Probing the effects of ligand structure on activity and selectivity of Cr(iii) complexes for ethylene oligomerisation and polymerisation

Abstract

A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH₂CH₂SC₁₀H₂₁)₂ (L¹) and O(CH₂CH₂SC₁₀H₂₁)₂ (L²)}, tripodal {MeC(CH₂SⁿC₄H₉)₃ (L³), MeC(CH₂SC₁₀H₂₁)₃ (L⁴)} and macrocyclic {(C₁₀H₂₁)[9]aneN₃ (L⁵), (C₁₀H₂₁)₃[9]aneN₃ (L⁶), with [9]aneN₃ = 1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl₃(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl₃(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me₃Al investigated similarly. Treatment with excess Me₃Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the neutral S and S/O-donor ligands.