The nature of the active site in bis(imino)pyridine iron ethylene polymerisation catalysts

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Abstract

Mössbauer and electron paramagnetic resonance (EPR) spectroscopic studies reveal that the iron centres in bis(imino)pyridine iron(II) olefin polymerisation pre-catalysts of the type LFeCl2 are oxidised upon treatment with methylaluminoxane (MAO) to give an active species that contains iron solely in the +3 oxidation state.

Introduction

In recent years there has been a phenomenal growth of interest in the discovery and development of late transition metal olefin polymerisation catalysts [1], [2], [3]. One of the most significant developments has been the discovery of highly active ethylene polymerisation catalysts based on iron, a metal that previously had no track record for catalysing polyethylene chain growth. The catalysts are stabilised by bis(imino)pyridine ligands [4], [5] and afford highly linear polyethylene, whose density, molecular weight and molecular weight distribution fall within the range of commercially relevant high-density polyethylenes (HDPEs) [6].

There has been much interest in understanding the mechanism by which these catalysts operate [7], [8], [9], [10]. It has generally been assumed that treatment of the dihalide Fe(II) pre-catalysts [LFeCl2] with excess methylaluminoxane (MAO) affords a cationic iron(II) alkyl species. However, considerable difficulties have been encountered in preparing well-defined cationic iron alkyl complexes bearing the bis(imino)pyridine ligand due, it has been presumed, to the instability of the dialkyl precursors. With a view to circumventing such dialkyl species, we have also investigated cationic precursors, but these too have failed to yield [LFe–R]+ products [11]. Furthermore, iron(III) pre-catalysts [LFeCl3] have been shown to afford catalysts with similar activities and polymer product characteristics to those generated from iron(II) precursors [5], [6] (Scheme 1).

In order to obtain more insight into the nature of the active species, we have carried out a series of Mössbauer and electron paramagnetic resonance (EPR) studies on the divalent [LFeCl2] and trivalent [LFeCl3] precursors 1 and 2 {L = 2,6-bis[(2,4,6-trimethylphenylimino)methyl]pyridine} and the species arising from their treatment with excess MAO. These results indicate that a cationic iron(II) alkyl is not the active species.

Section snippets

Experimental

Complexes 1 and 2 were prepared as previously described [6]. Solvents were dried by prolonged reflux over a suitable drying agent under a dinitrogen atmosphere, and freshly distilled and de-gassed prior to use. MAO was obtained from Aldrich Chemical Company. All other reagents were purchased from commercial suppliers and used as received.

57Fe Mössbauer spectra were recorded on finely ground powders under an argon atmosphere at ambient temperature using a Wissel MR-260 constant acceleration

Mössbauer studies

Mössbauer spectroscopy allows a probe of the 57Fe nucleus by γ-radiation and yields information on the oxidation state, spin state and coordination environment of the absorbing isotope in the bulk sample. For 57Fe, the first nuclear excited state is split into two sublevels by the presence of an electric field gradient across the nucleus, resulting in the observation of a quadrupole doublet. The Mössbauer spectra are characterised by two parameters, the isomer shift δ and quadrupole splitting ΔE

Conclusion

The near 100% conversion of the Fe(II) pre-catalyst into an Fe(III) species upon treatment with excess MAO, and the similarities of the Mössbauer and EPR spectra of catalysts derived from Fe(II) or Fe(III) precursors, suggest that similar Fe(III) species account for the active sites in these bis(imino)pyridine iron catalysts. The precise nature of the sites remains to be determined; possibilities include a dicationic iron(III) alkyl species [LFe–R]2+, or a chloro, alkyl species [LFe(Cl)(R)]+.

Acknowledgements

The authors are grateful to BP Chemicals for financial support. Mössbauer data were collected under the auspices of the University of London Intercollegiate Research Service. Dr. Udowole (QMW, London) is thanked for EPR measurements.

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