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

In ethylene polymerization by the Phillips catalyst, inorganic Cr(II) sites are believed to be activated by reaction with ethylene to form (alkyl)Cr-III 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)Cr-IV sites, and subsequent Cr-C bond homolysis gives (organo)Cr-III 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)Cr-IV site was found via spin crossing in a bis(ethylene)Cr-III 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)(5)(3+)] complexes. However, siloxane coordination also raises the barrier for the prior oxidative addition step to form the bis(alkyl)Cr-IV site. Thus, we suggest that hemilability in the silica "ligand" may facilitate the homolysis step while still allowing the oxidative addition of ethylene.