Spectroscopic and DFT study of tungstic acid peroxocomplexes

Article abstract of DOI:10.1021/jp066901x

The catalytic system formed by tungstic acid and its complexes with H ₂O₂ and phenylphosphonic acid has been analyzed from the experimental and theoretical points of view. Previous structural studies by XRD proved the validity of the DFT proposed models and methodology. Hydrogen peroxide reacts with tungstic acid to form a peroxo complex. Vibrational and electronic spectra showed significant changes upon interaction with H ₂O₂. The DFT and TD-DFT for IR and UV-vis calculations not only are in agreement with experimental data but also allow for a deeper characterization of the species formed in in situ conditions. A SCRF/PCM methodology was chosen to account for the solvent effect. The solvent effect of water was considered for geometry re-optimization of the structure and for the TD-DFT calculations.

Full text of DOI:10.1021/jp066901x

2166
J. Phys. Chem. A 2007, 111, 2166-2171
Spectroscopic and DFT Study of Tungstic Acid Peroxocomplexes
Laura Barrio, Jose´ M. Campos-Mart´ın, and Jose´ L. G. Fierro*
Instituto de Cata´lisis y Petroleoqu´ımica, CSIC, Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain
ReceiVed: October 20, 2006; In Final Form: January 9, 2007
The catalytic system formed by tungstic acid and its complexes with H2O2 and phenylphosphonic acid has
been analyzed from the experimental and theoretical points of view. Previous structural studies by XRD
proved the validity of the DFT proposed models and methodology. Hydrogen peroxide reacts with tungstic
acid to form a peroxo complex. Vibrational and electronic spectra showed significant changes upon interaction
with H2O2. The DFT and TD-DFT for IR and UV-vis calculations not only are in agreement with experimental
data but also allow for a deeper characterization of the species formed in in situ conditions. A SCRF/PCM
methodology was chosen to account for the solvent effect. The solvent effect of water was considered for
geometry re-optimization of the structure and for the TD-DFT calculations.
Introduction
substrate and forms an oxidized product 3. The monoperoxo
tungstate ion in 3 is reoxidized by H2O2 after returning to the
aqueous phase as the ion pair 4. This step may also occur at
the organic-aqueous interface or even in the organic phase to
some extent.
Oxidation reactions are a fundamental part of the chemical
industry and, more specifically, that portion related to converting
petroleum based materials into useful chemicals. Molecular
oxygen is the ideal oxidant, abundant and non-contaminating,
but its reactivity is difficult to control. Hydrogen peroxide seems
to be an attractive alternative for liquid phase oxidations because
the only byproduct is water. Because of their catalytic activity
and specialized redox behavior, tungstates have received special
attention in a variety of industrial, pharmaceutical, and biological
processes as they can be regarded as an environmentally friendly
alternative to traditional oxidation reactions that represents an
improvement in terms of pollution prevention.1
One the most attractive catalytic systems for H2O2 as an
oxidant is tungstic acid, a phosphorus based acid as a cocatalyst
and a quaternary ammonium salt as the phase transfer agent.2
These catalysts are very attractive because tungstate catalysts
are physiologically harmless, do not cause unproductive H2O2
decomposition, and can be applied to a wide variety of oxidation
reactions.1-6 A tungstic acid catalyst is useful with organic
substrates such as alcohols or aldehydes, but the addition of
phosphonic acid derivates is crucial for catalytic activity toward
olefinic or sulfide and sulfoxide substrates.2
Hydrogen peroxide is commercially available as an aqueous
solution. This fact implies the presence of two phases in the
reaction system, at least for most of the organic subtrates.
Because of this limitation, the best catalytic system of tungstic
acid consists of phase transfer catalysts (PTC). The bibliography
of ref 2 provides reliable data for the generally accepted
mechanism for this reaction with a PTC, as depicted in Scheme
1, and describes experimental findings in the oxidation of
organic substrates with hydrogen peroxide using tungsten
catalysts. In the aqueous phase, the catalyst precursor (HO)2WO2
is rapidly oxidized by H2O2 to a bisperoxotungstate compound,
which then reacts with phosphonic acid to form 1. This anion
moiety can easily be transferred to the organic phase by H+ -
Q+ ion exchange to form 2. Then, 2 oxidizes the organic
Because of the high industrial interest in such processes, it
is of paramount importance to identify the species present in
the reaction media and link them with the catalytic activity. To
better understand these catalytic processes, a more detailed
characterization of peroxotungstates is necessary. Computational
chemistry, combined with other characterization techniques, is
a key instrument for the identification of reaction species.
In light of the previous discussion, the present work was
undertaken with the aim of studying the interactions between
tungstic acid and hydrogen peroxide as a previous step to
understand the oxidation reaction mechanism. We studied
several chemical species involved in the oxidation reactions,
combining the experimental data from infrared (FT-IR) and
ultraviolet-visible (UV-vis) spectroscopy and the theoretical
calculations using the density functional theory (DFT) approach.
This combination of experimental and theoretical data was used
in the assignment of the IR and UV-vis bands. The information
found in this work will be helpful for identifying and analyzing
the properties of the species present in reaction conditions. These
species, which are quite difficult to find outside the reaction
environment, are responsible for the catalytic activity in
oxidation reactions. Therefore, they will be the main focus of
our study.
Experimental and Computational Methods
Tungstic acid ((HO)2WO2), phenylphosphonic acid (PhP(O)-
(OH)2), and hydrogen peroxide (50% solution) were purchased
from Sigma-Aldrich. All compounds were used without further
purification.
Infrared spectra were recorded at room temperature on a
Nicolet 510 FT-IR spectrophotometer provided with a KBr
beamsplitter and a DTGS detector, using KBr wafers containing
1% of the sample. For each spectrum, 100 scans were ac-
cumulated at a spectral resolution of 4 cm-1. UV-vis spectra
of the water diluted samples (10-5 M) were recorded on a Varian
Cary 5000 spectrophotometer.
* Corresponding author. E-mail: jlgfierro@icp.csic.es. Fax: +34
915854760.
10.1021/jp066901x CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/28/2007

Tungstic Acid Peroxocomplex Spectroscopic/DFT Study
J. Phys. Chem. A, Vol. 111, No. 11, 2007 2167
SCHEME 1: Proposed Mechanism of Phase Transfer Oxidation of Organic Compounds with Hydrogen Peroxide and
Tungstic Acid Catalysts
SCHEME 2: Structure of the Species Used in This Study
All calculations were performed with the Gaussian 03
program.7 For W, a modified LANL2DZ basis set with a (541,
541, 211) contraction was employed.8 This basis set includes
additional functions of the 4p shell that are needed to account
for charge transfer effects, which are so important in organo-
metallic chemistry. For H, C, O, and P, a 6-311+G(3d,2p) basis
set was chosen, in balance with the basis set employed for the
metal. A full geometry optimization was carried out with the
pure functional BPW91. DFT methodology was successfully
tested for studying tungsten oxide clusters.9 A BPW91 functional
has been previously shown to be a reliable model for describing
phosphoric tungsten complexes.10 The frequency analysis
performed revealed that all the structures were true minima.
Scheme 2 shows the analyzed structures.
The solvent effect of water was also studied under the self-
consistent reaction field approach by means of a polarizable
continuum model (PCM), as implemented in Gaussian 03.11 The
PCM allows for a reliable description of the solvent effect by
considering its dielectric constant and the solvent radius. To
mimic the solute molecule’s actual shape, the cavity was built
as the envelope of a series of interlocking atomic spheres.12
All the structures were fully re-optimized, taking into account
the presence of water. A TD-DFT analysis with solvent effect
was performed12 with the B3PW91 functional. The solvent effect
is included here at its full scale, not only by altering the
geometry of the system but also by taking into account
nonequilibrium solute-solvent effects during the light absorp-
tion process. The use of a hybrid functional was necessary for
an accurate prediction of electronic transitions.13 B3PW91 was
chosen among hybrid functionals due to the good behavior of
BPW91 for optimizing structures. Hybrid TD-DFT methodology
was recently proven to reproduce the UV-vis spectra of
polycyanotungsten complexes.14
Results and Discussion
In this study, the selected models are depicted in Scheme 2.
These clusters represent, from starting compounds such tungstic
acid ((HO)2WO2) or phenylphosphonic acid (PhP(O)(OH)2), the
complex formed between these two compounds (W(κ1-O-PhP-
(O)(OH)2(OH)2(O)2)) and the corresponding biperoxo complexes
(W(κ2-O-O)2(H2O)2(O) and W(κ2-O-O)2(H2O)(κ1-O-PhP(O)-
(OH)2)(O)). These latter species are very important in the
oxidation reaction mechanism because they are present in the
reaction medium in the oxidations with hydrogen peroxide and
are related to the reaction intermediates as described in the
bibliography of ref 2 (Scheme 1).
It is well-known that tungsten peroxo complexes can easily
dimerize. However, we have selected the monomeric peroxo
species because it is the principal species in the presence of
excess H2O2,16,17 and an excess of hydrogen peroxide occurs
when tungstic acid acts as a catalyst. For a better study of UV-
vis spectra, this experiment was performed with a low concen-
tration of compounds. In such dilute conditions, there is good

2168 J. Phys. Chem. A, Vol. 111, No. 11, 2007
Barrio et al.
Figure 1. Experimental and theoretical IR spectra for tungstic acid
and H2O2 treated samples.
TABLE 1: Selected Structural Data of (HO)2WO2 under C2
Figure 2. Experimental and theoretical IR spectra for phenylphos-
phonic acid, tungstic acid, and H2O2 treated samples.
Symmetry
bond distance (Å)
gas phase
water
experimental18
TABLE 3: Selected Structural Data of
WdO
W-OH
1.730
1.909
1.731
1.914
1.69
1.93
W(K2-O-O)2(H2O)2(O) Complex
bond distance (Å)
gas phase
water
experimental15
TABLE 2: Selected Structural Data of PhP(O)(OH)2
W-OH2
W-OO
WdO
2.346
1.953
1.723
2.374
1.950
1.728
2.30
2.00
1.64
bond distance (Å)
gas phase
water
experimental19
P-OH
PdO
P-C
1.621
1.478
1.807
1.614
1.494
1.804
1.55
1.50
1.77
TABLE 4: Selected Structural Data of
W(K2-O-O)2(H2O)(K1-O-PhP(O)(OH)2)(O) Complex
bond distance (Å)
gas phase
water
experimental20
evidence for the rapid formation of the monomeric species with
large formation constants. For instance, dimerization is not
apparent at concentrations below 0.02 M.17
W-OH2
W-OO
W-O
P-C
PdO
P-OH
2.487
1.962
1.717
1.784
1.486
1.759
2.402
1.969
1.729
1.778
1.490
1.724
1.97
1.62
First of all, we will analyze the tungstic acid system. The
main geometrical data obtained by our theoretical calculations
are compiled in Table 1. The optimization process yielded a C2
structure, very close to the one obtained experimentally by
XRD.18 The re-optimization step with solvent effects did not
produce any significant changes in the structure. The theoretical
model chosen along with the basis set employed seems to be
appropriate due to the good agreement between experimental
and theoretical structures.
The phenylphosphonic acid structure data are summarized
in Table 2. The correlation with experimental XRD structure19
is good, the major deviations being the P-OH bond distance.
As was obtained by tungstic acid, the effect of solvent did not
alter the geometry of the model.
1.48
acid with H2O2 (see Table 3). The tungsten-peroxo bond
distance determined by XRD15 was well-reproduced by our
model, and so was the metal-water molecule bond distance.
This result implies that the inclusion of two water molecules in
the model is consistent with the experimental observed structure.
Finally, the structure of the tungsten-phosphoric peroxo
complex is discussed. The experimental data20 and theoretical
results are summarized in Table 4. The metal-water molecule
bond distance is elongated when compared to the tungsten-
peroxo complex one, meaning that the interaction with water
is weaker now. There is no reported experimental value to
support such an effect. The tungsten-peroxo bond distance is
A more interesting feature is the structure of the tungsten
peroxo complex, which was obtained by the reaction of tungstic

Tungstic Acid Peroxocomplex Spectroscopic/DFT Study
J. Phys. Chem. A, Vol. 111, No. 11, 2007 2169
Figure 4. Experimental and theoretical UV-vis spectra of water
diluted samples treated with hydrogen peroxide (dotted line, H2O2
solution).
around 870 cm-1 22
. Additional bands in the range of 650-560
cm-1 belong to νasW(O-O) and νsW(O-O).22 A weak shoulder
at 1550 cm-1 can hardly be distinguished.
DFT calculations aimed at describing the vibrational structure
of the two species point to good agreement with the experi-
mental infrared spectra, as shown in Figure 1. The wavenumbers
of the characteristic infrared bands described previously fit those
predicted by the theoretical model fairly well. Additionally, the
DFT calculations provide some interesting clues that initially
could not be gained from inspection of the experimental infrared
spectra. An example illustrating the potential of the DFT
calculations is the prediction of a band at 1550 cm-1 attributed
to the vibration of the OH bending of water molecules
coordinated with the tungsten-biperoxocomplex that only
appears as a shoulder in the experimental spectra because it
was overshadowed by the vibration modes of H2O adsorbed in
the KBr pellet.
The phenylphosphonic acid infrared spectrum revealed the
characteristic vibration bands of this compound (Figure 2). The
most significant features are described subsequently: the peaks
at 690, 720, and 1438 cm-1 correspond to the vibration of the
aromatic ring,23 and the peaks between 920 and 950 cm-1 and
the peak at 1014 cm-1 are all assigned to P-OH bonds.23-25
The bands at 1014 and 1075 cm-1 can be assigned to P-O
stretching modes in the hydrogenophosphonate or phosphonate
ions.25 The bands at 1144 and 1212 cm-1 correspond to
stretching vibrations of coordinated and free PdO bonds,
respectively.23,25 The DFT calculated spectrum is in good
agreement with the experimental one and the assignments,
except the peak at 1144 cm-1, which cannot be predicted
because these species are not considered in our model.
The infrared spectrum of the complex between tungstic acid
and phenylphosphonic acid looks like the sum of individual
spectra of both compounds; however careful study shows some
differences (Figure 2). The peak of phenylphosphonic acid that
appears at 1212 cm-1, which corresponds to the free PdO bond
vibration, is not present in the complex spectrum, although the
Figure 3. Experimental and theoretical UV-vis spectra of water
diluted samples.
slightly reduced in the presence of phosphonic acid, and the
agreement with experimental value is excellent. All the other
relevant bond distances are quite similar to the previously
analyzed compounds. When available, correlation with experi-
mental XRD data is adequate.
Summing up the structural data for all the species considered,
the geometrical results obtained by our models are in reasonable
agreement with XRD data. This is a validation for the theoretical
method and basis set employed. Moreover, it also confirms the
validity of the structural models proposed from the complexes
formed by the reaction of H2O2 with tungstic acid. Once the
employment of our model was successfully tested by the
structural results, it was time to analyze the spectroscopic
behavior of the compounds.
The infrared spectrum of the (HO)2WO2 sample diluted in
potassium bromide revealed the characteristic vibration bands
of this compound (Figure 1). This spectrum shows a broad band
at 600-700 cm-1 attributed to W-O‚‚‚HO-W, W-O-H, and
W-O-W bond vibrations21,22 and a sharper band at 950 cm-1
assigned to WdO bond vibration.21,22 The peak observed at 1620
cm-1 is due to the OH bending of molecular water absorbed in
the KBr pellet. The spectrum of tungstic acid treated with
hydrogen peroxide shows clear differences. The intensity of
the bands attributed to the W-O and W-OH bond vibrations
was clearly lower, and additional bands were observed. The
ν(O-O) vibration appears as a clearly defined strong band at

2170 J. Phys. Chem. A, Vol. 111, No. 11, 2007
Barrio et al.
Figure 5. Molecular model and HOMO-LUMO involved in the less energetic electronic transition.
peak at 1144 cm-1 remains. This fact indicates an interaction
between oxygen with tungstic acid. Two new peaks (532 and
512 cm-1) are clearly present in the W(κ1-O-PhP(O)(OH)2(OH)2-
(O)2) spectrum, which are not in the individual spectra. DFT
results revealed that the vibrations in this region correspond to
a flexion of the phenylphosphonic acid coupled with a tungstic
acid one. The whole complex is involved simultaneously in these
vibrational signals. IR spectrum of the complex calculated by
DFT predicts the changes in the spectrum observed in the
experimental one.
weak band around 270 nm is not predicted because anionic/
dissociated species of the acids are not taken into account by
the model.
For the analysis of H2O2 treated samples, we employed an
excess of hydrogen peroxide to ensure the presence of isolated
biperoxotungten species.22 A blank spectrum of hydrogen
peroxide in water is shown for comparison purposes, although
this spectrum did not interfere with the bands under study. These
conditions simulate reaction conditions; when tungstic acid acts
as catalyst, there is an excess of hydrogen peroxide with respect
to (HO)2WO2. The H2O2 treated samples show different spectra
(Figure 4). Two new bands at 265 and 325 nm are observed
for the W(κ2-O-O)2(H2O)2(O) sample. These new bands are
attributed to the presence of peroxo moieties and coordinated
water molecules around the W center. But, the absorption band
at 325 nm was absent when phenyl phosphonic acid was added.
These observations and the differences observed between W(κ2-
O-O)2(H2O)2(O) and W(κ2-O-O)2(H2O)(κ1-O-PhP(O)(OH)2)(O)
samples are consistently simulated by TD-DFT results.
Theoretical results allow not only the assignment of experi-
mental absorption bands, but also to obtain a deeper knowledge
of the nature of the transition involved in the absorption. This
information can be gained by analyzing the molecular orbitals
implied in the transition. The orbitals with the highest participa-
tion in the less energetic transition are depicted in Figure 5.
For the W(κ2-O-O)2(H2O)2(O) complex, there is a HOMO-
LUMO electron jump, in which the peroxo oxygens donate their
electronic density to the tungsten atom during the transition.
Thus, the observed electronic transition is due to an absorption
in which the peroxo groups are involved. The peroxo moieties
are needed to obtain the UV-vis spectra. Therefore, not only
the experimental shift of the absorption band supports the
existence of the peroxo complex but also the theoretical results
are a good indicator of its presence.
The spectrum of the complex between tungstic acid and
phenylphosphonic acid treated with hydrogen peroxide showed
some clear differences (Figure 2). The ν(O-O) vibration appears
as a clear band at around 830 cm-1 22
. Additional bands around
560 cm-1 belong to νasW(O-O) and νsW(O-O).22 A series of
peaks associated with phenylphosphonic acid are also present
in addition to the band at 950 cm-1 assigned to the WdO
vibration.21,22
DFT calculations aimed at describing the vibrational structure
of the three species by way of a good agreement with the
experimental infrared spectra (Figure 2). The main differences
between experimental and theoretical vibration bands can be
attributed to the interaction between clusters in the solid, which
cannot be taken in the model employed in the calculations.
However, the infrared spectra of the complex between tungstic
acid and phenylphosphonic acid and its peroxo compound are
clearly predicted with the DFT models.
UV-vis spectra of the samples dissolved in water are
compared in Figure 3. The electronic spectra of (HO)2WO2
exhibited a band at 210 nm, which is characteristic of this
compound. The spectrum of phenylphosphonic acid is quite
similar with a band at 215 nm typical of the electronic spectrum
of aromatic compounds. When both compounds are present in
solution, a change in the spectrum is clearly observed: the band
shifts to a lower wavelength as a consequence of the interaction
between the two compounds. A weak band around 270 nm
present in the three spectra could be attributed to the presence
of an anionic/dissociated species of the acids. These experi-
mental observations are satisfactorily described by theoretical
calculations, and the shift to lower wavelengths observed in the
catalyst and cocatalyst cluster spectra is clearly predicted. The
A similar behavior was obtained for the W(κ2-O-O)2(H2O)-
(κ1-O-PhP(O)(OH)2)(O) complex. The first transition corre-
sponds to a HOMO-LUMO jump, where the electronic density
from the peroxo oxygens was transferred to the whole molecule.
The π electronic cloud was received by the neighboring W atom
but also by the distant aromatic structure from the phenylphos-
phonic acid. The probability, and hence the intensity, of an
electronic transition is proportional to the overlap between the

Tungstic Acid Peroxocomplex Spectroscopic/DFT Study
J. Phys. Chem. A, Vol. 111, No. 11, 2007 2171
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Conclusion
The good correlation found between experiment and theory
underlines the importance of studying this catalytic system from
both experimental and theoretical points of view. The structures
obtained by DFT calculations are in full agreement with
experimental XRD data. The validation of the proposed model
by the structural characterization allows trustworthy analysis
of theoretical spectroscopic data. Hydrogen peroxide reacts with
tungstic acid to form a peroxo complex, which can be character-
ized experimentally by UV-vis and IR spectroscopy. The
presence of coordinated water, as considered in the molecular
model proposed, is corroborated by the experimental spectra.
DFT calculations are able to reproduce the experimental UV-
vis and IR spectra. They can also provide additional information
about the structure of the observed complexes.
Acknowledgment. CTI (CSIC) and CESGA are acknowl-
edged for providing CPU time. L.B. gratefully acknowledges a
fellowship granted by Repsol-YPF (Spain).
Supporting Information Available: Cartesian coordinates
for all optimized structures in the gas phase and the basis set
input for the W atom. This material is available free of charge
via the Internet at http://pubs.acs.org.
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