A new superacid hafnium-based metal-organic framework as a highly active heterogeneous catalyst for the synthesis of benzoxazoles under solvent-free conditions

Article abstract of DOI:10.1039/c7cy01668a

A new crystalline porous superacid material was prepared by sulfation of a Hf-MOF constructed from a 6-connected Hf node and 1,3,5-benzenetricarboxylate. The new superacid MOF with the presence of sulfate groups coordinated to a Hf-oxo cluster as superacid sites was found to be an effective heterogeneous catalyst for solvent-free heterocyclization for benzoxazole synthesis.

Full text of DOI:10.1039/c7cy01668a

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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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DOI: 10.1039/C7CY01668A
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A new superacid hafnium-based metal-organic framework as a
highly active heterogeneous catalyst for the synthesis of
benzoxazoles under solvent-free condition
Received 00th January 20xx,
Accepted 00th January 20xx
Linh H. T. Nguyen, The T. Nguyen, Ha L. Nguyen, Tan L. H. Doan,a,b,* Phuong Hoang Tranb,*
a
b
a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new crystalline porous superacid material was prepared by Recently, Maleki et al. reported a green and efficient Ag@TiO
sulfation of a Hf-MOF constructed by 6-connected Hf node and nanocomposite for the efficient synthesis of benzoxazole derivatives
2
1
,3,5-benzenetricarboxylate. The new superacid MOF with the via the one-pot condensation of 2-aminophenol and several aromatic
8
presence of sulfate groups coordinated to Hf-oxo cluster as aldehydes.
superacid sites was found to be an effective heterogeneous catalyst
for solvent-free heterocyclization of benzoxazole synthesis.
In recent years, emerging as a new class of highly crystalline
porous materials, metal-organic frameworks (MOFs) have received
widespread attention for various potential applications, including gas
2-Aryl substituted benzoxazoles have attracted widespread attention
9
10
11
storage and separation, chemical sensing, environmental
due to their pharmaceutical properties such as anticancer, anti-
inflammatory, antifungal, and antiviral.1 The direct C-H bond
arylation of benzoxazoles using Pd-, Co-, or Cu-catalyzed has some
disadvantages such as expensive catalysts, volatile organic solvents,
strong bases, harsh reaction conditions, prolonged reaction times.2
Therefore, development of convenient and mild methods is still a
treatment,12 and heterogeneous catalysis.13 MOFs consist of
inorganic metal-oxo clusters as nodes connected to multi-topic
organic linkers, both constituents termed secondary building units or
SBUs, via metal coordination bonds to form extended frameworks
14
with permanent porosity. However, the stability of MOFs in
working conditions is an essential factor influencing the applicability
of these materials. Among them, Zr(IV) and Hf(IV)-based MOFs have
been well-known for their exceptional chemical and thermal
stability, the most notable being possibility to structural remaining in
water and acidic aqueous solutions. This property mainly results
3
desirable approach. Among them, the condensation of
aminobenzophenols with aldehydes has been known as a valuable
4
method for the synthesis of benzoxazoles. While good yields with a
variety of substrates and reagents can be obtained using
homogeneous and heterogeneous catalysts, these protocols often
suffer from various drawbacks such as the difficulty of recovery of
catalyst, reuse with loss of catalytic activity, the use of volatile
4+
from the high oxophilicity and the high positive charge density of Zr
4+
and Hf atoms leading to the strong coordination bond between
Zr(IV) or Hf(IV) and carboxylate O atoms of organic linkers in
carboxylate-based MOFs. With the highly robust structure, the
3a,5
organic solvent.
Consequently, these methods have generally
15
restricted the application of the industrial processes. Accordingly, a
greener and more efficient catalyst for the synthesis of benzoxazole
is needed. In 2014, Yang and co-workers reported a recyclable
possible uses of these MOFs have been developed and expanded in
many fields of industrial application. In particular, Zr- and Hf-MOFs
16
have emerged as potential materials for catalysis because the
materials can fulfill the criteria of the heterogeneous catalysts
including high surface areas, porosity, active-site uniformity, and
mesoporous
poly(melamine–formaldehyde)
as
a
green,
heterogeneous organocatalyst for the synthesis of benzoxazoles in
the presence of toluene as the solvent.6 Pinahi and co-workers
developed a greener ruthenium-catalyzed synthesis of benzoxazoles
using acceptorless dehydrogenative coupling reaction of primary
alcohols with 2‑aminophenol under heterogeneous conditions.7
17
highly crystalline and stable structure for ease of recycling. Also,
the catalytic activity of the MOFs can be increased by the
architectural designs of post-synthetic modification (PSM) on the
18
Zr(IV) or Hf(IV)-oxo clusters and organic linkers.
In 2014, the first reported superacid MOF based on a Zr-MOF,
6 4 4 2 6
MOF-808, comprised of Zr O (OH) (CO ) clusters and 1,3,5-
benzenetricarboxylate linkers (BTC), was prepared by soaking the
MOF in aqueous sulfuric acid to replace nonstructural formate by
a.Center for Innovative Materials and Architectures, Vietnam National University –
Hochiminh City, 721337, Vietnam. Email: dlhtan@inomar.edu.vn
Faculty of Chemistry, University of Science, Vietnam National University
Hochiminh City, 721337, Vietnam. Email: thphuong@hcmus.edu.vn
b.
–
†Electronic Supplementary Information (ESI) available: Synthesis and
characterization of MOFs, additional catalytic data, and spectra. See
DOI: 10.1039/x0xx00000x
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sulfate on the clusters.19 The sulfated MOF showed the catalytic
activity of both Bronsted and Lewis acid sites with high conversion
on Friedel–Crafts acylation, esterification, and isomerization. In this
study, we demonstrate a design strategy for further tailoring the
higher catalytic superacid MOF based on the hypothesis that Hf-
MOFs are considered as a stronger acid catalyst than Zr structural
analogues due to the higher oxophilic character of the Hf-oxo
View Article Online
DOI: 10.1039/C7CY01668A
20
clusters. Accordingly, we report the synthesis and structural
characterization of a novel superacid Hf-based MOF, termed VNU-
4
11-SO (VNU = Vietnam National University), that was prepared from
the hafnium(IV) analogue of MOF-808, named VNU-11. Application
of the superacid MOF as a heterogeneous catalyst was studied on
reaction for benzoxazole synthesis from heterocyclization of
substituted 2-aminophenols and aldehydes. The reaction studies
4
demonstrated that VNU-11-SO was more efficient heterogeneous
acid catalyst for the reaction under solvent-free condition than the
superacid Zr-MOF, common Lewis acids or Brønsted acids, and other
catalytically-active MOFs. Moreover, the new material showed no
significant loss of catalytic activity after recovered and reused over
five runs.
The precursor VNU-11 and the new superacid MOF were
synthesized via solvothermal reaction and sulfation process with
Fig. 1. The combination of octahedral metal building unit and triganular
slight modifications to previous reports, respectively. In particular, a organic linker (a) to form 3-dimensional structure of VNU-11 (b) that
contained two kinds of pore. The augmented of spn network (c) is also
presented for both VNU-11 and its isoreticular struture, VNU-11-SO (d).
2 2 3
mixture of HfOCl ·8H O and 1,3,5-benzenetricarboxylic acid (H BTC)
4
in N,N-dimethylformamide (DMF) with formic acid added as a
Color code: C, black; O, red; S, yellow; Hf, blue. The yellow and purple balls
represent for the cages of the structures.
o
modulator was heated at 110 C in an isothermal oven. By varying
the molar ratios of the metal salt to the linker and the length of
reaction time, the product could obtain in the form of single crystals 2 2] (2 kinds of vertex, 1 kind of edge, 2 kinds of face, and 2 kinds of
of VNU-11-SC (where SC = single crystal) or microcrystalline powder tiling) (Fig. 1b). VNU-11-SC-SO was produced by immersing VNU-11-
4
2
–
(
VNU-11-P; where P = powder). The as-synthesized VNU-11-P and SC into 0.1 M sulfuric acid solution. The formation of SO
4
was
proven by micro-elemental analysis (EA), which revealed the
chemical formula of VNU-11-SC-SO to be
Hf (SO O) , calcd: C, 10.95; H,
.25; S, 3.72% and found: C, 11.20; H, 1.18; S, 4.04% (SI, Section S3).
VNU-11-SC were rinsed in DMF and solvent-exchanged, but only the
prior was evacuated at room and elevated temperature under
vacuum due to maintaining crystallinity of the crystals for single
crystal X-ray diffraction analysis. The activated VNU-11-P or the
solvent-exchanged VNU-11-SC was soaked in 0.1 M sulfuric acid
solution to substitute sulfate groups for nonstructural formate
ligands remaining from the MOF synthesis. The sulfated samples
were washed with acetone and chloroform and then dried at
4
[
1
6
O
5.4(OH)2.52(BTC)
2
4
)
2.3(HCOO)0.08](H
2
8
Sulfate fragments replaced formates and were observed by SXRD
measurement. It should be noted that two O atoms in one of the
sulfate groups could not be located from the residual electron
density maps due to high thermal vibration or disorder. These
4
anions turned into the pores of VNU-11-SO leading to slightly reduce
19
ambient and high temperature under reduced pressure to obtain the the pore volume and pores space of VNU-11-SO compared to VNU-
4
activated superacid MOF.
4
11. Especially, the adamantane cage of VNU-11-SO was found to be
A single crystal of VNU-11 that displayed the octahedral shape 17.2 Å.
o
was solvothermally obtained through the reaction at 110 C for 7
days of three connected (3-c) organic linker, H
After activated in a vacuum condition, VNU-11-P and sulfated
microcrystalline powder MOF, named VNU-11-P-SO , were further
characterized. The excellent coincidence of PXRD patterns of the
original MOF and VNU-11-P-SO indicated that the crystallinity of the
material was retained after the sulfation (Fig. S1 and S2). The
porosity of the guest free samples was then accessed by nitrogen
adsorption isotherms, which exhibited permanent porosity and type-
IV behaviors corresponding to the presence of mesopores within the
structures (Fig. S10). According to the isotherms, the calculated BET
3
BTC, and 6-c of
Hf (OH) (CO cluster. Single crystal X-ray diffraction (SXRD)
analysis revealed that the crystal lattice of VNU-11 is a face-centered
cubic system with the parameter of a = 34.9801(12) Å, and Fd-3m
space group (No 227) (Tables S1 and S2 in the Supporting
Information). Six (6) coplanar terminal formates chelated directly to
Hf-based trigonal prism cluster and separated the links of the point
O (OH) (CO ) cluster into two directions: three
6 4 4 2 6
connections were located on one side, and three others were on
opposite side. Four (4) Hf-based trigonal prism building units were surface areas of VNU-11-P and the sulfated MOF were 1530 and 1440
connected to each other via connections of BTC linking units to m g , respectively. IR spectrum of the sulfated VNU-11-P showed
generate the small tetrahedral cage (4.8 Å). These sharing the presence of sulfate groups in the structure with identical signals
4
6
O
4
4
2 6
)
4
of extension of Hf
2
-1
tetrahedral building units were combined to form the overall
connectivity of VNU-11 with a large adamantane cage of 18.4 Å.
VNU-11 is isoreticular structure of MOF-808,16a which also belongs to
spn network comprised by 3,6-c binodal with the transitivity of [2 1
-1
in the range from 800 to 1500 cm (Fig. S11). Micro-elemental
1
analysis and integrated solution H-NMR spectrum of digested
sample performed to the determine chemical composition of VNU-
11-P-SO
4
revealed the disappearance of the formate groups and the
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Table 2. The solvent-free synthesis of benzoxazoles from 2-aminophenols and
replacement of 2.5 sulfate groups per Hf
Superacidity in the sulfated VNU-11 was determined by Hammett
indicator test with a value of H ≤ -14.5 that correspond with
6
cluster in the framework.
View Article Online
o
a
arylaldehydes catalyzed by VNU-11-P-SO at 140 C
4
DOI: 10.1039/C7CY01668A
o
superacid MOF-808-2.5SO
as well as sulfuric acid (H
demonstrated octahedral microcrystal morphology of VNU-11-P-SO
sample (Fig. S13 and S.15).
The catalytic activity of the prepared MOF catalyst was
investigated in the synthesis of benzoxazoles starting from 2-
aminophenols and aromatic aldehydes. The reaction of 2-
aminophenol and benzaldehyde was used for initial studies and after
optimization of temperature, time, and catalyst concentration (Table
4
and higher acidity than original VNU-11
= -12) (Table S3). SEM and TEM images
o
Entry
2-Aminophenol
Benzaldehyde
Time (h)
Isolated yieldb
%)
4
(
1
2
3
4
5
6
7
8
9
6
6.5
6
95 (94)c
91
90
o
S4-S6), the best result was obtained at 140 C for 6 h using 0.01 mmol
of VNU-11-P-SO . After screening various solvents and solvent-free
4
6
88
conditions, we found that the synthesis can be preceded even in the
absence of solvent (Table S7, Entry 8). As shown in Table 1, the
synthesis of benzoxazole did not proceed in the absence of the
catalyst (Table 1, Entry 1). The reactions took place in the presence
of Lewis acids or Brønsted acids provided a low yield of desired
products (Table 1, Entries 12-21). When various metal-organic
frameworks were used as catalysts, the yields of the product were
significantly improved (Table 1, Entries 3-11). Under described
5
87
6.5
5.5
5.5
6
90
92
o
condition at 140 C under solvent-free, the best catalytic
85
4
performance was obtained using VNU-11-P-SO (Table 1, Entry 2).
Table 1. Comparison of catalysts on the synthesis of 2-phenylbenzoxazolea
90
1
1
1
1
0
1
2
3
6.5
6
87
87
Entry
Type of
catalysts
Catalystb
Isolated
TOF d
s’
-1
yield (%)
(h )
5
91
1
2
3
4
5
6
7
8
9
Free-catalyst
-
5
-
VNU-11-P-SO
VNU-11-P
VNU-1
4
99
78
68
69
49
83
79
63
63
34
53
41
51
36
12
35
22
25
33
44
16.5
13.0
11.3
11.5
8.2
6
90
VNU-2
14
15
6
95
UiO-66
MOFs
MOF-808-2.5SO
MOF-808-P
MOF-177
HKUST-1
4
13.8
13.2
10.5
10.5
5.7
5
86
1
6
5
85
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
aReaction conditions:
benzaldehyde (1 mmol), and VNU-11-P-SO
C. Yield of the product was isolated by column chromatography
A
mixture of 2-aminophenol (1 mmol),
(0.01 mmol) was stirred at 140
ZIF-8
4
AlCl
FeCl
ZnCl
Cu(CH COO)
HfCl
SO
HCl
PO
COOH
TsOH
3
8.8
o
b
c
3
6.8
(acetone/hexane). 10 mmol scale reaction.
Metal salts
2
8.5
We next proceeded to evaluate the scope and limitation of VNU-
3
2
6.0
1
4
1-P-SO in the synthesis of benzoxazoles with a variety of aromatic
aldehydes and 2-aminophenol derivatives (Table 2). In general, all
the benzoxazoles were obtained in good to excellent yields using as
4
2.0
H
2
4
5.8
3.7
Brønsted
acids
4
low as 0.01 mmol of VNU-11-P-SO without solvent for 4-8 h.
H
3
4
4.2
Remarkably, aromatic aldehydes bearing electron-donating groups
such as methyl, tertbutyl, methoxy, and hydroxyl as well as electron-
withdrawing groups such as nitro, fluoro, bromo, and chloro afforded
the corresponding 2-arylbenzoxazoles in 85–95% yield for 4-6 h
under solvent-free conditions (Table 2 and Table S9). The effect of
the substituent in the reaction time and yields showed that as the
electron-withdrawing substituents in the benzaldehydes provided
CF
3
5.5
7.3
a
Reaction conditions:
A
mixture of 2-aminophenol (1 mmol),
o
benzaldehyde (1 mmol), and catalyst (0.01 mmol) was stirred at 140 C for
b
c
6
h. Chemical formulas of the MOFs were shown in Table S8. Yield of 2-
phenylbenzoxazole was isolated by column chromatography
acetone/hexane). TOF (turnover frequency): Moles of product formed
per mole of catalyst per hour.
d
(
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the high yield for short reaction times (Table S9, Entries 5-9). The 5. (a) R. Gupta, P. K. Sahu, P. K. Sahu, S. K. Srivastava and D. D.
View Article Online
DOI: 10.1039/C7CY01668A
analysis of the effect of the substituent in 2-aminophenol was also
investigated under current method. Table 2 indicated that the
catalyst proceeded well for the aminophenol derivatives, providing
the corresponding benzoxazoles in high yields (Table 2, Entries 5-16).
Agarwal, Catal. Commun., 2017, 92, 119. (b) M. A. Abdelgawad,
R. B. Bakr, H. A. Omar, Bioorg. Chem., 2017, 74, 82. (c) H. Naeimi,
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4745.
The synthesis of benzoxazole was also investigated on a 10 mmol 6. D. Yang, P. Liu, N. Zhang, W. Wei, M. Yue, J. You and H. Wang,
scale, and the yield is almost the same as on 1 mmol scale (94% vs. ChemCatChem, 2014, 6, 3434.
5%, entry 1 of Table 2). Thus, the current method can be potentially 7. A. Khalafi-Nezhad and F. Panahi, ACS Catal., 2014, 4, 1686.
applied to the industrial processes. The total disappearance of the 8. B. Maleki, M. Baghayeri, S. M. Vahdat, A. Mohammadzadeh and
starting materials was checked by thin layer chromatography (TLC) S. Akhoondi, RSC Adv., 2015, 5, 46545.
and gas chromatography-mass spectrometry (GC-MS). The by- 9. Y. He, W. Zhou, G. Qian and B. Chen, Chem. Soc. Rev., 2014, 43,
products were not found under current method. The products were 5657.
9
fully characterized by their melting points, NMR, and MS spectral 10. S. Qiu, M. Xue and G. Zhu, Chem. Soc. Rev., 2014, 43, 6116.
data, which were in total agreement with the values described in the 11. F.-Y. Yi, D. Chen, M.-K. Wu, L. Han and H.-L. Jiang, ChemPlusChem,
literature (Section S10).
The recyclability of the catalyst is an important feature for 12. E. Barea, C. Montoro and J. A. R. Navarro, Chem. Soc. Rev., 2014,
industrial applications. After the reaction, the VNU-11-P-SO catalyst 43, 5419.
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4
and ethanol, and reactivated under reduced pressure. The recovered
VNU-11-P-SO was reused in the reaction of aminophenol and
(b) H. L. Nguyen, T. T. Vu, D. Le, T. L. H. Doan, V. Q. Nguyen and
N. T. S. Phan, ACS Catal., 2017, 7, 338.
4
benzaldehyde for five times with only a slight loss of the reactivity 14. H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science,
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In conclusion, a new superacid MOF based on the 6-connected 15. (a) Y. Bai, Y. Dou, L.-H. Xie, W. Rutledge, J.-R. Li and H.-C. Zhou,
Hf-oxo cluster and the tritopic linker has been successfully
synthesized by sulfation method. The structural analysis for the MOF,
Chem. Soc. Rev., 2016, 45, 2327. (b) O. V. Gutov, W. Bury, D. A.
Gomez-Gualdron, V. Krungleviciute, D. Fairen-Jimenez, J. E.
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4
, by single crystal X-ray diffraction and material
characterization determined the presence of sulfate groups
coordinated to the Hf-oxo clusters of the framework, which played a 16. (a) H. Furukawa, F. Gándara, Y.-B. Zhang, J. Jiang, W. L. Queen,
role as superacid sites. The material exhibited exceptional
heterogeneous catalyst for free-solvent heterocyclization of 2-
arylbenzoxazoles synthesis with the high yield and ease of recycling.
The authors gratefully acknowledge the support of Vietnam
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Dr. Christopher A. Trickett, Dr. Juncong Jiang Dr. Hiroyasu Furukawa,
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valuable discussions and crystal structure analyses.
P. Li, J. T. Hupp and O. K. Farha, ACS Catal., 2017, 7, 997. (b) T. L.
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
Journal Name
COMMUNICATION
4
, was used as an efficient heterogeneous catalyst for free-solvent 2-aryVlbieewnAzrtoicxleaOzonllienes
A new superacid Hf-based MOF, termed VNU-11-P-SO
synthesis.
DOI: 10.1039/C7CY01668A
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