Co-Cluster-Based Metal–Organic Frameworks as Selective Catalysts for Benzene Tandem Acylation–Nazarov Cyclization to Benzocyclopentanone

Article abstract of DOI:10.1002/chem.201704749

One-step selective benzene acylation–Nazarov cyclization is an attractive, yet challenging method for the synthesis of the benzocyclopentanone skeleton, which is key intermediate of many natural products. Herein, two metal-cluster-based metal–organic frameworks (MCOFs) {(H₃O)₂[Co₄(pbcd)₂(μ₃-OH)₂]?CH₃CH₂OH?4 H₂O}_n (1; pbcd=9,9′-(propan-1,3-diyl)bis(9H-carbazole-3,6-dicarboxylic acid) and {[Co_5.25(mcd)₂(HCO₂)(μ₂-O)_0.5(μ₃-OH)_0.5(H₂O)₄]?6 H₂O?5 DMF}_n (2; mcd=9,9′-methylenebis(9H-carbazole-3,6-dicarboxylic acid) were developed to catalyze a tandem Nazarov cyclization reaction of 1,3-dimethoxybenzene with α,β-unsaturated carboxylic acids for the synthesis of cyclopentenone[b]benzenes. MCOFs 1 and 2, which were constructed from tetranuclear Co^II cluster [Co₄(μ₃-OH)₂] and hexanuclear Co^II cluster [Co₆(HCO₂)(μ₂-O)(μ₃-OH)], respectively, exhibit high catalytic activity arising not only from their suitable channel size and accessible catalytic sites, but also because of their high density of Lewis acidic sites within the frameworks and the synergic activity among Co^II ions. In contrast, {[Co₂(pbcd)(bpe)]?2 H₂O?2 DMF}_n (3; bpe=1,2-bis(pyridin-4-yl)ethane) containing binuclear Co^II and having large pore windows is a highly selective catalyst for obtaining exclusively the acylation products. Easy product separation, simple reaction procedures, and catalyst recycling make the catalyst system attractive. This work highlights the synergistic effect among ions of MCOFs in interacting with substrates in a sequential or cooperative manner to achieve tandem catalysis.

Full text of DOI:10.1002/chem.201704749

A Journal of
Accepted Article
Title: Co-Cluster-Based Metal-Organic Frameworks as Selective
Catalysts for Benzene Tandem Acylation-Nazarov Cyclization to
Benzocyclopentanone
Authors: Jie Wu, Mengjia Liu, Kuan Gao, Yanru Fan, Xiaoqing Guo,
Xiangru Meng, and Hongwei Hou
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704749
Link to VoR: http://dx.doi.org/10.1002/chem.201704749
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10.1002/chem.201704749
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Co-Cluster-Based Metal-Organic Frameworks as Selective
Catalysts for Benzene Tandem Acylation–Nazarov Cyclization to
Benzocyclopentanone
Mengjia Liu,[a] Kuan Gao,[a] Yanru Fan,[a] XiaoqingGuo,[a] Jie Wu,*[a] Xiangru Meng,*[a] Hongwei Hou*[a]
Abstract: One-step selective benzene acylation-Nazarov cyclization is an attractive yet challenging method for the synthesis of
benzocyclopentanone skeleton which is key intermediate of many natural products. Herein, we reported that two metal-cluster-based metal–
organic
frameworks
(MCOFs)
{(H3O)2[Co4(pbcd)2(μ3-OH)2]·(CH3CH2OH)·4H2O}n
(1)
and
{[Co5.25(mcd)2(HCO2)(μ2-O)0.5(μ3-
9,9'-methylenebis(9H-
OH)0.5(H2O)4]·6H2O·5DMF}n (2) (pbcd
=
9,9'-(propane-1,3-diyl)bis(9H-carbazole-3,6-dicarboxylic acid), mcd
=
carbazole-3,6-dicarboxylic acid)) were developed to catalyze tandem-Nazarov cyclization reaction of 1,3-dimethoxy benzene with α,β-
unsaturated carboxylic acids for the synthesis of cyclopentenone[b]benzene. MCOFs 1 and 2 were constructed from tetranuclear CoII cluster
[Co4(μ3-OH)2] and hexanuclear CoII cluster [Co6(HCO2)(μ2-O)(μ3-OH)], respectively, exhibiting high catalytic activity not only arising from their
suitable channel size and accessible catalytic sites, but also because of their high density of Lewis acidic sites within frameworks and the
synergic activity among CoII ions. In contrast, {[Co2(pbcd)(bpe)]·2H2O·2DMF}n (3) (bpe = 1,2-di(pyridin-4-yl)ethane) including binuclear CoII
with large pore windows was highly selective catalyst to obtain only acylation products. Easy product separation, simple reaction procedures,
and catalyst recycling make the catalytic system attractive. Our work highlights the synergistic effect among ions of MCOFs to interact with
substrates in a sequential or cooperative manner to achieve tandem catalytic products.
and network topologies, but also usually have some
unprecedented properties in contrast to MOFs constructed from
single metal as node, such as magnetism, luminescence and
porous absorption.[15] In particular, MCOFs can usually exhibit
Introduction
Metal−organic frameworks (MOFs) assembled from metal ions
better catalytic performance not only arising from their large
steric volume, better thermal stability and solvent resistance
properties than MOFs,[16] but also due to the fact that the
synergic activity among ions in MCOFs, mimicking the active
sites of many catalytic enzyme assemblies in nature, promoting
the reaction in a tandem or cooperative manner.
and organic linkers have been emerging as very promising
multifunctional materials,[1-3] which have led to many applications
including gas storage,[4] separation,[5] sensing,[6] drug delivery[7]
and heterogeneous catalysis.[8] In particular, due to their abilities
to create uniform active catalytic centers with identical
microenvironments around them throughout the solid and
unique features such as high surface areas, special confined
nanopore microenvironments, and extraordinarily broad
chemical and topological variety, MOFs give an ideal platform
for engineering solid catalysts to mimic the functions of natural
enzymes to facilitate many useful organic transformations.[9]
Although MOFs with periodically dispersed site-isolation or
adjacent active catalytic sites as single sites or cooperative
catalysts have been explored, precise assembly of multi-active-
sites into MOFs-based catalysts to work synergistically and
achieve tandem or cooperative catalysis without isolation of
intermediates is still important challenging. [10-12] To address this
problem, introducing metal clusters into MOFs has been
generated and developed into an ideal multinuclear platform to
investigate synergistic interactions with substrates during
catalytic process.[13,14] These metal−organic frameworks based
on metal cluster (MCOFs) with poly nuclear cluster not only
break through the geometrical and coordination limits of
classical mononuclear complexes to show intriguing architecture
On the other hand, our group has been devoted to explore
MOFs-based catalysts for tandem acylation-Nazarov cyclization
reactions.[17] The reaction is an acid-catalyzed acylation followed
by 4π-electrocyclic process to rebuild a new carbon-carbon
bond for the synthesis of cyclopentenone which is the key
intermediate in the synthesis of numerous biologically active
molecules or highly functional compounds.[18-20] Recently, on
account of the high catalytic activity, robustness, and excellent
selectivity of MOFs-based catalysts, we have reported the highly
efficient Zn-MOF and Ag-MOF for the synthesis of
cyclopenta[b]pyrrole by treatment of pyrrole derivatives with α,β-
unsaturated carboxylic acids.[21] In recently, we would like to
develop the MOFs-catalyzed tandem acylation-Nazarov
cyclization reaction of benzene and α,β-unsaturated carboxylic
acid for the formation of cyclopentenone[b]benzene, which is the
skeleton of a family of natural products taiwaniaquinones with
significantly antitumor activities and aromatase inhibitory
activities.[22-24] However, after a lot of attempts, we found that the
previous used homogeneous and heterogeneous Lewis acid
catalysts, such as [Ti(OiPr)4], AlCl3, [Sc(OTf)3], Zn-MOF, and Ag-
MOF could not promote the reaction because of the lesser
electron cloud density of benzene compared with that of
pyrrole.[25-26] Encouraged by the high density of Lewis acidic
sites and the synergic activity of MCOFs, we explored whether
the MCOFs could be used as heterogeneous catalysts to
promote the acylation-Nazarov cyclization reaction with benzene
derivatives.
[a]
M. Liu, K. Gao, Y. Fan, X. Guo, Prof. J. Wu, Prof. X. Meng, Prof.
H. Hou
The College of Chemistry and Molecular Engineering, Zhengzhou
University, Henan 450052 (P. R. China)
E-mail: houhongw@zzu.edu.cn
wujie@zzu.edu.cn
mxr@zzu.edu.cn
Herein, we designed three porous Co-based MOFs 1-3 under
solvothermal conditions and reported that the highly selective
Supporting information for this article is given via a link at the end
of the document.
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1,3-dimethoxy benzene tandem acylation-Nazarov cyclization
with α,β-unsaturated carboxylic acids could be achieved by
using two Co-cluster-based MOFs 1 and 2 as catalysts. In
contrast, 3 constituted by binuclear CoII units was a highly
selective catalyst to obtain only acylation product. Factors
influencing the performance were investigated, and a possible
catalytic mechanism of benzene acylation-Nazarov cyclization
catalyzed by Co-cluster-based MOFs was proposed. To the best
of our knowledge, this is the first demonstration that MCOFs
catalyze the tandem benzene acylation–Nazarov cyclization
reactions and highlight the great potential of developing MCOFs
as catalysts for the organic synthesis.
guest molecules. Co1, Co3 and symmetry-related Co1A, Co3A
associated together by two μ3-OH (O17 and O17A) and eight
carboxylate groups of eight pbcd4- ligands form a tetranuclear
[Co4(μ3-OH)2] cluster as second building unit (SBU) (Figure 1a).
Co2, Co4 and symmetry-related Co2A, Co4A are connected
together in the similar form to construct another kind of
tetranuclear [Co4(μ3-OH)2] cluster as SBU. The semi-flexible
H4pbcd ligands in 1 with two carbazole-3,6-dicarboxylic acids
linked by propyl are completely deprotonated and take two
different coordination modes to connect these {Co4}-based
SBUs, leading to the formation of a 3D zeolite structure with the
dimensions of the channels being 11.85 ×10.14 Å2 (considering
van der Waals radii) along the c axis. The flexible propyl group
of H4pbc facilitates the formation of the microporous structure
with dihedral angles between two carbazole rings approximately
77.12°. PLATON analysis gave the free void volume ratio as
26% in 1. N2 adsorption isotherm was recorded at 77 K, which
showed the saturated N2 uptake of 157 cm3·g−1 at P/P0 =1
(Figure S1e). The N2 adsorption isotherm is typical type-I
isotherms and show a steep rise at the low-pressure region,
which is a characteristic of microporous materials.[27] According
to the Brunauer−Emmett−Teller (BET) equation, the BET
surface area is 586 cm3g-1. The bivalent Co centers in 1 were
revealed by the X-ray photoelectron spectroscopy (XPS) (Figure
S1d). The relative intensities of the shake-up lines to the main
core level of both the Co 2P3/2 and 2P1/2 are the characteristic
that distinguish CoII from CoIII.
Results and Discussion
Crystal structure of 1
Single crystal X-ray diffraction analysis reveals that 1 is an
unusual 3D network based on the tetranuclear CoII cluster
Overall, the framework of 1 is anionic and is filled with [H3O]+
ions, which were further supported by the cation exchange
experiment listed in the supporting information. Given the
charge-balance consideration, there should be two [H3O]+ ions
per formula unit. In addition, four water molecules in 1 were
removed by the SQUEEZE routine in PLATON.[28] The final
molecular formula was determined by combing the single-crystal
structures, thermogravimetric analysis (TGA), elemental analysis
and the electron count of the SQUEEZE results.
Crystal structure of 2
Single crystal X-ray diffraction analysis reveals that 2 is a 3D
MOF based on hexanuclear CoII cluster [Co6(HCO2)(μ2-O)(μ3-
OH)] and crystallizes in triclinic space group P2(1)/n. As shown
in Figure 2, in the [Co6(HCO2)(μ2-O)(μ3-OH)] moiety, six CoII ions
are connected together by HCOO-, μ2-O and μ3-OH to form an
interesting hexanuclear CoII cluster. Additional, each
hexanuclear CoII cluster links with ligands mcd4- which are
completely deprotonated, forming a 3D network. The structure of
complex 2 has channels of 8.5 × 8.5 Å2 dimensions (considering
van der Waals radii) and the channels are occupied by the
coordinated water molecules. PLATON analysis gave the free
void volume ratio as 33% in 2 and suggested the microporous
characteristics of 2. The CO2 adsorption isotherm of 2 showed
that the values of CO2 adsorption was 30 cm3·g-1 at 273 K and
P/P0 = 0.03 and the pore width was 0.5 nm (Figure S2d).
Figure 1. (a) Ball-and-stick representation of the coordination environments of
Co1, Co2, Co3, and Co4 in 1 (hydrogen atoms and free water molecules are
omitted for clarity); (b) View of the 3D framework of 1 along the c axis.
[Co4(μ3-OH)2] and crystallizes in triclinic space group Pī. As
shown in Figure S1a, the asymmetric unit of 1 contains four
unique CoII ions, two pbcd4- ligands, two μ3-OH anions, and
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geometry. In 3, CoA and adjacent CoB are bound by four
carboxyl groups to form a dinuclear SBU. All carboxyl groups
from H4pbc are deprotonated and adopt chelate/bridge
coordination mode to bridge the SBUs, generating a 3D
framework.
Interestingly, with the ancillary ligand added, the pbcd4-
ligands in 3 are almost in a plane and the dihedral angles
Figure 2. (a) Ball-and-stick representation of the coordination environments of
Co1, Co2, Co3, Co4, Co5 and Co6 in complex 2 (hydrogen atoms and free
water molecule are omitted for clarity); (b) Polyhedron representation of the
hexanuclear Co clusters and infinite rod-shaped secondary building blocks
used to assemble complex 2; (c) View of the 3D framework of 2 and the
coordinated water molecules in the channel.
Crystal structure of 3
3 was obtained under the similar conditions of 1, except that 1,2-
Bis(4-pyridyl)ethane as the ancillary ligand was added. Different
with the metal-cluster based metal-organic frameworks of 1 and
2, single crystal X-ray diffraction analysis reveals that 3 consists
of a 3D MOF based on binuclear CoII units and crystallizes in
monoclinic space group P2/c. The asymmetric unit of 3 contains
a CoII ion, half of pbcd4- ligand and one 1,2-Bis(4-pyridyl)ethane
(Figure S3a). CoII is coordinated by four carboxylate oxygen
atoms of three pbcd4- ligands and two nitrogen atoms of two 1,2-
Bis(4-pyridyl)ethane to furnish an octahedral coordination
Figure 3. (a) Representation of dinuclear paddlewheel building unit (hydrogen
atoms and free water molecule are omitted for clarity; (b) 2D network formed
of dinuclear units connected by pbcd4- ligands; (c) 3D framework formed of 2D
network and bpe linkers.
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between two carbazole rings are approximately 135.33°, which
is tremendously different to that of 1. The flexible pbcd4- ligands
and ancillary ligands make the open channel sizes increased.
The square-like channels (~13.5 Å) and cuboid-like channels
(13.4×9.3 Å2) (considering van der Waals radii) can be observed
along the c axis and b axis, respectively. PLATON analysis gave
the free void volume ratio as 28% in 3, which suggests the
microporous characteristics of 3. The CO2 adsorption isotherm
of 3 showed that the values of CO2 adsorption was 25 cm3·g-1 at
273 K and P/P0 = 0.03 and the pore width was 0.6 nm (Figure
S3d).
the formation of cyclopentenone[b]benzenes. The excellent
match between the experimental and simulated powder X-ray
diffraction (PXRD) patterns of 1-3 proved the purity of the bulky
crystalline samples (Figure S1b, Figure S2b, Figure S3b).
Furthermore, the solvent-resistance property of the CoII MOFs
was examined by suspending crystals in boiling 1,2-
dichloroethane (DCE) because it was chosen as the reaction
solvent for the catalytic reaction. We found that 1-3 could keep
their original shapes in boiling DCE for more than 24 h and their
PXRD spectra excellently matched the simulated and the
synthesized PXRD patterns after treatment. It showed that
crystals of 1-3 had good solvent resistance properties in DCE.
To study the potential of MOFs 1–3 as heterogeneous
catalysts in the tandem acylation–Nazarov cyclization reaction,
we compared their activities under different reaction conditions,
using 1,3-dimethoxy benzene, trifluoroacetic anhydride (TFAA)
and tiglic acid as substrates in 1, 2-dichloroethene (DCE) at
reflux for 5 h. As shown in Table 1, 1 and 2 could promote the
catalytic tandem caylation-Nazarov cyclization reaction and the
desired cycloaddition products 8a were generated in yields of 71
and 74%, respectively (Table 1, entries 5 and 6). On the
contrary, 3 gave acylation product in the yield of 97% and no
cycloaddition product was obtained (Table 1, entry 7). It
indicated that 1-2 were novel catalysts for the acylation-Nazarov
cyclization reaction and 3 was highly selective catalyst to obtain
acylation product. For comparison, CoII salts were used as
homogeneous catalysts for this reaction by keeping the same
content of Co (30%). Using Co(OAC)2, Co(NO3)2, CoCl2, CoSO4
as homogeneous catalysts, the reaction leaded to a 76, 67, 84,
67% conversion of 1,3-dimethoxy benzene and gave
cycloaddition products 8a with yields of 27, 25, 35 and 24%
respectively (Table 1, entries 1-4). However, catalytic activity of
1-2 (Conv. of 98%) was much higher than that of CoII salts under
the same reaction conditions. Thus, we demonstrated that the
activity of 1-2 was greatly superior to those of the homogeneous
Analysis of thermal stability of 1-3
The thermal stability of 1-3 was evaluated by thermogravimetric
analysis (TGA). The result of 1 showed a weight loss of 10.19%
(calcd. 9.48%) in the range of 50–230°C attributed to the release
of ethanol molecules, coordinated and free water molecules, and
then the main framework of 1 began to collapse at 350°C
(Figure S1c). As for 2, three steps of weight loss were observed.
The first two steps of weight loss of 29.08% (calcd. 28.27%)
before 314°C were attributed to the adsorbed water and
coordinated water (Figure S2c). The third step of weight loss
began at 310°C, due to the collapse of framework. The curve of
3
revealed that the initiatory weight loss of 15.79%
(calcd.14.98%) in the range of 35-320°C, and framework of 3
was collapsed at the temperature of 320°C (Figure S3c).
Catalytic Performance of 1-3
The catalytic abilities of the CoII MOFs 1-3 were evaluated by
catalyzing the tandem acylation-Nazarov cyclization reaction of
1,3-dimethoxy benzene and α,β-unsaturated carboxylic acids for
Table 1. Optimization of the reaction conditions for the tandem acylation–Nazarov cyclization
OMe
OMe
Catalyst 30%
O
OMe
O
TFAA
+
HO
+
DCE, 5h
MeO
MeO
O
MeO
6
7a
8a
9a
Acylation Product
Nazarov cyclization product
Entry
Catalyst
TFAA
Conversion [%]
Yield [%] of isolated 8[c]
Yield [%] of isolated 9[c]
T/℃
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Co(OAc)2
Co(NO3)2
CoCl2
CoSO4
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
TFAA
none
none
none
90
90
90
90
90
90
90
90
90
90
25
90
90
90
76
67
84
67
98
98
97
>99
>99
71
41
0
27
25
35
24
71
49
42
49
43
27
24
97
20
18
1
2
3
74
n.o.[a]
79
dehydrated-1[b]
dehydrated-2[b]
81
5<
none
67
41
1
1
2
3
n.o.[a]
n.o.[a]
n.o.[a]
n.o.[a]
n.o.[a]
n.o.[a]
n.o.[a]
0
0
[a] n.o.=not observed.
[b] Dried in 363K.
[c] Isolated yields, based on the 6 used, average of two runs.
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CoII salts. Most importantly, heterogeneous catalysts 1-2
obviously showed superior activity to implement the tandem
acylation-Nazarov cyclization reaction rather than acylation
reaction, yielding Nazarov cyclization products as the main
products. Furthermore, the typical homogeneous and
could not promote the acylation-Nazarov cyclization reaction
using 1,3-dimethoxy benzene and α,β-unsaturated carboxylic
acids as substrates.
The catalytic reactions with 1 as heterogeneous catalyst at
different reaction temperatures (25 and 90 °C) were carried out.
heterogeneous Lewis acid catalysts ([Ti(OiPr)4], AlCl3, [Sc(OTf)3], At 25 °C, 41% conversion of 1,3-dimethoxy benzene was
and Zn-MOF, Ag-MOF) reported previously for the reaction achieved after 5 h and no cycloaddition product was obtained
Table 2. Dehydrated 1 and 2 catalyze tandem acylation–Nazarov cyclization for the synthesis of benzocyclopentanone[a]
OMe
Catalyst (30 mol%)
TFAA
DCE, Reflux
5h
a,b-unsaturated carboxylic acid
7
+
Nazarov cyclization product
Acylation product
9
+
MeO
HO
6
8
1 equiv
1.5 equiv
Ph
HO
HO
HO
O
O
7b
O
7a
O
7c
7d
α，β-Unsaturated carboxylic
acids 7a-d
Entry
Catalyst
Products
Yield [%] of isolated 8a-d[c]
Yield [%] of isolated 9a-d[c]
1
2
1
2
3
1
2
3
1
2
3
1
2
3
79
21
19
97
16
40
94
50
52
91
55
54
92
OMe O
OMe
O
+
81
MeO
MeO
9a
8a
3
n.o.[b]
4
84
OMe
OMe O
O
Ph
+
5
60
MeO
MeO
Ph
8b
9b
6
n.o.[b]
50
7
OMe
OMe O
O
+
8
48
MeO
MeO
9c
8c
9
n.o.[b]
45
10
11
12
OMe O
OMe
O
+
46
MeO
MeO
9d
8d
n.o.[b]
[a] Reaction conditions: 30 mol% catalyst, TFAA, DCE, heat at reflux, 5 h.
[b] n.o.=not observed.
[c] Isolated yields, based on the 6 used, average of two runs.
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(Table 1, entry 11). However, when the reaction temperature
increased to 90 °C, a complete conversion of 1,3-dimethoxy
benzene was obtained within 1 h and the yield of cycloaddition
product 8a increased to 71% after 5 h. It was clear that reaction
temperature significantly influenced the cycloaddition reaction
rate and selectivity.
They are shown to be excellent catalysts for the cyclization
reaction because of their high density of Lewis acidic sites within
the frameworks and the synergic activity among ions. The
periodically dispersed multinuclear CoII clusters may
synergistically influence the catalytic activities to promote
tandem or cooperative catalytic products, producing
cycloaddition products 8a-d. In addition, channel surfaces of 1
and 2 are occupied by the coordinated water molecules and
lattice water molecules that are easily removed from the host
materials during the reaction. The coordinatively unsaturated
metal sites (CUSs) inside MOFs can be readily tuned to adjust
interactions between MOFs and ketones, to activate C=O and
thereby to lower the reaction-energy barrier of the desired
chemical transformation.
Instead, when we use 3 as catalyst, the acylation products 9a-
d as the sole products are obtained and no Nazarov cyclization
products can be produced, though 3 has the biggest channels
(square-like channels ~13.5 Å and cuboid-like channels 13.4 ×
9.3 Å). In contrast to the multinuclear CoII clusters of 1 and 2,
single-crystal X-ray structure analysis reveals that 3 shows a 3D
framework based on binuclear CoII units bridged by carboxyl
groups. As a result, 3 can only serve as single-site catalyst to
provide the platform to ensure that the acylation reaction can be
executed.
The mechanism for the synthesis of cyclopentenone[b]
benzenes catalyzed by multinuclear CoII clusters has been
proposed tentatively (Scheme 1). We hypothesize that the
carbonyl groups from the mixture of anhydride (I) can be first
activated by the multinuclear CoII cluster to make them more
electrophilic. Subsequently, 1,3-dimethoxy benzene attacks II to
give intermediate III. Furthermore, the CoII cluster serves as
cooperative catalysis to activate the carbonyl generating a
pentadienyl cation, followed by 4π conrotatory electrocyclization,
proton loss, and finally the product (V) is obtained. The catalyst
can be recovered for the next catalytic cycle. Therefore, the high
yield of 8 may be arising from the easier proceeding 4π
conrotatory electrocyclization by introducing aryl group.
The dehydrated 1-3 were obtained by vacuum baking at 363K.
As shown in Figures S1c, S2c and S3c, according to the TGA
curves, we found that partial coordinated water molecules and
lattice solvent molecules were removed. The PXRD studies
demonstrated that the dehydrated MOFs could retain their
skeleton networks after dried in 363K (Figures S1b, Figures S2b
and Figures S3b). The dehydrated 1-2 were capable of
catalyzing the reaction to yields up to 79% and 81%,
respectively, suggesting that 1 and 2 should function as
dehydrated heterogeneous catalysts to promote this reaction
(Table 1, entries 8 and 9). The improvement of the catalytic
performance might be the porous MOFs. When the good
leaving solvent molecules were removed, it could facilitate
faster transport (diffusion) of molecular reactants and products
and simplify the contact between organic substrates and
catalytic active centers, which make these materials more
attractive heterogeneous catalysts for liquid-phase reactions.
However, using dehydrated 3 as catalyst, cycloaddition product
was still not obtained, and the acylation product was the sole
product.
Subsequently, with the optimized reaction conditions
established, we carried out the process catalyzed by Co-MOFs
with different α,β-unsaturated carboxylic acids as substrates.
This new catalytic system was found to be general for the
cyclization of 1,3-dimethoxy benzene with a variety of α,β-
unsaturated carboxylic acids, and the results were summarized
in Table 2. In all instances, heterogeneous catalyst 1 and 2
could promote the tandem acylation-Nazarov cyclization
reactions, resulting in 8a-d with good or moderate selectivity of
the cyclopentenone[b]benzenes products with yields ranging
from 45% to 84%. It was worth noting that the yields decrease in
the order 8a > 8c > 8d, corresponding to the increase of
substrate sizes, which indicated that the channel size of MOFs
and substrate size played a vital role in the reaction catalyzed by
heterogeneous catalysts.
The high catalytic activity of 1 and 2 may be attributed to their
specific frameworks and channel surface. Single-crystal
structural analyses reveal that 1 and 2 possess various channel
dimensions. 1 possesses the characteristic pore windows (11.85
×10.14 Å2) and 2 has relatively small one-dimensional (1D)
channels with square dimensions of 8.5 × 8.5 Å2, which are large
enough to accommodate α,β-unsaturated carboxylic acids (7a:
5.67 × 3.92 Å2； 7b: 8.75 × 3.83 Å2；7c: 6.37 × 4.51 Å2；7d:
6.40 × 4.91 Å2). The large channels of 1 and 2 provide enough
space to facilitate the transport of organic substrates and
products. Thus, substrates can access and react with both the
abundant coordinative defect sites of metal nodes on the
exterior solid surfaces and catalytic active sites in the interior of
the pore wall of crystals. More importantly, 1 and 2 are
composed of tetranuclear CoII cluster [Co4(μ3-OH)2] and
hexanuclear CoII cluster [Co6(HCO2)(μ2-O)(μ3-OH)], respectively.
O
Co
R2
R1
Co
O
O
O
O
+
HO
F3
C
O
CF3
F3
C
O
R2
O
O
O
R1
Co
Co
I
O
O
O
OMe
Co
Co
O
F3
C
R2
R1
R1
MeO
-H+
MeO
R2
O
V
OMe
Co
Co
II
OMe
O
O
+
R1
Co
O
MeO
O
F3
C
OH
H
R2
R1
Co
Co
O
H
Co
R2
OMe
O
MeO
-H+
R1
R2
+
MeO
OMe
III
IV
Scheme 1. The suggested mechanism.
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obtained with Bruker Avance-400 spectrometers. High-resolution mass
spectra (HRMS-ESI) were obtained on
spectrometer. FT-IR spectra were performed on a Bruker Vector 22 FT-
IR spectrophotometer using KBr pellets.
To prove the heterogeneous mechanism, atomic absorption
spectroscopy analysis of the filtrate firstly showed the presence
of slight leaching (<1 ppm) of cobalt from the crystalline sampes
of 1-3 during the reaction. Second, leaching test of 1-3 by
removing the solid catalysts after 1 h and continuing reaction for
another 4 h were performed. No further conversion was noted,
which demonstrated that no homogeneous catalyst species
existed in the reaction solution and 1-3 were true heterogeneous
catalysts.
We also examined the recyclability of 1-3 for this catalytic
reaction with 7a. In the recyclability experiments, compound 1-3
were readily recovered from the catalytic reaction via
centrifugation, and the recovered catalyst showed only slight
deterioration in four runs (Figures S1f, S2e, and S3e). It may
result from the fact that the pores and the coordinative defect
sites of metal nodes can be blocked during the catalysis,
deteriorating the catalytic efficiency. Moreover, the PXRD
patterns of the crystalline samples of 1-3 after the four round of
reaction completely matched the simulated 1-3 (Figure S1b, S2b,
S3b), and displayed that framework didn’t collapse, which
revealed that the framework could retain integrated after at least
four runs. The recycled catalysts exhibited activity similar to that
of the original catalysts in all of the runs, suggesting that no
irreversible decomposition of the catalysts occurred under the
reaction conditions.
a
MicroTM Q-TOF mass
Synthesis
{(H3O)2[Co4(pbcd)2(μ3-OH)2]·(CH3CH2OH)·4H2O}n (1):
A mixture of
[Co(NO3)2]∙6H2O (29 mg, 0.1 mmol), H4pbcd (16 mg, 0.03 mmol), DMF
(3 mL), H2O (3 mL) were placed in a 25 mL Teflon-lined stainless steel
container. The mixture was sealed and heated at 120 °C for three days.
After the mixture was cooled to 80 °C at a rate of 5 °C/h, dark purple
crystals of 1 were obtained with a yield of 57% (based on Co). Elemental
analysis calcd (%) for C64H50Co4N4O25: C 50.35, H 3.81, N 4.29; Found:
C 50.83, H 3.31, N 4.24; IR (KBr): ṽ=3439 cm-1 (-CH), 1705 cm-1 (C=O),
1602 cm-1 (C=C), 1295 cm-1 (C-N), 729 cm-1 (-CH2-).
{[Co5.25(mcd)2(HCO2)(μ3-O)0.5(μ3-OH)0.5(H2O)4]·6H2O·5DMF}n (2):
A
mixture of [Co(NO3)2]•6H2O (29 mg, 0.1 mmol), H4mcda (13 mg, 0.025
mmol), DMF (4 mL) and H2O (2 mL) was placed in a 25 mL Teflon-lined
stainless steel container. The mixture was sealed and heated at 160 °C
for three days. After the mixture was cooled to ambient temperature at a
rate of 5 °C/h, dark purple crystals of 2 were obtained in 79% yield
(based on Co). Elemental analysis calcd (%) for C74H80Co5.25N9O32.75: C
46.17, H 4.21, N 6.58; Found: C 46.06, H 4.15, N 6.54; IR (KBr): ṽ =3443
cm-1 (-CH), 1605 cm-1 (C=O), 1549 cm-1 (C=C), 1293 cm-1 (C-N), 725 cm-
1 (-CH2-).
{[Co2(pbcd)(bpe)]·2H2O·2DMF}n (3): A mixture of [Co(NO3)2]•6H2O (29
mg, 0.1 mmol), H4pbcd (16 mg, 0.03 mmol), 1,2-bis(4-pyridyl)ethane(8
mg, 0.05 mmol), DMF (3 mL) and H2O (3 mL) was placed in a 25 mL
Teflon-lined stainless steel container. The mixture was sealed and
heated at 120 °C for three days. After the mixture was cooled to 80 °C
temperature at a rate of 5 °C/h, pink crystals of 3 were obtained in 68%
yield (based on Co). Anal. Calcd for C61H60Co2N8O12: C, 60.35; H, 4.81;
N, 9.29; Found: C, 60.26; H, 3.94; N, 9.22; IR (KBr): ṽ=3427 cm-1 (-CH),
1674 cm-1 (C=C), 1607 cm-1 (C=N), 1564 cm-1 (C=C), 1280 cm-1 (C-N),
1223 cm-1 (C=O), 736 cm-1 (-CH2-).
Conclusions
In summary, we demonstrated that 1 and 2, two CoII-cluster
based MOFs, could selectively catalyze the acylation-Nazarov
cyclization reaction of 1,3-dimethoxy benzene and α,β-
unsaturated carboxylic acids to produce cyclopentenone[b]
benzenes due to their high density of Lewis acidic sites within
frameworks, and the synergic activity among CoII clusters. As to
3, only acylation products were observed, owing to the single-
site catalyst and the lower Lewis acidity of the dinuclear SBUs
compared to the clusters. Easy product separation, simple
reaction procedures, and catalyst recycling make the catalytic
system attractive. Moreover, this study highlights the impact of
synergistic effect in tuning the activity and/or selectivity of a
catalyst, and also display cooperative catalysis in forming
cyclopentenone[b]benzenes.
Crystal data collection and refinement
The data of the complex 1 were collected on a Rigaku Saturn 724 CCD
diffractomer (Mo Kα, λ =0.71073) at temperature of 293.15 K. Absorption
corrections were applied by using numerical program. The data were
corrected for Lorentz and polarization effects. The crystal diffraction data
for 2 were collected on a Bruker APEX-III CCD diffractometer at 100 K
using Cu Kα radiation (λ = 1.54178). Diffraction data collection for 3 was
finished on a Bruker Apex III CCD diffractometer (Mo Kα, λ = 0.71073).
The integration of the diffraction data, as well as the intensity corrections
for the Lorentz and polarization effects, were performed using the SAINT
program.[30] Semiempirical absorption correction was performed using
SADABS program.[31] The structures of 1-3 were solved by direct
methods using SHELXS-2014 and all the non-hydrogen atoms were
refined anisotropically on F2 by the full-matrix least-squares technique
with SHELXL-2014.[32] The hydrogen atoms except for those of water
molecules were generated geometrically and refined isotropically using
the riding model. The details of the crystal data for 1-3 are summarized in
Table S1, and selected bond lengths and angles are listed in Table S2.
CCDC-1572230, CCDC-1572218, and CCDC-1572290 contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
Materials and physical measurements
All reagents and solvents were commercially available and used as
received without further purification. Ligands were synthesized according
to the modified procedures reported previously.[29] Powder X-ray
diffraction (PXRD) analyses were performed on a Bruker D8 Advance X-
ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The thermal
decomposition characteristic of the samples was analyzed by TGA using
a Netzsch STA 449C thermal analyzer at a heating rate of 10℃min-1 in
air. Elemental analyses for C, H, and N were performed on an Elementar
Vario MICRO Elemental analyzer. 1H and 13C NMR spectra were
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10.1002/chem.201704749
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Typical procedure of
reaction for the synthesis of cyclopentenone[b]benzenes
a tandem acylation–Nazarov cyclization
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mixture of 1,3-Dimethoxy benzene (1.0 mmol, 1.0 equiv) and α,β-
unsaturated carboxylic acid (1.5 mmol, 1.5 equiv) in dry DCE (10 mL).
The solution was stirred under reflux for 5 h. The cool solvent was
evaporated in vacuo. Then the residue was dissolved in the EtOAC. The
organic phase was washed by distilled water and saturated aqueous
NaHCO3, dried by Na2SO4, filtered and evaporated in vacuo. The crude
residue was purified by column chromatography (silica gel, PE-EA, v/v =
3/1). Spectra of 1H NMR, 13C NMR and MS of compounds are listed in
the Supporting Information.
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Acknowledgements
This work was funded by the National Natural Science
Foundation (Nos. 21771163, 21671174 and 21371155), the
Science Foundation of Henan Provience (Nos. 162300410243)
and Project for the Leading Young Teachers in Henan Provincial
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Keywords: metal−organic frameworks
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Entry for the Table of Contents
FULL PAPER
Two MCOFs (1-2) have been
synthesized and used as novel
heterogeneous catalysts to selectively
catalyze tandem-Nazarov cyclization
reaction of 1,3-dimethoxy benzene
with α,β-unsaturated carboxylic acids
for the synthesis of
cyclopentenone[b]benzene. Complex
3 was highly selective catalyst to
obtain acylation product.
Mengjia Liu,[a] Kuan Gao,[a] Yanru Fan,[a]
Xiaoqing Guo,[a] Jie Wu,*[a] Xiangru
Meng,*[a] Hongwei Hou,*[a]
Page No. – Page No.
Co-Cluster-Based Metal-Organic
Frameworks as Selective Catalysts
for Benzene Tandem Acylation–
Nazarov Cyclization to
Benzocyclopentanone
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