Cationic 2,6-bis(imino)pyridine iron and cobalt complexes: synthesis, structures, ethylene polymerisation and ethylene/polar monomer co-polymerisation studies

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Abstract
The synthesis and characterisation of a series of cationic bis(imino)pyridine iron and cobalt complexes of the type [LMCl(D)]SbF6 (D = CH3CN or thf) and [LM(R2acac)]SbF6 (R = CH3, CF3, Ph) are described {L = 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine, M = Fe or Co}. The solid state structures of these five-coordinate complexes vary between square-based pyramidal and trigonal bipyramidal geometries, depending on the ligands used. Attempts to synthesise cationic metal alkyl complexes of the type [LMR]+ were unsuccessful. However, these complexes serve as highly active ethylene polymerisation catalysts when activated with MAO. Polymerisation activities are comparable to the activities obtained with neutral dichloride precursors [LMCl2] and the resulting polymer properties are nearly identical, suggesting that in all cases the same active species is being generated. The polymerisation activity is not inhibited by the presence of donors such as thf or CH3CN and these cationic precursors can be activated with less co-catalyst than is normally used for neutral dichloride pre-catalysts. As little as 10 equiv. TMA, in combination with 1 equiv. B(C6F5)3, afford a highly active polymerisation system. Co-polymerisation studies of ethylene with polar monomers such as methyl methacrylate (MMA) or styrene resulted in polymer production with high activities. However, in both cases no co-polymer is obtained. The activity of the catalyst is significantly reduced in the presence of methyl acrylate (MA) or 2-vinyl-1,3-dioxolane (VDO) and again no co-polymer is produced. Polar monomers such as vinyl acetate, acrolein and acrylonitrile deactivate the catalyst.

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