Synthesis, Characterization, and Catalytic Activities for the Polymerization of Olefins Promoted by Zirconium(III) and Titanium(III) Allyl Complexes

The reaction of early-transition-metal halides MCl₄ (M = Zr, Ti) with 2 equiv of the lithium allyl compound (^tBuMe₂SiCH)₂CHLi·TMEDA in toluene produced the corresponding reduced complexes [{(^tBuMe₂SiCH)₂CH}₂M(μ-Cl)₂Li·TMEDA] (M = Zr (1), Ti (2)), that were isolated as air-sensitive dark brown (Zr) and brick red (Ti) crystals in 48% and 42% yield, respectively. In addition, each reaction is associated with the formation of the dimer of the allyl ligand (3). Complexes 1 and 2 and dimer 3 were fully characterized spectroscopically, including solid-state X-ray diffraction studies. Complex 1 represents the first solid-state characterization for a Zr(III)−allyl complex. Oxidation of complexes 1 and 2 allows the formation of the corresponding neutral complexes {(^tBuMe₂SiCH)₂CH}₂MCl₂ (M = Zr (4), Ti 5)), which were isolated as air-sensitive light yellow (Zr) and orange (Ti) powders in 23% and 11% yields, respectively. The allylic moieties in complex 5 showed a dynamic interconverting coordination from η³ to η¹, causing an alternation between C₂ and C_2v symmetries of the complex. For the cationic complex 4 a similar dynamic interconverting coordination from η³ to η¹ was observed. The catalytic activities of complexes 1 and 2 activated with MAO in the polymerization of propylene and ethylene were studied. The activity of complexes 1 and 2 was compared to the corresponding activities presented by complexes 4 and 5, respectively, indicating that during the polymerization process, Zr(III) and Ti(III) are oxidized to the corresponding cationic Zr(IV) and Ti(IV) species. For propylene, an elastomeric polypropylene was obtained, whereas for ethylene high-density polyethylene was produced. When the η³ coordination modes of the ligands are operative in the complex, a cationic racemic octahedral compound is formed, which is responsible for the polymerization of the isotactic domains, whereas when the η¹ coordination modes of the ligands are effective, a cationic tetrahedral complex is obtained, accounting for the atactic domains. The formations of isotactic−atactic domains within a polymer chain give rise to elastomeric polypropylene.