In situ cross-linked chitosan Cu(I) or Pd(II) complexes as a versatile, eco-friendly recyclable solid catalyst

Article abstract of DOI:10.1016/j.molcata.2010.10.024

Herein we report a simple ultrasound-assisted procedure to synthesize solid supported Cu(I) and Pd(II) catalysts. We prepared cross-linked chitosan derivatives (CS-Cu, CS-Pd) by reacting hexamethylene diisocyanate and chitosan in the presence of the metal

Full text of DOI:10.1016/j.molcata.2010.10.024

Journal of Molecular Catalysis A: Chemical 334 (2011) 60–64
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
In situ cross-linked chitosan Cu(I) or Pd(II) complexes as a versatile, eco-friendly
recyclable solid catalyst
Katia Martinaa, Silke E.S. Leonhardtb, Bernd Ondruschkab, Massimo Curinic, Arianna Binelloa,
Giancarlo Cravottoa,∗
a Dipartimento di Scienza e Tecnologia del Farmaco, University di Torino, Via Pietro Giuria 9, 10125 Torino, Italy
b Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Lessingstr. 12, 07743 Jena, Germany
c Dipartimento di Chimica e Tecnologia del Farmaco, University of Perugia, Via del Liceo, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2010
Received in revised form 22 October 2010
Accepted 26 October 2010
Available online 3 November 2010
Herein we report a simple ultrasound-assisted procedure to synthesize solid supported Cu(I) and Pd(II)
catalysts. We prepared cross-linked chitosan derivatives (CS–Cu, CS–Pd) by reacting hexamethylene
diisocyanate and chitosan in the presence of the metal salt. The in situ polymerization in water under
sonochemical conditions was extremely fast and efficient. The catalysts were characterized by FT-IR, XPS,
ICP-MS, and TGA. The Cu(I)-loaded polyurethane/urea-bridged chitosan was used for the azide/alkyne
[3 + 2] cycloaddition, while those bearing Pd(II) complexes were used for Suzuki cross-couplings. In both
reactions these novel catalysts permitted the use of simpler procedures and the possibility to reuse the
catalyst several times with a minimal lost in activity. Our polyurethane/urea-bridged CS–Cu and CS–Pd
complexes are superior in term of activity, recyclability and ease of preparation when compared to the
adsorbed Cu or Pd/chitosan catalysts.
Key words:
Cross-linked chitosan
Ultrasound
Heterogeneous catalysis
Cu-catalyzed azide/alkyne click reaction
Pd-catalyzed Suzuki reaction
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
epichlorohydrin, diisocyanates or ethylene glycol glycidyl ether
have commonly been investigated [5–13]. Diisocyanates are an
In synthetic organic chemistry, one of the most direct way to
pursue Green Chemistry is the use of truly efficient catalytic reac-
tions. To address these issues, highly active immobilized metal
catalysts [1–3] are used to significantly facilitate these reactions
and to benefit from a much simplified reaction workup. The hetero-
geneous catalyst is often readily removed by filtration and can be
successfully recycled several times with no detectable metal leach-
ing or loss in activity. Polysaccharides have been widely applied
as a suitable architecture for solid catalysts. Chitosan (CS), the
N-deacetylated derivative of chitin, is an example of such polysac-
charides that is widely distributed in living organisms. Because
of the presence of a free-amino group, CS and its Schiff-base
derivatives are considered to be suitable solid supports for the
immobilization of a metal catalyst [4]. A limiting factor in the
application of CS is its poor chemical resistance and mechani-
cal strength, which significantly reduces the recycle life of this
biopolymer. Physical and chemical modifications have been used to
modify raw CS properties, in order to improve pore size, mechanical
strength, chemical stability, hydrophilicity and biocompatibility.
Modifications using cross-linking agents such as glutaraldehyde,
important class of cross-linking agents for CS owing to their high
reactivity towards amine as well as hydroxyl groups. However, the
high reactivity of common diisocyanates with water and their insol-
ubility in aqueous-acidic solutions represent an evident drawback.
To the best of our knowledge, diisocyanates have so far been uti-
lized to cross-link CS membranes [9,10], films [11], or to graft CS
with synthetic or natural polymers [12,13]. Here we present a novel
procedure for the preparation of in situ cross-linked CS–Cu(I) or
Pd(II) complexes as recyclable solid catalysts.
The synthesis of CS-supported Pd(II) has already been reported
in the literature as well as CS-imine derivatives used in C–C cou-
pling reactions [14,15]. A study on the structure of CS supported
ligand-free Pd(II) has already been published [16] and, based on our
knowledge, only a few reports refer to their application in coupling
reactions [17]. We have recently reported a number of Heck, Suzuki
and Sonogashira reactions catalyzed by Pd(II)-loaded CS derivatives
[18].
The physical properties of Cu–CS complexes are described in
the literature [19,20]. Other authors have reported the use of CS-
Schiff-base derivatives as Cu-chelating agents in heterogeneous
catalysis [21,22]. A few publications have focused on the appli-
cation of non-modified CS-supported copper [23,24]. Herein we
describe the simple and rapid preparation of cross-linked CS sup-
ported Cu(I) or Pd(II) under ultrasound irradiation. The specific
∗
Corresponding author.: Tel.: +39 011 6707684; fax: +39 011 6707687.
E-mail address: giancarlo.cravotto@unito.it (G. Cravotto).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.024

K. Martina et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 60–64
61
advantages of using sonication to remove the air bubbles in the
bulk of CS derivatives and breaking agglomerates to achieve regular
sized nanospheres, have been widely reported [23,25,26], as well
as its influence in the formation of clustering of metal nanopar-
ticles in the presence of chitosan [27]. All these reactions go to
completion in considerably shorter times and at lower temper-
atures, when compared to conventional methods, and positively
influenced the formation of nanostructure [25,27]. Two different
catalysts were prepared, characterized and applied in the Cu-
catalyzed azide/alkyne [3 + 2] cycloaddition (CuAAC) as well as in
Suzuki cross-coupling reactions. Their application under thermal
conditions and under microwave irradiation was investigated.
recovered catalyst was dried over night at 50 ◦C in a drying oven to
be recycled.
When required the product was purified by flash column chro-
matography. All isolated products were confirmed by 1H NMR and
GC-MS and were demonstrated to be more than 99% pure.
2.3. General procedure for Suzuki cross-coupling with
cross-linked CS–Pd(II) catalyst 1
The aryl halides (0.5 mmol) and the phenylboronic acid
(0.55 mmol) were dissolved in H2O/dioxane 9:1 (7.5 ml). CS–Pd (II)
catalyst 1 (1–0.5 mmol%) and triethylamine (1 mmol) were added.
The mixture was stirred at 70–100 ◦C (see Table 3) by conventional
heating or under microwave irradiation (90–100 ◦C, max power
150 W). The mixture was filtered to remove the catalyst and the
solid was washed twice with dioxane and three times with CH2Cl2.
1N HCl and water were added to the filtrate and the desired product
was extracted with CH2Cl2 without any further purifications.
The yield of biphenyl product form Suzuki cross-coupling was
determined by GC-MS.
2. Experimental
All chemicals were purchased from Sigma–Aldrich (solvents
from Carlo Erba SpA) and were used without further purification.
CS of medium molecular weight (from crab shells, 448877), 75–85%
deacetylated was used as the support material. The FT-IR spectra
were recorded with a Perkin-Elmer Fourier transform IR model
2000 spectrometer in the range of 4000–400 cm−1 at a resolu-
tion of 0.2 cm−1. TG curves were recorded by the TA Instrument
TGA 2050. The analyses were carried out at a constant heating
rate of 10 ◦C min−1 from 50 to 700 ◦C under atmospheric condi-
tions. The metal content in solution was determined by ICP-MS on
a Quadrupole-ICP-MS X Series II (Thermo Fisher Scientific) after
the digestion of CS–Pd(II) and CS–Cu(I) in HNO3 and aqua regia (1
HNO3/3 HCl). The oxidation state of the adsorbed palladium or cop-
per was measured by X-ray photoelectron spectroscopy (XPS) with
a Quantum 2000 (PHI Co., Chanhassen, MN, USA) with a focused
monochromatic Al K␣ source (1486.7 eV) for excitation.
3. Results and discussion
3.1. 3.1 Preparation and characterization of the catalysts 1–4
The cross-linked CS–Pd(II) 1 and CS–Cu(I) 2 were efficiently
prepared by reacting the CS with HDI while the cross-linked CS
derivatives 3 and 4 with hexamethylene 1,6-di(aminocarboxy-
sulfonate) (HDACS) in water. The latter was chosen because it is
stable under acidic conditions and easily activated by mild acidity
and heat (60–80 ◦C) [28,29]. The polymeric gels 3 and 4, however,
have been obtained in lower yield after a longer reaction time.
Moreover, the reaction with HDSA was hardly reproducible. The
lower viscosity of 3 and 4 also indicate a poorer reticulation degree
then confirmed by IR analysis. The TGA analysis also showed a lower
decomposition temperature. For all these reasons we decided to
perform all our experiments only with catalyst 1 and 2. All the
reactions were performed in a high-power ultrasound bath. We
suppose that the sonication breaks up intermicellar interaction
and may promote the formation of clustering of metal nanopar-
ticles. We observed that in absence of CuI or Pd(OAc)2 the reaction
failed even with a higher amount of the cross-linker. Nevertheless,
when the metal salt was added afterwards the viscosity steadily
increased until the formation of a turbid polymeric gel. This fact
is not unprecedented, as reported in the literature, the addition of
metals e.g. zinc strongly catalyzed the cross-linking reaction [30].
The IR spectra of plain CS, HDI cross-linked CS–Cu(I) 2 and
HDACS cross-linked CS–Cu(I) 4 were compared (Fig. 1). It is
expected that, with the increasing degree of reticulation, the bands
at around 1650 and 1570 cm−1, assigned to amino I and amino II
functional groups of CS, showed a better resolution. As depicted in
Fig. 1, the IR spectra reveal feature which are consistent with mod-
erate cross-linking for the derivative CS–Cu(I) 4 (HDACS derivative).
The carbonyl resonance at 1700 cm−1 becomes more prominent in
the IR spectrum of HDI cross-linked CS–Cu(I) 2, that evidences the
presence of urea or urethane linkages in CS and an higher degree of
reticulation. IR spectrum of HDI cross-linked CS–Pd(II) 1 was com-
parable to that of catalyst 2, while when the CS was cross-linked
with HDI in the absence of metal the IR was similar to that of plain
CS, that means no reticulation.
GC-MS analyses were performed on an Agilent Technologies
6850 Network GC System with 5973 Network Mass Selective Detec-
tor. NMR spectra were recorded on a Bruker 300 Avance (300 MHz
and 75 MHz for 1H and 13C, respectively) at 25 ◦C; chemical shifts
were calibrated to the residual proton and carbon resonance of the
solvent: CDCl3 (ıH = 7.26, ıC = 77.0).
2.1. Preparation of cross-linked CS–Pd(II) and CS–Cu(I) catalysts
(1–4)
CS (1.0 g) was dissolved in 0.1N HCl (68 ml) in a 250 ml three-
neck round bottom flask at 60 ◦C. A solution of Pd(OAc)2 (0.1 mmol)
or CuCl (0.5 mmol) in 0.1N HCl (12 ml) was added and stirred until a
clear solution was obtained. The mixture was sonicated (19.5 kHz,
30 W) and hexamethylene diisocyanate (HDI) (7.4 mmol, 1.2 ml)
was added dropwise at room temperature and sonicated for about
90 min (60 W) at 50 ◦C. A whitish polymeric (CS–Pd(II)) or a bluish-
pale violet gel (CS–Cu(I)) were obtained. 0.5 N NaOH was added
dropwise up to pH 10. The solid was filtered and washed with water,
acetone and diethylether. The residue was dried over night at 50 ◦C.
This procedure afforded the two Pd(II)- and Cu(I)-loaded
polyurethane/urea-bridged CS derivatives, 0.79 g (CS–Pd(II) 1) and
0.81 g (CS–Cu(I) 2) respectively.
In a variance of this procedure we replaced the HDI with HDACS
(1 g) [25], obtaining the cross-linked CS derivatives 3 and 4.
2.2. General procedure for CuAAC with cross-linked CS–Cu(I)
catalyst 2
CS–Cu(I) 2 catalyst (5–10 mol%) was added to the solution of
the azide (1 mmol) and the alkyne (1 mmol) in H2O/dioxane 8:2
(5 ml). The mixture was heated at 70 ◦C for 30 min. The mixture
was filtered to remove the catalyst and the filter cake was washed
twice with dioxane and three times with CH2Cl2. The solvent
was removed under vacuum to afford the triazole derivatives. The
Thermal gravimetric analysis (TGA) was used to measure and
compare thermal stability. Due to the loss in hydrogen bonding
caused by the derivatization of the amino group, the stability is
expected to decrease. As depicted in Fig. 2, TGA confirmed this
assumption, while CS is stable up to 307 ◦C, cross-linked CS–Pd(II)

62
K. Martina et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 60–64
In the absence of this product, measurements of the differences in
N1s and O1s binding energies in the polymer before and after the
coordination of the metal could not be accomplished.
The weight content of metal in different complexes analyzed by
ICP are 0.38% for cross-linked CS–Pd(II) 1, and 1.10% for cross-linked
CS–Cu(I) 2.
3.2. CuAAC with cross-linked CS–Cu(I) catalyst 2
The resulting cross-linked CS–Cu(I) catalyst 2 was applied
in the Huisgen cycloaddition (Scheme 1). Benzyl azide, phenyl
acetylene were mixed in H2O/dioxane 8:2 at 70 ◦C under mag-
netic stirring in the presence of 10 mol% cross-linked CS–Cu(I).
The reaction was complete after 30 min. Filtration and solvent
evaporation afforded pure triazole regioselectively and almost
quantitatively.
The reaction did not require any bases, ligands or reducing
agents, and metal leaching was negligible, as shown by ICP-MS
analysis of the triazole derivative. Compared to different copper
supported catalysts the cross-linked CS–Cu(I) 2 showed higher
reaction rate [31]. The reported Schiff-base CS–Cu-catalyst required
12 h to afford complete conversion of a similar click reaction [21].
Steric hindrance in either substrate can also reduce the reac-
tion rate. Several additional experiments were carried out using
the same conditions as listed in Table 2. After less than 30 min the
yields were all uniformly high for aromatic alkynes and aliphatic
alkynes, thus confirming the high catalytic activity of cross-linked
CS–Cu(I) 2 (Table 2, Entries 3 and 4).
Fig. 1. IR spectra of chitosan, HDI cross-linked CS–Cu(I) 2 and HDACS cross-linked
CS–Cu(I) 4.
Table 1
XPS data for CuCl, Pd(OAc)2, and the novel catalysts (eV).
Samples
Pd 3d5/2 eV
Cu 2p3/2 eV
CuCl
931.9
932.4
Cross-linked CS–Cu(I)
Pd(OAc)2
Cross-linked CS–Pd(II)
337.2
334.8
336.5
Numerous control experiments indicated the efficacy and the
robustness of the cross-linked CS–Cu(I) catalyst. After the comple-
tion of the reactions, the catalyst was filtered and washed twice
with dioxane and three times with CH2Cl2. The recovered cross-
linked CS–Cu(I) was reused for three cycles without any loss of
activity (Table 2, Entry 2). The copper content was found to be
almost the same in fresh and reused catalysts when checked by
ICP-MS. The treatment of phenyl acetylene and benzyl azide with
5 mol% mol of cross-linked CS–Cu(I) demonstrates that the reac-
tion can be performed at a lower catalyst concentration. This data
was confirmed when the reaction was repeated with 3-butyne-2-
ol and hexadecylazide (Table 2, Entries 4 and 5). The reaction yield
was only moderately affected when the reaction was performed in
ethanol, whereas, no conversion was observed in toluene (Table 2,
Entry 2).
1 and cross-linked CS–Cu(I) 2 (that presented a comparable TGA
profile) are stable up to 278 ◦C. The thermal decomposition tem-
perature (heating rate 10 ◦C min−1) decreased to 211 ◦C when CS
is cross-linked in the presence of HDSA. This indicates the higher
thermal stability of HDI cross-linked CS.
In order to confirm the presence of the metals and the coor-
dination of cross-linked CS with Cu or Pd, the catalysts were
characterized by X-ray photoelectron spectroscopy (XPS). The bind-
ing energy of Cu 2p3/2 in the cross-linked CS–Cu(I) 2 catalyst
increased of 0.5 eV compared with CuCl. As described in Table 1, Pd
3d5/2 signals of catalyst 1 could be divided into two varieties, indi-
cating either the presence of two different Pd species, Pd(0) and
Pd(II), or different coordination possibilities of Pd. The structure
of the catalysts was not investigated by XPS because a prepara-
tion of the cross-linked CS without the metal was not successful.
Fig. 2. Thermal stability of chitosan, HMDI cross-linked CS–Pd(II) 1 and HSACS cross-linked CS–Pd(II) 3.

K. Martina et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 60–64
63
Scheme 1. Synthesis of 1,2,3-triazole catalyzed by cross-linked CS–Cu(I) 2.
Table 2
CuAAC catalyzed by cross-linked CS–Cu(I) 2.
Entry
Alkyne
Azide
Triazole
Yield [%]
95
N
N
N
N3
N3
OH
1
OH
N
N
N
OH
2
3
94, 95a, 93b, 78c, –d
HO
N
N
15N
N
N
15 N3
95
15N
OH
15 N3
4
5
96, 95e
HO
N
N
N
N3
86e
Reaction condition: alkyne, azide, cross-linked CS–Cu(I) 2 10 mol%, H2O:dioxane 8:2, 70 ◦C, 30 min.
a
Yield obtained with recycled catalyst.
Yield obtained with the two cycles/twice recycled catalyst.
The reaction was performed in ethanol.
The reaction was performed in toluene.
b
c
d
e
Yield obtained with cross-linked CS–Cu(I) 2 5 mol%
Table 3
3.3. Suzuki cross-coupling with cross-linked CS–Pd(II) catalyst 1
Heterogeneous Suzuki coupling of 4-bromoacetophenone with phenylboronic acid.
The catalytic activity of cross-linked CS–Pd(II) catalyst 1 was
investigated. A model reaction with 4-bromoacetophenone and
phenylboronic acid was optimized using different reaction condi-
tions (Scheme 2). As shown in Table 3, the reaction was performed
under conventional heating condition and under ultrasound or
microwave irradiation. Satisfactory yields of biaryl product were
obtained at 90 ◦C without the addition of a phase transfer cata-
lyst. The catalytic activity of the complex with different amounts
(1 or 0.5 mol%) was studied and comparable yields were obtained
by increasing the reaction time. A yield of 90% was recovered
with 0.5 mol% in 18 h at 90 ◦C. The reaction time was signifi-
cantly reduced when the reaction was performed under microwave
irradiation, thus confirming its promoting effect on the Suzuki
cross-coupling in water. The feasibility of recycling the catalyst was
Entry
Catalyst 1 (mol%)
Temp. (◦C)
Time (h)
Yield (%)
1
1
70
70
50
90
90
90
100
90
90
18
18
5
4
18
1
2
18
18
90
57
12
98
96
92
89
77
72
2
0.5
0.5
1
0.5
1
0.5
1
1
3a
4
5
6b
7b
8c
9d
Reaction conditions: cross-linked CS–Pd(II) 1 H2O:dioxane 9:1.
a
Ultrasound irradiation (60 W).
Microwave irradiation (150 W).
Recycled catalyst.
Second recycle.
b
c
d
Scheme 2. Suzuki cross-coupling catalyzed by cross-linked CS–Pd(I) 1.

64
K. Martina et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 60–64
also examined. After the reaction the catalyst was simply recov-
ered by filtration and washed. However, a decrease of activity was
observed until the third reuse.
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Acknowledgements
Financial support from the Italian Ministry of Education, Univer-
sity and Research (PRIN 2008, 2008—A Green Approach to Process
Intensification in Organic Synthesis) is gratefully acknowledged.
We warmly thank Dr. T.F. Keller and R. Wagner (Institute of Mate-
rials Science and Technology – University Jena) for XPS-analyses.
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