Dramatic effect of heteroatom backbone substituents on the ethylene polymerization behavior of bis(imino)pyridine iron catalysts

Article abstract of DOI:10.1021/ic0490775

Bis(imino)pyridine iron complexes bearing ether and thioether backbone substituents have been synthesized and evaluated for the polymerization of ethylene. The methoxy derivative is inactive whereas bulky phenoxides or thioether derivatives afford activit

Full text of DOI:10.1021/ic0490775

Inorg. Chem. 2004, 43, 6511−6512
Dramatic Effect of Heteroatom Backbone Substituents on the Ethylene
Polymerization Behavior of Bis(imino)pyridine Iron Catalysts
Theo M. Smit, Atanas K. Tomov, Vernon C. Gibson,* Andrew J. P. White, and David J. Williams
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, U.K.
Received July 13, 2004
Scheme 1
Bis(imino)pyridine iron complexes bearing ether and thioether
backbone substituents have been synthesized and evaluated for
the polymerization of ethylene. The methoxy derivative is inactive
whereas bulky phenoxides or thioether derivatives afford activities
as high as the most active systems reported to date.
The discovery of highly active olefin polymerization
catalyts based on bis(imino)pyridine complexes of iron and
cobalt1 provided a significant impetus to the search for
polymerization systems based on the late transition metals.2
While many modifications to the ligand aryl substituents and
central donor moiety have been described,3 relatively little
attention has been given to the effect of changes to the
substituents at the imine carbon atoms. Here, we describe
iron catalysts that incorporate ether and thioether groups and
The ether and thioether derivatized proligands 1a-4a were
prepared by treatment of pyridine-2,6-dicarboxyimidoyl
dichloride with NaER according to Scheme 1. Their subse-
quent treatment with anhydrous iron(II)chloride afforded the
heteroatom-derivatized precatalysts 1b-4b.
The starting point for our catalytic studies was the methoxy
derivative 1b whose molecular structure was found to be
closely related to its ketimine relative, 5b (ER ) Me),1c with
a distorted trigonal bipyramidal geometry at the iron center
(Figure 1).
Contrastingly, though, 1b was found to be inactive for
ethylene polymerization under a variety of conditions of
temperature, pressure, and activator. A possible explanation
for this is attack on the ligand backbone by the Lewis acid
activator of a type seen in vanadium catalyst systems.4
However, this seems to be ruled out by the finding that free
ligand 1a is recovered quantitatively after treatment of 1b
with MAO. An alternative explanation is reversible binding
of the activator at the heteroatom leading to destabilization
of the catalyst, possibly via ligand dissociation, or decom-
position of the active iron-alkyl propagating species. In
order to probe the possibility of the activator binding to the
heteroatom, we targeted the 2,6-dimethylphenyl ether deriva-
tive 3b where it was envisaged that the increased steric
the surprising finding that small ether units afford catalyti-
cally inactive systems whereas large ether and thioether
substituents give catalysts with very high productivities.
* To whom correspondence should be addressed. E-mail: v.gibson@
imperial.ac.uk.
(1) (a) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P.
J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J.
Chem. Commun. 1998, 849-850. (b) Small, B. L.; Brookhart, M.;
Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049-4050. (c)
Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.;
Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan,
G. A.; Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem.
Soc. 1999, 121, 8728-8740.
(2) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38 428. Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem.
ReV. 2000, 100, 1169. Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283.
(3) Britovsek, G. J. P.; Baugh, S. P. D.; Hoarau, O.; Gibson, V. C.; Wass,
D. F.; White, A. J. P.; Williams, D. J. Inorg. Chim. Acta 2003, 345,
279-291. Abu-Surrah, A. S.; Lappalainen, K.; Piironen, U.; Lehmus,
P.; Repo, T.; Leskela, M. J. Organomet. Chem. 2002, 648, 55-61.
Britovsek, G. J. P.; Gibson, V. C.; Hoarau, O. D.; Spitzmesser, S. K.;
White, A. J. P.; Williams, D. J. Inorg. Chem. 2003, 42, 3454-3465.
Al-Benna, S.; Sarsfield, M. J.; Thornton-Pett, M.; Ormsby, D. L.;
Maddox, P. J.; Bres, P.; Bochmann, M. J. Chem. Soc., Dalton Trans.
2000, 4247-4257. Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B.
S.; Mastroianni, S.; Redshaw, C.; Solan, G. A.; White, A. J. P.;
Williams, D. J. J. Chem. Soc., Dalton Trans. 2001, 1639-1644.
Britovsek, G. J. P.; Gibson, V. C.; Mastroianni, S.; Oakes, D. C. H.;
Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J.
Inorg. Chem. 2001, 431-437. Chen, Y. F.; Chen, R. F.; Qian, C. T.;
Dong, X. C.; Sun, J. Organometallics 2003, 22, 4312-4321. Chen,
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(4) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G.; Wang, Q. Y. J.
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10.1021/ic0490775 CCC: $27.50
Published on Web 08/26/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 21, 2004 6511

COMMUNICATION
Figure 1. Molecular structure of 1b. Selected bond lengths (Å): Fe-
Cl(1) 2.301(2), Fe-Cl(2) 2.245(2), Fe-N(1) 2.096(6), Fe-N(7) 2.373(6),
Fe-N(9) 2.297(6), C(7)-N(7) 1.275(9), C(9)-N(9) 1.264(9).
Table 1. Ethylene Polymerizations Using 1b-5ba
precat
polymer
activity
b
b
run (µmol) yield (g) g/(mmol h bar)
Mn
Mw
Mw/Mn
Figure 2. Molecular structure of 4b. Selected bond lengths (Å): Fe-
Cl(1) 2.3300(14), Fe-Cl(2) 2.2490(14), Fe-N(1) 2.165(4), Fe-N(7) 2.216-
(4), Fe-N(9) 2.191(3), C(7)-N(7) 1.282(6), C(9)-N(9) 1.287(6).
1b (10.0)
2b (1.0)
3b (1.0)
4b (0.5)
5b (0.5)
0
0
2750
3500
35 000
30 000
2
3
4
5
11.0
14.0
70.0
60.0
6300 79 000
50 8000 432 000
24 000 398 000
34 400 212 000
12.5
8.5
16.6
6.2
0.29 Å out of the plane of its substituents, and there is a
significant lengthening of the Fe-N(1) bond [2.165(4) Å],
cf. those in 1b [2.096(6) Å] and the ketimine analogue 5b
[2.110(6) Å]. These distortions are attributed to the effect
of accommodating the sterically demanding thioether groups.
In an ethylene polymerization test under similar conditions,
4b gave an activity of 35 000 g mmol-1 h-1 bar-1, a value
that compares favorably with the ketimine standard 5b (run
5, Table 1). Examination of plots of ethylene uptake versus
time over 60 min runs (see Supporting Information) reveals
the contrasting behavior of the aryl ether and methylthioether
derivatives 3b and 2b, which possess very short catalyst
lifetimes, to the relatively long-lived nature of the bulky
arylthioether catalyst 4b. These results suggest that catalyst
deactivation is most probably enhanced by binding of
activator at the heteroatom within the ligand backbone.
Interestingly, 4b is found to have a longer catalyst lifetime
than the ketimine standard 5b and augurs well for the use
of the bulky thioether ligands to improve the performance
of iron-based polymerization catalysts.
a General conditions: 1-L stainless steel autoclave; precatalysts activated
with 100 equiv MAO prior to injection; 2 mmol MAO used as scavenger;
isobutane solvent; 1 h run; 4 bar of ethylene; 40 °C. b Determined by GPC.
demands of the aryl ether moiety would disfavor activator
binding at the ether oxygen atom. An ethylene polymerization
test in a 1-L stainless steel autoclave at 4 bar ethylene
pressure revealed an activity of 3500 g mmol-1 h-1 bar-1
(Table 1), affording linear polyethylene (by NMR) with a
Mw of 432,000 Da and a relatively broad molecular weight
distribution, typical of iron-based catalysts.
An alternative approach to disfavoring binding at the
heteroatom backbone substituent is to use a soft atom such
as sulfur which would be expected to bind much less strongly
to the hard aluminum activator centers. The methylthioether
derivative 2b was synthesized and found, unlike its ether
relative 1b, to be active for ethylene polymerization, giving
an activity of 2750 g mmol-1 h-1 bar-1, and with a
substantially lower molecular weight than for 3b.
In order to combine the electronic effect of a soft donor
atom with steric protection of the heteroatom, the 2,6-
dimethylphenyl thioether complex 4b was then targeted. Its
molecular structure, which has approximate Cs symmetry,
is substantially different to that of 1b, having a distorted
square pyramidal geometry at iron (Figure 2). The metal lies
0.41 Å out of the N3 plane and 0.47 Å out of the basal plane
(the atoms of which are coplanar to within 0.04 Å). A
particularly unusual feature of the structure is the substantial
inclination of ca. 22° of the pyridine ring to the basal
coordination plane. This out of plane tilt is accompanied by
a pyramidalization of the N(1) center, the nitrogen atom lying
The effect of these heteroatom containing ligand systems
on the performance of catalysts based on other transition
metals will be reported in due course.
Acknowledgment. BP Chemicals Ltd. is thanked for
financial support. Philippe Jehoulet is thanked for GPC
measurements.
Supporting Information Available: Crystallographic informa-
tion (in CIF format) and synthetic procedures and characterization
data for the ligands and complexes. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC0490775
6512 Inorganic Chemistry, Vol. 43, No. 21, 2004