The synthesis and applications of a micro-pine-structured nanocatalyst

Article abstract of DOI:10.1039/b814715a

Dendritic nanoferrites with a micro-pine morphology have been synthesized for the first time under microwave irradiation conditions without using any reducing or capping reagent; the nanoferrites were then functionalized and coated with Pd metal, which catalyzes various organic transformations. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b814715a

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The synthesis and applications of a micro-pine-structured nanocatalystw
Vivek Polshettiwar, Mallikarjuna N. Nadagouda and Rajender S. Varma*
Received (in College Park, MD, USA) 26th August 2008, Accepted 15th October 2008
First published as an Advance Article on the web 6th November 2008
DOI: 10.1039/b814715a
Dendritic nanoferrites with a micro-pine morphology have been
synthesized for the first time under microwave irradiation con-
ditions without using any reducing or capping reagent; the
nanoferrites were then functionalized and coated with Pd metal,
which catalyzes various organic transformations.
The self-assembly of nanoparticles, such as nanoclusters,
nanowires, nanobelts and nanotubes,1–3 is a modus operandi
for designing novel sensors, circuits and devices at the nano-
and microscale.4–7 Fractal structures are common in nature
across all areas, from self-assembled molecules, to the shapes
of coastlines, to the distribution of galaxies, and even to the
Fig. 1 TEM image of dendritic a-Fe2O3.
shapes of clouds.8 Dendritic hyperbranched structures are
formed by hierarchical self-assembly in a non-equilibrium
environment; techniques for assembling nanoparticles into
This material was synthesized using a hydrothermal techni-
que by simply heating K4[Fe(CN)6] in water at 150 1C under
MW irradiation conditions for 3 h, with a moderate yield
(40%). During the reaction, the [Fe(CN)6]4À ion, which has six
equivalent cyanide ions, dissociated in water and grew rapidly
along its six crystallographic directions, yielding the a-Fe2O3
particles. The morphology of these particles appeared to be
like a pine tree (Fig. 1), hence the term micro-pine particle.
Because of this unique morphology, these materials will have
significant applications in biomedical science and catalysis.
TEM images of the synthesized micro-pine a-Fe2O3 particles
show a single-crystal structure. A closer inspection of these
particles revealed well-defined and highly ordered branches
(Fig. 1(b) and (c)) distributed on both the sides. The TEM
observations also revealed that the micro-pine dendrites self-
assemble to form a six-fold symmetric structure (Fig. 1(a)).
Further structural characterization of this material was
carried out by powder XRD (Fig. 2(a)) and FT-IR
(Fig. 2(c)), which show that the a-Fe2O3 phase has a rhombo-
hedral structure (JCPDS 01-089-0596), with lattice parameters
of a = 0.50205 and c = 1.3457 nm.
superstructures include the use of sheer stress, solvent eva-
poration, lithography, templates, and optical, electrical and
magnetic fields.9–15 The study of self-assembled fractal pat-
terns in chemical synthesis has revealed that the size, shape
and chemical functionality of such structures make them
potential candidates for the design and invention of new
functional nanomaterials.1–7 In recent years, extensive re-
search has been performed on nanomaterials of various di-
mensions, such as magnetic metals and metal oxides, which
has led to substantial advances in nanomaterial synthesis.16,17
However, it is exigent to develop easy and sustainable ap-
proaches for building hierarchically self-assembled fractal
architectures of an assortment of materials.
Microwave (MW) chemistry has been widely used in syn-
thetic organic chemistry, with enhanced reaction rates, selec-
tivity and product yields.18 This technique is also useful for the
synthesis of high quality nanomaterials via direct MW heating
of their molecular precursors.19–22 However, to the best of our
knowledge, the hierarchical self-assembly of nanomaterials
under MW irradiation has not been reported. Engaged in
the development of greener and more sustainable pathways for
organic transformations18 and nanomaterials,23 herein we
report an easy and rapid synthesis of single-crystal a-Fe2O3
with a dendritic micro-pine nanostructure (Fig. 1) and its
catalytic application in various organic transformations under
MW irradiation conditions.
The effect of substrate concentration and reaction tempera-
ture on the morphology of the nanoparticles was also studied.
It was observed that with an increase in the concentration, the
sharpness of branched dendrite particles decreased and mate-
rial with slightly branched particles was formed (see the ESIw
for TEM images at various concentrations). The reaction
temperature plays a crucial role in this synthesis. At 150 1C,
moderate to good product yields were obtained in 3 h; how-
ever, below this temperature, the formation of nanoparticles
required extended reaction times.
Sustainable Technology Division, National Risk Management Research
Laboratory, U. S. Environmental Protection Agency, MS 443,
Cincinnati, Ohio 45268, USA. E-mail: polshettiwar.vivek@epa.gov.
E-mail: varma.rajender@epa.gov
w Electronic supplementary information (ESI) available: Experimental
procedures and spectral data of single-crystal a-Fe2O3, details of
various organic transformations, and catalyst recycling and leaching
tests. See DOI: 10.1039/b814715a
To investigate the utility of this new and highly structured
material, it was tested as a catalyst for organic transformations
involving various C–C coupling and hydrogenation reactions,
as these are significant processes in organic chemistry. The first
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alkynes, commonly called Suzuki, Heck and Sonogashira
reactions, respectively.24 The practical use of the ever increas-
ing number of tailor-made transition metal catalytic species is
connected with the problem of separation and reuse of the
rather costly catalyst systems. To overcome these problems,
we have explored this Pd-containing material as a hetero-
geneous catalyst for these coupling and hydrogenation
reactions (Scheme 2).
Micro-pine Pd displayed a high catalytic activity in Suzuki
reactions (ESI, Table 1w). Aryl halides with various functional
groups efficiently reacted with boronic acids to yield the
coupling products in good to excellent yields. 2-Iodothiophene
underwent a smooth reaction with various boronic acids,
providing
a useful pathway for the synthesis of aryl-
substituted thiophene heterocycles. The coupling of aryl halides
with alkenes (Heck reaction) occurred to yield the correspond-
ing products in good yields. A variety of substituted aryl
bromides and iodides reacted readily (ESI, Table 2w) and the
rates were barely influenced by the electronic effects of the
substituents on the aromatic ring of the halides, showing the
high activity of the catalyst. The utility of this nanocatalyst
was then explored in the Sonogashira reaction using a variety
of substrates (ESI, Table 3w), and excellent coupling products
were obtained within 45 min. The use of MW-assisted chem-
istry was due to the efficiency of the interaction of the polar
nanocatalysts with the MWs; the reaction mixture could be
rapidly heated under MW irradiation.18a,b
Fig. 2 The XRD (a and b) and FT-IR (c and d) of a-Fe2O3 and
functionalized a-Fe2O3, respectively.
step in the accomplishment of this goal was the functionaliza-
tion of the pine tips of synthesized a-Fe2O3 by an organic
functional group, which could then act as a ligand in metal-
catalyzed reactions (Scheme 1). The catalyst was prepared by
sonicating the nanoparticles with dopamine in water for 2 h,
followed by the addition of a Pd salt at basic pH. Material
with Pd nanoparticles on the tips of amine-functionalized
micro-pine ferrites was obtained in excellent yield. FT-IR
and XRD evidently confirmed the anchoring of dopamine
and Pd particles onto the ferrite surfaces (Fig. 2(b) and (d)).
The IR band at 2070 cmÀ1 is characteristic of linear CO
species adsorbed onto Pd and the band at 1407 cmÀ1 is due
to the C–N stretching vibration of the dopamine; XRD peaks
occurred at 2y = 151 (d = 5.05 A) and 251 (d = 3.56 A) for
the N–Pd complex. The weight percentage of Pd was found to
be 7.26% by ICP-AES analysis.
Metal-catalyzed hydrogenations of alkynes and alkenes,
which are generally catalyzed by homogeneous catalysts,
have received much interest in the past because of the immense
number of opportunities that exist to prepare high-value
products. Thus, there is an urgent need to develop less
expensive and easily available heterogeneous metal catalysts.
In view of this fact, we undertook the hydrogenation reactions
using the micro-pine Pd catalyst (ESI, Table 4w). The hydro-
genation of a range of unsaturated compounds was explored,
and the reactions proceeded smoothly at room temperature to
afford the desired products. Alkynes and alkenes were hydro-
genated to yield the corresponding alkanes in excellent yields.
Heterocyclic alkynes were also reduced in short reaction times
The formation of C–C bonds via Pd-catalyzed cross-
coupling reactions plays a crucial role in synthetic organic
chemistry. Particular attention has been paid to the coupling
reaction of aryl halides with boronic acids, alkenes and
Scheme 1 The synthesis of functionalized a-Fe2O3 with a Pd coating.
Scheme 2 Micro-pine Pd-catalyzed reactions.
Chem. Commun., 2008, 6318–6320 | 6319
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and this aspect of the protocol bodes well for the application
of this catalyst to the total synthesis of drug molecules. This
method can also be adopted for the reduction of nitro groups
to their corresponding amines, indicating the versatility of this
catalyst system.
tion conditions. Materials were readily prepared from inex-
pensive starting materials in water without using any reducing
or capping reagent. This synthetic concept could ultimately
enable the fine tuning of material responses to magnetic,
electrical, optical and mechanical stimuli. The nanoferrites
were then functionalized and coated with Pd metal, which
catalyzes various C–C coupling and hydrogenation reactions
with high yields. Also, the ease of recovery and high efficiency,
combined with the intrinsic stability of this material, make this
method robust and economic.
For practical applications of heterogeneous systems, the
lifetime of the catalyst and its level of reusability are very
important factors. To clarify this issue, we established a set of
experiments for the Suzuki reaction of iodobenzene and
phenylboronic acid using the micro-pine Pd catalyst. After
completion of the first reaction to afford the corresponding
product, the catalyst was recovered, washed with methanol
and finally dried at 50 1C. A new reaction was then performed
with fresh iodobenzene and phenylboronic acid under the
same conditions. The catalyst could be used at least five times
without any change in its activity. Similar experiments were
also conducted for the Heck (substrates: iodobenzene and
methyl acrylate), Sonogashira (substrates: iodobenzene and
phenyl acetylene) and hydrogenation (substrate: phenyl
acetylene) reactions. The nanocatalyst showed excellent
recyclability in all of the reactions and no catalyst deteriora-
tion was observed, confirming its high stability (see the ESIw).
The heterogeneity and metal leaching of this catalyst in the
Heck reaction of iodobenzene and methyl acrylate were
examined by the modified ‘hot filtration’ test. The reaction
was stopped at 25% conversion (10 min reaction time), and
after 1 min, the reaction mixture turned to a clear liquid as the
solid catalyst was deposited on the magnetic bar. Half of the
liquid reaction mixture was transferred to another reaction
tube. After an additional 20 min MW exposure at 100 1C, the
portion containing the nanocatalyst showed 95% conversion,
while the catalyst-free portion showed 30% conversion, evi-
dently proving the heterogeneity of catalyst. Metal leaching
was then studied by inductively coupled plasma-atomic emis-
sion spectroscopy (ICP-AES) analysis of the catalyst before
and after the reaction. The Pd concentration of the catalyst
was found to be 7.26% before the reaction and 7.12% after the
reaction, and there was no Pd detected in the final product,
confirming negligible Pd leaching. This is due to well-defined
amine binding sites located on the surfaces of the micro-
pine ferrites (Scheme 1), which act as pseudo-ligands by
non-covalent binding with the Pd through metal–ligand inter-
actions. The anchoring minimizes deterioration and metal
leaching, and allows efficient catalyst recycling. The most
important criterion in choosing a catalyst is the ease of metal
recovery. It would be preferable to use a micro-pine Pd
catalyst, provided that the reaction produces excellent yields
and that the catalyst leaves no remnants of metal in the end
product, since metal contamination is highly regulated by the
pharmaceutical industry. All of the above conditions were
satisfied by this recyclable micro-pine Pd catalyst.
V. P. is a postgraduate research participant at the National
Risk Management Research Laboratory administered by the
Oak Ridge Institute for Science and Education.
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