Discrete stacking of large aromatic molecules within organic-pillared coordination cages

Article abstract of DOI:10.1002/anie.200462171

Metal-hinged, organic-pillared cages accommodate a limited number (n) of π-stacked molecules in the cavity (see scheme): Discrete n + 2 aromatic stacking occurs by the accommodation of n large aromatic guests (n = 1-3). Owing to efficient donor-acceptor (D-A) interactions, A-D-A (n = 1), A-D-D-A (n = 2), and A-D-A-D-A (n = 3) stacks are observed, which bring about even-odd-number effects in the UV/Vis absorption spectra. (Figure Presented)

Full text of DOI:10.1002/anie.200462171

Communications
Stacking Interactions
Discrete Stacking of Large Aromatic Molecules
within Organic-Pillared Coordination Cages**
Michito Yoshizawa, Junichi Nakagawa,
Kazuhisa Kumazawa, Muneki Nagao, Masaki Kawano,
Tomoji Ozeki, and Makoto Fujita*
Aromatic stacking of p-conjugated planar molecules leads to
the exhibition of unique chemical and physical properties.
Discotic liquid crystals are, for example, a columnar assembly
[
1]
of aromatic compounds that contain long alkyl chains, and
organic electroconductive materials involve alternate charge-
transfer stacking of electron-donating and -accepting p-
[
2]
conjugated compounds. Whereas such infinite assemblies
have been thoroughly studied, precisely controlled discrete
assemblies composed of more than two p-conjugated mole-
[
3–6]
cules have been explored much less frequently.
As discrete
assemblies of a limited number of p-stacked molecules are
expected to show new properties different from those of
isolated or infinitely stacked p-systems, a general method for
constructing such structures has to be developed. Here, we
report the self-assembly of a metal-hinged, organic-pillared
cage with a large cavity that can accommodate two or more
large p-conjugated molecules. The cage consists of two large
organic panels (A), three rodlike pillars (B), and six metal
hinges (C; Scheme 1). The large, box-shaped cavity accom-
Scheme 1. Schematic representation of the organic-pillared cage
accommodating two or more p-conjugated guests. The ensemble self-
assembles from components A–D (A: aromatic panel; B: rodlike pillar;
C: metal hinge; D: aromatic guest).
[
*] Dr. M. Yoshizawa, J. Nakagawa, K. Kumazawa, M. Nagao,
Prof. M. Kawano, Prof. M. Fujita
Department of Applied Chemistry
School of Engineering
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Prof. T. Ozeki
Department of Chemistry and Materials Science
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
[
**] This work was supported by a CREST (Core Research for Evolution
Science and Technology) project from the Japan Science and
Technology Agency. We thank Prof. S. Adachi (KEK) for performing
the X-ray crystallographic measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1
810
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DOI: 10.1002/anie.200462171
Angew. Chem. Int. Ed. 2005, 44, 1810 –1813

Angewandte
Chemie
modates a limited number of p-conjugated molecules (D);
this number is strictly controlled by the length of the pillar
molecules.
the CSI mass spectrum shows a series of peaks for [1ꢁ(5) +
2
m+
(12Àm)PF + nDMF]
(e.g., m/z = 767.7 (m = 6, n = 12),
6
906.2 (m = 5, n = 9), and 1023.3 (m = 4, n = 1)), Figure 1b.
1
2+
To realize such a multistacked aromatic host–guest
system, we have designed the self-assembly of prismlike
Guest-free 1 was hardly detected, which suggests strong p–
p interactions between the host and the guest.
1
2+
12+
cage 1 from tridentate panel-like ligand 2, bidentate pillar
The selective formation of [1ꢁ(5) ]
from thirteen
2
II
2+
2+
ligand 3, and end-capped Pd complex 4 (Figure 1a). The
components (2 ꢁ 2 + 3 ꢁ 3 + 6 ꢁ 4 + 2 ꢁ 5) is due to two
dominant factors: the template effect of the aromatic guests
and ligand steric hindrance. Without the guests 5, the
1
2+
assembly of cage 1 occurs with considerable amounts of a
[
10]
2+
homotopic M L cage (composed of 2 and 4 ) and some
6
4
2
+
oligomeric products (composed of 3 and 4 ). The a-methyl
groups on the pyridyl pillars 3 provide a steric bulk that
prevents the coordination of two pillar ligands to the same
II
[11,12]
Pd center.
X-ray crystallographic analysis provided concrete evi-
1
2+
dence for the quadruple-stacking structure of [1ꢁ(5) ]
.
2
[
13]
When pyrene (6) was used as a large planar guest, a dark-
1
2+
reddish single crystal of [1ꢁ(6) ] was obtained after a few
2
weeks at room temperature by the slow diffusion of ethanol
1
2+
vapor into an aqueous solution containing [1ꢁ(6) ] . The
2
diffraction data were collected by synchrotron X-ray radia-
[
14]
tion (KEK, PF-AR beamline NW2) at À1848C. Despite
severe disorder of the solvents and counterions, the prismlike
1
2+
structure of 1
accommodating two molecules of 6 in the
cavity was successfully solved (Figure 2). The cage is twisted
by 368, which results in efficient quadruple aromatic stacking.
The interplane distances 2···6 and 6···6 are 3.4 and 3.3 ꢀ,
respectively.
Figure 1. a) Schematic representation of the self-assembly of
1
2+
1
[
1ꢁ(5) ] ; b) H NMR (500 MHz, D O, RT) spectrum after the combi-
2
2
2+
nation of 2, 3, 4 , and 5 in a 2:3:6:2 ratio at 1008C for 2 h (en=ethy-
1
2+
lenediamine); c) CSI mass spectrum of [1ꢁ(5) ] after anion
2
À
exchange with PF6
.
1
2+
resulting cage, as estimated by molecular modeling, can
accommodate two large aromatic compounds. The space
between the “floor” and the “roof” of the prism cage
Figure 2. X-ray crystal structure of [1ꢁ(6)2] (6: pyrene).
(
ꢀ 7.5 ꢀ) is approximately twice the thickness of planar
aromatic compounds. In fact, in the presence of coronene
5), we observed the quantitative self-assembly of a quad-
The use of elongated pillar ligand 7 increased the size of
[
7]
the cavity (Figure 3a). Indeed, the expanded prismlike cage
1
2+
(
8
was successfully assembled when the components 2, 7,
1
2+
2+
ruple-stacking structure [1ꢁ(5) ] . Four components, 2, 3,
and 4 were combined in the presence of guest 5. Surpris-
ingly, cage 8 binds two molecules of 5 and one molecule of
2
2
+
II
12+
4
6
, and 5, were combined in a 2:3:6:2 ratio in D O ([Pd ] =
0 mm). After stirring at 1008C for 2 h the suspension
2
[
8]
ligand 2 within the cavity. The uncoordinated 2 is sandwiched
1
2+
became clear and the color changed from pale yellow to deep
red. H NMR analysis of the solution indicated the formation
by two coronene guests such that an [8ꢁ(5·2·5)] structure is
1
formed. The yield of this assembly was optimal when 2, 7, and
1
2+
2+
of [1ꢁ(5) ]
as a single product (Figure 1b). The NMR
4 were combined in a 3:3:6 ratio in the presence of an excess
2
1
2+
[8]
spectrum agreed with the D3h symmetry of 1 and with the
amount of 5 (Figure 3a).
stoichiometry of the components. The signal of 5 (H ) is
The unique structure involving quintuple stacking
(2···5···2···5···2) was strongly supported by NMR spectroscopy
e
shifted strongly upfield due to encapsulation in the cavity of
1
2+
12+
1
1
. Remarkably, the [1ꢁ(5) ]
structure is stable even
and CSI mass spectrometry. Simple H NMR spectroscopy
2
[
9]
À
II
under CSI-MS conditions. After anion exchange with PF6
,
(Figure 3b) showed that the two Pd -coordinated panel
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Communications
Figure 4. UV/Vis spectra (H O, RT) of the multistacked aromatic
2
1
2+
12+
12+
assemblies [1ꢁ(5)2] , [8ꢁ(5·2·5)] , and [9ꢁ5]
.
1
2+
Figure 3. a) Quintuple-stacked, prismlike structure of [8ꢁ(5·2·5)]
2
+
which self-assembles from 2, 7, 4 , and 5 in a 3:3:6:2 ratio;
1
12+
b) H NMR (500 MHz, D O, RT) spectrum of [8ꢁ(5·2·5)]
.
2
ligands (2, signals H and H ) and two coronene guests (5,
a
b
signal H ) are equivalent. These results are in good agreement
h
with the expected D3h symmetry of the system. The middle
panel 2 is shifted significantly upfield (signals H and H at d =
f
g
7
.23 and 4.71 ppm, respectively) due to intercalation between
two molecules of 5. After counteranion exchange, the CSI
mass spectrum showed clear peaks for [8ꢁ(5·2·5) +
In summary, we have achieved quadruple and quintuple
stacking of large p-conjugated molecules by self-assembly.
Our strategy can, in principle, be extended to multiply
stacking p-systems. Such discrete stacking should produce
unique photochemical and electrochemical properties that
are uncommon for isolated or infinitely stacked p-systems.
Further investigations in these areas are now in progress.
À
m+
(
12Àm)PF6 + nDMF] . Note that the noncovalently asso-
12+
ciated [8ꢁ(5·2·5)] structure is still stable under the CSI-MS
[
8]
conditions.
1
2+
The efficient p-stacking in the [8ꢁ(5·2·5)] structure is
ascribed to donor–acceptor charge-transfer (CT) interactions
II
throughout the aromatic components. The Pd -coordinated
ligand 2 is highly electron deficient due to metal coordination
at the three pyridyl sites. Even the guest molecule of 2
possesses an electron-deficient triazine core because of the
electron-withdrawing effect of the pyridyl groups. Therefore,
electron-rich coronenes are apt to form a CT complex with
both coordinated and free molecules of 2 such that the
Received: October 1, 2004
Published online: February 16, 2005
Keywords: charge transfer · host–guest systems · palladium ·
.
pi interactions · self-assembly
1
2+
[
8ꢁ(5·2·5)]
structure is stabilized by A–D–A–D–A aro-
matic stacking (A: acceptor; D: donor).
[1] For a recent review of discotic liquid crystals, see: R. J. Bushby,
O. R. Lozman, Curr. Opin. Colloid Interface Sci. 2002, 7, 343.
The even–odd-number effect of the stacking aromatic
rings was clearly demonstrated by UV/Vis absorption
spectroscopy (Figure 4). The pyrazine-pillared prismlike
[
2] a) J. Ferraris, D. O. Cowan, V. Walatka, J. H. Perlstein, J. Am.
Chem. Soc. 1973, 95, 948 – 949; b) L. B. Coleman, M. J. Cohen,
D. J. Sandman, F. G. Yamagishi, A. F. Garito, A. J. Heeger, Solid
State Commun. 1973, 12, 1125 – 1132.
1
2+
cage 9 was found to bind coronene (5) to form an A–D–
1
2+
[4]
A stack with a [9ꢁ5]
structure. A weak CT band
[
3] a) L. D. Costanzo, S. Geremia, L. Randaccio, R. Purrello, R.
Lauceri, D. Sciotto, F. G. Gulino, V. Pavone, Angew. Chem. 2001,
113, 4375 – 4377; Angew. Chem. Int. Ed. 2001, 40, 4245 – 4247;
b) G. Moschetto, R. Lauceri, F. G. Gulino, D. Sciotto, R.
Purrello, J. Am. Chem. Soc. 2002, 124, 14536 – 14537.
(
shoulder) was observed for the A–D–A stack at around
4
4
50 nm. In contrast, a stronger CT absorption at around
À1
À1
12+
75 nm (e = 3500 m cm ) was observed for [1ꢁ(5) ]
,
2
which involves A–D–D–A stacking. As the HOMO level of
[
4] K. Kumazawa, K. Biradha, T. Kusukawa, T. Okano, M. Fujita,
Angew. Chem. 2003, 115, 4039 – 4043; Angew. Chem. Int. Ed.
the stacked (5) dimer is higher than that of 5, the charge
2
1
2+
transfer band is red-shifted. For [8ꢁ(5·2·5)] , the absorption
2003, 42, 3909 – 3913.
moves back to around 450 nm because of A–D–A-type
[
5] a) S.-S. Sun, A. Lees, Chem. Commun. 2001, 103 – 104; b) J. D.
Crowley, A. J. Goshe, B. Bosnich, Chem. Commun. 2003, 2824 –
1
2+
stacking similar to that of [9ꢁ5]
.
1
812
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2005, 44, 1810 –1813

Angewandte
Chemie
2825; c) J. M. C. A. Kerckhoffs, F. W. B. A. van Leeuwen, A. L.
Spek, H. Kooijman, M. Crego-Calama, D. N. Reinhoudt, Angew.
Chem. 2003, 115, 5895 – 5900; Angew. Chem. Int. Ed. 2003, 42,
5717 – 5722; d) K. Kumazawa, Y. Yamanoi, M. Yoshizawa, T.
Kusukawa, M. Fujita, Angew. Chem. 2004, 116, 6062 – 6066;
Angew. Chem. Int. Ed. 2004, 43, 5936 – 5940.
[
[
[
[
6] M. Fujita, N. Fujita, K. Ogura, K. Yamaguchi, Nature 1999, 400,
5
2 – 55.
1
2+
7] Molecular modeling of [1ꢁ(5) ]
was optimized by MM2
2
2
calculations with Cerius 3.5 (see Supporting Information).
1
2+
12+
8] Physical data of [1ꢁ(5) ] and [8ꢁ(5·2·5)] are given in the
2
Supporting Information.
9] CSI-MS analysis: K. Yamaguchi, J. Mass Spectrom. 2003, 38,
473 – 490.
[
10] a) M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K. Yamaguchi, K.
Ogura, Nature 1995, 378, 469 – 471; b) M. Yoshizawa, M.
Tamura, M. Fujita, J. Am. Chem. Soc. 2004, 126, 6846 – 6847;
c) M. Yoshizawa, S. Miyagi, M. Kawano, K. Ishiguro, M. Fujita,
J. Am. Chem. Soc. 2004, 126, 9172 – 9173; d) K. Nakabayashi, M.
Kawano, M. Yoshizawa, S. Ohkoshi, M. Fujita, J. Am. Chem.
Soc. 2004, 126, 16694 – 16695.
[
11] Side-chain-directed assembly: M. Yoshizawa, M. Nagao, K.
Umemoto, K. Biradha, M. Fujita, S. Sakamoto, K. Yamaguchi,
Chem. Commun. 2003, 1808 – 1809.
[
12] The following equilibrium is pushed toward the right-hand side
because of steric repulsion that arises on the left-hand side.
[
[
13] Other large p-aromatic guests (e.g., pyrene and perylene) were
1
2+
also efficiently incorporated within 1
to afford the corre-
sponding quadruple-stacking structures. See also the Supporting
Information.
1
2+
14] X-ray crystallographic data of [1ꢁ(6) ] : C244H272N O Pd ,
2
66 80
3
12
M = 6602.02, crystal dimensions 0.48 ꢁ 0.04 ꢁ 0.04 mm , ortho-
rhombic space group C222 , a = 20.4596(14) ꢀ, b = 35.453(3) ꢀ,
1
3
À3
c = 53.544(6) ꢀ, V= 38839(6) ꢀ , Z = 4, 1calcd = 1.129 gcm
,
F(000) = 13392, l(synchrotron radiation) = 0.6890 ꢀ, T=
89(2) K, reflections collected/unique 56652/25550 (Rint
=
0.1703). The structure was solved by direct methods
(
SHELXL-97) and refined by full-matrix least-squares methods
2
on F with 1376 parameters. R = 0.1579 (I > 2s(I)), wR =
1
2
0
.3822, GOF 1.122; max/min. residual density 0.621/
À3
À0.549 eꢀ
. The counterions and water molecules were
severely disordered. CCDC-251181 contains the supplementary
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.
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