The arylation of aldehydes with arylboronic acids using metal-organic framework Ni(HBTC)BPY as an efficient heterogeneous catalyst

Article abstract of DOI:10.1016/j.molcata.2012.08.015

A highly crystalline porous Ni(HBTC)BPY was synthesized from the reaction of 1,3,5-benzenetricarboxylic acid, nickel nitrate hexahydrate, and 4,4′-bipyridine by a solvothermal method. Physical characterizations of the solid catalyst were achieved by using several techniques, including X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Ni(HBTC)BPY exhibited high catalytic activity in the arylation of aldehydes with arylboronic acids to form diarylmethanols. The catalyst could be separated from the reaction mixture by simple filtration or centrifugation, and could be reused without a significant degradation in catalytic activity. No contribution from homogeneous catalysis of active species leaching into the liquid phase was detected.

Full text of DOI:10.1016/j.molcata.2012.08.015

Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The arylation of aldehydes with arylboronic acids using metal-organic
framework Ni(HBTC)BPY as an efficient heterogeneous catalyst
Nam T.S. Phan∗, Tung T. Nguyen, Anh H. Ta
Department of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2012
Received in revised form 17 August 2012
Accepted 20 August 2012
Available online 25 August 2012
A highly crystalline porous Ni(HBTC)BPY was synthesized from the reaction of 1,3,5-benzenetricarboxylic
acid, nickel nitrate hexahydrate, and 4,4ꢀ-bipyridine by a solvothermal method. Physical characteriza-
tions of the solid catalyst were achieved by using several techniques, including X-ray powder diffraction
(XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimet-
ric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and
nitrogen physisorption measurements. The Ni(HBTC)BPY exhibited high catalytic activity in the arylation
of aldehydes with arylboronic acids to form diarylmethanols. The catalyst could be separated from the
reaction mixture by simple filtration or centrifugation, and could be reused without a significant degra-
dation in catalytic activity. No contribution from homogeneous catalysis of active species leaching into
the liquid phase was detected.
Keywords:
Metal-organic framework
Arylation
Nickel catalyst
Heterogeneous
Ni(HBTC)BPY
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
recyclability and reusability [17]. At the same time, the catalyst
recovery also minimizes the contamination of the desired prod-
Diarylmethanols have attracted significant attention as versa-
tile intermediates in the synthesis of several biologically important
natural products as well as material science target molecules
[1–3]. Traditionally, these structures could be achieved by per-
forming the reduction of ketones or the addition of aldehydes
with organometallic reagents such as organolithium, organomag-
nesium, and organozinc compounds [1,2]. Compared with these
reagents, organoboron compounds exhibit several advantages,
including the significantly higher stability towards moisture and
air, the ease of handling and removal of boron-containing by-
products, the tolerance of various functional groups, and the
diversity of organoboron derivatives that are environmentally safer
than other organometallic reagents [4]. Miyaura and co-workers
previously demonstrated the first example of a rhodium-catalyzed
addition reaction of arylboronic acids to aldehydes [5], leading to
several reports on this organic transformation using other transi-
tional metal catalysts, such as palladium [6–10], nickel [2,11,12],
copper [1], iron [13], and dirhodium [14] complexes. However,
employing these homogeneous catalysts still suffers a number of
serious drawbacks, such as generation of a large amount of wastes,
tedious workup and difficult product purification [15,16]. From the
view of green chemistry, using heterogeneous catalysts would offer
several advantages, such as the ease of handling, simple workup,
ucts with hazardous or harmful metals [18,19]. To the best of our
knowledge, the arylation of aldehydes with arylboronic acids using
solid catalyst was not previously reported in the literature.
Metal-organic frameworks (MOFs) have been considered as
promising materials for their potential applications in several
fields, such as gas separation and storage, sensors, nonlinear
optics, drug storage and delivery, biomedical imaging, and catal-
ysis [20–28]. Combining some special properties of both organic
and inorganic porous materials, MOFs possess various advan-
tages such as high surface areas, well-defined structures, ability
to tune pore size, adjustable internal surface properties, the
ease of processability, and structural diversity [20–25,29–31].
Although catalysis is the youngest application of MOFs, several
MOFs have been employed as solid catalysts or catalyst supports
for a variety of organic transformations [32], including the cat-
alytic production of nitric oxide [33], ketalization reaction [34],
Henry reaction [35], Friedel–Crafts reaction between pyrroles and
nitroalkenes [36], Friedel–Crafts alkylation and acylation [37–39],
oxidation [40–45], alkene epoxidation [46–48], cycloaddition
of CO2 with epoxides [49], cyanosilylation [50,51], hydrogena-
tion [52], Suzuki cross-coupling [53,54], Sonogashira reaction
[55], transesterification reaction [56], Knoevenagel condensation
[57–59], aldol condensation [60,61], aza-Michael condensation
[62], 1,3-dipolar cycloaddition reactions [63], N-methylation of
aromatic primary amines [64], epoxide ring-opening reaction
[65–67], sequential alkene epoxidation/epoxide ring-opening reac-
tions [68], methanolytic kinetic resolution of styrene oxide [69],
∗
Corresponding author. Tel.: +84 8 38647256x5681; fax: +84 8 38637504.
E-mail address: ptsnam@hcmut.edu.vn (N.T.S. Phan).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.015

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N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
hydrolysis of ammonia borane [70], and cyclopropanation of alkene
[71]. In this work, we wish to report the utilization of a highly
porous metal-organic framework {Ni(HBTC)BPY} as an efficient
heterogeneous catalyst for the addition reaction of arylboronic
acids to aldehydes. The MOF catalyst could be easily isolated from
the reaction mixture by simple filtration, and could be reused with-
out significant degradation in activity.
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 Quanta-
chrome 2200e 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 under a nitrogen atmosphere. X-ray powder diffraction
(XRD) patterns were recorded using a Cu K␣ radiation source on
a D8 Advance Bruker powder diffractometer. Scanning electron
microscopy studies were conducted on a JSM 740 scanning electron
microscope (SEM). Transmission electron microscopy studies were
performed using a JEOL JEM 1400 transmission electron micro-
scope (TEM) at 100 kV. The Ni(HBTC)BPY samples were dispersed
on holey carbon 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 Bruker TENSOR37 instrument, with samples being
dispersed on potassium bromide pallets.
Gas chromatographic (GC) analyses were performed using a Shi-
madzu GC 17-A equipped with a flame ionization detector (FID)
and a DB-5 column (length = 30 m, inner diameter = 0.25 mm, and
film thickness = 0.25 ␮m). The temperature program for GC anal-
ysis heated samples from 100 to 200 ◦C at 25 ◦C/min and held
them at 200 ◦C for 1 min; then heated them from 200 to 250 ◦C
at 50 ◦C/min; and held them at 250 ◦C for 1 min. Inlet and detector
temperatures were set constant at 300 ◦C. n-Hexadecane was used
as an internal standard to calculate reaction conversions. GC–MS
analyses were performed using a Hewlett Packard GC-MS 5972
with an RTX-5MS column (length = 30 m, inner diameter = 0.25 mm,
and film thickness = 0.5 ␮m). The temperature program for GC–MS
analysis heated samples from 60 to 280 ◦C at 10 ◦C/min and held
them at 280 ◦C for 2 min. Inlet temperature was set constant at
280 ◦C. MS spectra were compared with the spectra gathered in
the NIST library.
Fig. 1. X-ray powder diffractogram of the Ni(HBTC)BPY.
2.3. Catalytic studies
A mixture of benzaldehyde (0.1 ml, 1 mmol), and n-hexadecane
(0.1 ml, 0.88 mmol) as an internal standard in n-butanol (4 ml)
was added into a 25 ml flask containing the Ni(HBTC)BPY cat-
alyst (0.04 g, 10 mol%) and K3PO4 (0.78 g, 3 mmol) as a base.
The catalyst concentration was calculated with respect to the
nickel/benzaldehyde molar ratio. The reaction mixture was stirred
for 15 min to disperse the Ni(HBTC)BPY catalyst in the liquid phase.
Phenylboronic acid (0.18 g, 1.5 mmol) was then added, and the
resulting mixture was stirred at 100 ◦C for 6 h. Reaction conversion
was monitored by withdrawing aliquots from the reaction mixture
at different time intervals, quenching with aqueous Na2CO3 solu-
tion (1% (w/w), 1 ml). The organic components were then extracted
into diethyl ether (2 ml), dried over anhydrous Na2SO4, analyzed by
GC with reference to n-hexadecane. The product identity was fur-
ther confirmed by GC–MS. The Ni(HBTC)BPY catalyst was separated
from the reaction mixture by simple centrifugation, washed with
copious amounts of n-butanol, dried under vacuum at room tem-
perature for 1 h, and reused if necessary. For the leaching test, a
catalytic reaction was stopped after 1 h, analyzed by GC, and cen-
trifuged to remove the solid catalyst. The reaction solution was then
stirred for a further 5 h. 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 Ni(HBTC)BPY was synthesized by a solvother-
mal method, using the reaction of 1,3,5-benzenetricarboxylic acid,
nickel nitrate hexahydrate, and 4,4ꢀ-bipyridine, according to a lit-
erature procedure [72]. The occluded DMF in the as-synthesized
Ni(HBTC)BPY sample was then exchanged with DCM so that the
excess solvent would be easily removed under vacuum. It was
found that the Ni(HBTC)BPY was produced in the form of green
crystals with a yield of 94% (based on 1,3,5-benzenetricarboxylic
acid). The Ni(HBTC)BPY was then characterized using a variety of
different techniques. A nickel loading of 2.27 mmol/g was achieved
for the material, as indicated by elemental analysis with AAS. The
XRD diffractogram of the Ni(HBTC)BPY exhibited the presence of a
very sharp peak below 10◦ (with 2Â of 7.5◦), indicating that a highly
crystalline material was produced (Fig. 1).
The SEM micrograph showed that a crystalline material was
obtained (Fig. 2). The TEM micrograph (Fig. 3) and nitrogen
physisorption measurements (Fig. 4) indicated that there existed
a porous structure, though the pore of the Ni(HBTC)BPY appeared
to be complex and different from that of conventional micro-
porous and mesoporous materials. Nitrogen adsorption/desorption
2.2. Synthesis of the metal-organic framework Ni(HBTC)BPY
In a typical preparation, a mixture of H3BTC (H3BTC = 1,3,5-
benzenetricarboxylic acid; 0.42 g, 2 mmol), 4,4ꢀ-bipy (4,4ꢀ-bipy =
4,4ꢀ-bipyridine; 0.47 g, 3 mmol), and Ni(NO3)2·6H2O (1.16 g,
4 mmol) was dissolved in DMF (DMF = N,Nꢀ-dimethylformamide;
40 ml) in a two 20 ml vials. The vial was heated at 100 ◦C in an
isothermal oven for 24 h. After cooling the vial to room temper-
ature, the solid product was removed by decanting with mother
liquor and washed with DMF (3× 10 ml). Solvent exchange was
then carried out with DCM (3× 10 ml) at room temperature. The
product was then dried at 140 ◦C for 6 h under vacuum, yield-
ing 0.80 g of the metal-organic framework Ni(HBTC)BPY as green
crystals (94% based on 1,3,5-benzenetricarboxylic acid). Elemental
analysis indicated a nickel loading of 13.34% (Ni(HBTC)BPY requires
13.95%) and a nitrogen loading of 6.00% (Ni(HBTC)BPY requires
6.60%).

N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
97
1400
1200
1000
800
600
400
200
0
Adsorption
Desorption
0
0.2
0.4
0.6
0.8
1
Relative Pressure (P/P0)
Fig. 5. Nitrogen adsorption/desorption isotherm of the Ni(HBTC)BPY.
Fig. 2. SEM micrograph of the Ni(HBTC)BPY.
Fig. 6. TGA of the Ni(HBTC)BPY.
Fig. 3. TEM micrograph of the Ni(HBTC)BPY.
Fig. 7. FT-IR spectra of 4,4ꢀ-bipyridine (a), 1,3,5-benzenetricarboxylic acid (b), and
the Ni(HBTC)BPY (c).
(Fig. 6). FT-IR spectra of the Ni(HBTC)BPY exhibited a significant
difference as compared to those of the 1,3,5-benzenetricarboxylic
acid and the 4,4ꢀ-bipyridine linkers (Fig. 7). In the spectra of
the 1,3,5-benzenetricarboxylic acid, there was a strong peak at
1721 cm−1, which was assigned for the C O stretching vibration in
free carboxylic acids (1760–1690 cm−1). The intensity of this peak
in the spectra of the Ni(HBTC)BPY was found to be significantly
Fig. 4. Pore size distribution of the Ni(HBTC)BPY.
isotherm data (Fig. 5) revealed a Langmuir surface areas of
1848 m2/g and a BET surface areas of 1427 m2/g. TGA result of
the Ni(HBTC)BPY indicated that the DMF was completely replaced
with the DCM, and that the material was stable up to over 300 ◦C

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N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
B(OH)2
100
OH
CHO
K3PO4
K2CO3
Na2CO3
Et3N
Ni(HBTC)BPY
n-butanol
+
80
60
40
20
0
Scheme 1. The addition reaction of phenylboronic acid to benzaldehyde using the
Ni(HBTC)BPY catalyst.
lower than the case of the free acid linker. Moreover, the spec-
tra of the Ni(HBTC)BPY exhibited the presence of a strong peak at
1610 cm−1, indicating the deprotonation of COOH groups in the
1,3,5-benzenetricarboxylic acid upon the reaction with nickel ions
[73,74]. The C N stretching vibration in the 4,4ꢀ-bipyridine was
observed at 1596 cm−1. It was found that this value was decreased
to 1539 cm−1 in the spectra of the Ni(HBTC)BPY, confirming the
coordination of the nitrogen with metal ions.
0
1
2
3
4
5
6
Time (h)
Fig. 9. Effect of base on reaction conversion.
3.2. Catalytic studies
The Ni(HBTC)BPY was assessed for its catalytic activity in the
addition reaction of phenylboronic acid to benzaldehyde to form
diphenylmethanol as the principal product (Scheme 1). Initial stud-
ies addressed the effect of temperature on the reaction conversion,
having carried out the addition reaction at 10 mol% catalyst, using
1.5 equiv. of phenylboronic acid in toluene at 80 ◦C, 90 ◦C, and
100 ◦C, respectively, and in the presence of 3 equiv. of K3PO4 as
a base. It was found that the reaction occurred slowly at 80 ◦C, with
only 14% conversion being observed after 6 h. As expected, increas-
ing the temperature led to a significant enhancement in the reaction
rate. The reaction carried out at 90 ◦C and 100 ◦C afforded 47% and
63% conversions after 6 h, respectively (Fig. 8). The temperature
range from 80 ◦C to 110 ◦C has been the most commonly used for the
addition reaction of arylboronic acids to aldehydes using different
types of catalysts. For example, the reaction was previously carried
out at 80 ◦C with nickel catalyst [2,11], 90 ◦C with ion catalyst [13],
100 ◦C with rhodium catalyst [75], and 110 ◦C with copper catalyst
[1]. Indeed, the reaction temperature would strongly depend on the
nature of the catalyst used for the reaction. The effective tempera-
ture range for the addition reaction using the Ni(HBTC)BPY catalyst
was therefore comparable with that previously mentioned in the
literature.
proceed readily in the presence of a variety of bases, such as K2CO3
[3,6], Cs2CO3 [75], CsF [9], KOH [14], NaOAc [1], K3PO4 [2], and
KOtBu [14]. We therefore decided to investigate the effect of differ-
ent bases on the reaction conversion. The Ni(HBTC)BPY-catalyzed
addition reaction was carried out in toluene at 100 ◦C for 6 h, using
1.5 equiv. of phenylboronic acid and 3 equiv. of K2CO3, Na2CO3,
K3PO4, and triethylamine, respectively. It was found that K3PO4
was superior to other bases, with 63% conversion being achieved
after 6 h. The reaction using K2CO3 afforded 52% conversion after
6 h, while only 30% conversion was observed for the case of
Na2CO3. Triethylamine was also found to be unsuitable for the
reaction using the Ni(HBTC)BPY catalyst, with only 23% conversion
being obtained after 6 h (Fig. 9). It was therefore decided to use
K3PO4 as the base for the reaction in further experiments.
For an organic transformation using a solid catalyst, the effect
of different solvents on the rate of the reaction could normally
be of extreme importance, depending on the nature of the cata-
lyst material [77,78]. Moreover, the reaction using a homogeneous
catalyst might also be significantly influenced by the polarity of
the solvent. Su and co-workers previously carried out the addi-
tion reaction of arylboronic acids to aromatic aldehydes using a
copper catalyst in several solvents, and demonstrated that toluene
was the best choice for the reaction among other common sol-
vents such as CH3CH2NO2, DMF, DMSO, dioxane, DCE, THF, and
t-butanol [1]. Trindade and co-workers used a dirhodium cata-
lyst for the addition reaction in several solvents, and pointed out
that t-amyl alcohol and methanol should be used for the reac-
tion instead of DME, toluene/H2O, DME/H2O, DME/t-butanol, and
CH3CN [14]. Zhou et al. also investigated the effect of solvent on the
addition reaction using a nickel catalyst, and showed that a mix-
ture of toluene/isopropanol was superior to isopropanol, toluene,
dioxane, ethanol, DME, and CH3CN [2]. We therefore decided to
investigate the effect of different solvents on the reaction conver-
sion, having carried out the reaction in different solvents at 100 ◦C,
using 1.5 equiv. of phenylboronic acid and 3 equiv. of K3PO4, in
the presence of 10 mol% Ni(HBTC)BPY catalyst. It was observed
that the solvent had a profound effect on the addition reaction of
phenylboronic acid to benzaldehyde using the Ni(HBTC)BPY cata-
lyst. Experimental results showed that xylene and DMF should not
be used for the reaction, as less than 30% conversion was obtained
after 6 h. The reaction carried out in toluene afforded 63% con-
version, while the n-butanol exhibited better performance with
80% conversion being achieved after 6 h (Fig. 10). It was there-
fore decided to employ n-butanol as the solvent for the reaction
in further studies.
As for most transition metal-catalyzed reactions of
organoboronic acids, the presence of at least 1 equiv. of a base
should be required to accelerate the transmetalation step in the
catalytic cycle of the reaction [76]. It was previously reported
that the addition reaction of arylboronic acids to aldehydes could
100
80oC
90oC
80
60
40
20
0
100oC
0
1
2
3
4
5
6
Time (h)
Fig. 8. Effect of temperature on reaction conversion.

N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
99
100
80
60
40
20
0
100
Toluene
Xylene
DMF
80
n-Butanol
60
40
20
0
10 mol% Ni(HBTC)BPY
Leaching test
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time (h)
Time (h)
Fig. 12. Leaching test indicated no contribution from homogeneous catalysis of
Fig. 10. Effect of solvent on reaction conversion.
active species leaching into reaction solution.
With these results in mind, we then investigated the effect of
catalyst concentration on the reaction conversion. The reaction
was carried out in n-butanol at 100 ◦C, using 1.5 equiv. of phenyl-
boronic acid and 3 equiv. of K3PO4, in the presence of 5 mol%,
7.5 mol%, 10 mol% Ni(HBTC)BPY catalyst, respectively. As expected,
decreasing the catalyst concentration from 10% to 5 mol% resulted
in a significant drop in the reaction rate, with 80% and 67% con-
versions being observed after 6 h for the former and the latter
case, respectively. It was also observed that the reaction using
7.5 mol% catalyst afforded 74% conversion after 6 h (Fig. 11). It
should be noted that approximately 5% conversion was observed
in the absence of the catalyst, probably due to the metal residue
present in the base used for the reaction. Indeed, elemental analy-
sis with AAS indicated that the K3PO4 contained 3 ppm copper and
6 ppm nickel. The catalyst concentrations used in this study were
comparable to those of several previous reports covering different
aspects of the addition reaction of arylboronic acids to aromatic
aldehydes, where the catalyst concentrations varied from less than
5 mol% to more than 10 mol%, depending on the nature of the cata-
lysts as well as the substrates. Indeed, the catalyst concentration of
5 mol% was previously employed for the reaction in several reports
[2,6,79–81], though lower or higher catalyst concentrations were
also used in other cases. Huang and co-worker carried out the reac-
tion in the presence of 1 mol% RhCl3·3H2O and 1 mol% t-Bu-Amphos
ligand [82]. Trindade also employed 3 mol% Rh2(OAc)4 in the pres-
ence of 3 mol% N-heterocyclic carbene ligand as catalyst for the
reaction [14]. However, higher catalyst concentrations were previ-
ously used for the reaction in other works, for example, 10% FeCl3
and 10% 2-(di-tert-butylphosphino)biphenyl ligand [13], 10 mol%
Cu(OAc)2 and 15 mol% dppf ligand [1], and 20% nickel phosphine
complexes [83].
For a liquid-phase organic transformation using a solid cata-
lyst, the possibility that some of active sites could dissolve into
the reaction solution during the course of the reaction should be
taken into account as these leached species could contribute signif-
icantly to the total conversion of the reaction. In this case, although
a solid catalyst is employed, the reaction would not proceed under
real heterogeneous catalysis conditions [78]. In order to confirm
if active nickel species leached from the solid Ni(HBTC)BPY cat-
alyst could play a significant role in the catalytic activity for the
addition reaction of phenylboronic acid to benzaldehyde, an exper-
iment was carried out using a simple centrifugation during the
course of the reaction. After the solid catalyst was removed from
the reaction mixture, if further conversion was still observed for
the solution phase, this would indicate that the active species were
from the solution rather than from the solid Ni(HBTC)BPY cata-
lyst. The Ni(HBTC)BPY-catalyzed addition reaction was carried out
in n-butanol at 100 ◦C for 6 h, using 1.5 equiv. of phenylboronic
acid and 3 equiv. of K3PO4, and in the presence of 10 mol% cata-
lyst. The organic phase was separated from the solid catalyst after
1 h reaction time by simple centrifugation. The solution was then
transferred to a new reactor vessel containing 3 equiv. of K3PO4,
and stirred for an additional 5 h at 100 ◦C with aliquots being sam-
pled at different time intervals, and analyzed by GC. The reaction
conversion calculated from GC determinations would show quanti-
tative information about residual, catalytically active species being
soluble in the solution during the course of the reaction. Experi-
mental result showed that no further conversion was detected for
the addition reaction in the liquid phase after the solid Ni(HBTC)BPY
catalyst was separated from the reaction mixture (Fig. 12). These
results indicated that the Ni(HBTC)BPY-catalyzed addition reaction
of phenylboronic acid to benzaldehyde could only proceed in the
presence of the solid catalyst, and there was no contribution from
leached active nickel species in the liquid phase.
100
80
60
10 mol%
7.5 mol%
5 mol%
40
20
0
To confirm the necessity of using the solid Ni(HBTC)BPY as a
heterogeneous catalyst for the addition reaction of phenylboronic
acid to benzaldehyde, three nickel salts including Ni(NO3)2, NiCl2,
and NiSO4 were also used as homogeneous catalysts for the reac-
tion. The reaction was carried out in n-butanol at 100 ◦C, using
1.5 equiv. of phenylboronic acid and 3 equiv. of K3PO4, in the
presence of 10 mol% nickel catalyst. Interestingly, it was found
that the Ni(HBTC)BPY-catalyzed addition reaction offered higher
0 mol%
0
1
2
3
4
5
6
Time (h)
Fig. 11. Effect of catalyst concentration on reaction conversion.

100
N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
Benzaldehyde
4-Chlorobenzaldehyde
2-Methylbenzaldehyde
4-Nitrobenzaldehyde
100
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
3-Methylbenzaldehyde
100
80
60
40
20
0
Ni(HBTC)BPY
NiCl2
Ni(NO3)2
NiSO4
80
60
40
0
1
2
3
4
5
6
20
0
Time (h)
Fig. 13. Effect of different catalysts on reaction conversion.
0
1
2
3
4
5
6
Time (h)
rate than that of the reaction using the homogeneous nickel cat-
alyst. NiSO4 was found to be unsuitable for the reaction, as a
conversion of only 16% was observed after 6 h. Ni(NO3)2, one
component of the Ni(HBTC)BPY, also exhibited significantly lower
activity for the reaction than the solid catalyst, with a conver-
sion of 48% being obtained after 6 h. It should be noted that the
reaction using 10 mol% the organic linker of the Ni(HBTC)BPY (i.e.
1,3,5-benzenetricarboxylic acid) gave approximately 5% conver-
sion, probably due to the nickel residue present in the base used
for the reaction. It was also observed that NiCl2 exhibited lower
activity than the Ni(HBTC)BPY, though the reaction still afforded
78% conversion after 6 h (Fig. 13). Indeed, nickel homogeneous cat-
alyst in the form of nickel phosphine complexes exhibited high
activity in the addition reaction of arylboronic acids to aromatic
aldehydes [83]. It was also reported that the reaction using nickel
salts as catalyst such as NiCl2, Ni(OAc)2, and Ni(ClO4)2 offered rea-
sonable conversions [2]. The higher activity of the Ni(HBTC)BPY
catalyst than that of homogeneous nickel catalysts still remains to
be investigated in further studies. However, a clear advantage of
using the Ni(HBTC)BPY as catalyst over conventional nickel salts
would be the reusability of the MOF-based material.
The study was then extended to the addition reaction of
phenylboronic acid to several aldehydes in the presence of
the Ni(HBTC)BPY catalyst, including benzaldehyde, 4-chloro-
benzaldehyde, 4-nitrobenzaldehyde, 2-methylbenzaldehyde, 3-
methylbenzaldehyde, 4-methylbenzaldehyde, and 4-methoxy-
benzaldehyde. The reaction was carried out in n-butanol at
100 ◦C for 6 h, using 1.5 equiv. of phenylboronic acid and 3 equiv.
of K3PO4, in the presence of 10 mol% Ni(HBTC)BPY catalyst. It
was found that the Ni(HBTC)BPY-catalyzed addition reaction of
phenylboronic acid to aromatic aldehydes was highly sensitive to
electronic effects of substituents present in the aldehyde struc-
ture. The reaction of phenylboronic acid and 4-chlorobenzaldehyde
afforded up to 95% conversion after 6 h, higher than the case
of benzaldehyde (i.e. 82% conversion). However, nitro group, a
strongly electron-withdrawing substituent, significantly deacti-
vated the aldehyde, with only 9% conversion being observed after
6 h. This observation was similar to that previously reported by
Trindade [14], Zhou [2], and Chen et al. [3], where dramati-
cally low yields were observed for aromatic aldehydes containing
strongly electron-withdrawing substituents like nitro group. In
contrast, Ueda and co-worker previously reported that tri(tert-
butyl)phosphine rhodium complex could quantitatively catalyze
the reaction of 4-nitrobenzaldehyde with phenylboronic acid [84].
The addition reaction of 4-methoxybenzaldehyde proceeded with
difficulty, affording 49% conversion after 6 h. The reaction of three
Fig. 14. Effect of different aldehydes on reaction conversion.
isomers of methylbenzaldehyde also occurred with slower rate
than that of benzaldehyde, with 61%, 74%, and 82% conversions
being obtained for the case of 3-, 2-, and 4-methylbenzaldehyde,
respectively (Fig. 14). This result was similar to that previously
reported by Zhou et al., where electron-rich aldehydes such as
4-methylbenzaldehyde and 4-methoxybenzaldehyde were less
reactive than benzaldehyde [2]. However, it should be noted that
Trindade [14] and Gois et al. [85] previously reported that electron-
donating groups activated the addition reaction of aromatic
aldehydes with phenylboronic acids using dirhodium complex cat-
alysts.
The effect of different arylboronic acids on the Ni(HBTC)BPY-
catalyzed addition reaction with aromatic aldehydes was
also investigated, having performed the reaction between
4-chlorobenzaldehyde with phenylboronic acid, 4-formyl-, 4-
methyl-, 4-fluoro-, and 4-chlorophenylboronic acid, respectively.
The reaction was carried out in n-butanol at 100 ◦C for 6 h, using
1.5 equiv. of arylboronic acids and 3 equiv. of K3PO4, in the pres-
ence of 10 mol% Ni(HBTC)BPY catalyst. It was found that the
addition reaction of 4-formylphenylboronic acid proceeded to
4-chlorobenzaldehyde with significantly slower rate than that
of other boronic acids, with only 56% conversion being observed
after 6 h. The reaction of 4-chlorophenylboronic acid occurred
more readily than that of 4-formylphenylboronic acid, affording
87% conversion after 6 h. As expected, 4-fluorophenylboronic
acid was found to be more reactive than 4-chlorophenylboronic
acid, improving the reaction conversion to 93%. The reaction
of 4-methylphenylboronic acid and phenylboronic acid pro-
ceeded readily, though 4-methylboronic acid was slightly more
reactive (Fig. 15). This observation was similar to that previ-
ously reported by Qin [6] and Yamamoto et al. [7,81], where
electron-deficient arylboronic acids are less reactive towards the
palladium-catalyzed addition reaction to aromatic aldehydes.
Indeed, it was previously reported that arylboronic acids con-
taining electron-withdrawing groups transmetalated more slowly
than electronneutral analogues [6], thus slowing down the reaction
with aromatic aldehydes. However, Zhou et al. used the nickel
catalyst for the reaction and demonstrated that higher yields were
obtained from the reactions of electron-deficient arylboronic acids
[2].
In the view of green chemistry, issues that should be seriously
considered when performing organic reactions using solid catalysts
are the ease of separation as well as the deactivation and reusability

N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
101
C6H5B(OH)2
4-CH3-C6H5B(OH)2
4-CHO-C6H5B(OH)2
4-F-C6H5B(OH)2
4-Cl-C6H5B(OH)2
100
80
60
40
20
0
Fig. 17. X-ray powder diffractogram of the fresh (a) and reused (b) Ni(HBTC)BPY.
0
1
2
3
4
5
6
Time (h)
Fig. 15. Effect of different arylboronic acids on reaction conversion.
of the catalysts. In the best case the solid catalyst can be separated
from the reaction mixture, recovered and reused several times
before it eventually deactivates completely. The Ni(HBTC)BPY cata-
lyst was therefore investigated for recoverability and reusability in
the addition reaction between 4-chlorobenzaldehyde and phenyl-
boronic acid over five successive runs, by repeatedly separating
the Ni(HBTC)BPY from the reaction mixture, washing it and then
reusing it. The reaction was carried out in n-butanol at 100 ◦C for 6 h,
using 1.5 equiv. of phenylboronic acid and 3 equiv. of K3PO4, in the
presence of 10 mol% Ni(HBTC)BPY catalyst. After the reaction was
complete, the Ni(HBTC)BPY catalyst was separated by simple cen-
trifugation, washed with copious amounts of n-butanol to remove
any physisorbed reagents, dried under vacuum at room tempera-
ture for 1 h. The recovered Ni(HBTC)BPY catalyst was then reused in
further reaction under identical conditions to those of the first run.
Experimental results showed that the activity of the Ni(HBTC)BPY
catalyst decreased slightly after each run. However, the catalyst
could be reused without a significant degradation in catalytic activ-
ity. Indeed, a conversion of 82% was still achieved in the 5th run
(Fig. 16). XRD result of the reused catalyst after the first run indi-
cated that the crystallinity of the Ni(HBTC)BPY catalyst remained
almost unchanged during the course of the reaction (Fig. 17). As
compared to the FT-IR spectra of the fresh Ni(HBTC)BPY, only a
Fig. 18. FT-IR spectra of the reused (a) and fresh (b) Ni(HBTC)BPY.
slight difference was observed on that of the reused Ni(HBTC)BPY
after the 1st run (Fig. 18). Further investigations would be needed
to clarify the reason of the catalyst deactivation in the addition
reaction of arylboronic acids to aromatic aldehydes using the
Ni(HBTC)BPY catalyst.
4. Conclusions
In summary, highly crystalline porous Ni(HBTC)BPY was
obtained from the reaction of 1,3,5-benzenetricarboxylic acid,
nickel nitrate hexahydrate, and 4,4ꢀ-bipyridine by a solvothermal
method in DMF. The Ni(HBTC)BPY was characterized using a vari-
ety of different techniques, including XRD, TEM, SEM, FT-IR, TGA,
AAS, and nitrogen physisorption measurements. The Ni(HBTC)BPY
catalyst exhibited high activity in the arylation of aldehydes with
arylboronic acids to form diarylmethanols. The Ni(HBTC)BPY cat-
alyst could be separated from the reaction mixture by simple
filtration or centrifugation, and could be reused without a signifi-
cant degradation in catalytic activity. To the best of our knowledge,
the addition reaction of arylboronic acids to aromatic aldehydes in
the presence of solid catalysts was not previously reported in the
literature. Our results indicate that the Ni(HBTC)BPY could be used
as an efficient recyclable catalyst for the reaction, thus offering a
route to green chemistry for the synthesis of diarylmethanols. This
achievement would contribute to the applications of metal-organic
frameworks in the field of catalysis, which would be interested to
the chemical industry.
100
80
60
40
20
0
1
2
3
Run
4
5
Fig. 16. Catalyst recycling studies.

102
N.T.S. Phan et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 95–102
Acknowledgements
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The Viet Nam National Foundation for Science and Technology
Development – NAFOSTED is acknowledged for financial support
through project code 104.99-2012.25.
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