Synthesis and catalytic activity of a poly(N,N-dialkylcarbodiimide)/ palladium nanoparticle composite: A case in the Suzuki coupling reaction using microwave and conventional heating

Article abstract of DOI:10.1039/b313210m

Poly(N,N-dialkylcarbodiimide) was found to be an effective polymeric ligand system for preparing and stabilizing palladium nanoparticles (1-5 nm). The composite material prepared in situ was found to be a robust catalyst for the Suzuki coupling reaction under microwave or regular heating.

Full text of DOI:10.1039/b313210m

Synthesis and catalytic activity of a
poly(N,N-dialkylcarbodiimide)/palladium nanoparticle composite: a case
in the Suzuki coupling reaction using microwave and conventional
heating†
Yubiao Liu, Chalermchai Khemtong and Jun Hu*
Department of Chemistry, The University of Akron, Akron, OH 44325, USA. E-mail: jhu@uakron.edu
Received (in West Lafayette, IN, USA) 19th October 2003, Accepted 8th December 2003
First published as an Advance Article on the web 23rd January 2004
Poly(N,N-dialkylcarbodiimide) was found to be an effective
polymeric ligand system for preparing and stabilizing palla-
dium nanoparticles (1–5 nm). The composite material prepared
in situ was found to be a robust catalyst for the Suzuki coupling
reaction under microwave or regular heating.
mide) is a more weakly coordinating ligand than the prevailing
polymers that have been used in these Suzuki couplings. For the
reactive intermediates in the catalytic processes, the weaker
coordination results in higher activity of the catalysts. For the
nanoparticles, more weakly coordinating ligands are less corrosive,
and therefore the more slowly the redistribution of the nanoparticles
to the larger size occurs in the reactions.
Recently we reported that poly(N,N-dialkylcarbodiimide), a helical
backbone synthetic polymer,1 is useful for synthesizing and
stabilizing transition metal nanoparticles.2 The unique supramo-
lecular chirality of the polymer and the remarkable stability of the
resulting nanocomposites prompted us to study the catalytic
activities of such new materials. While polymers have been used to
stabilize colloidal metal nanoparticles, only few of them are useful
as polymeric ligands to provide optimum stability and catalytic
activity. We examined the synthesis, stability and catalytic activity
of poly(N,N-dihexylcarbodiimide)/palladium nanoparticle com-
posite (PDHC–Pd). We report herein the remarkable catalytic
activity and stability of PDHC–Pd in the Suzuki coupling reaction.
In addition, we report that the catalyst is compatible with
microwave heating, which greatly accelerates the coupling reac-
tion.
The nanoparticles were prepared by NaBH4 reduction of
H2PdCl4 in a two phase mixture of poly(N,N-dihexylcarbodiimide),
toluene and water.† The light yellow PdCl422 ion was visible in the
22
organic layer before the reduction. Presumably, PdCl4
was
extracted into the organic phase by ion-pairing with the protonated
guanine units of the polymer backbone. Subsequent reduction of
the metal ions produced the PDHC–Pd nanocomposite. The
reduction reaction can be monitored by UV–vis absorption
spectroscopy. The peak at 376 nm of the organic phase before
reduction was assigned to the ligand-to-metal charge transfer
22
transition of the PdCl4 ion. This band disappeared after the
formation of the palladium nanoparticles. A broad increase in the
absorption coefficient from the ultraviolet to the visible region in
the spectrum indicated the formation of the palladium nano-
particles.12
The Suzuki coupling reaction is a powerful synthetic method for
preparing biaryls from arylboronates and aryl halides.3 Homoge-
neous transition metal catalysts such as Pd(PPh3)4 are most
effective in terms of mild reaction conditions, high yields and
shorter reaction times.4,5 Polymer stabilized colloidal palladium
nanoparticles have also been reported as catalysts for the Suzuki
coupling reaction.6 Poly(N-vinyl-2-pyrrolidone),7 polyoxometa-
lates,8 dendrimers and block copolymers have been used as
supporting matrices for stabilizing the colloidal nanoparticles.9 In
addition, nanosize palladium hollow spheres have also been
described as active catalysts.10 The most important potential
advantage of using colloidal nanoparticles for the Suzuki coupling
is to recover and reuse the catalysts. However, most of these
procedures require prolonged reaction times at elevated tem-
peratures and suffer lower yields. Furthermore, the reported
catalytic activity of the recovered catalysts is too low for them to be
reused for practical organic synthesis.7b Microwave heating has
recently been used as a method for increasing the reaction rates of
the Suzuki coupling reaction with homogenous catalysts.11 The
combination of using metal nanoparticles and microwave heating in
Suzuki coupling reaction was unprecedented.
The particle size and size distribution of the palladium
nanoparticles formed in the composite was analyzed with transmis-
sion electron microscopy (TEM). As shown in Fig. 1, narrowly
dispersed palladium nanoparticles were produced in the one-pot
synthesis. The average size of the palladium nanoparticles is about
3 nm and they are roughly spherical. Little change in the size of the
nanoparticles was observed as the molecular weight (Mw) of the
polymer used in the synthesis was varied from 42 000 to 84 000.
The PDHC–Pd catalyst displayed remarkable colloid stability in
storage and in the catalytic reactions. For example, they were found
to be stable on the lab shelf for months without any aggregation or
precipitation.
The catalytic activity of PDHC–Pd was examined in the Suzuki
coupling reaction.† As shown in Table 1, nearly quantitative yields
of biphenyl products for various substrates were observed in almost
all cases. The catalytic activity was preserved in the presence of a
From a mechanistic point of view, the most active catalytic
species in the nanoparticle catalyzed Suzuki coupling reactions
may be the polymeric coordination complexes rather than the
heterogeneous nanoparticles themselves. Examinations of the
morphology of the used nanoparticles in our lab and published by
others,7b indicated that: (1) there was an equilibrium between
nanoparticles and dissolved metal atoms in the reaction which led
to the dissolution of smaller nanoparticles and the growth of larger
ones; (2) the larger nanoparticles were catalytically inactive. We
found the selection of the polymer ligand for the catalyst to be
critical for achieving optimum activity. Poly(N,N-dialkylcarbodii-
† Electronic supplementary information (ESI) available: experimental
details and UV–vis spectrum. See http://www.rsc.org/suppdata/cc/b3/
b313210m/
Fig. 1 TEM analysis of palladium nanoparticles stabilized by poly(N,N-
dihexylcarbodiimide) (bar = 50 nm).
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C h e m . C o m m u n . , 2 0 0 4 , 3 9 8 – 3 9 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Table 1 Product yields for Suzuki cross coupling reaction catalyzed by
PDHC-Pd nanoparticles
Amount
of catalyst
(wt% wrt
halide)
Isolated
yield
(%)
Entry
R
X
RA
Time/h
1
2
3
4
5
6
7
8
H
H
H
I
I
Br
Br
H
H
H
H
H
H
HCO
Hc
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
20
40 min
23
24
18
19
20
20
97d
95e
98
95
84
90
99
98
Br
a
b
H
H
I
I
Fig. 3 Isolated yields of Suzuki coupling reactions of phenylboronic acid
and iodobenzene vs. number of recycles of the catalyst under microwave
heating at 100 °C for 40 min.
a 5,5A-Dibromo-2,2Abithiophene. b 3,5-Dibromopyridine. c Pd(PPh3)4 as
catalyst. d The yield from the first run, 2nd run yields 93% with TON 1600,
3rd run yields 95%, 4th run yields 92% and fifth run yields 90%.
e Microwave heating of the first run.
also be recycled for several times in the microwave heating
experiments. The activity of the recycled catalyst is shown in Fig.
3. With 40 min microwave heating in each cycle, we observed
lower yields for each of the consecutive reaction cycles, indicating
a significant decrease in the catalytic activity of the recycled
PDHC–Pd for the Suzuki coupling reaction when microwave
heating was employed. Narayanan et al. have observed a similar
decrease in the catalytic activity of polyvinylpyridine stabilized Pd
nanoparticle catalyzed Suzuki coupling reactions.7b Most likely the
intensive heating of the nanoparticles by microwave irradiation
promoted the size redistribution of the catalyst during the reactions.
However, we obtained good yields of the coupling reactions with
PDHC–Pd even after five recycles.
This work was supported by the US National Institute of Health
(DK61316-01) and the US National Science Foundation through
the NER program (DMR0210508). J.H. thanks the University of
Akron Research Foundation for a startup grant and a faculty
research fellowship. We thank Prof. S. Z. D. Cheng at the Polymer
Institute of the Univeristy of Akron for help with the TEM analysis.
We also thank Mr D. J. Giumento, Sr. of Personalchemistry Inc. for
technical support of the SmithCreator®.
Fig. 2 Isolated yields of Suzuki coupling reactions of phenylboronic acid
and iodobenzene vs. reaction time.
thio-containing substrate which is known to poison colloidal Pd
nanoparticle catalysts (entry 5). No obvious difference in the
catalytic activity of PDHC–Pd was observed when aryl iodides or
aryl bromides were used in the reactions. The turnover number of
the catalyst in the reactions was estimated to be over 1600 from the
substrate and catalyst ratios of the reactions. A distinctive
advantage of the polymer/nanoparticle composite over homoge-
neous catalysts such as Pd(PPh3)4 (entry 8) is that the catalyst can
be easily recovered by precipitation and filtration. The recovered
catalysts were found to be reactive in the coupling reactions even
after five reaction–recovery cycles (entry 1). While colloidal metal
catalysts tend to aggregate even in a single run of the Suzuki
coupling reaction,7 no significant aggregation of nanoparticles in
PDHC–Pd was observed in most of the reactions. A small amount
of black precipitate was observed in the coupling reaction between
5,5A-dibromo-2,2A-bithiophene and phenylboronic acid (entry 5),
presumably due to the poisoning of the catalyst by the thiophene
moiety.
Notes and references
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The PDHC–Pd catalyzed Suzuki coupling reaction between
phenylboronic acid and iodobenzene was also examined under
microwave heating conditions (entry 2). Excellent conversions
were achieved in about 40 min microwave irradiation. No
palladium black was observed in the reaction. In comparison, the
same coupling reaction required about 20 hours refluxing to
achieve the same degree of conversion (Fig. 2). The catalyst can
11 R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge and
J. Rousell, Tetrahedron Lett., 1986, 27, 279; A. Zhang and J. L.
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