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Publisher’s version / la version de l'éditeur:
Journal of the American Chemical Society, 128, pp. 5279-5282, 2006
Metal supported on dendronized magnetic nanoparticles: highly
selective hydroformulation catalysts
Abu-Reziq, Raed; Alper, Howard; Wang, Dashan; Post, Michael L.
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Metal Supported on Dendronized Magnetic Nanoparticles:
Highly Selective Hydroformylation Catalysts
Raed Abu-Reziq,†Howard Alper,*,†Dashan Wang,‡and Michael L. Post‡
Contribution from the Centre for Catalysis Research and InnoVation, UniVersity of Ottawa,
10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5, and Institute for Chemical Process and
EnVironmental Technology, National Research Council of Canada, 1200 Montreal Road,
Ottawa, Ontario, Canada K1A OR6
Received January 16, 2006; E-mail: email@example.com
Abstract: A method for homogenizing heterogeneous catalyst is described. The method is based on growing
polyaminoamido (PAMAM) dendrons on silica-coated magnetic nanoparticles. After the dendronizing
process, the silica-coated magnetic nanoparticles are more stable and more soluble in organic solvents.
The dendronized particles are phosphonated, complexed with [Rh(COD)Cl]2, and applied in catalytic
hydroformylation reactions. These new catalysts are proven to be highly selective and reactive.
During the past two decades, a great deal of attention has
been paid to developing methods for heterogenizing homoge-
neous catalysts in order to combine the advantages of both
homogeneous and heterogeneous catalysis.1Among these
methods, the binding of catalysts to organic polymer solids2or
inorganic solids3is widely used. Although the heterogenized
catalysts can be recycled and easily separated from the reaction
mixtures, they are significantly less reactive and selective than
their homogeneous counterparts. For this reason, there is a need
to find new methods and strategies in order to overcome these
Dendrimers, a class of macromolecules with special properties
and functions, have been utilized for several applications4
including homogeneous catalysis.5Interestingly, in many cases,
the soluble dendritic catalysts were found to be more efficient
or selective than the traditional analogues of metal complexes.
In recent years, dendrimers immobilized on silica or polymers
have been investigated intensively by our group6and others7-10
and applied to several catalytic organic transformations. These
new catalytic materials are sometimes highly efficient in terms
of reactivity and selectivity and are easily recyclable.
Magnetic nanoparticles that can be magnetized in the presence
of an external magnet have been studied extensively for various
biological applications such as magnetic resonance imaging,11
drug delivery,12biomolecular sensors,13bioseparation,14and
†University of Ottawa.
‡National Research Council of Canada.
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Published on Web 03/22/2006
10.1021/ja060140u CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 5279-5282 9 5279
magneto-thermal therapy.15Recent reports show that magnetic
nanoparticles are efficient supports for catalysts and can facilitate
their separation from the reaction media after magnetization with
a permanent magnetic field.16-19
We now describe a method based on combining the features
of magnetic nanoparticles and dendrimers. The objective of this
novel strategy is to homogenize heterogeneous catalysts as
follows: use magnetic nanoparticles to minimize the support
for the immobilization of the catalyst, while at the same time
keeping it easily separable. Second, by using dendrimers on a
support, one can enhance the solubility of the support in an
organic solvent. The dendrimerization process can produce
organic arms on the support enhancing its compatibility with
the medium, and the surrounding linked metallic complexes can
behave as real homogeneous catalysts. We demonstrate this by
growing a polyaminoamido (PAMAM) dendron on nanomag-
netite, Fe3O4, coated by a silica shell.
Results and Discussion
The magnetite nanoparticles of 8-12 nm were prepared by
coprecipitation of iron(II) and iron(III) ions in basic solution at
85 °C using the method described by Massart.20Our initial
attempts to grow PAMAM dendrimers directly on the magnetic
nanoparticles failed, due to coagulation of the particles and
solubility problems in organic media. These problems could be
solved by coating the magnetic nanoparticles with silica. The
process of the coating was performed by suspending the
magnetic particles in 2-propanol after coating with poly-
(vinylpyrrolidone) (PVP) and mixing with tetraethoxysilane to
start creating a silica shell under basic conditions via a sol-gel
process.21TEM analysis (see Supporting Information) confirms
the structure of the core-shell of the obtained particles, and
thus the core contains the nanomagnetite. The average size of
the coated particles is 50-60 nm. In addition, the particles
before and after coating with silica were analyzed by X-ray
powder diffraction. While the XRD pattern of the bare magnetic
particle shows characteristic peak positions and relative intensity,
the XRD pattern of the coated nanoparticles has the same
characteristic peaks with an additional broad peak between 2θ
) 15 and 2θ ) 30 which is attributed to the amorphous silica
shell (see Supporting Information). Silanation of these silica-
coated particles with (3-aminopropyl)triethoxysilane under reflux
in toluene for 24 h gave 0.25-0.27 mmol/g of amino groups
(determined by back-titration). PAMAM dendrons, up to
generation three, were constructed on the particles containing
amino groups as initiators by two subsequent steps for each
generation: Michael-type addition of methyl acrylate to produce
the amino propionate ester and amidation of the resulting ester
groups with ethylenediamine (see Scheme 1). During the
advance in the dendronizing process, it was noticed that the
solubility of the magnetic nanoparticles in organic solvents is
enhanced, and they become more stable for a prolonged time
in organic media.
The growth of the PAMAM dendrons was followed by
infrared spectroscopy and thermal gravimetric analysis. The IR
spectra G(0)-G(3) are shown in Figure 1. In the spectra of half
generations there is a well-defined absorption at 1738 cm-1
attributed to CO stretching of the ester groups. Amidation
affords the full-generations with the disappearance of the
absorption at 1738 cm-1in the spectrum of G(1), but G(2) and
G(3) show a small amount of ester groups. An absorption at
1668 cm-1is due to CO stretching of a secondary amide. This
absorption also appeared in the spectra of G(0) due to PVP
molecules that are encapsulated between the magnetite core and
the shell of silica.
TGA analysis (Figure 2) shows that there was an increase in
the organic content when the growth of the PAMAM dendrons
was increased to higher generations. The silica coated magnetite
contained 6.4% of organic material which belongs to the PVP
adsorbed on the magnetite nanoparticles. The organic content
(15) (a) Hiergeist, R.; Andra, W.; Buske, N.; Hergt, R.; Hilger, I.; Richter, U.;
Kaiser, W. J. Magn. Magn. Mater. 1999, 201, 420-422. (b) Jordan, A.;
Scholz, R.; Wust, P.; Fahling, H.; Felix, R. J. Magn. Magn. Mater. 1999,
(16) Yoon, T. J.; Lee, W.; Oh, Y. S.; Lee, J. K. New J. Chem. 2003, 27, 227-
(17) Stevens, P. D.; Fan, J.; Gardimalla, H. M. R.; Yen, M.; Gao, Y. Org. Lett.
2005, 7, 2085-2088.
(18) Stevens, P. D.; Li, G.; Fan, J.; Yen, M.; Gao, Y. Chem. Commun. 2005,
(19) Hu, A.; Yee, G. T.; Lin, W. J. Am. Chem. Soc. 2005, 127, 12486-12487.
(20) Massart, R. IEEE Trans. Magn. 1981, 17, 1247-1248.
(21) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183-186.
Scheme 1. Dendronizing Process of the Silica-Coated Magnetic Nanoparticles
Figure 1. FT-IR spectra of silica-coated nanomagnetite supported PAMAM
A R T I C L E S
Abu-Reziq et al.
5280 J. AM. CHEM. SOC.9VOL. 128, NO. 15, 2006
for the G(0), G(1), G(2), and G(3) steps was 7.3%, 13.3%,
14.1%, and 16.6%, respectively (for more details see Supporting
The dendrons on silica-coated nanomagnetite were phospho-
nated by reaction of the terminal amino groups with diphe-
nylphosphinomethanol prepared in situ from diphenylphosphine
with paraformaldehyde.6ICP analysis showed that the content
of phosphorus in G(0), G(1), G(2), and G(3) was 0.2, 0.16,
0.189, and 0.19 mmol/g, respectively. Although the amino
groups increased with the increase of the growth of the dendrons,
the phosphorus content was almost the same. We attribute the
incomplete phosphination reaction to steric effects resulting from
the growth of the dendrons to higher generations. The phos-
phonated dendrons were complexed by mixing at room tem-
perature with [Rh(cod)Cl]2in dry and degassed toluene for 5
h. The resultant complexes were tested in hydroformylation
reactions using a 1:1 mixture of carbon monoxide and hydrogen
pressurized to 1000 psi. The optimization of the reaction was
carried out using styrene as the model substrate and the rhodium-
complexed G(0) complex as the catalyst, and the results are
shown in Table 1. The reaction could be performed at room
temperature, but it was slow and needed 48 h for completion
(Table 1, entries 1 and 2). Nevertheless, the reaction was highly
regioselective for the branched aldehyde. Increases in temper-
ature decrease the regioselectivity of the reaction and increase
the reactivity of the catalyst (Table 1, entries 2-4), but this
decrease is not considered as significant and the reaction was
still highly selective to the branched aldehyde, even at 50 °C.
The solvent plays an important role in the reactivity and
selectivity of the catalyst; e.g., polar solvents negatively affect
the reaction resulting in reduction in reactivity and selectivity
(Table 1, entries 7-10), and nonpolar solvents are very efficient
media for the same reaction (Table 1, entries 4-6). The highest
reactivity and selectivity could be achieved by using dichlo-
romethane as the solvent. This solvent polarity effect was
observed even using toluene or benzene. Benzene is slightly
more polar than toluene and could decrease the conversion of
the reaction and the yield of the desired branched product (Table
1, entry 6).
The hydroformylation of various substrates with G(0)-G(3)
catalysts were carried out at 50 °C with a 1:1 mixture of carbon
monoxide and hydrogen under a total pressure of 1000 psi. The
results are shown in Table 2. For the hydroformylation of
styrene, the generation 1 catalyst was found to be the most
efficient (Table 2, entry 2), although the other catalysts were
still considered selective and reactive. In contrast to previous
studies6lin which there was a significant decrease observed in
activity and selectivity going to higher generations, the present
catalyst systems, at higher generations, essentially retained their
reactivity or selectivity. Apparently there is no significant
electronic effect for the hydroformylation reaction as shown by
results for the reactivity and selectivity of styrenes substituted
with electron-donating or -withdrawing groups (Table 2, entries
7-10). In the case of 4-vinylbenzoic acid (Table 2, entry 11)
the branched isomer was formed almost exclusively. The
hydroformylation of 2-bromostyrene was slow but highly
regioselective for the branched aldehyde (Table 2, entry 12),
and we believe that the decrease in reactivity in this case may
be due to steric effects. The substrates 4-vinylbiphenyl and
2-vinylnaphthalene undergo hydroformylation efficiently, in
terms of both reactivity and regioselectivity (Table 2, entries
13 and 15). In addition the lowest reactivity was observed for
the hydroformylation of 9-vinylanthracene (Table 2, entry 16).
Vinyl acetate and vinyl benzoate also gave the isomeric
aldehydes at low conversion (Table 2, entries 14 and 2), but
the selectivity toward the branched products was still high.
Figure 2. Weight loss as measured by TGA for silica-coated nanomag-
netite-supported PAMAM dendrons. (a) After coating with silica; (b) after
silanation; (c) G(1), (d) G(2); and (e) G(3).
Table 1. Hydroformylation of Styrene by a Rhodium-Complexed
G(0) Dendron Supported on Silica-Coated Magnetite
a1 mmol of styrene, 10 mL of solvent, 1000 psi of 1:1 H2:CO, 50 mg
of catalyst.bDetermined by1H NMR and GC.cDetermined by1H NMR.
Table 2. Hydroformylation of Various Substrates by
Rhodium-Complexed Dendrons Supported on Silica-Coated
a1 mmol of the substrate, 10 mL of dichloromethane, 50 °C, 1000 psi
of 1:1 H2:CO, 50 mg of catalyst, 16 h.bDetermined by1H NMR and GC.
cDetermined by1H NMR.
Metal Supported on Dendronized Magnetic Nanoparticles
A R T I C L E S
J. AM. CHEM. SOC. 9 VOL. 128, NO. 15, 2006 5281
In comparison with previous studies,6h,k,lthe new catalysts are Download full-text
much more reactive and selective. Unfortunately, our catalyst
is not selective for the hydroformylation of linear aliphatic
olefins, e.g., 1-octene produces isomeric aldehydes in a ratio
of 2.2:1 L:B.
Importantly, G(0) and G(1) catalysts can be recycled under
the same pressure, and at 40 °C, for up to five runs without
significant loss in activity or selectivity. In the fifth run the G(0)
catalyst was far less reactive (see Table 3). The recycling was
very simple, and thus after each run the catalyst was separated
magnetically, washed with dichloromethane, and after flushing
with nitrogen, used for the next run. After the reactions were
complete, the solution was reddish-brown in color, and after
magnetic separation, the solution became very clear.
In conclusion, PAMAM dendrons were grown for the first
time on silica-coated magnetic nanoparticles, for up to three
generations. After phosphination of the heterogenized dendron
with various generations, it was complexed with rhodium and
used as the catalyst for hydroformylation reactions. The reactiv-
ity and selectivity of the catalyst were very high. In addition,
the new dendronized magnetic nanoparticles were applied to
catalytic processes, but we believe they will find application in
many other domains.
Nanomagnetite particles of 8-12 nm were prepared according to
Massart’s method.20Coating of the nanoparticles with PVP was
achieved following the procedure of Lee and co-workers.22
Large-Scale Coating of the Magnetite Nanoparticles with Silica.
A solution of 1 g of PVP-coated magnetite nanoparticles was suspended
in 2 L of 2-propanol containing 50 mL of concentrated ammonia (28%).
The solution was divided into three portions, and each was sonicated
for 1 h. After combining the solutions, 5 mL (22.4 mmol) of
tetraethoxysilane (TEOS) in 100 mL of 2-propanol were added
dropwise, over a 3-h period, to the magnetite solution using a
mechanical stirrer. The stirring was continued for another 3 h, and then
the silica-coated nanoparticles were separated magnetically after
decantation of the solution and washed three times with TDW. The
final product (2 g) was obtained after drying at room temperature, under
a vacuum of 0.2 mmHg, for 24 h.
Silanation of the Silica-Coated Magnetite Nanoparticles. 10 g of
dry silica-coated magnetite powder were suspended in 200 mL of dry
toluene. After sonication for 35 min, a solution of 2.5 mL (10.68 mmol)
of (3-aminopropyl) triethoxysilane was added under mechanical stirring.
The solution was heated at 105 °C for 20 h. The particles were separated
by an external magnet after cooling to room temperature, washed 3
times with dry methanol, and dried under vacuum for 24 h. 0.25-0.27
mmol/g of amino groups was found (as determined by back-titration).
General Procedure for the Preparation of PAMAM Dendrons
on the Silica-Coated Magnetite Nanoparticles. 8 g of silanated
particles were suspended in 120 mL of dry methanol, and after
sonication for 35 min, 10 mL (111 mmol) of methyl acrylate were
added and the mixture was heated at 50 °C for 5 days. After cooling
to room temperature, the particles were separated magnetically, washed
3 times with dry methanol, and dried under a vacuum for 24 h to give
G(0.5). The dried material was then suspended again in 120 mL of
methanol and sonicated for 35 min. After the addition of 17.2 mL (257
mmol) of ethylenediamine dropwise at room temperature, the solution
was heated at 50 °C for 5 days. The magnetic material was magnetically
separated, washed 3 times with dry methanol, and dried under a vacuum
to give G(1). The second and third generations were prepared following
the same manner, using a 50-fold excess of methyl acrylate and a 125-
fold excess of methylenediamine with respect to the amount of amino
General Procedure for the Phosphination of PAMAM Dendrons
on the Silica-Coated Magnetite Nanoparticles. 10 g (53.7 mmol) of
diphenylphosphine were added to 1.7 g (54 mmol) of paraformaldehyde
in 50 mL of dry and degassed methanol. The solution was heated for
1 h under N2 at 65 °C. After the solution was cooled to room
temperature, 2 g of the silanated silica-coated magnetite nanoparticles
were added and the mixture was heated at reflux for 3 days. The
resulting phosphinated particles were magnetically separated and
washed 4 times with degassed methanol and dried under vacuum for
General Procedure for the Complexation of Phosphinated
PAMAM Dendrons on the Silica-Coated Magnetite Nanoparticles.
2 g of the phosphinated particles containing 0.4 mmol of phosphorus
and 0.1 g (0.2 mmol) of [Rh(cod)Cl]2were added to 40 mL of dry and
degassed toluene. The mixture was stirred for 5 h, and the particles
were magnetically separated, washed 3 times with dry toluene, and
dried under a vacuum for 24 h.
General Procedure for the Hydroformylation reaction. 50 mg
of the catalyst, which was homogenized in 10 mL of dichloromethane
by sonication for 10 min, and 1 mmol of appropriate substrate were
placed in a 45 mL glass-lined autoclave. After sealing, the autoclave
was purged 3 times with carbon monoxide and pressurized to 1000 psi
with a 1:1 of mixture of carbon monoxide and hydrogen. The autoclave
was placed in an oil bath preset to the desired temperature. After the
appropriate reaction time, the autoclave was cooled to room temperature
and the gases were released. The catalyst was separated magnetically,
and the solution was concentrated by evaporation of the solvent. The
catalyst was washed with dichloromethane, flushed with a nitrogen
stream, and used for subsequent cycles.
Acknowledgment. We are grateful to Sasol Technology and
to the Natural Sciences and Engineering Research Council of
Canada for support this research.
Supporting Information Available: DTA-TG curves for the
dendronized magnetic nanoparticles, XRD patterns, and TEM-
images for the silica-coated magnetic nanoparticles. This
material is available free of charge via the Internet at
(22) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M. H.; Lee, J. K.
Angew. Chem., Int. Ed. 2005, 44, 1068-1071.
Table 3. Recycling of the Rhodium-Complexed Dendrons
Supported on Silica-Coated Magnetite Nanoparticles for the
Hydroformylation of Styrenea
a1 mmol of styrene, 10 mL of dichloromethane, 1000 psi of 1:1 H2:
CO, 40 °C, 50 mg of catalyst, 20 h.bDetermined by1H NMR and GC.
cDetermined by1H NMR.
A R T I C L E S
Abu-Reziq et al.
5282 J. AM. CHEM. SOC.9VOL. 128, NO. 15, 2006