Tri- and tetra-dentate imine vanadyl complexes: synthesis, structure and ethylene polymerization/ring opening polymerization capability

Abstract

Reaction of the ligand 2,4-tert-butyl-6-[(2-methylquinolin-8-ylimino)methyl]phenol (L¹H) with [VOCl₃] in the presence of triethylamine afforded the complex [VOCl₂L¹] (1), whereas use of [VO(OnPr)₃] led to the isolation of [VO₂L¹] (2) or [VO₂L¹]·2/3MeCN (2·2/3MeCN). Reaction of 2-((2-(1H-benzo[d]imidazol-2-yl)quinolin-8-ylimino)methyl)-4,6-R¹,R²-phenols (R¹ = R² = ^tBu; L²H), (R¹ = R² = Me; L³H) or (R¹ = Me, R² = Ad; L⁴H) with [VO(OnPr)₃] afforded complexes of the type [L^2–4VO] (where L² = 3, L³ = 4, L⁴ = 5). The molecular structures of 1 to 3 are reported; the metal centre adopts a distorted octahedral, trigonal bipyramidal or square-based pyramidal geometry respectively. In Schlenk line tests, all complexes have been screened as pre-catalysts for the polymerization of ethylene using diethylaluminium chloride (DEAC) as co-catalyst in the presence of ethyltrichloroacetate (ETA), and for the ring opening polymerization (ROP) of ε-caprolactone in the presence of benzyl alcohol. All pre-catalyst/DEAC/ETA systems are highly active ethylene polymerization catalysts affording linear polyethylene with activities in the range 3000–10 700 g (mol h bar)⁻¹; the use of methylaluminoxane (MAO) or modified MAO as co-catalyst led to poor or no activity. In a parallel pressure reactor, 3–5 have been screened as pre-catalysts for ethylene polymerization in the presence of either DEAC or DMAC (dimethylaluminium chloride) and ETA at various temperatures and for the co-polymerization of ethylene with propylene. The use of DMAC proved more promising with 3 achieving an activity of 63 000 g (mol h bar)⁻¹ at 50 °C and affording UHMWPE (M_w ∼ 2 000 000). In the case of the co-polymerization, the incorporation of propylene was 6.9–8.8 mol%, with 3 exhibiting the highest incorporation when using either DEAC or DMAC. In the case of the ring opening polymerization (ROP) of ε-caprolactone, systems employing complexes 1–5 were virtually inactive at temperatures <110 °C; on increasing the CL : V ratio at 110 °C, conversions of the order of 80% were achievable.