Mesoporous Copper/Manganese Oxide Catalyzed Coupling of Alkynes: Evidence for Synergistic Cooperative Catalysis

Article abstract of DOI:10.1021/acscatal.6b00717

Copper oxide supported on mesoporous manganese oxide (meso Cu/MnO_x) was synthesized by an inverse micelle templated evaporation induced self-assembly procedure. Controlled aggregation of nanoparticles and a monomodal size distribution of mesopores with tunable structural properties were observed. The material possessed superior catalytic activity in the aerobic oxidative coupling of terminal alkynes. Excellent conversion (>99% in most cases) and selectivity were observed in both homocoupling and cross-coupling of alkynes using the optimized reaction conditions. Use of air as the sole oxidant, avoidance of any kind of additives, ease of product separation, great functional group tolerability, wide synthetic scope, and superior reusability (up to eighth cycle) are the notable features of our catalytic protocol. While the reaction mechanism was elucidated, a synergistic cooperative effect between the copper and manganese has been established, which is responsible for the superior catalytic activity. The labile lattice oxygen of the meso Cu/MnO_x played a vital role in deprotonation of the alkyne proton, as supported by TPD and TGA studies. Moreover, for the first time, we designed model complexes for the active sites of the catalyst by DFT calculations and provided a qualitative description of the coupling mechanism, which supports the experimental findings.

Full text of DOI:10.1021/acscatal.6b00717

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Article
Mesoporous Copper/Manganese Oxide-Catalyzed Coupling
of Alkynes: Evidence for Synergistic Cooperative Catalysis
Sourav Biswas, Kankana Mullick, Sheng-Yu Chen, David A. Kriz, Md Mahbubur Rahman
Shakil, Chung-Hao Kuo, Alfredo M Angeles-Boza, Angelo Raymond Rossi, and Steven L. Suib
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00717 • Publication Date (Web): 27 Jun 2016
Downloaded from http://pubs.acs.org on July 2, 2016
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Mesoporous Copper/Manganese Oxide-Catalyzed Coupling
of Alkynes: Evidence for Synergistic Cooperative Catalysis
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Sourav Biswas†, Kankana Mullick†, ShengꢀYu Chen†, David A. Kriz†, MD Shakil†, ChungꢀHao
Kuo†, Alfredo M. AngelesꢀBoza†, Angelo R. Rossi†* and Steven L. Suib†‡*
†Department of Chemistry, University of Connecticut, Uꢀ3060, 55 North Eagleville Rd., Storrs,
Connecticut 06269 (USA).
‡Institute of Materials Science, University of Connecticut, Uꢀ3060, 55 North Eagleville Rd.,
Storrs, Connecticut 06269 (USA).
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Abstract
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Copper oxide supported on mesoporous manganese oxide (meso Cu/MnOx) was synthesized by
an inverse micelle templated evaporation induced selfꢀassembly procedure. Controlled
aggregation of nanoparticles and a monomodal size distribution of mesopores with tunable
structural properties were observed. The material possessed superior catalytic activity in aerobic
oxidative coupling of terminal alkynes. Excellent conversion (>99% in most cases) and
selectivity were observed in both homo and crossꢀcoupling of alkynes using the optimized
reaction conditions. Use of air as the sole oxidant, avoidance of any kind of additives, ease of
product separation, great functional group tolerability, wide synthetic scope, and superior
reusability (up to 8th cycle) are the notable features of our catalytic protocol. While elucidating
the reaction mechanism, a synergistic cooperative effect between the copper and manganese has
been established, which is responsible for superior catalytic activity. The labile lattice oxygen of
the meso Cu/MnOx played a vital role in deprotonation of the alkyne proton as supported from
the TPD and TGA studies. Moreover, for the first time, we designed model complexes for the
active sites of the catalyst by DFT calculations and provided a qualitative description of the
coupling mechanism, which supports the experimental findings.
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Keywords: Alkyne, coupling, Copper, Manganese oxide, Density functional theory
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1. Introduction
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1,3ꢀdiyne moieties are a common feature in compounds with interesting medicinal, electronic,
and optical properties1,2. Pioneered by Glaser in 1869, by using stoichiometric amounts of copper
salt, oxidative homoꢀcoupling of terminal alkynes was considered the most straightforward route
for synthesizing diyne molecules3. Since then, many ligand/metal salt combinations have been
reported for alkyne coupling under homogeneous and mild reaction conditions4ꢀ10. From the
viewpoint of a green and environmental perspective, heterogeneous catalysts are more
advantageous over the homogeneous systems due to their easy separation, recyclability, and high
stability11ꢀ13. In this context, palladium based immobilized heterogeneous systems have been
successfully used14ꢀ16. However, the high price of palladium, tedious synthesis methodology, and
the use of copper as coꢀcatalyst in some cases hinder their practical application.
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Copper based heterogeneous catalysts, in particular supported copper catalysts, are specifically
17ꢀ24
attractive, as they provide similar or higher performance compared to palladium
.
Nevertheless, these catalysts often lacked reusability, require molecular oxygen or other
chemical oxidants, use high copper loadings, and need both base and ligand that are
indispensable for achieving high efficiency25ꢀ28. In addition, crossꢀcoupling of two different
terminal alkynes remains a challenging issue due to the competing homoꢀcoupling reactions.13,29
Alkyne preꢀfunctionalization, the presence of precious metals, and assistance of nitrogen based
ligands were identified as crucial factors to promote the transformation30,31. Moreover, current
understanding of the exact mechanism of the copper mediated alkyne coupling reaction still
remains unsatisfactory4. In spite of some efforts to describe the copper mediated coupling
reaction mechanism by computational techniques, proper design of catalytic active sites has not
been done32ꢀ34.Therefore, considering the aforeꢀmentioned disadvantages, design of copper based
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heterogeneous catalyst with proper explanation of reaction pathways for alkyne coupling
reactions is highly desirable. Ideally, the catalytic system will use air as the terminal oxidant,
avoid the use of additives, allow low catalyst loading, withstand proper reusability and show
high activity towards synthesis of symmetrical and asymmetrical diyne derivatives.
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Synergistic–cooperative catalysis involving two metal centers is an exciting phenomenon in
those organic transformations which cannot be accomplished by a single metal catalyst35,36. Joint
participation of dual metals having distinct properties is often required for achieving high
activity. The biggest challenge in this field is to exploit the relative compatibility of the two
metals in the same reaction medium. Mizuno et al. described a synergistic catalytic system for
alkyne homoꢀcoupling reactions using copper hydroxide on manganese oxideꢀoctahedral
molecular sieves (Cu(OH)x/OMSꢀ2)17. However, the catalyst was designed conceptually and no
clear understanding of the active sites has been provided. It is also difficult to estimate if crossꢀ
coupling products would be obtained with similar efficiency to homoꢀcoupling, since no example
of crossꢀcoupling has been reported.
Due to their intrinsic structural features, mesoporous nanostructured materials have been
recognized as popular heterogeneous catalytic systems37,38. They are ideal for providing a high
surface area scaffold to which active metals can be tethered. Further, tunable pore size offers
good flexibility for the transportation of organic moieties. Mesoporous manganese oxide
materials prepared by a soft templated method are powerful heterogeneous catalysts for diverse
oxidation reactions39ꢀ43. By tuning the nature of surface active manganese centers and the
mobility of labile lattice oxygen, the catalytic activity can be promoted by several orders of
magnitude. Herein, we designed mesoporous manganese oxide supported copper oxide for
oxidative homo and crossꢀcoupling of alkynes to 1,3ꢀdiyne derivatives. Use of air as the terminal
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oxidant, no pretreatment by basic additives, high turnover numbers, proper reusability, and high
functional group tolerability make our catalytic protocol superior to existing catalytic systems.
The mechanistic pathways and kinetic analysis were studied in detail. Additionally, we
performed density functional theory (DFT) computational techniques in order to establish a
correlation with the experimental results towards disclosing the active catalytic centers and
determined the role of manganese and labile lattice oxygen towards the activity.
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2. Experimental Section
2.1 Synthesis of mesoporous Cu/MnOx
The synthesis was performed following the synthetic procedure of University of Connecticut
(UCT) mesoporous materials44. Copper nitrate trihydrate (Cu(NO3)2.3H2O) was selected as the
dopant source in the synthesis. In a 120 mL beaker, 0.02 mol of manganese nitrate tetrahydrate
(Mn(NO3)2.4H2O), different amounts of Cu(NO3)2.3H2O, and (0.134 mol) 1ꢀbutanol were added.
To this solution (0.0034 mol) P123 (PEO20PPO70PEO20, molar mass 5750 g molꢀ1) and (0.032
mol) concentrated nitric acid (HNO3) were added and stirred at room temperature until the
solution became clear. The resulting clear blue solution was then kept in an oven at 120°C for 3
h under air. After reaction, the black material was washed with excess ethanol, centrifuged, and
dried in a vacuum oven overnight. The dried black powder was then subjected to a heat treatment
of 150°C for 12 h, cooled to room temperature, followed by heating at 250°C for 3 h under air
(named as meso Cu/MnOx).
2.2 Homo-coupling of terminal alkynes
In a typical homoꢀcoupling reaction, alkyne (0.5 mmol), meso Cu/MnOx (6 mol % with respect
to alkyne) and toluene (5 mL) were put in a 25 mL round bottom flask (two necked flask for time
dependent study, where the second neck was used as a sample port). The flask with the reaction
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mixture with a reflux condenser attached was immersed in a silicone oil bath preheated to 105°C.
The reaction mixture was refluxed under vigorous stirring (700 rpm) for the required time under
an air balloon. After reaction, the mixture was cooled, the catalyst was removed by filtration, and
Gas chromatography – mass spectra (GCꢀMS) was used to analyze the filtrate. The conversions
were determined based on concentration of alkynes. The selectivity was calculated based on
diyne as the only product. The products were isolated with silica gel column chromatography (nꢀ
hexane or a mixed solvent of nꢀhexane and dichloromethane was used as an eluent). Data of
diynes are summarized in the supporting information.
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2.3 Hetero-coupling of terminal alkynes
In a typical crossꢀcoupling reaction, alkynes with the required molar amounts, meso Cu/MnOx (6
mol % with respect to total alkyne amount), and toluene (5 mL) were put in a 25 mL round
bottom flask. The flask with the reaction mixture with a reflux condenser attached was immersed
in a silicone oil bath preheated to 105°C. The reaction mixture was refluxed under vigorous
stirring (700 rpm) for the required time under an air balloon. After reaction, the mixture was
cooled, the catalyst was removed by filtration, and GCꢀMS was used to analyze the filtrate. The
conversions were determined based on the concentration of limiting alkynes.
2.4 Calculations of reactive species on catalytic copper manganese oxide surfaces
The choice of model complexes, used in these calculations and necessary for providing an
adequate structural and energetic description of the active catalytic species, can be difficult,
especially for fleeting species resting on a solid surface where structures are not especially wellꢀ
defined. In these studies, the model compounds are simplified so that some ligands act as fixed
lattice sites and influence the local Mn and Cu environments. Other ligands are reactant species
or form shortꢀlived species bonded to metal centers, and consequently, are optimized because of
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their mobility and ability to follow a reaction pathway. The calculation details and geometries of
the model compounds and transient intermediates are given in the supporting information. The
model complexes, as herein described, should be able to provide a reasonably qualitative
description of a catalytic pathways, or reaction mechanism, and are capable of explaining
experimental results. The overarching goal of these calculations is to provide a qualitative
description of the mechanism for the coupling of terminal alkynes.
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3. Results
3.1 Physicochemical properties of meso Cu/MnOx
Typically, UCT mesoporous materials are formed by aggregation of monodispersed nanoparticle
in a random fashion, whereas the connection of the intraparticle voids is built the
mesostructure44. The presence of a low angle powder Xꢀray diffraction (PXRD) lines and a Type
IV adsorption isotherms for meso Cu/MnOx (Figure 1A ad 1C) indicate the preservation of
mesoporous structure at various copper loadings. The wide angle PXRD patterns (Figure 1B)
display diffraction lines corresponding to CuO at higher copper loading (10%), whereas no
diffraction lines for manganese oxide have been observed. This indicates a poorly crystalline
nature of manganese oxide at this calcination temperature (250°C). The surface areas were
calculated as 227 – 290 m2gꢀ1 (by Brunauer–Emmett–Teller (BET) methods) and 1% Cu
containing sample exhibited the highest surface area of 290 m2gꢀ1 (Table 1). The BarrettꢀJoynerꢀ
Halenda (BJH) adsorption pore size distribution (Figure 1D) indicated a narrow monomodal
pore size distribution for all the Cu containing materials. The pore diameters (Table 1) are within
the mesoporous range (3.0 – 3.4 nm).
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(a)
(b)
4000
3000
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1000
0
500
*
CuO
100/1
50/1
10/1
400
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*
*
100/1
50/1
10/1
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30
2 Theta
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2 Theta
(d)
(c)
0.8
100/1
50/1
10/1
180
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100
80
100/1
50/1
10/1
0.6
0.4
0.2
0.0
60
40
0.0
0.2
0.4
0.6
0.8
1.0
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Pore Diamater (nm)
Relative Pressure P/P
0
Figure 1. Structural characterization of meso Cu/MnOx with different Mn/Cu molar ratio. PXRD
patterns (A) Low angle (0.5° ꢀ8°), (B) Wide angle (5°ꢀ75°), (C) Nitrogen sorption isotherms and
(D) BJH adsorption pore size distributions.
Table 1. Structural parameters of meso Cu/MnOx
Mn/Cu
molar ratio
Surface Area BJH ads.
S BET (m2gꢀ1) pore size
(nm)
Pore
volume
(cc gꢀ1)
0.153
EDX
Cu mol %
Crystal
Structure
(PXRD)
n/a
200
290
227
270
2.8
2.9
3.4
3.4
n/a
1.0
3.8
6.0
Amorphous
100/1
0.244
0.202
0.301
Amorphous
Amorphous
CuO
50/1
10/1
n/a = not applicable.
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Figure 2. The FEꢀSEM images of meso Cu/MnOx with different Mn/Cu molar ratio (a) 100/1,
(b) 50/1 and (c) 10/1.
Figure 3. (a, b and c) The TEM images and TEMꢀEDX elemental maps (d) MnꢀL, (e) CuꢀL and
(f) OꢀL of meso Cu/MnOx with Mn/Cu molar ratio 10/1.
Figure 2 shows the morphological features of meso Cu/MnOx by field emission scanning
electron microscope (FEꢀSEM) having various Cu loadings. The materials are composed of
aggregated micronꢀsized rounded nanoparticles. Random packing of metal oxide particles was
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also observed by Transmission electron microscopy (TEM) (Figure 3) of 10% meso Cu/MnOx
material. Energy dispersive Xꢀray spectroscopy (EDX) was used to analyze the concentration of
each element in the material (Table 1, Figure S1). The high resolution TEMꢀEDX elemental
map analyses of Mn, Cu, and O (Figure 3) indicated a homogeneous distribution of CuO over
the MnOx nanoparticle throughout the material.
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In the case of metal oxide mediated oxidative reactions, oxidation states of active metals, and the
nature of surface oxygen species are two important parameters to be considered. A Cu 2p3/2
binding energy of 933.3 eV and the presence of shake up satellite peaks from Xꢀray
photoelectron spectra (XPS) studies confirmed the oxidation state of Cu as 2+17, which is in
agreement with the PXRD data (Figure 4a, Table S1). The Mn 2p XPS signal consists of two
peaks, which were assigned as Mn 2p3/2 and Mn 2p1/2 core levels (Figure 4b). The binding
energies of 641.5 eV and 653.2 eV can be attributed to the Mn3+ oxidation state40. On the other
hand, deconvolution of O 1s spectra (Figure 4c) specified the existence of three different
binding oxygen species, namely, structural or lattice oxygen (Os), surface adsorbed oxygen
(Oads), and adsorbed water or hydroxyl groups (Omw). In comparison to the reference catalyst
(meso MnOx), a shift of the O 1s peak at lower binding energy (530.2 eV in meso MnOx and
529.2 eV in meso Cu/MnOx) was observed (Figure 4d). This signifies an increase of mobility of
lattice oxygen due to copper incorporation. The deconvolution O 1s spectra also suggest a
predominance of lattice oxygen (Os) near the surface of the catalyst.
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5000
4500
4000
5000
(a)
(b)
*
Cu 2P
1/2
Mn 2p
3/2
4500
4000
3500
3000
2500
2000
1500
Mn 2p
1/2
*
Meso Cu/MnOx
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Cu 2P
3/2
Meso MnOx
640 635
965 960 955 950 945 940 935 930
Binding Energy (eV)
660
655
650
645
Binding Energy (eV)
O
O
O
(structural oxygen)
s
O 1S
(surface adsorbed oxygen)
ads
mw
(c)
(d)
3000
3000
(adsorbed water)
2500
2000
1500
2000
1000
0
Meso Cu/MnOx
Meso MnOx
528 526
536
534
532
530
533 532 531 530 529 528 527 526
Binding Energy (eV)
Binding Energy (eV)
Figure 4. XPS of meso Cu/MnOx (10% Cu): (a) Cu 2p (* indicates the shake up satellite peaks),
(b) Mn 2p, (c) deconvoluted O 1s and (d) comparison of the O 1s transitions of meso Cu/MnOx
and meso MnOx.
3.2 Optimization of reaction condition
The oxidative homoꢀcoupling of phenylacetylene was selected as the model reaction for
developing the optimal reaction conditions. The bare mesoporous manganese oxide was totally
inactive suggesting the absence of active sites for the coupling reaction. An increase of
conversion from 4 to 70% was observed by increasing the copper concentration from 1% to 10%
in the MnOx (Figure S2). The selectivity in all the cases were 100% as 1,3ꢀdiyne was the only
product. The 10% Cu incorporated sample was then selected for further studies. Changing the
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oxidant source from air (45% conversion) to oxygen (55% conversion) resulted in an increase of
activity [Entry 1 and 2, Table 2], whereas, reaction under nitrogen atmosphere displayed much
lower conversion (24%) [Entry 3, Table 2]. A significant enhancement of conversion (70%)
was achieved by addition of base [Entry 4, Table 2] and the combination of oxygen and base
displayed the best (85% conversion) activity [Entry 5, Table 2]. We then conducted the homoꢀ
coupling experiment with varied the amount of base (K2CO3) and found 1 equivalent of base as
the optimal amount (Table S2). On the other hand, the reaction did not proceed at all in the
presence of an acid [Entry 6, Table 2]. However, performing catalytic reactions under aerobic
conditions without any additives is much more desirable in terms of green and sustainable
chemistry. While performing further optimizations, an increment of catalyst loading from 3 to 6
mol% with respect to alkyne yielded the desired diyne quantitatively within 40 min (TOF 24.7)
[Entry 8-9, Table 2] without any additives under an air balloon. The reaction also proceeded
under nitrogen, though having a much lower TOF value of 3.9 [Entry 10, Table 2]. Therefore
the best optimized reaction conditions [Entry 9, Table 2] were used for the rest of the work.
The catalyst precursor Cu(NO3)2. 2H2O and other Cu salts (CuCl, CuCl2, CuSO4.5H2O) [Entry
1-4, Table S3] and commercial Cu oxide (CuO, Cu2O) [Entry 5-6, Table S3] did not produce
any coupling product. No reaction proceeded with a physical mixture of CuO and meso MnOx
[Entry 7, Table S3]. To determine the role of MnOx in the reaction, we selected mesoporous
Cu/TiO2 (prepared by the same UCT method, see supporting information for details) as a
reference catalyst for comparison. In contrast to meso Cu/MnOx (>99 % conversion), meso
Cu/TiO2 displayed much lower conversion (7 %) [Entry 8-9, Table S3], under identical
conditions. No reaction happened in the absence of catalyst [Entry 10, Table S3]. Therefore, the
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highly dispersed copper oxide species on mesoporous manganese oxide is indispensable for the
present aerobic oxidative coupling of terminal alkynes.
7
8
9
Table 2. Optimization of reaction conditionsa
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Entry
Cu (mol%)
Oxidant
Additives
Time
(min)
180
180
180
180
180
180
360
45
Conversionb
TOFc
(hꢀ1)
3
(%)
45
55
24
70
85
0
1
2
3
3
3
3
3
3
3
6
6
6
Air
O2
None
None
None
6
3
N2
2.7
7.8
9.4
0
d
4
Air
O2
K2CO3
d
5
K2CO3
6
Air
Air
Airf
Air
N2
PTSAe
None
None
None
None
7
80
94
>99
70
8.9
21
8
9
40
24.7
10
180
3.9
a Reaction conditions: phenylacetylene (0.5 mmol), catalyst (different mol% of meso Cu/MnOx),
toluene (5 mL), balloon of air/O2/N2 , 105°C. Conversions were determined by GCꢀMS and
based on phenylacetylene. TOF = TON/ time (h), TON = no of moles of phenylacetylene
converted per mol of Cu in the catalyst. 1 equivalent of K2CO3 was used. PTSA (pꢀ
Toluenesulfonic acid, 1 equivalent). f Open to atmosphere.
b
c
d
e
3.3 Oxidative homo-coupling of terminal alkynes
The methodology worked well for diverse alkyne derivatives such as aromatic [Entry 1-8 and
13, Table 3], aliphatic [Entry 9-12, Table 3], heterocyclic [Entry 8, Table 3], olefinic [Entry
7, Table 3], and silylic [Entry 10, Table 3]. In all cases, the reaction exhibited >99% selectivity,
as the corresponding 1,3ꢀdiyne was the only observed product. The isolated yields of the1,3ꢀ
diyne derivatives are given in Table 2 along with their H NMR and 13C NMR data (see
1
supporting information). Alkynes with electron releasing [Entry 2-4, Table 3] or electron
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withdrawing [Entry 5-6, Table 3] groups reacted smoothly to give the corresponding 1,3ꢀdiynes
with excellent yields. The position of the methyl group in the aromatic ring did not affect the
reaction rate [Entry 3-4 Table 3]. No dehalogenated product was observed in the case of
halogen substituted alkynes [Entry 5-7, Table 3]. The dimerization of a heterocyclic substituted
alkyne [Entry 8, Table 3] was conducted to afford high yield and selectivity but a much longer
reaction time (12 h) was required. Excellent activity (>99% conversion, >99% selectivity) in
oxidative coupling of an enyne (1ꢀethynylcyclohexene) [Entry 9, Table 3] revealed the superior
chemoꢀselectivity nature of the meso Cu/MnOx, since no reaction at the double bond was
identified by GCꢀMS. Moreover, aliphatic alkynes were successfully converted to the
corresponding diynes with similar turnover numbers (16.7) to the aromatic alkynes [Entry 10-
11, Table 3]. Great functional group tolerability was also observed with silylated alkyne [Entry
12, Table 3] and substrates containing reactive ester groups [Entry 13, Table 3].
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Table 3. Aerobic oxidative homoꢀcoupling of terminal alkynesa
3
4
5
6
7
8
9
meso Cu/MnOx
+ H2O
2
R
H
R
R
Air, toluene, 105°C
Conversionb, c
(%)
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Entry
1
Substrate
Time (h)
1
>99(97)
2
3
4
3
2
2
>99(85)
>99(83)
>99(91)
MeO
5
6
7
8
1
8
>99(98)
90(82) d
95(87) e
>99(85)
F
Cl
Br
6
12
N
9
2
2
>99(91)
>99(82)
10
n-C6H13
Si
11
12
6
2
>99(87)
>99(93)
13
O
O
4
>99(96)
a
Reaction conditions: alkynes (0.5 mmol), meso Cu/MnOx (6 mol% Cu with respect to amount
b
of alkyne), toluene (5 mL), 105°C, air balloon. Conversion was determined by GCꢀMS based
on the concentration of alkynes. Selectivity in all cases were 100% to diynes. Numbers in
parenthesis refer to yields of isolated products. c TON (no. of moles of alkynes converted / moles
of copper in catalyst) = 16.7. d TON = 15.0, e TON = 15.8.
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3.4 Cross coupling of alkynes
5
6
Table 4. Aerobic oxidative cross coupling of terminal alkynesa
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8
9
meso Cu/MnOx
R1
R2
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13
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R
H +
H
R1
2
Air, toluene,
105°C, 3 h
Entry
R1
R2
Rel.
ratio
Convb
(%)
Coupled products
selectivity (%)
R1ꢀ R1 R1ꢀ R2 R2ꢀ R2
1
2
3
4
5
6
7
1/1
1/2
1/3
1/3
1/3
1/1
1/1
>99
>99
80
17
38
12
11
20
20
20
52
23
12
36
32
18
35
25
50
F
54
92
57
n-C6H13
>99
>99
>99
62
45 d
55 e
F
MeO
a
Reaction conditions: alkynes total amount (0.5 mmol), meso Cu/MnOx (6 mol% Cu with
respect to total amount of alkynes), toluene (5 mL), 105°C, 3 h, air balloon. Conversion was
determined by GCꢀMS based on the concentration of limiting reagent. Selectivity was
b
c
calculated by GCꢀMS. d Isolated yield was 82% e Isolated yield was 87%.
As illustrated in Table 4, the catalyst was able to produce the desired asymmetrical diynes by
selecting two different terminal alkynes with good selectivity. We performed a conditional
screening with a focus on the molar ratio of partner alkynes to get the maximum selectivity
towards the asymmetric diynes [Table S4]. The catalyst exhibited crossꢀcoupling between
terminal alkynes of different types including aromatic and aromatic [Entry 1, 2, 6 and 7, Table
4], aromatic and aliphatic [Entry 3 - 5, Table 4], and aromatic and olefinic [Entry 4, Table 4].
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The best selectivity to asymmetrical diyne was obtained by selecting an excess of the aliphatic
alkynes over aromatic alkynes. On the other hand, selecting an equal molar amount of two
aromatic alkynes produced the maximum selectivity to the asymmetric diyne. We also compared
the crossꢀcoupling activity of our system with the best known copper based heterogeneous
catalyst [Cu(OH)x/OMSꢀ2] for alkyne coupling. As observed in table S5, meso Cu/MnOx
displayed better selectivity (62%) to the asymmetric diyne compared to Cu(OH)x/OMSꢀ2 (40%)
in crossꢀcoupling of phenylacetylene and 1ꢀoctyne.
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3.5 Reusability and heterogeneity
Stable reusability and negligible leaching of active species are two important factors for an
efficient heterogeneous catalytic system. We selected oxidative homoꢀcoupling of
phenylacetylene as the model reaction for a reusability study. After the reaction, the catalyst was
retrieved by filtration and was washed with excess solvent and ethanol (>90% recovery). Prior to
reuse the catalyst was reactivated at 250°C for 30 min to remove any adsorbed organic species.
Figure S3a shows that the catalyst can retain activity and selectivity even after the 8th reuse.
Moreover, no change in the PXRD pattern after the 8th cycle was observed (Figure S3b), which
confirms that the catalyst can retain the crystal structure even after multiple reuse cycles. To
verify any possible leaching of active Cu species in the solution, the catalyst was filtered and the
filtrate was analyzed by inductively coupled plasma optical emission spectrometry (ICPꢀOES). A
trace amount of Cu (1.9 ppb) and Mn (7.0 ppb) were detected in the filtrate. All of these results
signified the truly heterogeneous nature of our catalyst, which is active, stable, as well as
recyclable.
3.6 Kinetic study
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Kinetics of the reaction were then determined by conducting a time dependent study of the
model reaction (oxidative coupling of phenylacetylene). Periodic sampling was undertaken at
specific time intervals and conversion was determined by GCꢀMS. Kinetic experiments depicted
a first order rate equation with respect to alkyne (Figure S4) having a rate constant of 0.093 minꢀ
1. Kinetically relevant elementary steps in the reaction pathways were measured by changing the
acetylenic H to D in phenylacetylene. The obtained kinetic isotope effect (KIE) value was 0.9
(Figure S5), which signified that abstraction of the acetylenic proton of alkyne was very fast.
3.6 Role of manganese oxide
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A significant lower conversion (7% in air and 5% under nitrogen) was achieved when meso
Cu/TiO2 was selected instead of meso Cu/MnOx (94% in air and 28% under nitrogen). We used
XPS to probe the oxidation state of Cu and Mn in the unreacted catalysts and after reaction in air
and nitrogen atmospheres (Figure S6 and Table S6). The oxidation state of Cu was estimated as
2+ in both of the untreated materials (pink and blue lines), as shown in Figure S6. The oxidation
state of Cu persisted at 2+ (red and black lines) for meso Cu/MnOx irrespective of the reaction
environments (air or nitrogen). In contrast, reduction of Cu was observed (absence of satellite
peaks), when the Cu/TiO2 material was subjected to react under nitrogen (green line). These
results confirmed the reꢀoxidation of reduced Cu species occurs in the presence of MnOx even
under nitrogen atmosphere. To further establish the reꢀoxidation of reduced Cu species, we tested
the oxidation state of Mn in the meso Cu/MnOx before and after reaction. As revealed from XPS
(Figure S7), no change of oxidation state of Mn was detected when the coupling reaction was
performed under air, whereas a lowering of Mn oxidation state was observed (Table S6), when
the reaction was performed under nitrogen.
3.7 Comparison between experimental and calculated results
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3.7.1. Cu sites on the meso Cu/MnOx surface
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6
7
8
Because acetylenes have two π orbitals, π−[L4Cu(II)(H−C≡C−H)]2+ complexes are capable of
exhibiting enhanced stability. Tatsumi et al.45 showed that the DewarꢀChattꢀDuncanson model46
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can be applied to both double bonds, π⊥,π∗⊥ and πǀǀ,π∗ , which play an important role in bonding
ǀǀ
to mononuclear transition metal complexes. The π−[L4Cu(II)(H−C≡C−H)]2+ complex, as shown
in Figure 5a, appears to be more stable than the corresponding Cu(I) complex, which is quite
reasonable because the dz2 orbital is now only partially filled and can interact with the first π
system, i.e. πǀǀ,π∗ . The πǀǀ − dz2 is a threeꢀelectron stabilizing interaction, together with further
ǀǀ
stabilization from a favorable interaction with π∗ . The π∗ orbital from the second π system
⊥
ǀǀ
stabilizes a matched filled Cu dπ orbital. This corresponds to the transformation of
π−[L4Cu(II)(H−C≡C−H)] when H+ is abstracted to an π−[L4Cu(II)( –C≡C−H)] complex, shown in
Figure 5b which can then couple with a neighboring acetylide anion (H−C≡C:–).
Figure 5. (a) π−[L4Cu(II)(HꢀC ≡ CꢀH)]2+, and (b) π−[L4Cu(II)(C ≡ CꢀH)]+complexes. The
stability of the complex (a) relative to the separate components is 6.2 kcal molꢀ1. The acetylide
anion (−C ≡ CꢀH) in (b) was unstable with respect to a DFT calculation using a 6ꢀ31g* basis set,
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and prevented the determination of relative stability, but its value is larger than (3a). L stands for
H2O.
6
7
3.7.2 Mn sites on the meso Cu/MnOx surface
8
9
Vacant Mn sites on the Cu/MnOx surface play a complementary role with respect to the Cu
sites, especially when they are adjacent to one another. The Mn sites absorb O2, form stable
LnMnO2 intermediates, and are capable of abstracting H+ from neighboring Cu sites bonded to
H−C≡C−H molecules. In the case of Mn surface sites with LnMnO2 intermediate, it is important
to keep track of the spin on the Mn atom. The [LnMn(II)(O2)] (d5 : S = 6) complex is not stable,
whereas [LnMn(III)(O2)]+ (d4 : S = 5) forms a stable complex as shown in Figure 6a. The O2
abstraction of H+ by [LnMn(III)(O2)]+, does not alter the electronic configuration of the Mn(III)
in the complex. The complex [LnMn(III)(O2H)]2+ (d4: S = 5) remains stable as shown in Figure
6b below, eventually releasing the OOH– ion. The remaining [LnMn(III)]3+ site is eventually
reduced to [LnMn(II)]2+.
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Figure 6. (a) [LnMn(III)(O2)]+, and (b) [LnMn(III)(O2H)]2+, complexes. The stability of the
complex (a) relative to the separate components is 59.4 kcal molꢀ1. The hydroperoxyl anion
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(O2H–) was unstable with respect to a DFT calculation using a 6ꢀ31g* basis set, and prevented
the determination of relative stability, but its value is larger than (4a). L stands for H2O.
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7
3.7.3 Molecular analogues of reactive surface species
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9
Several model systems can be used to illustrate the oxidative coupling of alkynes on the
Cu/MnOx surface, and three are shown in Figure 7. For the models, it is assumed that nearby Mn
sites are capable of interacting with catalytically active Cu sites, and have the capability of
producing a coupled alkyne product. The model in Figure 7a is considered favorable because the
Mn atom, squeezed between two active Cu atoms, and bonded to two acetylide anions, is capable
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–
of interacting with the approaching anionic fragments (H−C≡C:– ↔ :C≡C−H), stabilizing the
incipient repulsive interaction, and subsequently absorbing the two discarded electrons, resulting
from the oxidation of the acetylide anions, into an available Mn(d) orbital. An advantage of
considering the model in Figure 7b is that the Cu sites are adjacent to each other (Mn sites are
also presumed to be adjacent to each Cu site), allowing direct coupling of the acetylide anions
and electron exchange with Mn atoms. Model Figure 7b is also analogous to an intermediate in
the solutionꢀphase GlaserꢀHay reaction. Finally, the model in Figure 7c is similar to that in
Figure 7b, but now includes O2 as the bridging ligand between Cu atoms. O2 is plentiful under
the experimental conditions, when air and O2 are available above the Cu/MnOx surface. A
dominant structure, supported by Cu atoms, may include the sideꢀon µꢀη2:η2ꢀperoxo motif47ꢀ49
shown in Figure 7c which provides some degree of confidence that this is a reasonably valid
model.
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Figure 7. Model systems [(a) Mn atom, squeezed between two active Cu atoms, (b) Cu sites are
adjacent to each other and (c) O2 as the bridging ligand between Cu atoms] on the Cu/MnOx
surface capable of oxidative coupling of alkynes.
4. Discussion
This study features the design of mesoporous copper incorporated manganese oxide material as
an efficient heterogeneous catalyst for aerobic oxidative coupling of alkynes to 1,3ꢀdiynes. The
material was synthesized by an inverse micelle templated solꢀgel synthesis procedure using
copper nitrate as the copper source. In this process, a pluoronic surfactant (P123) along with the
metal source (Mn and Cu) randomly packed in an acidic alcoholic solution (HNO3 + butanol) to
build the mesostructure. The nitrate ions pulled the positively charged manganese and copper
oxoꢀclusters into the core of the inverse micelles. The formation of NOx (by thermal
decomposition of nitrates) controls the whole solꢀgel process. Control of the dopant amount is
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crucial, since high amounts of dopants can destabilize the reaction gel and cause the surfactant to
precipitate. We have successfully employed 1 ꢀ10 mol % incorporation of copper without
disturbing the solution gel. The reaction system was then treated at 120°C for 3 h, followed by
extracting the surfactant with ethanol to get the mesoporous material (meso Cu/MnOx).
Chemisorbed species (nitrates and carboxylates) were removed from the material by a heat
treatment at 150°C for 12 h and 250°C for 3 h consecutively.
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As illustrated in Table 3, our system indicates excellent substrate scope and functional group
tolerability, giving the desired homoꢀcoupling product in excellent yields. For aromatic alkynes,
excellent activity was observed for both electron donation and electron withdrawing groups.
Halogen substituted aromatic alkynes gave the desired product with excellent selectivity (>99%)
without any dehalogenation, though dehalogenation and oxidative addition of halogen
substituted aromatic compounds has been shown to occur in copper mediated systems50. The
longer reaction time (15 h) to achieve efficiency for a heteroatom containing substrate can be
attributed to the poisoning of the active copper sites of the catalyst by coordination with Nꢀ
heteroatoms51. Homoꢀcoupling reactions with long chain aliphatic alkynes are known to be
relatively difficult and higher copper loading or longer reaction times are essential, as mentioned
in previous studies17,22,52. However, meso Cu/MnOx exhibited excellent performance in the
reaction with the long chain, inactive 1ꢀoctyne with no drop in turnover number versus aromatic
alkynes. The easy diffusion and transportation of 1ꢀoctyne in the mesoporous network may be
the reason behind the high activity. Excellent selectivity was also observed with a reaction of an
enyne, though manganese oxides are known to oxidize the double bond in catalytic conditions53.
The excellent selectivity was also demonstrated by the homoꢀcoupling reaction of a silylated
derivative without oxidation of the SiꢀH bond54.
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In crossꢀcoupling of alkynes, discrimination of partner alkynes to metal acetylide formation
has been considered the most important feature30. We hypothesized that proper choice of relative
alkyne amounts might provide the discrimination effect toward different alkynes. There were no
significant differences in the rate of the homoꢀcoupling of aromatic alkynes. Therefore, by
selecting an equal amount of aromatic alkynes, we observed the maximum selectivity towards
the asymmetric diynes. As observed in previous studies, aliphatic alkynes reacted faster to form
the metal acetylide species to provide the discrimination effect40. Similarly in our study,
asymmetric diynes were synthesized with good to excellent selectivity when aliphatic alkynes
were used in excess over aromatic alkynes.
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There are many existing hypotheses, but the alkyne coupling mechanism proposed by
Bohlmann et al. is considered as the most reasonable and acceptable55. Based on their
understanding, initially, a π coꢀordination species between the triple bond and copper (I) is
established, which further forms a dinuclear copper (II) acetylide species, and essentially
collapses to give the coupled product. However, considered as an oxidative reaction, little has
been discussed about the role of dioxygen in the mechanistic details. In general, copper
undergoes a one electron reduction and forms a Cuꢀacetylide species, which dimerizes to give
56
the corresponding diyne . As elucidated from DFT calculations, a schematic diagram, showing
how the energy decreases as the two separated acetylide anions for model Figure 7b readily
couple to form a coupled acetylene product is given in Figure S8. The relative energy (ꢀ84.6 kcal
molꢀ1) of the final optimized coupled product is reasonable for the formation of the CꢀC bond in
H−C≡CꢀC≡C−H, which is the bond joining the two acetylide fragments.
However, no change in copper oxidation state of meso Cu/MnOx was observed after the
reaction by XPS. Therefore, reꢀoxidation of reduced copper species has been included in the
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present oxidative coupling. The reꢀoxidation is well supported by disappearance of the satellite
peaks of Cu 2p in meso Cu/TiO2, confirmed the reduction of Cu(II) to Cu(I) in the present homoꢀ
coupling reaction under nitrogen atmosphere (Figure S6). In contrast, the reꢀoxidation was
efficiently promoted by MnOx even under nitrogen atmosphere. A reduction of Mn oxidation
state was also observed by XPS (Figure S7) which complemented the reꢀoxidation of reduced
Cu(I) species by Mn(III) species. On the other hand, under an aerobic atmosphere, the reduced
Mn species can effectively reoxidize57.
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The homoꢀcoupling reaction did not proceed with an internal alkyne (diphenylacetylene),
suggesting that the copper species is likely binding to terminal alkyne to form the copperꢀ
acetylide species. Since the addition of a base is unnecessary in our system, the catalyst itself
must have the ability to abstract the alkyne proton. An incremental increase of catalytic
performance was observed when the catalyst was subjected to a heat treatment from 250°C to
450°C, whereas, the activity again decreased at 550°C (Figure S9). As revealed from the XRD
studies, when the material was calcined at 250°C to 450°C, the manganese support was poorly
crystalline but transformed to highly crystalline Mn2O3 phase at 550°C (Figure S10).
Temperature programmed desorption (TPD) experiments were performed in order to determine
the nature of oxygen species of the material, as crystallinity has a remarkable effect in
accessibility of lattice oxygen of manganese oxides39. The material calcined at 250°C to 450°C
showed an oxygen desorption peak around 500°C which can be ascribed as desorption of lattice
oxygen (Figure S11). A transformation to lower temperatures of the lattice oxygen peak was
also observed with rising calcination temperature by O2ꢀTPD, which was complemented by
thermogravimetric (TGA) analyses (Figure S12). Thus, the effect of calcination temperatures on
catalytic performance can be correlated with the accessibility of labile lattice oxygen. Therefore,
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the role of labile lattice oxygen of copper manganese oxide is probably relevant to the
abstraction of alkyne hydrogen to from the alkynyl species. This results are also supported from
the models derived by DFT calculation, where the Mn sites of the catalyst can absorb O2,
forming stable intermediates, which are capable of abstracting H+ from neighboring Cu sites
bonded to H−C≡C−H molecules (Figure 6).
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Based on experimental findings, we propose a mechanism for oxidative coupling of alkynes by
meso Cu/MnOx (Scheme 1). First, the lattice oxygens of meso Cu/MnOx abstract an alkyne
proton to form the Cu(II) acetylide species (formed by weak π coordination between alkyne
triple bond and copper center) , which dimerizes to give the corresponding diyne and generates
Cu(I) species. The surface active Mn centers simultaneously involves a one electron reduction
and reꢀoxidizes the Cu(I) species to Cu(II). Under catalytic turnover conditions, the reduction of
active Mn species can lead to facile release of lattice oxygen58. The labile lattice oxygen reꢀ
oxidizes the Mn center back with production of H2O2, which could easily be decomposed over
manganese oxide and form water58ꢀ60. The supply of oxygen is crucial for the catalytic activity,
as loss of lattice oxygen should be replenished by oxygen from the air. This also was supported
by the observation that using a dinitrogen atmosphere instead of air diminished the reaction rate.
A kinetic isotope effect was hardly observed in the homoꢀcoupling of phenylacetylene (kH/kD =
0.9) indicating that the abstraction of the alkyne proton is not the rate determining step.
Therefore, the reꢀoxidation of reduced copper species is probably the rate determining step in the
present coupling reaction.
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Mn2+
Cu+
O2
R
H
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Mn3+
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H2O + 1/2 O2
H2O2
H+
Cu2+
2-
O2
R
2
Mn4+
H+
Scheme 1. Proposed mechanism of aerobic oxidative coupling of alkynes by meso Cu/MnOx.
The reaction mechanism was further validated by DFT calculations. A basic assumption for
any credible mechanism presumes that Cu atoms are randomly distributed on the Cu/MnOx
surface and are intertwined with neighboring Mn atoms, forming bonds with O atoms.
Air environment
1. Initially, the neutral H−C≡C−H molecule forms a weakly bound π − [L4Cu(II)(C2H2)]2+
complex (L = H2O).
2. Concomitantly, an adjacent LnMn(III)3+ species reacts with O2 to form [LnMn(III)(O2)]+.
The [LnMn(III)(O2)]+ intermediate is able to directly abstract an H+ and yield a
[π−L4Cu(II)(C≡CH)]+ and a [LnMn(III)(O2H)]2+ complex. LnMn sites exhibit a twofold role in
enhancing catalysis, by both changing its oxidation state and by yielding an acetylide anion.
3. [LnMn(III)(O2H)]2+ loses O2H– to form an [LnMn(III)]3+ complex.
4. In the final step, two acetylide anions from [π−L4Cu(II)(C≡CH)]+ species react to form a
diyne molecule (e.g., H −C≡C−C≡C−H) and residual L4Cu(II) surface species.
The overall equations for the postulated mechanism are given below:
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2 [π−LnCu(II)C2H2]2+ + 2 [LnMn(III)(O2)]+ → 2 [π−LnCu(II)(C≡CH)]+ + 2[LnMn(III)(O2H)] 2+
2 [LnMn(III)(O2H)]2+ → 2 [LnMn(III)]3+ + 2 OOH−
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2 OOH− → H2O + 1/2O2 + 2e−
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2 [π−LnCu(II)(C≡CH)]+ → 2 [LnCu(I)]+ + H−C≡C−C≡C−H
Nitrogen environment
Surface sites containing reactive species such as [LnMn(III)=O]+, are assumed to always be
available on the Cu/MnOx surface under nitrogen environments.
1. [LnMn(III)=O]+ reactive species abstracts an H+ from [π−LnCu(II)(C2H2)]2+ to form
[π−LnCu(II)(C≡CH)]+ and [LnMn(III)(OH)]2+.
2. [LnMn(III)(OH)]2+ decomposes to [LnMn(III)]3+ + OH–.
3. Two acetylide anions from [π−L4Cu(II)(C≡CH)]+ species react to form the coupled product
(H −C≡C−C≡C−H).
The yield of coupled alkyne product is reduced, when the Cu/MnOx surface is exposed to
nitrogen environments, and may be related to the formation of a weak [LnCu(II)(N2)]2+ complex
which inhibits the formation of the neutral [π−LnCu(II)(C2H2)]2+ complex (Figure S13).
Complex formation with nitrogen competes with H−C≡C−H for Cu(II) sites and decreases
catalytic activity. Similar to the LnCu(II) sites, the LnMn(III) sites can also form weakly bound
complexes with N2 which may hinder catalytic reaction of alkynes under nitrogen atmosphere
(Figure S14).
5. Conclusions
In summary, we report the fabrication of thermally stable and reusable mesoporous copper
supported manganese oxide materials (meso Cu/MnOx) for aerobic oxidative coupling of
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alkynes. The material was composed by aggregation of rounded nanoparticles with high surface
area (as high as 270 m2gꢀ1) and uniform mesoporous size (3.0 – 3.4 nm) distribution. The
alignment of copper over the manganese oxide was shown by PXRD refinement showing a
copper (II) oxide (CuO) phase at higher copper doping. Elemental mapping analysis by TEMꢀ
EDX confirmed a uniform distribution of copper oxide over the manganese oxide. Broad
substrate scope and excellent functional group tolerability were demonstrated for the oxidative
homo and crossꢀcoupling of terminal alkynes. While unravelling the mechanistic details, a
synergistic effect between copper and manganese has been established along with the
contribution of labile lattice oxygen. The high catalytic activity can be attributed to an electron
transfer pathway between dioxygen and alkyne through manganese and copper oxide. In
addition, we performed DFT calculations to provide a reasonably qualitative description of the
catalytic pathway by building model compounds in four/five or six ꢀ fold coordination to
manganese and copper atoms, together with their ligands and constrained by bonds in the lattice.
In case of copper active sites, a π coordination between the alkyne triple bond and copper
enhancing the acidity of alkyne proton has been shown, resembling the classical Bohlmann
mechanism. The proton was abstracted by the complex formed between manganese active sites
and dioxygen, which can be correlated with the experiments, where an exchange mechanism
between labile lattice oxygen and aerial oxygen was shown to be the factor behind the
deprotonation. After deprotonation, the copper (I) acetylide species was dimerized to produce the
diyne products as observed by the relative energies of the coupling of two acetylide moieties.
Therefore, the design of copper and manganese active sites along with the validity of the
coupling reaction mechanism was strengthened from both experimental and theoretical
perspectives. Finally, using air as the terminal oxidant, avoidance of additives, no pretreatment
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