The synthesis of a bifunctional copper metal organic framework and its application in the aerobic oxidation/Knoevenagel condensation sequential reaction

Article abstract of DOI:10.1039/c6dt01690a

A novel one-pot aerobic oxidation/Knoevenagel condensation reaction system was developed employing a Cu(ii)/amine bifunctional, basic metal-organic framework (MOF) as the catalyst. The sequential aerobic alcohol oxidation/Knoevenagel condensation reaction was efficiently promoted by the Cu₃TATAT MOF catalyst in the absence of basic additives. The benzylidenemalononitrile product was produced in high yield and selectivity from an inexpensive benzyl alcohol starting material under an oxygen atmosphere. The role of the basic functionality was studied to demonstrate its role in the aerobic oxidation and Knoevenagel condensation reactions. The reaction progress was monitored in order to identify the reaction intermediate and follow the accumulation of the desired product. Lastly, results showed that the yield was not significantly compromised by the reuse of a batch of catalyst, even after more than five cycles.

Full text of DOI:10.1039/c6dt01690a

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The synthesis of a bifunctional copper metal organic framework
and its application in the aerobic oxidation/Knoevenagel
condensation sequential reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Zongcheng Miaoa, Yi Luanb,*, Chao Qib and Daniele Ramellac
DOI: 10.1039/x0xx00000x
A novel one-pot aerobic oxidation/Knoevenagel condensation reaction system was developed employing a Cu(II)/amine
bifunctional, basic metal-organic framework (MOF) as the catalyst. The sequential aerobic alcohol oxidation/Knoevenagel
condensation reaction was efficiently promoted by the Cu3TATAT MOF catalyst in the absence of basic additives. The
benzylidenemalononitrile product was produced in high yield and selectivity from an inexpensive benzyl alcohol starting
material under oxygen atmosphere. The role of the basic functionality was studied to demonstrate its role in the aerobic
oxidation and Knoevenagel condensation reactions. The reaction progress was monitored in order to identify the reaction
intermediate and follow the accumulation of the desired product. Lastly, results showed the yield was not significantly
compromised by the reuse of a batch of catalyst, even after more than five cycles.
www.rsc.org/
diagnostics, gas separation, optics, biosensing and catalysis,
have been achieved employing MOF materials.5 As one of the
most promising inorganicꢀorganic hybrid materials for catalysis,
Introduction
Multicomponent reactions are attractive and practical tools in
chemistry as two or more synthetic steps are combined into
one. Compared to multiꢀstep synthetic processes, oneꢀpot
reactions improve efficiency, reduce production of waste, and
enable the rapid construction of more complex molecules from
simple, readily available starting materials.1 Several catalytic
systems have been designed to achieve efficient oneꢀpot
reactions. However, homogeneous catalysts, which are
commonly used in oneꢀpot multicomponent reactions, often
suffer from catalyst contamination and poor recyclability.2
Therefore, the development of heterogeneous catalysts that may
be systematically designed and facilely recycled is currently
one of the major foci of research on oneꢀpot multicomponent
reactions,, specifically for synthetic procedures, such as the
Knoevenagel condensation, which furnish synthetically useful
products.3
MOFs have emerged as highly practical and efficient platforms
to engineer multifunctional catalysts for multistep organic
transformations.6 MOF materials can function as ideal solid
platforms for catalytic design since both components, the metal
and the organic ligand, can be functionalized with many
advantageous features.7 So far, the utilization of MOF catalysts
in oneꢀpot multicomponent reactions is limited in the
literature.8 Although several multifunctional heterogeneous
catalysts have been obtained employing a “siteꢀisolation”
strategy, several drawbacks during their application in catalysis
strongly limit further utilization. The current multifunctional
catalytic systems in fact suffer often from weak catalytic
activity; the need for an extra light source, as well as tedious
preparation procedures.9 In addition, oneꢀpot multicomponent
reactions should exclude the use of expensive starting
materials.10 For example, despite being more expensive than
benzaldehyde itself, benzaldehyde acetal often serves as a
precursor for the generation of benzaldehyde.11 It would be
much more practical to use inexpensive starting materials to
initiate multistep reactions.
Recently, metalꢀorganic frameworks (MOFs), have drawn
great interest in several research areas because of their unique
and attractive properties, such as their highlyꢀordered porous
structure, high specific surface areas and great structural
tunability.4 Several utilizations, like gas storage, medical
In this study, a multifunctional Cu3TATAT catalyst was
designed, synthesized, and employed in an efficient oneꢀpot
multicomponent
reaction
involving
an
aerobic
a.Xijing University, Xi’an, Shaanxi Province, 710123, P.R. China
b.School of Materials Science and Engineering, University of Science and Technology
Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, P. R. China,
yiluan@ustb.edu.cn
oxidation/Knoevenagel condensation sequence of reactions
(Scheme 1). The Cu3TATAT catalyst was prepared in four
different morphologies and sizes to study the relationship
between its structure and catalytic activity. The novel
utilization of a Cu(II)ꢀpromoted aerobic oxidation for the
formation of aldehyde intermediates in a oneꢀpot reaction from
c. Temple University-Beury Hall, 1901, N. 13th Street, Philadelphia PA 19122, United
States
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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employing morphology control reagents. As shown in Fig. 1a,
the octahedral Cu3TATAT crystal at approximately 10 ꢁm scale
was obtained in the absence of an additive.13 In order to achieve
Cu3TATAT crystals at the nanoscale level, different mediators
have been added during the synthetic process, which could
strongly influence the dimension as well as the morphology.
When dodecanoic acid was employed as the additive, uniform
nanospheres with a diameter of 100 nm were observed by SEM
(Fig. 1b). While replacing dodecanoic acid with benzoic acid as
the additive gave a highly ordered crystalline Cu3TATAT
morphology, uniform and monodispersed octahedral crystals
with a diameter of 600 nm were observed(Fig. 1c). In addition,
a surfactant additive such as Pluronic® F127 provided a flowerꢀ
like Cu3TATAT crystal (Fig. 1d). The flowerꢀlike crystals were
assembled from a thin nanoplate at the size of about 5 ꢁm.
Scheme 1. The design and synthesis of Cu3TATAT MOF with
dual functionalities.
inexpensive aromatic alcohol starting materials has not been
previously reported in the literature. This newly developed
Cu3TATAT catalyst is compatible with various aromatic
alcohols under the studied conditions; it allows to convert them
to the desired Knoevenagel condensation products in good
yields and excellent selectivities. Furthermore, this oneꢀpot
reaction strategy can be extended to generate a nitroꢀvinyl
substituted Henry reaction product via nitromethane
nucleophilic addition.
Results and discussion
Fig. 2 PXRD of copper MOFs: a) Simulated Cu3TATAT using its
single crystal structure, b) Cu3TATAT-1, c) Cu3TATAT-2, d)
Cu3TATAT-3, e) Cu3TATAT-4, f) Cu3TATAT-3 after 5 recycles.
The crystallinity, structure and composition of the prepared
Cu3TATAT samples were characterized by powder Xꢀray
diffraction (PXRD). The addition of different categories of
mediators during the synthetic process does not affect the
internal crystalline structure, as shown in Fig. 2. All PXRD
patterns of the Cu3TATAT were measured at solvated state.
The patterns are in agreement with the simulated PXRD using
Accelrys Materials Studio using CIF file from CCDC 831219.
The PXRD results exclude the possibility that additional Cu
carboxylate complexes/clusters were being formed inside the
pores. Furthermore, the PXRD pattern results also indicate that
the internal crystal structures of Cu3TATAT samples 1ꢀ4 are
the same, despite the fact that different additives were
employed (Figure 2bꢀ2e). However, the existence of crystalline
defects is possible inside of Cu3TATAT samples 2ꢀ4 due to the
addition of external chemical additives. The characteristic
diffraction peaks at 2 theta values of about 4.05o, 4.66o, 6.61o,
7.68o, 8.09o, 10.16o, 12.13o and 13.24o are assigned to the (101),
(110), (112), (103), (202), (213), (303) and (224) planes of the
Cu3TATAT crystal, which is consistent with the literature.14
Fig. 1 SEM of Cu3TATAT produced with different additive: (a)
Cu3TATAT-1 (No additive), (b) Cu3TATAT-2 (Dodecanoic acid
additive), (c) Cu3TATAT-3 (Benzoic acid additive), (d) Cu3TATAT-
4 (Pluronic® F127 additive).
Nanosized MOF architectures have attracted much attention
as they typically offer significantly improved chemical and
physical properties and reactivity compared to the bulk ones.
The nanomorphology of a solid catalyst takes advantage of its
increased and accessible textural porosity; this leads to
increased activity in the catalytic system.12 As a result, the
preparation of MOFs with controllable size and morphology via
a facile route is crucial. Herein, we successfully synthesized a
series of Cu3TATAT in various sizes and morphologies
2 | J. Name., 2012, 00, 1-3
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Among the Cu3TATAT substrates studied, Cu3TATATꢀ3
Table 1 The N2 Adsorption/desorption Isotherms of Cu3TATAT demonstrated the optimal material morphology and largest BET
MOFs
surface area. A quick catalytic survey of the asꢀprepared
Cu3TATAT materials revealed that Cu3TATATꢀ3 provides the
highest yield and best recyclability. Cu3TATATꢀ4 only showed
moderate yield (70%) and poor recyclability due to the
relatively low crystalline structure and low BET surface area.
Furthermore, Cu3TATATꢀ1 and Cu3TATATꢀ2 were not as
good as Cu3TATATꢀ3 in terms of catalytic yield and
recyclability. As a result, Cu3TATATꢀ3 proved to be the
optimal copper MOF material for further catalytic evaluation
taking advantage of its high crystalline structure, large BET
surface area and efficient catalytic performance.
Sample
SBET
SLangmuir
Vpore
DAverage pore
(m2gꢀ1)
(m2gꢀ1)
(cm3gꢀ1) (nm)
Cu3TATATꢀ1 1307(35) 1393(40) 0.69
Cu3TATATꢀ2 1518(45) 1613(60) 0.72
Cu3TATATꢀ3 1520(45) 1620(60) 0.77
Cu3TATATꢀ4 1202(35) 1356(40) 0.62
2.12
1.91
2.03
1.94
The N2 adsorption/desorption isotherms of various
Cu3TATAT samples synthesized using different additives were
measured and the data is summarized in Table 1. The additive
free Cu3TATATꢀ1 sample gave a BET surface area of 1307.5
m2gꢀ1. It was observed that nanoscaled samples Cu3TATATꢀ2
and CuTATATꢀ3 had increased surface area (Table 1).
Cu3TATATꢀ2 with dodecanoic acid showed the BET area of
1518.1 m2gꢀ1, while switching the additive to benzoic acid in
Cu3TATATꢀ3 gave a BET surface area of 1520.9 m2gꢀ1. It was
expected that the flowerꢀmorphology assembled from nanoꢀ
plates could further increase the BET surface area. However,
the corresponding BET area decreased to 1202.9 m2gꢀ1; we
attribute this unexpected result to the low crystalline integrity
of Cu3TATATꢀ4.
The aerobic oxidation/Knoevenagel condensation sequential
reaction was systematically investigated employing various
catalysts and base additives and the results are summarized in
Table 2. No background reaction was observed in absence of
the catalyst and/or basic additive; an O2 balloon was used as the
oxygen source (Table 1, entry 1). Employing 8 mol% of
Na2CO3 as the base additive under mild reaction conditions, a
Cu(II) salt achieved 23% conversion of benzyl alcohol, leading
to 11% yield of the desired benzylidenemalononitrile product
(Table 2, entry 2). Cu(PhCOO)2 showed a much improved
conversion under the same reaction conditions, which gave a
32% yield of the benzylidenemalononitrile product (Table 2,
entry 3). This significant improvement in reaction efficiency is
observed for copper complex with carboxylate ligand, which
was also observed in our previous studies.15 Interestingly, the
Cu3(BTC)2 MOF promotes the oneꢀpot multicomponent
reaction efficiently in the presence of Na2CO3 (Table 2, entries
4 and 5). The use of Cu3(BTC)2 in the oxidation of benzyl
alcohol was studied in detail during our previous investigation
when 1.0 equiv. of Na2CO3 was used.15 The alcohol conversion
was compromised with only 0.08 equiv. of Na2CO3 due to its
low protonation/deprotonation equilibrium constant (Table 2,
entry 4). The conversion went up to 86% when increasing the
amount of Na2CO3 to 0.5 equiv. (Table 2, entry 5). In addition,
the conversion of benzyl alcohol was extremely low in the
absence of Na2CO3, which indicated the base additive plays a
crucial role (Table 2, entry 6). It would be ideal to employ an
amine organic base to serve as the base for the aerobic alcohol
oxidation, as well as the basic catalyst for the promotion of the
Knoevenagel condensation. The water byꢀproduct generated
during the oxidation and condensation reaction has no negative
impact on the catalytic activity of our catalyst.
Table
2 One-pot multicomponent reaction using various
catalyst
Base additive
(mol%)
Yield
(%)
Conv.
of 1
(%)
0
Entry
Catalyst
2
3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
ꢀ
ꢀ
0
8
Cu(NO3)2
Cu(PhCOO)2
Cu3(BTC)2
Cu3(BTC)2
Cu3(BTC)2
Cu(PhCOO)2
Cu(PhCOO)2
Cu3(BTC)2
Cu(PhCOO)2
Cu3TATATꢀ1
Cu3TATATꢀ2
Cu3TATAT-3
Cu3TATATꢀ4
ꢀ
Na2CO3 (8)
Na2CO3 (8)
Na2CO3 (8)
Na2CO3 (50)
23
46
53
86
<5
18
57
65
18
87
91
95
74
0
11
16 27
17 32
0
82
-
<5 <5
Melamine (2.7)
Melamine (10)
Melamine (10)
UiOꢀ66ꢀNH2 (8)
3
6
5
12
50
58
5
12
ꢀ
ꢀ
ꢀ
ꢀ
<5 85
<5 87
Melamine was chosen as the organic base candidate,
proceeding in low yield using the combination of 8 mol%
Cu(PhCOO)2 and 2.7 mol% melamine (8 mol% based on
aromatic nitrogen) in the oneꢀpot multicomponent reaction
(Table 2, entry 7). The nitrogenꢀbased organic base is less
efficient than the Na2CO3 inorganic base during the alcohol
0
95
<5 70
Melamine (50)
TEMPO (50)
0
0
0
0
16
ꢀ
0
a
Reaction conditions: catalyst (8 mol% based on copper), aerobic oxidation step. Increasing the melamine loading to 10
benzyl alcohol (1.0 mmol), malononitrile (1.5 mmol), TEMPO mol% improved the conversion of benzyl alcohol to 57%,
(0.5 equiv.), and base additive were stirred in 5 mL CH3CN at which corresponds to a 50% yield of the desired condensation
75 oC using 1 atm O2 balloon for 12 h.
product (Table 2, entry 8). The use of both Cu3(BTC)2 and 10
mol% melamine further increased the yield to 58% (Table 2,
entry 9). The successful implementation of melamine as the
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organic base provides the opportunity for organic base
integration into a catalyst structure. However, the use of basic
UiOꢀ66ꢀNH2 MOF as the basic additive was not successful due
to its weak basicity (Table 2, entry 10). This observation
excludes the possibility of using an aromatic amine on the MOF
structure as the base for the Knoevenagel condensation
reaction.16 In addition, it is also difficult to recycle the
homogeneous Cu(PhCOO)2 catalyst. Our Cu3TATATꢀ3 solid
catalyst was tested as
a bifunctional catalyst with the
incorporation of both Cu(II) and melamine catalytic moieties
(Table 2, entires 11ꢀ14). Cu3TATATꢀ3 gave a conversion of
95 % in the absence of base additive, taking advantage of the
basicity of the H6TATAT ligand (Scheme 1 and Table 2, entry
13). This result was better than its analogs, which gave 85 %,
87 % and 70 % yield, for sample 1, 2 and 4, respectively. We
believe that the relatively larger surface area and the more
unified crystalline structure assisted the catalytic process. The
aerobic oxidation reaction initiated from the deprotonation of
the benzyl alcohol, as promoted by the base. The further
deprotonation of the benzylic position was achieved using a
combination of copper(II), base, TEMPO and molecular
oxygen. The Cu3(BTC)2 and the Cu3TATAT promoted aerobic
oxidation reactions shared similar mechanisms, with the only
difference of employing different basic additives. Lastly, base
additive and TEMPO alone were shown to have no catalytic
oxidation activity to initiate the first step of the oneꢀpot
multicomponent reaction, even at 0.5 equiv. (Table 2, entries 15
and 16). The role of TEMPO in the Cu(II) promoted oxidation
has already been thoroughly studied in Semmelhack’s studies.17
TEMPO was being oxidized to nitrosonium ion while Cu(II)
was reduced to Cu(I). At this stage, the nitrosonium ion
generated from TEMPO oxidized benzyl alcohol to form the
corresponding aldehyde product. The regeneration of TEMPO
and Cu(II) was promoted by the oxygen in solution.
With the optimal catalytic reaction conditions in hand,
the oneꢀpot multicomponent reaction pathway was further
investigated by tracing the product distribution versus the
reaction time, while employing 8 mol% of Cu3TATATꢀ3 as the
catalyst (Fig. 3). According to GCꢀMS monitoring over the
reaction progress, the oneꢀpot reaction took place in two steps .
Inspection of the amount of reactant and product in the solution
as shown in Fig. 3 indicates that benzylidenemalononitrile was
efficiently generated via the oxidation of benzyl alcohol. The
concentration of the benzaldehyde intermediate went up
initially to reach a maximum after 6 h. Then the generation of
benzylidenemalononitrile became the dominant reaction
pathway and the oneꢀpot reaction was finished after 12 h.
According to the information reported in Fig. 3, intermediate
and product distribution exhibited a typical behavior of oneꢀpot
sequential reactions. The turnover number (TON) of
Cu3TATATꢀ3 was calculated to be 11.9, which is significantly
higher than other literature reported systems, in which the
reaction is performed in the absence of base additive(Table
S3).18
Fig. 3 Results of the catalytic activity of the Cu3TATAT-3
catalyst for the aerobic oxidation/Knoevenagel condensation
tandem reaction. Conversion vs reaction time of (a) benzyl
alcohol, (b) benzaldehyde and (c) benzylidenemalononitrile.
The role of basic additives during the nucleophilic addition
to the benzaldehyde intermediate has been further studied
employing various aromatic organic bases. These tests were
carried out under the optimal reaction conditions employing 8
mol% of the copper MOF catalyst, 10 mol% of base additive
(based on nitrogen), 1.0 mmol benzaldehyde, and 1.5 mmol
malononitrile, in 5 mL CH3CN. Melamine B1 was used first as
a basic catalyst for the Knoevenagel condensation reaction of
benzaldehyde and malononitrile. As shown in Fig. 4, melamine
demonstrated good catalytic activity for the benzaldehyde
conversion, by taking advantage of the large number of
aromatic amine and triazine moieties (Fig. 4, B1). Catalyst
Cu3TATATꢀ3 B5 was evaluated in the absence of the
Cu3(BTC)2 MOF and giving a slightly higher yield of B1; this
is likely due to the similarity of the core structure of
Cu3TATATꢀ3 to melamine.
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Table 3 One-pot reaction of various alcohols using Cu3TATAT-3
catalysta
Entry Substrate
1
Product
Conv. Sel.
%
%
90
99
2
3
97
86
99
99
4
5
78
72
99
99
6
93
99
7
8
69
12
99
99
aReaction conditions: Cu3TATAT-3 catalyst (8 mol% based on
copper), alcohol (1.0 mmol), malononitrile (1.5 mmol), and
TEMPO (0.5 equiv.), in CH3CN (5 mL) were stirred at 75 oC
using a 1-atm O2 balloon for 12 h.
With this information in hand, we can conclude that the
aromatic amino functional group in Cu3TATATꢀ3 does not
function effectively as a base. As a result, the triazine moiety in
Cu3TATATꢀ3 is responsible for the majority of reactivity in the
Knoevenagel condensation of the oneꢀpot multicomponent
Fig.
4 The effect of organic bases in the Knoevenagel
condensation reaction; conversion vs time of the Knoevenagel
condensation catalyzed by various bases.
reaction.
A variety of primary alcohols were evaluated
employing the synthesized Cu3TATATꢀ3 as the catalyst under
the optimal reaction conditions (Table 3). A methyl substituted
aromatic alcohol was compatible under the optimal reaction
conditions, providing 90% yield of the desired product (Table 3,
entry 1). Electronꢀrich aromatic alcohols, such as 4ꢀ
methoxybenzyl alcohol and 3ꢀmethoxybenzyl alcohol, were
compatible with the Cu3TATATꢀ3 catalyst under the
homogeneous reaction conditions.
Conversions of 97% and 86% were achieved for methoxy
substitutions at the paraꢀ and metaꢀpositions respectively
(Table 3, entries 2 and 3). An aromatic alcohol containing a
para electronꢀwithdrawing group also performed well under the
optimal reaction conditions despite its electronꢀdeficient nature
(Table 3, entry 4). Furthermore, heteroaromatic thiophenꢀ2ꢀ
ylmethanol reacted smoothly with malononitrile in the presence
of Cu3TATATꢀ3 and a moderate yield of 2ꢀ(thiophenꢀ2ꢀ
ylmethylene)malononitrile was obtained (Table 3, entry 5).
Similar reactivity was expected for pyridinꢀ2ꢀylmethanol,
which provided the malononitrile functionalized pyridine ring
in 93% conversion and 99% selectivity (Table 3, entry 6).
Furthermore, transꢀcinnamyl alcohol was evaluated and only a
However, whether or not the aromatic amine or triazine
moiety is responsible for the catalytic activity remains unclear
at this stage. 1,3,5ꢀTriazine B2 was evaluated to determine the
basic catalytic activity of the triazine core in bases B1 and B5
.
A slightly diminished reaction rate was observed in comparison
to the use of B1 as the base (Fig. 4, B2). This observation
strongly suggests the high activity and crucial role of the
triazine moiety during the Knoevenagel condensation reaction,
despite the fact that the triazine moiety is less efficient than the
aromatic amino moiety. Furthermore, aniline B3 was also tested
to study the catalytic activity of aromatic amines (Fig. 4, B3).
Aniline B3 showed the highest reaction efficiency when it was
employed in the same molar amount of nitrogen. This
observation also explains how melamine B1 behaved more
efficiently than triazine B2. Even though aniline showed good
activity in the Knoevenagel condensation reaction, the
deactivation of aniline through carboxylate groups significantly
reduced the catalytic efficiency during the condensation
reaction (Fig. 4, B4).
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moderate yield was achieved, presumably due to the poor Cu3TATATꢀ3 catalyst was assessed by PXRD and FTIR before
electron density of the hydroxyl group since it is relatively far and after oneꢀpot catalysis, to evaluate its structural integrity.
from the aromatic ring (Table 3, entry 7). Lastly, an alkyl
alcohol such as cyclohexane methylalcohol was also
compatible with the malononitrile nucleophile under the
optimal reaction conditions, despite the fact that the reaction
rate was slow and only a yield as low as 12% was achieved
after 12 hours of reaction time (Table 3, entry 8).
The recyclability of the Cu3TATATꢀ3 catalyst was
evaluated to demonstrate the advantage of bifunctional solid
catalysts. The reaction was carried out using benzyl alcohol and
malononitrile in presence of Cu3TATATꢀ3 under the optimal
reaction conditions as reported in Table 1. The recycled catalyst
showed no loss of reactivity over five runs (Fig. 5); the yield of
the desired 2ꢀbenzylidenemalononitrile product remained in
fact over 90% after five runs. The supernatant of the CH3CN
suspension after completion of the reaction showed no
reactivity to the benzyl alcohol in the presence of malononitrile
and further addition of melamine did not raise the consumption
of benzyl alcohol. This result suggests that there is no leakage
of the copper content. Addition of benzaldehyde to the filtered
Fig. 6 Conversion of benzyl alcohol as a function of reaction
time in a one-pot multicomponent reaction over: (black curve)
Cu3TATAT-3 and the reaction system after Cu3TATAT-3 was
filtered (red curve).
supernatant
did
not
lead
to
formation
of
benzylidenemalononitrile, indicating good stability of
Cu3TATATꢀ3 without detectable H6TATAT ligand leakage.
This further shows that only the H6TATAT ligand on the
heterogeneous Cu3TATATꢀ3 is responsible for the
Knoevenagel condensation reactivity.
Fig. 5 Recyclibility test of Cu3TATAT-3 in the one-pot reaction
Fig. 7 Proposed mechanism of the one-pot multicomponent
of benzyl alcohol and malononitrile.
reaction.
To further study the possibility of catalyst leaching of
Cu3TATATꢀ3 during the Knoevenagel condensation reaction, a
hot filtration test was performed and the catalytic reaction was
stopped after 4 h. Then the reaction solution was centrifuged
and the supernatant was further stirred for an additional 8 h.
The result in Fig. 6 shows the conversion of benzyl alcohol
paused after 4 h after the centrifugation of the solid catalyst.
This observation indicates the advantage of the stable
covalently bonded structure of the Cu3TATATꢀ3 MOF and that
there was no leakage of the copper or TATAT ligand. The
Powder XꢀRay Diffraction characterization of the recovered
Cu3TATATꢀ3 catalyst was carried out on fresh and recycled
catalysts, showing almost identical PXRD (Fig. 2f) profiles.
Furthermore, the FTꢀIR spectra of the fresh and recovered
Cu3TATATꢀ3 samples were also identical; we can conclude
that there is no structural alteration of the MOF framework (Fig.
S1). In addition, the BET surface area remained at 1514.9 m2gꢀ1
after 5 catalytic cycles (Fig S2).
A mechanism for the oneꢀpot multicomponent reaction
between aromatic alcohols and methylene nucleophiles has
6 | J. Name., 2012, 00, 1-3
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ARTICLE
been proposed (Fig. 7). The reaction initiates from the aerobic times. Finally, the obtained product was dried under vacuum at
oxidation of an aromatic alcohol using a Cu(II) catalyst in the 40 oC for 24 h.
presence of the basic H6TATAT ligand (step I). The aromatic
aldehyde is formed efficiently using the Cu3TATAT catalyst, Similarly, the syntheses of Cu3TATATꢀ3 and Cu3TATATꢀ4
through the first aerobic oxidation step. Secondly, the asꢀ were achieved using benzoic acid or Pluronic® F127 as an
formed aromatic aldehyde can further react with a methylene additive instead of dodecanoic acid and following a similar
nucleophile through a condensation reaction in the presence of procedure to that reported for Cu3TATATꢀ2.
the basic triazine moiety in the Cu3TATAT MOF. The initial
step of the Knoevenagel condensation reaction involves the
baseꢀpromoted deprotonation of the active methylene group for
the generation of a carbanion intermediate. This newly formed
by scanning electron microscopy (SEM, ZEISS SUPRA55).
carbanion will react with the carbonyl group on the aromatic
Characterization.
The morphologies of the resulting products were characterized
The samples for SEM detection were dispersed in ethanol and
sonicated for several minutes followed by supporting onto the
aldehyde via nucleophilic attack (step II). Lastly, the addition
intermediate
will
quickly
be
converted
to
the
silicon slice. The composite and structure of the crystals were
demonstrated by wideꢀangle Xꢀray powder diffraction (XRD,
M21X) with Cu Kα radiation at a scanning rate of 2 degrees per
minute (λ=0.154178 nm). The specific surface areas were
obtained through the Brunauer–Emmett–Teller (BET) method
(Micromeritics ASAP 2420 adsorption analyzer). All samples
were dried under vacuum at 120 ºC for 12 h before the
measurement. The porosity properties of the obtained samples
were determined by the adsorption branches of isotherms by
using the BarrettꢀJoynerꢀHalenda (BJH) model. Fourier
transform Infrared spectra (FTꢀIR) were also obtained using a
Nicolet 6700 spectrometer and the samples were prepared using
the potassium bromide (KBr) pellet technique. The thermal
decomposition characteristics of the samples were carried out
benzylidenemalononitrile product after the absorption of one
proton and loss of water (steps III and IV).
Experimental
Preparation of H6TATAT ligand.
H6TATAT (H6TATAT = 5,5',5''ꢀ(1,3,5ꢀtriazineꢀ2,4,6ꢀtriyl)tris
(azanediyl)triisophthalate) was synthesized according to
literature procedures.19 In a typical process, a 250ꢀmL threeꢀ
necked round bottom flask was charged with 7.6 g of 5ꢀ
aminoisophthalic acid (0.042 mmol), 2.68 g of NaOH (0.067
mmol) and 4.37 g of NaHCO3 (0.052 mmol). Then 70 mL of
H2O was added and the mix was vigorously stirred at 0 oC for 1
h. Then, a mixture of 1.84 g of cyanuric chloride (0.01 mmol)
in 35 mL of 1,4ꢀdioxane was added dropwise, followed by
using thermogravimetric analysis (TG) by
a
Netzsch
o
STA449F3 instrument at a heating rate of 10 C/min, under a
dry N2 flow. The element content of the samples was tested by
inductively coupled plasmaꢀatomic emission spectrometry
(ICPꢀAES, 715ꢀES). The chemical composition of the products
was analyzed by Xꢀray photoelectron spectrometry (XPS,
ESCALAB 250Xi), and the catalytic products were analyzed by
o
heating to, and maintaining at 100 C for 24 h. The resulting
mixture was acidified to pH=2 with a hydrochloric acid
solution. The product was collected by filtration and washed
with distilled water 3 times. Finally, the H6TATAT was dried
under vacuum at 40 oC for 24 h.
Gas
ChromatographyꢀMass
Spectrometry
(Agilent
Preparation of Cu3TATAT MOFs.
7890A/5975CꢀGC/MSD) with dodecane as an internal
standard.
In
a typical process, Cu3TATATꢀ1 was synthesized by
completely dissolving Cu(NO3)2 (0.164 g, 0.68 mmol) in a
mixture of DMA (2.0 mL), DMSO (2.0 mL), H2O (10 ꢁL) and
HBF4 (0.9 mL). Subsequently, H6TATAT (0.030 g, 0.049
mmol) was added. The mixture was transferred to a small vial
Conclusions
In summary, a general copper/amine bifunctional Cu3TATAT
MOF catalyst was developed for an efficient oneꢀpot
multicomponent reaction. An aerobic oxidation/Knoevenagel
condensation reaction sequence was optimized in presence of
the bifunctional Cu3TATAT catalyst. Four different
morphologies and sizes were achieved employing organic
additives and their corresponding catalytic activities were
studied in detail. Furthermore, the aromatic alcohol oxidation
and Knoevenagel condensation of the resultant aromatic
aldehyde with malononitrile were explored in a oneꢀpot
sequential reaction and it was found that various aromatic
alcohols are tolerated under the optimized reaction conditions.
The copper site was found to be responsible for the alcohol
oxidation and the organic amino functional group was found to
serve as a base in both, the aerobic oxidation and the
condensation reaction. The catalytic reactivity was further
o
and stirred at 90 C for 3 days. The blue crystal was collected
by filtration and washed with DMA 3 times. Finally, the
o
obtained product was dried under vacuum at 40 C for 24 h. In
order to tune the diameter and morphology of Cu3TATAT,
dodecanoic acid, benzoic acid and Pluronic® F127 were
employed as additives.
In
a typical process, Cu3TATATꢀ2 was synthesized by
completely dissolving dodecanoic acid (0.060 g, 0.3 mmol) and
Cu(NO3)2 (0.164 g, 0.68 mmol) in a mixture of DMA (2 mL),
DMSO (2 mL), H2O (10 ꢁL) and HBF4 (0.9 mL). Then,
H6TATAT (0.030 g, 0.049 mmol) was added. The mixture was
o
transferred to a small vial and heated at 90 C for 3 days. The
blue crystal was collected by filtration and washed with DMA 3
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Journal Name
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showing no significant loss of activity in the oneꢀpot
oxidation/Knoevenagel condensation reaction.
5
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We thank the National Natural Science Foundation of China
(No. 51503016), Fundamental Research Funds for the Central
Universities FRF-TP-15-001A2 and Key Laboratory of
Photochemical Conversion and Optoelectronic Materials,
TIPC, CAS. The authors gratefully acknowledge Kathryn E.
Summo for editorial reviews of the manuscript.
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8 | J. Name., 2012, 00, 1-3
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Table of Content
A novel one-pot aerobic oxidation/Knoevenagel condensation reaction system is developed
employing a Cu(II)/basic bifunctional metal-organic framework (MOF) as the catalyst. The
sequential aerobic alcohol oxidation/Knoevenagel condensation reaction can be efficiently
promoted by the Cu3TATAT MOF catalyst in the absence of base additives. The
benzylidenemalononitrile product was produced in high yield and selectivity from an
inexpensive benzyl alcohol starting material under oxygen atmosphere. The role of the basic
functionality has been systematically studied to demonstrate its utility in the aerobic oxidation
and Knoevenagel condensation reactions. The reaction progress has been monitored in order to
reveal the reaction intermediate and accumulation of the desired product. Lastly, this newly
developed catalytic system shows excellent recycling performance without significant
compromise of the reaction yield after five runs.