A Copper-Based Metal-Organic Framework Acts as a Bifunctional Catalyst for the Homocoupling of Arylboronic Acids and Epoxidation of Olefins

Article abstract of DOI:10.1002/asia.201501084

A copper(I)-based metal-organic framework ({[Cu₂Br₂(pypz)]_na.nH₂O} (Cu-Br-MOF) [pypz=bis[3,5-dimethyl-4-(4'-pyridyl)pyrazol-1-yl] methane] has been synthesized by using an elongated and flexible bridging ligand. The structure analysis reveals that each pypz ligand acts as a tritopic ligand connected to two Cu₂Br₂ dimeric units, forming a one-dimensional zig-zag chain, and these chains further connected by a Cu₂Br₂ unit, give a two-dimensional framework on the bc-plane. In the Cu₂Br₂ dimeric unit, the copper ions are four coordinated, thereby possessing a tetrahedral geometry; this proves to be an excellent heterogeneous catalyst for the aerobic homocoupling of arylboronic acids under mild reaction conditions. This method requires only 3 mol % of catalyst and it does not require any base or oxidant-compared to other conventional (Cu, Pd, Fe, and Au) catalysts-for the transformation of arylboronic acids in very good yields (98 %). The shape and size selectivity of the catalyst in the homocoupling was investigated. The use of the catalyst was further extended to the epoxidation of olefins. Moreover, the catalyst can be easily separated by simple filtration and reused efficiently up to 5 cycles without major loss of reactivity.

Full text of DOI:10.1002/asia.201501084

DOI: 10.1002/asia.201501084
Full Paper
Metal–Organic Frameworks
A Copper-Based Metal–Organic Framework Acts as a Bifunctional
Catalyst for the Homocoupling of Arylboronic Acids and
Epoxidation of Olefins
[
a]
Srinivasulu Parshamoni, Jyothi Telangae, Suresh Sanda, and Sanjit Konar*
Abstract:
A
copper(I)-based metal–organic framework
alyst for the aerobic homocoupling of arylboronic acids
under mild reaction conditions. This method requires only
3 mol% of catalyst and it does not require any base or oxi-
dant—compared to other conventional (Cu, Pd, Fe, and Au)
catalysts—for the transformation of arylboronic acids in very
good yields (98%). The shape and size selectivity of the cata-
lyst in the homocoupling was investigated. The use of the
catalyst was further extended to the epoxidation of olefins.
Moreover, the catalyst can be easily separated by simple fil-
tration and reused efficiently up to 5 cycles without major
loss of reactivity.
(
{[Cu Br (pypz)] ·nH O} (Cu—Br–MOF) [pypz=bis[3,5-dimeth-
2
2
n
2
yl-4-(4’-pyridyl)pyrazol-1-yl] methane] has been synthesized
by using an elongated and flexible bridging ligand. The
structure analysis reveals that each pypz ligand acts as a tri-
topic ligand connected to two Cu Br dimeric units, forming
2
2
a one-dimensional zig–zag chain, and these chains further
connected by a Cu Br unit, give a two-dimensional frame-
2
2
work on the bc-plane. In the Cu Br dimeric unit, the copper
2
2
ions are four coordinated, thereby possessing a tetrahedral
geometry; this proves to be an excellent heterogeneous cat-
[
4b]
Introduction
reuse of the expensive catalysts, 3) indispensability of addi-
[
4c]
tives such as stabilizing ligands and bases, and the 4) need
[
4d,11a]
Biaryls are an important class of organic compounds that
occur in many natural products and they have a wide variety
of applications in drugs, agrochemicals, dyes, semi-conductors,
of high temperatures.
Thus, the development of a robust, easily recoverable, and
recyclable heterogeneous catalyst is needed to overcome such
limitations. Recently, metal–organic frameworks (MOFs) have
emerged as versatile materials in the field of heterogeneous
[
1]
and asymmetric synthesis. Several synthetic methods (such
II
[2a]
as palladium (Pd )-catalyzed Suzuki reactions, modified Ull-
[
2b]
[2c]
mann reaction,
Hiyama–Kumada reaction, and the other
catalysis owing to their poor solubility in common solvents
[3a]
[3b]
[3c]
[3d]
[13]
metal catalysts such as Au, Cr, Rh, and V ) have been
developed so far for the construction of biaryl derivatives. In
and secondly tunable and uniform pore size.
One of the
most common approaches for the synthesis of catalytic MOFs
is to create an unsaturated metal coordination environment as
II
particular, the Pd -catalyzed homocoupling of boronic acids
[4]
[14]
has been extensively studied. Apart from the homocoupling
reactions, olefin epoxidation is a highly important oxidation re-
action, because the resultant epoxides are extensively used in
catalytically active sites. This method is more synthetically
challenging, but when the MOF is obtained, the handling and
recycling of the catalyst was significantly easier.
[5]
[6]
[7]
the manufacture of epoxy resins, paints, and surfactants.
In this study, we have developed a copper(I)-based 2D MOF,
namely, {[Cu Br (pypz)] ·nH O} (Cu-Br-MOF) [pypz=bis[3,5-di-
Various methods have been developed for the epoxidation of
olefins, amongst these are catalysts composed of metals such
2
2
n
2
methyl-4-(4’-pyridyl)pyrazol-1-yl] methane] by using a flexible
pypz bridging ligand (Scheme 1). In Cu-Br-MOF, the copper
ions are four coordinated and have a tetrahedral geometry,
and this proves to act as a heterogeneous catalyst for the aero-
bic homocoupling of arylboronic acids and the epoxidation of
olefins.
[
8]
[9]
[10]
[11]
[12]
as V, Mn, Fe, Co, and Cu and so forth.
However, most of the reported metal-catalyzed homocou-
pling reactions and epoxidations of olefins are homogeneous
and have major limitations such as 1) difficulty in separatio-
[4a,8a]
n,
2) large amounts of oxidants are needed to subsequent
[
a] S. Parshamoni, J. Telangae, S. Sanda, Prof. Dr. S. Konar
Molecular Materials Lab
Department of Chemistry, IISER Bhopal
Bhopal By-pass Road, Bhauri, Bhopal 462066, Madhya Pradesh (India)
Fax: (+91)755-6692339
E-mail: skonar@iiserb.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201501084.
Scheme 1. Ligand used in preparation of Cu-Br-MOF.
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Aerobic homocoupling of arylboronic acids, which can also
be promoted by a copper MOF, has been previously report-
this affords a distorted tetrahedral geometry around the metal
centre (t =0.88) (see Figure S4 in the Supporting Information).
4
[15]
ed. However, an advantage of our MOF over other existing
systems is that it transforms arylboronic acids into their corre-
sponding homo-biarlys with a low catalyst loading (3 mol%) in
shorter reaction times (3 h). In addition, the optimal catalyst
It was found that each pypz acts as a tritopic ligand con-
nected to two crystallographically different Cu Br dimeric
2
2
units and forms a 1D zig–zag chain along the b-axis (Figure 1a
and Figure S5 in the Supporting Information). These 1D chains
are further connected by a Cu Br unit to give a 2D framework
[
16]
Cu-Br-MOF was further used in the epoxidation of olefins
2
2
with low catalyst loadings. Moreover, the adsorption abilities
of the material towards selective CO2 sorption applications
were also investigated.
with a rhomboid-shaped channels along with a-axis with the
2
cross-section of 17.186(4)”13.799(1) Š (considering the distan-
I
ces between two Cu centers bridged by pypz ligands) (Fig-
ure 1b, Figure 2 and Figures S6–S7 in the Supporting Informa-
Results and Discussion
Structural Description of Cu-Br-MOF
Single-crystal structural analysis reveals that Cu-Br-MOF crystal-
lizes in tetragonal system with the space group P42/mbc and
I
the asymmetric unit contains two different Cu ions, two bro-
À
mides (Br ), half pypz ligand, and one lattice H O molecule
2
(
see Figure S1 in the Supporting Information). The molecular
structure of Cu-Br-MOF can be best described as if it consists
of two crystallographically different dibromo-bridged dimeric
Cu Br2 cores, Cu Br (1) and Cu Br (2) (Figure 1a). In the
2
2
2
2
2
Figure 2. Connolly surface of Cu-Br-MOF along the b-axis (inner surfaces:
blue; outer surfaces: gray).
tion). The existence of large rhomboid channels along with the
bent shaped spacer facilitates interweaving of one 2D nets to
generate a 2-fold interwoven 2D network (Figure 2 and Fig-
ure S8 in the Supporting Information). These channels are oc-
cupied by guest water molecules and are stabilized by hydro-
gen bonding interactions (O3w-H1A···Br2=2.44 Š; C14-
H14A···O3w=2.485 Š) with bromide anion and ÀCH of pyra-
3
zole ligand (see Figure S9 in the Supporting Information).
PLATON analysis reveals that Cu-Br-MOF has a pore-accessible
I
Figure 1. (a) Bridging mode of pypz ligand with Cu center, (b) Illustration of
D framework with rhomboid-shaped channels along the a-axis. Color code:
3
3
2
void volume of 216.5 Š out of 4635.1 Š , which represents
.7% per unit cell volume.
carbon (gray), nitrogen (blue), bromine (green), and copper (cyan).
4
PXRD and Thermal Stabilities
I
Cu Br (1) dimeric unit, the Cu centre adopts a distorted tetra-
2
2
hedral geometry by the contribution of two different bromide
anions and two pyrazolyl nitrogen atoms from one pypz
To check the phase purity of the bulk materials, a powder X-
ray diffraction (PXRD) experiment was carried out on Cu-Br-
MOF. All major peaks in the experimental PXRD matched well
with the simulated pattern, indicating equitable crystalline
phase purity of the Cu-Br-MOF (see Figure S10 in the Support-
ing Information). Thermo gravimetric analysis (TGA) analysis of
Cu-Br MOF shows a gradual weight loss of approximately
ligand in a chelating fashion (t =0.93) (see Figures S2 and S3
4
in the Supporting Information). In the Cu Br (2) dimeric unit,
2
2
I
the Cu centre is four coordinated by two pyridyl nitrogen
atoms from two different pypz ligands and the remaining two
positions are occupied by two different bromide anions and
Chem. Asian J. 2016, 11, 540 – 547
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3
wt% (calcd. 2.5 wt%) in the temperature range of 90–1408C,
dimethylformamide (DMF; 2 mL) in the presence of activated
Cu-Br-MOF (3 mol%) at room temperature and stirred for 3 h,
the yield of the homocoupling product was 98%. However, no
product was isolated in the absence of Cu-Br-MOF under the
same reaction conditions even after stirring for a long time (ca.
24 h).
which corresponds to the loss of one guest water molecule
from the framework and the dehydrated framework was found
to be stable up to 3408C (see Figure S11 in the Supporting In-
formation). The desolvated phase of Cu-Br-MOF was obtained
by heating the sample for 12 h at 908C under reduced pres-
sure. The PXRD pattern of the desolvated framework matches
well with the as-synthesized compound; this indicates the sta-
bility of the framework after removal of guest molecules (see
Figure S12 in the Supporting Information)
At the onset of this study, phenylboronic acid was chosen as
a model substrate to optimize the reaction conditions (solvent,
base, and the amount of catalyst) and all the reactions were
Table 1. Effect of solvents on the aerobic homocoupling reaction cata-
lyzed by Cu-Br-MOF.
Adsorption Studies
To examine the porous nature of the compound, adsorption
studies with N at 77 K and CO at 195 K were carried out. The
Entry
Solvent
t [h]
Yield [%]
2
2
1
2
3
4
5
6
7
8
9
DMF
DMSO
DMF+H O
3
3
3
98
60
65
12
32
41
7
11
Traces
12
Traces
Traces
8
desolvated framework of Cu-Br-MOF shows a diminished
3
À1
uptake for N , with an amount of 18 cm g . On the basis of
2
2
H
2
O
12
12
12
12
12
12
12
12
12
12
the N adsorption isotherm, the Brunauer–Emmett–Teller (BET)
2
2
À1
EtOH
surface area of Cu-Br-MOF is found to be 36 m g . However,
Cu-Br-MOF adsorbs a significant amount of CO with the
MeOH
Toluene
Xylene
DCM
DCE
1,4-dioxane
THF
2
3
À1
uptake amount of 55.25 cm g at 1 bar pressure (Figure 3).
10
1
1
1
1
2
3
CHCl
3
performed at room temperature (Table 1). Interestingly, Cu-Br-
MOF does not show any catalytic activity in non-polar solvents,
namely, toluene, xylene, dichloromethane (DCM), 1,2-dichloro-
ethane (DCE), 1,4-dioxane, tetrahydrofuran (THF), and chloro-
form (CHCl ). However, in contrast, the products are formed in
3
low-to-moderate yields in protic solvents such as EtOH, MeOH,
H O with phenol as a side product. The obtained phenol side
2
product might be due to the formation of the CÀO bond
through a peroxyboronate intermediate.
Figure 3. Gas adsorption isotherms of Cu-Br-MOF at low temperature, N
2
at
7
7 K and CO at 195 K.
2
When the reaction was carried out in a DMF/water (1:1) mix-
ture, biphenyl and phenol were formed in approximately 65%
and 15% yield, respectively. Based on the above solvent
screening, DMF was found to be the best choice among all the
solvents to catalyze the reaction without any side products.
To determine the role of oxygen, the reactions were also car-
ried out under nitrogen as well as in oxygen atmosphere. In-
terestingly, no product was obtained under an atmosphere of
nitrogen, but the reaction was complete in 3 h under an at-
mosphere of oxygen with the yield of 98%, which is similar to
the yield obtained in atmosphere. These results show that the
dioxygen might be playing a vital role as an oxidant during
the reaction.
The similar sorption behavior was also observed in some well-
[
17a]
known MOFs such as {[Ni(bpe) (N(CN) )](N(CN) )(5H O)} ,
2
2
2
2
n
[
17b]
[17c]
[
Cu(tba) ]n,
{[Zn(C H O ) (C S N H )]·5H O]}
n
(bpe=1,2-
2
10
2
8 0.5 10
-
2
2
8
2
bis(4-pyridyl)ethane, N(CN) =dicyanamide), tba=4-(1H-1,2,4-
2
[13i,17d,e,18d]
triazol-1-yl)benzoate) and others.
Furthermore, to check the structural stability of the frame-
work after adsorption of N , the adsorbed compound was sub-
2
jected to PXRD analysis. The PXRD pattern of the N adsorbed
2
sample matched well with those of the as-synthesized sample
(
see Figure S12 in the Supporting Information). This clearly in-
dicates that the framework remains intact after N adsorption.
2
The addition of a base can enhance the reaction efficiency
[
19]
for some catalytic systems (see Table S1 in the Supporting
Information). Consequently, we carried out the reaction in the
presence of different bases such as NaOH, KOH, Na CO , K CO ,
Catalytic Activity of Cu-Br-MOF in the Homocoupling of Ar-
ylboronic Acids
2
3
2
3
Previous studies have shown that coordinatively unsaturated
copper sites in the MOF can show interesting catalytic activi-
NEt , and cyclohexylamine. Surprisingly, these bases reduced
3
the yield of the desired product. Additionally, to check the
structural stability of the framework after addition of a base,
[18]
ties. When phenyl boronic acid (2 mmol) was added to N,N-
Chem. Asian J. 2016, 11, 540 – 547
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the recovered compound was subjected to PXRD analysis and
found to remain intact (see Figure S13 in the Supporting Infor-
mation).
[
a]
Table 3. The homocoupling of various arylboronic acids.
[b]
Entry Substrate
Product
t [h] Yield [%]
A screening of the catalyst loading revealed that the yield of
product increases from 65 to 98% when the amount of cata-
lyst increased from 1.0 to 3.0 mol%, respectively, but a further
increase in the amount of the catalyst led to no change in the
reaction yield (Table 2).
1
2
3
98
98
3
3
3
4
98
72
3
Table 2. Effect of catalyst loading on the aerobic homocoupling reac-
tions.
5
6
7
3
3
3
98
98
98
Entry
Amount of catalyst
Solvent
t [h]
Yield [%]
1
2
3
4
5
1 mol%
2 mol%
3 mol%
4 mol%
5 mol%
DMF
DMF
DMF
DMF
DMF
3
3
3
3
3
65
82
98
98
98
8
9
6
42
3
8
98
65
The homocoupling products were isolated in lower yields
under the similar reaction conditions with other copper sour-
ces, for example, Cu(NO ) ·3H O (41%), CuCl (ꢀ50%), CuBr
1
0
3
2
2
11
8
8
8
53
50
80
(
ꢀ50%), and Cu(Ac) ·H O (ꢀ30%). This demonstrates that Cu-
2
2
Br-MOF is much more efficient, as well as selective than the
commercially available copper salts as the catalyst. The major
advantage of this solid MOF is that it is easily separable from
the reaction mixture by simple filtration.
1
1
2
3
Once the reaction conditions were optimized, the scope of
this homocoupling reaction of aryl boronic acids was investi-
gated and the results are summarized in Table 3. As is evident
from the results, all meta- and para-substituted electron-rich
boronic acids gave the biaryls in excellent yields. But moderate
yields are observed with ortho-substituted arylboronic acids
1
1
4
5
8
75
40
10
(
Table 3, entries 4 and 8); this might be due to the steric hin-
drance caused by the ortho groups and also low-to-moderate
yields were obtained for the coupling of electron-deficient and
hetero arylboronic acids (Table 3, entries 10–12). To ascertain
that the reaction occurred in the channel of Cu-Br-MOF, we
carried out some homo coupling reactions for arylboronic
acids with larger substrates. As shown in Table 4, the yield of
the reaction steadily decreases with an increase in the size of
the substrate (Table 4, entry 1–3), probably due to its large di-
ameter that restricts access to the internal pores.
[
a] Reaction conditions: arylboronic acid (2 mmol), Cu-Br-MOF (39 mg),
DMF (2 mL), RT, 3 h. [b] With Cu-Br-MOF, yield determined by GCMS analy-
sis.
[
15a]
15b
over existing systems [(Cu (BTC) -MOF
or Cu(BTC)-MOF)
]
3
2
might be due to the easy replacement of the labile-coordinat-
[20a]
[20b]
À
I
The research groups of Demir,
[20c]
Yamamoto,
and Kabou-
ed Br anion and give more catalytically active Cu sites to en-
hance the reaction.
din
examined the Cu-mediated homocoupling of arylboron-
ic acids, and proposed a tentative mechanism for the transfor-
mation. However, at the present stage the mechanism of the
reaction is not yet obvious. The following observations may be
useful to speculate on the mechanism: (1) Copper in any oxi-
On the basis of these observations, a plausible reaction
pathway is proposed, as shown in Scheme 2. During the ho-
mocoupling reaction, in the first step the Cu Br bridge might
2
2
open up by the arylboronic acid. Double transmetallation of
I
II
I
I
dation state, Cu, or Cu , is catalytically active in the homocou-
pling reaction. (2) Dioxygen is essential for the reaction as an
Cu with two molecules of arylboronic acid affords a Ar-Cu-Ar,
II
which undergoes air oxidation to yield a Cu intermediate, and
II
oxidant. Therefore, the Cu state might be responsible for the
this undergoes subsequent reductive elimination to the sym-
metrical biaryl compound.
homocoupling reactions. (3) The bridging halide ion plays an
important role as Cu-Br-MOF. The effective transformation of
arylboronic acids into their corresponding homo-biarlys using
a low catalyst loading (3 mol%) in shorter reaction times (3 h)
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product (28%), however, the reaction was completed within
8 h when TBHP was used as an oxidant and the product was
obtained in 99% yield. Therefore, TBHP as an oxidant is crucial
for the reaction.
Table 4. Size and shape selectivity in the homocoupling of various aryl-
boronic acids.
[a]
Entry Substrate
Molecular size
Selectivity [%] Yield [%]
To explore the versatility of Cu-Br-MOF, various alkenes were
evaluated with different molecular dimensions. As shown in
Table 5 (entries 1–4), substrates participated well in this epoxi-
1
99
99
99
9
[a]
Table 5. Olefin epoxidations catalyzed by Cu-Br-MOF.
2
3
75
40
[b]
[b]
Entry Substrate
Product
t [h] Yield [%]
Selectivity [%]
>99
1
2
8
8
99
99
[
a] Reaction conditions: arylboronic acid (2 mmol), Cu-Br-MOF (39 mg)
DMF (2 mL), RT, 3 h.
>99
3
4
8
8
99
99
>99
>99
5
6
8
64
46
>64
>46
16
[a] Reaction conditions: olefin (0.5 mmol), Cu-Br-MOF catalyst (1 mol%),
TBHP (2.0 mmol), toluene (2.0 mL), [b] With Cu-Br-MOF, yield determined
by GCMS analysis.
dation reaction. Excellent yields (ꢀ99%) were achieved within
8
h without any byproduct. Epoxidation of styrene gave a the
product in moderate yield (64%; Table 5, entry 5) along with
benzaldehyde as a byproduct (36%). Similarly, epoxidation of
2-vinylnaphthalene required a longer reaction time and gave
poor yields (46%; Table 5, entry 6) and selectivity of the prod-
uct along with a-methyl-2-naphthalenemethanol (23%) and 2-
acetonaphthone (27%) as byproducts.
Scheme 2. A possible mechanism for the homocoupling of arylboronic
acids.
Epoxidation of Olefins
Encouraged by these preliminary results, the catalyst was fur-
ther extended to the epoxidation of olefins, as it is one of the
most important organic transformations and frequently used
as intermediates in industry. The catalytic activity of Cu-Br-MOF
towards selective epoxidation was evaluated, trans-b-methyl-
styrene (0.5 mmol) and tert-butyl hydroperoxide (TBHP) were
added to toluene (2 mL) in the presence of activated Cu-Br-
MOF (1 mol%) at 353 K and stirred for 8 h. The corresponding
product was obtained in high yield without any byproduct.
To confirm the role of catalyst, a blank reaction was per-
formed under similar conditions with trans-b-methylstyrene
and TBHP as an oxidant. No products were isolated even after
stirring for a long time (ꢀ24 h).
A plausible reaction pathway is proposed as shown in
Scheme 3. It can be predicted that the reaction proceeds
To determine the role of TBHP, the reactions were carried
out in the presence of 1atm. molecular oxygen instead of
TBHP. Using molecular oxygen as an oxidant prolonged the re-
action time (48 h) and gave poor yields and selectivity of the
Scheme 3. A possible mechanism for the epoxidation of olefins.
Chem. Asian J. 2016, 11, 540 – 547
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3
À1
through formation of a copper-peroxo species using TBHP as
with as-synthesized sample (ꢀ18 cm g ), and the BET surface
[21]
2 À1
an oxidant. Bhaumik et al. have reported a similar reaction
mechanism for epoxidation reactions of olefins over a Cu
Schiff base complex immobilized on mesoporous silica. There-
fore, a similar pathway is expected to be followed for the Cu-
Br-MOF in the liquid phase partial oxidation reactions.
area was found to be 30 m g .
Leaching is a common problem in heterogeneous catalysis,
especially in CÀC coupling reactions. The simplest and most ef-
fective test for leaching is in situ catalyst filtration and re-addi-
tion (Figure 4b). To ensure it, after 1 h of the reaction, Cu-Br-
MOF was filtered out from the reaction mixture and the reac-
tion was allowed to stir further without Cu-Br-MOF for 5 h and
no further product was observed. Furthermore, the filtered Cu-
Br-MOF was added to the above reaction mixture after 5 h of
standing without it, and a sudden increase in the yield was ob-
served after the addition of catalyst; this indicates that the
copper(I)-based MOF in the reaction is heterogeneous in
nature.
Recyclability and Leaching Test
The Cu-Br-MOF catalyst demonstrated excellent reusability for
aerobic homocoupling of arylboronic acids and epoxidation of
olefins. After the first reaction cycle, Cu-Br-MOF was recovered
by simple filtration, washed thoroughly with DMF, and then
dried in an oven at 1008C. The recovered frameworks were
subjected to PXRD analysis; this reveals that all major peaks
are well matched with the as-synthesized sample and indicates
structural stability of the framework (see Figures S14 and S15
in the Supporting Information) and can be reused at least for
five subsequent cycles without any significant loss in catalytic
activity (Figure 4a).
Conclusions
In conclusion, a copper(I)-based 2D MOF was found to be an
excellent heterogeneous catalyst for the homocoupling of aryl-
boronic acids and epoxidation of olefins. The shape and size of
the catalyst in the selectivity of the homocoupling was investi-
gated. Furthermore, the catalyst can be easily separated by
simple filtration and reused efficiently up to 5 cycles without
major loss in reactivity. Ongoing research in our laboratory is
aimed at developing new functionalized MOFs as catalysts for
the CÀC coupling reactions under progress.
Experimental Section
Materials
All the reagents and solvents for synthesis were purchased from
commercial sources and used as supplied without further purifica-
tion. CuCl, CuBr, 4-methylpyridine, hydrazine hydrate, dibromome-
thane, tert-butyl hydroperoxide (TBHP); all the olefins and arylbor-
onic acid were obtained from the Sigma–Aldrich Chemical Co.
India. Pypz ligand was obtained according to the literature proce-
[22]
dure.
Physical Measurements
Thermo gravimetric analysis (TGA) was recorded on a PerkinElmer
TGA 4000 instrument. Elemental analysis was carried out on an Ele-
mentar vario Micro Cube Elemental Analyzer. PXRD patterns were
measured on a PAnalytical Empyrian instrument by using CuKa radi-
ation.
Single Crystal X-ray Diffraction
Figure 4. (a) The kinetic profiles in five consecutive reaction cycles employ-
ing Cu-Br-MOF as the catalyst. (b) The leaching test (filtration test) indicated
no contribution from the homogeneous catalysis of the active species (A) in
the presence of Cu-Br-MOF. (B) The catalyst filtration (after 1 h).
Single crystal data for Cu-Br-MOF was collected on a Bruker APEX II
diffractometer equipped with a graphite monochromator and MoKa
(l=0.71073 Š, 296 K) radiation. Data collections were performed
using f and w scan. Non-hydrogen atoms are located from the dif-
ference Fourier maps, were refined anisotropically by full-matrix
2
[23]
least-squares on F , using SHELXS-97. All hydrogen atoms were
included in the calculated positions and refined isotropically using
a riding model. Determinations of the crystal system, orientation
matrix, and cell dimensions were performed according to the es-
tablished procedures. Lorentz polarization and multi-scan absorp-
tion correction were applied. All calculations were carried out
To ensure the structural stability and porosity of the frame-
work, N adsorption measurement was performed at 77 K on
2
recovered Cu-Br-MOF after five reaction cycles (see Figure S16
in the Supporting Information). Remarkably, the recovered
3
À1
sample shows a similar uptake (ꢀ16 cm g ) of N compared
2
Chem. Asian J. 2016, 11, 540 – 547
www.chemasianj.org
545
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
crude product was finally purified by column chromatography on
silica gel (100–200 mesh) with hexane/ethyl acetate (8:2, v/v) as
eluting solvent to give the desired product in up to 98% yield. The
recovered catalyst was thoroughly washed with DMF, and subse-
quently reused.
[
27]
Table 6. Crystallographic data for Cu-Br-MOF.
CCDC
1400148
Formula
21 2 2 6
C H24Br Cu N O
À1
Weight [gmol
Crystal shape
Color
]
663.36
Block
Green
0.46”0.34”0.26
Tetragonal
General experimental procedure for the Cu-Br-MOF catalyzed
epoxidation of olefins: A solution of olefins (0.065 mL, 0.5 mmol),
5.0–6.0m TBHP in decane (0.2 mL), and toluene (2 mL), was stirred
at 363 K with Cu-Br-MOF (15 mg) in powder form. The progression
and yield of the reaction were monitored by using GCMS analysis.
The recovered catalyst was thoroughly washed with DMF, and sub-
sequently reused.
Size
Crystal system
Space group
Cell length a [Š]
Cell length b [Š]
Cell length c [Š]
2
P4 /mbc
16.4226(8)
16.4226(8)
17.1859(12)
90
90
90
4635.07
8
298(2)
0.71073
5.310
1.901
2624.0
2137
Cell angle alpha [8]
Cell angle beta [8]
Cell angle gamma [8]
Acknowledgements
3
Cell volume V [Š ]
Cell formula units Z
Temperature [K]
SP and SS thank UGC and IISER Bhopal for the SRF fellowship
respectively, JT thanks IISER Bhopal for the INSPIRE fellowship.
The authors thank Jagadeesh Kalepu, Vinaykumar Kanchupalli
Dr. Prasath and Dr. H. S. Jena for helpful scientific discussions.
S.K. thanks CSIR, Government of India (Project No. 01/(2822)/
l (MoKa) [Š]
À1
m [mm
]
À3
Dc [gcm
]
Crystal_F_000
Measured reflections
Unique reflections
1
5/EMRII), and IISER Bhopal for generous financial and infra-
1782
0.0257
0.0598
[
a]
R
R
1
[I>2s(I)]
structural support.
[
b]
w
[I>2s(I)]
2
2
2
2 2 1/2
[
a] R
1
=Sj jF
o
jÀjF
c
j j/SjF
o
, [b] R
w
=[S{w(F
o
ÀF
c o
) }/S{w(F ) }] .
Keywords: arylboronic acids · copper · epoxidation · metal–
organic frameworks · olefins
[
24]
[25]
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using SHELXL 97,
PLATON 99,
and WinGXsystemVer-1.64.
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