A chromium precursor for the Phillips ethylene trimerization catalyst: (2-ethylhexanoate)2CrOH

Abstract

The conventional Phillips ethylene trimerization catalyst prepared by reacting Cr(EH)₃ (EH = 2-ethylhexanoate), 2,5-dimethylpyrrole (Me₂C₄H₂NH), Et₃Al, and Et₂AlCl in an aromatic hydrocarbon solvent was improved to obtain a congener composed of a new chromium precursor (EH)₂CrOH, (Me₂C₄H₂N)AlEt₂, and Et₃Al·ClAlEt₂. Reaction of CrCl₃ with 3 equiv. Na(EH) in water did not generate Cr(EH)₃, but unexpectedly produced (EH)₂CrOH. In comparison with the erratic catalytic performance of the original Phillips system, due to the ill-defined nature of the Cr(EH)₃ source (16 or 6.8 × 10⁶ g per mol-Cr h depending on the source), the improved system exhibited consistently high activity (54 × 10⁶ g per mol-Cr h). Reaction of (EH)₂CrOH with (Me₂C₄H₂N)AlMe₂·OEt₂ afforded the dimeric Cr(II)-complex (6) coordinated by (η⁵-Me₂C₄H₂N)AlMe₂(NC₄H₂Me₂) and μ₂–κ¹:η²-Me₂C₄H₂N ligands. 6 provided highly active species when activated with Et₃Al·ClAlEt₂.