Langmuir-Blodgett films of cyclopalladated ferrocenylimine: preparation, characterization, and application in Suzuki coupling reaction

Article abstract of DOI:10.1016/j.tet.2008.12.075

Langmuir monolayer and Langmuir-Blodgett (LB) films of cyclopalladated ferrocenylimine 1 were prepared and characterized. The surface pressure (π)-area (A) isotherm of complex 1 indicated the formation of highly condensed monolayer on the subphase. Ultraviolet-visible (UV-vis) and Fourier transform infrared (FTIR) spectroscopy showed that complex 1 monolayer was transferred successfully onto solid supports. Atomic force microscopy (AFM) image suggested that LB films transferred on the solid substrate were well-ordered, homogeneous. Cyclic voltammograms of LB films deposited on glassy carbon electrodes showed quasi-reversible oxidation/reduction waves of ferrocene moiety. From the average thickness of monolayer, the hydrocarbon chain could be fairly directed perpendicular to the substrates. Finally, LB films of complex 1 presented a largely improved catalytic efficiency for Suzuki reaction with respect to its cast films and homogeneous reactions under the same conditions. The results might have an implication on the catalytic mechanism of this reaction.

Full text of DOI:10.1016/j.tet.2008.12.075

Tetrahedron 65 (2009) 2599–2604
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Langmuir–Blodgett films of cyclopalladated ferrocenylimine: preparation,
characterization, and application in Suzuki coupling reaction
,
,
b,
c
,
c
a
,
,b,
Bing Mu a b, Tiesheng Li a
, Chenghuan Li , Pingping Liu b, Wei Shang c, Yangjie Wu a
*
*
a Department of Chemistry, Key Lab of Chemical Biology and Organic Chemistry of Henan Province, Zhengzhou University, Zhengzhou 450052, PR China
b Key Lab of Advanced Nano-information Materials of Zhengzhou, Zhengzhou University, Zhengzhou 450052, PR China
c Institute of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Langmuir monolayer and Langmuir–Blodgett (LB) films of cyclopalladated ferrocenylimine 1 were pre-
pared and characterized. The surface pressure ( )–area (A) isotherm of complex 1 indicated the for-
Received 24 September 2008
Received in revised form 15 December 2008
Accepted 16 December 2008
Available online 3 January 2009
p
mation of highly condensed monolayer on the subphase. Ultraviolet–visible (UV–vis) and Fourier
transform infrared (FTIR) spectroscopy showed that complex 1 monolayer was transferred successfully
onto solid supports. Atomic force microscopy (AFM) image suggested that LB films transferred on the
solid substrate were well-ordered, homogeneous. Cyclic voltammograms of LB films deposited on glassy
carbon electrodes showed quasi-reversible oxidation/reduction waves of ferrocene moiety. From the
average thickness of monolayer, the hydrocarbon chain could be fairly directed perpendicular to the
substrates. Finally, LB films of complex 1 presented a largely improved catalytic efficiency for Suzuki
reaction with respect to its cast films and homogeneous reactions under the same conditions. The results
might have an implication on the catalytic mechanism of this reaction.
Keywords:
Cyclopalladated ferrocenylimine
Suzuki–Miyaura reaction
Langmuir monolayer
Langmuir–Blodgett (LB) films
Ó 2008 Published by Elsevier Ltd.
1. Introduction
are also a variety of experimental techniques that can be employed
to characterize, identify, and estimate the nature of the surface
Palladium-catalyzed cross-coupling reactions such as Heck–
Mizoroki arylation, Suzuki–Miyaura reactions, Hartwig–Buchwald
amination, and Stille coupling have become an extremely powerful
method in organic synthesis for the formation of carbon–carbon or
carbon–heteroatom bonds.1 Among them, the Suzuki–Miyaura
cross-coupling reaction has been a common transformation for the
formation of carbon–carbon bonds because their products,2 in
particular biaryls, are recurring functional groups in natural prod-
ucts, pharmaceuticals, agrochemicals, ligands for asymmetric syn-
thesis and in new materials, such as liquid crystals.3 To reach a high
degree of efficiency in Suzuki–Miyaura reaction, previous studies
mainly focused on the development of new ligands with the steric
and electronic properties, notably phosphorus or N-heterocycle
carbene ligands.4
species.7 Therefore, flat surfaces obtained by the LB method offer
opportunities to elucidate the molecular structure of the anchored
species. Applications of ultrathin films are wide-ranging, and ca-
talysis is in no exception. Until now, there have been few studies of
LB films containing amphiphilic organometallic catalysts, which
were anchored at the solid surface for catalytic applications. To¨llner
et al. reported LB films of an amphiphilic rhodium bipyridine
complex for the hydrogenation of acetone presented largely im-
proved reaction activity and substrate selectivity with respected to
the same complex in solution, demonstrating a major influence of
ordered structure on a catalytic reaction.8 Abatti et al. reported that
porphyrin LB films were used in the heterogeneous catalysis of
cyclohexene oxidation, furnishing a yield two times higher than
that obtained for the homogeneous system, due to a suitable ori-
Langmuir–Blodgett (LB) films are built up from a condensed
monolayer formed on air–water interfaces by multiple depositions
onto solid supports, and the LB method is one of the best ways to
prepare thin films with a controlled thickness at a molecular size,
well-defined molecular orientation, highly ordered molecular ar-
ray, known surface coverage, and reproducible interface.5,6 There
entation and homogeneous distribution of the catalyst molecules in
9
´
the LB films. Benıtez et al. reported that organized monolayer film
of manganese(III) porphyrins prepared by a combination of Lang-
muir–Blodgett (LB) and self-assembled monolayer techniques
showed enhanced activity relative to homogeneous reaction to-
ward the epoxidation of cyclooctene, suggesting immobilization of
the catalyst and not only its modified structure, have an important
influence on its reaction activity.10 To the best of our knowledge, no
report is found on catalytic applications in coupling reactions using
organometallic LB films as catalysts.
* Corresponding authors. Tel./fax: þ86 371 67766667.
E-mail addresses: lts34@zzu.edu.cn (T. Li), wyj@zzu.edu.cn (Y. Wu).
0040-4020/$ – see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tet.2008.12.075

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B. Mu et al. / Tetrahedron 65 (2009) 2599–2604
During the past decades we have extensively studied the
40
30
20
10
cyclometalation including cyclopalladation of ferrocenylimine li-
gands, and cyclopalladated ferrocenylimines obtained have been
applied to the coupling reactions.11 We recently reported the
preparation of amphiphilic cyclopalladated ferrocenylimine
1
(Scheme 1) and its catalytic applications in homogeneous Heck and
Suzuki cross-coupling reactions at elevated reaction temperature or
room temperature.12,13 One of our interesting attempts was in-
troduction of hydrophobic chains onto cyclopalladated complex to
yield amphiphilic catalyst that was expected to form a stable
monolayer at the air–water interface. Whether the complex forms
LB films successfully is an interesting question. There is no report
on LB films prepared from organopalladated catalysts, because they
are usually unstable in water. Furthermore, it is interesting to us to
study whether LB films of amphiphilic cyclopalladated ferroceny-
limine as a catalyst can improve the efficiency of the coupling
reactions.
60
80 100
Area per Molecule (Å2)
120
Figure 1.
p
–A isotherm of complex 1 monolayer at 26ꢂ1 ꢃC.
R1
C
(Es)–surface pressure (
p
) isotherm of complex 1 monolayer, is
NR2
shown in Figure 2. The surface pressure obtained at the maximum
value of the static elasticity 22 mN/m, was considered as optimum
surface pressure of LB films deposition from water surface.
R1 = CH3
R2 = n-C12H25
Pd Cl
Fe
2
1
Scheme 1. The structure of cyclopalladated ferrocenylimine 1.
2.2. UV–visible spectroscopy
Herein, we first studied the behavior of monolayer of cyclo-
palladated ferrocenylimine 1 at the air–water interface, and then
structurally characterized its LB films on solid substrates by FTIR,
UV–vis, and the thickness studies on the molecular orientation,
aggregation, and structure at the level of functional groups in LB
films. The surface morphology of LB films was directly observed by
AFM. Furthermore, the deposition of LB films and the correspond-
ing spin-coated films on ITO substrates allowed a study of their
redox behavior by cyclic voltammetry. Finally, the catalytic activity
of complex 1 LB films as a catalyst for Suzuki reactions has been
examined, and compared to that of the corresponding cast films
and homogeneous catalyst under the same conditions.
The UV–vis spectra of complex 1 in CHCl3 solution and in LB
films are shown in Figure 3. Usually there are two peaks, w240 and
w440 nm, in the UV–vis spectra of ferrocene derivatives.15
A
strong, sharp absorption peak was observable with a maximum at
ca. 240 nm, which was attributable to the electron transition
p–
p
*
of the ferrocene chromophore of complex 1. However, the ab-
sorption peak in LB films was dramatically shifted to shorter
wavelengths compared with that in the solution, suggesting that
some interaction among molecules was due to the increase of the
p–
p energy transfer of cyclopentadiene in ferrocene moieties and
*
the close molecular packing such as H-aggregates, viz. the cyclo-
pentadienyl rings of cyclopalladated ferrocenylimine were parallel
to each other and tilted in LB films on quartz slides.16 When the
monolayer was transferred onto quartz substrate to form LB films,
2. Results and discussion
the shoulder band at ca. 280 nm attributed to the n–
*
p electron
transition of the ferrocenyl Schiff base chromophore in solution
showed a slight red shift, indicating the close packing of the mol-
ecules. The linear relationship between the number of layers and
the maximal absorbance in the UV–vis spectra indicated that the
monolayer was successfully deposited onto solid substrates.
2.1.
p–A Isotherm for cyclopalladated ferrocenylimine 1
Complex 1 having dodecyl chain was spread from a chloroform
solution (200
measure the
m
L, 5ꢀ10ꢁ4 M) on the ultrapure water surface to
p
–A isotherm at a compression speed of 5 mm/min at
26ꢂ1 ꢃC. Figure 1 shows the isotherms prepared by compression
after evaporation of solvent for 30 min and stabilization for ca. 1 h
in the uncompressed state. The p–A isotherm for complex 1 gave
200
150
100
50
a steep rise in surface pressure and exhibited high collapse pressure
36 mN/m, without phase transition, meaning that highly con-
densed monolayer was formed. The average surface area occupied
per molecule in the monolayer on the water surface of complex 1
was 96 Å2 by extrapolating the steepest tangent of the isotherm to
zero surface pressure.
In order to obtain directly optimum surface pressure for LB films
deposition of complex 1 monolayer, the static elasticity of the
monolayer film was calculated from
p–A isotherm (Fig. 1) by Eq. 1:
d
p
Es ¼ ꢁA
(1)
dA
The monolayer static elasticity is a measure of the monolayer
resistance to a change in area, thus a high static elasticity value is
associated with a monolayer that has a strong cohesive structure at
the surface and was the most stable, rigid.14,15 The static elasticity
40
10
20
Surface pressure (mN/m)
30
Figure 2. Es–p isotherm of complex 1.

B. Mu et al. / Tetrahedron 65 (2009) 2599–2604
2601
25 layers
5
21
17
13
9
0.6
0.6
0.4
0.2
= 192 nm
3
1
0.4
0.2
5
0
5
10
15
20
25
Number of Layers
-1
-3
-5
190
240
290
340
Wavelength (nm)
390
440
490
1
0.8
0.6 0.4
Potential (V)
0.2
0
Figure 3. UV–vis absorption spectra of complex 1 in chloroform solution (dotted line)
and in LB films with different layers (solid line), and the dependence of UV–vis spectra
for LB films on the number of layers deposited.
Figure 4. Cyclic voltammograms of complex 1 monolayer LB films (solid line) and cast
films (dotted line) deposited on glassy carbon substrates.
2.3. Infrared spectroscopy
monolayer film (
spin-coated films (
D
D
E¼73 mv) was smaller than that obtained for
E¼92 mv), indicating that the rate of electron
IR spectroscopy has proved to be a powerful tool to investigate
changes in molecular aggregation, orientation, and structure at the
level of functional groups in LB films.17–19 It is well-known that the
wavenumbers of the CH2 symmetric and antisymmetric modes can
be used to monitor the degree of conformational order of the alkyl
chain.20,21 When the hydrocarbon chain is highly ordered (trans-
zigzag conformation), the bands due to CH2 symmetric and anti-
symmetric modes appear near 2848 and 2918 cmꢁ1, respectively,
while, if conformational disorder is induced in the alkyl chain, these
bands shift to 2856 and 2927 cmꢁ1, respectively, depending upon
the extent of disordering.22
transfer from the LB films to the electrode and counterion move-
ment in LB monolayer film was fast on the time scale of the ex-
periment.25–28 This may be correlated with the partial hindrance of
the diffusion of the charged species through the spin-coated films
due to the formation of aggregates onto glassy carbon electrode, in
contrast to the LB monolayer film.29 Another effect observed was
that the half-wave potential E0 was more cathodic in the monolayer
film (E0¼460 mv) than that in spin-coated films (E0¼510 mv), in-
dicating the order arrangement in the LB films facilitated the oxi-
dation of iron(II).
In the present case, the vibrational position of the CH2 anti-
symmetric and symmetric in 40-layer LB films transferred onto
CaF2 substrate and KBr pellet are similar to each other, and they
appeared at ca. 2922 and 2852 cmꢁ1, respectively, suggesting the
existence of considerable gauche conformations in the hydrocarbon
chain. As for cyclopalladated complex 1 in the LB films, only one
long hydrocarbon chain resided near the ferrocene moiety so that
ample available space could enable it to rotate and tilt, producing
some gauche conformers in the chain. Therefore, the alkyl chains
could not pack densely to each other. The phenomenon is very
similar to that Shen et al. previously reported N,N-dimethylferro-
cenylmethylhexadecylammonium bromide LB films.15
2.5. Atomic force microscopy
AFM can be used to provide topographic images of the LB films,
giving a direct view of surface morphology with high spatial res-
olution, which is often indicative of the film quality.30 Figure 5
showed AFM image of one-layer LB films of complex 1 transferred
A characteristic C]N stretching band in the region of 1580,
1589 cmꢁ1 was observed for the KBr pellet and LB films, re-
spectively, suggesting that the complex maintained its structure in
LB films. However, the band shifted to the longer wavenumber in LB
films as compared to the KBr pellet. It could be tentatively assumed
that dimeric palladacycle underwent dissociation to monomeric
form in highly dilute solutions in coordinating solvents already at
room temperature.23,24
2.4. Electrochemical characteristics
Electrochemical features of complex 1 in LB monolayer film
were compared with those in spin-coated films. Figure 4 showed
cyclic voltammograms in LB monolayer film and spin-coated films
deposited onto glassy carbon electrodes in 0.1 M KClO4 at pH 1.5 at
scan rates of 100 mV sꢁ1. The voltammetric behavior of the LB
monolayer film was very similar to that in spin-coated films. All
voltammograms showed one quasi-reversible redox process, which
was ascribed to the Fc/Fcþ redox process in thin films. The sepa-
ration between cathodic and anodic peak potential for the LB
Figure 5. AFM image of one-layer LB films of complex 1 deposited onto freshly cleaved
mica at surface pressure of 22 mN/m.

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B. Mu et al. / Tetrahedron 65 (2009) 2599–2604
heating conditions, even with stirring, LB films could be desorbed
from substrates, and the ordered structure of LB films was influ-
enced.8,9 To assess the catalytic properties of LB films of cyclo-
palladated catalyst in Suzuki coupling reaction, we chose the
reaction of 4-iodobenzoic acid with phenylboronic acid as a model
reaction in aqueous medium at room temperature.
Initially, we investigated the effect of aqueous solvents, in-
cluding methanol, ethanol, propanol, isopropanol, butanol, t-bu-
tanol, pentanol, DMF, DMA, DME, acetonitrile, acetone, dioxane and
pure H2O, and bases (e.g., K3PO4, K2CO3, Na2CO3, KF$2H2O, KOH,
NaOtBu, KOtBu) on the reaction of 4-iodobenzoic acid with phe-
nylboronic acid in bulk solution with stirring, at room temperature
(0.5 mmol 4-iodobenzoic acid, 0.6 mmol phenylboronic acid and
base, with 0.1 mol % 1, yield determined by HPLC). As a result,
NaOtBu as the base and CH3CH2OH/H2O (1:1) as solvent gave the
best result for the reaction.
Fe
N
N
Cl
Fe
Pd
Pd
Cl
Figure 6. The model of cyclopalladated dimer 1 in LB films.
Under optimal reaction conditions, the LB films of complex 1
transferred onto hydrophilic glass substrates in the reaction 4-
iodobenzoic acid with phenylboronic acid exhibited high catalytic
activity, yielding 4-biphenylcarboxylic acid as the only observed
product [as analyzed by HPLC, yield based on 4-iodobenzoic acid
and the product] (Table 1, entries 1–5). With the increase of number
of LB films layers, the yields significantly increased, but the turn-
over numbers changed only slightly. However, To¨llner et al. have
reported that the catalytic activity of the rhodium complex LB films
as catalyst for the hydrogenation of carbon–oxygen double bonds
was dependent on the ordered structure and layer orientation, not
number of layers.8 Here, when the upper layer was oriented as
head-to-glass, reactions could still carry out, indicating that the
alkyl chains of the upper layer did not block the catalytically active
centers, and probably their packing was relatively loose to each
other in the LB films, in agreement with IR spectroscopy. Moreover,
the catalytic films were removed into the same, new reaction after
one batch of the reaction, and exhibited very low reaction activity,
suggesting desorption of LB films and not truly heterogeneous re-
action. LB films of complex 1 showed a significant improvement in
catalyst turnovers relative to its cast films and the homogeneous
reactions under the same conditions, as shown in Table 1. For the
cast films and homogeneous system, only by increasing the loading
of catalyst to 1.2ꢀ10ꢁ5 mmol and 8ꢀ10ꢁ6 mmol, respectively, could
onto freshly cleaved mica at 22 mN/m surface pressure. The image
obtained was smooth and homogeneous, indicating that the well-
ordered, uniform monolayer films were fabricated.
2.6. Film thickness
The thickness of the LB films was measured using the Vnkd-
8000 optical thickness measurer. The length of the straight dodecyl
chain including the C–N and Pd–N bonds is calculated to be 1.9 nm,
on the basis of the single crystal data reported by us previously,
assuming that the length was the theoretical height of the cyclo-
palladated complex molecule.12 Measured average thickness of
monolayer in the nine-layer LB films of complex 1 was ca. 2 nm. The
result indicated that the alkyl chain was oriented perpendicular to
the substrate. Based on the analysis above, we presumed the con-
formation of cyclopalladated complex 1 in LB films, indicated in
Figure 6.
2.7. Suzuki–Miyaura cross-coupling reactions
LB films deposited onto solid supports are held together by
noncovalent molecule–molecule interactions called the van der
Waals forces of attraction. Therefore, in organic solvent or under
Table 1
The cross-coupling reaction of 4-iodobenzoic acid with arylboronic acid catalyzed by catalyst 1 in LB films, cast films, and in solutiona
NaOtBu, in 30 °C air bath
CH3CH2OH/H2O=1/1, Cat. 1
+
I
B(OH)2
COOH R=H, CH3
COOH
R
R
Entry
Catalytic systems
LB films
Catalyst 1 (10ꢁ6 mmol)
Yieldsb (%)
TON
1
2
3
4
5
6
2.16 (1 layer)
4.32 (2 layers)
6.48 (3 layers)
8.64 (4 layers)
10.8 (5 layers)
8
15
23
52
2142
1605
2424
2733
2478
d
79
89 (73)c
Cast films
Trace
7
8
9
10
12
5
19
54
Trace
572
1341
d
Homogeneous
10
11
12
13
14d
15d
16d
6
7
8
9
6
13
46
50
314
543
1727
1658
2569
d
LB films
Cast films
Homogeneous
4.32 (2 layers)
6
4
37
No reaction
No reaction
d
a
Reaction conditions: 4-iodobenzoic acid 0.03 mmol, phenylboronic acid 0.06 mmol, base 0.12 mmol, solvent 3 mL (CH3CH2OH/H2O¼1:1), in 30 ꢃC air bath, 48 h without
stirring, in weighing bottle (40ꢀ25 mm). Cast films, LB films transferred glass slides (25 mmꢀ25 mmꢀ1 mm), homogeneous reaction with blank glass slides.
b
Yield determined by HPLC, based on 4-iodobenzoic acid and the product in two runs.
Isolated yield (the average value of three times).
4-Tolyboronic acid.
c
d

B. Mu et al. / Tetrahedron 65 (2009) 2599–2604
2603
high turnover number be obtained (Table 1, entries 8 and 12). In
addition, the coupling reaction of 4-iodobenzoic acid with 4-toly-
boronic acid, catalyzed by two-layer LB films gave the coupling
product with 37% yield and 2569 turnover number; however, the
coupling reaction catalyzed by cast films or homogeneous catalyst,
cannot proceed under the same reaction conditions (Table 1, entries
14–16). The catalytic activity could be highly dependent on the
ordered arrangement and homogeneous distribution of the catalyst
within LB films. All these results above implied that a quasi-ho-
mogeneous mechanism could occur, where firstly PdII in the LB
films was reduced to a Pd0 species still bound to a substrate surface
(possibly by the aqueous alcoholic solvent),31 then Pd was dissolved
as colloids or as complexes that had been leached from LB films, for
example, by oxidative addition of a 4-iodobenzoic acid to the
substate.32 Furthermore, synergistic effects may occur as a result of
interactions of catalyst molecules in close-packed LB films,8 which
could promote the reduction of PdII and provide better desorption,
leading to faster formation of colloids or complexes in solution.
More detailed investigations of the LB films catalytic mechanism
are in progress.
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 4-iodobenzoic acid and the product. The
HPLC conditions were a Kromasil C18 column (150ꢀ4.6 mm, 5
mm)
with methanol/0.5% acetic acid solution (60:40) as the mobile
phase, a flow rate 0.8 mL/min, column temperature 25 ꢃC, UV-de-
tection wavelength 272 nm (phenylboronic acid), 283 nm (4-toly-
boronic acid), and the sample size 20 mL. The purified product was
identified by 1H NMR, MS, and melting points with the literature
data.
3.3.1. 4-Biphenylcarboxylic acid
Mp: 223–225 ꢃC; 1H NMR (400 MHz, DMSO-d6)
d (ppm) 7.43–
7.45 (m, 1H), 7.49–7.53 (m, 2H), 7.73–7.75 (m, 2H), 7.81 (d, 2H,
J¼8.3 Hz), 8.03 (d, 2H, J¼8.3 Hz), 12.98 (s, 1H). MS: 197.0 [MꢁH]ꢁ.
3.4. Instrumentation
The experiment for monolayer spreading on water was per-
formed on a Langmuir–Blodgett system (KSV-5000-3, KSV In-
struments, Helsinki, Finland) equipped with computer controls.
Surface pressure was measured with a Pt Wilhelmy plate in air.
3. Experimental section
3.1. Reagents and materials
Distilled and deionized water with resistivity of 18.2 MU cm (MILI-
Q gradient MILIPORE CO., USA) was used for a subphase. FTIR
spectra were recorded on a Bruker VECTOR22 spectrophotometer.
UV–vis spectra were recorded with a Lambda 35 UV–vis spectro-
photometer (Perkin Elmer Inc. USA). A SPM-9500 J3 (Shimadzu
corporation, Japan) was employed for AFM measurements in air at
ambient temperature. Electrochemical experiments were carried
out with CHI650A electrochemical analyzer. Glassy carbon elec-
trode (ca. 3 mm) with deposited LB films was used as working
electrode. The reference electrode was a saturated calomel elec-
trode (SCE), while the counter electrode was a Pt wire. The elec-
Solvents were dried and freshly distilled prior to use. The
cyclopalladated ferrocenylimine 1 was prepared according to the
published procedures.12 All other chemicals were used as
purchased.
3.2. Procedures
Spreading solution was carefully spread onto the ultrapure
water surface, and the Langmuir monolayer was compressed after
evaporation of solvent for 30 min and stabilization for ca. 1 h in the
uncompressed state at a compression speed of 5 mm/min at
26ꢂ1 ꢃC. The Y-type LB films were obtained by the vertical lifting
method with a transfer ratio close to unity, at a constant surface
trolytic medium was H2O (Millipore Q-grade, 18.2 MU cm)
containing 0.1 M KClO4 (pH¼1.5). The thickness of the LB films was
measured with Vnkd-8000 (Aquila Instrument Ltd. UK). HPLC was
conducted on a Waters 600 liquid chromatograph. Melting points
were measured using a WC-1 microscopic apparatus and were
uncorrected. Mass spectra were measured on a LC-MSD-Trap-XCT
instrument. 1H NMR spectra were recorded on a Bruker DPX-400
spectrometer in DMSO-d6 with TMS as an internal standard. Spin-
coated films were prepared with KW-4A spin coater (Chemat
Technology, Inc).
pressure obtained by the static elasticity (Es)–surface pressure (p)
isotherm with a dipping speed of 5 mm/min. LB films were trans-
ferred onto CaF2 substrate (for IR spectroscopy), quartz substrate
(for UV–vis spectroscopy and thickness measurements), freshly
cleaved mica substrate (for AFM instruments), glassy carbon elec-
trode (for electrochemical study), and glass slide (for catalysis
study). The quartz, glass slides on which LB multilayer was de-
posited were cleaned in boiling concentrated HNO3, washed with
pure water, and then sonicated in ultrapure water for 30 min. Spin-
coated films were prepared from chloroform solution of cyclo-
palladated complex 1 by a spin-coated method with the first step
300 rpm for 6 s and the second step 2000 rpm for 15 s.
4. Conclusions
The surface behavior of cyclopalladated ferrocenylimine 1 at the
air–water interface and transferred onto solid substrates was in-
vestigated in detail. The surface pressure (p)–area (A) isotherm
indicated highly condensed monolayer was formed. UV–vis spectra
suggested the monolayer were transferred successfully onto solid
supports, and two dimensional H aggregates were formed. IR
spectra indicated that complex 1 maintained its structure in LB
films, and hydrocarbon chain attached to palladacycle was packed
loosely and, to some extent, was disordered. AFM image showed
that LB films transferred on the solid substrate were well-ordered,
homogeneous. LB films deposited on glassy carbon electrode
showed quasi-reversible oxidation/reduction peaks of ferrocene
moiety, and the separation between cathodic and anodic peak
potential was small compared with that in spin-coated films. From
the average thickness of monolayer, the hydrocarbon chain could
stand almost perpendicular to the substrate. Catalytic tests
revealed that LB films of complex 1 significantly improved the
catalyst turnovers for Suzuki reaction with respect to its cast films
and homogeneous reactions under the same conditions. Presently,
3.3. Suzuki reaction
A mixture of 4-iodobenzoic acid 0.25 mmol, arylboronic acid
0.5 mmol, NaOtBu 1 mmol was transferred to a 25 mL volumetric
flask to fix its quantity in aqueous ethanol (CH3CH2OH/H2O¼1:1).
Samples of 3 mL of this mixture were used for ‘homogeneous’ and
‘heterogeneous’ runs in weighing bottle (40ꢀ25 mm). The hetero-
geneous reactions contained a glass substrate (25ꢀ25ꢀ1 mm3)
with LB films or cast films, while the homogeneous reactions
contained a clean glass substrate of the same dimension with
a certain amount of 1 (10ꢁ3 M chloroform solution) added. The
reactions were carried out in 30 ꢃC air bath for 48 h without stir-
ring, then the mixture was acidified and the product was extracted
by ethyl ether for three times. The combined organic phase was
dried with MgSO4, filtrated, and solvent was removed on a rotary

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B. Mu et al. / Tetrahedron 65 (2009) 2599–2604
10. Benı´tez, I. O.; Bujoli, B.; Camus, L. J.; Lee, C. M.; Odobel, F.; Talham, D. R. J. Am.
Chem. Soc. 2002, 124, 4363.
the preparation of cyclopalladated complexes polymeric LB films
and self-assembled monolayer is underway to overcome these
weaknesses, such as thermal stabilities or poor resistance to dis-
solution by organic solvents. Further experiments are needed to
gain mechanistic understanding of the reaction and extend them to
other coupling reaction.
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We are grateful to the National Natural Science Foundation of
China (No. 20472074), the Innovation Found for Outstanding
Scholar of Henan Province (No. 0621001100), the Fund for Out-
standing Younger Scholar of Henan Province (074100510015), and
the National Natural Science Foundation of Henan Province
(0611020100, 2007150042) for their financial support.
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