PdII-directed dynamic assembly of a dodecapyridine ligand into end-capped and open tubes: The importance of kinetic control in self-assembly

Article abstract of DOI:10.1002/anie.200351397

Crystal engineering: A dodecapyridine ligand and six Pd^II ions self-assemble to give a mono-end-capped coordination tube that has significant stability, but is converted into a double open tube through crystallization. Once formed, there is no

Full text of DOI:10.1002/anie.200351397

Angewandte
Chemie
Kinetic Control of Self-Assembly
PdII-Directed Dynamic Assembly of a
Dodecapyridine Ligand into End-Capped and
Open Tubes: The Importance of Kinetic Control
in Self-Assembly**
Shohei Tashiro, Masahide Tominaga,
Takahiro Kusukawa, Masaki Kawano,
Shigeru Sakamoto, Kentaro Yamaguchi, and
Makoto Fujita*
Dynamic assembly deals with two or more self-assembled
units, which are on a relatively flat potential energy surface
and are interconverted into each other by the re-sorting or
reorganization of the component species.[1–3] When dynamic
structures have a cavity suitable for recognizing a guest, the
predominant assembly of one particular host structure is
induced by adding an appropriate guest. Such a guest-induced
assembly of a specific host framework is often seen in
biological events, and has been actively studied in both
hydrogen-bonded and coordination assembly systems.[2] Here
we report a unique dynamic assembly where, under a set of
identical thermodynamic conditions, the same guest induces
the formation of two different assemblies depending on
whether the assembly process experiences crystallization or
not.
The dynamic assembly discussed here stems from the
{(en)PdII} coordination block 1 (en = 1,2-ethanediamine) and
ligand 2, which has four tripyridine podands on a benzene-
tetracarboxylate scaffold.[4] In an analogy with the guest-
induced assembly of coordination nanotubes obtained from
oligo(3,5-pyridine) ligands and Pd units,[5] ligand 2 is expected
to yield the mono-end-capped coordination tube 312+ upon
complexation with 1 in the presence of an appropriate guest
(Scheme 1a).[6] This tube is designed to differentiate between
the ends of symmetrical rodlike molecules; namely, the end
that is accommodated at the bottom of the tube should show
different properties from the other end. We have found that,
in addition to expected structure 312+, the complexation of 1
and 2 also gives rise to the double open tube, 424+, in which
ligand 2 adopts an extended conformation (Scheme 1b). This
open tube is observed only at higher concentrations as a
[*] Prof. Dr. M. Fujita, S. Tashiro, Dr. M. Tominaga, Dr. T. Kusukawa,
Dr. M. Kawano
Department of AppliedChemistry, 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
Dr. S. Sakamoto, Prof. Dr. K. Yamaguchi
Chemical Analysis Center, Chiba University
Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
[**] This research was supportedby the CREST project of the Japan
Science andTechnology Corporation (JST), for which M.F. is the
principal investigator.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3267 – 3270
DOI: 10.1002/anie.200351397
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Communications
Scheme 1. Self-assembly of a) the mono-end-capped tube 312+, andb) the double open tube 424+. The template molecules necessary for the
formation of 312+ and 424+ are omittedfor clarity.
minor component, but is isolated in a pure form through slow
crystallization. Interestingly, once isolated, 424+ is not con-
verted into 312+ at lower concentrations, which indicates that
both structures are kinetically trapped at local minima of the
potential surface. Accordingly, we discuss here the self-
assembly and the host–guest behavior of the end-capped tube
312+ and the open tube 424+, as well
diastereotopic and thus observed as an AB quartet. In
addition to satisfactory NMR, the formation of complex ion
312+ was strongly supported by cold-spray ionization mass
spectrometry (CSI-MS).[8,9]
Observation by 1H NMR spectroscopy yielded some
structural aspects of the inclusion complex. As expected, the
as the importance of kinetic effects
in their self-assembly processes.
The quantitative assembly of 2
into end-capped tube 312+ was, in
fact, accomplished by the template
effect of rodlike guests such as 4,4’-
dimethylbiphenyl (5).[7] Without
the template, the complexation of
ligand 2 with [(en)Pd(NO3)2] (1) in
D2O resulted in a complex mixture
(Figure 1a). The addition of 5 to
the solution, however, induced the
assembly of a single product within
3 h at 708C (Figure 1b). In the tube
Figure 1. 1H NMR spectroscopic observation of the guest-templatedformation of 5ꢀ312+ (500 MHz,
D2O, 258C, TMS). a) An oligomeric mixture obtainedfrom 2 (1.7 mmol) and[(en)Pd(NO 3)2]
(10.2 mmol) in D2O (0.8 mL); b) the 5ꢀ312+ complex assembled after the addition of 5 (1.7 mmol).
structure, the side arms of 2 are
equivalent whereas the methylene
protons on each side arm are
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Angewandte
Chemie
two methyl groups of guest 5 accommodated in 312+ were
clearly discriminated, and were observed at d = À0.25 and
1.83 ppm (Figure 1b). The significant upfield shift of one
methyl group (approximately À2.6 ppm shifted from ordinary
chemical-shift region) suggests that this methyl group is
located on the benzene scaffold and surrounded by four
pyridine rings. Below 608C, these two methyl resonances did
not coalesce, which indicates that the guest is strongly bound
within the deep cavity of 312+, and the flipping of the guest
along its long axis is suppressed.[10] Aromatic protons of 5 were
also shifted upfield, being observed at d=4.47–4.86 ppm.
Asymmetric guests, such as sodium biphenylcarboxylate
(Na+6À) could be efficiently accommodated in a unidirec-
tional fashion.[7] Only one isomer was formed in which the
hydrophobic biphenyl group was included deep within the
tube and the hydrophilic carboxylate group was exposed
1
outside, as indicated by H NMR spectroscopy (Figure 2a).
Interestingly, disodium biphenyldicarboxylate (2Na+72À),
which was bound in an open tube in our previous study,[5]
was not included in 312+, which shows that the end-capped site
of 312+ provides an efficient hydrophobic pocket that cannot
bind the hydrophilic portion of the guest.
The self-assembly of inclusion complex 6Àꢀ312+ was
concomitant with the formation of a minor component at
high concentrations ([2]0 > 8 mm; Figure 2b). Fortunately,
this minor product was isolated as single crystals by slow
evaporation (over more than two weeks) of the solution. X-
ray crystallographic analysis showed that this product (424+) is
a double open tube, which consists of two molecules of
ligand 2 that are held together by twelve {(en)Pd} units
(Figure 3).[7,11] Open tube 424+ accommodates two molecules
of 6À, each of which is oriented so that its carboxylate group is
exposed outside the tube. An important feature is that the 16-
component self-assembled structure (two ligands, twelve
metals, and two guests) possesses a 3.0 nm molecular tube,
which is one of the longest among those structurally defined
by crystallography. The biphenyl moiety is surrounded by a
hydrophobic framework through efficient p–p and CH–p
interactions. The elemental analysis was consistent with a
formula of (Na6)2ꢀ4(NO3)24·45H2O.
Figure 3. Crystal structure of (6À)2ꢀ424+. For clarity, H atoms, water
molecules, andNO 3À ions are omitted.
in D2O at room temperature, we observed the (6À)2ꢀ424+
complex in a pure form (Figure 2c) although the 6Àꢀ312+
complex was the major product before isolation. More
surprisingly, the (6À)2ꢀ424+ complex remained unchanged in
solution, even after one month. These results clearly show
that the 6Àꢀ312+ and (6À)2ꢀ424+ complexes are not in
equilibrium despite the labile nature of PdII–pyridine coordi-
nation bonds. Slow conversion to the (6À)2ꢀ424+ complex only
occurs when 6Àꢀ312+ is crystallized at high concentrations.
It is noteworthy that two different assemblies (6Àꢀ312+
and (6À)2ꢀ424+) were obtained under a set of identical
thermodynamic conditions, dependent only on whether the
assembly process involved crystallization or not. We thus
The stability of the isolated (6À)2ꢀ424+ complex requires
special attention. When crystals of (6À)2ꢀ424+ were dissolved
Figure 2. 1H NMR spectra showing the conversion of 6Àꢀ312+ into (6À)2ꢀ424+ (500 MHz, D2O, 258C, TMS). a) [2]0 =2.1 mm; b) [2]0 =8.5 mm;
c) complex (6À)2ꢀ424+ isolatedas crystals.
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suggest the following: a) Complexes 6Àꢀ312+ and (6À)2ꢀ424+
,
[5] a) M. Aoyagi, K. Biradha, M. Fujita, J. Am. Chem. Soc. 1999,
121, 7457 – 7458; b) M. Aoyagi, S. Tashiro, M. Tominaga, K.
Biradha, M. Fujita, Chem. Commun. 2002, 2036 – 2037; c) M.
Tominaga, S. Tashiro, M. Aoyagi, M. Fujita, Chem. Commun.
2002, 2038 – 2039.
which are sustained by 12 and 24PdII–pyridine interactions,
respectively, are not thermodynamically trapped, but rather
kinetically trapped structures; b) complex 6Àꢀ312+ is kineti-
cally formed and trapped through intramolecular complex-
ation, although the more stable (6À)2ꢀ424+ complex also exists
on a potential energy surface; c) the interconversion of
6Àꢀ312+ into (6À)2ꢀ424+ does not occur at low concentrations,
but is slowly promoted at high concentrations, and much more
strongly facilitated by removing the (6À)2ꢀ424+ complex
through crystallization.
[6] a) I. Tabushi, K. Shimokawa, N. Shimizu, H. Shirakata, K. Fujita,
J. Am. Chem. Soc. 1976, 98, 7855 – 7856; b) S. Zhao, J. H. T.
Luong, J. Chem. Soc. Chem. Commun. 1995, 663 – 664; c) D.
Whang, J. Heo, J. H. Park, K. Kim, Angew. Chem. 1998, 110, 83 –
85; Angew. Chem. Int. Ed. 1998, 37, 78 – 80; d) T. Heinz, D. M.
Rudkevich, J. Rebek, Jr., Nature 1998, 394, 764 – 766; e) S. Saito,
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9630.
[7] For the physical properties of 5ꢀ312+, 6Àꢀ312+, and (6À)2ꢀ424+
,
Received: March 14, 2003 [Z51397]
see the Supporting Information.
[8] a) S. Sakamoto, M. Fujita, K. Kim, K. Yamaguchi, Tetrahedron
2000, 56, 955 – 964; b) Y. Yamanoi, Y. Sakamoto, T. Kusukawa,
M. Fujita, S. Sakamoto, K. Yamaguchi, J. Am. Chem. Soc. 2001,
123, 980 – 981.
Keywords: coordination modes · molecular recognition ·
nanotubes · palladium · self-assembly
.
[9] Complex 312+ was analyzed by CSI-MS to investigate the
accommodation of biphenylcarboxylate or naphthalene as a
guest. A series of [MÀ(NO3)n]n+ peaks were observed. See
Supporting Information.
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[10] At 808C, broad methyl signals were observed indicating that the
coalescence temperature was being approached (see the Sup-
porting Information).
[11] Crystal data for (6À)2ꢀ424+ (C202H218N72O119Pd14, Mw = 7048.08):
crystal dimensions 0.40 ” 0.20 ” 0.20 mm3, triclinic, P1, a =
ꢀ
16.494(4), b = 16.610(4), c = 31.977(8) Š, a = 78.438(5), b =
86.281(5), g = 70.333(4)8, V= 8082(3) Š3, Z = 1, 1calcd
=
1.438 mgmÀ3, F(000) = 3482, radiation: l(MoKa) = 0.71073 Š,
T= 113(2) K, reflections collected/unique 51758/35688 (Rint
=
0.0585). The structure was solved by direct methods
(SHELXL-97) and refined by full-matrix least-squares methods
on F2 with 1658 parameters. R1 = 0.1255 (I > 2s(I)), wR2 =
0.3444, GOF = 1.149, max./min. residual density 3.141/
À3.798 eŠÀ3. Several water molecules, nitrate molecules, and
1,2-ethanediamine ligands are disordered. Guest molecules sit
on two positions (1:1 ratio) in the cavity of a host molecule. The
thermal temperature factors of some disordered groups were
constrained. The bond lengths of severely disordered molecules
were chemically restrained. CCDC-204999 contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
[4] Ligand 2 was synthesized as follows: The Mitsunobu esterifica-
tion of tetracarboxylic acid 8 with alcohol 9 gave tetrabromide
precursor 10 (82%), which is subsequently treated with tribu-
tyltin bipyridine under the Stille coupling conditions
([Pd(PPh3)4] (10 mol%), toluene, reflux) to give 2 in 13% yield.
3270
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