Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide–Dioxygen Interconversion

Through nitrosylation of [Fe–S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe­(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2 –). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2 – by a series of DNICs [(NO)2Fe­(μ-MePyr)2Fe­(NO)2] (1) and [(NO)2Fe­(μ-SEt)2Fe­(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)]­[Fe­(μ-MePyr)4(μ-O)2(Fe­(NO)2)4] (2-K-crown) and a [Fe3+(MePyr) x (NO2) y (O) z ] n adduct, stoichiometric NO monooxygenation yielding NO2 – occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe­(NO)2}9-{Fe­(NO)2}10 electronic structure, [K-18-crown-6-ether]­[(NO)2Fe­(μ-MePyr)2Fe­(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO – )2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether]­[(NO)2Fe­(μ-SEt)2Fe­(NO)2] (3-red) display a reversible equilibrium of “3 + O2 – ⇋ 3-red + O2(g)”, which is ascribed to the covalent [Fe­(μ-SEt)2Fe] core and redox-active [Fe­(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2 – and O2 during reaction with O2 – (or O2).