Iron(II)-ethylene polymerization catalysts bearing 2,6-bis(imino)pyrazine ligands. Part I. Synthesis and characterization

Article abstract of DOI:10.1016/j.molcata.2006.07.005

The synthesis of a new series of 2,6-bis(imino)pyrazinyl ligands, [Ar{single bond}N{double bond, long}C{single bond}Pyz{single bond}C{double bond, long}N{single bond}Ar] where the aryl groups Ar = naphtyl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, and their iron(II) complexes is described starting from monoacetylpyrazine.

Full text of DOI:10.1016/j.molcata.2006.07.005

Journal of Molecular Catalysis A: Chemical 260 (2006) 210–214
Iron(II)–ethylene polymerization catalysts bearing
2,6-bis(imino)pyrazine ligands
Part I. Synthesis and characterization
L. Beauforta, F. Benvenutib, A.F. Noelsa,∗
a Laboratory of Macromolecular Chemistry and Organic Catalysis (CERM), University of Lie`ge, Sart-Tilman (B6a), 4000 Lie`ge, Belgium
b Solvay S.A., rue de Ransbeek 310, 1120 Bruxelles, Belgium
Received 10 March 2006; accepted 4 July 2006
Available online 23 August 2006
Abstract
The synthesis of a new series of 2,6-bis(imino)pyrazinyl ligands, [Ar N C Pyz C N Ar] where the aryl groups Ar = naphtyl, 2,6-
dimethylphenyl, 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, and their iron(II) complexes is described starting from monoacetylpyrazine.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Tridentate ligands; Bis(imino)pyrazine; Iron(II) complexes; Diacetylpyrazine
1. Introduction
vation with MAO. These 2,6-bis(imino)pyridine type catalysts
(Scheme 1) allowed the polymerization chemistry to enter the
“iron age” as described by Gibson and Wass [18]. The two great
advantages of these systems over existing homogeneous cata-
lysts are their low cost and the ease with which chemists can
derivatise the complexes. Here again, the catalyst structure is
the key of the activity.
It was demonstrated by modelling results that the electronic
density of a “Gibson’s type” iron catalyst could be drastically
altered by replacing the pyridine moiety by a pyrazine one.
This could result in different polymerization activities. So, we
report here on the synthesis of new tridentate bis(imino) Fe(II)
complexes incorporating the pyrazinyl moiety. Part 2 of this
series will describe the catalytic efficiency of these new precur-
sors for ethylene polymerization upon treatment with different
aluminium alkyls under homogeneous and heterogeneous con-
ditions.
The world-wide demand for valuable products that can be
produced in high yields and purities grows strongly year after
year. The driving force is a strong economic incentive to find
better and more cost effective catalyst systems that can pro-
duce these value-added products. New catalysts can provide the
potential to shift the production of expensive products to value-
added commodity products.
Beginning in the 1950s with the work and experiments of
Ziegler et al. [1–3], and Natta et al. [4,5], the interest in olefin
oligomerization and polymerization catalysts has grown dra-
matically over the years and still continues 50 years later. In
addition to the development of metallocene technology [7], the
search for catalysts that are more compatible with organic func-
tionalities has led to the development of new catalyst systems
based on late transition metals [6,8]. In 1995, Brookhart and
co-worker’s Nickel diimine complexes were synthesized and
tested successfully as olefin polymerization and oligomeriza-
tion catalysts [9–15] after activation with a co-catalyst such as
methylaluminoxane (MAO) [16]. Then in 1998, Bennett and co-
workers presented diiminopyridyl complexes of iron and cobalt
[17] as oligomerization and polymerization catalysts after acti-
2. Results and discussion
The synthesis of the iron complexes of the 2,6-bis(imino)-
pyrazine was envisaged in three steps, starting either from
pyrazine or from the commercially available monoacetyl-
pyrazine, according to the retrosynthetic route of Scheme 2.
The first step is the synthesis of the 2,6-diacetylpyrazine
backbone precursor (which contrary to 2,6-diacetylpyridine is
∗
Corresponding author.
E-mail address: AF.Noels@ulg.ac.be (A.F. Noels).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.07.005

L. Beaufort et al. / Journal of Molecular Catalysis A: Chemical 260 (2006) 210–214
211
The 2,6-bis(imino)pyrazine ligands 1–5 were then prepared
as yellow solids in good yields by the condensation of two
equivalents of the appropriate aniline with one equivalent of
2,6-diacetylpyrazine (Scheme 4) according to the experimental
details from the literature for the corresponding pyridine lig-
ands [17,23,24]. All of the syntheses described on Scheme 4
were conclusive except the one with 2,4,6-tritertbutylaniline that
did not afford the desired corresponding imine. Despite pro-
longed reaction times (more than 1 week in refluxing methanol),
the use of various acid catalysts and a large excess of aniline,
the diacetylpyrazine conversions remained lower than 5%. This
poor reactivity is most probably due to the large steric hin-
drance induced by the two ortho tert-butyl groups of this aniline.
Compounds 1–3 and 5 were characterised by NMR techniques,
infra-red and melting point.
Complexes 6–8 were then synthesized in relatively good
yields by refluxing a THF solution of the corresponding 2,6-
bis(imino)pyrazinyl ligand with FeCl2. All the iron complexes
are air stable, paramagnetic solids, all intensively coloured. In
particular, complexes 7 and 8 are blue in colour whereas com-
plex 6 is green. Because of their paramagnetic character, no
NMR spectra could be obtained for complexes 6–8 but elemen-
tal CHN analyses confirm their composition.
Scheme 1.
not commercially available). Next, Schiff base formation fol-
lows by reaction with differently substituted anilines. The last
step consists in coordinating the metal ions.
It is well known that if the Friedel-Crafts acylation is a
very useful method for the preparation of ketones in the ben-
zenoid series, it is of little value for diazaaromatic compounds.
Pyrazines (and especially pyrazine already substituted by an
electron with growing (EWG) group) are particularly unsuited to
direct electrophilic substitution due to the inductive effect of the
ring nitrogen atoms, and to their possible quaternization by the
Lewis acid catalyst [19]. In the past years, a variety of homolytic
substitutions of heteroaromatic bases, leading to C C bond for-
mation were developed [20], which on the whole reproduce the
Friedel-Crafts reaction, but with opposite reactivity and selectiv-
ity. We were inspired by a method reported by Minisci et al. [21]
who developed a two-phase system suitable for pyrazine monoa-
cylation. This system, based on the silver-catalyzed oxidative
decarboxylation of a keto acid by persulfate has been adapted to
the synthesis of 2,6-diacetylpyrazine. The acylation takes place
through a redox chain process that is generally valid for substi-
tution of heteroaromatic bases by nucleophilic carbon-centred
radicals [22]. This process allows the use of mild reaction con-
ditions that avoid decarbonylation of the acyl radicals.
In conclusion, we present here a new synthesis of 2,6-
diacetylpyrazine through a redox chain process. From this
molecule, new 2,6-bis(imino)pyrazine ligands of different steric
hindrance and their iron complexes can be readily synthesized
in good to fair yields.
3. Experimental
Thus, we considered synthesizing 2,6-diacetylpyrazine
through a direct diacylation of pyrazine by pyruvic acid and
silver thiosulfate according to Scheme 3 [21]. As it appeared
difficult to reach a maximum of conversion for the pyrazine ring
while favouring the diacetylated product, the starting pyrazine
product was then replaced by monoacetylpyrazine. So, the
diacetylpyrazine synthesis was undertaken according to the
same reaction protocol, in the presence of silver nitrate, pyruvic
acid, ammonium persulfate and sulphuric acid. The experiments
allowed us to isolate 2,6-diacetylpyrazine in about 50% yield
after purification by column chromatography. The infra-red
All manipulations were carried out under an atmosphere of
argon using standard Schlenk and cannula techniques. Argon
was purified by passage through columns of BASF R3-11 cat-
˚
alysts and 4 A molecular sieves. Solvents were refluxed over
an appropriate drying agent and distilled under argon prior to
use. 1H and 13C NMR spectra were recorded on a Bruker 400
spectrometer with TMS as internal standard. IR spectra were
recorded on a Perkin-Elmer FT-IR 1720X. Melting points (m.p.)
were determined with an electrothermal apparatus without fur-
thercorrection. Anilineswereuseddirectlyaspurchasedwithout
further purification.
1
spectrum, mass spectroscopy, H and 13C NMR analysis con-
firmed that the diacetylation took place in the 2 and 6 positions
of the pyrazine ring. In particular, the 1H NMR shift fitted well
the theoretical data obtained by simulation for the 2,6-isomer
(i.e. 9.25 ppm for the 2,6-isomer versus 8.40 and 8.25 ppm for
the 2,5- and the 2,3-isomers, respectively).
3.1. Synthesis of the 2,6-bis(imino)pyrazinyl ligands
3.1.1. Synthesis of the 2,6-diacetylpyrazine
In a three-necked 250 mL round-bottomed flask, one neck of
which is closed by a septum, 1 g (8.19 mmol) of acetylpyrazine
Scheme 2.

212
L. Beaufort et al. / Journal of Molecular Catalysis A: Chemical 260 (2006) 210–214
Scheme 3.
is introduced. The flask is flushed with argon and the
acetylpyrazine is dissolved in 100 mL of deionized water. The
solution is acidified by adding 0.87 mL (16.4 mmol) of 36 N
sulphuric acid. The flask is protected from light by an alu-
minium sheet and 2.163 g (24.5 mmol) of pyruvic acid are added
and the solution placed under magnetic stirring in a 50 ◦C oil
bath. A solution of 0.11 g (0.65 mmol) silver nitrate in 10 mL of
water and a solution of 5.6 g (24.5 mmol) of ammonium persul-
fate in 10 mL of water are then added separately dropwise and
simultaneously. The solution becomes rapidly red coloured. The
reaction mixture is cooled down, and the progress of the reac-
tion checked by TLC for unreacted pyrazine. A further amount
of pyruvic acid (2.16 g, 24.5 mmol), a solution of silver nitrate
(0.11 g, 0.65 mmol in 10 mL of water), together with a solution
of ammonium persulfate (5.6 g, 24.5 mmol in 10 mL of water)
are again added simultaneously but separately. The mixture is
heated at 50 ◦C for 2 h. After cooling down the reaction solu-
tion, the pH solution is adjusted to 12 by adding a 2 M solution
of sodium hydroxide; the mixture becomes brown. The aqueous
phase is extracted three times with portions of dichloromethane.
The organic fractions are collected and then dried on MgSO4.
Filtration and evaporation of the solvent yield a yellow solid.
The pure product is obtained by chromatography on silica (elu-
ant: ethyl acetate/pentane = 4/6). Yield: 0.62 g (46%); m.p. (◦C):
140; NMR 1H (CDCl3, ppm): 2.75 (s, 6H, CH3), 9.25 (s, 2H, H
pyrazine); NMR 13C (CD2Cl2, ppm): 198.7 (s, C O), 149.1 (s,
C2 pyrazine), 142.1 (s, C3 pyrazine), 25.7 (s, CH3); FT-IR (KBr,
cm−1): 3372.69, 3076.96, 2922.50, 1904.14, 1697.60, 1413.82,
1368.98, 1329.95, 1266.61, 1181.98, 1112.53, 1027.85, 937.76,
939.65, 647.76, 499.79, 431.05. Mass spectrum (m/z): 164
(molecular ion), 163, 135, 121, 120, 79, 78, 77, 52, 51, 50.
3.1.2. 2,6-Diacetylpyrazinebis(2,6-dimethylanil) (1)
In a 50 mL two-neck flask adapted with a condenser, 0.5 g
(3.0 mmol) of diacetylpyrazine is dissolved into 30 mL of anhy-
drous MeOH; 0.92 g (7.6 mmol) of 2,6-dimethylaniline is added
together with a few drops of acetic acid. The mixture is refluxed
during 2 days; the solution turns yellow. After one night under
reflux a solid appears in the solution and the progress of the
reaction can be checked by GC. After all the starting material
has reacted, the reaction mixture is cooled down. The mixture is
then filtrated and the solid washed with cold pentane. The filtrate
is then evaporated under vacuum and the yellow solid obtained
is crystallized in hot MeOH and water. Yield: 0.8 g (77%); m.p.
(◦C): 160; NMR 1H (CD2Cl2, δ, ppm): 9.6 (s, 2H, H pyrazine),
7.1 (d, 4H, C3H et C5H phenyl), 6.9 (m, 2H, C4H phenyl), 2.2 (s,
6H, CH3C N), 2.0 (s, 12H, CH3 phenyl); 13C NMR (CD2Cl2,
δ ppm): 166.0 (s, C N), 151.1 (s, C2 pyrazine), 148.3 (s, C1
phenyl), 141.5 (s, C3 pyrazine), 127.8 (s, C2 phenyl), 125.1 (s,
C3 phenyl), 123.2 (s, C4 phenyl), 17.5 (s, CH3 phenyl), 16.2 (s,
CH3 imine); FT-IR (KBr, cm−1): 3018 (s), 2921 (s), 2848 (s),
1635 (s), 1591 (s), 1470 (s), 1364 (d), 1253 (s), 1206 (s), 1167
(s), 1109 (s), 963 (s), 931 (s), 822 (s), 779 (s), 762 (s), 691 (s),
551 (s), 532 (s), 417 (s).
3.1.3. 2,6-Diacetylpyrazinebis(2,6-diisopropylanil) (3)
This product was synthesized following to the above pro-
cedure with 0.2 g (1.2 mmol) of diacetylpyrazine and 0.54 g
Scheme 4. Reagents and conditions: (i) EtOH, H+ and (ii) FeCl2, THF.

L. Beaufort et al. / Journal of Molecular Catalysis A: Chemical 260 (2006) 210–214
213
(3.04 mmol) of 2,6-diisopropylaniline. The colour of the prod-
suspended in hexane under agitation; the solid is finally filtered
under nitrogen, rinsed with hexane and dried under vacuum. In
this way, complex (6) is recovered as a green paramagnetic pow-
der. Yield: 354.6 mg (0.731 mmol, 72%); NMR: not recorded
due to the paramagnetic character of the complex. Anal. calc.
for C24H26N4·FeCl2: C, 58.0; H, 5.3; N, 11.3. Found: C, 60.1;
H, 5.6; N, 11.9.
1
uct is yellow. Yield: 0.28 g (48%); m.p. (◦C): 191.1; NMR H
(CD2Cl2, δ, ppm): 9.58 (s, 2H, pyrazine), 7.2 (d, 4H, C3H et C5H
phenyl), 7.12 (m, 2H, C4H phenyl), 2.8 (m, 4H, CH isopropyl),
2.2 (s, 6H, CH3C N), 1.16 (d, 24H, CH3 isopropyl); 13C NMR
(CD2Cl2, δ, ppm): 165.9 (s, C N), 151 (s, C2 pyrazine), 145.9
(s, C1 phenyl), 141.6 (s, C3 phenyl), 135.5 (s, C2 phenyl), 123.9
(s, C3 phenyl), 122.9 (s, C4 phenyl), 28.32 (s, CH isopropyl),
22.9 (s, CH3 isopropyl), 22.4 (s, CH3 isopropyl), 16.9 (s, CH3
imine); FT-IR (KBr, cm−1): 3055 (s), 2960 (s), 1632 (s), 1587
(s), 1470 (s), 1434 (s), 1370 (s), 1273 (s), 1170 (s), 1109 (s),
1024 (s), 937 (s), 821 (s), 766 (s), 699 (s).
3.2.2. [2,6-Diacetylpyrazinebis(2,6-diisopropylanil)]-
FeCl2 (7)
In a 10 mL Schlenk, equipped with a magnetic stir-
rer, 2,6-diacetylpyrazinebisimine(2,6-diisopropylanil) (100 mg;
0.2075 mmol) and 26 mg (0.205 mmol) of activated iron dichlo-
ride are introduced under nitrogen atmosphere, followed by
10 mL of anhydrous THF. A ball condenser is then set on
the Schlenk tube and the reaction medium is heated for 2 h
under reflux and under agitation. After having cooling down
the reaction medium at room temperature, the THF is evap-
orated under vacuum to the obtention of a dark blue residue.
This residue is suspended in the hexane under agitation;
the solid is finally filtered under nitrogen, rinsed with hex-
ane and dried under vacuum. In this way, we recover the
2,6-diacetylpyrazinebisimine(2,6-diisopropylanil) complex as a
very dark blue powder. Yield: 85 mg (0.1396 mmol) (68%);
NMR: not recorded due to the paramagnetic character of the
complex. Anal. calc. for C32H42N4·FeCl2: C, 63.1; H, 6.9; N,
9.2%. Found: C, 63.7; H, 6.5; N, 8.6%.
3.1.4. 2,6-Diacetylpyrazinebis(di-1-naphtylanil) (5)
This product was synthesized according to a procedure analo-
gous to the one above with 0.1 g (0.60 mmol) of diacetylpyrazine
and 0.22 g (1.5 mmol) of 2,6-naphtylamine. The colour of the
product is yellow. Yield: 0.13 g (51%); m.p. (◦C): 174.3; NMR
1H (CD2Cl2, δ, ppm): 9.7 (s, 2H, pyrazine), 7.9 (d, 1H, C4
naphtyl), 7.8 (d, 1H, C5 naphtyl), 7.7 (d, 1H, C8 naphtyl), 7.5 (m,
3H, C7, C6, C3 naphtyl), 6.8 (d, 1H, C2), 2.4 (s, 6H, CH3); 13
C
NMR (CD2Cl2, δ, ppm): 167 (s, C N), 151.4 (s, C2 pyrazine),
146.9 (C1 naphtyl), 141.8 (s, C3 pyrazine), 134.1 (s, C5 naphtyl),
127.9 (6s, C naphtyl), 113.2 (s, C2 naphtyl), 16.3 (s, CH3 imine);
FT-IR (KBr, cm−1): 3043 (s), 1637 (s), 1572 (s), 1505 (s), 1470
(s), 1390 (s), 1368 (s), 1259 (s), 1231 (s), 1191 (s), 1109 (s),
1079 (s), 1046 (s), 1023 (s), 935 (s), 865 (s), 804 (s), 780 (s),
735 (s), 713 (s), 636 (s), 610 (s), 574 (s), 423 (s).
3.2.3. [2,6-Diacetylpyrazinebis(dinaphtylanil)]-FeCl2 (8)
In a 10 mL Schlenk, equipped with a magnetic stirrer, 2,6-
diacetylpyrazinebis(di-1-naphtylanil) (5), (82 mg; 0.198 mmol)
and 25 mg (0.197 mmol) of activated iron dichloride are intro-
duced under nitrogen atmosphere, followed by 10 mL of anhy-
drous THF. A ball condenser is then set on the Schlenk tube and
the reaction medium is heated for 2 h under reflux and under agi-
tation. After having cooling down the reaction medium at room
temperature, the THF is driven away under vacuum to the obten-
tion of a dark blue residue. This residue is suspended in hexane
under agitation; the solid is finally filtered under nitrogen, rinsed
with hexane and dried under vacuum. In this way, complex (8) is
recovered as a dark blue, paramagnetic powder. Yield: 59.7 mg
(0.110 mmol, 56%). Anal. Calc. for C28H22N4·FeCl2: C, 62.1;
H, 4.1; N, 10.3%. Found: C, 63.2; H, 3.2; N, 9.8%.
3.1.5. 2,6-Diacetylpyrazinebis(2,4,6-trimethylanil) (2)
This product was also synthesized according to the above pro-
cedure with 0.1 g (0.60 mmol) of diacetylpyrazine and 0.20 g
(1.5 mmol) of 2,4,6-trimethylaniline. The colour of the prod-
1
uct is yellow. Yield: 0.15 g (62%); m.p. (◦C): 209.5; NMR H
(CD2Cl2, δ, ppm): 9.5 (s, 2H, pyrazine), 6.9 (s, 4H, phenyl),
2.3 (s, 6H, CH3C N), 2.2 (s, 6H, CH3 para aniline), 1.9 (s,
12H, CH3 ortho aniline); 13C NMR (CD2Cl2, δ, ppm): 166.2 (s,
C N), 151.2 (s, C2 pyrazine), 145.8 (s, C1 phenyl), 141.5 (s, C3
pyrazine), 132.5 (s, C2 phenyl), 128.5 (s, C3 phenyl), 124.9 (s,
C4 phenyl), 20.4 (s, CH3), 17.5 (s, CH3), 16.2 (s, CH3 imine);
FT-IR (KBr, cm−1): 3003 (s), 2916 (s), 1639 (s), 1474 (s), 1372
(s), 1269 (s), 1214 (s), 1175 (s), 1143 (s), 1109 (s), 1021 (s), 852
(s), 787 (s), 688 (s).
3.2. Preparation of iron complexes
Acknowledgements
3.2.1. [2,6-Diacetylpyrazinebis(2,6-dimethylanil)]-FeCl2
(6)
This work was generously supported by a BP-Solvay grant.
We also thank the FNRS, Brussels, for the purchase of major
instrumentation.
In a 50 mL Schlenk flask, equipped with a magnetic
stirrer, 2,6-diacetylpyrazinebis(2,6-dimethylanil) (1), (368 mg;
0.993 mmol) and 126.1 mg (0.995 mmol) of activated iron
dichloride are introduced under nitrogen atmosphere, followed
by 30 mL of anhydrous THF. A ball condenser is then set on the
Schlenk tube and the reaction medium is heated for 2 h under
reflux and under agitation. After having cooling down the reac-
tion medium at room temperature, the THF is evaporated under
vacuum to the obtention of a dark green residue. This residue is
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