A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur–Carbon Cross-Coupling Dual Catalyst

Article abstract of DOI:10.1002/anie.202101036

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur–carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

Full text of DOI:10.1002/anie.202101036

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 10820–10827
International Edition:
German Edition:
doi.org/10.1002/anie.202101036
doi.org/10.1002/ange.202101036
Covalent Organic Frameworks
Hot Paper
AVisible-Light-Harvesting Covalent Organic Framework Bearing
Single Nickel Sites as a Highly Efficient Sulfur–Carbon Cross-Coupling
Dual Catalyst
Hui Chen, Wanlu Liu, Andreas Laemont, Chidharth Krishnaraj, Xiao Feng, Fadli Rohman,
Maria Meledina, Qiqi Zhang, Rik Van Deun, Karen Leus, and Pascal Van Der Voort*
Abstract: Covalent Organic Frameworks (COFs) have re-
cently emerged as light-harvesting devices, as well as elegant
heterogeneous catalysts. The combination of these two proper-
ties into a dual catalyst has not yet been explored. We report
a new photosensitive triazine-based COF, decorated with single
Ni sites to form a dual catalyst. This crystalline and highly
porous catalyst shows excellent catalytic performance in the
visible-light-driven catalytic sulfur–carbon cross-coupling re-
action. Incorporation of single transition metal sites in a photo-
sensitive COF scaffold with two-component synergistic catalyst
in organic transformation is demonstrated for the first time.
metal-catalyzed cross-coupling reactions using copper, iron,
palladium, nickel, etc. (Scheme 1a).[3] Unfortunately, harsh
synthesis conditions are typically required, such as the
presence of strong bases and the need for high temperatures,
which often leads to a low functional group tolerance. Also,
highly specific and expensive ligands combined with high
catalyst loadings are required as thiols are prone to dimeriza-
tion and coordination to the transition-metals.[4] To overcome
these shortcomings, visible-light-driven organic transforma-
tions have received attention as they allow an environ-
mentally friendly and sustainable strategy to perform organic
reactions in very mild conditions.[5]
Introduction
Within this context, several photosensitive molecules
such as noble metal complexes (Ir[dF(CF3)ppy]2(bpy)PF6,
Ru(bpy)3]Cl2),[3d,6] organic dyes (Eosin Y),[7] and inorganic
semiconductors (TiO2, Bi2O3)[8] have been used as homoge-
neous photocatalysts for a variety of organic reactions
through a single electron transfer (SET) mechanism. In
2016, Johannes and co-workers employed Ir[dF(CF3)ppy]2-
(dtbbpy)PF6 as photocatalyst combined with an organome-
tallic Ni catalyst exhibiting a synergistic effect in S–C cross-
coupling reactions at room temperature.[6] Later on,
Molanderꢀs group developed a photoredox/Nickel dual cata-
lyst using [Ru(bpy)3](PF6)2 in thioetherification[3d] (Sche-
me 1b). In both studies, the catalysts exhibited a high
efficiency with yields of up to 95%. Nevertheless, the inherent
disadvantages of homogeneous catalysts such as their low
recyclability and high cost limit their industrial implementa-
tion. For this reason, there is an urgent need to develop
Organosulfur compounds such as methionine, gluta-
thione, biotin, etc. are widely present in various biological
systems, and play a crucial role in vital processes of living
organisms.[1] In addition to this, they are also often found in
artificial synthetic drugs such as potential HIV inhibitors,
esomeprazole, duloxetine hydrochloride, etc.[2] Because of
their broad applicability in biological processes and pharma-
ceuticals, the formation of sulfur- carbon bonds (S C bonds)
is of paramount importance in modern synthetic organic
chemistry. Traditionally, S C bonds are formed by transition
À
À
[*] H. Chen, W.-L. Liu, A. Laemont, C. Krishnaraj, Dr. X. Feng, Dr. K. Leus,
Prof. P. Van Der Voort
COMOC-Center for Ordered Materials, Organometallics and Catal-
ysis, Department of Chemistry, Ghent University
Krijgslaan 281-S3, 9000 Ghent (Belgium)
E-mail: Pascal.VanDerVoort@UGent.be
W.-L. Liu, Prof. R. Van Deun
L3-Luminescent Lanthanide Lab., Department of Chemistry
Ghent University
Krijgslaan 281-S3, 9000 Ghent (Belgium)
F. Rohman, Dr. M. Meledina
RWTH Aachen University, Central Facility for Electron Microscopy
52074 Aachen (Germany)
Dr. M. Meledina
Forschungszentrum Jülich GmbH, Ernst Ruska-Centre (ER-C 2)
52425 Jülich (Germany)
Q.-Q. Zhang
TJU-NIMS International Collaboration Laboratory, School of Materi-
als Science and Engineering, Tianjin University
No. 92 Weijin Road, Nankai District, Tianjin 300072 (P. R. China)
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.202101036.
À
Scheme 1. Various methods for the formation of S C bonds.
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heterogeneous visible-light-driven photocatalysts for the Results and Discussion
À
formation of S C bonds that can be easily recycled without
loss in activity and yield.
Initially, the model compound (marked as MC) was
synthesized to illustrate the possibility of acenaphthene-
In 2005, Omar Yaghi reported for the first time the
synthesis of a Covalent Organic Framework (COF) triggering
the development of several new structures.[9] COFs are
crystalline two or three-dimensional organic porous solids,
constructed from organic building blocks that are linked by
strong covalent bonds.[10] COFs have been widely recognized
as potential heterogeneous photocatalysts due to their
inherent light-harvesting and energy transition capabilities
as a consequence of their remarkable features including large
specific surface areas, p-p stacking interactions, long-range
order, and hierarchically integrated building blocks.[11] How-
ever, so far, photosensitive COFs have mostly been employed
in studying both half-reactions for water splitting and CO2
reduction.[12] Only a minority of COFs have been studied to
catalyze organic transformations,[13] of which all of them were
based on single components that served either as a photo-
sensitizer or as solid support. The application of COFs in two-
component or multi-component catalysis is up until now an
unexplored field. Inspired by these promising developments
on the use of COFs in photosynthesis, we hypothesized that
COFs might form an ideal platform to combine photoredox
and transition-metal catalysts to drive organic transforma-
tions. Herein, we report a novel triazine-based COF, which
not only acts as a photocatalyst but also as a support material
to incorporate nickel catalytic active sites (Scheme 1c). The
ordered structure with high porosity and the proximity of the
photosensitizing COF framework and the nickel catalytic
active sites significantly improves the catalytic efficiency, as it
facilitates the electron and thiol radical transfers from the
photosensitizer to the Ni catalytic active sites.
quinone and amine condensation (Scheme 2a). The success-
ful synthesis of MC is confirmed by H NMR and matched
1
with the literature report (Supporting Information, Fig-
ure S1).[14] Acenaphthenequinone was chosen because it
easily condenses with amine groups, allowing the construction
of robust, crystalline COF structures. Moreover, the resulting
1,2-Bis(phenylamino)-acenaphthene moiety can chelate tran-
sition metal ions for organometallic catalytic reactions.[15]
Based on this, the photosensitive triazine-based COF scaffold
(denoted as Ace-COF) was prepared under solvothermal
conditions in a sealed ampule through the condensation of
4,4’4’’-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA) and ace-
naphthenequinone (Ace). These two building blocks were
selected because their planar structure ensures high p-p
interactions between the layers to obtain a highly crystalline
COF (Figure 1a,b). Also, the use of these building blocks
ensures the presence of distinct electronic donor-acceptor
structures that offer topologically ordered D-A heterojunc-
tions with independent pathways for ambipolar electron and
hole transport resulting in enhanced photoconductivity and
photocatalytic activity.[13a,16] After a thorough screening of
several synthesis parameters including temperature, solvent,
and reaction time (Supporting Information, Table S1, Fig-
ure S3), it was observed that the optimal synthesis conditions
were as follows: condensation of 0.1 mmol TTA (35.5 mg) and
0.15 mmol Ace (27.3 mg) in an acetonitrile/1,4-dioxane/6 M
aqueous acetic acid (1.1 mL, 5:5:1 by vol.) solvent mixture at
a reaction temperature of 1208C for 3 days. The Ace-COF
exhibits remarkable chemical stability in common organic
Scheme 2. The synthesis process of a) model compound and b) Ace-COF-Ni.
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Figure 1. a) Top and b) side views of Ace-COF. c) PXRD pattern of experimental Ace-COF (red) and Ace-COF-Ni (olive), Pawley-refined (faint
yellow), difference (black), and the simulated PXRD pattern of Ace-COF AA eclipsed stacking (wine) and AB staggered stacking (blue). d) FT-IR
spectrum of Ace-COF and Ace-COF-Ni. e) XPS spectra of Ace-COF and Ace-COF-Ni. f) N 1s XPS spectra of Ace-COF and Ace-COF-Ni. g) Z-con-
trast HAADF-STEM image of Ace-COF-Ni: bright contrast features (some examples are labeled by the white arrows) correspond to single Ni sites
within the COF support.
solvents, HCl (1 M) and NaOH (1 M) aqueous. After soaking
the material in each of these media for 7 days, no change in
the PXRD pattern was observed, which indicates that the
crystallinity was preserved (Figure S4). Ni ions were intro-
duced into the Ace-COF scaffold through a simple post-
synthetic wet impregnation with NiCl2·6H2O (denoted as
Ace-COF-Ni, Scheme 2b).
similar to that of the pure Ace-COF (Figure 1c), indicating
that the crystalline structure of the COF is retained upon the
introduction of the Ni ions.
The Fourier transform infrared (FT-IR) spectra of the
Ace-COF-Ni and pristine Ace-COF (Figure 1d) exhibit
a typical vibration band at 1647 cmÀ1 which confirms the
=
successful formation of the imine bond (C N). Further
The crystalline structure of the Ace-COF and Ace-COF-
Ni compounds was determined by means of powder X-ray
diffraction (PXRD) analysis (Figure 1c). The relatively sharp
diffraction peaks reveal the good crystallinity of the materials.
The reflections at 4.08, 7.18, 8.08, 12.18, and 14.78 correspond
to the (100), (110), (200), (210), and (120) facets, respectively,
whereas the slightly broader peak at higher 2q ( ꢀ 258)
originates from the p-p stacking between the COF layers and
corresponds to the (001) plane. All the diffraction peaks
follow the PÀ6 space group that represents a hexagonal 2D
layered network. The structural simulation of Ace-COF
shows that an eclipsed AA stacking mode is preferred over
a staggered AB stacking. Pawley refinements of the exper-
imental PXRD profiles was carried out and the refinement
results yield unit cell parameters are a = b = 28.7668 Š, c =
3.5734 Š, and a = b = 908, g = 1208, which match well with the
predictions with good agreement factors (Rwp = 4.23% and
Rp = 5.08% ). The PXRD pattern of the Ace-COF-Ni is
structural information on the coordination of the Ni ions in
the Ace-COF was obtained employing X-ray photoelectron
spectroscopy (XPS). The XPS spectrum of Ace-COF-Ni
shows the presence of Cl, C, N, and Ni (Figure 1e). The
binding energy of the Ni 2p peak at 856 eV can be assigned to
Ni2+ (Figure S5). This value is similar to the reported value for
NiCl2·bpy (bpy = 2,2’-bipyridine),[12c] which indicates the
successful coordination of Ni ions with the framework. No
signals were found for any other Ni species, such as NiO and
metallic Ni. Moreover, a slight shift of the N 1s peaks to
higher binding energy was observed in the Ace-COF-Ni
material in comparison to the N 1s XPS spectrum of the
pristine Ace-COF (Figure 1 f). This shift can be ascribed to
the coordination of the nitrogen atoms to Ni2+ and is
consistent with literature reports.[12c] Scanning electron
microscopy (SEM) analyses show spherical morphologies of
Ace-COF-Ni (Figure S6). In the high-resolution transmission
electron microscopy (HR-TEM) images, no Ni nanoparticles
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were observed (Figure S7). The energy-dispersive X-ray
Therefore, the Ace-COF-Ni was examined in the visible-
(EDX) mapping images of the Ace-COF-Ni give clear
evidence for the presence of C, N, Cl, and Ni which are
homogeneously distributed in the COF matrix (Figures S7
and S8). Bright contrast features in the Z-contrast HAADF-
STEM image correspond to single atoms spread within the
Ace-COF supporting material (Figure 1g), some examples
are marked by the white arrows). Ni as the heaviest element
in the Ace-COF-Ni, the bright contrast features highlighted
by the white arrows in the Z-contrast HAADF-STEM images
can be safely attributed to single sites of Ni sitting within the
Ace-COF network. The Ni content, determined by induc-
tively coupled plasma mass spectrometry (ICP-MS) is
1.03 mmolgÀ1.
The surface area of the Ace-COF and Ace-COF-Ni
compound was determined by measuring the Argon adsorp-
tion isotherm at 87 K of the activated samples. As shown in
Figure S9a, a sharp increase in the gas uptake is observed at
low relative pressures (P/P0 < 0.1) indicating the presence of
micropores. The Brunauer–Emmett–Teller (BET) surface
area and total pore volume (at P/P0 = 0.97) decreased from
1238 m2 gÀ1 and 0.85 cm3 gÀ1 for Ace-COF to 825 m2 gÀ1 and
0.61 cm3 gÀ1 for Ace-COF-Ni, respectively. The pore sizes of
both the Ace-COF and Ace-COF-Ni were calculated to be
0.97 nm in diameter using Ar at 87 K quenched solid density
functional theory (QSDFT) carbon model (Figure S9b). From
these observations, it is clear that, although the interior
cavities of the Ace-COF-Ni are partially occupied by Ni ions,
the Ace-COF-Ni structure exhibits a permanent open struc-
ture, ensuring a good diffusion of the reactants to the Ni
active sites. Besides a permanent porosity, thermal stability is
also very important for its practical application as a hetero-
geneous catalyst. As indicated in Figure S10, the thermo-
gravimetric analysis (TGA) shows that both Ace-COF and
Ace-COF-Ni possess excellent thermal stability, up to 4508C
under a nitrogen atmosphere.
light-driven S–C cross-coupling reaction to evaluate its
potential as a dual catalyst. First, iodobenzene (1a) and
thiophenol (2a) were used as model substrates for the
optimization of the reaction conditions (Table 1). More
specifically, under an Ar atmosphere, a reaction mixture of
iodobenzene (0.5 mmol) (1a), thiophenol (0.75 mmol) (2a),
2 mol% Ace-COF-Ni, and pyridine (1 mmol) in anhydrous
acetonitrile (5 mL) was irradiated with 34W blue LEDs (420–
430 nm). After 24 hours, an excellent yield (> 95%) towards
the corresponding S–C cross-coupled product phenyl sulfide
(3a) was obtained at room temperature (Table 1, entry 1).
From the blank tests, it was noted that no reaction occurred in
the absence of light, the absence of pyridine, or the absence of
Ace-COF-Ni (Table 1, entries 2–4). When the model com-
pound MC-Ni instead of the Ace-COF-Ni was added into the
reaction mixture, no detectable product of phenyl sulfide was
observed, which suggests that the photosensitive triazine-
based Ace-COF scaffold is essential (Table 1, entry 5). When
using Ace-COF instead of Ace-COF-Ni, no product was
detected, implying that Ni also plays a crucial role in this
cross-coupling reaction (Table 1, entry 6). Interestingly, when
using a physical mixture of either Ace-COF and NiCl2·6H2O
or Ace-COF and the model compound MC-Ni as the catalyst
(Table 1, entries 7 and 8), also significant amounts of the
product was observed (58%, 26%), albeit much lower than
with the Ace-COF-Ni. This might be due to the in situ
formation of Ace-COF-Ni by Ace-COF scaffold and
NiCl2·6H2O. In the case of the mixture of MC-Ni and Ace-
COF, the COF will act as the required photosensitizer to
allow the reaction to proceed, which has been reported
previously.[17] Based on these results, it is clear that both
photosensitive triazine-based Ace-COF scaffold and Ni are
Table 1: Ace-COF-Ni dual-catalyzed S–C cross-coupling: influence of
reaction parameters.
The optical properties of Ace-COF-Ni and Ace-COF
were assessed to verify the feasibility of using Ace-COF-Ni to
catalyze reactions under visible light irradiation solely.
UV/Vis absorption experiments are carried out at room
temperature, the UV-vis spectra indicate that the Ace-COF
and the Ace-COF-Ni can absorb light in the UV and visible
regions (Figure S11). However, the Ace-COF-Ni model
compound (abbreviated as MC-Ni, Scheme 2a) only absorbs
UV light. The optical band gaps of Ace-COF, Ace-COF-Ni,
and MC-Ni were analyzed to be 1.74 eV, 1.83 eV, and 2.85 eV,
respectively. Ace-COF and Ace-COF-Ni have a much smaller
band gap than the model compound, this can be explained by
the introduction of the electron-accepting triazine unit, the
extended imine conjugation in the x and y direction of the
COF structure, and enhanced p-conjugation between the
COF layers. In comparison with the previously reported
photocatalytic COFs, such as LZU-190, LZU-191, and
LZU-192 (optical band gaps are 2.02 eV, 2.38 eV, and
2.10 eV, respectively).[13b] Ace-COF-Ni and Ace-COF show
enhanced absorption in the visible light range. This implies
that the Ace-COF-Ni is a promising platform for visible-light-
driven organic transformation reactions.
Entry
Variation from the standard conditions
Yield [%][b]
1
2
3
4
5
6
7
Standard conditions[a]
No light (dark)
>95
No Product
No Product
No Product
No Product
No Product
58
No pyridine
No Ace-COF-Ni
MC-Ni instead of Ace-COF-Ni
Ace-COF instead of Ace-COF-Ni
Ace-COF mixed NiCl2·6H2O
instead of Ace-COF-Ni
Ace-COF mixed MC-Ni
instead of Ace-COF-Ni
0.5 mol% Ace-COF-Ni
1 mol% Ace-COF-Ni
2 mol% Ace-COF-Ni
8
26
9
10
11
35
73
>95
[a] Standard conditions: Under an Ar atmosphere, 1a (0.50 mmol), 2a
(0.75 mmol), 2 mol% Ace-COF-Ni, pyridine (1 mmol), and 5 mL of
99.8% anhydrous acetonitrile, then 34 W blue LED irradiation for 24 h at
R.T. [b] yield was determined by 1H NMR spectroscopy with CH3NO2 as
an internal standard.
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required to perform the S–C cross-coupling reaction. A
significant increase in the yield was observed upon increasing
the amount of catalyst. When the amount of catalyst was
increased from 0.5 to 1 and 2 mol%, the yield increased from
35 to 73 and 95%, respectively (Table 1, entries 9–11). This
observation further corroborates the key role of the Ace-
COF-Ni catalyst for this model reaction. A screening of
several solvents (Table S3) showed that polar solvents (DMF,
CH3OH, DMSO, etc.) are more beneficial for the reaction
whereas nonpolar solvents (toluene, hexane, etc.) showed
a negative influence on the reaction thermodynamic or/and
kinetic control. This may be explained by the Hughes–Ingold
rules,[18] that state that polar solvents enhance the production
of polar compounds. This is definitely the case for the
photocatalytic S–C cross-coupling reaction, as many inter-
mediates are polar, ionic, or radical.
substrates (Table 1, entry 1) were chosen to evaluate the
recyclability of the Ace-COF-Ni catalyst. As can be seen from
Figure 2a, the catalyst could be recovered and reused for at
least five cycles without loss of catalytic performance. After
five cycles of catalysis, no Ni leaching was detected by ICP-
MS. The XPS spectrum of Ace-COF-Ni shows that the Ni 2p
peak at 855 eV is not changed (Figure S5) and the Far-
[19]
À
infrared spectra indicate the presence of the Ni Cl bond
(Figure S12). Also, no apparent change in the PXRD patterns
of the Ace-COF-Ni material. All the evidence indicates that
the structure of Ace-COF-Ni was preserved (Figure S13).
In order to obtain insights into the reaction mechanism,
photophysical and electrochemical measurements were per-
formed. In the first instance, to determine whether there is an
electron transfer between the excited state of Ace-COF-Ni
Hence, anhydrous acetonitrile was Table 2: Substrate scope of Ace-COF-Ni catalyzed cross-coupling between aryl iodides and thiols.
chosen as the optimum solvent for
further reactions.
In a final stage, the scope of
substrates was extended to examine
the wide applicability of the Ace-
COF-Ni catalyst in S–C cross-
coupling reactions. Diverse aryl io-
dides containing either electron-
withdrawing or electron-donating
groups, such as methyl, methoxy,
formyl, carbomethoxy, or cyano
groups, and three different aryl
thiols bearing hydrogen, methyl, or
methoxy groups were chosen as
substrates. The reactions were per-
formed under the optimized reac-
tion conditions in the presence of
2 mol% Ace-COF-Ni (Table 2).
For each S–C cross-coupling reac-
tion, an excellent yield (79–96%) of
the corresponding coupling product
was obtained. When comparing 3a,
3d, and 3g, it is noted that there is
no significant influence of the posi-
tion of the substituent on the result-
ing yield. Also, for substrates that
possess electron-neutral or elec-
tron-rich substituents, a satisfactory
yield was obtained (3j, 3m, and
3p). In addition to this, not only aryl
thiols but also alkyl thiols gave the
desired thioethers in good yield (3w
and 3x). From these observations, it
can be concluded that the Ace-
COF-Ni catalyst can be used to
convert a wide range of substrates
and that there is no significant
influence of the functional groups
on the resulting activity. Another
important aspect of its practical
[a] Under Ar atmosphere, 1 (0.50 mmol), 2 (0.75 mmol), 2 mol% Ace-COF-Ni, pyridine (1 mmol), and
implementation is the recyclability
5 mL of anhydrous CH3CN, blue LED irradiation for 24 h at R.T. [b] Iodobenzene (1 mmol). [c] Yield (%)
of the catalyst. The model was determined by 1H NMR spectroscopy with CH3NO2 as an internal standard.
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Figure 2. a) Assessment of the reusability of Ace-COF-Ni. The reusability tests were carried out under identical conditions (Table 1, entry 1).
b) EPR spectroscopy under various conditions. The standard conditions are the same as in Table 1, entry 1. c) Steady-state emission quenching of
Ace-COF-Ni* with thiol. Inset: Stern–Volmer analysis of the results. d) The CV curve of the Ace-COF-Ni model compound MC-Ni versus SCE in
CH3CN in the presence of 0.1 M pyridine.
(marked as Ace-COF-Ni*) and the thiophenol or the aryl
iodide, steady-state emission quenching of Ace-COF-Ni*
with varying thiol and aryl iodide concentration was per-
formed. From this experiment, it was observed that an
increase in the concentration of thiophenol and aryl iodide
resulted in a weaker fluorescence intensity for which the
Stern–Volmer analysis exhibited an excellent linear regres-
sion (Figure 2c; Figure S14). The quenching efficiencies for
Furthermore, we investigated the types of radicals pro-
duced during the reaction by electron paramagnetic reso-
nance (EPR) spectroscopy. Under an Ar atmosphere, a mix-
ture of 0.5 mmol thiophenol (2a), 2 mol% Ace-COF-Ni,
1 mmol pyridine, 5 mL anhydrous CH3CN, and 5,5-dimethyl-
1-pyrroline N-oxide (DMPO) as a radical trap was stirred for
20 minutes in the dark. Hereafter, a small amount of the
mixture was transferred into a capillary. The EPR spectra of
thiophenol and aryl iodide were quantified by the Stern– this mixture were recorded under different conditions. As
Volmer equation: (I0/I) = 1 + ksv[Q], resulting in a quenching
constant ksv for thiophenol and aryl iodide of 1.459 Æ
0.005 MÀ1 and 0.282 Æ 0.008 MÀ1, respectively. The thiophenol
is thus almost five times more effective than the aryl iodide in
quenching the Ace-COF-Ni* luminescence. This might be
because the aryl iodide quenches the Ace-COF-Ni* lumines-
cence by energy transfer rather than electron transfer.[20]
Moreover, time-resolved emission spectroscopy shows that
the lifetime of the Ace-COF-Ni* is ꢀ 5 ms as determined by its
emission at 485 nm. Interestingly, upon the addition of the
thiophenol, the excited state lifetime is significantly decayed.
More specifically, when the thiol concentration amounts to
0.5 M, the excited state lifetime of Ace-COF-Ni* is only
ꢀ 3.5 ms (Figure S15). In conclusion, the combined time-
resolved emission spectroscopy and steady-state emission
quenching experiments indicate that the initial step in the
photocatalytic process involves the reductive quenching of
Ace-COF-Ni* by thiophenol to generate the thiophenol
radical.
shown in Figure 2b, no radical signal was observed without
light irradiation, Ace-COF-Ni, or Pyridine. However, a sextet
signal with a g = 2.006 (AN = 1.33 mT, AH = 1.47 mT) was
observed under light irradiation, indicating that a sulfur-
centered radical was produced.[21] It further confirms the
conclusion of the time-resolved emission spectroscopy and
steady-state emission quenching experiments that a reductive
quenching of Ace-COF-Ni* by thiophenol occurs to generate
the thiophenol radical.
To rationalize the dependence of the oxidation state of Ni
in the cross-coupling reaction, electrochemical studies on the
Ace-COF-Ni model compound MC-Ni in MeCN were
performed. Figure S16 shows the cyclic voltammetry (CV)
curve of MC-Ni with two distinct reduction peaks at À1.81 V
(R1) and À1.44 V (R2), versus a saturated calomel electrode
(SCE) in MeCN, which correspond to the Ni0/Ni0CÀ and NiII/
Ni0 couples, respectively.[22] However, when pyridine is
present, a new reduction peak R3 at À0.98 V and a new
oxidation peak O1 at À1.08 V is observed besides the
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reduction peaks R1 and R2, which can be ascribed to a pyridine Conclusion
stabilized NiI-species[22a,23] (Figure 2d). As the NiII/Ni0 couple
reduction potentials of MC-Ni are very close to the reduction
potential of the triazine-based COF (À1.5 V versus SCE).[13a]
It is rather difficult to accurately ascertain the thermodynamic
preference towards reduction to Ni0. Nevertheless, as the
reduction potential of the triazine-based COF is more
negative than the NiI-species, this indicates that the triazine-
based COF can easily reduce NiII to NiI. This result supports
that the NiI-species are the thermodynamically and kinetically
active species in the catalytic cycle.
In summary, we have developed a novel imine-linked
triazine-based COF dual catalyst. The photoactive COF acts
as the photocatalyst while the incorporated single nickel sites
act as the active transition metal species for the visible-light-
driven S–C cross-coupling reaction. The resulting Ace-COF-
Ni exhibits high catalytic activity, broad substrate adaptabil-
ity, and outstanding recyclability and stability due to the
ordered structure and proximity of the photosensitizer and
the nickel catalytic active sites. This work demonstrates, for
the first time, the ability to incorporate transition metal single
sites in a photosensitive Ace-COF scaffold and to form a dual
catalyst to synergistically perform organic transformations.
Based on the obtained photophysical and electrochemical
analyses described above, we propose the following mecha-
nism for the visible-light-driven dual-catalytic S–C cross-
coupling reaction (Scheme 3). In this dual-catalytic process,
the Ace-COF cycle and Ni cycle are connected to each other
through both electron and radical transfers. Upon visible-light
irradiation, the photosensitive Ace-COF-Ni generates an
excited state Ace-COF-Ni*. This is followed by a single
electron transfer (SET) oxidation of the thiol through the
photoexcited Ace-COF-Ni*, which produces both the thiol
radical cation (I) and the Ace-COF-NiCÀ (II) complex. In the
presence of pyridine, the thiol radical cation (I) is deproton-
ated and converted to the thiol radical (III). A SETreduction
of the Ace-COF-Ni by Ace-COF-NiCÀ (II) delivers a NiI-
halide (V) while at the same time the Ace-COF-Ni is
regenerated. The thiol radical (III) then rapidly combines
with the NiI-halide (V) to form a NiII-sulfide complex (VI).
This NiII-sulfide complex (VI) is then again reduced to a NiI-
sulfide complex (VII) by Ace-COF-NiCÀ, which in the
following step undergoes an oxidative addition of the aryl
iodide to produce a NiIII-complex. Through a facile reductive
elimination process, the targeted S–C cross-coupled product
is formed and a NiI-halide (V) is released. Oderine et al.
found a similar electron transfer when studying the homoge-
neous Ir[dF(CF3)ppy]2(dtbbpy)PF6 combined with organo-
metallic Ni catalyst for the same cross-coupling reaction.[6]
Acknowledgements
We thank Prof. Dirk Poelman (UGent) for helping with the
Solid-state UV-vis measurements and thank Prof. Henk
Vrielinck (UGent) for far-infrared spectral measurements.
H.C. and W.L.L. gratefully acknowledge the Chinese Scholar-
ship Council (CSC) for financial support. M. M. gratefully
acknowledges the Verbundvorhaben iNEW: Inkubator Nach-
haltige Elektrochemische Wertschçpfungsketten with the
funding number 03SF0589A for financial support. C.K. and
P. V.D.V acknowledges support from the Research Board of
Ghent University (GOA010-17, BOF GOA2017000303).
A.L. acknowledges financial support from a Ghent University
BOF doctoral grant 01D12819. K.L. is thankful for financial
support from Ghent University.
Conflict of interest
The authors declare no conflict of interest.
Keywords: cross-coupling reactions ·dual catalysts ·
single nickel sites ·sulfur–carbon bonds ·
visible-light-driven photocatalysis
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Manuscript received: January 22, 2021
Accepted manuscript online: February 4, 2021
Version of record online: April 6, 2021
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