Self-assembly of a supramolecular, three-dimensional, spoked, bicycle-like wheel

Article abstract of DOI:10.1002/anie.201302362

Where there′s a wheel, there′s a way: The terpyridine-based title system has been synthesized through a facile self-assembly process. Two tris(terpyridine) ligands possessing angles of either 120°or 60°between adjacent tpy units were mixed with a stoichio

Full text of DOI:10.1002/anie.201302362

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Communications
3D Metallomacrocycles
Where there’s a wheel, there’s a way: The
terpyridine-based title system has been
synthesized through a facile self-assem-
bly process. Two tris(terpyridine) ligands
possessing angles of either 1208 or 608
between adjacent tpy units were mixed
with a stoichiometric amount of Zn2+
(2:6:12) to generate the desired coordi-
nation-driven bicycle-like wheel (90%
yield).
X. Lu, X. Li, Y. Cao, A. Schultz, J.-J. Wang,
C. N. Moorefield, C. Wesdemiotis,*
S. Z. D. Cheng,
G. R. Newkome*
&&&& — &&&&
Self-Assembly of a Supramolecular,
Three-Dimensional, Spoked, Bicycle-like
Wheel
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DOI: 10.1002/anie.201302362
3D Metallomacrocycles
Self-Assembly of a Supramolecular, Three-Dimensional, Spoked,
Bicycle-like Wheel**
Xiaocun Lu, Xiaopeng Li, Yan Cao, Anthony Schultz, Jin-Liang Wang, Charles N. Moorefield,
Chrys Wesdemiotis,* Stephen Z. D. Cheng, and George R. Newkome*
The function and properties of materials and biological
organisms are not only related to the chemical structure and
connectivity, but also through higher-order domain structure,
such as in the magnetic domain of magnetic materials[1] as
well as the secondary and tertiary structures of proteins and
DNA.[2] Weak interactions such as hydrogen bonding and
coordination exist widely in biological systems and are vital
for biological metabolism and many other essential func-
tions.[3] These supramolecular interactions[4] have attracted
attention both in biology[5] as well as in numerous other
fields,[6] such as supramolecular catalysis, chemical sensing,
and molecular electronics.[7]
2,2’:6’,2’’-Terpyridine (tpy) has been a widely used ligand
for the creation of such motifs, partly because of its ability to
coordinate diverse metals. There are numerous examples of
tpy-based supramolecular systems, from 2D-based macro-
cycles and grids[8] to 3D arrangements, such as cages and
prisms.[9]
A two-dimensional, tpy-based supramolecular spoked
wheel was previously reported.[10] In this three-component
ensemble, two different terpyridine ligands and one type of
metal were self-assembled. This 2D wheel structure is more
rigid than macrocyclic hexagons because of its fixed space-
filling centerpiece, which also serves as a template for the
outer ligands. Notably, very few supramolecular spoked-
wheel systems have been reported,[10,11] since the self-assem-
bly of multiple components can be a synthetic challenge that
requires more precise control over the geometry and con-
nectivity.[12]
To further functionalize the well-established tpy-based
spoked-wheel assembly, the backbone and connectivity
components of its original structure were redesigned. The
framework, in this case, includes two parts: the spokes (S3)
and rims (R3; core and outer ligands, respectively; numbers
equate to the available tpy units). The core that originally
consisted of the single, hexakis(terpyridine) S6 was replaced
by two aromatic cores functionalized with three terpyridine
ligands at 1208 that adopt
a staggered conformation
(Scheme 1). Thus, the new construct involves the two
tris(terpyridine)s S3, six rim units R3 in which the three
tris(terpyridine)s are separated by angles of 608, and twelve
metals in a precise 2:6:12 ratio, respectively. The two central
tris-tpy ligands are stacked with a common perpendicular axis
to impart the 3D bicycle-wheel motif. b-Glucose moieties
were attached to the tris-tpy rim component R3 to increase
the solubility of the desired complex.
The synthesis of rim ligand R3 (Scheme 2) started with
bromination of 2,6-dimethoxyphenol to afford 1, followed by
etherification with benzyl bromide to give 2. Next, 3 was
prepared (67%) through a Suzuki cross-coupling reaction
between 2 and 4’-boronatophenyl-2,2’:6’,2’’-terpyridine[13] by
utilizing [PdCl2(PPh3)2] as a catalyst. The benzyl group in 3
was removed with ammonium formate in the presence of
a Pd/C catalyst to afford 4, which underwent alkylation with
N-(6-bromohexyl)phthalimide[14] to give the imide 5. Depro-
tection of the phthalimide with hydrazine then gave the free
amine 6. Subsequent treatment with 2,3,4,6-tetra-O-acetyl-b-
d-glucopyranosyl isocyanate[15] afforded (66%) the desired
tris-tpy ligand R3, which was fully characterized by NMR
spectroscopy and MS. Its 1H NMR spectrum exhibited signals
for two sets of protons in the aromatic region with an
integration ratio of 2:1 for the tpy units, as well as one set of
signals corresponding to protons of the alkyl linker and
glucose. The full assignment of the signals was confirmed by
2D COSY and ROESY NMR spectroscopy.
The core S3, prepared by a similar Suzuki cross-coupling
reaction[13] with 2,4,6-tribromomesitylene[16] (Scheme 2),
exhibited signals for only one set of tpy-based protons in
the aromatic region of the 1H NMR spectrum and its identity
was confirmed by the single charged signal at m/z 1042.49 in
the MALDI-TOF mass spectrum.
The 3D wheel C1 (Scheme 3) was synthesized by mixing
a precise stoichiometric ratio (6:2:12) of ligands R3, S3, and
Zn(NO3)2·6H2O in MeOH and stirring at 708C for 1 h
(Scheme 1). After cooling the mixture to 258C, excess
NH4PF6 was added to afford a light-yellow precipitate,
[*] X. Lu, Dr. Y. Cao, Dr. A. Schultz, Dr. J.-J. Wang, Dr. C. N. Moorefield,
Prof. C. Wesdemiotis, Prof. S. Z. D. Cheng, Prof. G. R. Newkome
Department of Polymer Science
Department of Chemistry, The University of Akron
302 Buchtel Common, Akron, OH 44325 (USA)
E-mail: wesdemiotis@uakron.edu
newkome@uakron.edu
Homepage: http://www.dendrimers.com
Prof. X. Li
Department of Chemistry and Biochemistry
Texas State University-San Marcos
601 University Drive, San Marcos, TX 78666 (USA)
Dr. J.-J. Wang
College of Chemistry, Beijing Institute of Technology
Beijing, 100081 (China)
[**] We gratefully acknowledge support from the National Science
Foundation (CHE-1151991, G.R.N; DMR-0821313 and CHE-
1012636, C.W.) and support from the Ohio Board of Regents. X.L.
acknowledges support from the Research Enhancement Program of
Texas State University–San Marcos.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302362.
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2.00 ppm,
which
were
assigned to the rim methoxy
groups of R3 and the methyl
groups from internal ligand
S3, respectively. The inte-
gration ratio of these two
signals is 2:1, thus indicating
that the ratio of R3 to S3 is
3:1, which is also consistent
with the desired structure.
Only one set of signals were
seen for the alkyl linker and
glucose protons, which fur-
ther confirmed that only one
self-assembled structure is
formed. The 2D NOESY
NMR spectrum (see the
Supporting
Information)
showed typical cross-peaks
for all of the adjacent pro-
tons in the tpy units. The
cross-peak between the pro-
tons at 7.60 and 7.40 ppm
arises from the NOE effect
between PhA-Hk and PhB-Hk
from different branches in
ligand R3; this cross-peak
Scheme 1. Synthesis of the supramolecular 3D bicycle wheel C1 and 2D spoked wheel C2.
was used to distinguish the
tpyB and tpyC units, both of
which have equal integra-
tion. The full assignment of the
1H NMR spectrum was confirmed by
2D COSY and 2D NOESY NMR
spectroscopy.
Complex C1 was also characterized
by ESI-MS coupled with traveling-
wave ion mobility (TWIM) mass spec-
trometry (Figure 2).[8b,10,18] The ESI
mass spectrum of C1 showed a series
of signals with charge states from 8 + to
17 + . Each charge state was derived by
À
the loss of a different number of PF6
Scheme 2. Synthetic route to ligands S3 and R3; reagents and conditions: a) 4’-boronatophenyl-
2,2’:6’,2’’-terpyridine,[13] Na2CO3, toluene/H2O/tBuOH (3:3:1), 908C, 48 h; b) Br2, CH2Cl2, 08C,
12 h; c) C6H5CH2Br, K2CO3, MeCN, 708C, 18 h; d) HCO2NH4, Pd/C, DMF, 908C, 3 h; e) K2CO3,
N-(6-bromohexyl)phthalimide,[14] DMF, 808C, 24 h; f) hydrazine hydrate, EtOH, 708C, 12 h;
g) 2,3,4,6-tetra-O-acetyl-b-d-glucopyranosyl isocyanate,[15] CH2Cl2, 258C, 24 h.
units. The isotope patterns of each
charge state agree well with the corre-
sponding simulated isotope pattern. No
other structures or aggregates were
detected in the ESI mass spectrum,
thus showing that the 3D bicycle-like
which was washed thoroughly with water. Complex C1 was
isolated (91%) as a light-yellow powder with PF6 as the
wheel C1 is the only product. ESI-TWIM-MS further
confirmed the structural assignment, and each charge state
showed a narrow drift time distribution, thus indicating that
no structural conformer or isomer was present.
À
counterion after drying in vacuo at 508C.
1
The H NMR spectrum (Figure 1) of the 3D wheel C1
exhibited three sets of signals for tpy units in the aromatic
region with an integration ratio of 2:1:1, which was consistent
with the subunit of the desired structure bound by the dotted
lines in Scheme 3. All of the signals for the 6,6’’-protons of the
tpy units were shifted upfield because of the electron-
shielding effect, which is typical for pseudooctahedral terpyr-
idine–metal complexes.[17] There are two singlets at 3.85 and
Collision cross-sections (CCSs)[10,18c,19] of the ions sepa-
rated by TWIM can also be derived. To compare the structure
size, the Zn2+-based 2D spoked wheel C2 (Scheme 1), an
analogue of complex C1, was also synthesized and fully
characterized by NMR spectroscopy and ESI-MS (see the
Supporting Information). The CCSs for both C1 and C2 are
listed in Table 1. For all the charge states probed, the CCSs of
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Figure 2. A) ESI mass spectra for complex C1; B) 2D ESI-TWIM-MS
plots (m/z versus drift time) for complex C1. The charge states of
intact assemblies are marked; C) representative energy-minimized
structure of the 3D bicycle wheel C1 (protons and glucose moieties
were omitted for clarity).
Scheme 3. Structure of the supramolecular 3D bicycle wheel C1.
Table 1: Experimental collision cross-sections (CCSs) of supramolecular
architectures C1 and C2.
Z
CCSs [ꢀ2]
3D wheel C1
2D wheel C2
8+
1815.6
1841.0
9+
1824.2
1861.5
10+
11+
average
1760.8
1570.7
1742.8Æ118.1
1819.4
1679.0
1800.2Æ82.6
ZnII-tpy} connectivity. Its 2D analogue spoked wheel C2 was
also synthesized for comparison. These two new supramolec-
ular structures open the door to more complex, three-
dimensional, one-step, self-assembly of different macromo-
lecular architectures.
Figure 1. 1H NMR spectra (500 MHz) of ligands S3 and R3 in CDCl3
and bicycle wheel C1 in CD3CN (see Scheme 3 for proton assign-
ments).
C1 are consistently smaller than those of C2 despite the
slightly higher mass of C1. This trend provides evidence that
C1 has a somewhat more compact architecture, consistent
with its more globular 3D shape compared with the flat (and
more extended) shape of C2. Transmission electron micros-
copy (TEM) images were also acquired (see the Supporting
Information) by casting a dilute solution of either complex
(ca. 10À7 m) in MeCN on carbon-coated Cu grids (200 mesh).
The average diameters for both structures are about (6.0 Æ
1.0) nm, consistent with the optimized molecular model
(Figure 2); the TEM resolution is evidently too low to
detect the small size/shape differences between C1 and C2,
as revealed by TWIM-MS.
Received: March 20, 2013
Published online: && &&, &&&&
Keywords: coordination modes · N ligands · self-assembly ·
.
supramolecular chemistry · zinc
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Thus, the first supramolecular, 3D, bicycle-like wheel C1,
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through a simple self-assembly procedure by utilizing {tpy-
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