Geometrically Constrained Cofacial Bi-Titanium Olefin Polymerization Catalysts: Tuning and Enhancing Comonomer Incorporation Density

A series of shape-persistent bis-[C(sp³)-chelating] Ti₂ (plus Zr₂ and Hf₂) complexes with a rigid linker component (xanthene or dibenzofuran) are presented. These structurally diverse assemblies display limited yet different conformational flexibility, and crucially, such geometric constraints confer access to a range of intermetallic separations and orientations to potentially enhance catalytic activity and cooperative effects. For ethylene polymerizations, the Ti₂ catalysts (in conjunction with trityl borate) exhibit greater efficiencies and produced polymers with higher M_w than mononuclear controls, which is significant considering the more crowded environment for cofacial bimetallic sites. Proficient 1-hexene incorporations were observed for ethylene-(α-olefin) copolymerization reactions. The F-substituted m-aryl/dibenzofuran-linked catalyst (5), which is revealed by NMR analysis to be conformationally dissimilar to its F-absent congener, produced copolymers with higher M_w and elevated 1-hexene incorporation levels (up to 44%), when compared with its mono-Ti control (19%). These results suggest that catalyst frameworks with suitably adjustable conformations and Ti···Ti distances can facilitate bimetallic enchainment interactions with α-olefin substrates and their insertion.