Copper-coordination polymer-controlled Cu@N-rGO and CuO@C nanoparticle formation: Reusable green catalyst for A3-coupling and nitroarene-reduction reactions

Article abstract of DOI:10.1039/c7dt02119d

The intriguing structural properties of coordination polymers (COPs), together with the huge variety of metal ions and organic linkers to choose from, make COPs potential precursors for fabricating carbon-encapsulated metal and metal oxide nanoparticles (NPs). Herein, we have studied the role of the COP structural assembly, prepared through making subtle changes to the ligand structure, on the formation of NPs in a carbon matrix. Cu-COPs (Cu-COP-1-Cu-COP-7), generated using different amino acid-based reduced Schiff base phenolic chelating ligands, exhibited crystalline structures with differing structural organization in the solid state. Interestingly, the calcination of Cu-COP-1 and Cu-COP-5 at 330 °C produced pure CuNPs, whereas Cu-COP-2, Cu-COP-3, Cu-COP-4 and Cu-COP-7 gave CuONPs encapsulated by carbon matrix. The calcination of Cu-COP-6 produced both CuNPs and CuONPs together in the carbon matrix. The formation of CuNPs and CuONPs in the carbon matrices was unambiguously confirmed by PXRD and XPS studies. The sizes and morphologies of the Cu/CuONPs were analyzed using HR-TEM and FE-SEM. BET studies revealed higher surface areas with small pores for the CuNPs encapsulated by carbon and lower surface areas with higher porosity for the CuONP-carbon matrix. Raman spectra revealed the formation of a nitrogen-doped reduced graphene oxide (N-rGO) carbon matrix in CuNPs-1. The N-rGO coverage and high surface area with small pores provided CuNPs-1 with good stability in strong acid (10 M H₂SO₄). Importantly, the fabricated N-rGO-encapsulated CuNPs-1 and carbon-covered CuONPs-4 nanocomposites were used as green catalysts in solvent-free neat A³-coupling and nitroarene-reduction reactions, respectively. The products were confirmed using ¹H-NMR spectra. The recovered CuNPs-1 and CuONPs-4 catalysts, after the completion of the reactions, also showed similar catalytic activity at up to five cycles, without significant loss of catalytic activity. Thus, the present studies demonstrate the influence of Cu-COP structural assembly on the formation of Cu/CuONPs as well as the carbon matrix, and open the possibility of fabricating functional nanomaterials from the vast number of available COPs with intriguing structural motifs.

Full text of DOI:10.1039/c7dt02119d

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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Journal Name
DOI: 10.1039/C7DT02119D
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Copper-Coordination Polymers Controlled Cu@N-rGO or CuO@C
Nanoparticles Formation: Reusable Green Catalyst for A-3 Coupling
and Nitroarene Reduction
Vadivel Vinod Kumar,a) Rajamani Rajmohan,a) Pothiappan Vairaprakash,a) Mariappan Mariappanb) and
5
Savarimuthu Philip Anthony*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The choice of metal ions and organic linkers together with intriguing structural assembly make
coordination polymers (COPs) as potential precursors for fabricating carbon encapsulated metal and
10 metal oxide nanoparticles (NPs). Herein, we have studied the role of COPss structural assembly that was
prepared by subtle change in the ligand structure on the formation of NPs in the carbon matrix. CuꢀCOPs
(CuꢀCOPsꢀ1 ꢀ 7) generated using different amino acid based reduced Schiff base phenolic chelating
ligands showed crystalline structure with different structural organization in the solid state. Interestingly,
calcination of CuꢀCOPꢀ1 and CuꢀCOPꢀ5 at 330 °C produced pure CuNPs whereas CuꢀCOPꢀ2, 3, 4 and 7
15 gave CuONPs encapsulated by carbon matrix. The calcination of CuꢀCOPꢀ6 produced both Cu and
CuONPs together in the carbon matrix. The formation of Cu and CuONPs in the carbon matrices were
unambiguously confirmed by PXRD and XPS studies. The size and morphologies of Cu/CuONPs were
analyzed using HRꢀTEM and FEꢀSEM. BET studies revealed higher surface area with small pores for
CuNPs encapsulated by carbon and lower surface area with higher porosity for CuOꢀcarbon matrix.
20 Raman spectra revealed the formation of nitrogen doped reduced graphene oxide (NꢀrGO) carbon matrix
in CuNPsꢀ1. The NꢀrGO coverage and high surface area with small pores provided CuNPsꢀ1 good
stability in strong acid (10 M H2SO4). Importantly, the fabricated NꢀrGO encapsulated CuNPsꢀ1 and
carbon covered CuONPsꢀ4 nanocomposites have been used as green catalyst in solvent free neat A3ꢀ
coupling and nitroarene reduction reactions, respectively. The products were confirmed by 1HꢀNMR
25 spectra. The recovered CuNPsꢀ1 and CuONPsꢀ4 catalysts after the completion of reactions have also
shown similar catalytic activity up to five cycles without significant lose of catalytic activity. Thus the
present studies demonstrated the influence of CuꢀCOPs structural assembly on the formation of
Cu/CuONPs as well as carbon matrix and open the possibility of fabricating functional nanomaterials
from vast available COPs with intriguing structural motifs.
45 as glucose and phenolꢀformaldehyde resin has often been used as
carbon source in pyrolysis/thermolysis method to encapsulate
metal and metal oxide NPs.14ꢀ16 Nꢀdoped carbon encapsulated
nickel NPs synthesized using inꢀsitu formed polymer from
Glycoluril and terephthaldehyde exhibited high acid resistant and
50 enhanced catalytic activity in the direct conversion of
nitrobenzene to aminophenol.17
Coordination polymers (COPs) with intriguing and tunable
network structure constructed using limitless choice of metal ions
and organic bridging ligands have been investigated extensively
55 for diverse applications such as heterogeneous catalysis, drug
delivery, gas storage, optical, sensing and separation.18,19 In
recent years, the use COPs as precursors for generating metal and
metal oxide NPs encapsulated carbon matrix or as templates for
structural reproduction received significant interest especially
60 after Xu group report of highly porous carbon fabrication using
30 Introduction
Fabricating nanocomposites particularly encapsulating metal and
metal oxide nanoparticles (NPs) by carbon matrices such as
graphene not only increases the stability and dispersion of NPs
but also showed improved catalytic activity and/or some new
35 properties owing to large surface area and synergistic effect of
nanomaterials and the carbon matrix.1ꢀ6 For example, NPs
inserted carbon framework has been utilized as CO2 capture
materials, anode materials for lithiumꢀion batteries and electroꢀ
catalysts for oxygen reduction reaction (ORR).6ꢀ10 Nitrogen
40 doped carbon supported transition metal nanoparticles including
Pt, Fe, Fe3O4 and CoNPs have been extensively studied for the
ORR.6ꢀ11 PdNPsꢀgraphene nanocomposites exhibited strong
catalytic activity for the Suzuki and SuzukieꢀMiyaura crossꢀ
coupling reaction.12,13 Organic polymeric capping molecules such
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COPꢀ5 by thermolysis.20ꢀ25 The regularly arranged metal nodes in
the COPs can be transformed into dispersed metal/metal oxide 60 green catalyst in neat A3ꢀcoupling reactions and carbon
CuNPsꢀ1@NꢀrGO nanocomposites have been demonstrated as
NPs inꢀsitu by thermolysis induced carbonization, while the
diverse organic bridging ligands tend to produce highly porous
carbon matrices.26ꢀ30 Xu group first reported the conversion of
COPꢀ5 to highly porous carbon in which the in situ formed
encapsulated CuONPsꢀ4 has been utilized for reducing various
organic nitroarene compounds. The product of the organic
reactions were isolated and confirmed by NMR spectra. The
reusability of the CuNPsꢀ1 and CuONPsꢀ4 catalysts has also been
5
metallic Zn from thermolysis of ZnO vaporized at 1000 °C.20 65 evaluated. Thus the present study indicates that the structural
Poddar et al showed the electrochemical reduction potential of
assembly of COPs might play significant role in the formation of
metal ions decides the formation of pure metal or metal oxide
metal or metal oxide NPs as well as carbon matrix.
10 NPs during thermolysis of COPs in N2 atmosphere.31 Copper NPs
(CuNPs) dispersed in carbon matrix synthesized from COPs have
been used as peroxidase mimic for colorimetric sensing of
ascorbic acid.32 Carbon encapsulated Cu/Cu2O NPs were
fabricated using Cu3(BTC)2 COPs after treating with resorcinol
15 and formaldehyde.33 The controlled pyrolysis of Ru loaded
Cu3(BTC)2 COPs under innert atmosphere produced highly
catalytically active Cu/Ru@C catalyst.34 The porous structure of
COPs has also been exploited for synthesizing morphology
preserved nanoporouscarbon matrix dispersed with metal and
20 metal oxide NPs.35 Thus thermolysis of COPs at high temperature
under inert atmosphere produced pure metals while in air resulted
in the formation of metal oxide NPs in the carbon matrix. Most of
the COPsꢀderived synthesis of nanocomposites focused only on
the porosity control of carbon matrix that encapsulate the inꢀsitu
25 formed metal/metal oxide NPs. However, the ability of COPs
structural assembly that can be easily modulated by subtle change
of organic ligand structure to control the formation of metal/metal
oxide NPs in the carbon matrix has never been explored.
Amino acid based reduced Schiff base molecules have been
30 shown to produce interesting COPs topology including helical
assembly with transition metal ions.36ꢀ39 Zinc and copper COPs of
alanine based phenolic chelating ligand exhibited rare
topochemical conversion of single crystal to single crystal
transformation.36,40 Further, the structural versatility of natural
35 amino acid has been utilized to modulate COPs assembly by
subtle change in the structure. Hence we envisage that the facile
coordination and subtle structural change of amino acid based
reduced Schiff base molecules mediated CuꢀCOPs assembly
could be potential candidates to investigate the role of COPs
40 structure on the formation of metal/metal oxide NPs in the carbon
matrix. Herein, we report the fabrication of carbon encapsulated
Cu/CuONPs nanocomposites depend on the structural assembly
of CuꢀCOPs synthesized using amino acid based reduced Schiff
base ligands by calcination at 330 °C under normal atmosphere
45 (Scheme 1). PXRD studies of CuꢀCOPs indicated crystalline
materials with different solid state structure. Interestingly, the
calcination of CuꢀCOPꢀ1 and CuꢀCOPꢀ5 produced CuNPs
encapsulated in the carbon matrix whereas CuꢀCOPꢀ2, 3, 4 and 7
produced CuONPs in the carbon matrix. CuꢀCOPꢀ6 produced
50 both Cu and CuONPs in the carbon matrix. The structure, phase
purity, size and morphologies of formed Cu/CuONPs in the
carbon matrix were confirmed by PXRD, XPS, HRꢀTEM and FEꢀ
SEM studies. BET studies revealed higher surface area with small
pores for CuNPs encapsulated carbon whereas lower surface area
55 with higher porosity for CuONPs. Raman spectra studies revealed
the formation of NꢀrGO matrix in CuNPsꢀ1. The higher surface
area with smaller pores along with NꢀrGO encapsulation offered
CuNPs enhanced stability in strong acid (H2SO4). Importantly,
Materials and methods
Cu(CH3COO)2.2H2O, amino acids, salicylaldehyde, NaOH,
70 NaBH4 and solvents were purchased from Merck India and used
as received. Nitroarenes, phenylacetylene, substituted aromatic
aldehyde and amines were obtained from SigmaꢀAldrich.
Nitroarene compounds, 1, 2, 7 and 8, was synthesized by
following the reported procedure.41,42 Amino acid based reduced
75 Schiff base phenolic chelating ligands and CuꢀCOPs were
prepared according to the reported procedure.36 Typically, Cuꢀ
COPs was prepared by dissolving reduced Schiff base phenolic
chelating ligand (1.0 mmol) in water (10 mL) in presence of
LiOH (1.0 mmol) and adding copper acetate dihydrate (1.0
80 mmol) under vigorous stirring at room temperature. Dark green to
cyan colored precipitate depend on the ligand structure was
formed within 30 min to 1 h. The precipitate was filtered, washed
with water followed by ethanol and dried. CuꢀCOPsꢀ1 to Cuꢀ
COPsꢀ7 (1 g) were then transferred to seven different small silica
85 crucibles and calcinated at 330 °C for 12h in high temperature
furnace that resulted in the formation of Cu/CuONPs
encapsulated in the carbon matrix.
Cu and CuONPs Characterization
Cu and CuO NPs size and morphologies were characterized using
90 high resolution transmission electron microscopy (HRTEM,
JEOL, JEMꢀ2100) and field emission scanning electron
microscope (FEꢀSEM, Q400 SEM) equipped with an energyꢀ
dispersive Xꢀray spectroscopy analyzer. The phase and structure
of the nanocomposites were analyzed by powder Xꢀray
95 diffraction (XRD, D8 Advanced, Bruker). Raman spectra were
recorded using Renishaw laser Raman Spectrometer (Invia 11),
633 nm laser, 1 cm−1 spectral resolution. Functional groups of
CuꢀCOPs and calcined Cu/CuONPs were explored by FTIR
spectroscopy (Varian 3100 FTꢀIR spectrometer). The samples
100 were grounded with KBr and then pressed into pellets for FTIR
measurements. Chemical state of elements in Cu and CuONPs
were analyzed by Xꢀray photoelectron spectroscopy (Thermo
Scientific KꢀAlpha). Surface area and porosity of Cu and
CuONPs encapsulated in carbon matrices were analyzed using
105 nitrogen adsorption desorption isotherm measurements (BET
surface area analyzerꢀ ASAP 2020).
Catalytic studies of CuNPs
A mixture of aldehyde (0.75 mmol, 1.5 eq), amine (0.75 mmol,
1.5 eq), phenylacetylene (0.5 mmol, 1.0 eq) and CuNPsꢀcarbon
110 nanocomposites (5 mg) was stirred at 70 °C for 8h. After 8h,
reaction mixture was brought to room temperature and diluted
with dichloromethane (5 mL), centrifuged and the catalyst was
recovered as residual fraction. The supernatant was purified by
2
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column chromatography (ethyl acetateꢀhexane mixture). The
(200) and (220) crystalline planes of faceꢀcenteredꢀcubic
40 copper (JCPDS#03ꢀ1018) and indicated the formation of
CuNPs (Fig. 1a). No additional peaks relating to CuO or Cu2O
were observed. Whereas PXRD pattern of the black powder
obtained from CuꢀCOPꢀ2, 3, 4 and 7 can be indexed to
monoclinic phase of CuONPs (JCPDS#45ꢀ0937). The peaks
45 at 2θ 35.47, 38.67, 48.52, 57.92, 61.37, 65.97 and 68.02° are
assigned to (002), (111), (ꢀ202), (202) (ꢀ113), (022) and (ꢀ
220) plans of monoclinic phase (Fig. 1b). Surprisingly, black
powder of CuꢀCOPꢀ6 showed peaks corresponds to both Cu
and CuONPs (Fig. 1a). Along with CuNPs peaks at 2θ 43.32,
50 50.47 and 74.17° CuONPs peaks at 2θ 35.47, 38.67° were
also observed. According to our knowledge, this is the first time
CuꢀCOPs produced Cu/CuONPs by calcination under mild
condition in air (Table 1).
product was confirmed by 1HꢀNMR spectra.
Catalytic studies of CuONPs
Nitroarene compound (1 mmol) was dissolved in ethanol (10 mL)
and CuONPsꢀ4 (0.05 mmol) was added. The solution was
vigorously stirred at room temperature for 15 min. To this
mixture, NaBH4 (1.5 mmol) was added in portionꢀwise over a
period of 30 min. The resultant mixture was stirred for another 2h
at room temperature. The yellow colour of nitroarene was slowly
10 disappeared that indicates the reduction of nitro group. After
completion of reaction, reaction mixture was centrifuged and the
catalyst was recovered as residual fraction. The supernatant
ethanol solution was removed using rotary evaporator. The
product was confirmed by 1HꢀNMR spectra.
5
15 Result and Discussion
CuꢀCOPꢀ1 to CuCOPꢀ7 were prepared by stirring aqueous
solution of copper acetate and reduced Schiff base ligands in
presence of base according to the reported procedure (Scheme
1).36 The change of organic ligand structure produced Cuꢀ
20 COPs powder with different color (green, light green and
cyan). PXRD studies of CuꢀCOPs clearly indicated the
formation of crystalline materials with different structural
assembly (Fig. S1). FTIR spectra showed clear sharp peaks
for organic functional groups of CuꢀCOPs (Fig. S2a). The
25 calcination of CuꢀCOPs at 330 °C for 12 h in presence of air
produced black colored precipitate (Scheme 1). FTIR spectra
showed the disappearance of sharp organic functional group
peaks of CuꢀCOPs after calcination that suggest the
conversion of organic molecules to carbon at higher
30 temperature (Fig. S2b). PXRD studies revealed the conversion
of copper ions of the CuꢀCOPs into Cu/CuONPs inside the
carbon matrix depend on the ligand structure (Fig. 1).
Interestingly, the calcined black powder obtained from Cuꢀ
COPꢀ1 and CuꢀCOPꢀ5 showed peaks at 2θ 43.32, 50.47 and
Fig. 1. PXRD patterns of calcined CuꢀCOPs black powders.
55 Table 1. Cu/CuO NPs synthesis from CuꢀCOPs.
S. No Precursor
Reaction condition
400 oC, N2, 2 h
Product
CuNPs@C
Ref
32
1
Cu3(BTC)2
900 oC, N2, 1 h
900 oC, Air, 1 h
800 °C, 3 h, Ar atm.
Cu3(BTC)2 + resorcinol +
HCHO. 600 °C, 4 h, N2 atm.
CuNPs@C
[Cu3(TMA)2
(H2O)3]n
Cu3(BTC)2
2
31
CuONPs@C
Cu/Ru@C
3
4
34
33
Cu3(BTC)2
Cu/Cu2O@C
35 Scheme 1. Preparation of CuꢀCOPs using amino acid based reduced
Schiff base ligands and subsequent conversion to Cu/CuONPs
encapsulated in carbon matrices by calcination.
CuNPs@Nꢀ
rGO or
CuO@C
CuꢀCOPs
(1ꢀ7)
5
330 oC, 12 h, Air
This Work
74.17° that correspond to the diffractions from the (111),
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The formation of Cu and CuONPs in the carbon matrix has
also been confirmed by Xꢀray photoelectron spectroscopy (XPS).
form of copper. CuONPs obtained from CuꢀCOPꢀ3, CuꢀCOPꢀ
4 and CuꢀCOPꢀ7 have also showed similar XPS pattern for
CuNPsꢀ1 showed Cu 2p3/2 peak at 931.23 eV that can be 30 Cu2+, C1s and O1s region (Fig. S7ꢀ9). Cu/CuONPsꢀ6 that
attributed to zeroꢀvalence states of Cu (Fig. 2a).43 The peak at
951.13 eV assigned to Cu 2p1/2. The absence of Cu(II) peak at
934 eV as well as two satellites peak around 945 eV further
confirmed the formation of pure metallic CuNPs.44 Similarly
showed presence of both pure Cu and CuONPs by PXRD
exhibited XPS peaks at 934.08 and 954.08 eV including
satellites peaks at 942.28 and 944.58 eV for oxidized form of
copper (Fig. S10). The C1s signal appeared at 285.38, 286.58
5
CuNPsꢀ5 also exhibited zero valance Cu peak at 930.96 eV 35 and 289.18 eV whereas O1s signal was observed at 530.0 and
(2p3/2) and 951.16 eV (2p1/2) (Fig. S3). The high resolution signal 531.62 eV.
The size and morphology of Cu/CuONPs were investigated
10 of C1s region of CuNPsꢀ1 showed peaks at 282.88, 284.88,
285.08 and 289.78 that can be assigned to C–C/C=C, C–OH/C–
O, C=O and C=O–OH groups of carbon matrix (Fig. S4a).
using high resolution transmission electron microscopy (HRꢀ
TEM). CuNPsꢀ1 and CuNPsꢀ2 showed dispersion smaller CuNPs
CuNPsꢀ5 also showed similar signals in the C1s region (Fig. 40 in the spherical carbon matrix (Fig. 3). The magnified images
S4b). The O1s spectra were deconvoluted to two peaks at 530.0
15 and 531.9 eV for CuNPsꢀ1 and 529.96 and 532.96 eV for CuNPsꢀ
5, Fig. S5). The XPS spectra of CuONPsꢀ2 showed two peaks,
one at 939.98 and other at 953.98 eV, with 20.0 eV splitting that
clearly showed the encapsulation of crystalline CuNPs in the
carbon matrix (Fig. 3b, d). CuNPs size in the carbon matrices
varied between 2ꢀ5 nm and size of carbon matrix ranged from 50
to 100 nm. Interestingly, Cu/CuONPsꢀ6 exhibited clear spherical
could be assigned to Cu(2p3/2) and Cu(2p1/2) and attributed to the 45 amorphous carbon nanoparticles encapsulated Cu and CuONPs
Cu2+ oxidation state (Fig. 2b).45 Further the appearance of
20 shakeup satellites centered at 941.78 and 943.18 eV indicate
(Fig. 4). The high magnification confirmed the formation of
crystalline NPs (Fig. 4d). Similarly, CuONPsꢀ2, 3, 4 and 7 also
the showed formation of crystalline NPs in the carbon matrix
(Fig. 5,6). The size of the CuONPs are relatively bigger (10ꢀ15
50 nm) compared to CuNPsꢀ1. FEꢀSEM image of CuNPsꢀ1
showed micron sized blocks whereas CuONPsꢀ4 exhibited
spherical NPs (Fig. S11). EDX spectra also confirmed the
presence of Cu in both CuNPs and CuONPs samples (Fig.
S12). It is noted that both the samples revealed presence of O,
55 C and N that could be from carbon matrices.
Fig. 3. HRꢀTEM images of (a,b) CuNPsꢀ1 and (c,d) CuNPsꢀ5
encapsulated in the carbon matrix.
The formation of Cu/CuONPs encapsulated in the carbon
matrices from CuꢀCOPs might be explained based on the
60 subtle structural change of organic ligand that resulted in the
formation of different network topologies in the solid state.36ꢀ
Fig. 2. High resolution XPS spectra of Cu2p (a) CuNPsꢀ1 and (b)
CuONPsꢀ2 encapsulated in the carbon matrix.
the oxidized form of copper. The C1s spectra showed peaks at
284.58 and 288.38 eV (Fig. S6a). The O1s binding energy
25 peaks are appeared at 530.17, 532.77 and 535.00 eV (Fig.
S6b). Thus XPS spectra of CuNPs and CuONPs clearly
showed different peak for zero valence state and oxidized
40
Single crystal structures of CuꢀCOPs generated from
alanine, tryptophan and tyrosine based reduced Schiff base
phenolic chelating ligand showed a similar coordination
65 networks (Fig. S13a,b).36,39 Cu adopted distorted square
4
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pyramidal geometry via phenolate, carboxylate and nitrogen
coordination. The apical position was occupied by water
carbon. The copper and oxygen content in CuONPs matched
1:1 ratio except CuONPsꢀ2. The slightly higher percentage of
molecule and carbonyl oxygen of neighbouring molecule that 40 oxygen could be from the carbon matrix. Similarly, nitrogen
leads to 1D polymer. All three COPs showed helical structural
formation in the crystal lattice. The inclusion of two water
molecules in the crystal lattice was the only difference
exhibited by copperꢀtyrosine based phenolic chelating ligand
present in CuNPs samples is relatively higher than
CuONPssamples. It is noted that all CuꢀCOPs formed with 1:1
molar ratio of metal and ligand. Alanine based phenolic
reduced Schiff base ligand consist of less carbon mass
5
based COPs (Fig. S13c). The COPs further supported by Nꢀ 45 compared to all other ligands used for CuꢀCOPs. However,
H…O hydrogen bonds in the crystal lattice. In contrast, Cu
CuꢀCOPꢀ1 produced CuNPsꢀ1 showed higher carbon
10 with histidine based phenolic chelating ligand produced
tetrameric cage structure (Fig. 13d).39 The cage was built by
four copper and four ligand molecules. Copper displayed
square pyramidal geometry via coordinating phenolate,
carboxylate, amine nitrogen and histidine nitrogen from
15 neighbouring ligand. The apical position was occupied by
pyridine molecule. The cage is further linked via NꢀH…O
hydrogen bonds in the crystal lattice. Single crystal structures
for other ligands based CuꢀCOPs were not reported. Our
attempts to grow single crystals of CuꢀCOPs with other
20 ligands have also not been successful. But PXRD pattern of
CuꢀCOPs showed crystalline materials with different
molecular packing in the solid state. Hence, it was concluded
that helical structural arrangement of CuꢀCOPꢀ1, CuꢀCOPꢀ5
and CuꢀCOPꢀ6 from alanine, tryptophan and tyrosine based
25 ligands might lead to the formation of CuNPs whereas nonꢀ
helical arrangement in CuꢀCOPꢀ7 produced CuONPs.
However, structural studies of other COPs (CuꢀCOPsꢀ2ꢀ4) are
required to further substantiate the conclusion. Thus, subtle
change of ligand structure modulated the CuꢀCOPs assembly
Fig. 5. HRꢀTEM images of (a,b) CuONPsꢀ2 and (c,d) CuONPsꢀ3
encapsulated in the carbon matrix.
30 Fig. 4. HRꢀTEM images of (aꢀd) CuꢀCuONPsꢀ6 encapsulated in the
carbon matrix.
that influenced on the formation Cu/CuONPs. Further, atomic
weight percentage of Cu/CuONPs obtained from XPS
analysis has also revealed an interesting trend (Table S1).
35 CuNPsꢀ1 and CuNPsꢀ5 showed high content of carbon and
low content of copper whereas CuONPs samples showed
higher content of copper, oxygen and relatively low amount of
Fig. 6. HRꢀTEM images of (a,b) CuONPsꢀ4 and (c,d) CuONPsꢀ7
50 encapsulated in the carbon matrix.
compared to CuꢀCOPꢀ2, 3, 4, 6 and 7. Raman spectra of
CuNPsꢀ1 showed an intense Gꢀband at 1585 cmꢀ1,
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corresponding to the sp2ꢀhybdridized carbon atoms in the
hexagonal framework (Fig. 7).46 The Dꢀband (sp3-carbon)
Cu/CuONPs encapsulated in the carbon matrix provided
opportunity to use as reusable catalyst for organic chemical
corresponds to the outꢀofꢀplane breathing mode of the sp2 carbons 45 reactions. Propargylamines are versatile building block in
appeared at 1339 cmꢀ1 that indicates the presence of defects due
to oxidation.47 The relatively low intensity of D band compared
to G band suggest the formation of NꢀrGO with less defects in the
structure.48 In contrast, Raman spectra of CuONPsꢀ4 did not
natural product synthesis,49,50 key intermediates in the
synthesis of biologically active compounds51ꢀ53 and precursors
in the synthesis of nitrogen containing heterocyclic
compound.54 The syntheses of propargylamines are broadly
5
show any clear peak. Thus the solid state structural assembly 50 categorized into three methods namely (i) traditional method;
of COPs influenced on the transformation of CuꢀCOPs to
amination of propargylic halides, propargylic phosphates or
propargylic triflates, using stoichiometric amounts of highly
moisture sensitive reagents under strictly controlled reaction
conditions,55,56 (ii) classical method; from acetylene and imine
10 Cu/CuONPs as well as carbon matrix. However more studies
with COPs that differ in the solid state structure are required
to establish the complete mechanism of transformation.
Interestingly, carbon encapsulated CuNPs (CuNPsꢀ1 and 55 exploiting high acidity of
a
terminal alkyne using
CuNPsꢀ5) exhibited good stability in strong acids such as
15 concentrated H2SO4 (10 M). It did not produce the typical blue
color of CuSO4 even after standing 2 h in H2SO4. The PXRD
stoichiometric quantities of moisture sensitive organometallic
compounds such as butyllithium57 and (iii) transition metal
catalyzed multiꢀcomponent coupling reaction (MCR) of an
aldehyde, an alkyne and an amine (A3 coupling).58 Transition
60 metal catalyzed A3 coupling reactions to access
propargylamines are superior than other method due to (i) use
of catalytic amount of metal catalysts, (ii) mild conditions,
(iii) very high atom economy of 98% as only byproduct is
water and (iv) very low Eꢀfactor. Although various transition
65 metal catalysts have been used for A3 coupling reactions,59ꢀ62,
NꢀrGO encapsulated inexpensive CuNPs catalyst under
solvent free condition has never been explored.63ꢀ65
The catalytic behavior of the CuNPsꢀ1 was explored in A3
coupling reaction to access various propargylamines (Scheme. 2
70 and 3). In our initial foray, a mixture of anisaldehyde, phenyl
acetylene and morpholine was treated with catalytic amount of
CuNPsꢀ1 at 30 °C. Even a prolonged reaction time (> 48 h)
resulted only in trace amount of the corresponding
propargylamine (Table 1, entry 1). In an optimization process of
75 reaction at various temperature (40, 50, 60 and 70 °C), better
conversion was obtained at 70 °C and the corresponding
Fig. 7. Raman spectra of carbon encapsulated CuNPsꢀ1 and CuONPsꢀ4.
pattern of acid washed CuNPsꢀ1 further confirmed the stability
20 of CuNPs in strongly acidic condition. The PXRD pattern of
H2SO4 immersed CuNPsꢀ1 clearly showed typical CuNPs pattern
(Fig. S14). However, addition of H2SO4 into CuONPsꢀ4 produced
immediate blue color that suggests the formation of CuSO4. To
get the insight on the high stability of CuNPs in strongly acidic
25 condition, we have studied the porous properties of CuNPsꢀ1 and
CuONPsꢀ4 by the nitrogen isotherm measurements. CuNPsꢀ1 and
CuONPꢀ4 was chosen as representative sample. CuNPsꢀ1 posses
a BET specific surface area (SBET) of 4.0 m2/g whereas
CuONPsꢀ4 showed less than 1.0 m2/g specific surface area.
30 Further, BET analysis of CuNPsꢀ1 revealed relatively small pore
size (10.04 nm) compared to CuONPsꢀ4 (622.97 nm). It is noted
that SEM images of CuNPsꢀ1 showed formation of carbon sheet
that completely covered CuNPs whereas spherical NPs was
observed for CuONPs. Thus comparatively small pore size and
35 complete coverage of CuNPs by NꢀrGO might be responsible for
enhanced stability in strong acid compared to CuONPs with low
content of carbon.
80
85
90
The use of metal and metal oxide NPs especially
encapsulated by solid matrix such as silica and carbon as
40 catalyst for organic chemical transformation received
increased interest in recent years because of their enhanced
surface area, catalytic activity and stability. The fabrication of
Scheme 2. Molecular structures of various propargylamines synthesized
95 using CuNPsꢀ1 catalyst in the green reaction.
6
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formation of product in 95% yield, in a CuNPsꢀ1 catalyzed
reaction of 4ꢀchlorbenzaldehdye with piperidine and
phenylacetylene (Table 2, entry 9). In all experiments, the
catalyst was recovered by following a simple protocol; after
45 completion of reaction, the reaction mixture was diluted with
dichloromethane (5 mL), centrifuged for 15 min and catalyst was
recovered as residual fraction. The recovered catalyst showed
similar catalytic activity without any deterioration in yields up to
Scheme 3. General scheme of CuNPsꢀ1 catalysed A3 reactions.
5
Table 2. Solvent free synthesis of propargylamines
Table 3. Comparison of nanocatalysts efficiency for A3 coupling reaction.
Amine
Entry
Ar =
Yield (%)f
R1
R2
S. No
1
Catalyst
CuNPs@starch
microparticles
Reaction condition
THF, 60 oC, 20 h
Product
94
Ref
66
1a
2
3
4
5
6
7
8
4ꢀMeOꢀC6H4
4ꢀMeOꢀC6H4
4ꢀMeꢀC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
4ꢀClꢀC6H4
4ꢀClꢀC6H4
4ꢀClꢀC6H4
4ꢀClꢀC6H4
4ꢀClꢀC6H4
ꢀ(CH2)2O(CH2)2ꢀ
ꢀ(CH2)2O(CH2)2ꢀ
ꢀ(CH2)2O(CH2)2ꢀ
ꢀ(CH2)2O(CH2)2ꢀ
Trace
60
62
65
67
70
76
80
95
93
87
89
84
2
3
CuNPs@PMO
CHCl3, 60 oC, 24 h
Toluene, 110 °C, 6 h
97
96
67
68
CuNPs@COPꢀ
5ꢀC
C6H5
H
H
4ꢀBrꢀC6H4
ꢀ(CH2)2NMe(CH2)2ꢀ
ꢀ(CH2)5ꢀ
Cu2O@CuFe2O
4
90 oC, 1 h
96
69
4
Fe3O4@Cu(I)
Pybox
CuNPsꢀ1
5
6
CH2Cl2, 35 oC, 36 h, Ar
70 oC, 8 h
91
95
70
9
ꢀ(CH2)5ꢀ
ꢀ(CH2)5ꢀ
ꢀ(CH2)5ꢀ
ꢀ(CH2)5ꢀ
10b
11c
12d
13e
This Work
50
5 cycles tested (Table 2, entries 9ꢀ13). Further the catalytic
activity of the catalyst was explored in the synthesis of
propargylamine with quaternary carbon. Due to the steric
crowding around the quaternary carbon, the product was
ꢀ(CH2)5ꢀ
a
e
b
c
d
Reaction at 30 °C. Recycled Catalyst (II cycle). Recycled Catalyst (III cycle). Recycled Catalyst (IV cycle).
f
Recycled Catalyst (V cycle). isolated yield.
55 obtained in lower yield of 56% (Scheme 2). The products of
1
all the reaction were confirmed by HꢀNMR (Fig. S15). It is
noted that A3 coupling reactions have already been
demosntrated using Cu, Cu2O and Fe3O4 with copper complex
(Table 3).66ꢀ70 However, most of the catalysts used organic
60 solvents as reaction medium. In contrast, solvent free green
method has been demonstrated using CuNPsꢀ1 and
encapsulation of CuNPs by NꢀrGO provided good stability
and reusability.
10 propargylamine was obtained in 60 % yield (Table 2, entry 2).
The tentative mechanism of A3 coupling mediated by CuNPsꢀ1 is
described in Scheme S1. The adsorption of acetylene on CuNPsꢀ1
that was covered by NꢀrGO might increased the labile nature of
CꢀH bond and facilitated deprotonation by weak base. Further,
15 iminium ion adsorption, electron transfer and CꢀC bond
formation results in the formation of products.
Similarly, nitroarene reduction using CuONPs catalyst has
65 attracted considerable interest because of their low cost and
easy fabrication with controlled size and shape compared to
the presently employed costly PdNPs catalysts for organic
chemical reactions.71,72 The conversion of aromatic nitro
compounds to their corresponding amines has also been an
70 active field of research in industry since amines serve as
intermediates for several important pharmaceuticals, polymers
and herbicides.73 The catalytic activity of the CuONPsꢀ4 was
investigated for nitroarene reduction reaction for several
nitroaromatic compounds (Scheme 4, Table 2). CuONPsꢀ4
The reactivities of other aldehydes (pꢀtolualdehyde and
benzaldehyde) were tested at this temperature (Table 2, entries 3
and 4). Among various aldehydes, better conversion was
20 obtained with benzaldehyde and the product was obtained in 65%
yield (Table 2, entry 4). Further, the reaction was tested for
various aromatic, aliphatic, 1° and 2° amines. In a reaction with
morpholine, corresponding propargylamine was obtained in 65 %
yield (Table 2, entries 4). Aromatic amines such as aniline and 4ꢀ
25 bromoaniline showed similar reactivity and the corresponding
propargylamines were obtained in 65% and 70% yields 75 was chosen as representative example to demonstrate the
catalytic activity. The nitroarene reductions were performed
by adding catalytic amount of CuONPsꢀ4 powder into
methanol solution of nitroarene and NaBH4 at room
temperature under vigorous stirring. The disappearance of
80 yellow color of nitroarene solution in 2h indicated the
conversion of nitro to amine. The product (66 to 96 % of
benzaldehyde) were tested at this temperature (Table 2, entries 3
and 4). Among various aldehydes, better conversion was
obtained with benzaldehyde and the product was obtained in 65%
30 yield (Table 2, entry 4). Further, the reaction was tested for
various aromatic, aliphatic, 1° and 2° amines. In a reaction with
morpholine, corresponding propargylamine was obtained in 65 %
yield (Table 2, entries 4). Aromatic amines such as aniline and 4ꢀ
bromoaniline showed similar reactivity and the corresponding
35 propargylamines were obtained in 65% and 70% yields
respectively (Table 2, entries 5 and 6). Nꢀmethylpiperazine and
piperidine showed enhanced reactivity and the corresponding
propargylamines was obtained up to 80% yield (Table 2, entries 7
and 8). Electron withdrawing substituent present in aldehyde
40 facilitated the better conversion to product, as we observed the
yield) was obtained as solids after evaporating the solvents
1
and structure was confirmed HꢀNMR spectra (Fig. S16). It is
noted that NaBH4 alone did not exhibit any reduction of
85 nitroarene even standing at room temperature for 24h.
CuONPsꢀ4 catalyst was recovered after completion of the
reaction by centrifuging. The recovered catalyst exhibited
similar catalytic activity in the nitroarene reduction up to four
more cycle of reactions (Table 4, entry 9). In a typical
ꢀ
90 mechanism, the BH4 donor transfers electron to nitroarene
acceptor after both adsorbed on the surface of CuONPs and
facilitated the conversion of nitro to amines.74 Thus CuONPs,
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C7DT02119D
good yield in A3 coupling reactions. This could be due to the
difference in the copper content and covering matrix of NPs.
XPS analysis revealed only 1.18 % copper in CuNPsꢀ1 with
graphene coverage where 84 % percentage of CuO was
25 present in CuONPsꢀ4 (Table S1).
Table 5. Comparison of nanocatalyst efficiency for nitroarene coupling
reaction.
S. No
1
Catalyst
Reaction condition
Product
95
Ref
74
Cu/SiO2@NiFe2 NaBH4, H2O : MeOH (1:1)
O4
RT, 40 min
NaBH4, EtOH: H2O (1:2)
50 °C, 1.5 h
2
3
PdCu/graphene
95
85
75
76
HCOONH4, Ethylene glycol
120 °C, 12 h
CuNPs
CuBr2, NaBH4, EtOH, RT, 5
h
NaBH4, EtOH, 2h, RT
4
5
CuNPs
85
90
77
CuO@C
This Work
Conclusion
30 In conclusion, we have demonstrated the influence of Cuꢀ
COPs structural assembly and organic ligand structure on the
formation of Cu/CuONPs encapsulated in the carbon matrix
by calcination at 330 °C. PXRD pattern of CuꢀCOPs
confirmed the crystallinity with different structural assembly
35 in the solid state. Interestingly calcination of CuꢀCOPꢀ1 and
CuꢀCOPꢀ5 produced pure CuNPs without using any inert
atmosphere. CuꢀCOPꢀ2, 3, 4 and 7 produced CuONPs in the
carbon matrix. In contrast, calcination of CuꢀCOPꢀ6 resulted
in the formation of Cu and CuONPs together in the carbon
40 matrix. The formation of Cu and CuONPs in the carbon
matrices were confirmed by PXRD and XPS studies. The size
and morphologies were analyzed by HRꢀTEM and FEꢀSEM.
Raman spectra of CuNPsꢀ1 clearly indicated the formation of
NꢀrGO carbon matrix. BET analysis showed higher surface
45 area with small pores for NꢀrGO encapsulated CuNPsꢀ1 and
lower surface area with higher porosity for carbon
encapsulated CuONPsꢀ4. The encapsulation of CuNPs by Nꢀ
rGO with higher surface area and smaller pores leads to good
stability in strong acid. The fabricated CuNPsꢀ1 and CuONPsꢀ
50 4 nanocomposites have been used as green catalyst in neat
A3ꢀcoupling reactions and nitroarene reductions. The product
Scheme 4. (a) Conversion of nitroarene to nitroamine by CuONPsꢀ4
catalyst and (b) structure of different nitroarene compounds explored for
the nitro reductions.
5
Table 4. Nitroarene reductions by CuONPsꢀ4.
Nitro
arene
Yield
(%)a
Nitro
arene
Yield
(%)a
Nitro
arene
Yield
(%)a
1
2
3
4
66
70
75
80
6
7
68
75
90
88
11
12
13
14
95
96
86
89
8
1
9b
was isolated and confirmed by HꢀNMR. Both CuNPsꢀ1 and
CuONPsꢀ4 catalyst has been recovered after completing of the
reactions and explored the reusability up to five times that
5
82
10
90
15
94
aIsolated yield. bRecycled CuONPs-4 catalyst was used for four more 55 showed similar catalytic activity. Thus carbon encapsulated
cycle with 9 (86-84 % yield). Entry 12-15 was confirmed by comparing
with commercially available corresponding amine.
Cu/CuONPs has been fabricated by makes use of CuꢀCOPs
structural assembly and employed as green catalyst for A3
coupling and nitroarene reduction reactions.
which are easily accessible from abundantly available copper
10 salts, could be a potential catalyst for replacing costly Pd
catalyst. CuNPs catalyst including PdCu bimetallic catalyst
have been previously reported for nitroarene reduction that
yielded good conversion (Table 5).75ꢀ78 Although, CuONPs
have also been reported for nitroarene reduction, it is mostly
15 demonstrated using absorption studies without isolating the
product. The nitroarene reduced product using CuONPsꢀ4
catalyst has been isolated using column chromatography.
Further carbon coverage of CuONPs offered reusable catalyst.
It is noted that CuNPsꢀ1 did not show significant conversion
20 in nitroarene reduction. Similarly, CuONPsꢀ4 did not produce
Acknowledgements
60 Financial support from the Nanomission, Department of Science
and Technology, New Delhi, India (DSTꢀNanomission scheme
no. SR/NM/NSꢀ1053/2015) is acknowledged with gratitude. P.V.
thanks the DST for DSTꢀFast Track Grant (No. SB/FT/CSꢀ
003/2014). TMM acknowledges DST, India for financial support
65 by fast track scheme (SB/FT/CSꢀ093/2012). The authors thank
8
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DOI: 10.1039/C7DT02119D
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70 31 R. Das, P. Pachfule, R. Banerjee and P. Poddar, Nanoscale., 2012, 4,
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the SASTRA University for providing lab space and NMR
facility.
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613401, Tamil Nadu, India. Fax: +914362264120; Tel: +914362264101;
E-mail: philip@biotech.sastra.edu
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Page 11 of 11
Dalton Transactions
View Article Online
DOI: 10.1039/C7DT02119D
Copper-Coordination Polymers Controlled Cu@N-rGO or CuO@C Nanoparticles
Formation: Reusable Green Catalyst for A-3 Coupling and Nitroarene Reduction
The structural assembly of copper coordination polymers (Cu-COPs) based on reduced amino
acid based Schiff base ligands gave Cu/CuO NPs in nitrogen doped reduced graphene oxide (N-
rGO)/carbon matrix that has been used as green catalyst for A3 coupling and nitroarene
reductions.