Electronic Structure of Six-Coordinate Iron(III)-Porphyrin NO Adducts: The Elusive Iron(III)-NO(radical) State and Its Influence on the Properties of These Complexes

This paper investigates the interaction between five-coordinate ferric hemes with bound axial imidazole ligands and nitric oxide (NO). The corresponding model complex, [Fe(TPP)(MI)(NO)](BF4) (MI = 1-methylimidazole), is studied using vibrational spectroscopy coupled to normal coordinate analysis and density functional theory (DFT) calculations. In particular, nuclear resonance vibrational spectroscopy is used to identify the Fe-N(O) stretching vibration. The results reveal the usual Fe(II)-NO+ ground state for this complex, which is characterized by strong Fe-NO and N-O bonds, with Fe-NO and N-O force constants of 3.92 and 15.18 mdyn/angstrom, respectively. This is related to two strong pi back-bonds between Fe(II) and NO+. The alternative ground state, low-spin Fe(III)-NO(radical) (S = 0), is then investigated. DFT calculations show that this state exists as a stable minimum at a surprisingly low energy of only similar to 1-3 kcal/mol above the Fe(II)-NO+ ground state. In addition, the Fe(II)-NO+ potential energy surface (PES) crosses the low-spin Fe(Ill)-NO(radical) energy surface at a very small elongation (only 0.05-0.1 angstrom) of the Fe-NO bond from the equilibrium distance. This implies that ferric heme nitrosyls with the latter ground state might exist, particularly with axial thiolate (cysteinate) coordination as observed in P450-type enzymes. Importantly, the low-spin Fe(III)-NO(radical) state has very different properties than the Fe(II)-NO+ state. Specifically, the Fe-NO and N-O bonds are distinctively weaker, showing Fe-NO and N-O force constants of only 2.26 and 13.72 mdyn/angstrom, respectively. The PIES calculations further reveal that the thermodynamic weakness of the Fe-NO bond in ferric heme nitrosyls is an intrinsic feature that relates to the properties of the high-spin Fe(III)-NO(radical) (S = 2) state that appears at low energy and is dissociative with respect to the Fe-NO bond. Altogether, release of NO from a six-coordinate ferric heme nitrosyl requires the system to pass through at least three different electronic states, a process that is remarkably complex and also unprecedented for transition-metal nitrosyls. These findings have implications not only for heme nitrosyls but also for group-8 transition-metal (III) nitrosyls in general.