Transition metal complexes with pyrazine amine ligand: Preparation, structure and carbon dioxide copolymerization behavior

Article abstract of DOI:10.1016/j.molstruc.2019.05.013

Three complexes [Fe(L)₂](ClO₄)₂ (1)、[Ni(L)₂](ClO₄)₂ (2) and [Co(L)₂](ClO₄)₂ (3) were synthesized by the reaction of N,N’-(4-methylpyridin-2-yl)pyrazine-2,6-

Full text of DOI:10.1016/j.molstruc.2019.05.013

Accepted Manuscript
Transition Metal complexes with Pyrazine Amine Ligand: Preparation, structure
and Carbon dioxide copolymerization behavior.
Wen-Zhen Wang, Ya-Chao Xu, Li Wang, Lei-Lei Li, Li Xia, Xin-Gang Jia, Gene-
Hsiang Lee, Shie-Ming Peng
PII:
S0022-2860(19)30565-4
10.1016/j.molstruc.2019.05.013
MOLSTR 26520
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
26 December 2018
06 May 2019
Please cite this article as: Wen-Zhen Wang, Ya-Chao Xu, Li Wang, Lei-Lei Li, Li Xia, Xin-Gang Jia,
Gene-Hsiang Lee, Shie-Ming Peng, Transition Metal complexes with Pyrazine Amine Ligand:
Preparation, structure and Carbon dioxide copolymerization behavior., Journal of Molecular
Structure (2019), doi: 10.1016/j.molstruc.2019.05.013
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ACCEPTED MANUSCRIPT
Transition Metal complexes with Pyrazine Amine Ligand:
Preparation, structure and Carbon dioxide copolymerization
behavior.
Wen-Zhen Wanga,* (王 文 珍 ), Ya-Chao Xu (徐 亚 超 ), Li Wang (汪 力 ),
a
a
a
a
a
Lei-Lei Li (李磊磊), Li Xia (夏力), Xin-Gang Jia (贾新刚), Gene-Hsiang
b
b
Lee (李锦祥) and Shie-Ming Peng (彭旭明)
aSchool of Chemistry and Chemical Engineering, Xi’an Shiyou University,18 Dian-zi-er Road, Xi’an, 710065, PR China.
bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan, 106, China.

Corresponding authors: Email: wzwang@xsyu.edu.cn
Abstract
Three complexes [Fe(L)
reaction of N,N'-(4-methylpyridin-2-yl)pyrazine-2,6-diamine (L) with Fe(ClO
Co(ClO ·6H O) at room temperature, respectively. They were characterized by elemental analysis, IR
2
](ClO
4
)
2
(1)、 [Ni(L)
2
](ClO
4
)
2
(2) and [Co(L)
2
](ClO
4
)
2
(3) were synthesized by the
4
)
2
·6H O, (Ni(ClO ·6H O or
2
4
)
2
2
4
)
2
2
spectroscopy and single crystal X-ray diffraction. The catalytic behavior of the complexes for the synthesis of
propylene carbonate from propylene oxide and carbon dioxide were studied. The results show that all of them
-1
-1
-1
showed excellent catalytic activities with TOF as high as 2104 h , 2824 h and 2605 h , respectively. Complex 2
that exhibit higher catalytic activity was taken to explore the influence of time, pressure and temperature to
optimize the reaction condition in detail.
Keywords: Coordination complexes; Carbon dioxide; Catalyst for Propylene carbonate; Transition metal; X-ray single crystal;
1
. INTRODUCTION
The fixation of carbon dioxide to convert into a green chemical propylene carbonate (PC)
provides an important synthesis route with industrial application value. In 2001, to improve the
catalytic activity of metal complexes, Robert[1] reported the synthesis of cyclic carbonates from
epoxides and CO
2
catalyzed by Salen ligand. In 2005, Kim et al.[2] reported the synthesis of propylene
carbonate catalyzed by zinc complex. Although a variety of catalyst systems have been constructed for
this reaction, it still meets some limitations such as low catalyst reactivity with turnover frequencies

ACCEPTED MANUSCRIPT
-1
o
(
TOFs) rarely exceeding 300 h , high reaction temperatures (＞130 C) and high pressures conditions.
[3]
[4,5]
Ghosh et al . used Co (Ⅲ) complexes as catalyst for synthesis of propylene carbonate. Wang
reported the synthesis of [trans-Ru(py) Cl ] and Zn[bis(2-hydroxybenzene)phthaldimine] complexes
was developed to catalyze the coupling reaction of CO and propylene oxide to yield the cyclic
4
2
2
carbonate. However, TOF was lower than this article. It is necessary to develop more effective catalytic
systems for this reaction especially under mild condition.
The variety of coordination mode of multidentate ligands is a fascinating topic. In particularly,
oligo-α-pyridylamine ligands (Scheme I) have been designed for the synthesis of extended metal atom
chain complexes (EMACs) in several studies.[6,7] In EMACs the supporting ligands are coordinated to
metal in all-syn form, whereas all-anti form style was observed in mononuclear complexes [8]. The
ligands that contain nitrogen-containing heterocycles and flexible -N-H- bond(s) easily form a spiral
structure with numerous hydrogen bonds, while various nitrogen forms can coordinate with various
metals (M) to form one-, two-, and three-dimensional structures. The ligands of mononuclear
complexes exhibit helical structure, and the dimers are paired via hydrogen bonds between -NH groups.
Double-stranded structures can be constructed by the complexation of two ligands around metal
[9]
ions. In crystallization, the factors that affect the structure of a complex and packaging of molecules
are intricate and are still not clearly understood. Different structures can be affected by various factors,
such as solvents, temperature, solubility, acidity, etc.[10,11] Further, nitrogen-containing heterocyclic
ligands have proven to be versatile and can be used to construct various spatial structures with different
coordination modes.[12-14]
Recently, oligo-α-pyridylamine ligands containing pyrazine, pyrimidine, pyrrole, pyrazole, and
naphthyridine are employed to prepare series of coordination polymers with a metal in place of
traditional oligo-α-pyridylamine ligands.[15-17] By providing more nitrogen atoms in aromatic ring(s),
the pyrazine ligands exhibit various coordination modes and are especially versatile in the construction
of coordination polymers. According to previous reports, the modification of ligands can tune the
electronic structure, improve the function of complexes. and therefore has attracted increasing
attention.
In this work, we report the synthesis and characterization of three complexes namely
[
Fe(L)
2
(H
2
O)](ClO
4
)
2
(1), [Ni(L)
2
](ClO
4
)
2
(2) and [Co(L)
2
(H
2
O)](ClO
4
)
2
(3) based on
a
pyrazine-regulated ligand N,N'-di(4-methylpyridin-2-yl)pyrazine-2,6-diamine (L) (Scheme II), nickel

ACCEPTED MANUSCRIPT
(II) complex. Furthermore, the catalytic performance of these complexes for the synthesis of propylene
carbonate from propylene oxide and carbon dioxide were studied. The results showed that these metal
complexes exhibit excellent catalytic activities with high TOFs under relatively mild conditions.
2
Scheme I. Oligo-α-pyridylamine ligands (n = 1, H tpda)
2
Scheme II. N,N'-di(4-methylpyridin-2-yl)pyrazine-2,6-diamine (L=H dmpzda)
2
EXPERIMENTAL
.1 Materials and measurements
Starting materials were obtained from commercial sources and used without further purification
2
unless otherwise noted. The solvent benzene was dried over Na and freshly distilled prior to use.
Elemental analysis (C, H and N) was performed on a Vario MICRO of Germany. FT-IR spectrum were
-1
recorded in KBr pellets on a VERTEX-70 of Germany from 4000 to 400 cm range. Germany Bruker
APEX II CCD diffractometer.
Caution! Perchlorate salts are potentially explosive and should only be used in small quantities
and handled with the necessary precautions.
2
.2
Synthesis
of
H
2
dmpzda (L)
N,N'-(4-methylpyridin-2-yl)pyrazine-2,6-diamine)(L) was prepared according to the literatures.[18]
The reaction of 4-methyl-2-aminopyrazine (4.75 g, 0.044 mol) and 2,6-dichloropyridine (2.98 g,
t
0
.020 mol) in the presence of Pd
2
(dba)
3
(0.336 g, 2 mol%), BINAP (0.498 g, 4 mol%) and Bu ONa
(6.53 g, 0.068 mol) in benzene (75 mL) for 48 h gave a crude product L. Purification by column
chromatography over silica gel (dichloromethane-acetone, 20:1) gave pure L as light pink crystals (4.2

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3
-1
-1
4
4
3
g, 72% yield); UV/Vis (CH OH) λmax/nm [ε/ (dm •mol •cm )]: 206 (2.91×10 ), 262 (3.10×10 ), 359
4
-1
(
2.32×10 ); IR (KBr) m/cm 3343w, 3256 w, 3051 w, 1604 m, 1578 m, 1534 m, 1517 m, 1482 m,
418 vs, 1380 s, 1344 m, 1290 s, 1264 m, 1242 w, 1207 m, 1195 s, 1168m, 1116 w, 988 m, 862 m, 806
m, 520 w, 448 w;elemental analysis (%) C16 : calc. C 65.73, H 5.52, N 28.75; found: C 65.13, H
.23,N 28.95.
.3 Preparation of the complexes
The preparation processes for complexes 1-3 are similar; hence, a general procedure is given for 1.
.3.1 Synthesis of [Fe(L) ](ClO (H O)(CH OH)(1)
A mixture of L (0.029g, 0.1 mmol) and Fe(ClO ·6H
1
16 6
H N
5
2
2
2
4
)
2
2
3
4
)
2
2
O (0.043g, 0.12 mmol) was dissolved in
methanol (50 mL) and stirred at room temperature for 24 h. Then, the deep purple solid powder were
obtained after removal of solvent methanol. Slow diffusion of the filtrate with ether produced crystals
2
suitable for X-ray diffraction. Yield: 0.039 g, 54.16%; EA (%) C33H38Cl FeN12O10: calcd. C 44.54, H
3
−1
−1
4
3
1
.27, N 18.90; found: C 44.37, H 4.38, N 19.01; UV-Vis (CH
2
Cl
2
) λmax / nm (ε/dm mol • cm ) =
3
3
4
-1
56(1.19× 10 ), 272 (2.06× 10 ), 216 (6.15 × 10 ); IR (KBr) v/ cm =3474 (vs), 2061 (w), 1634 (s),
510 (w), 1473 (m), 1384 (m),1238(w), 1161 (w),1120 (m), 648(s), 480(s).
2
.3.2 Synthesis of [Ni(L)
The synthetic method for complex 2 is similar to that of complex 1 except for the replacement of
Fe(ClO ·6H O by Ni(ClO ·6H O (0.044g, 0.12 mmol). The light pink crystals were obtained with a
yield of 42.58% (0.031 g). EA (%) C32 10: calcd. C 43.75, H 4.10, N 19.14; found: C
2 4 2 2
](ClO ) 2(H O)(2)
4
)
2
2
4
)
2
2
H
2
36Cl NiN12O
3
−1
−1
3
4
1
5
3.84, H 4.04, N 19.16; UV-Vis (CH
2
Cl
2
) λmax / nm (ε/dm mol • cm ) = 371 (5.04× 10 ), 299 (6.35×
3
4
-1
0 ), 216 (5.42 × 10 ); IR (KBr) v/ cm =3442(vs),2061(w),1636(s), 1152(m), 1121(m), 624(m),
43(m).
2
.3.3 Synthesis of [Co(L)
The synthetic method for complex 3 is similar to that of complex 1 except for the replacement of
Fe(ClO ·6H O by Co(ClO ·6H O (0.044 mg, 0.12 mmol). The luminous yellow crystals were
obtained with a yield of 50.58% (0.037 g). EA (%) C34 14: calcd. C 40.62, H 3.88, N
2 4 2 3
](ClO ) 2(CH OH)(3)
4
)
2
2
4
)
2
2
H
3
39Cl CoN12O
3
−1
−1
1
1
1
2
2 2
6.72; found: C 40.65, H 3.79, N 16.63;UV-Vis (CH Cl ) λmax / nm (ε/dm mol • cm ) = 373(5.31×
3
3
4
-1
0 ), 301(4.70× 10 ), 219 (4.42× 10 ) ; IR (KBr) v/ cm =3449 (vs), 2065 (w), 1634 (s), 1472 (w),
384 (m), 1151 (m),1121 (m), 548(m).
. 4 General procedure for the reaction of epoxides and CO
2

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A 100 mL stainless-steel reactor was charged with purified propylene oxide (PO) and catalyst. The
air in the reactor was replaced by CO
pressure by CO . Then, the reactor was pressurized with carbon dioxide and heated to the desired
temperature by stirring. After reaction, the autoclave was cooled down to room temperature, and the
CO pressure was released by opening the outlet valve. The solid residue was separated from the
2
for 3 times, and the pressure in reactor was adjusted to desired
2
2
reaction mixture by filtration. The product propylene carbonate (PC) was obtained through distillation
of the filtrate under reduced pressure. A qualitative analysis of the liquid products was performed on IR
spectrum. For quantitative determination, the products were analyzed on an Agilent 6890 Plus GC with
flame ionization detection.
3
. Results and Discussions
3
.1 Crystal structure determination
The chosen crystals were mounted on a glass fiber. Data collections were carried out on a
BRUKER SMART Apex CCD at 100K for 1 and 3 and on a NONIUS Kappa CCD diffractometer at
50 K for 2 using a Mo-Ka radiation (k =0.71073 A˚ ) and a liquid nitrogen low-temperature controller.
1
Data reduction was performed on the the Bruker SAINT. All the structures were solved by using the
SHELXS-97[19] and refined with SHELXL-97[20] by full-matrix least squares on F values. The detailed
crystal data are listed in Table 1.
2
CCDC reference numbers 1886256 (1), 1867827 (2), 1867887 (3).
Table 1. Crystal data for complexes 1-3
Compound
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
1
2
3
C
33
H
38Cl
2
FeN12
O
10
C
32
H
36Cl
2
NiN12
O
10
34 3 14
C H39Cl CoN12O
889.50
878.34
1004.03
Triclinic
P-1
Monoclinic
Monoclinic
C2/c
P21/c
32.7801 (11)
11.7678.(4)
19.5254 (7)
94.622(2)
7507.4(4)
8
33.0540 (11)
12.1019 (5)
19.2889 (6)
95.039(2)
7686.1(5)
8
10.974 (2)
11.310 (2)
17.433(3)
79.052(11)
2027.4(7)
1
b [Å]
c [Å]
β [°]
V (Å3)
Z
-3
ρ calcd (Mg·m )
1.535
1.518
1.628
Absorption
coefficient
0
.617
0.716
0.705
1012
-1
(
mm )
F(000)
Crystal size (mm3)
3552
0.18 × 0.16× 0.1
3632
0.25×0.15 ×0.1
0.18× 0.1 × 0.06

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θ range for data collection
°]
1
.25 to 25.03
1.237 to 27.499
1.24 to 25.03
[
-
38≤h≤38, 0≤k≤13,
-42≤h≤42, -11≤k≤15,
-25≤l≤25
-12≤h≤13, -12≤k≤13,
Index ranges
0
≤l≤23
0≤l≤20
Reflections collected
Independent reflections
Goodness-of-fit on F2
6607
32203
7120
6
607 [R(int) =
8777 [R(int) =
0.0566]
7120 [R(int) = 0.0723]
0
.0000]
1.312
1.026
1.120
Final
R
indices
R
1
= 0.0789, wR
2
=
=
R
R
1
1
= 0.0806, wR
0.2283
2
2
=
=
R
1
= 0.0749, wR
2
=
=
[I>2sigma(I)]
0.1879
0.1920
R
1
= 0.1237, wR
.2438
0.0076(4)
2
= 0.1339, wR
0.2669
R
1
= 0.1128, wR
0.2298
2
R indices (all data)
0
Extinction coefficient
n/a
0.039(3)
1
.486 and -1.289
2.212 and -1.163
Largest diff. peak and hole
1.455 and -1.142 e.Å-3
-3
-3
e.Å
e.Å
Table 2. Selected bond lengths (Å) and angles (°) for complexes 1~3
Complex 1
Fe-N(2)
1.968(5)
1.974(5)
Fe-N(1)
2.082(4)
1.992(6)
Fe-N(13)
Fe-N(3)
1.997(6)
1.999(5)
Fe-N(12)
Fe-N(11)
N(2)-Fe-N(12)
N(2)-Fe-N(1)
178.5(2)
89.3(2)
N(1)-Fe-N(11)
N(2)-Fe-N(13)
89.5(2)
89.6(2)
N(2)-Fe-N(3)
N(12)-Fe-N(3)
89.2(2)
91.8(2)
N(12)-Fe-N(1)
N(12)-Fe-N(13)
89.3(2)
N(1)-Fe-N(3)
178.1(2)
8
9.7(2)
N(2)-Fe-N(11)
N(12)-Fe-N(11)
91.9(2)
89.3(2)
N(1)-Fe-N(13)
N(11)-Fe-N(13)
91.4(2)
N(11)-Fe-N(3)
N(13)-Fe-N(3)
89.5(2)
89.7(2)
178.3(2)
Complex 2
Ni-N(3)
Ni-N(5)
2.077(4)
2.080(4)
Ni-N(11)
Ni-N(7)
2.083(4)
2.083(4)
Ni-N(9)
Ni-N(1)
2.086(4)
2.088(4)
N(3)-Ni-N(5)
N(3)-Ni-N(11)
N(5)-Ni-N(11)
N(3)-Ni-N(7)
N(5)-Ni-N(7)
86.17(16)
94.19(16)
88.89(16)
91.73(15)
90.55(16)
N(11)-Ni-N(7)
N(3)-Ni-N(9)
N(5)-Ni-N(9)
N(11)-Ni-N(9)
N(7)-Ni-N(9)
174.00(16)
178.18(16)
95.16(16)
87.08(16)
87.03(16)
N(3)-Ni-N(1)
N(5)-Ni-N(1)
N(11)-Ni-N(1)
N(7)-Ni-N(1)
N(9)-Ni-N(1)
86.72(15)
172.83(16)
90.75(16)
90.55(16)
91.97(15)
Complex 3
Co-N(22)
Co-N(2)
1.926(5)
1.931(5)
Co-N(21)
Co-N(3)
1.941(5)
1.953(4)
Co-N(23)
Co-N(1)
1.957(5)
1.961(5)
N(22)-Co-N(2)
N(22)-Co-N(21)
178.6(2)
88.1(2)
N(21)-Co-N(3)
N(22)-Co-N(23)
91.89(19)
88.95(19)
N(22)-Co-N(1)
N(2)-Co-N(1)
91.90(19)
89.05(19)

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N(2)-Co-N(21)
N(22)-Co-N(3)
N(2)-Co-N(3)
90.94(19)
90.41(19)
88.66(19)
N(2)-Co-N(23)
N(21)-Co-N(23)
N(3)-Co-N(23)
92.07(19)
176.73(19)
89.47(19)
N(21)-Co-N(1)
N(3)-Co-N(1)
N(23)-Co-N(1)
89.28(19)
177.44(19)
89.48(19)
Crystallographic data of 1-3 are listed in Table 1. Selected bond lengths and bond angles for these
complexes are listed in Table 2. In our design, we introduce a oligo-α-pyridylamido derivative named
L with different metal ions to construct three novel complexes. For this N-riched ligand, abundant
nitrogen sources could act as Lewis base combined with multiple heterometallic Lewis-acid sites,
which is an attractive means to enhance catalytic reactivity and selectivity. In our ligand, six nitrogen
atoms of the ligandcan coordinate with metal atoms, including two pyrazine nitrogen atoms, two amido
nitrogen atoms and two methylpyridine nitrogen atoms. However, only three nitrogen atom on the
heterocycle were coordinated with metal ions at room temperature condition. The crystal structures
description of complexes 1-3 were exhibited as follows.
The crystal structure as shown in Fig. 1 indicate that complex 1 crystallize in monoclinic system
and C2/c space group. The crystal structure of 1 contained one Fe (II) ion, two ligands, one free water
molecule, one free methanol molecules and two free perchlorate anions. Ligand L is a tridentate ligand
with three aromatic nitrogen atoms as coordinating atoms, all of which are anti-anti coordination.[21]
2
Due to the presence of steric groups, the pyrazine and methylpyridine groups of the H dmpzda ligand
are not in same plane and are twisted. Two ligands wraped around the metal ion in a spiral fashion to
form mononuclear hellcats. As reflected in Fig. 1, the central Fe (II) atom was six-coordinated with
octahedral coordination geometry, six nitrogen atoms (N1, N2, N3, N11, N12, N13) from the
methylpyridin and pyrazine rings of ligand L The four nitrogen atoms (N2, N11, N12, N13)
constructed the equatorial plane, and the axial position was occupied by two nitrogen (N1, N3) from
the different methylpyridin ring. The nitrogen atoms in the amide groups were not coordinated with Fe
2
(II) ion is coordinated with six aromatic nitrogen atoms from different ligands H dmpzda, and nitrogen
atom from the amide groups is coupled with the central metal atom to form four stable six-membered
rings, each ring has two carbon atoms, three nitrogen atoms and a Fe (II) atom. The Fe-N distances
were in the range 1.968-1.999 Å with an average value of 1.987 Å, and the length of Fe-Npyrazine bond is
shorter than Fe-Npyridyl bond (Fe-N(2) 1.968(5) Å, Fe-N(12) 1.974(5) Å). The N-Fe-N bond angles
ranged from 89.2º to 178.5º (Table 2). Compared to the standard octahedral geometry, the degree of
distortions was not significant. The ligand L acted as a tridentate ligand that coordinated to the Fe (II)
atom, where coordination geometry are consistent with data previously reported for a similar complex,

ACCEPTED MANUSCRIPT
[Fe(H
2
tpda)
2
]Cl
2
(H
2
tpda=N,N'-Di(pyridin-2-yl)pyridine-2,6-diamine).[22]
Fig. 1 The molecular structure and crystallographic numbering scheme for complex 1.(perchlorate anions are omitted for
clarity)
Complex 2 belongs to the monoclinic system, C2/c space group and the crystal structure is shown
in Fig. 2. The smallest unit consists of one Ni (II) ion, two L ligands, two free water molecule, and two
free perchlorate anions. The six-coordinated octahedral structure was formed via the coordination of
the central metal Ni (II) with two nitrogen-atoms of pyrazine (Ni-N(3) 2.077(4) Å, Ni-N(9) 2.086(4) Å)
and four nitrogen atom of pyridine (Ni-N(1) 2.088(4) Å, Ni-N(5) 2.080(4) Å, Ni-N(7) 2.083(4),
Ni-N(11) 2.083(4)). The nitrogen atoms in the amide groups were not coordinated to the metal. The
N-Ni-N bond angles ranged from 86.17º to 178.18º (Table 2), and the bond angles of N-Ni-Npyrazine
(
N(3)-Ni-N(9) 178.18º) is more-than N-Ni-Npyridyl bond angles (N(5)-Ni-N(1) 172.83º, N(11)-Ni-N(7)
74.00º). Compared to the complex 1, the degree of distortions was more obvious. Similar with
complex 1, Ni (II), nitrogen-atoms and carbon atoms to form four stable six-membered rings, in which
1

ACCEPTED MANUSCRIPT
the length of Ni-N bond is longer than C-N and N-N bonds (Table 2).
Fig. 2 The molecular structure and crystallographic numbering scheme for complex 2 (perchlorate anions are omitted for clarity)
Complex 3 belongs to triclinic system and P-1 space group and the crystal structure is shown in
Fig. 3. The smallest unit consists of one Co (III) ion, two L ligands, one free methanol molecule, one
free methoxyl group and three free perchlorate anions. The six-coordinated octahedral structure was
formed via the coordication of the central metal Co (III) with two nitrogen-atoms of pyrazine (Co-N(2)
1
1
2
.931(5) Å, Co-N(22) 1.926(5) Å), four nitrogen atom of pyridine (Co-N(1) 1.961(5) Å, Co-N(3)
.953(4) Å, Co-N(22) 1.926(5), Co-N(23) 1.957(5)), and the average value is 1.945 Å. Similar to 1 and
, the nitrogen atoms in the amide groups were not coordinated to the metal. The N-Co-N bond angles
ranged from 88.1º to 178.6º (Table 2). Compared to the complex 2, the distortions of 3 was not
significant, but more obvious than that of complex 1.

ACCEPTED MANUSCRIPT
Fig. 3 The molecular structure and crystallographic numbering scheme for complex 3.(perchlorate anions are omitted for
clarity)
3
.2 The catalytic activity test
The catalytic activity of metal complexes (1~3) and n-tetrabutyl ammonium bromide to synthesize
propylene carbonate were shown in Table 3.
Table 3 Conversions and TOFs of complexes 1~3 on the synthesis of propylene carbonate (PC)
-
1
Catalyst
Yield (%)
14.93
TOF(h )
33.17
2104
4
Bu NBr
1
2
3
/Bu
/Bu
/Bu
4
4
4
NBr
NBr
NBr
47.36
63.54
2824
58.62
2605
Reaction conditions: catalyst 0.03 mmol, temperature: 100 ℃, pressure: 1.5 MPa, reaction time: 1.5 h, [PO]=15mL(0.2mol),
PO]/[Bu NBr]=1000:3
[
4
To explore the influence of different metal ions on the catalytic activities for the synthesis of
propylene carbonate from propylene oxide and carbon dioxide, complexes 1-3 were used as catalysts
combined with Bu NBr as co-catalyst to catalyze this reaction at the same reaction condition. As
shown in Table 3, under the conditions of 100 ℃, 1.5 Mpa and 1.5 h, all of these complexes show
4

ACCEPTED MANUSCRIPT
-1
-1
-1
excellent catalytic activities with TOF as high as 2104 h , 2824 h and 2605 h , respectively. Among
them, complex 2 exhibit highest catalytic activity than both complexes 3 and 1. Considering only
different metal ions of these complexes and reported literatures, we speculated that the correlation with
the catalytic activity were mainly depend on the different radius of the metal ion: the smaller the radius,
the higher the density of the charge distribution, and the stronger the Lewis acidity, the more stable
metal-ligand coordination bond can be formed.
Based on the present results and the reported studies of catalytic reaction mechanisms, a plausible
4
pathway is proposed: n-Bu NBr nucleophilic attack propylene oxide ring opening reaction with ternary
cyclic active intermediate, and finally closed ring to obtain propylene carbonate. Metal complex can
directly activate carbon dioxide, and then efficiently synthesize propylene carbonate under the
4
synergistic effect of the cocatalyst n-Bu NBr, and obtain relatively high conversion frequency. In the
synthesis reaction of carbon dioxide and epoxide to carbonate, propylene oxide has a high activity and
easily to take ring-opening reaction. Carbon dioxide is thermodynamically stable, and the molecular
structure determines that it is a weak electron donor and a strong electron acceptor. Therefore, the
activation of carbon dioxide is particularly important in the whole reaction. When the transition metal
is connected to the atom N having a lone pair of electrons, the Ni-N bond is formed. The lone pair
electron is easily filled into the empty orbit of the carbon atom to activate the carbon dioxide, so the
catalytic activity is greatly improved.
To further optimize the reaction condition of this catalytic system, the effect of various reaction
parameters on PC formation was investigated in detail using complex 2 as a selected catalyst.
Representative results are summarized in Table 4.
o
Reactions at different temperatures ranged from 80 to 160 C (Table 4, entries 9-13) revealed that
-1
the catalyst also shows excellent catalytic activity with relative high TOFs (1229 h ) even under mild
conditions. In addition, the results showed that the highest PC yield of 63.54% with a TOF of 2824 h-1
was obtained under 100 ℃ for 1.5 h, indicating 100 ℃ was the optimal temperature for the
cycloaddition reaction. We also examined the influence of CO
2
pressure in the range of 0.5～2.5 MPa
on the cycloaddition reaction. As shown in Table 4, the PC yield increases with the increase of CO
2
-1
pressure. The reaction could be realized at 0.5 MPa with a TOF of 2099 h , and the yield increased by
1
2.41% as the pressure changed from 0.5 MPa to 1.5 MPa (entry 1 to entry 3, Table 4). Further
2
increase of CO pressure results in the decrease the yield of PC. With extending the reaction time, the

ACCEPTED MANUSCRIPT
yields showed increasing trends and the TOF adverse trends. Thus, the optimal reaction condition was
o
determined at 100 C, 1.5 MPa and reaction for 1.5 h.
2
Table 4. Reation of CO converting to propylene carbonate in the presence of complex 2 under various conditions
a
b
-1
Entry
P (MPa)
0.5
Time (h)
1.5
1.5
1.5
1.5
1.5
1
Temp (℃)
100
Conversion (%)
TOF (h )
2099
2523
2824
2741
2651
3618
2281
1901
1634
1229
1311
1138
968
1
2
3
4
5
6
7
8
9
47.24
56.77
63.54
61.68
59.65
54.27
68.43
71.30
73.05
55.34
59.03
51.23
43.56
1.0
100
1.5
100
2.0
100
2.5
100
1.5
100
1.5
2
100
1.5
2.5
3
100
1.5
100
1
1
1
1
0
1
2
3
1.5
1.5
1.5
1.5
1.5
80
1.5
120
1.5
140
1.5
160
a Calculated from the mass of propylene carbonate distilled from reaction products.
b Turnover frequency (TOF) = moles of PO consumed per mole of catalyst per hour.
4
Conclusions
In summary, three metal complexes 1-3 were synthesized from the selected ligand
N,N'-(4-methylpyridin-2-yl)pyrazine-2,6-diamine(L) and metal ions (Fe(II), Ni(II), Co(III)). Their
catalytic activities were evaluated in the synthesis of the propylene carbonate from propylene oxide and
-1
carbon dioxide. All of these complexes show excellent catalytic activities with TOF as high as 2104 h ,
824 h-1 and 2605 h , respectively. The different catalytic activities of these complexes were Ni
-1
2
complex > Co complex > Fe complex. Moreover, complex 2 that exhibit highest catalytic activity was
taken to optimize the reaction condition in detail, which exhibit good yields with high TOF under the
reaction conditions of 100℃, 1.5 MPa and 1.5 h.

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ACCEPTED MANUSCRIPT
Dear Reviewers,
Thank you for you thoughtful, helpful, and most kind review of manuscript. Your comment
and suggestions have been incorporated into the revised draft. Specific revisions are noted below.
Manuscript
Revised manuscript
Catalytic Activity of Transition Metal Pyrazine Transition Metal complexes with Pyrazine
Amine Complex for Conversion of CO
Propylene Carbonate
2
to
Amine Ligand: Preparation, structure and
Carbon dioxide copolymerization behavior.
Added keywords
X-ray single crystal;
Carbon dioxide is thermodynamically stable,
and the molecular structure determines that it
is a weak electron donor and a strong electron
acceptor.
Carbon dioxide is stable in thermodynamics
and inert in dynamics, and a powerful electron
acceptor as well.