Synthesis of Novel Group 4 Complexes Bearing the Tropidinyl Ligand: Investigations of Dynamic Behavior, Reactivity, and Catalytic Olefin Polymerization

Abstract

A new class of group 4 complexes bearing the tropidinyl (Trop) ligand were synthesized. This novel anionic ligand, easily derived from the natural product tropine, is regarded as an analogue of the well-studied Cp ligand (Cp = cyclopentadienyl) and features a bicyclic ring system in which an allyl function donates 4π electrons and an amine function donates 2σ electrons to the metal center. The synthesis of the group 4 complexes involved transmetalation of a stannylated allyl functionality with a group 4 metal halide with concomitant elimination of Me3SnCl. Reaction of endo-stannylated tropidine (1) and ZrCl4, CpZrCl3, or CpTiCl3 afforded the complexes (Trop)2ZrCl2 (2), (Cp)(Trop)ZrCl2 (3), and (Cp)(Trop)TiCl2 (4), respectively. These dihalide complexes were alkylated using methyllithium to generate (Trop)2ZrMe2 (5), (Cp)(Trop)ZrMe2 (6), (Cp)(Trop)TiMe2 (7), or benzylmagnesium chloride to give (Trop)2ZrBn2 (8) and (Cp)(Trop)ZrBn2 (9). Complexes 2, 3, 5, and 8 were characterized by X-ray crystallography. Complexes 5 and 6 were observed to undergo protonolysis with various acids (HA) to give mixed [Zr]MeA complexes. Reaction of complex 6 with isonitriles (ArNC) gave the η2-iminoacyl complex (Cp)(Trop)ZrMe[N(Ar)CMe] (13), whose structure was confirmed by X-ray crystallography. All the complexes displayed dynamic behavior, and the various exchange processes were investigated by variable-temperature NMR spectroscopy. The dihalide complexes 2, 3, and 4 in the presence of modified methylaluminoxane (MMAO) were active catalysts for the polymerization of ethylene. The catalyst system 3/MMAO displayed a very high ethylene polymerization activity and was similar to that observed for Cp2ZrCl2/MMAO. High molecular weight polypropylene was generated using 4/MMAO, and polypropylene oligomer was obtained using 3/MMAO. The cationic complex [(Cp)(Trop)ZrMe][MeB(C6F5)3] (14) was formed upon reaction of the dimethyl complex 6 with B(C6F5)3 and was shown to be an active ethylene polymerization catalyst.