Engineering Multifunctionality in Hybrid Polyoxometalates: Aromatic Sulfonium Octamolybdates as Excellent Photochromic Materials and Self-Separating Catalysts for Epoxidation

Article abstract of DOI:10.1021/acs.inorgchem.7b01143

Engineering multifunctionality in hybrid polyoxometalates (hybrid POMs) is an interesting but scarcely explored topic. Herein, we set about engineering two important materials properties, viz., photochromism and self-separating catalysis, in a hybrid POM by modulating the counterion motif. A series of six aromatic sulfonium counterions have been developed on the basis of an aromatic sulfonium counterion motif that allows structural and electronic fine-tuning by changing substituents at multiple locations. Using the aromatic sulfonium counterions and sodium molybdate, six new aromatic sulfonium octamolybdate hybrids (1-6) having formulas (HPDS)₄[Mo₈O₂₆] (1), (HMPDS)₄[Mo₈O₂₆] (2), (MPDS)₄[Mo₈O₂₆] (3), (APDS)₄[Mo₈O₂₆] (4), (AMPDS)₄[Mo₈O₂₆] (5), and (MAPDS)₄[Mo₈O₂₆] (6) (where HPDS = (4-hydroxyphenyl)dimethylsulfonium, HMPDS = (4-hydroxy-2-methylphenyl)dimethylsulfonium, MPDS = (4-methoxyphenyl)dimethylsulfonium, APDS = (4-(allyloxy)phenyl)dimethylsulfonium, AMPDS = (4-(allyloxy)-2-methylphenyl)dimethylsulfonium and MAPDS = (4(methacryloyloxy)phenyl)dimethylsulfonium) have been synthesized, and their structures were confirmed by single crystal X-ray diffraction and ESI-MS analyses. Hybrids 1-6 acted as good solid-state photochromic materials exhibiting color change from white to purple under UV illumination (350 nm), and we show here that the photochromic properties of hybrids 1-6 could be modulated by changing the substitutions on the counterion motif. A coloration kinetic half-life (t_1/2) of 0.33 min was achieved with this class of hybrid POMs. Hybrids 1-6 exhibited excellent self-separating catalytic properties toward the epoxidation of olefins, yielding up to 99% epoxide product with good selectivity in short time. The substituents on the aromatic sulfonium counterions helps to fine-tune the electronic, lipophilic, and solubility properties of the hybrids and thereby their catalytic properties. Moreover, we used ESI-MS analyses to understand the mechanism of catalysis exhibited by octamolybdates 1-6 in the presence of H₂O₂, and we succeeded in identifying a hitherto undetected intermediate, tetraperoxo-octamolybdates, shedding more light on the epoxidation mechanism.

Full text of DOI:10.1021/acs.inorgchem.7b01143

Article
pubs.acs.org/IC
Engineering Multifunctionality in Hybrid Polyoxometalates:
Aromatic Sulfonium Octamolybdates as Excellent Photochromic
Materials and Self-Separating Catalysts for Epoxidation
Ashwani Kumar, Abhishek Kumar Gupta, Manisha Devi, Kenneth E. Gonsalves,
and Chullikkattil P. Pradeep*
School of Basic Sciences, Indian Institute of Technology Mandi, Kamand 175 005, Himachal Pradesh, India
S
* Supporting Information
ABSTRACT: Engineering multifunctionality in hybrid polyoxome-
talates (hybrid POMs) is an interesting but scarcely explored topic.
Herein, we set about engineering two important materials properties,
viz., photochromism and self-separating catalysis, in a hybrid POM
by modulating the counterion motif. A series of six aromatic
sulfonium counterions have been developed on the basis of an
aromatic sulfonium counterion motif that allows structural and
electronic fine-tuning by changing substituents at multiple locations.
Using the aromatic sulfonium counterions and sodium molybdate, six
new aromatic sulfonium octamolybdate hybrids (1−6) having
formulas (HPDS)4[Mo8O26] (1), (HMPDS)4[Mo8O26] (2),
( M P D S ) 4 [ M o 8 O 2 6
] ( 3 ) , ( A P D S ) 4 [ M o 8 O 2 6 ] ( 4 ) ,
(AMPDS)4[Mo8O26] (5), and (MAPDS)4[Mo8O26] (6) (where HPDS = (4-hydroxyphenyl)dimethylsulfonium, HMPDS =
(4-hydroxy-2-methylphenyl)dimethylsulfonium, MPDS = (4-methoxyphenyl)dimethylsulfonium, APDS = (4-(allyloxy)phenyl)-
dimethylsulfonium, AMPDS = (4-(allyloxy)-2-methylphenyl)dimethylsulfonium and MAPDS = (4(methacryloyloxy)phenyl)-
dimethylsulfonium) have been synthesized, and their structures were confirmed by single crystal X-ray diffraction and ESI-MS
analyses. Hybrids 1−6 acted as good solid-state photochromic materials exhibiting color change from white to purple under UV
illumination (350 nm), and we show here that the photochromic properties of hybrids 1−6 could be modulated by changing the
substitutions on the counterion motif. A coloration kinetic half-life (t1/2) of 0.33 min was achieved with this class of hybrid
POMs. Hybrids 1−6 exhibited excellent self-separating catalytic properties toward the epoxidation of olefins, yielding up to 99%
epoxide product with good selectivity in short time. The substituents on the aromatic sulfonium counterions helps to fine-tune
the electronic, lipophilic, and solubility properties of the hybrids and thereby their catalytic properties. Moreover, we used ESI-
MS analyses to understand the mechanism of catalysis exhibited by octamolybdates 1−6 in the presence of H2O2, and we
succeeded in identifying a hitherto undetected intermediate, tetraperoxo-octamolybdates, shedding more light on the epoxidation
mechanism.
class of inorganic materials possessing enormous diversity in
their size, structure and properties. Because of their unique
properties, POMs have been widely used as precursors in the
development of materials that are relevant to optical, magnetic,
catalytic, biological, and electronic applications.16−19 Despite
their widespread use in materials chemistry, POMs have only
scarcely been explored in the development of multifunctional
materials.20
It is well-known that the properties of a hybrid POM can be
engineered by systematically varying its counterions.21 In the
present work, we have attempted to combine two diverse
materials properties, viz., photochromism and self-separating
catalysis, in a simple hybrid POM through molecular design
approach using a new class of electronically and structurally
INTRODUCTION
■
The design and development of multifunctional materials that
exhibit two or more properties of interest are gaining attention
in recent years.1−3 Theranostics, which combine diagnosis and
therapy, are examples of multifunctional materials.4 The
different properties of a multifunctional material can be
brought in by two different components or a single component,
and these properties may simply coexist in the system or
interact with each other generating new properties.5−7 In
general, properties which cooperatively interact giving rise to
synergistic effects are of considerable interest.8,9 Multifunc-
tional materials are often built on platforms like metal
nanoparticles, metal organic frameworks, polymeric materials,
and nanocomposites.10−13 In addition, simple molecular
systems have also been reported as multifunctional materi-
als.14,15 Polyoxometalates (POMs), discrete, anionic metal
oxide clusters of early transition metal ions, represent a vast
Received: May 12, 2017
© XXXX American Chemical Society
A
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tunable aromatic sulfonium counterions. Photochromic materi-
als, which exhibit reversible light-induced switching between
two optically distinguishable states, are important in a wide
range of technological and marketable applications.22−25 POMs
have already been recognized as an important class of
precursors in the development of photochromic materials.25,26
A majority of the reported photochromic POM-hybrids are
organoammonium-POMs;27−29 however, in recent years,
sulfonium-POMs have also been reported as photochromic
materials.30,31 Meanwhile, self-separating catalysts, which
combine the properties of homogeneous and heterogeneous
catalysts in a single entity, are becoming important in many
industrial applications.32,33 In self-separating catalysis, the
catalyst precursor is insoluble in the reaction medium but
forms soluble catalytically active species under the action of a
reagent.34 However, when the reagent gets completely used up,
the catalyst returns to its original structure and precipitates out
of the reaction mixture. The quest for self-separating catalysts is
therefore driven by both economic and environmental reasons.
Although self-separating catalysts have been developed from a
variety of materials, there are only a few reports on POM based
self-separating catalysts so far.34,35 Moreover, to the best of our
knowledge, a multifunctional hybrid POM exhibiting photo-
chromism and self-separating catalysis is quite unprecedented.
In this regard, we were interested in exploring octamolybdate
([Mo8O26]4−) cluster-based systems, as octamolybdate belongs
to an important class of isopolyoxometalates and can exist in a
variety of isomeric forms.36 Over the last few decades,
octamolybdate hybrids have been successfully employed for a
variety of applications including photochromism and catalysis.25
Herein, we have designed and developed a series of
electronically and structurally tunable aromatic sulfonium
counterions (a−f) based on a fundamental counterion motif
A as shown in Chart 1. We imagined that the introduction of an
(MPDST), (iv) (4-(allyloxy)phenyl)dimethylsulfoniumtriflate
(APDST), (v) (4-(allyloxy)-2-methylphenyl)dimethyl-
sulfoniumtriflate (AMPDST), and (vi) (4(methacryloyloxy)-
phenyl)dimethylsulfoniumtriflate (MAPDST), respectively, see
Chart 1. The reaction of these counterions (a−f) with the
commercially available sodium molybdate in deionized water at
pH 4.0 resulted in a series of hybrid POMs, and using these
hybrids, we show here that the photochromic and catalytic
properties of a hybrid POM can be modulated by tuning the
counterion motif.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
ASILs HPDST, APDST, and MAPDST were synthesized and
characterized following reported procedures.30,38 The new
ASILs HMPDST, MPDST, and AMPDST were also
1
synthesized following similar synthetic procedures. The H
NMR spectrum of AMPDST exhibited a multiplet at δ = 6.12−
1
6.07 ppm due to the allyl *CHCH2 proton. The H NMR
spectra of HMPDST, MPDST, and AMPDST exhibited singlet
peaks at 3.03, 3.07, and 3.13 ppm, respectively, due to the six
methyl protons of the sulfonium moiety. The 19F NMR spectra
of HMPDST, MPDST, and AMPDST showed peaks at δ =
−79.26, −77.66, and −78.81 ppm, respectively, due to the
triflate counterion. ESI-MS spectra of HMPDST, MPDST, and
AMPDST exhibited peaks at m/z 169.060, 169.070, and
209.096, respectively, confirming their formation (Figures S1−
S8, Supporting Information). The FT-IR spectrum of
HMPDST showed a peak at 3255 cm−1 due to the phenolic
−OH group and a few peaks in the range 3110−2900 cm−1
corresponding to the aromatic C−H vibrations. The IR peaks
exhibited by HMPDST, MPDST, and AMPDST at 1221, 1238,
and 1240 cm−1 respectively are assigned to the CF3 group of
the triflate counterion moiety38 (Figure S9, Supporting
Information). The phase transition behaviors of HMPDST,
MPDST, AMPDST and MAPDST were analyzed by differ-
ential scanning calorimetry (DSC) and polarized optical
microscopy techniques; see Figures S10−S11, Supporting
Information. The DSC analyses showed melting points of 89,
48, 59, and 97 °C for HMPDST, MPDST, AMPDST, and
MAPDST, respectively. Polarized optical microscopy images of
HMPDST, MPDST, AMPDST, and MAPDST exhibited ionic
liquid textures under a crossed polarizer after 24 h at room
temperature. DSC and polarized optical microscopy studies
therefore show that the behaviors of the ASILs HMPDST,
MPDST, AMPDST, and MAPDST are quite similar to those of
the previously reported ASILs HPDST and APDST.30
Chart 1. Synthesis of Aromatic Sulfonium Counterions
aromatic ring in the counterion motif will provide flexibility to
introduce substituents. In these counterions, we have attempted
to fine-tune the electron availability on the sulfonium S atom by
introducing substituents at two different locations, viz., phenolic
oxygen (R1) and aromatic ring (R2) of the counterion motif A.
We have also attempted structural fine-tuning by introducing
allyl or methacryl groups in three counterions (d−f) for
modulating the solubility, lipophilicity and thereby the catalytic
properties of their hybrids. Interestingly, the triflate salts of the
counterions a−f were found to act as ionic liquids.37 The
various counterions used in this study are labeled as (a) HPDS,
(b) HMPDS, (c) MPDS, (d) APDS, (e) AMPDS, and (f)
MAPDS, which, as mentioned earlier, are the cationic units of
the respective aromatic sulfonium ionic liquids (ASILs) (i) (4-
hydroxyphenyl)dimethylsulfoniumtriflate (HPDST), (ii) (4-
hydroxy-2-methylphenyl)dimethylsulfoniumtriflate
(HMPDST), (iii) (4-methoxyphenyl)dimethylsulfoniumtriflate
Treatment of the ASILs HPDST, HMPDST, MPDST,
APDST, AMPDST, and MAPDST (1.5 equiv) with sodium
molybdate (1 equiv) in deionized water at pH 4.0 led to the
formation of a new series of aromatic sulfonium octamolyb-
dates (HPDS)4[Mo8O26] (1), (HMPDS)4[Mo8O26] (2),
(MPDS)4 [Mo8 O2 6
] (3), (APDS)4 [Mo8 O2 6 ] (4),
(AMPDS)4[Mo8O26] (5), and (MAPDS)4[Mo8O26] (6),
respectively. The hybrids 1−6 were characterized by IR,
NMR, ESI-MS, and single-crystal X-ray diffraction analyses.
The IR spectra of hybrids 1−6 showed bands at 3100−2900
cm−1 due to C−H stretching vibrations and bands at 1590,
1497, and 1420 cm−1 due to the aromatic rings of the
counterions. Hybrids 1 and 2 showed IR bands at 3510 and
3503 cm−1 respectively due to the phenolic −OH group.
Hybrid 6 exhibited an IR band at 1744 cm−1 due to the CO
group of the MAPDS counterion. The IR bands observed at
B
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938, 902, 835, 718, 662, 556, 520, and 443 cm−1 for hybrids 1−
6 are assigned to the Mo−O−Mo and MoO vibrations of the
cluster27 (Figure S12, Supporting Information). Hybrids 1−6
surrounded by four units of the corresponding aromatic
sulfonium counterions as expected. The octamolybdate cluster
exists in β-isomeric form42 in all the hybrids 1−6, see Figure 2.
1
exhibited H NMR peaks at 3.19, 3.17, 3.22, 3.22, 3.21, and
3.28 ppm, respectively, due to the methyl protons of the
1
sulfonium moieties. Hybrids 1 and 2 exhibited H NMR peaks
at 10.73 and 10.67 ppm, respectively, due to the phenolic −OH
group of the counterions. Hybrid 3 exhibited a singlet at 3.86
ppm due to the −OCH3 protons. The allylic protons of the
hybrids 4 and 5 appeared in the range of 6.06−6.00 ppm, see
Figures S13−S18, Supporting Information.
ESI-MS Analyses. For ESI-MS analyses, samples were
prepared by dissolving small quantities of the hybrids 1−6 in
minimum amounts of dimethylformamide followed by dilution
with acetonitrile (MeCN). The spectra were recorded in
negative-ion mode. The most important peak envelope
observed for hybrids 1−6 correspond to the “half cluster”
moiety, {Mo4O13}.39 For example, the mass peaks observed for
hybrids 1−6 at m/z values 746.6167, 761.6451, 761.4343,
786.6593, 801.4933, and 815.4375 can be assigned to the
corresponding “half-cluster” moieties [Mo4O13(HPDS)]1−,
[ M o 4 O 1 3 ( H M P D S ) ] 1 −
,
[ M o 4 O 1 3 ( M P D S ) ] 1 −
and
,
[Mo4 O1 3 (APDS)]1 − [Mo4 O1 3 (AMPDS)]1 −
,
,
[Mo4O13(MAPDS)]1−, respectively. In addition, all the major
peaks observed in the mass spectra of hybrids 1−6 could be
satisfactorily assigned to the corresponding molecular formulas
with varying combinations of counterions. The negative-ion
mode ESI-MS spectrum of a representative hybrid, hybrid 2, is
given in Figure 1. Here, the major peaks observed at m/z values
Figure 2. Crystal structures of the hybrids: (a) hybrid 1; (b) hybrid 2;
(c) hybrid 3; (d) hybrid 4; (e) hybrid 5; (f) hybrid 6. Thermal
ellipsoids are shown at 50% probability level. Solvent molecules are
omitted for clarity. Color code: MoO6 − purple octahedra; S − yellow;
O − red; C − gray; H − dark gray.
Figure 1. Negative-ion mode ESI-mass spectrum of hybrid 2. The
spectrum was recorded in MeCN at a concentration ∼10−6 M. The
inset is an expansion of the peak centered at 761.6451 to show the 1−
charge state.
The asymmetric unit (ASU) of hybrid 1 consists of half of the
[β-Mo8O26]4− cluster, two HPDS counterions, one DMF
molecule and one water molecule. The ASU of hybrid 2
contains half of the [β-Mo8O26]4− cluster surrounded by two
HMPDS counterions and one DMSO molecule. In the case of
hybrid 3, the ASU consists of half of the [β-Mo8O26]4− cluster
surrounded by two MPDS counterions and one water
molecule. The ASU of hybrid 4 contains half of the [β-
Mo8O26]4− cluster and two APDS counterions. In the case of
hybrid 5, the ASU consists of a [β-Mo8O26]4− cluster and four
AMPDS counterions. Finally, the ASU of hybrid 6 consists of
half of the [β-Mo8O26]4− cluster and two MAPDS counterions.
The Mo−O bond distances in hybrids 1−6 are comparable to
those reported for [β-Mo8O26]4− based hybrids.41 The average
S−C distances and C−S−C bond angles of the sulfonium
counterions of the hybrids 1−6 are in the ranges of 1.783(5)−
1.793(5) Å, 103.3(3)−101.6(3)°; 1.772(5)−1.790(5) Å,
102.4(3)−103.2(3)°; 1.763(5)−1.786(7) Å, 100.5(3)−
104.5(3)°; 1.778(6)−1.785(8) Å, 102.8(4)−103.5(4)°;
1.790(1)−1.821(8) Å, 100.6(5)−103.3(5)°; and 1.764(5)−
1.789(6) Å, 100.6(3)−103.5(3)°, respectively. The C−S−C
bond angles in hybrids 1−6 are less than 109°, which reveal
592.5720, 614.5535, 761.6451, 929.7102, and 1400.1745 are
assigned to the formulas [Mo4O13H]1−, [Mo4O13Na]1−,
[Mo4O13(HMPDS)]1−, [Mo4O13(HMPDS)2]1−, and
[Mo8O26(HMPDS)(Na)2]1−, respectively, see Figure 1. Mass
spectral data for the remaining hybrids are presented in Figures
S19−S29 and Tables S1−S6 of Supporting Information. The
ESI-MS data of hybrids 1−6 show the existence of some
reduced clusters as well in the samples.40,41
Single-Crystal X-ray Diffraction Analyses. The single
crystals of hybrids 1−6 were grown from dimethylformamide
(DMF) or DMF:dimethylsulfoxide(DMSO) (1:1, v/v) solvent
mixture by slow evaporation method. The crystal and structure
refinement data for the hybrids 1−6 are presented in Table S7,
Supporting Information. Hybrids 1, 3 and 6 crystallized in
monoclinic P21/C space group, while hybrids 2, 4, and 5
crystallized in triclinic P-1 space group. Single-crystal XRD data
revealed that the hybrids 1−6 contain octamolybdate cluster
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that the sulfonium centers of these hybrids are in trigonal
pyramidal geometry.30 Thermal ellipsoidal plots and some
representative crystal packing diagrams of hybrids 1−6 are
given in Supporting Information, Figures S30−S31.
∼100 min of irradiation while hybrid 6 took ∼160 min to attain
the absorbance saturation.
EPR Analysis. The color changes observed in hybrids 1−6
upon UV irradiation are assumed to be due to the
photoreduction of the Mo6+ (d0) sites of the octamolybdate
cluster to Mo5+ (d1).28 The photoreduction of Mo6+ sites in
hybrids 1−6 is evidenced by EPR spectroscopy. The EPR
spectra of the UV-irradiated samples of hybrids 1−6 exhibited
characteristic EPR spectra of Mo5+ (4d1) with g values 1.925,
1.908, 1.920, 1.912, 1.922, and 1.919, respectively, see Figure 4.
The EPR results observed for hybrids 1−6 are in good
agreement with the literature reports on similar systems.30,31
Solid-State Photochromism. The solid-state photochro-
mic properties of hybrids 1−6 were evaluated under ambient
conditions using diffused reflectance spectroscopy (DRS).
White microcrystalline powders of hybrids 1−6 exhibited
optical band gaps of 3.60, 3.58, 3.31, 3.41, 3.45, and 3.56 eV
respectively in their ground state following the order 1 > 2 > 6
> 5 > 4 > 3, see Figure S32, Supporting Information. Under UV
excitation at 350 nm, the hybrids 1−6 exhibited color change
from white to purple accompanied by the growth of two broad
absorption bands at around λmax values 560 and 472 nm.
Hybrids 2−5 exhibited faster photoresponse showing a visual
color change within 10 s of UV irradiation, while the hybrid 1
took comparatively longer time, ∼ 30 s, to display a visual color
change. At the same time, a visual color change occurred in
hybrid 6 only after 8 min of UV irradiation. The absorption
band of a representative hybrid, hybrid 3, is shown in Figure 3a
Figure 4. Powder X-band EPR spectra (recorded at 150 K) of (a)
hybrid 1, (b) hybrid 2, (c) hybrid 3, (d) hybrid 4, (e) hybrid 5, and (f)
hybrid 6 after irradiation with 350 nm UV lamp at 9.199, 9.196, 9.195
9.179, 9.180, and 9.178 GHz spectrometer frequencies, respectively.
Kinetics of Coloration. The photocoloration kinetics of
hybrids 1−6 were calculated by analyzing their reflectance vs
irradiation time (R(t) vs t) plots. Kinetics studies showed that
in all the cases, the reflectance value decreases sharply at the
beginning and eventually gets flattened out. This behavior is
expected based on the depletion of the Mo6+ sites in the
reaction medium with time, following a pseudo second order
kinetics law, R(t) = (a/bt + 1) + R(∞).31 The R(t) vs time
plots of hybrids 1−6 are given in Figure S34 and the
parameters related to the coloration kinetics are listed in
Table S8, Supporting Information. The coloration kinetics half-
life (t1/2) values calculated for hybrids 1−6 were 1.48, 0.83,
0.33, 0.64, 0.69, and 15.26 min, respectively, which show that
the speed of the photoinduced color change in these hybrids
follow the order 3 > 4 > 5 > 2 > 1 > 6.
Figure 3. (a) K−M-transformed reflectivity of hybrid 3 after
irradiation with 350 nm UV lamp at various time intervals. Inset:
the color of the hybrid corresponding to the time shown in minutes;
(b) 1/R(t) − R(∞) vs time plots for hybrids 1−6.
and similar spectra for other hybrids are presented in Figure
S33, Supporting Information. The absorption spectra of the
photoirradiated samples of hybrids 1−6 matched very well with
those of the similar compounds reported earlier.27,31 In the case
of hybrids 1−5, the absorbance saturation were attained within
It is well-known that for two hybrid POMs, i and j, having
the same type of POM cluster and same photogenerated
absorption bands, the ratio of their photocoloration rate
D
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constants, ki/kj, can be calculated from the ratio of the slopes
(Bi/Bj) of their respective 1/[R(t) − R(∞)] vs time plots
(Figure 3b) in order to compare their photocoloration
speeds.28 The photocoloration rate constant ratios of hybrids
1−5 calculated against that of hybrid 6 were 4.79 for k1/k6, 5.86
for k2/k6, 18.71 for k3/k6, 9.62 for k4/k6, and 8.63 for k5/k6 (see
Table S8, Supporting Information). These values clearly show
the faster rate of photocoloration in hybrids having MPDS,
APDS, AMPDS, HMPDS, and HPDS counterions compared to
hybrid 6 having MAPDS counterions. Hybrid 3 with MPDS
counterion exhibited fastest photocoloration compared to other
hybrids and its photocoloration speed was 18.71 times higher
than that of hybrid 6. Similarly, hybrid 4 with APDS counterion
exhibited a photocoloration speed 9.62 times higher than that
of hybrid 6. The ratio of the photocoloration rate constants of
hybrid 3 and 4 (k3/k4) is 1.94, suggesting that the
photocoloration of hybrid 3 is ∼2 times faster than that of
hybrid 4. A number of factors such as the type of the POM
unit, properties of the organic moiety, nature of the interaction
between the POM and the organic units, experimental
conditions etc. may affect the rate of the photocoloration
exhibited by a hybrid POM. As all the hybrids 1−6 contain the
same cluster, [β-Mo8O26]4−, the type of the POM cluster is not
a variable in the present case. Similarly, all the hybrids contain
the same number of counterions per cluster and the nature of
the interaction between the cluster and the counterions are also
same in hybrids 1−6. The analyses of the crystal packing
diagrams of the hybrids 1−6 show that in all these hybrids, the
sulfur atoms of the sulfonium counterions are oriented toward
the cluster and the positioning of the sulfur atoms with respect
to the cluster are comparable in the series. The sulfonium sulfur
to the cluster oxygen distances (S···O distances) in hybrids 1−6
vary in the range 2.841−3.252 Å, which are shorter than or
close to the sum of the van der Waals radii of O and S atoms
(3.32 Å).43 Considering all these, we believe that the electronic
properties of the counterions a−f, which in turn affect the
electron availability to the cluster on photoexcitation, is a
dominant factor in determining the photochromic behaviors of
the hybrids 1−6.
Role of Counter Ions in Photochromism. Dessapt et al.
have reported the mechanism of the photochromism exhibited
by sulfonium-POM hybrids.31 According to that mechanism,
the UV irradiation causes excitation of an electron from the oxo
ligand to the Mo6+ site of the octamolybdate cluster in hybrids
1−6. Concurrently, a lone-pair electron on the sulfonium S
atom gets strongly polarized toward the POM cluster to
stabilize the electron deficient oxo ligand. Thus, the photo-
generated hole on the POM cluster partially moves onto the
sulfonium S atom of the counterion, while the POM cluster
exhibits color due to the photoreduced Mo sites. The hybrid
maintains the color even after switching-off the UV source due
to charge segregation. Therefore, the electron availability on the
sulfonium S atom to stabilize the photoexcited POM cluster is
very crucial in determining the photochromic behavior of a
sulfonium-POM hybrid (see Figure S36, Supporting Informa-
tion for a suggested mechanism for the observed photo-
chromism).
1) is expected to increase the electron availability on the
sulfonium S atom, enhancing the photochromic behavior of the
hybrid. Hybrid 6 exhibited the highest t1/2 value in the series
probably due to the presence of an ester group at the para
position of the sulfonium center, which has poor electron
donating ability compared to −OH or −OR functional groups.
The t1/2 values exhibited by hybrids 2 and 3 (0.83 and 0.33
respectively) reveal that a −CH3 substituent on the phenolic
oxygen of the counterion motif A (as R1 substituent) is more
effective than a −CH3 substituent at the ortho position of the
aromatic ring (as R2 substituent), see the structure of HMPDS
and MPDS in Chart 1. The t1/2 values of hybrids 4 and 5 were
somewhat similar, 0.64 and 0.69 min, respectively, which show
that the presence of an additional −CH3 group as R2
substituent in AMPDS (see Chart 1) plays only a marginal
role in the photochromic behavior of the hybrid. However, in
the case of hybrids 1 and 2 with HPDS and HMPDS
counterions respectively, an additional −CH3 substituent at R2
position of the counterion (Chart 1) leads to a considerable
improvement in the photocoloration speeds (1.48 and 0.83
min, respectively). The t1/2 values exhibited by hybrids 1 and 4
were 1.48 and 0.64 min, respectively, which suggest that an
alkyl substituent at R1 position of the counterion motif A (−OR
group) is more effective than a −H substituent at the same
position (−OH group). Based on the t1/2 values exhibited by
hybrids 1, 2, 4, and 5, we can assume that −CH3 group as R1
substituent has a dominant role compared to the same group as
R2 substituent in controlling the photochromic behaviors of a
hybrid. However, in the absence of R1 substituent, the effect of
R2 substituent dominates.
A comparison of the best t1/2 values reported so far for
various hybrid POMs are given in Table S9, Supporting
Information. According to this table, the best t1/2 values
reported for organoammonium-{Mo12} cluster hybrids27 are in
the range 51.81−5.44 min; while that for organoammonium-
{Mo8O27}28 hybrids are in the range 37.0−0.8 min. Using
aliphatic sulfonium counterions, t1/2 values of 4.84 and 30 min
have been achieved for clusters {Mo3O8} and {Mo8O26}
respectively.31 The lowest t1/2 value reported so far for an
aromatic sulfonium-POM hybrid is 1.01 min, reported for
(APDS)4[SiMo12O40].30 However, using the same APDS
counterion, a better t1/2 value of 0.64 min is achieved in the
present study for (APDS)4[Mo8O26] (hybrid 4). This reveals
that the choice of the POM cluster is very crucial in deciding
the photochromic properties of a hybrid POM. To the best of
our knowledge, the best t1/2 value achieved so far for a POM-
hybrid is 0.37 min, reported for an organoammonium-{Mo6}
hybrid.29 In the present study, we could achieve a t1/2 value of
0.33 min for an aromatic sulfonium octamolybdate, which
reveals the potential of the counterion motif A (Chart 1) in
controlling the photochromic behavior of a hybrid POM. The
power of the UV source used in our experiments was 8 W,
which is lower than those used (12 W) in some of the examples
cited above. The photochromic behavior exhibited by hybrids
1−6 are reversible in nature, however, the observed color
fading over time is relatively slow. The observed color fading
can be correlated with the reoxidation of the Mo5+ centers by
atmospheric oxygen. It has been reported earlier that in
sulfonium-based POM hybrids, the electron-pair donation from
sulfonium S to the photoexcited cluster is stabilized even under
dark conditions and because of this reason, the lone pair of
electrons are not able to come back to their original position,
hence slowing down the bleaching processes.31 We expect the
The observed order (3 > 4 > 5 > 2 > 1 > 6) of the
photoinduced color change exhibited by hybrids 1−6 may be
explained based on the substituent effect of their counterions.
Hybrid 3 with MPDS counterion exhibited the lowest t1/2 value
in the series. In this case, a −CH3 substitution on the phenolic
oxygen (as R1 substituent on the counterion motif A, see Chart
E
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same reason behind the sluggish reoxidation behavior exhibited
by photoreduced hybrids 1−6 as well.
Catalytic Epoxidation. Some of the hybrid POMs are
known to act as oxidation catalysts.44,45 Therefore, we decided
to test the catalytic properties of hybrids 1−6 using the
epoxidation of olefin as a model reaction. Epoxides are
important precursors for a variety of industrially relevant
materials including glycols, glycol ethers, alkanol-amines, and
polymers.46 Although various POM-hybrids are reported to act
as epoxidation catalysts, reports on octamolybdates as
epoxidation catalysts are rare in the literature.35,47
The epoxidation reactions were carried out in MeCN at 60
°C using 30% H2O2 as the oxidant and cyclooctene as the
substrate, see Scheme 1. Powdered samples of catalysts 1−6
Scheme 1. cis-Cyclooctene Epoxidation with H2O2 Catalyzed
by Hybrids 1−6
Figure 6. Time-dependent yields of cyclooctene oxide catalyzed by
hybrids 1−6 at 60 °C with 1.5 mol % of H2O2.
Table 1. Epoxidation of cis-Cyclooctene Catalyzed by
Hybrids 1−6
a
b
c
entry
catalyst
yield (%)
1
2
3
4
5
6
hybrid-1
hybrid-2
hybrid-3
hybrid-4
hybrid-5
hybrid-6
87
73
82
93
97
99
were insoluble in the reaction medium initially; however, on
addition of 1.5 mmol of H2O2 reagent to the reaction mixture,
the catalysts changed color from white to yellow and became
soluble yielding a homogeneous yellow solution, see Figure 5.
a
Reaction conditions: cis-cyclooctene (0.3 mmol); MeCN (3 mL);
b
n(H2O2):n(cyclooctene) = 1.5; Time = 1.5 h at 60 °C; Catalyst
(0.003 mmol); Yields (%) were calculated from GC.
c
Information. Therefore, the current catalytic system possesses
the advantages of both homogeneous and heterogeneous
catalysis. We believe that the aromatic sulfonium counterions
play a crucial role in regenerating the catalysts after the
consumption of the oxidant H2O2 and also in protecting the
catalytic system from oxidative damages.48
Our studies showed that the hybrids 1−6 exhibit excellent
selectivity in catalysis as they did not yield any diol products
along with the epoxide product, as revealed by GC and NMR
analyses. Hybrids 4−6 in general gave higher yields (>90%)
compared to hybrids 1−3. Hybrid 6 exhibited the best catalytic
activity among all the hybrids tested, as it yielded >99%
conversion within 1.5 h with excellent selectivity. A control
experiment conducted in the absence of the catalyst did not
yield considerable amount of the product. Similarly, control
experiments performed using commercially available
(NH)6Mo7O24.4H2O as the catalyst yielded only 10% epoxide
product even after 6 h. These control experiments therefore
highlight the importance of the aromatic sulfonium-POM
hybrids 1−6 in the observed catalysis.
We were interested in analyzing the reaction products of
hybrids 1−6 with H2O2 in MeCN using ESI-MS analysis for
identifying the catalytic intermediates. The results of these
analyses are discussed in detail for hybrid 6 here, which
exhibited the best catalytic performance among all the hybrids
tested. The negative-ion mode ESI-MS spectrum of the clear
yellow solution obtained on addition of 30% H2O2 to a
suspension of hybrid 6 in MeCN at 60 °C is shown in Figure 7.
The spectrum shows the species [Mo(O)(O2)2] at m/z =
176.8612 that has already been identified in similar catalytic
reactions.35 Interestingly, in addition to this known species, we
Figure 5. (a) cis-Cyclooctene in MeCN; (b) white powder of hybrid 6
added to the reaction mixture; (c) reaction mixture after addition of
H2O2 (homogeneous mixture); (d) beginning of the self-precipitation
of hybrid 6 from the reaction medium; (e) self-separated hybrid 6 after
completion of the reaction.
The dissolution of the catalyst and the formation of a yellow
clear solution indicate the formation of some reactive
intermediates, which could be the real catalytic species of the
reaction.35
The progress of the catalytic reactions were monitored by gas
chromatography (GC) at various time intervals, and the results
are summarized in Figure 6. The reaction having hybrid 6 as the
catalyst yielded 69% epoxide product within 5 min and >99%
yield within 90 min. Similarly, catalysts 1−5 also yielded
epoxide product, and the results are summarized in Figure 6
and Table 1. Interestingly, it was observed that the catalysts
start to precipitate out of the reaction mixture within 15 min of
the start of the reaction and the precipitation gets completed by
∼1.5 h in all the cases. IR analyses showed that the white
powders thus obtained are indeed of the respective catalysts
used. Therefore, the hybrids 1−6 undergo solid−liquid−solid
phase transfer during the catalysis reaction and hence act as
self-separating catalysts. The catalytic reaction gets completed
by ∼1.5 h, and at the end of the reaction, the self-separated
catalysts 1−6 can be separated by simple filtration. The self-
separated catalysts 1−6 could be reused at least 5 times without
any significant loss in activity, see Figure S44, Supporting
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Figure 7. Negative-ion mode ESI-MS of the yellow solution obtained
by reacting hybrid 6 with 30% H2O2 in MeCN.
could observe peaks at m/z = 1438.5207, 1729.5622, and
1837.4060, that could be satisfactorily assigned to the formulas
[Mo8O22(O2)4(C8H14)Na2H]1−, [Mo8O22(O2)4(H2O)2-
(MAPDS)2H]1−, and [Mo8O22(O2)4(H2O)2(C8H14)-
(MAPDS)2H]1−, respectively. Observation of these peaks
indicate the formation of tetraperoxo-octamolybdate inter-
mediates in the reaction medium from octamolybdate catalyst
and H2O2 reagent. To the best of our knowledge, identification
of such tetraperoxo-octamolybdate intermediates in octamo-
lybdate based catalysis is unprecedented. Interestingly, two out
of the three tetraperoxo-octamolybdate intermediates identified
in this study have cyclooctene unit (C8H14) in their formulas,
resembling the intermediates proposed by Sharpless et al.49 in
the case of oxo-diperoxo Mo(VI) catalysts, see Figure S38,
Supporting Information.
There have only been a couple of attempts earlier to identify
the active intermediates of the epoxidation reaction using
octamolybdate catalysts. An intermediate tetraperoxo-octamo-
lybdate with the formula (Hdmpz)4[Mo8O22(O2)4(dmpz)2]·
2H2O had been reported during the preparation of the
oxodiperoxo species [MoO(O2)2(dmpz)2], where dmpz =
3,5-dimethylpyrazol.50 Another reported peroxo-octamolybdate
is [NH4]4[Mo8O24(O2)2(H2O)2]·4H2O.51 Recently, Zhou et al.
had identified oxo-diperoxomolybdenum, [MoO(O2)2], as the
possible active itermediate in the epoxidation reaction using
ESI-MS analyses. However, they did not observe any peroxo-
octamolybdates in their ESI-MS studies.35 In the present study,
for the first time, we were able to identify the species
tetraperoxo-octamolybdate in a catalytic epoxidation reaction
formed by the reaction between octamolybdate catalyst and
H2O2 reagent. We believe that, in addition to the oxo-
diperoxomolybdenum species [MoO(O2)2], the newly identi-
fied tetraperoxo-octamolybdate could also be playing significant
roles in the catalysis exhibited by the octamolybdates in the
presence of H2O2. The reaction products of hybrids 1−5 with
H2O2 in MeCN also exhibited similar ESI-MS spectra; the
details are given in Figures S39−S43 and Tables S10−S14,
Supporting Information. The tetraperoxo-octamolybdate inter-
mediates were also detected in these cases.
Figure 8. (a) FT-IR spectra of pure hybrid 6; (b) hybrid 6 after
reaction with H2O2 (active species); (c) recycled hybrid 6.
band appeared at 582 cm−1 corresonding to the Mo(O2)2
species,52 see Figure 8b.
These results suggest the disintegration of some of the
clusters upon reaction with H2O2. The reaction mixture also
showed a new band at 860 cm−1 corresponding to (O−O)
vibrations33 and another band at 626 cm−1, probably due to the
tetraperoxo-octamolybdate50 intermediates. The FT-IR results
therefore corroborate very well with the ESI-MS results and
confirm the formation of intermediates such as MoO(O2)2 and
peroxo-octamolybdates in the catalytic reaction medium. The
recycled hybrid exhibited spectral features similar to that of the
original hybrid, see Figure 8c, confirming the regeneration of
the octamolybdate hybrid in the reaction medium after the
complete comsumption of the reagent H2O2.
Furthermore, control experiments were performed to check
if the H2O2 reagent has any effect on the alkenyl group and/or
on the sulfonium sulfur of the counterions during catalysis. The
1
mass and H NMR spectra of the recovered hybrid 6 catalyst
showed no change compared to those of the original hybrid 6,
confirming that the oxidant H2O2 has no effect on the
sulfonium counterions, see Figures S45−S48 and Table S16,
Supporting Information. The differences in the catalytic
activities exhibited by hybrids 1−6 may be explained on the
basis of the differences in their counterions.34,48 Hybrids 4−6
exhibited better catalytic performances compared to hybrids 1−
3, probably because of the presence of relatively long allyl/
methacryl groups as R1 substituents, that are more lipophilic
compared to −H and −CH3 groups. These long carbon chain
groups may also help in solubilizing the active species in MeCN
at a faster rate compared to the hybrids 1−3.
Inorder to check if the photochromic and catalytic properties
can coexist in hybrids 1−6, the recoverd catalysts were
subjected to photochromic studies again. A similar color
change from white to purple on UV exposure was observed in
all the cases. Similarly, the photoreduced purple hybrids were
tested again as catalysts. The purple hybrids become white
under experimental conditions and behave quite similar to the
original hybrids.53 These studies confirm that the catalysis and
photochromism can coexist in these hybrids, see Figure S49,
Supporting Information for a model DRS spectrum of the
recovered catalyst.
We have also recorded the FT-IR spectrum of the yellow
reaction product of hybrid 6 and H2O2 in MeCN and
compared it with the spectra of hybrid 6 before and after
catalysis, see Figure 8. Hybrid 6 exhibited MoO and Mo−
O−Mo vibrations at 940 and 652−903 cm−1 respectively, see
Figure 8a. However, after reaction with H2O2, the intensity of
the Mo−O−Mo peak at 903 cm−1 was reduced, and a new
G
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Epoxidation of Bioderived Olefins. We were interested
in checking the applicability of hybrids 1−6 as self-separating
catalysts for the epoxidation of bioderived olefins54,55 as well.
This is because, the fatty acid alkyl esters (FAAE) have a wide
range of industrial applications as plasticizers/stabilizers in the
production of polyvinyl chlorides (PVC) and have relevance as
intermediates in the production of biodegradable lubri-
cants.56,57 The industrially used process (Prileshajev reaction)
for the epoxidation of unsaturated plant oils is known to have
certain drawbacks such as the formation of byproducts,
corrosion of the equipment and the necessity of a neutralization
processes downstream.57 The use of high-valent transition-
metal-ion-based catalysts is perceived as a solution to prevent
such drawbacks.54 We tested hybrid 6 as a model catalyst for
the epoxidation of bioderived olefins using methyl oleate (MO)
or ethyl oleate (EO) as the substrates; see Scheme 2.
confirmed the formation of the epoxide product of MO, see
Figure S51, Supporting Information. NMR analyses did not
show the formation of any side products in this reaction.
Similarly, the 1H NMR, 13C NMR, and ESI-MS analyses of EO
and its epoxidation products also revealed the selective
formation of the expected epoxide product, see Figure 9c,d
and Figures S52−S53, Supporting Information. These studies
therefore show the potential of hybrids 1−6 to act as self-
separating catalysts for the epoxidation of bioderived olefins.
CONCLUSIONS
■
In this study, electronically and structurally tunable aromatic
sulfonium counterions have been used to engineer multi-
functionality in hybrid POMs. Electronic and structural
properties of a series of aromatic sulfonium octamolybdates
have been systematically fine-tuned by introducing appropriate
substitutions on the counterion motif, which in turn helped to
control the overall photochromic and catalytic properties of the
hybrids. In this way, we could achieve a t1/2 value of 0.33 min
for one of the aromatic sulfonium-octamolybdate, which is
among the best t1/2 values reported so far for any POM based
photochromic material. The aromatic sulfonium octamolyb-
dates reported here combine the properties of homogeneous
and heterogeneous catalysts in a single entity and act as good
self-separating catalysts for the epoxidation of olefins. The
catalytic properties of these hybrids could be fine-tuned by
introducing lipophilic substituents on the counterion motif, and
the catalysts could be reused for at least 5 times without any
significant loss of activity. Moreover, the photochromic and
catalytic properties are shown to coexist in these hybrids. ESI-
MS analyses performed on the reaction mixture of aromatic
sulfonium octamolybdates and H2O2 revealed a hitherto
undetected intermediate, tetraperoxo-octamolybdate, shedding
more lights on the epoxidation mechanism of octamolybdates.
Aromatic sulfonium counterions play significant roles in
protecting the catalytic species from oxidative damages during
the epoxidation reaction and also in the regeneration of the
catalyst after the reaction. In short, a new approach to develop
multifunctional POM-organic hybrids using tunable counter-
ions has been successfully explored in this study. Currently, we
are in the process of developing new multifunctional POM
materials using the concepts delineated in this study.
Scheme 2. Epoxidation of Bioderived Olefins (R = CH3 and
C2H5) Catalyzed by Hybrid 6
The reaction was monitored by TLC analyses, which
revealed complete consumption of the starting materials within
6 h of the reaction. The catalyst was precipitated out of the
reaction mixture during this time, which was separated out and
1
the products thus obtained were subjected to H NMR and
ESI-MS analyses. The 1H NMR spectra of MO and its
epoxidation product are given in Figure 9a,b, respectively.
EXPERIMENTAL SECTION
■
Materials and Methods. 4-methylmercaptophenol and 3-methyl-
4-(methylthio)phenol were purchased from Alfa Aesar. 4-
Methoxyphenyl(methyl)sulfane, silver trifluoromethanesulfonate, allyl
bromide, cis-cyclooctene, and cyclooctene oxide were purchased from
Sigma-Aldrich. Na2MoO4·2H2O was purchased from Merck. Methyl
oleate and ethyl oleate were purchased from TCI India. All the
solvents used were of spectroscopic grade. Acetonitrile was distilled
over CaH2 and stored over 4 Å molecular sieves prior to the use.
Physical Measurements. FT-IR spectra were recorded on Agilent
Technologies Cary 600 Series instrument. 1H, 13C, and 19F NMR
spectra were recorded on Jeol-JNM 500 MHz NMR spectrometer
using CDCl3/D2O/CD3CN/DMSO-d6 solvents and TMS as internal
standard. ESI-MS spectra were recorded on Bruker HD compact
instrument. DRS spectra were recorded on PerkinElmer UV/vis/NIR
Lambda 750 spectrophotometer. Differential scanning calorimetry
(DSC) experiments were performed on a Netzsch (Model STA 449
F1 JUPITER) instrument at a heating rate of 5 °C/min under N2
atmosphere. Polarized optical microscopy images of the ASILs were
recorded on Nikon ECLIPSE LV 100 POL microscope. Luzchem
photoreactor model LZC-4 V with lamp LZC-UVA (λexc = 350 nm, P
= 8 W) was used for the photoirradiation of POM hybrids. The X-
1
Figure 9. H NMR spectra: (a) methyl oleate (MO); (b) MO after
epoxidation; (c) ethyl oleate (EO); (d) EO after epoxidation.
Comparison of these spectra reveals that the peak observed at
5.33 ppm for MO due to the −HCCH− protons is shifted to
3.36−3.39 ppm in the product. Similarly, the peak due to the
−CH2 protons adjacent to the alkene group of MO was shifted
from 1.99 to 1.43 ppm in the product. The 13C NMR spectral
analyses showed that the alkene carbon peak of MO at 129
ppm is shifted to 58 ppm in the product, confirming the
formation of the epoxide, see Figure S50 of the Supporting
Information. ESI-MS analyses of the reaction products
H
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band electron paramagnetic resonance (EPR) spectra were recorded at
150 K on JEOL Model JES FA200 instrument.
g, 4.86 mmol) in dry THF was added commercially available allyl
bromide (0.42 mL, 4.86 mmol) at 65 °C. Stirring was continued at the
same temperature for 12 h. After completion of the reaction
(monitored by TLC), the solvent was removed under reduced
pressure to yield a residue. The residue was dissolved in EtOAc (15
mL) and washed with a saturated solution of ammonium chloride. The
organic layer was dried over anhydrous Na2SO4 and concentrated
under reduced pressure. Purification by silica gel column chromatog-
raphy using a gradient solvent system of 0.5−1% ethyl acetate in
hexane yielded (4-(allyloxy)-2-methylphenyl)(methyl)sulfane as color-
less oil. Yield: 0.57 g, 90%. 1H NMR (500 MHz, CDCl3): δ ppm 7.18
(d, 1H, J = 8.25 Hz, ArH), 6.78 (d, 1H, J = 2.75 Hz, ArH), 6.75 (dd,
1H, J1 = 2.75 Hz, J2 = 8.25 Hz, ArH), 6.07−6.01 (m, 1H, CH2−
CH*CH2), 5.40 (dd, 1H, J1 = 17.17 Hz, J2 = 1.40 Hz, CHCH2*
(trans)), 5.28 (dd, 1H, J1 = 11.00 Hz, J2 = 1.40 Hz, CHCH2* (cis)),
4.51 (d, 2H, J = 5.45 Hz, −OCH2), 2.40 (s, 3H, −SCH3), 2.37 (s, 3H,
−ArCH3). 13C NMR (125 MHz, CDCl3): δ ppm 156.91, 138.87,
133.22, 129.03, 128.23, 117.60, 116.78, 112.66, 68.83, 20.43 17.84.
ESI-MS: calcd for C10H12OS [M]+, 194.077; found, 194.100.
X-ray Crystallography. Single crystal X-ray diffraction data of
hybrids 1−6 were collected on an Agilent Super Nova diffractometer,
equipped with multilayer optics, monochromatic dual source (Cu and
Mo) and Eos CCD detector, using Mo Kα (0.71073 Å) radiation at
293 K (for hybrid 1) and at 150 K (for hybrids 2−6). Data acquisition,
reduction and analytical face-index based absorption correction were
performed by using CrysAlisPRO program.58 The structure was solved
with ShelXS and refined on F2 by full matrix least-squares techniques
using ShelXL program provided in Olex2 (v.1.2) program package.59,60
Anisotropic displacement parameters were applied for all the atoms,
except hydrogen atoms and some less intensely scattered carbon
atoms. Data CCDC: 1536033−1536038 contain the supplementary
crystallographic data of hybrids 1−6 respectively.
In general, hybrids 1−6 are crystalline in nature; however, due to
their weakly diffracting nature, the quality of data were poor in some
cases. As a result of this, the probabilities of finding less intensely
scattered atoms, especially those from the solvent or counterion
moieties, were less and such atoms had to be refined isotropically.
Because of this, Checkcif alerts of “isotropic non-H atoms in main
residue(s)” were generated in some cases. Other Checkcif alerts like
“short interaction D···A contacts”, “Large Hirshfeld Difference”,
“Solvent Accessible Void” etc. might have arisen due to disorders in
the structure. “Short contacts” between disordered fragments are to be
expected. Due to the poor quality of diffraction data and small
unresolved disorders, alerts like “high “MainMol” Ueq as compared to
neighbors”, and high “ADP max/min Ratio”, were generated in hybrid
6. In addition, “ADDSYM” suggested possible pseudo/new space
group for hybrid 5, but our attempts to refine and solve the structure
in the suggested space group were unsuccessful.
Synthesis of AMPDST. AMPDST was synthesized using a
1
procedure similar to that used for HMPDST. Yield: 0.97 g, 70%. H
NMR (500 MHz, D2O): δ ppm 7.83 (d, 1H, J = 8.95 Hz, ArH), 7.14
(dd, 1H, J1 = 2.75 nad J2 = 8.90 Hz, ArH), 7.03 (d, 1H, J = 2.75 Hz,
ArH), 6.12−6.07 (m, 1H, CH2−CH*CH2), 5.42 (d, 1H, J = 17.15,
−CCH2* (trans)), 5.34 (d, 1H, J = 11 −CCH2* (cis)), 4.67 (d,
2H, J = 5.5 Hz, −OCH2), 3.13 (s, 6H, −S(CH3)2), 2.53 (s, 3H,
ArCH3). 13C NMR (125 MHz, D2O): δ ppm 162.13, 143.49, 131.68,
129.35, 120.87, 118.36, 117.53, 117.36, 115.81, 114.61, 69.10, 28.34,
18.90. 19F NMR (470 MHz, D2O): δ ppm −78.81 (3F, s, −CF3). ESI-
MS: calcd for C9H13OS+ [M]+, 209.100; found, 209.096. FT-IR
(cm−1): 3107 (w), 3028(sh), 2932 (m), 1583 (sh), 1495 (sh), 1456
(m), 1421 (m), 1240 (vs), 1158 (sh), 1026 (sh).
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cifdata.
Gas Choromatography (GC) Analyses. Gas chromatograms
were recorded on an Agilent 7890A series gas chromatograph system
equipped with a HP-5 column. Flame ionization detector (FID) and
N2 carrier gas were used for the sample analyses. Agilent open lab
control panel software A.01.05 (1.3.19.115) was used to analyze the
results and to operate the GC system.
Synthesis of MAPDST. MAPDST was synthesized according to a
literature procedure.38 Yield: 0.74 g, 83%. 1H NMR (500 MHz, D2O):
δ ppm 8.04 (d, 2H, J = 8.20 Hz, ArH), 7.52 (d, 2H, J = 8.90 Hz, ArH),
6.41 (s, 1H, −CCH2* (trans)), 5.95 (s, 1H, −CCH2* (cis)), 3.25
(s, 6H, −S(CH3)2), 2.03 (s, 3H, −CH3). 13C NMR (125 MHz, D2O):
δ ppm 167.60, 155.05, 134.88, 131.32, 129.48, 124.58, 122.44, 28.64,
17.26. 19F NMR (470 MHz, D2O): δ ppm −79.22 (3F, s, −CF3). ESI-
MS: calcd for C9H13OS+ [M]+, 223.079; found, 223.081. FT-IR
(cm−1): 3102 (w), 3032 (sh), 2928 (w), 1732 (sh), 1636 (w),
1584(sh), 1500 (sh), 1430 (w), 1248 (sh), 1168 (m), 1029 (sh).
General Procedure for the Synthesis of Hybrids 1−6. Hybrids
1−6 were synthesized by using a general procedure as follows:
Na2MoO4·2H2O (0.50 g, 2.06 mmol) was dissolved in 5 mL of water,
and the pH of the reaction mixture was adjusted to 4.0 with 4 M HCl.
Aromatic sulfonium counterion (3.09 mmol), which was dissolved in 5
mL of water, was added dropwise to the reaction mixture with
continuous stirring for 30 min. The white precipitates of hybrids 1−6
thus obtained were collected through filtration and washed
successively with cold H2O, EtOH, acetone, and diethyl ether followed
by air drying. The single crystals of hybrids 1−6 were obtained from
DMF or DMSO/DMF (1:1, v/v) solvent mixture via slow evaporation
method.
Synthesis and Characterisations of Aromatic Sulfonium
Ionic Liquids (ASILs). Synthesis of HMPDST. To an ice-cooled and
stirred solution of 3-methyl-4-(methylthio)phenol (0.50 g, 3.24 mmol)
and AgOTf (1.25 g, 4.86 mmol) in anhydrous acetonitrile was added
methyl iodide (0.30 mL, 4.86 mmol) under N2 atmosphere. The
stirring was continued at 0 °C for 2 h. Subsequently, the stirring was
stopped for 30 min to settle down the silver iodide (AgI). The AgI was
filtered off and the solvent was removed under reduced pressure to
afford HMPDST as a viscous solid. Yield: 0.81 g, 78%. 1H NMR (500
MHz, CD3CN): δ ppm 8.61 (s, −OH), 7.74 (d, 1H, J = 8.95 Hz,
ArH), 7.00 (dd, 1H, J1 = 2.75 and J2 = 8.90 Hz, ArH), 6.89 (d, 1H, J =
2.75 Hz, ArH), 3.03 (s, 6H, −S(CH3)2), 2.49 (s, 3H, ArCH3). 13C
NMR (125 MHz, CD3CN): δ ppm 163.01, 144.81, 131.24, 119.23,
117.40, 114.00, 29.69, 19.93. 19F NMR (470 MHz, CD3CN): δ ppm
−79.26 (3F, s, −CF3). ESI-MS: calcd for C9H13OS+ [M]+, 169.069;
found, 169.0607. FT-IR (cm−1): 3255 (vs), 3026 (sh), 2910 (w), 1610
(w), 1513 (sh), 1488 (w), 1420 (w), 1221 (vs), 1160 (sh), 1021 (sh).
Synthesis of MPDST. MPDST was synthesized using a procedure
1
(HPDS)4[Mo8O26] (Hybrid 1). Yield: 62% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 10.73 (1H, s, ArOH), 7.88 (2H, d, J =
8.25 Hz, ArH), 7.03 (2H, d, J = 8.95 Hz, ArH), 3.19 (6H, s,
−S(CH3)2). FT-IR (cm−1): HPDS+ cation, 3516 (sh), 3084 (w), 3012
(sh), 2935 (w), 1585 (sh), 1497 (sh), 1423 (w), 1285 (sh), 1242 (sh),
1172, 1093, 1044, ν(MoO, Mo−O−Mo), 944 (sh), 902 (vsh), 838
(sh), 707 (m), 657(br), 556 (m), 520 (m), 450 (w).
1
similar to that used for HMPDST. Yield: 0.83 g, 80%. H NMR (500
MHz, CD3CN): δ ppm 7.84 (d, 2H, J = 9.6 Hz, ArH), 7.21 (d, 2H, J =
6.90 Hz, ArH), 3.89 (s, 1H, ArOCH3), 3.07 (s, 6H, −S(CH3)2). 13C
NMR (125 MHz, CD3CN): δ ppm 165.43, 133.01, 118.37, 117.32,
56.86, 29.96. 19F NMR (470 MHz, CD3CN): δ ppm −77.66 (3F, s,
−CF3). ESI-MS: calcd for C9H13OS+ [M]+, 169.069; found, 169.070.
FT-IR (cm−1): 3102 (m), 3032(sh), 2910 (w), 1590 (sh), 1503 (sh),
1470 (w), 1428 (w), 1238 (vs), 1158 (sh), 1023 (sh).
Synthesis of (4-(Allyloxy)-2-methylphenyl)(methyl)sulfane. (4-
(Allyloxy)-2-methylphenyl)(methyl)sulfane was synthesized by adopt-
ing a published procedure.30 To a stirred solution of 3-methyl-4-
(methylthio)phenol (0.50 g, 3.24 mmol) and cesium carbonate (1.58
1
(HMPDS)4[Mo8O26] (Hybrid 2). Yield: 72% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 10.67 (1H, s, ArOH), 7.96 (1H, d, J =
8.90 Hz, ArH), 6.94 (1H, dd, J1 = 2.40 and J2 = 8.60 Hz, ArH), 6.83
(1H, d, J = 2.75 Hz, ArH), 3.17 (6H, s, −S(CH3)2), 2.48 (3H, s,
ArCH3). FT-IR (cm−1): HMPDS+ cations, 3509 (sh), 3155(w), 3012
(sh), 2921 (w), 1598 (sh), 1575 (sh), 1425 (w), 1314 (sh), 1046 (m),
ν(MoO, Mo−O−Mo), 943 (sh), 903 (vsh), 842 (sh), 702 (sh), 651
(br), 544 (m), 521 (m), 445 (w).
I
DOI: 10.1021/acs.inorgchem.7b01143
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
(MPDS)4[Mo8O26] (Hybrid 3). Yield: 67% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 8.01 (d, 2H, J = 9.60 Hz, ArH), 7.26
(d, 2H, J = 8.95 Hz, ArH), 3.86 (s, 3H, ArOCH3), 3.22 (s, 6H,
−S(CH3)2). FT-IR (cm−1): APDS+ cations, 3090 (w), 3016 (sh), 2923
(w), 1594 (sh), 1494 (sh), 1423 (w), 1315 (w), 1177 (m), ν(MoO,
Mo−O−Mo), 940 (sh), 900 (v, sh), 831 (sh), 698 (m), 662 (br), 552
(m), 520 (sh), 443 (w).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01143.
Characterization details of the compounds reported in
this study (PDF)
1
(APDS)4[Mo8O26] (Hybrid 4). Yield: 68% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 8.01 (d, 2H, J = 8.90 Hz, ArH), 7.27
(d, 2H, J = 8.95 Hz, ArH), 6.06−6.00 (m, 1H, −CH2−CH*=CH2),
5.41 (dd, 1H, J1 = 1.70, J2 = 18.90 Hz, −CHCH2* (trans)), 5.29
(dd, 1H, J1 = 1.63 and J2 = 10.65 Hz, −CHCH2* (cis)), 4.70 (d,
2H, J = 5.5 Hz, ArOCH2), 3.22 (s, 6H, −S(CH3)2). FT-IR (cm−1):
APDS+ cations, 3095 (w), 3012 (sh), 2927 (w), 1590 (sh), 1497 (sh),
1426 (w), 1332 (w), 1176 (m), ν(MoO, Mo−O−Mo), 938 (sh),
906 (v, sh), 830 (sh), 704 (w), 656 (b), 551 (m), 521 (m), 443 (w).
Accession Codes
CCDC 1536033−1536038 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
(AMPDS)4[Mo8O26] (Hybrid 5). Yield: 63% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 8.08 (d, 2H, J = 8.90 Hz, ArH), 7.19
(d, 1H, J = 8.95 Hz, ArH), 7.08 (s, 1H, ArH), 6.06−6.00 (m, 1H,
−CH2−CH*CH2), 5.40 (dd, 1H, J1 = 1.37 and J2 = 17.17 Hz,
−CHCH2* (trans)), 5.28 (d, 1H, J = 10.35 Hz, −CHCH2*
(cis)), 4.68 (d, 2H, J = 4.8 Hz, ArOCH2), 3.21 (s, 6H, −S(CH3)2),
2.55 (s, 3H, ArCH3). FT-IR (cm−1): APDS+ cations, 3088 (w), 3027
(sh), 2929 (w), 1597 (sh), 1484 (sh), 1423 (w), 1298 (sh), 1183 (m),
ν(MoO, Mo−O−Mo), 938 (sh), 903 (v, sh), 837 (sh), 707 (sh),
652 (br), 552 (m), 515 (m), 449 (w).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pradeep@iitmandi.ac.in; Fax: +91 1905 267 009; Tel:
+91 1905 267 045.
ORCID
Chullikkattil P. Pradeep: 0000-0002-5811-8509
Notes
The authors declare no competing financial interest.
1
(MAPDS)4[Mo8O26] (Hybrid 6). Yield: 60% based on Mo. H NMR
(500 MHz, DMSO-d6): δ ppm 8.15 (d, 2H, J = 8.95 Hz, ArH), 7.58
(d, 2H, J = 8.90 Hz, ArH), 6.32 (s, 1H, −CCH2* (trans)), 5.96 (s,
1H, −CCH2* (cis)), 3.28 (s, 6H, −S(CH3)2), 2.01 (s, 3H, −CH3).
FT-IR (cm−1): APDS+ cations, 3104 (w), 3027 (sh), 2927 (w), 1631
(w), 1581 (m), 1494 (m), 1415 (m), 1317 (sh), 1169 (sh), ν(MoO,
Mo−O−Mo), 940 (sh), 903 (v, sh), 835 (sh), 718 (sh), 652 (br), 549
(m), 516 (sh), 443 (w).
ACKNOWLEDGMENTS
■
C.P.P. thanks IIT Mandi for infrastructural facilities (AMRC,
IIT Mandi) and SAIF, IIT Madras for EPR analyses. A.K.
thanks UGC, Govt. of India for a Senior Research Fellowship.
Catalytic Activities of Hybrids 1−6 toward the Epoxidation
of cis-Cyclooctene. Three milliliters of an acetonitrile (MeCN)
solution of cis-cyclooctene (0.30 mmol) containing 0.003 mmol of
catalysts (hybrid 1/2/3/4/5 or 6) and 0.45 mmol of H2O2 was stirred
in a round-bottom flask equipped with a reflux condenser at 60 °C.
Conversion of the reaction was monitored by GC analyses using
internal standard method (phenyl bromide).61 Aliquots (30 μL) were
withdrawn after 5, 30, 60, and 90 min time intervals for time-
dependent product analyses. Each time, the aliquots were diluted with
1 mL of MeCN and dried over sodium sulfate. Internal standard (2.0
μL) was added to the resulting solution, filtered through a nylon-6
filter (0.22 μm), and subjected to gas chromatography (GC) analyses.
Reuse of the Catalyst. The recovered catalyst was washed
successively with ethanol, acetone, and diethyl ether and finally dried
in an oven at 40 °C. The resulting catalyst was characterized by IR
spectroscopy, NMR, and HRMS analyses. The catalyst was reused up
to 5 catalytic cycles. The catalyst was found to be robust after each
cycle and the yields obtained in consecutive cycles are given in Figure
S44. The NMR and mass spectrum of the recovered catalyst hybrid 6
are given in Figures S45−S48 and Table S16, Supporting Information.
General Procedure for the Epoxidation of Fatty Acid Alkyl
Ester (FAAE). MO/EO (0.5 mmol) was dissolved in 3 mL of MeOH.
To this solution, 0.005 mmol of hybrid 6 as catalyst was added
followed by the addition of 1.5 equiv of 30% H2O2. The catalyst gets
dissolved in the reaction mixture, and the color becomes yellow. The
reaction mixture was stirred at 60 °C for 6 h, and the reaction
dynamics were monitored from time to time using TLC measurements
revealed complete consumption of the starting material within 6 h.
Subsequently, the self-separated catalyst was filtered out. The reaction
mixture was dried over Na2SO4, the solvent was evaporated under
reduced pressure, and the product was extracted with hexane (3 × 5
mL). Hexane was removed under reduced pressure to afford the
epoxide product of MO/EO. The product was analyzed by NMR and
ESI-MS techniques.
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