Ferric Heme-Nitrosyl Complexes: Kinetically Robust or Unstable Intermediates?

We have determined a convenient method for the bulk synthesis of high-purity ferric heme-nitrosyl complexes ({FeNO}6 in the Enemark-Feltham notation); this method is based on the chemical or electrochemical oxidation of corresponding {FeNO}7 precursors. We used this method to obtain the five- and six-coordinate complexes [Fe(TPP)(NO)]+ (TPP2- = tetraphenylporphyrin dianion) and [Fe(TPP)(NO)(MI)]+ (MI = 1-methylimidazole) and demonstrate that these complexes are stable in solution in the absence of excess NO gas. This is in stark contrast to the often-cited instability of such {FeNO}6 model complexes in the literature, which is likely due to the common presence of halide impurities (although other impurities could certainly also play a role). This is avoided in our approach for the synthesis of {FeNO}6 complexes via oxidation of pure {FeNO}7 precursors. On the basis of these results, {FeNO}6 complexes in proteins do not show an increased stability toward NO loss compared to model complexes. We also prepared the halide-coordinated complexes [Fe(TPP)(NO)(X)] (X = Cl-, Br-), which correspond to the elusive, key reactive intermediate in the so-called autoreduction reaction, which is frequently used to prepare {FeNO}7 complexes from ferric precursors. All of the complexes were characterized using X-ray crystallography, UV-vis, IR, and nuclear resonance vibrational spectroscopy (NRVS). On the basis of the vibrational data, further insight into the electronic structure of these {FeNO}6 complexes, in particular with respect to the role of the axial ligand trans to NO, is obtained.