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

Biological N-2 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 N-2 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 N-2, 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-N-2 complex. The Fe=NNH2+ complex is redoxactive, 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 N-2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N-2 reduction may be operative in biological N-2 fixation.