Synthesis of monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex

Article abstract of DOI:10.1039/b411656a

We successfully synthesized monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex.

Full text of DOI:10.1039/b411656a

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COMMUNICATION
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Synthesis of monodisperse chromium nanoparticles from the thermolysis
of a Fischer carbene complex{
Seung Uk Son,a Youngjin Jang,a Ki Youl Yoon,a Changhua An,a Yosun Hwang,b Je-Geun Park,b
Han-Jin Noh,c Jae-Young Kim,c Jae-Hoon Parkd and Taeghwan Hyeon*a
Received (in Cambridge, UK) 27th September 2004, Accepted 11th October 2004
First published as an Advance Article on the web 26th November 2004
DOI: 10.1039/b411656a
We successfully synthesized monodisperse chromium nano-
particles from the thermolysis of a Fischer carbene complex.
The development of uniform nanometer-sized particles has been a
major research focus because of their many technological and
fundamental properties.1 Various nanoparticles of transition
metals and their oxides have been synthesized, and their magnetic
and catalytic properties characterized. In the synthesis of transition
metal nanoparticles, the thermolysis of metal carbonyl com-
pounds, such as Co2(CO)8,2 Fe(CO)5,3 and W(CO)6,4 have been
frequently used. Recently, tailored organometallic precursors were
used to synthesize high-quality nanoparticles. For example, the
Chaudret group demonstrated the shape-controlled synthesis of
ZnO5 and Co6 nanoparticles via an organometallic chemical
approach.
Scheme 1 Proposed mechanism.
size-controlled synthesis of Cr nanoparticles. It was found that
trioctylphosphine (TOP) is the best surfactant in the current
synthesis of Cr nanoparticles.
The following describes the typical synthesis of 2.5 nm sized Cr
nanoparticles. 0.1 g of chromium Fischer carbene complex, which
was synthesized using a method reported elsewhere,11 was
dissolved in 3 mL of TOP. The carbene solution was injected
into a 3 mL TOP solution at 300 uC under an argon atmosphere.
After the injection, the reaction temperature was reduced to 250 uC.
The reaction temperature was slowly increased to 300 uC and the
resulting solution was aged at this temperature for 1.5 h. During
this process, the colour of the reaction mixture turned black. The
reaction mixture was cooled to room temperature and then poured
into 30 mL of ethanol, resulting in precipitation. A powder form of
the nanoparticles was retrieved by centrifugation, which was
Although chromium and chromium oxide nanoparticles are
expected to exhibit interesting magnetic7 and catalytic8 properties,
there have been very few reports9 on the synthesis of monodisperse
chromium and chromium oxide nanoparticles. Consequently, we
attempted to synthesize chromium nanoparticles from the thermal
decomposition of Cr(CO)6 in the presence of various surfactants.
However, we were not able to synthesize Cr nanoparticles because
Cr(CO)6 is thermally stable up to 300 uC and readily sublimes at
temperatures .100 uC. Therefore, a chromium–carbene complex
was employed as a precursor for the synthesis because it is a
thermally labile zero-valent organo-chromium compound.10
In the past, the chemistry of the Fischer carbene complexes has
been extensively pursued by organic and organometallic chem-
ists.11 Intensive research interest has been focused on identifying
novel organic thermal reactions of Fischer carbene complexes,
including radical dimerization,12 carbene transfer reactions13 and
cyclization.11 These unique thermal reactions result from the
unique activity of the de-ligated carbene mode at high tempera-
tures. In addition, the thermal decomposition behaviour of metal
carbene complexes has been a very interesting research topic.13 In
contrast to the previous reports that focused on the organic part of
the carbene complexes, this study examined the metal fragment. It
was reasoned that the metal fragments could make a good metal
source for the synthesis of chromium nanoparticles if a Cr–carbene
complex was thermally decomposed in the presence of a suitable
surfactant (Scheme 1). This report demonstrates the successful
use of a chromium Fischer carbene complex as a precursor for the
{ Electronic supplementary information (ESI) available: Fig. S1: HRTEM
image of and ED pattern of 6.0 nm Cr nanoparticles. See http://
www.rsc.org/suppdata/cc/b4/b411656a/
Fig. 1 TEM images of Cr nanoparticles with particles sizes of (a) 2.5 nm,
(b) 4.5 nm and (c) 6.0 nm. XRD patterns of 4.5 nm Cr nanoparticles
(d) freshly synthesized, and (e) after annealing at 500 uC under argon.
*thyeon@plaza.snu.ac.kr
86 | Chem. Commun., 2005, 86–88
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state of coordinated metals, we expect that these carbene
complexes would make good precursors for the synthesis of
nanoparticles that cannot be readily prepared using conventional
precursors.
Table 1 Size control experiments
(TOP/DOE)
ml injected
TOP in reaction
pot/ml
Size of
nanoparticles/nm
Entry
1
2
3
3/0
4/1
2/3
3
4
4
2.5 (¡0.3)
4.5 (¡0.4)
6.0 (¡0.4)
We thank the Korean Ministry of Science and Technology for
the financial support through the National Creative Research
Initiative Program. The work at POSTECH was supported by
KOSEF through the eSSC at POSTECH and KISTEP through
the X-ray/particle-beam Nanocharacterization Program.
redispersed in chloroform. The TEM image shows monodisperse
2.5 nm sized nanoparticles (Fig. 1(a)). The X-ray diffraction
(XRD) pattern (Fig. 1(d)) and high-resolution TEM image (see
ESI{) shows that the nanoparticles are poorly crystalline. In order
to characterize clearly the crystal structure of the nanoparticles, the
sample were annealed at 500 uC under an argon atmosphere. The
XRD pattern after the annealing clearly showed two peaks at 2h
44 and 64u, which correspond to (110) and (200) reflections of
cubic Cr structure (JCPDS #06-0694, Im3m space group. The FT-
IR spectrum of the synthesized nanoparticles showed a C–P
vibration peak at 1460 cm21, demonstrating that the nanoparticles
were stabilized by TOP. The thermal behaviour of chromium
Fischer carbene complexes in TOP solvent was investigated in
order to obtain some insight in the nanoparticle formation
mechanism. The chromium Fischer carbene complex was readily
transformed to compound 1, which was characterized by 1H, 13C,
31P NMR, IR spectroscopy and elemental analysis.14
Seung Uk Son,a Youngjin Jang,a Ki Youl Yoon,a Changhua An,a
Yosun Hwang,b Je-Geun Park,b Han-Jin Noh,c Jae-Young Kim,c
Jae-Hoon Parkd and Taeghwan Hyeon*a
aNational Creative Research Initiative Center for Oxide Nanocrystalline
Materials and School of Chemical Engineering, Seoul National
University, Seoul, 151-744, Korea. E-mail: thyeon@plaza.snu.ac.kr;
Fax: 82-2-886-8457; Tel: 82-2-880-7150
bDepartment of Physics, Sungkyunkwan University, Suwon, 440-746,
Korea
cPohang Accelerator Laboratory, Pohang University of Science and
Technology, Pohang, Kyungbuk, 790-784, Korea
dDepartment of Physics, Pohang University of Science and Technology,
Pohang, Kyungbuk, 790-784, Korea
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It was reasoned that one carbonyl ligand of the chromium
Fischer carbene was easily substituted by TOP. This can be
contributed to the unique activity of Fischer carbene complexes.15
It is noteworthy that Cr(CO)6 showed no reactivity to TOP at
300 uC.
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Next, an attempt was made to control the size of the chromium
nanoparticles. It is well known that the use of two surfactants with
a different binding ability is a useful method for controlling the
size of nanoparticles.16 This study screened different types of
surfactants and found that dioctyl ether (DOE) is a good
surfactant since it was observed to reduce the decomposition rate
of the carbene complex. When 0.1 g of the carbene precursor
dissolved in 1 mL of dioctyl ether and 4 mL of TOP were injected
into 4 mL TOP at 300 uC, 4.5 nm sized Cr nanoparticles were
produced (Fig. 1(b) and Table 1). When 0.1 g of the precursor
dissolved in 3 mL of dioctyl ether and 2 mL of TOP were injected
in 4 mL of TOP at 300 uC, we obtained 6.0 nm sized nanoparticles
(Fig. 1(c)). When .3 mL dioctyl ether was employed, nano-
particles were not produced.
The synthesized Cr nanoparticles were highly sensitive to air.17
They were easily oxidized to form Cr2O3 when exposed to air. It is
well known that pure chromium metal is easily oxidized to Cr2O3,
which is known to be the most thermodynamically stable form of
chromium oxide. Our attempts to prepare the other controlled
chromium oxide species such as CrO2 are in progress.
In conclusion, we demonstrated the successful use of a
chromium Fischer carbene complex as a precursor for the
synthesis of monodisperse chromium nanoparticles. Because the
carbenes are usually very reactive and do not change the oxidation
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2001, 1544; (c) L. Xu, W. Chen and J. Xiao, Organometallics, 2000, 19,
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70 uC. The color of solution turned from red to yellow. The product was
isolated by flash chromatography (eluent hexane–ether 5 1:4) 65%
isolated yield. Characterization data: 1H NMR (CDCl3, 500 MHz): 7.31
(d, J 5 7.4 Hz, 2H), 7.25 (t, J 5 7.2 Hz, 2H), 7.15 (t, J 5 7.1 Hz, 1H),
3.35 (s, 3H), 2.14 (m, 6H), 1.34–1.24 (m, 36H), 0.87 (t, J 5 7.0 Hz, 9H)
ppm; 13C NMR (CDCl3, 125 MHz): 14.0, 16.8, 17.3, 21.7, 22.5, 28.7,
28.8, 30.7, 30.9, 31.6, 123.1, 127.0, 129.0, 130.3, 131.1, 157.4, 222.5,
228.0, 301.8 ppm; 31P NMR (CDCl3, 202 MHz) 34.4 ppm. Anal. Calc.
for C36H59O5P1Cr1: C, 66.03; H, 9.08. Found: C, 66.46; H, 8.78%. IR
11 (a) K. H. Do¨tz, E. O. Fischer, P. Hofmann, F. R. Kreissel, U. Schubert
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(neat) nCO 1957, 1897 cm21
.
15 Although clear experimental evidence was not obtained, it is believed
from previous reports on the thermal reactions of the Fischer carbene
complex that the carbene transfer reactions appeared to begin at a high
reaction temperature.
12 J. Barluenga, F. Rodr´ıguez and F. J. Fan˜ana´s, Chem. Eur. J., 2000, 6,
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14 To a one-neck flask were added trioctyl phosphine (2 mL), and the
chromium Fischer carbene complex (0.2 g). The solution was heated at
17 CAUTION! Sometimes the well washed nanoparticles will combust
in air!.
88 | Chem. Commun., 2005, 86–88
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