Selective trimerisation and polymerisation of ethylene: Halogenated chromium triazacyclohexane complexes as probes for an internal 'halogen effect'

A range of triazacyclohexane (R₃TAC) complexes of CrCl₃ with unsaturated N-substituents R have been prepared. The complexes with R = allyl (3f) and CH₂CH(CH₂CH = CH₂)₂ (3h) have been characterised by X-ray crystallography. Addition of HX or X₂ (X = Cl or Br) across the double bonds of the complexed ligand gives access to β- and γ-branched N-substituents containing three to twelve organic halogen atoms. The complexes with R = CH₂CHBrCH₂Br (3k), CH₂CH₂CX(ⁿC₅H₁₁)₂ (X = Cl (3n) and Br (3o)) have been characterised by X-ray crystallography. N-substituent chlorinated complexes can also be prepared via alkyl chloride containing amines as demonstrated for a crystal structure of a complex with R = (CH₂)₃Cl (3e). Complexes with β-branched N-substituents give high selectivity for ethylene trimerisation to 1-hexene while complexes with γ-branched N-substituents are good ethylene polymerisation catalysts. The halogenated polymerisation catalysts give higher activities than their hydrogenated analogues, especially chlorine containing species. Chlorinated trimerisation catalysts demonstrated significantly improved 1-hexene selectivities, though activity was similar to hydrogenated alternatives. This lends support to the concept of a positive 'halogen effect' on the catalysis.