Topology Exploration in Highly Connected Rare-Earth Metal-Organic Frameworks via Continuous Hindrance Control

Article abstract of DOI:10.1021/jacs.9b00122

The structural diversity of highly connected metal-organic frameworks (MOFs) has long been limited due to the scarcity of highly connected metal clusters and the corresponding available topology. Herein, we deliberately chose a series of tritopic linkers

Full text of DOI:10.1021/jacs.9b00122

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Article
Topology Exploration in Highly Connected Rare-Earth Metal–
Organic Frameworks via Continuous Hindrance Control
Yutong Wang, Liang Feng, Weidong Fan, Kun-Yu Wang, Xia Wang, Xiaokang
Wang, Kai Zhang, Xiurong Zhang, Fangna Dai, Daofeng Sun, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Apr 2019
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Topology Exploration in Highly Connected Rare-Earth Metal–
Organic Frameworks via Continuous Hindrance Control
Yutong Wang†,‡, Liang Feng§,‡, Weidong Fan†, Kun-Yu Wang§, Xia Wang†, Xiaokang Wang†, Kai
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Zhang†, Xiurong Zhang†, Fangna Dai†, Daofeng Sun*†, and Hong-Cai Zhou*§#
† School of Materials Science and Engineering, College of Science, China University of Petroleum (East China), Qingdao,
Shandong 266580, China
§ Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
# Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003, United
States
ABSTRACT: The structural diversity of highly connected metal-organic frameworks (MOFs) has long been limited due to the
scarcity of highly connected metal clusters and the corresponding available topology. Herein, we deliberately chose a series of tritopic
linkers with multiple substituents to construct a series of highly connected rare-earth (RE) MOFs. The steric hindrance of these
substituents can be systematically tuned to generate various linker rotamers with tunable configurations and symmetries. For example,
the methyl functionalized linker (L-CH3) with C2v symmetry exhibits larger steric hindrance, forcing two peripheral phenyl rings
perpendicular to the central one. The combination of C2v linkers and 9-connected RE6 clusters leads to the formation of a new
fascinating (3,9)-c sep topology. Unlike Zr-MOFs exhibiting Zr6 clusters in various linker configurations and corresponding different
structures, the adaptable RE6 clusters can undergo metal insertion and rearrange into new RE9 clusters when connected to an
unfunctionalized linker (L-H) with C1 symmetry, giving rise to a new (3,3,18)-c ytw topology. More interestingly, by judiciously
combining the linkers with both small and bulky substituents through mixed-linker strategies, a RE9-based MOF with an engaging
(3,3,12)-c flg topology could be obtained as a result of continuous steric hindrance control. In this case, the two mixed linkers adopt
configurations with moderate steric hindrances. Molecular simulation demonstrates that the combination of substituents with various
steric hindrances dictate the resulting MOF structures. This work provides insights into the discovery of unprecedented topologies
through systematic and continuous steric tuning, which can further serve as a blueprint for the design and construction of highly
complicate porous structures for sophisticated applications.
will force the two carboxylates and phenyl rings into a
perpendicular configuration, leading to the formation of PCN-
700 (PCN = porous coordination network) with a bcu net, rather
than UiO-67 (UiO = University of Oslo)18 with a fcu net.
Speaking of 3-connected linkers, the structural diversity based
on highly connected clusters, including Zr6 or RE6 (RE = rare
earth) can also be expanded by simply tuning steric hindrance
and linker configuration. For example, starting from coplanar
linkers with D3h symmetry like benzene-1,3,5-tricarboxylate
(BTC) or 4,4′,4″-s-triazine-2,4,6-triyltribenzoate (TATB),
(3,6)-c spn topology can form with 6-connected clusters
(Figure 1f).19-21 Similarly, modification of the central phenyl
ring of 1,3,5-benzenetribenzoate (BTB) with bulky methyl
groups can force the three peripheral phenyl rings perpendicular
to the central one, leading to the formation of (3,8)-c the
topology, although the linker still remains in D3h symmetry.22 In
contrast, directly constructing MOFs from 6-connected Zr6
clusters and BTB3– ligands with C1 symmetry results in an
interpenetrated Zr-BTB MOF.23 Recently, Yaghi and
coworkers reported a sky net based (3,12)-c Zr-MOFs based on
tritopic linkers with Cs symmetry (Figure 1a).24 Additionally,
highly connected MOFs based on tetratopic linkers can also be
used for advanced topology generation: by tuning the flexible
INTRODUCTION
As a promising class of porous crystalline materials,
metal−organic frameworks (MOFs) have attracted growing
attention due to their high tunability over their precisely
controlled pore environments.1-7 The linker length and
connectivity, the number and type of functional groups on the
linker, the metal node connectivity and nuclearity, and the
overall topology can be well manipulated.8-10 Among MOFs,
highly connected networks have been of a prime interest in
recent years, because they reveal underlying principles for the
rational design and construction of framework materials.11-13 In
addition, the resulting structures usually possess outstanding
chemical stability, given the fact that harsher conditions are
needed to break the higher connectivities in the frameworks
during the dissociation process.10
To enhance the structural diversity of highly connected
metal-organic frameworks, several strategies based on the
ligand design have been reported.9, 14-15 For example, the steric
control over ditopic linker based Zr-MOFs have been
systematically explored by our group by introducing bulky
substituents on the 2- and 2′-positions of BPDC (biphenyl-4,4′-
dicarboxylate).16-17 The steric hindrance of these substituents
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conformations of ligands, three types of structures with flu, scu,
and csq topologies can be obtained from various ligand
rotamers with D2, C2h and C2v, respectively.25-26 The formation
of these (4,8)-connected Zr-tetracarboxylate-based MOFs can
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Figure 1. Illustration of various topologies in RE-MOFs based on different linker symmetries: (a) 12-c Zr6 clusters are linked to tritopic
linkers with Cs symmetry into sky-MOFs; (b) The linking of 12-connected RE9 clusters with linkers possessing moderate steric hindrance
(C2v symmetry) leads to the formation of a flg net; (c) Employment of a C1 3-c ligand prompted the formation of a novel 18-c RE9 cluster
and its reticulation into a new highly connected (3,3,18)-c ytw net; (d) As expected, use of a C2v 3-c ligand resulted in the discovery of
a (3,18)-c gea net reported by Eddaoudi and coworkers; (e) The combination of C2v 3-c ligands and 9-connected RE6 clusters generates
(3,9)-c sep net; (f) The selection of D3h 3-c linkers can induce the formation of (3,6)-c spn net. The inorganic clusters, C, O and N are
represented by turquoise, black, red and blue, respectively, and H atoms are omitted for clarity. The linker functional groups and its
triangle version in the nets are represented by green.
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rise to a large library of novel and intricate polynuclear clusters,
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leading to the formation of sophisticated frameworks and their
corresponding topologies.11-13, 21, 29, 31-34 For instance, Eddaoudi
and coworkers reported a series of hexanuclear RE6 clusters-
based frameworks through reticular chemistry design.11-12
When linked to tritopic linkers that exhibited no compatible
network, this hexanuclear RE6 cluster tended to undergo
rearrangements and form nonanuclear RE9 clusters that present
distinct coordination behaviors. For example, gea-MOF-1, with
a (3,18)-connected net, can be constructed from the 18-
connected RE9 clusters and BTB ligands (Figure 1d), while
aea-MOF-1 and pek-MOF-1 can be built from 12-connected
RE9 clusters and 3-connected organic ligands with lower
symmetry.13, 32 In addition, mixed-linker or multivariate (MTV)
strategies have recently been employed in RE-MOFs to amplify
their structural diversity and function integrity.21, 31 However, it
still remains challenging to uncover elaborate and
unprecedented topologies, or control the formation of highly
connected MOFs through rational design.
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Herein, we deliberately chose a series of tritopic linkers with
multiple substituents to construct highly connected rare-earth
(RE) MOFs. The steric hindrance of these substituents can be
systematically tuned to generate various linker rotamers with
tunable configuration and symmetries. For example, the methyl
functionalized linker (L-CH3) with C2v symmetry exhibits larger
steric hindrance, forcing two peripheral phenyl rings
perpendicular to the central one. The combination of C2v linkers
and 9-connected RE6 clusters leads to the formation of a new
fascinating (3,9)-c sep topology (Figure 1e). Unlike Zr-MOFs
exhibiting preserved Zr6 clusters in numerous structures with
various linker configurations, RE6 clusters can undergo metal
insertion and rearrange into new RE9 clusters when connected
to the nonfunctionalized linker (L-H) with C1 symmetry, giving
rise to a new (3,3,18)-c ytw topology (Figure 1c). More
interestingly, by judiciously combining the linkers with both
bulky and low steric substituents through mixed-linker
strategies, a RE9-based MOF with an engaging (3,3,12)-c flg
topology was obtained as a result of continuous steric hindrance
control (Figure 1b). In this case, two mixed linkers adopt
configurations with moderate steric hindrances. The
abovementioned torsion angles between the central phenyl ring
and the peripheral ring can be well tuned, while the torsion
angles between the peripheral phenyl ring and the coordinating
carboxylate groups are negligible, less than 10o in all cases.
Molecular simulations demonstrate that the combination of
substituents with various steric hindrances dictates the resulting
MOF structures. This work provides insights into the discovery
of unprecedented topologies through systematic and continuous
steric tuning, which can further serve as blueprints for the
design and construction of highly complicate porous structures
for sophisticated applications.
Figure 2. Structural illustration of PCN-918 with (3,3,18)-c
ytw topology. (a) The distorted elongated triangular orthobicupola
(eto) found in PCN-918 with a 18o dihedral angle between the 3-
membered and 6-membered rings. For comparison, 18-c RE9 eto
clusters found in gea-MOF-1 are presented with all four rings
parallel. (b) The assembly of 18-c RE9 clusters and 3-c ligands
with bulky substituents (C1) into a (3,3,18)-c ytw net. (c)
Illustration of cluster packing and pore arrangement in PCN-918.
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be systematically tuned by altering the substituent positions on
linkers. So far, the steric control over linker configuration in Zr-
MOFs has been proven as a successful strategy because of their
predictable and well-maintained Zr6 clusters. However, this
structural uniformity also poses a challenge to the discovery of
new and intricate topologies especially in highly connected
networks for sophisticated applications.27 Therefore, it is
important to find highly connected MOFs with relatively labile
and dynamic substituents for previously reported Zr6 clusters,
which can still be selectively bonded to linkers with controllable
configurations and symmetries.
RESULTS AND DISCUSSION
Design, Synthesis and Structural Description. Herein we
selected six tritopic ligands with various substituents (-H, -F, -
OCH3, -NH2, -CH3, -Cl) to construct RE-MOFs that exhibit
multiple unprecedented topologies. As shown in Figure 1, the
functional groups with various levels of steric hindrance are
introduced at the -R position, tuning the rotation of two
peripheral phenyl rings near the central one.
RE based clusters are suitable candidates for the design and
construction of intricate topologies because the RE hard-sphere
behavior allows for multiple directionality of coordinated
ligands.28-30 Typically, each RE cation can coordinate with six
to twelve terminal ligands while each Zr cation can only
coordinate with eight. Moreover, the moderate RE-O bond
strength, along with the higher ion coordination numbers, gives
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Solvothermal reactions between L-Cl, -CH3 or -NH2 and viewed as triangular nodes (Figure 2b). The overall structure
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Eu(NO3)3·6H2O in the presence of 2-fluorobenzoic acid (2-
FBA) for 24 h yielded large colorless crystals with hexagonal
prism morphology, PCN-909 (also named UPC-909). With 2-
FBA as a modulator, hexanuclear RE clusters can easily form
in situ and further be utilized for the construction of extended
frameworks.13 The formula of PCN-909 was determined by
single-crystal X-ray diffraction. In contrast, under the same
condition, the reaction between Eu(NO3)3·6H2O and L-H, -F, or
-OCH3 yields large cuboid crystals with a new ytw net, donated
as PCN-918 (also named UPC-918). The powder X-ray
diffraction (PXRD) patterns also indicate the phase purity of the
aforementioned products. Notably, PCN-909-NH2 formed by
L-NH2 was often obtained in accompaniment with another
crystalline cuboid phase, donated as PCN-912 (also named
UPC-912).
was analyzed to be a 3,3,18-connected net with a point symbol
of {43}6{442.672.839} as determined by TOPOS 4.0 (Figure
2c)35.
For clarity, the cluster structure of PCN-918 was described
and compared with its analogous cluster found in gea-MOF-1.
In gea-MOF-1, the RE9 cluster is capped by 18 carboxylates
arranged in an elongated triangular orthobicupola manner. As
shown in Figure 2a, the 18 carboxylates consist of two 3-
membered rings at the top and bottom sides and two 6-
membered rings in the middle. These rings are parallel to each
other in gea-MOF-1. However, in PCN-918, the 18
carboxylates are linked in a more distorted way, leading to an
18o dihedral angle between the 3-membered and 6-membered
rings.
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Interestingly, these results indicate that when linkers with
low steric hindrance, for example H, F and OCH3, are used to
construct extended frameworks, PCN-918 is always obtained as
a pure phase, named as PCN-918-H, -F and -OCH3,
respectively. L-H, with only -H as a substituent at the -R
position tends to adopt a C1 symmetry in the frameworks due to
asymmetric rotation of C-C bonds between the two peripheral
phenyl rings and the central one. Substituting H with F or
OCH3, substituents with similar steric properties, has negligible
effects on the dihedral angle change between the two peripheral
phenyl rings.
PCN-918 represents a rare MOF composed of metal clusters
with a high connectivity number (18-connected). PCN-918
crystallized in the orthorhombic space group Pmmn (Table S1).
Crystallographically, it contains an 18-connected RE9 cluster
[RE9(μ3-OH)8(μ2-OH)3(O2C–)18] and a tritopic linker with C1
symmetry. Each RE9 cluster is connected by eighteen adjacent
L-R linkers, which can be simplified into a 3,3,18-connected
net with ytw topology (Figure 2). Topologically, the 18-
connected metal clusters can be regarded as distorted elongated
triangular orthobicupola (eto) nodes and tritopic linkers can be
Figure 3. Proposed pathway for the evolution from 12-connected RE6 clusters to 18-connected RE9 clusters. (a) Comparison of
18-c RE9 in gea-MOF-1 and PCN-918. In PCN-918, oxygens from two out of three highlighted carboxylates coordinate with three RE
cations, acting as a bridging oxygen. (b) Proposed metalation, linker migration, and installation process during the rearrangement of RE
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This distorted dihedral angle in the RE9 clusters of PCN-918
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originates from the nonidentical coordination environments for
the carboxylates forming 3-membered rings. In gea-MOF-1,
each oxygen from these carboxylates coordinates with one RE
cation. However, in PCN-918, only one carboxylate out of these
acts in this manner, while the oxygen in the other two
carboxylates tends to coordinate with three RE cations, with one
oxygen atom acting as a bridging oxygen (Figure 3). This
coordination behavior is quite similar to what we have observed
previously in the case of cluster metalation, where cooperative
cluster metalation and ligand migration in Zr-MOFs have been
captured through single-crystal X-ray diffraction.36 This led us
to propose a complementary path explaining the evolution from
the RE6 cluster to the RE9 cluster. The in situ formed RE6
clusters from the 2-FBA modulated synthesis have been well
documented in literatures.13 Due to the adaptable nature of the
RE clusters, insertion of three RE cations around the RE6
clusters without decomposing the clusters can be easily
achieved. The insertion of more exposed RE sites is believed to
induce the linker migration and installation, leading to the
formation of the rearranged 18-connected RE9 clusters.16
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Modification of central phenyl ring of L-H with bulky methyl
or chloride groups can force two peripheral phenyl rings
perpendicular to the central one, leading to the formation of
PCN-909. Single-crystal X-ray analysis revealed that PCN-909
crystallized in the hexagonal space group P63mc (Figure 4). It
contains a 9-connected RE6 cluster [RE6(OH)8(O2C−)9] and a
tritopic linker with C2v symmetry. Crystallographically, nine
tritopic linkers bridged three neighboring RE6 clusters, which
can be simplified into a 3,9-connected net with sep topology
(Figure 4c). Topologically, the 9-connected metal clusters can
be regarded as triangular cupola nodes and the tritopic linkers
can be viewed as triangularnodes (Figure 4a-b). The triangular
cupola, representing 9-connected RE6 clusters, can be viewed
as half a cuboctahedron (12-connected RE6 clusters) and part of
an elongated triangular orthobicupola (eto, 18-connected RE9
clusters). The overall structure was analyzed to be a 3,9-
connected net with a point symbol of {42.6}3{46.624.86} as
determined by TOPOS 4.0 (Figure 4c-e).
The PCN-909 framework here represents the high
adaptability of RE polynuclear clusters; when the linker
configuration is altered, local missing-linker defects might be
found on the corresponding RE cluster, allowing for the
formation of an extended ordered framework. The points of
extension (n-connectivity) in the parent 12-c RE6 clusters
formed in the presence of 2-FBA is reduced to nine, with this
being the first known example of this process in RE-MOFs, to
the best of our knowledge. Particularly, in the 12-c RE6 cluster,
three of the twelve carboxylate moieties, originally linking as
points of extension in MOFs with fcu topology, are replaced by
terminal carboxylates (2-FBA or formate anions from the DMF
decomposition). These terminal ligands can be further replaced
by soaking in water, allowing for the remaining nine connecting
carboxylates to arrange in a triangular cupola manner.
Figure 4. Structural illustration of PCN-909 with (3,9)-c sep
topology. (a) The triangular cupola, representing 9-connected RE6
clusters, can be viewed as half a cuboctahedron or a part of an
elongated triangular orthobicupola (eto); (b) The assembly of 9-c
RE6 clusters and 3-c ligands with bulky substituents (C2v) into a
(3,9)-c sep net. (c-e) The hexagonal close packing of the 9-c RE6
clusters of PCN-909 was observed with all triangular cupola
clusters placed in same direction. Layers 1 and 2 from the
hexagonal packing are represented in yellow and purple,
respectively.
Interestingly, PCN-909 has a similar topological structure as
observed in gea-MOF-1, although the connectivity and
configuration of clusters in both MOFs are quite different from
each other. Hexagonal close packing of the 9-c RE6 clusters of
PCN-909 was observed in a manner similar to the packing of
the 18-c RE9 clusters in gea-MOF-1. As indicated in Figure 4c,
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all 9-c RE6 clusters, simplified as triangular cupola units, are The pure single-crystalline cuboid phase of PCN-912 was
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arranged in same direction. The staggered arrangement of
clusters in two layers are linked by the tritopic linkers with C2v
symmetry. Note that the linkers in gea-MOF-1 also exhibit C2v
symmetry, but with a lower dihedral angle between the central
phenyl ring and the two peripheral ones.
formed with a moderate connectivity number (12), as compared
to the parent PCN-909 with a low connectivity number (9) and
PCN-918 with a high connectivity number (18). Single-crystal
X-ray analysis revealed that PCN-912 crystallizes in the
tetragonal space group I41/acd. PCN-912 contains a 12-
connected RE9 cluster [RE9(μ3-OH)12(μ3-O)2(O2C−)12] and a
tritopic linker with C2v symmetry. Crystallographically, the 12-
connected metal clusters can be regarded as hexagonal prism
nodes and the tritopic linkers can be viewed as triangular nodes
(Figure 6). The overall structure was analyzed to be a 3,3,12-
connected net with a point symbol of {418.630.818}{42.6}2{43}2
as determined by TOPOS 4.0 (Figure 6b-c)35.
Topology Exploration by Multivariate Strategy. The
preliminary results here enable us to tune the formation of
various phases with unprecedented topologies by simply
controlling the steric effects of linkers. However, RE-MOFs
constructed from single linkers with a simple variation of
functional groups can only achieve a few discrete energy states
caused by steric exclusion effects. Alternatively, a mixed-linker,
or so-called multivariate (MTV) strategy, allows for the
continuous creation of energy states associated with tunable
steric hindrances.37-38
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The 12-connected nonanuclear RE9 cluster, reported
In the case of Zr-based PCN-605, 606 and 608 series, where
the Zr6 clusters are stable enough to preserve the configuration
without undergoing rearrangement; by doping a small amount
of linker with a more free configuration into a more restricted
linker, it will force the linker into a more restricted
conformation.26 However, in the case of RE-based MOFs, the
RE cluster is more labile and prone to undergo rearrangement
to access frameworks with minimized energy based on the
formation of the RE-carboxylate bond being an exothermic
process. We expect that by mixing linkers with both more and
less sterically hindered substituents in a controllable manner we
can achieve a continuous tunability of the steric hindrances of
the overall system, which might further lead to the discovery of
unique structures and topologies. Various ratios of the less
hindered L-H were then incorporated into mixtures with the
other five linkers with different functional groups (-F, -OCH3, -
NH2, -CH3 and -Cl, respectively) during solvothermal reactions.
Remarkably, by doping a certain amount of linker (L-H, C1
symmetry) with low steric hindrance with a bulky linker (L-CH3,
C2v symmetry with 90o dihedral angle), it will force both linkers
into a less favorable state (L-H and -CH3, C2v symmetry with
67o dihedral angle), associated with the rearrangement of the
RE clusters and the formation of the PCN-912 framework.
Figure 5. Continuous tuning of linker steric hindrance
through multivariate (MTV) strategy. By mixing a certain
amount of linker (L-H, C1 symmetry) with low steric hindrance
into a bulky linker (L-CH3, C2v symmetry with 90o dihedral
angle), it will force both linkers into a less stable state (L-H and -
CH3, C2v symmetry with 67o dihedral angle), associated with the
rearrangement of RE clusters and formation of PCN-912.
previously by Eddaoudi and coworkers, is linked by 12
carboxylates arranged in a hexagonal prism way.32 The points
of extension in the parent 18-c RE9 clusters is reduced to twelve,
with the three carboxylate moieties missing on the top and
bottom side of the RE9 clusters, respectively (Figure 6a). In the
structure of PCN-912, each two neighboring RE9 clusters are
perpendicular to each other due to the rotation of the RE9
clusters caused by the length variation of the tritopic linkers
(Figure 6c). Noticeably, both of the linkers are forced into a
confined state with 67o dihedral angle between the central
phenyl ring and the two peripheral ones. The ratio of L-CH3 and
L-H found in the mixed-linker PCN-912(Eu)-CH3 was
calculated from the 1H NMR of digested MOF samples (Table
1
S6). As indicated from the H NMR results, the ratio of L-H
found in the structure of final product PCN-912(Eu)-CH3
increases as the ratio of L-H in the precursor increases.
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A phase diagram summarizing this multivariate strategy is
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shown Figure S25, illustrating the power of multivariate
strategy to functionalize targeted frameworks, limiting the
growth of the impure phase during the solvothermal reactions
and allowing for the discovery of unprecedented structures. For
instance, the synthesis of the single-linker PCN-909-NH2
always contains a small amount of PCN-912-NH2 impurity as
indicated by PXRD patterns; by mixing L-NH2 and L-H, MTV-
PCN-918-NH2 can be obtained without impurities.
To further verify the versatility of topology exploration in
RE-MOFs via continuous hindrance control, PCN-9XX(RE,
XX = 09, 12 and 18) analogues with various RE metals (i.e.,
Eu, Y, Ce, Yb, Tb, Dy) have also been successfully synthesized
under similar reaction conditions, as indicated by PXRD
patterns (Figure S11-13).
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Formation Mechanism of Multiple Topologies. Unlike Zr-
MOFs exhibiting preserved Zr6 clusters in numerous structures
with various linker configurations, RE6 clusters are more
adaptable, with the cluster form being dependent on the linker
configuration. Our work suggests that when highly connected
polynuclear RE clusters are linked to various linkers, crystalline
extended structures with preserved cluster configurations and
connectivities are formed initially. However, in order to reach
topologies that result in low energy structures, these RE cluster
can undergo three types of alterations: a) the reduction of points
of extension (n-connectivity) in the parent clusters, b) the
preservation or c) increase of points of extension in the parent
clusters by cluster metalation and rearrangement. By
systematically modifying the central phenyl ring on L-H, bulky
groups can force the rotation of the two peripheral phenyl rings,
leading to various unprecedented topologies.
The effects of ligand hinderance on steric control have been
investigated through density functional theory (DFT)
calculations. The energies of selected linkers in PCN-909,
PCN-912, and PCN-918 are calculated and summarized in
Figure 7. For clarity, all energies are normalized through
subtracting the energy of the unconstrained structure of a given
ligand. Owning to the constraints from RE6 or RE9 clusters in
confined frameworks, ligands in the three phases with different
topologies demonstrate distinct torsion angles between the
peripheral and central phenyl rings. By comparing the relative
energies of linkers in PCN-909 and PCN-918, ligands with
bulky groups such as -CH3, -Cl, and -NH2 tend to be obtained
with a larger torsion angle, while for ligands modified with
groups such as -H, -F and -OCH3, a smaller torsion angle is
favored, which confirms the point that steric bulk in a ligand is
beneficial for the formation of PCN-909.
Figure 7. Relative energies of ligands with various
configurations in PCN-9XX series possessing different
topologies. The relative energy for a given ligand is normalized
through subtracting the energy of its unconstrained structure.
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The energy difference of L-CH3 in PCN-912 and PCN-909 is energies of the ligands is only one perspective needed to
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~5.9 kJ/mol, while the difference is ~7.1 kJ/mol for L-H in
understand how ligand sterics controls the topologies of RE-
MOFs. Other factors such as solvent molecules, metal node
nuclearity and connectivity should also contribute to the
variation on RE-MOF topologies.
Stability, Porosity and Gas Sorption Studies. The thermal
stability of PCN-909, PCN-912 and PCN-918 were determined
by thermogravimetric analysis (TGA, Figure S57-63). The
initial weight loss before 120 °C is attributed to the removal of
the solvent molecules in the pores, whereas the weight loss from
200 to 300 °C can be attributed to the removal of strongly
coordinated DMF or DMA cations trapped insides the pores.
The decomposition of PCN-909, 912 or 918 starts at 550 °C,
leading to the removal of organic linkers and structural collapse.
In addition, the chemical stability of PCN-9XX(RE) was
examined by soaking the MOFs in various solvents for 24 h.
PXRD patterns suggest that the PCN-912 and PCN-918 have
excellent stability due to the retained crystallinity in most
solvents, including DMF, acetone, CH3OH, EtOH, CH3CN, and
water (Figure S17-19). PCN-909 with much lower connectivity
number demonstrate good stability only in organic solvents,
however, it will gradually get decomposed when exposed to
water (Figure S6-8). The presence of various functional groups
was also supported by infrared (IR) spectroscopy (Figure S26-
28).
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The permanent porosity of the PCN-9XX(RE, XX = 09, 12
and 18) series was confirmed by N2 adsorption isotherms
measured at 77 K (Figure S29-34). Prior to the gas sorption
measurements, PCN-9XX series samples were rinsed
thoroughly using DMF and fresh acetone for three days before
thermal activation at 100°C under vacuum. PCN-918(Eu)
exhibits a saturated uptake of 216 cm3 g-1 at 1 atm, with the
other PCN-9XX(Eu) series also showing similar N2 adsorption
isotherms, with slightly lower surface areas due to the presence
of bulkier substituents.
Due to the intrinsic permanent porosity and channel
environments of PCN-9XX(RE), their potential application for
light hydrocarbon (CH4, C2H2, C2H4, C2H6, C3H6, and C3H8)
adsorption (Figure S35-48) and separation (Figure S49-56) has
been investigated systematically. It was found that PCN-
909(Tb)-NH2 shown a much higher heats of adsorption for C3
light hydrocarbons, indicating stronger affinities for these gases
to be adsorbed on the framework. We further conducted ideal
solution adsorbed theory (IAST) for binary equimolar
components to study the potential for the separation of CH4
from C3 light hydrocarbons.32 At 1 bar and 298 K, the
selectivities for C3H8 and C3H6 over CH4 are 96.9 and 97.6
respectively, making PCN-909(Tb)-NH2 a promising adsorbent
for the efficient removal of C3 light hydrocarbons from natural
gas streams at room temperature.
Figure 6. Structural illustration of PCN-912 with (3,3,12)-c flg
topology. (a) The hexagonal prism, representing 12-connected RE9
clusters, can be viewed as a part of an elongated triangular
orthobicupola (eto) by removing six linkers on the top and bottom
side of a 18-connected RE9 clusters. (b) The assembly of 12-c RE9
clusters and mixed 3-c ligands (C2v) into a (3,3,12)-c flg net. (c)
Illustration of cluster packing and pore arrangement in PCN-912.
CONCLUSIONS
In conclusion, we report a highly tunable RE-MOF system
that contains a series of tritopic linkers with multiple
substituents. Depending on the sizes of these substituents, the
linkers can generate various rotamers with tunable
configurations and symmetries that have resulted in the
discovery of three unprecedented and fascinating topologies.
The combination of methyl functionalized linker (L-CH3) with
C2v symmetry and the 9-connected RE6 clusters led to the
PCN-912 and PCN-918. This result indicates that the formation
of PCN-912 only needs to cross a relative low energy barrier,
from a linker perspective. Yet, calculation on the relative
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formation of a (3,9)-c sep topology, while the linked RE9 Synthesis of PCN-909(Eu)-Cl. Eu(NO3)3 (15 mg), H3L-Cl
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clusters with a nonfunctionalized linker (L-H) with C1
symmetry, gave rise to a new (3,3,18)-c ytw topology. More
interestingly, continuous steric hindrance control can be
achieved by judiciously combining the linkers with both bulky
and compact substituents, leading to the formation of a RE9-
based MOF with an engaging (3,3,12)-c flg topology. This work
indicates that highly complicate networks can be rationally
designed and constructed through systematic and continuous
steric tuning, which will bring insights into the design and
construction of tailored porous frameworks for sophisticated
applications.
(10 mg) and 1 mol/L 2-fluorobenzoic acid DMF solution (3 mL)
were charged into a 10 mL vial. The mixture was heated in 115
ºC oven for 2 days. After cooling down to room temperature,
the colorless crystals of PCN-909(Eu)-Cl were harvested (yield:
66%).
Synthesis of PCN-912(Eu)-P%CH3 (P = 35%, 50% and
65%). Eu(NO3)3 (15 mg), H3L-CH3 (3.5, 5, and 6.5 mg,
respectively), H3L (6.5, 5, and 3.5 mg, respectively), and 1
mol/L 2-fluorobenzoic acid DMF solution (3 mL) were charged
into a 10 mL vial. The mixture was heated in 115 ºC oven for 2
days. After cooling down to room temperature, the colorless
crystals of PCN-912(Eu)-P%CH3 were harvested.
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EXPERIMENTAL SECTION
Synthesis of MTV-PCN-9XX(Eu)-P%L-R (XX = 09, 12
and 18). Eu(NO3)3 (15 mg), H3L-R (p mg), H3L (10-p mg), and
1 mol/L 2-fluorobenzoic acid DMF solution (3 mL) were
charged into a 10 mL vial. The mixture was heated in 115 ºC
oven for 2 days. After cooling down to room temperature,
crystals of MTV-PCN-9XX(Eu)-P%L-R were harvested.
Materials and Instrumentation. All reagents and solvents
used in the synthetic studies were commercially available and
used as supplied without further purification. The ligands were
synthesized through the procedures in Supporting Information
Section 1. 1H NMR spectra were obtained on an Inova 500
MHz spectrometer. Single crystal X-ray diffraction
experiments were carried out on a SuperNova diffractometer
equipped with mirror Cu-Kα radiation (λ = 1.54184 Å) and an
Eos CCD detector under 150 K. Powder X-ray diffraction
(PXRD) was carried out with a Bruker D8-Focus Bragg–
Brentano X-ray Powder Diffractometer equipped with a Cu
sealed tube (λ = 1.54178 Å) at 40 kV and 40 mA. Gas sorption
measurements were conducted on a Micrometritics ASAP 2020
system. PCN-909, 912 and 918 (or UPC-909, 912 and 918) are
named based on the cluster connectivity.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available
free of charge on the ACS Publications website at
DOI:10.1021/jacs.xxxxxx.
Text, tables, and figures giving experimental procedures for the
1
syntheses of the ligands, PXRD, gas adsorption isotherms, H
NMR spectra, and other additional information (PDF)
X-ray crystallographic details of the structures (CIF)
AUTHOR INFORMATION
‡ Y. T. Wang and L. Feng contributed equally to this work.
Synthesis of PCN-918(Eu). Eu(NO3)3 (15 mg), H3L (10 mg)
and 1 mol/L 2-fluorobenzoic acid DMF solution (3 mL) were
charged into a 10 mL vial. The mixture was heated in 115 ºC
oven for 2 days. After cooling down to room temperature, the
colorless crystals of PCN-918(Eu) were harvested (yield: 66%).
Corresponding Author
*dfsun@upc.edu.cn
Synthesis of PCN-918(Eu)-F. Eu(NO3)3 (15 mg), H3L-F (10
mg) and 1 mol/L 2-fluorobenzoic acid DMF solution (3 mL)
were charged into a 10 mL vial. The mixture was heated in 115
ºC oven for 2 days. After cooling down to room temperature,
the colorless crystals of PCN-918(Eu)-F were harvested (yield:
65%).
*zhou@chem.tamu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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 Relevant to
Clean Energy Technologies, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences under Award Number DE-
SC0001015. The PXRD characterization and analysis were
funded by the Robert A. Welch Foundation through a Welch
Endowed Chair to HJZ (A-0030). We gratefully acknowledge
Prof. Michael O'Keeffe and Mr. Gregory S. Day for the help
and insightful discussion.
Synthesis of PCN-918(Eu)-OCH3. Eu(NO3)3 (15 mg), H3L-
OCH3 (10 mg) and 1 mol/L 2-fluorobenzoic acid DMF solution
(3 mL) were charged into a 10 mL vial. The mixture was heated
in 115 ºC oven for 2 days. After cooling down to room
temperature, the colorless crystals of PCN-918(Eu)-OCH3 were
harvested (yield: 65%).
Synthesis of PCN-909(Eu)-NH2. Eu(NO3)3 (15 mg), H3L-
NH2 (10 mg) and 1 mol/L 2-fluorobenzoic acid DMF solution
(3 mL) were charged into a 10 mL vial. The mixture was heated
in 115 ºC oven for 2 days. After cooling down to room
temperature, the colorless crystals of PCN-909(Eu)-NH2 were
harvested (yield: 64%).
Synthesis of PCN-909(Eu)-CH3. Eu(NO3)3 (15 mg), H3L-
CH3 (10 mg) and 1 mol/L 2-fluorobenzoic acid DMF solution
(3 mL) were charged into a 10 mL vial. The mixture was heated
in 115 ºC oven for 2 days. After cooling down to room
temperature, the colorless crystals of PCN-909(Eu)-CH3 were
harvested (yield: 67%).
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