One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

Abstract

In ethylene polymerization by the Phillips catalyst, inorganic Cr(II) sites are believed to be activated by reaction with ethylene to form (alkyl)CrIII sites, in a process that takes about 1 h at ca. 373 K. The detailed mechanism of this spontaneous self-initiation has long remained unknown. It must account both for the formation of the first Cr–C bond and for the one-electron oxidation of Cr(II) to Cr(III). In this study, we used density functional theory to investigate a two-step initiation mechanism by which ethylene oxidative addition leads first to various (organo)CrIV sites, and subsequent Cr–C bond homolysis gives (organo)CrIII sites capable of polymerizing ethylene. Pathways involving spin crossing, C–H oxidative addition, H atom transfer, and Cr–C bond homolytic cleavage were explored using a chromasiloxane cluster model. In particular, we used classical variational transition theory to compute free energy barriers and estimate rates for bond homolysis. A viable route to a four-coordinate bis(alkyl)CrIV site was found via spin crossing in a bis(ethylene)CrII complex followed by intramolecular H atom transfer. However, the barrier for subsequent Cr–C bond homolysis is a formidable 209 kJ/mol. Increasing the Cr coordination number to 6 with additional siloxane ligands lowers the homolysis barrier to just 47 kJ/mol, similar to reported homolysis paths in molecular [CrR(H2O)53+] complexes. However, siloxane coordination also raises the barrier for the prior oxidative addition step to form the bis(alkyl)CrIV site. Thus, we suggest that hemilability in the silica “ligand” may facilitate the homolysis step while still allowing the oxidative addition of ethylene.