Molecular Pivot-Hinge Installation to Evolve Topology in Rare-Earth Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201910717

Linker desymmetrization has been witnessed as a powerful design strategy for the discovery of highly connected metal–organic frameworks (MOFs) with unprecedented topologies. Herein, we introduce molecular pivot-hinge installation as a linker desymmetrization strategy to evolve the topology of highly connected rare-earth (RE) MOFs, where a pivot group is placed in the center of a linker similar to a hinge. By tuning the composition of pivot groups and steric hindrances of the substituents on various linker rotamers, MOFs with various topologies can be obtained. The combination of L-SO₂ with C_2v symmetry and 12-connected RE₉ clusters leads to the formation of a fascinating (4,12)-c dfs new topology. Interestingly, when replacing L-SO₂ with a tetrahedra linker L-O, the stacking behaviors of RE-organic layers switch from an eclipsed mode to a staggered stacking mode, leading to the discovery of an intriguing hjz topology. Additionally, the combination of the RE cluster and a linker [(L-(CH₃)₆)] with more bulky groups gives rise to a flu topology with a new 8-c inorganic cluster. The diversity of these RE-MOFs was further enhanced through post-synthetic installation of linkers with various functional groups. Functionalization of each linker with acidic and basic units in the mesoporous RE-based PCN-905-SO₂ allows for efficient cascade catalytic transformation within the functionalized channels.

Full text of DOI:10.1002/anie.201910717

Angewandte
Research Articles
Chemie
International Edition: DOI: 10.1002/anie.201910717
German Edition: DOI: 10.1002/ange.201910717
Metal–Organic Frameworks Very Important Paper
Molecular Pivot-Hinge Installation to Evolve Topology in Rare-Earth
Metal–Organic Frameworks
Liang Feng+, Yutong Wang+, Kai Zhang, Kun-Yu Wang, Weidong Fan, Xiaokang Wang,
Joshua A. Powell, Bingbing Guo, Fangna Dai, Liangliang Zhang, Rongming Wang,
Daofeng Sun,* and Hong-Cai Zhou*
Abstract: Linker desymmetrization has been witnessed as
a powerful design strategy for the discovery of highly
connected metal–organic frameworks (MOFs) with unprece-
dented topologies. Herein, we introduce molecular pivot-hinge
installation as a linker desymmetrization strategy to evolve the
topology of highly connected rare-earth (RE) MOFs, where
a pivot group is placed in the center of a linker similar to
a hinge. By tuning the composition of pivot groups and steric
hindrances of the substituents on various linker rotamers,
MOFs with various topologies can be obtained. The combi-
nation of L-SO2 with C2v symmetry and 12-connected RE9
clusters leads to the formation of a fascinating (4,12)-c dfs new
topology. Interestingly, when replacing L-SO2 with a tetrahedra
linker L-O, the stacking behaviors of RE-organic layers switch
from an eclipsed mode to a staggered stacking mode, leading to
the discovery of an intriguing hjz topology. Additionally, the
combination of the RE cluster and a linker [(L-(CH3)6)] with
more bulky groups gives rise to a flu topology with a new 8-c
inorganic cluster. The diversity of these RE-MOFs was further
enhanced through post-synthetic installation of linkers with
various functional groups. Functionalization of each linker
with acidic and basic units in the mesoporous RE-based PCN-
905-SO2 allows for efficient cascade catalytic transformation
within the functionalized channels.
connectivity of building organic linkers.[10,11] Therefore,
rational linker design is critical to expand the diversity and
properties of MOFs with targeted properties and applications.
Among ditopic, tritopic, tetratopic, hexatopic and octa-
topic carboxylate linkers commonly used for MOF construc-
tion, tetratopic carboxylate linkers have been of a prime
interest in recent years because of their highly tunable
geometries and shapes.[12] Both Zhouꢀs and Yaghiꢀs groups
have reported the synthesis of Zr-MOFs based on a tetratopic
linker tetrakis(4-carboxyphenyl)porphyrin (TCPP) with
a square shape (Figure 1a).[13,14] The resulting materials show
very diverse topologies, including ftw, csq, shp, scu, she, and
sqc, due to the dynamic connection between Zr6 clusters with
varying connectivities and 4-connected TCPP linkers. De-
symmetrizing the square TCPP linker into a pyrene-based
ligand with rectangle shape leads to the formation of
mesoporous NU-1000 with csq topology, similar to the
PCN-222 structure (Figure 1b).[15] The structural tunability
can be further enhanced by desymmetrization from the
rectangular linker to a trapezoidal linker, where a defective
MOF can be created with three crystallographically distinct
pockets (Figure 1d).[16,17]
Alternatively, the effects of linker geometry and flexibility
controlled by the bulkiness of functional groups can be
utilized to enhance the diversity of highly connected MOFs.
For example, by altering the positions of linker substituents,
the formation of Zr-tetratopic carboxylate MOFs with various
topologies can be controlled (Figure 1c).[18] Recently we also
reported that by utilizing more adaptable rare-earth (RE)
clusters, more diverse structures and topologies can be easily
obtained through continuous hindrance control.[19] Yet in all
the aforementioned highly connected MOF examples, the
structural diversity, flexibility and the resulting component
tunability are highly limited by the rigid ligand backbone.
Consequently, the enhancement of structural control over the
multi-level assembly of sophisticated porous MOFs with high
Introduction
Metal–organic frameworks (MOFs), as a well-developed
class of porous crystalline materials, epitomize a highly
tunable structure through judicious linker design and cluster
selection.[1–9] The formation of metal clusters with selected
connectivity and nuclearity can be observed during the
construction of MOF structures, for example, 4-, 6-, 8-, 10-,
11- and 12-connected Zr6 and Zr12 clusters can be observed in
various Zr-MOFs, however, the formation of these inorganic
clusters is highly dependent on the geometry, length and
[*] Y. Wang,[+] K. Zhang, W. Fan, X. Wang, B. Guo, Dr. F. Dai, Dr. R. Wang,
Prof. Dr. D. Sun
Prof. Dr. H.-C. Zhou
Department of Materials Science and Engineering, Texas A&M
University, College Station, Texas 77843-3003 (USA)
College of Science, School of Materials Science and Engineering,
China University of Petroleum (East China)
Qingdao, Shandong 266580 (China)
Dr. L. Zhang
Xi’an Institute of Flexible Electronics, Northwestern Polytechnical
University, Xi’an 710072 (China)
E-mail: dfsun@upc.edu.cn
L. Feng,[+] K.-Y. Wang, J. A. Powell, Prof. Dr. H.-C. Zhou
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201910717.
E-mail: zhou@chem.tamu.edu
&&&&
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 1. Illustration of the desymmetrization concept in the topology discovery of highly connected MOFs based on different linker symmetries:
a) 4-c square porphyrinic linkers are linked to 6-c Zr6 clusters into PCN-224. b) The linking of 4-c rectangle pyrene linkers with 8-connected Zr6
clusters leads to the formation of NU-1000. c) Employment of rotamers with various steric hindrances prompted the formation of a series of
MOFs with various topologies; d) Use of a trapezoidal ligand resulted in the discovery of PCN-609 reported with three different pockets for
installation; e) Introduction of molecular pivot groups (R3) and substituents (R1 and R2) into rotamers can induce the formation of a series of
novel flexible topologies. Zr, C, O and N atoms are represented by turquoise, black, red and blue, respectively, and H atoms are omitted for
clarity.
connectivities require new design concepts and guidelines.
Herein, we introduce a powerful strategy, molecular pivot-
hinge installation, to tailor topology and diversity in MOFs by
placing a pivot group in the center of a linker similar to
a hinge (Figure 1e).[20] This joint pivot provides tunable
flexibility for the two dangling hinges, allowing for the
formation of ligands with various geometries and conforma-
tions. By tuning the composition of pivot groups and steric
hindrances of the substituents on various linker rotamers,
MOFs with various topologies can be obtained. For example,
the linker L-SO2 with SO2 as a pivot group with C2v symmetry
exhibits a 104.38 dihedral angle between the two central 6-
membered rings, while the tetrahedral linker L-O with O as
a pivot group has a 119.28 dihedral angle and results in an
entirely different topology. Herein, we choose RE based
MOFs as suitable candidates for the construction of novel
topologies due to the multiple directionality of RE clusters.
The combination of L-SO2 and 12-connected RE9 clusters
leads to the formation of a new fascinating (4,12)-c dfs
topology. The isoreticular structure of PCN-905-SO2 can also
be obtained when the RE9 clusters were connected to a similar
ligand (L-CH2) with CH2 as a pivot group. When replacing L-
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
SO2 with the aforementioned tetrahedral linker L-O, the
stacking behaviors of RE-organic layers switched from an
eclipsed mode to a staggered stacking mode, leading to the
discovery of a fascinating hjz topology. In addition, the
combination of the RE cluster and a linker [(L-(CH3)6)] with
more bulky groups gives rise to a flu topology with a new 8-c
inorganic cluster. The diversity of these RE-MOFs was
further enhanced through post-synthetic installation of link-
ers with various functional groups. The open pockets in PCN-
905-SO2 enables the precise placement of functional groups
within a multivariate pore environment. The installation of
various linkers also leads to tunable torsion angles around the
pivot groups, and the dihedral angles between the central
phenyl ring and the peripheral ring. Functionalization of each
linker with acidic units and base units in the mesoporous RE-
based PCN-905-SO2 allows for efficient cascade catalytic
transformation within the functionalized channels. This work
provides novel design strategies for the systematic design and
construction of unprecedented topologies through molecular
pivot-hinge installation, which shall further serve as blue-
prints for the discovery of highly complicate porous structures
for future sophisticated applications.
Results and Discussion
Design, Synthesis and Structural Description
Herein we selected four tetratopic ligands with various
molecular pivot groups (R3) and substituents (R1 and R2) to
construct RE-MOFs that exhibit multiple unprecedented
topologies (R1 =-H, -CH3; R2 =-NH2, -CH3; R3 =-SO2-,
-CH2-, -O-). As shown in Figure 1e, the molecular pivot
groups with varying rotation degrees are introduced at the
linker center, tuning the rotation of the two dangling hinges
around the central pivot. Meanwhile, the functional groups
with various levels of steric hindrance are placed at the R1 and
R2 position, tuning the rotation of two peripheral phenyl rings
near the central one.
Solvothermal reactions between L-SO2 and Eu-
(NO3)3·6H2O in the presence of 2-fluorobenzoic acid (2-
FBA) for 24 h yielded large colorless crystals with cuboid
morphology, PCN-905-SO2 (also named UPC-905-SO2, Fig-
ure 2a). The modulator, 2-FBA, can induce the in situ
formation of polynuclear RE clusters (RE6 or RE9), which
can further be utilized for the construction of extended
MOFs.[21] The formula of PCN-905-SO2 was determined by
single-crystal X-ray diffraction and the phase purity of the
aforementioned products was accessed by powder X-ray
diffraction (PXRD) patterns (Figure S19). Additionally, un-
der the same condition, the reaction between Eu-
(NO3)3·6H2O and L-CH2 yields large cuboid crystals with
the same topology, because of the similar conformations and
geometries of L-SO2 and L-CH2 (Figure 2b).
Figure 2. Structural illustration of PCN-905-SO2 with (4, 12)-c dfs
topology. a,b) Optical images of PCN-905-SO2 (a) and PCN-905-CH2
(b) single crystals. c) The assembly of 12-c RE9 clusters and 4-c ligands
with a SO2 pivot substituent (C2v) into a (4, 12)-c dfs net. d) The
structural illustration of PCN-905-SO2. e) Linker conformations of L-
SO2 (C2v symmetry) in the confined framework of PCN-905-SO2.
f) Illustration of open pockets within the PCN-905-SO2 structure and
the large mesoporous channels formed by the assembly of RE9 clusters
and L-SO2.
PCN-905-SO2 represents a rare RE-MOF with meso-
porosity and flexibility. PCN-905-SO2 crystallized in the
hexagonal space group P6/mmm (Table S1). Crystallograph-
ically, it contains a 12-connected RE9 cluster [RE9(m3-OH)12-
(m3-O)2(O2C-)12] and a bent tetratopic linker with C2v
symmetry (Figure 2c,d). The 12-connected RE9 clusters can
be viewed as hexagonal prism nodes and the bent tetratopic
linkers can be regarded as 4-connected joint triangular nodes.
The overall structure was analyzed to be a 4,12-connected dfs
net with a point symbol of {442.624}{46}3 as determined by
&&&&
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
TOPOS 4.0 (Figure 6b,c).[22] Note that only four (4, 12)-edge
constructed from 2D layers pillared by 12-c RE9 clusters,
transitive nets (ftw, ith, shp and tam) have been previously
reported and this study uncovers a novel topology (dfs)
constructed from 12-connected clusters and 4-connected
linkers.
where the 2D layers are composed of the nonanuclear clusters
arranged in a honeycomb (hcb) lattice pattern. The hcb layers
are further intercalated by a periodic array of the 12-
connected RE9 clusters in the aea net while they are directly
pillared by the bent tetratopic linkers without the participa-
tion of secondary RE9 clusters as pillars in the dfs net
(Figure 3b). In other words, the dfs net can be viewed as an
evolution process including the deconstruction of an aea net,
pillar cluster elimination to generate a series of isolated hcb
layers and subsequent direct reassembly of these hcb layers
into the dfs net. Alternatively, the (4, 12)-c dfs net can also be
viewed as an extended version of the (4, 6)-c stp net observed
in PCN-600 with doubly linked linkers.[24]
In the structure of PCN-905-SO2, each two neighboring
RE9 clusters are parallel to each other and the assembly of
these clusters leads to the formation of two types of channels.
It contains large 1D open channels, with a diameter of 2.2 nm
(Figure 2 f). Noticeably, the linkers are forced into a confined
configuration with a 104.38 dihedral angle between the two
central phenyl rings, and a 55.88 dihedral angle between the
central phenyl ring and the two peripheral ones (Figure 2e).
Interestingly, the underlying dfs topology of PCN-905-SO2
can be viewed as an evolved network from aea-MOF-
1 (Figure 3). The aea-MOF-1 reported by Eddaoudi and co-
workers is built from 12-connected RE9 clusters and 3-
connected organic ligands (Figure 3a).[23] The aea net is
When replacing the pivot group with an -O- unit, the
linker symmetry can be altered and decreased to C1 accord-
ingly (Figure 4). In the case of PCN-905-SO2 constructed from
C2v linkers, each linker was connected to four RE9 clusters
Figure 3. Stepwise topological evolution from an aea net to a may net via deconstruction, elimination, reassembly and installation processes.
a) The formation of a may net can be viewed as an evolution process including the deconstruction of a (3, 12, 12)-c aea net, pillar cluster
elimination to generate a series of isolated hcb layers and subsequent direct reassembly of these hcb layers into the (4, 12)-c dfs net. Further
installation of linear linkers generates a (4, 14)-c may net. b) Topological transformation pathway for the evolution from aea-MOF-1 to PCN-905-
SO2, and further to PCN-905-SO2-BDC.
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 4. Structural illustration of PCN-906-O with a (4,12,12)-connected hjz topology. a) The assembly of 12-c RE9 clusters and 4-c ligands with
an O pivot substituent (C1) into a (4,12,12)-c hjz net. The structural illustration of PCN-906-O and the simplified presentation of hjz net were
presented. b) Installation of a SO2 pivot group leads to an eclipsed packing mode of hcb layers in PCN-905-SO2, while a shift of RE9 clusters was
observed due to the linker configuration change, leading to a staggered stacking mode of these hcb layers in PCN-906-O. c) Optical images of
PCN-906-O single crystals. d) Topological presentation of PCN-906-O with staggered hcb layer structures.
that are arranged in the same plane. However, when the
linker configuration changes to a tetrahedral geometry, the
arrangement of RE9 clusters and the overall topology become
more complicated. Solvothermal reactions between L-O and
Eu(NO3)3·6H2O in the presence of 2-fluorobenzoic acid (2-
FBA) for 24 h yielded large colorless crystals with turtle-shell
morphology, PCN-906-O (also named UPC-906-O, Fig-
ure 4c). However, these crystals shown only weak diffraction
intensity, indicating that the organic components were highly
disordered, although the RE metal clusters could be easily
distinguished. To illustrate the overall structure of PCN-906-
O, we constructed a structural model based on the cluster
locations and single crystal unit cell parameters (Figure S13).
The phase purity of the aforementioned products was further
accessed by PXRD patterns. PCN-906-O crystallized in the
ꢀ
triclinic space group P1. Crystallographically, it contains a 12-
connected RE9 cluster [RE9(m3-OH)12(m3-O)2(O2C-)12] as
observed in PCN-905 and a bent tetratopic linker with C1
symmetry (Figure 4a). The 12-connected RE9 clusters can be
viewed as hexagonal prism nodes, which are further con-
&&&&
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
nected by a tetrahedra organic linker. The overall structure
was analyzed to be a new (4,12,12)-connected hjz net with
a point symbol of {424.630.812}{442.624}{46}6 as determined by
TOPOS 4.0 (Figure 4d).[22] Note that the hjz net can also be
viewed as a pillared assembly of hcb layers by the tetrahedra
linker L-O. In PCN-905-SO2, the hcb layers adapt an eclipsed
mode, therefore forming a mesoporous channel. However, in
PCN-906-O, a shift of RE9 clusters was observed due to the
linker configuration change, leading to a staggered stacking
mode of these hcb layers (Figure 4b,d). This staggered mode
in PCN-906-O also reduces the pore size of the MOF. The
linker L-O in PCN-906-O exhibits a configuration with
a 119.28 dihedral angle between the two central phenyl rings,
while the dihedral angles between the central phenyl ring and
the two peripheral rings vary from 57.68 to 76.68 (Figure 4a).
Alternatively, the underlying hjz net of PCN-906-O can be
derived from a (4, 6, 6)-c net. For example, the construction of
6-connected triangular prism cluster and 4-connected linker
lead to the formation of a novel txs net with a point symbol of
{43.612}{43.63}3{49.66} as determined by TOPOS 4.0 (Fig-
ure S16). After the linker insertion onto the 6-connected
RE9 clusters, the transformation from a txs net to a hjz net can
be accessed.
Modification of phenyl ring of L-CH2 with bulky methyl
groups can generate larger dihedral angle between the two
hinges and force two peripheral phenyl rings perpendicular to
the central one. Use of the bulky L-(CH3)6 ligand resulted in
the discovery of PCN-908-(CH3)6 with flu topology built from
newly discovered 8-c inorganic clusters (Figure 5a). Single-
crystal X-ray analysis revealed that PCN-908-(CH3)6 crystal-
ꢀ
lized in the triclinic space group P1 (Figure 5b). It contains
a novel 8-connected RE6 cluster [RE6(OH)8(O2C-)9] and
a tetratopic linker with C1 symmetry (Figure 5d). Crystallo-
graphically, eight tetratopic linkers bridged four neighboring
RE6 clusters, which can be simplified into a (4, 8)-connected
net with flu topology (Figure 5d). Topologically, the 8-
connected metal clusters can be regarded as twisted cuboid
nodes while the tetratopic linkers can be viewed as pivot-
hinge nodes. The overall structure was analyzed to be a (4, 8)-
connected net with a point symbol of {412.612.84}{46}2.
The PCN-908-(CH3)6 framework here contains an unusu-
al RE polynuclear clusters [RE6(OH)8(O2C-)8], where in the
equatorial plane, only one oxygen from the carboxylate group
on the linkers was coordinated to RE metals (Figure 5b). It
should be noted that in the previously reported flu-MOFs
such as PCN-605, a regular cuboid node without equatorial
carboxylate coordination was usually observed.[18] Due to the
adaptable behaviors of RE clusters, when the linker config-
uration is altered, local missing-linker defects were found on
the RE6 cluster of PCN-908-(CH3)6, allowing for the forma-
tion of an extended ordered framework.
Figure 5. Structural illustration of PCN-908-(CH3)6 with a (4, 8)-c flu
topology. a) Optical images of PCN-908-(CH3)6 single crystals. b) The
assembly of 8-c RE6 clusters and 4-c ligands with multiple bulky methyl
groups (C1) into a (4, 8)-c flu net. c) The structural illustration of PCN-
908-(CH3)6. d) The simplified presentation of a (4, 8)-c flu network
constructed from 8-connected RE metal nodes and 4-connected linker
nodes.
Enhancing Diversity of RE-MOFs by Linker Installation
Noticeably, the addition of multiple bulky methyl sub-
stituents on the two central phenyl rings can generate much
larger dihedral angles between the two central phenyl rings
(117.28) and between the central phenyl ring and the two
peripheral rings (78.0–89.18, Figure 5c). This result is con-
sistent with our previous study on the steric effect of various
functional groups, where -CH3 groups tend to induce a larger
torsion angle than -NH2.[19]
The flexible feature of the ligand and the defective nature
of the RE cluster enable the precise placement of functional
groups within a multivariate pore environment.[25] For in-
stance, the connectivity of 12-c RE9 clusters in the structure of
PCN-905-SO2 can be further increased to eighteen, as seen in
the structure of gea-MOF-1 and PCN-918. The missing-linker
defects in the open pocket therefore enables the insertion of
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
a 2-connected linear linker and a 4-connected bent tetratopic
linker with C2v symmetry. The overall structure was found to
be a 4,14-connected may net with a point symbol of
{454.636.8}{46}3 as determined by TOPOS 4.0 (Figure 6c).[22]
[22]
It should be noted that the points of extension in the
parent nonanuclear RE9 cluster, originally linked by twelve
carboxylates to form a hexagonal prism before installation, is
increased to fourteen after insertion, with one carboxylate
moiety inserted on each of the top and bottom side of the RE9
clusters (Figure 6a). It should also be noted that the insertion
of BDC linker slightly altered the unit cell parameters, the
dihedral angle between the two central phenyl rings (from
104.38 to 103.18), and the dihedral angle between the central
phenyl ring and the two peripheral rings (from 55.78 from
54.58 or 67.58, Figure 6b).
To further access the versatility of molecular pivot-hinge
installation in RE-MOFs, PCN-905(RE) analogues with
various RE metals (i.e., Eu, Y, Yb, Tb, Dy) have been
successfully prepared under similar solvothermal reaction
conditions, as indicated by PXRD patterns (Figure S18, 22). It
should be noted that attempts to explore topologies of mixed-
linker RE-MOFs by combining linkers with varying confor-
mations were conducted, however, either pure MOFs doped
with secondary linkers or MOF mixtures were obtained after
solvothermal reactions.
Stability, Porosity and Catalytic Studies
The thermal stability of PCN-90X (RE, X = 5, 6 and 8)
were analyzed by thermogravimetric analysis (TGA, Fig-
ure S30–34). The decomposition of PCN-90X(RE) starts at
around 3508C, indicated by the weight loss of the corre-
sponding organic linkers. In addition, the chemical stability of
PCN-905(Eu)-SO2 and PCN-905(Eu)-SO2-BDC was exam-
ined by soaking the MOFs in various solvents for 24 h. PXRD
patterns indicate that the PCN-905(Eu)-SO2 and PCN-905-
(Eu)-SO2-BDC have excellent chemical stability in most
solvents, including DMF, acetone, CH3OH and CH2Cl2 as
evidenced by the well-maintained crystallinity (Figure S20,
23). PCN-905(Eu)-SO2 shown relatively poor water stability,
as observed by the broadened PXRD peaks after soaking in
water for 24 h, due to the presence of open metal sites and
lower connectivity number, while PCN-905(Eu)-SO2-BDC
exhibited improved water stability when exposed to water.
The permanent porosity of the PCN-90X(RE) series was
confirmed by N2 sorption isotherms measured at 77 K (Fig-
ure S26–S29).
The open pockets in PCN-905-SO2 enable the precise
placement of functional groups within a multivariate pore
environment. Taking advantage of the mesoporous nature
and tailored pore environment of RE-based PCN-905-SO2,
linkers containing Brønsted acidic sites were post-syntheti-
cally anchored on the coordinatively unsaturated RE sites.
This acid functionality can work in tandem with basic -NH2
units installed on the pivot hinge linker to perform efficient
cascade catalytic transformations within the functionalized
channels (Figure 7).[27–29] This system avoids the inefficiency
observed with homogeneous systems wherein the acid and
Figure 6. Structural illustration of PCN-905-SO2-BDC with (4, 14)-c
may topology. a) The hexagonal prism, representing 12-connected RE9
clusters, can be extended to a new 14-connected RE9 building block, by
inserting one carboxylate moieties on the top and bottom side of the
RE9 clusters, respectively. A 14-connected RE9 cluster with one hexag-
onal cone on the top and bottom side of the RE9 clusters, respectively,
was obtained. b) Structural illustration of PCN-905-SO2 before and
after installation of BDC linkers. The open pockets in PCN-905-SO2
enables the precise placement of linkers with various functional groups
within a multivariate pore environment. c) The illustration of may
topology and PCN-905-SO2-BDC structure.
linkers with various lengths and functionalities into the
exposed RE sites (Figure 6a).[26]
Considering the size of open pocket in the structure of
PCN-905-SO2, linear linkers including BDC and their func-
tionalized versions BDC-A (A =-SO3H, -NH2, -Br and -NO2)
were chosen as precursors (Figure 6b). Solvothermal reac-
tions between these linear linkers and PCN-905-SO2 in the
presence of DMF solvents for 24 h yielded large colorless
crystals with cuboid morphology (Figure S11). There is no
notable change on the morphology or volume after the linker
installation as observed from optical images.
The structures of these installed RE-MOFs were further
analyzed by single-crystal X-ray diffraction and the corre-
sponding phase purity of the products was accessed by PXRD
patterns (Figure S25). PCN-905-SO2 inserted with BDC,
PCN-905-SO2-BDC, crystallized in the hexagonal space
group P63/mcm (Table S1). Crystallographically, it contains
a 14-connected RE9 cluster [RE9(m3-OH)12(m3-O)2(O2C-)14],
&&&&
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
various functional groups in PCN-905-SO2 enables the precise
functionalization of the pore environment. Functionalization
of each linker with acidic units and basic units in the
mesoporous RE-based PCN-905-SO2 allows the efficient
cascade catalytic transformation of benzaldehyde dimethyl-
acetal within the functionalized channels. This work provides
novel design strategies for the systematic design and con-
struction of unprecedented topologies through molecular
pivot-hinge installation, which will further guide the discovery
of periodic porous framework structures with intrinsic
cooperativity.
Figure 7. One-pot tandem reaction of benzaldehyde dimethylacetal
and malononitrile by bifunctional mesoporous PCN-905(Eu)-SO2-SO3H
catalyst.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (NSFC Grant Nos. 21875285,
21571187), Taishan Scholar Foundation (ts201511019). The
gas adsorption–desorption studies of this research were
supported as part of the Center for Gas Separations, an
Energy Frontier Research Center funded by the U.S. Depart-
ment of Energy, Office of Science, Basic Energy Sciences
under Award Number DE-SC0001015. The PXRD character-
ization and analysis were funded by the Robert A. Welch
Foundation through a Welch Endowed Chair to H.-C.Z. (A-
0030). We gratefully acknowledge Prof. Michael OꢀKeeffe for
the help and insightful discussion.
base catalytic sites neutralize each other. By treating amino-
functionalized PCN-905-SO2 with a secondary acidic linker
BDC-SO3H, a bifunctional acid-base mesoporous catalyst
was obtained and studied for its activity and recycability in
a tandem deprotection–aldol reaction, hydrolysis of an acetal
and a Knoevenagel condensation.
The bifunctional PCN-905-SO2-SO3H quantitatively con-
verts 1 to 2 after 1 h, as determined by gas chromatography,
while product 3 is obtained with 95.5% yield. The remaining
4.5% of the final reaction mixture was identified as the
intermediate benzaldehyde 2 (Table S4, entry 2). This dem-
onstrates the high activity and selectivity of the tandem
reaction. The high activity and selectivity was well-main-
tained across three catalytic cycles (Table S4, entry 2–4). In
contrast, the use of PCN-905-SO2 without inserted acidic
ligands does not lead to the full conversion of reactants, only
achieving 54.7% conversion of 1 and a 52.1% yield of 3
(Table S4, entry 1). The partial hydrolysis of 1 that was
observed is likely promoted by the Lewis acidic RE centers,
however the low conversion should be attributed to the lack
of strong acid sites within the PCN-905-SO2. The bifunctional
catalyst can also convert reactants with various functional
groups to 3 with high conversion and yield percentage
(Table S4, entries 5 and 6).
Conflict of interest
The authors declare no conflict of interest.
Keywords: heterogeneous catalysis · linker desymmetrization ·
metal–organic frameworks · structural evolution · topology
[1] S. Yuan, L. Feng, K. Wang, J. Pang, M. Bosch, C. Lollar, Y. Sun, J.
Qin, X. Yang, P. Zhang, Q. Wang, L. Zou, Y. Zhang, L. Zhang, Y.
Fang, J. Li, H. C. Zhou, Adv. Mater. 2018, 30, 1704303.
[2] H. C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673 –
674.
Conclusion
[3] A. Kirchon, L. Feng, H. F. Drake, E. A. Joseph, H. C. Zhou,
Chem. Soc. Rev. 2018, 47, 8611 – 8638.
[4] J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T.
Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459.
[5] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M.
OꢀKeeffe, O. M. Yaghi, Science 2002, 295, 469 – 472.
[6] S. M. Cohen, Chem. Rev. 2012, 112, 970 – 1000.
[7] M. Li, D. Li, M. OꢀKeeffe, O. M. Yaghi, Chem. Rev. 2014, 114,
1343 – 1370.
[8] E. A. Dolgopolova, A. J. Brandt, O. A. Ejegbavwo, A. S. Duke,
T. D. Maddumapatabandi, R. P. Galhenage, B. W. Larson, O. G.
Reid, S. C. Ammal, A. Heyden, J. Am. Chem. Soc. 2017, 139,
5201 – 5209.
In conclusion, we report a design strategy, molecular
pivot-hinge installation, to prepare RE-MOFs with novel
topologies and enhanced diversity. Depending on the compo-
sitions of the pivot groups and the sizes of linker substituents,
various linker rotamers can be generated with a range of
geometries and conformations. The combination of -SO2- or
-CH2- functionalized linkers (L-SO2 and L-CH2) with C2v
symmetry and the 12-connected RE9 clusters led to the
formation of a new fascinating (4, 12)-c dfs topology, while the
choice of a tetrahedra linker L-O leads to the discovery of
a novel hjz topology with a staggered stacking mode. Addi-
tionally, the linked RE6 clusters with a bulkier linker [(L-
(CH3)6)] with C1 symmetry gave rise to a (4, 8)-c flu topology.
More interestingly, post-synthetic installation of linkers with
[9] L. Feng, K.-Y. Wang, G. S. Day, H.-C. Zhou, Chem. Soc. Rev.
2019, 48, 4823 – 4853.
[10] Y. Bai, Y. B. Dou, L. H. Xie, W. Rutledge, J. R. Li, H. C. Zhou,
Chem. Soc. Rev. 2016, 45, 2327 – 2367.
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
[11] H. S. Cho, J. Yang, X. Gong, Y.-B. Zhang, K. Momma, B. M.
Weckhuysen, H. Deng, J. K. Kang, O. M. Yaghi, O. Terasaki, Nat.
Chem. 2019, 11, 562 – 570.
[21] V. Guillerm, L. J. Weselinski, Y. Belmabkhout, A. J. Cairns, V.
DꢀElia, L. Wojtas, K. Adil, M. Eddaoudi, Nat. Chem. 2014, 6,
673 – 680.
[22] V. A. T. P. Blatov, Commission on Crystallographic Computing,
IUCr, 2006.
[23] D. Alezi, A. M. P. Peedikakkal, L. J. Weselinski, V. Guillerm, Y.
Belmabkhout, A. J. Cairns, Z. J. Chen, L. Wojtas, M. Eddaoudi,
J. Am. Chem. Soc. 2015, 137, 5421 – 5430.
[24] K. Wang, D. Feng, T.-F. Liu, J. Su, S. Yuan, Y.-P. Chen, M. Bosch,
X. Zou, H.-C. Zhou, J. Am. Chem. Soc. 2014, 136, 13983 – 13986.
[25] L. L. Zhang, S. Yuan, L. Feng, B. B. Guo, J. S. Qin, B. Xu, C.
Lollar, D. F. Sun, H. C. Zhou, Angew. Chem. Int. Ed. 2018, 57,
5095 – 5099; Angew. Chem. 2018, 130, 5189 – 5193.
[26] H. Jiang, J. T. Jia, A. Shkurenko, Z. J. Chen, K. Adil, Y.
Belmabkhout, L. J. Weselinski, A. H. Assen, D. X. Xue, M.
OꢀKeeffe, M. Eddaoudi, J. Am. Chem. Soc. 2018, 140, 8858 –
8867.
[12] W. G. Lu, Z. W. Wei, Z. Y. Gu, T. F. Liu, J. Park, J. Park, J. Tian,
M. W. Zhang, Q. Zhang, T. Gentle, M. Bosch, H. C. Zhou, Chem.
Soc. Rev. 2014, 43, 5561 – 5593.
[13] D. W. Feng, Z. Y. Gu, Y. P. Chen, J. Park, Z. W. Wei, Y. J. Sun, M.
Bosch, S. Yuan, H. C. Zhou, J. Am. Chem. Soc. 2014, 136, 17714 –
17717.
[14] W. Morris, B. Volosskiy, S. Demir, F. Gandara, P. L. McGrier, H.
Furukawa, D. Cascio, J. F. Stoddart, O. M. Yaghi, Inorg. Chem.
2012, 51, 6443 – 6445.
[15] P. Deria, J. E. Mondloch, E. Tylianakis, P. Ghosh, W. Bury, R. Q.
Snurr, J. T. Hupp, O. K. Farha, J. Am. Chem. Soc. 2013, 135,
16801 – 16804.
[16] J. D. Pang, S. Yuan, J. S. Qin, M. Y. Wu, C. T. Lollar, J. L. Li, N.
Huang, B. Li, P. Zhang, H. C. Zhou, J. Am. Chem. Soc. 2018, 140,
12328 – 12332.
[17] X. Zhang, B. L. Frey, Y. S. Chen, J. Zhang, J. Am. Chem. Soc.
2018, 140, 7710 – 7715.
[18] J. D. Pang, S. Yuan, J. S. Qin, C. P. Liu, C. Lollar, M. Y. Wu, D. Q.
Yuan, H. C. Zhou, M. C. Hong, J. Am. Chem. Soc. 2017, 139,
16939 – 16945.
[19] Y. T. Wang, L. Feng, W. D. Fan, K. Y. Wang, X. Wang, X. K.
Wang, K. Zhang, X. R. Zhang, F. N. Dai, D. F. Sun, H. C. Zhou,
J. Am. Chem. Soc. 2019, 141, 6967 – 6975.
[27] M. J. Climent, A. Corma, S. Iborra, M. J. Sabater, ACS Catal.
2014, 4, 870 – 891.
[28] L. Feng, Y. Wang, S. Yuan, K.-Y. Wang, J.-L. Li, G. S. Day, D.
Qiu, L. Cheng, W.-M. Chen, S. T. Madrahimov, ACS Catal. 2019,
9, 5111 – 5118.
[29] E. Merino, E. Verde-Sesto, E. M. Maya, M. Iglesias, F. Sꢁnchez,
A. Corma, Chem. Mater. 2013, 25, 981 – 988.
Manuscript received: August 21, 2019
[20] C.-K. Koo, B. Lam, S.-K. Leung, M. H.-W. Lam, W.-Y. Wong, J.
Am. Chem. Soc. 2006, 128, 16434 – 16435.
Accepted manuscript online: September 13, 2019
Version of record online: && &&
,
&&&&
&&&&
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Metal–Organic Frameworks
&
Molecular pivot-hinge installation is
introduced as a linker desymmetrization
strategy to evolve the topology of rare-
earth metal–organic frameworks (MOFs),
where a pivot group is placed in the
center of a linker similar to a hinge. By
tuning the composition of pivot groups
and steric hindrances of the substituents
on various linker rotamers, MOFs with
various topologies can be obtained.
L. Feng, Y. Wang, K. Zhang, K.-Y. Wang,
W. Fan, X. Wang, J. A. Powell, B. Guo,
F. Dai, L. Zhang, R. Wang, D. Sun,*
H.-C. Zhou*
&&&& — &&&&
Molecular Pivot-Hinge Installation to
Evolve Topology in Rare-Earth Metal–
Organic Frameworks
Angew. Chem. Int. Ed. 2019, 58, 2 – 11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!