Structural, spectroscopic, and computational study of an octahedral, non-heme {Fe-NO}6-8 series:: [Fe(NO)(cyclam-ac)]2+/+/O

The reaction of low-spin [(FeCl)-Cl-II(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)-H-. (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, Mossbauer, 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 Mossbauer 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 NOdivided by, 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.