Inclusion of anionic guests inside a molecular cage with palladium(ii) centers as electrostatic anchors

Full text of DOI:10.1002/anie.200902717

Communications
DOI: 10.1002/anie.200902717
Anion Recognition
Inclusion of Anionic Guests inside a Molecular Cage with
Palladium(II) Centers as Electrostatic Anchors**
Guido H. Clever, Shohei Tashiro, and Mitsuhiko Shionoya*
Anion recognition and binding play an important role in
biological processes, such as ATP metabolism and trans-
membrane transport of anions to regulate osmotic pressure in
the cell. Furthermore, anion recognition has emerged to be a
major area of supramolecular chemistry, as exemplified by
systems such as selective anion sensors and anion-templated
[
1]
molecular architectures.
In supramolecular assemblies such as nanoscopic cages
[
2]
and capsules, metal ions often act as ꢀglueꢁ for the sponta-
[
3]
neous but directed assembly of organic building blocks. In
some cases, coordinatively labile sites of metal centers
provide a platform for direct binding to guest species through
[
1]
ligand-exchange reactions. In contrast, positively charged
metal complexes can also interact with anionic species
through pure electrostatic interactions.
Accordingly, the precise positioning of such metal centers
within molecular assemblies that have a hollow space would
allow for specific attractive interactions with anionic guest
II
molecules. In the system presented herein, two Pd ions are
bound by four bis(monodentate) ligands to give stable square-
8
II
II
planar complexes of the d Pd ions. The two Pd centers are
spaced 1.7 nm apart and are directly linked by the rigid,
banana-shaped ligands, thus allowing this construct to quan-
titatively bind one dianionic guest by coulombic interactions
[
4]
(
Figure 1a).
The synthesis of the ligands is based on consecutive
cycloaddition reactions to build up the backbone structure 3
from norbornene 1 and oxadiazole 2, as reported by Warrener
[
5]
et al. (Figure 1b). A further Diels–Alder reaction with
subsequent oxidative aromatization provided tetraester 4,
which was converted to ligand 5 in high yield. Upon heating
the ligand with [Pd(CH CN) ](BF ) in acetonitrile, the cage
3
4
4 2
compound 6 formed quantitatively, as shown by the ESI mass
spectrum and the NMR spectra (Figure 3 and the Supporting
1
Information). In the H NMR spectrum, two of the four
pyridine signals are shifted downfield, as can be assumed for
II
the complexation of the double positively charged Pd ions,
[
*] Dr. G. H. Clever, Dr. S. Tashiro, Prof. Dr. M. Shionoya
Department of Chemistry, Graduate School of Science
The University of Tokyo
Figure 1. a) Representation of the binding of a guest, 1,1’-ferrocene
II
bis(sulfonate), inside the cage formed by two Pd centers (light gray)
and four rigid, banana-shaped ligands (dark gray). b) Synthesis of cage
7
-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5841-8061
E-mail: shionoya@chem.s.u-tokyo.ac.jp
6
(see the Supporting Information).
[
**] G.C. thanks JSPS and the Alexander von Humboldt Foundation for a
postdoctoral scholarship. This work was supported by Grants-in-Aid
from MEXT of Japan, the Global COE Program for Chemistry
Innovation, and the Photon Factory Program Advisory Committee
and two resonances are shifted upfield, which is most likely
due to the position of these protons relative to the aromatic
planes of the neighboring pyridine rings. An EXSY NMR
experiment with a 1:4 mixture of the cage and the free ligand
at 293 K showed no cross-peaks between the corresponding
(
proposal no. 2008R-18).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902717.
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Chemie
state, see the Supporting Information) and two outside the
cage. We found that the inner counterions can be selectively
and quantitatively exchanged in solution by guest molecules
that contain two negatively charged sulfonate groups at a
À
À
4
suitable distance that matches the BF –BF distance found
4
inside one of the two cages in the unit cell (d(B–B) = 0.9 nm).
A screening of possible guests by molecular modeling studies
yielded the known compound 1,1’-ferrocene bis(sulfonate) 7
as a potential candidate, which showed a maximal S–S
[
6]
distance of 0.8 nm. Indeed, upon titration of a 0.70 mm
solution of the cage compound 6 in CD CN with a solution
3
Figure 2. Side view (left) and space-filling model of the top view
right) of the crystal structure of 6. Solvent molecules are omitted for
clarity. C gray, O red, N blue, F green, Pd fuchsia, B brown.
of bis(tetrabutylammonium)-1,1’-ferrocene bis(sulfonate) 7,
the signals of the cage shift until one equivalent of the guest is
added (Figure 3).
(
Most indicative is the signal at d = 8.41 ppm, which
corresponds to the inward-pointing hydrogen atoms attached
i
to the pyridine rings (H , blue in Figure 3). After 0.5 equiv-
alents of the guest molecule were added, this signal split up
into two broadened resonances, which indicated uptake of the
guest inside half of the available cage molecules. The signal
broadening might indicate an exchange between the ꢀfilledꢁ
and the ꢀemptyꢁ cages on the NMR timescale.
Upon addition of one equivalent of 1,1’-ferrocene bis(sul-
fonate), the inclusion complex was quantitatively formed, as
indicated by the sharpening of the signals in the NMR
spectrum. Additionally, the guest signals were identified in
the NMR spectrum at d = 3.31 and 3.60 ppm (Figure 3, red),
and thus undergo a significant upfield shift with respect to the
signals of the free guest molecule (d = 4.17 and 4.39 ppm,
Figure 3, green) at the same concentration because of the
[
7]
magnetic shielding of the surrounding cage structure.
A
1
comparison of the H DOSY NMR spectra of the free and
encapsulated guest likewise supports the quantitative forma-
tion of the inclusion complex (see the Supporting Informa-
tion).
Finally, the redox potential of the ferrocene guest 7 was
1
Figure 3. H NMR spectra (500 MHz, CD CN, 293 K) of a) ligand 5,
3
probed in a cyclic voltammetry experiment. The encapsulated
b) cage 6, c) 6 with 0.5 equiv 7, d) 6 with 1 equiv 7, and e) 7. *=water
and solvent.
II/III
guest molecule shows an anodic shift of 37 mV for E (Fe
)
1
/2
with respect to free 1,1’-ferrocene bis(sulfonate), thus imply-
ing that the reduced state of the guest 7 is stabilized when it is
included within the cage (Figure 4). The observation of an
anodic shift is in good agreement with a previous report on
signals of the coordinated and uncoordinated ligands; thus a
slow exchange of the free and bound ligands can be
anticipated (see the Supporting Information). The hydro-
dynamic radius of the molecule in solution was estimated to
[
8]
the interaction of 7 with a cationic polymer, and a similar
[
7a]
encapsulation study by Fujita and co-workers. The result
À10
2
À1
be about 1.1 nm (diffusion coefficient D = 5.0 ꢂ 10 m s )
1
by H DOSY NMR spectroscopy, which is in good agreement
with the structure of the 2 nm ball-shaped cage determined by
single-crystal X-ray analysis (Figure 2).
The number and multiplicity of the signals in the solution
H NMR spectrum indicate a D4h cage symmetry, which is
1
probably the averaged result of a fast flipping mechanism
between two degenerated conformations, each with C4h
symmetry. In the crystal, all of the cages are fixed in this C4h
symmetry. The stiff, banana-shaped structure of the ligands,
which assembles around the two metal ions, generates a
hollow space that is amenable to guest incorporation through
the four large portals. Each metal ion is flanked by two
À
counteranions. In total, two BF4 ions are located inside the
Figure 4. Cyclic voltammograms of the free (green) and encapsulated
À1
cavity (two different arrangements were found in the solid
guest (red) at 293 K, 0.15 mm, 0.1m TBAP, scan rate 0.1 Vs
.
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Communications
can be explained by the electron-withdrawing effect of the
positively charged Pd complexes that make up the cage.
Keywords: anion recognition · cage compounds ·
electrostatic interactions · host–guest systems ·
supramolecular chemistry
.
II
The system presented here is topologically similar to the
[9]
so-called molecular gyroscopes. We believe that the binding
principle that underlies our system will prove versatile in
different areas of nanoassembly since the ligand synthesis is
simple, the formation of the cage is spontaneous and
quantitative under the given conditions, a template is not
required, and the aromatic sulfonate guests are easily
accessible. The binding of the guest molecules has proven to
be quick, quantitative, and easily observable by NMR
spectroscopy and mass spectrometry.
[
1] a) Anion Receptor Chemistry (Eds.: J. L. Sessler, P. A. Gale, W.-S.
Cho), RSC Publishing, Cambridge, 2006; b) J. W. Steed, J. L.
Atwood, Supramolecular Chemistry, 2nd ed., Wiley, Chichester,
2009.
[
2] a) S. J. Dalgarno, N. P. Power, J. L. Atwood, Coord. Chem. Rev.
2008, 252, 825; b) L. C. Palmer, J. Rebek, Org. Biomol. Chem.
2004, 2, 3051; c) S. Hiraoka, Y. Sakata, M. Shionoya, J. Am. Chem.
Soc. 2008, 130, 10058; d) C. Hastings, D. Fiedler, R. G. Bergman,
K. N. Raymond, J. Am. Chem. Soc. 2008, 130, 10977; e) N. Takeda,
K. Umemoto, K. Yamaguchi, M. Fujita, Nature 1999, 398, 794;
f) Z. Liu, Y. Liu, S. Zheng, Z. Yu, M. Pan, C. Su, Inorg. Chem.
We imagine that our system will be useful for the
construction of higher-order functional nanoassemblies in
which various building blocks that contain multiple anionic
groups can be joined in a controlled manner. Currently, we
are extending this binding mode to generate rotaxanes and
higher-order aggregates of individual cage molecules.
2007, 46, 5814; g) C. J. Kuehl, Y. K. Kryschenko, U. Radhak-
rishnan, S. R. Seidel, S. D. Huang, P. J. Stang, Proc. Natl. Acad.
Sci. USA 2002, 99, 4932.
[3] a) D. J. L. Tranchemontagne, Z. Ni, M. OꢁKeeffe, O. M. Yaghi,
Angew. Chem. 2008, 120, 5214; Angew. Chem. Int. Ed. 2008, 47,
5
136; b) D. M. Vriezema, M. C. Aragones, J. A. A. W. Elemans,
J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte, Chem. Rev.
005, 105, 1445; c) M. Albrecht, S. Mirtschin, M. de Groot, I.
Experimental Section
2
Cage compound 6 was prepared in quantitative yield by heating a
mixture of the ligand 5 (2.1 mg, 2.8 mmol) and a solution of [Pd-
Janser, J. Runsink, G. Raabe, M. Kogej, C. A. Schalley, R. J.
Froehlich, J. Am. Chem. Soc. 2005, 127, 10371.
4] S. Hiraoka, K. Harano, M. Shiro, Y. Ozawa, N. Yasuda, K.
Toriumi, M. Shionoya, Angew. Chem. 2006, 118, 6638; Angew.
Chem. Int. Ed. 2006, 45, 6488.
5] R. N. Warrener, D. N. Butler, L. G. Liu, D. Margetic, R. A.
Russell, Chem. Eur. J. 2001, 7, 3406.
6] a) N. Lawrence, G. Tustin, M. Faulkner, T. Jones, Electrochim.
Acta 2006, 52, 499; b) Spartan 06 Software, Wavefunction Inc.,
Irvine CA, 2006.
(
CH CN) ](BF ) (1.4 mmol, 93 mL of a 15 mm stock solution in
3 4 4 2
[
CD CN) in CD CN (930 mL) at 708C for 30 min in a closed vial to
3
3
yield 1.0 mL of a 0.70 mm solution of 6. Single crystals suitable for X-
ray crystallographic analysis were grown from this solution by slow
evaporation of the solvent. CCDC 729445 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[
[
The host–guest complex was formed by titrating a solution of the
[
7] a) W.-Y. Sun, T. Kusukawa, M. Fujita, J. Am. Chem. Soc. 2002,
guest 1,1’-ferrocene bis(sulfonate) 7 (8.75 mm) in CD CN into 500 mL
3
124, 11570; b) W. S. Jeon, K. Moon, S. H. Park, H. Chun, Y. H. Ko,
of a 0.70 mm solution of cage 6 in CD CN in an NMR tube. The NMR
3
J. Y. Lee, E. S. Lee, S. Samal, N. Selvapalam, M. V. Rekharsky, V.
Sindelar, D. Sobransingh, Y. Inoue, A. E. Kaifer, K. Kim, J. Am.
Chem. Soc. 2005, 127, 12984.
spectra were recorded immediately after briefly shaking the solution.
The cyclic voltammograms were measured at a concentration of
0
.15 mm analyte and 0.1m tetra(n-butyl)ammonium perchlorate
[
[
8] D. J. Walton, C. E. Hall, A. Chyla, Synth. Met. 1991, 45, 363.
9] K. Skopek, M. Hershberger, J. Gladysz, Coord. Chem. Rev. 2007,
À1
(TBAP) in CH CN with a scan rate of 0.1 Vs (ALS-CH Instruments,
3
Model 630 A). The formation of the host–guest complex under the
conditions required for the electrochemical analysis was confirmed by
251, 1723.
1
H NMR spectroscopy.
Received: May 21, 2009
Published online: August 13, 2009
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Angew. Chem. Int. Ed. 2009, 48, 7010 –7012