The recyclable cyclopalladated ferrocenylimine self-assembly catalytic film and investigation of its role in the mechanism of heterogeneous catalysis

Article abstract of DOI:10.1039/c4ra02540g

An efficient, reusable and stable catalyst nano-sheet film (Si-CDI-Pd) was developed, in which cyclopalladated ferrocenylimines were grafted onto silicon, glass and quartz surfaces by covalent bonds. Water contact angle, ultraviolet-visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), low-angle X-ray diffraction (LAXD) and cyclic voltammetry (CV) were used to characterize the structural and compositional information of the modified surfaces. The immobilized catalyst films were tested for the Suzuki-Miyaura reaction and displayed high activity for the preparation of various biaryls at elevated temperatures in neat water without ligands. It also presented good stability and reusability. It can be reused at least 8 times with little Pd leaching into the crude product. The reasonable and feasible reaction mechanism of the heterogeneous Suzuki-Miyaura reaction based on the results of AFM, XPS, and CV tests of different reaction times were explored in detail, in which a cycle of Pd^II to Pd ⁰ and Pd⁰ to Pd^II on the surface was clearly detected and illustrated. In this approach, Pd⁰ on the surface of nano-sheet films as an active surface to catalyze the coupling reaction of aromatic halides and borophenylic acid proceeded via a mechanism of surface-catalyzed process.

Full text of DOI:10.1039/c4ra02540g

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The recyclable cyclopalladated ferrocenylimine
self-assembly catalytic film and investigation of its
role in the mechanism of heterogeneous catalysis†
Cite this: RSC Adv., 2014, 4, 26413
*
*
Zhihua Fu, Tiesheng Li, Xiaohang He, Jie Liu and Yangjie Wu
An efficient, reusable and stable catalyst nano-sheet film (Si-CDI-Pd) was developed, in which
cyclopalladated ferrocenylimines were grafted onto silicon, glass and quartz surfaces by covalent bonds.
Water contact angle, ultraviolet-visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS),
atomic force microscopy (AFM), low-angle X-ray diffraction (LAXD) and cyclic voltammetry (CV) were
used to characterize the structural and compositional information of the modified surfaces. The
immobilized catalyst films were tested for the Suzuki–Miyaura reaction and displayed high activity for the
preparation of various biaryls at elevated temperatures in neat water without ligands. It also presented
good stability and reusability. It can be reused at least 8 times with little Pd leaching into the crude
product. The reasonable and feasible reaction mechanism of the heterogeneous Suzuki–Miyaura
reaction based on the results of AFM, XPS, and CV tests of different reaction times were explored in
detail, in which a cycle of PdII to Pd0 and Pd0 to PdII on the surface was clearly detected and illustrated.
In this approach, Pd0 on the surface of nano-sheet films as an active surface to catalyze the coupling
Received 23rd March 2014
Accepted 14th April 2014
DOI: 10.1039/c4ra02540g
reaction of aromatic halides and borophenylic acid proceeded via
a mechanism of surface-
www.rsc.org/advances
catalyzed process.
immobilized on a support could be easily removed by ltration
leaving products virtually free of palladium residues.6
Introduction
The Suzuki reaction has been the most studied palladium-
catalyzed reaction in neat water under heterogeneous catalysis7
to evaluate and compare the catalytic activity due to the excel-
lent stability of boronic acids in aqueous media as well as the
versatility of the cross-coupling toward functional groups. A
variety of supports appropriately functionalized for a high
affinity with a palladium catalyst have been proposed. Palla-
dium supported on organic supports (polyaniline, PS-PEG,
polymeric imidazole, polyvinylpyrrolidone, chitosan, etc.),8
hybrid organic–inorganic supports (Pad/SiO2, Pad/PMO, Pad/
SBA-15, silica, zeolite and metal oxide-supported ionic liquids,
etc.)9 and inorganic supports (Pad/C, metal oxides, sepiolite,
hydroxyapatite, hydrotalcite, mesoporous silica, etc.)10 have
been applied to the cross-coupling reaction. However, there are
still some problems to solve, such as the high loading of cata-
lysts, high reaction temperature, addition of organic solvents,
and leakage of the catalyst into the reaction media.11 The
mechanistic studies are certainly one of the most striking
weaknesses of the current studies in heterogeneous catalysis.12
It is anticipated that help in improving the catalytic systems will
come from a careful examination of the heterogeneous
mechanism.
Palladium catalyzed C–C coupling reactions, including Suzuki,
Heck, Sonogashira as well as other reactions, have gained a
predominant place in contemporary organic chemistry in
recent decades.1 The versatility and robustness of these
processes have enabled major progress in total synthesis,
materials development, medicinal chemistry and ne-chemical
industries.2 Palladium catalysis usually allows selective reac-
tions with high turnover numbers (TONs) and turnover
frequencies (TOFs) under rather mild conditions.3 Most of these
studies have been focused on homogeneous catalysis which
provides excellent activity and selectivity. Despite its remarkable
usefulness, homogeneous catalysis suffers from a number of
drawbacks which lie in the removal and the reuse of the catalyst.
Indeed, contamination of advanced chemical intermediates by
palladium residues poses an acute issue for large-scale
synthesis, especially in the pharmaceutical industry where
metal contaminations are closely monitored.4 Moreover,
economic and environmental views make palladium recycling
crucial.5 From these perspectives, heterogeneous catalysis
seems particularly well suited since the palladium metal
The College of Chemistry and Molecular Engineering, Zhengzhou University, 75 Daxue
Road, Zhengzhou 450001, P.R. China. E-mail: lts34@zzu.edu.cn; wyj@zzu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02540g
In the primary mechanism of the coupling reactions cata-
lyzed by solid-supported Pd it is accepted that Pd catalysts
catalyze in a homogeneous way by leaching Pd species,13 which
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can re-deposit when the reaction comes to an end. Rapid Suzuki–Miyaura reaction. By characterizing, identifying, and
development of sophisticated experimental techniques and estimating the nature of the surface species, we will provide
theoretical modelling14 in recent years has allowed an unprec- strong evidence that the heterogeneous Suzuki–Miyaura reac-
edented insight into the molecular nature of the very complex tion catalyzed by catalytic nano-sheet lm proved to be a surface
reactions taking place on the catalyst surface as well as leaching catalytic process.
of Pd species.15 For example, Crudden and MacQuarrie
demonstrated the thermally controlled redistribution and
dissolution of palladium on the surface of Pd foil during the
Suzuki reaction.16 There was considerable circumstantial
Results and discussion
Preparation of catalyst nano-sheet lms (Si-CDI-Pd)
evidence that in the latter materials, surface sites may play a
Preparation of cyclopalladated ferrocenylimines nano-sheet
lms which graed onto silicon, glass and quartz surfaces was
illustrated in Scheme 1. The wafers were hydrophilically treated
by piranha solution rst and were functionalized with (3-ami-
nopropyl) triethoxysilane. Then, amino-functionalized wafers
were immersed in a THF solution of N,N0-carbonyldiimidazole
(CDI) to yield CDI-functionalized wafers which were washed and
then immersed in a THF solution of cyclopalladated ferroce-
direct role in the catalytic cycle.17 Both Ellis18 and Shao19 showed
evidence for the surface-catalyzed Suzuki–Miyaura reaction over
palladium nanoparticles. The most compelling evidence for
surface-driven Suzuki coupling derives from an ingenious
experiment in which Davis and co-workers used a palladium
nanoparticle coated AFM probe to initiate spatially controlled
coupling over surface-tethered aryl halides or styrene.20
Nonetheless, despite many microscopic and kinetic studies
being carried out, unequivocal evidence for surface-catalyzed
nylimines for 48 h to obtain Si-CDI-Pd.
The amount of Pd in Si-CDI-Pd was measured by inductively
cross-coupling chemistry remain elusive. Clearly, the study of
palladium based coupling reactions will continue and potential
coupled plasma atomic emission spectroscopy (ICP-AES). The
Pd content of the catalyst was found to be ꢀ1.0 ꢁ 10ꢂ6 mmol
breakthroughs in the elucidation of the dynamics of palladium
cmꢂ2 in the solid slides. The composition and morphology of
and the determination of the type(s) of palladium species which
the surfaces obtained along with the immobilization process at
are truly catalytically active. That will aid in the rational design
each step were characterized by water contact angle, UV-Vis,
of future generations of catalysts. Therefore, discovery of novel
XPS, AFM and CV.
supports and development of suitably sensitive analytical
Water contact angle can reect surface wettability which was
caused by either a chemical gradient or a gradient in the
topographical surface structure.24 Water contact angles on
modied solid surfaces are shown in Fig. 1.
Aer pre-treatment with “piranha” solution, the silicon and
quartz substrate became strongly hydrophilic because it had an
methods will be a key point in the context of research priority.
Thin lms21 (Langmuir–Blodgett (LB) lms, self-assemble lms,
etc.) on solid slides can be characterized, identied, and the
nature of the surface species by a variety of experimental tech-
niques cab be determined. That enables thin lms to be an ideal
model system to study surface and interface behaviour, with
extremely low water contact angle (5ꢃ), which gave rise to water
which we can gain a more in-depth insight into the catalytic
contact angles of 55ꢃ aer APTES treatment. The water contact
behaviour and mechanisms at the nanomolecular level.
angles of modied solid surfaces of Si-CDI-Pd changed from 58ꢃ
Our recent studies have found that molecule orientation and
for modied by CDI to 53ꢃ for graed cyclopalladated ferroce-
morphology of cyclopalladated ferrocenylimine in LB lms
nylimines. The changes in water contact angles of modied
could affect the catalysis efficiency in the heterogeneous
solid surfaces indicated that the surface conguration had
Suzuki–Miyaura reaction, which clearly demonstrated that
signicant changed, which could clearly reect the chemical
ordered structures have a major inuence on the catalytic
gradient of each step. It also meant that the catalyst lms on
solid substrates were obtained by the self-assembly method.
UV-vis absorption spectra of modied quartz plates along with
the immobilization process offered further evidence of the
reaction.22 However, the LB lms desorbed from the solid slides
to some extent when it was placed in organic solvents or under
heating conditions, which affected the experimental results.
Then cyclopalladated arylimine self-assembled lms were
synthesis of Si-CDI-Pd (Fig. S1†).
synthesized to overcome these disadvantages.23 Indeed, Pd
In order to understand the more detailed information of the
catalysts immobilized on solid slides exhibited improved cata-
catalyst structure, composition, the inuence of the supporting
substrate with the prospect of obtaining even more efficient
lytic activity and stability. Nevertheless, the structure of the Pd
catalysts in the lm was unclear, which made it difficult to
discuss the catalytic mechanism. So, rational design of suitably
catalysts should be closely linked to deeper insights into the
mechanisms involved at the nanomolecular level. We thus
reasoned that using a catalyst with unied, controlled, and
tunable characteristics, one could detect the structural changes
associated with oxidative addition more readily and more reli-
ably to shed light on the Pd surface reaction mechanism.
Herein, we attempted to gra cyclopalladated ferrocenylimine
catalysts onto solid surfaces by covalent bonds and evaluate the
activity and recyclability of the resulting composites in the sheet films (Si-CDI-Pd).
Scheme
1 Preparation of cyclopalladated ferrocenylimine nano-
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Fig. 3 High-resolution XPS spectra of the Pd3d5, Cl1s, Fe2p3, N1s,
C1s, and Si2p of the silicon surface at different steps of the surface
modification.
Fig.
1 Water contact angles on modified solid surfaces. Si–OH
(hydrophilic silicon wafers), Si-APTES (silicon wafers treated by 3-
aminopropyl triethoxysilane), Si-CDI (silicon wafers modified by car-
bonyldiimidazole), Si-Cat (silicon wafers linked by cyclopalladated
ferrocenylimines).
that CDI could be introduced. High resolution XPS narrow scan
for the Pd3d level showed 3d3/2 and 3d5/2 at 343.2 eV and 337.9
eV, Cl2p level at 198.0 eV and Fe2p at 708.4 eV BE, which
indicated that the cyclopalladated ferrocenylimine was attached
(Fig. 3). AFM investigation of the catalyst-graed surface
(Fig. 2d) presented evidence of high-density arrays and an
orderly structure regardless of a few clusters, resulting in a
lower rms of the surface rms down to 1.0 nm.
Electrochemical measurements of Si-CDI-Pd on ITO elec-
trodes were carried out in a conventional three-electrode
conguration using a saturated calomel electrode as the refer-
ence electrode and Ag/AgCl as the counter electrode. Fig. 4
shows cyclic voltammograms of catalyst lms on an ITO elec-
trode in 1 M HCl solutions at scan rates of 10 mV sꢂ1. The cyclic
voltammograms of cyclopalladated ferrocenylimines in THF
solution are shown in Fig. S2.† All the potential values are listed
in Table S2.† The voltammograms showed well-dened surface
waves consisting of symmetric oxidation and reduction waves.
These peaks were ascribed to the Fc/Fc+ redox process in thin
lms, showing that the cyclopalladated ferrocenylimines were
located on the electrode.
catalysis, AFM images (Fig. 2) and XPS spectra (Fig. 3) were
measured at different steps of the surface derivatization process
of Si-CDI-Pd.
The XPS spectra covered the Br 3d, Si 2p, Cl 2p, C 1s, N 1s, Fe
2p3 and Pd 3d5 energy ranges. The O 1s core level is not shown
here because it gave no useful information. The obtained XPS of
the clean silicon wafer only gave two Si 2p peaks (99.1 eV BE,
103.0 eV BE) attributed to bulk elemental Si and silicon oxide is
also not displayed. The binding energies (BEs) obtained for all
core-level spectra presented here are summarized in Table S1.†
The composition and morphology of the silicon wafer
immobilization process of Si-CDI-Pd are shown below. The AFM
images acquired on the clean wafer (Fig. 2a) revealed a at and
evenly aligned surface with 0.41 nm rms. And then a smooth
and homogeneous surface with a ca. 0.56 nm rms image
(Fig. 2b) was observed aer silanization. The binding energies
of C1s, N1s, and Si2p contributed at about 228.4 eV, 399.1 eV,
99.1 eV and 102.6 eV respectively at this step. Although no new
element was detected in the spectrum (Fig. 3) except for a
shoulder spectrum 289.1 eV BE of C1s, the AFM images
obtained aer CDI addition (Fig. 2c) exhibited a higher rms
than that observed aer silanization (1.5 nm), which indicated
The potential separation (DEp ¼ Epa ꢂ Epc) between the
anodic and cathodic peaks of the Si-CDI-Pd were smaller than
that obtained for catalysts in solution (Table S2†), indicating
that the rate of electron transfer from the catalyst lms to the
electrode was related to the structure of cyclopalladated ferro-
cenylimines lms graed on glass electrodes. The low
Fig. 2 AFM images of the silicon wafers at the immobilization process.
(a) Hydrophilic treatment, (b) treated by 3-aminopropyl triethoxysilane,
(c) modified by carbonyldiimidazole, (d) linked by cyclopalladated Fig. 4 Cyclic voltammograms of Si-CDI-Pd on ITO glass (solid and
ferrocenylimines.
dash dot line) and hydrophilicity treated ITO glass (dotted line).
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electroactivity of the catalyst lms is due to the blocking of complexes, which require 2.4–5 mol% of metal.13 There are clear
counterion permeation to the redox-active sites by the highly advantages on xing the orientation of a catalyst in terms of its
oriented ferrocene moieties. Furthermore, the lms were stable ability to inuence the activity and selectivity.
against peeling off during the potential scans since almost no
variation in the cyclic voltammograms were observed aer bromides were performed in the presence of catalyst lms
repeated successive potential scans.25
(Table 2). Good to excellent yields of products (71–99%) were
The coupling reactions of arylboronic acids with various aryl
Unobvious layered structures of fabricated lms proved by obtained (Table 2, entry 1–6), even in the cases of electron-rich
low-angle X-ray diffraction (LAXD) spectrum indicated that (R ¼ 4-OMe: entry 2, 4-NH2: entry 5) or bulky (entry 6) substrates.
mostly catalysts were graed onto the surface of the solid A coupling reaction of o-substituted aryl bromides gave the
substrate (Fig. S3†). CV and AFM investigation of the catalyst- products in lower yield probably due to the electronic effects
graed surface presented the evidence of high-density arrays (entries 7 and 8). For arylboronic acid, the conversion was
and an orderly structure. The ordered structure and the stability excellent (entries 9–12), except for pyrimidine which was
of catalyst in Si-CDI-Pd demonstrated by these measurements obtained in moderate yield (64%, entry 11), indicating that Si-
may inuence the catalysis performance of the nano-sheet lm, CDI-Pd was efficient for the Suzuki–Miyaura reaction in a
which prompted us to explore the surface chemical reactions in heterogeneous system. The conditions used above were also
the heterogeneous catalysis system.
competitive in terms of the ligand and solvent.17 For each
substrate, no ligands conferred the higher activity to Pd in neat
water.
Nevertheless, there has been little discussion about whether
the surface could inuence the catalytic properties in the
reported Suzuki reaction involving SAMs21 or not,15 which was
an important factor for the heterogeneous catalytic system. The
absence of a negative effect of the surface might be due to the
presence of the orderly arranged interface, which could
enhance the performance of the coupling by creating favourable
interactions with aryl substrates.23
Heterogeneous catalytic properties of Si-CDI-Pd
As mentioned earlier, the Suzuki cross-coupling reaction
usually serves as a model for characterization of new catalysts
and optimization of reaction conditions. Heterogeneous cata-
lysts based on different organic and inorganic supports have
been reported for this versatile and powerful method of C–C
bond formation.2 To our knowledge, only a few examples of self-
assembled lm catalysts have been reported but all of them
were with a higher catalyst loading.11
The catalytic activity of Si-CDI-Pd was evaluated in the
Suzuki–Miyaura reaction (Table 1). The catalytic activity of
The recyclability and stability of Si-CDI-Pd
cyclopalladated ferrocenylimines in homogeneous was unfav- Recycling experiments were carried out by using the round
ourable at the low catalyst concentration (Table 1, entry 4). The bottomed asks in which Pd catalysts were graed onto the
Suzuki–Miyaura reactions catalyzed by blank quartz plates internal surface. That is to say that the asks are not only the
cannot proceed under the same reaction conditions (Table 1, reactors but the catalysts, which simplied the reaction opera-
entry 5). Controlled trials (ESI†) clearly showed that it was a tions23 as depicted in Scheme 2.
heterogeneous catalytic process. Si-CDI-Pd displayed a much
In the case of Si-CDI-Pd, although a decreased yield was
higher catalytic activity even with less catalyst loading, which observed aer eight reaction cycles, a moderate yield was still
yielded 99% 4-methylbiphenyl as the only product compared obtained aer tenth cycle. There was no catalyst deterioration
with their analogous cyclopalladated ferrocenylimines in observed, conrming the high recyclability and stability of the
homogeneous, indicating that the well-ordered and higher heterogeneous catalysts (Fig. 5b). ICP-AES measurement
density catalysts were more efficient for the catalytic system. showed trace amounts of Pd-leaching during the recycling
These conditions were very competitive, especially compared to process (Table S4†). However, the reaction time had to be pro-
MNP-supported dendritic catalysts noncovalently graed Pd longed to 18 h aer six cycles. A similar observation was
Table 1 The Suzuki–Miyaura reaction of phenylboronic acid with 4-bromotoluene catalysed by catalyst or blank quartz platea
Entry
Catalytic system
Catalyst/10ꢂ6 mmol
Time/h
Product
Yieldb (%)
TON
1
Si-CDI-Pd
7.6
1000
500
50
12
24
24
48
48
p-CH3-Ph-Ph
p-CH3-Ph-Ph
p-CH3-Ph-Ph
p-CH3-Ph-Ph
p-CH3-Ph-Ph
99
90
73
Trace
16 283
113
182
—
2
Homogeneous
Homogeneous
Homogeneous
Quartz plate
3c
4
5
—
No reaction
—
a
Reaction conditions: 4-bromotoluene 0.125 mmol, PhB(OH)2 0.15 mmol, K2CO3 0.15 mmol, n-Bu4cNBr(TBAB) 0.15 mmol, H2O 3 mL, 80 ꢃC, 12 h.
b
Quartz plate (30 ꢁ mm ꢁ 10 mm ꢁ 1 mm). Yields determined by HPLC, based on the products. PhB(OH)2 0.3 mmol.
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Table 2 Suzuki reaction of arylboronic acid with aryl halides catalysed by Si-CDI-Pd (round-bottomed flask)a
Entry
Ar–Br
Ar–B(OH)2
Product
Yieldb (%)
1
2
3
4
5
6
p-NO2-Ph
p-CH3O-Ph
p-NC-Ph
p-F3C-Ph
p-H2N-Ph
1,3-Dimethy-Ph
2-Bromo pyridine
1-Bromon-NP
p-Me-Ph
p-Me-Ph
p-Me-Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
p-NO2-Ph-Ph
p-CH3O-Ph-Ph
p-NC-Ph-Ph
p-F3C-Ph-Ph
p-H2N-Ph-Ph
1,3-Dimethy-Ph-Ph
2-Bromo-Py-Ph
99
97
98
71
97
80
60
34
85
99
64
96
7
8c
9
1-Bromon-NP-Ph
p-OCH3-Ph
1,3-Diuoro-Ph
Pyrimidine
5-(Methoxycarbonyl)-2-methyl-Ph
p-OCH3-Ph-Ph-(p-Me)
1,3-Diuoro-Ph-Ph-(p-)Me
Pyrimidine-Ph-(p-)Me
5-(Methoxycarbonyl)-2-methyl-Ph-Ph-(p-)Me
10
11
12
p-Me-ph
a
Reaction conditions: phenylboric acid 0.15 mmol, aryl halides 0.125 mmol, catalyst, TBAB 0.15 mmol, K2CO3 0.15 mmol, H2O 3 mL, 80 ꢃC, 12 h,
round-bottomed ask 5 mL. Isolated yield. c NP: naphthyl.
b
interfaces with surface active centers to reveal the reaction
mechanism at these active sites. However, the investigations of
mechanism were usually conned to the structural changes and
morphology analysis before and aer the catalysis.19,23 The lack
Scheme 2 Illustration of the reaction operations used in flask reactors.
of detailed research on the heterogeneous catalytic process
leaves questions about the generality and validity of such
mechanistic studies. It is well known that the changes of surface
during the catalytic process are the key factor for elucidating the
catalytic mechanism. Thus, our aim is focused on insight into
what the nature of the activities is by utilizing more realistic
procedures which could capture part of the complexity inherent
to heterogeneous catalysis.
Plots of conversion versus reaction time for the Suzuki
reaction was shown in Fig. 6. Quantitative conversions were
obtained within 12 h. The corresponding analysis of CV
(Fig. S5†), AFM images and water contact angles acquired at
different reaction times were precisely investigated (Fig. 7).
Signicant morphology changes of silicon wafer graed with
catalysts were observed during catalytic processes, with which a
Fig. 5 Recycling experiments of the Suzuki–Miyaura reaction cata-
detailed analysis of the catalytic processes is presented.
lyzed by Si-CDI-Pd. (a) Isolated yield.
presented by other researchers of retained high yield with
decreasing activities (or lengthened reaction time) upon recy-
cling of catalysts.26 We speculated that it could be caused by the
overlay on the surface of the catalyst lms by reactants and
products, or the inactivation of the Pd active center in the
catalytic process, which could be further proved by CV tests
(Fig. S4†).
Plausible heterogeneous mechanism
Researchers have used surface science methods27 and metal
single crystals28 to explore elementary processes in heteroge-
Fig. 6 Isolated yield of Suzuki–Miyaura reaction catalyzed by Si-CDI-
neous catalysis, which illustrated the importance of the Pd at different reaction times.
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Fig. 7a showed smooth, ordered and uniform surface
morphologies of the catalyst lms with low rms (1.0 nm), in
which the water contact angles were 55ꢃ. However, considerable
restructuring of the catalyst surfaces were observed during
reaction. Fig. 7b–e shows the surface morphologies of Si-CDI-Pd
at different reaction times, in which remarkable morphological
changes of the catalyst lm were detected. These changes
indicated the occurrence of catalytic processes. It meant that a
sequential adsorption and connecting process occurred on the
catalyst lm. The confused aggregation phenomenon on the
catalyst lm surface with higher rms (29.8 nm) appeared
(Fig. 7e) at 6-h catalysis, which revealed that the drastic catalytic
process occurred. A uniform, homogeneous and ordered
pattern reappeared in Fig. 7f when the catalytic reaction was
completed. These results indicated that Si-CDI-Pd as a hetero-
geneous catalyst catalyzed coupling of aromatic halides and
borophenylic acid proceeded via a surface-catalyzed process.
The morphological changes of the catalyst lms were
remarkable, which could be illustrated by the surface changes
made by the catalytic reaction.29 There was an interaction
between the reactants and the support catalyst lms which was
consistent with previous studies that PdNP high reactivity
resulted in surface changes on PdNPs and supporting H-CNTs
during the Suzuki reaction.18 Signicant morphology changes of
solid slices graed with catalysts were observed during catalytic
processes, which could be illustrated by the surface changes
made by the catalytically active Pd on the catalyst lms.27 The
speculation of surface morphology changes of Si-CDI-Pd in
different catalysis times based on AFM images and water
contact angles were precisely investigated to try to elucidate the
processes proposed (Fig. 8a).
Fig. 8 (a) The speculation of Si-CDI-Pd at different catalysis times in
the Suzuki–Miyaura reaction. (b) High-resolution XPS spectra of Pd3d,
B1s and Br3d of Si-CDI-Pd at 0 h (black), 0.5 h (red), 1 h (green), 6 h
(blue) and 12 h (gray).
and 338.3 eV BE corresponding to the Pd3d core level. Reduced
PdII to catalytic active Pd0 could be seen in the whole process of
catalysis reaction and oxidation state of Pd0 to PdII in reaction
with PhBr was also conrmed by XPS spectra.
Pd0 (BE 335.8, 341.1 eV) was found in the XPS spectrum of Si-
CDI-Pd aer 0.5 h catalysis, indicating that the formation of
active Pd0 was considered at the beginning of the catalytic cycle.
The decrease of PdII (BE 343.3, 338.1 eV) intensity was found to
give direct evidence of reduction of PdII to Pd0 in the presence of
phenylboronic acid in Suzuki reactions30 The broadening
FWHM of the PdII peak (1.98 eV BE and 1.79 eV BE) suggested
that different oxidized forms of palladium are presented.23
Boron related peaks (192.5 eV BE) in the B1s spectrum and
bromine in Br3d spectrum (70.0 eV BE) suggested that
substrates sequential adsorption and surface synergy interac-
tion occurred on the catalyst lm.
Studies of XPS spectra may provide more information about
the dynamics of catalytically active species under reaction
conditions, which may contribute to a better understanding of
catalysis pathways. Analysis of the surfaces of the different
samples using XPS was carried out to determine what the
relationship was between the chemical changes and the obvious
morphological changes accompanied in the catalyst lms.
The XPS analysis of the palladium illustrated the cycle of PdII
to Pd0 and Pd0 to PdII of the cyclopalladated ferrocenylimines
on the surface of Si-CDI-Pd (Fig. 8b). It was examined by the
changes of palladium oxidation state during the catalysis
process. PdII in Si-CDI-Pd before reaction was detected at 343.5
The Pd peak at 342.1 eV BE and 336.8 eV BE appeared in the
XPS spectrum aer 1 h catalysis, which was attributed to the co-
existence of Pd0 species and PdII species (even of PdI (ref. 31)).
This could be explained as a result of incomplete reduction of
PdII to Pd0 or Pd0 oxidation additions on the surface. Mean-
while, the boron related peak (191.7 eV BE) and bromine
observed (68.6 eV BE) which are assigned to the oxidation
additions (Pd(PhBr)Lx),32 are presented as the occurrence of
oxidative additions in the catalytic process.
Main Pd0 (BE 340.5, 335.2 eV) and minor PdII (BE 343.2,
337.6 eV) are shown in Fig. 2d. The major of Pd0 demonstrated
the evidence that the oxidative addition of aryl halide to Pd0 (in
the surface) was a key stage of the Suzuki reaction as well as
other reactions catalysed by palladium with aryl halides as
substrates. The minor peak at 337.6 eV BE was reasonably
assigned to the presence of Pd–Br species or intermediate
[Bu4N]2[PdBr4]. This species on the surface might arise from
residual bromide from the catalytic process, which is known to
be catalytically active at forming Br–Pd–aryl intermediates.33
Fig. 7 AFM images of Si-CDI-Pd at different reaction times in the
Suzuki–Miyaura reaction. (a) 0 h, (b) 0.5 h, (c) 1 h, (d) 3 h, (e) 6 h, and (f)
12 h. Insets are water contact angles.
26418 | RSC Adv., 2014, 4, 26413–26420
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The bromine in the Br3d spectrum reduced to 68.3 eV BE which possible to identify new catalysts and reaction mechanisms
was very close to that found for [Pd(PhBr)Lx] which also showed which is very important since this oen provides a rational way
continuous oxidative additions in the catalytic process.
As we expected, the shape of the Pd3d core-level spectrum
was changed back to its initial state (343.1 eV BE and 337.8 eV
BE). It indicated that the integrity of the catalyst was retained as
PdII with little damage suffered in the catalysis process.34
to improve catalytic activity and selectivity.
Acknowledgements
We are grateful to the National Natural Science Foundation of
A cycle of PdII to Pd0 and Pd0 to PdII on the surface of Si-CDI- China (no. 20973157), Research and Development Foundation
Pd could be clearly illustrated with respect to the results of CA, of Zhengzhou (094SGZG23056); Prof. Minghua Liu, Penglei
AFM and XPS analysis. In this approach, Si-CDI-Pd as hetero- Chen for XPS measurements; Prof. Luyuan Mao for AFM
geneous catalyst catalyzed the coupling of aromatic halides and analysis.
borophenylic acid proceeded via a mechanism of surface-cata-
lyzed reactions as follows: First, PdII was reduced by activation
Notes and references
of ArB(OH)2 in situ to Pd0 on the surface of the catalyst lm.
Next, activated Ar0X reacted with Pd0 via oxidative addition to
yield [Ar0PdIIX] (still on the surface) which interacted with
ArB(OH)2 in a synergic way to form intermediates. Finally, the
coupling product was transferred to the solution and Pd0 was
deposited back on the support as PdII aer the catalytic process.
Because of the complexity of interactions occurring simulta-
neously on the surface, we thought that the whole catalytic
process was a synergy interaction between the catalyst and the
substrates to give the target molecules.
1 Cross-Coupling Reactions. A Practical Guide, Topics in
Current Chemistry, ed. N. Miyaura, Springer Verlag, Berlin,
2002; Metal Catalyzed Cross-Coupling Reactions, ed. A. de
Meijere and F. Diederich, Wiley-VCH, Weinheim, 2nd edn,
2004; S. L. Buchwald, Acc. Chem. Res., 2008, 41, 1439.
2 C. Torborga and M. Beller, Adv. Synth. Catal., 2009, 351, 3027;
G. Zeni and R. C. Larock, Chem. Rev., 2006, 106, 4644;
D. Wang, D. Denux, J. Ruiz and D. Astruc, Adv. Synth.
Catal., 2013, 355, 129; V. Farina, Adv. Synth. Catal., 2004,
346, 1553; X. F. Wu, P. Anbarasan, H. Neumann and
M. Beller, Angew. Chem., Int. Ed., 2010, 49, 9047.
Conclusions
´
3 J.-C. Hierso, M. Beauperin and P. Meunier, Eur. J. Inorg.
In this article, an efficient, reusable and stable Pd nano-sheet
lm was developed by immobilized cyclopalladated ferroceny-
limines onto a solid slice. The order, orientation structure
catalysts in Si-CDI-Pd were demonstrated by AFM measure-
ments. Si-CDI-Pd was tested in the Suzuki–Miyaura reaction and
displayed a higher catalytic activity even with a lower catalyst
loading35 for the preparation of various biaryls at elevated
temperature in neat water without ligands. There were clear
advantages on xing the orientation of a catalyst in terms of its
ability to inuence the activity and selectivity.
Chem., 2007, 3767.
4 M. Butters, D. Catterick, A. Craig, A. Curzons, D. Dale,
A. Gillmore, S. P. Green, I. Marziano, J.-P. Sherlock and
W. White, Chem. Rev., 2006, 106, 3002; J. Magano and
J. R. Dunetz, Chem. Rev., 2011, 111, 2177–2250.
5 J. A. Gladysz, Pure Appl. Chem., 2001, 73, 1319; A. Fukuoka
and P. L. Dhepe, Chem. Rec., 2009, 9, 224.
´
6 A.
´
Molnar,
Chem.
Rev.,
2011,
111,
ˇ
2251;
A. Dhakshinamoorthy, M. Opanasenko, J. Cejka and
H. Garcia, Adv. Synth. Catal., 2013, 355, 247.
Moreover, Si-CDI-Pd enabled the facile reaction and recovery
of the catalyst by simply removing and washing, which was
satises the requirement of green, sustainable and economical
chemistry. Good stability and reusability are presented; it was
reused at least 8 times with little Pd leaching into the crude
product, which could meet the specication limits in the
pharmaceutical industry without requiring tedious chromato-
graphic purication.
The reasonable and feasible reaction mechanism based on
the results of different reaction times was explored in detail.
Si-CDI-Pd acted as a heterogeneous catalyst to catalyse the
Suzuki–Miyaura reaction carried through a surface-catalysis
process. The cycle of PdII to Pd0 and Pd0 to PdII on the surface
was clearly detected and illustrated. There was also evidence
that the oxidative addition of aryl halide to Pd0 (in the surface)
was a key stage of the Suzuki reaction as well as other reactions
catalysed by palladium catalysts. Such a system, in addition,
offers opportunities for characterizing surface catalysis in a
simple and convenient way, which is crucial for detecting
dynamic changes on catalytically active species and under-
standing catalysis pathways. Following this approach, it is
7 M. Lamblin, L. Nassar-Hardy, J.-C. Hierso, E. Fouquet and
F.-X. Felpin, Adv. Synth. Catal., 2010, 352, 33; B. Yuan,
Y. Pan, Y. Li, B. Yin and H. Jiang, Angew. Chem., Int. Ed.,
2010, 49, 4054; S. Ogasawara and S. Kato, J. Am. Chem.
Soc., 2010, 132, 4608; J. Wei, J. Jiao, J. Feng, J. Lv, X. Zhang,
X. Shi and Z. Chen, J. Org. Chem., 2009, 74, 6283;
´
R. Franzen and Y. J. Xu, Can. J. Chem., 2005, 83, 266.
8 For polyaniline: P. Boomi, H. G. Prabu and J. Mathiyarasu,
Eur. J. Med. Chem., 2014, 72, 18. For polymeric imidazole:
Y. M. Yamada, S. M. Sarkar and Y. Uozumi, J. Am. Chem.
Soc., 2012, 134, 3190, For poly(vinylpyrrolidone): Y. Li,
E. Boone and M. A. El-Sayed, Langmuir, 2002, 18, 4921. For
chitosan: J. J. E. Hardy, S. Hubert, D. J. Macquarrie and
A. J. Wilson, Green Chem., 2004, 6, 53.
9 For Pd/SiO2: M. Kim, J. C. Park, A. Kim, K. H. Park and
H. Song, Langmuir, 2012, 28, 6441, For Pd/PMO: B. Karimi,
D. Elhamifar, J. H. Clark and A. J. Hunt, Org. Biomol.
Chem., 2011, 9, 7420. For Pd/SBA-15: J. Zhi, D. Song, Z. Li,
X. Lei and A. Hu, Chem. Commun., 2011, 47, 10707. For
silica: Z. Chen, Z.-M. Cui, F. Niu, L. Jiang and W.-G. Song,
Chem. Commun., 2010, 46, 6524.
This journal is © The Royal Society of Chemistry 2014
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View Article Online
RSC Advances
Paper
10 For Pd/C: H. Sakurai, T. Tsukuda and T. Hirao, J. Org. Chem., 25 K. Sakakibara, H. Kamitakahara, T. Takano and
2002, 67, 2721. For metal oxides: P. D. Stevens, G. Li, J. Fan, F. Nakatsubo, Biomacromolecules, 2007, 8, 1657.
M. Yen and Y. Gao, Chem. Commun., 2005, 4435. For 26 Z. Fu, N. Zhang, J. Liu, T. Li, W. Xu, F. Wang, T. Wang,
sepiolite: K. Shimizu, R. Maruyama, S. Komai, T. Kodama
and Y. Kitayama, J. Catal., 2004, 227, 202. For
Z. Zhai, L. Liu, L. Mao and Y. Wu, J. Colloid Interface Sci.,
2013, 394, 409.
´
hydroxyapatite: R. M. Mohamed and E. S. Baeissa, Appl. 27 C. Coperet, M. Chabanas, R. Petroff Saint-Arroman and
Catal., A, 2013, 464–465, 218. For hydrotalcite: M. Mora,
J.-M. Basset, Angew. Chem., Int. Ed., 2003, 42, 156;
X. Huang, Y. Li, Y. Chen, E. Zhou, Y. Xu, H. Zhou, X. Duan
and Y. Huang, Angew. Chem., Int. Ed., 2013, 52, 2520.
28 S. Schauermann, N. Nilius, S. Shaikhutdinov and
H.-J. Freund, Acc. Chem. Res., 2013, 46, 1673; Q. Fu,
F. Yang and X. Bao, Acc. Chem. Res., 2013, 46, 1692.
´
´
C. Jimenez-Sanchidrian and J. R. Ruiz, J. Mol. Catal. A:
Chem., 2008, 285, 79. For mesoporous silica: W. Fang,
J. Yang and J. Gong, Adv. Funct. Mater., 2012, 22, 842.
11 L. Jin and J. Liebscher, Chem. Rev., 2007, 107, 133.
12 N. T. S. Phan, M. V. D. Sluys and C. W. Jones, Adv. Synth.
Catal., 2006, 348, 609.
¨
¨
29 K. Kohler, W. Kleist and S. S. Prockl, Inorg. Chem., 2007, 46,
1876; F. Y. Zhao, M. Shirai, Y. Ikushima and M. Arai, J. Mol.
Catal. A: Chem., 2002, 180, 211.
13 J.-S. Chen, A. N. Vasiliev, A. P. Panarello and J. G. Khinast,
Appl. Catal., A, 2007, 325, 76; Z. Niu, Q. Peng, Z. Zhuang,
W. He and Y. Li, Chem.–Eur. J., 2012, 18, 9813.
30 (a) S. S. Soomro, F. L. Ansarib, K. Chatziapostoloua and
¨
14 L. Xue and Z. Lin, Chem. Soc. Rev., 2010, 39, 1692.
K. Kohlera, J. Catal., 2010, 273, 138; (b) B. Crociani,
´
15 M. Perez-Lorenzo, J. Phys. Chem. Lett., 2012, 3, 167.
S. Antonaroli, A. Marini, U. Matteoli and A. Scrivanti,
16 S. MacQuarrie, J. H. Horton, J. Barnes, K. McEleney,
Dalton Trans., 2006, 2698.
H.-P. Loock and C. M. Crudden, Angew. Chem., 2008, 120, 31 G. R. Cairns, R. J. Cross and D. Stirling, J. Mol. Catal. A:
3324; Angew. Chem., Int. Ed., 2008, 47, 3279. Chem., 2001, 172, 207.
17 B. M. Choudary, S. Madhi, N. S. Chowdari, M. L. Kantam and 32 F. C. Loh, K. L. Tan and E. T. Kang, Eur. Polym. J., 1991, 27,
B. J. Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127. 1055.
18 P. J. Ellis, I. J. S. Fairlamb, S. F. J. Hackett, K. Wilson and 33 J. Zou, S. G. Stewart, C. L. Raston and K. S. Iyera, Chem.
A. F. Lee, Angew. Chem., Int. Ed., 2010, 49, 1820. Commun., 2011, 47, 1803.
19 L. Shao, B. Zhang, W. Zhang, S. Y. Hong, R. Schlçgl and 34 The boron related peaks (192.5 eV BE) were due to
D. S. Su, Angew. Chem., 2013, 125, 2168; Angew. Chem., Int.
Ed., 2013, 52, 2114.
20 J. J. Davis, K. S. Coleman, K. L. Busuttil and C. B. Bagshaw,
J. Am. Chem. Soc., 2005, 127, 13082; J. J. Davis,
adsorption of excessive ArB(OH)2; the bromine related
peaks (68.1 eV BE) might arise from residual bromide
associated with the oxidative addition step aer the
catalysis.
C. B. Bagshaw, K. L. Busuttil, Y. Hanyu and K. S. Coleman, 35 About ppm Pd sufficient for cross-coupling reaction:
J. Am. Chem. Soc., 2006, 128, 14135.
21 A. K. Kakkar, Chem. Rev., 2002, 102, 3579.
22 Z. Fu, T. Li, B. Mu, L. Mao, G. Li, W. Xu and Y. Wu, J. Mol.
Catal. A: Chem., 2012, 200, 363.
23 N. Zhao, T. Li, Z. Zhai, J. Qiu, W. Xu, H. Liu and Y. Wu,
ChemCatChem, 2013, 5, 1481.
S. D. Walker, T. E. Barder, J. R. Martinelli and
S. L. Buchwald, Angew. Chem., Int. Ed., 2004, 43, 1871;
M. R. Biscoe, B. P. Fors and S. L. Buchwald, J. Am. Chem.
Soc., 2011, 133, 16707; B. P. Fors, D. A. Watson,
M. R. Biscoe and S. L. Buchwald, J. Am. Chem. Soc., 2008,
130, 13552; T. E. Barder, S. D. Walker, J. R. Martinelli and
S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, 4685.
24 J. S. Sharp, D. J. Farmer and J. Kelly, Langmuir, 2011, 27,
9367.
26420 | RSC Adv., 2014, 4, 26413–26420
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