Synthesis of vanadium(III) complexes bearing iminopyrrolyl ligands and their role as thermal robust ethylene (co)polymerization catalysts

Article abstract of DOI:10.1039/c0dt01650k

Bis(imino)pyrrolyl vanadium(iii) complexes 2a-e [2,5-C₄H ₂N(CHNR)₂]VCl₂(THF)₂ [R = C ₆H₅ (2a), 2,6-Me₂C₆H₃ (2b), 2,6-ⁱPr₂C<

Full text of DOI:10.1039/c0dt01650k

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 3490
www.rsc.org/dalton
PAPER
Synthesis of vanadium(III) complexes bearing iminopyrrolyl ligands and their
role as thermal robust ethylene (co)polymerization catalysts†
Jing-Shan Mu,a,b Yong-Xia Wang,a Bai-Xiang Lia and Yue-Sheng Li*a
Received 28th November 2010, Accepted 27th January 2011
DOI: 10.1039/c0dt01650k
Bis(imino)pyrrolyl vanadium(III) complexes 2a–e [2,5-C4H2N(CH NR)2]VCl2(THF)2 [R = C6H5 (2a),
2,6-Me2C6H3 (2b), 2,6-iPr2C6H3 (2c), 2,4,6-Me3C6H2 (2d), C6F5 (2e)] and bis(iminopyrrolyl)
vanadium(III) complex 4f [C4H3N(CH N-2,6-iPrC6H3)]2VCl(THF) have been prepared in good yields
from VCl3(THF)3 by treating with 1.0 and 2.0 equivalent deprotonated ligands in tetrahydrofuran
(THF), respectively. These complexes were characterized by FTIR and mass spectra as well as
elemental analysis. Structures of 2c and 4f were further confirmed by X-ray crystallographic analysis.
DFT calculations indicated the configurations of 2a–e with two nitrogen atoms of the chelating ligand
coordinating with vanadium metal centre were more stable in energy. These complexes were employed
as catalysts for ethylene polymerization at various reaction conditions. On activation with Et2AlCl,
these complexes exhibited high catalytic activities (up to 22.2 kg mmol-1 h-1 bar-1) even at high
V
temperature, suggesting these catalysts possessed remarkable thermal stability. Moreover, high
molecular weight polymer with unimodal molecular weight distributions can be obtained, indicating
the polymerization took place in a single-site nature. The copolymerizations of ethylene and 1-hexene
with precatalysts 2a–e and 4f were also explored in the presence of Et2AlCl. Catalytic activity,
comonomer incorporation, and properties of the resultant polymers can be controlled over a wide
range by tuning catalyst structures and reaction parameters.
form to prepare efficient vanadium metal catalysts for precise,
controlled olefin polymerization. Recently, we reported a series
of vanadium(III) complexes bearing b-enaminoketonato18a–d and
Introduction
Among the transition metals complexes, vanadium-based catalysts
salicyladiminato18c–f ligands, which were active towards ethylene
(co)polymerization in the presence of alkylaluminium compounds
and produced high molecular weight (co)polymers. More re-
cently, we introduced a new family of vanadium(III) contain-
ing mono(iminopyrrolyl) ligands.19 These complexes were also
efficient catalysts for ethylene polymerization. However, at high
polymerization temperature, broad molecular weight distribution
polymers were obtained since the fast deactivation or chain
transfer to aluminium occurred. Thus, this work was initially
aimed at introducing the steric bulk around the mental center
to stabilize vanadium(III) complexes and improve the thermal
stability.
In this paper, we described the synthesis and characterization of
new vanadium(III) complexes bearing bis(imino)pyrrolyl ligands
2a–e and bis(iminopyrrolyl) chelate ligand 4f (see Scheme 1),
and explored the potential application in ethylene polymeriza-
tion and the copolymerization of ethylene with 1-hexene. As
expected, these new precatalysts could effectively restrain active
vanadium center from deactivation and control chain transfer re-
action, therefore, high molecular weight polymers with unimodal
molecular weight distributions can be easily obtained at high
temperature.
have played a critically important role in producing high molecular
weight polyethylene with narrow molecular weight distribution as
well as ethylene/a-olefin or functional a-olefin copolymers with
high comonomer incorporations.1 However, catalyst deactivation
at high polymerization temperature is an issue in polymerization
with vanadium(III) complexes due to reduction of catalytically
active vanadium species to low-valent, less active or inactive
species. Therefore, the design and synthesis of new vanadium(III)
complexes with high thermal stability in olefin polymerizations
have attracted considerable attention.2–8
Nitrogen-based polydentate ligands have been widely used in
olefin polymerization catalysis over the past decades. A key
attraction of these ligands is their availability and amenability
to modification via straightforward Schiff-base condensation
procedures.9–17 We became attracted to building on this plat-
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, China. E-mail: ysli@ciac.jl.cn; Fax: +86-431-85262039
bGraduate School of the Chinese Academy of Sciences, Changchun Branch,
Changchun, 130022, China
† CCDC reference numbers 802187–802188. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt01650k
3490 | Dalton Trans., 2011, 40, 3490–3497
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 Crystallographic data and collection parameters, and refinement parameters
2c
4f
Empirical formula
fw
C38H54Cl2N3O2V
706.68
185(2) K
C38H50ClN4OV
665.21
185(2) K
Monoclinic
C2/c
11.0190(9)
14.8900(12)
22.0694(18)
3534.5(5)
90
102.5500(10)
90
Temperature
Cryst. syst.
Space group
Triclinic
¯
P1
˚
a/A
9.3696(7)
˚
b/A
15.8826(12)
16.5052(13)
2345.1(3)
95.5610(10)
106.3050(10)
90.2220(10)
2
1.001
0.354
752
0.32 ¥ 0.17 ¥ 0.11
1.29 to 26.04
13264/9045 [Rint = 0.0217]
Semi-empirical from equivalents
0.9621 and 0.8952
Full-matrix least-squares on F2
R1 = 0.0492, wR2 = 0.1194
R1 = 0.0706, wR2 = 0.1289
0.339 and -0.337 e
˚
c/A
3
˚
V/A
a (◦)
b (◦)
g (◦)
Z
4
D
calc/Mg m3
1.250
0.391
1416
0.24 ¥ 0.18 ¥ 0.08
1.89 to 26.03
9803/3489 [Rint = 0.0254]
Semi-empirical from equivalents
0.9709 and 0.9109
Full-matrix least-squares on Fs2
R1 = 0.0513, wR2 = 0.1405
R1 = 0.0594, wR2 = 0.1464
1.008 and -0.435 e
Abs. coeff./mm-1
F(000)
Crystal size/mm
q range (deg)
Reflections collected/unique
Abs. Corr.
Max. and min. transmn
Refinement method
Final R indices [I > 2s(I)]
R indices (all data)
Largest diff. peak and
-3
˚
hole/A
crystallization from a mixture of THF and hexane at room
temperature. The mono(iminopyrrolyl) vanadium(III) complex
[C4H2N(CH N-2,6-iPrC6H3)]VCl2(THF)2 (I) was also synthe-
sized for comparison.19 These complexes were identified by FTIR
and mass spectra as well as elemental analysis. The severe line
broadening in the 1H NMR spectra indicates that these complexes
are paramagnetic species.18
Crystals of 2a–e and 4f suitable for crystallographic analysis
were grown from the chilled concentrated THF–hexane mixture
solution. The ratio of THF–hexane was adjusted in the range of
1 : 1 to 1 : 3, according to the solubility of the complex. A dark red
block microcrystal of 2c and 4f suitable for X-ray crystallographic
analysis was grown from the chilled concentrated THF–hexane
mixture solution. The crystallographic data together with the
collection and refinement parameters are summarized in Table 1.
Molecular structures for 2c and 4f are shown in Fig. 1 and 2.
Complexes 2c and 4f fold six-coordinate distorted octahedral
geometry around the V metal center, which are similar to
mono(iminopyrrolyl) complex I.19 In complex 2c, the equatorial
positions are occupied by nitrogen and chlorine atoms of the
chelating bis(imino)pyrrolyl ligand, and two additional oxygen
atoms of two THF molecules are coordinated on the axial position
(Fig. 1). However, one nitrogen atom of imino-nitrogen coordi-
nates to vanadium, another one bents away from the metal center,
which is somewhat analogous to that [2,5-C4H2NH(CH N-2,6-
iPrC6H3)2]Zr(NMe2)3 reported by Bochmann.17 Compared with
complex I, 4f showed a small but significant lengthening in the V–
Scheme 1 General synthetic route of vanadium(III) complexes 2a–e
and 4f.
Results and discussion
Synthesis and characterization of iminopyrrolyl vanadium(III)
complexes (2a–f)
˚
˚
A general synthetic route of new bis(imino)pyrrolyl vanadium
complexes used in this study is shown in Scheme 1. The
bis(imino)pyrrolyl ligands 1a–e and bis(iminopyrrolyl) chelate
ligand 1f were deprotonated by 1.0 equiv. of NaH, followed
by treating with VCl3(THF)3 in THF at room temperature.
Then the pure complexes 2a–e and 4f were isolated by re-
N(1) bond distance (4f, 2.0483(19) A; I, 2.037(3) A) since the two
coordinated ligands repulsed each other (Fig. 2).
As shown in Scheme 2, density functional theory (DFT)
calculations indicated that extra energies of 4.4–15.8 kcal mol-1
were needed for complexes 2a–e to dissociate into 3a–e and
THF. Thus, the configurations with two nitrogen atoms of the
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 3490–3497 | 3491
©

View Article Online
Fig. 1 Molecular structure of complex 2c. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity. Selected bond
◦
˚
distances (A) and bond angles ( ): V(1)–O(2), 2.0462(15); V(1)–N(1), 2.1012(19); V(1)–O(1), 2.0543(15); V(1)–N(2), 2.1646(18); V(1)–Cl(1), 2.3450(7);
V(1)–Cl(2), 2.3225(7); O(2)–V(1)–N(1), 88.64(7); O(2)–V(1)–O(1), 172.79(7); N(1)–V(1)–O(1), 87.38(7); O(2)–V(1)–N(2), 93.82(7); N(1)–V(1)–N(2),
78.65(7); O(1)–V(1)–N(2), 91.28(7); O(2)–V(1)–Cl(1), 88.21(5); N(1)–V(1)–Cl(1), 98.24(5); O(1)–V(1)–Cl(1), 86.42(5); N(2)–V(1)–Cl(1), 176.23(5);
O(2)–V(1)–Cl(2), 93.93(5); N(1)–V(1)–Cl(2), 165.75(5); O(1)–V(1)–Cl(2), 91.40(5); N(2)–V(1)–Cl(2), 87.19(5); Cl(1)–V(1)–Cl(2), 95.85(3).
Fig. 2 Molecular structure of complex 4f. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity.
◦
˚
Selected bond distances (A) and bond angles ( ):V(1)–N(1), 2.0483(19); V(1)–N(1)A, 2.0483(19); V(1)–O(1), 2.135(2); V(1)–N(2), 2.1697(19);
V(1)–N(2)A, 2.1697(19); V(1)–Cl(1), 2.2671(10); N(1)–V(1)–N(1)A, 173.40(11); N(1)–V(1)–O(1), 86.70(6); N(1)A–V(1)–O(1), 86.70(6); N(1)–V(1)–N(2),
79.62(7); N(1)A–V(1)–N(2), 100.31(7); O(1)–V(1)–N(2), 89.34(5); N(1)–V(1)–N(2)A, 100.31(7); N(1)A–V(1)–N(2)A, 79.61(7); O(1)–V(1)–N(2)A,
89.34(5); N(2)–V(1)–N(2)A, 178.68(10); N(1)–V(1)–Cl(1), 93.30(6); N(1)A–V(1)–Cl(1), 93.30(6); O(1)–V(1)–Cl(1), 180.0; N(2)–V(1)–Cl(1), 90.66(5);
N(2)A–V(1)–Cl(1), 90.66(5).
potentially tridentate ligands coordinating with vanadium metal
centre are much more stable in energy. The theory calculations
results were in accordance with X-ray crystallographic analysis.
of Et2AlCl and promoter Cl3CCOOEt,25 as shown in Table 2. All
these complexes 2a–e and 4f exhibited notable catalytic activities
for ethylene polymerization, and the polymers prepared were
linear polyethylene confirmed by 13C NMR spectra and possessed
relatively high molecular weights with unimodal molecular weight
distributions confirmed by GPC.
Ethylene polymerization catalyzed by new vanadium complexes
The substituent in the ligands and reaction temperature
considerably influence polymerization behaviors. As shown in
Fig. 3, complexes 2a–e exhibited a temperature-dependence of
catalytic activity with a maximum at about 50 ◦C. Notably,
The bis(imino)pyrrolyl ligands act as a bidentate rather than
tridentate ligand, which is different form the structures of initial
design; nevertheless, complexes 2a–e have been investigated as
thermal stability ethylene polymerization catalysts in the presence
3492 | Dalton Trans., 2011, 40, 3490–3497
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 2 Ethylene polymerization catalyzed by 2a–e and 4fa
T/◦C
Yield/g
Activityb
Mwc/104
Mw/Mn
c
Entry
Catalyst
Al/V (molar ratio)
1
2
3
4
5
6
7
2a
2b
2c
2c
2c
2c
2c
2c
2c
2d
2e
4f
I
4000
4000
4000
4000
4000
2000
6000
4000
4000
4000
4000
4000
4000
50
50
25
50
75
50
50
50
50
50
50
50
50
0.47
0.59
0.44
0.56
0.37
0.33
0.57
0.75
1.41
0.32
0.36
0.21
0.65
28.2
35.4
26.4
33.6
22.2
19.8
34.2
30.0
28.2
19.2
21.6
12.6
39.0
4.55
5.03
12.4
6.03
1.33
8.84
4.46
7.66
9.08
4.94
3.64
6.23
3.36
2.00
2.03
2.28
2.10
1.91
2.20
1.99
2.22
2.18
1.98
2.04
1.87
1.80
8d
9e
10
11
12
13
a Reaction conditions: 0.2 mmol catalyst, ETA/V (molar ratio) = 500, Vtotal = 50 mL, 1 atm ethylene pressure, in toluene for 5 min. b Activity in kg
mmolv-1 h-1. c Determined by GPC in 1,2,4-trichlorobenzene versus polystyrene standard. d 0.1 mmol catalyst, 15 min. e 0.1 mmol catalyst, 30 min.
Scheme 2 DFT calculation results.
Fig. 3 Effects of ligand structures on activities of catalysts 2a–e and 4f
from 25 ◦C to 75 ◦C. Reaction conditions: 0.2 mmol of catalyst, ETA/V
(molar ratio) = 500, Al/V (molar ratio) = 4000, Vtotal = 50 mL, 1 atm
ethylene pressure, in toluene for 5 min.
bis(iminopyrrolyl) complex 4f shows the highest activity at 75 ◦C,
maybe due to the deactivation of the bimolecular species being
hindered by the steric hindrance. The selection of the substituent
at imine nitrogen in the ligands sensitively affected not only
the activity for ethylene polymerization but also the thermal
stability of complexes. The catalytic activity ratio of 75 ◦C to
50 ◦C (A75/A50) was monitored to describe the thermal stability of
catalysts. Just as expected, the value of A75/A50 was improved from
0.48 (2a) to 0.61 (2b) and 0.72 (2c) with increasing of the steric
hindrance. In combination with electron withdrawing groups as 2e,
the effect of five fluorine groups at imine group became remarkable
and the activities changed slightly with increasing polymerization
temperature (A75/A50 = 0.88). Compared with bis(imino)pyrrolyl
complexes 2a–e, bis(iminopyrrolyl) complex 4f displayed the better
thermal stability, which is in accordance with the highest value of
A75/A50 (1.06). In addition, although the catalytic activity of I was
higher than that of complexes 2c and 4f at lower temperature, the
A75/A50 values of 2c (0.72) and 4f (1.06) are much higher than that
of I (0.58), indicating the notable stability of 2c and 4f than I.
It can be seen from the above results that electronic withdrawing
and steric effects were beneficial to improve the thermal stability
of catalysts.
reaction temperature. The molecular weights were enhanced with
increasing the steric hindrance of ligands (4f > 2c > 2d, 2b > 2a, 2e
> I) (see Table 2), indicating that the chain transfer reactions were
hindered due to the protection of steric hindrance. In addition,
although the molecular weights of the polymers decreased at high
reaction temperature, the molecular weight distributions were kept
in the range of 1.78–2.38, suggesting the characteristics of single-
site catalysts. As a comparison, the molecular weight distributions
of I was seriously broadened with increasing the temperature due
to some deactivation (see Fig. 4).
The polymerization reactions carried out with different Al/V
molar ratios by 2c are listed in Table 2. The increase in Al/V ratio
from 2000 to 6000 caused change either in the catalytic activity
or the molecular weights for the resultant polymers. These results
suggested that the dominant chain-transfer pathway was chain-
transfer to aluminium alkyls under these conditions (entries 4, 6
and 7 in Table 2). To investigate the catalytic lifetime of complex
2c, ethylene polymerizations were conducted for 5, 15, and 30 min
at 50 ◦C (entries 4, 8 and 9 in Table 2). The relationship between
the polymerization time and the polymer yield indicates that the
2c/Et2AlCl catalyst has a catalytic lifetime of at least 30 min.
The molecular weight and molecular weight distributions
were considerably influenced by the substituent in ligands and
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 3490–3497 | 3493
©

View Article Online
Table 3 Ethylene/1-hexene copolymerizations with different complex/Et2AlCl systemsa
◦
T/◦C
Yield/g
Activity/kg mmolv h-1
Hexene incorp.
Mwb/103
Mw/Mn Tmc/ C
-1
b
Entry
Catalyst
1-Hexene/mol L-1
1
2a
2b
2c
2c
2c
2c
2c
2c
2d
2e
4f
I
0.2
0.2
0.2
0.2
0.2
0.6
0.6
0.6
0.2
0.2
0.2
0.2
25
25
25
25
25
25
50
75
25
25
25
25
0.20
0.29
0.18
0.22
0.31
0.21
0.75
0.45
0.17
0.23
0.10
0.32
4.80
6.96
4.32
5.28
7.44
5.04
5.63
3.38
4.08
5.52
2.40
7.68
4.20
3.88
3.10
3.00
3.04
8.80
9.80
16.1
3.64
2.22
2.00
3.45
25.9
37.3
155.2
50.2
45.1
20.7
8.6
1.86
1.87
3.88
1.82
1.81
1.76
1.89
1.80
1.84
1.90
2.06
1.89
114.6
115.8
117.4
117.1
117.6
100.7
76.8
2
3d
4e
5
6
7
8
9
10
11
12
3.2
—
24.8
26.1
45.8
28.6
115.8
120.1
122.3
116.8
a Reaction conditions: 0.5 mmol catalyst, Al/V (molar ratio) = 4000, ETA/V (molar ratio) = 500, Vtotal = 50 mL, 1 atm ethylene pressure, in toluene
polymerization for 5 min. b Weight-average molecular weight determined by GPC in 1,2,4-trichlorobenzene versus polystyrene standard. c Determined by
DSC. d Al/V (molar ratio) = 500. e Al/V (molar ratio) = 2000.
molecular weight distribution of the resultant polymers also
decreased with the increase of Et2AlCl dosage (PDI = 3.88,
Al/V = 500; PDI = 1.81, Al/V = 4000). A significant increase
in 1-hexene incorporation of 2c/Et2AlCl system was found if
the copolymerization was conducted at high temperature and
high 1-hexene initial concentration (entries 6–8 in Table 3). The
level of 1-hexene incorporation can approach 16 mol% in the
resultant copolymer. Although the molecular weights of the
resultant copolymers decreased with temperature or 1-hexene
concentration, the molecular weight distributions remained con-
stant, indicating a single-site catalytic behavior. Complex 4f show
much lower catalytic activity and the 1-hexene incorporations
(ca. 2.0%) than 2a–e towards ethylene/1-hexene copolymerization.
Compared with complex I, analogues 2c and 4f offered much high
molecular weights copolymers under the same polymerization
conditions.
Fig. 4 Effect of ligands structures on molecular weight distributions from
25 ◦C to 75 ◦C. Reaction conditions: Reaction conditions: 0.2 mmol of
catalyst, ETA/V (molar ratio) = 500, Al/V (molar ratio) = 2000, Vtotal
=
The microstructures of ethylene/1-hexene copolymers are estab-
lished by 13C NMR in o-C6D4Cl2 at 135 ◦C, with the assignment of
the microstructure following previous work reported by Hsieh and
Randall.20 As shown in Fig. 5, the resultant copolymer possessed
isolated 1-hexene inserted unit ([EHE] assigned as Tdd) among the
repeated ethylene insertions, and the alternating sequence ([HEH]
assigned as Sbb) was also present with a low extent. The resonances
ascribed to two 1-hexene repeating unit ([EHH] Tbd) were seen in
the spectrum, but no peaks due to block-type sequence of 1-hexene
repeating units were observed even with 16.1 mol% comonomer
incorporation (assigned as Saa). In addition, a trace amount of
resonances due to so-called pseudo random (Sab, Sbg and Tgd)
sequences was present.
The triad distributions data and reactivity ratios re and rh for
each catalyst were calculated at 50 ◦C according to 13C NMR using
the method of Hsieh and Randall,20 as shown in Table 4. The
steric hindrance and electron effect of the ligands exerted obvious
influence on the value of re, which manifested as the capability
of incorporation of 1-hexene. For complexes 2a–d containing
electron donating imino groups, the 1-hexene selectivity increases
in the order of decreasing steric bulk at the 2,6-aryl positions,
iPr < Me < H (2c < 2b, 2d < 2a, respectively) at the same
feed ratio of comonomers. Specifically, the re values of these
complexes decreased, while rh values increased from iPr to H
50 mL, 1 atm ethylene pressure, in toluene for 5 min.
Ethylene/1-hexene copolymerization catalyzed by new vanadium
complexes
The copolymerizations of ethylene/1-hexene were also studied
and the representative results are summarized in Table 3. In
the presence of cocatalyst Et2AlCl and promoter Cl3CCOOEt,
catalysts 2a–e and 4f show highly activities towards ethylene/1-
hexene copolymerization and produce copolymers with high
molecular weight and unimodal molecular weight distributions.
The 13C and 1H NMR spectroscopy revealed that the signals
belonging to chain-end double bonds were not detected for low
molecular weight copolymers, indicating that chain transfer to
aluminium was the dominant chain-transfer pathway under these
reaction conditions. Both the catalytic activity and the molecular
weight were dependent upon the Al/V molar ratio employed,
the highest catalytic activity appeared when the Al/V (mol/mol)
equals ca. 4000 for 2c/Et2AlCl catalyst system, whereas the
molecular weights of the resultant copolymers decreases sharply
with the increase of Al/V molar ratio (entries 3–5), indicating
chain transferred reaction to Al occurring. In addition, the
3494 | Dalton Trans., 2011, 40, 3490–3497
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 4 Triad distributions and reactivity ratios for ethylene/1-hexene copolymerization
Triads
Entry
Cat.
[H]feed
[H]copolymer
[EHE]
[EHH]
[HHH]
[HEH]
[HEE]
[EEE]
re
rh
1
2
3
4
5
6
7
2a
2b
2c
2d
2e
4f
I
0.677
0.677
0.677
0.677
0.677
0.677
0.677
0.057
0.052
0.039
0.046
0.031
0.033
0.054
0.037
0.035
0.036
0.037
0.030
0.027
0.036
0.020
0.017
0.003
0.010
0.001
0.006
0.018
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.094
0.088
0.076
0.083
0.061
0.061
0.090
0.849
0.860
0.885
0.870
0.909
0.906
0.856
40.2
43.2
50.5
46.0
64.1
64.7
42.2
0.10
0.09
0.02
0.06
0.01
0.05
0.09
Fig. 5 The typical 13C NMR spectra for poly(ethylene-co-1-hexene)s obtained by catalyst 2c.
substituents. The influence of the electronic withdrawing effects
on 1-hexene selectivity was investigated by comparison of the
copolymerization behavior of complexes 2a and 2e. Compared to
complex 2a (rh = 0.10, re = 40.2), complex 2e showed a much higher
tendency to incorporate ethylene (re = 64.1), and lower tendency
to incorporate 1-hexene (rh = 0.01). However, the coordination
and insertion velocity of 1-hexene by complex I was relatively
higher than complexes 4f and 2c. This was manifested in the higher
value of re and lower values of rh for 2c (re = 50.5, rh = 0.02) and
4f (re = 64.7, rh = 0.05) compared with those for I (re = 42.2,
rh = 0.09).
bility was improved with increasing the steric hindrance ef-
fect and electron withdrawing effect in N-aryl moiety of the
ligands.
In addition, these complexes also exhibited high catalytic
activity for ethylene/1-hexene copolymerization. Changing the
substituents in ligands significantly influenced the activity for
ethylene polymerization and molecular weight of resultant poly-
mers as well as the selectivity of 1-hexene. To further investigate
the copolymerization system, the reactivity ratios of comonomers
were determined using 13C NMR methods of Hsieh and Randall.
All vanadium(III) catalysts showed a significantly higher reactivity
toward ethylene (re = 40–65) as compared to 1-hexene (rh
=
0.01–0.1), and the 1-hexane units should be randomly and
homogeneously distributed in the copolymer chains. The steric
hindrance and electronic effect of the ligands exert obvious
influence on the value of re and rh, which manifested as the
copolymerization capability of catalysts. The microstructures and
then the mechanism of the chain transfer reaction were also
investigated by 13C NMR spectra. The molecular weights of the
resultant copolymers decreased sharply with the increase of Al/V
molar ratio and the unsaturated end groups were not detected in
NMR spectra, indicating that chain-transfer to alkylaluminium
was the dominant chain-transfer pathway in the copolymerization.
Conclusion
In summary, we have prepared and characterized a series of
vanadium(III) complexes bearing iminopyrrolyl ligands (2a–e
and 4f). These complexes displayed promising thermal stability
and high activity towards ethylene polymerization in the pre-
sented of Et2AlCl as a cocatalyst and Cl3CCOOEt as reactiva-
tion reagent even at high temperature. The resultant polymers
possessed high molecular weights with unimodal distributions,
strongly suggesting that these polymerizations proceeded with
a single catalytically active species. Especially, the thermal sta-
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 3490–3497 | 3495
©

View Article Online
We believe that the results through this study would introduce
important information for designing efficient transition metal
catalysts for olefin polymerization.
[2,5-C4H2N(CH N-2,6-iPrC6H3)2]VCl2(THF)2 (2c). Yield:
76.0%. IR (KBr pellets): n(C N) 1566, 1625 cm-1. EI-MS (70
eV): m/z = 705 [M+]. Anal. calcd for C38H54Cl2N3O2V: C, 64.59;
H, 7.65; N, 5.95%. Found: C, 64.46; H, 7.59, N, 5.91%.
Experimental
[2,5-C4H2N(CH N-2,4,6-MeC6H2)2]VCl2(THF)2 (2d). Yield:
70.0%. IR (KBr pellets): n(C N) 1569, 1624 cm-1. EI-MS (70 eV):
m/z = 621 [M+]. Anal. calcd for C32H42Cl2N3O2V: C, 61.74; H, 6.75;
N, 6.75%. Found: C, 61.64; H, 6.69, N, 6.76%.
General procedures and materials
All manipulation of air- and/or moisture-sensitive compounds
were carried out under a dry argon atmosphere by using standard
Schlenk techniques or under a dry argon atmosphere in an
MBraun glovebox unless otherwise noted. All solvents were
purified from an MBraun SPS system. Commercial ethylene was
directly used for polymerization without further purification.
NMR of the ligands and vanadium complexes were obtained on
a Bruker 300 MHz spectrometer at ambient temperature, with
DMSO and CDCl3 as the solvent. The IR spectra were recorded on
a Bio-RadFTS-135 spectrophotometer. Elemental analyses were
recorded on an elemental Vario EL spectrometer. The 13C NMR
data of copolymers were obtained on a Varian Unity-400 MHz
spectrometer at 110 ◦C with o-C6D4Cl2 as a solvent. The DSC
measurements were performed on a Perkin-Elmer Pyris 1 Differ-
ential Scanning Calorimeter at a rate of 10 ◦C min-1. The weight-
average molecular weight (Mw) and the polydispersity index (PDI)
of polymer samples were determined at 150 ◦C by a PL-GPC 220
type high-temperature chromatograph equipped with three Plgel
10 mm Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB)
was employed as the solvent at a flow rate of 1.0 mL min-1. The
calibration was made by polystyrene standard EasiCal PS-1 (PL
Ltd). Ethyl trichloroacetate (ETA) were purchased from Aldrich,
dried over calcium hydride at room temperature and then distilled.
VCl3(THF)3 was purchased from Aldrich. Diethylaluminium chlo-
ride was purchased from Albemarle Corporation, triisobutylalu-
minium, triethylaluminium, trimethylaluminium were purchased
from Albemarle Corporation. The other reagents and solvents
were commercially available. The ligands used were synthesized
according to the literature methods.17,21
[2,5-C4H2N(CH NC6F5)2]VCl2(THF)2 (2e). Yield: 56.0%. IR
(KBr pellets): n(C N) 1569, 1626 cm-1. EI-MS (70 eV): m/z =
717 [M+]. Anal. calcd for C26H20Cl2F10N3O2V: C, 43.45; H, 2.79;
N, 5.85%. Found: C, 43.36; H, 2.84, N, 5.91%.
[C4H3N(CH N-2,6-iPrC6H3)]2VCl(THF) (4f). To a slurry of
NaH (0.06 g, 2.5 mmol) in THF (10 mL) was added a solution
of ligand 1f (508 mg, 2.0 mmol) in THF (10 mL) at -30 ◦C. The
resulting suspension was warmed to room temperature and stirred
for 4 h. Next, a solution of VCl3(THF)3 (373 mg, 1.0 mmol) in THF
(20 mL) was slowly transferred to the yellow mixture, and the red
brown solution was stirred for 12 h. The reaction mixture was
condensed and filtered to remove NaCl and the excess NaH. The
filtrate was concentrated and recrystallized in THF–n-hexane and
yielded 501 mg of the pure complex as a brown solid (75.0%).
IR (KBr pellets): n(C N) 1624 cm-1. EI-MS (70 eV): m/z = 664
[M+]. Anal. calcd for C38H50ClN4OV: C, 61.35; H, 7.52; N, 11.01%;
Found: C, 61.42; H, 7.53; N, 11.12%.
General procedure for ethylene polymerization. Polymerization
was carried out under atmospheric pressure in toluene in a 150 mL
glass reactor equipped with a mechanical stirrer. The reactor was
charged with 45 mL of toluene, and then the ethylene gas feed
was started followed by equilibration at desired polymerization
temperature. After 5 min, a solution of Et2AlCl in toluene and a
solution of ETA in toluene were added. Subsequently, a toluene
solution of the catalyst was added into the reactor with vigorous
stirring (900 rpm) to initiate polymerization. The total volume of
the liquid phase was 50 mL. After a prescribed time, alcohol (10
mL) was added to terminate the copolymerization reaction, and
the ethylene gas feed was stopped. The resulted mixture was added
to acidic alcohol. The solid polymer was isolated by filtration,
washed with alcohol and acetone, and dried at 60 ◦C for 24 h in a
vacuum oven.
Synthesis of vanadium complexes
[2,5-C4H2N(CH NC6H5)2]VCl2(THF)2 (2a). To a slurry of
NaH (0.03 g, 1.25 mmol) in THF (10 mL) was added a solution
of ligand 1a (273 mg, 1.0 mmol) in THF (10 mL) at -30 ◦C. The
resulting suspension was warmed to room temperature and stirred
for 4 h. Next, the yellow mixture was slowly transferred to a
solution of VCl3(THF)3 (374 mg, 1.0 mmol) in THF (20 mL),
and the red brown solution was stirred for 12 h. The reaction
mixture was condensed and filtered to remove NaCl and the
excess NaH. The filtrate was concentrated and recrystallized in
THF–n-hexane and yielded 421 mg of the pure complex as a
brown solid (78%). Compounds 2b–e were prepared analogously.
IR (KBr pellets): n(C N) 1569, 1626 cm-1. EI-MS (70 eV): m/z =
537 [M+]. Anal. calcd for C26H30Cl2N3O2V: C, 57.99; H, 5.58; N,
7.81%. Found: C, 58.06; H, 5.49, N, 7.89%.
General procedure for ethylene/1-hexene copolymerization
Copolymerization was carried out under atmospheric pressure in
toluene in a 150 mL glass reactor equipped with a mechanical
stirrer. The reactor was charged with 30 mL of toluene and the
prescribed amount of 1-hexene, and then the ethylene gas feed
was started followed by equilibration at desired polymerization
temperature. After 5 min, a solution of Et2AlCl in toluene and a
solution of ETA in toluene were added. Subsequently, a toluene
solution of catalyst was added into the reactor with vigorous
stirring (900 rpm) to initiate copolymerization. The total volume
of the liquid phase was 50 mL. After a prescribed time, alcohol (10
mL) was added to terminate the copolymerization reaction, and
the ethylene gas feed was stopped. The resulted mixture was added
to acidic alcohol. The solid polymer was isolated by filtration,
[2,5-C4H2N(CH N-2,6-MeC6H3)2]VCl2(THF)2 (2b). Yield:
71.0%. IR (KBr pellets): n(C N) 1565, 1625 cm-1. EI-MS (70
eV): m/z = 593 [M+]. Anal. calcd for C30H38Cl2N3O2V: C, 60.61;
H, 6.40; N, 7.07%. Found: C, 60.46; H, 6.49, N, 7.01%.
3496 | Dalton Trans., 2011, 40, 3490–3497
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
washed with alcohol and acetone, and dried at 60 ◦C for 24 h in a
vacuum oven.
6 (a) Z. Janas, L. B. Jerzykiewicz, R. L. Richards and P. Sobota, Chem.
Commun., 1999, 11, 1105–1106; (b) Z. Janas, L. B. Jerzykiewicz, S.
Przybylak, R. L. Richards and P. Sobota, Organometallics, 2000, 19,
4252–4257; (c) Z. Janas, D. Wisniewska, L. B. Jerzykiewicz, P. Sobota,
K. Drabent and K. Szczegot, Dalton Trans., 2007, 2065–2069; (d) D.
Homden, C. Redshaw, L. Warford, D. L. Hughes, J. A. Wright, S. H.
Dale and M. R. J. Elsegood, Dalton Trans., 2009, 8900–8910; (e) A.
Arbaoui, C. Redshaw, D. M. Homden, J. A. Wright and M. R. J.
Elsegood, Dalton Trans., 2009, 8911–8922.
7 (a) C. Cuomo, S. Milione and A. Grassi, J. Polym. Sci., Part A: Polym.
Chem., 2006, 44, 3279–3289; (b) S. Milione, G. Cavallo, C. Tedesco and
A. Grassi, J. Chem. Soc., Dalton Trans., 2002, 1839–1846.
8 M. Bialek and O. Liboska, J. Polym. Sci., Part A: Polym. Chem., 2010,
48, 471–478.
9 (a) Y. Nakayama, H. Bando, Y. Sonobe, Y. Suzuki and T. Fujita, Chem.
Lett., 2003, 32, 766–767; (b) Y. Nakayama, H. Bando, Y. Sonobe and
T. Fujita, J. Mol. Catal. A:Chem., 2004, 213, 141–150; (c) H. Makio, N.
Kashiwa and T. Fujita, Adv. Synth. Catal., 2002, 344, 477–493; (d) M.
Mitani, T. Nakano and T. Fujita, Chem.–Eur. J., 2003, 9, 2396–2403.
10 J. Tian, P. D. Hustad and G. W. Coates, J. Am. Chem. Soc., 2001, 123,
5134–5135.
11 (a) V. C. Gibson, S. Mastroianni, C. Newton, C. Redshaw, G. A. Solan,
A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 2000,
1969–1971; (b) D. J. Jones, V. C. Gibson, S. Green and P. J. Maddox,
Chem. Commun., 2002, 1038–1039.
Crystallographic studies
Crystals for X-ray analysis were obtained as described in the
preparations. The crystallographic data, collection parameters,
and refinement parameters are listed in Table 1. The crystals were
manipulated in a glovebox. The intensity data were collected with
the x scan mode (186 K) on a Bruker Smart APEX diffractometer
with CCD detector using Mo-Ka radiation (l = 0.71, 073 E).
Lorentz, polarization factors were made for the intensity data and
absorption corrections were performed using SADABS program.
The crystal structures were solved using the SHELXTL program
and refined using full matrix least squares. The positions of
hydrogen atoms were calculated theoretically and included in the
final cycles of refinement in a riding model along with attached
carbons.
DFT calculations
12 C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A.
Bansleben and M. W. Day, Organometallics, 1998, 17, 3149–3151.
13 J. Houghton, S. Simonovic, A. C. Whitwood, R. E. Douthwaite, S. A.
Carabineiro, J. C. Yuan, M. M. Marques and P. T. Gomes, J. Organomet.
Chem., 2008, 693, 717–724.
In order to predict the structures of complexes 2a–e, we employed
density functional theory (DFT) calculations for the dissociation
energies by using the Amsterdam Density Functional (ADF)
program package.22 The structures and energies are obtained
based on the local density approximation augmented with Becke’s
nonlocal exchange corrections23 and Perdew’s nonlocal correlation
correction.24 A triple STO basis set was employed for V, while all
other atoms were described by a double-z plus polarization STO
basis. The 1s electrons of the C, N and O atoms, as well as the
1s–2p electrons of Cl and V atoms, were treated as frozen core.
Finally, first-order scalar relativistic corrections were added to the
total energy of the system.
14 (a) H. Tsurugi, T. Yamagata, K. Tani and K. Mashima, Chem. Lett.,
2003, 32, 756–757; (b) H. Tsurugi, Y. Matsuo, T. Yamagata and K.
Mashima, Organometallics, 2004, 23, 2797–2805; (c) Y. Matsuo, K.
Mashima and K. Tani, Organometallics, 2001, 20, 3510–3518.
15 (a) V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G. A. Solan,
A. J. P. White and D. J. Williams, Chem. Commun., 1998, 1651–1652;
(b) V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White
and D. J. Williams, J. Chem. Soc., Dalton Trans., 2002, 4017–4023.
16 (a) Y. Yoshida, S. Matsui, Y. Takagi, M. Mitani, T. Nakano, H. Tanaka,
N. Kashiwa and T. Fujita, Organometallics, 2001, 20, 4793–4799; (b) S.
Matsui, T. P. Spaniol, Y. Takagi, Y. Yoshida and J. Okuda, J. Chem.
Soc., Dalton Trans., 2002, 4529–4531; (c) S. Matsui, Y. Yoshida, Y.
Takagi, T. P. Spaniol and J. Okuda, J. Organomet. Chem., 2004, 689,
1155–1164.
Acknowledgements
17 D. M. Dawson, D. A. Walker, P. M. Thornton and M. Bochmann,
J. Chem. Soc., Dalton Trans., 2000, 459–466.
The authors are grateful for subsidy provided by the National Nat-
ural Science Foundation of China (Nos. 20734002 and 20923003).
18 (a) L. M. Tang, J. Q. Wu, Y. Q. Duan, L. Pan, Y. G. Li and Y. S. Li,
J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 2038–2048; (b) J. Q.
Wu, B. X. Li, S. W. Zhang and Y. S. Li, J. Polym. Sci., Part A: Polym.
Chem., 2010, 48, 3062–3072; (c) J. Q. Wu, L. Pan, Y. G. Li, S. R. Liu
and Y. S. Li, Organometallics, 2009, 28, 1817–1825; (d) J. S. Mu, J. Y.
Liu, S. R. Liu and Y. S. Li, Polymer, 2009, 50, 5059–5064; (e) J. Q. Wu,
L. Pan, N. H. Hu and Y. S. Li, Organometallics, 2008, 27, 3840–3848;
(f) J. Q. Wu, L. Pan, S. R. Liu, L. P. He and Y. S. Li, J. Polym. Sci., Part
A: Polym. Chem., 2009, 47, 3573–3582.
19 B. C. Xu, T. Hu, J. Q. Wu, N. H. Hu and Y. S. Li, Dalton Trans., 2009,
8854–8863.
20 E. T. Hsieh and J. C. Randall, Macromolecules, 1982, 15, 1402–1406.
21 Y. Matsuo, H. Tsurugi, T. Yamagata, K. Tani and K. Mashima, Bull.
Chem. Soc. Jpn., 2003, 76, 1965–1968.
22 (a) E. J. Baerends, D. E. Ellis and P. Ros, Chem. Phys., 1973, 2, 41–
51; (b) E. J. Baerends and P. Ros, Chem. Phys., 1973, 2, 52–59; (c) G.
te Velde and E. J. Baerends, J. Comput. Phys., 1992, 99, 8498–8503;
(d) C. G. Fonseca, O. Visser, J. G. Snijders, G. te Velde, E. J. Baerends,
in Methods and Techniques in Computational Chemistry, METECC-95,
E. Clementi, G. Corongiu, ed.; STEF, Cagliari, Italy, 1995, p 305.
23 A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 3098–3100.
24 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822–8824.
References
1 For reviews see: (a) S. Gambarotta, Coord. Chem. Rev., 2003, 237, 229–
243; (b) H. Hagen, J. Boersma and G. van Koten, Chem. Soc. Rev.,
2002, 31, 357–364; (c) L. S. Boffa and B. M. Novak, Chem. Rev., 2000,
100, 1479–1493; (d) C. Redshaw, Dalton Trans., 2010, 39, 5595–5604;
(e) K. Nomura and S. Zhang, Chem. Rev., DOI: 10.1021/cr100207h.
2 (a) Y. L. Ma, D. Reardon, S. Gambarotta and G. Yap, Organometallics,
1999, 18, 2773–2781; (b) D. Reardon, F. Conan, S. Gambarotta, G. Yap
and Q. Y. Wang, J. Am. Chem. Soc., 1999, 121, 9318–9325.
3 (a) H. Hagen, C. Bezemer, J. Boersma, H. Kooijman, M. Lutz, A. L.
Spek and G. van Koten, Inorg. Chem., 2000, 39, 3970–3977; (b) H.
Hagen, J. Boersma, M. Lutz, A. L. Spek and G. van Koten, Eur. J. Inorg.
Chem., 2001, 117–123.
4 (a) A. K. Tomov, V. C. Gibson, D. Zaher, M. Elsegood and S. H.
Dale, Chem. Commun., 2004, 1956–1957; (b) C. Redshaw, L. Warford,
S. H. Dale and M. R. J. Elsegood, Chem. Commun., 2004, 1954–
1955.
25 (a) D. L. Christman, J. Polym. Sci., Part A-1, 1972, 10, 471; (b) H.
Hagen, J. Boersma and G. van Koten, Chem. Soc. Rev., 2002, 31, 357–
364.
5 (a) E. Brussee, A. Meetsma, B. Hessen and J. H. Teuben, Chem.
Commun., 2000, 497–498; (b) E. Brussee, A. Meetsma, B. Hessen and
J. H. Teuben, Organometallics, 1998, 17, 4090–4095.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 3490–3497 | 3497
©