Theoretical Study on a Multicenter Model Based on Different Metal Oxidation States for the Bis(imino)pyridine Iron Catalysts in Ethylene Polymerization

We have analyzed by DFT methods the reactivity shown by the bis(imino)pyridine catalysts based on the oxidation states, II or III, of the Fe atom. Although it has been proposed that both Fe-based species could be involved in the olefin polymerization process, there is some controversy over which of them is the more active one. Previous theoretical studies based on conceptual DFT descriptors showed that the Fe(III) cationic species was more active than the Fe(II) one. The results obtained in the present work, based on the analysis of energy profiles for the ethylene insertion and β-hydrogen chain transfer steps calculated from DFT, confirm these observations, showing that unlike the Fe(II) species, the Fe(III) oxidation state would be more active in olefin polymerization and would yield polymers with a low molecular weight fraction. This result is in agreement with experimental observations showing the characteristic bimodal polymer molecular weight distribution obtained with these catalysts.