Heiner Winkler

Iron-containing proteins and related analogs — complementary Mössbauer, EPR and magnetic susceptibility studies

The three methods covered by this review — Mossbauer spectroscopy, electron paramagnetic resonance and magnetic susceptibility-provide a powerful set of tools for detailed studies of electronic structure and molecular magnetism of iron-containing proteins and related analogs. The interpretation of measured data is based on the spin-Hamiltonian concept which is described in detail. A short introduction to the principles of the three methods as well as a basic description of the spectrometers is given. The major part of the review deals with applications, e.g. to mononuclear iron complexes and to spin-coupled iron complexes. Wherever possible, the complementarity in applying the three methods is described.

Mössbauer and electron paramagnetic resonance study of the double‐exchange and Heisenberg‐exchange interactions in a novel binuclear Fe(II/III) delocalized‐valence compound

In this paper we present the characterization by UV‐VIS, Mossbauer, and EPR spectroscopy of [L2Fe2(μ‐OH)3](ClO4)2⋅2CH3OH⋅2H2O, with L=N,N’,N‘‐trimethyl‐1,4,7‐triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange‐coupled delocalized‐valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mossbauer spectroscopy. Mossbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double‐exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV‐VIS, Mossbauer, and EPR spectroscopy are for the double‐exchange parameter B=1300 cm−1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero‐field splitting D=4 cm−1 and E≊0 cm−1, and for the hyperfine parameters ΔEQ =−2.14 mm s−1, Ax,y =−21.2 T, Az =−27 T, and δ=0.74 mm s−1. Fr...

Structure and dynamics of biomolecules studied by Mössbauer spectroscopy

M?ssbauer spectroscopy not only offers information about the structural and electronic properties of iron centres in biomolecules but also about their dynamic behaviour. In order to apply this nuclear method to biologically relevant iron centres knowledge of nuclear and molecular physics is required. This review introduces the basic physical concepts of 57 Fe-M?ssbauer spectroscopy. The various oxidation and spin states of iron are discussed in a simple orbital frame. Aspects of the ligand field theory of paramagnetic molecules are introduced, and a review covering the iron centres in proteins which have been presently characterized is given. The review covers M?ssbauer studies starting from heme proteins, continuing with the above-mentioned proteins containing di-nuclear iron clusters, iron storage proteins and iron-sulfur proteins, and concludes with the complex iron clusters of nitrogenase. The physical properties of the different forms of iron centres as well as their biological relevance are elaborated upon. The effects of electronic spin dynamics on the M?ssbauer spectra are reviewed. In addition, the dynamics of the iron ion itself and how it can be studied by M?ssbauer spectroscopy is discussed. An introduction into the recently developed technique using synchroton radiation which is also called M?ssbauer spectroscopy in the time domain shall give an impression about the future developments in this field of spectroscopy.

Vibrational spectrum of the spin crossover complex [Fe(phen)2(NCS)2] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.

Generation of oxoiron(IV) tetramesitylporphyrin π-cation radical complexes by m-CPBA oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at −78 °C. Evidence for the formation of six-coordinate oxoiron(IV) tetramesitylporphyrin π-cation radical complexes FeIV=O(tmp)X (X=Cl−, Br−), by Mössbauer and X-ray absorption spectroscopy

Abstract The generation of six-coordinate oxoiron(IV) tetramesitylporphyrin π-cation radical complexes by m -CPBA ( meta -chloroperbenzoic acid) oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at −78 °C was investigated. UV–Vis and EPR spectroscopies indicate that the axial ligand present in the ferric starting derivatives is retained in the high-valent iron complexes. Indirect evidence for the formation of six-coordinate oxoiron(IV) tetramesitylporphyrin complexes Fe IV =O(tmp )X (X=Cl − , Br − ) by m -CPBA oxidation of FeX(tmp) (X=Cl − , Br − ) in butyronitrile at −78 °C was also obtained by Mossbauer spectroscopy. Direct confirmation of the presence of a halide ion as second axial ligand of iron in these high-valent iron species was obtained by X-ray absorption spectroscopy. The EXAFS spectra of the samples obtained by m -CPBA oxidation of FeX(tmp) (X=Cl − , Br − ) were refined using two different coordination models including both four porphyrinato-nitrogens and the axial oxo group. The two models include (model I) or exclude (model II) the axial halogen. The statistical tests indicate the presence of a halide ion as second axial ligand of iron in both derivatives. The refinements led to the following bond distances: Fe IV O(tmp )Cl ( 3 ): Fe–O=1.66(1), Fe–Cl=2.39(2) and Fe–N p =1.99(1) A; Fe IV =O(tmp )Br ( 4 ): Fe–O=1.65(1), Fe–Br=2.93(2), Fe–N p =2.02(1) A. The lengthening of the Fe–X (X=Cl − , Br − ) distances relative to those occurring in the ferric precursor porphyrins is, most probably, related to the strong trans influence of the oxoiron(IV) fragment present in 3 and 4 .

Exchange interactions, charge delocalization, and spin relaxation in a mixed‐valence di‐iron complex studied by Mössbauer spectroscopy

Exchange interactions and charge transfer in the Fe2+Fe3+ pair of the mixed valence [Fe2S2(dimethylmethanebisbenzimidazolate)2]3− trianion have been studied by analysis of Mossbauer spectra in the temperature range of 1.5–180 K and in applied fields of 10 mT, 0.35 T, and 6.2 T. The low‐temperature spectra reveal a ground state with total spin St=1/2 and hyperfine parameters intermediate between values for a Fe2+Fe3+ localized mixed‐valence pair and a fully delocalized system where the two iron atoms are equivalent. A consistent set of hyperfine parameters has been derived by fitting the spectra with a stochastic relaxation model taking into account spin relaxation in the St=1/2 state and electron hopping between the iron ions. An interpretation of the values of the hyperfine parameters has been given by solving a spin Hamiltonian, which includes antiferromagnetic and double exchange in an asymmetric Fe2+Fe3+ pair and which allows partial electron delocalization. Using the value a2=0.8 for the delocalizati...

Iron sulfur and iron molybdenum sulfur compounds: Comparison of molecular orbital calculations and spin‐Hamiltonian analysis of Mössbauer spectra

The electronic structures of mononuclear Fe–S complexes with a FeIIS4 core and of binuclear Fe–Mo–S complexes containing the FeS2Mo core have been calculated by a semiempirical molecular orbital method (iterative extended Huckel theory), followed by a spin‐orbit coupling calculation on the five highest occupied iron‐like molecular orbitals. Fine structure and hyperfine structure tensors and parameters (g, D, E, A, and electric field gradient) have been calculated and compared with data from spin‐Hamiltonian analysis of Mossbauer measurements. For the mononuclear complex anions [Fe(SPh)4]2− and [Fe(dts)2]2− it was found that Vzz is negative, D positive, and that the magnetic anisotropy places the preferred direction of the hyperfine magnetic field perpendicular to the Vzz direction in agreement with spin‐Hamiltonian results. The similarity of parameters of [Fe(SPh)4]2− and reduced rubredoxin (Rdred) confirms the suggestion that this anion has a ground electronic state practically identical to Rdred. Th...

Mössbauer and EXAFS studies of bacterioferritin fromStreptomyces olivaceus

Mössbauer spectra of bacterioferritin fromStreptomyces olivaceus in zero field show essentially a transition from a quadrupole doublet to a six-line pattern at a temperature around 7–8 K and a distribution of hyperfine fields from 40 to 50 T. The EXAFS data reveal 3–4 phosphorus atoms in the second coordination shell. These features are found to be typical for various bacterioferritins.

Open-framework iron phosphates: Syntheses, structures, sorption studies and oxidation catalysis{

The iron phosphates [C4H12N2][FeII(H2O)6](HPO4)2 (1), 3∞{[NH4][FeIII2(OH)(PO4)2(H2O)]·H2O} (2) and 3∞{[C4H12N2][FeIII3(PO4)3(HPO4)(H2O)]·∼0.25H2O}, (3) were synthesized by hydrothermal methods and their single-crystal X-ray structures were determined. While compound 1 is only an extended hydrogen bonded network of its three ionic building blocks, compounds 2 and 3 are three-dimensional open-framework materials albeit of different porosity. The structure of 2 corresponds to the mineral sphenicidite. The iron building blocks in 3 are pairs of distorted edge-sharing {FeO6} octahedra and a five-coordinated iron atom, {FeO5}, with a mostly trigonal-bipyramidal polyhedron. The oxidation-state assignment of 2 was backed by 57Fe Mossbauer spectroscopy. Thermogravimetric analysis (TGA) of 2 and 3 shows clearly separated steps of weight loss due to the loss of water of crystallization, aqua ligand and amine template molecules. X-ray powder diffractometry proved that the empty host-frameworks were still intact after heating to 215 °C. The porous empty frameworks of 2 and 3 could be employed as sorbents towards alkanes, alcohols, chlorinated halocarbons, amines and ethers. The larger-porous framework of 3 (but not that of 2) was found to be a catalyst for the highly regioselective oxidation of n-pentane to 3-pentanol with air at 15 bar and 100 °C.

Rapid spin-state interconversion in the Bis-complex of tris-(1-pyrazolyl) methane with Fe(II) studied by Mössbauer spectroscopy

Dynamic spin equilibrium is observed in a complex of the [Fe(II)-N6] type above room temperature. The Mössbauer lineshapes as function of temperature can be understood by means of the random-frequency-modulation model. Taking into accout the different Lamb-Mössbauer factors of the low- and high-spin state yields true populations of the two spin states. The transition rates follow rather well an Arrhenius law. With appropriate assumptions an activation energy ΔELH=18(1) kJmol−1 is deduced.