Heterobimetallic Al-Cl-Cr intermediates with relevance to the selective catalytic ethene trimerization systems consisting of CrCl3(THF) 3, the aminophosphorus ligands Ph2PN(R)P(Ph)N(R)H, and triethylaluminum

Abstract

We report in this paper the isolation and characterization of novel heterobimetallic Cr/Al complexes based on the PNPN ligand along with an assessment about their relevance as intermediates in the ethane selective trimerization process consisting of CrCl3(THF)3, the aminophosphorus ligands Ph2PN(R)P(Ph)N(R′)H (1a, R = R′ = i-Pr; 1b, R = R′ = c-Hex; 1c, R = n-Hex; R′ = i-Pr). Different reactions between these ligands or their aluminated products with different chromium salts and Et3Al gave the binuclear P,P′-coordinated chromium(II)-chloro-aluminum-amidophosphane complexes [(μ(Cr,Cr)-Cl)(μ(Cr, Al)-Cl)Cr{κ2P,P-P(Ph2)N(R)P(Ph)N(R)Al(Et 2)}]2 (2a, R = R′ = i-Pr; 2b, R = R′ = c-Hex; 2c, R = n-Hex; R′ = i-Pr). By using Ph2PN(i-Pr)P(Ph)N(i-Pr)H (1a) the corresponding complex [(μ(Cr,Cr)-Cl)(μ(Cr,Al)-Cl) Cr{κ2P,P-P(Ph2)N(i-Pr)P(Ph)N(i-Pr)Al(Me 2)}]2 (2d) was also obtained. Complexes 2a,c,d were characterized by X-ray analysis. The formation and role of these compounds are discussed and identified as an important step during the activation of the catalytic system. Studies reveal that the activity of the described trimerization reaction can be enhanced by an optimized Cr/Al/Cl ratio without affecting the high selectivity to 1-hexene, thus clarifying the importance of chlorine as part of the catalytic species. © 2011 American Chemical Society.