1,2,3-Triazole framework: a strategic structure for C-H?X hydrogen bonding and practical design of an effective Pd-catalyst for carbonylation and carbon-carbon bond formation

Article abstract of DOI:10.1039/d1ra03356e

1,2,3-Triazole is an interesting N-heterocyclic framework which can act as both a hydrogen bond donor and metal chelator. In the present study, C-H hydrogen bonding of the 1,2,3-triazole ring was surveyed theoretically and the results showed a good agreement with the experimental observations. The click-modified magnetic nanocatalyst Pd@click-Fe₃O₄/chitosan was successfully prepared, in which the triazole moiety plays a dual role as both a strong linker and an excellent ligand and immobilizes the palladium species in the catalyst matrix. This nanostructure was well characterized and found to be an efficient catalyst for the CO gas-free formylation of aryl halides using formic acid (HCOOH) as the most convenient, inexpensive and environmentally friendly CO source. Here, the aryl halides are selectively converted to the corresponding aromatic aldehydes under mild reaction conditions and low Pd loading. The activity of this catalyst was also excellent in the Suzuki cross-coupling reaction of various aryl halides with phenylboronic acids in EtOH/H₂O (1?:?1) at room temperature. In addition, this catalyst was stable in the reaction media and could be magnetically separated and recovered several times.

Full text of DOI:10.1039/d1ra03356e

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1,2,3-Triazole framework: a strategic structure for
C–H/X hydrogen bonding and practical design of
an effective Pd-catalyst for carbonylation and
carbon–carbon bond formation†
Cite this: RSC Adv., 2021, 11, 20812
a
Maryam Dehdashti Jahromi,b
*
Fatemeh Mohammadsaleh,
Abdol Reza Hajipour,
Seyed Mostafa Hosseinic and Khodabakhsh Niknam
cd
a
*
1,2,3-Triazole is an interesting N-heterocyclic framework which can act as both a hydrogen bond donor
and metal chelator. In the present study, C–H hydrogen bonding of the 1,2,3-triazole ring was surveyed
theoretically and the results showed a good agreement with the experimental observations. The click-
modified magnetic nanocatalyst Pd@click-Fe3O4/chitosan was successfully prepared, in which the
triazole moiety plays a dual role as both a strong linker and an excellent ligand and immobilizes the
palladium species in the catalyst matrix. This nanostructure was well characterized and found to be an
efficient catalyst for the CO gas-free formylation of aryl halides using formic acid (HCOOH) as the most
convenient, inexpensive and environmentally friendly CO source. Here, the aryl halides are selectively
converted to the corresponding aromatic aldehydes under mild reaction conditions and low Pd loading.
The activity of this catalyst was also excellent in the Suzuki cross-coupling reaction of various aryl
halides with phenylboronic acids in EtOH/H2O (1 : 1) at room temperature. In addition, this catalyst was
stable in the reaction media and could be magnetically separated and recovered several times.
Received 29th April 2021
Accepted 4th June 2021
DOI: 10.1039/d1ra03356e
rsc.li/rsc-advances
chemistry framework in synthesis routes in terms of the separation
and catalyst-recycling.4,5 Therefore, the design and development of
1. Introduction
a suitable support as catalyst carrier capable of improving the
catalyst characteristics, especially for metal-based catalysts, by
a cooperative effect between the metal complex and the support, is
still an important challenge for successful catalysts.6
Nowadays, new advances in the chemical synthesis and catal-
ysis areas are strongly tied to green chemistry. The goal is the
utilization of environmentally friendly reagents and a set of
principles that minimize or eliminate contamination and
pollution in the design of new chemical processes. Many efforts
have been directed towards the development of bio-based
materials in the synthesis routes due to the limited avail-
ability of fossil fuel and growing environmental concerns.1,2
Academic and industrial discoveries of efficient and selective
catalysts for a wide variety of organic reactions have had a huge
inuence on minimizing the costs of manufacturing and waste
disposal.3 Heterogenization of homogeneous catalysts on solid
supports has received signicant attention in the catalysis
researches and follows the majority of the principles of the green
Chemistry is knowledge of wonders, and in the depths of this
science, marvelous mysteries can be discovered. 1,2,3-Triazoles are
interesting class of N-heterocyclic compounds, which are typically
prepared by Cu(I)-catalyzed azide–alkyne 1,3-dipolar cycloaddition
(CuAAC) reaction recognized as a highly important example of
“click chemistry”.7,8 1,2,3-Triazole produced in the click reaction
offers properties beyond just the sum of its components, the azide
and alkyne. It has shown various successful applications in
different domains such as anion recognition,9–11 catalysis.12,13
surface modication and material science.14,15 This nitrogen-rich
aromatic entity has a great dipole moment (ꢀ5 D), and can act
as the metal chelator and C–H hydrogen bond donor interacting
with electron-rich partners such as anions (Scheme 1).
Several research groups have reported 1,4-disubstituted-
1,2,3-triazoles as appropriate host for the complexation of
different anions and have employed them in anion recognition
where many weak C–H/Xnꢁ interactions were accrued through
hydrogen bonding and electrostatic interactions.10,11 Indeed,
the large dipole moment of 1,2,3-triazoles and extrinsic polari-
zation of a carbon atom in the 5-position afford hydrogen bonds
aDepartment of Chemistry, Faculty of Nano and Bio Science and Technology, Persian
Gulf University, Bushehr, Iran. E-mail: f.mohammadsaleh@gmail.com; niknam@
pgu.ac.ir
bFaculty of Engineering, Jahrom University, Jahrom, Iran
cPharmaceutical Research Laboratory, Department of Chemistry, Isfahan University of
Technology, Isfahan 84156, Islamic Republic of Iran
dDepartment of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, 53706-1532 WI, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra03356e
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Magnetic nanoparticles have been a topic of great interest in
recent years, as their characteristics make them appropriate for
use in various elds such as physics, medicine, biology, mate-
rials and catalysis science.27,28 The advantage of magnetic cata-
lysts is the convenient separation and recovery from the
reaction mixture using an external magnet. However, Fe3O4
nanoparticles have a tendency to aggregate because of strong
magnetic dipole–dipole attractions between particles. Coating
Fe3O4 nanoparticles with inorganic/organic polymers increases
the stability of Fe3O4-based catalysts in reaction medium.
In continuation of our efforts towards the design of new and
versatile catalysts employing the 1,2,3-triazole frame-
works,17,29–31 in this work, the metal coordination ability of 1,2,3-
triazole ring was used for surface modication and new catalyst
design and the triazole-modied core–shell Fe3O4/chitosan was
synthesized as a sustainable solid support for the immobiliza-
tion of palladium catalyst. Herein, chitosan (CS) was used as an
effective stabilizer agent for Fe3O4 nanoparticles (MNPs), in
which polymer chains and Fe3O4 particles are connected to each
other covalently via click reaction. The detailed route is shown
in Scheme 2. This covalently-clicked connection increases the
strength of the resulted solid support, and also the presence of
nitrogen-rich triazole unites with excellent chelating ability to
metal center could cause an increased capability of metal-
immobilization for the support. The incorporation of palla-
dium(II) complex into the click-Fe3O4/chitosan (click-MNPs/CS)
gave the Pd@click-MNPs/CS, which could be successfully
applied as a magnetic heterogonous, efficient, and green cata-
lyst for the formylation of aromatic halides using formic acid as
the CO-source in the presence of DCC activator.
Scheme 1 The interactions of 1,2,3-triazole framework with metal
ions and anions, (I),16 (II),9 (III),13,17 (1V).13
involving C–H groups. Apart from anion complexation as
hydrogen-bond-donor, the nitrogen-rich 1,2,3-triazols have the
ability to coordinate a metal center as the N-donor ligands.8,18 Such
characteristics introduce triazoles as important and powerful
candidates in the catalysis eld, particularly in metal-based cata-
lysts. The 1,2,3-triazole as a high stable linker have attracted much
attention for surface modications and catalyst immobilization,
and as the effective N-donor ligand can coordinate with metal ions
and stabilize catalytic species on the surface of solid supports.19–21
Recently, these units have great tendency to coordination with
many metal ions like Cu, Re, Pd, and etc.22,23
Polysaccharides have been widely applied as a suitable
architecture for solid catalysts. Chitosan (CS), the N-deacety-
lated derivative of chitin, is a sample of such polysaccharides
that is widely spread in living organisms. Due to the presence of
reactive functional groups, free-amino groups and hydroxyl
groups, CS and its composite-derivatives are considered to be
proper solid supports for the immobilization of a metal catalyst.
A main limiting problem in the consuming of CS in the catalysis
elds is poor chemical resistance and mechanical strength,
which signicantly reduces the recycle life of the catalysts based
on this biopolymer. Accordingly, it is oen necessary to use
stronger ligands or metal/metal oxide nanoparticles to modify
fresh CS properties. Physical and chemical modications
improve pore size, mechanical strength, chemical stability,
hydrophilicity and biocompatibility of the CS-based composites
for catalytic applications. Several studies have been conducted
toward developing chitosan coated magnetic nanoparticles.
Miserez et al.24 prepared a composite of catechol-functionalized
chitosan with superparamagnetic iron oxide (g-Fe2O3) nano-
particles that represented a signicant improvement in func-
tionality of chitosan-based biomaterials. Zhao et al.25 reported
a mesostructured Fe3O4/chitosan composite as a pH-responsive
drug-delivery system. Movassagh et al.26 have reported magnetic
porous chitosan-thienyl imine palladium(II) complex as an
efficient and magnetically recoverable catalyst for the Mizoroki–
Heck reaction.
The general term ‘carbonylation’ was expressed particularly
for the various transformations that incorporate carbon
Scheme 2 Synthesis of Pd@click-Fe3O4/chitosan catalyst. Reaction
conditions: (a) 3-azidopropyltrimethoxysilane, ethanol, 40 ꢂC, 18 h, N2;
(b) propargyl bromide, DMF, 60 ꢂC, 24 h; (c) Cu2O, H2O/DMF, 60 ꢂC, 3
d; (d) toluene, Pd(OAc)2, r.t., 2 d.
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monoxide into the organic compounds. Recently, carbonylation
has become a powerful strategy for the preparation of carbonyl-
containing organic combinations.32–34 To date, many
researchers have focused their studies on the new CO gas-free
carbonylation systems35–37 and reported the various carbonyl
sources such as formic acid,38,39 Mo(CO)6,40 oxalic acid36 and N-
formylsaccharin.41 Formic acid is an eco-friendly, cheap and
available carbonyl source and can generate a carbon monoxide
(CO) molecule in situ during the reaction process.42 The formic
acid-assisted carbonylation catalyzed by palladium catalysts38,39
is an efficient approach for constructing carbonyl compounds.
In another part of this work, the Pd@click-MNPs/CS was
employed as a recyclable catalyst in the Suzuki–Miyaura cross-
coupling of diverse aryl halides with phenylboronic acids in
green solvent at room temperature. We also have studied the
hydrogen bonding of triazole C–H by 1HNMR analysis of 1-
benzyl-4-phenyl-1H-1,2,3-triazole in DMSO and CDCl3 solvents.
In this part, the effects of the solvent and its polarity on the
Fig. 1 FTIR spectra of Fe3O4 nanoparticles (MNP) (a), pure chitosan
(CS) (b), alkyne-functionalized chitosan (c), azide-functionalized Fe3O4
(d) and click-MNPs/CS (e).
1
most stable structure of this compound and HNMR signal of
triazole proton in the gas phase, DMSO and in CDCl3 solvent
were theoretically investigated.
groups graed on the surface of Fe3O4 particles to obtain the
1,2,3-triazole functionalized-MNPs/CS. The cycloaddition reac-
tion of alkyne and azide groups by the click process was
conrmed using FTIR analysis, which revealed disappearance
of the absorption bands at 2100 cmꢁ1 and 2099 cmꢁ1 attributed
to the alkyne sections and N3, respectively (Fig. 1e). In addition,
the IR absorption spectrum of click-MNPs/CS (Fig. 1d) reveals
characteristic bands of both species, i.e., Fe3O4 particles43,44 and
chitosan (CH, CO, CN, and OH stretching), conrming
successful attachment of Fe3O4 nanoparticles with chitosan
backbone.
2.1.2. Synthesis and characterization of Pd/support
sample. The resulting click-MNPs/CS was further treated with
Pd(OAc)2 in toluene at room temperature to afford the desired
solid catalyst Pd@click-MNPs/CS, meanwhile, the Pd loading
measured by inductively coupled plasma atomic emission spec-
trometry (ICP-AES) was obtained 1.16%. Also, the Fe content of
catalyst was 23.72% as analyzed by the ICP technique.
2. Results and discussion
2.1. Synthesis and characterization of composite
Due to ambivalent character of triazoles, they can act as both an
N-donor ligand and hydrogen donor, showing the complexa-
tions with both metal cations and anions. 1,2,3-Triazols as the
nitrogen-rich heterocycles have developed as interesting
candidates for use in transition-metal catalysis. In this part, the
novel magnetic catalyst Pd@click-MNPs/CS containing 1,2,3-
triazole unit was synthesized through a ‘‘click’’ reaction, in
which 1,2,3-triazole moiety acts as a strong linker and a Pd-
chelator. 1,2,3-Triazole having several N-donor positions can
be strongly coordinated to palladium(II) ions and stabilize the
catalytic sites in the solid catalyst matrix.
2.1.1. Synthesis and characterization of support with 1,2,3-
triazole motives. Herein, we have prepared the covalently
modied click-MNPs/CS as a triazole-containing stable support
for the immobilization of palladium(II) species.
XRD. Chemical phase analysis of the as-prepared material
was performed by X-ray reective diffraction (XRD) and Fig. 2
exhibits the XRD patterns of virginal CS and the catalyst Pd@click-
MNPs/CS. The XRD-diffractions of Fe3O4 nanoparticles are clearly
FTIR spectroscopy. In the rst stage of our strategy, the chi-
tosan and Fe3O4 were respectively functionalized with alkyne
and azide groups. The presence of these groups were conrmed
by the appearance of a sharp absorption band at 2100 cmꢁ1 in
FT-IR spectrum of alkynylated chitosan attributed to the alkyne-
groups, and also the N3 vibration at 2099 cmꢁ1 in spectrum of
azide-functionalized Fe3O4 nanoparticles (Fig. 1c and d). In
addition, the intensity of the band at 1602 cmꢁ1 (N–H bending
vibration) is decreased in the spectrum of the alkynylated chi-
tosan material compared to the pure chitosan spectrum; which
conrms that alkyne group has been attached into the amine
groups of chitosan aer the reaction between chitosan and
propargyl bromide.
In the next step, the resulting azide-functionalized Fe3O4
particles were treated with alkynylated chitosan in the presence
of Cu2O catalyst in mixed solvent H2O/DMF. During the “click”
reaction, alkyne sections of alkynylated-CS react with azide Fig. 2 XRD patterns of virginal chitosan (a) and Pd@click-MNPs/CS (b).
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Fig. 5 Room temperature magnetization curves of pure Fe3O4 (a) and
Pd@click-MNPs/CS (b).
Fe3O4 cores are surrounded by a grey polymer shell of about 3–6 nm
thick and the average size of the Fe3O4 cores is about 10–20 nm
(Fig. 4a). Aer incorporating of palladium, the Pd nanoparticles are
not clearly distinguishable from Fe3O4 particles (Fig. 4b).
VSM. The magnetic properties of Pd@click-MNPs/CS were
evaluated by a vibrating sample magnetometer (VSM) at room
temperature. As depicted in Fig. 5, the magnetization diagram
of the as-prepared material exhibits a strong magnetic property,
which was also conrmed by easy and complete attraction using
an external magnet.
Fig. 3 FE-SEM and SEM-EDX of Pd@click-MNPs/CS.
distinguishable and in good agreement with the crystalline Fe3O4
particles reported in literatures.18,45 Additionally, the broad peak
from 2q ¼ 15ꢂ to 25ꢂ is consistent with an amorphous chitosan
polymer. No characteristic peaks for metallic Pd particles are
detected which shows the construction of Pd(II)-complexes in the
matrix of the click-MNPs/CS composite.
FESEM. Fig. 3 displays the eld emission scanning electron
microscopy (FE-SEM) images of the catalyst Pd@click-MNPs/CS.
The FE-SEM micrographs clearly show polymer section and
Fe3O4 particles which have been distributed in the polymer
matrix. By the SEM-EDX analysis the presence of C, Fe and Pd
was approved in the catalyst (Fig. 3). In addition, the element
distribution in the surface of the catalyst was studied using the
meaningful pictures provided by the EDX mapping (Fig. 1S†).
TEM. The TEM images of the click-MNPs/CS support and the
catalyst Pd@click-MNPs/CS are shown in Fig. 4. The dark nano-
2.1.3. Behavior of 1,2,3-triazole motives in solutions
Interaction of 1-benzyl-4-phenyl-1H-1,2,3-triazole with solvents.
The 1,2,3-triazole C–H hydrogen bonding and the effects of
solvent on the 1HNMR spectrum of triazole moiety were
conrmed experimentally and also with computational results.
The 1-benzyl-4-phenyl-1H-1,2,3-triazole was prepared according
to the our previous work.46 The 1HNMR spectrum of this
triazole-containing compound shows the distinctive singlet of
triazole C5–H at 8.83 ppm and 7.59 ppm in DMSO and CDCl3
solvents, respectively. The C5–H proton of 1,2,3-triazole is easily
distinguished from the other protons. As shown in the HNMR
spectra (Fig. 2S†), the chemical shi of the triazole C5–H in
DMSO has been moved to downeld relative to the CDCl3
solvent. Based on the previous reports, the polarity and the large
dipole moment (ꢀ5 D) of 1,4-substituted-1,2,3-triazoles, that is
almost aligned with the triazole C–H bond, make an electro-
positive C–H position which can act as an active hydrogen bond
donor for anion (Xꢁn) binding.8 We believe that the dielectric
constant (3) and Gutmann donor number (DN) of the solvents,
exert a large effect on the polarization of the triazol C5–H bond
and the acidity of the C5–H proton.9,47 The Gutmann donor
number of a solvent exhibits a quantitative amount of the
solvent's ability to interact with positive charge. DMSO is a very
polar solvent, having a strongly polarized S–O bond, and can
enter into hydrogen bonding and dipole–dipole interactions.
The dielectric constant (3: 46.7) and the Gutmann donor
number (29.8) of DMSO is higher than that of CDCl3 (3: 4.8, DN:
4.0). The high ability of the DMSO as a strong electron donor to
interact with the acidic C5–H proton of triazole through
hydrogen bonding (C–H/DMSO) is typically well associated
1
with the downeld shi of the HNMR resonance of this CH
Fig. 4 TEM images of click-MNPs/CS support (a) and Pd@click-MNPs/
CS (b).
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proton, and similar correlations have been previously described
by other groups for C–H/anion interactions.9
In order to further study of the C5–H triazole interaction, the
1H NMR spectra of the 1-benzyl-4-phenyl-1H-1,2,3-triazole
compound in the gas phase, in DMSO and in CDCl3 solvent
have been investigated theoretically and the effect of the
solvents on the most stable structure of the compound was
evaluated. All the calculations have been carried out using
Gaussian 09 Quantum Chemistry package.48 DFT methods
employing Becke, 3-parameter, Lee–Yang–Parr (B3LYP) func-
tional49 were used for the calculations. 6-311++G (2df, p) basis
set was employed. The calculations in the presence of solvents
have been done by the SMD method.50 At rst, the structure of 1-
benzyl-4-phenyl-1H-1,2,3-triazole compound was optimized in
three different cases, in the gas phase, in DMSO and in CDCl3
solvent. As shown in Fig. 6, the selected dihedral angle (28N–
29C–7C–8C) is 53.48, 57.23 and 72.88 in the gas phase, in the
Fig. 7 The interactions between DMSO and the triazole C–H proton.
CDCl3 and in the DMSO solvent, respectively, which displays the was different in the different solvents with diverse polarity such
inuence of the solvent and its polarity on the optimized as DMSO and CDCl3 and in the gas phase.
structure. The size of these dihedral angles may well conrm
the existence of the interactions between DMSO and the triazole
C–H proton (Fig. 7). Then, the 1H NMR spectra of the compound
2.2. Catalytic properties of Pd-composite
in three different cases have been calculated. The obtained In this catalyst system, the covalently-attachment of CS chains
1
results showed that the HNMR signal of triazole C–H proton as organic section with inorganic magnetic nanoparticles via N-
rich triazole connectors could provide a high stability and
a desirable dispersion of Pd sites in the catalyst texture. More-
over, the hydrophilic nature of CS polymer provides a means of
good dispersion of magnetic nanoparticles in polar medium for
developing this system as a heterogeneous Pd catalyst for
organic reactions in water-containing and polar reaction media.
2.2.1. Carbonylation of aryl halides with formic acid.
Catalytic activity of the Pd@click-MNPs/CS was evaluated in the
formic acid-based carbonylation of aryl halides. The optimiza-
tion process of our carbonylation methodology was initiated
using 4-iodoanisole as the aryl halide substrate. The effects of
the reaction conditions such as solvent, base, temperature and
catalyst amount were examined on the yields of the model
reaction (Table 1).
In order to respect the principles of green chemistry, only
eco-friendly solvents such as mixed EtOH/H2O (1 : 1, v/v) and
polyethylene glycol (PEG-200) were tested in experiments and
PEG gave the best result. The concentration of base, formic acid
and DCC was also surveyed in this reaction system and found to
be important. Table 1 shows a summary of the optimization
experiments, in which the optimized conditions were recognized
as iodobenzene (1 equiv.), HCOOH (10 equiv.), DCC (2 equiv.),
catalyst loading (0.3 mol%), Na3PO4 (1 equiv.) in PEG-200 at 105 ꢂC
(Table 1, entry 4). We then explored the scope and limitations of
this transformation and several aryl halides were studied (Scheme
3). The aryl iodides and bromides were active in this reaction
system and aryl boronic acids were found to be inactive. The
electronic effects of aryl halides functionalized with various groups
Fig. 6 Optimized structure of 1-benzyl-4-phenyl-1H-1,2,3-triazole
compound in the gas phase and in different solvents (DMSO and
CDCl3) calculated at B3LYP/6-311++G(2df,p) level of theory applying
SMD method. The selected dihedral angle (28N–29C–7C–8C) is
53.48, 57.23 and 72.88 in the gas phase, in the CDCl3 and in the DMSO
solvent, respectively.
such as nitro, methoxy and methyl were also examined and it was
found that aryl halides substituted with electron-donating groups
compared to the electron-withdrawing substituents gave better
yields. The methoxy- and methyl-aryl iodides afforded the corre-
sponding aldehydes in excellent yields; however, low yield was
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Table 1 Optimization of the reaction conditions for the carbonylation couplinga
Entry
Solvent
Base
Catalyst [mol%]
DCC
Temp. (ꢂC)
Yieldc [%]
1
2
3
4
5
6
7
8
H2O/EtOH (1 : 1)
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
Na3PO4 (0.5 equiv.)
Na3PO4 (0.5 equiv.)
Na3PO4 (0.5 equiv.)
Na3PO4 (1 equiv.)
K2CO3 (0.5 equiv.)
K2CO3 (0.5 equiv.)
K2CO3 (0.5 equiv.)
K2CO3 (0.5 equiv.)
K2CO3 (0.5 equiv.)
—
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.1
0.3
—
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
—
1 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
Reux
105
60
10
80
20
85
80
25
50
60b
10
60
—
85
—
—
105
105
105
105
105
105
105
105
105
105
105
9
10
11
12
13c
14d
K2CO3 (0.5 equiv.)
DABCO (2 equiv.)
Na3PO4 (1 equiv.)
Na3PO4 (1 equiv.)
0.3
—
—
PEG
a
Reaction conditions:: 4-iodoanisole, HCOOH, DCC, catalyst, base, solvent (3 mL), 3 h. b Formic acid: 4 equiv. c Pure chitosan was used as catalyst.
Fe3O4/chitosan composite was used as catalyst.
d
observed using nitro-aryl iodides as the substrate. The Scheme 3 times, while in the most previous reports, about 3-5 mol% of Pd
depicts a number of aldehydes synthesized by the Pd@click-MNPs/ catalysts combined with the phosphine ligands promote the
CS catalyst.
reaction process.36,39,51–54 Most of the reported formic acid-assisted
Compared with the previous works reported by other formylation reactions have been performed under homogeneous
researchers on the pd-catalyzed carbonylation reactions, the catalytic conditions; meanwhile, some researchers have recently
most noteworthy advantages of our formylation methodology are directed their studies toward the use of heterogeneous Pd-catalysts
eco-friendly, mild, phosphine-free conditions, reusability and low for CO gas free formylation reactions.55,56
Pd-loading (0.3 mol%) of catalyst, good yields and short reaction
Based on our results and the other reports52,54 we have
proposed a detailed mechanism for formic acid-assisted for-
mylation reaction of aryl halides using Pd@click-MNPs/CS
catalyst, in which the carbon monoxide molecule is formed in
situ from the reaction of formic acid with DCC activator
(Scheme 4). According to this mechanistic model, the free CO
generated in the reaction media interacts with palladium-
catalyst and forms an acyl-Pd intermediate in the catalytic
cycle, next, followed by decarboxylation process and the
reductive elimination reaction, the desired product is obtained.
In this reaction system, the hydrogen source of the aldehyde
products is considered to be formic acid and indeed, formic
acid plays a dual role as both CO and hydrogen source to give
the aldehyde groups.
The carbonyl group is one of the most common of the
functional groups, and the carbonyl containing compounds are
probably the most important class of organic molecules.
Therefore, the construction of efficient catalytic systems for
selective synthesis of these compounds will be noteworthy to
follow in the near future.
2.2.2. Suzuki cross-coupling reaction. Encouraged by the
obtained results in the carbonylation reaction, we also investi-
gated the potential of our Pd@click-MNPs/CS catalyst in Suzuki
cross-coupling reaction. At rst, the reaction conditions were
Scheme
catalyst.
3 The formylation reaction using Pd@click-MNPs/CS
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was examined (Table 2). It was observed that the base has not
a signicant effect on the yields in this reaction system (Table 2,
entries 4–7). Based on the results, the optimized conditions
were chosen as follows: EtOH/H2O (1 : 1, v/v) as the solvent,
K2CO3 as the base with catalyst loading 0.2 mol% at room
temperature (Table 2, entry 6).
Aer optimization, we examined the reactivity of various
types of aryl halides in this catalytic reaction system and it was
found that the different biaryl derivatives were successfully
synthesized by Pd@click-MNPs/CS catalyst using the Suzuki
cross-coupling process (Table 3).
The electronic effects on the reaction times and yields were
evaluated and electron-rich as well as electron-poor aryl iodides
proceeded excellently to give the biaryl products. However, the
reaction of nitro-aryl bromides afforded low yields at room
temperature (Table 3 entry 14), while in contrast, methoxy-aryl
bromide required shorter reaction times and gave brilliant
yields (Table 3 entry 9). In the case of 4-nitrobromobenzene,
when the reaction performed at room temperature, the corre-
sponding coupled product was achieved in 25% of yield aer
20 h, however, the yield of the product increased to 95% when
the reaction was conducted at 70 ꢂC for 1 h. Various functional
groups such as cyano, methoxy, halogen, and carbonyl
substituted on the aryl halides were compatible with this pd-
Scheme 4 Proposed mechanism of the carbonylation reaction.
Table 2 Optimization of the reaction conditions for the Suzuki–
Miyaura couplinga
Table 3 Scope of aryl halides in Suzuki–Miyaura coupling reaction
with phenylboronic acid a
Entry
Solvent
Base
Catalyst [mol%]
Yieldc [%]
1
2
3
4
5
6
H2Ob
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na3PO4
KOH
0.4
0.4
0
0.2
0.1
0.2
0.2
—
85
100
—
100
97
100
98
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
t
[h]
Entry
R (X)
T [ꢂC]
Yieldb [%]
1
2
3
4
5
6
7
8
4-OCH3 (I)
4-NO2 (I)
4-CH3 (I)
4-COCH3 (I)
3-NO2 (I)
H (I)
4-COCH3 (Br)
3-COCH3 (Br)
4-OCH3 (Br)
4-COH (Br)
4-CN (Br)
4-Cl (Br)
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
80
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
70
1
1
2
2
1
1
2
20
1
2
1
1
3
20
1
20
3
100
100
96
100
100
95
7
8d
9e
K2CO3
K2CO3
—
—
—
a
Reaction conditions: 4-iodoanisole (0.1 mmol), phenylboronic acid
b
(0.12 mmol), H2O/EtOH (1 : 1, v/v) 2 mL, base (2 equiv.), 1 h. In the
presence of TBAB (1 equiv.). GC yield. Pure chitosan was used as
catalyst. e Fe3O4/chitosan composite was used as catalyst.
95
50
c
d
9
100
95
100
100
95
25
95
—
—
10
11
12
13
14
15
16
17
2-Cl (Br)
optimized using the cross-coupling of 4-iodoanisole with phe-
nylboronic acid as the model reaction. Due to environmental
concerns, only EtOH and water were tested as the solvent (Table
2). Using pure water as the solvent in the presence of TBAB,
moderate yield was observed (Table 2, entry 1). While, in the
presence of aqueous alcohol (H2O/EtOH, 1 : 1 v/v) as the
solvent, excellent results were achieved (Table 2, entries 2–7).
The superiority of the aqueous mixed solvents could be related
to the better solubility of the organic reagents and the inorganic
phase in the reaction medium. Next, the inuence of numerous
reaction factors such as base, temperature and catalyst loading
4-NO2 (Br)
4-NO2 (Br)
2-NO2 (Br)
H (Cl)
70
r.t.
18
r.t.
6
90
a
Reaction conditions: aryl halide (1 mmol), Phenylboronic acid (1.2
b
mmol), K2CO3 (2 mmol), solvent 3 mL. GC yield.
20818 | RSC Adv., 2021, 11, 20812–20823
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however, under the same reaction conditions, the use of reac-
ꢂ
tion temperature (80 C) increases the yield of the correspond-
ing coupled product up to 100%. Under these conditions, the
reaction of 1,4-diiodobenzene substrate with naphthalene-1-
boronic acid and naphthalene-2-boronic acid was also investi-
gated. As shown in Scheme 5, both iodide groups in 1,4-diio-
dobenzene were completely reacted with naphthalene-1-
boronic acid and the corresponding coupled product 1,4-
di(naphthalen-1-yl)benzene (a) was formed in excellent yield.
However, in the reaction of 1,4-diiodobenzene with
naphthalene-2-boronic acid, the Suzuki product 2-(4-iodo-
phenyl)naphthalene (c) was formed as the main product.
We proposed a possible mechanism for the Suzuki–Miyaura
coupling of 1,4-diiodobenzene with arylboronic acids using
Pd@click-MNPs/CS as the catalyst, which is in consisted with
previous reports.57 As shown in Scheme 6, a Pd(0)/Pd(II) catalytic
cycle via an oxidative addition/reductive elimination mecha-
nistic pathway has been reported, in which a complex Pd(II)
intermediate undergoes reductive elimination to expel the
coupled products.
Scheme 5 Reaction conditions: 1,4-diiodobenzene (1 equiv.), aryl-
boronic acid (2.2 equiv.), catalyst (0.4 mol%), K2CO3 (4 equiv.), H2O/
EtOH (1 : 1, v/v), 80 ꢂC.
catalyst system. Aryl chlorides were inactive in this system. The
steric effects was evaluated and it was observed that an
increasing hindrance in the vicinity of the leaving group using
2-bromonitrobenzene (Table 3, entry 16) led to a fall in the yield,
however, the reaction by 2-bromochlorobenzene required long
times, giving excellent conversion (Table 3, entry 13).
We also studied the efficiency of catalyst using 1,4-diiodo-
benzene as the substrate with various boronic acids (Scheme 5).
It was found that the treatment of 1,4-diiodobenzene (1 equiv.)
with phenylboronic acid (2 equiv.) in the presence of K2CO3 as
the base (4 equiv.), H2O/EtOH (1 : 1, v/v) as the solvent with
catalyst loading 0.4 mol% at room temperature affords the
coupled product 1,1':40,100-terphenyl (b) in 75% yield aer 2 h,
The present catalyst system was compared with published
reaction conditions reported by other groups for the Suzuki
coupling of 4-bromoanisole with phenylboronic acid, and the
results are summarized in Table 4. The present catalyst can be
one of the best catalysts in terms of low reaction time, lower
temperature, green media, easy separation and efficient
recycling.
2.3 Efficiency and stability of Pd-composite
To obtain more information about the selectivity and reactivity
of Pd@click-MNPs/CS catalyst, we designed a reaction mixture
containing the Suzuki and carbonylation reaction
Scheme 6 The Suzuki reaction mechanism of 1,4-diiodobenzene.
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Table 4 Comparison of the catalytic activity with literature examples for the Suzuki reaction between 4-bromoanisole and phenylboronic acid
Entry
Catalyst Ref
Reaction conditions
Yield (%)
1
2
3
4
5
Pd74Cu73 DENs58
HEC-NHC-Pd59
PET@IL/Pd60
Catalyst (1 mol%), EtOH/H2O (3 : 1), K2CO3, MW 50 ꢂC, 0.75 h
Catalyst (0.4 mol%), EtOH/H2O (3 : 2), K2CO3, 60 ꢂC, 2 h
Catalyst (0.1 mol%), H2O, K2CO3, 55 ꢂC, 45 min
99.7
84
80
91.3
100
MOP-BPY(Pd)61
Present work
Catalyst (0.1 mol%), MeOH/H2O (1 : 1), Na2CO3, 80 ꢂC, 12 h
Catalyst (0.2 mol%), EtOH/H2O (1 : 1), K2CO3, rt, 1 h
Scheme 7 Reaction selectivity of Pd@click-MNPs/CS for the Suzuki
Fig. 9 The FT-IR spectra of fresh (a) and recovered (b) catalyst.
and carbonylation reaction.
at least four times without signicant decrease in the catalytic
activity.
The FT-IR spectra of the fresh and recovered catalyst from
Suzuki reaction have been shown in Fig. 9. As can be seen, the
recycled catalyst exhibited characteristic peaks of the chitosan
(N–H, O–H, C–H, C–O) and Fe3O4 (Fe–O–Fe) similar to those of
the fresh one, which indicate the stability of catalyst structures
during the reaction process in aqueous media.
Fig. 8 The reusability results of the catalyst. Suzuki: 4-iodoanisole,
carbonylation 4-iodotoluene as the substrates under the optimized
conditions.
3. Conclusions
The present work is a study on the potential of 1,2,3-triazole
frameworks in C–H/X hydrogen bonding and metal-based
catalyst design. The triazoles prepared from azides and
alkynes are compounds beyond the sum of their clicking
building blocks. These N-heterocyclic frameworks offer the
interactions with both metal cations and anion species. The
environmentally friendly nanocatalyst Pd@click-MNPs/CS
exhibited a good catalytic activity in CO gas-free carbonyla-
tion reaction of aryl halides and HCOOH to prepare a diversity
of aromatic aldehydes in moderate to excellent yields. This
heterogeneous nanocatalyst was also evaluated in Suzuki
cross-coupling reaction of diverse aryl halides and phenyl-
boronic acid in aqueous solvent at room temperature. The
utilize of triazole entities as the C–H hydrogen bond donor,
stable linker and powerful N-donor ligand, especially for the
surface modication, covalently attachments and the catalyst
immobilization is noteworthy to follow in the future
researches due to effective and facile construction of these
frameworks.
experimental conditions (Scheme 7). It was found that by
employing iodobenzene (1 equiv.), phenylboronic acid (1.2
equiv.), Na3PO4 (2 equiv.), catalyst (0.3 mol%), DCC (2 ꢂequiv.)
and HCOOH (10 equiv.) in the PEG-200 solvent at 105 C, the
benzaldehyde was obtained as the only reaction product in
98% yield aer 3 h.
The recyclability of heterogeneous catalyst is a key factor that
is directly related to the catalyst stability and is a very essential
topic from both the economic and environment points of view,
especially for costly pd-based catalysts. Therefore, we carried
out further investigations about the reusability and recovery of
the Pd@click-MNPs/CS catalyst using the carbonylation and
Suzuki cross-coupling reaction. Aer completing each reaction,
the magnetic catalyst could be efficiently separate from the
reaction media by an external magnet and reused for the next
run. As demonstrated in Fig. 8, the catalyst could be recycled for
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4.5. Synthesis of Pd@click-MNPs/CS
4. Experimental
The click-MNPs/CS (0.1 g) was sonicated for 5 min and mixed with
palladium acetate (0.008 g) in dry toluene (10 mL). The reaction
mixture was stirred at room temperature for 2 days. Then, the
magnetic phase was separated by a permanent magnet, washed with
toluene and acetone several times and dried at room temperature.
4.1. Synthesis of Fe3O4 nanoparticles
A solution of Iron(III) chloride and Iron(II) chloride (1 : 2 molar
ratio) was prepared in a three-necked round bottom ask using
deiꢂonized (DI) water under nitrogen with vigorous stirring at
85 C.62 Then, ammonia solution was added gradually into the
above mixture while the reaction mixture was stirred strongly
until pH reached about 9. Aer 2 h stirring at 85 ꢂC, a black
precipitate (Fe3O4) was magnetically collected, washed with
distilled water and ethanol and dried.
4.6. General procedure for the carbonylation reaction
Aryl halide (1 mmol), Na3PO4 (1 mmol), catalyst (0.3 mol%),
DCC (2.0 mmol) and PEG-200 (3.0 mL) were mixed in a round-
bottom ask. Then, formic acid (10 mmol) was added to the
reaction mixture, immediately sealed, and the mixture was heated
in an oil bath at 105 ꢂC. Aer the completion of the reaction
process and cooling, the CH2Cl2 was added to the reaction mixture
and the catalyst was separated by a permanent magnet, washed
with CH2Cl2/acetone and then reused for recycling experiments.
The crude product was puried by silica gel-column chromatog-
raphy to obtain the corresponding aldehyde product.
4.2. Synthesis of azide-decorated Fe3O4 nanoparticles
At rst, the 3-azidopropyltrimethoxysilane was prepared using
3-chloropropyltrimethoxysilane.19
3-Chloropropyltrimethoxysilane (0.92 mL, 5 mmol), tetrabuty-
lammonium bromide (TBAB, 0.32 g, 1 mmol), NaN3 (0.46 g, 7
mmol) and dry acetonitrile (30.0 mL) were mixed under
nitrogen atmosphere. The resulting mixture was reuxed for
24 h. Aer completion and cooling, the acetonitrile was evap-
orated using rotary. The residue was then treated with Et2O (20
mL), ltered and washed with Et2O. The etheric phase was
evaporated to obtain 3-azidopropyltrimethoxysilane, and the
product was mixed with 50 mL ethanol for the next step reaction
with Fe3O4 nanoparticles.
The fresh prepared Fe3O4 particles (1 g) were ultrasonically
suspended in ethanol and then the solution of 3-azidopropyl-
trimethoxysilane in ethanol was added. The mixture was heated
at 40 ꢂC under nitrogen for 18 h. Then, the obtained azide-
functionalized magnetic particles were magnetically separated
and washed thoroughly with ethanol and dried under vacuum.
The production of 3-azidopropyltrimethoxysilane and the
successful graing of azide groups on the magnetic particles
surface were approved using FT-IR analysis.
4.7. General procedure for Suzuki–Miyaura coupling
reaction
Aryl halide (1 mmol), K2CO3 (2.0 mmol), phenylboronic acid (1.2
mmol), catalyst (0.2 mol%) and 3 mL H2O/EtOH (1 : 1) were trans-
ferred into a round-bottom ask. The reaction mixture stirred at
room temperature and monitored by gas chromatography (GC).
Aer completion, the reaction mixture was diluted with EtOH and
the catalyst was magnetically recovered by a permanent magnet,
washed with EtOH and then reused. The corresponding biaryl
product was extracted with EtOAc several times and the organic
phase was collected, dried over CaCl2, ltered and concentrated. The
puried product was obtained by recrystallization or column chro-
matography (EtOAc : n-hexane) on silica gel, and characterized.
Conflicts of interest
4.3. Synthesis of alkyn-functionalized chitosan
All authors declare that they have no conict of interest asso-
Propargyl bromide (8 mmol) was added dropwise, while stirring
to a mixture of DMF (10 mL) and 0.3 g chitosan (MW: 190 000–
310 000, DD: 84%) in a round-bottom ask. The mixture was
stirred at room temperature for 2 h and heated at 60 ꢂC for 20 h.
Next, the reaction mixture was cooled to room temperature and
added gradually to a beaker containing distilled water to
precipitate the alkynylated chitosan. The resulting solid was
collected by centrifugation, washed thoroughly with acetone
several times and dried. FT-IR (KBr, cmꢁ1): 3432 (OH), 2926–
2850 (aliphatic C–H), 2100 (C^C), 1072 (C–O).
ciated with this publication.
Acknowledgements
We would like to thank the Iran National Science Foundation,
INSF, for the support of the work with the proposal no.
95007799. The authors also acknowledge the Persian Gulf
University Research Council and Isfahan University of Tech-
nology (IUT), IR of Iran.
Notes and references
4.4. Synthesis of click-MNPs/CS
Azide-functionalized Fe3O4 particles (0.1 g) and alkynylated chito-
san (0.2 g) were mixed in 15 mL H2O/DMF (1 : 1), and then Cu2O
(0.015 g) was added as the catalyst. The reaction mixture was
stirred vigorously and heated in an oil bath at 60 ꢂC for 3 d. Aer
completion, the solid particles were magnetically separated,
washed several times with MeOH, H2O and ammonia solution (to
remove copper impurities) and then, dried under vacuum.
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