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 N2. Here we show that a nitrogen-fixing Fe–N2 catalyst can be protonated to form a neutral Fe­(NNH2) hydrazido(2−) intermediate, which, upon further protonation,
heterolytically cleaves the N–N bond to release [FeIVN]+ and NH3. These observations provide
direct evidence for the viability of a Chatt-type (distal) mechanism
for Fe-mediated N2-to-NH3 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.