A mononuclear nonheme {FeNO}6 complex: synthesis and structural and spectroscopic characterization

Abstract

While the synthesis and characterization of {FeNO}^7,8,9 complexes have been well documented in heme and nonheme iron models, {FeNO}⁶ complexes have been less clearly understood. Herein, we report the synthesis and structural and spectroscopic characterization of mononuclear nonheme {FeNO}⁶ and iron(iii)-nitrito complexes bearing a tetraamido macrocyclic ligand (TAML), such as [(TAML)Fe^III(NO)]^- and [(TAML)Fe^III(NO₂)]^2-, respectively. First, direct addition of NO_(g) to [Fe^III(TAML)]^- results in the formation of [(TAML)Fe^III(NO)]^-, which is sensitive to moisture and air. The spectroscopic data of [(TAML)Fe^III(NO)]^-, such as ¹H nuclear magnetic resonance and X-ray absorption spectroscopies, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (S = 0). We also provide alternative pathways for the generation of [(TAML)Fe^III(NO)]^-, such as the iron-nitrite reduction triggered by protonation in the presence of ferrocene, which acts as an electron donor, and the photochemical iron-nitrite reduction. To the best of our knowledge, the present study reports the first photochemical nitrite reduction in nonheme iron models.

Scheme 1. Schematic illustration showing different pathways of nitrite reduction.

Fig. 1. X-ray crystal structures of (a) 1 and (b) 2 showing 50% probability ellipsoids. Cations such as tetraphenylphosphonium, tetrabutylammonium, hydrogen atoms and ether molecules are omitted for clarity. Colors for atoms: Fe, orange; N, blue; O, red; C, white (see Tables S1 and S2, ESI†).

Fig. 2. Zero-field Mössbauer spectrum (black opened circles) and fit (blue line) of 1 measured at 77 K. The isomer shift of –0.19 mm s^–1 and quadrupole splitting of 3.29 mm s^–1 are in good agreement with parameters previously reported for this complex in ref. 14 (also see Table S3, ESI†).

Fig. 3. (a) Fourier transform and EXAFS (inset) of 1 (blue line) with fits (black line). (b) XANES data of 1 (blue line) and [(PaPy₃)Fe^III(NO)]²⁺ (black line) for comparison. Inset shows the expansion of the pre-edge region.

Fig. 4. (a) UV-vis spectra of [Fe^III(TAML)]^– (0.10 mM, black line) and [(TAML)Fe^III(NO₂)]^2– (0.10 mM, red line) under an inert atmosphere in CH₃CN at –40 °C. Insets show the isotopic distribution patterns of the peaks at m/z of 495.0 for 2–¹⁴NO₂ (left panel) and m/z of 496.0 for 2–¹⁵NO₂ (right panel), which were generated by Na¹⁴NO₂ and Na¹⁵NO₂, respectively. (b) X-band EPR spectrum of 2 (1.0 mM) recorded in CH₃CN at 5 K.

Fig. 5. UV-vis spectral changes observed in the photoirradiation reaction of 2 (0.10 mM, red line) under an inert atmosphere in acetone at –40 °C.

Scheme 2. Schematic representation showing the generation of [(TAML)Fe^III(NO)]^–.