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

Abstract

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 N_2-to-NH_3 conversion (N_2RR), nor have M-N_xH_y complexes been derived from protonation of their M-N_2 precursors. To help delineate factors for N_2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N_2RR, and compare their activities with an isostructural Fe complex. The Os system is most active, and liberates more than 120 equiv NH_3 per Os center in a single batch experiment using Cp*_2Co and [H_2NPh_2][OTf] as reductant and acid source. Isostructural Ru and Fe complexes generate little NH_3 under the same conditions. Protonation of Os-N_2– affords a structurally characterized Os=NNH_2+ hydrazido species that mediates NH_3 generation, suggesting it is a plausible intermediate of the catalysis. Inactive Os hydrides are characterized that form during catalysis.