N(2) Binding to an Iron-Sulfur-Carbon Site

Nitrogenases are found in some microorganisms, and these enzymes convert atmospheric N(2) to ammonia, thereby providing essential nitrogen atoms for higher organisms. Some nitrogenases reduce atmospheric N(2) at the FeMoco, a sulfur-rich iron-molybdenum cluster(1–5). The iron centers that are coordinated to sulfur and carbon atoms in FeMoco have been proposed as the substrate binding sites, based on kinetic and spectroscopic studies(5,6). Studies on the enzyme indicate that iron atom Fe6 and possibly also adjacent belt iron sites are involved.(5–8) In the resting state, the central Fe sites (including Fe6) have identical environments consisting of three sulfides and a carbide. Addition of electrons to the resting state causes the FeMoco to react with N(2), but the geometry and bonding environment of N(2)-bound species remain unknown(5). In this manuscript, we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe-S bond and binds N(2). The product is the first synthetic Fe–N(2) complex in which iron has bonds to sulfur and carbon atoms, providing a model for N(2) coordination in the FeMoco. Our results demonstrate that breaking an Fe-S bond is a chemically reasonable route to N(2) binding in the FeMoco, and show structural and spectroscopic details for weakened N(2) on a sulfur-rich iron site.