Solar and visible-light active nano Ni/g-C3N4photocatalyst for carbon monoxide (CO) and ligand-free carbonylation reactions

Article abstract of DOI:10.1039/d0cy01717e

In this study, we investigate the amino and alkoxycarbonylation reaction between various substituted aryl halides, benzyl iodides, and iodocyclohexane with different types of amines and alcohols in the absence of carbon monoxide gas and ligands. Similar reactions are carried out at high temperatures, in the presence of appropriate ligands, stoichiometric amounts of bases, and gaseous carbon monoxide, which endanger the health of organic chemists. We present a novel method that does not utilize ligands, bases, gaseous CO, and special conditions. This procedure is a redox reaction carried out by new economic nano Ni/g-C₃N₄at room temperature and under visible light. Mo(CO)₆was used toin situgenerate CO, to resolve the problems caused by the use of CO gas. This protocol has the ability to be used on a gram scale by using a continuous flow reactor.

Full text of DOI:10.1039/d0cy01717e

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Solar and visible-light active nano Ni/g-C3N4
photocatalyst for carbon monoxide (CO) and
Cite this: DOI: 10.1039/d0cy01717e
ligand-free carbonylation reactions†
*
Mona Hosseini-Sarvari
and Zahra Akrami
In this study, we investigate the amino and alkoxycarbonylation reaction between various substituted aryl
halides, benzyl iodides, and iodocyclohexane with different types of amines and alcohols in the absence of
carbon monoxide gas and ligands. Similar reactions are carried out at high temperatures, in the presence
of appropriate ligands, stoichiometric amounts of bases, and gaseous carbon monoxide, which endanger
the health of organic chemists. We present a novel method that does not utilize ligands, bases, gaseous
CO, and special conditions. This procedure is a redox reaction carried out by new economic nano Ni/g-
C3N4 at room temperature and under visible light. MoIJCO)6 was used to in situ generate CO, to resolve the
problems caused by the use of CO gas. This protocol has the ability to be used on a gram scale by using a
continuous flow reactor.
Received 1st September 2020,
Accepted 17th November 2020
DOI: 10.1039/d0cy01717e
rsc.li/catalysis
benzocaine and tetracaine are commonly used as pain
relievers for local anesthesia or in cough drops15 (Fig. 1
Introduction
The use of transition metals as catalysts for the synthesis of
carbonylated organic compounds is a useful and valuable
method in carbonylation reactions.1 For this reason, many
attempts have been made in previous decades to use
transition metals as catalysts in carbonylation reactions to
convert organic compounds with halide2 or pseudohalide3
substitutions to carboxylic acids. Nowadays, synthesized
molecules containing amide and ester-based scaffolds have
attracted the attention of many chemists, because they are
structural units found in natural compounds and are used in
the preparation of pharmaceutical, agrochemical, aromatic,4,5
and biologically active compounds.6 In fact, one of the most
essential chemical linkages that make up the structural
backbone of peptides and proteins is the amide bond.
Besides, heterocyclic amides can be used as specific species
in the central nervous system's active structures.7 For
example, as a vasoconstrictor agent, midodrine is employed
in the treatment of hypotension8,9 (Fig. 1 compound A),
loracarbef is an antibiotic10 (Fig. 1 compound B), and
procainamide11 is utilized as an antiarrhythmic prescription
(Fig. 1 compound C). Also, the carbonylation reaction is an
effective method to synthesize a wide range of aromatic esters
and their derivatives.12–14 Like amides, esters play a vital role
in the pharmaceutical industry. For example, esters such as
compounds D and E). Ethyl eicosapentaenoic acid is a drug
that is used along with changes in diet to treat
hypertriglyceridemia, for adults with hypertriglyceridemia ≥
150 mg dl−1 (ref. 16 and 17) (Fig. 1 compound F).
The Schmidt reaction,18 Schotten–Baumann reaction19
and Ugi reaction20 are the conventional methods to
synthesize amides and esters. Due to the importance of these
compounds, various synthetic methods have been developed.
However, the synthesis of carbonyl compounds via reactions
between aryl halides and an appropriate nucleophile using a
transition metal as a catalyst is an operational method.
Therefore, recently, a great number of non-photochemical21–31
Department of Chemistry, Shiraz University, Shiraz 7194684795, I.R. Iran.
E-mail: hossaini@shirazu.ac.ir
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01717e
Fig. 1 Some biologically active amide and ester compounds.
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Fig. 3 IR spectra of pure g-C3N4 and nano Ni/g-C3N4.
hybrid method36 reported by Ryu et al.,37 metal-catalyzed
carbonylation as a mild experimental method replaced free
radical-based methods. The use of coupling agents such as
aryl halides in multicomponent radical carbonylation
reactions under light irradiation has led to the synthesis of a
wide range of carbonyl compounds using radical species.
Recently, Xiao and Lu,32 Jacobi von Wangelin, and Gu33
groups reported the use of organic dyes causing
environmental problems. Additionally, the synthesis of
various functionalized carbonyl groups using carboxylic acids
and aryl diazonium salts in the presence of iridium and
ruthenium-based metal complexes as photocatalysts is a toxic
and costly method.34–37 Besides in 2018, He and co-workers
reported a Pd catalyzed carbonated Suzuki reaction at 80 °C
using CO from CO2 photoreduction by ruthenium.38
Therefore, the increasing efforts of the modern chemical
industry for green and renewable reaction conditions are
challenged to revise the reaction conditions. Low catalyst
loading, lower reaction temperatures, separation and
reutilization of the catalysts, and ligand-free catalytic systems
are some of the significant targets because of environmental
and economic demands. Heterogeneous semiconductors as
environmentally friendly materials having the following
benefits are used to replace other homogeneous catalysts: (1)
separation and recovery of the catalyst by centrifugation or
filtration (2) to take advantage of ligand-free catalytic systems
is a cost-effective method.39
Scheme
1 The synthesis of amides and esters derived from aryl
halides and nucleophiles in previous reports.21–37
Fig. 2 a) UV-vis diffuse reflectance spectra of g-C3N4 and nano Ni/g-
C3N4. (b) Photoluminescence emission spectra of g-C3N4 and nano Ni/
g-C3N4.
and photochemical32–37 techniques have been developed to
synthesize amides and esters (Scheme 1).
Non-photochemical methods are limited to the use of
ligands or bases and gaseous carbon monoxide (CO) and are
carried out at high temperatures.21–31 Coinciding with
advances in radical carbonylation, particularly the light/metal
Fig. 4 EDX spectrum of nano Ni/g-C3N4.
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Fig. 5 XRD patterns of g-C3N4 (a) and nano Ni/g-C3N4 (b) at room
temperature.
Fig.
7
a) TEM image of nano Ni/g-C3N4. b) The distribution of
nanoparticle sizes.
recombination rate have caused this compound to have
relatively little photocatalytic activity.49 Therefore, these
problems can be solved by doping a co-catalyst on g-C3N4
nanosheets.50 Nickel, like palladium,51 can catalyze the cross-
coupling reaction. The binding of nickel with photoredox
catalysts can lead to a variety of conversions, making it a
viable substitute for rare metals.52 Ni is a cheap metal that is
found in abundance and has high activity and good stability.
Due to the effective separation of electron–hole pairs, Ni
loading on g-C3N4 is an efficient method. This property is
due to the change in surface band bending, which kinetically
Fig. 6 SEM image of nano Ni/g-C3N4.
Heterogeneous photocatalyst graphitic carbon nitride (g-
C3N4) is one of the most profitable polymers among the
collection of metal-free.40 Contrary to semiconductors TiO2,
ZnO and WO3, g-C3N4 materials with a lower bandgap (TiO2:
3.2 eV,41 ZnO: from 3.10 eV to 3.37 eV,42 WO3 about 2.6 to 3.0
eV (ref. 43) versus g-C3N4: 2.7 eV) and ability to absorb visible
light in the 450–460 nm region are widely considered. Due to
the ease of preparation of these compounds from natural
and available materials such as urea and melamine, graphitic
carbon nitrides are economically viable and environmentally
safe.44,45 Besides, g-C3N4 is a heterogeneous semiconductor,
which, in addition to optical activity, has other advantages
such as non-toxicity, thermal stability, chemical stability, low
cost, significant electrical properties, and a large surface area.
Additionally, g-C3N4 has lately found
a wide range of
utilization in materials science, catalysis, electronic, and
optical fields as a result of its particular structure and
incomparable properties.46–48 The association of nitrogen
atoms in the carbon architecture provides anchor sites for
the immobility of active species and metal nanoparticles
when g-C3N4 is employed as
a heterogeneous catalyst
support, which in return leads to the enhancement of
catalytic efficiency. Therefore, this choice is beneficial and
economical compared to other semiconductors. Despite all
these advantages, features such as the low specific surface
area of pure g-C3N4 and the high electron–hole
Fig. 8 Nitrogen adsorption–desorption isotherms of nano 6 wt% Ni/g-
C3N4 and pure g-C3N4.
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Table 1 BET results for nano Ni/g-C3N4 and pure g-C3N4
Surface area
BET surface area (m2 g−1) (Ni/g-C3N4)
12.884
15.233
4.653
17.241
35.051
7.98
0.1129
0.1147
BJH adsorption cumulative surface area of pores (m2 g−1) (Ni/g-C3N4)
BET surface area (m2 g−1) (pure g-C3N4)
BJH adsorption cumulative surface area of pores (m2 g−1) (pure g-C3N4)
Mean pore diameter (nm) (Ni/g-C3N4)
Pore size
Pore size distribution (nm) (Ni/g-C3N4)
Pore volume
Single point adsorption volume of pores (cm3 g−1) (Ni/g-C3N4)
BJH adsorption cumulative volume of pores (m2 g−1) (Ni/g-C3N4)
Table 2 Solvent, base, and atmosphere optimizationa
Entry
Solvent
Base
Atmosphere
Yield (%)
1
2
3
4
5
6
7
8
THF
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DABCO
Bu3N
Et3N
Cs2CO3
K3PO4
K2CO3
KOH
—
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Ar
72
80
35
Trace
0
0
0
10
68
68
64
55
52
Trace
Trace
83
48
81
DMF
DMSO
CH3CN
EtOH
CHCl3
EtOAc
H2O
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
9
10
11
12
13
14
15
16
17
18
19b
—
—
—
O2
Air
15
a
Reaction conditions: iodobenzene (1 mmol), aniline (1.5 mmol), MoIJCO)6 (1 mmol), nano Ni/g-C3N4 (0.008 g, 6 wt%), base (3.0 mmol), solvent
(1.5 mL), room temperature, 11 W white LED (wavelength in the range of 400–750 nm), 24 h. b Without any MoIJCO)6.
Table 3 Amount of photocatalysta
Entry
Ni/g-C3N4 (mg)
Yield (%)
Entry
Ni/g-C3N4 (mg)
Yield (%)
1
2
3
4
5
6
3
6
8
10
12
14
51
62
83
90
90
87
7b
10
10
—
10
10
78
65
0
0
10
8c
9
10d
11e
a
Reaction conditions: iodobenzene (1 mmol), aniline (1.5 mmol), MoIJCO)6 (1 mmol) and photocatalyst in DMF (1.5 mL), at room temperature,
b
c
11 W white LED (wavelength in the range of 400–750 nm), air atmosphere, 24 h. Nano Ni/g-C3N4 (8 wt% Ni), 24 h. Nano Ni/g-C3N4 (4 wt%
Ni), 36 h. Pure g-C3N4, 36 h. e Pure NiCl2, 36 h.
d
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characterized nano Ni/g-C3N4 as heterogeneous nanosheets.
These nanosheets are biocompatible and effective
photocatalysts that catalyze radical reactions between aryl
halides/alkyl halides and amines/alcohols in the presence of
MoIJCO)6 as a source of CO gas54 to create a new method for
the synthesis of amides and esters as a transformation that
according to our knowledge has not previously been
described. This is a simple method carried out at room
temperature under the radiation of a blue LED lamp or
sunlight; it does not use any base, ligand, and organometallic
reagent, and it is not necessary to employ special conditions
or control CO gas release. It is noted that the use of MoIJCO)6
as a harmless source of CO gas eliminated the problems
caused by the use of CO gas, and made it possible to
synthesize amides and esters under mild conditions and
using visible light-activated catalysts. Therefore, to eliminate
the hazards of carbon monoxide in the carbonylation
reactions of aryl electrophiles, in the presence of a Ni
photocatalyst, MoIJCO)6 was used to replace CO gas, and we
predicted that this redox carbonylation mechanism would
attract the attention of chemists in most scientific and
applied fields.
Fig. 9 Aminocarbonylation reaction under a) different light and b)
light on and off conditions. aIsolated yield.
Results and discussion
To achieve the aim of photocarbonylation, we first
synthesized Ni/g-C3N4 nanoparticles with an approximate size
of 4.6 nm using the photodeposition technique.55 Since
semiconductors are efficient, economical, and easy-to-use
compounds, this method could be useful for gram scales.
After synthesizing nano Ni/g-C3N4, we investigated the
characteristics of this nanocatalyst via UV-visible, PL, FT-IR,
EDX, XRD, SEM, TEM, BET, and nitrogen adsorption–
desorption techniques (Fig. 2–8 and Table 1).
UV-vis diffuse reflectance spectroscopy was used to
determine the optical absorption of g-C3N4 and 6 wt% Ni/g-
C3N4. As shown in Fig. 2a, when Ni was loaded on g-C3N4, the
absorption peak of modified g-C3N4 gradually shifted to the
more extended wavelength region. The Kubelka–Munk
function56 was applied to calculate the optical band-gap of
both the unmodified and modified g-C3N4. As shown in
Fig. 2, g-C3N4 and nano Ni/g-C3N4 have band-gaps of 2.76 and
2.38, respectively. This reduction in the band-gap that occurs
after Ni loading is due to the increased interactions between
Ni species and g-C3N4 and reaffirms that Ni loading on
g-C3N4 regulates the electronic structure.57 In addition, nano
Ni/g-C3N4 has stronger visible light absorption than non-
modified g-C3N4, which is in acceptable accordance with the
shift of these compounds (Fig. 2a). To investigate the effect
of Ni doping on the recombination performance of
photocarriers, the photoluminescence emission spectra of
the pure g-C3N4 and nano Ni/g-C3N4 samples were
investigated. As presented in Fig. 2b, the pure g-C3N4 sample
shows a strong emission peak at around 450 nm, which is
related to the bandgap for the recombination of
photogenerated electrons–holes. The nano Ni/g-C3N4 sample
Fig. 10 Aminocarbonylation under natural sunlight.
increases the rate of surface reactions and causes this
compound to perform better photocatalytically than
unmodified g-C3N4.53 Based on previous research into the
development of inexpensive new photocatalysts that are
catalytically more efficient and simpler, as well as to design
more cost-effective processes, we have prepared and
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Table 4 A broad range of synthesized aromatic and aliphatic amidesa b
,
Table 4 (continued)
4b (96%)
4d (90%)
4f (89%)
4a (93%)
4c (95%)
4u (87%)
4w (90%)
4v (87%)
4x (86%)
4e (87%)
4g (93%)
4y (88%)
4z (86%)
a
Reaction conditions: aryl halide (1 mmol), amine (1.5 mmol),
Mo(CO)6 (1 mmol), nano Ni/g-C3N4 (10 mg, 6 wt% Ni), DMF (1.5 mL),
room temperature, 11 W blue LED (wavelength in the range of 400–
495 nm), air atmosphere, 48 h. b Yields refer to isolated yield.
4h (94%)
4j (84%)
4i (89%)
4b (96%)
presents a similar PL pattern, but the emission intensity is
quenched together with a small redshift. The PL emission
intensity of pure g-C3N4 was several times higher than that of
6
wt% Ni/g-C3N4. This observation showed that the
recombination of photogenerated electrons and holes was
prohibited undoubtedly by Ni, which would be exactly
desirable for the photocatalytic activity.
4l (90%)
Band gap energy (E) = h × C/λ
h = Planck's constant = 6.626 × 10–34 Joules second,
C = speed of light = 3.0 × 108 meters per second,
4m (92%)
λ = cut off wavelength nano Ni/g-C3N4 = 520 × 10–9 meters.
λ = cut off wavelength g-C3N4 = 450 × 10–9 meters.
4n (92%)
4p (89%)
4r (80%)
1 eV = 1.6 × 10–19 Joules (conversion factor)
The chemical functional groups of the catalyst were
determined by FT-IR and Fig. 3 shows the results of g-C3N4
and nano Ni/g-C3N4, respectively. The spectra showed strong
IR absorption bands, which were attributed to the typical
molecular structure of g-C3N4. The broad absorption at 3100–
3300 cm−1 belonged to the remaining N–H components and
O–H bonds, corresponding to uncondensed amino groups
and H2O molecules adsorbed on the surface. The broad
absorption at 900 and 1800 cm−1 was attributed to the
polycondensation structure of g-C3N4, which was ascribed to
the vibration characteristics of the s-triazine ring units and
heptazine heterocyclic ring units.58 In the FT-IR spectra, pure
4o (95%)
4q (89%)
4t (84%)
4s (84%)
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g-C3N4 and nano Ni/g-C3N4 exhibited similar absorption
bands. These results presented that the structure of g-C3N4
did not change during the photodeposition of Ni.
We employed EDX analysis to detect the presence of
nickel. The data obtained from the energy dispersive X-ray
analysis confirmed that there was not any P element, which
indicated that no phosphorus compounds were produced on
the surface (Fig. 4). Furthermore, the induced coupled
plasma (ICP) analysis revealed that only 6 wt% Ni in nano Ni/
g-C3N4 is present.
H3 hysteresis loops suggested that the slit-like pores were
based on the aggregates of g-C3N4 sheets.61 It can be
observed that the surface area of Ni/g-C3N4 (12.884 m2 g−1) is
larger than that of pure g-C3N4 (4.653 m2 g−1) with the
insertion of Ni. It is commonly known that a broad surface
area is always desirable to the photocatalytic efficiency due to
its abundant active sites. So with surface area increasing, the
photocatalytic activity of Ni/g-C3N4 is improved.
After the exact structure of the photocatalyst was
identified and characterized, we used this catalyst in
carbonylation reactions to evaluate its practical efficiency
and obtain better results in the synthesis of amides from
aryl and alkyl halides. Therefore, firstly we chose the
Fig. 5 shows the XRD patterns of pure g-C3N4 and Ni/g-
C3N4 nanoparticles. The small signal at 2θ = 13.08 implies
the interplanar length related to the interlayer packing of
pure g-C3N4 (Fig. 5a). The peaking signal at 2θ = 27.41
indicates the specified interlayer peak related to graphitic
systems. Pure g-C3N4 and g-C3N4 containing Ni exhibited a
specific signal at 2θ = 27.41; it demonstrates that the
structural features of g-C3N4 were preserved well in the
process of doping nickel. In addition, the signal intensity of
pure g-C3N4 decreased slightly when Ni was doped on it,
which indicated that the surface features of g-C3N4 were
tuned by Ni. The weakening of this peak suggested that the
Ni on the surface of g-C3N4 slightly prevented the diffraction
of the crystal structure of g-C3N4.54 It can be observed that
for Ni-loaded g-C3N4, new signals related to the (111), (200),
and (220) crystal planes of Ni appear at 2θ = 44.5°, 51.8°, and
76.4°, respectively (Fig. 5b), which are compatible with the
standard data of metallic Ni (JCPDS 87-0712). The average
size of nano Ni/g-C3N4 was calculated using the Scherrer
formula from 2θ = 27.41, and was obtained to be around 4.6
nm (see ESI† Table S2).
Fig. 6 and 7 show the scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) images and the
particle size distribution histogram of nano Ni/g-C3N4. The
TEM image showed that the range of particle sizes is
approximately between 5 and 9 nm, which is compatible with
the data of the XRD pattern, and the particles have cubic and
spherical shapes. The cubic morphology causes the increase
in absorption in the visible light range.59 By increasing the
surface/sub-surface defect ratio, the possibility of charge-
carrier separation can be elevated, which is achieved with the
sub-10 nm size of the particles.60 The diagram of the
approximate size range of nano Ni/g-C3N4 is shown in
Fig. 7b.
reaction
between
iodobenzene
(1a),
molybdenum
hexacarbonyl (2a), and aniline (3a) as a pattern system, and
investigated different factors to optimize the reaction
conditions of amide synthesis (4a). Tables 2 and 3 and Fig. 7
and 8 indicate the optimized results. We studied the effects
of various factors, including the solvent, base, atmosphere,
time, photocatalyst loading, and the source of light. First, we
irradiated an iodobenzene solution in THF using a white
LED (11 W) at room temperature and under an air
atmosphere, in the presence of Ni/g-C3N4 nanoparticles
(0.008 g) and the DBU base. After 24 h, phenyl benzamide
(4a) was obtained with a yield of 72% (Table 2, entry 1).
Therefore, firstly, we studied the effects of solvents and
found out that in the presence of THF or DMF, our desired
product was produced with excellent yields (72% yield and
80% yield) (Table 2, entries 1 and 2). However, when we used
other solvents, the yield of the product fell significantly
(entries 3–9). Hence, DMF is the best solvent for this
reaction. In the proposed mechanism, we explain why DMF
is a better solvent for such reactions. Since we assumed that
bases play an important role in this reaction, we examined
several inorganic and organic bases to optimize the reaction.
Table 2 includes the results of optimizing bases: DBU has
the highest yield, then next are DABCO, tributylamine, and
triethylamine (Table 2, entries 9–11), Cs2CO3 and K3PO4 have
average yields (Table 2, entries 12 and 13), and there are no
products produced by K2CO3 and KOH (Table 2, entries 14
and 15). We also investigated the carbonylation reaction
under base free conditions and found high efficiency under
base free conditions (Table 2, entry 16). Therefore, any C–N
bond generated under base free conditions is proof of the
existence of Ni (0) and confirms our proposed mechanism.
In the next step, we investigated the effects of the
atmosphere (Table 2, entries 17 and 18). In the atmosphere
of oxygen and air, almost the same results were obtained,
but the reaction efficiency in the atmosphere of argon
decreased. These results further evidence our proposed
mechanism, which will be discussed in the following. As
shown in entry 19, when we removed the CO source
(MoIJCO)6), the product was obtained with a 15% efficiency,
which implies that CO could be generated by photocatalytic
dissociation of DMF. This result is additional proof
confirming our proposed mechanism.
Table 1 shows the results of the BET surface area and pore
volume measurements for nano Ni/g-C3N4. The BET surface
area of nano Ni/g-C3N4 was obtained to be 12.884 m2 g−1, and
the BJH adsorption surface area of pores was 15.233 m2 g−1.
As shown in Table 1, the size distribution and single point
adsorption volume of pores were obtained to be 7.98 nm and
0.1129 cm3 g−1, respectively. Fig. 8 presents the nitrogen
adsorption–desorption isotherms of nano Ni/g-C3N4 and pure
g-C3N4. The adsorption–desorption isotherms of all the
samples exhibit typical type V curves with H3 hysteresis loops
suggesting the presence of mesopores. The high-pressure
hysteresis loop (P/P0 > 0.9) is related to macropores. The type
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Table 5 A broad range of synthesized aromatic and aliphatic estersa b
,
of the catalyst and in the presence of pure g-C3N4 (Table 3,
entries 9 and 10). In the presence of pure NiCl2, a 10% yield
of product 4a was obtained, indicating the catalytic role of
NiIJII) (Table 3, entry 11).
Influence of light
Since the yield of the desired product depends on selective
wavelengths of LEDs, we expose the reaction to different light
conditions, including long and short wavelengths with the
same intensity. As shown in Fig. 9a, the amide production
efficiency increased to 93% when the reaction was irradiated
with a blue LED (11 W). Therefore, higher frequency photons
can be used to produce photoelectrons–holes, and increase
the efficiency of the product. The results showed that under
dark conditions, only a 10% yield of product 4a is produced,
which indicates the catalytic role of nickel (0). By studying
the results of the reaction in the presence of light and
without light, it was found that light is an essential factor in
the reaction and has a significant effect on the efficiency of
the reaction. The results show that the reaction progresses as
long as it is exposed to radiation, and by stopping the
radiation, the reaction stops (Fig. 9b).
6a (98%)
6b (94%)
6c (83%)
6d (92%)
6e (90%)
6f (85%)
Reaction under sunlight
6g (85%)
6i (88%)
6h (89%)
6j (89%)
Besides, on a summer day, we investigated the operation of
nano Ni/g-C3N4 under the sun outside the laboratory. This
experiment was carried out in August, in Shiraz, at a
temperature of 26–37 °C and a zenith angle of 68.5°. The
results showed that this photocatalyst has a high capacity to
absorb sunlight (Fig. 10).
After optimizing the reaction conditions, we were able to
synthesize a wide range of amides. Table 4 shows the
photoaminocarbonylation reaction for a wide range of aryl
halides with aliphatic and aromatic substitutions. We found
that nano Ni/g-C3N4 could be used to produce a wide range
of different substrates with excellent efficiency. The results
indicated that the amidation reaction between iodobenzene
and primary amines with electron-withdrawing and electron-
donating groups on the anilines, as well as secondary amines
such as morpholine, piperazine, pyrrolidine, and piperidine,
is well done, and the desired amides are synthesized with
high efficiency (entries 4a–f). Also, Table 4 shows that both
iodobenzene with electron-withdrawing groups such as –NO2
and iodobenzene with electron-donating groups such as –Me
can be used in our proposed technique (entries 4g–j). Not
surprisingly, aryl bromides were less reactive than aryl
iodides. Thus, selective amidation of chloro-bromo or chloro-
iodo derivatives gives impressive results (entries 4k–n). Under
optimized conditions, heteroaromatic iodides also showed
favorable results. Both N-phenyl thiophene-2-carboxamide
and morpholinoIJthiophene-2-yl)methanone were obtained
from 2-iodothiophene with 95% and 89% efficiencies,
respectively (entries 4o–p). Similarly, N-(thiazol-2-yl)-
benzamide was obtained from thiazol-2-amine (entry 4q). In
our proposed method, in addition to aromatic halides,
6l (84%)
6n (81%)
6p (77%)c
6k (84%)
6m (90%)
6o (80%)
a
Reaction conditions: aryl halide (1 mmol), alcohol (1.5 mmol),
MoIJCO)6 (1 mmol), nano Ni/g-C3N4 (0.01 g, 6 wt% Ni), DMF (1.5 mL),
at room temperature, 11 W blue LED (wavelength in the range of
b
400–495 nm), air atmosphere, 48 h. Yields refer to isolated yield.
c
Yield of product 6p was determined by HPLC analysis.
In addition, we optimized the photocatalyst amounts and
applied an optimum of 0.01 g of Ni/g-C3N4 nanoparticles in
this research work (Table 3, entry 4). We also examined the
effect of different amounts of nickel (entries 7 and 8). A study
of the reaction showed a 0% conversion both in the absence
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Fig. 12 Hot filtration test.
designed
a model reaction between iodobenzene (1a),
molybdenum hexacarbonyl (2a), and benzyl alcohol (5a) in
the presence of nano Ni/g-C3N4 as a photocatalyst, and then
optimized the reaction conditions (solvent, base, atmosphere,
catalyst). According to the results, the best reaction
conditions for the synthesis of benzyl benzoate (6a) are very
close to the optimal conditions for the preparation of amides.
Therefore, we can carry out this reaction using 0.01 g of nano
Ni/g-C3N4, under base-free conditions and at room
temperature in the presence of DMF, and under irradiation
with a blue LED (11 W), and in air. In addition, the effects of
LED light and sunlight on the reaction are shown in Fig. S1–
S3 (see the ESI†). This reaction is carried out in the presence
of various aryl halides and aliphatic alcohols, so the yield of
the obtained products is satisfactory (Table 5). By using
iodobenzene and 4-nitroiodobenzene in the presence of
aliphatic alcohols, various esters were obtained. In the
esterification reaction, two-substituted aryl halides with
chloride, bromine, or iodine could be selected, and a
selective product could be obtained from its corresponding
aryl halide (entries 6l and m). Finally, using our proposed
method, we could obtain excellent results for synthesizing
benzyl cyclohexane carboxylate, 2-nitrobenzyl cyclohexane
carboxylate, and methylcyclohexane carboxylate from
iodocyclohexane (entries 6n–p). The production of high-
efficiency carboxylic acid derivatives, using a simple method,
is useful for synthesizing some beneficial compounds. This is
a new pathway for the synthesis of amides and esters through
photocatalytic carbonylation, in the absence of CO gas, using
aliphatic and aromatic halides, and in the presence of
molybdenum hexacarbonyl as the CO source.
Fig. 11 Recyclability of nano Ni/g-C3N4 in a) aminocarbonylation and
b) alkoxycarbonylation reactions.
Table
6 Catalyst recyclability for a) aminocarbonylation and b)
alkoxycarbonylation reactions
a) Aminocarbonylation
Runs
Weight of the catalyst (mg)
Nickel leaching (wt%)
1
2
3
4
5
6
10
0.000
0.003
0.005
0.007
0.009
0.011
9.6
9.3
8.9
8.5
8.2
b) Esterification
Runs
Weight of the catalyst (mg)
Nickel leaching (wt%)
1
2
3
4
5
6
10
0.000
0.004
0.007
0.009
0.011
0.014
9.5
9.1
8.7
8.4
8.0
Catalyst recyclability
aliphatic halides with C-sp3 also provided favorable results
for synthesizing amides (entries 4r–z).
Nano Ni/g-C3N4 can be used multiple times without a
significant reduction in reaction efficiency (Fig. 11), so it can
be said that nano Ni/g-C3N4 is a recyclable heterogeneous
photocatalyst with useful function. At the end of each cycle,
the catalyst was recovered using a centrifuge, washed with
water and ethanol, and then dried in an oven at 80 °C. Even
after 6 runs, no significant reduction in its catalytic activity
was observed. Details about the weight of the photocatalyst
Synthesis of esters
The promising data from the synthesis of amide derivatives
persuaded us to use the photocatalyst to form a C–O bond.
To evaluate the application of the photocatalyst, we first
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Fig. 14 Flow reactor. L = length of the filled column, din = internal
diameter of the column, F = flow rate, tr = residence time. 5 mmol
scale, 82% yield.
UV-vis, FT-IR, and PL spectra. According to Fig. 13 the XRD
spectra disclosed no remarkable differences between fresh
nano Ni/g-C3N4 and the recovered nano Ni/g-C3N4 affirming
that the crystal structures of the photocatalyst were not
changed during these recovery cycles (Fig. 13c). Also the PL
emission intensity and UV-vis absorption spectra show that
no significant difference was observed between fresh and
recyclable nano Ni/g-C3N4 (Fig. 13a and d).
Scale-up of continuous flow
Features such as easy access to equipment and reactants, as
well as the simple technique used, led to the investigation of
large-scale reactions using a continuous-flow photoreactor.
Given that in the laboratory and on a small scale, we were
able to collect a significant amount of product using 0.557
ml of iodobenzene, we can use this method on a large scale.
Next, we investigated the operation of this method with a
continuous flow column, glass beads, silica gel, and nano Ni/
g-C3N4. As expected, a suitable and reusable continuous-flow
photoreactor was obtained. By shortening the reaction time
(8 h), the practical capacity of the reaction was determined
(Fig. 14). As a result, a continuous-flow photoreactor is a
viable option for large-scale use. To do this, 50 mg of Ni/g-
C3N4 nanoparticles and 4 g of silica gel (particle size 70–230
μm) were gently ground in an agate mortar for 5 minutes. An
80 cm long glass tube (with an inner diameter of 1 cm) was
fitted with a cotton filter at an outlet to keep the solid
mixture in the reactor. The tube was filled with 8 g of glass
beads (d = 0.5 cm) and a pre-prepared mixture of nano Ni/g-
C3N4 and silica gel. DMF was added for rinsing. Blue LED
wires were wrapped outwards. Iodobenzene (0.557 ml, 5
mmol), aniline (0.684 ml, 7.5 mmol), and MoIJCO)6 (1320 mg,
5 mmol) were dissolved in DMF (40 ml), and the resulting
solution was gradually added to the tube. The flow rate was 1
mL h−1, controlled using a plunger with a residence time of tr
Fig. 13 The characterization of the recovered nano Ni/g-C3N4: (a)
UV-vis, (b) FT-IR, (c) XRD, and (d) photoluminescence spectra.
and the wt% of the nickel leaching after each run are given
in Table 6 and the results show that only trace amounts of
nickel were found in the collected solution after each run
(see the ESI† for more information on the leaching test).
Additionally, the reaction stopped completely after hot
filtration with efficiency and time of about 50% and 12 h,
respectively, indicating that the reaction was performed on
the surface of a heterogeneous catalyst (Fig. 12). Further, the
photostability of nano Ni/g-C3N4 was surveyed using XRD,
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Table 7 Comparison of using MoIJCO)6 instead of CO gas for the synthesis of some compounds similar to compounds 4c, 6d and 6i
Catalyst
Compds.
Conditions
Yield (%)
Ref.
Nano Ni/g-C3N4
Eosin Y
Nano Ni/g-C3N4
Eosin Y
Nano Ni/g-C3N4
Pd/SiO2
6d
6d
6i
6i
4c
4c
MoIJCO)6/visible light
Gaseous CO/visible light
MoIJCO)6/visible light
Gaseous CO/visible light
MoIJCO)6
95
86
88
57
95
77
This work
33
This work
33
This work
22
Gaseous CO/heat
= 8 h. During operation, the reactor was irradiated with an
air-cooled LED module. After 8 hours, we collected the
product in the outlet. Upon completion of the reaction, the
solvent was evaporated at room temperature. The crude
product was obtained as a powder and purified by column
chromatography to give the desired product (82%).
Finally, a comparative study of using MoIJCO)6 instead of
CO gas with other photochemical and non-photochemical
reported catalysts for the synthesis of some compounds
similar to compounds 4c, 6d and 6i was made, which
revealed that MoIJCO)6 is an efficient alternative to CO gas
(Table 7).
g-C3N4 and also it could capture the O2˙− radicals acquired
from the CB of g-C3N4.
Besides, according to Table 2 entry 17, the reaction did
not progress under the Ar atmosphere as well as only a trace
amount of the target product was obtained without the use
of the photocatalyst (Table 3 entry 9). For this reason, light,
activated oxygen (O2˙−), and a photocatalyst are required to
process this reaction. Also, in a new test, triethanolamine
(TEA), methanol, and ethylenediaminetetraacetic acid were
added as hole scavengers, and it was again observed that the
desired product was obtained in trace amounts. These
observations confirmed the possibility that the reaction took
place through a radical pathway. In addition, the fact that the
product does not form under dark conditions confirms that
visible light stimulates the carbonylation reaction.
Control experiments and mechanism suggestion
According to previously reported articles64–67 and our
experimental results, the proposed mechanism for amino
and alkoxycarbonylation is shown in Fig. 16. By light
irradiation, charge separation occurs, and electron/hole pairs
(e−/h+) are generated on the g-C3N4 surface. The electron in
the CB of g-C3N4 is effective for the generation of superoxide
radical anion O2˙−. This superoxide radical anion reacted with
amines or alcohols (3 or 5) by abstracting proton generated
HX and molecular O2. The corresponding RY anion is
trapped by NiIJII) species II generating organometallic adduct
III. In the catalytic cycle of nickel, the oxidative addition of
To increase our insight into this mechanism, some
additional studies were made, as shown in Fig. 15.
Tetramethylpiperidinyl-1-oxy (TEMPO·) is a stable nitroxide
radical which acts as an electron-trapping agent for
semiconductors.62,63 When 2 mmol of TEMPO was added to
the reaction mixture, the yield of the target molecule was
severely decreased. It could trap electrons from the CB of
Fig. 15 Control experiments: a) aminocarbonylation, b) alkoxycarbonylation.
Fig. 16 Proposed mechanism of carbonylation.
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nickel (0) (which exists on the catalyst surface and has been
proven by characterizing the photocatalyst and other
Ultimate product purification was done by silica gel column
chromatography utilizing petroleum ether/ethyl acetate.
experimental results) to aryl halide
1 resulted in the
production of intermediate I. These nickelIJII) species yield
intermediate II through the insertion of CO that is released
from MoIJCO)6. Finally, products 4 or 6 were obtained by
reductive elimination of intermediate III, and Ni (0) was
regenerated for the next catalytic cycle. According to Table 2,
the results of optimizing solvents showed that DMF provides
the best efficiency. Based on the reports published by Feng
and Li, C–N bond cleavage in DMF may occur through
electron transfer to the hole. As a result, CO gas and
dimethylamine are produced.68 Dimethylamine can serve as
a base to balance the HX produced from RYH and O2˙−. That
is why it is possible to obtain 15% of the desired product
without using any MoIJCO)6. DMF is a suitable species for
electron transfer to the hole (Table 2, entry 19).
General procedure for alkoxycarbonylation in the presence
of the nano Ni/g-C3N4 photocatalyst. To a Pyrex round-
bottom flask set with a magnetic stir bar, aryl iodide (1
mmol), alcohol (1.5 mmol), MoIJCO)6 (1 mmol), nano Ni/g-
C3N4 (0.01 g, 6 wt%) and DMF (1.5 mL) were added and the
reactant mixture was irradiated with an 11 W blue LED lamp
(λ > 440 nm, distance 3.0 cm) under air conditions at normal
temperature. The reaction was stirred for a suitable time at
room temperature (25 °C) and checked using TLC. After the
termination of the reaction, ethyl acetate (2 mL) was added
to the reaction mixture and heterogeneous nano Ni/g-C3N4
was entirely isolated applying centrifugation, then, the
organic phases were dried and dehydrated over MgSO4 and
the solvent was vaporized under reduced pressure to acquire
the crude. Ultimate product purification was done by silica
gel column chromatography utilizing petroleum ether/ethyl
acetate.
General procedure for using TEMPO in the presence of
the nano Ni/g-C3N4 photocatalyst. To a Pyrex round-bottom
flask set with a magnetic stir bar, aryl iodide (1 mmol), amine
or alcohol (1.5 mmol), MoIJCO)6 (1 mmol), nano Ni/g-C3N4
(0.01 g, 6 wt%), TEMPO (2 mmol), and DMF (1.5 mL) were
added and the reactant mixture was irradiated with an 11 W
blue LED lamp (λ > 440 nm, distance 3.0 cm) in an air
atmosphere at room temperature. The reaction was stirred
for a suitable time (24 h) at room temperature (25 °C) and
checked using TLC and GC. As expected, a trace amount of
the target product was obtained in the presence of TEMPO.
Experimental section
Chemicals materials were acquired from Fluka, Aldrich,
Acros, and Merck, and all solvents were obtained from
commercial sources and dried employing standard
procedures.
Catalyst preparation
Preparation of g-C3N4. g-C3N4 was prepared according to
the procedure reported by Li.69 Typically, a Coors high-
alumina crucible was charged with 10 g of melamine powder,
and then placed in a 673 in3 muffle furnace (115 V/14 Amps)
at a heating rate of 20 °C min−1 to 500 °C in 2 h. For
formation of a better structure, it was put at 520 °C for 2 h.
Preparation of nano Ni/g-C3N4. Nano Ni/g-C3N4 was
prepared by an altered photodeposition procedure.51 10 mg
g-C3N4, 3 mL triethanolamine (TEOA), 250 μL NiCl2 (0.1 M)
solution, 2 mL NaH2PO2 (0.1 M) aqueous solution, and 4.9
mL H2O were blended in a round bottom flask at room
temperature. The system was stirred for 40 min under a
nitrogen atmosphere. Then, the mixed solution was
irradiated under UV-vis light (300 W Xe lamp) for 30 min.
The final step involved washing collected precipitates with
distilled water to remove the residues of reactants.
Conclusion
General procedure for aminocarbonylation in the presence
of the nano Ni/g-C3N4 photocatalyst. To a Pyrex round-
bottom flask set with a magnetic stir bar, aryl iodide (1
mmol), amine (1.5 mmol), MoIJCO)6 (1 mmol), nano Ni/g-C3N4
(0.01 g, 6 wt%) and DMF (1.5 mL) were added and the
reactant mixture was irradiated with an 11 W blue LED lamp
(λ > 440 nm, distance 3.0 cm) in an air atmosphere at room
temperature. The reaction was stirred for a suitable time at
room temperature (25 °C) and checked using TLC. After the
termination of the reaction, ethyl acetate (2 mL) was added
to the reaction mixture and heterogeneous nano Ni/g-C3N4
was entirely isolated applying centrifugation, then, the
organic phases were dehydrated over MgSO4 and the solvent
was vaporized under reduced pressure to acquire the crude.
Heterogeneous
semiconductors
are
an
eco-friendly
replacement for photocatalyst reactions and sewage
treatment. A class of extracted metal-free polymers from
inexpensive natural resources like graphitic carbon nitrides
are heterogeneous semiconductors with great potential for
organic photosynthesis. In this study, we demonstrated that
graphitic carbon nitrides grafted with a nickel catalyst can
induce selective C–N and C–O cross-coupling of alkyl halides
and aryl halides with amines and alcohols, by using MoIJCO)6
as the source of CO gas under visible light that
alkoxycarbonylation has not been reported so far. This makes
it possible to operate carbon monoxide coupling reactions
under benign and safe conditions, without the presence of
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toxic gas to avoid the problematic use of gaseous CO.
Further, this methodology has experimental simplicity, is
additive, base, and ligand-free, and has a simple workup at
ambient temperature. Amides and esters were obtained in
good to excellent yields and selectivity by this procedure
which could be done using a continuous flow reactor to
scale-up, appropriate for industry applications.
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23 J. R. Martinelli, D. A. Watson, D. M. M. Freckmann, T. E.
Barder and S. L. Buchwald, J. Org. Chem., 2008, 73,
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There are no conflicts to declare.
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Acknowledgements
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