Colloidal cobalt nanoparticles: a highly active and reusable
Pauson–Khand catalyst†
Sang-Wook Kim,a Seung Uk Son,b Su Seong Lee,a Taeghwan Hyeon*a and Young Keun Chung*b
a School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Seoul
151-742, Korea. E-mail: thyeon@plaza.snu.ac.kr
b School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742,
Korea. E-mail: ykchung@plaza.snu.ac.kr
Received (in Cambridge, UK) 21st August 2001, Accepted 18th September 2001
First published as an Advance Article on the web 11th October 2001
A new Pauson–Khand catalyst based on colloidal cobalt
nanoparticles has been developed; the catalyst is highly
effective for many intra- and inter-molecular Pauson–
Khand reactions and can be recycled and reused many times
without losing catalytic activity.
A transmission electron microscopic (TEM) image (see ESI†)
confirmed that the particles are well separated and that they are
nearly monodisperse, having a mean diameter of 8 nm.
The intramolecular P–K reaction of an enyne was investi-
gated as a test reaction [eqn. (1)]. Table 1 summarizes the results
of this reaction under various reaction conditions.
The development of uniform nanometer size particles has been
intensively pursued because of their technological and funda-
mental scientific interest.1 They have a characteristic high
surface-to-volume ratio, and consequently a large fraction of the
metal atoms are at the surface and hence are available for
catalysis.2 Many colloidal transition metal nanoparticles, espe-
cially noble metals, have been applied as catalysts for the
hydrogenation of olefins,3 for carbon–carbon coupling reac-
tions and for other reactions.4 However, colloidal nanoparticles
of first-row transition metals have seldom been applied as
catalysts. Here we present the application of colloidal cobalt
nanoparticles for catalytic Pauson–Khand reactions.
(1)
Entry 1 shows that colloidal cobalt is very active with an
isolated yield of product of 97% at 130 °C and a CO pressure of
5 atm. The colloidal cobalt catalyst is more active than
heterogeneous catalysts based on metallic cobalt supported on
mesoporous silica or charcoal.7 Using these heterogeneous
catalysts, a CO pressure of > 20 atm was required to achieve a
yield of > 95% under otherwise identical reaction conditions.
To check the recyclability, the catalyst was separated and reused
several times (entries 2–5). The results shown in Table 1
confirm that the catalyst maintained its high activity even after
five cycles of recycling and reuse. A TEM image of the catalyst
after running four reaction cycles showed that the particle size
of the cobalt nanoparticles was unchanged and confirmed that
no particle agglomeration was observed (ESI†). However, when
either the reaction temperature was decreased to 110 °C or the
CO pressure was lowered to 3 atm, the catalytic activity was
found to decrease significantly (entries 6–9). Thus, to preserve
a high catalytic activity, the reaction temperature and CO
pressure have to be maintained at a minimum of 130 °C and 5
atm of CO. Nevertheless, the colloidal cobalt catalyst still
sustained a degree of activity at 3 atm of CO pressure and
130 °C, with 45% yield of product being obtained.
Pauson–Khand (P–K) reactions, the cycloaddition of alkynes
with alkenes and carbon monoxide to cyclopentenones, have
been recognized as one of the most important methodologies in
the synthesis of cyclopentenone derivatives.5 Several research
groups, including ourselves, have developed the catalytic
version of P–K reactions.6 Various catalysts applied in P–K
reactions include the most popular dicobalt octacarbonyl and
several other carbonyl-ligand containing organometallic com-
pounds. Very recently, we demonstrated the first heterogeneous
catalytic P–K reactions using metallic cobalt supported on
either mesoporous silica or charcoal.7 Even though these
heterogeneous metallic cobalt catalysts exhibit good perform-
ance for many intramolecular and intermolecular P–K reactions,
they require extreme reaction conditions of CO pressures > 20
atm and temperatures > 120 °C. For diverse applications of P–K
reactions, the development of recyclable heterogeneous cata-
lysts able to work in much milder conditions is desirable.
Several synthetic methods have been applied in the fabrica-
tion of colloidal cobalt nanoparticles, including the thermal and
sonochemical decomposition of organometallic compounds,8
and the reduction of metal salts.9 In this report, we have applied
a recently developed synthetic method to fabricate cobalt
nanoparticles. This method comprises the formation of seed
particles at low temperature and further aging at high tem-
perature. For the synthesis of cobalt nanoparticles, Co2(CO)8
(2.04 g, 6 mmol), oleic acid (1.68 g, 6 mmol), trioctylphosphine
(0.6 mL) and dioctyl ether (20 mL) were mixed with stirring at
room temperature. The mixture was heated slowly to reflux and
the refluxing was maintained for 30 min, after which the
reaction mixture was allowed to cool to room temperature. A
black precipitate was obtained by adding 50 mL of ethanol
followed by washing with 20 ml of acetone and drying in vacuo.
Table 1 Intramolecular Pauson–Khand reactions under various con-
ditionsa
Entry
Catalyst
T/°C
Pressure/atm Yieldb (%)
1
2
3
4
5
6
7
8
9
Colloidal cobalt
Recovered from #1
Recovered from #2
Recovered from #3
Recovered from #4
Colloidal cobalt
Colloidal cobalt
Colloidal cobalt
Colloidal cobalt
130
130
130
130
130
120
110
130
130
5
5
5
5
5
5
5
3
1
97
94c
94c
94c
95c
89
26
45
0
† Electronic supplementary information (ESI) available: experimental
details for the catalytic Pauson–Khand reaction. Transmission electron
micrographs of colloidal cobalt nanoparticles before and after reaction. See
a Reaction conditions: colloidal cobalt (25 mg, 45 wt% cobalt), allyl-
propargyl diethylmalonate (0.42 mmol), THF, 12 h.b Isolated yield. c Yield
obtained from GC.
2212
Chem. Commun., 2001, 2212–2213
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107577m
Kim
