CO2-Cross-Linked Frustrated Lewis Networks as Gas-Regulated Dynamic Covalent Materials

Article abstract of DOI:10.1002/anie.201812365

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli-responsive adaptive materials. Herein we first report a gas-responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil-like m

Full text of DOI:10.1002/anie.201812365

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Accepted Article
Title: CO2-Cross-Linked Frustrated Lewis Networks as Gas-Regulated
Dynamic Covalent Materials
Authors: Qiang Yan, Liang Chen, Renjie Liu, and Xiang Hao
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CO
2
-Cross-Linked Frustrated Lewis Networks as Gas-Regulated
Dynamic Covalent Materials
†
†
Liang Chen, Renjie Liu, Xiang Hao, and Qiang Yan*
Abstract: The design of structurally dynamic molecular networks can
induce polymer micellization.[16] Theoretical calculation disclosed
that the dissociation energy of this bridging bond is only ~18 kcal
offer strategies for fabricating stimuli-responsive adaptive materials.
-
1 [15b]
Here we first report a gas-responsive dynamic gel system based on
frustrated Lewis pair (FLP) chemistry. Two trefoil-like molecules with
bulky triphenylborane and triphenylphosphine groups are synthesized
as complementary Lewis acid and base with trivalent sites. They can
mol ,
which is much weaker than that of normal B-O (124 kcal
-
1
-1 [17]
mol ) and C-P bond (63 kcal mol ). This suggests that such a
2
CO -bridging interaction is of high plasticity and might become a
potential candidate for construction of dynamic covalent system.
together bind CO
network via the bonding interactions between FLPs and CO
CO -bridged dative linkages are proved to be dynamic covalent bonds,
2
gas molecules and further form a cross-linked
2
. Such
2
which endow the frustrated Lewis network with adaptable behaviors
and unprecedented gas-regulated viscoelastic, mechanical and self-
healing performance. This study is an initial attempt to apply the FLP
concept in materials chemistry, but we believe that this strategy will
open a promising future for gas-sensitive smart materials.
Structurally dynamic materials that can adapt to the environment
and possess ideal responsive and self-healing functions continue
[
1]
to receive attention. The emergence of these materials provides
chances to revolutionize technologies in sensors, actuators, and
biomedical applications. A major strategy to access such adaptive
systems adopts dynamic covalent chemistry, where a specific
chemical bond can be reversibly cleaved and restructured under
[2]
external stimuli. A variety of dynamic covalent linkages such as
[
3]
[4]
[5]
[6]
disulfide, diselenide, vitrimer, and borate ester motifs, as
well as Schiff-base, epoxy-acid, thiol-Michael,[9] and Diels-
Alder adducts, have been used for creating dynamic networks
that respond to pH, light and redox. Beyond these stimuli, carbon
[
7]
[8]
[
10]
2
dioxide (CO ) gas is also an ideal trigger for responsive materials
because it is mild, abundant, easy-to-handle and contamination-
[
11]
free to the system.
2
Current studies focused mainly on CO -
mediated molecular assembly[ and CO
12]
[13]
2
-switchable surfaces.
However, gas-sensitive dynamic covalent chemistry has yet been
explored. Here we address this unmet challenge and report a new
dynamic molecular materials that can be activated by CO .
2
Scheme 1. Trefoil-like frustrated Lewis pair (B3 and P3) as the trivalent building
blocks to fabricate CO
2
-cross-linked frustrated Lewis networks with reversible
-bridging dynamic covalent linkages.
CO /thermal responsiveness via CO
2
2
Recently, our group has strong interest in introducing frustrated
Lewis pair (FLP) chemistry into advanced materials. FLP is a new
organic concept that refers to a pair of sterically encumbered
Lewis acid-base that prevents Lewis adduct formation but offers
Inspired by these, we design two molecular trefoils, appending
respectively with three p-phenyl-bis(pentafluorophenyl)borane
(B3) and p-phenyl-dimesitylphosphine pendants (P3), as trivalent
FLP acceptor and donor (The synthesis and characterization in
the Supporting Information, Scheme S1 and Figure S8-S21). It is
[
14]
latent reactivity to small molecules, especially gas molecules
[
15]
(
e.g. H
and phosphorus (P)-bearing polymers as a macromolecular FLP.
They can bind CO to form active B-OC(=O)-P bridging bond and
2 2
and CO ). In a previous study, we developed boron (B)-
expected that CO
constituting a new form of molecular network, in which CO
function as the cross-linkers to interconnect the complementary
FLPs (Scheme 1). Since these CO -bridging molecular nodes are
2
molecules could be bound by the B3 and P3,
2
2
would
[
*]
L. Chen, R. J. Liu, X. Hao and Prof. Dr. Q. Yan
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science
Fudan University
2
dynamic covalent, this FLP-based gel system, termed frustrated
Lewis network, would manifest unique gas-regulated viscoelastic,
mechanical and self-healing performance.
Shanghai 200433 (China)
E-mail: yanq@fudan.edu.cn
Our first aim was to probe whether such a Lewis acid-base pair
could be gelated by CO . Mixing equimolar amounts of B3 and P3
2
in toluene (1.0 mM), neither solution color nor state had obvious
change (Figure 1a, left), implying that the two molecules are too
[†
]
These author contributed equally to this work
Supporting information for this article is given via a link at the end of
the document
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This viewpoint was validated by 31P and 11B NMR spectroscopy.
Prior to gas stimulus, the sol showed a sharp 31P peak at -27.2
11
ppm and a broad B signal at 78 ppm, ascribed to the phosphine
and borane groups (Figure 1b-c, bottom panel). However, in the
3
1
gel with CO
5
2
, the P signal gave a remarkable downfield shift to
2.5 ppm (∆δ = 79.7 ppm); meanwhile, the 11B resonance shifted
reversely to upfield region of -2 ppm (∆δ = -80 ppm) and further
widening (Figure 1b-c, middle panel). These NMR variation
[
15b]
2
coincides with the case of CO -bound FLP small molecules,
confirming the CO -driven cross-linking effect. Furthermore, we
2
wondered whether these crosslinks are reversible and dynamic.
o
If heating up the gel to 60 C, it can rapidly dissociate into a sol
31
11
(
Figure S1). NMR showed that its P and B peaks had identical
shifts with the original sol (Figure 1b-c, top panel). This sol-gel
transition can be repeated under alternating CO /thermal stimuli
Figure S2), indicating that the crosslinking points are capable of
reversible broken and reforming. To further elucidate the dynamic
2
(
exchange of CO
2
-bridging bonds in an equilibrium state, we used
a de-cross-linking method to monitor this process.[ An mono-
18]
functionalized Lewis acid B1 was synthesized and added to the
B3•CO
the network. This FLP metathesis can be tracked by the variation
of gel molecular weight (Figure 1d-e). The B3•CO •P3 gel had a
2
•P3 gel in a molar ratio of 3:1 to substitute the B3 units in
2
high molecular weight of 340 kDa measured by gel permeation
chromatography (GPC, as indicated by a peak at low elution time);
however, upon addition of B1, the gel was gradually fluidized, and
2
the GPC trace that corresponds to B3•CO •P3 was depressed in
intensity, whereas a new set of peaks at high elution time region
appeared and enhanced. Their molecular weights were evaluated
to be ~3.1 and ~1.6 kDa, respectively, close to that of the three-
arm small complex (B1•CO
1.3 kDa), indicating the exchangeability of FLP moieties. In view
of the reversible and dynamic natures of CO -bridged bonding,
-dependent
2
•P3, 2.7 kDa) and cleaved B3 species
(
2
we speculated that this network would possess CO
physical and chemical properties.
2
2
We next explored the relationship between CO stimulus level
and the gel viscoelastic and mechanical behaviors. Viscosity tests
first provided insight into the effects of CO
2
concentration on the
gel network growth.[ When increasing the CO
level from 0 mM
2
19]
to 3 mM (0‒3 equiv. with respect to B3), the systemic viscosity (η)
exponentially grew from 0.2 to 1040 Paꞏs, and the η curve well
satisfied the relationship, η = kꞏ[CO
α and slope k were determined to be 2.4 and 75, respectively
Figure 2a, red line). This result illustrates that CO concentration
is crucial to manipulating the dynamic network growth. The higher
CO level is, the more bonding nodes generated and the larger
covalent grids formed (Figure S3). In a control experiment, if using
univalent B1 in place of trivalent B3 and kept other conditions the
same, no gel formed due to lack of the crosslinks (Figure 2a, blue
line).
2
]α, where the power exponent
Figure 1. a) Photographs showing CO
B3•CO •P3 frustrated Lewis network and its reversible sol-gel transition under
alternating CO
structural changes (B3+P3) under different conditions: in the absence of CO
2
-induced gelation of B3 and P3 to form
2
(
2
3
1
11
2
/thermal stimuli. b) P NMR and c) B NMR studies of the FLP
2
(
(
bottom panel), after CO
top panel). d) De-cross-linking reaction showing the dynamic exchange of the
•P3 + B1 ↔ B1•CO •P3 + B3) by using mono-
functionalized B1 Lewis acid. e) GPC traces monitoring the FLP metathesis over
time. The peak at low elution time (9 min) corresponds to B3•CO •P3 molecular
2
treatment (middle panel) and upon heating to 60 o
C
2
frustrated Lewis network (B3•CO
2
2
2
network (340 kDa). With the increase of exchanging reaction time, the peak is
depressed and shifts to high elution time region (16-18 min) that corresponds to
small B1•CO •P3 complex and B3.
2
2
CO -dependent dynamic mechanical performance of the gels
were monitored by rheological method. Figure 2b compared the
changes in storage (G’) and loss (G’’) moduli versus frequency
under different CO
CO , all three gels could maintain their solid-like structures in an
oscillatory shear, as evidenced by the G’ much higher than the G’’
over the entire frequency range. Notably, when CO /FLP ratio is
stoichiometry (3:1), the storage moduli can reach a maximum of
4.5 MPa (Figure S4), suggesting its high mechanical strength. In
2
levels (1, 2 and 3 mM). With the increase of
bulky to form common Lewis adduct. However, when the mixture
was exposed to CO gas (pCO2 = 1.0 atm, flow rate = 1.0 mL s ),
2
the initial faint yellow solution faded and changed to an opaque,
self-standing gel within 1 min (Figure 1a, right). A possible reason
2
is that CO functions as a cross-linker to induce the FLP gelation.
2
-
1
2
~
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effective enhancement. This effect is proportional to the CO
2
content. For the case of 3.0 mM CO
tensile strength and 410% elongation at break (Figure 2c),
showing a considerable increase than the case of 1.0 mM CO
These results further supported that CO gas is a key factor to
control the gel mechanical performance. Moreover, we found that
even after three periods of alternating thermal/CO stimuli, the
mechanical parameters of regenerated gel (using the gel
containing 3 mM CO as a studied model) only dropped less than
% (Figure S6), presaging that this CO -cross-linked network is
2
, the gel could attain 3.4 MPa
2
.
2
2
2
8
2
not only mechanically strong and ductile, but also recyclable and
reproducible.
Figure 3. a) Self-healing process of the frustrated Lewis network: (i) two cutting
samples (one loaded with red pigment), (ii) cutting surfaces were treated with
Figure 2. CO
network: a) Viscosity change as a function of CO
B3 and P3 mixture (1:1 molar ratio), and the control mixture of univalent B1 and
trivalent P3 (3:1 molar ratio). b) Storage modulus (G’, solid symbols) and loss
2
-regulated physicochemical properties of the frustrated Lewis
stimulus level from trivalent
2
CO and contact each other, (iii) self-healed for 30 min under ambient condition,
2
(iv) the healed gel under stretching, and (v) under loading experiment. b) Stress-
strain curves of original and mended gel samples under various healing time. c)
Plot of the gel recovery efficiency versus healing time. d) Stress-strain curves
modulus (G’’, open symbols) variation of CO
various CO contents (1 mM, red; 2 mM, blue; 3 mM, green) versus frequency
at room temperature. c) Stress-strain curves of CO -cross-linked gel specimen
with various CO contents (1 mM, red; 2 mM, blue; 3 mM, green).
2
-cross-linked gel specimen with
of original and mended gel samples under various CO
2
stimulus level. e) Plot of
2
the gel recovery efficiency versus external CO concentration. Error bars in c)
2
2
and e) denote the standard deviations from at least three experiments.
2
In view of the satisfactory mechanical properties of our gels, we
finally examined their self-healing ability. To this end, we selected
the optimal gel (binding 3 mM of CO ) to perform experiments.
2
Two gel samples, one of which contained a little red pigment for
better visibility, were prepared and cut into two same pieces using
addition, temperature-dependent moduli were recorded from 20
C to 70 C at a heating rate of 2.0 C min . Both G’ and G’’
o
o
o
-1
decreased with elevated temperature, and a crossover appeared
o
at 58 C (Figure S5). This was identified as the sol-gel transition
o
point, which is in good agreement with the result of NMR (60 C).
a knife (Figure 3a, i). The cutting surfaces were swept by CO
2
for
On the other hand, tensile tests were also carried out on these
gels. From the stress-strain curves, it is clear that even adding a
1
min (excess of gas) and then put together immediately without
pressing (Figure 3a, ii). The in-contact gels were kept at room
temperature. After 30 min the two halves completely merged into
one single piece and the scars had almost vanished (Figure 3a,
2
small amount of CO (1 mM), the gel mechanics such as tensile
strength (0.6 MPa) and elongation at break (240%) obtained
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iii). The resulted gel column can bear a large stretching to a long
extension (370% elongation) without any fractures (Figure 3a, iv),
and can hang weights over 250 g (Figure 3a, v). To quantitatively
assess the self-healing efficiency, we compared the stress-strain
properties between the original and mended samples at varying
healing times (Figure 3b). The results indicate that longer healing
time led to better repair (Figure 3c). A 98% recovery of the original
fracture stress (tensile strength) only took 2 h. This rapid self-
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2
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[
14b]
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we believe that this strategy of combining FLP
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This work was financially supported by National Natural Science
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Keywords: carbon dioxide • dynamic covalent bond • frustrated
6
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2
CO -Cross-Linked Frustrated Lewis
Networks as Gas-Regulated Dynamic
Covalent Materials
2
2
CO weaves network: CO can act as a “gas glue” to cross-link trefoil-like
frustrated Lewis molecular pair into a new form of molecular network. This CO
2
-
bridged cross-linking are dynamically covalent, which endows the gel network with
unique gas-modulated viscoelastic, mechanical and self-healing performance.
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