Development of pyridine based: O -aminophenolate zinc complexes as structurally tunable catalysts for CO2 fixation into cyclic carbonates

Article abstract of DOI:10.1039/c7nj01656e

Zinc complexes of ZnL^APIPX (L = 2,4-di-tert-butyl-6-(((E)-2-(((E)-pyridin-2-lmethylene)amino)benzylidene)amino)phenol) bearing different axial ligands (X = OAc, Cl, Br, I) have been successfully synthesized and characterized. It is evident from X-ray crystallography analysis that ZnL^APIPX exists as a distorted square-pyramid in which Zn^II is coordinated by an o-aminophenolate-based ligand and an X moiety in the axial position. These easily synthesized complexes have been shown to perform as effective catalysts for the coupling of epoxides and carbon dioxide to generate cyclic carbonates. Their axial ligand was found to play an important role in enhancing their catalytic activity. The ZnL^APIPI complex represents a versatile bifunctional catalyst due to the synergistic effect of both an electrophilic zinc ion and a nucleophilic iodide in one molecule to achieve a high-yield of cyclic carbonates, while ZnL^APIPOAc in cooperation with a co-catalyst affords a productive system for this coupling reaction. The effects of reaction variables such as temperature, time and pressure were systematically investigated. This solvent free route is relevant to the context of green chemistry as it provides an atom-efficient protocol for the conversion of CO₂ into valuable cyclic carbonates.

Full text of DOI:10.1039/c7nj01656e

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PAPER
Development of pyridine based o-aminophenolate
zinc complexes as structurally tunable catalysts
Cite this:DOI: 10.1039/c7nj01656e
for CO fixation into cyclic carbonates†
2
a
b
c
Z. Alaji,
E. Safaei * and A. Wojtczak
APIP
Zinc complexes of ZnL
X (L = 2,4-di-tert-butyl-6-(((E)-2-(((E)-pyridin-2-lmethylene)amino)benzylidene)-
amino)phenol) bearing different axial ligands (X = OAc, Cl, Br, I) have been successfully synthesized and
APIP
characterized. It is evident from X-ray crystallography analysis that ZnL X exists as a distorted square-
II
pyramid in which Zn is coordinated by an o-aminophenolate-based ligand and an X moiety in the axial
position. These easily synthesized complexes have been shown to perform as effective catalysts for the
coupling of epoxides and carbon dioxide to generate cyclic carbonates. Their axial ligand was found to play an
APIP
important role in enhancing their catalytic activity. The ZnL I complex represents a versatile bifunctional
Received 13th May 2017,
Accepted 28th July 2017
catalyst due to the synergistic effect of both an electrophilic zinc ion and a nucleophilic iodide in one
APIP
molecule to achieve a high-yield of cyclic carbonates, while ZnL OAc in cooperation with a co-catalyst
affords a productive system for this coupling reaction. The effects of reaction variables such as temperature,
time and pressure were systematically investigated. This solvent free route is relevant to the context of green
DOI: 10.1039/c7nj01656e
rsc.li/njc
2
chemistry as it provides an atom-efficient protocol for the conversion of CO into valuable cyclic carbonates.
Introduction
The expected global increase in CO
2
emissions from coal-fired
power plants demands more sustainable methods of carbon
1
–5
dioxide capture, storage, and utilization.
In nature, large-
2
Scheme 1 Synthesis of polymeric or cyclic carbonates from CO and
scale capture and storage are achieved by conversion of CO to
2
epoxides.
carbonates by a series of cascade reactions carried out by
6
metalloenzymes. In particular, the zinc metalloenzyme carbo-
nic anhydrase, fulfilling the requirement of an excellent catalyst Cyclic carbonates are valuable synthetic targets because of their
1
5
for reversible hydration of CO
2
into carbonate with a high relevant properties. High molecular dipole moments, high dielec-
7
turnover number of 10 , plays an important role in various tric constants and high boiling temperatures make cyclic carbonates
7
,8
16–21
biological systems.
inexpensive and abundant C
useful chemicals and as a fluid in numerous applications can applications for the production of additives in lubricants,
The use of carbon dioxide as a safe, suitable as highly green polar aprotic solvents
and as ion-
22,23
1
feedstock for the synthesis of carriers for lithium-ion batteries.
In addition, they find
3
,24
contribute to a sustainable chemical industry and concomi- as precursors for pharmaceutical intermediates in drugs,
In this and more specifically as monomers for the preparation of
regard, the atom efficient synthesis of either valuable polymeric polyurethanes, polycarbonates, and polyglycerol, and also
1
,6,9,10
tantly reduce CO emissions into the atmosphere.
2
2
5
26
27
2
8,29
carbonates or cyclic carbonates through the cycloaddition of as precursors for diastereo selective formation of diols.
1,11–14
CO
2
to epoxide is of great importance in industry (Scheme 1).
2
In addition, since CO is such an inert molecule, these
processes are significantly energy demanding. Therefore,
the reactivity may be greatly enhanced by the judicious choice
3
0
a
b
Institute for Advanced Studies in Basic Sciences (IASBS), 45137-66731, Zanjan, Iran
Department of Chemistry, College of Sciences, Shiraz University, 71454, Shiraz,
Iran. E-mail: e.safaei@shirazu.ac.ir
of catalysts. Typically, for the promotion of CO /epoxide coupling,
2
binary catalytic systems including a Lewis acid and a nucleophile
are employed. The Lewis acid activates the epoxide towards ring
opening resulting from the attack of the nucleophile to the less
substituted carbon atom. This reaction is followed by CO
tion which leads to ring closure, via a backbiting mechanism
c
Faculty of Chemistry, Nicolaus Copernicus University, 87-100 Torun, Poland
†
Electronic supplementary information (ESI) available: The characterization
1
13
data, and copies of H, C NMR spectra, and MALDI-MS spectra. CCDC
497259, 1497260 and 1497261. For crystallographic data in CIF or other electro-
nic format see DOI: 10.1039/c7nj01656e
2
inser-
1
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3
1
APIP
APIP
producing the cyclic carbonate and regenerating the nucleophile.
or I anions (Fig. 1). Complexes ZnL
OAc and ZnL
Br crystal-
APIP
In this regard, metal complexes as catalysts (Lewis acid) in combi- lize in the triclinic space group P1%, while ZnL
I crystallizes in
nation with auxiliary nucleophiles play a pivotal role in activation the monoclinic P2 /n space group. The diffraction experiment
1
3
2
APIP
and utilization of CO as a source of chemical carbon. Successful and the structure refinement data of ZnL
X (X = OAc, Br, I) are
2
9
,33
28,34–40
examples of metal complexes of magnesium, aluminium,
presented in Table 1. A water molecule with the occupancy set to
coupled mainly with 50% is found in complex ZnL
4
1,42
43
44
45–47
APIP
chromium,
porphyrins,
cobalt, zinc, and iron
OAc. In all complexes, the
4
8–50
39,51–55
56–59
salen
and salophen
have been coordination sphere of ZnN OX has a distorted square pyramid
3
developed. Among these, Zn-based complexes exhibit some geometry. Selected bond lengths and angles are summarized in
APIP
advantages such as low toxicity, low price, and high stability Table 2. The four-dentate L
ligand coordinates in a square-
6
0,61
towards oxidation.
salen complexes, organocatalysts,
Recently, bifunctional catalysts of metal planar manner and an anion X acts as a monodentate axial
62–65
66–69
and ionic liquids
pre- ligand. The overall shape of all molecules is a bowl formed by
7
0–72
APIP
sented high activity and selectivity. Several Co(salen)X,
Al(salen)Cl
the L
ligand with the central Zn(II) ion slightly shifted above
the N O pyramid base towards the axially coordinated anion
have been prepared which contain quaternary onium salts or (0.631, 0.728 and 0.726 Å) for ZnL
quaternary phosphonium salts tethered to the salen ligand, respectively. In the coordination N
2,59
73,74
and m-oxo-bimetallic salen aluminium complexes
3
APIP
X (X = OAc, Br, I)
7
5
3
O plane of all complexes,
phenolate group are the
APIP
thereby affording a single component catalyst. In bifunctional the Zn–O distances involving the L
catalysts, it is unnecessary to use a co-catalyst, and also easy to shortest (Table 2). This value is a little larger than the Zn–O
3
5
58
explore the mechanism and reuse the catalyst.
bond distance reported in Zn(salpyr) complexes. In all com-
Despite these advancements, further improvements are still plexes, the Zn–N bonds formed by N1 adjacent to the phenol
needed to synthesize versatile catalysts to achieve high catalytic moiety are shorter than those formed by pyridyl N3. While, the
activity both in terms of control and selectivity with binary and Zn–N2 bonds are the longest within the N O plane, which
3
APIP
bifunctional systems.
reflects the rigidity of L
acting as a four-dentate ligand. In
APIP
We were interested in developing discrete complexes with a ZnL
OAc, the Zn–O2 bond length formed by acetate is
nucleophilic axial X group promoting the catalytic activity in the 1.991(4) Å which is slightly longer than the Zn–O1 bond length
absence of a co-catalyst. This accomplishment is achieved by the of phenolate (1.977(3) Å).
APIP
subtle tuning of the axial ligand into an iodide anion to offer a
In ZnL
Br, the significant effect of the bulk imposed by
APIP
robust zinc complex of the o-aminophenolate ligand as a bifunc- the Br ligand is clearly visible. The dihedral angles in ZnL Br
tional catalyst for this coupling reaction. Metal complexes of are larger than those found in ZnL OAc. In ZnL I, the
o-aminophenolate ligands bearing various substituents are an corresponding angles are almost identical to those in ZnL OAc.
important class of compounds, because they allow the tuning of An analogous architecture is also found for the ZnL Cl complex
APIP
APIP
APIP
APIP
steric and electronic properties of the catalyst, thus promoting not reported here due to its low quality structure (Fig. S1, ESI†).
7
6–80
a variety of catalytic reactions.
In this article, we report one
APIP
Catalytic activity of ZnL
X (X = OAc, Cl, Br, I)
pot and easy synthesis of pyridine based o-aminophenolate zinc
complexes with different axial ligands (OAc, Cl, Br, I) (Scheme 2). The conversion of styrene oxide with CO2 to produce styrene
The catalytic activity of these complexes for the coupling of carbonate was chosen as a typical reaction to examine the catalytic
epoxides and carbon dioxide to produce cyclic carbonates in activity of the Zn-based catalysts (Scheme 3 and Table 3).
the absence of organic solvents is evaluated. The effect of the
First, screening experiments were performed at 100 1C, with
axial ligand on the catalytic activity of the different complexes for the initial CO2 pressure of 1.0 MPa, for 4 h with 0.5 mol%
APIP
the coupling of epoxides and carbon dioxide has been examined catalyst loading of ZnL
in the presence of a co-catalyst.
X (X = OAc, Cl, Br and I) without any
1
8,81–84
APIP
co-catalyst. Among the zinc-based catalysts, ZnL
I permitted
an excellent yield of up to 94% of cyclic styrene carbonate 1b
(
Table 3, entry 6).
In contrast, the catalysts containing bromide and chloride
Results and discussion
APIP
X-Ray crystallography of ZnL
X (X = OAc, Br, I) complexes
ligands showed a dramatic decrease in the yield of styrene
The asymmetric part of all structures is constituted by a carbonate 1b in 4 hours (Table 3, entries 2 and 4); while they
APIP
molecular complex [Zn(L
)(X)], where X denotes acetate, Br afforded moderate yields of 70% and 45% at a long reaction
APIP
Scheme 2 One-pot synthesis of ZnL
X (X = OAc, Cl, Br, I) complexes.
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interactions, makes the iodide anion more prone to leave the Zn
center and act as a nucleophile. Reversible interactions between
the halide anions and the Zn ions are a prerequisite for catalytic
activity, because the halide is involved in nucleophilic ring-
opening of the epoxide substrate and Zn–X dissociation also
allows for coordination of the epoxide to the Lewis acidic Zn
center. The non-polar nature of the tert-butyl groups was
important to ensure that the catalyst dissolved in the epoxide
8
5,86
and showed high levels of catalytic activity.
examined the temperature effect on ZnL
7
(
We also
I performance at
0 1C. It obtained a poor yield of 22% styrene carbonate
Table 3, entry 7), implying that activation of the axial ligand
to act as a nucleophile probably requires a high temperature of
APIP
5
6,66,87,88
100 1C.
Performing the reaction using ZnI
2
under the
APIP
same reaction conditions of ZnL
poor yield of 3% (Table 3, entry 8).
I at 100 1C also gave a very
TBAI was also examined and afforded a yield of 70% under
the same reaction conditions (Table 3, entry 9). It is worth
APIP
noting that the ZnL
I catalyst was recovered by isolating the
formed cyclic carbonate and drying the catalyst in vacuo. It was
reused for the subsequent run without significant loss in its
catalytic activity (Table 3, entry 10).
Rieger and co-workers presented the tetraamine–iron
complex as a one component catalyst comprising an easily
modified ligand motif with iron as the metal centre. This iron
catalyst was active in cyclic carbonate synthesis through the
coupling of CO with propylene oxide using a catalyst loading of
2
8
7
1
.5 mol% at a CO pressure of 1.5 MPa at 2 h. Recently Kleij
2
and co-workers reported that a tetraoxo bis-Zn (salphen) supra-
molecular host can bind various divalent metal salts, thereby
providing access to tri-nuclear bifunctional systems that incor-
porate both Lewis acid sites and dynamically bound nucleo-
philic anions. This catalytic system was capable of catalyzing
the coupling reaction of terminal epoxides with CO
2
using high
amounts of catalyst loading (2.5 mol%) at a relatively long
8
8
reaction time of 18 h. In comparison with bifuntional catalytic
systems in which the axial ligand performs as a nucleophile, our
APIP
catalysis system (ZnL
I) could afford excellent results with a
LAPIP
APIP
LAPIP
Fig. 1 Molecular structure of Zn
OAc, ZnL Br, and Zn
I. Hydrogen
broad substrate scope tolerance using a low amount of catalyst
loading (0.5 mol%) at 4 h. This result indicates the significant
effect of the o-aminophenolate-iminopyridine ligand, which
atoms have been omitted for clarity. Thermal ellipsoids are set at 50%
probability.
facilitates the solubility as well as the leaving ability of the iodide
APIP
time of 12 h (Table 3, entries 3 and 5). For the bifunctional anion in the ZnL
I complex. It has been demonstrated that
Zn-based catalyst, the yield of 1b increased with the order the Lewis acidic properties of the metal can be tuned by the
À
À
À
of the atomic number of the halide Cl o Br o I . A change electronic properties of the ligand, which may influence the
8
9,90
in the axial anion to the more nucleophilic iodide increases the reactivity and catalytic behavior of the complex.
styrene carbonate formation, while the less-nucleophilic acetate
APIP
On the basis of the catalytic activity of ZnL X (X = OAc, Cl,
nearly caused the loss of catalyst activity at a 4 h reaction time. Br, and I), the coordination–insertion mechanism can be
91
Comparison of the structures reveals that the length of the axial inferred. The metal complex initiates the coupling reaction by
Zn–X bond increases in the same trend (Table 2). Additionally, in coordinating the epoxide followed by the attack of a nucleophilic
APIP
APIP
ZnL Br and ZnL I complexes, the axial ligand has no intra- group (nucleophilic axial anion or added co-catalyst), leading to
APIP
molecular non-bonding interactions with the L
ligand. On the epoxide ring-opening and formation of a metal-bound alkoxide.
APIP
contrary, in ZnL OAc the C29 methyl group of acetate forms CO insertion provides a metal-bound carboxylate, followed by the
2
additional interactions with atoms in the Zn1–O1–C1–C6–N1 production of cyclic carbonate via a backbiting pathway.
chelate ring. Most likely, the longest bond distance of Zn–I in
the ZnL I molecule, as well as it having no intramolecular halide ligand by the epoxide substrate could also be approved
The proposed mechanism including the replacement of the
APIP
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APIP
Table 1 Crystal data and structure refinement for ZnL
X (X = OAc, Br, I)
APIP
APIP
APIP
Identification code
ZnL
OAc
ZnL
Br
ZnL
I
Empirical formula
Formula weight
Temperature, K
C
29
H
34
N
3
O
3.50Zn
C
27
H
30BrN
3
OZn
27 3
C H30IN OZn
604.81
293(2)
545.96
293(2)
557.82
293(2)
Wavelength, Å
Crystal system, space group
Unit cell dimensions, Å, 1
0.71073
0.71073
0.71073
Triclinic, P1%
a = 10.1927(13)
b = 10.2059(12)
c = 14.7907(19)
a = 94.834(10)
b = 98.779(11)
g = 114.583(12)
1364.0(3)
Triclinic, P1%
a = 7.9476(3)
b = 11.5809(5)
c = 14.2737(6)
a = 83.054(4)
b = 76.725(4)
g = 89.507(3)
1269.02(10)
Monoclinic, P2
a = 13.7585(6)
b = 12.5082(4)
c = 15.7328(6)
1
/n
b = 110.107(4)
3
Volume, Å
2542.49(18)
4, 1.580
2.204
À3
À1
Z, calculated density, mg m
Absorption coefficient, mm
F(000)
2, 1.329
0.937
574
2, 1.460
2.566
572
1216
Crystal size, mm
Reflections collected/unique
Absorption correction
0.53 Â 0.31 Â 0.17
8580/5709 [R(int) = 0.0566]
Analytical
0.661 Â 0.326 Â 0.186
8731/5668 [R(int) = 0.0384]
Analytical
0.599 Â 0.229 Â 0.062
17 638/5858 [R(int) = 0.0313]
Analytical
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F
0.8586 and 0.6369
5709/0/334
1.069
0.686286 and 0.338022
5668/0/298
0.882
0.869034 and 0.492954
5858/0/298
0.918
2
Final R indices [I 4 2s(I)]
R indices (all data)
R
R
1
= 0.0611, wR
= 0.0851, wR
2
2
= 0.1653
= 0.2129
R
R
1
= 0.0375, wR
= 0.0639, wR
2
= 0.0796
= 0.0859
R
R
1
= 0.0284, wR
= 0.0493, wR
2
= 0.0569
= 0.0601
1
1
2
1
2
APIP
Table 2 Selected bond lengths [Å] and angles [1] for the ZnL
X
Table 3 Conversion of styrene epoxide and CO
2
into the corresponding
APIP
(
X = OAc, Br, I) complexes
styrene carbonate using ZnL
X (X = OAc, Cl, Br, I)
APIP
APIP
APIP
a
b
c
ZnL
OAc
ZnL
Br
ZnL
I
Entry
Catalyst
T (1C)
Yield (%)
Time (h)
TON
APIP
Zn1–O1
Zn1–N1
Zn1–N3
Zn1–N2
Zn1–O2
Zn1–Br1
Zn1–I1
O1–Zn1–O2
O2–Zn1–N1
O2–Zn1–N3
O2–Zn1–N2
O1–Zn1–Br1
N1–Zn1–Br1
N3–Zn1–Br1
N2–Zn1–Br1
O1–Zn1–I1
N1–Zn1–I1
N3–Zn1–I1
N2–Zn1–I1
1.977(3)
1.960(2)
2.106(2)
2.121(2)
2.170(2)
1.9616(14)
2.1164(18)
2.1257(19)
2.1621(18)
1
2
3
4
5
6
7
8
9
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
OAc
100
100
100
100
100
100
70
100
100
100
5
10
45
30
70
94
22
3
4
4
12
4
12
4
4
4
4
4
10
20
90
APIP
APIP
APIP
APIP
APIP
APIP
2.086(3)
2.110(4)
2.174(3)
1.991(4)
Cl
Cl
Br
Br
I
60
—
—
—
140
188
44
6
140
180
—
—
2.4002(4)
—
—
—
—
—
2.5765(3)
I
111.90(13)
117.07(14)
105.44(15)
94.73(13)
—
—
—
—
—
—
—
—
ZnI
2
d
TBAI
ZnL
70
90
APIP e
10
I
a
Solvent free-reaction conditions: styrene oxide (10 mmol, 1.140 mL),
catalyst (0.5 mol%), p(CO ) = 1.0 MPa. Selectivity for the cyclic
2
carbonate product 499% in all entries (unless stated otherwise)
determined by GC using biphenyl as the internal standard and GPC.
Turnover number (TON): mole of styrene carbonate per mole of
catalyst. Tetrabutylammonium iodide (TBAI). Reused catalyst.
—
—
—
—
—
—
—
112.25(6)
111.95(6)
116.75(6)
99.24(6)
—
b
c
114.73(5)
117.60(5)
106.81(5)
101.16(5)
d
e
—
—
—
labile and can be easily substituted by the substrate, which
seems to correlate with the catalytic activity data.
APIP
Substrate scope of ZnL
I catalytic activity
APIP
To explore the generality of ZnL
I as a bifunctional catalyst
, the substrate
scope was investigated with 0.5 mol% of this catalyst, and a CO
Scheme 3 Coupling reaction of styrene oxide and CO
to produce in the coupling reaction of epoxides and CO
2
2
styrene carbonate.
2
pressure of 1.0 MPa at 100 1C (Scheme 4). Different kinds of
terminal epoxides were tolerated in this catalysis system. Alkyl,
by the structural data. That is, acetate is involved in a much aryl-substituted and glycidol-derived ether and ester substrates
more complexed network of intramolecular interactions, there- were converted into the corresponding cyclic carbonates with
fore it is not easy to be released from the coordination sphere. excellent yields.
It may be hypothesized that with the increasing length of the
Epichlorohydrin, 8b, was converted into 9b, and a bis-
axial Zn–X bond and more relaxed molecules, as found in epoxide substrate was also cleanly converted into its bis-
APIP
APIP
complexes of ZnL
Br and ZnL
I, the X ligand is more carbonate product 11b in good yield (80%).
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Table 4 Conversion of styrene epoxide and CO
styrene carbonate catalyzed by ZnL OAc catalysts under various conditions
2
into the corresponding
APIP
b
Catalyst Co-catalyst
Entry (mol%) (mol%)
p(CO
2
)
Yield Time
a
e
T (1C) (MPa) (%)
(h)
TON
c
f
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
(2)
(2)
(1)
(1)
TBAB (2)
100
100
100
100
100
100
100
100
100
1
1
1
1
1
1
1
1
1
1
1
1
0.5
0.1
1
1
1
70
95
85
10
70
90
97
95
60
80
20
96
80
40
90
80
82
15
97
85
50
30
2
2
2
2
2
2
2
2
2
2
2
4
4
4
2
2
2
2
3
3
2
3
35
48
85
10
35
180
485
950
d
TBAB (2)
TBAB (2)
—
TBAB (2)
TBAB (2)
TBAB (2)
TBAB (1)
TBAB (1)
(2)
(0.5)
(0.2)
(0.1)
—
—
0
1
2
3
4
5
6
7
8
9
0
1
2
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.1)
(0.05)
—
TBAB (0.5) 100
TBAB (0.1) 100
800
200
960
800
400
900
800
820
150
970
1700
—
TBAB (1)
TBAB (1)
TBAB (1)
TBAI (1)
TBAB (1)
70
70
70
70
70
70
70
70
70
70
70
g
TBAC (1)
h
DMAP (1)
TBAI (1)
TBAI (0.5)
TBAI (1)
TBAI (0.5)
1
1
1
1
—
1
—
a
Solvent free-reaction conditions (unless otherwise stated): styrene
oxide (10 mmol, 1.140 mL), catalyst, co-catalyst. Selectivity for the cyclic
b
carbonate product 499% in all entries. GC yield using biphenyl as
c
the internal standard. Reaction conditions: styrene oxide (2 mmol,
0
1
.228 mL), catalyst (0.0218 g, 2 mol%), TBAB (0.0128 g, 2 mol%), p(CO
2
) =
d
.0 MPa, 100 1C, CH Cl (2 mL). Reaction conditions styrene oxide
2 2
(
2 mmol, 0.228 mL), catalyst (0.0218 g, 2 mol%), TBAB (0.0128 g,
e
2
2 3
mol%), p(CO ) = 1.0 MPa, 100 1C, CH CN (2 mL). Turnover number
f
(TON): mole of styrene carbonate per mole of catalyst. Tetrabutyl-
g
ammonium bromide (TBAB). Tetrabutylammonium chloride (TBAC).
h
4-Dimethylaminopyridine (DMAP).
(
Table 4, entries 1 and 2). To afford an atom-economic and
Scheme 4 Substrate scope for the synthesis of organic carbonates
environmental friendly reaction, we examined solvent-free
conditions as well.
To set a catalytic benchmark, different co-catalyst to catalyst
ratios were tested in this coupling reaction. These catalytic tests
APIP
APIP
1
b–11b using ZnL
I. General conditions: epoxide (10 mmol), ZnL
I
a
(0.5 mol%), T = 100 1C, p(CO
2
) = 1.0 MPa. GC yield using biphenyl as the
internal standard. Isolated yield obtained after column chromatography
only for this substrate). Selectivity for the cyclic carbonate product 499%
b
(
in all examples determined by GC using biphenyl as the internal standard showed some general trends with a number of promising results
1
and H NMR.
(Table 4, entries 3–11).
In our work, it was confirmed (Table 4, entry 8) that
APIP
0.1 mol% ZnL
OAc catalyst in combination with 1 mol%
APIP
Catalytic activity of ZnL
OAc
tetrabutylammonium bromide (TBAB) afforded an excellent
Given that the zinc complexes containing Br, Cl and acetate as yield of 95% in 2 hours for the production of styrene carbonate
4
2,51,92
axial ligands, similar to most of the metal complexes,
are 1b with high selectivity. The low yield of product formation at a
not capable of efficient conversion of epoxides to the corres- high catalyst loading amount (Table 4, entry 5) can be attributed
ponding cyclic carbonates in the absence of a co-catalyst, to the poor solubility of the catalyst in styrene oxide.
APIP
APIP
we were motivated to employ ZnL
OAc in combination with
Using ZnL
OAc without a nucleophilic moiety showed a
a co-catalyst as a binary catalyst. Werner pointed out that a poor yield (Table 4, entry 4), and using TBAB or TBAI alone
nucleophilic species for the ring opening of the epoxide (Table 4, entries 9, 21 and 22), gave yields of 60%, 50% and
is essential. Furthermore, Lewis acidic homogeneous metal 30% respectively. These experiments, in agreement with other
complexes accelerate the addition of CO
2
to epoxides, but there reports elucidate the importance of a nucleophile for this
5
2,92
is only very low or even no catalytic activity if there is no catalytic activity.
6
2
nucleophile or co-catalyst.
The first screening experiments were performed at 100 1C, ture had the most decisive effect on the model reaction (Fig. 2).
with an initial CO pressure of 1.0 MPa, for 2 h with 2 mol%
So the effect of temperature was examined during 2 hours of
Regarding the investigated reaction parameters, tempera-
2
catalyst loading in dichloromethane and acetonitrile, resulting reaction time. However, even at a temperature of 50 1C, a
in moderate and good yields of styrene carbonate, respectively relatively good yield of 34% cyclic styrene carbonate 1b was
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Fig. 2 Effect of the reaction temperature on the yield of styrene carbo-
APIP
nate. Reaction conditions: styrene oxide (10 mmol, 1.140 mL), ZnL
OAc
(
0.0055 g, 0.1 mol%), TBAB (0.0322 g, 1 mol%), p(CO ) = 1.0 MPa, time = 2 h.
2
observed. Moreover, at 70 1C, a moderate yield of 51% was
obtained. The excellent yield of 95% was obtained at the
temperature of 100 1C. Upon further raising the temperature
to 130 1C, no significant enhancement in styrene carbonate
production was observed. Having explored the temperature
effect, it was also found that this coupling reaction at 70 1C
at the extended time of 4 hours affords an excellent yield of 96%
(Table 4, entry 12); thus, other experiments to determine the
dependence of yield on carbon dioxide pressure and co-catalyst
amount were performed at 70 1C. Darensbourg showed that the
formation of the cyclic carbonate was thermodynamically
favoured over the formation of the polycarbonate, as the kinetic
4
1,84
product, which is favoured at lower temperatures.
There- Scheme 5 Substrate scope for the synthesis of organic carbonates
APIP
1
b–11b using ZnL
OAc/TBAI. General conditions: epoxide (10 mmol),
fore we carried out GPC analysis for the reaction mixture of
Table 4, entry 12, to get more information about the reaction
content. It did not exhibit any polymer chain when the reaction
was performed at a mild temperature of 70 1C (Fig. S2, ESI†).
Furthermore, the carbon dioxide pressure impact was inves- second number in brackets, is isolated yield obtained after column
tigated at 0.1 MPa, 0.5 MPa and 1.0 MPa (Table 4, entries 12–14).
APIP
ZnL
4 2
OAc (0.1 mol%), NBu I (1 mol%), T = 70 1C, p(CO ) = 1.0 MPa.
Selectivity towards the cyclic carbonate product was 499% determined by
1
GC using biphenyl as the internal standard and H NMR; in all cases the first
a
number, is GC yield using biphenyl as the internal standard and the
b
chromatography.
In the model reaction, even at a carbon dioxide pressure of
0
.1 MPa, 40% of epoxide 1a was converted into 1b with no side
product. Styrene carbonate 1b was obtained in good yield for a 0.1 mol% ZnL
CO pressure of 0.5 MPa and it was enhanced to an excellent were isolated. The corresponding carbonates 4b and 5b are
yield of 96% at a CO pressure of 1.0 MPa. Considering the full potential building blocks for homo- and copolymerization
conversion of 1a and the high yield of cyclic carbonate 1b, the products with a cyclic carbonate unit in the backbone. We
CO pressure of 1.0 MPa was chosen for further investigations. isolated these products in yields of up to 85% under the
APIP
OAc and 1 mol% TBAI at 70 1C. All products
2
2
9
3
2
To clarify the significant effect of the co-catalyst, the reaction reaction conditions. (In some cases, the effects arising from
time was set to 2 h. Among the employed co-catalysts, DMAP isolation through column purification resulted in lower iso-
exhibited a very poor effect on the model reaction conversion lated yields). A number of Zn-based binary catalytic systems for
with a low yield (15%), while onium salts had a positive effect on chemical fixation of CO
styrene carbonate formation (Table 4, entries 15–18). TBAI gave developed.
2
into cyclic carbonates have been
For example, Klije et al. presented a highly
the highest yield of 90% of 1b, so it was selected for further active Zn(salphen) catalyst for production of organic carbonates
studies. Full conversion of 1a using TBAI is achieved in 3 h (Table 4, in a green CO medium in a solvent-free, CO -rich environ-
entry 19). Decreasing the amount of catalyst to 0.05 mol% and ment. They suggested that the solvent free condition provides
using TBAI as a co-catalyst yielded a very good turnover number of an efficient interaction between CO and the reagents improv-
700 in 3 h (Table 4, entry 20). From parameter screening, the ing the catalyst performance. Very recently, Godard and
standard reaction conditions were defined as 70 1C, a CO pressure co-workers reported zinc complexes containing pyrrolidine-
4
4,57,60,94
2
2
2
9
4
1
2
of 1.0 MPa and a co-catalyst to catalyst ratio of 10.
To investigate the applicability of the ZnL
based ligands as robust catalysts in the coupling of CO
OAc/TBAI terminal and internal epoxides with TBAI as a co-catalyst. They
2
with
APIP
system, the substrate scope was also examined (Scheme 5). It demonstrated that the Zn complex with an N4 (pyrollidine)
showed high efficiency for different substrates at conditions of coordination environment exhibited a high turnover number of
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Paper
Table 5 Conversion of cyclohexene epoxide and CO
ponding cyclohexene carbonate under various conditions
2
into the corres- Due to the hindrance of cyclohexene oxide as explained above
and the larger size of iodide, we employed other Zn-based
b
catalysts bearing bromide and chloride as axial ligands.
The yield of cyclohexene carbonate was increased to 81% and
Co-catalyst
Catalyst (mol%) (mol%)
Yield
T (1C) (%)
a
c
Entry
Sel.
TON
APIP
APIP
APIP
APIP
APIP
APIP
APIP
APIP
APIP
APIP
APIP
APIP
90%, as expected, when ZnL
respectively.
Br and ZnL
Cl were utilized
1
2
3
4
5
6
7
8
9
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
ZnL
—
OAc (0.1) TBAI (1)
OAc (0.1) TBAI (1)
OAc (0.1) TBAB (1)
OAc (0.2) TBAB (2)
OAc (0.2) TBAC (2)
70
100
100
100
100
100
70
45
57
74
88
59
22
60
76
81
90
55
499 450
499 570
499 740
499 440
499 295
APIP
We observed that hindered zinc complexes (ZnL X, X = Cl,
Br, I) permitted the homopolymerization of cyclohexene epoxides
to polyether. Gel permeation chromatography established that a
low molecular weight of polymer is formed (Table 5, entries 6–11)
(Fig. S3, ESI†). The polyether was also identified by H NMR
analysis (Fig. S4, ESI†). That is, homo-polymerization of epoxides
requires a site on zinc for epoxide binding. Indeed, halide
I (0.5)
I (0.2)
I (0.2)
Br (0.2)
Cl (0.2)
—
490
44
TBAB (2)
TBAB (2)
TBAB (2)
TBAB (2)
TBAB (2)
490 300
490 380
495 405
495 450
100
100
100
100
96
1
1
1
0
1
485
—
a
Solvent free-reaction conditions (unless otherwise stated): cyclohex- derivative zinc complexes were found to be the most active to
) = 1.0 MPa;
ene oxide (10 mmol, 1.0 mL), catalyst, co-catalyst, p(CO
time = 10 h. GC yield using biphenyl as the internal standard.
Selectivity for the cyclic carbonate product determined by H NMR
and GPC. As confirmed by H NMR, cis-cyclohexene carbonate was the to provide a carbonate intermediate.
only observed isomer. Turnover number (TON): mole of styrene
carbonate per mole of catalyst.
2
leave a site on zinc for homopolymerization of epoxides. On the
b
1
other hand, a small substituent (e.g., OAc) allows for CO
2
insertion
1
97
c
For sure, in the absence of steric inhibition, CO
more facile than epoxide ring-opening. We also found that
not only the conversion of cyclohexene oxide decreased to 55%
2
insertion is
98
up to 1560 for styrene oxide (80 1C, 30 bar, 16 h). Compared to when we utilize TBAB without any Zn-based catalyst, but also
APIP
the reported catalytic systems, ZnL
OAc/TBAI operates at the amount of undesired polyether increased (Table 5, entry 11
a lower CO pressure (10 bar), and 70 1C in a short reaction and Fig. S3, ESI†).
2
time of 3 h to obtain a good turnover number of 1700 for
styrene oxide.
In all cases, the cis-isomer of cyclohexene carbonate was the
only product, in agreement with a mechanism in which an
Non-terminal cyclohexene epoxide is often considered to be external nucleophilic anion displaces the metal-bound carbonate
96,99
a challenging substrate to be converted to a cyclic carbonate. intermediate.
The steric hindrance of nucleophilic ring opening of the
cyclohexene epoxide usually causes lower yields and, as a
consequence, more drastic reaction conditions are required.
Moreover, the structure of cyclic cyclohexene carbonate con-
sists of a six-membered ring interconnected to a five-membered
ring and is, therefore, geometrically strained. So, we studied
the reaction of cyclohexene oxide catalysed by the new zinc
Conclusions
Facile one-pot synthesis of new zinc complexes of an
o-aminophenolate–iminopyridine hybrid ligand bearing different
axial anions is presented to perform as efficient catalysts for CO
fixation into valuable compounds of cyclic carbonates.
2
complexes in detail (Table 5).
X-ray crystallography analysis revealed that the zinc complexes
APIP
Under typical reaction conditions (ZnL
OAc/TBAI at
II
APIP
exist as distorted square-pyramid Zn structures (ZnL X, X =
OAc, Br, I) with an N O coordination environment at the equator-
70 1C), we obtained a low yield of cyclohexene carbonate
3
(Table 5, entry 1). By increasing the temperature to 100 1C,
ial region and a mono-dentate X ligand in the axial positions. By
tailoring the nature of the axial ligand, the resulting complexes
were studied to perform as binary and bifunctional catalysts for
the coupling reaction of carbon dioxide (CO ) with epoxides,
2
leading to the formation of cyclic carbonates.
Under solvent free conditions, ZnL
with TBAI or TBAB was found to be a versatile catalyst to convert
different epoxides including cyclohexene oxide to their corres-
the yield improved (Table 5, entry 2). Having employed TBAB as
a co-catalyst and increased the amount of the catalyst and co-
catalyst, cyclohexene carbonate was afforded in a high yield
(Table 5, entry 4). This is attributed to the larger size of the
iodide compared with bromide, which may prove unfavourable
in the nucleophilic attack and ring-opening of a strictly
APIP
OAc in combination
9
5
hindered cyclohexene oxide.
Using tetrabutylammonium
chloride (TBAC) as a co-catalyst did not improve the cyclo-
hexene carbonate yield much, attributed to the poor leaving
ability of the chloride anion, and to its stronger interaction with
the tetrabutylammonium cation compared to bromide and
ponding cyclic carbonates in high yield and excellent selectivity,
APIP
while ZnL
I as a one component catalyst could afford a high
yield of cyclic carbonates with no addition of onium salts as the
co-catalyst.
9
6
iodide anions (Table 5, entry 5). Then we tried to evaluate
APIP
the efficiency of ZnL
I as a one-component catalyst in
cyclohexene carbonate synthesis; however, the yield was only
moderate at 22% (Table 5, entry 6).
Experimental section
General information
Using TBAB as a co-catalyst at 70 1C, the yield of cyclohexene
carbonate increased to 60%, and with further increase in tem- All chemicals were purchased from Merck and Aldrich companies
perature to 100 1C, the yield improved to 76% (Table 5, entry 8). and used as received. Manipulations were performed under
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aerobic conditions. 3,5-DTBQ (3,5-di-tert-butylcyclohexa-3,5-diene- of Zn (OAc)
2
Á2H
2
O (0.219 g, 1 mmol), ZnCl
2
(0.136 g, 1 mmol),
1
2 2
,2-dione) was synthesized according to a modified literature ZnBr (0.225, 1 mmol) and ZnI (0.319, 1 mmol) were added to
100
procedure. FT-IR spectra were recorded in the solid state on a the reaction mixture immediately. Then, 2 equivalents of NEt3
FT-IR Bruker Vector 22 spectrophotometer in the 400–4000 cm
À1
and 10 mL CH CN was added to induce the precipitation of the
3
APIP
APIP
APIP
APIP
range. NMR spectra were recorded at 400 MHz on a Bruker DRX ZnL
OAc, ZnL
Cl, ZnL
Br and ZnL
I complexes after
spectrometer. The chemical shifts were referred to TMS using the 2 h of stirring, respectively. Finally, the precipitate was filtrated
residual signals from the solvent. Mass spectra (positive ion) were and crystallized in dichloromethane/acetonitrile for further
obtained on a Bruker Microflex LT MALDI-TOF MS instrument. purification.
Elemental analyses were carried out on Elementar Vario EL III.
APIP
ZnL
OAc
X-ray structure determination
The filtrate was left undisturbed to afford dark brown micro-
APIP
Brown crystals of ZnL
OAc complex suitable for the X-ray crystals by evaporation from dichloromethane/acetonitrile
diffraction experiment were obtained from the methanol solution (0.382 g, 70% yield). X-ray quality dark brown single
APIP
APIP
solution. ZnL
Br and ZnL
Cl
I complexes were crystallized crystals were grown from the methanol solution of the complex.
À1
from the CH CN–CH
3
2
2
solution. For all compounds, the X-ray n max(KBr)/cm : 3415 (O–H), 2946 (C–H), 1606 (CQO), 1593
data were collected with an Oxford Sapphire CCD diffracto- (CQC), 1469 (CQN), 740 (Q C–H bending) (Fig. S5, ESI†).
1
meter using MoKa radiation l = 0.71073 Å, at 293(2) K, by the
6
H NMR (400 MHz, DMSO-d ) d 9.09 (s, 1H), 9.00 (s, 1H), 8.93
o–2y method. Structures have been solved by direct methods (d, J = 4.90 Hz, 1H), 8.39 (td, J = 7.72, 1.65 Hz, 1H), 8.18
2
and refined with the full-matrix least-squares method on F
with the use of the SHELX2014
(d, J = 7.64 Hz, 1H), 8.06–7.97 (m, 2H), 7.76–7.70 (m, 1H), 7.65–
1
01
program package. The 7.61 (m, 2H), 7.41 (d, J = 2.33 Hz, 1H), 7.08 (d, J = 2.35 Hz, 1H),
analytical absorption correction was applied (RED171 package 1.59 (s, 3H), 1.45 (s, 9H), 1.29 (s, 9H) (Fig. S6, ESI†). C NMR
1
3
1
02
of programs
3 2
Oxford Diffraction, 2000) (Table 1). No extinc- (100 MHz, (CD ) SO): d 29.8, 30.0, 32.1, 32.2, 34.4, 35.5, 109.7,
tion correction was applied. Hydrogen atoms were located from 121.03, 124.5, 128.9, 129.2, 129.6, 129.7, 132.0, 132.3, 133.5,
the electron density maps and their positions were constrained 135.5, 138.5, 141.9, 145.0, 147.4, 149.9, 153.08, 161.1, 162.7
in the refinement.
(Fig. S7, ESI†). Elemental analysis: calculated: C, 64.74; H, 6.37;
Gel permeation chromatography (GPC) analyses were carried N, 7.81. Found: C, 64.95; H, 6.81; N, 7.58.
out at 40 1C using a Waters Breeze system equipped with a
APIP
ZnL
Cl
1
515 Refractive index detector. The samples were filtered before
analysis. Tetrahydrofuran was used as the eluent.
After the catalytic test, the reaction products were analysed crystals by evaporation from dichloromethane/acetonitrile
The filtrate was left undisturbed to afford dark brown micro-
1
13
À1
by means of H NMR and C NMR, FT-IR, and GPC.
solution (0.384 g, 75% yield). n max(KBr)/cm : 2948 (C–H),
1
The samples of reaction mixture for analysis by H NMR 1594 (CQC), 1470 (CQN), 745 (Q C–H bending) (Fig. S8, ESI†).
1
were prepared by adding 2 mL of deuterated chloroform to
6
H NMR (400 MHz, DMSO-d ) d 9.16 (s, 1H), 9.04 (s, 1H), 8.89
the reaction mixture while stirring to dissolve the reagents, (d, J = 4.88 Hz, 1H), 8.44 (td, J = 7.73, 1.62 Hz, 1H), 8.23 (d, J =
products and the catalyst. An aliquot of 50 mL of the obtained 7.65 Hz, 1H), 8.08 (dd, J = 8.32, 5.55 Hz, 1H), 8.03 (dd, J = 5.88,
solution was diluted with 1 mL of CDCl and analysed (cyclo- 3.45 Hz, 1H), 7.82 (dd, J = 5.86, 3.36 Hz, 1H), 7.71–7.63 (m, 2H),
3
hexene oxide (d = 3.1 ppm), carbonate linkages of poly (cyclo- 7.45 (d, J = 2.31 Hz, 1H), 7.10 (d, J = 2.33 Hz, 1H), 1.46 (s, 9H),
1
3
3 2
hexene carbonate) (broad signal at d = 4.65 ppm), polyether 1.30 (s, 9H) (Fig. S9, ESI†). C NMR (100 MHz, (CD ) SO) d 29.8,
linkages of homo-polymerization (broad signal at d = 3.45 ppm), 31.1, 32.1, 34.4, 35.5, 109.8, 121.2, 124.7, 128.8, 129.6, 130.0,
trans-cyclohexene carbonate (d = 4.0 ppm), and cis-cyclohexene 130.1, 132.1, 133.8, 135.7, 138.6, 142.2, 144.2, 147.35, 149.5,
carbonate (d = 4.64 ppm)). The cyclic carbonate samples for 153.0, 160.5, 162.60 (Fig. S10, ESI†). MALDI-MS m/z: 512.987
FT-IR analysis were prepared by placing a small drop of isolated (Fig. S11, ESI†). Elemental analysis, calculated: C, 63.17; H,
products diluted with CH Cl on a KBr plate. The distinctive 5.89; N, 8.19. Found: C, 63.28; H, 5.49; N, 8.55.
2
2
À1
absorption band at 1793–1804 cm is observed for the cyclic
APIP
9
6
ZnL
Br
carbonate.
The filtrate was left undisturbed to afford dark brown micro-
APIP
Synthesis of ZnL
X (X = OAc, Cl, Br, I)
crystals by evaporation from dichloromethane/acetonitrile
APIP
À1
ZnL
X (X = OAc, Cl, Br, I) complexes were synthesized by the solution (0.418 g, 75% yield). n max(KBr)/cm : 2948 (C–H),
following one-pot reaction method. Firstly, to a solution of 1594 (CQC), 1470 (CQN), 745 (Q C–H bending) (Fig. S12, ESI†).
1
3
2
,5-DTBQ (0.22 g, 1 mmol) in acetonitrile (4 mL) was added
6
H NMR (400 MHz, DMSO-d ) d 9.20 (s, 1H), 9.06 (s, 1H), 8.89
-aminobenzyl amine (0.122 g, 1 mmol), and the reaction (d, J = 4.87 Hz, 1H), 8.45 (t, J = 7.72 Hz, 1H), 8.24 (d, J = 7.65 Hz,
solution was stirred for 30 min at room temperature in the 1H), 8.09 (dd, J = 2.76, 1.15 Hz, 1H), 8.04 (dd, J = 5.89, 3.43 Hz,
presence of air. After 30 minutes, it afforded a yellow precipitate 1H), 7.87–7.82 (m, 1H), 7.71–7.65 (m, 2H), 7.47 (d, J = 2.30 Hz,
AP
AP
(
HL ). To a stirred suspension of HL in acetonitrile was 1H), 7.12 (d, J = 2.32 Hz, 1H), 1.47 (s, 9H), 1.30 (s, 9H) (Fig. S13,
1
3
added pyridine-2-carboxaldehyde (0.096 mL, 1 mmol). For ESI†). C NMR (100 MHz, (CD
3 2
) SO) d 29.8, 32.10, 34.4, 35.55,
synthesis of different zinc complexes, various zinc precursors 109.9, 121.2, 124.8, 128.7, 129.8, 130.1, 130.2, 132.0, 132.3,
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1
1
34.0, 135.8, 138.7, 142.38, 144.0, 147.2, 149.5, 153.4, 160.7, facility and Prof. Tim Storr’s collaboration for performing
62.3 (Fig. S14, ESI†). MALDI-MS m/z: 557.146 (Fig. S15, ESI†). MALDI-MS analyses.
Elemental analysis: C, 58.13; H, 5.42; N, 7.53. Found: C, 58.28;
H, 5.47; N, 7.68.
References
APIP
ZnL
I
1
2
D. J. Darensbourg, Inorg. Chem., 2010, 49, 10765–10780.
D. Tian, B. Liu, Q. Gan, H. Li and D. J. Darensbourg,
ACS Catal., 2012, 2, 2029–2035.
A.-A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96,
951–976.
S. H. Kim and S. H. Hong, ACS Catal., 2014, 4, 3630–3636.
O. Martin, M. Hammes, S. Mitchell and J. P ´e rez-Ram ´ı rez,
Energy Environ. Sci., 2014, 7, 3640–3650.
The filtrate was left undisturbed to afford dark brown micro-
crystals by evaporation from dichloromethane/acetonitrile
solution (0.483 g, 80% yield). n max(KBr)/cm : 2948 (C–H),
À1
3
1
594 (CQC), 1471 (CQN), 745 (Q C–H bending) (Fig. S16, ESI†).
1
H NMR (400 MHz, DMSO-d
6
) d 9.29 (s, 1H), 9.14 (s, 1H), 8.86
4
5
(d, J = 4.85 Hz, 1H), 8.50 (td, J = 7.74, 1.57 Hz, 1H), 8.28 (d, J =
7.65 Hz, 1H), 8.17–8.11 (m, 1H), 8.11–8.06 (m, 1H), 7.93–7.85
(
2
m, 1H), 7.76–7.66 (m, 2H), 7.52 (d, J = 2.26 Hz, 1H), 7.18 (d, J =
.27 Hz, 1H), 1.51 (s, 9H), 1.31 (s, 9H) (Fig. S17, ESI†). C NMR
1
3
6 G. Fiorani, W. Guo and A. W. Kleij, Green Chem., 2015, 17,
375–1389.
1
(100 MHz, (CD
3
)
2
SO) 29.9, 32.0, 34.5, 35.58, 110.2, 121.3, 125.1,
7
8
9
S. Schenk, J. Notni, U. Kohn, K. Wermann and E. Anders,
Dalton Trans., 2006, 4191–4206.
M. Mondal, S. Khanra, O. Tiwari, K. Gayen and G. Halder,
Environ. Prog. Sustainable Energy, 2016, 35, 1605–1615.
T. Ema, Y. Miyazaki, S. Koyama, Y. Yano and T. Sakai,
Chem. Commun., 2012, 48, 4489–4491.
1
1
28.4, 130.1, 130.4, 130.6, 132.0, 132.6, 134.7, 136.5, 138.8,
42.8, 143.9, 147.23, 149.4, 154.4, 161.5, 161.8 (Fig. S18, ESI†).
MALDI-MS m/z: 603.173 (Fig. S19, ESI†). Elemental analysis,
calculated: C, 53.62; H, 5.00; N, 6.95; found: C, 53.85; H, 5.56;
N, 6.66.
1
0 M. Peters, B. K o¨ hler, W. Kuckshinrichs, W. Leitner,
P. Markewitz and T. E. M u¨ ller, ChemSusChem, 2011, 4,
Typical experimental procedure for cyclic carbonate synthesis
APIP
using ZnL
I as the catalyst
1
216–1240.
1 Q. Liu, L. Wu, R. Jackstell and M. Beller, Nat. Commun.,
015, 6, 5933.
The coupling reactions were carried out in a 40 mL stainless
steel autoclave equipped with a magnetic stir bar. In a typical
reaction, the reactor was charged with an appropriate amount
of catalyst and epoxide. After the reactor was pressurized with
CO2 to a desired pressure, the reaction mixture was heated
to the desired temperature (100 1C) while stirring at about
1
1
2
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