Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: Sterically controlled monomer enchainment

Article abstract of DOI:10.1039/c9dt00452a

In this study, a series of 2-R-6-(1-(alkylamino)methyl)pyridine-iron complexes [alkyl: (CPh₃) Fe1_H; (CHPh₂) Fe2_H; (CHPh₂) Fe3_Me; (CHMePh) Fe4_H; (CH₂Ph) Fe5_H; (CHMe₂) Fe6_H; (C₆H₁₁) Fe7_H; (CH₂(4-OMe)Ph) Fe8_H; (CH₂(4-CF₃)Ph) Fe9_H; (CH₂(2,4,6-Me₃)Ph) Fe10_H; (CH₂Ph) Fe11_Me] were synthesized and well characterized by ATR-IR spectroscopy, HRMS spectroscopy and elemental analysis. In addition, Fe3_Me, Fe4_H, Fe7_H and Fe11_Me were characterized by X-ray diffraction analysis: Fe3_Me and Fe11_Me adopted distorted tetrahedral geometries in the solid state while Fe4_H and Fe7_H were found in dimeric or polymeric forms respectively in which chlorides acted as bridging ligands. The catalytic capacities of these iron complexes were investigated for isoprene polymerization. Upon activation with a MAO cocatalyst, the catalytic activities of complexes varied as a function of the steric and electronic influences of substituents. In general, the catalysts bearing the least steric groups and electron-withdrawing groups exhibited relatively high activities. An outstanding activity of 190.6 × 10⁴ g·mol^-1·h^-1 was obtained by Fe5_H [CH₂Ph]. Moreover, changes in the steric hindrance around the metal center showed a notable effect on the selectivity of monomer enchainment. In particular, most of the polymers obtained by these complexes bearing flexible frameworks were in favor of 3,4-enchainment.

Full text of DOI:10.1039/c9dt00452a

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Synthesis and characterization of aminopyridine
iron(II) chloride catalysts for isoprene
polymerization: sterically controlled
monomer enchainment†
Cite this: DOI: 10.1039/c9dt00452a
Chuyang Jing,‡a,b Liang Wang,‡a Qaiser Mahmood,a Mengmeng Zhao,a
a
Guangqian Zhu,a,b Xianhui Zhang,a,b Xiaowu Wang *a and Qinggang Wang
*
In this study, a series of 2-R-6-(1-(alkylamino)methyl)pyridine-iron complexes [alkyl: (CPh3) Fe1H; (CHPh2)
Fe2H; (CHPh2) Fe3Me; (CHMePh) Fe4H; (CH2Ph) Fe5H; (CHMe2) Fe6H; (C6H11) Fe7H; (CH2(4-OMe)Ph) Fe8H;
(CH2(4-CF3)Ph) Fe9H; (CH2(2,4,6-Me3)Ph) Fe10H; (CH2Ph) Fe11Me] were synthesized and well character-
ized by ATR-IR spectroscopy, HRMS spectroscopy and elemental analysis. In addition, Fe3Me, Fe4H, Fe7H
and Fe11Me were characterized by X-ray diffraction analysis: Fe3Me and Fe11Me adopted distorted tetra-
hedral geometries in the solid state while Fe4H and Fe7H were found in dimeric or polymeric forms
respectively in which chlorides acted as bridging ligands. The catalytic capacities of these iron complexes
were investigated for isoprene polymerization. Upon activation with a MAO cocatalyst, the catalytic activi-
ties of complexes varied as a function of the steric and electronic influences of substituents. In general,
the catalysts bearing the least steric groups and electron-withdrawing groups exhibited relatively high
activities. An outstanding activity of 190.6 × 104 g·mol−1·h−1 was obtained by Fe5H [CH2Ph]. Moreover,
changes in the steric hindrance around the metal center showed a notable effect on the selectivity of
monomer enchainment. In particular, most of the polymers obtained by these complexes bearing flexible
frameworks were in favor of 3,4-enchainment.
Received 31st January 2019,
Accepted 26th April 2019
DOI: 10.1039/c9dt00452a
rsc.li/dalton
ing from 3,4-enchainment of the monomer is an important
component in synthetic rubber upon selective crosslinking
Introduction
Polyisoprene is experiencing ever-growing demands for its which can be beneficial for preventing abrasion of tyres. These
applications in tyres, gloves, tubes, shape memory and the materials could also find application in high performance
medicinal industries.1,2 Indeed these materials are replacing rubbers with wet-skid resistance and low-rolling resistance
the natural rubber due to their excellent flexible and ductile thread.3,5 Although polymerizations mediated by early tran-
properties similar to the volcanized rubber, and their proces- sition metal catalysts or conventional Ziegler–Natta catalysts
sing and recycling properties also make them superior over have been developed for more than 60 years,6,7 there have
natural rubber. In principle, isoprene polymerization has four been exciting recent advances in terms of late transition metal
types of monomer enchainments namely cis-1,4- and trans- catalyzed polymerization. Since the discovery of α-diimino-
1,4-, 1,2- and 3,4-. The degree and type of enchainment of the Ni(II)/-Pd(II)8,9 in 1995 and bis(imino)pyridine-Fe(II)/-Co(II)10,11
monomer define the end-use applications of the polymer.3,4 catalysts in 1998 for olefin polymerization, late transition
More specifically, polyisoprene with side-chain olefins result- metal catalysts have attracted great attention from both acade-
mia and industries.12–15 Up to now, most studies have been
focused on ethylene oligo/polymerization using late transition
metal precatalysts,16–20 in particular N,N,N-tridentate
aKey Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and
cobalt21–23 and iron24–28 and N,N-bidentate nickel29–33 and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China.
palladium34–37 complexes. Among them, iron complexes have
been found as highly active precatalysts for ethylene oligo/
E-mail: wangxw@qibebt.ac.cn, wangqg@qibebt.ac.cn
bCenter of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing, 100049, China
polymerization and featured an expedient preparation and an
†Electronic supplementary information (ESI) available. CCDC 1884491 (Fe7H),
eco-friendly and abundant metal.38,39 In the last few decades,
1884670 (Fe3Me), 1884671 (Fe11Me) and 1884672 (Fe4H). For ESI and crystallo-
well-defined iron based catalysts have also been developed for
graphic data in CIF or other electronic format see DOI: 10.1039/c9dt00452a
regio- and/or stereoselective polymerization of dienes.40,41 N,N-
‡These authors contributed equally.
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Chelates such as α-diimine,42,43 (imino)pyridine44–46 and
In this work, we explored a new family of aminopyridine-
bipyridine47–50 were elegant examples in the field of diene iron complexes as precatalysts for isoprene polymerization. By
polymerization. However, low productivity, poor selectivity and reducing the imine bond of the iminopyridine ligand frame-
low molecular weight of the resultant polydiene polymer were work, we prepared a series of aminopyridine-iron catalysts and
characteristics of iron mediated diene polymerization. In spite additionally probe the effects of different steric and electronic
of these, there has been considerable and growing interest to substituents toward isoprene polymerization (Scheme 1). A
develop new ligand motifs for improving the catalytic perform- detailed polymerization study was performed and the influ-
ance of the iron based catalysts. Some elegant examples of ences of different reaction parameters such as Al/Fe and IP/Fe
iron catalysts for isoprene polymerization have been developed ratios, reaction temperature and run time were also ascer-
by Ritter44 and Chen45 groups. The resulting polyisoprene pos- tained. In addition to the catalytic investigation and polymer
sessed a high content of cis-1,4 motifs with a low amount of characterization, synthesis and characterization of ligands and
side-arm olefin groups. Very recently, our group focused on their complexes were reported.
establishing the correlations of the ligand architecture of iron
based catalysts with their catalytic performance for isoprene
polymerization. We observed that the introduction of an elec-
tron-withdrawing group (CF3) into the N-aryl imines of imino-
Results and discussion
pyridine-iron complexes showed a positive effect on the cata-
lytic performance in terms of activity.4 Subsequent work
expanded the ligand backbone to an iminoimidazole motif51
and the iron salt was extended to Fe(acac)2.52 Other studies
have shown that the mode of coordination with the metal
Synthesis and characterization of ligands and their iron(II)
complexes
A
series of 2-R-6-(1-(alkylamino)methyl)pyridine ligands
bearing different alkyl substituents have been prepared using
a two-step procedure as represented in Scheme 2. The inter-
mediate compounds 2 could be obtained by the Schiff base
condensation of (2-methyl)picolinaldehyde with the corres-
ponding anilines under mild conditions in good to excellent
yields. The efficiency of this reaction was substantially
improved by performing the reaction in the presence of acti-
vated 4 Å molecular sieves. Subsequent reduction of 2 with
NaBH4 provided the respective ligands. All organic compounds
were characterized by 1H/13C NMR, IR spectroscopy and
HRMS-ESI analysis. The 1H/13C NMR spectra were supportive
to the proposed structures of the ligands. Finally, the target
iron(II) complexes were obtained by the reaction of equimolar
ligands with anhydrous FeCl2 in DCM at room temperature.
The structures of all the complexes were well determined by
the IR, HRMS-ESI and elemental analysis. Further confir-
mation of Fe3Me, Fe4H, Fe7H and Fe11Me was provided in the
form of single crystal X-ray structures.
center plays
a crucial role in controlling the catalytic
performance:53–55 the modes of coordination such as amide–
metal (C–(R)N− → M) and imine–metal (CvN → M) (including
nitrogen containing heterocycles such as pyridine) have been
well studied. However, amine–metal (C–(R)N → M) with
coordination of an amine to iron metal was rarely been
explored toward isoprene polymerization. It is tempting to
explore the different modes of coordination of the side arm to
concurrently achieve high activities and subtle control over the
microstructure of the polymer.56–58
In the IR spectra, the stretching vibration for the N–H
bonds in complexes appeared in the range of 3193–3296 cm−1
,
which were relatively lower in wavenumber than those
observed for the free ligands (3310–3325 cm−1), indicating the
coordination of an sp3 nitrogen atom with iron metal.
Scheme 1 Structural variations in imino- and aminopyridine-iron(II)
pre-catalysts.
Scheme 2 Synthesis of 2-R-6-(1-(alkylamino)methyl)pyridine ligands and their iron(II) complexes.
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Single crystals of complexes Fe3Me, Fe4H, Fe7H and Fe11Me on the geometry. In consequence, Cl1–Fe1–Cl2 bond angles
suitable for X-ray diffraction analyses were obtained by slow are more open for Fe3Me (123.57°) than Fe11Me (120.41°).
evaporation of their dichloromethane solution. Since the mole- These structural features are very similar to the previously
cular structures of Fe3Me and Fe11Me are similar, these will reported iron complexes.49,59,60 There are modest variations in
therefore be discussed together.
the iron–nitrogen bond lengths in both iron complexes with
The molecular structures of Fe3Me and Fe11Me are mono- Fe1–N2sp3 [2.134 Å for Fe3Me and 2.145 Å for Fe11Me], which
nuclear species in which both sp3 and sp2 nitrogen belonging are slightly longer than Fe1–N1sp2 [2.112 Å for Fe3Me and
to the aminopyridine ligand serve as donor atoms. This geo- 2.107 Å for Fe11Me]. This indicates the unequal strength of
metry is best described as distorted tetrahedral with chloride coordination bonds with the metal center. Such differences in
ligands filling two other coordination sites. The molecular bond lengths are essentially similar to the reported imino-
structure of Fe3Me and Fe11Me can be seen in Fig. 1 and 2 pyridine-iron(II) complexes in which Fe–Nimine bond lengths
14,44,45
respectively. The N1–Fe1–N2 bite angles in both complexes are are longer than Fe–Npyridine
comparable at 80.63° for Fe3Me and 79.91° for Fe11Me high-
.
By contrast, the molecular structure of Fe4H reveals a chlor-
lighting some distortions imposed by the N,N-chelating ligand ide bridged centrosymmetric binuclear species as illustrated in
Fig. 3. In this dimeric structure, each iron core is bound by
two nitrogen atoms (sp3 and sp2) belonging to the aminopyri-
dine ligand motif, one chloride per iron center bridges the
iron centers. The remaining chloride serves as a monodentate
ligand and each iron center adopts a distorted square pyrami-
dal geometry. In this geometry, nitrogen (N1 and N2) and brid-
ging chloride atoms (Cl1 and Cl1a) form the basal plane while
Cl2 and Cl2a individually occupy the axial positions.
The solid state structure of Fe7H is a multicore system in
which each core was bridged by two chloride atoms with an
Fe⋯Fe separation of 3.735 Å. In addition, each iron center was
ligated with two nitrogen atoms (i.e. sp3 and sp2 atoms)
belonging to the neutral chelating L7H and four Cl atoms
leading to a distorted octahedral geometry conferred to each
metal center. In particular, the Namine atom and one bridging
chloride occupy the axial coordination site with an N2–Fe1–
Cl1 angle of 170.90° reflecting non-linear behavior of the
bond. While Npyridine and the rest of the three chloride atoms
form a basal plane of the octahedron with the iron atom lying
Fig. 1 Molecular structure of complex Fe3Me: all hydrogen atoms have
at 0.096 Å out of the plane. It is worth mentioning that one
bridging chloride atom occupies the axial position of the octa-
hedral geometry of one metal center and is also part of the
basal plane of the adjacent octahedral geometry. The plane of
the chelate ring, N–C–C–N–Fe, is significantly distorted from
been omitted for clarity. Atoms are drawn at the 50% probability level.
Selected bond lengths (Å) and angles (°): Fe1–N1 2.112(3), Fe1–N2
2.134(3), Fe1–Cl1 2.2190(12), Fe1–Cl2 2.2406(11), Cl1–Fe1–Cl2 123.57(5),
N1–Fe1–N2 80.63(12).
Fig. 2 Molecular structure of complex Fe11Me: all hydrogen atoms have
been omitted for clarity. Atoms are drawn at the 50% probability level.
Selected bond lengths (Å) and angles (°): Fe1–N1 2.107(2), Fe1–N2
2.145(2), Fe1–Cl1 2.2241(9), Fe1–Cl2 2.2374(9), Cl1–Fe1–Cl2 120.41(4),
N1–Fe1–N2 79.91(8).
Fig. 3 Molecular structure of complex Fe4H: all hydrogen atoms have
been omitted for clarity. Atoms are drawn at the 30% probability level.
Selected bond lengths (Å) and angles (°): Fe1–N1 2.117(2), Fe1–N2
2.245(2), Fe1–Cl1 2.3868(9), Fe1–Cl2 2.2791(9), Fe1–Cl1a 2.5668(8)
Cl1–Fe1–Cl2 131.90(3), N1–Fe1–N2 77.32(8).
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the plane of the pyridine ring due to the presence of the sp3
As shown in Table 1, the bond distances of Fe–N(amine)
hybridized nitrogen atom. Interestingly, the bond lengths of and Fe–N(pyridine) are all longer than the corresponding Fe–
Fe1–Cl1 or Fe1–Cl2 are significantly shorter than those of their N bond distances in Ritter’s complex (CCDC 853130†), respect-
counterpart Fe1–Cl1a or Fe1–Cl2a[Cl1–Fe1 (2.401 Å), Cl2–Fe1 ively, indicating that the donating ability of the sp3 N-donors is
(2.438 Å), Cl1a–Fe1 (2.591 Å), Cl2a–Fe1 (2.690 Å)], indeed con- lower than that of the sp2 N-donors leading to a reduction in
sistent with the previously reported N,N-bidentate iron com- the electron density around the metal center. Hence, the
plexes in which bond lengths between bridging chloride and aminopyridine-ligated iron center is more electrophilic than
the metal center are found to be nearly 2.400 Å (Fig. 4).46,60,61
the iminopyridine–ligated iron center (Fe(L)2+). The same con-
clusion was drawn in palladium complexes that the α-diamine-
ligated palladium center is more electrophilic than the
α-diimine-ligated palladium center.53
Isoprene polymerization
Screening of Fe1H–Fe7H with MAO. To probe the capacity of
the prepared iron complexes for isoprene polymerization, MAO
was used as a co-catalyst. Initially, the catalytic potential of
complexes Fe1H–Fe7H was studied and the resulting polymeriz-
ation data are given in Table 2. The observed catalytic activities
of these catalysts appended with different substituents varied
in the order Fe5H [CH2Ph] = Fe6H [CHMe2] = Fe7H [C6H11] >
Fe4H [CHMePh] > Fe2H [CHPh2] > Fe1H [CPh3] > Fe3Me
[CHPh2]. This decreasing trend of activity clearly showed that
steric factors are influential and the catalysts bearing the least
steric bulky group tended to give higher activities. The com-
plexes Fe5H [CH2Ph] = Fe6H [CHMe2] = Fe7H [C6H11] bearing
less sterically hindered groups exhibited standout performance
in terms of activity as high as 6.8 × 104 g mol−1 h−1 with >99%
conversion (Table 2, entries 5–7). This activity dramatically
reduced to 4.7 × 104 g mol−1 h−1 when relatively more bulky
groups were installed in the catalysts (Table 2, entry 4). The
lower activity of 2.3 × 104 g mol−1 h−1 was observed when a
triphenyl group was attached to the N-alkyl unit (Table 2,
entry 1). This decreasing trend of activities as a function of
steric hindrance suggests that more bulky groups occupying
the space around the metal center in the active species slow
down the rate of coordination and insertion of the monomer,
thus lower activity was observed.23–25 As part of our continuous
efforts to examine the steric effect, a methyl group was
installed at the ortho position of pyridine and its effect on the
polymerization of isoprene was studied. Introduction of a
Fig. 4 Molecular structure of complex Fe7H: all hydrogen atoms have
been omitted for clarity. Atoms are drawn at the 30% probability level.
Selected bond lengths (Å) and angles (°): Fe1–N1 2.161(8), Fe1–N2
2.252(7), Fe1–Cl1 2.401(3), Fe1–Cl2 2.438(3), Fe1–Cl1a 2.591(3), Fe1–
Cl2a 2.690 (3), N1–Fe1–N2 75.4(3), Cl1–Fe1–Cl1a 94.53(10), Cl1–Fe1–
Cl2a 82.80(10), Cl1a–Fe1–Cl2a 170.72(11), Cl1–Fe1–Cl2 96.55(9),
Cl2–Fe1–Cl1a 84.24(8), Cl2–Fe1–Cl2a 104.83(9).
Table 1 Crystal parameters of iron complexes with different N–Fe
lengths
Fe3Me Fe4H
Fe7H
Fe11Me Py-imine Fea
Fe–Npyridine
Fe–Namine/Fe–Nimine 2.134
2.112
2.107 2.117 2.161
2.145 2.245 2.252
2.103
2.129
a (E)-2,4,4-Trimethyl-N-(pyridin-2-ylmethylene)pentan-2-amine iron(II)
chloride.44
Table 2 Fe1H–Fe7H/MAO promoted isoprene polymerizationa
Microstructurec (%)
Entry
Cat.
Yield%
Activityb
cis-1,4
trans-1,4
3,4
Mn d (× 104)
PDId
1
2
3
Fe1H
Fe2H
Fe3Me
33
68
13
2.3
4.7
0.8
53
9
69
0
69
4
47
22
27
11.2
2.0
12.3
0.2
2.6
3.9
1.7
3.7
1.6
1.6
4.7
3.9
2.1
1.7
4
5
6
7
Fe4H
Fe5H
Fe6H
Fe7H
70
4.7
6.8
6.8
6.8
46
29
41
44
8
22
10
4
46
49
49
52
>99
>99
>99
8.1
11.1
a Polymerization conditions: solvent: 5 mL toluene; complex: 10 μmol; isoprene: 20 mmol; time: 2 h; T: 25 °C; MAO/Fe: 500. b 104 g mol−1 h−1.
c Determined by 1H NMR and 13C NMR. d Determined by gel permeation chromatography (GPC).
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methyl group on the ortho position of the pyridine moiety
exhibited a negative effect on the catalytic activity as Fe3Me
[CHPh2] displayed the lowest activity of 0.8 × 104 g mol−1 h−1
with only 13% conversion (Table 2, entry 3). Interestingly,
introducing a bulky group near the active species favored the
enhancement of the molecular weight of the resultant
polymer. For instance, Fe3Me [CHPh2] bearing a methyl group
at the sixth position of pyridine afforded polyisoprene with the
highest molecular weight up to 12.3 × 104 and Fe1H bearing a
bulky CPh3 substituent produced polyisoprene with a compar-
able molecular weight (11.2 × 104). It is assumed that bulky
groups protect the active species which would not only align
the coordination and insertion of a monomer but also
promote the chain propagation as compared to the chain
transfer to the aluminum species.62,63 The molecular weight
distributions (PDI) of the obtained polymers varied from
narrow to moderately broad and were bimodal as can be seen
in Fig. S49.†
Fig. 6 1H and 13C NMR spectra of the representative sample of polyiso-
prene obtained by Fe2H/MAO (Table 2, entry 2).
The microstructural properties of the obtained polyisoprene
have been determined by 1H and 13C NMR spectroscopy
(recorded in deuterated chloroform at room temperature) and
confirmed by the characteristic peaks listed in the
literature.64,65 According to the characteristic peaks that
1
appeared in the H and 13C NMR spectra, in general, most of
the obtained polyisoprene possessed nearly an equal amount
of cis-1,4 and 3,4 units (Table 2 & Fig. 5). For example, the
polymer obtained by complex Fe4H [CHMePh] equally favors
cis-1,4 (46%) and 3,4 (46%) monomer insertion. Interestingly,
Fe2H [CHPh2] and Fe3Me [CHPh2] catalyzed isoprene polymer-
ization showed essentially different selectivity of monomer
insertion: the Fe2H [CHPh2] based polymer was composed of
69% trans-1,4 units, while 69% cis-1,4 monomer insertion was
observed in the case of Fe3Me [CHPh2] promoted polymeriz-
ation (Fig. 6 & 7). These polymerization results indicated that
how fine tuning of the ligand structure gives different micro-
structures of the polymer.
Fig. 7 1H and 13C NMR spectra of the representative sample of polyiso-
prene obtained by Fe3Me/MAO (Table 2, entry 3).
Screening of the reaction conditions using Fe5H/MAO. In
order to ascertain the effects of reaction parameters such as
Al/Fe ratio, IP/Fe ratio and reaction temperature on isoprene
polymerization, Fe5H was used as a test precatalyst in combi-
nation with the MAO cocatalyst and the resulting polymeriz-
ation data are summarized in Table 3. Firstly, temperature was
fixed at 25 °C, and the Al/Fe ratio consistently decreased from
500 to 200 (Table 3, entries 1–3), resulting in 500 being identi-
fied as the best Al/Fe ratio with the catalytic activity of Fe5H/
MAO reaching a peak in activity of 81.6 × 104 g mol−1 h−1 with
>99% conversion (Table 3, entry 1). On decreasing the Al/Fe
ratio to 200, activity as well as conversion consistently reduced
to 62.4 × 104 g mol−1 h−1 with a conversion of 77% (Table 3,
entry 3). Nevertheless, this polyisoprene revealed no change in
the selectivity of monomer insertion on decreasing the Al/Fe
ratio. Changes in the Al/Fe ratio showed no regular influence
on the molecular weight and PDI of the resultant polymer. On
increasing the IP/Fe ratio from 2000 to 10 000, nonetheless the
conversion of the monomer reduced to 47% but the activity
Fig. 5 Microstructural properties of polyisoprene obtained by Fe1H–
Fe7H/MAO (Table 2, entries 1–7).
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Table 3 Screening of the reaction conditions using Fe5H/MAO and Fe8H–Fe11Me/MAO promoted isoprene polymerizationa
Microstructuree (%)
Entry
Cat.
Al/Fe
IP/Fe
2000
T/°C
Yield %
Activityd
cis-1,4
trans-1,4
3,4
Mn f (× 104)
PDIf
1
2
3
4
Fe5H
Fe5H
Fe5H
Fe5H
Fe5H
Fe5H
Fe5H
Fe8H
Fe9H
Fe10H
Fe11Me
500
300
200
500
500
500
500
200
200
200
200
25
25
25
25
50
−40
−78
25
25
25
>99
87
77
47
71
67
Trace
65
81.6
70.8
62.4
190.6
58.0
9.1
29
28
28
29
32
43
—
18
32
36
44
23
25
25
17
20
7
—
38
19
17
15
48
47
47
54
48
50
—
44
49
47
41
4.1
4.8
4.5
6.0
3.8
8.3
—
4.1
5.2
3.7
6.3
4.0
4.2
4.0
2.4
7.3
2.7
—
3.2
4.1
5.2
3.2
2000
2000
10 000
2000
2000
2000
2000
2000
2000
2000
5
6b
7c
8
—
53.4
74.4
56.7
0.7
9
91
70
11
10
11c
25
1
a Polymerization conditions: solvent: 5 mL toluene; complex: 10 μmol; time: 10 min. b 1 h. c 2 h. d 104 g mol−1 h−1. e Determined by H NMR and
13C NMR. f Determined by gel permeation chromatography (GPC).
dramatically improved to 190.6 × 104 g mol−1 h−1 over a run observed that higher temperatures showed no notable effect
time of 10 min (Table 3, entry 4). With the Al/Fe and IP/Fe on the selectivity of monomer insertion, however, a decrease
ratios fixed at 500 and 2000 respectively, reaction temperatures in the reaction temperature led to an improvement in the cis-
were varied between 50 °C and −78 °C (Table 3, entries 1 & 1,4 selectivity from 29% to 43% at 25 °C and −40 °C respect-
5–7). According to the data in Table 3, a peak in activity of ively. Meanwhile, trans-1,4 selectivity consistently reduced and
81.6 × 104 g mol−1 h−1 was observed at 25 °C (Table 3, entry 1), no substantial change in the 3,4 selectivity was observed on
while a higher temperature of 50 °C led to a decrease in elevating the reaction temperature.
activity (Table 3, entry 5) reflecting lower thermal stability of
Screening of Fe8H–Fe11Me with MAO. In order to establish a
the prepared iron complexes. It can be ascribed to the less relationship between structural variations in the precatalysts
coordinating ability of the amine moiety than the imine and their catalytic performance, the remaining four precata-
moiety. Indeed similar observations were found for iron-cata- lysts, Fe8H [R1 = H; R2 = OMe], Fe9H [R1 = H; R2 = CF3], Fe10H
lyzed olefin polymerization.24,25 Polymerization tests per- [R1 = Me; R2 = Me], and Fe11Me [R1 = H; R2 = H] were also
formed at −40 °C and −78 °C also resulted in lower activities, employed for isoprene polymerization (Table 3, entries 8–11).
indeed a trace amount of polymer was observed at −78 °C. Typical polymerization tests were performed in 5 mL toluene
This drop in activity can be attributed to the lower activation with an IP/Fe ratio of 2000 at 25 °C over a run time of
process of iron species at such
a lower temperature. 10 minutes. When the polymerization tests were performed
Expectedly, the molecular weight of polymers gradually with Fe8H–Fe10H, quantitative conversions were obtained with
decreased on elevating the reaction temperature (Fig. 8). It can the Al/Fe ratio fixed at 500 (ESI† section 1). For comparing the
be ascribed to an increased rate of chain transfer reaction and activities of different catalysts, the Al/Fe ratio decreased to 200.
termination as compared to the chain propagation at higher The overall activity was decreased in the order of Fe9H [R1 = H;
temperatures.24,25 Regarding the microstructural properties of R2 = CF3] > Fe5H [R1 = H; R2 = H] > Fe10H [R1 = Me; R2 = Me] >
the polymers prepared at different temperatures, it was Fe8H [R1 = H; R2 = OMe] > Fe11Me [R1 = H; R2 = H]. After
careful examination of the polymerization results, it was found
that catalytic activities gradually increased with respect to the
electron-withdrawing nature of the para substituents: Fe9H
[R1 = H; R2 = CF3] was found as the standout performer exhibit-
ing the highest activity of 74.4 × 104 g mol−1 h−1 with 91% con-
version (Table 3, entry 9). By contrast, Fe8H [R1 = H; R2 = OMe]
bearing an electron-donating group (i.e. OMe) showed a nega-
tive impact on the activity (Table 3, entry 8). Similar to the
findings described above (Table 3, entry 3), the introduction of
the methyl group at the sixth position of pyridine again
showed a negative effect on the catalytic performance as it led
to a lower activity of 0.7 × 104 g mol−1 h−1 with 11% conversion
(Table 3, entry 11), suggesting that the methyl group covers
more space around the active species which in turn reduces
the insertion and coordination of the monomer or in part, due
to the interaction of hydrogen of this methyl group with the
Fig. 8 Changes in the conversion (%), Mn and PDI of polyisoprene
obtained at different reaction temperatures (Table 3, entries 1 and 5–7).
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regard to the active species involved in the polymerization
needs to be addressed, whether it is cation pyridine-amine
(AS I), cation pyridine-amide (AS II) or neutral pyridine-amide
(AS III) active species on activation with MAO co-catalysts
(Scheme 3). For this confirmation, we synthesized the pyri-
dine-amide iron complex with an identical substituent to the
Fe2H obtained by the reaction of anhydrous FeCl2 with an
ionic ligand which was deprotonated by n-butyllithium base
(see the details of complex synthesis in ESI,† section 2). Later,
the catalytic potential of Fe12H for isoprene polymerization
was studied under similar conditions to those used for Fe2H
based polymerization. The polymerization results exhibited
activity up to 6.8 × 104 g mol−1 h−1 (full conversion) with a 1/1
ratio of cis-1,4 and 3,4 units, which is essentially different
from the selectivity observed for the Fe2H promoted polymeriz-
ation. This comparison precluded the involvement of active
species (AS III) in the polymerization performed using the
Fe2H catalyst. Our conclusion was also supported by ethylene
polymerization using quinolinolato-amine titanium complexes
by Jin66 and pyridine-amine nickel complexes by Wu.55
Therefore, we proposed the active structure of the pyridine-
amine Fe complexes as AS I, while the deprotonation of the
catalyst by AlMe3 as AS II can’t be ruled out. Probably, the
active species AS I and AS II could co-exist and were involved
in polymerization.
Furthermore, the title aminopyridine-iron catalysts resulted
in different microstructures of polyisoprene from those of the
previously reported iminopyridine-iron chloride catalyzed iso-
prene polymerization, indicating a different activation process
which results in different regioselectivity. For iron complexes
with bidentate nitrogen ligands, the crystal analysis showed
that the bond distances of Fe–Namine are longer than the Fe–
Nimine bond distances, indicating that the electron donating
ability of the sp3 N-donors is lower than that of the sp2
N-donors, which leads to the lower electron density around the
metal center. Hence, the aminopyridine-ligated iron center is
more electrophilic than the iminopyridine-ligated iron center
(Fe(L)2+). Such changes affect the binding interaction between
the metal and the coordination donor as well as the opening
environment near the metal center. Based on this, the iso-
prene monomer would coordinate to the iron atom through
the η4 bond including cis-η4 or trans-η4 rather than the η2 bond
as shown in Scheme 4. The insertion of the coordinated
monomer leads to the formation of anti-η3 or syn-η3 allyl-Fe
intermediates, respectively. The allylic unit has two reactive
sites, C1 and C3. The 1,4 unit is formed by insertion of the
Fig. 9 Microstructural properties of polyisoprene obtained by Fe5H,
Fe8H–Fe11Me/MAO (Table 3, entries 3 and 8–11).
active metal center.23–25 Regarding the properties of the
polymer, the molecular weights fall in the range of 3.7–6.3 ×
104 and a more sterically hindered precatalyst produced polyi-
soprene with a relatively higher molecular weight. This fact is
not ambiguous, indeed consistent with the literature.23–25 In
addition, broad and bimodal molecular weight distributions
are the characteristics of the resultant polymer. Regarding the
microstructural properties of the resultant polymer, from a
general point of view, 3,4 unit enriched polymers were
observed in all cases with the value of about 50% (Fig. 9).
According to what has been previously reported, the polyiso-
prene catalyzed by iminopyridine-iron catalysts possessed a
high content of 1,4 motif with only about 20% content of
3,4 motif.44,45 Besides, comparatively more sterically hindered
precatalysts favored cis-1,4 enchainment rather than trans-1,4.
For instance, the Fe11Me based polymer bearing a methyl
group at the sixth position showed high selectivity of cis-
1,4 monomer insertion which is higher in comparison with
the value of 29% observed in the Fe5H based polymer.
Mechanistic investigation of enriched 3,4-polyisoprene by
aminopyridine-iron complexes
In general, the deprotonation of the amine group by a strong
base, such as nBuLi, is feasible. The production of the amide
group can react with the metal to generate an amide–metal
combination model.54 Meanwhile, we have proved that the L2H
1
ligand can be deprotonated by excess AlMe3 through H NMR
in C6D6 (ESI Fig. S51†). Therefore, a crucial problem with
Scheme 3 Possible active species during the polymerization.
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weight of polymers can be controlled in the range from 2.0 ×
104 to 11.2 × 104 with narrow to broad molecular weight distri-
butions in the range of 1.7–7.3. Possible mechanistic consider-
ation of enriched 3,4-polyisoprene catalyzed by aminopyridine-
iron(II) complexes was provided.
Experimental
General procedures
All manipulations of air and/or moisture sensitive compounds
were performed using standard Schlenk techniques. Toluene,
dichloromethane (DCM), tetrahydrofuran (THF) and hexane
were purchased from Sinopharm Chemical Reagent, dried over
sodium benzophenone ketyl (toluene, THF) or calcium hydride
(DCM, hexane) and distilled under an argon atmosphere prior
to use. 1H and 13C NMR spectra were recorded on a Bruker
Avance III 400 MHz spectrometer using CDCl3 as the solvent
and trimethylsilane (TMS) as the internal reference. Chemical
shifts and coupling constants were given in ppm and in Hz
respectively. Attenuated total reflection-infrared (ATR-IR) spec-
troscopy was performed using a Thermo Scientific Nicolet
iN10. The Greek alphabet ‘ν’ stands for the stretching vibration
wavenumber. Elemental analysis was carried out on a Vario EL
III elemental analyzer at the Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences. Mass spectra for new
organic compounds were recorded using a maXis II of Bruker
Daltonics Corporation, while for Fe(II) complexes, the mass
spectra were recorded using an ACQUITYTM UPLC & Q-TOF
MS Premier at the Analytical Center of Shanghai Jiao Tong
University. X-ray diffraction data were collected on a Smart
1000 diffractometer with a Mo-Kα X-ray source (λ = 0.71073 Å)
or Cu-Kα radiation (λ = 1.54178 Å). Molecular weights and poly-
dispersity indices (PDI) of polyisoprene were measured by high
temperature gel permeation chromatography (HGPC, PL-GPC
220, Agilent Technologies) using trichlorobenzene as the
eluent and polystyrenes as standards. Isoprene was purchased
from Aladdin Industrial Corporation, dried over CaH2 and dis-
tilled prior to use. The MAO which was purchased from Aike
Reagent contains 10% AlMe3 and was used without further
purification. All other reagents were purchased from commer-
cial sources and used without further purification.
Scheme 4 Possible mode of formation of polyisoprene.
incoming monomer at C1, while a 3,4 unit is formed by inser-
tion of the incoming monomer at C3. In the case of Fe5H
mediated polymerization, the cis-η4 type of coordination–inser-
tion of monomer was expected as 3,4 unit enriched polyiso-
prene was obtained. Similar findings have been reported in
rare earth and cobalt systems.67,68
Conclusions
Synthesis of ligands
In conclusion, novel iron(II) complexes based on aminopyri-
dine ligands were synthesized and well characterized. The
2-(1-(Triphenylmethylamino)methyl)pyridine, L1H. L1H was
molecular structures of complexes Fe3Me, Fe4H, Fe7H and prepared using a two-step procedure.69 In the first step,
Fe11Me were characterized by single crystal X-ray diffraction in triphenylmethylamine (2.74 g, 10.55 mmol) and activated 4 Å
the solid state. All complexes displayed two combination molecular sieves (2.00 g) were added into the solution of 2-pyr-
fashions (pyridine (sp2) and amine (sp3)) leading to a distinc- idinecarboxaldehyde (1.13 g, 10.55 mmol) in dry DCM
tive effect on isoprene polymerization. Upon activation with a (20.0 mL) and stirred at room temperature. After overnight stir-
MAO cocatalyst, all catalysts showed moderate to high activities ring, all the volatiles were evaporated under reduced pressure
with a maximum value of 190.6 × 104 g mol−1 h−1 obtained by and the crude product was washed twice with cold methanol
Fe5H [CH2Ph] toward isoprene polymerization. Introduction of affording an orange solid (3.10 g, 84% yield). In the second
substituents on the amine moiety and to the ortho-position of step,70 the resulting imine (0.52 g, 1.49 mmol) was dissolved
pyridine exhibited a notable effect on the activity, meanwhile in methanol, and NaBH4 (0.57 g, 15.00 mmol) was added. The
monomer enchainment favored in the 3,4 unit. The molecular solution was stirred overnight. The product was extracted by
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DCM. The organic layer was dried over MgSO4 and filtered. (1.01 g) yield was obtained in the first and second step,
The solvent was evaporated under reduced pressure to give the respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.61–8.52 (m,
crude product as a white solid, which was further purified by 1H), 7.63 (td, J = 7.8, 1.8 Hz, 1H), 7.39–7.29 (m, 5H), 7.24 (d,
silica gel column chromatography (0.32 g, 61% yield). 1H NMR J = 6.8 Hz, 1H), 7.18–7.13 (m, 1H), 3.93 (s, 2H), 3.84 (s, 2H),
(400 MHz, CDCl3, 298 K): δ 8.49 (ddd, J = 4.8, 1.8, 0.8 Hz, 1H), 2.19 (s, 1H). 13C NMR (100 MHz, CDCl3, 298 K): δ 159.9, 149.4,
7.64 (td, J = 7.8, 1.8 Hz, 1H), 7.58–7.51 (m, 6H), 7.46 (d, J = 7.8 140.2, 136.5, 128.5, 128.4, 127.1, 122.4, 122.0, 54.6, 53.6. FTIR
Hz, 1H), 7.31–7.24 (m, 6H), 7.21–7.16 (m, 3H), 7.12 (ddd, J = (KBr, cm−1): 3313 (ν (N–H)), 3061, 3027, 2920, 2838, 1952,
7.6, 4.8, 1.2 Hz, 1H), 3.52 (s, 2H), 2.59 (s, 1H). 13C NMR 1591, 1454, 1433, 1362, 1120, 994, 753, 699. HRMS-ESI (m/z):
(100 MHz, CDCl3, 298 K): δ 160.5, 149.2, 146.1, 136.6, 128.8, [M + H]+ calcd for C14H17N2, 199.1235; found, 199.1231.
128.0, 126.5, 122.0, 121.8, 71.2, 49.7. FTIR (KBr, cm−1): 3313
2-(1-(1-Methylethylamino)methyl)pyridine, L6H. Using the
(ν (N–H)), 3054, 2923, 1956, 1592, 1569, 1428, 1354, 1109, similar procedure established for the synthesis of L1H, L6H
1031, 773, 708. HRMS-ESI (m/z): [M + H]+ calcd for C25H23N2, was obtained as a pale yellow solid. 92% (4.07 g) and 66%
351.1861; found, 351.1854.
(1.34 g) yield was obtained in the first and second step,
2-(1-(Diphenylmethylamino)methyl)pyridine, L2H. Using the respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.55 (ddd, J =
similar procedure established for the synthesis of L1H, L2H 5.0, 1.8, 1.0 Hz, 1H), 7.70 (td, J = 7.8, 1.8 Hz, 1H), 7.60 (dt, J =
was obtained as a pale yellow solid. 88% (2.54 g) and 74% 7.8, 1.2 Hz, 1H), 7.23 (ddd, J = 7.6, 4.8, 1.2 Hz, 1H), 4.20 (s,
(1.51 g) yield was obtained in the first and second step, 2H), 3.21 (hept, J = 6.6 Hz, 1H), 1.36 (d, J = 6.6 Hz, 6H). 13C
respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.54 (ddd, J = NMR (100 MHz, CDCl3, 298 K): δ 153.4, 149.4, 137.3, 123.8,
4.8, 1.8, 1.0 Hz, 1H), 7.61 (td, J = 7.6, 1.8 Hz, 1H), 7.47–7.40 (m, 123.4, 49.5, 49.5, 20.3. FTIR (KBr, cm−1): 3312 (ν (N–H)), 2964,
4H), 7.33–7.25 (m, 5H), 7.23–7.17 (m, 2H), 7.14 (ddd, J = 7.6, 2926, 2854, 1592, 1570, 1473, 1434, 1174, 995, 756. HRMS-ESI
4.8, 1.2 Hz, 1H), 4.87 (s, 1H), 3.87 (s, 2H), 2.52 (s, 1H). 13C (m/z): [M + H]+ calcd for C9H15N2, 151.1235; found, 151.1230.
NMR (100 MHz, CDCl3, 298 K): δ 159.8, 149.4, 143.9, 136.4,
2-(1-(Cyclohexylamino)methyl)pyridine, L7H. Using the
128.6, 127.5, 127.1, 122.6, 122.0, 77.5, 53.5. FTIR (KBr, cm−1): similar procedure established for the synthesis of L1H, L7H
3325 (ν (N–H)), 3044, 2818, 1958, 1591, 1566, 1450, 1342, 1149, was obtained as a pale yellow solid. 93% (5.70 g) and 94%
993, 743, 704. HRMS-ESI (m/z): [M + H]+ calcd for C19H19N2, (1.78 g) yield was obtained in the first and second step,
275.1548; found, 275.1546.
1
respectively. H NMR (400 MHz, CDCl3, 298 K): δ 8.54 (dt, J =
4.8, 1.2 Hz, 1H), 7.63 (td, J = 7.6, 1.8 Hz, 1H), 7.32–7.28 (m,
2-Methyl-6-(1-(diphenylmethylamino)methyl)pyridine, L3Me
.
Using the similar procedure established for the synthesis of 1H), 7.14 (ddd, J = 7.4, 4.8, 1.2 Hz, 1H), 3.93 (s, 2H), 2.54–2.44
L1H, L3H was obtained as a pale yellow solid. 68% (1.10 g) and (m, 1H), 2.15 (s, 1H), 1.924 (ddt, J = 9.8, 3.8, 1.8 Hz, 2H), 1.74
99% (1.00 g) yield was obtained in the first and second step, (dt, J = 12.2, 3.6 Hz, 2H), 1.65–1.57 (m, 1H), 1.31–1.09 (m, 5H).
1
respectively. H NMR (400 MHz, CDCl3, 298 K): δ 7.52 (s, 1H), 13C NMR (100 MHz, CDCl3, 298 K): δ 160.2, 149.3, 136.5, 122.4,
7.48–7.42 (m, 4H), 7.30 (t, J = 7.6 Hz, 4H), 7.22 (d, J = 7.4 Hz, 121.9, 56.7, 52.5, 33.6, 26.2, 25.1. FTIR (KBr, cm−1): 3310
2H), 7.08 (d, J = 7.6 Hz, 1H), 7.01 (d, J = 7.6 Hz, 1H), 4.88 (s, (ν (N–H)), 3065, 3009, 2926, 2852, 1592, 1570, 1433, 1371,
1H), 3.83 (s, 2H), 2.53 (s, 3H). 13C NMR (100 MHz, CDCl3, 1126, 1048, 755. HRMS-ESI (m/z): [M + H]+ calcd for C13H18N2,
298 K): δ 159.1, 158.1, 144.1, 136.7, 128.6, 127.6, 127.1, 121.5, 191.1548; found, 191.2980.
119.4, 77.5, 77.2, 76.8, 67.2, 53.6, 24.6. FTIR (KBr, cm−1): 3319
2-(1-(4-Methoxyphenylmethylamino)methyl)pyridine, L8H.
(ν (N–H)), 3060, 3025, 2921, 2829, 1953, 1593, 1577, 1492, Using the similar procedure established for the synthesis of
1452, 1342, 1117, 1028, 753, 702. HRMS-ESI (m/z): [M + H]+ L1H, L8H was obtained as a pale yellow solid. 73% (3.10 g) and
calcd for C20H21N2, 289.1705; found, 289.1701.
91% (0.31 g) yield was obtained in the first and second step,
2-(1-(1-Phenylethylamino)methyl)pyridine, L4H. Using the respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.59–8.51 (m,
similar procedure established for the synthesis of L1H, L4H 1H), 7.64 (tt, J = 7.6, 1.8 Hz, 1H), 7.35–7.19 (m, 3H), 7.15 (ddd,
was obtained as a pale yellow solid. 92% (2.00 g) and 80% J = 7.6, 4.8, 1.2 Hz, 1H), 6.92–6.81 (m, 2H), 3.91 (s, 2H), 3.79 (s,
(0.67 g) yield was obtained in the first and second step, 3H), 3.78 (s, 2H), 2.14 (s, 1H). 13C NMR (100 MHz, CDCl3,
1
respectively. H NMR (400 MHz, CDCl3, 298 K): δ 8.55 (d, J = 298 K): δ 159.9, 158.8, 149.4, 136.5, 132.4, 129.6, 122.5, 122.0,
4.8 Hz, 1H), 7.60 (td, J = 7.8, 1.8 Hz, 1H), 7.40–7.30 (m, 4H), 113.9, 55.4, 54.6, 53.0. FTIR (KBr, cm−1): 3312 (ν (N–H)), 3007,
7.26–7.22 (m, 1H), 7.20 (d, J = 7.8 Hz, 1H), 7.14 (ddd, J = 7.6, 2934, 2835, 2058, 1884, 1611, 1510, 1301, 1247, 1176, 1034,
4.8, 1.2 Hz, 1H), 3.83 (q, J = 6.6 Hz, 1H), 3.75 (s, 2H), 2.19 (s, 816, 758. HRMS-ESI (m/z): [M + H]+ calcd for C14H17N2O,
1H), 1.42 (d, J = 6.6 Hz, 3H). 13C NMR (100 MHz, CDCl3, 229.1341; found, 229.1335.
298 K): δ 159.9, 149.4, 145.5, 136.5, 128.6, 127.1, 126.9, 122.6,
2-(1-(4-Trifluoromethylphenylmethylamino)methyl)pyridine,
122.0, 58.2, 53.2, 24.6. FTIR (KBr, cm−1): 3317 (ν (N–H)), 3066, L9H. Using the similar procedure established for the synthesis
2964, 2925, 1950, 1591, 1450, 1433, 1369, 1128, 994, 761, 701. of L1H, L9H was obtained as a pale yellow solid. 79% (3.9 g)
HRMS-ESI (m/z): [M + H]+ calcd for C14H17N2, 213.1392; found, and 93% (0.93 g) yield was obtained in the first and second
213.1387.
1
step respectively. H NMR (400 MHz, CDCl3, 298 K): δ 8.57 (dt,
2-(1-(Phenylmethylamino)methyl)pyridine, L5H. Using the J = 5.0, 1.4 Hz, 1H), 7.64 (td, J = 7.6, 1.8 Hz, 1H), 7.58 (d, J = 8.0
similar procedure established for the synthesis of L1H, L5H Hz, 2H), 7.49 (d, J = 8.0 Hz, 2H), 7.29 (d, J = 7.8 Hz, 1H), 7.17
was obtained as a pale yellow solid. 68% (2.5 g) and 62% (m, J = 7.6, 4.8, 1.2 Hz, 1H), 3.92 (s, 2H), 3.91 (s, 2H), 2.13 (s,
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1H). 19F NMR (376 MHz, CDCl3, 298 K): δ −62.39. 13C NMR
2-Methyl-6-(1-(diphenylmethylamino)methyl) pyridylferrous
(100 MHz, CDCl3, 298 K): δ 159.5, 149.5, 144.4, 136.6, 129.4 (q, chloride, Fe3Me. Using the same procedure and molar ratio of
J = 32.2 Hz), 128.5, 125.4 (q, J = 3.8 Hz), 124.4 (q, J = 273.0 Hz), reactants as described for Fe1H, but ligand L3Me was used
122.5, 122.2, 54.6, 53.1. FTIR (KBr, cm−1): 3313 (ν (N–H)), instead of L1H, Fe3Me was obtained as a white powder
3068, 3013, 2924, 2839, 1922, 1592, 1434, 1327, 1162, 1123, (236.0 mg, 82% yield). ATR-IR (cm−1): 3193 (ν (N–H)), 1606,
1066, 818, 757. HRMS-ESI (m/z): [M
C14H14F3N2, 267.1109; found, 267.1103.
+
H]+ calcd for 1578, 1494, 1466, 1454, 1433, 1383, 1169, 1083, 1032, 1019,
989, 939, 819, 790, 750. HRMS (ESI, m/z): [M − FeCl2 + H]+
2-(1-(2,4,6-Trimethylphenylmethylamino)methyl)pyridine, L10H. calcd for C40H41Cl2FeN4, 703.2058; found, 703.2051. Anal.
Using the similar procedure established for the synthesis of calcd for C20H20Cl2FeN2: C 57.86, H 4.86, N 6.75; found: C
L1H, L4H was obtained as a pale yellow solid. 89% (4.0 g) and 57.09, H 4.87, N 6.47.
69% (0.64 g) yield was obtained in the first and second step
2-(1-(1-Phenylethylamino)methyl)pyridylferrous chloride, Fe4H.
respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.59–8.53 (m, Using the same procedure and molar ratio of reactants as
1H), 7.65 (td, J = 7.6, 1.8 Hz, 1H), 7.38 (d, J = 7.8 Hz, 1H), described for Fe1H, but ligand L4H was used instead of L1H,
7.19–7.12 (m, 1H), 6.83 (s, 2H), 3.99 (s, 2H), 3.77 (s, 2H), 2.33 Fe4H was obtained as a yellow powder (160.0 mg, 50% yield).
(s, 6H), 2.24 (s, 3H). 13C NMR (100 MHz, CDCl3, 298 K): ATR-IR (cm−1): 3247 (ν (N–H)), 3026, 1954, 1606, 1571, 1487,
δ 160.3, 149.3, 137.2, 136.6, 136.5, 133.6, 129.1, 122.5, 122.0, 1430, 1383, 1293, 1152, 1077, 1042, 1021, 961, 855, 762. HRMS
55.6, 47.3, 21.0, 19.6. FTIR (KBr, cm−1): 3323 (ν (N–H)), 3006, (ESI, m/z): [M − FeCl2 + H]+ calcd for C28H33Cl2FeN4, 551.1432;
2916, 2860, 2388, 1590, 1570, 1433, 1375, 1094, 1047, 850, 756. found, 551.1445. Anal. calcd for C14H16Cl2FeN2: C 49.60,
HRMS-ESI (m/z): [M + H]+ calcd for C16H21N2, 241.1705; found, H 4.76, N 8.26; found: C 48.82, H 4.83, N 8.30.
241.1700.
2-(1-(Phenylmethylamino)methyl)pyridylferrous chloride, Fe5H.
2-Methyl-6-(1-(phenylmethylamino)methyl)pyridine, L11H. Using the same procedure and molar ratio of reactants as
Using the similar procedure established for the synthesis of described for Fe1H, but ligand L5H was used instead of L1H,
L1H, L4H was obtained as a pale yellow solid. 69% (2.4 g) and Fe5H was obtained as a yellow powder (270.0 mg, 83% yield).
88% (0.44 g) yield was obtained in the first and second step ATR-IR (cm−1): 3250 (ν (N–H)), 3026, 2942, 1605, 1571, 1488,
respectively. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.52 (t, J = 7.6 1442, 1335, 1306, 1149, 1088, 1021, 995, 912, 880, 765. HRMS
Hz, 1H), 7.29–7.39 (m, 4H), 7.23 (s, 1H), 7.12 (d, J = 7.6 Hz, (ESI, m/z): [M − FeCl2 + H]+ calcd for C26H29Cl2FeN4, 523.1119;
1H), 7.01 (d, J = 7.6 Hz, 1H), 3.89 (s, 2H), 3.85 (s, 2H), 2.54 (s, found, 523.1122. Anal. calcd for C13H14Cl2FeN2·1/4CH2Cl2: C
3H), 2.10 (s, 1H). 13C NMR (100 MHz, CDCl3, 298 K): δ 159.1, 45.96, H 4.22, N 8.09; found: C 45.70, H 4.35, N 8.49.
158.1, 140.4, 136.8, 128.5, 128.4, 127.1, 121.6, 119.3, 54.8, 53.7,
2-(1-(1-Methylethylamino)methyl)pyridylferrous
chloride,
24.6. FTIR (KBr, cm−1): 3318 (ν (N–H)), 3062, 3027, 2920, 2837, Fe6H. Using the same procedure and molar ratio of reactants
1593, 1577, 1456, 1155, 1118, 1028, 780, 742. HRMS-ESI (m/z): as described for Fe1H, but ligand L6H was used instead of L1H,
[M + H]+ calcd for C14H17N2, 213.1392; found, 213.1385.
Fe6H was obtained as a yellow powder (240.0 mg, 87% yield).
ATR-IR (cm−1): 3241 (ν (N–H)), 2968, 2931, 1605, 1571, 1486,
1444, 1388, 1322, 1291, 1151, 1139, 1101, 1055, 1026, 985, 971,
Synthesis of ferrous chloride complexes
2-(1-(Triphenylmethylamino)methyl)pyridylferrous chloride, 908, 857, 815, 766. HRMS (ESI, m/z): [M − FeCl2 + H]+ calcd for
Fe1H. In a glovebox, to a solution of ligand L1H (200 mg, C18H29Cl2FeN4, 427.1119; found, 427.1108. Anal. calcd for
0.57 mmol) in CH2Cl2 (10 mL), anhydrous FeCl2 (72.3 mg, C9H14Cl2FeN2: C39.03, H 5.10, N 10.11; found: C 38.75, H 5.15,
0.57 mmol) was added. The resulting suspension was stirred N 9.52.
for 48 h at room temperature. The precipitate was collected by
2-(1-(Cyclohexylamino)methyl)pyridylferrous chloride, Fe7H.
filtration under an argon atmosphere, washed with distilled Using the same procedure and molar ratio of reactants as
hexane (10 mL × 2) and dried under vacuum to afford Fe1H as described for Fe1H, but ligand L7H was used instead of L1H,
a gray white solid (223.0 mg, 82% yield). ATR-IR (cm−1): 3296 Fe7H was obtained as a yellow powder (250.0 mg, 66% yield).
(ν (N–H)), 3053, 1613, 1488, 1446, 1163, 1029, 976, 904, 870, ATR-IR (cm−1): 3245 (ν (N–H)), 2939, 2846, 1604, 1572, 1483,
774, 735, 705. HRMS (ESI, m/z): [M − FeCl2 + H]+ calcd for 1450, 1384, 1293, 1239, 1159, 1102, 1091, 1012, 995, 978, 932,
C50H45Cl2FeN4, 827.2371; found, 827.2386. Anal. calcd for 903, 895, 842, 771, 732. HRMS (ESI, m/z): [M − FeCl2 + H]+
C25H22Cl2FeN2: C 62.92, H 4.65, N 5.87; found: C 61.83, H calcd for C24H37Cl2FeN4, 507.1745; found, 507.1754. Anal.
4.69, N 5.80.
calcd for C12H18Cl2FeN2: C 45.46, H 5.72, N 8.84; found: C
2-(1-(Diphenylmethylamino)methyl)pyridylferrous chloride, 45.19, H 5.76, N 8.83.
Fe2H. Using the same procedure and molar ratio of reactants
2-(1-(4-Methoxyphenylmethylamino)methyl)pyridylferrous
as described for Fe1H, but ligand L2H was used instead of L1H, chloride, Fe8H. Using the same procedure and molar ratio of
Fe2H was obtained as a white powder (228.0 mg, 79% yield). reactants as described for Fe1H, but ligand L8H was used
ATR-IR (cm−1): 3209 (ν (N–H)), 3030, 2870, 1607, 1475, 1445, instead of L1H, Fe8H was obtained as a yellow powder
1283, 1153, 1040, 984, 943, 810, 774, 766. HRMS (ESI, m/z): (170.0 mg, 61% yield). ATR-IR (cm−1): 3249 (ν (N–H)), 2939,
[M − FeCl2 + H]+ calcd for C38H37Cl2FeN4, 675.1745; found, 2836, 2354, 1698, 1610, 1572, 1513, 1486, 1441, 1324, 1301,
675.1735. Anal. calcd for C19H18Cl2FeN2: C 56.89, H 4.52, N 1251, 1180, 1050, 1032, 994, 899, 815, 765. HRMS (ESI, m/z):
6.98; found: C 56.85, H 4.66, N 7.05.
[M − FeCl2 + H]+ calcd for C28H33Cl2FeN4O2, 583.1331; found,
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Table 4 Crystal data and structure refinement for Fe3Me, Fe4H, Fe7H and Fe11Me
Fe3Me
Fe4H
Fe7H
Fe11Me
Formula
Formula weight
Temperature/K
C20H20Cl2FeN2
415.13
298(2)
C28H32Cl4Fe2N4
678.08
298(2)
(C12H18Cl2FeN2)n
317.03n
293(2)
C14H16Cl2FeN2
339.04
298(2)
Crystal system
Space group
Triclinic
P1
Monoclinic
C2/c
Orthorhombic
Pbca
Monoclinic
P21/c
ˉ
a/Å
b/Å
c/Å
α/°
8.9981(8)
10.7524(9)
11.3021(11)
89.495(3)
66.7300(10)
79.902(2)
986.77(15)
2
15.1785(13)
9.3366(8)
22.6274(18)
90
103.782(2)
90
3114.3(5)
4
1.446
9.1540(8)
6.8673(6)
42.281(3)
90
90
90
2657.9(4)
8
1.585
12.62
1312
7.0439(7)
13.0684(11)
16.9522(12)
90
90.7610(10)
90
1560.4(2)
4
1.443
β/°
γ/°
V/Å3
Z
Dcalc/Mg m−3
1.397
1.04
428
Absorption coefficient/mm−1
F(000)
1.30
1392
1.30
696
Crystal size/mm
Θ limits/°
0.22 × 0.15 × 0.10
3.8–22.4
5011/3435
0.048
0.16 × 0.11 × 0.10
2.5–25.0
7713/2739
0.038
0.30 × 0.15 × 0.05
4.2–66.1
13 972/2242
0.115
0.48 × 0.42 × 0.40
2.4–25.0
7388/2760
0.029
Reflections collected/unique
Rint
Goodness of fit on F2
Final R indices [I > 2σ(I)]
1.011
1.04
1.09
1.06
R1 = 0.0539
wR2 = 0.1171
R1 = 0.0804
wR2 = 0.1264
R1 = 0.034
wR2 = 0.066
R1 = 0.059
wR2 = 0.072
R1 = 0.099
wR2 = 0.223
R1 = 0.1370
wR2 = 0.2580
R1 = 0.035
wR2 = 0.092
R1 = 0.0570
wR2 = 0.092
R indices (all data)
583.1343. Anal. calcd for C14H16Cl2FeN2O·1/4CH2Cl2: C 45.49, X-ray crystallographic studies
H 4.42, N 7.45; found: C 45.74, H 4.55, N 7.91.
For complexes Fe3Me, Fe4H, Fe7H and Fe11Me, a single crystal
suitable for X-ray analysis was sealed into a fine-focus sealed
tube, and the intensity data of the single crystal were collected
on the CCD-Bruker Smart system. All determinations of the
unit cell and intensity data for Fe3Me, Fe4H and Fe11Me were
performed with graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å); however, Xcalibur/Gemini was used with Cu-Kα
radiation (λ = 1.54178 Å) for crystal Fe7H. The structures were
solved by direct methods and refined by full-matrix least-
squares on F2. All hydrogen atoms were placed at calculated
positions. Structural solution and refinement were performed
using the SHELXL-2018/3.71 Crystal data and structure refine-
ment for Fe3Me, Fe4H, Fe7H and Fe11Me are shown in Table 4.
2-(1-(4-Trifluoromethylphenylmethylamino)methyl)pyridyl-
ferrous chloride, Fe9H. Using the same procedure and molar
ratio of reactants as described for Fe1H, but ligand L9H was
used instead of L1H, Fe9H was obtained as a yellow powder
(241.0 mg, 82% yield). ATR-IR (cm−1): 3251 (ν (N–H)), 2941,
1606, 1571, 1436, 1329, 1164, 1115, 1066, 1020, 986, 900, 815,
765. HRMS (ESI, m/z): [M − FeCl3]+ calcd for C28H26ClF6FeN4,
623.1100; found, 623.1107. Anal. calcd for C14H13Cl2F3FeN2: C
42.79, H 3.33, N 7.13; found: C 42.46, H 3.27, N 7.25.
2-(1-(2,4,6-Trimethylphenylmethylamino)methyl)pyridylferrous
chloride, Fe10H. Using the same procedure and molar ratio of
reactants as described for Fe1H, but ligand L10H was used
instead of L1H, Fe10H was obtained as a yellow powder
(160.0 mg, 70% yield). ATR-IR (cm−1): 3290 (ν (N–H)), 2979, General procedure for isoprene polymerization
2948, 1608, 1573, 1486, 1441, 1363, 1312, 1151, 1080, 1051,
In a typical experiment, a reactor was heated, dried in a
1022, 981, 894, 869, 813, 761. HRMS (ESI, m/z): [M − FeCl2 +
H]+ calcd for C32H41Cl2FeN4, 607.2058; found, 607.2070. Anal.
calcd for C16H20Cl2FeN2: C 52.35, H 5.49, N 7.63; found: C
51.36, H 5.51, N 7.55.
vacuum, and recharged with nitrogen three times. When the
reactor cooled down, iron(II) complexes (10 μmol), toluene
(5 mL), isoprene (2 mL, 20 mmol) and MAO cocatalyst were
added into the reactor in order. The reaction was employed for
the corresponding time and was then quenched with a diluted
HCl solution of methanol (MeOH/HCl = 50/1). The polymer
was collected by filtration and washed with methanol several
times and dried at room temperature for 24 h under vacuum
until constant weight was reached.
2-Methyl-6-(1-(phenylmethylamino)methyl)pyridylferrous
chloride, Fe11Me. Using the same procedure and molar ratio
of reactants as described for Fe1H, but ligand L11Me was used
instead of L1H, Fe11Me was obtained as a yellow powder
(277.0 mg, 87% yield). ATR-IR (cm−1): 3235 (ν (N–H)), 2941,
1608, 1575, 1469, 1340, 1275, 1164, 1094, 1051, 1022, 998, 876,
797, 746. HRMS (ESI, m/z): [M − FeCl2 + H]+ calcd for
C28H33Cl2FeN4, 551.1432; found, 551.1442. Anal. calcd for
C14H16Cl2FeN2: C 49.60, H 4.76, N 8.26; found: C 48.63, H
4.57, N 8.09.
Conflicts of interest
There are no conflicts to declare.
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22 J. Guo, B. Wang, J. Bi, C. Zhang, H. Zhang, C. Bai, Y. Hu
Acknowledgements
and X. Zhang, Polymer, 2015, 59, 124–132.
Generous support by the Major Science and Technology 23 Z. Wang, G. A. Solan, Q. Mahmood, Q. Liu, Y. Ma,
Innovation Program of Shandong Province (2018CXGC1105),
the CAS Hundred Talents Program (Y5100719AL), the Young
X. Hao and W.-H. Sun, Organometallics, 2018, 37, 380–
389.
Taishan
(tsqn201812112), the “135” Projects Fund of CAS-QIBEBT
Director Innovation Foundation, DICP& QIBEBT United 25 Q. Mahmood, E. Yue, J. Guo, W. Zhang, Y. Ma, X. Hao and
Scholars
Program
of
Shandong
Province 24 Q. Mahmood, J. Guo, W. Zhang, Y. Ma, T. Liang and
W.-H. Sun, Organometallics, 2018, 37, 957–970.
Foundation (UN201701) and the National Natural Science
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W.-H. Sun, Polymer, 2018, 159, 124–137.
26 Y. Nakayama, Y. Baba, H. Yasuda, K. Kawakita and
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