Solvent-free cycloaddition of CO2 and propylene oxide to cyclic carbonates using different ligand metal complexes

Article abstract of DOI:10.1080/15533174.2016.1228673

Tetradentate ligand metal complexes salen-Co(III)OBzF₅, salen-CH₃Co(III)OBzF₅, salen-CH₃Co(III)Br, and salen-ClCo(III)Br were synthesized. These complexes were used as catalysts for the chemical fixation of CO

Full text of DOI:10.1080/15533174.2016.1228673

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Solvent free cycloadition of CO2 and Propylene Oxide to Cyclic carbonates using different
ligand metal complexes
Shilpa Narang1, Rajeev Mehta2,*, S.N.Upadhyay3
1School of Chemistry and Biochemistry, Thapar University, Patiala- 147004, Punjab, India
2Department of Chemical Engineering, Thapar University, Patiala-147004, Punjab, India
3Department of Chemical Engineering, IIT BHU, UP, India
*Corresponding author: rajeevmehta33@yahoo.com
This work was supported in part by All India Council of Technical Education, New Delhi (India)
under Project No. 8023/RID/RPS/068/11/12.
Abstract
Tetradentate ligand metal complexes salenCo(III)OBzF5, salen-CH3Co(III)OBzF5, salen-
CH3Co(III)Br and salen-ClCo(III)Br have been synthesized. The complexes were used as
catalyst for the chemical fixation of CO2 and propylene oxide to cyclic carbonates.
Tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, [PPN]+Cl-
([PPN]+ = bis(triphenylphosphine)iminium) and tetrabutyl ammonium bromide were used as co-
catalysts in the reactions. Additionally, the catalytic activity of napthenCoSO4 and salenCoSO4
was also explored using the above co-catalysts. The conversion of propylene oxide to cyclic
carbonate was found to increase with an increase in temperature and pressure of carbon dioxide.
It is significant to note that cyclic carbonate product could be obtained at temperature as low as
60oC and pressure of 20 bar in just 2 h. Further, it has been observed that introduction of
electron donating methyl substituent increases the nucleophilic character of the catalyst and
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reaction conversion. On the contrary, electron withdrawing substituent like chlorine decreased
the cyclic carbonate conversion thereby making it a weaker nucleophilic catalyst.
Keywords
carbon dioxide; propylene oxide; cyclic carbonate; catalyst; co-catalysts; ligand metal complexes
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INTRODUCTION
Ligand metal complexes have myriad applications in various reactions such as oxygenation,
hydrolysis, electro-reduction and decomposition etc [1-3]. The metal core in the complexes can
provide Lewis acid center and thus they can act as catalysts for a number of reactions. One such
reaction is coupling reaction between carbon dioxide and heterocyclics such as epoxides to
synthesize cyclic carbonates. Synthesis of cyclic carbonates from CO2 has turned out to be of
great interest since CO2 is an inexpensive and greener source [4-7]. Further, there have also been
continued efforts to use CO2 as a carbon source in different organic reactions (such as synthesis
of methanol, salicylalic acid, urea etc) and one such reaction is the addition of CO2 to epoxides
to produce cyclic carbonates [6-7]. Cyclic carbonates are products of great commercial importance
as they are used for various purposes such as electrolytic elements of lithium secondary batteries,
polar aprotic solvent and monomers for synthesis of polycarbonates and chemical intermediates
for preparing medicines or agricultural chemicals [8-11]
.
CO2 is a stable molecule with bond angle of 180o and thus it becomes a challenge to use it in
chemical reactions. Thus, its reaction with appropriate reactants such as reactive heterocyclics in
the presence of suitable catalysts for the reaction under milder conditions (temperature, pressure
and reaction time) is of great interest. Many studies have been carried out to synthesize cyclic
carbonates using various ionic liquids, inorganic salts, organometallic and ligand metal
complexes [12-21]. It has been observed that most of the reported studies deal with the catalyst
systems which are either expensive or involves a tedious methodology to prepare. The present
work is an attempt to synthesize cyclic carbonates in solvent free system by fixing CO2 with
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propylene oxide in the presence of various ligand metal complexes as catalysts and different co-
catalysts. Interestingly, the catalysts used in the reactions are economical, achiral and offers easy
route of preparation without any further purification.
In our previous work [22], we had reported preliminary catalytic studies of two different ligand
metal complexes (salenCoSO4 and naptheCoSO4) for cycloaddition of propylene oxide and
carbon dioxide. This further motivated us to comprehensively investigate the effect of more
number of new complexes with different electron densities and nucleophilic groups on this
cycloaddition reaction. Additionally, the enhanced catalytic activity of the previously
synthesized complexes has been observed by changing the co-catalysts ([PPN]+Cl-, tetrabutyl
ammonium bromide, hexdecyltrimethyl ammonium bromide and tetradecyltrimethyl ammonium
bromide). These quaternary ammonium ions were used as co-catalysts since they bear bromide
ion as nucleophilic group which can open up the epoxide ring. There are number of studies
[21, 23-24]
available which uses different type of salen cobalt based catalysts
in such CO2 based
fixation reactions. It is noteworthy that the use of co-catalysts as hexdecyltrimethyl ammonium
bromide and tetradecyltrimethyl ammonium bromide has not much been explored in such
reactions. As formed out of studies, the catalytic activity of different ligand metal complexes
used in various type of reactions can also be controlled by simply changing the electron density
around the catalyst.
Experimental
Materials:
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Propylene oxide, DMAP, [PPN]+Cl- ([PPN]+ = bis(triphenylphosphine)iminium), 4-chloro-o-
phenylenediamine,
4-methyl-o-phenylenediamine
were
purchased
from
Aldrich.
Salicylaldehyde and o-phenylenediamine were purchased from Loba Chemicals India.
Tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetrabutyl
ammonium bromide, cobaltous acetate and cobalt sulphate were purchased from SD Fine
Chemicals, India. Ethanol was purchased from E Merck (Germany) and pentaflourobenzoic acid
was purchased from High Media.
Measurements:
1HNMR was also used for the identification and quantification of the obtained crude product.
Both 1HNMR and 13C NMR spectra were recorded using CDCl3 as solvent and tetramethysilane
as an internal standard on Jeol ECS-400 Spectrophotometer. FTIR measurements were carried
out using Agilant Cary-660 spectrophotometer from 650 to 4000 cm-1. ATR mode was used for
all the measurements. CHNS analysis was carried on Thermo Scientific Flash 2000 Organic
Elemental Analyzer equipped with Themal conductivity detector.
Experimental
Procedure for the Synthesis of catalysts:
One equivalent of R-phenylenediamine (R=H, CH3 and Cl) and two equivalents of
salicylaldehyde or napthaldehyde were dissolved in ethanol. The refluxing and stirring of the
reaction was carried for 3-4 hours and then the mixture was cooled to room temperature. The
precipitate thus obtained were filtered, washed with cold ethanol and finally dried in oven. The
corresponding reaction scheme is shown in Figure 1. Complexation of the ligand was carried out
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separately with cobalt by refluxing with continuous stirring a 1:1 molar mixture of the
synthesized ligand and the corresponding salt dissolved in methanol for 3 h. The solid products
thus obtained were filtered using vacuum filtration. In order to bind pentafluorobenzoate or
bromine to the metal axial site, one equivalent of ligand cobalt complex and one equivalent of
pentafluorobenzoic acid/lithium bromide were taken and stirred open to air (for 18 h) using
1
13
toluene as solvent. The obtained products were analysed using HNMR, C NMR, CHNS and
FTIR.
1H NMR, 13C NMR and CHNS analysis of the Ligands:
N,N′-bis-(salicylaldehyde)-4-methyl-o-phenylenediamine (salen-CH3)
Salicylaldehyde (0.86 ml, 0.008 mol) with 3,4-diaminotoluene (0.5g, 0.004 mol) were used for
synthesizing this ligand by employing the general procedure.
Bright orange precipitates. (1H NMR, 400 MHz, CDCl3): δ 8.61(s, CH=N); 6.88-7.38 (m, Ar-
H); 11.01 (s, OH), 2.42 (s, 3H, CH3) (13C NMR, 100 MHz, CDCl3): 163.67, 161.42, 142.56,
139.99, 133.38, 132.26, 128.40, 120.46, 119.03, 117.63, 21.23 CHNS: Calculated for
C21H18N2O2: C: 75.4; H: 5.38; N: 8.38 Found: C:75.8; H: 5.49; N: 8.49
N,N′-bis-(salicylaldehyde)-4-chloro-o-phenylenediamine (salen-Cl)
For synthesizing this catalyst salicylaldehyde (0.86 ml, 0.008 mol) and 4-chloro-o-
phenylenediamine (0.568g, 0.004 mol) were taken and the general procedure as mentioned above
was followed.
Brown coloured product. (1H NMR, 400 MHz, DMSO): δ 12.7 (s, CH=N); 7.0-8.06 (m, Ar-H);
13.3 (s, 1H, OH) (13C NMR, 100 MHz, DMSO): 157.97, 152.96, 131.76, 126.31, 123.05,
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122.49, 118.96, 117.34, 112.54, 111.14 CHNS: Calculated for C20H15N2O2: C:67.3; H: 4.2; N:
7.85, Found: C:65.8; H:3.88; N: 8.04.
N,N′-bis-(2-hydroxy-1-naphthyldehyde)-o-phenyleneldiamine (napthen)
The precursors 2-hydroxy-1-naphthaldehyde (1g, 0.0058 mol) and o-phenylenediamine (0.313g,
0.0029 mol) were taken for the synthesis of the ligand using the general procedure.
Bright Yellow precipitates. (1H NMR, 400 MHz, DMSO): δ 9.5 (s, CH=N); 6.86-8.9 (m, Ar-H);
15.1 (s, OH) (13C NMR, 100 MHz, DMSO): 168.54, 157.22, 138.51, 136.65, 132.99, 128.92,
127.28, 126.84, 123.50, 121.42, 120.47, 119.61, 109.20 Napthen: Calculated for C28H20N2O2:
C: 80.0; H: 4.76; N: 6.66 Found: C: 81.8; H: 4.86; N: 6.83.
FTIR spectra of the catalysts:
The FTIR spectra of the SalenCoOBzF5 catalyst are shown in Figure 2. The band around 1275
cm−1 attributed to C−O stretch in salen shifted to higher frequency by about 30−40 cm−1 (around
1310 cm-1) which confirms the participation of oxygen in the C−O−M bond. The band for the
ligand at 1616 cm−1 due to C=N stretching shifted to a lower frequency of 1572 cm−1 and the
band due to C-N at 1480 cm-1 shifted to a lower frequency by about 20 cm-1 (around 1460 cm-1)
further confirming the coordination of nitrogen with the metal.
In case of FTIR spectra of Salen-CH3 CoBr and Salen-CH3 CoOBzF5 (Figure 3) the band around
1268 cm−1 attributed to C−O in Salen-CH3 shifted to higher frequency by about 30−40 cm−1
(around 1305 cm-1) confirming the participation of oxygen in the C−O−M bond. The band for
the ligand around 1613 cm−1 corresponding to C=N stretching shifted to a lower frequency of
1597 cm−1 and the band due to C-N at 1484 cm-1 shifted to a lower frequency by about 20 cm-1
(around 1457 cm-1) further confirming the coordination of nitrogen with the metal.
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FTIR spectra of Salen-Cl Co(III)Br catalyst (Figure 4) also shows band around 1245 cm−1
attributed to C−O stretch in Salen-Cl which shifted to higher frequency around 1260 cm-1 on
binding to cobalt metal confirming the formation of C−O−M bond. The band due to C=N strech
in the ligand around 1635 cm−1 shifted to a lower frequency of 1610 cm−1 and the band
corresponding to C-N stretch about 1480 cm-1 shifted to a lower frequency around 1460 cm-1
thereby confirming the coordination of nitrogen atom of the ligand with the metal. Similar
pattern was observed in case of Salen CoSO4 and Napthen CoSO4 spectra (Figure 5) [19]
.
These results indicate that Salen Co(III)OBzF5, Salen-CH3 Co(III)OBzF5, Salen Co(III)Br and
Salen-Cl Co(III)Br catalysts have been successfully synthesized.
Procedure for the Synthesis of cyclic carbonates :
A mixture of catalyst and co-catalyst was dispersed in propylene oxide (25 ml) and added to 100
ml reactor. The reaction was then carried out at the desired temperature and CO2 pressure of 20
bar. The mixture was stirred for a desired period of time with an overhead mechanical stirrer.
After a predetermined time, the reactor was cooled and the remaining CO2 was vented in a fume
1
hood. Finally, a small aliquot of the resultant product was taken from the reactor for H NMR
analysis. The obtained product can be purified by dissolving it in dichloromethane and washing
with aqueous HCl. The corresponding reaction and reactor setup for the reaction are given in
Figure 6 & 7.
The proposed mechanism for the chemical fixation of propylene oxide and CO2 is shown in
Figure 8 and is close agreement with the literature [24]. It is supposed that the metal ion in the
salen/napthen metal complexes acts as electrophilic metal center which coordinates to propylene
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oxide units. Further, the ring opening of epoxide ring is assisted by nucleophile (Br- or Cl-) of
the catalyst or the co-catalyst. It is supposed that the electropositive part of the co-catalyst reacts
with CO2 molecule to form carbamate salt which on further forms cyclic carbonate.
RESULTS AND DISCUSSION
Table 1 gives conversion of propylene oxide and CO2 to cyclic carbonate using various
catalyst/co-catalyst systems. The actual state of monomers under high pressure and temperature
conditions should be known before attempting to analyse the reaction data. Using the Clausius-
Clapeyron equation, it can be easily shown that at CO2 pressure of 20 bar and temperature of
100oC, the boiling point of propylene oxide is more than 200oC which is nearly two times higher
than the reaction temperature hence propylene oxide must exist in liquid state though its boiling
point at atmospheric pressure is 35oC. Thus, it is expected that at polymerization temperatures of
60, 80 and 100oC, propylene oxide will exist in liquid state under high pressure. The reaction
+
between propylene oxide and CO2 with salenCoSO4/Bu4 Br- and napthenCoSO4/[PPN]+Cl- as
catalyst systems (Table 1, Entry 1,2, 15 & 16) shows that on increasing the temperature the
conversion to cyclic carbonate increases. It has been observed that DMAP as a co-catalyst does
not give any conversion with either salenCoSO4 or napthenCoSO4 (Table 1, Entry 3, 4 and 14).
Among the explored catalysts salen-CH3CoBr/[PPN]+Cl was found to be the most active catalyst
(Table 1, Entry 17). It was observed that the conversion value increased from 40% to around
50% on increasing the pressure from 10 to 20 bar keeping the reaction time constant (Table 1,
+
Entry 11 and 12). Respective conversions with salen CoOBzF5/Bu4 Br- were found to increase
with an increase in reaction time from 56.8% in 2 h to 67.3% conversion in 4 h (Table 1, Entry 8
and 9).
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In order to study the steric effect on the catalyst, electron donating group was introduced on the
catalyst salen-CH3 Co(III)OBzF5 and its effect on the reaction was investigated. It is thought that
methyl substituent will increase the electron density on the ligand and thereby will decrease the
Lewis acid character of the Co(III) metal center by making the lone pair of electrons on nitrogen
more available to cobalt and thus enhance the Lewis base or nucleophilic character of the
pentafluorobenzoate group. Thus, it was observed that salen-CH3CoBr bearing electron donating
methyl group and relatively strong nucleophile as bromide ion was able to enhance activity of
the catalyst and gave 67.0% conversion with [PPN]+Cl- as co-catalyst (Table 1, Entry 17). The
obtained conversion was found to be significant in comparison to the conversion obtained with
salenCo(III)OBzF5 catalyst for the same reaction time of 2h (Table 1, Entry 13). Further,
introduction of electron withdrawing group decreased the electron density on the ligand by
making the lone pairs of electrons on nitrogen of diamine moiety less available to cobalt metal
center. This turns cobalt metal into a strong Lewis acid metal center and its conjugate bromide
into a relatively weak nucleophile and thus decreases the conversion of propylene carbonate
(Table 1, S.No. 18 and 19).
1H NMR, 13C NMR , HSQC and FTIR of cyclic carbonates:
1H NMR spectra of propylene carbonate confirms the formation of cyclic carbonates in all the
samples (Figure 9). A doublet around 1.49 ppm corresponds to methyl protons, triplet around
4.0 and 4.5 ppm corresponds to CH2 protons, and multiplet about 4.8 ppm refers to CH protons.
The multiplet at 4.8 ppm confirms the coupling of CH with both methyl and methylene
hydrogens.
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13C NMR spectra of propylene carbonate (Figure 10). The chemical shift around19 ppm
corresponds to primary carbon and that around 70 ppm and 73 ppm corresponds to secondary
and tertiary carbons respectively. The shift due to carbonyl carbon appears around 155 ppm.
HSQC spectrum further clearly confirms the formation of propylene carbonate (Figure 11). The
shift around1.5 ppm corresponds to primary carbon at 19 ppm. The shift around 4.2 and 4.5 ppm
corresponds to secondary carbon at 70 ppm and the one around 4.8 ppm corresponds to tertiary
carbon around 73 ppm.
The obtained propylene carbonate was also characterized by FTIR giving bands due to C=O at
around 1790 cm−1 and C−O stretching at about 1180 cm−1 which are the characteristic bands of
propylene carbonate and thereby confirm the formation of the product (Figure 12).
CONCLUSION
Cyclic carbonate was successfully synthesized from propylene oxide and CO2 using Salen CoX,
Napthen CoX (X=SO4, OBzF5) and Salen-CH3 CoBr and Salen-Cl CoBr as catalyst.
Tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, [PPN]+Cl- and
tetrabutyl ammonium bromide and DMAP (dimethylaminopyridine) were used as co-catalysts in
the reaction. It was observed that % conversion of propylene oxide to cyclic carbonates increases
with an increase in temperature and pressure. Higher monomer to catalyst ratio does not give
any homoether linkages and led to an increase in conversion in same reaction time. It was
observed that introduction of electron donating group such as methyl substituent will increase the
electron density on the ligand and will thereby increase the propylene carbonate conversion by
increasing the nucleophilic activity of the catalyst. On the other hand, introduction of electron
withdrawing group on the ligand will decrease the nucleophilic activity of the catalyst and thus
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will decrease the propylene carbonate conversion. Thus the best activity was achieved using
Salen-CH3 CoBr/[PPN]+Cl catalyst systems and least catalytic activity was achieved by Salen-Cl
CoBr/[PPN]+Cl-
.
Notes
The author declares no financial interest.
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Table 1: Reactions between Propyleneoxide and CO2 using different catalyst/co-catalyst
systems
Entry Catalyst/Co-catalyst
Temperature
Reaction
%
(oC)
Time (h)
Conversion
24.4
SalenCoSO4/ Bu4N+Br-
60
2
2
2
1.
SalenCoSO4/ Bu4N+Br-
100
60
50.7
2.
SalenCoSO4/DMAP
No
3.
conversion
No
SalenCoSO4/DMAP
80
2
4.
conversion
13.6
SalenCoSO4/[PPN]+Cl-
60
100
80
2
2
2
5.
6.
7.
SalenCH3-CoSO4/[PPN]+Cl-
SalenCoSO4/Hexadecyl ammonium
bromide
19.1
50.0
SalenCo(III)OBzF5/ Bu4N+Br-
SalenCo(III)OBzF5/ Bu4N+Br-
SalenCo(III)OBzF5/Tetradecyltri
methylammonium bromide
SalenCo(III)OBzF5/[PPN]+Cl-
#SalenCo(III)OBzF5/[PPN]+Cl-
SalenCo(III)OBzF5/[PPN]+Cl-
NapthenCoSO4/DMAP
80
80
80
2
4
2
56.8
67.3
36.3
8.
9.
10.
80
80
2
2
2
2
50.2
40.9
11.
12.
13.
14.
100
60
55.6
No
conversion
No
NapthenCoSO4/[PPN]+Cl-
60
2
15.
Conversion
51.8
NapthenCoSO4/ [PPN]+Cl-
SalenCH3-Co(III)Br /[PPN]+Cl-
100
100
2
2
16.
17.
67.0
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SalenCl-Co(III)Br/[PPN]+Cl-
SalenCl- Co(III)Br/ Bu4N+Br-
100
100
2
2
20.0
23.0
18.
19.
Mo/Io =2000:1, Pressure=20 bar, % Conversion has been calculated through 1HNMR.
# Pressure= 10 bar,*Mo/Io =1000:1
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Figure 1: Ligand used for the synthesis of cyclic carbonates. (A) [ N,N′-bis-(salicylaldehyde)-o-
phenylenediamine] (salen), [ N,N′-bis-(salicylaldehyde)-4-methyl-o-phenylenediamine] (salen-
CH3) and [ N,N′-bis-(salicylaldehyde)-4-chloro-o-phenylenediamine] (Salen-Cl) (B) [N,N′-bis-
(2-hydroxy-1-naphthyldehyde)-o-phenyleneldiamine] (Naphthen)
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Figure 2: FTIR analysis of Salen, SalenCo and SalenCoOBzF5
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Figure 3: FTIR spectra of Salen-CH3, Salen-CH3Co, Salen-CH3Co(III)OBzF5, Salen-CH3
CoSO4 and Salen-CH3CoBr
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Figure 4: FTIR spectra of Salen-Cl, Salen-Cl Co and Salen-Cl CoBr
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Figure 5: FTIR Spectra of Napthen and NapthenCoSO4
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Figure 6: Formation of Cyclic carbonate from Propylene Oxide and CO2
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Figure 7: Diagram of reactor used to carry out synthesis of cyclic carbonates
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Figure 8: Probable mechanism for the formation of Cyclic carbonate
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Figure 9: 1HNMR of cyclic carbonate
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Figure 10: 13C NMR spectra of cyclic carbonate
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Figure 11: HSQC spectra of cyclic carbonate
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Figure 12: FTIR spectra of pure cyclic carbonate
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