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

Abstract

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 FeIII-OOH and FeIV═O species unstable. Density functional theory calculations further disclosed that, in the presence of Sc(OTf)3, the Sc3+ adducts of FeIII-OOH and FeIV═O species are capable of epoxidizing olefin as well as FeV═O species, thus opening multiple channels for oxygenation. In particular, in the pathway of cyclooctene epoxidation, the FeIV═O/Sc3+ adduct-mediated epoxidation is more energetically favorable than that of the FeV═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.