| Abstract
| - In a previous study, we analyzed the electronic structure of S = 3/2 {FeNO}7 model complexes[Brown et al. J. Am. Chem. Soc. 1995, 117, 715−732]. The combined spectroscopic data and SCF-Xα-SW electronic structure calculations are best described in terms of FeIII (S = 5/2) antiferromagneticallycoupled to NO- (S = 1). Many nitrosyl derivatives of non-heme iron enzymes have spectroscopic propertiessimilar to those of these model complexes. These NO derivatives can serve as stable analogues of highlylabile oxygen intermediates. It is thus essential to establish a reliable density functional theory (DFT)methodology for the geometry and energetics of {FeNO}7 complexes, based on detailed experimentaldata. This methodology can then be extended to the study of {FeO2}8 complexes, followed by investigationsinto the reaction mechanisms of non-heme iron enzymes. Here, we have used the model complex Fe(Me3TACN)(NO)(N3)2 as an experimental marker and determined that a pure density functional BP86 with10% hybrid character and a mixed triple-ζ/double-ζ basis set lead to agreement between experimentaland computational data. This methodology is then applied to optimize the hypothetical Fe(Me3TACN)(O2)(N3)2 complex, where the NO moiety is replaced by O2. The main geometric differences are an elongatedFe−O2 bond and a steeper Fe−O−O angle in the {FeO2}8 complex. The electronic structure of {FeO2}8corresponds to FeIII (S = 5/2) antiferromagnetically coupled to O2- (S = 1/2), and, consistent with the extendedbond length, the {FeO2}8 unit has only one FeIII−O2- bonding interaction, while the {FeNO}7 unit has bothσ and π type FeIII−NO- bonds. This is in agreement with experiment as NO forms a more stable FeIII−NO- adduct relative to O2-. Although NO is, in fact, harder to reduce, the resultant NO- species forms amore stable bond to FeIII relative to O2- due to the different bonding interactions.
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