Mechanism and origins of enantioselectivity for [BMIM]Cl ionic liquids and ZnCl2 co-catalyzed coupling reaction of CO2 with epoxides

Article abstract of DOI:10.1016/j.molcata.2014.01.024

Aiming at gaining more insight into the high catalytic activity of ZnCl₂/[BMIM]Cl co-catalysts and elucidating the origination about the product enantioselectivity for the coupling reaction of CO₂ with epoxides, a mechanistic study has been conducted by performing density functional theory calculations. The calculated results indicate a new stable complex [BMIM]ZnCl₃ is probably formed via the dissociation of the in situ generated [BMIM]₂ZnCl₄ complex in the reaction system. This complex combined with another Cl^- jointly assists the break of CO bond of propylene oxide (PO), which is the rate-determining step for the coupling reaction, and the corresponding barrier (28.0 kcal mol ^-1) is effectively lowered in comparison with the reaction promoted only by ZnCl₂ (65.9 kcal mol^-1) or [BMIM]Cl (33.1 kcal mol^-1). [BMIM]⁺ takes part in the reaction by directly or indirectly stabilizing the intermediates and transition states via hydrogen bonding interaction with O of PO or Cl^- in the reaction system. The observed product enantioselectivity probably originates from the formation of an interesting intermediate which provides nearly equal opportunities for inserted CO₂ to attack the chiral carbon atom of PO on both sides and hence facilitates the formation of both R-product and S-product.

Full text of DOI:10.1016/j.molcata.2014.01.024

Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Mechanism and origins of enantioselectivity for [BMIM]Cl ionic
liquids and ZnCl2 co-catalyzed coupling reaction of CO2 with epoxides
Fang Wang, Chuanzhi Xu, Zhen Li, Chungu Xia∗, Jing Chen
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2013
Received in revised form 9 January 2014
Accepted 24 January 2014
Available online 3 February 2014
Aiming at gaining more insight into the high catalytic activity of ZnCl2/[BMIM]Cl co-catalysts and eluci-
dating the origination about the product enantioselectivity for the coupling reaction of CO2 with epoxides,
a mechanistic study has been conducted by performing density functional theory calculations. The cal-
culated results indicate a new stable complex [BMIM]ZnCl3 is probably formed via the dissociation of the
in situ generated [BMIM]2ZnCl4 complex in the reaction system. This complex combined with another
Cl− jointly assists the break of C O bond of propylene oxide (PO), which is the rate-determining step for
the coupling reaction, and the corresponding barrier (28.0 kcal mol−1) is effectively lowered in compari-
son with the reaction promoted only by ZnCl2 (65.9 kcal mol−1) or [BMIM]Cl (33.1 kcal mol−1). [BMIM]+
takes part in the reaction by directly or indirectly stabilizing the intermediates and transition states via
hydrogen bonding interaction with O of PO or Cl− in the reaction system. The observed product enantio-
selectivity probably originates from the formation of an interesting intermediate which provides nearly
equal opportunities for inserted CO2 to attack the chiral carbon atom of PO on both sides and hence
facilitates the formation of both R-product and S-product.
Keywords:
Mechanism
Enantioselectivity
CO2 coupling reaction
DFT
ZnCl2/[BMIM]Cl
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
activity, stability, or the requirement of cosolvent, high pressure
and/or high temperature.
Due to the growing concern on the greenhouse effect, the fix-
ation and utilization of primary greenhouse gas carbon dioxide
(CO2) have received considerable attention in recent years. It has
been found CO2 can be used as C1 building block in many organic
synthesis reactions, such as reforming with CH4 to yield CO and
H2 [1,2], hydrogenation to prepare methanol and formate [3,4],
cycloaddition onto epoxides to produce cyclic carbonate [5–12],
and so on. Among these reactions, CO2 cycloaddition onto epoxides
leading to the formation of five-membered cyclic carbonate which
can be widely used as chemical intermediates for the synthesis of
ethylene glycol esters, hydroxyalkyl derivatives, carbamates, poly-
mers, and pharmaceutical, is of particular interest and has been the
subject of extensive studies [13–15]. So far, a number of catalysts
have been developed for the coupling of CO2 with epoxides, such
as organometallic compounds [16], alkali metal salts [17], metal
oxides [18,19], zeolites [20,21], oxychlorides [22], phosphonium
salts [23], metallic salen complexes [24–26], amine [27] and so on
[28]. However, most of the catalyst systems suffer from either low
Recently, several literatures have reported that ionic liquids
present moderate activity toward the cycloaddition of CO2 with
propylene oxide (PO) [29–31]. Stirringly, our experimental results
demonstrated the activity and selectivity of ionic liquids can be
dramatically enhanced when Lewis acidic compounds, for exam-
ple, zinc halides, are co-present in the catalyst system, although
zinc halides themselves hardly have any reactivity for the cycload-
dition reaction [6,7,32]. Moreover, this composite catalyst system
is air stable, easily synthesized, extremely robust, environmentally
less hazardous and free of co-solvent. However, the catalytic mech-
anism is not clear. Additionally, it is worthy to note that there is a
chiral carbon atom in PO molecule. Our further experimental stud-
ies, which will be exhibited in this paper, show an enantioselective
formation of cyclic carbonate when different reaction substrates
are selected: the cyclic carbonate with ee value up to 99.7% is
obtained when (S)-PO or (R)-PO is chosen, while nearly racemic
cyclic carbonate mixture is obtained no matter (S)-epichlorohydrin
(S)-ECH or (R)-ECH is selected. The actual origin about this exciting
observation is also not well understood.
Up to date, the mechanistic details for the coupling of CO2
with epoxides mediated by heterobimetallic Ru–Mn complexes
[33], LiBr salt [34], Re(CO)5Br complexes [35], copper cyanomethyl
(NCCH2Cu) [36], Ni(PH3)2 [37], KI/NH3 [38] and KI/hydroxyl
∗
Corresponding author. Tel.: +86 931 4968089; fax: +86 931 4968129.
E-mail address: cgxia@lzb.ac.cn (C. Xia).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.024

134
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
Table 1
The free energy barriers (in kcal mol−1) calculated by B3LYP and M06 functionals
for the elementary steps along path IIc involved in Fig. 6.
O
3
Path I
Path II
O
O
ZnCl2/[BMIM]Cl
R=CH3, CH2Cl
1
+
C
O
*
R
O
3
19 → TS19–20
21 → TS21–22
22 → TS22–23
*
R
2
O
B3LYP
M06
28.0
25.0
3.2
4.8
16.1
18.6
1
2
Scheme 1. Coupling of CO2 with epoxides to produce cyclic carbonate.
without any symmetry constraints. The geometries for the transi-
tion states were located employing the synchronous transit-guided
quasi-Newton (STQN) method [49]. Vibrational frequency analysis
was performed to confirm the nature (minima or first-order saddle
points) of the stationary points at the same level. Zero-point energy
corrections were carried out for all calculated energies. The intrin-
sic reaction coordinate (IRC) [50] was conducted in both directions
(forward and reverse) from the transition states to the correspond-
ing local minima to identify the minimum-energy paths. To better
understand how composite catalysts control the coupling reaction,
the electronic structures for relevant stationary points were ana-
lyzed based on natural bond orbital (NBO) analysis [51–56]. To take
the entropy effects into account, the Gibbs free energies (ꢀG) were
used in the following discussion.
Additionally, to ascertain the reliability of the chosen func-
tional and basis sets, the structures included in the preferred path
of the ZnCl2/[BMIM]Cl-mediated reaction were re-optimized at
the M06/6-31 + G(d,p) level. To obtain more reliable energies of
Zn(II) species (ZnCl2x−x, x = 2–4), additional single-point energy cal-
culations were performed at the MP2/6-311 + G(d,p) level for the
stationary points optimized at the B3LYP/6-31 + G(d,p) level.
substances [11] have been investigated employing density func-
tional theory (DFT). In particular, the mechanism for the coupling
reaction of CO2 with propylene oxide (PO) only mediated by
alkylmethylimidazolium chlorine ([Cnmim]Cl, n = 2, 4, and 6)
ionic liquids has also been studied at the B3PW91/6-31G(d,p)
level by Sun and Zhang [39]. However, conclusions obtained from
these calculations are not transferable to our present reaction
systems. Therefore, to better understand how the composite
catalysts (ZnCl2/[BMIM]Cl) assist the coupling reaction of CO2
with PO and why different reaction substrates lead to different
product enantioselectivity, a detailed mechanistic study is quite
necessary.
It is well known that density functional theory (DFT) has become
a valuable tool for exploring the reaction mechanisms in recent
years. Therefore, to rationalize the interesting but inexplicable
experimental findings for the title reaction: (1) the detailed mech-
anism; (2) the role of the ZnCl2/[BMIM]Cl composite catalysts; (3)
the origination about the product enantioselectivity, a DFT mech-
anistic investigation has been conducted for the reaction shown in
Scheme 1. To compare the activity of the composite catalysts with
the separate one, the reaction mediated only by ZnCl2 or [BMIM]Cl
has also been considered. We expect the present studies could
clarify the mechanism of the coupling reaction, provide deeper
understanding for the critical role of the composite catalysts, elu-
cidate the origination for different product enantioselectivity, and
give guidance and inspiration for the exploitation of more efficient
and selective catalysts for the coupling of CO2 with PO.
3. Results and discussion
3.1. Calibration
To ascertain the reliability of the B3LYP functional with the
chosen basis sets for describing the present system, benchmark
calculations for the reaction mediated by ZnCl2/[BMIM]Cl along
path IIc were performed using the M06 functional, the hybrid func-
tional of Truhlar and Zhao [57] which was found to show good
performance for non-bonding interactions. The crucial parame-
ters of the optimized geometries for the transition states obtained
at M06/6-31 + G(d,p) level are shown in the parentheses in Fig. 5
to be compared with those calculated at the B3LYP/6-31 + G(d,p)
level. Moreover, the free energy barriers for the elementary steps
contained in Fig. 6 are also shown and compared with the B3LYP cal-
culated ones in Table 1. It can be seen that the B3LYP functional not
only gives the similar geometries but also the comparable barriers
and similar potential energy surface tendencies (energy profiles are
shown in Fig. S1 in the supplementary material) to M06 functional,
which ensure the quality and reliability of the B3LYP/6-31 + G(d,p)
level for describing the systems investigated in present study. Thus,
the results discussed in the following sections are based on the
B3LYP/6-31 + G(d,p) level.
2. Experimental and computational methods
2.1. Experimental
1-Butyl-3-methylimidazolium halides were prepared accord-
ing to the procedure reported in the literature [40]. CO2 with a
purity of 99.99%, zinc halides, (R or S)-PO, and (R or S)-ECH were
commercially available.
For a typical reaction of CO2 with epoxides, ZnBr2 (11.3 mg,
0.05 mmol), [BMIM]Br (65 mg, 0.3 mmol) and epoxides (PO: 20 ml,
0.286 mol; or ECH: 23.61 g, 0.255 mmol) were charged into a
100 ml stainless steel autoclave equipped with a magnetic stirrer
in sequence. The 3.0 MPa of CO2 was filled into the reaction ves-
sel which was then heated to 120 ◦C for 4 h. Once the reaction is
completed, the vessel was cooled to room temperature and the
gas was released slowly. Then the liquid phase was analyzed by
HP7890A/5975C GC–MS. The unreacted substrate was then dis-
tilled out from the mixture, and the product was obtained. The
enantiomeric purity of the mixture was analyzed by HP7890A GC
equipped with a chiral DEX120 chromatography column.
3.2. Zn(II)-complexes
To better understand the roles of ZnCl2/[BMIM]Cl played for
the coupling reaction, we firstly investigated the possible active
Zn(II) species existed in the reaction systems. The molar ratio
of ZnCl2/[BMIM]Cl reported in our previous studies is 1:6 [32],
demonstrating excess Cl− ions are concluded. Recently, bis(1-
butyl-3-methylimidazolium) zinc tetrahalide was reported to be
formed in situ in the mixture of [BMIM]X and ZnY2 (X, Y = Cl or
Br) and was suggested as the active complex for the title reac-
tion by Kim’s group [5,29]. Based on these facts, three possible
2.2. Computational details
All the DFT calculations were carried out with the Gaussian 09
software package [41]. The hybrid Becke functional (B3) [42,43] for
electron exchange and the correlation functional of Lee, Yang, and
Parr (LYP) [44] with 6-31 + G(d,p) [45–48] basis set was selected for
the geometry optimization. All geometries were fully optimized
2−
chlorozincate clusters ZnCl2, ZnCl3−, and ZnCl4
are considered

F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
135
˚
geometry and one of the lengthened Zn Cl bond (2.379 A) of
2−
ZnCl4 in [BMIM]2ZnCl4 suggest the interaction of [BMIM]2ZnCl4
with PO will make it dissociate into [BMIM]ZnCl3 and [BMIM]Cl.
This is also stood by the small dissociation energy, which is only
12.2 kcal mol−1. Then the Zn center of [BMIM]ZnCl3 coordinates to
O atom of PO and activates the PO subsequently. Thus, [BMIM]Cl
and [BMIM]ZnCl3 are supposed to be the real main catalytic active
species for the title reaction. In the following−section, to save the
computational cost, we just take Cl−, ZnCl3 and one [BMIM]+
cation into account.
3.3. Mechanistic details for the cycloaddition of CO2 with PO
For all cases, both paths commence with the broken of C1
O
bond (as indicated in Scheme 1, denoted as path I) and C2 O bond
(denoted as path II) were considered. In the following section,
these two pathways will be denoted as Ia, and IIa for ZnCl2-
mediated reaction, Ib for [BMIM]Cl-mediated reaction, Ic and IIc
for ZnCl2/[BMIM]Cl-mediated reaction. It is worth noting here that
the uncatalyzed coupling reaction of CO2 with PO has been studied
by Zhang’s [39], Wu’s [36], and Han’s [11] groups previously. All
of their results consistently indicate the reaction without catalysts
proceeds via a concerted one-step mechanism and the barrier to be
surmounted are as high as ∼60 kcal mol−1, confirming the observed
difficulty of the uncatalyzed coupling reaction. For simplicity, the
mechanism of uncatalyzed reaction was not calculated in this work.
Fig. 1. Optimized geometries for Zn(II) species (ZnCl2x−x, x = 2–4).
in present study. The calculated results exhibit that the stable con-
figurations of ZnCl2, ZnCl3−, and ZnCl42− are linear, triangular, and
tetrahedral, respectively, as depicted in Fig. 1. To test the stability
of these chlorozincate clusters, the possible dissociation forms of
2−
−
ZnCl4
and ZnCl3 are investigated. As illustrated in Scheme 2,
it is exoth−ermic by 52.3 and 2.6 kcal mol−1 to dissociate ZnCl4
2−
2−
into ZnCl3 and into ZnCl2, respecti−vely, suggesting ZnCl4 is not
stable and the formation of ZnCl3 is energetically more facile.
For ZnCl3−, its dissociation into ZnCl2 and Cl− is found endother-
3.3.1. ZnCl2-mediated reaction
mic by 49.8 kcal mol−1, indicating ZnCl3 is stable and difficult to
−
We first consider the ZnCl2-mediated reaction. As shown in
Figs. 2 and 3, both pathways start with the formation of similar
weakly bound complexes 1 and 5 from ZnCl2 and PO, which leads
to an energy release by 5.9 and 5.8 kcal mol−1, respectively. IRC cal-
culations indicated that these two complexes are converted into
2 and 6 via TS1–2 and TS5–6 with the energy barriers of 47.3 and
36.8 kcal mol−1, respectively. The optimized geometrical param-
eters and the transition vector (corresponding to the imaginary
frequency of 551i and 197i cm−1) for TS1–2 and TS5–6 obviously
decompose. Conv−ersely, the combination of ZnCl2 and Cl− will eas-
ily produce ZnCl3
.
To further confirm the relative stability of Zn(II) species, a more
reliable single-point energy calculation was performed at MP2/6-
311 + G(d,p) level. The obtained dissociation energies are shown
in parentheses in Scheme 2. Obviously, both methods, B3LYP/6-
31 + G(d,p) and MP2/6-311 + G(d,p), give the similar trends for the
−
stability of ZnCl42−, ZnCl3 and ZnCl2.
However, the influence of the [BMIM]+ cation on the stabil-
ity of these chlorozincate clusters cannot be excluded. Therefore,
the geometries of [BMIM]ZnCl3 and [BMIM]2ZnCl4 have also been
optimized, as shown in Fig. 1. The calculated results indicate the
formation of [BMIM]2ZnCl4 from [BMIM]Cl and ZnCl2 is exother-
mic by 38.9 kcal mol−1. Moreover, [BMIM]2ZnCl4 is energetically
12.2 kcal mol−1 more stable than [BMIM]ZnCl3, which is different
demonstrated the breaking of C O bond. Additionally, the C2
H
bond in TS1–2 is also been activated, and the transition vector clearly
indicate the migration of this H atom from C2 to C1, which is similar
in TS5–6. In the following step, the upcoming CO2 molecules elec-
trophilically attack O atom in 2 and 6, yielding the intermediates 3
and 7. Subsequently, 3 and 7 evolve into the product-like interme-
diates 4 and 8, complexes of ZnCl2 with propylene carbonate (PC),
which lie below the entrance by 7.5 and 5.8 kcal mol−1, respectively.
The saddle point connecting 3 and 4 is TS3–4 with an imaginary fre-
quency of 403i cm−1, and it is TS7–8 (290i cm−1) linked 7 and 8. The
barriers to be surmounted from 3 to TS3–4, and from 7 to TS7–8 are
as high as 83.1 and 65.9 kcal mol−1, which are even higher than that
for the uncatalyzed coupling reaction, demonstrating the insertion
of CO2 and the formation of PC is very energetically demanding.
The release of ZnCl2 from PC completes the whole reaction, and the
−
from the above-mentioned stability order of the isolated ZnCl3
2−
2−
and ZnCl4
species, indicating ZnCl4
can be effectively stabi-
lized by two [BMIM]+ cations. Thus, we conjecture [BMIM]2[ZnCl4]
complex is most likely formed in situ from the mixture of ZnCl2
and [BMIM]Cl with the molar ratio of 1:6, and both [BMIM]Cl and
[BMIM]2[ZnCl4] ionic liquids are probably co-present in the reac-
tion system. It is worthy to note this supposition is supported by
the experimental findings. Recently, Seddon group has validated
the formation of bis(1-ethyl-3-methylimidazolium) zinc tetra-
halide ([EMIM]2ZnCl4) when the molar ratio of ZnCl2/[BMIM]Cl is
equal to or less than 1:3 using Raman spectroscopy and differen-
tial scanning calorimetry [58]. However, the distorted tetrahedral
overall reaction is exothermic by 5.2 kcal mol−1
.
Above results clearly indicate the mechanism for the title reac-
tion mediated by ZnCl2 has been changed in comparison with the
uncatalyzed reaction, where each path have two elementary steps:
one relates to the ring-opening of PO; the other involves the CO2
insertion and ring-closure forming PC, which is a concerted process
and is also the rate-determining step for the reaction. In view of the
Gibbs free energy profiles shown in Fig. 3, it is obviously concluded
that no path is kinetically and thermodynamically favorable owing
to the high barriers to be overcome (83.1 kcal mol−1 along path Ia
and 65.9 kcal mol−1 along path IIa), which is consistent with the
experimental findings that ZnCl2 scarcely has any reactivity for the
coupling reaction of CO2 with epoxides [6]. When comparing the
1
ZnCl3 + Cl
ZnCl2 + 2Cl
kcal mol
2
ZnCl4
1
kcal mol
1
ZnCl3
ZnCl2 + Cl
kcal mol
2−
−
.
Scheme 2. Possible dissociation forms of ZnCl4 and ZnCl3

136
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
Fig. 2. Optimized geometries for the intermediates and transition states included in the cycloaddition of CO2 to PO promoted by ZnCl2 along paths Ia and IIa. The selected
distances are labeled with the unit in Å. The gray, red, green, purple, and white balls represent C, O, Cl, Zn, and H atoms, respectively. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
charges of Zn and O loaded in the whole process, small changes are
found. This combined with the variations of the distances between
Zn and O (see Fig. 2) indicate the role of ZnCl2 toward the reaction
is to weakly stabilize the intermediates and transition states along
the reaction paths.
are similar with those reported by Zhang’s group. For simplifica-
tion, only the main conclusions drawn out from our calculations
are summarized herein.
The calculated results demonstrate three steps are involved in
the whole reaction: ring-opening of PO, CO2 insertion and then
ring-closure forming PC, which is different from the one-step
uncatalyzed reaction and the two-step ZnCl2-promoted reac-
tion. The relative barriers for these processes are 33.1, 17.0, and
17.4 kcal mol−1, indicating the rate-determining step relates to the
ring-opening of PO, where it is the simultaneous CO2 insertion and
ring-closure of PC for the reaction mediated by ZnCl2. The bar-
rier to be surmounted for the rate-determining step is significantly
lowered in comparison with the uncatalyzed reaction and the
ZnCl2-mediated reaction. The catalytic activity of [BMIM]Cl results
from the cooperative actions of cations and anions, where cations
stabilize the intermediates and transition states via hydrogen-
bonding interaction and anions make the ring-opening of PO much
easier via nucleophilic attack on C atom of PO. These results well
rationalize the experimental findings about the moderate activity
of [BMIM]Cl ionic liquids.
3.3.2. [BMIM]Cl-mediated reaction
In order to ascertain the co-catalytic role and to explain the
co-catalytic mechanism of [BMIM]Cl, the sole effect of [BMIM]Cl
without ZnCl2 should be first evaluated for the coupling reaction.
Especially, the mechanistic details about the coupling reaction of
CO2 with PO in the presence of [Cnmim]Cl (n = 2, 4, and 6) ionic
liquids has been systematically investigated at the B3PW91/6-
31G(d,p) level by Zhang’s group [39]. One of the preferable
three-step pathway was chosen to be re-optimized in present work
(denoted as path Ib, see Fig. S2) at the B3LYP/6-31 + G(d,p) level and
then used to compare the catalytic activity with the ZnCl2/[BMIM]Cl
composite catalysts. Being different from Zhang’s report, the Gibbs
free energy herein was used to consider the entropy effect toward
the reaction, as shown in Fig. 4. The obtained mechanistic details

F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
137
G (kcal mol-1)
3.3.3. ZnCl2/[BMIM]Cl-mediated reaction
TS
453.8-4
(26.8)
The mechanistic details for the ZnCl2-promoted and the
[BMIM]Cl-promoted coupling reaction have been investigated. And
the calculated results indicate ZnCl2 shows little activity along a
two-step pathway and [BMIM]Cl exhibits moderate activity along
a three-step pathway. Next, we will explore the detailed mecha-
nism and analyze the co-catalytic role of ZnCl2/[BMIM]Cl composite
catalysts.
TS1-2
41.4
Path Ia
Path IIa
(31.9)
TS7-8
TS5-6
31.0
(21.5)
37.6
(18.9)
8
PC+ZC
-5.2
(-13.1)
1
PO+CO2+ZC
-5.8
0.0
-5.9
We first examined the reaction along path Ic. As illustrated in
Fig. 5 on the reaction entrance, a stable complex 14 is formed from
(-26.1)
PC+ZC
(-13.5)
7
5
−
4
the interaction of [BMIM]Cl and ZnCl3 with PO, which is energet-
6
-23.3
(-37.1)
-5.8
ically more stable by 54.4 kcal mol−1 than the isolated reactants.
And then this complex is converted into intermediate 15, a ring-
opened oxy species, which lies 32.1 kcal mol−1 lower in energy
than the separate reactants, via transition state TS14–15 with a bar-
-7.5
-28.3
(-13.3)
(-26.3)
(-36.0)
3
2
-32.0
-37.3
(-44.9)
(-46.9)
rier of 26.4 kcal mol−1. In TS14–15, ZnCl3 interacts with O atom
−
and Cl− nucleophilically attacks the C1 atom. The imaginary fre-
quency of TS14–15 is 299 cm−1, and the corresponding transition
vector indicated by vibration analysis as well as the subsequent
IRC calculations manifest that the C2 O bond of PO is stretching
and the C1 Cl bond is shrinking. The geometrical parameters of
15 demonstrate the cleavage of C2 O bond. Especially, comparing
the distance between Zn and O atoms in this process, it is found the
Fig. 3. Gibbs free energy profiles for the cycloaddition of PO with CO2 promoted
by ZnCl2 along paths Ia and IIa. The sum of free energies of free PO, CO2 and ZnCl2
(ZC) is taken as the zero point energy, which is in kcal mol−1. The relative electronic
energies are also given (see values in parentheses).
G (kcal mol-1)
TS9-10
˚
˚
˚
length shortens from 2.239 A in 14 to 2.022 A in TS14–15 and 1.954 A
−
in 15, indicating the interaction between ZnCl3 and ring-opened
oxy species is strengthened gradually. Moreover, from structures
14 to 15 via TS14–15, the charge located on O atom is more and more
negative (−0.556, −0.751 and −0.889 e in 14, TS14–15 and 15) while
it is more and more positive for Zn atom (0.885, 0.962 and 0.999
e in sequence), which makes the electrostatic interaction between
O and Zn become more and−more strong, confirming the enhanced
interaction between ZnCl3 and ring-opened oxy species. As for
C1, it receives 0.073 e from Cl− in TS14–15. The increased negative
charge both on C1 and O atoms enlarges their electrostatic repul-
sion interaction, which facilitates the break of C1 O bond much
easier.
33.1
(20.8)
TS12-13
27.1
(5.4)
TS11-12
18.0
(-3.2)
12
9
11
9.7
3.1
PC+IL
-1.7
(-13.1)
6.9
(-11.4)
10
1.0
(-6.8)
(-11.4)
13
0.0
(-8.9)
-4.0
PO+CO2+IL
(-24.7)
Additionally, the O atom enriched with negative charge is ben-
eficial to its following nucleophilic attack on C atom of CO2. Thus,
when a CO2 is introduced, intermediate 17 is readily formed vi−a
TS16–17. The insertion of CO2 makes the interaction between ZnCl3
and ring-opened oxy species decreases gradually, as illustrated by
Fig. 4. Gibbs free energy profiles for the cycloaddition of PO with CO2 promoted by
[BMIM]Cl. The sum of Gibbs free energies of free PO, CO2 and [BMIM]Cl is taken as
the zero point energy, which is in kcal mol−1. The relative electronic energies are
also given (see values in parentheses).
Fig. 5. Optimized geometries for the intermediates and transition states included in the cycloaddition of CO2 to PO promoted by ZnCl2/[BMIM]Cl along paths Ic and IIc. The
selected distances are labeled with the unit in Å. The gray, blue, red, green, purple, and white balls represent C, N, O, Cl, Zn, and H atoms, respectively. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this article.)

138
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
of the Cl− anion, the ZnCl3 anion and the [BMIM]+ cation. In the
−
G (kcal mol-1)
−
PO+CO2+IL+ZnCl2+Cl-
ring opening step, ZnCl3 polarizes the C O bonds via the electro-
static attraction interaction with the O atom of PO, which combined
with the nucleophilic attack of Cl− on C1 atom, making the break
of C O bonds occur much easier. After the introduction of CO2, the
consecutive increase of Zn O bond distances in the related inter-
0.0
TS17-18
-17.9
(-59.0)
TS16-17
-20.3
(-59.6)
TS
-2119.-200
(-51.1)
16
-29.1
(-65.9)
17
-20.8
(-59.4)
−
20
mediates and transition states indicates the interaction of ZnCl3
TS22-23
-30.6
TS14-15
-28.0
(-58.1) 15
-32.1
with ring-opened oxy species decreases gradually, which facilitate
the final release of the catalyst systems more facile. On the other
hand, [BMIM]+ stabilizes the reaction system via hydrogen bond-
ing interactions with the Cl− anion or with the O atom of PO. These
actions result in the lowered energy barriers, which can explain
the effective catalytic activity of the ZnCl2/[BMIM]Cl composite
systems.
(-59.7)
-26.0
(-64.3)
TS21-22
-32.1
(-71.8)
21
-35.3
(-73.8)
23
Path IIc
Path Ic
19
-41.4
(-79.3)
22
-42.1
(-81.6)
-49.0
(-61.6)
(-75.8)
14
-54.4
(-79.7)
18
-63.6
Based on the results discussed above, we found that the
title reaction proceeds along different pathways when different
catalysts are selected. It occurs along a concerted pathway for
uncatalyzed reaction, while ZnCl2-mediated reaction was pre-
dicted to follow a two-step mechanism involving ring opening of
PO and simultaneous CO2 insertion and intramolecular ring closure.
In the case of [BMIM]Cl and ZnCl2/[BMIM]Cl catalyzed reaction, a
three-step mechanism, contained ring opening of PO, CO2 inser-
tion and intramolecular ring closure forming cyclic carbonate, is
favorable. Moreover, the calculated results indicate ZnCl2 has no
catalytic effect toward the title reaction due to the high barrier to
be overcome (65.9 kcal mol−1), while [BMIM]Cl ionic liquids show
moderate catalytic activity which reduce the energy barrier for
the rate-determining step to 33.1 kcal mol−1. Once we combine
the ZnCl2 and [BMIM]Cl together, it is surprisingly found that a
new stable [BMIM]2ZnCl4 complex is formed. The dissociation of
[BMIM]2ZnCl4 generates the real active component [BMIM]ZnCl3.
And the cooperative action of [BMIM]ZnCl3 and [BMIM]Cl makes
-62.7
(-97.8)
(-87.9)
PC+ILZnCl3-
Fig. 6. Gibbs free energy profiles for the cycloaddition of PO with CO2 promoted by
ZnCl2/[BMIM]Cl. The sum of Gibbs free energies of free PO, CO2, ZnCl2 and [BMIM]Cl
is taken as the zero point energy, which is in kcal mol−1. The relative electronic
energies are given in round brackets.
the distance between Zn and O atom in 16, TS16–17 and 17. The last
step of the reaction refers to an intramolecular ring closure of 17
forming the cyclic carbonate 18 with TS17–18 as the transition state.
This process is accompanied by the leaving of ZnCl3− and Cl− owing
to the electrostatic and hydrogen bond interactions for these two
species with [BMIM]+ cation. Then the catalysts regenerated with
the dissociation of PC.
As can be seen in Fig. 6, the C O bond cleavage in PO, CO2
insertion, and intramolecular ring close to afford PC, all together
contribute to a very large barrier (36.5 kcal mol−1, 14 → TS17–18) in
path Ic, which is even larger than that for [BMIM]Cl-mediated reac-
tion (33.1 kcal mol−1). And the rate-determining step is identified
as the ring closure forming the final product.
the energy barrier reduce to much lower values, 28.0 kcal mol−1
.
This cooperative action is related to the nucleophilic attack of Cl− on
C1 or C2 atom, the electrostatic interaction between Zn and O atom
of PO, and the direct stabilization of the intermediates and transi-
tion states by [BMIM]+ via hydrogen bonding interaction with the
O atom of PO or the Cl− of [BMIM]Cl, which synergistically facilitate
the PO ring-opening much easier.
In the case of path IIc, as shown in Figs. 5 and 6, the reaction
proceeds with the similar elementary steps as in path Ic, i.e., the
ring-opening of PO is followed by the insertion of CO2, and the sub-
sequent intramolecular ring closure leads to the formation of PC.
The release of the catalyst complex from PC, which can be reused
for the PO turnover, completes the whole reaction. The whole pro-
cess is exothermic by 63.6 kcal mol−1. From the Gibbs free energy
profiles, it is found the energy barriers for these three steps are
28.0, 3.2 and 16.1 kcal mol−1, respectively, illustrating the rate-
determining step refers to the ring-opening of PO, which is similar
to that for [BMIM]Cl-mediated reaction, but different from path Ic,
Zn-mediated and uncatalyzed reaction. By comparing the energy
barriers of paths Ic and IIc, it is obviously found that path IIc is
favored over path Ic (36.5 kcal mol−1). Moreover, the energy bar-
3.4. Enantioselectivity
The coupling reactions of CO2 with PO and with ECH have
been carefully examined by experiment. As depicted in Table 1,
the products (S)-PC (or (R)-PC) with high ee value (up to 99.7%)
were obtained when (S)-PO (or (R)-PO) was chosen as the reaction
substrate. However, if the (S)-ECH (or (R)-ECH]) which bearing a
CH2Cl substituent group is selected, the products chloropropene
carbonate (CC) obtained are nearly racemic, indicating the cou-
pling reaction has different enantioselectivity for the substrates
with different substituted groups. To better understand this inter-
esting phenomenon, further DFT calculations have been performed
(Table 2).
rier to be overcome for the rate-determining step, 28.0 kcal mol−1
,
is lower by 5.1 and 37.9 kcal mol−1 than those for [BMIM]Cl- and
ZnCl2-mediated reaction, indicating the combination of [BMIM]Cl
and ZnCl2 contribute significantly to reduce the energy barrier.
Above-mentioned results demonstrate the promotion effect of
this composite catalyst derives from the ternary cooperative action
For the reaction of PO with CO2, as can be seen in Fig. 5, along
path Ic, O5 atom nucleophilic attack on C1 via TS17–18 leads to the
Table 2
Cycloaddition of CO2 with PO and with ECHa.
Substrate
Product
Yield (%)
(S)-PC/CC selectivity (%)
(R)-PC/CC selectivity (%)
% ee
(S)-PO
(R)-PO
(S/R)-PO
(S)-ECH
(R)-ECH
(S/R)-ECH
(S)-PC
(R)-PC
(S/R)-PC
(S/R)-CCb
(R/S)-CC
(S/R)-CC
>99
>99
>99
97.6
99.8
98.8
>99
Trace
>99
50.0
51.7
52.1
51.4
99.7
99.7
0.0
3.4
4.2
Trace
50.0
48.3
47.9
48.6
2.8
a
Reaction condition: epoxides (20 ml), [BMIM]Cl ionic liquids (65 mg), ZnBr2 (1113 mg); temperature 120 ◦C, time: 4 h; pressure: 3.0 MPa.
CC, chloropropene carbonate.
b

F. Wang et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 133–140
139
while [BMIM]Cl ionic liquids show moderate catalytic activity
which reduce the energy barrier for the rate-determining step to
33.1 kcal mol−1. In the case of ZnCl2/[BMIM]Cl composite catalysts,
a new stable Zn(II) species, ZnCl3−, is found formed from the disso-
ciation of the in situ formed ZnCl4− speciation. Thus, [BMIM]Cl and
[BMIM][ZnCl3] ionic liquids are supposed to coexist in the reaction
system and their cooperative action makes the energy barrier
reduce to much lower values, 28.0 kcal mol−1. This cooperative
action is related to the nucleophilic attack of Cl− on C1 or C2 atom,
the electrostatic interaction between Zn and O atom of PO, and
the direct stabilization of the intermediates and transition states
by [BMIM]+ via hydrogen bonding interactions with the O atom
of PO or the Cl− anion of [BMIM]Cl, which synergistically facilitate
the PO ring-opening much easier. As for the enantioselectivity
of cyclic carbonate, an interesting intermediate 27, where both
groups connected to the chiral carbon atom are chloromethyl, is
formed along one path, which provides nearly equal opportunities
for inserted CO2 to attack the chiral carbon atom on both sides and
hence facilitates the formation of both R-product and S-product.
The present results well rationalize the experimental findings.
Fig. 7. Optimized geometry of the intermediate 27.
formation of (S)-PC. The other transition state TS17–18ꢀ, where O5
atom nucleophilically attacks C4 yielding the (R)-PC, has also been
located (see Fig. 5). However, it is energetically 64.8 kcal mol−1
higher than that of TS17–18, indicating TS17–18ꢀ is more destabilized
in comparison to TS17–18. The calculated geometrical parameters
exhibit the formation of C4 O5 bond is accompanied by the break
of C4 H6 bond. Moreover, to stabilize the released H6 atom, H7
atom is found leaving from C8 to afford H2 with H6. All of these
actions contribute a high energy to TS17–18ꢀ, making the formation
of R-type product be not preferred, which coincides with the high
ee values found in experiment when PO is selected as the reaction
substrate.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (973 Program, No. 2011CB201404), National Key
Technologies R&D Program of China (Project No. 2011BAE17B00)
and National Natural Science Foundation of China (Project No.
21203218). We gratefully acknowledge computing resources and
time on the Supercomputing Center of Cold and Arid Region Envi-
ronment and Engineering Research Institute of Chinese Academy
of Sciences. We also thank Prof. Dongju Zhang and Prof. Siwei Bi for
helpful discussions.
As for the reaction of ECH with CO2, the calculated results indi-
cate this reaction also proceeds along two possible pathways, Id
and IId, just as the case of PO with CO2, where each pathway occurs
with the similar elementary steps, i.e., the cleavage of C O bond of
(R)-ECH, CO2 insertion, and the intermolecular ring closure forming
(R/S)-CC (see Figs. S3 and S4). Moreover, both the rate-determining
steps along paths Id and IId are related to the broken of (R)-ECH
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.01.024.
C
O bond with the comparable barriers of 23.0 and 22.3 kcal mol−1
,
respectively. What’s interesting, the insertion of CO2 results in the
formation of intermediate 27 along path Id, where both groups
connected to the chiral carbon atom are chloromethyl, which is
not possessed for PO, as illustrated in Fig. 7. This provides nearly
equal opportunities for inserted CO2 to attack the carbon atom on
both sides, i.e., 27 can transform to 28 via TS27–28 with a barrier
of 12.5 kcal mol−1, or transform to 29 via TS27–29 with a−barrier of
15.9 kcal mol−1. The dissociation of [BMIM]Cl and ZnCl3 from 28
and 29 releases the products (R)-CC and (S)-CC, respectively. For
reaction along path IId, the sole product obtained is the (R)-CC.
Comparing the intermediates 28, 29 and 34, it is found that 28 and
29 are energetically comparable while 34 is less stable than 29 by
11.9 kcal mol−1. Hence, we conjecture the probability for the forma-
tion of 28 and 29, i.e., (R)-products and (S)-products, is comparable,
while the formation of 34 is less favorable although both paths have
quite similar energy barriers for the rate-determining step. Conse-
quently, racemic products are obtained for the coupling reaction
of CO2 with ECH, which is in agreement with the experimental
findings.
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