The Mechanism of N–N Double Bond Cleavage by an Iron(II) Hydride Complex

The use of hydride species for substrate reductions avoids strong reductants, and may enable nitrogenase to reduce multiple bonds without unreasonably low redox potentials. In this work, we explore the NN bond cleaving ability of a high-spin iron­(II) hydride dimer with concomitant release of H₂. Specifically, this diiron­(II) complex reacts with azobenzene (PhNNPh) to perform a four-electron reduction, where two electrons come from H₂ reductive elimination and the other two come from iron oxidation. The rate law of the H₂ releasing reaction indicates that diazene binding occurs prior to H₂ elimination, and the negative entropy of activation and inverse kinetic isotope effect indicate that H–H bond formation is the rate-limiting step. Thus, substrate binding causes reductive elimination of H₂ that formally reduces the metals, and the metals use the additional two electrons to cleave the N–N multiple bond.