An M2L4 molecular capsule with an anthracene shell: Encapsulation of large guests up to 1nm

Article abstract of DOI:10.1021/ja2037029

A new M₂L₄ molecular capsule with an aromatic shell was prepared using two Pd(II) ions and four bisanthracene ligands. The self-assembled capsule possesses a cavity with a diameter of ～1 nm that can encapsulate medium-sized spherical and planar molecules as well as a very large molecule (C₆₀) in quantitative yields. The encapsulated guests are fully segregated and shielded from the external environment by the large anthracene panels.

Full text of DOI:10.1021/ja2037029

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pubs.acs.org/JACS
An M L Molecular Capsule with an Anthracene Shell: Encapsulation
2
of Large Guests up to 1 nm
4
†
†
‡
†
,†
Norifumi Kishi, Zhiou Li, Kenji Yoza, Munetaka Akita, and Michito Yoshizawa*
†
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Bruker AXS, 3-9 Moriya-cho, Kanagawa-ku, Yokohama 221-0022, Japan
‡
S Supporting Information
b
ABSTRACT: A new M L molecular capsule with an
2
4
aromatic shell was prepared using two Pd(II) ions and four
bisanthracene ligands. The self-assembled capsule possesses
a cavity with a diameter of ∼1 nm that can encapsulate
medium-sized spherical and planar molecules as well as a
very large molecule (C ) in quantitative yields. The en-
60
capsulated guests are fully segregated and shielded from the
external environment by the large anthracene panels.
Figure 1. Cartoons of (a) the M
reported herein and (b) a previous M
2
L
4
capsule with large aromatic panels
cage with a wirelike framework.
2 4
L
and suitable for the encapsulation of large guest molecules (e.g.,
fullerenes); and finally (3) the attachment of exterior functional
groups to control the solubility and crystallinity was readily
achievable.
Anthracene was selected for the large aromatic panels of the
ligands because it can be found in the partial structure of many
fullerene frameworks. To make a curved anthracene wall, we
designed the new bridging bispyridine ligand 1 having two
anthracene rings connected by a m-phenylene spacer (Figure 2a).
The phenylÀanthryl and anthrylÀpyridyl bonds of ligand 1 are
hindered by the ortho substituents, restricting bond rotation and
ullerenes, the well-known class of spherical molecules com-
F
posed of curved aromatic shells with diameters of ∼1 nm, are
1
the archetypical molecular capsules. Within their confines,
encapsulated small molecules can exhibit unusual properties that
emerge from the effects of isolation as well as interactions with
the aromatic rings. However, the enclathration of guest molecules
within fullerenes is nontrivial, so we designed a new molecular
capsule possessing extended aromatic frameworks intended to
2
engender unique chemical phenomena within its cavity. Co-
ordinative self-assembly is one of the most advanced methods for
3
the facile preparation of large capsulelike supramolecules, and
forcing the aromatic subunits to adopt orthogonal conformations
there are many examples of coordination capsules assembled
from organic ligands with small aromatic rings (e.g., benzene and
8
(
Figure 2b). As a result, 1 is a rigid, bent ligand with anthracene
4
,5
walls (Figure 2c). The two pendant methoxyethoxy groups on
the m-phenylene ring serve to increase the solubility in protic
solvents. Accordingly, the simple combination of Pd(II) ions and
the ligands in a 1:2 ratio favors the exclusive formation of an
M L capsule.
naphthalene rings). Capsules assembled from ligands with
6
extended aromatic panels, however, remain uncommon. Herein
we report the preparation of a novel molecular capsule with an
M L composition bearing large aromatic panels and metal ions
2
4
2
4
(
Figure 1a). The large capsule cavity is fully enclosed yet can
Bisanthracene ligand 1 was prepared in five steps from 9-
bromoanthracene in 55% overall yield using Negishi and SuzukiÀ
Miyaura cross-coupling protocols. When ligand 1 (13.7 μmol),
PdCl (DMSO) (7.0 μmol), and AgNO (14.3 μmol) were
quantitatively encapsulate a variety of neutral guest molecules
with diameters of up to ∼1 nm.
We focused on molecular M L coordination cages because of
2
4
2
2
3
their simple composition and highly symmetric structures.
Accordingly, many M L analogues have been reported since
combined in DMSO-d (0.5 mL) at 100 °C for 3 h, a single
6
2
4
7
product, 2, was formed quantitatively (Figure 3a), as demon-
the first example by Steel et al. in 1998. However, the structures
typically consist of wirelike frameworks (Figure 1b), and there-
fore, their molecular recognition or hostÀguest interactions have
been limited to anions or molecules with lone pairs mainly via
9
strated by NMR spectroscopy and mass spectrometry (MS). In
1
the H NMR spectrum, the shifts of the pyridyl signals as well as
the considerable upfield shift of the inner m-phenylene signal
7
gÀi
(H ; Δδ = À1.15 ppm) indicated the formation of a discrete
b
metal coordination.
To create a truly functional molecular
2
M L complex. At room temperature (rt), only six sharp signals
2
4
host for neutral guests with various sizes and shapes, we
in the aromatic region were observed (Figure 3c), but four
additional signals appeared and sharpened at 100 °C (Figure 3d).
COSY and NOESY analyses showed that the original sharp
refreshed the M L motif with the following design specifications
2
4
and features: (1) extended aromatic panels were incorporated
into the ligand framework to provide an aromatic shell (Figure 1a)
that demarcates the enclosed cavity and facilitates stronger
hostÀguest interactions; (2) the rigid and curved ligand was
designed to give a capsule cavity that is large (∼1 nm in diameter)
Received: April 22, 2011
Published: June 27, 2011
r 2011 American Chemical Society
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dx.doi.org/10.1021/ja2037029
|J. Am. Chem. Soc. 2011, 133, 11438–11441

Journal of the American Chemical Society
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Figure 4. X-ray crystal structure of molecular capsule 2: (a) ball-and-
stick representation without the hydrogen atoms (yellow balls represent
Pd atoms); (b) space-filling representation. The counteranion and
solvent molecules have been omitted for clarity.
Figure 2. (a) Bridging bidentate ligand 1 and (b, c) views of the
optimized structure (DFT calculation, B3LYP/6-31G* level): (b) side
view; (c) front view.
1
Figure 5. H NMR spectra (400 MHz, 2:1 CD
3
OD/D
complex (orange b,
signals from encapsulated guest 3 or 4; *, solvent signals).
2
O, rt) of (a)
capsule 2, (b) the 2⊃3 complex, and (c) the 2⊃(4)
2
crystals of capsule 2 were obtained by slow concentration of a
9
MeOH/H O solution, and X-ray crystallographic analysis re-
2
vealed that four curved ligands 1 bridge two Pd atoms to provide
the expected M L structure with a virtual D symmetry(Figure 4).
2
4
4
Each Pd atom has the standard square-planar geometry and is
bound by the four pyridines in a twisted conformation that brings
the anthracene panels into contact and couples their restricted
Figure 3. (a) Schematic representation of the self-assembly of mole-
1
motion, as observed in the broadened H NMR spectrum at
1
cular capsule 2. (bÀd) H NMR spectra (400 MHz, aromatic region) of
room temperature (Figure 3c). The large interior cavity of the
(
b) ligand 1 in CDCl at rt and of 2 in DMSO-d at (c) rt and (d) 100 °C.
3 10
3
6
capsule, with a volume of ∼580 Å , is defined by a PdÀPd
distance of 1.36 nm and a 1.12 nm diagonal between the
signals came from the pyridyl and m-phenylene protons (Hg,h,i,j
opposing m-phenylene hydrogen atoms (H ). Solvent molecules
b
À
and H , respectively) of 2 (Figures S16 and S18 in the
(seven MeOH and one H O) and a counteranion (NO ) were
a,b
2
3
Supporting Information). The broadened signals came from
protons located on the anthracene moieties (Hc,d,e,f) of 2. The
sharpening at higher temperatures is consistent with a tightly
wound closed-shell framework wherein the cooperative motion
of individual anthracene moieties is restricted and slow on the
NMR time scale at room temperature. A single band with a
present within the cavity. We emphasize that the eight anthra-
cene panels of 2 form a roughly spherical aromatic shell, fully
enclosing and segregating the cavity interior from the exterior
environment (Figure 4b).
Molecular capsule 2 presents an isolated hydrophobic cavity
suitable for encapsulating guest molecules through hydrophobic
interactions. The solubility of capsule 2 in aqueous solution is
À10
2
À1
diffusion coefficient (D) of 8.51 Â 10
m s was present in
1
1
the H DOSY NMR spectrum, from which the diameter of the
increased because of the pendant hydrophilic chains, and the H
M L complex was calculated to be ∼1.5 nm (Figure S21).
NMR spectrum in 2:1 CD OD/D O revealed desymmetrization
2
4
3
2
Prominent signals in the electrospray ionizationÀtime of flight
of the host framework. The anthracene protons H split into
cÀf
(
ESIÀTOF) MS spectrum at m/z 786.0, 1068.6, and 1634.0
eight sharp signals from 6.55 to 8.02 ppm (Figure 5a), which
suggests the anthracene panels of 2 were tightly wound, com-
pressing the capsule and decreasing the hydrophobic surface area
presented. When a slight excess of [2,2]paracyclophane (3), a
spherical guest with a diameter of 0.67 nm, was suspended in the
CD OD/D O solution (6.0 mM) of capsule 2 at 60 °C for 3 h, the
À n+
were assigned to the respective [2 À nNO
] (n = 4, 3, 2) ions,
3
corroborating the formation of a [Pd(NO ) ] (1) complex
3
2 2
3
4
with a molecular weight of 3391.9 Da (Figure S22). Therefore,
the formation of M L molecular capsule 2 was well-supported
2
4
by the solution-state NMR and MS studies.
X-ray crystallographic analysis provided the final evidence for
3
2
1:1 hostÀguest complex 2⊃3 was obtained in nearly quantitative
the formation of an M L molecular capsule. Pale-yellow single
yield, as revealed by NMR and MS analyses (Figures S24ÀS26).
2
4
1
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dx.doi.org/10.1021/ja2037029 |J. Am. Chem. Soc. 2011, 133, 11438–11441

Journal of the American Chemical Society
COMMUNICATION
based on 2) was suspended in a CD CN solution of 2 (4.5 mM)
3
at 80 °C for 3 h. The color of the solution dramatically changed
from pale-yellow to red-purple (Figure 7a), even though C is
6
0
11
insoluble in acetonitrile. The appearance of a new broadened
9
absorption band at 500À600 nm in the UVÀvis spectrum,
corresponding to a typical charge-transfer band for C due to π-
60
stacking interactions, substantiated the encapsulation of C by
6
0
13
capsule 2. A single C NMR signal at 141.2 ppm was easily
observed; no signal for free C60 was detected (Figure 7b). The
upfield shift of this signal (Δδ = ∼2 ppm relative to C in
Figure 6. X-ray crystal structure of the 2⊃(4) complex: (a) stick (2)
2
6
0
and space-filling (4) representation; (b) space-filling representation of
DMSO) provided further evidence for the encapsulation of C .
6
0
2
2
⊃(4) . The anions and solvent molecules have been omitted for clarity.
ESIÀTOF MS analysis unambiguously confirmed the 2 C
3
60
product composition with a molecular weight of 4109.0 Da
À n+
(
[2⊃C60 À nNO3
]
with n = 4, 3, 2) (Figure 7c,d). Peaks
corresponding to either the empty capsule or free C were not
6
0
observed, reflecting the high stability of the 2⊃C complex.
6
0
Furthermore, the isolated 2⊃C composite remained intact
60
over several weeks in CH CN at room temperature. The 2⊃C
3
60
complex is soluble in various organic solvents, and while en-
capsulated within capsule 2, very high concentrations (∼20 mM)
11,12
of C in MeOH or 2:1 MeOH/H O were obtained.
Notably,
60
2
capsule 2 showed selective molecular recognition capabilities and
could bind only C60 of complementary size and shape from a
13
mixture of C and C .
6
0
70
In summary, we have designed and prepared a new M2L4
molecular capsule with a spherical aromatic shell using two
Pd(II) ions and four bisanthracene ligands. The self-assembled
capsule possesses a cavity with a diameter of ∼1 nm that is capable of
encapsulating not only medium-sized spherical ([2,2]paracyclophane)
and planar (1-methylpyrene) molecules but also a very large
molecule (C ) in quantitative yields. It is worthy of note that
Figure 7. (a) Schematic representation of the quantitative encapsula-
60
13
tion of C60 by molecular capsule 2 in CD CN. (b) C NMR (100 MHz,
3
these molecular recognitions typically were not found for pre-
CD
3
CN, rt) and (c) ESIÀTOF MS spectra (CH
3
CN) of the 2⊃C60
vious M L coordination cages with wirelike frameworks. The
À 4+
3
]
2 4
complex. (d) Expansion and simulation of the [2⊃C60 À 4NO
encapsulated guest molecules are fully segregated and shielded
from the external environment by the large aromatic panels. The
pendant hydrophilic chains endow enhanced solubility and
facilitate the remarkable solubilization of encapsulated fullerene
C60 in protic solvents. This new class of molecular hosts with
extended aromatic π surfaces holds great promise, and potential
new properties and chemical reactions in their large cavities are
signals.
1
The methylene H NMR signals of the encapsulated 3 were
desymmetrized and strongly shifted upfield to À0.19 and À0.44 ppm
9
(
Δδ = À3.5 ppm) (Figure 5b). Capsule 2 also bound 2 equiv
max
of the large, planar guest molecule 1-methylpyrene (4) to give the
14
complex 2⊃(4) in quantitative yield. Sets of methyl and
2
currently under investigation.
1
aromatic H NMR signals of 4 were found at À1.25 and 2.75À
4
.82 ppm, respectively, indicative of encapsulation by 2 (Figure 5c).
’
ASSOCIATED CONTENT
Supporting Information.
1
The methyl H NMR signal exhibited a huge upfield shift (Δδ =
À4.2 ppm) due to enhanced shielding by the aromatic anthra-
cene panels. The aromatic signals of 2 underwent a slight shift,
but the symmetry of the capsule framework remained unchanged
S
Experimental procedures,
b
physical properties, and crystallographic data (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
9
after guest encapsulation. Quantitative formation of a 1:2 hostÀ
1
guest complex was confirmed by H NMR integration and
ESIÀTOF MS analysis: only signals at m/z 894.0, 1212.7, and
’
AUTHOR INFORMATION
À n+
]
3
1
850.0, assigned to the [2⊃(4) À nNO
(n = 4, 3, 2)
2
species, respectively, were observed (Figure S29).
Corresponding Author
yoshizawa.m.ac@m.titech.ac.jp
The detailed molecular structure of 2⊃(4) was confirmed by
2
9
X-ray crystallographic analysis. The crystal structure shows two
stacked molecules of 4 with an interplanar distance of 3.5 Å
resting between the two Pd(II) centers (Figure 6a). The stacked
dimer interacts with the framework of 2 through complementary
CHÀπ interactions: CH(4)Àπ(anthracene) and CH(pyridine)À
’
ACKNOWLEDGMENT
This work was supported by the Japanese Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT) via
Grants-in-Aid for Scientific Research on Innovative Areas
“Coordination Programming” (Area 2107, No. 21108011) and
the global COE program (GCOE) “Education and Research
Center for Emergence of New Molecular Chemistry” and by the
π(4). The large planar guests (4) are fully shielded by the
2
anthracene framework of capsule 2 (Figure 6b,c).
The molecular host properties of capsule 2 encouraged us to
examine the encapsulation of C . Black C powder (3 equiv
60
60
1
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Journal of the American Chemical Society
COMMUNICATION
Japan Society for the Promotion of Science (JSPS) via “Funding
Program for Next Generation World-Leading Researchers”. We
thank Prof. Dr. Kari Rissanen for helpful discussions.
M.; Suenaga, H.; Shinkai, S. Chem. Commun. 1999, 1403–1404. (b) Rio, Y.;
Nierengarten, J.-F. Tetrahedron Lett. 2002, 43, 4321–4324.
(13) The binding preference of capsule 2 for the guests employed
here is 4 > C60 > 3 . C . Investigations of further molecular recognition
70
(e.g., organometallic and chiral guests) and the mechanisms of guest
’
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(
(
9) See the Supporting Information.
10) The cavity size of 2 (∼580 Å ) is smaller than that of previous
3
3
3
large coordination capsules [e.g., ∼900 Å (ref 5b), ∼1150 Å (ref 5d),
3
3
1
340 Å (ref 6c), and ∼2700 Å (ref 5e)], but 2 shows good ability to
encapsulate large guest molecules, as discussed in detail later in the
manuscript.
(
11) The solubility of C60 in MeCN or MeOH is extremely low
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J. Phys. Chem. 1993, 97, 3379–3383.
12) To the best of our knowledge, there have been no reports on
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except for a few covalent hosts. See: (a) Ikeda, A.; Hatano, T.; Kawaguchi,
(
(
2
,
1
1441
dx.doi.org/10.1021/ja2037029 |J. Am. Chem. Soc. 2011, 133, 11438–11441