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Light-Directed Assembly of Nanoparticles.

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force-arising from the drive to lower the thin film free energy
by thickening-is sufficient to continue dragging along particles
only up to a certain point, when the contact line of the hole
becomes pinned due to particle- substrate interactions.
In determining the effect of the particles on the film, we note
that capillary forces[”’ between particles-arising from the deformation of the liquid surface due to the presence of particlesare weak because the film thickness is small enough for disjoining pressure to dominate over hydrostatic pressure (gravity).
For nanometer thicknesses, the effective capillary length 1, or
decay length of the capillary forces, is short-ranged-on the
order of angstroms. More explicitly, instead of the usual 1=
(y/Apg)1’2(Ap = density of liquid-density of air, g = acceleration due to gravity) we have 1= (yt4/AH)’’2.[201
This expression
implies that 1 is several Angstroms for yx 50 dynescm-’,
A,,% k,T, and t the in nanometer range. Finally, since the effective spreading coefficient for the liquid on the particle is approximately an order of magnitude smaller than the spreading coefficient of the liquid on the solid substrate, the liquid prefers to
spread on the solid substrate, rather than “riding up” the particle. It follows that the particle has little effect on the liquid, and
consequently capillary forces are effectively negligible.
Consider particles, previously immersed in the wetting thin
film, which are in the rim of an advancing hole. Because of
downward dispersional attractions between particles and the
substrate, they can move along with the expanding rim only if
the force opening the hole is strong enough to overcome the
static friction arising from particle-substrate and particle-particle interactions. Taking into account the former effect, we let
F,, dispr21] denote the downward dispersional force attracting an
individual particle to the substrate, and let K (of order unity) be
the associated friction coefficient. The lateral force opposing
motion of each particle is the product of these quantities. The
thickening force per particle was shown above to be inversely
proportional to hole size R, which decreases as the rim advances. It follows that the rim will be pinned when the hole
radius re‘aches a value Rpindetermined by the balance between
the lateral friction and outward thickening forces [Eq. (2)]:
Since F,, disp depends only on the size r of the individual particles for a given solution of nanocrystals-that is, for a specific
particle size r, solvent, and substrate (and hence for a fixed set
ring size R,,,from Equaof values for S, t,, and t,)--the
tion ( 2 ) is expected to vary inversely with concentration 4. Furthermore, for a given 4, all ring sizes should be the same (even
though their associated holes began to grow at different times),
if the particle size distribution is sufficiently narrow (see Figure 2 ) .
It was also observed that when the concentration of particles
is too small-such that the center-center ring separation becomes smaller than the ring diameter-both rings and compact
structures are observed, consistent with the idea that holes percolate and crowd particles into the interstices, where they then
form compact structures. Other physical properties, such as annulus width and surface coverage of rings, can also be controlled, and we are currently trying to quantify such control,
through both experiment and theory.
Received: November 14,1996 [Z9770IE]
German version: Angew. Chem. 1997,109,1119-1122
Keywords: nanostructures
1080
- self-assembly - thin films
0 VCH Verlagsgesellschaft mbH, 0-69451 Weinheim, 1997
[l] N. Herron, J. C. Calabrese, W. E. Farneth, Y. Wang, Science 1993,259, 14261428.
[2] T. Vossmeyer, G. Reck, L. Katsikas, E. T. K. Haupt, B. Schulz, H. Weller,
Science 1995,267, 1476-1479.
[3] A. Eychmuller, A. Mews, H. Weller, Chem. Phys. Lett. 1993, 208, 59-62.
141 C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 1995, 270, 13351338.
[5] P. C. Ohara, D. V. Leff, W M. Gelbart, J. R. Heath, Phys. Rev. Lett. 1995, 75,
3466- 3469.
[6] L. Motte, F. Billoudet, M. P. Pileni, J. Phys. Chem. 1995, 99, 1642516429.
[7] M. Giersig, P. Mulvaney, Langmuir 1993, 9, 3408-3413.
[S] R. L. Whetten, J. T. Khoury, M. Alvarez, S. Murthy, I. Vezmar, 2.L. Wang,
P. W. Stephens, C. L. Cleveland, W. D. Luedtke, U. Landman, Adv. Muter.
1996,8,428.
[9] W. D. Luedtke, U. Landman, J. Phys. Chem. 1996,100, 13323-13329.
[lo] J. R. Heath, C. M. Knobler, D. V. Leff, J. Phys. Chem. 1997,10f,189-197.
[Ill T. Takagahara, Surf Sci. 1992,267,310-314.
[I21 C. R. Kagan, C. B. Murray, M. Nirmal, M. G. Bawendi, Phys. Rev. Lett. 1996,
76, 1517-1528.
[I31 E. Adachi, A. S. Dimitrov, K. Nagayama, Langmuir 1995, ff, 1057-1060.
[14] R. D. Deegan, 0. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, T. A. Witten
(University of Chicago), personal communication.
[IS] J. A. Moriarty, L. W. Schwartz, J. Colloid Int. Sci. 1993, f6f, 335-342, and
references therein.
[16] M. Elbaum, S. G. Lipson, Phys. Rev. Lett. 1994, 72, 3562-3565.
[I71 P. DiBenedetti, Metastable Liquidr, Princeton University Press, 19%.
[IS] F. Brochard-Wyart, J. M. di Meglio, D. Quere, P. G. de Gennes, Langmuir
1991, 7, 335-342; P. G. de Gennes, Rev. Mod. Phys. 1985,57,827-863, and
references therein.
[I91 F. Brochard-Wyart, J. Daillant, Can. J. Phys. 1990,68, 1084-1088.
[20] P. A. Kralchevsky, I. B. Ivanov, K. Nagayama, J. Colloid In?. Sci. 1992, f5f,
79-94, and references therein.
[21] S. A. Safran, Statistical Thermodynamics of Surfaces. Interfaces. and Membranes, Addison-Wesley, 1994, and references therein.
Light-Directed Assembly of Nanoparticles”*
Tobias Vossmeyer, Erica DeIonno, and
James R. Heath*
A major challenge associated with the application of semiconductor and metal nanocrystals is the development of methods
for placing them into chemically and structurally complex environments.[’] In particular, applications of nanocrystals in
devices often require precise spatial positioning of the particles
onto a substrate. Recent approaches have utilized shadow-mask
evaporation or lithographic/etching technologies to prepare a
pattern of two different materials. Particles are then selectively
grown or attached to one of the two materials. One example is
the selective heteroepitaxial growth of germanium dots on
silicon microsurfaces patterned through an SiO, overlayer.[21
[*I Prof. Dr. J. R. Heath
Molecular Design Institute, Lawrence Berkeley Laboratory
and
Deptartment of Chemistry and Biochemistry
University of California at Los Angeles
405 Hilgard Avenue, Los Angeles, California 90095-1569 (USA)
Fax: Int. code +(310)206-4038
e-mail: heath@chem.ucla.edu
Dr. T. Vossmeyer, E. DeIonno
University of California at Los Angeles
[**I This work was supported by the Deutsche Forschungsgemeinschaft (DFG),
the Packard Foundation (Contract No. DE-AC03-76SF00098), the Office of
Naval Research (Order No. N00014-95-F-0099),and by the Director, Office of
Energy Research, Office of Basic Energy Research, Division of Materials Sciences, of the U. S. Dept. of Energy. We acknowledge S.-W. Chung for taking
the SEM images and Dr. L. Brandt, Dr. J. J. Shiang, V. Z. Doan, and E. Newman for their contributions. We are grateful for the valuable discussions with
Prof. Dr. H. Ringsdorf.
0570-0833~97~36lO-lO80
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Angew. Chem. Int. Ed. Engl. 1997, 36, No. 10
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posed to light and yielded a pattern of free and protected amino
groups. This pattern could be monitored by fluorescence microscopy after treating the free amino groups with ATTO-TAG,
a specific fluorescent marker for primary amino groups.
In order to bind gold nanocrystals to the free amino groups
of the pattern, we prepared a solution of 12-aminododecanecapped gold particles (2.6 nm diameter Au cores) in toluene.[*l
These particles have been shown to be kinetically, rather than
thermodynamically, stabilized. They readily undergo ligand exchange with the surface-bound amino groups, and thus, a selective binding of the particles onto the patterned substrate surface
is achieved. This kind of two-phase ligand exchange is further
favored, because it is coupled with a precipitation of the particles onto the sample surface. After the patterned substrates
had been kept overnight in the gold particle solution, binding of
particles to the substrate was sometimes observable by eye, but
not always. To amplify particle binding, we treated the surfacebound gold crystals with 1&octanedithiol to yield free, surfacebound thiol groups on areas where gold particles had already
been attached to the surface. Following dithiol treatment, the
slides were dipped again into the particle solution to bind more
gold nanocrystals onto previously bound particles. This dithiol
amplification could be repeated several times. I n this way, the
micropattern contrast was easily enhanced until it was readily
visible by eye or through an optical microscope. An optical
micrograph of an amplified pattern recorded with a CCD camera
is shown in Figure 2a. The micromask structure is clear, indicating the successful light-directed assembly of gold particles.
Micropatterns prepared on silicon wafers were imaged with a
scanning electron microscope (SEM). Such images are shown in
Figures 2b- and 2c. Figure 2c reveals that the
technique is not completely selective to the lightexposed surface, as there
hv > 340 nm
are some Au nanocrystal
Nvoc
grains assembled on the
I
unexposed regions of the
COOH
surface. The micropattern
+
was
also imaged with an
co co co co
co co co co
atomic force microscope
NHZNH2NH2NH2
NH NH NH NH
+ NH NH MI NH
(AFM) operating in conI
1
1
1
1
I1
tact mode. These images
confirmed that the assembly technique is not 100 %
spatially selective but provided no new information.
The spatial selectivity of
the assembly process was
quantified by preparing
macroscopic patterns on
glass slides that could be
probed by UViVis absorption spectroscopy. In Figure 3 we compare three absorption spectra. Spectrum a is that of a particle
1
1
1
1
co co co co
solution in toluene. Spectra b and c were taken
from the same glass slide,
Figure 1. Reaction scheme for the light-directed assembly of Au nanoparticles onto glass and silicon substrates. An amino-funcand correspond to areas
tionahzed substrate was first treated with NVOC-GLY (I) to give a photosensitive surface. Upon irradiation through a microchip
that were exposed to light
mask (11) a pattern of free and protected amino groups was obtained and treated with a solution of amine-stabilized Au nanopar(b)
or kept in the dark
ticles (111) The Au-particles assembled by ligand exchange on the areas with exposed free amino groups. To amplify selective
(c) prior to binding gold
particle binding. dithiols were coupled onto the surface-bound Au particles (IV). Renewed dipping of the sample into a solution
of Au particles ( V ) led to crosslinking between dissolved and previously bound particles (Vl).
nanoparticles. To amplify
A second example involves the selective attachment of particles,
through bifunctional organic ligands, onto a gold electrode pattern on silicon.[3]Bifunctional ligands have found general use in
binding particles to
Other than by physically removing these ligands from selected portions of the substrate by
laser or electron-beam ablation, this chemistry is not spatially
selective. However, in 1991 Fodor and coworkers described how
photolabile protecting groups attached to glass substrates can
be used for light-directed, combinatorial solid-phase peptide
In this communication we describe an extension of
synthesi~.'~]
this technique that allows the light-directed assembly of gold
nanoparticles onto glass and silicon substrates. The initial assembly process, while spatially selective, is not particularly efficient. However, the attached particles themselves then serve as
a chemically selective "substrate" for subsequent attachment of
bifunctional ligands. We show that this selectivity can be utilized
to amplify the initial photochemically defined pattern to produce arbitrarily thick, patterned films of particles.
To prepare the light-sensitive substrates, we first treated
cleaned silicon or glass slides with 3-aminopropyldimethylethoxysilane. The choice of the monoalkoxysilane is critical for
avoiding silane polymerization and for maintaining a smooth
substrate surface. The amino groups were then treated with the
cdrboxyk acid group of nitroveratryloxycarbonylglycine
(NVOC- GLY) to give an NVOC-terminated sample surface
(Figure 1 ) . NVOC is a photoremovable protecting group for
primary amino groups that is used in solid-phase peptide synthesis." 'I A pattern was defined photochemically by exposing
the substrate to UV/Vis radiation .(; > 340 nm) through a microchip mask. This removed NVOC groups from the areas ex-
- 1111
u
7"i""l"
u
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nus, the light-directed assembly of CdS-, CdSe-, and other semiconductor or metal particles should be feasible. Thus, by stepwise deprotecting and derivatizing the surface groups, it should
be feasible to build up structures consisting of different kinds of
nanoparticles. It will be an interesting challenge to investigate
the electronic, optical, and catalytic properties of such structures. It will also be a very interesting experiment to decrease the
dimensions of the prepared patterns down to the submicron
scale by using optical nearfield and solid imersion lens techniques.
a)
-
30 prn
b)
-
Experimental Sect ion
30 prn
C)
1
0
pn
Figure 2. a) Optical microscope image of Au nanoparticles assembled on a glass
substrate. The particles assemble preferrentially on areas which have been exposed
to light through a microchip mask. These areas appear as the darker parts of the
image. To obtain a good contrast, the dithiol amplification procedure described in
the text was carried out four times. b) SEM image of Au nanoparticles assembled
on a silicon substrate. In this image, the lighter parts represent the areas on which
the gold particles assembled. The resolution of light-directed particle binding is at
least 3 pm. Dithiol amplification was carried out three times. c) SEM image at
higher magnification. The image reveals some nonselective binding.
-A/nm
800 700
600
I " '
'
'
500
'
400
1
hvleVFigure 3. UV/Vis absorption spectra of Au nanoparticles. Spectrum a was taken of
a solution of Au particles in toluene. Spectra band c were taken of a macroscopically
assembled pattern of the particles on a glass substrate to determine the degree of
nonselective particle binding. Dithiol amplification was carried out three times. The
plasmon absorption around 2.1 eV in spectrum b, taken on an area of the pattern
that has been exposed to light prior to particle binding, is about seven times stronger
than that of spectrum c, taken on an area that was kept in the dark. This result
implies about 15% nonselective particle binding. For more details, see text;
A = absorbance.
Preparation of photosensitive glass and silicon substrates: microscope cover slides
and pieces of silicon wafers were cleaned as described in ref. 191 and stored under
deionized water (18.2MRcm) until needed. To treat the substrates with 3aminopropyldimethylethoxysilane we used the experimental setup described by
Haller (101. Thedried substrates were put intoa 3 . 0 m ~solution of the silane in dry
toluene and heated at retlux under argon overnight. Following standard procedures
for solid-phase peptide synthesis [ l l ] , NVOC-GLY (which was prepared as described in ref. [7]) was coupled to the surface amino groups. For this reaction
diisopropylcarbodiimide (DIC) was used to activate the free NVOC-GLY carboxy
group. The photosensitive substrates were stored under argon at - 18°C and used
for the lithographic experiments within one week. All preparations with the lightsensitive NVOC group were carried out in the dark.
Photolytical deprotection of surface bound NVOC-glycine: The photosensitive
substrates were covered with a microchip or macroscopic mask and irradiated with
a 1000 W tungsten halogen lamp for about 40 min. To avoid sample damage by UV
irradration with short wavelengths, a 340nm longpass filter was used. The success
of removing the NVOC groups could he tested by treating the prepared free amino
groups with ATTO-TAG (Molecular Probes) [12]. The slide was imaged with a
Reichert-Jung MET fluorescence microscope equipped with a RCA silicon intensified camera.
Preparation of gold nanoparticles and coupling to surface amino groups. gold
nanocrystals stabilized with dodecylamine molecules (2.6 nm metal-core diameter)
were prepared according to reaction Scheme 1 described in reference 181. In order to
treat the lithographically patterned substrates, we dissolved these particles in
toluene (concentration corresponded to an optical density of 0.5 for a 0.5 cm pathlength at 550 nm). The substrates were kept in this solution overnight under ambient conditions. To amplify particle binding the slides were washed with toluene and
acetone and then immersed into a solution of 100 pL (0.5 mmol) 1.8-octanedithiol
in 5 mL acetone for 1 h. After washing the substrates with toluene. they were treated
again with a fresh particle solution overnight. This amplification process could be
repeated several times. To investigate the ratio of nonspecific binding, UV/Vis
spectra ofa macroscopic pattern on a glass slide were taken with a Hewlett Packard
8451 A Diode Array Spectrophotometer.
Sample immaging of selectively bound gold particles: SEM images were taken with
a Cambridge 360 scanning electron microscope operated at 15 kV. The optical
microscope images were taken with a remodeled Reichert-Jung Polyvar Infrapol
microscope in reflection mode, equipped with a CCD camera.
Received: November 29, 1996 [Z9835IE]
German version: Angew. Chem. 1997, 109.1123-1125
Keywords: clusters
the macroscopic pattern, the dithiol treatment was carried out
three times. The plasmon band around 2.1 eV and 2.4 eV is the
optical signature of the gold particles and is detected in all three
samples. In spectrum b the maximum absorption strength of the
plasmon band is about seven times that of spectrum c, suggesting a ratio of about 15% unspecific binding. The shift and
energy broadening of the plasmon band for the substrate-bound
particles is probably due to differences in the dielectric environment and particle -particle interactions. In fact, the band shown
in spectrum b is shifted towards lower energies than the band in
spectrum c, suggesting stronger particle-particle interactions in
the thicker film.
The method described here for the light-directed assembly of
nanoparticles is a general approach that should make the preparation of complex nano/micro structures possible. Since the surface-bound amino group can be derivatized to give a thiol termi-
1082
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gold - nanostructures
*
photochemistry
[ I ] a ) H Weller, Angew. Chem. 1996, 108, 1 1 59; Angew. Chem. Int. Ed. Engl. 1996,
35, 1079; b) A. P. Alivisatos, K. P. Johnsson, X . Peng, T. E. Wilson, C. J.
Loweth, M. P. Bruchez. Jr., P. G Schultz, Nature 1996. 382, 609; c) C. A.
Mirkin. R. L. Letsinger, R. C. Mucic, J. J. Storhoff, ibid. 1996, 382, 607.
d) C. B. Murray, C. R. Kagan. M. G. Bawendi, Science 1995, 270. 1335;
e) S. A. Harfenist, 2. L. Wang, M. M. Alvarez, I. Vezmar, R. L. Whetten, J.
P h w Chem. 1996, 100, 13904; f ) M. Brust, D. Bethell, D. J. Schiffrin. C. J.
Kiely, A h . M u f w . 1995, 7, 795; g) P. Ohara, D. V. Leff. J. R. Heath. W. M.
Gelbart, P h w Rev. Leir. 1995, 75, 3466
[2] J. R. Heath, R. S. Williams, J. J. Shiang, S. J Wind, J. Chu, C. D'Emic, W,
Chen. C. L. Stanis. J J. Bucchignano, J. Phys. Chem. 1996. 100, 3144.
[3] D. L. Klein. P. L. McEuen, J. E. Bowen Katdri, R. Roth, A. P. Alivisatos,
Appl. P h u . Lett. 1996, 68, 2574.
141 a) V. L. Colvin, A. N. Goldstein, A. P. Alivisatos, J. Am. Chem. Soc. 1992, 114,
5221 ; b) S. Peschel, G Schmid, Angew. Chem. 1995,107,1568; Angew Chem.
Inf. Ed. Engl. 1995, 34. 1442.
(51 a) S. P. A. Fodor, J. L. Read, M. C. Pirrung, L. Stryer, A. T. Lu, D. Solas,
Science 1991,251,767;b) see also G. von Kiedrowski, Angew Chem. 1991,103,
839; Angen. Chem. Int. Ed. Engl. 1991.30. 822.
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Angex Chem Int Ed Engi 1997. 36, No I 0
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[6] a) C. Y. Cho, E. J. Moran, S. R. Cherry. J. C. Stephans, S. P. A. Fodor, C. L.
Adams, A. Sundaram, J. W. Jacobs, P. G. Schultr, Science 1993, 261, 1303;
b) A. Patchornik, B. Amit, R. B. Woodward, J. Am. Chen7. Soc. 1970,92,6333.
[7] A. Patchornik, B. Amit, R. B. Woodward. Peptides, Puoc. Eur. Pept. Symp..
North-Holland. Amsterdam. 1971, 12.
[8] D. V. Leff, L. Brandt, J. R. Heath, Langmirir 1996, 12, 4723.
[9] M. R. Linford, P. Fenter, P. M. Eisenberger, C. E. D. Chidsey, J1 Am. Chem.
Sor. 1995, 117, 3145.
[lo] 1. Haller, J. A m . Client. Sue. 1978, 100, 8050.
[I I] See, for example, J. M. Stewart, J. D. Young, SoiidPhuse Pepride Syn/hesis, 2nd
ed., Pierce Chemical Company, Rockford, IL, 1984.
[I21 W. T. Muller, D. L. Klein, T. Lee, J. Clarke, P. L. McEuen, P. G. Schultr,
Science 1995, 268, 272.
The [Cu,(O,CMe),(H,O)zl/(PY),CO System
as the Source of an Unusual Heptanuclear
Complex and a Novel Dodecanuclear
"Flywheel" Cluster
Vasilis Tangoulis, Catherine P. Raptopoulou,
Sofia Paschalidou, Evangelos G. Bakalbassis,"
Spyros P. Perlepes,'" and Aris Terzis"
Polynuclear transition metal complexes exhibit a fascinating
variety of unusual symmetries and structural patterns.['] The
synthetic techniques that provide access to these complexes,[21
the biological relevance of some of these compounds,[31and the
magnetic properties associated with a large number of interacting paramagnetic centers in a single cluster[41 add to their
interest.
Reactions between [Cu,(O,CMe),(H,O),] and (py),CO (di2-pyridyl ketone) have proved to be a rich source of interesting
polynuclear copper(@ complexes. For example, in water the
system [Cu,(0,CMe),(H,0),]/(py)2CO/C10~ yields the acetato-bridged, dicubane complex 1,15]whereas in MeOH 2 is obtained,['' in which the four Cu" atoms are coplanar with an ideal
rhombic arrangement. Suspecting that we had merely scratched
the surface of this chemistry, we decided to investigate the products obtained from the [Cu,(0,CMe),(H,0)2]/(py)2C0 reaction system in the absence of ClO,. Here we report on the
[*I
Prof. Dr. E. G. Bakalbassis
Laboratory of Applied Quantum Chemistry. Department of Chemistry
Aristotle University of Thessaloniki
GR-54006 Thessaloniki (Greece)
Fax: Int. code +(31)997738
e-mail : bakalbas(i calliope.ccf.auth.gr
Prof. Dr. S. P. Perlepes, S. Paschalidou
Department of Chemistry
University of Patras
GR-26500 Patras (Greece)
Fax: Int. code +(61)997118
Dr. A. Terzis, V. Tangoulis, Dr. C. P. Raptopoulou
Institute of Materials Science
NRCPS "Demokritos"
GR-15310 Aghia Paraskevi Attikis (Greece)
Fax: Int. code +(1)6519430
e-mail: terzis(a cyclades.nrcps.ariadne-t.gr
[**I This work was supported by the Greek General Secretariat of Research and
Technology, the Greek Secretariat of Athletics (OPAP), and Athina Athana-
siou.
Anwbz. Chern. Int. Ed. Engl. 1997, 36, N o . 10
8
copper(I1) complexes 3 and 4, which have unusual structural
features, and describe the magnetic properties of 3, which was
obtained in pure form.
[C~,I(PY),CO,~,(M~CO,),(OH),I
H,O 0 5 MeCN 3
[CU, 2{(PY)2CO,l,(MeCO,)I
21
MeCN 4
Treatment of [Cu,(O,CMe),(H,O),] with one equivalent of
(py),CO in hot MeCN afforded a deep blue solution. Layering
of this solution with Et,O/n-hexane ( l / l ) gave a mixture of
well-formed green cubes, violet-blue prisms, and sky blue
needles. The difference in color and shape of the crystals allowed
manual separation of the three materials. The green and violetblue crystals proved to be complexes 3 and 4,respectively, as
shown by single-crystal X-ray crystallography; the sky blue
needles, which analyzed as [CU(O,CM~){(~~)~C(OH)O}].
MeCN, have yet to be obtained in X-ray quality. With the
identities of 3 and 4 established, we sought preparative routes to
pure materials by slightly modifying the molar ratio of [Cu,(O,CMe),(H,O),] and (py),CO. This has so far been successful
only for 3, by employing low concentrations of the reactants.
Compounds 3 and 4 are the first metal complexes to contain
doubly deprotonated ions of the hydrated gem-diol form of
(py),CO as ligands. This is a consequence of the high MeCO;
to (py),C(OH), ratio (4:1, 4.7:l) used in the reactions.
Figure 1 shows the molecular structure of the neutral heptanuclear complex 3,[71which has one p 3 :y1 :y 2 : y2 :y'-bridging
Figure 1 , Central structural unit of 3. Most carbon atoms of the pyridine rings have
been omitted for clarity. A twofold axis runs through Cul and C6. Distances [A]:
Cu-Cu 3.04-6.44, 0 - 0 1.91-2.30, Cu-N 1.99 2.01.
(py),CO:- dianion (A), two p4:q1:q2:y3:y'-bridging (py),C0;- dianions (B), four s y n ~ y n - p:,y1 :q'-MeCO; groups, two
.B
A
monodentate MeCO; ions, and two p3-OH ions as ligands. A
twofold crystallographic axis passes through Cul and C6. The
four Cu" centers Cul -Cu4 occupy the corners of a distorted
tetrahedron, and as a result of the twofold axis, the seven metal
ions form two tetrahedra sharing a common apex at Cul. The
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