Flexible Zirconium Metal-Organic Frameworks as Bioinspired Switchable Catalysts

Article abstract of DOI:10.1002/anie.201604313

Flexible metal–organic frameworks (MOFs) are highly desirable in host–guest chemistry owing to their almost unlimited structural/functional diversities and stimuli-responsive pore architectures. Herein, we designed a flexible Zr-MOF system, namely PCN-700 series, for the realization of switchable catalysis in cycloaddition reactions of CO₂with epoxides. Their breathing behaviors were studied by successive single-crystal X-ray diffraction analyses. The breathing amplitudes of the PCN-700 series were modulated through pre-functionalization of organic linkers and post-synthetic linker installation. Experiments and molecular simulations confirm that the catalytic activities of the PCN-700 series can be switched on and off upon reversible structural transformation, which is reminiscent of sophisticated biological systems such as allosteric enzymes.

Full text of DOI:10.1002/anie.201604313

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604313
German Edition: DOI: 10.1002/ange.201604313
Metal–Organic Frameworks Hot Paper
Flexible Zirconium Metal-Organic Frameworks as Bioinspired
Switchable Catalysts
+
+
Shuai Yuan , Lanfang Zou , Haixia Li, Ying-Pin Chen, Junsheng Qin, Qiang Zhang,
Weigang Lu,* Michael B. Hall,* and Hong-Cai Zhou*
Abstract: Flexible metal–organic frameworks (MOFs) are
highly desirable in host–guest chemistry owing to their almost
unlimited structural/functional diversities and stimuli-respon-
sive pore architectures. Herein, we designed a flexible Zr-MOF
system, namely PCN-700 series, for the realization of switch-
for structure characterization by means of crystallography,
therefore maximizing the understanding of correlation
[
6]
between applied stimuli and ensuing properties. Addition-
ally, the inherent cavities and dynamic behaviors of flexible
MOFs are reminiscent of sophisticated biological systems,
such as regulatory enzymes, in which stimuli induce con-
formational changes and variations of catalytic activities.
High stability and dynamic pore architecture are indis-
able catalysis in cycloaddition reactions of CO with epoxides.
2
Their breathing behaviors were studied by successive single-
crystal X-ray diffraction analyses. The breathing amplitudes of
the PCN-700 series were modulated through pre-functionali-
zation of organic linkers and post-synthetic linker installation.
Experiments and molecular simulations confirm that the
catalytic activities of the PCN-700 series can be switched on
and off upon reversible structural transformation, which is
reminiscent of sophisticated biological systems such as allos-
teric enzymes.
pensable in switchable MOF catalysts. The Zr cluster, in this
6
regard, is a promising building unit for the construction of
switchable MOF catalysts. The dihedral angles between the
Zr clusters and carboxylate linkers vary from 0 to 14.58 in
6
various Zr-MOFs, suggesting extensive flexibility in the Zr-
[
7]
carboxylate junction. In addition, the chemical inertness of
ZrÀO bonds make the Zr-MOFs robust platforms for a variety
[8]
of catalytic applications. Although Zr-MOFs are one of the
research focuses in recent years, the reported Zr-MOFs
usually show limited breathing amplitude that mainly resulted
I
n nature, enzyme activity is often modulated through
feedback loops and a variety of trigger-induced effects.
Inspired by nature, chemists have been developing catalysts
[
9]
from linker flexibility. This promotes us to develop flexible
Zr-MOFs for the realization of switchable catalysis.
A Zr-MOF with flexible bcu topology, namely PCN-700-
[
1]
whose activity can be controlled by external stimuli. Such
systems are capable of alternating the environment of their
active centers, which in turn regulates the reaction rate and
[
12b]
Me , was selected as a prototype MOF.
PCN-700-Me2
2
[2]
selectivity, functioning as allosteric catalysts.
Flexible
crystalizes in the tetragonal space group P4 /mmc. Each Zr
2
6
metal–organic frameworks (MOFs) are suitable platforms
for the development of artificial switchable catalysts in
consideration of their inherent cavities and dynamic behav-
cluster is coordinated to eight Me -BPDC linkers (Me -
2 2
BPDC = 2,2’-dimethylbiphenyl-4,4’-dicarboxylate) and eight
À
terminal -OH /H O groups. The overall framework can be
2
[3]
iors. They are capable of responding to various chemical and
physical stimuli, such as light, pressure, temperature, or guest
simplified into a flexible bcu net, which is expected to shrink
along the c-axis while expanding within the ab-plane (Fig-
ure S4 in the Supporting Information). Breathing behaviors
of MOFs are usually triggered by the removal or introduction
of guest molecules. Bearing this in mind, we carried out
successive single-crystal X-ray diffraction (SC-XRD) analyses
[4]
molecules. A prominent example is the well-established
breathing behavior”, in which the framework experiences
a reversible unit-cell dimensional change as a result of host–
“
[
5]
guest interactions. Different from other porous materials,
such as zeolites and activated carbons, flexible MOFs respond
to the stimuli with retention of high regularity, which allows
on PCN-700-Me during desolvation to generate “snapshots”
2
of the breathing process. Crystallographic data clearly shows
that PCN-700-Me2 exhibits a “scissor-jack-like” behavior,
shrinking along c-direction by tweaking the metal–linker
angle (Figure 1a). A significant decrease in the length of c-
axis (from 14.92 ꢀ to 11.24 ꢀ) and a slight increase in the
length of a/b-axis (from 24.35 ꢀ to 24.84 ꢀ) were observed
upon guest removal. The flexible Zr-carboxylate connection,
acting as a hinge, is primarily responsible for the breathing
[
+]
[+]
[
*] S. Yuan, L. Zou, Dr. H. Li, Y.-P. Chen, Dr. J. Qin, Dr. Q. Zhang,
Prof. Dr. M. B. Hall, Prof. Dr. H.-C. Zhou
Department of Chemistry
Texas A&M University
College Station, TX 77843 (USA)
E-mail: hall@science.tamu.edu
zhou@chem.tamu.edu
behavior. As shown in Figure 1e,f, PCN-700-Me undergoes
2
Dr. W. Lu
a large conformational change that is associated with the
bending of Zr-O-C angle (from 1338 to 1308) and, more
intuitively, the varying of dihedral angle between the equa-
torial plane of O-Zr-Zr-O and the plane of carboxylate (from
Department of Chemistry, Blinn College
Bryan, TX 77805 (USA)
E-mail: weiganglu@yahoo.com
+
[
] These authors contributed equally to this work.
1
08 to 328). The bending of Zr–carboxylate bond affords
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
a closer packing along c-direction, which gives rise to
a shrinkage of the c-axis. A closer investigation indicates
1002/anie.201604313.
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7
00 system. Since PCN-700-Me undergoes unit-cell dimen-
2
sional contraction along c-axis, which involves a rotation of
the CÀC bond between phenyl rings, we speculate that the
framework flexibility can be tuned by changing substituents
on phenyl rings of the linker. To tune the breathing amplitude,
linkers with different substituents were selected, namely
H Me -BPDC (2,2’,6,6’-tetramethylbiphenyl-4,4’-dicarboxylic
2
4
acid), H (CF ) -BPDC (2,2’-bis(trifluoromethyl)biphenyl-
2
3 2
4
,4’-dicarboxylatic acid), and H Me-BPDC (2-methylbi-
2
phenyl-4,4’-dicarboxylic acid; Table 1). As expected, H Me -
2
4
BPDC and H (CF ) -BPDC give rise to MOFs with increased
2
3 2
Table 1: Tuning the breathing amplitude by linker modification.
Linker
Topology DMF Dry Dc [%]
c [ꢀ] c [ꢀ]
bcu
15.4 14.1 9.22
15.0 12.1 24.0
14.9 11.2 33.0
H Me -BPDC
2
4
bcu
H (CF ) -BPDC
2
3 2
bcu
fcu
Figure 1. a),b) Schematic representations of PCN-700-Me before and
2
after desolvation. Insets: photos of microscope images of the respec-
H Me -BPDC
2 2
tive single crystals. c),d) Structures of PCN-700-Me before and after
2
desolvation. e),f) Linker conformation before and after desolvation.
g) An overlap of structural conformations during desolvation.
26.7 26.7
0
H Me-BPDC
2
that the breathing motion of PCN-700-Me is a collective
2
result. Along with the bending of Zr–carboxylate bond, we
also observed a rotation of the CÀC bond between two phenyl
structural rigidity because of the elevated steric hindrance.
rings, which alleviates the steric hindrance within the
structure by arranging the two methyl groups on adjacent
linkers as far apart as possible. As a collateral effect of the
Among this series, PCN-700-Me exhibits the highest degree
4
of rigidity while only small changes of c-axis length (15.4 ꢀ to
3
3
14.1 ꢀ) and unit cell volume (9008 ꢀ to 8170 ꢀ ) were
observed upon desolvation (Table 1 and Table S3). Intui-
tively, linkers with less-bulky substituents tend to form more
flexible MOF structures, as demonstrated in MIL-53 and
rotation of CÀC bond, Zr clusters tilt about 148 (Figure 1d).
6
An overlap of different conformations during desolvation
(
Figure 1g) underlines the fact that the bending of Zr–
[
11]
carboxylate junction and the conformational change of the
organic linker together account for the breathing behavior of
PCN-700-Me2.
MIL-88 systems. However, this is not the case in PCN-700
system. The substituents on the phenyl rings are required to
be bulky enough to induce two off-plane carboxylate groups.
It should be noted that the axial breathing amplitude of
As far as steric hindrance is concerned, H Me-BPDC is
2
PCN-700-Me is much larger than the volumetric breathing
expected to generate an even more flexible MOF than PCN-
2
amplitude. The nearly uniaxial breathing behavior can be
directly observed on a real crystal under optical microscope.
One particular PCN-700-Me2 crystal was measured with
a length of 126 mm along c-axis, which shrank to 93 mm
700-Me . However, only an fcu net (UiO-67 analogue) was
2
obtained by using H Me-BPDC. PCN-700-Me is by far, the
2
2
most flexible MOF that we obtained with the bcu topology.
The breathing amplitude of PCN-700-Me can also be
2
(
Figure 1a,b) upon desolvation (c-direction is determined by
manipulated through linker installation. It has been proved
that dicarboxylate linkers with different lengths can be
Apex 2, Figure S1). These values match very well with unit-
cell parameters determined by SC-XRD. Such high elasticity
in macroscopic single crystals is rarely observed to our
[
12]
installed between Zr6 clusters (Scheme S5).
Four linear
linkers with different lengths, namely FA (fumarate), BDC
(1,4-benzenedicarboxylate), NDC (2,6-napthalene dicarbox-
ylate), and BPDC (4,4’-biphenyldicarboxylate), were selected
and installed in PCN-700 respectively. To carry out linker
[
10]
knowledge.
The SC-XRD investigation of the breathing mechanism
manifests the structure-property correlations, which further
enable us to judiciously modulate the flexibility of the PCN-
installation, PCN-700-Me crystals were soaked in a DMF
2
2
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original configuration. The catalytic
activity of PCN-700-Me was turned
2
on and off for four times, demon-
strating a successful reversible con-
trol of the catalytic activity towards
CO fixation reaction (Figure S42).
2
To elucidate the mechanism of
the switchable catalytic activity, con-
trol experiments were further con-
ducted. The difference in catalytic
activity of PCN-700-o and PCN-700-
c could be possibly attributed to
1) different diffusion rates of sub-
strate and 2) different Lewis acid
sites. To eliminate the influence of
substrate diffusion, Vmax was mea-
Figure 2. a) Representations and b) structures of PCN-700-Me with different linkers installed.
Insets: photos of the microscope view of the corresponding single crystal.
2
solution of linear linker at 758C for 24 h. The subsequently
installed linkers are unambiguously observed by SC-XRD. As
illustrated in Figure 2a, the subsequently installed linkers
support the MOF structure just like the “jack screws” support
the scissor jack. The channel size along the a-axis directly
correlates to the linker length as shown by SC-XRD (Fig-
Table 2: Catalytic reaction of CO
2
and epoxides.
ure 2b) and reinforced by PXRD patterns and N isotherms
2
(
Figure S5–S32). It is worth pointing out that the linker
installation can induce a much larger unit cell dimension
change (62% contraction along c-axis) than the removal of
solvent (32% contraction along c-axis), indicating that
a larger breathing amplitude can be achieved by stronger
guest–host interactions, such as coordination bonds (see
Supporting Information for details).
The inherent cavities and dynamic behaviors of flexible
MOFs allow the cavity environment to be tuned by external
stimuli, which could in turn influence the catalytic activity of
the encapsulated catalyst (Figure S38). Nevertheless, flexible
MOFs for switchable catalysis have not been widely explored.
This could be ascribed to the fact that the close-conformation
of flexible MOFs can be opened up by solvent molecules
under common catalytic conditions. In contrast to most
[
a]
[b]
À1 [c]
Entry
R
Catalyst
–
Conversion [%]
TON
TOF [h ]
1
2
3
4
5
6
7
8
9
10
Me
Me
Me
Me
Ph
Ph
Et
Et
CH Cl
CH Cl
2
15.1
67.9
93.2
29.1
83.3
37.0
91.7
27.5
92.3
30.2
–
–
ZrCl
131.8
244.0
76.2
218.0
96.9
240.0
72.0
241.6
79.0
13.2
24.4
7.6
21.8
9.7
24.0
7.2
24.2
7.9
4
700-o
700-c
700-o
700-c
700-o
700-c
700-o
700-c
2
1
[
a] Conversion calculated from the H NMR spectra. [b] Turnover number
(product (mmol)/metal (mmol)). [c] Turnover frequency (product
(mmol)/metal (mmol)/time (h)).
flexible MOFs, PCN-700-Me shows a very limited depend-
2
ence on the solvents (Table S2). The desolvated PCN-700-
Me2 maintains its shrunken structure in DMF at room
temperature and is only reversed by treating with trifluoro-
acetic acid/DMF solution. These observations suggest that
PCN-700-Me2 solid can be considered as “rigid” under
common catalytic reaction conditions. The closed conforma-
tion and opened conformation are expected to show different
catalytic activity in the same solvent.
sured which represents the maximum rate achieved by
catalysts at saturating substrate concentrations. Vmax of
PCN-700-o and PCN-700-c was calculated to be 42.9 and
6.95 mm min respectively, showing a clear difference in
maximum reaction velocity. Usually, Vmax depends on the
À1
efficiency and concentration of active sites, which is believed
À
[13]
to be the -OH /H O groups on the Zr clusters. To prove
2
6
As a proof of concept, the cycloaddition reaction of CO2
with epoxides was selected to evaluate the Lewis-acid
this, control experiments were carried out under the same
condition using UiO-67 as a Lewis acid, which has no
[13]
À
catalytic activity of PCN-700-Me2.
The performances of
accessible -OH /H O groups on the clusters. The Vmax(UiO-
2
À1
as-synthesized PCN-700-Me (denoted as PCN-700-o, stands
67) was tested to be 4.54 mm min , which is about 10 times
2
for open conformation) and desolvated PCN-700-Me2
smaller than that of V (PCN-700-o), indicating the prom-
max
À
(
denoted as PCN-700-c, stands for closed conformation)
samples were evaluated with different epoxides at 508C under
atm CO pressure and solvent-free condition. As shown in
inent influence of -OH /H O groups. In PCN-700 structure,
2
À
there are two pairs of symmetrically independent -OH /H O
2
1
groups on one Zr cluster along c-direction and b-direction,
2
6
À
Table 2, the desolvated sample, PCN-700-c, shows promi-
nently decreased performance. Yet, the catalytic activity of
PCN-700-c can be restored by trifluoroacetic acid/DMF
treatment which expands the shrunken structure to the
respectively. The -OH /H O groups along c-direction and b-
2
direction in PCN-700-o are both accessible by substrates
À
(Figure 3a), whereas -OH /H O groups in PCN-700-c are
2
only accessible from the b-direction, because of the close
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catalysis within flexible MOF systems will not only lead to
a new generation of catalysts, but also open up a field of study
intersecting with both crystalline porous materials and
switchable catalysts.
Acknowledgements
The gas adsorption-desorption studies of this research was
supported by the Center for Gas Separations Relevant to
Clean Energy Technologies, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award
Number DE-SC0001015. Structural analyses were supported
as part of the Hydrogen and Fuel Cell Program under Award
Number DE-EE-0007049. H. Li and M. B. Hall acknowledge
the National Science Foundation (CHE-1300787) and the
Welch Foundation (A-0648) for financial support and the
Texas A&M Supercomputing Facility for providing comput-
ing resources. We also acknowledge the financial support of
the U.S. Department of Energy Office of Fossil Energy,
National Energy Technology Laboratory (DE-FE0026472).
S.Y. also acknowledges the Texas A&M Energy Institute
Graduate Fellowship funded by ConocoPhillips and Dow
Chemical Graduate Fellowship. We thank Mr. Mathieu Bosch
for his proofreading and feedback.
Figure 3. Structure of PCN-700-o (a) and PCN-700-c (b) showing the
À
accessibility of the active -OH /H O centers (which are shown in red).
2
packing of ligand and clusters along the c-direction, leaving
no room for substrates (Figure 3b).
The halved active site in PCN-700-c is expected to reduce
the Vmax by half. However, the experimental data suggested
that the Vmax (PCN-700-c) is actually reduced by 84%
compared to that of V (PCN-700-o). We propose that the
max
À
-
OH /H O groups along c-direction are more active than
those along b-direction, thus burying the -OH /H O groups
2
À
2
along c-direction caused a dramatic decrease in catalytic
activity for PCN-700-c. Indeed, we observed that the -OH /
À
H O groups along c-direction selectively deprotonate to react
2
À
with basic metal hydroxides, whereas the -OH /H O groups
Keywords: bioinspired catalysts · epoxides · flexible MOFs ·
2
along the b-direction tend to act as bases to react with
molybdic acid (Figure S46). To further support our hypoth-
esis, we used density functional theory (DFT) to calculate the
pore opening · switchable catalysts
[
14]
relative acidities of Zr models of PCN-700-o and PCN-700-c
6
(
see Supporting Information, Section S9). The calculated
À
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7
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linkers. Consistent with this explanation, the asymmetric
character is more notable in PCN-700-c than that in PCN-700-
o, which explains the larger DpK value of PCN-700-c-Zr
a
6
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Received: May 4, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Metal–Organic Frameworks
S. Yuan, L. Zou, H. Li, Y.-P. Chen, J. Qin,
Q. Zhang, W. Lu,* M. B. Hall,*
H.-C. Zhou*
&&&& — &&&&
Flexible Zirconium Metal-Organic
Frameworks as Bioinspired Switchable
Catalysts
An open and shut case: The inherent
cavities and dynamic behavior of flexible
MOFs, bear a close resemblance to
regulatory enzymes. Switchable activa-
tion and deactivation of their catalytic
properties occurs on pore opening and
closing by solvation/solvent removal.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!