Alkyl-ligated iron–sulfur clusters in the [Fe4S4]3+ charge state have been proposed as short-lived intermediates in a number of enzymatic reactions. To better understand the properties of these intermediates, we have prepared and characterized the first synthetic [Fe4S4]3+–alkyl cluster. Isolation of this highly reactive species was made possible by the development of an expanded scorpionate ligand suited to the encapsulation of cuboidal clusters. Like the proposed enzymatic intermediates, this synthetic [Fe4S4]3+–alkyl cluster adopts an S = 1/2 ground state with g iso > 2. Mössbauer spectroscopic studies reveal that the alkylated Fe has an unusually low isomer shift, which reflects the highly covalent Fe–C bond and the localization of Fe3+ at the alkylated site in the solid state. Paramagnetic 1H NMR studies establish that this valence localization persists in solution at physiologically relevant temperatures, an effect that has not been observed for [Fe4S4]3+ clusters outside of a protein. These findings establish the unusual electronic-structure effects imparted by the strong-field alkyl ligand and lay the foundation for understanding the electronic structures of [Fe4S4]3+–alkyl intermediates in biology.

We report the synthesis and characterization of the first terminal imido complex of an Fe-S cluster, (IMes) 3 Fe 4 S 4 = NDipp (2; IMes = 1,3-dimesitylimidazol-2-ylidene, Dipp = 2,6-diisopropylphenyl), which is generated by oxidative group transfer from DippN 3 to the all-ferrous cluster ( IMes) 3 Fe 4 S 4 (PPh 3 ). This two-electron process is achieved by formal one-electron oxidation of the imido-bound Fe site and one-electron oxidation of two IMes-bound Fe sites. Structural, spectroscopic, and computational studies establish that the Feimido site is best described as a high-spin Fe 3+ center, which is manifested in its long FeÀN(imido) distance of 1.763(2) . Cluster 2 abstracts hydrogen atoms from 1,4-cyclohexadiene to yield the corresponding anilido complex, demonstrating competency for CÀH activation.Scheme 1. Synthesis of (IMes) 3 Fe 4 S 4 =NDipp (2). Abbreviations: Mes = 2,4,6-trimethylphenyl; Ar = 3,5-dimethylphenyl; Dipp = 2,6-diisopropylphenyl.

Iron‐sulfur clusters are well‐established targets in biological nitric oxide (NO) chemistry, but the key intermediate in these processes—a mononitrosylated [Fe4S4] cluster—has not been fully characterized in a protein or a synthetic model thereof. Here, we report the synthesis of a three‐member redox series of isostructural mononitrosylated [Fe4S4] clusters. Mononitrosylation was achieved by binding NO to a 3 : 1 site‐differentiated [Fe4S4]+ cluster; subsequent oxidation and reduction afforded the other members of the series. All three clusters feature a local high‐spin Fe3+ center antiferromagnetically coupled to 3[NO]−. The observation of an anionic NO ligand suggests that NO binding is accompanied by formal electron transfer from the cluster to NO. Preliminary reactivity studies with the monocationic cluster demonstrate that exposure to excess NO degrades the cluster, supporting the intermediacy of mononitrosylated intermediates in NO sensing/signaling.

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We report the synthesis and characterization of the first terminal imido complex of an Fe-S cluster, (IMes) 3 Fe 4 S 4 = NDipp (2; IMes = 1,3-dimesitylimidazol-2-ylidene, Dipp = 2,6-diisopropylphenyl), which is generated by oxidative group transfer from DippN 3 to the all-ferrous cluster ( IMes) 3 Fe 4 S 4 (PPh 3 ). This two-electron process is achieved by formal one-electron oxidation of the imido-bound Fe site and one-electron oxidation of two IMes-bound Fe sites. Structural, spectroscopic, and computational studies establish that the Feimido site is best described as a high-spin Fe 3+ center, which is manifested in its long FeÀN(imido) distance of 1.763(2) . Cluster 2 abstracts hydrogen atoms from 1,4-cyclohexadiene to yield the corresponding anilido complex, demonstrating competency for CÀH activation.Scheme 1. Synthesis of (IMes) 3 Fe 4 S 4 =NDipp (2). Abbreviations: Mes = 2,4,6-trimethylphenyl; Ar = 3,5-dimethylphenyl; Dipp = 2,6-diisopropylphenyl.