Catalytic conversion of dinitrogen (N₂) to ammonia (NH₃) with side-on bound molecular complexes and associated mechanistic studies of their stepwise reduction are relevant to gaining a better understanding of the mechanism of both the industrial (Haber-Bosch) and the biological (nitrogenase) processes, as molecular catalytic N₂ to NH₃ conversion currently exclusively involves reduction and protonation of terminal or bridging end-on bound metal-N₂ complexes. In that regard, low-valent multimetallic complexes of uranium are of great interest due to their ability to bind and transform chemically inert small molecules such as dinitrogen^[1a], however, advanced synthetic techniques required to access and to stabilise these species renders their isolation challenging. Recently, we reported a rare triply reduced N₂^3– moiety reported by stabilisation in a diuranium aryloxide complex, which upon further reduction results in dinitrogen cleavage and gives the U₄N₄ cubane and the U₆N₆ edge-shared cubane clusters – unprecedented high nitrogen-content species and molecular models for the binary uranium nitride UN_x that produce quantitative ammonia upon protonation with acid.^[1b]

More recently, here, using a uranium triamidoamine complex [{U^IV(Tren^DMBS)}₂(μ-η²:η²-N₂)] (Tren^DMBS = {N(CH₂CH₂NSiMe₂^tBu)₃}^3–), catalytic N₂ to NH₃ conversion involving side-on bound N₂ binding was identified for the first time.^[2] Stoichiometric reactions reveal stepwise reduction of N₂ from free-N₂ to bridging side-on bound forms and subsequently to bridging nitrides, uniquely accessing four different states of side-on bound N₂ for the same molecular system. This reveals the roles of N₂, N₂^2–, N₂^3–, N₂^4–, and N^3– in the catalytic conversion of N₂ to NH₃ when involving side-on bridging N₂. The recognition that side-on bound N₂ can be catalytically activated and converted into NH₃ through polymetallic cooperativity provides conceptual homogeneous-heterogeneous links, to e.g. Haber-Bosch processes, and offers new vistas for the elaboration of molecular catalytic transformations involving N₂ and likely other small molecules.

[1] a) N. Jori, L. Barluzzi, I. Douair, L. Maron, F. Fadaei-Tirani, I. Zivkovic and M. Mazzanti; J. Am. Chem. Soc., 2021, 143, 29, 11225–11234; b) M. S. Batov, I. del Rosal, R. Scopelliti, F. Fadei-Tirani, I. Zivkovic, L. Maron and M. Mazzanti, J. Am. Chem. Soc. 2023, 145, 48, 26435–26443;

[2] M. S. Batov, H. T. Partlow, L. Chatelain, J. A. Seed, R. Scopelliti, I. Zivkovic, R. W. Adams, S. T. Liddle and M. Mazzanti, Nat. Chem. 2025, accepted, in press