Stoichiometric self-assembly of shape-persistent 2D complexes: A facile route to a symmetric supramacromolecular spoked wheel

Article abstract of DOI:10.1021/ja203645m

An approach to multicomponent coordination-driven self-assembly of the first terpyridine-based, shape-persistent, giant two-dimensional D_6h supramacromolecular spoked wheel is reported. Mixing core T6, rim T3, and Zn^II or Cd^II

Full text of DOI:10.1021/ja203645m

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Stoichiometric Self-Assembly of Shape-Persistent 2D Complexes: A
Facile Route to a Symmetric Supramacromolecular Spoked Wheel
Jin-Liang Wang,†,§ Xiaopeng Li,‡,§ Xiaocun Lu,† I-Fan Hsieh,† Yan Cao,† Charles N. Moorefield,†
Chrys Wesdemiotis,*,†,‡ Stephen Z. D. Cheng,† and George R. Newkome*,†,‡
†Department of Polymer Science and ‡Department of Chemistry, The University of Akron, Akron, Ohio 44325, United States
S Supporting Information
b
synthetic route and increase the yield.11 Nevertheless, a supra-
ABSTRACT: An approach to multicomponent coordina-
tion-driven self-assembly of the first terpyridine-based,
shape-persistent, giant two-dimensional D6h supramacro-
molecular spoked wheel is reported. Mixing core T6, rim
T3, and ZnII or CdII ions in a stoichiometric ratio (1:6:12)
permitted the selective generation of a highly symmetric
spoked wheel in 94% isolated yield via geometric and
thermodynamic control. The products were characterized
by a combination of traveling-wave ion mobility mass
spectrometry and NMR techniques together with TEM
imaging, which agreed with computational simulations.
molecular spoked wheel with a continuous outer wheel rim based
solely on noncovalent interactions has not been reported to date
because of a dearth of appropriate supporting spoke and rim
elements for preorganization.12 To address this problem, we
herein describe a stoichiometric self-assembly algorithm for the
construction of a shape-persistent 2D supramacromolecular
spoked wheel possessing D6h symmetry in nearly quantitative
yield in a one-pot reaction.
The design rationale for multicomponent coordination-driven
self-assembly of the molecular spoked wheel [T61ꢀCd12ꢀT36]
(Figure 1) is outlined in Scheme 1. Formation of this giant wheel
possessing sixfold symmetry required only CdII ions and two
different building blocks. The three-armed outer ligand T3 was
designed to incorporate (1) three tpy groups that simultaneously
generate half of one spoke and one-sixth of the outer wheel and
(2) three methoxy moieties (2:1 ratio) to enhance the solubility
and serve as markers to permit easy analysis of the inherent D6h
symmetry. The hub T6 instills D6h symmetry as well as the other
half of each directed molecular spoke upon formation of the
<tpyꢀCdIIꢀtpy> linkages. In view of the high stability of highly
symmetric structures having multiple (>2) interlocking connec-
tions within each ligand,3c we hypothesized that the stability of
this symmetric wheel would be enhanced oversimpler bypro-
ducts on the basis of the three or six noncovalent interactions
within each ligand. Additionally, we assumed these multiple inter-
actions within the designed geometry would facilitate the for-
mation of a single, discrete architecture.
Boronic acid 2 (Scheme 1) was synthesized (64%) from
unprotected 4-formylphenylboronic acid by a known procedure.13
Ligand T3 was prepared (61%) from 2 and 1 by a threefold
Suzuki coupling reaction using Pd(PPh3)4 as the catalyst. Inter-
estingly, the 1:2 ratio of singlets (4.14 and 3.73 ppm; Δδ=0.41ppm)
for the methoxy groups in T3 were easily discernible from those
of 1 (3.94 and 3.90 ppm; Δδ = 0.04 ppm) because of the upfield
shift caused by the adjacent phenyl rings together with steric
congestion. Moreover, the aromatic region of T3 showed two
sets of protons in a 2:1 ratio belonging to the tpy branches and
the phenyl groups (Figure 2). The core T614 was synthesized
(41% isolated yield) by a sixfold reaction utilizing Suzuki
coupling of hexabromobenzene and 2 catalyzed by Pd(PPh3)4.
Since compositional reversibility and the strength of the
<tpyꢀMIIꢀtpy> linkages is important for kinetically controlled
generation of heteroleptic complexes, either ZnII or CdII is the
he spontaneous and precise self-assembly of multiple dis-
T
tinct subunits is used in Nature to create many functional
biological systems with well-defined geometries.1 To simulate
and simplify Nature's complexity, coordination-driven self-as-
sembly has been developed as a strategy for constructing
abiological supramolecular two-dimensional (2D)2 and three-
dimensional (3D)3 architectures with precise shapes and sizes.
However, use of multicomponent systems possesses the inherent
weakness that multiple equilibria can generate multiple similar
structures.4 These assemblies mainly rely on specific stoichiom-
etry, the shape information instilled in the components, and
the reaction conditions.5 Although such structures are easily
designed, construction of a single discrete structure within a
mixture by multicomponent self-assembly is still a synthetic
challenge.6
In the coordination-driven approach to supramolecular chem-
istry, 2,20:60,200-terpyridine (tpy)-based building blocks have
attracted considerable interest because of their inherent ability
to bind transition-metal ions strongly and the relative ease of
functionalizing connectors for multinuclear assemblies.7 Hex-
americ ring-shaped structures are found in many functional biolo-
gical systems, such as the family of ATPases, DNA hexameric
helicases, and HIV capsid.8 These biological superstructures have
inspired the use of 3,5-bis(terpyridine)benzene ligands posses-
sing a 120° angle to construct a family of utilitarian metalloma-
crocycles based on <tpyꢀMIIꢀtpy> connectivity.9 In addition to
the dominant hexagonal shape, other macromolecular family
members (squares to cyclododecamers) have also recently been
generated using this simple one-step strategy.10 Thus, the con-
cept of a molecular wheel consisting of a hub unit, stable directed
spokes, and a circular rim has been used for the synthesis of large,
monodisperse covalent macrocycles, as this can simplify the
Received: May 2, 2011
Published: June 09, 2011
r
2011 American Chemical Society
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Figure 2. 1H NMR spectra (500 MHz) of ligands T3 and T6 in CDCl3
and [T61ꢀCd12ꢀT36] in CD3CN.
appear as two singlets at 4.13 and 3.84 ppm in a precise 1:2 ratio.
Comparison with the 1H NMR spectra of free T6 and T3 reveals
three sets of diagnostic absorptions for the outer (red) and two
inner (black and blue) 30,50-tpy protons (singlet) and 3,300-tpy
protons (doublet) in the expected ratio, which are shifted
downfield as a result of the lower electron density upon
coordination. The other peaks can be assigned on the basis of
the 2D COSY spectra. The 6,600-tpy proton signals exhibit a
dramatic upfield shift relative to the corresponding peaks for the
free ligands due to the shielding effect of the metal centers.
Interestingly, the 30,50-tpy protons in the rim show a greater
downfield shift than the two sets of tpy protons in the spokes,
which might result from the added electron deficiency in the
slightly bent outer macrocyclic rim. Moreover, the 13C NMR
spectrum of [T61ꢀCd12ꢀT36], which contains three peaks for
the 30,50-tpy carbons and two peaks for the methoxy groups, is
in full agreement with the wheel's high degree of symmetry.
The observed NMR data definitely excluded the presence of
both homo-oligomeric byproducts (which were found with
[T61ꢀZn12ꢀT36]) and the free ligands. The related macrocycle
[T2ꢀCd]4 was subsequently prepared for comparison (see S13).
In addition to NMR spectroscopy, ESI-MS coupled with
traveling-wave ion mobility (TWIM) spectrometry,10,16 a variant
of ion mobility spectrometry,17 was applied to validate the proposed
structure. [T61ꢀCd12ꢀT36] has a molecular weight of 13303.2 Da.
The formation of the assembly was confirmed by preliminary
ESI-MS, which served as a first-level MS analysis (Figure 3A); a
complex with the composition of [T61ꢀCd12ꢀT36] was the ex-
clusive isolated product on the basis of the mass-to-charge ratios
of the peaks for the major charge states observed (Z = 7þ to
14þ) and the corresponding isotope patterns (Figure S5). In
order to separate any superimposed fragments and determine
whether overlapping isomers or conformers of the assembly existed,
ESI-TWIM-MS was applied as the second level of MS analysis.
As shown in Figure 3B, intact [T61ꢀCd12ꢀT36] was the pre-
dominant species observed; a few minor fragments were found,
but no isomers or conformers were detected. To examine the
stability of this spoked wheel, a gradient tandem MS (gMS2) ex-
periment16a was performed on the assembly ions with charges of
11þ (m/z 1063.4) by subjecting them to collisionally activated
dissociation before ion mobility separation at collision energies
ranging from 6 to 40 eV (see the SI and Figure S6B). After
initially losing the PF6 units, the 11þ complex ions dissociated
further and disappeared completely when the trap voltage reached
40 V, which corresponds to a center-of-mass collision energy
(Ecm) of 0.14 eV. Application of the gMS2 method to the 11þ
Figure 1. Structure of the supramacromolecular spoked wheel [T61ꢀ
Cd12ꢀT36]; the triangle denotes the smallest subunit.
Scheme 1. Synthetic Route to the Ligands and
[T61ꢀCd12ꢀT36].
most appropriate metal choice.7,9,15 The initial efforts utilized
core T6, rim T3, and Zn(NO3)2 6H2O in a precise stoichio-
3
metric ratio of 1:6:12 to form the desired molecular wheel
[T61ꢀZn12ꢀT36]; although it did form, some homo-oligomeric
byproducts were evident by NMR and electrospray ionization
mass spectrometry (ESI-MS) analysis [see Figures S1ꢀS4 (SI)].
Therefore, CdII reagents were used to see whether the weaker
bonds would favor the formation of the desired wheel in higher
purity. The reaction of core T6, rim T3, and Cd(NO3)2 4H2O
3
(1:6:12) in CHCl3/MeOH solution at 25 °C for 30 min followed
by the addition of excess NH4PF6 generated the anticipated
wheel [T61ꢀCd12ꢀT36] in 94% isolated yield. This assembled
product showed excellent solubility in MeCN, acetone, and N,N-
dimethylformamide and was stable at ambient temperatures.
1
The H NMR spectrum of [T61ꢀCd12ꢀT36] (Figure 2)
shows a sharp and very simple pattern, indicating the formation
of a discrete assembly with a high degree of structural symmetry.
The installed unique methoxy protons of [T61ꢀCd12ꢀT36]
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Table 1. Experimental and Theoretical Collision Cross Sec-
tions (CCSs) of Supramolecular Spoked-Wheel Architectures
CCS (Å2)
Z
exptl
exptl avg
calcd avg
[T61ꢀCd12ꢀT36]
6þ
7þ
1578.6
1621.2
1658.8
1643.7
1546.0
1463.6 (8.9),a
1464.0 (3.2),b
1750.8 (24.2),c
1801.3 (8.2)d
8þ
1609.7 (46.7)
9þ
10þ
[T61ꢀZn12ꢀT36]
7þ
8þ
1621.8
1638.3
1610.0
1541.3
1602.9 (42.6)
9þ
10þ
a PA value obtained using DriftScope 2.1. b PA value obtained using
MOBCAL. c TJ value obtained using MOBCAL. d EHSS value obtained
using MOBCAL.
Figure 3. (A) ESI mass spectrum and (B) 2D ESI-TWIM-MS plot (m/z
vs drift time) for [T61ꢀCd12ꢀT36]. The charge states of intact assemblies
are marked.
average experimental CCS for charge states 6þ to 10þ (1609.7 Å2).
The theory provides a somewhat overestimated value (by
∼8.8%) because it employs structures without counterions (the
precise location of the counterions is unknown). Obviously, the
attractive interactions added by the counterions reduce the CCS,
albeit only slightly because of the rigidity of the spoked wheel
complex.
Transmission electron microscopy (TEM) characterization
provided both sizes and shapes of individual molecules upon
deposition of a dilute (∼10ꢀ6 M) MeCN solution of [T61ꢀ
Cd12ꢀT36] on carbon-coated grids (Cu and Ni, 400 mesh).
Individual hexagonal-shaped particles with an average diameter
of 6.5 ( 1.0 nm were observed, consistent with the optimized
molecular model (Figure 4).
The ligands in dilute CHCl3 exhibited ligand-centered (LC)
π f π* transitions at 286ꢀ291 nm (Figure S12). In comparison
with these ligands, the absorption spectra of [T61ꢀCd12ꢀT36]
and [T2ꢀCd]420 (S13) in dilute MeCN showed another distinct
absorption band at ∼323 and ∼321 nm, respectively, which is
assigned to intraligand charge transfer (ILCT). Moreover, the
molar extinction coefficient dramatically increased from in going
from ligand T3 to [T61ꢀCd12ꢀT36], consistent with the
corresponding increase in the number of ligands and CdII metal
centers in the molecular wheel. The obvious red shift of the
emission maxima in dilute solution for T3 (392 nm) and T2
(398 nm) relative to T6 (368 nm) may be ascribed to electron-
donating effects. On the other hand, the emission of
[T61ꢀCd12ꢀT36] exhibited an obvious red shift and peaked
at 462 nm. It is noteworthy that excitation of [T61ꢀCd12ꢀT36]
at either 284 or 323 nm exclusively gave the same band,
suggesting a highly efficient intramolecular energy transfer
process.21 The macrocycle [T2ꢀCd]4 exhibited a significantly
weaker emission maximum at ∼465 nm. Notably, both com-
plexes showed strong fluorescence in thin films, and their
emission maxima were red-shifted (36 nm for [T2ꢀCd]4,
50 nm for [T61ꢀCd12ꢀT36]) because of the formation of
aggregates in the excited state. The enhanced fluorescence of
[T2ꢀCd]4 relative to its extremely low emission in solution is
attributed to aggregation-induced emission.22
ions of [T61ꢀZn12ꢀT36] showed complete dissociation at
Ecm = 0.14 eV as well (Figure S4C). From the TWIM-MS data,
it was possible to derive the collision cross sections (CCSs) of the
ions being separated by TWIM (see Figure S7).18 The CCSs for
spoked-wheel ions in various charge states are shown in Table 1.
Only slight fluctuations were observed as the charged changed
from 6þ to 10þ for [T61ꢀCd12ꢀT36] (1609.7 ( 46.7 Å2) or
from 7þ to 10þ for [T61-Zn12ꢀT36] (1602.9 ( 42.6 Å2),
indicating that such 2D architectures are rigid and shape-persistent.
In sharp contrast, our previously reported doughnut-shaped macro-
cycles, which were generated from a single ligand, showed significant
differences in CCS for different charge states,10 suggesting the
existence of various distinct conformers differing in compactness.
Thus, [T61ꢀCd12ꢀT36] and [T61ꢀZn12ꢀT36] must be more
rigid and shape-persistent than the related simpler macrocycles.
Undoubtedly, the sturdy supporting spoke T6 stabilizes and
stiffens the assembly, leading to only minor changes in CCS with
charge state. A similar insensitivity of CCS to charge state has
been reported by Bowers and co-workers17b for rigid prism
stuctures. It is noteworthy that TWIM-MS not only differentiates
ions by mass, charge, and shape but also quantitatively and
precisely probes the rigidity of these species through the depen-
dence of the CCS on the charge state.
The structural information deduced from TWIM-MS was
corroborated by molecular modeling. Theoretical CCSs for 150
candidate structures of the [T61ꢀCd12ꢀT36] spoked wheel
(obtained from molecular mechanics/dynamics simulations) were
calculated using the projection approximation (PA), trajectory
(TJ), and exact hard sphere scattering (EHSS) methods.19 The
resulting plots of CCS versus relative energy (Figure S8) revealed
that each of these three methods renders a group of tightly bunched
CCSs with a small standard deviation, as listed in Table 1. Most
rigorous is the TJ method, which considers both long-range
interactions and momentum transfer between the ions and the
gas in the ion mobility region, thus providing the most realistic
CCS predictions.19 The CCS obtained for bare [T61ꢀCd12ꢀ
T36] using the TJ model (1750.8 Å2) agrees fairly well with the
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’ AUTHOR INFORMATION
Corresponding Author
wesdemiotis@uakron.edu; newkome@uakron.edu
Author Contributions
§These authors contributed equally.
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The authors gratefully acknowledge support from the Na-
tional Science Foundation (DMR-0812337 and DMR-0705015
to G.R.N.; DMR-0821313 and CHE-1012636 to C.W.; DMR-
0906898 to S.Z.D.C.). Funding from the Ohio Board of Regents
is also acknowledged.
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