Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization

Ethylene tetramerization catalyst systems comprising a Cr(III) complex containing PNP ligands and methylaluminoxane (MAO) are useful for the production of 1-octene. However, a concern with these systems is the use of expensive MAO in excess. Herein, we report a catalytic system that avoids the use of MAO. Metathesis of CrCl₃(THF)₃ and [(CH₃CN)₄Ag]⁺[B(C₆F₅)₄]^- afforded [L₄Cr^IIICl₂]⁺[B(C₆F₅)₄]^- (L = CH₃CN or tetrahydrofuran (THF)), which was converted to [(PNP)CrCl₂L₂]⁺[B(C₆F₅)₄]^-, where PNP is iPrN(PPh₂)₂ (1) or [CH₃(CH₂)₁₆]₂CHN(PPh₂)₂ (2). The molecular structures of [(THF)₄Cr^IIICl₂]⁺[B(C₆F₅)₄]^- and [1-CrCl₂(THF)₂]⁺[B(C₆F₅)₄]^- were unambiguously determined by X-ray crystallography. The cationic (PNP)Cr^III complexes paired with [B(C₆F₅)₄]^- anions, that is, [(PNP)CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-, exhibited high activity in chlorobenzene when activated with common trialkylaluminum species (Me₃Al, Et₃Al, and iBu₃Al). The activities and selectivity were comparable to those of the original MAO-based Sasol system (1-CrCl₃/MAO). When activated with Et₃Al or iBu₃Al, the Cr complex, [2-CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-, which bears long alkyl chains, showed high activity in the more desirable methylcyclohexane solvent (89 kg/g-Cr/h) and much higher activity in cyclohexene (168 kg/g-Cr/h). Other advantages of the [2-CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-/Et₃Al system in cyclohexene were negligible catalyst deactivation, formation of only a negligible amount of polyethylene side product (0.3%), and formation of fewer unwanted side products above C10. The [B(C₆F₅)₄]^- anion is compatible with trialkylaluminum species once it is not paired with a trityl cation. Hence, [(PNP)CrCl₂(CH₃CN)₂]⁺[B(C₆F₅)₄]^-/Et₃Al exhibited a significantly higher activity than that of a previously reported system composed of [Ph₃C]⁺[B(C₆F₅)₄]^-, that is, 1/CrCl₃(THF)₃/[Ph₃C]⁺[B(C₆F₅)₄]^-/Et₃Al.