Visible-Light-Driven Photocatalytic Oxidation of Organic Chlorides Using Air and an Inorganic-Ligand Supported Nickel-Catalyst Without Photosensitizers

Article abstract of DOI:10.1002/cctc.201800629

Engineering photoredox-triggered chemical transformation via visible light has been an emerging area in organic synthesis. However, most of the well-established photocatalysts are based upon either transition metal complexes involved with noble metals and organic ligands or photosensitive organic dyes, the development of pure inorganic molecular photocatalysts that could provide better stability and durability is greatly retarded. Herein we discover that the Anderson polyoxometalate (POM) Na₄[NiMo₆O₁₈(OH)₆] (1), which consists of pure inorganic framework built from a central Ni^II core supported by six Mo^VIO₆ inorganic scaffold/ligands, can be used as a powerful photocatalyst. Upon irradiation with visible light (>400 nm), the compound can catalyze, in high efficiency, a wide range of reactions, including the oxidative cross-coupling reaction of chlorides with amines, as well as oxidation of chlorides using molecular oxygen, affording various imines, aldehydes, and ketones, respectively in high selectivity and good yields. Owing to the robust inorganic framework, this catalyst exhibits excellent stability during the catalysis and reusability with little loss of the catalytic activity, thus providing an alternative without use of complicated organic ligands and expensive noble metal-based photosensitizers.

Full text of DOI:10.1002/cctc.201800629

DOI: 10.1002/cctc.201800629
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Visible-Light-Driven Photocatalytic Oxidation of Organic
Chlorides Using Air and an Inorganic-Ligand Supported
Nickel-Catalyst Without Photosensitizers
+
[a, b, c]
+ [a]
+ [a]
[a]
[a]
[a]
Han Yu ,*
and Yongge Wei*
Jingjing Wang , Yongyan Zhai , Mengqi Zhang, Shi Ru, Sheng Han,*
[b, c]
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[
2]
Engineering photoredox-triggered chemical transformation via
visible light has been an emerging area in organic synthesis.
However, most of the well-established photocatalysts are based
upon either transition metal complexes involved with noble
metals and organic ligands or photosensitive organic dyes, the
development of pure inorganic molecular photocatalysts that
could provide better stability and durability is greatly retarded.
Herein we discover that the Anderson polyoxometalate (POM)
Na [NiMo O (OH) ] (1), which consists of pure inorganic frame-
decades, organo-Ni complexes in combination with photo-
[1a]
sensitizers (e.g. Ru and Ir-based polypyridyl complexes) have
been explored extensively for various chemical transformations
in organic synthesis (Figure 1b). In general, this catalytic system
4
6
18
6
II
VI
work built from a central Ni core supported by six Mo O
6
inorganic scaffold/ligands, can be used as a powerful photo-
catalyst. Upon irradiation with visible light (>400 nm), the
compound can catalyze, in high efficiency, a wide range of
reactions, including the oxidative cross-coupling reaction of
chlorides with amines, as well as oxidation of chlorides using
molecular oxygen, affording various imines, aldehydes, and
ketones, respectively in high selectivity and good yields. Owing
to the robust inorganic framework, this catalyst exhibits
excellent stability during the catalysis and reusability with little
loss of the catalytic activity, thus providing an alternative
without use of complicated organic ligands and expensive
noble metal-based photosensitizers.
The utilization of solar energy as a clean and renewable energy
resource for driving synthetic transformations has become a
[1]
surgent thrust in green chemistry. During the last two
+
+
+
[
a] Dr. H. Yu, J. Wang, Y. Zhai, M. Zhang, S. Ru, Prof. S. Han
School of Chemical and Environmental Engineering
Shanghai Institute of Technology
100 Haiquan Road
Shanghai 201418 (P.R. China)
E-mail: scihanyu@163.com
Figure 1. Imines synthesis systems: (a) Organic-ligand supported metal
catalysis systems. (b) Organic-nickel photoredox catalysis system. (c)
Inorganic-ligand supported nickel catalysis system.
hansheng654321@sina.com
+
[b] Dr. H. Yu, Prof. Y. Wei
Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of
Education
relies on the ability of metal-complexes and noble metal
photosensitizers to engage in electron transfer processes with
Department of Chemistry
Tsinghua University
Beijing 100084 (P.R. China)
[3]
organic substrates upon photoexcitation with visible light.
E-mail: yonggewei@tsinghua.edu.cn
[4]
+
Despite remarkable advantages of this system, shortcomings
such as poor stability of the organometallic compounds derived
from the delicate organic ligands, use of precious metals and
the difficulty to recover and reuse these homogeneous photo-
[
[
c] Dr. H. Yu, Prof. Y. Wei
State Key Laboratory of Natural and Biomimetic Drugs
Peking University
Beijing 100191 (P. R. China)
E-mail: ygwei@pku.edu.cn
[
5]
+
catalysts restrict their practical applications.
] These authors contributed equally to this work.
[6]
Polyoxometalates (POMs), a class of discrete metal-oxo
clusters analogous to semiconductor photocatalysts such as
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201800629
ChemCatChem 2018, 10, 1–7
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!

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WO3 and TiO , have shown photocatalytic activity towards
characterizations using XRD, FT-IR, and TEM are performed.)
2
degradation of organic pollutants, chemical transformation and
energy conversion. Owing to their pure inorganic structures,
POMs are thermally and oxidatively more stable than the
frequently utilized organometallic complexes and organic dyes,
under visible-light irradiation using O as the sole oxidant. The
2
[13]
Ni-POM cat. 1, Na [NiMo O (OH) ]
(Figure 1c), is easily
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prepared by simple one step route from cheap and commonly
*
*
available Ni(NO ) 6H O and Na MoO 2H O at room temper-
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[6g]
II
thus providing better durability and reusability.
In this
ature. The presence of Ni core enables cat. 1 to efficiently
VI
context, POMs are promising candidates as alternative photo-
catalysts compared with the traditional photocatalytic system
mentioned above. However, akin to their metal oxide counter-
parts, most POMs can only adsorb UV light due to the large
HOMO-LUMO gap energy. Although the electronic adsorption
properties can be precisely tuned by changing their constituent
elements and structures, there are only a few examples of
visible-light-active POM catalysts that are employed in chemical
transformation and most of them are based on polyoxotung-
states. Therefore, it is highly desirable to develop POM-based
photocatalysts compatible with visible light and especially
those constructed from other POMs scaffold such as polyox-
omolybdate given the structural diversity of POMs clusters.
Imines and their derivatives are essential intermediates for
the synthesis of pharmaceutically, biologically active com-
adsorb visible light and the supported {Mo O } inorganic
6
24
ligand not only shows quick photoredox response but also
II
greatly enhance the Lewis acidity of Ni , thus avoiding the use
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of complicated organic ligands and expensive photosensitizers
generally involved in photocatalysis (Figure 2). Furthermore, the
[7]
pounds and fine chemicals. Recently, several green method-
ologies have been developed to synthesize imines directly via
one pot protocol of oxidative coupling in stark contrast to the
traditional condensation of amines with carbonyl compounds.
Several reports showed the self-coupling of amines to
symmetric imines can be facilely promoted by a series of
Figure 2. Photocatalytic oxidation of organic chlorides to aldehydes or
ketones and oxidation coupling of organic chlorides with amines to imines
with Ni catalyst.
pure inorganic framework of this catalyst guarantees the
sustainable stability and reusability in the above three oxidative
reactions. To our best knowledge, cat. 1 represents the first
example of molybdenum-based POM that serves as a highly
efficient heterogeneous visible-light-active photocatalyst for
chemical transformation.
[8]
homogeneous and heterogeneous visible-light photocatalysts.
Moreover, direct catalytic oxidative coupling of alcohols and
amines has emerged as one promising and green synthesis
[9]
pathway to prepare both symmetric and asymmetric imines.
Since alcohols are readily generated from hydrolysis of halides
[10]
in industrial processes, direct cross-coupling of halides with
amines via photoredox catalysis will be an economical and eco-
friendly way to afford imines with more atomic economy. In
2014, Mizuno and co-workers reported that tetranuclear cerium
(III)-containing silicotungstate (CePOM) can work as an effective
photocatalyst for the self-oxidative coupling of amines to
Initially, we examined the photocatalytic activity of the
inorganic-ligand supported Ni-POM cat. 1 for the oxidative
cross-coupling reaction of chlorides with amines in acetonitrile
under irradiation by white LED light (40 W) using O (1 atm,
2
balloon) as the sole oxidant (Table S1). When benzyl chloride
and benzylamine were chosen as the model substrates, 92%
yield with 95% selectivity were obtained for N-benzylideneben-
zylamine (entry 1). However, without addition of cat. 1, the
starting materials were hardly converted into the product
[11]
symmetric imines with visible light. Inspired by this pioneer-
ing work, we aim to develop POM-based photocatalysts that
can be used to prepare imines from direct cross-coupling of
chlorides with amines. To do this, the Anderson-type polyox-
*
(entry 2). It should be noted that when Ni(NO ) 6H O or
3
2
2
*
omolybdate {MMo } (M=hetero metal centers) are chosen as
Na MoO 2H O was used as the catalyst alone, no oxidation
2 4 2
6
potential candidates as we have recently shown they are
versatile catalysts towards a series of oxidation transformation
product was observed even at prolonged reaction time up to
48 h (entries 3 and 4). In addition, the reaction scarcely occurred
in the absence of light (entry 5). Next, different solvents were
screened, and acetonitrile gave the best result (entries 6–11).
The influence of the catalyst loading on the product yield was
also investigated. Increasing or reducing the catalyst loading
gave a slightly lower yield (entries 12–14), which demonstrated
that altering the catalyst loading had a negative effect on
conversion. Switching from a pure oxygen atmosphere to air
under the same atmospheric pressure also led to 79% yield
(entry 15). When the reaction was carried out under nitrogen
atmosphere, a very low yield was obtained (<5%), implying a
stoichiometric aldehyde oxidation (entry 16). Compared other
and in particular {CuMo } has been used to catalyze aerobic
6
oxidation of amines to corresponding imines under mild
[
12]
condition. Furthermore, the catalytic activity and the elec-
tronic structures of Anderson-type POMs could be facilely
tuned by changing the central M atoms. We thus envision that
judicious choice of the central metal may lead to visible-light-
active polyoxomolybdate-based photocatalysts that are appli-
cable to synthesize imines via oxidative coupling.
Herein we demonstrate the oxidative cross-coupling re-
action of chlorides with amines, as well as oxidation reactions
of various chlorides photocatalyzed by Ni-POM catalyst 1 (to
demonstrate the successful formation of NiPOM, extensive
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catalysts reported (Figure 1a), the NiÀPOM cat. 1 show excellent
selectivity and activity for the synthesis of imine (Table S3).
With the optimal conditions in hand, the general applic-
ability for oxidative coupling reaction of various amines and
halides was examined, as shown in Table 1. The benzyl chloride,
chloride with aniline under analogous reaction conditions
yielded 92% of the corresponding imines (compound 4n).
Cyclic and aliphatic amines all reacted efficiently with benzyl
chloride to produce the imine products in excellent yields
(compounds 4o–4m). Notably, the most challenging coupling
of aliphatic chlorides and aliphatic amines was also promoted
by cat. 1 to afford the corresponding imines, albeit at relatively
lower but reasonable yields (compounds 4p and 4q).
Table 1. Photocatalytic oxidative coupling reactions of cross-coupling of
[
a,b]
organic halide and amines.
To further expand the applicability of our Ni-photocatalyst,
a series of substituted organic halides were oxidized with O2
under irradiation of white LED light (40 W). As shown in Table 2,
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Table 2. Photocatalytic oxidative of organic halides to aldehydes and
[a,b]
ketones.
[a] Reaction conditions: 1.0 mol% Cat. 1, 1.0 mmol amines, 1.0 mmol
halides, O (1 atm, balloon), 2.0 mL CH CN, 40w LED at room temperature
2
3
for 24 h; [b] determined by GC and confirmed by GC-MS.
benzyl bromide and benzyl iodide were smoothly cross-coupled
with benzylamine to yield N-benzylidenebenzylamine with a
yield of 92%, 95% and 90%, respectively (compound 4a).
When coupling with benzyl amine, benzyl chloride with
electron donating substituents generally resulted in higher
yields than substrates bearing electron-withdrawing substitu-
ents, probably because of higher nucleophilicity of the former
(compounds 4b–4f). Reaction of benzylamine with heterocycle
chloride such as 2-(chloromethyl) thiophene and 3-(chlorometh-
yl) pyridine produced the corresponding imines in 90% and
92% yields, respectively (compounds 4 g and 4 h). In view of
more challenging aliphatic chloride, (chloromethyl) cyclohexane
and 1-chloropentane gave corresponding imines in 76%, and
80% yields, respectively (compounds 4i and 4j). Switching from
benzyl amine to other amines, the catalytic coupling of benzyl
[a] Reaction conditions: 1.0 mol% Cat. 1, 1.0 mmol halides, O2 (1 atm,
balloon), 2.0 mL CH CN/H O (1:1), 40w LED, at room temperature for 12 h;
b] determined by GC and confirmed by GC-MS. Values in parentheses are
the isolated yields.
3
2
[
various primary and secondary benzyl chlorides and bromides
can be selectively oxidized to the corresponding aldehydes and
ketones in good to high yields under the optimal conditions
(Table S2). The catalytic system worked efficiently with the
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benzyl halides (compounds 6a and 6p) and electron-donating
substituted benzyl halides (compounds 6b–6e and 6 s) to
afford the corresponding aldehydes or ketones in excellent
yields, whereas electron-withdrawing substrates (compounds
6h–6m, 6q and 6r) gave a slightly lower yield. These results
indicate that electron-rich aryl halides are more reactive than
electron-deficient substrates toward oxidation. Notably, for the
cyclic and aliphatic primary and secondary benzyl chlorides, the
reactions also generated the corresponding aldehydes and
ketones with excellent yields (compounds 6n, 6o and 6x).
Reaction of heterocycle chloride such as 2-(1-chloroethyl)
thiophene and 2-(1-chloroethyl) furan produced 93% and 91%
of the corresponding imines, respectively (compounds 6u and
6v). The successful photooxidation of halides to corresponding
aldehydes or ketones also confirmed that the key intermediate
during cross-coupling of halides with amine is aldehyde.
not as an oxidant and molecular O is a crucial component for
2
the oxidative coupling reactions.
After removal of light from the reaction mixture and heated
at 608C, the starting materials were hardly converted into the
product (Figure 3b), indicating that Ni-catalyst 1 reacts with
molecular O to generate a new active species, with which the
2
reaction proceeds smoothly. These results are also consistent
with UV-Vis spectra of the NiPOM (0.1 mm) upon irradiation
with visible light in the presence of benzyl chloride (1.0 mm)
and benzylamine (1.0 mm) in acetonitrile under Ar (1 atm)
(Figure 4). According to previous reports, the molecular oxygen
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Then, we investigated the recyclability of photoactive
NiÀPOM catalyst in three oxidation reactions under visible-light
irradiation (Figure S4). After the reaction, the solid Ni-catalyst
was isolated by filtration and used directly for the subsequent
photocatalytic oxidation without further purification. In all
cases, catalyst 1 maintains high efficiency up to the sixth cycles
without obvious loss of catalytic activity. The recycled cat. 1
was analyzed by FT-IR and XRD after reusing for six times
(Figure S5 and S6). The FT-IR and XRD spectra of cat. 1 before
and after the reaction were almost identical to each other,
indicating cat. 1 kept better activity and morphology after
recycling. Moreover, the scale-up of the three reactions could
be carried out smoothly also in gram-scale with high yields
(Figure S7). This demonstrates the current photocatalytic
systems can be readily scaled up to an industrial level.
Figure 4. (a) UV-Vis spectra of the NiPOM (0.1 mm) upon irradiation with
visible light in the presence of benzyl chloride (1.0 mm) and benzylamine
(
(
1.0 mm) in acetonitrile under Ar (1 atm) (blue line) and of (b) without NiPOM
black line).
is converted into a superoxide radical by acquiring an excited
[14]
To unveil the mechanism, some control experiments were
conducted. When Ni-catalyst 1 was used stoichiometrically
under inert atmosphere, trace of the N-benzylidenebenzylamine
was obtained (Figure 3a), which indicates that catalyst 1 was
electron from the conduction band of a photosensitizer, and
the super-oxide radical can abstract a hydrogen atom at the
benzylic position of a benzylamine radical cation to produce
benzylimine and hydrogen peroxide (H O ) as a byproduct.
2
2
To confirm that a superoxide radical was involved in the Ni-
photocatalyzed oxidation of the amine, p-benzoquinone (BQ),
[15]
which is well-known as a superoxide scavenger, was added
[14]
to the reaction mixture (Figure 3c).
The product yield
decreased significantly to 5% in the presence of BQ, indicating
that the formation of a superoxide radical is an essential step in
Ni-photocatalyzed oxidative cross-coupling reaction of halides
with amines. Therefore, we analyzed the benzylamine coupling
1
reaction mixture during the middle of the reaction by H NMR
to confirm the production of H O ; in the NMR spectrum, the
2
2
peak for H O clearly appeared at 10.96 ppm (Figure S8 in the
2
2
Supporting Information).
On the basis of these results, we propose a possible
reaction mechanism as shown in Figure 5. In the initial step, the
Ni-catalyst activates molecular oxygen to afford an active
III
VI
species Ni ÀPOM (Mo ) and a superoxide radical under
irradiation by white LED light, which undergoes the one
II
V
electron transfer to give Ni ÀPOM (Mo ), and subsequently
transforms into Ni-catalyst by O in a photoredox cycle
2
(Figure S9, the cyclic voltammetry curve of catalyst 1 in the
Supporting Information). Molecular oxygen is activated on the
defect sites of the catalyst, and subsequently transformed into
Figure 3. Experimental studies providing insight into the mechanism of the
Ni-catalyzed the oxidative cross-coupling of chlorides and amines.
ChemCatChem 2018, 10, 1–7
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Keywords: Polyoxometalates · chlorides · imine derivative ·
photocatalytic · nickel catalysts
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In conclusion, we have successfully realized the oxidative
cross-coupling of chlorides and amines, and as well as oxidation
of chlorides, to afford imines, aldehydes and ketones, respec-
tively, using a NiÀPOM catalyst by the visible-light-induced
photo-catalysis, all attained with high efficiency and selectivity
under atmospheric pressure at room temperature. The NiÀPOM
photocatalyst can be recycled and reused for multiple runs
without obvious decrease in catalytic efficiency owing to robust
inorganic framework. Compared with homogeneous Ni-photo-
catalytic system, this inorganic-ligand supported NiÀPOM
photocatalyst avoids the use of complicated organic ligands
and expensive photosensitizers, thus showing great potentials
in practical application. The strategy described here will be
helpful for further development of other POM-based photo-
catalysts. Further study on extending the applicability of
NiÀPOM for more photoredox-driven chemical transformations
and designing novel photocatalysts by tuning the structures at
a molecular level is currently underway in our laboratory.
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Manuscript received: April 12, 2018
Version of record online: &&&, &&&&
ChemCatChem 2018, 10, 1–7
www.chemcatchem.org
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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COMMUNICATIONS
Best supporting role: A practical and
efficient environmentally benign
catalytic protocol of oxidation of
cross-coupling reaction of chlorides
with amines, as well as oxidation of
halides was developed with
Dr. H. Yu*, J. Wang, Y. Zhai, M. Zhang,
S. Ru, Prof. S. Han*, Prof. Y. Wei*
1 – 7
Visible-Light-Driven Photocatalytic
Oxidation of Organic Chlorides
Using Air and an Inorganic-Ligand
Supported Nickel-Catalyst Without
Photosensitizers
inorganic-ligand supported Ni-photo-
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catalyst Na [NiMo O (OH) ] using
4
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molecular oxygen and visible light.
This method utilizes mainly an
inorganic polymolybdate ligand to
2
+
support the Ni ion and 1 atm O2
gas as the sole oxidant, which can
smoothly proceed to afford various
imines and aldehydes or ketones
with high yields and selectivity,
avoiding the use of complicated
organic ligands and expensive noble
metal-based photosensitizers.