H2 Formation Holds the Key to Opening the Fe Coordination Sites of Nitrogenase FeMo-cofactor for Dinitrogen Activation

The present quantum-mechanical and molecular-mechanics study reveals the crucial roles of H₂ formation, of H₂S shift and of N₂ bond expansion in the nitrogenase process of the reduction of N₂ to NH₃. Proton and electron transfers to the Fe(C@Fe₆S₉)Mo unit of the FeMo-co complex weaken the Fe-S and Fe-H bonds and expose the Fe coordination sites, coupled with energy release due to H₂ generation. Thereby the two sites Fe2 and Fe6 become prepared for stronger N₂ adsorption, expanding and attenuating the ǀN≡Nǀ bond. After subsequent detachment of H₂S from its Fe binding site into a holding site of the rearranged protein residue, the Fe6 site becomes completely unfolded, and the N₂ triple bond becomes completely activated to an ‑N=N- double bond for easy subsequent hydrogenation to NH₃. We explain in particular, why the obligatory H₂ formation is an essential step in N₂ adsorption and activation