A novel 12-molybdovanadate nanocluster: Synthesis, structure investigation and its application as an efficient heterogeneous sulfoxidation catalyst

Article abstract of DOI:10.1016/j.inoche.2017.07.002

Tetraethylammonium salt of a new Keggin-type 12-molybdovanadate nanocluster, [(C₂H₅)₄N]₄[VMo₁₂O₄₀] (1) was synthesized via reaction between sodium tungstate, ammonium vanadate and tetraethylammonium bromide in acidic medium. Compound 1 was characterized by X-ray crystallography, FT-IR, UV–Vis and cyclic voltammetry and then applied as an efficient heterogeneous catalyst to oxidation of various organosulfides to sulfoxides with H₂O₂ at room temperature with 81–100% conversion and 60–99% selectivity. Nanocluster 1 was also shown to display excellent recyclability – it can be reused more than 10 times.

Full text of DOI:10.1016/j.inoche.2017.07.002

Accepted Manuscript
A novel 12-molybdovanadate nanocluster: Synthesis, structure
investigation and its application as an efficient heterogeneous
sulfoxidation catalyst
Mojtaba Amini, Ayda Sheykhi, Hadi Naslhajian, Arshad Bayrami,
S. Mojtaba Bagherzadeh, Małgorzata Hołyńska
PII:
DOI:
Reference:
S1387-7003(17)30479-3
doi: 10.1016/j.inoche.2017.07.002
INOCHE 6699
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
3 June 2017
30 June 2017
2 July 2017
Please cite this article as: Mojtaba Amini, Ayda Sheykhi, Hadi Naslhajian, Arshad
Bayrami, S. Mojtaba Bagherzadeh, Małgorzata Hołyńska , A novel 12-molybdovanadate
nanocluster: Synthesis, structure investigation and its application as an efficient
heterogeneous sulfoxidation catalyst, Inorganic Chemistry Communications (2016), doi:
10.1016/j.inoche.2017.07.002
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ACCEPTED MANUSCRIPT
A novel 12-molybdovanadate nanocluster: synthesis, structure investigation
and its application as an efficient heterogeneous sulfoxidation catalyst
Mojtaba Amini,a Ayda Sheykhi,a Hadi Naslhajian,a,b Arshad Bayrami,c S. Mojtaba
Bagherzadeh,c Małgorzata Hołyńskad
aDepartment of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran.
bDepartment of Chemistry, University of Tehran, Tehran, Iran.
c Chemistry Department, Sharif University of Technology, Tehran, P.O. Box 11155-3615, Iran
dFachbereich Chemie and Wissenschaftliches Zentrum für Material wissenschaften (WZMW), Philipps-
Universität Marburg, Marburg, Germany.
Abstract
Tetraethylammonium salt of
a
new Keggin-type 12-molybdovanadate nanocluster,
[(C2H5)4N]4[VMo12O40] (1) was synthesized via reaction between sodium tungstate, ammonium
vanadate and tetraethylammonium bromide in acidic medium. Compound 1 was characterized by
X-ray crystallography, FT-IR, UV-Vis and cyclic voltammetry and then applied as an efficient
heterogeneous catalyst to oxidation of various organosulfides to sulfoxides with H2O2 at room
temperature with 81–100% conversion and 60–99% selectivity. Nanocluster 1 was also shown to
display excellent recyclability – it can be reused more than 10 times.
Keywords: 12-molybdovanadate, nanocluster, sulfoxidation
Introduction
Metal–oxygen clusters, namely polyoxometalates (POMs), are anionic clusters of oxygen-
bridged early transition metal cations in their highest oxidation states (configuration d0 or d1)
with (heteropolyoxometalates) or without (isopolyoxometalates) heteroatoms (such as B, Al, Ge,
*
Corresponding author. Tel.: +98 41 37278900; Fax: +98 41 37276066; E-mail address:
mamini@maragheh.ac.ir (M.A.)
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Si, P and many more) [1-11]. Importance of this class of nanosized inorganic compounds is due
to their unmatched properties such as highly tunable nature, molecular and electronic structural
diversity, as well as extreme variability of compositions that leads to applications in the fields of
materials science, catalysis, medicinal chemistry, energy storage, electrochemistry, magneto
chemistry and nano-materials [12-23]. One set of important properties of POMs is the ability to
proceed in multiple, consecutive and reversible multi-electron charge transfer reactions, tunable
acidity and redox properties, inherent resistance to oxidative decomposition, high thermal
stability and impressive sensitivity to light and electricity. Therefore these fascinating properties
of POMs lead to the applications of them as promising catalysts [24, 25].
Sulfoxides and sulfones are the most important intermediates for preparing of many organic and
biologically relevant compounds. They are applied in synthetic chemistry, medicine, biology,
catalysis and material science. One of the best routes to the synthesis of sulfoxides and sulfones
is oxidation of organosulfur compounds [26-28]. Catalytic oxidation (especially selective
oxidation) is of basic importance in the development of chemical synthesis [29-31]. Most of the
oxidation processes require catalysts; among them metal-based catalysts (e.g. POMs) are the
most notable [24]. The use of ecologically safe oxidants such as molecular oxygen from air and
hydrogen peroxide, which are low-waste-producing, to selective oxidation organic compounds
has attracted great interest [26, 29, 32]. The combinations of these green oxidants and POMs can
oxidize carbon, nitrogen and as well as sulfur sites in organic compounds [24]. In addition, use
of heterogeneous POMs catalysts can be a response to the problem of catalyst reusability in
catalytic oxidations with hydrogen peroxide [33].
Herein, we report the synthesis and characterization of a novel 12-molybdovanadate nanocluster,
[(C2H5)4N]4[VMo12O40] (1) by a simple synthetic procedure. Further application of 1 as an
efficient heterogeneous catalyst toward sulfide oxidation to sulfoxide under mild and green
conditions is presented.
Experimental
Chemicals and materials
All chemicals and solvents were obtained from commercial sources without further purification.
Distilled water was used in the present work.
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Preparation of [(C2H5)4N]4[VMo12O40] (1)
[(C2H5)4N]4[VMo12O40] was synthesized as follows: a solution of Na2MoO4·2H2O (0.5g, 2.07
mmol), NH4VO3 (0.25 g, 2.14 mmol) and concentrated HClO4 (2.5 mL) in a 40 mL mixture of
acetonitrile:water (1:1) was heated at 70 °C for 24 h. To the filtrate tetraethylammonium bromide
(0.43 g, 2.05 mmol) was added and the obtained orange-yellow precipitate was washed with
water and ethanol several times, and then it was dissolved in acetonitrile. After several days,
slow evaporation of this solution at room temperature resulted in yellow crystals. Single crystals
were separated and one of them was selected for an X-ray diffraction study.
Typical procedure for the oxidation of sulfide
The oxidation reaction was carried out in a glass tube as the reaction vessel with a magnetic
stirring bar. We chose oxidation of methylphenyl sulfide (MPS) by 30% H2O2 in the presence of
1 as a model reaction. To a mixture of 0.002 mmol of 1 and 0.5 mmol sulfide in 1 mL mixture of
CH3OH/CH2Cl2 (1:1) 1 mmol H2O2 was added dropwise at room temperature and immediately
the time of reaction was counted. The reaction progress was monitored by TLC and finally
analyzed by gas chromatography. For the investigation of the influence of the reaction
conditions, amount of catalyst, type and amount of oxidant, solvent type and reaction time were
varied.
Results and Discussion
Complex characterization
The crystal data and structural refinements of [(C2H5)4N]4[VMo12O40] are summarized in Table
1. Compound 1 crystallizes in the monoclinic space group P21/n and as shown in Fig. 1. It
comprises four Et4N+ cations per one [VMo12O40]4- anion. Single crystal X-ray diffraction
studies demonstrate that the anion contains a {Mo12O36} shell that encapsulates a {VO4} moiety
in its center. Nevertheless, there is a common problem for POMs clusters, and that is the four
oxygen atoms in the central VO4 tetrahedron that are disordered over eight positions [34, 35].
In the compound 1 the distances of each of V-O bonds are from 1.58 to 1.62 Å and the average
length of Mo-Ot bonds is 1.65 Å (range from 1.63(1) to 1.66(1) Å). The bond lengths of multiple
bridging oxygen with molybdenum vary from 1.80(1) to 2.45(2) Å with average length of 2.07
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Å. The angles of O-V-O bond around the central V atom, involving its surrounding oxygen
atoms, are from 69.6 to 180.0º that demonstrate there is a distortion tetrahedral geometry of
vanadium coordination sphere [36].
Fig. 1. Ball-and-stick representation of the Keggin anion [VMo12O40]4-
Table 1. Selected X-ray data for [(C2H5)4N]4[VMo12O40]
[(C2H5)4N]4[VMo12O40]
Formula
C32H86Mo12N4O44V
2433.26
Formula weight
Temperature [K]
100(2)
0.71073
Monoclinic
P21/n
14.236 (6)
13.248 (6)
17.522 (8)
96.69(2)
3282.12
λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
β [°]
V [Å3]
Z, ρcalc [g cm-3]
2, 2.462
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μ [mm-1]
F(000)
rflns: total
Abs. corr.
2.445
2370.0
5779
numerical
Min., max.
transmission factors
R1 [I > 2σ(I)]
0.646,0.746
0.0713
0.2166
wR2 (all data)
In order to study of the electrochemical properties of nanocluster 1, the cyclic voltammetric
behavior of 1 in a 0.5 M H2SO4 + 0.5 M Na2SO4 solution was investigated. As shown in Fig. 2.
three pairs of redox peak are observed in the potential range +0.8 to -0.5 V. Redox peaks I–I’, II–
II’ and III–III’ ascribed to the reduction and oxidation processes of Mo centers. Because of the
weak signal of the V atom in the compound 1, this signal embedded in the reduction and
oxidation peaks of Mo and we could not observe redox peaks of V [34, 37].
Fig. 2. Cyclic voltammogram of 1 in a 0.5 M H2SO4 + 0.5 M Na2SO4 aqueous solution at a scan rate of
100 mV/s.
Fig. 3 shows UV-visible spectrum for 0.05 mM of 1 in acetonitrile that are measured from 200 to
600 nm. Nanocluster 1 salt is insoluble in protic solvents such as water, methanol and ethanol,
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and is soluble in polar aprotic solvents for example acetonitrile and acetone. It was found two
absorptions in UV-visible of compound 1 solution. The sharp absorption at 219 nm is attributed
to Ot→Mo charge-transition absorption and weaker absorption at 300 to 350 nm is corresponded
to Mo→Oc and Mo→Oe charge transfer transitions [36].
Fig. 3. UV-visible spectrum of 0.05 mM 1 in acetonitrile. Path length; 1.0 cm.
Catalytic effects
We investigated the catalytic effects of 1 for the oxidation of MPS with H2O2 at room
temperature as a model reaction. To find optimum reaction conditions the oxidation of MPS
varying factors such as the amount of the catalyst, type of solvent, oxidant type and amount and
reaction time were studied and the results are shown in Table 2. The influence of the catalyst
amount (0.0004, 0.001, 0.002 and 0.003 mmol of 1) on oxidation of MPS was studied (entries 1-
4). It was found that 0.002 mmol of catalyst results in excellent conversion and selectivity (100%
and 82%, respectively). Solvent type has remarkable effect on oxidation of sulfides, thus
influence of various solvents was studied (entries 5-10). Ethyl acetate resulted in the lowest
conversion (42%) and conversion of MPS in dichloromethane, acetonitrile, methanol, ethanol
and water was from 54 to 85%. The mixture of MeOH:CH2Cl2 (1:1) yielded the best conversion
(100%) and 82% sulfoxide selectivity, thus was chosen as reaction solvent. We also studied
various oxidants effect in sulfoxidation of MPS. UHP yielded low conversion (42%) but tert-
butyl hydroperoxide (TBHP) and hydrogen peroxide could completely convert MPS to product,
but selectivity of sulfoxide in H2O2 was more than for TBHP (82% vs. 78%). Comparison of the
entries 13 and 14 with the entry 3 demonstrates the importance of the oxidant’s amount. When
the amount of H2O2 was decreased from 1 to 0.5 mmol, the conversion of MPS decreased
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drastically from 100 to 67%, but with a further increase of H2O2 to 1.5 mmol, the selectivity to
sulfoxide decreased from 82 to 64%.
Table 2. Oxidation of MPS under various conditions, catalyzed by 1a
Entry
Cat.
amount
(mmol)
0.0004
0.001
0.002
0.003
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
Solvent
Oxidant
Oxidant
amount (mmol)
Conversionb Selectivityc
(%)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
MeOH/CH2Cl2
MeOH/CH2Cl2
MeOH/CH2Cl2
MeOH/CH2Cl2
CH2Cl2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
UHP
TBHP
H2O2
H2O2
1
1
1
1
1
1
1
1
1
1
1
1
0.5
1.5
64
95
100
99
85
54
81
42
62
56
42
100
67
96
89
85
82
80
89
92
88
83
87
61
71
78
99
64
CH3CN
MeOH
EtOAc
EtOH
H2O
MeOH/CH2Cl2
MeOH/CH2Cl2
MeOH/CH2Cl2
MeOH/CH2Cl2
a Reaction conditions: MSB (1 mmol), solvent (1 mL), time 150 min., r.t.
b The GC yield (%) is measured relative to the starting sulfide.
c Selectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone)) × 100.
Subsequently, different kinds of sulfides oxidation to the corresponding products were
investigated at room temperature. The result of sulfoxidation of various sulfides including
diphenyl sulfide, benzyl phenyl sulfide, dibenzyl sulfide, divinyl sulfide, dimethyl sulfide and
dioctyle sulfide in a mixture of MeOH:CH2Cl2 (1:1) in the presence of 1/H2O2 system is
summarized in Table 3. It was found that excellent conversion and selectivity for sulfoxidation
of all checked sulfides were obtained, so that and yields and sulfoxide selectivities were between
81 to 100% and from 60 to 99%, respectively.
The presence of peroxo-species in our catalyst system and therfore nucleophilic attack of
thioethers on one oxygen atom of the peroxido ligand, leads to the selective oxidation of sulfides
to the corresponding sulfoxides [38]. Selectivities of aromatic sulfoxides were obtained at over
81% (entries 1-4). On the contrary, oxidation of aliphatic sulfides (entries 6 and 7) display
edexcellent selectivities (99%) except for entry 5 (60%).
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Table 3. Oxidation of sulfides to sulfoxides using the 1/H2O2 systema
Conversionb
Selectivityc
Entry
1
Substrate
(%)
(%)
100
82
86
2
81
3
4
91
90
84
81
S
S
5
6
98
99
60
99
H3C
C8H17
CH3
S
7
90
99
C8H17
a Reaction conditions: sulfide (1 mmol), H2O2 (1 mmol), MeOH/CH2Cl2 (1 mL), time: 150 min., r.t.
b The GC yield (%) is measured relative to the starting sulfide.
c Selectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone)) × 100.
The FT-IR spectrum of the fresh and reused 1 is shown in Fig. 4. The peaks in the range of 600-
1100 cm-1 are attributed to vibrations of the Keggin primary structure [39]. The peaks at 794,
872, 950 and 1060 cm-1 correspond to the asymmetry vibration Mo-Oc (corner-sharing oxygen
connecting Mo3O13 units), Mo-Oe (edge-sharing oxygen connecting Mo), Mo=Ot (terminal
oxygen bonding to Mo atom) and V-Oi (internal oxygen connecting V and Mo), respectively.
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Fig. 4. IR spectra of a) pure catalyst 1 b) recycled catalyst 1
Table 4 shows comparison of our catalytic system in the oxidation of MPS by H2O2 with other
recently reported methods. Our catalytic system is superior than other works in terms of reaction
conditions and [(Et)4N]4[VMo12O40]/H2O2 does not endure from the unpleasant reaction
conditions, such as using high amount of catalyst, oxidant (entries 2 and 3 ) and solvent (entries
3
and 4). Also some reported catalytic systems show lower conversion than
[(Et)4N]4[VMo12O40]/H2O2 system (entry 4).
Table 4. Oxidation of methyl phenyl sulfide with different Keggin-type POMs.
Entry Catalyst
Conditions
Conversion Selectivity Ref.
(%)
(%)
82
1
2
[(Et)4N]4[VMo12O40]
0.002 mmol cat., 1 mmol MPS, 100
1 mmol H2O2, 1 mL
This
work
MeOH/CH2Cl2 (1:1), 150 min
(TBA)4[PW11CrO39]
0.0163 mmol cat., 1 mmol
100
-
[40]
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MPS, 6.5 mmol H2O2, solvent
free, 10 min
3
4
H3PMo12O40
3% in mmol cat., 0.7 mmol
MPS, 12 mmol H2O2, 5 mL
CH3CN, 60 min
95
98
98
[41]
[42]
(TBA)4[γ-HPV2W10O40] 0.001 mmol cat., 1 mmol MPS, 80
1 mmol H2O2, 3 mL CH3CN/t-
BuOH (1:1), 30 min
Recycling of the catalyst is one of the most important advantages when heterogeneous catalyst is
used. Fig. 5. shows recyclability of the compound 1 for oxidation of MPS with hydrogen
peroxide for 150 min. at room temperature. In order to ensure repeated use of the compound 1
after each run the catalyst was separated by centrifuging and then washed with methanol and
dried. It was found that the catalyst is able to be used for ten cycles without considerable loss in
conversion. As shown in Fig. 4(b) the IR spectra of the fresh and reused catalyst are the same
and show no vibrational changes that reveal that our catalyst has an excellent stability during the
oxidation process.
Fig. 5. Recycling study of 1 in the oxidation of MPS
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Conclusion
In this work we prepared and characterized a heterogeneous Keggin-type polyoxometalate,
[(C2H5)4N]4[VMo12O40] with metal heteroatom (vanadium). The nanocluster 1 was shown to
display excellent activity toward selective oxidation of sulfides to corresponding sulfoxides by
hydrogen peroxide as an environmentally friendly oxidant at room temperature. The catalyst 1
can be also separated easily with centrifuging and used more than ten times with no significant
loss of its activity.
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Graphical Abstract-P
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ACCEPTED MANUSCRIPT
Graphical Abstract-S
The catalytic reactivity of
a
new Keggin-type 12-molybdovanadate nanocluster,
[(C2H5)4N]4[VMo12O40], on oxidation of sulfides was studied.
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ACCEPTED MANUSCRIPT
Highlights
1) A new Keggin-type 12-molybdovanadate nanocluster was synthesized.
2) Nanocluster is applied for the oxidation of sulfides.
2) Excellent conversion and selectivity has been achieved.
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