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}](3)N [X - Cl, R - Me, Cp' - Cp (1); X = Cl, R = Bu-t, Cp' = Cp (4), Cp* (5), (BuC5H4)-Bu-t (6), 1,2,4-Me3C5H2 (7); X = Me, Cp' = Cp*, R = Me (8), Bu-t (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}](3)N 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}](3)N (8) showed the higher catalytic activities than the related mononuclear analogue, Cp*TiCl2(O-2-R-4,6-Me3C6H2) (R - Me, Bu-t); the activity by 8 in the presence of AliBu(3)-[Ph3C][B(C6F5)(4)] cocatalysts was higher than that in the presence of MAO.