Synthesis, properties, and reactivity of a series of non-heme {FeNO} ^7/8 complexes: Implications for Fe-nitroxyl coordination

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(LN₄^R)(NO)] (R = C₆H₄ or Ph for 3; and R = 4,5-Cl₂C₆H₂ or PhCl for 4) and K[Fe(LN₄^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 KC₈ 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 CF₃SO₃^-)) to form [Fe(TPP)NO] in stoichiometric yield via reductive nitrosylation.