New palladium catalyst immobilized on epoxy resin: Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1002/aoc.3389

A new thiol-functionalized epoxy resin as a support for palladium(II) complexes has been synthesized in good yields. A palladium catalyst was 'heterogenized' by anchoring [PdCl₂(PhCN)₂] complexes to these thiol-functionalized polymers via ligand exchange reaction. These new palladium catalysts were tested in Mizoroki-Heck coupling and hydrogenation reactions. The activity of the complexes in terms of yield is comparable to that of homogeneous PdCl₂(PhCN)₂. The stability and a good recycling efficiency of these catalysts make them useful for prolonged use. The constant and good selectivity of the supported catalysts during recycling experiments indicate that they could be useful for practical application in many organic reactions. To characterize the heterogeneous complexes before and after use, X-ray photoelectron spectroscopy, infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray microscopy, atomic absorption spectroscopy and time-of-flight secondary ion mass spectrometry were applied. Density functional theory calculations were also used to better understand the structures of the obtained palladium complexes. Polythiourethanes contain three atoms, oxygen, nitrogen and sulfur, capable of coordinating to transition metals. We examined the possibility of intra- A nd intermolecular binding for both cis and trans palladium complexes.

Full text of DOI:10.1002/aoc.3389

Full paper
Received: 15 June 2015
Revised: 31 July 2015
Accepted: 19 August 2015
Published online in Wiley Online Library: 26 October 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3389
New palladium catalyst immobilized on epoxy
resin: synthesis, characterization and
catalytic activity
Natalia Sienkiewicza*, Krzysztof Strzeleca, Piotr Pospiechb, Marek Cyprykb
and Tomasz Szmechtyka
A new thiol-functionalized epoxy resin as a support for palladium(II) complexes has been synthesized in good yields. A palladium
catalyst was ‘heterogenized’ by anchoring [PdCl2(PhCN)2] complexes to these thiol-functionalized polymers via ligand exchange
reaction. These new palladium catalysts were tested in Mizoroki–Heck coupling and hydrogenation reactions. The activity of the
complexes in terms of yield is comparable to that of homogeneous PdCl2(PhCN)2. The stability and a good recycling efficiency of these
catalysts make them useful for prolonged use. The constant and good selectivity of the supported catalysts during recycling
experiments indicate that they could be useful for practical application in many organic reactions. To characterize the heterogeneous
complexes before and after use, X-ray photoelectron spectroscopy, infrared spectroscopy, scanning electron microscopy, energy
dispersive X-ray microscopy, atomic absorption spectroscopy and time-of-flight secondary ion mass spectrometry were applied.
Density functional theory calculations were also used to better understand the structures of the obtained palladium complexes.
Polythiourethanes contain three atoms, oxygen, nitrogen and sulfur, capable of coordinating to transition metals. We examined the
possibility of intra- and intermolecular binding for both cis and trans palladium complexes. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: heterogeneous catalysts; properties and characterization; epoxy resins; polythiourethanes; structure–property relations;
hydrogenation reaction; Mizoroki–Heck reaction
Introduction
only the specific immobilization of enzymes through their thiol
groups by thiol–disulfide interchange, but also enzyme stabilization
through covalent attachment.[5] Epoxy resins with amine and mer-
captan chelating groups have been prepared by the suspended
condensation polymerization of 2-chloroethoxymethyl thiirane
and diamines. The resins show high affinity for noble metal ions
and Hg(II), and predominantly adsorb Pd(II) or Hg(II) in the presence
of Cu(II), Zn(II) and Mg(II) ions.[6] Dodecylthiolate-stabilized palla-
dium nanoparticles have been shown to be active catalysts for
the formation of carbon nanotubes.[7] Furthermore, they have been
demonstrated to be stable and recyclable catalysts in the Suzuki–
Miyaura C–C coupling reaction of halogenoarenes and phenylboronic
acid,[8] while thiolated β-cyclodextrin-stabilized palladium nanoparti-
cles have shown good activity in the hydrogenation of allylamine.[9]
Palladium nanoparticles stabilized by BINAP–thioether derivatives
have also been used in (quasi) homogeneous C–C coupling
reactions.[10] The presence of a chelating diphosphine ligand provides
Epoxy resins have a set of unique properties that are not commonly
found in other plastic materials, such as excellent mechanical
strength, outstanding chemical, moisture and corrosion resistance,
good thermal, adhesive and electrical properties, dimensional sta-
bility (i.e. low shrinkage upon curing) and a lack of volatile
emissions.[1–3] Epoxy resins, because of these characteristics, are
also used as carriers for catalysts in many important organic reac-
tions, in the industrial production of several commodity com-
pounds, as well as in the synthesis of many intermediates, fine
chemicals and pharmaceuticals. For example, catalysts with unprec-
edented long-term activities in the range of at least some months
have been obtained by the use of thermosetting epoxy resins as
supports. These catalysts are prepared in a convenient one-step
procedure employing metal complexes that act simultaneously as
polymerization indicators as well as precursors for catalytically ac-
tive species in the resulting polymers. With respect to polymeric
supports, it should be pointed out that about 65% of the reviewed
heterogenized catalyst systems are based on polystyrene,
polymethacrylate and poly(ethylene glycol). Furthermore, hybrid
catalyst systems comprising organic polymers together with inor-
ganic components, such as recently reported titanium catalysts
based on polystyrene–silica or molybdenum catalysts supported
on epoxy resin–silica composites, offer new possibilities for the
development of efficient polymer-supported catalysts.[4] Thiol-
functionalized epoxies have proved to be very convenient for stabi-
lizing enzymes by multipoint immobilization. Their use allows not
*
Correspondence to: Natalia Sienkiewicz, Institute of Polymer and Dye Technology,
Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16,
90-924 Lodz, Poland. E-mail: natalia.sienkiewicz@p.lodz.pl
a Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of
Technology, Stefanowskiego 12/16, 90-924, Lodz, Poland
b Center of Molecular and Macromolecular Studies, Polish Academy of Sciences,
Sienkiewicza 112, 90-363, Lodz, Poland
Appl. Organometal. Chem. 2016, 30, 4–11
Copyright © 2015 John Wiley & Sons, Ltd.

New palladium catalyst immobilized on epoxy resin
remarkable stability to the nanoparticles, preventing their aggrega-
tion throughout the process. Furthermore, the nanoparticles could
be subsequently isolated and reused without any loss of catalytic
activity.
Figure 1. Difunctional polythiourethane (DPTU).
In our earlier works, we used thiirane resins cured with
polythiourethane (PTU) hardeners which are very useful and versa-
tile polymeric supports for the preparation of heterogenized palla-
dium catalysts for the Heck reaction.[11] The use of PTUs as curing
agents enables one to obtain supports that do not need any further
functionalization. The different coordination sites in the polymer
promote the formation of active catalytic species mainly as finely
dispersed metal particles with mixed valence states. The chemical
structure of the PTU oligomers used can greatly affect the activity
of the catalysts. Polymers or metal–organic frameworks as some
of the essences of crystal engineering and coordination chemistry
have attracted considerable attention from chemists, physicists
and materials scientists for the development of hybrid crystalline
solids with novel supramolecular structures and physicochemical
properties. Supported-metal catalysts are among the most impor-
tant materials in heterogeneous catalysis.
In this report, we present a method for the coordination of a
palladium catalyst on a support prepared from an epoxy resin cured
with thiol-terminated PTU. In recent decades there has been consid-
erable progress in the application of quantum chemical calculations
to define the structures of catalysts, as well as to solve problems in
homogeneous and heterogeneous catalysis.[12–15] The rapid
increase in computing power and the development of computa-
tional algorithms allow one to perform calculations of the structures
and properties of transition metal complexes. It is known that com-
putational methods can help to explain the mechanisms of catalysis
involving transition metals. In the work reported here, density func-
tional theory (DFT) methods were applied to better understand the
geometries of the investigated catalytic systems. The oligomeric
PTUs, apart from their good curing characteristic of epoxides, allow
the introduction of new binding ligands into the structure of the
polymer. The activity of these new catalysts was investigated in
Mizoroki–Heck and hydrogenation reactions.
Figure 2. Multifunctional polythiourethane (MPTU).
was transferred onto a Teflon® mould and cured at a room
temperature over a period of 1–2 h. For all cured samples,
0.01% of 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) was
used as an accelerator. The cured resin was frozen by immer-
sion in liquid nitrogen and mechanically ground to powder.
The particle size was about 0.5 mm which was determined
using sieve analysis.
Preparation of heterogenized palladium complexes
A typical procedure was as follows. On the prepared resin matrix,
we immobilized the palladium complex PdCl2(PhCN)2 from toluene
solution, using a ligand-exchange process. To a known amount of
suitable resin (0.5 g) in a round-bottom flask, PdCl2(PdCN)2 dis-
solved in 10 ml of toluene was added. The palladium content for
all supported catalysts was fixed below the metal maximum uptake
capacity for a polymer, which was also confirmed from atomic
absorption spectroscopy (AAS) analysis of the colourless toluene
solutions after the immobilization process. The mixture was stirred
at room temperature for 3 days, with a change of the compound
colour from dark brown to slightly yellowish indicating that the
palladium complex had reacted with the prepared polymer. The
yellow product was filtered off, extracted with toluene under nitro-
gen in order to remove any complex that was non-chemically
bound to the polymer matrix, and finally dried under vacuum.
Experimental
Materials
All organic solvents used in this study were supplied by Sigma-
Aldrich and were used without further purification. The commercially
available epoxy resin (diglycidyl ether of bisphenol A) Epidian-5 (EP5),
having epoxide number of 0.487, was obtained from Organika-
Sarzyna, Poland. The viscosity of the resin was 23.9 N s mꢀ2 at 25°C.
PTU hardeners were prepared by the reaction of hexamethylene
diisocyanate and pentaerythritol tetrakis(3-mercaptopropionate)
and 3,6-dioxa-1,8-octanedithiol which were purchased from Sigma-
Aldrich and used without further treatment. PdCl2(PhCN)2 was
prepared from PdCl2 (Sigma-Aldrich, 98%) and benzonitrile (Sigma-
Aldrich, 99%).[16]
Structural characterization
FT-IR spectra were recorded with a BIORAD FT-IR 175C spectropho-
tometer in an air atmosphere. A polymer sample was mixed with
KBr using the snap-reflection technique for DRIFT measuring. Bond-
ing nature of the catalyst surface was investigated using X-ray
photoelectron spectroscopy (XPS). XPS measurements were made
using a VG ESCALAB 210 spectrometer with Mg Kα (1253.6 eV)
excitation from an X-ray tube (reduced power, 10 kV, 10 mA). The
pressure in the spectrometer chamber was about 10ꢀ9 mbar. Sam-
ples were pressed to pellets under a pressure of 100 kbar for 10 min
before measurements. The C 1s, N 1s, O 1s, Cl 2p, S 2p and Pd 3d
core level spectra were recorded. The analyser pass energy was
set at 20 eV. A take-off angle of 90° was used in all XPS studies.
Curve fitting was performed using the ECLIPSE data system soft-
ware. This software describes each of the components of a complex
envelope as a Gaussian–Lorentzian sum function. The background
was fitted using nonlinear model function proportional to the
Preparation of new thiol-functionalized epoxy resin
The carrier was prepared by mixing the epoxy resin (3 g) with
either difunctional PTU (DPTU; Fig. 1) or multifunctional PTU
(MPTU; Fig. 2) hardener (40 wt%). The Fourier transform infrared
(FT-IR), 1H NMR and gel permeation chromatography analyses
of the cross-linked resins were performed in our previous
work.[17] After obtaining homogenous consistency, the mixture
Appl. Organometal. Chem. 2016, 30, 4–11
Copyright © 2015 John Wiley & Sons, Ltd.
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N. Sienkiewicz et al.
integral of the elastically scattered electrons. All binding energies
were referenced to the C 1s neutral carbon peak at 284.6 eV.
The Heck and hydrogenation reaction products were quantitatively
analysed using GC with a Hewlett-Packard 5990 II gas chromatograph
equipped with a thermal conductivity detector. The GC column was
an HP-50+ (cross-linked 50% PhMe silicone; 30 m × 0.63 mm × 1.0 μm
film thickness). Reagents were identified based on previously deter-
mined retention times. The Heck reaction yield was determined from
the loss of iodobenzene relative to a standard (toluene) at a time. The
retention times of reagents for the above conditions of operation of
the column were determined experimentally: iodobenzene,
9.991 min; methyl acrylate, triethylamine, N-methylpyrrolidone,
12.693 min; toluene, 2.141 min; trans-methyl cinnamate, 20.132 min.
GC operating conditions: injector and detector temperature, 210°C;
carrier gas, helium. Temperature programme: the column was main-
point energies were obtained using the 6-311+G(2d,p) basis set for
all atoms, except Pd for which the LANL2DZ basis set incremented
by four sets of 4f polarization functions was applied.[21]
Mizoroki–Heck and hydrogenation reactions
A typical Heck reaction of iodobenzene and methyl acrylate was car-
ried out in a 50 ml laboratory reactor. The reactor was charged with
0.05 g of supported palladium catalyst, 0.28 ml of iodobenzene,
0.23 ml of methyl acrylate, 0.15 ml of toluene, 0.35 ml of
triethylamine and 1.00 ml of N-methylpyrrolidone (NMP). The reac-
tion was conducted at 80°C for 1.5 h. The reaction of bromobenzene
with styrene was carried out in a 50 ml laboratory reactor. This reac-
tion was carried out under standard conditions (0.1 mol% of sup-
ported palladium catalyst, 10 mmol of bromobenzene, 15 mmol of
styrene, 12 mmol of sodium acetate (NaOAc) as a base and 10 ml
of NMP). The reaction was conducted at 140°C for 6 h. The reaction
of iodobenzene and n-butyl acrylate was carried out in a 50 ml
laboratory reactor. The reactor was charged with 0.1 mol% of sup-
ported palladium catalyst. The Heck coupling of 2 mmol of
iodobenzene with 2.8 mmol of n-butyl acrylate in 3.00 ml of NMP
was used as a model system. Triethylamine (2.5 mmol) was addition-
ally used as a base. The reaction was conducted at 130C for 30 min.
The hydrogenation reactions were carried out in a 100 ml Parr
reactor (model 4593) with a glass liner. A typical hydrogenation run
was as follows. The reactor was charged with 0.2 g of supported pal-
ladium catalyst, 5 ml of 96% ethanol, 0.25 ml of toluene and 0.25 ml
of cinnamaldehyde or crotonaldehyde. The reactor was flushed
several times with pure hydrogen and then the hydrogen pressure
and temperature were adjusted to the required level: 40 bar and
75°C for cinnamaldehyde and 90°C for crotonaldehyde. The product
was identified by matching retention times with those of pure com-
pounds. In both recycling tests, when reactions were completed, the
catalyst was separated, washed with toluene (3 × 25 ml), vacuum
dried and used for the next reaction. The product was identified by
matching retention times with those of authentic samples.
tained at 50°C for 2 min, then heated to 150°C at a rate of 5°C minꢀ1
.
For the hydrogenation reaction, the GC operating conditions were:
injector and detector temperature, 210°C; carrier gas, helium. Temper-
ature programme: the column was maintained at 50°C for 3 min, then
heated to 140°C at a rate of 70°C minꢀ1. The retention times of
reagents for the above conditions of operation of the column were
determined experimentally: trans-cinnamic aldehyde, 6.948 min;
toluene, 2.117 min; ethanol, 0.657 min.
The amount of palladium loaded in the support was analysed
using AAS with a PerkinElmer 3030 atomic absorption spectrome-
ter. A sample of a polymer-supported catalyst was heated to
600°C and then dissolved in aqua regia. The resulting solution
was diluted and assayed using AAS.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS)
measurements were made using an ION-TOF GmbH instrument
(ToF-SIMS IV) equipped with a 25 keV pulsed Bi1 primary ion
gun in the static mode (primary ion dose about 1.5 × 1011 ions
cmꢀ2). The analysed area corresponded to
a square of
500 μm× 500 μm in the case of secondary ion mass spectra
collecting and surface imaging. For each prepared sample, three
spectra from different surfaces areas were obtained. To obtain a
plain surface of catalysts (then better mass resolution could be
achieved), powder samples were tableted before the measure-
ments. In order to compare the quantity of palladium present
on the surface of ‘fresh’ and ‘used’ catalysts, the number of counts
of selected ions obtained from collected mass spectra was nor-
malized on the basis of the value of total counts.
The morphology of samples was investigated with a scanning
electron microscopy (SEM) instrument (JEOL 5500 LV), working at
high vacuum and an accelerating voltage of 10 kV. Energy disper-
sive X-ray spectroscopy (EDX) SEM microscope (JEOL JSM 840A,
Japan) was used to observe the elemental distribution. The white
points in the figures denote Pd atoms. Pore size distribution param-
eters were determined by application of the BET method using a
Sorptomatic 1900 instrument (Fisons).
Mercury test
The mercury poisoning experiment was performed using 500-fold
excess of Hg(0) relative to palladium catalyst. Supported catalyst
was stirred with mercury for 24h prior to reaction. The same exper-
iment was also conducted as a reference, using as catalyst the
homogeneous complex PdCl2(PhCN)2. In this case, Hg(0) was intro-
duced at the beginning, together with other reactants.
Results and discussion
Characterization of polymer supports and immobilized Pd
catalysts
PTU hardeners were prepared and characterized according to our
earlier paper.[17] By the incorporation of thiol groups into the epoxy
matrix we obtain a polymer chain with ligands able chelate the pal-
ladium complex via ligand exchange reaction. This procedure leads
to a controlled introduction of bonding groups to the support
structure.
FT-IR spectroscopy was used to confirm the chemical structure of
polymer matrices and polymer-supported catalysts. Figure 3 shows
FT-IR spectra of epoxy resin cured with MPTU and catalyst
EP5/MPTU/Pd. The presence of thiourethane linkages is confirmed
by the characteristic peak at 1506cmꢀ1. The lack of absorption
Computational studies
The calculations were performed using the Gaussian 03 and Spartan
’08 programs.[18,19] The starting structures for geometry optimization
were obtained from conformational search using the MMFF method
with Spartan. Geometry optimizations (Gaussian 03) in the gas
phase were carried out using the M06-2X density functional[20]
and the 6-31G* basis set for H, C, O, Cl, N and S and the effective core
potential basis set LANL2DZ for Pd. All equilibrium structures were
identified by frequency calculations. Thermal corrections to the elec-
tronic energies for 298 K were scaled by 0.96. More accurate single
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Appl. Organometal. Chem. 2016, 30, 4–11

New palladium catalyst immobilized on epoxy resin
(oxygen, nitrogen, sulfur atoms) participate in binding Pd to epoxide
support. The XPS spectra of EP5/PTU/Pd catalysts show that palla-
dium appears predominantly in the form of Pd(II). This indicates that
these polymeric support chains might stabilize catalytically active
monovalent metastable species with a binding energy larger than
that of Pd metal. Since the epoxide matrix cured with PTU hardener
possesses coordination sites of different coordination ability, it seems
likely that the zero-valent palladium, Pd(0), comes from the reduc-
tion of Pd(II) coordinated to three possible coordination sites.[22,23]
Figure 3. FTIR spectra of (a) EP5/MPTU and (b) EP5/MPTU/Pd.
peaks of S–H vibration at 2545 cmꢀ1 and isocyanate group at
2272 cmꢀ1 shows the quantitative polyaddition of thiol and isocya-
nate groups. In our view, the peak at 1085 cmꢀ1 in the spectrum of
EP5/MPTU/Pd catalyst indicates that oxygen atoms of the epoxy
ring can participate in the metal coordination to the polymer ma-
trix. The stretching vibration of the C¼O group of the urethane
combined with strong absorption of N–H at 1256 cmꢀ1 and the
presence of characteristic urethane bands at 1607 cmꢀ1 give proof
for the urethane cross-linking process. The shifts in the intensity ra-
tio of these bands can indicate an interaction between the nitrogen
atom and the transition metal. The characteristic vibration of the
Pd–S bond cannot be detected in the FT-IR spectrum.
XPS analysis of the outer surface of the particles makes it possible
to obtain relevant information about the metal coordination to the
polymer matrix and the mechanism of the catalytic reaction. The ob-
served XPS (Table 1) binding energy peak of Cl 2p3/2 in the region of
198.22–200.81 eV indicates the presence of PdCl2 structure. The
fractions of palladium nanoclusters are also observed, which form
due to the overlapping side of the d atomic orbitals. The XPS data
summarized in Table 1 show that three possible coordination states
Quantum chemical calculations
DFT methods were employed to get a deeper insight into the struc-
tures of palladium–thiourethane complexes. We performed calcula-
tions of the structures and binding energies for the simplified
models (Scheme 1).
PTUs contain three atoms, oxygen, nitrogen and sulfur, capable of
coordinating to transition metals. We examined the possibility of in-
tramolecular (Scheme 1(B)) and intermolecular (Schemes 1(A) and
(C)) binding for both cis and trans palladium complexes (Scheme 2).
The enthalpy of formation of PdCl2L2 from PdCl2 and model
fragments of thiourethane chains strongly depends on
the coordinating atom involved, ranging from ꢀ83.8 kcal molꢀ1 for
S–Pd–S to ꢀ72.5 kcal molꢀ1 for O–Pd–O to ꢀ72.6 kcal molꢀ1 for
N–Pd–N systems. The Pd–Cl and Pd–L bond lengths and Cl–Pd–L
angles (where L = S-, N-, O-coordinating ligands) in optimized struc-
tures are consistent with the literature for similar systems. Small mo-
lecular crystal structures of complexes of PdCl2 with sulfur[24,25] or
nitrogen[26,27] ligands are well characterized, but, to our knowledge,
there are no such structures involving carbonyl oxygen ligands.
Therefore, we compared similar systems (Pd is coordinated to four
oxygens in β-diketones or acetic acids).[28,29] The comparison is pre-
sented in Table 2.
Table 1. Binding energy values from XPS measurements
Catalyst
Element
O 1s
Binding energy (eV)
Chemical states
EP5/MPTU/Pd
533.61
534.89
404.13
163.28
164.70
200.81
—
C¼O
C–O–C
N–C(O)
S–H
N 1s
S 2p
S–C
Cl 2p
PdCl2
Pd(0)
Pd(II)
Pdδ+
Pd 3d5/2
337.03
335.19
336.02
337.01
335.04
533.31
534.75
536.78
402.68
163.33
164.59
198.22
—
EP5/MPTU/Pd
after use
Pd 4f7/2
O 1s
Pd(0)
Pd(II)
Pdδ+
EP5/DPTU/Pd
C¼O
C–O–C
–O– epoxy ring
N–C(O)
S–H
N 1s
S 2p
S–C
Scheme 1. Structure of binding of palladium with polymer: (A)
intermolecular – head-to-head; (B) intramolecular – head-to-head; (C)
intermolecular – head-to-tail.
Cl 2p
PdCl2
Pd(0)
Pd(II)
Pdδ+
Pd 3d5/2
337.62
336.34
336.61
337.50
335.38
EP5/DPTU/Pd
after use
Pd 3d5/2
Pd(0)
Pd(II)
Pdδ+
Scheme 2. Ligand exchange between PdCl2(PhCN)2 and fragments of PTU
chain.
Appl. Organometal. Chem. 2016, 30, 4–11
Copyright © 2015 John Wiley & Sons, Ltd.
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N. Sienkiewicz et al.
Table 2. Selected bond distances (Å) and angles (°) for trans form of ex-
amined structure
Catalytic study
Mizoroki–Heck reaction
DFT calculation
Literature data
The activity and stability of the supported catalysts were tested in
Mizoroki–Heck (Scheme 4) and hydrogenation (Scheme 5) reac-
tions. The Heck reaction is one of the most important methods
for carbon–carbon bond formation in organic chemistry. It is used
in a wide variety of organic transformations and thus it now be-
longs to an indispensable set of palladium-catalysed cross-coupling
reactions. Therefore activity and stability of the supported catalysts
were tested in Mizoroki–Heck coupling of iodobenzene and methyl
acrylate (Scheme 4). The only observed product is methyl trans-
cinnamate. Kinetics of reaction was followed with GC, by withdraw-
ing samples at certain time intervals (Fig. 4). The prepared catalysts
were also tested for other substrates. All catalysts used show high
activity and selectivity in the Heck coupling of bromobenzene with
styrene (conversions: 73–82%; selectivities: 83–85%). Moreover,
activities in the transformation of activated bromobenzene are also
satisfactory. In the reactions complete E selectivity is observed,
whereas in the transformations of styrene, products of a coupling
are found. It is also seen that activities depend on palladium
loadings and parallel metal dispersion. For very low palladium con-
centrations of 0.1 mol%, trans-stilbene is observed as the main
product in all cases. The selectivity is very similar for both catalysts
(trans-stilbene:cis-stilbene:1,1-diphenylethene= 90:2:7) and compa-
rable to that for typical homogeneous systems.
Pd–L
Pd–Cl
L–Pd–Cl
Pd–L
Pd–Cl
L–Pd–Cl
O ligands
N ligands
S ligands
2.09
2.36
88.97
91.06
86.69
93.3
2.05
2.02
2.07
—
89.9a
90.1a
88.91
91.12
84.47
95.53
2.15
2.42
2.36
2.37
2.3
87.43
92.66
2.32
2.29
a–O–Pd–O angle.
The distance from cross-linking node in the net also significantly
affects the Pd–L bond strength. The closer the node is, the greater is
the steric hindrance, and the lower is the binding energy. The influ-
ence of the distance from the cross-linking node has been tested
for the longer chain structure and palladium coordination to S
ligands. Enthalpy of ligand exchange is ꢀ15.7 kcal molꢀ1 and
is slightly smaller than that of intermolecular binding
(ꢀ18.6 kcal molꢀ1). The head-to-head or head-to-tail arrangement
of thiourethane chain fragments has little meaning (Schemes 1(A)
and (C)). It is worth noting that cis-PdCl2L2 is less stable than the
trans form by 10 kcal molꢀ1 for oxygen and sulfur and by about
4 kcal molꢀ1 for nitrogen (Table 3; Scheme 2). To model the interac-
tions of Pd with thiourethanes, we have calculated the enthalpy of
ligand exchange between PdCl2(PhCN)2 and fragment PTU chains
containing appropriate groups (Table 3; Scheme 2). The dimeric
structures of PdCl2 with sulfur are known[24] but there is no informa-
tion on the corresponding systems with nitrogen and oxygen
atoms (Scheme 3). Calculations show that such connections are
possible, but the formation of the ‘dimeric’ complex in a reaction
analogous to that shown in Scheme 2 is less favourable than the
formation of the ‘monomeric’ complex (Scheme 1). However, in
excess of PdCl2, the formation of the dimer seems probable. The dif-
ference in enthalpy of formation of one Pd–Cl and one Pd–L bond
The scope of iodobenzene and n-butyl acrylate to give n-butyl
cinnamate was also examined for the Heck cross-coupling reaction.
In this case we obtain a high efficiency. The reaction proceeds
smoothly to completion within 30 min at 130°C to afford the desired
product in 79% isolated yield. Employing the optimized catalytic
Scheme 4. Model Heck reaction.
and enthalpy of formation the two Pd–L bonds is 7–15kcal molꢀ1
.
Table 3. Enthalpy of ligand exchange for the tested structures
ΔH (kcal molꢀ1
N–Pd–N
)
Structure
O–Pd–O
ꢀ7.28
S–Pd–S
ꢀ18.63
trans-form Head-to-head
(Scheme 1(A))
ꢀ7.43
ꢀ7.05
18.58
ꢀ4.01
0.46
Scheme 5. Hydrogenation of cinnamaldehyde.
trans-form Head-to-tail
(Scheme 1(C))
ꢀ7.65
17.92
3.31
ꢀ19.95
trans-form Intramolecular
(Scheme 1(B))
ꢀ4.49
(ꢀ15.7a)
ꢀ8.83
cis-form Head-to-head
(Scheme 2)
Dimeric structure (Scheme 3)
1.15
ꢀ6.76
aLonger chain structure (see supporting information).
Figure 4. Kinetics of reaction of iodobenzene with methyl acrylate
catalysed by EP5/MPTU/Pd and kinetics of reaction catalysed by
PdCl2(PhCN)2.
Scheme 3. Structure of palladium chloride dimer.
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New palladium catalyst immobilized on epoxy resin
conditions, iodobenzene with n-butyl acrylate provide excellent
yields of the desired products in very short reaction times. Products
of both reactions were followed using GC. The results confirm that
prepared catalyst could be used in many organic reactions. In this
work we present detailed results only for reaction of iodobenzene
and methyl acrylate because this reaction gives good results, does
not need to be conducted at temperatures above 80°C and we ob-
serve only one product of the reaction.
Table 5. Hydrogenation of crotonaldehyde over EP5/MPTU/Pd and
EP5/DPTU/Pda
Distribution of reaction
products
Catalyst
Run Conversion CAL SAL SOL UOL DA
(%)
(%) (%) (%)
(%) (%)
Homogeneous
PdCl2(PhCN)2
EP5/MPTU/Pd
1
98
14
86
—
—
—
1
2
3
4
5
1
2
3
4
5
98
98
98
97
97
96
96
96
95
95
25
25
18
10
7
11
11
11
10
10
9
12
15
21
27
30
11
16
24
29
31
28
29
31
34
35
27
25
29
37
39
24
20
19
19
18
28
27
21
12
12
Hydrogenation reaction
The second reaction investigated, the selective hydrogenation of
α,β-unsaturated carbonyl compounds to their corresponding unsat-
urated alcohols, is an important step in the preparation of various
fine chemicals such as fragrances for the perfume and pharmaceu-
tical industries. Hydrogenation reaction of cinnamaldehyde can
give three products, namely hydrocinnamaldehyde (HCALD),
cinnamyl alcohol (CALC) and hydrocinnamyl alcohol (HCALC). The
mostly desired product is the α,β-unsaturated alcohol (CALC) which
is difficult to produce in a catalytic hydrogenation of α,β-
unsaturated aldehydes because the hydrogenation of C¼C bonds
in the presence of noble metals is thermodynamically and kineti-
cally favoured over the hydrogenation of the C¼O bonds. The
results (Table 4) show that the morphology, chemical structure
of the polymeric matrix and way of immobilization influence the
selectivity of this reaction. The selectivity of this reaction strongly
depends on the types of binding groups present in the polymer.
Results presented in Table 4 show that for both tested catalysts
there is a significant difference in the product distribution
between the first and fifth runs. This is probably caused by easier
access of substrates to the active sites in the open pores of the
catalyst.
EP5/DPTU/Pd
25
23
16
10
5
9
10
12
13
aConditions: temperature, 90°C; time, 8 h; solvent, 96% ethanol; overall
volume, 5.5 ml. Unidentified reaction products are not shown.
towards UOL is increased after each catalytic cycle. The higher
selectivity to UOL is the result of a large increase in the rate of the
hydrogenation of the C¼O group. A higher selectivity to UOL
means a low selectivity for the hydrogenation of the C¼C bond.
What is more, a fast conversion of CAL to UOL also means a low
yield of DA.
The nitrogen BET adsorption method results (Table 6) indicate
that the investigated supports have different pore size distributions
which probably arise from differences in their cross-link densities.
The EP5/MPTU resin exhibits a lower value of average pore size,
lower surface area and lower porosity than EP/DPTU support. For
all supports, immobilization of palladium precursor results in a sim-
ilar loss in surface area and corresponding decrease in mean pore
size diameter. The characterization of palladium distributions on
the surface of the catalysts was performed using SEM and EDX for
samples of catalysts after fifth use (Fig. 5). The noticeable different
microporous structures of the DPTU- and MPTU-cured resins pre-
sented in Fig. 5 can influence the activity of catalyst by the steric de-
mands of a catalytic centre bound to these supports. From the
presented SEM-EDX images of the catalysts it is difficult to precisely
The activity and selectivity of the prepared catalysts were also
examined for liquid-phase hydrogenation of crotonaldehyde
(CAL). The reaction gives a mixture of saturated aldehyde (butyral-
dehyde – SAL), saturated alcohol (butanol – SOL) and crotyl alcohol
(UOL). Also the diacetal 1,1-diethoxybutane (DA) is produced in the
side reaction between CAL and ethanol used as a solvent. Results
presented in Table 5 show that in both cases the catalyst selectivity
Table 4. Hydrogenation of cinnamaldehyde over EP5/MPTU/Pd and
EP5/DPTU/Pda
Distribution of reaction
products
Catalyst
Run
1
Conversion
(%)
HCALD
(%)
HCALC
(%)
CALC
(%)
Table 6. Characteristics of polymer supports and supported Pd cata-
lysts obtained using BET methods
Homogeneous
PdCl2(PhCN)2
EP5/MPTU/Pd
98
13
76
11
EP5/
EP5/
EP5/
EP5/
MPTU
MPTU/Pd
DPTU
DPTU/Pd
1
2
3
4
5
1
2
3
4
5
98
98
97
97
97
96
96
96
95
95
9
8
7
3
3
7
6
5
2
3
66
65
73
79
80
60
63
78
83
95
25
27
20
18
17
33
31
17
15
2
Surface area
(m2 gꢀ1
88
24
82
19
59
9
51
3
)
Average pore
diameter (nm)
EP5/DPTU/Pd
Volume of pores (%)
Macro w > 50 nm
Meso 2 < w < 50 nm
Micro w < 2 nm
16.9
67.3
15.8
0.22
15.3
66.1
18.6
0.33
13.8
19.3
66.9
0.31
11.2
17.1
71.7
Pore specific volume
(cm3 gꢀ1
Porosity
)
aConditions: temperature, 75°C; time, 4 h; solvent, 96% ethanol; overall
volume, 5.5 ml. Unidentified reaction products are not shown.
0.25
0.36
0.12
0.15
Appl. Organometal. Chem. 2016, 30, 4–11
Copyright © 2015 John Wiley & Sons, Ltd.
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N. Sienkiewicz et al.
Table 7. Catalyst activity in Heck reaction
Catalyst
Run 106Pd+ intensity Conversion Pd content
of emissiona
(%)b
(mmol gꢀ1
)
c
Homogeneous
PdCl2(PhCN)2
EP5/MPTU/Pd
1
—
98
—
1
2
3
4
5
1
2
3
4
5
7.4 × 10ꢀ3
96
96
95
94
93
85
84
82
80
79
0.0019
—
—
—
—
—
—
5.2 × 10ꢀ3
4.7 × 10ꢀ3
—
0.0013
0.0017
—
EP5/DPTU/Pd
—
—
—
3.5 × 10ꢀ3
—
0.0011
aNormalized intensity of 106Pd+ selected from the mass spectra of
palladium catalysts (ToF-SIMS).
Figure 5. SEM micrographs and EDX Pd-mapping of surface of thiol resin-
supported palladium catalysts after fifth use.
bConversion of iodobenzene after 1.5 h. Temperature: 80°C. Substrates:
iodobenzene, 0.28 ml (2.5 mmol); methyl acrylate, 0.23 ml (2.5 mmol).
Solvent: toluene, 0.15 ml (1.4 mmol); triethylamine, 0.35 ml (2.5 mmol);
NMP, 1.00 ml (10.4 mmol). Supported catalyst: 0.05 g. The only observed
product was methyl trans-cinnamate.
characterize Pd clusters or nanoparticles. However, X-ray
microprobe analysis shows that Pd is homogeneously dispersed
throughout the particles of the EP5/MPTU/Pd and EP5/DPTU/Pd
catalysts, suggesting the pore structure in these samples is not
favourable for stabilization of Pd (Fig. 6). There are agglomerates
of Pd (2–20 nm) dispersed on the surface of resin which indicate
that different metal particle size distribution is influenced by the
morphology of the support used. This can be also an explanation
for the unequal catalyst performances.
cAmount of palladium loaded in the support determined using AAS.
occur in their structure during the catalytic reaction. Selective mer-
cury poisoning was used to exclude the possibility of the reaction
being catalysed by unbound palladium metal which is the result
of elution of the complex from the surface of the carrier. This
method is commonly used to define the nature of a homogeneous
or heterogeneous reaction.[30–33] The EP5/MPTU/Pd and EP5/DPTU/
Pd catalysts were exposed to mercury and then subjected to the
catalytic tests. The examined catalytic activity is similar to that
against mercury poisoning. The mercury test was also done for
the homogeneous complex PdCl2(PhCN)2. In this case, addition of
Hg(0) to the homogeneous reaction mixture causes the reaction
to stop. The results indicate that catalysis is not associated with
leached precursor complexes or in situ formed palladium
nanoclusters, which would be deposited to the polymer matrix
after the reaction. The mercury poisoning test shows that metal
complex eluted or in situ formed palladium nanoclusters do not
affect the catalytic activity. Mercury poisoning occurs by a
permanent adsorption on and reaction of mercury with catalyst
which results in a catalytically inactive compound.
The possibility of recycling five times the immobilized palladium
catalysts demonstrates their high stability despite the changes that
ToF-SIMS and AAS were applied to examine the deactivation
process (Table 7). The ToF-SIMS method enables the evaluation
of the chemical composition of the catalyst surface. This measure-
ment is useful to define the nature of interactions between metal-
lic phases and supports as well as the distribution of metal or
other catalyst components on the surface. In this case, catalysts
after the fifth use were investigated and compared to the fresh,
unused samples. Only a very small decrease of the amount of
surface-available palladium is observed; however, the decrease
of palladium content is at such a level that it does not significantly
affect the catalyst activity.
Conclusions
The new thiol-functionalized epoxy resin is a very useful polymeric
support for the preparation of heterogenized palladium catalysts.
Figure 6. SEM-EDX composition of Pd.
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New palladium catalyst immobilized on epoxy resin
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structure of the prepared supports, as well as the other effects of
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and sulfur atoms which can bind transition metals. The chemical
structure of the matrix and coordination surrounding the metal
centres have a significant effect on catalytic properties of the
heterogenized catalysts. Such thiol-functionalized epoxy resin is
being explored for many different applications, especially in
catalysis, where palladium can effectively catalysts a range of
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Acknowledgments
This research was supported in part by PL-Grid Infrastructure.
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Supporting information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
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