Nitrogen Fixation via a Terminal Fe(IV) Nitride

Abstract

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N₂. Here we show that a nitrogen-fixing Fe-N₂ catalyst can be protonated to form a neutral Fe(NNH₂) hydrazido(2-) intermediate, which, upon further protonation, heterolytically cleaves the N-N bond to release [Fe^IV≡N]⁺ and NH₃. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N₂-to-NH₃ conversion. The physical oxidation state range of the Fe complexes in this transformation is buffered by covalency with the ligand, a feature of possible relevance to catalyst design in synthetic and natural systems that facilitate multiproton/multielectron redox processes.

Figure 1

(A–D) Collected Mössbauer data; raw data are shown as circles, and the simulated data as a solid black line with individual subspectra plotted in gray, red, and blue. (A) Spectrum of [(P₃^B)Fe(N₂)]²⁻ prepared in situ from [(P₃^B)Fe(N₂)]⁻. (B) Spectrum of (P₃^B)Fe(NNMe₂). (C, D) Freeze-quench Mössbauer spectra from the reaction of [(P₃^B)Fe(N₂)]²⁻ with excess TfOH, showing conversion to (P₃^B)Fe(NNH₄) (red subspectrum, ~90%) after mixing for 15 min (C), and subsequent formation of [(P₃^B)Fe≡N]⁺ (blue subspectrum, ~60%) after mixing for 120 min (D). Collected Mössbauer and XAS characterization data for these species are given in tables E and F.

Figure 2

Collected XAS data. (A) Pre-edge regions of the XANES spectra of [(P₃^B)Fe(N₂)]²⁻ (black), (P₃^B)Fe(NNH₂) (red), (P3^B)Fe(NNMe₂) (red circles), and [(P₃^B)Fe≡N]⁺ (blue:). (B) TD-DFT-predicted spectra of [(P₃^B)Fe(N₂)]²⁻ (black), (P₃^B)Fe(NNH₂) (red), and [(P₃^B)Fe≡N]⁺ (blue). (C–E) Phase-uncorrected EXAFS data for (C) (P₃^B)Fe(NNMe₂), (D) (P₃^B)Fe(NNH₂), and (E) [(P₃^B)Fe≡N]⁺. The data are plotted in black, with simulations in blue.

Figure 3

(A) Frontier Kohn—Sham orbitals computed for [(P₃^B)Fe≡N]⁺. (B) Löewdin population analysis of the empty 1a₁ frontier orbital of [(P₃^B)Fe≡N]⁺. (C) Geometric analysis of the bonding in [(P₃^B)Fe(N₂)]²⁻, (P₃^B)Fe(NNH₂), and [(P₃^B)Fe≡N]⁺. The Fe—B distances from DFT models are given, along with the average Fe—P bond length from the EXAFS data. Shown in black are the Mayer bond orders (the average in the case of Fe—P).

Scheme 1