Suzuki coupling of iodo and bromoarenes catalyzed by chitosan-supported Pd-nanoparticles in ionic liquids

Article abstract of DOI:10.1016/j.jorganchem.2013.11.033

Palladium nanoparticles supported on chitosan (Pd-NPs@Chitosan) have been used for the first time in Suzuki cross coupling of bromo and iodoarenes carried out in ionic liquids. The reaction proceeded smoothly in molten tetrabutylammonium bromide (TBAB) affording unsymmetrical biaryls in good to excellent yields. A low catalyst loading (0.1%) is used and the catalyst can be recycled without deactivation.

Full text of DOI:10.1016/j.jorganchem.2013.11.033

Accepted Manuscript
Suzuki coupling of Iodo and Bromoarenes Catalyzed by Chitosan-Supported Pd-
Nanoparticles in Ionic Liquids
Pietro Cotugno, Michele Casiello, Angelo Nacci, Piero Mastrorilli, Maria Michela
Dell’Anna, Antonio Monopoli
PII:
S0022-328X(13)00845-0
10.1016/j.jorganchem.2013.11.033
JOM 18378
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 September 2013
Revised Date: 4 November 2013
Accepted Date: 25 November 2013
Please cite this article as: P. Cotugno, M. Casiello, A. Nacci, P. Mastrorilli, M.M. Dell’Anna, A. Monopoli,
Suzuki coupling of Iodo and Bromoarenes Catalyzed by Chitosan-Supported Pd-Nanoparticles in Ionic
Liquids, Journal of Organometallic Chemistry (2013), doi: 10.1016/j.jorganchem.2013.11.033.
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ACCEPTED MANUSCRIPT
Palladium nanoparticles supported on chitosan (Pd-NPs@Chitosan) have been used in Suzuki cross
coupling of bromo and iodoarenes carried out in ionic liquids. A low catalyst loading (0.1%) is used
and the catalyst can be recycled without deactivation.

ACCEPTED MANUSCRIPT
Highlights
ꢀ
ꢀ
Palladium nanoparticles supported on chitosan have been used for the first time in Suzuki
cross coupling of bromo and iodoarenes in ionic liquids.
The reaction proceeded smoothly in molten tetrabutylammonium bromide affording
unsymmetrical biaryls in good to excellent yields.
ꢀ
ꢀ
A low catalyst loading (0.1%) is used and the catalyst can be recycled without deactivation.
Hot-filtration test has been done in order to confirm a heterogeneous catalysis.

Suzuki coupling of Iodo and Bromoarenes Catalyzed by Chitosan-
Supported Pd-Nanoparticles in Ionic Liquids
a
a
a,b
c
ACCEPTED MANUSCRIPT
Pietro Cotugno, Michele Casiello, Angelo Nacci , Piero Mastrorilli , Maria Michela
c
,a
Dell’Anna and Antonio Monopoli*
a
Department of Chemistry, Università degli Studi di Bari "Aldo Moro", Via Orabona, 4,
0126 Bari – Italy. E-mail: antomono@libero.it
CNR – ICCOM, Department of Chemistry, Università degli Studi di Bari “Aldo Moro”, Via
Orabona, 4, 70126 Bari – Italy
7
b
c
DICATECh, Politecnico di Bari, via Orabona, 4, 70126 Bari - Italy

Abstract. Palladium nanoparticles supported on chitosan (Pd-NPs@Chitosan) have been used
for the first time in Suzuki cross coupling of bromo and iodoarenes carried out in ionic
liquids. The reaction proceeded smoothly in molten tetrabutylammonium bromide (TBAB)
affording unsymmetrical biaryls in good to excellent yields. A low catalyst loading (0.1%) is
used and the catalyst can be recycled without deactivation.
K eywords: Suzuki coupling; heterogeneous catalyst; palladium nanoparticles; chitosan; ionic
liquids
Introduction
Biaryl scaffolds are ubiquitous in nature and are widely used as structural subunit in
molecules of interest in agrochemical, pharmaceutical, and material fields.[1] Since from the
discovery of the Ullmann homocoupling of aryl halides promoted by stoichiometric amount
of copper at high temperatures,[2] the search for more efficient syntheses of biaryl
compounds has attracted many efforts by synthetic chemists for a long time. However, the
Ullmann reaction is considered of narrow scope, being limited to the preparation of
symmetrical biaryls, while for unsymmetrical compounds, a certain number of metal
catalyzed reactions has been discovered in the last 40 years, in which palladium chemistry is
used predominantly [3].
In this context, the Suzuki cross-coupling reaction is considered the most efficient way
of access to biaryls. The main advantages of this process are essentially: i) mild reaction
conditions (generally room temperature), ii) commercial availability of boronic acids and
aromatic halides, iii) water and functional groups tolerance, iv) high stereo- and regio-
selectivity. In addition, boron-containing by-products are much easier to handle and to be
removed from the reaction medium, in comparison with other organometallic reagents.
Finally, a very low catalyst loading are often needed and is comfortable to work under
heterogeneous conditions. [4]
As far as this latter topic is concerned, many robust palladium-based catalysts for
Suzuki coupling have been developed in recent years. [5-11] Among them, palladium

nanoparticles (Pd-NPs) have recently gained popularity, [12] and a number of solid supports
have been used for their immobilization such as double-layered hydroxide (LDH), [13]
diatomite, [14] alumina-based oxides, [15] MgO, [16] and polyaniline nanofibers. [17]
ACCEPTED MANUSCRIPT
Moreover, many efforts have been done for searching greener reaction conditions, and a
special attention have been paid to eco-friendly solvents like water [18-19] and ionic liquids.
[
20-21]
In this context, in the last decade we also investigated a number of organic and
inorganic matrixes for anchoring metal nanocatalysts suitable to work in these
environmentally benign media. [22-24] Among the several supports examined, chitosan
(poly[β-(1–4)-2-amino-2-deoxy-D-glucan]) displayed better performances providing a prompt
anchorage to Pd nanoparticles generated electrochemically. [24] The so formed heterogeneous
catalyst was very active in the Heck olefination of haloarenes in ionic liquids, being capable
of converting iodoarenes in only 15 min with an extensive recycle (about ten runs).
Chitosan is a cheap and non-toxic polymer obtained by deacetylation of chitin, that
has been used in agrochemical, food and pharmaceutical fields. [25] The use in heterogeneous
catalysis is encouraged by its easy and clean elimination by incineration (or dissolution in
acidic water solutions) that can allow a quantitative metal recovery. [26]
Due to its strong affinity for palladium, [27] chitosan has been used as supramolecular
ligand for the Suzuki coupling, [28-31] but in a sole example Pd nanoparticles have been used
as the catalyst source, [32] and to the best of our knowledge no protocols of Suzuki reactions
promoted by Pd-NPs on chitosan have been reported in ionic liquids media.
In this context, as a part of our program aimed at finding new eco-sustainable catalytic
protocols, [33-40] we decide to extend these findings to the synthesis of biaryls. Here, we
report the first application of a chemically generated Pd nanocomposite on chitosan to the
Suzuki coupling carried out in ionic liquids.

E xperimental Procedures
Materials. Ionic liquids [tetrabutylammonium bromide (TBAB), tetrabutylammonium acetate
(
TBAA), tetrabutylammonium chloride (TBAC), 1-butyl-3-methylimidazolium bromide
ACCEPTED MANUSCRIPT
(Bmim Br) and N-Butylpyridinium bromide (Bupy Br)], aryl halides and Pd acetate were
Sigma-Aldrich reagents. Chitosan flakes (medium molecular weight of about 400,000) were
purchased from Fluka and utilized as received. The structures of the biaryl compounds were
1
13
determined by comparison with authentic samples, or by MS, H-NMR and C-NMR spectra.
Procedure for the preparation of Pd-NPs supported on chitosan (Pd-NPs@Chitosan)
The nano-composite was prepared according to a procedure previously described. The
catalyst has also been deeply characterized by means of XPS, cross-sectional TEM and
ETAAS analyses. [24]
Summarizing, Pd-NPs on chitosan were electro-synthesised a in a three-electrode cell by
means of the sacrificial anode technique. In a typical experiment 200mg of chitosan were
suspended in 5 mL of an acetonitrile/ tetrahydrofuran (1/3) mixture containing TBAB as base
electrolyte (0.1M) under nitrogen atmosphere. A potential of+1.5 V (vs. reference) was
applied to the working electrode during the process, that was coulometrically controlled and
stopped at an electrolysis charge value of 200 C. Evaporation under vacuum of the solvent
leaves a grey solid that is stored under nitrogen. The Pd concentration on chitosan was
determined by Atomic Absorption Spectroscopy (ETAAS) after acid mineralization and it
was found equal to 0.35% w.
The so-prepared nanocomposite is a cortex-type catalyst presenting Pd nanoparticles with an
average core of diameter of about 3 nm, well-dispersed onto the surface of chitosan flakes.
[
24]
General procedure for Suzuki coupling catalyzed by Pd-NPs@Chitosan in ILs
A 5 mL screw cap vial was charged with tetrabutylammonium bromide (0.8 g, 2.5 mmol),
haloarene (1 mmol), boronic acid (1.1 mmol), K CO (1.5 mmol), H O (200 µL), and 30 mg
2
3
2
of the catalyst Pd-NPs@Chitosan (loading 0.1 mol%). The resulting mixture was heated
under stirring at 90 °C for 5 hrs.
Then, after cooling at room temperature, 3 mL of n-hexane were added to the reaction and the
mixture was stirred for 1 min. Next, after centrifugation, the supernatant solution containing
the biaryl product was removed with glass pipette and the operation was repeated twice. The
organic phases were collected and added of n-decane as an external standard to evaluate
conversions, yields and selectivities reported in Tables 1-2.

In case of recycle experiments, the remaining mixture was washed and centrifuged with water
to remove inorganic by-products, leaving a mix containing the catalyst that was softly dried
under a nitrogen steam and, upon addition of fresh reagents, repeatedly recycled under one-
ACCEPTED MANUSCRIPT
pot conditions. Because of the catalytic system proved to be air-stable, no care was taken to
exclude oxygen during operations.
R esults and Discussion
As reported in the above section, the catalyst was obtained by electro-chemical
reduction of Pd acetate in the presence of flakes of chitosan and tetrabutylammonium bromide
(TBAB) (see experimental section) [24].
Under these conditions, the nanoparticles metallic core is surrounded by a stabilising
-
2-
layer composed by tetrabutylammonium cations mixed with Br and [PdBr ] anions that
4
counterbalance the positive charges in the shell. The core-shell nanostructured catalyst is
chemisorbed on the chitosan surface by means of coordination bonds with amino and
hydroxyl groups of the polymer surface (Figure 1).
-
+
Br
4
R N
+
+
R
Br
4
N
R
4
N
-
-
Br
+
+
Pd 0
R N
4
4
R N
-
Br
+
+
R
4
N
R
4
N
-
+
Br R
N
4
-
NHR
Br CH
2
OH
O
HO
O
O
O
O
HO
n
CH
2
OH
NHR
Figure 1. A representation of the core-shell structure of Pd nanoparticles chemisorbed on chitosan in
tetraalkylammonium-based ionic liquids
Preliminary experiments aimed at optimizing the reaction conditions, were calibrated
for processing 1 mmol of bromobenzene and p-tolylboronic acid as model substrates, with a
catalyst loading of 0.1 mol%. A temperature of 90 °C and a reaction time of 5 hours were
found as the maximum values after which no further reaction progress was observed (Table
1
).
The blank experiments, carried out in the absence of catalyst or in the presence of the
sole support, gave no conversion, thus showing that the catalytically active material is related
to palladium nanoparticles.
From data reported in Table 1 immediately emerged the scarce performances
displayed by molecular solvents traditionally used in Suzuki coupling (toluene or DMF), even

when used in the presence of water as co-solvent, which is known to have a beneficial effect
on the coupling (Table 1, runs 1-4). Similarly, ionic liquids based on imidazolium and
pyridinium cations like [Bmim]Br (1-Butyl-3-methylimidazolium bromide) and [BuPy]Br (1-
ACCEPTED MANUSCRIPT
butylpyridinium bromide) furnished disappointing results (Table 1, runs 5-7).
These findings are in line with our previous ones on the Heck coupling, [24] in which
+
+
ionic liquids bearing planar aromatic cations, like [Bmim] and [BuPy] , proved to be less
efficient than the ones possessing tetrahedral cations, such as the tetraalkylammonium salts,
probably due to the superior ability of these latter in stabilizing the nanoparticles core.
Among the quaternary ammnonium salts, tetrabutylammonium bromide (TBAB)
displayed the best performances if compared with the analogous molten salts bearing acetate
and chloride anions (Table 1, runs 8-12).

Table 1
Optimization of reaction conditions in the Suzuki coupling of bromobenzene and p-
a)
tolylboronic acid catalysed by Pd-NPs@Chitosan
ACCEPTED MANUSCRIPT
Br
B(OH)2
PdNPs@CS (0.1%)
Solvente/Base
H C
+ H C
CH3
+
3
3
90 °C, 5 h
cross-coupling product
homocoupling product
CH3
b)
c)
Run
Solvent
Base/Additive
Conv . (%)
Sel. (%)
1
2
3
4
5
6
7
8
9
DMF
NaOH
22
35
35
40
8
89
45
93
97
95
96
97
97
97
96
97
98
97
96
98
DMF
K CO3
2
DMF
K CO /H O
2
3
2
Toluene
K CO3
2
[Bmim]Br
[Bmim]Br
[BuPy]Br
TBAA
K CO3
2
K CO /H O
10
14
51
35
25
58
88
48
98
98
2
3
2
K CO3
2
TBAA
TBAA
K CO3
2
10
11
12
13
14
15
TBAC
K CO3
2
TBAB
TBAA
TBAB
K CO3
2
d)
TBAA
K CO /H O
2 3 2
d)
TBAB
K CO /H O
2 3 2
d)
TBAB
Cs CO /H O
2 3 2
a)
Reaction conditions: p-tolyl boronic acid (1.1 mmol), bromobenzene (1 mmol), base (1.5
mmol), catalyst Pd-NPs@Chitosan (0.1 mol%), solvent (1 mL or 0.8 g of molten salt TBAA or
b)
TBAB), heated under stirring for 5 hours. As determined by GLC using n-decane as external
c)
d)
standard. Percentage of the coupling respect to the homocoupled product. Reaction carried
out with addition of 200 µL of water.
The nature of the base, as already reported by us, [22,35] strongly affected the
coupling, and in these cases better results were obtained by using inorganic bases such as
K CO or Cs CO (Table 1, runs 12, 14-15).
2
3
2
3
As expected, the addition of small amounts of water had a beneficial effect on
conversion, probably due to two main reasons: i) the increase of the solubility of inorganic

-
bases into the reaction mixture and ii) the formation in situ of the good nucleophile OH
which would activate boronic acid by forming boronate species (Table 1, runs 13-15).
On the basis of this preliminary screening, the optimal reaction conditions for Suzuki
ACCEPTED MANUSCRIPT
coupling promoted by Pd-NPs on chitosan could be established as follows: molten TBAB as
the solvent, Pd catalyst (0.1 mol%), K CO (1.5 eq), H O as an additive (200 µL), 90 °C as
2
3
2
the temperature. Potassium carbonate was preferred to cesium carbonate due to the toxicity of
the latter one.
With this protocol in hand, we extended the coupling to various aryl halides and
boronic acids. Results reported in Table 2 show a good catalytic activity of the system Pd-
NPs@Chitosan, which can promote the coupling of haloarenes bearing both electron-donating
and electron-withdrawing groups with good to excellent yields. Only a slight decrement of
yield was observed in the case of the less reactive p-bromotoulene and p-bromoanisole (Table
2
, runs 8-9).
In case of iodides, the temperature was lowered to 70° C in order to limit the
undesirable side-reaction of homocoupling of boronic acid (Table 2, runs 1-2). Unfortunately,
chlorobenzene proved to be unreactive under our conditions (Table 2, run 11).

Table 2
a)
Suzuki cross-coupling reaction catalyzed by Pd-NPs@Chitosan in TBAB
R1
R2
1
R2
PdNPs@chitosan (0.1%) R
X +
AC
B
C
(O
E
H
P
)
TED MANUSCRIPT
2
TBAB, T °C, 5 h
1
2
Yield
Run
X
R
R
T
Product
b)
(%)
1
2
3
4
5
6
7
8
9
1
1
I
H
OCH3
OCH3
H
70
OCH3
OCH3
96
c)
I
H
90
70
70
70
90
90
90
90
90
90
77
I
H
98
I
CH3
H
CH3
NO2
95
98
97
92
88
91
95
<1
I
NO2
H
Br
Br
Br
Br
Br
Cl
H
H
H
OCH3
H
OCH3
OCH3
CH3
OCH3
H
CH3
H
0
1
C(O)CH3
H
C(O)CH3
CH3
CH3
a)
Reaction conditions: boronic acid (1.1 mmol), halobenzene (1 mmol), K CO (1.5 mmol),
2
3
catalyst PdNPs@Chitosan (0.1 mol%), solvent (0.8 g of molten TBAB), heated under stirring for
b)
c)
5
hours. As determined by GLC using n-decane as external standard. A 20% of biphenyl was
detected.
To evaluate the catalyst robustness we carried out appropriate recycling experiments. After
each run, the supported nanocatalyst was washed by centrifugation, firstly with hexane

(3x3mL) and then with water to remove both organic and inorganic residues. Then, the solid
was used in the next run without isolation and further manipulations.
Results in figure 2 show that Pd-NPs@CS nanocomposite underwent a loss of activity
ACCEPTED MANUSCRIPT
after the third cycle, probably due to the leaching of Pd during the reaction or the loss of
material during the recycling procedure.
I
B(OH)2
CH3
PdNPs@Chitosan (0.1%)
H C
+
3
TBAB, K CO3
2
70 °C, 5 h
100
75
50
25
0
1
2
3
4
5
6
n. of cycles
Figure 2. Recycling experiments in the Suzuki coupling catalyzed by PdNPs@Chitosan in TBAB.
Reaction conditions: Iodotoluene (1.0 mmol), phenylboronic acid (1.1 mmol), K CO (1.5 mmol),
2
3
H O 200 µL, catalyst PdNPs@Chitosan (0.1 mol%), TBAB (0.8 g) at 70 °C. Recycle number 6 was
2
obtained by weighing the recovered catalyst coming from the fifth recycle, then adding fresh reagents
in a proper amount to realize a Pd loading of 0.1 mol% (see ref. [41]). Reaction conditions: PhI (0.5
mmol), phenylboronic acid (0.55 mmol), K CO3 (0.75 mmol), H O 100 µL, catalyst
2
2
PdNPs@Chitosan (16 mg, 0.1 mol%), TBAB (0.4 g) at 70 °C
However, being the catalyst loading very low (0.1 mol%) and the yield averaged on
three runs of 80% ca., an overall TON of 2400 can be estimated. In order to ascertain if the
loss of activity was due to the leaching of palladium during the reaction, a hot filtration test
was performed.
Data in figure 3 clearly show that catalytic activity dropped down when the Pd-

NPs@Chitosan nanocomposite was removed from the reaction mixture (see dashed line in
Figure 3), thus confirming that catalysis is heterogeneous in nature and that the metal leaching
can be neglected. Consequently, the decrement of catalytic activity can be attributed to a
ACCEPTED MANUSCRIPT
progressive loss or inactivation of palladium nanoparticles during the recycling manipulations.
In order to ascertain which of the two explanations actually occurs, at the end of the fifth
cycle, the catalyst was removed, washed, dried under vacuum and weighted, giving 18 mg of
nanocomposite and clearly evidencing a loss of catalytic material. As a further confirmation,
the addition of fresh reagents calibrated on the remaining 18 mg of PdNPs@Chitosan [41],
after 5 hours gave the desired biphenyl in 91% of conversion (Fig. 2, run 6), still showing a
good activity of the catalyst and excluding its deactivation. Experiments are in progress to
improve the recycling procedure and minimize the loss of active material.
100
80
60
40
20
0
0
1
2
3
4
5
6
7
time (h)
Figure 3. Hot-Filtration test. Reaction conditions: Iodotoluene (1.0 mmol), phenylboronic acid (1.1
mmol), K CO (1.5 mmol), H O (200 µL), catalyst Pd-NPs@Chitosan (0.1 mol%), TBAB (0.8 g) at
2
3
2
7
0 °C. Continue line: reaction performed as in table 2, run 2. Dashed line: reaction results after
catalyst removal at about 50% of conversion.
The role of Pd colloids in the catalytic cycle and their behaviour at the presence of
tetraalkylammonium salts has been the subject of many pioneering studies [42], and results of
our hot filtration test, which account for a heterogeneous catalysis, may appear in contrast
with some of them. Indeed, Reetz et al. [42d] demonstrated the easy solubilisation of Pd
+
2-
clusters stabilized by Oct N (HCO ) as a consequence of the oxidative insertion into the C-I
4
bond of iodobenzene, with formation of soluble aryl-palladium species. This means that in
our reactions the leaching of the metal into the ionic medium would be expected and
consequently a homogeneous catalysis would occur.
It should be noted that the question homogeneous vs heterogeneous nature of catalysis in
the case of metal nanoparticles has been long debated, and we do not have an exhaustive

explanation for the present case. We can speculate, as also reported by others [43], that
catalysis should occur on the nanoparticles surface, but if Pd-aryl species are released into the
reaction medium as an effect of the oxidative addition, then the Pd atoms may re-deposit on
ACCEPTED MANUSCRIPT
colloid due to molecular collisions after the closure of cycle. In this case, catalysis would be
homogeneous in nature and clusters should act only as a catalyst reservoir.
Conclusions
In this work we have reported for the first time the use of a Pd nanoparticle-based
catalyst supported on chitosan to the Suzuki coupling carried out in ionic liquids. Excellent
yields and selectivities have been achieved in relatively short reaction times (5 hrs) on a wide
array of iodo- and bromo-arenes bearing both electron-donating and electron-withdrawing
groups.
Satisfactory activity and good TONs (2400 ca.) were achieved in a one-pot manner and
the catalyst may be reused provided that careful manipulations are carried out to prevent
accidental loss of active material. The gradation of the aryl bromide conversion slightly
depended on the electronic effect of substituent in para-position to the bromine. The studies
also revealed that the palladium leaching into the solution during the reaction is negligible
and therefore the catalysis is heterogeneous in nature.
Remarkably advantageous conditions like the low catalyst loading (0.1%), the use of
cheap and eco-friendly support (chitosan) and of a green solvent (TBAB, an ionic liquid)
provide to the method great benefits in terms of safety, economy and sustainability. In
addition, the use of water as an additive proved to be particularly advantageous for its ability
to dissolve boron side-products coating the catalyst surface.
Therefore, we believe that it can compete with the most efficient known protocols, and
due to its simple operating procedure we can anticipate that it will find wide applicability.
A cknowledgments
Partial financial support of this work by the Ministry of University and Scientific Research of
Italy (MIUR) and by University of Bari is gratefully acknowledged.
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