Individual Iron Research Articles

A Trinuclear High-Spin Iron(III) Complex with a Geometrically Frustrated Spin Ground State Featuring Negligible Magnetic Anisotropy and Antisymmetric Exchange.

The trinuclear high-spin iron(III) complex [Fe3Cl3(saltagBr)(py)6]ClO4 {H5saltagBr = 1,2,3-tris[(5-bromo-salicylidene)amino]guanidine} was synthesized and characterized by several experimental and theoretical methods. The iron(III) complex exhibits molecular 3-fold symmetry imposed by the rigid ligand backbone and crystallizes in trigonal space group P3̅ with the complex cation lying on a crystallographic C3 axis. The high-spin states (S = 5/2) of the individual iron(III) ions were determined by Mößbauer spectroscopy and confirmed by CASSCF/CASPT2 ab initio calculations. Magnetic measurements show an antiferromagnetic exchange between the iron(III) ions leading to a geometrically spin-frustrated ground state. This was complemented by high-field magnetization experiments up to 60 T, which confirm the isotropic nature of the magnetic exchange and negligible single-ion anisotropy for the iron(III) ions. Muon-spin relaxation experiments were performed and further prove the isotropic nature of the coupled spin ground state and the presence of isolated paramagnetic molecular systems with negligible intermolecular interactions down to 20 mK. Broken-symmetry density functional theory calculations are consistent with the antiferromagnetic exchange between the iron(III) ions within the presented trinuclear high-spin iron(III) complex. Ab initio calculations further support the absence of appreciable magnetic anisotropy (D = 0.086, and E = 0.010 cm-1) and the absence of significant contributions from antisymmetric exchange, as the two Kramers doublets are virtually degenerate (ΔE = 0.005 cm-1). Therefore, this trinuclear high-spin iron(III) complex should be an ideal candidate for further investigations of spin-electric effects arising exclusively from the spin chirality of a geometrically frustrated S = 1/2 spin ground state of the molecular system.

Catalytic Carbonyl-Olefin Metathesis of Aliphatic Ketones: Iron(III) Homo-Dimers as Lewis Acidic Superelectrophiles.

Catalytic carbonyl-olefin metathesis reactions have recently been developed as a powerful tool for carbon-carbon bond formation. However, currently available synthetic protocols rely exclusively on aryl ketone substrates while the corresponding aliphatic analogs remain elusive. We herein report the development of Lewis acid-catalyzed carbonyl-olefin ring-closing metathesis reactions for aliphatic ketones. Mechanistic investigations are consistent with a distinct mode of activation relying on the in situ formation of a homobimetallic singly bridged iron(III)-dimer as the postulated active catalytic species. These "superelectrophiles" function as more powerful Lewis acid catalysts that form upon association of individual iron(III)-monomers. While this mode of Lewis acid activation has previously been postulated to exist, it has not yet been applied in a catalytic setting. The insights presented are expected to enable further advancement in Lewis acid catalysis by building upon the activation principle of "superelectrophiles" and to broaden the current scope of catalytic carbonyl-olefin metathesis reactions.

Dynamics of pH on the oxidation of [formula omitted] with iron(III) chelates in anoxic conditions

A fundamental study about a redox process for the removal of hydrogen sulfide by means of iron(III) organometallic complexes was instigated to evaluate its prospective application for the abatement of total reduced sulfurs in the pulp and paper industry. More specifically, the oxidation kinetics of the hydrosulfide species ( HS - ) in alkaline solutions ( pH = 9 –11) with iron(III) trans-1,2,-diaminocyclohexanetetraacetate (cdta) complexes was investigated at room temperature and near zero ionic strength. Simultaneous measurements of iron(III)-cdta ( C Fe ( III ) 0 : 50– 300 mmol / m 3 ) and HS - ( C HS - 0 : 30– 125 mmol / m 3 ) were used to clarify the reaction mechanism as well as the role of individual iron(III)-cdta species ( Fe 3 + cdta 4 - , Fe 3 + OH - cdta 4 - ) and sulfur species ( HS - , S n 2 - , S 8 ) on the kinetics. Fe 3 + OH - cdta 4 - , being the dominating iron(III)-cdta species at pH > 10 , was found to be less reactive than Fe 3 + cdta 4 - towards the polysulfides ( S n 2 - ; n = 2 –8). Oligomerization of HS - into polysulfides accelerates the HS - conversion rate until elemental sulfur ( S 8 ) appears in the solution. It was determined that elevated alkaline pH decreases the initial HS - conversion rate, stretches the induction period for S 8 formation, and generally inhibits the overall reaction.

Porosity appearance in cobalt ferrite and changes under the thermal treatment of co-precipitated iron(II) and cobalt(II) oxalates

Some aspects of porosity appearance and peculiarities of its change during the thermal treatment of the co-precipitated iron(II) and cobalt(II) oxalates at different temperatures (400–800 °C) have been considered. The porosity of individual iron(III) and cobalt(II) oxides at 400–600 °C and separate oxides porosity contribution to the overall porosity of the cobalt(II) ferrite has been studied as well.

Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy

The knowledge of mineralogy and molecular structure of As is needed to better understand the stability of As in wastes resulting from processing of gold ores. In this study, optical microscopy, scanning electron microscopy, electron microprobe, X-ray diffraction and X-ray absorption fine structure (XAFS) spectroscopy (including both XANES and EXAFS regimes) were employed to determine the mineralogical composition and local coordination environment of As in gold ores and process tailings from bench-scale tests designed to mimic a common plant practice. Arsenic-bearing minerals identified in the ores and tailings include iron (III) oxyhydroxides, scorodite (FeAsO 4·2H 2O), ferric arsenates, arseniosiderite (Ca 2Fe 3(AsO 4) 3O 2·3H 2O), Ca-Fe arsenates, pharmacosiderite (KFe 4(AsO 4) 3(OH) 4·6–7H 2O), jarosite (K 2Fe 6(SO 4) 4(OH) 12) and arsenopyrite (FeAsS). Iron (III) oxyhydroxides contain variable levels of As from trace to about 22 wt% and Ca up to approximately 9 wt%. Finely ground ore and tailings samples were examined by bulk XAFS and selected mineral grains were analyzed by microfocused XAFS (micro-EXAFS) spectroscopy to reconcile the ambiguities of multiple As sources in the complex bulk EXAFS spectra. XANES spectra indicated that As occurs as As 5+in all the samples. Micro-EXAFS spectra of individual iron (III) oxyhydroxide grains with varying As concentrations point to inner-sphere bidentate-binuclear arsenate complexes as the predominant form of As. There are indications for the presence of a second Fe shell corresponding to bidentate-mononuclear arrangement. Iron (III) oxyhydroxides with high As concentrations corresponding to maximum adsorption densities probably occur as nanoparticles. The discovery of Ca atoms around As in iron (III) oxyhydroxides at interatomic distances of 4.14–4.17 Å and the coordination numbers suggest the formation of arseniosiderite-like nanoclusters by coprecipitation rather than simple adsorption of Ca onto iron (III) oxyhydroxides. Correlation of Ca with As in iron (III) oxyhydroxides as determined by electron microprobe analysis supports the coprecipitate origin for the presence of Ca in iron (III) oxyhydroxides. The samples containing higher abundances of ferric arsenates released higher As concentrations during the cyanidation tests. The presence of highly soluble ferric arsenates and Ca-Fe arsenates, and relatively unstable iron (III) oxyhydroxides with Fe/As molar ratios of less than 4 in the ore and process tailings suggests that not only the tailings in the impoundment will continue to release As, but also there is the potential for mobilization of As from the natural sources such as the unmined ore.

Mössbauer studies on iron(II)-substituted yeast metallothionein.

Iron(II)-substituted yeast metallothionein has been studied with Mössbauer spectroscopy. The iron in the protein is in the high-spin ferrous state. A maximum metal content of four iron(II)/molecule has been determined, with the four metal ions forming a diamagnetic cluster due to the antiferromagnetic exchange interaction between Fe2+ via bridging thiolates. In the case where the iron titration gives a value of less than four iron(II)/apoprotein, the metal ions are magnetically noninteracting, with each individual iron(II) behaving like iron(II) in reduced rubredoxin.

Structure of iron(III) and formation of iron(III) thiocyanato complexes in N,N-dimethylacetamide

The solvation structure of the iron(III) ion in N,N,-dimethylacetamide (DMA) has been determined by means of EXAFS (extended X-ray absorption fine structure) spectroscopy and the complex formation between iron(III) and thiocyanate ions has been calorimetrically and spectrophotometrically studied in DMA containing (C 2H 5) 4NClO 4 (0.4 mol dm −3) as a constant ionic medium at 25°C. The iron(III) ion is surrounded with six DMA molecules and has an octahedral [Fe(dma) 6] 3+ structure. Calorimetric and spectrophotometric titration data were well explained in terms of the formation of [Fe(SCN) n ] 3− n) ( n = 1−6) and their formation constants, reaction enthalpies and entropies were determined. Electronic spectra of individual iron(III) thiocyanato complexes in DMA were also extracted. The complexation of iron(III) with thiocyanate ions in DMA is more favorable than that in N,N-dimethylformamide (DMF). The favorable formation is mainly due to the more negative reaction enthalpies in DMA than those in DMF, which is resulting from the larger steric interaction between the ligating DMA molecules around the iron(III) ion than the DMF ones.

Structure of iron(III) ion and its complexation with thiocyanate ion in N, N-dimethylformamide

Complexation of iron(III) with thiocyanate ions has been calorimetrically and spectrophotometrically investigated in N, N-dimethylformamide (DMF) containing 0.4 mol/dm 3 (C 2H 5) 4NClO 4 or 1 mol/dm 3 NH 4ClO 4 as a constant ionic medium at 25°C. Calorimetric titration data were well explained in terms of the formation of [Fe(SCN) n ] (3- n)+ ( n = 1–5), and their formation constants, reaction enthalpies and entropies were determined. Electronic spectra of individual iron(III) thiocyanato complexes were also determined. The stepwise thermodynamic quantities changed monotonously, i.e. Δ G° 1 < Δ G° 2 < Δ G° 3 < Δ G° 4, < Δ G° 5, Δ H° 1 > Δ H° 2 > Δ H° 3 > Δ H° 4 > Δ H° 5, Δ S° 1 > Δ S° 2 > Δ S° 3 > Δ S° 4 > Δ S° 5. This suggests that no extensive desolvation occurred at any step of complexation. On the basis of these thermodynamic quantities, it is postulated that the [Fe(SCN) n ] (3- n)+ ( n = 1–5) complexes have a six-coordinate octahedral structure as well as the [Fe(dmf) 6] 3+ ion, the octahedral structure of which has been confirmed by the EXAFS (extended X-ray absorption fine structure) method.