Reversible Structural Transformations of Metal–Organic Frameworks as Artificial Switchable Catalysts for Dynamic Control of Selectively Cyanation Reaction

Article abstract of DOI:10.1002/chem.201901118

The synthesis of molecular-level artificial switchable catalysts, of which activity in different chemical processes can be switched by controlling different stimuli, has provided a new paradigm to perform mechanical tasks and measurable work. In this work, to obtain highly effective and regioselective artificial switchable catalysts, a hierarchical anion-pillared framework {(H₃O)[Cu(CPCDC)(4,4′-bpy)]}_n (1; H₃CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylic acid, 4,4′-bpy=4,4′-bipyridine), including free [H₃O]⁺ ions as guest molecules, was constructed. Upon dissolve–exchange–crystallization behavior, fascinating reversible structural transformations proceeded between anion framework 1 and neutral 2D stair-stepping framework {[Cu(CPCDC)(4,4′-bpe)]}_n (2; 4,4′-bpe=4,4′-vinylenedipyridine). Moreover, frameworks 1 and 2 can act as heterogeneous artificial switchable catalysts to selectively promote the direct cyanation reaction of terminal alkynes and azobisisobutyronitrile. The results indicated that 1 and 2 exhibited excellent selectivity to generate vinyl isobutyronitrile skeletons or propiolonitrile frameworks, respectively, as unique products. Furthermore, indicating paper, GC-MS, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis demonstrated that the reversible structural transformations endowed 1 and 2 with well-defined platforms to stabilize the isobutyronitrile and CN sources through the different catalytic pathways.

Full text of DOI:10.1002/chem.201901118

A Journal of
Accepted Article
Title: Reversible Structural Transformations of Metal-Organic
Frameworks (MOFs) as Artificial Switchable Catalysts for
Dynamic Control of Selectively Cyanation Reaction
Authors: Hongwei Hou, Chao Huang, Gaoxiang Li, Lin Zhang, Yingying
Zhang, and Liwei Mi
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To be cited as: Chem. Eur. J. 10.1002/chem.201901118
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Reversible
Structural
Transformations
of
Metal-Organic
Frameworks (MOFs) as Artificial Switchable Catalysts for
Dynamic Control of Selectively Cyanation Reaction
Chao Huang,[a] Gaoxiang Li,[a] Lin Zhang,[a] Yingying Zhang,[a] Liwei Mi,*[a] and Hongwei Hou*[b]
Abstract: Synthesis of molecular-level artificial switchable catalysts,
of which activity in different chemical processes can be switched by
a stimuli-controlled, have provided a new paradigm to perform
mechanical tasks and measurable work. In this work, to obtain the
highly effective and regioselective artificial switchable catalysts, a
hierarchical anion-pillared framework 1 including free [H3O]+ ions as
guest molecules was constructed. Upon dissolving-exchange-
artificial switchable catalysts have been attempted by employing
solid materials to regulate the functional structures, but the
uncontrollability, low surface area and instability of present solid
materials restricts their catalytic performance, resulting into the
limitation of their industrial applications.[9,11,12] Therefore,
immobilizing the atomistically well-defined catalytic active sites
into the reversible structural transformations materials, such as
porous coordination polymers (PCPs) or metal-organic
frameworks (MOFs), could be a shortcut way to prevent the
undesired multimolecular catalysts deactivation and achieve
higher catalytic efficiency and better recycling property than
present artificial switchable catalysts but has not yet been
obtained.
crystallization
behavior,
fascinating
reversible
structural
transformations proceeded between an anion framework 1 and a
neutral 2D stair-stepping framework 2. Moreover, 1 and 2 can be
acted as heterogeneous artificial switchable catalysts to selectively
promote the direct cyanation reaction with terminal alkynes and
azobisisobutyronitrile (AIBN). The results indicated 1 and 2 exhibited
excellent selectivity to generate vinyl isobutyronitrile skeletons or
propiolonitrile frameworks as the unique products, respectively.
Furthermore, indicating paper, GC-MS, EDS and XPS analysis
demonstrated that the reversible structural transformations endow 1
and 2 with well-defined platform to stabilize the isobutyronitrile and
CN source via the different catalytic pathways.
As one dramatic variety of porous solid material, MOFs have
provided
a
well-defined platform to engineer anchoring
molecular catalysts and demonstrated great potential in MOF-
based heterogeneous catalysis areas because of their
accessible and uniform catalytic sites, massive and
functionalized structural variations.[13-18] MOFs with site-isolation
and adjacent active catalytic sites via direct use or post-synthetic
functionalization as single-site and cooperative catalysts to
catalyze chemical transformations have been investigated.[19-27]
Taking advantage of their frequently-occurred reversible
structural transformations via single-crystal to single-crystal
processes, the unique flipping motion between the structural and
topology connections are almost similar to each other, making it
a highly appropriate choice for building molecular devices, such
as switching or sensing materials.[28-32] Meanwhile, solvent-
induced dissolving-exchange-crystallization behaviors are
another important but difficult approach to regulate the reversible
structural transformations because the connection modes
between metal ions and ligands are very difficult to reedit.[30]
Furthermore, coordination geometry of the catalytic sites, the
size of the channel and the electronegativity of the frameworks
in MOFs-based dissolving-exchange-crystallization process are
more easily realized, resulting from the fact that their controllable
structural transformation behavior can turn on/off the catalytic
activity of molecular catalysts.[33-36] Moreover, to mimic the
actions of channel enzymes, particular effort has been dedicated
to adjust and control the reversible structural transformations as
heterogeneous catalysts to discern the molecular structure
stimuli-controlled process, which can give a more distinct
catalytic mechanism and richer perspective on the functional
performances to illuminate the fundamental man-made catalytic
chemistry.[22,33,37,38] In addition, the reversible structural
transformations of MOFs could be envisaged as a scaffold
similar to the passive transport in enzymes, of which the
arrangement of second and tertiary structures could be acquired
to realize the trigger-induced effects.[39-41] Despite the
construction of MOFs-based catalysts and reversible structural
transformations of MOFs have been realized successfully,[15-18,31-
Introduction
Recently increasing attention has been attracted by artificial
switchable catalysts because of their broad adaptabilities, from
the macromolecular chemistry to enantioselectivity and other
synthetic
chemistry.[1-3]
Incorporating
stimuli-controlled
characteristics into artificial switchable catalysts can afford an
attached „bio-like‟ level of control over chemical transformations,
which are analogous to modulate by a series of trigger-induced
effects and feedback loops of enzyme activity through the
transformations processes with the necessary temporal and
spatial control and without unnecessary disturbance from other
reaction pathways.[4-7] Although artificial switchable catalysts can
be commonly triggered in the “on” or “off” states to execute
reactions in synthesis that are extremely difficult or impossible to
achieve chemo-, regio-, size-, shape-, and stereoselectivity in
other ways, the instability of most artificial switchable catalysts
upon recycling ineluctably results in the distinct catalytic activity
8-10]
loss during the reaction system.[3-5,
Meanwhile, several
[a]
[b]
Dr.C. Huang, G. LI, Dr. L. Zhang, Dr. Y. Zhang, Prof. L. Mi
Center for Advanced Materials Research, Zhongyuan University of
Technology, Zhengzhou 450007 (P. R. China)
E-mail: mlwzzu@163.com
Prof. H. Hou
The College of Chemistry and Molecular Engineering, Zhengzhou
University, Henan 450052 (P. R. China)
E-mail: houhongw@zzu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
32]
MOFs-based switchable catalysts using atomically precise
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structures are still at an early stage. Therefore, new rational
synthetic strategies are needed to achieve MOFs-based artificial
switching catalysts in a predictable fashion.
Herein, we exhibit how the reversible structural
transformations of MOFs can serve as artificial switching
catalysts for the creation of an enzyme-like active center to
selectively execute the direct cyanation reaction with different
reaction pathways. Upon dissolving-exchange-crystallization
process, reversible structural transformations visible to the
framework (Figure 1c). Furthermore, XPS measurements of blue
crystals 1 evidenced that the observation of Cu 2p3/2 peak at
934.68 eV was well in agreement with the CuII cations (Figure
1b).[42, 43] In addition to XPS spectrum, PXRD of the as-prepared
crystalline sample displayed that the sample was pure (Figure
S1b).
naked-eye occurred between
a hierarchical anion-pillared
framework 1 including free [H3O]+ ions as guest molecules and a
neutral 2D stair-stepping framework 2. The reversible structural
transformations qualified 1 and 2 as heterogeneous artificial
switchable catalysts to selectively promote the direct cyanation
reaction of terminal alkynes and azobisisobutyronitrile (AIBN),
producing vinyl isobutyronitrile skeletons and propiolonitrile
frameworks as the unique products, respectively. Moreover, a
series of experiments (indicating paper, GC-MS, EDS and XPS
analysis) revealed that the reversible structural transformations
of 1 and 2 could provide intact well-defined platform to stabilize
the isobutyronitrile and CN source by different catalytic
pathways.
Results and Discussion
Synthesis and Structural Characterization of Copper(II)
Complex 1.
To design and construct artificial switchable catalysts for tuning
the activity and selectivity, we chose a Y-shaped and rigid 9-(4-
carboxyphenyl)-9H-carbazole-3,6-dicarboxylic acid (H3CPCDC)
as shape-persistent ligand, which can serve as a premade
building unit to dictate the self-assembly of highly ordered layer
structures. Furthermore, by virtue of highly ordered layer unit,
they can be bridged by suitable dipyridyl terminated connecting
ligands to form hierarchical pillared MOFs with accessible
internal void, atomically precise nanometer scale open channels,
and good fabricability. To illustrate the principle, direct reaction
of Cu(NO3)2 with H3CPCDC led to exclusive formation of highly
ordered two-dimensional (2D) layer structure. We utilized the
layer structure, as a premade, shape persistent unit, and rigid
4,4ʹ-bipyridine (4,4ʹ-bpy) ligands to construct the hierarchical
pillared Cu-MOF. The pillared Cu-MOF was arranged by
coordination bonds between Cu ions and 4,4ʹ-bpy terminated
ligands. Indeed, reaction of Cu(NO3)2 with H3CPCDC in the
presence of excess 4,4ʹ-bpy ligands and nitric acid in
dimethylformamide (DMF)/H2O at 85 ºC for 2 days afforded
{(H3O)[Cu(CPCDC)(4,4ʹ-bpy)]}n (1) as blue single crystals with
free [H3O]+ ions encapsuled in an anion-pillared framework
(Figure 1a). The finally formula of 1 was determined through
Figure 1. Crystal structures of 1: (a) The image of the blue crystal of 1. (b)
XPS spectrum of 1. (c) View of the hierarchical anion-pillared framework along
b axis with large channel (15.35 ×11.08 Å2) (considering van der Waals radii).
Hydrogen atoms are omitted for clarity. (d) The 2D layer unit linked by
CPCDC3- ligands with an inner sphere 11 Å diameter. (e) The cage in the 1. (f)
The cage unit ([Cu6(CPCDC)2(4,4ʹ-bpy)3]6-) with an inner sphere about 11 Å
diameter.
In 1, the asymmetric unit included one Cu ion, one 4,4ʹ-bpy
ligand and one CPCDC3- ligand (Figure S1c). The Cu1 ion was
five coordinated with three O atoms (O1, O4, and O5) from three
CPCDC3- ligands and two pyridyl-N atoms (N2 and N3) from two
4,4ʹ-bpy ligands, featuring a distorted tetragonal pyramidal
geometry. The carboxyl groups in the H3CPCDC ligands were
completely deprotonated and taking a monodetate coordination
mode in 1, which can be proved by the O 1s peak in XPS
spectrum (Figure S1d).[44] Furthermore, these deprotonated
carboxyl groups can be linked by Cu ions to form a 2D layer unit,
revealing a 42-membered macrocycle and a cavity adequate to
fixate a sphere with a diameter of 11 Å (considering van der
Waals radii) (Figure 1d). Such layer unit was bridged by 4,4ʹ-bpy
ligands through the coordination bonds of Cu ions and pyridyl-N
single-crystal
X-ray
diffraction
analysis
(SCXRD),
thermogravimetric analysis (TGA, Figure S1a), X-ray
photoelectron spectroscopy (XPS), elemental analysis (EA), and
charge-balance considerations. SCXRD study indicated that
complex 1 crystallized in the triclinic space group P-1 and
possessed
a
three-dimensional (3D) open anion-pillared
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atoms to provide hierarchical pillared framework. A prominent
structural characteristic in pillared framework consisted of cages
made up of Cu ions, 4,4ʹ-bpy and CPCDC3- ligand (Figure 1e).
Each cage ([Cu6(CPCDC)2(4,4ʹ-bpy)3]6-) included six Cu ions
connected by three 4,4ʹ-bpy and two CPCDC3- ligands to
generate [Cu6(CPCDC)2(4,4ʹ-bpy)3]6- units, resulting in an inner
sphere of about 11 Å diameter (Figure 1f). Each of these cages
was surrounded by the adjacent cages via a face-sharing mode
to shape a 3D framework, in which a large open rectangle
channel (15.35 × 11.08 Å2, considering van der Waals radii)
along b direction was given. Moreover, the CO2 adsorption
isotherm of 1 demonstrated that the value of CO2 adsorption
was 60 cm3·g -1 at 273 K and the channel width was 1.0 nm
(Figure S1e).
In additon, the anionic nature of 1 can also be further
supported by cation-exchange capability. Cation-exchange
experiment was performed using 0.05 mol/L solution of NaCl in
H2O and heated at 85 ºC . After 12h, the success incorporation of
guest cations (Na+) was verified by XPS and mapping images
analysis (Figures S1d and S2a-c). The observation of the Na 1s,
Cu 2p3/2, and O 1s peaks in XPS revealed that the coordination
mode of carboxyl groups were the same in 1 during the
exchange processes, which suggested the cation-exchange was
occured in the guest molecule.42-44 Moreover, PXRD analysis
indicated that there were no significant changes among the
simulated, synthesized and cation-exchanged patterns, which
proved that the cation-exchanged framework had the same
structure with 1 (Figure S2d). Therefore, given the charge-
balance consideration, the pillared MOF of 1 was anionic
framework filled with one [H3O]+ ions per formula unit.
included one Cu ion, one HCPCDC2- ligand, and two 4,4ʹ-bpe
ligands (Figure S3c). The coordination sphere of Cu1 ion
included three O atoms (O1, O2, and O5) from two HCPCDC2-
ligand and two pyridyl-N atoms (N2 and N3) from two 4,4ʹ-bpe
ligands, giving a slightly distorted tetragonal pyramidal geometry.
In 2, two carboxyl groups of H3CPCDC ligands were
deprotonated and taking two different coordination modes, which
can be testified by the O 1s peak in XPS spectrum (Figure S1d).
One deprotonated carboxyl group adopted
coordination mode to bridge Cu ion (Cu1 or Cu1a) whereas the
other deprotonated carboxyl group adopted chelating
a monodetate
a
coordination mode to bridge Cu ion (Cu1 or Cu1a), generating a
1D chain structure (Figure S3d). Furthermore, such 1D chain
can be linked by 4,4ʹ-bpe ligands via the coordination bonds of
Cu ions and pyridyl-N atoms to construct hierarchical 2D stair-
stepping structure. In additon, the hierarchical 2D stair-stepping
unit, stabilized by π···π interactions between the adjacent
carbazole and pyridine rings (3.85 Å), build a 3D supermolecule
framework with an open pore (Figure S3e). In addition, the CO2
adsorption isotherm of 2 revealed that the value of CO2
adsorption was 66 cm3·g -1 at 273 K and the channel width was
1.1 nm (Figure S3f).
Dissolution-recrystallization
Structural
Transformation
Behavior Between 1 and 2.
Experiment aimed at appraising the hierarchical pillared Cu-
MOF 1 demonstrated that a reversible structural transformation
can be acquired through dissolving-exchange-crystallization
process. When the fresh blue single crystals of 1 was immersed
in 4,4ʹ-vinylenedipyridine (4,4ʹ-bpe, 0.1 mmol) solution (3 mL
DMF, 2 mL H2O, and 3 drops HNO3) for 5 minutes at room
temperature, the blue crystals of 1 were quickly dissolved and
blue solution was obtained, which could be easily observed by
naked eye (Figure 2a). Remarkably, after standing for about 36
h, the yellow-green crystals of 2 with retention of single
crystallinity, which were to be determined by XPS, SCXRD,
PXRD and TGA (Figures S3a-b), were achieved from the blue
solution. As shown in Figure 2b, yellow-green 2 demonstrated
the Cu 2p3/2 peak at 934.5 eV in the XPS spectrum, suggesting
the only existence of CuII in 2.[42-44] Furthermore, SCXRD
analysis of the yellow-green crystal indicated the new crystalline
category formulated as [Cu(HCPCDC)(4,4ʹ-bpe)]n (2) with a
distinctive 2D stair-stepping framework (Figure 2c). It was
noteworthy that the 3D hierarchical anion-pillared framework of 1
may undergo a spontaneous dissolving-exchange-crystallization
process to form 2D stair-stepping structure.
Figure 2. Crystal structures of 2: (a) The structural transformation process
from 1 to 2. (b) XPS spectrum of 2. (c) View of the 2D stair-stepping structure.
Hydrogen atoms are omitted for clarity. (d) The [Cu4(HCPCDC)2(4,4ʹ-bpe)2] flat
with the channel size of 13.71 ×13.80 Å2 (considering van der Waals radii).
Prominently, the transformation processes from the 3D
hierarchical anion-pillared framework of 1 into the 2D stair-
stepping structure of 2 were reversible. Treatment of the fresh
single crystals of 2 with 0.1 mmol 4,4ʹ-bpy solution (3 mL DMF, 2
mL H2O, and 3 drops HNO3) for 5 minutes at room temperature
indicated that the yellow-green crystals were rapidly dissolved
and the blue solution was generated again. When the re-
solvation of the blue solution was standing at 85 ºC for 24 h, the
blue crystals were re-occupied with retention of single
crystallinity, which was accompanied by a color change from
Unlike 1, 2 crystallized in the monoclinic space group C2/c
and contained large open channels (13.71
considering van der Waals radii) (Figure 2d). The complex
×
13.80 Å2,
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yellow-green to blue (Figure S4a). SCXRD analysis revealed a
triclinic P-1 space group in which the unit cell parameters of 1a
was very close to those of 1 (Table S2), that is, the framework of
1a should be exactly consistent with that of 1. Correspondingly,
the Cu 2p3/2, and O 1s peaks in XPS verified that the
coordination environment of Cu ions and carboxyl groups were
the same in 1 during the transformation processes (Figures S4b
and S1d). In addition, the phase purity of the re-occupied blue
found that 1 and 2 could keep stable in boiling solvents, which
could be verified by the unit cell parameters and PXRD patterns
of samples 1 and 2 after 48 h. Then, to investigate the optimal
catalytic performances of artificial switchable catalysts 1 and 2
for directing cyanation reaction, phenylacetylene (3a) and AIBN
(4) were chosen as coupling partners to explore the reaction
conditions (Table S3). After comprehensive screening of
solvents, atmosphere, and temperature, the standard conditions
samples was identified by the PXRD of 1 and simulated patterns, were achieved by utilizing 1 and 2 as catalysts (5 mol %) under
which demonstrated that they matched exactly and had the
same framework (Figure S4c). Thus, based on the SCXRD, EA,
TGA, XPS, and charge-balance considerations, the finally
formula of {(H3O)[Cu(CPCDC)(4,4ʹ-bpy)]}n (1a) was provided
(Figure S4d).
Even more noticeably, the re-occupied blue crystals of 1a
could be redissolved in 4,4ʹ-bpe solution to afford blue solution,
which can regrow the yellow-green crystals of 2a in a dissolving-
air at 90 ºC , producing the desired vinyl isobutyronitrile (5a) and
propiolonitrile (6a) products in 95 and 94%, respectively. It
demonstrated that
1 and 2 served as excellent artificial
switchable catalysts to perform the cyanation reaction with high
selectivity, whereas Cu(NO3)2 was less selective to generate a
mixture of vinyl isobutyronitrile (5a, 21%) and propiolonitrile (6a,
31%) under the same reaction conditions (Table S3, entry 10).
Under inert gases (argon or nitrogen), vinyl isobutyronitrile
skeletons can be obtained (< 10 %) (Table S3, entries 8 and 9).
The direct cyanation reaction proceeded moderately efficient in
EtOH (5a, 32%; 6a, 43%) and toluene (5a, 37%; 6a, 44%)
(Table S3, entries 2 and 5), however, dioxane, DMF, and CHCl3
were unsuitable solvents (Table S3, entries 1, 4 and 6). When
the reaction temperature increased from 25 to 90 ºC, the yields
of 5a and 6a were obviously improved, since the acquisition of
CN source and isobutyronitrile group from AIBN needed
heatment (Table S3, entry 7). Therefore, In view of stability,
efficiency, selectivity, and environmentality, we validated 1 and
2/CH3CN/90 °C /air as the standard conditions to execute the
cyanation reactions.
exchange-crystallization
process.
XPS
measurement
demonstrated that the coordination modes of Cu ions and
carboxyl groups were consistent with 2 during the transformation
processes (Figures S5a and S1d). Furthermore, unit-cell
parameters and PXRD determinations revealed that 2a kept as
single crystals with the same framework as 2 (Figure S5b). The
transformations from the 3D hierarchical anion-pillared
frameworks (1 and 1a) into the 2D stair-stepping structures (2
and 2a) in this work indicated totally reversible dissolving-
exchange-crystallization process. Although
a
number of
examples of recrystallization process of MOFs were reported,
the transformation of the electronegativity in MOFs frameworks
via dissolving-exchange-crystallization fashion has not been
reported. Complexes 1 and 2 revealed the first example of the
transformation of the electronegativity dependent reversible
dissolving-exchange-crystallization behavior.
Subsequently, this artificial switchable catalytic system was
found to be general for the cyanation reactions with a variety of
substituted terminal alkynes (3a-e), and the results were
summarized in Table 1. In all situations, heterogeneous catalyst
1
possessed excellent selectivity to generate vinyl
Artificial Switchable Catalysts for the Direct Cyanation of
Alkynes with AIBN.
isobutyronitrile skeletons as the single products with the yields
ranging from 90 to 95 % (5a-e), meanwhile heterogeneous
catalyst 2 had the high selectivity to produce propiolonitrile
frameworks as the unique products with the yields ranging from
88 to 94 % (6a-e). Furthermore, we were pleased to discover
that both the electron-withdrawing (3b and 3d) and electron-
donating groups (3c and 3e) at the para positions of
cyclobenzene could be well tolerated to give the unique
regioisomer of desired products (5b-e and 6b-e) with remarkable
yields. In addition, 4-biphenylacetylene (3e), a bulky substrate,
was selected to test the influence of the substrate size in the
direct cyanation reaction under the same reaction condition,
providing 5e (85%) and 6e (82%) in moderate yields as the sole
product (Table 1, entries 14 and 15). The yields of 5e and 6e
were slightly lower than the yields of 5a-d and 6a-d, which
revealed that the increasing size of substrates would decrease
the diffusion rate of the substrates and products without any
restrictions on the selectivity. Moreover, to investigate the
practicality of this artificial switchable catalytic system, a 10.0
mmol scale reactions were performed by using 3a as substrate,
and 5a and 6a were isolated in the excellent yields of 91 and
90 %, respectively. In comparison, the homogeneous catalyst
Cu(NO3)2, which was commonly utilized in a copper-mediated
Direct cyanation of alkynes into highly valuable cyanide
compounds with AIBN which was a safe cyanide source,
represents one of the most graceful and environmentally friendly
pathways to avoid the metal waste and toxicity.[45,46] Although
AIBN is also widely used as a radical initiator to generate CN
source and isobutyronitrile to react with alkynes to produce
highly valuable cyanide compounds, the highly effective and
regioselective formation of the propiolonitrile or vinyl
isobutyronitrile compounds as the single regioisomer product is
particularly difficult. To accomplish high selectivity catalysis on
this chemical transformation, we took advantage of MOFs-based
heterogeneous catalysts with high selectivity to construct an
artificial stimuli-controlled molecule catalyst platform to mimic the
functions of natural enzymes, in which the immobilized active
centers (Cu ions) inside the hierarchical framework 1 and 2 were
utilized to mimic the roles of stimuli-driven dynamic control of
selectivity. The catalytic performances of artificial stimuli-
controlled molecule catalyst platform were evaluated by terminal
alkynes and AIBN. The solvent resistance properties of 1 and 2
were first tested by suspending samples in boiling dioxane,
EtOH, CH3CN, DMF, toluene, CHCl3, and DCM for 48 h. It was
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cyanation reaction, produced a mixture of vinyl isobutyronitrile
and propiolonitrile products under the same reaction conditions .
Only harsh conditions (e.g., oxygen-free, high pressure,
precious metals and toxic cyanide source) could afford the vinyl
isobutyronitrile frameworks as the unique products (yields, 45-
62%).[47]
isobutyronitrile and CN source during the reaction process.
SCXRD analysis indicated that
1 and 2 demonstrated a
hierarchical framework with the channel size of 15.35 ×11.08 Å2
and a distinctive 2D stair-stepping framework with the channels
size of 13.71 ×13.80 Å2, offering appropriate space to facilitate
the transport reactant and product molecules. Furthermore, the
anion-pillared framework of 1 with free [H3O]+ ions as cations
encapsuled in the channel also contains the [Cu6(CPCDC)2(4,4ʹ-
bpy)3]6- cages, which can act as the tunable and functionalized
platform and provide proton to stabilize isobutyronitrile radical
and react with substrates to produce the vinyl isobutyronitrile
skeletons. Whereas, the neutral framework of 2 only included
the neutral [Cu4(HCPCDC)2(4,4ʹ-bpe)2] flat, which was not able
to further prevent isobutyronitrile radical cleavage to form CN
radical to produce the propiolonitrile skeletons. In order to further
clarify the conversion process of AIBN in the artificial switchable
catalytic system of 1 and 2, experiments were conducted with 3a
to gain insight during different reaction times. The cyanide anion
was first examined by indicating paper, which reacted with picric
acid to indicate rose-red color (details in Supporting
Information).[48] As shown in Figure S6, the color of the indicator
paper was not changed with 1 as catalysts at 90 °C for 20
minutes, whereas the color of the indicator paper was changed
rose-red color with 2 as catalysts under the same reaction
conditions. The results revealed that AIBN could produce the CN
sources to generate the propiolonitrile skeletons 6a by using 2
as catalysts. Furthermore, GC-MS was used to detect the
reaction process. As shown in Figure S7, by using 1 as catalysts,
the isobutyronitrile was found in this process, which
demonstrated that the intact well-defined platform could stabilize
the isobutyronitrile to provide the vinyl isobutyronitrile skeletons
5a. However, only the acetone molecules, which were given by
oxidizing the isobutyronitrile radicals in air, were found by using
2 as catalysts under the same conditions (Figure S8). Moreover,
by adding 2.0 equiv of TEMPO, the direct cyanation reaction of
3a was completely inhibited with 1 or 2 as catalysts, displaying a
radical pathway may be involved during this procedure.
Therefore, for the above experiment results, a plausible catalytic
Table 1. Artificial switchable catalysts 1 and 2 catalyzed the direct cyanation
reactions to produce vinyl isobutyronitrile and propiolonitrile frameworks.[a]
Yield % of
Yield % of
Entry
1
Catalysts
Alkynes
5a-e[b]
6a-ef[b]
1
95, 91[c]
--
94, 90[c]
21
--
2
3
2
--
Cu(NO3)2
24
95
--
4
1
5
2
94
20
--
6
Cu(NO3)2
22
94
--
7
1
8
2
93
20
--
9
Cu(NO3)2
21
93
--
10
11
12
13
14
15
1
2
90
17
--
process for using
1 and 2 as catalysts were proposed,
Cu(NO3)2
18
85
--
respectively (Scheme S1). By using 1 as catalysts, the reaction
was started by heating AIBN to generate the isobutyronitrile
radical I, which can stabilized by the the [Cu6(CPCDC)2(4,4ʹ-
bpy)3]6- cages. Meanwhile, the arylacetylene interacted with
active sites (Cu) to form the possible intermediate (III).
Subsequently, III could undergo a protonation to produce the
product (IV). Finally, IV was given the proton to generate the
desired product 3a-e and the catalyst regenerates. In contrast,
when 2 was used as catalysts, the isobutyronitrile radical (I),
which could not have well-defined platform in 2 to stabilize these,
could be oxidized by air to give CN radical (II) and release
acetone molecules as byproduct. In the meantime, the
arylacetylene was interacted with Cu centers to form the
(phenylethynyl)copper intermediate (V). V went through a single-
electron transfer process to give VI. Finally, the elimination
reaction of VI occured to afford the desired procucts 6a-e and
regenerated catalyst 2.
1
2
82
13
Cu(NO3)2
15
[a]Reaction conditions: 3a (1.0 mmol), 4 (1.5 mmol), catalyst 1 or 2 (0.05 mmol),
CH3CN (10 mL), 90 ºC (8h). [b]Isolated yield of the product after 8h. [c]Reaction
conditions: 3a (10.0 mmol), 4 (15.0 mmol), catalyst 1 or 2 (0.5 mmol), CH3CN
(30 mL), 90 ºC (8h).
The distinct and different selectivity of the artificial switchable
catalytic system demonstrated that the direct cyanation
reactions catalyzed by artificial switchable catalysts 1 and 2
might come through a different approach. Since the active
centres in 1 and 2 are arranged in the framework regularly with
identical distances, the reversible structure transformations can
endow 1 and 2 with intact well-defined platform to stabilize the
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samples of 1 and 2 without stirring and grinding for 1 h, the
reaction system was centrifuged to remove 1 and 2 with the
mother solution continuted to react for 5 h. The results revealed
that the conversion held virtually unchanged during this time.
Therefore, these results unambiguously displayed that 1 and 2
were turly heterogeneous catalysts. Furthermore, the long-term
stability and recylability of the artificial switchable catalysts 1 and
2 were investigated. 1 and 2 were easily isolated from the
reaction system by centrifugation and remained stable after at
least ten cycles. Moreover, PXRD patterns of the isolated
samples after ten runs closely matched with that of pristine 1
and 2 and indicated no structure collapse and decomposition,
which revealed the artificial switchable catalysts 1 and 2 could
hold intact after at least ten cycles (Figure S9).
Conclusions
In summary, we report a dramatic transformation of the artificial
switchable catalytic system upon dissolving-exchange-
crystallization process. The reversible structural transformations
distinct to the naked-eye proceeded between a hierarchical
anion-pillared framework 1 including free [H3O]+ ions as guest
molecules and a distinctive 2D stair-stepping neutral framework
2. For the direct cyanation reaction of terminal alkynes and AIBN,
the reversible structural transformations qualified 1 and 2 as the
excellent and high selectivity catalysts to generate vinyl
isobutyronitrile or propiolonitrile framework as the single product,
respectively. The results revealed that the artificial switchable
catalysts 1 and 2 could provide intact well-defined platform to
stabilize the isobutyronitrile and CN source by the different
catalytic pathways. This work reveals that the reversible
structural transformations of 1 and 2 not only give an insight into
the intrinsic mechanism of the direct cyanation reaction, but also
provide a particular pathway to adjust and control artificial
switchable catalytic system to realize structure-function
relationships in MOF materials.
Figure 3. (a) Elemental mapping images of pristine 1 (0 h). (b) Elemental
mapping images of 1 after reacting for 4 h. (c) Elemental mapping images of 1
after reacting for 8 h. (d) XPS spectrum of 1 during the reaction process (0, 4,
and 8 h) (e) Elemental mapping images of pristine 2 (0 h). (f) Elemental
mapping images of 2 after reacting for 4 h. (g) Elemental mapping images of 2
after reacting for 8 h. (h) XPS spectrum of 2 during the reaction process (0, 4,
and 8 h).
In addition, to further demonstrate the reaction process,
mapping experiments were carefully executed with 3b to detect
the variations of elements in the artificial switchable catalysts 1
and 2 during the different reaction times (0, 4, and 8 h). The
mapping images distinctly revealed the distribution of C, O, N,
Cu and F elements, with the source of F element attributed to
the substrate 3b during the reaction process (Figures 3a-c).
Moreover, to observe the tuning of catalytic microenvironments
of the artificial switchable catalysts 1 and 2, XPS was used to
supervise the catalytic active sites during the different reaction
times (0, 4, and 8 h) (Figures 3e-g). The detection of the Cu
2p3/2 peak in 1 and 2 revealed that the valence of Cu retained
solely CuII in the reaction systems, which invariably held the
catalytic activity to promote the direct cyanation reaction
(Figures 3d and 3h).
Experimental Section
Materials and Physical Measurements
All reagents were commercially available without further purification. 9-(4-
carboxyphenyl)-9H-carbazole-3,6-dicarboxylic acid (H3CPCDC) was
synthesized with the procedure modified from the literature.[41] 1H NMR
was performed on the Bruker Avance-400 spectrometers. Powder X-ray
diffraction (PXRD) was executed with the Bruker D8 Advance X-ray
diffractometer with Cu Kα1. X-ray photoelectron spectroscopy (XPS)
determination was measured in an ESCALAB 250Xi-type instrument. The
Elementar Vario MICRO elemental analyzer was utilized to register C, H,
and N. The thermal stability of 1-2 was conducted by Netzsch STA 449C
thermal analyzer in air. Inductively coupled plasma-atomic emission
spectrometry (ICP-AES) analyses were conducted by a Thermo Scientic
ICP 6000 spectrometry. Bruker-ALPHA spectrophotometer was used to
test FT-IR spectra with KBr pellets in 400-4000 cm-1 region. X-ray
photoelectron spectroscopy (XPS) determination was studied by an
ESCALAB 250Xi-type instrument.
The heterogeneous natures of the artificial switchable
catalysts 1 and 2 were first examined by ICP-AES, and only
slight leaching (1: 0.81 ppm, 2: 0.78 ppm) of the reaction filtrates
were observed. Next, the filtrations experiments were carried out.
After 21 and 19% conversion of 3a in the existence of large
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10.1002/chem.201901118
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Synthesis of {(H3O)[Cu(CPCDC)(4,4ʹ-bpy)]}n (1)
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21701201, 21601212,
21671205, and 21671174), Technological Project of Henan
Province (182102210528), Thousand Talents Program of
Zhongyuan, and Young Backbone Teacher of Zhongyuan
University of Technology.
Cu(NO3)2·3H2O (0.048 g, 0.2 mmol), H3CPCDC (0.037 g, 0.1 mmol),
4,4ʹ-bipyridine (4,4ʹ-bpy, 0.015 g, 0.1 mmol), DMF (3 mL), H2O (2 mL)
and HNO3 (3 drops) was placed in a 10 mL vial. The vial was sealed and
heated at 85 °C for two days, and blue blocky crystals of 1 were obtained
with a yield of 56% (based on Cu). Anal. Calcd (%) for C31H21CuN3O7: C,
60.93 %; H, 3.46 %; N, 6.88 %. Found: C, 60.92 %; H, 3.45 %; N, 6.91 %.
IR (KBr, cm-1): 3428 (vs)，2930 (w), 1605 (vs), 1510 (m), 1473 (m), 1386
(vs), 1232 (vw), 1025 (w), 919 (w), 780 (s), 726 (vw).
Keywords: metal-organic frameworks, reversible structural
transformations, multifunctional catalysts, artificial switchable
catalysts, cyanation reaction
Synthesis of {[Cu(CPCDC)(4,4ʹ-bpe)]}n (2)
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Reversible structural transformations
Chao Huang, Gaoxiang Li, Lin Zhang,
Yingying Zhang, Liwei Mi,* and Hongwei
Hou*
took place between
a
hierarchical
and
anion-pillared framework
1
a
neutral stair-stepping framework 2,
which endowed them as artificial
Reversible
Structural
Transformations of Metal-Organic
Frameworks (MOFs) as Artificial
Switchable Catalysts for Dynamic
Control of Selectively Cyanation
Reaction
switchable
cyanation reaction to generate vinyl
isobutyronitrile or propiolonitrile
catalysts
for
direct
frameworks as the single product,
respectively.
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