Observation and identification of the catalytically active species of bis(phenoxy-imine) group 4 transition metal complexes for olefin polymerization using 1H NMR spectroscopy

Abstract

Solution structures of bis(phenoxy-imine) group 4 transition metal complexes (FI Catalysts) were investigated using 1H NMR spectroscopy. At least two isomers exist in equilibrium for FI Catalysts precursors, bis[N-(3-tert-butylsalicylidene)anilinato]zirconium(IV) dichloride (1), and bis[Ar-(3,5-dicumylsalicylidene)anilinato]zirconium(IV) dichloride (2), while bis[N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato]titanium(IV) dichloride (3) exhibits only one isomer under the conditions examined. Upon activation with MAO, all FI Catalysts (1-3) generate two species at ambient temperature judging from some key signals in the 1H NMR. When temperature is raised (up to 75 °), one species (1a-3a) converts irreversibly to the other species (1b-3b). The resulting species, 1b-3b, are stereochemically rigid, in contrast to precursors 1 and 2. Species 3b, derived from a living FI Catalyst, exhibited virtually no reactivity toward olefin insertion. The imine protons of species 1b-3b are temperature and solvent polarity sensitive. Two possibilities are proposed for the assignment of species 1b-3b, i) heterobinuclear complexes of group 4 metal and alkylaluminum with methyl and/or chlorine as bridging groups and ii) phenoxy-imine ligated aluminum complexes whose ligands are transferred from the group 4 metal. The latter is more probable from the separate synthesis of LAlMe2 (L: phenoxy-imine ligand). When 3 was activated with MAO in the presence of olefins, a new imine signal was observed. This species (3c for ethylene and 3d for propylene) is thermally more robust than 3a toward transformation to 3b and assignable to the living propagating species.