Optically active polyoxometalate-based silica nanohelices: Induced chirality from inorganic nanohelices to achiral POM clusters

Article abstract of DOI:10.1002/chem.201801905

In order to investigate the principle of chiral induction from nanometric silica helices to polyoxometalate (POM) clusters, a series of optically active silica POM-based nanohelices (NANOPOMs) have been prepared by electrostatic grafting and direct adsorption of a-Keggin polyoxometalate [a-PW₁₂ O₄₀]³ to well-defined left-and right-handed silica nanohelices. UV/Vis, Raman, DRIFT, TEM, HR-TEM, EDS and circular dichroism (CD) spectroscopy were used to characterize these NANOPOMs, and confirm the presence of POM clusters as well as their interactions with the helical support. The optical activity of the left-handed and right-handed NANOPOMs has been proven by CD spectroscopy. Their CD spectra are mirror images of one another, showing cotton effects at around 214 and 276 nm, this last contribution corresponding to the oxygen-to-tungsten charge-transfer bands of Keggin polyoxoanions. The CD signal of POM clusters is strongly enhanced for NANOPOMs built by adsorption of POM onto silica nanohelices, indicating a better induced optical activity to POM clusters. These nanohelices are stable, recoverable and active catalysts in the oxidation of sulfides. To the best of our knowledge, the present research represents the first examples of optically active POM-containing silica nanohelices in which achiral POM clusters have been grafted onto silica nanohelices, and display chiroptical effects.

Full text of DOI:10.1002/chem.201801905

A Journal of
Accepted Article
Title: Optically Active Polyoxometalate-based Silica Nanohelices:
Induced Chirality from Inorganic Nanohelices to Achiral POM
Clusters
Authors: Sylvain Nlate, Mariam Attoui, Emilie Pouget, Reiko Oda,
David Talaga, Gwénaëlle Le Bourdon, and Thierry Buffeteau
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801905
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Supported by

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Optically Active Polyoxometalate-based Silica Nanohelices:
Induced Chirality from Inorganic Nanohelices to Achiral POM
Clusters.
[
a, b]
[a]
[a]
[b]
[b]
Mariam Attoui,
Emilie Pouget, Reiko Oda, David Talaga, Gwénaëlle Le Bourdon, Thierry
[
b]
[a]
Buffeteau,* and Sylvain Nlate*
Abstract: In order to investigate the principle of chiral induction from
nanometric silica helices to polyoxometalate (POM) clusters, a
series of optically active silica POM-based nanohelices
POMs are efficient catalysts for many reactions through either
[
4]
homogeneous or heterogeneous processes.
Due to the
advantage of POMs and the importance of chirality, tremendous
efforts have been dedicated to the development of chiral POMs-
based materials. Structures of most POM clusters are generally
symmetric, increasing their difficulty to crystallize in a non-
centrosymmetric space group. Thus, the access to chiral POM-
based structures is very challenging. In this context, a variety of
(
NANOPOMs) have been prepared by electrostatic grafting and
3-
direct adsorption of -Keggin polyoxometalate [-PW12
40
O ] to well-
defined left- and right-handed silica nanohelices. UV-vis, Raman,
DRIFT, TEM, HR-TEM, EDS and circular dichroism (CD)
spectroscopy were used to characterize these NANOPOMs, and
confirm the presence of POM clusters as well as their interactions
with the helical support. The optical activity of the left-handed and
right-handed NANOPOMs has been proven by CD spectroscopy.
Their CD spectra are mirror images of one another, showing cotton
effects at around 214 and 276 nm, this last contribution
corresponding to the oxygen-to-tungsten charge-transfer bands of
Keggin polyoxoanions. The CD signal of POM clusters is strongly
enhanced for NANOPOMs built by adsorption of POM onto silica
nanohelices, indicating a better induced optical activity to POM
clusters. These nanohelices are stable, recoverable and active
catalysts in the oxidation of sulfides. To the best of our knowledge,
the present research represents the first examples of optically active
POM-containing silica nanohelices in which achiral POM clusters
have been grafted onto silica nanohelices, and display chiroptical
effects.
[5]
chiral POM have been studied, and most of them are usually
unstable either in solution or in the solid state, and their
enantiomers are difficult to isolate from each other because of
rapid racemization in solution, and only a few of spontaneous
[
6]
resolution of chiral POMs were obtained.
Among
chiral
POMs-based materials reported so far, helical self-assembled
supramolecular architectures have received more attention in
the last few years because of their attractive structural feature
and potential applications. Many approaches are proposed for
the preparation of POM-based helical materials generally
obtained by chirality induction through attachment of chiral unit
as a chirality transfer agent onto the POM, or by spontaneous
symmetry breaking of POM upon crystallization in the absence
of any chiral source, the latter being the strategy used most for
[
7]
the preparation of chiral POM-based materials. However, the
spontaneous resolution of POMs is difficult because of their
rapid racemization in solution, partial hydrolysis, and fluxional
behavior. Despite these advances, we are still lacking an
effective method for the construction of helical structures based
on POMs. Exploring new POM-based nanostructures remains
an interesting and challenging topic for chemists.
Introduction
The quest for new and highly optically active materials is
currently one of the primary challenges in chemical synthesis.
[
1]
In this context, chiral polyoxometalates (POMs)-based materials
which can combine the functionality of both POMs and chiral
materials have attracted particular attention in recent years, due
to their various structure, remarkable properties, and potential
applications in the fields of catalysis, bio-medicine, optics and
The organization of Gold nanoparticles (GNPs), or dyes onto
helical structures have shown interesting chiroptical properties,
[
8]
offering a useful approach for the design and formation of
optically active smart materials. Other materials such as
polypyrrole threads and transition-metal complexes had also
[
2]
[9]
nanomaterials.
Polyoxometalates are distinctive inorganic
been organized onto various helical structures. Meanwhile, to
metal oxide clusters that are the source of fascinating
architectures with an almost unmatched range of physical and
the best of our knowledge, POM-based helical structures built by
adsorption or electrostatic grafting of POM clusters onto well-
defined helical structures (POM@helices) have never been
[3]
chemical properties, resulting in a wide variety of applications.
They are able to form dynamic structures ranging in size from
described.
In
general,
nanostructures
obtained
by
[
2d]
nano- to micrometer scale. Due to their rich redox chemistry,
supramolecular organization of organic molecules are very
[10]
sensitive to chemical, mechanical and thermal perturbation.
Thus, much effort has been made to develop more stable
[
8a,11]
[
a]
M. Attoui, Dr E. Pouget, Dr R. Oda, Dr S. Nlate
Chimie et Biologie des Membranes et des Nanoobjets (CBMN),
CNRS, UMR 5248,
inorganic chiral nanostructures.
We have shown that
organic self-assembled nanohelices based on di-cationic gemini
surfactants with tartrate counterions can be used as a template
for sol-gel condensation of tetraalkoxysilane to give inorganic
Université de Bordeaux-Bordeaux INP,
Allée St Hilaire, Bat B14, 33607 Pessac, France
E-mail: s.nlate@iecb.u-bordeaux.fr
[
12]
silica counterparts.
The organization of GNPs onto the
surface of these silica helical structures give stable GNP
nanohelices in which the plasmonic chiratity is induced by the
chiral arrangement of nanoparticles, as a result of the transfer
of optical properties from the helical silica template to GNPs.
Based on these results, and due to the interest of chiral hybrid
nanomaterial in one hand and POM chemistry on the other hand,
[
b]
Dr D. Talaga, G. Le Bourdon, Dr T. Buffeteau
Institut des Sciences Moléculaires (ISM), CNRS, UMR 5255
Université de Bordeaux
[
8a]
351 Cours de la libération, 33405 Talence
Supporting information for this article is given via a link at the end of
the document.
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we sought to prepare POM-based silica nanohelices
tetramethyl) ethylene diammonium] noted here after as 16-2-16.
In the presence of tartrate counterions they form nanometrical
helical structures in water, as previously described by Oda et
(
NANOPOMs) by grafting -Keggin polyoxometalate [-
3-
PW12
O
40
]
to silica nanohelical templates with well-defined
[
12]
morphologies, in order to investigate their chiroptical properties.
The grafting of POMs onto a template can be achieved through
various methods depending on the type of template. Since
POMs are negatively charged, the first strategy to organize them
on the support relies on the counterions exchange with positively
al.
Organic self-assembled nanohelices (L)-2 and (D)-2 with
well-defined helical pitch, diameter and handedness are
obtained. Right-handed helices are formed with (L)-tartrate
whereas left-handed helices are obtained with (D)-tartrate.
Organic nanohelices (L)-2 and (D)-2 were then used as template
for the sol-gel condensation of tetraethyl orthosilicate (TEOS) to
obtain the corresponding inorganic silica nanohelices (L)-3 and
[
13]
charged materials, even if the impregnation of Keggin type
heteropolyacids on the surface of silica materials is very solvent
[
14]
[12]
dependent,
and electrostatic interactions do not avoid the
(D)-3, using the previous reported procedures.
leaching of POM clusters from the template in polar media. On
the other hand, the coupling of POM units with chiral species
through covalent bonding gives stable POM-based hybrid
Electrostatic grafting approach to POM-based silica
nanohelices.
[
15]
materials that showed interesting optical activities. A few years
[16]
ago, Wu et al. have reported stable chiral three-component
supramolecular POM-based systems in which the chirality is
transferred from an alpha-cyclodextrine as a chiral source, to
cationic dyes via the bridge of an azobenzene-grafted Anderson-
type polyoxometalate cluster, and recently shown that this
chirality can be modified by modulating the interaction of POMs
The polyanionic feature of POMs allows them to interact with
many positively charged molecules, resulting in various POM-
based nanostructures and functional materials. Thus, our first
approach to organize POM clusters on silica nanohelices
involved the counterion exchange. As shown in scheme 1 for
right-handed helices, the first step involves the surface
functionalization of silica nanohelices (L)-3 and (D)-3, by
aminopropyl triethoxysilane (APTES) to yield the APTES
[
16b]
with cyclodextrins.
To date, silica NANOPOM helical
structures built by electrostatic bonding or adsorption of POM
clusters onto silica nanohelical templates, as well as their
chiroptical activity investigation have, to the best of our
knowledge, never been reported.
In this article, we report the preparation and characterization
of optically active POM-based silica nanohelices constructed by
electrostatic and direct adsorption coupling of achiral POM
clusters onto chiral silica nanohelices. UV-vis, Raman, diffuse
[
8b]
functionalized silica nanohelices (L)-4 and (D)-4. The amount
of effective amine groups at the surface of silica nanohelices (L)-
4 and (D)-4 has been estimated by UV spectroscopy titration,
2
[8a]
and found to be 0.7-1 amine group per nm . The reaction of
amine functionalized silica nanohelices with the -Kegging
heteropolyacid H PW12O40 in water leads to the corresponding
3
right-handed (L)-5 and left-handed (D)-5 silica NANOPOM
helices, in which anionic POM clusters form ion pair with the
surface ammonium groups of silica nanohelices. It is noteworthy
that partial direct reaction of POM clusters with the residual
silanol groups of APTES functionalized silica nanohelices (L)-4
and (D)-4 are also considered. However, we believe that the
high diluted reaction medium used combined to the basicity of
amino groups of APTES will probably favor the formation of the
reflectance
infrared
Fourier
transformation
(DRIFT)
spectroscopy as well as TEM and HR-TEM have been used to
demonstrate the presence of the POM clusters at the surface of
silica nanohelices and to get information about the interactions
between the two. In addition, the chiroptical properties of silica
NANOPOM helices have been investigated by CD spectroscopy
to reveal the chirality induction from silica nanohelices to the
POM clusters. A particular effort is given to investigate the
effects of the grafting approach on the stability and chiroptical
properties of NANOPOMs. We also investigate the catalytic
efficiency and the recovery capacity of these NANOPOM helices
for the oxidation of model sulfides.
3-
ammonium salt of -Keggin polyoxometalate [-PW12O40] .
Direct adsorption of POM clusters onto silica nanohelices.
We have also grafted POM onto silica nanohelices by a direct
adsorption of POM clusters onto silica nanohelices. In this case,
POM clusters are directly bounded to silica nanohelices, and
thus closer to the chiral source, which should be beneficial for
the chirality induction. As shown in scheme 1, the treatment of
silica nanohelices (L)-3 and (D)-3 with an aqueous solution of
Results and Discussion
Preparation of POM-based silica nanohelices.
3
the heteropolyacid H PW12O40 provides right-handed (L)-6 and
Silica NANOPOM helices were obtained by electrostatic
coupling and direct adsorption of POM clusters onto silica
nanohelices as summarized in scheme 1. For this purpose, we
left-handed (D)-6 silica NANOPOM helices in which achiral POM
clusters are directly adsorbed onto nanohelices. Whereas the
nature of the interactions is no clearly established, Lefebvre et
[
19]
3-
al., have investigated the nature of direct interactions between
Keggin-type heteropolyacids and silica supports using
have selected the -Keggin polyoxometalate [-PW12
40
O ]
as
[
17]
a
model POM because of its catalytic properties
and its ability
combination of physicochemical methods and density functional
theory (DFT) calculations. These studies revealed that POM
species are linked to the silica support by proton interactions
with two or three silanol groups, or via three covalent bonds
to be used in acidic or basic form. Silica nanohelices used as
chiral source for POM clusters were prepared by the self-
assembly of (L)-1 and (D)-1 cationic bis-quaternary ammonium
[
18]
Gemini
surfactants
[(N,N’-dihexadecane-N,N,N’,N’-
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formed by deshydroxylation of these two species, as main
interactions. Thus, in the present work, these interactions can
be considered for the adsorption of acidic POMs. Therefore,
hydroxyl groups of silanols (Si-OH) at the surface of silica
nanohelices (L)-3 and (D)-3 react with the heteropolyacid
H
3
PW12O40 through proton interactions, followed in some case
by the deshydroxylation process.
Scheme 1. Schematic approaches of the synthesis of right-handed POM-based silica nanohelices (L)-5 (electrostatic grafting), and (L)-6 (adsorption of POM)
Characterization of silica NANOPOM helices
TEM, HR-TEM, STEM and EDX analyses of silica NANOPOM
helices.
In order to investigate the morphology of silica NANOPOM
helices,
Transmission
Electron
Microscopy
(TEM)
measurements were performed for all the silica nanohelices
described in scheme 1. The helical nanostructures are
evidenced for all the silica materials prepared in this work, as
shown in Figure 1 for silica NANOPOM helices (L)-5 (Figure 1a)
and (L)-6 (Figure 1b), obtained by electrostatic grafting and
adsorption of POM, respectively. Compared to the TEM images
of silica nanohelices (L)-3 and APTES functionalized silica
nanohelices (L)-4 (Figure S1a and S1b, in the Supporting
Information), no discernible change was observed in the
morphology and size of NANOMPOM helices (L)-5 and (L)-6,
indicating a high stability of silica helices during the grafting
process.
Figure 1. TEM images of: a) Silica NANOPOM helices (L)-5 and b) Silica
NANOPOM helices (L)-6.
Interestingly, the high-resolution (HR) TEM images of silica
NANOMPOM helices (L)-5 (Figure 2a) and (L)-6 (Figure 2b)
obtained respectively by electrostatic grafting and direct
adsorption of POMs, as well as their scanning transmission
electron microscopy (STEM) images shown in Figure 2c and 2d,
respectively, indicate the presence of POM clusters (black dots)
within silica nanohelices. The diameter sizes of POM clusters in
the range of 1 to 2.8 nm are consistent with that of the Keggin
3-
[20]
40
tri-anionic POM [PW12O ] alone or as small aggregates.
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Figure 3. a) STEM images of silica NANOPOM helices (L)-6; b) Elemental
mappings of tungsten (W) and silicon (Si); c) Elemental mapping of Si; d)
Elemental mapping of W.
Considering that the tungsten element comes only from the
POM tri-anion, we have estimated the amount of POM within
silica nanofibers, and found average values of 20w% and 27w%
for NANOPOM helices 5 and 6, respectively. These values
Figure 2. HR-TEM images of a) Silica NANOPOM helices (L)-5 and b) Silica
NANOPOM helices (L)-6, and STEM images of c) Silica NANOPOM helices
(
L)-5 and d) Silica NANOPOM helices (L)-6.
2
correspond to 0.18 and 0.26 molecule of POM per nm at silica
Interestingly, the images obtained from electrostatic grafting and
adsorption of POMs show differences. The electrostatically
grafted NANOPOM helices 5 show relatively homogeneous
grafting of POM on the silica surfaces. They are not very dense,
and no strong aggregation is observed. On the other hand, the
surfaces of adsorbed NANOPOM helices 6 show denser grafting
of POMs forming clusters. Interestingly, in some areas of silica
nanohelices, POM units are closely packed and well-arranged
following the helicity of the silica template.
helix surface indicating that the direct adsorption approach leads
to higher surface density of POM clusters. In order to optimize
the amount of POM at the surface of silica nanohelices,
adsorption of POMs was repeated for four times to the same
silica sample. No significant change in the amount of POM
adsorbed after the first experiment was observed from EDX
measurements, indicating that the maximum of POM clusters
grafted on silica nanohelices is almost reached after the first
experiment.
The elemental energy dispersive X-ray (EDX) analysis was
performed to confirm the presence of POM clusters within silica
nanohelices. The EDX spectra of silica NANOPOM helices (L)-5
UV-Visible spectroscopy
(
Figure S2 in the Supporting Information), and (L)-6 (Figure S3
UV-vis spectroscopy was used to investigate the grafting of
POM clusters onto silica nanohelices. The UV-vis spectra of
silica NANOPOM helices (L)-5 or (D)-5 and (L)-6 or (D)-6 as well
in the Supporting Information), show elemental tungsten,
indicating the presence of the Keggin tri-anion [PW12O ] in the
40
3-
structure of silica nanohelices. The elemental mappings of silica
NANOPOM helices (L)-6 (Figure 3) confirm that the black dots
correspond to POM clusters.
3
as the parent heteropolyacid H PW12O40 are shown in Figure 4.
An absorption band around 260 nm is observed for the parent
POM, assigned to O  W charge-transfer band of Keggin
[
21]
polyoxometalate.
Absorption profiles for silica NANOPOM
helices (L)-5 and (L)-6 are similar to the one of the parent POM,
but the maximum of the O  W charge-transfer band is shifted
to 270 nm for (L)-6. The determination of the position of this
band for (L)-5 is more difficult, since it appears as a shoulder of
the intense band at 200 nm. However, O  W charge-transfer
band are observed at lower energy for silica NANOPOM helices,
due to the interactions of the POM units with silica nanohelices.
Finally, the intensity of the POM bands is higher for (L)-6 than
for (L)-5 due to the higher amount of POM clusters for (L)-6.
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Figure 4. UV-visible spectra of silica NANOPOM helices (L)-5 or (D)-5, (L)-6
or (D)-6 as well as the parent POM H PW12O40, in water.
3
Raman spectroscopy analysis.
The grafting of POM clusters onto silica nanohelices was also
investigated using Raman spectroscopy. The bare Raman
spectra recorded for (L)-3, (L)-4 and (L)-5 are reported in the
Supporting Information (Figure S4). These spectra display a
small fluorescence background which has been removed by a
baseline correction to give the corrected spectra presented in
Figure 5a. The Raman spectrum of 5 shows the two most
intense bands characteristic of the Keggin tri-anionic POM
3-
-1
-1
[
PW12
O
(O
40
]
at 979 cm and 219 cm , attributed to the stretching
c
stand for terminal oxygen) and bending WO W
Figure 5. Raman spectra of: (a) Silica nanohelices (L)-3, APTES
functionalized silica nanohelices (L)-4, silica NANOPOM helices (L)-5; and (b)
the parent POM H PW12O40 and silica NANOPOM helices 5 and 6.
3
WO
d
d
vibrations (“intra” bridges between edge-sharing octahedra, i.e.
[
22]
-1
inside W
3
O
13), respectively.
The band observed at 489 cm
[
23]
can be assigned to bending vibration of Si-O-Si groups,
-
1
whereas the small bands in the 1460-1410 cm spectral range
are related to APTES. The Raman spectra of silica NANOPOM
helices with electrostatic grafting (L)-5 and adsorption grafting
DRIFT spectroscopy
The diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopy was used to investigate the bonding surface
interaction between POM entities and silica nanohelices, and to
confirm the presence of POM clusters in the structure of
NANOPOM helices. As shown in Figure 6a, characteristic bands
(
L)-6 are reported in Figure 5b, and compared with the one of
the POM alone. The position of the band related to the
stretching vibration of W-O groups is observed at lower
wavenumbers for (L)-5 (ca 980 cm ) than that measured for the
d
-
1
-
1
-1
free POM alone (ca two sharp bands at 1008 and 990 cm ).
This shift associated with the broadening of the W-O band is
of the free POM appear in the 1100-500 cm spectral range.
-
1
d
4
The specific bands of the PO tetrahedron appears at 1010 cm ,
-
1
the consequence of the electrostatic interactions of the POM
with the silica nanohelices. On the other hand, the Raman
spectrum of (L)-6 seems to be the addition of the contributions of
Raman spectra of POM clusters adsorbed to silica nanohelices
by covalent and proton interactions (two sharp bands at 1006
(asymmetric stretching vibration,  PO) and 596 cm (bending
as
vibration, OPO). The asymmetric stretching vibrations of W-O
-1
-1
bonds are observed at 983 cm (asWO
d
), 891 cm (asWO
b
W
of “inter” bridges between corner-sharing octahedra) and 800
-1
-1
cm (asWO
c
W). Finally, the band observed at 526 cm is
W and WO W vibrations. For silica
-
1
and 990 cm , and a broad band similar to that observed with
assigned to the bending WO
b
c
-
1
(
L)-5, ca 980 cm ).
NANOPOM helices (L)-6, most of these bands overlap with the
strong bands related to silica nanohelices (L)-3 (i.e. asSiOSi at
-
1
-1
-1
1
086 cm , SiOH at 956 cm , 
s
SiOSi at 803 cm and SiOSi
-
1
at 467 cm ). Indeed, it is possible to observe the bands
-
1
-1
associated with the asWO
d
b
(975 cm ), asWO W (898 cm ) and
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-
1

asWO
c
W (805 cm ) vibrations, confirming the presence of POM
Figure 6. DRIFT spectra of: (a) Silica nanohelices 3, silica NANOPOM helices
-1
3
6, and the parent POM H PW12O40 in the 1400-400 cm region and (b) Silica
entities in the structure of silica NANOPOM helices (L)-6. As
shown in Figure 6b, the DRIFT spectra of silica nanohelices (L)-
nanohelices 3 and silica NANOPOM helices 6 built by adsorption of POM, in
-1
the 4000-3000 cm region.
3
(or (D)-3) and their POM-based counterparts (L)-6 (or (D)-6)
reveal the characteristic OH vibration for hydroxyl groups of
-
1
silanols at 3739 cm . Interestingly, the adsorption of POM
clusters onto silica nanohelices is manifested by the decrease of
the intensity of the hydroxyl band. Indeed, as reported by
Chiroptical properties of (L)-5, (D)-5, (L)-6 and (D)-6
NANOPOM helices
[
19]
In order to reveal the induced chirality of the POMs after being
grafted onto the silica nanohelices, the chiroptical properties of
right-handed ((L)-5 and (L)-6) and left-handed ((D)-5 and (D)-6)
silica NANOPOM helices were investigated by circular dichroism
Lefebvre et al.,
the direct linkage of POM clusters to silica
support results in various interactions with silanols, and thus the
decrease of the amount of free hydroxyl groups at the surface of
silica. For silica NANOPOM helices (L)-5 (or (D)-5), built by
electrostatic grafting of POM clusters onto APTES functionalized
silica nanohelices, the DRIFT spectrum shows the decrease of
the 3739 cm band upon APTES functionalization of silica
nanohelices (L)-3 or (D)-3 (Figure S5 in the Supporting
Information). No additional decrease of the band assigned to
hydroxyl groups is observed upon grafting of POMs, indicating
that surface hydroxyl groups of APTES-silica nanohelices (L)-4
or (D)-4 are less involved upon electrostatic grafting of POMs.
Thus, the decrease of the intensity of the hydroxyl band upon
direct adsorption of acidic POMs to silica template could be
partially attributed to the interactions between POMs and silanol
groups.
(CD) in aqueous suspensions. As shown in Figure 7a, b, the CD
spectra of NANOPOM helices (L)-5 and (D)-5, as well as (L)-6
and (D)-6 are mirror images of one another, confirming the
enantiopurity of these NANOPOM helices. Overall, right-handed
helices exhibit positive signals in the 190-350 nm spectral range,
while opposite spectra with negative signals were obtained for
the left-handed counterparts. The CD signals of silica
NANOPOM helices (L)-5 and (D)-5 built by electrostatic grafting
approach, show opposite significant Cotton effects at around
-
1
2
14 nm and signals with very low intensity in the 240-290 nm
spectral range (Figure 7a). For NANOPOM helices (L)-6 and
D)-6 obtained by direct adsorption of POM, higher Cotton
(
effects with maxima at 214 and 276 nm were obtained as shown
in Figure 7b. In the two cases, the CD signals comes from the
induced chirality to POM clusters since the CD signals of APTES
functionalized silica nanohelices (Figure 7a) and silica
nanohelices (Figure 7b) is close to zero in the 220-350 nm
spectral range. The small opposite signals observed at 190-210
nm range are due to the residual tartrate trapped in the silica
nanohelices. The induced circular dichroism (ICD) of POMs is
certainly associated with the helical organisation of POM
clusters onto the silica nanohelices. To the best of our
knowledge, silica NANOPOM helices (L)-5, (D)-5 and (L)-6, (D)-
6
represent the first examples in which achiral POM clusters
have been grafted onto inorganic helical templates and display
chiroptical effects, as a result of the chirality induction from
enantiopure silica helices. Interestingly, as shown in figure 7b,
the ICD signal of POM clusters is strongly enhanced for
NANOPOM helices (L)-6 and (D)-6 in which POM clusters are
directly adsorbed onto the silica matrix. The decrease of the
induced optical activity obtained with silica NANOPOM helices
(L)-5 and (D)-5, built by the electrostatic grafting approach, may
be explained both by the presence of the amino propyl spacer
between POM clusters and the surface of the chiral source
(silica nanohelices), and by the lower amount of POMs as
revealed by EDX analyses. Indeed, in the direct adsorption
approach, POMs are more densely assembled, and well-
arranged following the helicity of the silica template.
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-
4
around 1.12 x 10 ) than the electrostatically grafted silica
-
5
NANOPOM helices 5 (values around 3.3 x 10 ). It is interesting
-
4
to note that the g-factor values (1.12 x 10 ) obtained for
NANOPOM 6 at 270 nm after the first grafting experiment is
-
4
similar to the one obtained (1.07 x 10 ), after four times grafting,
indicating that the maximum amount of POM clusters grafted on
silica helices is probably reached after the first experiment.
Importantly, the UV-vis and CD spectra of NANOPOM helices 5
and 6 showed no discernible changes over several months,
highlighting the induced chirality stability of these materials. This
feature is very important for the use of these silica NANOPOM
helical systems in asymmetric catalysis.
Catalytic tests of silica NANOPOM helices 5 and 6 in
oxidation reactions.
The catalytic performance of silica NANOPOM helices 5 and 6
has been investigated in this study. We have selected the
oxidation of methyl p-tolylsulfide 7 and methyl phenyl sulfide 8
2 2 3
with H O (35%) in CH CN (Scheme 2).
Scheme 2. Catalytic oxidation of methyl p-tolylsulfide 7 and methyl phenyl
2 2
sulfide 8 catalyzed by silica NANOPOM hybrids 5 and 6 in CH CN, using H O .
3
The reaction kinetics were monitored over time by plotting the
1
ratio between the intensity of the disappearing H NMR signal of
Figure 7. (a) CD of APTES functionalized silica nanohelices (L)-4 and (D)-4
and silica NANOPOM helices (L)-5 and (D)-5 (electrostatic grafting approach);
CH
3
S group of the substrate and the growing peak of the
(
b) CD of silica nanohelices (L)-3 and (D)-3 and silica NANOPOM helices (L)-6
corresponding sulfoxides (CH SO) and sulfones (CH SO ). As
3
3
2
and (D)-6 (adsorption approach).
shown in Table 1, silica NANOPOM helices 5 and 6 oxidize
methyl p-tolylsulfide and methylphenylsulfide to the
7
8
corresponding sulfoxides 9 and 10, with a small amount of
sulfones 11 and 12, respectively. In all cases, the selectivity to
sulfoxide increased with decreasing reaction temperature, with
yield ranging from 39 to 75% after 30 to 60 min, depending of
the substrate and the reaction temperature. Catalyst 5 appeared
to be the much efficient catalytic system in these conditions.
With NANOPOM 5, 86% conversion was obtained after 30 min
in the oxidation of sulfide 7 at 0°C, whilst 1 h is necessary to
afford 85% conversion with NANOPOM 6 in similar conditions
Indeed, the amino propyl spacer increases the distance between
POM clusters and the chiral source, and along with the lower
global density of POM at silica surface thus probably lessens the
chirality transfer to the POM units. In contrast, the stronger
induced optical activity of POM clusters in NANOPOM (L)-6 and
(D)-6 could be attributed to the strong interaction, higher surface
density of POM and the proximity of POM clusters with the chiral
nanohelices, resulting in the stabilization of the chiral
microenvironment around POM clusters.
(Table 1, entries 4-5). This result demonstrates higher catalytic
The optical activity of chiral systems can also be measured
through the anisotropic factor (g-factor), which is defined by the
ratio between the molar circular dichroism and the molar
activities and reaction kinetics of NANOPOM 5 compared to 6.
This is probably because 6 has higher steric hindrance around
the catalytic sites and lower hydration of POM, both because
they are aggregated and much closer to the silica surface
compared to 5 which has a spacer group between the active
POM clusters and the surface of silica nanohelices. Thus the
NANOPOM 6 likely has slower reaction kinetics in the oxidation
of less polar substrates such as sulfide 7. The presence of a
spacer group between POM clusters and the silica nanohelical
promotes catalytic oxidation in one side, but weakens the chiral
[
8a,24]
absorptivity.
Thus, in order to evaluate the induced optical
activity of POM clusters in silica NANOPOM helices, the g-
factors have been calculated at two wavelengths, corresponding
to the maxima of the bands observed in the 190-350 nm spectral
range. As shown in Table S1 in the Supporting information, the
g-factors of silica NANOPOM helices 6 in which POMs are
adsorbed on silica support are almost four times larger (values
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Chemistry - A European Journal
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environment around the POM clusters on the other side (Figure
structure of the recovered NANOPOM helices was verified by
TEM as well as UV-Vis and Raman spectroscopy before a
second catalytic cycle was performed. The UV/Vis (Figure S6 in
Supporting Information), as well as the TEM images (Figure S7)
and Raman spectra (Figure S8) are similar to those obtained
before the first catalytic reaction, indicating that no sign of
decomposition of NANOPOM catalyst was observed. Compared
to the first reaction (Table 1, entry 6), similar conversion rate
(41%) and selectivity to sulfoxide (39%) (Table 1, entry 10) were
obtained after 60 min in the oxidation of 7 with the recovered
NANOPOM helices 6 in the second catalytic reaction. These
results reveal the stability and easy recuperation of the
NANOPOM system built by direct adsorption of POM clusters
7
). This result represents a serious drawback for the potential
use of these chiral systems. The comparison of Left- and Right-
handed NANOPOM helices 6 in the oxidation of sulfide 8
showed no obvious difference in the reaction kinetics as
summarized in Table 1, entries 7-8. It is interesting to note that
all the performed oxidation reactions did not proceed in the
absence of POM clusters, when carried out in similar conditions
(Table 1, entries 1-3). To evaluate the effect of chiroptical
properties of NANOPOM helices 5 and 6 in the oxidation of
sulfides 7 and 8, the enantiomeric excess (ee) of sulfoxides 9
and 10 were measured by chiral HPLC using a Chiracel ASH
column, UV detection (254 nm), eluting with a mixture of hexane
-
1
and 2-propanol (1:1) at a flow rate of 0.5 mL.min . Unfortunately, onto the silica template, contrary to the system built by
no significant enantioselectivity (ee) was observed, even at low
temperatures. If the presence of CD indicates that the POM
catalyst present a chiral organization, the lacking of ee in the
oxidation of sulfides 7 and 8 may arise by the fact that these
molecules are may be too small to recognise this chirality. The
oxidation of larger molecules like peptides should be ideal to
promote enantioselectivity with these catalytic systems. Further
work for this purpose and for the development of new systems
with greater potential for chiral induction is in progress.
electrostatic grafting in which a slight decreased was observed
in the reaction kinetics with the recovered NANOPOM helices 5
(Table 1, entries 4 and 9). This is probably due to the leaching of
POM units usually observed for similar NANOPOM hybrids in
[
25]
polar media.
Conclusions
This work reported our initial efforts to construct hybrid
NANOPOM helices by grafting achiral POM clusters onto silica
nanohelices by electrostatic or covalent bonding. The structures
of these silica NANOPOM helices were identified by TEM and
HR-TEM as well as UV-vis, DRIFT and Raman spectroscopy.
The induced chirality of POM was demonstrated by electronic
circular dichroism measurements. Such POM hybrid materials
are expected to display an induction of optical activity in POM
clusters, efficiency in the reaction kinetics and the recovery of
the NANOPOM hybrid.
The present research is an important milestone between POM
chemistry and chiral nanomaterials based on silica nanohelices.
In comparison to the electrostatic grafting approach, direct
adsorption of the acidic POM onto silica nanohelices results in
the enhancement of the induced optical activities of POM
clusters, probably due to the proximity and the arrangement of
POM clusters onto the chiral source. These NANOPOM helices
oxidize sulfides to the corresponding sulfoxides, but no
significant ee observed in the condition used up to now.
However, to the best of our knowledge, the present work
represents the first example in which achiral POM clusters have
been grafted onto silica nanohelices, displaying a chiroptical
activity due to the chirality transfer from silica nanohelices. This
Table 1. Oxidation of methyl p-tolylsulfide 7 and methyl phenyl sulfide 8
a
3 2 2
catalyzed by silica NANOPOM hybrids 5 and 6 in CH CN, using H O .
b
b
Entry
Substrate
catalysts T[°C]
t[min]
Conv. Products
c
[
%]
2
(SO) (SO )
1
2
3
4
5
6
7
8
7
L-(3)
0
240
240
240
30
2
9 (2) 11 (0)
9 (2) 11 (0)
9 (2) 11 (0)
9 (72) 11 (14)
9 (75) 11 (10)
9 (39) 11 (2)
10 (65) 12 (6)
10 (64) 12 (0)
9 (62) 11 (10)
9 (39) 11 (2)
-35
0
2
L-(4)
L-(5)
L-(6)
2
0
86
85
41
71
64
72
41
0
60
-35
-35
-35
0
60
8
7
L-(6)
D-(6)
L-(5)
L-(6)
45
45
d
9
30
d
10
-35
60
result is encouraging and promising since it provides
CN (1mL). [b] Reactions were monitored by H NMR. [c] convenient and effective way to prepare chiral POM-based
a
[
2 2
a] Reaction conditions: catalyst (0.2 mol%), substrate (500 eq), 35% H O (550
1
eq), solvent CH
3
1
Conversion determined from the relative intensities of the H NMR signals of the
substrate and the product. [d] Catalytic reaction with the recovered catalysts L-5
and L-6.
nanohelices for potential applications in asymmetric
transformations. Due to the variety of existing POM structures
and our expertise in preparation of organic and inorganic
nanohelical objects, we expect to find new POM-based
nanohelical systems with greater potential for chiral induction,
especially in asymmetric reactions. An approach in which POM
clusters are incorporated into nanohelices during sol gel
supramolecular organization is expected to result in NANOPOM
helices with more pronounced chiroptical properties. Further
work in this area is in progress.
Recovery of silica NANOPOM helices.
In order to test the stability and the recovery potential of silica
NANOPOM helices 5 and 6 in catalytic experimental conditions,
the catalyst was separated from the reaction mixture by
centrifugation after the reaction, giving a clear supernatant of
3
CH CN and a white solid of silica NANOPOM helices. The
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Chemistry - A European Journal
10.1002/chem.201801905
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water to remove unreacted POM clusters, then freeze-dried for 1 day.
The final samples were kept in 20°C before any characterization and
measurements.
Experimental Section
Materials
Silica NANOPOM helices (L)-6 and (D)-6 built by direct adsorption of
POM clusters onto silica nanohelices (L)-3 and (D)-3.
Reagent-grade solvents and all chemicals were used as received.
Organic oxidation products were identified by correlation to authentic
samples.
In a typically procedure, 1 ml of phosphotungstic acid solution (10 mM)
was added to 5 mg of silica nanofibers dispersed in 3 ml of Milli-Q water.
The mixture was stirred at 20˚C with a roller for 4 h. The reaction mixture
was then washed with Milli-Q water (at least 10 times), in order to
remove all free POM clusters, including those absorbed at the surface of
silica nanohelices, following by the freeze-dried for 1 day. The final
samples were kept in 20°C before any characterization and
measurements.
Synthesis of Gemini tartrate (L)-1 and (D)-1 and organic nanohelices
(
L)-2 and (D)-2.
The 16-2-16 Gemini amphiphile with tartrate counterions (L)-1 and (D)-1
[26]
were prepared according to a previously reported procedure. To obtain
the organic gel (L)-2 and (D)-2, at the concentration of 1mM, 3.6 mg of
1
6-2-16 L- or D-tartrate powder were dissolved into 5ml of ultra pure
water. The solution is heated up to 60˚C (above the Krafft temperature
which is 43˚C for 16-2-16 tartrate) for 20 minutes to obtain a complete
dissolution and incubated at 20˚C. The helical organic structures were
observed after 1 day. The morphology of self-assembly was monitored
with Transmission Electron Microscopy (TEM), after a negative staining
with 3wt% uranyl acetate.
UV Measurement
The UV-Vis spectra were recorded with a Cary 300 UV-vis spectrometer
between 190 and 400 nm, with 1.0 nm data interval and 300 nm/min
scan rate. Quartz cuvette with optical path length 1cm was used for the
measurements.
.
Preparation of silica nanohelices (L)-3 and (D)-3
DRIFT measurements
After the preparation of organic gels, the later were used as template to
prepare silica nanofibers through a sol-gel transcription method. In a
typical procedure, 500 μL of tetraethoxysilane (TEOS) were added tot the
The DRIFT spectra were recorded with
spectrometer using a DRIFT accessory (Harrick Praying Mantis ). Each
spectrum was obtained at a resolution of 4 cm by coadding 200 scans
in a dry-air atmosphere. The sample (0.5 mg) was mixed with 50 mg of
calcinated (550 °C) KBr. The pure KBr was used as background. The
spectral intensities were converted into Kubelka-Munk units.
a Nicolet 6700 FTIR
TM
-
1
1
0 mL of 0.1 mM aqueous solution of L- tartaric acid (pH 3.8) and
prehydrolyzed at 20°C by stirring on the roller-mixer for 7 h. In parallel, a
solution of 1mM 16-2-16 Gemini surfactant with L-tartrate counterion is
aged between 2 days for the formation of helices. Equal volume of
prehydrolyzed TEOS (5 mL) in 0.1 mM aqueous solution of L-tartaric acid
and organic gel (5 mL) are mixed and stirred at 20˚C with a roller-mixer
overnight. Once the transcription is completed, the mixture thoroughly is
washed five times with isopropanol in order to eliminate the excess of
TEOS and the organic template. Finally, the suspension of silica fibers is
stored in pure ethanol. The morphology of the later was checked with
TEM. Basically, we obtained 5 mg of silica helices from 1 mM of organic
gel.
Raman measurements
Raman spectra were recorded with
a LabRam HR800 confocal
spectrometer (HORIBA), using for excitation 514.5 nm radiation from an
argon ion laser and equipped with an air-cooled CCD detector (ANDOR)
cooled to -85°C. The laser power at the samples was 10 mW and the
spectra were accumulated 4 times during 900 s. The spectral resolution
given by the entrance slit of 100 μm and the 600 lines/mm grating was
Amine functionalized silica nanohelices (L)-4 and (D)-4
-1
6
.8 cm . A DuoScan device composed of two mirrors was activated in
order to prevent any sample degradation. Using this device, the
excitation beam scan a 10x10µm sized area with a 50x objective (NA
0.75) in the confocal regime. The Raman spectra were recorded between
For electrostatic grafting of POM clusters at the surface of silica
nanohelices, the silica nanohelices were functionlized with (3-
[8b]
-1
Aminopropyl)triethoxysilane (APTES). Generally, 20mol of APTES in
absolute ethanol was added per 1mg of silica nanohelices in ethanol.
The reaction mixture was sonicated in ultrasonic bath for 5 min and
200 and 1600 cm .
Transmission Electron Microscopy (TEM) measurements
heated for 15
h at 70˚C. After completion, the modified silica
nanostructure was washed 3 times with absolute ethanol and then 2
times with water. This procedure can be repeated once to improve the
coverage of silica surface with amine groups. The quantity of amine
groups at the surface of silica nanohelice was estimated as reproted in
TEM measurements were performed at room temperature on a Philips
EM 120 electron microscope operating at 120 kV, and the images were
collected by 2k × 2k Gatan ssCCD camera. Drops of diluted dispersion of
the hybrids were deposited on a carbon-coated 200/400-mesh copper
grids. Hydrophilic grids were prepared for the observation of samples
dispersed in water by treating the carbon-coated copper grids in
BioForce Nanosciences UV/Ozone ProCleaner 220 for 20 min. For the
sample preparation, 5µL of silica suspension were drop-casted onto the
grid and blotted after 1 min. Samples were dried in air.
[8a]
ref.
Silica NANOPOM helices (L)-5 and (D)-5 built by electrostatic
coupling of POM clusters with amine functionalized silica
nanohelices.
Typically, 3 equiv. of phosphotungstic acid (vs APTES) was dissolved in
HR-TEM measurements
1
ml ultrapure water (0.01M). In parallel, 5mg of modified silica nano-
structures was dispersed in 3 ml of ultrapure water and sonicated for 5
min in ultrasonic bath. The two solutions were mixed and stirred at 20˚C
with a roller-mixer for 4 h. After completion, the silica NANOPOM helices
The clusters of POM grafted on the silica nanohelices was observed by a
JEOL 2200FS high resolution transmission electron microscope
operating at 200 kV and several images was taken using the STEM
(
L)-5 and (D)-5 were washed at least 10 times by centrifugation ultrapure
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Chemistry - A European Journal
10.1002/chem.201801905
FULL PAPER
mode with probe size: spot 1.5 nm. EDX (Energy Dispersive X-ray
Spectrometry) and element mapping were performed on the JEOL
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nanohelices. The atomic percentage of the tungsten W (L) and silicon Si
(
K), in the different samples was estimated by EDS. For each sample
several areas of the sample were studied and the average of percentage
of tungsten and silicon was calculated. % H 40 = % W/12; %Silica
% Si. To obtain the % of POM in weight, the molar percentage of POM
3
PW12O
[3]
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Circular Dichroism measurements
4
Electronic circular dichroism (ECD) measurements were performed on a
Jasco J-815 CD spectrometer. The scan rate was 20 nm/min and 50
scans were applied to each sample. The concentration of hybrids applied
for these experiment was 0.2 mg/ml. All CD experiments were carried out
in Milli-Q water with a quartz cuvette (1cm path length) at 25˚C and with
stirring. The baseline corrections of the ECD spectra were performed by
subtracting the two opposite-enantiomers ECD spectra and divided by
two in case of silica nanohelices (L)-3 and (D)-3 and APTES
functionalized silica nanohelices (L)-4 and (D)-4. This baseline procedure
removes the experimental artefacts coming from the ECD spectrometer.
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[
5]
[
[
6]
7]
General procedure for catalytic reactions and catalyst recycling
[
[
8]
9]
The Silica NANOPOM catalyst was dispersed in acetonitrile (1 mL) and
the sulfide (500 equiv) was added to the mixture. An aqueous solution of
H O (35% in water, 550 equiv) was added to the reaction mixture at the
2 2
appropriate temperature. The latter was stirred and monitored by 1H
NMR spectroscopy. Upon completion of the reaction, the catalyst was
separated from the medium by centrifugation and washed with ethanol
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(3x5 mL) and water (3x5 mL). The recovered catalyst was freeze dried,
[
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and then analysed by UV visible spectroscopy, Raman spectroscopy and
TEM before its use in a new catalytic experiment. The enantiomeric
excess were determined by chiral HPLC using Chiracel ASH column and
UV detector (254 nm), eluting with hexane/2-propanol (1:1) at a flow of
-
1
0
1
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Financial support from the University of Bordeaux and the
CNRS is gratefully knowledged. We thank IECB technology
platforms and S. Tan, M. Decossas and E. Morvan (CBMN) for
TEM and P NMR experiments, S. Buffière (ICMCB, CNRS)
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Entry for the Table of Contents
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4
2
Rigth handed
Left handed
M. Attoui, Dr E. Pouget, Dr R. Oda, Dr
D. Talaga, Dr G. Le Bourdon, Dr T.
Buffeteau and Dr S.Nlate
POM
0
H O
2
-2
-
4
2
00 300
Wavelength(nm)
400
Silica Nanohelice
Silica NANOPOM helice
Page No. – Page No.
Optically Active Polyoxometalate-
based Silica Nanohelices: Induced
Chirality from Inorganic Nanohelices
to Achiral POM Clusters.
Silica NANOPOM helices: Left- and right-handed silica NANOPOM helices have
been prepared by electrostatic and direct adsorption of -Keggin polyoxometalate
to silica nanohelices. The presence of POM clusters and their interactions with the
helical template are confirmed by various characterization techniques. The induced
chirality from silica nanohelices to POM clusters is evidenced by CD spectroscopy.
These NANOPOM helices are robust and easy to handle.
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