Synthesis, properties, and reactivity of a series of non-heme {FeNO}(7/8) complexes: implications for Fe-nitroxyl coordination.

Abstract The biochemical properties of nitroxyl (HNO/NO − ) are distinct from nitric oxide (NO). Metal centers, particularly Fe, appear as suitable sites of HNO activity, both for generation and targeting. Furthermore, reduced Fe–NO − /Fe–HNO or {FeNO} 8 (Enemark–Feltham notation) species offer unique bonding profiles that are of fundamental importance. Given the unique chemical properties of {FeNO} 8 systems, we describe herein the synthesis and properties of {FeNO} 7 and {FeNO} 8 non-heme complexes containing pyrrole donors that display heme-like properties, namely [Fe(LN 4 R )(NO)] (R = C 6 H 4 or Ph for 3 ; and R = 4,5-Cl 2 C 6 H 2 or PhCl for 4 ) and K[Fe(LN 4 R )(NO)] (R = Ph for 5 ; R = PhCl for 6 ). X-ray crystallography establishes that the Fe–N–O angle is ~ 155° for 3 , which is atypical for low-spin square-pyramidal {FeNO} 7 species. Both 3 and 4 display ν NO at ~ 1700 cm − 1 in the IR and reversible diffusion-controlled cyclic voltammograms (CVs) (E 1/2  = ~− 1.20 V vs. Fc/Fc + (ferrocene/ferrocenium redox couple) in MeCN) suggesting that the {FeNO} 8 compounds 5 and 6 are stable on the CV timescale. Reduction of 3 and 4 with stoichiometric KC 8 provided the {FeNO} 8 compounds 5 and 6 in near quantitative yield, which were characterized by the shift in ν NO to 1667 and ~ 1580 cm − 1 , respectively. While the ν NO for 6 is consistent with FeNO reduction, the ν NO for 5 appears more indicative of ligand-based reduction. Additionally, 5 and 6 engage in HNO-like chemistry in their reactions with ferric porphyrins [Fe III (TPP)X] (TPP = tetraphenylporphyrin; X = Cl − , OTf − (trifluoromethanesulfonate anion or CF 3 SO 3 − )) to form [Fe(TPP)NO] in stoichiometric yield via reductive nitrosylation.