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 (O 2 - ). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O 2 - by a series of DNICs [(NO) 2 Fe(μ- Me Pyr) 2 Fe(NO) 2 ] ( 1 ) and [(NO) 2 Fe(μ-SEt) 2 Fe(NO) 2 ] ( 3 ). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether) 2 (NO 2 )][Fe(μ- Me Pyr) 4 (μ-O) 2 (Fe(NO) 2 ) 4 ] ( 2-K-crown ) and a [Fe 3+ ( Me Pyr) x (NO 2 ) y (O) z ] n adduct, stoichiometric NO monooxygenation yielding NO 2 - occurs without the transient formation of peroxynitrite-derived • OH/ • NO 2 species. To study the isoelectronic reaction of O 2(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) 2 Fe(μ- Me Pyr) 2 Fe(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 {Fe III (NO - ) 2 } 9 units and S = 5/2 Fe 3+ center. In contrast to DNICs 1 and 1-red , DNICs 3 and [K-18-crown-6-ether][(NO) 2 Fe(μ-SEt) 2 Fe(NO) 2 ] ( 3-red ) display a reversible equilibrium of " 3 + O 2 - ⇋ 3-red + O 2(g) ", which is ascribed to the covalent [Fe(μ-SEt) 2 Fe] 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 O 2 - and O 2 during reaction with O 2 - (or O 2 ).