Conversion of high-spin iron(iii)-alkylperoxo to iron(iv)-oxo species via O-O bond homolysis in nonheme iron models

The mechanism of the alkylperoxo O-O bond cleavage of low-spin iron(iii)-alkylperoxo species has been well established in nonheme iron models. In contrast, the alkylperoxo O-O bond cleavage in nonheme high-spin iron(iii)-alkylperoxo species binding an axial ligand has yet to be elucidated. Herein, we report the synthesis and characterization of mononuclear nonheme high-spin iron(iii)-alkylperoxo complexes each bearing an N-tetramethylated 13-membered macrocyclic ligand (13-TMC), [Fe^III(OOC(CH ₃)₃)(13-TMC)]²⁺ and [Fe ^III(OOC(CH₃)₂C₆H₅)(13- TMC)]²⁺. The high-spin iron(iii)-alkylperoxo complexes were converted to an iron(iv)-oxo complex at a fast rate upon addition of thiocyanate (NCS^-) via the formation of a short-lived intermediate. This intermediate was identified as a high-spin iron(iii)-alkylperoxo complex binding a thiocyanate ion as an axial ligand by characterizing it with various spectroscopic methods and density functional theory (DFT) calculations. We have also provided strong evidence that conversion of the high-spin iron(iii)-alkylperoxo complex to its corresponding iron(iv)-oxo complex occurs via O-O bond homolysis. Thus, we have concluded that the role of the axial ligand binding to a high-spin iron(iii)-alkylperoxo complex is to facilitate the alkylperoxo O-O bond cleavage via the "push effect", which has been well established in heme enzymes. To the best of our knowledge, the present study reports the first clear example showing the O-O bond homolysis of a high-spin iron(iii)-alkylperoxo complex and the axial ligand effect on the alkylperoxo O-O bond cleavage in nonheme iron models.