Triazinetriamine-derived porous organic polymer-supported copper nanoparticles (Cu-NPs@TzTa-POP): an efficient catalyst for the synthesis of: N -methylated products via CO2fixation and primary carbamates from alcohols and urea

Article abstract of DOI:10.1039/d0nj02798g

In recent times, carbon dioxide fixation has received much attention for its potential application as an abundant C1 source and a range of important fine chemicals can be manufactured via this fixation. Here, a copper nanoparticle-decorated porous organic polymer-based (Cu-NPs@TzTa-POP) material was prepared by a simple in situ process. The catalyst was characterized by various techniques such as UV-vis spectra, FTIR spectra, HR-TEM, PXRD, N2 adsorption-desorption, TG-DTA, XPS, and AAS analysis. The synthesized heterogeneous catalyst showed excellent activity in an atmospheric carbon dioxide fixation reaction to produce N-methylated products from aromatic/heterocyclic amines in the presence of polymethyl-hydrosiloxane (PMHS) as the reducing agent at 80 °C within 12 h of the reaction. Through this catalytic N-methylation reaction, we obtained 98% yield of the product with turnover frequency ranging from 18 to 42 h-1. The catalyst is also very stable for the formation of primary carbamates from alcohols using the eco-friendly carbonylating agent, urea. Diverse alcohols (such as benzylic alcohols, phenols, heterocyclic alcohols, as well as aliphatic alcohols) showed much acceptance to this catalytic reaction and produced moderate to excellent yields of the respective carbamate products under ambient reaction conditions. Moreover, Cu-NPs@TzTa-POP is effortlessly recyclable and reusable without the extensive loss of active copper metal centres for many catalytic rounds (up to six catalytic rounds were examined).

Full text of DOI:10.1039/d0nj02798g

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Received 00th January 20xx,
Accepted 00th January 20xx
TriazineTriamine derived Porous Organic Polymer supported Copper
Nanoparticles (Cu-NPs@TzTa-POP): An efficient catalyst for
synthesis of N-methylated products via CO2 fixation and primary
carbamates from alcohols and urea
DOI: 10.1039/x0xx0
www.rsc.org/
Najirul Haque a,# Surajit Biswas a,# Priyanka Basu a, Imdadul Haque Biswas a, Resmin Khatun a, Aslam
Khanb and Sk Manirul Islam a *
Abstract
Nowadays, carbon dioxide fixation has received much more attention for its imminent application as an abundant C1
source and a range of important fine chemicals which can be manufactured via this fixation. Here, Cu NP decorated porous
organic polymer-based (Cu-NPs@TzTa-POP) material was prepared by a simple in-situ process. The catalyst was
characterized by UV-Vis spectra, FTIR spectra, HR-TEM, PXRD, N2 adsorption-desorption, TG-DTA, XPS, AAS analysis, etc.
The synthesized heterogeneous catalyst showed excellent activity in atmospheric carbon dioxide fixation reaction to
produce N-methylated products from aromatic/ heterocyclic amines in the existence of polymethyl-hydrosiloxane (PMHS)
as reducing agent at 80°C within 12 h of reaction. Through this catalytic N-methylation reaction, we got up to 98% yield of
the product with turnover frequency from 18 to 42 h-1. Besides, the catalyst is also very stable for the formation of primary
carbamates from alcohols using eco-friendly carbonylating agent urea. A diverse type of alcohols (such as benzylic
alcohols, phenols, heterocyclic alcohols as well as aliphatic alcohols) showed very much acceptance to this catalytic
reaction and produced moderate to the excellent yield of respective carbamate products under ambient reaction
conditions. Moreover, Cu-NPs@TzTa-POP is effortlessly recyclable and reusable without extensive loss of active copper
metal centre for many catalytic rounds (up to six catalytic rounds were examined).
CO2 fixation in amines will be very attractive. Xia and co-workers
published a paper where N-Methylation of amines with carbon
Introduction
Carbon dioxide, is the main green house gas, is also considered as
cheap, abundant, renewable, sustainable and non-toxic carbon fed
stock for the production of fuels and valuable organic compounds1.
The catalytic carbon dioxide reduction into energy-enrich small
compounds like CO, HCOOH, CH4, and CH3OH has been in rapid
improvement in last decade2. Not only that, fixation of carbon
dioxide via reduction onto amines in order to make a new C–N bond
for the formation of methylamines or formamides has also gaining a
growing attention today.3 Methylamines are broadly used as
solvents and in synthesis of agrochemicals, medicines, dyes,
perfumes, and so on.4 Since 1970, hydrogen has been utilized for
reductive CO2 fixation in amines to produce methylamines or
formamides.5 But drastic reaction conditions like high pressure of
hydrogen and carbon dioxide, high temperature of reaction and
costly metal-based catalysts are generally required in these
methodologies. Recently scientists have found that, catalytic CO2
reduction could be performed easily under mild conditions by using
boranes6 or hydrosilanes7 as hydrogen source. Till date various
catalytic systems have been already reported where hydrosilanes is
used for CO2 reduction in selective N-methylation reaction.8 In most
of the cases, phenylsilane was used as reductive agent but it is
more costly than PMHS (polymethylhydrosiloxane), which is
copious chemical waste having no toxicity, cheap, and impressive
resistant to moisture.9 Hence use of PMHS as reducing agent for
dioxide using PMHS as reductant in presence of DBU (base)
catalyst.10 Here the catalytic process was performed under
homogeneous conditions. As a result, recyclability and reusability of
the catalyst remains challenging. Henceforth we have designed and
synthesized a trizinetriamine derived porous organic polymer frame
work incorporated copper nanoparticles (Cu-NPs@TzTa-POP) which
produced N-methylated product via catalytic reductive fixation of
CO2 in amines using PMHS as a reducing agent under ambient and
heterogeneous reaction conditions.
Carbamates are very essential class of organic compounds that
usually found in drugs and medicines (such as albendazole,
retigabine, flupirtine, and physostigmine etc.),11 agricultural
products (as antiviral agents, bactericides, pesticides and
herbicides),12 cosmetics, organic fine chemicals, biologically active
compounds and polymer compounds.13 Again carabamtes are used
in organic synthesis for the protection of amino groups of amino
acids in peptides synthesis chemistry.14 Traditionally carbamates are
synthesized from amines and alcohols using phosgene (COCl2) as
carbonyl source.15 But this protocol suffer from the limitations of
long reaction duration, low atom economy, large amount of toxic
waste production, utilization of harmful reagent (phosgene) and
insufficiency for N-unsubstituted (primary) carbamates etc.15,16
During current few years, immense attentions have been paid to
discover eco-friendly pathways that use other carbonyl sources
such as isocyanides,17 cyanate or isocyanate salts,18 azides,19 N-
acylbenzotriazoles,20 carbonate esters,21 1,1,1-trichloromethyl
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal,
India
bKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh,
11451, Saudi Arabia
# Authors contributed equally
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formate
(diphosgene),22
trichloroacetylchloride,23
1,1- polymethylhydrosiloxane (PMHS) was added. UVniedweArrticsletiOrrnilninge
carbonyldiimidazole (CDI),24 dialkylazodicarboxylate,25 CO226, and condition 20 mg of Cu-NPs@TzTa-POP cataDlyOstI: w10a.1s0m39i/xDe0dNiJn0t2o79th8Gis
amides27 in place of phosgene for carbamate synthesis. Although reaction solution and the temperature was raised to 80oC. The
alternate carbonyl source is used in these protocols, these system was fitted with a CO2 balloon. Thin layer chromatography
processes have the disadvantages of multi-step procedures, drastic method was used to observe advancement of the reaction. After
reaction conditions, utilization of toxic metal-based catalysts, strong completion of reaction, the mixture was diluted with ethyl acetate
bases and expensive ligands. Now our interest is to develop an eco- and washed with brine solution several times. Lastly dry sodium
friendly, proficient and cost effective protocol for production of sulfate was used to absorb the excess water present in the filtered
carbamate compounds. The use of polyurea/urea as carbonyl mixture. Finally the desired product was obtained via performing
sources for the production of carbamates is considered as safe and purification by thin layer chromatography (pet ether: ethyl acetate
green.28 Beller and co-workers published carbamoylation reaction 10:1) method.
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for primary carbamates production from urea and alcohols by
General procedure of catalytic synthesis of primary carbamates
utilizing eco-friendly Lewis-acidic FeBr2 catalyst under
from urea
homogeneous conditions.29 But use of homogeneous FeBr2 catalyst
leads to reusability and recyclability problems. Inaloo et. al.
reported couple of publications where polyurea/urea was
employed as carbonyl sources for this derivatives preparation.30
Although heterogeneous catalysts are used in those reports, the
reacting conditions are not mild in all those cases. Recently our
group synthesized a polymer supported heterogeneous catalyst,
[FeII(Anthra-Merf)] that can synthesize primary carbamates via
carbamoylation reaction of urea with alcohols at 120°C in 6.5 h31.
Here Cu-NPs@TzTa-POP catalyst is used in synthesis of primary
carbamates from urea and alcohols at 80°C within 4 h of reaction.
In a 25 ml RB, 4 mmol of urea was treated with 5 mmol of alcohol
derivatives in 1,4-dioxane (4 ml) solvent. 20 mg of catalyst Cu-
NPs@TzTa-POP were added to the mixture. After 4 hours of
continuous stirring at 80oC, the reaction mixture was filtered to
separate the catalyst. With the filtrate, work up was performed by
ethyl acetate and brine solution for several times. At last, the
remaining water part was absorbed by sodium sulphate and
1
purified organic layer was preserved and analyzed by H NMR and
IR spectra.
Results and discussion
Characterization of Cu-NPs@TzTa-POP material
FT-IR spectra
Therefore
a trizinetriamine derived porous organic polymer
embedded copper nanoparticles (Cu-NPs@TzTa-POP) catalyst is
synthesized for the production of N-methylated products as well as
primary carbamates under mild reaction conditions with high yield
of isolated products.
The FTIR spectrum of ligand TzTa-POP and Cu-NPs@TzTa-POP
catalyst are described in Figure 1. The corresponding stretching
frequencies provide a proof behind the structure of the obtained
porous organic polymer skeletons. In FTIR spectra of TzTa-POP, the
bands observed at 1622 and 1507 cm-1 were arises due to the
triazine rings in the synthesized polymer frame.32 The stretching
frequencies at 1255 and 1074 cm-1 were found because of the
occurrence of aromatic ether of the benzoxazine ring in polymer
moiety for C-O-C asymmetric and symmetric stretching vibrations,
respectively.33 Again the characteristics vibrations of phenyl ring
connected to oxazine ring appeared at 945 cm-1.33 All these
respective bands were also seen in the spectra of Cu-NPs@TzTa-
POP catalyst but in slightly shift their position validates the
attachment of copper nanoparticles over the TzTa-POP.
Experimental
Synthesis of Copper Nanoparticles decorated TriazineTriamine
derived Porous Organic Polymer catalyst (Cu-NPs@TzTa-POP)
500 mg of Triazine Triamine based Porous Organic Polymer (TzTa-
POP) was sonicated in 100 ml distilled water till the polymer
dispense uniformly. After that, 3 ml aqueous solution of copper
acetate was added drop by drop to the solution under moderate
stirring and the temperature was raised to 80oC. After 1h of stirring,
500 µL of hydrazine was added to the reaction mixture and stirring
was continued for another 2h. The solid product obtained was
washed with water and ethanol and dried under vacuum. The
details procedure of synthesis of the catalyst is depicted here in
Scheme 1 as pictorial representation.
Scheme 1. Pictorial presentation of synthesis of Cu-NPs@TzTa-POP catalyst.
General procedure of catalytic synthesis of N-methylated products
In a 50 ml round bottom flask, 5 mmol of respective amine was
dissolved in 5 ml acetonitrile solvent. In this mixture 2 equivalent of
Fig. 1 FT-IR spectra of TzTa-POP and Cu-NPs@TzTa-POP catalyst.
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PXRD of Cu-NPs@TzTa-POP catalyst
satellite peaks were detected to somehow validateVtiehweArpticuleriOtynlionef
metallic Cu/Cu (II). We can attribute particular components
DOI: 10.1039/D0NJ02798G
The crystalline nature of the synthesized Cu-NPs@TzTa-POP catalyst
was defined through powder X-ray diffraction analysis experiment.
This represented some strong peaks in the range of 5 to 90o 2θ
value (Figure 2). In this figure the Bragg reflection peaks appeared
at 43.0083o, 50.1654o, 73.9181o 2θ value, with the corresponding
planes (111), (200) and (220) of Cu(0) nanoparticles respectively.
From this lattice planes it is confirmed that copper formed a face
centered cubic lattice (fcc).34 The existence of a broad peak near 35°
2θ value suggest existence of TzTa-POP framework in the catalyst.
TzTa-POP material showed a broad peak centred at 32° 2θ value
indicates amorphous nature of the material (Figure S1).
comprising the line of Cu 2p3/2 to the states Cu0 and Cu+ (Fig. S2,
ESI†). Having regard to the XPS results demonstrating the
coexistence of Cu0/1+ as a major species and Cu2+ as a minor species
on the surface of the as-obtained POP, one is willing to concede
that such combined species of various oxidation states appear on
the surface of the material. The formation of Cu NPs was assessed
by XRD technique, which supports the synthesis of Cu NPs owing to
the absence of Cu(I) diffraction peaks (i.e. Cu2O). We can conclude
that the analysis of XRD and XPS precisely indicates the chemical
integrity of Cu NPs decorated POP.36 The experimental result gave
carbon, nitrogen, oxygen 1s core level binding energy peak at 284.2,
398.3, 532.3 eV respectively.
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Fig. 2 Powder X-ray diffraction pattern of Cu-NPs@TzTa-POP catalyst.
High Resolution Transmission Electron Microscopic (HR TEM)
analysis
The HR TEM micrographs of Cu-NPs@TzTa-POP catalyst in Figure
3(a-d) represent with various magnifications. The images
substantiate that the material holds multilayered structure with
homogeneously spread pores on its surface. From these images it is
confirmed that in catalyst copper nanoparticles deposited on its
surface. The calculated d-spacing (Figure 3d) marked by red circle is
0.2332 nm which corresponds to (111) planes of Cu(0) NPs.35
Different size of copper nano particles (ranging from 2-20 nm) are
distributed on surface of TzPa-POP (figure 3e). The particle size
distribution plot is given in figure 3f.
Energy dispersive X-ray analyses (EDAX) of Cu-NPs@TzTa-POP
catalyst
For further clarification of presence of copper metal on synthesized
porous organic polymer surface we have carried out EDAX analysis
of the catalyst. Figure 4 firmly demonstrates the existence of
elements carbon, oxygen and nitrogen along with copper metal.
Fig. 3 HR TEM images (a-f) of Cu-NPs@TzTa-POP catalyst with different
magnification.
XPS analysis
The determination of oxidation state of Cu and the attachment of
Cu metal with the other elements in the synthesized Cu-NPs@TzTa-
POP catalyst was properly searching out through XPS analysis. A full
range XPS analysis is given in Figure 5 whereas; the narrow range
analysis is represented in Figure 6a-d for copper, carbon, nitrogen,
oxygen respectively. In the spectrum the strong binding energy
peak are observed at 952.2 and 932.4 eV, assigned the two
different Cu(0) level energy, Cu 2p1/2 and Cu 2p3/2 respectively. The
difference of the two binding energy peaks was represented in
Table 1.34 These core levels could be resolved into two, Cu (0)/Cu (I)
as a major and Cu(II) as minor species. In the Cu 2p spectrum, no
Fig. 4 EDAX spectrum of Cu-NPs@TzTa-POP catalyst.
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homogeneously dispersed the materials in acetonitrile solvent
View Article Online
individually. A sharp peak appear at ~207DaOnId: 10~.2100369/nDm0NfJo0r27b9o8tGh
ligand and catalyst moiety due to presence of π-π* transition.32
Many other absorption peaks indicate other transitions hold by the
highly conjugative systems.
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Fig. 5 Full range X-ray photoelectron spectroscopy of Cu-NPs@TzTa-POP
catalyst.
Fig. 7 adsorption desorption curve of Cu-NPs@TzTa-POP at 77 K. Pore size
distribution plot (inset).
Fig. 6 Narrow-range XPS spectra of Cu-NPs@TzTa-POP containing elements
(a) Cu 2p, (b) C 1s, (c) N 1s, and (d) O 1s.
Table 1. XPS data of catalyst Cu-NPs@TzTa-POP
Cu2p3/2
peak
(eV)
Cu2p1/2
peak (eV)
Peak difference (eV)
Sample name
Cu (2p1/2-2p3/2
)
Fig. 8 UV-Vis spectroscopy of TzTa-POP and Cu-NPs@TzTa-POP catalyst.
Cu-NPs@TzTa-POP
952.2
932.4
19.8
TGA analysis of TzTa-POP ligand and Cu-NPs@TzTa-POP catalyst
N2 Adsorption-Desorption study
To investigate thermal stability of porous organic polymer and TzTa-
POP incorporated copper nano particle (Cu-NPs@TzTa-POP),
thermogravimetric analysis was done under N2 atmosphere with
the heating rate of 10 °C/minute (Figure 9). For TzTa-POP support
the weight loss occur from 100 °C to 256 °C but for Cu-NPs@TzTa-
POP catalyst the weight loss occur in the range of 115 °C to 455 °C.
It concludes from the thermogram that beyond 455 °C the
decomposition of catalyst is completely done and the stability of
catalyst increase with the incorporation of Cu nanoparticle onto
TzTa-POP surface.
In order to measure the pore size, surface area and pore volume of
the synthesized catalyst we have carried out BET analysis of the Cu-
NPs@TzTa-POP. The obtained isotherm is typical type IV isotherm
having a hysteresis loop in the pressure range of 0.8<p/p0<1.0
clearly indicates that the material is mesoporous in nature (Figure
7).33,37 Through BJH method, pore size distribution curve was
calculated and the result showed that the catalyst have the average
pore size of 19 nm (inset of figure 7). The multi point BET surface
area and total pore volume of Cu-NPs@TzTa-POP are 228.501 m²/g,
0.927 cc/g respectively.
Atomic absorption spectroscopy analysis
UV-Vis spectroscopy of Cu-NPs@TzTa-POP catalyst
Atomic absorption spectroscopy analysis was monitored to
measure the amount of copper metal present on TzTa-POP surface.
The UV-Vis spectra of both TzTa-POP ligand and Cu-NPs@TzTa-POP
catalyst is represented in Figure 8. The data was recorded by
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1,4-dioxane
Tolune
DMF
DMSO
MeCN
DCM
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r.t.
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It was observed that nearly 4.86% of the metal is loaded on the
catalyst surface.
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10
Reaction occurred at atmospheric CO2 pressure, N-methylaniline (5 mmol),
PMHS (2 equivalent), Cu-NPs@TzTa-POP Catalyst (15 mg).
The reaction was harmonized by changing the amount of reducing
agent and catalyst loading (Table 3). With enhancing catalyst
amount from 10 to 20 mg, GC yield of the product was also rising
from 49% to >99% (Table 3, Entries 1-3). In absence of catalyst, the
reaction did not take place (Table 3, Entry 4). The amount of PMHS
was used from 0.5 equivalents to 2.0 equivalents for the reaction
(Table 3, Entries 5-8). Using 0.5 equivalents of PMHS, very little
amount of product was appeared (Table 3, Entry 5). With increasing
the amount of reducing agent, the yield of product was also
increases and attain optimum value by utilization of 2 equivalents
PMHS (Table 3, Entry 8). No product was obtained when the
reaction was monitored without reducing agent (Table 3, Entry 9).
When TzTa-POP without copper incorporation was used as catalyst
42% yield of desired product was observed (Table 3, Entry 10) and
trace amount of product was obtained when copper acetate salt
was used as catalyst (Table 3, Entry 11). Recently we have prepared
a covalent organic framework supported copper (0) nanoparticles
(Cu-NPs@COF) through similar methodology and used as efficient
catalyst for CO2 fixation reaction to produce 2-oxazoliinones and
benzimidazoles.38 In this catalysis the same Cu-NPs@COF material
was used as catalyst in order to order to compare the efficiency of
our present catalyst and the TriazineTriamine derived Porous
Organic Polymer support. Cu-NPs@COF catalyst produced 81% yield
of product (Table 3, Entry 12). Again the support of the catalyst Cu-
NPs@COF i.e. COF yielded 38% of the product (Table 3, Entry 13).
Since both support of the catalysts; TzTa-POP and COF gave
considerable amount of desired product (42% and 38%
respectively) whereas Copper(II)acetate produced only trace
amount of product. So there is definitely a contribution of the
support TzTa-POP on the catalytic reaction.
Fig. 9 TGA plot of TzTa-POP and Cu-NPs@TzTa-POP catalyst.
Catalysis
Reaction conditions optimization for benzimidazolone synthesis
Catalytic N-methylation reaction via carbon dioxide fixation in
presence of Cu-NPs@TzTa-POP catalyst
Reducing agent
Cu-NPs@TzTa-POP
NH
N
+ CO2 (Balloon)
Solvent, Temperature
Time
Scheme 2. Catalytic N-methylation reaction of amines through carbon
dioxide fixation.
Catalytic N-methylation reaction of amines was carried out in
presence of our synthesized Cu-NPs@TzTa-POP catalyst under 1
atmospheric CO2 pressure in attendance of one reducing agent
(Scheme 2). N-methylaniline was chosen as model substrate to
optimize the catalytic reaction’s conditions. Primarily we commence
the reaction by using 5 mmol of N-methylaniline, 2 equivalent of
polymethylhydrosiloxane (PMHS) as reducing agent under CO2 (1
atm) at 80°C in dioxane (5 ml) solvent with the help of 15 mg Cu-
NPs@TzTa-POP catalyst. It was very inspiring that the reaction was
worked and we got 76% GC yield of N,N-dimethylaniline after 12h
of reaction. Then solvent effect on the reaction was scrutinized by
using various type of solvents (Table 2, Entries 1-8). From table 2 it
is very clear that almost all solvents gave more or less amount of
corresponding product but acetonitrile (MeCN) showed the
maximum efficiency as solvent in this reaction (Table 2, Entry 5).
Using MeCN as solvent the reaction was proceed at RT (room
temperature), after 12h of reaction we got only 25% yield of the
product (Table 2, Entry 9). Again the same reaction was performed
at 50°C, although better yield of product (69%) (Table 2, Entry 10)
was obtained as compare to the previous reaction but maximum
product yield was achieved from 80°C temperature reaction (Table
2, Entry 5).
Table 3. Effect of catalyst loading and PMHS amount on the N-methylation
reaction
Entry
Catalyst
Amount
of
GC yield of product
amount (mg)
PHMS (equiv.)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
10
15
20
-
20
20
20
20
20
20a
20b
20c
20d
2.0
2.0
2.0
2.0
0.5
1.0
1.5
2.0
-
2.0
2.0
2.0
2.0
49
72
>99
-
Trace
34
53
>99
-
42
Trace
81
38
Reaction conditions: N-methyl aniline (5 mmol), PMHS, CO2 (1 atm), Cu-
NPs@TzTa-POP catalyst, 80°C, 12h, aTzTa-POP as catalyst, bCopper acetate
salt as catalyst, cCu-NPs@COF as catalyst, dCOF as catalyst.
Table 2. Effect of solvents and temperature in the catalytic N-methylation
reaction of N-methylaniline
Entry
Solvent
Temperature (°C)
Time (h)
GC yield (%)
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94.3
95
26.19
The rate of the reaction with respect to time in presence as well as
in absence of Cu-NPs@TzTa-POP catalyst (Figure 10) was also
studied in this circumstance. From the figure it is clearly noticeable
that the reaction was performed in faster rate and completed
within 12h in presence of catalyst. Whereas non-existence of
catalyst in reaction mixture, the reaction was not progressed
smoothly. After 15h of reaction, only 5% (approximately) yield of
product was appeared.
N
[91.3]View Article Online
NH
DOI: 10.1039/D0NJ02798G
F
F
2c
1c
9.5
100
65
98
[93.7]
35.09
18.05
NH
N
1d
2d
5
12
97
10
11
12
13
14
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18
19
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42
43
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45
46
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49
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51
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53
54
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56
57
58
59
60
[61.3]
NH
N
1e
2e
6
7
8
8
100
100
98
[94.3]
41.67
41.67
NH
O
N
O
1f
2f
H
97
[92.0]
N
N
O
O
2g
1g
aReaction conditions: substrate (5 mmol), PMHS (2 equiv.), Cu-NPs@TzTa-
POP catalyat (20 mg, 0.015 mmol based on copper metal), CO2 (1 atm),
MeCN (5 ml), 80°C; bGC conversion; cGC yield; TOF (turn over frequency)=no.
of moles of the substrate being converted per mole of active site of the
catalyst/time (in h).
Scheme 3 represents the probable mechanistic pathway for the
formation of N-methylated products via carbon dioxide fixation of
amines. The depicted mechanism is basically supported from the
previously published literature.3c,39 Initially copper metal of the
catalyst activated Si–H bond of PMHS. Then carbon dioxide is
incorporated into the bond. Since polymethylhydrosiloxane
contains multiple Si–H bonds, it successfully provides hydrogen to
the reaction medium for the reduction of carbonyl group to
produce desired product.
Fig. 10 Study of rate of N-methylation reaction in presence and absence of
catalyst.
Catalytic N-methylation reaction was carried out in presence of 20
mg Cu-NPs@TzTa-POP catalyst under atmospheric CO2 pressure at
80°C in MeCN solvent with various aromatic and heterocyclic
amines. All aromatic amines as well as heterocyclic amines
smoothly go through this reaction and furnishes very good yield of
desired product (Table 4). The given data of each entry are the
mean value of three successive reactions. The collected results of all
reactions are given in Table S1. Very high yield of desired product
was obtained from N-methylaniline and N,4-dimethylaniline
through this catalytic reaction (Table 4, Entries 1-2). Electron
withdrawing group containing N-methylaniline (like 4-fluoro-N-
methylaniline) also produced very high yield (91.3%) of product in
this reaction (Table 4, Entry 3). Again N-ethylaniline gave 93.7%
yield of respective N-methyl product (Table 4, Entry 4) whereas
diphenylamine produced relatively lower yield (61.3%) of the
product due to presence of sterically crowding phenyl group (Table
4, Entry 5). The heterocyclic amines (such as morpholine, pyrrolidin-
2-one) also very efficiently performed this catalysis reaction and
gave us >92% yield of corresponding N-methyl product (Table 4,
Entries 6,7).
Table 4. Substrates scope of catalytic N-methylation reactiona
E
n
t
Amines
Products
Time
(h)
Conv.
Yield
TOF
(h-1)
(%)b
(%)c [%
of
r
y
isolated
yield]
Scheme 3. Probable route of catalytic synthesis of N-methylated products.
1
12
10
100
100
>99
[96.3]
27.78
Catalytic synthesis of primary carbamates from alcohol and urea
in presence of Cu-NPs@TzTa-POP catalyst
NH
N
1a
1b
O
2a
Cu-NPs@TzTa-POP
O
2
>99
[95.3]
33.33
NH
N
+
OH
R
Solvent, Temperature
Time
NH2
NH2
NH2
OR
2b
6 | J. Name., 2012, 00, 1-3
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Scheme 4. Catalytic synthesis of primary carbamates in presence of Cu- Fig. 11 Effects of catalyst loading on synthesis of benzyl carbamate from
View Article Online
NPs@TzTa-POP catalyst.
benzyl amine and urea.
DOI: 10.1039/D0NJ02798G
Primary carbamates were synthesized by using various alcohols and By enhancing the catalyst loading, isolated yield of the product was
urea in presence of the Cu-NPs@TzTa-POP catalyst (Scheme 4). The also enhanced and highest yield of the product (98%) was attained
standardization of the catalysis reaction was monitored by taking by using 20 mg of catalyst. Utilization of 25 mg catalyst was not
benzyl alcohol as model substrate. First of all we performed a affecting the further increment of product yield.
catalytic reaction by using benzyl alcohol (5 mmol), urea (4 mmol),
The rate of the reaction with respect to time in presence and
Cu-NPs@TzTa-POP catalyst (15 mg) at 80°C in MeCN (4 ml) solvent.
absence of Cu-NPs@TzTa-POP catalyst (Figure 12) was also
After 4 h of reaction, 56% isolated product of benzyl carbamate was
observed under the optimized reaction conditions. From the figure
obtained (Table 5, Entry 1). So this obtained result of catalysis
it is clearly observable that the reaction was performed in faster
reaction inspired us to optimize the reaction by varying solvents,
rate and completed within 4 h in presence of catalyst but the
temperature, time and catalyst loading. Table 5 represents the
reaction did not progress smoothly in absence of catalyst. After 6h
effect of solvents as well as temperature in the catalytic reaction.
of reaction, only 5% (approximately) yield of product was appeared.
From table 5 it is clear that 1,4-dioxane is the most excellent solvent
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9
10
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for the reaction (Table 5, Entry 4). Under solvent free condition we
got only trace amount of desired product (Table 5, Entry 10). Then
temperature optimization was examined. At room temperature
only 24% of benzyl carbamate was appeared. With increasing
reaction temperature the product yield was also improved and
maximum product yield was observed at 80°C (Table 5, Entry 4). At
90°C temperature the product yield was reduced due to the
formation of bicarbamate products (Table 5, Entry 14).
Under optimized reaction conditions the catalytic reaction was
carried out in presence of copper acetate salt (20 mg) and TzTa-POP
(20 mg) as catalyst in two separate reactions. 36% isolated yield of
benzyl carbamate was obtained in use of TzTa-POP support as
catalyst whereas copper salt produced trace amount of product.
The same reaction was performed in presence of our previously
synthesized Cu-NPs@COF and its support (COF) as catalyst. Cu-
NPs@COF synthesized 89% of desired product and COF produced
31% of the product. The summarized results for both catalytic
reaction in presence of different catalysts and their supports
definitely indicates the Synergistic Effect of support and copper
nano particles.40
Table 5. Effect of solvents and temperature in the catalysis reaction
Entry
Solvent
Temperature
Isolated yield of benzyl
(°C)
80
80
80
80
80
80
80
80
80
80
80
r.t.
50
70
90
carbamate (%)
1
2
3
4
5
6
7
8
8
MeCN
DCM
Benzene
1,4-Dioxane
DMF
56
43
68
88
71
77
73
55
Trace
Trace
Trace
24
63
78
DMSO
o-Xylene
Toluene
n-Bu2O
9
H2O
10
11
12
13
14
No solvent
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
85
Reaction performed by taking benzyl amine (5 mmol), Urea (4 mmol), Cu-
NPs@TzTa-POP catalyst (15 mg), 4 h.
Fig. 12 Study of rate of benzyl carabamate formation in presence and
absence of catalyst.
After regulating the solvent and reaction temperature, we
inspected the loading of catalyst on the same catalysis reaction
(Figure 11). Without catalyst very trace amount of product was
produced in 4 h of reaction.
Therefore the catalytic formation of carbamates from various
alcohols and urea was performed in presence of 20 mg Cu-
NPs@TzTa-POP catalyst at 80°C in dioxane solvent. Both aromatic
alcohols (benzyl alcohols and phenols) as well as aliphatic alcohols
efficiently go through this catalytic reaction and produced
moderate to high amount of corresponding product within 4 h of
reaction (Table 6). The given data of each entry are the mean value
of three successive reactions. The collected results of all reactions
are given in Table S2. Benzyl alcohol and 4-methoxy benzyl alcohol
(having electron donating group) gave very high yield of desired
product (96.7%) in this catalytic reaction (Table 6, Entries 1,2)
where as mild electron withdrawing group containing benzyl
alcohol (such as 4-bromo benzyl alcohol) also gave very good
amount of corresponding carbamate (91.7%) (Table 6, Entry 3).
Strong electron withdrawing group containing benzyl alcohol (i.e. 4-
nitro benzyl alcohol) produced relatively lower (82.3%) of 4-nitro
benzyl carbamate product (Table 6, Entry 4). Phenol and its
derivatives (both electron donating and withdrawing group
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NH2
DOI: 10.1039/D0NJ02798G
14
95.7
containing) were formed relatively less amount of carbamates
product (56% to 77%) as compare to benzyl alcohols (Table 6,
Entries 5-8). Phenylethyl alcohol gave 95.7% of corresponding
desired product (Table 6, Entry 9). Then tolerance of different types
of aliphatic alcohols (like propyl, isopropyl, butyl and pentyl
alcohols etc.) were checked, all of them produced >88% yield of
respective products (Table 6, Entries 11-14). Not only that, we also
examined the efficiency of furan-2-ylmethanol (a hetero cyclic
compound) in this catalytic reaction and got 72.7% isolated yield of
Furan-2-ylmethyl carbamate (Table 6, Entry 10).
OH
View Article Online
O
O
4n
3n
aReaction conditions: alcohols (5 mmol), urea (4 mmol), Cu-NPs@TzTa-POP
catalyst (20 mg, 0.015 mmol based on copper metal), 1,4-dioxane (4 ml),
80°C, 4 h.
By the support of earlier reported publications28,31 a plausible
mechanism for the synthesis of primary carbamates from urea and
alcohol is depicted in Scheme 5. Initially urea is activated by the
catalyst through O-coordination to the copper centre of the catalyst
(intermideate I). The electrophilic character of the carbonyl carbon
is increased via this coordination.41 Nucleophilic attacks of alcohol
to the electron deficient carbonyl centre leads to the intermediate
II.30b Transfer of a proton from alcohol to an amino group produces
intermediate III. Then intermediate III gives rise to the final product
after elimination of NH3 molecule and regeneration of the catalyst.
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59
60
Table 6. Substrates scope of catalytic formation of carbamatesa
Entry
1
Alcohols
Carbamates
Isolated
Yield (%)
96.7
NH2
OH
O
O
3a
3b
4a
OH
OH
NH2
O
2
3
96.7
91.7
Tests of heterogeneity for Cu-NPs@TzTa-POP catalyst
MeO
Br
O
MeO
Br
Hot filtration test in catalytic primary carbamates formation
reaction was performed to check the heterogeneity nature of the
Cu-NPs@TzTa-POP catalyst. Firstly, catalytic primary carabamates
synthesis reaction from benzyl alcohol and urea was allowed to run
under optimization reaction condition for 2h.
4b
4c
NH2
O
O
3c
OH
NH2
O
4
82.3
O2N
O
O2N
3d
3e
3f
4d
O
5
6
77.0
81.7
OH
O
NH2
O
4e
OH
OH
MeO
Me
O
MeO
Me
NH2
4f
O
7
8
78.3
56.3
O
O
NH2
4g
NO2
3g
3h
NO2
OH
O
NH2
Scheme 5. Probable route of catalytic synthesis of primary carbamates from
urea and alcohols.
4h
OH
O
9
95.7
After 2 h of reaction advancement, the catalyst was filtered out.
The desired product yield was observed to be 57%. Then the
residue reaction mixture was permitted to continue for another 2h
without presence of Cu-NPs@TzTa-POP. It was found that the
reaction did not make any progress without catalyst. AAS
analyzation was done with the filtered catalyst and the filtrate
reaction mixture. In filtrate, no trace amount of Cu metal was
found. In recovered catalyst, same amount of copper metal loading
(i.e. 4.86%) was arises as that of fresh catalyst. AAS analysis also
been done with the catalyst after the first run of catalytic cycle. It
showed no leaching of Cu metal. The recycled catalyst after 6th cycle
was also analyzed by AAS method. It showed a negligible amount of
metal leaching. For further confirmation of heterogeneity, FT-IR,
XRD and XPS analysis were done with the recycled Cu-NPs@TzTa-
POP catalyst. FT-IR and XRD data of the reused catalyst showed no
significant change in peak position from the freshly prepared Cu-
NPs@TzTa-POP (Figure S38, S39). In XPS analysis of recovered
catalyst, no considerable alteration of peak position in the copper
O
NH2
3i
4i
O
10
11
72.7
91.7
OH
O
O
NH2
O
3j
4j
OH
O
O
NH2
O
3k
4k
OH
12
13
88.3
93.7
O
NH2
3l
4l
NH2
OH
O
O
4m
3m
8 | J. Name., 2012, 00, 1-3
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nanoaprticles was found (Figure 13). The characteristics binding
View Article Online
DOI: 10.1039/D0NJ02798G
energy peaks were observed at 932.1 eV, 951.9 eV which
represents to Cu(0) Cu 2p3/2 and Cu 2p1/2, respectively and the peak
difference (19.8 eV) is also same as that of synthesized catalyst. This
result clearly confirms the unchanged oxidation state of copper
nanoparticles in utilized catalyst.42 All these analytical data clearly
indicates the heterogeneity of Cu-NPs@TzTa-POP catalyst.
Recyclability of Cu-NPs@TzTa-POP catalyst
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The reusability and recyclability of Cu-NPs@TzTa-POP catalyst was
screened for both catalytic reactions of N-methylation and primary
carbamate formation reactions. After completion of every catalytic
cycle, the catalyst was detached from the reaction mixture via
simple filtration technique. Then recovered catalyst was washed
properly with methanol and acetone for several times. After
washing the catalyst, it was first air dried and then dried under
vacuum. Next it was utilized for another catalytic cycle. Figure 14
evidently elucidates that Cu-NPs@TzTa-POP catalyst was
sufficiently stable after subsequent employ of six cycles. Atomic
absorption spectra analysis was performed to determine the copper
amount present on TzTa-POP surface after six catalytic rounds, the
result depicted that very negligible amount of copper metal was
loosed from the surface. The recycled catalyst showed almost the
same copper loading as that of fresh catalyst (≈4.78). Hence we can
firmly conclude that Cu-NPs@TzTa-POP catalyst is heterogeneous
catalyst which is easily recyclable and reusable.
Fig. 14 Graphical representation of recyclability test of Cu-NPs@TzTa-POP
catalyst.
Conclusion
A
newly designed heterogeneous copper nanoparticles
incorporated porous organic polymer (Cu-NPs@TzTa-POP) catalyst
was synthesized and properly characterized by different
instrumental methods. The low cost easily producible catalyst is
very competent in the chemical fixing of atmospheric CO2 in order
to produce N-methylated compounds from aromatic and
heterocyclic amines under moderate reaction conditions. Various
N-methylated products were obtained up to 98% isolated yield with
turnover frequency value from 18 to 42 h-1. Again the catalyst is
also very proficient to manufacture primary carbamates from eco-
friendly carbonylating agent urea and alcohols. Different benzylic
alcohols, phenols, heterocyclic alcohols as well as aliphatic alcohols
were very much tolerant to the catalytic reaction and produced
moderate to high yield of respective carbamates under ambient
conditions. Furthermore, outstanding catalyst recyclability and
reusability are observed without substantial loss of active Cu metal,
demonstrating the porous Cu-NPs@TzTa-POP material as a capable
vibrant heterogeneous catalyst for both thermal catalytic reactions.
Acknowledgments
S.M.I. is thankful to the Department of Science and Technology
(DST-SERB, Project No. EMR/2016/004956), New Delhi, Govt. of
India, and Board of Research in Nuclear Sciences (BRNS), Project
reference No: 37(2)/14/03/2018- BRNS/37003, Govt. of India, and
Council of Scientific and Industrial and Research, CSIR (project
reference no 02(0284)2016/EMR-II dated 06/12/2016) New Delhi,
Govt. of India for providing financial support. PB is thankful to the
NFOBC for UGC, New Delhi for her fellowship. SB acknowledges
University Grants Commission for D. S. Kothari Post Doctoral
Fellowship (Award letter no. F.4-2/2006 (BSR)/CH/16-17/0026).A.K.
is grateful to the Researchers Supporting Project number (RSP-
2020/127), King Saud University, Riyadh, Saudi Arabia for the
support. We acknowledge the Department of Science and
technology (DST) and University Grant Commission (UGC), New
Delhi, Govt. of India for providing support to the Department of
Chemistry, University of Kalyani under FIST, PURSE and SAP
program.
Fig. 13 XPS spectra of used Cu-NPs@TzTa-POP catalyst for Cu 2P level.
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Conflicts of interest
There are no conflicts to declare.
* Author to whom correspondence should be addressed.
Dr. S M Islam, Dept of Chemistry, University of Kalyani, E-mail:
manir65@rediffmail.com
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Notes and references
6.
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DOI: 10.1039/D0NJ02798G
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Graphical Abstract
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TriazineTriamine derived Porous Organic Polymer supported Copper
Nanoparticles (Cu-NPs@TzTa-POP): An efficient catalyst for synthesis of N-
methylated products via CO2 fixation and primary carbamates from alcohols and
urea
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Najirul Haque a,# Surajit Biswas a,# Priyanka Basu a, Imdadul Haque Biswas a, Resmin Khatun a, Aslam
Khanb and Sk Manirul Islam a *
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India
bKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi Arabia
#These authors contributed equally
Copper nanoparticles incorporated TriazineTriamine derived Porous Organic Polymer-based catalyst was
synthesized for catalytic production of N-methylated amines via CO2 fixation reaction and primary
carbamates formation from urea and alcohols.