Synthetic models of the nitrogenase FeMo cofactor

The FeMo cofactor (FeMoco), the key active site in the Mo-based nitrogenase, is one of the most complicated metalloenzyme molecules. Synthesis of the FeMoco model cluster ([MoFe₇S₉C]) is essential to understanding its function in dinitrogen binding, activation, and conversion. However, the complex framework of the FeMoco cluster, which features a unique trigonal prismatic [Fe₆C] moiety comprising a µ₆-bridging carbide, has made the synthesis of the cluster a persistent challenge. In this work, two analogous mimics of FeMoco have been synthesized, using a cluster-coupling synthetic strategy facilitated by the fabrication of unsaturated ligand/metal coordination. The incorporation of a µ₆-X (X = C^4- or N^3-) to construct the characteristic triangular prismatic [Fe₆(µ₆-X)] moiety, replicating that in FeMoco, has been achieved synthetically. The two mimics have similar key structural parameters to FeMoco in natural nitrogenase, but differ from the FeMoco structure in two major aspects: the µ₂-bridging ligands and the metal atoms capping the [Fe₆S₉C] cores (Mo/Fe in FeMoco vs. Mo/Mo or W/W in the synthetic models). Quantum chemical studies indicate that the electronic ground states of these clusters resemble those observed for FeMoco, with maximized antiferromagnetic coupling among the iron centers. A future systematic study on the physical and chemical properties of a family of mimics with programmed variations of key structural elements can provide a valuable comparison and facilitate a better understanding of the structure and function of FeMoco.