Pd nanosized particles supported on chitosan and 6-deoxy-6-amino chitosan as recyclable catalysts for Suzuki-Miyaura and Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.apcata.2010.12.002

Several chitosan and 6-deoxy-6-amino chitosan-Schiff base ligands (1-4) were prepared by condensation of either 2-pyridinecarboxaldehyde or 2-(diphenylphosphino)benzaldehyde with the amino group(s) on chitosan and its derivative (6-deoxy-6-amino chitosan). The supported ligands were reacted with [PdCl₂(COD)] to form chitosan-supported Pd^II catalysts (5-8). All the supported catalysts were air- and moisture-stable and have been characterized using elemental analysis, ICP-MS, UV-vis, FT-IR, PXRD, TGA, ³¹P solid state NMR and TEM. As models for the heterogenized catalysts (5 and 6), mononuclear Pd^II complexes (9 and 10) were also prepared via the Schiff-base condensation reaction of 1,3,4,6-tetra-O-acetyl- β-d-glucosamine hydrochloride to form 1,3,4,6-tetra-O-acetyl-β-d- glucos-2-pyridylimine and 1,3,4,6-tetra-O-acetyl-β-d-glucos-2- (diphenylphosphino)imine which were subsequently reacted with [PdCl ₂(COD)]. Complexes (9 and 10) and their precursors were characterized by ¹H and ³¹P NMR, UV-vis, FT-IR spectroscopy and elemental analysis. Catalytic Suzuki-Miyaura and Heck carbon-carbon cross-coupling reactions were carried out using the supported Pd catalysts and their mononuclear analogues. The immobilized and homogeneous catalysts showed high activity for both the Suzuki-Miyaura and Heck cross-coupling reactions in organic and aqueous media. Homogeneous catalysts (9 and 10) decomposed during the first run, while the supported catalysts could be recycled and reused up to five times.

Full text of DOI:10.1016/j.apcata.2010.12.002

Applied Catalysis A: General 393 (2011) 231–241
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Pd nanosized particles supported on chitosan and 6-deoxy-6-amino chitosan as
recyclable catalysts for Suzuki–Miyaura and Heck cross-coupling reactions
Banothile C.E. Makhubela, Anwar Jardine, Gregory S. Smith∗
Department of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Several chitosan and 6-deoxy-6-amino chitosan-Schiff base ligands (1–4) were prepared by condensation
of either 2-pyridinecarboxaldehyde or 2-(diphenylphosphino)benzaldehyde with the amino group(s)
on chitosan and its derivative (6-deoxy-6-amino chitosan). The supported ligands were reacted with
[PdCl2(COD)] to form chitosan-supported PdII catalysts (5–8). All the supported catalysts were air- and
moisture-stable and have been characterized using elemental analysis, ICP-MS, UV–vis, FT-IR, PXRD,
TGA, 31P solid state NMR and TEM. As models for the heterogenized catalysts (5 and 6), mononuclear
PdII complexes (9 and 10) were also prepared via the Schiff-base condensation reaction of 1,3,4,6-tetra-
O-acetyl-␤-d-glucosamine hydrochloride to form 1,3,4,6-tetra-O-acetyl-␤-d-glucos-2-pyridylimine and
1,3,4,6-tetra-O-acetyl-␤-d-glucos-2-(diphenylphosphino)imine which were subsequently reacted with
[PdCl2(COD)]. Complexes (9 and 10) and their precursors were characterized by 1H and 31P NMR, UV–vis,
FT-IR spectroscopy and elemental analysis.
Catalytic Suzuki–Miyaura and Heck carbon–carbon cross-coupling reactions were carried out using the
supported Pd catalysts and their mononuclear analogues. The immobilized and homogeneous catalysts
showed high activity for both the Suzuki–Miyaura and Heck cross-coupling reactions in organic and
aqueous media. Homogeneous catalysts (9 and 10) decomposed during the first run, while the supported
catalysts could be recycled and reused up to five times.
Received 18 June 2010
Received in revised form
30 November 2010
Accepted 1 December 2010
Available online 9 December 2010
Keywords:
Chitosan
Suzuki–Miyaura
Heck
Heterogeneous PdII catalysis
Schiff-base
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
result especially in the pharmaceutical industry [1,2]. Therefore,
there is a need to develop improved and practical strategies for
Palladium-catalyzed cross coupling reactions of organo-halides
with olefins (Heck) and organo-boronic acids (Suzuki) for
carbon–carbon bond formation are extremely useful to the chemi-
cal industry and in research. Since their discovery they have evolved
into a general technique in preparing biologically active function-
alized biphenyls which are important intermediates or products in
drug discovery, pharmaceuticals and agricultural compounds [1,2].
Historically, palladium complexes such as [Pd(OAc)2] and
[Pd(PPh3)2Cl2] have been widely used as homogeneous catalyst
systems in cross coupling reactions [1]. However, these homo-
geneous catalytic systems suffer from problems associated with
the separation and recovery of the active catalyst as well as
instability at high temperatures. These drawbacks have so far pre-
cluded their industrial exploitation [3]. Also from the perspective
of process development, homogeneous catalysts require expen-
sive phosphine ligands (to generate the active catalyst) which are
often not available in bulk. Metal contamination of the products
is inevitable when using homogeneous catalysts, an undesirable
recycling active catalysts for economical and environmental stew-
ardship reasons [3].
Most of the problems related to homogeneous catalysts can
be solved by immobilizing the catalyst or catalyst precursor on
polymer supports with good solvation attributes [4–6]. For exam-
ple, supporting transition metal catalysts on insoluble polymer
supports can improve their stability without compromise in the
activity and selectivity of the homogeneous catalyst. Supported cat-
alysts alsoallowsimplifiedrecoveryandreuse ofthecatalystaswell
as physical separation of the active site, thus minimizing catalyst
self-destruction [6a–b].
Due to the inherent advantages of heterogenizing homogeneous
catalysts through immobilization on solid supports, a great deal of
effort has been devoted to these developments [4–6]. However,
the majority of these reported catalysts are based on synthetic
organic polymer supports (polystyrene, poly(ethylene)glycol, etc.)
and inorganic supports (silica, alumina or other metal oxides).
Some of these supported catalysts, such as [Pd(PPh3)4]-crosslinked
polystyrene-bound and Pd0 on alumina, are commercially available
[5a–c]. Recent efforts in the development of cleaner and sustainable
chemistry have lead to the use of biopolymers as catalyst sup-
ports. Biopolymers are readily available in nature and can be used
∗
Corresponding author. Fax: +27 21 650 5195.
E-mail address: Gregory.Smith@uct.ac.za (G.S. Smith).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.12.002

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B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
Fig. 1. Idealized structures of chitin and chitosan. (Chitosan is rarely 100% deacetylated, so it is more accurately described as a copolymer intermediate between chitin and
chitosan.).
as suitable supports for many reagents and catalysts, offering the
advantages of being renewable, biodegradable and non-toxic. As
such, biopolymers such as cellulose, gelatine and starch have been
investigated as catalyst supports [6].
a Perkin-Elmer Spectrum One FT-IR spectrophotometer. Elemental
analyses were conducted with a Thermo Flash 1112 Series CHNS-
O Analyzer. Inductively coupled plasma-mass spectrometry was
obtained using a Perkin-Elmer Elan600 quadrupole ICP-MS with
a Cetax LSX-200 UV laser module. TEM imaging was done on a
JEOL 1200EXII CRYO TEM. Catalysis products were analysed using
a Varian 3900 GC. 1H and 31P NMR spectra were recorded on a Var-
ian XR400 MHz spectrometer using tetramethylsilane (TMS) as the
internal standard (for 1H) and H3PO4 as the external standard (for
31P).
Chitosan is produced by deacetylation of the abundant biopoly-
mer chitin, a key constituent of the exoskeletons of crustaceans and
the cell walls of algae (Fig. 1) [4,6,7]. It has been shown to exhibit
interesting biopesticidal, antifungal and anticancer properties and
has been used successfully in food and water treatment [6a,7a–e].
Chitosan can be readily transformed into films or fibres and has
found applications as adsorbents for metals, in medicine and in
drug delivery [4,6,7]. Its chirality, insolubility (in many organic sol-
vents) and capability of being cast into films and fibres from dilute
acid makes chitosan an excellent candidate as a support for cata-
lysts. In this respect, several catalytic systems using chitosan as a
support are known [6,8]. Functionalization of chitosan to provide
coordination sites has been carried out and this has provided cat-
alysts for oxidation, cyclopropanation of olefins, Suzuki and Heck
cross coupling reactions [4,7,9].
We now present our work on the synthesis, characterization and
evaluation of chitosan- and 6-deoxy-6-amino chitosan-supported
Schiff base PdII complexes and mononuclear PdII complexes in the
catalytic Suzuki–Miyaura and Heck cross coupling reactions. Our
aim was to use chitosan and 6-deoxy-6-amino chitosan-supports
to aid catalyst recovery, reuse and stability. Chitosan-supported PdII
iminopyridyl catalysts, were reported to catalyze Heck and Suzuki
cross-coupling reactions [4b,9f]. This work has expanded on this by
preparing new iminophosphine and iminopyridyl PdII supported
catalysts for Suzuki and Heck cross-coupling reactions. Chitosan
has been modified to 6-deoxy-6-amino chitosan for increased solu-
bility purposes and application as a gene carrier [10d]. Furthermore,
the synthetic methodology for this has been improved to obtain
higher amine loading [10a]. Increased solubility and metal loading
was expected of the modified chitosan, therefore the activity and
efficiency of catalysts (7 and 8) were compared to (5 and 6) and
these in turn to homogeneous catalysts (9 and 10).
2.1. General procedure for the synthesis of 6-deoxy-6-amino
chitosan- and chitosan-Schiff base ligands (1–4)
Chitosan and the corresponding aldehydes (30 mmol) were
refluxed in methanol (30 ml) and acetic acid (3 ml) for 10 h. After
completion of the reaction, the chitosan-Schiff base ligands were
collected by filtration and washed thoroughly with distilled water,
ethanol then acetone (50 ml each), respectively. The product was
then dried under vacuum at 60 ◦C for 8 h.
6-Deoxy-6-amino chitosan-Schiff base ligands (3) and (4) were
prepared by a similar procedure as above.
2.1.1. Preparation of chitosan-2-pyridylimine (1)
Compound (1) was prepared using chitosan (1.90 g) and 2-
pyridinecarboxaldehyde (2.87 ml, 30.08 mmol). The product was
obtained as a light brown solid. Yield by mass (1.60 g, 84%). FT-
IR (KBr): 3422 ꢀ(OH), 2881 ꢀ(C–H), 1649, ꢀ(C N), 1590 ꢀ(pyridyl
C
N), 1569 ꢀ(C C), 1153–1070 ꢀ(pyranose), 776 cm−1 ꢀ(aromatic
C–H). Elemental analysis, Found: C, 54.79; H, 6.02; N, 10.39.
2.1.2. Preparation of chitosan–2(diphenylphosphino)imine (2)
Compound (2) was prepared using chitosan (0.500 g) and
2-(diphenylphosphino)benzaldehyde (0.160 g, 0.56 mmol). The
product was obtained as a light brown solid. Yield by mass (0.725 g,
69%). FT-IR (KBr): 3429 ꢀ(OH), 2917 ꢀ(C–H), 1642, ꢀ(C N), 1577
ꢀ(aromatic C C), 1158–1024 ꢀ(pyranose), 665 cm−1 ꢀ(aromatic
C–H). Elemental analysis, Found: C, 44.63; H, 6.92; N, 7.03.
2. Experimental
2.1.3. Preparation of 6-deoxy-6-amino chitosan-2-
pyridylimine (3)
Low molecular weight chitosan (Cat. No. 44,886-9, deacetylation
75–85%), 1,5-cyclooctadiene (COD), 2-pyridinecarboxaldehyde
and 2(diphenylphosphino)benzaldehyde were purchased from
Sigma–Aldrich and used as received. 6-Deoxy-6-amino chitosan
[10a] and 1,3,4,6-tetra-O-acetyl-␤-d-glucosamine hydrochloride
[11a] were prepared according to reported procedures. All solvents
were obtained commercially and distilled under N2 prior to use.
Methanol, ethanol, dichloromethane and acetone were dried over
calcium hydride. PdCl2 was obtained from Johnson Matthey and
[PdCl2(COD)] [11b], was prepared according to a literature proce-
dure.
Compound (3) was prepared using 6-deoxy-6-amino chitosan
(400 mg) and 2-pyridinecarboxaldehyde (1.20 ml, 12.53 mmol).
The product was obtained as a brown solid. Yield by mass (241 mg,
60%). FT-IR (KBr): 3434 ꢀ(OH), 2919 ꢀ(C–H), 1648 ꢀ(C N), 1591
ꢀ(pyridyl C N), 1569 ꢀ(aromatic C C), 1154–1069 ꢀ(pyranose),
777 cm−1 ꢀ(aromatic C–H). Elemental analysis, Found: C, 50.08; H,
5.89; N, 9.10.
2.1.4. Preparation of 6-deoxy-6-amino 2(diphenylphosphino)
imine (4)
Compound (4) was prepared using 6-deoxy-6-amino chitosan
(500 mg) and 2-(diphenylphosphino)benzaldehyde (142 mg,
0.490 mmol). The product was obtained as an orange solid.
UV–vis spectra were obtained at ambient temperature using a
Varian Cary 50 Conc. UV–vis spectrophotometer as glycerol mulls.
Powder X-ray diffraction (PXRD) data was collected on a Bruker D8
Advanced diffractometer. IR spectra were recorded as KBr disks on

B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
233
Yield by mass (324 mg, 65%). FT-IR (KBr): 3436 ꢀ(OH), 2913
ꢀ(C–H), 1639 ꢀ(C N), 1154–1023 ꢀ(pyranose), 697 cm−1
ꢀ(aromatic C–H). Elemental analysis, Found: C, 45.66; H, 6.22;
N, 7.28.
(m) ꢀ(pyridyl C N), 1569 (m) ꢀ(aromatic C C), 1220–1206 (vs)
ꢀ(pyranose), 779 cm−1 (s) ꢀ(aromatic C–H). 1H NMR (400 MHz,
DMSO-d6, 25 ◦C): ı (ppm) = 8.66 (d, 1H, J = 4.8 Hz, Arpyr), 8.36 (s,
1H, imine), 8.00 (m, 1H, Arpyr), 7.76 (m, 1H, Arpyr), 7.36 (m, 1H,
Arpyr), 6.00 (d, 1H, J1,2 = 9.5 Hz, H-1), 5.48 (t, 1H, J3,4 = 9.7 Hz, H-3),
5.18 (t, 1H, J2,3 = 8.37 Hz, H-2), 4.16 (dd, 2H, J5,6 = 1.9, 12.4 Hz, H-6),
4.00 (m, 1H, H-4), 3.60 (dd, 1H, J4,5 = 9.5 Hz, H-5), 1.98–2.21 (4s,
12H, OAc).
2.2. General procedure for the synthesis of 6-deoxy-6-amino
chitosan- and chitosan-Schiff base PdII catalysts (5–8)
The 6-deoxy-6-amino chitosan- and chitosan-Schiff base lig-
ands were stirred with a solution of [PdCl2(COD)] in acetone, at
room temperature over 48 h. After the reaction, the supported
Schiff base catalysts were collected by filtration, washed with dis-
tilled water, ethanol then acetone (50 ml each), respectively then
dried under vacuum at 60 ◦C for 8 h.
[PdCl2(COD)] (131 mg, 0.459 mmol) was added to the 1,3,4,6-
tetra-O-acetyl-␤-d-glucosyl-2-pyridylimine
ligand
(200 mg,
0.459 mmol) in dry CH2Cl2 (25 ml) and this stirred at room
temperature. After 12 h, the solvent was removed by rotary
evaporation to afford an orange solid which was washed with
n-hexane (3 × 25 ml) and dried under vacuum for 2 h. Com-
pound (9); Yield (180 mg, 90%). Mp.: decomp. without melting
at 248 ◦C. FT-IR (KBr): 2957 (s) ꢀ(C–H), 1754 (vs) ꢀ(C O), 1624
(s) ꢀ(C N), 1593 (m) ꢀ(pyridyl C N), 1569 (m) ꢀ(aromatic C C),
1224–1214 (vs) ꢀ(pyranose), 770 cm−1 (s) ꢀ(aromatic C–H).
2.2.1. Preparation of chitosan–2-pyridylimine–Pd catalyst (5)
Catalyst (5) was prepared using chitosan-Schiff base (1) (500 mg,
0.025 mmol) and [PdCl2(COD)] (21 mg, 0.075 mmol). The product
was obtained as a yellow solid. Yield by mass (482 mg, 96%). FT-
IR (KBr): 3418 ꢀ(OH), 2881 ꢀ(C–H), 1651 ꢀ(C N), 1591 ꢀ(pyridyl
1H NMR (400 MHz, DMSO-d6, 25 ◦C):
ı (ppm) = 9.03 (d, 1H,
J = 5.3 Hz, Arpyr), 8.80 (s, 1H, imine), 8.36 (m, 1H, Arpyr), 8.17
(m, 1H, Arpyr), 7.91 (m, 1H, Arpyr), 6.44 (d, 1H, J1,2 = 8.37 Hz,
H-1), 5.44 (m, 1H, H-3), 5.21 (t, 1H, J2,3 = 9.1 Hz, H-2), 4.24 (dd,
2H, J4,5 = 4.1, 12.6 Hz, H-6), 4.12 (m, 1H, H-4), 3.99 (m, 1H, H-
5, 1.92–2.29 (4s, 12H, OAc). Elemental analysis calculated for
C
N), 1569 ꢀ(aromatic C C), 1152–1068 ꢀ(pyranose), 776 cm−1
ꢀ(aromatic C–H). Elemental analysis, Found: C, 48.82; H, 5.92; N,
9.05. ICP-MS (Pd, mmol g−1): 0.044.
2.2.2. Preparation of chitosan–2(diphenylphosphino)imine–Pd
catalyst (6)
C20H24Cl2N2O9Pd: C, 39,14; H, 3.94, N, 4.56, Found: C, 39.33; H,
3.10; N, 4.77.
Catalyst (6) was prepared using chitosan-Schiff base (2) (350 mg,
0.035 mmol) and [PdCl2(COD)] (30 mg, 0.105 mmol). The product
was obtained as a pale yellow solid. Yield by mass (310 mg, 88%). FT-
IR (KBr): 3429 ꢀ(OH), 2920 ꢀ(C–H), 1639 ꢀ(C N), 1576 ꢀ(aromatic
2.2.6. Preparation of 1,3,4,6-tetra-O-acetyl-ˇ-d-glucosyl-2
(diphenylphosphino)imine palladium(II) complex (10)
C
C), 1154–1023 ꢀ(pyranose), 669 cm−1 ꢀ(aromatic C–H). Ele-
1,3,4,6-Tetra-O-acetyl-␤-d-glucosamine
hydrochloride
(280 mg, 0.806 mmol) was treated with Et3N (122 mg,
mental analysis, Found: C, 44.54; H, 5.65; N, 5.10. ICP-MS (Pd):
0.054 mmol g−1
.
1.209 mmol) in dry CH2Cl2
(25 ml) for 15 min. 2-
(Diphenylphosphino)benzaldehyde (233 mg, 0.806 mmol) was
then added to the solution and the contents were stirred for
24 h at room temperature. Anhydrous magnesium sulphate
was transferred to the stirred solution and the mixture was
filtered, the solvent was removed by rotary evaporation to
give a yellow solid of the 1,3,4,6-tetra-O-acetyl-␤-d-glucosyl-
2(diphenylphosphino)imine ligand which was dried under vacuum
for 2 h. Yield (218 mg, 78%). Mp.: 109–111 ◦C. FT-IR (KBr): 2977 (s)
ꢀ(C–H), 1755 (vs) ꢀ(C O), 1697 (s) ꢀ(C N), 1674 (s) ꢀ(aromatic
2.2.3. Preparation of 6-deoxy-6-amino chitosan-2-
pyridylimine-Pd catalyst (7)
Catalyst (7) was prepared using 6-deoxy-6-amino chitosan-
Schiff base (3) (200 mg, 0.02 mmol) and [PdCl2(COD)] (17 mg,
0.06 mmol). The product was obtained as a dark yellow solid. Yield
by mass (218 mg, 91%). FT-IR (KBr): 3419 ꢀ(OH), 2918 ꢀ(C–H), 1639
ꢀ(C N), 1595 ꢀ(pyridyl C N), 1566 ꢀ(aromatic C C), 1152–1066
ꢀ(pyranose), 775 cm−1 ꢀ(aromatic C–H). Elemental analysis, Found:
C
C), 1254–1228 (vs) ꢀ(pyranose), 700 cm−1 (s) ꢀ(aromatic
C, 44.08; H, 5.70; N, 7.42. ICP-MS (Pd): 0.121 mmol g−1
.
C–H). 1H NMR (400 MHz, DMSO-d6, 25 ◦C):
ı
(ppm) = 7.97
(s, 1H, imine), 6.98–7.52 (br m, 14H, Arpyr, Ph), 5.46 (d, 1H,
J1,2 = 8.5 Hz, H-1), 5.03 (m, 1H, H-3), 4.11 (m, 1H, H-2), 4.08 (d, 2H,
J5,6 = 2.0 Hz, H-6), 3.82 (m, 1H, H-4), 3.03 (m, 1H, H-5), 2.03–2.17
2.2.4. Preparation of 6-deoxy-6-amino chitosan–2
(diphenylphosphino)imine–Pd catalyst (8)
Catalyst (8) was prepared using 6-deoxy-6-amino chitosan-
Schiff base (4) (350 mg, 0.035 mmol) and [PdCl2(COD)] (15 mg,
0.052 mmol). The product was obtained as a dark orange solid. Yield
by mass (100 mg, 67%). FT-IR (KBr): 3428 ꢀ(OH), 2920 ꢀ(C–H), 1638
ꢀ(C N), 1576 ꢀ(aromatic C C), 1152–1021 ꢀ(pyranose), 667 cm−1
ꢀ(aromatic C–H). Elemental analysis, Found: C, 42.08; H, 6.50; N,
1
(4s, 12H, OAc). 31P{ H} NMR (121 MHz, DMSO-d6, 25 ◦C): ı
(ppm) = −13.01, (s).
[PdCl2(COD)] (184 mg, 0.646 mmol) was added to the 1,3,4,6-
tetra-O-acetyl-␤-d-glucosyl-2(diphenylphosphino)imine ligand
(400 mg, 0.646 mmol) in dry CH2Cl2 (30 ml) and this stirred
at room temperature. After 12 h, the solvent was removed by
rotary evaporation to afford a yellow solid which was washed
with n-hexane (3 × 25 ml) and dried under vacuum for 2 h. Yield
(401 mg, 92%). Mp.: 146–147 ◦C. FT-IR (KBr): 2946 (s) ꢀ(C–H),
1747 (vs) ꢀ(C O), 1676 (s) ꢀ(C N), 1573 (m) ꢀ(aromatic C C),
1242–1211 (vs) ꢀ(pyranose), 693 cm−1 (s) ꢀ(aromatic C–H). 1H
NMR (400 MHz, DMSO-d6, 25 ◦C): ı (ppm) = 8.61 (s, 1H, imine),
7.36–7.52 (br m, 14H, Arpyr, Ph), 5.93 (d, 1H, J1,2 = 8.20 Hz, H-1),
5.39 (m, 1H, H-3), 5.21 (t, 1H, J2,3 = 9.7 Hz, H-2), 4.17 (dd, 2H,
J4,5 = 4.1, 12.0 Hz, H-6), 4.00 (m, 1H, H-4), 3.54 (m, 1H, H-5),
4.12. ICP-MS (Pd): 0.123 mmol g−1
.
2.2.5. Preparation of 1,3,4,6-tetra-O-acetyl-ˇ-d-glucosyl-2-
pyridyliminepalladium(II) complex (9)
1,3,4,6-tetra-O-acetyl-␤-d-glucosamine hydrochloride (500
mg, 1.440 mmol) was treated with Et3N (218 mg, 2.160 mmol) in
dry CH2Cl2 (30 ml) for 15 min. 2-Pyridinecarboxaldehyde (154 mg,
1.440 mmol) was then added to the solution and the contents
were stirred for 24 h at room temperature. Anhydrous magnesium
sulphate was transferred to the stirred solution and the mixture
was filtered, the solvent was removed by rotary evaporation to
give a white solid of the 1,3,4,6-tetra-O-acetyl-␤-d-glucosyl-2-
pyridylimine ligand. Yield (243 mg, 90%). Mp.: 49–52 ◦C. FT-IR
(KBr): 2960 (s) ꢀ(C–H), 1754 (vs) ꢀ(C O), 1650 (s) ꢀ(C N), 1588
1
1.92–2.05 (4s, 12H, OAc). 31P{ H} NMR (121 MHz, DMSO-d6,
25 ◦C): ı (ppm) = 35.99, (s). Elemental analysis calculated for
C33H34Cl2NO9PPd: C, 49,74; H, 4.30, N, 1.76, Found: C, 49.18; H,
4.58; N, 1.77.

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2.3. General procedure for the Suzuki–Miyaura reaction
catalysts (5–8) evidenced partial complexation of the Pd, through
decreases in the percentage of CHN content in the materials
(Tables 1 and 2). Approximately 20% of the nitrogens in the low
molecular weight chitosan used in these experiments were acety-
lated. Therefore, 2.10 mmol g−1 are present as free NH2 units [4b].
Of these supported ligands, 7% (1) and 6% (2), have been function-
alized with Schiff base groups. This corresponds to 0.150 mmol g−1
(1) and 0.120 mmol g−1 (2), final loading of the ligand sites (Table 1).
The free NH2 functionalities in 6-deoxy-6-amino chitosan
amounts to 2.99 mmol g−1. Of these 7% (3) and 6% (4) were func-
tionalized with 2-pyridylimine and 2(diphenylphosphino)imine
groups, respectively. Loading of the imine ligands in (3) and (4)
was higher than that of (1) and (2), this was expected due to the
presence of more NH2 sites in 6-deoxy-6-amino chitosan (Table 1).
From the analyses, it can be seen that not all NH2 sites have reacted
to form imines, suggesting that many are inaccessible given that
chitosan is a weakly swelling polymer at pH 3–4.5 in methanol, as
15% of the polymer is still acetylated.
The general procedure used for the Suzuki–Miyaura cross-
coupling reactions was as follows:
A quantity equivalent to (4.47 × 10−4 mmol) of either one of the
Pd catalyst (5–10) was suspended in the corresponding solvent
(xylene or EtOH:H2O, 50:50) (20 ml) and the arylhalide (5.1 mmol),
phenyl boronic acid (7.5 mmol), base (10 mmol) and n-decane
(4.4 mmol) were added. The reaction mixture was then heated at
the appropriate temperature and followed by GC using n-decane
as the internal standard. The conversion was calculated relative to
arylhalide, the limiting reagent of the reaction.
In the case of the Pd leaching tests, 4 molar equivalents of
mercury was also added to the reaction mixture. For catalyst recy-
cling experiments, the recovered catalysts was washed with water,
ethanol and acetone (80 ml each), respectively, oven dried at 80 ◦C
for 8 h and reused in the subsequent run.
In addition to elemental analysis, the loading of ligand sites
was also determined using UV spectrophotometry, by measur-
ing the absorbance of increasing amounts (0.5–11.0 × 10−6 M,
in methanol) of the aldehydes (2-pyridinecarboxaldehyde and
2(diphenylphosphino)benzaldehyde) which gave linear (R = 0.995)
calibration curves. Thus, the absorbances of the supported imines,
once cleaved with dilute HCl were measured and used to determine
the imine loading (Table 1) [10b].
The degree of substitution (DS) was calculated from the ele-
mental analyses results using Eq. (1) [12]. This equation slightly
overestimates the values obtained by UV analysis, potentially due
to the residual water molecules in the biopolymer (Table 1).
2.4. General procedure for the Heck reaction
A quantity equivalent to (0.00437 mmol, 0.00044 mol%) of either
one of the Pd catalysts (5–8) was suspended in DMF (20 ml) and
arylhalide (5.3 mmol), methyl acrylate or styrene (5.3 mmol), tri-
ethylamine (10 mmol) and n-decane (4.4 mmol) were added. The
reaction mixture was then heated at 130 ◦C and followed by GC
using n-decane as the internal standard. The conversion was calcu-
lated relative to arylhalide.
3. Results and discussion
ꢁC/N
3.1. Catalyst characterization
DS =
,
(1)
{MC/N(1 − DA)]
Stirring chitosan Schiff base ligands (1–4) with [PdCl2(COD)]
precursor complex in acetone over 48 h (Schemes 1 and 2) resulted
in the respective formation of several chitosan-supported PdII cat-
alysts (5–8). The chitosan catalysts (5–8) were obtained in good
to moderate yields. The yellow to dark yellow supported-catalysts
proved to be air- and moisture-stable and have been stored on the
bench-top for up to 12 months without disintegration. The cata-
lysts were characterized using elemental analysis, ICP-MS, UV–vis,
FT-IR, TGA, PXRD and TEM.
where ꢁC/N is the C/N percentage ratio differences of these ele-
ments in the derivatives and in the original starting material
(chitosan or 6-deoxy-6-amino chitosan), MC/N is the ratio of C to
N molar mass and DA is the degree of acetylation of the starting
material [12].
The metal loading was determined using ICP-MS, and the
results showed Pd loadings of 0.044 (5), 0.054 (6), 0.121 (7) and
0.123 mmol g−1 (8) (Table 2). Thus for the supported Pd catalysts
approximately 27% (5), 45% (6), 60% (7) and 62% (8) of the ligands
are occupied by metal. Generally, the Pd loading levels on the sup-
Comparison of the elemental analyses results of the supported
Schiff base ligands (1–4) (Schemes 1 and 2) with the corresponding
Scheme 1. Outline for the preparation of supported Schiff base ligands (1 and 2) and supported catalysts (5 and 6).

B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
235
Scheme 2. Outline for the preparation of supported Schiff base ligands (3 and 4) and supported catalysts (7 and 8).
Table 1
Microanalyses, yields and degree of substitution (DS) of chitosan-Schiff base ligands.
Biopolymer/chitosan-ligand
Elemental analysis EA (%)
Loading(DS)(mmol g−1
)
Yield (%)
C
H
N
C/N
EA
UVa
Chitosan
6-Deoxy-6-amino chitosan
1
2
3
4
40.92
44.35
54.79
44.63
50.08
45.66
6.89
7.04
6.02
6.92
5.89
6.22
7.85
5.21
5.45
5.27
6.34
5.50
6.27
2.10b
2.99c
0.15
0.12
0.20
0.18
–
–
0.13
0.12
0.18
0.14
–
–
84
69
60
65
8.14
10.39
7.03
9.10
7.28
a
Absorbance is dependent on conc, solvent and pH [10b].
Free NH2 determined using literature [4b].
Loading of NH2 in 6-deoxy-6-amino chitosan previously determined by a fluorescamine assay [10a].
b
c
range of 1559–1590 and 695–817 cm−1 which were not present
in the starting materials (chitosan or 6-deoxy-6-amino chitosan).
They were attributed to the C C and C–H stretching vibrations in
the aromatic ring of the Schiff-base ligands.
Upon complexation of the chitosan-Schiff bases with Pd, subtle
shifts in the imine stretching vibrations were observed. This is pos-
sibly due to low degree of incorporation of the Pd. Also, overlapping
of the imine bands coordinated to Pd with those that are uncoordi-
nated may occur since most of the imine sites were left unaltered
during complexation.
ported catalysts are comparable to similar catalysts in literature
[4a–c].
UV–vis studies were conducted on complexes (9) and (10),
in order to confirm the molecular PdII loading onto catalysts (5)
and (6). Similar to complexes (9) and (10) (absorbance peaks
at 360 nm and 330 nm, respectively), catalysts (5) and (6) dis-
played absorbance peaks at 358 nm and 332 nm, respectively
(Fig. 2(a)–(d)). The presence of similar, though weak absorbance
peaks for catalysts (5) and (6), suggests that a low loading
of Pd complexes on the chitosan structure has been attained
[4a].
The chitosan-based catalysts and their corresponding chitosan-
Schiff base ligands proved to be insoluble in most common solvents
including THF, EtOH, MeOH, acetone and water, but showed slight
swelling in DMSO.
The IR spectral data for the chitosan-Schiff base derivatives
(1–4) showed characteristic imine (C N) stretching vibrations in
the range of 1631–1648 cm−1. There were also new bands in the
Table 2
Microanalyses, yields and Pd content of chitosan-Schiff base catalysts.
Chitosan-catalyst
Metal
Elemental analysis EA (%)
Pd loading (mmol g−1
ICP value
)
Yield (%)
C
H
N
C/N
5.39
8.73
5.94
5
6
7
8
Pd
Pd
Pd
Pd
48.82
44.54
44.08
42.08
5.92
5.65
5.70
6.50
9.05
5.10
7.42
4.12
0.044
0.054
0.121
0.123
96
88
91
67
10.21

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B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
Fig. 2. UV–vis spectra showing absorbance peaks of supported catalysts: 5 (a) and 6 (b) and complexes 9 (c) and 10 (d).
Characteristic Pd0 peaks (2ꢂ = 40◦, 45◦, 68◦ and 80◦) were
not observed in the diffractograms of catalysts (5–8), implying
that Pd exists in an amorphous state on the chitosan structure
(Fig. 4(a)–(d)) [6f]. It was interesting to see that the crystallinity of
chitosan was of fairly good quality showing well resolved specific
peaks at 2ꢂ = 10◦ and 20◦ [6e–f, 9e]. As expected the 6-deoxy-6-
amino chitosan has lower crystallinity, due to the reaction steps
involved in introducing the second amino group (Fig. 4(c) and (d)).
The supported-Pd catalysts (5–8) were analysed thermally by
thermal gravimetric analysis (TGA) in air. Thermal analysis of the
catalysts showed that the catalysts are stable up to 240 ◦C, fol-
lowing which a significant weight loss due to degradation of the
TEM images of catalysts (5) and (6) taken at 1000 nm showed
that these catalysts formed evenly dispersed nano-sized Pd parti-
cles across the biopolymer (Fig. 3(a) and (b)). The majority of these
particles lie in the 15–20 nm range. Catalysts (7) and (8) formed
nano-sized Pd particles of average size 80–120 nm (Fig. 3(c) and
(d)). The existence of Pd particles visible through TEM imaging may
imply intermolecular interactions between angstrom size molec-
ular PdII sites on the biopolymer resulting in nanosized particles.
Willocq et al. have also seen Pd and Ru particles of size range 2–6 nm
on phosphine modified active carbon, in which surface coordina-
tion of Pd and Ru organometallic complexes was achieved through
the phosphines on the active carbon [13a–b].
Fig. 3. TEM images of supported catalysts 5 (a), 6 (b), 7 (c) and 8 (d).

B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
237
Fig. 4. Powder X-ray diffraction diagrams of catalysts (5–8) (patterns a–d, respectively).
biopolymer was observed. Thus the catalysts would be capable
of withstanding high reaction temperatures up to slightly below
240 ◦C. A weight loss of 3.5–5.0% occurs before degradation and is
attributed to the desorption of water [9c–d]. These thermal analysis
results were the same for all the supported catalysts (5–8).
Solid state 31P NMR spectroscopy of the precursor
chitosan–iminophosphine ligand (2) showed
a signal at ı
−18.5 ppm (Fig. 5(a)) This further indicates that the ligand
has been successfully anchored to the chitosan support as this
shift corresponds with the chemical shift obtained for the soluble
ligand seen at ı −13.0 ppm. A resonance due to phosphine oxide
was also present at ı 35.0 ppm [14a]. The complex [PdCl2(COD)]
was reacted with the chitosan–iminophosphine and the solid state
31P NMR spectrum of the resulting chitosan-supported complex
(6) showed a decrease in intensity of the peak at ı −18.5 ppm,
and the appearance of a resonance at ca ı 20.0 ppm assigned to
the Pd complex (Fig. 5(b)). This effect was previously observed
in phosphine–Rh complexes attached to a peptide synthesis
resin [14b]. These NMR experiments indicated that there are
changes on the chitosan structure upon the formation of the
chitosan-supported Schiff base ligand (2) and subsequent complex
formation. To this effect, we have reason to believe molecular PdII
complexes do exist on the chitosan and 6-deoxy-6-amino chitosan
supports, and it would thus be fair to assume the same for the
chitosan-supported iminopyridyl catalyst (5 and 7).
3.2. Catalytic studies
3.2.1. Suzuki–Miyaura cross-coupling reactions
The activities of catalysts (5–8) were examined in the
Suzuki–Miyaura reaction of aryl halides with phenylboronic acid
(Scheme 3).
Under conditions similar to those reported in literature (143 ◦C)
[4b], catalysts (5) and (7) gave yields of 87% and 85%, respec-
tively (Table 3, entries 5 and 6), while catalyst (6) afforded 65%
of the biphenyl. On varying the reaction temperature (143 ◦C,
130 ◦C and 95 ◦C), it was observed that the optimal temperature
for Suzuki–Miyaura cross-coupling using catalysts (6 and 8) was
130 ◦C. Subsequently, all other reactions were performed at this
temperature, resulting in good catalytic activities (4.4 × 10−4 mol%
Pd used) as evidenced by moderate to high TONs (Table 3). Our cat-
Fig. 5. Solid state 31P NMR spectra of (a) chitosan–iminophosphine ligand (2) and
(b) chitosan-supported Pd complex (6): 31P one-pulse experiments were performed
at a 31P frequency of 15 kHz at room temperature. Chemical shifts were referenced
to Na2HPO4 at ı = 0 ppm. Signals arising from side bands are marked with an asterisk
(*).

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B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
Table 3
Suzuki–Miyaura cross coupling reactions catalyzed by supported Pd catalysts (5–8).
Entry
Catalysta
R
X
Base
Conversion (%)b
TON [(molProd.)/(molPd)]c
Selectivity (%)
1
2
6
8
5
6
5
7
6
6
6
6
8
6
7
8
6
7
13
6
5
8
7
6
6
7
9
10
H
H
H
H
H
H
H
p-NO2
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
I
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
98
96
82
48
87
85
65
76
26
19
75
5
1144
1116
959
558
1016
1144
763
904
320
221
879
56
>99
>99
99
98
99
99
98
99
97
97
98
97
97
97
97
98
99
98
99
99
99
>99
>99
>99
99
3
4d
5e
6e
7e
8
9
p-OMe
10
11
12
13
14
15
16
17
18
19
20
21
22f
23f
24f
25
26
p-Ac
p-Bun
H
H
H
H
H
H
H
H
H
H
H
p-Ac
p-NO2
H
1
8
12
88
34
29
71
21
32
26
34
100
99
87
84
100
397
342
823
249
346
309
405
1167
1160
1015
1010
1167
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
>99
a
Reactions carried out in xylene (20 ml) over 6 h, with 5.1 mmol of arylhalide, 7.5 mmol of phenylboronic acid, 10 mmol of base and 4.47 × 10−4 mmol Pd catalyst
(loading = 0.044 (5); 0.054 (6); (7) 0.121 and (8) 0.123 mmol g−1) at 130 ◦C unless stated otherwise.
b
GC conversions obtained using n-decane as an internal standard and are based on the amount of arylhalide employed in relation to authentic standard biphenyl or
substituted biphenyl.
c
Maximum TON from reaction at 4 h.
d
Reaction carried out at 95 ◦C.
e
Reaction carried out at 143 ◦C.
Reaction carried out in EtOH:H2O (1:1, 20 ml).
f
alysts’ TON values are comparably higher than those reported in
literature, that reported the use of more catalyst (by mol%) [4b,15].
Generally, the iminophosphine supported catalysts afforded bet-
ter yields than their iminopyridyl counterparts and this can be
attributed to the better stabilizing nature of phosphorus donor
ligands as compared to nitrogen donor ligands. When substituted
bromobenzenes were employed (entries 8–11) a good yield of the
coupled product was obtained for the substrate containing p-NO2
and p-Bun substituents (TONs 904 and 879, respectively). The p-
OMe and p-Ac containing substrates gave low yields as the catalysts
decomposed to a black species, possibly Pd0 particles. As shown
in Table 3 (entries 12–17), the arylbromide gave good yields fol-
lowed by aryliodide while the less reactive arylchloride gave the
poorest product yield. Thus, unexpectedly the order of reactivity
follows: Br > I > Cl. In addition to the bond dissociation energies,
we suspect as possible halide size contribution to this different
result, however further extensive experiments involving the effect
of halide would be necessary to confirm this. When sodium carbon-
ate was used as the base, conversions dropped by approximately
60% under these reaction conditions (entries 18–21). Reactions
performed in EtOH:H2O (1:1 ratio) drastically improved prod-
uct yields to as high as 100% (entries 22–24), indicating that the
chitosan-supported catalyst works well in aqueous conditions. The
homogeneous complexes (9 and 10, Fig. 6) displayed good activity
(TONs 1010 and 1167, respectively) for the Suzuki reactions (entries
25–26), but showed decomposition to a black species minutes into
the reaction. This suggests that the chitosan support plays a cru-
cial stabilizing role in the supported Schiff-base catalysts in these
reactions.
On following the yields to biphenyl over time, it was observed
that the reaction is almost complete in the first 15 min when using
supported catalysts (5) and (6) (Fig. 7). Catalysts (7) and (8) exhib-
ited lower activity and required longer time for near completion.
The homogeneous catalysts (9 and 10) showed similar activity as
the chitosan-supported ones, with the iminophosphine complex
being slightly more active than its iminopyridyl counterpart. This
phenomenon is also evident when comparing the activities of cat-
alysts (5–8).
3.2.2. Reusability of catalysts
Catalysts (5–8) were recovered by simple filtration and washed
with water, ethanol then acetone and oven dried. The catalysts
Scheme 3. Outline for the Suzuki–Miyaura reactions carried out using catalysts
(5–8).
Fig. 6. Structure of homogeneous catalysts: samples 9 and 10.

B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
239
Fig. 7. Variation of the biphenyl formation with time on stream achieved over cat-
alysts (5–10): Reactions carried out in xylene (20 ml) over 6 h, with 5.1 mmol of
arylhalide, 7.5 mmol of phenylboronic acid, 10 mmol of base and 4.47 × 10−4 mmol
Fig. 10. Effect of removing Pd catalyst (6) from reaction (hot filtration test cat-
alyst (6) removed after 5 min): Reactions carried out in xylene (20 ml) over 6 h,
with 5.1 mmol of arylhalide, 7.5 mmol of phenylboronic acid, 10 mmol of base and
4.47 × 10−4 mmol Pd catalyst (loading = 0.054 (6) mmol g−1) at 130 ◦C unless stated
otherwise. GC conversions obtained using n-decane as an internal standard and
are based on the amount of arylhalide employed in relation to authentic standard
biphenyl or substituted biphenyl.
Pd catalyst (loading = 0.044 (5); 0.054 (6); (7) 0.121 and (8) 0.123 mmol g−1) at 130 ◦
C
unless stated otherwise. Similar considerations were taken for reactions involving
catalysts (9 and 10). GC conversions obtained using n-decane as an internal stan-
dard and are based on the amount of arylhalide employed in relation to authentic
standard biphenyl or substituted biphenyl.
3.2.3. Pd leaching tests
A hot filtration test, whereby the catalyst was filtered 5 min
after the reaction started, followed by the filtrate taken back to the
reaction temperature, stopped the catalytic reaction. This, effec-
tively proves the heterogeneous nature of the supported catalyst
and without a measurable homogeneous contribution (Fig. 10). Fur-
thermore, no Pd was detected in the filtrate by ICP-MS analysis in
this mixture together with samples from entries 1, 3 and 6 (Table 3).
Coordination of Pd to the biopolymer-supported Schiff-base ligands
has shown no evidence of Pd leaching into the products. This pro-
vides evidence of chitosan acting as a good scaffold for PdII catalyst
precursors [1,2,4].
A test was performed to establish whether the reactions were
catalyzed by Pd0 or PdII species. A reaction using catalyst (6) in
the presence of mercury resulted in no inhibition of catalytic activ-
ity (94% yield). This result suggested predominantly heterogeneous
catalysis.
Fig. 8. Variation of conversion with the number of runs achieved for catalysts
(5–8): Reactions carried out in xylene (20 ml) over 6 h, with 5.1 mmol of arylhalide,
7.5 mmol of phenylboronic acid, 10 mmol of base and 4.47 × 10−4 mmol Pd catalyst
(loading = 0.044 (5); 0.054 (6); (7) 0.121 and (8) 0.123 mmol g−1) at 130 ◦C unless
stated otherwise. GC conversions obtained using n-decane as an internal standard
and are based on the amount of arylhalide employed in relation to authentic stan-
dard biphenyl or substituted biphenyl.
3.2.4. Heck cross-coupling reactions
could be reused at least five times without significant loss of activity
(Fig. 8). Loss of activity in the 6th run may be due to poison-
ing or fouling of the catalysts by waste inorganics and unreacted
reagents therefore blocking the active Pd sites. Sintering of the
nanosized Pd particles (thus decreasing the catalyst surface area)
also occurs as evidenced by TEM images of catalyst (5 and 6)
after five cycles (Fig. 9(a) and (b)). This is possibly induced by
the fairly high temperature at which the reactions were carried
out.
Heck reactions were also investigated under similar temper-
ature conditions as utilised for the Suzuki–Miyaura reactions
(Scheme 4). The reactions using catalysts (5 and 7) with
iodobenzene and methyl acrylate produced 98% and 99% trans-
methylcinnamate, respectively, in 4 h at 130 ◦C (Table 4, entries
1 and 3). Coupling of styrene and iodobenzene was also accom-
plished, giving 99% and 90% of trans-stilbene, respectively, with
good TONs of 1209 and 1086 (entries 2 and 4). Thus, the afore-
mentioned catalytic reactions lead to the regiospecific formation
Fig. 9. TEM images of used supported catalysts: 5 (a) and 6 (b).

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B.C.E. Makhubela et al. / Applied Catalysis A: General 393 (2011) 231–241
Table 4
Heck cross coupling reactions catalyzed by supported Pd catalysts (5–8).
Entry
Catalysta
X
Y
R
Conversion (%)b
TON [(molProduct)/(molPd)]
Selectivity (%)
1
2
3
4
5
6
7
5
6
7
8
8
7
5
I
I
I
I
I
I
Br
H
H
H
H
OMe
OMe
H
CO2Me
Ph
CO2Me
Ph
Ph
CO2Me
CO2Me
98
99
99
90
20
62
8
1110
1150
1209
1086
237
>98
>99
>99
>99
>99
>99
>99
757
93
a
Reactions carried out in DMF (20 ml) over 4 h, with 5.3 mmol of iodobenzene, 5.3 mmol of methyl acrylate or styrene, 10 mmol of triethylamine and 4.47 × 10−4 mmol Pd
catalyst (loading = 0.044 (5); 0.054 (6); (7) 0.121 and (8) 0.123 mmol g−1) at 130 ◦C unless stated otherwise.
b
GC conversions obtained using n-decane as an internal standard and is based on the amount of iodobenzene employed in relation to authentic standard trans-methyl
cinnamate or trans-stilbene.
Acknowledgements
We would like to thank the Anglo Platinum Corporation, the Uni-
versity of Cape Town and the National Research Foundation (NRF)
of South Africa for financial support.
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The supported-Pd catalysts exhibited high activity in the
Suzuki–Miyaura and Heck cross-coupling reactions in xylene and
aqueous ethanol solvents, with no Pd detected in the reac-
tion products. The supported catalysts displayed good activity at
low Pd loading (4.4 × 10−4 mol%) and in aqueous media. Though
the mononuclear model catalysts also showed good activity for
Suzuki–Miyaura cross-coupling, they could not be recycled and
reused, thus the chitosan environment plays an essential stabi-
lizing role for the supported catalysts in the coupling reactions.
The supported catalysts could be easily separated and recov-
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times. The combination of advantages displayed by the supported
catalysts such as: ease of preparation, high catalytic activity,
stability, reusability, versatility (organic or aqueous solvent catal-
ysis) and no measurable Pd leaching, prove that these catalysts
should be considered as a viable alternative in cross-coupling reac-
tions on efficiency, environmental stewardship and economical
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