Control of symmetry in active cationic ansa-zirconocene species: catalyst preparation, characterization and ethylene–norbornene copolymerization

New bis-hydrocarbyl complexes of methylene bridged ansa-metallocenes [H2C(C5Me2H2)2]ZrR2 {R = Me (1), CH2Ph (2), CH2SiMe3 (3), Ph (4)} have been prepared. They form catalytically active intermediates with borane or borate depending on solvent and Zr–R group. Specifically, [H2C(Me2C5H2)2]Zr(CH2Ph)2 (2) produced an ion pair upon treatment with B(C6F5)3 whereas [H2C(Me2C5H2)2]Zr(CH2SiMe3)2 (3) produced a zwitterionic species, identified by 1H, 13C, and 19F NMR spectroscopy. Copolymerization of ethylene and norbornene for the metallocene dichloride/MAO and bis-hydrocarbyl complex/borate systems was compared.