Research Article

pubs.acs.org/acscatalysis

Palladium Nanoparticle-Embedded Polymer Thin Film “Dip Catalyst”

for Suzuki−Miyaura Reaction
E. Hariprasad and T. P. Radhakrishnan*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
*
S Supporting Information

ABSTRACT: Hallmarks of a successful catalyst include

simplicity of design and low cost of fabrication, high efficiency,
facile recovery and extensive reusability, amenability to
monitoring between reuses, and ease of scale up. Even though
the number of palladium nanoparticle based catalysts reported
for the Suzuki−Miyaura reaction has grown exponentially in
recent years, the aforesaid criteria are rarely met in a single
system. We present a palladium nanoparticle-embedded polymer
thin film which functions as a highly efficient and reusable “dip
catalyst” for the Suzuki−Miyaura reaction. The multilayer free-
standing nanocomposite thin film is fabricated using a simple in
situ process through thermal annealing of a spin-coated film of poly(vinyl alcohol) (PVA) containing the palladium precursor.
Fabrication parameters of the Pd-PVA film are optimized for enhanced catalyst efficiency. The catalyst is shown to produce very
high yield, turn over number and turn over frequency in the prototypical reaction of iodobenzene with phenylboronic acid. The
“dip catalyst” film is easily retrieved from the reaction system and reintroduced in successive batches; the high efficiency is
retained beyond 30 cycles. The thin film structure enabled convenient catalyst monitoring by spectroscopy and microscopy
between reruns. Efficient use of the catalyst up to 5 mmol scale reaction is demonstrated. A simple figure-of-merit is formulated
to quantify the catalyst performance, and the present catalyst is evaluated in the context of those reported earlier. Preliminary
exploration of the utility of the thin film catalyst in the Suzuki−Miyaura reaction with several substrates as well as in the Heck
and Sonogashira coupling reactions is carried out.
KEYWORDS: palladium nanoparticle, polymer−metal nanocomposite, thin film, dip catalyst, Suzuki−Miyaura reaction

■ INTRODUCTION
Efficiency of a catalyst is generally enhanced under homoge-
through the repeat cycles, impairing the reaction yields and
limiting the reuse. An optimal solution that combines a simple
neous conditions as it is molecularly dispersed. However the and cheap fabrication of a highly efficient catalyst with effective
same factor hampers its recovery and reuse. Heterogeneous recovery and reuse, remains elusive.
formulation facilitates recycling, however at the cost of The concept of a “dip catalyst” that we have developed
efficiency as the reaction is confined to the interface region. recently16 is relevant in this context. The high efficiency and
Metal nanoparticles tend toward molecular level efficiency, and extensive reusability of a catalyst based on silver nanoparticle-
deployment in a suitable framework can improve the embedded poly(vinyl alcohol) (Ag-PVA) thin film fabricated
reusability, paving the way to harnessing the advantages of through a simple in situ protocol,17 in the reduction of 4-
homogeneous and heterogeneous catalysis. This general nitrophenol by sodium borohydride was demonstrated in that
concept has motivated extensive explorations. As an example study. Easy fabrication, convenient recycling, and facile
of enormous contemporary interest, we take the case of monitoring of the “dip catalyst” thin film between uses, are
palladium catalyzed cross coupling reactions, focusing on the highlights of this approach. Its generality and versatility
nanoparticle catalysts and the issue of catalyst recycling. Even stem from the wide variations possible in the catalyst-polymer
though catalyst recovery through nanofiltration1 and magnetic
combinations. Moving beyond the earlier “proof-of-principle”
separation2 have been used, the procedures are often laborious
or the catalyst fabrication elaborate, making the process exploration, we now demonstrate the utility of the approach in
expensive. Some of the supports that have been used to load a reaction of wide applicability in organic synthesis, the
the palladium nanoparticle catalyst are mesoporous organosilica palladium catalyzed Suzuki−Miyaura coupling. Choice of this
and grafted silica foam,3,4 layered double hydroxide and clays,5,6 reaction also allows an appraisal of our concept against the
zeolites and molecular sieves,7−9 various metal oxides,9,10
activated or nitrogen doped carbon,9,11,12 and polymeric Received: March 8, 2012
networks, capsules, and resins.9,13−15 However, most of these Revised: May 3, 2012
approaches are prone to catalyst leaching or degradation Published: May 14, 2012

© 2012 American Chemical Society 1179 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186
ACS Catalysis Research Article

background of a large collection of nanoparticle catalysts rpm for 10 s followed by 6000 rpm for 10 s. After heating the
reported earlier. film at 90 °C for 30 min, an aqueous solution of PVA was spin-
A survey of the extensive literature on palladium catalyzed coated at 500 rpm for 10 s followed by 3000 rpm for 10 s and
Suzuki−Miyaura reaction in particular and the subject of heated at 90 °C for 30 min; a second coating of PVA was added
catalysis in general, reveals the critical need to evaluate a to increase the thickness. The final layer was formed by spin-
catalyst on the basis of multiple parameters. It is desirable to coating the K2PdCl4−PVA solution as before, and the film was
have an integrated view of the various relevant factors including heated at 130 °C for 4 h. It was found that longer heating did
the turn over number (TON) or turn over frequency (TOF), not enhance the extent of reduction of the Pd(II) ions or the
the number of reuse cycles, scale of the reaction, the stability of the film. The film was peeled off the glass substrate
temperature at which it is carried out (hence the energy and wrapped around a Teflon frame. It was then dipped in
input), the solvent/special atmosphere required, and the cost of toluene to dissolve the PS layer and yield the free-standing 3-
catalyst fabrication. While quantification of some of these are layer Pd-PVA/PVA/Pd-PVA film fixed on the frame. The film
straightforward, others are complicated. An attempt to evolve a was finally washed in water to remove the unreduced K2PdCl4
figure-of-merit (FOM) that takes into account as many of the and the byproduct KCl formed during the in situ reduction of
clearly quantifiable factors as possible appears to be worthwhile. K2PdCl4 by PVA; the observation of KCl and its removal have
Even though FOMs have been invoked for the specific purpose been discussed in ref 19.
of designing some catalysts,18 rarely have they been employed Characterization of the Catalyst Film. The thickness of
for the direct comparison of catalysts available for a chosen the film coated on the glass substrate was measured using an
reaction. In the present study, we have made a preliminary Ambios Technology XP-1 Profilometer. Measurement carried
effort toward this goal which allowed us to make a meaningful out after each spin-coating/heating step provided the thickness
comparison of our “dip catalyst” with those reported earlier for of the film at that stage. Cross section samples were prepared
the same reaction. by fixing the free-standing film in Araldite resin and cutting into
Extending our in situ method for the synthesis of dendritic 50 nm thin sections with a Leica ultramicrotome. Field
palladium nanostructures in a PVA film,19 we have fabricated a
emission scanning electron microscope (FESEM) imaging with
multilayer Pd-PVA thin film catalyst. The multilayer structure
energy dispersive X-ray spectroscopy (EDXS) was carried out
makes the free-standing film robust while facilitating easy access
on a Carl Zeiss model Ultra 55 microscope. Transmission
of the reagents to the ligand-free catalyst nanoparticles
electron microscopy (TEM) was carried out using a Tecnai G2
embedded and stabilized within the polymer matrix. Experi-
ments are carried out to optimize the Pd/PVA ratio and the FEI F12 TEM at an accelerating voltage of 200 kV. Electronic
reaction conditions for the Suzuki−Miyaura coupling of absorption spectra were recorded on a Varian Model Cary 100
iodobenzene and phenylboronic acid to yield biphenyl. Under UV−visible spectrometer. Chemical composition of the film
optimal conditions, nearly 100% yield is obtained, and the yield was analyzed using a Varian 720-ES Inductively Coupled
and reaction time shows negligible change over 30 reuses. Very Plasma-Optical Emission Spectrometer (ICP-OES). Sample for
large TONs and TOFs can be realized by using tiny pieces of the analysis was prepared by dissolving a known weight of the
the catalyst film, and the reaction is found to be facile at scales free-standing film in 100 mL of 60% nitric acid.
up to 5 mmol. An FOM of the catalyst is defined incorporating Catalysis Studies. All reactions were carried out under
the values of average TOF, number of reuses, scale of the normal atmosphere; inert conditions were not required. The
reaction, and the temperature condition required. Using this procedure followed in a typical reaction is as follows. Two
FOM, we compare the Pd nanoparticle based catalysts reported mmol of the base was introduced into a reaction tube in which
for this reaction including our “dip catalyst”. Finally, the utility a magnetic stirring bar was placed. This was followed by the
of our catalyst in the Suzuki−Miyaura reaction with a range of addition of 15 mL of the solvent and 1 mmol of the aryl halide.
substrates, as well as in the Heck and Sonogashira reactions is The catalyst film (thickness ∼1 μm, total surface area = 35 cm2,
demonstrated. total weight = 5.62 mg, Pd content = 0.6 μmol)20 wrapped on a

■
Teflon frame was introduced. After adding 1.1 mmol of
EXPERIMENTAL SECTION phenylboronic acid, the reaction tube was closed with a stopper
Fabrication of the Catalyst Film. The catalyst film was and keck clip, and introduced into an oil bath preheated to the
fabricated by extending the protocol developed in our required temperature. The reaction mixture was stirred with the
laboratory earlier.19 Glass substrates were cleaned by washing magnetic bar. Formation of the product was monitored through
and ultrasonication in isopropyl alcohol for 10 min; Gas Chromatograph-Mass Spectrometer (Shimadzu model
subsequently they were dried in a hot air oven. A few drops QP2010) analysis of 0.2 mL samples of the reaction mixture
of a solution of polystyrene (PS) (Aldrich, average molecular retrieved periodically from the reaction tube. The maximum
weight = 280 kDa) in toluene (1 g in 8 mL) was spin-coated error in the determination of the reaction time is ∼15%; the
using a Laurell Technologies Model WS-400B-6NPP/LITE/8K error is lower for longer reaction times. When the reaction was
Photoresist Spinner operated at 1000 rpm for 10 s, and dried at completed, the catalyst film was taken out and the reaction
90 °C for 15 min. Aqueous solutions of K2PdCl4 (Aldrich, mixture was filtered; after adding a small amount of silica
purity = 99.99% on metal basis) and PVA (Aldrich, average (Merck, 100−200 mesh), the filtrate was evaporated
molecular weight = 85−146 kDa, hydrolysis = 99+ %) were completely. The residual product was purified through column
mixed in the required proportions; for example, 80 mg of chromatography and analyzed by NMR (Bruker 500 MHz).
K2PdCl4 dissolved in 2 mL of water was mixed with 200 mg of The catalyst film taken out of the reaction mixture was dipped
PVA dissolved in 4 mL of water to prepare a film with a Pd/ briefly in diethyl ether and washed with dichloromethane to
PVA weight ratio, x = 0.13. Millipore Milli-Q water (resistivity remove organic residues, dipped in water and isopropanol to
= 18.2 MΩ cm) was used in all operations. The K2PdCl4−PVA remove the base, and finally dried in vacuum for 30 min. The
solution was coated on top of the PS layer by spinning at 500 film was ready for reuse.
1180 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186
ACS Catalysis

■
Research Article

RESULTS AND DISCUSSION layers are consistent with those noted above. Because of the
The thickness of the different layers of the Pd-PVA/PVA/Pd- high density of particles in the outer two layers, individual ones
PVA catalyst film was determined by profilometer measure- are not resolved, but the low density in the middle layer reveals
ments on several samples. Values obtained for the film with x = spherical particles ∼15−30 nm in size. The FESEM image of
0.13 (in the Pd-PVA layers) are 0.18 (0.03), 0.53 (0.05), and the cross-section sample combined with EDXS analysis across
0.17 (0.06) μm for the Pd-PVA, PVA, and Pd-PVA layers, the layer thickness shows a lower concentration of Pd in the
respectively.20 Electronic absorption spectra of the film, middle layer of the film.20 The microscopy images are thus
recorded through the different stages of fabrication are shown consistent with the Pd-PVA/PVA/Pd-PVA coating sequence.
in Figure 1a. The first layer (K2PdCl4−PVA) shows the peaks A wide range of solvents and bases have been employed in
the Suzuki−Miyaura reaction. The catalyst film is robust
enough to be used in any of these solvents. However, as the
swelling of the polymer film in the solvent enhances the catalyst
activity by improving access of the reactants to the catalyst, we
have focused in particular on aqueous and alcoholic solvents.
We have chosen a film with x = 0.13 and a reaction temperature
of 80 °C (basis of these choices are discussed below) to
investigate the effect of different solvents and bases. All the
reactions were carried out using 1 mmol of iodobenzene and
1.1 mmol of phenylboronic acid. Observations with the
different solvents (using K2CO3 as the base) are collected in
Table 1. The rates and yields are poor with pure water and

Table 1. Time and Yield of the Reaction of Phenylboronic

Acid with Iodobenzene in Different Solvents Using K2CO3
as the Base and the Thin Film Catalyst (Pd-PVA Layer, x =
0.13)a
solvent time (h) yield (%)
water 5 8
2b 100b
water−ethanol (10:1) 5 22
2b 100b
water−ethanol (1:1) 5 58
Figure 1. (a) Electronic absorption spectra of the catalyst film
1.5b 100b
recorded through the different stages of spin coating and heating.
toluene 3 45
TEM images of the heated (b) 1-layer film, (c) 2-layer film, and (d)
final 3-layer film cross section. N,N-dimethylacetamide 2 100
ethanol 1.5 100
a
due to the precursor salt; after the short heating step there is Reaction conditions: 1 mmol of iodobenzene, 1.1 mmol of
phenylboronic acid, 2 mmol of K2CO3, temperature = 80 °C. bWith
little change in the spectrum. The visible change upon coating
0.01 mmol of tetrabutylammonium bromide.
the second layer (PVA) is due to increased light scattering and
the localized surface plasmon resonance (LSPR) absorption of
water−ethanol mixtures, but enhanced considerably by the
the Pd nanostructures formed in the film; the latter may result
addition of tetrabutylammonium bromide. However, as the
from the higher net ratio of PVA to Pd that enhances the
base dissolves in the solvent, the product has to be isolated by
chemical reduction of the Pd2+ ions.19 The LSPR peak becomes
extraction. Ethanol is an optimal choice of solvent, as there is
prominent on heating the 2-layer film for 30 min. As expected,
no need of a phase-transfer agent, the reaction is fast, work up
the precursor peaks appear clearly again on coating the third
of the reaction mixture is simple, and the yields are quantitative.
layer (K2PdCl4−PVA). The subsequent heating leads to the
We have also explored the utility of different bases; Table 2 lists
emergence of the strong LSPR absorption of palladium
the yields obtained for the reaction run in ethanol for 1.5 h.
nanoparticles extending into the visible region; changes become
negligible after 4 h of heating. The spectrum of the final film
dipped briefly in water20 indicates the removal of the soluble Table 2. Yield of the Reaction of Phenylboronic Acid with
precursor left unreacted in the film. TEM images of the catalyst Iodobenzene in Ethanol Using Different Bases and the Thin
film are shown in Figure 1b−d. The first layer shows dendritic Film Catalyst (Pd-PVA Layer, x = 0.13)a
structures due to the precursor and some Pd formed on
heating;20 this is consistent with the observation in the base yield (%) remarks
precursor-to-palladium nanocrystal transformation investigated triethylamine 60 convenient to use
in detail earlier.19 Upon coating the second layer (PVA), the sodium acetate 85 convenient to use
film is too thick to get a clear TEM image; however, one can potassium phosphate 100 hygroscopic
visualize some morphological changes; spherical particles sodium hydroxide 100 hygroscopic
∼100−300 nm in diameter are observed. The increased net potassium carbonate 100 convenient to use
PVA/Pd ratio and the extended heating contribute to the a
Reaction conditions: 1 mmol of iodobenzene, 1.1 mmol of
morphology change.19 The TEM image of the cross section of phenylboronic acid, 2 mmol of base, temperature = 80 °C, time =
the final 3-layer film reveals the layer structure; thickness of the 1.5 h.

1181 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186

ACS Catalysis Research Article

Even though K2CO3, K3PO4, and NaOH provided similar

quantitative yields, K2CO3 is preferred as it is nonhygroscopic
and easy to handle.
Using ethanol as the solvent and K2CO3 as the base, we have
carried out several batches of exploratory reactions to
determine the optimal value of x of the catalyst film and the
reaction temperature. It may be noted that only a single
product is observed in all the reactions and test runs excluding
iodobenzene or phenylboronic acid showed negligible homo-
coupling reaction. Figure 2 shows the yield as a function of

Figure 3. Turn over frequency (TOF) of the Suzuki−Miyaura reaction

of phenylboronic acid with iodobenzene in ethanol at different
temperatures, using K2CO3 as the base and the thin film catalysts with
different Pd/PVA weight ratios (x) in the Pd-PVA layer; reaction
conditions: 1 mmol of iodobenzene, 1.1 mmol of phenylboronic acid,
2 mmol of K2CO3.

Figure 2. Progress of the Suzuki−Miyaura reaction of phenylboronic

acid with iodobenzene in ethanol using K2CO3 as the base and the thin
film catalyst with different Pd/PVA weight ratios (x) in the Pd-PVA
layer; reaction conditions: 1 mmol of iodobenzene, 1.1 mmol of
phenylboronic acid, 2 mmol of K2CO3, temperature = 80 °C. Inset:
Progress of the reaction using the catalyst with x = 0.13 monitored at
shorter time intervals.

time, when catalyst films with different x values are employed;

considerable improvement is observed when x is increased from
0.07 to 0.13, but there is little enhancement beyond. It is seen
that with x = 0.13, 100% conversion is achieved in 1.5 h. The
TONs for the different catalyst films are not influenced by the
reaction temperature. However, the TOFs increase substantially
at 80 °C for the films with x = 0.13 and 0.15 (Figure 3). The
slightly higher TOF obtained using the film with x = 0.13 can
be attributed to the lower catalyst content and the only
marginal increase in the time for 100% conversion.
On the basis of the experiments above, we have chosen the
catalyst with x = 0.13 (in the Pd-PVA layers), ethanol as the
solvent, K2CO3 as the base, and a reaction temperature of 80
°C to demonstrate the recycling capability of our “dip catalyst”.
These reactions were run using 1 mmol of iodobenzene and 1.1
mmol of phenylboronic acid. Nearly 100% yield was obtained
in the first run in 1.5 h. The catalyst film was taken out, washed,
dried (Experimental Section) and reinserted for the next run. It Figure 4. (a) Yield obtained at 1.5 h reaction time in repeated runs of
is found that the same film can be reused extensively. The the Suzuki−Miyaura reaction of phenylboronic acid with iodobenzene
yields obtained at 1.5 h in the 30 cycles we have run are shown in ethanol using K2CO3 as the base and the same piece of thin film
in Figure 4a; it may be noted that 100% yield can be obtained catalyst (Pd-PVA layer, x = 0.13); reaction conditions: 1 mmol of
even in the last few cycles by slight increase in the reaction iodobenzene, 1.1 mmol of phenylboronic acid, 2 mmol of K2CO3,
time. Yield of the isolated product from the combined reaction temperature = 80 °C. (b) Kinetics plot showing the yield as a function
of time for selected reaction runs.
mixtures of the 30 cycles is 89%. The ease of redeployment of
the catalyst and the quantitative conversion observed over such
a large number of cycles testifies to the versatility of the “dip indicates that the Pd content is 0.6 μmol.20 This implies a TON
catalyst” concept. The activity at the end of these runs suggests of 1667 in the runs with 100% yield; the total TON for the 30
that the recycling can indeed be continued further. ICP-OES cycles is the high value of 4.96 × 104. The average TOF works
analysis of the catalyst film with x = 0.13 used in these reactions out to be 1102 h−1. We have monitored the progress of the
1182 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186
ACS Catalysis Research Article

reaction at various points during the repeat cycles. As seen in

Figure 4b, the kinetics remains nearly unchanged through the
large number of repeated uses.
TEM and X-ray photoelectron spectroscopy (XPS) studies
we have carried out earlier19 have unambiguously proved that
the Pd-PVA formed through extended thermal annealing does
still contain some amount of unreacted Pd2+. Even though our
catalyst film is washed in water following the 4 h heating
process, it is possible that it contains trace Pd2+. This may
indeed be advantageous for the Suzuki−Miyaura reaction as
Pd2+ has been implicated in the catalytic cycle.21 To rule out
the possibility of Pd2+ alone being the active catalyst, we have
attempted the reaction with unheated films of K2PdCl4−PVA
(with x = 0.13). Under identical reaction conditions as above,
we find that the reaction hardly proceeds; less than 10% yield is
obtained even after 2 h.20 An experiment in which the Pd-PVA/
PVA/Pd-PVA film was removed after 30 min of reaction
showed the yield of ∼40% reached in that time remaining
unchanged for several hours afterward, clearly ruling out the
possibility of Pd leached from the film catalyzing the reaction.20
The extremely low concentration (5 ppb) of Pd detected by
ICP-OES analysis in the reaction mixture is clearly insufficient
to catalyze the reaction. ICP-OES analysis of a control sample
containing all the reagents including the base but no catalyst Figure 5. (a) Electronic absorption spectra and (b) FESEM images
(scale bar = 100 nm) of the same thin film catalyst (Pd-PVA layer, x =
film showed the Pd content to be <1.0 ppb; this is significant in 0.13) after different number of uses (zero corresponds to the fresh
view of the report22 of Pd impurity in the base catalyzing the film) in the Suzuki−Miyaura reaction of phenylboronic acid with
Suzuki−Miyaura reaction. An important advantage of the “dip iodobenzene in ethanol using K2CO3 as the base; reaction conditions
catalyst” is the ease with which the catalyst can be monitored for each run: 1 mmol of iodobenzene, 1.1 mmol of phenylboronic acid,
during the reuse cycles; this is difficult with most of the catalyst 2 mmol of K2CO3, temperature = 80 °C.
immobilization schemes reported earlier. Such examinations
provide useful insight into the basis of the durability of the system is the scale of the reaction that it can be employed in.
catalyst. The electronic absorption spectra and FESEM images Figure 6 shows the reaction times for obtaining quantitative
of the catalyst film before the first use and after 15 and 30
cycles of usage are shown in Figure 5. The spectrum shows
faint signs of Pd2+ generation, but negligible impact on the
nanoparticle content, ruling out once again any significant
leaching. The microscopy images show that the film
morphology is affected very little; but small particles emerge
near the surface after extensive recycling. In view of their sizes
(compare with Figure 1d) and the EDXS analysis which again
indicates that the Pd content is nearly unchanged,20 we believe
that these are Pd nanoparticles. Retention of the morphology of
the nanoparticles is a significant observation, as it is different
from the case of spherical particles aggregating into needles in
solution based catalysts.23
Very large TONs of the order of 105−107 have been reported
for the Suzuki−Miyaura reaction in recent times.13,24−31
However, specialized catalyst preparation including high
temperature treatments or special reaction conditions,24−27 Figure 6. Time for completion (100% yield) of the Suzuki−Miyaura
catalyst degradation or decline of yield with reuse13,28 or reaction of phenylboronic acid with different amounts of iodobenzene
relatively low reaction yields29−31 are involved in most cases. in ethanol using K2CO3 as the base and different quantities (areas) of
To test the high TONs that we can realize, the reaction was the thin film catalyst (Pd-PVA layer, x = 0.13); reaction conditions: 1.1
carried out with a very small piece of the catalyst film equiv of phenylboronic acid, 2 equiv of K2CO3, temperature = 80 °C.
containing 0.014 μmol (14 ppm) of the catalyst; 5 mmol of
iodobenzene and 6 mmol of phenylboronic acid were used. conversion in the reactions of 1−5 mmol of iodobenzene, using
Because of the very small size, the film could not be fixed on a catalyst films with different areas and hence palladium content.
Teflon scaffold and hence could not be used in several cycles. It is seen that with the catalyst film with an area of 35 cm2, even
However, in just 2 runs, a total TON of 7.2 × 105 could be a 5 mmol scale reaction is satisfactorily completed in a short
realized, and a high TOF of 1.1 × 104 h−1. In addition to the time; yield of the isolated product is 91%.
efficient circulation in the reaction tube, the smaller size of the As mentioned earlier, a fair appraisal of the performance of a
film is likely to enhance the accessibility of the Pd nanoparticles catalyst for a specific reaction should take into account several
embedded within the film contributing to the enhanced TON parameters. Using the clearly quantifiable ones among them we
and TOF. An important factor for the development of a catalyst define the following FOM,
1183 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186
ACS Catalysis Research Article

(TOF)av × N × S Table 3. Time Required for 100% Conversion in the

FOM = Reaction of Phenylboronic Acid with Different Aryl Halidesa
298 + |ΔT |
in Ethanol Using K2CO3 as the Base and the Thin Film
where (TOF)av is the average TOF, N the number of runs Catalyst (Pd-PVA layer, x = 0.13)b
(cycles), S the maximum scale of reaction demonstrated, and
ΔT the magnitude of the deviation in the reaction temperature X R time (h)
from the ambient (taken as 298 K). We have carried out a I H 1.5
search of the reports on the Suzuki−Miyaura reaction from the I 4-COCH3 1.5
year 2000, employing Pd nanoparticles for the specific reaction I 4-NO2 1.5
of iodobenzene with phenylboronic acid. Taking into account I 4-NH2 1.5
all the reports which have provided unambiguous information I 4-CH3 1.5
on the parameters listed above, the FOM of the various I 4-OCH3 2.0
catalysts have been estimated. The complete list is provided in I 2-OCH3 3.5
the Supporting Information. Those catalysts which exhibit Br H 2.5
FOMs ≥ 2.0 are represented schematically in Figure 7. As a Br 4-COCH3 2.5
Br 4-NO2 2.5
Br 4-NH2 3.0
Br 4-CH3 3.5
Br 4-OCH3 10.0
Br 2-OCH3 20.0
a
X = halogen; R = substituent on aromatic ring. bReaction conditions:
1 mmol of aryl halide, 1.1 mmol of phenylboronic acid, 2 mmol of
K2CO3, temperature = 80 °C.

Figure 7. Schematic representation of the figure-of-merit (FOM) of

various reported catalysts (refs 15, 32−57) and the present thin film
catalyst (Pd-PVA layer, x = 0.13) used for the Suzuki−Miyaura
reaction of iodobenzene with phenylboronic acid (see text for the
definition of FOM and details of the data set).

result of the simultaneous realization of the several beneficial

features, our “dip catalyst” scores highest in the list. It is indeed
quite possible that the relevant parameters of the various
catalysts are not optimized in the reported studies and their
FOMs could be enhanced; it is also likely that higher FOMs
can be realized for reactions with other substrates or Pd
catalysts that are not based on nanoparticles.
To evaluate the general applicability of our thin film catalyst
we have explored the Suzuki−Miyaura reaction with iodo and
bromobenzenes bearing various electron donating and with-
drawing groups in different positions. Similar conditions as
above were employed: ethanol solvent, K2CO3 base, 80 °C, and
ambient atmosphere. The observations collected in Table 3
show that the catalyst is very effective in all cases except with Figure 8. Progress of the (a) Heck reaction of styrene and (b)
methoxy substituted bromobenzene. It does not work well with Sonogashira reaction of phenylacetylene, with iodo and bromoben-
chlorobenzene and its derivatives. Preliminary studies suggest zenes using ethanol as the solvent, K2CO3 as the base, and the thin
that our catalyst film works well in other C−C coupling film catalyst (Pd-PVA layer, x = 0.13); reaction conditions: 1 mmol of
halobenzene, 1.1 mmol of styrene or phenylacetylene, 2 mmol of
reactions as well. Progress of the Heck and Sonogashira K2CO3, temperature = 80 °C.
reactions of styrene and phenylacetylene with iodo and
bromobenzenes yielding stilbene and diphenylacetylene,
respectively, catalyzed by our film is shown in Figure 8.
These observations point to the feasibility of extending the
■ CONCLUSIONS
We have developed a simple protocol for the fabrication of a
application of the nanocomposite thin film catalyst to a number multilayer Pd-PVA thin film using commercially available Pd
of organic transformations. precursor and aqueous medium for mixing with the polymer,
1184 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186
ACS Catalysis Research Article

and spin-coating/mild thermal annealing steps for the in situ (12) Deshmukh, A. A.; Islam, R. U.; Witcomb, M. J.; van Otterlo, W.
generation of Pd nanoparticles inside the film. The thin film A. L.; Coville, N. J. ChemCatChem 2010, 2, 51−54.
“dip catalyst” is gainfully employed in the Suzuki−Miyaura (13) Ogasawara, S.; Kato, S. J. Am. Chem. Soc. 2010, 132, 4608−4613.
reaction of iodobenzene with phenylboronic acid, providing a (14) Ley, S. V.; Ramarao, C.; Gordon, R. S.; Holmes, A. B.; Morrison,
A. J.; McConvey, I. F.; Shirley, I. M.; Smith, S. C.; Smith, M. D. Chem.
very high yield, TON and TOF, and the possibility of scale up.
Commun. 2002, 1134−1135.
Extensive recycling capability and the unique advantage of (15) Zhang, P.; Weng, Z. H.; Guo, J.; Wang, C. C. Chem. Mater.
convenient catalyst monitoring between reuse cycles are 2011, 23, 5243−5249.
demonstrated. A simple FOM is defined to integrate the (16) Hariprasad, E.; Radhakrishnan, T. P. Chem.Eur. J. 2010, 16,
various significant and quantifiable parameters related to the 14378−14384.
catalyst performance. An appraisal of the present catalyst in the (17) Ramesh, G. V.; Porel, S.; Radhakrishnan, T. P. Chem. Soc. Rev.
context of the others reported in the recent literature is 2009, 38, 2646−2656.
presented on the basis of the FOM. The “dip catalyst” with its (18) Breuer, C.; Lucas, M.; Schütze, F.; Claus, P. Comb. Chem. High
ease and low cost of fabrication, convenience of deployment Throughput Screening 2007, 10, 59−70.
and recycling, and high catalytic efficiency is expected to be (19) Porel, S.; Hebalkar, N.; Sreedhar, B.; Radhakrishnan, T. P. Adv.
extremely useful in organic reactions of great synthetic utility. Funct. Mater. 2007, 17, 2550−2556.

■
(20) See Supporting Information.
(21) Xu, J.; Wilson, A. R.; Rathmell, A. R.; Howe, J.; Chi, M.; Wiley,
ASSOCIATED CONTENT B. J. ACS Nano 2011, 5, 6119−6127.
*
S Supporting Information (22) Leadbeater, N. E. Nature Chem. 2010, 2, 1007−1009.
Details of catalyst film characterization and figure-of-merit (23) Hu, J.; Liu, Y. Langmuir 2005, 21, 2121−2123.
estimation (11 pages). This material is available free of charge (24) Okumura, K.; Matsui, H.; Tomiyama, T.; Sanada, T.; Honma,
via the Internet at http://pubs.acs.org. T.; Hirayama, S.; Niwa, M. ChemPhysChem 2009, 10, 3265−3272.

■
(25) Luzyanin, K. V.; Tskhovrebov, A. G.; Carias, M. C.; da Silva, M.
F. C. G.; Pombeiro, A. J. L.; Kukushkin, V. Y. Organometallics 2009,
AUTHOR INFORMATION 28, 6559−6566.
Corresponding Author (26) Okumura, K.; Tomiyama, T.; Okuda, S.; Yoshida, H.; Niwa, M.
*E-mail: tprsc@uohyd.ernet.in. Phone: 91-40-2313-4827. Fax: J. Catal. 2010, 273, 156−166.
91-40-2301-2460. (27) Chen, L.; Yang, Y.; Jiang, D. J. Am. Chem. Soc. 2010, 132, 9138−
9143.
Funding (28) Rao, G. K.; Kumar, A.; Ahmedz, J.; Singh, A. K. Chem. Commun.
Financial support from the Department of Science and 2010, 46, 5954−5956.
Technology, New Delhi and infrastructure support from the (29) Kopylovich, M. N.; Lasri, J.; da Silva, M. F. C. G.; Pombeiro, A.
Centre for Nanotechnology at the University of Hyderabad are J. L. Dalton Trans. 2009, 3074−3084.
acknowledged with gratitude. E.H. thanks the CSIR, New (30) Takemoto, T.; Iwasa, S.; Hamada, H.; Shibatomi, K.;
Delhi, for a senior research fellowship. Kameyama, M.; Motoyama, Y.; Nishiyam, H. Tetrahedron Lett. 2007,
48, 3397−3401.
Notes (31) Jiang, N.; Ragauskas, A. J. Tetrahedron Lett. 2006, 47, 197−200.
The authors declare no competing financial interest.

■
(32) Ul Islam, R.; Witcomb, M. J.; van der Lingen, E.; Scurrell, M. S.;
Van Otterlo, W.; Mallick, K. J. Organomet. Chem. 2011, 696, 2206−
ACKNOWLEDGMENTS 2210.
We thank Dr. M. Lakshman, Mr. M. Durga Prasad and Mr. M. (33) Polshettiwar, V.; Nadagouda, M. N.; Varma, R. S. Chem.
Commun. 2008, 6318−6320.
Laxminarayana for help with the ultramicrotoming, TEM (34) Wu, L.; Li, B.-L.; Huang, Y.-Y.; Zhou, H.-F.; He, Y.-M.; Fan, Q.-
imaging, and FESEM imaging, respectively.

■
H. Org. Lett. 2006, 8, 3605−3608.
(35) Diallo, A. K.; Ornelas, C.; Salmon, L.; Aranzaes, J. R.; Astruc, D.
REFERENCES Angew. Chem., Int. Ed. 2007, 46, 8644−8648.
(1) Datta, A.; Ebert, K.; Plenio, H. Organometallics 2003, 22, 4685− (36) Zhong, L.-S.; Hu, J.-S.; Cui, Z.-M.; Wan, L.-J.; Song, W.-G.
4691. Chem. Mater. 2007, 19, 4557−4562.
(2) Rosario-Amorin, D.; Wang, X.; Gaboyard, M.; Clérac, R.; Nlate, (37) Zhang, M.; Zhang, W. J. Phys. Chem. C 2008, 112, 6245−6252.
S.; Heuzé, K. Chem.Eur. J. 2009, 15, 12636−12643. (38) Gopidas, K. R.; Whitesell, J. K.; Fox, M. A. Nano Lett. 2003, 3,
(3) Yang, H.; Han, X.; Li, G.; Ma, Z.; Hao, Y. J. Phys. Chem. C 2010, 1757−1760.
114, 22221−22229. (39) Sun, Q.; Zhu, L. F..; Sun, Z. H.; Meng, X. J.; Xiao, F.-S. Sci.
(4) Ungureanua, S.; Deleuze, H.; Babot, O.; Achard, M. -F.; Sanchez, China, Chem. DOI: 10.1007/s11426-011-4491-8.
C.; Popa, M. I.; Backov, R. Appl. Catal., A 2010, 390, 51−58. (40) Wan, L.; Cai, C. Catal. Lett. 2011, 141, 839−843.
(5) Shiyong, L.; Qizhong, Z.; Zhengneng, J.; Huajiang, J.; Xuanzhen, (41) Tamami, B.; Ghasemi, S. J. Mol. Catal. A: Chem. 2010, 322, 98−
J. Chin. J. Catal. 2010, 31, 557−561. 105.
(6) Ramchandani, R. K.; Uphade, B. S.; Vinod, M. P.; Wakharkar, R. (42) Shiyong, L.; Qizhong, Z.; Huajiang, J. Chin. J. Chem. 2010, 28,
D.; Choudhary, V. R.; Sudalai, A. Chem. Commun. 1997, 2071−2072. 589−593.
(7) Djakovitch, L.; Koehler, K. J. Am. Chem. Soc. 2001, 123, 5990− (43) Zhu, M.; Diao, G. J. Phys. Chem. C 2011, 115, 24743−24749.
5999. (44) Zhang, Z.; Wang, Z. J. Org. Chem. 2006, 71, 7485−7487.
(8) Durap, F.; Rakap, M.; Aydemir, M.; Ö zkar, S. Appl. Catal., A (45) Chandrasekhar, V.; Narayanan, R. S.; Thilagar, P. Organo-
2010, 382, 339−344. metallics 2009, 28, 5883−5888.
(9) Biffis, A.; Zecca, M.; Basato, M. J. Mol. Catal. A: Chem. 2001, 173, (46) Wang, J.; Song, G.; Peng, Y. Tetrahedron Lett. 2011, 52, 1477−
249−274. 1480.
(10) Köhler, K.; Wagner, M.; Djakovitch, L. Catal. Today 2001, 66, (47) Cao, M.; Lin, J.; Yanga, H.; Cao, R. Chem. Commun. 2010, 46,
105−114. 5088−5090.
(11) Zhao, F.; Bhanage, B. M.; Shirai, M.; Arai, M. Chem.Eur. J. (48) Cao, M.; Wei, Y.; Gao, S.; Cao, R. Catal. Sci. Technol. 2012, 2,
2000, 6, 843−848. 156−163.

1185 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186

ACS Catalysis Research Article

(49) Das, D. D.; Sayari, A. J. Catal. 2007, 246, 60−65.

(50) Deshmukh, K. M.; Qureshi, Z. S.; Bhatte, K. D.; Venkatesan, K.
A.; Srinivasan, T. G.; Rao, P. R. V.; Bhanage, B. M. New J. Chem. 2011,
35, 2747−2751.
(51) Zhi, J.; Song, D.; Li, Z.; Lei, X.; Hu, A. Chem. Commun. 2011,
47, 10707−10709.
(52) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921−
4925.
(53) Chen, Z.; Cui, Z.-M.; Niu, F.; Jiang, L.; Song, W.-G. Chem.
Commun. 2010, 46, 6524−6526.
(54) Makhubela, B. C. E.; Jardine, A.; Smith, G. S. Appl. Catal., A
2011, 393, 231−241.
(55) Borhade, S. R.; Waghmode, S. B. Beilstein J. Org. Chem. 2011, 7,
310−319.
(56) Zhou, P.; Wang, H.; Yang, J.; Tang, J.; Sun, D.; Tang, W. RSC
Advances 2012, 2, 1759−1761.
(57) Martins, D. L.; Alvarez, H. M.; Aguiar, L. C. S. Tetrahedron Lett.
2010, 51, 6814−6817.

1186 dx.doi.org/10.1021/cs300158g | ACS Catal. 2012, 2, 1179−1186