Highly trans-1,4 selective polymerization of 1,3-butadiene initiated by iron(III) bis(imino)pyridyl complexes

Article abstract of DOI:10.1016/j.ica.2011.03.047

A series of 2,6-bis(imino)pyridyl iron(III) complexes of the general formula [2,6-(ArNCMe)₂C₅H₃N]FeCl₃ (Ar = -C₆H₅, 3a; 2-MeC₆H₄, 3b; 2-EtC₆H₄, 3c; 2-ⁱPrC₆H₄, 3d; cyclohexyl, 3e; 4-MeC₆H₄, 3f; 4-ⁱPrC ₆H₄, 3g; 4-FC₆H₄, 3h and 4-CF ₃C₆H₄, 3i), activated by alkylaluminum, MAO or MMAO, have been investigated in 1,3-butadiene polymerization. Iron(III) complex (3a), with the least steric hindrance around the metal center, gives polymer up to 99% in yield in 4 h (butadiene to iron ratio = 1000), and trans-1,4 selectivity about 94.7% at room temperature in toluene, while those (3b-3d) bearing alkyl substituents at the 2-position of each N-aryl ring exhibit much lower catalytic activity and tunable trans-1,4 selectivity. Introduction of an alkyl group at the 4-position (para-position, 3f and 3g) exerts a slightly beneficial effect on the trans-1,4 selectivity, while electronegative groups at the same position (3h and 3i) affect negatively on the activity. The effects of temperature, types of cocatalyst and Al/Fe molar ratio on the polymerization behavior are investigated. More importantly, a mechanism for forming trans-1,4 structure is also proposed. Crown Copyright

Full text of DOI:10.1016/j.ica.2011.03.047

Inorganica Chimica Acta 373 (2011) 47–53
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Inorganica Chimica Acta
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Highly trans-1,4 selective polymerization of 1,3-butadiene initiated by iron(III)
bis(imino)pyridyl complexes
Dirong Gong a,b, Xiaoyu Jia a,b, Baolin Wang a,b, Feng Wang a,b, Chunyu Zhang a, Xuequan Zhang a,
,
⇑
Liansheng Jiang a, Weiming Dong a
a Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2,6-bis(imino)pyridyl iron(III) complexes of the general formula [2,6-(ArN@CMe)2C5H3N]FeCl3
(Ar = –C6H5, 3a; 2-MeC6H4, 3b; 2-EtC6H4, 3c; 2-iPrC6H4, 3d; cyclohexyl, 3e; 4-MeC6H4, 3f; 4-iPrC6H4, 3g;
4-FC6H4, 3h and 4-CF3C6H4, 3i), activated by alkylaluminum, MAO or MMAO, have been investigated in
1,3-butadiene polymerization. Iron(III) complex (3a), with the least steric hindrance around the metal
center, gives polymer up to 99% in yield in 4 h (butadiene to iron ratio = 1000), and trans-1,4 selectivity
about 94.7% at room temperature in toluene, while those (3b–3d) bearing alkyl substituents at the 2-
position of each N-aryl ring exhibit much lower catalytic activity and tunable trans-1,4 selectivity. Intro-
duction of an alkyl group at the 4-position (para-position, 3f and 3g) exerts a slightly beneficial effect on
the trans-1,4 selectivity, while electronegative groups at the same position (3h and 3i) affect negatively
on the activity. The effects of temperature, types of cocatalyst and Al/Fe molar ratio on the polymerization
behavior are investigated. More importantly, a mechanism for forming trans-1,4 structure is also
proposed.
Received 1 December 2010
Received in revised form 28 February 2011
Accepted 16 March 2011
Available online 24 March 2011
Keywords:
Iron(III) catalysts
2,6-Bis(imino)pyridine
Polybutadiene
Trans-1,4 selectivity
Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
an important component in the iron-based catalyst in our group
and by Luo, with which syndiotactically enriched 1,2-polybutadi-
The development of well-defined transition metal catalysts for
highly regio- and/or stereoselective polymerization of butadiene
has attracted considerable attention in polydienes areas [1–15],
because the resultant polymers find wide applications in gloves,
tube, tire industry, etc. Among the transition metals, high active
and selective iron-based catalysts featured an expedient prepara-
tion, low toxicity and high tolerance to moisture and air are highly
anticipated. However, low activity, poor selectivity and extremely
low molecular weight of resultant product are generally the tough
issues in iron-based catalysts. In spite of these, there has been a
considerable and growing interest in designing and synthesizing
ancillary electron donor or ligand to improve catalytic performance
over the past decade, and partial success has been achieved in
some systems. The activity has been improved by introducing
bidentate ligands such as 1,10-phenanthroline or 2,20-bipyridine
into iron dichloride complexes, but the formed catalysts only gives
equivalent cis-1,4 and 1,2-polybutadiene at room temperature,
high selectivity (1,2-selectivity: 91%) can be available only at
ꢀ78 °C at the expense of activity [16,17]. To date, progresses have
been made since dialkylphosphite and phosphate are invented as
ene (1,2-: 87–95%, pentad rrrr: 65–95%) can be obtained in butadi-
ene polymerization [18–24], albeit by a poorly structurally defined
catalyst and an unclear mechanism of formation of the active site.
In comparison with the versatile cobalt- and vanadium-based cat-
alysts, research on the iron counterparts has been relatively lim-
ited. Until recently, a few on with high 1,4-selectivity and some
with 1,2-selectivity have started to emerge [25–29]. Therefore,
organometallic iron-based catalysts with well-designed structure
for high 1,4-selective polymerization of butadiene are quite chal-
lenging and highly desired.
Over the past decade, the field of olefin polymerization catalysts
has undergone an important renewal with the discovery of extre-
mely high active iron(II) and cobalt(II)chloride carrying 2,6-
bis(imino)pyridyl ligands [30–33]. Encouraged by this initiation,
N,N,N-type auxiliary ligands have received an upsurge in research
interest and numerous corresponding metal complexes have
emerged in the olefin field [34–40]. The impressive activity of late
transition metal complexes bearing 2,6-bis(imino)pyridyl ligands,
especially, more remarkable activity of iron derivatives, in ethylene
polymerization indicates the significance of such kind of ligands in
olefin polymerization, presumably, the finding that may be rele-
vant for the diene polymerization area. Study on iron-complex
bearing such ligands for butadiene polymerization, seems to be
⇑
Corresponding author. Tel.: +86 431 8526 2303; fax: +86 431 8526 2307.
E-mail address: xqzhang@ciac.jl.cn (X. Zhang).
0020-1693/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.047

48
D. Gong et al. / Inorganica Chimica Acta 373 (2011) 47–53
ter. Elemental analyses were recorded on an elemental Vario EL
spectrometer. The proportion of 1,2, cis-1,4 and trans-1,4 units of
polymer was determined by IR spectra, 1H NMR and 13C NMR
[43,44]. The molecular weights (Mn) and molecular weight distri-
butions (Mw/Mn) of polymer were measured at 30 °C by gel perme-
ation chromatography (GPC) equipped with a Waters 515 HPLC
pump, four columns (HMW 7 THF, HMW 6E THF ꢁ 2, HMW 2
THF) and a Waters 2414 refractive index detector. THF was used
as eluent at a flow rate of 1.0 mL/min. The molecular weight of
polymer was determined using the polystyrene calibration. DSC
measurements were performed on a Perkin–Elmer Diamond differ-
ential scanning calorimeter at a heating rate of 10 °C/min.
2.2. Syntheses and characterizations of ligands (2a–2i) and complexes
(3a–3i)
2.2.1. Syntheses and characterizations of ligands (2a–2i)
The ligands 2a–2i were prepared by condensation reaction of
excess appropriate anilines or amine with of 2,6-diacetylpyridine
according to the literatures [33,37].
Scheme 1. The structures of complexes (3a–3i).
2.2.2. Preparation of complexes (3a–3i)
The complexes 3a–3i were synthesised by reaction of FeCl3 with
the corresponding ligands in THF. A typical synthetic procedure for
complex 3a can be described as follows. Anhydrous iron(III) chlo-
ride powder (0.16 g, 1.0 mmol) and 2,6-bis[1-(phenylimi-
no)ethyl]pyridine (0.31 g, 1.0 mmol) were added into an oxygen
and moisture free flask containing cold THF (8 mL), then cooled
to 0 °C for minimizing the undesired reduction of iron(III) to iro-
n(II). The red precipitate (0.46 g, 97.9%) was collected after stirring
for 1 h, followed by filtering, washing with heptane (3 ꢁ 5 mL) and
drying in vacuum.
quite limited [25,29,41,42]. Although the additional neutral do-
nors, such as dialkylphosphite and phosphate, as demonstrated
previously, can also act as effecitve ligands in occupying unsatu-
rated coordination site of metal center as a potential shelter for
deactivating poison, as well as in stiffening the structure of the
complex, thus, facilitating regio- and/or stereoselective polymeri-
zation, the latter (N,N,N-ligand) is more attractive due to more con-
strained environment and more tunable features endowed with
such tridentate framework, which may deliver a clear image on
the relationship between steric and electronic characteristics of
substituents and polymerization behaviors, especially, the
stereoselectivity.
We are interested in the 2,6-bis(imino)pyridyl ligand from two
aspects essential for selective polymerization, i.e. stabilizing the
central metal by the large electron conjugated system and inducing
specific selectivity by constraining the central metal through the
rigidness of ligand framework. Here, iron(III) complexes with steric
and electronic modified 2,6-bis(imino)pyridyl ligands (Scheme 1),
a new type of precursor, are synthesised for butadiene polymeriza-
tion in the presence of cocatalysts. The facile modification of the li-
gand by varying the substituent on the iminoaryl ring, allows us to
control the steric and electronic effects on the iron center, ulti-
mately, paves the way to access the desired catalysts.
2.2.2.1. {2,6-Bis-[1-(phenylimino)ethyl]pyridine}FeCl3 (3a). Yield:
97.9%. IR(KBr, cmꢀ1): 3079, 1626(vC@N), 1586, 1486, 1372, 1266,
1229, 1027, 777, 694. Anal. Calc. for C21H19N3FeCl3: C, 53.03; H,
4.03; N, 8.84.Found: C, 53.30; H, 3.89; N, 8.68%.
2.2.2.2. {2,6-Bis-[1-(2-methylphenylimino)ethyl]pyridine}FeCl3
(3b). Yield: 92.8%. IR (KBr, cmꢀ1): 3058, 1622 (vC@N), 1586,
1486, 1451, 1372, 1268, 1031, 755. Anal. Calc. for
C
23H23N3FeCl3: C, 54.85; H, 4.61; N, 8.34. Found: C, 54.50; H,
4.50; N, 8.68%.
2.2.2.3. {2,6-Bis-[1-(2-ethylphenylimino)ethyl]pyridine}FeCl3
(3c). Yield: 85.4%. IR (KBr, cmꢀ1): 2972, 2875, 1623 (vC@N),
1484, 1457, 1373, 1265, 1032, 748. Anal. Calc. for
2. Experimental
C25H27N3FeCl3: C, 56.47; H, 5.12; N, 7.90. Found: C, 56.80; H,
5.00; N, 7.68%.
2.1. General considerations
2.2.2.4. {2,6-Bis-[1-(2-isopropylphenylimino)ethyl]pyridine}FeCl3
(3d). Yield: 84.6%. IR (KBr, cmꢀ1): 2964, 2867, 1623 (vC@N),
1585, 1485, 1444, 1372, 1266, 1030, 756. Anal. Calc. for
All manipulations for air- or/and moisture-sensitive compounds
were carried out under a nitrogen atmosphere using standard
Schlenk techniques. Anhydrous FeCl3 and all anilines were pur-
chased from Alfa Aesar. 2,6-Diacetylpyridine was purchased from
Aldrich. Triisobutylaluminum (AliBu3), triethylaluminum (AlEt3),
methylaluminoxane (MAO) and modified methylaluminoxane
(MMAO) were commercially products from AkzoNoble. Toluene
and tetrahydrofuran (THF) were freshly distilled in the presence
of sodium and benzophenone. Polymerization grade butadiene
was supplied from Jinzhou Petrochemical Corporation and purified
by passing through four columns packed with 4 Å molecular sieves
and KOH. Other chemicals were used as received unless otherwise
noted. 1H NMR (400 MHz) and 13C NMR (100 MHz) were recorded
on a Varian Unity spectrometer in CDCl3 at room temperature. IR
spectra were performed on BRUKE Vertex-70 FIR spectrophotome-
C
27H31N3FeCl3: C, 57.93; H, 5.58; N, 7.51. Found: C, 57.60; H,
5.70; N, 7.88%.
2.2.2.5. {2,6-Bis-[1-cyclohexylimino)ethyl]pyridine)}FeCl3 (3e). Yield:
77.5%. IR (KBr, cmꢀ1): 2963, 2868, 1621 (vC@N), 1584, 1485, 1444,
1368, 1267, 1209, 785. Anal. Calc. for C21H31N3FeCl3: C, 51.72; H,
6.41; N, 8.62. Found: C, 51.50; H, 6.60; N, 8.78%.
2.2.2.6. {2,6-Bis[1-(4-methylphenylimino)ethyl]pyridine}FeCl3
(3f). Yield: 87.5%. IR (KBr, cmꢀ1): 2921, 1620 (vC@N), 1584,
1505, 1372, 1267, 1232, 1028, 850, 817. Anal. Calc. for
C
23H23N3FeCl3: C, 54.85; H, 4.60; N, 8.34. Found: C, 54.99;
H, 4.79; N, 8.15%.

D. Gong et al. / Inorganica Chimica Acta 373 (2011) 47–53
49
Table 2
2.2.2.7. {2,6-Bis[1-(4-isopropylphenylimino)ethyl]pyridine}FeCl3.
(3g). Yield: 90.5%. IR (KBr, cmꢀ1): 2960, 1624 (vC@N), 1585,
1558, 1372, 1266, 1116, 857, 814. Anal. Calc. for
Selected bond distances (Å) and angles (°) of complexes 3a, 3d and 3f.
3a 3d 3f
C
27H31N3FeCl3: C, 57.93; H, 5.58; N, 7.51. Found: C, 57.64; H,
Fe–N1
Fe–N2
Fe–N3
Fe–Cl1
Fe–Cl2
Fe–Cl3
2.118(5)
2.155(5)
2.151(5)
2.2427(19)
2.361(2)
2.321(2)
2.119(4)
2.177(3)
2.185(4)
2.2517(13)
2.3164(14)
2.3789(13)
73.18(14)
73.74(14)
146.72(14)
175.90(11)
105.48(11)
107.77(10)
83.21(11)
90.70(12)
88.91(10)
92.97(5)
2.1163(19)
2.1794(19)
2.178(2)
2.2354(7)
2.3622(7)
2.3323(7)
5.64; N, 7.79%.
2.2.2.8. {2,6-Bis[1-(4-fluorophenylimino)ethyl]pyridine}FeCl3
(3h). Yield: 65.5%. IR (KBr, cmꢀ1): 1623 (vC@N), 1588,
1501, 1373, 1267, 1155, 1096,1029, 759, 817. Anal. Calc.
for C21H17F2N3FeCl3: C, 49.30; H, 3.35; N, 8.21. Found: C,
49.04; H, 3.59; N, 8.09%.
N1–Fe–N3
N1–Fe–N2
N3–Fe–N2
N1–Fe–Cl1
N3–Fe–Cl1
N2–Fe–Cl1
N1–Fe–Cl3
N3–Fe–Cl3
N2–Fe–Cl3
Cl1–Fe–Cl3
N3–Fe–Cl2
N2–Fe–Cl2
N1–Fe–Cl2
Cl1–Fe–Cl2
Cl3–Fe–Cl2
72.9(2)
73.5(2)
73.33(7)
73.56(7)
146.86(7)
177.61(6)
104.58(6)
108.56(6)
86.25(5)
89.44(5)
87.11(6)
94.92(3)
90.16(5)
88.67(5)
85.65(5)
93.25(3)
171.66(3)
146.13(19)
176.00(16)
109.37(14)
104.42(14)
87.57(14)
86.74(14)
87.62(14)
95.81(7)
89.13(15)
91.41(15)
83.63(14)
93.05(7)
2.2.2.9. {2,6-Bis[1-(4-trifluoromethylphenylimino)ethyl]pyridine}FeCl3
(3i). Yield: 58.5%. IR (KBr, cmꢀ1): 3011, 1611 (vC@N), 1587, 1376,
1322, 1267, 1235, 1166, 1066, 871, 831, 668. Anal. Calc. for
C23H17F6N3FeCl3: C, 45.17; H, 2.80; N, 6.87. Found: C, 45.54; H,
2.70; N, 6.64%.
86.01(12)
89.83(11)
88.82(11)
94.97(5)
2.3. X-ray structure determinations
171.05(7)
171.97(5)
Data collections were performed at ꢀ88.5 °C on a Bruker SMART
APEX diffractometer with a CCD area detector, using graphite
monochromated Mo Ka radiation (k = 0.71073 Å). The determina-
tion of crystal class and unit cell parameters was carried out by
the SMART program package. The raw frame data were processed
using SAINT and SADABS to yield the reflection data file. The structures
were solved by using SHELXTL program. Refinement was performed
on F2 anisotropically for all non-hydrogen atoms by the full-matrix
least-squares method. The hydrogen atoms were placed at the cal-
culated positions and were included in the structure calculation
without further refinement of the parameters. Crystal data and col-
lection parameters as well as the selected bond distances and an-
gles are collected in Tables 1 and 2, respectively.
Table 3
Influence of substituents of iminoary moieties of complex on polymerization of
butadiene.
Entrya Complex Yield
(%)
Mn
Mw
Mn
/
Microstructure (%)
Tm
b
(ꢁ10ꢀ4
)
Trans-
Cis-
1,2
1,4
1,4
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
3i
99
25
8
10
5
99
97
64
43
2.9
25.3
29.5
27.8
23.6
3.7
3.1
3.0
3.3
2.0
2.9
2.6
2.7
2.7
2.2
2.0
2.1
2.0
94.7
43.2
30.0
10.0
8.3
95.1
96.8
93.4
92.7
0
5.3 101
13.6
29.6
43.6
27.6
0.2
0.1
0.4
0.6
43.2
–
–
–
–
40.4
46.4
64.1
2.4. Procedure for butadiene polymerization
4.7 109
3.1 111
6.2
6.7
98
99
A typical procedure for the polymerization is as follows (entry 1
in Table 3): a toluene solution of butadiene (5 mL, 2.0 mol/L) was
added to a moisture free ampoule bottle preloaded with complex
3a (4.7 mg, 0.01 mmol). AliBu3 (0.4 mL, 1.0 mol/L) was injected to
initiate the polymerization at 20 °C. After 4 h, methanol was added
a
Polymerization conditions: complex, 0.01 mmol; AliBu3, 0.4 mmol; butadiene,
0.01 mol; toluene, 5 mL; temperature, 20 °C; time, 4 h.
b
Melting temperature, determined by DSC.
Table 1
Crystal data and data collection parameters.
3aꢂ0.5C6H6
3dꢂ2CH2Cl2
3fꢂ0.5C6H6
Formula
C21H19Cl3N3Feꢂ0.5C6H6
C27H31Cl3N3Feꢂ2CH2Cl2
C23H23Cl3N3Feꢂ0.5C6H6
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
514.65
729.60
542.70
monoclinic
P2(1)/c
13.369(3)
9.5751(19)
19.502(4)
90
monoclinic
P2(1)/c
15.9012(10)
12.5736(8)
18.4593(11)
90
monoclinic
P2(1)/n
12.2348(5)
14.8542(6)
14.2180(6)
90
a
(°)
b (°)
103.652(4)
90
2425.9(8)
4
1.409
0.968
112.4350(10)
90
3411.3(4)
4
1.421
1.014
91.3540(10)
90
2583.23(18)
4
1.395
0.913
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
1056
1500
1120
Crystal size (mm)
h Range (°)
Noumber of reflections collected
Number of independent reflections (Rint
Number of data/restraints/parameters
Goodness-of-fit (GOF) on F2
R1 (I > 2sigma(I))
0.12 ꢁ 0.09 ꢁ 0.06
1.57–25.07
12 330
4278 (0.1308)
4278/3/282
0.963
0.38 ꢁ 0.09 ꢁ 0.08
2.01–25.36
17 705
6228 (0.0938)
6228/0/367
1.061
0.22 ꢁ 0.19 ꢁ 0.08
1.98–26.07
14 145
5108 (0.024)
5108/0/302
1.051
)
0.0742
0.0691
0.0371
wR2
0.1125
0.1685
0.9988

50
D. Gong et al. / Inorganica Chimica Acta 373 (2011) 47–53
Table 4
to the system to quench the polymerization. The mixture was
poured into a large quantity of methanol containing 2,6-di-tert-bu-
tyl-4-methylphenol (1.0 wt.%) as a stabilizer. After filtration and
drying under vacuum at 40 °C to constant weight, polybutadiene
was collected (0.53 g, 99%).
Effects of temperature on polymerization of butadiene with complex 3a.
b
Entrya Temp
Yield
(%)
Mn
Mw
Mn
/
Microstructure (%)
Tm
(°C)
(ꢁ10ꢀ4
)
Trans-
Cis-
1,2
1,4
1,4
10
11
12
13
14
ꢀ20
0
20
40
60
13
93
99
78
40
1.7
2.3
2.9
2.6
2.5
1.4
2.3
2.0
1.9
1.8
84.7
85.0
94.7
94.5
93.5
2.3
2.6
0
0
0
13.0
12.4
5.3 101
5.5 100
90
82
3. Results and discussion
3.1. Syntheses and characterization of ligands and complexes
6.5
97
a
Polymerization conditions: complex 3a, 0.01 mmol; AliBu3, 0.4 mmol; butadi-
2,6-Bis(iminoethyl)pyridine derivatives (2a–2i) are readily pre-
pared by condensation of anilines with 2,6-diacetylpyridine in
CH2Cl2 in the presence of a calculated amount of acid. Complexes
(3a–3i) are synthesised by reaction of anhydrous iron(III) chloride
with the corresponding ligands in THF. All complexes are charac-
terized by the elemental analyses and IR spectra. The elemental
analyses results reveal that the components of all complexes are
in accord with the formula FeCl3L. The IR spectra of the free ligands
show that the C@N stretching frequencies appear at 1648–
ene, 0.01 mol; toluene, 5 mL; time, 4 h.
b
Melting temperature, determined by DSC.
Table 5
Effects of Al/Fe molar ratio and type of Al on polymerization of butadiene with
complex 3a.
Entrya Type
of Al
Al/Fe
(equiv) (%)
Yield Mn
Mw
Mn
/
Microstructure (%) Tm
Trans- Cis- 1,2
b
(ꢁ10ꢀ4
)
1633 cmꢀ1
, which shift toward lower frequencies to 1626–
1611 cmꢀ1 in the complexes together with greatly reduced inten-
sity, indicating the coordination interaction between the imino
nitrogen atoms and the metal ions occurs. Complexes 3a, 3d and
3f are further characterized by X-ray crystallography and the crys-
tal structures of 3f is depicted in Fig. 1.
1,4
1,4
15
16
17
18
19
20
21
22
23
24
AliBu3
AliBu3
AliBu3
AliBu3
MAO
MAO
MMAO
MMAO 100
AlEt3
AlEt3
20
40
80
150
50
100
50
98
99
99
2.8
2.9
2.7
2.8
2.9
2.7
3.1
2.9
1.4
1.0
2.1
2.0
1.9
1.9
2.0
1.9
1.8
2.0
1.3
1.9
94.7
94.7
94.6
94.7
94.8
94.0
95.4
94.1
95.5
94.9
0
0
0
0
0
0
0
0
0
0
5.3 103
5.3 101
5.4 103
98
5.2 101
6.0 103
98
99
4.5 100
5.1 97
97
5.3
100
100
99
99
90
The conformers of the complexes 3a, 3d and 3f are isomor-
phous, all having approximate Cs symmetry about a plane formed
by the iron atom, the three chlorine groups and the pyridyl nitro-
gen atom. The geometry around the iron atom could be described
as a distorted octahedron with the equatorial plane formed by
three nitrogen atoms, one chlorine ligand (Cl(1))and the two axial
Fe–Cl bonds. The deviations of the metal ion from the plane formed
by the coordinated nitrogen atoms in 3a, 3d and 3f are 0.1127,
0.0940 and 0.0338 Å, respectively. Three chlorine ligands are in a
meridional geometry. The Fe–Cl bond distances located at trans po-
sition (Fe–Cl(1)) to pyridine are shorter than those at cis position to
pyridine (Fe–Cl(2)) in three complexes. The Fe–Cl(2) bond distance
in 3d is shorter than the corresponding distances in 3a and 3f,
while Fe–Cl(3) bond in 3d is longer than its analogs. The Fe–N(pyr-
idyl) bond distances of complexes are about 0.035–0.070 Å shorter
than that of Fe–N(imino), and Fe–N(imino) bonds in 3a are about
0.03 Å shorter than the counterparts in 3d and 3f. The planes of
the phenyl rings are all oriented essentially orthogonal to the coor-
dination planes of the bis(imino)pyridine unit. The two imino C@N
4.6
5.9
20
40
92
a
Polymerization conditions: complex 3a, 0.01 mmol; butadiene, 0.01 mol; tolu-
ene, 5 mL; temperature, 20 °C; time, 4 h.
b
Melting temperature, determined by DSC.
bonds have typical double bond character with C@N bond lengths
from 1.277(7) to 1.291(3) Å.
3.2. Butadienepolymerization behaviors
As reflected in Tables 3–5, the catalytic productivity, micro-
structure and molecular weight of polymer are significantly af-
fected by the nature of the substituent on the ligand frame,
consistent with the cases of ethylene polymerization and oligo-
merization with transition metals bearing such kind of ligands
[33]. In the meanwhile, catalytic performances are less affected
by the variation of polymerization conditions, such as Al/Fe molar
ratio, types of cocatalyst and polymerization temperature.
3.2.1. Substituent at the ortho-position
Iron(III) complex 3a, possessing the least steric hindrance
around the iron atom among all the complexes is the most active
one, polymerizing butadiene in nearly 100% yield. The obtained
polybutadiene possesses 94.6% trans-1,4 chain structure (Fig. 2)
with molecular weight of 2.9 ꢁ 104 g/mol and molecular weight
distribution (Mw/Mn) about 2.0, indicating polymerizations are
probably performed by a single active site. Interestingly, incorpora-
tion of sterically bulky alkyl groups onto the 2-positions of each N-
aryl moiety remarkably influences the polymerization behaviors.
With the group at 2-position of each phenyl ring of the ligand in
complexes gradually varying from 2-H (3a), 2-methyl (3b), 2-ethyl
(3c) to 2-isopropyl (3d), both the yield and trans-1,4 selectivity de-
crease, from 99% to 10% in the yield and 94.6% to 10% in the trans-
1,4 selectivity, while the molecular weight of yielded polymer
jumps from 2.9 ꢁ 104 g/mol (3a) to 25.3 ꢁ 104 g/mol (3b), and then
Fig. 1. ORTEP view of the complex 3f, drawn at 35% of probability. Hydrogen atoms
and the benzene molecule were omitted for clarity.

D. Gong et al. / Inorganica Chimica Acta 373 (2011) 47–53
51
species, therefore, the trans-1,4selectivity varies only in a rather
narrow range. Referring to catalytic activity, the alkyl substituent
at the 4-position also has a negligible effect on the electronic nat-
ure of the active species, and the activity. Electron withdrawing
groups, however reduce the electron density of the metal center
and probably destablize the active site, diminishing the catalytic
activity and reducing the polymer yield.
Combined with the effects of the 2-positoned substituents, it
can be concluded that bulkiness at the 2-positions plays an impor-
tant role in affecting the activity and selectivity, while tho seat the
4-positions, regardless of the nature and size, influence the poly-
mer yield only.
3.2.3. Butadiene polymerization behavior by using complex 3a
Besides the structure of the complexes, the polymerization con-
ditions play an important role in controlling the activity, selectivity
as well as the properties of the resulted polymer [17,19,21,48,49].
Herein, the influence of the reaction temperature is investigated by
using complex 3a as a precursor. The obtained results in Tables 4
show some interesting differences with the aforementioned iron
catalysts [16–19]. The activity is dependent on the polymerization
temperature. Maximal yield up to 99% is noticed at 20 °C, followed
by a decrease with increasing temperature. It is most deduced that
the lifetime of active site reaches maximum at 20 °C, then de-
creases with increasing temperature due to the deactivation of
the active site. It is reported that for most iron-based catalysts,
both catalytic selectivity and molecular weight of the resulting
polymer decreases with increasing polymerization temperature,
especially above room temperature, distinctively, both trans-1,4
selectivity and molecular weight of the obtained polymer increase
with the polymerization temperature rising from ꢀ20 to 20 °C, and
are kept as it increases further. The results demonstrate that the
active site displays a superior tolerance to that found in the iron
catalysts with bidentate ligands, and the latter has been shown
that optimal activity and selectivity could not be reached simulta-
neously, for instance, selective polymerization is only performed at
extremely low temperature (ꢀ78 °C), while the high activity ap-
pears at room temperature only [16–19]. It turns out that the li-
gand system employed here is capable of stabilizing the
activated metal center in spite of the severe temperature variation.
With complex 3a as a precursor, the effects of the Al/Fe molar
ratio and the types of the alkyl aluminum on polymerization
behavior are investigated (Table 5). With a variation of the Al/Fe
molar ratio from 20 to 150, little influence on the selectivity, the
activity, or even the Mn and Mw/Mn of the polymer can been seen,
differentiating from the reported iron systems where either the
activity or the molecular parameters of the polymer is governed
by the cocatalyst concentration [18–21]. Similarly, no significant
difference with respect to polymerization behaviors is apparent
when AliBu3, MMAO, MAO, or AlEt3 is used as cocatalyst, respec-
tively, with an exception that the Mn of the polymer observed in
the AlEt3 activated system is almost half of that found in the pres-
ence of AliBu3. These results suggest the catalyst exhibits superior
tolerance to the natures of the alkyl aluminum activators.
Fig. 2. The 13C NMR of trans-1,4 polybutadiene (entry 1 in Table 3).
becomes insensitive to the size of the group at the 2-position. The
decreased catalytic activity can possibly be ascribed to increased
difficulty for coordination of butadiene monomer to the iron cen-
ter, as the space around the metal center becomes more crowded
[45] and the substantial increased molecular weight of polymer
can probably be attributed to the suppressed b-hydride elimina-
tion and favoring chain propagation rate over chain transfer rate.
Similar results have been shown in ethylene oligomerization and
polymerization
using
catalysts
bearing
2,6-bis[(iminoa-
ryl)ethyl]pyridine ligand with the bulky group at the ortho-posi-
tions of the N-arylgroups [33,46,47]. Surprisingly, trans-1,4
selectivity decreases with the increment of the bulkiness of the
group at the 2-positions though the correlation need to be ex-
plained further.
Interestingly, replacement of the phenyl group in 3a with cyclo-
hexyl, a nonconjugated substituent, in both N-aryl moieties of the
ligand (3e) also witnesses a substantial decay in the activity and a
remarkable decline in trans-1,4selectivity. The results may imply
that the large and rigid
the catalytic activity and high trans-1,4 selectivity. On one hand,
the large -system of ligand system exerting as an electroning
p-system is essential in maintaining both
p
withdraw moiety, increases the Lewis-acidity of the metal center,
thus, facilitates coordination of butadiene monomer and impacts
positively on the polymerization activity, on the other hand, the
cyclohexyl group is more flexible compared to the phenyl group,
and such flexibility may go against the trans-1,4 selectivity.
3.2.2. Substituent at the 4-position
The effects of 4-substituents on the iminophenyl rings on poly-
merization behavior were also evaluated while leaving the 2-posi-
tions on each iminoary ring open. As seen from entries 5 and 6 in
Table 3, complexes 3f and 3g with methyl and isopropyl group at
the4-positions, respectively, show marginally higher trans-1,4
selectivity (95.1% and 96.8%, respectively) than the benchmark
complex 3a, while the activity is maintained as high as its analog
3a. Interestingly, while the high trans-1,4 selectivity is kept, the
yield decreases to 64% and 43% when electronegative groups, F
(3h) and CF3 (3i), are introduced, respectively. Therefore, the activ-
ity follows the trends of 4-H(3a) ꢃ 4-methyl (3f) ꢃ 4-isopropyl
(3g) > 4-F (3h) > 4-F (3i), while trans-1,4 selectivity is in the order
of 4-CF3 (3h) < 4-F (3i) < 4-H(3a) < 4-methyl (3f) < 4-isopropyl
(3g). It can be deduced that the size of the substituents at the 4-po-
sition has a rather feeble effect on the steric nature of the active
3.2.4. Mechanism study on trans-1,4 selectivity
It has been reported that there may be two types of active spe-
cies which undergo different polymerization mechanisms in buta-
diene polymerization [50–56], as shown in Scheme
2 [57],
according to the structure of the ligands. Iron catalysts such as
the bidentate counterparts FeCl2(bipyridine)2 [19] and FeEt2(bipyr-
idine)2 [18] tended to produce polybutadiene with mixed cis-1,4
and 1,2 selectivity, through the insertion of the monomer in an
alternative mode to the iron anti-g -
3-allyl intermediate by cis-g4
coordination of butadiene with the central metal (Path 1) at room
temperature. It is worthy to note that one bipyridine is assumed to

52
D. Gong et al. / Inorganica Chimica Acta 373 (2011) 47–53
Scheme 2. The proposed mechanism for butadiene polymerization.
disassociate from metal center during the polymerization [19],
otherwise, the electron number in iron(II) active species would
surpass 18e, and such disassociation has also been deduced in
CrCl2 complexes with two bidentated phosphine ligands [6]. In
comparing to complexes bearing a bidentate ligand, one more
coordination site in the corresponding metal is occupied in case
of those with a tridentate ligand, as the iron(III) is assumed to be
reduced to the Fe(II) in the presence of cocatalyst. Thus, the syn-
activator types and polymerization temperature. The high trans-
1,4 selectivity can be ascribed to the predominately trans-g
2coordi-
nation of butadiene to the metal center.
Acknowledgments
The authors appreciate financial supports from the National Sci-
ence and Technology Infrastructure Program (2007BAE14B01-06
and 2007BAE32B01) and The Fund for Creative Research Groups
(50621302).
g
3-allyl polymer chain resulting from one double bond coordinated
intermediate (trans-
g
2coordination) (Scheme 2, path 2) is mainly
produced, giving, ultimately, a trans-1,4 chain structure dominated
polybutadiene. It is presumed that, by employing a tridentate li-
gand, the coordination mode of iron with monomer and the confor-
Appendix A. Supplementary material
mation of
p-allyl polymer intermediate have been altered, and in
CCDC 713160, 713933, 740169 contain the supplementary crys-
tallographic data for 3a, 3d and 3f, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2011.03.047.
turn, the microstructure of resulting polymer switches to trans-
1,4 dominated.
4. Conclusion
Bis(imino)pyridyl iron(III) complexes have been synthesised,
characterized and butadiene polymerization has been examined
and discussed. The effects of the substituents at the 2-positions
and the 4-position of the N-aryl moieties on the catalytic activity,
selectivity and molecular weight of the resulting polymer have
been explored from the viewpoint of both steric and electronic nat-
ures of the substituents. It has been proved that the complexes
bearing a 2,6-bis(iminoethyl)pyridine tridentate ligand as precur-
sors have a high tolerance to the variations of Al/Fe molar ratio,
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