Recasting Nitrogenase’s Carbide Role as a Beating Heart of Steel: A Joint Inorganic and Organic Perspective for µ6Carbide-Iron Bonding

Nitrogenase’s cofactor is distinguished by a trigonal prismatic interstitial carbide (μ6C4-), an architectural motif without parallel in other biological systems. The enzymatic significance of nitrogenase’s carbide remains unknown, although nitrogenase catalyzes a wide range of transformations that include being biological analogues of the industrial Haber-Bosch (N2-to-NH3) process. A 13C ENDOR study revealed negligible hyperfine coupling at the carbide, suggesting an inert geometric and electronic structure that preserves its trigonal prismatic framework and antiferromagnetic coupling of the coordinated sites (J. Am. Chem. Soc., 2022, 144, 18315-18328). In contrast to this “heart of steel” interpretation, the alternative “beating heart” model proposes a flexible, rather than inert, structure that stabilizes the cofactor during catalysis. Here, we establish the theoretical basis of the carbide’s bonding using valence bond (VB) and molecular orbital (MO) theory, showing that both approaches predict its preferential trigonal prismatic geometry and antiferromagnetic coupling of the coordinated Fe-centers. We assign the carbide in FeMoco’s resting state to six equivalent σ-bonds of half bond order, formed through sp2-hybridization, thereby reconciling perspectives from both inorganic and organic chemistry. Using broken-symmetry density functional theory (BS-DFT) with a quantum mechanics/molecular mechanics (QM/MM) model, we further examine FeMoco’s low-lying electronic configurations and show that the carbide retains its trigonal prismatic geometry, while its local σ/π bonding and hybridization adapt to the spin coupling of the Fe centers. Altogether, our findings suggest that the carbide imparts an inert geometric framework alongside a dynamic electronic structure that enables the catalytic reduction of diverse substrates.