Bifunctional coordination polymers as efficient catalysts for carbon dioxide conversion

Article abstract of DOI:10.1002/aoc.5202

The multidentate ligand H₂L upon complexation with Zn (II) and Cd (II) provide a one-dimensional polymeric networks. These coordination polymers (CPs) CP-1 and CP-2 containing Zn (II) and Cd (II) metals respectively are well characterized. The

Full text of DOI:10.1002/aoc.5202

Received: 4 June 2019
DOI: 10.1002/aoc.5202
Revised: 18 July 2019
Accepted: 1 August 2019
F U L L P A P E R
Bifunctional coordination polymers as efficient catalysts for
carbon dioxide conversion
1
,2
1,2
3
Rajendran Arunachalam | Eswaran Chinnaraja
| Arto Valkonen |
3
1,2
Kari Rissanen | Palani S. Subramanian
1
Inorganic Materials and Catalysis
The multidentate ligand H L upon complexation with Zn (II) and Cd (II) pro-
Division, CSIR‐Central Salt and Marine
Chemicals Research Institute (CSIR‐
CSMCRI), Bhavnagar 364002, Gujarat,
India
2
vide a one‐dimensional polymeric networks. These coordination polymers
(CPs) CP‐1 and CP‐2 containing Zn (II) and Cd (II) metals respectively are well
2
characterized. The single crystal structural analysis confirms the formation of
one‐dimensional coordination polymer with zigzag fashion in CP‐1 and ladder
chain CP‐2. Both the CPs are applied as catalysts to synthesize various cyclic
carbonates from epoxides and carbon dioxide. The catalysts are giving better
conversion under solvent‐free and additive‐free condition using 10 bar CO2
and 100 °C as optimized pressure and temperature. The detailed kinetic exper-
iments suggesting the first order kinetics, the energy of activation (Ea) is calcu-
lated for this catalytic conversion.
Academy of Scientific and Innovative
Research (AcSIR), Ghaziabad 201002,
India
3
Department of Chemistry, University of
Jyvaskyla, Jyväskylä FI‐40014, Finland
Correspondence
Palani S. Subramanian, Inorganic
Materials and Catalysis Division, CSIR‐
Central Salt and Marine Chemicals
Research Institute (CSIR‐CSMCRI),
Bhavnagar‐ 364002, Gujarat, India.
Email: siva@csmcri.res.in
KEYWORDS
activation energy, CO
2
utilization, coordination polymers, cyclic carbonates, reaction kinetics
Funding information
Academy of Finland, Grant/Award Num-
ber: 314343; CSIR for Indus Magic; DST‐
SERB New Delhi, Grant/Award Number:
SR/S1/IC-23/2011
1
| INTRODUCTION
CO to cyclic carbonate or polycarbonates from epoxide
2
[
13]
or diol is considered potential. Most of the homogenous
catalytic systems thus reported are found to be salen and
The alarmingly increasing carbon dioxide level in the
atmosphere by industrial and other pollutants leads to
[
14–
salphen complexes with a wide variety of metal ions.
[1]
16]
drastic climate change globally. This freely available C‐
source in the atmosphere on one side causing serious
health hazardous, it can also be effectively converted into
In this series, the Zn‐salphen catalyst in combination
1
with tetrabutylammonium iodide (TBAI) as co‐catalyst,
reported by A. W. Kleij and co‐workers give maximum
[2–4]
[17]
useful value‐added products
for mankind. To this pur-
conversion of 94% at 45 °C in 18 hr at 10 bar CO2.
pose, the inert and thermodynamically stable CO might
be made reactive by some judicious choice of catalysts.
In this regard researchers worldwide are attempting to
The same group has reported a modified Zn‐salphen cata-
lyst with 80% conversion adapting the same reaction time,
but at reduced temperature 25 °C, and pressure of 2 bar
2
[
5]
[
6]
[18]
convert CO into many value‐added products such as
CO2. Similarly the Zn‐clusters reported by K. Mashima,
2
[7]
[8]
[9]
carboxylic acids (acetic acid , formic acid ), alcohols
with almost similar ambient condition (25 °C, 1 atm) gives
[10,11]
[19]
(methanol, ethanol), organic carbonates
(diethyl car-
94–99% conversion with reaction time ranging 6–20 hr.
[12]
bonate, dimethyl carbonate) and hydrocarbons
etc.
In the same trend, the Al‐salen chiral complex by M.
Among the various products, the catalytic conversion of
North although works at ambient conditions, the use of
Appl Organometal Chem. 2019;e5202.
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1 of 12
https://doi.org/10.1002/aoc.5202

2
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ARUNACHALAM ET AL.
+
chiral moiety and long reaction time are remain unad-
dressed.
[Zn L ]H and the m/z = 899.90 (calc. 899.08) for CP‐2
2
2
[20–22]
+
is attributed to [Cd L ]H (S1‐S2). This mono positive
2
2
Similarly, the Al – triphenolate based catalyst by A.W.
Kleij and co‐workers giving 96% conversion with 960
ions obtained by successive de‐protonation of the labile
–NHs of ligand H L indicate a monomeric units of the
2
[23]
TOF ; an addition of trace amount of TBAI as a co‐
polymers. The elemental analysis establishes that the
polymeric units CP‐1 and CP‐2 exists with one and five
water molecules respectively. The absorption spectra of
CP‐1 and CP‐2, show an absorption maxima at 290 nm
correspond to ligand centered transitions along with a
weak shoulder at 265 nm, attributable to LMCT transi-
tions (Figure 1).
−
1
catalyst was reported to enhance its TOF to 36000 h
and suggests it is highly efficient.
[23]
Thus the simple
metal complexes with Al, and Zn mentioned above are
[
24]
working as catalysts under ambient conditions,
the
metal complexes with various supramolecular assemblies
[
25]
[26–30]
such as MOF,
coordination polymers
are consid-
[
31]
ered important for such catalytic applications,
since
they are composed with multi‐metallic active sites. With
this in mind, we in the present report describe the synthe-
size of two important one‐dimensional coordination poly-
mers (CPs) of Zn (II) and Cd (II) complex with
2
.1 | Structural description by single
crystal XRD
multidentate ligand H L and their application as catalyst
2
The suitable crystals of CP‐1 were obtained by diffusion of
acetonitrile into its aqueous solution. The monoclinic crys-
tal structure showed six‐coordination and established a
distorted octahedral geometry around Zn (II) ion as shown
in Figure 2. The two N O cleft from H L coordinated to the
in converting CO to cyclic carbonate through cycloaddi-
2
tion reaction. More strikingly both these catalysts work-
ing efficiently without any co‐catalyst provides 87–99%
of terminal cyclic carbonate with highest selectivity gains
2
2
significance in view of CO utilization.
2
metal centre are almost orthogonal to each other showing
an angle value between N O‐Zn coordination planes of
2
about 83°. In the complex each ligand is oriented in the
polymeric form along M‐M and the motif is extended along
the x‐axis. The crystallographic structure establishes that
the complex is composed of a tetracationic monomeric unit
and are extended in a polymer chain. Each tetracationic
2
| RESULTS AND DISCUSSION
The multidentate ligand H L with glutarate‐di‐aza spacer
was synthesized following our recent reports.
ligand H L upon complexation with Zn (NO ) and Cd
2
[32]
The
4+
[Zn (H L) ] dimeric unit is counterbalanced by four
2
3 2
2
2
2
(
NO ) in 1:1 metal–ligand ratio leads to the formation
nitrate ions in the crystal lattice. Along the polymer the
metal ions are separated by 8.802(2) Å.
3
2
of a one dimensional coordination polymers, CP‐1 and
CP‐2 respectively (Scheme 1).
Colourless crystals of complex CP‐2, {[Cd (H L)
2
2
The ESI‐MS spectra of the CPs provide an intense peak
correspond to the monomeric unit. The intense peak at
m/z = 803.71 (calc. 803.13) for CP‐1, is attributed to
(NO ) ](NO ) } , were obtained by diffusion of isopropyl
alcohol into its aqueous solution. Low symmetry struc-
ture revealed eight‐coordination around both two Cd
2
3 2 3 2 n
SCHEME 1 Synthesis (i) {[Zn
2
(H
2 2
L) ]
(
NO
NO
3
)
)
4
}
}
n
(CP‐1) (ii) {[Cd
(CP‐2)
2
(H
2
L)
2
3 2
(NO ) ]
(
3
2
n

ARUNACHALAM ET AL.
3 of 12
charges are counterbalanced by two nitrates in the crystal
lattice. Further details of crystallographic analysis of CP‐1
and CP‐2 are shown in Supplementary Material (S3‐S7).
A close scrutinizatin of the crystal packing and supra-
molecular interaction revealed extensive hydrogen bond-
ing between the one dimensional CP strand and counter
nitrate anions/lattice water in both CP‐1 and CP‐2. Thus,
in the case of CP‐1, strong N‐H … O and N‐H … N inter-
actions are felicitated by amide hydrogen H9 and H9A
involving a bifurcated H‐bonding with O1, N2 and O1A,
N2A respectively of the nitrate anion. As depicted in the
Figure (S8), packing diagram viewed down c‐axis clearly
showed the offset π … π stacking of the pyridyl rings of
the ligand moiety between the adjacent strand from
either side with measured centroid … centroid perpendic-
ular distance between the pyridyl ring was 3.53 and 4.1 Å
respectively. H‐bonding interaction in CP‐2 reveal that
amide hydrogen as a donor makes N‐H … O contacts with
the nitrate oxygens and lattice water molecule. Thus,
amide hydrogens H9, H9A is involved in a bifurcated N‐
H…O contact with nitrate oxygens O1C, O3C and O1E,
O2E, while H9B is making only one contact with nitrate
oxygen O1B. However the amide hydrogen H9C is mak-
ing N‐H … O contact with the lattice water molecule.
Hydrogen atoms of the lattice water molecule O1F is
making bifurcated O‐H … O contact with nitrate oxygens
as well as other lattice water molecules. In addition to the
mentioned N‐H..O and O‐H..O interactions, the pyridyl
rings of the ligand moiety is involved in offset π … π
stacking interaction between the adjacent strands in
extending the supramolecular three dimensional network
and stabilize the molecule in the crystal lattice. As shown
in Figure (S9), the centroid … centroid distance between
the offset π … π stacked ring is in the range 3.69 to
4.42 Å, the perpendicular distance between the ring plane
(3.7 Å) is within the stacking limit (S8‐S9).
2
FIGURE 1 UV–Vis spectra for H L, CP‐1 and CP‐2 in water
−
5
(
3 × 10 M)
(II) centers. The Cd coordination spheres consist half
units of two different H L ligands and one bidentate
2
nitrate anion. The structure show approximately 72°
angular separation in N O‐Cd coordination planes of
2
ligands and N O‐Cd to O ‐Cd angles of about 67 and
2
2
6
4° for the first coordination sphere, respectively. For
the second sphere the angular separation of the ligands
is identical, but the coordination angles to the nitrate
show slightly deviating values of about 71 and 63°. The
bond distances and angles around the two independent
Cd (II) centers are almost the same and the overall geom-
etry around both Cd (II) centers resembles triangular
dodecahedron (S7). The H L ligands are assembled in a
2
side‐by‐side orientation with metal axis possessing inter-
metallic Cd − Cd distances of 8.3753(7) and 8.5267(7) Å
(Figure 3). One half of the ligand moiety coordinating
on one Cd center, the other half of the ligand coordinated
to the next Cd center in the polymeric chain through
their N2O clefts. Thus two such ligands forming ladder
type coordination with alternative metal center assemble
a one dimensional polymeric chain in z‐axis. In addition
to two coordinated nitrates the remaining positive
Followed by this, the PXRD pattern for CP‐1 and CP‐2
were recored to determine the bulk purity. The peak posi-
tions showed good match with the simulated data, but
the peak broadening is observed in the experimental
2 2 2 n
FIGURE 2 Crystal structure of CP‐1 [Zn (H L) ] with 50% probability factors for thermal ellipsoid. The anions are omitted for clarity

4
of 12
ARUNACHALAM ET AL.
FIGURE 3 Crystal structure of CP‐2 [Cd
2
(H
2
L)
2
(NO
3
)
2
]
n
with 50% probability factors for thermal ellipsoid. The anions and solvents are
omitted for clarity
pattern attributed to low crystallinity of the powder sam-
ples due to loss of weakly bound water molecules (S10).
The thermo gravimetric analysis was performed for
both the polymers CP‐1 and CP‐2 in order to understand
their thermal stability. The CP‐1 shows the weight loss of
attributed for their dynamic water loss. Followed by this,
the ligand molecules decomposed from 220 to 600 °C
(55.2%) and beyond 600 °C, the cadmium nitrate
decomposed to give cadmium oxide. This similar thermal
behaviour shown by both these CPs clearly establish that
these polymeric networks are stable up to 220 °C and
then the decomposition of organic ligands occurs beyond
220 °C (S11).
1
.9% up to 200 °C correspond to 0.5 H O molecules and
2
matches to the calculated 1.5%. Followed by this, a major
weight loss of 61.22% in the range 220–250 °C is attrib-
uted to the decomposition of H L (calculated 62%). Then
2
the remaining zinc nitrate undergoes decomposition and
gives zinc oxide in the temperature range 300–700 °C.
In the case of CP‐2 the thermal decay correspond to
1
2.2 | H NMR for coordination polymers
1
6
.83% weight loss is attributed to the elimination of five
In the H NMR spectra (Figure 4), the ligand (H L) and
2
water molecules in the temperature range up to 200 °C
matches almost with the calculated values 7.2% agrees
well with the crystal data. The minimal difference
between the experimental and calculated value may be
their coordination polymers CP‐1 and CP‐2 (S13‐S14),
show the signals in the range of 8.6 to 7.3 δ are corre-
spond to the pyridyl and azomethine hydrogen of the
ligand. The distinctive singlets at 11.5 δ attributed to –
1
FIGURE 4 H NMR for (a) ligand, (b) CP‐1 c) CP‐2 in DMSO‐d
6

ARUNACHALAM ET AL.
5 of 12
NHs are confirmed by D O exchange. The methylene
change from its narrow pattern to broad pattern indicate
their close proximity with the metal center (Figure 5).
2
α1,α2
β
α1 α2
groups (−
CH ‐ CH ‐
CH ‐) in the spacer of the
2
2
2
ligands H L depicting three well‐resolved resonances at
2
2
.77, 2.36 and 1.94δ respectively, the integral ratio of all
of them are equivalent to two hydrogens (Figure 4).
Although the H NMR of ligand and its CPs, showing
2.3 | Cycloaddition of carbon dioxide into
cyclic carbonate
1
no difference in the aliphatic region correspond to CH2
spacers, a systematic variable temperature study per-
formed in the range 20–70 °C on these complexes derives
interesting information about the conformation of the
ligand strands (Figure 5).
As stated above both the CPs possessing multimetallic
active sites, similar to that of MOFs we inspired to exam-
ine their catalytic activity. Accordingly, both the CPs
were applied as catalysts for the synthesis of cyclic car-
bonates from epoxide. Initially, the CP‐1 and CP‐2 were
screened for their catalytic performance using styrene
oxide as a substrate under solvent‐free condition using
The resonance at 1.94 δ in the up‐field appeared for
middle CH group in both CP‐1 and CP‐2 shows no
2
change in the chemical shift as well as a spectral pattern
with respect to the effect of varying temperature indicate
that they are away from the metal centers. However upon
increasing the temperature the peaks at 2.77, 2.36 δ attrib-
10 bar CO (S17). A model reaction performed initially
2
in the absence of catalyst showed no conversion of sty-
rene oxide to styrene carbonate (Table 1, Entry 1). But
when added tetra butyl ammonium bromide (TBAB), a
utable to terminal CH s are showing a very distinct
2
1
FIGURE 5 VT H‐NMR (DMSO‐d
6
) for
(
a) CP‐1 and (b) CP‐2 (aliphatic region)

6
of 12
ARUNACHALAM ET AL.
a
TABLE 1 Screening of catalyst
formation of both cyclic carbonate and polycarbonate.
But the present catalyst producing 100% cyclic carbonate,
it is noteworthy to mention that this catalyst is highly
selective towards cyclic carbonate. Although both the cat-
alysts adapted in solvent‐free condition seems to be effi-
cient, based on the yield the CP‐1 giving 99% yield was
found better, than the CP‐2.
b
c
d
−1
Entry
Catalyst
Yield %
TON
TOF h
1
2
3
4
5
Blank
TBAB
No reaction
‐‐‐
‐‐‐
‐‐‐
‐‐‐
Identifying the CP‐1 as an efficient catalyst, next we
attempted to optimize various other parameters such as
68
99
99
87
e
CP‐1
493
493
432
123
123
108
temperature and pressure. The pressure of the CO was
2
varied from 2–12 bar and their respective conversions
and TOF calculated are plotted in Figure 6a. During the
reaction, the initial and final pressure of the carbon‐
dioxide were measured, which showed a fall in the pres-
sure of approximately 4 bar indicating consumption of
CP‐1
CP‐2
a
All reactions were carried out using 100 mmol of styrene oxide, 0.2 mmol of
catalyst, 10 bar carbon‐dioxide, at 100 °C for 4 hr under solvent free
condition.
b
CO during the reaction. So in the initial pressure ranging
Isolated yield,
2
c
2 to 4 bar, the conversion was found very low, but upon
increasing the pressure beyond this range the conversion
was found increased significantly. As noticed in the above
plot, the maximum conversion 98% was achieved at
TON = mmol of product formed/mmol of catalyst used,
d
TOF = TON/reaction time,
TBAB used.
e
sudden raise in the yield to 68% (Table 1, Entry 2) was
observed. The combination of CP‐1 (0.2 mmol) and
TBAB, together raised the conversion to 99% within 4 hr
10 bar CO , and 100 °C.
2
Keeping the CO pressure constant at 10 bar, the tem-
2
perature was varied up to 120 °C from 30 °C in view of its
optimization (Figure 6b). The reactions performed at
room temperature (30 °C) and 60 °C giving trace amount
of conversion around less than 10%, a raise in the temper-
ature to 80 °C, raised the conversion of styrene epoxide to
76%. A further increase in the temperature to 100 °C pro-
vides a further enhancement in the conversion to 99% as
(Table 1, Entry 3).
Surprisingly in our successive attempt adding CP‐1
0.2 mol %) without any co‐catalysts, the conversion has
(
remained as 99% styrene carbonate (Table 1, Entry 4).
Similarly, when applied CP‐2 (0.2 mol%) as catalyst, the
conversion was achieved as 87% of cyclic carbonates
(Table 1, Entry 5). This minimum amount 0.2 mmol of
shown in Figure 6b. So the 10 bar CO at 100 °C giving
2
catalyst CP‐1 and CP‐2 required to catalyse 100 mmol
highest conversion, the optimized reaction temperature
substrate (1:500 ratio of catalyst: substrate) encouraged
is ultimately fixed as 100 °C for CP‐1.
us to adopt these catalyst in such CO reactions for fur-
2
ther investigation. In addition, the reduction in the reac-
tion time, non‐requirement of co‐catalyst at 100 °C are
considered advantages for effective usage of this catalyst
in the synthesis of cyclic carbonate from epoxide. Gener-
2.4 | Screening of substrates
The efficient conversion of neat styrene epoxide to sty-
ally, in such CO reactions, there is a possibility for the
rene carbonate shown by CP‐1 inspired us to explore its
2
FIGURE 6 (a) Pressure optimization, (b) Temperature optimization parameters using 0.2 mmol CP‐1, 100 mmol styrene oxide for 4 hr.
1
The conversion was determined from H NMR; TOF = TON/4

ARUNACHALAM ET AL.
7 of 12
catalytic role on various substituted terminal and internal
epoxides adapting the above‐optimized reaction condi-
tions. Except few, most of the aliphatic substituted termi-
nal epoxides used here is giving better conversion and the
results are summarized in Chart 1.
epoxides such as cyclohexene oxide (k) and 4‐vinyl‐1‐
cyclohexene oxide (l), giving no conversion, the terminal
epoxides (a‐j) are effectively converted into respective car-
bonate. Thus the CP‐1 is found effective in catalysing the
cycloaddition reaction for terminal epoxides. (S17‐S19).
All these reactions were carried out using 100 mmol of
epoxide, 0.2 mmol of CP‐1, 10 bar carbon dioxide, at 100
2
.5 | Reaction kinetics
o
C for 4 h under solvent‐free condition. Conversions
1
were determined using H NMR for reaction mixture.
As this catalyst is working efficiently, we aimed to under-
stand the reaction path by studying its kinetics. Accord-
ingly, we performed a series of kinetic experiments
Yield here represent the isolated yield after column
chromatography.
[33–36]
In Chart 1 adapting alkyl substituted epoxides, in the
case of propylene oxide, 98% yield was obtained (b); with
epoxy butane the yield was 94% (e); with epoxy hexane it
was further reduced to 66% (h); and with tertiary butyl
epoxide it is found very low, i.e., 10% (i). The decreasing
trend of conversion here shows that as the aliphatic sub-
stituent's increases, the respective conversions decreases.
The maximum TOF was obtained in the conversion of
propylene carbonate as 216/h and 192/h for glycidol car-
bonate and 190/h using epichlorohydrin carbonate. The
catalyst is found to give a yield ranging 44 to 99% with
using epichlorohydrin (Epo) as a model substrate, under
the solvent‐free condition and the conversions were
1
determined using H NMR (Figure 7).
The general equation for determining the rate of a
reaction is described as
a
b
c
Rate ¼ k½Epoꢀ ½CO2ꢀ ½CP−1ꢀ
(1)
Firstly the reaction runs without any co‐catalyst or addi-
tive, secondly the concentration of CO and concentra-
2
tion of CP‐1 remains unaltered. Hence the equation‐1
can be re‐written as
1
00% selectivity on various substrates without any exter-
nal co‐catalysts. The catalyst is giving 72% yield with allyl
glycidyl ether (g), 44% yield in case of phenyl glycidyl
ether (d) and 86% yield with glycidol (f). In the case of
a
b
c
Rate ¼ kobs½Epoꢀ ; where k ¼ ½CO ꢀ ½CP−1ꢀ
(2)
obs
2
1
,2,7,8‐diepoxy octane, the 90% yield (j) was obtained.
Where a, b and c represents the order of the reaction with
Based on the conversion shown in Chart 1, the cyclic
respect to Epo, CO and CP‐1. Initially, to determine the
2
order of the reaction, a series of experiments were con-
ducted using 0.06 mmol of catalyst, as a function of reac-
tion time with 30 min interval. The respective
conversions of epichlorohydrin to epichlorohydrin car-
bonate were calculated. The plot derived between ln[Epo]
and reaction time providing a straight line indicates that
the reaction follows first‐order kinetics and, it gives an
understanding that the rate of the reaction depends
mainly on the concentration of epoxide. When the plot
is drawn between ln[Epo] versus reaction time ranging
0
to 330 min at every 30 min interval, this linear least
2
square fit show a significant deviation in its R value.
However, in the range 150 to 330 min, the plot drawn ver-
sus conversion giving an excellent linear least square fit
2
R = 0.99, it is convincing to conclude that the reaction
follows two step kinetic process. In the initial period
ranging 0–150 min, the reaction follows zero‐order kinet-
ics (S20). But the best straight line obtained during 150–
330 min the reaction obeys first‐order kinetic is obvious
to understand (Figure 8). This is because, initially the
epoxide acts as solvent and substrate, but after forming
the cyclic carbonate as the main product, the reaction fol-
lows first‐order kinetics. It is noteworthy to mention that
a similar behavior was observed by M. North's in the case
[
37–39]
CHART 1 Screening of substrates using CP‐1
of Al‐heteroscorpionate complex.

8
of 12
ARUNACHALAM ET AL.
1
3
FIGURE 7 H NMR (CDCl ) for the reaction mixture with a mixture of epoxide and a cyclic carbonate
Similarly to evaluate the efficiency of the catalyst, a set
of experiments were carried out by varying the catalyst
concentration of CP‐1, from 0.02 mol% to 0.2 mol%. All
the reactions were conducted using 10 bar CO , 100 mmol
2
of epichlorohydrin at 100 °C for 3 hr and the respective
data are plotted in Figure 9. The kinetic plot in Figure 9
indicates that the rate of the reaction increases as the cat-
alyst concentration increases. The plot derived between
the concentrations of the catalyst ln[cat] versus the rate
of the reaction ln[rate] provides an additional informa-
tion about the efficiency of the catalyst CP‐1.
2.6 | Activation energy
FIGURE 8 Kinetic plot for ln[Epo] with time for determining the
order of the reaction. ( y = −0.015x‐0.177, R = 0.99)
The energy required for the conversion of epoxide to
cyclic carbonate via transition state is known as activa-
tion energy. Accordingly, the activation energy in the
conversion of epoxide to cyclic carbonate was established
using the Arrhenius equation. The activation energy for
2
the CO insertion into epichlorohydrin catalyzed by CP‐
2
1
was determined by varying the range of temperature
from 60 to 120 °C. The plot drawn between the rate and
temperature clearly provide a straight line and obeys the
Arrhenius equation as shown in Figure 10. So the slope
of the straight line [−(Ea/R)] is used to derive Ea. The
conversion of epichlorohydrin to epichlorohydrin carbon-
ate, catalyzed by CP‐1 and CP‐2, (0.06 mmol) at 10 bar
CO , and temperature varied from 80 to 120 °C. The acti-
2
vation energy for CP‐1 is calculated as 37.98 KJ/mol and
for CP‐2 is 39.78 KJ/mol (Figure 10).
In view of activation energy of various catalysts used
we made a comparative analysis with reported catalysts.
In this regard the Aluminium unsymmetrical Schiff base
catalyst used for conversion of epoxide to cyclic carbonate
reported with 34 KJ/mol of Ea in the temperature range
FIGURE 9 The kinetic plot as a function of the catalyst using
ln[rate] Vs ln[cat] ( y = 1.77x‐8.22, R = 0.99)
2

ARUNACHALAM ET AL.
9 of 12
2
FIGURE 10 Arrhenius plot for determining the activation energy for CP‐1 ( y = −4568.65x + 0.52, R = 0.9696; Ea = 37.98 KJ/mol) and
2
CP‐2 ( y = −4785.10x + 0.74, R = 0.9877; Ea = 39.78 KJ/mol)
[34]
8
0–150 °C.
The same catalyst in combination with
TBAB the respective activation energy calculated was 23
KJ/mol which showed a significant reduction. Various
catalytic systems reported in the literature are possessing
[40]
activation energy in the range 35–70 KJ/mol.
The
highest activation energy 98.4 KJ/mol was reported for
the iron amino‐bis (phenolate) complex catalyst, for the
conversion of propylene oxide in combination with
[41]
TBAB.
It is understandable that when a catalyst is effi-
cient, that the actual activation energy required for the
catalytic conversion needs to be minimum. Keeping this
in view the present catalysts CP‐1 and CP‐2 possessing
3
7.9 and 39.7 KJ/mol respectively calculated for the con-
version of epoxide to cyclic carbonates at 80–110 °C gives
hope that they are efficient when compared to various
catalysts mentioned above Scheme 1.
2.7 | Mechanism
SCHEME 2
A probable mechanism for the cycloaddition
The CPs are bifunctional in nature with Lewis acidic sites
of the metal center and the basic nature of the ligand. The
catalytic mechanism in the reaction is explained in four
steps as shown in Scheme 2. The labile NH proton on
the ligand leads to enhance the activity of the catalyst
without any external additives. Based on the kinetic
experiments, the mechanism for the catalytic conversion
is described as follows. The epoxide is activated by the
metal centers interacting via an oxygen atom and leads
to the formation of metal–alkoxide in step 1. The epoxide
oxygen is electron rich, when compared to ketonic oxy-
gen of the ligand, which favors the metal –alkoxide inter-
reaction
center and the labile NH in the ligand are involved in the
catalytic reaction. The metal‐alkoxide intermediate is
quite stable for rapid CO attack, leading to the CO
insertion to form the carbonate intermediate. This nucle-
ophilic attack on the carbonate in step 4 facilitate the ring
closure and produce the end product as cyclic carbonate
2
2
[
42–45]
as depicted in Scheme 2.
Interestingly a comparative analysis with respect to the
present catalysts (CP‐1 and CP‐2) against reported zinc
containing catalysts are considered noteworthy in view
of its reaction time, CO2 pressure and temperature.
Among the various zinc catalyst reported in the literature
−
action more feasible. Simultaneously, the nitride (N ) ion
of the ligand activates the CO , implying that the carbon
2
+
atom has a partial positive charge (δ ) and the oxygen
−
[46]
[47]
atoms with a partial negative charge (δ ).Thus each metal
such as ZnBr2 , Zn‐CMP
although are found

10 of 12
ARUNACHALAM ET AL.
efficient in view of their high conversion and short reac-
tion time (1 hr), they all require harsh reaction condition
CHNS/O analyzer. The X‐ray data were collected at
123 K on an Agilent SuperNova single‐source diffractom-
eter equipped with an Eos CCD detector using mirror‐
such as 17–30 bar CO pressure, and temperature ranging
2
1
1
20–140 °C and additive such as HBGB (Hexa butyl
monochromated Mo‐Kα (λ = 0.71073 Å) radiation. H
13
guanidinium bromide), TBAB, etc. The Zn –Cluster by
and C NMR spectra were recorded on a Bruker Avance
II 500 or 600 MHz FT‐NMR spectrometer. Chemical
shifts for proton resonances are reported in ppm (δ) rela-
tive to tetramethylsilane. All the catalytic products were
[
19]
[48]
Mashima
and Zn –azatrane complex
required more
[
49]
than 24 hr. The Zn‐porphyrin complexes by Ema
although provides yield up to 92% with short reaction
1
time such as 3 hr, with 17 bar CO pressure at 120 °C.
established based on the H NMR spectra.
2
Thus both CP‐1 and CP‐2 giving excellent conversion
up to 99%, the advantage is that these catalysts works
without any additive and comparatively moderate condi-
tions such as 4 hr reaction time, 100 °C temperature
4.2 | Synthesis of coordination polymers
and 10 bar CO pressure (S21) Chart 1.
2
4
.2.1 | CP‐1. {[Zn(H L)](NO ) }
2
3 2 n
3
| CONCLUSIONS
Methanolic solution of ligand H L (1137 mg, 3.36 mmol)
2
and zinc (II) nitrate hexahydrate (1000 mg, 3.36 mmol)
were mixed and stirred. The transparent solution of the
ligand turns turbid leading to the formation of a precipi-
tate. The reaction mixture was heated at 60 °C for 2 hr
and then allowed to stir at RT for 14 hr. The white precip-
itate obtained was filtered, washed with cold methanol
and dried. White solid. Yield. 85% (1810 mg). IR (KBr)
ν = 3478 (‐OH), 3134, 2792 (‐NH), 1642 (int, C=O),
The ligand H L upon complexation with Zn (NO ) and
2
3 2
Cd (NO ) has provided two important coordination poly-
3
2
mers CP‐1 and CP‐2. The single crystal X‐ray structures
of these CP‐1 and CP‐2 clearly indicates the formation
of coordination polymers with zinc and cadmium. Both
these coordination polymers are applied as a catalyst for
cycloaddition of carbon dioxide to epoxides. Among
them, the CP‐1 in solvent‐free condition produced better
yield when compared to CP‐2 without any external addi-
tives. Further, the optimization and substrate screening
were carried out using CP‐1, which gave better conver-
sion with aliphatic substituted terminal epoxides without
use of any external additives. The kinetic experiments
performed with epichlorohydrin as a substrate with
1
554 (int, CH=N), 1480, 1397, 1315, 1231, 1194, 1156,
−
1
1100, 1036, 1015, 948, 780, 643, 520 cm . UV–Vis [water,
−
1
λ, nm, (ε/M )]: 290(79286), 265(53593), 205(78576). Ele-
mental analysis: {[Zn(H L)](NO ) (H O)} calc. For
2
3 2
2
C H N O Zn. Calc (Expt.). C, 37.41 (37.44), H, 3.69
1
7
20 8 9
+
(
(
3.80), N, 20.53 (20.93)%. ESI m/z = 803.13(calc) 803.71
found) for [Zn L ]H .
+
2
2
0
.06 mmol of the catalyst, suggests the reaction follows
first‐order kinetics. From the Arrhenius plot, the activa-
tion energy for CP‐1 and CP‐2 is calculated as 37.9 and
3
9.7 KJ/mol. This detailed study gives an understanding
4
.2.2 | CP‐2. {[Cd (H L) (NO ) ]
2
2
2
3 2
that the CP‐1 needs less energy than CP‐2 for promoting
the cyclic carbonate is matches to our experimental
results.
(NO ) (H O) }
3 2 2 5 n
Methanolic solution of ligand H L (548 mg, 1.6208 mmol)
2
and cadmium (II) nitrate hexahydrate (500 mg,
1.6208 mmol) were mixed and stirred. The transparent
solution of the ligand turns slowly turbid then to precipi-
tate. The reaction mixture is heated at 60 °C for 2 hr and
then allowed to stir at RT for 14 hr. The resultant precip-
itate obtained was filtered, washed with methanol and
dried. Solid, Yield: 73% (770 mg). IR (KBr) ν = 3460
(‐OH), 3154 (‐NH), 1668 (‐C=O), 1562 (‐CH=N), 1340,
4
4
| EXPERIMENTAL SECTION
.1 | Materials and methods
All the chemicals were purchased from Aldrich & Co. IR
spectra were recorded using KBr pellets (1% w/w) on a
PerkinElmer Spectrum GX FT‐IR spectrophotometer.
Electronic spectra were recorded on a Shimadzu UV
−
1
1303, 1148, 1037, 938, 778, 627, 513 cm . UV–Vis [water,
−
1
3
101PC spectrophotometer. Electrospray ionization mass
λ, nm, (ε/M )]: 290(86766), 265(62733), 206(84800). Ele-
spectrometry (ESI MS) measurements were carried out
on a Waters QTof‐micro instrument for all of these com-
plexes upon dissolving in methanol–water solvents.
CHNS analyses were done using a PerkinElmer 2400
mental analysis: {[Cd (H L) (NO ) ](NO ) (H O) } calc.
2
2
2
3 2
3 2
2
5 n
For C H Cd N O . Calc (Expt.). C, 32.94 (32.52), H,
3
4
46
2 16 21
+
3.74 (3.71), N, 18.08 (18.33)%. ESI m/z = 899.90 (calc)
899.08 (found) for [Cd L ]H .
+
2
2

ARUNACHALAM ET AL.
11 of 12
4
4
.3 | Crystal data
.3.1 | CP‐1
SERB New Delhi (project No SR/S1/IC‐23/2011) for
financial support. A. V. also kindly acknowledges the
Academy of Finland (grant no. 314343) for financial sup-
port. We are thankful to ADCIF for the instrument
support.
CCDC‐1894863. {[Zn(H L)](NO ) } : C H N O Zn
2
3 2 n
17 18 8 8
(
M
=
527.76 g/mol); monoclinic, P2 /c (14),
1
a = 8.8025(18) Å, b = 28.507(6) Å, c = 8.7358(17) Å,
ORCID
o
o
o
3
α = 90 , β = 100.13(3) , γ = 90 , V = 2157.9(8) Å ,
3
−1
Z = 4, ρcalc = 1.624 g/cm , μ = 1.202 mm ,
F (000) = 1080, 13252 reflections collected, of which
Eswaran Chinnaraja
2316
https://orcid.org/0000-0003-0617-
3
910 independent, R = 0.1179, 46 restraints, 344 param-
Palani S. Subramanian https://orcid.org/0000-0002-9035-
0973
int
2
eters, GOF on F = 1.020, R /wR [I ≥ 2σ(I)] = 0.0939/
1
2
0
.2442, R /wR2 (all data) = 0.1420/0.2925, largest di.
1
3
peak/hole (e./Å ) = 0.664/−0.715.
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5