Tetrameric Lanthanide-Substituted Silicotungstate {Ln8Si4W40} Nanoclusters: Synthesis, Structural Characterization, Electrochemistry, and Catalytic Application for Oxidation of Thioethers

Article abstract of DOI:10.1002/ejic.202001096

All the title nanoclusters 1–10 with the molecular formula [(Ln₂SiW₁₀O₃₈)₄(W₃O₈)(OH)₄(H₂O)₂]^26? [Ln^III =Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), Tm (8), Yb (9), and Y (10)] have been synthesized and isolated as mixed sodium and potassium salts. These nanoclusters are characterized by various analytical techniques such as FT-IR, UV/Vis, Photoluminescence, Single crystal X-ray diffraction, Electrochemistry, ICP-AES and Thermogravimetric analysis, and Powder X-ray diffraction. The {Ln₈Si₄W₄₀} complexes show high efficiency and selectivity for the oxidation of thioethers using H₂O₂ as green oxidant. It is worth mentioning that the catalyst can be recovered even after five cycles of reaction with only a slight loss in its activity.

Full text of DOI:10.1002/ejic.202001096

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doi.org/10.1002/ejic.202001096
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Tetrameric Lanthanide-Substituted Silicotungstate
{
Ln Si W } Nanoclusters: Synthesis, Structural
8
4
40
Characterization, Electrochemistry, and Catalytic
Application for Oxidation of Thioethers
[a]
[a]
[b]
[b]
Imran Khan, Vivek Das, Anne-Lucie Teillout, Israël-Martyr Mbomekallé,
1
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[b]
[c]
[a]
Pedro de Oliveira, Subash Chandra Sahoo, and Firasat Hussain*
All the title nanoclusters 1–10 with the molecular formula
crystal X-ray diffraction, Electrochemistry, ICP-AES and Ther-
mogravimetric analysis, and Powder X-ray diffraction. The
{Ln Si W } complexes show high efficiency and selectivity for
2
6À
III
[
(Ln SiW O ) (W O )(OH) (H O) ] [Ln =Sm (1), Eu (2), Gd (3),
2 10 38 4 3 8 4 2 2
Tb (4), Dy (5), Ho (6), Er (7), Tm (8), Yb (9), and Y (10)] have been
synthesized and isolated as mixed sodium and potassium salts.
These nanoclusters are characterized by various analytical
techniques such as FT-IR, UV/Vis, Photoluminescence, Single
8
4
40
the oxidation of thioethers using H O as green oxidant. It is
2
2
worth mentioning that the catalyst can be recovered even after
five cycles of reaction with only a slight loss in its activity.
7
6À [10]
Introduction
molecular formula [Ce As (H O) W148O524
et al. reported the interaction of a monolacunary Keggin-type
]
.
In 1991, Qu
1
6
2
2
36
Polyoxometalates (POMs) are metal-oxygen nanoclusters that
consist of early transition metals such as Mo (VI), W (VI), and V
silicotungstate with early lanthanide ions to form [Ln(β -
2
1
3À
III
SiW O )] polyanions (Ln =La, Ce, Pr, Nd, Sm, Gd, and Er).
1
1
39
[11]
(V) in their highest oxidation state. Due to the flexibility in their
However, no structural evidence was provided either. Mialane
structure, size, charge density, and redox potential, POMs show
various interesting applications in many fields like medicine,
material science, catalysis, magnetism, clinical chemistry, photo-
et al. synthesized the lanthanide-substituted monolacunary
Keggin-type silicotungstate polyanions [Ln(α-SiW O )] (Ln =
8
À
III
1
1
39
Yb, Nd, Eu, and Gd). They found that these isolated compounds
were strongly affected by the nature of the lanthanide
[1–8]
chemistry, and analytical chemistry.
The lacunary POMs can
[12]
be derived from their parent compounds such as Keggin-type
cations. Gouzerh et al. in 2002, reported a cerium-containing
nÀ
V
,
V
IV
IV
19À
33 4
ions [XW O ] (X = As P , Ge , Si , etc.), and these lacunary
antimonate [Ce Sb W O (H O) (SbW O ) ]
and arsenotung-
12
40
3
4
2
8
2
10
9
2
5À [13]
POMs consist of multiple oxygen donor atoms as well as
multiple coordination sites which are more susceptible to
construct a variety of structures. It is known that lanthanide-
containing POMs exhibit structural varieties and have impres-
sive properties that are useful in various application fields as
mentioned above. A wide range of research papers has been
published on lanthanide-substituted lacunary POMs. In the year
state [Ce(H O) As W O140]
.
Niu et al. reported a one-dimen-
2
5
4
40
sional (1D) structure with the formula K [(Pr(H O) SiW O )
3
2
4
11 39
[14]
(NaPr (H O) )(Pr(H O) SiW O )]·13.5H O.
The crystal struc-
2
2
12
2
4
11 39
2
ture of this complex consists of two distinct units of [Pr-
5
À
7+
(H O) SiW O ] and one [NaPr (H O) ]
subunit, that is
2
4
11 39
2
2
12
further linked with the help of praseodymium and sodium
cations via bridging and terminal oxygen atoms of the Keggin
8
À
1
971, Peacock and Weakley reported the synthesis of the first
ion [α-SiW O ] , giving rise to a complete one-dimensional
11 39
lanthanide-containing polyoxometalates. However, structural
structure. In 2006, Niu et al. also synthesized 1D and 2D
structures of three new organic-inorganic hybrid monolacunary
Keggin-type silico and germanotungstate of the formula H
[9]
characterization evidence was not provided. Pope and co-
workers reported cerium-containing arsenotungstate of
a
{
[
[Sm(H O) (DMF) ] [Sm(H O) (DMF)][Sm(H O) (α-SiW O )] },
2 5.5 0.5 2 2 2 2 3 11 39 2
Sm(H O) ] [Sm(H O) ] H {Sm(H O) -[Sm(H O) (DMSO)(α-
2
6 0.25
2
5 0.25 0.5
2
7
2
2
[a] I. Khan, V. Das, Dr. F. Hussain
Department of Chemistry, Faculty of Science,
University of Delhi,
SiW O )]}·45H O,
(H O) [Dy(H O) (DMSO)(α-GeW O )]}·25H O. All three com-
pounds show good thermal stability and magnetic properties.
ln 2007, Kortz et al. have reported twenty cerium-containing
and
[Dy(H O) ] [Dy(H O) ] H {Dy-
2 4 0.25 2 6 0.25 0.5
1
1
39
2
[15]
2
7
2
2
11 39
2
North Campus, Delhi110007, India
E-mail: fhussain@chemistry.du.ac.in
b] Dr. A.-L. Teillout, Dr. I.-M. Mbomekallé, Prof. P. de Oliveira
Institut de Chimie Physique, UMR 8000 CNRS,
Faculté des Sciences d’Orsay,
[
germanotungstates
Ce Ge W100O376(OH) (H O) ]
of
the
formula
5
6À [16]
[
.
Patzke et al. isolated three
2
0
10
4
2
36
Université Paris-Saclay,Bâtiment 350,
new lanthanide-substituted silicotungstates based on open
Wells-Dawson fragments [Ln (H O) SiW O ] (Ln = Eu, and
2 2 7 18 66
Tb) and a sandwich-type monolacunary Keggin-type [Eu(α-
9
1405 Orsay Cedex, France
1
0À
III
[
c] Dr. S. C. Sahoo
Department of Chemistry & Center of Advanced Studies in Chemistry,
Panjab University,
Chandigarh160014, India
1
3À
[17]
SiW O ) ]
polyanion.
reported a sandwich-type carbonate encapsulated samarium-
containing silicotungstate of the formula [(β-
In 2008, Khoshnavazi et al. have
1
1
39 2
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1
3À
III
SiW O ) (H OSm) CO ] , which consists of a planar triad of
formula K Na [(Ln SiW O ) (W O )(OH) (H O) ] [Ln = Sm
9
34 2
2
3
3
11
15
2
10 38 4
3
8
4
2
2
1
2
3
4
5
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0
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three lanthanide ions. Further, they also synthesized samarium-
containing arseno and germanotungstate analogs; [(α-
(1a), Eu (2a), Gd (3a), Tb (4a), Dy (5a), Ho (6a), Er (7a), Tm
(8a), Yb (9a), Y (10a)]. Later, all the polyanions were
characterized by various analytical techniques such as FT-IR,
UV/Vis, TGA and SC-XRD. These polyanions were further
investigated for different applications in fields such as catalysis,
photoluminescence, and electrochemistry.
1
1À
13À
3
AsW O ) (H OSm) CO ]
respectively.
and [(β-GeW O ) (H OSm) CO ]
,
9
34 2
2
[
3
3
9
34 2
2
3
18–20]
A new class of lanthanide-containing phos-
3
0À
III
photungstates [(PLn W O ) (W O )] (Ln = Y, and Eu) was
2
10 38 4
3
14
[21]
reported by Francesconi and co-workers. Later, Khoshnavazi’s
group showed that in aqueous solution, the carbonate
encapsulated
samarium-containing
sandwich-type
[(α-
1
3À
SiW O ) (H OSm) CO ] complex convert into a new class of
Structure of the polyanion
9
34 2
2
3
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
rare-earth metal-substituted polyoxometalate of the formula
2
6À [22]
[
(SiW Sm O ) (W O (OH) )(H O) )]
.
In 2014, Hussain et al.
Herein, we have reported the synthesis of lanthanide-substi-
tuted silicotungstates, which are stable both in the solid-state
as well as in an aqueous solution. All the compounds are
structurally characterized by single crystal X-ray diffraction
analysis which reveals that the nanoclusters are isostructural
and crystallize in the monoclinic crystal system having space
10
2
38 4
3
8
4
2
2
have reported a series of lanthanide-containing phosphotung-
states which resembles a similar structure reported by Frances-
coni. The structural formula of the complex is
2
6À
III
[
(Ln PW O ) (W O )(OH) (H O) ]
(Ln = Sm, Eu, Gd, Tb, Dy,
2
10 38 4
3
8
4
23]
2
2
[
Ho, Er, Tm, Yb, and Y). Thereafter, germanotungstate analogs
were reported by Hussain et al. in 2019.
[24]
8À
In addition with
group P-1. The complexes 1a–10a consist of four [SiW O ]
10
36
3
+
POMs, recently many other coordination complexes were also
reported for its applications in the field of catalysis and
anions, eight Ln cations, and seven extra tungsten atoms,
which form a [Ln W O ] network. The complexes are formed
6
+
8
7
30
[25–28]
8À
magnetism.
Henceforth, we utilized a similar strategy to
by four [SiW O ] anions connected by a network of LnÀ O
1
0
36
obtain such isostructural lanthanide-substituted POMs by using
bonds. Eight lanthanide ions and seven additional tungsten
+
6
8À
10 36
a trilacunary Keggin-type Na [A-α-SiW O ]·16H O precursor.
atoms in [Ln W O ] network connected to the [SiW O ]
1
0
9
34
2
8
7
30
We have changed the reaction parameters and optimized the
reaction conditions to isolate a series of ten lanthanide-
give
rise
to
a
complete
tetrameric
2
6À
[(Ln SiW O ) (W O )(OH) (H O) ] structure (Figure 1).
2
10 38 4
3
8
4
2
2
substituted
silicotungstates
of
the
formula
If the extra tungsten unit is removed from the structure,
then it can be seen as an assembly of two asymmetric units.
Each asymmetric unit consists of four lanthanide ions and two
2
6À
III
[
(
(Ln SiW O ) (W O )(OH) (H O) ]
3), Tb (4), Dy (5), Ho (6), Er (7), Tm (8), Yb (9), Y (10)]. Although
[Ln = Sm (1), Eu (2), Gd
2
10 38 4
3
8
4
2
2
8
À
8À
10 36
Khosnavazi’s group have reported the Sm complex, it was
obtained as a by-product. The complexes reported here are
obtained as a major product by employing a simple one-pot
procedure under mild reaction conditions. We have investi-
gated the photoluminescence property of complexes and are
observed only in the case of the Sm, Eu, and Dy complexes.
Moreover, we have also investigated the catalytic applications
for the oxidation of thioethers. The screening of all the catalysts
is performed under optimized conditions. However, we have
selected {Dy Si W } POM as the model catalyst and checked for
distorted Keggin-type [SiW O ] units. Each of the [SiW O ]
10 36
asymmetric units are substituted with two lanthanide ions
(Figure 2) and these lanthanide ions are connected to Keggin-
type ions via LnÀ O linkages. The selected bond lengths and
bond angles for LnÀ O linkages are shown in Table S1 and
Table S2 respectively. Bond Valence Sum (BVS) calculations
were also performed for all the oxygen atoms to find out what
sort of ligand they are: oxo, hydroxo or water molecules. The
BVS values for the oxygen atoms bridging between lanthanide
ions are found within the range of 0.95 to 1.25. This indicates
8
4
40
the substrate scope only due to the high yield of this complex.
Furthermore, due to the redox properties of POMs, electro-
chemistry of the synthesized complexes was carried out.
Results and Discussion
During the syntheses of compounds 1a–10a, several prereq-
uisite parameters such as pH, concentration of the solution,
reaction temperature and time have to be taken care of. Herein,
we have used 0.2 mmol of trilacunary Keggin-type Na [α-
1
0
SiW O ]·16H O precursor and reacted with 0.1 mmol of
9
34
2
lanthanide nitrate salt in 0.25 M aqueous KCl solution. The
reaction has been carried out at 80°C for 1 h. The pH of the
reaction mixture is found to be around 4.5. At this pH, the
1
0À
trilacunary Keggin-type [α-SiW O ] anion easily transforms to
9
34
8
À
the dilacunary Keggin-type [α-SiW O ] ion. Colorless block-
1
0
36
Figure 1. Polyhedral representation of the complex 1a–10a. (Color code:
dark green: lanthanide; pink: silicon; dark blue: μ À OH; yellow: μ À OH; aqua:
shaped crystals were isolated after two-three days. The isolated
crystals were salt of mixed potassium and sodium with the
3
2
terminal water molecule; plum: polyhedra).
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silicotungstates {Dy Si W } and observed that all the complexes
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4
40
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
show excellent catalytic activity (Table S4). Further, for the
catalytic reactions, we have taken {Dy Si W } as the model
8
4
40
catalyst due to its high yield during the synthesis. The selectivity
and conversion to sulfoxide (RSOR’) and sulfone (RSO R’) were
2
investigated by Gas Chromatography using the internal
standard method (GC spectra Figure S12). Initially, biphenyl
sulfide has been used as a standard substrate, which is oxidized
into biphenyl sulfoxide (PhSOPh) and biphenyl sulfone
(
PhSO Ph).
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
It was observed that the substrate was completely con-
Figure 2. (a) Asymmetric unit consists of two distorted Keggin-type parts
sumed in 40 min and produced PhSO Ph, with 100% selectivity
6
À
2
[Eu
2
SiW10
O
38
]
connected to each other by EuÀ O bonds. (b) Coordination
at 50°C in 1 mL of acetonitrile, 0.27 mol% of the catalyst
environment of lanthanide ions (Color code: dark green: lanthanide; pink:
silicon; dark blue: μ
plum: polyhedra).
3
2
-OH; yellow: μ À OH; aqua: terminal water molecule;
{Dy Si W } and 4 mmol of H O . We have also oxidized various
8
4
40
2
2
thioethers to the corresponding oxygenated products by using
H O and {Dy Si W } as the catalyst. When the reaction is
2
2
8
4
40
carried out in the absence of a catalyst (Table S5, Entry 3), the
conversion is only about 20% and the selectivity of the major
that the oxygen atoms labeled as O73 and O112 are μ - and μ -
2
3
[29]
hydroxo linkages respectively. For the terminal oxygen atom
O162, the BVS value is found in the range of 0.25–0.36,
suggesting it to be a water molecule. The BVS values for oxygen
atoms bridging between lanthanide and tungsten atoms are
found to be in the range of 1.506–1.707, which suggests that
they are oxo linkages. Henceforth, according to BVS values the
polyanion has an overall charge of À 26, which is compensated
with sodium and potassium cations.
product (PhSO Ph) is also very low. However, as the catalyst
2
amount is varied from 0.14–0.27 mol%, we obtain its optimized
amount to be 0.27 mol%, with 100% selectivity of sulfone
(Table 1, Entry 3). Under the same conditions, we have also
investigated the catalytic activity of the lanthanide salt Dy-
(NO ) ·xH O and the trilacunary POM precursor Na [A-α-
3
3
2
10
SiW O ]·16H O in the absence of any catalyst, and the
9
34
2
selectivity is found to be 34% and 98% respectively (Table S3,
Entries 4 and 5). It can be observed that the silicotungstate
Na [A-α-SiW O ]·16H O precursor also shows good catalytic
The lanthanides Eu3 and Eu4 are seven-coordinated with a
capped Trigonal prismatic geometry, and the coordination sites
1
0
9
34
2
of Eu3 are occupied by one μ -OH hydroxo and six oxygen
activity, but the amount of this precursor used is very high
(1.08 mol%), and the selectivity towards the sulfone decreases
upon decreasing the mol% of the precursor. Thereafter, we
have performed the oxidation reaction in the absence of H O ,
3
atoms connected to the parent Keggin ion. The coordination
sphere of Eu4 is occupied by one μ -OH and six oxygen atoms
2
of the Keggin ion. Eu6 and Eu8 are eight-coordinated having a
Square-antiprismatic geometry, and the coordination sphere of
2
2
which results in less conversion to the desired product (Table 1,
Entry 6). The small conversion of sulfone and sulfoxide was
obtained due to the oxidation in presence of atmospheric
Eu8 is fulfilled with one μ -OH, one terminal water molecule,
3
and six neighboring oxygen atoms from tungsten polyhedra.
[33]
Eu6 is linked with one μ -OH and seven oxygen atoms from
oxygen. This indicates that the catalyst {Dy Si W } shows an
8 4 40
3
tungsten polyhedra.
excellent catalytic property in the presence of H O . Generally,
2 2
POMs activate H O by forming peroxy species. The amount of
2
2
oxidant also plays an important role in the oxidation reaction,
since as the oxidant amount increases the selectivity for the
desired product sulfone also increases (Table S5). The optimized
ratio of substrate and the oxidant is found to be 1:8 (Table S6).
The reaction temperature also affects the selectivity of the
biphenyl sulfone. At low temperature, the selectivity for
Catalysis
The catalytic oxidations of organic compounds by using mild
conditions have both a potential utility and wide-spread
interest. The selective oxidation of thioether compounds is
interesting due to the versatile application of their oxidized
PhSO Ph is low, but increases with temperature, reaching an
2
sulfone (RSO R’) and sulfoxide (RSOR’) forms in various fields
optimized value at 50°C (Table 1, Entries 3, 4 and 5). All these
2
such as sulfoxidation reactions, oxido-transfer reagents, bio-
parallel experiments indicate that the compound {Dy Si W }
8
4
40
[30–32]
chemistry, chemical industry, and medicinal chemistry.
H O has been used as an oxidant in the reaction and is also
shows an excellent catalytic activity for the oxidation of
thioethers.
2
2
known as a “green” oxidant, as the side product of the H O
We have also checked the catalytic performance in various
solvents, and it is observed that the selectivity and the
conversion of the oxidation reaction are more pronounced in
polar solvents as compared to non-polar ones (Table 2).
Generally, this happens due to the poor solubility of the catalyst
in non-polar solvents. The oxidation reaction has also been
carried out in water as a green solvent, but since sulfur alkyl
and aryl thioethers are not soluble in water, the catalysis
2
2
oxidation reaction is water. Polyoxometalates have been
investigated as catalysts due to their versatile features such as
redox properties, proton-transfer reactions, and structural
variety. Here, we have utilized POM complexes as catalysts for
the oxidation reaction of a standard substrate like biphenyl
sulfide in the presence of H O as the oxidant. We have
2
2
investigated the catalytic behavior of the lanthanide-substituted
Eur. J. Inorg. Chem. 2021, 1071–1081
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doi.org/10.1002/ejic.202001096
[
a]
8 4
Table 1. Oxidation of biphenyl sulfide by using catalyst {Dy Si W40}.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[
b]
[c]
Entry
Temperature
H
2
O
2
[mmol]
Catalyst loading
[mol %]
Conversion [%]
Selectivity [%]
(PhSOPh
Time
[min]
[
°C]
2
PhSO Ph)
1
2
3
4
5
6
7
50
4
4
4
4
4
0
4
0.14
0.20
0.27
0.27
0.27
0.27
0.27
90
99
100
100
30
10
100
20
5
0
25
70
80
44
80
95
100
75
30
20
56
40
40
40
40
40
40
30
50
50
40
25 (RT)
50
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
50
[a] Reaction conditions: biphenyl sulfide 0.5 mmol, oxidant H
2
O
2
8 4
(30% aqueous), 0–4 mmol, catalyst {Dy Si W40} loading 0.14–0.27 mol%, temperature 30–
0°C, 40 min, acetonitrile 1 mL. [b] Conversion based on substrate consumption and determined by GC-FID using an internal standard technique. [c] The
5
2
selectivity (PhSOPh/ PhSO Ph) is determined by GC.
[a]
Table 2. Effect of the solvent on the reaction.
[c]
Entry Solvent Conversion
Selectivity [%]
[
b]
[%]
2
PhSOPh PhSO Ph
1
2
3
4
5
DMF
50
14
52
100
50
84
57
78
0
16
43
22
100
80
Toluene
H
2
O
Acetonitrile
Acetonitrile+H
(50:50)
2
O
20
[
2 2
a] Reaction conditions: Substrate biphenyl sulfide 0.5 mmol, oxidant H O
(30% aqueous), 4 mmol, catalyst 0.27 mol%, temperature 50°C, 40 min,
solvent 1 mL. [b] Conversion based on substrate consumption and
determined by GC-FID using an internal standard technique. [c] The
2
Selectivity (PhSOPh/ PhSO Ph) is determined by GC.
reaction is not much effective. In addition, under the optimized
reaction conditions, we have performed the catalysis reaction of
different aromatic and alkyl thioethers. Alkyl thioethers are
most reactive towards the oxidation into their corresponding
sulfone due to the less steric hindrance of the alkyl group, and
Figure 3. Catalytic runs for the oxidation reaction of thioethers under
standard conditions.
conducted, the catalyst {Dy Si W } having been removed after
8
4
40
1
00% selectivity is obtained (Table 3, entry 9). We have also
10 minutes by filtration, and the reaction allowed to continue. It
was observed that there is a small conversion to the product,
suggesting that the reaction mechanism is heterogeneous. In
this heterogeneous system, the catalyst was collected by
filtration after the completion of the reaction, washed with
ethyl acetate, dried, and reused in the next run.
The recycled catalyst can be used up to five consecutive
runs, and thereafter the conversion yield and selectivity
decrease slightly (Figure 3). The recycled catalyst was charac-
terized by FT-IR and it was observed that the FT-IR spectrum
was similar to that of the fresh catalyst (Figure S13). The FT-IR
spectrum of the recycled catalyst shows major characteristic
peaks at around 998 cm , 931 cm , 854 cm , 757 cm and
700 cm , similar to the FT-IR spectrum of the fresh catalyst.
Hence, this indicates that the catalyst remains stable after five
runs.
We have compared our catalyst with some reported results
and concluded that {Dy Si W } shows a better catalytic activity
examined that the conversion is still 100% even on increased
steric hindrance, except in the few cases where the selectivity
of the sulfone is decreased. Various substituents attached to the
aromatic ring also affects the selectivity for the sulfone. The
strong electron-withdrawing group-containing substrates, such
as 4-nitrothianisol and 4-cynothianisol, show less selectivity, as
shown in Table 3 (Entries 3 and 6), due to the decrease in the
electron density on the benzene ring. The electron-donating
group-containing substrates, such as 4-methoxythioanisol and
4
-methylthioanisol, as shown in Table 3 (Entries 4 and 10), show
excellent selectivity which might be due to the high electron
density on the sulfur atom in these substrates. We have also
studied the oxidation reaction for an aromatic system such as
dibenzothiophene, for which an excellent selectivity is obtained
as shown in Table 3 (Entry 8). Furthermore, a recycling test was
also carried out to evaluate the catalytic performance of the
catalyst during the oxidation reaction of the biphenyl sulfide
under optimal conditions. A hot filtration experiment was
À 1
À 1
À 1
À 1
À 1
8
4
40
towards the oxidation of thioethers. Notably, our synthesized
Eur. J. Inorg. Chem. 2021, 1071–1081
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[
a]
8 4
Table 3. Selective oxidation of various alkyl and aryl derivative thioethers by using the catalyst {Dy Si W40}.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[
c]
Entry
Substrate
Conversion
Selectivity [%]
RSOR’
Time
[min]
[
b]
[
%]
RSO
100
2
R’
1
2
3
4
100
100
100
100
0
40
40
60
50
0
100
80
20
0
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
100
100
91
5
6
7
100
100
100
0
9
0
30
70
30
100
8
9
100
100
100
0
0
0
100
100
100
50
30
50
1
0
[a] Reaction conditions: Substrate 0.5 mmol, oxidant H
2
O
2
(30% aqueous), 4 mmol, catalyst 0.27 mol%, temperature 50°C, solvent 1 mL. [b] Conversion based
on substrate consumption and determined by GC-FID using an internal standard technique. [c] The selectivity (RR’SO/ RR’SO ) is determined by GC.
2
catalyst requires less time, a smaller amount of solvent, and
shows a better selectivity for the sulfone. For instance, the
oxidation of biphenyl sulfide (Table 3, entry 1) using {Dy Si W }
satisfactory stability at pH 5.0 and 3.0. Meanwhile, the com-
pounds Ho (6a), Tm (8a) and the ligand had only limited
stability over time at pH 5.0 and poor stability at pH 3.0. The
voltammogram of the Na [A-α-SiW O ]·16H O compound (Fig-
ure S4) is composed of two reversible waves. The first (noted (a)
in Figure S4) is multi-electronic and has a shoulder. The latter
(noted (#) in Figure S4) is in the same potential range as the
first reduction wave of SiW11 under the same experimental
conditions (CV not shown), indicating that the ligand decays
and evolves into SiW . The second reduction wave (noted (b) in
Figure S4) seems to indicate a reversible bi-electronic transfer.
In addition, a shoulder (noted (*) in Figure S4) is observed in
the reverse scan, which is not the case when the potential
window is restricted to the study of just the first wave. This
observation suggests that a partial degradation of the com-
pound occurs at more negative potentials with the formation of
8
4
40
as catalyst shows 100% selectivity for sulfones, and the
1
0
9
34
2
À 1
turnover frequency (TOF) is found to be 517.3 h at 50°C,
which is much higher than some of the reported catalysts. In
2
014,
Yang
et al.
reported
a
catalyst,
Na K [Zr O (OH) (H O) (W O OH) -
10
22
24 22
10
2
2
2
1
2
(
GeW O ) (GeW O ) ]·85H O, with a turnover frequency of
9 8 31 2 2
34 4
À 1
[34]
2
2.5 h and a selectivity for biphenyl sulfone of 73% at 60°C.
1
1
Thereafter, in 2019, they have also reported the
NH ) Na K [Zr (μ -O) (μ-H) (ox) (SiW O ) ]·23H O catalyst with
(
4 3 5 6 4 3 2 2 2 10 37 2 2
selectivity for biphenyl sulfone of 42% and a turnover
frequency of 42 h at 60°C.
À 1
[35]
[36,37]
Electrochemistry
blue derivatives.
In addition, it should be noted that the
stability of this compound over time at pH 5.0 is not marked
and its decomposition kinetics increases as the pH decreases, as
expected for a lacunary derivative. Its electrochemical character-
ization at pH below 4.0 was not possible. At pH 5, the study of
the peak current as a function of the square root of the scan
rate is consistent with a diffusion-controlled system (Figure S5).
The compounds 1a–10a (except Eu (2a)) exhibit a similar
The electrochemical study of the compounds 1a–8a as well as
the lacunary species Na [A-α-SiW O ]·16H O (ligand) was
carried out either in lithium acetate buffer at pH 5.0 and 4.0, or
in lithium sulphate buffer at pH 3.0 and 2.0. On the one hand,
on a 3 hour time scale and under the present experimental
conditions of cyclic voltammetry, the compounds Sm (1a), Eu
1
0
9
34
2
(
2a), Gd (3a), Tb (4a), Dy (5a), Er (7a) and Yb (9a) showed
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electrochemical behaviour in cyclic voltammetry at pH 5.0
the cathodic peak of the first wave, suggesting that an
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
(Figure S6) as illustrated in Figure 4.
adsorption process followed by the decomposition of the
compound on the electrode surface could be operative upon
reaching the second wave. This hypothesis is corroborated by
the observation of a peak potential difference of less than
60 mV in the second wave. The evolution of the cathodic peak
current of the first wave as a function of the square root of the
scan rate is linear, again being consistent with a diffusion-
controlled system (Figure S8). The electrochemical behaviour as
a function of the pH of the target compounds is shown in
Figure 5 and summarized in Table S3. As the pH decreases, the
cyclic voltammogram waves of 3a are shifted to positive values,
indicating that the electron transfer is accompanied by proton
transfer. Unfortunately, it is difficult to evaluate the evolution of
the apparent redox potential of each wave as a function of the
pH, because the anodic peak is poorly defined for pH values
below 5.0.
The cyclic voltammograms exhibit two reversible waves, the
0
first one having an apparent redox potential (E ’) around
0
À 860 mV, and the other one a E ’ around À 960 mV. The
electrochemical signals depend on the nature of the working
electrode. The best defined signals were obtained on edge-
plane pyrolytic graphite (EPG) electrodes (Figure S7). The
electrochemical signal of the target compounds is related to
6
+
the delocalized reduction of their W centers. Indeed, the
lanthanides do not have an electrochemical signature in the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[6a,38]
potential window explored except for europium.
A con-
trolled-potential coulometry study indicated that there are 16
electrons involved in the total reduction of a molecule of POM
carried out at a potential corresponding to the first wave of the
voltammogram. The shape of the anodic peak of the first wave
depends on the vertex potential. When both waves are swept, a
shoulder is visible after the anodic peak of the first wave. This
shoulder is not observed when the scan is reversed right after
If a rapid electron transfer is assumed at all pH values, then
the peak potential difference (ΔE=Epa1À Epc1) should be con-
stant. Thus, plotting the evolution of Epc1 as a function of the
pH for the different compounds gives slopes close to À 80 mV/
pH unit. This result does not allow to draw a conclusion on the
value of the ratio “number of protons exchanged/number of
electrons exchanged” involved in the redox process of the first
wave of the cyclic voltammogram for each compound. The
cyclic voltammogram of Eu (2a) shows waves of greater
intensity than Sm (1a) (and other similar compounds, Fig-
ure S6). The first process is composite with a shoulder at
À 0.770 V vs SCE on the cathodic wave and a splitting on the
anodic wave (Figure 6). The shoulder at À 0.770 V vs SCE may
3
+
be attributed to the electrochemical signal of the Eu
ions.
[7,39,40]
Indeed, in this potential range, the lacunary species does
not exhibit any electrochemical signal, and the shoulder thus
observed is similar to the signal observed with EuCl (Figure S9).
3
Figure 4. Cyclic voltammograms of 3a (black and grey lines) and of the
ligand (dotted line) recorded in 0.4 M LiCH
3
COO/CH
3
COOH buffer, pH=5.0,
The study of the electrochemical signal as a function of the
scan rate shows again that the compound is freely diffusing in
at v=0.1 V/s. POM concentrations: [3a]=0.195 mM and [ligand]=0.78 mM.
Figure 5. Cyclic voltammograms of 3a as a function of the pH, recorded in
.4 M LiCH COO/CH COOH at pH=5.0 (black) and 4.0 (red), and in 0.2 M
Na SO /H SO buffer at pH=3.0 (green) and 2.0 (blue), at 0.1 V/s. POM
0
3
3
Figure 6. Cyclic voltammograms normalized with respect to the concen-
tration of 1a (dashed line), 2a (grey line) and the ligand (black line),
recorded in 0.4 M LiCH COO/CH COOH at pH=5.0, at v=0.1 V/s.
3 3
2
4
2
4
concentration: [3a]=0.195 mM.
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solution (Figure S10). As the pH decreases, the waves in the
cyclic voltammogram of Eu (2a) are shifted to positive values
indicating that the electron transfer is accompanied by proton
transfer (Figure 7). Unfortunately, it is difficult to evaluate the
evolution of the apparent redox potential of each wave as a
function of the pH, because the anodic peak is poorly defined
for pH values below 5.0. The Epc1 values evolve by about
À 77 mV per pH unit. Again, this result does not allow to draw a
conclusion on the value of the ratio “number of protons
exchanged/number of electrons exchanged“ involved in the
redox process of the first wave of the cyclic voltammogram.
Oxford diffractometer operated at 50 kV and 40 mA (Mo Kα1
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
radiation). Thermogravimetric analysis was performed on a DTG-60
TG/DTA instrument (Shimadzu) in the temperature range 30–400 C.
The catalytic study of the oxidation of thioethers was performed
using Gas chromatography (GC Plus 2010 Shimadzu). Solid state
UV/Vis spectra were recorded with a Thermo Scientific Evolution
300 spectrometer. Photoluminescence spectra were obtained with
the help of a Fluorolog Horiba Jobin Yvon spectrometer. Elemental
analysis was performed with an ICP-AES ARCOS instrument from M/
s Spectro Germany. Electrochemical data was obtained using an
Autolab PGSTAT 128N driven by a PC with the NOVA software. A
one-compartment cell with a standard three electrode configura-
tion was used for cyclic voltammetry experiments. The reference
electrode was a saturated calomel electrode (SCE) and the counter
electrode, a platinum plate of large surface area. The reference
electrode was separated from the bulk electrolyte solution via a
fritted compartment filled with the same electrolyte. The working
electrode was a 3 mm diameter disk made of edge-plane pyrolytic
graphite (EPG - Mersen). Before each run, the working electrode
was polished with a 1 μm grain size diamond paste (DP Diamond-
Struers) during 2 min and washed in an ultrasonic bath of ethanol
during 2 min. Prior to each experiment, solutions were thoroughly
deaerated with pure argon. A positive pressure of this gas was
maintained during subsequent work. All experiments were per-
formed at room temperature, which is controlled and fixed for the
lab at 20°C. Pure water obtained with a Milli-Q Integral 5
purification set was used throughout. All reagents were of high-
purity grade and were used as purchased from Sigma Aldrich
°
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Conclusion
We report a series of ten lanthanide-substituted silicotungstates
III
of formula K Na [(Ln SiW O ) (W O )(OH) (H O) ] [Ln =Sm
11
15
2
10 38 4
3
8
4
2
2
(
(
1a), Eu (2a), Gd (3a), Tb (4a), Dy (5a), Ho (6a), Er (7a), Tm
8a), Yb (9a), Y (10a)]. All the isolated complexes were
characterized by various analytical techniques such as FT-IR,
UV/Vis spectroscopy, TGA, ICP-AES, SC-XRD, and P-XRD. Single
crystal X-ray diffraction reveals that all the complexes are
isostructural and crystallize in the monoclinic crystal system
with the space group P-1. Complexes 1a, 2a, and 5a show
excellent photoluminescence properties. The complexes were
used as catalysts for the oxidation of various thioethers and
show good catalytic activity. The electrochemistry properties of
the complexes were also investigated.
without further purification: CH COOH (glacial acetic acid),
3
LiCH COO, Na SO and H SO . Powder X-ray diffraction was carried
3
2
4
2
4
out in high resolution Rigaku X-ray diffractometer employing Cu-Kα
radiation (λ=1.5406 Å) at 298 K in the range of 2θ=5°–30°.
Synthesis of K Na [(Sm SiW O ) (W O )(OH) (H O) ]·58H O
1
1
15
2
10 38 4
3
8
4
2
2
2
(
1a)
Experimental Section
0
.0889 g of Sm(NO ) ·6H O (0.2 mmol) was dissolved in 25 mL of
3 3 2
Materials and Methods
0.25 M potassium chloride solution. Subsequently, 0.2546 g of
Na [A-α-SiW O ]·16H O (0.1 mmol) was added to the above
1
0
9
34
2
The trilacunary Na10[A-α-SiW O34]·16H O precursor was prepared
9 2
[41]
solution with continuous stirring and the reaction mixture was
heated at 80°C for 1 h. Thereafter, the reaction mixture was cooled
at room temperature and filtered. The filtrate was left for slow
evaporation in order to obtain block-shaped crystals after 2 days.
The crystals were collected with a yield of 60 mg (23% based on
Na [A-α-SiW O ]·16H O). FT-IR: �n =997 (s), 936 (s), 897 (s), 853 (s)
according to the published literature. The FT-IR spectra of all the
complexes were recorded on Perkin-Elmer BX spectrometer on KBr
À 1
pellets in the range of 400–1400 cm . Liquid UV/Vis spectroscopy
was investigated on Analytic Jena Specord 250 spectrometer. Single
crystal X-ray diffraction data were obtained by using an Xcalibur
1
0
9
34
2
À 1
cm . Elemental analysis (%) calcd (found): Na 2.52 (2.83), K 3.14
(4.01), W 57.68 (58.10), Si 0.82 (0.86). The number of water
molecules was calculated by TGA.
Synthesis of K Na [(Eu SiW O ) (W O )(OH) (H O) ]·62H O
1
8
8
2
10 38 4
3
8
4
2
2
2
(
2a)
The above synthetic protocol procedure was followed by using
Eu(NO ) ·5H O (0.0856 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
3
3
2
3 3
2
Crystals were collected with a yield of 55 mg (21% based on
Na [A-α-SiW O ]·16H O). FT-IR: �n =997 (s), 950(s), 882 (s), 851 (s),
1
0
9
34
2
À 1
750 cm . Elemental analysis (%) calcd (found): Na 1.32(1.58), K 5.06
(6.00), W 56.86 (57.60), Si 0.81 (0.90). The number of water
molecules was calculated by TGA.
Synthesis of K Na [(Gd SiW O ) (W O )(OH) (H O) ]·59H O
1
6
10
2
10 38 4
3
8
4
2
2
2
Figure 7. Cyclic voltammograms of Eu (2a) normalized with respect to the
(3a)
concentration as a function of the pH, recorded in 0.4 M LiCH
CH COOH at pH=5.0 (black) and 4.0 (red), and in 0.2 M Na SO
at pH=3.0 (green) and 2.0 (blue), at 0.1 V/s.
3
COO/
/H SO
buffer
The above synthetic protocol procedure was followed by using
Gd(NO ) ·6H O (0.0903 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
3
2
4
2
4
3
3
2
3 3
2
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Crystals were collected with a yield of 57 mg (22% based on
Na [A-α-SiW O ]·16H O). FT-IR: �n =999 (s), 946(s), 885 (s), 856 (s),
Synthesis of K Na [(Yb SiW O ) (W O )(OH) (H O) ]·56H O
11 15 2 10 38 4 3 8 4 2 2 2
(9a)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
0
9
34
2
À 1
745 cm . Elemental analysis (%) calcd (found): Na 1.66 (1.80), K
The above synthetic protocol procedure was followed by using
Yb(NO ) ·5H O (0.0898 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
4.50 (4.90), W 56.91 (58.03), Si 0.81 (0.92). The number of water
molecules was calculated by TGA.
3
3
2
3 3
2
Crystals were collected with a yield of 59 mg (23% based on
Na [A-α-SiW O ]·16H O). FT-IR: �n =1008 (s), 946 (s), 896 (s), 853 (s),
1
0
9
34
2
À 1
7
65 (w) cm . Elemental analysis (%) calcd (found): Na 2.49 (2.60), K
Synthesis of K Na [(Tb SiW O ) (W O )(OH) (H O) ]·60H O
1
5
11
2
10 38 4
3
8
4
2
2
2
3.11 (4.01), W 57.08 (58.12), Si 0.81 (0.83). The number of water
molecules was calculated by TGA.
(4a)
The above synthetic protocol procedure was followed by using
Tb(NO ) ·5H O (0.087 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
3
3
2
3 3
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Crystals were collected with a yield of 72 mg (28% based on
Synthesis of K Na [(Y SiW O ) (W O )(OH) (H O) ]·55H O
1
1
15
2
10 38 4
3
8
4
2
2
2
Na [A-α-SiW O ]·16H O). FT-IR: �n =1002 (s), 941(s), 896 (s), 855 (s),
(10a)
1
0
9
34
2
À 1
760 cm . Elemental analysis (%) calcd (found): Na 1.82 (1.92), K
.22 (4.75), W 56.85 (57.20), Si 0.81 (0.87). The number of water
The above synthetic protocol procedure was followed by using
Y(NO ) ·6H O (0.0766 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
4
molecules was calculated by TGA.
3 3
2
3 3
2
Crystals were collected with a yield of 67 mg (26% based on
Na [A-α-SiW O ]·16H O). FT-IR: n=1003 (s), 940 (s), 895 (s), 855 (s),
�
1
0
9
À 1
34
2
760 (w) cm . Elemental analysis (%) calcd (found). Na 1.61 (1.67), K
.36 (3.69), W 59.89 (60.95), Si 0.85 (0.90). The number of water
Synthesis of K Na [(Dy SiW O ) (W O )(OH) (H O) ]·68H O
1
9
7
2
10 38 4
3
8
4
2
2
2
3
(5a)
molecules was calculated by TGA.
The above synthetic protocol procedure was followed by using
Dy(NO ) ·H O (0.0877 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
3
3
2
3 3
2
Crystals were collected with a yield of 77 mg (30% based on
X-ray crystallography
Na10[A-α-SiW
7
O
34]·16H
O). FT-IR: �n =1000 (s), 940 (s), 890 (s), 857 (s),
61 (m), 715 (w) cm . Elemental analysis (%) calcd (found): Na 1.14
9
2
À 1
Single crystals of 1a–10a suitable for X-ray diffraction were
mounted on a capillary tube for indexing and intensity data
collection at 192 K. An Xcalibur Oxford diffractometer was used
(
1.81), K 5.25 (5.60), W 55.90 (57.10), Si 0.79 (0.87). The number of
water molecules was calculated by TGA.
which operated at 50 kV and 40 mA (Mo K , λ=0.71073 Å, graphite
α
[42]
monochromated).
Pre-experiment, data collection and data
reduction [ were performed with the Oxford program suite
Synthesis of K Na [(Ho SiW O ) (W O )(OH) (H O) ]·57H O
2
0
6
2
10 38 4
3
8
4
2
2
2
43]
CrysAlisPro. An empirical adsorption correction based on symme-
(6a)
[
44]
try equivalent reflections was applied using SCALE3 ABSPACK.
The above synthetic protocol procedure was followed by using
Ho(NO ) ·5H O (0.0882 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
The crystal structure was solved by direct methods using SHELXS-
[45]
3
3
2
3 3
2
2008, which located all the heavy metal atoms (W and Ln) and
Crystals were collected with a yield of 57 mg (22% based on
enabled us to locate all the positions of other non-hydrogen atoms
(O, K, Na, Si) from the difference Fourier maps. The last cycles of
refinement included atomic positions, anisotropic thermal parame-
ters for all the heavy metal atoms and isotropic thermal parameters
for all other non-hydrogen (O) atoms. Some of the tungsten atoms
were refined with partial occupancies and isotropic due to disorder.
Due to disorder, the R-factor for some of the crystals are higher
than usual but, the structure has been solved with utmost precision
as possible. Some of the oxygen atoms are disordered water
molecules and as a result the structure has solvent accessible voids.
The data obtained are Full-matrix least squares structure refinement
Na [A-α-SiW O ]·16H O). FT-IR: �n =1005 (s), 950 (s), 880 (s), 857 (s),
10
9
34
2
À 1
7
5
55 (m) cm . Elemental analysis (%) calcd (found): Na 0.99 (1.11), K
.59 (6.02), W 56.55 (57.02) Si 0.80 (0.87). The number of water
molecules was calculated by TGA.
Synthesis of K Na [(Er SiW O ) (W O )(OH) (H O) ]·60H O
16
10
2
10 38 4
3
8
4
2
2
2
(7a)
The above synthetic protocol procedure was followed by using
Er(NO ·5H O (0.0886 g, 0.2 mmol) instead of Sm(NO ·6H O.
)
3
)
3
3
2
3
2
2
against jF j was performed using the SHELXL-2015 package of
Crystals were collected with a yield of 62 mg (24% based on
[45]
programs. The crystal data is shown in Table 4.
Na [A-α-SiW O ]·16H O). FT-IR: �n =1002 (s), 943 (s), 888 (s), 859 (s),
1
0
9
34
2
À 1
7
50 (w) cm . Elemental analysis (%) calcd (found): Na 1.64 (1.89), K
.47 (5.02), W 56.51 (58.10), Si 0.80 (0.86). The number of water
Deposition Numbers 1979347 (for 2a), 2045339 for (3a), 1979348
for (4a), 2045340 (for 5a), 2045341 (for 6a), 1979349 (for 7a),
2045342 (for 8a), 2045343 (for 9a), and 1979350 (for 10a) contain
the supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Struc-
tures service www.ccdc.cam.ac.uk/structures.
4
molecules was calculated by TGA.
Synthesis of K Na [(Tm SiW O ) (W O )(OH) (H O) ]·57H O
1
1
15
2
10 38 4
3
8
4
2
2
2
(8a)
The above synthetic protocol procedure was followed by using
Tm(NO ) ·5H O (0.089 g, 0.2 mmol) instead of Sm(NO ) ·6H O.
3 3
2
3 3
2
FTIR Spectroscopy
Crystals were collected with a yield of 54 mg (21% based on
Na [A-α-SiW O ]·16H O). FT-IR: �n =1006 (s), 943(s), 892 (s), 857 (s),
FT-IR spectra of all the complexes 1a–10a were recorded on a
PerkinElmer BX spectrometer using KBr pellets. All complexes show
similar spectral patterns. The finger-print region of POM ligands is
1
0
9
34
2
À 1
7
60 (w) cm . Elemental analysis (%) calcd (found): Na 2.49 (2.86), K
.10 (3.48), W 56.97 (57.15), Si 0.81 (0.89). The number of water
3
À 1
molecules was calculated by TGA.
in the range from 1000–400 cm and in this region four major
characteristic vibration bands (asymmetric) are obtained, which are
attributed to ν (SiÀ O ), terminal ν (WÀ O ), corner sharing ν
as
a
as
t
as-
(
WÀ O À W), and edge-sharing ν (WÀ O À W). The asymmetric vibra-
b
as
c
Eur. J. Inorg. Chem. 2021, 1071–1081
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© 2021 Wiley-VCH GmbH

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Table 4. Crystal data and refinement.
Eu (2a)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Gd (3a)
Tb (4a)
Dy (5a)
Dy
14142.25
Triclinic
P-1
19.7120(3)
22.3933(4)
27.7268(5)
84.5880(1)
84.5880(10)
68.833(2)
11341.3(4)
2
3.888
25.832
10994
0.10×0.08×0.03
141951
42761
0.1649
1.038
Ho (6a)
8 4 43 225 122
Ho Si W O H
13979.64
Triclinic
P-1
19.6104(2)
22.3285(2)
27.5388(2)
84.5980(10)
84.2650(10)
68.5000(10)
11141.66(2)
1
3.870
25.241
11390
0.10×0.08×0.03
177151
45487
0.0625
1.045
Empirical formula
Formula weight
Crystal system
Space group
K
18Na
8
Eu
8
Si
4
W
43
O
228
H
132
K
16Na10Gd
8
Si
4
W
43
O
227
H
126
K
15Na11Tb
8
Si
4
W
43
O
228
H
128
K19Na
7
8
Si
4
W
43
O
236
H
144
K
20Na
6
13901.77
Triclinic
P-1
13889.79
Triclinic
P-1
13905.10
Triclinic
P-1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
19.6970(4)
22.5010(5)
27.7261(5)
85.016(2)
84.577(2)
68.450(2)
11360.1(4)
1
19.6503(6)
22.5510(7)
27.7175(7)
85.001(2)
84.641(2)
68.218(3)
11337.5(6)
1
3.898
25.41
11196
0.08×0.05×0.02
153475
42889
0.1631
1.154
19.6505(4)
22.4392(4)
27.5480(5)
84.717(2)
84.288(2)
68.323(2)
11211.9(4)
2
3.952
24.824
11546
0.07×0.03×0.02
155783
42498
0.0980
1.060
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
3
V/Å
Z
À 3
c
D /gcm
3.798
24.181
À 1
μ/mm
F
000
11224
3
Crystal size/mm
Reflns collected
Indep reflns
0.08×0.04×0.02
147746
43012
R
int
0.1107
1.038
0.0808
0.2288
2
Goodness-of-fit on F
[a]
R
1
[I>2σ(I)]
0.1255
0.3574
0.0693
0.1687
0.1323
0.3176
0.0505
0.1316
[
b]
wR (all data)
2
Er (7a)
Tm (8a)
Yb (9a)
Y (10a)
Empirical formula
Formula weight
Crystal system
Space group
K
16Na10Er
8
Si
4
W
43
O
228
H
128
K
11Na15Tm
8
Si
4
W
43
O
225
H
130
K
11Na15Yb
8
Si
4
W
43
O
222
H
120
8 4 43 223 126
K11Na15Y Si W O H
13987.88
Triclinic
P-1
13874.75
Triclinic
P-1
13849.52
Triclinic
P-1
19.6283(3)
22.2290(3)
27.6410(4)
84.6040(10)
84.6120(10)
69.2110(10)
11201.3(3)
1
3.952
24.824
11264
0.06×0.05×0.02
177317
45587
0.1306
1.075
13198.50
Triclinic
P-1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
19.6576(3)
22.3426(3)
27.6670(5)
84.7030(10)
68.9950(10)
110.653(1)
10998(3)
2
19.6577(6)
22.2947(6)
27.6328(7)
84.669(2)
84.780(2)
69.086(3)
11241.8(6)
2
3.859
25.318
11256
0.09×0.07×0.02
137356
38224
0.1633
1.044
19.6091(4)
22.3151(5)
27.5576(6)
84.933(2)
84.527(2)
68.512(2)
11150.8(4)
1
3.733
24.465
10884
0.08×0.06×0.02
155995
45566
0.1098
1.028
3
V/Å
Z
À 3
c
D /gcm
3.908
25.134
À 1
μ/mm
F000
11454
3
Crystal size/mm
Reflns collected
Indep reflns
Rint
0.07×0.03×0.02
151119
42714
0.1026
1.053
0.0708
0.2089
2
Goodness-of-fit on F
[
a]
R
1
[I>2σ(I)]
0.1205
0.2886
0.2211
0.2369
0.0698
0.1840
[b]
wR2 (all data)
={Σ[w(F 2o – F ) ]/Σ[w(F ) ]}
2
2
2
2 1/2
.
[
a] R =Σj jF
1
o
j – jF
C
j j/ΣjF
O
j [b] wR
2
c o
À 1
tion bands at 1000–1005 cm are ascribed to ν (SiÀ O ) and the
which two are in the UV region and the others belong to the visible
range. Two absorbance peaks in the UV region are ascribed to
as
a
À 1
bands at around 935–940 cm are assigned to ν (W=O ), these
as
t
6
2
6
4
4
bands being considered as pure vibrational bands of the POM
ligands. The vibration frequency in the range of 885–890 cm is
H5/2! D and H ! G , H
transitions at 370 and 390 nm
1/2
5/2
11/2
11/2
À 1
respectively. Other seven absorbance peaks6 are assigned
À 1
6
4
6
5
6
4
attributed to νas(WÀ O
À W), and the peaks at 850–855 and 800 cm
as
H
5/2! K11/2
,
H
M
5/2! P3/2
,
P
3/2
,
H
! G9/2
,
,
b
5/2
4
6
4
4
6
4
4
may be assigned to ν (WÀ O À W) (Figure S1).
I15/2, H ! F
,
,
H5/2! I11/2,
I
as
c
5/2
5/2
17/2
13/2
6
4
6
4
H5/2! G and H ! F transitions at 405, 425, 445, 465, 485,
7/2
5/2
3/2
[46,47]
5
10 and 530 nm respectively.
Five excitation peaks for the Eu
UV/Vis Spectroscopy
(2a) complex are observed in the UV/Vis range which are at 362,
7 5 7 5
3
82, 395, 416 and 465 nm corresponding to the F ! D , F ! L ,
0
4
0
7
The liquid UV/Vis spectra of all the complexes 1a–10a show two
characteristic absorption bands: a strong band at around 240 nm
and another broad band at around 190–200 nm. The higher energy
absorption band may be attributed to pπ-dπ charge transfer
7
5 7 5 7 5
F ! L , F ! D and F ! D transitions respectively. The absorb-
0
6
0
3
0
2
ance spectrum of the complex Dy (5a) shows a total of five
absorbance peaks, three of them being very weak at 429, 454 and
6
4
79 nm. They involve excitations from the H
ground state to
1
5/2
transitions of terminal O !W bonds, and the lower energy band is
t
various excited states, two transitions being highly intense at 758
and 808 nm.
due to pπ-dπ charge transfer transitions of Ob,c!W bonds (Fig-
ure S2). We have also recorded the solid UV/Vis spectra of which
the absorbance peaks are seen in three complexes (Figure S3). The
complex Sm (1a) shows nine characteristic absorbance peaks in
Eur. J. Inorg. Chem. 2021, 1071–1081
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1079
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Full Papers
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3
+
Photoluminescence spectroscopy
which are influenced by the chemical environment of the Eu ion.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
5 7
The transition assigned as D ! F is very weak, because it is
0
3
Since polyoxometalates are metal oxide nanoclusters, they have
ligand to metal charge transfer properties. Initially, Yamase and co-
workers investigated the photoluminescence (PL) properties of all
[51,52]
forbidden according to the Judd-Ofelt theory.
the complex 5a shows three characteristic emission peaks at 485,
574 and4 664 which are assigned
The PL spectra of
[
48,49]
lanthanide-substituted polyoxometalates.
All the title polyan-
4
6
6
4
6
as F ! H , F9/2! H and F ! H respectively (Figure 10).
9
/2
15/2
13/2
9/2
11/2
ions were studied and some of them exhibit good photolumines-
cence properties on photo-excitation at room temperature. The
complex ion 1a shows four emission peaks at 565, 601, 648 and
4
6
The transition corresponding to F ! H at 575 nm is related to
9
/2
13/2
3+
the electric dipole environment of the Dy ion. It has the highest
intensity and is responsible for the yellow emission of the complex
7
08 nm on photo-excitation at 377 nm (Figure 8). These peaks can
be assigned to the corresponding transitions
respectively. All peaks are
[53,54]
5a.
4
6
4
6
4
6
G5/2! H , G ! H and G ! H
5/2
7/2
9/2
5/2
11/2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
observed in the visible region. A peak at 565 nm is known as zero-
Experimental method for catalytic oxidation of thioethers by
catalyst {Dy Si W }
zero band and another two transitions at 648 and 708 nm are
8
4
40
4
6
4
6
attributed to G5 ! H and G ! H , known as magnetic dipole
/2
5/2
7/2
9/2
[
46–50]
and electric dipole transitions, respectively.
The photolumines-
The catalytic oxidation reactions of thioethers was performed in a
25 mL round bottom flask connected to a refluxing condenser
under magnetic stirring at room temperature which was heated
cence spectra of the complex 2a shows five emission peaks at 584,
5
7
5
94, 625, 656 and 704 nm. All five peaks are assigned as D ! F ,
0
0
5
7 5 7 5 7 5 7 5 7
(50
°
C). The desired amount of the catalyst was added into the flask,
D ! F , D ! F , D ! F , D ! F , and D ! F respectively
0
1
0
2
0
3
0
3
0
4
(
Figure 9).
along with 1 mL of acetonitrile solution containing thioether and
oxidant. The reaction vessel was sealed and stirred into a thermo-
statted oil bath. Reaction progress was monitored by TLC and gas
chromatography. After the reaction, the vessel was cooled to room
temperature and extracted with ethyl acetate for GC analysis. The
thioether oxidation products (sulfoxide and sulfone) were identified
with GC-MS and quantified using gas chromatography with internal
standard techniques.
5
7
The transition assigned as D ! F is magnetic dipole in nature and
0
1
3
+
the intensity is independent from the environment of the Eu ion,
but directly affected by the crystal-field strength of the attached
ligands. Transitions D ! F and D ! F are electric dipole ones,
5
7
5
7
0
2
0
4
Thermogravimetric Analysis
The thermogravimetric analysis (TGA) of all the complexes 1a–10a
were performed in the range of 30–400
°
C under inert conditions
(
N2 gas) at a heating rate of nitrogen gas of 5°C/min. The TGA
curves of the complexes 1a–10a show a single step weight loss of
7
.5, 7.9, 7.5, 7.6, 8.8, 7.2, 7.5, 7.3, 7.5, 7.6% respectively, which are
attributed to the loss of both lattice and coordinated water
molecules (Figure S11).
Powder X-ray Diffraction
The X-ray powder diffraction patterns have been obtained for all
the nanoclusters 1a–10a which has been shown in Figure S14. It
Figure 8. The photoluminescence spectrum of the complex Sm (1a),
excitation at 377 nm.
Figure 9. The photoluminescence spectrum of the complex Eu (2a),
excitation at 460 nm.
Figure 10. The photoluminescence spectrum of the complex Dy (5a),
excitation at 370 nm.
Eur. J. Inorg. Chem. 2021, 1071–1081
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doi.org/10.1002/ejic.202001096
has been observed that powder patterns for all the complexes
[18] R. Khoshnavazi, E. Naseri, S. Tayamon, A. Ghiasi-Moaser, Polyhedron
011, 30, 381.
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
2
perfectly matches suggesting that all ten synthesized complexes
are isostructural in nature. We have also compared the powder
pattern of the complex with the simulated pattern obtained from
single crystal X-ray diffraction (Figure S15). Both the patterns were
almost similar with slight shifting in peaks which we believe might
be due to the change in crystalline material to amorphous material
[
[
20] R. Khoshnavazi, S. Gholamyan, J. Coord. Chem. 2010, 63, 3365.
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Francesconi, Angew. Chem. 2001, 113, 4155; Angew. Chem. Int. Ed. 2001,
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while drying and pressing.
[
[
[
[
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H. E. Ooyama, M. Sadakane, F. Hussain, ChemistrySelect 2019, 4, 12668.
25] J.-X. Li, Z.-X. Du, L.-Y. Xiong, L.-L. Fu, W.-B. Bo, J. Solid State Chem. 2021,
293, 121799.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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4
4
4
4
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3
Instrumentation Centre (USIC) for the instrumentation facilities
and IIT Bombay for ICP-AES. I. K. and V. D. thank CSIR for
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Electrochemistry
·
Heterogeneous catalysis
·
Lanthanides · Photoluminescence · Polyoxometalates
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Manuscript received: December 4, 2020
Revised manuscript received: February 19, 2021
Accepted manuscript online: February 23, 2021
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