Active-site-accessible, porphyrinic metal-organic framework materials

Article abstract of DOI:10.1021/ja111042f

On account of their structural similarity to cofactors found in many metallo-enzymes, metalloporphyrins are obvious potential building blocks for catalytically active, metal-organic framework (MOF) materials. While numerous porphyrin-based MOFs have alrea

Full text of DOI:10.1021/ja111042f

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pubs.acs.org/JACS
Active-Site-Accessible, Porphyrinic MetalꢀOrganic Framework
Materials
Omar K. Farha,* Abraham M. Shultz, Amy A. Sarjeant, SonBinh T. Nguyen,* and Joseph T. Hupp*
Department of Chemistry and the Institute for Catalysis in Energy Processes, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, United States
S Supporting Information
b
Information (SI) for complete references). Remarkably, and despite
ABSTRACT: On account of their structural similarity to
cofactors found in many metallo-enzymes, metalloporphyrins
are obvious potential building blocks for catalytically active,
metalꢀorganic framework (MOF) materials. While numerous
porphyrin-based MOFs have already been described, versions
featuring highly accessible active sites and permanent micro-
porosity are remarkably scarce. Indeed, of the more than 70
previously reported porphyrinic MOFs, only one has been shown
to be both permanently microporous and contain internally
accessible active sites for chemical catalysis. Attempts to general-
ize the design approach used in this single successful case have
failed. Reported here, however, is the synthesis of an extended
family of MOFs that directly incorporate a variety of metallo-
porphyrins (specifically Al3þ, Zn2þ, Pd2þ, Mn3þ, and Fe3þ
complexes). These robust porphyrinic materials (RPMs) feature
large channels and readily accessible active sites. As an illustrative
example, one of the manganese-containing RPMs isshowntobe
catalytically competent for the oxidation of alkenes and alkanes.
considerable effort, only one has been shown to be both catalytically
competent and porous with respect to chemical reactants.9
The problems encountered in building robust, catalytic MOFs
from porphyrins have been several. In the early report from Robson,
large cavities and pores were obtained, but the framework irreversibly
collapsed upon removal of solvent.8,10 Fortunately, a recently de-
scribed “activation” strategy involving solvent exchange with liquid
CO2, followed by supercritical drying,11 has proven effective in
preventing the collapse of a variety of MOFs having high initial
solvent content.12 Several years after Robson’s report, Suslick and co-
workers13 succeeded in using 5,10,15,20-tetrakis(4-carboxyphe-
nyl)porphyrin to build zeolite structural analogues from cobalt and
manganese salts (PIZA series). The Co(porphyrin)-based PIZA-1
showed remarkable size selectivity with respect to small-molecule
sorption (water, methanol, ethanol, 1-butylamine, among others),
and the Mn(porphyrin)-based PIZA-3 was shown to be capable of
oxidative catalysis. However, these materials feature very small
channels, and the authors concluded that catalysis of the substrates
examined (larger than those used for sorption studies with PIZA-1)
occurs only on the exterior surface. Choe and co-workers14 have used
a paddlewheel coordination motif with the same tetracarboxylate
porphyrin to construct 2D metalꢀorganic sheets that are pillared in
the third dimension by dipyridyl struts.15 While these materials are
permanently microporous, unfortunately the metal ions in the
porphyrin struts behave as structural nodes, preventing their use as
catalysts. (Exceptions are MOFs constructed from Pd(II)porphyrins.
The Pd(II) ions, however, are coordinatively saturated by the porphy-
rin itself, so lack affinity for candidate substrate molecules.15) Similar
active-site blocking has been encountered in several other studies.16
Finally, also of interest is a very recent report by Choe and co-
workers on the use of 2,20-methyl-4,40-bipyridine as a pillaring
strut.17 The methyl substituents were intended to prevent coordina-
tion of the strut at the porphyrinic metal site, while still allowing for
coordination by metal ions contained in the paddlewheel nodes.
Single-crystal X-ray structural studies show that this clever approach
indeed does work, in the sense that the desired control over pillar
coordination is achieved. Unfortunately, the evacuated compounds
showed almost no accessible internal surface area (81 m2/g based
on CO2 adsorption), implying framework collapse.18,19
he already sizable field of metalꢀorganic framework (MOF)
T
chemistry continues to expand at a remarkable pace,1 in part
because of the seemingly limitless number of potential structures, but
also because of myriad potential applications.2 Prominent among
these applications is chemical catalysis.3 Like zeolites (by far the most
widely used catalysts industrially) MOFs promise catalysis-friendly
features such as large internal surface areas, extensive micro- and/or
mesoporosity, and crystallographically well-defined cavities and por-
tals of molecular dimensions. In addition, because they enlist the
chemistry of carbon they offer enormous tunability with respect to
chemical functionality and composition. One attractive approach
to the construction of catalytic MOFs, therefore, is to incorporate as
struts (or less commonly, as nodes) molecules that have already been
proven effective as catalysts in homogeneous solution environments.
Examples include metallo-salens4 and Ti-BINOLate complexes.5
Also seemingly attractive would be MOFs containing metallo-
porphyrins, as metalated porphyrins and their congeners are
ubiquitous in metallo-enzymology, functioning, for example, as
the key active sites in a range of oxidases,6 peroxidases,6 isomerases,7
dehalogenases,7 and transferases.7 Indeed, one of the earliest
intentional attempts (Robson and co-workers) to create a crystalline
and permanently microporous MOF utilized copper porphyrins as
building blocks.8 While no catalytic studies were described, the
report emphasized the potential of metallo-porphyrinic MOFs as
catalysts. Since then, 74 additional two- or three-dimensional
porphyrinic MOFs have been described (see Supporting
We have recently succeeded in building a noninterpenetrated,
pillared-paddlewheel MOF by combining 1,2,4,5-tetrakis(4-carbox-
yphenyl)benzene and 5,15-dipyridyl-10,20-bis(pentafluorophenyl)-
porphyrin with a Zn salt. This crystalline compound (ZnPO-MOF)
Received: December 8, 2010
Published: March 29, 2011
r
2011 American Chemical Society
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Figure 1. Using the free-base dipyridyl porphyrin, the highly porous
Zn(porphyrin)-based material ZnPO-MOF can be easily synthesized (top
right). However, starting with the Mn(porphyrin) yields only a 2D material
(bottom right), in which the Mn in the porphyrin center acts as a structural
node (and the tetratopic carboxylate strut is not incorporated). The
coordinatively saturated metal center of the 2D material is useless for catalysis.
Figure 2. Structure of RPM materials. Top left: A schematic representa-
tion of a generic RPM unit cell, based on sheet formation by the tetraacid
ligand (L1) pillaring by a dipyridyl strut (L2). The gray-black spheres are the
paddlewheel-coordinated zinc nodes. Top center and right: Structures of
the porphyrinic struts used to synthesize the RPM series (M1 = 2H, Pd,
Al(OH) or Fe(Cl), M2 = 2H or Mn(Cl)); Bottom: Crystallography-
derived stick representations of the unit cells of three representative RPMs
(yellow polyhedra = Zn, yellow = Zn, brown = Fe, purple = Mn, teal = Al,
red = O, green = F, blue = N, gray = C, white = H). Hydrogen atoms and
disordered solvent molecules have been omitted for clarity.
featured a high degree of porosity and contained fully reactant-
accessible metalloporphyrin sites.9 Remarkably, despite extensive
prior synthesis efforts, ZnPO-MOF was the first metalloporphyrin-
based MOF to show catalytic activity in the interior of the material.
Unfortunately, our attempts to extend the approach to catalytically
more interesting metals such as Fe(III) were uniformly unsuccessful.
For example, efforts to employ other metallo-porphyrins as building
blocks for catalytic MOFs were undermined by the propensity of
these components to form instead networks in which the intended
active site serves as an auxiliary node. Figure 1 shows an example
involving a Mn(III)porphyrin as a building block. A second tack
involved efforts to prepare a free-base analogue of the previously
synthesized catalytic MOF, with the aim of postsynthetically metallat-
ing it. These efforts were thwarted by the effectiveness of the
porphyrin in sequestering zinc ions that we hoped instead would
form framework nodes. (It is worth noting an alternative approach by
Eddaoudi and co-workers.20 They electrostatically encapsulated free,
cationic metalloporphyrins within cavities of anionic MOFs.)
carboxylate- and pyridyl-porphyrin struts. By using a solvent mixture
(1:1 v/v DMF/EtOH) in which the tetracarboxylate has good
solubility, and the dipyridyl has poor, the growing 2D metal/
carboxylate sheets have access to a low effective concentration of
the dipyridyl subunit. This results in pillar coordination only at the
more favorable sites, i.e. paddlewheel sites.
With the aforementioned strategies in mind, we found that static
heating of L1 and Zn(NO3)2 6H2O in DMF at 80 °C for 2 h,
3
followed by addition of 0.03 M HNO3 in ethanol and L2 and heating
of the resulting suspension at 80 °C for 20 h, resulted in the
formation of block-shaped crystals, suitable for single-crystal X-ray
diffraction. The sequence of ligand addition is important; suitable
materials are not obtained if the order is reversed. We speculate that
delaying the addition of the dipyridyl strut until after the tetra-
carboxylate strut has begun to assemble with zinc ions (as coordi-
natively unsaturated nodes) prevents the formation of 2D
aggregates of the dipyridyl unit. Several isostructural materials were
made in which the metals in L2 and L1 were varied (Figure 2). L1
was employed as an Al3þ, Pd2þ, or Fe3þ complex, or simply as a
free-base porphyrin, while L2 was used as either the Mn3þ complex
or the free-base. For both kinds of struts, use of a free-base porphyrin
in the synthesis resulted in Zn2þ complexes in the RPM. Analysis of
single-crystal data revealed noncatenated frameworks with formula
of Zn2(L1)(L2). As expected, L1 bridges the ZnII dimers and forms
flat two-dimensional sheets that are, indeed, pillared by L2. The
purity of bulk samples was confirmed via PXRD, and the proposed
metal ratios were corroborated via ICP-OES.
In this report, we describe the successful synthesis of MOFs that
can incorporate a variety of metalloporphyrins (specifically Al3þ
,
Zn2þ, Pd2þ, Fe3þ, and Mn3þ complexes) as components of well-
defined, crystalline, highly porous, and stable materials. These robust
porphyrinic materials (RPMs) are effective catalysts for the oxidation
of alkenes and alkanes and are highly stableunderoxidativeconditions
in comparison to homogeneous catalyst analogues.
Due to the tendency of catalytically interesting metals to favor
penta- or hexa-coordination, the synthesis of MOFs with available Fe-
or Mn-porphyrin sites has not been straightforward until now. As
noted above, simply targeting pillared paddlewheel MOFs with
metalloporphyrin struts typically results in ligation of both the
paddlewheel and porphyrin metal sites. Our synthetic approach
employed two strategies to avoid this problem. First, we took
advantage of the steric bulk to create a structure with as few pillars
as possible. This was accomplished by using a tetracarboxylated
porphyrin ligand (L1) in conjunction with a bulky dipyridyl porphyrin
pillar (L2). The building block L2 was chosen, in part, because of the
known ability of electron-withdrawing groups such as fluorine to
greatly increase the activity of metalloporphyrins for oxidative
catalysis.21 We have previously shown that the use of tetracarboxy-
lated ligands can produce MOFs particularly sensitive to sterics,
producing noninterpenetrated structures.22 Here, the steric bulk
prevents pillar coordination at the metalloporphyrin site. The second
element of our approach was to exploit differences in solubility of the
Six representative M1M2-RPMs (M1 designates the metal in
L1 and M2 designates the metal in L2) were synthesized and then
analyzed by single crystal X-ray diffraction to confirm their struc-
tures. (For FeMn-RPM, the structure was disordered near the
centers of the dipyridyl struts; see SI.) Each member of this series
possesses the same framework with differences only in the identities
of the porphyrin metal sites. For Fe- and Mn-porphyrins, the metal
center axially ligates a chloride ion to achieve charge balance, while
the Al-porphyrin axially ligates a hydroxide ion.
The crystal structures of all materials in the RPM series reveal
large channels in three directions (see Figure 2) that are occupied
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soluble analogue of iodosylbenzene, the MOF was found to be an
effective catalyst for both the epoxidation of styrene and hydro-
xylation of cyclohexane. The RPM material showed much
greater stability than an analogous homogeneous catalyst.
While the homogeneous Mn complex of 5,10,15,20-tetrakis-
(pentafluorophenyl)porphyrin showed complete catalyst deacti-
vation after 780 epoxidation turnovers (Figure S24A), ZnMn-RPM
functioned for 2150 turnovers, stopping only because of deple-
tion of oxidant. (Figure S24A shows an induction period for
catalysis by ZnMn-RPM. This behavior is attributed to slow
diffusion of reactants and/or oxidants into the MOF. Consistent
with this interpretation, the induction time is considerably
shortened when smaller crystallites, obtained by mechanically
crushing the crystals, of ZnMn-RPM are used.) No catalytic
activity was observed in the reaction solution after removal of the
MOF by filtration (in a run where oxidant still remained),
confirming that the catalytic reaction is heterogeneous.
Simple filtration and rinsing of the heterogeneous catalyst enabled
recovery of the MOF, which was further tested for catalytic activity;
the tests revealed activity very similar to that of the initially synthesized
material, albeit at roughly two-thirds the earlier rate. (Note that the
usual mechanism for catalyst deactivation, oxo-bridged dimer forma-
tion, is not available to the framework-constrained porphyrins. We
speculate that the decrease in activity instead is due to partial blockage
of MOF pores by insoluble polymeric remnants of the consumed
oxidant.)24 In separate experiments, oxidation of cyclohexane pro-
ceeded in 20% yield (1 mol % catalyst) to give a mixture of
cyclohexanol and cyclohexanone (83:17 alcohol/ketone; see SI).
To check further for permeation of the putatively porous RPMsby
reactants such as styrene, we exposed ZnZn-RPM to a chloroform
solution of an essentially identically sized compound, 4-vinyl-pyridine
(Figure S24B, 1) that is capable of axially binding to available
porphyrinic Zn(II) sites. The test solution also contained the much
larger pyridine-functionalized probe molecule, 2(Figure S24B). Follow-
ing overnight exposure to an equimolar solution of 1 and 2, the
MOF sample was rinsed repeatedly with chloroform and then
digested in aq. D2SO4. Consistent with the large size of 2 (i.e.,
Figure 3. (a) Photograph of ZnMn-RPM crystals. (bꢀd) Space-filling
representations of the crystal structure of ZnMn-RPM showing channels in
three directions (yellow = Zn, red = O, green = F, blue = N, gray = C, white =
H, purple = Mn). Disordered solvent molecules have been omitted.
by a substantial amount of disordered solvents. The metal-to-
metal distance between cofacial struts of L1 is 22 Å, while the
distance between cofacial L2's is 16.6 Å. Figure 3 shows a space-
filling model of the crystal structure of ZnMn-RPM along the
various channels. Importantly, RPMs are composed of a single
independent framework (noncatenated), implying a high degree
of porosity. This is in stark contrast to the majority of MOF
materials, where researchers routinely encounter two- and three-
fold catenation, with examples of considerably higher degrees of
catenation being known.23 Noncatenated structures such as
obtained for the RPM materials are particularly important for
catalytic applications, due to the need for large channels and
pores to allow diffusion of reactants into the MOF environment.
The immobilization of L1 and L2 with large separation distances
between the struts should preclude the commonly observed (in
homogeneous solution) deactivation of porphyrin-based oxida-
tion catalysts via μ-oxo dimer formation.
The hoped-for porosity of the RPM materials was confirmed via
TGA measurements, which uniformly revealed 45ꢀ60% mass loss
due to solvent at ∼110 °C (Figure S22A). Furthermore, ZnMn-RPM
displayed permanent microporosity by gas adsorption (CO2 at 273
K); NLDFT analysis provided a surface area of 1000 m2/g. This
high porosity is critical for catalysis of reactions involving dissolved
species (see below); reagents and substrates that are large (relative
to typical gas-phase reactants) must be able to access the metall-
oporphyrin sites in the pores. All materials exhibited high stability,
showing no sign of framework decomposition until 425 °C. Further
analysis of a representative framework, ZnMn-RPM, indicates that this
material is quite robust and can be resolvated, after solvent removal by
heating at 80 °C under dynamic vacuum, with almost no loss of
capacity for solvent.
1
greater than the channel widths in ZnZn-RPM), H NMR
measurements revealed uptake of only trace amounts, presum-
ably present due to binding to Zn(II)porphyrin sites defining the
exterior of the MOF. In contrast, the same NMR measurements
indicated an extensive uptake of 1, roughly 60 times that of the
control molecule, 2. Taken together with the TGA experiments
described above and the particle-size-dependent induction
behavior, these results strongly support the notion that styrene
(and cyclohexane) can access the interiors of catalytic RPMs.
In conclusion, we have found that, with the appropriate experi-
mental strategies and design of the organic building blocks, a sizable
range of metalloporphyrins can now be incorporated into porous
MOFs with retention of open coordination sites necessary for the
active-site-based applications for which metalloporphyrins are often
used. For example, we have observed that a porous RPM material
containing a Mn-porphyrin is catalytically active for routine oxidation
reactions, specifically, alkene epoxidation and alkane hydroxylation
(albeit, with limited selectivity). The ability to synthesize RPMsfrom
a variety of metalloporphyrin struts opens up possibilities for the
design of a wide assortment of porphyrinic MOF materials suitable
for catalysis and/or chemical separations. In particular, the ability to
incorporate two distinct metalloporphyrins in a single material is
highly intriguing as it could enable the synthesis of MOFs containing
two distinct catalysts competent to accelerate, sequentially or
cooperatively, multiple steps of complex reactions. While the focus
ZnMn-RPM was selected to demonstrate/illustrate the cata-
lytic activity of RPMs containing oxidation catalysts. Using a
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here is mainly on MOF synthesis and characterization, our ongoing
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have preserved the porosity of this and other delicate porphyrinic MOFs,
it is important to note that the SCD technique has only recently been
utilized in the MOF field; see refs 11 and 12.
’ ASSOCIATED CONTENT
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of starting materials and MOFs, including single-crystal X-ray
diffraction data in CIF format, as well as a detailed description of
catalysis conditions. This information is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
j-hupp@northwestern.edu; stn@northwestern.edu; o-farha@
northwestern.edu
’ ACKNOWLEDGMENT
We thank Dr. Rebecca Jensen for providing L1-Pd. We gratefully
acknowledge the NU-ICEP, NU-NSEC, DTRA (Grant HDTRA-
09-1-0007), and the AFOSR for financial support. We acknowledge
the NU-IMSERC for use of analysis instrumentation. We thank
ChemMatCARS and LS-CAT for use of the APS, a facility
supported by the U.S. DOE, Office of Science, Office of Basic
Energy Sciences (No. DE-AC02-06CH11357). ChemMatCARS
Sector 15 is principally supported by the NSF/DOE (NSF/CHE-
0822838). Use of the LS-CAT Sector 21 was supported by the
Michigan Economic Development Corporation and the Michigan
Technology Tri-Corridor (Grant 085P1000817).
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(18) The substantial (but not complete) degradation of crystallinity
following solvent evacuation (as indicated by powder measurements)
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see ref 17) likewise are suggestive of framework collapse. It is con-
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protocol.
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area is less than expected from the material’s crystal structure, it should
be noted that the detailed activation protocol was not optimized; see SI.
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(24) 1 mg of catalyst (the amount typically used) produces nearly
400 mg of depleted oxidant. The depleted oxidant readily aggregates to
form an insoluble polymer. An additional 400 mg of insoluble polymer is
formed via spontaneous degradation of the oxidant (and thus requiring
the oxidant to be used in ca. 2-fold excess).
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