Ethylene polymerization in the presence of iron(II) 2,6-bis(imine)pyridine complex: Structures of key intermediates

Article abstract of DOI:10.1023/A:1010451213400

The structures of intermediates generated by the activation of 2,6-bis[1-(2,6-dimethylphenylimino)ethyllpyridineiron(II) chloride (1) with various cocatalysts, methylalumoxane (MAO), trimethylaluminum (TMA), and TMA in combination wilh B(C₆F₅)₃ and Ph₃CB(C₆F₅)₄, is studied by ¹H and ²HNMR spectroscopy. The 1/AlMe₃ system exhibits a higher catalytic activity in ethylene polymerization than the 1/MAO system. The activity of the latter decreases sharply with a decrease in the amount of AlMe₃ in MAO. Neutral Fe(II) complexes rather than cationic intermediates are suggested to be active components in both catalytic systems.

Full text of DOI:10.1023/A:1010451213400

Kinetics and Catalysis, Vol. 42, No. 2, 2001, pp. 147–153. Translated from Kinetika i Kataliz, Vol. 42, No. 2, 2001, pp. 165–171.
Original Russian Text Copyright © 2001 by Talzi, Babushkin, Semikolenova, Zudin, Zakharov.
Ethylene Polymerization in the Presence
of Iron(II) 2,6-Bis(imine)pyridine Complex:
Structures of Key Intermediates1
E. P. Talzi, D. E. Babushkin, N. V. Semikolenova, V. N. Zudin, and V. A. Zakharov
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received August 22, 2000
Abstract—The structures of intermediates generated by the activation of 2,6-bis[1-(2,6-dimethylphe-
nylimino)ethyl]pyridineiron(II) chloride (1) with various cocatalysts, methylalumoxane (MAO), trimethylalu-
minum (TMA), and TMA in combination with B(C6F5)3 and Ph3CB(C6F5)4, is studied by 1H and 2HNMR spec-
troscopy. The 1/AlMe3 system exhibits a higher catalytic activity in ethylene polymerization than the 1/MAO
system. The activity of the latter decreases sharply with a decrease in the amount of AlMe3 in MAO. Neutral Fe(II)
complexes rather than cationic intermediates are suggested to be active components in both catalytic systems.
1
INTRODUCTION
Synthesis
of 2,6-Bis[1-(2,6-Dimethylphenylimino)ethyl]pyridine
Iron(II) complexes with tridentate bis(imine)pyri-
dine ligands activated by methylalumoxane (MAO) are
stable and very active catalysts of ethylene oligomer-
ization and polymerization [1–4]. However, experi-
mental data on the nature of the active component of
these catalytic systems are lacking. In this work, we
present the first H and H NMR characterization of
the iron complexes generated by the activation of
2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II)
chloride (1) by various cocatalysts: MAO, trimethylalu-
minum, AlMe3/B(C6F5)3, and AlMe3/CPh3B(C6F5)4. In
addition, data on ethylene polymerization in the pres-
ence of these catalytic systems are presented.
2,6-Diacetylpyridine (1.0 g, 6.1 mmol) and 2,6-dim-
ethylaniline (3 ml, 24.4 mmol) were placed in a con-
stant-temperature flask with a magnetic stirrer filled
with methanol (30 ml). Then, 99% formic acid
(5 drops) was added, and the solution was stirred at
50°ë overnight. Afterwards, the solution was cooled in
a vessel with ice. A lightyellow precipitate was filtered
off, washed with cold methanol and pentane, and dried
in a vacuum. The yield was 1.70 g (75%).
1
2
Synthesis of 2,6-Bis[1-(2,6-
Dimethylphenylimino)ethyl]pyridineiron(II) Chloride
The obtained tridentate ligand (0.39 g, 1.05 mmol)
was dissolved in tetrahydrofuran (25 ml) in an argon
atmosphere, and FeCl2(H2O)4 (0.2 g, 1.0 mmol) was
added. The color of the solution turned immediately
from yellow to dark blue. The mixture was stirred at
room temperature in an argon atmosphere in a closed
flask for several hours. The precipitate formed was fil-
tered off in air, washed with diethyl ether and pentane,
and dried in a vacuum. 2,6-Bis[1-(2,6-dimethylphe-
nylimino)ethyl]pyridineiron(II) chloride (1) was obtained
as a violet-green powder in 100% yield (0.5 g).
EXPERIMENTAL
Reagents and Solvents
Methylene dichloride, methylene dichloride-d2, and
toluene-d8 were dried with P2O5 and distilled. Tetrahy-
drofuran (Aldrich) was kept above KOH, distilled, and
shaken with LiAlH4. Pentane (analytical grade) was
treated with P2O5, filtered, kept above sodium, and dis-
tilled. Diethyl ether was treated with KOH and P2O5
and distilled. 2,6-Diacetylpyridine, 2,6-dimethyla-
niline, and FeCl2(H2O)4 were used as received (Ald-
rich). Methylalumoxane was commercially available
from Witko (Berghamen). Operations with solvents
along with their storage was carried out in a vacuum.
All experiments were carried out in hermetically sealed
evacuated glassware.
Preparation of MAO and AlMe3 Samples
The MAO-1 sample was commercially available
from Witko as a solution in toluene with a total alumi-
num concentration of 1.8 mol/l (~0.5 mol/l aluminum
was in the form of AlMe3).
The MAO-2 sample was prepared by drying the
MAO-1 sample in a vacuum at 20°ë. The solid product
obtained was dissolved in purified deuterated solvents
1
Proceedings of the Seminar in Commemoration of professor
Yu.I. Ermakov, Novosibirsk.
0023-1584/01/4202-0147$25.00 © 2001 MAIK “Nauka /Interperiodica”

148
TALZI et al.
(the total concentration of Al in the solution was reaction started after the tube containing complex 1 was
0.1 mol/l, including that in the form of AlMe3 of broken. The ethylene pressure and temperature were
0.01 mol/l).
kept constant during polymerization. The polymeriza-
tion conditions are presented in more detail in Table 2.
The MAO-3 sample was prepared by drying the
MAO-1 in a vacuum of sample at 50°ë. A solution of
MAO-3 in toluene (the total concentration of Al was
0.1 mol/l, including that in the form of AlMe3 of
0.003 mol/l) was used for ethylene polymerization.
RESULTS AND DISCUSSION
1H NMR Spectrum of Initial Complex 1
1
1
The H NMR spectrum of the initial complex 1
According to the H NMR data, the liquid fraction
(0.003 mol/l, CD2Cl2) (Fig. 1a) contains six narrow
paramagnetically shifted signals (∆ν1/2 = 20–50 Hz),
which can unambiguously be assigned to the corre-
sponding protons of the tridentate ligand on the basis of
the data on the integral intensity and relative distance to
the disturbing paramagnetic center (Table 1, spectrum 1).
obtained by drying the MAO-1 sample in a vacuum is a
solution of AlMe3 in toluene (Al concentration
0.4 mol/l). This fraction was used as an AlMe3 sample.
Al(CD3)3 (99% D) was prepared by the interaction
of CD3I (99% D) with aluminum powder at 80°ë and
then with metallic sodium at 100°ë followed by the
vacuum distillation of the target product.
Interaction of 1 with MAO
New complex 2 (Fig. 1b) is formed immediately in
the reaction of 1 with MAO-2 in CD2Cl2 at room tem-
perature ([1] = 0.003 mol/l, Al/Fe = 50). All signals of
the initial tridentate ligand are retained in the 1H NMR
spectrum of complex 2, but their position changes sub-
stantially (Table 1, spectrum 2). In addition, a new sig-
nal (X) with a chemical shift of 38.8 ppm appears in the
spectrum. Its intensity corresponds to the presence of
two additional (as compared to complex 1) methyl
groups in complex 2. As shown below, signal X belongs
to the AlMe2 fragment. Complex 2 is stable, and its
concentration changes insignificantly over 5 h at room
temperature.
When toluene-d8 is used as a solvent instead of
CD2Cl2, complex 2 dominates the 1/åÄé-2 system
only at relatively low Al/Fe ratios (lower than 50). At
higher ratios (higher than 500), new complex 3 domi-
nates the solution (Fig. 2). The signals from complexes 2
and 3 are designated in Fig. 2 by letters with the corre-
sponding figures in parentheses. Note that, when the
CD2Cl2 solvent is replaced by toluene, the position of
the signals from complex 2 changes insignificantly. The
1H NMR spectrum of complex 3 exhibits signals of the
initial ligand (Table 1, spectrum 5). The signal of pro-
tons of Ar–Me in complex 3 (12 H) is masked by an
intense signal from MAO. As in the case of complex 2,
the spectrum of complex 3 has an additional signal at
30.9 ppm (6 H) from the AlMe2 fragment designated in
Fig. 2 as X (3).
Preparation of 1/MAO, 1/AlMe3, 1/Al(CD3)3,
1/AlMe3/B(C6F5)3, and 1/AlMe3/CPh3B(C6F5)4
Samples and NMR Measurements
The desired amounts of complex 1 and a solution of
MAO-2, AlMe3, Al(CD3)3, AlMe3/B(C6F5)3, or
AlMe3/CPh3B(C6F5)4 in toluene-d8 or CD2Cl2 were
mixed directly in an NMR tube in a vacuum at a low
temperature (–80 to –30°C). Then, the tube was sealed
from the vacuum system and placed in a spectrometer.
The concentration of Fe in the 1/AlMe3 and 1/MAO-2
catalytic systems was 0.0003–0.003 mol/l, and that of
aluminum was 0.5–3.0 mol/l. The concentrations of
B(C6F5)3 and CPh3B(C6F5)4 in the 1/AlMe3/B(C6F5)3 and
1/AlMe3/CPh3B(C6F5)4 catalytic systems were approxi-
mately equal to the concentration of 1 (0.001 mol/l),
and that of AlMe3 was 0.1 mol/l.
2
1H and H NMR spectra were recorded at 400.13
and 61.4 MHz using a Bruker MSL-400 spectrometer
in the spectral range of 125 kHz. The accumulation fre-
quency was 20 Hz (1H) or 2.5 Hz (2H), the storage num-
ber was 5000–20000, and 20°–40°-pulses with a dura-
tion of 5 µs (1H) or 10 µs (2H) were used. The chemical
1
shifts of H were timed from the signals of residual
CHDCl2 (5.27 ppm) or CHD2C6D5 (2.1 ppm (CH3)) in
2
deuterated solvents. The chemical shifts of H were
read off from the deuterium signal of the solvent (the
natural concentration of 2H). The error in the determi-
nation of chemical shifts was ±0.1 ppm for lines with a
width of at most 200 Hz.
Interaction of 1 with AlMe3 and Al(CD3)3
Ethylene Polymerization
When 1 reacts with AlMe3 in CD2Cl2 (Al/Fe = 100)
Ethylene was polymerized in a 1-liter autoclave. at a low temperature (–80 to 0°C), the starting complex
Complex 1 (0.001 g, 2.0 × 10–6 mol) in an evacuated is almost completely transformed into new complex 4.
sealed glass tube was placed into the autoclave. The Complex 4 is less stable in the 1/AlMe3 system in
autoclave was evacuated at 50°ë and cooled to 20°ë. CD2Cl2 than complexes 2 and 3 in the 1/åÄé system
Then, a solution containing the calculated amount of in toluene, and its signals in the spectrum disappear at
the cocatalyst in toluene (150 ml) was poured into the 0°ë. The chemical shifts of all five signals of complex 4 in
autoclave. When temperature reached 35°ë, ethylene CD2Cl2 are unexpectedly close to the corresponding
was admitted to the reactor to a pressure of 5 atm. The signal from complex 3 in toluene-d8 (Table 1). Thus,
KINETICS AND CATALYSIS Vol. 42 No. 2
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ETHYLENE POLYMERIZATION
149
ë
D
F
E
A
(a)
B
D
F
ë
X
E
A
(b)
120
80
40
0
δ, ppm
1
Fig. 1. H NMR spectra of (a) starting complex 1 and (b) complex 2 generated in the reaction of 1 with MAO-2 in CD Cl at 20°C
2
2
([1] = 0.003 mol/l, Al/Fe = 50).
complexes 3 and 4 are either identical or very close in neutral complexes. To establish their structure, we stud-
their structures. Since complex 4 is formed in the ied the 2H NMR spectrum of complex 4, which is gen-
1/AlMe3 system in which the formation of cation-like erated in the reaction of 1 with Al(CD3)3 at –30°ë
structures is impossible, we assume that 3 and 4 are (Fig. 3). This spectrum contains two signals with equal
Table 1. Chemical shifts (ppm) in 1H NMR spectra of complexes 1–6*
C
D
E
F
X
Spec-
trum no.
A
B
Complex Solvent
T, °C
(Py–Hm) (Py–Hp)
(Ar–Hm) (Ar–Me) (Ar–Hp) N=C(Me) (Al–Me)
1
2
1
2
2
2
3
3
4
5
5
6
6
CD2Cl2
20
20
82.9
130.3
126.6
153.9
107.2
127.0
125.0
148.4
165.9
141.8
157.3
32.6
15.9
8.5
8.1
–
12.1
–17.6
–14.7
–25.0
–
–10.8
–18.8
–19.8
–25.2
–10.9
–13.6
–13.3
–21.2
–24.1
–21.3
–24.1
–20.0
19.7
21.6
21.6
20.0
19.4
18.4
–
n/a
CD2Cl2
6.1
38.8
38.3
51.4
30.9
37.3
35.5
42.7
45.7
44.2
51.5
3
Toluene-d8
Toluene-d8
Toluene-d8
Toluene-d8
CD2Cl2
20
–
4
–10
20
–
5
–36.1
16.0
18.2
18.3
7.7
–
6
–10
–10
10
–44.9
–
7
–42.8
–
8
CD2Cl2
–
–
–
–
–35.9
–45.0
–23.2
–29.5
9
CD2Cl2
–10
10
–
10
11
CD2Cl2
7.6
6.8
19.5
19.2
CD2Cl2
–10
Note: Designations: Py–H , two hydrogen atoms in the meta positions of pyridyl; Py–H , the hydrogen atom in the para-position
m
p
of pyridyl; Ar–H , four hydrogen atoms in the meta positions of the aromatic cycle; Ar–Me, 12 hydrogen atoms of methyl groups
m
at the aromatic cycle; Ar–H , two hydrogen atoms in the para positions of the aromatic cycle; N=C(Me), six hydrogen atoms
of methyl groups at the N=C bond; and Al–Me, AlMe fragment.
p
2
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150
TALZI et al.
C(3)
E(3)
F(3)
D(2)
E(2)
F(2)
A(3)
X(2)
B(3)
X(3)
A(2)
120
80
40
0
–40
δ, ppm
1
Fig. 2. H NMR spectrum of complexes 2 and 3 generated in the reaction of 1 with MAO-2 in toluene-d at 20°C ([1] = 0.0003 mol/l,
8
Al/Fe = 500).
integral intensities: one signal (47 ppm) from the erably from those of the same signals in complex 4 at
Al(CD3)2 fragment and another (610 ppm) from two the same temperature (Table 1). As shown above, the
2
Fe(II)–CD3 fragments. The H NMR signal from the reaction of 1 with AlMe3 creates complex 4. Therefore,
CD2 group of the (TMEDA)FeII(CD2–Ph)2 complex, complexes 5 and 6 are generated in the reaction of 4
where TMEDA is Me2N(CH2)2NMe2, has previously with B(C6F5)3 and CPh3B(C6F5)4, respectively. A typi-
been observed at 1000 ppm [5] in agreement with our cal reaction of the activators under consideration
data. We failed to observe the 1H NMR signal from the with the methylated metal complex is the elimina-
Fe(II)–Me fragment due to a larger paramagnetic tion of the methyl anion to form ion pairs [6–8].
2
broadening of the line than that in the H NMR spec- That is why complexes 5 and 6 are probably the ion
trum. The spectroscopic data suggest that 3 and 4 rep- pairs [LFe(II)(µ-Me)2AlMe2]+[MeB(C6F5)3]– and
resent the neutral complex LFe(II)(Me)(µ-Me)2AlMe2, [LFe(II)(µ-Me)2AlMe2]+[B(C6F5)4]–, respectively.
and 2 is the LFe(II)(Cl)(µ-Me)2AlMe2 complex, where
L is the starting bis(imine)pyridine ligand (Fig. 4). We
Ethylene Polymerization in the Presence of the 1/åÄé,
failed to detect the signal of the terminal methyl group
1/AlMe3, and 1/AlMe3/B(C6F5)3 Catalytic Systems
of complexes 3 and 4 in the 1H and 2H NMR spectra.
The data on ethylene polymerization are presented
in Table 2. The activity of the 1/åÄé catalyst
Interaction of 1 with AlMe3/B(C6F5)3
decreases dramatically (almost 20-fold) with a
and AlMe3/CPh3B(C6F5)4
decrease in the residual trimethylaluminum concentra-
Immediately after the mixing of reactants in the tion in MAO (Table 2, rows 1 and 2). At equal Al/Fe
1/AlMe3/B(C6F5)3 system at –20°ë, complex 1 trans- molar ratios, the activity of the 1/AlMe3 system was
forms into new complex 5, and in the much higher than that of the 1/åÄé-1 system and,
1/AlMe3/CPh3B(C6F5)4 system new complex 6 is especially, 1/åÄé-3 (Table 2, rows 1–3). A decrease
formed (Fig. 5) (the ratio of reactants is 1 : 100 : 1). The in the concentration of AlMe3 in the polymerization
chemical shifts of the corresponding signals of com- medium leads to a decrease in the catalytic activity of the
plexes 5 and 6 are close to each other and differ consid- system (rows 3–5).
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ETHYLENE POLYMERIZATION
151
CHDCl2
Al2Me6
Al–Me
Al–(µ-Me)–Fe
600
500
400
300
200
100
0
δ, ppm
2
Fig. 3. H NMR spectrum of complex 4 generated in the reaction of 1 with Al(CD ) in CD Cl at –50°C ([1] = 0.001 mol/l, Al/Fe = 100).
3 3
2
2
B
A
A
F
F
N
D
D
2: L = Cl–
N
N
C
C
Fe
L
E
E
Me
3, 4: L = Me–
Me
Me
C
C
D
D
Al
X
X
Me
+
B
A
A
F
F
N
D
D
55:: [[MMeeBB((CC6FF5))3[]––
N
N
C
C
6
5 3
Fe
E
E
Me
6: [B(C6F5)4]–
Me
Me
C
C
D
D
Al
X
X
Me
Fig. 4. Assumed structures of complexes 2–6.
KINETICS AND CATALYSIS Vol. 42 No. 2 2001

152
TALZI et al.
C
F
D
X
E
A
120
80
40
0
δ, ppm
1
Fig. 5. H NMR spectrum of complex 1 in CD Cl at 10°C after its reaction with AlMe /CPh B(C F ) ([1] = 0.003 mol/l, Al/Fe = 100).
2
2
3
3
6 5 4
When B(C6F5)3 is added to the 1/AlMe3 system, the
The participation of the 1/AlMe3 and
catalyst activity somewhat increases (rows 5 and 6). 1/AlMe3/B(C6F5)3 catalysts with the ratio Al/Fe = 100
The kinetic curves of ethylene polymerization in the gives polymers with close molecular weights (see melt
presence of 1/MAO, 1/AlMe3, and 1/AlMe3/B(C6F5)3 indices in Table 2, rows 5 and 6). It is well known that
are very close. All these systems are characterized by AlMe3 is an efficient chain carrier, and an increase in its
the high initial activity, which drops 15 min after the concentration leads to a decrease in the molecular
beginning of polymerization. The polymer samples weight of the polymer. This can be the reason for an
have an analogous linear structures and close numbers of increase in the melt index of the polymer with an
methyl groups (~3 CH3 groups per 1000 carbon atoms) and increase in the Al/Fe molar ratio from 100 to 500
terminal vinyl groups (~0.8 per 1000 carbon atoms).
(Table 2, rows 3 and 5).
Table 2. Ethylene polymerization in the presence of complex 1 with various cocatalysts (solvent toluene, 35°C, ethylene
pressure 5 atm, time 60 min, [Fe] = 1.3 × 10–5 mol/l)
Activity, (kg PE)
No.
Cocatalyst
Al/Fe
Yield of polyethylene, g
MI*, g/10 min
(mol Fe)–1 h–1 atm–1
1
2
3
4
5
6
MAO-1
500
500
500
250
100
100
1
37.2
2.0
3720
200
13.4
–
MAO-3
AlMe3
59.0
11.4
5.1
5900
1140
510
>100
32
AlMe3
AlMe3
26
AlMe3 + B(C6F5)3
7.2
720
23
* Melt indices of polyethylene samples measured at 190°C and a load of 2.16 kg.
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ETHYLENE POLYMERIZATION
153
Possible Structures of Active Polymerization Centers
lytic and NMR data indicate that the neutral rather than
cationic complexes are the key intermediates of the
1/åÄé and 1/AlMe3 catalytic systems.
The catalytic data (Table 2) indicate that the
1/AlMe3 and 1/MAO catalytic systems contain proba-
bly the same active component, which is generated in
the reaction of 1 with AlMe3. Complex 4 (Fig. 4) is
most likely the closest precursor of these active species.
To transform complex 4 into a species that carries poly-
merization, the coordination site for an ethylene mole-
cule must be freed. The mechanism for this transforma-
tion is still unclear. The active components are neutral
rather than cation-like structures because the formation
of these structures in the 1/AlMe3 system is improba-
ble. The authors of [9] have recently shown that the
neutral Ni(II) complex with a Schiff base containing a
methyl ligand is capable of carrying ethylene polymer-
ization without cocatalyst additives and is not worse
than the best known catalytic systems in its activity.
ACKNOWLEDGMENTS
The authors are grateful to L.G. Echevskaya for
studying of the polymer structure.
REFERENCES
1. Small, B.L., Brookhart, M., and Bennett, A.M.A., J. Am.
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4. Britovsek, G.J.P., Bruce, M., Gibson, V.C., et al., J. Am.
Chem. Soc., 1998, vol. 121, no. 38, p. 8728.
According to NMR data, the structures of com-
plexes 5 and 6 differ from that of complex 4. However,
the data in Table 2 do not suggest that the
1/AlMe3/B(C6F5)3 catalytic system contains any active
centers different from those in the 1/AlMe3 system.
5. Hill, D.H. and Sen, A., J. Am. Chem. Soc., 1988,
vol. 110, no. 8, p. 1651.
6. Bochmann, M. and Lancaster, S.L., Angew. Chem. Int.
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CONCLUSION
1
2
Thus, the use of H and H NMR spectroscopy
revealed the structure of the Fe(II) complexes generated
in the activation of the Fe(II)-based catalysts of ethylene
polymerization with various cocatalysts (MAO, AlMe3,
AlMe3/B(C6F5)3, and AlMe3/CPh3B(C6F5)4). The cata-
9. Younkin, T.R., Connor, E.F., Hendersen, J.I., et al., Sci-
ence, 2000, vol. 287, no. 3, p. 460.
KINETICS AND CATALYSIS Vol. 42
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2001