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X-ray Absorption Spectroscopy and Reactivity of Thiolate-Ligated FeIII−OOR Complexes

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journal contribution
posted on 18.10.2010, 00:00 by Jay Stasser, Frances Namuswe, Gary D. Kasper, Yunbo Jiang, Courtney M. Krest, Michael T. Green, James Penner-Hahn, David P. Goldberg
The reaction of a series of thiolate-ligated iron(II) complexes [FeII([15]aneN4)(SC6H5)]BF4 (1), [FeII([15]aneN4)(SC6H4-p-Cl)]BF4 (2), and [FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (3) with alkylhydroperoxides at low temperature (−78 °C or −40 °C) leads to the metastable alkylperoxo-iron(III) species [FeIII([15]aneN4)(SC6H5)(OOtBu)]BF4 (1a), [FeIII([15]aneN4)(SC6H4-p-Cl)(OOtBu)]BF4 (2a), and [FeIII([15]aneN4)(SC6H4-p-NO2)(OOtBu)]BF4 (3a), respectively. X-ray absorption spectroscopy (XAS) studies were conducted on the FeIII−OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (∼7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the FeIII−OOR species range from 7121−7125 eV and correlate with the nature of the thiolate donor. Extended X-ray absorption fine structure (EXAFS) analysis of the iron(II) complexes 13 in CH2Cl2 show that their solid state structures remain intact in solution. The EXAFS data on 1a3a confirm their proposed structures as mononuclear, 6-coordinate FeIII−OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe−O bond distances obtained from EXAFS for 1a3a are 1.82−1.85 Å, significantly longer than other low-spin FeIII−OOR complexes. The Fe−O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe−O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a3a show an important influence of the thiolate donor.