EPR and Mössbauer Spectroscopy and DFT Analysis of a High-Spin FeIV-oxo Complex

摘要

The kinetically competent oxidant in non-heme iron enzymes is a high-spin Fe^IV-oxo species, which are not as well characterized as the intermediate-spin species of heme systems. The present work gives a detailed characterization of the structurally similar [Fe^IVH3buea(O)]^-, [Fe^IIIH3buea(O)]^2- and [Fe^IIIH3buea(OH)]^- (H3buea = tris[(N'-tert-butylureaylato)-N-ethylene]aminato) complexes using Mössbauer and dual-frequency/dual-mode electron paramagnetic resonance (EPR) spectroscopies. The [Fe^IVH3buea(O)]^- complex has a high-spin (S = 2) configuration imposed from the C3-symmetric ligand. EPR spectroscopy of the [Fe^IVH3buea(O)]^- complex is the first documented example of an EPR signal from an Fe^IV-oxo complex, demonstrating the ability to detect and quantify Fe^IV species with EPR spectroscopy. Quantitative simulations allowed determination of the zero-field parameter, D = +4.7 cm^-1, and the species concentration. Density functional theory calculations of the zero-field parameter are found to be in agreement with the experimental value and indicate the major contribution to the D-value is from spin–orbit coupling of the ground state with an excited S = 1 electronic configuration at 1.2 eV. ¹⁷O isotope enrichment experiments allowed a determination of the hyperfine constant ¹⁷⁰Az = 10 MHz for [Fe^IVH3buea(O)]^- and ¹⁷⁰Ay = 8 MHz, ¹⁷⁰Az = 12 MHz for [Fe^IIIH3buea(OH)]^-. The isotropic hyperfine constant (¹⁷⁰Aiso = -16.8 MHz) was derived from the experimental value to allow a quantitative determination of the spin polarization (ρp = 0.56) of the oxo p-orbitals of the Fe-oxo bond in [Fe^IVH3buea(O)]^-. This is the first experimental determination for non-heme complexes, and indicates significant covalency in the Fe-oxo bond. High-field Mössbauer spectroscopy gave an ⁵⁷Fe >Adip tensor of (+5.6, +5.3, -10.9) MHz and A_iso = -25.9 MHz for the [Fe^IVH₃buea(O)]^- complex, and DFT calculations are in agreement with the nuclear parameters of the complex.