Immobilized Pd on Eggshell Membrane: A powerful and recyclable catalyst for Suzuki and Heck cross-coupling reactions in water

Article abstract of DOI:10.1016/j.jorganchem.2020.121496

A stable heterogeneous palladium catalysts, the waste eggshell membrane (ESM) anchored Pd complex, was synthesized by a simple and green technique. The catalyst obtained can display an outstanding catalytic activity for Suzuki and Heck coupling reactions in water media. The reactions of various aryl halides with aryl boronic acids and terminal alkenes provided the corresponding products in moderate to excellent yields. More importantly, this novel and efficient catalyst with high stability can be easily reused for at least twelve times with no decrease of the catalytic activity, performing an endurable and wide tolerance of the substrates. Facile synthesis, high catalytic activity, and recyclability make these catalysts interesting for further studies.

Full text of DOI:10.1016/j.jorganchem.2020.121496

Journal of Organometallic Chemistry 925 (2020) 121496
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Immobilized Pd on Eggshell Membrane: A powerful and recyclable
catalyst for Suzuki and Heck cross-coupling reactions in water
Shang Wua,∗, Hongyan Jianga, Hong Zhanga,∗, Lianbiao Zhaoa, Peilin Yuan a, Ying Zhanga,
Qiong Sua, Yanbin Wang a, Lan Wua, Quanlu Yang b,∗
a Key Laboratory for utility of Environment-Friendly Composite Materials and Biomass in university of Gansu Province, College of Chemical Engineering,
Northwest Minzu University, Lanzhou, Gansu 730030, People’s Republic of China
b College of Chemical Engineering, Lanzhou University of Arts and Science, Beimiantan 400, Lanzhou, Gansu 730000, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A stable heterogeneous palladium catalysts, the waste eggshell membrane (ESM) anchored Pd complex,
was synthesized by a simple and green technique. The catalyst obtained can display an outstanding cat-
alytic activity for Suzuki and Heck coupling reactions in water media. The reactions of various aryl halides
with aryl boronic acids and terminal alkenes provided the corresponding products in moderate to excel-
lent yields. More importantly, this novel and efficient catalyst with high stability can be easily reused
for at least twelve times with no decrease of the catalytic activity, performing an endurable and wide
tolerance of the substrates. Facile synthesis, high catalytic activity, and recyclability make these catalysts
interesting for further studies.
Received 17 June 2020
Revised 20 July 2020
Accepted 25 August 2020
Available online 27 August 2020
Keywords:
Eggshell membrane
Pd catalysts
Heterogeneous catalyst
Suzuk cross-coupling reaction
Heck cross-coupling reaction
Water
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
phase. For instance, carbon nano-tubes have been found a serious
hazard to health through the assessment of its toxicity, and meso-
Palladium-catalyzed cross-coupling reactions have emerged
as a tremendously powerful synthetic tool in organic synthesis
and fine chemical fields [1,2]. Among them, the Suzuki and Heck
reactions are the most important and highly effective protocols
for the construction of C–C bonds [3a–7]. In the past few years,
numerous effective and selective homogeneous palladium catalytic
systems have been developed for such reactions. However, most of
these methods not only suffer from expensive and non-renewable
palladium catalysts, but also require phosphine ligands, which are
equally unrecoverable and sensitive to oxygen and moisture [8-10].
Therefore, the development of ligand free heterogeneous catalysts
for C–C bond forming reactions would be an important topic of
interest especially to current industrial research.
porous silicas are unstable under aqueous and basic conditions for
cross-coupling catalysis [13–15].
In this regard, the use of environmentally friendly supports is
very promising due to their low cost and nontoxicity [161–17b].
Therefore, some natural biopolymers, such as chitosan, [18–20]
cellulose, [21a–22] wool, [23–27] etc. have been used as effi-
cient polymer supports in several important palladium catalyzed
transformations. Among them, we have focused on the natural
waste material, eggshell membranes (ESM). ESM are constructed
of interwoven protein fibers, which could be used as a solid phase
ligand with no need for further functionalization. Furthermore,
the fibrous nature of the ESM could offer a large surface area and
abundant functional groups (on the ordered amino acid chains)
for immobilization of catalysts particles. These features support a
high dispersion of palladium particles and prevent the aggregation
of Pd-black, which was regarded as the most critical problem to
the performance of palladium-catalyzed conversions.
From this point of view, various organic and inorganic solid
supports bearing amine groups have been developed for ligation
with palladium [11a–12b]. However, most of these supporters
were synthesized by tedious steps with low loading of palladium,
showing toxicity, severe catalyst leaching, and the poor swell
property prevented the reactions to take place in the neat aqueous
Focused on the environmentally benign organic reactions,
water-containing media are popular due to reduction of burden for
organic solvent disposal [28-30]. Remarkable, amino acids are am-
phiphilic molecules, which existed in the surface of the ESM. So it
is possible to take organic reactions in aqueous phase.
∗
Corresponding authors.
E-mail addresses: chwush84@163.com (S. Wu), 641124768@qq.com (H. Zhang),
yangquanlu2002@163.com (Q. Yang).
https://doi.org/10.1016/j.jorganchem.2020.121496
0022-328X/© 2020 Elsevier B.V. All rights reserved.

2
S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
Herein, we report a new biopolymer supported palladium cat-
alyst, ESM-Pd (II), in neat water for the Suzuki and Heck cou-
pling reactions under atmospheric conditions. Notably, the catalyst
is easy to separate from the reaction mixture by simple filtration,
and retains good activity for at least twelve successive runs with-
out any additional activation treatment.
2. Results and discussion
2.1. Synthesis and characterization of the catalysts
Commercial fresh eggs were gently broken, and the con-
tents were removed. The white semipermeable eggshell membrane
(ESM) was carefully peeled and washed with distilled water several
times. Then, the clean ESM was dried in the ambient air at room
temperature (r. t.), and cut to pieces (4 mm2). (Fig. 1, a)
1.0 g of ESM pieces were immersed in aqueous solutions of
Pd(OAc)2 and PdCl2, respectively, (20 mL, 0.6 mmol of Pd). The
mixture was stirred at r.t. for 5 h. The metal salt solutions slowly
decolorized and transparent, and the white ESM was turned to a
light yellow for Pd(OAc)2 solution and a brown color for PdCl2 so-
lution. Subsequently, the samples were filtered, washed with de-
ionized water (3 × 20 mL) and acetone (3 × 20 mL), dried for
24 h at room temperature to obtain ESM-Pd(OAc)2 and ESM-PdCl2
complex, respectively. (Fig. 1, b and c)
Fig. 3. XRD patterns of ESM, ESM-Pd(OAc)2 and ESM-PdCl2 complex catalyst.
NH2 has shifted to a higher wave number (3447 cm−1 and 3441
cm−1), similarly, the peak corresponding to the C=O stretching vi-
bration moved to 1610 cm−1 and 1602 cm−1 The above is indi-
.
cated that coordination or ionic bonds were formed by the con-
nection of N atoms (in –NH2) and –NH–CO– with Pd atoms in the
ESM-Pd(OAc)2 and ESM-PdCl2.
Fig. 3 shows the X-ray diffraction (XRD) patterns of ESM, ESM-
Pd(OAc)2 and ESM-PdCl2. It can be seen that natural ESM is to-
tally amorphous whereas ESM proteins composed of alanine, ser-
ine, and other amino acids, may form crystalline domains in the
membranes resulting in the peak around 20.6°In the ESM-Pd(OAc)2
and ESM-PdCl2, both of them have crystalline structure of Pd with
peaks emerging at 39.8°, 46° and 67.8°, which can be indexed to
the Pd (1 1 1), (2 0 0), (2 2 0) planes, respectively. The above in-
formation further reveals that palladium was anchored onto the
surface of ESM.
The palladium content in ESM-Pd(OAc)2 and ESM-PdCl2, were
determined by means of inductively coupled plasma equipped
with atomic emission spectrometry (ICP-AES) and amounted to be
2.64 wt % (0.248 mmol/g) and 8.12 wt% (0.763 mmol/g).
Fourier transform infrared spectroscopy (FT-IR) spectra of ESM,
ESM-Pd(OAc)2 and ESM-PdCl2 were shown in Fig. 2. In the ESM,
the following characteristic peaks were observed: 3415 cm−1 for
the –NH stretching vibration bands of –NH2, 1591 cm−1 for C=O
bending vibration of –NH–CO–, and 1375cm−1 for aliphatic C–H
deformation, respectively. Compared with that of ESM, in the ESM-
Pd(OAc)2 and ESM-PdCl2, the –NH stretching vibration bands of –
To accurately establish the chemical composition of the surface,
the ESM supported Pd catalyst were analysed by X-ray photoelec-
tron spectroscopy (XPS; setting reference C1s = 284.5 eV). Fig. 4
displayed the XPS peaks of the Pd3d core level in the ESM-Pd(OAc)2
and ESM-PdCl2.The binding energy of the Pd3d 3/2 and Pd3d 5/2 in
the ESM-Pd(OAc)2 decrease 0.51 eV and 0.49 eV, respectively. And
for the ESM-PdCl2, the differences of binding energy values were
-0.61 eV and -0.66 eV. The results mean the increase in its elec-
tron density for the ESM-supported Pd catalyst. In Fig. 5, two ma-
jor peaks with binding energies at 400.15 eV and 400.84 eV, corre-
sponding to–NH2 and –NH–CO– groups in ESM respectively (Fig. 5,
a). The N1s binding energies of –NH2 in ESM-Pd(OAc)2 and ESM-
PdCl2, were found to be 0.55 eV and 0.55 eV lower compared with
that of ESM, respectively. Likewise, the difference between –NH–
CO– in ESM and ESM-Pd(OAc)2 was 0.59 eV and that between
ESM and ESM-PdCl2 is 0.53 eV (Fig. 5, b and c). Furthermore, the
XPS peaks of S were fitted, including four major peaks: peak with
binding energies of 163.41 eV was assigned to –SH groups, peak at
164.65 eV corresponded to–S–S–, whereas the peaks at 169.50 and
168.27 eV were assigned to –SOx– (x=2–3) group (Fig. 6, a). And
the S2p binding energy changes for –SH group upon moving from
ESM to ESM-Pd(OAc)2 was −0.79 eV, that for the –S–S– group was
−0.49 eV, that for the –SOx– group were −0.13 eV and −0.12 eV
(Fig. 6, b). In the same way, the difference for the –SH, –S–S– and
–SOx–group between ESM and ESM-PdCl2 were -0.78 eV, -0.51 eV,
-0.47 eV and -0.83 eV, respectively (Fig. 6, c). The result indicated
that coordination or ionic bonds were formed by the connection of
N atoms (in –NH2 and –NH–CO–) and S atoms (in –SH, –S–S– and
–SOx–) with Pd atoms in ESM-Pd(II) catalysts.
Fig. 1. (a) ESM, (b) ESM-Pd(OAc)2 and (c) ESM-PdCl2 complex catalyst.
The catalysts were also characterized by scanning electron mi-
croscopy (SEM). The images of the natural ESM, ESM- Pd(OAc)2 and
Fig. 2. FT-IR spectra of ESM, ESM-Pd(OAc)2 and ESM-PdCl2 complex catalyst.

S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
3
Fig. 4. XPS spectra of (a) ESM-Pd(OAc)2 and Pd(OAc)2, (b) ESM-PdCl2 and PdCl2.
Fig. 5. XPS spectra of N1s (a) ESM, (b) ESM-Pd(OAc)2 and (c) ESM-PdCl2.
Fig. 6. XPS spectra of S2P (a) ESM, (b) ESM-Pd(OAc)2 and (c) ESM-PdCl2.
ESM- PdCl2 are given in Fig. 7. The natural ESM is a macroporous
network composed of interwoven and coalescing shell membrane
fibres ranging in diameter from 1 to 3 um (Fig. 7, a and b). As dis-
played in Fig. 7 c, EDX spectrum suggested none of metal existed in
the supporter. In ESM- Pd(OAc)2 (Fig. 7, d) and ESM-PdCl2 (Fig. 7,
g), obvious aggregation of palladium cannot be observed, even at
a high magnification. (Fig. 7, e and h), but the EDX measurement
proved that the presence of Pd on the surface of fibres (Fig. 7, f
and i). Additionally, a homogeneous distribution of the palladium
was revealed by EDS mapping of Pd (Fig. 7, j and k). These results
indicated that palladium was uniformly dispersed onto the surface
of ESM.
mobenzene (1a) with phenyl boronic acid (2a) as model substrate
in the reactions (Table 1).
As is known, water is safe, readily available and environment-
benign, therefore it was selected as the only solvent in the re-
action. Meanwhile, in Suzuki coupling reactions, base plays very
important role in the transformation. It was found that Cs2CO3,
K3PO4, TBAB and Et3N resulted in poor conversion, while K2CO3
and NaOAc improved the yield up to 88% and 91%, respectively, un-
der the same reaction conditions (entries 1–6). However, after the
reaction, the catalyst was comminuted in the presence of K2CO3
in the mixture, and it was inactivated in the reused test (entry 5).
The results indicated NaOAc was a suitable base to obtain a high
yield of the coupling product (entry 6). In addition, variation of
temperature in the range of 20–50 °C revealed that the optimum
temperature for the reaction was 40 °C (entry 10 versus entries
6–9). Time trials were taken out thereafter (entry 3, entry 5–7). To
our surprise, an excellent yield (92%) of the product was obtained
in a comparatively very short reaction time of only 60 min (entry
2.2. Suzuki reaction in water
Our initial study focused on the aryl halides with aryl boric
acids, firstly, the reaction conditions were optimized. We used bro-

4
S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
Fig. 7. (a), (b) SEM image of ESM, and (c) the corresponding EDX spectrum of the ESM; (d), (e) SEM images of ESM-Pd(OAc)2, and (f) the corresponding EDX spectrum of the
ESM-Pd(OAc)2; (g), (h) SEM images of ESM-PdCl2, and (i) the corresponding EDX spectrum of the ESM-PdCl2; (j), (k) the corresponding EDS mapping of Pd in ESM-Pd(OAc)2
and ESM-PdCl2 catalyst.
14); and prolonged reaction would not increase the yields (entry
10). Then we observed the dosage of ESM-Pd(OAc)2 complex (en-
tries 14–17). Obviously, 10 mg (0.62 mol%) catalyst (entry 17) can
provide fine yields for the product 3a, and for the higher amounts
of catalyst, the desired product was afforded in a nearly quantita-
tive yield (entry 14).
lyst doses, not only on the amount of usage, but also on the cat-
alyst activity. In a controlled experiment, as expected, we did not
observe any product in the presence of eggshell membrane only
(entry 23). Finally, for ESM-Pd(0) catalyst with the same amount
(0.62 mol%) of palladium, the catalyst system presented poor activ-
ity (entry 24). The results reflected the essential role of the ESM-
Pd(OAc)2 catalyst for product yield.
In order to evaluate the superiority of ESM-Pd(OAc)2 complex,
the other palladium salt such as Pd(OAc)2 (entry 18), PdCl2 (entry
19), Pd(acac)2 (entry 20) and Pd2(dba)3 (entry 21) was tested with
the same amount (0.62 mol%) of palladium. It was noticed that
palladium compounds gave lower yield than ESM-Pd(OAc)2 cat-
alyst. Finally, comparison between ESM-Pd(OAc)2 and ESM-PdCl2
(entry 22) indicated apparently that the former was more active
within a shorter reaction time. Hence the ESM-Pd(OAc)2 complex
performed better in the reaction than the other palladium cata-
With the optimized reaction conditions in hand, the catalytic
activity was tested in Suzuki coupling reaction of aryl halides with
arylboronic acids (Scheme 1). The cross-coupling products were
isolated in good to excellent yields with a variety of electron-
withdrawing and electron-donating groups both in the aryl bro-
mides and chlorides as well as aryl boronic acids (3a, 3c-3 h, 3j-3n,
3s-3v). In addition, the catalytic system can be used for selective
coupling of bromo groups keeping fluoro and chloro functionalities

S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
5
Table 1
Optimization of conditionsa.
Entry
Catalystb
Base
Temperture
Time
Yieldc
1
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (20 mg)
ESM-Pd(OAc)2 (5 mg)
ESM-Pd(OAc)2 (8 mg)
ESM-Pd(OAc)2 (10 mg)
Pd(OAc)2 (0.62 mol%)
PdCl2 (0.62 mol%)
Cs2CO3
K3PO4
TBAB
50oC
50oC
50oC
50oC
50oC
50oC
20oC
25oC
30oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
40oC
2h
12%
16%
49%
33%
88%
91%
46%
65%
73%
90%
42%
56%
79%
92%
33%
61%
91%
38%
29%
38%
26%
49%
0%
2
2h
3
2h
4
Et3N
2h
5
K2CO3
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
2h
6
2h
7
2h
8
2h
9
2h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2h
15min
30min
45min
60min
60min
60min
60min
60min
60min
60min
60min
60min
60min
60min
Pd(acac)2 (0.62 mol%)
Pd2(dba)3 (0.62 mol%)
ESM-PdCl2 (0.62 mol%)
ESM (20 mg)
ESM-Pd(0) (0.62 mol%)
0%
a
–
Reaction conditions: 1a (0.4 mmol), 2a (0.6 mmol), base (0.6 mmol), water
(3 mL)-
b– Catalyst amount: 10 mg of ESM-Pd(OAc)2 (Pd 2.64 wt %) equals to 0.62 mol%
c– Isolated yield by column chromatography.
intact (3d-3e, 3r-3 w). Fortunately, for the aryl chlorides, the cou-
pling reactions with the electron withdrawing groups (-NO2) gave
a slight lower product yield than chlorobenzene (3a, 3i).
Remarkably, -ortho substitute groups were less reactive than the
-meta and -para substituents (3b, 3o, 3q, 3r). This fact indicates
that the steric hindrance of coupling partners has a definite in-
fluence on this coupling reaction. In addition, the Suzuki reaction
of sterically hindered 1-naphthaleneboronic acid afforded 72% and
70% yield of 3p and 3 w, respectively.
a
Scheme 1. Scope of the Suzuki reactions with ESM–Pd complex catalyst. Reac-
tion conditions: 1 (0.4 mmol), 2 (0.6 mmol), ESM-Pd(OAc)2 (10 mg, Pd 2.64 wt %),
b
NaOAc (0.6 mmol), water (3 mL); Isolated yields of 3 by column chromatography.
Reaction conditions: 1 (0.4 mmol), 2 (1.0 mmol), ESM-Pd(OAc)2 (20 mg, Pd 2.64 wt
%), NaOAc (1.0 mmol), water (3 mL).
Table 2
a
Optimization of conditions
.
2.3. Heck reaction in water
The Heck cross-coupling reaction, as another typical Pd-
catalyzed carbon–carbon bond formation has occupied a special
place in both fundamental research and industrial application. And
encouraged by ESM-Pd(OAc)2 complex in Suzuki reaction, we ex-
plore terminal alkenes as new partner of coupling reaction with
aryl halides. Based on the optimized conditions of Suzuki reaction,
the effect of catalysts, temperature and reaction time were stud-
ied through the reaction of bromobenzene 1a with styrene 4a. Sur-
prisingly, ESM-PdCl2 was more active than ESM-Pd(OAc)2 with the
same amount (0.62 mol%) of palladium for Heck reaction (entries 1,
2). And 6 mg (1.14 mol%) of ESM-PdCl2 can be sufficient for cataly-
sis (entries 2–5). As illustrated in Table 2, lower temperature often
results in incomplete conversion of the starting materials (entries
4, 6–10). And for the effects of reaction time, there is gradual in-
crease of the product yield with prolonged reaction time (entries
9, 11–15). Similarly, ESM-Pd(0) was inactive in Heck reaction (entry
16). Therefore, the excellent result was obtained when the coupling
reaction was carried out with 10 mg of the catalyst using NaOAc as
a base in water at 70 °C for 20 h.
b
c
Entry
Catalyst
Temperture
Time
Yield
1
ESM-Pd(OAc)2 (10 mg, 0.62 mol%)
ESM-PdCl2 (3.3 mg, 0.62 mol%)
ESM-PdCl2 (5 mg, 0.954 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (8 mg, 1.526 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-PdCl2 (6 mg, 1.14 mol%)
ESM-Pd(0) (1.14 mol%)
25oC
25oC
25oC
25oC
25oC
40oC
50oC
60oC
70oC
80oC
70oC
70oC
70oC
70oC
70oC
70oC
24h
24h
24h
24h
24h
24h
24h
24h
24h
24h
2h
22%
36%
41%
52%
53%
59%
68%
79%
93%
93%
18%
39%
55%
74%
91%
0%
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
6h
12h
16h
20h
24h
a– Reaction conditions: 1a (0.4 mmol), 4a (0.6 mmol), NaOAc (0.6 mmol), water
(3 mL)
b– Catalyst amount: 10 mg of ESM-Pd(OAc)2 (Pd 2.64 wt %) equals to 0.62 mol%,
6 mg of ESM-PdCl2 (Pd 8.12 wt %) equals to 1.14 mol%
c– Isolated yield by column chromatography.
Encouraged by our initial studies, we then investigated the re-
activity of aryl halides (1) with different olefin substrates (4) un-

6
S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
Fig. 8. The recycling test of ESM-Pd(OAc)2 and ESM-PdCl2 catalyst in Suzuki and
Heck coupling reactions.
der the optimized conditions (Scheme 2). The experimental results
showed that the electronic effects of either coupling partner pro-
duced no significant impact on the reaction yields. It was worth
noting that the coupling of aryl chlorides with phenylboronic acid
can proceed smoothly under the same conditions. For chloroben-
zene and 4-nitrochlorobenzene, the former provided higher prod-
uct yield (92%, 88% and 78%).
Different alkynes were tolerated well, styrene derivatives and
acrylate esters afford the desired cross-coupling products in high
to excellent yields. In general, the coupling reactions with acry-
lates as the olefin were faster than styrene. In particular, 1,1-
diphenylethylene, which has some discernible steric-encumbrance,
may slightly influence the conversion (5j, 5l, 5i).
a
2.4. Hot filtration test and recycling ability of the catalyst
Scheme 2. Scope of the Heck reactions with ESM–Pd complex catalyst. Reaction
conditions: 1 (0.4 mmol), 4 (0.6 mmol), ESM-PdCl2 (6 mg, Pd 8.12 wt%), NaOAc
(0.6 mmol), water (3 mL); Isolated yields of 3 by column chromatography.
Leaching of the metal ions is a serious problem for supported
metal catalysts, and it prevents catalyst separation and recycling.
To explore the leaching of the catalyst (ESM-Pd(OAc)2 for Suzuki
reaction, and ESM-PdCl2 for Heck reaction), hot filtration test was
performed by coupling of bromobenzene (1a) with phenylboronic
acid (2a) and styrene (4a) under the optimized conditions, re-
spectively. After 50% of the coupling reaction had completed (the
Suzuki reaction for 20 min, and the Heck reaction for 11 h), the
catalyst was removed and filtrate was allowed to react further. In
the absence of the catalyst, No further reaction was observed even
after a prolonged reaction time (GC) suggesting that the palladium
catalyst remains on the support during the reaction. In fact, only
56 and 71 parts per billion palladium were detected in the reaction
phase for Suzuki and Heck reaction (by ICP-MS), indicating quite a
slender metal leaching of the catalyst during the reactions and the
palladium atom stable remains in the ESM during the reaction.
Catalyst recycling is essential from economic, environmental
and industrial points of view. As an important aspect of hetero-
geneous catalysis, the recyclability of the catalyst was tested in
the model Suzuk and Heck cross-coupling reactions. After the fresh
catalytic cycle, the mixture was cooled to room temperature, then
the catalysts were simply separated by filtration, washed thor-
oughly with water and acetone, dried in the ambient air, and
reused for the next cycles. It was noteworthy that ESM-Pd(OAc)2
and ESM-PdCl2 can be recycled up to twelve times for Suzuki
and Heck reaction without significant loss of activity, respectively
(Fig. 8). Additionally, by the end of each reaction, ICP-AES analysis
of the catalyst was carried out, and no appreciable change was ob-
served (2.64% to 2.59% for ESM-Pd(OAc)2, and 8.12% to 7.99% for
Fig. 9. XPS spectra of (a) fresh and recycling ESM-Pd(OAc)2, (b) fresh and recycling ESM-PdCl2.

S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
7
Fig. 10. (a) SEM image of the reused ESM-Pd(OAc)2 catalyst (after the Suzuki reaction), (b) the corresponding EDX spectrum; (a) SEM image of the reused ESM-PdCl2 catalyst
(after the Heck reaction), (b) the corresponding EDX spectrum.
ESM-PdCl2). These results revealed that the ESM-Pd(II) is stable
and can be renewed for frequent use, the total TON value is 1728
and 961, respectively.
To further investigate the reused catalyst, XPS measurements
for the Pd
peak were also performed (Fig. 9). The spectra
3d
showed that there was no obvious shift change between the fresh
and recycling catalyst. It demonstrated that the presence of Pd
Ⅱ
species was Pd , during the coupling reaction. The results provided
the direct evidences for the high recyclability and stability of ESM-
Pd(OAc)2 complex.
Subsequently, the morphology of the catalysts after the twelfth
cycle of the reaction were observed through SEM (Fig. 10), which
clearly showed the surface of reused catalysts was rougher than
the fresh, either ESM-Pd(OAc)2 or ESM-PdCl2 catalyst (Fig. 10, a
and c). Probably, it was resulted form the etching of the base that
is essential for the coupling reaction. Moreover, the EDX spectrum
proved the presence of Pd on ESM indicating that Pd was stable on
ESM and did not leach out during the reaction (Fig. 10, b and d).
2.5. Proposed mechanism for ESM-Pd(OAc)2 the Suzuki reaction
Based on the above XPS observations, and also in accordance
with previous literature reports, [31,32] a summary of processes
occurring during the catalytic reaction was proposed (Scheme 3).
Firstly, the palladium atom of Pd salt coordinated with ESM, and
then the reaction was catalyzed through the usual addition step
with aryl halides (1a) forming a Pd(II) intermediate complex B.
Subsequently, Ar–Pd–X complex B was coupled with partner (2a or
4a) to obtain Ar–Pd(II)–R intermediate complex C in the presence
of base. Finally, elimination of intermediates C yielded the desired
product, accompanied by the generation of Pd(II) species A, which
continued catalyzing the coupling reaction in the next run.
Scheme 3. Plausible mechanism of the C–C bond forming reactions catalyzed by
the ESM–Pd (II) catalyst.
3. Conclusions
In summary, ESM-Pd(II) complex has been developed as
a
novel, green and highly efficient catalyst for no ligand Suzuki and
Heck coupling reactions in water. The biopolymer supported cat-
alyst system shows several superior features as following: natural
biopolymer as a solid ligand and catalyst support, mild reaction

8
S. Wu, H. Jiang and H. Zhang et al. / Journal of Organometallic Chemistry 925 (2020) 121496
conditions, water as the solvent, and the high to excellent yield
of the products. More importantly, the catalyst shows outstanding
stability and reusability, it can be separated simply, effectively and
reused twelve times (ESM-Pd(OAc)2 for Suzuki reaction and ESM-
PdCl2 for Heck reaction) without any activity decrease. To the best
of our knowledge, it is the first time that ESM-Pd(II) complex is
used as a reusable catalyst for C–C bond forming reactions. A fur-
ther study is continued in our laboratory to investigate the detailed
mechanism and extend the application of the system to other cou-
pling transformations are under way in our laboratory.
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Acknowledgments
This work was financially supported by the National Natu-
ral Science Foundation of China (No. 21962017, 21968032 and
31760608), the Fundamental Research Funds for the Central Univer-
sities (31920200042, 31920200086, 31920200002, 31920190016),
the project of cultivating higher education teaching achievements
in Gansu Province (No. 2019GSJXCGPY-10), and the Northwest
Minzu University’s Double First-class and Characteristic Develop-
ment Guide Special Funds-Chemistry Key Disciplines in Gansu
Province (No. 11080316). We also thank Key laboratory for Util-
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