Expanding applications of copper-based metal-organic frameworks in catalysis: Oxidative C-O coupling by direct C-H activation of ethers over Cu2(BPDC)2(BPY) as an efficient heterogeneous catalyst

Article abstract of DOI:10.1016/j.jcat.2013.06.006

A crystalline porous metal-organic framework Cu₂(BPDC) ₂(BPY) was synthesized and characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis

Full text of DOI:10.1016/j.jcat.2013.06.006

Journal of Catalysis 306 (2013) 38–46
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Journal of Catalysis
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Expanding applications of copper-based metal–organic frameworks in
catalysis: Oxidative C–O coupling by direct C–H activation of ethers over
Cu2(BPDC)2(BPY) as an efficient heterogeneous catalyst
⇑
Nam T.S. Phan , Phuong H.L. Vu, Tung T. Nguyen
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:
A crystalline porous metal–organic framework Cu2(BPDC)2(BPY) 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) spectra, atomic absorption
spectrophotometry (AAS), hydrogen temperature-programmed reduction (H2-TPR), and nitrogen physi-
sorption measurements. The Cu-MOF could be used as an efficient heterogeneous catalyst for the cop-
per-catalyzed cross-dehydrogenative coupling reaction of ethers with 2-carbonyl-substituted phenols.
The Cu2(BPDC)2(BPY) exhibited significantly higher catalytic activity than that of other Cu-MOFs such
as Cu3(BTC)2, Cu(BDC), and Cu(BPDC). The Cu-MOF also offered advantages over several copper salts such
as CuCl2, CuCl, CuI, Cu(NO3)2, and Cu(OAc)2 in terms of the catalytic activity as well as the catalyst reus-
ability. To the best of our knowledge, the cross-dehydrogenative coupling reaction of ethers with 2-car-
bonyl-substituted phenols via C–H activation under oxidative conditions using a heterogeneous catalyst
was not previously mentioned in the literature.
Received 13 April 2013
Revised 6 June 2013
Accepted 8 June 2013
Keywords:
Metal–organic framework
Cross-dehydrogenative
C–H activation
Heterogeneous catalyst
Cu2(BPDC)2(BPY)
Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction
cols should be investigated to improve the green credentials of
the reaction [34]. For the development of greener processes, reac-
Transition metal-catalyzed cross-coupling reactions have at-
tracted significant attention as a powerful method for the forma-
tion of carbon–carbon and carbon–heteroatom bonds [1–5].
Conventional approaches require the preparation of prefunctional-
ized starting materials, being a major concern from atom-econom-
ical and environmental aspects [6–8]. Apparently, the cross-
dehydrogenative coupling reaction by the direct activation of C–
H bonds would eliminate such drawbacks and thus could offer
shorter and more efficient synthetic schemes [6,9]. Li and co-work-
ers previously explored the homogeneous copper-catalyzed
cross-dehydrogenative coupling via C–H activation under oxida-
tive conditions to efficiently achieve C–C and C–X (X = O, N) bonds
[6,7,10,11]. Recently, several homogeneous copper [10–13]-, iron
[14–19]-, palladium [20–22]-, or ruthenium [23–27]-based cata-
lysts have been employed for this transformation in the presence
of oxidants such as organic peroxides, H2O2, and oxygen. Moreover,
molecular iodine [28,29] and N-tetrabutylammonium iodide (nBu4-
NI) [30–33] have also been reported as efficient transition metal-
free catalytic protocols for the cross-dehydrogenative couplings.
In terms of green chemistry, more environmentally benign proto-
tions using heterogeneous catalysts should be targeted in terms of
the ease of handling, simple workup, recyclability, and reusability
[35]. Furthermore, using solid catalysts also decreases contamina-
tion of the desired products with hazardous or harmful metals
[36,37].
Metal–organic frameworks (MOFs) have recently emerged as
promising materials for their potential applications in gas separa-
tion and storage, sensors and luminescence, drug storage and
delivery, templated low-dimensional material preparation, and
catalysis [38–43]. Although the application of MOFs in catalysis
is a young research area, MOFs have been employed as solid cata-
lysts or catalyst supports for a variety of organic transformations
[44,45], ranging from carbon–carbon [46–51] to carbon–hetero-
atom [52–55] forming reactions. Among several popular MOFs,
copper-based materials such as Cu3(BTC)2 [56–59] and Cu(BDC)
[60,61] previously offered high activity in various organic transfor-
mations due to their unsaturated open copper metal sites. In this
work, we wish to report a copper-catalyzed cross-dehydrogenative
coupling reaction of ethers with 2-carbonyl-substituted phenols
using Cu2(BPDC)2(BPY) as an efficient heterogeneous catalyst. The
Cu2(BPDC)2(BPY) also offered significantly higher catalytic activity
than that of other Cu-MOFs such as Cu3(BTC)2, Cu(BDC), and
Cu(BPDC). Indeed, Kappe and co-workers have recently reported
the first example of this transformation using Cu(OAc)2 as a
⇑
Corresponding author. Fax: +84 (8) 38637504.
E-mail address: ptsnam@hcmut.edu.vn (N.T.S. Phan).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.06.006

N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
39
homogeneous catalyst [62]. To the best of our knowledge, the cou-
pling reaction of ethers with 2-carbonyl-substituted phenols in the
presence of a heterogeneous catalyst was not previously men-
tioned in the literature.
decanting with 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 0.103 g of Cu2(-
BPDC)2(BPY) in the form of blue crystals (67.5% based on copper
nitrate).
2. Experimental
2.3. Catalytic studies
2.1. Materials and instrumentation
In a typical experiment, a mixture of 2-hydroxybenzaldehyde
(0.143 ml, 1.0 mmol) and n-hexadecane (0.1 ml) as an internal
standard in 1,4-dioxane (4 ml, 50 mmol) was added into a 25-ml
flask containing the pre-determining amount of Cu2(BPDC)2(BPY)
catalyst and tert-butyl hydroperoxide (70 wt.% in water;
0.436 ml, 3.0 mmol) as an oxidant. The catalyst concentration
was calculated with respect to the copper/2-hydroxybenzaldehyde
molar ratio. The reaction mixture was stirred at 100 °C for 2 h.
Reaction conversion was monitored by withdrawing aliquots from
the reaction mixture at different time intervals, quenching with
water (1 ml). The organic components were then extracted into
diethyl ether (2 ml), dried over anhydrous Na2SO4, and analyzed
by GC with reference to n-hexadecane. The product identity was
further confirmed by GC–MS. To investigate the recyclability of
Cu2(BPDC)2(BPY), the catalyst was separated from the reaction
mixture by simple centrifugation, washed with copious amounts
of methanol, dried under air, and reused if necessary. For the leach-
ing test, a catalytic reaction was stopped after 20 min, analyzed by
GC, and centrifuged to remove the solid catalyst. The reaction solu-
tion was then stirred for a further 40 min. Reaction progress, if any,
was monitored by GC as previously described.
All reagents and starting materials were obtained commercially
from Sigma-Aldrich and Merck and were used as received without
any further purification unless otherwise noted. Nitrogen physi-
sorption measurements were conducted using a Micromeritics
2020 volumetric adsorption analyzer system. Samples were pre-
treated by heating under vacuum at 150 °C for 3 h. A Netzsch Ther-
moanalyzer STA 409 was used for thermogravimetric analysis
(TGA) with a heating rate of 10 °C/min under a nitrogen atmo-
sphere. X-ray powder diffraction (XRD) patterns were recorded
using a Cu Ka radiation source on a D8 Advance Bruker powder dif-
fractometer. 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 Cu2(-
BPDC)2(BPY) sample was dispersed on holey carbon grids for TEM
observation. Elemental analysis with atomic absorption spectro-
photometry (AAS) was performed on an AA-6800 Shimadzu. Fou-
rier transform infrared (FT-IR) spectra were obtained on a Nicolet
6700 instrument, with samples being dispersed on potassium bro-
mide pallets. The chemisorption experiments were studied in a
Micromeritics 2020 analyzer. For hydrogen temperature-pro-
grammed reduction (H2-TPR), the sample was outgassed at
100 °C for 30 min with helium, then cooled down to room temper-
ature, and exposed to 50 ml/min of 10% H2/Ar as the temperature
ramped at 2.5 °C/min to 600 °C. The amount of hydrogen consump-
tion was determined from TCD signal intensities, which were cali-
brated using an Ag2O reference sample.
3. Results and discussion
3.1. Catalyst synthesis and characterization
In this work, the Cu2(BPDC)2(BPY) was synthesized in a yield of
67.5% by a solvothermal method, according to a modified literature
procedure [63]. The material was then characterized by a variety of
different techniques. The X-ray diffraction patterns of the Cu2(-
BPDC)2(BPY) (Fig. S1) showed the presence of a sharp peak at
2h = 6°, being consistent with the simulated pattern of single crys-
tals previously reported by James and co-workers [63]. Elemental
analysis by AAS indicated that a copper loading of 2.5 mmol/g
was obtained. The SEM micrograph of the Cu2(BPDC)2(BPY) re-
vealed that well-shaped, high-quality cubic crystals were formed
(Fig. S2). Moreover, the TEM observation showed that the Cu2(-
BPDC)2(BPY) possessed a porous structure (Fig. S3). However,
nitrogen physisorption measurements indicated a complicated
pore structure for the Cu2(BPDC)2(BPY). A pore size distribution
of the Cu2(BPDC)2(BPY) revealed two peaks in the range of 5–
15 Å (using the Horvath–Kawazoe method) and another peak at a
pore size between 15 Å and 50 Å (using the Dubinin–Astakhov
method) (Fig. S4). A type I adsorption isotherm with a hysteresis
loop was achieved for the Cu2(BPDC)2(BPY). Langmuir surface areas
of 1547 m2/g were achieved for the material, as calculated from
nitrogen adsorption–desorption isotherm data (Fig. S5). TGA result
indicated that the material was stable up to over 300 °C (Fig. S6).
FT-IR spectra of the Cu2(BPDC)2(BPY) exhibited a significant differ-
ence as compared to those of the H2BPDC and the BPY, showing the
coordination of copper cations and organic linkers (Fig. S7). The H2-
TPR experiment revealed the nature of copper species within Cu2(-
BPDC)2(BPY) structure. Two broad reduction peaks being at 340 °C
and 410 °C could be attributed to the reduction of Cu2+ and Cu+
ions, respectively (Fig. S8). Although these assignments were pre-
viously reported in several copper-based catalytic systems
Gas chromatographic (GC) analyses were performed using a
Shimadzu GC 2010-Plus equipped with a flame ionization detector
(FID) and an SPB-5 column (length = 30 m, inner diame-
ter = 0.25 mm, and film thickness = 0.25 lm). The temperature
program for GC analysis held samples at 100 °C for 1 min, heated
them from 100 to 180 °C at 40 °C/min, held them at 180 °C for
1 min, heated them from 180 to 280 °C at 50 °C/min, and held them
at 280 °C for 2 min. Inlet and detector temperatures were set con-
stant at 280 °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 a RTX-5MS column
(length = 30 m, inner diameter = 0.25 mm, and film thick-
ness = 0.5 lm). 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.
2.2. Synthesis of the metal–organic framework Cu2(BPDC)2(BPY)
In a typical preparation, a solid mixture of H2BPDC (H2-
BPDC = 4,40-biphenyldicarboxylic acid; 0.1039 g, 0.4 mmol), bpy
(bpy = 4,40-bipyridine; 0.033 g, 0.2 mmol), and Cu(NO3)2ꢀ3H2O
(0.105 g, 0.4 mmol) was dissolved in
a mixture of DMF
(DMF = N,N0-dimethylformamide; 30 ml), pyridine (0.3 ml), and
methanol (3 ml). The resulting solution was stirred at 70 °C for
5 min and then distributed to four 20-ml vials. The vials were then
heated at 120 °C in an isothermal oven for 24 h. After cooling the
vials to room temperature, the solid product was removed by

40
N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
O
100
80
60
40
20
0
O
O
O
Cu2(BPDC)2(BPY)
O
+
O
OH
O
tert-butyl hydroperoxide
Scheme 1. The cross-dehydrogenative coupling reaction of 2-hydroxybenzalde-
hyde and 1,4-dioxane using Cu2(BPDC)2(BPY) as a solid catalyst.
0 mol%
1 mol%
3 mol%
5 mol%
[64,65], further investigations would be needed to elucidate the
nature of copper sites in the MOF structure.
0
20
40
60
80
100
120
Time (min.)
3.2. Catalytic studies
Fig. 2. Effect of catalyst concentration on reaction conversion.
The Cu2(BPDC)2(BPY) was assessed for its catalytic activity in
the cross-dehydrogenative coupling reaction by studying the cou-
pling of 2-hydroxybenzaldehyde and 1,4-dioxane to form 2-(1,4-
dioxan-2-yloxy)benzaldehyde as the principal product (Scheme 1).
Initial studies addressed the effect of temperature on the reaction
conversion, having carried out the coupling reaction at 5 mol% cat-
alyst, using the 2-hydroxybenzaldehyde:1,4-dioxane molar ratio of
1:50, in the presence of three equivalents of tert-butyl hydroperox-
ide as an oxidant, at 80 °C, 90 °C, and 100 °C, respectively. Aliquots
were withdrawn from the reaction mixture at different time inter-
vals and analyzed by GC, giving kinetic data during the course of
the reaction. It was found that the coupling reaction proceeded
with difficulty at 80 °C, affording a conversion of only 15% after
120 min. As expected, increasing the temperature led to a signifi-
cant enhancement in reaction rate. A conversion of more than
99% was achieved after 120 min for the reaction carried out at
90 °C. Furthermore, it was observed that the coupling reaction
could proceed to quantitative conversion after 80 min at 100 °C
(Fig. 1). Indeed, in the first example of the homogeneous copper-
catalyzed coupling reaction of 2-hydroxybenzaldehyde and 1,4-
dioxane, a full substrate conversion was achieved after 180 min
at 100 °C [62].
With this result in mind, we then decided to investigate the
effect of catalyst concentration on the reaction conversion. The
reaction was carried out at 100 °C, using the 2-hydroxybenzalde-
hyde:1,4-dioxane molar ratio of 1:50, in the presence of three
equivalents of tert-butyl hydroperoxide as the oxidant, with
1 mol%, 3 mol%, and 5 mol% catalyst, respectively. It was found that
the coupling reaction of 2-hydroxybenzaldehyde and 1,4-dioxane
could proceed readily in the presence of a catalytic amount of
the Cu2(BPDC)2(BPY). Using 1 mol% catalyst, a reaction conversion
was obtained after 120 min. As expected, when increasing the cat-
alyst concentration from 1 mol% to 3 mol%, the reaction rate went
up significantly, affording more than 99% conversion after 100 min.
The reaction rate could be improved with qualitative conversion
being achieved after 80 min for the reaction using 5 mol% catalyst
(Fig. 2). It should be noted that almost no reaction occurred in the
absence of the Cu2(BPDC)2(BPY), indicating the necessity of using
the Cu-MOF as catalyst for the cross-dehydrogenative coupling
reaction. Indeed, Kappe and co-workers previously reported that
the cross-dehydrogenative coupling reaction of 2-hydroxybenzal-
dehyde and 1,4-dioxane could occur readily in the presence of
5 mol% Cu(OAc)2 catalyst [62]. In some other oxidative C–O cou-
pling reactions by direct C–H activation, the copper catalyst con-
centration required for the reaction could vary from 1 mol% to
10 mol%, depending on the nature of the catalyst as well as that
of the substrate. These reactions included the coupling of N,N-dial-
kylformamides with b-keto esters and 2-carbonyl-substituted phe-
nols (1 mol% CuCl [66], 5 mol% CuBr2 or 5 mol% Cu(OAc)2 [8]), and
the coupling of carboxylic acids with N,N-dialkylformamides
(10 mol% Cu(ClO4)2ꢀ6H2O) [67]. The catalyst concentrations used
for the cross-dehydrogenative coupling via C–H activation in this
study were therefore comparable with those in the literature.
Another factor that should be considered for the cross-dehy-
drogenative coupling reaction using the Cu2(BPDC)2(BPY) catalyst
is the reagent molar ratio. The coupling of 2-hydroxybenzaldehyde
with 1,4-dioxane could proceed under solvent-free condition, and
a large excess of 1,4-dioxane should be required to act as the reac-
tion medium. The reaction was carried out at 100 °C, using 3 mol%
catalyst, in the presence of three equivalents of tert-butyl hydro-
peroxide as the oxidant, with the 2-hydroxybenzaldehyde:1,4-
dioxane molar ratio of 1:50, 1:37.5, and 1:25, respectively. It was
found that decreasing the reagent molar ratio resulted in a drop
in the reaction conversion. However, the reaction using 25 equiva-
lents of 1,4-dioxane could still proceed to 95% conversion after
120 min, while this value could be improved to 98% for the case
of 37.5 equivalents. Increasing the reagent molar ratio to 1:50
led to an enhancement in the reaction rate, affording more than
99% conversion after 100 min (Fig. 3). For organic transformations
using a solid catalyst, reaction solvents could have a profound
100
100
80
1:37.5
1:50
1:25
80
60
60
80 ºC
90 ºC
100 ºC
40
20
0
40
20
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min.)
Time (min.)
Fig. 1. Effect of temperature on reaction conversion.
Fig. 3. Effect of reagent molar ratio on reaction conversion.

N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
41
Solvent-free
Acetonitrile
Toluene
Ethylacetate
oxygen in air were found to be unsuitable for the coupling reaction,
with a conversion of 29%, 34%, 7%, and 12%, respectively, being ob-
served after 120 min (Fig. 5). Indeed, tert-butyl hydroperoxide was
previously employed as the oxidant in the first example of the
homogeneous copper-catalyzed coupling reaction of 2-hydroxy-
benzaldehyde and 1,4-dioxane [62]. Chang and co-workers previ-
ously used several oxidants for the cross-dehydrogenative
coupling of N,N-dialkylformamides with b-keto esters and 2-car-
bonyl-substituted phenols and also showed that tert-butyl hydro-
peroxide offered significantly better performance than other
oxidants [66].
100
80
60
40
20
0
Moreover, it was also found that the concentration of the tert-
butyl hydroperoxide exhibited a profound influence on the reac-
tion rate, having carried out the coupling reaction in the presence
of one, two, and three equivalents of tert-butyl hydroperoxide as
the oxidant, respectively. The coupling reaction using three equiv-
alents of tert-butyl hydroperoxide could proceed to completion
after 100 min, while 94% conversion was obtained after 120 min
for the case of two equivalents. Decreasing the amount of the oxi-
dant to one equivalent resulted in a drop in the reaction rate, with
79% conversion being observed after 120 min (Fig. 6). To verify the
necessity of the oxidant in the cross-dehydrogenative coupling
reaction between 2-hydroxybenzaldehyde and 1,4-dioxane, ascor-
bic acid or phloroglucinol as the antioxidant was added to the reac-
tion mixture after the first 20 min. If the reaction conversion was
not observed in the presence of the antioxidant, this behavior
would indicate that the catalytically active metal sites could be
poisoned or the radicals in the catalytic cycle could be decom-
posed. The coupling reaction was carried out at 100 °C, using
3 mol% catalyst, in the presence of three equivalents of tert-butyl
hydroperoxide as the oxidant, with the 2-hydroxybenzalde-
hyde:1,4-dioxane molar ratio of 1:50. After 20 min, 0.15 mol% of
ascorbic acid as the antioxidant was then added to the reaction
mixture, and the mixture was stirred for an additional 100 min
at 100 °C with aliquots being sampled at different time intervals
and analyzed by GC. It was found that the coupling reaction was
significantly affected by the antioxidant. Similarly, adding phloro-
glucinol as the antioxidant to the reaction mixture also led to a
dramatic drop in the reaction rate (Fig. 7). As the total amount of
the copper was 20 times higher than that of the ascorbic acid or
the phloroglucinol, it could be proposed that the reaction stopped
owing to the interaction of the radicals in the catalytic cycle with
the antioxidant. It should be noted that almost no reaction oc-
curred in the absence of the Cu2(BPDC)2(BPY) but with the pres-
ence of tert-butyl hydroperoxide, thus emphasizing the function
of the copper sites in the catalytic cycle.
0
20
40
60
80
100
120
Time (min.)
Fig. 4. Effect of co-solvent on reaction conversion.
100
TBHP
H2O2
AgNO3
MnO2
PhI(OAc)2
O2
80
60
40
20
0
0
20
40
60
80
100
120
Time (min.)
Fig. 5. Effect of oxidant on reaction conversion.
influence on the reaction rate, depending on the nature of the cat-
alyst [68,69]. We therefore decided to investigate the effect of dif-
ferent co-solvents on the coupling reaction using the
Cu2(BPDC)2(BPY) catalyst. The reaction was carried out in the mix-
ture of 1,4-dioxane and the co-solvent (1:1 by volume) at 100 °C,
using 3 mol% catalyst, in the presence of three equivalents of
tert-butyl hydroperoxide as an oxidant, with the 2-hydroxybenzal-
dehyde:1,4-dioxane molar ratio of 1:50. It was found that the reac-
tion rate was decreased significantly in the presence of the co-
solvent, confirming the advantages of the solvent-free condition.
Indeed, reaction conversions of 92%, 68%, and 38% were observed
after 120 min for the transformation carried out in the mixture of
1,4-dioxane and toluene, acetonitrile, and ethyl acetate, respec-
tively (Fig. 4).
In order to check the possibility that the adsorption of the 2-
(1,4-dioxan-2-yloxy)benzaldehyde product on the active copper
sites in the Cu2(BPDC)2(BPY) could hinder the reaction, a reaction
As for other cross-dehydrogenative coupling reaction by the di-
rect activation of C–H bonds [6], the presence of at least one equiv-
alent of an oxidant should be required for the coupling of
2-hydroxybenzaldehyde with 1,4-dioxane using the Cu2(BPDC)2
(BPY) catalyst. It was therefore decided to investigate the effect
of different oxidants on the reaction conversion. The coupling reac-
tion was carried out at 100 °C, using the 2-hydroxybenzalde-
hyde:1,4-dioxane molar ratio of 1:50, with 3 mol% catalyst, in the
presence of three equivalents of an oxidant including tert-butyl
hydroperoxide, hydrogen peroxide, AgNO3, MnO2, PhI(OAc)2, and
oxygen in air, respectively. Experimental results showed that
tert-butyl hydroperoxide should be the oxidant of choice for the
Cu2(BPDC)2(BPY)-catalyzed reaction of 2-hydroxybenzaldehyde
with 1,4-dioxane, affording more than 99% conversion after
100 min. The reaction using PhI(OAc)2 as the oxidant proceeded
with significantly more difficulty, though 49% conversion was still
observed after 120 min. Hydrogen peroxide, AgNO3, MnO2, and
100
80
60
40
3 equiv.
2 equiv.
1 equiv.
20
0
0
20
40
60
80
100
120
Time (min.)
Fig. 6. Effect of oxidant concentration on reaction conversion.

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N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
100
80
60
40
20
0
ane (molar ratio of 1:50) was added to the reaction mixture after
20 min. It was observed that the addition of the substrates to the
reaction mixture did not affect the reaction rate significantly
(Fig. 9). These observations suggested that the adsorption of a large
amount of the 2-(1,4-dioxan-2-yloxy)benzaldehyde product on the
copper sites of the solid catalyst could inhibit the coupling reac-
tion. However, the fact that the original reaction mixture could
proceed to 100% conversion led us to believe that there would be
an adsorption–desorption equilibrium during the course of the
reaction. Indeed, Kholdeeva and co-workers previously employed
MOF-based catalysts for the oxidation reaction and also reported
similar observation, in which the reaction stopped completely after
the product was added to the reaction mixture [70]. However, fur-
ther studies on the adsorption–desorption equilibrium of the prod-
uct on the Cu-MOF catalyst would be needed.
3 mol% Cu2(BPDC)2(BPY)
Adding 0.15 mol% ascorbic acid
Adding 0.15 mol% phloroglucinol
0
20
40
60
80
100
120
Time (min.)
Fig. 7. Adding antioxidant to the reaction mixture after 20 min.
To clarify the advantages of using the Cu2(BPDC)2(BPY) as cata-
lyst for the cross-dehydrogenative coupling reaction, the catalytic
activity of the Cu2(BPDC)2(BPY) was compared with that of other
Cu-MOFs, including Cu3(BTC)2, Cu(BDC), and Cu(BPDC). The cou-
pling reaction was carried out at 100 °C, using 3 mol% catalyst, in
the presence of three equivalents of tert-butyl hydroperoxide as
the oxidant, with the 2-hydroxybenzaldehyde:1,4-dioxane molar
ratio of 1:50. Interestingly, it was found that the Cu2(BPDC)2(BPY)
could offer significantly higher activity than other three Cu-MOFs,
with more than 99% conversion being achieved after 100 min.
Although the well-known Cu3(BTC)2 previously exhibited high
activity in several copper-catalyzed organic reactions [54,71–77],
the cross-dehydrogenative coupling reaction using this Cu-MOF
proceeded to only 48% conversion after 120 min. Ether cyclic sol-
vents were previously reported to be able to coordinate with active
copper centers in the Cu3(BTC)2, thus significantly blocking catalyt-
ically active sites [78]. Low conversions were also observed for the
cross-dehydrogenative coupling reaction using Cu(BDC) or
Cu(BPDC) as catalyst (Fig. 10). Indeed, Tannenbaum and co-work-
ers previously proposed that the desolvated Cu(BDC) would pos-
sess a compact structure, and this phase could be converted into
a lamellar form in solvents containing carbonyl groups such as
DMF and NMP [79]. Similarly, Corma and co-workers also demon-
strated that the transformation of Cu(BDC) from the catalytically
inactive compact structure to the lamellar form upon DMF adsorp-
tion could generate the catalytically active phase [76]. In the cross-
dehydrogenative coupling reaction between 2-hydroxybenzalde-
hyde and 1,4-dioxane, the Cu(BDC) structure would remain com-
pact in excess 1,4-dioxane, thus offering lower activity. The
performance of the Cu(PBDC) as catalyst in the cross-dehydrogena-
tive coupling reaction could be explained based on similar reasons.
In the Cu2(BPDC)2(BPY) structure, the BPDCs act as grid-forming
ligands and the BPYs as pillars [63], thus preventing the formation
of catalytically inactive compact structure. To gain insight into the
solution at 20% conversion of 2-hydroxybenzaldehyde was pre-
pared and was then added into a second reaction mixture in the
presence of 3 mol% Cu2(BPDC)2(BPY) catalyst after 20 min. The
resulting mixture was stirred for another 100 min at 100 °C, with
aliquots being sampled at different time intervals, and analyzed
by GC. Interestingly, it was found that the coupling reaction
stopped immediately, indicating the inhibition role of the product.
It should be noted that the coupling reaction using 3 mol% catalyst
could proceed to completion after 100 min at 100 °C. In a second
experiment, the Cu2(BPDC)2(BPY)-catalyzed coupling reaction
was carried out for 40 min, affording 55% conversion. The solution
at 20% conversion of 2-hydroxybenzaldehyde was then added, and
the resulting mixture was stirred for another 80 min at 100 °C. It
was also observed that adding product to the reaction mixture
stopped the cross-dehydrogenative coupling between 2-hydroxy-
benzaldehyde and 1,4-dioxane immediately. Interestingly, adding
the solution at 20% conversion of 2-hydroxybenzaldehyde to the
Cu(NO3)2-catalyzed coupling reaction did not change the reaction
rate significantly (Fig. 8). To further verify this observation, two
controlled experiments were also carried out at 100 °C using
3 mol% catalyst, in the presence of three equivalents of tert-butyl
hydroperoxide as the oxidant, with the 2-hydroxybenzalde-
hyde:1,4-dioxane molar ratio of 1:50. In the first experiment, the
pre-purified 2-(1,4-dioxan-2-yloxy)benzaldehyde product was
added to the reaction mixture after 20 min. Experimental result
showed that almost no further reaction was observed after the
addition of the product to the reaction mixture. In the second
experiment, the mixture of 2-hydroxybenzaldehyde and 1,4-diox-
Cu2(BPDC)2(BPY)
Cu(NO3)2
Cu2(BPDC)2(BPY)/adding product
Cu(NO3)2/adding product
Cu2(BPDC)2(BPY)/adding product
100
100
80
80
60
40
20
0
60
40
20
0
Adding product
Adding substrates
Cu2(BPDC)2(BPY)
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min.)
Time (min.)
Fig. 8. Adding product to the reaction mixture when using Cu2(BPDC)2(BPY) or
Cu(NO3)2 as catalyst.
Fig. 9. Adding substrates to the reaction mixture using Cu2(BPDC)2(BPY) as catalyst.

N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
43
Cu2(BPDC)2(BPY)
Cu(NO3)2
CuCl
Cu(OAc)2
Cu2(BPDC)2(BPY)
Cu3(BTC)2
Cu(BPDC)+BPY
Cu(BPDC)
Cu(BDC)
BPY
CuCl2
CuI
100
80
60
40
20
0
100
80
60
40
20
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min.)
Time (min.)
Fig. 11. Effect of different copper salts as catalyst on reaction conversion.
Fig. 10. Effect of different Cu-MOFs as catalyst on reaction conversion.
problem. Therefore, the reaction would not proceed under real het-
erogeneous catalysis condition [69]. In order to determine whether
active copper species dissolved from the solid Cu2(BPDC)2(BPY)
catalyst contribute to the total conversion of the coupling reaction
of 2-hydroxybenzaldehyde and 1,4-dioxane, a control experiment
was carried out using a simple centrifugation during the course
of the reaction. Apparently, if the reaction conversion was still ob-
served after the solid catalyst was separated from the reaction
mixture, this behavior would confirm that the reaction would not
occur under real heterogeneous catalysis condition. The coupling
reaction was carried out at 100 °C, using 3 mol% catalyst, in the
presence of three equivalents of tert-butyl hydroperoxide as the
oxidant, with the 2-hydroxybenzaldehyde:1,4-dioxane molar ratio
of 1:50. The Cu2(BPDC)2(BPY) catalyst was removed from the reac-
tion mixture after 20 min reaction time by simple centrifugation.
The liquid phase was then transferred to a new reactor vessel, stir-
red for an additional 100 min at 100 °C with aliquots being sam-
pled at different time intervals, and analyzed by GC. It was found
that almost no further conversion was observed after the Cu2(-
BPDC)2(BPY) catalyst was separated from the reaction mixture
(Fig. 12). These observations would indicate that cross-dehydroge-
native coupling reaction between 2-hydroxybenzaldehyde and 1,4-
dioxane could only proceed in the presence of the solid Cu2(-
BPDC)2(BPY) catalyst, and no contribution from catalytically active
species soluble in the solution was detected.
possible reasons for the higher activity of the Cu2(BPDC)2(BPY) in
comparison with other Cu-MOF catalysts, a controlled experiment
was also carried out at 100 °C, using 3 mol% Cu(BPDC) and
1.5 mol% BPY ligand as the catalyst system, in the presence of three
equivalents of tert-butyl hydroperoxide as the oxidant, with the 2-
hydroxybenzaldehyde:1,4-dioxane molar ratio of 1:50. Interest-
ingly, it was observed that the presence of the BPY ligand in the
reaction mixture led to an enhancement in the reaction rate, with
72% conversion being obtained after 120 min. It should be noted
that the Cu(BPDC)-catalyzed coupling reaction afforded only 44%
conversion after 120 min in the absence of the BPY ligand and that
the BPY itself was inactive for the reaction. However, using
Cu(BPDC) and BPY ligand (molar ratio of 2:1) as the catalyst system
still offered lower activity for the coupling reaction than the case of
the Cu2(BPDC)2(BPY) (Fig. 10). It could be proposed that the com-
bination of Cu(BPDC) and BPY ligand might produce Cu2(BPDC)2(-
BPY) phase during the course of the coupling reaction [80],
resulting in a medium catalytic performance. An amount of the
BPY would not be coordinated with the Cu(BPDC); therefore, lower
catalytic activity was observed as compared to that of the Cu2(-
BPDC)2(BPY) [80,81]. However, further investigations would be
necessary to elucidate the reaction mechanism of the Cu2(BPDC)2(-
BPY)-catalyzed cross-dehydrogenative coupling reaction of 2-
hydroxybenzaldehyde with 1,4-dioxane.
To further emphasize the advantages of employing the Cu2(-
BPDC)2(BPY) catalyst in the cross-dehydrogenative coupling reac-
tion, we also carried out the reaction in the presence of several
copper salts as catalyst, including CuCl2, CuCl, CuI, Cu(NO3)2, and
Cu(OAc)2, respectively. The coupling reaction was carried out at
100 °C, using 3 mol% catalyst, in the presence of three equivalents
of tert-butyl hydroperoxide as the oxidant, with the 2-hydroxy-
benzaldehyde:1,4-dioxane molar ratio of 1:50. As expected, the
Cu(OAc)2-catalyzed coupling reaction could afford 87% conversion
after 120 min. Interestingly, it was observed that higher conver-
sions were obtained for the reaction using CuCl2 or CuCl as catalyst
for the first 60 min. However, the reaction did not proceed any fur-
ther, and conversions of 86% and 90%, respectively, were observed
after 120 min. The Cu(NO3)2-catalyzed coupling reaction could af-
ford 92% conversion after 120 min, while CuI exhibited signifi-
cantly lower activity, with 55% conversion being observed after
120 min (Fig. 11). The fact that the Cu2(BPDC)2(BPY)-catalyzed
coupling reaction could proceed to completion after 100 min
therefore would offer advantages over that using copper salts as
catalyst.
Although several copper salts exhibited reasonable conversions
in the coupling reaction between 2-hydroxybenzaldehyde and 1,4-
dioxane, it was apparent that the catalyst could not be reused. For
the development of more environmentally benign processes, the
ability to recover and reuse the catalyst should be taken into
100
80
60
40
3 mol% Cu2(BPDC)2(BPY)
20
Leaching test
0
0
20
40
60
80
100
120
Time (min.)
For liquid-phase organic reaction, the possibility that some of
catalytically active sites on the solid catalyst could dissolve into
the solution during the course of the reaction might be a serious
Fig. 12. Leaching test indicated no contribution from homogeneous catalysis of
active species leaching into reaction solution.

44
N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Run
Fig. 13. Catalyst recycling studies.
accounts. It is expected that the solid catalyst can be separated
from the reaction mixture and reused several times before it even-
tually deactivates completely. The Cu2(BPDC)2(BPY) catalyst was
therefore investigated for recoverability and reusability in the cou-
pling reaction between 2-hydroxybenzaldehyde and 1,4-dioxane
over eight successive runs, by repeatedly separating the Cu-MOF
catalyst from the reaction mixture, washing it, and then reusing
it. The coupling reaction was carried out at 100 °C, using 3 mol%
catalyst, in the presence of three equivalents of tert-butyl hydro-
peroxide as the oxidant, with the 2-hydroxybenzaldehyde:1,
4-dioxane molar ratio of 1:50. After the coupling reaction was
complete, the Cu2(BPDC)2(BPY) catalyst was separated by simple
centrifugation, then washed with copious amounts of methanol
to remove any physisorbed reagents, and dried at room tempera-
ture under air. The recovered Cu2(BPDC)2(BPY) was then reused
in further reactions under identical conditions to those of the first
run. It was observed that the Cu2(BPDC)2(BPY) catalyst could be
recovered and reused several times without a significant degrada-
tion in catalytic activity. Indeed, a conversion of 88% was still
achieved in the 8th run (Fig. 13). The XRD result of the reused Cu2(-
BPDC)2(BPY) catalyst indicated that the crystallinity of the Cu-MOF
could be maintained during the course of the reaction (Fig. 14).
Furthermore, the FT-IR spectra of the reused Cu2(BPDC)2(BPY)
showed a similar absorption as compared to that of the fresh
catalyst (Fig. 15). These observations confirmed that the Cu2
(BPDC)2(BPY) catalyst offered excellent reusability in the coupling
reaction between 2-hydroxybenzaldehyde and 1,4-dioxane.
Fig. 15. FT-IR spectra of the fresh (a) and reused (b) Cu2(BPDC)2(BPY) catalyst.
2-Hydroxybenzaldehyde
Methylsalicylate
2-Methylphenol
2'-Hydroxyacetophenone
2-Nitrophenol
Phenol
100
80
60
40
20
0
0
20
40
60
80
100
120
Time (min.)
Fig. 16. Effect of different phenols on reaction conversion.
reaction between 1,4-dioxane and 2-hydroxybenzaldehyde, meth-
ylsalicylate, 2-methylphenol, 20-hydroxyacetophenone, 2-nitro-
phenol, and phenol, respectively, was carried out at 100 °C, using
3 mol% catalyst, in the presence of three equivalents of tert-butyl
hydroperoxide as the oxidant, with the reagent molar ratio of
1:50. It was found that the phenol structure exhibited a profound
effect on the coupling reaction. Phenol was almost unreactive in
the Cu2(BPDC)2(BPY)-catalyzed coupling reaction with 1,4-diox-
ane, with only 7% conversion being observed after 120 min. The
presence of a substituent at the ortho position led to a significant
improvement in the reaction rate. Conversions of 48%, 85%, 87%,
90%, and more than 99% were obtained for the coupling reaction
of 2-nitrophenol, 2-methylphenol, 20-hydroxyacetophenone, meth-
ylsalicylate, and 2-hydroxybenzaldehyde, respectively (Fig. 16).
These observations indicated the importance of the substituent
at the ortho position in the phenol structure. However, further
investigations would be needed to clarify the effect of the phenol
structure on the coupling reaction. The coupling reaction between
2-hydroxybenzaldehyde and 1,4-dioxane, diethyl ether, dim-
ethoxymethane, and tetrahydrofuran, respectively, was also inves-
tigated. Owing to the low boiling point of diethyl ether,
dimethoxymethane, and tetrahydrofuran, the reaction was carried
out at reflux temperature instead of 100 °C. As a sequence, the
reaction resulted in significantly lower conversion than the case
of 1,4-dioxane (Fig. 17).
The study was then extended to the coupling reaction between
different phenols with different ethers, respectively. In the first
experiment series, the Cu2(BPDC)2(BPY)-catalyzed coupling
Fig. 14. X-ray powder diffractograms of the fresh (a) and reused (b) Cu2(BPDC)2(-
BPY) catalyst.

N.T.S. Phan et al. / Journal of Catalysis 306 (2013) 38–46
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4. Conclusions
In summary, the metal–organic framework Cu2(BPDC)2(BPY)
was synthesized by a solvothermal method and was characterized
by several techniques including XRD, SEM, TEM, FT-IR, TGA, AAS,
H2-TPR, and nitrogen physisorption measurements. The Cu-MOF
could be used as an efficient heterogeneous catalyst for the cop-
per-catalyzed cross-dehydrogenative coupling reaction of ethers
with 2-carbonyl-substituted phenols. Excellent conversions were
obtained in the presence of a catalytic amount of the Cu-MOF.
The Cu2(BPDC)2(BPY) exhibited significantly higher catalytic activ-
ity than that of other Cu-MOFs such as Cu3(BTC)2, Cu(BDC), and
Cu(BPDC). The Cu2(BPDC)2(BPY) also offered advantages over sev-
eral copper salts such as CuCl2, CuCl, CuI, Cu(NO3)2, and Cu(OAc)2
in terms of the catalytic activity as well as the ability to recover
and reuse the catalyst. The Cu2(BPDC)2(BPY) could be recovered
and reused several times without a significant degradation in cat-
alytic activity. The cross-dehydrogenative coupling reaction could
only occur in the presence of the solid Cu-MOF catalyst, and no
contribution from leached copper sites present in the solution
was detected. To the best of our knowledge, the cross-dehydroge-
native coupling reaction of ethers with 2-carbonyl-substituted
phenols via C–H activation under oxidative conditions using a het-
erogeneous catalyst was not previously mentioned in the
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Acknowledgment
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