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Nanostructured Metal Oxide Clusters by Oxidation of Stabilized Metal Clusters with Air.

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Nanostructured Metal Oxide Clusters by
Oxidation of Stabilized Metal Clusters with Air
Manfred T. Reetz,* Stefan A. Quaker, Martin Winter,
Jorg A. Becker, Rolf Schiifer, Ulrich Stimming, Andrea
Marmann. Roland Vogel, and Toyohiko Konno
Dedicated to Ptofi.ssot. Heini A . Starth
on the occusion of his 70th hirtliituj~
Nanostructured metal sulfides and oxides are useful as catalysts in organic and inorganic chemistry, and as materials with
special electronic and optical properties.['] The size-dependent
semiconductor properties of these materials are of special interest in microelectronics (quantum dots). Metal chalcogenides are
usually obtainable by precipitation o r sol-gel processes, in
which a wide variety of compounds, for example, polyphosphate, can be used as stabilizers.['. Although there are several
interesting approaches to control the particle size of metal sultides and oxides in the nanometer range, it is usually necessary
to use chromatographic enrichment to obtain monodisperse
We have recently described two electrochemical methods for
preparing R,N+X--stabilized metal and bimetal clusters.["']
In some cases it was possible to control the size of the clusters
within the range 1.5-6 nm by varing the current density and
solvent.[3. It was shown by high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy
(STM) that the metal clusters are stabilized by a monomolecular
layer of the ammonium salt.[61We now report that in the case of
fairly readily oxidizable metals such as cobalt, iron, or nickel the
R,N+X --stabilized clusters can be quantitatively converted into the corresponding nanostructured metal oxide clusters
(Scheme 1 ) .
Scheme 1. Idealized representation of the oxidation o f (C,H,,),N-X -stabilized
metal clusters with 0, (dark spheres. metal atoms: light spheres. oxygen atoms).
tion. Accordingly, the amount of oxygen consumed turned out
to be exactly that required for the stoichiometric formation of
cobalt(I1) oxide.[81The UV/Vis spectra of the material before
and after the reaction with 0, also indicate complete oxidation
(Fig. 1 ) . The solution of Co clusters shows unstructured absorption in the UViVis spectrum (359-800 nm). increasing with
increasing wavelength, which is typical for metal clusters
(Fig. l a ) . This decreases by a factor of 10 during the course of
the oxidation. and additional bands appear between 600 and
750 nm (Fig. 1 b).
To obtain a better insight into the oxidation process, the magnetic properties of the Co clusters were investigated. Since
R,N Br--stabilized Co clusters are s~perparamagnetic,['~
property should change during the oxidation. Indeed, the magnetic susceptibility, as measured by a magnetic (Gouy) balance,
falls away completely during the course of the reaction with 0,
(Fig. 2). parallel to the 0, uptake.['] After complete oxidation
the T H F solution of COO clusters is diamagnetic.
If a 0 . 5 solution
of (C,H,,),N+Br--stabilized cobalt clusters"] (3.6 nm mean diameter by TEM) is allowed under exclusion of moisture to react slowly with oxygen, the color of the
solution changes from blackish-brown to olive-green within a
few hours. To exclude the possibility that only a surface layer of
oxide on the cobalt clusters is formed, the progress of the oxidation reaction was monitored by measuring the oxygen consump[*] Prof. M . T Reetz. D r . S A. Quaker. DiplLChem. M Winter
Max-Planck-lnstitut fur Kohlenforschung
Kaiscr~Wilhelm-PlatzI . D-45470 Mulheim a n dei- Ruhr (Geriiiany)
F a x . Int. code +(208)3062-985
D r habil. J. A. Bccker. Dipl.-Chein. R Schiifer
Fachbereich Physikalischc Chemie der Universitit Marburg (Germany)
Prof. U. Stimming. Dipl.-Chem. A . Marmann. Dr. R. Vogel
Forschungszentrurn Julich GmbH. lnstitut fur Energirverf~ihrenstechiiik
Julich (Geriiiany)
Dr. T. Konno
Institute of M e t a l s Research. Tohoku Univci-sity. Scndai (Japan)
Fig. 1 UV Vis apectra of(C,H,-),N . Br -stabilized C o cliizters (ti) and COO cluslei-s ( h ) in THE
Fig 2 . Change of inagnctic susceptibility
with time i i i the rei~ction of
(C,H,-),N+Bi --stabilircd C'oclustcrs i n T H F with air(*.- ):exponential interpolation (
For further characterization. the cobalt oxide clusters were
investigated by HRTEM. In the TEM images (Fig. 3) single
clusters are visible, which are similar in mean diameter and
shape to the Co clusters in their initial state.["l The lattice
planes are also clearly visible. In the electron diffraction image
the diffraction rings characteristic of nanocrystalline materials
can be identified (Fig. 3 inset). Analysis of these revealed the
presence of a face-centered cubic structure. Of the known bulk
Co clusters in THF, theclusters become efficiently fixed onto the
support. Within 45 min the impregnation is complete. as shown
by photographs of sections of the pellets (Fig. 5 , top). During
and after the immobilization, the Co clusters are oxidized by air,
resulting in an olive-green coloration. Fixation of preformed
COOclusters on a support proceeds differently, penetration being only slight even after 45 min, (less than 200 pm; Fig. 5. bottom). This corresponds to a morphology that is desirable for
shell catalysts.[14.' 1 It is likely that the Co clusters d o not migrate very far into the pores of the A120, particles on a microscopic scale (cortexlike morphology).['41 although this still
needs to be clarified.
Ftg. 3 Tt.M image ol'owidized Cocluater\. The inset s h o w theelectron dit'l'fraction
I I l l age
cobalt oxides COO. Co,O,, and Co,O,, only the unit eel! of the
monoxide is face-centered cubic. The lattice constant found
from the electron diffraction rings (Fig. 3 inset) was found to be
4.249 A. which agrees well with the known value for bulk COO
(4.2495 A).[''.
To answer the question whether the cobalt oxide clusters in
solution are in fact stabilized by a layer of ammonium salt
an STM study was carried out. The oxidized
metal clusters were deposited on a specially prepared. faceted
gold s u r f ~ c by
~ "dip
~ coating. The STM image shows that the
cobalt oxide clusters are located on the steps of the surface
(Fig. 4). The clusters have an average diameter of 6.0 nm. As the
corresponding TEM images for the metal oxide core gave a
diameter of approximately 3.6 nm, the result of the combined
STM TEM study is consistent with the existence of a stabilized
outer layer of sui-factant.['l
Fig 3. Section thiough AI,O, pellets (dtanietcr 3 1 mm)aftct- i~iiiiiersion111 0.5\1
T H F soIution of ( C , H - ) , N ' Br -stabilized c'o clusters ( top) atid COO clusters
(bottom) for 5. 15. 30. and 45 mrn.
Other readily oxidizable R,N+X--stabilized metal clusters
can also be converted by oxygen into the corresponding metal
oxide clusters without forming insoluble bulk metal oxides. Examples include (C,H ,),N 'Br--stabilized iron oxide clusters
(2-3 nm) and nickel oxide clusters (2-3 nm), which respectively form reddish-brown and green solutions in THE Finally,
bimetal clusters (for example, F ~ / C O ) [can
~ I also be oxidized by
oxygen. The exact characterization of these materials and their
possible use in catalysis are subjects of further investigation.
Rrceivud: March 19. I V Y 6 [ZXY4YlE]
Get-man version: Aiigeii. C/imi 1996. /IN. 2228 2230
Keywords: clusters * cobalt compounds . magnetic properties
nanostructures scanning tunneling microscopy
H.Wellcr. Aii<yotic C h i . 1993. /fLi. 43: Aii,qrii. C/imi. / / i i Ed Eii,q/. 1993. 32,
41 : A. Henglein. &,r. Biiii.\oi,qu.r Phi,\ Chwi. 1995. YY. YO3. < j . Nimtr. P.
Marquard. H.Gleiter. J. Cri.\~.G r o i d i 1988. 86. 66.
[ I ] A . Fojtik. H.Wcller. U . Koch. A. Henglrin. B w Biiiwii,yc\ Phi \ C / w i i i 1984.
XN. 969
[3] M T. R e e t ~ .W. Helbig. J h i Chmii So<. 1994. i/6.7401. M.T. Reetz. S. A.
Quatacr. .Aii,yc,i,. C / i c v i i . 1995. 107. 2461 . Aii,qeii C / i ~ i i iii.i i E d Eii,q/. 1995. 34.
1240: M T Reetz. U: Hclb~g.S A. Q ~ ~ t ~ sC e' / i ri v.i i . .Mo/<,r 1995. 7. 2227.
[4] R,N'X
-\cabilii.ed metal cIustci-\ h a w been knoun S o i 'I long lime: .I Kiwi.
M .Griitrel. J A i i i ( ' / i c i i i . S o . 1979, ilt/. 7114. J. Bluni. 1' Sasson. A Zoran.
J. .Mo/ Curol. 1981, I / . 293. M Boutonnet. J. Kirling. P Stciitus. G . Mait-e.
( ' o / / i m / \ .Ski-/. 1982. i. 3OY. N Toshiiiia. T. Takahashi. H . H i t - a i . C h i i i . Lr,rr
1985. 1245: M Boutonnet. J Kirling. R.Touroudc. G. Maire. P Stenitis. :Ipp/.
C < i / u / .1986. 20. 163: K Mcguro. M. Tot-iyuka. K Esumi. Bii// ( ' h e i i i . So(.Jpi.
Pii/)iii Ski.
1988. 6/.341: J Wiesner. A. Wokaun. H . Hofl'niann. i+oc (d/.
1988. 76. 271: N. Satoh. K . Kimura. Bid/. C / i w .Yo<. ./p 1989. 62,1758. H
Bonncmann. W. Brijoux. R. Brinkmann. E DinJUs. T. Jotissen. B Korall,
. 4 / i q w C / i ~ ~ i1991.
/03. 1344. Air,qP'I (%(,111. I l l / . Ed. E i y / 1991. 30. 1311. &.
Toshima. T Takahashi. B i d Umi.S i c J p . 1992. 65. 400
151 M. T Rcetz. R. Breinhauei-. M.Wintcr. unpublithed re.;ult\
Fig 4
STM inioge oi' COO clusters
If solutions of COO clusters are allowed to come into contact
with moisture in the air. light green precipitates are formed.
TEM images reveal particles 100 nm to several millimeters in
size formed by agglomeration of cobalt oxide clusters.
Both the preformed Co clusters and the COOclusters can be
immobilized o n solid supports. For example, if dried AJ,O,
pellets (Johnson Matthey 11 838) are immersed in solutions of
[6] M. T. Reetz. W. Helbig. S. A. Quaiser. U. Stimming. N. Breuer. R. Vogel,
Science 1995. 267, 367.
[7] J. A. Becker. R. Sch2fer. R. Festag, W. Ruland. J. H. Wendorff, J Pebler, S. A.
Quaiser. W. Helbig. M. T. Reetz, J C h m Plivs. 1995. 103. 2520.
[S] It is also possible intentionally t o oxidize the Co clusters partially with substoichiometric amounts of Oi
[9] We thank C. Herwig, H. Petersen. S. Zdhn. and M. Becker (University of
Marburg) for performing the measurements.
[lo] A slight increase in particle size (by about 18%. estimated from the properties
of bulk Co and COO)would be expected. but such small differences cannot be
detected with certainty within the limits of accuracy of the TEM analysis.
[ I l l N. C. Trombs. H. P Rooksby. Nururi, 1950, 165.412.
[12] I n bulk COOthe proportion ofcobalt is usually substoichiometric because of
lattice defects. and the required charge neutralization is achieved by an increase
in the oxidation state of a few cobalt ions in the lattice from + 2 to +3.
This material is thus a homeotopic mixed crystal of COO and a little (green)
Co20, (N.Wiberg. LehrhLdi rler Airorguiirscheii Cheniie, de Gruyter. Berlin,
91st--100th ed.. 1985, p. 127, 1148). We cannot exclude the presence of small
amounts of C O , ~ , in the COOclusters. Interestingly, the electron diffraction
images show the presence of not only Co and COObut also C O ~ O(corundum
structure. hexagonal close packing) in the samples obtained by partial oxidation (extent, for example, 50%) [8].
[13] Some surfactant-stabilized metal oxides in microemulsions are known: J. B.
Nagy, A. Claerbout in SurJuctants i n Solution, Vol. 11 (Eds.: K. L. Mittal,
D. 0. Shah), Plenum, New York, 1991, p. 363; J. B. Nagy. Colloids Strrfuces
1989, 35, 201 ; A. Claerbout, J. B. Nagy, Stud. Surf: Scr. Cum/. 1991, 63. 705.
See also N. Moumen, M. P. Pilini, Chcni. Murrr. 1996.8, 1128. and references
[t4] M. T. Reetz. S. A. Quaiser, R. Breinbauer, B. Tesche. Angeu-. Cli~rii.1995. 107,
2956; A n p i . C h m In[. Ed. Engl. 1995, 34. 2728.
(151 E. F. Gallei. H.-P. Neumann. Chetn. h g . Terli. 1994, 66. 924.
transition metal containing calix[4]arenes1' with rigid well-defined receptor cavities for the development of supramolecular
materials and catalysts. We report herein the first resolution of
chiral metallocalix[4]arenes. This method makes use of tungsten
alkoxides,[' 31 which may be efficiently transformed into Lewis
acidic oxotungsten(vi) calix[4]arene complexes or dichlorotungsten(vr) caIi~[4]arenes.[~",
Among many resolution methods, we have concentrated on
the use of diastereomeric intermediates to resolve the chiral
metallocalix[4]arenes. Since the racemic R,W=O complex can
be considered as a conceptual analog of a R,C=O, the complex
R,W(OR*), was chosen to provide separable diastereomers
similar to classical resolutions of chiral ketones via
diastereotneric ketals. After separation, removal of the alkoxy
groups produces resolved enantiomers. To test the accessibility
and stability of cyclic tungsten(v1) calix[4]arene a l k o x i d e ~ , [ ' ~ ~
we synthesized trans-1,2-cyclohexanediol-basedchelates to produce complexes 2 a and 2 b . Two synthetic procedures were
developed (Scheme 1 ) . In one route, dichlorotungsten(vr)
p-tBu-calix[4]-arene (1 a) was heated to reflux with doubly deprotonated cyclohexanediol in toluene for four hours to give
2a in 96 YOyield. Alternatively. oxotungsten(v1) calix[4]arene
(Ib) is transformed to 2 b in 10% yield when treated with
silylated cyclohexanediol in refluxing toluene for two days. The
Chiral Metallocalix[4jarenes:
Resolution by Diastereomeric
Tungsten(v1) Alkoxides**
Bing Xu, Partrick J. Carroll, and
Timothy M. Swager*
2 a: Y= rBu
Efficient preparations of chiral supramolecular systems are critical to the design of new materials and catalysts. Calixarenes['l present one of
the most versatile platforms for the construction
Scheme 'Y1 nthesis of 2 .
of supramolecular receptor assemblies with welldefined structures and specific functions.['] Recent applications include the use of calixarenes as highly specific
H N M R spectra displayed by these calix[4]arenes are diagnosl i g a n d ~ , [sensors,[41
nonlinear optical c h r o m ~ p h o r e s ,porous
tic for the formation of 2. Due to lower symmetry of 2, the
monolayers,'61 and bowlic liquid crystals.[71 Chiral calix[4]methylene groups of the calix[4]arene core are inequivalent and
arenes have been produced by unsymmetric substitution of the
produce two sets of coupled geminal protons for a total of four
doublets. Both 2a and 2 b can be purified by chromatography
lower rim,['] by the addition of chiral appendages,['] and by the
incorporation of unsymmetric substitution patterns on the
on a silica gel column without any decomposition.
phenyl rings.['*] rneta-Substituted systems with C, symmetry of
With methods to prepare tungsten(v1) dialkoxy calix[4]arenes
in hand, we endeavored to synthesize diastereomeric complexes
the latter type have been prepared as racemates,"obl and a route
in which both the calix[4]arene and dialkoxy ligands are chiral
to analogs with a variety of functional groups has recently been
(Scheme 2). We employed 3,4-dimethylcalix[4]arene,which had
reported.[' In spite of the interest in chiral calix[4]arenes, few
been previously reported by Bohmer et al., since it has an interresolutions have been developed. We have been investigating
esting inherently chiral structure with C, symmetry.['0b1Reaction of 3,4-dimethylcalix[4]arene with WCI, in benzene pro[*I Prof. T. M Swager.[+' Dr. B Xu."' Dr. P. J. Carroll
the dichlorotungsten(v1) complex 3 in 81 YOyield. The
Department of Chemistry
'H N M R spectrum contained four doublets for the calix[4]arene
University of Pennsylvania
methylene protons. which is consistent with 3 having a elliptical
Philadelphia, PA 19104-6323 (USA)
Racemic 3 reacts
cone conformation with C ,
['I New address:
Department of Chemistry
with (I S,2S)-tvans-l,2-~yclohexanediol
under the same condiMassachusetts Institute of Technology
tions used to prepare 2 a to give a mixture of the diastereomers
Cambridge, MA 02139 (USA)
and 5. The diastereomeric relationship is apparent from the
Fax: Int. code +(617)253-7929
N M R resonances for the methylene protons (eight doublets,
[**I This work was supported by the Office of Naval Research and a Camille and
1). The two diastereomers 4 and 5 can be separated by
Henry Dreyfus Teacher Scholar Award.
VCH Ver/ugs~e,srll.s~liu/t
m h H . 0-69451 Weiiiheun. IY96
OS70-0833/Y6~3518-1094$ 15.00+ .25/0
Angen.. Chem. Inr. Ed. Engl. 1996, 35, No. 18
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air, oxidation, oxide, clusters, stabilizer, metali, nanostructured
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