Template-directed synthesis of a covalent organic capsule based on a 3 nm-sized metallocapsule

Article abstract of DOI:10.1021/ja903324r

(Figure Presented) A 4 nm-sized covalent organic capsule having 24 pyridyl groups was synthesized in extremely high yield (43% in three steps) using an octahedron-shaped metallocapsule as a template molecule. The entity was fully characterized by NMR and

Full text of DOI:10.1021/ja903324r

Published on Web 07/31/2009
Template-Directed Synthesis of a Covalent Organic Capsule
Based on a 3 nm-Sized Metallocapsule
Shuichi Hiraoka,*,†,‡ Yoshitaka Yamauchi,† Ryo Arakane,† and Mitsuhiko Shionoya*,†
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan
Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received April 24, 2009; E-mail: hiraoka@chem.s.u-tokyo.ac.jp; shionoya@chem.s.u-tokyo.ac.jp
Molecular capsules have been of continuing interest to many
researchers because they provide a specific, confined nanospace as
a site for recognition and catalysis.1a,2 Since the birth of a covalent
organic capsule, carcerand, by Cram et al.,2 a number of excellent
molecular capsules have been reported to exhibit the usefulness of
the isolated internal space. Because of their synthetic facility, many
of them are constructed by self-assembly with the aid of reversible
chemical bonding such as H bonding and metal coordination.1 In
contrast, covalent organic capsules, which have a higher chemical
and thermal stability than self-assembled capsules, are synthetically
more difficult.3 At this stage, template-directed synthesis is expected
to be a rational strategy for constructing elaborate 3D architectures.4
Among a series of possible template species, metal-assisted self-
assembled molecules have great advantages, such as their relative
stability and the fact that the relative position of the metal ligands
is precisely defined by directional coordination bonding. Previously,
we reported 3 nm-sized octahedron-shaped metallocapsules
[M618]12+ constructed from eight tris(monodentate) ligands 1 and
six divalent transition-metal ions M2+ (M ) Mn, Co, Fe, Ni, Cu,
Zn, Cd, Hg, Pd, Pt) (Figure 1).5 The six metal ions and eight ligands
1 occupy the six vertexes and eight faces of the octahedron,
respectively, and as a result, two methyl groups on two ligands
lying side-by-side come close to each other. Accordingly, the
introduction of terminal olefins at these positions with an appropriate
linker would lead to cross-linkage of the adjacent metal ligands by
olefin metathesis reactions. To achieve this, we designed the
modified tris(monodentate) ligands 2-4, in which three terminal
olefins are attached to benzene rings in 1 through alkoxy linkers
having different chain lengths. In view of its high thermal stability
and inertness to Grubbs catalyst, a Pd(II) capsule was chosen. Here
we demonstrate the highly efficient synthesis of covalent organic
capsule 6 (from 3) starting from Pd638 ·(BF4)12 via the following
three steps (Figure 1): (1) olefin metathesis reaction between the
nearest-neighbor olefins on the Pd(II) capsule; (2) removal of the
Pd2+ ions; and (3) reduction of the internal olefins. The 24 pyridyl
groups of the resulting organic capsule 6 were fully methylated to
afford 5 nm-sized 7·I24, which was found to be larger than the 4
nm-sized neutral capsule 6.
3a, respectively. The olefin metathesis reactions of hexanuclear capsules
having different chain lengths were then carried out at a capsule
concentration of 0.5 mM in the presence of Grubbs catalyst 5 in CH2Cl2
at 293 K. Each reaction was monitored by ESI-TOF MS (Figure S6
in the Supporting Information). The 12-point metathesis reactions of
the capsules having longer alkoxy chains, [Pd6L8]12+ (L ) 3, 4),
completed within two days, while the reaction did not proceed in the
case of [Pd628]12+ having shorter alkoxy linkers.6
Figure 1. Schematic representation of the formation of covalent organic
capsules based on a template-directed approach.
Next, we investigated the effect of concentration on the metath-
esis reaction for the [Pd638]12+ and [Pd648]12+ capsules. In the case
of [Pd338]12+, decreasing the concentration of the capsule to 0.08
mM led to completion of the reaction within 1 day, and the ESI-
TOF MS spectrum of this reaction mixture showed only one species
for the capsule, which lost 12 C2H4 molecules (Figure 3b). On the
other hand, the metathesis reaction of [Pd648]12+ at the lower
concentration was found to be too slow (Figure S7).
To determine the optimal chain length of the linker for the olefin
metathesis reaction, three ligands having different chain lengths, 2-4,
were synthesized. The metallocapsules Pd6L8 ·(BF4)12 (L ) 2-4) were
quantitatively constructed by mixing L and Pd(CH3CN)4 ·(BF4)2 in a
4:3 ratio in a 1:1 CH3CN/CHCl3 mixed solvent at 70 °C for 10 h. The
1
formation of the hexanuclear Pd(II) capsules was confirmed by H
1
NMR and ESI-TOF MS measurements. For example, H NMR and
ESI-TOF MS spectra of Pd638 ·(BF4)12 are shown in Figures 2b and
The olefin metathesis reaction with the Pd638 ·(BF4)12 capsule was
then performed under the optimal conditions, after which the six Pd2+
ions were removed from the resulting complex by the addition of
ethylenediamine. The subsequent reduction of the internal olefins by
† The University of Tokyo.
‡ PRESTO.
9
11646 J. AM. CHEM. SOC. 2009, 131, 11646–11647
10.1021/ja903324r CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
a sulfonylhydrazide reagent produced the desired covalent organic
capsule 6 in 43% yield in three steps from Pd638 ·(BF4)12. The organic
capsule 6 was fully characterized by NMR and MS measurements.
The 1H NMR spectrum of 6 in CDCl3 displayed only 12 proton signals
in the aromatic region (Figure 2c), indicating a highly symmetrical
structure. This was also confirmed by the 13C NMR spectrum. All of
the proton and carbon signals were fully characterized by H-H COSY,
HMQC, and HMBC measurements (Figures S1-S3). It is noteworthy
that no signals for methyl groups were observed in these spectra,
indicating that the cross-linking of the terminal olefins was complete
at all 12 positions.7 The MALDI-TOF MS spectrum showed only one
intense signal at m/z 7833.4, which is consistent with 6+ (Figure 3c).
The finding that the solution structure of the capsule is flexible
prompted us to conduct N-methylation of the 24 pyridyl groups in 6
to form the polycationic capsule 724+. It was expected that in this
molecule, the electrostatic repulsion between the positive charges on
the nitrogen atoms would destabilize the folded conformation, causing
724+ to be larger than 6. N-methylation of 6 with CH3I gave 7·I24
quantitatively. The 1H NMR spectrum of 7·I24 in DMSO-d6 became
relatively sharp (Figure 2e). As expected, 7·I24 was found to have a
significantly larger hydrodynamic radius of 2.4 nm (Figure S5), which
is comparable to that of the fully expanded capsule structure as
estimated from molecular modeling.9
In summary, a covalent organic capsule 6 was synthesized from a
template capsule, Pd638 ·(BF4)12, in extremely high yield. The molecular
size of the capsule can be enlarged by N-methylation of the 24 pyridyl
groups of 6. Research on molecular recognition and chemical reactions
utilizing both neutral and polycationic nanosized capsules as platforms
is currently underway.
Acknowledgment. We thank A. Sato of JASCO International Co.,
Ltd., for measuring the high-resolution FT MS spectrum of 6 and H.
Sato of Bruker Biospin Japan Co. Ltd. for measuring 1H and 19F DOSY
spectra of 7·(TfO)24. This work was supported by Grants-in-Aid from
MEXT of Japan and the Global COE Program for Chemistry
Innovation.
Supporting Information Available: Synthetic procedures, NMR
spectra, and ESI-TOF MS spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 2. 1H NMR spectra (500 MHz, 293 K). (a) 3 in CDCl3; (b)
Pd638 ·(BF4)12 in CD2Cl2; (c) 6 in CDCl3; (d) 6 in DMSO-d6; (e) 7·I24 in
DMSO-d6.
References
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Figure 3. (a, b) ESI(+)-TOF MS spectra: (a) Pd638 ·(BF4)12; (b) measured
after the olefin metathesis reaction. (c) MALDI-TOF MS spectrum of 6.
R-Cyano-4-hydroxycinnamic acid (CHCA) was used as a matrix.
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(6) The template effect of the metallocapsule is obvious, since the same reaction
of ligand 3 at low concentration ([3] ) 4.3 mM) resulted in no reaction,
whereas the reaction at higher concentration ([3] ) 33 mM) gave only
insoluble material that may be a polymeric structure.
(7) When the metathesis reaction was incomplete, signals for the methyl groups
of the linkers in the N-methylated capsule were found at 0.74 and 13.6 ppm
in the 1H and 13C NMR spectra, respectively.
The molecular size of the capsule in solution was estimated by 1H
DOSY measurements (Figure S4). The hydrodynamic radius of 6 in
CDCl3, rh ) 1.9 nm, is larger than that of the original Pd(II) capsule
(rh ) 1.5 nm), indicating that the eight panels are farther apart from
each other in 6 than in the metallocapsule. In contrast, the 1H NMR
spectrum of 6 in DMSO-d6 was extremely broadened, suggesting a
compacted structure as the result of condensation of the immiscible
apolar alkoxy chains and phenylene groups in the polar medium.8
(8) The DOSY measurement of 6 in DMSO-d6 failed because of the extremely
broadened 1H NMR spectrum.
(9) A detailed discussion of the larger molecular size of the polycationic capsule
is provided in the Supporting Information.
JA903324R
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