Controlled isoprene polymerization mediated by iminopyridine-iron (II) acetylacetonate pre-catalysts

Article abstract of DOI:10.1002/aoc.4836

A ligand controlled stereoselective polymerization of isoprene has been developed. A series of (aryl/alkyl)-iminopyridine iron (II) acetylacetonate complexes: (aryl?=?Ph Fe1; alkyl?=?CH₂Ph Fe2, CH (Ph)₂ Fe3, CH (Me)₂ Fe4, C (Me)₃ Fe5, C (Me)₂CH₂C(Me)₃ Fe6), has been prepared in which steric and electronic substituents systematically modified to investigate their influences for isoprene polymerization. The molecular structure of representative complex Fe2 was confirmed by single crystal X-ray diffraction and, revealed a distorted octahedral geometry at iron center. On treatment with methylaluminoxane (MAO), Fe1–Fe6 displayed low (Fe5 & Fe6) to high activities (Fe1–Fe4) with quantitative monomer conversion (>99%) for isoprene polymerization producing polyisoprene of high molecular weight (up to 2.0?×?10⁵?g/mol) and unimodal molecular weight distribution (1.4–3.3). Specifically, complex Fe2 (alkyl?=?CH₂Ph) displayed the highest activity of 7.0?×?10⁶?g (mol of Fe)^?1?h^?1 with 85% conversion of monomer over run time of 10?min at 25?°C. While, Fe6 catalyzed polyisoprene possessed high content of trans-1,4 unit (up to 87%). Furthermore, the influence of the reaction parameters and the nature of the ligands on the catalytic activities and microstructural properties of the polymer were investigated in detail.

Full text of DOI:10.1002/aoc.4836

Received: 4 December 2018
DOI: 10.1002/aoc.4836
Revised: 4 January 2019
Accepted: 7 January 2019
F U L L P A P E R
Controlled isoprene polymerization mediated by
iminopyridine‐iron (II) acetylacetonate pre‐catalysts
1
1
1
1,2
1,2
Mengmeng Zhao | Liang Wang | Qaiser Mahmood | Chuyang Jing | Guangqian Zhu
|
1
,2
1
1
Xianhui Zhang | Xiaowu Wang
| Qinggang Wang
1
Key Laboratory of Biobased Materials,
A ligand controlled stereoselective polymerization of isoprene has been devel-
oped. A series of (aryl/alkyl)‐iminopyridine iron (II) acetylacetonate com-
plexes: (aryl = Ph Fe1; alkyl = CH Ph Fe2, CH (Ph) Fe3, CH (Me) Fe4, C
Qingdao Institute of Biomass Energy and
Bioprocess Technology, Chinese Academy
of Sciences, Qingdao 266000, China
2
2
2
2
Center of Materials Science and
(
Me) Fe5, C (Me) CH C(Me) Fe6), has been prepared in which steric and
3 2 2 3
Optoelectronics Engineering, University
of Chinese Academy of Sciences, Beijing
electronic substituents systematically modified to investigate their influences
for isoprene polymerization. The molecular structure of representative complex
Fe2 was confirmed by single crystal X‐ray diffraction and, revealed a distorted
octahedral geometry at iron center. On treatment with methylaluminoxane
100049, China
Correspondence
Qinggang Wang and Xiaowu Wang, Key
Laboratory of Biobased Materials,
Qingdao Institute of Biomass Energy and
Bioprocess Technology, Chinese Academy
of Sciences, Qingdao 266000, China.
Email: wangqg@qibebt.ac.cn;
(MAO), Fe1–Fe6 displayed low (Fe5 & Fe6) to high activities (Fe1–Fe4) with
quantitative monomer conversion (>99%) for isoprene polymerization produc-
5
ing polyisoprene of high molecular weight (up to 2.0 × 10 g/mol) and
unimodal molecular weight distribution (1.4–3.3). Specifically, complex Fe2
wangxw@qibebt.ac.cn
6
−1
(
alkyl = CH Ph) displayed the highest activity of 7.0 × 10 g (mol of Fe)
h
2
Funding information
−1
with 85% conversion of monomer over run time of 10 min at 25 °C. While,
National Natural Science Foundation of
China, Grant/Award Number: 21702215;
CAS Hundred Talents Program, Grant/
Award Number: Y5100719AL; Major Sci-
ence and Technology Innovation Program
of Shandong Province, Grant/Award
Number: 2018CXGC1105
Fe6 catalyzed polyisoprene possessed high content of trans‐1,4 unit (up to
87%). Furthermore, the influence of the reaction parameters and the nature
of the ligands on the catalytic activities and microstructural properties of the
polymer were investigated in detail.
KEYWORDS
Iminopyridine ligand, iron (II) acetylacetonate complex, isoprene polymerization
1
| INTRODUCTION
through shape memory functional materials to medicinal
industries. For example, cis‐1,4 polyisoprene characteris-
As a valuable alternative material to the natural rubber,
polyisoprene has emerged as one of the most promising
tically resemble to the natural rubber being structurally
similar, therefore is highly demanded as alternative mate-
[1]
[2]
thermoplastic elastomers in the field. The excellent pro-
cessing and recycling properties of these synthetic poly-
mers make them superior over the classical volcanized
rubber. These properties can easily be controlled by
adjusting the microstructure of the polymer, particularly
by controlling the molecular weight and stereoselectivity
of the polymer and thus these polymers have found
widespread applications ranging from the tire industry,
rial for the tire industry. Meanwhile, trans‐1,4 polyiso-
prene is widely used as shape memory functional
materials due to having the similar properties to gutta‐
[
3]
percha rubber. Considering this industrially valuable
material, researchers are engrossed to develop new and
efficient methods for controlling the properties of iso-
prene polymerization. For this homogeneous transition
metal complexes have attracted much attention in term
Appl Organometal Chem. 2019;e4836.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 10
https://doi.org/10.1002/aoc.4836

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ZHAO ET AL.
of high activities and subtle control over the properties of
the polymer for olefin polymerization. For the isoprene
polymerization, titanium, lithium and rare earth metal
binary and ternary catalyst systems. These catalytic sys-
tems exhibited high activities with good stereoselectivity
and molecular weight.
[
40]
[4–7]
based catalysts
have been extensively studied in aca-
Generally, it is noted that counteranion plays key role
in regulating the catalytic activity, selectivity and mecha-
demia as well as in industry. Compared to the early tran-
sition metal catalysts or conventional Ziegler‐Natta
catalysts, few studies are found in which late transition
metal complexes used as catalysts for the isoprene poly-
[
41–43]
nism of transition metal catalyzed reaction.
In the
previous studies, Fe (acac) /additional donor/reducing
3
agent ternary systems have been reported for the isoprene
[8–10]
merization.
The discovery of α‐diimine‐Pd (II)−/‐Ni (II)
bis (phenylimino)pyridine‐Co (II)/Fe (II) based cata-
polymerization that exhibited moderate activity and
[11–23]
[44–46]
and
selectivity.
With a view to gain deeper insight and
to study the effect of auxiliary ligand into the catalytic
performance of iminopyridine iron complexes, in this
contribution, we reported a series of well‐defined
iminopyridine iron acetylacetonate catalysts for isoprene
polymerization. A detailed polymerization study is per-
formed to investigate the effect of different reaction con-
ditions on catalytic performance and polymer properties.
In addition, detail microstructural analysis of obtained
[24–27]
lysts
for the polymerization of olefins were found
as instrumental in the development in the late transition
based catalyst for olefin polymerization. Since then, the
well‐defined late transition metal complex began to
attract attention from scientific researcher in academia
as well as from industrial community to use these com-
plexes as catalysts for olefin polymerization, due to their
expedient preparation, high reactivity, single catalytic
behavior and controlled stereoselectivity. Among them,
iron metal based catalyst is an excellent choice because
of its low toxicity, low cost, friendly environment and sig-
nificant natural abundance. However, less attention has
given to the iron based catalysts for diene polymerization
because of their lower activities and selectivities. Since
1
13
polymer is performed by H/ C NMR spectroscopy.
Herein reported catalysts exhibited substantial enhanced
catalytic activities and controlled polymer properties.
2 | EXPERIMENTAL
2.1 | Materials
1
970, studies have focused on the complexing agent to
stabilized the active species and prevent them being
reduced, but the catalytic activity and selectivity were
All experiments were carried out under argon atmo-
sphere by using standard Schlenk techniques or in
[28–35]
low.
In 2002, Porri and his coworkers disclosed
(
bipy) FeEt complexes as stereospecific catalyst for iso-
glovebox. Fe (acac) , methylaluminoxane (MAO, 1.5 M
2
2
2
prene polymerization and produced highly selective 3,4
solution in toluene), triethylaluminum (AlEt , 1.0 M solu-
3
[36]
polyisoprene.
Later, Ricci et al. investigated iron (II)
tion in toluene) and triisobutylaluminum (Al(i‐Bu)3,
1.0 M solution in toluene) and starting materials of alde-
hyde and amines for the synthesis of ligands were pur-
chased and used without further purification. Toluene
was refluxed over sodium and distilled and stored over
molecular sieves under nitrogen. Hexane and dichloro-
methane were refluxed over calcium hydride and distilled
and stored over molecular sieves under argon conditions.
Commercial available isoprene was purchased and puri-
fied by distillation over calcium hydride before use. The
chloride catalysts bearing aliphatic and aromatic N,N‐
bidentate ligand framework for the polymerization of iso-
prene and other 1,3‐dienes. These catalyst displayed high
activities and generated polyisoprene with different
[37]
microstructural properties.
In 2012, the group of Ritter
employed iminopyridine Fe (II) chloride complexes in
conjunction with methylaluminoxane and [Ph C]
3
[
B(C F ) ] as a ternary catalyst system for isoprene and
6 5 4
other 1,3‐diene polymerization. At very low loading of
iron catalysts as low as 0.02% displayed high activity for
isoprene polymerization and produced polymer with dif-
ferent microstructure ranging from highly selective
molecular weights (M ) and molecular weight distribu-
n
tions (M /M ) of polymers were measured by high tem-
w
n
perature gel permeation chromatography (GPC) using a
PL‐GPC 220 chromatography and maintained at 150 °C
by using trichlorobenzene as eluent and polystyrene as
standard. NMR spectra were conducted on a Bruker
[38]
trans‐1,4 to cis‐1,4 polyisoprene.
Elsewhere, Chen
group also explored catalytic potential of iminopyridine
iron chloride complexes for isoprene polymerization and
exhibited moderate cis‐1,4 selectivity with high molecular
1
13
Advance 400 spectrometer at 298 K. H NMR and
C
[39]
weight.
As part of our on‐going project to establish the
NMR spectra of ligands and polyisoprene were recorded
correlation between the ligand framework and its effect
on catalytic performance, we have examined the fluori-
nated aryl‐iminopyridine Fe (II) complexes and systemat-
ically studied the polymerization of isoprene by using
in CDCl and trimethylsilane as internal reference. The
3
polyisoprene microstructure of the 1,4 and 3,4 ratio was
1
determined from H NMR of the 1,4 = CH signals at
5.15 ppm and the 3,4 = CH signal at 4.7 ppm. The
2

ZHAO ET AL.
3 of 10
1
3
trans/cis‐1,4 stereoisomer ratio was determined from
C
J = 7.6 Hz, 1H), 7.72–7.70 (m, 1H), 7.29–7.25 (m, 1H),
3.65–3.59 (m, 1H), 1.26 (d, J = 6.4 Hz, 6H); C NMR
13
NMR of the –CH signals of cis‐1,4 at 23.8 ppm and
3
trans‐1,4 at 16.3 ppm. Mass spectra for Fe complexes were
detected using ACQUITYTM UPLC & Q‐TOF MS Pre-
mier. Elemental analysis was recorded using Vario EL
III elemental analyzer. X‐ray diffraction data was per-
formed on Smart 1000 diffractometer with Mo K‐alpha
X‐ray source (λ = 0.71073 Å) at 150 K. Attenuated total
reflection‐ infrared (ATR‐IR) spectroscopy was conducted
using Thermo Scientific Nicolet iN10.
(100 MHz, CDCl , 298 K) δ: 159.3, 154.9, 149.5, 136.6,
124.6, 121.4, 61.5, 24.1.
3
2
L5
.2.5 | 2‐(tert‐butyliminomethyl)pyridine
1
Colorless oil, 1.2 g, 72% yield; H NMR (400 MHz, CDCl ,
3
2
98 K) δ: 8.63–8.62(m, 1H), 8.36 (s, 1H), 8.02 (d, J = 8.0 Hz,
H), 7.74–7.70 (m, 1H), 7.30–7.26 (m, 1H), 1.31 (s, 9H);
1
13
2.2 | Synthesis and characterization of
C NMR (100 MHz, CDCl , 298 K) δ: 156.6, 155.7,
3
ligands (L1–L6)
149.4, 136.7, 124.5, 121.1, 58.0, 29.7.
All the ligands are known and can be prepared using a
[38]
1
13
2.2.6 | 2‐(2,4,4‐trimethylpentan‐2‐
yliminomethyl)pyridine L6
procedure reported in the literature.
The H and
C
NMR spectra of these ligands agreed with reported ones.
1
Light yellow oil, 1.6 g, 80% yield; H NMR (400 MHz,
CDCl , 298 K) δ 8.64–8.59 (m, 1H), 8.34 (s, 1H), 8.04–
2
.2.1 | 2‐(phenyliminomethyl)pyridine L1
3
8
.02 (m, 1H), 7.74–7.70 (m, 1H), 7.30–7.27 (m, 1H), 1.71
1
13
Yellow oil, 2.0 g, 58% yield; H NMR (400 MHz, CDCl ,
(s, 2H), 1.33 (s, 6H), 0.95 (s, 9H); C NMR (100 MHz,
CDCl , 298 K) δ 155.9, 155.8, 149.2, 136.6, 124.3, 120.7,
61.6, 56.5, 32.1, 31.8, 29.6.
3
2
1
98 K) δ: 8.75–8.69 (m, 1H), 8.61 (s, 1H), 8.21 (d, J = 8.0 Hz,
H), 7.84–7.80 (m, 1H), 7.46–7.35 (m, 3H), 7.31–7.27
3
13
(m, 3H); C NMR (100 MHz, CDCl , 298 K) δ 160.6,
3
1
54.5, 150.9, 149.6, 136.7, 129.2, 126.7, 125.1, 121.9, 121.1.
2.3 | Synthesis and characterization of
iron complexes
2
.2.2 | 2‐(benzyliminomethyl)pyridine L2
The corresponding ligand and Fe (acac) were added in
2
1
Yellow oil, 2.5 g, 68% yield; H NMR (400 MHz, CDCl ,
anhydrous dichloromethane (10 ml) at room tempera-
ture. The reaction mixture was stirred for 24 hr at same
temperature. After that, reaction mixture concentrated
to 2 ml and hexane was then added to induce precipita-
tion and the precipitate collected by filtration, washed
with hexane and dried under vacuum to give the corre-
sponding complex.
3
2
98 K) δ 8.64–8.63 (m, 1H), 8.49 (s, 1H), 8.06 (d, J = 7.6 Hz,
H), 7.74–7.69 (m, 1H), 7.74–7.69 (m, 6H), 4.87 (s, 2H);
1
13
C NMR (100 MHz, CDCl , 298 K) δ 162.9, 154.6, 149.4,
3
1
38.7, 136.6, 128.6, 128.2, 127.2, 124.8, 121.4, 65.0.
2.2.3 | 2(diphenylmethyliminomethyl)
pyridine L3
2
.3.1 | 2‐(phenyliminomethyl)pyridyliron
1
White solid, 1.1 g, 75% yield; H NMR (400 MHz, CDCl ,
(II) acetylacetonate Fe1
3
2
(
4
98 K) δ: 8.58 (d, J = 4.4 Hz, 1H), 8.53 (s, 1H), 8.19
d, J = 8.0 Hz, 1H), 7.67–7.64 (m, 1H), 7.40 (d, J = 7.2 Hz,
H), 7.31 (t, J = 7.2 Hz, 4H), 7.23–7.20 (m, 3H), 5.70
−
1
Brown solid, 75 mg, 44% yield. ATR‐IR (cm ): 1582
ν(C=N), 1514, 1491, 1395, 1257, 1014, 920, 784, 762. TOF‐
MS‐ES+ (m/z): calcd. For [C H FeN O ] : 436.1085,
13
+·
(s, 1H); C NMR (100 MHz, CDCl , 298 K) δ: 162.0,
3
22 24
2 4
1
1
54.7, 149.3, 143.3, 136.4, 128.5, 127.7, 127.1, 124.8,
21.5, 77.7.
found: 436.1095. Anal.: calcd. For C H FeN O ·H O: C,
58.16; H, 5.77; N, 6.17; found: C, 57.82; H, 5.75; N, 6.15.
22 24 2 4 2
2
L4
.2.4 | 2‐(isopropyliminomethyl)pyridine
2.3.2 | 2‐(benzyliminomethyl)pyridyliron
(II) acetylacetonate Fe2
1
−1
Colorless oil, 2.1 g, 68% yield; H NMR (400 MHz, CDCl ,
Brown solid, 189 mg, 54% yield. ATR‐IR (cm ):
3
2
98 K) δ: 8.61 (d, J = 4.40 Hz, 1H), 8.37 (s, 1H), 7.96 (d,
1578ν(C=N), 1511, 1400, 1360, 1014, 921, 773, 763. TOF‐

4
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ZHAO ET AL.
+
·
MS‐ES+ (m/z): calcd. For [C H FeN O ] : 450.1242,
2.4 | Polymerization procedure
23
26
2 4
found: 450.1244. Anal.: calcd. For C H FeN O ·H O:
23
26
2
4
2
C, 58.99; H, 6.03; N, 5.98; found: C, 59.18; H, 6.12; N,
.98. X‐ray quality crystals were obtained from slow
evaporation of dichloromethane.
The polymerization of isoprene in toluene was carried out
in a 25 ml Schlenk reactor. In a typical process, the reac-
tor was dried in vacuum and refilled back with argon for
more than three times. The iron complex was weighed in
glove box and then added into a Schlenk reactor. The
required amount of solvent, cocatalyst and isoprene was
sequentially added into the reactor under argon atmo-
sphere. After the required time of reaction, the polymeri-
zation quenched with HCl solution in methanol
5
2
.3.3 | 2‐(diphenylmethyliminomethyl)
pyridyliron (II) acetylacetonate Fe3
−
1
Brown solid, 80 mg, 39% yield. ATR‐IR (cm ):
581ν(C=N), 1513, 1449, 1398, 1012, 919, 768, 740, 710.
(
methanol/HCl = 50/1). The resulting suspension was
1
poured into a large volume of methanol containing 2,6‐
bis(1,1‐dimethylethyl)‐4‐methylphenol (BHT) as the sta-
bilizing agent. The polymer was collected by filtration
and washed with ethanol for several times and dried
under vacuum at room temperature until no change
was observed in weight. The polymer yields were deter-
mined by gravimetry.
+·
TOF‐MS‐ES+ (m/z): calcd. For [C H FeN O ] :
29
30
2 4
5
26.1555, found: 526.1564. Anal.: calcd. For
C H FeN O ·H O: C, 63.98; H, 5.92; N, 5.15; found: C,
29
30
2
4
2
6
3.62; H, 6.07; N, 5.01.
2
.3.4 | 2‐(isopropyliminomethyl)
pyridyliron (II) acetylacetonate Fe4
3
| RESULTS AND DISCUSSION
.1 | Synthesis and characterization of
−
1
Brown solid, 160 mg, 67% yield. ATR‐IR (cm ):
581ν(C=N), 1511, 1442, 1394, 1259, 1013, 921, 775,760.
3
1
+·
iron complexes (Fe1‐Fe6)
TOF‐MS‐ES+ (m/z): calcd. For [C H FeN O ] :
19
26
2 4
4
02.1242, found: 402.1250. Anal.: calcd. For
A series of 2‐(aryl/alkyliminomethyl)pyridyliron (II) ace-
C H FeN O ·H O: C, 54.30; H, 6.72; N, 6.67; found: C,
5
19
26
2
4
2
tylacetonate complexes: (aryl = Ph Fe1; alkyl = CH Ph
2
5.30; H, 6.12; N, 6.73.
Fe2, CH (Ph)2 Fe3, CH (Me)2 Fe4, C (Me)3 Fe5, C
(
Me) CH C(Me) Fe6), has been prepared in moderate
2 2 3
yields (39–67%) by the treatment of Fe (acac) with the
2
corresponding 2‐(aryl/alkyliminomethyl)pyridine ligands
2
.3.5 | 2‐(tert‐butyliminomethyl)
under argon atmosphere (Scheme 1). The ligands were
pyridyliron (II) acetylacetonate Fe5
[38]
prepared by two‐step procedure reported elsewhere
1
13
−
1
and characterized by H/ C NMR spectroscopy. The
identity and purity of complexes were determined by
ATR‐IR and mass spectroscopies, and elemental analysis.
Additionally, the molecular structure of Fe2 has been
established by X‐ray diffraction analysis.
Yellow solid, 155 mg, 63% yield. ATR‐IR (cm ):
582ν(C=N), 1511, 1442, 1394, 1259, 1014, 921, 776,755.
1
+
·
TOF‐MS‐ES+ (m/z): calcd. For [C H FeN O ] :
20
28
2 4
4
16.1398, found: 416.1409. Anal.: calcd. For
C H FeN O ·H O: C, 55.31; H, 6.96; N, 6.45; found: C,
20
28
2
4
2
X‐ray quality crystals of complex Fe2 was obtained by
the slow evaporation of its dichloromethane solution
under air at room temperature, indicating obtained crys-
tals were air and moisture stable. The prospective view
of molecular structure of Fe2 is shown in Figure 1. The
structure of Fe2 is mononuclear species having hexa‐
coordinate geometry best described as distorted octahe-
dron: the N2imine, O1acac, O2acac and O4acac atoms form
the basal square plane while N1pyridine and O3acac atoms
occupy the apical position. The O3‐Fe1‐N1 angle
[163.71(9)°] is modestly bend from the linear, it might
be due to the ring strain induced by chelation of
iminopyridine and acetylacetone anions with metal cen-
ter. More distortions are observed in the O2‐Fe1‐N2 angle
5
5.79; H, 6.78; N, 6.29.
2
.3.6 | 2‐(2,4,4‐trimethylpentan‐2‐
yliminomethyl)pyridyliron (II) acetylace-
tonate Fe6
−
1
Green solid, 152 mg, 41% yield. ATR‐IR (cm ):
1
582ν(C=N), 1511, 1442, 1394, 1259, 1014, 921, 776,755.
+
·
TOF‐MS‐ES+ (m/z): calcd. For [C H FeN O ] :
24
36
2 4
4
72.2024, found: 472.2022. Anal.: calcd. For
C H FeN O ·H O: C, 58.78; H, 7.81; N, 5.71; found: C,
5
24
36
2
4
2
7.95; H, 7.26; N, 5.63.

ZHAO ET AL.
5 of 10
SCHEME 1 Synthesis of 2‐(aryl/alkyliminomethyl)pyridine ligands and their iron (II) acetylacetonate complexes
3.2 | Isoprene polymerization
In order to determine the catalytic potential of the iron
II) acetylacetonate (Fe1–Fe6) in isoprene polymeriza-
tion, preliminary tests were performed using solely
Fe2 with range of different aluminumalkyl
(
a
cocatalysts, including methylaluminoxane (MAO),
triisobutylaluminum (Al(i‐Bu) ) and triethylaluminum
3
(
AlEt ). Typically, the tests were performed at 25 °C in
3
toluene (5 mL) with the ratio [Fe]/[Ip]/[Al] of 1/2000/
00 over different period of time to reach the maximum
5
yield; the results of this initial screening are presented in
Table 1. Examination of the data, reveals Fe2 is active
with only MAO cocatalyst offering quantitative yield
(
>99%) over run time of 10 min, while with other two
cocatalysts, was essentially inactive (Table 1, entries 2 &
). On the basis of these findings, subsequent more
FIGURE 1 The molecular structure of complex Fe2
3
in‐depth studies focused on the use of MAO as the
co‐catalyst for the activation of all other complexes
(Fe1, Fe3–Fe6). Worthy to mention that this is first
example of iron (II) catalysts in which well‐defined
iminopyridine iron (II) acetylacetonate complex used for
olefin polymerization.
With the intent to establish a correlation of structural
variations in the pre‐catalysts with their catalytic perfor-
mance, all the iron complexes were employed for iso-
prene polymerization and the resulting polymerization
data are collected in the Table 2 (Figure 2). Upon activa-
tion with MAO cocatalyst, the pre‐catalysts (Fe1–Fe6)
displayed good to excellent activities (Table 2, entries
[
158.95(8)°], however, O4‐Fe1‐O1 angle exhibited more
linear features [176.49(8)°]. In addition, modest varia-
tions in the bite angles of acetylacetonate anion was
observed as O1‐Fe1‐O2 is smaller than O3‐Fe1‐O4 angle
of [83.74(8)° vs. 86.24(8)° respectively] can be ascribed
to the steric hindrance induced by the bulky N‐aryl
group. In comparison with structurally related iron com-
plexes bearing chloride auxiliary ligands instead of acetyl-
acetonate reported by Ritter and his co‐workers,
significant variations in the band angles and lengths
were observed. For instance, the bite angle N1‐Fe1‐N2
[38]
some
[
74.57(9)°] in Fe2 is found to be smaller than that was
observed in the iron (II) chloride analogue [78.3(2)°].
1–6). On the other hand, Fe (acac) was found to be
2
Similarly, the Fe‐Nimine and Fe‐Npyridine bond distances
essentially inactive toward the isoprene polymerization
highlighting the key role of ligand in stabilizing the active
species and thereby high activities (Table 2, entry 7).
The overall catalytic activities of Fe1–Fe6 decreased
[
Fe1‐N1 2.312(2) Å and Fe1‐N2 2.224(2) Å respectively]
are all longer than that have been reported for iron (II)
chloride complex [Fe1‐N2A 2.121(6) Å and Fe1‐N1A
2
.119(5) Å) respectively], possibly strong negative elec-
in the order: Fe1 [Ph] ∼ Fe2 [CH Ph] ∼ Fe3 [CH
2
tronic effect of chloride as compared to the acetylaceto-
nate is influential to induce these structural variations.
(Ph) ] ∼ Fe4 [CH (Me) ] > Fe5 [C (Me) ] > Fe6 [C
2
2
3
(Me) CH C(Me) ]. Though complexes Fe1–Fe4 were
2
2
3

6
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ZHAO ET AL.
a
TABLE 1 Isoprene polymerization using Fe2 with different co‐catalysts
e
b
c
d
Microstructure(%)
Yield
(%)
Act.
M
n
M
M
w
/
−
5
−4
d
Entry
Cocat.
Time
(×10
)
(×10
)
n
trans‐1,4
cis‐1,4
3,4
1
2
3
MAO
10 min
16 h
>99
0
8.2
6.7
2.2
10
‐
36
‐
54
‐
Al(i‐Bu)
3
‐
‐
‐
‐
‐
‐
AlEt
3
16 h
0
‐
‐
‐
a
General conditions: catalyst Fe2; [Ip] = 4.0 mol/L; [Fe]/[Ip]/[cocat.] = 1/2000/500; toluene (5 ml); reaction temperature 25 °C.
b
Isolated yield.
Polymer (mol of Fe)
c
−1 −1
g
h .
d
Determined by gel permeation chromatography (GPC).
e
1
13
Determined by H and C NMR.
a
TABLE 2 Isoprene polymerization using Fe1–Fe6/MAO and Fe (acac)
2
/MAO
e
b
c
d
Microstructure(%)
Yield
(%)
Act.
M
n
M
M
w
/
−
5
−4
d
Entry
Cat.
Time
(×10
)
(×10
)
n
trans‐1,4
cis‐1,4
3,4
1
2
3
4
5
6
7
Fe1
10 min
10 min
10 min
10 min
2 h
>99
>99
>99
>99
>99
55
8.2
8.2
8.2
8.2
0.7
0.4
‐
4.3
6.7
6.5
5.8
1.7
1.2
‐
3.3
2.2
2.7
2.7
3.0
2.0
‐
0
51
36
55
39
12
3
49
54
42
45
14
10
‐
Fe2
10
3
Fe3
Fe4
16
74
87
‐
Fe5
Fe6
2 h
Fe (acac)
2
16 h
‐
‐
a
General conditions: [Ip] = 4.0 mol/L; [Fe]/[Ip]/[MAO] = 1/2000/500; toluene (5 ml); reaction temperature 25 °C.
b
c
Isolated yield.
Polymer (mol of Fe)
−1 −1
g
h .
d
Determined by gel permeation chromatography (GPC).
e
1
13
Determined by H and C NMR.
the iron active species in favour of high activities. Mean-
while, it is also noted that the electronic effect through
resonance also affect the molecular weights of resultant
polymer: Fe2 [CH Ph] and Fe3 [CH (Ph) ] based poly-
2
2
mers exhibited the highest molecular weight (Table 2,
entries 2 & 3). As can be seen in the GPC curves in
Figure 3, molecular weight distributions are unimodal
(
M /M = 2.0–3.3) suggesting single‐site catalytic behav-
w
n
ior of title iron complexes systems. The microstructures of
1
13
the resultant polyisoprene were analyzed by H/ C NMR
1
spectroscopy and their H NMR spectra can be seen in
the Figure 4. In general, all complexes showed poor to
good regio‐selectivities. The Fe1–Fe4 based polyisoprene
mainly composed of cis‐1,4 and 3,4 units, particularly
polymer obtained using Fe1 possessed nearly equal
amount of cis‐1,4 (51%) and 3,4 (49%) units. However,
in case of Fe5 and Fe6 based polymer contained notable
amount of trans‐1,4 units (74% and 87% respectively)
with small amount of cis‐1,4 and 3,4 units (Table 2,
entries 5 & 6). In comparison with structurally related
FIGURE 2 Activities and conversion of isoprene polymerization
using different precatalysts (Table 2, entries 1–7)
the standout performers showing the highest activities
5
−1 −1
(
up to 8.2 × 10 g
(mol of Fe) h ). The precise
Polymer
explanation behind these differences in catalytic perfor-
mance remain unclear but would be relate to the negative
inductive effect of imino group increasing the stability of

ZHAO ET AL.
7 of 10
for 100% conversion (Table S1, entries 1–4). Similar
results were found when compared the activities with
[
38]
[39]
catalysts reported by the groups of Ritter
and Chen.
It can be assumed that titled catalysts bearing acetylaceto-
nate instead of halide would have impact on the activa-
tion process and stability of active species; therefore
higher activities were achieved. Concerning microstruc-
tural properties, comparatively poor regio‐selectivity of
monomer enchainment was obtained with the titled
pre‐catalysts. Meanwhile, the catalytic activities of
pre‐catalysts (Fe1–Fe4) are substantially higher than
[
9,39,40]
reported cobalt halide analogues.
Further polymerization tests were performed using
the Fe2 and Fe6 to examine the effect of different amount
of cocatalyst and reaction temperature. The resulting
polymerization data is collected in the Table 3. Changes
in the amount of cocatalyst showed moderate effect on
the catalytic activities and properties of the resulting poly-
mer. When Al/Fe2 ratio was decreased from 500 (Table 2,
entry 2) through 100 to 50 and temperature kept at 25 °C,
catalytic activities remained unchanged (Table 3, entries
FIGURE 3 GPC curves of polyisoprene obtained using Fe1‐Fe6/
MAO catalyst system (Table 2, entry 1–6)
1
–2). On further decreasing the Al/Fe ratio to 10, the
5
activity slightly decreased to 8.0 × 10 g
Fe)
(mol of
Polymer
−
1
−1
h
(Table 3, entry 3), but still it maintained high
activity even on small amount of cocatalyst loadings.
However, no polymer at all was obtained with Al/Fe ratio
of 5 (Table 3, entry 4). The number molecular weight con-
sistently increased with decrease of cocatalyst loading,
5
but reached to a lower value of 1.1 × 10 g/mol at an
Al/Fe ratio of 50 (Table 3, entry 2). On further decreasing
of Al/Fe ratio, number molecular weight again increased
5
to 2.0 × 10 g/mol (Table 3, entry 3) and the variations in
number molecular weight might be due to the mass
transport problems as the catalyst becomes increasingly
embedded in the polymer. In the similar way, negligible
influence of variations in the cocatalyst loadings on
the molecular weight distributions and regio‐selectivities
of the resultant polymer was observed. With the fixed
Al/Fe ratio at 50, thermal stability of Fe2/MAO was
examined by performing polymerization tests at different
temperatures namely −30 and 50 °C (Table 3, entries
5 & 6). Interestingly, catalytic activities remained high
1
FIGURE 4 H NMR spectra of the polyisoprene obtained with
Fe1–Fe6 (Table 2, entries 1–6)
iron (II) chloride complexes bearing N‐octamyl group
reported by Chen et al., Fe6 showed slightly lower cata-
lytic performance regarding activity, molecular weight
and distribution. However, polyisoprene showed better
trans‐1,4 regio‐selectivities (trans‐1,4 units of 87% vs.
cis‐1,4 units of 77%). This would be due to the different
activation process as title complexes contained acetylace-
tonate instead of chloride, stability of active species and
different counter ion.
In comparison with previously reported related iron,
herein reported catalysts showed a better catalytic perfor-
mance in term of activities. For instance, the precatalysts
bearing very similar ligand support to the previously pre‐
catalysts reported by our group showed lower activates as
[39]
5
−1 −1
(activity: 8.2 × 10 g
(mol of Fe) h ) as was
Polymer
observed at 25 °C indicating high thermal stability of
active species either polymerization perform at very low
temperature of −30 °C or at high temperature of 50 °C.
The molecular weight of polyisoprene was reduced from
5
−1
5
−1
11.5 × 10 g mol to 8.6 × 10 g mol on raising the reac-
tion temperature from 25 °C to 50 °C. It would be due to
the high rate of chain transfer reaction at elevated tem-
perature. For the obtained polyisoprene, cis‐1,4−/3,4‐
units with ratio of 1/1 was the major part of the resultant
polyisoprene when the reaction temperature decreased to
[
40]
required 2 h reaction time for complete conversions,
while iron catalysts reported herein only required 1 min

8
of 10
ZHAO ET AL.
a
TABLE 3 Isoprene Polymerization under different reaction conditions
e
b
c
d
Microstructure(%)
[
Fe]/
Time
(min)
Yield
(%)
Act.
M
n
M
w
/
−
5
−4
d
Entry
Cat.
[MAO]
(×10
)
(×10
)
M
n
trans‐1,4
cis‐1,4
3,4
1
2
3
4
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe2
Fe6
Fe6
Fe6
Fe6
1/100
1/50
1/10
1/5
10
>99
>99
98.5
0
8.2
8.2
8.0
‐‐
12.8
11.5
19.6
‐‐
1.7
1.9
1.8
‐‐
9
37
39
41
‐‐
54
52
53
‐‐
10
9
10
6
10
‐‐
f
5
1/50
1/50
1/50
1/500
1/500
1/500
1/500
10
>99
>99
85
8.2
8.2
69.3
0.02
0.1
0.3
0.3
10.7
8.6
2.3
2.1
1.8
1.4
1.9
1.9
1.7
0
45
38
38
32
7
55
49
53
32
13
10
9
g
6
10
13
9
h
7
10
11.9
0.6
f
8
120
120
180
300
2.5
36
80
86
86
g
9
19.8
75
0.7
1
0
1
1.5
4
1
92
1.9
5
a
General conditions: [Fe]/[Ip] = 1/2000, [Ip] = 4.0 mol/L; toluene (5 ml), 25 °C.
b
Isolated yield.
Polymer (mol of Fe)
c
−1 −1
g
h .
d
Determined by gel permeation chromatography (GPC).
e
1
13
Determined by H and C NMR.
f
30 °C.
g
50 °C.
h
[Fe]/[Ip] = 1/20000, [Ip] = 4.0 mol/L; toluene (50 ml), 25 °C.
−
30 °C. These finding suggesting that higher temperature
favour trans‐1,4 selectivity which can be switched to
cis‐1,4/3,4 selectivity by decreasing the reaction tempera-
ture. Meanwhile, when the catalyst loading of Fe2 was
decreased to 0.005%, catalytic activities substantially
5
5
improved from 8.2 × 10 to 69.3 × 10 g
Fe) h .
(mol of
Polymer
−
1
−1
Considering the high selectivities obtained by Fe6, the
influence of reaction temperature and time on the cata-
lytic performance of this catalyst was additionally studied
(Table 3, entries 8 & 9). The change in temperature
showed a negative effect on the catalytic activities as well
as the properties of the resultant polymer (Table 3, entry 8
&
9). At −30 °C, the selectivity of trans‐1,4 units signifi-
cantly reduced from 87% (25 °C) to 36% (Table 3, entry 8).
Similarly, this selectivity slightly reduced to 80%, when
polymerization performed at 50 °C (Table 3, entry 9).
On prolonging the reaction time to 3 h and 5 h while
other conditions including Al/Fe ratio and temperature
fixed at 500 and 25 °C, monomer conversion gradually
improved to 92% over run time of 5 h highlighting longer
lifetime of the active species. Meanwhile, no effect of
reaction time was observed on the properties of the
obtained polymer (Figure 5).
FIGURE 5 Variation in conversions of isoprene and trans‐
,4 units of polymer over different run times (Table 2, entry 6 &
1
Table 3, entries 10 & 11)
for these iron catalyzed isoprene polymerization as given
in the Scheme S1 (see Supporting information). The poly-
mer obtained by title iron catalysts being different from
the previously reported iron chloride catalyzed isoprene
[
39]
Bearing in mind the microstructural properties of the
obtained polyisoprene, we proposed a general mechanism
polymerization,
indicates a different activation process
thus lead to different counterions. Upon activation with

ZHAO ET AL.
9 of 10
MAO, a heterobimetallic intermediate I or II (Scheme S1)
would form during the activation process: two metals
centers (Fe and Al species) connected through
acetylacetone in which one oxygen atom of acetylacetone
coordinated to iron center and remaining oxygen atom
of isoprene polymerization mechanism is currently in
progress in our laboratories.
ACKNOWLEDGEMENTS
[10]
bonded to aluminum center.
The type of monomer
Generous support by the Major Science and Technology
insertion and subsequent monomer enchainment are
the key steps that determine the end structural properties
of the polymer. Following the initial coordination of
monomer at vacant site of iron center either through
Innovation
Program
of
Shandong
Province
(2018CXGC1105), CAS Hundred Talents Program
(Y5100719AL), Young Taishan Scholars Program of
Shandong Province, “135” Projects Fund of CAS‐QIBEBT
Director Innovation Foundation, DICP& QIBEBT United
Foundation (UN201701) and National Natural Science
Foundation of China (21702215) is gratefully
acknowledged.
4
4
cis‐η or trans‐η coordination (III and IV in Scheme S1)
and insertion of coordinated monomer to Fe‐alkyl bond
3
3
leading to the formation of anti‐η or syn‐η allyl‐Fe inter-
mediates (V and VI in Scheme S1) respectively, and then
multiple coordination–insertion steps proceed leading to
chain growth (propagation step). In the case of multiple
ORCID
4
cis‐η type coordination‐insertion (VII), the produced
polyisoprene would be expected to be cis‐1,4‐unit if no
isomerization of syn‐η allyl type of chain end occurs.
Xiaowu Wang https://orcid.org/0000-0001-7916-0100
Qinggang Wang https://orcid.org/0000-0003-1851-7078
3
If multiple coordination‐insertion of isoprene was hap-
pened as in VIII (Scheme S1), then trans‐1,4 enriched
polymer is likely; if as is in IX or/and X (Scheme S1),
then 3,4‐polyisoprene is likely to achieve. In the case
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[
2