Cu-tetracatechol metallopolymer catalyst for three component click reactions and β-borylation of α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1039/d0cc05823h

Phenol-metal coordination polymers are used in applications such as catalysis, sensing and separation science. In addition, combining eco-friendly conditions with economical and handling advantages of the polymeric catalyst is of interest to the community

Full text of DOI:10.1039/d0cc05823h

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DOI: 10.1039/D0CC05823H
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Cu-Tetracatechol metallopolymer catalyst for three component
click reactions and -borylation of , -unsaturated carbonyl
compounds
Received 00th January 20xx,
Accepted 00th January 20xx
ab
a
a
b
a
DOI: 10.1039/x0xx00000x
Saurabh Joshi, Yip Yong Jie, Tankut Türel, Sandeep Verma* and Suresh Valiyaveettil*
The phenol-metal coordination polymers are used in applications its derivatives are known to generate functional materials
such as catalysis, sensing and separation science. In addition, owing to their intrinsic properties such as redox behaviour,
combining eco-freindly conditions with economical and handling adhesiveness and strong coordinating ability with oxophilic
1
0-12
advantages of the polymeric catalyst is of interest to the metal ions (e.g., Fe, Cu).
community. Here, we report a simple one pot synthesis of
tetracatechol based ligand and its coordination polymer with organic synthesis with a number of applications.
Multicomponent reactions (MCRs) are powerful tools in
1
3-15
In
copper ions. The Cu polymer showed electrochemical potential addition, the use of water as a solvent makes the synthesis of
1
6
with the band gap of 1.01 eV. The BET surface area of the functional molecules more eco-friendly in nature. Click
2
-1
3 -1
metallopolymer was 91.19 m g with 0.14 cm g pore volume
.
reaction and alkyl boronic esters are widely used in synthetic
The polymer catalyst was used in one pot three component click organic chemistry, polymer chemistry, biomedical sciences,
1
7-19
reaction and in borylation of unsaturated carbonyl compounds synthesis of agrochemicals and pharmaceuticals.
with a maximum 99% conversion in water and good turnover Moreover, the catalyst which can drive these organic
efficiency even after 4 repetitive catalysis cycles. The Cu- transformations in an environmental friendly manner with
tetracatechol catalyst offer several advantages such as high added economic benefits is fascinating for both academic and
activity, easy handling, scalable, recyclable and cost effectiveness.
industrial perspective. In this communication, we present the
design, one pot synthesis and full characterization of a
tetracatechol ligand and its copper coordination polymer.
Catechol derivative 1 was designed and synthesized to explore
the formation of a metallopolymer through interfacial and bulk
polymerizations (Scheme 1, Fig. S1-S3, ESI). For insight into the
structure of the ligand, good quality single crystals were grown
in DCM/MeOH mixture (1:1), with a few drops of water added,
by slow evaporation. The crystal structure of the ligand is
shown in Fig. 1A. As expected, the four catechol groups are
organized to form a network structure through complexation
with copper ions. The copper-tetracatechol complex polymer
was prepared in one step and fully characterized. Copper is
chosen due to its oxophilicity, biological relevance and
The metallopolymers are considered as supramolecular
architecture by combining the orientation of the functional
groups on organic ligand and the dimensionality of the metal
1
coordination. Due to the presence of a large number of
transition metal ions with different coordination geometries,
polymers with interesting structures and properties have been
reported. Such polymers are used in biomedical applications,
fabrication of sensors, energy storage devices and as catalysts
2
-5
for organic transformations. Moreover, the structure and
properties can be fine-tuned using different external stimuli
such as applied potential or light to make smart stimuli
6
,
7
responsive functional materials.
Many higher order
structures such as 2D-polymer layers, 3D-metal–organic
frameworks and tunable metallopolymer based gels have been
20, 21
catalytic properties.
The Cu-tetracatechol polymer formed
upon mixing the ethanolic solution of copper acetate with the
ethanolic solution of tetracatechol ligand at 5 °C. Since the
polymerisation was instantaneous, low temperature influences
the kinetic parameters and drives the formation of the desired
and reproducible structure of the polymer.
8
,9
reported using metal coordination polymers. Catechol and
a. Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543. Email – chmsv@nus.edu.sg
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar
Interfacial polymerisation was carried out at the interface of
b.
the aqueous solution of copper acetate and ethyl acetate (EA)
Pradesh 208016, India. Email – sverma@iitk.ac.in
1
1
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available:
See solution of tetracatechol ligand (Fig. S4, ESI). After 24 h of
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Journal Name
2
6
accompanied by LMCT transitions (Fig. S4, ESI). The broad
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-
1
DOI: 10.1039/D0CC05823H
absorbance around 3300 cm (-OH) in the FTIR spectra of the
2
2
ligand disappeared after polymerization. Also, the observed
-1
peaks at  535,  600 &  680 cm indicates the presence of
1
1, 27
Cu–O bonds (Fig. S8, ESI).
They may be assigned to
symmetrical stretching of Cu-O-C bond of the polymer while
Scheme 1. Synthesis of tetracatechol ligand 1 and polymerisation with Cu (II) ions. The
plausible structure of the polymer is given in Figure 1B.
-1
-1
the peaks at 2900 cm and 1460 cm corresponds to aromatic
2
8
C-H stretching vibrations.
The copper based polymers/compounds are used in
polymerisation, the film was deposited on a quartz
2
9,
catalysis owing to redox behaviour between Cu(II) and Cu(I).
1
1
substrate (Fig. S5, ESI) and used for further characterisation.
3
0
Here, we explored the potential use of the polymer as a
As expected, the Cu-metallopolymer showed low solubility in
most organic solvents and water, reflecting its high molecular
weight with a network structure (Fig. 1B). The divalent
character of Cu centres in the catechol complex has been
3
1
catalyst for a click reaction. A three component [3+2]
cycloaddition was tested with the help of our Cu–tetracatechol
polymer. All components; catalyst, alkene, alkyl halide, sodium
ascorbate and sodium azide (Table 1, entry 1a) were directly
added in water/t-BuOH (3:1) mixture and stirred at 50 °C for
2
2-25
established by earlier studies by different groups.
The high
molecular weight of the polymer was confirmed by gel
permeation chromatographic (GPC) analysis of the soluble
fraction of the polymer in dimethylformamide (DMF) solvent.
The analysis of GPC traces gave a molecular weight of  428
kDa &  568 KDa, for polymers obtained from interfacial and
bulk polymerization, respectively (Fig. S6 and S7, ESI). The
thermogravimetric analysis (TGA) showed the polymer is
stable up to 190 °C, followed by sharp weight loss. The first
derivative of the TGA curve revealed one peak at 230 °C, which
corresponds to the maximum degradation temperature (Fig.
2
.5 h. The progress of the reaction was also monitored using
thin layer chromatography. After cooling the mixture, the
crude product was extracted in dichloromethane (DCM) and
the percentage conversion was calculated by comparing the
integration of peaks of product to that of reactants in the NMR
spectra of the crude products/mixture. The Cu-tetracatechol
polymer catalyst showed a conversion efficiency of 99% in 2.5
h (Fig. S9, ESI).
Table 1: Three component cyclization of alkyl bromide, sodium azide, and alkyne by
Cu-tetracatechol polymer.
1
C). The UV-Vis spectrum of polymer in DMF showed two
peaks at 445 & 660 nm for the Cu-metallopolymer obtained
from bulk polymerization, while maxima at 430 & 690 nm for
the polymer film obtained from liquid-liquid interface
polymerization (Fig. 1D). The observed peaks correspond to
the absorbance of Cu-catechol complex and charge transfer
peaks from catechol ligands to the Cu-metal centres along the
polymer backbone.26
R2
Br
Polymer (10 mg)
R1
+
+
NaN3
N
Water : t-BuOH (3:1)
N
N
R1
R2
o
50 C, 2.5 h
S.No
R1
R2
% Conversion
1
2
3
4
a
a
a
a
H
99
58
99
15
H
H
NO2
I
5
6
a
a
30
51
O
O
S
H
O
Standard reaction condition: alkyne (0.5 mmol), sodium azide (0.5 mmol), alkyl
bromide (0.5 mmol), in water and t-butyl alcohol (1.5/0.5 mL each) with Cu
polymer (10 mg). % conversion was calculated using NMR spectroscopy.
Even though a small amount (3 mg) of the catalyst was
sufficient to achieve a conversion efficiency of 99%, we
selected an optimum amount of catalysts (10 mg) to offer
maximum recovery and reusability of the polymer. Control
experiments without the polymer catalyst did not convert the
reactants into product fully within 2.5 h (Fig. S10, ESI).
The surface charge density (zeta potential) of the solid
polymer particles dispersed in water was -9.32 mV, indicating
an electron rich polymer backbone (Fig. S11, ESI). To further
support the high catalytic activity of the electron rich polymer,
the same click reaction was carried out without sodium
ascorbate (Table1, entry 1a) and achieved 99% conversion
Fig. 1 Crystal structure of ligand 1 (red – oxygen, black – carbon and blue – hydrogen)
(A). The plausible network structure of metallopolymer (B). TGA trace with first
derivative of the polymer (C), UV-Vis spectra (D) for interfacial film - solid state (——),
solutions of polymer film (—▲—) and bulk polymer (——) in DMF.
The UV-Vis spectra of solid state film showed two major
peaks at 456 nm and 675 nm because of ligand to metal
2
6
charge transfer (LMCT) transitions (Fig. 1D). Moreover the
obtained film was green in colour owing to complexation
2
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efficiency (Fig. S12, ESI). Also, the effect of substituents on the catalyst was quantified with respect to control rVeieawcAtritoicnle Ownilitnhe
reactants was explored and showed that aromatic alkynes no catalyst added. Control reaction did DnOoIt: 1p0r.o10c3e9e/Dd0wCCit0h58h2i3gHh
gave a better product yield as compared to aliphatic t-butyl conversion of reactants to product (Fig. S16, ESI). Our Cu-
alkyne (Table 1, entry 1-3). In addition, a significant reduction tetracatechol polymer showed good catalytic activity, high
in the conversion efficiency was observed in the presence of conversion rate, good reproducibility and high recycling
electron withdrawing and bulky groups (Table 1). The efficiency for the two important organic reactions (click and
reproducibility and reusability of the polymer were also tested borylation, Fig. 2).
in successive reactions. After each experiment (Table 1, entry Since LMCT transitions are accompanied with the shift in
1
a), the solid polymer catalyst was collected by centrifugation partial electron density from ligand to metal, redox potential is
and used for the next reaction. The polymer showed 99% expected to differ for polymer from its monomer. The cyclic
conversion efficiency for 4 repeated cycles (Fig. S12, ESI). Two voltammogram (CV) of tetracatechol showed reversible redox
positive control reactions were also carried out with CuSO
4
properties of the molecule with the reduction peak at -1.12 V
and sodium ascorbate, and Cu(I) without reducing agent. All and oxidation at 0.99 V (Fig. 3A). Only one type of redox
reactions give the same product confirming the catalysis by Cu potential can be attributed to the symmetry of the molecule,
ions (Fig. S13, ESI).
where all the catechol groups behave in the same manner. The
It is known that copper catalyses the -borylation of ,- HOMO-LUMO calculations for the tetracatechol ligand (1) gave
3
2
unsaturated carbonyl compounds to yield alkyl boronates. To a band gap of 2.11 eV. Subsequently, the corresponding
further prove the ability of designed Cu-tetracatechol polymer positions of HOMO and LUMO orbitals of the tetracatechol
as a catalyst, all reactants- acrylate, boronpinacolate and ligand (1) were calculated as -5.34 eV and -3.23 eV,
amine base (Table 2) along with 5 mg polymer, were mixed in respectively (Fig. 3A).
3
mL of water and stirred at room temperature in air for 3 h.
Table 2: -borylation of -unsaturated carbonyl compound by Cu – tetracatechol
polymer in water
Polymer (5 mg)
O
O
O
O
O
O
DMAP (0.05 eq.)
O
R1
R1
+
B
B
B
O
O
Water (3 mL), RT
O
R2
R2
3
h
S.No
R
1
R
2
% Conversion
1
2
3
b
b
b
H
99
99
99
CH3
H
CH3
Fig. 2 % conversion of reactant into product during repetitive catalytic cycles for three
component click reaction () and for -borylation of -unsaturated acrylate ()
using Cu-tetracatechol polymer.
CH2CH3
CH3
4
5
b
b
98
89
Cyclic voltammetry (CV) studies have been conducted with
solution of Cu-tetracatechol polymer in DMSO. CV traces
H2C
H
st
showed an oxidation potential at 0.205 V, while the 1
Standard reaction conditions: bis(pinacolato)diboron (1.0 equ.), acrylate (1.0
equ.), and DMAP (0.05 equ.) in water (3 mL) with 5 mg of solid Cu – tetracatechol
nd
reduction at -0.81 V and 2 reduction potential at -1.21 V (Fig.
polymer and stirred at RT for 3 h. % conversion was calculated using % conversion 3B). Two reduction potentials can be assigned to the redox
was calculated using NMR spectroscopy.
33
chemistry of catechol ligand. Using the CV data, HOMO and
LUMO orbital of the Cu-tetracatechol polymer was calculated
as -4.55 eV and -3.54 eV, respectively (Fig. 3B). Similarly, band
gap for the polymer gave an interesting low value of 1.01 eV.
This is due to the strong interactions of orbitals of both
partners and enhanced LMCT between the ligand and metal
centre along the polymer backbone. Such low band gap also
suggests that electrons can reach LUMO easily and can
participate in the catalytic reactions. The plausible mechanism
for the observed catalytic activity is shown in Fig. S17, ESI. Cu
NMR spectra were taken at different time periods to
monitor the product formation and to calculate the %
conversion. The alkene protons of acrylate starting material
disappeared from the spectrum and two new triplets appeared
in the upfield region of the spectrum, indicating the product
formation (Fig. S14 and S15, ESI). The NMR calculations
showed 99% conversion of reactant (Fig. S14, ESI). Different
acrylates were used to explore the variability of the reaction
and excellent % conversion was observed for all acrylates
indicating efficient catalysis (Table 2). The reusability and
reproducibility of the catalyst was checked by collecting the
products and reactants from the reaction mixture by liquid-
liquid extraction followed by adding fresh reagents (Table 2,
entry 1a) without any processing to regenerate the catalyst.
The catalyst gave >99 % conversion up to four cycles as
monitored by NMR spectroscopy (Fig. S15, ESI). The use of
(I) species is required for the reaction to proceed and formed
in situ via electron transfer from ligand to Cu (II) centres along
34
the polymer chain. The Cu (I) formed reacts with the alkyne
group to form Cu acetylide, which then reacts with the azide
group in a [3+2] cycloaddition to give Cu-triazole adduct.
During the work up, the Cu-triazole gets protonated and
release the copper ion, which becomes available for continuing
29
the catalytic cycle.
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A. Kumar, S. Bawa, K. Ganorkar, S. K. Ghosh and A.
DOI: 10.1039/D0CC05823H
K. Y. Zhang, S. Liu, Q. Zhao and W. Huang, Coord. Chem. Rev.,
5
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8
9
1
To further gain the details of the mechanism, the porosity
of the polymer was measured using Brunauer–Emmett–Teller
View Article Online
Bandyopadhyay, Inorg. Chem., 2020, 59, 1746-1757.
2
-1
(
BET) isotherm, which showed a surface area of 91.19 m g
2
016, 319, 180-195.
3
-1
with 0.14 cm g pore volume (Fig. S18, ESI). This suggests that
the catalytic reactions are taking place at the catalytic sites
along the pore surfaces of the Cu-tetracatechol catalyst. The
mechanism for the borylation reaction is given in the earlier
literature reports.32 This includes the role of amine added to in
situ deprotonate nucleophilic water-boron adduct and form
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Fig. 3 Cyclic voltammetry for tetracatechol ligand (A) and polymer (B) in DMSO.
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obtained from the CV measurements. The BET analysis gave a
2
2
2
-1
3 -1
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used as a catalyst for three component click reaction and
borylation of unsaturated carbonyl compounds. In both cases,
a maximum (>99 %) conversion efficiency of reactant into
product was observed in repeated cycles indicating a high
catalytic turnover efficiency of the polymer. The catalyst is
eco-friendly, easy to synthesize, recyclable and showed wide
applicability. Such low cost tendering catalysts are useful and
needed in many water based organic transformations.
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