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

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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 Hosseini^cand 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.^4,5Therefore, 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.³Heterogenization 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”.^7,81,2,3-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,^9–11catalysis.^12,13

surface modication and material science.^14,15This 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/X^nꢁinteractions were accrued through

hydrogen bonding and electrostatic interactions.^10,11Indeed,

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.^27,28The 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

Fe₃O₄nanoparticles 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,^17,29–31in 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 Fe₃O₄nanoparticles (MNPs), in

which polymer chains and Fe₃O₄particles 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.^8,18Such

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.²⁴prepared 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.²⁵reported

a mesostructured Fe₃O₄/chitosan composite as a pH-responsive

drug-delivery system. Movassagh et al.²⁶have 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.^32–34To date, many

researchers have focused their studies on the new CO gas-free

carbonylation systems^35–37and reported the various carbonyl

sources such as formic acid,^38,39Mo(CO)₆,⁴⁰oxalic acid³⁶and N-

formylsaccharin.⁴¹Formic acid is an eco-friendly, cheap and

available carbonyl source and can generate a carbon monoxide

(CO) molecule in situ during the reaction process.⁴²The formic

acid-assisted carbonylation catalyzed by palladium catalysts^38,39

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 CDCl₃solvents.

In this part, the eﬀects of the solvent and its polarity on the

Fig. 1 FTIR spectra of Fe₃O₄nanoparticles (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 CDCl₃solvent

were theoretically investigated.

groups graed on the surface of Fe₃O₄particles 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 cm^ꢁ1and 2099 cm^ꢁ1attributed

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., Fe₃O₄particles^43,44and

chitosan (CH, CO, CN, and OH stretching), conrming

successful attachment of Fe₃O₄nanoparticles with chitosan

backbone.

2.1.2. Synthesis and characterization of Pd/support

sample. The resulting click-MNPs/CS was further treated with

Pd(OAc)₂in 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 Fe₃O₄nanoparticles are clearly

FTIR spectroscopy. In the rst stage of our strategy, the chi-

tosan and Fe₃O₄were 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 cm^ꢁ1in

FT-IR spectrum of alkynylated chitosan attributed to the alkyne-

groups, and also the N₃vibration at 2099 cm^ꢁ1in spectrum of

azide-functionalized Fe₃O₄nanoparticles (Fig. 1c and d). In

addition, the intensity of the band at 1602 cm^ꢁ1(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).

Fe₃O₄cores are surrounded by a grey polymer shell of about 3–6 nm

thick and the average size of the Fe₃O₄cores is about 10–20 nm

(Fig. 4a). Aer incorporating of palladium, the Pd nanoparticles are

not clearly distinguishable from Fe₃O₄particles (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.^18,45Additionally, the broad peak

from 2q ¼ 15^ꢂto 25^ꢂis 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

Fe₃O₄particles 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.⁴⁶The 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 (X^ꢁn) binding.⁸We 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.^9,47The 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 CDCl₃solvent

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.⁴⁸DFT methods

employing Becke, 3-parameter, Lee–Yang–Parr (B3LYP) func-

tional⁴⁹were 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.⁵⁰At 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.

CDCl₃and 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 CDCl₃and 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 CDCl₃and 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 coupling^a

Entry

Solvent

Base

Catalyst [mol%]

DCC

Temp. (^ꢂC)

Yield^c[%]

1

2

3

4

5

6

7

8

H₂O/EtOH (1 : 1)

PEG

Na₃PO₄(0.5 equiv.)

Na₃PO₄(1 equiv.)

K₂CO₃(0.5 equiv.)

—

0.3

0.1

0.3

—

2 equiv.

—

1 equiv.

2 equiv.

Reux

105

60

10

80

20

85

80

25

50

60^b

10

60

—

85

—

105

9

10

11

12

13^c

14^d

K₂CO₃(0.5 equiv.)

DABCO (2 equiv.)

Na₃PO₄(1 equiv.)

0.3

—

PEG

a

Reaction conditions:: 4-iodoanisole, HCOOH, DCC, catalyst, base, solvent (3 mL), 3 h. ^bFormic acid: 4 equiv. ^cPure 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 reports^52,54we 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,

K₂CO₃as 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 coupling^a

Table 3 Scope of aryl halides in Suzuki–Miyaura coupling reaction

with phenylboronic acid ^a

Entry

Solvent

Base

Catalyst [mol%]

Yield^c[%]

1

2

3

4

5

6

H₂O^b

K₂CO₃

Na₃PO₄

KOH

0.4

0

0.2

0.1

0.2

—

85

100

—

100

97

100

98

H₂O/EtOH (1 : 1)

t

[h]

Entry

R (X)

T [^ꢂC]

Yield^b[%]

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.

80

r.t.

70

1

2

1

2

20

1

2

1

3

20

1

20

3

100

96

100

95

7

8^d

9^e

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. ^eFe₃O₄/chitosan composite was used as catalyst.

95

50

c

d

9

100

95

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)

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.

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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.⁵⁷As 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 K₂CO₃as

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

Pd₇₄Cu₇₃DENs⁵⁸

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), Na₂CO_3,80 ^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 Fe₃O₄nanoparticles

A solution of Iron(^III) chloride and Iron(II) chloride (1 : 2 molar

ratio) was prepared in a three-necked round bottom ask using

dei_ꢂonized (DI) water under nitrogen with vigorous stirring at

85 C.⁶²Then, 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 CH₂Cl₂was 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 Fe₃O₄nanoparticles

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 Fe₃O₄nanoparticles.

The fresh prepared Fe₃O₄particles (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, cm^ꢁ1): 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 Fe₃O₄particles (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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Article Doi

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

DOI: 10.1039/d1ra03356e

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Article abstract of DOI:10.1039/d1ra03356e

Full text of DOI:10.1039/d1ra03356e

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