Aerobic homocoupling of arylboronic acids catalysed by copper terephthalate metal-organic frameworks

Article abstract of DOI:10.1039/c4gc00056k

Copper terephthalate MOF is utilised as an environmentally benign, efficient and reusable heterogeneous catalyst to effect the aerobic homocoupling of arylboronic acids yielding the corresponding symmetrical biphenyls under mild reaction conditions. This method tolerates various substituents present in arylboronic acids such as halogens, cyano and nitro groups. The catalytic performance has been compared with that of other copper based MOFs namely MOF-101, [Cu(pdc)₂]NH₂Me₂, [Cu ₂(ndc)₂ted]_n and [Cu(H₂L)] _n as well as with other copper salt catalysts. Sheldon test confirmed the heterogeneity of the catalyst, which can be reused under optimized conditions with only a minor loss in its activity. A mechanism for the homocoupling reaction is also proposed. The simplicity of catalyst preparation, its stability, substrate selectivity, easy recovery and regeneration designate possible utilization of this catalytic system in a multitude of catalytic reactions and industrial processes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00056k

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Aerobic homocoupling of arylboronic acids
catalysed by copper terephthalate metal–organic
frameworks†
Cite this: Green Chem., 2014, 16,
865
2
a
a,b
a,b
Pillaiyar Puthiaraj, Palaniswamy Suresh and Kasi Pitchumani*
Copper terephthalate MOF is utilised as an environmentally benign, efficient and reusable heterogeneous
catalyst to effect the aerobic homocoupling of arylboronic acids yielding the corresponding symmetrical
biphenyls under mild reaction conditions. This method tolerates various substituents present in arylboro-
nic acids such as halogens, cyano and nitro groups. The catalytic performance has been compared with
2 2 2 2 2 n 2 n
that of other copper based MOFs namely MOF-101, [Cu(pdc) ]NH Me , [Cu (ndc) ted] and [Cu(H L)] as
well as with other copper salt catalysts. Sheldon test confirmed the heterogeneity of the catalyst, which
can be reused under optimized conditions with only a minor loss in its activity. A mechanism for the
homocoupling reaction is also proposed. The simplicity of catalyst preparation, its stability, substrate
selectivity, easy recovery and regeneration designate possible utilization of this catalytic system in a multi-
tude of catalytic reactions and industrial processes.
Received 12th January 2014,
Accepted 24th February 2014
DOI: 10.1039/c4gc00056k
www.rsc.org/greenchem
zeolites, these MOFs have the potential for a more flexible
rational design through control of the architecture,
functionalization of the pores and substantially higher metal
Introduction
Metal–organic frameworks (MOFs) have emerged as a hot
topic in heterogeneous catalysis. They are composed of metal
ions as nodes and organic ligands as linkers, creating infinite
polymeric frameworks with regular void spaces that can
accommodate small gas and solvent molecules in their pores.
Over the past decade, chemists have focused only on design
and synthesis of new MOFs and studying their role in gas
storage, separations and sensors. MOFs have recently
emerged as a particular class of functional materials owing to
their high inner surface area, tenability of pore size, chemical
tenability and topologies. MOFs contain a large percentage of
loading, which offers the opportunity to significantly reduce
1
the overall amount of catalyst, provided that the internal metal
7
centres are accessible to the substrates.
2
2
Aryl C–C (sp –sp ) bond formation is an important and
challenging process in both synthetic and industrial points of
view. Symmetrical and unsymmetrical biaryls are important
structural motifs exhibiting a wide variety of physical and
2
3
8
chemical properties with versatile applications in drugs, agro-
9
chemicals, dyes, semi-conductor and optically active ligands.
These couplings have been achieved by palladium catalysed
processes such as Suzuki reaction, modified Ullmann reaction
4
5
transition metals that can act as catalytic sites provided that
1
0
and Hiyama–Kumada reaction. Particularly, palladium cata-
they have free coordination positions not compromised in the
construction of MOFs. These materials are expected to have
many similarities from the structural point of view with zeo-
lites and related microporous solids. Consequently MOFs are
attractive catalysts for organic transformations, though,
however, only a relatively limited number of studies have
focussed on the investigation of the catalytic activity of these
lysed homocoupling of boronic acids has been extensively
1
1
reported. However there are limitations in these palladium
catalysed methods: (1) palladium is expensive and additional
1
1c,i,j
ligands are needed to stabilize the palladium species,
(2)
stoichiometric amount of oxidants are needed to restore the
1
1g,h
catalytically active palladium(II) species,
to improve the yields,
(3) require a base
1
1c,i, j
11h
and (4) need of high temperature.
1
a,c,6
MOFs.
Another advantage is, compared to conventionally
Consequently many research groups have reported the homo-
used microporous and mesoporous inorganic materials like
1
2
coupling of boronic acids also by other metals. The synthetic
use of boronic acids are more common as they are more stable
1
3
and less toxic than other organometallic reagents. Generally
a
14
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
homocoupling of arylboronic acids is also very slow. In con-
b
Centre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj
15
cordance with the principles of green chemistry, air is an
University, Madurai 625021, India. E-mail: pit12399@yahoo.com
ideal oxidant because of its abundance, low cost, safety, lack of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4gc00056k
1
6
1
toxic by-products and it is cheap for industrial applications.
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Recently, Singh et al. reported the clay encapsulated Cu(OH)
x
Initially phenylboronic acid was selected as a model sub-
1
7a
catalysed homocoupling of arylboronic acids,
Xiao et al. strate in the presence of different catalysts under various con-
reported the Mg–Al oxides stabilised gold nanoparticles cata- ditions with the aim of optimizing the yield and the results are
1
7b
lysed homocoupling reaction and Sakurai et al. reported the summarized in Table 1. All reactions were conducted at room
gold nanoparticles catalysed homocoupling of arylboronic temperature (RT) in air. No product was detected either in the
1
7c
acids under acidic conditions. Fe
3 4
O nanoparticle-supported absence of catalyst or in an inert atmosphere (Table 1, entries
Cu(II)-β-cyclodextrin complex is also employed as a reusable 1 and 32). When the reaction was conducted using Cu(BDC)
1
8
catalyst for homocoupling of arylboronic acids. However, pre- MOF in DMSO, a 75% yield of homocoupling product
vious investigations of copper-mediated coupling reactions for (Table 1, entry 2) was observed. When the reaction was con-
2 4 2
the synthesis of biaryls required a stoichiometric amount of ducted using other copper sources like Cu(OAc) , CuSO ·5H O
1
2
catalyst and ligand, and a higher reaction time. Although and CuCl respectively (Table 1, entries 3–5) a lower yield of
2
homogeneous catalysts have been extensively investigated for homocoupling products was observed (42, 18 and 31%). When
coupling reactions, the challenges are very significant as the the amount of Cu(OAc)
catalysts are expensive, metal contamination in the reaction reduced to 50 mol%, the homocoupled product was also
mixture, cannot be reused and the products are difficult to sep- reduced to 34, 12, 22%. When Cu(NO ·3H O was used as
2 4 2 2
, CuSO ·5H O and CuCl catalysts was
3
)
2
2
arate from the reaction mixture, chances of undesired product catalyst, no product was observed but in the presence of ter-
formation due to the requirement of ligands and additional ephthalic acid as an additive, a very low yield was noticed
1
9
reagents.
In order to overcome these problems, MOFs (Table 1, entries 6 and 7). This observation of a significantly
provide unique advantages as these heterogeneous materials decreased yield with other Cu(II) sources, eliminates the possi-
are easily prepared and are cheap. Recently, Yaghi et al. bility of a Cu(0)/Cu(II) cycle in the present study as the major
3 2
reported the Cu (BTC) catalysed homocoupling of arylboronic reaction pathway. These results indicate that the catalytic
2
0
acids with external cyclohexylamine base and observed the centres might be ligated by the terephthalic acid, which
arylated Chan–Lam cross coupling by-products. Corma et al. cannot be easily approached by the substrates. When the reac-
reported the Cu(BDC) MOF as catalyst for three-component tion was conducted using Cu(BDC) MOF in DMF, an excellent
2
1
couplings of amines, aldehydes and alkynes. Recently Phan yield of homocoupling product (Table 1, entry 8) was obtained.
et al. reported the use of Cu(BDC) MOF as catalyst for the One of the main reasons for the high catalytic activity of
2
2
modified Friedlander reaction. To the best of our knowledge, copper terephthalate MOF is that it is a porous material and
2
3
there are no reports describing the copper terephthalate MOF has a high surface area, which is favourable for the accessi-
catalysed homocoupling of arylboronic acids. bility of reactants to the active metal sites of MOF. To gain an
In this study, we have demonstrated copper terephthalate insight into the role of the substituents, the presence of
Cu(BDC) MOF as an efficient heterogeneous reusable catalyst heteroatoms and the size of the linkers, the following four
towards the aerobic homocoupling of arylboronic acids additional MOFs namely MOF-101, [Cu(pdc) ]NH Me ,
2
2
2
2 2 n 2 n
without losing the crystallinity and structure of the Cu(BDC) [Cu (ndc) ted] and [Cu(H L)] , were synthesised (see ESI† for
MOF (Scheme 1). The Cu(BDC) catalysed protocol offers their structure and characterization) and their catalytic activity
several advantages, compared to the conventional approach in in the homocoupling reaction was studied (Table 1, entries
the formation of biphenyl derivatives and the observed results 9–12). With an ortho-substituent namely bromine as in
are discussed below.
MOF-101, a small decrease in yield was noticed (71%), which
may be attributed to steric effects. When a heteroatom is
present in the linker as in [Cu(pdc) ]NH Me MOF, the
2 2 2
decrease in the yield of homocoupled product was significant,
probably due to the presence of an alternative binding site.
Results and discussion
With more bulkier and flexible linkers as in [Cu
and [Cu(H L)] , and the presence of more heteroatoms as in
Cu (ndc) ted] and [Cu(H L)] , the homocoupling reaction
did not proceed at all, indicating clearly that steric effects and
the presence of heteroatoms play a very significant role in the
above reaction. It is also likely that in these cases, the linkers
themselves bind strongly to the central copper metal atom
thereby affecting the overall reaction. When the reaction was
2 2 n
(ndc) ted]
Herein, we report the heterogeneous copper terephthalate
MOF catalysed homocoupling of arylboronic acids in air at
room temperature. To the best of our knowledge, the MOF
catalysed aerobic homocoupling of arylboronic acids has not
been reported so far. In addition, Cu(BDC) MOF is shown as
an efficient and environmentally-friendly heterogeneous, reu-
sable catalyst. The various factors determining the reactivity of
boronic acids under different experimental conditions are dis-
cussed below.
2
n
[
2
2
n
2
n
also conducted with MOF-101, [Cu(pdc) ]NH Me , [Cu (ndc) -
2
2
2
2
2
n 2 n
ted] and [Cu(H L)] , MOFs containing an identical amount of
copper species as Cu(BDC), the homocoupled product was
obtained in only 76 and 62%, and there was no reaction with
the other two MOFs (Table 1, entries 13–16). Interestingly,
Cu(BDC) MOF showed no catalytic activity in non-polar sol-
vents, namely toluene, xylene, DCM, DCE, dioxane, THF and
Scheme 1 Homocoupling of arylboronic acids.
2866 | Green Chem., 2014, 16, 2865–2875
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a
Table 1 Optimization of reaction conditions
b
Yield [%]
Entry
Catalyst
Additives
Solvent
Time
Biphenyl
Phenol
1
2
—
Cu(BDC)
Cu(OAc)
CuSO ·5H
CuCl
Cu(NO
Cu(NO
Cu(BDC)
MOF-101
[Cu(pdc)
[Cu (ndc)
[Cu(H L)]
MOF-101
[Cu(pdc)
[Cu
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
DMF
DMF
DMF
DMF
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
—
75
—
—
—
Traces
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
74
29
38
—
—
—
—
—
12
c
i
2
42 (34)i
c
4
2
O
18 (12)i
2
31 (22)
3
)
)
2
·3H
·3H
2
O
O
—
10
97
71
60
—
—
76
3
2
2
Terephthalic acid
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
2
]NH
ted]
2
Me
2
2
2
2
n
2
n
d
e
f
DMF
DMF
DMF
DMF
2
]NH
ted]
2
Me
62
—
—
2
(ndc)
L)]
2
n
g
[Cu(H
2
n
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
Cu(BDC)
DMF
16, 14, 12, 10
97, 82, 68, 48
Toluene
Xylene
DCM
DCE
Dioxane
THF
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
—
—
—
—
—
—
—
8
26
42
49
37
22
62
—
68
CHCl
3
2
H O
Methanol
Ethanol
DMF
DMF
DMF
DMF
DMF
2
DMF–H O
Na
CO
Cs CO
NEt
2
CO
3
3
K
2
2
3
3
h
a
b
c
d
Reaction conditions: phenylboronic acid (2 mmol), catalyst (100 mg), solvent (2 mL), RT. Isolated yield. 100 mol% of catalyst. 155 mg of
e
f
g
h
i
2
catalyst. 130 mg of catalyst. 105 mg of catalyst. 120 mg of catalyst. Nitrogen atmosphere. 50 mol% of catalyst; H L = (Z)-4-((2-
hydroxynaphthalen-1-yl) methyleneamino)benzene-1,3-dioic acid; ndc = 1,4-naphthalenedicarboxylic acid; ted = triethylenediamine; BDC = 1,4-
benzenedicarboxylic acid.
CHCl3 for the homocoupling reaction of arylboronic acid
The biaryl yield was also found to increase rapidly with an
(Table 1, entries 18–24). As the aryl substrates are more soluble increase in the amount of catalyst. As the amount of catalyst
in nonpolar solvents, their binding to Cu(BDC) MOF is very was increased to 100 mg, biphenyl was obtained in highest
slow, resulting in the absence of homocoupling. When the yield and the further addition of catalyst had no obvious
reaction was conducted in protic polar solvents like water, effects on the yield of biphenyl (Fig. 1). Thus the optimized
methanol and ethanol, the biphenyl product was obtained in conditions for the homocoupling of arylboronic acid are use of
only 8, 26 and 42% yields respectively, along with consider- Cu(BDC) MOF as a catalyst in DMF under air for 16 h at room
able amounts of phenol (Table 1, entries 25–27). When protic temperature.
solvents were used, the conversion of the C–B bond into a C–O
This Cu(BDC) MOF promoted aerobic homocoupling reac-
bond via a peroxyboronate intermediate was promoted result- tion was also successfully extended to various substituted aryl-
1
7c
ing in a considerable amount of phenol formation. Gener- boronic acids. As depicted in Table 2, this reaction worked very
ally, as the base can improve the reaction yield for some well for a wide range of substrates with both electron-donating
2
4
catalytic systems, we have used different bases for this reac- and electron-withdrawing substituents. It can be concluded
tion and only 20–60% yields were observed (Table 1, entries that the nature of the substituent, either electron-donating or
2
8–31). When carried out in DMF–water (1 : 1) mixture, 68% withdrawing, did not show any significant change in the
yield of the biphenyl and 12% of phenol were observed overall yield. Both para- and meta-substituted arylboronic acids
(Table 1, entry 33).
gave homocoupling products in excellent yields (Table 2,
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1-boronic acid, 2,6-difluorophenylboronic acid and 2,4,6-tri-
methylphenylboronic acid did not give the corresponding
homocoupling products (Table 2, entries 22–24) as the steri-
cally hindered boronic acids did not bind with MOF metal
centers. The biphenyl and their derivatives were characterised
by m.p. and NMR techniques.
To verify whether catalysis by Cu(BDC) MOF is truly hetero-
geneous or due to some leached copper species present in the
filtrate, the Sheldon test was performed. The reaction was
carried out under the optimized conditions and the copper ter-
ephthalate MOF catalyst was filtered from the reaction mixture
at 48% of biphenyl formation (Table 3). After removal of MOF
catalyst, the filtrate was further stirred for an additional 6 h
and no further biphenyl product was observed. The absence of
metal leaching was also confirmed from atomic absorption
spectroscopy analyses of the filtrate from the reaction mixture
and also of the filtrate from a stirred solution of Cu(BDC) in
DMF under identical reaction conditions. Thus the atomic
absorption spectroscopic data clearly demonstrate that
Cu(BDC) MOF is truly heterogeneous in nature.
Fig. 1 Dependence of yield on the amount of catalyst for homo-
coupling of phenylboronic acid.
Table 2 Cu(BDC) MOF catalysed homocoupling of various arylboronic
a
acids
Reusability of the Cu(BDC) MOF was also studied. After
completion of the reaction, the catalyst was recovered by fil-
tration and washed with ethyl acetate, heated with 2 mL of
fresh DMF at 100 °C for 2 h, activated under vacuum at room
temperature for 4 h, which was subsequently reused and the
results are presented in Fig. 2. It is clear that the decrease in
b
Boronic
Entry acid (R)
Yield
(%)
Products
1
2
3
4
5
6
7
4-CH
4-OCH
4-F–C
4-Br–C
4-Cl–C
4-CN–C
4-OCF –C
3
–C
6
H
H –
6 4
–
4
–
4
–
4,4′-Dimethylbiphenyl
4,4′-Dimethoxybiphenyl
4,4′-Difluorobiphenyl
4,4′-Dibromobiphenyl
4,4′-Dichlorobiphenyl
4,4′-Dicyanobiphenyl
4,4-Di(trifluoromethoxy)-
biphenyl
95
96
94
93
87
89
95
3
–C
6
H
4
6
H
6 4
H –
6
4
H –
3
6
H
4
–
Table 3 Sheldon test
a
8
4-CF –C
3 6
H
4
2 2
–C H –
1,1′-(1E,3E)-1,3-Butadiene-
92
Yield (%)
1,4-diylbis[4-(trifluoromethyl)-
benzene
Catalyst
10 h
48
(10 + 6) h
48
Reused catalyst
97
9
1
1
1
1
1
1
1
4-C
3-Cl–C
3-NO –C
3-OCH
3-CH –C
3,4-(CH
3,4-F –C
3-Cl-4-F–C
2
H
5
–C
6
H
–
4
–
4,4′-Diethylbiphenyl
3,3′-Dichlorobiphenyl
3,3′-Dinitrobiphenyl
3,3′-Dimethoxybiphenyl
3,3′-Dimethylbiphenyl
3,3′,4,4′-Tetramethylbiphenyl
3,3′,4,4′-Tetrafluorobiphenyl
3,3′-Dichloro-4,4′-
difluorobiphenyl
2,2′-Dibromobiphenyl
2,2′-Dimethylbiphenyl
2,2′-Dimethoxy-5,5′-
fluorobiphenyl
88
90
98
89
96
93
98
97
0
1
2
3
4
5
6
H
6 4
Cu(BDC)
2
6
H –
4
3
–C
6
H
4
–
a
Reaction conditions: phenylboronic acid (2 mmol), Cu(BDC)
100 mg), DMF (2 mL), RT.
3
6 4
H –
) –C H –
3 2 6 3
H –
6 3
(
2
6 3
H –
17
18
19
2-Br–C
2-CH –C
6 3
2-OCH -5-F–C H –
6 4
H –
49
52
35
3
6 4
H –
3
20
21
22
23
4-Pyridinyl-
3-F-4-pyridinyl–
1-Naphthyl-
4,4′-Bipyridyl
3,3′-Difluoro-4,4′-bipyridyl
—
62
55
—
—
2
2,5-F -4-OCH
3
–
—
6 2
C H –
24
2,4,6-(CH ) –C
3 3 6
2
H –
—
—
a
Reaction conditions: arylboronic acid (2 mmol), Cu(BDC) MOF
b
(100 mg), DMF (2 mL), RT, 16 h. Isolated yield.
entries 1–16). But in the case of ortho-substituted arylboronic
acids only lower yields (Table 2, entries 17–19) are observed.
The hetero arylboronic acids also gave only moderate yields
(Table 2, entries 20 and 21), as the heteroatom is likely to bind
with the metal centre. Bulkier substrates such as naphthalene- Fig. 2 Reusability of Cu(BDC) catalyst experiments.
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transmetallation of Cu(II) with two molecules of arylboronic
acid affords a Ar–Cu(II)–Ar, which undergoes air oxidation to
yield a Cu(III) intermediate, reductive elimination of which
releases the homocoupled product Ar–Ar. An alternative
pathway involving a Cu(0)/Cu(II) cycle is ruled out on the basis
of the control experiments discussed earlier.
With a rigid framework as in the present MOF, a bimetallic
mechanism is also likely to yield the homocoupled product.
In the IR spectrum of the copper containing MOF,
recorded with the reaction intermediate, the DMF carbonyl
stretching frequency 1663 cm− had disappeared and the
B–OH band shift from 3480 to 3442 cm− peak was observed
1
1
(
3
Fig. 4). In the original catalyst, there is no peak around
442 cm− (Fig. 5b).
1
Data on reaction conditions, activity and efficiency of the
various other catalysts employed earlier for the homocoupling
of arylboronic acids are given in Table 4. Comparison of the
results indicates that our catalytic system (entry 27) exhibits
better catalytic activity compared to conventional catalysts
such as Cu, Pd, Fe and Au. These systems require additional
base (entries 1, 3, 10, 11–17, 20 and 24–26), external additives
Fig. 3 Powder XRD patterns of fresh and reused catalyst.
the activity of the catalyst resulted in a minor loss of product (entries 3, 5, 9, 11, 12, 15 and 25), additional oxidants (entries
yield even after five times of its reuse.
3, 5, 9, 11, 12, 15, 25 and 26) and higher reaction time (entries
Comparison of the powder XRD patterns of the fresh, three 2, 5, 6, 11, 20, 22, 24, 26 and 25).
and five times reused MOF were also recorded (Fig. 3) which
clearly show that the reused catalyst exhibits a similar powder
XRD pattern. The specific surface area and average pore
volume for the fresh and reused Cu(BDC) MOF were measured
using BET surface area analysis. The BET isotherms show a
Experimental section
General methods
2
−1
2
−1
small decrease in the surface area (590 m g and 564 m g
for fresh Cu(BDC) and fifth time used Cu(BDC) respectively).
3
−1
The pore volume of the fresh and reused MOF is 0.268 cm g
All reactions were carried out under aerobic conditions. All
3
−1
and 0.264 cm g from the BET analysis. These BET surface chemicals were used without further purification as commer-
area analyses and powder XRD pattern results clearly demon- cially available unless otherwise noted. NMR spectra were
strate that no appreciable change in the structural integrity is recorded at 500, 400 and 300 MHz (mentioned in respective
1
observed during/after the reuse.
NMR data itself) on a Bruker spectrometer. All H NMR and
1
3
To demonstrate the potential utility of this method for pre-
C NMR spectra were measured in CDCl3 with TMS as the
parative purposes, the reaction was also carried out in the opti- internal standard. The powder XRD pattern of the catalyst
mized reaction conditions on a 1.2 gram scale, giving 94% sample was measured with a SHIMADZU XD-D1 and X’Pert
yields which are comparable to those obtained for a small- PANalytical Diffractometer instrument using a Cu Kα radiation
scale reaction (reaction condition: phenylboronic acid, 10 mL at room temperature. Brunauer Emmett Teller (BET) surface
DMF, RT, 16 h).
By analogy with these reports and previous report,
area analysis was carried out using the Quantachrome-Auto-
sorb at 77 K. Elemental analysis with atomic absorption spec-
1
2a–d,18,20,34
a plausible reaction pathway is proposed as shown in trophotometry (AAS) was performed on Varian AA 240. IR
Scheme 2. The copper terephthalate MOF shows a larger spectral analyses were performed using a JASCO FT/IR-410
surface area and the presence of the copper atom and its instrument by the KBr pellet technique in the range of
exposed apical coordination sites might also lead to catalytic 4000–500 cm− . Thermogravimetric analysis (TGA) was per-
1
2
3
applications. Terephthalate ligands are coordinated in a formed with a TGA/Shimadzu Thermal Analyser under a nitro-
bidentate bridging fashion to Cu(II) dimer, separated vertically gen atmosphere in the temperature range 50–570 °C in a flow
by 2.63 Å. Each Cu(II) is also coordinated to a molecule of DMF of nitrogen at a heating rate of 10 °C min− . Electrospray
to give the Cu(II) atom a square-planar geometry. The catalyti- ionization mass spectrometry (ESI-MS) analyses were recorded
cally active copper present in the channel walls of MOF can in LCQ Fleet, Thermo Fisher Instruments Limited, US. ESI-MS
promote the homocoupling reaction of various arylboronic was performed in negative ion mode. The collision voltage and
acids. During the homocoupling reaction, in the first step the ionization voltage were −70 V and −4.5 kV, respectively, using
solvent molecule may be replaced by the arylboronic acid. The nitrogen as atomization and desolvation gas. The desolvation
proposed mechanism involves a Cu(I)/Cu(III) cycle. Double temperature was set at 300 °C.
1
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Scheme 2 Plausible mechanism for the homocoupling of arylboronic acid.
2870 | Green Chem., 2014, 16, 2865–2875
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MCM-41 where broader peaks were normally observed on their
2
5
diffractograms. Elemental analysis by atomic absorption
spectroscopy (AAS) indicated a copper loading of 3.87 mmol
−
1
g
. The FT-IR spectra of the Cu(BDC) MOF exhibited the pres-
−
1
ence of a strong peak at 1603 cm , which was lower than the
value for CvO stretching vibration observed in free carboxylic
acids (1760–1690 cm− ) (Fig. 5a and b). This strong peak was
due to the stretching vibration of carboxylate anions present in
1
−
1
the material. After removal of DMF, the 1603 cm peak was
−
1
shifted to 1576 cm and the DMF carbonyl frequency peak
1665 cm− ) disappeared (Fig. 5c). Thermogravimetric analysis
1
(
(
TGA) was carried out to examine the stability of the frame-
work. For the as-synthesised samples, the TGA of Cu(BDC)
showed a loss of ligated form of DMF molecules in the temp-
erature range 160–260 °C. After 260 °C, the phase remains
completely changed, which was stable upto over 370 °C
(
Fig. 6). The overall powder XRD patterns, IR spectrum and the
Fig. 4 FT-IR spectra of phenylboronic acid (a) and reaction intermedi-
ate (b).
thermal stability of the copper terephthalate MOF were in
good agreement with the literature.
2
3
The Cu(BDC) MOF was dissolved in HNO , diluted and ana-
3
lysed with AAS and the results show that the copper concen-
−1
tration in Cu(BDC) MOF was found to be 3.87 mmol g . To
see if there was any metal leaching, the following control
experiment was performed. Cu(BDC) MOF (0.100 g) was added
to a mixture of phenylboronic acid (2 mmol), in DMF (2 mL)
and stirred at RT for 16 h. After completion of reaction, the
reaction mixture was filtered. The filtrate and reused Cu(BDC)
3
MOF (dissolved in HNO ) were analysed by AAS. The copper
concentration in the filtrate was found to be BDL (below
detectable limit). In the reused Cu(BDC) MOF (dissolved in
HNO
3
), the copper concentration was found to be 3.86 mmol
−
1
g
. This result shows that no leaching of copper has taken
place from the solid to the liquid phase. This proves that the
catalyst is heterogeneous in nature.
Fig. 5 FT-IR spectra of 1,4-benzenedicarboxylic acid (a), Cu(BDC) (b)
and Cu(BDC) after removal of DMF (c).
Synthesis of MOF-101
2
-Bromoterephthalic acid was prepared according to the pub-
3
8
Synthesis of copper terephthalate MOF [Cu(BDC)]
lished procedure. The prepared 2-bromoterephthalic acid
was characterized by ESI-MS spectrum (ESI-MS: m/z calcd for
C H BrO : 243.94; found, 243.00 (M − H)). MOF-101 was pre-
Cu(BDC) MOF was prepared according to the procedure
2
3
8
5
4
reported by Tannenbaum et al. Equimolar quantities of
3
9
pared according to the published procedure. An equimolar
copper nitrate trihydrate (15 mmol) and terephthalic acid
amount of 2-bromoterephthalic acid and Cu(NO ·2.5H O in
3
)
2
2
(15 mmol) in 300 mL DMF were used. This solution was
DMF in a capped vial at room temperature gave blue crystals of
the MOF-101. The prepared MOF-101 was characterized by
FT-IR and powder XRD techniques (see ESI†). The FT-IR
spectra of the MOF-101 exhibited the presence of a strong
peak at 1623 cm− , which was lower than the value for the
placed in a closed flask in an oven at 110 °C for 36 h. It was
observed the blue crystals precipitated in a yield of 75% which
is comparable to that of the reported procedure.
2
3
1
Characterization of copper terephthalate MOF [Cu(BDC)]
CvO stretching vibration observed in free carboxylic acids
The Cu(BDC) MOF was characterized using a variety of 1704 cm− . This strong peak was due to the stretching
1
different techniques. A very sharp peak below 15° (with 2θ of vibration of carboxylate anions present in the material. The
1
0.27) was observed on the powder X-ray diffractogram of the DMF solvent carbonyl stretching frequency band appears to be
Cu(BDC) MOF, indicating that a highly crystalline material 1666 cm− for a molecule that is coordinated to the copper
1
(
Fig. 3). Furthermore, the powder XRD patterns of the metal centre. These powder XRD pattern and FT-IR spectra
3
9
Cu(BDC) MOF exhibited a better crystallinity as compared to were in good agreement with that reported. Elemental analy-
that of silica-based materials such as SBA-15, SBA-16 and sis by AAS indicated a copper loading of 2.48 mmol g
−
1
.
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Green Chemistry
a
Table 4 Comparison with reported catalytic systems for homocoupling of arylboronic acids
Temp.
(°C)
Time
(h)
Yield
(%)
Entry
Catalyst
Additive
Base
CO
Solvent
Oxidant
References
1
2
Au/CeO
Pd/C
2
—
—
K
2
3
Toluene
—
Air
60
75
15
1
100
94
12h
26
—
2
H O–2-propanol
(
9 : 1)
i
3
Pd(OAc)
2
NaClO
4
Pr
2
NEt
ACN–H
2
O (7/1)
TEMPO
Air
Electro
oxidation
70
100
90
RT-90
50
RT
—
88
11e
4
5
6
7
8
9
3 4
Fe O –Cu
2
-β-CD
—
PS-NMe
—
—
—
—
—
—
—
—
—
DMF
Water
DMF
DMF
DMF
2-Propanol
n-Propanol–H
(1/2)
24
6
2
14
1
4
90
90
83
83
80
83
99
18
27
28
12c
12b
12a
11i
PdCl
2
2
CO
Air
Air
Air
Air
Air
, O
2 2
Pd(OAc)
Cu -β-CD
CuSO /MS
Cu(OAc)
2
: 2PPh
3
2
4
1,10-Phenanthroline
—
10
Cyclopalladated
ferrocenylimine
PdNPs/Te-DPs
K
3
PO
4
·7H
2
O
2
O
RT
12
11
Tris-HCl buffer,
—
H
2
O
Air
100
24
90
11f
pH 8.9
[Bmim]PF
—
—
1
1
1
2
3
4
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
6
K
K
K
2
2
2
CO
CO
CO
3
H
2
O
—
Air
TEMPO
60
RT
Electro
oxidation
RT
3
15
—
92
91
86
29
30
11g
3
3
Acetone–H
ACN–H
2
O
2
O
15
PdCl
2
p-Toluenesulfonyl
Na
2
CO
3
H
2
O
N
2
12
95
31
chloride
—
—
—
—
—
—
—
16
17
18
19
20
21
22
23
24
25
26
27
AuCl
AuNPs : L
CuCl
Au/chitosan
Au-PEG
Cu(OAc)
Au/MAO
Au-CNPs
Au-(MCM-41)
Pd(OAc)
K
K
—
—
NaOH
—
—
—
K
Na
K
2
CO
CO
3
EtOH
Air
Air
Air
Air
—
50
24
24
4
76
86
93
96
60
78
90
86
99
99
97
97
32
33
12d
17c
35
34
17b
36
2
3
2
H O
RT
RT
30
MeOH
2
H O
7
EtOH–H
DMF
MeOH
Toluene–H
Xylene
DMF
PEG-400
DMF
2
O
80
25
24
12
7
24
2.1
48
16
2
O
O
2
RT
100
70
130
80
140
RT
2
—
—
2
O
Air
—
—
Air
Air
2
CO
3
12g
11h
37
2
p-Benzoquinone
2
SO
4
—
Cu(BDC)
I
2
2
CO
3
—
—
Present work
a
ACN = acetonitrile, TEMPO = 2,2,6,6-tetramethylpiperidinyloxy, CD = cyclodextrin, PPh
= butylmethylimidazolium hexafluorophosphate, L = m-ferrocenyl
benzoic acid, PEG = polyethylene glycol, MAO = Mg–Al mixed oxides, CNPs = carbon nanoparticles.
3
= triphenylphosphine, PS = polystyrene, MS = molecular
sieves, Te = Thermosynechococcus elongatus, DPs = DPs protein, [Bmim]PF
6
Synthesis of copper pyridine-2,5-dicarboxylate MOF
[Cu(pdc) ]NH NMe
Cu(pdc) ]NH NMe was prepared according to the previously
(
2
2
2
)
[
2
2
2
4
0
reported procedure and the prepared MOF was characterized
by FT-IR and powder XRD techniques (see ESI†). The FT-IR
spectra of the [Cu(pdc) ]NH NMe exhibited the presence of a
2
2
2
strong peak at 1652 cm− , which was lower than the value for
1
CvO stretching vibration observed in free carboxylic acids
732 cm− . This strong peak was due to the stretching
1
1
vibration of carboxylate anions present in the material. The
carboxylate ion is coordinated to the copper metal centre.
These characterization results were in good agreement with
4
0
reported crystal powder XRD simulated patterns. Elemental
−
1
analysis by AAS indicated a copper loading of 2.92 mmol g
.
Synthesis of [Cu (ndc) ted] MOF
2
2
n
[
Cu (1,4-naphthalenedicarboxylate) ted] was prepared accord-
2 2 n
4
1
ing to the reported procedure. The prepared MOF was
characterized by FT-IR and powder XRD techniques (see ESI†).
The prepared [Cu (ndc) ted] was characterized by FT-IR and
Fig. 6 TGA of Copper terephthalate MOF.
2
2
n
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powder XRD techniques (see ESI†). The FT-IR spectra of the advantages: low cost, higher stability, substrate selectivity,
[
1
Cu (ndc) ted] exhibited the presence of a strong peak at easily separated and recyclability. The leaching test and re-
2 2 n
615 cm− , which was lower than the value for CvO stretching cycling experiment clearly demonstrate that the copper MOF is
1
−
1
vibration observed in free carboxylic acids 1702 cm . The car- truly heterogeneous in nature.
boxylate ion is coordinated to the central copper metal. This
strong peak was due to the stretching vibration of carboxylate
anions present in the material. These results were in good
4
1
Acknowledgements
agreement with those reported. Elemental analysis by AAS
−
1
indicated a copper loading of 3.68 mmol g
.
PP gratefully acknowledges financial support from University
Grants Commission (UGC), New Delhi, for UGC-BSR-SRF.
KP thanks DST, New Delhi for financial support.
Synthesis of [Cu(H L)]n
2
(Z)-4-((2-Hydroxynaphthalen-1-yl)methyleneamino)benzene-1,3-
dioic acid (H L) was prepared according to the reported pro-
2
4
2
cedure. The prepared H
2
L was characterized by ESI-MS spec-
trum (ESI-MS: m/z calcd for C H NO : 335.08; found, 334.11
1
9
13
5
References
(
M − H)). [Cu(H L)] was prepared according to the reported
2
n
4
2
procedure. The prepared MOF was characterized by FT-IR
and powder XRD techniques (see ESI†). The FT-IR spectra of
the [Cu(H L)] exhibited the presence of a strong peak at
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598 cm− , which was lower than the value for the CvO
stretching vibration observed in free carboxylic acids
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1
717 cm− . This strong peak was due to the stretching
vibration of carboxylate anions present in the material. The
carboxylate ion is coordinated to the copper metal centre.
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Elemental analysis by AAS indicated a copper loading of
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3
.26 mmol g−
.
General experimental procedure for the Cu(BDC) MOF
catalysed homocoupling reaction of arylboronic acid
4
5
N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933.
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(2 mmol) was taken in 2 mL of DMF under an air atmosphere.
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diluted with ethyl acetate (5 mL) and then the catalyst removed
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pressure, the resulting crude product was finally purified by
column chromatography on silica gel (60–120 mesh) with
petroleum ether and ethyl acetate as eluting solvent to give the
desired product upto 97% yield. The recovered catalyst was
thoroughly washed with ethyl acetate and heated with 2 mL of
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temperature for 4 h, which was subsequently reused.
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and highly efficient heterogeneous catalyst for promoting the
aerobic homocoupling of arylboronic acid involving Cu(BDC)
MOF as the catalyst at room temperature without any additives
and bases. The reaction employs environmentally benign air
as oxidant under mild reaction conditions. The catalyst is
stable, shows no metal leaching and the reused catalyst exhibi-
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other catalysts, framework-immobilization confers multiple
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