Formation and Nature of the Active Sites in Bis(imino)pyridine Iron-Based Polymerization Catalysts

Abstract

The activation of 2,6-bis(2,6-diisopropylphenylimino)ethylpyridine iron-based catalysts of ethylene polymerization with methylalumoxane (MAO) and aluminum trialkyls has been studied in detail by 1H NMR and EPR spectroscopy. Neutral catalytically active species are formed in LiPrFeCl2/Al(Alk)3 systems. They are complexes [LiPr(−)Fe(+)(μ-Me)2AlMe2] (6) or [LiPr(−)Fe(+)(μ-iBu)(μ-X)Al(iBu)2] (7 or 8, X = iBu or Cl), depending on the activator used (either AlMe3 or Al(iBu)3). On the contrary, when "AlMe3-free" methylalumoxane (PMAO) is used as the activator, catalytically active ion pairs of the type [LFeII(μ-Me)2AlMe2]+[MeMAO]− (3) are formed. Intermediates 6 and 8 are relatively unstable and decay within minutes at room temperature, giving rise to an EPR-active iron species with a signal at g = 2.08, presumably of the type L′FeI−Alk (L′FeI(μ-Alk)2Al(Alk)2) in the low-spin state S = 1/2 (where L′ is a modified bis(imino)pyridine ligand). Polymer properties for the different catalyst/activator systems are presented.