Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H₂O₂: DFT Calculations Reveal Multiple...

Nonredox metal ions play significant roles in a wide range of biological and chemical oxidations in which they can modulate the oxidative reactivity of those redox metal ions. With environmentally benign H₂O₂ as oxidant, the influence of nonredox metal ions on an iron(II) complex mediated olefin epoxidation was investigated through experimental studies and theoretical calculations. It was found that adding nonredox metal ions like Sc³⁺ can substantially improve the oxygen transfer efficiency of the iron(II) complex toward cyclooctene epoxidation even in the presence of certain amount of water. In ¹⁸O-labeling experiments with ¹⁸O water, the presence of Sc³⁺ provided a higher ¹⁸O incorporation in epoxide. In UV-vis studies, it was found that the presence of Sc³⁺ makes both Fe^III-OOH and Fe^IV═O species unstable. Density functional theory calculations further disclosed that, in the presence of Sc(OTf)₃, the Sc³⁺ adducts of Fe^III-OOH and Fe^IV═O species are capable of epoxidizing olefin as well as Fe^V═O species, thus opening multiple channels for oxygenation. In particular, in the pathway of cyclooctene epoxidation, the Fe^IV═O/Sc³⁺ adduct-mediated epoxidation is more energetically favorable than that of the Fe^V═O species (12.2 vs 17.2 kcal/mol). This information may implicate that the presence of certain nonredox metal ions can facilitate these redox metal ions mediating biological and chemical oxidations happening at a relatively low oxidation state, which is more energetically accessible.