Ligand-Directed Conformational Control over Porphyrinic Zirconium Metal-Organic Frameworks for Size-Selective Catalysis

Article abstract of DOI:10.1021/jacs.1c03960

Zirconium-based metal-organic frameworks (Zr-MOFs) have aroused enormous interest owing to their superior stability, flexible structures, and intriguing functions. Precise control over their crystalline structures, including topological structures, porosity, composition, and conformation, constitutes an important challenge to realize the tailor-made functionalization. In this work, we developed a new Zr-MOF (PCN-625) with a csq topological net, which is similar to that of the well-known PCN-222 and NU-1000. However, the significant difference lies in the conformation of porphyrin rings, which are vertical to the pore surfaces rather than in parallel. The resulting PCN-625 exhibits two types of one-dimensional channels with concrete diameters of 2.03 and 0.43 nm. Furthermore, the vertical porphyrins together with shrunken pore sizes could limit the accessibility of substrates to active centers in the framework. On the basis of the structural characteristics, PCN-625(Fe) can be utilized as an efficient heterogeneous catalyst for the size-selective [4 + 2] hetero-Diels-Alder cycloaddition reaction. Due to its high chemical stability, this catalyst can be repeatedly used over six times. This work demonstrates that Zr-MOFs can serve as tailor-made scaffolds with enhanced flexibility for target-oriented functions.

Full text of DOI:10.1021/jacs.1c03960

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Article
Ligand-Directed Conformational Control over Porphyrinic
Zirconium Metal−Organic Frameworks for Size-Selective Catalysis
Liting Yang,# Peiyu Cai,# Liangliang Zhang,# Xiaoyi Xu, Andrey A. Yakovenko, Qi Wang, Jiandong Pang,
Shuai Yuan, Xiaodong Zou, Ning Huang,* Zhehao Huang,* and Hong-Cai Zhou*
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ABSTRACT: Zirconium-based metal−organic frameworks (Zr-
MOFs) have aroused enormous interest owing to their superior
stability, flexible structures, and intriguing functions. Precise control
over their crystalline structures, including topological structures,
porosity, composition, and conformation, constitutes an important
challenge to realize the tailor-made functionalization. In this work, we
developed a new Zr-MOF (PCN-625) with a csq topological net, which
is similar to that of the well-known PCN-222 and NU-1000. However,
the significant difference lies in the conformation of porphyrin rings,
which are vertical to the pore surfaces rather than in parallel. The
resulting PCN-625 exhibits two types of one-dimensional channels with
concrete diameters of 2.03 and 0.43 nm. Furthermore, the vertical
porphyrins together with shrunken pore sizes could limit the
accessibility of substrates to active centers in the framework. On the
basis of the structural characteristics, PCN-625(Fe) can be utilized as an efficient heterogeneous catalyst for the size-selective [4 + 2]
hetero-Diels−Alder cycloaddition reaction. Due to its high chemical stability, this catalyst can be repeatedly used over six times. This
work demonstrates that Zr-MOFs can serve as tailor-made scaffolds with enhanced flexibility for target-oriented functions.
MOFs with different topologies, such as csq, fcu, ftw, reo, scu,
and spn.37,38 Consequently, many kinds of Zr-MOFs have
been developed, including UiO-66,39 PCN-222,40 NU-1000,41
PCN-223,42 and PCN-224,43 which possess various pore
geometries and dimensions. The 8-connected PCN-222 and
NU-1000 exhibit remarkable hydrothermal and chemical
robustness in comparison with other MOFs. In the two
MOFs, the Zr6 nodes are connected with tetracarboxylate
ligands, offering four coordination sites on each node as
terminal and bridging hydroxyl units. Moreover, the large sizes
of porphyrin- and pyrene-based tetracarboxylate ligands cause
a long distance (>1.0 nm) to exist between adjacent Zr6
clusters, permitting the insertion or grafting of small guest
species into their porous skeletons. Consequently, three types
of distinct pores are generated within the csq-net framework.
The trigonal and hexagonal channels are formed along the a
axis, while the rhombic channels are arrayed along the c axis.
INTRODUCTION
■
Over the past decades, metal−organic frameworks (MOFs)
have emerged as new types of crystalline materials, which allow
the assembly of different organic ligands with metal ions or
metal clusters by coordination bonds.1−3 On the basis of
reticular chemistry, both organic ligands and metal species can
be considered as multitopic building blocks, which can be
judiciously selected and integrated into MOFs to tune their
properties for various purposes.4 Due to their periodically
ordered and atomically accurate structures, excellent porosity,
flexible building blocks, and high tunability of properties, such
as topologies, pore sizes, particle sizes, and acidity or
alkalinity,5 MOFs have been widely exploited for a large
number of applications, including light emission,6−8 gas
adsorption,9−11 gas separation,12−14 heterogeneous cataly-
sis,15−26 drug delivery,27−29 proton conduction,30−33 and
optoelectronics.34−36
Among the reported MOF materials, zirconium-based
MOFs (Zr-MOFs) represent one class of specific examples
with relatively high stability and structural flexibility.
Particularly, the connectivity of the [Zr6O4(OH)8(COO)8]
(Zr6) cluster can be well controlled through regulating the
synthetic conditions. Up to now, five types of Zr6 nodes with
different connectivities have been reported, namely 12-, 10-, 8-,
6-, and 4-connected, leading to the generation of numerous Zr-
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Figure 1. Schematic diagrams for the construction of PCN-625 and PCN-222. (a) Construction of PCN-625 using an 8-connected Zr6 cluster and
rectangular, 4-connected BBCPPP ligand. (b) Synthetic route for the construction of PCN-625 under solvothermal conditions. (c) The csq
topology diagram of PCN-222. (d) Crystal structure of PCN-222 viewed along the c axis.
Recently, the Zr6 node has been broadly utilized as a scaffold
for the construction of MTV-MOFs on the basis of its tunable
connectivity and remarkable stability.44 As a robust and
universal platform, Zr-MOFs enable the integration of metal
ions, metal clusters, terminal ligands, and multicarboxylate
linkers into the coordinatively unsaturated Zr6 clusters. As a
result, a series of more complex Zr-MOFs were developed on
the basis of a postsynthetic retrosynthesis strategy, which
exhibited plenty of opportunities to build potential architec-
tures that have been impossible or difficult to implement.
These special advantages give Zr-MOFs additional properties,
further widening their application range.45 Although many Zr-
MOFs with the csq topology have been developed, all of them
possess a similar pore environment, in which the cores of
ligands are parallel to the channel walls. From the perspective
of architectural design, it is of great interest to create different
pore environments to accommodate specific guest species or
satisfy new requirements. In this work, we developed a new
type of porphyrin-based Zr-MOF with porphyrin rings vertical
to the pore walls under the premise of maintaining the csq
topology. Two kinds of isostructural MOFs with porphyrins
were synthesized and unambiguously characterized. Due to the
decreased pore sizes and increasing steric hindrance, the
resulting PCN-625(Fe) exhibited highly size selective and
efficient catalysis toward the [4 + 2] hetero-Diels−Alder
cycloaddition of aldehydes with dienes. By a combination of
the advantages of superior stability and high crystallinity, PCN-
625(Fe) can be recycled over six times without significant loss
of its activity.
EXPERIMENTAL SECTION
■
Materials. All of the chemicals were directly used after purchase
from reagent companies. 3,5-Dibromobenzaldehyde, 4-carboethox-
ybenzeneboronic acid, pyrrole, anhydrous indium(III) chloride,
trifluoroacetic acid (TFA), dienes, and boron trifluoride diethyl
etherate (BF3·Et2O) were purchased from Aldrich Chemical Co.
Tetrakis(triphenylphosphine)palladium, potassium carbonate, and
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) were purchased
from Ark Pharm, Inc. 1,4-Dioxane, zirconium chloride, and N,N-
dimethylformamide (DMF) were purchased from Tokyo Chemical
Industry Co., Ltd. (TCI). Iron(III) meso-tetraphenylporphine
chloride (FeTPPCl) and aldehydes were purchased from Adamas.
All of the air-sensitive operations were conducted under the
protection of nitrogen on a standard Schlenk device.
Characterizations. Synchrotron powder X-ray diffraction was
carried out on the 17-BM beamline at the Advanced Photon Source
(λ = 0.45236 Å), Argonne National Laboratory, Argonne, IL, US.
Powder X-ray diffraction (PXRD) measurements were performed on
a Bruker D8-Advance X-ray diffraction system equipped with a Cu
sealed tube (λ = 1.54178 Å). Proton nuclear magnetic resonance (1H
NMR) analysis was conducted on a Mercury 400 NMR spectrometer.
Nitrogen sorption isotherms were collected on a Micromeritics ASAP
2020 instrument at 77 K. Thermogravimetric analysis (TGA) data
were collected on a Shimadzu TGA-50 thermogravimetric analyzer at
a heating rate of 10 °C min−1 under a nitrogen atmosphere. Elemental
analysis was carried out on a vario EL cube Elementar. The surface
morphologies of MOF samples were recorded with an FEI Quanta
600 scanning electron microscope (SEM). The conversion and yield
values were determined on a Shimadzu QP2100 plus gas chromato-
graph−mass spectrometer (GC-MS). Inductively coupled plasma-
optical emission spectroscopic (ICP-OES) data were recorded using a
Thermo iCAP6300 spectrometer.
Synthesis of Ligands. The synthesis routes and characterization
results of bis[3,5-bis(4-carboxyphenyl)phenyl]porphyrin (BBCPPP)
B
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and {bis[3,5-bis(4-carboxyphenyl)phenyl]porphyrinato} iron (Fe-
BBCPPP) are described in the Supporting Information.
and PCN-625(Fe) exhibited a morphology of regular
hexagonal prisms, which is in good accord with those of csq-
net MOFs (Figure S1). These results indicated that PCN-625
exhibited a topological and crystalline structure similar to that
of PCN-222 (Figure 1c,d). Elemental analyses of PCN-625
and PCN-625(Fe) agreed closely with theoretical values
(Table S1). Due to the small crystal size of PCN-625, cRED
collection was employed to resolve its crystal structure. Figure
2 shows the reconstructed 3D reciprocal lattice from the cRED
Synthesis of PCN-625. ZrCl4 (20 mg, 0.086 mmol) and BBCPPP
(10 mg, 0.011 mmol) were added to 2 mL of DMF. Then TFA (100
μL) was added to the solution as an acid modulator. The mixture was
ultrasonically dissolved in a 5 mL high-pressure vessel, which was
sealed and heated at 120 °C for 72 h. After the vial was slowly cooled
to room temperature, purple crystallites were collected by filtration
and rinsed with DMF and acetone. The obtained product was dried
under vacuum, affording a yield of 77%.
Transmission Electron Microscopic (TEM) Analysis. The
MOF samples for TEM characterization were dispersed in acetone.
A droplet of the suspension was transferred onto a carbon-coated
copper grid. The observation was carried out on a JEOL JEM2100
microscope, operated at 200 kV (Cs 1.0 mm, point resolution 0.23
nm). The images were recorded with a Gatan Orius 833 CCD camera
(resolution 2048 × 2048 pixels, pixel size 7.4 μm) under low-dose
conditions. Electron diffraction patterns were recorded with a
Timepix QTPX-262k pixel detector (512 × 512 pixels, pixel size 55
μm, Amsterdam Sci. Ins.).
Continuous Rotation Electron Diffraction (cRED) Collection.
The data were collected using the software Instamatic.46−48 A single-
tilt tomography holder was used for the data collection, which could
tilt from −70° to +70° in the TEM. The aperture used for cRED data
collection was about 1.0 μm in diameter. The speed of the goniometer
tilt was 0.45° s−1, and the exposure time was 0.5 s per frame. Data
were collected within 4 min to minimize the beam damage and to
maximize the data quality. The covered tilt angle was 103.6°.
Gas Sorption Measurement. Before the nitrogen sorption
measurement, the freshly prepared PCN-625 (∼100 mg) samples
were first washed with DMF (10 mL × 3) and acetone (10 mL × 3),
respectively. After that, the PCN-625 samples were soaked in
anhydrous acetone (20 mL) for 24 h to completely exchange the
nonvolatile DMF and remove residual metal ions or ligands. The
samples were collected by centrifuging and activated by drying under
vacuum at 100 °C prior to the nitrogen sorption analysis. Finally, the
Zr-MOF samples were subjected to sorption measurement at 77 K.
Heterogeneous Catalysis. Activated PCN-625(Fe) (0.5 μmol)
and AgBF4 (1.0 μmol) were added to 2 mL of dry toluene in a 25 mL
two-necked flask equipped with a Graham condenser. The mixture
was stirred at room temperature for 5 h under a nitrogen atmosphere.
After that, a toluene solution (4 mL) containing aldehydes (0.5
mmol) and diene (2.0 mmol) was added under a nitrogen
atmosphere. The mixture was sealed and stirred at 80 °C. After
filtration, the combined filtrate was concentrated by rotary
evaporation and separated by column chromatography. The pure
products were confirmed by NMR spectrometry.
Figure 2. 2D slice cuts from the reconstructed 3D reciprocal lattice of
PCN-625 showing the (a) 0kl, (b) h0l, and (c) hhl planes. (d)
Projection of the reconstructed 3D reciprocal lattice along [001]. The
inset shows the crystal from which the cRED data were collected.
data, revealing that PCN-625 has a primitive unit cell with
parameters of a = 39.5 Å, b = 38.3 Å, c = 24.1 Å, α = 90.8°, β =
89.8°, and γ = 119.2°. As the lattice parameters a and b are
very close and γ is around 120°, the possible crystal system
could be hexagonal.
The two-dimensional (2D) slice cuts of the 3D reciprocal
lattice at the 0kl (Figure 2a), h0l (Figure 2b), and hhl (Figure
2c) planes exhibited similar diffraction patterns, which
confirms the hexagonal unit cell. No special reflection
conditions can be deduced. Therefore, there are several
Recycling of Catalyst. After each cycle, the PCN-625(Fe)
catalyst was collected by filtration and thoroughly rinsed with
tetrahydrofuran (10 mL × 3). Then PCN-625(Fe) was soaked in 20
mL of DMF overnight to remove any residual substrates or products
trapped in the framework. Finally, PCN-625(Fe) was filtered, and
washed with acetone (10 mL × 3), and dried under vacuum for 2 h
for the use in the next cycle. All of the cycling experiments were
performed under a nitrogen atmosphere in toluene with 2 mol %
AgBF4 as additive.
possible space groups for PCN-625: P6 (No. 168), P6
174), P6/m (No. 175), P622 (No. 177), P6mm (No. 183),
P62m (No. 189), P6m2 (No. 187), and P6/mmm (No. 191).
̅
(No.
̅
̅
The space group P6/mmm with the highest symmetry was
chosen for further structural determination on the basis of the
projection of the reconstructed 3D reciprocal lattice along
[001] (Figure 2d).
Due to the low crystallinity of the crystals, the resolution of
the data set is low (∼3.3 Å). A structure model was built
according to the chemical and structural information (unit cell
and space group). The optimization of the geometry was
applied by using a pseudopotential plane-wave method within
the DFT framework by the Accelrys Materials Studio package.
After the creation of the initial structure, the geometry was
quickly optimized in the FORCITE module with a Dreiding
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. In this
work, we developed a new porphyrin-based Zr-MOF, denoted
PCN-625, which was constructed with BBCPPP and Zr6
clusters as building blocks under typical solvothermal
conditions in the presence of TFA as a modulator (Figure
1a,b). The iron porphyrin ligand Fe-BBCPPP was also
employed for the construction of PCN-625(Fe) under the
same conditions. Similarly to other porphyrin-based Zr-MOFs,
PCN-625 was obtained as deep purple crystals after a 72 h
reaction. As shown in SEM images, the crystals of PCN-625
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force field. The structure model of PCN-625 was further
refined against synchrotron powder X-ray diffraction (Figure
3). High-resolution PXRD data of PCN-625 was collected at
Figure 4. (a) Representative crystal structure of PCN-625 viewed
along the c axis. (b) The crystal unit cell of PCN-625 viewed along
the c axis. (c) The crystal unit cell of PCN-625 viewed along the b
axis. The longer distance between adjacent porphyrin rings is about
10.8 Å.
size along the c axis was calculated as 2.1 nm (Figure S4),
which is much smaller than those of PCN-222 and NU-1000
on account of the vertical porphyrin rings.35,36 PCN-625(Fe)
showed a BET surface area of up to 2600 m2 g−1 (Figure S3)
and the same mesopore size as that of PCN-625 (Figure S5).
We further investigated the chemical stability of PCN-625
under different conditions, including water, boiling water,
hexane, acetone, methanol, hydrochloric acid (6 M),
concentrated hydrochloric acid, and sodium hydroxide (2
M) aqueous solutions. After soaked in these solvents for 72 h,
the intensity of PCN-625 remained almost unchanged except
in base solution (Figure S6). It was found that the PXRD
pattern of base-soaked PCN-625 largely decreased, indicating
decomposition under alkaline conditions. In addition, the
residual mass percentage of PCN-625 was also evaluated
(Figure S7). We found that the residual mass percentage of the
soaked MOF samples in water, boiling water, hexane, and
acetone was close to 100%. However, in methanol, hydro-
chloric acid (6 M), and concentrated hydrochloric acid, the
values were determined to be 97%, 96%, and 95%, respectively.
This result demonstrated that a small amount of PCN-625
decomposed after treatment in methanol, hydrochloric acid (6
M), and concentrated hydrochloric acid. The excellent
framework stability of PCN-625 was also confirmed by N2
sorption isotherms after the aforementioned treatment (Figure
S8). All of the MOF samples maintained their original BET
surface areas without significant loss. TGA measurement
results indicated that PCN-625 is thermally stable up to 360
°C (Figure S9). Such high chemical stability is comparable
with those of other Zr-MOFs.
Heterogeneous Catalysis in [4 + 2] Hetero-Diels−
Alder Cycloaddition Reaction. By a combination of the
advantages of high crystallinity, rich porosity, remarkable
stability, and specific conformation, PCN-625(Fe) is consid-
ered to be an excellent candidate for selective heterogeneous
catalysis. (1) The periodically ordered metalloporphyrin offers
an ideal platform with atomically distributed catalytic sites,
which is desirable for the development of high-performance
catalysts. (2) The uniformly aligned mesoporous channels
facilitate high-rate mass transfer, leading to an enhanced
catalytic efficiency. (3) The exceptional chemical stability over
a wide range of pH enables its feasibility and recyclability
under substandard conditions. (4) The vertical porphyrin rings
Figure 3. Rietveld refinement of PXRD patterns for PCN-625 (red
curve, simulated; black curve, experimental; blue curve, difference
profile; green bars, peak positions).
room temperature at 298 K (λ = 0.45236 Å). Rietveld
refinement was performed using TOPAS Academic V5.1. The
background of PCN-625 was fitted well with an 32nd-order
Chebychev polynomial. The refinement was conducted using a
Pearson VII type peak profile function, followed by the
refinement of unit cells and zero shift. The Zr−O distances
were soft-restrained to 2.20 Å. The guest species in the pores
could not be located due to their partial occupancies and low
symmetry. Instead, five oxygen atoms were added at random
positions inside the pores to compensate for the contributions
of the guest species, and refined subsequently. Finally, the R
value was converged to Rp = 0. 0592, Rwp = 0. 0951, Rexp = 0.
0730, and GOF = 1.302. The crystallographic details are
shown in Table S2. PCN-625(Fe) exhibited almost the same
PXRD patterns as those of PCN-625 (Figure S2), revealing
they have the same crystal structures.
As shown in Figure 4a, each Zr6 cluster links to eight
BBCPPP ligands, while each BBCPPP ligand links to four Zr6
clusters. Furthermore, the remaining terminal −OH groups on
Zr centers supply the possibility to introduce additional
functionalities. The resultant MOF exhibits a Kagome csq 3D
net with two sets of 1D channels running along the c axis. One
channel is mesoporous (20.3 Å), and the other is ultra-
microporous (4.3 Å). Four porphyrins and two Zr6 clusters
form a nanocage (Figure 4b,c). The longer distance between
adjacent porphyrin rings is about 10.8 Å, and the shorter
distance is about 3.6 Å. PCN-625(Fe) displayed the same
PXRD pattern as PCN-625, indicating both possess an
identical crystalline structure except for the center metals.
Porosity and Chemical Stability. Moreover, PCN-625
exhibited high porosity and excellent chemical stability. The
Brunauer−Emmett−Teller (BET) surface area of PCN-625
was determined to be 2658 m2 g−1 (Figure S3), which is close
to those of PCN-222 and NU-1000. In addition, the mesopore
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could largely limit the access of substrates to catalytic metal
centers, leading to a size- or shape-dependent catalysis.
Under the optimized conditions, we further expanded the
substrate scope to other aldehydes and dienes with different
dimensions (Tables S5 and S6). As shown in Table 1, when
We investigated the catalytic performance of PCN-625(Fe)
for the [4 + 2] hetero-Diels−Alder cycloaddition using
aldehydes and dienes as substrates.49 This reaction is broadly
utilized for the synthesis of pyrans, which constitute an
important class of compounds involved in the field of fine
chemicals and biological pharmaceuticals.50 As was previously
reported, the cationic Fe(III) porphyrins can efficiently
activate electron-poor aldehydes, facilitating the chemo-
selective cycloaddition reaction with less reactive dienes. It
was found that some kinds of weakly coordinating counter-
Table 1. Catalytic Performance of PCN-625(Fe) in the [4 +
2] Hetero-Diels−Alder Cycloaddition Reaction Using
a
Dienes and Aldehydes under Optimized Conditions
−
−
anions, such as SbF6 and BF4 , were necessary for high-
performance catalysis.51 The valence of iron in PCN-625(Fe)
was determined as +3 according to the XPS profile (Figure
S10), which exhibited two characteristic signals at 712.6 and
725.5 eV for Fe 2p3/2 or Fe 2p1/2 peaks, respectively. To
accomplish this goal, an in situ anion metathesis using AgSbF6
−
−
and AgBF4 to displace the Cl− with SbF6 and BF4 could be
implemented in the catalysis process, generating the weakly
coordinating PCN-625(Fe-SbF6) and PCN-625(Fe-BF4),
respectively.
To screen the optimum catalytic conditions, we employed
benzaldehyde (1c) and 2,3-dimethyl-1,3-butadiene (2a) as
model compounds in the presence of PCN-625(Fe) as the
catalyst (Table S3). Similarly to the reported homogeneous
catalytic system,46 the combination of toluene as the solvent, 1
mol % of PCN-625(Fe) as the catalyst, 2 mol % AgBF4 as the
additive, 80 °C, and 8 h afforded the best catalytic
performance. With other solvents, including tetrahydrofuran,
acetonitrile, and 1,4-dioxane, the yield was quite inferior
(Table S3). Upon the screening of counteranions, the yield
reached 99% with AgBF4 as the additive. Other iron catalysts,
such as FeCl3, FeCl2, FeF3, FeCl3·2bpy, and FeCl3·2tmeda
species, as well as AgSbF6 led to trace or lower yields. The
reaction temperature, time, and amount of catalyst can make a
difference in the catalytic performance (Table S3). To exclude
other interference factors, we conducted three other control
experiments with free-base PCN-625, a Zr6 cluster, and NaBF4
additive as catalysts for the selected model reaction. However,
none of the species worked for the cycloaddition reaction.
Moreover, the catalytic process would immediately terminate
upon removal of PCN-625(Fe) by filtration, indicating that no
homogeneous catalytic sites exist in the solution. To
investigate the slight difference between conversion and
yield, the used PCN-625(Fe) was hydrolyzed using a
concentrated NaOH solution (6 M) after one catalytic cycle.
A small percentage (2−3%) of products was detected in the
digested solution, indicating that small portions of the products
were probably trapped in the porous skeleton of PCN-
625(Fe). In addition, approximately 1−2% of unknown
byproducts were also detected in the reaction. Therefore, the
difference between the conversion and yield can be attributed
to the combined contribution of the porous texture of PCN-
625(Fe) and side reactions. The enhanced catalytic perform-
ance can be attributed to the following two reasons: (1) the
electron-withdrawing carbonate groups could decrease the
electron density on active iron sites, which was confirmed by
the DFT calculations (Table S4); (2) the Zr6 cluster is also an
electron-deficient unit, which could further reduce the
peripheral electron density around the active sites and thus
improve the activity.
a
Reaction conditions: PCN-625(Fe) catalyst (1 mol %), aldehydes
(0.5 mmol), and dienes (2.0 mmol) in 4.0 mL toluene at 80 °C.
aliphatic aldehydes with small dimensions were employed as
substrates, such as acetaldehyde (1a; 6.2 × 3.6 Å2) and
cyclopropanecarboxaldehyde (1b; 6.1 × 3.7 Å2), which can
diffuse freely to approach the active sites, satisfactory yields
were achieved (90% and 92% after 5 h, respectively). In
contrast, the yield for the transformation of 2-naphthaldehyde
(1d; 11.2 × 7.1 Å2) was calculated to be 56% after 5 h (Table
S7). A complete conversion with a yield of 92% was realized
after 24 h under the same conditions (Table S7). A yield of
86% was achieved using 1-naphthaldehyde (1e; 10.2 × 7.6 Å2)
and 2a as substrates. Notably, very low yields were observed
for the bulky 4-phenoxybenzaldehyde (1f; 14.7 × 7.1 Å2) and
1-pyrenecarboxaldehyde (1g; 13.3 × 7.3 Å2) after 24 h. To
evaluate the reactivity of these aldehydes, FeTPPCl was
employed as a homogeneous catalyst under the same
conditions. It was found that most of the aldehydes (1a−f)
exhibited similar reactivities in the homogeneous system
(Table S8). However, the reaction with 1g afforded low
conversion and yield values of 23% and 16%, respectively,
which could be attributed to its low solubility in toluene.
Therefore, the low conversion of 1f suggested that the 10.8 Å
nanopore was not wide enough to accommodate and then
activate its bulky molecule, while the low conversion of 1g
could be ascribed to the combined contribution of its large
molecule size and low solubility in the heterogeneous system.
To investigate the electronic effects of substitutes on dienes, 2-
methyl-1,3-butadiene (2b; 6.8 × 3.6 Å2) and 2-phenyl-1,3-
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butadiene (2c; 11.5 × 7.4 Å2) were employed to react with 1c,
furnishing cycloadducts 3cb,cc in 94% and 83% yields,
respectively. In comparison with 2a−c having less electron-
donating methyl and phenyl groups, which seems to make no
difference to the reactivity of dienes. Furthermore, 1-(1-
methylene-2-propenyl)-4-phenylbenzene (2d; 15.2 × 10.6 Å2)
and 2,3-dibenzyl-1,3-butadiene (2e; 16.9 × 8.5 Å2) were
employed for the synthesis of 3cd,ce. However, no product
was detected owing to their enormous molecule sizes.
and dispersion interactions can facilitate the reaction in MOF
catalysis systems.
Recyclability. The robust skeleton of PCN-625(Fe)
enables its repeated use in the cycloaddition reaction. We
investigated its recyclability using the model reaction with 1c
and 2a under similar conditions (Table S12). It was found that
the PCN-625(Fe) catalyst can be recycled up to six times with
only a modest loss of its reactivity (Figure S12). The catalyst
can be simply separated by filtration and recovered by rinsing
and soaking with some solvents, such as toluene, acetone, and
tetrahydrofuran. The leaching amount of active iron species
was further analyzed using ICP-OES measurements (Table
S5). Although the amount of loss slowly increases along with
the cycle number, only 3% of the iron was leached out after six
cycles (Table S5). Interestingly, a significant loss can be
detected after three runs. Furthermore, the influence of
repeated utilization on the crystallinity and porosity of PCN-
625(Fe) was evaluated. The PXRD measurement revealed that
the catalyst could retain its high crystallinity over the whole six
runs without a significant loss of intensity (Figure S13).
However, its surface area gradually decreased after the third
cycle, which is highly consistent with the leaching results
(Figure S14).
The catalytic activities of PCN-625(Fe-BF4), PCN-625, Fe-
BBCPPP, AgBF4, and AgSF6 were evaluated using 1c and 2a as
substrates under the optimized conditions (Table S9). PCN-
625(Fe-BF4) exhibited outstanding performance with a yield of
99%, which is the same as that of the in situ PCN-625(Fe)
system. However, other catalysts, including PCN-625, Fe-
BBCPPP, AgBF4, and AgSF6, were proved inactive, indicating
each one cannot work alone for the cycloaddition reaction.
Similarly to PCN-625(Fe), FeTPPCl can work effectively in
the presence of AgBF4 as an additive, suggesting that the
unsaturated irons serve as active centers in the reaction.
To disclose the advantage of the specific conformation in
PCN-625(Fe), control experiments with PCN-222(Fe)40 and
PCN-223(Fe)42 as catalysts were conducted with 1a,f as
aldehydes (Table S10), which have a large difference in size.
PCN-222(Fe) exhibited almost the same catalytic performance
toward 1a,f, while PCN-625(Fe) and PCN-223(Fe) showed
much higher catalytic efficiency toward 1a than toward 1f.
Moreover, additional control experiments with a mixture of 1a
and 1f (molar ratio 1:1) were also conducted (Table S10).
Interestingly, for the PCN-625(Fe) catalytic system, almost all
of 1a was consumed while only a small portion (<2%) of 1f
was involved. In contrast, PCN-222(Fe) only exhibited a
medium efficiency for the transformation of 1a and 1f with
conversion values of 58% and 53%, respectively. Thus, the size-
selective advantage of PCN-625(Fe) can be attributed to the
conformation change. Unlike PCN-222, which has active metal
sites totally exposed to the 3.6 nm mesopores and can
accommodate a broad range of substrates, PCN-625(Fe)
exhibited an obvious catalytic selectivity.
CONCLUSION
■
In summary, we have rationally designed and synthesized a
new type of porphyrin-based Zr-MOF material, in which the
porphyrin ring adopted a vertical configuration to the pore
surface. This specific configuration leads to the generation of
shrunken cavities and channels, which can act as a nanoreactor
for catalysis. In addition, this Zr-MOF exhibits high porosity
and remarkable robustness under various conditions. In
combination with the advantages of high crystallinity, rich
porosity, distinct configuration, and excellent stability, PCN-
625(Fe) displayed highly efficient size-selective catalysis
toward the [4 + 2] hetero-Diels−Alder cycloaddition reaction
with aldehydes and dienes as substrates. Importantly, the
development of this particular Zr-MOF platform constitutes a
further step forward tailor-made synthesis and functionaliza-
tion.
The size-selective catalysis for [4 + 2] hetero-Diels−Alder
cycloaddition was investigated using PCN-625(Fe), due to the
specific conformation. To evaluate the catalytic performance of
PCN-625(Fe), we compared its activity with those of other
reported catalysts for the cycloaddition between 1c and 2a
(Table S11). It was found that PCN-625(Fe) exhibited a
performance superior to those of the reported UNLPF-16-
FeIII,52 FePp−H-HCP,53 TrBF4,54 [Fe(TPP)]BF4,51,55 Sc-
(OPf)3,56 and AlCl3,57 which required higher amounts of
catalysts or longer reaction time but afforded lower yields.
Meanwhile, due to their similar skeletal structures, its catalytic
performance was almost the same as that of PCN-223(Fe).42
The reaction mechanism in this study was conjectured to be
by five-coordinate and five-coordinate routes (Figure S11),
which was proposed by Chung for the Fe-catalyzed hetero-
Diels−Alder reaction. A concerted asynchronous process with
the quartet and sextet states occurred in the two-mode
reaction. In the five-coordinate mode, one of the coordinated
aldehydes in an axial position dissociated with the formation of
pentacoordinated iron species. The unsaturated iron would
facilitate the chemospecific cycloaddition between diene and
the inert CO bond. However, the dienes could directly react
with aldehydes in the saturated iron complex in the six-
coordinate route. In addition, an oriented external electric field
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c03960.
■
sı
Detailed synthesis procedures, characterization data for
new compounds, and materials (PDF)
Accession Codes
CCDC 2087538 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Ning Huang − MOE Key Laboratory of Macromolecular
Synthesis and Functionalization, State Key Laboratory of
Silicon Materials, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China; Research Center for Intelligent
F
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pubs.acs.org/JACS
Article
Sensing, Zhejiang Lab, Hangzhou 311100, People’s Republic
of China; orcid.org/0000-0002-7021-8705;
Email: nhuang@zju.edu.cn
Zhehao Huang − Berzelii Centre EXSELENT on Porous
Materials, Department of Materials and Environmental
Chemistry, Stockholm University, 10691 Stockholm, Sweden;
orcid.org/0000-0002-4575-7870;
Research Fellowship under Grant No. DGE: 1252521. Use of
the Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357.
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Liting Yang − MOE Key Laboratory of Macromolecular
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Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China
Peiyu Cai − Department of Chemistry, Texas A&M
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Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c03960
Author Contributions
#L.Y., P.C., and L.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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N.H. acknowledges support by the research start-up fund of
Zhejiang University. Z.H. and X.Z. acknowledge support by
the Swedish Research Council (VR, 2016-04625, 2017-04321)
and the CATSS project from the Knut and Alice Wallenberg
Foundation (KAW, 2016.0072). H.-C.Z. acknowledges sup-
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Office of Basic Energy Sciences (DE-SC0001015), the Robert
A. Welch Foundation through a Welch Endowed Chair to H.-
C.Z. (A-0030), and the National Science Foundation Graduate
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