Urea postmodified in a metal-organic framework as a catalytically active hydrogen-bond-donating heterogeneous catalyst

Article abstract of DOI:10.1039/c3cc42531b

New functionally diverse urea-derived MOF hydrogen-bond-donating heterogeneous catalysts were achieved via postsynthetic modification, which exhibit excellent catalytic activity and very broad substrate scopes for the Friedel-Crafts alkylation reactions.

Full text of DOI:10.1039/c3cc42531b

Chemical Communications
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Volume 49 | Number 70 | 11 September 2013 | Pages 7671–7754
ISSN 1359-7345
COMMUNICATION
Yong-Zhou Hu, Xin-Yuan Liu et al.
Urea postmodified in a metal–organic framework as a catalytically active
hydrogen-bond-donating heterogeneous catalyst
1359-7345(2013)49:70;1-W

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Urea postmodified in a metal–organic framework
as a catalytically active hydrogen-bond-donating
heterogeneous catalyst†
Xiao-Wu Dong,a Tao Liu,a Yong-Zhou Hu,*a Xin-Yuan Liu*b and Chi-Ming Chec
Cite this: Chem. Commun., 2013,
49, 7681
Received 8th April 2013,
Accepted 10th May 2013
DOI: 10.1039/c3cc42531b
www.rsc.org/chemcomm
New functionally diverse urea-derived MOF hydrogen-bond-donating into MOFs.5 In this field, Cohen et al. reported seminal work on the use
heterogeneous catalysts were achieved via postsynthetic modification, of MOFs bearing amino groups anchored with isocyanates to generate
which exhibit excellent catalytic activity and very broad substrate urea units within the structure.6 Stock et al. elegantly demonstrated the
scopes for the Friedel–Crafts alkylation reactions.
only example of MIL-101 with ethyl isocyanate to produce the urea-
derived MIL-101 in good yields.7 Despite their great advantages, these
Metal–organic frameworks (MOFs) are some of the most newly emer- reactions typically involved the introduction of only small groups into
ging, widely studied porous materials, exhibiting potential applications MOFs, whereas processes that facilitate the construction of urea-based
in a wide range of areas including gas storage,1a separation,1b sensing1c MOFs with functionally diverse and reasonably large substituents in
and biomedicine.1d In particular, since the pioneering studies carried excellent yields remain largely unexplored. Moreover, no systematic
out by Kim et al.,2a Cohen et al.,2b Hupp et al.,2c Lin et al.2d and others, study on the applications of such urea-based MOFs as the HBD
MOFs have increasingly gained popularity as some of the most heterogeneous catalysts has appeared in the literature to date. The
prominent and powerful heterogeneous catalysts.3 Despite the great attractive features of MIL-101, including thermal and chemical stability
success of the MOF heterogeneous catalysts, wide applications of these and broad apparent surface areas with large pores and windows,8
catalysts are impeded by drawbacks such as unsatisfactory yields, poor render it a useful platform for the development of highly efficient
chemo- and/or stereoselectivities, and limited substrate scopes.3 In this heterogeneous catalysts.9 We wondered about the possibility of the use
context, Farha, Hupp, and Scheidt et al. have recently reported seminal of postsynthetic modification of MIL-101 with isocyanates providing
work4 on the synthesis of a novel urea-containing MOF as a hydrogen- highly porous and functionally diverse urea-derived MOFs, which
bond-donating (HBD) heterogeneous catalyst, which has shown an might act as the key functional group capable of two-point hydrogen
increased catalytic activity in comparison with similar HBD homo- bonding through acidic N–H bonds as HBD catalysts.10 Furthermore,
geneous catalysts. However, the incorporation of relatively large sub- such highly robust MOFs would not only possess spatially distinct
strates for the catalytic Friedel–Crafts reaction in such a heterogeneous hydrogen-bonding sites to prevent the oligomerization of the active
manner remains a significant challenge. Thus, from the perspective of catalyst,11 but also possess accessible pores/windows large enough to
developing a useful heterogeneous catalyst with high efficiency, the admit reasonably large substrates and products to promise high
design of a novel and efficient MOF-derived HBD catalyst with catalytic activity. With these concerns, we herein report the preparation
large enough accessible pores/channels to tolerate differently of the functionally diverse urea-containing MIL-101s in nearly quanti-
sized substrates would still be particularly significant.
tative conversions via postsynthetic modification. More significantly,
In recent years, postsynthetic modification (PSM) reactions have they exhibit better catalytic activity and much broader substrate scopes
been proven to be a powerful tool for introducing functional groups bearing reasonable sizes and diverse functional groups than similar
homogeneous12 and known heterogeneous MOF HBD catalysts4 for the
a ZJU-ENS Joint Laboratory of Medicinal Chemistry, College of Pharmaceutical
Friedel–Crafts alkylation reactions, making them promising for
advanced catalytic applications.
Sciences, Zhejiang University, Hangzhou, 310058, P. R. China.
E-mail: huyz@zju.edu.cn; Fax: +86 571-8820-8460
To determine the generality of this reaction for the synthesis of
functionally diverse urea-containing MIL-101 heterogeneous catalysts,
following a modified protocol described by Stock et al.,7 three different
isocyanates were treated with Cr-MIL-101-NH2 in CH3CN at 120 1C in
a sealed tube for 12 h. Surprisingly, in all cases examined, isocyanates
bearing various substituents, such as aliphatic and aryl groups with
bulky, branched substituents, were successfully anchored onto the
MIL-101 walls via covalent incorporation to afford Cr-MIL-101-UR1–3
b Department of Chemistry, South University of Science and Technology of China,
Shenzhen, 518055, P. R. China. E-mail: liuxy3@sustc.edu.cn;
Fax: +86 755-8624-5672
c Department of Chemistry and State Key Laboratory of Synthetic Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China.
E-mail: cmche@hku.hk
† Electronic supplementary information (ESI) available: Experimental proce-
dures, product characterizations of new compounds and Fig. S1–S7. See DOI:
10.1039/c3cc42531b
c
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Table 1 Optimization of the urea-catalysed Friedel–Crafts reactiona
Entry
Catalyst
Solvent
Time (h)
Yieldb (%)
1
2
3
4
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR3
Cr-MIL-101-UR1
Cr-MIL-101-UR2
DMF
THF
36
36
36
60
24
24
24
24
24
24
48
24
Trace
26
27
96
95
90
73
72
87
Acetone
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
5c
6d
7e
8c
9c
10
11
12c
f
—
Trace
22
79
Scheme 1 Schematic representation of (a) postmodification of Cr-MIL-101-NH2
with isocyanates to generate Cr-MIL-101-UR1–3; (b) the ligand structure and
[Cr3O(CO2)6] cluster nodes.
Cr-MIL-101
4
a
Reactions were conducted with 1A (0.1 mmol), 2a (0.2 mmol), and
b
catalyst (15 mol%) in the solvent (0.15 mL). Determined using
c
d
1H NMR. A 1 : 5 ratio (1A : 2a) was used. The loading of the catalyst
in nearly quantitative conversion (Scheme 1). They were quantified
using 1H-NMR spectroscopy upon digestion of the modified MOFs in
dilute base (Fig. 1a), together with various techniques including IR
(Fig. S1, ESI†) and ESI-MS spectroscopy (Fig. S2, ESI†). This result was
attributed to the larger accessible pores/windows of MIL-101 as
compared to other MOFs,6,13 thus rendering the introduction of
functional groups more favorable. The catalyst loading of Cr-MIL-
101-UR1–3 is 2.61, 2.48 and 1.85 mmol gÀ1, which was determined
from elemental analysis data, respectively (ESI†). Powder X-ray diffrac-
tion (PXRD) data and scanning electron microscopy (SEM) images,
similar to those of Cr-MIL-101, confirmed the retention of the parent
framework structure with an octahedron-like morphology and the
crystallinity of the resultant urea-derived MOFs (Fig. S3 and S4, ESI†).
They also displayed chemical stability with good tolerance to different
polar solvents, and thermal stability comparable to other MIL-101
derivatives, as determined by thermogravimetric analysis (TGA,
Fig. S5, ESI†). Examination of the gas sorption behavior of these
MOFs (Fig. 1b) indicated that all of the functionalized MOFs
were still highly porous even after postmodification (BET surface
area, 1409.9 m2 gÀ1 for Cr-MIL-101-UR1, 1178.6 m2 gÀ1 for Cr-MIL-
101-UR2, 829.3 m2 gÀ1 for Cr-MIL-101-UR3).
e
f
was 10 mol%. The loading of the catalyst was 5 mol%. No catalyst.
No product was detected in DMF with 15 mol% of Cr-MIL-101-UR3 at
60 1C for 36 h (Table 1, entry 1). Surprisingly, the screening of solvents
revealed CH3CN to be optimal to give the desired product 3Aa in 96%
yield (entries 1–4), suggesting that the solvent had a significant effect
on the conversion of this reaction. Next, the molar ratio of 1A/2a was
screened, and the reaction time was remarkably reduced from 60 to
24 h by increasing the molar ratio from 1 : 2 to 1 : 5 (entries 4 and 5).
We next studied this reaction under the optimal reaction conditions
in the presence of Cr-MIL-101-UR3 with different loadings, which
could be reduced from 15 to 10 mol% without remarkably affecting
the yield of 3Aa (entries 5–7). All of the other modified urea-containing
MOF heterogeneous catalysts were then examined, and Cr-MIL-101-
UR3 bearing proper steric bulkiness and electron-withdrawing
substituents was noted to be beneficial for the reaction (entries 5, 8
and 9). A control experiment with 15 mol% of unmodified MIL-101 as
the catalyst gave only 22% yield under the optimized reaction
conditions (presumably catalysed by the Lewis acidic chromium
center) (entry 11), demonstrating that the catalytic activity arises
mainly due to the presence of urea groups. Notably, the removal
of Cr-MIL-101-UR3 by filtration after 12 h resulted in no further
conversion, unambiguously revealing that the catalyst in this
reaction is heterogeneous in nature. Most importantly, the same
reaction performed with 1-(3,5-bis(trifluoromethyl)phenyl)-3-phenyl-
urea 4, which is prone to oligomerization as observed in the solid
state,11 as a catalyst under homogeneous conditions, resulted in a
relatively lower yield (79%) in comparison with Cr-MIL-101-UR3
(entry 12). The better reactivity in the case of the heterogeneous
catalyst may partially originate from the catalyst spatial isolation
achievable in such a confined porous MOF environment, which is
also in agreement with the experimental findings with other results.4
With the optimized conditions in hand, we set out to explore
the scope of this protocol with respect to other electron-rich
nucleophiles. Changing the nucleophiles from N-methylpyrrole
to reasonably larger substrates did not have a significant influence
on the reactivity. For example, treating N,N-dimethylaniline (2b),
The catalytic activity of these urea-containing MIL-101 was first
assessed by using Cr-MIL-101-UR3 as the catalyst in the Friedel–Crafts
alkylation between trans-b-nitrostyrene and N-methylpyrrole (Table 1).
Fig. 1 (a) 1H-NMR data of the linker molecules of the digested compounds (from
top to bottom): Cr-MIL-101 (red), Cr-MIL-101-NH2 (blue), Cr-MIL-101-UR1 (orange)
and Cr-MIL-101-UR2 (purple), Cr-MIL-101-UR3 (green). (b) N2 sorption properties for
MIL-101 (purple), MIL-101-UR1 (red), MIL-101-UR2 (green), MIL-101-UR3 (black).
c
7682 Chem. Commun., 2013, 49, 7681--7683
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Table 2 Cr-MIL-101-UR3-catalysed Friedel–Crafts alkylationa
selectivity for such a heterogeneous catalyst. Finally, upon testing
the recyclability of the urea-containing MOF heterogeneous
catalyst, we found that the solids of Cr-MIL-101-UR3 can be
easily isolated from the reaction suspension by centrifugation
and can be reused at least four times with little or no loss of
activity (Fig. S6, ESI†) while retaining its crystallinity as verified
by PXRD (Fig. S7, ESI†).
In summary, we have developed a simple and efficient method
for the synthesis of new functionally diverse urea-derived MOF
heterogeneous catalysts via postsynthetic modification. The result-
ing robust MOFs with large pores as HBD catalysts have shown
remarkable catalytic activities in the Friedel–Crafts reactions,
including much higher reaction activity versus the homogeneous
catalyst, and much broader substrate scopes versus the known
heterogeneous catalyst. Further studies toward the synthesis of
chiral MOF HBD catalysts and their application to the asymmetric
reactions are underway.
Entry
1
1
2
3
Yieldb (%)
95
R1 = Ph(1A)
2
R1 = Ph(1A)
93
3
4
5
6
7
8
9
10
11
R1 = Ph(1A)
3Ad
3Bd
3Cd
3Dd
3Ed
3Fd
3Gd
3Hd
3Id
94
90
81
90
88
91
92
93
65
R1 = 4-MeO-Ph (1B)
R1 = 2-MeO-Ph (1C)
R1 = 4-CF3-Ph (1D)
R1 = 4-Cl-Ph (1E)
2d
2d
2d
2d
R1 = 2-naphthalene (1F) 2d
Financial support from NSFC (No. 81001359), and the China
Postdoctoral Science Foundation (No. 201003734), as well as
from South University of Science and Technology of China
(Talent Development Starting Fund) is greatly appreciated.
R1 = 2-furyl (1G)
2d
2d
2d
R1 = 2-thiophene (1H)
R1 = propane (1I)
12
2d
2d
40 (80c)
17 (76c)
Notes and references
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3 For selected reviews on the MOF heterogeneous catalysts, see:
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13
a
Reactions were carried out with 1 (0.1 mmol), 2 (0.12 mmol), Cr-MIL-101-
UR3 (15 mol%), CH3CN (0.15 mL). b The isolated yield is based on 1. c The
isolated yield in parentheses refers to the reaction catalyzed by 4.
indole (2c), and 1-methylindole (2d) with 1A even with a molar
ratio of 1 : 1.2 gave the expected products 3Ab–Ad in 93–95%
yields, respectively, which is in sharp contrast to the result
obtained for other MOF catalysts4 (Table 2, entries 1–3). Next a
variety of nitrostyrenes were further examined. Both electron-
donating and -withdrawing groups on the phenyl ring were well
tolerated affording the expected products (3Bd–Ed) in good yields
(entries 4–7). (E)-1-(2-Nitrovinyl)naphthalene (1F) containing an
extended conjugated system gave the corresponding product in
91% yield as well (entry 8). Heteroaromatic nitroalkenes were
amenable to this protocol affording the expected products in
excellent yields (entries 9 and 10). Aliphatic nitroalkene was also
compatible with the reaction conditions, albeit displaying rela-
tively sluggish reactivity (entry 11). Next, we turned our attention
toward the longer-chained or larger nitroalkenes as substrates for
this reaction. For 1J and 1K, the expected products 3Jd and 3Kd
were obtained in only 40 and 17% yields, respectively (entries 12
and 13). However, the corresponding products were obtained in
good yields (3Jd : 80%; 3Kd: 76%) with 4 as the catalyst (Table 2,
entries 12 and 13). The different sequences of catalytic activities
corresponding to Cr-MIL-101-UR3 in a heterogeneous manner
and free urea 4 in a homogeneous manner demonstrated that
catalysis in the case of the heterogeneous catalyst mainly took
place within the pores of the MOF, thus exhibiting reagent size 13 C. Volkringer and S. M. Cohen, Angew. Chem., Int. Ed., 2010, 49, 4644.
c
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Chem. Commun., 2013, 49, 7681--7683 7683