. After a two-step dehydration, the compound melts at 464°C and, apparently, depolymerization takes place during this process. The characteristics of the pyrolysis residues, collected at different temperatures, were spectroscopically investigated.

Full text of DOI:10.1023/A:1011588904771

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 177–183
SPECTROSCOPIC AND THERMAL BEHAVIOR OF
Na [(VO) (P O ) ]⋅7H O
6
3
2
7 3
2
*
C. I. Muglia, E. G. Ferrer and E. J. Baran
Centro de Química Inorgánica (CEQUINOR/CONICET, UNLP), Facultad de Ciencias Exactas,
Universidad Nacional de La Plata, C. Correo 962, 1900-La Plata, Argentina
(
Received September 1, 2000; in revised form January 18, 2001)
Abstract
The electronic (absorption and reflectance) as well as the infrared and Raman spectra of
Na
6
[(VO)
3
(P
2
O
7
)
3 2
]⋅7H O were recorded and discussed. Its thermal behavior was investigated by
means of TG and DTA measurements. After a two-step dehydration, the compound melts at 464°C
and, apparently, depolymerization takes place during this process. The characteristics of the pyroly-
sis residues, collected at different temperatures, were spectroscopically investigated.
Keywords: spectroscopic behavior, thermal behavior, trimeric oxovanadium(IV) diphosphate
Introduction
Vanadium phosphates exhibit a wide range of structures partly due to the different
oxidation states that this metal adopts and the variety of ways in which phosphate tet-
rahedral and vanadium–oxygen polyhedra can be interlinked. Interest on these com-
pounds arises from the fact that most of them are potentially useful as catalysts or pre-
cursors of catalysts for selective oxidations [1–8].
On the other hand, interaction of vanadium cations with different kind of phos-
phates are also of biological significance [9–11] and generated renewed interest in the
last years in relation to similar processes occurring in natural systems, in which phos-
phate groups form part of more complex structures or ligands [12].
In order to attain a wider insight into the peculiarities of some complex vanadium
phosphates in this paper we present results on the spectroscopic and thermal behavior
of a solid oxovanadium(IV) diphosphate trinuclear species, Na [(VO) (P O ) ]⋅7H O,
6
3
2
7
3
2
which has been briefly mentioned in literature [13, 14] but has never been character-
ized in detail.
*
Author to whom all correspondence should be addressed.
1
418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
©
2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

1
78
6 3 2 7 3 2
MUGLIA et al.: Na [(VO) (P O ) ]⋅7H O
Experimental
VO(ClO ) solution were obtained by reactions between equimolecular aqueous solu-
tions of VOSO and Ba(ClO ) as described earlier [15]. To 40 mL of a 0.1 M solution
4
2
4
4 2
of this salt, 40 mL of an 0.1 M solution of sodium diphosphate were dropwise added,
under continuous stirring. The resultant solution was reduced to half volume by evap-
oration on a water bath. After cooling, 40 mL of absolute ethanol was added. The blue
precipitate was filtered off, washed several times with ethanol and finally dried dur-
ing 24 h at 60°C. The content of vanadium was determined by the tungstophospho-
vanadic method [16], typical values for different samples ranged between
1
5.3–15.7% (calculated, V=15.49%). Previous reports on this complex suggest the
presence of eighteen [13] or twelve [14] molecules of water. The difference with the
now reported stoichiometry probably arises from different precipitation and/or dry-
ing conditions of the material.
Electronic absorption spectra were recorded with a Hewlett-Packard 8452 di-
ode-array spectrophotometer, using 10 mm quartz cells. The reflectance spectra were
obtained with a Shimadzu UV-300 spectrophotometer, using MgO as a reference. In-
frared spectra were recorded with a Perkin Elmer 580 B spectrophotometer, using the
KBr pellet technique. Raman spectra were obtained with a Bruker IF6 66 FTIR in-
strument provided with a FRA 106 Raman accessory and using the 1064 nm line of a
Nd:YAG laser for excitation. ESR spectra were measured at 25°C with a Bruker-300
spectrometer, using 34.01 GHz (Q-band).
Thermogravimetric (TG) and differential thermoanalytical (DTA) measure-
ments were performed with a Shimadzu system (models TGA-50 and DTA-50),
–
working under a constant O flow (50 mL min ) and at a heating rate of 5°C min .
1
–1
2
Al O was used as a DTA standard. Sample quantities ranged between 15 and 20 mg.
2
3
Results and discussion
Electronic spectra
The electronic absorption spectrum of an aqueous solution of the complex shows
–1
–
1
–1
–1
bands at 810 (ε=43.3 L mol cm ) and 595 nm (ε=18.2 L mol cm ) and a shoulder
–
1
–1
at ca 370 nm (ε=9 L mol cm ), located on the tail of a strong charge transfer band
situated in the UV region. These are the three typical electronic transitions expected
2
+
for the complexed VO cation under C -symmetry, i.e. b →e, b →b and b →a re-
spectively [17].
4
v
2
2
1
2
1
2
A comparison with the spectrum of the [VO(H O) ] species [17] shows a slight
+
2
5
displacement of the higher intensity band to lower energies in the phosphate complex
from 760 to 810 nm), whereas for the second transition a reverse displacement is ob-
(
served (from 625 to 595 nm), indicating a somewhat higher 10 Dq value in the pres-
ent case. Also, the band intensities are appreciable higher in the phosphato than in the
aqua complex.
J. Therm. Anal. Cal., 65, 2001

MUGLIA et al.: Na
6
[(VO)
3
(P
2
O
7
)
3
]⋅7H
2
O
179
The electronic reflectance spectrum of the powdered solid is practically identi-
cal to the solution spectrum, confirming the presence of the same complex species in
both phases. It shows the bands at 815, 605 and 375 nm.
Vibrational spectra
–
1
The infrared spectrum of the compound in the range between 1300 and 300 cm is
shown in Fig. 1. The Raman spectra were rather poor due to strong fluorescence ef-
fects, which generate a high background. Notwithstanding a number of characteristic
Raman lines, which show good reproducibility in different measured samples, were
also useful for the vibrational analysis. The proposed assignment is presented in Ta-
ble 1, and is briefly commented as follows:
–
strong and broad bands between 3600–3200 cm (O–H stretchings) and at 1652 cm
The vibration of the water molecules (not shown in Fig. 1) are found as rather
–
1
–1
(
OH-bending).
–1
Fig. 1 Infrared spectrum of Na
6
[(VO)
3
(P
2
O
7
)
3
2
]⋅7H O, between 1300 and 300 cm
4
−
–
in Na P O [18, 19] and in some heavy metal diphosphates [20], but lie at lower fre-
The internal vibrations of the P O moieties are found in similar ranges than
2 7
4
2
7
quencies than in (VO) P O [19].
–
2
2
7
An important splitting of the ν (PO ) vibration is observed, which is surely
as
3
generated by the cyclic structure that involves coordination of two of the oxygen at-
oms to the metal center, with a concomitant decrease of the strength of these two P–O
bonds and the reinforcement of the remaining (non-coordinated) P–O bond.
–
1
–
The separation of the two P–O–P bridge stretchings (∆ν=163 cm ) is smaller
than that found in most of the other investigated diphosphates [18, 20]. This is also a
consequence of the cyclic structure.
J. Therm. Anal. Cal., 65, 2001

1
80
6 3 2 7 3 2
MUGLIA et al.: Na [(VO) (P O ) ]⋅7H O
– The characteristic ν(V=O) stretching is the most intense line in the Raman
–1
spectrum (968 cm ) and is found in the same position in the infrared spectrum, par-
tially overlapped by the components of the ν (PO ) mode (Fig. 1).
s
3
–
1
The spectral range below 650 cm is more difficult to assign. In this region,
–
one expects deformational and torsional modes of the PO groups. The bending mode
3
of the P–O–P bridges probably lies out of the measured range [20]. Besides, the V–O
bands are expected in this region. Such metal-to-ligand vibrations are often found to
–
lie between 600 and 400 cm [21–23]. A comparison of our infrared spectrum with
1
–
1
that of Na P O ⋅10H O, suggests that the weak band at 453 cm could be assigned to
one of these modes.
4
2
7
2
Table 1 Assignment of the vibrational spectrum of Na
6
[(VO)
3
(P
2
O
7
)
3
2
]⋅7H O
–
1
–1
Infrared/cm
525 s
430 vs
Raman/cm
Assignment
3
3
1
1
ν(OH)
3
240 m
652 m
δ(HOH)
1
199 vs
176 m
1180 m
1138 m
1092 m
1038 s
968 vs
890 w
753 m
631 w
585 w
1
110 sh
093 vs
3
νas(PO )
1
1
007 vs, 990 sh
68 m
04 vs
ν
s
(PO
ν(V=O)
as(POP)
(POP)
3
)
9
9
ν
7
41 s
22 s
ν
s
6
5
5
77 w
61 w
3
PO -deformations
5
5
38 vs
22 sh
and torsions
(see tex)
520 w
352 m
4
93 w, 477 sh
53 w
86 s, 357 sh
4
ν(V–O) (?)
3
vs: very strong; s: strong; m: medium; w: weak; sh: shoulder
To conclude, the vibrational spectroscopic results are in agreement with the pre-
viously reported ESR data [11, 13, 14] that suggested the formation of a cyclic struc-
ture in which each diphosphate anion acts as a bidentate bridging ligand (also [24]).
J. Therm. Anal. Cal., 65, 2001

MUGLIA et al.: Na
6
[(VO)
3
(P
2
O
7
)
3
]⋅7H
2
O
181
Thermal behavior
The thermogravimetric (TG) and differential thermal analysis (DTA) traces are
shown in Fig. 2.
Fig. 2 TG and DTA traces for the thermal decomposition of Na
6
[(VO)
3
(P
2
O
7
)
3
2
]⋅7H O
The figure presents two clearly defined steps; the first one between 20 and 66°C,
related to a mass loss of 7.5%, and the second, which extends up to 180°C and implies
a mass diminution of 5.3%. In the range between 180 and 300°C, usually another
small mass loss of about 0.5–0.7% is observed. The analysis of the two principal
steps shows that the first implies a loss of four H O molecules (calcd 7.29%), whereas
2
the three remaining water molecules are lost in the second step (calcd 5.47%). This
behavior suggests that the first four molecules are crystalline hydration water where-
as the other three are probably coordinated to the metal centers.
Four weak DTA-signals located respectively at 66, 185, 464 and 585°C accom-
panies the degradation. The first two are clearly related to the two dehydration steps.
The third one could be related to the melting of the anhydrous solid, as confirmed by
independent studies in a crucible furnace, using greater amounts of the complex. The
last signal cannot be explained with certainty.
Fused products, collected from furnace experiments at temperatures of 460 and
00°C, show a dramatic color change, presenting a dark green tint.
In order to analyze more deeply the characteristics of these products, we have re-
6
corded its IR and electronic spectra. The IR spectra of the blue powders collected af-
ter heating at 200 and 350°C and corresponding to the anhydrous product are similar
to that shown in Fig. 1, but the fine structure of the bands is practically lost. Addi-
tionally, in the samples collected after heating at 350°C the higher energy band
–
1
(
ca 1200 cm ) shows a notable intensity diminution.
In the spectra of the green products collected after the thermal treatments at 460
and 600°C some other important changes are observed, as it can be seen in Fig. 3. The
first two bands, appear collapsed in one unique and broad feature centered at about
J. Therm. Anal. Cal., 65, 2001

1
82
6 3 2 7 3 2
MUGLIA et al.: Na [(VO) (P O ) ]⋅7H O
Fig. 3 Infrared spectrum of the thermolysis product of the complex, collected at 600°C,
between 1300 and 300 cm
–1
–
120 cm (with shoulders at 1150 and 1090 cm ), whereas the intensity of the next
1
–1
1
group of bands diminishes dramatically, presenting two peaks, of medium and weak
–
intensity at 1030 and 990 cm respectively. The following two bands are centered at
1
–
1
95 and 734 cm , whereas the last block of band also shows important changes in re-
8
spect to that of the original species. In general, these spectra suggest the presence of
mixtures of Na P O and (VO) P O [19]. Therefore, one can assume that during the
4
2
7
2
2
7
melting, depolymerization of the cyclic anhydrous complex takes place probably
generating a mixture of the type:
2
Na [(VO) (P O ) ]→3Na P O +3(VO) P O
6
3
2
7 3
4
2
7
2
2
7
If this proposal is correct, the last observed DTA signal (585°C) is probably re-
lated to a phase transformation or to a new ligand rearrangement of the vanadyl
diphosphate. This speculation is supported by the electronic spectra of these green
materials. The spectra of these phases, collected after heating at 460 and 600°C, al-
though similar, show some differences. That obtained at the melting temperature
–
1
–1
presents absorption bands at 796 (ε=40), 615 (ε=26) and 402 nm (ε=20 L mol cm ),
whereas that collected at 600°C shows the bands at 804, 625 and 408 nm, with similar
intensities as those found above. These differences, although small, point out changes
2
+
on the coordination sphere of the VO cation.
We have attempted to attain a wider insight into the characteristics and eventual
differences between the initial blue complex and the green material collected after
melting, by ESR spectroscopy. Although the signals are very broad, the two spectra
show some differences, confirming the existence of two different complex species.
2
+
The green species presents the typical hyperfine splitting of VO (8 lines). From this
–
4
–2
spectrum we estimate an A value of 195 Gauss (176⋅10 cm ) and g=1.934, values
which are similar to those found in some monomeric species in the VO /P O sys-
tem [11] supporting our proposal of a depolymerization process during the fusion of
2
+
4−
2
7
J. Therm. Anal. Cal., 65, 2001

MUGLIA et al.: Na
6
[(VO)
3
(P
2
O
7
)
3
]⋅7H
2
O
183
the material. The spectrum of the original trimeric species is much broader, and its
hyperfine structure is more difficult to analyze.
*
* *
This work was supported by CONICET, CIC-Provincia de Buenos Aires and ANPCyT (PICT
0119). The authors are indebted to Prof. Dr. T. Rojo (Universidad del Pais Vasco, Bilbao, Spain)
0
for the ESR measurements. C. I. M. benefits from a fellowship for advanced students from
CIC-Provincia de Buenos Aires. E. G. F. and E. J. B. are members of the Research Career from
CONICET.
References
1
2
3
P. L. Gay and K. Kourtakis, Sciencs, 267 (1995) 661.
E. Bordes and P. Courtine, J. Catal., 57 (1979) 236.
A. Martin, B. Lücke, H. Seeboth, G. Ladwig and E. Fischer, React. Kinet. Catal. Lett., 38
(
1989) 33.
J. W. Johnson, D. C. Johnston, A. J. Jacobson and J. F. Brody, J. Am. Chem. Soc., 106 (1984)
123.
4
8
5
6
7
E. Bordes, P. Courtine and J. W. Johnson, J. Solid State Chem., 55 (1984) 270.
G. Centi, Catal., Today, 5 (1993) 16.
C. C. Torardi, Z. G. Li, H. S. Horowitz, W. Liang and M. H. Whangbo, J. Solid State Chem.,
1
19 (1995) 349.
8
J. Kalousová, J. Votinsky, L. Benes, K. Melanova and V. Zima, Collect. Czech. Chem.
Comm., 63 (1998) 1.
9
P. A. M. Williams and E. J. Baran, Biol. Trace Elem. Res., 36 (1993) 143.
1
0 E. J. Baran, in Metal Ions in Biological Systems (H. Sigel, A. Sigel, Eds.), Vol. 21, Dekker,
New York 1995, p. 129.
1
1
1 P. Buglyó, T. Kiss, E. Alberico, G. Micera and D. Dewaele, J. Coord. Chem., 36 (1995) 105.
2 J. J. R. Fraústo da Silva and R. J. P. Williams, The Biological Chemistry of the Elements, Clar-
endon Press, Oxford 1991.
1
3 C. C. Parker, R. R. Reeder, L. B. Richards and P. H. Rieger, J. Am. Chem. Soc., 92 (1970)
5
230.
1
1
1
1
1
1
2
2
2
2
2
4 A. Hasegawa, Y. Yamada and M. Miura, Bull. Chem. Soc. Jpn., 42 (1969) 846.
5 K. M. Jones and F. Larsen, Acta Chem. Scand., 19 (1965) 1205.
th
6 H. Onishi, Photometric Determination of Trace Metals, part 2, 4 Ed., Wiley, New York 1984.
nd
7 A. B. P. Lever, Inorganic Electronic Spectroscopy, 2 Ed., Elsevier, Amsterdam 1984.
8 E. Steger and B. Kässner, Z. Anorg. Allg. Chem., 355 (1967) 131.
9 I. Kanesaka, K. Ozaki and I. Matsuura, J. Raman Spectr., 26 (1995) 997.
0 E. Steger and B. Kässner, Spectrochim. Acta, 24A (1968) 447.
1 E. J. Baran, A. H. Jubert and A. L.Rocha, J. Raman Spectr., 20 (1989) 801.
2 P. Kögerler, E. G. Ferrer and E. J. Baran, Monatsh. Chem., 127 (1996) 801.
3 A. C. González-Baró and E. J. Baran, Monatsh. Chem., 128 (1997) 323.
4 G. Micera and D. Sanna, in Vanadium in the Environment (J. O. Nriagu, Ed.), Part 1, Wiley,
New York 1998, p. 131.
J. Therm. Anal. Cal., 65, 2001