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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands. [ABSTRACT FROM AUTHOR]

Biological [Fe-S] clusters are increasingly recognized to undergo proton-coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate-ligated [4Fe-4S] cluster [Fe4S4(SAr)4]2− ( 1, SAr=S-2,4-6-( iPr)3C6H2) leads to thiol dissociation, reversibly forming [Fe4S4(SAr)3L]1− ( 2) and ArSH (L=solvent, and/or conjugate base). Solutions of 2+ArSH react with the nitroxyl radical TEMPO to give [Fe4S4(SAr)4]1− ( 1ox) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe4S4(SAr)3(HSAr)]1− ( 1-H). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states. [ABSTRACT FROM AUTHOR]