文摘
High-spin FeIV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 [FeIVH3buea(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 17OAz = 10 MHz for [FeIVH3buea(O)]鈭?/sup> and 17OAy = 8 MHz, 17OAz = 12 MHz for [FeIIIH3buea(OH)]鈭?/sup>. The isotropic hyperfine constant (17OAiso = 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 Adip tensor of (+5.6, +5.3, 鈭?0.9) MHz and Aiso = 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.