Synthesis of novel group 4 complexes bearing the tropidinyl ligand: Investigations of dynamic behavior, reactivity, and catalytic olefin polymerization

A new class of group 4 complexes bearing the tropidinyl (Trop) ligand were synthesized. This novel anionic ligand, easily derived from the natural pro duct tropine, is regarded as an analogue of the well-studied Cp ligand (Cp = cyclopentadienyl) and features a bicyclic ring system in which an allyl f unction donates 4 pi electrons and an amine function donates 2 sigma electr ons to the metal center. The synthesis of the group 4 complexes involved tr ansmetalation of a stannylated allyl functionality with a group 4 metal hal ide with concomitant elimination of Me3SnCl. Reaction of endo-stannylated t ropidine (1) and ZrC4, CpZrC4, or CpTiCl3 afforded the complexes (Trop)(2)Z rCl2 (2), (Cp)(Trop)ZrCl2 (3), and (Cp)-(Trop)TiCl2 (4), respectively. Thes e dihalide complexes were alkylated using methyllithium to generate (Trop)( 2)ZrMe2 (5), (Cp)(Trop)ZrMe2 (6), (Cp)(Trop)TiMe2 (7), or benzylmagnesium c hloride to give (Trop)(2)ZrBn2 (8) and (Cp)(Trop)ZrBn2 (9). Complexes 2, 3, 5, and 8 were characterized by X-ray crystallography. Complexes 5 and 6 we re observed to undergo protonolysis with various acids (HA) to give mixed [ Zr]MeA complexes. Reaction of complex 6 with isonitriles (ArNC) gave the et a(2)-iminoacyl complex (Cp)(Trop)ZrMe[N(Ar)CMe] (13), whose structure was c onfirmed by X-ray crystallography. All the complexes displayed dynamic beha vior, and the various exchange processes were investigated by variable-temp erature NMR spectroscopy. The dihalide complexes 2, 3, and 4 in the presenc e of modified methylaluminoxane (MMAO) were active catalysts for the polyme rization of ethylene. The catalyst system 3/MMAO displayed a very high ethy lene polymerization activity and was similar to that observed for Cp2ZrCl2/ MMAO. High molecular weight polypropylene was generated using 4/MMAO, and p olypropylene oligomer was obtained using 3/MMAO. The cationic complex [(Cp) (Trop)ZrMe][MeB(C6F5)(3)] (14) was formed upon reaction of the dimethyl com plex 6 with B(C6F5)(3) and was shown to be an active ethylene polymerizatio n catalyst.