Dynamic Electronic Structure of the FeMoco Enables Mechanistic Coupling between N2 Binding and H2 Evolution in Nitrogenase

The mechanistic coupling of N2 binding and H2 evolution through reductive elimination from the E4 intermediate constitutes a critical step in the nitrogenase catalytic cycle. Despite extensive mechanistic investigations, how the Fe centers within the FeMoco cofactor dynamically reorganize their local electronic structures to facilitate this coupled process remains unresolved. Here, using density functional theory (DFT) combined with intrinsic atomic orbital/intrinsic bond orbital (IAO/IBO) and natural bond orbital (NBO) analyses, we uncover the electronic structure evolution of FeMoco along the coupled N2-binding/H2-evolution reaction coordinate. Comparative analyses of reaction energetics, with appropriate transition states, support an "intermediate" mechanism over purely "associative" or "dissociative" pathways, revealing a key intermediate E4(H2N2) in which N2 and H2 are simultaneously coordinated to the same Fe center. Detailed molecular-orbital analyses demonstrate that the coupled reaction is enabled by highly synchronized electronic reorganization across neighboring Fe centers, which dynamically redistribute electron density to stabilize concurrent H2 release and N2 activation. In particular, cooperative Fe-Fe electronic coupling allows the reactive Fe site to adapt its local bonding interactions. These findings provide an electronic-level rationale for the functional necessity of the multinuclear FeMoco architecture and reveal how orchestrated electronic flexibility within the cluster governs the critical mechanistic step of nitrogenase catalysis.