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(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 degrees for 3, which is atypical for low-spin square-pyramidal {FeNO}(7) species. Both 3 and 4 display nu(NO) at similar to 1700 cm(-1) in the IR and reversible diffusion-controlled cyclic voltammograms (CVs) (E-1/2 = similar to-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 nu(NO) to 1667 and -1580 cm(-1), respectively. While the nu(NO) for 6 is consistent with FeNO reduction, the nu(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. (C) 2012 Elsevier Inc. All rights reserved.