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Z. Fu et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 293–299
CuCl (0.5 mmol), and 15 mL of the mixture (methanol/water: 4:1)
proceed for 4.5 h at room temperature under nitrogen atmosphere.
At last, the PHAM-grafted substrates were immersed in 40 mL of
dried THF with N,Nꢀ-carbonyldiimidazole (12 mmol) and a solu-
tion of cyclopalladated ferrocenylimines (0.001 g) in THF 48 h to
get Pd/PBs.
2.2. General procedure for the Suzuki reaction of aryl halide with
phenylboronic acid
Aryl halide (0.125 mmol), phenylboronic acid (0.15 mmol),
K2CO3 (0.15 mmol) and n-Bu4NBr (0.15 mmol) were combined
with 3 mL water in a small round-bottomed flask used for “homoge-
neous” and “heterogeneous” runs. The reaction mixture was stirred
at 80 ◦C for 12 h, then, the mixture was acidified, and the product
was extracted by ethyl ether for three times. The combined organic
phase was dried with MgSO4, filtered, and solvent was removed
on a rotary evaporator. Crude product was transferred into a 10 mL
volumetric flask to fix its quantity in ethyl acetate. The yields were
determined by high performance liquid chromatography (HPLC),
based on the peak area ratio between the products. The HPLC con-
ditions were a Zorbax eclipse Xdb C18 column (150 × 4.6 mm 5 m)
with methanol/acetonitrile as the mobile phase, and a flow rate of
1 mL/min.
Scheme 1. The structure of cyclopalladated ferrocenylimines functionalized poly-
mer brushes film (Pd/PBs).
the catalytic activity. The Suzuki reaction had been the most stud-
ied in neat water under heterogeneous palladium catalysis [11].
Meanwhile, versatile transformations using Pd catalysts usually
had been developed [12]. Our team had done a lot works in past
decades about the synthesis and catalysis of cyclopalladated fer-
coupling reactions in organic solvent or aqueous medium [13,14].
Recently, we prepared cyclopalladated arylimine functionalized
polymer brushes and preliminarily discussed the catalytic activity
and recyclability in heterogeneous Suzuki reaction [15]. However,
the functionalized polymer brushes catalysts showed decreasing
activities (lengthened reaction times) with high yield in recycle
studies because of the intertwining and aggregation of long-chain
octadecyl in cyclopalladated arylimine, which hindered the inter-
actions of reactants and the active center of Pd on polymer brushes
film. Therefore, rational design of suitable catalysts should be
closely linked to deeper insights into mechanisms involved at a
nanomolecular level.
2.3. Atomic absorption spectroscopy (AAS) analysis
The amounts of cyclopalladated ferrocenylimines immobilized
on solid substrates were measured by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) (ICAP 6000 Series, Thermo
Scientific). The sample treatments were as follows: Pd/PBs (quartz
plate or round-bottomed flask) was boiled in concentrated nitric
acid, and then, residual solid was dissolved with 2 M hydrochloric
acid and transferred into a 10 mL volumetric flask to fix its quantity
in water.
As described above, catalytic polymer brushes film on solid sub-
strates is suitable to study heterogeneous catalysis because they
can characterize, identify, and estimate the nature of the surface
species by various experimental techniques. We thus reasoned that
by using a catalyst with unified, controlled, and tunable charac-
oxidative addition more readily and more reliably to shed light on
the Pd surface reaction mechanism. Herein, we attempted to graft
cyclopalladated ferrocenylimines catalyst onto polymer brushes
solid surfaces by covalent bond (Scheme 1). Then we evaluated
the activity and recyclability of the resulting composites in Suzuki
reaction and further explored the reasonable and feasible reaction
mechanism of heterogeneous reaction.
3. Results and discussion
3.1. Preparation of cyclopalladated ferrocenylimine
functionalized polymer brushes film
The cyclopalladated ferrocenylimines functionalized polymer
brushes film (Pd/PBs) grafted on silicon, glass and quartz surfaces
had been synthesized as previous work [15]. The amounts of Pd
in Pd/PBs were measured by inductively coupled plasma atomic
emission spectroscopy (ICP-AES). The Pd content of the catalytic
film on the solid slide was found to be 1.2 × 10−6 mmol cm−2
.
The composition and morphology of the surfaces obtained along
with the immobilization process at each step (showed in Sup-
porting Information) were characterized by water contact angle
(WCA), ultraviolet–visible spectroscopy (UV–vis), X-ray photoelec-
tron spectroscopy (XPS), atomic force microscope (AFM), low-angle
X-ray diffraction (LAXD) and cyclic voltammetry (CV). The ordered,
oriented structure and unobvious layered structure in Pd/PBs had
been demonstrated by the characterization methods. These results
indicated that mostly catalysts were grafted onto the surface of
polymer brushes on the solid substrates, which prompted us to
explore the surface chemical reactions in heterogeneous catalysis
system.
2. Experimental
The cyclopalladated ferrocenylimines functionalized polymer
brushes films were prepared according to the published proce-
dures [15]. The details about the major steps of the processing
are as follows: hydrophilic treatment wafers by pirhana solu-
tion were functionalized with 3-aminopropyl triethoxysilane
(2.1 mmol). Then, amino-functionalized wafers were immersed
in a solution of 2-bromoisobutyryl bromide (3.3 mmol) in 25 mL
of dry dichloromethane containing TEA (7.0 mmol) to react
12 h and obtained initiator-functionalized wafers. Next, initiator
functionalized wafer was placed into the flask which included N-
hydroxymethyl acrylamide (50 mmol), 2,2ꢀ-bipyridine (1.0 mmol),
3.2. Heterogeneous catalytic properties of Pd/PBs
As mentioned above, Suzuki reaction usually served as a model
for characterization of new catalysts. Heterogeneous catalysts
based on different organic and inorganic supports have been
reported for this versatile and powerful method of C C bond