Iron(II) coordination compounds with ω-alkenyl substituted bis(imino)pyridine ligands: Self-immobilizing catalysts for the polymerization of ethylene

Article abstract of DOI:10.1016/j.molcata.2006.07.078

Coordination compounds of iron(II)chloride with bis(imino)pyridine ligands bearing ω-alkenyl substituents were synthesized and characterized. They can be activated with methylalumoxane (MAO) and then be used as self-immobilizing catalysts for the polymerization of ethylene. The produced polyethylene has a monomodal molecular weight distribution which is in contrast to polyethylenes that are produced with the unsubstituted catalysts.

Full text of DOI:10.1016/j.molcata.2006.07.078

Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
Iron(II) coordination compounds with ␻-alkenyl substituted
bis(imino)pyridine ligands: Self-immobilizing catalysts
for the polymerization of ethylene
Marcus Seitz, Wolfgang Milius, Helmut G. Alt∗
Laboratorium fu¨r Anorganische Chemie, Universita¨t Bayreuth, Universita¨tsstraße 30, D-95440 Bayreuth, Germany
Received 10 February 2006; received in revised form 25 July 2006; accepted 25 July 2006
Available online 14 September 2006
Dedicated to Professor Max Herberhold’s 70th birthday (August 02, 2006).
Abstract
Coordination compounds of iron(II)chloride with bis(imino)pyridine ligands bearing ␻-alkenyl substituents were synthesized and characterized.
They can be activated with methylalumoxane (MAO) and then be used as self-immobilizing catalysts for the polymerization of ethylene. The
produced polyethylene has a monomodal molecular weight distribution which is in contrast to polyethylenes that are produced with the unsubstituted
catalysts.
© 2006 Published by Elsevier B.V.
Keywords: Bis(imino)pyridine; Iron; Catalysis; Polymerization; Self-immobilization
1. Introduction
called reactor fouling [10]. This term describes the precipitation
of the polymer on the walls of the reactor during the polymeriza-
tion process. It is found in slurry as well as in gas phase reactors.
The growing layer of polymer inhibits the reaction control and
prevents a continuous process. In order to prevent reactor fouling
many methods have been developed in order to heterogenize sin-
gle site catalysts [11]. For instance, the approaches include the
use of heterogeneous cocatalysts [12] or heterogenization of the
catalysts on a support [13]. Concerning metallocene compounds
Peifer and Alt developed a very elegant approach to prevent
reactor fouling, the so called self-immobilization [14]. Here the
catalystprecursorisfunctionalizedwithan␻-alkenylsubstituent
and as an olefin it can be copolymerized with the ethylene. The
catalytic centers are immobilized in the growing polymer chain
and therefore become heterogeneous preventing the reactor foul-
ing. A first approach to transfer this idea to bis(imino)pyridine
complexes was made by Herrmann and co-workers in 2002, who
used ␻-alkenyl substituents at the imino carbon atom [15].
Coordination compounds of late transition metals with
bis(imino)pyridine ligands are known since the mid of the last
century [1–3]. But their use as catalyst precursors for olefin
polymerization was not discovered until Brookhart and Gibson
first described such experiments in 1998 [4,5]. Because of the
reduced sensitivity towards air and moisture compared to met-
allocene compounds and the expected tolerance towards polar
functionalgroups, thisclassofcatalystprecursorsreceivedmuch
attention in the following years. The strong relations between
substituents on the ligand backbone and the performance of the
catalysts allow the tailoring of the catalyst precursors accord-
ing to demands [6,7]. It is found that substituents on the aniline
part of the ligand backbone show the strongest influence [8].
For instance, by changing the pattern of substitution from 2-
alkyl to 2,6-alkyl, the catalysts no longer produce oligomers
with 2–40 carbon atoms but high molecular weight polyethy-
lene [9]. Unfortunately the bis(imino)pyridine complexes suffer
from a problem known from other single site catalysts, the so
2. Results and discussion
∗
Because of the strong influence of substituents on the ani-
line part of the bis(imino)pyridine complexes, we have devel-
Corresponding author. Fax +49 921 552157.
E-mail address: helmut.alt@uni-bayreuth.de (H.G. Alt).
1381-1169/$ – see front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.molcata.2006.07.078

M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
247
Scheme 1. Synthesis of the ␻-alkenyl substituted ligand precursors.
oped a six step synthesis to place an ␻-alkenyl substituent at
position 4 of the pyridine ring. The reaction starts from che-
lidamic acid which is esterified with butanol. The ␻-alkenyl
substituent is introduced via an ether bridge in position 4. The
following steps are used to convert the ester groups first into
acetyl functions and then by a double Schiff base condensation
into the desired ligand precursor. Scheme 1 shows the reaction
pathway.
Using the synthetic route shown in Scheme 1 the following
ligand precursors were synthesized (Scheme 2):
The characterization of the ligand precursors was per-
formed by GC/MS analysis and NMR spectroscopy (see Sec-
tion 4 for data). The ␻-allyloxy substituted derivative with
isopropyl substituents on the aniline parts was further char-
acterized by X-ray diffraction. Fig. 1 shows the molecular
structure.
Scheme 2. Ligand precursors synthesized.

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M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
Table 1
Coordination compounds synthesized
Number
R
Rꢀ
MLn
1
2
3
4
5
6
2,6-Diisopropyl
2,6-Dimethyl
2,6-Diisopropyl
2,6-Diisopropyl
2,6-Diisopropyl
2,6-Dimethyl
Allyloxy
Allyloxy
␻-Pentenyloxy
Butyloxy
H
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
Fig. 1. Molecular structure of an ␻-allyloxy substituted ligand precursor.
H
The X-ray data clearly shows that the ligand precursor crys-
tallizes in the (E,E) conformation with typical C N imino
˚
bond lengths of 1.271 and 1.275 A. As may be seen by the
Table 2
high-temperature factors of the atoms of the strongly vibrat-
ing terminating group and by the relatively high R-values, the
crystal quality was poor. So, the molecular structure of the
ligand precursor could be well established but we will not
discuss details (bond lengths and angles) of this compound
(Fig. 2).
The ligand precursors with substituents in position 4
of the pyridine ring were applied for the coordination of
iron(II)chloride. For reasons of comparison two ligand precur-
sors were used which are known from literature [16] (Table 1).
Due to the paramagnetism of the iron complexes and their
poorsolubility in prevalentsolventsused forNMRspectroscopy,
the coordination compounds were characterized by mass spec-
trometry. The molecular structure of complex 5 was determined
by X-ray analysis (Fig. 2).
Data of the X-ray analysis of compound 5
Crystal data
Selected bond lengths
Crystal system
Space group
Monoclinic
P 21/c
N1
N2
Fe
Fe
2.205(22)
2.083(5)
˚
Unit cell
a = 13.7871(28) A
N3
Fe
Fe
Fe
Cl1
Cl2
2.191(6)
2.247(27)
2.351(25)
˚
b = 15.1742(30) A
˚
c = 18.6711(37) A
β = 102.49(3)◦
This structure is identical to structures already published
except that the dimensions of the unit cell differ because of the
inclusion of n-butanol [5,16]. Table 2 shows some selected data
of the X-ray analysis.
Scheme 3. Activation of the coordination compounds.
All coordination compounds were activated with MAO
according to Scheme 3 which is in agreement to the mech-
anism proposed for the activation of metallocene com-
pounds [17,18] and is accepted for theoretical investiga-
Fig. 3. Activities of the catalysts; polymerization conditions: catalyst precur-
sor in 500 ml pentane, activation with MAO (Al: Fe = 2000:1), 10 bar ethylene
pressure, 60 ◦C, 1 h.
Fig. 2. Molecular structure of 5.

M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
249
3. Summary and conclusions
A suitable reaction pathway to synthesize coordination com-
pounds with bis(imino)pyridine ligands bearing ␻-alkenyl sub-
stituents was described starting from chelidamic acid. From the
six step synthesis the desired ligand precursors are obtained in
moderate yields and the coordination of iron(II)chloride pro-
ceeds almost quantitatively.
Thepolymerizationexperimentsshowthattheintroductionof
␻-alkenyl substituents in position 4 of the pyridine rings gener-
atescatalystprecursorswhicharecapableofself-immobilization
and can prevent the reactor fouling efficiently. Due to the
incorporation of the active centers into the growing chains the
polymerization reaction shows lower activities and because of
the inhibition of the chain transfer to the aluminum centers,
polyethylenes with monomodal molecular weight distributions
are obtained.
Fig. 4. HT-GPC data of polyethylenes obtained with catalyst precursors 2 and
6.
tions concerning the bis(imino)pyridine catalyst precursors
[19,20].
The catalysts obtained were used for the polymerization of
ethylene in a 1l autoclave. The reactions were conducted over
1 h, at a temperature of 60 ◦C and an ethylene pressure of 10 bar.
Fig. 3 summarizes the results.
4. Experimental
The series of polymerization experiments shows that the cat-
alysts with substituents on position 4 of the pyridine ring have
lower activities than the unsubstituted derivatives by a factor
of 3.
4.1. General methods
All manipulations of air and water sensitive compounds
were performed using standard Schlenk techniques. Therefore,
argon was purified by passage over BTS catalyst and molec-
With all the catalyst precursors tested it is found that the
sterically more demanding isopropyl substituents on the aniline
parts of the ligand backbone inhibit the coordination of ethylene
leading to lower activities compared to the methyl substituted
compounds [11]. With the series of coordination compounds dif-
fering only in the substituents on position 4 of the pyridine ring,
the butyloxy substituted derivative shows the highest activity.
The compound without the alkenyl function is not immobilized
inthegrowingpolymerchainsasthiscatalystshowsreactorfoul-
ing. Only when using coordination compounds 1, 2 and 3 which
have ␻-alkenyl substituents the polymer does not precipitate
on the reactor walls and the stirrer. Besides this advantage, the
self-immobilization of the catalyst precursors also has an influ-
ence on the characteristics of the polymers produced. Fig. 4
compares the molecular weight distributions of polyethylenes
obtained with catalyst precursors with and without an ␻-allyloxy
substituent in position 4 of the pyridine ring.
As can be seen from the HT-GPC analysis, the self-
immobilized catalyst produces polyethylene with a monomodal
molecular weight distribution. The average molecular weights
are Mn = 8902 and Mw = 70902 g/mol and the polydispersity is
7.96. In contrast to this the non-functionalized derivative pro-
duces polyethylene with bimodal molecular weight distribution.
In this case, the average molecular weights are Mn = 6672 and
Mw = 90950 g/mol, with a polydispersity of 13.63. Since it is
known, that the low molecular weight fraction of polyethylenes
produced with late transition metal bis(imino)pyridine catalysts
results from a chain transfer reaction to the aluminum centers of
the cocatalyst [6], this behavior can be explained by the steric
demand of the alkyl groups surrounding the self-immobilized
catalysts. Because the active centers are incorporated in the
growing chains, the interaction with the MAO counterions is
sterically hindered and therefore the chain transfer is disfavored
resulting in a monomodal molecular weight distribution [6].
˚
ular sieves 4 A. All solvents used were purchased in techni-
cal grade and purified by distillation over Na/K alloy under
argon atmosphere. Methylalumoxane (MAO) was supplied by
Witco GmbH, Bergkamen, as 30% solution in toluene (aver-
age molecular weight 1100 g/mol, aluminum content: 13.1%,
3.5% as trimethylaluminum). All other chemicals were com-
mercially available or were synthesized according to literature
procedures.
NMR spectra were recorded on a Bruker ARX 250 spec-
trometer at a temperature of 25 ◦C. CDCl3 served as solvent.
1
The chemical shifts (δ) in the H NMR spectra are referenced
to the residual proton signal of the solvent (δ = 7.24 ppm for
chloroform) and in 13C NMR spectra to the solvent signal
(δ = 77.0 ppm for chloroform-d1). MS spectra were recorded
with a VARIAN MAT 8500 mass spectrometer (direct inlet
system, electron impact ionization 70 eV). In addition, a
Hewlett–Packard 5917A mass spectrometer was routinely used
to record MS spectra and in combination with a Hewlett–
Packard Series II 5890 gas chromatograph to record GC/MS
spectra.
Molecular weight determinations of the polyethylene sam-
ples were performed using a Millipore Waters 150 C HT-GPC
with refractometric detection (RI Waters 401). The polymer
samples were dissolved in 1,2,4-trichlorobenzene (flow rate
1 ml/min) and measured at 150 ◦C.
4.2. Synthesis of the ␻-alkenyl substituted ligand
precursors
15.42 g (0.084 mol) chelidamic acid were dissolved in 600 ml
toluene. Fifty millitres 1-butanol and 3 ml concentrated sulfu-
ric acid were added and the mixture was kept at reflux over

250
M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
night. Afterwards neutralization was done by adding an aqueous
NaHCO3-solution. The organic layer was separated and washed
with water. After drying over Na2SO4 the solvent was removed
by distillation in vacuo. The chelidamic acid dibutyl ester was
obtained as a white powder with 87% yield. 3.22 g (0.045 mol)
of it were dissolved in DMF and 18.35 g (0.133 mol) of potas-
sium carbonate were added. After the drop wise addition of
0.045 mol of the desired bromide the mixture was heated to
65 ◦C and stirred for 20 h. Afterwards the solvent was removed
almost completely and the residue was suspended in water. This
mixture was extracted three times with dichloromethane. The
organic layers were combined and washed with water. Then they
were dried over Na2SO4 and the solvent was removed by distilla-
tion in vacuo. The resulting substituted chelidamic acid dibutyl
esters were suspended in 100 ml ethanol and 100 ml aqueous
KOH solution were added. The mixture was kept at reflux for
2 h. After acidification with 5% HCl, the products were obtained
by filtration as white powders. These were dried in vacuo and
were mixed carefully with an excess of thionylchloride. After
2 h of refluxing the remaining SOCl2 was distilled off and the
products were obtained as white powders. In order to convert
these diacidic dichlorides into the desired ketones a cuprate
reaction analogous to the literature procedures of Paquette and
co-workers [21] and Marino and Linderman [22] was chosen.
Therefore, a suspension of 3.32 g (17.42 mmol) CuI in diethyl
ether was cooled to 0 ◦C and an equimolar amount of methyl
lithium was added drop wise. 8.81 mmol of the correspond-
ing diacidic dichloride were added drop wise as suspension in
diethyl ether at 0 ◦C. After this, the mixture was allowed to
warm up to room temperature within 4 h. By addition of satu-
rated NH4Cl solution, the reaction was quenched and the solvent
was removed in vacuo. The remaining residue was extracted
with dichloromethane several times and the extracts were com-
bined and dried over Na2SO4. After the removal of the solvent
brown solids were obtained which were recrystallized from
ethanol.
Substituent in position 4 of the pyridine ring
Yield (%)
Allyloxy
␻-Pentenyloxy
Butyloxy
40
38
43
The corresponding 2,6-diacetylpyridine compound was dis-
solved in toluene and p-toluenesulfonic acid was added as a
catalyst. A 2.5-fold excess of the desired amine was added and
the solution was heated to reflux at a Dean–Stark trap. After
the end of the reaction monitored by GC, the reaction solution
was neutralized with aqueous NaHCO3 solution. The organic
layer was dried over Na2SO4 and the ligand precursors were
crystallized at −20 ◦C. The bis(imino)pyridine compound was
recrystallized from absolute ethanol.
Substituent in position 4
of the pyridine ring
Substituent at the aniline moiety
Yield (%)
Allyloxy
Allyloxy
␻-Pentenyloxy
Butyloxy
None
2,6-Diisopropyl
2,6-Dimethyl
2,6-Diisopropyl
2,6-Diisopropyl
2,6-Dimethyl
71
62
58
29
67
58
None
2,6-Diisopropyl
The compounds were characterized by NMR spectroscopy.
Compound
1H-NMRa
13C-NMRb
8.12(s,2H), 7.16(m,6H), 6.09(m,1H), 5.43(dd,2H),
4.79(d,2H), 2.77(sept,4H,3JHH = 2.5 Hz), 2.27(s,6H),
1.17(d,24H,3JHH = 2.5 Hz)
Cq: 173.2, 165.9, 160.4, 142.9, 139.1; CH: 132.6, 123.4,
121.8, 104.3, 28.8; CH2: 118.1, 70.6; CH3: 23.7, 17.7
8.19(s,2H), 7.04(m,6H), 6.07(m,1H), 5.51(m,2H),
4.82(m,2H), 2.27(s,6H), 2.08(s,12H)
Cq: 174.2, 162.7, 161.9, 150.2, 129.0; CH: 132.6, 128.1,
124.3, 103.6; CH2: 117.2, 68.7; CH3: 18.2, 16.9

M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
251
Compound
1H-NMRa
13C-NMRb
7.10(m,6H), 5.86(m,1H), 5.74(s,2H), 4.91(m,2H),
3.82(t,2H, 3JHH = 6.2 Hz), 2.87(sept,4H,3JHH = 2.3 Hz),
2.13(s,6H), 2.04(m,2H), 1.54(m,2H), 1.19(d,24H,
3JHH = 2.3 Hz)
Cq: 174.9, 166.7, 160.8, 142.9, 138.8; CH: 141.0, 123.8,
122.5, 103.4, 26.3; CH2: 115.4, 67.9, 30.5, 28.9; CH3:
23.3, 16.4
7.09(m,6H), 5.81(s,2H), 3.97(t,2H,3JHH = 6.3 Hz),
2.86(sept,4H,3JHH = 2.4 Hz), 2.13(s,6H), 1.69(m,4H),
1.18(d,24H, 3JHH = 2.4 Hz), 0.99(t,3H,3JHH = 7.3 Hz),
Cq: 174.8, 166.7, 160.7, 142.9, 138.9; CH: 124.8, 122.4,
103.6, 26.3; CH2: 67.8, 31.2, 19.2; CH3: 23.3, 16.4, 13.7
8.31(d,2H,3JHH = 7.18 Hz), 7.99(t,1H,3JHH = 7.18 Hz),
7.18(m,6H), 2.96(sept,2H, 3JHH = 7.17 Hz), 2.33(s,6H),
1.19(d,24H,3JHH = 7.17 Hz)
Cq: 167.7, 155.9, 147.3, 122.9; CH: 137.6, 136.6, 124.4,
123.8, 25.5; CH3: 28.1, 21.9
8.23 (d,2H), 7.86(t,1H), 7.04(d,2H), 6.81(t,2H),
6.52(d,2H), 2.35 (s,12H), 2.13(s,6H)
Cq: 167.2, 155.1, 148.7, 130.4; CH: 138.4, 127.9, 125.4,
123.0; CH3: 18.0, 16.5
a
250.13 MHz, 25 ◦C, in chloroform-d1, δ [ppm] rel. chloroform (7.24).
62.9 MHz, 25 ◦C, in chloroform-d1, δ [ppm] rel. chloroform-d1 (77.0).
b
4.3. Synthesis of the coordination compounds
The solution was concentrated in vacuo and the products
were crystallized at −20 ◦C. After washing with pentane and
drying the coordination compounds were obtained as blue
powders.
0.1–0.3 g of the corresponding ligand precursor were dis-
solved in dry n-butanol. One equivalent of iron(II)chloride
was added and the mixture was stirred at room temperature.
The compounds were characterized by mass spectrometry.
Compound
Yield (%)
m/z (%)
88
664(M+•), 629(17), 537(41), 522(100), 496(13), 364(6),
297(5), 237(6), 186(9), 174(5), 160(8), 144(3), 132(3), 122(2)

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M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
Compound
Yield (%)
m/z (%)
89
552(M+•), 517(6), 425(53), 410(100), 384(29), 356(9), 326(5),
280(9), 239(11), 146(25), 131(12), 105(26), 77(12)
90
692(M+•), 657(9), 566(47), 551(100), 523(11), 497(8), 405(9),
202(26), 186(13), 176(11), 160(17), 144(15), 122(18), 91(10)
92
679(M+•), 644(5), 553(31), 538(100), 510(10), 496(8),
380(13), 202(17), 186(31), 122(20)
86
93
608(M+•), 573(13), 558(7), 532(4), 481(37), 466(100),
309(12), 265(9), 249(32), 238(33), 202(73), 186(36), 177(61),
144(27), 132(21), 91(28)
495(M+•), 460(3), 370(11), 369(38), 368(11), 355(27),
354(100), 224(15), 223(20), 209(28), 146(23), 105(23), 79(12),
77(11)
4.4. Activation of the catalyst precursors
4.5. Polymerization of ethylene
The activated complex was added to a 1l steel autoclave
An amount of 10 mg of the corresponding complex was
suspended in pentane and activated with an excess of MAO
(Al/Fe = 2000/1). The activated catalyst was used for ethylene
polymerization within 15 min.
¨
(Buchi), filled with 500 ml n-pentane. The polymerizations were
performed with constant ethylene pressure of 10 bar (99.98%
ethylene, dried over aluminum oxide) and at a temperature of

M. Seitz et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 246–253
253
60 ◦C. After a period of 1 h, the autoclave was cooled to room
temperature and the pressure was reduced. The polymerization
mixture was filtered. The remaining polymer was washed with
half concentrated hydrochloric acid, dried and weighed.
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