Structural insights into nonheme alkylperoxoiron(III) and oxoiron(IV) intermediates by x-ray absorption spectroscopy

Transient mononuclear low-spin alkylperoxoiron(l I I) and oxoiron(IV) complexes that are relevant to the activation of dioxygen by nonheme iron enzymes have been generated from synthetic iron(II) complexes of neutral tetradentate (TPA) and pentadentate (N4Py, Bn-TPEN) ligands and structurally characterized by means of Fe K-edge X-ray absorption spectroscopy (XAS). Notable features obtained from fits of the EXAFS region are Fe-O bond lengths of 1.78 Angstrom for the alkylperoxoiron(ill) intermediates and 1.65-1.68 Angstrom for the oxoiron(IV) intermediates, reflecting different strengths in the Fe-O pi, interactions. These differences are also observed in the intensities of the 1s-to-3d transitions in the XANES region, which increase from 4 units for the nearly octahedral iron(II) precursor to 9-15 units for the alkylperoxoiron(III) intermediates to 25-29 units for the oxoiron(IV) species.