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(LN4R)(NO)] (R = C6H4 or Ph for 3; and R = 4,5-Cl2C6H2 or PhCl for 4) and K[Fe(LN4R)(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 νNO at ~1700cm−1 in the IR and reversible diffusion-controlled cyclic voltammograms (CVs) (E1/2=~−1.20V 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 KC8 provided the {FeNO}8 compounds 5 and 6 in near quantitative yield, which were characterized by the shift in νNO to 1667 and ~1580cm−1, respectively. While the νNO for 6 is consistent with FeNO reduction, the νNO for 5 appears more indicative of ligand-based reduction. Additionally, 5 and 6 engage in HNO-like chemistry in their reactions with ferric porphyrins [FeIII(TPP)X] (TPP = tetraphenylporphyrin; X = Cl−, OTf− (trifluoromethanesulfonate anion or CF3SO3−)) to form [Fe(TPP)NO] in stoichiometric yield via reductive nitrosylation.