Heterogenization of Trinuclear Palladium Complex into an Anionic Metal-Organic Framework through Postsynthetic Cation Exchange

Article abstract of DOI:10.1021/acs.organomet.9b00286

The innate modular nature of metal-organic frameworks (MOFs) enables postsynthetic modification of the crystalline framework, thereby resulting in novel properties. Anionic MOFs are an interesting category of frameworks since their pore environment can be modified using a simple ion-exchange process. In this work, we demonstrate that via directly ion exchanging an anionic metal-organic framework can not only be the host for a palladium trinuclear transition metal complex but also gain catalytic capability as a hybrid system in the semireduction of internal alkynes. The confined pore space within the MOF structure and the thiol groups of the cluster successfully minimize the detrimental aggregation of palladium during the catalytic process, thereby resulting in a heterogeneous recyclable catalyst system.

Full text of DOI:10.1021/acs.organomet.9b00286

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Heterogenization of Trinuclear Palladium Complex into an Anionic
Metal−Organic Framework through Postsynthetic Cation Exchange
Junyu Ren, Pui Ching Lan, Meng Chen, Weijie Zhang, and Shengqian Ma*
Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States
S
* Supporting Information
ABSTRACT: The innate modular nature of metal−organic frameworks
(MOFs) enables postsynthetic modification of the crystalline framework,
thereby resulting in novel properties. Anionic MOFs are an interesting
category of frameworks since their pore environment can be modified
using a simple ion-exchange process. In this work, we demonstrate that via
directly ion exchanging an anionic metal−organic framework can not only
be the host for a palladium trinuclear transition metal complex but also
gain catalytic capability as a hybrid system in the semireduction of internal
alkynes. The confined pore space within the MOF structure and the thiol
groups of the cluster successfully minimize the detrimental aggregation of
palladium during the catalytic process, thereby resulting in a heteroge-
neous recyclable catalyst system.
complexes into MOFs (ZJU-28 and MIL-101-SO3) via cation
exchanging.11 Subsequently, Rosseinsky and co-workers
presented a recyclable heterogeneous [catalyst]+@[MOF]−
system for a Diels−Alder catalyst.12 This approach avoids
covalent tethering which requires intense synthesis that has
deleteriously affects to the catalytic system. Besides, complexes
were partially immobilized via complementary electrostatic
interaction instead of fully depending on the size limitations.
Although the above-mentioned MOF-supported catalytic
species have been developed, research on multinuclear
complex encapsulation in MOFs is still limited.
Compared with the single-site transition metal catalysts,
trinuclear metal−aromatic complexes are an interesting
category of material in catalytic field.13 They are metal
analogues of the cyclopropenyl cation, with their all-metal
aromaticity involving d-type atomic orbitals. Among the
transition metal family, palladium is a good candidate to
generate aromatic bonding patterns involving unfilled d
orbitals. The triangles are formed by three Pd atoms, with
three identical 60° angles and the same metal−metal bonds.
The coexistence of thiol groups and phosphines stabilizes the
Pd core while leaving metal sites accessible for substrates.
Palladiums and heteroatoms were nearly coplanar (largest
dihedral angles remaining below 2°). The phosphines on the
palladium centers point in the same direction, and sulfur
substituents point in the opposite direction, with a perfect
alternation (Figures S1 and S2). With a positively charged
metal core, they act as a Lewis acid, while the delocalized
INTRODUCTION
■
Much attention has been focused on the heterogenization of
homogeneous catalyst on solid supports in past decade, due to
their integration showing the potential to combine the
advantages of both homogeneous and heterogeneous catal-
ysis.1,2 Loading onto supports could prevent the metal atoms
or small clusters from aggregating, which leads to catalyst
deactivation. Activated carbon, zeolites, and organic polymers
are the three most popular hosts for the molecular catalyst and
result in numerous efficient and recyclable catalyst systems.3
However, most of these are amorphous materials, and it is
difficult to decipher the structure−property relationships as
well as host−guest interactions.4
As an emerging class of porous materials with high
crystallinity nature, metal−organic frameworks (MOFs)
feature great amenability to design, high surface areas, tunable
pore sizes, and tailorable functionality, and this class has long
been proposed as a host for the heterogenization of
homogeneous catalysts.5,6 Catalyst@MOFs hybrids inherit
the features of active and selective catalytic sites, program-
mable chemical environment, and easy postreaction separation
from the parent constituents. In addition, metal−support
interactions are well-known to isolate the active centers and
prevent them from aggregating, by which means the catalyti-
cally active centers could be isolated within the pore space and
thus decrease multimetallic decomposition.7,8 Direct encapsu-
lation without covalent interaction is one common strategy for
heterogenization.9 Either MOFs are built around the catalysts
(bottle-around-ship strategy), or molecular catalysts are
synthesized in situ and limited inside the framework (ship-in-
the-bottle strategy). An alternative method was first reported
by encapsulating cationic catalysts into anionic cages.10
Sanford’s group then presented the incorporation of cationic
Special Issue: Organometallic Chemistry within Metal-Organic
Frameworks
Received: April 30, 2019
© XXXX American Chemical Society
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HOMO contributes Lewis basic properties to the triangular
core. In comparison with the mononuclear complexes, they
show more bonding modes and different mechanism as
catalysts.14
lighter red. Fresh stock solution was added several times until a
comprehensive color change of the crystals was observed under
microscope (Figure 1). Optical microscopy of the cross section
Research on new catalytic species for the semihydrogenation
of acetylene to ethylene is the subject of present research
interest.15 The reaction is widely applied in industry,
particularly in the case of industrial polymerization of ethylene
to polyethylene to purify the feedstock from acetylene, which
would otherwise poison the polymerization catalyst.16 Transfer
hydrogenation (TH) reaction, referring to the addition of
hydrogen to a molecule from a non-H2 hydrogen source, is
becoming the center of research to access various hydro-
genated compounds, mainly due to (i) no need for hazardous
pressurized H2 gas or elaborate experimental setups, (ii) easy
handling of the hydrogen donors, and (iii) ready accessibility
of the catalysts that are involved.17
For noble-metal−aromatic clusters, heterogenization would
enable separation and recyclability which has a profound
significance in industry. In this work, we report the
encapsulation of two trinuclear cationic palladium clusters
(Pd3 cluster-1 and Pd3 cluster-2, Supporting Information,
Section S3) inside the pores of the anionic metal−organic
framework bio-MOF 100 (Scheme 1), generating bio-MOF
Figure 1. Optical microscopy images of (a)bio-MOF 100 and (b)bio-
MOF 100-1.
(Figure S4) showed Pd3 complexes spreading across the whole
crystal rather than just the periphery of crystal. Through ICP-
mass spectrometry (MS) analysis, the Pd/Zn ratio was
measured after digestion in diluted HCl, by which the
exchange result was further confirmed. Results showed the
Pd contents are, respectively, 5.16 and 6.40 wt % in the bio-
MOF 100-1 and bio-MOF 100-2 complexes with 15.2 and 14%
of the H2NMe2+ counterion being replaced. From the aspect of
+
the charge balance, one complex replaces one H2NMe2 .
Scheme 1. Illustrative Scheme of the Encapsulation of
Palladium Cluster within bio-MOF 100 for Heterogeneous
Chemo-Selective Semireduction of Internal Alkynes
However, if the size differences between Pd cluster and
+
H2NMe2 were considered, then the upper limit for the
replacement is about 15%, because the diameter of the
complex (∼16 Å) is bigger than the H2NMe2+ cations (∼3 Å),
as the ICP-MS data proved.
NMR spectroscopy was also applied to verify the
incorporation of the Pd3 cation in anionic framework by
digesting Pd3@bio-MOF 100 complex in DCl/dmso-d6
(Figure 2). Since the Pd3 cation is too stable for the digestion
100-1 and bio-MOF 100-2. Catalytic performance parameters
regarding stability, activity, and product selectivity were
evaluated for the title sample. The catalytic activity of the
cluster@MOF system is comparable to that of the homoge-
neous counterpart. It is also interesting to observe that the
catalytic behavior of the trinuclear palladium cluster was
altered after encapsulated into the confined MOF structure.
This strategy liberates the metal catalyst and the MOF from
intense synthetic modification. The obtained cluster@MOF
complexes are proven to be heterogeneous recyclable catalysts
for the semihydrogenation of alkynes.
1
Figure 2. H NMR spectrum of bio-MOF 100-1 digested in DCl/
dmso-d6.
conditions, a holistic analysis of the encapsulated guests were
performed to analyze the postencapsulation complex. Quanti-
tative study of cation exchange was done through comparing
integrations of relative peak areas from the pyridine ring of
adenine (8.52 ppm, 1H) and methyl group on cluster (2.09
ppm, 9H). For bio-MOF 100-1 and bio-MOF 100-2, the
replacement ratio was determined to be 22 and 21% according
to the formula of bio-MOF 100 (Zn8(ad)4(BPDC)6O2·
4Me2NH2). The encapsulation ratios determined by NMR
RESULTS AND DISCUSSION
Encapsulation via Cation Exchange. First, cation-
■
exchange experiments were performed to replace the original
+
counterion H2NMe2 . Bio-MOF 100 crystals were soaked into
CHCl3 solutions of Pd3 complexes for 1 week to ensure certain
degree of cation exchange. Transparent crystals were found to
transform to a deep color, while the solvents’ color turned to a
B
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spectrum differ from the ICP-MS data, which could be
attributed to the strength of the cluster. It did not completely
decompose, and part of it was filtered together with the organic
ligand before the ICP-MS test.
Comparison of PXRD patterns of Pd3@MOF with pure bio-
MOF 100 reveals that the structure remains intact after the
cation-exchanging process (Figure 3). Also, it shows that the
(Table 1, entry 1) indicates that bio-MOF 100 does not
catalyze this reaction. Under standard reaction conditions
Table 1. Investigation of bio-MOF 100-1 and bio-MOF 100-
a
2 in the Semireduction of Internal Alkynes
b
b
entry substrate
catalyst
t (h) conversion
selectivity
1
2
3
4
5
a and b
a
a
b
b
bio-MOF 100
bio-MOF 100-1
bio-MOF 100-2
bio-MOF 100-1
bio-MOF 100-2
Pd complex-1
Pd complex-2
Pd complex-1
Pd complex-2
96
96
48
36
36
96
10
10
10
0%
95%
94%
91%
89%
88%
90%
95%
96%
95%
94%
94%
96%
92%
90%
98%
99%
6
7
a
b
a
Reaction conditions: 30 mg Pd3@bio-MOF 100 catalyst, 45 μL
formic acid(1.2 mmol), 167 μL triethylamine(1.2 mmol), 3.5 mL
THF, 70 °C. Determined by H NMR.
Figure 3. PXRD patterns of (a) bio-MOF 100, (b) bio-MOF 100-1,
and (c) bio-MOF 100-2.
b
1
Pd3@MOF material could retain its structure after storing for a
long time. The N2 sorption studies (Figure 4) showed that in
(Table 1, entry 2), the model reaction using 1-phenylpropyne
as substrate, 5.16 wt % bio-MOF 100-1 (5.16 wt % based on
Pd content) as the catalyst, and HCO2H/NEt3 as the hydrogen
source, gave 95% yield in THF after heating at 70 °C for 96 h.
The product shows good Z selectivity (95%) together with
1
(E)-phenylpropene (5%) (determined by H NMR, Figure
S8). Then, the substrate was extended to diaryl-substituted
acetylenes (substrate b). A conversion rate of 91% was
achieved by bio-MOF 100-1 to the product with 94%
selectivity toward [Z]-stilbene (Table 1, entry 4). Compared
with the homogeneous counterpart, the Pd3@MOF system
shows variations for catalytic behavior, which could be
attributed to the long-range chemical environment surround-
ing the metal clusters. The adsorption and binding behavior of
substrates on the metal site could be altered after encapsulating
the palladium complex into a confined space. Last, the alkane
was not observed even prolonging the heating time for another
24 h.
Figure 4. N2 sorption isotherms at 77 K of bio-MOF 100, bio-MOF
100-1, and bio-MOF 100-2.
Recycling experiments were performed (Figure 5) and show
that bio-MOF 100-1 could be recycled 2 times with 74% yield
for the second cycle and 69% for the third cycle (determined
by 1H NMR, Figures S9 and S10). For bio-MOF 100-2, it was
observed that the second and third recycling percents are 72
and 64%, respectively (Figures S12 and S13). The products
showed Z-preferred selectivity in all recycling experiments.
From the PXRD patterns, it was observed that the overall
structure of the MOF did not change after 3 catalytic cycles.
Compared with the homogeneous counterparts, the Pd3@
MOF version shows the ability as heterogeneous catalyst which
can be recovered from the liquid phase while retaining
respectable catalytic activity. The decrease of catalytic activity
could attribute to slowly decomposition of metal complexes, as
already been noted in homogeneous catalysis where the loss of
activity is more pronounced. Besides, defects formed inside the
framework could also bring a negative effect to the catalytic
system.
comparison with bio-MOF 100 a decrease in the Brunauer−
Emmett−Teller surface area (from 3115 to 2793/2646 m2 g−1)
was observed because of the incorporation of the Pd3 cluster.
Semireduction of Internal Alkynes. Selective hydro-
genation of alkynes to alkenes, instead of alkane, is of cardinal
significance in industry.18 It is indispensable in the
petrochemical industry and the manufacture of pharmaceut-
icals and fine chemicals. However, selective hydrogenations
with discrimination between alkenes and alkanes are very
challenging. With the palladium core being partially poisoned
by thiol groups, trinuclear palladium clusters are good
candidates for the selective hydrogenation reaction. In this
contribution, bio-MOF 100-1 and bio-MOF 100-2 were
studied for the semireduction of internal alkynes. The yield
and selectivity were calculated according to the integration
ratio of 1H NMR spectra. As expected, the control experiment
C
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hexafluoroantimonate (0.067 mmol, 23 mg) was then introduced into
the mixture and stirred for an additional 1 h. The deep red mixture
was filtered by a 0.45 μm syringe membrane filter, and the solvent was
removed under vacuum. Raw product was washed three times by
CH3Cl/n-hexane (1:30 v/v) mixture to remove impurities. Last, the
pure cluster was dried under vacuum. The purity of the complex was
1
characterized by H NMR spectra in CDCl3.
Preparation of Pd3@bio-MOF 100 Catalyst. In a 10 mL
scintillation vial, bio-MOF 100 crystals were soaked in CHCl3 (5
mL). Then, a solution of Pd3 cluster in CHCl3 (0.01 M, 5 mL) was
added into the MOF suspension. The solution was changed every day
within 1 week. At the end, the crystals were washed with fresh CHCl3
(4 × 10 mL). The as-obtained Pd3@MOF was then stored in CHCl3
solution for further use.
General Procedure for Semireduction of Internal Alkynes.
The Pd3@MOF (30 mg, 0.0048 mmol) was put into a 25 mL Schlenk
tube equipped with a small magnetic stir bar. A HCO2H/NEt3 stock
solution in THF (HCO2H/NEt3 = 1:1, 4 equiv, 3.5 mL) was then
added to the tube, and substrates (0.3 mmol, 1 equiv) were added to
the tube after 10 min. The Schlenk tube was capped and taken out of
the glovebox and heated in an oil bath for the noted time. The catalyst
was separated by centrifugation. The product was isolated as indicated
in the Supporting Information.
Characterization of the Pd3@bio-MOF 100 Catalysts. Powder
X-ray diffraction (PXRD) was performed on a Bruker AXS X-ray
diffractometer. Cu Kα radiation of 40 mA, 40 kV, Kλ = 0.15418 nm,
2θ scanning range of 2−40°, a scan step size of 0.02°, and a time of 3 s
per step. The samples were ground to smaller particles and placed on
a zero-background silicon holder. Brunauer−Emmett−Teller surface
areas were determined by N2 adsorption/desorption isotherms at 77
K using automatic volumetric adsorption equipment (Micromeritics
ASAP2020). Pretreatment was done by using super critical CO2
method. Digestion analysis was performed by 1H NMR, approx-
imately 3 mg of dried MOF material was digested with sonication in
600 μL of DMSO-d6 and 10 μL of DCl. The NMR tests were
performed on a Varian Unity Inova 400 spectrometer.
Recycling Experiments. The reaction mixture was centrifuged
after the reaction, and the liquid layer was siphoned out. The residual
solid was then washed with THF solution and centrifuged. This
process was repeated twice followed by drying of the solid in nitrogen
flow. The HCO2H/NEt3 stock solution in THF and substrate were
then added to the glovebox for the next reaction cycle
Figure 5. PXRD patterns of (a) bio-MOF 100-1 after each reaction
cycles. (b) Recycling performances of bio-MOF 100-1. Blue columns
= 1-phenylpropyne as substrate, red columns = diphenylethyne as
substrate. (c) Recycling performances of bio-MOF 100-2. Blue
columns = 1-phenylpropyne as substrate, red columns = diphenyle-
thyne as substrate.
Leaching tests were performed as described in the
Experimental Section. It was proven that the reaction did
not proceed after the removal of Pd3@MOF catalyst. In the
catalytic reactions, a HCO2H/NEt3 stock solution was made in
advance, and the pH was carefully maintained at 7 to minimize
the potential detrimental effect to the MOF structure as well as
the encapsulated Pd3 cluster.
CONCLUSION
■
In summary, we demonstrated the successful incorporation of
two trinuclear palladium complexes into the anionic MOF, bio-
MOF 100, through a direct cation-exchanging method.
Electrostatic interaction between the cation complex and the
anionic framework stabilizes the palladium complex through
Coulombic force, thereby rendering it with good recyclability
as well as respectable catalytic activity for the semireduction of
internal alkynes. The obtained complexes are proven to be
heterogeneous recyclable catalysts for the semihydrogenation
of alkynes, which cannot be achieved for their homogeneous
counterparts. Moreover, the thiol groups of the cluster and the
confined pore space within the MOF minimize the detrimental
aggregation of palladium during the catalytic process. Further
studies in our lab will aim to design heterogeneous catalytic
systems based on a MOF platform for other types of chemo-
selective catalysis and to gain a better understanding of the
mechanisms.
Pd3@bio-MOF 100 Catalyst Leaching Test. In a 25 mL Schlenk
tube, the Pd3@MOF (30 mg), HCO2H/NEt3 stock solution in THF,
and substrate were added. The tube was then taken out of glovebox
and heated at 70 °C. After 12 h, the reaction was cooled to room
temperature. In the glovebox, the solution was filtered. Half of the
filtrate was removed and dried for NMR analysis. The remaining
solution was transferred to another 25 mL Schlenk tube and heated at
70 °C for another 36 h. Then, the yield was again determined via
NMR. The result shows that the reaction did not go any further after
Pd3@MOF was removed.
EXPERIMENTAL SECTION
■
Preparation of bio-MOF 100 and Pd3 Cluster Catalyst.
Material bio-MOF 100 was synthesized according to the method
described in previous work with little modulation.19 In brief,
Zn(NO3)2·6H2O(0.05M, 7.5 mL), adenine (0.05M, 2.5 mL),
H2BPDC (0.1M, 2 mL), H3−BTB(0.1m, 0.5 mL), HNO3 (1M, 600
μL), and 1 mL of H2O were mixed and divided into 7 Pyrex tubes.
Then, the tubes were sealed on a Schlenk line and heated in a 135 °C
oven for 24 h. The as-obtained crystals were washed with fresh DMF
and put into a 100 °C oven for another 24 h to get rid of most of the
uncoordinated ligand. Pd3 clusters were synthesized completely
according to previous work.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00286.
Synthetic procedures of related compounds; SEM
1
images; H NMR spectra of related compounds (PDF)
Synthesis of Trinuclear Palladium Cluster. In a N2-filled
glovebox, Pd(dba)2 (115 mg, 0.2 mmol) was completely dissolved in
CHCl3(20 mL). Then, phosphine (0.2 mmol, 63 mg for tris(4-
fluorophenyl) phosphine, 61 mg for tris(4-methylphenyl) phosphine)
and disulfide (0.1 mmol, 25 mg for p-tolyl disulfide and 10 mg for
methyl disulfide) were added also under nitrogen atmosphere. The
resulting mixture was stirred at room temperature for 4 h. Silver
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sqma@usf.edu.
ORCID
Shengqian Ma: 0000-0002-1897-7069
D
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the NSF (DMR-1352065) and the University
of South Florida for financial support of this work.
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