Three powerful dinuclear metal-organic catalysts for converting CO2 into organic carbonates

Article abstract of DOI:10.1039/c6dt02755e

Developing efficient catalysts for converting carbon dioxide (CO₂) into varied organic carbonates is an important scientific goal. By using the NH₂-functionalized tripodal ligand 2-((bis(2-aminoethyl)amino)methyl)phenol (HL), three dinuclear metal-organic complexes [Zn(L)]₂·2ClO₄ (1), [Cu(L)]₂·2ClO₄·2H₂O (2) and [Cd(L)]₂·2ClO₄ (3) have been successfully isolated and structurally characterized using single-crystal X-ray diffraction analyses. Considering the dinuclear metal centers and the NH₂-functional groups in the structures, 1-3 were investigated as catalysts for converting CO₂ into organic carbonates, and the results show that 1-3 exhibit an outstanding ability for converting CO₂ into varied organic carbonates at atmospheric pressure (0.1 MPa). The catalytic system also displays a wide substrate scope and high catalytic activity, and the reaction mechanism has been proposed herein.

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Three powerful dinuclear metal-organic catalysts for converting
CO into organic carbonates†
2
Received 00th January 20xx,
Accepted 00th January 20xx
Dan Zhao, Xiao-Hui Liu, Zhuang-Zhi Shi, Chen-Dan Zhu, Yue Zhao, Peng Wang and Wei-Yin Sun*
DOI: 10.1039/x0xx00000x
Developping efficient catalysts for converting carbon dioxide (CO
goal. By using NH -functionalized tripodal ligand 2-((bis(2-aminoethyl)amino)methyl)phenol (HL), three dinuclear metal-
organic complexes [Zn(L)] ·2ClO (1), [Cu(L)] ·2ClO ·2H O (2) and [Cd(L)] ·2ClO (3) have been successfully isolated and
structurally characterized by single-crystal X-ray diffraction analyses. Considering the dinuclear metal centers and the NH
functional groups in the structures, 1 - 3 were investigated as catalysts for converting CO into organic carbonates and the
results show that 1 - 3 exhibit outstanding ability for converting CO into varied organic carbonates at atmospheric
2
) into varied organic carbonates is a significant scientific
2
www.rsc.org/
2
4
2
4
2
2
4
2
-
2
2
pressure (0.1 MPa) and the catalytic system also displays a wide substrate scope and high catalytic activity. The reaction
mechanism has been proposed.
both single catalysts and coꢀcatalysts to promote the cycloaddition of
epoxides with CO2 including alkali metal/organic phosphorus
catalysts, organic alkali catalysts, ionic liquids, and metalꢀorganic
Introduction
Carbon dioxide (CO ) as a renewable feedstock and its
effective fixation and high valueꢀadded utilization deserve
worldwide research efforts from the perspective of
establishment of sustainable society. Chemical fixation and
conversion of CO show great promise for the recycling of CO2
into valuable chemicals due to its advantages such as an
abundant, sustainable, nontoxic and renewable C feedstock.
However, the utilization of CO2 still encounters major
challenges that originate from its high thermodynamic stability
and kinetic inertness. Recent progress in the field of catalytic
transformations with CO has led to a spectacular increase in
the application of this C synthon providing access to important
and functional chemical intermediates, and catalytic reactions
are considered to be essential for widening and deepening the
synthetic utility of CO2.
Varied catalytic reactions for the transformation of CO2 into
chemicals have emerged, among them the cycloaddition of CO2
with epoxides to form cyclic carbonates is one of the most effective
strategies. Cyclic carbonates as low cost and accessible monomers
applied in industry and commerce. They are also important
2
5
complexes. Various metal complexes have successfully developed
with advantages of low toxicity, low cost and tunable Lewis acidity
for the synthesis of cyclic organic carbonates. More recently, Kleij et
al. used an amino triphenolate scaffold ligand and designed an Al(III)
1
2
complex for the coupling reactions between CO
2
and epoxides with
turnover frequency (TOF) up to 36000 h . Ema and coworkers
have developed series of bifunctional diporphyrinꢀ and
−
1 6
2
1
a
triporphyrinꢀZn catalysts which cooperatively activate epoxide via
7
multiple catalytic sites.
To overcome the thermodynamics of CO
activators are attracting everꢀincreasing attention in green chemistry.
The assistance of organic bases able to activate CO could further
2 2
, organic bases as CO
2
1
2
enhance the catalytic efficiency, thus allowing the incorporation of
3
CO into cyclic carbonates smoothly under mild reaction conditions.
2
For instance, 4ꢀ(dimethylamino)pyridine (DMAP), 1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (TBD) or PPh
3
could fix CO
adduct. In addition, it has been proposed that the
dinuclear metal centers can activate both epoxide and CO , and
2
to afford
8
baseꢀCO
2
2
accordingly promote nucleophilic attack of the alkoxide to the
9
4
carbon atom of the activated CO molecule.
2
intermediates for pharmaceuticals in biomedical applications.
Given the foregoing statement, we envisaged metal complexes
with amino phenolate tripodal organic ligand, namely 2ꢀ((bis(2ꢀ
Recently, different kinds of catalysts have been investigated as
1
0
aminoethyl)amino)methyl)phenol (HL, Chart S1). Such N, N, N,
Oꢀchelating ligand may show inductive effects and favor the high
oxophilic nature of the coordinated metal center to promote the
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Collaborative Innovation Center of Advanced Microstructures,
Nanjing University, Nanjing 210023, China. E-mail address: sunwy@nju.edu.cn; Tel:
cycloaddition of epoxides with CO
dinuclear metalꢀorganic complexes [Zn(L)] ·2ClO4 (1),
Cu(L)] ·2ClO ·2H O (2) and [Cd(L)]
and used as catalysts for converting CO
2
. Herein, three wellꢀdefined
10
+
86 25 89683485
Electronic Supplementary Information (ESI) available: X-ray crystallographic data
2
†
in CIF format, structure illustrations for organic ligand HL and HL-PA, PXRD
[
2
4
2
2
4
·2ClO (3) have been isolated
1
13
patterns, figures of HPLC, characterization data and H NMR and C NMR spectra,
CCDC 1441720-144172 and 1492227-1492229. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/x0xx00000x
2
into organic carbonates
under atmospheric pressure. The results show that even at low
catalyst loading (0.16 mol%), the complexes still show a fairly high
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catalytic activity for a broad substrate scope.
temperature until the complete consumption of aldehyde
monitored by TLC. The mixture was poured into cold water (60
mL), and extracted with ethyl acetate (30 mL x 3). The
combined organic layers were washed with water (30 mL) and
Experimental
4
brine (30 mL x 2), dried over MgSO . Then the crude epoxide
General information
was purified by flash chromatography (hexane/EtOAc = 8:1).
Catalytic reaction process
Chemicals were obtained from commercial suppliers and used
as received without further purification. All reactions were
carried out under carbon dioxide atmosphere unless otherwise
noted. Ligand HL was synthesized according to the previously
reported procedure. Reactions were monitored by thinꢀlayer
chromatography (TLC) on silica gel plates (GF254), and the
analytical TLC was performed on preꢀcoated, glassꢀbacked
silica gel plates. Powder Xꢀray diffraction (PXRD) analyses
The catalytic reactions were carried out in an autoclave which
can regulate the pressure. The air in the autoclave was replaced
by CO gas (the purity of CO is 99.999%). The catalytic
2 2
reactions in 0.1 Mpa were performed in 10 ml sealed test tube
equipped with magnetic stirrer and charged with the
corresponding catalyst and depressurization with rubber bladder
1
0
2
in which filled with CO (0.1 MPa) to control the reaction
were performed on
a
Shimadzu XRDꢀ6000 Xꢀray
1
13
pressure. After the reaction, the autoclave and the test tube
cooled to room temperature, and then slowly released the
reaction gas. Then the reaction was quenched by placing the
autoclave and the test tube in ice water. The final target product
was obtained from the liquid phase mixture by flash
chromatography.
diffractometer with Cu Kα (λ = 1.5418 Å) radiation. Hꢀ, Cꢀ
NMR spectra were recorded on BrukerꢀDRX (300 MHz, 400
MHz) instruments at room temperature. High resolution mass
spectra were recorded on AGILENT 6540QTOF for Mass
Spectrometry. Elemental analyses for C, H, and N were
performed on Elementar Vario MICRO at the analysis center of
Nanjing University. HPLC analyses were carried out on
DIONEX Ultimate 3000.
X-ray crystallography
Crystallographic data collections for 1 - 3
)ꢀ2a were carried out on a Bruker Smart Apex CCD area
detector diffractometer with graphiteꢀmonochromated MoꢀKα
radiation ( = 0.71073 Å) at 293(2) K using the ꢀscan
technique. The structures were solved by direct methods and
, L3-Zn, (R)ꢀ2a and
Synthesis of complexes 1 - 3. A mixture of M(ClO
4 2 2
) ·6H O
(
S
(
M = Zn for ; Cu for and Cd for ; 0.1 mmol) and HL (0.1
1
2
3
mmol) was dissolved in 10.0 mL distilled water. The pH of the
resulting solution was adjusted to about 7~8 using 0.5 M NaOH
λ
ω
2
and then saturated NaClO
few minutes, the filtered precipitate was dissolved into CH
Crystals suitable for Xꢀray crystallographic analysis were
obtained by slow evaporation of the CH CN solvent (yields
and about 50% for ). Anal.
Zn ): C, 35.41; H, 4.86; N,
4
·H
2
O solution was added. After a
refined on
F by the fullꢀmatrix leastꢀsquares methods using the
1
2
3
CN.
SHELXTL package. The detailed crystal data and structures
refinement for 1 - 3 are given in Table 1. The detailed crystal
3
data and structure refinements for L3-Zn, (R)ꢀ2a and (S)ꢀ2a are
given in Table S1.
about 60% for
Calcd (%) for
1
1
; about 70% for
2
3
(C22
H
36
N
6
O
10Cl
2
2
Table 1 Crystallographic data for 1 - 3
1
2
3
1.26; found: C, 35.65; H, 4.87; N, 11.32. Anal. Calcd (%) for
Cu ): C, 33.94; H, 5.18; N, 10.79; found: C,
4.01; H, 5.21; N, 10.82. Anal. Calcd (%) for
Cd ): C, 31.45; H, 4.32; N, 10.00; found: C,
1.69; H, 4.51; N, 10.32.
(C22
H
40
N
6
O
12Cl
2
2
1
2
3
3
formula
C
22
H
36
N
6
O
10Cl
2
Zn
2
C
22
H
40
N
6
O
12Cl
2
Cu
2
22 36 6 2 2
C H N O10Cl Cd
(C
22
H
36
N
6
O
10Cl
2
2
fw
746.21
778.58
Monoclinic
P2 /n
840.27
Monoclinic
P2 /c
3
9
b,11
crystal system Monoclinic
space group P2 /c
Preparation of epoxides
1
1
1
Substrates 1e, 1f, 1g and 1o: Trimethylsulfonium iodide (24
mmol), sodium hydride (60% in oil, 24 mmol) were dissolved T (K)
in DMSO (15 mL) and THF (11 mL) under nitrogen a (Å)
293(2)
293(2)
293(2)
7.637(10)
17.001(10)
11.812(10)
98.518(10)
1516.7(3)
2
16.857(2)
10.426(10)
17.971(2)
95.160(2)
3145.6(6)
4
7.6227(15)
16.863(3)
11.676(2)
99.041(5)
1482.2(5)
2
atmosphere. The mixture was stirred for 20 minutes and then
b (Å)
cooled to 0 °C, after that aldehyde (12 mmol) in THF (5 mL)
was added and the solution was stirring at room temperature
until consumption of aldehyde monitored by TLC. The
remaining residue was poured into water (20 mL) and extracted
c (Å)
β(°)
3
V(Å )
with ethyl acetate (30 mL x 3). The combined organic layers
Z
ꢀ3
were washed with water (30 mL) and brine (30 mL x 2), dried D (g cm )
1.634
1.644
1.883
c
4
over MgSO . Further purification was carried out by column
F(000)
768
1608
840
chromatography (hexane/EtOAc = 10:1).
θ for data
collection (°)
2
.12 ꢀ 25.09
1.59 ꢀ 25.00
2.71 ꢀ 27.56
Substrates 1h, 1i, 1j, 1k, 1l, 1m and 1n
:
a
Trimethylsulfonium iodide (20 mmol), sodium hydride (60% in
oil, 20 mmol) were dissolved in DMSO (15 mL) under nitrogen
atmosphere. After stirring for 20 minutes, the corresponding
aldehyde (12 mmol) dissolved in DMSO (20 mL) was added
dropwise within 20 min. Then the reaction was stirring at room
R
1
, [I > 2σ (I)] 0.0720
0.0517
0.1435
0.0387
0.1052
1.075
b
wR
2
[I > 2σ (I)] 0.2248
GOF
1.089
1.046
a
b
2
o
2
c
2
2
o
2
1/2
R
1
= ∑║F
o
│─│F
c
║/∑│F
o
│. wR
2
= {∑[w(F
─F
) ]/∑[w(F
) ]}
2
| J. Name., 2012, 00, 1-3
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Results and discussion
loading is a key factor for the catalytic reaction, we investigated the
use of reduced amounts of LꢀZn, it was delighted to find that when
catalyst dosage decreased to 0.04 mol% and the coꢀcatalyst dosage
decreased to 1.4 mol%, formation of 2a with high yields could still
be achieved (Table 2, entries 17).
Moreover, the formation of 2a decreased when the reaction
temperature was lower than 100 °C under both high (0.4 MPa) and
atmospheric pressure (0.1 MPa) (Table 2, entries 19ꢀ20 and entries
Structural description
Crystallographic analyses reveal that 1 ꢀ 3 are dinuclear structures
and consisting of two L ligands, two metal ions obtained by
treatment of HL and corresponding M(ClO
evaporation of the CH CN solution. The differences in the structures
of 1 - 3 are the free solvent molecules (none for 1 and 3, two H O for
). The synthetic route and Xꢀray structures of 1 ꢀ 3 are shown in
4 2 2
) ·6H O through slow
3
2
2
2
3ꢀ24). Once the reaction temperature is higher than 100 °C, the
Scheme 1. The pure phase of 1 ꢀ 3 is confirmed by powder Xꢀray
diffraction (PXRD) measurements, each PXRD pattern of the asꢀ
synthesized sample is consistent with the simulated one indicating
the phase purity of the products (Figure S1).
formation of 2a was more than 99% in 0.4 MPa and the NMR yield
of 2a can be up to 92% in 0.1 MPa (Table 2, entries 21ꢀ22, entries
2
5ꢀ26). In brief, the experimental results showed that the optimum
reaction conditions for 0.4 MPa are 100 °C, 2 h with the catalyst
loading of 0.04 mol% and NBu Br loading of 1.40 mol% (Table 2,
entry 17), while those for 0.1 MPa are 100 °C, 6 h with the catalyst
4
loading of 0.16 mol% and NBu
entry 25).
4
Br loading of 1.8 mol% (Table 2,
a,b
Table 2 Optimization of reaction conditions
2
Cycloaddition of CO with varied epoxides: In the view of the
high catalytic activity of LꢀZn and a viable cycloaddition reaction in
hand (Table 2, entry 25), the scope of epoxides were then examined
under atmospheric pressure (0.1 MPa). As illustrated in Table 3,
various substituted styrene oxides without (Table 3, 2a) and with
electronꢀwithdrawing groups (Table 3, 2b-2d, 2h) or electronꢀ
Scheme 1 Synthetic route of the three dinuclear metal catalysts and donating groups at the pꢀposition (Table 3, 2e-2g) gave the
their crystal structures
corresponding organic carbonates in good yields, indicating the high
versatility of this catalyst system. The reactions between the styrene
Cycloaddition reactions
oxides with mꢀmethyl, methoxy and trifluoromethyl groups and CO
into give organic carbonates 2i, 2j and 2k in 84%, 84% and 74% yield,
Br as coꢀ respectively. The combined results clearly demonstrate the catalytic
2
1
ꢀ 3 were initially investigated as catalysts for converting CO
2
organic carbonates under mild conditions using NBu
4
13
catalyst. Initial investigation was carried out under reaction potential of LꢀZn showing the amplified scope for the conversion of
conditions with 0.16 mol% loading of 1 ꢀ 3 as well as other catalysts substituted styrene oxide either in pꢀposition or in mꢀposition.
at 100 °C, 0.4 MPa for 2 h. By using the catalysts 1 ꢀ 3 (LꢀZn However, styrene oxide with oꢀmethyl, methoxy and trifluoromethyl
represents for 1, LꢀCu represents for 2 and LꢀCd represents for 3) groups gave obviously different results. The substrate with oꢀ
4
with NBu Br (1.80 mol% loading), the formation of 4ꢀphenylꢀ1,3ꢀ methoxy group provides the respective product 2m also in good
dioxolanꢀ2ꢀone (2a) are 100%, 72% and 82%, respectively (Table 2, yield, however, the ones with oꢀmethyl and oꢀtrifluoromethyl gave
entries 1 ꢀ 3). When the reaction was catalyzed by other catalysts, products with rather poor yields, especially the substrate with oꢀ
such as perchlorate salts (Table 2, entries 4ꢀ6), the mixtures of trifluoromethyl only showed 31% yield because of the electronic and
14
perchloride salts and HL (Table 2, entries 7ꢀ9), organic ligand HL steric effects exerted by the oꢀsubstituents. To further exemplify
Table 2, entry 10), the formation of 2a was no more than 60% the catalytic tolerance of LꢀZn, reactions of epoxides with steric
under the same conditions. When only using NBu Br as catalyst, the hindrance were performed. The steric aromatic terminal epoxide
formation of 2a was poor (Table 2, entry 11). In addition, without forms the corresponding product 2o in good yield, while the
using NBu Br in the reaction, the formation of 2a was trace (Table 2, cyclohexene oxides bearing steric hindrance afford the
(
4
4
entries 12ꢀ13). The results clearly indicate that under the same corresponding organic carbonates 2p and 2q in moderate yields. The
conditions, catalysts LꢀZn, LꢀCu and LꢀCd have superior catalytic experimental results confirmed that LꢀZn has a fairly high catalytic
activities compared to the other catalysts in the conversion of 1a into activity for a broad substrate scope.
2
a. Particularly, LꢀZn shows the highest catalytic activity due to the
a, b, c
4
a
Table 3 Substrate scope
better Lewis acidity of Zn(II) than the other metal ions.
Catalyst loading and reaction temperature: In order to
optimize reaction conditions, the catalyst loading (Table 2, entries
1
4ꢀ18) and reaction temperature (Table 2, entries 19ꢀ22 for 0.4 Mpa;
Mechanism investigation
entries 23ꢀ26 for 0.1 Mpa) were also investigated. The catalyst
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2
The function of NH -group
2
In order to verify whether the NH ꢀgroups in the organic amino
ligands play a role in the catalytic process, parallel experiments were
performed. As listed in Table 4, compared with the reaction of
HL/[NBu
L3/[NBu
metal complexes ZnꢀL also show higher catalytic activity (92%) than
L3ꢀZn (78%), these combined results clearly imply that the NH
groups in the structure can improve the catalytic activity of the
catalytic system. Presumably the NH ꢀgroup can act as Lewis base to
4
Br] which can afford 21% yield, L2/[NBu
4
Br] and
4
Br] can only give 8% and 5% yields, respectively, and the
2
ꢀ
2
activate carbon dioxide by weakly multipoint interactions and
accordingly improve the catalytic activity of the catalytic system.
Scheme 3 Stereospecific S
N
2 (a) and S
N
1 (b) reactions in the
cycloaddition reaction.
The experiment results support the most likely course in the
conversions is stereospecific S 2 reaction: ringꢀopening by an S
reaction, CO insertion, and finally a second S 2 displacement of the
Table 4 Parallel experiments with different catalysts.
N
N
2
2
N
S 1 or S 2 reaction
N
N
alkylꢀbromide by the linear carbonate to form the cyclic product, as
shown in Scheme 3a. Meanwhile, considering the low ee value
(
actually losing ee from > 99% to around 80%), it can be supposed
that an alternative mechanism (through an pathway) is
N
S 1
competitive but is actually not too surprising considering the
relatively high reaction temperature (100 ºC), as shown in Scheme
3
b.
Proposed catalytic mechanism
Taken together, the above results point toward the following reaction
sequence in the catalytic cycle (Scheme 4). The dinuclear metal
centers act as Lewis acid sites to activate the epoxide through MꢀO
coordination. The ringꢀopening of the epoxide was accelerated by
Lewis acid site with the intervention of the tetrabutylammonium
bromide to generate an intermediate metal ion bound alkoxide by an
S 2 reaction. Then, the NH ꢀgroup acts as Lewis base to activate
N 2
carbon dioxide by weakly multipoint interactions according the
experiment of protecting the amino unit in the ligand. Finally the
Scheme 2 Stereochemical fidelity in chiral styrene oxides
To further understand the mechanism of the cycloaddition reaction,
stereospecific reactions were carried out. From Table 3, the reaction
of styrene oxide (1a) only gave one diastereomer 2a in 79% yield.
Remarkably, according to the analysis by HPLC (Figure S2), when
substrates (R)ꢀ1a and (S)ꢀ1a were employed (both ee > 99%), the
products with corresponding chiral quaternary center were generated
in better stereochemical fidelity with ee values 80% for (R)ꢀ2a and
CO
alkoxide into the CO
regenerated the catalyst.
2
insertion into the MꢀO bond and intramolecular transferred the
2
to give the carbonate still by S 2 pathway and
N
15
8
2
1% for (S)ꢀ2a, as shown in Scheme 2. Moreover, the products (R)ꢀ
a and (S)ꢀ2a were structurally characterized by singleꢀcrystal Xꢀray
diffraction analyses using CuꢀKα radiation (λ = 1.54178 Å) at 173(2)
K. The Flack parameters for (R)ꢀ2a and (S)ꢀ2a are 0.05(7) and 0.11(7)
respectively implying the absolute stereochemistry of these two
structures (Table S1).
Scheme 4 Proposed catalytic cycle
4
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Chen, S. R. Yang, C. R. Qi and H. F. Jiang, RSC Adv., 2013,
i) M. Taherimehr, A. Decortes, S. M. AlꢀAmsyar, W.
Lueangchaichaweng, C. J. Whiteoak, E. C. EscuderoꢀAdán, A. W.
Kleij and P. P. Pescarmona, Catal. Sci. Technol., 2012, , 2231; (j) M.
Taherimehr, A. Decortes, S. M. AlꢀAmsyar, W. Lueangchaichaweng,
C. J. Whiteoak, E. C. EscuderoꢀAdán, A. W. Kleij and P. P.
3, 2167;
Conclusions
(
In summary, we have presented three wellꢀdefined, highꢀ
activity dinuclear metal catalysts based on hydroxyl/aminoꢀ
functionalized organic ligand that have been demonstrated to be
versatile catalysts and show fairly high catalytic activity for
2
Pescarmona, Catal. Sci. Technol., 2012, 2, 2231; (k) C. J. Whiteoak,
2
synthesis of cyclic carbonates from CO and epoxides under
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substrate scope and functionality tolerance on substituted
terminal epoxides at low catalyst loading (0.16 mol%) under
atmospheric pressure (0.1 MPa). Mechanism for catalytic
reaction was finally proposed.
6
7
8
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation
of China (grant nos. 21331002 and 21573106) for financial support
of this work. This work was also supported by a Project Funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institutions.
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J. Name., 2013, 00, 1-3 | 5
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Page 6 of 9
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DOI: 10.1039/C6DT02755E
a,b
Table 2 Optimization of reaction conditions
b
Catalyst
NBu
(mmol)
4
Br 1a
2a
T
Time
(h)
P
Conversion
(%)
Entry
o
(
mmol)
(mmol) (mmol) ( C)
(MPa)
a
1
2
3
4
5
6
7
8
9
L-Zn
LꢀCu
LꢀCd
0.016
0.016
0.016
0.016
0.016
0.016
0.18
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
2.5
2.5
2.5
2.5
10
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
80
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
6
6
6
6
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.1
0.1
0.1
0.1
>99
a
a
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.18
0.18
─
7.2
8.2
2.3
1.7
2.1
5.8
4.3
5.3
5.6
2.7
0.5
0.2
10
72
82
23
17
21
58
43
53
56
27
5
4 2
Zn(ClO )
Cu(ClO
Cd(ClO
4
)
2
2
4
)
Zn(ClO
Cu(ClO
Cd(ClO
HL
4 2
) +HL 0.016
4
)
)
2
+HL 0.016
+HL 0.016
0.016
4
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
6
─
HL
0.016
0.016
0.013
0.006
0.004
0.004
0.003
0.016
0.016
0.016
0.016
0.004
0.004
0.004
0.004
a
LꢀZn
─
2
a
LꢀZn
0.18
0.18
0.16
0.14
0.14
0.18
0.18
0.18
0.18
0.045
0.045
0.045
0.045
>99
>99
>99
>99
89
85
91
>99
>99
84
88
92
88
a
LꢀZn
10
a
LꢀZn
10
a
L-Zn
10
a
LꢀZn
8.9
8.5
9.1
10
a
LꢀZn
a
LꢀZn
90
a
L-Zn
100
110
80
a
LꢀZn
10
a
LꢀZn
2.1
2.2
2.3
2.2
a
LꢀZn
90
a
L-Zn
100
120
a
2
LꢀZn
a
LꢀZn represents the complex 1; LꢀCu represents the complex 2; LꢀCd represents the complex
b
1
3
.
The conversion was determined by H NMR analysis of the crude reaction mixture using
hexamethylcyclotrisiloxane as the internal standard.

Page 7 of 9
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View Article Online
DOI: 10.1039/C6DT02755E
a, b, c
Table 3 Substrate scope
O
O
1 2
R = PhꢀR
2
2
2
2
2
2
2
2
a, R
b, R
c, R
d, R
e, R
f, R
2
= H, 79% (93%)
= F, 80% (93%)
2i, R
2j, R
2
2
= Me, 84% (96%)
= OMe, 84% (91%)
= CF , 74% (93%)
O
2
2
= Cl, 80% (96%)
= Br, 78% (91%)
= tBu, 71% (90%)
= Me, 75% (89%)
= OMe, 76% (91%)
2k, R
2
3
R2
2
2
2l, R
2m, R
2n, R
2
= Me, 64% (76%)
= OMe, 78% (89%)
= CF , 31% (62%)
2
2
g, R
2
2
2
3
h, R
3
= CF , 72% (92%)
2p, 61% (76%)
2q, 52% (73%)
2o, 76% (92%)
a
b
Conditions: Varied epoxides (2.5 mmol). The NMR yield in parentheses was determined by
1
H NMR analysis of the crude reaction mixture using hexamethylcyclotrisiloxane as the
c
internal standard. Isolated yield.

Dalton Transactions
Page 8 of 9
View Article Online
DOI: 10.1039/C6DT02755E
Table 4 Parallel experiments with different catalysts
a
Catalyst
mmol)
1a
(mmol)
2a
(mmol)
NMR yield
(%)
(
HL 0.004
LꢀZn 0.004
L2 0.004
L3 0.004
L3ꢀZn 0.004
2.5
2.5
2.5
2.5
2.5
0.53
2.30
0.20
0.13
2.15
21
92
8
5
78
a
1
The NMR yield in parentheses was determined by H NMR analysis of the crude reaction
mixture using hexamethylcyclotrisiloxane as the internal standard.

Page 9 of 9
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DOI: 10.1039/C6DT02755E
Three powerful dinuclear metal-organic catalysts for
converting CO into organic carbonates
2
Dan Zhao, Xiao-Hui Liu, Zhuang-Zhi Shi, Chen-Dan Zhu, Yue Zhao, Peng Wang and Wei-Yin
Sun*
Hydroxyl/amino-functionalized dinuclear metal catalysts show high catalytic activity
2
for synthesis of cyclic carbonates from CO and epoxides under mild conditions.