Nitrogen-doped graphene stabilized copper nanoparticles for Huisgen [3+2] cycloaddition "click" chemistry

Article abstract of DOI:10.1039/c9cc02057h

Nitrogen-doped reduced graphene oxide (NRGO) stabilized copper nanoparticles are designed to assist Cu(i)-catalyzed Huisgen [3+2] cycloaddition "click" chemistry (CuAAC). This study demonstrates a robust route for the synthesis of vastly dispersed heterog

Full text of DOI:10.1039/c9cc02057h

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DOI: 10.1039/C9CC02057H
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Nitrogen-doped graphene stabilized copper nanoparticles for
Huisgen [3+2] cycloaddition “click” chemistry
R.V.Siva Prasanna Sanka,a Balaji K,a Yves Leterrier,b Shyam Pandey, a Monika Srivastava,c Anurag
Received 00th January 20xx,
Accepted 00th January 20xx
Srivastava,c Wolfgang H. Binder,d Sravendra Rana,*a and Véronique Michaud*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Nitrogen-doped reduced graphene oxide (NRGO) stabilized copper distribution of particles over a large area. However, catalyst
nanoparticles are designed to assist Cu(I)-catalyzed Huisgen [3+2] recyclability and the necessity of an external ligand/base often
cycloaddition “click” chemistry (CuAAC). This study demonstrates a limits their applicability.
robust route for the synthesis of vastly dispersed heterogeneous
catalyst (NRGO/Cu2O), achieving CuAAC at low temperature
without any external additive (oxidizing/reducing agent) with high
stability and recyclability. Underlying mechanisms are analysed
using DFT calculations, confirming the experimental results.
Cu(I)-catalysed Huisgen [3+2] cycloaddition reaction between
alkynes and azides “click” reaction (CuAAC)1–5 has become a
valuable tool owing to its ability to be employed in broad range
of applications, in particular in materials, bioorganic, and
organic science.6–17 The click reactions are regioselective,
functional-group-tolerant, and can be performed under Scheme 1 Schematic illustration of click chemistry promoted by
different reaction conditions using wide range of solvents.8,18
nitrogen doped graphene supported copper nanoparticles.
However, after completion of reaction, the removal of copper
Owing to their attractive properties such as high stability,
electrical conductivity, porosity, and their performance for
electron capture and transport, graphene supported metal
nanoparticles/catalysts present outstanding catalytic activity
compared to other carbon/polymer supported catalysts. 30,31 To
tune the intrinsic reactivity of the metal particles, the doping of
heteroatom effectively alters the nature of the designed
catalytic systems. N-doping of graphene oxide (GO) is
cooperative, leading to a stronger interaction of metal particles
and thus preventing the agglomeration of nanoparticles.32
Further, the N-doping is not only helpful to reduce the surface
energy but also helpful to enhance the electrical conductivity as
well as generating active sites, thus demonstrating a promising
catalyst with improved catalytic performance.33,34 Only a few
articles report use of N-doped carbon nanomaterials supported
catalyst and their relevance in different applications,35,36
however, according to our knowledge the NRGO-Cu(I) (N-doped
reduced graphene oxide) catalysed “click” chemistry has not
been explored yet.
catalyst as well as used oxidizing/reducing agent remains a
challenge and thus hampering its utilization, in particular in
electronics and biological applications.19 The separation of
catalyst from the reaction mixture was strongly facilitated by
synthesizing the heterogeneous copper catalyst on
commercially available scaffolds, including resins, linear and
crosslinked polymers,20,21 charcoal, graphene,22–25 or other-
solid supports.26–29 The support materials are also helpful to
reduce the agglomeration as well as to achieve a uniform
a. University of Petroleum & Energy Studies (UPES), School of Engineering, Energy
Acres, Bidholi, Dehradun, 248007, India, Email: srana@ddn.upes.ac.in
b. Laboratory for Processing of Advanced Composites (LPAC), Institute of Materials,
Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne,
Switzerland, Email: veronique.michaud@epfl.ch
c. Advanced Materials Research Group, CNT Lab, ABV - Indian Institute of
Information Technology and Management, Gwalior-Madhya Pradesh, 474015,
India
d. 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)
Here we explore a robust approach to improve the
dispersion, stability, and recyclability of copper catalyst via
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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NRGO-Cu), where a histogram shows the particle size distribution for
immobilization of copper nanoparticles Cu(I) on N-doped
reduced graphene oxide nanosheets. The synthesized
heterogeneous catalyst (NRGO-Cu(I)) is used to perform click
reaction at room temperature without addition of any co-
catalyst (oxidizing/reducing agent).
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the NRGO-Cu particles (Figure 1c). A similar particle size was
DOI: 10.1039/C9CC02057H
observed for the TRGO/Cu(I). (for TEM image, see Fig. S1) A 7.35 x
10-7 and 7.16 x 10-7 mol mg-1 loading of Cu content for NRGO-Cu(I)
and TRGO-Cu(I), respectively, was determined via FAAS analysis.
Elemental analysis was also made for the prepared catalysts (Table
S1). XPS technique was performed to further evidence of the
chemical composition of the nanoconjugates including the oxidation
state of copper as well as copper interaction with doped nitrogen
(Fig. S2-4). The XPS peaks correspond to C 1s (285.1 eV), O 1s (531.6
eV) and Cu 2p were observed, where the high resolution XPS (Fig. 1d)
reveals the peak energy of Cu(I), confirmed by the presence of peak
satellites at 932.3 eV and 952.3 eV.39 Furthermore, the data confirms
the absence of Cu(II), as no peak satellites at 942 eV and 962 eV was
observed. A small associated peak observed at 932.7 eV (higher by
0.3 eV than that for N-free carbon; ((NRGO/Cu(I), Figure S4), could
be attributed to a stronger interaction of the Cu(I) with N-doped
RGO.36
Hummer’s method was applied for the preparation of graphene
oxide (GO).37 GO (100 mg) was dispersed in water (30 mL) by
ultrasonication. Subsequently, Copper acetate(II) (20 mg, 0,11 mmol)
was added and the mixture was vigorously sonicated for 5 min, the
mixture was stirred at room temperature overnight. After several
washings with water and acetone the Cu2+-GO was dried in vacuum
oven at 40 degree C. 100 mg Cu2+-GO was sonicated in ethanol
resulting a uniform suspension (thick suspension). 200 mg of
Melamine (2:1 ratio of N precursor to GO) is added to the above
suspension and sonicated (~ 15 minute) further to obtain a uniform
suspension. Ethanol from the resulting suspension was evaporated
slowly at room temperature under constant stirring to obtain a
powder residue. Residue has to be grinded and then dried. The dried
powder is heat treated in a tubular furnace under Ar atmosphere.
The temperature of the furnace was raised from room temperature
to 600 °C with a heating rate of 20 °C/minute, followed by holding
the temperature at 600 °C for 10 minutes. The furnace is cooled
down to room temperature under Ar atmosphere. The resulting
samples was washed with DI water and dried. (see Supporting
Information). For studying the impact of N-doping on click reaction,
pure thermally reduced GO (TRGO/Cu(I)) based conjugates were also
prepared as per our earlier reported method.38 X-ray photoelectron
spectroscopy (XPS), Transmission electron microscopy (TEM),
Elemental analysis, Flame atomic absorption spectroscopy (FAAS)
were performed to investigate the chemical composition, oxidation
state, and morphology of the prepared nanoconjugates.
After confirming the present form of copper (CuI) and
immobilized amount, the catalytic activity of the NRGO/Cu and
TRGO/Cu was investigated through a click model reaction between
phenyl acetylene and benzyl azide (Table 1). A 100% reaction yield
was achieved for the NTRGO-Cu(I) catalyst with only 2 mol% catalyst
loading in THF, whereas with similar amount loading only 70%
conversion was obtained for the TRGO-Cu(I) catalyst after 48 h at
room temperature. As a reference, click reaction was also performed
in presence of commercially available copper catalyst Cu2O powder
and Cu on charcoal (Cu/C), and in that case only very little conversion
was observed (Table 1).
Table 1: Performance of different Cu-catalysts: phenyl acetylene and
benzylazide in THF at room temperature
Entry Conditions
Yield (%)
0
1
Cu2O (2 mol%) in THF, 48 h, r.t.
2
3
4
5
6
7
Copper on charcoal (2 mol%), in THF, 48 h, r.t.
TRGO without copper, in THF, 48h, r.t.
TRGO/Cu (I) (2 mol%), in THF, 12h, r.t.
TRGO/Cu (I) (2 mol%), in THF, 48h, r.t.
NRGO/Cu (I) (2 mol%), in THF, 12h, r.t.
NRGO/Cu (I) (2 mol%), in THF, 48h, r.t.
0
0
3
70
17
100
As shown in Table 1, compared to TRGO/Cu(I) a higher reaction
rate was observed for NRGO/Cu(I) (after 12 h; Table 1, 17% and 3 %
for NRGO/Cu(I) and TRGO/Cu(I), respectively). The initially obtained
higher efficiency for NRGO/Cu(I) may be attributed to the formation
of coordination of N-doped rGO with Cu+ ions,40 including a high
binding energy of the Cu nanoparticles to N-doping carbon
support,32,35,41 The N-doping is also helpful to reduce the surface
energy as well as generating active sites, enhancing the catalytic
performance.33,34,42 Surprisingly, 70% and 100% reaction yield was
obtained after 48 h for TRGO/Cu(I) and NRGO/Cu(I), respectively. The
enhancement in catalytic activity with time could be due to
formation of N-ligands (triazole) as a source of autocatalysis, as
discussed earlier.17,43
The recyclability and reusability testing for the synthesized catalyst
(NRGO-Cu(I)) was carried out using the azide/alkyne click model
reaction under the optimized conditions. The NRGO-Cu(I) catalyst
was reused ten times and observed around 90% yield after 10th cycle,
whereas washing and recyclability of catalyst was performed under
open air environment, which strongly supports the stability and
recyclability for the synthesized N-doped heterogeneous catalysts.
However, almost 40% reduction was observed for the nitrogen free
Fig. 1 (a,b) TEM images of the NRGO/Cu(I) catalyst. (c) Copper particle size
and distribution. (d) XPS of Cu element.
Figure 1 a-b shows the TEM image of NRGO/Cu(I), displaying
uniformly dispersed copper nanoparticles onto the surface of N-
doped reduced graphene oxide. The TEM images were also found
helpful to observe the average copper nanoparticles size (~20 nm for
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TRGO-Cu(I) catalyst. TEM images were observed to check the been investigated for copper ion (Cu+) adsorption on three different
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morphology of the recycled catalyst.
sites: bridge (B), at the top of a carbon atom (C), and hollow (H).
DOI: 10.1039/C9CC02057H
Alkyne-PDMS-Alkyne + N3-PDMS-N3
Click Polymerization
NRGO/Cu(I)
TRGO/Cu(I)
Cu(PPh3)3F
W/O catalyst
50
100
150
200
250
Temperature (0C)
Fig. 3 DSC measurements for the bulk click reaction with different catalysts
at 5 0C/min heating rate.
The computed adsorption energies are -8.545eV, -8.537eV, and -
8.427 eV for B, C and H site respectively which indicates that the
bridge site is energetically the most favorable adsorption site on the
graphene sheet (Fig. 4(a,b), Fig. S7). It has been observed that on
interaction with Cu+ ion, fermi level shifts into conduction band (Fig.
4c) and peaks appeared at the fermi level in TDOS profile (Fig. S8)
which inferred that Cu+ ion adsorbed sheet is electron rich and may
act as a catalyst in CuAAC reaction as reported in earlier studies,47
whereas, Cu+ ion is adsorbed on NRGO with adsorption energy -8.77
eV which infer strong interaction between NRGO and Cu+ ions (Fig.
4b), validates our experimental studies. On adsorption of Cu+ ion, a
bond of length 2Å has been formed between carbon and the ion. The
analysis found that by doping with nitrogen, Fermi level shifts into
the conduction band which is in good agreement with earlier
studies;44 on adsorption of Cu+ ion an indirect band gap of ~0.29 eV
has been introduced (Fig. 4d; Fig. S9). In order to understand the high
reaction rate that was experimentally obtained, Mulliken charge
population has been calculated, showing that the charge transfer of
0.073 e takes place on N after adsorption of Cu+ ion; this could
explain the high reaction rate.
Fig. 2 (a,b) catalyst recycling within ten reaction cycles for NRGO/Cu(I) and
TRGO/Cu(I), respectively. (c,d) TEM image of Cu particles after 10 cycles for
NRGO/Cu(I) and TRGO/Cu(I), respectively.
A unchanged morphology of NRGO-Cu(I) was observed after 10
cycles (Fig. 2c), whereas agglomeration of Cu particles was observed
for TRGO-Cu(I) (Fig. 2d), which might be the key factor for the
reduction of catalytic activity for the synthesized catalyst
(TRGO/Cu(I)).38 Again, the high binding energy of Cu particles to N-
doping graphene might be the reason to prevent the agglomeration
of copper nanoparticles for nitrogen doped graphene oxide sheets.
Metal leaching for reaction product (after 5, and 10 cycles) was
studied by FAAS measurements including hot filtration; however, the
detectable amount of copper was too low for accurate detection.
To evaluate the performance of synthesized catalysts, bulk click
chemistry was also evaluated by differential scanning calorimetry
(DSC) (Fig. 3). For the formation of a covalent network, sufficient
molecular mobility is required. In order to keep diffusion high, PDMS
based liquid components (bivalent azide and alkyne with high
mobility; low molecular weights) were prepared (for NMR, Fig. S5-6).
Further studies have been carried out to evaluate the interaction
of cluster containing 3 Cu+ ions (CU-3) and 5 Cu+ ions (CU-5) (Fig.
4e,f). On adsorption of CU-3, the cluster shows physisorption with
adsorption energy -31.07 eV and binding distance 2.76 Å while for
adsorption of CU-5, physisorption with adsorption energy and
binding distance -59.31 eV and 2.79 Å respectively is found. Further,
the band structure and density of states have also been explored in
Fig. S10. It has been found that the band gap has reduced when
increasing the size of the cluster given in Table S2. Similar results
have been observed from density of state profile of the system as
well. Mulliken population has been calculated to understand the
reaction rate for CU-3 and CU-5, and it is found that .156 e and .094e
charge has been transferred on nitrogen respectively showing high
reaction rates.
0
DSC thermograms at 5 C/min for different catalysts are plotted in
Fig. 3 (1 mol% of catalyst per functional group). Thermal click
crosslinking (W/O catalyst) happened at a high temperature with a
0
0
0
Tonset at 110 C and a Tp at 173 C, whereas, a lower Tonset (at 45 C)
was observed for homogeneous commercial catalyst (Cu(PPh3)3F),
however, crosslinking reaction was not completed at a certain
temperature, which might be related to the inhomogeneous
dispersion of catalyst in PDMS matrix. The TRGO/Cu(I) catalyst
0
0
reduced the reaction temperatures to 50 C (Tonset) and 65 C (Tp),
respectively, whereas, a room temperature Tonset and 59 0C (Tp) was
observed for NRGO/Cu(I) catalyst. The DSC data again confirms the
better catalytic activity of NRGO/Cu(I) catalyst.
To further analyse the effect of N-doping on the click reaction,
Density Functional Theory (DFT) based calculations were performed
using Synopsis Atomistix Toolkit (ATK) virtual nanolab (VNL).44–46 The
aim was to quantify the interaction of Cu+ ion with the pristine RGO
as well as NRGO, and also to observe the impact of Cu+ ion cluster
size on the NRGO for CuAAC reaction. A 5x5 optimized graphene has
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In summary, we have developed a highly stable and recyclable
heterogeneous catalyst (NRGO/Cu(I)), which performs click reaction
under both solvent and bulk conditions. Due to an increase in
electron density on the nitrogen atom doping including the
formation of coordination of N-doped rGO with Cu+ ions, nitrogen
doped graphene supported copper particles demonstrate a higher
reaction yield at room temperature without adding any external
ligand/base. The incorporation of graphene based catalyst can also
be advantageous to enhance the physical, mechanical and
conductive properties of the cross-linked materials.
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We gratefully acknowledge the financial support from the Swiss
National Foundation Scientific Exchange Grant Switzerland
(IZSEZO_178681/1) as well as Science and Engineering Research
Board (SERB-DST), Government of India (Grant No.
ECR/2016/001355); WHB acknowledges a grant from the DFG within
the priority programme SPP 1568.
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Conflicts of interest
There are no conflicts to declare.
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