Selective ethylene oligomerisation using supported tungsten mono-imido catalysts

Abstract

A series of substituted phenyl mono-imido complexes of the type W(NR)Cl₄(THF) (R = C₆H₅, 2,6-Me-C₆H₃, 3,5-Me-C₆H₃, 2,4,6-Me-C₆H₂, 4-OMe-C₆H₄, 2,6-F-C₆H₃ and 3,5-CF₃-C₆H₃) have been synthesised and characterised. Reaction of these complexes with solid polymethylaluminoxane (sMAO) leads to immobilisation and in situ methylation of the chloride positions on the surface of the support. Reaction of W(NR)Cl₄(THF) with trimethylaluminium (TMA) yields the trimethyl complexes W(NR)Me₃Cl. Immobilisation of the isotopically labelled W{N(2,6-F-C₆H₃)}(¹³CH₃)₃Cl on sMAO furnished the supported complex with two identifiable methyl resonances in the ¹³C–{¹H} solid state CPMAS spectrum (45 and 56 ppm), with the latter matching the unsupported complex, confirming retention of the structure on the surface. The sMAO-supported complexes (W : Al = 1 : 150) were tested for their propensity to dimerise ethylene (1 bar) in d₆-benzene at 100 °C and compared with the previously reported sMAO-W{N(2,6-ⁱPr-C₆H₃)}Cl₄(THF) (sMAO-1.a). Complexes with electron deficient imido groups were shown to be the most active, and increased steric bulk in the ortho positions is also an important factor, with sMAO acting as a support, scavenger and activator. sMAO-W{N(3,5-CF₃-C₆H₃)}Cl₄(THF) was the most active, demonstrating a turnover frequency of 5.65 mol_C2H4 mol⁻¹_W h⁻¹ and a selectivity towards 1-butene of 91% after 8 h.