Nickel-catalyzed oxidative coupling of alkynes and arylboronic acids using the metal-organic framework Ni2(BDC)2(DABCO) as an efficient heterogeneous catalyst

Article abstract of DOI:10.1039/c3cy01053h

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. Ni₂(BDC)₂(DABCO) could be used as an efficient and recyclable heterogeneous catalyst for the cross-coupling reaction between phenylboronic acids and phenylacetylenes in the presence of molecular oxygen in air as the stoichiometric oxidant. High conversions of the cross-coupling products were achieved with no homo-coupling products being detected by GC. Ni₂(BDC)₂(DABCO) exhibited significantly higher catalytic activity in the coupling reaction than that of other Ni-MOFs such as Ni₃(BTC)₂ and Ni(HBTC)(BPY). Zn ₂(BDC)₂(DABCO) and Co₂(BDC)₂(DABCO) were found to be completely inactive for the transformation, while the reaction using Cu₂(BDC)₂(DABCO) as the catalyst led to the formation of a 10% homo-coupling product. To the best of our knowledge, a nickel-based catalyst was not previously used for the oxidative coupling reactions between phenylacetylenes and phenylboronic acids. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01053h

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Nickel-catalyzed oxidative coupling of alkynes and aryl boronic acids using metal-
organic framework Ni (BDC) (DABCO) as an efficient heterogeneous catalyst
2
2
*
Thanh Truong, Chung K. Nguyen, Thi V. Tran, Tung T. Nguyen, Nam T. S. Phan
Department of Chemical Engineering, HCMC University of Technology, VNU-HCM,
2
68 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
*
Email: ptsnam@hcmut.edu.vn
Ph: (+84 8) 38647256 ext. 5681
Fx: (+84 8) 38637504
Graphic abstract
The first Ni-catalyzed oxidative coupling of alkynes and aryl boronic acids

Catalysis Science & Technology
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Nickel-catalyzed oxidative coupling of alkynes and aryl boronic acids using metal-
organic framework Ni (BDC) (DABCO) as an efficient heterogeneous catalyst
2
2
*
Thanh Truong, Chung K. Nguyen, Thi V. Tran, Tung T. Nguyen, Nam T. S. Phan
Department of Chemical Engineering, HCMC University of Technology, VNUꢀHCM,
2
68 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
*
Email: ptsnam@hcmut.edu.vn
Ph: (+84 8) 38647256 ext. 5681
Fx: (+84 8) 38637504
Abstract
A crystalline porous metalꢀorganic framework Ni (BDC) (DABCO) was synthesized,
2
2
and characterized by Xꢀray powder diffraction (XRD), scanning electron microscopy
SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA),
(
Fourier transform infrared (FTꢀIR), atomic absorption spectrophotometry (AAS), and
nitrogen physisorption measurements. The Ni (BDC) (DABCO) could be used as an
2
2
efficient and recyclable heterogeneous catalyst for the crossꢀcoupling reaction between
phenylboronic acids with phenylacetylenes in the presence of molecular oxygen in air as
the stoichiometric oxidant. High conversions of the crossꢀcoupling products were
achieved with no homo-coupling products being detected by GC. The
Ni (BDC) (DABCO) exhibited significantly higher catalytic activity in the coupling
2
2
reaction than that of other NiꢀMOFs such as Ni (BTC) and Ni(HBTC)(BPY).
3
2
Zn (BDC) (DABCO) and Co (BDC) (DABCO) were found to be completely inactive for
2
2
2
2
the transformation, while the reaction using Cu (BDC) (DABCO) as catalyst led to the
2
2
formation of 10% homoꢀcoupling product. To the best of our knowledge, a nickelꢀbased
1

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catalyst was not previously used for the oxidative coupling reactions between
phenylacetylenes and phenylboronic acids.
Keywords: Metalꢀorganic framework; nickelꢀcatalyzed; alkynylation; heterogeneous
catalysts.
1
. Introduction
Arylalkyne moieties have emerged as versatile intermediates for the synthesis of several
pharmaceutical candidates and agrochemicals, as well as a variety of functional organic
1
ꢀ5
materials
. These structures are conventionally synthesized from the palladiumꢀ
catalyzed Sonogashira crossꢀcoupling reactions between aryl halides and terminal
6
ꢀ9
alkynes in the presence of CuI as coꢀcatalyst and phosphines as ligand . Many
modifications of the Sonogashira protocol have been developed to improve the efficiency
of this transformation. However, homoꢀcoupling byꢀproducts of the reactants were
normally observed. Zou and coꢀworkers previously reported the oxidative coupling
reaction of terminal alkynes with arylboronic acids catalyzed by a Pd(dppf)Cl and Ag O
2
2
1
0
system . Yang and Wu extended the scope to the cyclopalladated ferrocenylimine
catalyzed crossꢀcoupling reaction of arylboronic acids/esters with terminal alkynes in the
1
1
presence of Ag O as the additive . Subsequently, Li and coꢀworkers developed this
2
protocol to the crossꢀcoupling reactions between a variety of terminal alkynes and
1
2
numerous boronic acids in the presence of Pd(OA ) and Ag O mixture . Bao and coꢀ
C 2
2
workers offered an alternative protocol by using porous palladium nanospheres in
2

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1
3
conjunction with Ag O and triphenylphosphine as the catalyst system . To overcome
2
problems associated with palladiumꢀbased catalysts, palladiumꢀfree cross coupling
1
4
15, 16
reactions have been investigated, using CuBr/racꢀBINOL
or Cu O
as catalysts.
2
Although interesting results have been achieved, the development of a greener protocol
for this transformation should be targeted.
Metalꢀorganic frameworks (MOFs) have recently attracted significant attention as a new
1
7ꢀ25
class of porous materials with potential applications in several fields
. Although the
application of MOFs in catalysis has just emerged as a young research area, MOFs have
26, 27
been used as heterogeneous catalysts for a variety of organic transformations
,
2
8ꢀ33
34ꢀ37
ranging from carbonꢀcarbon
to carbonꢀheteroatom
forming reactions. However,
reports on nickelꢀbased MOFs as catalysts for organic transformations have been very
3
8ꢀ41
limited in the literature
. Kim and coꢀworkers previously demonstrated that nickel
nanoparticles embedded in the pores of a mesoporous MOF exhibited high activity in the
4
2
hydrogenolysis of nitrobenzene and the hydrogenation of styrene . similarly, Chou and
coꢀworkers pointed out that nickel nanoparticles supported on MOFꢀ5 could be a good
4
3
catalyst for the hydrogenation reaction of crotonaldehyde . Subsequently, Jiang and coꢀ
workers synthesized a nickel(salphen)ꢀbased MOF and used it as a heterogeneous catalyst
4
4
for the synthesis of cyclic carbonates . Recently, Canivet and coꢀworkers reported the
application of a nickel complex grafted within the MILꢀ101 framework as a catalyst for
4
5
the liquidꢀphase ethylene dimerization to selectively form 1ꢀbutene . In this work, we
wish to report the application of the NiꢀMOF Ni (BDC) (DABCO) as an efficient and
2
2
selective heterogeneous catalyst for the oxidative crossꢀcoupling reaction between
3

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phenylboronic acid with phenylacetylene. The Ni (BDC) (DABCO) catalyst could be
2
2
facilely separated from the reaction mixture, and could be reused without a significant
degradation in activity. To the best of our knowledge, a nickelꢀbased catalyst was not
previously used for this transformation.
2
. Experimental
2
.1. Materials and instrumentation
All reagents and starting materials were obtained commercially from SigmaꢀAldrich 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
o
vacuum at 150 C for 3 h. A Netzsch Thermoanalyzer STA 409 was used for
o
thermogravimetric analysis (TGA) with a heating rate of 10 C/min under a nitrogen
atmosphere. Xꢀray powder diffraction (XRD) patterns were recorded using a Cu Kα
radiation 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 Ni (BDC) (DABCO) sample
2
2
was dispersed on holey carbon grids for TEM observation. Elemental analysis with
atomic absorption spectrophotometry (AAS) was performed on an AAꢀ6800 Shimadzu.
Fourier transform infrared (FTꢀIR) spectra were obtained on a Nicolet 6700 instrument,
with samples being dispersed on potassium bromide pallets.
4

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Gas chromatographic (GC) analyses were performed using a Shimadzu GC 2010ꢀPlus
equipped with a flame ionization detector (FID) and an SPBꢀ5 column (length = 30 m,
inner diameter = 0.25 mm, and film thickness = 0.25 ꢁm). The temperature program for
o
o
o
GC analysis heated samples from 120 to 130 C at 40 C/min; heated from 130 to 180 C
o
o
o
at 40 C/min and held them at 180 C for 0.5 min; heated them from 180 to 280 C at 50
o
o
C/min and held them at 280 C for 2 min. Inlet and detector temperatures were set
o
constant at 280 C. nꢀHexadecane was used as an internal standard to calculate reaction
conversions. GCꢀMS analyses were performed using a Hewlett Packard GCꢀMS 5972
with a RTXꢀ5MS column (length = 30 m, inner diameter = 0.25 mm, and film thickness =
o
0
.5 ꢁm). The temperature program for GCꢀMS analysis heated samples from 60 to 280 C
o
o
at 10 C/min and held them at 280 C for 2 min. Inlet temperature was set constant at 280
o
C. MS spectra were compared with the spectra gathered in the NIST library.
2
.2. Synthesis of the metal-organic framework Ni (BDC) (DABCO)
2 2
4
6
In a typical preparation , a solid mixture of H BDC (H BDC = 1,4ꢀbenzenedicarboxylic
2
2
acid; 0.415 g, 2.5 mmol), DABCO (DABCO = 1,4ꢀdiazabicyclo[2.2.2]octane; 0.168 g,
.5 mmol), and Ni(NO ) .6H O (0.58 g, 2 mmol) was dissolved in DMF (DMF = N,N’ꢀ
1
3
2
2
dimethylformamide; 15 ml). The resulting solution was distributed to two 20 ml vials.
o
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 mother
liquor and washed in DMF (3 x 10 ml) for 3 days. Solvent exchange was carried out with
methanol (3 x 10 ml) at room temperature for 3 days. The material was then evacuated
5

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o
under vacuum at 140 C for 6 h, yielding 0.425 g of Ni (BDC) (DABCO) in the form of
2
2
green crystals (76% yield).
2
.3. Catalytic studies
The Ni (BDC) (DABCO) was used as a catalyst for the nickelꢀcatalyzed crossꢀcoupling
2
2
of phenylacetylene and phenylboronic acids to achieve diphenylacetylenes as the
principal product. In a typical experiment, a preꢀdetermined amount of
Ni (BDC) (DABCO) was added to the flask containing a mixture of phenylacetylene
2
2
(0.110 ml, 1 mmol), phenylboronic acid (0.244 g, 2 mmol), 1,8ꢀdiazabicycloundecꢀ7ꢀene
(DBU) (0.299 ml, 2 mmol), 2,2’ꢀbipyridine (0.031 g, 0.2 mmol) as ligand, and nꢀ
hexadecane (0.1 ml) as internal standard in N,Nꢀdimethylformamide (DMF) (5 ml). The
catalyst loading as calculated based on the molar ratio of nickel/phenylacetylene. The
o
reaction mixture was stirred at 120 C for 180 min. The reaction conversion was
monitored by withdrawing aliquots from the reaction mixture at different time intervals,
quenching with an aqueous KOH solution (5%, 1 ml), drying over anhydrous Na SO ,
2
4
analyzing by GC with reference to nꢀhexadecane, and further confirming product identity
by GCꢀMS. To investigate the recyclability of Ni (BDC) (DABCO) in the reaction
2
2
between phenylacetylene and phenylboronic acid, the catalyst was filtered from the
reaction mixture after the experiment, washed with copious amounts of DMF and
o
methanol, dried under air at 85 C for 1 h, and reused if necessary. For the leaching test,
a catalytic reaction 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. Reaction
progress, if any, was monitored by GC as previously described.
6

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3
. Results and discussion
Scheme 1. The crossꢀcoupling reaction of phenylacetylene and phenylboronic acids using
the Ni (BDC) (DABCO) as catalyst.
2
2
The Ni (BDC) (DABCO) was synthesized by a solvothermal method, using the reaction
2
2
of nickel nitrate hexahydrate and 1,4ꢀbenzenedicarboxylic acid in DMF, according to a
4
6
literature procedure . The material was then characterized by several techniques,
including XRD, SEM, TEM, TGA, FTꢀIR, AAS, and nitrogen physisorption
measurements (Fig. S1 – Fig. S7). The NiꢀMOF was used as a solid catalyst for the
nickelꢀcatalyzed crossꢀcoupling of phenylacetylene and phenylboronic acid to achieve
diphenylacetylene as the principal product (Scheme 1). Initial studies addressed the effect
of temperature on the reaction conversion. The crossꢀcoupling reaction was carried out in
DMF using 10 mol% Ni (BDC) (DABCO) as catalyst, in the presence of two equivalents
2
2
of DBU as a base and 20 mol% 2,2’ꢀbipyridine as a ligand, with the
o
o
o
phenylacetylene:phenylboronic acid molar ratio of 1:2, at 80 C, 100 C, and 120 C,
respectively. 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
o
found that the reaction carried out at 120 C could afford 86% conversion after 180 min.
o
As expected, decreasing the reaction temperature to 100 C resulted in a significant drop
7

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in the reaction rate, with 76% conversion being observed after 180 min. The reaction
o
proceeded with dificulty at 80 C, though 51% conversion was still obtained after 180
min (Fig. 1). Moreover, it was observed that a phenylacetylene/phenylboronic acid molar
ratio of 1/2 should be used. Using more than two equivalents of phenylboronic acid was
unnecessary as the reaction rate was not enhanced significantly (Fig. 2).
Fig. 1. Effect of temperature on reaction conversions.
8

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Fig. 2. Effect of phenylacetylene:phenylboronic acid molar ratio on reaction conversion.
The effect of percent of catalyst was then investigated. It was found that the percent of
catalyst exhibited a significant effect on the reaction conversion. The reaction using 10
mol% Ni (BDC) (DABCO) as catalyst could proceed to 86% conversion after 180 min.
2
2
Decreasing the catalyst loading to 7.5 mol% resulted in 78% conversion, while the
reaction using 5 mol% catalyst afforded 69% conversion after 180 min (Fig. 3).
Previously, the oxidative crossꢀcoupling reaction between terminal alkynes with
arylboronic acids could occur in the presence of different amounts of catalyst, depending
on the nature of catalyst as well as the nature of the substrates. These catalyst systems
1
2
included 0.01 mol% Pd(OAc) and 150 mol% Ag O , 0.5 mol% cyclopalladated
2
2
4
7
ferrocenylpyridine and 100 mol% Ag O , 1 mol% cyclopalladated ferrocenylimine and
2
1
1
1
00 mol% Ag O , 1 mol% palladium nanoparticles in conjunction with 10 mol%
2
9

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1
3
triphenylphosphine and 100 mol% Ag O , 5 mol% Pd(OAc) and 15 mol% TEMPO
2
2
4
8
15, 16
under electrooxidation condition with Ag anode , and 10 mol% Cu O
.
2
Fig. 3. Effect of catalyst loading on reaction conversions.
The oxidative crossꢀcoupling reaction between terminal alkynes with arylboronic acids
4
9, 50
were previously carried out in different solvents
. Yang and Wu carried out the
reaction with cyclopalladated ferrocenylimine and Ag O as catalyst in dichloromethane
2
1
1
,
while Li and coꢀworkers reported that acetonitrile was the solvent of choice for the
1
2
reaction using Pd(OAC) and Ag O mixture as catalyst . For Cu Oꢀcatalyzed coupling
2
2
2
reactions, Rao and coꢀworkers demonstrated that the transformation should be carried out
1
6
in chloroform , whereas Li and coꢀworkers pointed out that the reaction proceeded
1
5
readily in a mixture of dioxane and isopropanol . The crossꢀcoupling reactions were
carried out in DMF, DEF, DMAc, DMSO, and NMP as solvent. It was found that DEF
was not suitable for the reaction, affording only 33% conversion after 180 min. This
1
0

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value could be improved to 67% and 72% for the reaction carried out in DMAc and
NMP, respectively. Both DMF and DMSO could be used as solvent for the reaction, with
8
6% conversion being achieved after 180 min (Fig. 4).
Fig. 4. Effect of different solvents on reaction conversion.
Li and coꢀworkers previously employed a series of bases for the reaction between
terminal alkynes and boronic acids in the presence of Pd(OAC) and Ag O mixture, and
2
2
1
2
reported that best results were achieved with K CO as the base . However, Yang and
2
3
Wu tested various bases for the palladiumꢀcatalyzed oxidative coupling, and pointed out
1
1
that the transformation should be carried out in the presence of KOAc . Using Cu O as
2
catalyst, Li and coꢀworkers also screened several bases for the arylation transformation,
1
5
and achieved best results with tꢀBuOK . Rao and coꢀworkers did not employed a base
for the Cu Oꢀcatalyzed reaction between terminal alkynes with arylboronic acids, though
2
1
6
the presence of pyridine as an additive was required . In this work, a series of bases
1
1

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were employed for the Ni (BDC) (DABCO)ꢀcatalyzed oxidative coupling reaction,
2
2
including DBU, triethylamine, piperidine, K PO , K CO , Na CO , NaOAc, and tꢀBuOK,
3
4
2
3
2
3
respectively. Experimental results indicated that among these bases, only DBU should be
used for the transformation, with 86% conversion being achieved after 180 min. The
transformation proceeded with difficulty in the presence of other bases (Fig. 5).
Moreover, it was also found that using less than two equivalents of the base resulted in a
significant drop in the reaction rate, with 38% conversion being detected after 180 min
for the reaction using one equivalent of DBU. Employing more than two equivalents of
DBU for the reaction did not accelerate the crossꢀcoupling significantly. It should be
noted that no product was detected for the reaction in the absence of the base (Fig. 6).
Fig. 5. Effect of different bases on reaction conversion.
1
2

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Fig. 6. Effect of base concentration on reaction conversion.
It was decided to investigate the effect of ligand nature on the reaction conversion,
having used 1,10ꢀphenanthroline, 2,2’ꢀbipyridine, ethylenediamine, and acetylacetone as
o
ligand, respectively. The reaction was carried out in DMF at 120 C using 10 mol%
Ni (BDC) (DABCO) as catalyst, in the presence of two equivalents of the base and 20
2
2
mol% ligand, with the phenylacetylene:phenylboronic acid molar ratio of 1:2. It was
observed that ethylenediamine and acetylacetone were ineffective for the reaction, with
less than 5% conversion being detected after 180 min. The reaction conversion could be
improved to 60% after 180 min in the presence of 1,10ꢀphenanthroline as ligand, while
the reaction using 2,2’ꢀbipyridine as ligand could proceed to 86% conversion after 180
min (Fig. 7). Moreover, in the absence of the ligand, only 26% conversion was afforded.
Decreasing the amount of ligand to one equivalent resulted in a drop in the reaction rate,
though 77% conversion was still obtained after 180 min. However, it was found that
1
3

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using three equivalents of the ligand also slowed down the reaction, with 78% conversion
being detected after 180 min (Fig. 8).
Fig. 7. Effect of different ligands on reaction conversion.
Fig. 8. Effect of ligand concentration on reaction conversions.
1
4

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Table 1. Reaction scope with respect to alkynes
Entry
1
Alkynes
Product
Convesion (%)
86
2
67
3
4
81
47
Conditions: DMF (5 mL), alkynes (1 mmol), arylboronic acids (2 mmol).
Reaction scope with respect to various alkynes was shown in Table 1. Alkynylations by
phenyl boronic acids were conducted under optimal conditions. Phenylacetylenes with
methyl or methoxy group substituents were active and arylated products were obtained in
6
7 % and 81 %, respectively (entries 2 and 3). Interestingly, alkynylation by aliphatic
terminal alkyne was also possible and arylation of 1ꢀoctyne afforded moderate yield
entry 4). It is worth to mention that arylation of aliphatic alkynes, in many cases, require
(
5
1
strict conditions due to their facile isomerization .
We then expanded the research to the Ni (BDC) (DABCO)ꢀcatalyzed oxidative coupling
2
2
reaction using different phenylboronic acids. It was observed that 4ꢀ
1
5

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carboxyphenylboronic acid was not suitable for the system due to the interaction with the
base in the reaction mixture. 4ꢀFormylphenylboronic acid was found to be more reactive
than phenylboronic acid, affording 91% conversion after 180 min. The presence of a
halogen atom in the phenylboronic acid slowed down the reaction, with 69% and 51%
conversions being observed after 180 min of 4ꢀchlorophenylboronic acid and 4ꢀ
florophenylboronic acid, respectively. 4ꢀMethoxyphenylboronic acid and 4ꢀ
ethylphenylboronic acid were also found to be less reactive than phenylboronic (Table 2).
Bao and coꢀworkers previously used porous palladium nanospheres in conjuction with
Ag O and triphenylphosphine as the catalyst system for the oxidative cross coupling
2
reactions and demonstrated that no obvious differences were detected between the case of
1
3
arylboronic acids with electronꢀwithdrawing and with electronꢀdonating groups .
Table 2. Arylation of phenylacetylene by various aryl boronic acids
Entry
1
Boronic acids
Product
Convesion (%)
92
2
< 2
3
4
70
51
1
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5
6
65
30
Conditions: DMF (5 mL), alkynes (1 mmol), arylboronic acids (2 mmol).
To highlight the advantages of using the Ni (BDC) (DABCO) catalyst, the catalytic
2
2
activity of the Ni (BDC) (DABCO) was compared with that of other NiꢀMOFs including
2
2
Ni (BTC) and Ni(HBTC)(BPY), and that of other M (BDC) (DABCO) including
3
2
2
2
Zn (BDC) (DABCO), Cu (BDC) (DABCO), and Co (BDC) (DABCO). Interestingly, it
2
2
2
2
2
2
was observed that Ni (BTC) and Ni(HBTC)(BPY) were significantly less active than the
3
2
Ni (BDC) (DABCO), affording 51% and 59% conversions after 180 min. Under the
2
2
same condition, the reaction using Ni (BDC) (DABCO) could proceed to 86%
2
2
conversion after 180 min. These results would confirm the necessary of the DABCO
linker of the NiꢀMOF structure in the crossꢀcoupling reaction. Moreover,
Zn (BDC) (DABCO) and Co (BDC) (DABCO) were found to be inactive, with no trace
2
2
2
2
amount of the product being detected by GC. In contrast, the reaction using
Cu (BDC) (DABCO) as catalyst could proceed 72% conversion after 180 min (Fig. 9).
2
2
However, this copperꢀcatalyzed transformation led to the formation of 10% homoꢀ
coupling product in the product mixture. It should be noted that no homoꢀcoupling
product was detected by GC in the Ni (BDC) (DABCO)ꢀcatalyzed oxidative reaction.
2
2
These observations would emphasize the advantages of using the Ni (BDC) (DABCO)
2
2
catalyst.
1
7

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Fig. 9. Effect of different MOFs catalysts on reaction conversions.
To gain insights into the reaction pathway of the Ni (BDC) (DABCO)ꢀcatalyzed
2
2
oxidative transformation, the role of the air was clarified. The reaction was carried out in
o
DMF at 120 C using 10 mol% NiꢀMOF as catalyst, in the presence of two equivalents of
the base and 20 mol% ligand, with the phenylacetylene:phenylboronic acid molar ratio of
1
:2. After 30 min under air with 50% conversion, argon was introduced to the system to
o
replace the air, and the reaction mixture was heated at 120 C with magnetic stirring for
another 60 min. Interestingly, no further reaction was observed in the absence of air, with
5
0% conversion was obtained after 90 min. It should be noted that the reaction could
proceed to 81% conversion after 90 min under air. After 90 min, air was introduced back
o
to the system, and the reaction mixture was heated at 120 C with magnetic stirring for
additional 90 min. It was found that the reaction rate was significantly enhanced in the
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presence of air, as compared to the case of argon (Fig. 10). We also investigated whether
contaminated water in air could play an important role on reaction. Specifically, only
trace amount of product was detected by GC when the reaction was carried out under
argon atmosphere with added H O (0.5 mmol). These results would confirm the role of
2
the oxygen as an oxidant. Furthermore, we applied pure oxygen, tertꢀbutyl
hydroperoxide, tertꢀbutyl perbenzoate, and cumyl peroxide, respectively, as the oxidants
for the reaction. However, it was observed that most of phenylacetylene was converted to
benzoic acid in the presence of these oxidants, while the crossꢀcoupling product was
detected only in trace amounts. Apparently, employing atmospheric oxygen as the
stoichiometric oxidant should be the best choice for economical and environmentally
benign processes.
Fig. 10. Effect of argon and water on reaction conversions.
In further mechanistic studies, a radical trapping reagent, (2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀ
yl)oxy (TEMPO), was added to the reaction mixture. If the reactions proceed through a
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radical pathway, it is likely that the catalytic cycle would be terminated in the presence of
TEMPO. The reaction was carried out with added TEMPO (1 equiv.). It was observed
that similar conversions were obtained for reactions with and without TEMPO (Fig. 11).
With all of these results, the reaction mechanism was proposed (Scheme 2).
Fig. 11. Effect of TEMPO on reaction routes
Our proposed mechanism is consistent with experimental data and similar to ChanꢀLam
5
2
reaction mechanism . Deprotonation of phenylacetylenes followed by nucleophilic
attack to form phenylacetylide Ni(II) species. Transmetalation with aryl boronic acid
affords diaryl Ni(II) intermediates. This step was previously reported to be facile in the
5
3
presence of base . Atmospheric oxygen is required to oxidize diaryl Ni(II) to diaryl
Ni(III) complexes before reductive elimination. It is wellꢀknown that reductive
5
4
elimination is much faster from high oxidation state metal species . Nickel (I)
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intermediates formed by reductive elimination reacts with oxygen oxidant to regenerate
Ni(II) complexes and finish the catalytic cycle. The intermediacy of Ni(II)/Ni(III)
5
5
catalytic cycles was recently proposed in the literature .
Ar1 Base
Ni(II)
]
[
Oxidation
Air
Deprotonation
(
I)
[
Ni ]
Ar2
Ar1
[
Ni(II)
]
Ar
Reductive elimination
2
Ar ꢀB(OH)2
2
(III)
Ar1
Ar Ni
Transmetalation
2
(II)
Ar1
Oxidation
Ar Ni
Air
Scheme 2. Proposed reaction mechanism through Ni(II)/Ni(III) catalytic cycle
In order to confirm if active nickel species dissolved from the solid Ni (BDC) (DABCO)
2
2
catalyst could contribute to the total conversion of the oxidative coupling reaction
between phenylacetylene and phenylboronic acid, a control experiment was carried out
using a simple filtration during the course of the reaction. The coupling reaction was
o
carried out in DMF at 120 C using 10 mol% Ni (BDC) (DABCO) as catalyst, in the
2
2
presence of two equivalents of the base and 20 mol% ligand, with the
phenylacetylene:phenylboronic acid molar ratio of 1:2. The Ni (BDC) (DABCO) catalyst
2
2
was separated from the reaction mixture after 30 min reaction time by simple filtration.
The reaction solution was then transferred to a new reactor vessel, and stirred for an
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1

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o
additional 150 min at 120 C with aliquots being sampled at different time intervals, and
analyzed by GC. It was found that almost no further conversion was detected after the
Ni (BDC) (DABCO) catalyst was removed from the reaction mixture (Fig. 12). This
2
2
result could confirm that the reaction between phenylacetylene and phenylboronic acid
could only occur in the presence of the solid Ni (BDC) (DABCO) catalyst, and the
2
2
contribution of catalytically active species soluble in the solution, if any, was negligible.
Fig. 12. Leaching test indicated no contribution from homogeneous catalysis of active
species leaching into reaction solution.
The Ni (BDC) (DABCO) catalyst was also investigated for recoverability and reusability
2
2
over six successive runs, by repeatedly separating the NiꢀMOF catalyst from the reaction
mixture, washing it and then reusing it. The coupling reaction was carried out in DMF at
o
1
20 C using 10 mol% Ni (BDC) (DABCO) as catalyst, in the presence of two
2
2
equivalents of the base and 20 mol% ligand, with the phenylacetylene:phenylboronic acid
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molar ratio of 1:2. After the crossꢀcoupling reaction was completed, the
Ni (BDC) (DABCO) catalyst was separated by simple filtration, washed with copious
2
2
amounts of DMF and methanol to remove any physisorbed reagents, and dried under air
o
at 120 C for 1 h. The recovered NiꢀMOF was then reused in further reactions under
identical conditions to those of the first run. It was observed that the
Ni (BDC) (DABCO) catalyst could be recovered and reused several times without a
2
2
significant degradation in catalytic activity. Indeed, a conversion of 86% was still
th
th
achieved in the 4 run. However, lower conversion was observed from the 5 run (Fig.
3). The recovered NiꢀMOF was also characterized by FTꢀIR and XRD. The FTꢀIR
spectra of the reused Ni (BDC) (DABCO) exhibited a similar absorption as compared to
1
2
2
that of the fresh NiꢀMOF (Fig. 14). It was observed that the crystallinity of the reused Niꢀ
MOF was slightly different to that of the fresh catalyst. However, XRD result indicated
that the reused Ni (BDC) (DABCO) was still highly crystalline (Fig. 15).
2
2
Fig. 13. Catalyst recycling studies
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Fig. 14. FTꢀIR spectra of the fresh (a) and reused (b) Ni (BDC) (DABCO) catalyst.
2
2
6
4
3
1
400
800
200
600
0
(
a)
(
b)
5
10
15
20
-Theta-Scale
25
30
2
Fig. 15. Xꢀray powder diffractograms of the fresh (a) and reused (b) Ni (BDC) (DABCO)
2
2
catalyst.
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The concern about the actual catalyst in the reaction was also addressed. AAS analysis
indicated that 29.6 ppm of copper and less than 5 ppm of palladium were presented in the
catalyst. To test the possibility that contaminated palladium and copper can promote the
formation of desired product, several control experiments were conducted using identical
conditions (Fig.16). In particular, reactions with 10 % of CuI or Cu(OAc) afforded less
2
than 60 % conversion under optimal conditions. Noticeably, only trace amount of product
was detected in similar reaction using 0.1 % Pd(OAc) . These results revealed that the
2
copper and palladium catalysis is unlikely and NiꢀMOF is actual catalyst for the
transformation.
Fig. 16. Control experiments using homogeneous copper and palladium catalysts
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4
. Conclusions
In summary, the metalꢀorganic framework Ni (BDC) (DABCO) was synthesized, and
2
2
characterized by several techniques including XRD, SEM, TEM, FTꢀIR, TGA, AAS, and
nitrogen physisorption measurements. The NiꢀMOF could be used as an efficient
heterogeneous catalyst for the oxidative crossꢀcoupling reaction between phenylboronic
acid with phenylacetylene in the presence of molecular oxygen in air as the
stoichiometric oxidant. High conversion to the crossꢀcoupling product was achieved with
no homoꢀcoupling product being detected by GC. The Ni (BDC) (DABCO) exhibited
2
2
significantly higher catalytic activity in the oxidative cross coupling reactions than that of
Ni (BTC) , Ni(HBTC)(BPY), Zn (BDC) (DABCO), Cu (BDC) (DABCO), and
3
2
2
2
2
2
Co (BDC) (DABCO). Moreover, although Cu (BDC) (DABCO) could proceed to
2
2
2
2
reasonable conversion, approximately 10% homoꢀcoupling product was observed. The
Ni (BDC) (DABCO) could be recovered and reused several times without a significant
2
2
degradation in catalytic activity. The fact that no homoꢀcoupling product was detected for
the Ni (BDC) (DABCO)ꢀcatalyzed coupling reaction was therefore advantageous. To the
2
2
best of our knowledge, a nickelꢀbased catalyst was not previously used for the oxidative
reaction between phenylacetylene and phenylboronic acid.
Acknowledgements
The Viet Nam National Foundation for Science and Technology Development ꢀ
NAFOSTED is acknowledged for financial support through project code 104.99ꢀ2012.25.
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