Catalytic efficacy of an oxido-peroxido tungsten(VI) complex: Synthesis, X-ray structure and oxidation of sulfides and olefins

Article abstract of DOI:10.1080/00958972.2013.794383

An oxido-peroxido tungsten(VI) complex [WO(O₂)L(CH ₃₀H)] using salicylidene benzoyl hydrazine as a tridentate ONO donor Schiff base (H₂L) has been synthesized and characterized by elemental analysis, IR, 1H NMR, molar conductance data, and single-crystal X-ray analysis. The complex was used as a catalyst for epoxidation of olefins and oxidation of sulfides. The results show that epoxides and sulfoxides were produced in high yield, turnover number, and selectivity.

Full text of DOI:10.1080/00958972.2013.794383

Journal of Coordination Chemistry, 2013
Vol. 66, No. 11, 1897–1905, http://dx.doi.org/10.1080/00958972.2013.794383
Catalytic efficacy of an oxido-peroxido tungsten(VI) complex:
synthesis, X-ray structure and oxidation of sulfides and
olefins
MOJTABA AMINI*†, MOJTABA BAGHERZADEH*‡, BAGHER EFTEKHARI-SIS†,
ARKADY ELLERN§ and L. KEITH WOO§
†Faculty of Science, Department of Chemistry, University of Maragheh, Maragheh, Iran
‡Chemistry Department, Sharif University of Technology, Tehran, Iran
§Chemistry Department, Iowa State University, Ames, IA, USA
(Received 7 September 2012; in final form 23 January 2013)
An oxido-peroxido tungsten(VI) complex [WO(O2)L(CH3OH)] using salicylidene benzoyl
hydrazine as a tridentate ONO donor Schiff base (H2L) has been synthesized and characterized by
elemental analysis, IR, 1H NMR, molar conductance data, and single-crystal X-ray analysis. The
complex was used as a catalyst for epoxidation of olefins and oxidation of sulfides. The results
show that epoxides and sulfoxides were produced in high yield, turnover number, and selectivity.
Keywords: Oxido-peroxido complex; Tungsten; Olefin epoxidation; Sulfide oxidation
1. Introduction
Oxido and peroxido transition metal compounds have played an important role in oxida-
tion of organic substrates. A variety of peroxido complexes of various metals are known to
catalyze the oxidation of olefins, arenes, phenols, alcohols, phosphines, and sulfides [1–3].
The catalytic activity of peroxido metal complexes is influenced by the type of metal, the
number of peroxido ligands attached to the catalyst and the nature of the remaining ligands
in the coordination sphere [3–6].
Current interest in tungsten is due to the importance of the metal as an essential trace
element that participates in a number of important enzymatic reactions [7]. There are a
few reports on preparation and catalytic activity of oxido-peroxide tungsten(VI) complexes
in oxidation reactions [8–10].
We recently reported a highly efficient method for oxidation of olefins and sulfides by
using an oxido-peroxido Mo(VI) complex [MoO(O2)L(CH3OH)] with salicylidene benzoyl
hydrazine (H2L) as ligand [11]. In order to study the effect of metal on catalytic reactivity,
herein we report the synthesis, characterization, and comparative catalytic activity of a
new oxido-peroxido W(VI) complex [WO(O2)L(CH3OH)] in oxidation of olefins and
sulfides under the same condition as given in our previous work (scheme 1).
*Corresponding authors. Email: mamini@maragheh.ac.ir (M. Amini); bagherzadeh@sharif.edu (M. Bagherzadeh)
Ó 2013 Taylor & Francis

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Scheme 1. Epoxidation of olefins and oxidation of sulfides in the presence of [WO(O2)L(CH3OH)].
2. Results and discussion
2.1. Complex characterization
[WO(O2)L(CH3OH)] can be synthesized in high yield by treating a H2O2 solution of
freshly precipitated tungsten(VI) oxide hydrate, WO3·nH2O, with the Schiff base salicylid-
ene benzoyl hydrazine.
The oxido-peroxido W(VI) complex is yellow, stable to air and light and soluble in
methanol, ethanol, acetonitrile, methylene chloride, DMF, and DMSO. The analytical data
for the new complex agree very well with the proposed molecular formula.
The IR spectrum shows a broad absorption at 3129 cmÀ1 attributed to ν(O–H) of a methanol
coordinated to tungsten and a strong absorption at 1604 cmÀ1 assigned to ν(C=N) of the imine.
The ν(W=O) and ν(O–O) bands appear, respectively, at 960 cmÀ1 and 885 cmÀ1 [8, 12].
1
The H NMR spectrum of the complex has been recorded in d6-DMSO. A sharp singlet
for OH proton of phenolic group of the ligand at δ = 11.54 ppm is absent in the complex
and also a significant downfield shift (Δδ = 0.94 ppm) of the signal for the azomethine
(ÀCH = NÀ) proton in the complex relative to the corresponding ligand demonstrates the
coordination through the oxygen of the deprotonated phenol and the azomethine nitrogen
[11, 13]. The complex shows multiplets at δ 6.75–8.60 ppm due to the aromatic protons.
1
Coordinated methanol displays two signals at δ = 3.17 and 4.08 ppm in the H NMR spec-
trum of complex [14, 15].
To confirm coordination of the Schiff base, as well as the stereochemistry of the com-
plex, the structure has been determined by X-ray crystallography. An ORTEP view is
shown in figure 1. The crystal lattice is stabilized by intermolecular hydrogen O–H…N
bonds between hydrogen of coordinated methanol and N2 of Schiff base ligand from
neighboring complexes to form an infinite 1-D polymeric array along the α axis (figure 2).
The summary of single-crystal X-ray structure refinement is shown in table 1. The com-
plex crystallizes in the monoclinic crystal system and space group P21/c. The structure is
mononuclear, consisting of discrete monomeric units of [WO(O2)(L)(CH3OH)], with a dis-
torted pentagonal-bipyramidal arrangement around tungsten. The structure shows that the
ligand is coordinated to tungsten in the expected tridentate O, N, and O in addition to
oxido, peroxido, and one methanol. The ligand binds the metal center at O, N, and O
forming five-membered and six-membered chelate rings with a bite angle of O(1)–W(1)–N
(1) = 72.15(9)° and O(2)–W(1)–N(1) = 81.44(10)° and a bond length of W(1)–O(1) = 2.013
(2) (Å), W(1)–N(1) = 2.172(2) (Å), and W(1)–O(2) = 1.967(2) (Å).

Oxido-peroxido tungsten(VI)
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Figure 1. ORTEP diagram of [WO(O2)L(CH3OH)] with thermal ellipsoids drawn at 50% probability.
Figure 2. Depicting intermolecular hydrogen bonded 1-D polymeric array along the a axis for [WO(O2)(L)
(CH3OH)].

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Table 1. Crystal data and structure refinement for [WO(O2)L(CH3OH)].
Empirical formula
Formula weight
Temperature
C15H14N2O6W
502.12
173(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
P 21/c
Unit cell dimensions
a = 10.830(8) Å, α = 90°
b = 9.872(8) Å, β = 105.325(9)°
c = 14.611(11) Å, γ = 90°
1507.(2) Å3
Volume
Z
4
Density (Calcd)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to h = 28.70°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
2.209 Mg cmÀ3
7.704 mmÀ1
956
0.12 Â 0.24 Â 0.24 mm3
1.95–28.70°
À14 6 h 6 14, À13 6 k 6 13, À19 6 l 6 18
14,336
3763 [R(int) = 0.0433]
96.6%
Multiscan
0.4583 and 0.2593
Full-matrix least-squares on F2
3763/0/218
1.095
R1 = 0.0228, wR2 = 0.0521
R1 = 0.0242, wR2 = 0.0526
2.111 and À1.653
2
Rint = Σ|Fo À Fo2(mean)|/Σ[Fo2].
R1 = Σ||Fo| À |Fc||/Σ|Fo|.
GOOF = S = {Σ[w(Fo À Fc2)2]/(n À p)}1/2
.
2
wR2 = {Σ[w(Fo À Fc2)2]/Σ[w(Fo2)2]}1/2
.
2
w = 1/[σ(Fo2) + (aP)2 + bP], where P is [2Fc2 + Max(Fo2, 0)]/3.
2.2. Catalytic reactivity
Selective oxidation of sulfides to the corresponding sulfoxides is a fundamental transfor-
mation both in laboratory synthesis and industrial production [16]. In order to compare cat-
alytic reactivity of complex with oxido-peroxido Mo(VI) complex, we used the same
optimization condition [11].
[WO(O2)(L)(CH3OH)] was a catalyst for the oxidation of various sulfides and olefins. A
series of structurally diverse sulfides were subjected to the oxidation using the complex as
catalyst and UHP as oxidant. Aliphatic and aromatic sulfides underwent clean and selective
oxidation to the corresponding sulfoxide under air, in impressive yields and turnover num-
ber (TON). The results are summarized in table 2. Aromatic sulfides undergo oxidation
more easily than aliphatic substrates. Significantly, the maximum selectivity of 88–100%
was achieved at 81–100% of conversion for the substrates (table 2).
To gain insight into the catalytic mechanism, reaction progress was monitored using
UV–vis spectrometry (figure 3). Monitoring the UV–vis spectra of the catalytic reaction
mixture shows no evident changes in absorption peaks, which suggests that the oxidation
state of the W(VI) moiety remains unchanged in the reaction. According to this study and
also by comparison with the literature, a mechanism for oxidation of various sulfides to
sulfoxides using [WO(O2)L(CH3OH)] was proposed (scheme 2). The reaction proceeds
with nucleophilic attack of the uncoordinated sulfide on one oxygen of the peroxido,

Oxido-peroxido tungsten(VI)
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Table 2. Oxidation of sulfides catalyzed by [WO(O2)L(CH3OH)]a.
Substrate
S CH3
Conversion (%)b (TON)c
100 (93)
Selectivity (%)d
94
100 (93)
100 (93)
100 (93)
100 (93)
92
90
88
95
S CH2CH3
S C
H2
C S C
H2 H2
S
CH3CH2–S–CH2CH3
CH3(CH2)2–S–(CH2)2CH3
CH3(CH2)7–S–(CH2)7CH3
81 (76)
83 (77)
83 (77)
100
100
100
aThe molar ratios for catalyst: substrate: UHP are 1 : 93 : 167. The reactions were performed in 70 : 30 mixture of
CH3OH /CH2Cl2 (1.2 mL) under air at room temperature.
bThe GC conversion (%) are measured relative to the starting sulfide after 35.25 min.
cTON = (mM of sulfoxide + mM of sulfone)/mM of catalyst.
dSelectivity to sulfoxide = (sulfoxide %/(sulfoxide % + sulfone %)) Â 100.
Figure 3. UV–vis spectra of [WO(O2)L(CH3OH)] solution (5 Â 10À4 M) in 70 : 30 mixture of CH3OH/CH2Cl2
(1/1 v/v), Inset shows the electronic spectrum of reaction mixture (catalyst : UHP : SMe2; 1 : 100 : 200) in the same
solvent which does not show any changes during the time.

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M. Amini et al.
L= salicylidene benzoyl hydrazine
L
O
O
W
O
R
R
S
R'
R'
OH2
O
S
L
UHP
O
W
O
Scheme 2. Plausible mechanism for catalytic oxidation of sulfides to sulfoxides with UHP as an oxidant.
Table 3. Catalytic oxidation of olefins catalyzed by [WO(O2)L(CH3OH)]a.
Substrate
Product
Conversion (%)b (TON)c
93 (277)
Selectivity (%)d
100
O
100 (298)
85 (253)
97 (289)
93 (277)
100 (298)
91 (271)
98 (292)
100
100
100
100
100
100
100
O
O
OH
OH
O
CH3
O
CH3
O
O
O
aThe molar ratios for catalyst: substrate: NaHCO3 : H2O2 are 1 : 298 : 117 : 1170. The reactions were performed in
(70 : 30) mixture of CH3OH/CH2Cl2 (1.2 mL) under air at room temperature.
bThe GC conversion (%) are measured relative to the starting olefin after 74.5 min.
cTON= (mM of product)/mM of catalyst.
dSelectivity to epoxide = (epoxide %/(epoxide % + aldehyde %)) Â 100.
yielding sulfoxide and generating the related molybdenum(VI) dioxido complex. The pri-
mary catalyst is then regenerated with reaction of dioxido complex and UHP oxidant,
which reenters the cycle.

Oxido-peroxido tungsten(VI)
1903
The epoxidation of olefins to epoxides is a valuable transformation for organic synthesis
since epoxides are highly important intermediates in the preparation of various chemically
and biologically active compounds [17–19]. High-valent metal oxido-peroxido species
have demonstrated the ability to catalyze the epoxidation of alkenes, via homogeneous as
well as heterogeneous routes [10, 20]. Therefore, this study was further extended to the
epoxidation of several different linear and cyclic olefins using the complex (table 3). The
complex showed high potential and selectivity as catalyst in the epoxidation of olefins at
room temperature in the presence of NaHCO3 as a promoter and H2O2 as a terminal
oxidant. Indene and cyclooctene could be almost quantitatively converted into the
corresponding epoxides after a reaction time of 74.5 min. The result of catalytic studies
using oxido–peroxido W(VI) catalyst reveals that the efficiency of [WO(O2)L(CH3OH)]
toward all substrates is almost the same with maximum conversion (85–100%), TON, and
selectivity (100%).
The results have shown that [MoO(O2)(L)(CH3OH)] and [WO(O2)(L)(CH3OH)] are
almost equally effective in catalysis of sulfide oxidation and epoxidation of olefins.
3. Experimental
Chemicals and solvents were purchased from Fluka and Merck Chemical companies. Salicy-
lidene benzoyl hydrazine (H2L) has been synthesized according to published procedure [21].
Elemental analysis (carbon, hydrogen, and nitrogen) was obtained from a Carlo ERBA
Model EA 1108 analyzer. FT-IR spectrum was obtained using a Unicam Matson 1000 FT–
IR spectrophotometer using KBr disks at room temperature. Molar conductance was deter-
mined in methanol (ca. 10À3 M) at room temperature using a Toa CM 405 conductivity
1
meter. The H NMR spectra of the complex in d6-DMSO were obtained using a Bruker
FT–NMR 500 MHZ spectrometer. The products of oxidation reactions were determined
and analyzed by HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary col-
umn (phenyl methyl siloxane 30 m…320 lm…0.25 lm) and flame-ionization detector.
3.1. Preparation of [WO(O2)L(CH3OH)]
An aqueous solution (10 cm3) of Na2WO4·2H2O (0.825 g, 2.5 mM) was acidified with HCl
solution (6 M), and a white precipitate of WO3·nH2O was obtained. It was filtered off and
washed several times with water and ethanol. The precipitate was transferred quantitatively
into a beaker and dissolved in H2O2 (30%, 5 cm3) by stirring at room temperature (25 °C)
to get a clear and colorless solution. Salicylidene benzoyl hydrazine (H2L, 2.5 mM) dis-
solved in a minimum volume of methanol (ca. 15 cm3) was added to the above solution
with stirring (5 min), and a light-yellow solid separated out. The solid was filtered off,
washed thoroughly with water, methanol, and diethyl ether, and dried in vacuo. The com-
pound crystallized from CH3OH to give yellow crystals. Yield 0.93 g (74%). C15H14N2O6W
(Mw = 502.12): Calcd C 35.05, H 2.79, N 5.62; found C 35.1, H 2.76, N 5.62.
3.2. General procedures for epoxidation of olefins
To a solution of olefins (0.28 mM), NaHCO3 (0.11 mM), and catalyst (9.4 Â 10À4 mM)
in MeCN (0.5 mL) was added H2O2 (1.1 mM, 30% H2O solution) as oxidant. After the

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M. Amini et al.
reaction was over, for products analysis, the solution was subjected to multiple ether
extraction and the extract was concentrated to 0.5 cm3 by distillation in a rotary evaporator
at room temperature, and then, a sample (2 μL) was taken from the solution and analyzed
by GC. The retention times of the peaks were compared with those of commercial stan-
dards, and chlorobenzene was used as an internal standard for GC yield calculation.
3.3. General procedures for oxidation of sulfides
To a solution of sulfide (0.28 mM) and catalyst (0.003 mM) in 70 : 30 mixture of CH3OH/
CH2Cl2 (1.2 mL) was added urea hydrogen peroxide (UHP) (0.5 mM) as oxidant. The mix-
ture was stirred at room temperature, and the progress of the reaction was monitored by
GC by removing small samples of the reaction mixture. To establish the identity of the
products unequivocally, the retention times and spectral data were compared to those of
commercially available compounds using chlorobenzene as an internal standard.
4. Conclusion
We have synthesized and characterized an oxido-peroxide W(VI) complex and have used
it as catalysts in olefin and sulfide oxidation. The complex is catalytically active in the oxi-
dation of sulfides and epoxidation of olefins to their respective oxidation products under
homogeneous conditions. This study affords new impetus to designing new oxido-peroxido
W(VI) complexes as homogeneous oxidation catalysts.
Supplementary material
The CIF file of crystal structure of complex has been deposited with the CCDC, No.
794733. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: +44 1223–336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
MB thanks the Research Council of Sharif University of Technology for financial support
of this work and LKW thanks the National Science Foundation for funding of this work.
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