cis-Dihydroxylation by Synthetic Iron(III)–Peroxo Intermediates and Rieske Dioxygenases: Experimental and Theoretical Approaches Reveal the Key O–O Bond Activation Step

Abstract

Dioxygen (O₂) activation by iron-containing enzymes and biomimetic compounds generates iron–oxygen intermediates, such as iron-superoxo, -peroxo, -hydroperoxo, and -oxo, that mediate oxidative reactions in biological and abiological systems. Among the iron–oxygen intermediates, iron(III)–peroxo species are less frequently implicated as active intermediates in oxidation reactions. In this study, we present the combined experimental and theoretical investigations on cis-dihydroxylation reactions mediated by synthetic mononuclear nonheme iron–peroxo intermediates, demonstrating the importance of supporting ligands and metal centers in activating the peroxo ligand toward the O–O bond homolysis for the cis-dihydroxylation reactions. We found a significant ring size effect of the TMC ligand in [Fe^III(O₂)(n-TMC)]⁺ (TMC = tetramethylated tetraazacycloalkane; n = 12, 13, and 14) on the cis-dihydroxylation reactivity order: [Fe^III(O₂)(12-TMC)]⁺ > [Fe^III(O₂)(13-TMC)]⁺ > [Fe^III(O₂)(14-TMC)]⁺. Additionally, we found that only [Fe^III(O₂)(n-TMC)]⁺, but not other metal–peroxo complexes such as [M^III(O₂)(n-TMC)]⁺ (M = Mn, Co, and Ni), is reactive for the cis-dihydroxylation of olefins. Using density functional theory (DFT) calculations, we revealed that electron transfer from the Fe d_xz orbital to the peroxo σ*(O–O) orbital facilitates the O–O bond homolysis, with the O–O bond cleavage barrier well correlated with the energy gap between the frontier molecular orbitals of d_xz and σ*(O–O). Further computational studies showed that the reactivity of the synthetic [Fe^III(O₂)(12-TMC)]⁺ complex is comparable to that of Rieske dioxygenases in cis-dihydroxylation, providing compelling evidence of the potential involvement of Fe(III)–peroxo species in Rieske dioxygenases. Thus, the present results significantly advance our understanding of the cis-dihydroxylation mechanisms by Rieske dioxygenases and synthetic nonheme iron–peroxo models.