title: 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-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.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech) , Pasadena, California 91125, United States.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Rittle
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech) , Pasadena, California 91125, United States.
authors.forename: Jonas C
authors.initials: JC
authors.lastname: Peters
mesh_terms: D000641:Ammonia / Q000138:chemical synthesis / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D006834:Hydrazines / Q000138:chemical synthesis / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D056831:Coordination Complexes; D006834:Hydrazines; C029424:hydrazine; D000641:Ammonia; D007501:Iron; D009584:Nitrogen