N–N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex

N–N
bond cleavage of hydrazines on transition metals is
of considerable importance in understanding the mechanism of biological
nitrogen fixation under ambient conditions. We found that a metal–ligand-bifunctional
complex of iron with a pincer-type ligand bearing two proton-responsive
pyrazole arms catalyzes the disproportionation of hydrazine into ammonia
and dinitrogen. The NH groups in the pyrazole ligands and hydrazines
are crucial for the reaction, which most likely occurs through multiple
and bidirectional proton-coupled electron transfer between the iron
complex and hydrazine. The multiproton-responsive pincer-type ligand
also stabilizes the intermediate diazene complex through a hydrogen-bonding
network, as revealed by structural characterization of a κ1N-phenylhydrazine complex.