Click chemistry promoted by graphene supported copper nanomaterials

Article abstract of DOI:10.1039/c4cc07774a

A facile and robust approach is provided for the synthesis of highly dispersed copper nanoparticles immobilized onto graphene nanosheets, useful as a recyclable and reusable heterogeneous catalyst with excellent catalytic activity to achieve Cu(i)-catalyz

Full text of DOI:10.1039/c4cc07774a

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ARTICLE TYPE
Click chemistry promoted by graphene supported copper nanomaterials
Ali Shaygan Nia,a Sravendra Rana,a Diana Dӧhler,a Xavier Noirfalise,b Alice Belfioreb and Wolfgang H.
Binder*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
from photocatalytic hydrogen production to lithium ion batteries
50 have been discussed.20-21 Recently, few reports have appeared
regarding the preparation of Cu/GO nanoconjugates and their
relevance in hydrogen generation, supercapacitors, as well as in
other applications.22-25 However according to our knowledge their
application in CuAAc catalyzed “click” chemistry has not been
55 explored yet.
For the preparation of graphene oxide (GO), Hummer’s
method was applied,26 whereas ion exchange with Cu2+ was
achieved by dispersing GO in water,27 followed by addition of
copper acetate under vigorous stirring for overnight. Afterwards
60 Cu(II)/GO was reduced under Ar at 600 0C in oven to obtain
TRGO/Cu(I). We first investigated the morphology and chemical
composition of our nanoconjugates by TEM, EDX, XPS, and
FAAS.
A facile and robust approach is provided for the synthesis of
highly dispersed copper nanoparticles immobilized onto
graphene nanosheets, useful as a recyclable and reusable
heterogeneous catalyst with excellent catalytic activity to
10 achieve Cu (I)-catalyzed [3+2] cycloaddition ‘click’ chemistry.
Since its development by Sharpless1-2 and Meldal3-4, the Cu(I)-
catalyzed [3+2] cycloaddition reaction5 between terminal
acetylenes and azides (“click” reaction (CuAAc)) has emerged as
a strategy for the rapid and efficient assembly of molecules with
15 diverse functionality on both laboratory and production scales.6-9
Click reactions are modular, tolerant of a wide range of
functional groups, simple to perform, insensitive to reaction
solvents irrespective to their polar/non-polar or protic/aprotic
character.10-11 However, in order to enhance the catalytic activity,
20 the presence of a co-catalyst is required such as bases (mainly
amines), auxiliary ligands, and oxidizing or reducing agents
depending on the used Cu sources (CuII/Cu0). Furthermore, the
recyclability, reusability, and easy removal of the copper catalyst
is often severely limited. Therefore, the development of
25 recyclable and stable heterogeneous copper catalysts with
improved catalytic activity devoid of any oxidizing/reducing
agents is highly desirable.
Herein, we report a facile and robust approach for the synthesis
of highly dispersible, recyclable, and reusable Cu (I)
30 nanoparticles decorated onto graphene nanosheets, useful as a
catalyst for “click” chemistry without any co-catalyst (Scheme 1).
Graphene, a single layer of two-dimensional sp2-hybridized
carbon has attracted significant research interest due to its unique
electrical and thermal conductivity, including exceptional
35 mechanical, optical and chemical properties.12-14 In contrast to
single layer graphene sheets conveniently prepared by mechanical
exfoliation, graphene oxide (GO) and reduced graphene oxide
(rGO)15-16 are easily available by controlled chemical reactions
and can be produced on a large scale. Also due to the presence of
40 functional groups on GO and rGO surfaces, further chemical
reaction onto their surfaces is possible. On account of their large
surface area, unique interaction with metal particles17-18, and their
performance for electron capture, transport, as well as prevention
of supported nanoparticles agglomeration due to their scaffold
45 behaviour, graphene supported catalysts have represented
outstanding catalytic activity compared to other carbon supported
catalysts.19 Several reports for the preparation of different metal
particles-GO heterostructures and their wide range of applications
65
70
75
Scheme 1 Schematic illustration of click chemistry promoted by graphene
supported copper nanoparticles.
Fig. 1a shows the TEM image of TRGO/Cu(I) conjugates,
80 where highly dispersed uniform in size Cu nanoparticles onto the
surface of graphene nanosheets were obtained. From the TEM
images the average particle size is about 25 nm for Cu particles
(Fig. 1b). HRTEM was performed to measure the lattice plane
distance in a single Cu nanoparticle (Fig. 1c,d), where 0.2385 ±
85 0.01 nm as lattice plane distance was obtained, that could be due
to presence of CuO (110)28 or Cu2O (111)29, windcative that for
the TRGO/Cu conjugates the Cu is in the form of Cu (I) or Cu
(II). To confirm the existing form of Cu, quantitation of
nanoparticles was also achieved by STEM-EDXS (Fig. S1, ESI†)
90 indicating that Cu and O are presented in the ratio of 68.8:31.2 (≈
2:1), which strongly supports that the particles consist of Cu2O,
and the valency of Cu is Cu(I) as required for their use in “click”
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chemistry. Further evidence of the chemical composition of the
nanoconjugates was obtained by X-ray photoelectron
whereas the washing and recycling of the catalyst was
accomplished in open air environment, supportive of the stability
spectroscopy (XPS) (Fig. 2a), where the peaks correspond to C 1s 60 and recyclability of the synthesized catalyst (Table 1, Fig. 3a).
(285.1 eV), O 1s (531.6 eV) and Cu 2p. The high-resolution XPS
(Fig. 2b) reveals that Cu 2p3/2 peaks located at 932.37 eV and
952.31 eV correspond to the peak energy of Cu (I).30 Also there
are no peak satellites at 942eV and 962 eV and neither a peak at
934.8 eV which would be related to Cu(II). The Cu content of the
sample was also determined by analysis on a flame atomic
The reduction of reaction yield after several repetitions could be
due to agglomeration of Cu particles, as observed by TEM (Fig.
3b)
5
65 Table 1 Performance of various Cu-catalysts at different reaction
conditions
10 absorption spectroscopy (FAAS) as 7.55 x 10-7 mol/mg loading.
Entry
conditions
conversion
1
2
3
4
5
6
7
8
TRGO without copper, in THF, 40°C, 48 h
Copper on charcoal (2 mol%), in THF, 40°C, 48 h
Cu2O (2 mol%),in THF, 40°C, 48 h
TRGO/Cu (2 mol%), in THF, 40°C, 48 h
TRGO/Cu (2 mol%),in THF, room temperature,48h
Recycled catalyst in air (first cycle)
Recycled catalyst in air (fourth cycle)
TRGO/Cu, 1:1 water/methanol, room temperature,
24 h
0
3
0
99
70
99
55
99
15
20
25
30
70
75
Fig. 3 (a) Catalyst recycling and stability in four cycles of click
chemistry. (b) TEM image of Cu particles after the fourth run.
Fig. 1 (a) TEM image of the TRGO/Cu catalyst. (b) Copper particles size
and distribution. (c, d) HRTEM of TRGO/Cu(I) and calculation of the
lattice plane distance.
80
For kinetic investigation, NMR studies have been performed.
Therefore aliquots were taken at intervals from the reaction,
filtered to remove the catalyst and conversion of the reaction was
determined by the resonance of the CH2-moiety of benzyl azide
(educt) in comparison to the CH2-moiety of the click product
35
The catalytic activity of the TRGO/Cu catalyst was
investigated in the azide/alkyne click chemistry between benzyl
azide and phenyl acetylene under different reaction conditions in
high yields (up to 99%) as a model system. Also the model
reaction for commercial available Cu on charcoal (Cu/C)31 and
85 (Fig. S2, ESI†). Equation (1)32 was used for kinetic study and
calculation of k’=1.33 M-1.3.h-1, which was in the same range as
those reported previously by Finn et al.33 using CuSO4 and
sodium ascorbate as catalytic system.
40 Cu2O powder as a reference was done, but a very low click
conversion was obtained by these commercial catalysts (Table 1).
90 rate = k’ [alkyne]1.3±0.2[azide]1±0.2 (k’ = k[Cu]2)
Equation (1)
45
Furthermore Table 2 shows the different types of click reactions
with different alkynes for TRGO/Cu catalyst, and even with a
highly sterically hindered alkyne (entry 3) and linear hydroxy
95 alkynes (entry 4) a high conversion was achieved.
To evaluate the performance of synthesized TRGO/Cu catalyst
for bulk click chemistry including high molecular weight
molecules, melt rheology was carried out for trivalent azide- and
alkyne-functionalized polyisobutylenes (PIBs, Mn 5500 g∙mol-1
100 and 6900g∙mol-1 respectively) (Scheme 2), suitable for
crosslinking via “click” chemistry at room temperature, being
designed for self healing materials.9 For crosslinking experiments
an azide-functionalized trivalent PIB and an alkyne-
functionalized trivalent PIB were mixed with the TRGO/Cu
50
Fig. 2 (a) X-ray photoelectron spectroscopy of TRGO/Cu. (b) high
resolution XPS of elemental Cu.
55
The recyclability of the catalyst was studied using the same
alkyne/azide click model reactants, After four repetitions, a
reduction (~30%) of the reaction yield was observed (Fig. 3a),
2
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catalyst (2 mol%, 0.7 mass %) and crosslinking was investigated 25 the complete crosslinking of the polymer. In contrast, in
on a rheometer plate. Fig. S3a† shows the rheology result of the
“click” reaction of polymers at 20 C, where at the beginning of
the reaction, the sample clearly possesses liquid like character;
the loss modulus is higher than the storage modulus (G’’ > G’).
experiments with Cu/C and without catalyst no gel point was
observed even after 1000 minutes, which again supports the high
activity of the synthesized TRGO/Cu catalyst.
0
5
In summary, we have presented a facile and robust approach
After some time (520 minutes), both moduli increase and a 30 for the immobilization of Cu (I) catalyst onto graphene
crossover was observed (gel point), which is indicative of
nanosheets obtaining a highly active TRGO/Cu(I)-catalyst for the
Cu(I)-catalyzed alkyne-azide “click” cycloaddition reaction. The
TRGO/Cu-catalyst shows high stability at standard reaction
conditions (air) with excellent recyclability and reusability.
35 Furthermore, the TRGO/Cu catalyst shows excellent performance
for bulk click reactions as proven via melt-rheology. Thus, Cu
nanoparticles immobilized graphene nanosheets can function as
an effective catalyst in click chemistry under both solvent and
bulk conditions, including low and high molecular weight
40 molecules.
Table 2 Click reaction of benzyl azide with different alkynes with 2
10 mol% TRGO/Cu at 40°C after 48 h.
Entry
1
Alkyne
Azide
Conversion
99
2
3
90
97
We gratefully acknowledge the financial support from the EU-
IASS project: Improving the Aircraft Safety by Self-Healing
Structure and Protecting Nanofillers, European Union Seventh
45 Framework Programme (FP7/20072013), Grant Agreement No.:
313978 as well as the grant DFG-BI 1337/8-1 and DFG-Bi
1337/8-2 within the SPP 1568 (“Design and Generic Principles of
Self-Healing Materials).
4
5
90
80
Notes and references
50 a Institute of Chemistry, Chair of Macromolecular Chemistry, Faculty of
Natural Sciences II (Chemistry, Physics and Mathematics), Martin-Luther
University Halle-Wittenberg,von Danckelmann-Platz 4, Halle 06120
(Germany), E-mail: wolfgang.binder@chemie.uni-halle.de;
Fax: +49 345 55 27392
55 b Materia Nova Research Center, 1 Avenue Nicolas Copernic, B7000
Mons, Belgium
†Electronic Supplementary Information (ESI) available: Experimental
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