Self-assembled inverse dendrimer

Article abstract of DOI:10.1246/cl.2011.726

Bent bridging ligands appending dendrons (G-1, G-2, and G-3) at their concaves are complexed with Pd(II) ions to selfassemble into M ₁₂L₂₄ spherical shells confining inversely branched dendrimers. Their structures were fully characte

Full text of DOI:10.1246/cl.2011.726

doi:10.1246/cl.2011.726
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Published on the web June 4, 2011
Self-assembled Inverse Dendrimer
Qing-Fu Sun, Sota Sato, and Makoto Fujita*
1
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
CREST, Japan Science and Technology Agency (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
7
2
(
Received April 14, 2011; CL-110317; E-mail: mfujita@appchem.t.u-tokyo.ac.jp)
Bent bridging ligands appending dendrons (G-1, G-2, and
G-3) at their concaves are complexed with Pd(II) ions to self-
assemble into M12L24 spherical shells confining inversely
branched dendrimers. Their structures were fully characterized
by NMR, CSI-TOF-MS, and AFM.
Dendrimers are structurally well-defined, hyperbranched
polymers with branches stretching out from the core to the
1
­3
They are typically synthesized by divergent4 or
outside.
5
convergent covalent synthesis, or sometimes, by noncovalent
self-assembly methods, where the hyperbranched dendrons are
used as building blocks. If the branches are anchored onto the
inner wall of a capsular molecule with a defined structure to stretch
out from the shell to the core, the product is a new type of
6
dendrimer that can be called an “inverse dendrimer.” Previously,
we have shown that giant hollow coordination M12L24 spheres
with a precise chemical structure and uniform diameter are
spontaneously self-assembled by simply mixing 12 Pd(II) ions and
4 bent bidentate ligands.7,8 Here, we report that by appending the
Scheme 1. Schematic representation of the self-assembly of inverse
dendrimers and the structures of three generations of ligands 1a­1c.
2
dendrons at the concave of the bent ligands, inverse dendrimers
with controlled interior density are efficiently prepared, demon-
strating the high potential of metal-directed self-assembly to create
a new class of nanosized molecular materials (Scheme 1).
Three generations of Frechet-type oligo(benzyl ether) den-
drons were attached at the concave of bent bipyridyl ligands before
complexation. The ligands were synthesized in two steps with
reasonable yields: the dendrons were connected to 2,6-dibromo-
phenol by Williamson ether synthesis, and then two 4-pyridyl-
ethynyl groups were introduced by Sonogashira coupling.9
When ligand 1b (10 ¯mol) was treated with Pd(BF4)2
(5 ¯mol) in CD3CN (1.0 mL) at 50 °C for 3 h, the quantitative
self-assembly of a single and highly symmetric product was
1
confirmed by H NMR (Figure 1). From this NMR and MS
analysis (as discussed later), the structure of the product was
deduced to be a spherical inverse dendrimer 2b. The downfield
shift (¦¤ = 0.34) of the ¡-proton on the pyridyl group is
characteristic for the coordination on Pd(II) ions. Meanwhile, all
the other signals for the dendritic substituents are found to be
clearly upfield shifted. The upfield shift is presumably due to the
crowded packing of benzyl groups inside the sphere and, in part,
the magnetic shielding from the M12L24 shell.
Figure 1. 1H NMR spectra of ligand 1b (500 MHz, CD3CN, 300 K)
and complex 2b (500 MHz, CD3CN, 300 K).
molecular weight of sphere 2c amounts to 48292.90, yet a series
¹
n+
of [2c ¹ (BF4 )n]
(n = 9­17) peaks were clearly observed.
The M12L24 composition of sphere 2b was established by
CSI-TOF mass spectrum in terms of exact mass, multivalency,
and high-resolution isotope patterns. As shown in Figure 2,
Because of the huge molecular weight, highly resolved isotope
patterns were not observed (Figure S18 ).
To estimate the molecular size, DOSY (diffusion ordered
spectroscopy) measurements were performed. Obtained similar
9
¹
n+
prominent peak series corresponding to [2b ¹ (BF4 )n] (n =
­16) were observed, from which the molecular weight was
¹
10
2 ¹1
7
diffusion coefficients D = 2.57 © 10 m s for spheres 2a and
¹
10
2 ¹1
calculated to be 27917.08 Da, confirming the M12L24 component.
The high-resolution mass spectrum showed the highest peak to be
2b and calibrated D = 2.53 © 10 m s
for sphere 2c in
9
CD3CN (see SI ) proved that all the spheres are almost the same
size, but sphere 2c is slightly larger than spheres 2a and 2b. The
characteristic behavior for sphere 2c was also observed in the
+
in a 12 charged state.
In the same way, spheres 2a and 2c were self-assembled and
1
1
characterized by H NMR and MS. It is worth mentioning that the
H NMR spectrum, which showed severely broadened signals
Chem. Lett. 2011, 40, 726­727
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Figure 4. Cartoon presentation for the segment density in the normal
left) and inverse (right) dendrimers.
(
enlargement of the spherical framework itself is in progress, and
we envision that the inverse dendritic architecture can be applied
for material syntheses of uniform-sized nanoparticles which have
controlled internal density.
Figure 2. CSI-TOF mass spectrum of sphere 2b with an inset showing
+
the isotope pattern for the 12 charged peak.
Q.-F. S. thanks JSPS for the research fellowship, the Global-
COE program, MEXT, Japan. We also thank Dr. N. Iwasaki
(Bruker Daltonics) for the support of CSI-TOF-MS measure-
ments.
References and Notes
1
2
3
Dendrimers and Other Dendritic Polymers in Wiley Series in
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Figure 3. AFM image of sphere 2b on mica.
(
Figures S9­S129­11). The apparently different line width and
slightly small D value of 2c can be understood by the optimized
structure of these inverse dendrimers (Figure S209 ). Taking the
furthest Pd£Pd distance as the diameter of the hollow shell, the
occupancies of the dendron segments are calculated to be 31%,
,10
4
5
7
4
6%, and 169% for 2a, 2b, and 2c, respectively (Materials Studio
.4). This suggests that some of the branches in 2c “break” out of
the spherical “prison” through the square or triangle “windows”
of the framework. The over-staffed nature of the G-3 dendrimer,
which makes benzyl groups packed in high density, together
with the large molecular weight (48292.90 Da for [Pd12(C125-
6
7
M. Tominaga, K. Suzuki, M. Kawano, T. Kusukawa, T. Ozeki, S.
Sakamoto, K. Yamaguchi, M. Fujita, Angew. Chem. 2004, 116, 5739;
M. Tominaga, K. Suzuki, M. Kawano, T. Kusukawa, T. Ozeki, S.
Sakamoto, K. Yamaguchi, M. Fujita, Angew. Chem., Int. Ed. 2004,
2
4+
¹
H102N2O15)24] ¢24BF4 ), explains the line broadness and a
slightly larger molecular diameter.
43, 5621.
8
M. Tominaga, K. Suzuki, T. Murase, M. Fujita, J. Am. Chem. Soc.
2005, 127, 11950; S. Sato, J. Iida, K. Suzuki, M. Kawano, T. Ozeki,
M. Fujita, Science 2006, 313, 1273; T. Murase, S. Sato, M. Fujita,
Angew. Chem. 2007, 119, 1101; T. Murase, S. Sato, M. Fujita,
Angew. Chem., Int. Ed. 2007, 46, 1083; K. Suzuki, M. Kawano, S.
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The solid-state structure of 2b on a surface was observed by
atomic force microscopy (AFM). A representative image of the
surface pattern is shown in Figure 3, where most of the inverse
dendrimers maintained their spherical shapes. Height of the
inverse dendrimer was roughly 5 nm, consistant with the diameter
of the M12L24 framework.
9
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
1
Our reverse dendrimers are characteristic compared with
normal” ones in the following aspects: (1) Whereas the segment
10 All the behaviors of 2a­2c in CSI-MS, H NMR, DOSY, and AFM
were consistent with those of M12L24 spherical complexes whose
shell frameworks were unambiguously determined by X-ray crys-
tallography. A remarkable kinetic stabilization of the resultant
M12L24 spherical structures has been discussed (where the ligand
“
density of common dendrimers gradually increases from the core
to the outer sphere, that of the inverse dendrimers drastically
increases from the outer to inner sphere and then drops to nearly
zero at the core (Figure 4); (2) Defined by the rigid M12L24 shell,
the size, shape, and molecular weight are perfectly uniform;
3) The precursor dendrons are summed up through the self-
assembly the M12L24 shell.
In summary, the present study describes the self-assembly of
newly defined inverse dendrimers by taking the advantage of
an around 5 nm-sized M12L24 spherical rigid framework. The
6
exchange is 10 times slower than that in mononuclear Pd(II)­
pyridine complex). Also, mathematical constraint allows only five
structures for MnL2n spheres (n = 6, 12, 24, 30, and 60), and the n
values critically change when the structural parameters of the ligand
are modulated, thus forming a unique single structure rather than a
mixture with different n values. Based on these previous findings, we
confirmed the stable, unique formation of 2a­2c.
(
11 Because of this broadening, further studies may be required to fully
characterize the structure 2c.
Chem. Lett. 2011, 40, 726­727
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/