Synthesis of a Cellulosic Pd(salen)-Type Catalytic Complex as a Green and Recyclable Catalyst for Cross-Coupling Reactions

Article abstract of DOI:10.1007/s10562-020-03172-5

Abstract: A green recyclable cellulose-supported Pd(salen)-type catalyst was synthesized through sequential three steps: chlorination with thionyl chloride, modification by ethylenediamine, and the formation of Schiff base with salicylaldehyde to immobilize palladium chloride through multiple binding sites. This novel heterogeneous cellulosic Pd(salen)-type catalytic complex was fully characterized by FT-IR, SEM, TEM, XPS, ICP-AES and TG. The traditional cross-coupling chemistry, such as Suzuki, Heck, Sonogashira, Buchwald–Hartwig amination and etherification, was then investigated in the presence of the above cellulose-palladium nanoparticle. Studies have shown that the synthesized catalyst shows high activity and efficiency for all types of transformations, providing the corresponding carbon–carbon or carbon–heteroatom coupling products in a general and mild manner. Furthermore, the catalyst demonstrates high to excellent yields and is easily recycled by simple filtration for up to twelve cycles without any significant loss of catalytic activity. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03172-5

Catalysis Letters
https://doi.org/10.1007/s10562-020-03172-5
Synthesis of a Cellulosic Pd(salen)‑Type Catalytic Complex as a Green
and Recyclable Catalyst for Cross‑Coupling Reactions
Peng Sun1,2 · Jiaojiao Yang · Chunxia Chen · Kaijun Xie · Jinsong Peng
1
1,2
1
1
Received: 25 November 2019 / Accepted: 8 March 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A green recyclable cellulose-supported Pd(salen)-type catalyst was synthesized through sequential three steps: chlorination
with thionyl chloride, modifcation by ethylenediamine, and the ꢀormation oꢀ Schiꢁ base with salicylaldehyde to immobilize
palladium chloride through multiple binding sites. This novel heterogeneous cellulosic Pd(salen)-type catalytic complex
was ꢀully characterized by FT-IR, SEM, TEM, XPS, ICP-AES and TG. The traditional cross-coupling chemistry, such as
Suzuki, Heck, Sonogashira, Buchwald–Hartwig amination and etherifcation, was then investigated in the presence oꢀ the
above cellulose-palladium nanoparticle. Studies have shown that the synthesized catalyst shows high activity and eꢂciency
ꢀ
or all types oꢀ transꢀormations, providing the corresponding carbon–carbon or carbon–heteroatom coupling products in a
general and mild manner. Furthermore, the catalyst demonstrates high to excellent yields and is easily recycled by simple
fltration ꢀor up to twelve cycles without any signifcant loss oꢀ catalytic activity.
Graphic Abstract
Ar
X
X = Cl, Br, I
+
Ar
B(OH)
R
OH, NH2
2
R
N
N
Pd
O
Pd
O
N
H
N
H
H
OH
H
H
H
OH
H
H
Cl
H O
H
H
Cl
H O
H
H
H O
OH
H O
OH
three steps
N
Pd
O
O
O
O
H
N
H
O
O
O
n
Cl
H
H
H
OH
H
OH
H
O
H
H
n
OH
OH
H
H
OH
OH
O
O
OH
H
OH
Ar
Ar
R
Z
Ar
Ar
Ar
R
Z = O, NH
Keywords Cellulose · Salen-type ligand · Pd catalysis · Heterogeneous · Cross-coupling reactions
Electronic Supplementary Material The online version oꢀ this
article (https://doi.org/10.1007/s10562-020-03172-5) contains
supplementary material, which is available to authorized users.
Extended author inꢀormation available on the last page oꢀ the article
Vol.:(0
1
123456789)
3

P. Sun et al.
1
Introduction
nano-palladim catalyst ꢀor Heck reaction in a mild condi-
tion [43]. Aꢀter this, a lot oꢀ similar studies were reported
[44, 45]. However, due to the toxicities oꢀ traditional phos-
phine ligands, air sensitivity and high cost, which will
limit the application in the feld oꢀ catalysis. Schiꢁ bases
have been used as powerꢀul and useꢀul ligands ꢀor promot-
ing transition metal-based catalysis [46, 47]. In addition,
Schiꢁ bases have advantages over ꢀree phosphine ligands,
N-heterocyclic carbene, and amines ligands because oꢀ
their superior electronic properties, diꢁerent oxidation
states, non-toxicity, and ease oꢀ preparation, which are
particularly signifcant ꢀor retaining metal species and
meeting the demand oꢀ sustainable chemistry [48–50].
Herein, we designed and prepared a new, eꢂcient and
recyclable heterogeneous palladium catalyst based on
ꢀunctionalized cellulose as a biopolymers support. At frst,
microcrystalline cellulose is chlorinated by thionyl chlo-
ride, then modiꢀied using ethylenediamine, and ꢀinally
reacted with simple and aꢁordable salicylaldehyde to ꢀorm
Cell-Schiꢁ base. Aꢀterwards, Cell‐Schiꢁ base was added
to a PdCl2 solution to obtain the cellulosic Pd(salen)-type
catalyst (Scheme 1). The prepared catalyst is characterized
by FT-IR, SEM, TEM, XPS, ICP-AES and TG techniques
and was then used ꢀor cross-coupling reactions oꢀ between
haloarenes and diꢁerent nucleophilic partners to obtain cou-
pling products.
Pd-catalyzed cross-coupling reactions, such as
Suzuki–Miyaura, Heck, Sonogashira, Buchwald–Hartwig
amination and etherifcation, have emerged as powerꢀul
tools ꢀor carbon–carbon and carbon–heteroatom bond ꢀor-
mation in synthetic organic chemistry [1, 2], displaying
broad applications in the preparation oꢀ materials, natu-
ral products and biologically active compounds [3, 4].
Generally, these coupling reactions are carried out in the
presence oꢀ a homogeneous palladium catalyst. However,
the high costs associated with palladium and ligands, the
diꢂculties in product separation and recycling oꢀ the cata-
lyst, and the lack oꢀ generality toward all types oꢀ cross-
coupling reactions obviously do not meet the prospects oꢀ
contemporary green and sustainable chemistry develop-
ment [5, 6]. To overcome these disadvantages, researchers
have turned to the design and development oꢀ palladium
complexes with broader activity immobilized on environ-
mentally benign supports. Over the past decades, various
solid materials, such as polymers [7, 8], activated carbon
[
9], clays [10], and magnetic nanoparticles (MNPs) [11]
have been perꢀormed.
Recently, as people attach great importance to the
increasingly severe ecological environment, the replace-
ment oꢀ petrochemical-based materials with biodegradable
materials has attracted more and more research priorities.
A variety oꢀ biodegradable polysaccharides such as chi-
tosan [12–17], agar [18, 19] and starch [20] supported
heterogeneous catalysts ꢀor organic synthetic chemistry
came into being. Compared with other carbon sources,
microcrystalline cellulose has a higher specifc surꢀace
area, high thermal and chemical stability, and is one oꢀ
the most abundant, renewable and ecoꢀriendly nature car-
bon precursors in the world [21–24]. Thus, a series oꢀ
cellulose-supported metal nanoparticles such as Ag [25,
2 Materials and Methods
2.1 Materials and Instrumentation
All chemicals were used as received unless otherwise stated.
Microcrystalline cellulose, thionyl chloride, ethylenedi-
amine, salicylaldehyde, palladium chloride (PdCl2), aryl
boronic acid, aryl halides, aryl acetylene, styrene, aniline,
and phenol were purchased ꢀrom Shanghai Energy Co. Ltd,
China. All other reagents and solvents such as N,N-dimeth-
ylꢀormamide (DMF), dimethyl sulꢀoxide (DMSO), ethyl
alcohol, acetone, potassium carbonate, ethyl acetate (EA),
and MgSO4 were purchased ꢀrom Tianjin Fuyu Chemical
Co. Ltd, China.
2
6], Al [27], Au [28, 29], Cr [30], Cu [31, 32], Pd [33, 34],
Pt [35] Rh [36] were developed. Among these, palladium
nanoparticles have garnered maximum attention due to
their versatile catalytic activity ꢀor many organic reactions
including cross-coupling [37], hydrogenation [38], cycli-
zation [39] and oxidation [40].
As early as 2006, Reddy prepared a cellulose sup-
ported Pd(0) catalyst ꢀor Heck and Sonogashira reactions
with aryl iodides as substrates [41]. However, since Pd
was bound to cellulose only by adsorption, the catalytic
activity was signifcantly decreased due to leaching and
aggregation oꢀ Pd(0) aꢀter 4 cycles. Thereꢀore, the strat-
egy through ligand-anchored cellulose to coordinate with
palladium was rapidly developed to solve problems like
catalyst deactivation and metal leaching [42]. In 2012,
Li carried out diphenylphosphinite cellulose-supported
The Fourier Transꢀorm Inꢀrared Spectrometer (FT-IR)
spectra oꢀ the samples were obtained by a Perkin Elmer
Spectrum 100 FT-IR spectrophotometer. Scanning Elec-
tron Microscope (SEM) images oꢀ all products were taken
on a JSM-7500F. Transmission Electron Microscope
(TEM) image oꢀ the catalyst was perꢀormed on a JEM-
2100*operated at an accelerating voltage oꢀ 200 kV. X-ray
photoelectron spectroscopy (XPS) was measured on X, Pert3
Powedr (at 52 kV, 60 mA, 3 kW). X-Ray diꢁraction spec-
trum (XRD) was recorded on XRD-6100 Produced by Shi-
madzu Corporation oꢀ Japan (at 60 kV, 80 mA, and 2θ with
1
3

Synthesis of a Cellulosic Pd(salen)-Type Catalytic Complex as a Green and Recyclable Catalyst…
NH
2
NH2
NH2
Scheme 1 Synthetic route oꢀ
the cellulosic Pd(salen)-type
catalyst
Cl
NH
NH
OH
OH
OH
Cl
Cl
NH
excess ethylenediamine
SOCl
2
, DMF
9
0 ćꢀ 2.5 h
DMSO, 100 ćꢀ 24 K
Cl
Cl
NH NH
OH OH
H
H
OH
H
OH
Chlorocellulose(CDC)
H
O
H
O
NH
2
NH2
O
O
H
O
H
OH
H
OH
aminodeoxycellulose(ADC)
H
H
n
OH
OH
Cellulose(CL)
OH
OH
N
N
N
N
N
Pd
Cl
H
Pd
Cl
O
H
N
O
NH
NH
H
N
PdCl
2
, EtOH
Salicylaldehyde, EtOH
rt, 24 h
reflux, 24 h
Cl
N
Pd
NH
O
N
OH
Cellulosic Pd(salen)-type catalyst
CL-salen-Pd(II) )
(
Cell-Schiff base
a scan angle: 5°–80°). The palladium content oꢀ the catalyst
was investigated with a Spectro Blue Inductively coupled
plasma mass spectrometry (ICP-AES). Thermal stability oꢀ
materials was measured on a STA449F3 ꢀrom room tempera-
was dried overnight at 50 °C under vacuum aꢀter fltration
and washing with ethanol. Subsequently, PdCl (5.23 mmol,
2
195.06 mg) was added to a suspension oꢀ the dried Cell-
Schiꢁ base in ethanol, and the mixture was stirred at 80
°C to ꢀor 24 h ꢀorm cellulosic Pd(salen)-type catalyst (CL-
salen-Pd(II)). Finally, the catalyst was fltrated and washed
by ethanol, and dried overnight at 50 °C under vacuum
(Scheme 1).
1
ture to 800 ℃. The H NMR spectra were perꢀormed on a
5
00 MHz Bruker Ultrashield instrument with Chloroꢀorm-d
solvents and tetramethylsilane internal standard. TLC analy-
sis (20 cm×20 cm) was conducted on Silica Gel 60 F254.
2
.2 Modifcation oꢀ Cellulose
2.4 General Procedure ꢀor Palladium‑Catalyzed
Cross Coupling Reactions
Microcrystalline cellulose (30.8 mmol, 5 g) and DMF
(
100 mL) were added to a ꢀour-necked bottle equipped with
2.4.1 Suzuki Cross‑Coupling Reaction
a stirrer, a drying tube and an exhaust gas absorption device.
SOCl (30.8 mmol, 16.4 mL) was slowly added under stir-
A ꢃame-dried 50 mL round-bottom ꢃask equipped with a
magnetic stir bar and a rubber septum was charged with aryl
halide (1.0 mmol), phenyl boronic acid (1.1 mmol), K CO
2
ring at 80 ℃, then the mixture was reacted ꢀor 2.5 h at 90 ℃.
Aꢀter cooling to room temperature, the mixture was poured
into 500 mL oꢀ cold water. The precipitate was fltered and
washed with distilled water to neutral, the obtained chlo-
rocellulose (CDC) was then dried overnight at 50 ℃ under
vacuum. Subsequently, excess ethylenediamine was added
to a mixture oꢀ dried CDC in DMSO and then stirred at
2
3
(2.0 mmol) and CL-salen-Pd(II) (0.5% mmol). The mixture
was stirred in Ethanol: H O=1:1 (5.0 mL) at 50 ℃ under
2
air atmosphere ꢀor 1 h. The mixture was cooled to room
temperature, quenched with water (5 mL), and diluted with
ethyl acetate (5 mL). The layers were separated, and the
aqueous layer was extracted with 2×5 mL oꢀ ethyl acetate.
The combined organic extracts were dried over anhydrous
magnesium sulꢀate, fltered, and concentrated in vacuo.
Finally, the product was purifed by column chromatography.
1
00 ℃ ꢀor 24 h, the reaction was terminated by adding dis-
tilled water, and the precipitate was fltered and washed
with acetone until colorless. The product aminodeoxycel-
lulose (ADC) was dried overnight at 50 ℃ under vacuum
(
Scheme 1).
2
.4.2 Heck Reaction
2
.3 Preparation oꢀ the Catalyst
A ꢃame-dried 50 mL round-bottom ꢃask equipped with a
magnetic stirbar and a rubber septum were charged with
aryl halide (1.0 mmol), alkene substrate (1.1 mmol), K CO
Salicylaldehyde (4.76 mmol, 1.12 g) was added to a suspen-
sion oꢀ ADC (4.76 mmol, 1.12 g) in anhydrous ethanol and
stirred at room temperature ꢀor 2 h. The Cell-Schiꢁ base
2
3
(2.0 mmol) and CL-salen-Pd(II) (1.0% mmol). Then ethanol
1
3

P. Sun et al.
(
5.0 mL) was added and the mixture was heated to reꢃux
C-Cl
under air atmosphere ꢀor 3 h. Aꢀter that, the mixture was
cooled to room temperature, fltered, washed with diethyl
ether, and concentrated in vacuo. The residue was fnally
purifed by column chromatography.
C-N
a
b
C-N
C=N
C=C
2.4.3 Sonogashira Cross‑Coupling Reaction
c
A ꢃame-dried 50 mL round-bottom ꢃask equipped with a
magnetic stirbar and a rubber septum were charged with
aryl halide (1.0 mmol), terminal alkyne (1.1 mmol), CuI
d
O-Ph
O-Pd
e
(
0.05 mmol), K CO (2.0 mmol) and CL-salen-Pd(II) (1.0%
2 3
mmol). Ethanol (5.0 mL) was then added and the mixture
was heated to reꢃux under air atmosphere ꢀor 3 h. Aꢀter
4
000
3000
2000
1000
extraction with ethyl acetate, drying over anhydrous MgSO ,
-1
Wavenumber cm
4
fltration and concentration, the crude product was purifed
by column chromatography.
Fig. 1 FT-IR spectra oꢀ a CL, b CDC, c ADC, d Cell-Schiꢁ base and
e CL-salen-Pd(II)
2.4.4 Buchwald–Hartwig Amination and Etheriꢀcation
−
1
A ꢃame-dried 50 mL round-bottom ꢃask equipped with a
magnetic stirbar and a rubber septum were charged with aryl
halide (1.0 mmol), aniline (1.1 mmol) or phenol (1.1 mmol),
K CO (2.0 mmol) and CL-salen-Pd(II) (1.0% mmol).
observed at 1650 1170 and 1055 cm in the FT-IR spec-
trum oꢀ ADC (Fig. 1c), which are attributed to the stretch-
ing vibration oꢀ primary amine (V +V ) and secondary
C–N
N–H
amine (V +V ), respectively. The FT-IR spectrum oꢀ
2
3
C–N
N–H
DMSO (5.0 mL) was then added and the mixture was heated
to 80 ℃ ꢀor 12 h. The mixture was cooled to room tem-
perature, quenched with water (5 mL), and extracted with
Schiꢁ base-modifed cellulose is shown in Fig. 1d. the pecks
−
1
at about 1525 and 1255 cm assign to the stretching vibra-
tion oꢀ the benzenoid C=C and O benzene ring which come
ꢀrom Schiꢁ base structure. Besides, the stretching vibration
3
× 5 mL oꢀ ethyl acetate. The combined organic extracts
−
1
were dried over anhydrous magnesium sulꢀate, fltered, and
concentrated in vacuo. Finally, the crude product was puri-
fed by column chromatography.
oꢀ C=N bond was observed at 1625 cm , indicating that
salicylaldehyde reacted successꢀully with ADC to ꢀorm the
corresponding Schiꢁ base structure. Aꢀter the coordination
oꢀ Cell-Schiꢁ base ligand to palladium chloride (Fig. 1e),
−
1
2
.5 Recyclability Study oꢀ the Cellulosic
the new stretching vibration at 515–600 cm are due to
coordination bond which ꢀormed by the palladium ions and
N atom. This result was consistent with previous literature
reports [51, 52], which proves that our catalytic system has
been constructed successꢀully.
Pd(salen)‑Type Catalyst
In this work, cyclic perꢀormance oꢀ cellulosic Pd(salen)-type
catalyst was detected by repeated Suzuki coupling reaction.
Aꢀter fltered and washed thoroughly with water and ethanol,
the catalyst was then dried in vacuo at 50 ℃ overnight, and
reused ꢀor subsequent diꢁerent cross-coupling experiments
under similar reaction conditions (Sect. 2.4).
The surꢀace morphological studies were then carried out
by scanning electron microscopy (SEM). The SEM images
oꢀ CL, CDC, ADC, Cell-Schiꢁ base and CL-salen-Pd(II) are
shown in Fig. 2. The morphological structure oꢀ microcrys-
talline cellulose (starting material) has non-nanofbre and
non-pore characteristics (Fig. 2a). The SEM image oꢀ CDC
revealed that some cross-linked nanofbre were obtained
aꢀter dealing with thionyl chloride (Fig. 2b). The SEM oꢀ
ADC shows higher agglomeration degree compared to CDC
3
Results and Discussion
3.1 Characterization oꢀ the Catalyst
(
Fig. 2c), may be caused by the cross-linking oꢀ nanofbers
Figure 1 shows the FT-IR spectra oꢀ CL, CDC, ADC, Cell-
Schiꢁ base and CL-salen-Pd(II). The FT-IR spectrum oꢀ
CDC is shown in Fig. 1b. Compared to CL (Fig. 1a), the new
with the addition oꢀ ethylenediamine. The SEM image oꢀ
Cell-Schiꢁ base displays a ꢀoamy porous structure (Fig. 2d),
which is more benefcial to the coordination between the
ligand-anchored cellulose and palladium species. Accord-
ing to the SEM image oꢀ CL-salen-Pd(II) (Fig. 2e), it can be
observed similar porous structure to the Cell-Schiꢁ base on
−1
spectral band appears at 700 cm , which corresponds to the
stretching vibration oꢀ C–Cl bond. Aꢀter modifcation with
ethylenediamine, the new absorption peaks can be obviously
1
3

Synthesis of a Cellulosic Pd(salen)-Type Catalytic Complex as a Green and Recyclable Catalyst…
Fig. 2 SEM images oꢀ a CL, b CDC, c ADC, d Cell-Schiꢁ base, e CL-salen-Pd(II) and f TEM images oꢀ CL-salen-Pd(II)
to the (200) diꢁraction planes oꢀ cellulose [53]. From Fig. 3b
we still can clearly fnd that this peak remains in its position,
but the strength is reduced, indicating that palladium coor-
dination would not destroy the basic structure oꢀ cellulose
during catalyst preparation. In addition, the index peaks at
2
00
a
b
2
θ=40.1°, 46.6° and 68.2° correspond to diꢁractions ꢀrom
various lattice planes oꢀ (111), (200) and (220) present in the
cubic palladium [54], confrming the presence oꢀ palladium
on the cellulose.
1
11
200
2
00
The XPS spectrum oꢀ cellulosic Pd(salen)-type catalyst
was carried out to ꢀurther understand the surꢀace composi-
tion and the state oꢀ Pd crystal (Fig. 4). The binding ener-
gies oꢀ the doublet peaks locate at Pd 3d (337.1 eV) and
2
20
5
/2
10
20
30
40
50
60
70
80
Pd 3d (342.4 eV) respectively, which can be attributed
3
/2
to the Pd(II) state [55]. These results indicated that the Pd
species on Cell-Schiꢁ base was oꢀ the Pd(II) state and no
Pd(0) state existed in this cellulosic Pd(salen)-type catalytic
complex. The inset image shows the survey spectrum oꢀ the
cellulosic Pd(salen)-type catalytic complex. The signal oꢀ Pd
was weaker than those oꢀ elements C, N and O because Cell-
Schiꢁ base was not ꢀully covered by Pd NPs. Besides, this
2
-Theta(deg)
Fig. 3 XRD patterns oꢀ a Cell-Schiꢁ base and b CL-salen-Pd(II)
our catalyst. The slightly cross-linked structure demonstrates
the successꢀul ꢀormation oꢀ the cellulosic Pd(salen)-type
catalytic complex. From the TEM image oꢀ the CL-salen-
Pd(II) (Fig. 2ꢀ), it is obvious that the palladium is uniꢀormly
distributed on the surꢀace oꢀ the catalyst, and its particle size
is uniꢀorm, ranging ꢀrom 5 to 10 nm.
−1
new ligand anchored cellulose chelated near 1.725 mol kg
oꢀ palladium, as determined by ICP-AES analysis.
The thermal stability oꢀ supported catalyst has a great
inꢃuence on the activity and reusability oꢀ the catalyst.
Thereꢀore, the thermal stability oꢀ CL, Cell-Schiꢁ base and
CL-salen-Pd(II) were investigated by Thermal Gravity Anal-
ysis. As illustrated in Fig. 5a, the decomposition temperature
The crystal structure oꢀ Cell-Schiꢁ base and the cellu-
losic Pd(salen)-type catalyst were investigated using XRD
(
Fig. 3). The wide diꢁraction peak at 2θ=22.5° is reꢀerred
1
3

P. Sun et al.
3
.2 Catalytic Activity oꢀ the Cellulosic
Pd(salen)‑Type Catalyst ꢀor Diꢁerent
Cross‑Couplings
Pd3d5/2
37.1
O1s
3
C1s
N1s
Cl2p
0
200
400
600
800
1000 1200 1400
To evaluate the catalytic perꢀormance oꢀ cellulosic
Pd(salen)-type catalyst, Suzuki, Heck, and Sonogashira
carbon–carbon cross-coupling reactions were chosen
as model reactions, and the results are summarized in
Tables 1, 2, and 3. At frst, it was used in Suzuki cross-
couplings (Table 1). In general, this catalyst is eꢂcient
Bonding energy (eV)
Pd3d3/2
3
42.4
ꢀor the synthesis oꢀ biaryl products 3 in high to excellent
yields. The nature oꢀ aryl halides was very important to
the reaction outcome. Both aryl bromides and iodides dis-
played excellent reactivity, however, the use oꢀ aryl chlo-
rides aꢁorded inꢀerior results than their iodo or bromo ana-
logues (3aa, Table 1). No desired product was obtained,
probably attributed to poorer tendency oꢀ C–Cl bond to
undergo oxidative addition by the CL-salen-Pd(II) in mild
reaction conditions. Arylboric acids with both electron-
3
30
335
340
345
350
Bonding Energy(eV)
Fig. 4 XPS spectrum oꢀ the cellulosic Pd(salen)-type catalytic com-
plex
donating (–OMe) and electron-withdrawing (–CF , –Cl)
3
groups can smoothly undergo the Suzuki cross-coupling
reaction, moreover, heterocyclic boronic acid (2-thio-
pheneboronic acid) was also suitable substrate to aꢁord
the corresponding product in 85% yield. When 1,4-dibro-
mobenzene was used as a substrate, terphenyl product can
be obtained in 94%. It is worth noting that an aqueous
Suzuki cross-coupling was examined in the presence oꢀ
this cellulosic Pd(salen)-type catalyst, biphenyl product
3
aa can be isolated in 97% yield.
The activity oꢀ this catalyst was then investigated in Heck
cross-couplings (Table 2). Similar to Suzuki cross-coupling
process, aryl iodides and bromides were suitable substrates,
and excellent yields were obtained ꢀor all the cases. The mild
reaction conditions were compatible with various ꢀunction-
alities including methoxy, chlorine, and ester. Electron-rich
and electron-defcient groups oꢀ aryl halides displayed simi-
lar results in yields. No ester hydrolysis was observed when
alkene coupling partners had ester residues.
Fig. 5 TG curves oꢀ a CL, b Cell-Schiꢁ base and c CL-salen-Pd(II)
The Sonogashira coupling reactions oꢀ various aryl
iodides and aryl bromides with terminal alkynes were
fnally explored using this cellulosic Pd(salen)-type cata-
lyst (Table 3). Diꢁerent ꢀrom the above Suzuki and Heck
reactions, aryl chloride was also applicable, giving 7aa in
96% yield. For aryl iodides, the coupling reaction proceeded
smoothly and provided the corresponding products in good
to excellent yields. For aromatic terminal alkynes, electron
withdrawing or donating substituents (Br, CN and OMe)
were tolerated, aꢀꢀording products 7ab–7ad in 93–95%
yields. Hetero-aromatic 2-ethynylpyridine can also undergo
well to give 7ae in 78% yield. Finally, both aliphatic ter-
minal alkyne and ester propiolate were suitable substrates
to provide the corresponding products in 83% and 87%,
respectively.
oꢀ CL is at 320 ℃. Nevertheless, the weight loss oꢀ the Cell-
Schiꢁ base (Fig. 5b) starts at around 255 ℃, demonstrat-
ing cellulose is successꢀully modifed. Finally, according
to the TG curve oꢀ CL-salen-Pd(II) (Fig. 5c), it could be
observed that the initial weight loss oꢀ the catalyst is near
to 100 ℃ (3.5%), which may be due to the weight loss oꢀ
water trapped on its surꢀace. Aꢀter that The TG curve oꢀ the
catalyst slowly decreased ꢀrom 255 ℃, which corresponds
to the Schiꢁ base curve. The above results indicate that this
cellulosic Pd(salen)-type catalyst has good thermal stabil-
ity ꢀrom room temperature to 255 ℃. It can be concluded
that our catalyst is an eꢁective catalyst to carry out diꢁerent
cross-coupling reactions at 80 ℃ without any decomposition
under experimental conditions.
1
3

Synthesis of a Cellulosic Pd(salen)-Type Catalytic Complex as a Green and Recyclable Catalyst…
Table 1 Suzuki cross-coupling reaction oꢀ aryl halides and arylboronic acids
a
Reaction conditions: 1.0 mmol aryl halide, 1.1 mmol arylboronic acid, 11.1 mg oꢀ CL-salen‐Pd(II) (0.5 mol %), 2.0 mmol K CO , 5.0 mL sol-
2
3
vent, 50 ℃, 1 h
Isolated yields
a
Water was used as the solvent
Table 2 Heck reaction oꢀ aryl halides with alkenes
Reaction conditions: 1.0 mmol aryl halide, 1.1 mmol alkene, 22.3 mg oꢀ CL-salen‐Pd(II) (1.0 mol % Pd), 2.0 mmol K CO , 5.0 mL ethanol,
2
3
reꢃux, 3 h
Isolated yields
The catalytic activity oꢀ the cellulosic Pd(salen)-type
catalyst was ꢀurther investigated ꢀor carbon-heteroatom
bond ꢀormation, such as Buchwald–Hartwig amination and
etherifcation reaction (Fig. 6). Iodobenzene 1a was utilized
to react with phenol and aniline at 80 ℃ in DMSO, diphenyl
ether 9aa and diphenyl amine 9ab can be isolated in 93%
1
3

P. Sun et al.
Table 3 Sonogashira cross-coupling reaction oꢀ aryl halides with terminal alkynes
Reaction conditions: 1.0 mmol aryl halide, 1.1 mmol terminal alkyne, 22.3 mg oꢀ CL-salen‐Pd(II) (1.0 mol% Pd), 0.05 mmol CuI, 2.0 mmol
K CO , 5.0 mL ethanol, reꢃux, 3 h
2
3
Isolated yields
Fig. 6 CL-salen‐Pd(II)
OH
catalyzed Buchwald–Hartwig
amination and etherifcation
X
8
a
b
CL-salen-Pd(II)
K CO , DMSO, 80
Z
+
2
3
ഒ
9
aa Z=O, X=I,93%
ab Z=N, X=I,78%
1a
9
NH2
8
and 78% yields respectively. The above results demonstrated
our catalyst was versatile toward all types oꢀ cross-coupling
reactions.
in Table 4, entries 13–18, our catalyst showed some exten-
sive improvement in reaction conditions, such as reaction
temperature, solvent and yield. It is worth noting that when
comparing the synthesized sample with CNC-BIA-Pd cata-
lyst (Table 4, entries 19–20), it can be observed the ꢀact
that the catalyst activity still needs to be improved in the
Buchwald–Hartwig etherifcation reaction. Interestingly,
we report the frst example oꢀ cellulose-supported Pd cata-
lysts as eꢂcient and sustainable catalyst systems ꢀor Buch-
wald–Hartwig amination reactions. Finally, it can be con-
cluded that the catalyst is a desirable alternative because
it not only shows particularly good activity in traditional
carbon–carbon coupling but also plays an important role in
the ꢀormation oꢀ carbon-hetero bonds.
The compared results achieved in the work with other
biopolymer-based catalysts ꢀor the Suzuki, Heck, Sonoga-
shira Buchwald–Hartwig amination and etherifcation cou-
pling reactions, which are summarized in Table 4. In com-
parison with other catalysts in Suzuki reaction, it is clear
that each method has its own advantages. However, our work
has some merits, such as lower reaction temperatures, high
yields and without using any other instruments like micro-
wave ovens (Table 4, entries 1–6). As ꢀor the Heck reaction,
taking iodobenzene reacting with styrene as an example,
the results are listed in Table 4, entries 7–12. These data
show that our catalyst is more eꢂcient than other catalysts,
producing high reaction yields with obviously lower reaction
temperature and utilization oꢀ green solvent. To compare
the eꢂciency oꢀ our catalyst with those oꢀ other catalysts
reported ꢀor the Sonogashira reaction, we chose the reac-
tion between iodobenzene and phenylacetylene. As shown
3.3 Recyclability Study oꢀ the Cellulosic
Pd(salen)‑Type Catalyst
To check the heterogeneity oꢀ catalyst, a hot fltration test
on the cellulosic Pd(salen)-type catalyst using the model
1
3

Synthesis of a Cellulosic Pd(salen)-Type Catalytic Complex as a Green and Recyclable Catalyst…
Table 4 Catalytic perꢀormance
Entry
Catalyst
Condition
Yield (%)
Reꢀerences
oꢀ diꢁerent Pd-based catalysts
in coupling reactions
a
a
a
a
a
a
b
b
b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
Cell-Pd(0)
Cell-OPPh -Pd(0)
CL-Sc-Pd(II)
CL-DETA-Pd
Cell-Sb-Pd(II)
CL-salen‐Pd(II)
Cell-Pd(0)
Cell-OPPh -Pd(0)
PdNPs@CNCs
CMC-Pd
K CO /H O,100℃
83
95
99
96
99
99
100
91
75
85
87
99
98
95
96
87
96
98
96
93
78
[56]
[44]
[49]
[51]
[57]
This work
[41]
[43]
[58]
[59]
[60]
This work
[41]
[45]
[61]
[62]
[63]
This work
[64]
2
3
2
K CO /EtOH, 80℃
3
2
3
K CO /Solvent-ꢀree, MW
2
3
K CO /EtOH: H O (1:1),75℃
2
3
2
K CO /EtOH: H O (1:1),70℃
2
3
2
K CO /EtOH: H O (1:1),50℃
2
3
2
Et N/CH CN, 120℃
3
3
Bu N/DMF, 110℃
2
3
K CO /CH CN: H O (1:1),100℃
2
3
3
2
b
b
b
c
c
c
c
c
c
d
d
e
II
0
1
2
3
4
5
6
7
8
9
0
1
K PO ·3H O/DMF, 110℃
3
4
2
PdNP@CNXL
CL-salen‐Pd(II)
Cell-Pd(0)
NaOAc/CH CN: H O (1:1),100℃
3
2
K CO /EtOH, Reꢃux
2
3
Et N/CH CN, Reꢃux
3
3
Cell-O PPh -Pd(0)
Et N/DMF, 80℃
2
3
3
PdNPs@XH
Pd-MNPSS
Pd/Cu@MMT/CS
Cell‐halꢀ-salen‐Pd(II)
CNC-BIA-Pd
CL-salen‐Pd(II)
CL-salen‐Pd(II)
Et N/CH CN, 90℃
3
3
K CO /H O, 100℃
2
3
2
PPh ,Na CO /DME: H O (4:1) 80℃
3
2
3
2
K CO /EtOH, Reꢃux
2
3
K CO /DMSO, 80℃
2
3
K CO /DMSO, 80℃
This work
This work
2
3
K CO /DMSO, 80℃
2
3
a
Suzuki coupling reaction
Heck coupling reaction
Sonogashira coupling reaction
Buchwald-Hartwig etherifcation reaction
Buchwald-Hartwig amination reaction
b
c
d
e
Suzuki-coupling reaction was perꢀormed. Aꢀter reacting
or one hour under optimized reaction conditions, the reac-
The recyclability oꢀ the cellulosic Pd(salen)-type cat-
alyst was investigated by repetitive experiment on the
model Suzuki–Miyaura cross-coupling reaction. (Fig. 7).
The CL-salen‐Pd(II) was recycled by simple centriꢀuga-
tion, washing with water to remove the salt aꢀter reaction
completion. The catalytic activity decreased slightly aꢀter
the second cycle however, the yields oꢀ Suzuki cross-
coupling reaction can still reach up to 90% aꢀter twelve
times cycles with a slight loss oꢀ activity. In addition, the
palladium content oꢀ the reaction materials beꢀore and
ꢀ
tion mixture was fltered under thermal conditions, and
then the fltrate was heated under the same reaction condi-
tions. Interestingly, no ꢀurther reactions were observed at
all. Besides, the ICP-AES analysis oꢀ the fltrate revealed
that no palladium species were leached out into the aque-
ous solution. Thereꢀore, the results veriꢀy that the catalytic
process is heterogeneous catalysis.
Fig. 7 Recycling test oꢀ the CL-
salen‐Pd(II)
1
3

P. Sun et al.
Fig. 8 SEM image oꢀ CL-salen‐Pd(II) aꢀter the 12th runs
aꢀter the cycle by ICP-AES was also analyzed. It could be
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Aꢀliations
Peng Sun1,2 · Jiaojiao Yang · Chunxia Chen · Kaijun Xie · Jinsong Peng
1
1,2
1
1
2
*
*
Chunxia Chen
Material Science and Engineering College, Northeast
Forestry University, Harbin 150040, Heilongjiang,
People’s Republic oꢀ China
ccx0109@neꢀu.edu.cn
Jinsong Peng
jspeng1998@neꢀu.edu.cn
1
College oꢀ Chemistry, Chemical Engineering and Resource
Utilization, Northeast Forestry University, Harbin 150040,
Heilongjiang, People’s Republic oꢀ China
1
3