A Dinuclear Iron(II) Persulfide Complex Reacts with O₂ to Give Sulfite: Relevance to Persulfide Dioxygenases

Persulfides (RSSH) have been proposed as key players in biochemical transformations that often involve iron, including iron-sulfur cluster assembly, H₂S regulation, post translational modifications, and mitochondrial sulfur oxidation. An example of the latter is found in the O₂-mediated oxidation of glutathione persulfide to sulfite dianion (SO₃^2-) catalyzed by ETHE1, a nonheme iron persulfide dioxygenase (PDO). The iron-mediated mechanism of persulfide oxidation by PDOs remains poorly understood, and there are no synthetic analogues to date. Herein, we report the synthesis, characterization, and O₂ reactivity of a rare iron(II)-alkylpersulfide complex. The adamantyl persulfide anion (AdSS-) was isolated and characterized by X-ray diffraction as a complex with potassium 18-crown-6 [K(18-crown-6)][AdSS], and employed in the synthesis of a new dinuclear iron(II) complex, [(Fe^II(Me₃TACN))₂(μ₂-SSAd)₃][OTf] (1). Complex 1 was characterized by single crystal X-ray diffraction (XRD), UV-vis, ¹H/¹⁹F NMR, and ⁵⁷Fe Mössbauer spectroscopy. Reaction of 1 with O₂ in CH₃CN affords a diiron(III) oxo-bridged complex [(Fe^III(Me₃TACN))₂(μ-O)(μ₂-SO₄)(μ₂-SO₃Ad)][OTf] (2) identified by XRD, and SO₃^2- (∼0.5 equiv per Fe₂). Isotopic labeling studies using ¹⁸O₂ and H₂¹⁸O, supported by control experiments and ESI-MS analysis, indicate that SO₃^2- production proceeds via an iron-centered S-oxygenation mechanism similar to that proposed for persulfide dioxygenases.