Exploring ortho-(4,4′-dimethoxybenzhydryl) substitution in iron ethylene polymerization catalysts: Co-catalyst effects, thermal stability, and polymer molecular weight variations

Article abstract of DOI:10.1002/aoc.6259

Six different types of 4,4′-dimethoxybenzhydryl-substituted bis(arylimino)pyridine-iron(II) chloride complex—[2-{{2,6-((p-MeOPh)₂CH)₂-4-MeC₆H₂}N=CMe}-6-(ArN=CMe)C₅H₃N]FeCl₂ (Ar =

Full text of DOI:10.1002/aoc.6259

Received: 3 March 2021
DOI: 10.1002/aoc.6259
Revised: 22 March 2021
Accepted: 29 March 2021
F U L L P A P E R
Exploring ortho-(4,40-dimethoxybenzhydryl) substitution in
iron ethylene polymerization catalysts: Co-catalyst effects,
thermal stability, and polymer molecular weight variations
Tian Liu1,2
| Yanping Ma1
| Gregory Solan1,3
| Tongling Liang1
|
Wen-Hua Sun1,2,4
1Key Laboratory of Engineering Plastics
and Beijing National Laboratory for
Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences,
Beijing, China
2CAS Research/Education Center for
Excellence in Molecular Sciences,
University of Chinese Academy of
Sciences, Beijing, China
3Department of Chemistry, University of
Leicester, Leicester, UK
4State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou
Six different types of 4,40-dimethoxybenzhydryl-substituted bis(arylimino)pyri-
dine-iron(II) chloride complex—[2-{{2,6-((p-MeOPh)2CH)2-4-MeC6H2}
N=CMe}-6-(ArN=CMe)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3 (Fe1), 2,6-Et2C6H3
(Fe2), 2,6-iPr2C6H3 (Fe3), 2,4,6-Me3C6H2 (Fe4), 2,6-Et2-4-MeC6H2 (Fe5),
2,6-((p-MeOPh)2CH)2-4-MeC6H2 (Fe6)—are reported. The molecular struc-
tures of Fe2 and Fe3 emphasize the unsymmetrical nature of the N,N,N-
chelating ligand and the steric protection exerted by the ortho-(4,40-
dimethyoxybenzhydryl) groups. A range in catalytic activities for ethylene
polymerization was observed when Fe1–Fe6 were treated with either
methylaluminoxane (MAO) or modified methylaluminoxane (MMAO). In par-
ticular, Fe1/MAO bearing the least bulky N-2,6-dimethylphenyl group
exhibited a very high activity of 1.11 × 107 g (PE) mol−1 (Fe) h−1 at
70ꢀC/10 atm over a 30-min run time that remained high even after 60 min, an
observation highlighting its appreciable catalyst lifetime and thermal stability.
In addition, the polyethylene formed by this catalyst class displayed desirable
characteristics such as high linearity and high molecular weight (Mw range:
5.17–7.62 × 105 g mol−1). Indeed, the molecular weight of the polymer pro-
duced using Fe1 (with MAO or MMAO) exceeds that obtained using related
benzhydryl-containing bis(imino)pyridine-iron catalysts. As a general feature,
activation of Fe1–Fe6 with MMAO led to lower activity and lower molecular
weight polyethylene, especially for runs performed at 1 atm ethylene pressure.
Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou, China
Correspondence
Wen-Hua Sun, Key Laboratory of
Engineering Plastics and Beijing National
Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China.
Email: whsun@iccas.ac.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21871275
K E Y W O R D S
bis(imino)pyridyl-iron catalyst, ethylene polymerization, high molecular weight polyethylene,
ortho-(4,40-dimethoxybenzhydryl) substitution
1 | INTRODUCTION
been considered as a compatible supporting ligand for this
type of catalysis in which the conjugated diiminopyridine
The polymerization of ethylene using late transition metal
catalysts centered on iron and cobalt (A, Chart 1)[1,2] has
remained a fertile field of research since the latter part of
the 1990s.[3–20] In this regard, 2,6-bis(imino)pyridines have
core can stabilize transition states during the polymeriza-
tion process.[21–23] In addition, the ligand and the central
metal form two rigid five-membered chelate rings, which
contribute to the overall stability of the catalyst.[24,25]
Appl Organomet Chem. 2021;e6259.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6259

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LIU ET AL.
Perhaps more significantly, the steric and electronic effects
exerted by strategically placed substituents on the ligand
frame can affect not only the catalytic performance but
also the properties of the polymer product.[16,19,26–38]
As a consequence, many studies have been devoted
to modifying the N-aryl substituents belonging to
the 2,6-bis(imino)pyridine ligand with more recent
progress identifying the beneficial effects of introducing
benzhydryl (CHPh2) groups. For example, B (Chart 1)[39]
and C (Chart 1)[40] have seen both increases in catalytic
activity and thermal stability. In addition, modification of
the benzhydryl substituents has witnessed some notable
developments with fluorinated D (Chart 1)[32] and
E (Chart 1)[16] demonstrating a favorable effect on the
catalytic performance. Elsewhere, the introduction of
electron-donating methoxy groups to the benzhydryl
periphery has seen some promising results for other types
of late transition metal catalysts.[41]
Key Points
• Symmetrical and unsymmetrical bis(imino)
pyridine-iron (II) chlorides
• Highly active precatalysts for ethylene
polymerization
• Linear polyethylenes with high molecular
weights
• Unusual influence of co-catalyst on the ther-
mal stability
• Dimethoxybenzhydryl groups and their benefi-
cial effects on PE molecular weight
2 | EXPERIMETAL
To explore the effects of introducing methoxy-
substituted benzhydryl groups into an iron polymeriza-
tion catalyst, we target in this work bis(arylimino)
pyridine-iron precatalysts bearing one fixed N-2,6-bis
(4,40-dimethoxybenzhydryl)-4-methylphenyl group, and
the other aryl group varied in terms of its steric/
electronic profile (Fe1–Fe5). It is envisioned that
this latter variation will allow some fine control over
catalytic performance. To complement this work,
the symmetrical iron(II) chloride counterpart, Fe6,
containing two sterically encumbered N-2,6-bis(4,40-
dimethoxybenzhydryl)-4-methylphenyl groups, is also
disclosed. These complexes are then subjected to an in-
depth ethylene polymerization study in which catalyst
optimization will be undertaken by varying the type/
amount of co-catalyst, ethylene pressure, reaction
temperature, and run time.[35,42–49] In addition, the prop-
erties of the resulting polyethylenes will also be thor-
oughly examined in terms of molecular weight (Mw),
dispersity (Mw/Mn), melting temperature (Tm) and
crystallinity (Xc).
2.1 | General considerations
All manipulations involving air- and moisture-sensitive
compounds were carried out using standard Schlenk
techniques under an inert nitrogen atmosphere. Toluene
was heated to reflux over sodium and distilled under
nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M
solution in toluene) and modified methylaluminoxane
(MMAO, 1.93 M in n-heptane) were purchased from
Albemarle (USA). High-purity ethylene was purchased
from Beijing Yansan Petrochemical Co. (Beijing, China)
and used as received. Other reagents were purchased
from Aldrich (Beijing, China), Acros (Beijing, China), or
1
local suppliers (Beijing, China). The H and 13C NMR
spectra of the free ligands were recorded on a DMX
400 MHz instrument (Bruker, Karlsruhe, Germany) at
ambient temperature with TMS as an internal standard.
The FT-IR spectra were recorded on a System 2000 FT-IR
spectrometer (PerkinElmer, Shanghai, China), and ele-
mental analysis was carried out with a Flash EA 1112
CHART 1 Developments in
benzhydryl substitution in
bis(imino)pyridyl-iron
(II) chloride precatalysts (A−E)

LIU ET AL.
3 of 19
microanalyzer (Thermo Electron SPA, Beijing, China).
The molecular weight (Mw) and molecular weight distri-
bution (Mw/Mn) of the polyethylene were determined
with an Agilent PL-GPC 220 instrument (Beijing, China)
at 150ꢀC with 1,2,4-trichlorobenzene as the eluting sol-
vent. The melting points of the polyethylenes were mea-
sured from the fourth scanning run on a PerkinElmer
TA-Q2000 differential scanning calorimeter (DSC) under
a nitrogen atmosphere. In a typical procedure, a sample
of about 5.0 mg was heated up to 160 ꢀC at a heating rate
of 20 ꢀC min−1, maintained for 2 min at 160 ꢀC to remove
the thermal history and then cooled to –20ꢀC at a
rate of 20ꢀC min−1. The 13C NMR spectra of the
polyethylenes were recorded on a Bruker DMX 300 MHz
instrument at 100ꢀC; sample preparation involved dis-
solving a weighed amount of polyethylene (80–100 mg)
in 1,1,2,2-tetrachloroethane-d2 (2 mL) at elevated
temperature with TMS as an internal standard. Com-
pounds (E)-1-(6-(1-((aryl)imino)ethyl)pyridin-2-yl)ethan-
1-one (aryl = 2,6-dimethylphenyl, 2,6-diethylphenyl,
2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, 2,6-diethyl-
4-methylphenyl, and 2,6-bis(4,40-dimethoxybenzhydryl)-
4-methylaniline) were prepared using literature
procedures.[16,32]
(100 MHz, CDCl3, TMS): δ 167.5, 158.0, 157.9, 155.4,
155.1, 148.9, 136.8, 136.5, 135.5, 132.9, 131.7, 130.9,
130.5, 128.5, 128.1, 125.6, 123.2, 122.5, 122.1, 113.8,
113.5, 55.4, 50.5, 21.5, 18.1, 17.3, 16.6. FT-IR (cm−1):
2945 (w), 2831 (w), 2,160 (w), 2016 (w), 1645 (νC=N
,
m), 1607 (νC=N, m), 1577 (m), 1505 (m), 1456 (m),
1365 (m), 1301 (m), 1239 (s), 1209 (m), 1175 (m), 1105
(m), 1033 (s), 869 (m), 826 (m), 813 (m), 766 (m),
738 (m), 686 (m). Anal. calcd for C54H53N3O4
(808.04): C, 80.27; H, 6.61; N, 5.20%. Found: C,
79.93; H, 6.53; N, 5.14%.
b. Ar = 2,6-Et2C6H3 (L2). Based on the procedure and
molar ratios described for the synthesis of L1 but with
(E)-1-(6-(1-((2,6-diethylphenyl)imino)ethyl)pyridin-2-yl)
ethan-1-one as the ketone-imine, L2 was prepared as a
1
light green powder (0.38 g, 29%). H NMR (400 MHz,
CDCl3, TMS): δ 8.41 (d, J = 8.0 Hz, 1H, Py-H), 8.13
(d, J = 8.0 Hz, 1H, Py-H), 7.85 (t, J = 8.0 Hz, 1H, Py-H),
7.13 (d, J = 8.0 Hz, 2H, Ph-H), 7.05 (t, J = 6.0 Hz, 1H,
Ph-H), 6.94 (t, J = 8.0 Hz, 8H, Ph-H), 6.78
(d, J = 8.0 Hz, 4H, Ph-H), 6.72 (d, J = 8.0 Hz, 4H,
Ph-H), 6.67 (s, 2H, Ph-H), 5.20 (s, 2H, −CH−), 3.79
(s, 6H, −OCH3), 3.72 (s, 6H, −OCH3), 2.43–2.35 (m, 4H,
−CH2−), 2.19 (s, 3H, −CH3), 2.15 (s, 3H, −CH3), 1.16
(t, J = 8.0 Hz, 9H, −CH3). 13C NMR (100 MHz, CDCl3,
TMS): δ 157.9, 157.8, 136.6, 136.4, 135.3, 132.7, 131.2,
130.7, 130.3, 128.3, 125.9, 123.3, 122.3, 121.9, 113.6,
113.4, 77.3, 77.0, 76.7, 55.2, 50.4, 24.6, 21.4, 17.1, 16.8,
13.7. FT-IR (cm−1): 2967 (m), 2928 (m), 2163 (w), 2016
(w), 1642 (νC=N, m), 1608 (νC=N, m), 1582 (m), 1508 (s),
1457 (m), 1366 (m), 1300 (m), 1246 (s), 1177 (m), 1118
(m), 1075 (m), 1038 (m), 827 (m), 767 (m),
733 (w). Anal. calcd for C56H57N3O4 (836.09): C,
80.45; H, 6.87; N, 5.03%. Found: C, 80.21 H, 6.76; N,
4.98%.
2.2 | Syntheses of 2-{{2,6-((p-
MeOPh)2CH)2-4-MeC6H2}N=CMe}-
6-(ArN=CMe)C5H3N (L1–L5)
a. Ar = 2,6-Me2C6H3 (L1). A solution of (E)-1-(6-(1-((2,-
6-dimethylphenyl)imino)ethyl)pyridin-2-yl)ethan-1-one
(0.74 g, 2.78 mmol) and 2,6-bis(4,40-dimethoxybenzhydryl)-
4-methylaniline (2.00 g, 3.74 mmol) in toluene (10 mL)
was stirred and heated to reflux. After 0.5 h, a catalytic
amount of p-toluenesulfonic acid was added, and the
reaction mixture further heated to reflux for 9 h. On
completion of the reaction (checked by silica TLC),
the mixture was allowed to cool to room temperature.
The mixture was filtered, and all volatiles removed
using a rotary evaporator. The residual solid was puri-
fied by basic alumina column chromatography using
petroleum ether/ethyl acetate (v/v = 100/1) as the
eluent, affording L1 as a light green solid (0.53 g,
24%). 1H NMR (400 MHz, CDCl3, TMS): δ 8.42
(d, J = 8.0 Hz, 1H, Py-H), 8.12 (d, J = 8.0 Hz, 1H,
Py-H), 7.84 (t, J = 8.0 Hz, 1H, Py-H), 7.08
(d, J = 8.0 Hz, 2H, Ph-H), 6.94 (t, J = 8.0 Hz, 10H,
Ph-H), 6.78 (d, J = 8.0 Hz, 4H, Ph-H), 6.72
(d, J = 8.0 Hz, 4H, Ph-H), 6.67 (s, 2H, Ph-H), 5.19
(s, 2H, −CH−), 3.78 (s, 6H, −OCH3), 3.72 (s, 6H,
−OCH3), 2.19 (s,3H, −CH3), 2.14 (s, 3H, −CH3), 2.07
(s, 6H, −CH3), 1.18 (s, 3H, −CH3). 13C NMR
c. Ar = 2,6-iPr2C6H3 (L3). Based on the procedure and
molar ratios described for the synthesis of L1 but with
(E)-1-(6-(1-((2,6-diisopropylphenyl)imino)ethyl)pyridin-
2-yl)ethan-1-one as the ketone-imine, L3 was prepared
as a light green powder (0.13 g, 18%). 1H NMR
(400 MHz, CDCl3, TMS): δ 8.39 (d, J = 7.6 Hz, 1H,
Py-H), 8.10 (d, J = 8.0 Hz, 1H, Py-H), 7.84
(t, J = 7.8 Hz, 1H, Py-H), 7.17 (d, J = 7.2 Hz, 2H,
Ph-H), 7.10 (t, J = 7.6 Hz, 1H, Ph-H), 6.93
(t, J = 7.6 Hz, 8H, Ph-H), 6.78 (d, J = 8.4 Hz, 4H,
Ph-H), 6.71 (d, J = 8.4 Hz, 4H, Ph-H), 6.67 (s, 2H,
Ph-H), 5.18 (s, 2H, −CH−), 3.78 (s, 6H, −OCH3), 3.71
(s, 6H, −OCH3), 2.81–2.74 (m,2H, −CH−), 2.17
(d, J = 8.0 Hz, 6H, −CH3), 1.17 (t, J = 5.0 Hz, 15H,
−CH3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.9,
167.1, 157.9, 157.8, 155.3, 154.9, 146.5, 145.9, 136.7,
136.3, 135.8, 135.3, 132.7, 131.5, 130.7, 130.3, 128.3,
123.6, 123.0, 122.3, 121.9, 113.6, 113.4, 55.2, 50.3, 28.3,

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LIU ET AL.
23.3, 22.9, 21.4, 17.1. FT-IR (cm−1): 2959 (m), 2834 (w),
Anal. calcd for C57H59N3O4 (850.12): C, 80.53; H,
7.00; N, 4.94%. Found: C, 80.40; H, 6.98; N, 4.87%.
1976 (w), 1652 (m), 1640 (νC=N, m), 1607 (νC=N, m),
1,581 (m), 1508 (s), 1451 (m), 1382 (m), 1364 (m), 1301
(m), 1238 (s), 1173 (m), 1113 (m), 935 (w), 833 (m),
823 (m), 798 (m), 781 (m), 768 (m), 758 (m), 736 (m),
684 (w), 658 (w). Anal. calcd for C58H61N3O4
(864.14): C, 80.62; H, 7.12; N, 4.86%. Found: C,
80.40; H, 7.11; N, 4.71%.
2.3 | Synthesis of 2,6-{{2,6-((p-
MeOPh)2CH)2-4-MeC6H2}N=CMe}2C5H3N
(L6)
d. Ar = 2,4,6-Me3C6H2 (L4). Based on the procedure and
molar ratios described for the synthesis of L1 but with
(E)-1-(6-(1-((2,4,6-trimethylphenyl)imino)ethyl)pyridin-
2-yl)ethan-1-one as the ketone-imine, L4 was prepared
as a light green powder (0.15 g, 24%). 1H NMR
(400 MHz, CDCl3, TMS): δ 8.39 (d, J = 0.8 Hz, 1H, Py-
H), 8.10 (d, J = 8.1 Hz, 1H, Py-H), 7.83 (t, J = 8.0 Hz,
1H, Py-H), 6.96–6.91 (m, 10H, Ph-H), 6.78
(d, J = 8.8 Hz, 4H, Ph-H), 6.71 (d, J = 8.8 Hz, 4H,
Ph-H), 6.68 (s, 2H, Ph-H), 5.19 (s, 2H, −CH−), 3.78
(s, 6H, −OCH3), 3.72 (s, 6H, −OCH3), 2.30 (s, 3H,
−CH3), 2.19 (s, 3H, −CH3), 2.14 (s, 3H, −CH3), 2.03
(s, 6H, −CH3), 1.18 (s, 3H, −CH3). 13C NMR (100 MHz,
CDCl3, TMS): δ 157.8, 136.6, 136.3, 135.3, 132.7, 132.2,
131.5, 130.7, 130.3, 128.6, 128.3, 125.3, 122.3, 121.9,
113.6, 113.4, 77.3, 77.0, 76.7, 55.2, 50.4, 21.4, 20.8, 17.9,
17.1, 16.4. FT-IR (cm−1): 3667 (w), 2994 (m), 2951 (m),
A
solution of 1,10-(pyridine-2,6-diyl)bis(ethan-1-one)
(0.12 g, 0.74 mmol) and 2,6-bis(4,40-dimethoxybenzhydryl)
-4-methylaniline (1.00 g, 1.87 mmol) in toluene (10 mL)
was stirred and heated to reflux. After 0.5 h, a catalytic
amount of p-toluenesulfonic acid was added, and the
reaction mixture further heated to reflux for 9 h. On
completion of the reaction (checked by silica TLC), the
mixture was allowed to cool to room temperature. The
mixture was filtered, and all volatiles removed using a
rotary evaporator. The residual solid was purified by basic
alumina column chromatography using petroleum ether/
ethyl acetate (v/v = 100/1) as the eluent, affording L6 as a
light green solid (0.22 g, 24%). 1H NMR (400 MHz, CDCl3,
TMS): δ 8.11 (d, J = 8.0 Hz, 2H, Py-H), 7.79 (t, J = 7.6 Hz,
1H, Py-H), 6.95–6.90 (m, 16H, Ph-H), 6.78 (d, J = 8.4 Hz,
8H, Ph-H), 6.66 (d, J = 9.2 Hz, 12H, Ph-H), 5.18 (s, 4H,
−CH−), 3.78 (s, 12H, −OCH3), 3.61 (s, 12H, −OCH3), 2.18
(s, 6H, −CH3), 1.01 (s, 6H, −CH3). 13C NMR (100 MHz,
CDCl3, TMS): δ 169.8, 157.7, 155.0, 145.7, 136.3, 135.0,
132.6, 131.4, 130.5, 130.1, 128.2, 121.9, 113.5, 113.2, 77.2,
76.9, 76.6, 55.1, 50.2, 21.2, 16.9. FT-IR (cm−1): 2834 (w),
1638 (νC=N, m), 1607 (νC=N, m), 1580 (m), 1507 (s), 1458
(m), 1420 (w), 1364 (w), 1301 (m), 1242 (s), 1173 (m), 1113
(m), 1033 (m), 962 (w), 828 (m), 769 (w), 739 (w). Anal.
calcd for C83H79N3O8 (1246.56): C, 79.97; H, 6.39; N,
3.37%. Found: C, 79.66; H, 6.43; N, 3.28%.
2905 (m), 2159 (w), 2018 (w), 1973 (w), 1,637 (νC=N
,
m), 1607 (νC=N, m), 1577 (m), 1507 (m), 1454 (m), 1366
(m), 1298 (m), 1243 (s), 1174 (m), 1116 (m), 1075 (m),
1035 (m), 826 (m), 730 (w). Anal. calcd for C55H55N3O4
(822.06): C, 80.36; H, 6.74; N, 5.11%. Found: C,
80.34; H, 6.84; N, 5.04%.
e. Ar = 2,6-Et2-4-MeC6H2 (L5). Based on the procedure
and molar ratios described for the synthesis of L1 but
with (E)-1-(6-(1-((2,6-diethyl-4-methylphenyl)imino)
ethyl)pyridin-2-yl)ethan-1-one as the ketone-imine,
L5 was prepared as a light green powder (0.64 g, 44%).
1H NMR (400 MHz, CDCl3, TMS):
δ
8.41
2.4 | Syntheses of [2-{{2,6-((p-
MeOPh)2CH)2-4-MeC6H2}N=CMe}-
6-(ArN=CMe)C5H3N]FeCl2 (Fe1–Fe6)
(d, J = 7.6 Hz, 1H, Py-H), 8.12 (d, J = 7.2 Hz, 1H,
Py-H), 7.84 (t, J = 7.8 Hz, 1H, Py-H), 6.94
(t, J = 8.4 Hz, 10H, Ph-H), 6.78 (d, J = 8.4 Hz, 4H,
Ph-H), 6.71 (d, J = 8.8 Hz, 4H, Ph-H), 6.67 (s, 2H,
Ph-H), 5.19 (s, 2H, −CH−), 3.78 (s, 6H, −OCH3), 3.72
(s, 6H, −OCH3), 2.43–2.35 (m, 4H, −CH2−), 2.34
(s, 3H, −CH3), 2.18 (s, 3H, −CH3), 2.15 (s, 3H, −CH3),
1.15 (t, J = 7.6 Hz, 9H, −CH3). 13C NMR (100 MHz,
CDCl3, TMS): δ 157.9, 157.8, 136.6, 136.4, 135.3, 132.7,
132.4, 131.1, 130.7, 130.3, 128.3, 126.7, 122.2, 121.9,
113.6, 113.4, 77.3, 77.0, 76.7, 55.2, 50.4, 24.6, 21.4, 21.0,
17.1, 16.7, 13.9. FT-IR (cm−1): 3653 (w), 2960 (m),
a. Ar = 2,6-Me2C6H3 (Fe1). To a solution of ligand L1
(0.10 g, 0.12 mmol) in freshly distilled ethanol
(10 mL) was added FeCl2 4H2O (0.020 g, 0.11 mmol)
under a nitrogen atmosphere. Immediately, the color
of the solution changed from yellow to blue. The reac-
tion mixture was stirred for 12 h at room temperature
to form a precipitate. The precipitate was collected by
filtration and washed with diethyl ether (3 × 5 mL)
yielding Fe1 as a blue powder (0.08 g, 78%). FT-IR
(cm−1): 2948 (w), 2834 (w), 2027 (w), 1608 (νC=N, m),
1579 (νC=N, m), 1509 (s), 1460 (m), 1372 (m), 1300
(m), 1251 (s), 1211 (m), 1175 (m), 1110 (m), 1033 (m),
869 (m), 833 (m), 812 (m), 769 (m), 733 (m), 661 (w).
2925 (w), 2158 (m), 2025 (m), 1968 (m), 1,639 (νC=N
,
m), 1,607 (νC=N, m), 1575 (m), 1508 (s), 1455 (m),
1360 (m), 1298 (m), 1250 (s), 1175 (m), 1115 (m), 1080
(m), 1036 (m), 867 (m), 829 (m), 764 (w), 736 (m).

LIU ET AL.
5 of 19
Anal. calcd for C54H53Cl2FeN3O4(934.78): C, 69.38; H,
(w), 1298 (m), 1249 (s), 1177 (m), 1110 (m), 1032 (m),
830 (m), 776 (m), 734 (m), 688 (w). Anal. calcd for
C83H79Cl2FeN3O8 (1373.30): C, 72.59; H, 5.80; N,
3.06%. Found: C, 72.22; H, 5.77; N, 3.01%.
5.72; N, 4.50%. Found: C, 69.00; H, 5.47; N, 4.38%.
b. Ar = 2,6-Et2C6H3 (Fe2). The preparation of Fe2 was
carried out using a procedure and molar ratios similar
to that described for Fe1, but with L2 (0.10 g,
0.12 mmol) as the bis(imino)pyridine. On work-up,
Fe2 was isolated as a blue powder (0.080 g, 76%). FT-
IR (cm−1): 2958 (w), 2,834 (w), 2160 (w), 2026 (w),
1607 (νC=N, m), 1580 (νC=N, m), 1508 (s), 1458 (m),
1371 (m), 1299 (m), 1,249 (s), 1176 (m), 1110 (m),
1033 (m), 834 (m), 809 (m), 769 (m), 734 (m). Anal.
calcd for C56H57Cl2FeN3O4(962.83): C, 69.86; H,
5.97; N, 4.36%. Found: C, 69.98; H, 6.06; N, 4.15%.
c. Ar = 2,6-iPr2C6H3 (Fe3). The preparation of Fe3 was
carried out using a procedure and molar ratios similar
to that described for Fe1, but with L3 (0.10 g,
0.12 mmol) as the bis(imino)pyridine. On work-up,
Fe3 was isolated as a blue powder (0.06 g, 55%). FT-IR
(cm−1): 2955 (w), 2834 (w), 2158 (w), 2024 (w), 1607
(νC=N, m), 1579 (νC=N, m), 1509 (s), 1459 (m), 1370
(m), 1300 (m), 1248 (s), 1175 (m), 1108 (m), 1033 (m),
834 (m), 810 (m), 773 (m), 736 (w). Anal. calcd for
C58H61Cl2FeN3O4(990.89): C, 70.30; H, 6.21; N, 4.24%.
Found: C, 69.89; H, 6.16; N, 4.17%.
2.5 | X-ray crystallographic studies
Single crystals of Fe2, Fe3, and Fe30 suitable for the X-
ray determinations were grown by layering diethyl ether
onto a dichloromethane solution of the corresponding
compound at room temperature. Using mirror-
monochromatic Cu-Kα radiation (λ = 1.54184 Å) at
169.99 K, cell parameters were obtained by global refine-
ment of the positions of all collected reflections.
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 calcu-
lated positions. Structural solution and refinement
were performed by using the Olex2 1.2 package and
SHELXTL.[50] During the structural refinement, the
solvent was squeezed (Fe30) with PLATON software.[51]
Details of the X-ray structure determinations and refine-
ments are provided in Table S1.
d. Ar = 2,4,6-Me3C6H2 (Fe4). The preparation of Fe4
was carried out using a procedure and molar ratios
similar to that described for Fe1, but with L4 (0.10 g,
0.13 mmol) as the bis(imino)pyridine. On work-up,
Fe4 was isolated as a blue powder (0.08 g, 76%). FT-
IR (cm−1): 2946 (w), 2834 (w), 2159 (w), 2027 (w),
1608 (νC=N, m), 1579 (νC=N, m), 1509 (s), 1459 (m),
1373 (m), 1299 (m), 1251 (s), 1175 (m), 1111 (m), 1034
(m), 860 (m), 833 (m), 813 (m), 776 (w), 735 (w). Anal.
calcd for C55H55Cl2FeN3O4(948.81): C, 69.62; H,
5.84; N, 4.43%. Found: C, 69.21; H, 5.81; N, 4.35%.
e. Ar = 2,6-Et2-4-MeC6H2 (Fe5). The preparation of Fe5
was carried out using a procedure and molar ratios
similar to that described for Fe1, but with L5 (0.10 g,
0.12 mmol) as the bis(imino)pyridine. On work-up,
Fe5 was isolated as a blue powder (0.07 g, 65%). FT-IR
(cm−1): 2931 (w), 2834 (w), 2160 (w), 2016 (w), 1608
(νC=N, m), 1580 (νC=N, m), 1509 (s), 1459 (m), 1373
(m), 1300 (m), 1249 (s), 1173 (m), 1111 (m), 1032 (m),
862 (m), 833 (m), 812 (m), 775 (w), 735 (w), 689 (w).
Anal. calcd for C57H59Cl2FeN3O4 (976.86): C, 70.08; H,
6.09; N, 4.30%. Found: C, 69.39; H, 6.05; N, 4.18%.
f. Ar = 2,6-((p-MeOPh)2CH)2-4-MeC6H2 (Fe6). The
preparation of Fe6 was carried out using a procedure
and molar ratios similar to that described for Fe1, but
with L6 (0.10 g, 0.08 mmol) as the bis(imino)pyridine.
On work-up, Fe6 was isolated as a blue powder
(0.08 g, 81%). FT-IR (cm−1): 2955 (w), 2834 (w), 1608
(νC=N, m), 1580 (νC=N, m), 1509 (s), 1459 (m), 1374
2.6 | Typical procedure for ethylene
polymerization
2.6.1 | Ethylene polymerization at 1 atm
ethylene pressure
The polymerizations at 1 atm ethylene pressure were
carried out in a Schlenk vessel. Under ambient ethyl-
ene atmosphere (approximately 1 atm), Fe1 (2.0 μmol)
was added followed by toluene (30 mL), and then the
required amount of co-catalyst (MAO, MMAO) intro-
duced by syringe. The solution was then stirred at the
required temperature under 1 atm of ethylene pres-
sure. After 30 min, the vessel was vented, and the
solution quenched with 10% hydrochloric acid in
ethanol. The polymer was washed with ethanol,
dried under reduced pressure at 60ꢀC, and then
weighed.
2.6.2 | Ethylene polymerization at 5 or
10 atm ethylene pressure
The polymerizations at 5 or 10 atm of ethylene pressure
were performed in a stainless steel autoclave (250 mL)
equipped with an ethylene pressure control system, a

6 of 19
LIU ET AL.
mechanical stirrer, and a temperature controller. The
autoclave was evacuated and refilled with ethylene three
times. When the required temperature was reached, the
iron complex (1.5 μmol) was dissolved in toluene (25 mL)
in a Schlenk tube and then injected into the autoclave
under an atmosphere of ethylene (approximately 1 atm),
followed by the addition of more toluene (25 mL). The
required amount of co-catalyst (MAO and MMAO) was
then injected, followed by more toluene taking the total
volume of toluene to 100 mL. The autoclave was immedi-
ately pressurized with 5 or 10 atm pressure of ethylene,
and the stirring commenced. After the required reaction
time, the reactor was cooled with a water bath, and the
excess ethylene pressure released. The reaction was
quenched with 10% hydrochloric acid in ethanol, and the
polymer collected. Following washing with ethanol, the
polymer was dried under reduced pressure at 60ꢀC and
weighed.
dimethoxybenzhydryl)-4-methylaniline was condensed
with the corresponding monoketone to form the target
ligands L1–L5 in good yield (Scheme 1). Notably, this
4,40-dimethyoxybenzhydryl-substituted aniline was new
and was readily synthesized by a zinc(II) chloride
catalyzed Friedel–Crafts alkylation of p-toluidine.[32] By
contrast, the symmetrical bis(imino)pyridine, 2,6-{{2,6-((p-
MeOPh)2CH)2-4-MeC6H2}N=CMe}2C5H3N (L6), was pre-
pared in one step by the acid-catalyzed reaction of
2,6-diacetylpyridine with two molar equivalents of 2,6-bis
(4,40-dimethoxybenzhydryl)-4-methylaniline. All the new
compounds have been fully characterized by 1H/13C NMR
and FT-IR spectroscopy as well as by elemental analysis.
The iron complexes, [2-{{2,6-((p-MeOPh)2CH)2-
4-MeC6H2}N=CMe}-6-(ArN=CMe)C5H3N]FeCl2
(Ar =
2,6-Me2C6H3 (Fe1), 2,6-Et2C6H3 (Fe2), 2,6-iPr2C6H3
(Fe3), 2,4,6-Me3C6H2 (Fe4), 2,6-Et2-4-MeC6H2 (Fe5),
2,6-((p-MeOPh)2CH)2-4-MeC6H3 (Fe6)), were then gener-
ated in good yield by the interaction of L1–L6 with one
molar equivalent of FeCl2 4H2O in ethanol at room tem-
perature (Scheme 1). All six ferrous complexes were air
stable in the solid state but displayed some signs of oxida-
tion on prolonged standing in solution in the air (vide
infra). Each iron complex has been characterized by FT-
IR spectroscopy and elemental analysis. In addition, the
structures of Fe2 and Fe3 have been elucidated by single-
crystal X-ray diffraction.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization of
Fe1−Fe6
The unsymmetrical bis(imino)pyridines, 2-{{2,6-((p-
MeOPh)2CH)2-4-MeC6H2}N=CMe}-6-(ArN=CMe)C5H3N
(Ar = 2,6-Me2C6H3 (L1), 2,6-Et2C6H3 (L2), 2,6-iPr2C6H3
(L3), 2,4,6-Me3C6H2 (L4), 2,6-Et2-4-MeC6H2 (L5)), have
been prepared in two steps (Scheme 1).[39,40] Firstly, the
ketone-imine compounds, 2-(O=CMe)-6-(ArN=CMe)
C5H3N (Ar = 2,6-Me2C6H3, 2,6-Et2C6H3, 2,6-iPr2C6H3,
2,4,6-Me3C6H2, 2,6-Et2-4-MeC6H2, 2,6-((4-OCH3-C6H4))2-
4-CH3C6H2), were prepared by treating 2,6-diacetylpyridine
with the corresponding aniline using procedures
detailed in the literature.[16] Secondly, 2,6-bis(4,40-
Single crystals of Fe2 and Fe3 suitable for the X-ray
diffraction determinations were grown under a nitrogen
atmosphere by the slow layering of diethyl ether onto a
dichloromethane solution of the corresponding complex
at room temperature. In both cases, the unit cell contains
two independent molecules (A and B) that are essentially
the same that differ only in the positioning of the FeCl2
unit within the N,N,N cavity. On a separate occasion,
attempts at growing crystals of Fe3 gave green crystals of
SCHEME 1 Synthesis of L1–L6 and their
iron(II) chloride complexes Fe1–Fe6

LIU ET AL.
7 of 19
TABLE 1 Selected bond lengths (Å) and angles (o) for Fe2 and
Fe3
Fe2
Fe3
Bond lengths (Å)
Fe(1)–N(1)
2.204(2)
2.073(2)
2.245(2)
2.3141(8)
2.2433(8)
2.244(2)
2.066(2)
2.187(3)
2.3074(9)
2.2554(9)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–Cl(1)
Fe(1)–Cl(2)
Bond angles (ꢀ)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
N(1)–Fe(1)–Cl(2)
N(2)–Fe(1)–Cl(2)
N(3)–Fe(1)–Cl(2)
N(1)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(1)
N(3)–Fe(1)–Cl(1)
Cl(2)–Fe(1)–Cl(1)
73.34(8)
139.46(8)
73.17(8)
100.66(6)
153.43(7)
99.17(6)
99.43(6)
91.29(6)
103.45 (6)
115.28(3)
72.56(9)
137.78(9)
73.54(10)
98.27(6)
152.07(8)
99.98(8)
102.39(6)
93.05(8)
103.82(8)
114.82(4)
FIGURE 1 ORTEP diagram of Fe2 with the thermal ellipsoids
at the 30% probability level. All hydrogen atoms have been omitted
for clarity
Fe2 (molecule A) and Fe3 (molecule A) are shown in
Figures 1 and 2; selected bond distances and angles are
presented for both in Table 1.
The structures are similar, distinguishable only in the
2,6-substitution pattern of one of the N-aryl groups
(2,6-diethylphenyl for Fe2 and 2,6-diisopropylphenyl for
Fe3), and hence will be discussed together. Both comprise
a single iron center surrounded by three nitrogen atoms
deriving from the N,N,N-chelating ligand and two chlo-
rides to complete a five-coordinate geometry. These geom-
etries can be best viewed as distorted square pyramidal,
and the degree of distortion quantified in terms of τ5 value
(with τ5 = 0 perfectly square pyramidal and τ5 = 1 per-
fectly trigonal bipyramid),[52–54] with Fe2 being 0.233 and
Fe3 0.238. More specifically, the three nitrogen atoms and
one chloride atom (Cl2) form the basal plane and Cl1 the
apical position. The iron atom itself is housed
at a distance of 0.530 Å above the basal plane for Fe2
and 0.567 Å for Fe3, which mimics that observed
in related (N,N,N)FeCl2 species.[16,19,39] The central
Fe–Npyridine bond length [2.0730(2) (Fe2), 2.0660(4) (Fe3)
Å] is shorter than the exterior Fe–Nimine ones [2.204(2)–
2.245(2) Å], which can be attributed to a combination of
the good donor properties of the central pyridine and the
constraints of this ligand class; similar binding properties
of the bis(imino)pyridine ligand have been observed in
structurally related iron complexes.[55,29–34] In addition,
there is some modest variation in the exterior Fe-Nimine
FIGURE 2 ORTEP diagram of Fe3 with the thermal ellipsoids
at the 30% probability level. All hydrogen atoms have been omitted
for clarity
Fe30 in which an oxo-bridged diferric species, L3FeCl
(μ-O)FeCl3, formed as one of the two independent mole-
cules in the unit (see Figure S1 and Table S1). Views of

8 of 19
LIU ET AL.
distances that can, in part, be credited to the steric and
electronic differences between the N-aryl groups. There
are also some differences in the Fe–Cl bond distances
with the Fe−Cl2basal bond length (2.2433(8) Fe2, 2.2554
(9) Å Fe3) shorter than the Fe−Cl1axial one (2.3141
(8) Fe2, 2.3074(9) Å Fe3). The imine bond lengths are
comparable and typical of those expected for this func-
tional group (N(1)–C(2): 1.294(3) (Fe2), 1.289(4) Å (Fe3)
vs. N(3)–C(8): 1.288(3) (Fe2), 1.282(8) Å (Fe3)). As is
seen in many bis(imino)pyridine-iron complexes, the
N-aryl rings are positioned almost perpendicularly to the
central pyridine ring with dihedral angles of 80.91ꢀ and
77.67ꢀ for Fe2 and 78.24 and 75.52ꢀ for Fe3.[19,36–38,56] It
is plausible that this inclination of the N-aryl groups
and the consequent positioning of the bulky ortho-
substituents shield the space about the active center in
the catalyst itself, thereby inhibiting chain transfer and
leading higher catalytic activity and thermal stability
(vide infra). Furthermore, within the diiminepyridine
core, there is some twisting as is evidenced by the N1–
C2–C3–N2 and N3–C8–C7–N2 torsion angles of 2.65ꢀ
(Fe2), 2.66ꢀ (Fe3) and −1.81ꢀ (Fe2), and −1.84ꢀ (Fe3),
respectively; a similar distortion has also been noted in
their bis(imino)pyridine-cobalt analogues.[19,35,42–44]
iron complexes, all six complexes gave satisfactory
microanalytical data.
3.2 | Catalyst evaluation for ethylene
polymerization
So as to explore the catalytic performance of the six iron
complexes in ethylene polymerization, Fe1 was selected
as the test precatalyst to establish a viable set of operating
conditions with MAO and MMAO employed as the co-
catalysts.[17–20] Typically, the polymerizations runs using
each aluminoxane co-catalyst were performed in toluene
over 30 min at initially 1 atm and then at 10 atm ethylene
pressure to allow a comparison of pressure effects.
a. Catalytic evaluation using Fe1–Fe6 with MAO at
PC2H4 = 1 atm
With the pressure of ethylene set at 1 atm, Fe1 in
combination with MAO was screened to examine the
influence of reaction temperature and the molar equiva-
lents of MAO on catalytic activity and polymer proper-
ties. Once the optimal set of conditions for Fe1 were
established, these would be applied to Fe2–Fe6; the full
set of results are collected in Table 2.
In the first instance, the influence of temperature on
the polymerization using Fe1/MAO was investigated
with the Al:Fe molar ratio set at 2000:1. On increasing
the reaction temperature from 30 to 60ꢀC (Entries 1–4,
When compared with the corresponding free ligand,
the ν(C=N)imine stretching frequencies in the IR spectra
for Fe1−Fe6 are each shifted to lower wavenumber by
approximately 30 cm−1, in line with effective coordina-
tion of the two imine nitrogen atoms.[20] To further
support the proposed (N,N,N)FeCl2 composition for the
TABLE 2 Ethylene polymerization results using Fe1–Fe6/MAO at PC2H4 = 1 atma
c
c
d
Entry
Precat.
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
T (ꢀC)
30
Al:Fe
2000
2000
2000
2000
1000
1500
1750
2500
2000
2000
2000
2000
2000
t (min)
30
Mass of PE (g)
1.92
Act.b
1.92
3.72
3.01
0.83
0.68
2.41
2.99
3.29
1.69
0.89
2.72
1.70
Trace
Mw
Mw/Mn
Tm (ꢀC)
Xcd (%)
78.6
73.5
81.2
77.1
75.6
72.4
76.4
79.9
71.7
58.6
76.7
70.0
-
1
6.37
5.84
5.22
4.49
6.74
6.28
5.48
4.67
6.90
7.40
5.51
4.94
-
7.0
135.1
135.4
134.8
133.4
134.8
135.3
135.5
134.6
134.7
134.5
134.8
135.2
-
2
40
30
3.72
6.5
3
50
30
3.01
6.1
4
60
30
0.83
8.5
5
40
30
0.68
11.1
12.4
7.1
6
40
30
2.41
7
40
30
2.99
8
40
30
3.29
9.2
9
40
30
1.69
6.0
10
11
12
13
40
30
0.89
6.9
40
30
2.72
7.9
40
30
1.70
12.6
-
40
30
Trace
aConditions: 2.0 μmol of iron precatalyst, 30 mL toluene, 1 atm ethylene.
bValues in units of 106 g (PE) mol−1 (Fe) h−1
cDetermined by GPC, and Mw: 105 g mol−1
dDetermined by DSC, Xc = 100 × ΔHf (Tm)/[ΔHꢀf (Tꢀm)], where ΔHꢀf (Tꢀm) = 248.3 J g −1
.
.
.

LIU ET AL.
9 of 19
FIGURE 3 For Fe1/MAO at PC2H4 = 1 atm: catalytic activity and molecular weight of the polymer (a) as a function of reaction
temperature and (b) as a function of the Al:Fe molar ratio (Entries 1–8, Table 2)
Table 2), the catalytic activity attained a maximum value
of 3.72 × 106 g of PE (mol of Fe)−1 h−1 at 40ꢀC
(Figure 3a). In comparison with other iron catalysts at
1 atm pressure of ethylene, the catalytic activity can be
regarded as remarkably high.[56] When the temperature
was raised above 40ꢀC, the activity gradually decreased,
reaching a minimum value of 0.83 × 106 g of PE (mol of
Fe)−1 h−1 at 60ꢀC (Entry 4, Table 2). This lowering in cat-
alytic activity at higher temperature can be accredited to
partial deactivation of the active species and the lower sol-
ubility of ethylene at the higher temperature. In terms of
molecular weight of the polyethylene, the highest value of
6.37 × 105 g mol−1 was obtained at 30ꢀC, and as the tem-
perature was raised to 60ꢀC, it progressively decreased
from 6.37 to 4.49 × 105 g mol−1. This downward trend in
molecular weight can be attributed to increased chain
transfer at the higher temperature.[39,57–60] In all cases, the
melting temperatures (Tm) of the polymers remained
above 130ꢀC with only modest variations across the run
temperature range. For example, the representative DSC
curve of the polyethylene (Entry 2, Table 2) was shown in
Figure S2 with a Tm value as 135.4ꢀC, suggesting that the
polymers possessed a linear backbone.
Secondly, with the temperature of the polymerization
fixed at 40ꢀC, the Al:Fe molar ratio using Fe1/MAO was
varied between 1000:1 and 2500:1 (Entries 2 and 5–8,
Table 2). The uppermost catalytic performance of
3.72 × 106 g of PE (mol of Fe)−1 h−1 was achieved with a
Al:Fe ratio of 2000:1 (Figure 3b). It may be that at lower
ratios, the amount of co-catalyst was insufficient to fully
activate the catalyst, leading to lower activity. On the
other hand, at ratios that exceeded 2000:1, chain transfer
of the polymer from the active iron species to aluminum
could explain the decrease in catalytic activity. As for the
molecular weight of the polymer, this was found to
decrease from 6.74 to 4.67 × 105 g mol−1 as the molar
ratio of Al:Fe was increased, a finding that is in line with
chain transfer to aluminum occurring at a faster rate
than chain growth.[40,41,48]
Thirdly, using the optimal conditions established for
Fe1/MAO at PC2H4 = 1 atm (run temperature of 40ꢀC,
Al:Fe ratio of 2000:1, reaction time of 30 min), the
remaining precatalysts Fe2–Fe6 were additionally
screened for ethylene polymerization (Entries 9–13,
Table 2). Collectively, these catalysts exhibited activities
in the range, 0.89–3.72 × 106 g of PE (mol of Fe−1 h−1
)
and followed the order Fe1 (2,6-di (Me)) > Fe4 (2,4,6-tri
(Me)) > Fe5 (2,6-di (Et)-4-Me) > Fe2 (2,6-di (Et)) > Fe3
(2,6-di (iPr)) > Fe6 (2,6-di((4-OMeC6H4)2CH)-4-Me).
Based on this trend, it is evident that the catalytic perfor-
mance was affected by a combination of the steric hin-
drance and electronic properties of the ligand.[57–60] For
example, the least sterically bulky Fe1 displayed the
highest catalytic activity, whereas the molecular weight
of the polyethylene falls in the midrange (Mw range:
4.94–7.40 × 105 g mol−1). By comparison, the more bulky
isopropyl-containing Fe3 is less active but produced poly-
ethylene falling at the top end of the molecular weight
range.[61,62] The reason for the relatively high activity of
Fe1 is likely due to the greater accessibility to the active
site allowing for more facile monomer coordination and
insertion, resulting in a higher polymerization rate. By
comparison, Fe6, bearing the greatest steric hindrance of
this series of precatalysts, proved inactive. As a general
observation, all catalysts produced highly linear polymers
(Tm range: 134.5–135.4ꢀC) with relatively narrow dis-
persities (Mw/Mn range: 6.0–12.6).
b. Catalytic evaluation using Fe1–Fe6 with MAO at
PC2H4 = 10 atm

10 of 19
LIU ET AL.
TABLE 3 Ethylene polymerization results using Fe1–Fe6/MAO at PC2H4 = 10 atma
c
c
d
Entry
1
Precat.
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
T (ꢀC)
40
50
60
70
80
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
Al:Fe
2500
2500
2500
2500
2500
1500
2000
3000
3500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
t (min)
30
30
30
30
30
30
30
30
30
5
Mass of PE (g)
4.38
Act.b
5.84
8.28
10.11
11.11
5.08
7.79
9.49
11.03
10.41
18.72
13.68
9.05
7.82
3.53
0.31
4.92
4.47
4.95
3.96
0.24
Mw
Mw/Mn
11.7
6.2
Tm (ꢀC)
Xcd (%)
61.3
57.0
60.0
66.3
65.1
59.0
62.3
65.4
62.7
72.8
63.2
62.2
61.5
60.4
67.1
68.9
61.7
75.0
65.0
54.2
7.62
7.43
7.04
5.85
5.17
6.26
5.82
5.53
5.24
4.83
5.69
5.85
5.94
5.46
2.83
4.74
6.42
4.06
4.33
7.49
135.8
136.0
135.8
135.3
135.3
135.8
135.4
135.2
135.4
135.8
135.6
135.4
135.2
135.9
134.7
135.0
135.5
134.5
135.3
135.6
2
6.21
3
7.58
9.4
4
8.33
12.7
14.5
9.9
5
3.81
6
5.84
7
7.12
10.6
9.8
8
8.27
9
7.81
14.0
16.3
11.9
8.4
10
11
12
13
14e
15e,f
16
17
18
19
20
2.34
15
45
60
30
30
30
30
30
30
30
5.13
10.18
11.73
2.65
4.0
6.4
0.23
13.9
15.6
12.3
17.0
8.2
3.69
3.35
3.71
2.97
0.18
7.9
aConditions: 1.5 μmol of iron precatalyst, 100 mL toluene, 10 atm ethylene.
bValues in units of 106 g (PE) mol−1 (Fe) h−1
cDetermined by GPC, and Mw: 105 g mol−1
.
.
dDetermined by DSC, Xc = 100 × ΔHf (Tm)/[ΔHꢀf (Tꢀm)], where ΔHꢀf (Tꢀm) = 248.3 J g −1
.
e1 atm of ethylene.
f5 atm of ethylene.
FIGURE 4 For Fe1/MAO at PC2H4 = 10 atm: (a) plots of catalytic activity and molecular weight of the polymer versus reaction
temperature and (b) GPC traces of the polymer molecular weight as a function of temperature (Entries 1–5, Table 3)

LIU ET AL.
11 of 19
The ethylene pressure was then raised to 10 atm, and
(Mw range: 6.26–5.24 × 105 g mol−1) with a modest initial
increase, followed by a decrease as the molar ratio of Al:
Fe was increased (Figure 5).[40,41,48]
the catalyst optimization again performed using Fe1/
MAO; the results are collected in Table 3. When the run
temperature was increased from 40 to 80ꢀC with the Al:
Fe ratio fixed at 2500:1 (Entries 1–5, Table 3), the activity
gradually increased to a maximum value of 11.11 × 106 g
of PE (mol of Fe)−1 h−1 at 70ꢀC (Figure 4a). Above 70ꢀC,
an abrupt decline in activity was observed, which could
be down to catalyst deactivation.[39,57–60] Furthermore,
the polyethylene produced over the temperature range
40–70ꢀC exhibited a steady drop in molecular weight (Mw
To investigate the catalytic lifetime of the active spe-
cies derived from Fe1/MAO at 70ꢀC, the duration of the
polymerization run was varied between 5 and 60 min
(Entries 4 and 10–13, Table 3). Inspection of the data
reveals an upmost value in activity of 18.72 × 106 g of
PE (mol of Fe)−1 h−1 occurred after 5 min (Figure 6a).
When this reaction time was extended, the activity grad-
ually decreased with time. It would seem plausible that
for this catalyst, only a short time frame was needed to
fully generate the active species, whereas partial deacti-
vation sets in from the commencement of the polymeri-
zation.[16,29,41] Nonetheless, this catalyst still maintained
a high catalytic activity of 7.82 × 106 g of PE (mol of
Fe)−1 h−1 at the 1-h mark (Entry 13, Table 3), highlight-
ing the good lifetime of this catalyst. With reference to
the molecular weight of the resulting polyethylenes,
these were found to increase with time, reaching a
maximum value of 5.94 × 105 g mol−1 at 60 min
(Figure 6b).
To investigate the effect of pressure variation on Fe1/
MAO when operating under its optimal conditions at
PC2H4 = 10 atm, the polymerization runs were addition-
ally performed at 5 atm and 1 atm (Entries 4, 14, and
15, Table 3). It was found that the catalytic activity and
the molecular weight of the polyethylene rapidly dropped
with a decrease in pressure, reaching an activity low point
of 0.31 × 106 g of PE (mol of Fe)−1 h−1 at 1 atm and a
molecular weight of 2.83 × 105 g mol−1 (Entry 15, Table 3).
As a comparison between the data collected in Tables 2
(1 atm) and 3 (10 atm), it is evident that the activity of the
range: 7.62–5.85 × 105 g mol−1
)
that reduced to
5.17 × 105 g mol−1 on raising the run temperature to
80ꢀC (Figure 4b). When compared with the runs at
1 atm, all of the polymers obtained at higher pressure,
across the same temperature range, exhibited a higher
range in molecular weights. Consistent to above observa-
tion, the Tm values of obtained polyethylenes are
observed by the DSC measurement above 135ꢀC, such as
shown in Figure S3 (entry 4, Table 3), confirming the
high linearity of resultant polyethylenes.
As with the variable temperature studies, variations
in the Al:Fe molar ratio showed some noticeable effects
on the catalytic activities and the type of properties dis-
played by the polyethylenes. When the Al:Fe molar ratio
using Fe1/MAO was increased from 1500:1 to 3500:1
with the temperature kept constant at 70ꢀC (Entries
4 and 6–9, Table 3), an Al:Fe ratio of 2500:1 was found to
be optimal. Notably, when the molar ratio of Al:Fe was
increased to 3000:1 (Entry 8, Table 3), the activity only
slightly reduced, highlighting the stability of the catalyst
above a critical amount of MAO. In addition, only
modest variations in molecular weight were observed
FIGURE 5 For Fe1/MAO at PC2H4 = 10 atm: (a) plots of catalytic activity and molecular weight of the polymer versus Al:Fe molar ratio
and (b) GPC traces of the polymer molecular weight as a function of the molar ratio (Entries 4 and 6–9, Table 3)

12 of 19
LIU ET AL.
FIGURE 6 For Fe1/MAO at PC2H4 = 1 atm: (a) plots of catalytic activity and molecular weight of the polymer versus reaction time and
(b) GPC traces of the polymer molecular weight as a function of reaction time (Entries 4 and 10–13, Table 3)
FIGURE 7 For Fe1–Fe6 with MAO as co-catalyst and PC2H4 = 10 atm: (a) bar chart showing catalytic activity and molecular weight of
the polymer versus the type of iron precatalyst and (b) GPC traces of the polymer molecular weight as a function of the iron precatalyst
(Entries 4 and 16–20, Table 3)
catalyst under high pressure is superior, whereas the poly-
ethylene displays a higher molecular weight.
(Figure 7a). This order resembles that seen at 1 atm with
the least sterically bulky Fe1 and Fe4 the most active,
whereas the most bulky Fe6 the least. Some variation in
the order is seen for Fe2, Fe3, and Fe5, but this likely
reflects the modest variations in activities for these three
precatalysts seen at 1 atm pressure. Nevertheless, steric
and electronic properties of the ligands again prove
highly influential. In terms of electronic effects, compari-
son of the pairs Fe1 > Fe4 and Fe2 > Fe5 indicates that
the electron-donating effect of the para-methyl group on
the N-aryl ring has an adverse effect on the catalytic
performance of the precatalyst.[49,55] With regard to
Under the optimal reaction conditions obtained using
Fe1/MAO at 10 atm ethylene pressure (run temperature
of 70ꢀC, the Al:Fe molar ratio of 2,500:1), the five other
precatalysts (Fe2–Fe6) were evaluated for the polymeri-
zation of ethylene (Entries 16–20, Table 3). When Fe1 is
included in the comparison, a broad range in activities
was observed (0.24–11.1 × 106 g of PE (mol of Fe)−1 h−1)
that follows the order Fe1 (2,6-di (Me)) > Fe4 (2,4,6-tri
(Me)) > Fe2 (2,6-di (Et)) > Fe3 (2,6-di (iPr)) > Fe5
(2,6-di (Et)-4-Me) > Fe6 (2,6-di((4-MeOC6H4)2CH)-4-Me)

LIU ET AL.
13 of 19
the polymer molecular weight, a wide range (7.49–
4.06 × 105 g mol−1) was observed with Fe6/MAO for-
ming the highest molecular weight polymer
(Figure 7b).[61,62] In all cases, the polymers displayed
fairly broad dispersities (Mw/Mn range: 7.9–17.0).
two iron catalysts, with Fe1/MMAO displaying values
hundreds of times lower than with Fe1/MAO. A possible
explanation for this finding may be due the fact that
MMAO is more likely to initiate chain transfer to alumi-
num.[39,57–59] In addition, the polyethylene produced by
Fe1/MMAO displayed a narrower dispersity (Mw/Mn
range: 1.3–1.6) and a slightly lower melting temperature
range in line with the lower molecular weight polymer
(Tm range: 123.3–130.6ꢀC); the DSC image is shown in
Figure S4 for the polyethylene (Entry 2, Table 4).
To examine the influence of the number of molar
equivalents of co-catalyst, the Al:Fe molar ratio of
MMAO was varied between 1000:1 and 3000:1 (Entries
2 and 5–8, Table 4) with the reaction temperature fixed at
40ꢀC. A ratio of 2000:1 was identified as the optimum
with a peak activity of 3.11 × 106 g of PE (mol of
Fe)−1 h−1 (Entry 2, Table 4). At the lowest ratio of 1000:1,
the activity was 2.57 × 106 g of PE (mol of Fe)−1 h−1
(Entry 5, Table 4), which is larger to that seen with Fe1/
MAO when conducted using a comparable equivalents of
co-catalyst, highlighting the differences in the critical
amount of MAO and MMAO needed to deliver high
activity. Once again, the polyethylenes were of low
molecular weight (Mw range: 1.0–1.2 kg mol−1), narrow
molecular weight distribution (Mw/Mn range: 1.3–1.9)
with melting temperatures in the range 127.3–132.3ꢀC
that are in accord with highly linear polymers.
c. Catalytic evaluation using Fe1–Fe6 with MMAO at
PC2H4 = 1 atm
To complement the MAO study, Fe1–Fe6 were also
the subject of an ethylene polymerization investigation
with MMAO as co-catalyst at both 1 and 10 atm C2H4.
Once again, Fe1 was used as the test precatalyst to allow
an optimization of the reaction parameters. The results of
the polymerization tests performed at lower ethylene
pressure (1 atm) will be discussed initially.
Using a similar catalytic optimization approach to that
described earlier, the polymerization runs were first per-
formed at temperatures between 30 and 60ꢀC with the Al:
Fe molar ratio fixed at 2000:1 and the ethylene pressure at
1 atm (Entries 1–4, Table 4). The activity was observed to
peak at 3.11 × 106 g of PE (mol of Fe)−1 h−1 at 40ꢀC
(Entry 2, Table 4), which is consistent with the optimum
temperature seen using Fe1/MAO at 1 atm but with a
slightly reduced activity (cf. 3.72 × 106 g of PE [mol of
Fe]−1 h−1). Compared with Fe1/MAO, Fe1/MMAO dis-
played higher activity at 30ꢀC but a more rapid decrease
at 50ꢀC, reflecting the superior thermal stability of Fe1/
MAO. Perhaps more significantly are the differences in
molecular weights of the polymers obtained with these
With the optimum catalytic conditions established for
Fe1/MMAO (temperature of 40ꢀC, Al:Fe ratio of 2000:1),
the remaining iron complexes (Fe2–Fe6) were screened for
TABLE 4 Ethylene polymerization results using Fe1–Fe6/MMAO at PC2H4 = 1 atma
c
c
d
Entry
Precat.
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
T (ꢀC)
30
Al:Fe
2000
2000
2000
2000
1000
1500
2500
3000
2000
2000
2000
2000
2000
t (min)
30
Mass of PE (g)
2.45
Act.b
2.45
3.11
1.04
0.92
2.57
2.72
2.42
1.14
2.38
1.54
2.73
2.63
Trace
Mw
Mw/Mn
1.3
1.6
1.3
1.3
1.9
1.9
1.8
1.3
1.2
1.2
1.7
1.5
-
Tm (ꢀC)
Xcd (%)
61.3
54.7
41.4
11.3
65.0
56.7
47.9
35.8
56.1
70.1
63.6
61.9
-
1
0.94
1.16
0.99
0.99
1.16
1.12
1.06
0.97
0.88
1.37
1.08
1.29
-
130.6
130.6
128.5
123.3
132.3
130.0
129.3
127.3
131.2
133.6
131.5
130.8
-
2
40
30
3.11
3
50
30
1.04
4
60
30
0.92
5
40
30
2.57
6
40
30
2.72
7
40
30
2.42
8
40
30
1.14
9
40
30
2.38
10
11
12
13
40
30
1.54
40
30
2.73
40
30
2.63
40
30
Trace
aConditions: 2.0 μmol of iron precatalyst, 30 mL toluene, 1 atm ethylene.
bValues in units of 106 g (PE) mol−1 (Fe) h−1
cDetermined by GPC, and Mw: 105 g mol−1
dDetermined by DSC, Xc = 100 × ΔHf (Tm)/[ΔHꢀf (Tꢀm)], where ΔHꢀf (Tꢀm) = 248.3 J g −1
.
.
.

14 of 19
LIU ET AL.
TABLE 5 Ethylene polymerization results using Fe1–Fe6/MMAO at PC2H4 = 10 atma
c
c
d
Entry
1
Precat.
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
T (ꢀC)
40
50
60
70
80
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
Al:Fe
2500
2500
2500
2500
2500
1000
1500
2000
3000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
t (min)
30
30
30
30
30
30
30
30
30
5
Mass of PE (g)
3.42
Act.b
4.56
4.91
5.51
4.95
3.92
3.52
4.65
7.75
4.00
19.84
10.43
6.55
6.02
4.76
0.89
5.20
4.67
6.27
4.76
0.21
Mw
Mw/Mn
9.4
Tm (ꢀC)
Xcd (%)
61.1
68.2
81.2
78.1
91.1
60.3
72.7
77.9
76.5
86.7
61.5
74.4
78.5
76.7
65.6
76.0
75.2
80.3
66.5
60.1
5.08
4.77
2.60
1.81
0.80
3.52
3.88
2.68
2.37
1.74
3.81
4.21
4.24
1.54
0.99
3.12
3.38
2.94
4.16
5.06
136.1
135.3
134.2
133.4
132.2
136.1
136.3
134.2
134.7
135.1
136.6
134.2
135.6
133.2
123.3
134.1
136.5
134.7
136.5
135.8
2
3.68
12.5
20.6
16.4
8.2
3
4.13
4
3.71
5
2.94
6
2.64
5.9
7
3.49
4.6
8
5.81
10.3
9.4
9
3.00
10
11
12
13
14e
15e,f
16
17
18
19
20
2.48
10.8
5.4
15
45
60
30
30
30
30
30
30
30
3.91
7.37
22.9
23.5
4.9
9.03
3.57
0.67
1.3
3.90
22.9
11.8
18.2
6.2
3.50
4.70
3.57
0.16
9.1
aConditions: 1.5 μmol of iron precatalyst, 100 mL toluene, 10 atm ethylene.
bValues in units of 106 g (PE) mol−1 (Fe) h−1
cDetermined by GPC, and Mw: 105 g mol−1
.
.
dDetermined by DSC, Xc = 100 × ΔHf (Tm)/[ΔHꢀf (Tꢀm)], where ΔHꢀf (Tꢀm) = 248.3 J g −1
.
e1 atm of ethylene.
f5 atm of ethylene.
ethylene polymerization at 1 atm ethylene pressure (Entries
On increasing the polymerization temperature from
9–13, Table 4). Overall, the range in catalytic activities was
less than that seen with MAO (trace to 3.11 × 106 g of PE
[mol of Fe]−1 h−1) but found to follow the same order: Fe1
(2,6-di (Me)) > Fe4 (2,4,6-tri (Me)) > Fe5 (2,6-di (Et)-
4-Me) > Fe2 (2,6-di (Et)) > Fe3 (2,6-di (iPr)) > Fe6. It
would appear that irrespective of the aluminoxane co-
catalysts employed, the steric/electronic differences in these
precatalysts played a key role in controlling activity. As with
Fe6/MAO, Fe6/MMAO was inactive for ethylene polymeri-
zation (Entry 13, Table 4). With respect to the polymers,
these were of high linearity (Tm range: 130.6–133.6ꢀC), low
molecular weight (Mw range: 0.88–1.37 kg mol−1), and
narrow dispersity (Mw/Mn range: 1.2–1.7).
40 to 80ꢀC (Entries 1–5, Table 5), the maximum level of
activity was found to be 5.51 × 106 g of PE (mol of
Fe)−1 h−1 at 60ꢀC (Figure 8a). Compared with Fe1/MAO
under the same pressure, the activity for Fe1/MMAO
across this temperature range was noticeably lower,
highlighting a potential drawback of MMAO as co-catalyst
for activating this particular class of precatalyst. Likewise,
the range in molecular weights of the resulting
polyethylene was markedly reduced (Mw range: 0.80–
5.08 × 105 g mol−1), which can be credited to this
co-catalyst increasing the possibility of chain transfer from
iron to aluminum.[39,57–59] Nevertheless, compared with
published iron catalysts adopting similar structural types,
the current system still exhibited a significant improve-
ment in activity, thermal stability, and molecular weight of
the resulting polyethylene, regardless of whether MAO or
MMAO is used as co-catalyst.[63] Inspection of the GPC
traces in Figure 8b illustrates this downward trend in
molecular weight of polymer as the temperature is
raised and moreover highlights modality variations with
d. Catalytic evaluation using Fe1–Fe6 with MMAO at
PC2H4 = 10 atm
The ethylene pressure was then increased to 10 atm
and the optimum conditions again determined using
Fe1/MMAO; the results are compiled in Table 5.

LIU ET AL.
15 of 19
FIGURE 8 For Fe1/MMAO: (a) plots of catalytic activity and molecular weight of the polymer versus reaction temperature and (b) GPC
traces of the polymer molecular weight as a function of temperature (Entries 1–5, Table 5)
FIGURE 9 For Fe1/MMAO at PC2H4 = 10 atm: (a) plots of catalytic activity and molecular weight of the polymer versus Al:Fe molar
ratio and (b) GPC traces of the polymer molecular weight as a function of the molar ratio (Entries 3 and 6–9, Table 5)
unimodal polymers evident at 40 and 80ꢀC but broader
bimodal-like distributions at 60 and 70ꢀC. It would seem
likely that different chain transfer pathways are occurring
across the temperature range.[61]
Subsequently, the effect of varying the Al:Fe molar
ratio from 1000:1 to 3000:1 was probed with the tempera-
ture fixed at 60ꢀC (Entries 3 and 6–9, Table 5). A peak in
catalytic activity was achieved of 7.75 × 106 g of PE (mol
of Fe)−1 h−1 at an Al:Fe molar ratio of 2000:1 (Figure 9a).
In terms of the polymer molecular weight, this was
shown to decrease as the ratio of Al:Fe increased on
account of a more rapid rate of chain transfer from iron
to aluminum (Figure 9b).[40,41,48] It is also worthily
mentioned the high linear polyethylene obtained and
confirmed by the DSC measurement (Figure S5 for sam-
ple obtained by Entry 3, Table 5). Besides that, 13C
NMR spectrum of the polyethylene obtained using Fe1/
MAO at 70ꢀC (Entry 4, Table 4) was recorded in
1,1,2,2-tetrachloroethane-d2 at 100ꢀC and shown in
Figure S6 providing the direct evidence.
With the temperature set at 60ꢀC and the Al:Fe molar
ratio at 2000:1, the lifetime of Fe1/MMAO was studied
by conducting the polymerization runs over 5, 15, 30, 45,
and 60 min (Entries 8 and 10–13, Table 5). The highest
activity of 19.84 × 106 g of PE (mol of Fe)−1 h−1 was
achieved after 5 min after which it gradually decreased,

16 of 19
LIU ET AL.
reaching its lowest value of 6.02 × 106 g of PE (mol of
Fe)−1 h−1 after 60 min (Entries 10 and 13, Table 5). As
would be expected, the molecular weight of the polyeth-
ylene progressively increased, reaching its highest value
of 4.24 × 105 g mol−1 after 1 h. On reducing the ethylene
pressure to 5 atm and then 1 atm, the activity for Fe1/
MMAO and the molecular weight of the polymer gener-
ated were lowered (Entries 8, 14, 15, Table 5). Similarly,
inspection of the data presented in Table 4 (1 atm) and
Table 5 (10 atm) supports these pressure–activity and
pressure–molecular weight correlations.
Finally, under the favored operating conditions
established for Fe1/MMAO, the effect of ligand structure
on the performance of the iron catalyst was explored by
investigating the remaining precatalysts Fe2–Fe6
(Entries 16–20, Table 5). In comparison with the catalytic
performance with MAO (Entries 16–20, Table 3), a
similar trend in performance was observed at 10 atm eth-
ylene: Fe1 (2,6-di (Me)) > Fe4 (2,4,6-tri (Me)) > Fe2
(2,6-di (Et)) > Fe5 (2,6-di (Et)-4-Me) > Fe3 (2,6-di (iPr))
> Fe6 (2,6-di((4-OMeC6H4)2CH)-4-Me). Indeed, the
effect of steric hindrance and electronic properties on the
catalytic activity of the precatalyst was further under-
lined.[49,55,64] The range in molecular weights (Mw range:
2.68–5.06 × 105 g mol−1) for the polymers was narrower
than with MAO, whereas the dispersities were similar
(Mw/Mn range: 6.2–22.9).
3.3 | Mechanical properties of the
polyethylene
To confirm the linearity of the polymers suggested by the
values of the melting temperatures and to further explore
any other structural properties of the polyethylenes, the
polymeric materials obtained using Fe1/MAO (Entry
4, Table 3; Mw = 5.85 × 105 g mol−1) at 70ꢀC/10 atm and
Fe1/MMAO (Entry 8, Table 5; Mw = 2.68 × 105 g mol−1
)
at 60ꢀC/10 atm were selected as representative examples
and characterized by high-temperature 13C NMR spec-
troscopy (recorded at 100ꢀC in 1,1,2,2-tetrachloroethane-
d2) (see Figures 10 and S5). In both cases, a sharp singlet
at δ 30.00 was clearly visible that can be ascribed to the
carbon resonance for the −(CH2)n− repeat unit of a lin-
ear polyethylene; this high linearity is a characteristic for
iron polymerization catalysis.[65] No signals for the poly-
mer chain ends could be detected, which is likely due to
the high molecular weight of the polymer.
3.4 | Comparison of the current iron
catalysts with previously reported
examples
To permit a comparison of the polymers produced by
benzhydryl-containing bis(imino)pyridine-iron catalysts,
FIGURE 10 13C NMR spectrum of the polyethylene obtained using Fe1/MMAO at 60ꢀC/10 atm (Entry 8, Table 5); recorded in
1,1,2,2-tetrachloroethane-d2 at 100ꢀC

LIU ET AL.
17 of 19
FIGURE 11 Comparative molecular weight data for the polyethylenes obtained using F (Fe1) and previously reported iron precatalysts
B–E with MAO and MMAO as the co-catalysts; all polymerization runs were performed at PC2H4 = 10 atm under optimal conditions
Figure 11 collects together the molecular weight data for
polyethylenes generated using Fe1 (F, Chart 1) along
with that for B–E (Chart 1).[16,32,39,40] All these polymeri-
zation tests were carried out under optimized reaction
conditions using MAO or MMAO as activator at
PC2H4 = 10 atm. As a key finding from these data,
F incorporating the 4,40-dimethoxybenzhydryl groups
generates the highest molecular weight polyethylene of
the series with either co-catalyst. By comparing the clos-
est structural comparators, B (benzhydryl) and D (4,40-
difluorobenzhydryl), the positive effect of the introduc-
tion of methoxy groups to the benzhydryl periphery is
further emphasized. It would seem reasonable that given
the remoteness of the methoxy group from the active cen-
ter of the catalyst, the molecular weight enhancement is
electronic in origin. Nevertheless, the likely covalent
interactions of the methoxy group with the aluminoxane
co-catalyst may also play a role.
structurally related ferrous chloride precatalysts (Fe1–
Fe6) successfully prepared, characterized, and fully eval-
uated. In combination with MAO or MMAO, Fe1–Fe6
displayed a broad range in catalytic activities and pro-
duced highly linear polyethylenes with molecular
weights up to 7.62 × 105 g mol−1. As a key finding, the
least sterically encumbered precatalyst of the series, Fe1,
achieved the highest activity at 10 atm ethylene pressure
with MAO or MMAO; even at 1 atm, exceptionally high
activity was achieved. On the other hand, the most bulky
Fe6 proved the least active catalyst on account of the hin-
drance to ethylene coordination and insertion. In terms
of electronic variations, the para-methyl-containing Fe4
and Fe5 displayed lower activity than their para-protio-
containing analogues Fe1 and Fe2. Moreover, the pres-
sure and type of aluminoxane co-catalyst employed had
marked effects on the molecular weight of the polyethyl-
enes produced. When compared with polymers obtained
using a series of structurally related iron catalysts bearing
benzhydryl substitution (B–E, Chart 1), the propensity of
this class of iron catalyst class to form higher molecular
weight polymer has been demonstrated.
4 | CONCLUSIONS
The effect of introducing 2,6-bis(4,40-dimethoxyben
zhydryl)-4-methyl-substitued N-aryl groups to a bis
(arylimino)pyridine-iron ethylene polymerization catalyst
has been the subject of this investigation with six
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (No. 21871275). G.A.S. thanks the

18 of 19
LIU ET AL.
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[18] S. Zhang, Q. Xing, W.-H. Sun, RSC Adv. 2016, 6, 72170.
[19] Q. Mahmood, J. Guo, W. Zhang, Y. Ma, T. Liang, W.-H. Sun,
Organometallics 2018, 37, 957.
[20] C. Huang, A. Vignesh, C. Bariashir, Q. Mahmod, Y. Ma, Y.
Sun, W.-H. Sun, J. Polym, Sci., Part a: Polym. Chem 2019, 57,
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Chinese Academy of Sciences for a President's Interna-
tional Fellowship for Visiting Scientists.
AUTHOR CONTRIBUTIONS
Tian Liu: Data curation; investigation. Yanping Ma:
Investigation. Gregory Solan: Data curation. Tongling
Liang: Investigation. Wen-Hua Sun: Project administra-
tion; supervision.
CONFLICT OF INTEREST
All authors declared that they have no conflict of
interest.
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The data of this manuscript are available from the
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ORCID
Tian Liu https://orcid.org/0000-0002-7563-0838
Yanping Ma https://orcid.org/0000-0003-3106-9724
Gregory Solan https://orcid.org/0000-0002-9833-6666
Tongling Liang https://orcid.org/0000-0002-0669-9751
Wen-Hua Sun https://orcid.org/0000-0002-6614-9284
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
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