High-spin FeIII Research Articles

Inferences from the 1H-NMR Spectroscopic Study of an Antiferromagnetically Coupled Heterobinuclear Fe(III)−(X)−Cu(II) S = 2 Spin System (X = O2-, OH-)

This study considers how the electronic relaxation rate enhancement effects of strong antiferromagnetic coupling in the FeIII−X−CuII unit (X = O2-, OH-) of complexes [(F8-TPP)FeIII−O−CuII(TMPA)]+ (1), and [(F8-TPP)FeIII−OH−CuII(TMPA)]+ (2) (F8-TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2−); TMPA = tris(2-pyridylmethyl)amine) are manifested as observable upfield-shifted resonances of the TMPA moiety in the 1H-NMR spectra. The pyrrole resonances appear at 65 and 69 ppm, respectively, for 1 and 2, consistent with an S = 2 ground state derived from antiferromagnetic coupling of high-spin FeIII (S = 5/2) and CuII (S = 1/2) through the bridging ligand X. Paramagnetic mononuclear complexes [Co(TMPA)(CH3CN)]2+ (S = 3/2) (X-ray structure reported) and [Cu(TMPA)(CH3CN)]2+ (S = 1/2) demonstrate downfield-shifted peaks consistent with a σ contact shift mechanism. Assignments for all complexes were achieved via 1H- and 2H-NMR spectroscopy of appropriately synthesized methylated and deuteriated derivatives. In [Cu(TMPA)(CH3CN)]2+, the observed ligand peaks (298 K) are broad; in 1, however, they are considerably sharper and upfield-shifted to −61, −7, 4.5, −21, and −104 ppm, corresponding to pyridyl 6-H, 5-H, 4-H, and 3-H and aminomethyl −CH2−, respectively, of TMPA. The observance of these upfield peaks is a consequence of enhancement of the electronic relaxation rate for CuII due to antiferromagnetic coupling with the faster relaxing FeIII. This observation represents the prototype of a FeIII−X−CuII S = 2 spin state hitherto only theoretically predicted. The attenuation of δ for a particular hydrogen as its distance (through bonds) from CuII increases correlates with a σ contact shift mechanism. Only one peak per type of pyridyl H is observed (from 220 to 300 K for 1 and from 220 to 270 K for 2), indicating dynamic behavior of the Cu−TMPA moiety in solution. Both 1 and 2 exhibit pseudo-Curie temperature dependence manifested as augmentation of δ in both upfield and downfield directions as temperature is lowered. Linear Curie and anti-Curie plots of the pyrrole and TMPA chemical shifts (from 220 to 300 K for 1 and from 220 to 270 K for 2) imply a predominantly S = 2 spin state in each case; i.e., the extent of antiferromagnetic coupling is strong.

Control of Spin State by Ring Conformation of Iron(III) Porphyrins. A Novel Model for the Quantum-Mixed Intermediate Spin State of Ferric Cytochromec‘ from Photosynthetic Bacteria

(2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride FeIII(OETPP)Cl and (2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride FeIII(OMTPP)Cl complexes have been synthesized and characterized by 1H NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature 1H NMR studies confirm that the conformational distortions are maintained in solution with ΔG⧧ = 15.8 and 10.1 kcal/mol for ring inversion for Fe(OETPP)Cl and Fe(OMTPP)Cl, respectively. EPR (g⊥ = 5.2−5.3 at 77 K), magnetic moments (μeff = 4.7−5.2 μB at 300 K), and structural data (Fe−Np = 2.03 A, Fe−Cl = 2.24−2.25 A) all indicate that unlike high-spin FeIII(TPP)Cl and FeIII(OEP)Cl (S = 5/2), FeIII(OETPP)Cl and FeIII(OMTPP)Cl these complexes are of the uncommon quantum-mixed S = 5/2, 3/2 intermediate-spin state. Saddle-shaped ring deformations lower the symmetries of the complexes into C2v. Other than the nonaxial symmetric E...

A Phenoxo-Bridged Diferric Complex with Two Different Coordination Numbers in Two Distinct Coordination Sites

The bimetallic complex [Fe2L(μ-OCH3)(OCH3)](BPh4) (1) has been obtained from the unsymmetrical heptadentate ligand H3L, which provides two different coordination environments for the two iron ions. X-ray data reveal a doubly bridged (one phenoxo and one methoxo) diferric complex containing one five-coordinated and one six-coordinated iron center. The two high spin FeIII centers are weakly antiferromagnetically coupled (J = −13 cm-1), and two redox processes are observed.

An Exchange-Coupled Complex with Localized High-Spin FeIV and FeIII Sites of Relevance to Cluster X of Escherichia coli Ribonucleotide Reductase

An Exchange-Coupled Complex with Localized High-Spin FeIV and FeIII Sites of Relevance to Cluster X of Escherichia coli Ribonucleotide Reductase

Soluble Substituted μ‐Oxo(phthalocyaninato)iron(III) Dimers

AbstractAttempts to prepare various peripheral tetra‐ and octasubstituted (phthalocyaninato)iron derivatives RnPcFe [n = 4: R = CH2OEt (6e), CO2Et (6h), CO2‐n‐C6H13 (6l), O(2‐Et‐n‐C6H13) (6j), OCH2C(CH3)3 (6k), OCH2C(CH3)2CH2Ph (6l); n = 8: R = CH2CH(CO2Me)CH(CO2Me)CH2 (6f), CH2CH(CO2Et)‐CH(CO2Et)CH2 (6g), O‐n‐C8H17 (6m), O(2‐Et‐n‐C6H13) (6n)] by starting from the corresponding substituted phthalonitriles led to substituted (μ‐oxo)bis[(phthalocyaninato)iron(III)] compounds [RnPcFe]2O. The tert‐butyl‐ and ethyl‐substituted systems [tBu4PcFe]2O (6c) and [Et4PcFe]2O (6d) were reinve‐stigated. UV/Vis, FD mass, Mössbauer, NMR as well as ESR spectroscopy was used to characterize the complexes [RnPcFe]2O to furnish evidence for the presence of Fe‐O‐Fe moieties in [RnPcFe]2O. The UV/Vis data reported for [RnPcFe]2O as well as their spectral behavior in pyridine correspond to unsubstituted [PcFe]2O. Mössbauer spectra of [RnPcFe]2O show that the complexes were obtained as a mixture of two isomeric μ‐oxo compounds A (δFe = 0.22 mm s‐1, ΔEQ = 1.33–1.39 mm s‐1) and B (ΔFe = 0.33–0.36 mm s‐1, ΔEQ = 0.39–0.53 mm s‐1), whose Mössbauer parameters are comparable to [PcFe]2O μ‐oxo(2) and μ‐oxo(1), respectively. Mössbauer spectral data of [RnPcFe]2O indicate high‐spin (S = 5/2) FeIII centers. NMR‐spectra of [RnPcFe]2O (n = 4: R = tBu, Et, O(2‐Et‐n‐C6H13), OCH2C(CH3)2CH2Ph; n = 8: R = O‐n‐C8H17, O(2‐Et‐n‐C6H13) give further evidence for μ‐oxo bridged structures.

Iron(III) complexes of a tetradentate tripodal ligand N,N?,N?-tris (2-benzimidazolyl-methyl)amine

FeIII complexes of a tetradentate ligand with pendant benzimidazolyl groups have been synthesized and characterized. Room temperature Mossbauer spectra depict a quadruple split doublet in the case of NOinf3p− as co-ligand, while a nearly symmetrical one line spectrum is obtained for complexes with Cl−as co-ligand. The isomer shift values are towards the lower end of the range found for other high spin FeIII complexes. 1H-n.m.r. spectra of the complexes reveal relatively broad linewidths with large isotropic shifts. Paramagnetically shifted resonances are observed in the range of −10.0–+70.0 p.p.m.

Has the Affinity of High Spin FeIII for sp3 Nitrogen Donors Been Underestimated?

Has the Affinity of High Spin FeIII for sp3 Nitrogen Donors Been Underestimated?

β″-(bedt-ttf)4[(H2O)Fe(C2O4)3]·PhCN: the first molecular superconductor containing paramagnetic metal ions

β″-(bedt-ttf)4[(H2O)Fe(C2O4)3]·PhCN [bedt-ttf = bis(ethylenedithio)tetrathiafulvalene] is a superconductor [Tc 8.5(3) K]; from 9–300 K its magnetic susceptibility is dominated by the high-spin FeIII and obeys the Curie–Weiss law (C= 4.375 emu K mol–1, θ=–0.2 K).

Temperature dependence of theg=6 EPR linewidth in high spin FeIII myoglobin samples

The temperature dependence of theg=6 line of high spin FeIII aqueous mixed water-glycerol myoglobin samples has been investigated by EPR spectroscopy. The trend with the temperature of the linewidth has been analyzed by taking into account both a temperature-independent contribution from the frozen structural heterogeneity (conformational substate distribution) and the relaxation processes. The temperature-dependent line broadening process due to the spin-lattice relaxation has been singled out by simulation of the experimental data and put into relationship to the density of the vibrational states. The effect on the vibrational modes of the protein-solvent system, as induced by the addition of large amount of the glass-making solvent, glycerol, is discussed.

Hydrothermal synthesis and structural characterisation of SrFe3(PO4)3(HPO4), an iron(III) phosphate containing FeO5 trigonal bipyramids and dimers of face-sharing FeO6 octahedra

A new iron(III) phosphate, SrFe3(PO4)3(HPO4), has been synthesised hydrothermally and characterised by single-crystal X-ray diffraction and Mossbauer spectroscopy. The compound crystallises in the monoclinic space group P21/c, with a= 8.476(2), b= 7.298(1), c= 19.501(3)A, β= 94.96(2)°, and Z= 4. The structure consists of channels parallel to the a axis where the Sr atoms are located. The framwork is composed of dimers of face-sharing FeO6 octahedra, discrete FeO5 trigonal bipyramids, phosphate tetrahedra, and hydrogenphosphate groups. Room-temperature Mossbauer spectroscopy confirms the presence of high-spin FeIII.

Hexanuclear Ferric Complex with a Novel S = 5 Ground State

The iron complex 1 shows novel, magnetic properties. Its spin ground state is S = 5, even though all six iron centers are high-spin FeIII ions. Magnetic measurements revealed an increase in μeff ith decreasing temperature. Complex 1 was also characterized by X-ray structure analysis.

Resonance Raman investigation of ferric iron in horseradish peroxidase and its aromatic donor complexes at room and low temperatures

Resonance Raman (RR) spectra of the acidic form of FeIII horseradish peroxidase (HRP) were obtained at room and low temperatures using B- and Q-band excitation. At 296 K, HRP exhibits two sets of porphyrin skeletal stretching frequencies which are attributed to a thermal mixture of 5- and 6-coordinate high-spin FeIII states. When the temperature is lowered, the observed bands shift to higher frequencies, and these are assigned to intermediate- and low-spin states. Addition of 40% glycerol has no effect on the spectra at 296 K, but at 20 K, all four frequency sets are observed corresponding to the two forms observed at room and low temperature in the absence of glycerol. The 296 K RR spectrum of the HRP-hydroquinone complex is similar to that of free HRP, but conversion to the intermediate- and low-spin states is complete at a higher temperature than in the free enzyme. Addition of benzohydroxamic acid (BHA) to HRP shifts the RR frequencies to those corresponding to a 6-coordinate high-spin species at both room and low temperature. Two upsilon (C = C) stretching modes are observed for HRP and its donor complexes, indicating that the vinyl groups are inequivalent. On BHA binding, one of the vinyl modes and upsilon 37 (Eu) are enhanced, suggesting symmetry lowering of the heme site.

Resonance Raman spectroscopy of cytochrome c peroxidase single crystals on a variable-temperature microscope stage

Good quality resonance Raman (RR) spectra have been obtained for cytochrome c peroxidase single crystals (0.2 x 0.5 x 1 mm) lying on their 110 faces on a microscope stage. Crystal orientation and polarization effects are observed which differentiate the RR bands on the basis of the symmetries of the porphyrin vibrational modes. The measured depolarization ratios are accurately calibrated for isolated bands of both totally symmetric and non totally symmetric modes by using a model of D4h chromophores in an oriented gas using the crystal structure atomic coordinates. The calculations indicate that the electronic transition moments are approximately along the lines connecting the methine bridges, suggesting an electronic steering effect of the vinyl groups. Deviations are observed for bands associated with the porphyrin v10 and the vinyl C = C stretching modes, which may be due to their near-resonant interaction. The band frequencies correspond to those of a five-coordinate high-spin FeIII heme, as previously observed in solution, consistent with the X-ray structure showing the Fe atom to be out of the heme plane on the proximal side with a distal water molecule located at a nonbonded distance, 2.4 A. The temperature dependence of the RR spectrum was determined with a Joule-Thompson cryostat on crystals sealed in glass capillaries. As the temperature is lowered, the spectrum converts to one characteristic of a low-spin FeIII heme. The conversion, which is readily reversible, is quite gradual. It is detectable at -50 degrees C but is incomplete even at -190 degrees C. A temperature effect on the protein structure is proposed which permits the Fe atom to approach the heme plane and bind the distal water molecule, or the distal histidine.

G̃-tensor calculation for FeIII triads: Application to the 3Fe clusters in iron–sulfur proteins

The g̃ tensor of a triad of high spin FeIII ions coupled by antiferromagnetic exchange interactions, is calculated in the case where the fine structure terms are not negligible compared to the exchange parameters. Using some simplifying assumptions concerning the relative orientations of the magnetic axes, we obtain the expressions of the g components in closed form. A simple ‘‘g strain’’ model based on these expressions is used to simulate the EPR spectra given by the 3Fe clusters of four different iron sulfur proteins. In each case, a good simulation is only observed if the three exchange parameters are nearly equal, and of the order of −20 cm−1. These conclusions are in good agreement with our earlier estimations which were obtained by an independent method [J. P. Gayda, P. Bertrand, F-X. Theodule, and J. J. G. Moura, J. Chem. Phys. 77, 3387 (1982)].

Raman and infrared spectra of cytochrome c peroxidase-carbon monoxide adducts in alternative conformational states.

Resonance Raman (RR) spectra are reported for CO-bound cytochrome c peroxidase (CCP). At low pH, two forms are observed: form II, with nu Fe-C = 530 cm-1 and delta FeCO = 585 cm-1, and form I, with nu Fe-C = 495 cm-1 and no detectable delta FeCO. They appear to have coincident nu CO infrared bands, at 1922 cm-1. These low-pH forms, similar to those observed for horseradish peroxidase (HRP), are attributed to tilted, H-bonded CO and perpendicular CO, respectively. The frequencies differ between the two proteins, a weaker H bond to CO being indicated for CCP. As with HRP, the equilibrium between forms I and II is shifted toward the latter at increasing CO concentrations, suggesting that secondary binding of CO perturbs the distal residues. At high pH [8.4, tris(hydroxymethyl)aminomethane buffer] the form II fraction converts to another form, II', with nu FeC = 503 cm-1, delta FeCO = 575 cm-1, and nu CO = 1948 cm-1; a tilted, non-H-bonded geometry is suggested. If phosphate buffer is used, however, form II (H bonded) persists at pH 8.4. This result establishes a role for phosphate in stabilizing the H-bonded form of the enzyme; it is suggested that phosphate binds near the distal imidazole and substantially increases its pKa. The conformational state is also influenced by aging. Fresh protein contains purely high spin FeIII heme, as monitored by the high-frequency RR spectrum, and yields form II almost exclusively at elevated CO concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron spin resonance studies of iron(III) complexes of ethylenediaminetetra-acetate and N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetate in co-ordinating solvents

Remarkable solvent dependence of the apparent peak-to-peak linewidth (ΔHpp) has been observed for the rhombic type of e.s.r. spectra of the high-spin FeIII(edta)(edta = ethylenediaminetetra-acetate) and FeIII(hedta)[hedta =N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetate] complexes in a series of co-ordinating solvents. Moreover, the order of the solvents which gave the characteristic ΔHpp for each complex was different for the two complex systems: i.e. H2O dimethyl sulphoxide (dmso) > NN-dimethylformamide (dmf) > CH3OH for FeIII(edta) and H2O > CH3OH > dmf dmso for FeIII(hedta). Solvent dependence of the band maxima (λmax.) corresponding to the d–d transition has been also observed for the same series of iron(III) complex–solvent systems although the λmax. values obtained were less sensitive to the solvent than the ΔHpp values. In this case, the solvent dependence of λmax. for FeIII(edta) and FeIII(hedta) was similar: λmax. was smaller for H2O and CH3OH (<600 nm) than for dmf and dmso (>700 nm). The co-ordination behaviour of the solvent molecules to these iron(III) complexes is discussed in relation to both the ΔHpp and λmax. values. It is revealed that in the case of rhombic high-spin iron(III) complexes wit such edta and hedta, ΔHpp values obtained by e.s.r. spectroscopy reflect small changes in the co-ordination circumstances; λmax. values obtained from electronic spectra did not.