Nitrogen Fixation via a Terminal Fe(IV) Nitride

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N_2. Here we show that a nitrogen-fixing Fe–N_2 catalyst can be protonated to form a neutral Fe(NNH_2) hydrazido(2−) intermediate, which, upon further protonation, heterolytically cleaves the N–N bond to release [Fe^(IV)≡N]^+ and NH_3. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N_2-to-NH_3 conversion. The physical oxidation state range of the Fe complexes in this transformation is buffered by covalency with the ligand, a feature of possible relevance to catalyst design in synthetic and natural systems that facilitate multiproton/multielectron redox processes.