A Mössbauer Study of [Fe(edta)(O2)]3−Agrees with a High-Spin FeIII Peroxo Complex

Abstract

An isotopically enriched [ 57 Fe(edta)(O 2 )] 3− complex, formed by the addition of H 2 O 2 to a 57 Fe III -edta (edta = ethylenediaminetetraacetic acid) complex at elevated pH (pH = 11.3), was studied by Mössbauer spectroscopy. All Mössbauer parameters were determined for this nonheme high-spin ferric peroxo complex with an η 2 -FeO 2 (side-on) arrangement by a numerical treatment of applied field Mössbauer data. The peroxo complex exhibits an isomer shift δ /Fe = 0.65(1) mm s −1 and a quadrupole splitting ∆E Q = +0.72(2) mm s −1 (T = 4.2 K). The isomer-shift value is very similar to the value of

Small axial and transverse magnetic field systems for a57Fe M ssbauer study of Kramers systems

“…These spectra were simultaneously fitted within the spin Hamiltonian formalism in the slow relaxation limit, using A field-direction dependence of the Mössbauer spectrum is clearly seen (figure 6) which signifies that [Fe(EDTA)(O 2 )] 3− is EPR-active, in agreement with a previous study (Neese and Solomon 1998). The set of parameters obtained for [Fe(EDTA)(O 2 )] 3− , in particular the isomer shift and the quadrupole splitting values, suggests by a comparison with model complexes (Girerd et al 2000) that the peroxo group is coordinated with the iron ion in a side-on way (Horner et al 2002).…”

Small axial and transverse magnetic field systems for a57Fe M ssbauer study of Kramers systems

“…These spectra were simultaneously fitted within the spin Hamiltonian formalism in the slow relaxation limit, using A field-direction dependence of the Mössbauer spectrum is clearly seen (figure 6) which signifies that [Fe(EDTA)(O 2 )] 3− is EPR-active, in agreement with a previous study (Neese and Solomon 1998). The set of parameters obtained for [Fe(EDTA)(O 2 )] 3− , in particular the isomer shift and the quadrupole splitting values, suggests by a comparison with model complexes (Girerd et al 2000) that the peroxo group is coordinated with the iron ion in a side-on way (Horner et al 2002).…”

Mössbauer Characterization of an Unusual High-Spin Side-On Peroxo−Fe3+ Species in the Active Site of Superoxide Reductase from Desulfoarculus baarsii. Density Functional Calculations on Related Models

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands(17,19,42,43).Obviously, the SOR active site differs from these model compounds by the presence of an axial thiolate ligand. This ligand is expected to lower the isomer shift by covalency effects.However, to the best of our knowledge no high-spin  2 -O 2 Fe 3+ complexes mimicking the peculiar [N 4 S] SOR environment have been characterized so far in the literature by Mössbauer spectroscopy.…”

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4 Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands (17,19,42,43).…”

“…This difference is explained by covalency effects associated to the thiolate ligand in complexes 4 and 5 (44). Taking these covalency effects into consideration for thiolato-bound peroxo-Fe 3+ species would lower the expected isomer shift value from the 0.61-0.65 mm/s range (17,19,42,43) to ca. 0.55 mm/s, indeed close to the experimentally determined value of the SOR peroxoiron species.…”

End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands: Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

“…Table shows that the parameters extracted from the analysis of data for 1b resemble those of other η 2 -peroxoiron(III) complexes. ,, The δ values (range 0.61−0.67 mm/s) are somewhat higher than is typical for a high-spin iron(III) center and appear to be a signature for side-on peroxo coordination. Such larger δ values are also associated with some (μ-1,2-peroxo)diiron(III) complexes, , but other (μ-1,2-peroxo)diiron(III) complexes have isomer shifts in the normal range for high-spin iron(III) centers. − The electronic basis for the increased δ values in these complexes is not currently understood.…”

Small axial and transverse magnetic field systems for a57Fe M ssbauer study of Kramers systems

“…These spectra were simultaneously fitted within the spin Hamiltonian formalism in the slow relaxation limit, using A field-direction dependence of the Mössbauer spectrum is clearly seen (figure 6) which signifies that [Fe(EDTA)(O 2 )] 3− is EPR-active, in agreement with a previous study (Neese and Solomon 1998). The set of parameters obtained for [Fe(EDTA)(O 2 )] 3− , in particular the isomer shift and the quadrupole splitting values, suggests by a comparison with model complexes (Girerd et al 2000) that the peroxo group is coordinated with the iron ion in a side-on way (Horner et al 2002).…”

Mössbauer Characterization of an Unusual High-Spin Side-On Peroxo−Fe3+ Species in the Active Site of Superoxide Reductase from Desulfoarculus baarsii. Density Functional Calculations on Related Models

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands(17,19,42,43).Obviously, the SOR active site differs from these model compounds by the presence of an axial thiolate ligand. This ligand is expected to lower the isomer shift by covalency effects.However, to the best of our knowledge no high-spin  2 -O 2 Fe 3+ complexes mimicking the peculiar [N 4 S] SOR environment have been characterized so far in the literature by Mössbauer spectroscopy.…”

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4 Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands (17,19,42,43).…”

“…This difference is explained by covalency effects associated to the thiolate ligand in complexes 4 and 5 (44). Taking these covalency effects into consideration for thiolato-bound peroxo-Fe 3+ species would lower the expected isomer shift value from the 0.61-0.65 mm/s range (17,19,42,43) to ca. 0.55 mm/s, indeed close to the experimentally determined value of the SOR peroxoiron species.…”

End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands: Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

“…Table shows that the parameters extracted from the analysis of data for 1b resemble those of other η 2 -peroxoiron(III) complexes. ,, The δ values (range 0.61−0.67 mm/s) are somewhat higher than is typical for a high-spin iron(III) center and appear to be a signature for side-on peroxo coordination. Such larger δ values are also associated with some (μ-1,2-peroxo)diiron(III) complexes, , but other (μ-1,2-peroxo)diiron(III) complexes have isomer shifts in the normal range for high-spin iron(III) centers. − The electronic basis for the increased δ values in these complexes is not currently understood.…”

Small axial and transverse magnetic field systems for a57Fe M ssbauer study of Kramers systems

“…These spectra were simultaneously fitted within the spin Hamiltonian formalism in the slow relaxation limit, using A field-direction dependence of the Mössbauer spectrum is clearly seen (figure 6) which signifies that [Fe(EDTA)(O 2 )] 3− is EPR-active, in agreement with a previous study (Neese and Solomon 1998). The set of parameters obtained for [Fe(EDTA)(O 2 )] 3− , in particular the isomer shift and the quadrupole splitting values, suggests by a comparison with model complexes (Girerd et al 2000) that the peroxo group is coordinated with the iron ion in a side-on way (Horner et al 2002).…”

Mössbauer Characterization of an Unusual High-Spin Side-On Peroxo−Fe3+ Species in the Active Site of Superoxide Reductase from Desulfoarculus baarsii. Density Functional Calculations on Related Models

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands(17,19,42,43).Obviously, the SOR active site differs from these model compounds by the presence of an axial thiolate ligand. This ligand is expected to lower the isomer shift by covalency effects.However, to the best of our knowledge no high-spin  2 -O 2 Fe 3+ complexes mimicking the peculiar [N 4 S] SOR environment have been characterized so far in the literature by Mössbauer spectroscopy.…”

“…3+ -peroxo Species with an Axial Thiolate Ligand. The set of Mössbauer and spin-Hamiltonian parameters that we have obtained for the peroxo-iron species of the E47A mutant of the SOR from D. baarsii match the properties reported for side-on peroxo-Fe 3+ model compounds (eg [( 2 -O 2 )Fe(EDTA)] 3or [( 2 -O 2 )Fe(N 4 Py)] + ) except for its isomer shift value which is slightly lower :  = 0.54 mm/s vs 0.61 -0.65 mm/s for complexes with nitrogen ligands (17,19,42,43).…”

“…This difference is explained by covalency effects associated to the thiolate ligand in complexes 4 and 5 (44). Taking these covalency effects into consideration for thiolato-bound peroxo-Fe 3+ species would lower the expected isomer shift value from the 0.61-0.65 mm/s range (17,19,42,43) to ca. 0.55 mm/s, indeed close to the experimentally determined value of the SOR peroxoiron species.…”

End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands: Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

“…Table shows that the parameters extracted from the analysis of data for 1b resemble those of other η 2 -peroxoiron(III) complexes. ,, The δ values (range 0.61−0.67 mm/s) are somewhat higher than is typical for a high-spin iron(III) center and appear to be a signature for side-on peroxo coordination. Such larger δ values are also associated with some (μ-1,2-peroxo)diiron(III) complexes, , but other (μ-1,2-peroxo)diiron(III) complexes have isomer shifts in the normal range for high-spin iron(III) centers. − The electronic basis for the increased δ values in these complexes is not currently understood.…”