Urea-containing metal-organic frameworks as heterogeneous organocatalysts

Article abstract of DOI:10.1039/c5ta03835a

Two novel pillared metal-organic frameworks (MOFs) containing a urea-functional group are introduced. Herein, the urea functional group was incorporated into the MOF backbone by preparing a urea-ditopic ligand. These frameworks (TMU-18 and TMU-19) were fabricated using the synthesized urea-containing ligand, 4,4′-bipyridine (bipy) and 1,2-bis(4-pyridyl)ethane (bpe), and using zinc nitrate as the metal source. Subsequently, TMU-18 and TMU-19 were characterized by X-ray diffraction, IR spectroscopy, elemental analysis, scanning electron microscopy (SEM) and thermogravimetric analysis. Furthermore, their potential efficiency as organocatalysts was evaluated in the regioselective methanolysis of epoxides.

Full text of DOI:10.1039/c5ta03835a

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Urea Containing Metal–organic Frameworks as
Heterogeneous Organocatalysts
Alireza Azhdari Tehrani,‡a Sedigheh Abedi,‡a Ali Morsali*a, Jun Wangb and Peter C. Junkb
a
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115ꢀ
175, Tehran, Iran
b
College of Science Technology and Engineering, James Cook University, Townsville Qld,
4811, Australia
ABSTRACT:
Two novel pillared metalꢀorganic frameworks (MOFs) including ureaꢀfunctional groups are
introduced. Herein, urea functional groups were incorporated into the MOF backbone by
preparing ureaꢀditopic ligand. These frameworks (TMU-18 and TMU-19) were fabricated using
the synthesized ureaꢀcontaining ligand, 4,4'ꢀBipyridine (bpy) and 1,2ꢀBis(4ꢀpyridyl)ethane
(bpe), respectively using zinc nitrate as metal source. Subsequently, TMU-18 and TMU-19 were
characterized by Xꢀray diffraction, IR spectroscopy, elemental analysis, scanning electron
microscopy (SEM) and thermogravimetric analysis. Furthermore, their potential talent as
organocatalysts was evaluated in the regioselective methanolysis of epoxides.
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Introduction
Supramolecular organocatalysis is an interdisciplinary research area that includes elements from
2
organic chemistry, supramolecular chemistry and biochemistry.1,
The design of a
supramolecular catalyst is based on using hydrogen bonding and other intermolecular
interactions in recognition and activation of substrates for triggering a variety of chemical
transformations.3, 4 However, supramolecular catalysis often suffer from drawbacks such as the
lack of catalyst recycling and low efficiency due to the selfꢀaggregation (selfꢀquenching) of the
catalyst.3 The heterogenization of supramolecular organocatalysts may be a logical solution to
overcome these obstacles in extending the applicability of these systems.5ꢀ7 Recently, metalꢀ
organic frameworks (MOFs) were introduced as promising candidates for applications in diverse
areas.8ꢀ10 Compared to other porous materials, MOFs have given to the chemist the opportunity
of tuning the topology, pore size and functionality by rational selection of the organic linkers and
inorganic metal centers. Owing to this feature, MOFs with uniform and permeable pores and
channels have shown to be particularly promising for catalysis.11ꢀ14 According to the catalytic
active sites, these frameworks may be categorized into four distinct groups, namely metalꢀ
organic frameworks with coordinatively unsaturated metal sites (group I), MOFs with
metalloligands (group II), MOFs with functional organic sites (group III) and metal nanoparticles
embedded in the MOF cavities (group IV). Among these, MOFs with catalyticallyꢀactive
functional organic sites have received less attention due to the synthetic complexities in
providing guestꢀaccessible functional organic groups in the pore surface of frameworks.15 In this
regard, different organic functional groups, such as proline, amide, binol and pyridyl were
successfully incorporated into MOFs.16 Recently, both CrꢀMILꢀ101 and IRMOFꢀ3 are decorated
with activated urea and thiourea functions, respectively using postꢀsynthesis modification
2

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method.17, 18 The appealing idea of preparing heterogeneous ureaꢀbased MOF catalysts was
proposed recently by Farha, Hupp and Scheidt et al.6 They have examined the catalytic activity
of the ureaꢀbased MOF for the FriedelꢀCrafts reaction between pyrroles and nitroalkenes. In
addition, although high conversion was obtained in the case of small substrates, the yield of the
addition product was low (39%) at the optimum reaction condition. They could also successfully
synthesize a Zrꢀbased MOF containing urea function used for Morita–Baylis–Hillman reaction.19
Therefore, designing such a catalytic system based on urea containing MOF is in the early stages
and needs further work and development. Ring opening of epoxides is one of the most important
reactions produced vital intermediates in organic synthesis. There are some reports for this
reaction catalysed by MOF containing Lewis acid sites, especially with Cu, Fe and Hf metal
nodes.20ꢀ23 In this regards, there is no report of ureaꢀcontaining MOFs as hydrogenꢀbond catalyst
in this reaction.
In this paper, we report an extension of these studies aimed at investigating the organocatalytic
role of ureaꢀcontaining MOFs in the activation of epoxides. Our design is based on two
following assumptions: (i) synthesis a ditopic oxygenꢀdonor ligand containing a urea functional
group, that are capable of forming dual hydrogen bonds with organic substrates. (ii) selection of
an appropriate pillaring strut able to form MOFs with dinuclear paddleꢀwheel SBUs, in which
five of the six coordination positions of each Zn(II) ion are anticipated to be occupied for
network propagation and the sixth is located inside the zinc cluster. Accordingly, two novel ureaꢀ
containing MOFs synthesized by combining the ditopic urea “strut”, a pillaring strut, and
Zn(NO3)2.6H2O using the solvothermal method at 90˚C for 120 h to give suitable Xꢀray quality
crystals of [Zn2(ubl)2(bipy)]·DMF (TMU-18) and [Zn2(ubl)2(bpe)]·DMF (TMU-19), where ubl
3

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(ureaꢀbased ligand) is 4,4'ꢀ(carbonylbis(azanediyl))dibenzoic acid, bipy and bpe are 4,4'ꢀ
bipyridine and 1,2ꢀbis(4ꢀpyridyl)ethane, respectively, Figure 1.
Experimental Section
Apparatus and Reagents
All starting materials, including 1,1'ꢀCarbonyldiimidazole, 4ꢀAminobenzoic acid were purchased
from commercial suppliers (SigmaꢀAldrich, Merck) and used as received. The infrared spectra
were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500ꢀ
4000 cmꢀ1 using the KBr disk technique. Elemental analyses (carbon, hydrogen, and nitrogen)
were performed using an ECS 4010 CHN made in Costech, Italy. Melting points were obtained
by a Bamstead Electrothermal type 9200 melting point apparatus and corrected.
Thermogravimetric analyses (TGA) of the compounds were performed on a computerꢀcontrolled
PLꢀSTA 1500 apparatus. The 1HꢀNMR spectrum was recorded on a Bruker ACꢀ250 MHz
spectrometer at ambient temperature in d6ꢀDMSO and CDCl3. Xꢀray powder diffraction (XRPD)
measurements were performed using a Philips Xpert diffractometer with monochromated CuꢀKα
radiation (λ = 1.54056 Å). The samples were also characterized by field emission scanning
electron microscope (FEꢀSEM) SIGMA ZEISS and TESCAN MIRA (Czech) with gold coating.
Single-Crystal Diffraction.
Xꢀray crystal structure determinations: Crystals in viscous paraffin oil were mounted on
cryoloops and intensity data were collected on the Australian Synchrotron MX1 beamline at 100
K with wavelength (λ = 0.71073 Å). The data were collected using the BlueIce24 GUI and
processed with the XDS25 software package. The structures were solved by conventional
methods and refined by fullꢀmatrix leastꢀsquares on all F2 data using SHELX9726 or SHELX2014
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in conjunction with the XꢀSeed27 or Olex228 graphical user interface. Anisotropic thermal
parameters were refined for nonꢀhydrogen atoms and hydrogen atoms were calculated and
refined with a riding model.
Crystallographic data: TMUꢀ18: C83H63N13O21Zn4, M= 1839.94 g molꢀ1, Triclinic, Pī, a=
21.179(4) Å, b= 22.560(5) Å, c= 23.138(5) Å, α= 105.30(3)˚, β= 114.19(3)˚, γ= 102.88(3)˚, V=
9013(4) Å3, Z=2, ρcalc= 0.678 g cmꢀ3, λ= 0.71073, T=100 K, R1= 0.0688, wR2= 0.1817, S= 0.897,
ccdc=1041981; TMUꢀ19: C21H16N3O5Zn, M= 455.74 g molꢀ1, orthorhombic, Pnna, a= 21.035(4)
Å, b= 16.064(3) Å, c= 29.891(6) Å, V= 10100(3) Å3, Z=8, ρcalc= 0.599 g cmꢀ3, λ= 0.71073,
T=100 K, R1= 0.0786, wR2= 0.2428, S= 1.103, ccdc=1041982.
Synthesis of urea ditopic ligand.
The urea ditopic ligand was synthesized in three steps, starting from commercially available 4ꢀ
aminobenzoic acid. Detailed synthetic procedures and characterization of the synthesized
frameworks are provided in the experimental section and supporting information.
Synthesis and activation of TMU-18
Zn(NO3)2.6H2O (0.297 g, 1 mmol), 4,4'ꢀ(carbonylbis(azanediyl))dibenzoic acid (0.300 g, 1
mmol) and 4,4'ꢀBipyridine (0.078 g, 0.5 mmol) were dissolved in 20 mL DMF. The mixture was
placed in teflonꢀlined stainless steel autoclaves and heated to 90 ˚C for 72 h and then it was
gradually cooled to room temperature over 48 h. The crystals were obtained in a 72% yield. The
products were characterized by different techniques such as powder Xꢀray diffraction (PXRD),
IR spectroscopy, elemental analysis and SEM microscopy. (Before activation) FTꢀIR (KBr
pellet, cmꢀ1): 3320 (br), 1657 (vs), 1604 (vs), 1530 (vs), 1398 (vs), 1308 (m), 1230 (m), 1171
5

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(m), 856 (w), 779 (m), 626 (w), 500 (w). Anal. calcd for ZnC23H23N4O7: C, 51.84; H, 4.35; N:
10.51, Found: C, 49.96; H, 4.79, N: 10.85.
The sample was activated by immersing the crystals of TMU-18 in anhydrous chloroform
followed by heating at 80°C in vacuum for 24 h. The activation was also confirmed by PXRD
and FTꢀIR spectroscopy.(after activation) FTꢀIR (KBr pellet, cmꢀ1): 3390 (br), 1604 (vs), 1532
(vs), 1400 (vs), 1308 (s), 1228 (m), 1172 (m), 856 (w), 779 (m), 628 (w), 500 (w). Anal. calcd
for ZnC20H14N3O5: C, 54.38; H, 3.19; N: 9.51, Found: C, 54.12; H, 3.82, N: 9.94.
Synthesis and activation of TMU-19
Zn(NO3)2.6H2O (0.297 g, 1 mmol), 4,4'ꢀ(carbonylbis(azanediyl))dibenzoic acid (0.300 g, 1
mmol) and 1,2ꢀBis(4ꢀpyridyl)ethane (0.092 g, 0.5 mmol) were dissolved in 20 mL DMF. The
mixture was placed in teflonꢀlined stainless steel autoclaves and heated to 90 ˚C for 72 h and
then it was gradually cooled to room temperature over 48 h. The crystals were obtained in a 60%
yield. The products were characterized by different techniques such as powder Xꢀray diffraction
(PXRD), IR spectroscopy, elemental analysis and SEM microscopy. (Before activation) FTꢀIR
(KBr pellet, cmꢀ1): 3346 (br), 1655 (vs), 1605 (vs), 1532 (s), 1397 (s), 1308 (m), 1231 (m), 1172
(m), 855 (m), 780 (m), 622 (w), 500 (w). Anal. calcd for ZnC24H25N4O7: C, 52.71; H, 4.61; N:
10.25, Found: C, 50.99; H, 4.83, N: 11.05.
The sample was activated by exchanging the DMF molecules with chloroform and then
evacuating at room temperature for 8 h. The FTꢀIR spectroscopy confirmed that some of the
DMF molecules are removed from TMUꢀ19, while the rest of them could be necessary to
stabilize the MOF framework. (after activation) FTꢀIR (KBr pellet, cmꢀ1): 3346 (br), 1606 (vs),
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1527 (s), 1391 (s), 1306 (m), 1227 (m), 1169 (m), 855 (m), 778 (m), 619 (w), 505 (w). Anal.
calcd for ZnC21H16N3O5: C, 55.34; H, 3.54; N: 9.22, Found: C, 55.17; H, 3.88, N: 9.96.
Catalysis Experiments.
In a typical reaction, the ureaꢀbased MOFs (25 mg, ca. 0.05 mmol equiv of urea species) were
added to a CH3OH solution (3 mL) containing the epoxide substrate (0.2 mmol). The reaction
mixture was stirred at 60 ̊C for the indicated times mentioned in Table 1 of the manuscript. After
the required reaction time, the reaction mixture was cooled to room temperature and analysed by
GC analysis using internalꢀstandard method.
Catalyst Recycling.
The reusability of TMU-18 and TMU-19 was tested for the methanolysis of styrene oxide. After
stirring for 40 h, the heterogeneous mixture was allowed to settle completely followed by
decanting the supernatant liquid. The TMU-18 and TMU-19 catalysts were filtered off after 40 h
reaction, washed with excess MeOH and respectively dried at 80 ̊C and under vacuum at room
temperature. The recovered catalyst was then reused without further purification for the second
run with fresh styrene oxide and methanol.
Typical Procedure for the Methanolysis of Styrene Oxide.
The prepared urea MOF (25 mg, ca. 0.05 mmol) catalysts is suspended in a MeOH (3 mL)
solution of styrene oxide (25 mg, 2 mmol) and stirred at 60 ̊C for 140 h. Then, the solid catalyst
was filtered off. The reaction mixture was analyzed using gas chromatography. Then excess of
solvent was removed under reduced pressure to give the corresponding product. The major
1
product (2ꢀMethoxyꢀ2ꢀphenylethanol) is determined by NMR. HNMR (250 MHz, CDCl3):
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7.26ꢀ7.39 (m, Ph, 5H), 4.3 (dd, J = 3.74, 8.1 Hz, 1H), 3.58ꢀ3.72 (m, CH2OH, 2H), 3.31 (s, OMe,
3H), 2.91 (bs, OH, 1H).
Typical Procedure for the Methanolysis of other Epoxides.
The prepared urea MOF catalysts is suspended in a MeOH (3 mL) solution of epoxide (25 mg)
and stirred at 60 ̊C for 110 h. Then, the solid catalyst was filtered off. Determination of the major
product was performed based on an internalꢀstandard method. All standards samples for
determination of the major products in the reaction mixture were prepared using the separated
methanolysis reactions of the epoxides in the presence of catalytic amount of HCl. Since the
prominent product in this acidic condition is the adduct owing to steric reasons, their retention
time for GC analysis was revealed in this manner. For this reaction, 2 drop of the concentrated
HCl was added to the solution of epoxides (2 mmol) and MeOH (5 ml). The reaction solution
was stirred at 60 ̊C for 1 h. The progress of the reaction was monitored using GC analysis.
Results and Discussion
Structural analysis and characterization
TMU-18 and TMU-19 were synthesized by combining the ditopic urea ligand, a pillaring strut,
and Zn(NO3)2.6H2O using the solvothermal method at 90˚C for 120 h to give suitable Xꢀray
quality crystals. Xꢀray crystallography analyses reveal that TMU-18 and TMU-19 crystallize in
triclinic Pī and orthorhombic Pnna, respectively. In these compounds, the coordination geometry
around the Zn(II) can be described as distorted octahedral, with four sites occupied by oxygen
atoms of four different urea ligand carboxylate groups in an approximately square configuration
and the fifth site occupied by a nitrogen atom of bipy/bpe ligand (Figure 2a and 2d). The
remaining coordination site of each metal center is located inside the zinc paddleꢀwheel cluster.
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The ZnꢀZn distances are 2.930(1) and 2.9319(7) Å for TMU-18 and TMU-19, respectively. Both
compounds are composed of paddleꢀwheel dinuclear zinc carboxylate clusters (Zn2(COO)4)
bridged by the urea struts to form a two dimensional square grid. The 2D square grids are further
linked to each other by pillaring bipy/bpe forming a 3D framework which can be described a
doublyꢀinterpenetrated pcu network, Figure 2. Both compounds possess large channels (along
the bcꢀplane with aperture size of 13.5 × 9.9, for TMU-18 and along the bꢀaxis with aperture size
of 12.1 × 10.6 Å for TMU-19, including van der Waals radii of the atoms), Figures 2(c) and 2(f).
Also, the calculated void space per unit cell for disorderꢀ and guestꢀfree TMU-18 and TMU-19
frameworks are 66.3% (5977.6 Å3) and 69.6% (7034.1 Å3), respectively.29 Xꢀray crystallography
analysis reveals that the NꢀH groups of TMU-18 are involved in NꢀH•••O hydrogen bonding
interactions with the oxygen atom of the N,Nꢀdimethylformamide (DMF) molecule. In the case
of TMU-19, the DMF molecule cannot be located in the electronꢀdensity map, due to the severe
disorder and therefore was squeezed out with the help of PLATON squeeze.29 However,
spectroscopic analyses suggest that TMU-18 and TMU-19 may have similar chemical
compositions.
Thermogravimetric analysis (TGA) indicates that TMU-18 has a much better thermal stability
compared to TMU-19. The TGA data of TMU-18 shows an initial weight loss (4%, after heating
to 120˚C) which is attributed to the loosely bound water molecule. The other weight loss
occurred between 120 and 200˚C (13%) corresponding to the removal of DMF. In contrast to
TMU-18, TGA analysis of TMU-19, indicates a large mass loss in two steps in the range of 30
to 260˚C, indicating its low thermal stability. Thus, in order to activate the potential catalytic
sites of TMU-18, the crystals were immersed in anhydrous chloroform for 72 h, filtered and
vacuumꢀdried at 80˚C for 24 h. The activation was confirmed by FTꢀIR spectroscopy and PXRD
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analysis (see the ESI). An attempt to activate TMU-19 by the same procedure was unsuccessful
probably due to the low thermal stability of this compound at elevated temperatures.
Accordingly, this compound was activated by exchanging the DMF molecules with chloroform
and then evacuating at room temperature for 8 h. The FTꢀIR spectroscopy confirmed that some
of the DMF molecules are removed from TMU-19, while the rest of them may be necessary
to stabilize the MOF framework.
Catalytic studies of TMU-18 and TMU-19
The ringꢀopening reaction of epoxides, by alcoholic compounds known as “alcoholysis”, was
chosen as a probe to study the catalytic activity of the synthesized ureaꢀcontaining MOFs. The
alcoholysis reaction of epoxides is facile providing 1,2ꢀbifunctional compounds such as 1,2ꢀ
diols, βꢀamino alcohols and other interesting compounds for the pharmaceutical and
agrochemical points of view.30, 31 Actually, the activation of the oxygen atom of the epoxides is
catalytically performed using either Lewis or Brønsted acids. Methanolysis of epoxides which
was traditionally attained using corrosive sulfuric acid,32 has been widely investigated in
heterogeneous catalytic systems including polymers and silicaꢀbased materials.33, 34 In recent
years, some noteworthy attempts for the methanolysis reaction using MOFs has been reported, in
which the epoxide activation is mostly achieved by the function of metal Lewis acidity centers
within the MOF structures.20, 22, 23 Generally, the coordination talent of the solvent typically
MeOH either acts as a nucleophile on the one side, and the requisite free metal center sites within
the MOF catalyst, on the other side led to the weak structural stability of these kinds of catalysts
even those provided excellent catalytic activity and selectivity.35 Therefore, incorporating the
linkers containing Brønsted acids36 or hydrogenꢀbond donating (HBD) moieties upon a MOF
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structure which involves locked metal centers such as paddleꢀwheel nodes can hopefully provide
a MOF catalytic system that would be durable even for recycling purposes. According to the
only valuable report presented by Hupp and coworkers, incorporating a urea strut within MOF
structures can significantly enhance is HBD ability preventing the intrinsic unproductive selfꢀ
quenching behavior of the urea units.6
Regarding to the above synthesized ureaꢀcontaining MOFs of TMU-18 and TMU-19, we
subsequently examined their potential as organocatalyst for methanolysis reaction of epoxides.
The ring opening of the styrene oxide in MeOH as a probe reaction was selected to explore the
reaction conditions. No methanolysis reaction proceeded at room temperature. Moreover, in the
presence of mixed solvent systems including toluene, CH2Cl2, CHCl3, THF and CH3CN in
combination with MeOH (1:1 ratio), no additional product was observed during 48 h reaction
with styrene oxide. The alcoholysis reaction in the presence of 25 mg (0.05 mmol) of these ureaꢀ
based MOFs and net MeOH as solvent gave 5% conversion as obtained by GC. However, in the
absence of catalyst, when the reaction temperature was increased to 60 ̊C, during 24 and 110 h,
the reaction proceeded with 14 and 19% conversions (Table 1, entries 1 and 2) while within 24 h,
in the presence of TMU-18 and TMU-19, 35 and 31% of styrene oxide was converted,
respectively (entry 4). This observation clearly revealed the catalytic effect of the prepared ureaꢀ
containing MOFs. In addition, the same reaction runs were carried out for optimization of the
catalyst and also solvent amounts. During the survey of the reaction conditions in the presence of
25 mg of the styrene oxide, the best results were obtained by using 25 mg of the catalysts (~25
mol%, indicated by ICP analysis) and at 60 ˚C in 3 ml of MeOH. Through the optimized reaction
conditions 45 and 41% of the corresponding products were respectively formed after 40 h (entry
6). It is noteworthy to mention that 15% of methanolysis reaction took place in the presence of
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40 mol% of urea powder as catalyst, in a homogeneous system (entry 3). It should be noted
that using the original ureaꢀcontaining ligand could not be appropriate in the control reaction. In
this case, the reaction proceeds through the role of this two carboxylic acid groups over the urea
species. Considering the paddleꢀwheel nodes established by Xꢀray analysis, in these ureaꢀbased
heterogeneous catalytic systems the selfꢀquenching phenomena raised from aggregation of
urea molecules in homogeneous systems has been suppressed through accommodation into MOF
structures. Further investigation of heterogeneous character of the catalytic system as well as
stability of the structures was carried out using a hot filtration test in addition to ICP analysis.
After 40 h of the methanolysis reaction of the styrene oxide, the reaction mixtures were
centrifuged and the catalysts were filtered off. Then, the supernatant of methanolic liquids were
left stirred at 60 ̊C. Interestingly, within 40 h of further reaction time, no distinguishable changes
were recognized in the reaction conversion using GC analysis.
Moreover, in other sets of the same reactions, the catalysts were filtered off after 40 h, washed
thoroughly with MeOH and subsequently the filtrates were examined by ICP analysis. 0.13 and
0.19% of residual zinc was identified respectively which significantly confirmed more than 99%
of the zinc metal center do not leach into reaction mixture during the methanolysis conditions.
Not only all of these observations confirm reliable chemical stability of the prepared MOFs, but
also they reject the Lewis acid catalytic role of metal species for epoxide activation. Finally,
these observations may confirm the HBD character of the urea moieties through the MOF
structure.
The productivity of both catalysts was evaluated by determination of the reaction selectivity for
the conversion of styrene oxide to 2ꢀmethoxyꢀ2ꢀphenylethanol as the major product in our
catalytic system. As Table 1 shows, after 40 h reaction time, 95% selectivity was calculated for
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the mentioned major product (entry 6). When the reactions prolonged to 140 h, with TMU-18,
quantitative conversion of styrene oxide was obtained with 98% selectivity for the 2ꢀmethoxyꢀ2ꢀ
phenylethanol that was characterized by GC and NMR (ESI). During the same time 95%
conversion with 98% selectivity was achieved in the presence of TMU-19 catalyst (entry 7).
The observed catalytic activity as well regioselectivity achieved in methanolysis of styrene oxide
encouraged us to examine other epoxides in the reaction. The reaction of three epoxides
including γꢀphenoxypropylene oxide, allyl(2,3ꢀepoxypropyl)oxide and cyclohexene oxide were
screened with both catalysts using the same optimum reaction conditions (Table 1, entries 10ꢀ
18). However, the reactivity changed in methanolysis of these less reactive substrates especially
cyclohexene oxide and the corresponding products which were formed with relatively moderate
yield and regioselectivity, even in the presence of twice the amount of the catalysts (entries 12,
15 and 18). Actually, increasing the amount of catalyst, in the case of styrene oxide, did not lead
to doubling of catalytic activity (entry 9) and a little improvement was observed (110 h vs. 140 h
for completion the reaction). Although more detailed studies need to identify the real cause of
this issue, this may be explained by engaging or hiding of some urea functional groups within the
complexities of the bulk matrix of the framework. We also checked the reaction with a bulky
substrate, tꢀbutyl styrene oxide, to clarify the catalysis occurred within the pores of the
frameworks. As tabulated in Table 1 (entry 19), the methanolysis of tꢀbutyl styrene oxide
proceeded negligibly even after 140 h.
In order to further investigate the comparison of the catalytic performance of these
heterogeneous urea catalytic systems, the timeꢀconversion for both catalysts were plotted
and compared with the control methanolysis reaction of styrene oxide, Figure 3. Although both
catalyst systems are carried out with significant diversity relative to the control reaction and
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moderately implement the reaction within 140 h, the TMU-18 catalyst shows somewhat higher
activity than TMU-19 catalyst. The comparison of the PXRD diffraction peaks of the catalysts
upon activation, represented in Figure S1(ESI), indicates little change occurred in the TMU-19
structure. Accompanied with the lower thermal stability of TMU-19 indicated by TG analysis,
these results demonstrate higher catalytic performance of the TMU-18 compared to TMU-19
thereafter it was also confirmed through the recycling experiment.
Moreover, to evaluate durability and the catalyst recycling ability, TMU-18 and TMU-19
catalysts were filtered off after 40 h reaction, washed with excess MeOH and dried at 80 ̊C and
under vacuum at room temperature, respectively. Methanolysis reaction of styrene oxide with
these recovered catalysts proceeded with more diminished catalytic reactivity in the case of
TMU-19 structure (Table 1, entry 8). In addition, the comparison of the PXRD patterns of these
catalysts clearly show that a decrease in crystallinity occurred in both recovered catalyst
structures (ESI). As shown in Figure S1 (ESI), the prominent changes of the TMU-19 structure
takes place after its activation, while activated TMU-18 structure has more similar PXRD to the
simulated pattern. Considering the instability observed after extraction of entire DMF molecules
in TMU-19, these data confirm the foundation role of the solvent molecules in preserving the
whole skeleton. Although the solvent exchange with the epoxide molecules propels the
methanolysis reaction, it seems that remaining DMF within the pores slightly suppresses the
orgaocatalytic activity of this structure.
The foundation role of the solvent within the pores was additionally proved for both MOFs by
immersing them in deionized water. The catalysts were removed after 48 h, washed with CHCl3
and subsequently the aliquots were monitored using GC. Interestingly no segregated residues of
the organic pillars were detected for both samples. Furthermore, preserving of the PXRD
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patterns of these two water treatment samples along with their PXRD patterns after 140 h of
methanolysis reaction of styrene oxide (Figure S2) explicitly reveals the aforementioned effect.
In addition, this data may show the genuine heterogeneous character and actual chemical
stability in the reaction.
Conclusion
Two new pillared metalꢀorganic frameworks containing urea functional groups were synthesized
aiming at their application as heterogeneous organocataysts. Structural analysis revealed that
both TMU-18 and TMU-19 could be described as doublyꢀinterpenetrated pcu network. These
frameworks were characterized by different techniques and were further utilized as
organocatalyst in the methanolysis of epoxides. Compared with the previous report,6 in which
the number of electron withdrawing carboxyl groups per urea units is doubled, the results of the
methanolysis reaction indicate that TMU-18 and TMU-19 have weaker hydrogen bond donating
ability, while they are more chemically stable MOF structures. In addition, the elucidation of
additional structural features of these organocatalysts to design more active ureaꢀbased MOFs
structures is an ongoing project in our laboratory and needs further work and refinement.
ASSOCIATED CONTENT
Supporting Information.
Experimental, details, PXRD patterns, TGA, IR spectroscopy, SEM images “This material is
available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
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*Phone: +98ꢀ21ꢀ82884416. Fax:+98ꢀ21ꢀ82884416. Eꢀmail: Morsali_a@modares.ac.ir (A.
Morsali)
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was funded by Iran National Science Foundation (INSF) (grant number: 92024961),
Iran Nano technology Initiative Council (INIC) and Tarbiat Modares University.
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Figure 1. Synthesis of TMU-18 and TMU-19 from urea containing dicarboxylate ligand and
bipy/bpe as pillar ligand.
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Figure 2. Paddleꢀwheel dinuclear zinc carboxylate clusters (a) and (d), Representation of the
cubic structure (b) and (d) and a view along the pore direction (c) and (f). The two
interpenetrating frameworks are shown in red and blue. All guest molecules were omitted for
clarity. The left top insert illustrates the simplified interpenetration in TMU-18 and TMU-19,
respectively.
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Figure 3. Comparison of time conversion plot for methanolysis of styrene oxide catalyzed by
TMU-18, TMU-19 and the reaction control system. Conditions: styrene oxide (25 mg, 0.2
mmol), catalyst (25 mg, 0.05 mmol of urea moiety), 60
(without catalyst)
̊C, methanol (3 mL); reaction control
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Table 1. Methanolysis of epoxides by urea-containing MOFs
O
OCH3
OH
urea-MOF
R
R
MeOH, 60 °C
Conversion Conversion
Entry
Substrate
Major Product
Time [h]
[%]a
TMU-18
14
19
15
[%]a
TMU-19
1
2
3
4
5
6
7
8
24b
110b
24c
24
40
110
OCH3
OH
35
31
O
45 (95)f
78 (96)
100 (98)
39 (95)
100 (98)
22
41 (95)
72 (97)
95 (98)
29 (94)
100 (98)
140
40d
110e
110b
55
9
OCH3
10
11
12
13
14
15
16
17
18
O
O
O
OH
40
78 (96)
26
37
64 (81)
16
33
67 (91)
110e
110b
55
OCH3
OH
O
O
35
53 (77)
O
110e
110b
55
OCH3
OH
O
34
51 (78)
19
48 (72)
110e
40
140
<5
9
<5
6
19
a
GC yield using internal-standard method; Conditions: styrene oxide (25 mg, 0.2 mmol),
catalyst (25 mg, 0.05 mmol of urea moiety, ~ 25 mol%), 60
C, methanol (3 mL). b reaction
̊
c
d
control (without catalyst). with 40 mol% urea in homogeneous system. with recycled
catalysts. with 50 mg (0.1 mmol) of the catalysts. The data in parenthesis are the
selectivity calculated for the major product.
e
f
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Table of contents
Urea Containing Metal–organic Frameworks as
Heterogeneous Organocatalysts
Two novel pillared MOFs including urea-functional group are introduced and their talent as
organocatalysts were evaluated in methanolysis of epoxides.
1