Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.     

Single-Crystal Fe Q-Band ENDOR Study of the 4 Iron–4 Sulfur Cluster in its Reduced [4Fe–4S] State

⁵⁷Fe Q-band ENDOR has been used to study the [4Fe–4S]¹⁺ state created by γ irradiation of single crystals of the synthetic model compound [N(C₂H₅)₄]₂[Fe₄S₄(SCH₂C₆H₅)₄] enriched in ⁵⁷Fe. This compound is an excellent biomimetic model of the active sites of many 4 iron–4 sulfur proteins, enabling detailed and systematic studies of its oxidized [4Fe–4S]³⁺ and reduced [4Fe–4S]¹⁺ paramagnetic states. Taking advantage of the fact that Q-band ENDOR, in contrast with X-Band ENDOR, allows for a very good separation of the ⁵⁷Fe transitions from those of the protons, the complete hyperfine tensors of the four iron atoms for the [4Fe–4S]¹⁺ species has been measured with precision. For each iron atom, the electron orbital and electron spin isotropic contributions have been determined separately. Moreover, it is remarkable that two ⁵⁷Fe hyperfine tensors attributed to the ferrous pair of iron atoms are very different. In effect, one tensor presents a much larger anisotropic part and a much smaller isotropic part than those of the other. This difference has been interpreted in terms of a differential electron orbital hyperfine interaction among the two ferrous ions.     

Study of [Fe(pyim)3] complexes: dynamic spin equilibrium on encapsulation in zeolite Y

Tris complex of Fe^II2(2′-pyridyl)imidazole has been encapsulated in the supercages of zeolite Y and characterized by using powder XRD, FTIR, Mössbauer spectroscopy, variable temperature magnetization and MAS NMR techniques and results have been compared with those obtained for this complex with ClO₄⁻ and SO₄²⁻ as anions. At room temperature, the [Fe(pyim)₃](ClO₄)₂ complex exhibited low spin state, while [Fe^II(pyim)₃]SO₄ exhibited the existence of both low and high spin states. The encapsulated [Fe^II(pyim)₃]²⁺ complex exhibited a broad quadrupole doublet characterized by isomer shift, δ=+0.55 mm/s and quadrupole splitting ΔE_q=1.26 mm/s. The magnetization measurements carried out for the encapsulated [Fe^II(pyim)₃]²⁺ complex showed a systematic decrease in its values with decreasing temperature down to 75 K with no indication of thermal hysteresis effects. These results suggest the existence of a dynamic spin state equilibrium between the high and low spin states for the encapsulated [Fe^II(pyim)₃]²⁺ complex with time constant comparable to the characteristic Mössbauer time scale of ⁵⁷Fe nuclei.     

Study on the ferromagnetic state in iron mixed-valence complexes, A[FeIIFeIII(dto)3] (A = (n-CnH2n + 1)4N; dto = C2S2O2) by means of Mössbauer spectroscopy

We have investigated the ferromagnetic states for (n-C_nH_2n?+?1)₄N[Fe^IIFe^III(dto)₃] (n = 3–6; dto = C₂O₂S₂) by means of ⁵⁷Fe Mössbauer spectroscopy. The major component of the spin configuration in the ferromagnetic states for n = 3 and 4 is the low-temperature phase (LTP) with the Fe^III (S = 5/2) and Fe^II (S = 0) states. The high-temperature phase (HTP) of n = 4 remains by more than 20%, which is consistent with two ferromagnetic transitions (T_C = 7 & 13 K). Moreover, it was revealed that the Mössbauer spectra in the ferromagnetic states for n = 5 and 6 correspond to the HTP consisting of the Fe^II (S = 2) and Fe^III (S = 1/2) states.     

Dimeric Fe (II, III) complex of quinoneoxime as functional model of PAP enzyme: Mössbauer, magneto-structural and DNA cleavage studies

Purple acid phosphatase, (PAP), is known to contain dinuclear Fe₂ ^?+?2,?+?3 site with characteristic Fe^?+?3 ← Tyr ligand to metal charge transfer in coordination. Phthiocoloxime (3-methyl-2-hydroxy-1,4-naphthoquinone-1-oxime) ligand L, mimics (His/Tyr) ligation with controlled and unique charge transfers resulting in valence tautomeric coordination with mixed valent diiron site in model compound Fe-1: [μ-OH-Fe₂ ^?+?2,?+?3 (o-NQ_CH3ox) (o-NSQ_CH3ox)₂ (CAT) H₂O]. Fe-2: [Fe^?+?3(o-NQ_CH3ox) (p-NQ_CH3ox)₂]₂ a molecularly associated dimer of phthiocoloxime synthesized for comparison of charge transfer. ⁵⁷Fe Mössbauer studies was used to quantitize unusual valences due to ligand in dimeric Fe-1 and Fe-2 complexes which are supported by EPR and SQUID studies. ⁵⁷Fe Mössbauer spectra for Fe-1 at 300 K indicates the presence of two quadrupole split asymmetric doublets due to the differences in local coordination geometries of [Fe^?+?3]_A and [Fe^?+?2]_B sites. The hyperfine interaction parameters are δ _A = 0.152, (ΔE _Q)_A = 0.598 mm/s with overlapping doublet at δ _B = 0.410 and (ΔE _Q)_B = 0.468 mm/s. Due to molecular association tendency of ligand, dimer Fe-2 possesses 100% Fe^?+?3(h.s.) hexacoordinated configuration with isomer shift δ = 0.408 mm/s. Slightly distorted octahedral symmetry created by NQ_CH3ox ligand surrounding Fe^?+?3(h.s.) state generates small field gradient indicated by quadrupole split ΔE _Q = 0.213 mm/s. Decrease of isomer shifts together with variation of quadrupole splits with temperature in Fe-1 dimer compared to Fe-2 is result of charge transfers in [Fe₂ ^?+?2,?+?3 SQ] complexes. EPR spectrum of Fe-1 shows two strong signals at g ₁ = 4.17 and g ₂ = 2.01 indicative of S = 3/2 spin state with an intermediate spin of Fe^?+?3(h.s.) configuration. SQUID data of $\chi _m^{{\rm corr}} \mbox{.T}$ were best fitted by using HDVV spin pair model S = 2, 3/2 resulting in antiferromagnetic exchange (J = ?13.5 cm^???1 with an agreement factor of R = 1.89 × 10^???5). The lower J value of antiferromagnetic exchange leads to Fe⁺³μ-(OH) Fe^?+?2 bridging in Fe-1 dimer instead of μ-oxo bridge. The intermolecular association through H-bonds may lead to weakly coupled antiferromagnetic interaction between two Fe-2 molecules having Fe^?+?3(h.s.) centers. Using S = 5/2, 5/2 spin pair model we obtained best-fitted parameters such as J = ?12.4 cm^???1, g = 2.3 with R = 3.58 × 10^???5. Synthetic strategy results in non-equivalent iron sites in Fe-1 dimer analogues to PAP enzyme hence its reconstitution results in pUC-19 DNA cleavage activity, as physiological functionality of APase. It is compared with nuclease activity of Fe-2 RAPase.     

57Fe Mössbauer spectroscopy on the cyclic spin-cluster Fe6(tea)6(CH3OH)6

We present ⁵⁷Fe Mössbauer spectroscopy experiments on the cyclic spin-cluster Fe₆(tea)₆(CH₃OH)₆ (tea = triethanolaminato(-3)). In former studies, the spin cluster has been treated as a homogenous, quasi-one-dimensional spin S=5/2 Heisenberg antiferromagnet. Our experiments reveal spectra, which consists of two different quadrupolar doublets. In consequence, there are two different Fe sites among the hexanuclear iron spin-cluster.     

Laser induced breakdown spectroscopy surface analysis correlated with the process of nanoparticle production by laser ablation in liquids

Linkage isomerism is the coexistence of iso-compositional molecules or solids differing by connectivity of the metal to a ligand. In a crystalline solid state, the rotation is possible for asymmetric ligands, e.g., for cyanide ligand. Here we report on our observation of a phase transition in anhydrous RbMn[Fe(CN)₆] (nearly stoichiometric) and on the effect of linkage isomerism ensuing our interpretation of the results of Mössbauer study in which we observe the iron spin state crossover among two phases involved into this transition. The anhydrous RbMn[Fe(CN)₆] can be prepared via prolonged thermal treatment (1 week at at 80 °C) of the as-synthesized hydrated RbMn[Fe(CN)₆]·H₂O. The latter compound famous for its charge-transfer phase transition is a precursor in our case. As the temperature is raising above 80 °C (remaining below 100 °C) we observe RbMn[Fe(CN)₆] that inherited its F-43 m symmetry from RbMn[Fe(CN)₆]·H₂O transforming to a phase of the Fm-3 m symmetry. In the latter, more than half of Fe^3?+? ions are in high-spin state. We suggest a plausible way to explain the spin-crossover that is to allow the linkage isomerism by rotation of the cyanide ligands.     

Magnetic properties of the cyclic spincluster Fe6:bicine

The magnetic properties of the cyclic compound [Fe₆(bicine)₆] LiClO₄ ^. 2MeOH are reported. The cluster Fe₆(bicine)₆ forms an antiferromagnetically coupled ring structure of Fe ^III ions. The magnetic susceptibility is measured between 2 and 300 K and yields the exchange coupling of J/k _B = - 27.5±0.5 K. The field dependence of the magnetic moment is studied at 3 and 6 K in magnetic fields up to 5 T. The zero-field splitting of the first excited spin states with S = 2 and 3 are determined by ESR at 94 GHz. The intra-molecular interactions of the Fe ^III ions are analyzed and the on-site anisotropy of the Fe ^III due to the ligand-configuration is determined to d /k _B = - 0.633±0.008K. Received 28 October 2002 / Received in final form 22 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: bernd@piobelix.physik.uni-karlsruhe.de     

The analysis of spin Hamiltonian and crystal field tensors for Fe3+ in crystals of LiCaAlF6 and LiSrAlF6

In single crystals of LiCaAlF₆ and LiSrAlF₆ doped with Fe³⁺ the trigonal EPR spectra with multiplicityK _M = 1 were observed due to Fe³⁺ substituted for Al³⁺. The spin Hamiltonian parameters describing the fine structure and the superhyperfine structure were determined. It is argued that the tensorsB ₂ andB ₄ of the spin Hamiltonian for Fe³⁺ ions are essentially determined by the quadratic contributions of the crystal field at the substitution site. The signs and the relative values of the elements in the spin Hamiltonian tensorB ₄ of rankL = 4 for Fe³⁺ are determined by the irreducible tensor product [V ₄ ?V ₄]₂ of the crystal field tensorV ₄ of rankL = 4 at the substitution sites. The ratio between the invariant sum of the spin Hamiltonian tensorB ₄ for Fe³⁺ in oxygen octahedra [FeO₆] and that in fluorine octahedra [FeF₆] is directly proportional to the fourth power of the ratio between the effective charges of surrounding ions. The sign of the spin Hamiltonian parameterB ₂₀ corresponds to the sign of the element [V ₄ ?V ₄]₂₀ in the irreducible tensor product [V ₄ ?V ₄]₂ of rankL = 2.     

X-Band Pulsed ENDOR Study ofFe-Substituted Sodalite— The Effect of the Zero-Field Splitting

X-band (∼9.3 GHz) pulsed ENDOR measurements were carried out on⁵⁷Fe-substituted sodalite (FeSOD) which contains only one type of Fe(III) (S=) located at a framework site. The ENDOR spectrum recorded atg= 2 shows three doublets corresponding to the sixM_Smanifolds. The assignment of these signals was confirmed by hyperfine-selective and triple ENDOR experiments. The components of each of the doublets had different intensities, reflecting the different populations of the EPR energy levels at the measurement temperature, 1.8 K. ENDOR spectra were recorded at magnetic fields within the EPR powder pattern, and the field dependence observed showed an anisotropic behavior, unexpected from the isotropic character of the⁵⁷Fe(III) hyperfine coupling. This dependence was attributed to the high-order effects of the zero-field splitting (ZFS) interaction on the ENDOR frequencies. Three different theoretical approaches were used to account for the dependence of the ENDOR spectrum on the ZFS interaction. The first involves the exact diagonalization of the total spin Hamiltonian, the second uses third-order perturbation approximations, and the third employs an effective nuclear Hamiltonian for each of theM_Smanifolds. The simulations showed that the ENDOR signals of theM_S= ±5/2 (ν_±5/2) manifold are the least sensitive to the magnitude of the ZFS parameterDand are therefore the most appropriate for the determination ofa_iso. It is shown that at X band anda_isovalues of about 30 MHz, the perturbation approach is valid up toDvalues of 500 MHz if all three doublets are concerned. However, if only the ν_±5/2doublet is considered, then this approach is valid forD< 1000 MHz. The third approach was found inappropriate fora_isovalues of ∼30 MHz. Using the method of exact diagonalization together with orientation selectivity, the trends observed in the experimental spectra could be reproduced. The ENDOR spectra of the⁵⁷Fe-substituted zeolites ZSM5, L, and mazzite showed broad and ill-defined peaks since the ZFS of Fe(III) in these zeolites is significantly larger than that of FeSOD. Because this broadening is a high-order effect, it can be significantly reduced at higher spectrometer frequencies.     

EPR of trivalent iron centers in a BaF2: Fe crystal

An orthorhombic paramagnetic Fe³⁺ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF₂: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F^? ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba²⁺ cation by a Fe²⁺ cation, which transforms into a Fe³⁺ (⁶ A _1g ) cation under x-ray irradiation and shifts into a neutral position along the C₂ axis of the cubic coordination shell of the replaced host cation.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Probing electronic and structural properties of the reduced [2Fe−2S] cluster by orientation-selective1H ENDOR spectroscopy: Adrenodoxin versus rieske iron-sulfur protein

The¹H electron-nuclear double resonance (ENDOR) spectra in frozen buffer solutions of the reduced [2Fe?2S] clusters in adrenodoxin (Adx) and in the “Rieske” iron-sulfur protein (ISP) from the bovine mitochondrial bc1 complex were measured at low temperatures (5–20 K) and analyzed by spectra reconstruction. A single paramagnetic species with iron valence states (II) and (III) connected uniquely to the cluster irons was found in both proteins. For Adx, the experimental spectra from 23 field positions across the nearly axial (g _max=2.0241,g _int=1.9347, andg _min=1.9331) electron paramagnetic resonance (EPR) spectrum were analyzed. Four larger hyperfine couplings were assigned to the cysteine β-protons near the Fe(III) ion. Transfer into the crystal structure showed that the Fe(III) ion was coordinated to the residues Cys55 and Cys92. The spin density was estimated as +1.60 for the Fe(III) and ?0.6 for the Fe(II) ion, respectively. Theg-tensor direction with respect to the cluster showed strong similarities with the earlier assignment inArthospira platensis ferredoxin (Canne C., Ebelshauser M., Gay E., Shergill J.K., Cammack R., Kappl R., Hüttermann J.: J. Biol. Inorg. Chem. 5, 514, 2000). An Adx mutant (T54A) exhibiting a change (70 mV) in redox potential showed no significant influence at the [2Fe?2S] cluster. The Rieske ISP was subjected to the same analysis. The ENDOR spectra from 35 field positions across the rhombic (g _max=2.028,g _int=1.891, andg _min=1.757) EPR spectrum were simulated. Three major proton contributions were identified from the orientation behavior. Two were assigned to cysteine β-protons and one to a β-proton of His141. In contrast to Adx, the direction of theg _max-component was found to lie roughly in the FeS-core plane and the largest proton coupling occurred alongg _int. The spin population was estimated as about +1.6 for the oxidized and ?0.55 for the reduced iron.     

3-Methyl and N-Methyl-substitution in heptanuclear complexes effects multistability investigated by Mössbauer spectroscopy

The precursors [Fe^(III)(^N???R???L)Cl] (^N???R???LH₂ = N,N ^′ -bis(2’-hydroxy-3’- methyl-benzyliden)-1,7-diamino-4-R-4-azaheptane, R = H, methyl(Me)) are high-spin (S?=?5/2) complexes. The Lewis-acidic precursors are combined with Lewis-Base-bridging-units [M(CN) _x ] ^y??? (M = Fe^(II), Ru^(II), Co^(III)) to form heptanuclear star-shaped [M{CN–Fe^(III)(^N???R???L)} _x ]Cl _y molecular switches. The starshaped compounds are high-spin systems at room temperature. On cooling to 20 K some of the compounds exhibit multistability, i.e. several iron(III) centers within a molecule switch the spin state as shown by Mössbauer spectroscopy.     

Effect of N-substitution in multinuclear complexes allows interplay between magnetic states and multistability investigated by Mössbauer spectroscopy

A series of pentadentate ligands N–X–⁵LH₂ (X?=?H, Methyl, Benzyl)?=?N–X–saldptn (4-X-N,N′-bis(1-hydroxy-2-benzylidene)-1,7-diamino-4-azaheptane) has been prepared by a Schiff base condensation between 1,7-diamino-4-X-azaheptane and salicylaldehyde. Complexation with Fe(III) yields a series of high-spin (S?=?5/2) complexes of [Fe^III(N–X–⁵L)Cl]. Such precursors were combined with [Mo(CN)₈]^4? and a series of blue nonanuclear cluster compounds [Mo^IV{(CN)Fe^III(N–X–⁵L)}₈]Cl₄ resulted. Such star-shaped nonanuclear compounds are high-spin systems at room temperature. On cooling to 10 K some of the iron(III) centers switched to the low-spin state as proven by Mössbauer spectra, i.e. multiple electronic transitions. Parts of the compounds perform a high-spin to high-spin transition. Under light irradiation the populations are altered slightly.     

Carbon-13 EPR Hyperfine Interaction in the Complex [Rh(CN)6]in a KCl Host Lattice

The hyperfine interaction with¹³C in nonenriched [Rh(CN)₆]⁴⁻, in a KCl host lattice at 7 K, is measured by continuous-wave X- and Q-band electron paramagnetic resonance, allowing, for the first time, a comparative study of spin densities on the carbon of the axial cyanide in this complex and in the similar [Co(CN)₆]⁴⁻complex. From the experimental data corrected for dipolar interactions,A_iso= 79.80 × 10⁻⁴cm⁻¹andA_anis= 0.76 × 10⁻⁴cm⁻¹.     

Magnetic and Mössbauer spectra observed for mixed-metal magnets NBu4 {\bf Fe}^{\bf II}_{\bf n}MA II 1 − n [FeIII(OX)3] (MA=Mn, Fe)

Mixed-metal molecular-based magnets NBu₄ ${\rm Fe}^{\rm II}_{\rm n}$ M_A ^II _1???n[Fe^III(OX)₃] (M_A=Mn, Fe) were investigated by magnetic and Mössbauer measurements. The magnetic susceptibility of NBu₄ ${\rm Fe}^{\rm II}_{0.07}{\rm Mn}^{\rm II}_{0.93}$ [Fe^III(OX)₃] can be fitted to a Curie-Weiss law with a Weiss paramagnetic Curie temperature of θ?=??114.76 K. The negative Weiss constant indicates an intramolecular antiferromagnetic coupling interaction between the adjacent Fe(II) and Fe(III) ions through the oxalate bridge. In the complex NBu₄Fe^II[Fe^III(OX)₃], the Mössbauer results indicate that the Fe^II and Fe^III sublattices experience spontaneous magnetizations. The compound contains two different spin carriers; i.e. Fe^II(S = 2), Fe^III(S = 5/2). Two magnetic sublattices are defined. The appearance of nuclear Zeeman splittings suggests that long range magnetic ordering takes place below 50 K.     

Synthesis of ultrasmall magnetic iron oxide nanoparticles and study of their colloid and surface chemistry

Colloidal nanoparticles of Fe₃O₄ (4 nm) were synthesized by high-temperature hydrolysis of chelated iron (II) and (III) diethylene glycol alkoxide complexes in a solution of the parent alcohol (H₂DEG) without using capping ligands or surfactants: [Fe(DEG)Cl₂]²⁻+2[Fe(DEG)Cl₃]²⁻+2H₂O+2OH⁻→Fe₃O₄+3H₂DEG+8Cl⁻ The obtained particles were reacted with different small-molecule polydentate ligands, and the resulting adducts were tested for aqueous colloid formation. Both the carboxyl and α-hydroxyl groups of the hydroxyacids are involved in coordination to the nanoparticles’ surface. This coordination provides the major contribution to the stability of the ligand-coated nanoparticles against hydrolysis.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.