Sequential Transformation of Zirconium(IV)-MOFs into Heterobimetallic MOFs Bearing Magnetic Anisotropic Cobalt(II) Centers

Article abstract of DOI:10.1002/anie.201808568

Heterometallic metal–organic frameworks (MOFs) allow the precise placement of various metals at atomic precision within a porous framework. This new level of control by MOFs promises fascinating advances in basic science and application. However, the rational design and synthesis of heterometallic MOFs remains a challenge due to the complexity of the heterometallic systems. Herein, we show that bimetallic MOFs with MX₂(INA)₄ moieties (INA=isonicotinate; M=Co²⁺ or Fe²⁺; X=OH^?, Cl^?, Br^?, I^?, NCS^?, or NCSe^?) can be generated by the sequential modification of a Zr-based MOF. This multi-step modification not only replaced the linear organic linker with a square planar MX₂(INA)₄ unit, but also altered the symmetry, unit cell, and topology of the parent structure. Single-crystal to single-crystal transformation is realized so that snapshots for transition process were captured by successive single-crystal X-ray diffraction. Furthermore, the installation of Co(NCS)₂(INA)₄ endows field-induced slow magnetic relaxation property to the diamagnetic Zr-MOF.

Full text of DOI:10.1002/anie.201808568

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Accepted Article
Title: Sequential Transformation of Zr(IV)-MOFs into Heterobimetallic
MOFs Bearing Magnetic Anisotropic Co(II) Centers
Authors: Shuai Yuan, Jun-Sheng Qin, Jian Su, Bao Li, Jialuo Li,
Wenmiao Chen, Hannah Drake, Peng Zhang, Daqiang Yuan,
Jinglin Zuo, and Hong-Cai Zhou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808568
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Sequential Transformation of Zr(IV)-MOFs into Heterobimetallic
MOFs Bearing Magnetic Anisotropic Co(II) Centers
Shuai Yuan,[b]‡ Jun-Sheng Qin,[b]‡ Jian Su,[a] Bao Li,[b] Jialuo Li,[b] Wenmiao Chen,[b] Hannah F. Drake,[b]
Peng Zhang,[b] Daqiang Yuan,[c] Jinglin Zuo,*[a] and Hong-Cai Zhou*[b,d]
Abstract: Heterometallic metal–organic frameworks (MOFs) allow
the precise placement of various metals at atomic precision within a
porous framework. This new level of control by MOFs promises
fascinating advances in basic science and application. However, the
rational design and synthesis of heterometallic MOFs remains a
challenge due to the complexity of the heterometallic systems. Herein,
we show that bimetallic MOFs with MX2(INA)4 moieties (INA =
isonicotinate; M = Co2+ or Fe2+; X = OH-, Cl-, Br-, I-, NCS-, or NCSe-)
can be generated by the sequential modification of a Zr-based MOF.
This multi-step modification not only replaced the linear organic linker
with a square planar MX2(INA)4 unit, but also altered the symmetry,
unit cell, and topology of the parent structure. Single‐crystal to single‐
crystal transformation is realized so that snapshots for transition
process were captured by successive single‐crystal X‐ray diffraction.
Furthermore, the installation of Co(NCS)2(INA)4 endows field-induced
slow magnetic relaxation property to the diamagnetic Zr-MOF.
two or more inorganic secondary building units (SBUs) represent
a unique class of MOFs.[3] The combination of multiple inorganic
SBUs has contributed to the structural diversity and functional
complexity of the MOF materials in particular. Furthermore, the
ability to arrange different metals with atomic precision within a
porous framework has promoted fascinating developments in
basic science and application. For example, the combination of
diverse metal clusters in heterometallic MOFs can lead to
emergent synergistic effects in catalysis and gas adsorption.[4]
However, it is challenging to synthesize heterometallic MOFs due
to their structural and compositional complexity.
Among the limited number of heterometallic MOFs with
different metal nodes, most of the structures are synthesized in a
“one-pot” reaction using bifunctional linkers that span two or more
different metal clusters. A handful of porous heterometallic
frameworks were reported using bifunctional linkers such as
pyrazolecarboxylate, isonicotinate (INA) and their derivatives.[5]
Very recently, this “one-pot” strategy was further developed by the
Rosi group in a comprehensive work.[6] In their work, they used
bifunctional INA linker to connect hard-acid metal cations (Zr4+,
Hf4+, and Dy3+) and soft metal cations (Co2+, Ni2+, Cu+, Cu2+, and
Cd2+) into a series of bimetallic MOFs with controlled topologies.
The synthetic difficulty of heterometallic MOFs can be attributed
to the sensitivity of the cluster formation in the reaction conditions.
In many cases, the incompatibility of the formation conditions of
different clusters has prevented the integration of a variety of
different inorganic SBUs into MOFs during a “one-pot” reaction.
Stepwise synthetic methods have been explored in order to
take control of the formation of different metal clusters
Metal–organic frameworks (MOFs), also known as porous
coordination polymers (PCPs), are an emerging class of porous
materials consisting of metal ions or clusters linked together in a
network by coordinating organic ligands.[1] Because of their
intrinsically porous structures and unlimited tunability, MOFs have
attracted considerable research interest in the fields of gas
storage, separation, heterogeneous catalysis, chemical sensing,
light harvesting, and drug delivery and magnetic materials.[2]
Among the various MOF topologies, heterometallic MOFs with
[a]
J. Su, Prof. Dr. J. Zuo
independently within
a heterometallic MOF. For example,
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Collaborative Innovation
Center of Advanced Microstructures
Nanjing University
Nanjing 210093, P. R. China
E-mail: zuojl@nju.edu.cn
preformed clusters terminated by pyridyl groups can be used as
starting materials to generate heterometallic MOFs, combining
various soft metal cations in the process.[7] Postsynthetic
modification (PSM) brings new opportunity to generate
heterometallic MOFs from monometallic MOFs as templates.
Using this method, heterometals can be anchored onto the
chelating sites of organic ligands or incorporated into the
inorganic SBUs by metathesis.[8] Furthermore, recent
developments utilizing Zr-MOFs have enabled the application of
a wider range of PSM reactions due to their high stability and
strong tolerance towards defects.[9] Metal cations and metal-
organic complexes can be attached to the coordinatively
unsaturated Zr6 clusters without losing the structural integrity.[10]
However, most of the modification processes traditionally used in
PSM rely on the metalation of the ligands or the replacement of
metal cations of the inorganic SBUs. Neither of these strategies
alter the topology of the parent framework. Therefore, the
structural diversity of heterometallic MOFs cannot be fully
[b]
Dr. S. Yuan, Dr. J.-S. Qin, Dr. B. Li, J. Li, W. Chen, H. F. Drake, Dr.
P. Zhang, Prof. Dr. H.-C. Zhou
Department of Chemistry
Texas A&M University
College Station, TX 77843, United States
E-mail: zhou@chem.tamu.edu
[c]
[d]
Dr. D. Yuan
State Key Laboratory of Structure Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
Prof. Dr. H.-C. Zhou
Department of Materials Science and Engineering
Texas A&M University
College Station, TX 77840, United States
[⁺]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201808568
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explored by the traditional PSM method. Recently, our group has
developed a series of methods to sequentially install metal
clusters and organic linkers into a preformed Zr-MOF, which leads
to the discovery of various multi-component coordination
assemblies.[11] Along this line, we sought to enrich the structures
and functions of Zr-MOFs by the incorporation of heterometals. In
this study, we introduced the MX2(py)4 units (py = pyridine; M =
Co2+ or Fe2+; X = OH-, Cl-, Br-, I-, NCS-, or NCSe-) into Zr-MOFs
because of their interesting electronic and magnetic properties.
For example, Co(NCS)2(py)2 and its derivatives have been
studied as single-ion magnets and the Fe(NCS)2(py)2 moieties
potentially exhibit spin crossover behavior.[12] To connect the
MX2(py)4 moieties and the Zr6O4(OH)4(COO)12 clusters, the
bifunctional INA was selected as the linker. Topologically, the
combination of a square planar MX2(py)4 with a cuboctahedral
Zr6O4(OH)4(COO)12 cluster will give rise to a network with ftw
topology. The ftw network topology can be viewed as two sets of
interlaced fcu networks (Figure 1k, colored by blue and yellow
respectively) connected through square planar nodes. Therefore,
we hypothesise that the fcu network could be postsynthetically
transformed into a ftw structure by replacing the linear linker with
square planar linker.
To prove our hypothesis, PCN-160-imine (Figure 1a), a
scaffold MOF with fcu topology, was initially constructed from Zr6
clusters
and
CBAB
linker
(4-carboxybenzylidene-4-
aminobenzoate, Figure 1g) according to the literature.[11b] The
CBAB linker (13.1 Å) in PCN-160-imine was subsequently
exchanged with Co-isonicotinate moiety (Co-INA2, Figure 1h) with
a similar length (13.7 Å) forming PCN-160-CoCl2 (Figure 1b). The
Co2+ center of PCN-160-CoCl2 was coordinated to two INA
ligands, two Cl-, and two solvent molecules in a trans orientation.
The neutral solvent molecules on the Co2+ center was further
replaced by a pair of INA ligands, giving rise to a square planar
metaloligand (Co-INA4, Figure 1i). The incorporated INA
coordinated to Co2+ through pyridyl groups, leaving the
carboxylates poised for the binding of Zr4+. Twelve uncoordinated
carboxylate groups around the large octahedral cage of the
framework formed a pocket to accommodate a 12-connected Zr6
cluster. As a result, a new Zr6 cluster was formed at the center of
the octahedral cage of PCN-160-CoCl2 after the treatment of the
parent MOF with Zr4+ and INA. The final product, namely PCN-
161-CoCl2 (Figure 1c), represents a new structure with a ftw
topology. In contrast to the parent fcu network that has large
octahedral cages and small triangular cages, the ftw daughter
structure only has one type of cubic cage (Figure 1d-f). A
comparison of the two networks shows that the tetrahedral cages
in the fcu network were augmented into cubic cages by four newly
formed Zr6 clusters (colored by yellow). Meanwhile, the large
octahedral cage was segmented into eight cubic cages by one Zr6
cluster and twelve INA linkers. The transformation of the fcu net
into a ftw net (Figure 1j,k) enhanced the symmetry of the resulting
framework. Ultimately, a smaller unit cell with halved edge length
can be assigned to the PCN-161-CoCl2. As result of the high
symmetry, the newly formed Zr6 clusters are crystallographically
identical to that of the original Zr6 clusters.
Notably, the scaffold MOF maintained single crystallinity
throughout the multi-step modifications. This allows us to monitor
the structural transformation process by single crystal X-ray
diffraction (SC-XRD, Table S1). Single crystals of PCN-160-imine
were incubated in a DMF solution of INA and CoCl2 at 75°C and
sampled for SC-XRD study every hour. Once the occupancy of
Co reached 100%, the solution was decanted and replaced by a
DMF solution of INA and Zr4+. The samples were analyzed by SC-
XRD to give a series of snapshots of the structural transformation
process. As shown in Figure 2a-c, the treatment of PCN-160-
imine with INA and CoCl2 solution leads to the incorporation of the
Co-INA2 moieties into the framework. The occupancy of Co was
refined to 0.4 in intermediate 1, indicating that 40% of the CBAB
linkers were replaced by the Co-INA2 moieties within 1 h of
exchange (Figure 2b). The linker exchange process almost
finished after 2 h when the occupancy of Co reached ~100%
(Figure 2c). The cell-edge length also slightly increased when the
shorter CBAB linker was replaced by the longer Co-INA2 linker. It
should be noted that the imine bond in CBAB was critical for the
successful linker exchange. It has been demonstrated by our
previous work that the imine bonds in PCN-160-imine can partially
dissociate leading to the easier replacement of the ligand by
acetate when treated with acetic acid/DMF solutions. Since
isonicotinic acid (pKa = 4.96) exhibits similar acidity to acetic acid
Figure 1. Single crystal structure of PCN-160-imine (a), PCN-160-CoCl2 (b),
and PCN-161-CoCl2 (c) with small tetrahedral cages (d, e) and cubic cages (f)
respectively. (g-i) Linkers and their topological representations. The
transformation of fcu topology (j) into ftw topology (k).
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(pKa = 4.76), the INA and CoCl2 solution in DMF is expected to
promote the CBAB dissociation and thereby facilitate the Co-INA2
incorporation. This hypothesis was further supported by control
experiments using an isostructural MOF with stable azobenzene-
4,4′-dicarboxylate (AZDC) as the linker.[13] No obvious Co-INA2
incorporation was observed by SC-XRD after 2 h.
After 1 h of incubation, the peaks of postsynthetically incorporated
Zr atoms were clearly observed at the center of the pore within
the MOF corresponding to a 20% occupancy (Intermediate 2,
Figure 2d). The structural transformation was almost completed
after 2 h when the occupancy of the incorporated Zr increased to
100% (Figure 2e). Meanwhile, the cell-edge decreased by half as
a result of the increased symmetry of PCN-161-CoCl2. The size
and morphology of the crystals remained unchanged throughout
the transformation (Figure S1). The gradual formation of the Co
centers and the Zr6 clusters monitored by SC-XRD
unambiguously demonstrated that the structural transformation
occurs in a single-crystal-to-single-crystal manner rather than
crystal dissolution and reformation. It should be noted that the
transition process of small crystals was expected to be faster than
that of large crystals. To diminish the effect of crystal sizes, single
crystals with similar size were picked each time from the same
batch of sample for SC-XRD.
PXRD patterns of the bulk materials correspond well to those
of the SC-XRD results (Figure S2). The replacement of CBAB with
Co-INA2 do not significantly affect the PXRD patterns with the
exception of the slightly shifted diffraction peaks to lower angles.
This is inline with the unit cell expansion caused by the Co-INA2
incorporation. The splitting of peaks at 5.9 degrees indicated the
generation of a small amount of PCN-161-CoCl2 without the
addition of Zr4+ in the solution. This was tentatively attributed to
the slightly dissolved PCN-160-imine that provided extra Zr4+. The
addition of Zr4+ led to the conversion of PCN-160-CoCl2 into PCN-
161-CoCl2 as indicated by the decreased peak at 5.2 degrees and
enhanced peak at 5.9 degrees. The structural transformation
completed after 4 h, generating a pure PCN-161-CoCl2 phase.
The 1H-NMR spectra of the digested samples indicate the gradual
replacement of CBAB by INA along with the structural
transformation (Figure S3). Element mapping by scanning
electron microscopy coupled with energy dispersive X-ray
spectroscopy (SEM/EDX) shows the uniform distribution of Co
and Cl throughout the crystal (Figure S4). The Zr/Co/Cl ratios
determined by SEM/EDX are comparable to the SC-XRD results
(Table S2). The Co/Zr ratios in bulk samples were further
monitored by inductively coupled plasma mass spectrometry
(ICP-MS). The compositions of samples determined by 1H-NMR,
SEM/EDX and ICP-MS agree with the structural model
Figure 2. Unit cell parameters, structures, and schematic representation of (a)
PCN-160-imine, (b) intermediate 1, (c) PCN-160-CoCl2, (d) intermediate 2, and
(e) PCN-161-CoCl2 based on SC-XRD results. The disorders were deleted for
clarity.
By incubating PCN-160-CoCl2 crystals in the DMF solution of
INA and Zr4+, the gradual growth of Zr6 clusters was observed.
Figure 3. Single crystal structures of PCN-161-MX2 with ten different MX2(INA)4 moieties.
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determined by SC-XRD (Table S2).
ion.[16] The D and E values to some extent indicate the
existence of magnetic anisotropy in the sample, which could
be further validated by the results of magnetic dynamic
relaxation. Firstly, the temperature dependence of the
alternating current (AC) magnetic susceptibility measurements
were carried out at 999 Hz in a 2 Oe ac field and zero dc field
which did not show slow magnetic relaxation signals (Figure
S9). To suppress the fast quantum tunneling of magnetization
through the spin-reversal barrier, the dc field was increased to
1000 Oe. Although no apparent peaks could be observed, the
dissociated AC curves under different frequencies (1–999 Hz)
in a 2 Oe ac field obviously indicate the existence of slow
magnetic relaxation (Figure 4b and S10). This result clearly
confirms that the behavior of single-ion magnets has been
successfully attached into diamagnetic MOFs via the
introduction of cobalt ions with the specific coordination
environment though stepwise modification.
The N2 adsorption/desorption isotherms indicated
a
decreased porosity and BET surface areas for the MOFs after the
structural transformation (Figure S5). This is attributed to the
partially occupied pore space and increased material density after
the incorporation of Co-INA2 and Zr6 clusters. In addition, the
stability of MOF decreased when replacing the strong covalent
bond with a labile Co-pyridyl coordination bond. This is reflected
by the structural decomposition of PCN-160-CoCl2 intermediate
upon solvent removal. Further incorporation of Zr6 clusters into
PCN-160-CoCl2 intermediate reinforced the structure by reducing
the pore size and increasing the connectivity of Co2+ center.
Therefore, PCN-161-CoCl2 shows a much higher surface area
than that of PCN-160-CoCl2. However, the total N2 uptake of
PCN-161-CoCl2 is still lower than the simulations (Figure S6),
possibly due to the partial decomposition of the MOFs during the
modification or solvent removal.
This synthetic method allows for the incorporation of a wide
variety of MX2(py)4 units (M = Co2+ or Fe2+; X = OH-, Cl-, Br-, I-,
NCS- or NCSe-) into Zr-MOFs. Single crystals of the PCN-161
structures with Co(OH)2(INA)4, CoCl2(INA)4, CoBr2(INA)4,
CoI2(INA)4, Co(NCS)2(INA)4, Co(NCSe)2(INA)4, FeCl2(INA)4,
FeBr2(INA)4, FeI2(INA)4, and Fe(NCS)2(INA)4 were successfully
obtained under identical modification conditions using respective
MX2 (Figure 3). The PXRD of these materials matched well with
the simulated data, which further proved their phase purity (Figure
S7). Note that a “one-pot” synthesis of PCN-161-CoCl2 using a
mixture of ZrCl4, CoCl2, and INA is possible by the judicious
control of the synthetic conditions as reported very recently.[6]
However, the “one-pot” synthesis usually ends up with a
thermodynamically stable product (PCN-161-CoCl2), whereas the
kinetic intermediate (PCN-160-CoCl2) cannot be isolated. More
importantly, it is difficult to introduce Co2+ centers into a MOF with
other counterion such as NCS- and NCSe- due to the counterions
exchange and decomposition under the solvothermal condition
necessary for MOF synthesis.
The magnetic properties of PCN-161-Co(NCS)2 was
investigated because the Co(NCS)2(py)4 moieties may exhibit
single ion magnetic behavior. Variable temperature direct-current
(dc) magnetic susceptibility for the samples was recorded at the
applied dc field of 1000 Oe and in the temperature range from 1.8
to 300 K (Figure S8). As shown in Figure 4a, the χMT value was
determined to be 2.96 cm3 K mol−1 at room temperature, which
is larger than the theoretical value for a spin-only Co2+. This
result can be ascribed to the unquenched orbital
contribution.[14] The χMT values almost keep constants along
with the temperature decreasing and begin to get smaller
below 100 K, which might be caused by the thermal
depopulation via Zeeman splitting and zero-field splitting.[14-15]
Low-temperature magnetization data for the sample was also
collected at 2, 3 and 4 K. At 2 K, the magnetization could achieve
3.05 NμB at 7 T (Figure 4a, inset), which is consistent with the
theoretical value for a spin-only Co2+ ion. Furthermore, these
experimental curves had been fitted by the utilization of the PHI
program with an anisotropic spin Hamiltonian (with gx = gy,
Equation S1). The simulated parameters are listed in Table S3,
and are similar to other values with the node of single cobalt
Figure 4. (a) Variable-temperature dc susceptibility data of the PCN-161-
Co(NSC)2. Solid red lines indicate the best fits with the PHI program. Inset: Field
dependence of the magnetization for the suspensions of the PCN-161-
Co(NSC)2 at 2, 3 and 4 K. (b) Temperature dependence of the out-of-phase (χ′′)
ac susceptibility data at different frequencies in 2 Oe ac field oscillating at 1-999
Hz with 1000 Oe applied dc field for PCN-161-Co(NCS)2.
In conclusion,
synthesized by sequential linker exchange and cluster
incorporation within scaffold Zr-MOF. The postsynthetic
a series of heterometallic MOFs was
a
modification process not only changed the composition of the
MOFs by introducing MX2(py)4 (py = pyridine; M = Co2+ or Fe2+; X
= OH-, Cl-, Br-, I-, NCS-, or NCSe-) units and a new Zr6 cluster, but
it also altered the symmetry and unit cell of the structure. The
MX2(py)4 moieties endowed intriguing magnetic and electronic
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Mater. 2017, 29, 1606134; h) J.-L. Liu, Y.-C. Chen, Y.-Z. Zheng, W.-Q. Lin,
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field-induced slow magnetic relaxation behavior of PCN-161-
Co(NCS)2. Considering the versatility of this method and the
unlimited tunability of MOFs, the synthesis of other heterometallic
MOFs with unexplored properties are envisioned. Apart from
providing a method to construct heterometallic MOFs, this work
also revealed the topological connection between two different
frameworks and the possibility of interconversion of these MOFs
through postsynthetic modifications.
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This work was supported by National Key R&D Program of China
(No. 2018YFA0306004) and the National Natural Science
Foundation of China (No. 21631006). The gas adsorption-
desorption studies for this research were supported by 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, Office of Basic Energy Sciences
under Award Number DE-SC0001015. Structural analyses were
supported by the Robert A. Welch Foundation through a Welch
Endowed Chair to HJZ (A-0030). The authors also acknowledge
the financial supports of U.S. Department of Energy Office of
Fossil Energy, National Energy Technology Laboratory (DE-
FE0026472) and National Science Foundation Small Bussiness
Innovation Research (NSF-SBIR) under Grant No. 1632486. The
Distinguished Scientist Fellowship Program (DSFP) at KSU is
gratefully acknowledged for supporting this work.
Keywords: metal–organic framework • heterometallic MOFs •
single‐crystal to single‐crystal transformation • postsynthetic
modification
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10.1002/anie.201808568
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
A
series of heterometallic metal–
S. Yuan, J.-S. Qin, J. Su, B. Li, J. Li, W.
Chen, H. F. Drake, P. Zhang, D. Yuan,
J. Zuo,* and H.-C. Zhou*
organic frameworks (MOFs) was
synthesized by sequential modification
of a Zr-based MOF. The multi-step
modification altered the symmetry,
topology, and unit cell of the framework
while maintaining its single crystallinity.
Furthermore, the postsynthetically
introduced Co(NCS)2(pyridine)4 moiety
endows field-induced slow magnetic
relaxation property to the diamagnetic
Zr-MOF.
Page No. – Page No.
Sequential Transformation of Zr(IV)-
MOFs into Heterobimetallic MOFs
Bearing Magnetic Anisotropic Co(II)
Centers
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