Visible-Light Promoted C–O Bond Formation with an Integrated Carbon Nitride–Nickel Heterogeneous Photocatalyst

Article abstract of DOI:10.1002/anie.202016511

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CN_x) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CN_x is demonstrated for C–O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60–92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CN_x can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Full text of DOI:10.1002/anie.202016511

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8494–8499
International Edition:
German Edition:
doi.org/10.1002/anie.202016511
doi.org/10.1002/ange.202016511
Heterogeneous Catalysis
Visible-Light Promoted C–O Bond Formation with an Integrated
Carbon Nitride–Nickel Heterogeneous Photocatalyst
Arjun Vijeta, Carla Casadevall, Souvik Roy, and Erwin Reisner*
Abstract: Ni-deposited mesoporous graphitic carbon nitride
(Ni-mpg-CNx) is introduced as an inexpensive, robust, easily
synthesizable and recyclable material that functions as an
integrated dual photocatalytic system. This material overcomes
the need of expensive photosensitizers, organic ligands and
additives as well as limitations of catalyst deactivation in the
existing photo/Ni dual catalytic cross-coupling reactions. The
dual catalytic Ni-mpg-CNx is demonstrated for C–O coupling
between aryl halides and aliphatic alcohols under mild
condition. The reaction affords the ether product in good-to-
excellent yields (60–92%) with broad substrate scope, includ-
ing heteroaryl and aryl halides bearing electron-withdrawing,
-donating and neutral groups. The heterogeneous Ni-mpg-CNx
can be easily recovered from the reaction mixture and reused
over multiple cycles without loss of activity. The findings
highlight exciting opportunities for dual catalysis promoted by
a fully heterogeneous system.
intermediate that produces an excited-state Ni*II species upon
light-induced energy transfer from the photosensitizer. These
NiIII or Ni*II species can then thermodynamically drive the
RE step efficiently.[7–9] This dual catalysis approach has not
only facilitated traditional cross-coupling reactions, but also
enabled direct coupling of radical species generated under
mild reaction conditions.[10,11] However, this approach has
been mostly limited to precious Ru- and Ir-based homoge-
neous photocatalysts.[12]
Carbon nitride (CNx) has emerged as a promising light-
harvesting material for applications in dual nickel/photo-
catalysis owing to its low-cost, non-toxicity and facile syn-
thesis (Scheme 1a).[13,14] The tunable redox potential, excel-
lent photostability and light harvesting ability of CNx make it
an appealing candidate for photocatalysis,[15] which has
already been proven useful for challenging organic trans-
formations,[16–18] solar water splitting[19,20] and CO2 reduction
reactions.[21–23] However, nickel-based catalysis with CNx
relies on a homogenous coordination complex such as
pyridine-based Ni catalysts, which introduces fragility, diffu-
sion-limited charge transfer, gradual formation of inactive
nickel-black and challenges in product isolation.[24]
The heptazine-units and amine functional groups within
CNx contain intrinsic coordination sites, which provide
a robust scaffold for binding Ni2+ and enable direct electronic
communication between the light-harvesting units and Ni-
active sites.[25,26] Previous studies have shown that metal
doping in CNx improves the photocatalytic activity of CNx for
solar fuels production by facilitating charge separation.[27,28]
Despite its potential to enhance activity, robustness, and
recyclability, the synergic effect of Ni-deposition on CNx is
still underexplored in organic synthesis. Only a very recent
report demonstrated an assembly of carbon nitride with Ni as
a dual catalytic system for C–O coupling, but it still required
imidazole as an auxiliary ligand for the activating Ni catalysis,
and quinuclidine as a sacrificial electron donor.[29]
T
he development of sustainable methodologies for organic
synthesis is essential to establish a green chemical industry in
a future circular economy. Transition-metal-catalyzed cross-
coupling reactions are essential tools in fine-chemical syn-
thesis, predominately employing homogeneous palladium
catalysts.[1,2] Abundant nickel has emerged as a more sustain-
able alternative,[3] but its lower electronegativity makes the
reductive elimination (RE) step challenging,[4] particularly in
the coupling reactions of carbon and an electronegative
À
À
heteroatom to form C O and C N bonds.
Recent studies have demonstrated that the RE step can be
enhanced by combining a photosensitizer with nickel catalysis
via a dual photocatalysis approach (Scheme 1a).[5,6] The
strategy involves: i) photoinduced electron transfer from the
light absorber to the Ni catalyst yielding a reduced NiI species
that can undergo oxidative addition to generate a high energy
NiIII species, or ii) oxidative addition to Ni0 generating a NiII
Herein, we report nickel-deposited mesoporous carbon
nitride (Ni-mpg-CNx) as an integrated single-entity photo-
catalyst to perform organic C–O coupling reactions between
simple alcohols and various aryl halides under visible-light
irradiation (Scheme 1b). Kinetic studies provide mechanistic
insights into the dual catalytic role of Ni-mpg-CNx and the
robustness and applicability of Ni-mpg-CNx is demonstrated
by continuous recycling experiments.
Mpg-CNx has been synthesized following a slightly modi-
fied reported procedure by heating cyanamide with silica as
a hard template in air at 5508C, followed by etching with
aqueous ammonium difluoride.[23] Ni-mpg-CNx was prepared
by heating a mixture of NiCl2 and mpg-CNx in acetonitrile at
808C under microwave treatment in the presence of triethyl-
[*] A. Vijeta, Dr. C. Casadevall, Dr. S. Roy, Prof. E. Reisner
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge CB2 1EW (UK)
E-mail: reisner@ch.cam.ac.uk
Dr. S. Roy
Current address: School of Chemistry, University of Lincoln
Joseph Banks Laboratories, Lincoln LN6 7DL (UK)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016511.
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
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Scheme 1. Comparison between a) state-of-the-art homogeneous Ni dual systems for photoredox catalysis and b) the integrated Ni-mpg-CNx
heterogeneous photocatalyst for C–O coupling reported in this study.
amine to facilitate the deposition of Ni. For comparative
studies, Ni2+ was also deposited on two other types of carbon
nitride (non-mesoporous CNx and NCN-functionalized CNx)
under the same conditions (see Experimental Details in the
Supporting Information).
The deposition of Ni on mpg-CNx was confirmed by
inductively coupled plasma optical emission spectrometry
(ICP-OES), X-ray photoelectron spectroscopy (XPS), scan-
ning electron microscopy (SEM), and transmission electron
microscopy (TEM) (Figure 1 and Figures S1–S7). ICP-OES
shows that 4.14 Æ 0.99 wt% Ni was deposited on mpg-CNx
(Table S1). The XPS spectrum of Ni-mpg-CNx consists of two
peaks in the Ni 2p region at 855 (Ni 2p3/2) and 872 eV (Ni
2p1/2), which confirm the presence of Ni2+ sites on mpg-CNx
(Figure 1A). The absence of a peak at ꢀ 852 eV indicates the
absence of a metallic Ni0 species.[30] The N 1 s peak of
unmodified mpg-CNx at ꢀ 398 eV can be deconvoluted into
three peaks centered at 397.0, 398.6 and 399.6 eV, assigned to
=
C N-C (pyridinic N), N(C)3 (quaternary N) and C-N-H
(uncondensed amine) (Figure S6).[31] Upon nickel modifica-
tion, a new peak appears in the N 1s region at 397.9 eV, which
can be tentatively attributed to Ni-coordinated pyridinic
N.[32,33] The C 1s XPS spectra of mpg-CNx and Ni-mpg-CNx
could be deconvoluted into near-identical components with-
out any significant shifts (Figure S7). Analysis of the compo-
sition of the near-surface region by XPS shows a lower Ni
content of 2.2 wt% at the catalyst surface. Energy-dispersive
X-ray (EDX) analysis of TEM images indicates uniform
distribution of Ni throughout the material (Figure 1B,C).
Powder X-ray diffraction (pXRD), attenuated total reflec-
tance infrared (ATR-IR) and UV/Vis spectra of Ni-mpg-CNx
are similar to bare mpg-CNx, suggesting that Ni deposition did
not affect the optical properties and composition of the
material (Figure S8–S10). Overall, we speculate that Ni-mpg-
CNx features active Ni2+ sites coordinated to the pyridinic
groups in CNx and the material retains the photocatalytic
activity of the parent mpg-CNx.
Figure 1. A) Normalized XPS Ni2p region; (s) denotes satellite peak.
B) TEM image with C) EDS mapping of Ni in Ni-mpg-CNx. D) pXRD,
E) ATR-IR and F) UV/Vis DRS of mpg-CNx, Ni-mpg-CNx (as synthe-
sized), Ni-mpg-CNx post-catalysis, and Ni-mpg-CNx obtained after
performing catalysis with NiCl2 (salt)+mpg-CNx.
4-bromobenzonitrile in ethanol, which also serves as a cou-
pling partner, with sodium hydroxide as a base (Table 1). The
desired coupling product 1 was obtained in 75% yield upon
Our investigation into C–O coupling using Ni-mpg-CNx
powder as an integrated solid-state photocatalyst started with
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Table 1: Screening of carbon nitride, co-catalyst and control experiments.
Entry
Deviation
Product[a]
1
2
3
none
75% (73%)[b]
28%
8%
Ni-CNx
Ni-NCNCNx
4
5
6
7
8
9
10
11
12
13
NiCl2 (1 wt%) + mpg-CNx
NiCl2 (2 wt%) + mpg-CNx
NiCl2 (5 wt%) + mpg-CNx
FeCl2 (5 wt%) + mpg-CNx
CoCl2 (5 wt%) + mpg-CNx
no light
no base
no photocatalyst
mpg-CNx
61%
70%
70%
n.d.
n.d.
<1%
<1%
n.d.
2%
n.d.
NiCl2 (5 mol%)
Standard conditions: 4-bromobenzonitrile (100 mM), Ni-mpg-CNx
(5 mg), [NaOH] (200 mM in EtOH, 2 mL), 420 min irradiation at
l=447Æ20 nm and 408C under N2. NiCl2 wt% refers to weight
percentage of mpg-CNx, n.d.=not detected. [a] 1H-NMR yield using
1,3,5-trimethoxybenzene (50 mmol) as an internal standard. [b] Isolated
yield.
irradiation of the reaction mixture in a blue LED photo-
reactor (l = 447 Æ 20 nm, 1.03 W @ 700 mA per LED)[34] for
18 hours under an inert atmosphere (entry 1). Other Ni-
deposited CNx materials with similar optical and redox
properties provided a significantly lower yield of 1 (28% for
pristine CNx and 8% for NCNCNx, entries 2 and 3, Table 1).[35]
The superior activity of mpg-CNx can be attributed to its
higher surface area, greater number of defects and available
pyridyl as well as amine groups for Ni coordination.[36]
Performing the reaction upon separate addition of mpg-
CNx and different wt% of NiCl2 resulted in a similar yield of
1 (entries 4–6, Table 1). This result shows that adding NiCl2 in
situ leads to its deposition on mpg-CNx as confirmed by ICP-
OES (Table S1) and post-catalysis characterization by XPS,
TEM, pXRD, ATR-IR and UV/Vis spectra confirmed the
formation of Ni-mpg-CNx (Figure 1 and Figures S2 and S5).[13]
The amount of NiCl2 (2 wt%, 0.085 mol%) used here is > 100
times less than the generally used amount for homogeneous
Ni catalyst (10 mol%) in dual catalysis. This approach
provides us with an alternative and straightforward route to
the synthesis of an integrated Ni-mpg-CNx photocatalyst.
Replacing NiCl2 with Co and Fe salts showed no reactivity
(entries 7 and 8, Table 1). The use of other commonly used
solvents resulted either in a lower yield or various side
products (Table S2). Various other organic and inorganic
bases resulted in a reduced yield (Table S3), whereas lowering
the loading of NaOH from 2 to 1.2 equivalent slightly
improved the yield of 1 by 7% (Table S4). Control experi-
ments showed that no significant cross-coupling product was
observed in the absence of light, base, mpg-CNx or Ni
(entries 9–13, Table 1).
Scheme 2. Substrate scope for C–O coupling. Isolated yields are given,
except where * indicates NMR yields. #Indicates where hydrolysis of
the ester bond is the major byproduct. Standard catalytic conditions:
Aryl halide (200 mM), Ni-mpg-CNx (10 mg), [NaOH] (240 mM in
alcohol, 2 mL) irradiation at l=447Æ20 nm and 408C under N2.
enes containing nitrile, ketone, aldehyde, ester, amide, and
sulfone substituents provide the coupling products 2–8 in 78–
92% yield. The high yield may be the result of the enhanced
reaction rate that reduces the formation of byproducts. Small
amounts of dehalogenated product, the corresponding phenol
and occasional hydrolysis of the functional group on the aryl
halides are commonly observed byproducts during this
coupling reaction. The ortho- and meta-acetyl substituted
bromobenzene (9 and 10) were also tolerable, but reacted
slower than the para-analogue (3) and the presence of acetyl
group at ortho-position did not allow the reaction to go to full
conversion.[13] An additional bromo- and chloro-substituted
aryl bromide gave the monosubstituted ether products 11 and
12, respectively, without any 1,4-diether product as the
resulting etherification product deactivates the aryl halide
for the second etherification. The reaction also proceeds with
electron-rich aryl bromides but requires a significantly longer
reaction time to form 13 and 14. Heteroaryl bromide
including pyridine, quinoline and benzofuranone have effec-
tively participated in the reaction to give a good yield of the
corresponding coupling products 15–17. With respect to the
alcohol substrate, we observed reactivity for primary alcohols
including ethanol and n-propanol to yield 1 and 18. The
deuterated aryl ether 19 can also be prepared using the Ni-
mpg-CNx photocatalyst. Overall, the observed reactivity
We subsequently investigated the scope of the C–O
coupling reaction using different aryl and heteroaryl halides
(Scheme 2). The para-substituted electron-deficient bromoar-
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trend correlates with other transition metal catalyzed cou-
pling reactions.[37]
We further studied the coupling protocol between meth-
anol and various aryl iodide and chlorobenzene substrates.
Consistent with homogeneous transition metal catalysis
reactions, a general inverse correlation between reaction
À
rate and strength of C X bond (I < Br < Cl) was observed for
a given functional group of aryl halides towards the coupling
reaction. Consequently, the aryl iodides were the fastest to
react to give the coupling products 2a–4a followed by aryl
bromide coupling products 2–4 and then the aryl chlorides
2b–4b. Chlorobenzenes required 608C to achieve full con-
version (Scheme 2).
The use of an integrated Ni-mpg-CNx photocatalyst
powder allowed for easy and quantitative recovery (up to
94% recovered material per run) of the Ni-mpg-CNx after
catalysis from the reaction crude upon simple centrifugation–
washing cycles (see Supporting Information for details). The
recovered heterogeneous material could be reused for further
cycles to give coupling product 3 with no need of re-addition
of any of the catalytic components (Ni or mpg-CNx) and
without loss of activity up to three cycles after which a slower
rate of coupling was observed (Figure 2). The decrease in
rate/activity could be due to an overall leaching of 20% Ni
from Ni-mpg-CNx after the fourth recycling or migration of
surface-activated Ni species to the bulk (Table S1).[38] The
recovered material displayed unchanged spectroscopic fea-
tures and pXRD diffraction pattern (Figure 1).
Figure 2. Recycling studies of the C–O coupling reaction. Catalytic
conditions: 4-bromoacetophenone (200 mM), Ni-mpg-CNx (10 mg),
[NaOH] (240 mM in MeOH, 2 mL), 420 min irradiation at
l=447Æ20 nm and 408C under N2.
ln[substrate]. Although a more complex mechanism cannot
be excluded, this suggests an approximately first order rate at
initial reaction times (first 10 min, Figure S11). This is in
agreement with the first step being the oxidative addition of
the substrate to the reduced Ni-mpg-CNx (Figure 3). Kinetic
analysis based on the Eyring equation shows a kinetic barrier
(2.81 Æ 0.31 kcalmolÀ1, Table S6) that agrees with the
observed fast reaction times at 408C (Figure 2). Moreover,
Mechanistic studies were performed using 3-bromoaceto-
phenone as a model substrate (Figures S11-13 and section 3 in
the Supporting Information). First, kinetic studies varying the
concentration of the substrate while keeping other parame-
ters constant showed a slope 0.77 Æ 0.04 on plotting lnk versus
the negative value obtained for DS° (À70.0 Æ 1 calmolÀ1 KÀ1
)
supports an associative mechanism.[11,39]
Figure 3. Proposed catalytic cycle for the C–O coupling reactions catalyzed by Ni-mpg-CNx.
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Computational studies at the B3LYP/6-31G* level of
Acknowledgements
theory have also been performed (see section 4 in the
Supporting Information for details) and we propose a catalytic
cycle for this transformation based on our experimental
evidence, computational calculations and previously reported
results with molecular systems (Figure 3). Briefly, upon
visible light excitation mpg-CNx generates a photoexcited
electron-hole pair. The photogenerated holes are quenched
by the alcohol substrate (potential of mpg-CNx valence
band =+ 1.7 V vs. SHE; conduction band = À1.0 V vs. SHE,
and MeOH Eox = + 1.54 V vs. SHE).[17,23] Simultaneously, the
photoexcited electron from the mpg-CNx matrix is transferred
to NiII center to yield NiI as supported by DFT calculations
(Eredtheor = 0.33 V vs. SHE), since further reduction to Ni0 is
We thank the financial support from the Cambridge Trust and
Science and Engineering Research Board (SERB), India,
OMV, BBSRC (BB/S00159X/1) and the European Commis-
sion for Horizon 2020 Marie Skłodowska-Curie Individual
Fellowships (890745-SmArtC, GAN890745 and CO2RED,
GAN745604). Dr. Erwin Lam and Dr. Christian Pichler are
acknowledged for their useful feedback and helpful discus-
sion.
Conflict of interest
not accessible according to the DFT calculations (Ered
=
The authors declare no conflict of interest.
theor
À1.51 V vs. SHE). This is followed by oxidative addition and
ligand exchange, where NiIII intermediates are invoked.
Finally, reductive elimination completes the cycle (Figure 3
and Schemes S2,S3).[11,29]
Keywords: carbon nitride · cross-coupling reaction ·
dual catalysis · heterogeneous catalysis · photocatalysis
The external quantum efficiency (EQE) for the coupling
reaction with Ni-mpg-CNx was calculated by assuming that
one photon is consumed for a molecule of product (although
we note that the number of photons required is not
unequivocally known, see below).[40] We obtained an EQE
of 2.26 Æ 0.1% at l = 447 (see Supporting Information for
more detail). Light-dark experiments showed no product
formation in the dark following an irradiation period (Fig-
ure S14), which suggests that even though the Ni catalytic
cycle is ideally self-sustainable, continuous irradiation is
necessary to avoid the deactivation of NiI/NiIII species by
carbon nitride matrix. Further mechanistic studies are ongo-
ing to elucidate the complete photocatalytic cycle in more
detail.
In summary, we have developed an inexpensive, robust,
and easily synthesizable Ni-mpg-CNx heterogeneous material
to play a dual catalytic role. The Ni deposition was confirmed
by ICP, TEM and XPS, also indicates that the nickel is present
as a Ni2+ active site. UV/Vis, ATR-IR and SEM images
confirms that the morphology, composition, and optical
property of mpg-CNx is unaffected by nickel modification.
The dual catalysis of Ni-mpg-CNx is demonstrated for C–O
coupling between aryl halides and alcohols. The coupling has
demonstrated broad substrate scope with significant func-
tional group tolerance and adaptability to different halides.
Moreover, the observed reactivity trend is similar to a homo-
geneous transition metal catalyzed cross-coupling reaction.
The heterogeneous Ni-mpg-CNx materials are easily recov-
ered from the reaction mixture and have been reused several
times, suggesting a potential application of this material for
a dual catalytic approach in larger-scale industrial chemical
synthesis. Finally, kinetic studies have shed some light on the
mechanism and suggest an oxidative addition of the aryl
halide to the reduced Ni-mpg-CNx as the rate-determining
step of the reaction. We anticipate that this powdered
material will enable a more simple, sustainable, and versatile
dual photocatalysis approach in organic synthesis in the
future.
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Manuscript received: December 12, 2020
Accepted manuscript online: February 9, 2021
Version of record online: March 3, 2021
Angew. Chem. Int. Ed. 2021, 60, 8494 –8499
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 8499