Bis-[C(sp3)-chelating] Ti2 catalysts supported by arylene-1,4-diyl-2,3-X2 bridges for olefin copolymerisation: X substituents impose conformational cooperative effects

Abstract

The synthesis, spectroscopic characterisation and catalytic olefin polymerisation behaviour of a class of binuclear titanium bis(benzyl) complexes bearing bis-(pyridine-2-phenolate-6-methine)-[μ-(arylene-1,4-diyl-2,3-X₂)] ligands [X₂ = –C₄H₄– (1), F₂ (2), H₂ (3)], and mononuclear analogues, are described. These bimetallic catalyst frameworks are designed to exhibit a degree of conformational flexibility, which is regulated by steric effects and crucially permits tuning of intermetallic distances and geometry, yet their shape-persistent nature can also confer favourable entropic terms. Complexes 1–3 are characterised as two diastereomers [meso (RS) and rac (RR,SS)] in ratios of 1.32, 1.18 and 1.13 respectively, according to ¹H NMR spectroscopy. In contrast to 3, [¹H,¹H]-ROESY experiments for 1 and 2 revealed that the X₂ substituents can impose preferred conformations with syn orientations of Ti₂ centres and benzyl groups, thus implying that the activated catalysts would present binding sites with the same direction of access. For ethylene-(1-octene) copolymerisation reactions, in conjunction with [Ph₃C][B(C₆F₅)₄], catalyst 1 displayed superior efficiencies and produced polymers with higher M_w values and enhanced comonomer incorporation ratios (up to 41%), when compared with the mononuclear 5m (22%). These results are indicative of enhanced comonomer enchainment and cooperative reactivity by the Ti₂ sites.