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

Abstract

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.