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.

Figure 1

Spectroscopic data collected in situ on compound 5. (A) UV–visible absorbance spectra of 3, 2, and 5. Spectra were collected in 2-MeTHF at −80 °C. (B) Zero-field ⁵⁷Fe Mössbauer spectra of ⁵⁷Fe-enriched 5 as a 3 mM solution in 2-MeTHF prepared from 1 and collected at 80 K. The minor component (10%) was identified as complex 3 derived from competitive oxidation.

Figure 2

X-ray diffraction crystal structure of 5 and core-atom structures of 5′, 6, and 8 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms (excepting the N–H′s), the BAr^F₂₄ counteranion of 5, the triflate counteranion of 6, and cocrystallized solvent molecules have been removed for clarity. Refer to the Supporting Information for complete crystallographic details.

Figure 3

NMR spectra of 5′ recorded at −60 °C in 9:1 THF-d₈:CD₃CN. (A) ¹⁵N NMR spectrum of ¹⁵N-5′. (B) ³¹P{¹H} NMR spectrum of 5′. (C) Overlaid ¹H and ¹H{¹⁵N} spectra of ¹⁵N-5′. The central feature in the ¹H spectrum results from contamination of ¹⁵N-5′ with the natural abundance 5′.

Figure 4

(A) X-band EPR spectra of Fe═NNH₂ 7 and 7-d₂, derived from the in situ reduction of 5′ or 5′-d₂, respectively, with Cp*₂Co; Fe═NNMe₂ 8 and ¹⁵N-8 collected at 77 K in 2-MeTHF glasses. Signals derived from S = ¹/₂ Fe-N₂ 2 have been subtracted from the displayed spectra of 7 and 7-d₂ for clarity. (Inset) Prominent features of 8 that differ in ¹⁵N-8. These features arise from hyperfine coupling to single ³¹P and single ^14/15N nuclei of comparable magnitude. (B) ⁵⁷Fe Mössbauer spectra of in situ-prepared 7 and 8 obtained by subtracting out quadrupole doublet impurities from the raw data. A 50 mT magnetic field was applied (left) perpendicular and (right) parallel to the propagation of γ-beam. The solid lines are theoretical fits to an S = ¹/₂ spin Hamiltonian operating in the slow relaxation regime. Refer to the Supporting Information for a detailed discussion and the derived spin Hamiltonian parameters.

Scheme 1

Distal and Alternating Pathways for N₂ Reduction, and the Hybrid N₂ Reduction Pathway Emphasized Herein

Scheme 2

Functionalization of [SiP^iPr₃]Fe(N₂) Complexes

Scheme 3

Comparison of the Reaction Products Observed in the Reduction of (A) Fe═NNH₂⁺ 7 and (B) Mo═NNH₂ Supported by the Tri(amido)amine [HIPTN₃N]³⁻ Ligand Framework