Iron/iron oxide nanoparticles: A versatile support for catalytic metals and their application in Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1039/b922462a

Iron/iron oxide core-shell nanoparticles (Fe@Fe_xO_y) are a versatile support for immobilizing catalytic metals. Fe@Fe _xO_y decorated with Pd (Fe@Fe_xO_y/Pd) exhibits high catalytic activity toward Suzuki-Miyaura cross-coupling reactions in aqueous solution at room temperature in air. The Royal Society of Chemistry.

Full text of DOI:10.1039/b922462a

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Iron/iron oxide nanoparticles: a versatile support for catalytic metals
and their application in Suzuki–Miyaura cross-coupling reactionsw
Shuai Zhou, Melanie Johnson and Jonathan G. C. Veinot*
Received (in Berkeley, CA, USA) 27th October 2009, Accepted 19th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b922462a
Iron/iron oxide core–shell nanoparticles (Fe@FexOy) are a
versatile support for immobilizing catalytic metals. Fe@FexOy
decorated with Pd (Fe@FexOy/Pd) exhibits high catalytic
activity toward Suzuki–Miyaura cross-coupling reactions in
aqueous solution at room temperature in air.
with a reduction potential more positive than that of iron
(e.g., Co, Ni, Cu, Rh, Ru, Pd, Ag, Pt and Au).
The Suzuki–Miyaura cross-coupling reaction7 has become
arguably one of the most powerful synthetic methods for
preparing biaryl compounds, which constitute a wide range
of natural products,8 pharmaceuticals,9 and polymers.10
Homogeneous palladium complexes have been widely
explored as catalysts for this reaction.11 Despite the high
activity and selectivity of these systems, they have limited
utility in industrial processes because of challenges associated
with their removal, recovery, and recycling. Complicating
matters, many ligands used in catalyst design (e.g., phosphines)
are air sensitive, challenging to prepare and/or costly, limiting
suitability of such materials for large-scale application.12 To
overcome these issues, many heterogeneous catalysts have
been developed, including inorganic solid (e.g., charcoal,
zeolites and metal oxides) supported, and polymer-stabilized
systems.13 Palladium nanoparticles prepared in situ also show
promise.14
Nanomaterials find application as catalysts for chemical
transformations in a wide range of fields including the petro-
chemical industry,1 pollutant removal,2 and the production of
fine chemicals.3 Generally, these catalysts consist of nanoscale
active components (typically metals or metal oxides) dispersed
onto a solid support material. Recently, transition metals
supported on magnetic nanoparticles have emerged as promising
catalysts because of the combined advantages of high surface
area-to-volume ratio and facile magnetic recovery.4,5
In our earlier publications, we reported iron oxide capped
iron nanoparticles (Fe@FexOy) sequestered transition metal
ions from aqueous and organic media, as well as coordinating
environments.6 The accepted sequestration mechanism
involves coordination of metal ions to the iron oxide surface
followed by reduction of these coordinated ions to their
metallic state by the iron core (Scheme 1). This process gives
rise to Fe@FexOy particle surfaces bearing spectroscopically
detectable zero valent metals. Examples of catalytic metals
being deposited onto magnetic nanoparticles have appeared
in the literature, however reported procedures require the
addition of external reducing agents and surface modification
to realize deposition of the metals.5 Uniquely, our approach
does not use any reducing reagents or surface modification
because of the established reducing nature and coordinating
ability of the Fe@FexOy support. Highlighting the versatility
of our method, in principle, it is possible to deposit any metal
As a first demonstration of the catalytic capabilities of
metal decorated Fe@FexOy, we report the application of
Fe@FexOy/Pd prepared via direct reduction of Pd2+ by size
polydisperse Fe@FexOy as an efficient catalyst for the
Suzuki–Miyaura cross-coupling reaction. Adding to the
appeal of this catalyst, the reaction proceeds in aqueous
solution at room temperature in air. Furthermore, the
particles are conveniently recycled from the reaction mixture
via centrifugation, and reused three times without substantial
loss of activity.
Briefly, Fe@FexOy/Pd were prepared as follows: 0.01 mmol
of Pd(NO3)2, 2 mg of Fe@FexOy, and 5 mL of aqueous KOH
solution (pH 10.5) were mixed and sonicated for 20 minutes.
The reaction mixture was subsequently centrifuged, and the
black precipitate was washed with distilled water once and
directly used in the Suzuki–Miyaura coupling reactions under
investigation.
Fig. 1a shows a representative bright-field transmission
electron micrograph of Fe@FexOy/Pd, showing particle
diameters within the range of 3–45 nm. Individual Pd nano-
particles are not observed, although the presence of Pd was
confirmed by EDX (Fig. S6, ESIw). This suggests Pd domains
are small (o1 nm), making them undetectable by standard
imaging techniques.5c X-Ray photoelectron spectroscopy
(XPS) was used to determine the oxidation state of surface
borne palladium species. Fig. 1b shows the Pd 3d region of a
typical Fe@FexOy/Pd XP spectrum. Emissions at 335.4 and
340.7 eV tailing to higher binding energies are characteristic of
metallic Pd0.15 XPS analysis of the Fe 2p and O 1s regions
indicates the nanoparticle surface structure consists of mostly
Scheme
1 The accepted mechanism for deposition of metallic
domains onto the surface of Fe@FexOy.
Department of Chemistry, University of Alberta, Edmonton, Alberta,
Canada T6G 2G2. E-mail: jveinot@ualberta.ca;
Fax: +1 780-492-8231; Tel: +1 780-492-7206
w Electronic supplementary information (ESI) available: Experimental
procedures, TEM, XPS, EDX, GC-MS spectra, 1H NMR and
13C NMR of the coupling products. See DOI: 10.1039/b922462a
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coupling product in moderate yield at 80 1C (entry 13,
Table 1). The blank reaction of bromobenzene with phenyl-
boronic acid using Fe@FexOy without Pd resulted in no
reaction (entry 14, Table 1). Five-fold scale-up of the
reaction of bromobenzene with phenylboronic acid did not
compromise yield (i.e., 95%). The Fe@FexOy/Pd system
shows improved activity over commercial Pd/C powder under
identical reaction conditions (Table S1, ESIw). Adding to the
appeal of Fe@FexOy/Pd, residual Pd content in the crude
solid product (0.31 ppm, entry 1, Table 1) is far below the limit
mandated by the European Agency for the Evaluation of
Medicinal Products (i.e., 5 ppm).17
Fig. 1 (a) Bright-field TEM image of Fe@FexOy/Pd before reaction,
scale bar 20 nm. (b) Shirley background-subtracted high-resolution Pd
3d region of the XP spectrum of Fe@FexOy/Pd (dotted line), fit solid
lines indicate metallic Pd(0).
Catalyst recycling was performed to evaluate the reusability
of Fe@FexOy/Pd in the Suzuki–Miyaura coupling of
bromobenzene with phenylboronic acid. After each cycle, the
catalyst was separated from the reaction mixture by centri-
fugation, washed with distilled water and stored in an argon
atmosphere between reactions. As summarized in Table 2, we
observed only a slight loss of activity after the third reaction.
The mechanism for reactions catalyzed by heterogeneous
palladium is the subject of much scientific interest and the
exact identity of the active species remains unclear (i.e., is it
heterogeneous or quasi-homogeneous). Currently, it is widely
accepted that leached palladium species are the active catalysts
in most coupling reactions employing solid-supported
palladium.13a,18 Following reaction, the dissolved palladium
is subsequently redeposited onto the support. For the present
catalyst system, the leached palladium content in the filtrate
from a one hour reaction of bromobenzene with phenyl-
boronic acid was 1.87 ppm (i.e., 1.76% of initial catalyst
loading, as determined by inductively coupled plasma mass
spectrometry (ICP-MS)). In contrast, when the same quantity
of Fe@FexOy/Pd was reacted for one hour in the absence of
bromobenzene and phenylboronic acid, only 0.15 ppm of Pd
was detected. From this we conclude that Pd leaching is
facilitated by oxidative addition of bromobenzene to the Pd0
surface.13a,17 Comparing the Pd content (0.54 ppm vs. 1.87 ppm)
in the filtrate solutions from four hours (i.e., complete) and
one hour reactions supports the proposal that leached Pd is
redeposited onto Fe@FexOy after reaction. Reusing the
filtrate with 0.54 ppm Pd as catalyst for the reaction results
in 12% yield, illustrating that reactivity is in fact caused by the
Fe@FexOy/Pd particles themselves acting as ‘‘source’’ and
‘‘sink’’ for Pd.19
Fe3O4 and FeOOH (Fig. S3 and S4, ESIw).16 After 5 reactions,
we note no detectable differences of the catalyst by XPS, TEM
and EDX (Fig. S2, S6, and S7, ESIw) and conclude the
material is consistent throughout.
Reactions of various aryl halides with aryl boronic acids
were evaluated using 0.5 mol% Fe@FexOy/Pd in a H2O/EtOH
(1 : 1) solvent mixture at room temperature in air (Table 1).
Aryl bromides and iodides bearing either electron-withdrawing
or donating groups afforded cross-coupling products in high
yields (entries 1–5 and 9–12, Table 1). As expected, electron-
rich aryl boronic acids furnished coupling products in good
yields (entries 6–8, Table 1). Steric hindrance present in the
ortho-methylated aryl bromide and iodide influenced the
reactivity resulting in a lower yield (entries 5 and 12,
Table 1). No biphenyl was obtained from reaction of aryl
chlorides when the reaction was performed at room tempera-
ture, however activated 4-nitrophenyl chloride provided the
Table 1 A summary of Suzuki–Miyaura coupling reactionsa
Entry R1
X
R2
Time/h Yieldb (%) TOF/hÀ1
1
2
3
4
5
6
7
8
H
4-NO2
4-COMe Br
4-OMe
2-Me
H
H
H
H
4-NO2
4-OMe
2-Me
4-NO2
H
Br
Br
H
H
H
H
H
4
4
4
4
4
4
4
4
2
2
2
2
36
4
95
96
96
93
86
91
89
94
98
99
97
84
24
0
47.5
48
48
46.5
43
45.5
44.5
47
98
99
97
84
1.33
0
In conclusion, we have demonstrated that Fe@FexOy can
be used as a versatile support for immobilizing catalytic
metals. The immobilization is readily achieved via
Br
Br
Br 2-OMe
Br 3-OMe
Br 4-OMe
I
I
I
I
Cl
Br
Table 2 A summary of catalyst reuse for Suzuki–Miyaura coupling
reaction of bromobenzene with phenylboronic acida
9
H
H
H
H
H
H
10
11
12
13c
14d
Run
Time/h
Yieldb (%)
TOF/hÀ1
1
2
3
4
5
4
4
4
4
4
95
93
94
88
83
47.5
46.5
47
44
41.5
a
Reaction conditions: 2.0 mmol aryl halide, 2.4 mmol aryl boronic
acid, 5.0 mmol K2CO3, 0.5 mol% Fe@FexOy/Pd, H2O/EtOH (1 : 1),
b
room temperature. Isolated yield, average of minimum two inde-
c
a
b
pendent reactions with deviation r 2%. 80 1C, average of two
Reaction conditions: see Table 1. Isolated yield, average of two
independent tests.
independent reactions with 4% deviation. d 2 mg Fe@FexOy without Pd.
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coordination of metal ions to the iron oxide surface and
subsequent reduction of these coordinated ions to their
metallic state by the iron core. As an example of their catalytic
potential, Fe@FexOy decorated with spectroscopically
detectable metallic Pd domains was evaluated and found to
exhibit high catalytic activity toward Suzuki–Miyaura cross-
coupling reactions in aqueous solution at room temperature in
air. The present catalytic system effectively couples the
advantages of heterogeneous (e.g., low cost, air-stability, easy
separation, and good reusability) and homogeneous systems
(e.g., high yield), making it a promising material for practical
application. We are currently investigating other catalytic
metals and metal combinations in an effort to demonstrate
the versatility of the Fe@FexOy support.
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Natural Sciences and Engineering Research Council of
Canada (NSERC) and Department of Chemistry at University
of Alberta are thanked for funding. MJ acknowledges
scholarship from Alberta Ingenuity Fund. We thank
W. Moffat, C. Turk and D. Zinz at the Analytical &
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Chem. Commun., 2010, 46, 2411–2413 | 2413