Mechanistic Study on Catalytic Disproportionation of Hydrazine by a Protic Pincer‐Type Iron Complex through Proton‐Coupled Electron Transfer

Density functional theory calculations have been performed for the proposal of a plausible reaction pathway for disproportionation of hydrazine catalyzed by an iron complex bearing a multiproton‐responsive pincer‐type bis(pyrazole) ligand. The pyrazole arms in this ligand are capable of serving as both Brønsted acid and base. At the first stage of the catalytic cycle, a hydrazine molecule bound to the iron center is converted into two molecules of ammonia by two successive protonation steps from the pyrazole NH groups. The deprotonated pyrazolate arms later two protons from another hydrazine molecule to afford an iron–diazene complex, which is a possible intermediate leading to formation of dinitrogen and ammonia. This bidirectional proton transfer (pyrazole arm ↔ hydrazine) is coupled with electron shuttling along a different pathway (iron center ↔ hydrazine). Overall energy profiles of the proposed mechanism calculated in different spin states elucidate the importance of the spin‐state flexibility of this iron complex. Proton–electron shuttling between a metal–ligand bifunctional iron complex and hydrazine, which enables catalytic disproportionation of hydrazine into dinitrogen and ammonia, was investigated by DFT calculations. The proton‐coupled electron transfer processes are associated with singlet–triplet interconversion.