Iron-based ethylene polymerization catalysts supported by bis(imino)pyridine ligands: Derivatization via deprotonation/alkylation at the ketimine methyl position

Article abstract of DOI:10.1016/j.molcata.2006.10.028

Bis(imino)pyridine ligands, L [where L = 2,6-(ArNCR¹)₂C₅H₃N R¹ = Et, ⁱPr, CH₂CH₂Ph or CH(CH₂Ph)₂ and Ar = 2,4,6-(Me)₃C₆H₂ (MES) or 2,6-(ⁱPr)₂C₆H₃ (DIPP)] have been prepared by deprotonation of the parent ketimine ligand (R¹ = Me) using lithium diisopropylamide (LDA), followed by alkylation with the appropriate alkylhalide. The corresponding iron dichloride complexes LFeCl₂ are highly active ethylene polymerisation catalysts upon treatment with methylaluminoxane (MAO), with activities in the range of 3000-18,000 g/mmol bar h. The molecular weights (M_n) of the resultant polyethylenes lie in the range of 6500-24,000 with broad molecular weight distributions (16.5-38.0). The nature of the imine carbon substituent has a marked effect on the polymer molecular weight whereas the catalyst activity is largely unaffected by changes to this substituent.

Full text of DOI:10.1016/j.molcata.2006.10.028

Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
Iron-based ethylene polymerization catalysts supported by
bis(imino)pyridine ligands: Derivatization via
deprotonation/alkylation at the ketimine methyl position
Stuart McTavish, George J.P. Britovsek, Theo M. Smit, Vernon C. Gibson∗,
Andrew J.P. White, David J. Williams
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, UK
Received 3 July 2006; received in revised form 12 October 2006; accepted 16 October 2006
Available online 21 October 2006
Abstract
i
Bis(imino)pyridine ligands, L [where L = 2,6-(ArNCR1)2C5H3N R1 = Et, Pr, CH2CH2Ph or CH(CH2Ph)2 and Ar = 2,4,6-(Me)3C6H2 (MES)
or 2,6-(iPr)2C6H3 (DIPP)] have been prepared by deprotonation of the parent ketimine ligand (R1 = Me) using lithium diisopropylamide (LDA),
followed by alkylation with the appropriate alkylhalide. The corresponding iron dichloride complexes LFeCl2 are highly active ethylene poly-
merisation catalysts upon treatment with methylaluminoxane (MAO), with activities in the range of 3000–18,000 g/mmol bar h. The molecular
weights (Mn) of the resultant polyethylenes lie in the range of 6500–24,000 with broad molecular weight distributions (16.5–38.0). The nature of
the imine carbon substituent has a marked effect on the polymer molecular weight whereas the catalyst activity is largely unaffected by changes
to this substituent.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Iron; Ethylene; Imine; Polymerization; Pyridine
1. Introduction
utilized to prepare alkenyl-functionalized bis(imino)pyridine
iron complexes [16].
Since the first reports of active ethylene polymerization cat-
alysts based on iron and cobalt supported by bis(imino)pyridine
ligands [1–3] there have been numerous studies directed at
modifying the bis(imino)pyridine frame [1,4–21] especially the
groups attached to the imine nitrogen donors [4–13]. Relatively
little attention has been directed towards changes to the sub-
stituents at the imine carbon atoms. Bennett and co-workers
reported an imidazolyl substituted derivative that showed similar
activity to its ketimine analogue [14]. Recently, we described the
synthesis, characterization and polymerization behavior of cat-
alysts containing ether and thioether functionalities attached to
the imine carbon atom [15]. Here, we describe a straightforward
deprotonation/alkylation procedure that allows systematic vari-
ations at the imine carbon position. A related approach has been
2. Experimental section
2.1. General
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenk and cannula techniques or in
a conventional nitrogen-filled glovebox. Solvents were refluxed
over an appropriate drying agent, and distilled and degassed
prior to use. Elemental analyses were performed by the micro-
analytical services of University College London, Department of
Chemistry and Mikroanalytisches Labor Pascher. NMR spectra
were recorded on a Bruker spectrometer at 250 MHz (1H) and
62.9 MHz (13C) at 293 K; chemical shifts are referenced to the
residual protio impurity of the deuterated solvent. Mass spec-
tra were obtained using either fast atom bombardment (FAB)
or chemical ionization (CI). IR spectra were recorded on a
Perkin-Elmer Spectrum GXI instrument. Magnetic moments
∗
Corresponding author.
E-mail address: v.gibson@imperial.ac.uk (V.C. Gibson).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.10.028

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S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
(s), 1567 (s) cm−1. Anal. (C31H39N3·0.66H2O) calcd.: C, 79.98;
were obtained by the Evans’ NMR method (solvent, CH2Cl2;
reference cyclohexane) [22]. Polymer GPC analyses were car-
ried out at BP Chemicals, Ltd. on a Waters 150CV (columns sup-
pliedbyShodex(807, 806and804))usingpolyethylenestandard
reference material NBS1484a. Lithium diisopropylamide [23]
and 2,6-diacetylpyridinebis(2,4,6-trimethylanil)iron dichloride
(7b) [1] were prepared according to literature procedures.
n-Butyllithium was purchased from Fisher Scientific UK.
Research grade ethylene (BOC, Grade 3.50) was used for all
ethylene polymerisation experiments.
H, 8.73; N, 9.03; found, C, 79.97; H, 8.68; N, 8.77.
2.2.3. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH2CH2C6H5]2C5H3N} (3a)
Lithium diisopropylamide [2.2 equiv., 5.54 mmol; freshly
prepared from diisopropylamine (0.78 ml, 5.53 mmol) and
n-butyllithium (3.46 ml, 5.53 mmol, 1.6 M in hexanes) in
THF (8 ml)] was added dropwise to a solution of 2,6-
diacetylpyridinebis(2,4,6-trimethylanil) (1.00 g, 2.52 mmol) in
THF (50 ml) at −78 ◦C. The resulting dark solution was allowed
to warm to 0 ◦C with stirring (2 h). Benzylbromide (0.72 ml,
6.04 mmol, 2.4 equiv.) was then added and the ensuing solu-
tion was allowed to warm to room temperature (12 h) to give a
yellow solution. The solvent was removed under reduced pres-
sure and the residue was dissolved in 40 ml of diethylether.
The solution was then washed with water (3× 30 ml) and the
organic phase dried over MgSO4. The volatile components were
removed under reduced pressure and the residue was recrys-
tallised from ethanol to yield 3a as a yellow solid (0.88 g,
61 %). 1H NMR (CDCl3): δ 8.45 (d, 2H, 3J(HH) 7.9, Py-
Hm), 7.97 (t, 1H, Py-Hp), 6.90 (s, 4H, Ar-H), 7.26–6.83 (m,
10H, N CCH2CH2Ph), 3.06 (m, 4H, N CCH2CH2Ph), 2.78
(m, 4H, N CCH2CH2Ph), 2.31 (s, 6H, Ar-CMe), 2.03 (s, 12H,
2.2. Synthesis of preligands 1a–6a and complexes 1b–6b
2.2.1. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH2CH3]2C5H3N} (1a)
Lithium diisopropylamide [2.2 equiv., 2.77 mmol; freshly
prepared from diisopropylamine (0.39 ml, 2.77 mmol) and
n-butyllithium (1.73 ml, 2.77 mmol, 1.6 M in hexanes) in
THF (5 ml)] was added dropwise to a solution of 2,6-
diacetylpyridinebis(2,4,6-trimethylanil) (0.50 g, 1.26 mmol) in
THF (30 ml) at −78 ◦C. The resulting dark solution was allowed
to warm to 0 ◦C with stirring (2 h). Iodomethane (0.17 ml,
2.77 mmol, 2.2 equiv.) was then added and the ensuing solu-
tion was allowed to warm to room temperature (12 h) to form a
yellow solution. The solvent was removed under reduced pres-
sure and the residue was dissolved in 40 ml of diethylether. The
solution was washed with water (3 × 30 ml) and the organic
phase was dried over MgSO4. The volatile components were
removed under reduced pressure and the residue was recrys-
tallised from ethanol to yield 1a as a yellow solid (0.52 g,
98%). 1H NMR (CDCl3): δ 8.42 (d, 2H, 3J(HH) 7.9, Py-
Hm), 7.91 (t, 1H, Py-Hp), 6.91 (s, 4H, Ar-H), 2.72 (q, 4H,
3J(HH) 7.5, N CCH2CH3), 2.31 (s, 6H, CMe), 2.05 (s, 12H,
1
Ar-CMe). 13C NMR (CDCl3, { H}): δ 169.83 (N C), 154.93
(Py-Co), 145.60 (Ar-Cip), 141.02 (Ph-Cip), 137.27 (Ar-Co),
132.33 (Ar-Cp), 128.85 (Ph-Co), 128.36 (Ph-Cp), 128.12 (Ph-
Cm), 126.00 (Py-Cp), 125.12 (Py-Cm), 122.76 (Ar-Cm), 32.80
(N C-CH2CH2Ph), 20.76 (Ar-Mep), 18.09 (Ar-Meo), 31.75
(N C-CH2CH2Ph). MS (CI), m/z 578 [(M + H)+]. IR ν(C N)
1638 (s), 1567 (s) cm−1. Anal. (C39H43N3) calcd.: C, 84.57;
H, 7.53; N, 7.22; found, C, 84.56; H, 7.47; N, 7.29.
1
CMe), 1.04 (t, 6H, N CCH2CH3). 13C NMR (CDCl3, { H}):
2.2.4. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH(CH2C6H5)2]2C5H3N} (4a)
δ 171.71 (N C), 154.56 (Py-Co), 145.92 (Ar-Cip), 136.98 (Ar-
Co), 132.08 (Ar-Cp), 128.57 (Py-Cp), 125.17 (Py-Cm), 122.85
(Ar-Cm), 23.14 (N C–CH2CH3), 20.73 (Ar-Mep), 18.08 (Ar-
Meo), 11.30 (N C–CH2CH3). MS (CI), m/z 426 [(M + H)+].
IR ν(C N) 1634 (s), 1567 (s) cm−1. Anal. (C29H35N3·0.25H2O)
calcd.: C, 80.98; H, 8.32; N, 9.77; found, C, 80.92; H, 7.99;
N, 9.32.
Procedure as described for 3a, but employing 3a to give 4a
1
as a viscous yellow oil in 58 % yield. H NMR (CDCl3): δ
8.50 (broad, 2H, Py-Hm), 8.0 (broad, 1H, Py-Hp), 6.88 (broad,
m, 4H, Ar-H), 7.15–6.87 (broad, m, 20H, N CCH(CH2Ph)2),
3.45 (broad, 2H, N CCH(CH2Ph)2), 2.91 (broad, m, 8H,
N CCH(CH2Ph)2), 2.34 (broad, m, 6H, CMe), 2.05 (broad, m,
12H, CMe). MS (CI), m/z 758 [(M + H)+]. Pre-ligand 4a is a
highly viscous compound and satisfactory elemental analysis
could not be obtained.
2.2.2. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH(Me)2]2C5H3N} (2a)
Procedure as described for 1a, but employing 1a to give
1
2a as a yellow powder in 72% yield. H NMR (CDCl3): δ
8.02 (broad, d, 3J(HH) 7.8 2H, Py-Hm), 6.86, and 6.66 (broad,
m, 4H, Ar-H), 6.39 (broad, m, 1H, Py-Hp), 3.76, 3.50 and
2.91 (broad, 2H, CHMe2), 2.22 (broad, m, 6H, Ar-CMep),
2.05 (broad, m, 12H, Ar-CMeo), 1.45 and 1.36 (broad, m,
2.2.5. Preparation of {2,6-[(2,6-(CH(Me)2)2C6H3)
NCCH2CH3]2C5H3N} (5a)
Procedure as described for 1a using 2,6-diacetylpyri-
dinebis(2,6-diisopropylanil) to give 5a as a yellow powder in
96% yield. 1H NMR (CDCl3): δ 8.40 (d, 2H, 3J(HH) 7.8,
Py-Hm), 7.93 (t, 1H, Py-Hp), 7.1 (m, 6H, Ar-H), 2.84–2.69
(m, 8H, CHMe2 and N CCH2CH3), 1.18 (d, 24H, CHMe2),
1
12H, CHMe2). 13C NMR (CDCl3, { H}): δ 172.62, 172.13
and 171.62 (N C), 153.05 (Py-Co), 145.57, 144.92 and 144.50
(Ar-Cip), 137.62, 136.21 and 136.16 (Ar-Co), 132.56 (Ar-Cp),
128.61 (Py-Cp), 125.98 (Py-Cm), 123.15, 123.08 and 122.93
(Ar-Cm), 35.60, 34.79 and 34.30(N C–CH(Me)2), 23.78, 23.58
and 23.56 (N C–CH(Me)2), 20.61 (Ar–Mep), 18.48, 18.27 and
18.20 (Ar–Meo). MS (CI), m/z 454 [(M + H)+]. IR ν(C N) 1651
3
1.08 (t, 6H, J(HH) 8.8, N CCH2CH3). 13C NMR (CDCl3,
1
{ H}): δ 170.61 (N C), 154.48 (Py-Co), 146.05 (Ar-Cip),
137.44 (Ar-Co), 137.09 (Ar-Cp), 135.61 (Py-Cp), 123.45 (Py-
Cm), 122.83 (Ar-Cm), 28.28 (CHMe2), 23.61 (N C–CH2CH3),

S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
295
22.23 (CHMe2), 11.01 (N C–CH2CH3). MS (CI), m/z 510
[(M + H)+]. IRν(C N) 1637(s), 1570(s)cm−1. Anal. (C35H47N3)
calcd.: C, 82.46; H, 9.29; N, 8.24; found, C, 82.11; H, 9.22; N,
8.17.
6.5 (4H, N CCH2CH2Ph), 5.4 (4H, N CCH2CH2Ph), 5.3
(2H, N CCH2CH2Ph), 4.4 (4H, N CCH2CH2Ph), −2.4 (4H,
N CCH2CH2Ph). MS (FAB), m/z 703 [M+], 668 [M+ − Cl], 633
[M+ − 2Cl]. Anal. (C39H43N3FeCl2) calcd.: C, 68.83; H, 6.37;
N, 6.17; found, C, 68.73; H, 6.07; N, 5.72. μeff (Evans’ NMR
Method) 5.34 BM.
2.2.6. Preparation of {2,6-[(2,6-(CH(Me)2)2C6H3)
NCCH(Me)2]2C5H3N} (6a)
Procedure as described for 1a using 5a to give 6a as a yellow
powder in 70% yield. 1H NMR (CDCl3): δ 8.00 (broad, m, 2H,
Py-Hm), 7.99 (broad, m, 1H, Py-Hp), 7.15 (broad, m, 6H, Ar-
H), 2.78 (broad, m, 6H, CHMe2), 1.40–0.91 (m, 36H, CH(Me)2).
2.2.10. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH(CH2C6H5)2]2C5H3N}FeCl2 (4b)
Procedure as described for 1b using 4a and FeCl2 to give 4b
1
as a blue powder in 73% yield. H NMR (CD2Cl2, broad sin-
1
13C NMR (CDCl3, { H}): δ 172.67 and 171.61 (N C), 156.20
glets are observed in each case): 87.3 (2H, Py-Hm), 72.6 (1H,
Py-Hp), 25.7 (6H, Ar-Mep), 24.7 (8H, N CCH(CH2Ph)2), 18.0
(4H, Ar-Hm), 14.8 (12H, Ar-Meo), 7.8 (2H, N CCH(CH2Ph)2),
6.1 (8H, CH2Ph), 5.3 (4H, CH2Ph), 5.0 (8H, CH2Ph). MS
(FAB), m/z 883 [M+], 848 [M+ − Cl], 813 [M+ − 2Cl]. Anal.
(C55H55N3FeCl2·1.5H2O) calcd.: C, 72.45; H, 6.41; N, 4.61;
found, C, 72.33; H, 5.85; N, 4.49. μeff (Evans’ NMR Method)
5.00 BM.
(Py-Co), 153.28 and 152.32 (Ar-Cip), 145.78 (Ar-Co), 135.60
(Ar-Cp), 135.29 (Py-Cp), 123.46 (Py-Cm), 123.15 and 122.78
(Ar-Cm), 35.11 and 32.24 (N C–CH(Me)2), 28.26 and 28.03
(Ar-CHMe2), 23.43 and 22.13 (Ar-CHMe2), 20.59 and 19.64
(N C–CH(Me)2). MS (CI), m/z 538 [(M + H)+]. IR ν(C N) 1645
(s), 1567 (s) cm−1. Anal. (C37H51N3) calcd.: C, 82.63; H, 9.56;
N, 7.81; found, C, 81.98; H, 9.69; N, 7.80.
2.2.7. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
2.2.11. Preparation of {2,6-[(2,6-(CH(Me)2)2C6H3)
NCCH2CH3]2C5H3N}FeCl2 (5b)
NCCH2CH3]2C5H3N}FeCl2 (1b)
A suspension of 1a (0.275 g, 0.647 mmol) in n-butanol was
added dropwise at 80 ◦C to a solution of FeCl2 (1 equiv., 0.082 g,
0.647 mmol) in n-butanol (20 ml) to yield a blue solution. After
stirring at 80 ◦C for 12 h, the reaction was allowed to cool
to room temperature. The reaction volume was concentrated
and diethylether (30 ml) was added to precipitate the product
as a blue powder. The powder was subsequently washed with
diethylether (3× 10 ml) and dried under reduced pressure to
Procedure as described for 1b using 5a and FeCl2 to give
1
5b as a blue powder in 74% yield. H NMR (CD2Cl2, broad
singlets are observed in each case): δ 85.6 (1H, Py-Hp), 82.6
(2H, Py-Hm), 14.6 (4H, Ar-Hm), −0.7 (4H, N CCH2CH3),
−5.3 (12H, Ar-i-Pr-Me), −6.9 (12H, Ar-i-Pr-Me), −8.4 (6H,
N CCH2(Me)2), −10.9 (2H, Ar-Hp), −26.8 (4H, Ar-i-Pr-CH).
MS (FAB), m/z 635 [M+], 600 [M+ − Cl], 565 [M+ − 2Cl]. Anal.
(C35H47N3FeCl2) calcd.: C, 66.04; H, 7.44; N, 6.60; found, C,
65.73; H, 7.22; N, 6.45. μeff (Evans’ NMR Method) 5.20 BM.
1
yield 0.31 g (87%) of 1b. H NMR (CD2Cl2, broad singlets
are observed in each case): δ 84.8 (2H, Py-Hm), 44.5 (1H,
Py-Hp), 23.1 (6H, Ar-Mep), 15.5 (4H, Ar-Hm), 12.5 (12H, Ar-
Meo), 6.6 (4H, N CCH2CH3). −5.5 (6H, N CCH2CH3). MS
(FAB) m/z 551 [M+], 516 [M+ − Cl], 481 [M+ − 2Cl]. Anal.
C29H35N3FeCl2·0.5H2O calcd.: C, 61.07; H, 6.54; N, 7.36;
found, C, 61.37; H, 6.43; N, 6.69. μeff (Evans’ NMR Method)
5.09 BM.
2.2.12. Preparation of {2,6-[(2,6-(CH(Me)2)2C6H3)
NCCH(Me)2]2C5H3N}FeCl2 (6b)
Procedure as described for 1b using 6a and FeCl2 to give
1
6b as a blue powder in 71% yield. H NMR (CD2Cl2, broad
singlets are observed in each case): δ 88.3 (2H, Py-Hm), 82.4
(1H, Py-Hp), 17.6 (2H, N CCH(Me)2), 14.1 (4H, Ar-Hm),
−5.4 (12H, Ar-i-Pr-Me), −6.3 (12H, Ar-i-Pr-Me), −3.2 (12H,
N CCH(Me)2), −10.7 (2H, Ar-Hp), −29.7 (4H, Ar-i-Pr-CH).
MS(FAB), m/z663[M+], 628[M+ − Cl], 593[M+ − 2Cl]. Anal.
(C37H51N3FeCl2) calcd.: C, 66.87; H, 7.74; N, 6.32; found, C,
66.49; H, 7.43; N, 6.01. μeff (Evans’ NMR Method) 5.12 BM.
2.2.8. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
NCCH(Me)2]2C5H3N}FeCl2 (2b)
Procedure as described for 1b using 2a and FeCl2 to give
1
2b as a blue powder in 94% yield. H NMR (CD2Cl2, broad
singlets are observed in each case): δ 86.5 (2H, Py-Hm),
54.8 (1H, Py-Hp), 24.6 (6H, Ar-Mep), 20.3 (4H, Ar-Hm),
15.6 (2H, N CCH(Me)2), 10.8 (12H, Ar-Meo), −3.7 (12H,
N CCH(Me)2). MS (FAB), m/z 579 [M+], 544 [M+-Cl], 509
[M+ − 2Cl]. Anal. C31H39N3FeCl2·0.5H2O calcd.: C, 63.17; H,
6.84; N, 7.13; found, C, 63.01; H, 7.37; N, 6.25. μeff (Evans’
NMR Method) 5.03 BM.
2.3. X-ray crystallography
2.3.1. Crystal data for 1b
C29H35Cl2FeN3·CH2Cl2, M = 637.28, orthorhombic, Pna21
˚
(no. 33), a = 17.7689(11), b = 15.2630(6), c = 23.8344(11) A,
3
˚
V = 6464.0(6) A , Z = 8 (two independent molecules),
2.2.9. Preparation of {2,6-[(2,4,6-(Me)3C6H2)
Dc = 1.310 g cm−3
,
μ(Cu K␣) = 6.955 mm−1
,
T = 293 K,
NCCH2CH2C6H5]2C5H3N}FeCl2 (3b)
green platy needles; 5408 independent measured reflections,
F2 refinement, R1 = 0.066, wR2 = 0.133, 2986 independent
observed absorption-corrected reflections [|Fo| > 4σ(|Fo|),
2θmax = 128◦], 765 parameters. The structure of 1b was
shown to be a polar twin by a combination of R-factor tests
Procedure as described for 1b using 3a and FeCl2 to give 3b
as a blue powder in 79% yield. 1H NMR (CD2Cl2, broad singlets
are observed in each case): δ 83.1 (2H, Py-Hm), 56.3 (1H, Py-
Hp), 23.7 (6H, Ar-Mep), 15.4 (4H, Ar-Hm), 12.7 (12H, Ar-Meo),

296
S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
[R1+ = 0.0681, R1− = 0.0683] and by use of the Flack parameter
[x+ = +0.46(4), x− = +0.54(4)] CCDC 606439.
was also discounted on the grounds that an increased size of the
carbonyl substituents in the ligand precursor would likely prove
problematic in the subsequent amine condensation reactions.
The effect of increasing the size of the imino alkyl substituent on
the solution properties of 1a–6a is apparent from their 1H NMR
spectra. Thus, while the spectrum of 1a is sharp at room temper-
ature, the isopropyl derivative 2a affords a broadened spectrum,
consistent with restricted rotation. Similar broadening is also
seen for 4a, 5a and 6a.
2,6-Bis(imino)pyridyl iron(II) chloride pre-catalysts 1b–6b
can be synthesised in good yield by treatment of iron(II) chlo-
ride with the corresponding 2,6-bis(imino)pyridine ligand in
n-butanol at elevated temperature. 1b–6b are paramagnetic, with
room temperature magnetic moments (μeff) ranging from 5.0
to 5.5 BM (Evans’ NMR method [22]), corresponding to four
unpaired electrons and consistent with high spin iron(II) cen-
tres. Nevertheless, assignable contact-shifted 1H NMR spectra
can be obtained (see Experimental).
2.3.2. Crystal data for 2b
¯
˚
C31H39Cl2FeN3·CH2Cl2, M = 665.33, triclinic, P1 (no. 2),
a = 15.1126(9), b = 15.7923(9), c = 17.3838(14) A, α =
◦
3
˚
69.152(6), β = 69.167(7), γ = 69.367(7) , V = 3500.5(4) A ,
Z = 4 (two independent molecules), Dc = 1.262 g cm−3
,
μ(Cu-K␣) = 6.443 mm−1
, T = 293 K, dark blue prismatic
needles; 10,196 independent measured reflections, F2 refine-
ment, R1 = 0.080, wR2 = 0.198, 6218 independent observed
absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax = 120◦],
741 parameters. CCDC 606440.
2.3.3. Crystal data for 6b
C37H51Cl2FeN3, M = 664.56, monoclinic, Pn (no. 7),
˚
a = 9.7637(6),
b = 12.8384(4),
c = 14.8855(13) A,
β =
3
93.479(7)◦, V = 1862.5(2) A , Z = 2, Dc = 1.185 g cm−3
,
˚
μ(Cu-K␣) = 4.763 mm−1, T = 293 K, blue platy needles; 2932
independent measured reflections, F2 refinement, R1 = 0.046,
wR2 = 0.106, 2507 independent observed absorption-corrected
reflections [|Fo| > 4σ(|Fo|), 2θmax = 128◦], 389 parameters. The
absolute structure of 6b was determined by a combination of
R-factor tests [R1+ = 0.0457, R1− = 0.0743] and by use of the
Flack parameter [x+ = +0.017(13), x− = +0.983(13)]. CCDC
606441.
3.2. Molecular structures of 1b, 2b and 6b
The 2,6-bis(imino)pyridyl iron(II) chloride pre-catalysts
have been shown to assume both square-based pyramidal and
trigonal bipyramidal geometries depending upon the ligand sub-
stitution pattern of the aryl ring.[1] In order to further probe the
geometries adopted by this class of complex, structural determi-
nations were carried out on complexes 1b, 2b and 6b. Crystals
suitable for X-ray structural determinations were grown from
layered CH2Cl2-pentane solutions (1:1); the molecular struc-
tures are shown in Figs. 1-3, with selected bond distances and
angles collected in Table 1; see Fig. 4 for a numbered schematic
for this Table.
2.4. Ethylene polymerisation procedure
A 1 l stainless steel reactor was heated under a nitrogen
flow for 1 h at 85 ◦C and subsequently cooled to the temper-
ature required for the polymerization run. Isobutane (0.5 l), an
alkylaluminium scavenger (triisobutylaluminium, trimethylalu-
minium or MAO) and the desired pressure of ethylene were
introduced into the reactor and the mixture was stirred at the
polymerisation temperature for 30 min. A toluene solution of
the catalyst was then injected under nitrogen. The reactor pres-
sure was maintained constant throughout the polymerisation run
(60 min.) by computer controlled addition of ethylene. Runs
were terminated by venting off the volatile components. The
reactor contents were isolated, washed with aqueous hydrogen
chloride, methanol and then dried under vacuum at 50 ◦C.
All three structures are closely related; in each complex the
M–N(pyridine) bond, ranging from 2.084(4) to 2.126(9) A, is
˚
significantly shorter than the M–N(imino) bonds, which range
˚
from 2.207(5) to 2.272(11) A. The formal double bond charac-
ter of the imino linkages N7 C7 and N11 C11 are retained,
with C N distances in the range 1.267(18)–1.300(16) A. The
bond distances and angles compare well with those reported for
similar 2,6-bis(imino)pyridyl iron(II) chloride complexes in the
literature.[1] Regardless of the nature of the backbone or ortho-
aryl substituents, the planes of the phenyl rings are orientated
˚
3. Results and discussion
3.1. Synthesis and characterization
Ligands 1a-6a were prepared by a straightforward protocol
in which the methyl substituent of the ␣-imino carbon moi-
ety in I or II is deprotonated using freshly prepared LDA at
−78 ◦C, followed by alkylation using a primary alkylhalide at
0 ◦C (Scheme 1). This procedure can be repeated as necessary
to build up the desired substitution pattern at the ␣-imino carbon
position. Sequential deprotonation and alkylation of the imine
functionality was found to be superior to analogous treatment
of the ligand precursor, 2,6-diacetylpyridine. The latter route
Fig. 1. The molecular structure of one (I) of the two independent complexes
present in the crystals of 1b.

S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
297
Scheme 1.
Fig. 2. The molecular structure of one (I) of the two independent complexes
present in the crystals of 2b.
Fig. 3. Molecular structure of 6b.

298
S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
Table 1
◦
˚
Selected bond lengths (A), angles ( ) and derived parameters for 1b, 2b and 6b (see Fig. 4 for a numbered schematic)
1b (R = Et, Ar = MES)
Molecule I
2b (R = i-Pr, Ar = MES)
Molecule I
6b [R = i-Pr, Ar = DIPP]
Molecule II
Molecule II
Fe–Cl1
Fe–Cl2
Fe–N1
Fe–N7
Fe–N11
C7–N7
C11–N11
Cl1–Fe–Cl2
Cl1–Fe–N1
Cl1–Fe–N7
Cl1–Fe–N11
Cl2–Fe–N1
Cl2–Fe–N7
Cl2–Fe–N11
N1–Fe–N7
N1–Fe–N11
N7–Fe–N11
2.305(4)
2.275(5)
2.126(9)
2.232(11)
2.272(11)
1.300(16)
1.282(18)
112.03(17)
118.9(3)
96.7(3)
100.6(3)
129.0(3)
101.8(3)
98.0(3)
74.0(4)
72.5(4)
146.5(4)
0.04
2.300(5)
2.306(5)
2.105(11)
2.260(10)
2.240(11)
1.275(15)
1.267(18)
110.95(17)
120.1(3)
98.4(3)
100.2(3)
128.9(3)
101.3(3)
98.7(3)
72.7(4)
72.8(4)
145.5(4)
0.05
2.291(3)
2.274(3)
2.108(6)
2.264(6)
2.266(6)
1.294(9)
1.276(10)
112.24(13)
117.9(2)
99.6(2)
97.8(2)
129.9(2)
99.0(2)
101.3(2)
73.3(2)
72.6(2)
145.9(2)
0.05
2.296(2)
2.273(2)
2.091(5)
2.207(5)
2.213(5)
1.292(8)
1.294(9)
112.43(11)
108.2(2)
99.8(2)
102.9(2)
139.4(2)
99.17(15)
97.44(15)
73.7(2)
72.9(2)
144.0(2)
0.35
2.3150(17)
2.2446(16)
2.084(4)
2.224(4)
2.233(5)
1.292(7)
1.283(8)
113.87(9)
90.68(14)
99.52(13)
99.92(13)
155.42(15)
101.94(12)
100.51(14)
73.62(16)
72.72(17)
141.04(18)
0.52
˚
A (A) [a]
B (◦) [b]
C (◦) [c]
D (◦) [d]
5.1
99
95
3.6
100
91
2.4
90
93
7.3
95
90
4.6
85
94
[a] A is the deviation of the iron centre from the N3 plane. [b] B is the inclination of the N3 and pyridyl ring planes. [c and d] C and D are the torsion angles about
the N7–Ar and N11–Ar bonds respectively (ignoring the iron centre, using only the major occupancy orientations, and with an angle greater than 90◦ signifying a
twist away from the Cl1 centre).
essentially orthogonal to the plane formed by the three nitrogen
atoms (N3 plane). The principal difference between complexes
1b and 6b is the deviation of the iron atom from the N3 plane and
the associated distortions from trigonal bipyramidal to square
based pyramidal geometries.
The solid state structure of 1b revealed the presence of two
crystallographically independent molecules (I and II) in the
asymmetric unit (molecule I is shown in Fig. 1, and molecule
II in Fig. S2 in the supporting information). Though both
molecules exhibit some disorder (see Figures S7 and S8) the
major occupancy orientations are similar, having an r.m.s. fit of
˚
ca. 0.12 A. The metal coordination geometry in both I and II is
distorted trigonal bipyramidal with N(7) and N(10) in the axial
˚
˚
positions [the iron atoms lie ca. 0.01 A (molecule I) and 0.01 A
(molecule II) out of their respective {Cl(1),Cl(2),N(1)} planes].
Interestingly, in molecule I, and the major occupancy orienta-
tion of molecule II, the C(7) and C(10)-bound ethyl substituents
adopt an anti configuration, giving both molecules approximate
C2 symmetry about the Fe–N(1) vector. (The C(10ꢀ) ethyl group
of molecule II exhibits 60:40 disorder, with the minor orien-
tation having the group flipped into a syn configuration cf. the
C(7ꢀ) ethyl unit.)
As was seen for the structure of 1b, X-ray analysis of a crystal
of 2b also revealed two independent molecules (I and II) in the
asymmetric unit (molecule I is shown in Fig. 2, and molecule
II in Figure S10 in the supporting information). Unlike the case
seen for 1b, however, here the two independent molecules are
markedly different; whilst the iron centre in molecule I has a
distorted trigonal bipyramidal coordination geometry, that in
molecule II adopts a geometry intermediate between trigonal
bipyramidal and square-based pyramidal (Figures S12 and S13,
respectively). For molecule I, N(7) and N(11) occupy the axial
positions, and the {Fe,Cl(1),Cl(2),N(1)} equatorial plane is
˚
coplanar to better than 0.01 A. For molecule II, when viewed as
square-based pyramidal, Cl(1ꢀ) occupies the apical site, and the
ꢀ
ꢀ
ꢀ
ꢀ
˚
iron centre lies ca. 0.61 A out of the {Cl(2 ),N(1 ),N(7 ),N(11 )}
˚
basal plane which is coplanar to within only ca. 0.18 A. Both
molecules have approximate CS symmetry about a plane that
bisects the central pyridine ring and contains the iron and chlo-
rine atoms.
Fig. 4. Schematic representation of the metal coordination environment and
ligand backbone in the structures of complexes 1b, 2b and 6b.

S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
299
In the solid state structure of 6b (Fig. 3), the iron adopts
trimethylphenyl derivatives resulting in increased propagation
rates. A comparison of the polyethylene products obtained using
2b and 6b reveal an increase in the molecular weight for the 2,6-
diisopropylphenyl derivative, an effect that is less marked for the
ethyl pair 1b and 5b.
a distorted square-based pyramidal coordination environment
˚
with Cl(1) in the apical position, and the iron lying ca. 0.48 A out
of the {Cl(2),N(1),N(7),N(11)} basal plane (which is coplanar
˚
to within ca. 0.03 A). As was seen for both molecules in the
structure of 2b, here the complex has molecular CS symmetry
about a plane that bisects the central pyridine ring and contains
the iron and chlorine atoms.
Replacing one of the ketimine methyl protons of 7b with
alkyl substituents (1b and 3b) has only a minor effect on cat-
alyst performance. Virtually identical activities are obtained to
benchmark catalyst 7b with only minor differences in the molec-
ular weights of the resultant polyethylenes. Introducing a second
methyl to give isopropyl backbone substituents leads to a more
substantial increase in Mpk (82,000 for 2b cf. 47,000 for 7b),
whileforthedibenzylderivativeamuchhighermolecularweight
is seen (Mpk 422,000). Increasing the size of the imine carbon
substituent is expected to result in a greater rotational barrier
for the N-aryl rings (as indicated by NMR studies on ligands
1a–6a). Thus, the steric interaction between the large dibenzyl-
methine and the aryl substituents in 4b may effectively ‘lock’
the 2,4,6-trimethylphenyl ring perpendicular to the N3 coordi-
nation plane. This places the ortho aryl substituents above and
below the metal centre where they are anticipated to be most
effective at reducing the rate of chain transfer processes. The
imine carbon substituents in 1b–3b appear to be insufficiently
bulky for such purposes. As might be expected, the larger 2,6-
diisopropylphenyl rings can be ‘locked’ with smaller backbone
groups, such that replacing ethyl (5b) with isopropyl (6b) sub-
stituents leads to a more marked increase in polymer molecular
weight (5b; Mpk 26 000, 6b; Mpk 263,000) whereas the same
modification to the ligand backbone had less impact on the poly-
mer molecular weight obtained using mesityl bearing catalysts
(1b and 2b).
Significantly, it has been noted in earlier studies [1] that
bulky aryl substituents, e.g. 2,6-diisopropylphenyl, attached to
the imine nitrogen donors are required to afford high molecular
weight products. However, this is accompanied by a substan-
tial penalty on catalyst productivity compared with, for exam-
ple, smaller aryl substituents such as 2,4,6-trimethylphenyl.
The findings reported here show that molecular weight can
be dramatically raised by increasing the steric hindrance
at the ketimine carbon site, in combination with N-2,4,6-
trimethylphenyl donors, without any significant sacrifice over
catalyst productivity.
The transition from trigonal bipyramidal to square-based
pyramidal can be viewed as largely a twisting of the FeCl2 unit
along the N(1)· · ·N(10/11) vector. In complexes 1b(I), 1b(II)
and 2b(I) the pyridyl and N3 planes are near coplanar (inclined
by ca. 5, 4 and 2◦ respectively), and bisect the Cl–Fe–Cl angle.
In molecule 2b(II) the pyridyl and N3 planes are now slightly
inclined (ca. 7◦) and the FeCl2 unit is slightly twisted along
the N(7ꢀ)· · ·N(11ꢀ) vector such that Cl(2ꢀ) is much closer to the
ꢀ
˚
N3 plane than Cl(1 ) (ca. 0.81 and 2.62 A respectively). For
complex 6b this twisting has proceeded further with Cl(2) and
˚
Cl(1) now ca. 0.12 and 2.77 A, respectively out of the N3 plane.
The reasons for these changes of geometry are far from clear.
Since the altered groups are distal to the iron and chlorine atoms
a steric argument is unconvincing; equally tenuous would be
an argument based on electronic differences between ethyl and
isopropyl, and between mesityl and diisopropylphenyl. What is
clear is that square-based pyramidal and trigonal bipyramidal
geometries lie on a very shallow energy surface for these Fe(II)
complexes.
3.3. Ethylene polymerization studies
All of the complexes are active for ethylene polymerisa-
tion upon treatment with methylaluminoxane (MAO), afford-
ing highly linear polyethylenes with broad molecular weight
distributions (Table 2), typical of iron ethylene polymeri-
sation catalysts supported by bis(imino)pyridine ligands.[1]
Changing the aryl ring attached to imine nitrogen from 2,4,6-
trimethylphenyl to 2,6-diisopropylphenyl has a marked effect
on catalyst behaviour. Catalysts bearing 2,4,6-trimethylphenyl
substituents (1b–4b and 7b) are considerably more produc-
tive than their 2,6-diisopropylphenyl bearing relatives (5b and
6b). This is attributed to less congested active sites for 2,4,6-
4. Conclusions
Table 2
Results of ethylene polymerisation runs using pre-catalysts 1b–7ba
A straightforward synthetic strategy, based on deprotona-
tion and alkylation, has been employed to derivatise 2,6-
bis(imino)pyridyl ligands at the ␣-imino carbon position. For
ligands bearing bulky backbone substituents, slow rotation of the
aryl rings occurs on the NMR time scale at room temperature
due to steric interactions between the ortho-aryl and ␣-imino
carbon substituents. All of the pre-catalysts 1b–6b are highly
active ethylene polymerisation catalysts upon treatment with
MAO. Increasing the size of the backbone position is shown
to increase the molecular weight of the resultant polyethylene
due to an increase in the effective steric protection of the metal
centre, but without lowering the catalyst productivity.
b
b
b
Run Pre-cat. Yield
(g)
Activity
(g/mmol bar h)
Mn
Mw/Mn
Mpk
1
2
3
4
5
6
7
1b
2b
3b
4b
5b
6b
7b
34.2
35.6
32.5
33.6
6.3
17,100
17,800
16,250
16,800
3,160
10,000
12,000
6,500
15,000
14,000
24,000
9,400
16.5
38.0
30.4
31.2
25.4
22.8
22.0
35,000
82,000
26,000
422,000
26,000
263,000
47,000
6.0
32.1
3,000
16,050
a
Isobutanesolvent, 4 barethylene, catalystloading0.5 ␮mol, 100 equiv. MAO
activator, i-Bu3Al scavenger, 50 ◦C, reaction time 1 h.
b
Determined by GPC at 135 ◦C.

300
S. McTavish et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 293–300
[8] G.J.P. Britovsek, S. Mastroianni, G.A. Solan, S.P.D. Baugh, C. Redshaw,
Acknowledgements
V.C. Gibson, A.J.P. White, D.J.J. Williams, M.R.J. Elsegood, Chem. Eur.
J. 6 (12) (2000) 2221–2231.
[9] M.E. Blum, C. Folli, M. Do¨ring, J. Mol. Catal. A: Chem. 212 (1–2) (2004)
13–18.
[10] C. Amort, M. Malaun, A. Krajete, H. Kopacka, K. Wurst, M. Christ, D.
Lilge, M.O. Kristen, B. Bildstein, Appl. Organomet. Chem. 16 (9) (2002)
505–516.
The authors are grateful to BP Chemicals, Ltd. (now Ineos
Technologies) for financial support and to Crompton Co. for the
donation of MAO. Drs. J. Boyle and G. Audley are thanked for
NMR and GPC measurements, respectively.
[11] Z. Ma, H. Wang, J. Qiu, D. Xu, Y. Hu, Macromol. Rapid Commun. 22 (15)
(2001) 1280–1283.
Appendix A. Supplementary data
[12] A.S. Ionkin, W.J. Marshall, D.J. Adelman, B.B. Fones, B.M. Fish, M.F.
Schiffhauer, Organometallics 25 (2006) 2978–2992.
[13] V.C. Gibson, N.J. Long, P.J. Oxford, A.J.P. White, D.J. Williams,
Organometallics 25 (2006) 1932–1939.
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.molcata.2006.10.028.
[14] A.M.A. Bennett, (E.I. Du Pont de Nemours and Co., USA) WO9827124,
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