Aromatic N-arylations catalyzed by copper-anchored porous zinc-based metal-organic framework under heterogeneous conditions

Article abstract of DOI:10.1002/cctc.201400056

A highly porous Zn-based metal-organic framework (MOF) IRMOF-3 was covalently decorated with pyridine-2-aldehyde. The free amine group of IRMOF-3 upon condensation with pyridine-2-aldehyde affords a bidentate Schiff-base moiety in the porous matrix. The Schiff base moieties are availed to anchor copper(II) ions to display the catalyst's utility towards catalytic reactions. The catalyst was characterized by UV/Vis and IR spectroscopy, powder XRD spectrometry, SEM energy-dispersive X-ray spectrometry, and nitrogen sorption measurements. The catalyst exhibits excellent activity in catalyzing the N-arylation reaction of nitrogen-containing heterocycles with aryl bromides in DMSO medium, under mild condition (90°C) in the presence of Cs ₂CO₃. The porous catalyst demonstrates size selectivity towards substrate as a result of the presence of active sites inside the pores of the MOF. The anchored complex seems to be not leached or decomposed during the catalytic reactions up to five successive catalytic cycles, demonstrating practical advantages over homogeneous catalysis.

Full text of DOI:10.1002/cctc.201400056

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DOI: 10.1002/cctc.201400056
Aromatic N-Arylations Catalyzed by Copper-Anchored
Porous Zinc-Based Metal–Organic Framework under
Heterogeneous Conditions
Tanmoy Maity, Debraj Saha, and Subratanath Koner*[a]
A highly porous Zn-based metal–organic framework (MOF)
IRMOF-3 was covalently decorated with pyridine-2-aldehyde.
The free amine group of IRMOF-3 upon condensation with pyr-
idine-2-aldehyde affords a bidentate Schiff-base moiety in the
porous matrix. The Schiff base moieties are availed to anchor
copper(II) ions to display the catalyst’s utility towards catalytic
reactions. The catalyst was characterized by UV/Vis and IR
spectroscopy, powder XRD spectrometry, SEM energy-disper-
sive X-ray spectrometry, and nitrogen sorption measurements.
The catalyst exhibits excellent activity in catalyzing the N-aryla-
tion reaction of nitrogen-containing heterocycles with aryl bro-
mides in DMSO medium, under mild condition (908C) in the
presence of Cs2CO3. The porous catalyst demonstrates size se-
lectivity towards substrate as a result of the presence of active
sites inside the pores of the MOF. The anchored complex
seems to be not leached or decomposed during the catalytic
reactions up to five successive catalytic cycles, demonstrating
practical advantages over homogeneous catalysis.
Introduction
N-Arylazoles have been employed in a variety of useful appli-
cations for enzyme inhibitors,[1] ATP binding inhibitors,[2] ago-
nists or antagonists of G-protein-coupled and other recep-
tors,[3] preparation of key components in LED production,[4]
and for creating organometallic catalysts through ligand prep-
aration.[5] N-Arylated products are also registered pharmaceuti-
cals (e.g., celecoxib, midazolam, flumazenil, nilotinib, alprazo-
lam) and insecticides or fungicides (e.g., pyraclofos, pyraclos-
trobin) as they contain important structural motifs of numer-
ous natural products and biologically active pharmaceutical
agents.[6] Traditionally, these compounds have been synthe-
sized by nucleophilic aromatic substitution (SNAr) of NꢀH-con-
taining p-electron-rich nitrogen heterocycles with aryl halides
or by the classical Ullmann-type coupling at very high temper-
ature.[7] For this reason, transition-metal-catalyzed arylation of
nitrogen-containing heterocycles is one of the most efficient
and powerful tool for the synthesis of N-arylazole derivatives.[8]
However, expensive catalysts such as palladium and rhodium
complexes are used in current method for such kind of cou-
pling reactions.[8d,9] Recently, Buchwald and co-workers and
Taillefer and co-workers have initiated inexpensive copper cata-
lysts for the N-arylation of nitrogen-containing heterocycles
with aryl halides under homogeneous conditions.[10] Even
though homogeneous catalytic systems are known to exhibit
better reactivity than heterogeneous systems, for large-scale
applications in liquid-phase reactions, heterogeneous catalysts
are superior over homogenous systems with respect to their
easy recovery, recycling, and minimization of unnecessary toxic
wastes. Hence, development of heterogeneous systems by
grafting the metal on inorganic or organic supports have at-
tracted attention in recent years.[11] Nevertheless, example of
N-arylation reactions of nitrogen-containing heterocycles cata-
lyzed by CuII–MOF (MOF=metal–organic framework) under
heterogeneous condition are very rare in the literature.[11c,d]
In last few decades, MOFs have attracted considerable atten-
tion and displayed their potential in designing advanced multi-
functional materials. Control over the structure of the material
now can be achieved through rich knowledge of sophisticated
molecular engineering. Research in the area of fabricating
MOFs continues to be intriguing for their unique topology,
tunable properties and straightforward postsynthetic function-
alization of inner-pore surfaces.[12] Recent interest of MOF com-
pounds are focused on the catalysis,[13] solid-state lumines-
cence,[14] biotechnology,[15] molecular magnetism,[16] and fixa-
tion media for atmospheric gases.[17]
There are some excellent features of MOFs that make them
suitable in employing in heterogeneous catalytic reactions. The
high porosity of MOFs allows for fast mass transport and inter-
action with substrates; most of them display high thermal sta-
bility, and the active sites can be chemically tuned and modi-
fied like a homogeneous, single-site catalyst.[18] Therefore, new
strategies to improve the activity and selectivity of catalytic
systems exploiting their remarkable structural and chemical
flexibility deserve to be explored. Postsynthetic modifications
(PSM) of porous MOFs have been developed because of their
additional reactive metal sites.[19] Standing out from the vast
majority of metal–organic coordination polymers, isoreticular
metal–organic framework (IRMOF) is the class of highly porous
basic zinc carboxylates discovered by Yaghi et al.[20] Its proto-
[a] Dr. T. Maity, Dr. D. Saha, Dr. S. Koner
Department of Chemistry
Jadavpur University
188 Raja S. C. Mullik Road, Jadavpur, Kolkata-700032 (India)
E-mail: snkoner@chemistry.jdvu.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201400056.
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type is MOF-5, in which {Zn4O} building blocks are linked to-
gether by terephthalate bridges to form a primitive cubic
framework.[21] IRMOF-3 is a known MOF with a cubic topology
prepared from Zn(NO3)2·4H2O and 2-amino-1,4-benzene dicar-
boxylic acid (R3-BDC).[20a] It is such an MOF in which the amine
moieties are not intimately involved in coordination to the tet-
ranuclear Zn4O nodes and lined the pores of the framework.
With the advantage of free amino groups present in the frame-
work, anchoring of metal sites in MOF has been facilitated.
Thus IRMOF-3 represents a good model system for a postsyn-
thetic covalent modification study.
In the course of our continuing investigation on heterogene-
ous catalysis using MOFs, we have successfully employed lay-
ered metal carboxylates and vanadium(IV) hydrogen phos-
phate and lanthanide-based MOFs to catalyze olefin epoxida-
tion reactions under the heterogeneous condition.[22] We have
also developed alkaline earth metal based MOF systems that
efficiently catalyze aldol condensation reactions.[14d,23] Recently,
by availing postsynthesis modification strategy of MOF, we
have developed a new Pd-containing catalyst that exhibits
high activity toward CꢀC cross-coupling reactions.[24a]
Scheme 1. Generic scheme for the postsynthetic modification of IRMOF-3
for preparation of MOF containing the CuII Schiff-base moiety.
Herein we report a facile route of N-arylation of nitrogen-
containing heterocycles efficiently catalyzed by a heterogene-
ous system derived by organic modification of the MOF,
IRMOF-3, and subsequent anchoring of CuII into MOF through
complex formation. Notably, the MOF-based catalyst exhibits
shape selectivity in products by virtue of the restriction of pore
size in framework.
quantitative. Visible color change of the modified MOF from
yellow to green indicates copper(II) centers anchored through
the designed Schiff-base moiety in the framework.
Spectroscopic studies
Results and Discussion
Electronic spectra of IRMOF-3, IRMOF-3-PI, and IRMOF-3-PI-Cu
were measured in the solid state (see Figure S3). UV/Vis spectra
of both IRMOF-3 and IRMOF-3-PI featured a strong band at ap-
proximately 331 nm and the latter one showed an additional
weak transition near 412 nm. IRMOF-3-PI-Cu, however, exhibit-
ed a weak yet distinct band at approximately 749 nm, which
can be ascribed to d–d transition of metal electronic state typi-
cal for a Cu–Schiff-base complex.[27] The FTIR spectra of IRMOF-
3, IRMOF-3-PI, and IRMOF-3-PI-Cu were measured in the KBr
pellet (Figure 1). The IR vibration bands of IRMOF-3 found be-
tween approximately 3500 and 3400 cmꢀ1 and between ap-
proximately 1600 and 1500 cmꢀ1 are caused by free ꢀNH2
groups present in the framework. A moderately intense band
at approximately 3431 cmꢀ1 and a relatively strong band at ap-
proximately 1564 cmꢀ1 can be ascribed to the NꢀH stretching
and bending vibration mode of the ꢀNH2 group, respectively.
The IR spectrum of IRMOF-3-PI displays two new extra bands
at approximately 1653 cmꢀ1 and approximately 1616 cmꢀ1; and
the former band can be ascribed to the vibration band for the
azomethine group (>C=Nꢀ) the latter one for the vibration
mode of the C=N bond of the pyridine ring. Appearance of
these two new bands in the IR spectrum of IRMOF-3-PI indicat-
ing IRMOF-3 framework seems to indicate modification as
shown in Scheme 1.[24a] For IRMOF-3-PI-Cu, the vibration bands
of both azomethine group and C=N moiety of the pyridine
ring are shifted to the lower frequency range and overlap with
the asymmetric band of the carboxylato group. This indicates
Postsynthetic modification of IRMOF-3
IRMOF-3 (isoreticular MOF-3) is a highly porous MOF decorated
with pendant amine (ꢀNH2) groups previously developed by
Yaghi et al.[20,25] The amine moieties are uncoordinated and
available for further chemical treatment. Thus the free NH2
groups were modified through a simple condensation reaction,
producing a 2-pyridylimine (RꢀN=CꢀC5H4N) moiety that is a po-
tential bidentate ligand. For further exploitation, the modified
system as catalyst copper(II) centers were anchored in the
framework through complex formation. Reactions involved in
the process are summarized in Scheme 1. IRMOF-3 is known to
be hydrophilic and relatively unstable against moisture.[26] Thus
there is a possibility of structural disintegration of IRMOF-3-PI
(PI=Pyridyl Imine) in presence of hydrated CuII salt. However,
organic modification changes the nature of IRMOF-3 from hy-
drophilic to hydrophobic.[26b] This may be the reason to retain
its structural integrity after metalation using CuCl2·2H2O. To
optimize the catalytic efficacy of IRMOF-3-PI-Cu, we investigat-
ed the functionalization of IRMOF-3, by varying the amount of
pyridine-2-aldehyde and, subsequently, by varying the amount
of copper chloride in the Schiff-base complex formation (see
Supporting Information, Figure S1 and S2). Through this
method, a maximum of approximately 7% of the total ꢀNH2
groups was functionalized without losing the framework integ-
rity, and copper complexation by the Schiff-base complex was
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molecules and the second mass loss by decomposition of the
organically modified framework.
Nitrogen sorption studies
The nitrogen sorption isotherms of IRMOF-3, IRMOF-3-PI, and
catalyst IRMOF-3-PI-Cu are shown in Figure 2. All three materi-
als (IRMOF-3, IRMOF-3-PI, and catalyst IRMOF-3-PI-Cu) featured
Figure 1. IR spectra of 1) IRMOF-3, 2) IRMOF-3-PI, and 3) IRMOF-3-PI-Cu.
coordination of azomethine nitrogen with CuII. In addition, IR
spectrum of IRMOF-3-PI-Cu reveals two new bands at 487 and
418 cmꢀ1, which can be ascribed to the asymmetric CuꢀN
stretching mode.[24b] Two weak peaks at 363 and 306 are as-
cribed to symmetric CuꢀN stretching and CuꢀCl (terminal
chloro) vibrations, respectively (see Figure S4).[24b,c] Characteris-
tic bands for carboxylato vibration appeared in the range of
1430–1380 cmꢀ1 in all three samples.
Figure 2. Nitrogen sorption isotherms of 1) IRMOF-3, 2) IRMOF-3-PI, and
3) IRMOF-3-PI-Cu.
XRD studies
a type II (according to the IUPAC nomenclature) nitrogen sorp-
tion isotherm.[24a] BET surface area of all materials was calculat-
ed from those isotherms, the pore volume and average pore
diameter of samples was calculated by using the NLDFT
method (Figure S7). The nitrogen sorption experiment demon-
strated that the BET surface area of IRMOF-3 was 857 m2 gꢀ1
(pore volume=1.15 cm3 gꢀ1 and pore diameter=8.26 ꢁ), which
is little lower than that of similar other MOF systems.[24d,e,f]
Lower value of BET surface area may be owing to the presence
of very small amounts of DMF that remains even after CHCl3
treatment of IRMOF or to slight degradation of the framework
system. IRMOF-3-PI displayed a significantly smaller N2 uptake
with the BET surface area of 572 m2 gꢀ1 (pore volume=
0.81 cm3 gꢀ1 and pore diameter=7.59 ꢁ), indicating that the
framework is modified with the formation of Schiff base in
IRMOF-3 pores. Nevertheless, upon incorporation of Cu in the
framework only a slight change in the BET surface area
(539 m2 gꢀ1), and also for pore volume (0.73 cm3 gꢀ1) and pore
diameter (7.14 ꢁ) was observed. Apparently the space occu-
pied by the Schiff base moiety has not changed substantially
after complex formation with copper.
XRD patterns of IRMOF-3, IRMOF-3-PI, and catalyst IRMOF-3-PI-
Cu were measured to understand the authenticity and stability
of the frameworks (see Figure S5). The powder XRD pattern of
IRMOF-3 corresponds well with the literature data indicating
the desired material is obtained.[24a,28] Comparison of XRD pat-
terns of simulated IRMOF-3, as-synthesized IRMOF-3, IRMOF-3-
PI, and catalyst IRMOF-3-PI-Cu demonstrates the diffraction
lines remained almost intact upon modification. This clearly
shows that organic functionalization as well as metal complex
formation in IRMOF-3 had no effect on the structural integrity
of the framework.
Thermal measurements
Thermogravimetric (TG) analysis of the catalyst was performed
under nitrogen atmosphere. The TG curve indicates that upon
heating IRMOF-3-PI-Cu suffers a small weight loss between
room temperature and 708C (see Figure S6). The correspond-
ing differential thermal analysis (DTA) curve exhibits a broad
and weak endothermic peak in the above-mentioned tempera-
ture range. Thereafter, the TG curve remains flat up to approxi-
mately 2308C indicating no mass loss in this temperature
range. On further heating, the TG curve indicates a relatively
large mass loss, approximately 59%, in the temperature range
230–5108C, which corresponds to a sharp exothermic peak
centered at approximately 4538C in its DTA curve. The first
mass loss seems to be caused by removal of adsorbed solvent
SEM and EDX studies
SEM analysis was undertaken to study the external morpholo-
gy of the framework compound and its postsynthetically modi-
fied variety. SEM images of IRMOF-3 and the catalyst, IRMOF-3-
PI-Cu confirm that the surface morphology of frameworks re-
mained unaltered upon postsynthetic modification (Figure 3A
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than the soluble organic bases (e.g., Et3N). Cs2CO3
was the best one for this catalytic reaction in DMSO
(Table 1, entry 10) and the catalyst was inactive in the
absence of base (Table 1, entry 9). Besides, the reac-
tion temperature also has significant role in the bet-
terment of yield of the products. Isolated yield was
not up to the mark both in high (1108C) and low
(608C) temperatures. At low temperature the conver-
sion was lower (Table 1, entry 16), but at high tem-
perature the yield of desired product was still lower
because of the formation of undesired non-isolable
byproducts (Table 1, entry 17). Thus, the optimum
conditions of the catalytic reaction to achieve highest
yield of desired product is as follows: Cs2CO3 (base),
DMSO (solvent), and reaction temperature 908C. No-
tably, the reaction should be performed under N2 at-
mosphere as products yield decreases drastically in
air because of the presence of moisture (Table 1, en-
tries 15 and 16). Reactions in the absence of catalyst,
Figure 3. SEM image and EDX spectra, respectively, of A,C) IRMOF-3-PI (scale bar=1 mm);
and B,D) IRMOF-3-PI-Cu (scale bar=2 mm).
and 3B). Energy-dispersive X-ray (EDX) analysis of IRMOF-3 re-
vealed the presence of Zn and oxygen on the surface of frame-
work, and EDX analysis of the catalyst confirmed the presence
of copper and chloride in addition of Zn and oxygen (Fig-
ure 3C and 3D). The ratio of Cu/Cl obtained from EDX was
close to 1:2. This ratio indicates coordination of copper with
two chloride atoms as shown in Scheme 1. Platinum is present
in EDX elemental spectra as platinum was used in coating for
SEM/EDX analysis.
IRMOF-3, IRMOF-3 with CuCl2, IRMOF-3-PI and IRMOF-3-PI with
CuCl2 were also investigated under optimum condition to un-
derstand the competence of IRMOF-3-PI-Cu as a catalyst
(Table 1, entries 11–15).
After obtaining optimum conditions for catalytic reaction,
the efficacy of IRMOF-3-PI-Cu catalyst in the CꢀN cross-cou-
pling reaction was evaluated for a wide range of substrates.
Table 1. Optimization of reaction conditions for N-arylation of pyrazole
with bromobenzene.
EPR studies
The EPR spectrum of powdered IRMOF-3-PI-Cu catalyst was
measured at room temperature (see Figure S8). The principle g
values calculated by the usual methods from EPR spectra are
in agreement with those reported for other copper(II)–Schiff-
base complexes.[29a,b] The g and g values of IRMOF-3-PI-Cu
Entry[a] Catalyst
Base
Solvent
Yield[b] [%]
1
IRMOF-3-PI-Cu
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
Et3N
K3PO4·3H2O
Na3PO4·12H2O DMSO
–
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMF
83
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18[d]
19[e]
20
21[f]
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
–
toluene
acetonitrile
DMSO
DMSO
DMSO
j j
?
31
55
50
26
68
59
0
92
–
–
were calculated as 2.42 and 2.06, respectively. Continuous-
wave EPR studies have been performed extensively for cop-
per(II) complexes. Copper(II) systems with g >g >2.0023
j j
?
suggest that the unpaired electron occupies the orbital, which
is the characteristic of a square planar coordination environ-
ment of copper(II). In the case of immobilization of Cu(salen)
into NaY zeolite, a similar type of spectrum has been observed
in a previous study.[29c]
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
IRMOF-3
IRMOF-3+CuCl2
IRMOF-3-PI
24
–
Catalytic activity
IRMOF-3-PI+CuCl2 Cs2CO3
22
31[c]
79
94
31
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
IRMOF-3-PI-Cu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Initial optimization studies for CꢀN cross-coupling reactions
were undertaken by using pyrazole and phenyl bromide as
substrates under various reaction conditions as given in
Table 1. Solvents, bases, and temperature have significant influ-
ence on the reactivity of the catalyst in the reaction. To select
the most suitable solvent, several solvents were employed in
CꢀN coupling reaction. In DMSO, the highest yield was ob-
tained amongst all the solvents used (Table 1, entry 10).
Screening of bases revealed that inorganic bases, such as
K2CO3, K3PO4, Cs2CO3, Na2CO3, and Na3PO4, were more efficient
DMSO/H2O (19:1) 24
DMSO 39
[a] Reaction condition: bromobenzene (1.2 mmol), pyrazole (1.0 mmol),
base (2 mmol), catalyst (0.002 g), solvent (2 mL) at 908C for 20 h under N2
atmosphere (except entries 16 and 17 for which temperature was 60 and
110 8C, respectively). [b] Isolated yield. [c] Yield in 40 h. [d] Iodobenzene
was used instead of bromobenzene. [e] Chlorobenzene was used instead
of bromobenzene. [f] Reaction was performed in air.
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Desired products were isolated in moderate to good yields in
catalytic reactions (Table 2). Both iodobenzene and bromoben-
zene (Table 1, entries 18 and 10, respectively) afforded compa-
rable yields. However, iodobenzene generally exhibited higher
reactivity than bromobenzene in the cross-coupling reactions.
This may be attributed to the fact that the Iꢀ or I2 generated
from iodobenzene can act as an inhibitor during the catalytic
process as proposed in a recent report.[30] The catalyst is capa-
ble of activating chlorobenzene (Table 1, entry 19), which nor-
mally either remains inactive or may require drastic reaction
condition to activate the CꢀCl bond because of their relative
inertness. However, the yield in the case of chlorobenzene was
much lower than that for iodo- and bromobenzene. At first,
we extended the scope of the N-arylation reaction to different-
ly substituted pyrazole with substituted bromobenzene. It
seemed that substitution of the pyrazole ring at 3-position did
not obstruct the cross-coupling reaction (Table 2, entries 2, 11,
and 12). The N-arylation of 3-phenyl-1H-pyrazole and indazole
(fused pyrazole with benzene ring) gave moderate yield
(Table 2, entries 11 and 14). This reaction revealed high sensitiv-
ity towards the 5-substituent of pyrazole ring and lower yields
were obtained for 5-methylpyrazole (Table 2, entry 13) and 3,5-
dimethylpyrazole (entry 15), which may be attributed to the
large steric crowding with electron-donating ability of methyl
groups during the course of the cross-coupling reaction. Reac-
tions of all the asymmetric pyrazoles (3-phenyl-1H-pyrazole, 3-
methyl-1H-pyrazole, and 5-methyl-1H-pyrazole) with aryl bro-
mide always yielded only one regioisomeric product. Reactions
of pyrazole with p-NO2- and m-NO2-functionalized aryl bromide
gave excellent yields of desired products, whereas for sterically
hindered o-nitrobromobenzene a lower yield was obtained
(Table 2, entries 2–4). A moderately good yield was also ob-
tained in the case of p-methoxybromobenzene and p-methyl-
bromobenzene (entries 5 and 6). The catalyst is also capable of
activating the substrates containing functional groups such as
aldehyde, acetyl, chloride, etc. (Table 2, entries 6, 7, 8, 9 and
10). It is reasonable to conclude from Table 2 that electron-
withdrawing substituted aryl bromides enhance the reactivity
and electron-releasing group deactivates. The kinetic plot of
the progress of catalytic reactions (Figure 4) indicates that the
rate of formation of products increased steadily from the be-
Table 2. N-arylation reaction of different N-containing heterocycles with
substituted aryl bromide.
Entry[a] Amine
R
Product
Yield[b] [wt%] TOF[c] [hꢀ1
]
1
H
92, 90[d]
97, 95[d]
2
p-NO2
99
105
3
4
m-NO2
o-NO2
94
87
100
92
5
m-OMe
82
87
6
7
8
p-Me
86
95
92
91
101
98
p-COMe
p-Cl
9
p-CN
91
98
97
10
p-CHO
104
Figure 4. Kinetic plot for CꢀN cross-coupling reactions catalyzed by IRMOF-
3-PI-Cu.
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Table 2. (Continued)
Table 2. (Continued)
Entry[a] Amine
Entry[a] Amine
R
Product
Yield[b] [wt%] TOF[c] [hꢀ1
]
R
Product
Yield[b] [wt%] TOF[c] [hꢀ1
]
20
p-OMe
81
58
86
61
11
p-NO2
70
74
21
22
H
12
13
14
p-NO2
p-NO2
p-NO2
90
45
34
95
48
36
p-NO2
34
36
23
24
o-NO2
28
73
30
77
m-NO2
15
p-NO2
32
34
25
p-OMe
28
30
16
17
H
90, 87[d]
95, 92[d]
26
27
H
90, 88[d]
95, 93[d]
p-NO2
99
105
p-NO2
98
104
18
19
m-NO2
o-NO2
93
84
99
89
28
29
m-NO2
o-NO2
94
86
100
91
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using an equimolecular mixture of indazole (1 mmol) and pyra-
zole (1 mmol) with p-nitrobromobenzene (1.2 mmol) catalyzed
by IRMOF-3-PI-Cu under the optimum reaction conditions as
described earlier. After 20 h of reaction, pyrazole was almost
fully consumed, whereas maximum amount of indazole re-
mained inactivated. This clearly indicates that the activation of
the substrates depends on their shape. To get deeper insight
into the selective activation, we optimized the structures of
the substrates used in the catalytic reaction by DFT calculations
(see Figure S9). Evidently the dimension of the product of reac-
tion between indazole and p-nitrobromobenzene is very close
to the pore dimension of the catalyst as determined by nitro-
gen sorption analysis (see above).
Table 2. (Continued)
Entry[a] Amine
R
Product
Yield[b] [wt%] TOF[c] [hꢀ1
]
30
p-OMe
80
85
31
32
H
84, 82[d]
89, 87[d]
To justify this explanation, we studied all four reaction under
homogeneous condition maintaining optimized reaction con-
dition and data collected in Table S1. Products isolated from
homogeneous reactions were as expected and highly differed
from those with our modified catalyst. This evidence defends
our argument that larger substrates are facing difficulty to
enter into the channels in comparison to smaller ones
(Scheme 2).
p-NO2
90
95
33
34
m-NO2
o-NO2
87
78
92
83
35
36
p-OMe
72
87
76
92
H
Scheme 2. Test for shape selectivity of N-arylation using indazole and pyra-
zole with p-nitrobromobenzene.
[a] Reaction condition: Aryl bromide (1.2 mmol), N-containing heterocy-
cles (1.0 mmol), Cs2CO3 (2 mmol, 0.65 g) and IRMOF-3-PI-Cu (2 mg, Cu:
4.7ꢂ10ꢀ2 mol%) in 2 mL DMSO at 908C for 20 h under N2 atmosphere.
[b] Isolated yield. [c] molN-arylazolemolCuꢀ1 hꢀ1. [d] Fifth cycle.
Recovery and recycling of the catalyst
The catalyst IRMOF-3-PI-Cu is easily recoverable by filtration
and can be reused several times without any significant loss of
catalytic activity. As an amount of 0.002 g of solid catalyst was
not enough to recover, reuse, and finally characterize it, the
catalyst was collected from different batches of the same cata-
lytic cycle and average values of the data are reported here.
The cross-coupling reactions were performed with bromoben-
zene, pyrazole, and by using Cs2CO3 thus maintaining the opti-
mized reaction conditions. After the first cycle, the catalyst was
recovered by centrifugation and then washed thoroughly with
diethyl ether. The recovered catalyst was dried under vacuum
at 1208C overnight. Atomic absorption spectrometric (AAS)
analysis of the recovered catalysts confirmed the copper con-
tent of the IRMOF-3-PI-Cu remained the same. The per-
formance of the recycled catalyst in N-arylation reaction up to
five successive runs is shown in Figure 5. The catalytic efficien-
cy of the recovered catalyst remained almost the same in
ginning of the reactions, giving no noticeable induction
period.
Encouraged by the high efficacy of the catalyst for the reac-
tion of pyrazole as described above, we also examined the re-
activity of other N-containing heterocycles. Imidazole, benzimi-
dazole, 1,2,4-triazole, and 1,2,3-triazole were used as N-contain-
ing heterocycles to couple with substituted aryl bromide
(Table 2, entries 16–35). All of them followed the same type of
reactivity as observed for pyrazole. Aromatic amine (aniline)
could also be activated to yield the desired product with bro-
mobenzene (Table 2, entry 36).
We undertook few control experiments to look into the se-
lectivity of product in CꢀN cross-coupling reactions catalyzed
by IRMOF-3-PI-Cu. The coupling reaction was performed by
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in the above-described reactions. Though, this did not happen
for the present catalyst. Therefore, all the above tests convinc-
ingly demonstrated that there was no leaching of Cu species
occurring in the IRMOF-3-PI-Cu-catalyzed N-arylation reactions.
Attempts have been made in search of copper-catalyzed N-
arylation of heterocycles reactions, a few of which deserve par-
ticular mention. Most of them have been performed with CuI
systems and some with CuII.[33,34] Nevertheless, very few of
them have been used in catalytic reaction under heterogene-
ous conditions (see Table S2).[11c,d,34a,c,g] Choudary et al. used CuII
fluorapatite as a heterogeneous catalyst for N-arylation with
aryl halide as well as aryl boronic acid;[34a] however, with rela-
tively large metal loading (ꢁ8 mol%) compared to that of our
catalyst (ꢁ2.44ꢂ10ꢀ2 mol%). Rao et al. reported N-arylation of
imidazole and benzimidazole derivative with trans-b-iodostyr-
ene by recyclable CuO nanoparticles at relatively low tempera-
ture (808C).[34c] Wu et al. studied the cross-coupling reaction of
substituted arylboronic acid by a CuII-containing MOF in meth-
anol medium at room temperature.[11c] Zhao et al. reported
heterogeneous catalytic study by CuII–MOF system of arylbor-
onic acids with imidazole in methanol medium.[11d] Notably,
herein we for the first time disclosed size-selective CꢀN cross-
coupling reaction under heterogeneous condition with the
help of a porous postsynthetically modified Cu–MOF catalyst.
Besides, IRMOF-3-PI-Cu was capable to catalyze N-arylation re-
action of 1,2,3-triazole. The catalyst showed high catalytic ac-
tivity compared to other CuII salts and CuII complexes at opti-
mized conditions (see Table S1).
Figure 5. Comparison of catalytic activities of the catalyst for the cross-cou-
pling reaction between bromobenzene and different N-heterocycles for up
to five successive catalytic cycles.
every run. To check the stability of the recovered catalyst, it
was thoroughly characterized by powder XRD spectrometry, IR
spectral analysis, and gas adsorption study. Comparison of XRD
patterns (see Figure S5), IR spectra (see Figure S10), and gas
adsorption studies (see Figure S11) of the pristine and recov-
ered catalyst persuasively demonstrated structural integrity of
the compound retained after reactions. TEM measurements of
the recovered catalyst revealed no sign of formation of Cu
nanoparticles on the surface of the catalyst (see Figure S12).
Hot filtration test[31]
To test if copper was leaching out from the solid catalyst
during reaction, the liquid phase of the reaction mixture was
separated by filtration after 8 h of reaction, and the activity of
the supernatant solution was studied. As Cs2CO3 also filters out
during the filtration procedure and as the reaction does not
proceed without base, we added the optimal amount of
Cs2CO3 to filtrate. From the kinetic plot (see Figure S13) it was
noticed that after filtration, coupling reactions do not proceed
further. AAS analysis (sensitivity up to 0.001 ppm) of the super-
natant solution collected by filtration also confirmed the ab-
sence of copper ions in the liquid phase. The above results
suggested that Cu was not leached out from the solid catalyst
during the reaction.
Conclusions
We have successfully synthesized a new heterogeneous cata-
lyst by anchoring CuII Schiff-base moiety into porous isoreticu-
lar metal–organic framework (IRMOF-3) by a postsynthetic
method. The reusable catalyst exhibits high activity toward the
CꢀN cross-coupling reactions of a variety of azoles with aryl
bromides under relatively low temperature. The catalytic
system tolerates a broad range of functional groups. Bulkier
substrates that are large enough for diffusion through the
functionalized pores of the catalyst do not participate in the
catalytic reaction. Most significantly of all, easy recycling and
inner-wall modification is very important for further study of
this catalytic system. Further investigations on the application
of postsynthetically modified MOF catalysts in other organic re-
actions are currently on progress in our laboratory.
Three-phase test[32]
To ascertain if the reactions are truly heterogeneous, we have
performed the three-phase test for the N-arylation reaction by
using a designed aryl bromide in the optimal condition as de-
scribed in the Experimental Section (see also Scheme S1). The
isolated products in this process were 1-phenyl-1H-pyrazole
and p-bromoacetophenone. Notably, 1-acetophenone-1H-pyra-
zole was not detected. This clearly indicates that p-bromoace-
tophenone anchored on the MCM-41 did not participate in the
coupling reaction. If there was any leaching of Cu from the cat-
alyst, anchored p-bromoacetophenone would also participate
Experimental Section
Materials
2-Aminoterephthalic acid, pyridine-2-aldehyde, CuCl2·2H2O and all
other reagents were purchased either from Sigma–Aldrich or
Merck (India) and were used as received without further purifica-
tion. Required solvents were purchased from Merck (India) and
were distilled and dried before use.
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alyst in a glass batch reactor. To this solid mixture, nitrogen-con-
taining heterocycle (1.0 mmol) and dry DMSO solvent (2 mL) were
added (solid nitrogen-containing heterocycles were added directly
with the catalyst) and the reaction mixture was stirred at 908C in
an oil bath under nitrogen atmosphere. After cooling to RT, the
catalyst was first separated out by centrifugation and the solution
part was extracted with water and diethyl ether (2ꢂ15 mL). The or-
ganic layers thus collected were combined and washed with brine,
dried over Na2SO4, and concentrated in vacuum. The residue was
purified by column chromatography on silica gel (mesh 60–120)
using an n-hexane/ethyl acetate mixture as the eluent to give the
Measurements
Elemental analysis was performed by using the PerkinElmer 240C
elemental analyzer. FTIR spectra of KBr pellets were measured on
a PerkinElmer RX I FTIR spectrometer. The solid-state UV/Vis spec-
tral measurements were performed by using a Shimadzu UV/Vis
1700 spectrophotometer. Thermogravimetric differential thermal
analysis (TG–DTA) measurements were performed by using a Perki-
nElmer (Singapore) Pyris Diamond TG/DTA unit. The heating rate
was programmed at 48Cminꢀ1 with a protecting stream of N2
flowing at a rate of 150 mLminꢀ1. The powder XRD patterns of
samples were recorded on a Bruker D8 Advance diffractometer
using CuKa radiation at the desired temperature. The SEM image
and EDX spectra were analyzed by using a HITACHI S-4800 SEM in-
strument. The TEM measurement was undertaken on a JEOL-TEM-
2010 transmission electron microscope with an operating voltage
of 200 kV. The metal content of the sample was estimated on
a Varian Techtron AA-ABQ atomic absorption spectrometer. Gas
sorption isotherms were measured in the pressure range of 0–
1 bar by using an Autosorb iQ (Quantachrome Inc.) gas sorption
system. For all isotherms, warm and cold free-space correction
measurements were performed by using ultrahigh-purity He gas
(99.999% purity). N2 isotherms at 77 K were measured in a liquid-
nitrogen bath by using an ultrahigh purity grade (99.999%) gas
source. 1H NMR spectra were measured on a Bruker Avance DPX
300 NMR (300 MHz) spectrometer.
1
desired product. The product was analyzed by H NMR spectrome-
try and compared with literature value. The spectroscopic data
(1H NMR) of these products are consistent with the reported
values.
Hot filtration test
The reaction mixture of pyrazole (1 mmol, 0.068 g), Cs2CO3
(2 mmol, 0.65 g), and IRMOF-3-PI-Cu (0.002 g) in dry DMSO (2 mL)
with bromobenzene (1.2 mmol, 0.19 g) was stirred at 908C under
nitrogen atmosphere and the progress of the reaction was moni-
tored by TLC. After 8 h, the catalyst was separated at reaction tem-
perature by filtration (Whatman 1, pore size 11 mm); further Cs2CO3
(2 mmol, 0.65 g) was added to the solution and it was stirred at
that temperature for another 12 h, and the progress of the reaction
was examined by TLC analysis.
Syntheses
IRMOF-3, [Zn4O(ATA)3] (ATA=2-aminoterephthalate): Bulk samples
of IRMOF-3 were prepared by following the procedure reported
previously[24a,30] and were activated by guest exchange with
CHCl3.[22] After bulk-sample preparation and activation, the struc-
ture and purity of the material was confirmed by powder XRD
analysis. The yield was 46% (average). Elemental analysis was per-
formed on samples outgassed under vacuum (1208C, 12 h): calcd
(%) for Zn4O(ATA)3: C 35.46, H 1.93, N 5.24; found: C 35.4, H 1.9, N
5.1.
Preparation of PBA-MCM-41
The anchoring of (3-aminopropyl)-triethoxysilane (3-APTES) into
MCM-41 was achieved by stirring MCM-41 (0.1 g) with 3-APTES
(0.81 mmol, 0.18 g) in dry chloroform at RT for 12 h under nitrogen
atmosphere. The white solid, MCM-41-(SiCH2CH2CH2NH2)x thus pro-
duced was filtered and washed with chloroform and dichlorome-
thane. This solid was then heated at reflux with p-bromoacetophe-
none (50 mmol, 10 g) in ethanol (10 mL) for 3 h at 608C. The result-
ing yellowish solid PBA-MCM-41 was then collected by filtration
and dried in a desiccator (see Scheme S2).
IRMOF-3-PI, [Zn4O(ATA)3ꢀx(PITA)x] (PITA=2-pyridyl-imine terephtha-
late): Organic modification of IRMOF-3 was performed by using
pyridine-2-aldehyde (4 mmol, 0.428 g) in dry CH3CN (15 mL) as pre-
viously described.[24a] Elemental analysis was performed on samples
outgassed under vacuum (1208C, 12 h): calcd (%) for Zn4O(ATA)2.79-
(PITA)0.21: C 37.10, H 1.93, N 5.47; found: C 37.0, H 1.9, N 5.4. This
result corresponds to the functionalization of 7% of total amine
group present in IRMOF-3.
Three-phase test
A solution of bromobenzene (1.2 mmol, 0.24 g), pyrazole (1 mmol,
0.068 g), and CsCO3 (2 mmol, 0.65 g) in DMSO (2 mL) was stirred in
the presence of IRMOF-3-PI-Cu (0.002 g) and PBA-MCM-41 (1 g) at
908C for 20 h under N2 atmosphere. The reaction mixture was col-
lected by filtration and filtrate was then extracted with water/di-
ethyl ether and analyzed by TLC and 1H NMR spectrometry. The
residue obtained from the reaction mixture was then hydrolyzed
with 2n aqueous HCl solution (1.7 mL HCl in 8.5 mL H2O) for 4 h
under reflux. The resulting solution was neutralized, extracted with
dichloromethane, concentrated, and analyzed by 1H NMR spec-
trometry.
IRMOF-3-PI-Cu, [Zn4O(ATA)3ꢀx(PITA-CuCl2)x]: IRMOF-3-PI-Cu was pre-
pared by stirring a sample of 1 g of desolvated IRMOF-3-PI with
CuCl2 (0.5 mmol, 0.085 g) in dry CH3CN (12 mL) for 8 h under an
inert (nitrogen) atmosphere. The green solid thus formed was then
filtered and washed with CH3CN by using Soxhlet to remove un-
anchored copper species and dried in vacuum at 1208C. Elemental
analysis: calcd. (%) for Zn4O(ATA)2.79(PITA-CuCl2)0.21 (corresponding
to 7% amine functionalization and quantitative copper uptake): C
35.20, H 1.83, N 5.22; found: C 35.3, H 1.9, N 5.3. The amount of
copper in the final solid was determined by AAS analysis: calcd
(%): Cu 1.55; found: Cu 1.5 (2.36ꢂ10ꢀ2 mol%).
Preparation of Cu–salen complex
The complex was synthesized by following a previously reported
standardized procedure.[35] Ethylenediamine (1 mmol) and salicylal-
dehyde (2 mmol) were dissolved in ethanol (10 mL), and then
heated to boiling temperature. This was followed by the dropwise
addition of a solution of 1 mmol Cu(CH3COO)2 salt in 25 mL etha-
nol and 10 mL NaOH (2m) aqueous solution. The resultant solution
N-arylation coupling reactions
General procedure: At first, aryl halide (1.2 mmol) and Cs2CO3
(2 mmol, 0.650 g) were added to solid IRMOF-3-PI-Cu (0.002 g) cat-
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was stirred and heated at reflux for 1 h. After cooling, the solid
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Acknowledgements
Financial support from CSIR, New Delhi by a grant (Grant No.
01(2542)/11/EMR-II) (to S.K.) is gratefully acknowledged. T.M.
thanks the Council of Scientific and Industrial Research (CSIR),
New Delhi, for awarding him a fellowship. Authors also thank
the DST for funding the Department of Chemistry, Jadavpur Uni-
versity to procure a single-crystal X-ray diffractometer, an NMR
spectrometer and powder XRD system.
Keywords: copper
frameworks · nitrogen heterocycles · Schiff bases
·
cross-coupling
·
metal–organic
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Received: January 18, 2014
Revised: March 24, 2014
Published online on July 16, 2014
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