6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridylmetal (Fe and Co) complexes as highly active precatalysts for ethylene polymerization: Influence of metal and/or substituents on the active, thermostable performance of their complexes and resul

Article abstract of DOI:10.3390/molecules25184244

A series of 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridines and their iron(II) and cobalt(II) complexes (Fe1–Fe5, Co1–Co5) were synthesized and routinely characterized as were Co3 and Co5 complexes, studied by single crystal X-ray crystallo

Full text of DOI:10.3390/molecules25184244

molecules
Article
6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)-
iminopyridylmetal (Fe and Co) Complexes as Highly
Active Precatalysts for Ethylene Polymerization:
Influence of Metal and/or Substituents on the Active,
Thermostable Performance of Their Complexes and
Resultant Polyethylenes
Wenhua Lin 1,2 , Liping Zhang 1,*, Jiahao Gao 2, Qiuyue Zhang 2, Yanping Ma 2 , Hua Liu 2 and
Wen-Hua Sun 2,3,
*
1
School of Textiles Science and Engineering, Jiangnan University, Wuxi 214122, China;
6180709011@stu.jiangnan.edu.cn
2
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; gaojh514@163.com (J.G.);
zhangqiuyue@iccas.ac.cn (Q.Z); myanping@iccas.ac.cn (Y.M.); hualiu@iccas.ac.cn (H.L.)
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
3
*
Correspondence: 8323200210@jiangnan.edu.cn (L.Z.); whsun@iccas.ac.cn (W.-H.S.);
Tel.: +86-0510-8591-7602 (L.Z); +86-10-6255-7955 (W.-H.S.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Vincent Ritleng
Received: 29 August 2020; Accepted: 14 September 2020; Published: 16 September 2020
Abstract: A series of 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridines and their
iron(II) and cobalt(II) complexes (Fe1 Fe5 Co1 Co5) were synthesized and routinely characterized
–
,
–
as were Co3 and Co5 complexes, studied by single crystal X-ray crystallography, which individually
displayed a distorted square pyramidal or trigonal bipyramid around a cobalt center. Upon treatment
with either methyluminoxane (MAO) or modified methyluminoxane (MMAO), all complexes
displayed high activities regarding ethylene polymerization even at an elevated temperature,
enhancing the thermostability of the active species. In general, iron precatalysts showed higher
activities than their cobalt analogs; for example, 10.9
17.0
×
106 g(PE) mol−1 (Co) h−1 by Co4 and
×
106 g(PE) mol−1 (Fe) h−1 by Fe4. Bulkier substituents are favored for increasing the molecular
weights of the resultant polyethylenes, such as 25.6 kg mol−1 obtained by Co3 and 297 kg mol−1
obtained by Fe3. A narrow polydispersity of polyethylenes was observed by iron precatalysts
activated by MMAO, indicating a single-site active species formed.
Keywords: linear polyethylenes; cobalt precatalyst; iron precatalyst; thermostable and efficient
catalysis; correlation between structure and activity
1. Introduction
Late-transition metal precatalysts have made great progress toward ethylene polymerization since
the pioneering studies initiated by Brookhart and Gibson [
palladium precatalysts [ 12], iron and cobalt precatalysts have showed a higher activity in ethylene
polymerization and generated polyethylenes with highly linear structures [13 18]. However, the poor
1–6]. In addition to the α-diimino-nickel and
7–
–
thermostability of iron and cobalt precatalysts and broad molecular weight distribution of obtained
polyethylenes impeded further applications in the industry [19,20].
Molecules 2020, 25, 4244; doi:10.3390/molecules25184244
www.mdpi.com/journal/molecules

Molecules 2020, 25, 4244
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Though new frameworks have been developed for ligands supporting iron and cobalt species in
ethylene polymerization, the model precatalysts ( , Scheme 1), namely the Gibson–Brookhart catalyst [ ],
are extensively investigated and improved catalytic performances through finely turning the steric
and electronic influences of ligands used [21 31]. For example, benzhydryl-substituted precatalysts
, Scheme 1) maintained high activities at the reaction temperature up to 80 ◦C [32
33], which fixed
the critical problem regarding the thermostability of their analogs ( , Scheme 1) [ ]. The successful
precatalysts [32 33], in correcting the simulation results, provided evidence that the steric hindrance
of ligands enhanced the thermal stability but decreased the catalytic activity of their complexes [34].
In our benzhydryl-modification [32 33], the phenylethyl-substituted precatalysts ( , Scheme 1) also
improved the thermostability and maintained reasonable activities in ethylene polymerization [29].
Using naphthalenamine derivatives instead of anilines, the precatalysts ( , Scheme 1) also showed good
catalytic performances regarding both thermostability and catalytic activity [13 26], showing positive
influences in comparison to precatalyst 32 33]. Subsequently, 2-(1-phenylethyl)naphthalenamine
A
1–3
–
(B
,
A
1,2
,
,
C
D
,
B
[ ,
derivatives were recently developed for N,N-bidentate iron and cobalt complexes as active precatalysts
for diene polymerization [35], and the substituent at the ortho-position of the N-aryl group heavily
affected the catalytic performance. The 6-arylimino-2-(2-(1-phenylethyl) naphthalen-1-yl)imino pyridine
derivatives and their iron and cobalt chlorides (E, Scheme 1) have been extensively synthesized and
characterized. The title complexes show high activities in ethylene polymerization along with a good
thermostability. The metal (iron or cobalt) and steric influences of ligands have been investigated and
reported herein.
Ph
Ph
R1
N
R1
Ph
Ph
N
N
N
N
N
N
N
N
Fe
M
N
N
N
N
Cl
Cl
N
N
M
Ar
M
Ar
CHPh2
M
Ar
Ar
Cl
Cl
R2
R2
Cl
Cl
Cl
Cl
Ph2HC
Cl
Cl
Ph
Ph
Ph
Ph
Ph
M=Fe,Co
this work
M=Fe,Co
M=Fe,Co
M=Fe,Co
B
A
C
D
E
Scheme 1. Structural variations in bis(imino)pyridine-iron and cobalt chloride precatalysts (A–E).
2. Results
2.1. Synthesis and Characterization
The 6-arylimino-2-acetylpyridine derivatives (S1–S4, Scheme 2) were prepared according to
a previous procedure [36
form the bis(imino)pyridine ligands 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine
L1 L4, Scheme 2) with a reasonable isolated yield (Scheme 2). The 2,6-bis((2-(1-phenylethyl)
–38], and further reacted with 2-(1-phenylethyl)-1-naphthalenamine to
(
–
naphthalen-1-yl)-iminopyridine (L5) was obtained by reacting 2,6-diacetylpyridine with 2 equivalent
of 2-(1-phenylethyl)-1-naphthalenamine. All iron (II) and cobalt (II) complexes were prepared by the
stoichiometric reaction of the ligands (L1
and further characterized by FT IR spectroscopy. Compared to free ligands (L1
–
L5, Scheme 2) with FeCl2
·
4H2O or CoCl2
·
6H2O, respectively,
L5), the stretching
–
vibrations of C=N bonds in these iron and cobalt complexes have shifted to lower wavenumbers
(1632–1640 cm−1 vs. 1615–1621 cm−1), which is consistent with the effective coordination between the
metal (II) and imino nitrogen atoms [39]. The molecular structures of complexes Co3 and Co5 were
further confirmed by single-crystal X-ray diffraction.

Molecules 2020, 25, 4244
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R1
R1
NH2
R2
NH2
Ph
N
N
R1
R1
N
R1
MCl2
N
N
O
N
R1
N
N
N
p-TsOH
Toluene
Ethanol
M
Methanol
R1
R2
O
O
R2
R1
R2
Cl
Cl
Ph
S1-S4
L1-L4
Ph
Fe1-Fe4
or
Co1-Co4
NH2
Ph
Co1 Co2 Co3 Co4
Fe1 Fe2 Fe3 Fe4
Ph
Ph
N
N
MCl2
Ethanol
N
N
N
Ph
N
R1
R2
Me Et i-Pr Me
p-TsOH
Toluene
M
H
H
H
Me
Cl
or
Cl
Co5
Ph
Fe5
L5
Scheme 2. Synthesis route of the ligands (L1–L5) and their complexes Fe1–Fe5 and Co1–Co5.
2.2. X-ray Crystallographic Studies
Single crystals of Co3 and Co5 (Figure 1) of suitable quality for the X-ray determinations
were grown by the slow diffusion of diethyl ether into a solution of the corresponding complex in
dichloromethane at room temperature. Co3 and Co5 comprise a single cobalt center surrounded by
three nitrogen atoms of a tridentate ligand (N1, N2, and N3) and two chlorides (Cl1 and Cl2) to form
a pentacoordinate geometry. Three nitrogen atoms and a Cl2 atom of Co3 formed a basal plane, and the
cobalt atom lay at a distance of 0.586 Å above this basal plane (Figure 1a). The dihedral angle of the
naphthyl plane and the basal plane is 74.71◦, and 80.75◦ for the angle between the benzene plane
and the basal plane. Co5 displayed C2 molecular symmetry (Figure 1b), and two naphthyl planes
were close vertical to the pyridyl frame plane with dihedral angles are 78.50◦ and 85.39◦, respectively.
Two benzene substituents planes on naphthyl planes. Similar to many bis(imino)pyridine-cobalt
complexes, the Co-Npyridyl bond length of two complexes [Co3: 2.050(4) Å; Co5: 2.021(4) Å] is shorter
than their exterior Co-Nimino ones [2.170(4)-2.294(4) Å], indicating good donor property of the central
pyridine [13,14]. Other selected bond lengths and angles are listed in Table 1 and more detail can be
seen in Supplementary Materials.
(b)
(a)
Figure 1. ORTEP drawing of Co3
(
a) and Co5
(
b) with thermal ellipsoids at the 30% probability level.
Hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths and angles for Co3 and Co5.
Co3
Co5
Co3
Co5
Bond Lengths (Å)
Co1-N1
Co1-N2
CO1-N3
2.170(4)
2.050(4)
2.196(4)
2.294(4)
2.021(4)
2.212(4)
Co1-Cl1
Co1-Cl2
2.3093(16)
2.2555(16)
2.2511(15)
2.2357(14)
Bond Angles (deg)
N1-Co1-N2
N1-Co1-N3
N1-Co1-Cl1
N1-Co1-Cl2
N2-Co1-N3
74.16(16)
141.24(15)
101.50(12)
100.67(12)
74.01(17)
73.56(15)
149.89(14)
96.36(11)
96.71(11)
76.32(16)
N2-Co1-Cl1
N2-Co1-Cl2
N3-Co1-Cl1
N3-Co1-Cl2
Cl1-Co1-Cl2
89.35(12)
152.82(13)
99.67(12)
97.61(12)
117.74(7)
120.48(13)
124.83(13)
98.87(12)
100.34(11)
114.48(6)

Molecules 2020, 25, 4244
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2.3. Ethylene Polymerization
Previous achievements have approved the efficiency of both MAO and MMAO cocatalysts in
activating the iron or cobalt precatalysts for ethylene polymerization [40 43]. Therefore, the title metal
–
complexes have been screened for their catalytic performances towards ethylene polymerization with
the assistance of either methyluminoxane (MAO) or modified methyluminoxane (MMAO).
2.3.1. Ethylene Polymerization by Co1–Co5
Co2/MAO was first selected as a precatalyst system to optimize ethylene polymerization
parameters and the results were collected (Table 2). The temperature was the first considered
parameter. The polymerization reactivity was significantly increased from 1.89 to 8.32
×
106 g(PE)
mol−1 (Co) h−1 as the temperature elevated from 50 to 70 ◦C (run 1–3, Table 2), due to the reactant
molecule becoming more active at higher temperatures. The polymerization reactivity was then
decreased to 4.16
×
106 g(PE) mol−1 (Co) h−1 (run 4–5, Table 2) with a temperature that continued
to increase to 90 ◦C at a lower concentration of ethylene in toluene and decomposed cobalt active
species at this temperature [44].The molecular weight of the polyethylenes obtained reduced from 26.4
to 14.6 kg mol−1 as the polymerization temperature increased (run 1–5, Table 2), indicating a faster
termination of polymer chains at the increased temperature [45,46].
Table 2. Polymerization screening using Co1–Co5 a.
b
c
c
T (◦C)
t (min)
PE (g) Act.
Mw
Mw/Mn
Tm (◦C) d
Run
Precat.
Al/Co
1
2
3
4
5
6
7
8
9
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co2
Co1
Co3
Co4
Co5
Co1
Co2
Co3
Co4
Co5
2500
2500
2500
2500
2500
2000
2250
2750
3000
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
50
60
70
80
90
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
30
30
30
30
30
30
30
30
30
5
15
45
60
30
30
30
30
30
30
30
30
30
1.89
4.23
8.32
5.74
4.16
1.72
4.81
7.15
6.98
3.44
5.19
9.23
9.64
10.2
7.01
10.9
9.24
5.27
4.71
2.91
5.61
4.68
1.89
26.4
20.2
19.6
18.8
14.6
24.0
21.1
19.4
10.1
18.8
18.9
21.1
23.8
15.1
22.6
14.3
21.8
17.1
21.8
25.6
17.0
23.4
4.86
4.77
4.41
4.83
4.53
4.82
4.96
4.11
3.34
4.68
4.46
4.62
4.75
6.56
2.19
6.50
4.90
6.39
4.26
2.63
6.27
4.82
b
131.8
130.4
131.4
130.8
131.6
130.0
131.8
131.4
131.3
131.1
131.0
131.7
130.7
129.3
131.8
128.9
131.0
130.2
130.9
133.4
129.9
131.8
4.23
8.32
5.74
4.16
1.72
4.81
7.15
6.98
13.8
10.4
6.16
4.82
10.2
7.01
10.9
9.24
5.27
4.71
2.91
5.61
4.68
10
11
12
13
14
15
16
17
18 e
19 e
20 e
21 e
22 e
a
Conditions: 2 µmol cobalt, 100 mL toluene, 10 atm C2H4, methyluminoxane (MAO) as cocatal. Values in units of
106 g(PE) mol−1 (Co) h−1
(MMAO) as cocatal.
.
Determined using GPC, Mw: kg mol−1
.
Determined using DSC; e methyluminoxane
c
d
In comparison to the previous cobalt models described in Scheme 1 (A B D
, 60 ◦C; , 40 ◦C; , 30 ◦C),
the Co2 shows a higher thermostability with the optimum temperature (70 ◦C) due to the positive
influence of the bulky substituent at N-aryl groups. Moreover, the molecular weights of the resultant
polyethylenes are well maintained, with only 7% decreasing within the temperature from 60 to 80 ◦C.
The parameter of the Al/Co molar ratio was then optimized. The catalytic activity was increased
from 1.72 to 8.32
decreased to 6.98
×
106 g(PE) mol−1 (Co) h−1 as the Al/Co increased from 2000 to 2500, but slightly
106 g(PE) mol−1 (Co) h−1 as the ratio further increased to Al/Co = 3000, resulting in
×

Molecules 2020, 25, 4244
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the optimized molar ratio of Al/Co = 2500 (run 3, 6–9, Table 2). The molecular weight of the obtained
polyethylenes decreased from 24.0 kg mol−1 (Al/Co = 2000) to 10.1 kg mol−1 (Al/Co = 3000), and the
result was similar to previous reports [47,48].
The polymerization was conducted over different running times at 70 ◦C (run 3, 10–13, Table 2),
in order to evaluate the lifetime of the Co2/MAO. The catalytic reactivity was quite prominent
(13.8 × 106 g(PE) mol−1 (Co) h−1
) at 5 min, suggesting the active species were quickly formed [49].
Then the catalytic activity gradually decreased as the polymerization time prolonged, but still reached
up to 4.82 × 106 g(PE) mol−1 (Co) h−1 at 60 min.
Other precatalysts (Co1, Co3–Co5) were employed to catalyze ethylene polymerization at the
optimized polymerization conditions (r◦un 14–17, Table 2). All the complexes showed high activity
during ethylene polymerization at 70 C with an order Co4
> Co1 > Co5 > Co2 > Co3, which
was affected by both the electron effect and steric effect [50,51]. Co4 had most electron-donating
methyl groups but the lowest steric hindrance around the metal center resulted in the highest activity
while Co3 with bulky isopropyl groups displayed the lowest activity [30]. Co5 showed a closed
activity with Co4 and Co1 for bulky o-sec-phenyl-substituted aryl groups in ligands and donated
more electrons [52
was also affected by their ligand structure. The molecular weight of polyethylenes catalyzed by
cobalt complexes decreased in the order Co3 Co5 Co2 Co1 Co4, which was consistent
–54]. The molecular weight of the polyethylenes obtained by cobalt complexes
>
>
>
>
with the literature, which previously reported that the bulky steric substituent of the N-aryl group
usually produces a higher molecular weight of polyethylenes because bulky substituents prevent the
deactivation of active species [46,55]. Moreover, the current model of the cobalt precatalysts displays
a higher activity than previous models of cobalt complexes in Scheme 1. For example, the activity of
Co4/MAO goes up to 10.9 × 106 g(PE) mol−1 (Co) h−1
, while the activity of its analog (D) (Scheme 1) is
4.15 × 106 g(PE) mol−1 (Co) h−1 at their optimized conditions.
The system of the Co1–Co5/MMAO was employed to further explore the catalytic behavior during
the ethylene polymerization process. All of these complexes displayed a high activity at an elevated
temperature (70 ◦C). For the Co1
Co5/MMAO system, the catalytic activity decreased in the order
Co4 Co1 Co2 Co5 Co3, and the molecular weight of the polyethylenes obtained decreased
in the order Co3 Co5 Co2 Co1 Co4. The melting temperature of all the polyethylenes was
around 130 ◦C, suggesting that polyethylenes had a high linear microstructure [56
57]. To confirm
–
>
>
>
>
>
>
>
>
,
the microstructure of the polyethylenes obtained, the 1H NMR and 13C NMR of the polyethylenes
were carried out and the polyethylenes showed a high linear microstructure with the vinyl-end group
(Figure 2). This vinyl-end group was formed by β-H eliminate during ethylene polymerization.
Figure 2. The NMR spectrum of the polyethylenes obtained by Co2/MMAO at 70 ◦C (run 19, Table 2).

Molecules 2020, 25, 4244
6 of 17
2.3.2. Ethylene Polymerization by Fe1–Fe5
Subsequently, the ethylene polymerizations were conducted by the iron complexes with MMAO
as a cocatalyst. Initially, the Fe2/MMAO system was used to optimize the polymerization conditions
(run 1–4, Table 3). The polymerization activity first increased and then decreased by varying the
polymerization temp◦erature from 60 to 90 ◦C, and the highest activity (16.5
×
106 g(PE) m◦ol−1 (Fe) h−1
)
was observed at 70 C. A better thermostability with an optimized temperature of 70 C would be
a favorable advantage for industrial considerations [57]. From the GPC curves (Figure 3a), the molecular
weight decreased as the polymerization temperature increased, and all samples displayed a unimodal
distribution of molecular weight except for one sample obtained at 60 ◦C. The molecular distribution index
demonstratedthat there was a narrow distribution of molecular weights obtained at a higher temperature
(PDI = 1.60–2.45, run 2–4, Table 3) but a broad one observed at a lower temperature (PDI = 4.62,
run 1, Table 3). Such a phenomenon was common in catalytic systems—i.e., that there are multi-active
species at lower temperatures while single-active species at higher temperatures [58
,
59]—including
bulky substituents.
Table 3. Polymerization screening using Fe1–Fe5 a.
b
c
c
T (◦C)
t (min)
PE (g) Act.
Mw
Mw/Mn
Tm (◦C) d
Run
Precat.
Al/Fe
1
2
3
4
5
6
7
8
9
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe1
Fe3
Fe4
Fe5
Fe1
Fe2
Fe3
Fe4
Fe5
2500
2500
2500
2500
2000
2750
3000
2750
2750
2750
2750
2750
2750
2750
2750
2750
2750
2750
2750
2750
60
70
80
90
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
30
30
30
30
30
30
30
5
15
45
60
30
30
30
30
30
30
30
30
30
16.4
16.5
9.74
0.630
15.8
16.7
13.0
9.48
12.8
17.5
17.6
16.5
8.81
17.0
15.5
9.05
14.4
2.58
15.8
9.29
16.4
20.8
9.01
3.59
1.08
6.64
4.93
4.31
4.24
4.54
4.84
5.78
4.72
5.32
2.83
5.39
43.2
49.0
291
4.62
2.45
1.60
1.68
1.73
1.55
1.56
1.55
1.60
1.57
1.50
1.54
1.48
1.70
1.56
7.16
6.69
27.8
1.69
4.71
131.7
130.4
130.1
128.2
130.5
130.0
129.0
128.0
129.2
129.8
130.5
129.1
129.8
129.2
129.4
132.6
132.8
134.6
131.0
132.5
16.5
9.74
0.630
15.8
16.7
13.0
56.9
25.6
11.6
8.81
16.5
8.81
17.0
15.5
9.05
14.4
2.58
15.8
9.29
10
11
12
13
14
15
16 e
17 e
18 e
19 e
20 e
a
10.7
56.7
Conditions: 2
µ
c
mol iron complexes, 100 mL toluene, 10 atm C2H4, MAO as cocatal; b Values in units of 106 g(PE)
mol−1 (Fe) h−1
.
Determined using GPC, Mw: kg mol−1
.
Determined using DSC; e MMAO as cocatal.
d
The polymerization conditions were further optimized by varying the Al/Fe molar ratio from
2000 to 3000 (run 2, 5–7, Table 3). Similar to the previous cobalt complex behavior, the catalytic activity
initially increased from 15.8 to 16.5
2000 to 2500, and then decreased to 13.0
increased to 3000. The molecular weight decreased from 6.64 to 4.31 kg mol−1, and all the samples
displayed a narrow molecular weight distribution (Figure 3b), highlighting the single active site of the
Fe2/MMAO system at 70 ◦C.
×
106 g(PE) mol−1 (Fe) h−1 as the Al/Fe molar ratio increased from
×
106 g(PE) mol−1 (Fe) h−1 as the Al/Fe molar ratio further
The lifetime of Fe2/MMAO was assessed by varying the polymerization time from 5 to 60 min (run
6, 8–11, Table 3). It was worth mentioning that a high activity (up to 56.9 × 106 g (PE) mol−1 (Fe) h−1
was observed at 5 min, indicating that the active species was formed very quickly [57].
By prolonging the polymerization time from 5 to 30 min, the high activity was significantly
decreased to 16.5 × 106 g (PE) mol−1 (Fe) h−1). Additionally, the activity of Fe2/MMAO declined
)
to 8.81 × 106 g (PE) mol−1 (Fe) h−1
, further extending the polymerization time to 60 min. Such a result

Molecules 2020, 25, 4244
7 of 17
would be attributed to the higher viscosity of the polymerization solution which restricted active
species to coordinate with ethylene.
(b)
(a)
Figure 3.
(
a) GPC curves of the resultant polyethylenes by Fe2/MMAO various temperature (run 1–4,
Table 3); (b) GPC curves of the resultant polyethylene by Fe2/MMAO various ratio of Al/Fe (run 2 and
5–7, Table 3).
Fe1
–
Fe5/MMAO were then employed as catalysts to catalyze ethylene polymerization
(run 6, 12–15, Table 3). Fe1
–
Fe5/MMAO showed a higher activity (8.81–17.0 × 106 g(PE) mol−1 (Fe) h−1
) at
70 ◦C, which was higher than most of the previous analogs (
B
–
D
, ◦Scheme 1) (
B
, 70 ◦C, 9.51
×
106 g(PE)
mol−1 (Fe) h−1;
C
, 70 C, 10.9
×
106 g(PE) mol−1 (Fe) h−1;
D, 70 C, 15.8
×
106 g(PE) mol−1 (Fe) h−1;
> Fe5 > Fe3, and the order of activity
◦
Scheme 1). The activity decreased in the order Fe4
was affected by both the electron effect and steric effect of ligands. All samples showed a really narrow
distribution (PDI = 1.48–1.70) with low molecular weights (2.83–5.39 kg mol−1, Figure 4a), meaning
>
Fe1 ≈ Fe2
the single-site active species of Fe1–Fe5/MMAO. All the polyethylenes show a melting temperature in
the range of 129.1–134.6 ◦C, indicating the highly linear microstructure [56 57]. Their 1H NMR and
,
13C NMR spectra at an elevated temperature were measured and demonstrate the linear property of
resultant polyethylenes.
(b)
(a)
Figure 4.
(
a
) GPC curves of the resultant polyethylenes by Fe1
–
Fe5/MMAO (run 6, 12–15, Table 3);
(
b) GPC curves of the resultant polyethylene by Fe1–Fe5/MAO various ratio of Al/Fe (run 16–20,
Table 3).
The activities of iron precatalysts (8.81–17.0
generally higher than their cobalt analogs (7.01–10.9
×
106 g(PE) mol−1 (Fe) h−1
106 g(PE) mol−1 (Co) h−1
,
Fe1
–
Fe5/MAO) are
Co5/MAO).
×
,
Co1–

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For example, the Fe2/MAO at 70 ◦C reaches 16.7
×
106 g(PE) mol−1 (Fe) h−1, compared to the Co2/MAO
with 8.32
×
106 g(PE) mol−1 (Co) h−1. Meanwhile the resultant polyethylene catalyzed by Fe2/MAO
has a molecular weight of 4.93 kg mol−1, compared with the polyethylene catalyzed by Co2/MAO,
which has a molecular weight of 19.6 kg mol−1. Furthermore, the polyethylene catalyzed by Co2/MAO
significantly shows a vinyl-end group (Figure 2), but the polyethylene catalyzed by Fe2/MAO indicates
a saturated end group (Figure 5). The metal center characterizes the microstructure of resultant
polyethylene. Herein, polyethylenes with vinyl-ends are favorably produced by cobalt precatalysts.
Figure 5. The NMR spectrum of polyethylenes obtained by Fe2/MMAO at 70 ◦C (run 6, Table 3).
The Fe1
–
Fe5/MAO system was also employed to further investigate the ethylene polymerization
Fe2 Fe5 Fe1 Fe3
106 g(PE) mol−1 (Fe) h−1), and the molecular weight of polyethylenes decreased in the
Fe5 Fe2 Fe1 Fe5/MMAO, the molecular
Fe4 (291–10.7 kg mol−1). Different than Fe1
Fe5/MAO was higher with a broader molecular weight
behavior (run 16–20, Table 3). The activity decreased in the order Fe4
>
>
>
>
(15.8–2.58
order Fe3
×
>
>
>
>
–
weight of polyethylenes catalyzed by Fe1
–
distribution. These different ethylene polymerization behaviors affected by cocatalysts may be ascribed
to the different active species’ structure nature formed by different cocatalysts [60]. From the GPC
curves (Figure 4b), all the samples displayed a unimodal distribution, which illustrates a stable active
site during polymerization.
3. Materials and Methods
3.1. General Considerations
All the experimental manipulations of air- and/or moisture-sensitive compounds were carried out
under an atmosphere of nitrogen by the use of standard Schlenk techniques. Freshly distilled toluene
was used for the polymerization runs that had previously been dried over sodium for approximately
10 h before distillation under a nitrogen atmosphere. Methylaluminoxane (MAO, 1.46 M in toluene)
and modified methylaluminoxane (MMAO, 1.93 M in heptane) were provided by Albemarle Corp
(Charlotte, NC, USA). High purity ethylene was provided by the Beijing Yanshan Petrochemical
Company and used as received. Other reagents were purchased from Alderich (Beijing, China),
1
Acros (Beijing, China), or local suppliers. The H and 13C NMR spectroscopic measurements for
the organic compounds were performed on Bruker DMX 400 MHz instruments (Beijing, China) at

Molecules 2020, 25, 4244
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room temperature using tetramethylsilane (TMS) as an internal standard. All the chemical shifts
and coupling constants are given in ppm and Hz, respectively. Elemental analyses (C, H, and N)
were performed on a Flash EA 1112 microanalyzer (Beijing, China). The FT IR spectra were recorded
using a PerkinElmer System 2000 FT IR spectrometer (Beijing, China). The molecular weights and
molecular weight distributions (Mw/Mn) of the polyethylenes were measured using a PL-GPC220
instrument (Beijing, China) at 150 ◦C with 1,2,4-trichlorobenzene as the solvent. Data collection and
processing were performed using Cirrus GPC Software (Agilent PL-Cirrus Software, Beijing, China)
and Multi-Detector Software (Agilent PL-Cirrus Software, Beijing, China). The calibrants employed
for construction of the conventional calibration (Polystyrene Calibration KitS-M-10) were provided
by PL Company (Beijing, China). The true average molecular weights of the polyethylenes were
attained by inputting the Mark–Houwink constants of polyethylenes; K (0.727) and
α (40.6) were
provided by PL Company (Beijing, China). The sample was dissolved at a concentration of 1.0 mg
mL−1. The DSC traces and melting points of the polyethylenes were obtained from the second scanning
run on a PerkinElmer TA-Q2000 DSC analyzer (Beijing, China) under a nitrogen atmosphere. During
the procedure, a sample of about 4.0–6.0 mg was heated to 160 ◦C at a heating rate of 20 ◦C min−1
followed by 5 min at 150 ◦C to remove the thermal history and then cooled at a rate of 20 ◦C min−1 to
,
−
20 ◦C. The compound 2-arylimino-6-acetylpyridines (S1
reports [36–38].
–S4) was prepared according to our previous
3.2. Synthesis of 6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine (L1–L5)
3.2.1. Synthesis of 6-(2,6-Dimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine (L1
)
In this reaction, S1 (3.4 mmol) and 2-phenethyl-1-naphthylamine (2.9 mmol) were added into
a flask with 20 mL of toluene. When the temperature of the reactor reached 110 ◦C, 0.17 g p-TsOH was
added into this reactor. After 6 h, the product was purified by column chromatography on aluminum
1
oxide to afford the product as yellow solid in 14% yield. H NMR (400 MHz, CDCl3, TMS):
δ 8.61
(
d, J = 7.76 Hz, 1H); 8.52 (m, 2H); 7.77 (t, J = 7.99, 1H); 7.85 (d, J = 8.03 Hz, 1H); 7.59–7.48 (m, 3H);
7.43 (t, J = 6.90 Hz, 1H); 7.12–7.08 (m, 8H); 7.0–6.93 (m, 2H); 4.33 (m, 1H), 2.25 (d, J = 10.28 Hz, 3H);
2.18 (s, 3H); 2.08 (m, 9H); 1.70 (d, J = 7.23 Hz, 3H). 13C NMR (100 MHz, CDCl3, TMS):
170.08, 167.26,
δ
155.25, 148.75, 146.54, 144.86, 136.91, 132.80, 129.30, 128.98, 128.24, 127.91, 127.68, 125.82, 125.62, 125.42,
124.97, 124.03, 123.21, 122.98, 122.64, 122.31, 40.45, 39.08, 21.97, 17.96, 16.45. FT IR (cm−1): 3056 (m),
3023 (m), 2962 (m), 2928 (m), 2873 (m), 1698 (s), 1639 (s), 1601 (s), 1565 (s), 1507 (s), 1449 (w), 1361 (s),
1297 (m), 1258 (s), 1233 (m), 1180 (w), 1118 (s), 1094 (m), 1073 (w), 1022 (w), 955 (m), 902 (m), 863 (m),
814 (s), 797 (s), 765 (s), 744 (s), and 697 (s). Anal. Calcd for C35H33N3: C, 84.81, H, 6.71, N, 8.48; found:
C, 84.98, H, 6.50, N, 8.52%.
3.2.2. Synthesis of 6-(2,6-Diethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine (L2
)
Similar to the synthesis L1
MHz, CDCl3, TMS): 8.62 (d, J = 8.01 Hz, 2H); 8.49 (t, J = 7.21 Hz, 1H); 8.20–8.18 (m, 1H);
,
L2 was obtained as yellow solid in 18% yield. 1H NMR (400
δ
7.94–7.92 (m, 1H); 7.51–7.41 (m, 4H); 7.26–7.25 (s, 2H); 7.24–7.00 (m, 7H); 4.26 (m, 1H); 2.51–2.49
(m, 4H); 2.16 (d, J = 7.22 Hz, 3H); 1.62 (t, J = 7.32 Hz, 3H); 1.47 (s, 3H); 1.30–1.06 (m, 6H). 13C NMR (100
MHz, CDCl3, TMS):
δ 170.10, 166.86, 155.26, 147.79, 146.52, 136.92, 132.80, 131.17, 128.98, 128.23, 128.14,
128.04, 127.68, 125.95, 125.81, 125.62, 125.42, 124.98, 124.03, 123.34, 123.20, 122.99, 122.33, 122.27, 40.43,
24.64, 21.95, 16.81, 13.76. FT IR (cm−1): 3057 (s), 2964 (s), 2931 (s), 2872 (s), 1632 (s), 1567 (s), 1494 (s),
1453 (w), 1405 (w), 1360 (s), 1321 (m), 1300 (m), 1259 (s), 1246 (s), 1196 (w), 1149 (s), 1122 (s), 1101 (m),
1026 (w), 962 (m), 906 (m), 877 (m), 866 (m), 817 (s), 777 (s), 733 (s), 714 (s). Anal. Calcd for C37H37N3:
C, 84.86, H, 7.12, N, 8.02; Found: C, 84.55, H, 7.21, N, 8.24%.

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3.2.3. Synthesis of 6-(2,6-Diisopropylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridine
(L3)
1
Similar to the synthesis of L1
CDCl3, TMS):
2.79 (m, 5H); 1.71 (s, 3H); 1.16 (m, 15H). 13C NMR (100 MHz, CDCl3, TMS):
,
L3 was obtained as a yellow solid in a 15% yield. H NMR (400 MHz,
δ
8.63–8.49 (m, 3H); 8.03–7.95 (m, 2H); 7.69–7.35 (m, 4H); 7.19–7.09 (m, 8H); 4.34 (m, 1H);
170.15, 167.08, 155.30,
δ
155.10, 146.54, 144.91, 136.96, 135.83, 132.85, 129.04, 128.27, 128.18, 127.96, 127.72, 125.84, 125.74, 125.66,
125.46, 125.04, 124.08, 123.66, 123.41, 123.26, 123.06, 122.36, 40.45, 39.09, 28.40, 28.38, 28.34, 23.29, 17.16.
FT IR (cm−1): 3060 (m), 2960 (s), 2925 (m), 2869 (m), 1698 (s), 1632 (s), 1579 (w), 1567 (s), 1497 (s),
1453 (s), 1435 (w), 1361 (s), 1319 (m), 1300 (m), 1238 (s), 1190 (s), 1149 (w), 1121 (s), 1102 (w), 1057 (w),
995 (m), 744 (s), 733 (s), 713 (m), 699 (s), 674 (m). Anal. Calcd for C39H41N3: C, 84.89, H, 7.49, N, 7.62;
Found: C, 84.64, H, 7.56, N, 7.80%.
3.2.4. Synthesis of 6-(2,4,6-Trimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridine
(L4)
1
Similar to the synthesis L1
CDCl3, TMS):
,
L4 was obtained as yellow solid in an 18% yield. H NMR (400 MHz,
δ
8.64 (d, J = 7.12 Hz, 1H); 8.55 (t, J = 6.87 Hz, 2H); 8.01 (t, J = 7.77 Hz, 1H); 7.88
(d, J = 8.08 Hz,1H); 7.69 (m, 2H); 7.59 (t, J = 9.09 Hz, 1H); 7.53 (d, J = 8.42 Hz, 1H); 7.42 (m, 4H);
7.30 (m, 3H); 4.38 (m, 1H); 2.18 (s, 12H); 1.73 (d, J = 7.22 Hz, 3H); 1.61 (s, 3H). 13C NMR (100 MHz,
CDCl3, TMS): δ 170.30, 152.49,145.65. 137.41, 128.72, 128.27, 127.52, 126.38, 125.68, 125.55, 125.01, 124.86,
123.40, 122.69, 120.34, 118.32, 40.18, 25.51, 22.08, 21.77, 18.45, 18.39. FT IR (cm−1): 2964 (s), 2900 (s),
1698 (s), 1640 (s), 1565 (s), 1507 (m), 1491 (s), 1449 (s), 1406 (m), 1362 (s), 1297 (m), 1259 (m), 1233 (s),
1074 (m), 1025 (w), 954 (m), 902 (m), 864 (m), 815 (m), 798 (s), 744 (s), 732 (m), 698 (s), 672 (m). Anal.
Calcd for C36H35N3: C, 84.83, H, 6.92, N, 8.24; Found: C, 84.77, H, 6.85, N, 8.38%.
3.2.5. Synthesis of 2.6-bis(2-(1-Phenylethyl)naphthalen-1-yl)iminopyridine (L5)
The 2,6-diacetylpyridine (1 mmol) and 2-phenethyl-1-naphthylamine (2 mmol) were added into
a flask with 20 mL of toluene. When the temperature of the reactor reached 110 ◦C, 0.17 g of p-TsOH
was added into this reactor. After 6 h, the product was purified by column chromatography on
1
aluminum oxide to afford the product as a yellow solid in a 21% yield. H NMR (400 MHz, CDCl3,
TMS): 8.09–8.01 (m, 2H); 7.87–7.81 (m, 3H); 7.69–7.35 (m, 10H); 7.16–7.06 (m, 8H); 4.40–4.33 (2H);
1.74–1.72 (s, 6H); 1.54–1.53 (d, J = 7.20 Hz, 6H). 13C NMR (100 MHz, CDCl3, TMS):
δ 170.31, 169.62,
146.57, 145.73, 138.74, 137.38, 137.02, 133.11, 132.84, 128.73, 128.47, 128.17, 127.94, 127.71, 127.52, 126.38,
125.80, 125.49, 125.23, 124.94, 124.68, 124.09, 123.39, 122.83, 122.69, 122.55, 120.43, 118.45, 40.61, 40.31,
25.65, 21.78, 16.95. FT IR (cm−1): 3026 (m), 2959 (m), 2923 (m), 2870 (m), 1699 (m), 1640 (s), 1566 (m),
1492 (m), 1450 (m), 1413 (m), 1362 (s), 1297 (m), 1235 (s), 1120 (m), 1097 (m), 1067 (m), 1024 (m), 904 (m),
816 (s), 738 (s), 697 (s), 639 (s), 591 (s). Anal. Calcd for C45H39N3: C, 86.92, H, 6.32, N, 6.76; Found: C,
86.98, H, 6.55, N, 6.47%.
3.3. Synthesis of 6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridyl-cobalt dichloride (Co1–Co5)
3.3.1. Synthesis of 6-(2,6-Dimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-
cobalt dichloride (Co1)
In this reaction, L1 (0.2 mmol), CoCl2 6H2O (0.19 mmol) and 10 mL ethanol was added into
a Schlenk tube. After a reaction of 6 h, the ethanol was removed and the product was washed by ether.
Finally, the product was collected by a filter to afford a yellow solid in a 64% yield. FT IR (cm−1):
3399 (s), 3061 (m), 3022 (m), 2967 (m), 2918 (m), 2870 (m), 1685 (w), 1621 (s), 1587 (s), 1508 (m), 1491 (s),
1469 (m), 1450 (m), 1427 (w), 1370 (s), 1308 (w), 1262 (s), 1213 (s), 1162 (w), 1099 (s), 1060 (w), 1026
(w), 904 (s), 817 (s), 768 (s), 746 (s), 700 (s). Anal. Calcd for C35H33Cl2CoN3: C, 67.21, H, 5.32, N, 6.72;
Found: C, 67.42, H, 5.47, N, 6.64%.

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3.3.2. Synthesis of 6-(2,6-Diethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-
cobalt dichloride (Co2)
Similar to the synthesis of Co1,
Co2 was obtained as yellow solid in 57% yield. FT IR (cm−1):
3676 (m), 2969 (s), 2901 (s), 1620 (s), 1584 (s), 1507 (w), 1493 (m), 1449 (s), 1426 (w), 1393 (w), 1372 (s),
1321 (m), 1262 (s), 1208 (s), 1066 (w), 1057 (w), 1026 (s), 976 (w), 904 (m), 868 (s), 808 (s), 768 (s), 745 (s),
702 (s). Anal. Calcd for C37H37Cl2CoN3: C, 68.00, H, 5.71, N, 6.43; Found: C, 67.82, H, 5.89, N, 6.53%.
3.3.3. Synthesis of 6-(2,6-Diisopropylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridine-
cobalt dichloride (Co3)
Similar to the synthesis of Co1,
Co3 was obtained as a yellow solid in a 68% yield. FT IR (cm−1):
3473 (m), 2959 (s), 2922 (m), 2866 (m), 2160 (m), 1619 (s), 1582 (s), 1566 (w), 1508 (s), 1491 (s), 1451 (s),
1371 (s), 1260 (s), 1204 (s), 1176 (m), 1156 (w), 1100 (m), 1058 (m), 1024 (s), 946 (m), 829 (s), 797 (s),
805 (s), 773 (s), 767 (s). Anal. Calcd for C39H41Cl2CoN3: C, 68.72, H, 6.06, N, 6.16; Found: C, 68.69, H,
5.94, N, 6.22%.
3.3.4. Synthesis of 6-(2,4,6-Trimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-
cobalt dichloride (Co4)
Similar to the synthesis of Co1, )
Co4 was obtained as a yellow solid in a 67% yield. FT IR (cm−1
3389 (s), 3061 (w), 3027 (w), 2965 (w), 2918 (w), 1621 (s), 1586 (s), 1567 (w), 1508 (w), 1450 (s), 1428 (w),
1370 (s), 1262 (s), 1220 (s), 1157 (w), 1101 (m), 1061 (m), 1026 (s), 904 (m), 855 (s), 817 (s), 769 (s), 746 (s),
701 (s). Anal. Calcd for C36H35Cl2CoN3: C, 67.61, H, 5.52, N, 6.57; Found: C, 67.76, H, 5.74, N, 6.51%.
3.3.5. Synthesis of 2.6-bis((2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-cobalt dichloride (Co5)
Similar to the synthesis of Co1,
Co5 was obtained as a yellow solid in a 68% yield. FT IR (cm−1):
3473 (m), 3065 (m), 2959 (s), 2922 (m), 2865 (m), 2160 (m), 1619 (s), 1582 (s), 1567 (w), 1508 (m), 1491 (w),
1451 (s), 1430 (w), 1372 (s), 1336 (w), 1321 (w), 1311 (w), 1260 (s), 1205 (s), 1176 (w), 1156 (w), 1100 (m),
1058 (m), 1025 (s), 976 (w), 946 (w), 905 (m), 866 (m), 829 (s), 814 (s), 805 (s), 797 (s), 767 (s), 747 (s),
714 (s), 704 (s). Anal. Calcd for C45H39Cl2CoN3: C, 71.91, H, 5.23, N, 5.59; Found: C, 69.98, H, 5.24, N,
5.47%.
3.4. Synthesis of 6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridyl-iron dichloride (Fe1–Fe5)
3.4.1. Synthesis of
6-(2,6-Dimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-iron dichloride (Fe1
)
In this reaction, L1 (0.2 mmol), FeCl24H2O (0.19 mmol) and 10 mL of ethanol was added into
a Schlenk tube. After a reaction of 6 h, the ethanol was removed and the product was washed by ether.
Finally, the product was collected by a filter to afford blue solid in 41% yield. FT IR (cm−1): 3420 (s),
3066 (m), 2968 (m), 2932 (m), 2157 (w), 1647 (w), 1621 (s), 1589 (s), 1491 (s), 1450 (s), 1370 (s), 1337 (w),
1263 (m), 1203 (s), 1108 (w), 1060 (m), 1026 (m), 910 (w), 769 (s), 748 (s), 714 (s), 701 (s). Anal. Calcd for
C35H33Cl2FeN3: C, 67.54, H, 5.34, N, 6.75; Found: C, 67.39, H, 5.74, N, 6.51%.
3.4.2. Synthesis of 6-(2,6-Diethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridine-iron
dichloride (Fe2)
Similar to the synthesis of Fe1,
Fe2 was obtained as a blue solid in a 75% yield. FT IR (cm−1):
2970 (s), 2900 (s), 1621 (s), 1587 (s), 1584 (s), 1491 (s), 1449 (s), 1370 (s), 1262 (m), 1208 (s), 1057 (m), 1027
(m), 905 (w), 807 (m), 823 (m), 768 (s), 749 (s). Anal. Calcd for C37H37Cl2FeN3: C, 68.32, H, 5.73, N,
6.46; Found: C, 68.39, H, 5.86, N, 6.67%.

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3.4.3. Synthesis of 6-(2,6-Diisopropylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridine-
iron dichloride (Fe3)
Similar to the synthesis of Fe1,
Fe3 was obtained as a blue solid in a 90% yield. FT IR (cm−1):
2962 (s), 2900 (s), 1615 (s), 1580 (s), 1491 (s), 1450 (s), 1423 (m), 1372 (s), 1308 (m), 1268 (m), 1203 (s),
1099 (m), 1058 (m), 1027 (m), 828 (m), 812 (m), 797 (w), 767 (s), 748 (s), 714 (m), 704 (s). Anal. Calcd.
for C39H41Cl2FeN3: C, 69.04, H, 6.09, N, 6.19; Found: C, 69.18, H, 5.97, N, 6.25%.
3.4.4. Synthesis of 6-(2,4,6-Trimethylphenyl)imino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridine-
iron dichloride (Fe4)
Similar to the synthesis of Fe1,
Fe4 was obtained as a blue solid in a 46% yield. FT IR (cm−1):
2971 (s), 2901 (s), 1620 (s), 1588 (s), 1508 (m), 1492 (s), 1407 (m), 1394 (w), 1370 (s), 1250 (m), 1204 (s),
1066 (m), 1028 (m), 903 (m), 822 (m), 748 (s), 701 (s). Anal. Calcd. for C36H35Cl2FeN3: C, 67.94, H, 5.54,
N, 6.60; Found: C, 67.87, H, 5.65, N, 6.72%.
3.4.5. Synthesis of 2.6-bis(2-(1-phenylethyl)naphthalen-1-yl)iminopridine-iron dichloride (Fe5)
Similar to the synthesis of Fe1,
Fe5 was obtained as a blue solid in a 55% yield. FT IR (cm−1):
3058 (m), 3025 (m), 2960 (m), 2922 (m), 2868 (m), 1838 (s), 1619 (s), 1574 (s), 1492 (s), 1430 (s), 1365 (m),
1299 (s), 1241 (m), 1207 (w), 1132 (w), 1108 (m), 1077 (m), 1030 (m), 998 (m), 964 (w), 909 (w), 872 (m),
813 (w), 781 (s), 741 (s), 697 (s). Anal. Calcd for C45H39Cl2FeN3: C, 72.20, H, 5.25, N, 5.61; Found: C,
72.34, H, 5.48, N, 5.63%.
3.5. X-ray Crystallographic Studies
Single crystals of Co3 and Co5 suitable for the X-ray diffraction analysis were obtained by layering
a dichloromethane solution of the corresponding complex with ethyl ether at room temperature
under a nitrogen atmosphere. With graphite monochromated Mo-K
α radiation (λ = 0.71073 Å) at
170.00(10)K or CuK = 1.54184 Å) at 169.99(14)K, the cell parameters were obtained by the global
α (λ
refinement of the positions of all collected reflections. The intensities were corrected for Lorentz and
polarization effects and empirical absorption. The structures were solved by direct methods and
refined by full-matrix least-squares on F2. All hydrogen atoms were placed in calculated positions.
Structure solution and structure refinement were performed using the SHELXT-97 package [61
The free solvent molecules present within the crystal structures were removed by using the SQUEEZE
option of the crystallographic program PLATON [61 62]. Detail of the X-ray structure determinations
,62].
,
and refinements are provided in Table 4. X-ray crystallographic data in Calibration Index File (CIF) for
the Cambridge Crystallographic Data Centre (CCDC) 2024895 (Co2) and 2024896 (Co5) are available
free of charge from the Cambridge Crystallographic Data Centre.
3.6. General Procedure for Ethylene Polymerization under 10 Atm Pressure
The polymerization at an ethylene pressure of 10 atm was carried out in a 250 mL stainless steel
autoclave (Dalian Sanling Electronic Manufacture, Dalian, China) equipped with an ethylene pressure
control system, a mechanical stirrer, and a temperature controller. The autoclave was evacuated and
backfilled with ethylene three times. When the required temperature was reached, the precatalyst
(2.0 µmol) was dissolved in toluene (25 mL) in a Schlenk tube and injected into the autoclave containing
ethylene (1 atm) followed by the addition of more toluene (25 mL). The required amount of cocatalyst
(MAO and MMAO) and additional toluene was 100 mL. The autoclave was immediately pressurized
with an ethylene pressure of 10 atm and the stirring commenced. After the required reaction time,
the reactor was cooled with a water bath and the ethylene pressure vented. Following quenching
of the reaction with 10% hydrochloric acid in ethanol, the polymer was collected and washed with
ethanol and dried with a vacuum oven at 60 ◦C and weighed.

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Table 4. Crystal data and structure refinement for Co3 and Co5.
Co3
Co5
CCDC No.
Empirical formula
Formula weight
2024895
C78H82Cl4CoN6
1363.15
2024896
C45H39Cl2CoN3
751.62
Temperature/K
Crystal system
Space group
169.99(14)
triclinic
170.00(10)
monoclinic
P21/c
P1
10.9022(2)
11.4855(2)
17.2387(3)
74.805(2)
74.746(2)
68.494(2)
1904.26(7)
1
13.0704(7)
19.0887(9)
16.5809(7)
90
98.492(4)
90
4091.5(3)
4
1.220
a/Å
b/Å
c/Å
α/◦
β/◦
γ/◦
Volume (Å3)
Z
DCalcd. (g cm−3
)
1.189
5.034
714.0
µ (mm −1
F(000)
)
0.583
1564.0
0.3 × 0.25 × 0.19
MoKα (λ = 0.71073)
6.868 to 57.49
Crystal size (mm3)
Radiation
0.5 × 0.19 × 0.15
CuKα (λ = 1.54184)
5.406 to 151.256
2Θ range (◦)
−
13
≤
h
≤
13,
−
14
≤ 16
≤
k
≤
14,
−
21
≤
l
−16≤ h ≤15, −22 ≤ k ≤ 25, −22 ≤ l
Index ranges
≤ 18
Reflections collected
Independent reflections
23545
34131
10720 [Rint = 0.0381, Rsigma
0.0431]
=
9259 [Rint = 0.0740, Rsigma =
0.0817]
9259/0/464
1.031
Data/restraints/parameters
Goodness of fit on F2
10720/3/825
1.040
Final R indexes [I ≥ 2σ (I)]
Final R indexes (all data)
Largest diff. peak/hole (e Å−3
R1 = 0.0454, wR2 = 0.1119
R1 = 0.0505, wR2 = 0.1217
0.59/−0.35
R1 = 0.0852, wR2 = 0.2379
R1 = 0.1302, wR2 = 0.2606
1.39/−0.68
)
4. Conclusions
A series of iron and cobalt complexes supporting by 6-arylimino-2-(2-(1-phenylethyl)
naphthalen-1-yl)iminopyridineligands(L1–L5)weresynthesizedandcharacterized. Ontheactivationwith
eitherMMAOorMAO, cobaltprecatalystsshowedhighactivitiesupto 10.9 × 106 g(PE) mol−1 (Co) h−1 at
an elevated temperature. Their activities are in the order as Co4
the molecular weights of the resultant polyethylenes decreased in the order Co3
>
Co1
>
Co5
>
Co2
>
Co3, meanwhile
Co5 Co2 Co1
Co4, which was interpreted as the electron-donating ligands enhancing the catalytic activity and
the steric hindrance decreasing the catalytic activity, producing polyethylene with a higher molecular
weight. In comparison to the cobalt precatalysts, their iron analogs Fe1 Fe5/MMAO displayed higher
activities up to 17.0
106 g(PE) mol−1 (Fe) h−1 when polymerizing ethylene and produced highly linear
polyethylenes with lower molecular weights in the range of kg mol−1 and with a narrow polydispersity.
>
>
>
>
–
×
In the case of using MAO as the cocatalyst, the polyethylenes obtained by Fe1–Fe5/MAO showed
higher molecular weights up to 291 kg mol−1 with boarder polydispersity, indicating the multispecies
of active sites. All precatalysts achieved a good thermostability and were positively influenced by the
new modifications of the tridentate ligands.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/18/4244/s1,
X-ray crystallographic data for Co3 and Co5. CCDC: 2024895 (Co3), 2024896 (Co5).
Author Contributions: Design of the study and experiments, L.Z. and W.-H.S.; synthesis and catalysis, W.L., J.G.,
and Y.M.; the single crystal X-ray diffraction, Q.Z. and H.L.; manuscript, L.Z. and W.-H.S. All authors have read
and agreed to the published version of the manuscript.
Funding: This work was supported by the National Natural Science Foundation of China (21801098 and 21871275).

Molecules 2020, 25, 4244
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Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
ORTEP
CIF
Oak Ridge Thermal Ellipsoid Plot
Calibration Index File
GPC
MAO
Gel Permeation Chromatography
Methylauminoxane
MMAO Modified methylaluminoxane
PDI
Tm
Polydispersity index
Melting temperature
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Sample Availability: Samples of the organic compounds and complexes are available from the authors.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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