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: helmut.alt@uni-bayreuth.de (H.G. Alt)

110

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).

111

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

112

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)

113

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)

114

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.

115

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.

116

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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