The key finding is that a trisphosphine–borane Fe catalyst ([P3BFe][BArF4]) with a selectivity profile favouring NH3 in the dark shifts strongly towards N2H4 under blue light irradiation. Combining electrochemical and spectroscopic data, the authors propose a terminal hydrazido intermediate (P3BFe=NNH2) as selectivity-determining. The structure of the hydrazido complex of the catalyst is determined computationally to have a low-lying triplet excited state with increased spin at the proximal N atom (Nα). This causes partial bending of the hydrazido ligand and facilitates protonation at Nα. In the ground state, the intermediate is reduced and protonated through the established distal mechanism (protonation of the terminal N atom, Nβ) releasing NH3, while in the excited state it follows a distal-to-alternating hybrid pathway leading to N2H4. The rapid desorption of N2H4 makes clear that these are two diverging pathways rather than NH3 being produced by further reduction of N2H4. Increasing the electronic occupation of the excited state, and hence boosting the proportion of N2H4, can also be achieved through thermal excitation (at relatively higher temperatures) as well as electrochemical reduction. Of the attempted strategies, photoexcitation provides the most effective means to do so, with N2H4 even becoming the major product.
The crucial mechanistic understanding revealed in this study makes possible the further design of N2 reduction catalysts and reaction conditions that will maximize the desired product, whether that be NH3 or N2H4.
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