Structural, Spectroscopic, and Computational Study of an Octahedral, Non-Heme {Fe−NO}^6-8 Series: [Fe(NO)(cyclam-ac)]^2+/+/0

The reaction of low-spin [FeIICl(cyclam-ac)] with NO in CH3CN yields the octahedral, non-heme (nitrosyl)iron complex [Fe(NO)(cyclam-ac)](PF6) (2), S = 1/2, which can be one-electron oxidized with (tris(4-bromophenyl)ammoniumyl)hexachloroantimonate to the complex [Fe(NO)(cyclam-ac)](PF6)2 (1'), S = 0, or complex [Fe(NO)(cyclam-ac)]Cl(ClO4).H2O (1); (cyclam-ac)- represents the pentadentate monoanion of 1,4,8,11-tetraazacyclotetradecane-1-acetic acid. Similarly, in CH3CN solution 2 can be electrochemically or chemically reduced by one-electron to the neutral complex [Fe(NO)(cyclam-ac)]0 (3), S = 0. 1-3 represent members of the [FeNO]6-8 series, respectively. The electronic structure of these three species have been spectroscopically elucidated by EPR, UV-vis, Mössbauer, and IR spectroscopy. The crystal structures of 1 and 2 have been determined by X-ray crystallography. In addition, detailed density functional calculations have been performed for all three species taking into account the possibility that 3 is actually a protonated HNO or NOH species. For all complexes structures, energetics, IR parameters, and Mössbauer parameters as well as EPR parameters (g-values and hyperfine couplings) have been calculated using state-of-the art DFT methods which are compared to experiment. The results establish unequivocally that 3 is indeed the elusive [FeNO]8 species. Furthermore a detailed picture of the bonding in the low-spin non-heme iron nitrosyl series [FeNO]6, [FeNO]7, and [FeNO]8 has been developed. This allows a description of 1 as a low-spin ferrous complex containing an N-coordinated NO+, whereas 2 is a low-spin ferrous species with a NO* ligand and 3 is a low-spin ferrous complex with a coordinated NO-. On the basis of this description, all spectroscopic and geometric observables find a satisfactory interpretation.