Copper(ii)-containing tungstotellurates(vi): Syntheses, structures and their catalytic performances in selective oxidation of thioethers

Article abstract of DOI:10.1039/d0ra02609c

We report the syntheses and structures of two new copper(ii)-containing tungstotellurates(vi) Na12[TeVI2W8O38Cu2(H2O)2]·7H2O (Te2W8Cu2) and Na6[TeVIW6O24Cu(NH2CH2CO2)2]·6H2O (TeW6Cu). The two compounds were synthesized by a simple one-pot method and characterized by single-crystal X-ray diffraction (XRD), powder XRD, FT-IR spectroscopy, elemental analysis, and thermogravimetric analysis in the solid state. Furthermore, their catalytic properties for the selective oxidation of thioethers were also studied systematically. The catalytic experiment results indicate that the tungstotellurate(vi) Te2W8Cu2 is an effective heterogeneous catalyst for the selective oxidation of thioethers to sulfoxides or sulfones by an H2O2 oxidant at room temperature. Under the ambient conditions, Te2W8Cu2 can convert 99percent of methyl(phenyl)sulfane to sulfoxides or sulfones with 96percent or 99percent selectivity, respectively, and the utilization rate of H2O2 is up to 80percent. Furthermore, Te2W8Cu2 as a heterogeneous catalyst is stable in the reaction and could be reused at least five cycles with conserved activity.

Full text of DOI:10.1039/d0ra02609c

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Copper(II)-containing tungstotellurates(VI):
syntheses, structures and their catalytic
Cite this: RSC Adv., 2020, 10, 22515
performances in selective oxidation of thioethers†
a
b
b
a
a
*
*
Zhengguo Lin and Changwen Hu
*
Xuanyao Li, Peihe Li, Jinghai Liu,
We report the syntheses and structures of two new copper(II)-containing tungstotellurates(VI)
Na12[TeVI2W8O38Cu2(H2O)2]$7H2O (Te2W8Cu2) and Na6[TeVIW6O24Cu(NH2CH2CO2)2]$6H2O (TeW6Cu).
The two compounds were synthesized by a simple one-pot method and characterized by single-crystal
X-ray diffraction (XRD), powder XRD, FT-IR spectroscopy, elemental analysis, and thermogravimetric
analysis in the solid state. Furthermore, their catalytic properties for the selective oxidation of thioethers
were also studied systematically. The catalytic experiment results indicate that the tungstotellurate(VI)
Te2W8Cu2 is an effective heterogeneous catalyst for the selective oxidation of thioethers to sulfoxides or
sulfones by an H2O2 oxidant at room temperature. Under the ambient conditions, Te2W8Cu2 can
convert 99% of methyl(phenyl)sulfane to sulfoxides or sulfones with 96% or 99% selectivity, respectively,
and the utilization rate of H2O2 is up to 80%. Furthermore, Te2W8Cu2 as a heterogeneous catalyst is
stable in the reaction and could be reused at least five cycles with conserved activity.
Received 21st March 2020
Accepted 4th June 2020
DOI: 10.1039/d0ra02609c
rsc.li/rsc-advances
polyoxometalates (POMs) were used as effective catalysts for the
oxidation of thioethers using H2O2 as oxidant.6–8 In those re-
Introduction
ported procedures, low conversion of thioethers and the service
life and reusability of the POM catalysts also need to be further
improved. Consequently, based on the above reported results
and our previous work on the oxidation of sulphides,9 We
believe that more attractive catalytic systems can be developed
along this line to further increase the utility of these thioethers
oxidation reactions by using some new stable and recyclable
POM catalysts.
The selective oxidation of thioethers into the corresponding
sulfoxides is a very important process in organic synthesis
chemistry, because the sulfoxides are a type of essential
building block in pharmaceuticals, agrochemicals, and other
ne chemicals.1 Many signicant methods have been developed
for the synthesis of sulfoxides.2,3 Traditionally, the process was
carried out with a wide variety of oxidant agents, such as per-
acids, dioxiranes, NaIO4, MnO2, CrO3, SeO2, and PhIO.4
However, these oxidation reactions relied upon strong or
environmentally-unfriendly oxidants, some of which are
hazardous or toxic. Therefore, developing an environmentally
benign method and using easily available oxidants in thioethers
oxidation is attractive and needs to be further explored. H2O2
can be used as oxidant in the oxidation of thioethers because it's
a green oxidant and water is the only by-product.5 However, how
to improve the utilization ratio and reduce the consumption of
H2O2 with good catalytic selectivity in the oxidation reaction of
thioethers still remains a challenge. In recent years, some
As we all know, POMs are a large class of discrete anionic
metal–oxygen clusters of early transition metals with high
oxidation states, such as Mo, W, V, Nb, Ta,10–13 which present
abundant structural diversity, favourable electrical carrier
properties and great potentials in medicine, catalysis and
material science.14 The most signicant subset of POMs is the
heteropolyoxometalates that incorporate various anion
templates as heterogroups, such as pyramidal groups ({SbIVO3},
{BiIIIO3}, {SeIVO3}), tetrahedral groups ({PVO4}, {SiIVO4},
{GeIVO4}, {AsVO4}), and octahedral groups ({IVIO6}, {CrVIO6},
{TeVIO6}) et al.15–17 It is well-known that the heterogroups play
a key role in the structures and properties of these POMs,
especially the redox properties, which is very important to their
redox catalytic activities.18 Since heteropolytungstates are
usually synthesized in acidic media by the condensation of
molybdate or tungstate with relevant heteroanions,19 great
achievements have been made for Keggin and Dawson type
POMs by incorporating different tetrahedral heterogroups into
their anionic cluster structures.20 It is worth mentioning that
the POMs with octahedral hererogroups usually present
aKey Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of
Photoelectronic/Electrophotonic, School of Chemistry and Chemical Engineering,
Beijing Institute of Technology, Beijing 100081, China. E-mail: linzhengguo11@163.
com; cwhu@bit.edu.cn
bInner Mongolia Key Laboratory of Carbon Nanomaterials, Nano Innovation Institute
(NII), College of Chemistry and Materials Science, Inner Mongolia University for
Nationalities, Tongliao 028000, China. E-mail: lipeihe1988@aliyun.com
† Electronic supplementary information (ESI) available. CCDC 1991106 and
1991107. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d0ra02609c
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interesting redox properties and very few POMs with octahedral following the above procedure, led to blue crystals of TeW6Cu
heterogroups were reported, especially with the [TeVIO6]6ꢀ het- within 2–3 days (yield: 21% based on Te). Selected IR (2% KBr
erogroup.21 To the best of our knowledge, many Anderson– pellet, n/cmꢀ1): 1647(m), 1604(m), 1436(s), 1389(s), 1373(m),
Evans type [TeW6O24]6ꢀ POM derivatives were reported such as 1210(m), 1165(m), 1067(m), 959(m), 928(m), 903(w), 895(w),
{[Ni(H2O)3]2[TeW6O24]},21 {[Ca(H2O)4]2(TeW6O24)}22 and [4-H2- 699(m), 637(m), 551(m), 460(m), elemental analysis (%) for
methyl-imz]6[TeW6O24]$Te(OH)6.23 In 2010, Ali and coauthors Na6[TeW6O24Cu(NH2CH2CO2)2]$6H2O: calcd. C 2.32, N 1.352,
employed Anderson–Evans type POM {[Cu(en)2]3[TeW6O24]} as Na 6.66, W 53.2, Te 6.16, Cu 3.07; found C 2.32, N 1.608, Na
catalyst for the epoxidation of cyclohexene and styrene showed 7.15, W 53.1, Te 6.38, Cu 3.07.
good catalytic efficiency.24 Recently, we reported the self-
assembly of a series Ln(III)-containing tungstotellurates(VI)
with interesting photoluminescence properties.25
X-ray crystallography
Crystal data for all compounds were collected at 150 K on
a Bruker APEX 2 DUO CCD single-crystal diffractometer equip-
ped with a sealed Mo tube and a graphite monochromator (l ¼
In order to develop new stable and recyclable POM catalysts
for selective oxidation of thioethers, in this work, we prepared
two new copper(II)-containing tungstotellurates(VI) by a simple
one-pot synthetic method. The two tungstotellurates(VI)
Na12½TeV2IW8O38Cu2ðH2OÞ2ꢁ$7H2O (Te2W8Cu2) and Na6[TeVIW6-
O24Cu(NH2CH2CO2)2]$6H2O (TeW6Cu) were fully characterized
in solid state and their heterogeneous catalytic performances in
selective oxidation of thioethers were also studied systemati-
cally in the presence of 30% H2O2.
ꢀ
0.71073 A). The selected crystals were selected by examination
under mineral oil using a polarising microscope and mounted
in a Hampton cryoloop with oil and placed within one minute
under a stream of cold N2. The structure solution and rene-
ment were carried out by the SHELXTL program package
(Bruker), and all structures were solved by direct methods and
rened by the full-matrix least-squares method (Sw(|Fo|2
ꢀ
|Fc|2)2).27 The hydrogen atoms of waters were not incorporated
in the renements, and all other atoms were rened with
anisotropic thermal parameters. The crystal data and structure
renement details for the two compounds are discussed and
summarized in Table S1.† The crystallographic data have been
deposited to the Cambridge Crystallographic Data Centre
(CCDC) as entries CCDC-1991106, CCDC-1991107.
Experimental
Materials and methods
Na6TeW6O24$22H2O (TeW6) was prepared according to the
published procedure and characterized by IR spectroscopy.26 All
reagents were purchased from commercial sources, and used
without further purication. Elemental analyses were deter-
mined by inductively coupled plasma mass spectrometry (ICP-
MS) with PerkinElmer NexlON 350X spectrometer and the
Elemental Analyser (C, H, N). FT-IR spectra (KBr pellets) were
recorded with a Nicolet 170SX-FT/IR spectrometer. Thermog-
ravimetric analyses were carried out with a TG-DTA 6200 device
at a heating rate of 10 ꢂC minꢀ1 under nitrogen atmosphere.
Powder X-ray diffraction (PXRD) data were recorded on a Bruker
D8 instrument equipped with graphite-monochromatized Cu
Ka radiation (l ¼ 0.154060 nm; scan speed ¼ 8ꢂ minꢀ1; 2q ¼ 5–
50ꢂ) at room temperature.
Catalytic oxidation of thioethers
The selective oxidation of thioethers using different catalysts
were carried out as follows. Thioethers (0.5 mmol) and 30%
H2O2 (0.6 mmol or 1.25 mmol) were dissolved in EtOH/MeCN
(1 ml) in a pressure tube (10 ml), then added catalyst
(0.17 mol%) with a sealing cap at room atmosphere. The
reaction mixture was vigorously stirred until the completion
as indicated by GC. The reaction mixture was ltered through
a short pad of celite and quenched with water. Then the
mixture was extracted with dichloromethane (10 ml ꢃ 3),
dried over MgSO4, and evaporated under reduced pressure to
afford the crude product, which was further puried by ash
chromatography on silica gel with n-hexane/EtOAc to obtain
the corresponding sulfoxides or sulfones. In the recycle
experiments, the catalyst was ltered aer the reaction and
washed with dichloromethane and dried under vacuum at
Synthetic procedures
Synthesis of Na12½TeV2IW8O38Cu2ðH2OÞ2ꢁ$nH2O (n ¼ 7–38). A
mixture of Na2WO4$2H2O (1.5 g, 4.55 mmol), Te(OH)6 (0.1 g,
0.44 mmol), dimethylamine hydrochloride (1.5 g, 0.26 mmol),
glycine (0.025 g, 0.05 mmol), Cu(OAc)2$H2O (0.075 g, 0.02
mmol) were dissolved in water (20 ml). The suspended solution
was stirred for 2 hours. Aer ltration, the blue solution was
kept in open air for slow evaporation, pH value of the solution is
8.9. The green crystals obtained aer 1 week at room tempera-
ture (yield: 47% based on Te). Selected IR (2% KBr pellet, n/
ꢂ
50 C and used for the next cycle.
Results and discussion
Synthesis and structure description
cmꢀ1): 1631(m), 940(m), 884(s), 871(m), 689(m), 615(w), As described in the synthetic procedures above, the two cop-
elemental analysis (%) for Na12[Te2W8O38Cu2(H2O)2]$7H2O: per(II)-containing tungstotellurates(VI) Te2W8Cu2 and TeW6Cu
calcd Na 9.52, W 50.73, Te 8.80, Cu 4.38; found Na 8.80, W were prepared by simple one-pot reaction with a time-resolved
51.50, Te 8.89, Cu 4.70.
process in which the hydrated salts of the two compounds
Synthesis of Na6[TeVIW6O24Cu(NH2CH2CO2)2]$nH2O (n ¼ 6– were isolated successively from the same mother liquor as bulk-
20). The synthetic procedure is exactly extending process of that pure materials. The crystals of Te2W8Cu2 were isolated rstly
of Te2W8Cu2. Aer removal of the green crystals of Te2W8Cu2 and then the crystals of TeW6Cu. We noticed that the glycine
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ligand plays a very important role in the synthesis of the two
For compound TeW6Cu, X-ray diffraction analysis reveals
compounds although it is absent in the nal structure of that it consists of one dimensional organic–inorganic hybrid
Te2W8Cu2. Moreover, heteropolytungstates are usually formed polymeric chains having zig–zag architecture, which crystallizes
ꢁ
in condensation reactions of tungstate oxoanions with relevant in the triclinic lattice with space group P1 in form of sodium
heteroanions in an acid medium, but the two copper(II)-con- salts. As shown in Fig. 1b, the polymeric chain is formed by
6ꢀ
taining tungstotellurates(VI) were obtained straightly in the Anderson–Evans type [TeVIW6O24
]
polyoxoanion and
reaction solution without addition of appropriate acid and the {Cu(gly)2} complex via the multiple O–Cu–O bonds. The poly-
pH value is 8.9, which means the two compounds can be con- oxoanion [TeVIW6O24]6ꢀ in TeW6Cu is similar to the structures
structed in the presence of Te(OH)6 which can provide enough reported for other Anderson–Evans type anions with D3d
protons during the condensation reactions.
symmetry, in which a {TeO6} octahedron is surrounded by six
Single-crystal X-ray diffraction analysis reveals that edge-sharing {WO6} octahedra. The Te–O bond distances and
Te2W8Cu2 is a di-copper-containing tungstotellurate(VI) which O–Te–O bond angles are exhibited in Table S2,† and it can
crystallizes in the triclinic lattice with space group P1. As shown conclude that there is only mild distortion of the {TeVIO6}
ꢁ
12ꢀ
in Fig. 1a, the polyanion cluster ½TeV2IW8O38Cu2ðH2OÞ2ꢁ
octahedra. The CuII centers in the chain present a distorted
comprises two {TeW4O19} subunits linked via two ve- octahedral coordination sphere which is tetra-coordinated by
coordinate CuII ions in an anti-conformation. The {TeW4O19} two O atoms and two N atoms positioned in the equatorial
subunit is a divacant fragment of the well-known Anderson– plane from the carboxylate and amine groups of two glycine
Evans structure {TeW6O24}, in which four edge-sharing WO6 ligands, and the other two coordinate sites at the apical position
octahedron formed a slightly distorted di-vacant hexagon and are occupied by two O atoms from the neighbouring [TeVIW6-
the central octahedral cavity is occupied by a {TeVIO6} hetero- O24]6ꢀ polyoxoanions. The individual polymeric chains can be
ꢀ
group. The Te–O bond length are in the range of 1.881(4) A to further linked by sodium ions to form a 3D structure in solid
ꢀ
ꢀ
1.968(4) A, and the average distance is 1.928(4) A. The two state, as shown in Fig. S1b.†
{TeW4O19} subunits are further connected by two CuII ions via
multiple W–O–Cu and Te–O–Cu bonds. Each CuII center adopts
a distorted square-pyramidal coordination sphere which tetra-
Catalysis
coordinated by four terminal O atoms in the square plane Selective oxidation of thioethers to the versatile utility of sulf-
from two {TeW4O19} subunits, and the Cu–O bond length is oxides and sulfones is considered to be a topical interest in
ꢀ
between 2.092(2) to 2.108(2) A, which is comparable to the Cu–O organic synthesis. As mentioned above, POMs can be used as
bond lengths reported. The last coordinate site of CuII center is effective catalysts for the oxidation of thioethers using H2O2 as
occupied by one terminal H2O at the conic node position of the oxidant. In this work, in order to further develop a more
square-pyramidal. The two CuII ions are not within the same attractive catalytic system for the selective oxidation of thio-
planar of {TeW4O19} subunits and connected via two m3-O atoms ethers, we selected three different tungstotellurates(VI), the
from the {TeO6} heterogroups, the distance between the two classic Anderson–Evans type POM TeW6, the organic–inorganic
CuII ions is 3.031(1) A and the angle of Te–O–Cu is averaging hybrid complex TeW6Cu, and the di-copper-containing POM
ꢀ
126.2ꢂ, the other bond lengths and angles are summarized in Te2W8Cu2, as heterogeneous catalysts for this reaction and
12ꢀ
Table S2.† The individual ½TeV2IW8O38Cu2ðH2OÞ2ꢁ
polyanions optimized the reaction conditions systematically. Moreover, we
can be further linked by sodium ions to form a 3D structure in also veried that the Te2W8Cu2 is an effective heterogeneous
solid state, as shown in Fig. S1a.†
catalyst with good conversion and selectivity for the oxidation of
different thioethers to corresponding sulfoxides or sulfones at
room temperature.
In order to investigate the catalytic performance and effects
of the different catalysts for selective oxidation of thioethers,
Te2W8Cu2, TeW6Cu, TeW6, CuO and Cu(OAc)2 were used as
catalysts, and the catalytic reaction experiments of the oxidation
of thioethers were investigated systematically by using H2O2 as
oxidant under different conditions. At the beginning of our
studies, in order to nd out the best catalyst and optimize the
reaction conditions for this catalytic system, methyl phenyl
sulde (1a) was employed as the model substrate for the
oxidation reaction. The catalytic study results were summarized
in Table 1. Notably, when the reaction was carried out in the
absence of catalysts, only trace of product methyl phenyl sulf-
oxide (2a) was observed, and no methyl phenyl sulfone (3a) was
observed, and the intact substrate 1a can be recovered (Table 1,
entry 1). The copper(II) salts and oxides, such as Cu(OAc)2 and
CuO were rst evaluated in the reaction (1a to 2a), although the
selectivity of 2a was 99%, while the conversion of 1a were 19%
Fig. 1 Polyhedral/ball-and-stick representation of the core structures
of Te2W8Cu2 (a and b) and TeW6Cu (c). Color code: WO6, red octa-
hedron; TeO6, turquoise octahedron, Cu, green balls, O, red balls, C,
light gray balls, N, blue balls, H, white balls.
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Table 1 Optimization of reaction conditionsa
UR (%)
Conv.b (%) 2a Sel.b (%) 3a Sel.b (%) (H2O2)
Entry Catalyst
1c
2c
3c
4d
5e
6e
7f
None
TeW6Cu
Te2W8Cu2 99
Te2W8Cu2 91
TeW6Cu
3
89
99
86
96
96
7
0
14
4
2.5
64
80
73
74
79
79
4
99
93
99
79
Te2W8Cu2 99
Te2W8Cu2 99
1
21
a
Reaction conditions: 1a (0.5 mmol), catalyst (0.17 mol%), solvent (1
b
ml). Yields determined by GC analysis using 1-adamantanol as an
internal standard. H2O2 (30%) (1.2 equiv.), MeOH or EtOH (1 ml),
8 h. H2O2 (30%) (1.0 equiv.), MeOH or EtOH (1 ml), 8 h. H2O2
(30%) (2.5 equiv.), MeCN (1 ml), 6 h. H2O2 (30%) (2.0 equiv.), MeCN
c
d
e
f
(1 ml), 8 h. UR ¼ utilization rate.
and 31%, respectively. Considering the Anderson–Evans type
TeW6 is the basic structure of TeW6Cu, we also tested the
activity of TeW6 in the model reaction (1a to 2a). The results
show that the conversion of 1a was 88% and the selectivity of 2a
was 84%. Continuing experiments, the activity of TeW6Cu and
Te2W8Cu2 were also examined in the model reaction, as shown
in Table 1. Interestingly, according to the results of the catalysts
screening, the nal conversion of 1a and selectivity of 2a and 3a
indicate that Te2W8Cu2 is the best catalyst for the both selective
oxidation reactions of 1a to 2a and 1a to 3a (Table 1, entries 1–7)
under ambient conditions. The selectivity of 2a is up to 96%
when using MeOH or EtOH as solvent (Table 1, entry 3).
Compared to 1.0 equiv. H2O2 (30%), the 1.2 equiv. H2O2 (30%)
showed almost equal quality conversion rate (99%) of 1a (Table
1, entries 3–4). Furthermore, the selectivity of 3a is up to 99%
when using MeCN as solvent and H2O2 (30%) (2.5 equiv.) as
oxidant (Table 1, entry 6), which is much better than using H2O2
(30%) (2.0 equiv.) as oxidant. Based on the results summarized
in Table 1, the optimized reaction conditions for this catalytic
system are as follow: (1a to 2a): 0.17 mol% Te2W8Cu2, 1.2 equiv.
of H2O2 (30%) in 1.0 ml of MeOH or EtOH at room temperature
for 8 h; (1a to 3a) 0.17 mol% Te2W8Cu2, 2.5 equiv. of H2O2 (30%)
in 1.0 ml of MeCN at room temperature for 6 h.
The variation of different catalysts Te2W8Cu2, TeW6Cu,
TeW6, CuO and Cu(OAc)2 with reaction time in the oxidation
reaction is demonstrated in Fig. 2a, and their kinetic studies
indicated that if the reaction was carried out using Te2W8Cu2 as
catalyst, the reaction proceeded much faster than using other
catalysts, which means the Te2W8Cu2 present best activity for
this reaction. The initial velocity order of the different catalysts
is Te2W8Cu2 > TeW6Cu > TeW6 > CuO > Cu(OAc)2 > no catalyst
(41.70 > 23.18 > 18.88 > 17.50 > 6.79 > 1.17, conversion moles per
unit time). There are two possible reasons may explain these
Fig. 2 The conversion of thioanisole 1a with reaction time in the
oxidation of thioanisole reaction by the catalyst of Te2W8Cu2,
TeW6Cu, TeW6, CuO and Cu(OAc)2 (a). The initial velocity of the
reaction by using different catalysts (b). (Reaction conditions: thio-
anisole 1a (0.5 mmol), catalyst (0.17 mol%), 30% H2O2 (1.0 equiv.), EtOH
(1 ml), rt. All catalyst has the same number of moles of Cu, except
TeW6. 1-adamantanol as an internal standard).
results. Firstly, many other POMs were used as effective catalyst
for the oxidation of thioethers due to their excellent redox
properties, and the tungstotellurates(VI) Te2W8Cu2, TeW6Cu,
TeW6 with {TeVIO6} heterogroup can also present interesting
oxidation catalytic activity which is much better than the simple
cupric salts and cupric oxides for the selective oxidation of
thioethers. Sencondly, by introducing the CuII ions into the
structures of the tungstotellurates(VI) Te2W8Cu2 and TeW6Cu,
the CuII-containing tungstotellurates(VI) are more active than
the simple Anderson–Evans type tungstotellurates(VI) TeW6,
because the CuII centers can affect their redox properties and
offer the new active sites for the catalysts during the catalytic
reaction. More importantly, the structure and the coordination
environment of CuII in Te2W8Cu2 and TeW6Cu are different,
which can also affect their catalytic activity for selective oxida-
tion of thioethers. As proved by single crystal analyses, for
Te2W8Cu2, there are two CuII centers in the structure, and each
CuII center is ve-coordinated square-pyramidal coordination
sphere, which make it benecial to active the substrates (sulde
and H2O2). While in TeW6Cu, the CuII center is six-coordinated
with an octahedra coordination sphere, and the CuII center is
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coordination-saturated, resulting in strong coordination of the of thioethers. Moreover, the copper-containing tung-
ligand to Cu. The results make the CuII in TeW6Cu is less active stotellurates(VI) Te2W8Cu2 and TeW6Cu present better cata-
to the substrates. So the activity of Te2W8Cu2 is higher than lytic activity than the classic Anderson–Evans type
TeW6Cu. Based on the above results, Te2W8Cu2 can be proved tungstotellurates(VI) TeW6, which means introducing CuII
as the best optimal catalyst for the oxidation reaction under this centers in to the POM based complexes can improve the
conditions. Moreover, the heterogeneous nature of Te2W8Cu2 catalytic activity of tungstotellurates(VI), especially the
was also investigated by a leaching experiment. As shown in Te2W8Cu2 with two CuII centers. Because the heterogeneous
Fig. 3, when the conversion of 1a reached to about 38% at 0.5 h, catalyst Te2W8Cu2 shows best catalytic activities for the
the catalyst was ltered off and the ltrate continued to react selective oxidation of thioethers (Table 1), we studied
under the same conditions for 6 h, the reaction almost stopped systematically the catalytic performance of selective oxida-
and the conversion of 1a increased slightly due to the trans- tion of different thioethers to corresponding sulfoxides using
formation of 1a itself without catalyst under such reaction H2O2 (30%) (1.2 equiv.) as oxidant. A variety of organic sulfur
conditions. The result revealed that the catalyst of Te2W8Cu2 is compounds were subjected to this highly chemoselective
a heterogeneous catalyst in this reaction and stable without catalytic system. As shown in Table 2, all of the desired
leaching under the optimized conditions.
sulfoxides products were obtained in moderate to good yield.
As far as we know, the mechanism of catalytic oxidation Surprisingly, electron-donating as well as electron-
reaction using H2O2 as oxidant usually shows two different withdrawing substituted phenyl ring thioanisole gave the
pathways, the radical pathway and the peroxo species pathways. desired sulfoxides (2a–2f) in high yields with good selectiv-
To further understand the mechanism of our system, the ities and high H2O2 utilization rate. Ethyl phenyl sulde 1g
oxidation of thioethers using H2O2 as oxidant and Te2W8Cu2 as can be converted into ethyl phenyl sulfoxide 2g with 99%
catalyst, radical trap experiments on the oxidation of methyl(- conversion and 93% selectivity. The sterically more hindered
phenyl)sulfane were employed (ESI, Table S3†). The reaction substrate diphenylsulfane 1h were also investigated in this
was carried out under the optimized conditions, aer adding catalytic system. Fortunately, the corresponding product 3h
Ph2NH as oxygen-radical scavenger, 1,4-benzoquinone as was obtained in moderate yield. It is worth noting that less
superoxide (cO2ꢀ/cO2H scavenger) and tert-butyl alcohol as reactive dibutylsulfane 1i could be efficiently oxidized into
a hydroxyl radical scavenger,28 no obvious changes on the the desired sulfoxides 2i in high yields with 98% selectivity.
conversion and selectivity were observed (ESI, Table S3†). The
results indicate that the mechanism of the reaction is not
Table 2 Selective oxidation of thioethers to sulfoxidesa
a radical pathway, which is consistent with previous literature
that POMs tend to form peroxo-metal species in the presence of
H2O2.29 So, we proposed that an active peroxo species formed
during the reaction by using H2O2 as oxidant and Te2W8Cu2 as
catalyst, although it has not been tested in this work.
According to the study of the catalytic performance of
different catalysts for selective oxidation of thioethers, we
noticed that the tungstotellurates(VI) Te2W8Cu2, TeW6Cu and
TeW6 shown good catalytic potentials for this system, we
think the {TeVIO6} heterogroup may sufficiently affect the
redox property of the tungstotellurate(VI) catalysts which
might be good for their catalytic activity of selective oxidation
UR (%)
Selb. (%) (H2O2)
Entry Substrate 1
Conv.b (%) Product 2
1
2
R ¼ H
99
99
99
99
99
95
2a
2b
2c
2d
2e
2f
96
95
98
91
93
90
80
79
81
75
77
71
R ¼ Me
R ¼ MeO
R ¼ Ac
R ¼ Cl
3
4c
5c
6c
R ¼ NO2
7
R1 ¼ Et
R1 ¼ Ph
99
90
2g
2h
93
66
77
54
8d
9
R ¼ R1 ¼ nBu 99
2i
98
81
a
Fig. 3 Leaching test for the oxidation of thioanisole 1a to 2a by using
Te2W8Cu2 as catalyst. (Reaction conditions: 1a (0.5 mmol) Te2W8Cu2
(0.17 mol%), 30% H2O2 (1.2 equiv.), EtOH (1.0 ml), rt).
Reaction conditions: 1 (0.5 mmol), catalyst Te2W8Cu2 (0.17 mol%),
EtOH (1 ml), H2O2 (30%) (1.2 equiv.), 8 h. b Isolated yield. c 12 h. d 15 h.
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Furthermore, we also explored its catalytic activity for
selective oxidation of different thioethers to corresponding
sulfones using H2O2 (30%) (2.5 equiv.). As shown in Table 3,
a broad array of organic sulfur compounds worked well to afford
the corresponding sulfones in good to excellent yields (Table 3,
entries 1–9). It is worth noting that sterically hindered substrate
diphenylsulfane 1h and less reactive dibutylsulfane 1i gave the
corresponding sulfones 3h and 3i in 71% and 99% selectivity,
respectively. Interestingly, we noticed that the molar ratio of
H2O2 and Te2W8Cu2 plays a very important role for the selec-
tivity of thioethers to corresponding sulfoxides and sulfones.
This results are much better than other catalytic system using
H2O2 as oxidant, and in this work we have developed a good
method to improve the utilization ratio and reduce the Fig. 4 Recycle test for the oxidation of thioanisole 1a to 2a by
Te2W8Cu2. (Reaction conditions: 1a (0.5 mmol) Te2W8Cu2 (0.17 mol%),
30% H2O2 (1.2 equiv.), EtOH (1.0 ml) at rt.).
consumption of H2O2 with good catalytic selectivity in the
oxidation reaction of thioethers by using Te2W8Cu2 as catalyst.
It is interesting to see that different oxidized products can be
performed in high selectivity by using different solvents. The
reason could be that the hydrogen bond and interaction
between the reactants and solvents are different in ethanol and
acetonitrile, which could solvate H2O2 effectively and thereby
condition. Therefore, in order to the service life and reus-
ability of the catalyst, we also evaluated the recyclability and
stability of Te2W8Cu2 for the selective oxidation of thioethers.
As shown in Fig. 4, the catalytic reactivity of Te2W8Cu2
reduce its availability at the surface of catalyst.
remains without obviously decreasing the conversion and
As mentioned above, we have investigated the heteroge-
selectivity aer 5 cycles, which means the catalyst is recy-
neous nature of Te2W8Cu2 by a leaching experiment, which
clable and reusable aer the reaction. The results of
comparison of FT-IR spectra of the catalyst before and aer
the recycle reactions also conrms that the structure of
Te2W8Cu2 is stable without decomposition in the reaction
reveals that the catalyst is a heterogeneous catalyst in this
reaction and stable without leaching under the optimized
(ESI, Fig. S5†). This results indicate that the structure of
Table 3 Selective oxidation of thioethers to sulfonesa
Te2W8Cu2 is very favourable for this catalytic system. The
recyclability and stability of TeW6 and TeW6Cu were also
evaluated (ESI, Fig. S6 and S7†). Interestingly, the stability
order of the catalyst is Te2W8Cu2 > TeW6Cu > TeW6. The
results indicate that the new catalyst becomes more stable
than POMs when the CuII centers were introduced into the
UR (%)
POMs.
Entry Substrate 1
Conv.b (%) Product 3
Sel.b (%) (H2O2)
Conclusions
In summary, we have synthesized two new copper-containing
tungstotellurates(VI) Te2W8Cu2 and TeW6Cu by a simple one
pot reaction which were characterized by single-crystal X-ray
diffraction(XRD), powder XRD, FT-IR spectroscopy, elemental
analysis, and thermogravimetric analysis in solid state. The
catalytic properties of the tungstotellurates(VI) for the selective
oxidation of thioethers were also studied systematically. The
compound Te2W8Cu2 showed very high catalytic activity for the
selective oxidation of thioethers to sulfoxides (conversion: up to
99%, selectivity: up to 98%) and sulfones (conversion: up to
99%, selectivity: up to 98%) by H2O2 under ambient conditions.
In addition, the Te2W8Cu2 catalyst was stable in the environ-
mentally benign catalytic system and could be reused at least
ve cycles without a signicant loss of reactivity on the model
reaction. This work offer a new strategy for the selective oxida-
tion of thioethers by using tungstotellurates(VI) as heteroge-
neous catalyst, and the utilization ratio of the oxidant H2O2 is
up to about 80%.
1
2
R ¼ H
99
99
99
99
99
99
3a
3b
3c
3d
3e
3f
99
99
99
98
97
96
79
79
79
78
77
77
R ¼ Me
R ¼ MeO
R ¼ Ac
R ¼ Cl
3
4c
5c
6c
R ¼ NO2
7
8
R1 ¼ Et
R1 ¼ Ph
99
96
3g
3h
99
71
79
54
9d
R ¼ R1 ¼ nBu 99
3i
99
79
a
Reaction conditions: 1 (0.5 mmol), catalyst Te2W8Cu2 (0.17 mol%),
b
c
MeCN (1 ml), H2O2 (30%) (2.5 equiv.), rt, 6 h. Isolated yield. 10 h.
d
40 ꢂC, 10 h.
22520 | RSC Adv., 2020, 10, 22515–22521
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RSC Advances
10 P. A. Lorenzo-Luis, P. Gili, A. Sanchez, E. Rodriguez-
Castellon, J. Jimenez-Jimenez, C. Ruiz-Perez and X. Solans,
Transition Met. Chem., 1999, 24, 686.
Conflicts of interest
There are no conicts to declare.
11 J. Lu, X. Ma, P. Wang, J. Feng, P. Ma, J. Niu and J. Wang,
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Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (grant numbers 21871026, 21671019, and
21701010) for support. Inner Mongolia Natural Science Foun-
dation (2019BS02008). Scientic Research Projects of the Inner
Mongolian Higher Educational System (NJZZ20110). Inner
Mongolia University for Nationalities Doctoral Research Startup
Fund Project (BS245).
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