Mossbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Biomimetric Iron(IV)=Oxygen Species and High-Valent Diamond Core Intermediates

Mononuclear high-valent FeIV=O intermediates have recently been identified as the active oxidizing species in dioxygen activation and oxygen atom transfer reactions carried out by several nonheme Fe-dependent enzymes. Biomimetic efforts to synthesize the FeIV=O unit have successfully stabilized a variety of high-valent model complexes using assorted tetradentate and pentadentate N/S ligands. One of the most well characterized FeIV=O complexes, [FeIV(O)(TMC)(X)] 2+, has been used to generate several (S = 1) oxoiron(IV) species by substituting various ligands trans to the oxo group. Mossbauer spectroscopy, electronic absorption, Fe K-edge X-ray absorption, and resonance Raman evidence along with in depth density functional theory (DFT) studies reveal how the donor properties of the trans ligand modulate spectroscopic features and reactivity of the FeIV=O unit. Investigations into the mechanism of sulfur oxidation for the related Fe-(TMC-S) complex, which has a thiolate ligand similar to that of cytochrome P450, suggest that in the absence of base, addition of increasing equivalents of peracid to the FeII(TMC-S) starting material leads to the formation of a novel oxoiron(IV) sulfinate as confirmed by Mossbauer spectroscopy.;Additional efforts have been made to model structural and/or functional features of nonheme [Fe2(mu-O)2] diamond cores as observed in methane monooxygenase (MMO) and ribonucleotide reductase (RNR), both proposed to involve high-valent intermediates. In the work presented herein, spectroscopic characterization of the first synthetic complex with an [FeIV2(mu-O)2] core is reported, providing important details regarding the structure and oxidative behavior of MMO Intermediate Q. Further experiments have successfully isolated open-core variants including a [X-FeIII-O-Fe IV=O]+3 complex (X = -OH, -F) exhibiting a 1,000,000 fold increase in reactivity compared to the diamond core precursors. Detailed Mossbauer, electron paramagnetic resonance (EPR), and DFT studies reveal an S = 1/2 ground state for both variations of this complex resulting from a high-spin FeIII (S = 5/2) antiferromagnetically coupled to a high-spin (S = 2) FeIV site. Temperature dependent EPR studies yield an exchange coupling constant J = 90 +/- 20 cm-1 (H = JS1·S 2), which is in agreement with DFT analysis suggesting the S = 1 → S = 2 transition of the FeIV=O site is driven by strong antiferromagnetic exchange interactions.