Copper catalyzed oxidative homocoupling of terminal alkynes to 1,3-diynes: A Cu3(BTC)2 MOF as an efficient and ligand free catalyst for Glaser-Hay coupling

Article abstract of DOI:10.1039/c7ob02196h

A straightforward and efficient method has been demonstrated for the oxidative coupling of terminal alkynes using a simple Cu₃(BTC)₂-metal organic framework as a sustainable heterogeneous copper catalyst. A series of symmetrical 1,3-diynes bearing diverse functional groups have been synthesized in moderate to excellent yields via a Cu₃(BTC)₂ catalyzed Glaser-Hay reaction. The presence of the coordinatively unsaturated open Cu^II sites in Cu₃(BTC)₂ catalyzes the homocoupling in the presence of air, as an environment friendly oxidant without the use of external oxidants, ligands or any additives. The present methodology avoids stoichiometric reagents and harsher or special reaction conditions, and shows good functional group tolerance. The as-prepared catalyst could be separated easily by simple filtration and reused several times without any notable loss in activity. The hot filtration test has investigated the true heterogeneity of the catalyst. Additionally, the powder X-ray diffraction pattern of the reused catalyst revealed the high stability of the catalyst.

Full text of DOI:10.1039/c7ob02196h

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G
Copper Catalyzed Oxidative Homocoupling of Terminal Alkynes to
1,3-Diynes: Cu3(BTC)2 MOF as an Efficient and Ligand Free Catalyst
for Glaser-Hay Coupling
Nainamalai Devarajan, Murugan Karthik and Palaniswamy Suresh*
Supramolecular and Catalysis Lab, Dept. of Natural Products Chemistry,
School of Chemistry, Madurai Kamaraj University, Madurai-625021, India.
*ghemistry@gmail.com, suresh.chem@mkuniversity.org
Abstract
A straightforward and efficient method has been demonstrated for the oxidative
coupling of terminal alkynes using simple Cu3(BTC)2-Metal Organic Framework as a
sustainable heterogeneous copper catalyst. A series of symmetrical 1,3-diynes bearing
diverse functional group have been synthesized in moderate to excellent yields via
Cu3(BTC)2 catalyzed Glaser-Hay reaction. Presence of the coordinatively unsaturated
open CuII sites of Cu3(BTC)2 catalyzes the homocoupling in the presence of air, as an
environment friendly oxidant without the use of external oxidants, ligands or any
additives. Present methodology avoids stoichiometric reagents, harsher or special
reaction conditions, and shows good functional group tolerance. The as-prepared
catalyst could be separated easily by simple filtration and reused for several times
without any notable loss in activity. Hot filtration test has investigated the true
heterogeneity of the catalyst. Additionally, powder x-ray diffraction pattern of the
reused catalyst revealed the high stability of the catalyst.
Key Words: Cu3(BTC)2 MOF, 1,3-diynes, Glaser-Hay coupling, Homocoupling,
Heterogeneous catalysis.

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Introduction
Acetylenic compounds are significant core motifs of various natural products and
consist of a series of conjugated acetylenic units. In particular, conjugated 1,3- diynes
are prevalent, find wide applications in organic and natural products synthesis,1
bioactive molecules,2 conducting polymer materials,3 optical light emitting materials,4
and supramolecular chemistry.5 The most straight forward approach to get the 1,3-
diynes is the direct coupling of two terminal alkynes was developed by Glaser and
Hay.6 Since then, several attempts have been made to develop a mild and efficient
method for the synthesis of 1,3-diyenes using various transition metal catalysts7 and
supported catalysts.8 Though different metals have been used for the Glaser-Hay
coupling of terminal alkynes, copper based catalyst are considerably9 being explored
owing to the easy accessibility of copper catalysts and adequate reaction efficiency.
Most of the copper catalyzed coupling reactions are performed using a stoichiometric
amount of catalyst without ligand at elevated temperature or at the mild condition with
ligands in order to stabilize the copper species. Recently several attempts have been
made to achieve symmetrical 1,3-diynes under the mild condition,8a,10 however, each
method has its own demerits like the use of stoichiometric amount of catalyst, use of
organic ligands, external oxidants and lack of reusability. Several solid supported
catalytic systems have been developed for Glaser-Hay reactions.8 However,
preparation of solid support, stability, purification, reusability, and requirement of
external oxidant, makes the reaction system cumbersome to get pure 1,3-alkynes.
Thus, the development of robust, ligand and additional external oxidant free catalytic
system is important to overcome the practical difficulties for the synthesis of pure
conjugated diynes. These triggered us for the development of green processes in
Glaser-Hay coupling diynes.
Metal–Organic Frameworks (MOFs) are the porous coordination polymeric
materials, have gained wide attention in heterogeneous catalysis for their unique
tunable functionality, high surface porosity, thermal stability and the presence of
potentially active metal sites.11 Recently, transition metal based MOFs are prevalently
used as a catalytic material in synthetic organic transformations and slowly replacing

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their homogeneous counterparts, as it played a vital role in greening the organic
synthesis.12 Among them, copper based MOFs are indispensable, gained attention in
heterogeneous catalysis, owing to its simple preparations, high stability, variable, but
stable oxidation states and tunbable pore size.13 In organic synthesis, though, copper
mediated transformations replacing expensive noble metal catalyst, the separation, and
handling of copious quantities of copper-based waste produced from homogeneous
processes creating undesirable economic, environmental, health and safety issues.
Copper based MOF catalysts slowly rectify these adverse effects of homogeneous
copper catalysts.13 The study on the catalytic role of Cu-based MOFs is of particular
interest because of the presence of coordinatively unsaturated open metal sites14 that
mimic the coordination and electronic properties of N-heterocyclic Cu coordination
complexes in catalysis.15 The mesoporous Cu3(BTC)2 (BTC-benzene-1,3,5-
tricarboxylate) is one of the well-known copper MOF. Cu3(BTC)2 is composed of
dimeric Cu2 units coordinated to four BTC ligands forming paddle wheel building unit
with high surface area and large pore volume.16 The presence of coordinatively
unsaturated CuII open metal sites and its Lewis acidic nature can easily facilitate
various synthetic organic transformations like oxidation,17 cyclization,18
condensation19 and coupling reactions.20 Though Cu3(BTC)2 have been used to
catalyze various coupling reactions like C-C,20a,b C-O,20c C-N,20d,e and C-S,20f,g still, its
catalytic potential has not yet explored in synthetically important 1,3-alkyne coupling
reactions. Therefore, in the present work, a sustainable heterogeneous version of the
Glaser-Hay homocoupling reaction have been developed to synthesise symmetrical
1,3-diynes using Cu3(BTC)2 as an efficient and reusable solid copper catalyst under
additional external oxidant and ligand free condition.
Results and discussion
To develop a benign methodology for the synthesis of symmetrical 1,3-diynes using
Cu3(BTC)2 MOF (1) as a catalyst, phenylacetylene (2a) was taken as a model
substrate to screen the desired reaction parameters such as solvent, temperature, time,

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base and catalyst load. Few preliminary studies show that the homocoupling
proceeded when using 1 as catalyst and 1,4-dioxane as a solvent with K2CO3 at 85 °C
in an open vessel. The optimization results obtained by employing various solvents
and bases are listed in Table 1. From the observed results, it can be seen that the
solvents behaved perversely in the presence of 1 with yields in different ranges with
the use of highly
Table 1 Optimization of the Cu3(BTC)2 catalyzed on the Glaser-Hay Reactionsa
Yieldb
(%)
Entry Solvent
Base
1
2
3
4
5
6
7
8
Toluene
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
-
K2CO3
Na2CO3
Cs2CO3
TEA
DIPEA
Pyridine
Pyridine
Piperidine
-
Dichloromethane
1,2-Dichloroethane
THF
Acetonitrile
Chloroform
1,4-Dioxane
DMF
58
49
-
68
-
99
-
9
Methanol
72
83
16
12
87
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Ethanol
iPrOH
tBuOH
Solvent-free
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
c
-
-
21d
62
75
30
47
31
11e
35
aReaction conditions: 2a (1 mmol, 1.0 eq) 1 (20 wt %,
6.9 mol %), and base (1.2 mmol, 1.2 eq), solvent (2.0
b
mL) at 85 °C for 12 h under air. Isolated yield.
e
cWithout catalyst, dReaction under argon atm, 1
socked in pyridine

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polar solvents including DMF, the reaction was totally inert and in solvents like DCM,
DCE, chloroform, and ACN, the reaction under went with poor to moderate yield. By
using protic polar solvents, such as methanol and ethanol, the yield percentage is
improved, to some extent than that of iso-propanol and tert-butanol. However, in the
case of non-polar such as toluene and THF, no reaction occurred as like high polar
solvents. Furthermore, 1,4-dioxane exhibited significant superiority over other
solvents and reaction went to completion with a very good yield of 99 % of the
homocoupled product (entry 7, Table 1). To improve the greenness of the reaction and
without the use of harmful solvents, reaction has been carried out under the solvent-
free condition and gave a good yield of 87 % (entry 13, Table 1). Further, to fix the
role of base in homocoupling of terminal alkynes, various bases have been screened,
among which inorganic base K2CO3 resulting better yield in the presence of 1 without
any adverse effect during the product separation and no influence on catalyst stability.
Other organic bases like triethyl amine (TEA), Di-isopropyl ethyl amine (DIPEA),
pyridine and piperidine gave poor yield (entries 18-23, Table 1), as they block the
active copper sites of 1 through coordination of nitrogen and the catalyst was found to
be inactive towards coupling in the absence of a base. To clearly understand the role
of the active copper site in homocoupling, 1 was soaked in excess of pyridine for 48 h,
after the catalyst was filtered and dried. After that, coupling was carried out using the
pyridine adsorbed catalyst, whereas the drastic reduction in the yield was noted (entry
22, Table 1), which clearly demonstrates that the open Cu2+ sites catalyze the
homocoupling of terminal alkynes, when the active open metal sites were blocked by
external pyridine, the catalytic potential of 1 was suppressed. Similarly, 1 was
degassed under high vacuum using a glass oven for 48 h at 120 °C and then purged
with argon. Using this air and oxygen free catalyst, reaction was carried out under the
optimized condition, and a huge drop in the coupling product is noted (entry16, Table
1) which shows the importance of the presence of the air in 1 catalyzed coupling. The
coupling reaction was highly time and temperature dependent. Shorten the reaction
time directly affects the course of the reaction resulted in a moderate conversion, (Fig.
1a) whereas reaction time 12 h affords excellent yield of 99 %. Similarly, the product
conversion was

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Fig. 1 Optimization of time (a) and temperature (b) for Cu3(BTC)2 Catalyzed Glaser-
Hay coupling
susceptible to temperature changes, the decrease in temperature dramatically affect the
yield (Fig. 1b) within the optimized time, and at 85 °C the complete conversion has
been observed without any by-products. Thus, 85 °C has been chosen as the optimum
temperature for further reactions. With the screened parameters, the catalyst load for
the homocoupling was further optimized, since, it plays a crucial role in the
enhancement of product conversion and quantifies the efficiency of the catalyst used.
The increase in the catalyst load from minimum exhibited a phenomenal change in the
yield. Finally, the optimization results show that 6.9 mol % (20 wt %) catalyst was
sufficient to get maximum yield (99 %) of homocoupled product with the very low
copper load (4.3 mg, 0.06 mmol). No coupling product was observed in the absence of
Cu3(BTC)2. Finally, the combinations of 6.9 mol % of Cu3(BTC)2 with K2CO3 as a
base in 1,4-dioxane medium at 85 °C was screened as optimized reaction condition for
the synthesis of symmetrical 1,3-diynes.
Fig. 2 Optimization of catalyst (Cu3(BTC)2) load for Glaser-Hay coupling

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With the optimum reaction conditions in hand, we have expanded the scope of
1 catalyzed homocoupling to other terminal alkynes including aromatic and aliphatic
terminal alkynes and the results are summarized in Table 2. Various aromatic terminal
alkynes with either electron-donating or electron-withdrawing substituents smoothly
undergoing homocoupling and gave the corresponding 1,3-diynes in 42-99 % yields.
(entries 1-12, Table 2). Phenylacetylene having electron donating substituents such as
methyl, ethyl, tert-butyl, methoxy and propyloxy groups facilitate the coupling and
gave better yield within the stipulated reaction time. Meantime, the bulky substituents
o-methoxy (entry 6, Table 2) and p-benzyloxy (entry 8, Table 2) on phenylacetylene
decreased the transformation rate and required long reaction time. The same trend was
observed with m-amino substituted phenylacetylene with the moderate yield of 42 %
even after 24 h (entry 12, Table 2). On the other hand, electron-withdrawing
substituents such as chloro and fluoro gave good yield irrespective of their substituted
position and electron withdrawing nature. On the basis of the promising results with
aromatic alkynes, further attempts have been made on various non-aromatic terminal
alkynes, which act as an effective substrate for the oxidative homocoupling, with the
corresponding 1,3-diynes in yields of 35 to 92 % (entries 13-16, Table 2). However,
all non-aromatic terminal alkynes required long reaction time with poor to moderate
yield. These observations reveal that the present catalyst promotes the coupling of
both aromatic and aliphatic alkynes, between aromatic is more reactive.
Even though, 1 catalyzed the homocoupling of terminal alkynes with minimum
Cu load in the absence of any external ligands, in order to understand the role of
framework Cu, various copper salts namely, Cu(OAc)2.H2O, CuSO4.5H2O,
Cu(NO3)2.3H2O, Cu(BF4)2.H2O, Cu2O, CuCl, CuBr, and CuI have been used under
the same optimized condition in the absence of any external ligand and external
oxidant in an open vessel. All copper salts have invariably catalyzed the coupling,
resulting in the moderate yield with stoichiometric equivalents of the catalyst.
Likewise, to demonstrate the competence of the reactivity of the Cu2+ in 1, Glaser-Hay
coupling was carried out using various MOFs, such as Cu(tpa),
Cu(bpy)(H2O)(BF4)2(bpy) (bpy=bipyridine),

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Table 2 Cu3(BTC)2 catalyzed Glaser-Hay coupling of terminal alkynesa
Time Yieldb
Entry
1
Reactant
Product
(h)
(%)
2a
3a
12
99
2
3
4
5
2b
2c
2d
22
3b
3c
3d
3e
12
12
13
12
92
90
90
87
6
7
2f
3f
24
16
85
91
2g
3g
BnO
8
9
2h
2i
3h
3i
24
12
67
93
10
11
2j
3j
15
15
90
93
2k
3k
12
2l
3l
24
42
13
14
2m
2n
3m
3n
12
24
92
53

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15
16
2o
2p
3o
3p
24
24
40
35
aReaction conditions: 1 (6.9 mol %, 20 wt %), alkynes (1.0 eq) and K2CO3 (1.2 eq) 1,4-Dioxane
(2.0 mL) at 85 °C for under air. bIsolated yield.
Ni(BDC)(DABCO) MOF, (BDC-benzene dicarboxylic acid) (DABCO-1,4
diazabicyclo[2.2.2]octane), Fe(BTC), Fe-MIL-53 and IRMOF-3 under identical
condition. Among them, copper MOF Cu(tpa) was absolutely inert in Glaser-Hay
coupling as previously reported15 and another Cu-MOF showed poor reactivity (entry
2, Table 4). All other MOFs are highly inert in catalyzing the homocoupling under the
optimized condition. The observed results strongly suggest that the presence of high
surface area, porous nature and the catalytic center ligated by the trimesic acid are
responsible for the high catalytic activity of the 1. In order to confirm the above
observation, a control experiment has been performed using a stoichiometric mixture
of Cu(NO3)2.3H2O and trimesic acid which gave the poor yield (33 %, entry 7, Table
4). The above control experiments are strongly indicating the potentiality of
framework Cu2+ in 1 catalyzing the homocoupling reaction.
Table 3 Catalytic efficiency of different copper sources in Glaser-Hay Reactiona
Entry Catalyst
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
Cu(OAc)2.H2O
72
71
35
45
68
72
69
75
79
99
CuSO4.5H2O
Cu(NO3)2.3H2O
Cu(BF4)2.H2O
CuCl2.3H2O
Cu2O
CuCl
CuBr
CuI
Cu3(BTC)2-MOFc
aReaction conditions: Phenylacetylene (1
mmol, 1.0 eq) and K2CO3 (1.2 mmol, 1.2 eq)
Copper salt (1 eq.), 1,4-Dioxane (2.0 mL) at 85
b
c
°C for 12 h under air. Isolated yield, 6.9 mol
%

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Table 4 Role of different MOF in Glaser-Hay Reactiona
Yieldb
(%)
-
43
-
-
Entry
Various MOFs
Cu(tpa).dmf
Cu(bpy)(H2O)2(bpy)
Ni(BDC)(DABCO) MOF
Fe(BTC)
1
2
3
4
5
6
7
Fe-MIL-53
-
-
33
IRMOF-3
Cu(NO3)2.H2O:BTCc
aReaction conditions: Phenylacetylene (1.0
eq) and K2CO3 (1.2 eq), MOF catalyst 20 wt
%, 1,4-Dioxane (2.0 mL) at 85 °C for 12 h
b
c
under air. Isolated yield. As a 1:1 physical
mixture.
In heterogeneous catalysis, reusability determines the sustainability and robustness
of the catalyst as well as quantify the greenness of the catalytic process. In the present
work, reusability of the catalyst assessed by the stability of catalytically active copper on
framework upon completion of the reaction. For that analysis, catalyst 1 has been reused
in the homocoupling of terminal alkynes under optimized condition. The data as shown
in Fig. 3 obtained for the catalyst reused at least for five times indicates the high stability
without any retention. However, a mild drop in the yield was observed when the sample
was used for five successive runs, which might be due to the partial blocking of the active
sites of the 1 as a result of repeated reactions. Similarly, to ascertain the stability of the
reused catalyst, after every catalytic cycle, the sample was recovered by simple filtration
followed by washing and drying and subjected to FT-IR, PXRD and SEM analysis, such
that the structural and morphological changes on framework during the course of the
reactions can be analyzed. The FT-IR spectral data obtained for the reused 1 catalyst
showed the absorption bands which coincide well with the fresh catalyst. Added to that
no change in the IR stretching frequencies was observed (Fig. 4a). The absence of strong
absorption band corresponds to free carboxylic acid (1760–1690 cm-1) in reused catalyst
indicates that catalyst 1 does not decomposed into carboxylic acids during the course of
the reaction and retained its polymeric coordination framework structure even after five
consecutive experiments. Similarly, the powder XRD diffraction pattern of the reused

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catalyst was exactly matched well with the freshly synthesized MOF pattern without any
change in the peak position (Fig. 4b) indicates that the crystallinity and framework
structure of the catalyst was maintained without any deformation even after five times
repeated use. Further, the stability and crystalline morphology nature of the reused
catalyst was demonstrated using SEM characterization. Fig. 5 shows the SEM images of
both fresh and fifth time reused catalyst. It can be observed that the morphology of the
particle was highly crystalline in nature with a mild deformation from the octahedral
shape with porous and rough surface MOFs.
Fig. 3 Reusability profile of Cu3(BTC)2 in Glaser-Hay coupling. Reaction condition:
Phenylacetylene (1.0 eq), K2CO3 (1.2 eq), Cu3(BTC)2 (20 wt %, 6.9 mol %), and 1,4-
dioxane (2.0 mL) at 85 °C for 12 h under air; bIsolated yield.
Fig. 4 a) FT-IR spectra and b) Powder-XRD patterns of fresh and reused catalyst. (i)
fresh Cu3(BTC)2 MOF, (ii) first run, (iii) third run, and (iv) fifth run.

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Fig. 5 SEM images of a) fresh and b) reused Cu3(BTC)2 MOF after the fifth run.
In heterogeneous catalysis, especially in the case of MOF based catalysis,
during the course of the reaction some framework metals in MOF may leach to
solvent which influences the reaction and behave like a homogeneous catalysis.
Hence, control experiment was performed under the optimized condition, in order to
verify the true heterogeneity of the catalyst 1. The catalyst was separated from the
reaction mixture by simple filtration after 4 h at 85 °C reaction temperature, and the
mother liquor alone was allowed to stir to complete the optimized reaction time (Fig.
6). The product conversion, before and after filtration was quantified using high
performance
Fig. 6 Hot Filtration test for the true heterogeneity of Cu3(BTC)2-MOF in the Glaser-
Hay Reaction
liquid chromatography and no further product conversion was observed even after the
removal of catalyst from the reaction medium. The filtration test results strongly
evident that no leaching from the framework occurred and catalyst 1 was stable and

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catalyze the homocoupling of terminal alkynes in a heterogeneous manner. These
results were confirmed by inductively coupled plasma optical emission spectroscopy
(ICP-OES) measurement of the filtrate from the reaction mixture in dioxane in which
there is no evidence for metal leaching is observed demonstrating 1 MOF behaves
truly heterogeneous.
By analogy with previous reports21 a plausible mechanism has been proposed
which is shown in scheme 1 for the 1 catalyzed Glaser-Hay coupling. The presence of
a high surface area and exposed apical coordination sites on Cu2+ in 1 promote the
homocoupling of the terminal alkynes. The flexibility of the frameworks allows Cu2+
to coordinate to acetylide either by expanding the coordination sphere of framework
copper or by displacing the solvent molecules coordinated to unsaturated metal sites
without causing framework collapse. From the recent finding,22 the possibility to
R
Base
+
R
B H
R
O
O
O
O
O
½O2
O
Cu2+
Cu2+
2+
R
2+
R
½O2
Cu
O
Cu
O
O
O
O
O
-BH
O
O
(V)
H2O
(II)
R
H2O
R
Bimetallic
Mechanism
O
O
O
O
Monometallic
Mechanism
Cu3(BTC)2
O
O
O
O
Cu2+
O
2+
Cu2+
Cu3+
Cu
3+
3+
O
R
Cu
R
Cu
O
O
O
O
O
O
O
O
O
O
O
(I)
(VI)
(III)
R
Reductive
Elimination
O
O
O
O
Reductive
Elimination
O
O
O
Cu1+
Cu
O
Cu2+
1+
1+
O2
Cu
R
R
O
O
O
O
O
O
O2
R
R
R = -C6H5
(IV)
(VII)
Scheme 1 Plausible mechanism for the Cu3(BTC)2 MOF-catalyzed Glaser-Hay
coupling of terminal alkynes
generate reversible Cu2+/Cu+ entities in 1 could significantly support the following
proposed mechanism. The proposed mechanism involves the formations of Cu2+-
acetylide II after deprotonation of acetylene by base K2CO3. In the presence of O2 in

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the air, the Cu2+ center of 1 in II is oxidized readily to Cu3+ and forms the
intermediate III, which subsequently undergo reductive elimination to give the
coupled product and form IV where the Cu1+ species oxidized to Cu2+, catalyst
regenerated and enters to a new cycle. Due to the presence of the rigid paddle-wheel
coordination geometry of the 1, a bimetallic mechanism is also likely to yield the
homocoupled product.
Conclusions
In the present work, a simple, efficient and greener heterogeneous copper
catalyst for the synthesis of symmetrical 1,3-diynes has been demonstrated with good
to excellent yield. The catalyst, Cu3(BTC)2 1 catalyzes the homocoupling of terminal
alkynes under a mild condition with wider substrate scope and without any by-
products. This method is advantageous compared to the existing methodologies due to
the utilization of sustainable copper-MOF without any ligand or additives and using
air as an oxidant in open vessel with the minimum copper load. The Cu3(BTC)2 MOF
is highly stable under the optimized conditions and contribution from the leached
species is negligible. The robust nature of the catalyst is proved by simple filtration
which can be reused for five times without any significant loss in catalytic behaviours.
The highly reactive nature of the copper catalyst offers scope for the wide application
of natural products synthesis and medicinal chemistry.
Experimental section
General Information
All reagents and starting materials were purchased commercially from Sigma–
Aldrich, Alfa-Aesar, or Merck, and used as received without any further purification
1
13
unless otherwise noted. All H and C NMR spectra were measured either in CDCl3
with TMS as an internal standard by using a 300 MHz on a Bruker spectrometer. EI-
MS analysis carried out on NexION 300X; Perkin Elmer Inc., using N2 as a carrier gas

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with flow rate (1 mL·min-1), initial column temperature (100 °C), final column
temperature. (280 °C), progress rate (20 K·min-1), injection temperature (280 °C),
detection temperature (280 °C). The powder XRD patterns of the MOF samples were
recorded by using a XPERT-PRO instrument using CuKα radiation at RT. FT-IR
spectrum recorded by using a Shimadzu instrument from 4500 to 500 cm-1 and the
samples were dispersed using the KBr pellet technique. N2 physisorption
measurements were conducted by using a Micromeritics 2020 volumetric adsorption
analyzer system at –196 °C analysis bath temperature before the sample was
outgassed at 200 °C for 12 h. A Netzsch Thermo analyzer STA 409 was used for
thermogravimetric analysis (TGA) with a heating rate of 10 °C min-1 under a nitrogen
atmosphere at 50–700 °C. SEM images were recorded using a TESCAN VEGA3
instrument with SE detector and equipped with an EDAX energy-dispersive X-ray
spectroscopy (EDX) detector. ICP-OES analysis was carried out using a PerkinElmer
OPTIMA 5300 DV. HPLC analysis was carried out using Thermo Fisher Scientific
instrument using C18 column (phenomenox - 5µ) with a flow rate of 0.8 mL/min.
(0.01 mol ammonium acetate buffer and acetonitrile) using UV range of 254 nm.
Synthesis and Characterization of Cu3(BTC)2 MOF (1)
Cu3(BTC)2 was synthesized according to the reported literature.16,23
Cu(NO3)2.3H2O (2.69 g, 18.1 mmol) was dissolved in 1:1.33:1 ratio of
DMF:ethanol:deionized water (15:20:15 mL). Then trimesic acid (1.18 g, 11.2 mmol)
was dissolved in the same amount of solvents. Then both were mixed together and
stirred well to make a homogeneous mixture in a glass vial and placed in an air oven
at 85 °C for 24 h. After that, the vials were allowed to cool down to room temperature
naturally. The resulting mixture was filtered through a fluted filter paper (Whatman
filter paper No.3) and washed with excess of DMF to remove unreacted reactants. The
product was then immersed in a MeOH medium for 24 h. Then periodically changed
with fresh MeOH for three times. The resulting solid was dried under vacuum at 170
°C for 6 h to obtain deep purple crystals.

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The resulting crystalline 1 was characterized using FT-IR, PXRD, TGA, SEM,
surface area measurement and elemental analysis (ICP-OES, EDAX). In FT-IR
analysis of Cu3(BTC)2 showed the absence of strong absorption band at 1760–1690
cm−1 and the presence of peak at 1620 cm−1 is due to the carboxylate anion present in
the framework, which was lower than the C=O stretching vibrations of carboxylic
acids and confirms the deprotonation of carboxylic acid groups in 1,3,5-trimesic acid.
The thermal stability of Cu3(BTC)2 MOF was analyzed by thermogravimetric analysis
(TGA) and indicated that it was stable up to 300 °C. The first weight loss up to 100 °C
due to vaporization of solvent (MeOH), residing in the pore. The second step of
weight loss was in the temperature range of 300–550 °C due to decomposition of the
material. These results show that Cu3(BTC)2 is stable at temperature as high as 300
°C. The nitrogen physisorption measurements revealed the presence of porous
structure in Cu3(BTC)2. From the BET method, the surface area was measured as 838
m2/g with the pore width size of 0.6375 nm for Cu3(BTC)2 MOF were achieved. ICP-
OES analysis showed the copper loading was about 4.287 mmolg-1 and further
confirmed using EDAX results. The SEM micrograph of Cu3(BTC)2 shows that the
morphology of the particle was defined crystalline in nature and well-shaped
octahedral shape with porous and rough surface MOFs, which was in good agreement
with the previous study. The overall analyses of synthesized Cu3(BTC)2 MOF are
corroborated well with the reported values.
General Procedure for Cu3(BTC)2 catalyzed Glaser-Hay reaction
The C-C homocoupling of phenylacetylene (2a) was performed in oven-dried
glassware. Compound 2a (100 mg, 0.979 mmol, 1.0 eq) was added to 1,4-dioxane (2
mL). Then K2CO3 (162 mg, 1.20 mmol, 1.2 eq) was added followed by Cu3(BTC)2
MOF (20 mg (20 wt %), 6.9 mol %). The above mixture was stirred at 85 °C for 12 h
in an open atmosphere. The course of the reaction was monitored by TLC, and the
catalyst was recovered by simple filtration from the reaction mixture after completion
of the reaction. The catalyst was repeatedly washed with 1,4-dioxane and then with
ethyl acetate to remove the product and any unreacted materials, if present. The

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filtrate was evaporated to dryness under vacuum, and the resulting residue was diluted
with ethyl acetate followed by water wash. Then dried with anhydrous sodium sulfate
and concentrated under reduced pressure. The resulting product was purified by
passing over silica gel (100–200 mesh) using petroleum ether as an eluent to afford
1,4-diphenylbuta-1,3-diyne (3a), which was analyzed by using NMR spectroscopy
and EI-MS. After the product extracted from reaction media, Cu3(BTC)2 MOF was
repeatedly washed with DMF followed by immersed in methanol and dried in a hot-
air oven at 100 °C before reuse.
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
We gratefully acknowledge the financial support from the Department of Science and
Technology-Science and Engineering Research Board (DST-SERB), New Delhi, India
(Grant No. SR/FT/CS-53/2011), University Grants Commission, New Delhi, India
(UGC grant No.: 42-291/2013(SR)) and Council for Scientific and Industrial
Research, New Delhi, India (CSIR grant No.: 02(0191)14/EMR-II). We thank
University Grants Commission under University with Potential for Excellence
program (UGC-UPE), DST-FIST and DST-PURSE for the instrumental facility.
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Table of Content
Copper Catalyzed Oxidative Homocoupling of Terminal Alkynes to
1,3-Diynes: Cu3(BTC)2 MOF as an Efficient and Ligand Free Catalyst
for Glaser-Hay Coupling
Nainamalai Devarajan, Murugan Karthik and Palaniswamy Suresh*
An efficient and sustainable methodology for the synthesis of 1,3-diynes has been
demonstrated using framework copper present in Cu3(BTC)2 Metal Organic
Framework.