Highly dispersed palladium(II) in a defective metal-organic framework: Application to C-H activation and functionalization

Article abstract of DOI:10.1021/ja2094316

High reversibility during crystallization leads to relatively defect-free crystals through repair of nonperiodic inclusions, including those derived from impurities. Microporous coordination polymers (MCPs) can achieve a high level of crystallinity through a related mechanism whereby coordination defects are repaired, leading to single crystals. In this work, we discovered and exploited the fact that this process is far from perfect for MCPs and that a minority ligand that is coordinatively identical to but distinct in shape from the majority linker can be inserted into the framework, resulting in defects. The reaction of Zn(II) with 1,4-benzenedicarboxylic acid (H₂BDC) in the presence of small amounts of 1,3,5-tris(4-carboxyphenyl)benzene (H ₃BTB) leads to a new crystalline material, MOF-5(O_h), that is nearly identical to MOF-5 but has an octahedral morphology and a number of defect sites that are uniquely functionalized with dangling carboxylates. The reaction with Pd(OAc)₂ impregnates the metal ions, creating a heterogeneous catalyst with ultrahigh surface area. The Pd(II)-catalyzed phenylation of naphthalene within Pd-impregnated MOF-5(O_h) demonstrates the potential utility of an MCP framework for modulating the reactivity and selectivity of such transformations. Furthermore, this novel synthetic approach can be applied to different MCPs and will provide scaffolds functionalized with catalytically active metal species.

Full text of DOI:10.1021/ja2094316

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Highly Dispersed Palladium(II) in a Defective MetalÀOrganic
Framework: Application to CÀH Activation and Functionalization
†
†
†
†
†
Tae-Hong Park, Amanda J. Hickman, Kyoungmoo Koh, Stephen Martin, Antek G. Wong-Foy,
,†
,†,‡
Melanie S. Sanford,* and Adam J. Matzger*
†
‡
Department of Chemistry and Macromolecular Science and Engineering Program, University of Michigan, 930 North University
Avenue, Ann Arbor, Michigan 48109, United States
S Supporting Information
b
ABSTRACT: High reversibility during crystallization leads
to relatively defect-free crystals through repair of nonper-
iodic inclusions, including those derived from impurities.
Microporous coordination polymers (MCPs) can achieve a
high level of crystallinity through a related mechanism
whereby coordination defects are repaired, leading to single
crystals. In this work, we discovered and exploited the fact
that this process is far from perfect for MCPs and that a
minority ligand that is coordinatively identical to but distinct
in shape from the majority linker can be inserted into the
framework, resulting in defects. The reaction of Zn(II) with
1
,4-benzenedicarboxylic acid (H BDC) in the presence of
2
small amounts of 1,3,5-tris(4-carboxyphenyl)benzene
H BTB) leads to a new crystalline material, MOF-5(O ),
(
3
h
that is nearly identical to MOF-5 but has an octahedral
morphology and a number of defect sites that are uniquely
functionalized with dangling carboxylates. The reaction with
Figure 1. (a) Synthesis of BTB-incorporated MOF-5 crystals by
addition of H BTB to the reaction mixture of H BDC and Zn-
NO3)2 6H O. (b) Photographs of crystals showing the dependence
3
2
Pd(OAc) impregnates the metal ions, creating a hetero-
2
(
3
2
geneous catalyst with ultrahigh surface area. The Pd(II)-
catalyzed phenylation of naphthalene within Pd-impreg-
of the morphology upon the percentage of H
00 μm). Needle-shaped UMCM-1 crystals appear above 10 mol %
BTB.
3
BTB in the feed (scale bar:
1
nated MOF-5(O ) demonstrates the potential utility of an
h
H
3
MCP framework for modulating the reactivity and selec-
tivity of such transformations. Furthermore, this novel
synthetic approach can be applied to different MCPs and
will provide scaffolds functionalized with catalytically active
metal species.
7
MCP constructed from either linker. In the latter case, the
stoichiometry of the two linkers should be within a certain range
of linker mole ratios to obtain a pure phase; if this is not the case,
typically the ligand present in highest concentration determines
the structure of the resulting MCP. Although the identical
coordinating functionality causes the minority linker to be
incorporated to some extent, the high reversibility during crystal-
lization repairs such defect sites during synthesis. However, we
propose that if the driving force for crystal growth of the majority
linker is sufficient, the minority ligand can be inserted into the
framework, leading to defects with dangling ligands. In this work,
we utilized this approach to produce a new material with a powder
X-ray diffraction (PXRD) pattern nearly identical to that of the
MCP derived from the dominant linker and displaying compar-
able gas sorption behavior. This new material possesses a
significant number of defect sites that functionalize the pores
and show unprecedented reactivity.
rystalline microporous coordination polymers (MCPs), and
C
in particular the subset known as metalÀorganic frame-
works (MOFs), have shown extensive potential for use in
3
1
2
applications in gas storage, chemical separations, catalysis,
4
5
sensing, and drug delivery. These MCPs are constructed from
the self-assembly of metal ions/clusters and organic linkers under
reaction conditions where relatively perfect single-crystal growth
is favorable. Despite extensive success with single-linker systems
possessing high porosity, coordination copolymerization (as in
the use of mixed linkers) has presented tremendous opportu-
6
nities to control the functionality within the framework and to
7,8
build new materials with unparalleled surface areas. We have
reported that two linkers that are coordinatively and topologi-
cally identical and have similar reactivities can be randomly
The copolymerization of 1,4-benzenedicarboxylic acid (H BDC)
and 1,3,5-tris(4-carboxyphenyl)benzene (H BTB) (Figure 1) in
2
3
6
copolymerized into the parent MCP structure, whereas two
linkers that are coordinatively identical but distinct in shape can
Received: October 6, 2011
form a new, highly porous MCP structure very different from the
Published: November 28, 2011
r 2011 American Chemical Society
20138
dx.doi.org/10.1021/ja2094316
|J. Am. Chem. Soc. 2011, 133, 20138–20141

Journal of the American Chemical Society
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the presence of Zn(NO ) in N,N-diethylformamide (DEF) pro-
3
2
vides highly porous, needle-shaped Zn O(BDC)(BTB)
4
4/3
(
UMCM-1) crystals, whereas the same conditions with pure
7a
H BDC or H BTB afford MOF-5 or MOF-177, respectively.
2
3
The mole ratio of the two linkers determines the phase
obtained. For example, a pure phase of UMCM-1 was
obtained at an optimal range of H BDC:H BTB mole ratios
2
3
(
3:2 to 1:1), whereas a reaction mixture containing ∼10 mol %
H BTB with respect to H BDC provided octahedral crystals
3
2
of MOF-5 as determined by PXRD analysis; the dramatic
change in crystal shape was presumed to be an example of an
additive-induced morphological change. Here we report, quite
surprisingly, that these octahedral MOF-5 crystals differ from
MOF-5 in their chemical reactivity rather dramatically and in a
manner that is particularly useful for creating heterogeneous
catalysts with ultrahigh surface area.
Figure 1 shows the evolution of the crystal morphology with
the addition of increasing amounts of H BTB to the reaction
3
mixture of H BDC and Zn(NO ) 6H O in DEF. The addition
2
3 2
3
2
of H BTB gradually results in different morphologies from cubic
3
to octahedral. At 2 mol % H BTB relative to H BDC, truncated
3
2
cubic or truncated octahedral crystals appear, and at 5À10 mol %
Figure 2. (a) Selective impregnation of Pd(II) ions in MOF-5(O ) by
h
H BTB, most of the crystals exhibit a uniform octahedral shape.
addition of Pd(OAc)
2 2 2
in CH Cl . For comparison, deliberate mixtures
3
These noncubic crystals show a nearly identical PXRD pattern
and the same cubic space group with indistinguishable cell
of pristine MOF-5 or needle-shaped UMCM-1 with MOF-5(O )
h
crystals were tested. (b) Reduction by NaBH in DMF. (c) Growth of
4
7a
an IRMOF-3 shell layer on a Pd@MOF-5(O ) core using H ABDC and
h
2
parameters as MOF-5. Above 10 mol % H BTB, needle-shaped
3
Zn(NO
3
)
2
6H
2
O in DEF at 85 °C. Scale bars in (aÀc): 100 μm.
UMCM-1 appears concomitantly. The morphology change from
cubic to octahedral has been observed in inorganic nanocrystals
3
(d) TEM image of Pd@MOF-5(O
h
). Scale bar: 5 nm.
9
using a capping reagent. The functional groups on such reagents
weakly interact with specific crystal faces, leading to selective
changes in the relative rates of growth from different faces.
Through a mechanism that we ascribe to the presence of
uncoordinated carboxylate groups, defective MOF-5 produced
10
When benzoic acid derivatives having a single carboxylate were
with 10% BTB in the feed, designated as MOF-5(O ), adsorbs a
considerable amount of palladium ions from a dilute Pd(OAc)2
solution. A variety of catalytic functions employing different
h
added to MOF-5 crystallization, the additives, however, either
11a
terminated the crystal growth or formed holey defects in the
1
1b
cubic crystals. On the other hand, here a small amount of the
tritopic BTB linker appears to regulate the rate and direction of
framework extension and crystal growth of MOF-5 without
providing the UMCM-1 coordination copolymer.
oxidation states and significantly high affinity for H make Pd one
2
of most studied metals to be incorporated into porous solid
12
supports, including MCPs. When MOF-5(O ) crystals were
h
slowly shaken in a Pd(OAc) solution (8.9 mM) in CH Cl for
2
2
2
Despite the PXRD results, NMR measurements revealed the
inclusion of BTB in the MOF-5 framework after the crystals were
2 days, the colorless crystals became brown and the solution was
visibly depleted in Pd. After adsorption in a fresh Pd solution for
another 2 days, the crystals were washed with CH Cl three times
digested in DMSO-d containing DCl [see the Supporting
6
2
2
1
Information (SI)]. The quantity of BTB determined by H NMR
and immersed in CH Cl for 2 days, during which time fresh
2 2
analysis was closely correlated with the percentage of H BTB in
solvent was replenished three times to remove unbound Pd ions.
3
the starting feed. The solvothermal reaction conditions produce
diethylamine from thermal degradation of DEF that deproton-
ates H BDC and H BTB, leading to the formation of Zn O clus-
Optical images of Pd-ion-adsorbed MOF-5(O ), designated as
h
Pd(II)@MOF-5(O ), are shown in Figure 2. For comparison,
h
MOF-5(O ) crystals were mixed with pure MOF-5 or UMCM-1
2
3
4
h
ters. Whereas the carboxylates in BDC are neutralized in the
construction of the MOF-5 framework, only one of the three
and then placed in a Pd(OAc) solution; after 4 days, only MOF-
2
5(O ) became brown, whereas MOF-5 (cubes) and UMCM-1
h
BTB carboxylates may be incorporated into the Zn O cluster as a
(needles) remained nearly colorless. Such selective impregnation
4
1
2f
defect and interrupt the framework extension, leading to octahe-
dral crystals. However, the other two carboxylates of BTB cannot
extend the framework. The BET surface areas of 5 and 10% BTB-
suggests that the Pd adsorption is not significant in MOF-5
and UMCM-1 but that the dangling carboxylate groups of MOF-
5(O ) extensively interact with Pd(OAc) . The NMR spectrum
h
2
2
loaded material were 3070 and 2850 m /g, respectively, whereas
after digestion of Pd(II)@MOF-5(O ) in acidic DMSO-d
h 6
+
that of pure MOF-5 synthesized under the same conditions was
revealed the presence of [Pd(OAc)] that had exchanged onto
the dangling linkers. The Pd content of Pd(II)@MOF-5(Oh)
was 2.3 wt %, as determined by inductive coupled plasma
optical emission spectroscopy (ICP-OES). Therefore, the above
data strongly support the idea that BTB is incorporated into
the MOF-5 structure, giving rise to not only a morphology
change but also carboxylate functionality within the pores. While
the partial degradation of frameworks after Pd incorpora-
2
3
470 m /g (see the SI). Although physical entrapment of BTB
within MOF-5 as a possible mode of introducing carboxylate
functionality cannot be completely ruled out, the replacement of
H BTB with 1,3,5-tris(4-acetylphenyl)benzene, an isostere for
3
H BTB lacking carboxylate functionality, did not lead to incor-
3
poration into the structure, as judged by NMR spectroscopy of
the acid-digested framework; furthermore, a change in morphol-
ogy of the crystals was not observed.
1
2a
tion via chemical vapor deposition (CVD) and infiltration
2
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Journal of the American Chemical Society
COMMUNICATION
1
2b
methods often significantly reduce surface area, the PXRD and
surface area measurements showed that the crystallinity and
porosity of MOF-5(O ) were maintained after the Pd impregna-
h
2
tion. The very mild wet chemistry maintained a 2600 m /g BET
surface area for Pd(II)@MOF-5(O ). Treatment of Pd(II)@
h
MOF-5(O ) with NaBH in N,N-dimethylformamide (DMF)
h
4
2
+
reduced Pd ions to Pd nanoparticles within the framework,
giving a product designated as Pd@MOF-5(O ). The fairly
h
uniform brown color of cross-sectioned crystals and transmission
electron microscopy (TEM) studies suggest that the Pd nano-
particles are evenly distributed throughout the crystals (see the SI
and Figure 2d). However, the nearly identical PXRD pattern and
2
surface area (2570 m /g) indicate that the framework is still
intact after reduction, presenting well-dispersed metal catalysts in
a high-surface-area MCP.
Multifunctionality within a single MCP crystal can provide
unique applications in catalysts and sensors via regulation of the
13
diffusion rate and size of guest molecules. In this context,
coreÀshell-type crystals were constructed by heating a solution
of H BDC or 2-amino-1,4-benzenedicarboxylic acid (H ABDC)
Figure 3. Pd-catalyzed direct phenylation of naphthalene using Pd-
2
2
6
a
(II)@MOF-5(O
ratio of isomers (α:β) at the end of the reaction is given in parentheses.
The Pd loading of Pd(II)@MOF-5(O ) was 2.3 wt % as determined by
h 2
) (black b) or Pd(OAc) (red 2) as the catalyst. The
and Zn(NO ) 6H O to grow a Zn OL layer upon
3
2
3
2
4
3
Pd@MOF-5(O ) seed crystals. The distinct colors of the core
h
h
and shell indicate the Pd NPs are localized in the core of the
crystal (Figure 2c; also see the SI). The replacement of BDC with
ABDC presents amine-functionalized pores that can allow
further control of pore functionality as well as pore accessibility
ICP-OES, and the yield and selectivity were determined by GCÀMS
using an internal standard.
14
of the outer layer via postsynthetic modification strategies.
the same reaction conditions. In addition, nitrobenzene contain-
Notably, these coreÀshell crystals exhibit morphology evolution
from octahedral to polyhedral, suggesting that a recovery of the
native cubic shape is underway.
ing only Pd(II)@MOF-5(O ) was heated for 3À16 h, and the
h
solid was removed by hot filtration. Naphthalene and oxidant were
added to the filtrate, and the mixture was heated for 12 h, giving
only 2À8% of phenylated products. These results suggest that
diffusion of the Pd catalyst out of the solid support is negligible.
PXRD studies of solid recovered from the reactions (see the
To illustrate the utility of Pd(II)@MOF-5(O ) in catalysis, we
h
sought to investigate the material’s reactivity for a known Pd(II)-
catalyzed reaction. The CÀH phenylation of naphthalene using
diphenyliodonium tetrafluoroborate was chosen as a test reaction
because the reactivity and selectivity have been shown to exhibit a
SI) showed that the MOF-5(O ) framework gradually degraded
h
with reaction time, although Pd(II)@MOF-5(O ) did not lose
h
15
strong dependence on the catalyst structure. Notably, the use
crystallinity under the same conditions when either of the
reactants was not present in the solution. After 12 h, the crystal-
linity of the MCP completely disappeared, and the arylation also
terminated. A TEM study of solid collected from the reaction
showed that ∼20 nm Pd nanoparticles were spread over non-
crystalline solid, leading to broad Pd metal reflections in the
PXRD pattern at 40.2 and 46.5°. Such MCP decomposition
suggests that defect inclusion in more robust MCP crystals will be
useful in producing improved catalysts for direct CÀH arylation.
Additionally, only modest site selectivity was observed for the
of 5 mol % Pd(OAc) as the catalyst was shown to result in a low
2
yield and rapid catalyst deactivation. For example, the Pd(OAc)2-
catalyzed CÀH arylation of naphthalene in nitrobenzene at
1
20 °C nearly terminated after 30 min, providing a total yield
of 13% with 15:1 selectivity for the α isomer over the β isomer.
Employing longer reaction times (17 h) improved the yield only
slightly and lowered the selectivity (17% yield, α:β = 8:1;
Figure 3). We hypothesized that the MCP framework might
serve to promote this transformation by slowing the relative rate
of catalyst decomposition and/or by stabilizing Pd intermediates
during the catalytic cycle.
Pd(II)@MOF-5(O )-catalyzed naphthalene arylation, and ex-
h
ploration of the influence of pore size and shape is an attractive
future direction. Previous investigations of diimine Pd(II) cata-
lysts for this transformation suggest that further optimization of
the coordination environment of the palladium active site via
modification of the MCP crystal could also dramatically increase
Indeed, under otherwise identical conditions, the phenylation
of naphthalene catalyzed by Pd(II)@MOF-5(O ) (5 mol % Pd)
h
resulted in a greatly increased yield of phenylated naphthalenes
(
64%) after 12 h. In contrast to the Pd(OAc) -catalyzed reaction,
2
15
the yield of product steadily increased with time, indicating that
the selectivity.
Pd(II)@MOF-5(O ) remained catalytically viable significantly
In conclusion, defects in MOF-5 can be produced by the
h
longer than Pd(OAc) . Furthermore, the Pd(II)@MOF-5(O )-
addition of a small amount of H BTB during synthesis; this leads
2
h
3
based catalyst provided significantly different site selectivity
to a new material nearly identical to MOF-5 but having an
octahedral morphology. The resulting framework is uniquely
functionalized with carboxylates of the dangling linkers and
extensively differs from MOF-5 in reactivity toward Pd ions,
leading to heterogeneous catalysts with ultrahigh surface area.
This novel synthetic approach can be further applied to different
MCPs providing scaffolds for impregnation of catalytically active
metal species.
(
α:β = 3:1) in comparison with Pd(OAc) . This highlights the
2
exciting promise of the MCP framework for modulating the
selectivity of this and other transformations.
16
Next, potential loss of active Pd species from the crystals was
tested. Pd(II)@MOF-5(O ) crystals were removed from a
h
reaction mixture by hot filtration after heating for 3 h. The
isolated solution did not show any further catalytic activity under
2
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Journal of the American Chemical Society
COMMUNICATION
’
ASSOCIATED CONTENT
Long, J. R.; Yaghi, O. M. J. Am. Chem. Soc. 2010, 132, 14382–14384.
(
h) Hermannsd €o rfer, J.; Kempe, R. Chem.—Eur. J. 2011, 17, 8071–8077.
S
Supporting Information. Synthesis, characterization,
2
(i) Li, H.; Zhu, Z.; Zhang, F.; Xie, S.; Li, H.; Li, P.; Zhou, X. ACS Catal.
2011, 1, 1604–1612.
b
and N isotherm measurements of MCPs; procedure for Pd-
II)@MOF-5(O )-catalyzed phenylation of naphthalene; and
complete ref 5b. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) Zacher, D.; Schmid, R.; Woll, C.; Fischer, R. A. Angew. Chem.,
(
h
Int. Ed. 2011, 50, 176–199.
(14) Wang, Z.; Cohen, S. M. Chem. Soc. Rev. 2009, 38, 1315–1329.
(15) Hickman, A. J.; Sanford, M. S. ACS Catal. 2011, 1, 170–174.
(16) (a) Only a small background reaction was observed in the
’
AUTHOR INFORMATION
presence of unmetalated MOF-5(O
selectivity after 16 h). (b) The presence of MOF-5 crystals in a
homogeneous catalyst reaction with Pd(OAc) only slightly improved
the yield (21%) with a parallel selectivity (8:1) after 16 h (see the SI).
h
) (resulting in 7% yield, 5:1
Corresponding Author
mssanfor@umich.edu; matzger@umich.edu
2
’
ACKNOWLEDGMENT
This material is based upon work supported by the U.S.
Department of Energy (DE-SC0004888 to A.J.M. and A.G.W.-F.
and DE-FG02-08ER 15997 to M.S.S.). In addition, A.J.H. thanks
the National Science Foundation for a graduate fellowship.
’
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