Toward Iron‐Catalyzed Alkene Metathesis: Mapping the Reactivity and Deactivation Pathways of an Iron Metallacyclobutane

Iron-catalyzed alkene metathesis holds great promise as a sustainable alternative to its precious metal congeners, yet its development has been hindered by poor mechanistic understanding and rapid catalyst deactivation. Here, we report the combined computational and experimental identification of β-hydride elimination as a key decomposition pathway from an iron metallacyclobutane, an essential intermediate in metathesis catalysis. Using our previously reported PC_NHCP-ligated iron(0) complex [(PC_NHCP)Fe(N₂)₂], we observe under metathesis conditions the formation of an iron(II) allyl hydride product, consistent with our computational predictions of a low-energy β-hydride elimination pathway. Detailed spin-state-resolved DFT analysis reveals that while metallacyclobutane formation is feasible across multiple spin surfaces, subsequent reactivity is strongly governed by the singlet state. Coordination of N₂ is shown to inhibit metathesis and promote decomposition by raising the transition-state barrier for cycloreversion while facilitating β-hydride elimination. Subsequent calculations show that upon suppressing this decomposition channel productive metathesis is restored. These findings offer mechanistically grounded design principles for next-generation iron-based metathesis catalysts and highlight the importance of spin-state control, ligand environment, and substrate selection in overcoming catalyst deactivation and provide a foray into productive iron catalyzed alkene metathesis.