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

Abstract

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-NH_2NH₂⁺ 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.