Synthesis and structural analysis of aryloxo-modified trinuclear half-titanocenes, and their use as catalyst precursors for ethylene polymerisation

A series of trinuclear half-titanocenes, [Cp′TiX2{(O-2,4-R2C6H2)-6-CH2}]3N [X = Cl, R = Me, Cp′ = Cp (1); X = Cl, R = tBu, Cp′ = Cp (4), Cp* (5), tBuC5H4 (6), 1,2,4-Me3C5H2 (7); X = Me, Cp′ = Cp*, R = Me (8), tBu (9)] and the related bimetallic complexes, [Cp′TiCl2{(O-2,4-Me2C6H2)-6-CH2}][Cp′TiCl{(O-2,4-Me2C6H2)-6-CH2}2]N [Cp′ = Cp* (2), 1,2,4-Me3C5H2 (3)], have been prepared and identified. Structures of 1–5, 7 and 9 were determined by X-ray crystallography, and all complexes fold with distorted tetrahedral geometries around titanium. These complexes (2–9) are stable in solution except the Cp analogue (1), which presents as a mixture of the trinuclear analogue (1) and the (proposed) binuclear analogue, CpTiCl3, and CpTi[{(O-2,4-Me2C6H2)-6-CH2}]3N in solution; there is an equilibrium between 1 and the binuclear analogue (and CpTiCl3) depending on the temperature, solvent and concentration. The Cp* analogues (2, 8, 9), exhibited high catalytic activities for ethylene polymerisation in the presence of MAO cocatalyst, affording ultrahigh molecular weight polymers with uniform molecular weight distributions in most cases. [Cp*TiMe2{(O-2,4-Me2C6H2)-6-CH2}]3N (8) showed the higher catalytic activities than the related mononuclear analogue, Cp*TiCl2(O-2-R-4,6-Me3C6H2) (R = Me, tBu); the activity by 8 in the presence of AliBu3–[Ph3C][B(C6F5)4] cocatalysts was higher than that in the presence of MAO.