A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant

Abstract

A new nonheme iron(II) complex, Fe^II (Me₃ TACN)((OSi^Ph2 )₂ O) (1), is reported. Reaction of 1 with NO_(g) gives a stable mononitrosyl complex Fe(NO)(Me₃ TACN)((OSi^Ph2 )₂ O) (2), which was characterized by Mössbauer (δ=0.52 mm s^-1 , |ΔE_Q |=0.80 mm s^-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}⁷ complex with an S=3/2 spin ground state. The RR spectrum (λ_exc =458 nm) of 2 combined with isotopic labeling (¹⁵ N, ¹⁸ O) reveals ν(N-O)=1680 cm^-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm^-1 ). Complex 2 reacts rapidly with H₂ O in THF to produce the N-N coupled product N₂ O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N₂ O in the absence of an exogenous reductant.

Keywords: iron; nitric oxide; nitrous oxide; nonheme; reduction.

Figure 1.

Plausible mechanisms for N₂O formation by FDPnor.

Scheme 1.

Syntheses of Complexes 1 and 2

Figure 2.

(Left) Displacement ellipsoid plot (50% probability level) for 1 at 110(2) K. H atoms are omitted for clarity. Selected bond distances (Å): Fe1–O1, 1.9402(16), Fe1–O2, 1.9458(13), Fe1–N1, 2.265(6), Fe1–N2, 2.280(6), Fe1–N3, 2.212(6). (Top-right) Mössbauer spectrum of ⁵⁷Fe-1 (80 K). Data: black circles; best fit: red line.

Figure 3.

UV-vis spectra for 1 (1.4 mM) (black line) after addition of NO(g) to form 2 (green line) in THF. Inset: EPR spectrum of 2 in 2-MeTHF recorded at 8 K, 9.21 GHz.

Figure 4.

⁵⁷Fe Mössbauer spectrum (80 K in frozen THF) of 2. Experimental data are plotted as black dots and best fits are overlaid as solid lines. The major species (δ = 0.52 mm s⁻¹, |ΔE_Q| = 0.80 mm s⁻¹, 90 %) is shown as a green line. A second broad sub-component with δ = 0.6 mm s⁻¹, |ΔE_Q|= 1.00 mm s⁻¹, Г_R= Г_L = 6.0 (blue line) was added to represent the intermediate relaxation of the doublet at 80 K.

Figure 5.

Low-temperature RR spectra of 2-NO (black), 2-¹⁵NO (red), and 2-¹⁵N¹⁸O (blue) in THF obtained with a 458-nm laser excitation. The difference spectra are shown in green.

Figure 6.

Fe K-edge X-ray absorption near edge (XANES) spectra of [Fe^II(Me₃TACN)((OSi^Ph2)₂O)] (1, black line) and [Fe(NO)(Me₃TACN)((OSi^Ph2)₂O)] (2, red line).

Figure 7.

TD-DFT simulated UV-vis absorption spectrum for 2 (dashed purple line) overlaid with the experimental spectrum (red line). The computed spectral excitations are shown as black sticks. Electron difference density map for the computed transition (λ_max = 447 nm) is shown with the green and red regions indicating gain and loss of electron density respectively.

Scheme 2.

N₂O formation from 2.