New coordination complexes based on the 2,6-bis[1-(Phenylimino)ethyl] pyridine ligand: Effective catalysts for the synthesis of propylene carbonates from carbon dioxide and epoxides

Article abstract of DOI:10.3390/molecules23092304

We aimed to develop new effective catalysts for the synthesis of propylene carbonate from propylene oxide and carbon dioxide. A kind of M^x+LCl_x coordination complex was fabricated based on the chelating tridentate ligand 2,6-bis[1-(p

Full text of DOI:10.3390/molecules23092304

molecules
Article
New Coordination Complexes Based on the
2,6-bis[1-(Phenylimino)ethyl] Pyridine Ligand:
Effective Catalysts for the Synthesis of Propylene
Carbonates from Carbon Dioxide and Epoxides
Li Xia 1, Wen-Zhen Wang 1,*, Shuang Liu 1, Xin-Gang Jia 1, Ying-Hui Zhang 2, Lei-Lei Li 1,
Ya Wu 1, Bi-Yun Su 1, Shu-Bo Geng 2 and Wei Fan 1
1
School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China;
lxia9264@163.com (L.X.); liush@xsyu.edu.cn (S.L.); jiaxingang76@xsyu.edu.cn (X.-G.J.);
lll@xsyu.edu.cn (L.-L.L.); wuya@xsyu.edu.cn (Y.W.); subiyun@163.com (B.-Y.S.); vfan111@163.com (W.F.)
Department of Chemistry, Nankai University, TianJin 300071, China; zhangyhi@nankai.edu.cn (Y.-H.Z.);
2
shubogeng216@163.com (S.-B.G.)
*
Correspondence: wzwang@xsyu.edu.cn; Tel.: +86-133-8921-4744
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 12 July 2018; Accepted: 30 August 2018; Published: 10 September 2018
Abstract: We aimed to develop new effective catalysts for the synthesis of propylene carbonate
from propylene oxide and carbon dioxide. A kind of Mx+LClx coordination complex was fabricated
based on the chelating tridentate ligand 2,6-bis[1-(phenylimino)ethyl] pyridine (L). The obtained
products were characterized by elemental analysis, infrared spectroscopy, ultraviolet spectroscopy,
thermogravimetric analysis, and single-crystal X-ray diffraction. It was found that the catalytic
activity of the complexes with different metal ions, the same ligand differed and co-catalyst, where
the order of greatest to least catalytic activity was
2 > 3 > 1. The catalytic system composed of
complex
2
and DMAP proved to have the better catalytic performance. The yields for complex−21
◦
systems was 86.7% under the reaction conditions of 100 C, 2.5 MPa, and 4 h. The TOF was 1026 h
under the reaction conditions of 200 ◦C, 2.5 MPa, and 1 h. We also explored the influence of time,
pressure, temperature, and reaction substrate concentration on the catalytic reactions. A hypothetical
catalytic reaction mechanism is proposed based on density functional theory (DFT) calculations and
the catalytic reaction results.
Keywords: coordination complexes; carbon dioxide; catalyst for propylene carbonate; DFT calculations
1. Introduction
Recently, considerable attention has been paid to the fixation of carbon dioxide, due to its key role
as a greenhouse gas. The chemical fixation and conversion of carbon dioxide into valuable chemicals
is generally regarded as an excellent method from both the environmental protection and resource
utilization standpoints [
carbon dioxide has been involved in various organic reactions, especially the cycloaddition of epoxides
with carbon dioxide to produce carbonates [ 11]. This reaction is a standard “atom-economy” and
1–5]. As a naturally abundant, cheap, recyclable, and nontoxic carbon source,
6–
“green-chemistry” reaction to produce environmentally-friendly propylene carbonate that is widely
utilized as a special solvent in many chemical industries such as liquefied natural gas (LNG), the
textile printing industry, lithium batteries, and wood processing [12–15]. Many methods have been
developed for the synthesis of propylene carbonate, including urea alcoholysis, phosgene (carbonyl
chloride), chloropropanols, cycloaddition, and ester exchange. Among these methods, ester exchange
is the simplest and easiest one to operate, with low cost, high product yield, good selectivity, and high
Molecules 2018, 23, 2304; doi:10.3390/molecules23092304
www.mdpi.com/journal/molecules

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quality [16–18]. However, the reaction conditions are harsh, and the mechanism of the catalytic reaction
remains unclear. In the 1990s, Kruper [19] reported the synthesis of cyclic carbonates from epoxides
and CO2 catalyzed by metalloporphyrins. To improve the catalytic activity of metal complexes,
a salen ligand was used to replace the porphyrin ligand, and higher catalytic performance was
observed as expected [20]. In 2000, Kim et al. [21–23] reported the synthesis of propylene carbonate
catalyzed by zinc complexes. Similarly, Shi’s group [24] found that the catalytic systems comprising a
Schiff base/organic base and phenol/organic base components exhibit high activity in catalyzing the
synthesis of cyclic propylene carbonate from CO2. In 2017, ojas’s group developed a series of amidinate
aluminium complexes as catalysts for the conversion of carbon dioxide into cyclic carbonates [25].
In this work, three metal complexes based on 2,6-bis[1-(phenylimino)ethyl] pyridine (
Cl2] ( ), [Cr Cl3] ( ), and [Mn Cl2] 0.5(CH3CN) ( ) were constructed, and were investigated in
terms of their activity in catalyzing the cycloaddition reaction between propylene oxide and carbon
dioxide (Scheme 1) [26 28], in the presence of co-catalyst. Furthermore, the relationship between
L), namely
[CuL
1
L
2
L
·
3
–
the molecular structures and catalysis performance was analyzed based on experiment and density
functional theory (DFT) calculation, and a reaction mechanism is proposed.
Scheme 1. The synthesis of propylene carbonate (PC) from carbon dioxide and propylene oxide.
2. Results and Discussion
2.1. Crystal Structures
Complexes
The relevant crystallographic and structural modification data are collected in Table 1, and partial
data/parameters of the bond lengths and bond angles are summarized in Table 2. Complex is
shown in Figure 1, crystallized in the orthogonal system space group, Pbca, with Z = 16 and with two
independent molecules ( L and R) in the asymmetric unit. The molecules ( L and R) all contain
1–3 were analyzed by single-crystal X-ray crystallography (see Figures 1–3).
1
1
1
1
1
one Cu(II) ion, one bis(imino)pyridine ligand, and two coordinated chloride ions. The Cu(II) adopts a
pentacoordinated structure, in which one pyridine nitrogen atom and two chlorine atoms form the
basal plane (N2, C11 and C12) (or (N5, C32 and C33)) and two imino-nitrogen atoms occupy the apical
position with a N1–Cu–N3 (or (N4–Cu2–N6)) angle of 155.48(18)◦ (or 154.78(17)◦). The calculated
value of the
from square pyramidal (
L and R molecules all are distorted square pyramidal geometry, and the
perfectly square pyramidal than the L molecule. Although systems are unsuitable, the result can be
τ
factor [29
,
30] for Cu1 is 0.23, indicating a significant distortion of the metal surroundings
= 0) toward trigonal bipyramidal ( = 1). For Cu2, this value is 0.09. Thus, the
R molecule is nearer to
τ
τ
1
1
1
1
used a reference to roughly explain the degree of distortion for the molecules. The dihedral angles
between the three nitrogen-coordination plane and two phenyl rings are 70.97◦ (C1–C6) and 87.39◦
◦ ◦
1R, they are 59.78 (C22–C27) and 88.27 (C37–C42), respectively.
(C16–C21), respectively. For molecule
Meanwhile, these two phenyl rings are nearly perpendicular with a dihedral angle of 85.35 and 89.99
for L and R. The central metal copper atoms deviate from the three nitrogen-coordination planes by
all about 0.001 Å for L and R and deviate from the equatorial plane by about 0.006 Å and 0.005 Å
for L and R, respectively. In both L and R, the lengths of the Cu–Nimino bonds are all longer than
◦
◦
1
1
1
1
1
1
1
1
the Cu–Npyridyl bond. Moreover, N1–C7 (1.273(6) Å), N3–C14 (1.295(7) Å), N4–C28 (1.284(6) Å) and
N6–C35 (1.273(6) Å) are of typical double bond character.

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Figure 1. The molecular structure and crystallographic numbering scheme for complex 1.
Table 1. Crystal data for complexes 1–3.
Compound
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C21H19Cl2CuN3
447.83
Orthorhombic
C21H19Cl3CrN3
471.74
C22H20.5Cl2MnN3.5
459.76
Tetragonal
Triclinic
Pbca
I41/a
P1
15.632(8)
20.298(11)
25.070(13)
90.00
90.00
90.00
7955(7)
16
1.496
1.377
15.775(4)
15.775(4)
33.247(8)
90.00
90.00
90.00
8274(3)
16
1.515
0.953
8.988(3)
12.730(5)
20.143(7)
80.318(8)
79.252(7)
86.277(8)
2230.7(14)
4
b (Å)
c (Å)
◦
α ( )
◦
β ( )
◦
γ ( )
V (Å3)
Z
−3
1.369
0.845
944
ρcalcd (Mg·m
)
−1
µ (mm
F(000)
)
3664
3856
Crystal size (mm)
0.35 × 0.28 × 0.21
2.31–18.37
38,290
0.33 × 0.26 × 0.14
1.43–25.10
20,559
0.31 × 0.24 × 0.13
2.07–25.10
11,205
◦
θ
range for data collection ( )
Reflections collected
Independent reflections
Rint
7082
0.1513
3688
0.1302
7858
0.0723
Final R indices
[(I > 2σ(I)]
R indices
(all data)
R1 = 0.0571
wR1 = 0.1052
R2 = 0.1291
wR2 = 0.1316
0.958
R1 = 0.0613
wR1 = 0.1305
R2 = 0.1296
wR2 = 0.1651
1.011
R1 = 0.0807
wR1 = 0.1638
R2 = 0.1985
wR2 = 0.2245
0.951
GOF

Molecules 2018, 23, 2304
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◦
Table 2. Selected bond lengths (Å) and angles ( ) for complexes 1–3.
1
Cu1—N1
Cu1—N2
Cu1—N3
2.121(4)
1.950(5)
2.078(4)
Cu1—Cl2
N1—C6
N1—C7
2.3857(19)
1.429(7)
1.296(7)
N2—C13
N3—C14
N3—C16
1.335(6)
1.277(6)
1.434(6)
Cu1—Cl1
Cu2—N4
2.2340(18)
2.107(4)
N2—C9
Cu2—Cl3
1.337(6)
2.388(2)
N5—C34
N6—C35
N6—C37
1.324(6)
1.277(6)
1.435(6)
Cu2—N5
Cu2—N6
1.936(4)
2.086(4)
N4—C27
N4—C28
1.429(7)
1.285(6)
Cu2—Cl4
2.2076(18)
155.34(18)
100.62(13)
94.55(13)
77.76(18)
154.87(17)
101.51(13)
93.57(13)
78.17(19)
N5—C30
1.344(7)
N1—Cu1—N3
N1—Cu1—Cl1
N1—Cu1—Cl2
N2—Cu1—N1
N4—Cu2—N6
N4—Cu2—Cl4
N4—Cu2—Cl3
N5—Cu2—N4
N2—Cu1—N3
N2—Cu1—Cl1
N2—Cu1—Cl2
N3—Cu1—Cl1
N5—Cu2—N6
N5—Cu2—Cl4
N5—Cu2—Cl3
N6—Cu2—Cl4
77.64(18)
141.29(13)
109.45(13)
97.41(13)
77.50(18)
149.59(14)
103.48(14)
96.31(13)
N3—Cu1—Cl2
Cl1—Cu1—Cl2
95.23(13)
109.23(7)
N6—Cu2—Cl3
Cl4—Cu2—Cl3
98.07(13)
106.88(7)
2
Cl1—Cr1
Cl2—Cr1
Cl3—Cr1
2.3002(19)
2.3106(19)
2.3239(19)
2.109(5)
153.68(19)
100.10(14)
90.44(14)
89.14(14)
76.55(19)
Cr1—N2
Cr1—N3
N1—C6
2.001(5)
2.111(5)
1.441(7)
1.281(7)
77.1(2)
176.62(16)
87.15(14)
87.58(14)
106.21(14)
N2—C8
N2—C12
N3—C13
N3—C14
N3—Cr1—Cl2
N3—Cr1—Cl3
Cl1—Cr1—Cl2
Cl1—Cr1—Cl3
Cl2—Cr1—Cl3
1.333(7)
1.336(7)
1.295(7)
1.448(7)
88.06(14)
89.96(14)
92.43(7)
92.88(7)
174.67(8)
Cr1—N1
N1—C7
N1—Cr1—N3
N1—Cr1—Cl1
N1—Cr1—Cl2
N1—Cr1—Cl3
N2—Cr1—N1
N2—Cr1—N3
N2—Cr1—Cl1
N2—Cr1—Cl2
N2—Cr1—Cl3
N3—Cr1—Cl1
3
Mn1—N1
Mn1—N2
Mn1—N3
Mn1—Cl1
Mn2—N5
Mn2—N4
Mn2—N6
Mn2—Cl4
2.186(6)
2.297(7)
2.303(7)
2.337(3)
2.200(7)
2.258(7)
2.271(7)
2.328(3)
71.0(3)
Mn1—Cl2
N1—C9
N1—C13
N2—C6
Mn2—Cl3
N5—C30
N5—C34
N4—C27
2.331(3)
1.350(10)
1.347(10)
1.449(10)
2.341(3)
1.352(10)
1.324(10)
1.445(10)
142.3(2)
99.24(19)
99.6(2)
98.97(18)
141.6(6)
97.37(4)
101.9(3)
96.89(4)
N2—C7
N3—C14
N3—C16
1.278(10)
1.290(9)
1.417(8)
N4—C28
N6—C35
N6—C37
1.280(10)
1.293(10)
1.451(10)
N1—Mn1—N3
N1—Mn1—Cl1
N1—Mn1—Cl2
N2—Mn1—N1
N5—Mn2—N6
N5—Mn2—Cl4
N5—Mn2—Cl3
N4—Mn2—N5
N2—Mn1—N3
N2—Mn1—Cl1
N2—Mn1—Cl2
N3—Mn1—Cl1
N4—Mn2—N6
N4—Mn2—Cl4
N4—Mn2—Cl3
N6—Mn2—Cl4
N3—Mn1—Cl2
Cl1—Mn1—Cl2 118.28(10)
100.37(18)
116.29(10)
125.43(19)
71.3(3)
71.0(3)
N6—Mn2—Cl3
Cl4—Mn2—Cl3 116.91(11)
102.94(18)
116.26(18)
126.84(19)
70.3(3)
Complex
2 crystallizes in the tetragonal system, space group I41/a with Z = 16, and the crystal
structure is shown in Figure 2 The molecular structure contains one Cr (III) ion, one bis(imino)pyridine
ligand, and three coordinated chloride ions. The Cr (III) adopts a six-coordinated octahedral structure,
in which there is one pyridine nitrogen atom, three chlorine atoms, and two imino-nitrogen atoms.
The dihedral angles between the three nitrogen-coordination planes and two phenyl rings are
68.77◦ (C1–C6) and 49.17◦ (C14–C19), respectively. Meanwhile, these two phenyl rings are nearly
perpendicular, with a dihedral angle of 86.94◦. The central metal copper atoms deviate from the three
nitrogen-coordination planes and the equatorial plane by about 0.047 and 0.177 Å, respectively. The lengths

Molecules 2018, 23, 2304
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of Cr–Nimino bond (2.109(5) and 2.111(5) Å) are longer than Cr–Npyridyl bond (2.001(5) Å). Moreover, the
N1–C7 (1.281(7) Å) and N3–C13 (1.295(7) Å) bond lengths show typical double bond character.
Figure 2. The molecular structure and crystallographic numbering scheme for complex 2.
Complex
molecules ( L and
The molecules (
3
crystallizes in the triclinic system, space group P
1
with Z = 4 and with two independent
3
3R) and an acetonitrile molecule in the asymmetric unit, as shown in Figure 3.
3
L and
3R) all contain one Mn (II) ion, one bis(imino)pyridine ligand, and two
coordinated chloride ions. Like complex
1, the Mn (II) adopts a pentacoordinated structure, in which
one pyridine nitrogen atom and two chlorine atoms form the basal plane (N1, C11 and C12) (or
(N5, C32 and C33)) and two imino-nitrogen atoms occupy the apical position with a N2–Mn–N3 (or
(N4–Cu2–N6)) angle of 143.30◦ (or 154.78(17)◦).
Figure 3. The molecular structure and crystallographic numbering scheme for complex
3 (CH3CN
molecules are omitted for clarity).
The Mn1 and Mn2 have a τ value of 0.28 and 0.25, respectively. Thus, the 3L and 3R molecules
are all of distorted square pyramidal geometry, and their distortion levels are approximately the
same. The dihedral angles between the three nitrogen-coordination planes and two phenyl rings
3
are 65.17◦ (C1–C6) and 71.39◦ (C16–C21), respectively. For molecule R, they are 59.78◦ (C22–C27)

Molecules 2018, 23, 2304
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and 88.27◦ (C37–C42), respectively. Meanwhile, these two phenyl rings are nearly perpendicular
with a dihedral angle of 75.56◦ and 89.63◦ for
L and R. The central metal copper atoms deviate
from the three nitrogen-coordination planes and the equatorial plane by about 0.016 Å and 0.017 Å,
respectively. For molecule R, these values are 0.015 Å and 0.016 Å, respectively. Molecules L and
3
3
3
3
3R
both have Mn–Nimino bond lengths greater than the length of the Mn–Npyridyl bond. Moreover, N2–C7
(1.270(16) Å), N3–C14 (1.296(11) Å), N4–C28 (1.280(10) Å) and N6-C35 (1.293(10) Å) have typical
double bond character.
2.2. Cycloaddition of CO2 and Propylene Oxide
The cycloaddition of propylene oxide (PO) to CO2 was conducted in the presence of various
catalysts, and the results are listed in Table 3. No reaction was observed in the absence of co-catalyst and
the presence of only 4-dimethylaminopyridine (DMAP) (entries 1 and 5, Table 3), which is consistent
with that observed for some previous reports [31]. However, the reaction occurred when TBAB alone
presented. The phenomenon mainly stems from the fact that TBAB, as a quaternary ammonium
salt, is an effective catalyst for the catalytic synthesis of carbonates from carbon dioxide and alkylene
oxides [32
co-catalysts for the synthesis of propylene carbonate (entries 2–4 and 7–8, Table 3). The efficiency
of complexes decreased in the order . The highest turnover frequency (TOF) value
reached 373 h−1 under mild conditions for complex
. Moreover, complex /TBAB all showed well
,33]. Interestingly, complexes 1–3 demonstrated excellent catalytic performance with both
1
–
3
2 > 3 > 1
2
2
catalytic performance, the TOF value reached 348 h−1. Liu et al. [34] reproted aluminum (salen)
complexes as catalyst for the synthesis of propylene carbonate, and the TOF was as low as 189−h1−1
.
The chromium-salen complexes of Ramin et al. [35] showed propylene carbonate TOF of 90–330 h at
140 ◦C. Complex
reached 204 h−1, which is quite effective.
1 showed the lowest catalytic performance among complexes 1–3, but its TOF still
Table 3. Conversion and turnover frequency (TOF) of various catalysts in the cycloaddition of CO2 to
propylene oxide (PO).
Conversion a (%)
−1
TOF b (h
)
Entry
Catalyst
Co-Catalyst
1
2
3
4
5
6
7
8
9
none
1
2
3
DMAP
DMAP
DMAP
DMAP
none
TBAB
TBAB
TBAB
TBAB
0
0
43.1
74.6
62.3
0
28.6
40.8
69.5
61.4
216
373
312
0
143
204
348
307
2
none
1
2
3
Reaction conditions: temperature: 120 ◦C, pressure: 2.5 MPa, reaction time: 2 h, [PO] = 5.808 g, (100 mmol),
[PO]/[Cat.]/[Co
−
cat.] = 1000:1:1 a The conversion obtained by internal standard method in GC analysis. b Turnover
frequency (TOF) = moles of PO consumed per mole of catalyst per hour.
To further optimize the reactivity of this catalyst system, the effect of various reaction parameters
on PC formation was investigated using complex 2/DMAP as catalyst. Representative results are
summarized in Table 4. The coupling reactions were performed under pressures ranging from 2.0 MPa
◦
to 3.5 MPa at 120 C. As shown in Table 4, the pressure apparently influenced the PO and CO2
cycloaddition reaction at low pressures. The reaction could be realized at 2.0 MPa with a TOF of
337 h−1, and the yield increased by 15.5% as the pressure changed from 2.0 MPa to 3.0 MPa (entry
1 vs. entry 3, Table 4). With further increases in the operating pressure above 3.0 MPa, the yield
of the product increased slightly (entry 3 vs. entry 4, Table 4). For practical reasons, the following
experiments were conducted at a fixed pressure of 2.5 MPa.

Molecules 2018, 23, 2304
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The effect of the molar ratio of PO to complex
2
on the reaction was evaluated under otherwise
identical reaction conditions. The data in entries 2, 8, 11, and 12 in Table 4 show that the yield of PC
decreased markedly from 74.6% to 14.8% with increasing monomer usage. Regarding the effect of
reaction time (entries 5–7, Table 4), a prolonged reaction time was found to be beneficial for improving
the PC yield. In terms of the TOF, the appropriate reaction time was 3 h, in which the TOF reached
554 h−1 (entries 6 and 7, Table 4). The ratio of DMAP to complex
2 had a significant effect on the
TOF (entries 5, 13, and 14, Table 4) and showed the TOF increased as the number of equiv. of DMAP
increased, up to two equiv. (entries 5 and 13, Table 4). Increasing the DMAP concentration any further
resulted in a loss of activity, to the point where the reaction was almost completely shut down when
4 equiv. of DMAP were used (entry 14, Table 4). Entries 5, 8, 9, and 10 in Table 4 demonstrate the effect
of temperature on the reaction efficiency. The catalytic activity was clearly sensitive to◦temperature.
◦
The PC yield appeared to decrease as the temperature increased from 200 C to 220 C, probably
because of catalyst decomposition at higher temperatures. In fact, we found that complex 2 began to
break down at temperatures above 220 ◦C, as determined by thermogravimetric analysis. (see Figure
S1 in Supplementary Materials).
Table 4. Cycloaddition of PO to CO2 in the presence of complex 2/DMAP under various conditions.
◦
Conversion a (%)
−1
TOF b (h
)
Entry
[PO]/[2]/[DMAP] P (MPa)
Time (h)
Temp ( C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1000:1:1
1000:1:1
1000:1:1
1000:1:1
2000:1:1
2000:1:1
2000:1:1
2000:1:1
2000:1:1
2000:1:1
3000:1:1
4000:1:1
2000:1:2
2000:1:4
2.0
2.5
3.0
3.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2
2
2
2
2
3
4
2
1
1
2
2
2
2
120
120
120
120
100
100
100
120
200
220
120
120
120
120
67.3
74.6
82.8
84.2
69.5
83.1
86.7
62.2
51.3
41.6
25.4
14.8
72.3
15.3
337
373
429
421
695
554
434
622
1026
832
381
296
723
153
Reaction conditions: [PO] = 11.62 g (0.2 mol). a The conversion obtained by internal standard method in GC analysis.
b
Turnover frequency (TOF) = moles of PO consumed per mole of catalyst per hour.
2.3. Mechanism for the Cycloaddition Reaction of Propylene Oxide and CO2
Jacobsen et al. have shown that the nucleophilic ring opening of epoxides catalyzed by Cr(III)
salen complexes occurs via the catalyst activation of both the electrophilic epoxide and the incoming
nucleophile [36]. Previous reports on the synthesis of cyclic carbonate from CO2 and epoxides also
suggest the parallel requirement of both Lewis base activation of the CO2 and Lewis acid activation of
the epoxide [37
reaction mechanism is proposed as shown in Scheme 2 [31
mechanism, the alkylene oxide was activated by complexes (1a), and species
same time, the DMAP co-catalyst joined in the formation of species , which related to the activation of
CO2 (1b). Subsequently, species can then attack the activated epoxide species at the least sterically
hindered carbon ( ), leading to the formation of the dimeric intermediate which eventually yields
the cyclic carbonate product and ). This mechanism can explain the dramatic losses in activity
that accompany an increase, beyond an optimal concentration, in either DMAP molar equivalents or
,
38]. According to the above catalytic reaction result, one possible coordination-insertion
39]. Based on the ring-catalyzed reaction
was formed. At the
,
A
B
B
A
2
C
E
(3
4
CO2 pressure. Both of these situations would lead to an increased formation of
thereby reducing the reaction rate.
B at the expense of A,

Molecules 2018, 23, 2304
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Scheme 2. Proposed mechanism for the cycloaddition of propylene oxide to CO2 with DMAP.
In order to further explore the influencing factors of the reaction mechanism, DFT calculations [40
]
were performed on the three complexes. The frontier molecular orbitals of complexes , including
1–3
the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),
are shown in Figures S4–S6 (see Supplementary Materials), respectively. The negative HOMO orbital
energy values indicate that the electronic state of the complex was stable. The natural bond orbitals (NBO)
charges [41] for selected atoms are listed in Table S1 (see Supplementary Materials). The segmental results
of calculations are shown in Table 5 and the proprietary results of calculations are shown in Table S2 (see
Supplementary Materials). The electrostatic potential (ESP) [42] of complexes
respectively. It is apparent that the electrostatic potential surface of the metal ions was influenced by the
charge density distribution. The positive charge on the surface of complex was stronger than those on
complexes and . It is consistent for catalytic reaction results (entries 2, 3 and 4, Table 3), which indicating
that the stronger the acidity of Lewis acid, the higher catalytic reaction will be [43]. Moreover, Table 5
also reveals that the energy gap
1–3, as shown in Figures 4–6,
2
3
1
∆
E values (∆E = ELOMO − EHOMO) of the three complexes followed the
order from highest to lowest: 2 < 3 < 1. Since complex 2 had the lowest energy gap ∆E, catalytic reaction
occurred relatively easily for this system.
Figure 4. The electrostatic potential (ESP) of complex 1.

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Figure 5. The electrostatic potential (ESP) of complex 2.
Figure 6. The electrostatic potential (ESP) of complex 3.
Table 5. Metal atom charge distribution, highest occupied molecular orbital (HOMO), and lowest
unoccupied molecular orbital (LUMO) of complexes 1–3.
Complex
Metal Atom Charge Distribution
LUMO/eV
HOMO/eV
∆E/eV
1
2
3
0.460
0.454
0.697
−0.03090
−0.15833
−0.10583
−0.24552
−0.16290
−0.22280
0.21463
0.00457
0.11697
Some conclusions are obtained as follows according to the catalytic reaction results, DFT
calculations and mechanism. First, the larger the net charge of metal ions are, the easier the nucleophilic
attack on oxygen atom to activate propylene oxide (see 1a in Scheme 2). On the contrary, it is easier for
electrophilic attack on carbon atom to activate carbon dioxide (see 1b in Scheme 2). Therefore, only
the suitable charged metal in complexes can join in both procedures (1a and 1b) and give rise to
the catalytic activation. Second, the net charge of metal ions are not decisive factor, the energy gap
∆
E values (
∆
E = ELOMO
−
EHOMO) also play a important role that smaller ∆E values are, the higher
catalytic reaction activation (see Table 5). Third, the activation of carbon dioxide is the decisive step for
the catalytic reaction probably. Fourth, by properly fine-tuning the charge of the active metal center it
is possible to develop a highly active catalyst.
3. Materials and Methods
3.1. Chemical Materials
2,6-Diacetylpyridine, aniline, copper (II) chloride dihydrate (CuCl2
(III) chloride (CrCl3), and manganese (II) chloride tetrahydrate (MnCl2
commercial sources and used without further purification unless otherwise noted.
·
2H2O), anhydrous chromium
·
4H2O) were obtained from

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3.2. X-ray Crystallographic Studies
Single crystals with proper dimensions were selected for single-crystal X-ray diffraction
measurement, and the relevant diffraction data were collected on an APEX CCD II Bruker single
crystal diffraction meter (Bruker, Billerica, MA, USA). At 296(2) K, 38290, 20559, and 11205 X-ray
reflections were collected, respectively, for complexes
1, 2, and 3 using graphite monochromated
Mo K = 0.71073 Å) and scan mode. The crystal structures were refined using SHELXL-97
α
(
λ
φ-ω
(University of GÖttingen: GÖttingen, Germany) [44] analytical procedures, while the coordinates of
the non-hydrogen atom structure and anisotropic parameters were obtained using the full matrix least
square method in the SHELXL-97 program. The CCDC deposition numbers of the complexes are
1508337 (1), 1496950 (2), and 1860911 (3), respectively.
The following restrains were introduced to improve the quality of the crystal data. For complex
1
: (1) the phenyl ring defined by C16 > C21 was restrained by AFIX 66 instruction to give rational
C-C bond lengths; (2) Delu 0.01 N2 C7 instruction was used to confirm the two bonded atoms have
rational parameters. For complex : Alert level B suggested the structure exist a large error at low
3
angle diffraction point, we find the bad point related HKL (0 0 2) in the “most disagreed reflection” in
the .lst file, then add “omit 0 0 2” to the .ins file, and refine it.
3.3. Catalyst Characterization
The infrared data of the complexes were recorded on a VERTEX-70 Fourier transform infrared
spectrometer with a band range of 4000–400 cm−1, where the measured samples were dealt with
KBr tablet. The elemental analysis data of complexes were obtained by using a Vario MICRO
element analyzer from Elementar (Langenselbold, Germany). The thermogravimetric analysis data
of complexes were obtained using a Mettler Toledo thermogravimetric analysis system (Mettler
Toledo, Columbus, OH, USA). The UV spectrum data of complexes were obtained by using a UV-2600
ultraviolet spectrophotometer (Shimadzu, Kyoto, Japan).
3.4. Catalyst Preparation
3.4.1. Synthesis of the Ligand (2,6-bis[1-(Phenylimino)ethyl] pyridine) (L)
2,6-Diacetylpyridine (0.50 g, 3.06 mmol) and aniline (0.095 g, 1.02 mmol) were added to 100 mL
methanol. After the addition of several drops of formic acid, the reaction mixture was refluxed for 12 h
at 40 ◦C. A yellow powder was obtained by the filtration of crude yellow precipitate, evaporation of
the solvent and recrystallization from methanol. The yield was 70%. EA (%) C21H19N3: calcd. C 80.40,
H 6.06, N 13.40; found: C 80.36, H 6.19, N 13.31; UV-Vis (CH2Cl2)
λ
max/nm (
ε
/dm3 mol−1
·
cm−1) =
285 (6.39
×
104), 249 (1.10 105); IR (KBr, cm−1): 2981 (m), 1653 (vs), 1559 (m), 1492 (s), 1468 (s), 1407
×
(s), 1319 (m), 1217 (s), 1116 (m), 1071 (s), 955 (w), 932 (m), 872 (m), 820 (s), 770 (w), 740 (m), 633 (w), 547
(m), 451(w).
3.4.2. Synthesis of Complexes
[Cu
L
Cl2] complex
1
: The ligand
L
(0.116 g, 0.37 mmol) and CuCl2 2H2O (0.063 g, 0.37 mmol) were
·
dissolved in 30 mL CH3CN, which were stirred for 12 h at room temperature under the protection
of nitrogen. Then, a deep yellow solid powder was obtained after distillation of CH3CN solvent.
Finally, deep yellow crystals were gained after the recrystallization with CH3CN, with a yield of 76%.
EA (%) C21H19Cl2CuN3: calcd. C 56.27, H 4.24, N 9.38; found: C 56.19, H 4.33, N 9.42; UV-Vis (CH2Cl2)
−1
λ
max/nm (
ε
/dm3 mol
·
cm−1) = 284 (5.06
×
104), 246 (9.05
×
104); IR (KBr, cm−1): 3436 (m), 2919 (m),
1637 (vs), 1485 (m), 1384 (s), 1270 (m), 1233(s), 1209 (m), 872 (m), 820 (s), 692 (m), 633 (w).
[Cr Cl3] complex : The synthetic method for complex was similar to that of complex
for the replacement of CuCl2 2H2O by anhydrous CrCl3 (0.059 g, 0.37 mmol). Light green crystals were
obtained with a yield of 79%. EA (%) C21H19Cl3CrN3: calcd. C 53.45, H 4.03, N 8.91; found: C 53.38, H
L
2
2
1 except
·

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4.18, N 8.87; UV-Vis (CH2Cl2)
λ
max/nm (
ε
/dm3 mol−1
·
cm−1) = 246 (8.24
×
104); IR (KBr, cm−1): 2925
(s), 2851 (s), 1637 (vs), 1462 (s), 1383 (s), 1304 (m), 1256 (m), 1231 (w), 1042 (w), 859 (w), 823 (w).
[Mn Cl2] 0.5(CH3CN) complex : The synthetic method for complex was similar to that
of complex except for the replacement of CuCl2 2H2O by MnCl2 4H2O (0.073 g, 0.37 mmol).
L
·
3
3
1
·
·
Orange crystals were obtained with a yield of 71%. EA (%) C22H20.5Cl2MnN3.5: calcd. C 57.39,
−1
H 4.46, N 10.65; found: C 57.27, H 4.42, N 9.58; UV-Vis (CH2Cl2)
(1.71
105), 246 (2.25
λmax/nm (
ε
/dm3 mol
·
cm−1) = 280
×
×
105); IR (KBr, cm−1): 2918 (m), 2789 (m), 1627 (vs), 1593 (vs), 1485 (s), 1371 (m),
1257 (m), 1225 (s), 1169 (m), 1018 (s), 818 (w), 777 (m), 721 (s), 695 (w), 631 (w).
3.4.3. Catalytic Procedure
The detailed catalytic procedure was described in our previous work [45]. A 100 mL stainless-steel
reactor was charged with purified propylene oxide (PO) and catalyst. Then, the reactor was
pressurized to desired pressure with carbon dioxide and heated to the desired temperature by stirring.
After predetermined time of reaction, the autoclave was cooled down to room temperature, and
the CO2 pressure was released by opening the outlet valve. The solid residue was separated from
the reaction mixture by filtration. The product propylene carbonate (PC) was obtained through
distillation of the filtrate under reduced pressure. Finally, a qualitative analysis of the liquid products
was performed on FT-IR spectrum in Figure S5 (see Supplementary Materials). For quantitative
determination, the products were analyzed on an Agilent 6890 Plus GC with flame ionization detection.
The PC yield was obtained by internal standard method (Biphenyl as the internal standard substance).
3.5. Gaussian Calculation
All DFT calculations in this work were performed with the B3LYP functional by using the Gaussian
09W (Gaussian, Inc., Wallingford, CT, USA) program. The effective nuclear LanL2DZ basis set was
adopted for metal atoms, and the 6-31G (d, p) [46] basis set for other atoms.
1L and 3R were used as
calculation models for complexes and , respectively. It was found that the optimized geometric
1
3
structure was substantially similar to the experimental one. Partial predicted bond lengths and bond
angles are collected in Table S2 (see Supplementary Materials). Frequency calculations at the same
theoretical level confirmed that the optimized structures settled at a local minimum site of the potential
energy surface. The atomic charge distributions were calculated by natural bond orbital (NBO) analysis
to better understand the catalytic activity.
4. Conclusions
In this work, three different metal complexes
nitrogen-containing ligand 2,6-bis[(1-phenylimino)ethyl] pyridine (
1
–
3
were synthesized from the selected tridentate
). It is worth highlighting that
L
these widely used metal complexes for ethylene oligomerization were applied for the first time to the
synthesis of cyclic propylene carbonate from propylene oxide and carbon dioxide. The catalytic activity
of the complexes was evaluated in the synthesis of the propylene carbonate from propylene oxide
and carbon dioxide. The catalytic investigation revealed outstanding catalytic activity for [Cr
), with propylene carbonate (PC) yields >80% in the presence of 4-dimethylaminopyridine (DMAP),
outperforming complexes [Cu Cl2] ( ) and [Mn (CH3CN)0.5Cl2] ( ). A supposed catalytic reaction
LCl3]
(2
L
1
L
3
mechanism is proposed based on the DFT calculations and catalytic reaction results. It is a competitive
relationship between the activation of carbon dioxide (1b) and propylene oxide (1a), which the former
one could have a greater inflence for catalytic reaction. Moreover, the energy gap
∆E values can reveal
catalytic activation very intuitively. This work provides a simple method for future research.
Supplementary Materials: The following are available online. Figure S1: TG curves of complexes
Infrared spectra of ligand( ) and complexes ; Figure S3: UV-Vis spectra of ligand ( ) and complexes
Figure S4: HOMO and LUMO frontier molecular orbitals of Complex ; Figure S5: HOMO and LUMO frontier
molecular orbitals of Complex ; Figure S6: HOMO and LUMO frontier molecular orbitals of Complex ; Figure S7:
1–3; Figure S2:
L
1–
3
L
1–3;
1
2
3

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Infrared spectra of catalytic product; Table S1: NBO charges for some atoms; Table S2:Selected bond lengths (Å)
◦
and angles ( ) after optimizing for complexes 1–3; X-ray molecular structure reported for complexes 1–3. (CIF)
Author Contributions: Data curation, L.X. and L.-L.L.; Formal analysis, L.X., X.-G.J. and Y.W.; Funding acquisition,
W.-Z.W., S.L. and X.-G.J.; Investigation, W.F.; Methodology, L.X., Y.-H.Z., and S.-B.G.; Project administration,
W.-Z.W.; Software, X.-G.J.; Writing—original draft, L.X.; Writing—review & editing, W.-Z.W., S.L., Y.-H.Z. and
B.-Y.S.
Funding: Financial support for this study came from the Science Research Program funded by Shaanxi Provincial
Education Department (Program No. 16JK1598 and No. 17JK0606) and special supporting funds from the Xi’an
Shiyou University.
Conflicts of Interest: The authors declare no conflict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).