Towards applications of metal-organic frameworks in catalysis: C-H direct activation of benzoxazole with aryl boronic acids using Ni2(BDC) 2(DABCO) as an efficient heterogeneous catalyst

Article abstract of DOI:10.1039/c3cy00503h

A crystalline porous metal-organic framework Ni₂(BDC) ₂(DABCO) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Ni₂(BDC)₂(DABCO) could be used as an efficient heterogeneous catalyst for the nickel-catalyzed direct heterocycle C-H arylation reactions between azoles and arylboronic acids forming 2-arylbenzoxazoles as the principal product. This approach avoids the use of hazardous aryl halides as well as the formation of halide byproducts in the synthesis of aryl-substituted benzoxazoles. The Ni₂(BDC) ₂(DABCO) exhibited significantly higher catalytic activity than that of other Ni-MOFs such as Ni₃(BTC)₂, Ni(HBTC)(BPY), and that of some common nickel salts such as NiCl₂, Ni(NO ₃)₂, Ni₂SO₄, and Ni(OAc) ₂. To the best of our knowledge, application of Ni ₂(BDC)₂(DABCO) in the field of catalysis as well as the C-C cross coupling reaction via direct C-H functionalization using a nickel heterogeneous catalyst have not previously been reported in the literature. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00503h

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Towards applications of metal–organic
frameworks in catalysis: C–H direct activation
of benzoxazole with aryl boronic acids
using Ni2(BDC)2(DABCO) as an efficient
heterogeneous catalyst†
Cite this: Catal. Sci. Technol., 2014,
4, 369
*
Nam T. S. Phan, Chung K. Nguyen, Tung T. Nguyen and Thanh Truong
A crystalline porous metal–organic framework Ni2(BDC)2(DABCO) was synthesized and characterized by
X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectro-
photometry (AAS), and nitrogen physisorption measurements. The Ni2(BDC)2(DABCO) could be used as an
efficient heterogeneous catalyst for the nickel-catalyzed direct heterocycle C–H arylation reactions
between azoles and arylboronic acids forming 2-arylbenzoxazoles as the principal product. This approach
avoids the use of hazardous aryl halides as well as the formation of halide byproducts in the synthesis of
aryl-substituted benzoxazoles. The Ni2(BDC)2(DABCO) exhibited significantly higher catalytic activity than
that of other Ni-MOFs such as Ni3(BTC)2, Ni(HBTC)(BPY), and that of some common nickel salts such as
NiCl2, Ni(NO3)2, Ni2SO4, and Ni(OAc)2. To the best of our knowledge, application of Ni2(BDC)2(DABCO) in
the field of catalysis as well as the C–C cross coupling reaction via direct C–H functionalization using a
nickel heterogeneous catalyst have not previously been reported in the literature.
Received 15th July 2013,
Accepted 12th October 2013
DOI: 10.1039/c3cy00503h
www.rsc.org/catalysis
through direct C–H activation. Miura and co-workers discov-
ered that the combination of NiBr2 and nitrogen-containing
1. Introduction
Metal-mediated C-arylation of heterocycle compounds
through direct C–H activation has recently attracted signifi-
cant attention as a shorter and more efficient synthetic
approach than the conventional cross-coupling methods.1,2
The direct functionalization of C–H bonds would avoid or
minimize the preparation of prefunctionalized starting mate-
rials, thus improving atom-economics and environmental
aspects of the synthetic schemes.3–5 Enormous effort has
been devoted to achieve C–C bond formation through direct
C–H functionalization, employing palladium-, rhodium-,
ruthenium-, or iridium-based catalyst systems.6–13 Daugulis
and Do previously demonstrated a significant breakthrough
in the copper-catalyzed direct heterocycle C–H arylation reac-
tion of various heteroarenes with haloarenes using copper
salts as catalysts.2,14,15 Recently, nickel-based catalysts have
emerged as promising candidates for organic transformations
compounds such as 2,2′-bipyridine or 1,10-phenanthroline as
ligands could offer high activity for the direct coupling of azole
derivatives and organoboronic acids.16 This protocol avoids the
use of hazardous aryl halides as well as the formation of
halide byproducts. Lei and co-workers demonstrated a novel
oxidative arylation of cyclic ethers with arylboronic acid
employing Ni(acac)2 as the catalyst precursor in the presence
of triphenylphosphine (PPh3) as the ligand and di-tert-butyl
peroxide as the oxidant.17 For the development of greener pro-
cesses, however, heterogeneous catalysts should be targeted in
terms of the ease of handling, simple workup, recyclability
and reusability.18
Metal–organic frameworks (MOFs) have emerged as a new
class of porous materials with potential applications in several
fields including gas separation and storage, sensors and
luminescence, drug storage and delivery, templated low-
dimensional material preparation, and catalysis.19–24 Although
the application of MOFs in catalysis has been investigated only
for the past few years, several MOFs have been used as hetero-
geneous catalysts for a variety of organic transformations, rang-
ing from carbon–carbon to carbon–heteroatom forming
reactions.25–37 However, nickel-based MOFs as catalysts for
organic transformations have been rarely reported in the
Department of Chemical Engineering, HCMC University of Technology, VNU-HCM,
268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam.
E-mail: ptsnam@hcmut.edu.vn; Fax: +84 8 38637504;
Tel: +84 8 38647256 ext. 5681
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00503h
This journal is © The Royal Society of Chemistry 2014
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literature. Kim and co-workers previously demonstrated that
nickel nanoparticles embedded in the pores of a mesoporous
MOF could be used as a catalyst for the hydrogenolysis of nitro-
benzene or the hydrogenation of styrene.38 Similarly, Chou and
co-workers reported that nickel nanoparticles supported on
MOF-5 exhibited high activity in the hydrogenation reaction of
crotonaldehyde.39 Recently, Jiang and co-workers employed
a nickel(salphen)-based MOF as a heterogeneous catalyst for
the synthesis of cyclic carbonates.40 In this work, we report
the nickel-catalyzed direct heterocycle C–H arylation reaction
between azoles and arylboronic acids using Ni2(BDC)2(DABCO)
as an efficient heterogeneous catalyst. The Ni-MOF catalyst
could be facilely isolated from the reaction mixture and could
be reused without a significant degradation in activity. To the
best of our knowledge, the application of Ni2(BDC)2(DABCO) in
the field of catalysis as well as the C–C cross coupling reaction
via direct C–H functionalization using a nickel heterogeneous
catalyst have not previously been reported in the literature.
0.25 mm, and film thickness = 0.5 μm). The temperature pro-
gram for GC-MS analysis heated samples from 60 to 280 °C at
10 °C min−1 and held them at 280 °C for 2 min. The inlet tem-
perature was set to a constant value of 280 °C. MS spectra
were compared with the spectra in the NIST library.
2.2. Synthesis of the metal–organic framework
Ni2(BDC)2(DABCO)
In a typical preparation, a solid mixture of H2BDC (H2BDC =
1,4-benzenedicarboxylic acid; 0.332 g, 2 mmol), DABCO
(DABCO = 1,4-diazabicyclo[2.2.2]octane; 0.112 g, 1 mmol),
and Ni(NO3)2·6H2O (0.58 g, 2 mmol) was dissolved in DMF
(DMF = N,N′-dimethylformamide; 15 ml). The resulting solu-
tion was distributed among two 20 ml vials. The vials were
then heated at 100 °C in an isothermal oven for 48 h. After
cooling the vials to room temperature, the solid product was
removed by decanting with the mother liquor and washed in
DMF (3 × 10 ml) for 3 days. Solvent exchange was carried out
with methanol (3 × 10 ml) at room temperature for 3 days.
The material was then evacuated under vacuum at 140 °C for
6 h, yielding 0.385 g of Ni2(BDC)2(DABCO) in the form of
green crystals (69% yield).
2. Experimental
2.1. Materials and instrumentation
All reagents and starting materials were obtained commer-
cially from Sigma-Aldrich and Merck, and were used as
received without any further purification unless otherwise
noted. Nitrogen physisorption measurements were conducted
using a Micromeritics 2020 volumetric adsorption analyzer
system. Samples were pretreated by heating under vacuum at
150 °C for 3 h. A Netzsch Thermoanalyzer STA 409 was used
for thermogravimetric analysis (TGA) with a heating rate of
10 °C min−1 under a nitrogen atmosphere. X-ray powder dif-
fraction (XRD) patterns were recorded using a Cu Kα radia-
tion source on a D8 Advance Bruker powder diffractometer.
Scanning electron microscopy studies were conducted on a
S4800 Scanning Electron Microscope (SEM). Transmission
electron microscopy studies were performed using a JEOL
JEM 1400 Transmission Electron Microscope (TEM) at 100 kV.
The Ni2(BDC)2(DABCO) sample was dispersed on holey car-
bon grids for TEM observation. Elemental analysis with
atomic absorption spectrophotometry (AAS) was performed
on an AA-6800 Shimadzu. Fourier transform infrared (FT-IR)
spectra were obtained on a Nicolet 6700 instrument, with
samples being dispersed on potassium bromide pellets.
Gas chromatographic (GC) analysis was performed using a
Shimadzu GC 2010-Plus equipped with a flame ionization
detector (FID) and an SPB-5 column (length = 30 m, inner
diameter = 0.25 mm, and film thickness = 0.25 μm). The tem-
perature program for GC analysis heated samples from 120 to
130 °C at 40 °C min−1, then heated them from 130 to 180 °C
at 40 °C min−1 and held them at 180 °C for 0.5 min, then
heated them from 180 to 280 °C at 50 °C min−1 and held them
at 280 °C for 2 min. The inlet and detector temperatures were
set to a constant value of 280 °C. n-Hexadecane was used as
an internal standard to calculate reaction conversions. GC-MS
analysis was performed using a Hewlett Packard GC-MS 5972
with a RTX-5MS column (length = 30 m, inner diameter =
2.3. Catalytic studies
The Ni2(BDC)2(DABCO) was used as a catalyst for the
nickel-catalyzed coupling of benzoxazole and arylboronic
acids to achieve phenylbenzoxazole derivatives. In a typical
experiment, a pre-determined amount of Ni2(BDC)2(DABCO)
was added to the flask containing a mixture of benzoxazole
(0.119 g, 1 mmol), phenylboronic acid (0.244 g, 2 mmol),
K3PO4 (0.532 g, 2 mmol), 2,2′-bipyridine (0.031 g, 0.2 mmol)
as ligand, and n-hexadecane (0.1 ml) as internal standard in
N,N-dimethylacetamide (DMAc) (5 ml). The catalyst concen-
tration was calculated based on the molar ratio of nickel/
benzoxazole. The reaction mixture was stirred at 100 °C
for 180 min. The reaction conversion was monitored by
withdrawing aliquots from the reaction mixture at different
time intervals, quenching with an aqueous NaOH solution
(5%, 1 ml), drying over anhydrous Na2SO4, analyzing by GC
with reference to n-hexadecane, and further confirming the
product identity by GC-MS. To investigate the recyclability of
the Ni2(BDC)2(DABCO), the catalyst was filtered from the reac-
tion mixture after the experiment, washed with copious
amounts of DMF, dried at 140 °C under vacuum for 6 h, and
reused if necessary. For the leaching test, the catalytic reac-
tion was stopped after 30 min, analyzed by GC, and filtered to
remove the solid catalyst. The reaction solution was then
stirred for a further 150 min. The reaction progress, if any,
was monitored by GC as previously described.
3. Results and discussion
3.1. Catalyst synthesis and characterization
In this work, the Ni2(BDC)2(DABCO) was synthesized in a
yield of 69% by a solvothermal method, according to a
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slightly modified literature procedure.41 The Ni-MOF was
then characterized by several techniques. Elemental analysis
by AAS indicated a nickel loading of 3.55 mmol g−1. The X-ray
diffraction patterns of the Ni2(BDC)2(DABCO) (Fig. S1†)
exhibited the presence of very sharp peaks, being consistent
with the theoretical patterns from the single crystal data
previously reported in the literature.42,43 The SEM micro-
graph of the Ni-MOF showed the formation of a crystalline
powder (Fig. S2†). As expected, the TEM observation of the
Ni2(BDC)2(DABCO) revealed that a porous structure was
obtained (Fig. S3†). However, the pore structure of the Ni-MOF
was more apparently complex than that of conventionally used
microporous and mesoporous materials due to the bent BDC
linker molecules and the type of guest molecules in the pores.
Nitrogen physisorption measurements indicated that the
Ni2(BDC)2(DABCO) is a well-defined crystalline microporous
material, with a median pore width of 6.112 Å being detected
(Fig. S4†). Our results are consistent with previous reports.42
Calculations from nitrogen adsorption/desorption isotherm
data showed that a Langmuir surface area and BET surface
area of 2006 m2 g−1 and 1473 m2 g−1, respectively, were
achieved for the material (Fig. S5†). TGA result indicated that
the Ni-MOF was stable up to over 200 °C (Fig. S6†). FT-IR spec-
tra of the Ni2(BDC)2(DABCO) exhibited a significant difference
as compared to those of the H2BDC and the DABCO linkers,
revealing the deprotonation of –COOH groups in the H2BDC
upon the reactions with nickel cations (Fig. S7†).41
of view, it should be noted that carrying out the
Ni2(BDC)2(DABCO)-catalyzed coupling reaction at 120 °C led
to the significant decomposition of benzoxazole. Indeed, Liu
and co-workers also carried out the direct coupling reaction
of azoles with boronic acids using a mixture of 5 mol%
Pd(OAc)2 and 10 mol% Cu(OAc)2 as a bimetallic catalytic
system in the temperature range of 80–100 °C.44 However,
Miura and co-workers previously employed NiBr2 as a catalyst
and nitrogen-containing compounds such as 2,2′-bipyridine
or 1,10-phenanthroline as ligands for the direct coupling of
benzoxazole with 4-methylphenylboronic acid, and demon-
strated that a high yield could be obtained at 120 °C.16
For liquid-phase organic transformations using solid cata-
lysts, the effect of different solvents on the reaction rate is nor-
mally an important issue that should be seriously
considered.45,46 In the first example of the C–H arylation reac-
tion between benzoxazole and 4-methylphenylboronic acid
using NiBr2 as the catalyst, Miura and co-workers demon-
strated that DMAc offered better performance for the reaction
compared to other solvents such as dimethyl sulfoxide (DMSO),
DMF, and N-methyl-2-pyrrolidone (NMP).16 Itami and
co-workers reported that 1,4-dioxane offered better perfor-
mance compared to toluene, dimethoxyethane (DME), DMF,
and NMP for the Ni(OAc)2-catalyzed C–H arylation reaction of
azoles with haloarenes.47,48 It was therefore decided to investi-
gate the effect of different solvents on the reaction conversion,
having used DMAc, DMF, DEF, NMP, toluene, DMSO, and
1,4-dioxane as the reaction solvent. The coupling reaction
was carried out at 100 °C using the reagent molar ratio of
1 : 2, in the presence of 10 mol% Ni2(BDC)2(DABCO) catalyst
with two equivalents of K3PO4 as the base and 20 mol% of
2,2′-bipyridine as the ligand. It was found that toluene and
DMSO were completely ineffective for the reaction, with no
trace amount of the product being detected. 1,4-Dioxane
was also unsuitable for the reaction, affording only 11%
conversion after 180 min. DEF and NMP offered better
performance than 1,4-dioxane, with 27% and 18% conver-
sions, respectively, being observed after 180 min. The
Ni2(BDC)2(DABCO)-catalyzed coupling reaction carried out in
3.2 Catalytic studies
The Ni2(BDC)2(DABCO) was assessed for its catalytic activity
in the C–H arylation of benzoxazole with phenylboronic
acid to form 2-phenylbenzoxazole as the principal product
(Scheme 1). Initial studies addressed the effect of tem-
perature on the reaction conversion of benzoxazole to
2-phenylbenzoxazole. The coupling reaction was carried out
in DMAc, at a benzoxazole : phenylboronic acid molar ratio of
1 : 2, using two equivalents of K3PO4 as a base, in the pres-
ence of 10 mol% Ni2(BDC)2(DABCO) catalyst and 20 mol% of
2,2′-bipyridine as a ligand, at 80 °C, 90 °C, 100 °C, and
120 °C. Aliquots were withdrawn from the reaction mixture at
different time intervals and analyzed by GC, giving kinetic
data during the course of the reaction. It was observed that
the reaction proceeded with difficulty at 80 °C, affording a
conversion of 23% after 180 min. As expected, increasing the
reaction temperature resulted in a significant enhancement
in the reaction rate, with 61% and 75% conversions being
achieved after 180 min for the reactions carried out at 90 °C
and 100 °C, respectively (Fig. 1). From the experimental point
Scheme 1 The direct heterocycle C–H arylation reactions between
azoles and arylboronic acids using Ni2(BDC)2(DABCO) as a catalyst.
Fig. 1 Effect of temperature on reaction conversion.
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DMF could offer 60% conversion after 180 min. Among
these solvents, DMAc exhibited the best performance, and
should be the solvent of choice for the C–H arylation of
occurred in the absence of the Ni2(BDC)2(DABCO) catalyst,
confirming the necessity of using the Ni-MOF as catalyst for
the direct heterocycle C–H arylation reaction between azoles
and arylboronic acids (Fig. 4). Indeed, in the first example of
the Ni-catalyzed C–H arylation reaction between benzoxazole
and 4-methylphenylboronic acid, up to 10 mol% NiBr2 was
employed as catalyst.16 Lei and co-workers previously reported
the oxidative arylation of cyclic ethers with arylboronic
acids using 10 mol% Ni(acac)2 catalyst in the presence of
triphenylphosphine (PPh3) as the ligand and di-tert-butyl
peroxide as the oxidant.17 Moreover, Liu and co-workers
successfully performed the direct coupling reaction of azoles
with boronic acids using a mixture of 5 mol% Pd(OAc)2 and
benzoxazole
and
phenylboronic
acid
using
the
Ni2(BDC)2(DABCO) catalyst (Fig. 2). Moreover, it was found
that increasing the benzoxazole : phenylboronic acid molar
ratio to 1 : 2.5 could improve the reaction conversion to 86%
after 180 min (Fig. 3).
With these results in mind, we then decided to investigate
the effect of catalyst concentration on the reaction conver-
sion. The coupling reaction was carried out at 100 °C in
DMAc, using the reagent molar ratio of 1 : 2.5, in the presence
of two equivalents of K3PO4 as the base or 20 mol% of
2,2′-bipyridine as the ligand, with 5 mol%, 7.5 mol%, or
10 mol% catalyst. It was found that the C–H arylation reac-
tion between benzoxazole and phenylboronic acid using
10 mol% Ni2(BDC)2(DABCO) catalyst could proceed readily to
86% conversion after 180 min. As expected, decreasing the
catalyst concentration to 7.5 mol% resulted in a drop in the
reaction rate, with 73% conversion being obtained. The
coupling reaction using 5 mol% could still afford 65% con-
version after 180 min. It should be noted that no reaction
10 mol% Cu(OAc)2 as
a
bimetallic catalytic system.44
The Ni2(BDC)2(DABCO) catalyst concentrations used in the
arylation of benzoxazole with phenylboronic acid via C–H
activation in this study were therefore comparable to those in
the literature.
As for most metal-mediated arylation reactions through C–H
direct functionalization, the presence of at least one equivalent
of a base should be required to speed up the transmetallation
step in the catalytic cycle of the reaction. Indeed, in the first
example of the Ni-catalyzed C–H arylation reaction between
benzoxazole and 4-methylphenylboronic acid using NiBr2 cata-
lyst, Miura and co-workers screened different bases for the reac-
tion, and pointed out that K3PO4 should be the base of choice
for this transformation.16 Similarly, Liu and co-workers
performed the direct coupling reaction of azoles with boronic
acids using a mixture of 5 mol% Pd(OAc)2 and 10 mol%
Cu(OAc)2 as a bimetallic catalytic system, and also observed
that K3PO4 was the most effective base.44 However, You and
co-workers demonstrated that KF should be employed for the
C–H arylation reaction between azoles with boronic acids using
a mixture of 5 mol% Pd(OAc)2 and 10 mol% CuCl as a bimetal-
lic catalytic system.49 It was therefore decided to investigate the
effect of different bases on the reaction conversion, using
t
Na2CO3, K2CO3, K3PO4, CH3COONa, BuOK, piperidine, and
1,8-diazabicycloundec-7-ene (DBU). The C-arylation reaction
was carried out at 100 °C in DMAc, at the reagent molar ratio of
Fig. 2 Effect of different solvents on reaction conversion.
Fig.
3 Effect of benzoxazole : phenylboronic acid molar ratio on
reaction conversion.
Fig. 4 Effect of catalyst concentration on reaction conversion.
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1 : 2.5, using two equivalents of the base and 20 mol% of
2,2′-bipyridine as the ligand, in the presence of 10 mol%
Ni2(BDC)2(DABCO) catalyst. Na2CO3, CH3COONa, tBuOK,
K2CO3, and piperidine were found to be unsuitable for the
coupling reaction with lower than 10% conversion being
observed after 180 min. When employing DBU as the base for
the C–H arylation reaction, a conversion of 51% was obtained
after 180 min. Among these bases, K3PO4 exhibited the best
performance, producing the desired product in a conversion of
86% after 180 min (Fig. 5).
Another factor that should be considered for the C–H
arylation reaction between benzoxazole and phenylboronic
acid using the Ni2(BDC)2(DABCO) catalyst is the nature of the
organic ligand. In the first example of the Ni-catalyzed C–H
arylation reaction using NiBr2 catalyst, Miura and co-workers
screened several organic ligands, and reported that
1,10-phenanthroline and 2,2′-bipyridine could be used for
this transformation, although 2,2′-bipyridine exhibited better
performance than 1,10-phenanthroline.16 Liu and co-workers
also employed 1,10-phenanthroline or 2,2′-bipyridine as a
ligand for the reaction, and pointed out that a better
result was observed for the case of 1,10-phenanthroline.44
We then decided to investigate the effect of the ligand nature
on the Ni2(BDC)2(DABCO)-catalyzed C–H arylation reaction
between benzoxazole and phenylboronic acid, using
1,10-phenanthroline, 2,2′-bipyridine, triphenylphosphine,
ethylenediamine, and acetylacetone as the ligand. The C–H
arylation reaction was carried out at 100 °C in DMAc, at
the reagent molar ratio of 1 : 2.5, using two equivalents of
K3PO4 as the base and 20 mol% ligand, in the presence of
10 mol% Ni2(BDC)2(DABCO) catalyst. It was observed that
triphenylphosphine, ethylenediamine, and acetylacetone were
ineffective for the reaction, with less than 10% conversion
being detected. The reaction using 1,10-phenanthroline could
proceed to 56% conversion after 180 min. 2,2′-Bipyridine was
found to be more effective for the reaction, affording 86%
Fig. 6 Effect of different ligands on reaction conversion.
conversion after 180 min (Fig. 6). It should be noted that no
reaction occurred in the absence of the ligand, confirming
the necessity of the organic ligand in the Ni-catalyzed C–H
arylation reaction. Moreover, it was observed that the ligand
concentration also exhibited a slight effect on the reaction
conversion. Using a ligand concentration of 10 mol%, a con-
version of 83% was still obtained after 180 min. However,
increasing the ligand concentration to 30 mol% also led to a
slight drop in the reaction rate, with 78% conversion being
observed after 180 min (Fig. 7). Indeed, 20 mol% ligand was
previously employed for the C–H arylation reaction using
NiBr2 catalyst,16 and up to 30 mol% ligand was employed for
the case of the Pd(OAc)2/Cu(OAc)2 bimetallic catalytic
system.44
To emphasize the advantages of using Ni2(BDC)2(DABCO)
as catalyst for the C–H arylation reaction between
benzoxazole and phenylboronic acid, the catalytic activity of
the Ni2(BDC)2(DABCO) was compared with that of other
Ni-MOFs including Ni3(BTC)2 and Ni(HBTC)(BPY) (H3BTC =
1,3,5-benzenetricarboxylic acid, BPY = 4,4′-bipyridyl), and that
of common nickel salts including NiCl2, Ni(NO3)2, NiSO4, and
Ni(OAc)2. The C–H arylation reaction was carried out at 100 °C
Fig. 5 Effect of different bases on reaction conversion.
Fig. 7 Effect of ligand concentration on reaction conversion.
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the advantages of using the Ni2(BDC)2(DABCO) as a catalyst for
the transformation.
To verify the possibility that the adsorption of the
2-phenylbenzoxazole product on the catalytically active nickel
sites in the Ni2(BDC)2(DABCO) could hinder the C–H
arylation reaction between benzoxazole and phenylboronic
acid, a control experiment was carried out, adding the prod-
uct to the reaction mixture during the course of the reaction.
The reaction was carried out at 100 °C in DMAc, at the
reagent molar ratio of 1 : 2.5, using two equivalents of K3PO4
as the base and 20 mol% of 2,2′-bipyridine as the ligand, in
the presence of 10 mol% Ni2(BDC)2(DABCO) catalyst. After
30 min reaction time with 43% conversion being observed,
one equivalent of 2-phenylbenzoxazole was added to the reac-
tion solution. The resulting mixture was stirred for another
150 min at 100 °C, with aliquots being sampled at different
time intervals, and analyzed by GC. Interestingly, it was
found that the C–H arylation reaction stopped immediately
after the product was added, with 43% conversion being
detected after 180 min (Fig. 10). The fact that no further con-
version was observed upon adding the 2-phenylbenzoxazole
product to the reaction mixture would confirm the inhibition
role of the product in the Ni2(BDC)2(DABCO)-catalyzed C–H
arylation reaction between benzoxazole and phenylboronic
acid. Indeed, Kholdeeva and co-workers previously employed
Fe and Cr-containing MOFs as heterogeneous catalysts for
the oxidation reaction of cyclohexene and α-pinene with
molecular oxygen, and also reported a similar observation, in
which the reaction stopped completely after the product was
added to the reaction mixture.50 However, further studies on
the adsorption–desorption equilibrium of the product on the
Ni-MOF catalyst in the C–H arylation reaction are necessary.
As for several liquid-phase organic transformations, the
possibility that some of catalytically active sites on the solid
catalyst could dissolve into the solution during the course of
the reaction should be seriously addressed. The soluble spe-
cies might exhibit catalytic activity, and therefore the reaction
would not proceed under real heterogeneous catalysis
Fig.
conversion.
8 Effect of different nickel salts as catalysts on reaction
in DMAc, at the reagent molar ratio of 1 : 2.5, using two equiva-
lents of K3PO4 as the base and 20 mol% of 2,2′-bipyridine as
the ligand, in the presence of 10 mol% nickel catalyst. It was
observed that the coupling reaction using NiCl2, Ni(NO3)2
Ni2SO4, and Ni(OAc)2 could proceed to 62%, 51%, 54%, and
58% conversion, respectively, after 180 min (Fig. 8).
Interestingly, it was found that Ni2(BDC)2(DABCO) could
offer significantly higher activity than other two Ni-MOFs,
with 86% conversion being achieved after 180 min. The
Ni3(BTC)2-catalyzed C–H arylation reaction afforded 47%
conversion, while 61% conversion was observed after 180 min
for the case of Ni2(BDC)2(DABCO). Furthermore, the C–H
arylation reaction between benzoxazole and phenylboronic acid
was also carried out in the presence of Cu2(BDC)2(DABCO) or
Co2(BDC)2(DABCO) as the catalyst under similar reaction
conditions (Fig. 9). Interestingly, it was found that only 10%
conversion was detected after 180 min, thus emphasizing
Fig. 10 Adding product to the reaction mixture when using
Fig. 9 Effect of different MOFs as catalysts on reaction conversion.
374 | Catal. Sci. Technol., 2014, 4, 369–377
Ni2(BDC)2(DABCO) as the catalyst.
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conditions. In order to determine if active nickel species
dissolved from the solid Ni2(BDC)2(DABCO) catalyst contribute
to the total conversion of the C–H arylation reaction between
benzoxazole and phenylboronic acid, a control experiment was
carried out using a simple centrifugation during the course of
the reaction. If the C–H arylation reaction still proceeded to
higher conversion after the solid Ni-MOF was removed from
the reaction mixture, this behavior would demonstrate that the
coupling reaction would not occur under real heterogeneous
catalysis conditions, and there should be a contribution from
leached active nickel species to the catalysis process. The C–H
arylation reaction was carried out at 100 °C in DMAc, at the
reagent molar ratio of 1 : 2.5, using two equivalents of K3PO4 as
the base and 20 mol% of 2,2′-bipyridine as the ligand, in the
presence of 10 mol% Ni2(BDC)2(DABCO) catalyst. The solid
Ni-MOF catalyst was separated from the reaction mixture after
30 min reaction time by simple centrifugation. The reaction
solution was then transferred to a new reactor vessel, two
equivalents of K3PO4 were added, and the resulting mixture
was stirred for an additional 150 min at 100 °C with aliquots
being sampled at different time intervals, and analyzed by GC.
It was observed that almost no further conversion was
detected after the solid Ni-MOF catalyst was removed from the
reaction mixture (Fig. 11). The fact that no further reaction
conversion was obtained would indicate that the C–H arylation
reaction between benzoxazole and phenylboronic acid could
only occur in the presence of the solid Ni2(BDC)2(DABCO) cat-
alyst, and there should be no contribution from catalytically
active nickel species soluble in the liquid phase.
reusing it. The C–H arylation reaction was carried out at 100 °C
in DMAc, at the reagent molar ratio of 1:2.5, using two equiva-
lents of K3PO4 as the base and 20 mol% of 2,2′-bipyridine as the
ligand, in the presence of 10 mol% Ni2(BDC)2(DABCO)
catalyst. After the C–H arylation reaction was complete, the
Ni2(BDC)2(DABCO) catalyst was removed from the reaction mix-
ture by simple centrifugation, then washed with copious
amounts of DMF to remove any physisorbed reagents, and dried
at 140 °C under vacuum for 6 h. The recovered Ni-MOF was then
reused in further C–H arylation reaction between benzoxazole
and phenylboronic acid under identical conditions to those of
the first run. It was found that the Ni2(BDC)2(DABCO) catalyst
could be recovered and reused several times without a signifi-
cant degradation in catalytic activity. Indeed, a conversion of
77% was still obtained in the 7th run (Fig. 12). To confirm the
recoverability and reusability of the Ni2(BDC)2(DABCO) catalyst,
the recovered Ni-MOF was also characterized by XRD, pore size
distribution, and FT-IR. Indeed, the XRD and pore size distribu-
tion results of the reused Ni2(BDC)2(DABCO) catalyst showed
that the crystallinity and pore structure of the Ni-MOF remained
almost unchanged during the course of the C–H arylation reac-
tion (Fig. 13 and 14). Moreover, the FT-IR spectra of the reused
In order to achieve greener processes, the ability to recover
and reuse the solid catalyst should be seriously addressed. In
the best cases, the solid catalyst can be facilely separated from
the reaction mixture, and reused several times before it eventu-
ally deactivates completely. The recoverability and reusability of
the Ni2(BDC)2(DABCO) catalyst in the C–H arylation reaction
between benzoxazole and phenylboronic acid was investigated
over seven successive runs, by repeatedly separating the Ni-MOF
catalyst from the reaction mixture, washing it and then
Fig. 12 Catalyst recycling studies.
Fig. 11 Leaching test indicated no contribution from the
homogeneous catalysis of active species leaching into the reaction
solution.
Fig. 13 X-ray powder diffractograms of the fresh (a) and reused (b)
Ni2(BDC)2(DABCO) catalyst.
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Catalysis Science & Technology
Fig. 14 Pore size distribution of the fresh Ni2(BDC)2(DABCO) and that
recovered for the 5th time.
Fig. 16 Effect of different phenylboronic acids on reaction
conversion.
Ni2(BDC)2(DABCO) revealed similar absorption as compared to
that of the fresh catalyst (Fig. 15).
4-fluorophenylboronic acid proceeded slowly under these
conditions, with 6%, 19%, 25%, and 29% conversion being
detected after 180 min (Fig. 16).
The study was then extended to the Ni2(BDC)2(DABCO)-
catalyzed C–H arylation reaction between benzoxazole and dif-
ferent phenylboronic acids, including 4-formylphenylboronic
acid, 4-carboxylphenylboronic acid, 4-chlorophenylboronic
acid, 4-fluorophenylboronic acid, phenylboronic acid,
4-methoxyphenylboronic acid, and 4-ethylphenylboronic
acid. The C–H arylation reaction was carried out at 100 °C in
DMAc, at the reagent molar ratio of 1 : 2.5, using two equiva-
lents of K3PO4 as the base and 20 mol% of 2,2′-bipyridine as
the ligand, in the presence of 10 mol% Ni2(BDC)2(DABCO)
catalyst. It was found that the presence of a substituent on
the benzene ring in the phenylboronic acid led to a drop in
the reaction conversion. Among these reagents, it was
observed that phenylboronic acids with an electron-donating
group offered higher reactivity in the C–H arylation reaction
than those with electron-withdrawing substituents. The
Ni2(BDC)2(DABCO)-catalyzed C–H arylation reaction between
benzoxazole and 4-methoxyphenylboronic acid afforded 67%
conversion after 180 min, while 47% conversion was
obtained for the case of 4-ethylphenylboronic acid. The reac-
tion of benzoxazole with 4-carboxyphenylboronic acid,
4-formylphenylboronic acid, 4-chlorophenylboronic acid, and
4. Conclusions
In summary, the metal–organic framework Ni2(BDC)2(DABCO)
was synthesized by
a solvothermal method, and was
characterized by several techniques including XRD, SEM, TEM,
FT-IR, TGA, AAS, and nitrogen physisorption measurements.
The Ni2(BDC)2(DABCO) could be used as an efficient heteroge-
neous catalyst for the nickel-catalyzed direct heterocycle C–H
arylation reaction between azoles and arylboronic acids to form
2-arylbenzoxazoles as the principal product. This approach
avoids the use of hazardous aryl halides as well as the forma-
tion of halide byproducts in the synthesis of aryl-substituted
benzoxazoles. The Ni-MOF catalyst could be facilely isolated
from the reaction mixture, and could be reused without a
significant degradation in activity. The direct heterocycle C–H
arylation could only occur in the presence of the solid Ni-MOF
catalyst, and there was no contribution from leached active
sites present in the solution. The Ni2(BDC)2(DABCO) exhibited
significantly higher catalytic activity than that of other
Ni-MOFs such as Ni3(BTC)2, Ni(HBTC)(BPY), and that of some
common nickel salts such as NiCl2, Ni(NO3)2, Ni2SO4, and
Ni(OAc)2. Cu2(BDC)2(DABCO) and Co2(BDC)2(DABCO) were
found to be inactive for the C–H arylation reaction. To the best
of our knowledge, the application of Ni2(BDC)2(DABCO) in the
field of catalysis as well as the C–C cross coupling reaction via
direct C–H functionalization using a nickel heterogeneous
catalyst have not previously been reported in the literature.
Acknowledgements
The Ho Chi Minh City Department of Science and Technol-
ogy (Vietnam) is acknowledged for financial support through
contract no. 289/2012/HĐ-SKHCN.
Fig. 15 FT-IR spectra of the fresh (a) and reused (b)
Ni2(BDC)2(DABCO) catalyst.
376 | Catal. Sci. Technol., 2014, 4, 369–377
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25 A. Corma, H. García and F. X. Llabrés i Xamena, Chem. Rev.,
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