Selective catalytic behavior of a phosphine-tagged metal-organic framework organocatalyst

Article abstract of DOI:10.1002/chem.201404498

Steric hindrance by a metal-organic framework (MOF) is shown to influence the outcome of a catalytic reaction by controlling the orientation of its intermediates. This is demonstrated using an organocatalyst, phosphine MOF LSK-3, which is evaluated with t

Full text of DOI:10.1002/chem.201404498

DOI: 10.1002/chem.201404498
Full Paper
&
Metal–Organic Frameworks
Selective Catalytic Behavior of a Phosphine-Tagged Metal-Organic
Framework Organocatalyst
Xiaoying Xu,[a, b] Stephan M. Rummelt,[a] Flavien L. Morel,[a, b] Marco Ranocchiari,*[b] and
Jeroen A. van Bokhoven*[a, b]
Abstract: Steric hindrance by a metal–organic framework
(MOF) is shown to influence the outcome of a catalytic reac-
tion by controlling the orientation of its intermediates. This
is demonstrated using an organocatalyst, phosphine MOF
LSK-3, which is evaluated with the aid of molecular model-
ing and NMR spectroscopy techniques. This report is the
first application of phosphine MOFs in organocatalysis and
explores the potential of a framework steric hindrance to
impose selectivity on a catalytic reaction. These findings
expand the opportunities for control and design of the
active site in the pocket of heterogeneous catalysts.
Introduction
the molecular level. Due to their crystallinity, MOFs provide
a unique way to recycle a molecular catalyst while exploiting
the pocket around the active site. This concept has been dem-
onstrated in some significant publications.[22,23] For instance,
Lin and co-workers reported that the cavity of a MOF can
induce reversed enantioselectivity in asymmetric catalysis.[23]
Phosphines can be employed as organocatalysts for reac-
tions of activated alkenes and alkynes.[24] Triphenylphosphine
(PPh3) analogues have been supported on polystyrene and
successfully applied in the aza-Morita–Baylis–Hillman reac-
tion.[25–28] MOFs with free phosphine groups have also been re-
ported. A crystalline MOF containing a 4,4’,4’’-phosphinetriyltri-
benzoate (ptbc) linker and Zn4O clusters, PCM-1, was reported
by Humphrey and co-workers.[29] Another MOF based on the
same organic linker, PCM-10, was discovered by the same
group.[30] We reported the catalytic properties of a highly
stable ptbc–zirconium MOF called LSK-1, which was used to
coordinate single AuI atoms onto the framework with subse-
quent catalytic application.[31a] More recently, our group pub-
lished the synthesis of MOFs with MOF-5 and MIL-101 topolo-
gies and pendant diphenylphosphino groups.[31b] To our knowl-
edge, no application of phosphine MOFs in organocatalysis
has been reported to date. MOFs are ideal candidates to sup-
port phosphine organocatalysts because they can provide an
in-depth understanding of their catalytic behavior, as shown in
this contribution. Herein we demonstrate that the framework
of a MOF creates steric hindrance around the active site and
determines which molecules react in different phosphine-cata-
lyzed reactions. We confirm experimentally previous repeated
claims, both from our group[9] and others,[32,33] that the restrict-
ed space in a MOF cage mimics the pocket observed in the
active site of enzymes.
Metal–organic frameworks (MOFs)[1–3] have been used for inter-
esting applications in heterogeneous catalysis,[4,5] as well as in
gas storage[6,7] and drug delivery,[8] owing to their chemical/
structural versatility and big pores. Catalysis by MOFs is nowa-
days mainly focused on metal catalysis.[9] However, these mate-
rials can also be treated like organic molecules because organ-
ic functional groups can be easily introduced into the porous
and crystalline structure[10–12] and modified by post-synthetic
modification (PSM).[13] It is therefore quite straightforward to
think of them as pseudo-organocatalysts, and some examples
in the literature have already shown this.[14–18] The group of
Telfer reported a strategy to introduce a non-protected proline
derivative inside a non-interpenetrated IRMOF structure.[19]
Duan and co-workers used a MOF as a photoactive initiator for
a physisorbed organocatalyst in the asymmetric a-alkylation of
aldehydes.[20]
Organocatalytic functional groups can also be immobilized
in polymers or dendrimers, and on silica supports to produce
recoverable and recyclable catalysts.[21] One inevitable conse-
quence of the heterogenization of organocatalysts—and of
other molecular catalysts in general—in such amorphous sup-
ports is the loss of information on the position of the atoms at
[a] X. Xu, S. M. Rummelt, F. L. Morel, Prof. Dr. J. A. van Bokhoven
Department of Chemistry and Applied Biosciences
Laboratory of Chemical and Bioengineering
ETH Zurich, 8093 Zꢀrich (Switzerland)
E-mail: jeroen.vanbokhoven@chem.ethz.ch
[b] X. Xu, F. L. Morel, Dr. M. Ranocchiari, Prof. Dr. J. A. van Bokhoven
Department of Synchrothron Radiation
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institute, 5232 Villigen PSI (Switzerland)
E-mail: marco.ranocchiari@psi.ch
jeroen.vanbokhoven@psi.ch
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404498.
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ure 1A).[35] This synthetic procedure yielded a mixed-linker
MOF (MIXMOF) with a uniform distribution of functional
groups within a single crystal, whose structure is represented
schematically in Figures 1B and 1 C. Deliberate dilution of
phosphine groups within the framework reserves sufficient
space for catalytic processes to occur (see below).
Results and Discussion
Synthesis and Characterization of LSK-3
Our strategy was to synthesize a phosphine MOF (P-MOF) cata-
lyst that featured the structural and electronic properties of
PPh3, contained sufficient pore space to accommodate sterical-
ly hindered groups, and allowed the diffusion of reactants and
products during catalysis (Figure 1). PPh3 is classically em-
ployed as a ligand in homogeneous catalysis[34] and itself func-
tions as catalyst for Lewis-base catalyzed reactions.[24] We used
diphenylphosphino moieties to produce the cubic topology of
IRMOF-9,[10] derived from MOF-5. The first step was to synthe-
size the organic linker 2-(diphenylphosphino)-[1,1’-biphenyl]-
4,4’-dicarboxylic acid (1) via an in situ CÀC coupling followed
by a previously developed catalytic PÀC coupling with Pd/C as
catalyst (see the Supporting Information, Scheme S1).[35] We
produced our P-MOF (LSK-3), by the reaction of 1, [1,1’-biphen-
Single crystal X-ray crystallography of LSK-3 revealed
a doubly interpenetrated cubic framework with [Zn4O]6+ inor-
ganic units linked by biphenyl linkers (with and without P-
functionalization). LSK-3 crystallizes in the tetragonal space
group P42/ncm with lattice parameters a=17.2209(11) ꢁ and
c=34.229(2) ꢁ (see the Supporting Information, Figure S2). The
distance between the two interpenetrated networks (dFÀF) was
12.9 ꢁ, which is the largest among IRMOF-9 derivatives.[10,36]
A
large dFÀF is needed to accommodate the diphenylphosphino
groups, which are located exclusively on the linkers oriented in
the ab plane. This is a unique example of a MOF in which
mixed linkers of the same type and size are ordered and in
preferential directions of the unit cell. Surprisingly, whereas the
biphenyl rings in the c direction were highly disordered, those
in the ab plane were not, and the Zn-, P-, C-, and O-
atom positions in the ab plane could be described by
anisotropic displacement parameters.
.
yl]-4,4’-dicarboxylic acid (2), and Zn(NO3)2 4H2O in N,N-dime-
thylformamide (DMF) under solvothermal conditions (Fig-
ICP analysis of the material revealed a P:Zn ratio of
0.24, and 1H NMR spectroscopy after digestion of
LSK-3 in DCl/[D6]DMSO, showed the ratio of linkers 2
to 1 to be 2:1. This value was used to fix the P occu-
pation in the single crystal X-ray diffraction analysis.
The P amount, calculated by combining TGA with
1H NMR spectroscopy data after digestion, was
1.6 wt%. The combined characterization data con-
firmed the expected [Zn4O(1)(2)2] stoichiometry of
LSK-3, in agreement with previously reported IRMOF-
9 and IRMOF-10 structures.[10,19,36] Nitrogen physisorp-
tion measurements carried out at 77 K of the MOF
activated with supercritical CO2 showed the type I
isotherm with BET surface area SBET =865 m2gÀ1, and
micropore volume Vp =0.25 cm3gÀ1. The diphenyl-
phosphino-incorporating group on LSK-3 is stable
under dried and degassed conditions. The crystals ex-
hibit good chemical stability in solvents such as
CHCl3, dichloromethane, benzene, DMF and diethyl-
formamide.
Analysis of the Connelly surface of the material
(Figure 1E and Figure S6 in the Supporting Informa-
tion) showed that 2D channels with rectangular and
perpendicular pore openings of 6.1 ꢁꢂ8.4 ꢁ (Channel
2 in Figure 1D) and 6.9 ꢁꢂ8.4 ꢁ (Channel 1 in Fig-
ure 1D) are formed in the absence of diphenylphos-
phino moieties. These channels are pitted by cavities
Figure 1. A) Synthesis of LSK-3. B) Schematic representation of LSK-3. Only one of the
two interpenetrated frameworks is shown for clarity. For a crystal structure obtained
from X-ray diffraction data, see the Supporting Information. C) Schematic structure of the
two interpenetrated frameworks (each cube represents the same structure as in B).
D) Schematic representation of the pore structure of LSK-3. The green and red hollow
cubes are the two interpenetrated networks of the material. The blue parallelepipeds
represent the cavities where one or two PPh2 groups can be present. The yellow paralle-
lepipeds represent the cavities without phosphine groups. E) Connelly surface of LSK-3
with two (top) or one (bottom) PPh2 groups in one cage.
that could theoretically contain up to two diphenyl-
phosphino moieties (depicted in blue in Figure 1D),
and are interconnected by two types of voids of 5.9ꢂ
6.1ꢂ8.4 ꢁ and 6.7ꢂ6.9ꢂ8.4 ꢁ (depicted in yellow in
Figure 1D). Although the open space in the blue cav-
ities is decreased in the presence of diphenylphos-
phino groups (Figure 1E), it is clear that plenty of
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space remains for catalysis in the yellow cavities, which do not
contain phosphine groups and therefore permit free diffusion
of relatively large molecules. 31P MAS NMR spectroscopy shows
that all P atoms in LSK-3 are accessible to react with reactants
(see below).
produced the equivalent amount of product after 120 h. Wash-
ing of the MOF with CHCl3 allowed reuse of the material in
a second cycle, where it mediated 89% conversion of reactants
into product (Figure 2A) compared to stoichiometric reaction
of PPh3.
Encouraged by these findings we probed the activity of LSK-
3 in umpolung addition of ethyl 2,3-butanedienoate to malo-
nonitrile yielding ethyl 5,5-dicyanopent-2-enoate (4; Fig-
ure 2B). This reaction, when catalyzed by PPh3 (20 mol%) in
the homogeneous phase, affords the product in 73% yield.[38]
LSK-3 (20 mol% P) did not produce even a trace of product
under identical conditions. In an effort to further probe the im-
pediment to this reaction we investigated the Knoevenagel
condensation of benzaldehyde with malononitrile to give 2-
benzylidene malononitrile (5; Figure 2C),[39] a reaction that is
also catalyzed by amino MOFs.[40] After 110 h reaction time at
808C in CHCl3, the conversion of benzaldehyde to product was
98%. The catalyst was recycled four times by consecutive
washings with dichloromethane leading to stabilized produc-
tivity at about 70% conversion. PXRD revealed that the struc-
ture remains intact after four recycling steps. Knoevenagel con-
densation with our P-MOF catalyst works three times faster
than the homogeneous reaction catalyzed by PPh3, which con-
verted only 37% of benzaldehyde after 110 h at 808C.
Catalytic Application of LSK-3
LSK-3 was tested as catalyst in four different types of stoichio-
metric and catalytic reactions with substrates of different type
and size. A summary of all catalytic results is shown in
Figure 2. In all reactions described below, we used samples
with relatively big single crystals of size 0.5–1.0 mm (as mea-
Finally, we treated the Knoevenagel product 5 with ethyl
2,3-butanedienoate and performed a [3+2] cycloaddition that
we hoped would yield ethyl 4,4-dicyano-5-phenylcyclopent-1-
enecarboxylate (6) as product (Figure 2D). Although the reac-
tion is successfully mediated by PPh3 catalyst (20 mol%),[41]
LSK-3 (20 mol% P) did not produce any product after 60 h of
reaction at 808C in CHCl3.
Molecular Modeling and Intermediate Characterization
Figure 2. Reactions catalyzed by LSK-3. A) Coumarin synthesis; run 1: >99%
conversion, run 2: 89% conversion. B) Umpolung addition. C) Knoevenagel
condensation; run 1: 98% conversion, run 2: 89% conversion, run 3: 73%
conversion, run 4: 72% conversion. D) [3+2] cycloaddition.
A number of MOF catalytic systems have shown size selectivity
induced by their pore size.[42–44] However, comparison of the
relative size of reactants and products (see the Supporting In-
formation, Table S1) to the available pore opening and void
space (Figure 1D,E) in LSK-3 excludes the possibility that tradi-
tional size-selectivity plays a role. In fact, the molecules partici-
pating in the umpolung reaction without any apparent conver-
sion are actually smaller than those reacting in the successfully
catalyzed coumarin synthesis. We therefore performed geome-
try optimization, to check the relative stability of the inter-
mediates oriented in a different way within the MOF cage, and
solid-state 31P NMR spectroscopy to confirm the calculation’s
results. Both analyses revealed that the orientation of reaction
intermediates enable or hamper reaction to occur (see below).
Geometric optimizations were performed with Materials Studio
from Accelrys with the Forcite molecular modeling (MM) opti-
mization, which takes account only for steric effects and not
sured by optical microscopy) to minimize the ratio between
the phosphine on the surface and that inside the pores. In this
way, we would indirectly prove that, if catalysis were to occur,
it would do so within the pores and not just on the surface of
the crystal. After all catalytic reactions, the material was recov-
ered and analyzed either by powder X-ray diffraction (PXRD) or
by polarized-light optical microscopy, which showed the single
crystal integrity in all catalytic attempts below (see the Sup-
porting Information, Figure S4). Notably, the non-functionalized
IRMOF-9 failed to catalyze any of the reactions in which LSK-3
was active. We explored the synthesis of coumarin methyl 6-
acetyl-2-oxo-2H-chromene-4-carboxylate (3) from 4-hydroxya-
cetophenone and dimethyl acetylenedicarboxylate (Figure 2A),
which is a reaction requiring a stoichiometric amount of tri-
phenylphosphine (PPh3) in the homogeneous phase.[37] LSK-3
(100 mol% P) effected quantitative conversion to the desired
coumarin 3 in CHCl3 after 100 h at 808C. By comparison, a reac-
tion carried out with PPh3 at an identical phosphorus loading
31
electronic ones, followed by DMol3 DFT gradient-corrected
(GGA) correlation functional of Perdew–Burke–Ernzerhof
(PBE)[46] geometry optimization with the precise numerical
basis set DNP 3.5 (double numerical plus polarization). Similar
DFT calculations have already been successfully used in MOF
geometry optimization.[47] The presence of one or two phos-
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Figure 3. Mechanism and intermediates a1 and a2 of coumarin synthesis catalyzed by LSK-3. The yellow arrow indicates the reactive moiety of the intermedi-
ate. DE is the difference between the energy of the intermediates and that of the intermediate lowest in energy.
phine moieties in one cage did not change the overall out-
come. We therefore restrict the discussion here to one phos-
phine group.
left for 1 day at 808C under argon atmosphere. After approxi-
mately 30% conversion of alkyne had occurred, the NMR spec-
trum of the solid showed no free phosphine peak at
À12.6 ppm and a new peak at 30.1 ppm (see the Supporting
Information, Figure S8). The latter peak is the superimposition
of signals pertaining to the intermediate and the phosphine
oxide adduct, which is a byproduct arising from reaction of
the alkyne.[48] The quantitative disappearance of the free phos-
phine signal suggests that all phosphine sites are available for
reaction. After completion of the reaction, the portion of the
30.1 ppm signal that relates to the adsorbed intermediate re-
verted to the free phosphine (peak at À12.6 ppm). The phos-
phine oxide contribution of the 30.1 ppm signal remained and
contributed to catalyst deactivation. The analogous homoge-
neous reaction catalyzed by PPh3 was also monitored by NMR
spectroscopy in CD2Cl2. After 24 h of reactivity at 608C, the
31P NMR spectrum showed both the intermediate at 30.7 ppm
and PPh3 at À5.0 ppm (see the Supporting Information, Fig-
ure S11).
The mechanism of the phosphine-catalyzed coumarin syn-
thesis has previously been established in the literature
(Figure 3).[37] The first step involves the reaction of the phos-
phine with an alkyne to form a zwitterionic phosphonium salt,
followed by deprotonation of phenol and formation of the
phosphonium–alkene/phenolate ion couple. Subsequent aro-
matic substitution occurs to form the final product with cycli-
zation, elimination of methanol, and finally regeneration of the
phosphine. Geometry optimization of the zwitterionic inter-
mediate a revealed two possible configurations, a1 and a2
(Figure 3). These are distinguished by the relative orientation
of their double bonds. In the reactive intermediate a1, this
bond is oriented towards the framework, whereas in a2 the
double bond is oriented towards the pore. We note that a2 is
slightly more stable than a1 by 2.6 kJmolÀ1
.
31P MAS NMR and
1
liquid-state H NMR spectroscopy carried out under the reac-
tion conditions confirmed the formation of the P intermediate.
A stoichiometric amount of substrates and LSK-3 were loaded
in a sealed glass vessel with deuterated dichloromethane and
The umpolung addition mechanism starts with the forma-
tion of the zwitterionic intermediate (b in Figure 4), which de-
protonates malononitrile and before undergoing nucleophilic
Figure 4. Mechanism and intermediates b1 and b2 of umpolung addition catalyzed by LSK-3. The yellow arrow indicates the reactive moiety of the intermedi-
ate.
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addition. Geometry optimization of the zwitterionic intermedi-
ate b shows that two isomers, b1 and b2, are possible. In b1,
the double bond subject to further reaction is oriented to-
wards the pore, whereas for b2 this bond is oriented towards
the framework. The total energy of b2 is 28.9 kJmolÀ1 less
than that of b1, establishing b2 as the favored intermediate.
This disparity results from the steric hindrance by the frame-
work, which makes the most bulky part of the intermediate,
and therefore also the reactive carbon, point towards the
framework. This inhibits the nucleophilic attack of deprotonat-
ed malononitrile to the protonated b2. This time, the energy
difference is significant and explains why LSK-3 is an inactive
catalyst. Solid state NMR spectroscopy confirmed our calcula-
tions, identifying an intermediate without concomitant forma-
tion of product (see the Supporting Information, Figure S9)
and regeneration of free phosphine. 1H NMR spectroscopic
analysis of the reaction mixture indicated that no product is
formed at room temperature after heating for 1 h at 608C (Fig-
ure S13), nor after an extended two-week period under ambi-
ent conditions. The chemical shift of the intermediate differs
from that of the phosphine oxide adduct (31.2 ppm; Fig-
tion, the second substrate is small enough to react with a steri-
cally restricted reactive center (Figure 5C).
Conclusion
In summary, we have designed a novel MOF catalyst, LSK-3,
which features phosphine functionalization and IRMOF-9 topol-
ogy. This material demonstrated remarkable reactivity and was
rationalized by a combination of molecular modeling calcula-
tions and solid-state NMR spectroscopy. We have demonstrat-
ed that it is not the size of the reactants and products that de-
termines selectivity, but the steric hindrance by the framework,
which induces a selective orientation of reaction intermediates
and influences subsequent reactivity. In this way, we were able
to favor certain reactions while completely inhibiting others.
This is a unique feature that is observed neither in zeolites nor
in previous MOF studies. The modification of the local struc-
ture inside a MOF cage represents a powerful strategy for
tuning reaction selectivity to engineer the catalytic environ-
ment around the active site.
1
ure S10). In contrast, H and 31P NMR spectra of the homogene-
ous reaction catalyzed by PPh3 indicated parallel formation of
intermediate and product at room temperature. After heating
of the liquid phase reaction for 1 h at 608C the signal pertain-
ing to the intermediate disappeared with attendant reforma-
tion of free PPh3 (Figure S12).
Experimental Section
Synthesis of LSK-3
[1,1’-biphenyl]-4,4’-Dicarboxylic acid (2; 59 mg, 0.24 mmol) and
1 (52 mg, 0.12 mmol) were placed into a 20 mL glass vial. The vial
was flushed with Ar and a solution of Zn(NO3)2·4H2O (286 mg,
1.51 mmol) in degassed DMF (15 mL) was added. The vial was
tightly closed and kept in an oven at 858C for 72 h. The solvent
was decanted, the crystals were washed with degassed DMF (3ꢂ
5 mL) and degassed chloroform (5 mL) was added. Chloroform was
exchanged 3 times over three days and the crystals were stored in
degassed toluene.
Modes of reactivity between substrate and the diphenyl-
phosphino groups in LSK-3 are illustrated in Figure 5. Generally,
large substituents are oriented toward the pore by the frame-
work and consequently align reactive substituents in the same
direction. When the reactive part of the intermediate is also
the largest of the ancillary phosphine ligands—as is the case in
our example of coumarin synthesis (Figure 5A)—the intermedi-
ate is granted sufficient space to react with the second sub-
strate. In contrast, steric hindrance imposed by the framework
can impede further reactivity of the intermediate when the
small size of a substituent induces irreversible formation of
said intermediate, as for umpolung addition and [3+2] cyclo-
addition (Figure 5B). In the case of the Knoevenagel condensa-
Synthesis of coumarin 3 mediated by LSK-3
LSK-3 (59 mg, 0.05 mmol PPh2), 4-hydroxyacetophenone (6.8 mg,
0.05 mmol) and degassed CHCl3 (1.0 mL) were placed into a dried
argon-flushed 15 mL glass vial equipped with a septum. The sus-
pension was cooled to À58C with an ice-salt bath, and a solution
of dimethyl acetylenedicarboxylate (7.1 mg, 0.05 mmol) in de-
Figure 5. Schematic model that explains the reactivity of LSK-3 in the coumarin synthesis (A), umpolung and [3+2] cycloaddition (B), and Knoevenagel con-
densation (C). Once the first reactant (blue) approaches the active site in LSK-3 with the big substituent oriented towards the pore, it forms the intermediate
(red). If the reactive part of the intermediate is oriented towards the pores (A), the intermediate can react with the second reactant (yellow) and form the
product (orange). If the reactive part of the intermediate is oriented towards the framework, it reacts with the second reactant only if the latter is small
enough (B, C).
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reaction solution was warmed slowly to 808C and left at this tem-
perature for 100 h. After reaction, LSK-3 was washed with degassed
CHCl3 (6ꢂ2 mL) within three days. CHCl3 was removed from the so-
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Umpolung addition catalyzed by LSK-3.
LSK-3 (86 mg, 0.07 mmol PPh2), ethyl 2,3-butadienoate (36 mg,
0.35 mmol), malononitrile (23 mg, 0.35 mmol), and degassed ben-
zene (1.0 mL) were added into a dried argon-flushed 15 mL glass
vial equipped with a septum. The vial was sealed and the reaction
solution was warmed slowly to 608C and kept at the same temper-
ature for 8 h. LSK-3 was washed with degassed CHCl3 (6ꢂ2 mL)
within three days. No product was detected by GC-MS and liquid
1H NMR spectroscopy of the crude reaction mixture.
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frameworks · organocatalysis · phosphines · selectivity
catalysis
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Published online on October 3, 2014
Chem. Eur. J. 2014, 20, 15467 – 15472
www.chemeurj.org
15472
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