A route to metal-organic frameworks through framework templating

Article abstract of DOI:10.1021/ic3019937

A microporous metal-organic framework (MOF), PCN-922 [Cu ₄(ETTB)], containing a dendritic octatopic organic linker and a Cu₂-paddlewheel structural motif, has been synthesized by using a Zn₂-paddlewheel-based MOF as a template to prearrange the linkers for the Cu₂-based MOF target. PCN-922 shows permanent porosity and excellent gas adsorption capacity.

Full text of DOI:10.1021/ic3019937

Communication
pubs.acs.org/IC
A Route to Metal−Organic Frameworks through Framework
Templating
Zhangwen Wei, Weigang Lu, Hai-Long Jiang, and Hong-Cai Zhou*
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S
* Supporting Information
solvent molecules, the unsaturated Cu centers have been
demonstrated to enhance gas adsorption capacity because of
their reported interaction with gas molecules.8
ABSTRACT: A microporous metal−organic framework
(MOF), PCN-922 [Cu4(ETTB)], containing a dendritic
octatopic organic linker and a Cu2-paddlewheel structural
motif, has been synthesized by using a Zn2-paddlewheel-
based MOF as a template to prearrange the linkers for the
Cu2-based MOF target. PCN-922 shows permanent
porosity and excellent gas adsorption capacity.
However, sometimes it is nontrivial to obtain single crystals
of Cu-based MOFs, especially with extensive linkers, for
structure determination.6b,9 Direct synthesis of a Cu-based
MOF from solvothermal reactions between a copper(II) salt
and H8ETTB only yielded powders under various reaction
conditions including different solvents, reaction temperatures,
metal salts, and pH values (Table S1 in the Supporting
Information, SI). In contrast to the labile Zn−O, the more inert
Cu−O coordination bond may hamper defect repair during
crystal growth. On the other hand, thanks to the d10
configuration of the ZnII ion, the labile Zn−O coordination
bond may facilitate ligand exchange and defect repair during
crystal growth, assisting the access of single crystals for X-ray
diffraction (XRD) and structure determination.10 Meanwhile,
both the Zn2- and Cu2-paddlewheel SBUs, which possess very
similar coordination environments and geometric parameters,
are quite common in MOFs.9b,11 It has been demonstrated that
a Zn2-paddlewheel in a MOF can be substituted by a Cu2
paddlewheel.12 Preparing new metal−organic polyhedra by a
ligand-substitution strategy has also been reported.13 With
these considerations in mind, we set out to produce the Cu-
based MOF using a Zn-based MOF as a template to prearrange
the linkers.
The first step is to synthesize the MOF template containing
the Zn2-paddlewheel structural motif. As expected, the reaction
between H8ETTB and Zn(NO3)2·6H2O in N,N-diethylforma-
mide (DEF) in the presence of HBF4 produced a Zn-based
MOF, Zn4(ETTB)·4DEF·xS (S represents noncoordinated
solvent molecules), designated PCN-921 (PCN stands for
porous coordination network). A single-crystal XRD14 study
reveals that PCN-921 is a porous 3D framework composed of
ETTB linkers and Zn2 paddlewheels as desired. Subsequently,
the colorless crystals of PCN-921 were immersed in a N,N-
dimethylformamide (DMF) solution of Cu(NO3)2 at room
temperature for 4 days (Scheme 1) to ensure complete metal
replacement and permanent porosity (vide infra). The crystals
became green in color, and X-ray photoelectron spectroscopy
studies on a thoroughly washed and grinded MOF sample
clearly indicate that no significant amount of zinc was left in the
framework (Figure S11 in the SI). The resultant Cu-based
MOF, Cu4(ETTB)·4DMF·xS (PCN-922), was obtained as
green crystal. Single-crystal XRD studies have revealed that
etal−organic frameworks (MOFs) are a new class of
Mporous materials and have been intensively studied
during the last decades.1 Owing to their high specific surface
areas, diverse structures, tunable pore sizes, and functionalities,
MOFs have great application potential in gas storage and
separation,2 chemical sensing,3 catalysis,4 and drug delivery.5
MOFs are composed of organic linkers and metal or metal-
containing structural units, often called secondary building
units (SBUs). To construct highly porous MOFs for
applications, especially for gas storage, using dendritic ligands
was demonstrated to be a direct and effective method.6 Thus,
the acid of an octatopic carboxylate ligand, 4′,4‴,4′′′′′,4′′′′′′′-
ethene-1,1,2,2-tetrayltetrakis{([1,1′-biphenyl]-3,5-dicarboxy-
late)} (H8ETTB), has been designed and synthesized (Scheme
1). We are particularly interested in a Cu2-paddlewheel motif as
a SBU because it is superior to a Zn2-paddlewheel motif in
stability.7 Moreover, after removal of the terminally coordinated
Scheme 1. Synthesis of PCN-922 Using PCN-921 as a
Template
Received: September 12, 2012
Published: January 22, 2013
© 2013 American Chemical Society
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Communication
PCN-921 and PCN-922 are isostructural with only a slight
difference in cell parameters. In the following, only the
structure of PCN-922 will be illustrated in detail.
PCN-922 crystallizes in space group I4/mmm with a = b =
18.583(16) Å and c = 35.68(3) Å. In the structure of PCN-922,
the bond angle (107.96°) between the two single bonds at each
end sp2 carbon atom of the central ethylene fragment highly
deviates from the ideal value (120°). The six carbon atoms at
the center of the linker are coplanar; the dihedral angle between
the benzene ring of one isophthalic moiety and the central plane
is 90°. Each carboxylate group of an ETTB ligand connects one
Cu2-paddlewheel so that each ETTB ligand, in a rectangular
prismatic arrangement, connects eight Cu2 SBUs, leading to a
highly porous 3D network. The framework contains two types
of microporous cages with different sizes. The small one (14 Å
in diameter) is composed of 8 Cu2 SBUs and 8 ETTB linkers
(Figure 1a), while the large one (18 Å) is formed by 8 Cu2
Figure 2. (a) Zn2- and (b) Cu2-paddlewheel SBUs in PCN-921 and
PCN-922, respectively.
between a dxz or dyz orbital and a p π-donor ligand),
respectively.10b On the basis of the orbital diagrams constructed
from the AOM, the net stabilization energy for CuII in a square-
pyramidal coordination environment is 3eσ, and that for a
square-planar or tetrahedral coordination environment is 3eσ or
(4/3eσ + 8/9eπ), respectively (Figures S13−S15 in the SI). Thus,
the Cu2 paddlewheel is more stable than the Zn2 one in a
square-pyramidal coordination environment, shifting the meta-
thesis equilibrium toward the Cu-based MOF. Furthermore,
because eσ is often much larger than 2eπ, four-coordinated CuII
is more stable in square-planar than in tetrahedral structures,
which is why the Cu2 paddlewheel can survive the activation
process.
Calculations using the CALCSOLV routine in PLATON
software were performed to evaluate the porosities of PCN-921
and PCN-922.15 The results indicate that the solvent-accessible
volumes of fully desolvated MOFs (1.8 Å probe radius) are
66.0% for PCN-921 and 66.2% for PCN-922, respectively. To
confirm the porosities, both samples were degassed under
dynamic vacuum at 85 °C for 10 h each after solvent exchange
with methanol and dichloromethane. After activation, the
colorless PCN-921 became yellow, possibly a sign of crystal
decomposition. Indeed, it showed no N2 adsorption as
expected. Upon activation, the color of PCN-922 changed
from green to deep blue, indicating the generation of open
metal sites in Cu-based MOFs.6b,16 PCN-922 showed a type I
isotherm for N2 sorption at 77 K and 1 bar, revealing the
microporous nature of the framework, consistent with the
observed porosity from the crystal structure (Figure 3a).
Langmuir and Brunauer−Emmett−Teller surface areas are
2615 and 2006 m2 g−1, respectively.15 The total pore volume is
0.94 cm3 g−1, approaching the value (0.99 cm3 g−1) calculated
from the crystal structure using the PLATON routine.
The microporosity, high density of open metal sites, and
robustness of PCN-922 encouraged us to investigate its gas
adsorption capacity. It exhibited very high hydrogen and CO2
uptake capacities under low pressure. Hydrogen uptake of
PCN-922 can reach 2.3 wt % at 77 K and 1 bar (Figure 3b).
PCN-922 also exhibited high CO2 uptake (142.97 and 80.78
cm3 g−1 at 273 and 298 K, respectively; Figure 3c).2b,d
Meanwhile, the CH4 sorption isotherms indicate that PCN-922
has adsorption capacities of 29.3 and 20.56 cm3 g−1 at 273 and
298 K, respectively (Figure 3d).
Figure 1. (a) Large cage. (b) Small cage. (c) View of the structure of
PCN-222 along the c axis. (d) Topological view of PCN-922
(hydrogen atoms and disordered structural components are omitted
for clarity).
SBUs and 12 ETTB linkers (Figure 1b). The two types of cages
alternate in a 1:1 ratio, forming 1D channels of 9 Å running
along the c axis of the framework (Figure 1c). If the
paddlewheel SBU and ETTB linkers are viewed as 4- and 8-
connected nodes, respectively, the framework can be simplified
as a 4,8-c 2-nodal net with scu topology, which is the same as
that of other reported MOFs assembled with octatopic linkers
and 4-connected SBUs (Figure 1d).6
In the frameworks, both ZnII and CuII ions adopt distorted
square-pyramidal geometry, with each metal atom sitting above
the square base of the pyramid (Figure 2). The distances
between the metals and bases are 0.386 Å for Zn and 0.184 Å
for Cu, respectively, indicating that the coordination environ-
ment of ZnII undergoes more severe distortion than that of
CuII. Meanwhile, after removal of the apical solvent molecule,
the ZnII ion prefers a tetrahedral to a square-planar ligand
field.10b Therefore, Zn2-paddlewheel SBUs, upon removal of
coordinated solvates, often collapse. On the basis of a simple
angular overlap model (AOM), we can estimate the preference
In summary, a robust and porous MOF, PCN-922, which
could not be assembled directly from copper(II) salts and an
octatopic ligand ETTB by conventional one-step synthesis, has
been successfully synthesized with a framework-templating
strategy by using a Zn-based MOF as a template to prearrange
the linkers in place. PCN-922 has high gas uptakes and may be
used for carbon capture and hydrogen/methane storage.
2
energy in terms of eσ (based on an ideal overlap between the dz
orbital and a σ-donor ligand) and eπ (based on an ideal overlap
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ASSOCIATED CONTENT
* Supporting Information
Crystallographic data for PCN-921 and PCN-922 in CIF
format as well as other experimental details. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: zhou@mail.chem.tamu.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy
(Grant DE-SC0001015).
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