Ferrous rather than ferric species are the active sites in bis(imino)pyridine iron ethylene polymerization catalysts
Introduction
Despite the great interest in the development of late transition metal olefin polymerization catalysts [1], [2], [3], [4], [5] and in understanding the mechanism by which these catalysts operate [6], [7], [8], [9], [10], the nature of the active species formed upon treatment of iron pre-catalysts with methylaluminoxane (MAO) remains unclear. Very recently, based on Mössbauer and EPR spectroscopic studies, Gibson et al. concluded that the iron centers in bis(imino)pyridine iron(II) olefin polymerization pre-catalysts L1FeCl2 (1) {L1=2,6-bis[1-(2,4,6-trimethylphenylimino)methyl]pyridine} are oxidized upon treatment with MAO to give an active species that contains iron solely in the +3 oxidation state [10]. 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 [11], [12]. This fact was explained by nearly 100% conversion of Fe(II) and Fe(III) precursors into similar Fe(III) species upon treatment with excess MAO [10].
However, our previous 1H and 2H NMR spectroscopic study of L2FeCl2 (2) catalyst activated with MAO and Al2Me6 {L2=2,6-bis[(1-2,6-dimethylphenylimino)ethyl]pyridine} showed that closest precursors of active centers of polymerization may be ferrous complexes of the type L2FeCl(μ-Me)2AlMe2 or L2FeMe(μ-Me)2AlMe2 [9]. It is natural to expect that AlMe3 present in MAO solutions would reduce but not oxidize iron precursors. Similarly, reaction of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridine cobalt(II) pre-catalyst with AlMe3 gives rise to reduction of Co(II) into Co(I) [13], [14]. This background prompted us to reinvestigate by means of 1H NMR and EPR spectroscopy LFeCl3/MAO and LFeCl2/MAO catalytic systems in order to determine concentration of Fe(II) and Fe(III) species in the reaction solution. Complexes 2, and shown in Scheme 1 were studied.
Section snippets
Experimental
Complexes 2, and were prepared as previously described [12]. Methylene chloride-d2 was dried over P2O5 and distilled. Toluene-d8 was dried over molecular sieves (4 Å), purified by refluxing over sodium metal and distilled in dry argon. All solvents were stored and handled in vacuum. All experiments were carried out in sealed high vacuum systems using breakseal techniques. Two samples of MAO were used: commercial MAO from Witco as toluene solution (MAO-1, total Al content 1.8 and 0.5 M of
Results and discussion
Complex dissolved in CD2Cl2 ([]=3 × 10−3 M) displays isotropic EPR signal at room temperature (Fig. 1(a), g=1.995, width at half-height 150 G). Actually, is virtually insoluble in toluene, and “toluene solution” of L1FeCl3 reported in [10] probably should be regarded as a suspension in toluene displaying a powder-like spectrum (wide single line at geff=2). EPR spectrum of frozen solution of exhibits anisotropic signal typical for rhombic high spin Fe3+ complex [16] (S=5/2, g=9, 4.3, 2,
Conclusions
The closest precursors of the active centres of polymerization of the catalytic systems LFeCl3/MAO and LFeCl2/MAO, where L is bis(imino)pyridine ligand are ferrous complexes of the type [L(Me)Fe(II)(μ-Me)2AlMe2] or [LFe(II)(μ-Me)2AlMe2]+[Me-MAO]−.
Acknowledgements
This work was supported by the Russian Foundation for Basic Research (Grant No. 03-03-33034) and INTAS (Grant 00-841).
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