Molybdenum-Iron-Sulfur Cluster with a Bridging Carbide Ligand as a Partial FeMoco Model: CO Activation, EPR Studies,...

Nitrogenase enzymes catalyze the reduction of N₂ to NH₃ at a complex Fe-M (M = Mo, Fe, or V) cofactor (FeMco), which displays eight metal centers and sulfide and carbide bridges with a MFe₇S₈C composition. The role of the unusual μ₆-carbide ligand and its effects on the metal centers remain unclear. Here, we describe the transfer of a carbide ligand to a MoFe₃S₃ cluster supported by a bisphenoxide ligand from a previously reported terminal Mo carbide complex to yield a pentametallic cluster of MoS₃Fe₃CMo composition, which also displays a bridging CO that resembles the lo-CO form of nitrogenase. This cluster has an S = 1/2 spin state amenable to studies by pulse EPR spectroscopy, revealing a significantly larger carbide ¹³C hyperfine interaction (a_iso(¹³C) = 12.5 MHz) than any observed for various states of FeMoco studied by EPR thus far (|a_iso(¹³C)| = 0.89 to 2.7 MHz). This report provides a strategy for the synthesis of carbide-containing iron-sulfur clusters relevant to nitrogenase cluster modeling, as well as benchmarking information for the metal-carbon interactions by EPR methods.