Quantitative dynamic interconversion between AgI-mediated capsule and cage complexes accompanying guest encapsulation/release

Article abstract of DOI:10.1002/anie.200462394

(Chemical Equation Presented) An accommodating host: Capsule- and cage-shaped complexes are formed from Ag^I and disk-shaped trismonodentate ligands in 1:1 and 1.5:1 concentration ratios, respectively; their interconversion is reversible. The ca

Full text of DOI:10.1002/anie.200462394

Angewandte
Chemie
Host–Guest Systems
Quantitative Dynamic Interconversion between
I
Ag -Mediated Capsule and Cage Complexes
Accompanying Guest Encapsulation/Release**
Shuichi Hiraoka, Koji Harano, Motoo Shiro, and
Mitsuhiko Shionoya*
Multicomponent self-assembly leading to discrete molecular
[
1]
architectures through reversible hydrogen bonding or
[
2]
metal-coordination bonding has attracted a great deal of
attention. So far, many excellent examples of well-defined 3D
[
3]
[4]
[5]
[6]
structures, such as capsules, cages, boxes, and tubes,
have been reported, and their inner spaces have been
[
7]
[8]
efficiently used as recognition and reaction centers for
neutral molecules and ionic species. External stimuli-respon-
sive supermolecules are considered as molecular devices in
which conformational or configurational transitions of the
molecules allow on–off switching of their functions such as
motion, molecular recognition, and reaction control. How-
ever, to date, only a few examples have been reported on
quantitative dynamic interconversions between differentiated
self-assembled molecules formed from identical chemical
components. To realize such a dynamic system, metal-
mediated self-assembly has a great advantage in that the
coordination number and geometry of transition-metal ions
[
9]
can be reversibly changed by their oxidation states and
[
10,11]
concentration ratios of metal to ligand.
Such metal-
centered changes should allow dynamic interconversion
through metal–ligand exchanges between different kinds of
metal-assembled structures.
I
Recently, we established an Ag -mediated interconverti-
ble system with a disk-shaped trismonodentate ligand (L) in
which three 2-benzimidazolyl rings and three methyl groups
[
10]
are alternately attached to the central benzene ring. In this
system, a structural interconversion between a capsule-
shaped and a sandwich-shaped complex accompanies encap-
sulation/release of anionic molecules and was established by
varying the metal-to-ligand ratio. In the study described
herein, a disk-shaped trismonodentate ligand 1, which has
[
*] Dr. S. Hiraoka, K. Harano, Prof. Dr. M. Shionoya
Department of Chemistry, Graduate School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5841-8061
E-mail: shionoya@chem.s.u-tokyo.ac.jp
Dr. M. Shiro
Rigaku Corporation, 3-9-12 Matsubaracho
Akishima, Tokyo 196-8666 (Japan)
[
**] This work was supported by a Grant-in-Aid for The 21 st Century
COE Program for Frontiers in Fundamental Chemistry and by
Grants-in-Aid for Scientific Research (S) to M.S. (No. 16105001)
and for Scientific Research on Priority Areas, “Dynamic Complexes”
to S.H. (No. 420/16033215) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2727 –2731
DOI: 10.1002/anie.200462394
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2727

Communications
three 3-pyridyl and p-tolyl groups attached alternately to the
central benzene ring, was designed with the aim of construct-
ing larger capsule molecules that allow encapsulation/release
of organic guest molecules. Each pyridyl ring is almost
perpendicular to the central ring plane as a result of the steric
hindrance between the neighboring pyridyl and p-tolyl
groups. The coordination direction of each nitrogen donor
atom of the 3-pyridyl groups is thereby somewhat distorted
away from the central ring by 308. From a molecular-modeling
study, we expected that the combination of the ligand 1 with
I
Ag , which can assume both a three-coordinate trigonal-
planar and a two-coordinate linear geometry with mono-
dentate ligands, should generate two different 3D structures
with inner spaces when the metal-to-ligand ratios are 4:4 and
[
12]
6
:4. Moreover, the quantitative, reversible structural inter-
conversion between these two structures would provide an
excellent molecular encapsulation/release system if only one
of them binds preferentially to some given guest molecules.
Herein we present a quantitative interconversion between
Figure 1. Schematic representation of the interconversion between
4+
6+
I
[
Ag 1 ] capsule and [Ag 1 ] cage complexes by changing the [Ag ]/
4 4 6 4
+
4
[1] ratio from 1:1 to 1.5:1. The [Ag 1 ] capsule complex can entrap a
4
4
I
two Ag -containing molecular architectures, a capsule-shaped
neutral organic molecule such as adamantane in the inner space,
6
+
4+
6+
while the [Ag 1 ] cage complex cannot practically encapsulate the
6 4
[
Ag 1 ] and a cage-shaped [Ag 1 ] complex (Figure 1).
4
4
6 4
guest molecule. The encapsulation/release of the guest molecule is
These two complexes were self-assembled from trismono-
I
coupled with the reversible Ag -dependent capsule$cage interconver-
I
I
dentate disk-shaped ligands 1 and Ag by changing the 1/Ag
concentration ratios in the presence or absence of guest
sion. A front disk is opened to show clearly a guest molecule encapsu-
4+
lated in the inner space of [guestꢁAg 1 ]
.
4
4
4
+
molecules. Indeed, the [Ag 1 ] capsule complex could
4
4
accommodate a neutral organic molecule such as adamantane
in the inner space with a high affinity. On the other hand, as
soon as the capsule complex was converted into the cage-
I
of the Ag -bound aromatic ligands that form a self-assembled
capsulelike structure.
6
+
shaped counterpart, [Ag 1 ] , the included guest molecule
On the other hand, when 1.5 equivalents of AgPF were
added ([Ag ]/[1] = 1.5:1), the H NMR spectrum showed
another set of highly symmetrical signals (Figure 2b), which
indicate the quantitative formation of another Ag complex.
In this case, the signals for the p-tolyl proton H do not shift
upfield, which suggests that the ligand array should be
different from that of a complex formed from a 1:1 mixture
of Ag and 1. These results demonstrated that two highly
symmetrical structures were quantitatively formed from Ag
6
4
6
I
1
was immediately released. X-ray single-crystal analysis
4
+
revealed a [Ag 1 ] capsule structure in which an adaman-
4
4
I
tane molecule is trapped inside. Furthermore, the encapsula-
tion and release of the guest molecule could be dynamically
f
I
controlled by the quantitative Ag -dependent capsule$cage
interconversion.
1
I
H NMR titration experiments with ligand 1 and AgPF in
6
I
CD NO revealed the quantitative formation of two different
3
2
I
I
Ag complexes depending on the [Ag ]/[1] ratios. Upon
and 1 in 1:1 and 1.5:1 ratios. The interconversion between
these two thermodynamically stable complexes was fast and
reached equilibrium within a few minutes after changing the
addition of an equimolar amount of AgPF to a solution of
6
I
1
1
in CD NO ([Ag ]:[1] = 1:1), the signals for metal-free ligand
3
2
I
completely disappeared and one set of new signals simulta-
Ag /1 ratios.
neously appeared in a highly symmetrical pattern (Figure 2a).
ESI-TOF mass spectra confirmed the formation of
4+
6+
I
The signals for the p-tolyl ring moieties, H and H , are divided
[Ag 1 ] and [Ag 1 ] complexes with [Ag ]/[1] ratios of
e
f
4 4 6 4
I
into two sets, which indicate that the Ag ions are placed only
1:1 and 1.5:1, respectively (see Supporting Information). The
on one side of the disk-shaped ligand 1. Notably, the signals
ESI-TOF mass spectrum of a mixture of AgPF and 1 in a 1:1
6
for one of the p-tolyl protons (H ) and for the methyl protons
ratio showed a signal at m/z = 965.2, which was assigned to
f
3
+
(
H ) are shifted upfield (Dd = ꢀ2.0 and ꢀ0.4 ppm for H and
[Ag 1 ·PF ] . In contrast, the spectrum of a mixture of AgPF
g
f
4
4
6
6
H , respectively). This is probably due to the shielding effects
and 1 in a 1.5:1 ratio showed signals at m/z = 814.1 and 1133.9,
g
2
728
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Angew. Chem. Int. Ed. 2005, 44, 2727 –2731

Angewandte
Chemie
d = 2.3 ppm with downfield shifts of Dd = + 0.60 and
0.41 ppm, respectively, as a result of the deshielding effect
+
from the pyridyl and p-tolyl rings of the ligand part of the
inclusion complex (Figure 2c). Under these conditions,
approximately 91% of the guest molecules are included in
4
+
the [Ag 1 ] complex and the guest 3 moves in and out of the
4
4
interior slowly on the NMR timescale. The significant upfield
shift of H (Dd = ꢀ0.59 ppm) upon inclusion suggests that the
a
I
N–Ag distances are somewhat lengthened as the four ligands
move slightly apart from each other as a result of the
encapsulation of 3 accompanying an increase in the inner
[
13]
volume. Evidence for the formation of the 1:1 inclusion
4
+
complex, [3ꢁAg 1 ] , was provided by the ESI-TOF mass
4
4
spectrum, which showed a signal that was assigned to
4
+
[
3ꢁAg 1 ] at m/z = 721.7 (see Supporting Information).
4
4
Thermodynamic parameters were determined for the encap-
sulation of 3 into the inner space of the [Ag 1 ] by variable-
temperature H NMR measurements. The negative values
4
+
4
4
1
[14]
ꢀ
1
ꢀ1 ꢀ1
of DH = ꢀ63.3 kJmol and DS = ꢀ144 Jmol
K
in the
encapsulation process indicate the great contribution of
enthalpy-driven association to the stability through van der
4
+
Waals contacts between the [Ag 1 ] capsule and the guest 3
4
4
as well as by the solvophobic effects on the process. The
ꢀ
1
binding constant (K = [3ꢁAg 1 ]/([3]·[Ag 1 ]); m ) was thus
4
ꢀ1
4
4 4
4
determined to be 3.8 ꢀ 10 m at 273 K. A structurally similar
1
4+
Figure 2. a)–b) H NMR spectra (500 MHz) of [Ag 1 ] capsule and
[
4
4
4+
6
+
1-adamantanol was also encapsulated in the [Ag 1 ] capsule
4
4
3
Ag 1 ] cage complexes (CD NO , 293 K, [1]=5.9 mm);
6 4 3 2
ꢀ1
but the binding constant was decreased to 1.3 ꢀ 10 m , which
indicated that only 59% of 1-adamantanol was included
under the same conditions. As for 1-chloroadamantane with a
a) Ag 1 ·(PF ) and b) Ag 1 ·(PF ) . c)–e) Encapsulation of a guest mol-
4
4
6
4
6
4
6 6
4+
ecule 3 in the [Ag 1 ] capsule complex and encapsulation/release of
4
4
the guest molecule triggered by the capsule$cage interconversion
H NMR (CD NO , 273 K, [1]=10.8 mm)); c) [3ꢁAg 1 ]
1
4+
(
,
ꢀ1
3
2
4
4
larger Cl group, only a slight binding affinity (K < 1m ) was
[
Ag 1 ·(PF ) ]=[3]=2.7 mm. d) Upon addition of AgPF (2.0 equiv) to
4+
4
4
6
4
6
observed. These results indicate that the [Ag 1 ] capsule has
4
4
the sample for (c), encapsulated 3 was released from inside as a result
of the interconversion from the capsule into the cage. e) Upon addi-
tion of [2,2,2]cryptand 2 (2.0 equiv) to the sample for d), an Ag 1 cap-
a high selectivity for unsubstituted adamantane in the
[
15,16]
recognition of adamantane derivatives.
On the other
4
4
6
+
sule complex was formed and 3 was immediately encapsulated again
hand, the [Ag 1 ] cage complex did not include any guest
6
4
in the capsule. Filled circles and triangles denote the inclusion com-
molecules tested, probably owing to its larger inner space, the
absence of preferable interaction between the host and guest,
and the presence of the openings in the cagelike structure.
4
+
6+
plex [3ꢁAg 1 ] and the [Ag 1 ] cage complex, respectively.
4
4
6 4
I
The interconversion between the two Ag complexes was
4
+
3+
I
which are ascribable to [Ag 1 ·(PF ) ] and [Ag 1 ·(PF ) ] ,
fast, as expected from the labile nature of the Ag -centered
6
4
6
2
6
4
6 3
respectively. These mass spectral data and the symmetrical
patterns of the H NMR spectra overall suggest that the
ligand exchange reactions. Furthermore, under the condition
employed, the ratios of the two complexes were highly
dependent on the [Ag ]/[1] ratios (Figure 3a). This tendency
was also observed when an adamantane was added to the
solution, as shown in Figure 3b.
1
4
+
I
[
Ag 1 ] complex should assume a trigonal-pyramidal, cap-
4
4
I
sule-shaped structure in which four Ag ions are arranged in a
tetrahedral fashion and three pyridyl nitrogen donor atoms
coordinate to each Ag . Similarly, an octahedral structure was
proposed for the [Ag 1 ]
coordinate Ag ions are arranged in an octahedral fashion
I
Similar results were obtained when silver trifluorometha-
nesulfonate (AgOTf) was used instead of AgPF6 which
indicated that counteranions have no significant influence
on the interconversion process and on the encapsulation
6
+
complex in which six two-
6
4
I
so that half of the faces of octahedron are occupied by ligands
I
4+
1
and the other half is opened to form an Ag -linked cage-
property of the cationic part, [Ag 1 ] . Finally, X-ray analysis
4
4
[
12]
shaped complex.
of the neutral inclusion complex 3ꢁAg 1 ·(TfO) revealed a
4
4
4
1
From a molecular-modeling study in light of the H NMR
and ESI-TOF spectra, the Ag complexes should have the
capsule-shaped coordination structure in which one adaman-
tane molecule is accommodated in the inner space (Figure 4).
The capsule complex is composed of four pyridyl ligands 1
I
inner spaces for small molecules, and, in particular, the
4+
I
[
Ag 1 ] complex was expected to have an enclosed space
and four Ag ions, which are arranged 10.0 ꢁ apart from each
4
4
with an encapsulation function. Indeed, the investigation of
other in a tetrahedral fashion. Three pyridyl nitrogen atoms
from three disk-shaped ligands (Ag–N: 2.29 ꢁ (average)) and
an oxygen atom of TfO coordinate to each Ag center (Ag–
4
+
the ability of [Ag 1 ] for molecular inclusion revealed that
4
4
1
ꢀ
I
adamantane (3) is a suitable guest. The H NMR spectrum of
an equimolar mixture of 3 and the [Ag 1 ] capsule in
4
+
O: 2.50 ꢁ (average)) in a distorted tetrahedral geometry. The
4
4
I
CD NO at 273 K gave rise to signals for 3 at d = 2.4 ppm and
mean deviation of the Ag from the N plane is 0.391 ꢁ. One
3
2
3
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Communications
Figure 3. Effects of the presence of adamantane (3) on the inter-
4
+
6+
conversion between [Ag 1 ] and [Ag 1 ] . a) Plot of [Ag 1 ]/
4
4
6
4
4 4
(
[Ag 1 ]+[Ag 1 ]) (filled circles) and [Ag 1 ]/([Ag 1 ]+[Ag 1 ]) (open
4 4 6 4 6 4 4 4 6 4
I
1
circles) against [Ag ]/[1] determined from the H NMR integral ratios.
Solid lines are theoretically drawn for [Ag 1 ]/([Ag 1 ]+[Ag 1 ]) (blue)
and [Ag 1 ]/([Ag 1 ]+[Ag 1 ]) (red) as a function of [Ag ]/[1]. b) Plot of
4
4
4
4
6 4
I
6
4
4
4
6 4
(
[3ꢁAg 1 ]+[Ag 1 ])/([3ꢁAg 1 ]+[Ag 1 ]+[Ag 1 ]) (filled circles) and
4
4
4
4
4
4
4
4
6 4
I
[
Ag 1 ]/([3ꢁAg 1 ]+[Ag 1 ]+[Ag 1 ]) (open circles) against [Ag ]/[1]
6
4
4
4
4
4
6 4
1
determined from the H NMR integral ratios. Solid lines are theoreti-
I
cally drawn as a function of [Ag ]/[1]. A front disk is opened to clearly
show a guest molecule in the inner space for the [3ꢁAg 1 ] complex.
4
+
4
4
adamantane molecule is closely packed in the inner space and
surrounded by the four ligands. The hydrogen atoms of 3 are
close to p-tolyl H or pyridyl H within approximately 2.2–
e
a
1
3
.1 ꢁ, as suggested by the H NMR data for 3ꢁAg 1 ·(PF ) .
4
4
6 4
The distance between H of 3 and the central benzene ring of
h
1
is 2.85 ꢁ, thus indicating that there are no CH–p inter-
actions between them.
4
+
As described above, the [Ag 1 ]
capsule and the
4
4
6+
[
Ag 1 ] cage complexes are totally different from each
6 4
other in shape, volume, and internal structures, thus resulting
in the large difference in their affinity for guest molecules.
4
+
That is, an adamantane (3) is encapsulated in the [Ag 1 ]
4
4
6
+
capsule with high affinity, whereas the [Ag 1 ] cage has
6
4
practically no interaction with 3. We exploited this switching
system for the encapsulation/release of the guest molecule
through the interconversion between the capsule- and cage-
Figure 4. The crystal structure of 3ꢁAg
1 ·(TfO) . Color labels: red
4 4 4
carbon), turquoise (carbon of 3), blue (nitrogen), yellow (silver),
(
purple (oxygen), light green (sulfur), cyan (fluorine), and white (hydro-
gen). a) A space-filling model; b) the capsule structure is depicted as a
cylinder model. Hydrogen atoms of the [Ag 1 ] moiety are omitted
4
+
shaped structures. First, the [Ag 1 ] complex entrapped 3
4+
4
4
4
4
into its inner space to form a 1:1 complex (Figure 2c). Upon
for clarity.
addition of 2.0 equivalents of AgPF , the encapsulated 3 was
6
immediately released to the bulk solvent as a result of the
4
+
structural conversion from the [Ag 1 ] capsule into the
complex with an adamantane molecule. Finally, we have
established an excellent dynamic system for the encapsula-
tion/release of guest molecules by the use of metal-mediated
interconversion between the capsule and cage complexes
4
4
6+
[
Ag 1 ] cage (Figure 2d). When 2.0 equivalents of [2,2,2]-
cryptand 2 were added to the solution, two Ag ions of the
6 4
I
6+
4+
[
Ag 1 ] complex were trapped by 2, and then the [Ag 1 ]
6
4
4
4
I
capsule was reconstructed with simultaneous inclusion of 3 in
the capsule (Figure 2e). This encapsulation/release cycle
could be repeated at least three times without any loss of
efficiency within a few minutes for each encapsulation/release
constructed from trismonodentate ligands and Ag in a self-
assembled manner. These results would open up prospects for
exploiting nanocapsules for molecular transport systems. The
construction of larger metallocapsules with dynamic proper-
ties is currently ongoing.
[
17]
cycle.
I
In summary, we have developed an Ag -mediated struc-
tural on–off switching system in which the Ag -linked capsule
[Ag 1 ] ) and cage ([Ag 1 ] ) complexes are quantitatively
interconverted, depending on the ratios of Ag to 1. The
capsule complex only has a large affinity for small neutral
organic molecules such as adamantane, and its crystallo-
graphic analysis revealed the structure of the 1:1 inclusion
I
4
+
6+
(
4 4 6 4
Experimental Section
I
3
ꢁAg 1 ·(TfO) : A single crystal suitable for X-ray crystallographic
4
4
4
analysis was obtained from a saturated solution of Ag 1 ·(TfO) and 3
4
4
4
(
1:1 mixture) in CD NO at 253 K. Crystallographic data
3 2
(C185H173Ag F N O S ): M = 3810.18, colorless, 0.20 ꢀ 0.12 ꢀ
4
12 15 26 4
2
730
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Chemie
3
0
3
0
.07 mm , orthorhombic, space group Pccn, a = 16.414(11), b =
Aoyagi, K. Biradha, M. Fujita, J. Am. Chem. Soc. 1999, 121,
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3
2.34(2), c = 37.30(2) ꢁ, V= 19800(21) ꢁ , Z = 4, R (I>2s(I)) =
.1532, wR(F ) = 0.4282, GOF = 1.091; intensity data were measured
at 93.1 K on a Rigaku RAXIS-RAPID Imaging Plate diffractometer
with graphite monochromated MoKa (l = 0.71075 ꢁ) radiation;
1
2
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a
structure solution was carried out with the program PATTY (P.
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3+
[
[
12] The possibility of a sandwich-shaped [Ag 1 ] structure is ruled
3
2
out because the coordination direction of pyridyl nitrogen atoms
of 1 does not meet the requirements for the formation of
3+
[
Ag 1 ] , as suggested by its molecular-modeling study.
3 2
4+
13] The changes in the chemical shift of H signal of the [Ag 1 ]
a
4 4
capsule complex should be affected by both the bonding nature
of the Ag N bonds and the deshielding effects of the p-tolyl
rings in the neighboring ligands. Upon encapsulation of 3 in the
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[
Ag 1 ] complex, the Ag N bonds are lengthened so as to
4 4
4+
maximize van der Waals interaction between the [Ag 1 ]
4
4
113, 2690 – 2692; Angew. Chem. Int. Ed. 2001, 40, 2620 – 2622;
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[
3ꢁAg 1 ] should decrease the deshielding effects of the p-
4
4
tolyl rings.
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1
[
3
3
98, 796 – 799; g) L. R. MacGillivray, J. L. Atwood, Nature 1997,
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4+
4+
ratios of the [3ꢁAg 1 ] and [Ag 1 ] , and the thermodynamic
4
4
4 4
parameters (DH and DS) were calculated from the DG values
obtained at temperatures ranging from 273 to 333 K by the least-
square method. Experimental data are shown in the Supporting
Information.
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[
[
15] Several halogenated methanes were also encapsulated in the
9
ꢀ1
Ag414 complex. Their binding constants K (m ) at 273 K are
2
2
7
.5 ꢀ 10 (CBr ), 1.0 ꢀ 10 (CBr Cl ), 14 (CFBr ), 18 (CBrCl ),
4 2 2 3 3
3
and 1.4 (CCl ).
4
Yamaguchi, K. Ogura, Nature 1995, 378, 469 – 471.
19
16] F NMR spectra of the Ag 1 capsule and Ag 1 cage complexes
4
4
6 4
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6
9
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release of guest molecules in the hydrogen-bonded cage
triggered by the change of their components. However, the
reversible encapsulation/release process was not established by
this system.
1
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www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2731