24-Fold endohedral functionalization of a self-assembled M 12L24 coordination nanoball

Article abstract of DOI:10.1021/ja054069o

A ca. 5 nm-sized, roughly spherical hollow complex was assembled from 12 Pd²⁺ ions and 24 bis(4-pyridylethenyl)-substituted bent ligands. By putting a functional group at the curvature of the ligand, the 24 functional groups were precisely arra

Full text of DOI:10.1021/ja054069o

Published on Web 08/09/2005
24-Fold Endohedral Functionalization of a Self-Assembled M12L24
Coordination Nanoball
Masahide Tominaga, Kosuke Suzuki, Takashi Murase, and Makoto Fujita*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, CREST, Japan Science and
Technology Corporation (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received June 20, 2005; E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Biological giant hollow structures, such as a family of spherical
viruses, have highly functionalized shell interior for efficient
interaction with substances (e.g., DNA) which are stored within
the shells.1 Contrary to this, the voids of artificial hollow compounds
are, in general, too small to be functionalized.2 Endohedral
functionalization is limited only to polymer materials, such as
mesoporous silica gel3 or micelles,4 which do not have well-defined
structures and shapes. Here, we report the 24-fold endohedral
functionalization of a large hollow coordination cage. The shell of
Figure 1. 1H NMR spectrum of 2a (500 MHz, DMSO-d6, 300 K, TMS).
the cage consists of 12 metals and 24 ligands and has a roughly
spherical shape with the symmetry of cuboctahedron.5,6 Each ligand
has a bis(4-pyridyl)-substituted bent framework involving two
acetylene spacers. When complexed with Pd2+ ions, the ligand
assembles quantitatively into the M12L24 spherical complex. We
show that, by putting a functional group at the curvature of the
bent ligand, the 24 functional groups are precisely arrayed inward
within the spherical complex (Scheme 1).
Scheme 1. Self-Assembly of M12L24 Complexes with 24
Endohedral Functional Groups
Figure 2. CSI-MS spectrum of 2a (CH3CN:DMSO ) 20:1, OTf- salt).
region, the peak at m/z 870.1 was assigned to [2a-(OTf-)12
+
(DMSO)4]12+ (Figure 2). A clear AFM image was observed for 2b
(Figure 3), strongly supporting the formation of a 4-5 nm-sized
molecular particle and revealing the stability of the complex under
AFM conditions.
In the endohedral functionalization of the M12L24 sphere, the
acetylene spacer of the ligand plays two important roles. First, the
acetylene spacer expands the cavity of the complex. The diameter
and the longest Pd-Pd distance of the sphere are 4.6 and 3.5 nm,
respectively, being considerably larger than those of the spherical
complex without the acetylene spacer.5 Second, the acetylene spacer
prevents the ligand from taking unfavorable nonplanar conforma-
tion, which is caused by steric repulsion between the pyridyl groups
and the core benzene ring when the acetylene spacer is absent. For
example, ligand 3 having no acetylene spacer adopts twisted
conformation and does not assemble into the M12L24 complex upon
complexation with Pd(II) ions. At each Pd(II) center of the M12L24
complex, a perpendicular array of four pyridyl groups with respect
to the PdN4 plane seems to be essential to the self-assembly of the
spherical complex.
To align 24 functional groups at the interior surface of the
spherical complex, we prepared ligand 1a by the Sonogashira cross-
coupling reaction7 of 4-ethynylpyridine with 2,6-dibromotoluene.
When ligand 1a (0.02 mmol) was treated with Pd(NO3)2 (0.01
mmol) in DMSO-d6 (1 mL) for 4 h at 70 °C, the formation of a
1
single product was indicated by H NMR spectroscopic analysis
(Figure 1). The five proton signals (Ha-e) agree with the formation
of spherical complex 2a. The large downfield shifts of the protons
of pyridine rings (e.g., ∆δPyR ) 0.59 ppm) can be ascribed to the
-
metal-ligand complexation. After anion exchange from NO3 to
OTf-, cold-spray ionization mass spectroscopy (CSI-MS)8 clearly
confirmed an M12L24 composition with the molecular weight of
11919 Da as evident from prominent peaks of [2a-(OTf-)m
+
Thanks to the extraordinarily large cavity of the molecular sphere,
a variety of functional groups can be appended at the interior
(DMSO)n]m+ (m ) 6-16, n ) 0-12). For example, in the 12+
9
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J. AM. CHEM. SOC. 2005, 127, 11950-11951
10.1021/ja054069o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
were observed for the signal of -CH3 (∆δ ) 0.13 ppm). In contrast,
the signals of the shell of 2d remained almost unchanged. These
results indicated the selective binding of La(III) ions at the ether
oxygen atoms of the pseudo poly(ethylene oxide) core. The
complexation ratio of La(III) ion to each (OCH2CH2)4OMe chain
was estimated to be roughly 1:1 by the Job’s plot (Supporting
Information). This indicated that the sphere contained ca. 20 La-
(III) ions within the shell. Interestingly, the absorbed metal ions
were expelled by adding a coordinative solvent, such as DMSO.
Thus, upon the addition of DMSO (5 vol %), the broad signals of
the ethylene oxide chain were sharpened and the chemical shifts
of these protons became identical to that of La(III)-free 2d in 5
vol % DMSO/CD3CN.
Figure 3. AFM image of 2b on mica: (a) 3D image; (b) 2D image; and
(c) its height profile.
In summary, we have demonstrated the 24-fold functionalization
at the interior surface of a spherical complex. The facile endohedral
multi-functionalization is mainly indebted to two factors. The first
is the extraordinarily large cavity of the M12L24 spherical complex
that can accommodate 24 functional groups. The second is the
spontaneous and quantitative self-assembly of the spherical shell
from 36 components. By utilizing such a unique method for
“endohedral molecular coating”, well-defined nanospace surrounded
by various types of the interior surfaces can be provided at will, as
currently investigated in our laboratory.
Supporting Information Available: Preparation and physical
properties of 1a-1d, 2a-2d, and Job’s plot of 2d and La(III) ions.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) (a) Speir, J. A.; Munshi, S.; Wang, G.; Baker, T. S.; Johnson, J. E.
Structure 1995, 3, 63-78. (b) Douglas, T.; Young, M. Nature 1998, 393,
152-155. (c) Reinisch, K. M.; Nibert, M. L.; Harrison, S. C. Nature 2000,
404, 960-967. (d) Lucas, R. W.; Larson, S. B.; McPherson, A. J. Mol.
Biol. 2002, 317, 95-108. (e) Dragnea, B.; Chen, C.; Kwak, E.-S.; Stein,
B.; Kao, C. C. J. Am. Chem. Soc. 2003, 125, 6374-6375.
(2) (a) Boxter, P.; Lehn, J.-M.; DeCian, A.; Fischer, J. Angew. Chem., Int.
Ed. Engl. 1993, 32, 69-72. (b) Meissner, R. S.; Rebek, J., Jr.; de Mendoza,
J. Science 1995, 270, 1485-1488. (c) Fujita, M.; Oguro, D.; Miyazawa,
M.; Oka, H.; Yamaguchi, K.; Ogura, K. Nature 1995, 378, 469-471. (d)
Beissel, T.; Power, R. E.; Raymond, K. N. Angew. Chem., Int. Ed. Engl.
1996, 35, 1084-1086. (e) MacGillivray, L. R.; Atwood, J. L. Nature 1997,
389, 469-472. (f) Olenyuk, B.; Whiteford, J. A.; Fechtenko¨tter, A.; Stang,
P. J. Nature 1999, 398, 796-799.
(3) (a) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner,
K. M. Science 1997, 276, 923-926. (b) Lim, M. H.; Blanford, C. F.;
Stein, A. J. Am. Chem. Soc. 1997, 119, 4090-4091. (c) Inagaki, S.; Guan,
S.; Ohsuna, T.; Terasaki, O. Nature 2002, 304, 304-307. (d) Liu, N.;
Chen, Z.; Dunphy, D. R.; Jiang, Y.-B.; Assink, R. A.; Brinker, C. J. Angew.
Chem., Int. Ed. 2003, 42, 1731-1734. (e) Radu, D. R.; Lai, C.-Y.; Wiench,
J. W.; Pruski, M.; Lin, V. S.-Y. J. Am. Chem. Soc. 2004, 126, 1640-
1641.
(4) (a) Oehme, G.; Paetzold, E.; Selke, R. J. Mol. Catal. 1992, 71, L1-L5.
(b) Kataoka, K.; Togawa, H.; Harada, A.; Yasugi, K.; Matsumoto, T.;
Katayose, S. Macromolecules 1996, 29, 8556-8557. (c) Brettreich, M.;
Hirsch, A. Tetrahedron Lett. 1998, 39, 2731-2734. (d) Jin, R.-H. AdV.
Mater. 2002, 14, 889-892. (e) Kang, Y.; Taton, T. A. J. Am. Chem. Soc.
2003, 125, 5650-5651. (f) Adams, M. L.; Lavasanifar, A.; Kwon, G. S.
J. Pharm. Sci. 2003, 92, 1343-1355.
(5) Tominaga, M.; Suzuki, K.; Kawano, M.; Kusukawa, T.; Ozeki, T.;
Sakamoto, S.; Yamaguchi, K.; Fujita, M. Angew. Chem., Int. Ed. 2004,
43, 5621-5625.
(6) Related cuboctahedral complexes: (a) Moulton, B.; Lu, J.; Mondal, A.;
Zaworotko, M. J. Chem. Commun. 2001, 863-864. (b) Eddaoudi, M.;
Kim, J.; Wachter, J. B.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. J. Am.
Chem. Soc. 2001, 123, 4368-4369.
(7) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16,
4467-4470. (b) Nguefack, J.-F.; Bolitt, V.; Sinou, D. Tetrahedron Lett.
1996, 37, 5527-5530.
(8) CSI-MS is quite effective for analyzing the solution structures of metal
complexes: (a) Sakamoto, S.; Fujita, M.; Kim, K.; Yamaguchi, K.
Tetrahedron 2000, 56, 955-964. (b) Yamaguchi, K. J. Mass Spectrom.
2003, 38, 473-490.
(9) Both 2a and 2d showed the same diffusion coefficients (logD ) -9.62
in CD3CN), suggesting that the oligo(ethylene oxide) chains of 2d are
inside the sphere.
Figure 4. Molecular modeling of (a) 2b, (b) 2c (trans form), (c) 2c (cis
form), and (d) 2d optimized by a force-field calculation with Cerius2 3.5.
surface. Ligand 1b with a cyanophenyl appendix was assembled
into M12L24 sphere 2b upon complexation with Pd(II). At the core
of 2b, 24 cyano groups were highly concentrated (Figure 4a),
potentially making possible the insulation of metal or metal oxide
clusters therein via cyano coordination. Sphere 2c, assembled from
1c and Pd(II), included 24 photoresponsive azobenzene units (Figure
4b). A modeling study predicted considerable free volume change
of the cavity upon trans-cis isomerization (Figure 4c). Ligand 1d
with an oligo(ethylene oxide) chain was assembled into sphere 2d,
whose cavity was filled with a “pseudo-nanoparticle” of poly-
(ethylene oxide) (Figure 4d). This pseudo-nanoparticle, containing
120 ether oxygen atoms, has a well-defined structure with a
spherical shape (4.6 nm in diameter) and no size and molecular
weight distributions. The structures of 2b-2d were reliably
characterized by 1H NMR and CIS-MS (Supporting Information).9
Particularly interesting is that the poly(ethylene oxide) pseudo-
nanoparticle insulated within spherical complex 2d absorbed metal
ions quite efficiently. When complex 2d was mixed with rare earth
1
metal ions and alkaline earth metal ions, H NMR spectroscopic
studies indicated the formation of metal-ethyleneoxide complex
inside 2d. For example, when 2d (0.83 mM) was mixed with La-
(OTf)3 (12 molar equiv) in CD3CN, outstanding downfield shifts
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