Selective oxidation of organosulfurs with a sandwich-type polyoxometalate/hydrogen peroxide system

Article abstract of DOI:10.1016/j.poly.2020.114622

A dimeric tungstophosphate, Na₁₄[Co₄(PW₉O₃₅)₂].28H₂O (1), was synthesized via a simple one-pot method and characterized by single-crystal X-ray crystallography. Nanocluster 1 was exhibited to be able to selective oxidation of aliphatic and aromatic sulfides to sulfoxides in the presence of hydrogen peroxide at room temperature with high performance. Further investigations showed that this catalyst could reuse four times without a significant reduction in its activity for the oxidation of methyl phenyl sulfide.

Full text of DOI:10.1016/j.poly.2020.114622

Journal Pre-proofs
Selective oxidation of organosulfurs with a sandwich-type polyoxometalate/
hydrogen peroxide system
Hadi Naslhajian, Mojtaba Amini, Mohammad Amin Dastyar, Arshad
Bayrami, Mojtaba Bagherzadeh, S. Morteza F. Farnia, Jan Janczak
PII:
S0277-5387(20)30279-5
DOI:
Reference:
https://doi.org/10.1016/j.poly.2020.114622
POLY 114622
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
29 January 2020
31 March 2020
27 May 2020
Please cite this article as: H. Naslhajian, M. Amini, M. Amin Dastyar, A. Bayrami, M. Bagherzadeh, S. Morteza
F. Farnia, J. Janczak, Selective oxidation of organosulfurs with a sandwich-type polyoxometalate/hydrogen
peroxide system, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.2020.114622
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Selective
oxidation
of
organosulfurs
with
a
sandwich-type
polyoxometalate/hydrogen peroxide system
a,b
a,
a
c
Hadi Naslhajian, Mojtaba Amini, Mohammad Amin Dastyar, Arshad Bayrami, Mojtaba
c
b
d
Bagherzadeh, S. Morteza F. Farnia, Jan Janczak
a
Department of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran
b
School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6619, Tehran, Iran
c
Chemistry Department, Sharif University of Technology, P.O. Box 11155-3615, Tehran, Iran.
d
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-
950, Wrocław, Poland
Abstract
A dimeric tungstophosphate, Na [Co (PW O ) ].28H O (1), was synthesized via a simple one-
1
4
4
9
35 2
2
pot method and characterized by single-crystal X-ray crystallography. Nanocluster 1 was
exhibited to be able to selective oxidation of aliphatic and aromatic sulfides to sulfoxides in the
presence of hydrogen peroxide at room temperature with high performance. Further
investigations showed that this catalyst could reuse four times without a significant reduction in
its activity for the oxidation of methyl phenyl sulfide.
Keywords: Sandwich-type POM, Nanocluster, Selective oxidation, Sulfide
1
. Introduction
Polyoxometalates (POMs) as a broad series of metal-oxo clusters [1-5] have widespread
applications in catalytically systems, medical treatments and molecular magnetism because of
their high versatility and chemically tunable natures [6-9]. Among these POMs, lacunary
polyoxoanions are active inorganic ligands that can link to various transition metal or rare-earth
atoms [10]. Heteropolyoxotungstates are common groups of POMs which include W(VI) species
link to oxo ligands. Replacement of W(VI) addenda atoms with d-block transition metal ions
*
Corresponding authors. Tel.: +98 41 37278900; Fax: +98 41 37276066; E-mail address: mamini@maragheh.ac.ir
(
M.A.)
1

produce oxidatively resistant catalytic systems that are attractive compounds in catalysis studies
11]. Because of the thermal stability and catalytic activity of these transition-metal-substituted
[
POMs (TMSPs) in the oxidation of organic compounds, they are employed in some industrial
processes [12]. Sandwich-type POMs as the largest subclass of TMSPs include 2, 3, 4 or 6 metal
centers (such as Cu, Co, Zn, Cd, Mn and Ni) between 2 or 3 lacunary polyoxoanion [13] and
have some applications in magnetism, fluorescence, catalysis and radiochemistry fields [14].
TMSPs can also activate different oxidants to oxidation of a wide-ranging of organic compounds
[
15].
Hydrogen peroxide as an economically beneficial, available, easy to handle, safety in storage and
green oxidant has an important role in oxidation processes [16,17]. Recently, H O as an oxidant
2
2
has been widely applied for the oxidation of sulfides with different nonmetallic (e.g. enzymes,
organocatalysts, …) and metallic (e.g. Mo, Au, Ru, POMs, …) catalysts [18-22]. Nowadays,
catalytic selective oxidation is the basis for the synthesis of a large number of chemicals [23].
Sulfoxides are a valuable segment of synthetic chemical intermediates and the development of
methods for selective oxidation of sulfides to sulfoxides with hydrogen peroxide is still requested
[
24,25].
Herein, in continuation of our studies about the synthesis and application of POMs as oxidative
sulfoxidation catalysts [26,27], we report a cobalt core sandwich-type POM and its H O
2
2
mediated catalytic investigation in the selective oxidation of sulfides reactions.
2
. Experimental
2
.1. Chemicals and materials
All reagents were purchased from commercial sources and were utilized without further
purification.
2
.2 Synthesis of Na [P W Co O ].28H O (1)
14 2 18 4 70 2
The compound 1 was prepared according to the following process: Na WO .2H O (1.00 g, 3.03
2
4
2
mmol), Na HPO .2H O (0.28 g, 1.92 mmol) and Co(NO ) .6H O (0.29 g, 1.00 mmol) were
2
4
2
3 2
2
added to a 10 mL H O containing round-bottom flask. The pH of this mixture was adjusted in
2
6
.4 with 4 M HCl and then heated to 70 °C for 30 min. After cooling to room temperature, the
2

mixture was filtered and the filtrate transferred to an open beaker to evaporate slowly. After two
weeks the purple crystals were formed. The composition of the obtained crystals was checked
with energy dispersive spectrometry (EDS) as well as with a Perkin-Elmer 240 elemental
analyser. EDS spectra were acquired and analysed using an EDAX Pegasus XM4 spectrometer
with SDD Apollo 4D detector mounted on a FEI Nova NanoSEM 230 microscope. Anal. Calc.
for Co H Na O P W (MW = 5553.26 g/mol): W, 59.59; Co, 4.24, Na, 5.80; O, 28.23; P,
4
56
14 98
2
18
1
.12 and H, 1.02%;. Found: W, 60.10; Co, 4.28; Na, 5.72; O, 27.80; P, 1.10 and H, 1.00%.
2
.3. Crystal structure determination
Single-crystal X-ray data collection for 1 were performed using graphite-monochromated Mo-
Kα, λ = 0.71073 Å radiation on a four-circle к geometry KUMA KM-4 diffractometer with a 2D
CCD area detector at 100(1) K. The ω-scan technique with Δω = 1.0º for each image was used
for data collection. One image was used as a standard after every 40 images for monitoring of the
crystal stability and data collection, and no correction on the relative intensity variation was
necessary. Integration, scaling of the reflections, correction for Lorenz and polarization effects
and absorption corrections were performed using the CrysAlis Red program [28]. The structure
was solved by direct methods using SHELXT [29] and refined using SHEXL-2018 program
[
30]. It was not possible to localize the hydrogen atoms of water molecules as solvent molecules.
The final difference Fourier maps showed no peaks of chemical significance. The data collection
parameters, crystallographic data, and final agreement parameters are collected in Table 1. The
structures were visualized by using the DIAMOND 3.0 [31].
2
.4. General procedure for the oxidation of organosulfurs
For a typical procedure in the oxidation of sulfides, nanocluster 1 (0.0005 mmol) was added to a
mixture of the sulfide substrate (0.5 mmol), H O (1.0 mmol) and chlorobenzene (0.4 mmol) as
2
2
an internal standard in methanol (1.0 mL) and then was stirred 150 min at room temperature. The
reaction completion was monitored by Gas Chromatography (GC).
3
. Results and discussion
3
.1. Description of the crystal structure.
3

The common route for the synthesis of sandwich-type polyoxometalates is a preparation of
lacunary species of Keggin or Dawson-type polyoxoanions and then bonding of lacunary units
together by two, three or four transition metal cations [32-35]. Nevertheless,
.
Na [Co W P O ] 28H O was synthesized via a one-pot approach by using sodium tungstate,
1
4
4
18
2
70
2
cobalt nitrate and disodium hydrogen phosphate in the neutral condition. The molecular structure
of purple single-crystal X-ray analysis of compound 1 is given in Fig. 1. The lack of three WO6
3
−
9−
groups from a Keggin-type [PW O ] ion leads to [PW O ] anion as an important lacunary
1
2
40
9
34
2
+
polyoxoanion [10]. Incorporation of four Co cations between two trilacunary Keggin
14-
phosphotungstate obtains a tetranuclear sandwich-type [P W Co O ] anion.
2
18
4
70
Fig. 1. Ball-and-stick represent of sandwich-type structure of nanocluster 1
.
This heteronuclear cluster with the formula of Na [Co W P O ] 28H O crystallizes in the
1
4
4
18
2
70
2
ꢀ
centrosymmetric space group P of the triclinic system. The distances of Co–O bonds in
1
4-
centrosymmetric anionic [Co W Co P O ] nanocluster vary from 2.010 (4) to 2.252 (4) Å
4
18
4
2
70
whereas the W–O bond lengths are in the range of 1.710 (4) and 2.516 (3) Å (terminal oxygen
atoms connecting to tungsten atoms (W–O ) bond lengths are from 1.710 (4) to 1.736 (5) Å with
t
an average length of 1.721 Å in vs. of bridged oxygen atoms between tungsten atoms (W–Ob)
4

that fall from 1.758 (4) to 2.516 (3) Å with mean length of 1.997 Å). The PO tetrahedrons are
4
2
+
located inside the W O hemispheres connected together by four Co cations forming the
9
30
14-
[
Co W P O ] nanocluster. Three oxygen atoms of PO tetrahedron are linked to two W
4 18 2 70 4
atoms while the fourth oxygen atom is linked to the three Co atoms, so the P-O distances are
vary from 1.536 (4) to 1.551 (4) Å with an average distance of 1.543 Å that is shorter than
3
-
corresponding bond distances in the [PW O ] anion. P–O bond lengths in the Keggin-type
1
2
40
3
-
[
PW O ] are in the range of 1.581 (12) – 1.668 (12) Å [36]. Furthermore, O–P–O angles are
12 40
14-
in the range of 108.1 (2) and 111.0 (2)° with a mean angle of 109.3°. The [Co W P O ]
4
18
2
70
+
nanocluster in the crystal are interconnected via Na cations and water molecules forming a 3D-
network (Fig. 2).
Table 1. Experimental details and crystal data of nanocluster 1.
.
Na [Co W P O ] 28H O
1
4
4
18
2
70
2
Empirical formula
Formula weight (g/mol)
Temperature (K)
Wavelength(Å)
Crystal system
Space group
Co H Na O P W
4 56 14 98 2 18
5553.26
100
0.71073
Triclinic
ꢀ
P
Unit cell dimensions(Å/°)
a (Å)
11.4898 (3)
12.7884 (4)
17.2064 (5)
97.921 (3)
106.576 (3)
b (Å)
c (Å)
α (º)

(º)
γ (º)
111.297 (40)
2173.62 (13)
0.23 × 0.19 × 16
3
Volume(Å )
Crystal size [mm]
Z
1
-3
ρcalc [g cm ]
4.242
24.68
2520
-1
μ [mm ]
F(000)
Total / Unique / Obs. Refs [ I > 2σ(I)]
Rint
31382 / 8533 / 7641
0.0325
Abs. corr.
Multi-scan
Min., max. transmission factors
Final R indexes [I > 2σ (I)]
Final R indexes [all data]
goodness_of_fit, S
0.4673 / 1.000
R = 0.0231, wR = 0.0400
1
2
R = 0.0282, wR = 0.0413
1
2
1.001
2.482 / -1.143
ꢀ
3
Largest difference in peak/hole (e. Å )
5

14-
Fig. 2. Packing of [Co W P O ] nanoclusters in the crystal of 1.
4
18
2
70
3
.2. Catalytic effects
In order to test the catalytic activity of compound 1, the oxidation of MPS with H O in methanol
2
2
at room temperature was selected as a model reaction. Effect of several factors in the oxidation
of sulfides such as catalyst amount, type and amount of oxidant, the effect of solvent and
reaction time were investigated and the results were demonstrated in Fig. 3 and Table 2. The
amount of catalyst has dramatically affected the conversion and selectivity of sulfides oxidation
reactions. When the oxidation of MPS was carried out in the presence of hydrogen peroxide with
no catalyst usage, only 3% of the product was formed. The addition of catalyst to the reaction
mixture increased drastically the conversion of MPS to related sulfoxide and sulfone. As shown
in Fig. 3, when the catalyst amount increased from 0.0002 mmol to 0.0005 mmol, the conversion
of MPS to product increased from 73 to 99%, but the sulfoxide selectivity decreased from 81 to
6

7
5%. Further increase in the amount of the catalyst to 0.0008 mmol led a 2% decrease in the
conversion of MPS.
Fig. 3. Effect of catalyst dosage on conversion of MPS and selectivity to methyl phenyl sulfoxide product
Solvent has an important role in the sulfoxidation of MPS. Ethanol, water, 1,2-dichloroethane,
dichloromethane, methanol, 1:1 mixture of dichloromethane and methanol were chosen to
investigate the influence of solvents in the oxidation of MPS (Table 2, entries 1-6). Results
showed that nonpolar solvents (C H Cl and CH Cl ) have low conversions (25 and 32%) due to
2
4
2
2
2
the very low solubility of catalyst 1 in these solvents (entries 3 and 4). In contrast, methanol
illustrated the most performance among the polar solvents with 99% conversion and 75%
selectivity (entry 5). The catalytic activity of 1 with various oxidants was also studied. It was
observed that H O as an oxidant with 99% MPS conversion has much more efficiency than air
2
2
bobbling with 38% and urea hydrogen peroxide with 65% yield for the oxidation of MPS to
related sulfoxide and sulfone (entries 7 and 8). More investigations showed the using 1.0 mmol
of H O is the optimum amount of this oxidant with 99% conversion and 75% sulfoxide
2
2
selectivity. The using of 0.5 and 1.5 mmol hydrogen peroxide resulted 68% and 99%
conversions and 88% and 65% selectivities, respectively (entries 9 and 10). Finally, the influence
of reaction time in the yield and selectivity of the MPS Oxidation reaction at room temperature
was tested. After 45 minutes, MPS conversion and selectivity of methyl phenyl sulfoxide
reached to 78% and 80%, respectively. By increasing reaction time from 45 to 110 min., the
7

reaction completed and the selectivity decreased to 75%. When the reaction continued to 130
min., MPS completely converted to products, but the selectivity reduced to 69% (entries 11-13).
a
Table 2. Oxidation of MPS in various reaction conditions.
Amount of oxidant Time Conversion
Selectivity
Entry
Solvent
EtOH
Oxidant
b
c
(
mmol)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.5
1.0
1.0
1.0
(min)
110
110
110
110
110
110
110
110
110
110
45
(%)
60
82
25
32
99
82
65
38
68
99
78
94
99
(%)
1
2
3
4
5
6
7
8
9
H O2
80
69
90
81
75
62
85
96
88
65
80
76
69
2
H O
H O2
2
2
C H Cl
H O2
2
4
2
2
CH Cl2
H O2
2
2
MeOH
H O2
2
CH Cl :MeOH(1:1)
H O2
2
2
2
MeOH
UHP
O2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
H O2
2
1
1
1
1
0
1
2
3
H O2
2
H O2
2
H O2
90
130
2
H O2
2
a
b
c
Reaction conditions: catalyst: 0.0005 mmol; MPS: 0.5 mmol; solvent: 1.0 mL; r.t. Determined by GC.
sulfoxide selectivity = [sulfoxide%/(sulfoxide% + sulfone%)] × 100
Catalyst 1 was examined in the oxidation of various linear and aromatic substituted
organosulfurs involving of diphenyl sulfide, dibenzyl sulfide, benzyl phenyl sulfide, diethyl
sulfide, diallyl sulfide and dioctyl sulfide. The reaction condition for the oxidation of these
sulfides was chosen based on the MPS sulfoxidation condition (Table 3). For all sulfides,
conversions and selectivities were excellent. Aromatic sulfides catalytic oxidation was led to
8
9% to 99% conversions with 75% to 82% sulfoxide selectivities (entries 1-4) and aliphatic
sulfides are also showed 78-99% and 89-99% conversions and selectivities (entries 5-7),
respectively.
a
Table 3. oxidation of different organosulfurs by nanocluster 1.
b
c
Entry
Substrate
Main product
Conversion (%)
99
Selectivity (%)
75
1
2
85
82
8

3
4
92
89
76
81
5
6
91
99
99
98
7
78
89
a
Reaction conditions: catalyst: 0.0005 mmol; sulfide: 0.5 mmol; MeOH: 1.0 mL; H O : 1.0 mmol; time:
2
2
b
c
1
10 min; r.t. Determined by GC. sulfoxide selectivity = [sulfoxide%/(sulfoxide% + sulfone%)] × 100
3
.3. Recycling study
Recycling ability is a valuable property for catalyst in industrial processes, therefore, reusability
of nanocluster 1 for the oxidation of MPS in the presence of H O at room temperature was
2
2
investigated according to the following route; after completion of the oxidation of MPS reaction,
addition of ethyl acetate to the reaction mixture leads to precipitation of catalyst. After filtration,
the solid residue washed with diethyl ether and ethyl acetate several times and then dried in an
oven at 50 °C and then it used in a similar reaction subsequently with the same condition. This
procedure repeated for four cycles and the results are collected in Fig. 4. After four steps only
5
% decrease was detected in the conversions of MPS. In contrast, sulfoxide selectivity was
increased from 75 to 81%.
9

Fig. 4. Reusability of nanocluster 1 in the sulfoxidation of MPS
As shown in Fig. 5, there are no significant differences between the IR spectra of fresh and
reused catalyst 1 after four steps. In other words, the recycling process of nanocluster 1 displayed
good stability under reaction condition and showed a high catalytic activity without a remarkable
decreasing in the yield of oxidation of MPS.
Fig. 5. FT-IR analysis of a) fresh and b) reused POM 1
1
0

Table 4 shows the activity of some previously reported heteropolyoxometalates in comparison
with catalyst 1 in the oxidation of MPS. The activity of nanocluster 1 is comparable with other
catalysts and in some cases such as using a small amount of solvent (entries 2 and 3), low
catalyst/substrate ratio (entry 4) and short reaction time (entries 2 and 4) are better than others.
Furthermore, the obtained conversion of the reported catalysts (C H N )[Zn W O ], [TBA] [γ-
1
92 240 12
5
19 68
4
HPV W O ] (entries 2 and 3) are respectively 11% and 19% lower than the presented catalyst in
2
10 40
this work.
Table 4. comparison of catalytic activity of different heteropolyoxometalates for the oxidation of MPS
Entry Catalyst
Conditions
Conversion Selectivity Ref.
(
%)
(%)
75
1
2
3
4
Na [P W Co O ].28H O 0.0005 mmol cat., 0.5 mmol 99
present
work
1
4
2
18
4
70
2
MPS, 1.0 mmol H O , 1 mL
2
2
MeOH, 110 min
(C H N )[Zn W O ]
0.0004 mmol cat., 0.4 mmol 88
MPS, 0.4 mmol H O , 2 mL
85
98
82
[37]
[38]
[27]
1
92 240 12
5
19 68
2
2
CHCl , 360 min
3
[TBA] [γ-HPV W O ]
0.001 mmol cat., 1.0 mmol
MPS, 1.0 mmol H O , 3 mL
80
4
2
10 40
2
2
CH CN/t-BuOH 1:1, 30 min
3
[TBA] [VMo O ]
0.002 mmol cat., 1.0 mmol
MPS, 1.0 mmol H O , 1 mL
100
4
12 40
2
2
MeOH/CH Cl 1:1, 150 min
2
2
4
. Conclusion
A sandwich-type polyoxotungstate, Na [P W Co O ].28H O, has been synthesized via a
1
4
2
18
4
70
2
simple route at a neutral medium. Single crystal X-ray crystallography of nanocluster 1 gave
some valuable information about the crystalline structure of this complex. 1/H O catalytic
2
2
system showed high oxidative performance for sulfoxidation of sulfides at room temperature.
Nanocluster 1 needed 110 minutes to converted MPS to methyl phenyl sulfoxide completely
with 75% selectivity. Several aliphatic and aromatic sulfides were also applied to their oxidation
1
1

behavior studying in the presence of catalyst 1. Gas chromatography results revealed 78-99%
conversions and 75-99% selectivities to these sulfides oxidation reactions. A comparison
between FT-IR spectroscopy before and after reusing of compound 1 displayed no obvious
changes in their diagrams and it conformed high stability and activity of catalyst 1 after several
times using.
Acknowledgments
Authors thank the Research Council of the University of Maragheh and the University of Tehran
for financial support of this work.
Supplementary Data
The structure of 1 in the CIF format was deposited in Cambridge Crystallographic Data Center
with deposition number CCDC 1969073. This data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk
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Graphical abstract-S
A new cobalt core (tetra-cobalt-substituted) polyoxotungstate was used as an efficient catalyst
for selective oxidation of organosulfurs
Graphical abstract-P
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Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
CRediT author statement
Hadi Naslhajian: Conceptualization & Methodology, Mojtaba Amini: Supervision,
Mohammad Amin Dastyar: Methodology, Arshad Bayrami: Methodology, Mojtaba
Bagherzadeh: Software 1, S. Morteza F. Farnia; Supervision 2; Jan Janczak; Software 2
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