Exposed Equatorial Positions of Metal Centers via Sequential Ligand Elimination and Installation in MOFs

Article abstract of DOI:10.1021/jacs.8b04886

Metal-organic frameworks (MOFs) provide highly designable platforms to construct complex coordination architectures for targeted applications. Herein, we demonstrate that trans-coordinated metal centers with exposed equatorial positions can be placed in a MOF matrix. A Zr-based MOF, namely, PCN-160, was initially synthesized as a scaffold structure. Postsynthetic linker labilization was subsequently implemented to partially remove the original dicarboxylate linkers and incorporate pyridinecarboxylates. A pair of neighboring pyridyl groups was arranged at proper proximity within the framework to form trans-binding sites that accommodate different metal cations including Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Pd²⁺. Furthermore, the trans-coordinated Ni²⁺ sites in porous frameworks can be readily accessed by substrates along the equatorial plane, facilitating the catalysis as manifested by the superior activity in ethylene dimerization over that observed for a cis-chelated catalyst.

Full text of DOI:10.1021/jacs.8b04886

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Article
Expose Equatorial Positions of Metal Centers via
Sequential Ligand Elimination and Installation in MOFs
Shuai Yuan, Peng Zhang, Liangliang Zhang, Angel T Garcia-Esparza, Dimosthenis
Sokaras, Junsheng Qin, Liang Feng, Gregory S. Day, Wenmiao Chen, Hannah F.
Drake, Palani Elumalai, Sherzod Madrahimov, Daofeng Sun, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04886 • Publication Date (Web): 08 Aug 2018
Downloaded from http://pubs.acs.org on August 9, 2018
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Expose Equatorial Positions of Metal Centers via Sequential Ligand
Elimination and Installation in MOFs
Shuai Yuan,1‡ Peng Zhang,1‡ Liangliang Zhang,2 Angel T. Garcia-Esparza3,4, Dimosthenis So-
karas3, Jun-Sheng Qin,1 Liang Feng,1 Gregory S. Day,1 Wenmiao Chen,1 Hannah F. Drake,1 Palani
Elumalai,5 Sherzod Madrahimov,5 Daofeng Sun,2* and Hong-Cai Zhou1,6*
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1Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
2College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China
3Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, Menlo Park, California
94025, United States
4Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States
5Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
6Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United
States
ABSTRACT: Metal-organic frameworks (MOFs) provide highly designable platforms to construct complex coordination
architectures for targeted applications. Herein, we demonstrate that trans-coordinated metal centers with exposed equatorial
positions can be placed in a MOF matrix. A Zr-based MOF, namely PCN-160, was initially synthesized as a scaffold structure.
Postsynthetic linker labilization was subsequently implemented to partially remove the original dicarboxylate linkers and
incorporate pyridinecarboxylates. A pair of neighboring pyridyl groups was arranged at proper proximity within the framework
to form trans-binding sites that accommodate different metal cations including Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Pd2+.
Furthermore, the trans-coordinated Ni2+ sites in porous frameworks can be readily accessed by substrates along the equatorial
plane, facilitating the catalysis as manifested by the superior activity in ethylene dimerization than that observed for a cis-
chelated catalyst.
coordinative bonds, giving rise to topologically identical
INTRODUCTION
MOFs with diverse functionalities.9 Furthermore, metal
Metal-organic frameworks (MOFs) are an emerging class
cations and organic linkers can be exchanged by the virtue of
of crystalline porous materials constructed from metal-
the framework dynamics. This allows the formation of meta-
containing nodes and organic linkers.1 The structures and
stable MOFs that cannot be synthesized in a “one-pot”
reaction.10 Recent developments of stable MOFs bring new
structure-related properties of MOFs can be precisely
controlled at atomic precision by judicious design of both
opportunities to perform a wider range of PSM reactions
constituents.2 Due to their structural and functional
tunability as well as their ever-expanding application scope,
MOFs have become one of the most fascinating classes of
materials for both scientists and engineers.3 The ability to
control the framework structures in three dimensions has led
to fascinating supramolecular assemblies with unique
properties. For example, MOFs constructed with flexible
frameworks,4 exposed metal sites,5 and synergistic moieties6
exhibited unique cooperative gas adsorption behaviors.7
while maintaining the framework’s structural integrity.11 For
example, as stable and versatile platforms, Zr-MOFs allow
the incorporation of terminal ligands,12 dicarboxylate
linkers,13 metal cations,14 and metal-organic complexes onto
the coordinatively unsaturated Zr6 clusters. Most recently,
our group proposed the concept of “MOF retrosynthesis” to
construct complex MOFs by “layer-on” molecular
elaborations to preformed Zr-MOFs using a series of
postsynthetic strategies.6b Along this line, we seek to use
MOFs as the matrix to assemble coordination structures that
are difficult to synthesize in other systems, such as the trans-
coordinated metal complexes.
Despite numerous advantages, the rational design and
synthesis of targeted MOF structures are ultimately limited
by the lack of control over the framework assembly in a
conventional “one-pot” reaction. To overcome the inherent
limitations of the “one-pot” strategy, postsynthetic
modification (PSM) methods were developed.8 Functional
groups and metal complexes have been postsynthetically
anchored on the organic linkers through covalent or
Trans-chelating or trans-spanning ligands are bidentate
ligands that bind the opposite sites of a complex with square-
planar geometry at or near 180° angle.15 Although a far larger
number and variety of cis-chelating ligands than trans-
chelating ones have been developed for catalysis and their
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chemistry thoroughly explored, catalysts containing trans-
into 4-amino benzoic acid and 4-formylbenzoic acid through
hydrolysis by acetic acid/N,N-dimethylformamide (DMF)
solution to create missing-linker defects (Figure 1e). Since
isonicotinic acid (pKa = 4.96) exhibits similar acidity to
acetic acid (pKa = 4.76), we hypothesized that it would
replace the CBAB linker and form trans-binding sites (Figure
1f). In fact, metalloligands with similar trans-coordinated
geometries have been adopted in MOF synthesis.19 Therefore,
we expect that the proposed structure is feasible.
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chelating ligands may display alternative chemistry, activity
and selectivity in various catalytic processes. However, many
attempts to generate trans-chelating complexes have led to
the formation of coordination polymers in which bidentate
ligands act as bridging linkers. Although this problem can be
addressed by tuning the ligand rigidity and bulkiness,16
ligand synthesis remains a challenge. Herein, we show that
the trans-coordinated metal centers with exposed equatorial
positions can be created in three-dimensions using MOFs as
the matrix.
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RESULTS AND DISCUSSION
Figure 1. Structure of PCN-160 (a) and PCN-160-R%M with
trans-chelating ligands (b). Transformation of ligand frag-
ment in PCN-160 (c) by CBAB exchange (d), linker labializa-
tion (e), and installation of M-INA2 (f). These figures are
based on respective single crystal structures after removal of
the disordered fragments.
Creating Trans-Coordination Sites in MOFs. In the
literature, trans-chelating bis-pyridyl ligands are usually
constructed by connecting two 2-pyridyl groups by
judiciously designed spacers such as 1,2-dialkynylbenzene
(Figure S1a).16a, 16c We hypothesized that MOFs can act as
three-dimensional spacers for the alignment of two pyridyl
moieties in the trans-position (Figure S1b). For this purpose,
we used a previously reported Zr-based MOF, Zr6-AZDC or
PCN-160, as the matrix.17 It is noteworthy that Zr-AZDC
based MOFs were firstly reported in 2012 and reproduced
using different synthetic methods in many other literatures.17
By replacing the azobenzene-4,4 -dicarboxylate linker
(AZDC) with a pair of 4-pyridinecarboxylates (also known as
isonicotinate or INA), a bis-pyridyl sites at trans-position will
be formed (Figures 1a, b). Direct exchange of AZDC by INA
or M-INA2 was unsuccessful because of the inertness of the
AZDC linker. Our previous works have shown that AZDC can
be labilized by the exchange of an imine-based linker with
identical length, denoted as CBAB (4-carboxybenzylidene-4-
aminobenzate, Figures 1c, d).18 The CBAB can be dissociated
Figure 2. (a) Exchange process of CBAB by Ni-INA2 moni-
tored by UV-vis. (b) PXRD patterns of PCN-160 and PCN-160-
R%Ni. (c) N2 adsorption-desorption isotherms of PCN-160
and PCN-160-R%Ni at 77 K, 1 bar.
Monitoring Sequential Ligand Elimination and
Installation. As anticipated, Ni-INA2 moieties were
successfully incorporated into PCN-160 by CBAB exchange
and subsequent treatment with a solution of INA and NiCl2.
The final product was verified by single crystal X-ray
crystallography (Figure 1f, Table S1), powder X-ray diffraction
(PXRD), inductively coupled plasma mass spectrometry
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(ICP-MS), and H-NMR digestion experiments (Table S3).
The effect of CBAB linker and reaction time on the level of
incorporation of M-INA2 to PCN-160 structure was
systematically studied. For this purpose, first, a series of
PCN-160 samples with variable CBAB linker content were
synthesized through a linker exchange procedure reported in
literature (Figures S2-4). These samples are denoted as PCN-
160-R%, where R% stands for the percentage of the CBAB
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linker to the total amount of linkers in the sample (Table S2).
S4). The PXRD patterns and N2 sorption isotherms further
verified the maintained crystallinity and porosity. (Figures
S7-8). Although different metal sizes can affect the length of
M-INA2 linkers, the framework of PCN-160 possesses certain
flexibility that can tolerate the change caused by different
metals. X-ray photoelectron spectroscopy (XPS) showed that
all the metals incorporated into MOFs are in +2 oxidation
states (Figures S9-13).
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Crystals of PCN-160-R% were subsequently incubated in
DMF solution with an excess amount of INA and NiCl2 at
75ꢀ°C. Incorporation of the INA linker to PCN-160 structure
was monitored by ultraviolet–visible (UV-vis) spectroscopy
as both AZDC and INA linkers possess distinctive UV-vis
absorption peaks at 432ꢀnm and 264 nm respectively. The Ni-
INA2 exchange ratio in PCN-160, defined as the amount of
Ni-INA2 divided by the total amount of all the linkers
(CBAB+AZDC+Ni-INA2), was calculated based on the UV-vis
data of the digested MOFs. As shown in Figure 2a, the Ni-
INA2 content in the samples increased with time and leveled
off after 4 h, indicating that a dynamic equilibrium reached
between the MOF scaffold and the solution. The CBAB
ligands are important for the incorporation of Ni-INA2 as
manifested by the low exchange ratio and slow reaction
kinetics of PCN-160. After the treatment by INA and NiCl2,
the CBAB linker was completely replaced by Ni-INA2
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according to UV-vis and H-NMR data. The maximum Ni-
INA2 exchange ratios were roughly equal to the CBAB
content in the initial sample (Table S3). The size and
morphology of the MOF particles were maintained
throughout the postsynthetic modification, suggesting a
single crystal to single crystal transformation process. It is
important to note that the direct synthesis of MOFs using
CBAB (PCN-160-R%) or Ni-INA2 (PCN-160-R%Ni) as starting
materials was not successful due to the labile imine bonds or
Ni-pyridyl bonds that tend to be destroyed by the harsh
solvothermal synthesis necessary for Zr-MOF synthesis
(Figure S5).
Figure 3. Photos and microscope images of PCN-160 (a),
PCN-160-65% (b), and PCN-160-65%M (M = Mn, Fe, Co, Ni,
Cu, and Pd).
As revealed by the powder X-ray diffraction (PXRD)
patterns (Figure 2b), PCN-160-R%Ni samples maintained
crystallinity. They showed similar PXRD patterns to that of
the parent PCN-160 with slight changes of the peak
intensities and some shifting of the peak positions. The peak
intensity at 6.0ꢀdegrees decreased while the peaks at 10.4
degrees increased with the incorporation of Ni-INA2 which
matched well with simulations. The peaks of PCN-160-R%Ni
were slightly shifted to a lower angle as compared to that of
the parent PCN-160 (Figure S6), corresponding to a unit cell
expansion which resulted from the replacement of the
shorter AZDC linkers (13.1 Å) with longer Ni-INA2 linkers
(13.7 Å). After the incorporation of the Ni-INA2 linkers, the
PCN-160-R%Ni were still porous, although the total N2
uptake and BET surface area decreased (Figure 2c). This is
attributed to the increased material density and decreased
pore size after the Ni-INA2 ligand incorporation. In addition,
the partial decomposition of the framework after solvent
removal also accounts for the decreased surface area. Indeed,
PCN-160-R%Ni samples showed decreased mechanical
stability due to the relatively weak Ni-pyridyl bonds.
Coordination Environment of Metal Sites. Single
crystal structures of PCN-160-47%Ni and PCN-160-47%Pd
were successfully obtained, providing direct structural
evidence of the M-INA2 moieties in a trans-geometry (Table
S1). The existence of Ni and Pd was clearly refined with an
occupancy of 50%, consistent with the ICP-MS results.
However, the terminal ligands cannot be precisely refined
because of the disorder within the structure. To gain a better
understanding of the structure of the M-INA2 moieties,
single crystals of molecular M-INA2 complexes were isolated
by slow cooling of saturated solutions containing the
corresponding MCl2 salts and INA ligands. Both Ni-INA2 and
Cu-INA2 show a six-coordinate octahedral metal center
surrounded by two INA ligands, two Cl- ions, and two DMSO
molecules in a trans orientation (Figure S14). By adding 30%
water to precipitate the Ni-INA2 and Cu-INA2 crystals, the Cl-
, and DMSO ligands could be replaced by OH- and H2O
respectively. The identity of the coordinated solvent on the
metal center was further confirmed by thermogravimetric
analysis/mass spectrometry (TGA-MS) plots, which revealed
the loss of coordinated solvent molecules before 250ꢀ°C
(Figure S15). The coordination environment of Ni in PCN-
160-47%Ni was further studied by extended X-ray absorption
fine structure (EXAFS) at the Ni K-edge. EXAFS fitting
confirms the presence of four direct O/N ligands and two Cl
in the PCN-160-47%Ni MOF sample (Table S5 and Figure
S16a). The EXAFS technique does not have the sensitivity to
separate N from O, hence both ligands are treated with the
Versatility of M-INA2. The method for incorporating Ni-
INA2 linkers to PCN-160 structure can also be used to
incorporate a series of M2+ transition metals including Mn2+,
Fe2+, Co2+, Ni2+, Cu2+, and Pd2+ into the structure of a MOF.
The successful incorporation of different metals into the
framework can be directly observed by microscope images of
the respective single crystals (Figure 3). The compositions of
PCN-160-65%M (M = Mn, Fe, Co, Ni, Cu, and Pd) were
1
further determined by ICP-MS and H-NMR. Although the
same single scattering path. For comparison,
a
amount of incorporated M-INA2 varied for different samples,
the metal to INA ligand ratio was approximately 1:2. (Table
complementary fitting was performed on PCN-160-47%Ni
assuming only six O/N ligands without Cl-coordination;
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however, the unsuitability of that model was clear (Figure
substrates can more readily reach the trans-coordinated Ni
center from any direction within the equatorial plane,
facilitating the ethylene binding and accelerating the
reaction. Moreover, PCN-160-47%Ni can be recycled for at
least three runs by separation from the reaction mixture
through centrifugation at the end of each reaction without
significant loss of Ni content and crystallinity (Figure S21).
Elemental mapping at the microstructural level by scanning
electron microscopy (SEM) with energy dispersive X-ray
spectrometry (EDX) shows uniform distribution of Ni
throughout the crystal (Figure S22). The catalytic activity
evaluated by the amount of generated C4 only reduced by
14% after three cycles. (Table S7)
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S16b and Table S6). Furthermore, the pre-edge of the X-ray
absorption near edge structure (XANES) spectra also
indicates an octahedral local geometry (see comparison of
the reference standards NiO and NiCl2∙6H2O, with PCN-160-
47%Ni at Figure S18). The single crystal structures, TGA-MS,
XANES, and EXAFS manifest that the Ni center is six
coordinated to INA ligands, two Cl- ions, and two solvent
molecules in octahedral geometry. In contrast to the first-
row transition metals, Pd was determined to be four
coordinated with two INA and two Cl- ligands, leading to a
square planar geometry. Strictly speaking, only PCN-160-
R%Pd can be named as a trans-chelating complex, because
the trans-chelating ligands are defined as bidentate ligands
that span the trans-positions of square-planar complexes.
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Table 1. Ethylene dimerization catalyzed by Ni-
containing catalystsa
Very recently, the Rosi group also reported Zr-MOFs built
from Cu-(2-methyl-INA)2 complex as a supramolecular linear
dicarboxylate linker.19b This work is in line with our
hypothesis that M-INA2 can act as a linker fragment to
support the MOF structure. Compared with our stepwise
synthetic method, the one-pot strategy by Rosi group allows
facile preparation of heterometallic MOFs with various
structures. However, these structures are supported by M-
INA2 ligands exclusively, resulting in relatively labile
frameworks. PCN-160-R%M contains both stable AZDC
linker to form a robust matrix and M-INA2 ligands as
catalytic sites, which is more suitable for catalysis. Since the
stability and porosity of PCN-160-R%Ni decrease as R%
escalates, the exchange ratio was controlled up to 47% for
catalytic applications.
selectivity (%)
C4 intrinsic
entry
catalystsb
activityc (h-1)
1-C4
2-C4
C6 + C8
1d
2e
3d
4
PCN-160
0
0
0
0
none
0
0
0
0
NiCl2@PCN-160
Ni-INA2
171±30
81.1
9.9
21.1
19.5
29.0
15.8
9.0
3.0
6.4
4.3
1.8
304±50
288±20
455±40
3360±240
75.9
74.1
66.7
82.4
5
(bpy)NiCl2
UiO-67-50%Ni
PCN-160-47%Ni
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7e
Ethylene Dimerization Reaction. With a series of trans-
coordinated metal complexes with exposed equatorial
positions incorporated into MOFs, we further explored their
catalytic properties. The Ni catalyzed ethylene dimerization
reaction was selected as a model reaction in this study. The
ethylene dimerization reactions were carried out in a 25-mL
stainless steel reactor under 40 bar of ethylene in toluene at
25ꢀ°C with diethylaluminum chloride (Et2AlCl) as the
activator (Table 1, Table S7, and Figure S19). Both PCN-160
and the activator Et2AlCl showed no catalytic activity for
ethylene dimerization as controls, thus ruling out the
possibility of any background reaction in the absence of Ni2+
(entries 1 and 2). To eliminate the influence of Ni2+ trapped
in the MOF cavity, PCN-160 was incubated in NiCl2 solution
without INA ligands (NiCl2@PCN-160). It showed very low
catalytic activity (entry 3). The molecular Ni-INA2 and
(bpy)NiCl2 complexes displayed poor catalytic activity, due to
the low solubility and limited surface area which prevent the
exposure of active sites (entries 4 and 5). For comparison, the
catalytic performance of UiO-67-50%Ni with cis-chelating
Ni-2,2'-bipyridyl centers was also tested.20 Notably, we found
that PCN-160-47%Ni has an intrinsic activity of 3360 h-1 for
ethylene dimerization reaction, which is ~7 times higher
than that of UiO-67-50%Ni. The PCN-160-47%Ni and UiO-
67-50%Ni showed butene selectivity comparable to other
previously reported catalysts (entries 6 and 7).21 Since PCN-
160-47%Ni and UiO-67-50%Ni have similar Ni content, pore
size, and BET surface area (Figure S20), the superior catalytic
activity of PCN-160-47%Ni was attributed to its unique trans-
coordination environment of the Ni. The trans-coordinated
Ni sites in PCN-160-47%Ni are more sterically accessible than
those of the Ni-2,2'-bipyridyl sites in UiO-67-50%Ni. As such,
a Reactions were performed in toluene under 40 bar of ethylene at 25
°C. Et2AlCl (Al/Ni = 300) was used as activator and reaction time was
1 h. b Catalysts containing 5 μmol Ni were used unless otherwise
notified. Calculated based on the moles of butenes generated and
the Ni content in the catalyst. Standard deviations were calculated
c
d
based on three runs. MOF catalysts (5 mg) were used. The Ni
e
loading in NiCl2@PCN-160 is 0.4 wt%, as determined by ICP-MS.
e
Et2AlCl (1.5 mmol) was added. The first cycle of PCN-160-47%Ni
catalyst.
CONCLUSIONS
In conclusion, trans-coordinated metal centers were
created in MOFs by sequential ligand elimination and
installation of M-INA2 moieties. This method enables the
convenient incorporation of different M2+ transition metals
of various ratios into trans-binding sites within a porous
framework. Furthermore, we have shown that the trans-
coordinated metal sites in MOFs have intriguing properties
in catalytic processes due to the exposed equatorial
positions, as demonstrated by the remarkably improved
activity of PCN-160-47%Ni in catalytic ethylene dimerization.
Considering the versatility of this method and the unlimited
tunability of MOFs, many more trans-coordinated metal
complexes are envisioned. Apart from providing a method to
construct trans-coordinated complexes, this work also
highlights the vast opportunities of using a MOF matrix to
assemble coordination architectures that are otherwise
difficult or impossible to realize.
ASSOCIATED CONTENT
Supporting Information
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Tranchemontagne, D. J.; Mendoza-Cortes, J. L.; O'Keeffe, M.;
Text, tables, and figures giving experimental procedures for
the syntheses of the ligand, MOFs, PXRD, N2 adsorption
isotherms, TGA, and crystallographic data of the structure.
The Supporting Information is available free of charge on the
ACS Publications website.
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AUTHOR INFORMATION
Corresponding Author
* zhou@chem.tamu.edu; dfsun@upc.edu.cn
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‡S.Y. and P.Z. contributed equally. Original idea was
conceived by H.-C.Z. and S.Y.; experiments and data analysis
were performed by S.Y., P.Z., L.F., G.S.D., W.C. and L.Z.;
single crystal structures were refined by J.-S.Q.; catalysis was
performed by P.Z. and P.E.; EXAFS was performed by A.T.G.-
E. and D.S., manuscript was drafted by H.-C.Z, S.Y., P.Z.,
H.F.D., S.M., and D.S. All authors have given approval to the
manuscript.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The gas adsorption-desorption studies of 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), National Science Foundation
Small Business Innovation Research (NSF-SBIR) under Grant
No. (1632486), and NPRP award (NPRP9-377-1-080) from the
Qatar National Research Fund. The EXAFS experiments were
performed at the Stanford Synchrotron Radiation
Lightsource (SSRL) of the SLAC National Accelerator
Laboratory, and use of the SSRL is supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
under Contract No. DE-AC02-76SF00515. The EXAFS fitting
used resources of the National Energy Research Scientific
Computing Center, a DOE Office of Science User Facility
supported by the Office of Science of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. This
material is based upon work supported by the National
Science Foundation Graduate Research Fellowship under
Grant No. DGE: 1252521.
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