A Tale of Copper Coordination Frameworks: Controlled Single-Crystal-to-Single-Crystal Transformations and Their Catalytic C-H Bond Activation Properties

Article abstract of DOI:10.1002/chem.201501672

Metal-organic frameworks (MOFs), as a class of microporous materials with well-defined channels and rich functionalities, hold great promise for various applications. Yet the formation and crystallization processes of various MOFs with distinct topology, connectivity, and properties remain largely unclear, and the control of such processes is rather challenging. Starting from a 0D Cu coordination polyhedron, MOP-1, we successfully unfolded it to give a new 1D-MOF by a single-crystal-to-single-crystal (SCSC) transformation process at room temperature as confirmed by SXRD. We also monitored the continuous transformation states by FTIR and PXRD. Cu MOFs with 2D and 3D networks were also obtained from this 1D-MOF by SCSC transformations. Furthermore, Cu MOFs with 0D, 1D, and 3D networks, MOP-1, 1D-MOF, and HKUST-1, show unique performances in the kinetics of the C-H bond catalytic oxidation reaction.

Full text of DOI:10.1002/chem.201501672

DOI: 10.1002/chem.201501672
Communication
&
Metal–Organic Frameworks
A Tale of Copper Coordination Frameworks: Controlled Single-
Crystal-to-Single-Crystal Transformations and Their Catalytic CÀH
Bond Activation Properties
Yifa Chen,[a] Xiao Feng,[a] Xianqiang Huang,[a, b] Zhengguo Lin,[a] Xiaokun Pei,[a] Siqing Li,[a]
Jikun Li,[a] Shan Wang,[a] Rui Li,[a] and Bo Wang*[a]
In heterogeneous catalysis, catalysts with different pores/
channels and structural dimensions usually show drastically dif-
ferent activity and/or selectivity.[3] Metal–organic frameworks
(MOFs) with uniformly distributed metal ions, highly ordered
open channels and a heterogeneous nature are often consid-
ered as efficient heterogeneous catalyst candidates.[4] Different
structures or coordination environments of MOFs show distinct
catalysis performance and various catalytic stages often pres-
ent diverse coordination modes.[5] As a kind of heterogeneous
catalyst, copper-based MOFs usually contain active copper
sites that can undergo ad/desorption and redox cycles and
consequently these MOFs can be used as efficient catalysts in
various reactions with industrial importance.[6] Due to the
structural diversity of copper-based MOFs, SCSC transforma-
tions in such MOFs can serve as a tool to further explore the
catalytic kinetics in MOFs and better understand the underly-
ing principles.
Abstract: Metal–organic frameworks (MOFs), as a class of
microporous materials with well-defined channels and rich
functionalities, hold great promise for various applications.
Yet the formation and crystallization processes of various
MOFs with distinct topology, connectivity, and properties
remain largely unclear, and the control of such processes
is rather challenging. Starting from a 0D Cu coordination
polyhedron, MOP-1, we successfully unfolded it to give
a new 1D-MOF by a single-crystal-to-single-crystal (SCSC)
transformation process at room temperature as confirmed
by SXRD. We also monitored the continuous transforma-
tion states by FTIR and PXRD. Cu MOFs with 2D and 3D
networks were also obtained from this 1D-MOF by SCSC
transformations. Furthermore, Cu MOFs with 0D, 1D, and
3D networks, MOP-1, 1D-MOF, and HKUST-1, show unique
performances in the kinetics of the CÀH bond catalytic ox-
idation reaction.
In the past decades, much research has demonstrated that
SCSC transformations of MOFs can be triggered by external
stimuli, including physical[7a] and chemical[7b] processes. Physi-
cal stimuli can be light[7c] or heat[7d] and chemical ones are
mainly: 1) ad/desorption of guest molecules in the framework
resulting in structural transformation;[7e] 2) metal ions or organ-
ic linker substitution to achieve partially replaced[7f] or com-
pletely exchanged structures;[7g] and 3) post-synthetic modifi-
cation (PSM) of MOFs to acquire MOFs with altered functionali-
ties.[7h] Yet the investigation of SCSC transformations among
MOFs with identical metal ions but diverse structural dimen-
sions was rather rare. Some pioneering work by Kitagawa’s
and Chen’s groups showed that insertion of bidentate bridging
linkers into 2D MOFs can lead to structural transformations to
3D MOFs; Zhou’s and Rahul’s groups also reported structural
transformations from 0D coordination cages (metal–organic
polyhedra, MOP) to 3D MOFs.[8]
Single-crystal-to-single-crystal (SCSC) transformations are inter-
esting and critical processes in chemistry and material sci-
ence.[1] As an important solid-state transformation, SCSC is rare
since the majority of the solid-state processes that begin with
single crystals often result in polycrystalline products.[2] Cataly-
sis, for instance, will be benefited if one can identify the crystal
structures of the active species and pinpoint the catalytic cen-
ters with distinct coordination geometries. SCSC transforma-
tions will ultimately help in understanding the reaction mecha-
nism and guide the design of new catalysts.
[a] Y. Chen,+ Prof. X. Feng,+ Prof. X. Huang, Z. Lin, X. Pei, S. Li, J. Li, S. Wang,
R. Li, Prof. B. Wang
Key Laboratory of Cluster Science, Ministry of Education of China
Beijing Key Laboratory of Photoelectronic/Electrophotonic
Conversion Materials, School of Chemistry
Beijing Institute of Technology, Beijing, 100081 (P. R. China)
E-mail: bowang@bit.edu.cn
Herein, we pursue controlled stepwise SCSC transformations
in different Cu MOFs with distinct structural dimensions and
systematical investigation on their catalysis performance. We
decided to start from 0D copper MOF (MOP-1, Cu24(m-
BDC)24(DMF)14(H2O)10) and explore the structures and connec-
tion changes by SCSC transformations triggered by different
organic linkers. Specifically we achieved a 1D Cu MOF from
MOP-1 by using 1-methylimidazole and prepared copper-
based MOFs with 2D and 3D open frameworks from this 1D-
MOF by using 3,5-pyridinedicarboxylic acid (3,5-PDC) and bi-
phenyl-3,3’,5,5’-tetracarboxylic acid (BPTC), respectively. The
[b] Prof. X. Huang
Shandong Provincial Key Laboratory of Chemical Energy
Storage and Novel Cell Technology
School of Chemistry &Chemical Engineering
Liaocheng University, Liaocheng, 252059 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501672.
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ligand replacement of isophthalic acid in the MOP-1 structure
with topologically matching 1,3,5-benzenetricarboxylic acid
(BTC) and extended 1,3,5-tris(4-carboxyphenyl) benzene (BTB)
gave two typical 3D Cu MOFs. Furthermore, the unique catalyt-
ic kinetics of these Cu MOFs were also investigated and dis-
cussed.
Diamond-shaped crystals of MOP-1 were synthesized
through the solvothermal reaction of Cu(NO3)2·3H2O and 1,3-
benzenedicarboxylate (m-BDC) according to the literature
(Supporting Information, Figure S1).[9] The single-crystal X-ray
diffraction (SXRD) study reveals that MOP-1 is comprised of
Cu2(COOÀ)4 paddle-wheel units joined by ditopic m-BDC links.
Each unit in the as-prepared MOP-1 has two solvent terminal
ligands, respectively, directing to the center and outside of the
polyhedron (Scheme 1).
The conversion of 0D MOP-1 to 1D MOF was achieved by
employing monotopic 1-methylimidazole to unfold MOP-1 by
partially occupying its coordination sites (Figure 1a). After
soaking in 1-methylimidazole for 12 h, the color of the MOP-
1 crystal changed from lake blue to greenish blue (Scheme 1).
SXRD analysis reveals that the
Scheme 1. Single-crystal-to-single-crystal transformations and crystal struc-
tures of copper-based frameworks. The scale bar represented is 100 mm.
structure of the thus-obtained
crystals features
a 1D linear
structure (Figure 1, Table S1,
Supporting Information). The
paddle-wheel square tetracon-
nected dinuclear Cu unit in
MOP-1 transforms to a biconnect-
ed mononuclear center, which
displays a CuN3O2 tetrahedral en-
vironment. The copper atom is
coordinated to two oxygen
atoms from two carboxyl groups
of m-BDC and three nitrogen
atoms from 1-methylimidazole in
a terminal position. The depro-
tonated m-BDC takes a bis(biden-
tate) mode linking copper ions
to generate this 1D chain.
Based on the obtained trans-
formation, we performed a fur-
ther investigation into the
nature of the SCSC process. One
lake-blue crystal of MOP-1 (ca.
100 mm) was selected and put
into 1-methylimidazolesolution
under an optical microscope to
monitor the process (Figure 1). A
clear transformation was ob-
served: just in 1 min, bubble-like
granules were formed in the
crystal; after 3 min, bubble-like
granules grew up to cover all
the crystal with the color slightly
darkened; after 5 min, the crystal
transferred to dark blue with the
Figure 1. SCSC transformations from MOP-1 to 1D-MOF: a) Structure transformations from MOP-1 to 1D-MOF, Cu
(magenta), O (red), N (green), and C (grey); b) the crystal images of SCSC transformations at different time stages;
c) the FTIR of SCSC transformations at corresponding time stages; d) the PXRD of SCSC transformations at corre-
shrinking in size; after 10 min, sponding time stages. Shadow parts represent the positions that change obviously.
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small crystals were formed inside the crystal; then at 15, 60,
and 720 min these small crystals gradually grew up at the
same position. During the transformation, the corresponding
FTIR and powder X-ray diffraction (PXRD) data of these crystals
were collected at different time periods (Figure 1c,d). The
peak at 485 cmÀ1 can be attributed to the CuÀO stretching vi-
bration of MOP-1 and this peak gradually shifted to 446 cmÀ1
during the transformation. Ultimately a new CuÀN stretching
vibration peak appeared at 620 cmÀ1 due to the coordination
of 1-methylimidazole. Consistent with the structural environ-
ments of MOP-1 and 1D-MOF, the peak of C=O stretching at
1667 cmÀ1 significantly decreased. Also as shown in the PXRD
patterns, the peaks at 4.46 and 16.378, which can be attributed
to the spacing planes of (200) and (510) for MOP-1 gradually
disappeared; meanwhile, the diffraction peak of the (011) spac-
ing plane of 1D-MOF appeared at 8.578.
mersing the single crystals of 1D-MOF in the DMF solution of
tritopic 3,5-pyridinedicarboxylic acid (3,5-PDC) or tetratopic bi-
phenyl-3,3’,5,5’-tetracarboxylic acid (BPTC) at elevated temper-
ature led to a rapid exchange of the links and an increment of
the dimensionality of the framework from 1D to 3D. Sky-blue
cubic crystals (Cu-pdc-MOF) and dark-green rectangle crystals
(MOF-505) were obtained in high yield (67 and 80%, respec-
tively), and their phase purities were further confirmed by
PXRD patterns (Scheme 1, Supporting Information, Figures S3–
S4).[10] After the SCSC transformation, the bi-connected copper
centers in the 1D-MOF turned to triconnected centers bridged
by three 3,5-PDC linkers in a Cu-pdc-MOF and tetraconnected
paddle-wheel centers joined by four BPTC links in MOF-505, re-
spectively. The arrangements of metal ions and linkers in Cu-
pdc-MOF and MOF-505 yield 2D and 3D periodic scaffolds
(Scheme 2). 1D-MOF possesses a biconnected mononuclear
center with a copper atom coordinated to two oxygen atoms
from two carboxyl groups of m-BDC and three nitrogen atoms
from 1-methylimidazole in a terminal position. This unique co-
We further unfolded the 1D-MOF to construct higher dimen-
sional frameworks with improved porosities and potential
functionalities by a bridging-ligand-substitution strategy. Im-
Scheme 2. The dimensions and basic structures of MOP-1, HKUST-1, MOF-14, MOF-505, and Cu-pdc-MOF.
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ordination environment combined with the 1D linear structure
makes the 1D-MOF easily adopt new coordination modes with
different binding angles and ligand sterics.
20 to 30 mm were closely packed together to form chains,
which is quite different from the discrete single crystals pre-
pared by the direct solvothermal reaction of Cu(NO3)2·3H2O
and BTC (Figure S7, Supporting Information).
After having successfully converted 0D-MOP to 1D-MOF and
1D-MOF to 2D- or 3D-MOFs, we tried to directly unfold the 0D-
MOP to construct 3D-MOFs by using a similar bridging-ligand-
substitution strategy. As initial attempts, 3,5-PDC and BPTC
were used for the replacement of m-BDC in MOP-1. However,
no recognizable new crystalline products were observed. This
was probably due to the mismatch of the topologies and con-
nectivity of the substituent and therefore the stepwise trans-
formation from 0D to 1D and then to 3D is necessary.
Taking many intriguing features into account, the high crys-
tallinity and tunability of the active metal sites of MOFs make
them attractive heterogeneous catalysts.[12] The structural
transformation of single crystals with different dimensions has
a great impact not only on their density, porosity, stability, and
sorption abilities, but also on their catalytic activity. This moti-
vates us to have a comprehensive investigation on the influen-
ces in catalytic kinetics by varying Cu MOF dimensionalities.
Taking MOP-1, 1D-MOF, and HKUST-1 for example, the hypoth-
esis is shown in Scheme 3, that is, the copper sites in 0D and
1D structures are often more accessible than those in the 3D
structure and thus may favor the initial catalysis performance.
During the catalysis, 0D and 1D ones might possess a higher
conversion rate than the 3D structure at the beginning. Then,
with the continuance of substrate diffusion, the conversion
rate of 3D MOF will catch up at the middle state. The final cat-
alysis performance mainly depends on the opening active sites
in the structure (Scheme 3).
With this in mind, we turned to topologically matched tri-
topic 1,3,5-benzenetricarboxylic acid (BTC) and 1,3,5-tris(4-car-
boxyphenyl) benzene (BTB) as the substitution linkers for the
SCSC transformation from 0D to 3D. For the replacement reac-
tion, crystals of MOP-1 were added into the DMF solution con-
taining BTC or BTB and left to exchange at 1308C for 24 h. The
resulting light green and light greenish blue single crystals
were attributed to HKUST-1 and MOF-14, respectively, as evi-
denced by PXRD measurements (Figure S5–S6, Supporting In-
formation) and SXRD analyses.[11] HKUST-1 and MOF-14, similar
to MOP-1, were both constructed from copper-based paddle-
wheel clusters and the replacement of the ditopic links in
MOP-1 by a tritopic substituent endows the formation of 3D
networks. These two 3D MOFs crystallize in tbo and pto topol-
ogies, respectively (Scheme 2). Interestingly, single crystals of
HKUST-1 obtained from SCSC transformations with sizes from
Based on the above hypothesis, we decided to evaluate the
catalytic activities of three representative structures, MOP-1,
1D-MOF, and HKUST-1. Given the fact that the oxidation of
benzylic CÀH bonds is one of the key industrial processes in
organic syntheses and these copper-based structures encom-
pass numerous potential catalytic centers, we decided to eval-
Scheme 3. Possible catalysis procedures of 0D, 1D, and 3D-MOF structures during the catalysis reaction.
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phenone demonstrates unique catalysis activities for copper
MOFs with different dimensions. This investigation can be po-
tentially applied to yield various MOF structures with various
coordination modes and may help in understanding the cata-
lytic mechanism in MOFs. This may also shed light on new
MOF catalyst design for efficient catalytic reactions.
Acknowledgements
This work was financially supported by the 973 Program
2013CB834704; Provincial Key Project of China (grant no.
7131253); the National Natural Science Foundation of China
(Grant No. 21471018, 21404010, 21201018, 21401094); 1000
Plan (Youth)’.
Figure 2. Results of catalysis performance for the CÀH activation of diphe-
nylmethane to benzophenone (catalyst, 0.02 mmol based on copper; diphe-
nylmethane, 0.125 mmol; TBHP, 0.312 mmol; benzonitrile, 0.5 mL; 608C,
24 h).
Keywords: catalysis · CÀH bond oxidation · multidimensional
networks · metal–organic frameworks · SCSC transformations
uate the catalytic activity for the transformation of diphenyl-
methane to benzophenone.[13] The kinetic study results of
MOP-1, 1D-MOF, and HKUST-1 in the transformation of diphe-
nylmethane to benzophenone supported our hypothesis
(Figure 2): initially, from 0 to 1 h, both MOP-1 and 1D-MOF
showed high conversion rates while the conversion rate using
HKUST-1 was rather low; after 1 h, the conversion rate of 1D-
MOF was higher than those of MOP-1 and HKUST-1; at 6 h, the
conversion rate of HKUST-1 was approaching that of MOP-1.
The final conversion and selectivity at 24 h were calculated to
be MOP-1 (conv. 91.6%, selec. 92.5%), 1D-MOF (conv. 94.3%,
selec. 96.1%), and HKUST-1 (conv. 87.0%, selec. 93.9%; the by-
product was benzhydrol). Given the ratio of the active sites
inside the crystals versus those on the surface (crystal size
measures at >20 microns) and the superior catalytic per-
formance, we can conclude that most of the reaction should
happen inside these MOF crystals like in the pores (as in the
case of HKUST-1) and/or between the chains (as in 1D-MOF).
As a control experiment, an equivalent amount of Cu(NO3)2
was also used as the homogeneous catalyst for the same pro-
cess. The conversion, as shown in Figure 2, ramped up quickly
and reached its plateau (47.8%) in only two hours and was
maintained unchanged for rest of the reaction (24 h).[14] As
a homogeneous catalysis system, in general, the catalyst or co-
catalyst might lose activity if some irreversible products are
formed. In our catalytic condition, TBHP can form an irreversi-
ble complex with copper ions which consequently kill the cata-
lyst. This actually leads to the conversion plateau of copper ni-
trates. It is a great example in which heterogeneous catalysts,
like MOFs, can outperform their homogeneous counterparts.
In conclusion, we have demonstrated a versatile link-ex-
change strategy for the SCSC transformation among different
dimensions. We successfully replaced isophthalic acid in the
MOP-1 structure with BTC and BTB from single crystal to single
crystal. To synthesize MOFs with distinct linker connectivity
and topologies, we started from 0D MOP-1 to form a 1D-MOF
and further utilized this 1D structure to construct two 3D-
MOFs that cannot be obtained from MOP-1 directly. Further-
more, the kinetic catalysis study of diphenylmethane to benzo-
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Received: April 29, 2015
Published online on August 21, 2015
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