Nitrate-to-Nitrite-to-Nitric Oxide Conversion Modulated by Nitrate-Containing {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC)

Nitrosylation of high-spin [Fe­(κ2-O2NO)4]2– (1) yields {Fe­(NO)}7 mononitrosyl iron complex (MNIC) [(κ2-O2NO)­(κ1-ONO2)3Fe­(NO)]2– (2) displaying an S = 3/2 axial electron paramagnetic resonance (EPR) spectrum (g⊥ = 3.988 and g∥ = 2.000). The thermally unstable nitrate-containing {Fe­(NO)2}9 dinitrosyl iron complex (DNIC) [(κ1-ONO2)2Fe­(NO)2]– (3) was exclusively obtained from reaction of HNO3 and [(OAc)2Fe­(NO)2]– and was characterized by IR, UV–vis, EPR, superconducting quantum interference device (SQUID), X-ray absorption spectroscopy (XAS), and single-crystal X-ray diffraction (XRD). In contrast to {Fe­(NO)2}9 DNIC [(ONO)2Fe­(NO)2]– constructed by two monodentate O-bound nitrito ligands, the weak interaction between Fe(1) and the distal oxygens O(5)/O(7) of nitrato-coordinated ligands (Fe(1)···O(5) and Fe(1)···O(7) distances of 2.582(2) and 2.583(2) Å, respectively) may play important roles in stabilizing DNIC 3. Transformation of nitrate-containing DNIC 3 into N-bound nitro {Fe­(NO)}6 [(NO)­(κ1-NO2)­Fe­(S2CNEt2)2] (7) triggered by bis­(diethylthiocarbamoyl) disulfide ((S2CNEt2)2) implicates that nitrate-to-nitrite conversion may occur via the intramolecular association of the coordinated nitrate and the adjacent polarized NO-coordinate ligand (nitrosonium) of the proposed {Fe­(NO)2}7 intermediate [(NO)2(κ1-ONO2)­Fe­(S2CNEt2)2] (A) yielding {Fe­(NO)}7 [(NO)­Fe­(S2CNEt2)2] (6) along with the release of N2O4 (·NO2) and the subsequent binding of ·NO2 to complex 6. The N-bound nitro {Fe­(NO)}6 complex 7 undergoes Me2S-promoted O-atom transfer facilitated by imidazole to give {Fe­(NO)}7 complex 6 accompanied by release of nitric oxide. This result demonstrates that nitrate-containing DNIC 3 acts as an active center to modulate nitrate-to-nitrite-to-nitric oxide conversion.