Finite, spherical coordination networks that self-organize from 36 small components

Article abstract of DOI:10.1002/anie.200461422

Simple banana-shaped organic molecules self-organize into finite, spherical coordination networks with a diameter of up to 7 nm (see structure; green Pd, red O, blue N, gray C). These molecular spheres consist of 12 equivalent metal centers and 24 equival

Full text of DOI:10.1002/anie.200461422

Angewandte
Chemie
Self-Assembly
Finite, Spherical Coordination Networks that Self-
Organize from 36 Small Components**
Masahide Tominaga, Keisuke Suzuki, Masaki Kawano,
Takahiro Kusukawa, Tomoji Ozeki, Shigeru Sakamoto,
Kentaro Yamaguchi, and Makoto Fujita*
Highly symmetric structures often appear in nature as
revealed by, for example, the capsids of spherical viruses
that have icosahedral symmetry consisting of 60n identical
protein subunits.[1] The reason for the high symmetry lies
behind the principle that increasing the number of elements
with the same symmetry reduces the amount of independent
structural information, which is directly related to the length
of DNA. Thus, the self-organization of tiny subunits into a
giant biological molecule can be regarded as the process of
not only structural growth but of the amplification of
molecular information. We show herein that, through
metal–ligand interactions,[2,3] simple banana-shaped organic
molecules self-organize into finite, spherical coordination
networks with a diameter of up to 7 nm, which is in contrast to
the formation of two-dimensional (2D) infinite networks that
occurs with linear organic ligands. The spherical coordination
networks consist of 36 components, 12 equivalent metal
centers (M) and 24 equivalent ligands (L), and have cubocta-
hedron symmetry. By attaching a functional group (e.g., C60 or
porphyrin) to each ligand, 24 functional groups are aligned
equivalently at the periphery of the sphere.
Over the last decade, extensive studies have been made on
infinite coordination networks that are formed by the
complexation of exo-multidentate ligands with transition-
metal ions. A typical and simple example is given by a 2Dgrid
complex that forms from a rodlike ligand and a metal
(Figure 1a).[4] We expect that, if the ligand framework is
slightly bent, the coordination network will develop with a
[*] Dr. M. Tominaga, K. Suzuki, Dr. M. Kawano, Dr. T. Kusukawa,
Prof. M. Fujita
Department of AppliedChemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Dr. S. Sakamoto, Prof. K. Yamaguchi
Chemical Analysis Center, Chiba University
Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Dr. T. Ozeki
Department of Chemistry andMaterials Science
Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
[**] This research was supportedby the CREST project of the Japan
Science andTechnology Corporation (JST), for which M.F. is the
principal investigator. We thank S. Adachi (KEK) for supporting X-
ray crystallographic measurement. This work has been performed
under the approval of the Photon Factory Program Advisory
Committee (Proposal No. 2003G186).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461422
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Communications
but became sharp at higher temperatures, characteristic of
very large species whose motion is slow on the NMR time
scale. Downfield shift of the signals, particularly for Pya (Dd =
0.79 ppm; Py = pyridine), was ascribed to the metal–ligand
complexation. Diffusion-ordered NMR spectroscopy
(DOSY) showed a single band at the diffusion coefficiency
of 1.1 ” 10À10 m2 sÀ1, from which the diameter of the product
was roughly estimated to be 3.6 nm.[7] After anion exchange
from [NO3]À to [PF6]À ions, cold-spray ionization mass
spectrometry (CSI-MS)[8] clearly indicated an M12L24 compo-
sition with the molecular weight of 10330 Da by a series of
Figure 1. Schematic representation of the self-assembly of coordina-
[MÀ(PF6 )n]n+ (n = 6–13) peaks (Figure 3).[9] Fragmentation
À
tion networks from metal ions which favor a square-planar coordina-
tion geometry and different bridging ligands. a) Linear ligands are
expectedto self-assemble to give 2D gridcomplexes. b) Slightly bent
ligands are expected to self-assemble to give spherical finite com-
plexes.
in the MS measurement was hardly observed except the
dissociation of counteranions, which demonstrates the
remarkable stability of the product in solution. Elemental
analysis was also consistent with the M12L24 composition.
constant radius of curvature and a spherical finite network
will be obtained (Figure 1b), reminiscent of the formation of
graphite versus that of fullerene from sp2 hybridized carbon
atoms. Based on this idea, we designed ligands 1a–c and
Figure 3. CSI-MS spectrum showing the formation of M12L24 product
(PF6À salt).
From the detailed NMR and mass spectroscopic measure-
ments, the formation of the roughly spherical giant molecule
2a was deduced because of good agreements with the M12L24
composition and the equivalence of all the ligands (Fig-
ure 4a). The symmetry of 2a is dictated by a cuboctahedron,
which is formed by truncating each of the eight vertices of a
cube to generate eight triangular faces. The 12 equivalent
vertices and 24 equivalent edges of the cuboctahedron can be
superimposed on the 12 palladium(ii) centers and 24 bridging
ligands, respectively (Figure 4b). Related cuboctahedral
complexes with Cotton-type copper(ii) dinuclear tetracarbox-
ylate junctions have been reported,[10] but no solution
behavior is reported probably because of their poor solubility
in common polar solvents. Since these copper(ii) species are
generated by inert dinuclear tetracarboxylate formation,
instead of self-assembly, oligomeric products are produced
as well and the yields are moderate or not well determined. In
contrast, spherical complex 2a immediately assembles from
36 components in a quantitative yield thanks to moderately
labile palladium(ii)–pyridine interactions.
examined their complexation with naked palladium(ii) ions
which favor a square-planar coordination environment.[5]
When ligand 1a (0.02 mmol) was treated with Pd(NO3)2
(0.01 mmol) in [D6]DMSO (1.0 mL) at 708C for 4 h, the
quantitative self-assembly of a single product was detected by
1H NMR spectroscopy.[6] Only five signals are observed
indicating that all the ligands are located equivalently in the
product and have same inherent symmetry (Figure 2). The
resonance signals were relatively broad at room temperature
A scanning tunneling microscopy (STM) study revealed
the structural integrity of 2a (Figure 5a) even under STM
conditions. More significantly, the image obtained on a
graphite surface demonstrates that the spheres 2a behaves
as “molecular particles” with precise chemical structure and
uniformed dimension (height) of 3.5 nm (Figure 5b), which is
consistent with the DOSY measurement and molecular
model prediction.
Figure 2. The 1H NMR spectrum (aromatic region) of the product
assembledfrom Pd(NO 3)2 andligand 1a (2 equiv; 500 MHz,
[D6]DMSO, 258C, TMS).
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Angewandte
Chemie
is 3.4 nm. The shortest Pd–Pd sepa-
ration is 1.3 nm while the longest one
is 2.6 nm.
In the crystal, the spherical com-
plex 2b enjoys a cubic close-packed
structure. Each molecule of 2b is
linked to twelve neighboring spheres
by PdII-NO3 -PdII bridges, which
À
presumably stabilize the crystal of
2b despite approximately 80% void
space.
Ligand 1c is effectively an
expanded version of the framework
of 1a. From Pd(NO3)2 and ligand 1c
in 1:2 stoichiometry, we again
observed the self-assembly of
a
single and highly symmetric product,
2c (an analogue of 2a where ligand
1a is replaced by 1c) which was
assigned by CSI-MS measurement.
The molecular mass of 13982 Da for
2c was clearly demonstrated (see
Figure 4. a) Molecular structure 2a assembled from 24 bidentate ligands 1a and12 metal ions.
b) Schematic representation of the cuboctahedral frameworks of 2a.
Supporting Information). Molecular modeling of 2c by
Cerius2 program predicted a diameter of 5.2 nm.[12]
Figure 5. a) STM image of individual spheres 2a on the graphite at
room temperature. b) Height profile of the STM image.
Reliable evidence for the spherical M12L24 structure was
obtained by X-ray crystallographic analysis of 2b, which is an
analogue of 2a where ligand 1a is replaced by 1b.[11] Single
crystals of 2b were obtained by very slow vapor diffusion of
1,1,2-trichloroethane into a DMSO solution of 2b. With a
Figure 6. The crystal structure of sphere 2b. Counterions andsolvent
molecules are omittedfor clarity (green Pd, redO, blue N, gray C).
CCDdetector, MoK radiation (55 kV, 30 mA) afforded low
a
resolution data (only 874 unique reflections (> 2s(I))), which
were insufficient for solving the structure. The poor quality of
the data was due to severe disorder of solvent molecules and
anions in the extraordinarily large void within the spherical
framework of 2b. However, synchrotron X-ray radiation with
high flux and low divergence provided much higher quality of
data with 2717 unique reflections (> 2s(I)), from which the
spherical M12L24 structure of 2b was solved with all the heavy
atoms being refined anisotropically (Figure 6). The crystal
system is cubic and the cell volume is 108456(16) Š3.
Surprisingly, the framework of 2b occupies only 20% of the
cell volume (as estimated by Platon program), remaining
80% being occupied by disordered solvent molecules and
counterions. The diameter of a sphere in which 2b is inscribed
We also emphasize that, by attaching a functional group
on each ligand, 24 functional groups are aligned equivalently
at the periphery of the sphere. Metal–porphyrins are known
to collect light energy when they are aggregated as in light-
harvesting proteins or chlorophylls. To mimic these aggre-
gates, we synthesized ligand 1d in which a porphyrin unit is
attached on the backbone of 1a. By simply mixing this ligand
with Pd(NO3)2 in a 2:1 ratio in DMSO, porphyrin nanoball 2d
with regular arrangement of the 24 porphyrin units at the
periphery of the sphere was immediately assembled
(Figure 7). The NMR spectrum of this complex at 258C
shows the equivalence of the 24 attached porphyrin ligands.
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Communications
estimation of the dimensions of 2b, 2d, and 2e (3.4, 6.7, and
5.3 nm, respectively), which agree quite well with the X-ray
structure (3.4 nm for 2b) and refined structures with Cerius2
program (3.4, 7.4, and 6.0 nm for 2b, 2d and 2e, respectively).
Received: July 24, 2004
Published Online: September 28, 2004
Keywords: fullerenes · molecular spheres · palladium ·
.
porphyrinoids · self-assembly
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Angewandte
Chemie
[11] X-ray crystallographic analysis of 2b: The diffraction data was
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