A Density Functional Study on Ethylene Trimerization and Tetramerization Using Real Sasol Cr-PNP Catalysts.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Computational Chemistry

Background:

• Ethylene oligomerization is crucial for producing linear alpha-olefins like 1-hexene and 1-octene.
• Chromium-diphosphine complexes are known catalysts for selective ethylene trimerization and tetramerization.
• The metallacycle mechanism is the generally accepted pathway for these transformations.

Purpose of the Study:

• To elucidate the molecular-level mechanistic details of ethylene trimerization and tetramerization using DFT.
• To investigate the influence of ortho- and para-methoxyaryl substituents on chromium-diphosphine catalysts.
• To compare the potential energy surfaces (PES) for trimerization and tetramerization pathways.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model the catalytic systems.
• Potential energy surfaces (PES) were computed for ethylene trimerization and tetramerization.
• A specific chromium complex featuring a nitrogen-bridged diphosphine ligand with methoxyaryl substituents was studied.

Main Results:

• Both ortho- and para-methoxyaryl Cr-PNP catalysts exhibit similar potential energy surfaces for ethylene oligomerization.
• The para-methoxyaryl catalyst demonstrates a slightly reduced energy barrier (~2.6 kcal/mol) for tetramerization compared to the ortho-isomer.
• Selectivity towards trimerization or tetramerization is dictated by the competition between ethylene insertion and beta-hydride transfer.

Conclusions:

• The metallacycle mechanism, involving Cr(I)-Cr(III) intermediates, is the predominant pathway.
• Oxidative coupling of ethylene to form chromacyclopentane is the rate-determining step in the catalytic cycle.
• Substituent effects on the diphosphine ligand can subtly tune catalytic activity and selectivity.