Synthesis, Structural Characterization, and Reactivity of Zirconium Complexes Containing Trimethylenemethane-Based Ligands

Abstract

General synthetic routes to zirconium metallocene-like complexes containing derivatives of the dianionic trimethylenemethane (TMM) ligand are presented. One approach consists of reacting the dilithium salts of TMM, tribenzylidenemethane (TBM), tert-butyltribenzylidenemethane (t-Bu-TBM), and dibenzylidenemethylenemethane (DBM) with either Cp*ZrCl3 or CpZrCl3(DME). In the case of the small TMM fragment, the product is the zwitterionic Cp*(TMM)Zr(μ-Cl)2Li(TMEDA) (1). Larger TMM derivatives give discrete salts such as [Cp*(TBM)ZrCl2][Li(TMEDA)2] (2), [Cp(TBM)ZrCl2][Li(TMEDA)2] (3), [Cp(t-Bu-TBM)ZrCl2][Li(TMEDA)2] (4), [Cp*(t-Bu-TBM)ZrCl2][Li(TMEDA)2] (5), and [Cp*(exo-endo-DBM)ZrCl2][Li(TMEDA)2] (6). The reaction of TBM(LiTMEDA)2 with Cp*ZrCl2CH2Ph affords [Cp*(TBM)ZrCl(CH2Ph)][Li(TMEDA)2] (9); thus the retention of LiCl(TMEDA)2 by zirconium is strong. Structural characterization of these complexes reveals crowded environments around the zirconium, especially when both TBM and Cp* are coordinated. It is also possible to take advantage of intramolecular σ-bond metathesis reactions to convert coordinated allyl ligands to TMM-related fragments. For example, [Cp*(TMM)Zr]2(μ-CH2) (10) is derived from Cp*(η3-CH2C(Me)CH2)ZrMe2, and Cp*(TBM)ZrMe(THF) (12) is from Cp*(PhCH2C(CHPh)2)ZrMe2 (11). Formation of the methylpropargyl complex Cp*(TBM)Zr(η3-CH2CCMe) (13) from Cp*(TBM)ZrMe(THF) and 2-butyne instead of a butenyl derivative is a consequence of steric constraints. Activation of 2−6 with methylaluminoxane affords homogeneous catalyst mixtures for polymerization of ethylene and 1,5-hexadiene and copolymerization of ethylene with 1-hexene. There is a strong correlation between catalyst precursor structure and reactivity. Polyethylene can also be prepared by pressurizing a vessel containing only Cp*(TBM)ZrMe(THF).