Electron Paramagnetic Resonance and M枚ssbauer Spectroscopy and Density Functional Theory Analysis of a High-Spin FeIV鈥揙xo Complex

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• 作者：Rupal Gupta ; David C. Lacy ; Emile L. Bominaar ; A. S. Borovik ; Michael P. Hendrich
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：June 13, 2012
• 年：2012
• 卷：134
• 期：23
• 页码：9775-9784
• 全文大小：501K
• 年卷期：v.134,no.23(June 13, 2012)
• ISSN：1520-5126

文摘

High-spin Fe^IV鈥搊xo species are known to be kinetically competent oxidants in non-heme iron enzymes. The properties of these oxidants are not as well understood as the corresponding intermediate-spin oxidants of heme complexes. The present work gives a detailed characterization of the structurally similar complexes [Fe^IVH₃buea(O)]^{鈭?/sup>, [FeIIIH₃buea(O)]2鈥?/sup>, and [FeIIIH₃buea(OH)]鈭?/sup> (H₃buea = tris[(N鈥?tert-butylureaylato)-N-ethylene]aminato) using M枚ssbauer and dual-frequency/dual-mode electron paramagnetic resonance (EPR) spectroscopies. The [FeIVH₃buea(O)]鈭?/sup> complex has a high-spin (S = 2) configuration imposed by the C₃-symmetric ligand. The EPR spectra of the [FeIVH₃buea(O)]鈭?/sup> complex presented here represent the first documented examples of an EPR signal from an FeIV鈥搊xo complex, demonstrating the ability to detect and quantify FeIV species with EPR spectroscopy. Quantitative simulations allowed the determination of the zero-field parameter, D = +4.7 cm鈥?, and the species concentration. Density functional theory (DFT) calculations of the zero-field parameter were found to be in agreement with the experimental value and indicated that the major contribution to the D value is from spin鈥搊rbit coupling of the ground state with an excited S = 1 electronic configuration at 1.2 eV. 17O isotope enrichment experiments allowed the determination of the hyperfine constants 17OA_z = 10 MHz for [FeIVH₃buea(O)]鈭?/sup> and 17OA_y = 8 MHz, 17OA_z = 12 MHz for [FeIIIH₃buea(OH)]鈭?/sup>. The isotropic hyperfine constant (17OA_iso = 鈭?6.8 MHz) was derived from the experimental value to allow a quantitative determination of the spin polarization (蟻_p = 0.56) of the oxo p orbitals of the Fe鈥搊xo bond in [FeIVH₃buea(O)]鈭?/sup>. This is the first experimental determination for non-heme complexes and indicates significant covalency in the Fe鈥搊xo bond. High-field M枚ssbauer spectroscopy gave an 57Fe A_dip tensor of (+5.6, +5.3, 鈭?0.9) MHz and A_iso = 鈭?5.9 MHz for the [FeIVH₃buea(O)]鈭?/sup> complex, and the results of DFT calculations were in agreement with the nuclear parameters of the complex.}