The Elusive Mononitrosylated [Fe 4 S 4 ] Cluster in Three Redox States.

Iron-sulfur clusters are well-established targets in biological nitric oxide (NO) chemistry, but the key intermediate in these processes-a mononitrosylated [Fe 4 S 4 ] 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 [Fe 4 S 4 ] clusters. Mononitrosylation was achieved by binding NO to a 3 : 1 site-differentiated [Fe 4 S 4 ] + cluster; subsequent oxidation and reduction afforded the other members of the series. All three clusters feature a local high-spin Fe 3+ 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.