Poly(ethylene glycol) stabilized Co nanoparticles as highly active and selective catalysts for the Pauson-Khand reaction

Article abstract of DOI:10.1039/b702330h

PEG-stabilized cobalt nanoparticles were prepared by thermal decomoposition of [Co₂(CO)₈] in PEG and were shown to be highly active and selective catalysts, for intra- and intermolecular Pauson-Khand reactions (PKR), in organic solvents or aqueous media. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702330h

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Poly(ethylene glycol) stabilized Co nanoparticles as highly active and
selective catalysts for the Pauson–Khand reaction{
Jean-Luc Muller,a Ju¨rgen Klankermayera and Walter Leitner*ab
Received (in Cambridge, UK) 14th February 2007, Accepted 22nd March 2007
First published as an Advance Article on the web 11th April 2007
DOI: 10.1039/b702330h
PEG-stabilized cobalt nanoparticles were prepared by thermal
decomoposition of [Co2(CO)8] in PEG and were shown to be
highly active and selective catalysts, for intra- and intermole-
cular Pauson–Khand reactions (PKR), in organic solvents or
aqueous media.
The [2 + 2 + 1] cycloaddition of alkynes with alkenes and carbon
monoxide (CO) to form cyclopentenones is known as Pauson–
Khand reaction (PKR).1 The original reaction setup2 required a
Fig. 1 PEG-stabilized Co-nanoparticles.
stoichiometric application of cobalt complexes derived from
[Co2(CO)8], but catalytic versions were developed more recently.3,4
carbonyl absorptions, indicating a complete decomposition of the
metal carbonyl. In a first set of experiments the intramolecular
PKR (Scheme 1) catalyzed by the PEG-stabilized cobalt
nanoparticles was investigated. Under optimized reaction condi-
tions conversions of 1 were almost quantitative using catalyst
loadings of only 3.3 mol% (Table 1). By using higher catalyst
loadings a significant decrease of the pressure under otherwise
identical conditions was possible (Entry 3–4, Table 1).
Nevertheless the practical utility of this synthetic method is still
limited by problems of catalyst separation and the volatile and
toxic nature of cobalt carbonyls.1
Our group recently demonstrated that commercially available
Raney cobalt acts as active and recyclable catalyst for inter- and
intramolecular PKR.5 Raney cobalt can be efficiently separated
and reused by filtration and/or exploiting its ferromagnetic
properties. Chung and co-workers deposited cobalt particles by
thermal decomposition of [Co2(CO)8] onto various supports and
demonstrated the catalytic activities of the resulting materials for
the PKR.6,7 Colloidal cobalt nanoparticles in supported or
unsupported form were also used as reusable catalysts for this
reaction.8,9 Cobalt nanoparticles, for the intramolecular PKR in
aqueous solution, stabilized by sodiumdodecylsulfate (SDS) were
reported.10 All these nanoparticle catalysed systems require very
high catalyst loadings approaching stoichiometric ratios in many
cases. So far only heterobimetallic Co/M (M = Rh,11 Ru12)
nanoparticles on charcoal lead to active catalytic systems that
operate at loadings of a few mol%.
The scope of this new protocol was tested in a variety of
different intra- and intermolecular PKRs. Representative results
obtained under standard conditions are given in Table 2. Almost
quantitative conversions were achieved with both catalysts for
the malonate derived enynes 1 and 3 as well as for the diol 5. The
PEG-stabilized cobalt nanoparticles were also active for the
intramolecular PKR in water, giving high yields of cyclic products
(Scheme 2). Interestingly the decarboxylated compound 16 was
formed to a large extent under these conditions. In contrast, the
literature reported hydrogenation of the double bond in water, was
not observed.10,13
The intermolecular PKR of norbonene 7 with different alkynes
occurs also smoothly with both catalysts, also a higher catalyst
loading and and increased CO pressure was required. Again, protic
functional groups were tolerated (Table 2, entry 3, 5) and excellent
conversions and selectivities were obtained in all cases. The
performance of the two different types of nanoparticles was largely
identical for the various substrates.
Here we report that poly(ethylene glycol) PEG-stabilized cobalt
nanoparticles serve as readily available, highly active and partly
recyclable catalysts for the PKR. The catalysts are compatible with
organic solvents and aqueous solutions as reaction media. PEG-
stabilized cobalt nanoparticles were prepared by thermal decom-
position of [Co2(CO)8] in molten PEG at 80–100 uC. PEG of
molecular weight 1000 and 5000 was used. Transmission electron
microscopic (TEM) pictures confirmed that the particles stabilized
either with PEG1000 or PEG5000 were smaller than 1.6 nm and well
dispersed (Fig. 1). The IR spectra of the catalysts showed no
The workup of the reaction mixture is particularly straight
forward with the PEG5000-stabilized Co-nanoparticles. These
materials are readily separated from the reaction mixture using
aInstitut fu¨r Technische und Makromolekulare Chemie, RWTH Aachen,
Worringerweg 1, 52074, Aachen, Germany
bMax Planck Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
45470, Mu¨lheim an der Ruhr, Germany
{ Electronic supplementary information (ESI) available: An experimental
section for the synthesis and characterisation of the PEG-stab. cobalt
nanoparticles and the catalysis and characterisation of the products. See
DOI: 10.1039/b702330h
Scheme 1 Intramolecular PKR reaction with PEG-stabilized
Co-nanoparticles.
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Table 1 Pressure dependence of the PKR catalysed by the PEG-
stabilized Co-nanoparticles
Conversion
[%]
Selectivity
for 2 [%]
Entry
Co [mol%]
Pressure/bara
1
2
3
4
a
3
3
50
50
23
10
10
5
¢99.0
35.1
97.4
98.0
89.8
98.2
91.8
84.5
Pressure in the autoclave at 130 uC. Reaction conditions: All
reactions were performed in 4 mL THF, in a 10 mL window
equipped steel autoclave. Reaction time 16 h. Catalyst: PEG5000
stabilized Co-nanoparticles.
Scheme 2 Intramolecular PKR in water.
-
Table 3 Recycling of PEG5000-Co-nanoparticles with scCO2
either Et2O or scCO2 as extraction media for the products. Fair to
excellent yields of the pure products were obtained after separation
of the catalyst with Et2O (Table 2). NMR spectra of the products
showed that they contained no or only trace amounts of PEG in
the product so that a simple and effective procedure for the
product isolation and catalyst separation is available.
Contamination with cobalt was found to be in the range of 19 to
120 ppm for the examples in Table 2. The recyclability of these
PEG5000-Co catalysts was tested for both methods with different
products (Table 3 and 4). Pressurizing the reaction mixture with
scCO2 leads to precipitation of the PEG-stabilized nanoparticles,
similar to what has been observed for PEG-modified organome-
tallic complexes.14 The activity of the recycled catalyst remained
reasonable over three runs but decreased sharply in the fourth run
(Entry 4, Table 3). A comparable deactivation was observed using
the Et2O extraction protocol (Table 4). Nevertheless the selectivity
for the formation of the desired products 11 and 15 remained
uniformly high in both series of experiments. The total turnover
Entry
Product
Conversion [%]
Selectivity [%]
1
2
3
4
99.0
77.9
72.3
32.2
97.0
99.0
99.0
99.0
Reaction conditions: All reactions were performed in 2 mL THF, in
a 10 mL, window equipped steel autoclave at 130 uC and 35 bar CO
pressure. Reaction time 16 h. 14/Catalyst 20.0. Extraction conditions:
130 bar, 80 uC, 10 l/h, 2.5 h.
number (TON) achieved for the Et2O extraction and recycling
experiments amounts to 58.3 mol of 11 per mol of cobalt. This
corresponds to 203.8 g of 11 per gram of cobalt.
The deactivation of the catalytic system results partly from the
as yet noticeable cobalt leaching (up to 0.74% per cycle based on
the contamination in the product). Other factors such as changes
in the particle morphology can, however, not be excluded. TEM
Table 2 Inter- and intramolecular PKR with PEG-stabilized cobalt nanoparticles
PEG1000 stab. Co-nanoparticles
PEG5000 stab. Co-nanoparticles
Entry Substrate
1
Product
Conversion [%]
Selectivity [%]
98.0
Conversion [%] Selectivity [%] Yield [%]a
¢99.0
99.0
99.0
98.0
98.9
98.0
95.0
86.2
91.1
88.7
89.9
73.5
87.5
2
3
4
¢99.0
99.0
99.0
94.0
95.1
94.3
5
6
7
99.0
¢99.0
99.0
85.0
94.2
95.5
98.5
85.0
99.0
85.0
90.4
99.0
81.5
71.5
85.5
a
Yield of isolated product. Reaction conditions: all reactions were performed in 4 mL THF, in a 10 mL window equipped stainless steel
autoclave. Reaction time: 16 h. Entry 1–3: substrate/Co 30.0. CO pressure 23 bar at 130 uC. Entry 4: substrate/Co 30.0. CO pressure 35 bar at
130 uC. Entry 5–7: substrate/Co 20.0. CO pressure 35 bar at 130 uC.
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demonstrated, albeit further optimization is necessary to minimize
the metal leaching and catalyst deactivation.{
Table 4 Recycling of PEG5000-Co-nanoparticles with Et2O
Entry
Product
Conversion [%]
Selectivity [%]
J. L. M. thanks Luxemburg’s Ministry of Culture and Research,
for awarding a research scholarship. Furthermore we are grateful
to Dr. B. Tesche, A. Dreier and B. Spliethoff for the TEM
measurements and the group of Prof. Dr. A. Behr for the ICP
measurements.
1
2
3
4
98.5
98.0
69.0
25.8
85.0
83.0
82.6
97.0
Reaction conditions: All reactions were performed in 4 mL THF, in
a 10 mL, window equipped steel autoclave at 130 uC and 35 bar CO
pressure. Reaction time 16 h. 10/Catalyst 20.0.
Notes and references
{ Catalyst preparation. After degassing of PEG1000(OMe)2 (1.1 g, 1.1 mmol)
for 3 h at 80 uC, a solution of [Co2(CO)8] (170.0 mg, 0.5 mmol) in 10 mL
toluene was injected to the molten PEG under counter current flow of
argon. The reaction solution was heated to 100 uC for 20 min under
rigorous shaking and was concentrated afterwards over 1 h under reaction
conditions leading to a green black waxy precipitate. The PEG1000
-
stabilized cobalt nanoparticles were analysed by IR and TEM. Preparation
of PEG5000-stabilized Co nanoparticles is achieved using an identical
procedure.
1 For recent reviews see: (a) K. M. Brummond and J. Kent, Tetrahedron,
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2 I. U. Khand, G. R. Knox, P. L. Pauson, W. E. Watts and
M. I. Foreman, J. Chem. Soc., Perkin Trans. 1, 1973, 977–981.
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4 T. Shibata, Adv. Synth. Catal., 2006, 348, 2328–2336.
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6 S. W. Kim, S. U. Son, S. I. Lee, T. Hyeon and Y. K. Chung, J. Am.
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Fig. 2 PEG5000-stabilized Co-nanoparticles before reaction (A) and after
the recycling experiments (B).
micrographs of the same catalyst before and after the recycling
series (summarized in Table 4) show no significant increase in the
size of the primary particles, but indicate a reduced degree of
dispersion within the PEG matrix (Fig. 2). Possibilities to
ameliorate both leaching and agglomeration include the further
development of supported PEG-phase systems.15
7 S. U. Son, S. I. Lee and Y. K. Chung, Angew. Chem., 2000, 112,
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8 S. U. Son, K. H. Park and Y. K. Chung, Org. Lett., 2002, 4, 3983–3986.
9 S. W. Kim, S. U. Son, S. S. Lee, T. Hyeon and Y. K. Chung, Chem.
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In conclusion, we have shown that Poly(ethylene glycol) (PEG)
stabilized cobalt nanoparticles could be synthesized by thermal
decomposition of [Co2(CO)8] in molten PEG. These PEG-
stabilized cobalt nanoparticles are broadly applicable as catalysts
in intra- and intermolecular Pauson–Khand reactions, in organic
solvents or aqueous phase. The activities of these new
materials compare favourably with that of previously described
nanoparticle-based systems and heterogeneous catalysed PKR
reactions. The PEG-stabilized cobalt nanoparticles are easy to
handle, stable over weeks and can be effectively separated from the
reaction products by using diethyl ether or supercritical carbon
dioxide. The principle recyclability of the catalysts has been
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Chem. Commun., 2007, 1939–1941 | 1941