Dimeric Iron Research Articles

Solvent Influence on the Magnetic Properties of Iron(II) Spin‐Crossover Coordination Compounds with 4,4′‐Dipyridylethyne as Linker

AbstractNew iron(II) coordination polymers with solvent‐dependent spin‐crossover behaviour have been synthesised by using an N2O2 Schiff base like ligand and 4,4′‐dipyridylethyne as the bridging axial ligand in different solvents. The effects of different solvents and preparation methods (temperature, concentration) on the magnetic properties have been investigated. Annealing the coordination polymers prepared by the same method but in different solvents led to the same compound exhibiting the same three‐step spin crossover independent of the magnetic properties of the solvated species. The X‐ray structures of the axial ligand, an iron(II) dimer and one of the iron(II) coordination polymers are discussed. Powder X‐ray diffraction analysis was additionally used for further comparison of the compounds.

Special aspects of diffusion mechanisms and dynamic behavior of Mössbauer iron atoms on a nanoclay surface

Using a technique that is developed using Mossbauer spectroscopy, parameters of the diffusion motion and data on geography of the exchangeable iron complexes on a silica–alumina surface of clays were presented. The analyzed natural causes of the temperature behavior of the dynamic Mossbauer parameters allowed the pattern of the atoms' motion to be determined on the basis of various models of diffusion motion. For iron atoms adsorbed in the interwrapper space of the mineral, a jumplike translational diffusion is typical, which allowed determination of the sizes of the fluctuation hollow (r ~ 0.16 nm) necessary for the diffusion of atoms. A nonisotropic character of the diffusion for the interlayer and surface iron atoms caused by structural anisotropy of a clay substrate and by the structure of the iron complexes themselves was found. The surface diffusion of iron dimers occurs in a bounded area (L ~ 0.25 nm) and is rotational.

Crystal structure of a μ-oxo-bridged dimeric iron(III) complex

A dimeric [{Fe(5-ClL1)}2(μ-O)], [H2-5-ClL1 = N,N′-bis(5-chloro-2-hydroxybenzylidene)-2-methylpropane-1,2-diamine] tetradentate Schiff-base complex, 1, has been synthesized and its crystal structure has been determined by single crystal X-ray diffraction analysis. Structural analysis of complex 1 shows that the complex is a centrosymmetric dimer. Each of the Fe(III) ions has a five-coordinate geometry and one oxygen atom bridges two Fe(III) ions to form a μ-oxo structure. The geometry around iron atom can be described as a square based pyramid with the FeN2O2 coordination plane and oxo ligand.

New Synthetic Routes to Iron-Sulfur Clusters: Deciphering the Repair Chemistry of [2Fe-2S] Clusters from Mononitrosyl Iron Complexes.

The nitrosylation of inorganic protein cofactors, specifically that of [Fe-S] clusters to form iron nitrosyls, plays a number of important roles in biological systems. In some of these cases, it is expected that a repair process reverts the nitrosylated iron species to intact [Fe-S] clusters. The repair of nitrosylated [2Fe-2S] cluster, primarily in the form of protein-bound dinitrosyl iron complexes (DNICs), has been observed in vitro and in vivo, but the mechanism of this process remains uncertain. The present work expands upon a previous observation (Fitzpatrick et al. J. Am. Chem. Soc. 2014, 136, 7229) of the ability of mononitrosyl iron complexes (MNICs) to be converted into [2Fe-2S] clusters by the addition of nothing other than a cysteine analogue. Herein, each of the critical elementary steps in the cluster repair has been dissected to elucidate the roles of the cysteine analogue. Systematic variations of a cysteine analogue employed in the repair reaction suggest that (i) the bidentate coordination of a cysteine analogue to MNIC promotes NO release from iron, and (ii) deprotonation of the α carbon of the ferric-bound cysteine analogue leads to the C-S cleavage en route to the formation of [2Fe-2S] cluster. The [2Fe-2S] cluster bearing a cysteine analogue has also been synthesized from thiolate-bridged iron dimers of the form [Fe2(μ-SR)2(SR)4](0/2-), which implies that such species may be present as intermediates in the cluster repair. In addition to MNICs, mononuclear tetrathiolate ferric or ferrous species have been established as another form of iron from which [2Fe-2S] clusters can be generated without need for any other reagent but a cysteine analogue. The results of these experiments bring to light new chemistry of classic coordination complexes and provides further insight into the repair of NO-modified [2Fe-2S] clusters.

Solution structures of chloroquine–ferriheme complexes modeled using MD simulation and investigated by EXAFS spectroscopy

The interaction of chloroquine (CQ) and the μ-oxo dimer of iron(III) protoporphyrin IX (ferriheme) in aqueous solution was modeled using molecular dynamics (MD) simulations. Two models of the CQ-(μ-oxo ferriheme) complex were investigated, one involving CQ π-stacked with an unligated porphyrin face of μ-oxo ferriheme and the other in which CQ was docked between the two porphyrin rings. The feasibility of both models was tested by fitting computed structures to the experimental extended X-ray absorption fine structure (EXAFS) spectrum of the CQ-(μ-oxo ferriheme) complex in frozen aqueous solution. The docked model produced better agreement with experimental data, suggesting that this is the more likely structure in aqueous solution. The EXAFS fit indicated a longer than expected Fe–O bond of 1.87Å, accounting for the higher than expected magnetic moment of the complex. As a consequence, the asymmetric Fe–O–Fe stretch shifts much lower in frequency and was identified in the precipitated solid at 744cm−1 with the aid of the O18 isomer shift. Three important CQ-ferriheme interactions were identified in the docked structure. These were a hydrogen bond between the oxide bridge of μ-oxo ferriheme and the protonated quinolinium nitrogen atom of CQ; π-stacking between the quinoline ring of CQ and the porphyrin rings; and a close contact between the 7-chloro substituent of CQ and the porphyrin methyl hydrogen atoms. These interactions can be used to rationalize previously observed structure–activity relationships for quinoline–ferriheme association.

Reactivity Modes of an Iron Bis(alkoxide) Complex with Aryl Azides: Catalytic Nitrene Coupling vs Formation of Iron(III) Imido Dimers

The iron bis(alkoxide) complex Fe(OR)2(THF)2 (R = CtBu2Ph), 1, was found to have strikingly different reactivity with various aryl azides, ArN3. Azides with methyl or ethyl groups in the ortho positions of the phenyl ring react catalytically via nitrene coupling to give azoarenes, ArNNAr. Catalyst loading as low as 1 mol % yields clean, quantitative conversion of aryl azides to azoarenes at room temperature in as little as 4 h. A combination of two different aryl azides leads to the catalytic formation of all three possible azoarenes, including the asymmetric one. In contrast, reactions with aryl azides lacking ortho substituents yield stable dimeric iron imido complexes of the form (RO)(THF)Fe(μ-NAr)2Fe(THF)(OR) (Ar = 4-(trifluoromethyl)phenyl, 5; Ar = phenyl, 6; Ar = 3,5-dimethylphenyl, 7), which do not undergo catalytic nitrene coupling. The isocyanide adduct Fe(OR)2(CNR)2 (4, R = 2,6-dimethylphenyl) was obtained from the reaction of Fe(OR)2(THF)2 with two equivalents of isocyanide. No C–N bond formati...

Dinuclear iron(III) complexes with different ligation for ring opening polymerization of lactide

Dimeric iron(III) complexes bearing acetylacetonate (acac) ligands and tridentate chiral Schiff base ligands, which were synthesized from condensation reactions between salicylaldehyde and amino-indanol, were prepared and characterized by various methods, including infrared spectroscopy and mass spectrometry. X-ray analysis revealed that two iron centers are each coordinated octahedrally within the complexes, but are situated in different coordination environments; one is attached to two tridentate ligands with meridional isomerization, while the other is coordinated to two acac molecules and two of the oxygen atoms of the indanol moieties of the Schiff bases. The complexes showed higher activity than Fe(acac)3 for the ring opening polymerization of lactide.

Structure and magnetic properties of an unusual homoleptic iron(III) thiocyanate dimer.

We describe the structural and variable temperature magnetic susceptibility properties of an unusual homoleptic bimetallic iron(III) thiocyanate tetraanion. This work represents the first structurally characterized bis(μ-1,3-thiocyanato) dimer of iron(III). A weak antiferromagnetic exchange interaction is observed between the two iron(III) ions, which is supported by broken symmetry density functional theory (DFT) calculations.

Neutral diiron(III) complexes with Fe₂(μ-E)₂ (E = O, S, Se) core structures: reactivity of an iron(I) dimer towards chalcogens.

Three neutral bis(μ-chalcogenido)diiron(III) complexes, [{(N,N'-Pipiso)Fe(μ-E)}2] (Pipiso(-) = [(DipN)2C(cis-2,6-Me2NC5H8)](-), (Dip = C6H3Pr(I)2-2,6; E = O, S or Se) have been prepared by reactions of the iron(I) dimer [{(μ-N,N'-Pipiso)Fe}2] with O2, S8 or Se∞. Treating the μ-selenido compound [{(N,N'-Pipiso)Fe(μ-Se)}2] with O2 cleanly generated its μ-oxo counterpart, [{(N,N'-Pipiso)Fe(μ-O)}2]. X-ray crystallographic analyses of the compounds showed them to possess Fe2(μ-E)2 core structures with distorted square planar (E = O) or tetrahedral (E = S or Se) iron coordination geometries. Magnetic, (57)Fe Mössbauer spectroscopic and computational studies indicate medium to strong antiferromagnetic coupling between the two high-spin Fe(III) ions in all three compounds.

Photophysical and electrochemical studies of multinuclear complexes of iron(II) with acetate and extended conjugated N-donor ligands.

A dimeric iron(II) complex, trans-[Fe2(CH3COO)4(L1)2] (1), and a trinuclear iron(II) complex, [Fe3(CH3COO)4(H2O)4(L2)] (2), were studied as potential dye-sensitised solar cell materials. The structures of both complexes were deduced by a combination of instrumental analyses and molecular modelling. Variable-temperature magnetic susceptibility data suggested that 1 was made up of 56.8% high-spin (HS) and 43.2% low-spin (LS) Fe(II) atoms at 294 K and has a moderate antiferromagnetic interaction (J = −81.2 cm−1) between the two Fe(II) centres, while 2 was made up of 27.7% HS and 72.3% LS Fe(II) atoms at 300 K. The optical band gaps (E o) for 1 were 1.9 eV (from absorption spectrum) and 2.2 eV (from fluorescence spectrum), electrochemical bandgap (E e) was 0.83 eV, excited state lifetime (τ) was 0.67 ns, and formal redox potential (E′(FeIII/FeII)) was +0.63 V. The corresponding values for 2 were 3.5 eV (from absorption spectrum), 1.8 eV (from fluorescence spectrum), 0.69 eV, 2.8 ns, and +0.41 V.

Vibrational spectroscopy and theory of Fe2+(CH4)n (n = 1–3)

Vibrational spectra are measured for Fe2(+)(CH4)n (n = 1-3) in the C-H stretching region (2650-3100 cm(-1)) using photofragment spectroscopy, by monitoring the loss of CH4. All of the spectra exhibit an intense peak corresponding to the symmetric C-H stretch around 2800 cm(-1). The presence of a single peak suggests a nearly equivalent interaction between the iron dimer and the methane ligands. The peak becomes slightly blue shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and the complexes are in the octet spin state.

Interplay between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida interaction.

The interplay between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect is expected to provide the driving force for the emergence of many phenomena in strongly correlated electron materials. Two magnetic impurities in a metal are the smallest possible system containing all these ingredients and define a bottom-up approach towards a long-term understanding of concentrated/dense systems. Here we report on the experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of low-temperature scanning tunnelling spectroscopy combined with density functional theory and numerical renormalization group calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. It oscillates as a function of dimer separation revealing that it is related to indirect exchange interactions mediated by the conduction electrons.

Study of iron dimers reveals angular dependence of valence-to-core X-ray emission spectra.

Transition-metal Kβ X-ray emission spectroscopy (XES) is a developing technique that probes the occupied molecular orbitals of a metal complex. As an element-specific probe of metal centers, Kβ XES is finding increasing applications in catalytic and, in particular, bioinorganic systems. For the continued development of XES as a probe of these complex systems, however, the full range of factors which contribute to XES spectral modulations must be explored. In this report, an investigation of a series of oxo-bridged iron dimers reveals that the intensity of valence-to-core features is sensitive to the Fe–O–Fe bond angle. The intensity of these features has a well-known dependence on metal–ligand bond distance, but a dependence upon bond angle has not previously been documented. Herein, we explore the angular dependence of valence-to-core XES features both experimentally and computationally. Taken together, these results show that, as the Fe–O–Fe angle decreases, the intensity of the Kβ″ feature increases and that this effect is modulated by increasing amounts of Fe np mixing into the O 2s orbital at smaller bond angles. The relevance of these findings to the identification of oxygenated intermediates in bioinorganic systems is highlighted, with special emphasis given to the case of soluble methane monooxygenase.

Low temperature activation of S8, Se(red) and α-Te with [Cp(BIG)Fe(CO)2] radicals.

The bulky dimeric iron complex, [Cp(BIG)Fe(CO)2]2, readily activates elemental chalcogens (S8, Se(red) and α-Te) under mild conditions at room temperature. Six compounds containing Q2(2-) ligands (Q = S, Se) and a Te(2-) ligand, respectively, were isolated and completely characterized, including by X-ray diffraction techniques.

Stability and catalytic properties of μ-oxo and μ-nitrido dimeric iron tetrasulfophthalocyanines in the oxidation of Orange II by tert-butylhydroperoxide

A study of catalytic activity of μ-nitrido- and μ-oxo-dimeric iron tetrasulfophthalocyanines in the oxidation of Orange II by tert-butylhydroperoxide in aqueous solutions has been performed. It is shown that though in one catalytic cycle activity of μ-oxo-dimer is higher, stability of this complex in oxidative conditions is poor. μ-nitrido-dimer combines relatively good catalytic activity with very high stability in the presence of tert-butylhydroperoxide. The mechanisms of oxidative decomposition of dimers and catalytic oxidation of Orange II have been proposed on the base of kinetic results. The products of catalytic processes are shown to be bio-degradable non-toxic small organic compounds.

Magnetic properties of Cu3.9Fe3.4V6O24 with a lyonsite structure

Magnetic properties of the lyonsite-type phase, Cu3.9Fe3.4V6O24, that has been synthesized by a standard solid-state reaction method, were investigated by dc magnetization and electron paramagnetic resonance (EPR) techniques. Complex magnetic behavior and transition to the antiferromagnetic phase at liquid helium temperature have been revealed by measurements of dc susceptibility in ZFC and FC modes in the 2–300 K temperature range as well as static magnetization in magnetic fields up to 70 kOe. Strong antiferromagnetic interactions in clusters or chains of magnetic ions even at high temperatures has been deduced from the obtained value of the effective magnetic moment that was significantly smaller than expected for nominal valences of iron and copper ions. The presence of antiferromagnetic iron dimers in the high-temperature range, critical slowing down of spin fluctuations on approaching Neel temperature and the existence of magnetically isolated iron ions in the antiferromagnetic phase has been suggested from the temperature dependence of EPR parameters (g-factor, linewidth, integrated intensity) obtained by fitting the experimental spectrum with Lorentzian lineshape.

Crystal structure and thermal stability of A μ1,1-(OMe)-bridged dimeric Schiff base iron(III) complex

A new μ1,1-OMe-bridged dimeric iron(III) complex, [Fe2L2(μ1,1-OMe)2(NCS)2], where L is the deprotonated form of 2-[(2-ethylaminoethylimino)methyl]-5-methoxyphenol, has been prepared and structural characterized by elemental analysis, IR spectrum, and single crystal X-ray crystallography. The complex crystallizes in the monoclinic space group P21/c, with unit cell dimensions a = 10.156(1) A, b = 11.972(1) A, c = 14.256(2) A, β = 102.643(3)°, V = 1691.3(3) A3, Z = 2, R1 = 0.0394, and wR2 = 0.0922. Each Fe atom in the complex is in an octahedral coordination. The Fe...Fe distance is 3.102(1) A. The thermal stability of the complex was studied.

Multimetallic cooperativity in activation of dinitrogen at iron–potassium sites

The reaction of soluble iron-oxygen-potassium assemblies with N2 gives insight into the mechanisms of multimetallic N2 coordination. We report a series of very electron-rich three-coordinate, β-diketiminate-supported iron(I) phenoxide complexes, which are metastable but have been characterized under Ar by both crystallography and solution methods. Both monomeric and dimeric Fe-OPh-K compounds have been characterized, and their iron environments are very similar in the solid and solution states. In the dimer, potassium ions hold together the phenoxide oxygens and aryl rings of the two halves, to give a flexible diiron core. The reactions of the monomeric and dimeric iron(I) compounds with N2 are surprisingly different: the mononuclear iron(I) complexes give no reaction with N2, but the dimeric Fe2K2 complex reacts rapidly to give a diiron-N2 product. Computational studies show that the key to the rapid N2 reaction of the dimer is the preorganization of the two iron atoms. Thus, cooperation between Fe (which weakens the N-N bond) and K (which orients the Fe atoms) can be used to create a low-energy pathway for N2 reactions.

Low-valent Iron Complexes Stabilised by a Bulky Guanidinate Ligand: Synthesis and Reactivity Studies

A toluene-capped guanidinato iron(i) complex [(Pipiso)Fe(η6-toluene)] (Pipiso = [(DipN)2C(cis-NC5H8Me2-2,6)]–) was prepared by magnesium metal reduction of {[(Pipiso)FeII(µ-Br)]2} in toluene. The reactivity of the closely related FeI–FeI multiply bonded species, {[Fe(μ-Pipiso)]2} towards a range of unsaturated small molecule substrates was investigated, and found to be broadly similar to that of low-valent β-diketiminato iron complexes. That is, its reaction with CO yielded the iron(i) carbonyl complex [(Pipiso)Fe(CO)3], whereas reaction with CO2 formed the same product via an apparent reductive disproportionation of the substrate. In contrast, reaction between {[Fe(μ-Pipiso)]2} and CS2 led to reductive C=S bond cleavage and the isolation of {[(Pipiso)Fe]2(μ-S)(μ-CS)}. Different reactivity was seen with AdN3 (Ad = 1-adamantyl), which was reductively coupled by the iron(i) dimer to give iron(ii) hexaazenyl complex {[(Pipiso)Fe]2(μ-AdN6Ad)}.

Catalysis and molecular magnetism of dinuclear iron(iii) complexes with N-(2-pyridylmethyl)-iminodiethanol/-ate

The reaction of N-(2-pyridylmethyl)iminodiethanol (H2pmide) and Fe(NO3)3·9H2O in MeOH led to the formation of a dimeric iron(III) complex, [(Hpmide)Fe(NO3)]2(NO3)2·2CH3OH (1). Its anion-exchanged form, [(pmide)Fe(N3)]2 (2), was prepared by the reaction of 1and NaN3 in MeOH, during which the Hpmide ligand of 1 was also deprotonated. These compounds were investigated by single crystal X-ray diffraction and magnetochemistry. In complex 1, one iron(III) ion was bonded with a mono-deprotonated Hpmide ligand and a nitrate ion. The two iron(III) ions within the dinuclear unit were connected by two ethoxy groups with an inversion center. In 2, one iron(III) ion was coordinated with a deprotonated pmide ligand and an azide ion. The Fe(pmide)(N3) unit was related by symmetry through an inversion center. Both 1 and 2 efficiently catalyzed the oxidation of a variety of alcohols under mild conditions. The oxidation mechanism was proposed to involve an Fe(IV)=O intermediate as the major reactive species and an Fe(V)=O intermediate as a minor oxidant. Evidence for this proposal was derived from reactivity and Hammett studies, KIE (kH/kD) values, and the use of MPPH (2-methyl-1-phenylprop-2-yl hydroperoxide) as a mechanistic probe. Both compounds had significant antiferromagnetic interactions between the iron(III) ions via the oxygen atoms. 1 showed a strong antiferromagnetic interaction within the Fe(III) dimer, while 2 had a weak antiferromagnetic coupling within the Fe(III) dimer.