title: Electron paramagnetic resonance and Mössbauer spectroscopy and density functional theory analysis of a high-spin Fe(IV)-oxo complex.
abstract: High-spin Fe(IV)-oxo 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(IV)H(3)buea(O)](-), [Fe(III)H(3)buea(O)](2-), and [Fe(III)H(3)buea(OH)](-) (H(3)buea = tris[(N'-tert-butylureaylato)-N-ethylene]aminato) using Mössbauer and dual-frequency/dual-mode electron paramagnetic resonance (EPR) spectroscopies. The [Fe(IV)H(3)buea(O)](-) complex has a high-spin (S = 2) configuration imposed by the C(3)-symmetric ligand. The EPR spectra of the [Fe(IV)H(3)buea(O)](-) complex presented here represent the first documented examples 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 the determination of the zero-field parameter, D = +4.7 cm(-1), 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-orbit coupling of the ground state with an excited S = 1 electronic configuration at 1.2 eV. (17)O isotope enrichment experiments allowed the determination of the hyperfine constants ((17)O)A(z) = 10 MHz for [Fe(IV)H(3)buea(O)](-) and ((17)O)A(y) = 8 MHz, ((17)O)A(z) = 12 MHz for [Fe(III)H(3)buea(OH)](-). The isotropic hyperfine constant (((17)O)A(iso) = -16.8 MHz) was derived from the experimental value to allow a quantitative determination of the spin polarization (ρ(p) = 0.56) of the oxo p orbitals of the Fe-oxo bond in [Fe(IV)H(3)buea(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 (57)Fe A(dip) tensor of (+5.6, +5.3, -10.9) MHz and A(iso) = -25.9 MHz for the [Fe(IV)H(3)buea(O)](-) complex, and the results of DFT calculations were in agreement with the nuclear parameters of the complex.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Rupal
authors.initials: R
authors.lastname: Gupta
authors.forename: David C
authors.initials: DC
authors.lastname: Lacy
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar
authors.forename: A S
authors.initials: AS
authors.lastname: Borovik
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hendrich
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005296:Ferrous Compounds / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry*; D008958:Models, Molecular; D016877:Oxidants / Q000737:chemistry*; D017601:Oxygen Compounds / Q000737:chemistry*; D011789:Quantum Theory; D015204:Spectroscopy, Mossbauer
chemical_list: D005296:Ferrous Compounds; D058085:Iron Compounds; D016877:Oxidants; D017601:Oxygen Compounds