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

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-X 2 )] ligands [X 2 = -C 4 H 4 - (1), F 2 (2), H 2 (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 1 H NMR spectroscopy. In contrast to 3, [ 1 H, 1 H]-ROESY experiments for 1 and 2 revealed that the X 2 substituents can impose preferred conformations with syn orientations of Ti 2 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 [Ph 3 C][B(C 6 F 5 ) 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 Ti 2 sites.