Mechanistic DFT Study on Ethylene Trimerization of Chromium Catalysts Supported by a Versatile Pyrrole Ligand System

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.

Exploring Basic Components Effect on the Catalytic Efficiency of Chevron-Phillips Catalyst in Ethylene Trimerization

“…Furthermore, a two-step process that leads to 3 from 2 was also attempted, bearing a first H transfer; however, it was omitted from the analysis since kinetically it was found to be much less efficient. For the parent model system A, the energy profile calculated in Figure 1 is very similar to that calculated earlier by Liu and coworkers [12]. The rate for determining step size (rds) turned out to be the transition state leading to the formation of the 7-membered metallacycle 5A, with an upper barrier placed at 24.9 kcal/mol over intermediate 3A, which is slightly less efficient compared to the previous results of Liu [12].…”

“…For example, there is a compromise that the catalytic cycle is accomplished via the formation of a metallacycle pathway in which the chromium centre operates via a redox Cr(I/III) metallacyclic mechanism [11]. In fact, theoretical studies of the activation energies for different stages of the trimerization process have revealed that the model containing Cr (I/III) redox has lower energy levels than the Cr(II/IV) species, due to greater stability of the oxidation state in the Cr(I/III) system [12]. This finding was further supported by the experimental results from an electron paramagnetic resonance (EPR) investigation on the same catalytic system [13].…”

Exploring Basic Components Effect on the Catalytic Efficiency of Chevron-Phillips Catalyst in Ethylene Trimerization

“…For the parent model system A, the energy profile we calculate in Figure 1 is very similar to that 98 calculated earlier by Liu and coworkers [12]. The rate determining step (rds) turns out to be the 99 transition state leading to the formation of the 7-membered metallacycle 5A with an upper barrier 100 placed 24.9 kcal/mol over intermediate 3A, which is slightly more disfavoured with respect to 101 previous results of Liu [12].…”

“…The rate determining step (rds) turns out to be the 99 transition state leading to the formation of the 7-membered metallacycle 5A with an upper barrier 100 placed 24.9 kcal/mol over intermediate 3A, which is slightly more disfavoured with respect to 101 previous results of Liu [12]. The alternative undesired reactions that lead to 8 and 9 are found to be 102 less favoured kinetically by 8.8 and 1.6 kcal/mol, respectively.…”

MAO‐free and extremely active catalytic system for ethylene tetramerization

“…Even though a catalytic cycle involving cationic Cr I/III species currently prevails, [43][44][45][46][47] a catalytic cycle involving the Cr II/IV species has not been SCHEME 2 Preparation of catalyst precursors bearing [B(C 6 F 5 ) 4 ]-completely excluded. [9,44,48] It was also reported that the majority of the Cr III precursors were converted into EPR-silent Cr II species during the activation process with alkylaluminum. [46,47,49] In the elemental analysis data roughly agreed with the formula (calculated: C 53.50, H 4.34%; found: C 51.20, H 3.41%).…”