Highly ordered amphiphilic cyclopalladated arylimine self-assembly films for catalyzing Heck and Suzuki coupling reactions

Article abstract of DOI:10.1002/aoc.3467

A series of new cyclopalladated arylimine compounds (3a, 3b, 3c, 4a, 4b, 4c) were synthesized and characterized. Their catalytic properties for Heck and Suzuki coupling reactions in a homogeneous system were preliminarily investigated using water as solve

Full text of DOI:10.1002/aoc.3467

Full paper
Received: 6 December 2015
Revised: 13 January 2016
Accepted: 1 February 2016
Published online in Wiley Online Library: 17 May 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3467
Highly ordered amphiphilic cyclopalladated
arylimine self-assembly films for catalyzing
Heck and Suzuki coupling reactions
Yaqing Zhao, Na Zhao, Meiling Zou, Tiesheng Li*, Guoping Li, Jinpeng Li
and Yangjie Wu*
A series of new cyclopalladated arylimine compounds (3a–c, 4a–c) were synthesized and characterized. Their catalytic properties
for Heck and Suzuki coupling reactions in a homogeneous system were preliminarily investigated using water as solvent, in which
no ligands, air isolation or assistant solvents were needed in cross-coupling reactions. The optimization of the homogeneous
system provided a basis for research on the heterogeneous catalytic reaction catalyzed by ordered self-assembly films. Organized
monolayers of 3a–c were prepared and utilized as C–C coupling catalysts. Monolayers of 3a–c were deposited using Langmuir–
Blodgett techniques and analyzed using π–A isotherms, UV–visible and X-ray photoelectron spectroscopies and atomic force
microscopy, which showed near orientation on the surface and stability under the optimized experimental conditions suitable
for exploring Heck and Suzuki coupling reactions. The activity of immobilized 3c monolayer is enhanced relative to homogeneous
reaction, in which the ordered monolayers are efficient with a catalyst loading as low as 10ꢀ5 mol%, turnover number as high as 79
200 and turnover frequency as high as 2640hꢀ1. The catalytic efficiency is 100 times higher than that in the homogeneous case
using the same amount and ratio of reagent. The increased activity of immobilized 3c monolayer is due to a combination of its
structure and changes in conformation when deposited onto the substrate. The topographic changes of catalyst films, stability
of films and catalytic activity were investigated with atomic force microscopy, cyclic voltammetry, X-ray photoelectron spectros-
copy and inductively coupled plasma atomic emission spectrometry, from which a heterogeneous catalytic mechanism for Suzuki
coupling reaction is proposed. The study demonstrates that careful monolayer studies can provide useful models for the design
and study of supported molecular catalyst systems. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: cyclopalladated arylimines; Heck reaction; Suzuki reaction; Langmuir–Blodgett (LB) films; ordered monolayer
methods.[42–46] Beller et al.[47] first reported the use of
cyclopalladated complexes as catalysts for Heck and Suzuki
coupling reactions, and since then various palladacycles have
Introduction
C–C coupling reactions are versatile tools in synthetic chemistry
been reported. Milstein and co-workers described good perfor-
and of practical interest in industry.[1–3] Heck and Suzuki coupling
mance from orthopalladated imines and CN palladacycles for
reactions are the most thoroughly investigated.[4,5] Catalytic proce-
dures have been shown to be particularly attractive and versatile in
numerous organic transformations because they usually offer high
yields, good regio- and stereo-selectivity and excellent compatibil-
ity with many functional groups. Efficiency of these reactions has
been elegantly and widely demonstrated in the synthesis of natural
products, organic building blocks, pharmaceuticals and agricultural
derivatives.[6–9] The mechanism in homogeneous systems has been
well explored.[10–15] In recent years, much attention has been
focused on aqueous and heterogeneous switching of organic trans-
formations using amphiphilic material-supported transition metal
complexes or metal nanoparticles.[16–19] Meantime, palladium sup-
ported on carbon,[20–24] mesoporous material (MCM-41), molecular
sieves,[25–27] oxides (MgO, TiO2, ZnO, Al2O3),[28–31] basic zeolites and
sepiolites,[32–34] silica[35–39] and alumina[40,41] has been applied to
cross-coupling reactions. Also, cyclopalladated complexes have
been extensively studied because of advantages such as their
high activity in various types of coupling reactions, stability in
air, no need for ancillary ligands and simple synthetic
Heck[48] and Suzuki[49] reactions.
Our group obtained a series of cyclopalladated ferrocenylimines
which can be used for Heck, Suzuki, Sonogashira and Kumada reac-
tions, among others, in homogeneous systems, in which they
showed highly efficient catalytic ability, and their catalytic mecha-
nisms were also investigated.[50–62]
However, homogeneous catalysts with traditional phosphine-
based ligands are difficult to handle and remove from a reaction
*
Correspondence to: Tiesheng Li or Yangjie Wu, College of Chemistry and
Molecular Engineering, Key Laboratory of Biological Chemistry and Organic
Chemistry of Henan Province, Key Laboratory of Advanced Nano-information
Materials of Zhengzhou, Zhengzhou University, Kexuedadao 100, Zhengzhou
450001, PR China. E-mail: lts34@zzu.edu.cn
College of Chemistry and Molecular Engineering, Key Laboratory of Biological
Chemistry and Organic Chemistry of Henan Province, Key Laboratory of
Advanced Nano-information Materials of Zhengzhou, Zhengzhou University,
Kexuedadao 100, Zhengzhou 450001, PR China
Appl. Organometal. Chem. 2016, 30, 540–549
Copyright © 2016 John Wiley & Sons, Ltd.

Highly ordered amphiphilic cyclopalladated self-assembly films
mixture, and phosphorus ligands are often water- and air-
sensitive, which limits their applications in heterogeneous
catalysis.[28,63,64]
say, if the designed cyclopalladated imine LB films were used in
pure water, or their polymers, they should well assemble on a
substrate and would not be desorbed easily from the substrate.
In the study reported here, we took advantage of a well-
understood surface modification process to compare the
activity of surface-confined monolayer films of a series of
polymerizable cyclopalladated arylimine (3a–c, 4a–c; Scheme 1)
catalysts with their homogeneous behavior. Their catalytic
properties for Heck and Suzuki cross-coupling reactions in
homogeneous systems were initially investigated to determine
suitable experimental conditions for subsequent investigation
of their assembled films in heterogeneous catalysis. Through
careful molecular design and controlled deposition, monolayers
with known surface coverage and molecular orientation to the
interface were obtained. Using these controlled-orientation
surfaces for the example of Suzuki coupling reaction, increased
activity of the surface-confined catalyst relative to homoge-
neous reactions under the same conditions was observed. Their
catalytic mechanism and recycling for Suzuki reaction were also
investigated. In addition, to provide an insight into the specific
example of immobilized cyclopalladated complexes, the results
demonstrate the utility of studying well-characterized molecular
monolayers which can be used as models for supported molec-
ular catalyst systems.
It is well known that there are difficulties in investigating
mechanisms because active species cannot be determined in
heterogeneous systems,[65–67] which makes general conclusions
difficult to obtain in most cases because the mechanism of a
certain reaction is strongly affected by the conditions. Therefore,
contradictory results are sometimes reported.[68,69] Immobilized
catalysts can mirror their homogeneous behavior or the process
can alter a catalyst’s activity perhaps even changing the mecha-
nism whereby it operates. Detailed understanding of catalytic
behavior at interfaces is clearly desirable, but studies of sup-
ported catalysts can be riddled with uncertainties. Typical
high-surface-area supports are often not uniform, giving var-
iation in catalyst binding sites, aggregation and orientation
with respect to the surface. It is often difficult to determine if the
structure of a molecular catalyst remains intact upon adsorbing it
to a surface. If catalytic activity is observed with a supported system,
it is frequently impossible to know what species is responsible for the
catalysis. Therefore, procedures for studying molecular catalysts at a
uniform and reproducible interface can enhance the understanding
of their behavior in heterogeneous environments, which often pro-
vides a rational way to improve catalytic activity and selectivity.[70]
For this proposal, we placed cyclopalladated catalysts on a substrate
in the form of self-assembly films which have high catalytic activity
and recyclability, but the orientation and density could hardly be
controlled.[71–75]
Experimental
General methods
The Langmuir–Blodgett (LB) technique is a simple method for
assembling layers with well-defined molecular orientation and con-
trol over thickness at the molecular level to reproducibly produce
highly ordered molecular arrays.[76–79] One advantage of LB films
is that various experimental techniques can be used to study
reaction mechanisms. Recently, amphiphilic transition metal
catalysts on substrates for catalytic applications have been stud-
ied using the LB technique and have generated considerable
interest.[80] Töllner et al.[81] reported the hydrogenation of
carbon–oxygen double bonds catalyzed by active LB films of a
rhodium complex. Abatti et al.[82] used porphyrin LB films as
catalysts for alkene epoxidation, and doubled yields compared
with the homogeneous system. Benítez et al.[83] investigated
the catalytic properties of irconium phosphonate LB films and
self-assembled monolayers using the epoxidation of cis-
cyclooctene. Park et al.[84] reported catalytic CO oxidation by
LB films of Rh/Pt bimetallic nanoparticles, and Pasc-Banu et al.[85]
used pincer Mn LB films to catalyze olefin oxidation.
All solvents were obtained from commercial sources and used with-
out purification. The experiment for monolayer spreading on water
was performed with a LB system (KSV-5000-3, KSV Instruments,
Helsinki, Finland) equipped with computer controls. Surface pres-
sure was measured with a Pt Wilhelmy plate in air. Distilled and
deionized water with a resistivity of 18.2 MΩ cm (Milli-Q Gradient,
Millipore Co., USA) was used as a subphase. Melting points were
measured using a WC-1 microscopic apparatus and were uncor-
rected. 1H NMR and 13C NMR spectra were recorded using a Bruker
DPX-400 spectrometer in CDCl3 with tetramethylsilane as an inter-
nal standard. Elemental analyses were conducted with a varioEL III
system (Elementar GmbH). Mass spectra were measured with an
LC-MSD-Trap-XCT instrument. All Heck and Suzuki reactions were
accomplished without the protection of inert gas. An SPM-9500J3
(Shimadzu Corporation, Japan) was employed for atomic force
microscopy (AFM) measurements in air at ambient temperature.
In order to gain an insight into the mechanism for heteroge-
neous systems, amphiphilic cyclopalladated ferrocenylimines
were designed and synthesized.[4,5] The results indicated that
these amphiphilic compounds exhibited good catalytic activity
in homogeneous systems and their assembled films had higher
catalytic efficiency due to the regular molecular orientation. In
the research on heterogeneous mechanisms mentioned above,
the amphiphilic cyclopalladated ferrocenylimine LB films
desorbed from glass slides when they were placed in organic
solvents, stirring or under heating conditions, which would af-
fect the experimental results because of their lower molecular
weight, and result in recycling only once.[5,86,87] To overcome
this limitation, it is one of the key factors that a cyclopalladated
complex should be strongly adsorbed on the substrate under
experimental conditions. If a polymerizable group is introduced,
catalytic activity and recycling could be improved.[88] That is to
Scheme 1. Synthesis of cyclopalladated arylimine complexes (3a–c, 4a–c).
Appl. Organometal. Chem. 2016, 30, 540–549
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Y. Zhao et al.
HPLC was conducted with a Waters 600 liquid chromatograph. The
content of Pd in LB films was measured with atomic absorption
spectroscopy (AAS) using a Z-8000 polarized Zeeman atomic
absorption spectrophotometer (Hitachi Corporation, Japan). Induc-
tively coupled plasma atomic emission spectrometry (ICP-AES) was
carried out with an ICAP 6000 Series (Thermo Scientific). X-ray
photoelectron spectroscopy (XPS) data were obtained with an
ESCALab220i-XL electron spectrometer from VG Scientific using
300 W Al Kα radiation. The base pressure was approximately
3 × 10ꢀ9 mbar. The binding energies were referenced to the C 1 s
line at 284.8 eV from adventitious carbon. Crystal structures were
determined using an X’Pert PRO (PANalytical) with graphite
monochromated Cu Kα radiation (λ = 1.5418 Å) at room
temperature.
ultrasonically washed in acetone, ammonia, ethanol and pure water
in sequence.
Hydrophilic treatment
Silicon and glass wafers were put into a mixed solvent of concen-
trated nitric acid and concentrated sulfuric acid (2:1v/v) and heated
to boiling for 1–1.5h. Then, the wafers were washed with deionized
water. The ITO glass substrates were ultrasonically washed in
acetone, ammonia, ethanol and pure water in sequence.
ICP-AES analysis
After each reaction was over, the glass wafer was removed and the
reaction mixture was centrifuged and broken down with nitrolysis,
in which the Pd concentration was measured using ICP-AES.
Preparation of compounds (1a–c, 2a–c, 3a–c, 4a–ac)
Compounds 1a–c and 2a–c were prepared according to the
literature[5,56] and dimer compounds 3a–c, i.e. (PdCl(CH2¼
CHCH2OC6H6CH¼NR))2, were prepared as follows. A mixture of
dilithiumchloropalladate (10 ml, 0.1 mol lꢀ1), sodium acetate
(1 mmol), compound 2 (1.1 mmol) and methanol (10 ml) was stirred
at room temperature for 24 h. The mixture was then filtered,
distilled and subjected to column chromatography using
dichloromethane and petroleum ether (3:1), and yellow solid 3
was obtained. Compounds 3a–c were decomposed with
triphenyl phosphate to give monolayers 4a–c. Crystals of 4a–c
were also obtained. Characterization of 1, 2 and 3 and analysis
of crystals of 3a–c and 4a–c are presented in the supporting
information (content 1 and 2).
AAS analysis
Glass wafers (2.5 × 2.5 cm2) with 3c monolayer were put into a
mixed solvent of concentrated nitric acid and concentrated sulfuric
acid (2:1v/v) and heated to boiling for 1–1.5h. The wafers were
then washed with deionized water. The Pd concentration of the
solution was found to be 45.7mg mlꢀ1 as measured using AAS.
The Pd content of the catalyst was 2.0 × 10ꢀ10 mol cmꢀ2
.
Results and discussion
Catalytic activity of complexes 3a–c in homogeneous systems
It is very important for selecting suitable system, for example, cer-
tain C-C coupling reaction as temple and using solvent to catalytic
LB films. So, the catalytic activity of cyclopalladated arylmine com-
plexes 3a–c for homogeneous Heck and Suzuki coupling reactions
was systematically investigated. An initial survey of bases (0.1mol%
catalyst, DMF as solvent, at 140°C for 24 h) reveals that K3PO4,
NaOAc and KOAc are much better than other bases (Table S3,
entries 1, 5–9, 11, 12). There are nearly no products when the tem-
perature is decreased to 75°C with NaOAc and KOAc (Table S3,
entries 10 and 13). The yields are obviously improved when the
temperature is decreased from 140 to 75°C and the reaction time
is reduced from 24 to 12 h with K3PO4 (Table S3, entry 2). But as
the temperature decreased to 50°C or to room temperature, the
yields decrease rapidly (Table S3, entries 3 and 4). There are no
significant differences among the three catalysts with the same
solvent (DMF), base (K3PO4), reaction temperature (140°C) and
reaction time (24 h) (Table S3, entries 1, 14 and 15). Therefore, the
optimum reaction conditions are as follows: base, K3PO4; reaction
time, 12 h; reaction temperature, 75°C. Taking catalyst 3c as an
example, a wide range of electronically and structurally diverse aryl
halides can be cross-coupled efficiently under these optimized
reaction conditions. The results are summarized in Table S4.
Heck cross-coupling reactions
Aryl halide (0.5 mmol), styrene (1.5mmol) and catalyst 3 (0.1 mol%)
were combined with base (1mmol) and dimethylformamide (DMF;
1.5ml) or deionized water (1.5ml) in a small round-bottom flask.
The reaction mixture was stirred and then quenched. The organic
layer was separated and the aqueous layer was extracted three
times with ethyl acetate. The combined organic phase was dried
with MgSO4 and filtered, and the solvent was analyzed using gas
chromatography.
Suzuki cross-coupling reactions
Aryl halide (0.25 mmol), arylboronic acid (0.6mmol), catalyst 3
(0.1 mol%) and tetrabutylammonium bromide (0.3 mmol) were
combined with base (0.3 mmol) and deionized water (1.5ml) in a
small round-bottom flask. The reaction mixture was stirred and
then extracted three times with ethyl acetate. The combined
organic phase was dried with MgSO4 and filtered, and the solvent
was analyzed using gas chromatography.
The reaction of substrates with a strongly electron-donating
group, such as p-substituted group of bromobenzene, with styrene
gives low yields (Table S4, entries 4–6). On the contrary, high yields
are obtained with strongly electron-withdrawing p-substituted
group of bromobenzene (Table S4, entries 7, 9–12). For this reaction
system, the yield of 4-bromobenzenecarboxylic acid is only 5.5%
because there are many byproducts due to the carboxyl
group (Table S4, entry 8). The yields are low with electron-
withdrawing or electron-donating group in m-position (Table S4,
entries 13–16). The yields using substrates with o-substituted
Preparation of Langmuir monolayer and LB films
A chloroform solution containing the catalyst was carefully spread
onto an ultrapure water surface. After evaporation of solvent
(30 min) and stabilization for 1 h, the Langmuir monolayer was
compressed at a speed of 5 mmminꢀ1 at 20 1 °C. LB films were
transferred onto indium tin oxide (ITO) glass substrates (for electro-
chemical study) and glass slides (for AFM and catalysis study). The
glass slides were cleaned in boiling concentrated H2SO4 for 1 h
and then washed with pure water. The ITO glass substrates were
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Highly ordered amphiphilic cyclopalladated self-assembly films
electron-donating groups are also lower because of steric effects
(Table S4, entries 18 and 19). By contrast, the yields are higher with
o-substituted electron-withdrawing groups (Table S4, entries 17 and 20).
The results of Heck coupling with structurally diverse aryl ha-
lides with styrene in pure water are listed in Table S5. Different
results are obtained, in which the catalytic efficiency of 3c is
higher and independent of the substrate structure except chlo-
robenzene (Table S5, entry 3). Cross-coupling reactions of 1-
bromonaphthalene, 3-bromothiophene and 3-bromopyridine
with styrene catalyzed by 3c also proceed smoothly under the
same reaction conditions to afford the cross-coupling products
in high yields (Table S5, entries 12–14).
To investigate the activity of the catalysts 3a–c for other
cross-coupling reactions, the Suzuki reactions of arylboronic
acid with aryl halides were employed. Our initial survey of bases
for 4-bromotoluene with phenylboronic acid as the coupling
partners using 0.1 mol% loading of 3c with DMF as solvent re-
veals that K3PO4, NaOAcꢁ3H2O and K2CO3 are much better than
other bases (Table S6, entries 1, 7 and 8). Considering economic
factors, K2CO3 was adopted in subsequent research owing to its
low cost.
Reaction temperature, solvent and reaction time for 4-
bromotoluene with phenylboronic acid were also studied (Table
S7) using the appropriate base (K2CO3) and catalyst 3c. The
results show that DMF, tetrahydrofuran, toluene and water are
much better than other solvents (Table S7, entries 3–6) using
0.1 mol% of 3c and K2CO3 as base. When water is selected as
the solvent, n-Bu4NBr is present. The yields are obviously
decreased when the temperature is lowered from 80 to 60°C
in DMF (Table S7, entries 7–9). With the appropriate solvent
(water) and base (K2CO3), we set out to examine the scope
and limitations of the protocol based on both aryl halides and
arylboronic acids in the presence of 0.1 mol% of complex 3c,
at 80°C. The results are listed in Table S8.
The results of Suzuki coupling reaction with structurally
diverse arylboronic acid and aryl halides are listed in Tables S8
and S9. For activated and inactivated aryl bromides, excellent
yields are obtained (Table S8, entries 1–12). But the yields are
lower when the substrates are heterocyclic compounds (Table
S8, entries 13–15). For arylboronic acid, the conversion is excel-
lent with m-substituted and p-substituted compounds (Table
S9, entries 1–7). With heterocyclic substituents and alkyl
substituents, moderate yields are obtained (Table S9, entries 9,
10 and 12). However, the yields are also low when aryl halides
are o-substituted (Table S9, entries 8 and 11). These results show
that the amphiphilic cyclopalladated arylimines are efficient cat-
alysts for Suzuki coupling reaction of arylboronic acid with aryl
halides in homogeneous systems under the optimized reaction
conditions.
Figure 1. π–A isotherms of 3a–c at water–air interface at room
temperature.
The high collapse pressure can be attributed to the strong polar
groups and chain–chain interactions.[89] The average surface
areas occupied per molecule obtained by extrapolation of the
liner part of the isotherms of complexes 3a–c are 72, 106 and
67 Å2, respectively. The isotherms show a gradual transition,
indicating a slight structural alteration in the molecules. The
static elasticity–surface pressure (Es–π) curve is the differential
form of the change of surface pressure during compression
and would be better for determining the surface state of the
monolayer than the integral form of the π–A isotherm. It is de-
termined from the condensability of the arranged molecules.
To directly select optimum surface pressure for Langmuir mono-
layer and LB film formation, the static elasticity of the mono-
layer was calculated from π–A isotherms (Fig. 1). A high static
elasticity value is associated with a monolayer that has a strong
cohesive structure at the interface and is the most stable and
rigid.[90,91] The surface pressures obtained as 10–18, 25–40 and
10–20 mN mꢀ1 are determined as optimum transfer pressures
for 3a–c deposition, respectively, from air–water interface onto
substrates (Fig. S4).
UV–visible spectra can be used to verify whether the monolayer
can be transferred ordered and densely onto substrates. Film-
forming materials can be transferred with different surface configu-
ration when different surface pressures are selected.[86] UV–visible
absorption spectra of 3a–c LB films with different layers and
pressures were obtained and the best surface pressures were
chosen for their LB film deposition from air–water interface. In the
UV–visible spectra of complexes 3a–c in CHCl3 solution, the peak
(200–230 nm) belongs to the benzene derivatives. Figure 2 shows
the UV–visible spectra of 3a LB films with different numbers of
layers deposited at various surface pressures. When the surface
pressure is 18 mN mꢀ1, the absorption peak of benzene is incon-
spicuous and the absorption is very weak (Fig. 2(A)). The absorption
increases with increasing number of layers when the surface
pressures are12 and 15 mN mꢀ1. The liner relationship between
number of layers and absorbance at 12 and 15 mN mꢀ1 indicates
that the monolayer is successfully transferred onto solid substrates
(Figs. 2(B) and (C)). In contrast, the absorbance does not vary linearly
with the number of layers when the surface pressure is 18 mN mꢀ1
(Fig. 2(D)). Similar results are obtained for the UV–visible spectra of
complexes 3b and 3c (Figs S5 and S6). Although the absorbance is
liner with the number of layers (Figs. 2(B) and (C), S2(B) and (D) and
S3(B) and (C)), the orientation of the monolayer changes with a
change in surface pressure.
Characterization of 3a–c monolayer and LB films
In order to investigate the heterogeneous catalytic mechanism
of 3a–c LB films, the molecular arrangement at the air–water
interface was first studied. Complex 3a–c linked long alkyl chain
was spread from a chloroform solution (200 μl, 0.5 mg mlꢀ1) to
measure the π–A isotherms at a compression speed of
5 mm minꢀ1 at 20 0.5°C. Figure 1 shows the isotherms ob-
tained by compression after evaporation of solvent for 30 min
and stabilization for ca 1 h in the uncompressed state. The π–A
isotherms of complexes 3a–c exhibit collapse pressures of 58,
69 and 63 mN mꢀ1, respectively. All isotherms are reproducible.
Appl. Organometal. Chem. 2016, 30, 540–549
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Y. Zhao et al.
AFM can be used to provide topographic images of mono-
layers, giving a direct view of surface morphology with high
spatial resolution, which is often indicative of film quality.[92]
Figure 3 shows AFM images of the hydrophilic treated glass
slides and one-layer LB films of complex 3c transferred at sur-
face pressures of 10, 15, 20, 25 and 30 mN mꢀ1. All scan
ranges are 2.5 μm × 2.5 μm. The AFM images obtained after
one-layer LB films of complex 3c are transferred show obvious
signs of cluster formation and exhibit a higher Ra than that
observed after hydrophilic treatment (14.34 nm instead of
2.29 nm). The images obtained are smooth and homogeneous,
indicating that well-ordered and uniform monolayer films had
been fabricated.
The cyclic voltammetry curves of one-layer LB films of complex
3c deposited on ITO glass are shown in Fig. 4. The experiments
were carried out in 0.1M HCl solutions versus Ag/AgCl electrode
in three-electrode electrochemical cells. No redox peaks are
observed between ꢀ0.6 and +1.0 V versus Ag/AgCl electrode in
0.1 M HCl solution (Fig. 4, dotted curve), suggesting that the ITO
glass is hydrophilic treated. For LB films of complex 3c transferred
onto ITO glass, a pair of redox peaks is seen between ꢀ0.2
and +1.6 V versus Ag/AgCl electrode (Fig. 4, solid curve). There
is a reduction peak at around +0.6 V corresponding to the
reduction reaction from Pd2+ to Pd0 and an oxidation peak
at around +0.3 V corresponding to the oxidation reaction from
Pd0 to Pd2+, suggesting that complex 3c had been deposited
onto ITO glass.
XPS measurements of 3c LB films are shown in Fig. 5, in which
the peaks of elements Pd, N and C in the XPS spectrum can be seen
at 337.9, 399.8 and 284.8 eV, which is evidence that the complex 3c
is present in the LB films.
After pretreatment with piranha solution, the glass substrates
become strongly hydrophilic, having extremely low water contact
angle (6°), and then the water contact angles increase to 80° after
deposition of 3c monolayer (Fig. 6), which indicates that catalyst
is deposited on the substrate, with which 99% yield can be
obtained for Suzuki reaction (50°C, 24 h, water, K2CO3, n-Bu4NBr).
On the other hand, hydrophilic surface modified with
dimethyldichlorosilane has higher water contact angle (101°) and
has a contact angle of 65° after deposition of 3c monolayer (Fig. 6),
which indicates that catalyst is deposited on the substrate, with
which 95% yield can be achieved in catalyzing Suzuki reaction
(50°C, 24 h, water, K2CO3, n-Bu4NBr). This is evidence that the orien-
tation of the deposited cyclopalladated ferrocenylimine can be
influenced by different surfaces. The changes of water contact
angles on differently modified solid surfaces also indicate that the
configuration of catalyst monolayer had significantly changed,
which could clearly have an influence on the catalytic properties.
Catalytic activity of organized 3a–c monolayer and LB films in
heterogeneous systems
The catalytic performance of LB films of cyclopalladated arylimine
complexes 3a–c for Suzuki coupling reaction was systematically in-
vestigated. Suzuki reaction was taken as an example to investigate
the catalytic activity of complex 3a–c Langmuir monolayers under
mild reaction conditions (compared with the homogeneous case,
such as lower reaction temperature and shorter reaction time).
The results are summarized in Table 1. The yield of the reaction of
4-bromotoluene with phenylboronic acid is 92% in pure water
when catalyzed by 3c Langmuir monolayer prepared at 20 mN
Figure 2. UV–visible spectra of complex 3a LB films with different numbers
of layers deposited at various surface pressures: (A) 10 mN mꢀ1; (B) 12 mN
m
ꢀ1; (C) 15 mN mꢀ1; (D) 18 mN mꢀ1). Insets: number of layers deposited
versus absorption at certain wavelength.
m
ꢀ1 (Table 1, entry 5). The yield increases to 99% when the reaction
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Highly ordered amphiphilic cyclopalladated self-assembly films
Figure 4. Cyclic voltammograms of pure ITO glass (dotted line) and one-
layer LB films of complex 3c transferred onto ITO glass (solid line).
Figure 5. XPS spectrum of 3c LB film.
hydropillic
hydropillic
Figure 6. Water contact angle of 3c monolayer transferred onto glass
(surface pressure = 20 mN mꢀ1).
time is prolonged to 30 h (Table 1, entry 6) and changes when the
Langmuir monolayer is obtained at different surface pressures
(Table 1, entries 7–16). The results confirm that the catalyst
3a–c Langmuir monolayers can effectively catalyze Suzuki re-
action when the monolayers are obtained at 15 (3a), 35 (3b)
and 20 mN mꢀ1 (3c), respectively. For example, the π–A iso-
therm of 3c shows an increase in surface pressure near a
mean molecular area of 67 Å2 per molecule. This indicates that the
Figure 3. AFM images acquired from glass slides after (A) hydrophilic
treatment, and monolayer of complex 3c deposited at (B) 10, (C) 15,
(D) 20, (E) 25 and (F) 30 mN mꢀ1
.
Appl. Organometal. Chem. 2016, 30, 540–549
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Y. Zhao et al.
Table 1. Data for the Suzuki coupling reaction catalyzed by 3c
monolayera
Entry Solvent (H2O/ Temperature Catalyst/surface pressure Yield
C2H5OH)
(°C)
(mN mꢀ1
)
(%)b
1
1:1
2:1
50
50
60
60
50
50
50
50
50
50
50
50
50
50
50
50
3c/20
3c/20
3c/20
3c/20
3c/20
3c/20
3a/10
3a/12
3a/15
3a/18
3b/25
3b/30
3b/35
3c/10
3c/15
3c/25
23
13
2
3
1:1
86
4
2:1
30
5
6c
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
92
>99
12
7
8
88
9
90
10
11
12
13
14
15
16
5.4
53
79
93
3.0
55
4.8
aReaction conditions: PhB(OH)2 (0.3 mmol), 4-bromotoluene (0.25 mmol),
catalyst 3a–c Langmuir monolayer (2.5 cm × 1 cm), K2CO3 (0.3 mmol),
solvent (2.5 ml), 24 h, n-Bu4NBr (0.3 mmol), no stirring.
bYield determined by HPLC based on products.
c30 h.
Table 2. Amount of Pd in LB films after various recycling runsa
Entry
Solvent Stir Pd (×10ꢀ6 mmol) TON Yield (%)b
After first reaction
After first reaction
After first reaction
DMF No
H2O Yes
H2O No
No
No
Trace
28
6.29
5.03
4.77
No
3676
3308
2573
99
After second reaction H2O No
After third reaction H2O No
After fourth reaction H2O No
95
87
9.0
aReaction conditions: PhB(OH)2 (0.03 mmol), 4-bromotoluene
(0.025 mmol), K2CO3 (0.03 mmol), n-Bu4NBr (0.03 mmol), 50°C
for 24 h under air.
bYield determined by GC, based on product.
calculated catalyst loading of its LB film is as low as 2.5 × 10ꢀ10 mol
cmꢀ2 (measured as 2.0 × 10ꢀ10 mol cmꢀ2) from which its catalytic ef-
ficiency is 100 times higher than that in the homogeneous case, with
a turnover number (TON) as high as 79 200 and turnover frequency
(TOF) as high as 2640 hꢀ1. The catalytic results clearly demonstrate
that ordered structures have a major influence on catalytic perfor-
mance (Table 1). The observed increase in activity can be attributed
to a combination of enhanced catalyst lifetime and the altered con-
formation that the molecule adopts upon adsorption. However,
effects due to the local concentration of sites cannot be ruled out.
Figure 7. AFM images of monolayer of complex 3c deposited on glass
slides after (A) the first, (B) the second, (C) the third and (D) the fourth
catalytic run.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 540–549

Highly ordered amphiphilic cyclopalladated self-assembly films
Figure 8. Schematic of the arrangement of LB films transferred onto solid
slides (A) before catalytic reactions and after (B) the first and (C) the
second catalytic run.
Figure 9. Cyclic voltammograms before (solid line) and after (dashed line)
catalytic reactions of one-layer LB films transferred onto ITO glass.
Concerning the use of the catalyst for both industrial and
pharmaceutical applications, its recyclability is important. Taking
catalyst 3c LB films as an example, these LB films deposited onto
glass slides show great advantage in that Suzuki reaction can be
conveniently carried out without ligand and the catalyst is easily
separated. However, it is known that a disadvantage of LB films is
easy desorption from the solid slides when stirred or in organic so-
lution which go against recycling and elucidation of the mechanism
in heterogeneous systems. The influence of stirring and solvent was
investigated (Table 2). High yield can be obtained without stirring
and using water as solvent.
Figure 10. (A) XPS spectra of 3c monolayer on silicon wafers before
catalytic reaction (black curve) and after catalytic reaction (red curve). (B)
Pd 3d, (C) Br 3d and (D) Cl 2p XPS spectra before (red curve) and after
(black curve) catalytic reaction.
can also be used to improve the recycling ability compared to our
previous research.[86]
We examined the reuse of the catalyst LB films in Suzuki coupling
reaction between 4-bromotoluene and phenylboronic acid under
various conditions. Recycling of one-layer LB film catalyst of 3c
shows that three runs can be performed without any significant loss
of catalytic activity (99, 95 and 87%, respectively). However, there is
almost no coupling product in the fourth and fifth runs (9 and 11%),
which is probably because the LB film is no longer ordered as a
result of heating or is coated by reagents or desorbed from the solid
slides.
AFM images and cyclic voltammetry curves of LB films after
catalytic runs were used to investigate the topographic and electro-
chemistry properties. Taking catalyst 3c as an example, the surfaces
were investigated using AFM before and after catalytic reactions.
The AFM images of one-layer LB films of complex 3c deposited
onto glass slides after various reaction runs are shown in Fig. 7. All
scan ranges are 2.5μm × 2.5μm. The images indicate the height
of the clusters decreases from 10–11nm (after the first two runs)
to 1.6nm (after the third run) and to 0.9nm (after the fourth run),
probably because the alkyl chains bind to each other or aggregate
instead of forming an ordered arrangement (Fig. 8) resulting in a
quick decrease in catalytic activity (in the fourth run).
It should also be noted that new phenomena might arise from
the ability to organize molecular catalysts into controlled assem-
blies to enhance catalytic activity. Changes in molecular design
Appl. Organometal. Chem. 2016, 30, 540–549
Copyright © 2016 John Wiley & Sons, Ltd.
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Y. Zhao et al.
key step. The intermediates react with another substrate in a syner-
gic way to form coupling product and leaching Pd0 at the same
time. Finally, Pd0 re-deposits onto the 3c films and leaches again.
This is not real surface catalysis, but a leaching and re-deposition
mechanism.[68] The plausible mechanism is depicted in Scheme 2.
Conclusions
A series of cyclopalladated arylmine compounds 3a–c were synthe-
sized and characterized using infrared, UV–visible, 1H NMR and
mass spectroscopies. Their catalytic activity for Heck and Suzuki
cross-coupling reactions was investigated in both homogeneous
and heterogeneous systems.
For reactions of activated and inactivated aryl bromides with
arylboronic acids, excellent yields were obtained in pure water,
which showed that the catalytic efficiency was high in the homoge-
neous case. Especially, all the cross-coupling reactions need no
ligands, air isolation and assistant solvents, which mean the
catalysts are suitable for future research for their assembled films
in heterogeneous catalysis.
Scheme 2. Proposed catalytic mechanism.
Organized monolayer films of 3a–c were prepared using LB tech-
niques and used as C–C coupling catalysts. The results confirmed
that the yields were changed when the Langmuir monolayers were
obtained at different surface pressures. The immobilized 3c mono-
layer showed an enhanced activity relative to homogeneous reac-
tion, for which the results show that the ordered monolayers are
efficient with a catalyst loading as low as 10ꢀ5 mol%, and TON as
high as 79 200 and TOF as high as 2640 hꢀ1. The catalytic efficiency
is 100 times higher than that in the homogeneous case using the
same reagent amount and ratio. The increased activity of
immobilized 3c monolayer is a result of a combination of the
cyclopalladated arylimine structure and a change in conformation
when deposited onto the substrate. A heterogeneous catalysis
mechanism for Suzuki coupling reaction was also proposed. In
addition, to provide an insight into the specific example of
immobilized cyclopalladated complexes, the results demonstrate
that the utility of well-characterized molecular monolayers can be
used as models for supported molecular catalyst systems.
The cyclic voltammetry curve of LB films of complex 3c depos-
ited on ITO glass has obvious differences after the catalytic reac-
tions (Fig. 9). Reduction peak is inconspicuous and oxidation peak
decreases from +0.3 to ꢀ0.05 V. This indicates that the catalysts
after catalytic reaction are harder to reduce and easier to oxidize,
probably because the Pd catalysts change from oxidized state
(Pd2+) to reduced state (Pd0) and the LB films are no longer ordered
or are covered by reagents which are obstacles for the current.
XPS spectra of 3c monolayer on silicon were also measured
before and after catalysis to give an insight into the mechanism.
The results are shown in Fig. 10. For the monolayer before catalysis,
the peaks of elements Pd, N and C in the XPS spectrum (Fig. 10(A))
can be seen at 337.9, 399.8 and 284.8 eV, which show that the com-
plex 3c is present in the LB films. The ratio of I(Pd)/I(C 1 s) is very
small, indicating that Pd catalyst is covered with native carbon ele-
ment. However, peaks in the Br 3d spectrum (68.4, 188.3, 181.9,
168.6 eV; Fig. 10(C)) and Na 1 s spectrum (1071.7 eV) appear when
the reaction is over, indicating the substrates and the base (Na2CO3)
enter into the reaction. We suppose that the reagent coordinates
with Pd to generate intermediate in LB films in the whole stage.
Thus, we speculate that the reaction intermediates and products
coexist in the LB films in this process. Shifts of Pd 3d5/2 (from
337.9 to 338.6 eV) and Pd 3d3/2 (from 343.2 to 344.1 eV) assigned
to Pd2+ are observed in Fig. 10(B). The shoulder peak at
336.82 eV (Pd03/2) is assigned to Pd(0).[93] The shifts of the binding
energy indicate that the Pd catalyst changes from oxidized state
(Pd2+) to reduced state (Pd0), but there is a certain amount of Pd2
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (no. 20973157), Natural Science Foundation of Henan
(142300410223) and Fund of Innovation Training of Zhengzhou
University (2015xjm180) for financial support of this work, and
to Prof. Minghua Liu and Penglei Chen for XPS measurements
and Prof. Luyuan Mao for AFM measurements.
+
on the surface. There is an inconspicuous binding energy peak
of Cl 2p detected before the catalytic reaction (Fig. 10(D)), suggest-
ing that the Pd catalyst changes from oxidized state (Pd2+) to re-
duced state (Pd0). The Cl 2p peak appears at 197.1 eV after
catalysis (Fig. 10(D)) indicating that P¼Cl covalent bonds are
present.
Supporting experiments were designed. The substrate with 3c LB
monolayer was taken from the reaction system which was filtered
and new substrate was added. No coupling products are detected,
indicating that the catalytic mechanism is of heterogeneous nature.
According to the results discussed above, a reasonable heteroge-
neous catalytic mechanism is proposed. First, an amount of Pd2+ on
the surface of LB films generates Pd0 precursor, and then bromo
substrates combine with Pd0 to form intermediates which is the
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