Reliable evidence for the tetrahedral structure of 3·5 follows
from X-ray crystallographic analysis.‡ The structure clearly
showed that the four panels of 2b were linked with 1 in an
antiparallel fashion to form a distorted tetrahedron (Fig. 2).
Methyl groups (red) are exposed outside at every corner of the
tetrahedron minimizing the steric repulsion with neighboring
ligands. The tetrahedral guest 5 was almost fully insulated
within the framework of 3.
For ligand 2c, sterically demanding (MeO)Me2C- groups on
the edge allow only the parallel link of the ligands giving rise to
open cone structure 4 (Scheme 1). Again, guest molecules
assisted the efficient assembly. With a large guest, PhCOCOPh
(6), the quantitative formation of 4·6 complex was observed by
1H NMR spectrum (Fig. 1b) that displayed ten singlet signals
derived from panel 2c with its inherent symmetry (C2v).§
Highly upfield-shifted guest signals agree with the accommoda-
tion of 6 in the large cavity of the open cone. CSI-MS4 spectrum
also evidenced the formation of 4·6 with a series of prominent
peaks of [4·6–(NO32)n·(DMF)m]n+: (e.g., for n = 4, m/z 1026.1
(m = 0), 1044.4 (m = 1), and 1062.7 (m = 2)). It is noteworthy
that this inclusion complex does not dissociate under the CSI-
MS conditions despite the open cavity structure. Probably,
bulky substituents that hung over at the rim of the cone prevent
the facile escape of the included guest.
framework. Thus, the addition of mesitylene to an aqueous
solution of 4·6 resulted in the formation of a fully guest-
exchanged mesitylene-complex, which was, furthermore, con-
verted to the inclusion complex of 1,2-dibromoethane and CBr4
(4·5, Fig. 1c).† These results sharply contrast with the behavior
of the host from non-substituted ligand 2a where guest
exchange by CBr4 caused the smooth reorganization of the host
framework from open cone into tetrahedron.3
In summary, we have shown that the self-assembly pathways
of triangular ligands can be controlled by attaching sterically
demanding substituent(s) at appropriate positions of the ligand.
The principle demonstrated here potentially makes possible the
design and self-assembly of highly ordered, less symmetrical
architectures through specific orientation of component li-
gands.
Notes and references
‡
Typical procedure: To an aqueous suspension (1.0 mL) of (en-
)Pd(NO3)2 (1, 6.2 mg; 21.3 µmol) and ligand 2b (2.8 mg; 7.9 µmol), excess
(ca. 27 µmol) CBr4 (5) was added and the mixture was stirred for 24 h at
ambient temperature. After filtration, the resulting clear solution was
concentrated to precipitate included complex 3·5 in 77% isolated yield.
Complex 3·5: 1H NMR (500.13 MHz, D2O, 27 °C, TMS as external
standard): d = 10.48 (s, 4H), 10.44 (s, 8H), 9.48 (s, 8H), 8.89 (s, 8H), 8.57
(s, 8H), 8.15 (s, 8H), 8.11 (s, 4H), 3.12 (m, 32H), 3.00 (s, 24H), 1.57 (s,
24H), 1.56 (s, 24H); 13C NMR (125.77 MHz, D2O, 27 °C, TMS as external
standard): d = 161.1 (CH), 158.8 (CH), 149.4 (CH), 148.0 (CH), 145.3 (Cq),
138.2 (Cq), 137.5 (Cq), 136.2 (CH), 135.2 (Cq), 131.8 (Cq), 128.6 (CH),
126.2 (CH), 76.5 (Cq), 50.4 (CH3), 47.7 (CH2), 47.0 (CH2), 26.6 (CH3),
225.5 (Cq, 5).
Due to the presence of side chains (two (MeO)Me2C-
substituents) on the edge, ligand 2c is permitted to assemble into
only open cone 4 regardless of the size and shape of organic
guests. Large guest 6 could be smoothly exchanged by medium
to small size molecules without the transformation of the host
Crystal data for 3·5: C101H174N48O73Br4Pd8, M = 4399.72, monoclinic,
space group C2/c, a = 31.439(4), b = 44.137(6), c = 27.987(4) Å, b =
107.253(3)°, V = 37088 Å3, T = 173(2) K, Z = 8, Dc = 1.576 g cm23, l
= 0.71073 Å, 44275 reflections measured, 32651 unique (Rint = 0.4402)
which were used in all calculations. R1 = 0.1073 and wR2 = 0.2412. CCDC
b305129c/ for electronic files in .cif format.
§ Complex 4·6: 1H NMR (500.13 MHz, D2O, 27 °C, TMS as external
standard): d = 10.70 (s, 4H), 10.58 (s, 8H), 9.54 (s, 8H), 8.99 (s, 8H), 8.30
(s, 8H), 7.97 (s, 8H), 7.83 (s, 4H), 6.27 (s, 4H, 6), 6.07 (m, 6H, 6), 3.14 (m,
32H), 2.96 (s, 24H), 1.49 (s, 24H); 13C NMR (125.77 MHz, D2O, 27 °C,
TMS as external standard): d = 193.7 (Cq, 6), 160.6 (CH), 159.2 (CH),
148.9 (CH), 148.3 (CH), 145.2 (Cq), 137.2 (Cq), 136.9 (Cq), 135.7 (Cq),
135.5 (CH), 133.9 (CH, 6), 131.3 (Cq), 130.9 (Cq, 6), 128.1 (CH), 128.0
(CH, 6), 127.6 (CH, 6), 126.0 (CH), 76.4 (Cq), 50.3 (CH3), 47.8 (CH2), 46.9
(CH2), 26.6 (CH3), 26.1 (CH3); Elemental Analysis Calcd. for
C142H162N32O10Pd8P16F96·(H2O)17 (counter ions of 4·6 was replaced by
PF62): C, 28.56; H, 3.34; N, 7.51. Found: C, 28.75; H, 3.73; N, 7.26.
Fig. 1 1H NMR spectra (500 MHz, 27 °C, D2O, TMS as an external
standard) of (a) 3·5, (b) 4·6, and (c) 4·5.
1 A review of molecular paneling via coordination: M. Fujita, K.
Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa and K. Biradha, Chem.
Commun., 2001, 509.
2 For recent reviews of metal-directed assembly of 3D cage-like molecules:
(a) D. L. Caulder and K. N. Raymond, Acc. Chem. Res., 1999, 32, 975;
(b) S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853;
(c) F. A. Cotton, C. Lin and C. A. Murillo, Acc. Chem. Res., 2001, 34,
759; (d) F. Hof, S. L. Craig, C. Nuckolls and J. Jr. Rebek, Angew. Chem.,
Int. Ed., 2002, 41, 1488.
3 K. Umemoto, K. Yamaguchi and M. Fujita, J. Am. Chem. Soc., 2000, 122,
7150.
4 Coldspray ionization mass spectroscopy (CSI-MS):(a) S. Sakamoto, M.
Fujita, K. Kim and K. Yamaguchi, Tetrahedron, 2000, 56, 955; (b) S.
Sakamoto, M. Yoshizawa, T. Kusukawa, M. Fujita and K. Yamaguchi,
Org. Lett., 2001, 3, 1601.
Fig. 2 The crystal structure of 3·5 represented by (a) cylinder and (b) ball
models (Me = red, C = grey, N = blue, Pd = yellow, Br = cyan).
CHEM. COMMUN., 2003, 1808–1809
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