Homogeneous and noncovalent immobilization of NHC-Cu catalyzed azide-alkyne cycloaddition reaction

Article abstract of DOI:10.1016/j.mcat.2021.111452

A series of new pyrene tagged nitrogen heterocyclic carbene copper (NHC-Cu) molecular complexes were synthesized and characterized by ¹H- and ¹³C-NMR, high resolution ESI-mass spectrometry (HR-MS). The corresponding NHC-Cu complex was immobilized onto the surface of multi-walled carbon nanotubes (MWNTs) by using π-π stacking interaction, the noncovalent immobilization catalytic material MWNTs@NHC-Cu were characterized by FT-IR, XRD, XPS, thermogravimetry (TGA) and TEM. The new NHC-Cu complexes were tested both in the homogeneous phase and once immobilized onto multi-walled carbon nanotubes support for the azide-alkyne cycloaddition reaction. Both were active in the two-component click cycloaddition reaction of terminal alkynes and organic azides and three-component cycloaddition reaction that employs various terminal alkynes, NaN₃ and organic halides with the advantage of excellent yields, low catalyst dosage, short reaction time. The MWNTs@NHC-Cu catalyst could be separated from the reaction system at the end of the reaction and reused in another catalytic cycle, but with loss of product yield.

Full text of DOI:10.1016/j.mcat.2021.111452

Molecular Catalysis 504 (2021) 111452
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Homogeneous and noncovalent immobilization of NHC-Cu catalyzed
azide-alkyne cycloaddition reaction
,
,
,
,
,b,
Xiangjie Zhanga b, Bingyang Wanga b, Yanmei Lua b, Chungu Xiaa b, Jianhua Liua
*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China
b University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of new pyrene tagged nitrogen heterocyclic carbene copper (NHC-Cu) molecular complexes were syn-
thesized and characterized by 1H- and 13C-NMR, high resolution ESI-mass spectrometry (HR-MS). The corre-
sponding NHC-Cu complex was immobilized onto the surface of multi-walled carbon nanotubes (MWNTs) by
NHC-Cu
Carbon nanotube
Azide-alkyne cycloaddition reaction
Noncovalent immobilization
Recyclability
using π-π stacking interaction, the noncovalent immobilization catalytic material MWNTs@NHC-Cu were char-
acterized by FT-IR, XRD, XPS, thermogravimetry (TGA) and TEM. The new NHC-Cu complexes were tested both
in the homogeneous phase and once immobilized onto multi-walled carbon nanotubes support for the azide-
alkyne cycloaddition reaction. Both were active in the two-component click cycloaddition reaction of terminal
alkynes and organic azides and three-component cycloaddition reaction that employs various terminal alkynes,
NaN3 and organic halides with the advantage of excellent yields, low catalyst dosage, short reaction time. The
MWNTs@NHC-Cu catalyst could be separated from the reaction system at the end of the reaction and reused in
another catalytic cycle, but with loss of product yield.
1. Introduction
restricting its industrial application [22]. Undoubtedly, the simple sep-
aration of catalyst and product will bring more economic benefits in
The 1,3-dipolar cycloaddition between an azide and alkyne is a
prime example of a click chemistry reaction as a concept postulated by
Sharpless [1], which has been applied to the various field including
medicinal chemistry, materials, and biology [2–6]. Copper (I) is widely
used in azide-alkyne cycloaddition reaction as a catalyst because it has
several advantages, such as excellent regioselectivity, milder reaction
condition, usage of inexpensive and green reaction solvent including
aqueous, alcoholic, neat, etc [7,8]. However, the catalytic activity of
single copper salt is relatively low. So, various ligands are used in order
to improve the activities of the systems catalyzed by Cu (I) [8–14].
Among the various ligands, N-heterocyclic carbenes (NHCs) shows
excellent catalytic activity under mild conditions [15–17]. Because of its
industrial production. Therefore, the immobilization of homogeneous
catalysts has been a hot issue of research [23,24].
As an excellent supporting material, carbon nanotubes (CNTs) have
good optical, mechanical, chemical, electrical, and functional properties
[25–29]. One of the most commonly used functionalization methods for
carbon nanotubes is to use covalent bonds to achieve the heterogeneity
of homogeneous catalysts by using carboxylic acid groups or hydroxyl
groups, etc [30–33]. Despite this method achieving effective grafting, it
has obvious drawbacks. First, the modification of the covalent bond
leads to the destruction of the structure and properties of the CNTs. In
addition, the method will also lead to changes in the structure of the
catalyst resulting in potentially impaired activity. So an alternative way
was chosen—noncovalent interactions, although this interaction is
weak, the catalyst can also be recycled after being immobilized under
appropriate methods and fitting reaction conditions. Thus, it has become
a trend to immobilize homogeneous catalysts on carbon nanotubes
special electronic structure, it can be used as both a
σ electron donor and
a
π
electron acceptor, which can make it coordinate with various metals
and maintain considerable stability compared to phosphine ligands
[18]. Therefore, NHC-Cu is widely used to accelerate the azide-alkyne
cycloaddition reaction [8,19–21]. Despite the high activity and selec-
tivity of carbene complexes in terms of catalyzing various types of re-
actions, separating the catalyst from the product is a bottleneck
through
effect of homogeneous catalysts. People usually use aromatic small
molecules or some polymers to achieve bond stacking with carbon
π-π interaction to achieve the same and even enhanced catalytic
π-π
* Corresponding author at: State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, 730000, China.
E-mail address: jhliu@licp.cas.cn (J. Liu).
https://doi.org/10.1016/j.mcat.2021.111452
Received 3 December 2020; Received in revised form 29 January 2021; Accepted 1 February 2021
Available online 13 February 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
Scheme 1. Homogeneous pyrene tagged NHC-Cu molecular complexes and noncovalent immobilization of NHC-Cu onto multi-walled carbon nanotubes.
nanotubes to achieve immobilization [34–36]. Initial carbon nanotubes
complexes on carbon nanotubes, people have also successfully ach-
ieved immobilization of pyrene-modified complexes on reduced gra-
phene oxide. Cuni and Sabater synthesized pyrene-modified NHC-Pt,
NHC-Pd and NHC-Ru, respectively, then, loaded it onto reduced gra-
phene oxide, which can be reused several times in various organic
transformations [37,42]. In the heat filtration experiment, Researchers
did not find detectable leaching or boomerang effect to reveal the nature
of heterogeneity. These evidences show that the homogeneous catalyst
have a large number of sp2 hybridized carbon atoms and the stacking of
π-π bonds provides enough convenience. Among them, pyrene, as a
smaller aromatic group, is widely used because of its simple modifica-
tion method [37–42]. A lot of work on the immobilization of the ho-
mogeneous catalyst through the
π-π bond conjugation and the
realization of multiple cycles of the catalyst has been developed. Liu and
co-workers successfully synthesized a pyrene-tagged ruthenium carbene
and immobilized on carbon nanotubes via
π-
π
stacking and discovered
supported by the
catalyst.
π-π stacking effect can fully realize the recycling of the
that this π-π stacking is severely affected by temperature and solvent
polarity, which leads to the “boomerang effect” during the reaction [41].
Besides, carbon nanotubes can be easily recovered by using tetrahy-
drofuran to wash off the complex when it loses activity. Charles et al.
prepared a pyrene-tagged gold complex. The author successfully real-
ized the recycling of gold complexes through the influence of tempera-
On the basis of the previous reports, As a part of our ongoing efforts
to extend the scope of the use of recyclable catalyst in the azide-alkyne
cycloaddition reaction [43]. Herein, a series of new pyrene tagged
NHC-Cu (nitrogen heterocyclic carbene) molecular complexes and the
corresponding multi-walled carbon nanotubes (MWNTs) immobilized
ture on the stacking effect of
desorbed gold complex was re-adsorbed to the carbon nanotubes [39].
In addition to the stacking effect by immobilizing pyrene-modified
π
-
π
bonds. At low temperature, the
NHC-Cu catalytic material by using π-π stacking interaction noncovalent
were prepared and applied in the azide-alkyne cycloaddition reaction
(Scheme 1). Interestingly, the noncovalent immobilization of NHC-Cu
π-π
Scheme 2. The synthesis of pyrene tagged NHC ligand precursors and their copper complexes.
2

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
catalyst displayed the same activities with the homogeneous related
complexes under solvent-free conditions. Moreover, the noncovalent
immobilization of NHC-Cu catalyst could be separated from the reaction
system at the end of the reaction and reused in another catalytic cycle,
but without significant loss in the catalytic activity.
Table 1
Screening the solvent for azide-alkyne cycloaddition reaction catalyzed by NHC-
Cua.
2. Materials and methods
Entry
Solvent
Isolated yield (%)
2.1. Materials and characterizations
1
2
3
4
5
6
CH3OH
PhCH3
CH3CN
THF
66
35
15
50
93
39
Unless otherwise noted, all the reagents were of analytical grade and
were used as received. MWNTs (purity >95 %) were purchased from
Nanjing Xianfeng Nano Material Technology Co., Ltd. Nitrides were
prepared according to the reference [39]. Thermogravimetric analysis
(TGA) measurement was measured using NETZSCH STA449F3 under a
nitrogen atmosphere at an increased rate of 10 ◦C/min from room
temperature to 800 ◦C. 1H NMR and 13C NMR spectra were obtained by
using a Bruker AvanceTM III 400 MHz spectrometer using CDCl3 or
DMSO-D6 containing TMS as the solvent. The chemical shift unit is ppm.
The unit of coupling constant J is Hz. High resolution mass spectra
(HR-MS) were recorded with Bruker MicroTOF-Q II mass instrument
(ESI). ICP analysis was recorded using an Agilent 725-ES spectrometer.
FT-IR was performed at VERTEX 70v (Germany). Powder X-ray
diffraction (XRD) was characterized by Smartlab-SE (Japan). Fluores-
cence measurement was performed on F-7000 from Hitachi
High-Technologies Corporation. High-resolution transmission electron
microscopy images and energy-dispersive X-ray (EDX) mapping were
executed at the FEI Tecnai F20 transmission electron microscope.
Neat
H2O
[a] Reaction conditions: 1 mol% 4c, 1 mmol Benzyl azide, 1.2 mmol Phenyl-
acetylene, 3 mL solvent, 30 ◦C, reaction time 3 h.
6.5 Hz, 2 H).13C NMR (101 MHz, DMSO) δ 151.8, 131.1, 127.2, 126.4,
125.9, 124.9, 124.5, 124.2, 124.0, 120.7, 119.5, 110.2, 68.7, 31.8.
2.2.2. Preparation of 1-(2-(pyren-2-yloxy) ethyl)-1H-benzo[d]imidazole
(2)
A mixture containing benzimidazole (1.18 g, 10 mmol) and potas-
sium hydroxide (1.13 g, 12 mmol) dissolved in acetonitrile (50 mL) was
refluxed for 30 min, which was added compound 1 (3.26 g, 10 mmol)
and continued to reflux for 24 h. After cooling to room temperature,
20 mL of n-hexane was added to the reaction solution and filtered. The
filter was washed three times with n-hexane and dried in vacuo to obtain
a pale yellow solid. (2.7 g, 75 % yield) 1H NMR (400 MHz, CDCl3) δ 8.55
(s, 1 H), 8.29 – 8.16 (m, 4 H), 8.04 (ddd, J = 26.3, 18.8, 9.3 Hz, 4 H),
7.89 (t, J =8.9 Hz, 1 H), 7.72 (dd, J = 21.5, 8.3 Hz, 2 H), 7.40 – 7.32 (m,
1 H), 7.28 – 7.21 (m, 1 H), 5.01 – 4.85 (m, 2 H), 4.73 (t, J =5.0 Hz, 2 H).
13C NMR (101 MHz, CDCl3) δ 157.1, 148.6, 139.2, 136.4, 136.2, 132.4,
131.6, 131.1, 130.1, 129.7, 129.6, 129.3, 129.3, 128.9, 127.5, 126.7,
125.9, 124.6, 124.5, 124.2, 115.9, 114.9, 72.4, 49.1.
2.2. Procedure for synthesis of pyrene tagged NHC-Cu (nitrogen
heterocyclic carbene) molecular complexes
The pyrene tagged benzimidazolium salts used as NHC precursors
and the corresponding NHC-Cu molecular complexes were prepared
similarly by using the published procedures [44]. As shown in Scheme 2.
2.2.3. Synthesis of benzimidazole salt (3a, 3b, 3c, 3d)
2.2.1. Preparation of 2-(2-bromoethoxy) pyrene (1)
Compound 2 (2 mmol, 724 mg) in 20 mL of acetonitrile was added
2.2 mmol halogenated hydrocarbon and was refluxed for 24 h. The filter
was formed and washed with n-hexane, providing the desired product as
a yellow solid material (3a: 730 mg, 72 % yield; 3b: 800 mg, 75 % yield;
3c: 560 mg, 56 % yield; 3d: 630 mg, 60 % yield).
1-Hydroxypyrene (2.18 g, 10 mmol), Cs2CO3 (6.5 g, 20 mmol), 1,2-
dibromoethane (7.48 g, 40 mmol) and acetonitrile (35 mL) were
added into a 100 mL round bottom flask, which was stirred and refluxed
for 24 h. After cooling, water (50 mL) was added to the reaction
mixture, which was extracted with dichloromethane (3 × 30 mL) and
dried with magnesium sulfate. Finally, the obtained product was puri-
fied by column chromatography (Ethyl acetate: petroleum ether = 1:4)
to give yellow solid (2.3 g, 70 % yield). 1H NMR (400 MHz, DMSO) δ
8.44 (t, J =9.9 Hz, 1 H), 8.20 (dq, J = 17.1, 8.2 Hz, 4 H), 8.12 – 7.96 (m,
3 H), 7.74 (d, J =8.5 Hz, 1 H), 4.82 – 4.57 (m, 2 H), 4.04 (dd, J = 11.7,
3a：1H NMR (400 MHz, DMSO) δ 10.58 (s, 1 H), 8.40 (t, J =7.8 Hz,
1 H), 8.23 (ddd, J = 20.2, 13.5, 8.0 Hz, 5 H), 8.13 – 7.93 (m, 6 H), 7.81 –
7.73 (m, 2 H), 7.69 (dd, J = 13.4, 5.2 Hz, 1 H), 7.59 (dd, J = 6.5,
2.9 Hz, 2 H), 7.32 – 7.26 (m, 3 H), 5.91 (s, 2 H), 5.30 (t, J =4.6 Hz, 2 H),
4.90 (t, J =4.7 Hz, 2 H). 13C NMR (101 MHz, DMSO) δ 151.5, 143.2,
Scheme 3. The synthesis of immobilization of NHC-Cu by using
π-
π
stacking interaction of pyrene and multi-walled carbon nanotubes (MWNTs).
3

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
Fig. 1. The yield of 4c and MWNTs@NHC-Cu catalyzed click reaction at
different time points.
[a] Reaction conditions: 1 mol% 4c, 1 mol% MWNTs@NHC-Cu, 1 mmol Benzyl
azide, 1.2 mmol Phenylacetylene, neat, 30 ◦C.
133.90, 131.5, 131.1, 130.9, 130.7, 128.8, 128.7, 128.4, 127.2, 126.7,
126.5, 125.8, 125.0, 124.7, 124.6, 124.3, 123.9, 120.7, 119.1, 114.2,
113, 109.7, 66.2, 49.9, 46.7.
Fig. 2. TGA curves of MWNTs, MWNTs@NHC-Cu, NHC-Cu (4c).
3b：1H NMR (400 MHz, DMSO) δ 10.43 (s, 1 H), 8.39 (d, J =8.4 Hz,
1 H), 8.30 – 8.18 (m, 4 H), 8.05 (ddd, J = 25.8, 13.6, 7.6 Hz, 5 H), 7.78
(t, J =7.2 Hz, 2 H), 7.69 (t, J =7.5 Hz, 1 H), 7.57 (dd, J = 6.6, 2.8 Hz,
2 H), 7.33 – 7.25 (m, 3 H), 5.88 (s, 2 H), 5.29 (t, J =4.6 Hz, 2 H), 4.95 –
4.81 (m, 2 H). 13C NMR (101 MHz, DMSO) δ 151.5, 143.1, 133.8, 131.5,
131.1, 130.9, 130.7, 128.8, 128.6, 128.3, 127.1, 126.7, 126.4, 125.8,
125.0, 124.7, 124.3, 123.9, 120.6, 119.1, 114.2, 113.9, 109.7, 66.1,
49.8, 46.7.
19.0, 13.3.
3d: 1H NMR (400 MHz, DMSO) δ 10.19 (s, 1 H), 8.38 (d, J =8.2 Hz,
1 H), 8.26 – 8.16 (m, 4 H), 8.12 (d, J =8.2 Hz, 1 H), 8.02 (ddd, J = 24.5,
13.0, 6.7 Hz, 4 H), 7.84 – 7.69 (m, 3 H), 5.39 – 5.04 (m, 2 H), 4.84 (dd,
J = 22.8, 18.0 Hz, 2 H), 4.54 (t, J =7.1 Hz, 2 H), 1.84 (dt, J = 14.9,
7.3 Hz, 2 H), 1.24 – 1.20 (m, 2 H), 1.14 – 1.04 (m, 2 H), 1.02 – 0.92 (m,
2 H), 0.60 (t, J =7.2 Hz, 3 H). 13C NMR (101 MHz, DMSO) δ 151.5,
142.8, 131.3, 131.1, 130.9, 127.2, 126.7, 126.5, 126.4, 125.8, 125.0,
124.8, 124.7, 124.6, 124.3, 124.0, 123.8, 120.6, 119.2, 114.1 113.7,
109.7, 66.2, 46.7, 46.5, 30.4, 28.5, 25.3, 21.6, 13.6.
3c：1H NMR (400 MHz, DMSO) δ 10.39 (s, 1 H), 8.39 (t, J =15.2 Hz,
1 H), 8.22 (dd, J = 14.5, 8.5 Hz, 4 H), 8.14 (d, J =8.2 Hz, 1 H), 8.11 –
7.94 (m, 4 H), 7.85 – 7.68 (m, 3 H), 5.28 (d, J =4.3 Hz, 2 H), 5.02 – 4.77
(m, 2 H), 4.58 (t, J =7.0 Hz, 2 H), 1.97 – 1.73 (m, 2 H), 1.43 – 1.10 (m,
2 H), 0.78 (t, J =7.3 Hz, 3 H). 13C NMR (101 MHz, DMSO) δ 151.5,
142.9, 131.2, 131.0, 130.8, 127.1, 126.6, 126.4, 125.8, 124.9, 124.7,
124.5, 124.3, 123.9, 120.6, 119.1, 114.1, 113.8, 109.8, 66.3, 46.6, 30.5,
2.2.4. Synthesis of NHC-Cu-X’ (4a, 4b, 4c, 4d, 4e)
To a Schlenk tube under nitrogen was introduced 3a (0.2 mmol,
98 mg), copper(I) chloride (0.2 mmol, 20 mg) and sodium tert-butoxide
(0.2 mmol, 20 mg) followed by 5 mL dry THF. The mixture was stirred
at room temperature for 4 h. A suspension was filtered through a short
pad of Celite and the pad was washed with methylene chloride
(3 × 10 mL). Then the filtrate was concentrated by vacuum rotary
evaporation to a saturated solution, which was added n-hexane (30 mL)
to generate white solid washed with pentane (3 × 10 mL) to afford pure
complex 4a as off-white solid (46 mg, 42 % yield). Other complexes
were obtained by similar methods.
Table 2
Catalytic activity test under different conditions a.
Entry
Catalyst
Catalyst amount
t/h
Isolated Yield (%)
1
4a
1 mol%
2
2
2
2
2
3
6
4
2
18
5
–
78
83
85
74
23
93
97
98
98
94
96
NR
95
87
NR
2
4b
1 mol%
3
4c
1 mol%
4
4d
1 mol%
5
4e
1 mol%
6
4c
1 mol%
7
4c
0.25 mol%
0.5 mol%
2 mol%
8
4c
9
4c
10
11
12
13
14
15
CuI
1 mol%
3b + CuI
1 mol% CuI and 3b
1 mol% 3d
1 mol%
3b
MWNTs@NHC-Cu
MWNTs@NHC-Cu
MWNTs
3
4
–
0.5 mol%
1 mol%
[a] Reaction conditions: 1 mmol Benzyl azide, 1.2 mmol Phenylacetylene, re-
action temperature 30 ◦C, reaction time 3 h.
Fig. 3. XRD patterns of MWNTs (a) and MWNTs@NHC-Cu (b).
4

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
4c: off-white, 48 %yield, 1H NMR (400 MHz, DMSO) δ 8.07 (dd,
J = 14.0, 7.4 Hz, 4 H), 8.00 – 7.86 (m, 3 H), 7.80 (t, J =8.5 Hz, 2 H),
7.60 (d, J =8.6 Hz, 1 H), 7.43 (dd, J = 13.9, 7.3 Hz, 2 H), 7.35 – 7.23
(m, 3 H), 7.20 – 7.10 (m, 3 H), 5.48 (s, 2 H), 5.08 (t, J =8.5 Hz, 2 H),
4.78 (t, J =4.5 Hz, 2 H). 13C NMR (101 MHz, DMSO) δ 194.6, 151.6,
136.4, 134.3, 133.2, 130.9, 130.7, 128.4, 127.5, 127.3, 127.1, 126.9,
126.2, 125.9, 125.5, 124.7,124.5, 124.2, 124.0, 123.7, 123.2, 120.7,
118.9, 111.8, 111.5, 109.2, 67.4, 50.8, 47.0. HRMS (ESI): calcd for
C
64H48CuN4O2 [2M–2I–Cu]+: 967.3073, found 967.3043
4d: light-pink, 40 % yield, 1H NMR (400 MHz, DMSO) δ 8.08 – 7.94
(m, 5 H), 7.90 (t, J =7.6 Hz, 1 H), 7.76 (dt, J = 13.2, 9.1 Hz, 3 H), 7.61
(dd, J = 16.9, 8.3 Hz, 2 H), 7.47 (t, J =7.5 Hz, 1 H), 7.40 (t, J =7.6 Hz,
1 H), 5.11 (s, 2 H), 4.84 (d, J =4.4 Hz, 2 H), 4.27 (t, J =7.1 Hz, 2 H),
1.85 – 1.70 (m, 2 H), 1.21 (dd, J = 14.5, 7.5 Hz, 3 H), 0.78 – 0.65 (m,
3 H). 13C NMR (101 MHz, DMSO) δ 191.5, 151.5, 133.6, 133.2, 130.9,
130.8, 130.6, 126.8, 126.2, 125.9, 125.6, 124.6, 124.3, 124.0,
123.8,123.6, 123.4,123.2, 123.3, 123.1, 120.9, 120.6, 119.1, 111.8,
111.5, 111.3, 109.4, 67.9, 66.2, 47.4, 47.3, 47.1,46.3, 32.0, 19.5, 13.52.
HRMS (ESI): calcd for C58H52CuN4O2 [2M–2I–Cu]+: 899.3386, found
899.3362
4e: light-pink, 47 % yield, 1H NMR (400 MHz, DMSO) δ 8.13 – 7.90
(m, 6 H), 7.84 (d, J =9.0 Hz, 1 H), 7.81 – 7.72 (m, 2 H), 7.63 (d, J
=7.0 Hz, 2 H), 7.49 (t, J =7.6 Hz, 1 H), 7.42 (t, J =7.6 Hz, 1 H), 5.08 (s,
2 H), 4.82 (s, 2 H), 4.23 (t, J =6.8 Hz, 2 H), 1.81 – 1.63 (m, 2 H), 1.15 (s,
2 H), 1.08 – 0.89 (m, 4 H), 0.72 – 0.48 (m, 3 H). 13C NMR (101 MHz,
DMSO) δ 191.1, 151.6, 133.7, 133.2, 130.9, 130.7, 127.3,127.0, 126.9,
126.3, 125.9, 125.6, 124.7, 124.3, 124.0, 123.8, 123.3, 120.8, 119.1,
111.9, 111.4, 109.4, 47.5, 46.9, 30.7, 29.9, 25.8, 21.8, 13.6. HRMS
Fig. 4. FT-IR spectra of NHC-Cu (4c), MWNTs and MWNTs@NHC-Cu.
4a: off-white, 45 % yield, 1H NMR (400 MHz, DMSO) δ 8.25 – 8.14
(m, 1 H), 7.95 (dddd, J = 28.6, 21.0, 17.0, 8.1 Hz, 7 H), 7.77 (d, J
=8.9 Hz, 1 H), 7.71 (q, J =8.6 Hz, 1 H), 7.43 (t, J =7.9 Hz, 2 H), 7.34 –
7.22 (m, 3 H), 7.12 (dt, J = 14.4, 7.0 Hz, 3 H), 5.54 (s, 2 H), 5.09 (d, J
=11.5 Hz, 2 H), 4.70 (d, J =11.4 Hz, 2 H). 13C NMR (101 MHz, DMSO) δ
193.6, 151.5, 136.4, 134.0, 133.1, 130.9, 130.7, 128.4, 127.59, 127.32,
127.1, 126.9, 126.3, 126.2, 125.9 125.5, 124.7, 124.3, 124.2, 124.1,
124.0, 123.7, 123.3, 122.7, 121.9, 120.75, 119.00, 111.86, 111.61,
(ESI): calcd for
955.3996.
C
62H60CuN4O2 [2M–2I–Cu]+: 955.4012, found
2.3. Procedure for immobilization of NHC-Cu complex on MWNTs
The immobilization of NHC-Cu complex on MWNTs catalyst was
prepared similarly by using one-step synthesis process [39] which was
demonstrated in Scheme 3.
109.12, 67.2, 50.9, 47.2. HRMS (ESI): calcd for
C64H48CuN4O2
[2M–2Cl–Cu]+: 967.3073, found 967.2991
4b: white soild, 42 % yield, 1H NMR (400 MHz, DMSO) δ 8.13 – 7.82
(m, 7 H), 7.75 (dd, J = 26.7, 9.1 Hz, 2 H), 7.40 (t, J =8.1 Hz, 3 H), 7.30
(d, J =5.6 Hz, 3 H), 7.11 (dd, J = 14.5, 7.0 Hz, 3 H), 5.59 (d, J
=27.7 Hz, 2 H), 5.09 (s, 2 H), 4.70 (s, 2 H). 13C NMR (101 MHz, DMSO)
δ 191.6, 151.5, 136.3, 134.0, 133.1, 130.9, 130.7, 128.4, 127.6, 127.1,
126.9, 126.2, 125.9, 125.5, 124.8, 124.3, 124.0, 123.7, 123.3, 120.7,
119.0, 111.8, 111.6, 109.1, 67.2, 50.8, 47.1. HRMS (ESI): calcd for
First, 300 mg of carbon nanotubes was dispersed under ultra-
sonication in 50 mL of methylene chloride for 30 min. Then complex 4c
(120 mg) of dissolved with 50 mL of dichloromethane was added into
above mixture for further ultrasonication for 30 min and stirred over-
night. The resulting mixture was filtered and then washed with
dichloromethane (3 × 20 mL) to remove unloaded 4c, marked as
MWNTs@NHC-Cu. The loading of the complex measured by ICP-AES is
0.15 mmol/g.
C
64H48CuN4O2 [2M–2Br–Cu]+: 967.3073, found 967.3068
Fig. 5. Cu 2p XPS spectra of a: NHC-Cu(4c) b: MWNTs@NHC-Cu c: MWNTs@NHC-Cu after reaction and the MWNTs@NHC-Cu composite spectra.
5

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
2.4. General procedure for homogeneous cycloaddition reaction of azide
and alkyne
3. Results and discussions
3.1. Optimization of MWNTs@NHC-Cu reaction conditions
To a vial fitted with a screw cap, azide (1.0 mmol), alkyne
(1.2 mmol) and NHC-Cu complex catalyst were added and stirred at
30 ◦C. The reaction process was monitored by TLC. After the reaction,
the filtered solid product was washed with water and n-hexane and dried
at 60◦ C to get the corresponding product (No further purification is
required). For three-component click cycloaddition reaction, the reac-
tion was carried out by using organic halide (1.0 mmol), alkyne
(1.0 mmol) and NaN3 (1.2 mmol) as the substrates and 3 mL H2O as the
solvent. The purity of the crude product is more than 95 % by 1H NMR.
We have successfully synthesized five different types of NHC-Cu with
different side chain R groups and coordinating with different halogens,
which were characterized by NMR and HR-MS spectroscopy to clarify
their structure (Shown in Scheme 2). Due to the instability of NHC-Cu, a
large number of [Cu(NHC)2]+ species appear in high-resolution mass
spectrometry, which presents a confusion of ligands [45,46]. Therefore,
we further verify its structure by measuring the content of copper in the
compound by ICP-AES and through the anion mode of high-resolution
mass spectrometry to determine its coordinated halogen, indicating
that it was formed as NHC-Cu species. Through the activity test, it is
found that the conversion rate of azide-alkyne cycloaddition reaction is
lower in the presence of the solvent such as THF, CH3CN, CH3OH, but
pleasantly, there is a noticeable acceleration without solvent (Show in
Table 1).
2.5. General procedure for the cycloaddition reaction catalyzed by
MWNTs-supported 4c and the corresponding recycle
To a vial fitted with a screw cap, azide (1.0 mmol), alkyne
(1.2 mmol) and MWNTs@NHC-Cu were added and stirred at room
temperature. After the reaction was completed, the mixture was
extracted by 10 mL of dichloromethane to give the crude product and
was filtered to isolate the catalyst washed with dichloromethane
(3 × 2 mL). The crude product was obtained by rotary evaporation to
remove the solvent, washed with water and n-hexane and further dried.
The purity of the crude product is greater than 95 % by 1H NMR. The
separated catalyst was vacuum dried at 40◦ C, and then used for the next
cycle test.
Then, we tested the activity of the five kinds of NHC-Cu prepared
without solvent, and found that 4c had a better catalytic performance
(Show in Table 1). The substrate is completely converted within three
hours at 30 ◦C. We attribute it to two aspects. On the one hand, it is
possible that I was easier to leave the Cu(I)(active center) than Br and Cl
to form a copper acetylide intermediate [8,47]. On the other hand, the
benzene ring has greater rigidity and less steric hindrance than other
straight-chain alkanes, making the alkynyl group closer to the active
center. For the above reasons, 4c has better activity than other com-
plexes. Therefore, we choose 4c as the precursor of the load. The content
of 4c was determined by ICP to be 0.15 mmol/g.
Fig. 6. a, b: TEM image of the MWNTs. c, d: TEM image of the MWNTs@NHC-Cu. e: STEM images of the MWNTs@NHC-Cu. f, g, h: Corresponding EDS line scan
profiles for C, N, and Cu along the line.
6

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
We compared the catalytic activity before and after 4c immobiliza-
3.2.2. Study of XRD
tion and found that there was no obvious difference (Show in Fig.1).
MWNTs@NHC-Cu has a TOF value of 32.67 hꢀ 1 when the product is
close to complete conversion after three hours under mild conditions. a
This shows that the immobilization of the homogeneous catalyst
does not significantly affect the activity of the catalyst, which is
consistent with our original purpose. NHC-Cu is conjugated on the
XRD patterns of the functionalized MWNTs before and after 4c
immobilized. Both samples have an obvious diffraction peak at 26.1◦
(Show in Fig. 3) which is attributed to the (002) planes of crystalline
graphite-like materials [48]. We can see that there is a slight decrease in
the intensity of the peak after loading. This may be due to the complexes
loaded on the surface of the carbon nanotubes increasing the disorder of
the crystal lattice. This also confirms the successful loading of the
complex from another level. The absence of crystal face peaks of the
complex indicates that it does not show an aggregated form but was
evenly distributed.
surface of carbon nanotubes through π-π bonds and plays a catalytic role.
By comparing items 10 and 11 (Show in Table 2), we find that the ligand
has a significant acceleration effect. During the click reaction, (NHCH)+
[(Ph≡Cꢀ )CuX]- are easily formed [47]. Therefore, we believe that 3b
(NHCH)+ X- may act as a base and reacts with phenylacetylene to
remove a molecule of HX to obtain an (NHCH)+ [(Ph≡Cꢀ )CuX]-. Then,
this intermediate is subjected to the HX removal reaction to obtain
acetylide copper ligand Ph≡C-Cu-NHC to promote the reaction. The
amount of 4c also has a significant effect on the reaction.
3.2.3. FT-IR analysis
Although the absorption peak of NHC-Cu is mostly curtained off by
carbon nanotubes, after careful identification, we can still find the cor-
responding peak of NHC-Cu. NHC-Cu has a strong infrared absorption
peak at 1250 cmꢀ 1 (Show in Fig. 4), which is attributed to the stretching
– –
vibration of C O C, and correspondingly an enhanced absorption
3.2. Characterization of MWNTs@NHC-Cu
peak appears in MWNTs@NHC-Cu.
3.2.1. Discussion of TGA
TGA investigations on the thermal degradation of NHC-Cu, MWNTs,
MWNTs@NHC-Cu in nitrogen were the temperature range of r. t. –
800 ◦C at a heating rate of 10 ◦C/min. The thermogravimetric experi-
ment showed that the decomposition temperature of the NHC-Cu was
between 280 ◦C and 420 ◦C (Show in Fig. 2). The MWNTS@NHC-Cu had
a more significant weight loss at around 350 ◦C compared to the original
MWNTs. It showed that the NHC-Cu complex was successfully loaded on
the MWNTs.
3.2.4. XPS analysis
Through the above analysis, we confirmed that NHC-Cu was suc-
cessfully loaded onto MWNTs. Further, we characterized the state of the
NHC-Cu on MWNTs. XPS analysis was performed on the solid samples
and the complex 4c. The binding energies of a, b, and c are all around
933ev and 953ev (Show in Fig.5), and they are attributed to Cu 2p3/2
and Cu 3p1/2, respectively, which are consistent with what is reported in
the literature [49], indicating that there was no change during the whole
load process. Meanwhile, through composite spectra, we can also see
that the existence of the Cu, I, O, N, C, elements confirm the existence of
Fig. 7. i, g: TEM image after four runs of MWNTs@NHC-Cu. k: STEM images of the MWNTs@NHC-Cu after recycling. l, m, n: Corresponding EDS line scan profiles
for C, N, and Cu along the line.
7

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
Table 3
Cycloaddition reaction of azides and alkynes catalyzed by MWNTs@NHC-Cu a.
Entry
1
Azide
Alkyne
Product
Isolated yield (%)
82
2
3
91
86
4
5
90
87
6
7
8
90
84
95
9
94
77
10
11
12
89
97
13
93
14
15
90
92
17
18
92b
93b
[a] Reaction conditions: 1 mol% catalyst MWNTs@NHC-Cu, 1 mmol organic azide, 1.2 mmol alkyne, reaction temperature 30 ◦C, reaction time 3 h.
[b] 3 mol% catalyst MWNTs@NHC-Cu, reaction time 3 h.
8

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
4c. There is no change in the binding energy of Cu 2p before and after
the reaction, which is consistent with that before loading, indicating that
the complex remained intact, and the real catalyst is NHC-Cu which
returned to its original position through a completed catalytic cycle. Our
catalyst can exist stably throughout the loading and catalytic reaction
process without degradation.
3.2.5. TEM analysis
The supported catalyst and multi-walled carbon nanotubes were
further studied through TEM and Energy-dispersive X-ray (EDX) map-
ping. Comparison before (a, b) and after (c, d) loading, there was no
obvious difference and we did not found agglomeration of metal parti-
cles or complex molecules indicating that the complexes are uniformly
distributed on the carbon nanotubes at the molecular level (Show in
Fig. 6). But Energy dispersive X-ray (EDX) mapping and line-scan
analysis (f, g, h) can obtain the distribution, and Cu on the surface of
MWNTs. Due to the low copper content, the response signal value of
copper may be lower. However, some enhanced signals can still be seen
in the significant range of carbon and nitrogen signals which indicates
the presence of copper.
Fig. 9. The effect of catalyst removal on click reaction in ethanol solvent.
[a] Reaction conditions: 1 mol% catalyst MWNTs@NHC-Cu, 1 mmol azide,
1.2 mmol alkyne, reaction temperature 70 ◦C, ethanol as solvent.
After cycles of MWNTs@NHC-Cu, we conducted a TEM test on the
MWNTs@NHC-Cu. Through i and g (Show in Fig. 7), we did not find
obvious copper or copper ion aggregates on the surface of the carrier
after the reaction. Besides, the scanning transmission electron micro-
scopy (STEM) images and energy disperse spectroscopy mappings of
MWNTs@NHC-Cu reveals that the Cu and N components are distributed
throughout the catalyst. Although the signal value of copper is low, it
still shows a trend similar to that of nitrogen which shows that the NHC-
Cu is distributed on the carrier.
MWNTS@NHC-Cu. At the same time, we use the time-yield distribution
graph to determine the degree of inactivation.The supported catalyst
can be recycled four times without significant loss of activity. But in the
fifth cycle, there was an obvious loss of activity (Shown in Fig. 8a). The
unchanged binding energy of copper before and after the reaction in-
dicates that the state of copper present in the catalyst has not changed,
namely NHC-Cu(I). The resulting loss of activity is not due to changes in
the valence of copper. ICP analysis was realized after four runs, it is
found that the copper loss is close to 23 %. Therefore, it caused a sig-
nificant decrease in activity. We assumed that part of the reason for the
serious loss of copper is that the product encapsulates the complex
loaded on the carbon nanotubes so that it is easily lost with the product
during separation.
3.3. Substrate scope of the azide-alkyne cycloaddition reaction
For cycloaddition reactions of click, we found that the reaction be-
tween azide and acetylene, whether it is aliphatic or aromatic, has a
good yield under mild reaction conditions (Show in Table 3). The results
indicated that our catalyst has broad substrate adaptability, which is
consistent with the ideal of azide-alkyne cycloaddition reaction. The
reaction has good compatibility with various functional groups and is
prompt, convenient and efficient.
3.5. Hot filtration experiment
We conducted a hot filtration experiment on the MWNTs@NHC-Cu
with ethanol as the solvent a. In the early stage of the reaction, a hot
filtration experiment was performed on the reaction, and then the re-
action solution and the original reaction were monitored (Shown in
Fig. 9). It was found that the reaction solution did not react further,
3.4. Catalyst recycling
The cyclability of the catalyst is a very important characteristic of
heterogeneous catalysts. Therefore, we conducted a cycle test on the
Fig. 8. a: The recycling of the MWNTs@NHC-Cu (1 mol%). b: valid assessment by rates: the consecutive experimental runs 1–3 to examine the extent of deactivation
Reaction condition: neat, 30 ◦C, for 3 h, using a model reaction of phenylacetylene and azide.
9

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
Table 4
Three-component cycloaddition reaction catalyzed by 4c.
Entry
1
R1-X
Alkyne
Product
Isolated yield(%)
98a
2
3
4
90a
93a
94a
5
6
95a
93a
7
8
9
96a
90b
94b
10
11
92b
92b
12
13
14
95b
96b
88b
15
90c
16
17
92c
94c
10

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
[a] Reaction conditions: 3 mol% 4c, 1 mmol benzyl bromide (PhCH2Br), 1.2 mmol NaN3, 1.2 mmol alkyne, 3 mL H2O, reaction temperature 30 ◦C, reaction time 5 h.
[b] Reaction conditions: 3 mol% 4c, 1 mmol benzyl chloride (PhCH2Cl), 1.2 mmol NaN3, 1.2 mmol alkyne, 3 mL H2O, reaction temperature 30 ◦C, reaction time 20 h.
[c] Reaction conditions: 3 mol% 4c, 1 mmol RBr, 1.2 mmol NaN3, 1.2 mmol phenylacetylene, 3 mL H2O, reaction temperature 40 ◦C, reaction time 24 h.
nanotubes under the influence of the solvent [57–59]. Based on the
above two reasons, Significant copper loss occurred after the first cycle.
poor cyclicity occurred in the three-component cycloaddition reaction.
3.7. Possible mechanisms
Based on previous research [47,60–62], a possible reaction mecha-
nisms of cycloaddition reaction of azides and alkynes through NHC-Cu
Complexes (Shown in Fig. 11) was proposed. First, acetylide copper
ligand is formed by NHC-Cu dissociation and phenylacetylene depro-
tonation. Then, a stable six-membered ring is formed by reacting with
azide, acetylide copper ligand. Finally, this six-membered ring inter-
mediate underwent a rapid ring contraction to form another more stable
five-membered ring which It reacts with phenylacetylene to produce
a product and another molecule of acetylide copper ligand entering the
next catalytic cycle.
In addition, in comparison with the other recently reported Cu
Fig. 10. The recycling of the MWNTs@NHC-Cu(3 mol%). Reaction condition:
30 ◦C, for 5 h, using a model reaction of benzyl bromide, phenylacetylene and
Sodium azide.
catalyzed cycloaddition reaction for the synthesis of triazoles,
MWNTs@NHC-Cu has higher catalytic activity (Table 5).
4. Conclusions
which proved the heterogeneity of the catalyst and no detectable
obvious contribution came from the homogeneous catalyst.
Due to the potential dangers of organic azides in the synthesis pro-
cess, we tested the three-component click cycloaddition reaction that
uses organic halides and sodium azide for direct cycloaddition with
alkyne and found that our catalyst still has good catalytic activity for this
reaction (Table 4).
In summary, we have successfully prepared five novel pyrene-tagged
NHC-Cu molecular complexes and characterized by 1H- and 13C- NMR,
and high resolution ESI-mass spectrometry to clarify their structure.
Moreover, MWNTs supported NHC-Cu catalytic material was achieved
by via π-π stacking between pyrene and carbon nanotube. Both the NHC-
Cu molecular complexes and the immobilized homogeneous catalyst
onto carbon nanotube are demonstrated to be highly effective in the
two-component click cycloaddition reaction of terminal alkynes and
organic azides or three-component click cycloaddition reaction that
employs various terminal alkynes, NaN3 and organic halides with the
advantage of excellent yields. Especially, the immobilization catalyst
displayed competitive advantages, including no need to add other li-
gands and easy catalyst recycling, allowing several reuses with no sig-
nificant loss in catalytic activity. This may be due either to a facilitated
boomerang effect or to a gradual release of the active species in solution.
It nevertheless describes the first use of multi-walled carbon nanotubes
3.6. Three-component cycloaddition reaction
3.6.1. Catalyst recycling of three-component [3 + 2] cycloaddition
reaction
Then, we carried out a catalytic cycle on the MWNTs@NHC-Cu and
found that the circularity was short of (Shown in Fig. 10). We performed
an ICP measurement on the used catalyst. Nearly 20 % of copper is lost
after the first run, which resulted in a significant decrease in catalytic
activity. There may be two main reasons for the loss of copper(I) one is
the separation process between a product which are attached to the
catalyst and catalyst. The other is that NHC-Cu desorbed from carbon
as support for such NHC-Cu catalysts by
π-π interaction, as far as we
know, and its successful use in recycling methodologies for azide-alkyne
Fig. 11. Possible reaction mechanisms of azide-alkyne cycloaddition reaction.
11

X. Zhang et al.
Molecular Catalysis 504 (2021) 111452
Table 5
Comparison of MWNTs@NHC-Cu with other reported catalysts for the synthesis of triazoles.
Entry
Catalyst
Solvent
[Cu] (mol%)
Temp.(oC)
Time (h)
Yield (%)
Refs
1
2
3
4
5
6
PS[NEt3][OAc](CuI)
NRGO/Cu(I)
t-BuOH/H2O
THF
13
2
R.T
R.T
R.T
R.T
40
12
48
24
18
72
0.3
100
100
99
[50]
[51]
[52]
[53]
[54]
[55]
TRGO/Cu
water/methanol
water
2
Fe3O4/Si[CuI]
CRGO-Ima-CuI
Cu NPs
0.25
2
92
THF
99
[BMIM][BF4]/water
5
R.T
90
Poly(M1-co-M3)-
b-poly(M2)–Cu
MWNTs@NHC-Cu
7
8
water
neat
0.25
1
R.T
30
16
3
53
95
[56]
Present work
[15] M.L. Teyssot, A. Chevry, M. Traïkia, M. El-Ghozzi, D. Avignant, A. Gautier, Chem.-
cycloaddition reactions. Further work is being undertaken in order to
improve the performance of the immobilized homogeneous catalyst
onto carbon materials in other organic transformations.
Eur. J 15 (2009) 6322–6326, https://doi.org/10.1002/chem.200900727.
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Declaration of Competing Interest
[18] H. Jacobsen, A. Correa, A. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev. 253
(2009) 687–703, https://doi.org/10.1016/j.ccr.2008.06.006.
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10.1039/C1GC15871F.
The authors report no declarations of interest.
CRediT authorship contribution statement
[20] S.D. ıez-Gonz ́alez, S.P. Nolan, Angew. Chem. Int. Ed. 47 (2008) 8881–8884,
́
https://doi.org/10.1002/ange.200803289.
[21] S. Diez-Gonzalez, E.D. Stevens, S.P. Nolan, Chem. Commun. 39 (2008) 4747–4749,
https://doi.org/10.1039/B806806B.
Xiangjie Zhang: Conceptualization, Methodology, Writing - original
draft. Bingyang Wang: Conceptualization, Methodology. Yanmei Lu:
Validation. Chungu Xia: Supervision. Jianhua Liu: Supervision,
Funding acquisition, Writing - original draft, Writing - review & editing.
[22] D.J. Cole-Hamilton, Science 299 (2003) 1702–1706, https://doi.org/10.1126/
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We thank for the financial support of the following funders: National
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"Light of West China" Program.
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
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