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Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes

Fajardo, Javier, Peters, Jonas C.
Journal of the American Chemical Society 2017 v.139 no.45 pp. 16105-16108
ammonia, catalytic activity, hydrides, iron, nitrogen, osmium, reducing agents, ruthenium
Despite the critical role Ru and Os complexes have played in the development of transition metal dinitrogen chemistry, they have not been shown to mediate catalytic N₂-to-NH₃ conversion (N₂RR), nor have M-NₓHy complexes been derived from protonation of their M-N₂ precursors. To help delineate factors for N₂RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N₂RR, and compare their activities with an isostructural Fe complex. The Os system is most active, and liberates more than 120 equiv NH₃ per Os center in a single batch experiment using Cp*₂Co and [H₂NPh₂][OTf] as reductant and acid source. Isostructural Ru and Fe complexes generate little NH₃ under the same conditions. Protonation of Os-N₂– affords a structurally characterized Os=NNH₂⁺ hydrazido species that mediates NH₃ generation, suggesting it is a plausible intermediate of the catalysis. Inactive Os hydrides are characterized that form during catalysis.