Unraveling the Effects of H2, N Substituents and Secondary Ligands on Cr/PNP-Catalyzed Ethylene Selective Oligomerization

The mechanism of ethylene oligomerization catalyzed by Cr/PNP catalysts has been investigated using combined experimental and theoretical methodologies. The effects of hydrogen, N substituents, and secondary ligands were fully explored in order to tune the catalyst with better selectivity and activity. Our DFT calculations revealed that the associating/dissociating interactions of the hydrogen with the chromium center might be a key factor in promoting the activity of the Cr/PNP/H2 system, the N substituents show an apparent effect on the energy gap between two adjacent spin states (quartet and sextet) of the bis(ethylene)chromium adducts, which mostly leads to a change in the activation barrier in the reaction of oxidation coupling with a two-state reactivity, and the coordination of secondary ligands (phenyl butyl ether and ethyl benzoate) increases the hindrance of the chromium center, resulting in a shift of the rate-limiting step from the oxidative coupling of two coordinated ethylene molecules to metallacycle growth by insertion of a third coordinated ethylene. The turnover frequencies (TOFs) were computed using the energetic span model (ESM) on the basis of the Gibbs free energy profiles. The steric effects of the secondary ligands as well as the N substituents were evaluated using the Sambvca 2.0 software developed by Cavallo et al.