Visible-Light-Driven Oxidative Coupling Reactions of Amines by Photoactive WS2 Nanosheets

Article abstract of DOI:10.1021/acscatal.5b02798

Visible-light-driven photocatalysis has been emerging as an efficient and sustainable approach for chemical transformation in organic reactions, in which photostable and cost-effective photosensitizers are required to trigger and promote it. Monolayer WS<

Full text of DOI:10.1021/acscatal.5b02798

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Article
Visible Light-Driven Oxidative Coupling Reactions
2
of Amines by Photoactive WS Nanosheets
Faizan Raza, Jung Hyun Park, Hye-Rim Lee, Hye-In Kim, Su-Ji Jeon, and Jong-Ho Kim
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02798 • Publication Date (Web): 17 Mar 2016
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Visible Light-Driven Oxidative Coupling Reactions
of Amines by Photoactive WS Nanosheets
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Faizan Raza, Jung-Hyun Park, Hye-Rim Lee, Hye-In Kim, Su-Ji Jeon and Jong-Ho Kim*
Department of Chemical Engineering, Hanyang University, Ansan, 426-791, Republic of Korea.
ABSTRACT: Visible-light driven photocatalysis has been emerging as an efficient and
sustainable approach for chemical transformation in organic reaction, in which photo-stable and
cost-effective photosensitizers are required to trigger and promote it. Monolayer WS nanosheets
2
smaller than 120 nm were prepared by means of a modified liquid exfoliation method, and they
showed strong photoluminescence in the visible range of electromagnetic spectrum from 450 to
650 nm. These photoactive WS nanosheets were exploited as photocatalysts in the oxidative
2
coupling reactions of various amines under visible light irradiation. They showed excellent
photocatalytic activity and reusability without the loss of their catalytic activity in the visible light–
driven oxidative coupling reactions of various amines. In addition, the mechanism responsible for
WS nanosheets-catalyzed imine production under visible light irradiation was fully investigated.
2
KEYWORDS: aerobic oxidative coupling, amines, photocatalysis, tungsten disulphide, 2D
nanosheets, visible light-driven reaction
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INTRODUCTION
Visible light in the solar spectrum has been employed as a sustainable driving force for
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various chemical transformations in organic reaction. For efficient transformation to desired
products under visible light irradiation, the use of photosensitizers is essential to provide electron
or hole transfer to reactant chemicals.1e, 2 Organometallic compounds involving precious metals
have been widely utilized as photosensitizers in photocatalysis due to their long excited state
1
a
lifetimes. However, the weak long-term stability of organometallic photosensitizers and the
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limited abundance of their constituents precious metals limit their practical applications. Hence,
it still remains important and interesting to develop alternative photosensitizer materials that
address these issues.
Tungsten disulfide (WS
2
) is an emerging transition metal dichalcogenide (TMD) that has
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received great attention in various research fields. The optical properties of exfoliated WS
2
can
be modulated by varying its thickness; decreasing the number of layer; eventually leads to a
transition from indirect bandgap in bulk WS . Monolayer
to direct bandgap in monolayer WS 5
WS is able to emit photoluminescence (PL) and also strongly absorbs in the visible region of the
electromagnetic spectrum. Hence, monolayer WS could be a promising candidate to serve as a
2
2
2
2
photosensitizer in visible light-driven organic reaction. However, to our best knowledge, there has
been no report thus far regarding the chemical transformation of organic compounds
photocatalyzed by monolayer WS
2
. In particular, we are interested in exploring the use of WS as
2
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a photocatalyst in light-driven conversion of amines to imines because imine derivatives are
essential intermediates in the synthesis of pharmaceutical, agrochemicals, dyes, and fine
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chemicals. A few reports showed the photocatalytic transformation of amines to imines by several
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types of photosensitizers.7
Herein; we demonstrate the oxidative coupling reactions of various aryl amines photocatalyzed
by WS
2
nanosheets under visible light irradiation. The photoactive WS
2
nanosheets prepared by
simple liquid exfoliation showed great photocatalytic activity and reusability in the oxidative
coupling reactions of the various amines. The mechanism responsible for WS
2
-photocatalyzed
transformation of the amines to the corresponding imines was also investigated.
EXPERIMENTAL METHODS
Preparation of tungsten disulfide (WS
2
) nanosheets: photoactive WS
2
monolayer was prepared by
modified liquid exfoliation method. Bulk WS
2
(2 mg/mL) was sonicated in NMP by a tip sonicator
in an ice bath for 90 min at a power of 150 W. After more addition of 1 mL NMP to the dispersion
to make the final concentration of 1 mg/mL, the mixture was sonicated further for 60 min at the
same power in the ice bath. The resulting dispersion was then centrifuged at 5000 xg for 90 min,
and the supernatant was obtained.
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General procedure for oxidative coupling reactions of amines; a 0.1 mmol portion of amine
substrate was dissolved in 7 mL of acetonitrile, followed by addition of 3 mL of a dispersion of
WS
2
nanosheets (11 wt. %) in a Pyrex reactor equipped with a rubber septum in an oil bath placed
gas into the reaction solution, it was irradiated with 60 W
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on a hot plate. After purging with O
2
white LED lamps at 50 °C (a distance between the reactor and the lamp was 7 cm) as shown in
Figure S1. After reaction for a certain period of time, the product as well as the unreacted amine
were extracted with diethyl ether (3×2 mL). Then, the conversion and yield of the product were
quantified by Gas Chromatography (YL Instrument 6500GC) using the calibration curves prepared
by bromobenzene as an internal standard and each imine product.
Recyclability of WS
were extracted out with diethyl ether, and the remaining catalyst solution was centrifuged at 15000
rpm for 45 min by using a micro centrifuge. The WS sediment was then collected, followed by
addition of 3 mL NMP. The WS /NMP solution was sonicated with a tip sonicator for 30 min at a
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nanosheets; after the first round of reaction, all the reactant and the product
2
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power of 22 W in an ice bath. The dispersed WS
oxidative coupling reaction.
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nanosheets were used again in the next round of
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RESULTS AND DISCUSSION
Photoactive WS
2
nanosheets were prepared by means of a modified liquid exfoliation method
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based on the previously reported method, to obtain a smaller size of monolayer WS
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nanosheets,
in N-methyl-2-pyrrolidone (NMP) was carried out in
an ice bath. After centrifugation, the exfoliated WS nanosheets were characterized by atomic force
microscopy (AFM) and transmission electron microscopy (TEM) (Figure 1).
the sequential ultrasonication of bulk WS
2
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Figure 1. Characterization of exfoliated WS nanosheets: (a) AFM image; (b) height profile along
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the dash line in (a); (c) statistical analysis of lateral sizes; (d) TEM image; (e) lattice structure; (f)
SAED pattern; and (g) FFT pattern.
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The AFM image of the exfoliated WS
that they had a height of 1 nm, suggesting that most of the nanosheets were monolayers (Figure
a, b). Analysis of the lateral size distribution showed that majority of the WS nanosheets were
laterally smaller than 120 nm (Figure 1c). TEM analysis confirms the formation of WS mono-
layer with a clear lattice fringe of 2.5 Å (Figure 1d, e). In addition, SAED and FFT patterns clearly
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nanosheets and a corresponding height profile showed
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show the 2H hexagonal crystal structure of WS
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monolayer (Figure 1f, g).
Figure 2. Optical properties of exfoliated WS
2
nanosheets: (a) photographs of fluorescent WS
2
solution without and with light irradiation; (b) fluorescence emission spectra for various excitation
wavelengths; (c) absorption and photoluminescence excitation (PLE) spectra of exfoliated WS
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nanosheets; (d) Raman spectra of bulk WS
2
and exfoliated WS
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nanosheets
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Next, optical properties of the exfoliated WS
showed blue emission under ultraviolet (UV) light irradiation (Figure 2a). The PL maximum
wavelength of the WS nanosheets was bathochromically shifted from 435 to 575 nm as the
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nanosheets were investigated. The WS solution
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excitation wavelengths were gradually increased (Figure 2b). This shift can be attributed to a
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quantum confinement effect arising from various sizes of smaller WS
WS
wavelength at 439 nm in their PL excitation (PLE) spectra (Figure 2c). Raman spectra of the WS
nanosheets clearly showed a bathochromic shift in their E12g vibrational mode (from 348 to 352
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nanosheets. The obtained
2
nanosheets showed a maximum excitation wavelength at 365 nm and a maximum emission
2
-1
-1
cm ) and a hypsochromic shift in their A1g vibrational mode (from 420 to 417 cm ) relative to
those of bulk WS (Figure 2d). A decrease was observed in the distance between the two peaks for
these vibrational modes, indicating that the monolayer WS
feature for monolayer WS was also observed in its absorption spectrum in which the characteristic
excitonic peak of monolayer WS for the direct electronic transition at K point of Brillouin zone
2
1
0
2
nanosheets were well prepared. The
2
2
1
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appeared at 627 nm (1.97 eV) (Fig. 2c, black line). Note that the monolayer WS
2
nanosheets
exhibited strong absorption in the visible region of the electromagnetic spectrum.
We then examined the photocatalytic activity of the obtained WS nanosheets in the oxidative
2
coupling reaction of various amino compounds (Scheme 1). As shown in Table 1, the oxidative
coupling reaction of benzylamine (0.1 mmol) as a model substrate was carried out in the presence
of the WS
2
nanosheets (11 wt. %) and of molecular oxygen under irradiation by white LED light
(60 W). Benzylamine was converted into the corresponding product, N-benzylidene benzylamine,
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with high yield (94%) under very mild conditions (Table 1, entry 1). However, without addition
of WS
). In addition, the reaction scarcely occurred in the absence of light (Table 1, entry 3). In
particular, when 1T-phase of metallic WS nanosheets, which are not photoactive, were added to
the reaction (Table 1, entry 5), the oxidative coupling reaction was not promoted. As an additional
control experiment, bulk WS was also examined in the visible light-driven oxidative coupling
reaction of benzylamine. As expected, no benzylamine was converted into the product in the
presence of bulk WS under visible light (Table 1, entry 6). The inactivity of bulk WS in this
photocatalysis can be attributed to the fact that the band edge position of its conduction band is
much lower than the potential of molecular O because bulk WS with stacked multilayers (Figure
S4a) exhibits an indirect bandgap of 1.3 eV in the near-infrared range of electromagnetic
spectrum.11a, 11c, 11d In addition, bulk WS
showed no emission in visible range despite its strong
absorption of visible light (Figure S4b, and S4c). However, monolayer WS nanosheets exhibited
a direct band gap of 1.9 eV in visible range, and the band edge positon of their conduction band is
higher than the potential of molecular O
,12 which facilitates the transfer of photo-excited electrons
from WS nanosheets to O to produce superoxide that is an essential species in the photocatalzyed
oxidative coupling reaction of amines. All control experimental results clearly indicated that
semiconducting WS nanosheets of 2H-phase (Figure 1d, g and S2) were the essential photoactive
2
nanosheets, the starting compound was hardly converted into the product (Table 1, entry
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Scheme 1. Photocatalytic oxidative coupling of benzylamine into imine by WS nanosheets
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Table 1. Photocatalytic oxidative coupling reaction of benzylamine by WS
2
nanosheetsa
Entry
1
+
+
+
2
̶
3
+
̶
4
5
6
7
+
8c
+
+
+
2
9
+
WS
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̶
1T Bulk
hν
+
+
̶
+
+
9
+
+
+
+
O
2
+
+
Ar
10
Air
30
Yield(%)b 94 15 16 13
n.r
aReaction conditions: 0.1 mmol of amine, WS
mL NMP, 7 mL CH CN, O , 60 W white LED lamp, 50 C.
(11 wt. %), 3
2
0
3
2
b
Determined by GC using bromobenzene as an internal
standard. 0.1 mmol of p-benzoquinone was added as a
c
superoxide scavenger.
Further experiments were carried out to investigate the mechanism responsible for the WS -
2
photocatalyzed oxidative coupling of benzylamine. After removal of O from the reaction mixture
2
by purging with Ar gas, benzylamine was hardly converted into the product (Table 1, entry 7),
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indicating that molecular O
2
is a crucial component for the oxidative coupling reactions. According
to the previous reports, the molecular oxygen is converted into a superoxide radical by acquiring
an excited electron from the conduction band of a photosensitizers.7b, 7d, 13 To confirm that a
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superoxide radical was involved in the WS
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-photocatalyzed oxidation of the amine p-
benzoquinone (BQ) which is well known as a superoxide scavenger, was added in the reaction
1
4
(
Table 1, entry 8). The product yield decreased significantly to 2% in the presence of BQ,
indicating that the formation of a superoxide radical is an essential step in WS photocatalyzed
oxidative coupling of amines. We further investigated the PL response of the WS nanosheets to
benzylamine to indirectly confirm that benzylamine could be oxidized by hole transfer from the
valence band of the WS nanosheets (Figure S3). The PL intensity of the WS nanosheets gradually
decreased as the concentration of benzylamine increased. This PL quenching of the WS
nanosheets can be ascribed to electron transfer from benzylamine to the valence band of the
photoexcited WS
based on their potential energy.1c, 13b, 15 This reductive quenching of the WS
2
2
2
2
2
2
2
PL could induce oxidation of benzylamine to yield a benzylamine radical cat- ion as reported
previously.7d, 13b, 16 According to the previously reported mechanism, a superoxide radical can
abstract a hydrogen atom at the benzylic position of a benzylamine radical cation to produce
benzylimine and hydrogen peroxide (H
2
O
2
) as a by-product. Therefore, we analyzed the
1
benzylamine coupling reaction mixture during the middle of the reaction by H-NMR to confirm
the production of H
2
O
2
; the NMR spectrum, the peak for H
2
2
O clearly appeared at 10.95 ppm
(Figure S5).
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On the basis of the above experimental results, we proposed a mechanism responsible for the
oxidative coupling reactions of amines photocatalyzed by WS
2
nanosheets as shown in Scheme 2.
producing a
When WS nanosheets are photoexcited, their excited electrons are transferred to O
2
2
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superoxide radical while their holes are transferred to amine yielding amine radical cations.
Reaction between the superoxide radical and the amine radical cation forms the intermediate
imine, which can be activated by its interaction with a hole from WS
2
nanosheet to produce a
desired product after addition of another amine and removal of ammonia.
Scheme 2. Proposed mechanism for the oxidative coupling of benzylamine photocatalyzed by WS
2
nanosheets.
Next, the oxidative coupling reactions of various amines photocatalyzed by WS
2
nanosheets
were examined as listed in Table 2. Benzylamines bearing electron donating or withdrawing
substituents were converted into their corresponding products with high yields by WS
photocatalysis (Table 2, entries 1-8). However, the benzylamines with electron donating groups
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(
Table 2, entries 1-4) underwent photocatalyzed oxidative coupling more efficiently than those
with electron withdrawing groups (Table 2, entries 5-8). In addition, the benzylamine having a
methyl group at ortho-position showed a slight decrease in product yield (Table 2, entry 2) com-
pared to the para isomer (Table 2, entry 1) owing to steric hindrance. Thiophene-methylamine a
heterocyclic amino compound, that is known to poison metal catalysts, was also examined (Table
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, entry 9); it was converted into the corresponding product with a high yield, that was somewhat
lower than the yields for benzylamines. It was found that cyclohexylamine was not an active
substrate in WS -photocatalyzed oxidative coupling reactions (Table 2, entry 10) as compared to
2
benzylamines. Secondary dibenzylamine also yielded the corresponding imine with moderate
yield (Table 2, entry 12). In addition to the homocoupling reactions, an oxidative cross-coupling
reaction between benzylamine and heptylamine was also conducted in the presence of WS
nanosheets, showing moderate yield (Table 2, entry 13). These results suggest that photoactive
WS nanosheets are a promising photosensitizer that promotes the oxidative coupling reactions of
various amines.
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nanosheetsa
Table 2. Photocatalytic oxidative coupling reactions of various amines by WS
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t.
conv.
(%)b
sel.
entry
amine
product
(h)
(%)b
NH2
N
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30
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92
85
95
92
79
90
92
91
83
----
77
63
95
NH2
N
N
N
96
98
96
69
89
94
90
85
----
86
93
NH2
O
O
O
NH2
O
O
O
O
O
NH2
N
N
O
NH2
F
F
F
Cl
Cl
NH2
N
Cl
Cl
Cl
Cl
NH2
N
Cl Cl
Cl
S
S
S
NH2
N
NH2
1
0
----
NH2
11
N
N
Ph
N
1
2
H
NH2
3c
N
1
30
71
78
NH2
aReaction conditions: 0.1 mmol of amine, WS
NMP, 7 mL CH CN, O , 60 W white LED lamp, 50 C.
Determined by GC using bromobenzene as an internal standard.
Added 3eq. of heptylamine
(11 wt. %), 3 mL
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Finally, we investigated the recyclability of photoactive WS nanosheets in the oxidative
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coupling reaction of benzylamine under visible light irradiation. After each round of reaction, some
portion of WS nanosheets were lost due to their incomplete recovery according to quantification
of the collected catalysts using inductively coupled plasma-atomic emission spectroscopy (ICP-
AES) as shown in Table S1. However, there was no leaching of WS into the solution of the
product, indicating that WS nanosheets are chemically stable during the reaction. We also
analyzed the structure of the WS nanosheets with TEM after recycling them five times in the
reaction. As shown in Figure S6, WS nanosheets retained the same size and morphology after
reactions. In order to keep the amount of catalyst constant in every reaction cycle, we made up the
lost amount of WS nanosheets in the next round of reaction. As shown in Figure 3, benzylamine
was converted into the corresponding product with high yields in all rounds of reaction. This result
0
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9
0
1
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3
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indicates that the photocatalytic activity of the WS
2
nanosheets remained almost the same
throughout their use in all reaction cycles.
Figure 3. Recyclability of photoactive WS nanosheets in the photocatalytic oxidative coupling of
2
benzylamine
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CONCLUSION
In summary, we readily prepared small-sized photoactive WS
modified liquid exfoliation method, and successfully applied them as photocatalysts in the
oxidative coupling reactions of various amines. The WS nanosheets exhibited excellent
photocatalytic activity and recyclability in these reactions. The mechanism responsible for the
WS -photocatalyzed oxidative coupling of the amines was also fully investigated. To the best of
our knowledge, this is the first reported example for the use of WS nanosheets as a photosensitizer
in light-driven organic synthesis. The use of WS nanosheets-based photocatalysts can be
2
nanosheets by means of a simple
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expanded to other organic reactions driven by solar energy.
ASSOCIATED CONTENT
Supporting Information.
The supporting information is available free of charge on the ACS publication website at DOI:
http://pubs.acs.org. Materials, instrumentation, additional experimental procedures, XPS, PL and
NMR and FT-IR analysis for all imine products.
AUTHOR INFORMATION
Corresponding Author
*
To whom correspondence should be addressed; Jong-Ho Kim, kjh75@hanyang.ac.kr
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ACKNOWLEDGMENT
This work was supported by Samsung Research Funding Center of Samsung Electronics under
project number SRFC-TA1503-02. Faizan Raza greatfully acknowledges scholarship support
provided by the Higher Education Commission of Pakistan (H.E.C) through its HRDI-
UESTPs/UETs faculty development program.
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