A palladium(II)-clipped aromatic sandwich

Article abstract of DOI:10.1002/anie.200460868

A filling that makes the sandwich: Large aromatic molecules are sandwiched by two π-conjugated ligands (L) that are joined together by six {(en)Pd} ²⁺ units (M; en = 1,2-ethanediamine; see picture; guest red, {(en)Pd}²⁺ yellow, ligand blue). This host has an M₆L ₂ composition and is distorted by the concave shape of the ligand. Without a guest, it releases the distortion by forming the dimeric M ₁₂L₄ in a reversible fashion.

Full text of DOI:10.1002/anie.200460868

Communications
Self-Assembly
A Palladium(ii)-Clipped Aromatic Sandwich**
Kazuhisa Kumazawa, Yoshinori Yamanoi,
Michito Yoshizawa, Takahiro Kusukawa, and
Makoto Fujita*
Enclathration of large p-conjugated molecules by a synthetic
receptor is an interesting task because the properties of these
molecules, such as stability, reactivity, solubility, and photo-
[
1]
and electroresponse, can be controlled. To enclathrate large
p-conjugated molecules, however, a synthetic receptor with a
cavity is required, whose dimensions should be larger than
that of the p-conjugated guests. While there are many
[
2]
examples of three-dimensional receptors, large two-dimen-
[
3]
sional receptors have been less explored. Herein we
describe the self-assembly of a p-stacked host–guest system
in which large aromatic guests are sandwiched by metal-
clipped p-conjugated ligands. The ligand 1 is a roughly 2-nm-
sized hexagonal planar molecule that consists of ten aromatic
rings with six pyridyl donor sites at the periphery. Upon
complexation with [(en)Pd(NO ) ] (en = 1,2-ethanediamine)
3
2
this ligand is assembled to give large two-dimensional
receptors. In the presence of D -symmetric guests (2),
3
h
1
2+
12+
sandwich complexes [2ꢀ3] , where 3 has a composition
1
2+
of [{(en)Pd} (1) ] , are quantitatively assembled. The guest
6
2
molecule is wrapped by two ligands whose pyridyl donor sites
+
[*] K. Kumazawa, Dr. Y. Yamanoi, Dr. M. Yoshizawa, Dr. T. Kusukawa,
Prof. M. Fujita
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
+
[
] Present address:
Department of Chemistry and Materials Technology
Kyoto Institute of Technology
Sakyo-ku, Kyoto 606–8585 (Japan)
[
**] This work was supported by the CREST (Core Research for
Evolutional Science and Technology) project of Japan Science and
Technology Agency. K.K. thanks the JSPS fellowship for Japanese
Young Scientists.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460868
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Angewandte
Chemie
2
+
are fully clipped by six {(en)Pd} units at the rim of the
structure (Scheme 1).
1
12+
Figure 1. H NMR spectrum of [2aꢀ3] . Assignments of H –H are
a
j
given.
1
2+
Scheme 1. The self-assembly of [2ꢀ3] complex.
12+
Figure 2. CSI-MS spectrum of [2aꢀ3] (H O:CH CN:DMF=12:6:1,
2
3
RT).
An excess of 2a (suspension) was treated with 1
(
(
3.5 mmol) and [(en)Pd(NO ) ] (15 mmol) in D O:CD CN
under CSI-MS conditions, there was little indication for the
presence of either guest-free species or fragmented species,
3
2
2
3
2:1; 0.7 mL) for 2 h at 608C. After filtration of non-
1
12+
enclathrated 2a, a simple H NMR spectrum was obtained
from the clear solution, which indicated the formation of a
single product (Figure 1). The seven signals observed in the
aromatic region (Ha–g) agree with the D3h structure of 3
Highly upfield-shifted signals (H ), showing a typical phenyl
which shows the remarkable stability of the [2aꢀ3]
complex in solution.
When guest 2a was replaced by 1,3,5-triphenylbenzene
(2b), single crystals suitable for X-ray analysis were obtained
by slow diffusion of THF into an aqueous solution of the
1
2+
.
h–j
[
6]
splitting pattern, were assigned to guest 2a accommodated
complex. The X-ray analysis revealed the expected structure
1
2+
12+
2+
within the cavity of 3 . The [2aꢀ3] structure was strongly
where two molecules of 1 are clipped by six {(en)Pd} units
supported by NOESY experiment, which showed correlation
allowing the complete wrapping of the large planar guest
(Figure 3). The conformation of 1 is not planar but slightly
concave. The pyridine ring centers are situated above the face
of the core benzene ring at an average separation of 1.2 Š.
The 3-pyridyl groups are tilted by 31–428 with respect to
1
2+
[4]
between H of 3 and H of 2a.
f
j
Cold spray ionization mass spectroscopy (CSI-MS) clearly
1
2+
suggested the formation of [2aꢀ3] , it showed a series of
À
prominent peaks corresponding to [2aꢀ3 + (NO ) +
3
m
1
2Àm+
+
+
II
(
dmf)n]
(m = 5–9, n = 0–14). For example, in 4 and 5
adjacent phenyl groups so that they coordinate to Pd centers
regions, two intense peaks at m/z 872.3 and 729.4 were
with ideal bite angles (89–918). Thus, host distortion exists not
around the Pd centers but along the large framework of 1,
which suggests that close host–guest packing generates a
À
4+
II
assigned to [2aꢀ3 + (NO ) + (dmf) ]
and [2aꢀ3 +
3
8
2
À
5+
[5]
(
NO ) + (dmf) ] , respectively (Figure 2).
Note that,
3
7
5
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2
4+
Scheme 2. Self-assembly of the dimeric structure, 4
.
A smaller aromatic guest, triphenylene (5), is included in
2
4+
12+
the cavity of 4 (which is expanded compared to that of 3
)
2
4+
in a 1:2 ratio upon treating it with a solution of 4 for 2 h at
2
4+
6
08C. The formation of the [(5) ꢀ4] complex was revealed
2
by NMR spectroscopy with the highly upfield-shifted signals
of 5 (d = 6.9 and 6.0 ppm) and the slightly shifted 19 signals in
the aromatic region of the host (Figure 4b). Some minor
signals in Figure 4b may be assigned to other host–guest
1
2+
Figure 3. X-ray crystallographic structure of [2bꢀ3] . a) Space-filling
representation (top view), b) top view showing the D orientation of
3
guest 2b. Around the phenylgroup P, p–p stacking with pyridine rings
A and C, and CH–p contact with pyridine rings B and D are observed;
c) A side view.
2
4+
complexes, such as 1:1 complex [5ꢀ4] or monomer complex
1
2+
[
5ꢀ3] . CSI-MS also indicated the required stoichiometry
2
4+
for the [(5) ꢀ4] complex (see Supporting Information).
2
When guest 2a was suspended in the solution of the
2
4+
[
(5) ꢀ4]
complex, guest exchange took place at 608C
within 24 h concomitant with host monomerization. That is,
2
[
7]
greater distortion. The core benzene ring of 2b is tightly
stacked with the two ligands (3.5 Š van der Waals contact). In
the crystal, D symmetry is observed (not D ), each phenyl
2
4+
12+
[(5) ꢀ4] was converted into [2aꢀ3] , as shown by NMR
2
spectroscopy (Figure 4b–e). Clearly, the self-assembly of
3
3h
1
2+
24+
group of the guest being tilted by 30–368 allowing efficient
p–p and CH–p interactions with four surrounding pyridine
hosts 3 and 4 is dynamic owing to the labile nature of
1
2+
the PdÀN bond. The host–guest stabilization in [2aꢀ3]
1
2+
rings (Figure 3b). As a result, 2b is fully wrapped by 3
.
dominates over the distortion of the host framework, whereas
1
2+
24+
While [2ꢀ3]
complexes are stabilized by sufficient
host–guest interactions, the host framework itself is consid-
erably distorted. In the absence of a guest, therefore, less
less distorted 4 is favored when a guest is absent or less
efficiently trapped by the host.
In summary we have constructed a large p-conjugated,
expanded two-dimensional receptor by self-assembly. Exten-
sion of this study could ease the handling of very large p-
molecules, such as molecular graphites, which are in general
tedious to treat in solution because of their very poor
solubility. The chemical and physical properties of such
large p-molecules may be restricted within large two-dimen-
sional cavities.
2
4+
24+
distorted structure 4 is formed (Scheme 2). Compound 4
1
2+
is a dimer of 3 generated through the breaking of two PdÀN
2
4+
bonds. The high-yield formation of 4
NMR spectroscopy and CSI-MS spectrometry. When 1 and
(en)Pd(NO ) ] were mixed in a 1:3 ratio without guest
was confirmed by
[
3
2
molecules, 19 resonance signals, corresponding to half the
framework of 1, appeared in the aromatic region indicating
the reduction, after complexation, of the inherent D3h
symmetry of 1 into D2h symmetry (Figure 4a). CSI-MS
measurement showed the molecular weight of 6562 Da,
Received: June 4, 2004
1
2+
À
which is exactly twice as much as 3 [NO ] , supporting
Keywords: molecular recognition · palladium · pi interactions ·
3 12
.
2
4+
the proposed M L dimeric structure 4
.
receptors · self-assembly
1
2
4
5
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Angewandte
Chemie
1
24+
12+
24+
24+
Figure 4. H NMR spectroscopic monitoring of the conversion of [(5) ꢀ4] into [2aꢀ3] . The spectrum of a) dimeric host 4 , b) [(5) ꢀ4]
,
2
2
2
4+
c) the spectra upon the addition of 2 (1 equiv) to the [(5)
of 2 (1 equiv) to the solution, and stirring the mixture for an additional 2 h.
2
ꢀ4] solution after 1 h, d) after 24 h, e) The spectrum after filtration, further addition
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Angew. Chem. 2003, 115, 4039 – 4043; Angew. Chem. Int. Ed.
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S. Sun, A. Lees, Chem. Commun. 2001, 103 – 104.
[4] In the NOESY spectrum, not only host–guest correlation but also
the orientation of the ligand 1 (between H and H ) were assigned,
a
e
[
2] a) M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa,
K. Biradha, Chem. Commun. 2001, 509 – 518; b) M. M. Conn, J.
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1
2+
[6] Crystal data for [2bꢀ3] : C144H138N O Pd , M = 3927.85,
36
3
57
6
r
crystal dimensions 0.40 ” 0.20 ” 0.20 mm , monoclinic, P2(1)/n,
9
6, 149 – 175; d) R. W. Saalfrank, H. Glaser, B. Demleitner, F.
a = 20.5995(11),
16340.8(15) Š , Z = 4, 1calcd = 1.597 gcm
b = 26.3842(15),
c = 30.3963(16) Š,
V=
3
À3
Hampel, M. M. Chowdhry, V. Schünemann, A. X. Trautwein,
,
F(000) = 7951,
Angew. Chem. Int. Ed. 2004, 43, 5936 –5940
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Communications
l(MoKa) = 0.71073 Š, T= 173(2) K, 104788 reflections collected,
7340 independent reflections observed; 2079 number of param-
3
1
2+
eters; R = 0.0556; wR = 0.1403. CCDC-238928 (2bꢀ3 ) con-
1
2
tains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road Cambridge CB21EZ,
UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
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1
24, 14510–14511; b) S. Tashiro, M. Tominaga, T. Kusukawa, M.
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2
1
Eur. J. 2000, 6, 4510–4517; e) D. P. Funeriu, J.-M. Lehn, K. M.
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