Synthesis, structure and catalytic properties of [Cp*Cr(C6F5)(Bn)(THF)] toward ethylene in the presence of AlEt3

Treatment of Cp*CrCl(C₆F₅)]₂ with BnMgCl (Bn = benzyl) in Et₂O/THF affords Cp*Cr(C₆F₅)(Bn)(THF)] (1) which has been isolated in 72% yield. This compound whose magnetic moment is equal to of 4.037 μ_B has been characterized by NMR and single crystal X-ray analysis. Compound 1 alone does not polymerize ethylene when dissolved in toluene. However, addition of excess AlEt₃ to a solution of 1 in toluene leads to a catalytically active system which readily oligomerizes ethylene under standard conditions. Oligomerization experiments carried out with 1] = 10⁻³ M and AlEt₃] = 9 × 10⁻² M for 15 min lead to the production of ethylene oligomers with an activity of 280 kg mol Cr⁻¹ h⁻¹. The experimental molecular weight distribution observed at intermediate times during the reaction is satisfactorily accounted for by the Poisson distribution formula, which is indicative of a living polymerization system. These observations are in agreement with a catalytic cycle in which the growing alkyl chain is transferred from chromium to aluminum via a bimetallic complex in which the chromium and aluminum centers are bridged by an alkyl group and the growing polymer chain.