Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 8494–8499
International Edition:
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Heterogeneous Catalysis
Visible-Light Promoted C–O Bond Formation with an Integrated
Carbon Nitride–Nickel Heterogeneous Photocatalyst
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:
ꢀ 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.
8494
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 8494 –8499