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

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.