X-ray absorption spectroscopic studies of high-spin nonheme (Alkylperoxo)iron(III) intermediates

The reactions of iron(II) complexes [Fe(Tp ^t-Bu,i-Pr)(OH)] (1a, Tp ^t-Bu,i-Pr = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)-borate) , [Fe(B-Me ₂BPMCN)(OTf) ₂] (1b, 6-Me ₂BPMCN = N,N-bis((2-methylpyridin-6-yl)methyl)-N,N′-dimethyl-trans-1, 2-diaminocyclohexane), and [Fe(L ⁸Py ₂)(OTf)](OTf) (1c, L ⁸Py ₂ = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin Fe ^III-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.; et al. J. Am. Chem. Soc. 2004, 126, 16750-16761). These include (1) an intense 1s → 3d preedge feature, with an area around 20 units, (2) an edge energy, ranging from 7122 to 7126 eV, that is affected by the coordination environment, and (3) a 1.86-1.96 Å Fe-OOR bond, compared to the 1.78 Å Fe-OOR bond in low-spin complexes. These unique features likely arise from a flexible first coordination sphere in those complexes. The difference in Fe-OOR bond length may rationalize differences in reactivity between low-spin and high-spin Fe ^III-OOR species.