Responsive supramolecular polymer metallogel constructed by orthogonal coordination-driven self-assembly and host/guest interactions

Article abstract of DOI:10.1021/ja412249k

An emerging strategy for the fabrication of advanced supramolecular materials is the use of hierarchical self-assembly techniques wherein multiple orthogonal interactions between molecular precursors can produce new species with attractive properties. Herein, we unify the spontaneous formation of metal-ligand bonds with the host/guest chemistry of crown ethers to deliver a 3D supramolecular polymer network (SPN). Specifically, we have prepared a highly directional dipyridyl donor decorated with a benzo-21-crown-7 moiety that undergoes coordination-driven self-assembly with a complementary organoplatinum acceptor to furnish hexagonal metallacycles. These hexagons subsequently polymerize into a supramolecular network upon the addition of a bisammonium salt due to the formation of [2]pseudorotaxane linkages between the crown ether and ammonium moieties. At high concentrations, the resulting 3D SPN becomes a gel comprising many cross-linked metallohexagons. Notably, thermo- and cation-induced gel-sol transitions are found to be completely reversible, reflecting the dynamic and tunable nature of such supramolecular materials. As such, these results demonstrate the structural complexity that can be obtained when carefully controlling multiple interactions in a hierarchical fashion, in this case coordination and host/guest chemistry, and the interesting dynamic properties associated with the materials thus obtained.

Full text of DOI:10.1021/ja412249k

Communication
pubs.acs.org/JACS
Responsive Supramolecular Polymer Metallogel Constructed by
Orthogonal Coordination-Driven Self-Assembly and Host/Guest
Interactions
Xuzhou Yan,† Timothy R. Cook,‡ J. Bryant Pollock,‡ Peifa Wei,† Yanyan Zhang,∥ Yihua Yu,∥
Feihe Huang,*,† and Peter J. Stang*,‡
†State Key Laboratory of Chemical Engineering, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027,
P. R. China
‡Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States
∥Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062,
P. R. China
S
* Supporting Information
precursors used in gel formation possess internal cavities, the
ABSTRACT: An emerging strategy for the fabrication of
advanced supramolecular materials is the use of
hierarchical self-assembly techniques wherein multiple
orthogonal interactions between molecular precursors
can produce new species with attractive properties. Herein,
we unify the spontaneous formation of metal−ligand
bonds with the host/guest chemistry of crown ethers to
deliver a 3D supramolecular polymer network (SPN).
Specifically, we have prepared a highly directional dipyridyl
donor decorated with a benzo-21-crown-7 moiety that
undergoes coordination-driven self-assembly with a
complementary organoplatinum acceptor to furnish
hexagonal metallacycles. These hexagons subsequently
polymerize into a supramolecular network upon the
addition of a bisammonium salt due to the formation of
[2]pseudorotaxane linkages between the crown ether and
ammonium moieties. At high concentrations, the resulting
3D SPN becomes a gel comprising many cross-linked
metallohexagons. Notably, thermo- and cation-induced
gel−sol transitions are found to be completely reversible,
reflecting the dynamic and tunable nature of such
supramolecular materials. As such, these results demon-
strate the structural complexity that can be obtained when
carefully controlling multiple interactions in a hierarchical
fashion, in this case coordination and host/guest
chemistry, and the interesting dynamic properties
associated with the materials thus obtained.
resulting gels may possess additional functionalities based on
potential host/guest chemistry.3 Existing examples of cavity-
containing gelators are mainly based on traditional covalent
macrocycles, such as cucurbit[n]urils, cyclodextrins, calixarenes,
and so on.4 These precursors are typically formed by grafting
gelating moieties onto exiting covalent cores. An alternative
approach is to organize the gelators using a preliminary self-
assembly step, providing a facile route to metallacyclic cores that
maintain an internal cavity and orient pendant groups with high
directionality.5
Coordination-driven self-assembly is a well-established
method to construct supramolecular coordination complexes
(SCCs) based on the spontaneous formation of metal−ligand
bonds.6 This approach organizes metal acceptors and organic
donors to prepare well-defined metallacycles in which secondary
recognition moieties can be easily functionalized.7 Encouraged
by the versatility and functionality of traditional cavity-containing
gelators, we envisioned the preparation of gels based on the
polymerization of metallacycles formed via an initial self-
assembly step, wherein the desirable properties of the metal-
lacyclic cores may be preserved in the final material.3,8 However,
metallacyclic gels based on well-defined discrete SCC cores have
rarely been reported.
Crown ether-based host/guest interactions, which show good
selectivity, high efficiency, and reversibility, have been widely
employed in the construction of supramolecular polymeric
materials.9 When combined with coordination-driven assembly,
remarkable supramolecular topological architectures can be
prepared.10 For example, the Stang group previously reported
the formation of tris[2]pseudorotaxanes via metal-coordination
and crown ether-based molecular recognition.11 These examples
demonstrate the use of secondary host/guest interactions in the
formation of discrete supramolecular species. Herein, we unify
the themes of host/guest interactions, coordination-driven self-
assembly, and supramolecular polymerization through a
hierarchical orthogonal design strategy to prepare a supra-
he structural complexity and functional specificity demon-
Tstrated across multiple biological macromolecules provides
inspiration for exploiting related hierarchical self-assembly
methods to synthesize state-of-the-art supramolecular materials
with dynamic and tunable properties.1 One class of these
materials are supramolecular gels constructed from low
molecular weight precursors that can undergo intramolecular
interactions to deliver extended molecule-based networks that
oftentimes show stimuli-responsiveness, dynamic structural
properties, and attractive characteristics for tissue engineering,
drug-delivery, nanotechnology, etc.2 When the molecular
Received: December 2, 2013
© XXXX American Chemical Society
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Scheme 1. (a) Self-Assembly of B21C7-Functionalized
Discrete Metallacyclic Hexagon 1 and (b) Cartoon
Representation of the Formation of a Cross-Linked 3D
Supramolecular Polymeric Network from Self-Assembly of
Hexagon 1 and Bisammonium Salt 4
Figure 1. Partial (a,b) 31P and (c−e) 1H NMR spectra (CD2Cl2, 293 K)
of free building blocks 2 (a,e) and 3 (c), and hexagonal metallacycle 1
(b,d).
The formation of hexagonal metallacycle 1 was further
confirmed by electrospray ionization time-of-flight mass
spectrometry (ESI-TOF-MS). In the mass spectrum of 1, six
peaks were identified to support the formation of a [3 + 3]
assembly (Figure S9), including those corresponding to an intact
hexagonal core with charge states arising from the loss of
counterions [m/z = 1323.66 for [M − 4OTf]4+ (Figure 2a), m/z
= 1029.14 for [M − 5OTf]5+ (Figure 2b), and m/z = 832.79 for
[M − 6OTf]6+ (Figure 2c)]. All the peaks were isotopically
resolved and in excellent agreement with their calculated
theoretical distributions. PM6 semiempirical molecular orbital
molecular metallogel containing hexagonal cavities in an
extended network. A benzo-21-crown-7 (B21C7)-functionalized
120° dipyridyl ligand 3 self-assembles into a hexagonal
metallacycle 1 when mixed with a 120° acceptor 2 (Scheme
1a). These discrete hexagons subsequently form the first example
of a hexagonal-cored supramolecular polymer network (SPN)
upon cross-linking the B21C7 moieties with a bisammonium salt
4 (Scheme 1b).The cross-linked material exhibits dynamic gel
properties and sol−gel transitions based on temperature or the
introduction of competing cations for the crown-ether sites.
The B21C7-functionalized ligand 3 was synthesized by an
esterification reaction and a subsequent Pd-catalyzed Sonoga-
shira coupling (Scheme S1). Stirring a mixture of ligand 3 and
1,3-bis[trans-Pt(PEt3)2(OTf)2 ethynyl]benzene (2), in a 1:1
molar ratio in CD2Cl2 at room temperature for 8 h resulted in the
formation of self-assembled [3 + 3] hexagon 1 with pendent
B21C7 moieties at the vertices (Scheme 1a). Multinuclear NMR
(1H and 31P) analysis of the reaction mixture supported the
formation of a discrete, highly symmetric entity (Figure 1). The
31P{1H} NMR spectrum of 1 shows a sharp singlet at δ = 15.91
ppm with concomitant 195Pt satellites corresponding to a single
phosphorus environment (Figure 1, spectrum b). This peak is
shifted upfield from acceptor 2 by ca. 6.10 ppm. In addition, in
the 1H NMR spectrum of 1 (Figure 1, spectrum d), the protons
of the pyridyl groups showed downfield shifts (Δδ[H1] = 0.04
ppm; Δδ[H2] = 0.41 ppm) compared to those of ligand 3 (Figure
1, spectrum c), consistent with coordination of the N-atoms to
platinum centers.
Figure 2. Experimental (red) and calculated (blue) ESI-TOF-MS
spectra of hexagon 1: (a) [M − 4OTf]4+, (b) [M − 5OTf]5+, and (c) [M
− 6OTf]6+.
B
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Figure 3. Partial 1H NMR spectra [CD2Cl2/CD3CN (1:1 v/v), 293 K,
500 MHz]: (a) B21C7-functionalized hexagon 1; (b) a mixture of
hexagon 1 and bisammonium salt 4 (6.00 mM crown ether/ammonium
salt moieties); and (c) bisammonium salt 4. Here “c” and “u” denote
complexed and uncomplexed moieties, respectively.
Figure 4. (a) Concentration dependence of diffusion coefficient D
[CD2Cl2/CD3CN (1:1 v/v), 308 K, 500 MHz] of SPN. (b) Size
distributions of SPN at different concentrations. (c) Rheological
characterization (T = 278 K) of cavity-containing SPN metallogel. (d)
Size distributions of SPN before and after addition of KPF6 (peak A is
DB18C6⊃K+ and peak B is bisammonium salt 4).
methods were used to obtain further insight into the structural
characteristics of 1. Simulations indicated a planar hexagonal
framework with exohedral functionalization by the pendent
B21C7 units and an internal diameter of 2.90 nm (Figure S10).
Proton 1H NMR experiments provided important insight into
the complexation behavior of hexagon 1 and bisammonium salt 4
in solution. Comparisons of the 1H NMR spectra of hexagon 1,
bisammonium salt 4, and their mixture, reveal chemical shift
1
changes (Figure 3). As shown in Figure 3b, the H NMR
spectrum of the mixture is complicated and each of the protons
of the B21C7 and ammonium units are split into two sets of
signals, corresponding to the complexed and uncomplexed
species, due to the slow-exchange nature of host/guest
complexation relative to the proton NMR time scale.12
Downfield chemical shift changes were observed for ethylenoxy
protons Hα, Hβ, Hγ, and Hδ, while methylene protons H8 and H9
and ethylenoxy protons Hε shifted upfield after complexation, in
agreement with the well-known B21C7/dialkylammonium
complexation motif.13 These changes in chemical shifts indicated
that B21C7/ammonium salt complexation exists in the cross-
linked SPN, providing the intramolecular interactions necessary
for mixtures of 1 and 4 to exhibit gel properties at high
concentrations. The formation of an extended hexagonal
network upon host/guest interactions between 1 and 4 was
further investigated by concentration-dependent 1H NMR
experiments (Figure S11).
Figure 5. Formation of the cavity-containing SPN metallogel and its
dual-responsive gel−sol transitions.
salt units in the SPN increased from 3.00 to 6.00 to 12.0 mM, the
average hydrodynamic diameter (Dh) increased from 58.8 to 142
to 459 nm, indicating a concentration dependence on SPN size
(Figure 4b).
Next, the gelation properties of the SPN were studied.
Solutions of hexagon 1 in CH2Cl2 (60.0 mM B21C7 units) and
bisammonium salt 4 in CH3CN (60.0 mM ammonium salt
moieties) were prepared in two vials (Figure 5, left). Upon
addition of the bisammonium salt 4 solution into the solution of
1, the hexagonal cavity-cored SPN metallogel was formed
immediately (Figure 5, middle). This mixture contains 30.0 mM
B21C7/ammonium moieties. The good association between
B21C7 and ammonium moieties leads to the formation of
extended hexagonal networks and a dramatic increase of
viscosity, responsible for the observed gelation. The critical gel
concentration was determined to be 20.0 mM B21C7/
ammonium units at 4 °C.
Rheological characterization can give further insight to the
bulk material properties of the cross-linked supramolecular
assemblies. Therefore, a rheological analysis was carried out on
the gel at a concentration of 40.0 mM B21C7/ammonium salt
moieties at 278 K. The data revealed that the storage modulus
(G′) is much larger than the loss modulus (G″), and both of them
(G′ and G″) are independent of the angular frequency (ω),
To further confirm SPN formation, 2D diffusion-ordered 1H
NMR spectroscopy (DOSY)14 was performed to investigate the
self-aggregation between hexagon 1 and bisammonium salt 4. As
the concentration of B21C7/ammonium salt units increased
from 12.0 to 60.0 mM, the measured weight average diffusion
coefficient decreased from 1.48 × 10−10 to 1.51 × 10−11 m2 s−1
(D12.0mM/D60.0mM = 9.80) (Figure 4a), indicating the formation of
a high molecular weight SPN. The DOSY NMR experiments
were performed at 308 K because the SPN formed a gel in the
concentration range of 35.0−60.0 mM at 293 K. Moreover,
dynamic light scattering (DLS) measurements were conducted
to study the size distributions of the SPN at different
concentrations. As the concentration of the B21C7/ammonium
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thereby showing the formation of the hexagonal cavity-cored
SPN metallogel (Figure 4c). Moreover, the morphology of the
xerogel, prepared by a freeze-drying method, was examined by
scanning electron microscopy (SEM), which revealed extended
and interconnected fibers (Figure S12). As shown in Figure S12,
the thin and long fibers can further self-organize into 2D ribbon-
like fibrous structures that extend for several micrometers.
The reversible thermo- and cation-induced gel−sol transitions
can be visualized macroscopically (Figure 5, middle and right).
Qualitative characterization of the thermo-induced reversibility
of this metallogel is demonstrated with an inverted vial test.
Upon heating, the gel easily flows as the host/guest association
constant decreases at elevated temperatures, indicating a gel−sol
transition. This process is completely reversible as the gel
reforms upon cooling due to the restoration of host/guest
interactions. Moreover, the B21C7 moiety preferentially forms a
1:1 complex with potassium cation (K+),12 which provides a
route for deconstructing the B21C7/ammonium linkages.
Adding and removing K+ can trigger reversible gel−sol
conversions. DLS measurements showed that the addition of
KPF6 into a 3.00 mM B21C7/ammonium salt solution results in
a decrease of the average Dh from 58.8 to 5.62 nm, indicating that
the host/guest complexation is quenched. After addition of
enough dibenzo-18-crown-6 (DB18C6) to trap all the K+, the
B21C7/ammonium complex is re-formed, and the average Dh
recovers (Figure 4d). These reversible conversions can also be
monitored by 1H NMR experiments (Figure S13).
In summary, a B21C7-functionalized hexagonal metallacycle
was prepared with high efficiency by means of the directional-
bonding approach. This macrocyclic precursor could be extended
into a supramolecular polymer upon the formation of [2]-
pseudorotaxane host/guest linkages with a bisammonium salt.
The gel that resulted from high concentrations of this
supramolecular polymer exhibited dynamic properties, specifi-
cally thermo- and cation-induced sol−gel transitions. As this
design method preserves internal metallacyclic cavities through
the resulting SPN, such gels are promising candidates for
applications in catalysis, separation, absorption, etc. Moreover,
due to the stimuli-responsive nature of non-covalent interactions,
the SPN gel can also act as a degradable material triggered by
external stimuli. The design principles established here
encompass hierarchical orthogonal self-assembly with coordina-
tion-driven self-assembly, host/guest interactions, and supra-
molecular polymerization. Given the rich chemistry of supra-
molecular coordination complexes and the favorable properties
of emerging supramolecular polymers developed previously,1c,7
the unification of metal−ligand coordination with orthogonal
non-covalent interactions is a promising route to access novel
supramolecular polymers with fascinating characteristics. Further
efforts to organize non-covalent interactions between pendant-
functionalized metallacycles are ongoing.
ACKNOWLEDGMENTS
■
This work was supported by National Basic Research Program
(2013CB834502), the NSFC/China (21125417), and the State
Key Laboratory of Chemical Engineering. P.J.S. thanks the NSF
(1212799) for financial support.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
fhuang@zju.edu.cn; stang@chem.utah.edu
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/ja412249k | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX