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Abstract

Iron–sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron–sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron–sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH₂ bound to an iron–sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron–sulfur clusters like the FeMoco of nitrogenase.