A photo-driven polyoxometalate complex shuttle and its homogeneous catalysis and heterogeneous separation

Article abstract of DOI:10.1021/ja4057882

A "smart" core-shell complex is designed to combine a catalytic reaction and automatic separation through remote light control. Here, we present the induced amphiphilic behavior of a surfactant-encapsulated polyoxometalate complex with photoresponsive azobenzene units on the periphery. The reversible phase transfer of the complex shuttle between two incompatible phase termini, driven by a photoisomerization-induced polarity change, further facilitates the separation and recycle of the catalyst.

Full text of DOI:10.1021/ja4057882

Communication
pubs.acs.org/JACS
A Photo-driven Polyoxometalate Complex Shuttle and Its
Homogeneous Catalysis and Heterogeneous Separation
Yang Yang, Bin Zhang, Yizhan Wang, Liang Yue, Wen Li,* and Lixin Wu*
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
S Supporting Information
*
treatment filtration procedure, which restrains their possible
25
ABSTRACT: A “smart” core−shell complex is designed
to combine a catalytic reaction and automatic separation
through remote light control. Here, we present the
induced amphiphilic behavior of a surfactant-encapsulated
polyoxometalate complex with photoresponsive azoben-
zene units on the periphery. The reversible phase transfer
of the complex shuttle between two incompatible phase
termini, driven by a photoisomerization-induced polarity
change, further facilitates the separation and recycle of the
catalyst.
use in microfluidic systems and trace utilizations. Therefore,
more convenient approaches that can separate POM catalysts
from the reactions automatically through a remote control are
needed.
Light is an excellent trigger because of its noninvasive
character and the easy mediation of wavelength, intensity,
illuminated area, and duration. Wang et al. demonstrated the
recycle of a homogeneous ruthenium−carbene catalyst via the
light-controlled reversible transition of a nitrobenzospiropyran
25
unit. Azobenzene (Azo) derivatives possess a photosensitive
group and undergo a reversible isomerization upon light
26,27
irradiation.
In the trans state, the Azo group is in its linear
omogeneous catalysis has higher activity and selectivity
structure and its polarity is small due to the axial symmetry,
while in the cis state, the bent structure leads to an increased
dipolar moment due to its non-axial symmetry. This
configuration change provides a favorable feature for
controlling both spatial alignment and polarity of Azo-
containing compounds, because the former can be utilized for
reversible transformation of self-assembled structures and the
latter can be applied for assembly and disassembly of
H
than heterogeneous catalysis, but it is often inconvenient
1
for industrial purposes. In contrast, heterogeneous catalysis is
easier to run and separate, yielding a sustainable catalytic
process even if sophisticated post-treatment becomes necessary.
Consequently, automatic heterogenization of homogeneous
catalysts is of interest. If the catalytic reaction is controllable
through the catalyst’s response to environmental stimuli such as
pH, magnetism, temperature, light, external chemicals, and so
2
8,29
complexes.
Therefore, it can be predicted that, by selecting
2
on, the catalysis becomes “smart” and valuable. Polyoxome-
incompatible solvents, the Azo-group-containing POM com-
plexes can be applied to controllable phase separation systems,
with performance superior to that of most known phase-
talates (POMs) are discrete nano-sized metal oxide clusters
with abundant compositions and structures, and versatile
properties leading to potential applications in the fields of
medicine, photochromic materials, solar energy, and so
30−35
transfer methods for nanoparticles.
To realize an agile
recovery of POM-based catalysts, we design and prepare a
photoresponsive SEP complex in which the surface of the POM
is electrostatically modified with cationic surfactants bearing
Azo groups at the hydrophobic ends (Scheme 1). By alternating
irradiations with UV and visible lights, we successfully create a
deft method to recycle the homogeneous catalyst using this
POM complex as a reversible phase-transfer shuttle.
3
−6
forth.
Because of their specific redox properties, POMs
have also been widely used as effective catalysts for oxidations
of various compounds such as alkenes, alcohols, sulfides, and
7
−11
even water.
To improve the high lattice energy, the low
mass transfer, and the low compatibility with organic solvents,
the surfaces of POMs are often modified to increase their ability
12−17
A POM with zinc-sandwiched structure possessing catalytic
to adapt to different chemical environments.
Among the
36
activity, Na [WZn (H O) (ZnW O ) ] (its polyanion is
reported strategies, a simple and frequently used approach is to
replace the counterions of POMs with cationic amphiphiles,
generating surfactant-encapsulated POM (SEP) complexes.
12
3
2
2
9
34 2
abbreviated as Zn W O ), is selected as the polyanionic core,
5
19 68
1
8
and an Azo-ended cationic surfactant (its cation is abbreviated
29
as (AzoC ) N) is employed as the responsive shell. By mixing
When the complexes are applied for homogeneous catalysis,
6 2
the inorganic cluster aqueous solution and organic surfactant
chloroform solution, a photoresponsive complex, Azo-SEP, was
they show superior efficiency and selectivity compared to
19
POM-based heterogeneous catalysts. However, the separation
and recycle of POM catalysts from reaction systems remain a
key challenge. Many efforts have been made to realize both
high reaction efficiency and quick separation of POM
37
prepared through an ion-exchange reaction. Due to the
electrostatic interaction, the counterions of POM clusters were
easily replaced by hydrophobic cations. Unlike the Zn W O
5
19 68
2
0
21
23
cluster that is soluble in water, the obtained Azo-SEP complex
was immiscible in water but readily dissolved in common
complexes: nanofiltration, addition of poor solvent,
2
2
magnetic field, temperature-controlled phase separation,
2
4,19
and reaction-controlled phase separation
methods have
been developed to separate POM-based homogeneous
catalysts. But these recovery routes also involve a post-
Received: June 10, 2013
Published: September 16, 2013
©
2013 American Chemical Society
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Journal of the American Chemical Society
Communication
Scheme 1. Chemical Structures of Cationic Surfactant
isomerizes from trans to cis form, while under visible light
irradiation around 450 nm, the reverse change from cis to trans
+
−
(
AzoC ) N Br and a POM Cluster and Preparation of an
6 2
2
6,27
Azo-SEP Complex, as Well as Reversible Phase Transfer of
form takes places.
S10) verifies the reversible isomerization of Azo groups in both
UV−vis spectral characterization (Figure
the Complex between Toluene and H O/DMF Mixed
2
Solution
toluene and H O/DMF solutions. No obvious degradation is
2
found even after 10 cycles of photoisomerizations. For both
cases, the photostationary state can be reached in less than 2
min, though the transformation from trans to cis is faster than
the reverse process (Figures S11 and S12). The rapid
1
isomerization is further characterized by H NMR spectra
(
Figure S13), and the result shows that over 94% of the initial
Azo groups are in the trans form in polar solvent. After UV light
irradiation, 98% of the Azo groups isomerize into the cis form.
With subsequent visible light irradiation of the solution, only
7
0% of the cis-form Azo groups return to the trans form. This
non-equivalent back-transformation further indicates the polar
character of the complex with its Azo groups in cis state and the
non-polar character of the complex with its Azo groups in trans
state.
When the two immiscible solvents are mixed in one glass
vessel, as expected, the photomediated reversible phase transfer
of Azo-SEP is observed upon alternate UV and visible light
irradiations, as shown in Figure 1. At the initial state or upon
organic solvents, e.g., toluene, dimethylformamide (DMF),
tetrahydrofuran, and chloroform. The solubility change
indicates successful encapsulation, and the obtained complex
h a s a n a v e r a g e c h e m i c a l f o r m u l a o f
[
(AzoC ) N] Na [Zn W O ] (MW = 10 474.25), which is
6 2 9 3 5 19 68
1
identified by FT-IR, H NMR, elemental analysis, thermogravi-
metric analysis, and MALDI-TOF measurements. A 53.5% (in
w/w) mass loss before 700 °C (Figure S2, Supporting
Information) matches well with the theoretical value of
Figure 1. Digital photographs of reversible phase transfer of Azo-SEP
complex between toluene and H O/DMF (1:1 vol/vol) mixed
solution upon UV and visible light irradiations.
2
5
3.2%, estimated from the above chemical formula by assuming
that the organic component decomposes completely and the
POM transforms into its corresponding oxides. In the NMR
spectra taken in both CDCl and D O/DMF-d mixed solvents
visible light irradiation, the upper toluene phase is transparent
3
2
7
(
Figures S3 and S4), obvious proton peak shifting and
and yellow in color, while the bottom H O/DMF phase is
2
broadening are observed for the (AzoC ) N component in
colorless, indicative of the complex mainly existing in toluene.
Upon UV light irradiation, the upper toluene phase turns to
colorless, while concurrently the bottom phase becomes yellow,
indicating the phase transfer of the complex. It is known that
the polarity of Azo-SEP bearing cis-Azo groups is much higher
6
2
Azo-SEP. Compared with the signals of free surfactant, these
changes confirm the strong electrostatic interaction between
the cationic head and the anionic POM in both solutions. The
absorption bands of W−O and W−O−W vibrations (Table S1)
in FT-IR spectra of the complex (Figure S5) indicate the well-
kept frame structure of POM and support the strong
electrostatic interaction between surfactant and POM. The
MALDI-TOF result (Figure S6) around 10 475 confirms the
predicted chemical formula.
It has been reported that the hydrophobicity change resulting
from isomerization of the Azo group triggers opposite
assembling behaviors of Azo-SEP in strongly polar and weakly
polar solvents. Similarly, in the present study, the turbid
solution and bigger size of aggregates imply poorer solubility,
while the transparent solution and good dispersion reflect good
solubility (Figures S8 and S9). Typically, Azo-SEP in its trans
state dissolves well in the weakly polar solvent toluene but
27
than that of it bearing trans-Azo groups. So, at the initial state
and/or upon visible light irradiation, the trans-state Azo-SEP
complex with smaller polarity prefers to stay in the toluene
phase. After UV light irradiation, the Azo groups isomerize into
the cis form, the polarity becomes larger, and the complex is no
longer compatible with toluene. The increased polarity propels
the complex to depart from toluene and enter into the polar
solvent of H O/DMF. Upon visible light irradiation, the
2
2
9
formation of trans-Azo groups makes the polarity decrease,
which drives the complex to transfer back into the toluene
phase. Therefore, it is the photoisomerization-induced polarity
change of Azo groups that triggers the photoresponsive phase
transfer of Azo-SEP between toluene and H O/DMF phases.
2
poorly in the strongly polar solvent H O/DMF (1:1 vol/vol).
According to the UV−vis spectra (Figure 2), the phase-transfer
2
In contrast, Azo-SEP in its cis state displays the opposite
behavior in the two solvents. Importantly, because the two
solvents are often used in chemical reactions and are immiscible
with each other, we select them as phase termini for the
photosensitive phase-transfer study of the POM complex
shuttle. Upon 365-nm light irradiation, the Azo group
efficiency of the Azo-SEP complex can reach 98−99% when the
volume ratio of H O/DMF is increased to 2:1, while the value
2
is ca. 96−98% when the volume ratio remains at 1:1 (Figure
S14), and the values remain similar even after 10 transfer cycles.
The incomplete phase transfer of Azo-SEP is probably caused
by trace amounts of DMF in the toluene phase and toluene in
1
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Journal of the American Chemical Society
Communication
Figure 2. UV−vis spectra of Azo-SEP before and after phase transfer
in (a) toluene and (b) H O/DMF (2:1 vol/vol) mixed solution upon
2
UV and visible light irradiations: red, after one cycle; blue, after 10
cycles of UV and visible light irradiations.
Figure 4. Schematic catalytic and phase-transfer recycle of photo-
responsive catalyst Azo-SEP, driven by alternate UV and visible light
irradiations (reduced oxidant DMPC = dimethylphenylcarbinol).
the H O/DMF phase. The higher water content reduces the
2
amounts of DMF in the toluene phase (Figures S15−S17) and
toluene in the H O/DMF phase (Figures S18−S20). However,
2
further increasing the water content leads to poor phase
transfer due to the decreased solubility of the complex.
Next we demonstrate the use of photoresponsive phase
transfer and shuttle recycle of the Zn W O complex with
simple visible light irradiation, and the catalytic oxidation takes
place again. After several recycles, the catalyst is confirmed to
be quite stable, and no obvious decomposition is found
5
19 68
7
,36
high activity for the oxidation of sulfides to sulfones.
Phenothiazine (PH), a useful intermediate in organic synthesis,
is selected as the substrate to carry out the homogeneous
catalysis. Due to its hydrophobicity, PH was oxidized in toluene
at 30 °C with cumyl hydroperoxide (CHP) as the
homogeneous oxidizing agent. The UV−vis spectral change
of the reaction mixture as a function of reaction time clearly
reveals the oxidation process (Figure 3 and Figure S21). The
(
Figures S24 and S25). Using the same extraction solvent in the
subsequent recycles, after five recycles the recovered complex
catalyst is still at ca. 88.6−94.1% of its initial amount in the case
of the 2:1 volume ratio of H O/DMF mixed solvent (Figure
2
38
S26). Because of partial loss of the catalyst, the product
selectivity becomes low (Figure S27), but it can be recovered
by prolonging the reaction time or supplementing the catalyst.
To confirm the universality of such a concept, we utilize Azo-
ended surfactants to encapsulate POMs with different charges
and topological shapes, Keplerate-type (NH ) {(Mo)-
4
72
Mo O (H O) } {Mo O (SO )} (Mo132) and disk-like
5
21
2
6
12
2
4
4
30
3
9
K12.5Na [NaP W O ] (P W ), to prepare two new
1.5
5
30 110
5
30
complexes. As expected, upon UV and visible light irradiations,
reversible phase transfers of these two complexes between two
phases were also observed, as can be seen in Figure S28. We
also employed the (P W ) complex as the homogeneous
5
30
catalyst for the oxidation of PH (Figure S29) and successfully
recycled it through light-mediated reversible phase transfer.
In summary, we have demonstrated a “smart” photo-
responsive catalyst based on a POM hybrid complex, which
combines the advantages of both a homogeneous catalyst and
heterogeneous separation. The POM hybrid complex shows
high catalytic activity and can be easily recycled through a
simple photomediated phase transfer. The polarity change of
the complex induced by the photoisomerization of Azo groups
on the periphery is distinct enough to trigger the reversible
transfer of the complex shuttle between two immiscible
solvents. The present method provides a general strategy for
recycle of POM-based homogeneous catalysts and similar
systems.
Figure 3. UV−vis spectra of PH oxidation reaction in toluene solution
versus reaction time under the catalysis of Azo-SEP. The reaction
solution has been diluted 40-fold.
absorption band at 319 nm, deriving from the initial PH,
decreases as the reaction proceeds and finally disappears, while
the intensities of the bands at 306 and 339 nm corresponding
to the oxidized product apparently increase. At the same time,
the color of the reaction solution becomes a bit darker after the
oxidation (Figure 4). Due to the homogeneous catalytic
reaction, the reaction is quick and finishes when the spectrum
is no longer changing. According to the NMR result (Figure
S22), the substrate PH has been completely oxidized into its
product S,S-dioxide (PHDO) in 4 h. After addition of the same
ASSOCIATED CONTENT
Supporting Information
Experimental and characterization details, and supporting
figures. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
volume of H O/DMF mixed solvent as that of toluene followed
*
2
by UV light irradiation, the Azo-SEP catalyst transfers to the
water phase automatically due to the increased polarity of cis-
Azo-SEP, while the light pink product and residual oxidant
remain in the toluene phase (Figures S18 and S23). Through a
simple phase separation operation, the homogeneous catalyst
can be easily removed from the reaction phase. Furthermore,
after addition of the substrate solution, Azo-SEP catalyst can be
conveniently transferred back to the toluene phase through a
AUTHOR INFORMATION
Corresponding Authors
wenli@jlu.edu.cn
■
wulx@jlu.edu.cn
1
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Journal of the American Chemical Society
Communication
Notes
(34) Cheng, L.; Liu, A.; Peng, S.; Duan, H. ACS Nano 2010, 4, 6098.
̌
́
Z.; Prior, I.
(
35) Cioran, A. M.; Musteti, A. D.; Teixidor, F.; Krpetic,
A.; He, Q.; Kiely, C. J.; Brust, M.; Vinas, C. J. Am. Chem. Soc. 2011,
34, 212.
36) Haimov, A.; Cohen, H.; Neumann, R. J. Am. Chem. Soc. 2004,
26, 11762.
37) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673.
(38) The first recycle loses 1−2% twice from toluene to water and
back. After this cycle, there is no additional weight loss in the further
recycles of catalyst from H O/DMF to toluene phase. Therefore, each
The authors declare no competing financial interest.
̃
1
ACKNOWLEDGMENTS
■
(
1
(
This work is supported by the National Basic Research
Program (2013CB834503), NSFC (91227110, 21221063),
Ministry of Education of China (20120061110047), 111-
Project (B06009) for the collaboration with WVU, and Open
Project of SKL of Polymer Physics & Chemistry, CAS.
2
of the following recycles loses only 1−2% of catalyst in the transfer
process from toluene to water.
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