A copper(ii) metal-organic hydrogel as a multifunctional precatalyst for CuAAC reactions and chemical fixation of CO2 under solvent free conditions

Article abstract of DOI:10.1039/c6cc09039g

A copper(ii) metal-organic hydrogel has been synthesised and characterised. This hydrogel is an efficient, reusable precatalyst for CuAAC reactions and chemical fixation of CO₂ under solvent free conditions.

Full text of DOI:10.1039/c6cc09039g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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A Copper(II) Metal – Organic Hydrogel as Multifunctional
Precatalyst for CuAAC Reaction and Chemical Fixation of CO2
Under Solvent Free Condition
Received 00th January 20xx,
Accepted 00th January 20xx
Chandan Kumar Karana, Mohan Chandra Saua and Manish Bhattacharjee*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A copper(II) metal – organic hydrogel has been synthesised and attention of the researchers. Copper xerogel catalysed CuAAC
characterised. The hydrogel is an efficient, reusable precatalyst for have been reported.14,15 Another recent report deals with
CuAAC reaction and chemical fixation of CO2 under solvent free Cu(II) metal – organic nanoparticle catalysed CuAAC in the
condition.
presence of sodium ascorbate.27 Most of the reports deal with
Supramolecular functional materials constructed from metal catalysis by xerogel, which requires an extra step of drying the
ions and organic ligands are of immense importance due to gel. The catalytic activity of the as-synthesized gel is very rare.
their promising application in different fields including One of the major greenhouse gasses, CO2, is responsible for
sensing,1,2 photoluminescence,3 drug delivery,4 and catalysis.5 the increasing temperature and climate changes.28 The
Among the supramolecular materials, metallohydrogels6,7 are adsorption and chemical conversion of CO2 by the catalytic
still rare. Metal-organic gels (MOGs) are multiresponsive in reaction using porous MOFs, MOGs is an attractive
nature.8 Depending on the external stimuli these materials sequestration for the recycling of carbon on the earth.29 The
undergo sol-gel transition. These nano-fibrous MOGs have cycloaddition of carbon dioxide and an epoxide produces cyclic
potential as efficient catalysts due to their heterogeneous9-14 carbonate without any side product which is an atom-
nature and recycling ability.15,16 The short–range ordered economical example of green chemistry. Recently, a few
catalytically active repeating units are responsible for the MOFs30-32 and porous materials33 were reported as the
enhanced efficiency of these type of MOG catalysts.17 The efficient heterogeneous catalyst for the chemical conversion of
transition metal containing MOGs as a catalyst are much more CO2. To the best of our knowledge till date, there is no report
encouraging because of their low-cost, non-toxic nature and on MOG or the corresponding xerogel catalyzed conversion of
easy availability.
CO2 to cyclic carbonates.
Synthesis of triazoles through copper catalyzed azide-alkyne Recently we have embarked upon synthesis and properties of
cycloaddition (CuAAC) reaction is vital due to the wide range of metal-organic gels using low molecular weight gelator.34 We
bioactivities of triazoles and their various applications in have reported synthesis of metal-organic organogels using
medicinal chemistry,18,19,20 It is well known that regioselective Na2HL {H3L
=
carboxymethyl-(3,5-di-tert-butyl-2-hydroxy-
synthesis of 1,4-disubstituted 1,2,3-triazole derivatives by benzyl)amino acetic acid35}. Herein, we report the synthesis of
Huisgen 1,3-dipolar cycloaddition21,22 of organic azides and copper(II) containing multiresponsive metallohydrogel using
terminal alkynes are catalyzed by Cu(I) species. The Cu(II) salts Na2HL (Fig. S1a, ESI) and its properties, catalytic activity
and a reducing agent, such as sodium ascorbate is used for the towards 1,3-dipolar cycloaddition of organic azides and
in situ production of Cu(I) species.22-24 The heterogeneous terminal alkyne without any external reducing agent and
catalysis for azide-alkyne 1,3-dipolar cycloaddition reactions cycloaddition of CO2 to epoxides.
has much more advantages over the homogeneous catalysis25, The Cu-MOG was readily synthesised by mixing an aqueous
26
due to the easy separation of the catalyst. In recent years, solution of CuCl2 and aqueous Na2HL solution in 1:1 ratio. The
easily recoverable and recyclable heterogeneous catalyst, such gelation took place instantly as confirmed by the inverted vial
as MOFs,9 MOGs,10 nanocatalysts,11 nanoclusters,12 and method (Fig. S1, ESI). The minimum gelation concentration
polymer or silica supported13 catalysts have received the (MGC) of the Cu-MOG was found to be 1.14% (w/v) in water.
We have studied the gelation in various organic solvents.
However, only water was found to be effective for gelation.
The carboxylate and phenolate groups of the ligand form
bonds with the Cu(II) metal canter leaving donor sites for the
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hydrogen bonding interaction with water which is responsible BET surface area for Cu-MOG was measured to be 100.84 m2 g-
1
for metallohydrogel formation (Fig. S2a, ESI).
and exhibit a saturated sorption amount of N2 of 122 cm3 g-1
The Cu-MOG is stable at room temperature for months in the and can uptake 14 cm3 g-1 amount of CO2 gas at 273 K, which is
sealed vial without any deformation. The Cu-MOG shows a comparable with the reported MOGs (Table S2, ESI).
quick recovery during the shaking and resting cycle due to its The MALDI-TOF spectrum of the MOG shows a peak at m/z
thixotropic nature, and the process can be repeated for more 450 which corresponds to the 1 : 1 complex of copper along
than ten cycles. The Cu-MOG also exhibit chemical with two water molecules (Fig. S7 and S8, ESI).
responsiveness. The addition of few drops of aqueous NH3
Table
phenyltriazolea
1 Screening of experimental condition for the synthesis of 1-benzyl-4-
solution transforms the gel into a deep brown solution, and
the blue coloured gel can be regenerated by the addition of
few drops hydrochloric acid. In the presence of disodium salt
of ethylene diamine tetraacetic acid (Na2EDTA), the gel breaks
and transform into gelatinous solution due to complexation of
Cu(II) by Na2EDTA (Fig. S2b, ESI).
The hydrogel was investigated by various microscopic
techniques (Fig. 1). The field emission scanning electron
microscopic (FESEM) image of the Cu-MOG reveals the
entangled fibrillar structures. The optical microscopic (OM)
image of Cu-MOG shows the dendritic aggregation of the
metallohydrogel. The transmission electron microscopic (TEM)
image shows the entangled fibrillar nature of the gel. This
fibrillar structure is responsible for gel formation. The three-
dimensional view of atomic force microscopic (AFM) images
confirms that the Cu-MOG consists of an entangled fibrillar
network.
Entry
Cat.(mol%)
Solvent
Temp.
(℃)
rt
Yieldb (%)
1
2
3
4
5
6
7
8
9
Xerogel (4)
No catalyst
Xerogel (4)
Xerogel (2)
Xerogel (1)
Gel (2)
H2O
72
0
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
rt
rt
76
75
68
78
86
91
94c
rt
rt
rt
Xerogel (2)
Gel (2)
50
50
80
Gel (2)
aBenzyl azide 1a (1.0 mmol), phenylacetylene 2a (1.2 mmol) and Cu-MOG (11.3
mg). bIsolated yield. cThe ratio of 1,4 and 1,5 regioisomers was 68:26.
We investigated the catalytic activity of the Cu-MOG using
both gel material and xerogel material without any solvent.
Initially, 4 mol% catalyst was used in the cycloaddition reaction
between benzyl azide (1a) and phenylacetylene (2a). The
reaction was carried out using xerogel material at room
temperature in water without any reducing agent. After six
hours, 1-benzyl-4-phenyltriazole (3a) was isolated in 72% yield
(Table 1, entry 1). We have done the reaction under solvent
free condition using xerogel of Cu-MOG at room temperature
maintaining the same reaction condition (entry 3). This time,
we found the conversion to be moderate (76% yield). Then we
reduced the amount of catalyst and found that 2 mol% of the
catalyst is optimum for the reaction (entry 4). After that, we
tried the reaction using the metallogel material, and we found
the conversion to be similar (entry 6). Then, we have done the
Fig. 1 Different type of microscopic images of Cu-MOG; (a) FESEM, (b) OM, (c) TEM and
(d) AFM image with 3D view of the gel and height profile.
reaction at 50
The mechanical strength of the Cu-MOG was estimated by excellent (91% yield). Further, to get 100% yield, we increased
amplitude sweep (Fig. S3, ESI). The plot of G' and G'' against the temperature from 50 to 80 . But we got a mixture of
angular frequency ( ) reveals that the gap between G' and G'' 1,4 and 1,5 regioisomers in the ratio of 68:26 (entry 9).
℃, and the conversion was found to be
℃
℃
ω
was large for the entire region of frequency and their To explore the scope of the catalytic reaction under the
frequency-invariant character shows the viscoelastic nature of optimised condition, we have carried out the reactions
the hydrogel (Fig. S3, ESI). From the thermogravimetric between different azides and the alkynes with both electron
analysis (TGA) plot, we can conclude that the Cu-MOG is stable withdrawing (CF3) and donating (Me, OMe) groups, and the
up to 190
℃ (Fig. S4, ESI). To check the porosity of the Cu- corresponding 1,4-disubstituted triazoles were isolated in
MOG, we have performed the N2 gas sorption study with the moderate to excellent yield (Table 2). All the compounds have
xerogel. The permanent porosity of Cu-MOG was confirmed by been characterised by 1H and 13C NMR spectroscopy, which
the reversible N2 sorption measurements at 77K, which were found to be same as those of the reported compounds.
revealed type-III adsorption isotherm indicating the The turn over frequency (TOF) of the reaction is higher than
mesoporous nature of the material (Fig. S5, ESI). Further, the the corresponding reactions catalysed by the reported Cu(I)
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xerogel15 and competitive to some of the MOFs36 (Table S1,
ESI).
Table 2 Synthesis of various 1,4-disubstituted triazole derivatives
Entry
1
R1
R2
Yielda (%)
91
Bn
Ph
Scheme 1 Plausible mechanism for the reduction and oxidation of Cu(II) centre.
2
Bn
4-MeC6H4
4-OMeC6H4
4-CF3C6H4
C6H12
89
3
Bn
92
4
Bn
80
From the results, we can conclude that, without any reducing
agent, the Cu(II) is reduced to Cu(I) in the reaction medium
which is the active catalyst for the cycloaddition reaction. To
find out any possible role of oxygen, we studied the reaction of
1a with 2a under an argon atmosphere. Interestingly, we
found the conversion to be very poor (yield = 11 %), showing
that the oxygen has a vital role in the reaction. It has been
shown that Cu(II) is reduced to Cu(I) by 2,3-dihydroxybenzoic
acid (DBA) in aqueous solution via electron transfer from DBA
to Cu(II) with the formation of phenoxyl radical.37 Hence, from
the reported mechanism of reduction, we suggest that
through the intramolecular electron transfer, the phenoxyl
radical is formed, and Cu(II) bonded to the ligand carboxylate
group and the phenoxide group is reduced to Cu(I) species.
The XPS (Fig. S9, ESI) study of the Cu-MOG after the catalysis
confirms the reduction of Cu(II) to Cu(I). In the presence of
oxygen, the Cu(I) is oxidised to Cu(II) and the catalytic cycle
continues (Scheme 1).
5
Bn
Bn
56
6
CH2OH
51
7
Ph
Ph
84
8
Ph
4-MeC6H4
4-OMeC6H4
4-CF3C6H4
Ph
84
9
Ph
82
10
11
12
13
Ph
68
Cyclohexyl
Cyclohexyl
Cyclohexyl
62
4-OMeC6H4
4-CF3C6H4
51
84
aYield of the isolated product.
The heterogeneity is the most advantageous property of the
metallogel catalyst which can be very easily recovered and
reused. After the reaction, the reaction mixtures were washed
thoroughly with ethyl acetate (3 mL × 3 times) and Cu-MOG
was recovered by centrifugation. Then before use for the next
catalytic cycle, the Cu-MOG was dried in an oven at 50
℃ for 1
hour. During the process, the colour of the Cu-MOG remains
unchanged. After the first reaction, the xerogel of Cu-MOG
was used for the second, third, fourth and fifth cycle. During
the five catalytic cycles (Fig. 2) the Cu-MOG exhibits a good
catalytic activity (91%−76%) without significant loss.
a
Table 3 Cycloaddition of various epoxides with CO2
From the PXRD data (Fig. S6, ESI), we can conclude that the
integrity of Cu-MOG remains unaltered after the catalytic
reaction and the FESEM elemental analysis (Fig. S10, ESI)
confirms the uniform distribution of copper in the Cu-MOG
before and after the CuAAC reaction. The ICPMS measurement
of the reaction solution after the isolation of the gel clearly
indicates that, the leaching of copper is very negligible (54
ppb).
Entry
1
Substrate
Conversionb (%)
80
TON
400
2
3
4
74
61
62
370
305
310
aReaction conditions: epoxide (10 mmol), catalyst (0.02 mmol), TBAB (0.32 g, 10
mol%) under CO2 (1 bar), 300 K and 48 h. bFrom 1H NMR (Fig. S11-S22, ESI).
In a typical reaction, cycloaddition of the epoxide (10 mmol)
was carried out with carbon dioxide purged at 1 bar pressure
in the presence of 0.02 mol of the Cu-MOG and 10 mol%
Fig. 2 Recyclability study of Cu-MOG for five cycles.
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In conclusion, we have synthesised a new multiresponsive Cu-
MOG using the ligand, Na2HL which can behave as a reducing
agent for the reduction of Cu(II) to Cu(I). The MOG is an
efficient precatalyst for highly regioselective Huisgen 1,3-
dipolar cycloaddition reaction of azides and terminal alkynes
under solvent free condition. The expected 1,4-disubstituted
products were obtained in a moderate to excellent yield
without using any reducing agent. Also, the catalytic system
can work under click condition and afford triazoles with a
moderate conversion. Further, being a heterogeneous catalyst,
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of catalytic activity for at least more than five cycles.
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