A Tetranitrosyl [4Fe–4S] Cluster Forms En Route to Roussin’s Black Anion: Nitric Oxide Reactivity of [Fe₄S₄(LS₃)L′]^2–

Previous studies provide evidence that [4Fe–4S] clusters serve as targets of reactive nitrogen oxide species in biology. The products of this reaction range from dinitrosyliron complexes, [Fe­(NO)2L2]−, to Roussin’s black anion, [Fe4S3(NO)7]−. To date, the pathways by which these reactions occur have not been fully elucidated. In this study, we prepared the site-differentiated complexes [Fe4S4(LS3)­L′]2– (LS3 = 1,3,5-tris­(4,6-dimethyl-3-mercaptophenylthio)-2,4,6-tris­(p-tolylthio)­benzene; L′ = Cl, SEt, SPh, N3, 2-SPyr, Tp, S2CNEt2) to serve as synthetic models for biological [4Fe–4S] clusters and studied their reactivity toward NO­(g) and Ph3CSNO. The products were characterized by X-ray crystallography, mass spectrometry, and IR, electron paramagnetic resonance (EPR), and 1H NMR spectroscopy. In all cases reported here, the reactions proceed via formation of the S = 1/2 species [Fe4S4(NO)4]−, which ultimately converts to EPR-silent [Fe4S3(NO)7]−.