No abstract

To elucidate fundamental mechanistic aspects of the landmark Chevron–Phillips ethylene trimerization system, a detailed theoretical study has been carried out by DFT methods on an aluminum pyrrolyl chromium catalyst. Reaction pathways for selective ethylene oligomerization have been successfully located on the basis of the metallacycle mechanism. Consistent with experimental results, for the model system ethylene trimerization was proven to be energetically preferred in comparison to ethylene dimerization or further ring expansion toward the formation of higher α-olefins. The Cr(I/III) redox couple was found to be the most likely for the catalytic ethylene trimerization. A careful electronic configuration analysis has been conducted, and the ground state of all active species involved in the catalytic cycle is identified to be S = 3/2 except for the bare active species, which favors a high spin state of S = 5/2. The role of a pendant chlorine functionality has been investigated as well. Variable Cr–Cl bond distance and NBO charge analysis of every intermediate clearly exhibit the hemilabile behavior of the chlorine. This unique hemilability is considered to be a key factor for the selectivity toward 1-hexene formation.

Selective ethylene trimerization into 1-hexene, which is an important comonomer for production of copolymers with high performance, was first commercialized in 2003 using a chromium tris(2ethylhexanoate) (Cr(EH) 3 )-based catalyst. One milestone during the development of this catalyst is the addition of 1,2-dimethoxyethane (DME) to Cr(EH) 3 /partially hydrolyzed tri-isobutyl aluminum (PIBAO) catalyst, resulting in a transformation from ethylene polymerization into trimerization. However, the mechanism of this transformation is unknown. In this work, the role of DME in the switching mechanism from ethylene polymerization to trimerization was investigated by a density functional theory (DFT) method based on the 10 most plausible molecular models for the active species of Cr(EH) 3 /PIBAO catalyst with or without DME coordination. For neutral models (Cr(I)OR; Cr(I)OR/DME; Cr(I)R; Cr(I)R/DME; R refers to isobutyl ligand), those without DME coordination were found to be dominantly ethylene trimerization sites. After DME coordination, both the weak electron-donating effect and steric effect of DME increased the total energy barriers of metallocycle growth, leading to ethylene trimerization with dimerization as a side reaction. So neutral models mainly presented ethylene trimerization with or without DME coordination and were not consistent with experimental evidence of transformation. For cationic models (Cr(I) þ ; Cr(I) þ /DME; Cr(II) þ OR; Cr(II) þ OR/DME; Cr(II) þ R; Cr(II) þ R/DME), metallocycle growth leading to ethylene polymerization occurred on those models without DME. After DME coordination, the strong electron-donating effect of DME was found to facilitate metallocycle expansion, while the dominant role of the steric effect of DME made nine-membered ring formation unfavorable, and thus 1-hexene was liberated from the seven-membered ring. So all the cationic models presented a transformation from ethylene polymerization to trimerization after DME coordination and might be the most plausible active sites for the Cr(EH) 3 /PIBAO catalyst system. The results provided a much deeper understanding of the highly selective trimerization mechanism and for further development of new catalysts with high performance as well.