Successful Stabilization of the Elusive Species {FeNO}⁸ in a Heme Model

Nitroxyl (HNO/NO⁻) heme-adducts have been postulated as intermediates in a variety of catalytic processes carried out by different metalloenzymes. Hence, there is growing interest in obtaining and characterizing heme model nitroxyl complexes. The one-electron chemical reduction of the {FeNO}⁷ nitrosyl derivative of Fe^III(TFPPBr₈)Cl, Fe^II(TFPPBr₈)NO (1) (TFPPBr₈ = 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-[Tetrakis-(pentafluorophenyl)]porphyrin) with cobaltocene yields the significantly stable {FeNO}⁸ complex, [Co(C₅H₅)₂]⁺[Fe(TFPPBr₈)NO]⁻ (2). Complex 2 was isolated and characterized by UV−vis, FTIR, ¹H and ¹⁵N NMR spectroscopies. In addition, DFT calculations were performed to get more insight into the structure of 2. According to the spectroscopic and DFT results, we can state unequivocally that the surprisingly stable complex 2 is the elusive {FeNO}⁸ species. Both experimental and computational data allow to assign the electronic structure of 2 as intermediate between Fe^IINO⁻ and Fe^INO, which is contrasted with the predominant Fe^IINO⁻ character of known nonheme {FeNO}⁸ complexes. The enhanced stability achieved for a heme model {FeNO}⁸ is expected to allow further studies related to the reactivity of this elusive species.