Organocatalysis by site-isolated N-heterocyclic carbenes doped into the UIO-67 framework

Article abstract of DOI:10.1016/j.poly.2016.02.033

Two imidazolium-tagged biphenyldicarboxylates were synthesized and incorporated into the UiO-67 framework. Post synthetic exchange did not prove a useful route to these materials. Alternatively, mixed-linker synthesis of the solid afforded materials loaded with 6-7% imidazolium linker, corresponding to fewer than one active site per cage. Powder X-ray diffraction patterns for the doped metal-organic frameworks (MOFs) revealed that they were isostructural with UiO-67. Once activated, the N-heterocylic carbene (NHC) containing MOF was found to catalyze the transesterification of vinyl acetate with benzyl alcohol in good yield. The catalyst could be recycled with a modest drop in conversion. This is the first report of a NHC-doped MOF acting as an organocatalyst.

Full text of DOI:10.1016/j.poly.2016.02.033

Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Organocatalysis by site-isolated N-heterocyclic carbenes doped into the
UIO-67 framework
William T. Schumacher, Madeleine J. Mathews, Sean A. Larson, Carl E. Lemmon, Karin A. Campbell,
⇑
⇑
Brendan T. Crabb, Brent J.-A. Chicoine, Laurance G. Beauvais , Marc C. Perry
Department of Chemistry, Point Loma Nazarene University, San Diego, CA 92106, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Two imidazolium-tagged biphenyldicarboxylates were synthesized and incorporated into the UiO-67
framework. Post synthetic exchange did not prove a useful route to these materials. Alternatively,
mixed-linker synthesis of the solid afforded materials loaded with 6–7% imidazolium linker, correspond-
ing to fewer than one active site per cage. Powder X-ray diffraction patterns for the doped metal–organic
frameworks (MOFs) revealed that they were isostructural with UiO-67. Once activated, the N-heterocylic
carbene (NHC) containing MOF was found to catalyze the transesterification of vinyl acetate with benzyl
alcohol in good yield. The catalyst could be recycled with a modest drop in conversion. This is the first
report of a NHC-doped MOF acting as an organocatalyst.
Received 30 October 2015
Accepted 23 February 2016
Available online xxxx
Keywords:
Metal–organic frameworks
N-heterocyclic carbene
Organocatalysis
Transesterification
Bromination
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
were then combined with a variety of metal nodes. Treatment of
the azolium containing MOFs with an appropriate base would then
The discovery of the first stable N-heterocyclic carbenes (NHCs)
over 20 years ago led to a wide range of research using them as
ligands and catalysts [1–3]. Their facile tunability, both sterically
and electronically, has contributed to their successful application
to a range of homogenous catalytic reactions [3]. Incorporation of
these catalysts into porous frameworks would offer the advantages
of heterogeneous catalysts, such as ease of product isolation and
catalyst recyclability, while maintaining the tunability that made
them successful in homogenous catalysis.
Metal–organic frameworks (MOFs) are synthesized using
organic linkers and metal nodes yielding predictable structures
with tunable porosity. In fact, they are an ideal platform for con-
structing heterogenous catalysts as the organic linker can be
modified to incorporate a wide range of known catalysts [4,5].
Not surprisingly, NHCs have been incorporated into a number of
MOFs using a variety of metal nodes including: Cd, Co, Cu, Zn,
and Zr ions [6–14]. Typically, the NHC is incorporated by grafting
an imidazolium or benzimidazolium salt into the organic linker.
The azolium salts have been incorporated as a central structural
component of the linker or as appendages to known organic linkers
as demonstrated in a few examples (Fig. 1). These azolium salts
form the NHC. The linker syntheses are fairly linear making the
creation of a diverse set of NHC precursors with varying steric
and electronic properties cumbersome. Hupp and coworkers (3),
on the other hand, have a more flexible synthesis where the imida-
zolium component is incorporated near the end of the linker
synthesis via a simple SN2 reaction [13]. This synthesis should
allow a diverse set of N-alkyl and N-aryl groups to be incorporated
onto the imidazolium ring opposite the diacid linker although, to
date, only the synthesis of a linker containing an N-methyl was
reported, likely due to the difficulty making MOF solids with
linkers that are too bulky.
Bulky NHC precursors are potentially problematic when used as
the sole organic linker as they can lead to a dramatic reduction in
the solid’s porosity. However, it has been demonstrated that large
transition-metal containing groups can be incorporated into MOFs
by either post-synthetic exchange or by a doping synthesis in
which the metal-tagged linker is combined with a traditional
linker in the MOF synthesis [7,15–18]. In either of these cases a
relatively small percentage of the bulky linkers are incorporated
into the solid resulting in overall larger pore sizes making them
better candidates for applications such as catalysis. One example
demonstrating this possibility involved the incorporation of an
Ir-NHC linker into UIO-68 by both post synthetic exchange and
doping at 27% and 17% respectively [7].
⇑
Corresponding authors. Tel.: +1 619 8493251; fax: +1 619 8493452
(L.G. Beauvais). Tel.: +1 619 8492976; fax: +1 619 8493452 (M.C. Perry).
E-mail addresses: lbeauvais@pointloma.edu (L.G. Beauvais), mperry1@pointloma.
edu (M.C. Perry).
Although a variety of NHC containing MOFs have been synthe-
sized [10], only a few have actually resulted in useful catalysis.
http://dx.doi.org/10.1016/j.poly.2016.02.033
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
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W.T. Schumacher et al. / Polyhedron xxx (2016) xxx–xxx
2.2.2. Me2BPDC-MeBr (5)
Me2BPDC-ME (4) (1.14 g, 4 mmol), NBS (784 mg, 5.1 mmol),
and benzoyl peroxide (96 mg, 0.4 mmol) were combined in a pres-
sure tube. Benzene (24 mL) was added and the reaction was placed
in an oil bath at 85 °C overnight. The reaction was allowed to cool
and the volatiles were removed in vacuo. The crude residue was
purified via silica gel column chromatography using hexanes and
ethyl acetate as the eluent. Compound 5 was isolated as a white
solid (800 mg, 55%). The NMR spectra matched that of the known
compound.
2.2.3. H2BPDC-DiPPI (6a)
Compound 5 (268 mg, 0.80 mmol) was added to a pressure
tube along with 1-(2,6-diisopropylphenyl)imidazole (200 mg,
0.88 mmol). Freshly distilled THF (2 mL) was added, and the reac-
tion was heated to 70 °C overnight. The precipitate was collected
via vacuum filtration and was dried in vacuo yielding a fluffy white
solid (341 mg, 72%). A portion of this solid (180 mg, 0.30 mmol)
was then added to a round bottom flask along with 20 mL 0.1 M
LiOH and 4 mL of methanol. The reaction stirred at room tempera-
ture overnight. The solution was acidified with 10% HCl and white
solids precipitated from the reaction mixture. The solids were col-
lected via vacuum filtration and were dried in vacuo. 6a (154 mg,
98%) was isolated as a white solid. 1H NMR (DMSO-d6): 1.14 (6H,
d, J = 6.8 Hz), 1.16 (6H, d, J = 6.8 Hz), 2.21 (2H, sep, J = 6.8 Hz),
5.75 (2H, s), 7.46 (2H, d, J = 7.6 Hz), 7.58 (1H, d, J = 7.6 Hz), 7.64
(3H, m), 7.73 (1H, s), 7.97 (1H, s), 8.11 (4H, m), 9.60 (1H, s); 13C
NMR (DMSO-d6): 24.55, 24.67, 29.04, 51.66, 124.88, 125.24,
126.20, 129.60, 129.90, 130.55, 130.62, 131.34, 131.46, 131.84,
131.94, 132.38, 133.33, 139.27, 143.61, 145.11, 145.92, 167.47,
167.81; ESI-MS (M-Cl) 483.
Fig. 1. Representative NHC precursor linkers.
Reactions include the Ir-catalyzed isomerization of allyl alcohols
[7], copper-catalyzed hydroboration of CO2 [6], and Pd-catalyzed
Suzuki–Miyaura and Heck reactions. [11] To date no organocataly-
sis has been reported using NHC-containing MOF’s although NHC’s
are known to catalyze a variety of organocatalytic reactions [1,19].
Herein, we report the synthesis of two new building blocks that
include an NHC precursor and investigate two strategies for incor-
porating these linkers into UIO-67. We demonstrate that using a
mixed ligand synthesis results in an ideal doping level of around
8%. This doping level should correspond to approximately one
NHC per large cavity and, therefore, afford site-isolated catalysts.
Finally, these solids are probed for their ability to catalyze the
transesterification of vinyl acetate with benzyl alcohol.
2. Experimental
2.2.4. H2BPDC-DEPI (6b)
2.1. Measurement and materials
Compound 5 (153 mg, 0.46 mmol) was added to a pressure tube
along with 1-(2,6-diethylphenyl)imidazole (100 mg, 0.50 mmol).
Freshly distilled THF (2 mL) was added, and the reaction was
heated to 70 °C overnight. The precipitate was collected via vac-
uum filtration and was dried in vacuo yielding a fluffy white solid
(200 mg, 78%). A portion of this solid (200 mg, 0.35 mmol) was
then added to a round bottom flask along with 25 mL 0.1 M LiOH
and 4 mL of methanol. The reaction stirred at room temperature
overnight. The solution was acidified with 10% HCl and white
solids precipitated from the reaction mixture. The solids were col-
lected via vacuum filtration and were dried in vacuo. 6b (155 mg,
90%) was isolated as a white solid. 1H NMR (DMSO-d6): 1.07 (6H,
t, J = 7.2 Hz), 2.23 (4H, m), 5.73 (2H, s), 7.39 (2H, d, J = 7.6 Hz),
7.59 (4H, m), 7.87 (2H, d, J = 8.4 Hz), 8.04 (1H, s), 8.10 (3H, m),
9.45 (1H, s); 13C NMR (DMSO-d6): 15.67, 24.34, 51.80, 124.61,
125.75, 127.92, 129.82, 130.46, 130.61, 130.79, 131.85, 132.04,
132.88, 133.10, 139.08, 140.54, 141.28, 143.57, 145.07, 167.68,
167.98; ESI-MS (M-Cl): 455.
All reagents were of analytical grade and used as received.
UiO-67 was synthesized as previously described [20]. N-arylimida-
zoles were synthesized using a published procedure [21].
The 1H and 13C NMR spectra were collected at 400 and 100 MHz
respectively on an Agilent MercuryPlus 400 AS instrument. Mass
spectra were collected by the Mass Spectrometry Facilities in the
Department of Chemistry and Biochemistry at UC San Diego.
X-ray powder diffraction patterns were collected using Cu K
a
(k = 1.5418 Å) radiation on a Bruker D8 Advance diffractometer
equipped with a LynxEye detector.
2.2. Preparation of complexes
2.2.1. Me2BPDC-ME (4)
Methyl-4-bromo-3-methylbenzoate (2.06 g, 9 mmol) and
4-Methoxycarbonyphenylboronic acid (1.62 g, 9 mmol) were
added to a round-bottom flask and pumped into a glove box. Dry
DMF (40 mL) was added along with Pd(PPh3)4 (312 mg, 0.27 mmol)
and cesium carbonate (4.40 g, 13.5 mmol). The flask was fitted
with a septa, placed in an oil bath at 80 °C, and stirred overnight.
The flask was removed from the oil bath and allowed to cool to
room temperature at which time water (20 mL) and ethyl acetate
(20 mL) were added and a white precipitate was removed by
vacuum filtration. The filtrate was washed with brine (3 Â 50 mL)
and dried over sodium sulfate. The volatiles were removed in vacuo
and the crude residue was purified by silica gel column chromatog-
raphy using hexanes and ethyl acetate as eluent. The product 4 was
isolated as a white solid (1.77 g, 69%). The NMR spectra matched
that of the known compound.
2.3. Preparation of solids
2.3.1. [Zr6(l3-O)4(l3-OH)4(BPDC)5.4(6a)0.6] (MOF-1)
ZrCl4 (120 mg, 0.514 mmol) and benzoic acid (1.88 g, 30 equiv.)
were added to a 120 mL jar containing 20 mL of anhydrous DMF.
The solution was sonicated until all solids dissolved. The linkers
bpdc (62 mg, 0.26 mmol) and 6a (133 mg, 0.26 mmol) were added
and the solution sonicated for 5 min. The solution was heated to
120 °C over 4 h, kept at that temperature for 24 h, and cooled to
room temperature over 4 h. The solids were washed with DMF
(3 Â 10 mL) followed by EtOH (3 Â 10 mL) and THF (5 Â 10 mL).
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W.T. Schumacher et al. / Polyhedron xxx (2016) xxx–xxx
3
2.3.2. [Zr6(
l
3-O)4(
l
3-OH)4(BPDC)5.4(6b)0.6] (MOF-2)
by 1H NMR. Reaction progress was determined by comparison of
the integrated values of the –CH2– peak of benzyl alcohol
(d 4.6 ppm) with that of the –CH2– peak of the benzyl acetate
product (d 5 ppm).
ZrCl4 (120 mg, 0.514 mmol) and benzoic acid (1.88 g, 30 equiv.)
were added to a 120 mL jar containing 20 mL of anhydrous DMF.
The solution was sonicated until all solids dissolved. The linkers
bpdc (62 mg, 0.26 mmol) and 6b (117 mg, 0.26 mmol) were added
and the solution sonicated for 5 min. The solution was heated to
120 °C over 4 h, kept at that temperature for 24 h, and cooled to
room temperature over 4 h. The solids were washed with DMF
(3 Â 10 mL) followed by EtOH (3 Â 10 mL) and THF (5 Â 10 mL).
2.7. Recycling of catalyst
After the transesterification trial, the solid was rinsed with
anhydrous THF (3 Â 10 mL). An additional 10 mL of THF was added
and the solid was activated as described in activation of solids
above and the procedure for transesterification was repeated as
described.
2.4. Digestion of solids
Approximately 10 mg of MOF was dried under vacuum and
added to a vial containing 600 lL of DMSO-d6 and 10 lL of 70%
HF. The solution was sonicated until the solid dissolved and the
2.8. Verification of carbene formation
NMR spectra were obtained.
MOF-1 was activated, rinsed with THF (3 Â 10 mL), and then
reacted with an excess of Br2 in THF. After 20 min, the solids were
washed with THF (3 Â 10 mL) and digested for NMR analysis. A
negative control was also run in which unactivated MOF-1 was
suspended in THF (10 mL) and treated with excess Br2. After
20 min the solids were washed with THF (3 Â 10 mL) and digested
for NMR analysis.
2.5. Activation of solids
Approximately 20 mg (0.0094 mmol) of MOF material was
washed with THF (3 Â 10 mL) and stored in THF overnight. The
THF solution was exchanged once more, and 150 lL (0.30 mmol)
of lithium diisopropylamide solution (2.0 M in THF/hexanes) was
added to the solid. The solids were dispersed by sonication for
20 s. After 10 min, the solids were washed with THF (5 Â 10 mL)
and used for reactions immediately.
3. Results and discussion
3.1. Organic synthesis
2.6. Transesterification reactions
Although two syntheses of the brominated BPDC (4) have been
reported [13,22], we elected to use a modified procedure utilizing
commercially available starting materials (Scheme 1). A Suzuki-
Miyuari coupling from commercially available aryl bromide and
boronic acid allowed for the synthesis of the biaryl core (4).
Benzylic bromination using NBS yielded the brominated
In a typical reaction, benzyl alcohol (0.10 mL, 1.0 mmol) and
vinyl acetate (0.11 mL, 1.2 mmol) were dissolved in 1.0 mL of
THF. Approximately 20 mg of MOF was added and the reaction
was allowed to proceed for 10 min. A 50
mixture was removed and added to 550
l
L aliquot of the reaction
lL of CDCl3 and analyzed
Scheme 1. Synthesis of bulky imidazolium linkers.
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W.T. Schumacher et al. / Polyhedron xxx (2016) xxx–xxx
compound 5. Bulky N-aryl imidazoles were then added in THF at
elevated temperatures followed by hydrolysis of the pendant ester
groups to yield the desired substituted biphenyldicarboxylate(6) to
be incorporated into the MOF framework. We intentionally
incorporated bulky imidazolium salts to investigate the viability
of incorporating such large groups in the UIO-67 framework.
this strong base (Fig. 2). In order to verify that carbenes were
actually generated we treated the activated MOF with excess Br2
which is known to brominate the carbene carbon atom and should
produce MOF-1Br (Scheme 3) [28]. Digestion of the solid and NMR
analysis comparing the intensity of the imidazolium C–H peak to a
backbone C–H peak indicated that over 80% of all the imidazolium
rings had been brominated. Thus confirming generation of the
carbene species and indicating that nearly all of these active sites
are accessible.
3.2. Solid synthesis
The UiO-67 platform was chosen due to its crystallinity,
durability [23], and amenability to post-synthetic modification
[24,25], Our initial attempts to produce mixed linker solids focused
on using post-synthetic exchange to replace the BPDC linkers in
UiO-67 with our imidazolium-containing linkers 6a and 6b. These
post-synthetic exchange (PSE) reactions were attempted in water,
DMF, and mixtures thereof at 85 °C. Despite the success of this
procedure for UiO-66 [26] and UiO-68 [7], it did not work with
our building blocks. Characterization of the samples after PSE
revealed that only a minute amount of the linker was exchanged
and the resulting solid was amorphous. In fact others have
reported only small amounts of incorporation into UiO-67 using
PSE with large linkers [27]. Due to these poor results, we sought
to dope our imidazolium-containing building blocks into the
framework of UiO-67 during synthesis (Scheme 2).
Linkers 6a and 6b have the same length as BPDC and should
substitute for BPDC when both linkers are present for MOF synthe-
sis. The sterically demanding imidazolium moieties should limit
the amount of linker substituted in the framework and, ideally,
lead to site-isolated catalysts. This method had proven successful
for introducing sterically demanding reactive sites into UiO-67
[18,27] and UiO-68 [7,15]. Doped UiO-67 (MOF-1 and -2) with
3.3. Catalysis
We tested the catalytic activity of MOF-1 in the transesterifica-
tion of benzyl alcohol and vinyl acetate, a reaction which has been
used to test various NHC and NHC-like catalysts [29]. The reactions
were performed in THF using a slight excess of vinyl acetate and
reaction progress was followed using NMR spectroscopy. NHC
catalysts require activation with a base to deprotonate the imida-
zolium salt (Scheme 3). As expected, samples of UiO-67 and
MOF-1 did not catalyze the reaction (Table 1). The MOF-1 catalyst
was activated by treatment with LDA in THF for 10 min, and
washed five times with 10-mL aliquots of dry THF. Activated
MOF-1⁄ fully catalyzed the transesterification in 10 min and all
of the vinyl acetate was consumed. The catalyst was able to be
recycled although some moderate loss in activity was observed.
Surprisingly, the pure UiO-67 solid, after treatment with LDA,
catalyzed the reaction, though to a substantially lesser extent than
the doped sample. This activity is most likely due to residual LDA
trapped within the framework or deprotonated hydroxyl groups
on the Zr6 cluster. Previous studies have indicated that the
the general formula [Zr6(l3-O)4(l3-OH)4(BPDC)6Àx(L)x] were
synthesized by reacting ZrCl4 with a 1:1 mixture of BPDC and 6a
and 6b in DMF at 120 °C in the presence of benzoic acid. X-ray
powder diffraction (PXRD) analysis reveals that MOF-1 and
MOF-2 are isostructural with UiO-67 (Fig. 2). Unfortunately, pure
UiO-67 and the doped materials do not maintain their structure
for very long after removal from solution. Within 30 min of drying
the samples, powder patterns reveal an amorphous material. When
in solution, the frameworks are quite robust and the samples are
not allowed to dry before use.
NMR analysis of digested samples of MOF-1 and -2 reveals that
6–7% of the linkers contain an imidazolium moiety (Fig. 3). Based
on the structure of UiO-67, this corresponds to less than one
NHC precursor per large cage, which appears reasonable consider-
ing the size of the N-aryl groups. An equimolar concentration of
linker is required to achieve maximal doping into the framework.
To generate the NHC moiety, the solids were exposed to LDA for
10 min followed by washing with THF (Scheme 3). PXRD analysis
reveals that the solids maintain their structure upon exposure to
Fig. 2. Powder X-ray diffraction data for UiO-67 (black), MOF-1 (blue), MOF-2
(green), UiO-67 after activation with LDA (red), and MOF-1 after two cycles of
catalysis (purple). (Color online.)
Scheme 2. Doping synthesis of MOF-1 and -2.
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W.T. Schumacher et al. / Polyhedron xxx (2016) xxx–xxx
5
Fig. 3. Linker and digested MOF 1H NMR.
Scheme 3. Activation of MOF by NHC formation.
experiments will focus on introducing metal ions into these solids
and exploring a wider range of catalytic reactions. These heteroge-
neous catalysts are robust and recyclable and could offer an
alternative to traditional catalysts. The modularity of these linkers
will allow us to rapidly explore a range of ligand modifications and
compare the role of ligand. In addition, further studies of the
activated UiO-67 are necessary to determine the source of its
ability to catalyze this reaction and to determine if that material
could be an effective catalyst for other reactions.
Table 1
MOF-catalyzed transesterification.
O
O
MOF
O
Catalyst,
OH
+
+
O
H
O
THF, RT, 10 min.
MOF
Unactivated MOF
Activated MOF
% conversion
Second run
% conversion
% conversion
UIO-67
MOF-1
0
0
27%
80%
27%
72%
Acknowledgments
We would like to thank Research Associates (PLNU) and the
Inamori Foundation (M.J.M.) for funding, Prof. Seth Cohen (UCSD)
for use of the X-ray diffractometer, and the Mass Spectrometry
Center in the Department of Chemistry and Biochemistry at UCSD.
hydroxyl groups of the Zr6 cluster are reactive [30,31], and we plan
to investigate this phenomenon further.
4. Conclusions
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transesterification of vinyl acetate with benzyl alcohol. Further
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