Mixed N3S2-Ligated Nonheme Fe(IV)=O Species Balancing Stability and Oxidation Reactivity as a Platform for Nonheme Iron Oxidation Catalysis

Abstract

Mononuclear nonheme iron(IV)–oxo species supported by mixed nitrogen–sulfur (N/S) ligands remain scarce, despite the prevalence of sulfur donors in biological iron sites and their expected impact on ferryl reactivity and catalyst design. In this work, a seven-coordinate iron(II) complex, [(N3S2)FeII(ClO4)2], bearing a rigid 15-membered N3S2 macrocycle, is shown to rapidly generate a mononuclear nonheme FeIV=O intermediate upon reaction with m-chloroperbenzoic (m-CPBA) acid in acetonitrile. The FeIV=O species forms within ≤2 s and is thermally persistent (t1/2 = 4.3 h at 25 °C), albeit in partial yield (~39% FeIV=O by Mössbauer spectroscopy), placing it in an intermediate regime between highly reactive but short-lived ferryl species and more inert, long-lived analogues. The intermediate is characterized by Mössbauer spectroscopy (δ = 0.35 mm s−1, ΔEQ = 0.90 mm s−1, ΓFWHM = 0.30 mm s−1, relative area = 39.4%), EPR silence, a UV–vis absorption band at 428 nm, and cryogenic high-resolution ESI–MS (m/z 223.510, (N3S2)FeIV=O2+). Stopped-flow kinetic studies reveal saturation behavior that is well described by a pre-equilibrium oxidant-association model and subsequent O–O bond activation, with apparent activation parameters of ΔH‡ = 17.7 kJ mol−1 and ΔS‡ = −155 J mol−1 K−1, indicating a highly ordered transition state within the seven-coordinate N3S2 framework under the conditions employed. Functionally, the FeIV=O species mediates clean oxygen-atom transfer to triphenylphosphine (k2 = 8.1 × 10−2 M−1 s−1) with an effective turnover number of ~12 after correction for the FeIV=O yield, establishing that this mixed N/S platform is catalytically competent under mild conditions, though less reactive than state-of-the-art all-nitrogen systems. Collectively, these findings identify the seven-coordinate N3S2 macrocycle as a mixed-donor platform that moderately extends ferryl lifetime while retaining measurable oxo-transfer reactivity, providing mechanistic guidance for the development of nonheme iron oxidation catalysts that incorporate sulfur donors.