Cr/PCCP-catalysed selective ethylene oligomerization: analysis of various conformations and the hemilabile methoxy group

Abstract

In this work, the effects of the hemilabile methoxy group in Cr-based catalysts bearing (C₆H₅)₂–P(CH₂)₂P–(C₆H₅)₂ (PCCP) and (o-MeOC₆H₄)(C₆H₅)–P(CH₂)₂P–(C₆H₅)(o-MeOC₆H₄) (PCCP^OMe) ligands on ethylene tri- and tetramerization were systematically investigated by density functional theory (DFT) calculations. Through analysis of the ensemble of low energy conformers of the intermediates and the transition states, the key geometrical features are of great importance to understand the switch between ethylene trimerization and tetramerization induced by the hemilabile methoxy group. Given the fact that the migratory insertion of the coordinated ethylene molecule occurs outside the metallacycle, the energy barrier tends to increase for insertion of the fourth ethylene molecule in the presence of the methoxy group. However, the one-step β-H transfer takes place inside the metallacycle, and therefore it is less effected by introducing the methoxy group in the PCCP ligand. Starting from chromacycloheptane, the catalyst model A (Cr/PCCP) favours the migratory insertion of the fourth ethylene molecule resulting in the formation of 1-octene (14.9 kcal mol⁻¹ for expansion of chromacycloheptane vs. 16.3 kcal mol⁻¹ for decomposition of chromacycloheptane), while the catalyst model B (Cr/PCCP^OMe) results in the formation of 1-hexene through reductive elimination by 3,7 H-shift in chromacycloheptane (17.6 kcal mol⁻¹ for expansion of chromacycloheptane vs. 16.1 kcal mol⁻¹ for decomposition of chromacycloheptane). The hemilability of the methoxy group in the PCCP ligand is of crucial importance in tuning the selectivity of ethylene tri-/tetramerization, which could shed some light on the design of new catalysts for ethylene selective oligomerization.