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

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, bisN-(3-tert-butylsalicylidene)anilinato]zirconium(IV) dichloride ( 1 ), and bisN-(3,5-dicumylsalicylidene)anilinato]zirconium(IV) dichloride ( 2 ), while bisN-(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°C), 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.