An Fe-N₂ Complex That Generates Hydrazine and Ammonia via Fe═NNH₂: Demonstrating a Hybrid Distal-to-Alternating Pathway for N₂ Reduction.

Biological N2 fixation to NH3 may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e(-)/H(+) transfer steps of iron-ligated N2 in well-defined synthetic systems is hence of much interest but remains a significant challenge. While iron complexes have been recently discovered that catalyze the formation of NH3 from N2, mechanistic details remain uncertain. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate Fe═NNH2(+) species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N2(-) complex. The Fe═NNH2(+) complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = (1)/2 Fe═NNH2. At warmer temperatures, Fe═NNH2 rapidly converts to an iron hydrazine complex, Fe-NH2NH2(+), via the additional transfer of proton and electron equivalents in solution. Fe-NH2NH2(+) can liberate NH3, and the sequence of reactions described here hence demonstrates that an iron site can shuttle from a distal intermediate (Fe═NNH2(+)) to an alternating intermediate (Fe-NH2NH2(+)) en route to NH3 liberation from N2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N2 reduction may be operative in biological N2 fixation.