Stimuli-responsive supramolecular gels through hierarchical self-assembly of discrete rhomboidal metallacycles

Article abstract of DOI:10.1002/chem.201301385

A new class of supramolecular organometallic gels with a discrete rhomboidal metallacycle as the main skeleton was fabricated through hierarchical self-assembly (see figure). More importantly, by taking advantage of the dynamic nature of metal-ligand bonds, the stimuli-responsive gel-sol transition of the obtained gels was realized by disassembly and reassembly of the rhomboidal scaffold as controlled by bromide anions. Copyright

Full text of DOI:10.1002/chem.201301385

DOI: 10.1002/chem.201301385
Stimuli-Responsive Supramolecular Gels through Hierarchical Self-Assembly
of Discrete Rhomboidal Metallacycles
Guang-Zheng Zhao,[a] Li-Jun Chen,[a] Wei Wang,[a] Jing Zhang,[a] Guang Yang,[b]
De-Xian Wang,[c] Yihua Yu,[b] and Hai-Bo Yang*[a]
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Chem. Eur. J. 2013, 19, 10094 – 10100

COMMUNICATION
During the past few decades, supramolecular chemistry[1]
has witnessed tremendously rapid development in the con-
struction of discrete polygons and polyhedra by employing
coordination-driven self-assembly methodology.[2] More re-
cently, the applications and dynamic function of these
metallacycles can be expected to impart new methods for
stimuli-responsive sol–gel phase transitions induced by con-
trolled assembly and disassembly. Herein, we present the
construction of a new family of metallodendritic rhomboids
and their hierarchical self-assembly into supramolecular
gels. Remarkably, these hierarchically self-assembled supra-
molecular gels can undergo a reversible sol–gel phase transi-
tions based on the reversible assembly and disassembly of
rhomboidal metallacycles.
Previously, Fan and co-workers reported that peripherally
dimethyl isophthalate (DMIP)-functionalized poly(benzyl
ether) dendrons featured highly efficient gelation proper-
ties.[12] Thus we employed this family of poly(benzyl ether)
dendrons as gelators in this research. The [G-0]–[G-2] 1208
donor building blocks 1a–c, substituted with (DMIP)-func-
tionalized poly(benzyl ether) dendrons, were divergently
synthesized from 3,5-bis(pyridylethynyl)phenol and commer-
cially available dimethyl 5-hydroxylisophothalate as the
growth unit (Schemes S1 and S2 in the Supporting Informa-
tion). The self-assembled [G-0]–[G-2] metallodendritic
rhomboids 3a–c were prepared by simply mixing the donor
ligand 1a–c, respectively, with the 608 diplatinum(II) accept-
or 2 in a 1:1 ratio in CH2Cl2, followed by a precipitation
from aqueous solution of KPF6 (Scheme 1). Multinuclear
metalloACHTUNGTRENNUNGsupramolecular architectures have received growing
attention.[3] For instance, the nature of reversible metal–
ligand coordination interactions allows for self-organization
within multicomponent mixtures[4] and reversible supramo-
lecular transformations.[5] The reversible stimuli-responsive
self-assembly and disassembly of discrete polygons and poly-
hedra, however, have seldom been studied.[6] In particular,
the fabrication of stimuli-responsive, “smart” soft materi-
als[7] by taking advantage of the dynamic functions of dis-
crete metalloACHTUNGTRENNUNGsupramolecular polygons and polyhedra is still
extremely challenging.
Supramolecular organometallic gels represent
a new
family of soft material that has attracted considerable atten-
tion over recent years, because the presence of metal centers
allows for the photochemical, electrochemical, and catalytic
properties of the gels to be rationally tuned.[8,9] Of particular
interest are “smart”, stimuli-responsive, organometallic gels
based on the manipulation of metal–ligand interactions due
to their potential applications in, for example, sensors, drug
delivery and release, and actuators.[10] Nonetheless, to date
many metal–ligand coordination-based gels have been limit-
ed to supramolecular polymers. These coordination poly-
meric materials are often poorly defined both physically and
structurally, restricting their ability to be easily handled and
processed as discrete scaffolds. Inspired by successful exam-
ples of the nanoscale multifunctionalized supramolecular
metallacycles,[11] we envisioned that a new family of discrete
multigelator-moiety metallacycles can be efficiently pre-
pared through coordination-driven self-assembly, which can
undergo hierarchical self-assembly into nanostructured su-
pramolecular gels. This strategy would likely give rise to the
design and synthesis of novel organometallic gels with new
functionality arising from their unique interior cavities.
More importantly, the dynamic nature of supramolecular
[a] G.-Z. Zhao,+ L.-J. Chen,+ W. Wang, J. Zhang, H.-B. Yang
Shanghai Key Laboratory of Green Chemistry
and Chemical Processes
Department of Chemistry, East China Normal University
3363 N. Zhongshan Road, Shanghai 200062 (P. R. China)
Fax : (+86)21-6223-5137
Scheme 1. Self-assembly of [G-0]–[G-2] 1208 dendritic donors 1a–c and
acceptors 2 into rhomboids 3a–c.
E-mail: hbyang@chem.ecnu.edu.cn
[b] G. Yang, Y. Yu
NMR (1H and 31P) analysis of [G-0]–[G-2] assemblies 3a–c
exhibited very similar characteristics, which suggested the
formation of discrete, highly symmetric rhomboidal dendrit-
Shanghai Key Laboratory of Magnetic Resonance
Department of Physics, East China Normal University
Shanghai 200062 (P. R. China)
1
[c] D.-X. Wang
ic metallacycles. In the H NMR spectrum of each assembly,
National Laboratory for Molecular Sciences
CAS Key Laboratory of Molecular Recognition
and Function Institute of Chemistry
the a and b hydrogen atoms of the pyridine rings displayed
downfield shifts (a-H, 0.01–0.50 ppm; b-H, 0.36–0.54 ppm)
due to the loss of electron density that occurs upon coordi-
nation of the pyridine nitrogen atom with the PtII metal
center (Figures S1–S3 in the Supporting Information). The
31P{1H} NMR spectra of the [G-0]–[G-2] assemblies 3a–c
Chinese Academy of Sciences, Beijing 100190 (P. R. China)
[+] G.-Z. Zhao and L.-J. Chen contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301385.
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H.-B. Yang et al.
each displayed a sharp singlet (ca. 12.7 ppm) shifted upfield
from the starting platinum acceptor 2 by approximately
6.5 ppm (Figure S4 in the Supporting Information). Further
characterization by two-dimensional spectroscopic tech-
(3.34ꢁ10À3 molLÀ1, 298 K). The D values were observed to
decrease gradually with an increase of the molecular weight
(Table S1 and Figure S9–S11 in the Supporting Information),
in good agreement with the modeling results.
A
According to the “directional bonding” and “symmetry
interaction” models,[14] the [2+2] discrete rhomboidal struc-
tures can be self-assembled by combining two 1208 pyridyl
donor building blocks and two 608 diplatinum acceptors.[15]
Furthermore, recent studies have indicated that the addition
of functional groups at the vertices of individual building
units, such as ferrocene and crown ether moieties, does not
hinder the formation of [2+2] self-assembled metallosupra-
molecules.[16] The sharp NMR signals in both the 31P{1H}
mation of the discrete rhomboidal metallodendrimers. For
example, the presence of cross-peaks between the signals of
the PEt3 protons and the pyridine protons was found in the
2D NOESY spectrum, which supported the construction of
self-assembled rhomboids (Figures S5–S7).
Mass-spectrometric studies of rhomboidal dendritic
metallaACHTUNGTRENNUNGcycles 3a–c were performed by cold-spray ionization
mass spectrometry (CSI-TOF-MS),[13] which allows the as-
semblies to remain intact during the ionization process in
order to obtain the high resolution required for unambigu-
ous determination of individual charge states. In the CSI-
TOF-MS spectra of the [G-0]–[G-2] assemblies 3a–c, peaks
1
and H NMR spectra along with the solubility of these spe-
cies ruled out the formation of oligomers. Moreover, the
CSI-TOF-MS results show that only [2+2] rhomboidal
metallodendrimers are formed in each self-assembly.
attribute
G
The presence of peripherally DMIP-functionalized
poly(benzyl ether) dendrons stimulated us to investigate
their hierarchical self-assembly behavior in various organic
solvents. As demonstrated in Table S2 in the Supporting In-
formation, gelation was not observed for [G-0] and [G-1]
dendritic rhomboids 3a and 3b in all selected solvents, al-
though some ordered aggregated nanostructures were ob-
served in the scanning electron microscopy (SEM) investiga-
tion (Figure S16–S19 in the Supporting Information). By
contrast, [G-2] dendritic rhomboids 3c exhibited highly effi-
cient gelation in several pure and mixed organic solvents
(Figure 2). Interestingly, compared to dendritic precursor
1685.95), 3b (m/z=2014.78), and 3c (m/z=2671.64), in
which M represents the intact assemblies. These peaks were
isotopically resolved and they agree very well with the theo-
retical isotopic distribution (Figure 1).
Figure 2. A) Photograph of gel formation of 3c in some solvents at CGCs
(from left to right): a) acetone, b) acetone/water=5:3, c) tetrahydrofur-
an/water=3:1, d) benzene/chloroform=1:3, e) phenyl ether/chloro-
form=6:5, f) tetrahydrofuran. B) SEM image of the xerogel of 3c in ace-
tone/water=5:3 (2.3 mgmLÀ1, scale bar 2.0 mm).
Figure 1. Theoretical (top) and experimental (bottom) CSI-TOF-MS
spectra of [G-0]–[G-2] dendritic rhomboids A) 3a, B) 3b, and C) 3c.
1c, which did form gels in some solvents (Figure S12 in the
Supporting Information), dendritic rhomboids 3c even dis-
played stronger gelation ability under some certain condi-
tions. For example, rhomboid 3c could efficiently entrap the
solvent molecules to form a turbid gel in the mixed solvent
of acetone and water (v/v 5:3) with a very small critical gela-
tor concentration (CGC) of 2.3 mgmLÀ1, while only partial
gelation was observed for 1c in the same solvent. Moreover,
as shown in Table S2 in the Supporting Information, the gel
formed by 1c generally turned into a viscous fluid more
readily than that from 3c, which is indicative of that 3c has
a better thermal stability than 1c. The aggregation struc-
tures of the dendritic xerogels were investigated by both
SEM and transmission electron microscopy (TEM). Typical
PM6 semiempirical methods were employed to optimize
the geometry of all dendritic rhomboids (Figure S8 in the
Supporting Information). It was found that the rhomboids
3a–c possess a well-defined rhombus structure with an aver-
age internal cavity measuring 1.3ꢁ2.3 nm. Moreover, the ex-
ternal dimensions of rhomboidal structures increased with
increasing dendron generation: 3.0ꢁ3.8 nm for 3a; 3.0ꢁ
5.0 nm for 3b; 3.0ꢁ6.2 nm for 3c. In order to obtain further
structural information, the diffusion coefficients (D) of the
newly designed dendritic rhomboids were determined by
two-dimensional diffusion-ordered NMR (2D-DOSY) spec-
1
troscopy. The H DOSY NMR measurements for assemblies
3a–c in CD2Cl2 were carried out under similar conditions
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Stimuli-Responsive Supramolecular Gels
COMMUNICATION
fiberlike morphologies for both 3c and 1c in different or-
ganic solvents were observed in SEM and TEM investiga-
tions (Figure 2 and Figure S13–S15 in the Supporting Infor-
mation).
To gain further insight into the forces driving gel forma-
1
tion, concentration-dependent H NMR spectra of 3c were
recorded for 1.5–10.3 mgmLÀ1 solutions in a mixed solvent
of [D6]acetone and D2O (v/v 10:1). It was found that in-
creasing the concentration resulted in slight upfield shifts of
resonance signals for the aromatic protons on the peripheral
DMIP rings and the internal benzyl rings (Figure S20 in the
Supporting Information), indicating the formation of p–p
stacking interactions between dendrons in solution and the
gel state.[12] Moreover, the titration studies were carried out
for the [G-2] dendritic rhomboid 3c and ligand 1c. Emission
spectra revealed that the fluorescence intensity drastically
decreased when the volume ratio of tetrachloromethane to
chloroform was increased in the binary solvent system (Fig-
ure S21 in the Supporting Information). This finding sug-
gests that the p–p interactions are enhanced as the percent-
age of the less solubilizing solvent (CCl4) is increased. Based
on all above experimental results, we presume the formation
of supramolecular metallogels might be mainly driven by
the p–p stacking of the periphery dendrons.
Figure 3. Pictures, SEM images, and schematic structures of the reversible
stimuli-responsive gel–sol phase transition: A) gels of 3c in acetone/
water=5:3 (2.3 mgmLÀ1), B) after the addition of four equivalents of
Bu4NBr to the gels, and C) after subsequent addition of twelve equiva-
lents of AgPF6 to the turbid solution.
mately became a turbid solution (Figure 3). SEM imaging
revealed that the fibrous morphology in the original gels
was destroyed after the addition of Bu4NBr, and an irregular
microstructure formed. Interestingly, the reformation of the
gel was achieved upon treatment of the turbid solution of
3c with slight excess of silver hexafluorophosphate (AgPF6).
Notably, a small amount of black precipitate was generated
at the bottom along with the regeneration of gels.[19] SEM
investigation revealed that the resulting irregular micro-
structures were converted back into an interconnected net-
work, which is consistent with the typical morphology of
gels.
We assume that the stimuli-responsive gel–sol phase tran-
sition is essentially as a consequence of the halide-induced
disassembly and reassembly of the rhomboidal metallacycle
(Scheme 2). With the aim to provide the further support for
the stimuli-responsive disassembly and reassembly process,
an in situ multinuclear NMR (1H and 31P) investigation of
3c in CD2Cl2 was carried out. Upon adding Bu4NBr to the
solution of 3c in CD2Cl2, the typical proton signals of rhom-
boidal metallacycles disappeared and the resonance from
Having confirmed the construction of rhomboidal
metalloACHTUNGTRENNUNGdendrimers 3a–c and the hierarchical self-assembly
of 3c into the supramolecular gels, further investigation was
performed to investigate whether reversible stimuli-respon-
sive gel–sol phase transition could be realized via disassem-
bly and reassembly of rhomboidal scaffold based on the dy-
namic nature of metal–ligand bonds. Although some reports
of the disassembly of self-assembled palladium architectures
with 4-dimethylaminopyridine (DMAP) or pyridine (Py) as
a competing ligand exist,[6a,17] the addition of either DMAP
(4 equiv) or Py (4 equiv) to the organometallic gels 3c did
not lead to an observable gel–sol phase transition (Fig-
ure S22–23 in the Supporting Information). The lack of this
transformation upon addition of DMAP or Py was associat-
À
ed with the less labile nature of the Pt N bond compared to
[18]
À
À
the Pd N bond in solution at room temperature. Revers-
free ligand 1c and a new Pt Br complex 4 in a 1:1 ratio ap-
ible gel–sol transitions could be realized, however, with the
peared in the 1H NMR spectrum, indicating the complete
disassembly of rhomboidal metallacycles. It should be noted
that the increase in coupling of flanking 195Pt satellites (ca.
D1JP,Pt =61 Hz) was observed, which was in agreement with
addition and removal of bromide ions (BrÀ). Upon adding
four equivalents of tetrabutylammonium bromide (Bu4NBr)
into a gel sample of 3c, the gel gradually collapsed and ulti-
Scheme 2. Stimuli-responsive disassembly and reassembly of [G-2] supramolecular rhomboid 3c in CD2Cl2 solution.
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H.-B. Yang et al.
well-defined metallacycle at
their cores by means of coordi-
nation-driven self-assembly. It
was found that second-genera-
tion metallodendrimers are
able to hierarchically self-as-
semble into stable supramolec-
ular organometallic gels. Nota-
bly, this kind of supramolecular
gel adopts a discrete metallacy-
cle as the main skeleton, which
is different from many previ-
ous reports of coordination
polymeric gels. More impor-
tantly, the dynamic nature of
metal–ligand bond allows for
reversible
gel–sol
stimuli-responsive
phase transition
Figure 4. Partial A) 1H and B) 31P NMR spectra showing the disassembly and reassembly of rhomboid 3c
(CD2Cl2, 298 K): a) rhomboid 3c, b) after the addition of four equivalents of Bu4NBr to rhomboid 3c, c) after
subsequent addition of twelve equivalents of AgPF6 to the mixture. The assignments correspond to the num-
bering shown in Scheme 2.
through the reversible disas-
sembly and reassembly of dis-
crete rhomboidal metallacycles
induced by bromide ions.
These findings not only enrich
À
the formation of the new Pt Br complex 4. After the addi-
tion of AgPF6 to the mixture, the original signals in the H
the library of supramolecular metallogels, but also provide a
new avenue to the construction of novel “smart” soft
materials.
1
and 31P NMR spectra were restored, thus demonstrating
quantitative reassembly of the supramolecular rhomboid
(Figure 4). To obtain the direct evidence for the stimuli-re-
sponsive disassembly and reassembly process during the
gel–sol transitions, a multinuclear NMR (1H and 31P) investi-
gation of xerogels 3c was carried out. As expected, the addi-
tion and removal of bromide anion to the gel of 3c caused
the similar NMR resonances changes as those in the solu-
tion state (Figure S24 and S25 in the Supporting Informa-
tion). Again the sharp NMR signals in both the 31P{1H} and
1H NMR spectra along with the solubility of these species
ruled out the formation of oligomers during the process of
the gel–sol transformation.
Experimental Section
Synthesis of 3a: The dipyridyl donor ligand 1a (5.3 mg, 10.5 mmol) and
the organoplatinum 608 acceptor 2 (12.2 mg, 10.5 mmol) were weighed ac-
curately into a glass vial. CD2Cl2 (1 mL) was added to the vial and the re-
action solution was stirred at room temperature for 3 h to yield a homo-
geneous orange solution. Then acetone (3 mL) was added, followed by
the addition of a saturated aqueous solution of KPF6 (2 mL) into the re-
action vessel with continuous stirring (10 min) to precipitate the product.
The reaction mixture was centrifuged, washed several times with water,
and dried. The pale-yellow product 3a (16.8 mg, 96%) was collected and
redissolved in CD2Cl2 for NMR analysis. IR (neat): n˜ =3589, 2974, 2884,
2839, 2359, 2340, 2217, 1722, 1609, 1578, 1493, 1380, 1246, 1207, 1106,
The above H and 31P NMR experiments strongly support
1
the conclusion that rhomboidal metallacycles 3c can be re-
versibly disassembled and reassembled in a controlled stim-
uli-responsive manner by addition and subsequent removal
of bromide ions. It should be noted that, although the con-
trolled self-assembly and disassembly has been extensively
explored in the area of supramolecular polymers and molec-
ular devices,[20] there are very few examples of reversible
stimuli-responsive disassembly and reassembly of 2D dis-
crete metallacycles reported in literature.[5] More important-
ly, the well-controlled gel–sol phase transitions can be realiz-
ed through disassembly and reassembly processes based
on dynamic nature of metal–ligand bonds. Thus this work
presents a new family of supramolecular metallogel that can
undergo stimuli-responsive gel–sol phase transition induced
by the reversible disassembly and reassembly of discrete
supramolecular metallacyclic scaffolds.
1034, 1003, 837, 763, 677 cmÀ1 1H NMR (CD2Cl2, 400 MHz): d=9.09 (d,
;
J=4.8 Hz, 4H; Ha-Py), 8.72 (s, 4H; PhH), 8.65–8.63 (m, 6H; Ha-Py and
PhH), 8.37 (s, 4H; PhH), 7.91 (d, J=5.6 Hz, 4H; Hb-Py), 7.78 (d, J=
4.0 Hz, 4H; Hb-Py), 7.65 (s, 2H; PhH), 7.60–7.59 (m, 12H; ArH), 7.41 (s,
4H; PhH), 5.28 (s, 4H), 3.96 (s, 12H), 1.34–1.35 (m, 48H), 1.18–1.10 ppm
(m, 72H); 31P{1H} NMR (CD2Cl2, 161.9 MHz): d=12.73 ppm (1JPtÀP
=
2695.6 Hz); CSI-TOF-MS: m/z: 1685.95 [MÀ2PF6]2+; elemental analysis
calcd (%) for C138H180F24N4O10P12Pt4: C 45.25, H 4.95, N 1.53; found: C
45.18, H 5.19, N 1.78.
Synthesis of 3b: A similar procedure to that for 3a was carried out for
the synthesis of 3b; the use of 1b (17.8 mg, 21.42 mmol) and acceptor 2
(24.9 mg, 21.41 mmol) yielded 3b as a pale-yellow solid (40.5 mg, 95%).
IR (neat): n˜ =3599, 3095, 3048, 2950, 2359, 2334, 2216, 1724, 1608, 1578,
1493, 1338, 1246, 1206, 1117, 1063, 1003, 841, 760, 679 cmÀ1 1H NMR
;
(CD2Cl2, 400 MHz): d =.09 (d, J=6.0 Hz, 4H; Ha-Py), 8.73 (s, 4H;
PhH), 8.63 (d, J=5.6 Hz, 4H; Ha-Py), 8.27 (s, 4H; ArH), 7.92 (d, J=
6.0 Hz, 4H; Hb-Py), 7.87 (s, 8H; ArH), 7.77 (d, J=4.8 Hz, 4H; Hb-Py),
7.63–7.59 (m, 20H; ArH), 7.39 (s, 4H; PhH), 5.25 (s, 8H), 5.24 (s, 4H),
3.93 (s, 24H), 1.36 (br, 48H), 1.18–1.10 ppm (m, 72H); 31P{1H} NMR
(CD2Cl2, 161.9 MHz): d=12.59 ppm (1JPtÀP =2689.2 Hz); CSI-TOF-MS:
In summary, we have designed and constructed a new
family of supramolecular metallodendrimers featuring a
m/z: 2014.78 [MÀ2PF6]2+
;
elemental analysis calcd (%) for
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Stimuli-Responsive Supramolecular Gels
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C174H212F24N4O22P12Pt4: C 48.38, H 4.95, N 1.30; found: C 48.10, H 4.70,
N 1.51.
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Synthesis of 3c: A similar procedure to that for 3a was carried out for
the synthesis of 3c; the use of 1c (12.8 mg, 8.60 mmol) and acceptor 2
(10.0 mg, 8.60 mmol) yielded 3c as a pale-yellow solid (21.6 mg, 95%). IR
(neat): n˜ =3596, 3000, 2950, 2852, 2363, 2342, 2217, 1723, 1594, 1453,
1335, 1312, 1243, 1117, 1064, 1033, 1004, 837, 732, 678 cmÀ1 1H NMR
;
(CD2Cl2, 400 MHz): d=9.11 (d, J=4.8 Hz, 4H; Ha-Py), 8.74 (s, 4H;
PhH), 8.64 (d, J=4.0 Hz, 4H; Ha-Py), 8.24 (s, 8H; ArH), 7.91 (d, J=
4.8 Hz, 4H; Hb-Py), 7.82 (s, 16H; ArH), 7.75 (d, J=4.4 Hz, 4H; Hb-Py),
7.64–7.59 (m, 20H; ArH), 7.40 (s, 4H; PhH), 7.18 (s, 4H; PhH), 7.11 (s,
8H; ArH), 5.20–5.17 (m, 28H), 3.91 (s, 48H), 1.37 (br, 48H), 1.16–
1.12 ppm (m, 72H); 31P{1H} NMR (CD2Cl2, 161.9 MHz): d=12.70 ppm (s,
1JPtÀP =2707.0 Hz); CSI-TOF-MS: m/z: 2671.64 [MÀ2PF6]2+; elemental
analysis calcd (%) for C246H276F24N4O46P12Pt4·2CH2Cl2: C 51.57, H 4.78,
N 0.98; found: C 51.30, H 4.86, N 1.15.
Acknowledgements
This work was supported by NSFC/China (No. 21132005 and 91027005),
RFDP (No. 20100076110004) of Higher Education of China, Fok Ying
Tung Education Foundation (No. 131014), and the Program for New
Century Excellent Talents in University of the Ministry of Education of
China (NCET-09-0358).
Keywords: gels · metallacycles · platinum · self-assembly ·
supramolecular chemistry
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Received: April 12, 2013
Published online: July 10, 2013
10100
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