JJC Jordan Journal of Chemistry Vol. 3 No.2, 2008, pp....
Transcript of JJC Jordan Journal of Chemistry Vol. 3 No.2, 2008, pp....
Jordan Journal of Chemistry Vol. 3 No.2, 2008, pp. 109-145
JJC Halogen Substituted Bis(arylimino)Pyridine Transition Metal
Complexes as Catalysts for the Oligomerization and Polymerization of Ethylene
Marcus Seitz, Christian Görl, Wolfgang Milius, Helmut G. Alt*
Laboratorium für Anorganische Chemie, Universität Bayreuth, Postfach 10 12 51, Universitätsstraße 30, D-95440 Bayreuth, Germany
Received on Oct. 16, 2007 Accepted on Feb. 4, 2008
Abstract A series of 15 complexes containing 3d transition metals ranging from titanium to nickel
and halogen functionalized bis(arylimino)pyridine ligands was synthesized and characterized.
After activation with methylalumoxane (MAO), these catalysts oligomerized ethylene to give α-
olefins with 4-40 carbon atoms. The influence of the metal center, the halogen substituents, and
the reaction parameters on the product compositions are discussed. Some of the described
catalyst precursors showed the potential to isomerize α-olefins and to generate olefins with
uneven numbers of carbon atoms. Quantum mechanical calculations (DFT, B88LYP) helped
explaining the isomerization behaviour of 5-halide-2-methyl substituted transition metal
compounds. Relationships between the structure of the catalyst precursors or the parameters of
the reactions and the activity or the product compositions were confirmed by experiments.
Keywords: Bis(arylimino)pyridine; α-Olefins; Ethylene; Polymerization;
Oligomerization; Polyethylene
1. Introduction In the past few years, late transition metal complexes became more and more
interesting as catalyst precursors for the oligomerization and polymerization of α-
olefins. For example, α-diimine nickel complexes developed by Brookhart et al.[1] were
used for the polymerization of ethylene. These complexes also showed the potential to
incorporate polar monomers like acrylates. In 1998, Gibson[2,3] and Brookhart[4-6]
applied bis(imino)pyridine iron and cobalt complexes for the polymerization of
ethylene. Numerous transition metal complexes containing 2,6-bis(arylimino)pyridine
ligands are known in the literature since the 1960’s using especially the first row
transition metals iron[1-23], cobalt[7,11,13,15,19,22-26], nickel[19,27,28], zinc[15,19,29-30],
vanadium[19,31-33] and chromium[19,34-37] as central metals. The early work also focused
on other late transition metals like copper[38-41], ruthenium[42,43], rhodium[44-47], and
iridium[46] while some actual work deals with the synthesis of titanium, zirconium,
hafnium[48-50], and manganese[51-56] complexes. Especially bis(imino)pyridine iron * Corresponding author: Tel.: +49-921-552555; fax: +49-921-552044. E-mail-address: [email protected] (H.G. Alt)
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complexes proved to be very active catalysts after activation with a suitable co-catalyst
like MAO. 2,6-Bis(arylimino)pyridine iron complexes bearing substituents both at
positions 2 and 6 of the iminophenyl rings are known to produce only polyethylene. If
only one of the ortho positions of the imino nitrogens is substituted, these complexes
produce oligomer/polymer mixtures or pure oligomer mixtures depending on the size of
the substituent and the reaction conditions[4]. These oligomer mixtures consist of α-
olefins with 6-24 carbon atoms. α-Olefins are important resources for the synthesis of
a variety of fine chemicals or can be used as comonomers for polymerization
reactions. A great deal of theoretical work has been performed[7,25,57-60] investigating
mechanistic aspects of oligomerization and polymerization reactions with
bis(imino)pyridine complexes. The iron complex bis(5-chloro-2-methylphenyl-1-
ethylimino)pyridine iron(II) chloride[61] proved to be very active for the oligomerization
of ethylene (3660 kg prod./g Fe · h). Since bis(imino)pyridine complexes are suitable
candidates for structure-property relationships, this complex was chosen for further
investigations. Furthermore, a series of bis(imino)pyridine complexes containing the 3d
transition metals titanium, vanadium, chromium, manganese, iron, cobalt, and nickel is
prepared whereby the bis(imino)pyridine ligand is kept the same. With this series of
catalyst precursors the influences of different metal centers both on catalyst activities
and product compositions are analyzed. Additionally, the influence of different halogen
substituents at the ligand framework and at the metal center on the oligomerization
and polymerization behavior is investigated. A couple of highly active halogen
substituted bis(arylimino)pyridine iron complexes were already described in the
literature[8,9,12,13,62]. A new aspect concerning the ability of some of the
bis(imino)pyridine complexes to produce olefins with uneven numbers of carbon atoms
is discussed. To our knowledge, the described catalysts are the first ones which are
able to produce 1-alkenes with uneven carbon numbers from ethylene. Finally,
optimized catalysts are presented with regard to their activity and their potential to
produce olefins with uneven numbers of carbon atoms.
2. Materials and methods All experimental work was routinely carried out using Schlenk technique. Dried
and purified argon was used as inert gas. n-Pentane, diethyl ether, toluene und
tetrahydrofuran were purified by distillation over Na/K alloy. Diethyl ether was
additionally distilled over lithium aluminum hydride. Methylene chloride was dried with
phosphorus pentoxide and calcium hydride. Methanol and ethanol were dried over
molecular sieves. 1-Butanol (p. a.) was purchased from Merck and used without prior
distillation. Deuterated solvents (CDCl3, CD2Cl2) for NMR spectroscopy were stored
over molecular sieves (3Ǻ).
Methylalumoxane (30 % in toluene) was purchased from Crompton
(Bergkamen) and Albemarle (Baton Rouge, USA / Louvain – La Neuve, Belgium).
Ethylene (3.0) und argon (4.8/5.0) were supplied by Rießner Company (Lichtenfels).
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All other starting materials were commercially available and were used without further
purification.
2.1 NMR spectroscopy
The spectrometer Bruker ARX 250 was available for recording the NMR spectra. The
samples were prepared under inert atmosphere (argon) and routinely recorded at 25
°C. The chemical shifts in the 1H NMR spectra are referred to the residual proton
signal of the solvent (δ = 7.24 ppm for CDCl3, δ = 5.32 ppm for CD2Cl2) and in 13C
NMR spectra to the solvent signal (δ = 77.0 ppm for CDCl3, δ = 53.5 ppm for CD2Cl2).
2.2 Mass spectrometry
Mass spectra were routinely recorded at the Zentrale Analytik of the University of
Bayreuth with a VARIAN MAT CH-7 instrument (direct inlet, EI, E = 70 eV) and a
VARIAN MAT 8500 spectrometer. Post-processing and data analyses were performed
using the software “Maspec II32 Data System”.
2.3 GC/MS
GC/MS spectra were recorded with a HP 5890 gas chromatograph in combination with
a HP 5971A mass detector. A 12 m J&W Scientific fused silica column (DB1, diameter
0.25 mm, film 0.33 µm, flow 1ml/min) respectively 25 m J&W Scientific fused silica
column (DB5ms, diameter 0.25 mm, film 0.33 µm, flow 1ml/min) were used, helium
(4.6) was applied as carrier gas. Using a 12 m column, the routinely performed
temperature program started at 70 °C (2 min). After a heating phase of eleven minutes
(20K/min, final temperatur 290 °C) the end temperature was held for a variable time
(plateau phase).
At the Zentrale Analytik of the University of Bayreuth, GC/MS spectra were routinely
recorded with a HP5890 gas chromatograph in combination with a MAT 95 mass
detector.
2.4 Gas chromatography
For the analysis of organic compounds, especially oligomer mixtures, a PERKIN
ELMER Auto System gas chromatograph (column: HP1, 28 m, diameter 0.32 mm /
carrier gas helium, flow 5.7 ml/min, split 3.5 ml/min) was used. The standard
temperature program contained a starting phase at 50 °C (3 min), a heating phase of
50 minutes (heating rate 4 K/min, final temperatur 250 °C) and a plateau phase at 250
°C (37 min).
2.5 IR spectroscopy
For the recording of IR spectra, the compounds were levigated with dried cesium
iodide. Thereof, thin pellets were prepared applying a pressure of 10 bar. The pellets
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were introduced into a PERKIN ELMER Spectrum 2000 FT-IR instrument containing a
He-Ne-laser. The maximum resolution was 0.15 cm-1. IR absorptions in the range of
400-4000 cm-1 were recorded in steps of 1.0 cm-1. Fourier transformations and post-
processing was performed using the software “Spectrum for Windows” (PERKIN
ELMER).
2.6 X-ray analysis
The X-ray analyses were performed by Dr. Wolfgang Milius (University Bayreuth) using
a Siemens P4 diffractometer (radiation source: MoKα, λ = 0.71073 Å).
Crystal data for compound 1:
Empirical formula: C23 H21 Cl2 N3; Formula weight: 410.33; yellow prisms from
diethylether; monoclinic; space group P 21/c; a = 11.1358(22) Å; b = 15.7676(18) Å; c
= 12.3067(16) Å; α = 90°, β = 95.88(1)°, χ = 90°; Volume = 2149.5(6) Å3; Z = 4; d(calc) =
1.268 g/cm3; absorption coeffizient 0.315 mm-1; F(000) = 856; Theta range for data
collection 1.84-22.50°; Index ranges: -10<= h =>11, -4<= k =>16, -13<= l =>13;
reflections collected: 3429; independent reflections 2649 [R(int) = 0.0211];
completeness to theta = 22.50°: 94.4 %; refinement method: full-matrix least-squares
on F2; Goodness-of-fit 1.021; R1 [l>2σ(I)] = 5.24%, wR2 = 0.1312; R1 (all data) = 8.70
%, wR2 = 0.1531; extinction coefficient = 0.0097(15); largest diff. peak and hole: 0.259
and -0.174 e. Å3.
The crystal was sealed in a glass capillary and measured at 293 K.
Table 1. Atom coordinates (·10-4) and equivalent isotropic shift parameters (Å2 ·103) for
compound 1.
x y z U(eq)a)
Cl(1) 1165(1) 3844(1) 1902(1) 116(1)
Cl(2) -1717(2) -5024(1) 2601(1) 122(1)
N(1) -2775(3) -295(2) 786(2) 53(1)
N(2) -2455(3) 1931(2) 712(2) 56(1)
N(3) -3869(3) -2378(2) 1021(3) 63(1)
C(1) -3575(3) -898(2) 969(3) 51(1)
C(2) -4668(4) -719(2) 1348(3) 58(1)
C(3) -4941(4) 115(2) 1553(3) 61(1)
C(4) -4129(3) 740(2) 1361(3) 55(1)
C(5) -3059(3) 515(2) 970(3) 46(1)
C(6) -2141(3) 1156(2) 705(3) 51(1)
C(7) -953(4) 839(3) 419(4) 88(2)
C(8) -1636(3) 2572(2) 439(3) 54(1)
C(9) -1800(4) 2932(2) -596(3) 62(1)
C(10) -2798(4) 2622(3) -1427(4) 90(2)
C(11) -1003(4) 3567(3) -837(4) 77(1)
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x y z U(eq)a) C(12) -91(4) 3852(3) -91(4) 75(1)
C(13) 34(4) 3495(3) 933(4) 72(1)
C(14) -736(3) 2863(2) 1204(3) 63(1)
C(15) -3199(4) -1788(2) 736(3) 58(1)
C(16) -2066(4) -1905(3) 202(4) 90(2)
C(17) -3553(4) -3240(2) 848(4) 61(1)
C(18) -3993(4) -3664(2) -92(4) 67(1)
C(19) -4768(5) -3219(3) -990(4) 100(2)
C(20) -3686(4) -4513(3) -176(4) 81(1)
C(21) -2985(4) -4935(3) 630(5) 83(2)
C(22) -2587(4) -4506(3) 1555(4) 76(1)
C(23) -2860(4) -3652(3) 1684(4) 70(1)
a) U(eq) is defined as 1/3 of the track of the orthogonalized Uij tensor.
Table 2. Bond lengths [Å] and angles [°] in compound 1.
Bond lengths [Å] Angles [°]
Cl(1)-C(13) 1.733(5)
Cl(2)-C(22) 1.734(5)
N(1)-C(1) 1.337(4)
N(1)-C(5) 1.340(4)
N(2)-C(6) 1.271(4)
N(2)-C(8) 1.425(4)
N(3)-C(15) 1.264(4)
N(3)-C(17) 1.426(5)
C(1)-C(2) 1.377(5)
C(1)-C(15) 1.500(5)
C(2)-C(3) 1.378(5)
C(3)-C(4) 1.374(5)
C(4)-C(5) 1.377(5)
C(5)-C(6) 1.497(5)
C(6)-C(7) 1.489(5)
C(8)-C(14) 1.381(5)
C(8)-C(9) 1.390(5)
C(9)-C(11) 1.389(5)
C(9)-C(10) 1.512(6)
C(11)-C(12) 1.373(6)
C(12)-C(13) 1.374(6)
C(13)-C(14) 1.379(5)
C(15)-C(16) 1.493(5)
C(17)-C(18) 1.383(6)
C(17)-C(23) 1.384(6)
C(18)-C(20) 1.387(6)
C(18)-C(19) 1.504(6)
C(1)-N(1)-C(5) 118.4(3)
C(6)-N(2)-C(8) 119.7(3)
C(15)-N(3)-C(17) 119.9(3)
N(1)-C(1)-C(2) 122.6(3)
N(1)-C(1)-C(15) 115.4(3)
C(2)-C(1)-C(15) 122.0(3)
C(1)-C(2)-C(3) 118.5(3)
C(4)-C(3)-C(2) 119.4(4)
C(3)-C(4)-C(5) 118.9(3)
N(1)-C(5)-C(4) 122.1(3)
N(1)-C(5)-C(6) 115.4(3)
C(4)-C(5)-C(6) 122.4(3)
N(2)-C(6)-C(7) 125.2(3)
N(2)-C(6)-C(5) 116.9(3)
C(7)-C(6)-C(5) 117.8(3)
C(14)-C(8)-C(9) 120.5(4)
C(14)-C(8)-N(2) 120.8(4)
C(9)-C(8)-N(2) 118.6(4)
C(11)-C(9)-C(8) 117.6(4)
C(11)-C(9)-C(10) 122.0(4)
C(8)-C(9)-C(10) 120.4(4)
C(12)-C(11)-C(9) 122.6(4)
C(11)-C(12)-C(13) 118.5(4)
C(12)-C(13)-C(14) 120.7(4)
C(12)-C(13)-Cl(1) 120.0(4)
C(14)-C(13)-Cl(1) 119.3(4)
C(13)-C(14)-C(8) 120.1(4)
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Bond lengths [Å] Angles [°]
C(20)-C(21) 1.371(6)
C(21)-C(22) 1.359(6)
C(22)-C(23) 1.393(6)
N(3)-C(15)-C(16) 125.5(3)
N(3)-C(15)-C(1) 116.8(3)
C(16)-C(15)-C(1) 117.7(3)
C(18)-C(17)-C(23) 121.2(4)
C(18)-C(17)-N(3) 120.9(4)
C(23)-C(17)-N(3) 117.7(4)
C(17)-C(18)-C(20) 117.4(4)
C(17)-C(18)-C(19) 121.1(4)
C(20)-C(18)-C(19) 121.5(4)
C(21)-C(20)-C(18) 122.8(5)
C(22)-C(21)-C(20) 118.5(4)
C(21)-C(22)-C(23) 121.4(5)
C(21)-C(22)-Cl(2) 119.8(4)
C(23)-C(22)-Cl(2) 118.8(4)
C(17)-C(23)-C(22) 118.7(4)
2.7 Synthesis of the 2,6-bis(arylimino)pyridine compounds 1-3 To a solution of 0.82 g (5 mmol) 2,6-diacetylpyridine in 150 ml of toluene were
added 12,5 mmol (2,5 equivs.) of a substituted aniline and a few milligrams of para-
toluenesulfonic acid. The reaction mixture was heated under reflux for 8-24 hours
applying a Dean-Stark-trap. After cooling to room temperature, 200 ml of a saturated
sodium hydrogencarbonate solution were added, the organic phase was separated
and filtered over sodium sulfate and silica. The solvent was removed and 20 ml of
methanol were added. The imino compounds precipitated when stored at - 20 °C for
some days. After filtration and washing with cold methanol, the products were dried in
vacuo.
2.8 Synthesis of the mono(imino)pyridine compound 4
To a solution of 0.82 g (5 mmol) 2,6-diacetylpyridine in 150 ml of toluene were
added 5 mmol (1 equiv.) of 5-chloro-2-methylaniline and a few milligrams of para-
toluenesulfonic acid. The reaction mixture was heated under reflux for 8 hours applying
a Dean-Stark-trap. After cooling to room temperature, 200 ml of a saturated sodium
hydrogencarbonate solution were added, the organic phase was separated and filtered
over sodium sulfate and silica. The solvent was removed and 20 ml of methanol were
added. The imino compound precipitated when stored at - 20 °C over night. After
filtration and washing with cold methanol, the product was dried in vacuo.
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Table 3. NMR and MS data for compounds 1-4.
Com-pound
1H NMR a) 13C NMR b) MS [m/z] c)
1
8.39 d (2H, 3JHH = 7.94 Hz, PyH3),
7.90 t (1H, 3JHH = 7.94 Hz, PyH4),
7.15 d (2H, 3JHH = 7.92 Hz), 7.01
dd (2H, 3JHH = 7.92Hz, 4JHH = 3.17
Hz), 6.72 d (2H, 4JHH = 3.17 Hz),
2.36 s (6H, N=C-CH3), 2.08 s (6H,
Ar-CH3)
167.5 (Cq, C=N), 154.9 (Cq, PyC2/6),
150.8 (Cq, C-N), 136.9 (CH), 131.6
(Cq, C-Cl), 131.4 (CH), 125.5 (Cq, C-
Me), 123.4 (CH), 122.5 (CH), 118.0
(CH), 17.2 (Ar-CH3), 16.4 (N=C-CH3)
409 M°+
394 M - Me (100)
284 M – (Cl-Ph-
Me) (12)
125 (Cl-Ph-
Me)(40)
2
8.35 d (2H, 3JHH = 7.71 Hz, PyH3),
7.85 t (1H, 3JHH = 7.71 Hz, PyH4),
7.11 d (2H, 3JHH = 7.63 Hz), 6.99
dd (2H, 3JHH = 7.63 Hz, 4JHH = 3.02
Hz), 6.69 d (2H, 4JHH = 3.02 Hz),
2.28 s (6H, N=C-CH3), 2.04 s (6H,
Ar-CH3)
167.1 (Cq, C=N), 161.3 (Cq, PyC2/6),
150.7 (Cq, C-N), 137.1 (CH), 132.6
(Cq, C-Me), 131.5 (CH), 125.9 (Cq, C-
Br), 123.7 (CH), 121.0 (CH), 113.8
(CH), 17.8 (Ar-CH3), 16.4 (N=C-CH3)
499 M°+
484 M - Me (37)
328 M – (Br-Ph-
Me) (10)
169 (Br-Ph-
Me)(38)
3
8.41 d (2H, 3JHH = 8.36 Hz, PyH3),
7.92 t (1H, 3JHH = 8.36 Hz, PyH4),
7.17 d (2H, 3JHH = 8.85 Hz), 6.75
dd (2H, 3JHH = 8.85 Hz, 4JHH = 3.42
Hz), 6.45 d (2H, 4JHH = 3.42 Hz),
2.35 s (6H, N=C-CH3), 2.15 s (6H,
Ar-CH3)
167.5 (Cq, C=N), 163.4 (Cq, C-F),
155.0 (Cq, PyC2/6), 151.0 (Cq, C-N),
136.9 (CH), 131.2 (CH), 122.5 (CH),
122.3 (Cq, C-Me), 110.1 (CH), 105.2
(CH), 17.0 (Ar-CH3), 16.4 (N=C-CH3)
377 M°+
362 M - Me (100)
268 M – (F-Ph-
Me) (10)
109 (F-Ph-Me)(76)
4
8.41 d (2H, 3JHH = 8.05 Hz, PyH3),
7.87 t (1H, 3JHH = 8.05 Hz, PyH4),
7.15 d (2H, 3JHH = 7.87Hz), 7.03
dd (2H, 3JHH = 7.87 Hz, 4JHH =3.02
Hz), 6.71 d (2H, 4JHH = 3.02 Hz),
2.77 s (3H, O=C-CH3), 2.24 s (3H,
N=C-CH3), 2.03 s (3H)
199.3 (Cq, C=O), 169.6 (Cq, C=N),
155.3 (Cq, PyC2/6), 151.2 (Cq, C-N),
135.7 (CH, PyC4), 131.8 (Cq, C-Cl),
131.0 (CH), 129.1 (CH), 125.5 (Cq, C-
Me), 123.4 (CH), 122.7 (CH), 118.1
(CH), 25.5 (CH3), 17.6 (O=C-CH3),
16.4 (N=C-CH3)
286 M°+
271 M - Me (100)
251 M - Cl (16)
166 (37)
a) 25 °C, in CDCl3, rel. CHCl3, δ = 7.24 ppm
b) 25 °C, in CDCl3, rel. CDCl3, δ = 77.0 ppm
c) in brackets: intensity of the ion peak in relation to the base peak
2.9 General synthesis of 2,6-bis(arylimino)pyridine transition metal complexes 5-19
An amount of 0.5 mmol of the 2,6-bis(arylimino)pyridine compound was dissolved in 20
ml 1-butanol or 20 ml THF and reacted with 0.5 mmol of the desired water free metal
salt mostly resulting in an immediate colour change. The mixture was stirred for 3-5
hours at room temperature whereby the complexes precipitated. In case of the nickel
complexes, it was necessary to keep the reaction mixture under reflux. n-Pentane (10
ml) was added for complete precipitation. The complexes were filtered over a glass frit,
washed three times with 15 ml n-pentane, and dried in vacuo. If necessary, the
complexes were recrystallized from methanol or methylene chloride.
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Table 4. MS, IR, and elemental analysis data of the complexes 5-19.
Nr. complex MS
m/z (%)
IR ν(C=N) [cm-1]
Cexp [%]
Ctheor [%]
Hexp [%]
Htheor [%]
Nexp [%]
Ntheor [%]
5 N
NNCl Cl
Ti
Cl ClCl
564 M+• 529 M-Cl (3) 409 M-TiCl3 (29) 394 M- TiCl3-Me (100)
n.d. 48.69 48.93 3.61 3.75 7.30 7.44
6 N
NNCl Cl
V
Cl ClCl
567 M+• 532 M-Cl (2) 497 M-2Cl (3) 409 M-VCl3 (37) 394 M-VCl3-Me (100)
1578 48.31 48.67 3.75 3.73 7.29 7.40
7 N
NNCl Cl
Cr
Cl ClCl
568 M+• 533 M-Cl (3) 498 M-2Cl (7) 409 M-CrCl3 (21) 394 M-CrCl3-Me(100)
1654 48.41 48.58 3.79 3.72 7.24 7.39
8 N
NNCl Cl
Mn
Cl Cl
536 M+• 501 M-Cl (2) 409 M-MnCl2 (19) 394 M-MnCl2-Me (100)
1633, 1593 51.23 51.52 3.88 3.95 7.69 7.84
9 N
NNCl Cl
Fe
Cl Cl
537 M+• 502 M-Cl (1) 409 M-FeCl2 (21) 394 M-FeCl2-Me (100)
1626, 1593 51.28 51.43 3.99 3.94 7.76 7.82
10 N
NNCl Cl
Fe
Cl ClCl
571 M+• 536 M-Cl (20) 500 M-2Cl (25) 409 M-FeCl3 (45) 394 M-FeCl3-Me (100)
1633, 1593 48.06 48.25 3.67 3.70 7.22 7.34
11 N
NNCl Cl
Co
Cl Cl
540 M+• 505 M-Cl (6) 409 M-CoCl2 (37) 394 M-CoCl2-Me (100)
n.d. 51.02 51.14 3.85 3.92 7.63 7.78
12 N
NNCl Cl
Ni
Cl Cl
539 M+• 504 M-Cl (2) 409 M-NiCl2 (35) 394 M-NiCl2-Me (75)
n.d. 51.05 51.16 3.78 3.92 7.67 7.78
117
Nr. complex MS
m/z (%)
IR ν(C=N) [cm-1]
Cexp [%]
Ctheor [%]
Hexp [%]
Htheor [%]
Nexp [%]
Ntheor [%]
13 N
NNCl Cl
Fe
Br Br
625 M+• 546 M-Br (40) 409 M-FeBr2 (50) 394 M-FeBr2-Me (100)
1593 43.98 44.13 3.29 3.38 6.64 6.71
14 N
NNCl Cl
Fe
Br BrBr
705 M+• 625 M-Br (20) 546 M-2Br (35) 409 M-FeBr3 (50) 394 M-FeBr3-Me (100)
1591 39.05 39.13 3.06 3.00 5.84 5.95
15 N
NNCl Cl
Ni
Br Br
628 M+• (not visible)549 M-Br (5) 409 M-NiBr2 (46) 394 M-NiBr2-Me (100)
n.d. 43.66 43.93 3.30 3.37 6.55 6.68
16 N
NNBr Br
Fe
Cl Cl
625 M+• 590 M-Cl (1) 546 M-Br (20) 499 M-FeCl2 (59) 484 M-FeCl2-Me (94)
n.d. 43.97 44.13 3.32 3.38 6.54 6.71
17 N
NNF F
Fe
Cl Cl
504 M+• 469 M-Cl (1) 378 M-FeCl2 (10) 363 M-FeCl2-Me (100)
1605 54.34 54.79 4.17 4.20 8.19 8.33
18 N
N OCl
Fe
Cl Cl
413 M+• 378 M-Cl (3) 286 M-FeCl2 (42) 271 M-FeCl2-Me (100)
n.d. 46.22 46.47 3.70 3.66 6.59 6.77
19 N
NNBr Br
Fe
Cl ClCl
660 M+• (not visible)625 M-Cl (4) 590 M-2Cl (3) 546 M-Cl-Br (19) 499 M-FeCl3 (67) 484 M-FeCl3-Me (100)
n.d. 41.64 41.76 3.18 3.20 6.22 6.35
2.10 Oligomerization of ethylene at low pressure
An amount of 0.1 – 0.3 mmol of the desired complex was placed in a Schlenk
tube and suspended in 100 ml of toluene or n-pentane. After activation with methyl
alumoxane (30% in toluene), an ethylene pressure of 0.5 bar or 1.0 bar was applied
and the mixture was stirred for one hour at room temperature. The reaction was
stopped by releasing the pressure. The mixture was carefully poured into 100 ml of
diluted hydrochloric acid. When a polymer was obtained, it was separated by filtration
118
using a Büchner funnel. The polymer was washed with water and acetone and finally
dried in vacuo. The liquid organic phase was washed twice with 50 ml of water and
dried over sodium sulfate. The resulting solutions were analyzed by gas
chromatography. Including the weight increase of the solutions, the activities were
calculated by integration of the GC peaks.
3. Results and discussion 3.1 Synthesis and characterization of 2,6-bis(arylimino)pyridine compounds
Condensation reactions of 2,6-diacetylpyridine with 5-halogen-2-methyl substituted
anilines yielded the 2,6-bis(arylimino)pyridine compounds 1 – 3 (see Scheme 1).
NO O
2
toluenep-TosOH
reflux- 2 H2O
NN N
R R
R = Hal
NH2
R
Compound R Yield
1 Cl 78
2 Br 63
3 F 49
Scheme 1. Synthesis of 5-halogen-2-methyl substituted 2,6-bis(arylimino)pyridine
compounds.
Additionally, the monosubstituted compound 4 was prepared from 2,6-diacetylpyridine
and 5-chloro-2-methylaniline (Scheme 2) in a 61% yield.
NONCl
Scheme 2. Monosubstituted compound 4.
The compounds 1 – 4 were characterized by GC/MS, 1H NMR and 13C NMR
spectroscopy. The spectra of compound 1 are discussed representatively.
119
Scheme 3. Mass spectrum of 2,6-bis(5-chloro-2-methylphenyl-1-ethaneimino)pyridine
(1).
The molecule ion at m/z = 409 is clearly visible in the mass spectrum of 1
(Scheme 3). Due to the chlorine substituents, the peak shows a characteristic isotope
pattern which fits excellently to the theoretically calculated distribution (see Scheme 4).
Scheme 4. Isotope pattern of the molecular ion of compound 1.
The base peak at m/z = 394 results from the loss of one iminomethyl group.
Again, the characteristic isotope pattern can be observed. The peaks at m/z = 266 and
m/z = 243 can be explained by α-cleavage reactions starting from the imino nitrogen
atom and the nitrogen atom of the pyridine ring. The loss of one of the substituted
phenyl rings gives a peak at m/z = 284.
NNNCl Cl
284
125
M=409
243
166
394
M+
measured calculated
120
Scheme 5. 1H NMR spectrum of compound 1.
The 1H NMR spectrum of 1 (see Scheme 5) shows a dublet at δ = 8.39 ppm that
can be assigned to the meta protons (2,4) at the pyridine ring. The corresponding
triplet at δ = 7.90 ppm stems from the para proton (3) of the pyridine ring. The phenyl
protons appear at δ = 7.15 ppm (12,18), δ = 7.01 ppm (13,19), and δ = 6.72 ppm
(15,21). Finally, the two singlets at δ = 2.36 ppm and δ = 2.08 ppm can be assigned to
the iminomethyl groups (7,9) and the methyl groups at the phenyl rings (22,23).
5
43
2
1N6 8
9
N
7
N16 10
1718
19
2021 15
14
1312
112322
Cl Cl2,4
3
12,18
13,19
15,21
2,4
3 12,18
13,19 15,21
CHCl3
CHCl3
22,23
7,9
121
6,8 1,5 10,16 14,2011,17
CDCl3
3
12,18
15,21 2,4
13,19
22,23
7,9
5
43
2
1N6 8
9
N
7
N16 10
1718
19
2021 15
14
1312
112322
Cl Cl
Scheme 6. J-modulated 13C NMR spectrum of compound 1.
A J-modulated 13C NMR spectrum (Scheme 6) was recorded from compound 1.
The resonance signal at δ = 167.5 ppm can be assigned to the imino carbon atoms
(6,8). The quaternary carbons of the pyridine ring (1,5) give the peak at δ = 154.9 ppm
followed by the signal for the nitrogen-bonded carbon atoms of the phenyl rings (10,16)
at δ = 150.8 ppm. The para carbon atom of the pyridine ring (3) yields the signal at δ =
136.9 ppm. The signal for the chloro substituted carbon atoms (14,20) appears at δ =
131.6 ppm. At δ = 131.4 ppm, the signal for the carbon atoms 12 and 18 can be found.
The methyl substituted quaternary carbon atoms of the phenyl rings (11,17) give the
signal at δ = 125.5 ppm, while the meta-standing carbon atoms of the pyridine ring
(2,4) produce the signal at δ = 123.4 ppm. The unsubstituted ortho-carbon atoms of the
phenyl rings (15,21) appear at δ = 122.5 ppm, followed by the signal for the para-CH
groups (13,19) at δ = 118.0 ppm. The methyl groups at the phenyl rings (22,23) give
the signal at δ = 17.2 ppm, while the iminomethyl groups (7,9) yield the signal at δ =
16.4 ppm.
After crystallization from diethylether, single crystals of 1 were obtained which
were subjected to X-ray analysis.
122
Scheme 7. X-ray structure of compound 1.
The structure of 1 is analogous to already published structures showing other
substitution patterns at the iminophenyl rings. The crystal data can be found in the
Experimental part.
3.2 Synthesis of 2,6-bis(arylimino)pyridine transition metal complexes
Using the bis(arylimino)pyridine compounds 1-3 and the mono(imino)pyridine
compound 4, a series of coordination compounds including the 3d transition metals
from titanium to nickel was prepared (see Scheme 8 and Table 5). Titanium,
vanadium, and chromium were applied in the oxidation state +III, while manganese,
cobalt, and nickel were used in the oxidation state +II. In case of iron, both iron(II) and
iron(III) complexes were prepared. After dissolving the 2,6-bis(arylimino)pyridine
compound in 1-butanol, THF, or diethylether, the corresponding metal salt was added
resulting in an immediate color change. In most cases, the complexation reactions
were completed within three hours. The complexes could be isolated in very high
yields (80-95 %).
For the nickel complexes, (dme)NiBr2 and (dme)NiCl2 were prepared as starting
materials according to Nylander[63]. THF adducts of titanium(III)chloride,
vanadium(III)chloride, and chromium(III)chloride were prepared following a general
procedure[64,65]. All complexes were characterized by mass spectrometry, IR, and
elemental analysis. Additionally, the magnetic moments of the complexes were
determined using the Evans NMR method[66-68].
123
MXn
NN N
R R
M
Xn
1-BuOH,Et2O or THF
r.t., 3-5 h(reflux for
Ni complexes)
NN N
R R
R = HalX = Haln = 2; 3
Scheme 8. Synthesis of 2,6-bis(arylimino)pyridine transition metal complexes.
Table 5. Synthesized 2,6-bis(arylimino)pyridine transition metal complexes.
Nr. complex R M n X educt solvent colour yield[%]
5 N
NNCl Cl
Ti
Cl ClCl
Cl Ti 3 Cl TiCl3•
3 THF THF black 91
6 N
NNCl Cl
V
Cl ClCl
Cl V 3 Cl VCl3•
3 THF Et2O red 96
7 N
NNCl Cl
Cr
Cl ClCl
Cl Cr 3 Cl CrCl3•
3 THF THF
dark
green 87
8 N
NNCl Cl
Mn
Cl Cl
Cl Mn 2 Cl MnCl2•
2 THF THF yellow 89
9 N
NNCl Cl
Fe
Cl Cl
Cl Fe 2 Cl FeCl2 n-BuOH blue 92
124
Nr. complex R M n X educt solvent colour yield[%]
10 N
NNCl Cl
Fe
Cl ClCl
Cl Fe 3 Cl FeCl3 n-BuOH orange 93
11 N
NNCl Cl
Co
Cl Cl
Cl Co 2 Cl CoCl2 n-BuOH green 90
12 N
NNCl Cl
Ni
Cl Cl
Cl Ni 2 Cl NiCl2•
DME
THF
(boi-ling) orange 91
13 N
NNCl Cl
Fe
Br Br
Cl Fe 2 Br FeBr2 n-BuOH blue 88
14 N
NNCl Cl
Fe
Br BrBr
Cl Fe 3 Br FeBr3 n-BuOH dark
brown 86
15 N
NNCl Cl
Ni
Br Br
Cl Ni 2 Br NiBr2•
DME
THF
(boi-ling) orange 95
16 N
NNBr Br
Fe
Cl Cl
Br Fe 2 Cl FeCl2 n-BuOH blue 94
17 N
NNF F
Fe
Cl Cl
F Fe 2 Cl FeCl2 n-BuOH blue 85
125
Nr. complex R M n X educt solvent colour yield[%]
18 N
ONCl
Fe
Cl Cl
Cl Fe 2 Cl FeCl2 n-BuOH blue 90
19 N
NNBr Br
Fe
Cl ClCl
Br Fe 3 Cl FeCl3 n-BuOH dark
brown 87
Scheme 9. IR spectrum of complex 9.
The spectrum shows the charateristic ν (C=N) band at 1626 cm-1. Due to the
coordination of iron(II)chloride, the band is shifted to lower energy compared with the
ligand precursor (ν = 1638 cm-1). The bands at 1593 cm-1, 1484 cm-1, 1267 cm-1, and
811 cm-1 are characteristic for the substituted phenyl rings.
Scheme 10 shows the mass spectrum of complex 9. The molecular ion appears
at m/z = 537. The loss of FeCl2 results in the formation of the peak at m/z = 409
NNNCl Cl
Fe
Cl Cl
126
corresponding to the bis(arylimino)pyridine ligand. Peaks below this value can be
explained in analogy to ligand precursor 1.
Scheme 10. Mass spectrum of 9.
Analogously to the bis(arylimino)pyridine compound, the isotope pattern agrees very
well with the theoretically calculated distribution (Scheme 11).
NNNCl Cl
Fe
Cl Cl
C23H21Cl4FeN3537.10
M+
127
measured:
Calculated:
Scheme 11. Isotope pattern for the molecule ion of complex 9.
The magnetic moments of the transition metal complexes were determined
applying the Evans NMR method. Since the effective magnetic moments µeff directly
correspond with the electronic configuration, the electronic ground states of the 2,6-
bis(arylimino)pyridine metal complexes can be obtained[69] (see Table 6).
Table 6. Magnetic moments µeff and number of unpaired electrons in complexes
derived from ligand precursor 1.
complex metalcenter µeff
unpaired electrons
6 V(III) 2.97 2
7 Cr(III) 4.77 3
8 Mn(II) 5.92 5
9 Fe(II) 5.40 4
11 Co(II) 4.09 3
The knowledge of the electronic ground states plays an important role for “ab
initio” calculations concerning the theoretical investigation of the oligomerization
reactions.
128
3.3 Results of the homogeneous ethylene oligomerization and polymerization
After activation with methylalumoxane (MAO), the transition metal complexes 5-19 were used as catalyst precursors for the homogeneous oligomerization of ethylene.
The influences of different reaction parameters (metal, substituents at the ligand
framework, ethylene pressure, temperature, Al:M ratio) were investigated (Table 7). To
confirm the stability of the halogenated bis(arylimino)pyridine ligands against
trimethylaluminum/methylalumoxane, samples of the complexes were activated with
MAO. The mixtures were hydrolyzed after five minutes, worked up, and analyzed by
GC/MS revealing that the ligand systems remained unchanged. The oligomerization
products were characterized by GC and GC/MS and the Schulz-Flory coefficient α was
determined[70-73]. Data analysis was performed using a computer program which was
developed for this special purpose[74].
Table 7. Oligomerization and polymerization results for the complexes 5-19 (solvent:
250 ml toluene, activator: MAO, 1h).
Com- plex
p [bar]
T [°C] Al:M Activity
[g/g M·h] Activity
[kg/mol M·h] TOF
[mol C2H4/ mol Cat·h-1]
uneven olefins
[%]
Schulz-Flory coefficient α
5 0.5 25 250 61 2.9 82 - polymer
6 0.5 25 250 431 22.0 615 - 0.71
7 0.5 25 250 72 3.7 105 - 0.88
8 0.5 25 250 35 1.9 54 - polymer
9 1 25 250 504 28.1 782 4.4 0.77
9 0.5 0 250 1505 84.0 2337 2.7 0.80
9 0.5 25 250 473 26.4 734 3.6 0.83
9 0.5 50 250 433 24.2 672 3.4 0.73
9 0.5 75 250 25 1.4 39 - 0.77
10 1 25 250 783 43.7 1216 3.8 0.77
10 0.5 25 150 390 21.8 605 1.8 0.76
10 0.5 25 250 705 39.3 1095 2.5 0.78
10 0.5 25 350 584 32.6 907 3.9 0.82
10 0.5 25 500 462 25.8 717 6.8 0.76
10 0.5 25 750 1363 76.1 2116 6.0 0.82
10 0.5 25 1000 658 36.7 1022 5.3 0.81
10 0.5 25 1500 678 37.8 1053 3.4 0.83
129
Com- plex
p [bar]
T [°C] Al:M Activity
[g/g M·h] Activity
[kg/mol M·h] TOF
[mol C2H4/ mol Cat·h-1]
uneven olefins
[%]
Schulz-Flory coefficient α
10 0.5 25 2000 613 34.2 952 2.4 0.83
11 0.5 25 250 13 0.8 21 - polymer
12 0.5 25 250 80 4.7 131 - polymer
12 1 25 250 126 7.4 207 - polymer
13 0.5 25 250 297 16.6 461 7.3 0.80
13 1 25 500 198 11.0 307 19.2 0.78
14 0.5 25 250 342 19.1 531 5.2 0.80
15 0.5 25 250 71 4.2 117 - polymer
16 0.5 25 250 1690 94.3 2624 - 0.85
17 0.5 25 250 890 49.7 1382 2.8 0.82
18 0.5 25 250 3720 207.6 5775 - 0.90
19 1 0 750 2970 165.7 4611 - 0.80
The following mechanism is proposed for oligomerization and polymerization
reactions using bis(imino)pyridine transition metal complexes[57,58]:
methylationmethyl abstraction
coordination
N
N
N M
N
N
N MMeinsertion
N
N
N MMe
N
N
N MX
X
N
N
N MMe
N
N
N MMe
MAO
MAO-Me MAO-Me
MAO-Me MAO-Me MAO-Me
freecoordination
site
Scheme 12. Proposed mechanism of ethylene oligomerization and polymerization with
bis(imino)pyridine transition metal complexes.
As the main chain termination reactions, β-hydrogen elimination, β-hydrogen
transfer, or chain transfer to aluminum centers can be considered.
The GC spectrum of an oligomer mixture obtained with 9/MAO is shown in
Scheme 13.
130
Scheme 13. GC spectrum of an oligomer mixture obtained with 9/MAO.
The mixture consists of olefins with carbon numbers between 6 and 34. Since
the GC integrals are proportional to the molar amount of each component, the
logarithm of the GC integrals can be plotted against the carbon number to check
whether the obtained distribution matches an Anderson-Schulz-Flory distribution[75].
y = -0.0426x + 6.6744R2 = 0.9973
5.5
5.6
5.7
5.8
5.9
6
6.1
6.2
6.3
8 10 12 14 16 18 20 22 24 26 28
C-Atom-Zahl
log
[CnH
2n]
Scheme 14. Plot of the logarithmic GC integrals against the carbon numbers.
Polymerization conditions: 250ml toluene; MAO (Al : Fe = 250 : 1); 0.5 bar ethylene; 25°C; 1h.
C-10
C-12
C-14
C-18
C-22 C-26
C-12
C-9 C-11
τ [min]
τ [min]
Inte
nsitä
t [m
V] In
tens
ität [
mV
]
NNNCl Cl
Fe
Cl Cl
C-10
Number of carbon atoms
log[
CnH
2 n]
131
The coefficient of determination R2 is an indicator for the accordance with the
Anderson-Schulz-Flory theory. In Scheme 14, R2 is 99.7 %. For each distribution, the
characteristic Schulz-Flory coefficient α can be determined:
kpropagation
kpropagation + kterminationα =
mol (Cn+2)mol (Cn)
=
A higher coefficient α directly corresponds to an increased propagation
probability resulting in higher molecular weight products. The upper limit α = 1 is never
reached.
Interestingly, small amounts of olefins with uneven numbers of carbon atoms
could be detected by GC/MS analyses in some of the oligomer mixtures (see enlarged
part of Scheme 13). This result was proved by comparing the GC/MS data with the
results obtained for uneven numbered α-olefins used as references (1-nonene, 1-
undecene). The GC retention times are identical and the fragmentation patterns in the
mass spectra agree very well.
3.3.1 Influence of the metal center on the oligomerization activities and the
isomerization potentials
The influence of different metal centers in complexes bearing one and the same
bis(imino)pyridine ligand on the ethylene oligomerization and polymerization activities
is shown in Scheme 15.
61 7235
473
705
1380
431
0
100
200
300
400
500
600
700
800
Act
ivity
[g(P
E)/g
(M)*
h-1]
Scheme 15. Ethylene oligomerization activities of complexes 5-12 bearing
bis(imino)pyridine compound 1 as chelating ligand. Polymerization conditions: 250 ml
of toluene; (Al:M = 250:1); 0.5 bar ethylene; 25 °C; 1h.
The highest activities were obtained with the vanadium complex 6 and the iron
complexes 9 and 10 while the cobalt complex 11 showed the lowest activity in this
series. Only the iron catalysts 9 and 10 gave small amounts of uneven numbered
TiCl3 5
VCl3 6
CrCl3 7
MnCl2 8
FeCl2 9
FeCl3 10
CoCl2 11
NiCl2 12
132
olefins. The overall contents of these uneven numbered olefins in the obtained
mixtures are 3.6 % for 9/MAO and 2.5 % for 10, respectively. In case of the complexes
5 (Ti), 8 (Mn), 11 (Co), and 12 (Ni), only polymeric products were produced.
Differences in their potential to isomerize α-olefins can be found for the vanadium
complex 6, the chromium complex 7 and both iron complexes (Scheme 16).
Scheme 16. Parts of the GC spectra obtained for the oligomer mixtures using 6/MAO,
7/MAO, 9/MAO, and 10/MAO. The pictures show the isomers in the region of C-10 up
to C-12.
The chromium complex 7 showed the highest selectivity for α-olefins while both
iron complexes yielded also traces of other isomers of the even numbered alkenes and
small amounts of uneven numbered olefins. In contrast, the mono(imino)pyridine iron
complex 18 did not produce any uneven numbered oligomers but showed a quite high
activity (3720 g/g Fe · h-1). The vanadium complex 6 produced a whole series of
τ [min] τ [min]
τ [min] τ [min]
Inte
nsity
[mV
]
Inte
nsity
[mV
]
Inte
nsity
[mV
]
Inte
nsity
[mV
]
NNNCl Cl
V
Cl ClCl
6
NNNCl Cl
Cr
Cl ClCl
7
NNNCl Cl
Fe
Cl Cl
9
NNNCl Cl
Fe
Cl ClCl
46
133
isomers of even numbered olefins but no uneven numbered olefins were observed.
The following mechanism is assumed for the isomerization of α-olefins:
2-olefins
ethyleneinsertion
methyl side chains
furtherisomerization
internal olefins
ethyleneinsertion
ethyl, propyl side chains
β-hydrogenelimination
n
n
N
N
N MH
n
N
N
N M H
N
N
N M H
β-hydrogenelimination N
N
N MH
n
hydrogen transfer to the coordinated
olefin
n
N
N
N M
H
Scheme 17. Proposed mechanism for the isomerization reactions of α-olefins.
According to Brookhart[2,24,29,30] and Ziegler[57,58], a so-called “chain-running”
mechanism is supposed. The decisive reaction step is the β-hydrogen elimination,
since the interaction of the metal center and a β-hydrogen atom affects the degree of
isomerization. Using quantum chemical calculations, the degree of these β-agostic
interactions can be determined. Therefore, two transition structures were calculated for
the proposed active species of the complexes 6, 7, and 9, whereby the first structure is
always described with β-hydrogen interaction, the second one without β-hydrogen
interaction. The energy differences between the two species can be explained as the
degree of interaction. For the calculations, cationic catalyst species bearing a propyl
substituent at the metal center were used and the potential energies were minimized
using B88LYP. The structures of the catalyst precursors were optimized with MM3.
Substituents were introduced or exchanged keeping the main geometry constant. In
case of the vanadium complex 6, the active species described by Gambarotta[31] was
used (Scheme 18, I, addition of a methyl group to C-2 of the pyridine ring), while the
free coordination site in the chromium complex 7 is occupied by a methyl group
according to the active species presented by Esteruelas[34] (Scheme 18, II). Calculations for the activated iron complex 9 were performed applying the structure
parameters used by Gibson[76] and Ziegler[58] (Scheme 18, III). To rule out geometric
effects, an iron complex with an analogous structure to the vanadium complex was
used (Scheme 18, IV).
134
NN NCl Cl
V
Me
NN NCl Cl
V
H
Me
NN NCl Cl
Cr
NN NCl Cl
Cr
H
NN NCl Cl
Fe
NN NCl Cl
Fe
H
NN NCl Cl
Fe
Me
NN NCl Cl
Fe
H
Me
I
II
III
IV
Scheme 18. Structures of proposed active species (left: without β-agostic interaction;
right: with β-agostic interaction) of the complexes 6 (I), 7 (II), and 9 (III) used for the
calculations of β-agostic interaction energies.
All structures were optimized with MM3 to reduce the calculation times. The
energy calculations were performed starting the dGauss algorithm (CaChe 6.1)[77,78]
applying DFT on the basis of SCF optimizations. In dGauss, geometry optimizations on
the basis of the calculated energy gradients are carried out applying the Broyden-
Fletcher-Goldfarb-Shanno method[79]. For all SCF and gradient calculations the local
exchange potentials were used as described by Vosko, Wilk, and Nusair[80]. Non local
corrections were then computed on the basis of the resulting geometries (local spin
density geometry) and electron densities. For correlation, the „Becke ´88 Functional”[81]
135
is used including the additional specifications from Lee and Miehlich[82,83] for exchange
interactions.
DZVP[84] was used as a basic set for orbital calculations. For the corresponding
atoms the following orbitals were included into the calculations:
Table 8. Electron configurations for „ab initio“ calculations.
atom orbitals
H 1s,2s
C, N 1s, 2s, 2p, 3s, 3p, 3d
Cl 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p
V, Cr, Fe 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 5s
The energy contents of each structure were computed and the energies of the β-
hydride interactions were calculated as the energy differences between the structures
without and with β-agostic interactions (see Scheme 19 and Table 9).
I Chromium complex 6
II Vanadium complex 7
136
III Iron(II) complex 9
IV Iron(II) complex 9 (modified in analogy to the vanadium complex 7[45]
Scheme 19. Energy differences between the structures without (left) and with (right) β-
agostic interactions for the catalysts 7-9.
Table 9. Energy differences between the structures with and without β-agostic
interactions.
metal center ∆E kcal/mol
vanadium ± 0
chromium + 168.7
iron + 62.8
iron (V structure) + 133.5
It is clearly visible that an agostic interaction between the metal center and a
hydrogen atom at the β-position of the growing chain affects the geometry of the whole
molecule. These structural changes are responsible for the resulting energy
differences which are in good agreement with the oligomerization behavior of the
catalyst systems 6/MAO, 7/MAO, and 9/MAO. In case of the chromium complex 7, the
largest energy difference was found corresponding to a high selectivity towards α-
olefins, since β-hydrogen elimination and isomerization are energetically unfavorable.
Both calculated structures of the iron complex 9 also show an increased total energy
137
when β-agostic interactions are assumed. In contrast to the chromium complex 7,
isomerization reactions are not completely prevented but occur at a very low level. In
case of the vanadium complex 6, both structures are energetically equivalent.
Therefore, isomerization reactions can proceed on a large scale.
3.3.2 Formation of olefins with uneven numbers of carbon atoms
Among the synthesized transition metal complexes only iron complexes showed the
potential to produce olefins with uneven numbers of carbon atoms. Different reaction
pathways can be assumed for the formation of these unusual products. Siedle et al.[85]
proposed the transfer of methyl groups from MAO to the catalytically active species,
while the growing chain is transferred to an aluminum center (Scheme 20). This kind of
reaction would comply with a repeated activation step resulting in the formation of
uneven numbered olefins.
n
n
n
N
N
N Fe
N
N
N FeMe
AlR2MeAlR2
N
N
N FeMe
AlR2 Scheme 20. Reaction pathway proposed by Siedle et al. for the formation of olefins
with uneven numbers of carbon atoms.
To verify this mechanism, oligomerization reactions were carried out using the
catalyst systems 9/MAO and 10/MAO and 1-octene as the monomer. Both catalysts
produced dimers (hexadecene isomers) and trimers (tetracosene isomers) of 1-octene,
but there was no evidence for the formation of uneven numbered oligomers. Also,
saturated hydrocarbons which should be formed by hydrolysis of the alkylaluminum
species, could not be detected.
Another possible mechanism known in the literature[86,87] is the β-carbon
elimination pathway (Scheme 21). Usually this reaction is preferred by electron lacking
d0 complexes[88].
n
N
N
N Fe Me
N
N
N FeMe
nβ-carbon
eliminationisomerization
Scheme 21. Mechanism of the β-carbon elimination.
138
Since only the iron complexes produced oligomers with uneven carbon
numbers, this pathway does not seem to be relevant.
A plausible mechanism including the experimental results is proposed in
Scheme 22.
N
N
N Fen
isomer-ization
N
N
N Fen
N
N
N Fen
meta-thesis
Scheme 22. Metathesis reaction generating oligomers with uneven numbers of carbon
atoms.
At the beginning, 1-olefins are isomerized to give the corresponding 2-olefins.
The 2-olefins remain in the coordination sphere of the metal and undergo a metathesis
reaction with another coordinated ethylene molecule resulting in olefins with uneven
numbers of carbon atoms. Since the “chain-running” mechanism is energetically
hindered for the investigated iron complexes, 2-olefins must be the main products of
isomerization reactions. Their concentration is evidently high enough to undergo
metathesis reactions.
3.3.3 Influence of the halogen substituents at the metal center
For the preparation of complexes 13-15 metal bromides were applied instead of
the metal chlorides. Compared to the chloride complexes, the bromide complexes
showed lower activities (Scheme 23).
473
297
705
342
80 71
0
100
200
300
400
500
600
700
800
Act
ivity
[g(P
E)/g
(Fe)
*h-1
]
Scheme 23. Comparison of the ethylene oligomerization and polymerization activities
of bis(imino)pyridine metal chloride and bromide complexes. Polymerization
conditions: 250 ml of toluene; (Al:M = 250:1); 0.5 bar ethylene; 25 °C; 1h.
FeCl2 9
FeBr2 13
FeCl3 10
FeBr3 14
NiCl2 12
NiBr2 15
139
Contrarily, the overall contents of uneven numbered α-olefins increase when
changing the metal halide from chlorine to bromine (Table 10):
Table 10. Overall contents of uneven numbered α-olefins in the mixtures produced
with 9/MAO, 10/MAO, 13/MAO and 14/MAO.
Complex Content of uneven numbered olefins [%]
9 3.6
10 2.5
13 7.3
14 5.2
This result can be explained by steric effects. If a bromide ligand is transferred
to a MAO cage, the counter ions are better separated. Consequently, there is more
space around the catalytically active center leading to an increased rate of metathesis
reactions. The oligomer distributions are little influenced by the change from a metal
chloride to the corresponding metal bromide. The α values only vary in the range 0.78-
0.80.
3.3.4 Effect of different halogen substituents on the ligand frameworks
The influence of different halogen substituents on the ligand framework was
investigated with the iron(II) chloride complexes 9, 16 and 17 (Scheme 24).
890
473
1690
0
200
400
600
800
1000
1200
1400
1600
1800
Act
ivity
[g(P
E)/g
(Fe)
*h-1
]
Scheme 24. Polymerization activities of iron(II) chloride complexes bearing different
halogen substituents in their ligand frameworks. Polymerization conditions: 250 ml of
toluene; (Al:M = 250:1); 0.5 bar ethylene; 25 °C; 1h.
F Cl Br9 1617
140
Complex 16 with a bromo substituted ligand framework shows the highest activity
among these three complexes. The activities of the fluoro and chloro substituted
complexes 17 and 9 are apparently smaller. While the bromo substituted complex 16
did not give any uneven numbered olefins, the content of uneven numbered olefins in
the mixture produced with 17/MAO is 2.8 %. In case of the bromo substituted complex
16, there is not enough space around the metal center for metathesis reactions due to
the big halogen substituent. Since β-hydrogen elimination reactions or chain transfer
reactions to aluminum centers are also hindered, an increased activity and a higher
Schulz-Flory coefficient are observed. The Schulz-Flory coefficients α increase in the
row F (α = 0.82) < Cl (α = 0.83) < Br (α = 0.85) providing higher molecular weight
products by increasing the size of the halogen substituent.
3.3.5 Influence of the ethylene pressure
Ethylene was oligomerized and polymerized at 0.5 bar and 1.0 bar applying the
catalysts 9/MAO, 10/MAO, and 12/MAO. As can be seen in scheme 25, the activities
increase with increasing pressure.
473504
705
783
80126
0
100
200
300
400
500
600
700
800
900
Act
ivity
[g(P
E)/g
(Fe)
*h-1
]
Scheme 25. Activities of the catalyst systems 9/MAO, 10/MAO, and 12/MAO at
different ethylene pressures. Polymerization conditions: 250 ml of toluene; (Al:M =
250:1); 25 °C; 1h.
0.5 bar 1.0 bar 0.5 bar 1.0 bar 0.5 bar 1.0 bar
NNNCl Cl
Fe
Cl ClCl
10
NNNCl Cl
Ni
Cl Cl
12
NNNCl Cl
Fe
Cl Cl
9
141
In case of the iron catalysts, the increased ethylene pressure led to a higher
content of uneven numbered olefins. For 9/MAO, the amount increased from 3.6 % at
0.5 bar to 4.4 %, while for 10/MAO 3.8 % of uneven numbered olefins were detected
by GC compared with an amount of 2.5 % at 0.5 bar ethylene pressure. In contrast, the
Schulz-Flory coefficients α decrease with increasing pressure resulting in lower
molecular weight olefins (see Table 3). At higher ethylene pressure, there is a higher
probability for metathesis reactions, since the concentrations of both required educts
are increased.
3.3.6 Influence of the polymerization temperature
The catalyst 9/MAO was chosen to investigate the influence of the reaction
temperature on the oligomerization activity and the product composition.
1505
473 433
250
200400600800
1000120014001600
Act
ivity
[g(P
E)/g
(Fe)
*h-1
]
Scheme 26. Activities of the system 9/MAO at different reaction temperatures.
Polymerization conditions: 250 ml of toluene; (Al:M = 250:1); 0.5 bar ethylene; 1h.
At 0°C, the catalyst 9/MAO shows the highest activity (1505 g/g M · h). The
strong decrease in activity at higher temperatures can be explained with a faster
deactivation of the catalytically active species. In the range from 0°C to 50°C, the
amounts of uneven numbered olefins are quite similar. In contrast, at 75°C no uneven
numbered olefins were observed. The temperature dependence of these reactions can
be explained with different reaction orders for oligomerization (first order to monomer)
and metathesis reaction (first order to monomer and additionally a dependence from
the formation rate of the 2-olefins). Therefore, an optimum equilibrium between the
formation of 2-olefins and the metathesis reaction is reached at 25°C.
3.3.7 Influence of the aluminum/metal ratio
The ratio of co-catalyst to catalyst precursor is a very important parameter for all
oligomerization and polymerization reactions. Using the catalyst 10/MAO, the influence
of different aluminum/metal ratios on the activity and the product composition was
investigated.
0° 25°C 50°C 75°C
142
390
705584
462
1363
658 678 613
0
200
400
600
800
1000
1200
1400
1600
150:1 250:1 350:1 500:1 750:1 1000:1 1500:1 2000:1Ratio Al:Fe
Act
ivity
[g(P
E)/g
(Fe)
*h-1
]
Scheme 27. Activities of the catalyst 10/MAO at different Al:Fe ratios. Polymerization
conditions: 250 ml of toluene; 0.5 bar ethylene; 25 °C; 1h.
At a ratio Al:Fe = 750:1, the activity reaches its maximum value. Applying lower
values, there are not enough suitable aluminum centers available in the mixture
(possibly “free” TMA molecules), while at higher Al:Fe ratios side reactions become
more dominant resulting in deactivation of the catalytically active species. Similarly, the
overall content of uneven numbered olefins first increases and reaches a maximum at
a ratio Al:Fe = 500:1, while at higher ratios a gradual decrease can be observed (see
Scheme 28).
0
1
2
3
4
5
6
7
8
150:1 250:1 350:1 500:1 750:1 1000:1 1500:1 2000:1
Ratio Al : Fe
Ove
rall
cont
ent o
f une
ven
num
bere
d ol
efin
s [%
]
Scheme 28. Overall contents of uneven numbered oligomers at different Al:Fe ratios
applying the catalyst 10/MAO. Polymerization conditions: 250 ml of toluene; 0.5 bar
ethylene; 25 °C; 1h.
143
3.3.8 Optimization of the catalysts
Taking into account the structure-property relationships derived from the
experimental data, the catalysts were optimized with regard to higher activity and an
increased formation rate of uneven numbered olefins. To achieve higher activities, the
iron(III) chloride complex 19 was prepared containing bromo substituents at the ligand
framework. Under optimized conditions (Al:Fe = 750:1; 1 bar ethylene; 0°C), an activity
of 2970 [g products/g Fe · h] was obtained. As expected, the catalyst 19/MAO did not
produce uneven numbered olefins.
NNNBr Br
Fe
Cl ClCl
13
NNNCl Cl
Fe
Br Br
19
Scheme 29. Optimized catalyst precursors.
The highest amount of olefins with uneven numbers of carbon atoms was found
applying 13/MAO (Al:Fe = 500:1; 1 bar ethylene; 25°C). For this catalyst, the overall
content in the reaction mixture increased to 19.2 %.
4. Conclusion A series of bis(arylimino)pyridine transition metal complexes with halogen containing
ligand frameworks was prepared, characterized, and applied for catalytic ethylene
oligomerization and polymerization reactions. Some of these catalysts surprisingly
produced α-olefins with odd carbon numbers. As a possible explanation of this
unprecedented behavior, a combined isomerization/metathesis reaction pathway is
proposed. Optimized bis(arylimino)pyridine iron catalyst precursors are presented.
Acknowledgements We thank Saudi Basic Industries Corporation (SABIC, Riyadh, Saudi Arabia) for
the financial support.
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