Shape control of thermodynamically stable cobalt nanorods through organometallic chemistry

Article abstract of DOI:10.1002/1521-3773(20021115)41:22<4286::AID-ANIE4286>3.0.CO;2-M

The shape of things to come: Nanospheres, thermodynamically stable nanorods, and nanowires are selectively produced in solution from [Co(η³-C₈H₁₃) (η⁴-C₈H₁₂)] in the presence of oleic acid

Full text of DOI:10.1002/1521-3773(20021115)41:22<4286::AID-ANIE4286>3.0.CO;2-M

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coupling method was used to prepare (S)-4-phenylmandelic acid from (S)-
4-bromomandelic acid. The corresponding (S)-4-bromomandelic acid was
obtained from 4-bromoacetophenone bystandard procedures and was
resolved with 2. The substituted phenylethylamines were prepared from the
corresponding acetophenones bya standard Leuckardt procedure. A
Strecker reaction was used to prepare 4-methylphenylglycine, which was
reduced to 4-methylphenylglycinol.
Shape Control of Thermodynamically Stable
Cobalt Nanorods through Organometallic
Chemistry **
Frÿdÿric Dumestre, Bruno Chaudret,*
Catherine Amiens, Marie-Claire Fromen, Marie-
Josÿ Casanove, Philippe Renaud, and Peter Zurcher
Received: June 5, 2002
Revised: August 15, 2002 [Z19422]
The synthesis of nanoparticles is of fundamental impor-
tance for the development of novel technologies based on
nanomaterials. This is particularlytrue for magnetic nano-
materials which could be used, amongst other things, for high-
[1] L. Pasteur, C. R. Hebd. Seances Acad. Sci. 1848, 26, 535.
[2] a) R. A. Sheldon, Chirotechnology, Marcel Dekker, New York, 1993,
chap. 6; b) Chirality in Industry II (Eds.: N. A. Collins, G. N. Shel-
drake, J. Crosby), Wiley, Chichester, 1997.
[3] a) T. Vries, H. Wynberg, E. van Echten, J. Koek, W. ten Hoeve, R. M.
Kellogg, Q. B. Broxterman, A. Minnaard, B. Kaptein, S. van der Sluis,
L. A. Hulshof, J. Kooistra, Angew. Chem. 1998, 110, 2491; Angew.
Chem. Int. Ed. 1998, 37, 2349; b) Eur. Pat. Appl. , EP 0,838,448 (to
DSM).
[1]
densityinformation storage. These materials would ideally
consist of spatiallyseparated particles in the nanometer range
that would function as a single magnetic domain that exhibits
ferromagnetic behavior at room temperature, and could be
electronicallyisolated. In addition, the organization of the
particles in the solid state or in solution (self-assembly) should
be controllable as desired for the intended application.
Several methods are available for the production of
[4] J. Jacques, A. Collet, S. H. Wilen, Enantiomers, Racemates, and
Resolutions, Wiley, New York, 1981.
[5] The cyclic phosphoric acids (P mix) were developed as resolving
agents in our lab and have been patented: W. ten Hoeve, H. Wynberg,
J. Org. Chem. 1985, 50, 4508; Eur. Pat. , 180,276; US Pat. , 4,814,477.
[6] Arthur Conan Doyle, Silver Blaze, Strand Magazine, 1892.
[7] E. Fogassy, A. Lopata, F. Faigl, F. Darvas, M. çcs, L. Toke,
Tetrahedron Lett. 1980, 21, 647.
magnetic nanomaterials. Theycan be divided into physical
[2,3]
methods, which produce essentiallythin layers,
template
methods, which involves the growth of nanorods or nanowires
bydifferent approaches, frequentlyelectrochemical, within
the channels of inorganic or track-etch organic matrixes,[4] and
chemical methods involving synthesis in a solution of nano-
particles.[5,6] The latter maygenerallyallow, after size
selection, the formation of self-assembled superlattices. Sun
and Murray[5] have, for example, recentlyreported a high-
temperature preparation of 9 nm cobalt nanoparticles dis-
playing a long-range self-organization. This method has been
extended to the synthesis of Fe/Pt particles to increase the
[8] Reproducibilities are good; the experimentallydetermined error limit
of the S factor is ꢀ 5%.
[9] Analogous experiments with pure (S)-o-nitrophenylethylamine (5) as
the inhibitor led to a de value of 51% and an S factor of 0.55. With
pure (S)-p-nitrophenylethylamine as the inhibitor the de value was
62% and the S factor was 0.56. We conclude that use of the more
readilyavailable mixture instead of the pure materials is entirely
justified and allows direct comparison with the data given in ref. [3].
[10] In this experiment we used only o-nitrophenylethylamine instead of
the 1:1 mixture of o,p isomers for reasons of simplicity.
[11] The experimentallydetermined error limit is
experiment.
ꢀ 0.38C for this
magnetic anisotropyof the materials, with the goal of
[7]
[12] The dissolution temperatures differ somewhat, owing to the high
concentrations in this experiment (1.6m); the experimentallydeter-
mined error limit is ꢀ 0.78C.
producing high-densitymemories.
Self-organized cobalt
nanoparticles have also been reported bythe group of Pileni
[8]
byreverse micelle synthesis.
[13] A manuscript covering the concept of reverse resolutions is in
preparation: Dutch Resolution of Alaninol with (R)-Mandelic acid by
addition of (S)-or (R)-2-Amino-1-butanol; the Role of Nucleation
Inhibition; B. Kaptein, K. L. Pouwer, T. R. Vries, R. F. P. Grimbergen,
H. M. J. Grooten, H. L. M. Elsenberg, J. W. Nieuwenhuijzen, R. M.
Kellogg, Q. B. Broxterman, Chem. Eur. J., unpublished work.
[14] a) K. Kinbara, K. Oishi, Y. Harada, K. Saigo, Tetrahedron 2000, 56,
6651; b) K. Kinbara, Y. Harada, K. Saigo, Tetrahedron: Asymmetry
1998, 9, 2219.
In all cases, as a result of their small dimension, the particles
are superparamagnetic at room temperature and thus not
usable for manyapplications, such as magnetic recording. One
wayto increase the magnetic anisotropyof the particles is to
modifytheir shape. This problem has been addressed by
Alivisatos who initiallydemonstrated the importance of
reaction conditions, in particular the concentration of the
[15] a) L. Addadi, Z. Berkovitch-Yellin, N. Domb, E. Gati, M. Lahav, L.
Leiserowitz, Nature 1982, 296, 21; b) Z. Berkovitch-Yellin, L. Addadi,
M. Idelson, L. Leiserowitz, M. Lahav, Nature 1982, 296, 27; c) L.
Addadi, S. Weinstein, E. Gati, I. Weissbuch, M. Lahav, J. Am. Chem.
Soc. 1982, 104, 4610; d) D. Zbaida, M. Lahav, K. Drauz, G. Knaup, M.
Kottenhahn, Tetrahedron 2000, 56, 6645.
[*] Dr. B. Chaudret, F. Dumestre, Dr. C. Amiens
Laboratoire de Chimie de Coordination du CNRS
205, route de Narbonne, 31077 Toulouse Cÿdex 04 (France).
Fax : (þ 33)5-61-55-30-03
E-mail: chaudret@lcc-toulouse.fr
[16] K. Sakai, Y. Maekawa, K. Saigo, M. Sukegawa, H. Murakami, H.
Nohira, Bull. Chem. Soc. Jpn. 1992, 65, 1747.
F. Dumestre, Dr. P. Renaud
Digital DNA Labs, Semiconductor Products Sector, Motorola
le Mirail B.P. 1029, 31023 Toulouse Cÿdex (France)
M.-C. Fromen, Dr. M.-J. Casanove
CEMES-CNRS
29 Rue Marvig, BP 4347, F 31055 Toulouse, France.
Dr. P. Zurcher
Digital DNA Labs, Semiconductor Products Sector, Motorola
2100 E. Elliot Road, Tempe, AZ-85824 (USA)
[**] The authors thank CNRS and MOTOROLA S.P.S. for support, M.
Vincent Colliõre, Lucien Datas, and TEMSCAN service (Universitÿ
Paul Sabatier Toulouse) for transmission electron microscopy.
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precursors and the ratio of surfactants, for preparing CdSe
nanocrystals of various shapes including nanorods.[9] The same
methodology was employed for synthesizing cobalt nanorods,
which were found to be thermodynamically unstable and to
rearrange into spherical particles.[10] This result contrasts with
a recent report byHyeon and co-workers, who show that
spherical iron nanoparticles maybe used at 320 8C as initiators
for the growth of nanorods.[11] However, no systematic study
on the influence of ligand-type on the particle aspect ratio,
and hence on the magnetic properties of prepared metal
nanoparticles, has been reported so far.
transmission electron microscopy(TEM, sample 2 ; Fig-
ure 1A), which are the onlyspecies present after reacting
for 3 h. However, after 48 h, regular nanorods of approxi-
mately9 î 40 nm are the unique species in solution (sample 3 ;
Figure 1B). To determine the importance of dihydrogen, two
parallel reactions were carried out. In both cases the precursor
was decomposed under standard conditions. After 3 h the
reactors were evacuated, and the first reactor was pressurized
again to 3 bar with H2 whereas the second reactor was placed
under argon. After 48 h nanorods were found in the first
reactor and 4 nm nanoparticles in the second. The nature of
the precursor complex is also important since, under standard
conditions, the use of [Co2(CO)8] in place of [Co(h3-C8H13)(h4-
C8H12)] leads to polydisperse particles with a mean size of
approximately10 nm. The role of the ligands, with particular
respect to the concentration of oleic acid and the nature of the
amine, was then examined in order to understand the
formation of the particles. If the oleic acid concentration
was decreased (0.3 equivalents, with respect to Co), particles
of regular size (approximately3 nm) were formed, whereas if
the concentration was doubled (2 equivalents), verylong
(micron range) nanowires of 4 nm diameter were produced
(sample 4; Figure 1C). Replacing oleylamine by octyl-, dode-
cyl-, hexadecyl-, and octadecylamine causes a dramatic
change in the dimensions of the nanorods that are produced
(Figure 2). The use of octadecylamine yields nanorods of
comparable size, but with greater regularity, to those obtained
with oleylamine (47 î 6nm, Figure 2C); hexadecylamine
produces longer and thinner rods (120 î 5.5 nm, Figure 2E)
whereas octylamine yields smaller and wider ones (17 î
10 nm, Figure 2F). Therefore, particles displaying aspect
ratios between 1.7 and 22 can be controlled solelybythe
number of carbon atoms in the alkyl chain of the amine,
whereas a verylarge aspect ratio maybe obtained for the
nanowires prepared in the presence of excess oleic acid. The
variation of the length and diameter of the produced nano-
rods is summarized in Figure 2A,2B.
In our group we have previouslydescribed the synthesis of
magnetic nanoparticles using olefinic complexes as precur-
sors.[12,13] These compounds rapidlydecompose to give alkane
by-products which do not interact with the surface of the
particles under the reaction conditions. This methodology
allows the synthesis of magnetic nanoparticles that display the
same magnetic moment as particles of similar size in the gas
phase.[14] The absence of oxidation and of stronglycoordinat-
ing ligands at the surface of the particles mayalso favor size
and shape changes in solution, as recentlydemonstrated for
ruthenium,[15] and therefore an easier control of these
parameters bythe surrounding medium. In separate studies,
we have emphasized the role of amines for the formation of
nanorods or nanowires.[16] This technique has been used to
produce nickel nanorods, the surface magnetism of which is
[17]
not altered bythe presence of amine ligands.
In this paper we describe the synthesis and magnetic
properties of novel cobalt nanoparticles. We demonstrate that
by varying the synthesis parameters it is possible to selectively
prepare spherical nanoparticles of uniform size, nanorods, the
aspect ratio of which is controlled bythe alkyl chain-length of
the ligand, or even verylong nanowires. Finally, we report the
magnetic properties of these nanoparticle materials.
The decomposition reactions of [Co(h3-C8H13)(h4-
C8H12)][18] were carried out in anisole under a preesure of
3 bar H2 (initial pressure at room temperature in the reactor)
in the presence of ligands. The use of
a single ligand such as hexadecyl-
amine (1:1 Co/amine molar ratio)
leads to small, agglomerated nano-
particles at room temperature. Sim-
ilarly, the use of oleylamine
¼
(CH3(CH2)7CH CH(CH2)8NH2) at
1508C produces particles that dis-
playa large size distribution, where-
as monodisperse particles with a
mean size of 5 nm are obtained when
using oleic acid under the same
conditions (sample 1). These parti-
cles self-organize on a microscopy
grid into 2D and 3D crystalline
arrays. When both oleic acid and
oleylamine are used as ligands
(1 equivalent each, 1508C, and
Figure 1. Variation of size and shape with reaction time and the ratio of stabilizing agents. A, B) TEM
micrographs of nanoparticles and nanorods obtained with 1 mmol of oleylamine and 1 mmol of oleic acid
after 3 h and 48 h, respectively: Bar ¼ 30 nm; C) TEM micrograph of nanowires obtained with 1 mmol of
oleylamine and 1 mmol of oleic acid after 48 h: Bar¼ 30 nm; D, E, F) High resolution (HR)TEM images of
the nanoparticles, nanorods, and nanowires presented in A, B, and C, respectively: Bar ¼ 5 nm.
3 bar H2, standard conditions), the
synthesis initially produces nanopar-
ticles with mean dimensions of ap-
proximately3 nm, as shown by
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The former are superparamagnetic
at room temperature with a blocking
temperature of 20 K, whereas the
latter are ferromagnetic at room
temperature. Hysteresis loop meas-
urements at 2 K record, for both
samples, a saturation magnetization
(Ms) of 160 Am2 kgCoꢁ1, which corre-
sponds to a magnetic moment per
atom of m ¼ 1.71mB, identical to that
found in bulk cobalt. In addition, a
coercive field (Hc) of respectively
1100 and 8900 Oe is found for sam-
ples 2 and 3, hence demonstrating
the importance of the shape aniso-
tropyin such systems (see Figure 3).
This work emphasizes the role of
the organometallic precursor in the
synthesis of the nanomaterial. The
high decomposition rate of the pre-
Figure 2. Variation of the aspect ratio of nanorods with respect to the change in the alkyl chain length of
the amine. A) Evolution of nanorod length; B) Evolution of nanorod diameter; C F) TEM micrographs of
hcp cobalt nanorods obtained with oleic acid and octadecylamine, dodecylamine, hexadecylamine, and
octylamine, respectively: Bar¼ 30 nm.
These results demonstrate the possibilityto selectively
synthesize nanorods, but raise questions concerning the exact
growth mechanism and, in particular, the exact role of the
ligands. It has been previouslyproposed that the growth
mechanism of nanorods could be related to the preferred
coordination of the ligand along a face of a growing
crystal.[6,10,19] In the present case, the employed amine ligands
displaysimilar chemical properties, in terms of basicityand
affinityfor the metal, but form verydifferent nanoobjects.
This suggests that, in the present case, the nanorod synthesis is
controlled bysupramolecular organization and/or the dynam-
ics of the ligands in solution; it is conceivable that longer alkyl
chains could organize across longer distances. Oleylamine
displays a ™cis∫ configuration, which leads to an apparent rod
length comparable to that of octylamine, with less opportunity
for long-range organization; this could be a reason for the
relativelylow regularityof the rods obtained in this case.
Oleic acid can also playan important role in this organization
since its concentration relative to the amine has a strong
influence on the aspect ratio of the nanoobjects produced. In
addition, it presumablytransforms rapidlyin the medium into
the oleate anion bydeprotonation in the presence of amines,
which maylead to a stronglyassociated medium and/or
coordination to cobalt. The necessityof dihydrogen for the
synthesis of rods from nanospheres probably arises from the
necessityto remove the carboxylate groups from the surface
to allow coalescence and growth to occur. This, therefore,
suggests a strong coordination of oleate anions to the
cursor and, more importantly, the absence of by-products that
could potentiallyinteract with the surface allows the particles
to grow without losing their intrinsic magnetic properties.
These objects displaya saturation magnetization per cobalt
atom that is identical or veryclose to that of bulk cobalt,
which is consistent with the pure s-donor behavior of both the
amine and the acid ligands.[14,17] Furthermore, their magnetic
[20]
nanoparticles formed at the earlystage of the reaction.
All of the particles, rods, and wires described above consist
of a pure hexagonal close-packed (hcp) cobalt structure,
which displays no sign of oxidation as evidenced by wide-
angle X-rayscattering (WAXS) and high-resolution electron
microscopy(HREM) studies. The rods and wires are mono-
crystalline and adopt the c axis of the hcp structure as the
growth axis. Magnetic measurements were carried out on all
samples, in particular the 3 nm nanoparticles obtained after
3 h under standard conditions (sample 2) and nanorods
produced after 48 h under the same conditions (sample 3).
Figure 3. Plots of magnetization against applied magnetic field. a) Hyste-
resis loop of cobalt nanorods obtained with 1 mmol of oleylamine and
~
1 mmol of oleic acid: ( ) measured at 300 K. b) Hysteresis loops of cobalt
nanoparticles obtained with 1 mmol of oleylamine and 1 mmol of oleic
~
acid: (&) measured at 2 K, ( ) measured at 300 K.
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anisotropymaybe controlled bytheir aspect ratio, which gives
rise to ferromagnetic nanomaterials at room temperature. The
rods grow after the initial formation of spherical nano-
particles; the exact mechanism of this process is not known
but we note that the initial formation of nanoparticles,
followed byformation of nanorods or nanowires either
through coalescence of the initial particles[21] or upon using
the particles as nuclei for an anisotropic growth process, have
been veryrecentlyreported. [22] Further work will be necessary
to determine the exact mechanism that operates in our case.
In conclusion, we describe in this report a new and simple
method for the preparation of cobalt nanoparticles, nanorods,
and nanowires of uniform diameter that does not require a
special procedure or size selection. The magnetic nanowires
have no equivalent, whereas the rods differ from those
previouslydescribed bytheir uniformityof diameter, their
thermodynamic stability, and the possibility of fine tuning of
their aspect ratio. We further demonstrate that, under these
conditions, the nanomaterials maintain a magnetization at
saturation similar to bulk cobalt. This results from the choice
of ligands that do not display p-accepting behavior, which is in
agreement with previous research work from our group. All of
these aspects emphasize the role of an organometallic
approach in the design of precursors and ligands. Finally,
these objects mayfind use in manypractical applications such
as, for example, in data storage.
Generation of Reactivity from Typically Stable
ꢁ
Ligands: C C Bond-Forming Reductive
Elimination from Aryl Palladium(ii) Complexes
of Malonate Anions**
Joanna P. Wolkowski and John F. Hartwig*
Anions of 1,3-dicarbonyl compounds are some of the most
[1,2]
common ligands in transition-metal chemistry.
Theytypi-
callybind in an h2-O,O fashion,[3] have delocalized charge, and
donate electron densitymore weaklyto a metal center than
alkyls or alkoxides. They are usually supporting ligands that
are ancillaryto the site of reaction. Anions of 1,3-dicarbonyl
compounds are also common nucleophiles in metal-catalyzed
6]
allylic substitution,[4 but the facilityof this chemistryrelies
on external attack of the anion without coordination to the
metal center. If metal fragments could induce reactivityfrom
coordinated versions of these anions, then complexes of these
common ligands could serve as intermediates in catalytic
processes.
Complexes of malonate anions are likelyintermediates in
recently developed palladium-catalyzed arylations of malo-
nates.[7 11] Although palladium complexes of malonate anions
[12 16]
have been isolated previously,
their reactivityhas been
limited.[17 20] We report here reductive elimination of arylmal-
onate and acetylarylacetone from isolated aryl palladium
complexes of malonate and acetylacetone anions, respective-
ly. Our results suggest that the propensity of these complexes
to undergo reductive elimination depends criticallyon the
steric properties of the ancillaryphosphane ligand.
Received: July10, 2002
Revised: September 3, 2002 [Z19713]
[1] D. Weller, A. Moser, IEEE Trans. Magn. 1999, 35, 4423.
[2] Y. M. Kim, D. Choi, K. H. Kim, S. H. Han, H. J. Kim, IEEE Trans.
Magn. 2001, 37, 2288.
[3] M. Dumm, M. Zˆlfl, R. Moosb¸hler, M. Brockmann, T. Schmidt, G.
Bayreuther, J. Appl. Phys. 2000, 87, 5457.
[4] H. Cao, Z. Xu, H. Sang, D. Sheng, C. Tie, Adv. Mater. 2001, 13, 121.
[5] S. Sun, C. B. Murray, J. Appl. Phys. 1999, 85, 4325.
[6] V. F. Puntes, K. M. Krishnan, A. P. Alivisatos, Appl. Phys. Lett. 2001,
78, 2187.
[7] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science 2000, 287,
1989.
Our synthesis of aryl palladium malonates is summarized in
Scheme 1. Addition of dimethyl malonate or diethyl phenyl-
malonate to the basic PPh3-ligated palladium hydroxide
dimers 1a[21,22] and 1b generated the O,O’-bound palladium
dimethyl malonate complexes 2a c. Complexes 2a and 2c
were characterized byX-raydiffraction (Figure 1). No
unusual angles at the palladium center were found in 2a or
[8] J. Legrand, A. T. Ngo, C. Petit, M. P. Pileni, Adv. Mater. 2001, 13, 58.
[9] L. S. Li, J. Hu, W. Yang, A. P. Alivisatos, Nano Lett. 2001, 1, 349.
[10] V. F. Puntes, K. M. Krishnan, A. P. Alivisatos, Science 2001, 291, 2115.
[11] J. Park-Sang, K. Seungsoo, S. Lee, Z. G. Khim, K. Char, T. Hyeon, J.
Am. Chem. Soc. 2000, 122, 8581.
[12] T. Ould Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, M.
Respaud, J. M. Broto, Chem. Mater. 1999, 11, 526.
[13] M. Verelst, T. Ould Ely, C. Amiens, E. Snoeck, P. Lecante, A. Mosset,
M. Respaud, J. M. Broto, B. Chaudret, Chem. Mater. 1999, 11, 2702.
[14] M. Respaud, J. M. Broto, H. Rakoto, A. R. Fert, L. Thomas, B.
Barbara, M. Verelst, E. Snoeck, P. Lecante, A. Mosset, J. Osuna, T.
Ould Ely, C. Amiens, B. Chaudret, Phys. Rev. B 1998, 57, 2925.
[15] C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante,
M. J. Casanove, J. Am. Chem. Soc. 2001, 123, 7584.
Scheme 1. Synthesis of aryl palladium malonates.
[16] K. Soulantica, A. Maisonnat, F. Senocq, M. C. Fromen, M. J.
Casanove, B. Chaudret, Angew. Chem. 2001, 113, 3071; Angew. Chem.
Int. Ed. 2001, 40, 2983.
[17] N. Cordente, M. Respaud, F. Senocq, M. J. Casanove, C. Amiens, B.
Chaudret, Nano Lett. 2001, 1, 565.
[18] S. Otsuka, M. Rossi, J. Chem. Soc. A 1968, 2630.
[19] Z. A. Peng, X. Peng, J. Am. Chem. Soc. 2001, 123, 1389.
[20] J. S. Bradley, B. Tesche, W. Busser, M. Maase, M. Reetz, J. Am. Chem.
Soc. 2000, 122, 4631.
[*] Prof. J. F. Hartwig, J. P. Wolkowski
Department of Chemistry
Yale University
P.O. Box 208107, New Haven, CT 06520-8107 (USA)
Fax : (þ 1)203-432-3917
E-mail: john.hartwig@yale.edu
[**] We thank the National Institutes of Health for support of this work
(GM-58108) and Johnson-Mattheyfor a gift of palladium salts.
[21] Z. Tang, N. A. Kotov, M. Giersig, Science 2002, 297, 237.
[22] Z. A. Peng, X. Peng, J. Am. Chem. Soc. 2002, 124, 3343.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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