EPR properties of mixed-valent μ-oxo and μ-hydroxo dinuclear iron complexes produced by radiolytic reduction at 77 K

Abstract

Radiolytic reduction at 77 K of oxo/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shapes of the mixed-valent EPR signals depend strongly on the bridging ligands. Spectra of the Fe(II)OFe(III) species reveal an S = 1/2 ground state with small g-anisotropy as characterized by the uniaxial component (gz-gav/2 < 0.03) observable at temperatures as high as approximately 100 K. In contrast, hydroxo-bridged mixed-valent species are characterized by large g-anisotropy (gz-gav/2 > 0.03) and are observable only below 30 K. Annealing at higher temperatures causes structural relaxation and changes in the EPR characteristics. EPR spectral properties allow the oxo- and hydroxo-bridged, mixed-valent diiron centers to be distinguished from each other and can help characterize the structure of mixed-valent centers in proteins.

Dioxygen Activation and Methane Hydroxylation by Soluble Methane Monooxygenase: A Tale of Two Irons and Three Proteins

“…In MMOH ox , the high-spin ferric irons are antiferromagnetically coupled to form a diamagnetic ground state that is EPR silent. [98,100,117,118] These studies demonstrate that the diiron site of MMOH ox is affected by binding of MMOB but not of MMOR. EXAFS studies of MMOH ox showed no evidence for REVIEWS Soluble Methane Monooxygenase short 1.75 ± 1.80 FeÀO bonds characteristic of {Fe 2 -(m-oxo)} 4 units, and optical spectra similarly lacked absorbances at 350 and 500 ± 600 nm that are typical of oxo-bridged diferric complexes.…”

Structural, EPR, and Mössbauer Characterization of (μ-Alkoxo)(μ-Carboxylato)Diiron(II,III) Model Complexes for the Active Sites of Mixed-Valent Diiron Enzymes

“…D/J mixing) as previously assumed (see eq 8 and eq 11; g A = 2.0). 67 This conclusion applies both to weakly coupled systems, like the alkoxo-bridged complex 1 , and strongly coupled systems, such as those that contain an oxo bridge. …”

Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = 1/2 FeIII−O−FeIV═O Complexes. Superexchange-Mediated Spin Transition at the FeIV═O Site

“…As a means of determining the core structure of 1-OH relative to the HO-Fe IV -O-Fe IV =O open core of 4 , the latter was subjected to radiolytic reduction at 77 K,48 a temperature at which the ligands are expected to be essentially immobile, in order to compare the Mössbauer spectra of the radiolytically reduced species with those derived from ferrocene reduction of 4 . Figure 10A shows a 4.2 K Mössbauer spectrum of 4 prior to reduction, prepared as described previously 23.…”