High-spin Schiff-base dinuclear iron(III) complexes bridged by N-oxide ligands

Synthesis and structure of dinuclear complexes [{Fe^III(L⁵)}b{Fe^III(L⁵)}](BPh₄)₂, where L⁵ is a pentadentate Schiff-base ligand, b is a bidentate N-oxide bridging ligand based on bipyridine, is reported. Magnetic behavior is investigated in terms of the magnetic susceptibility, magnetization, and Mössbauer spectroscopy revealing that the complexes are high-spin over the whole temperature region.     

Biotechnological application of homo- and heterotrinuclear iron(III) furoates for cultivation of iron-enriched Spirulina

New trinuclear iron(III) furoates with the general formula [Fe₃O(α-fur)₆(R-OH)₃]X, where α-fur C₄H₃OCOO, R = CH₃ (1), C₂H₅ (2), n-C₃H₇ (3), n-C₄H₉ (4), X = NO₃⁻ (1-4); [Fe₃O(α-Fur)₆(DMF)(CH₃OH)₂]NO₃ (5); [Fe₃O(α-Fur)₆(H₂O)(CH₃OH)₂]Cl (6); [Fe₂MO(α-Fur)₆(L)(H₂O)₂], where L = THF (7-9), DMF (10-12), M = Mn²⁺ (7, 10), Co²⁺ (8, 11), Ni²⁺ (9, 12) and [Fe₂MO(α-Fur)₆(3Cl-Py)₃], where M = Mn²⁺ (13), Co²⁺ (14), Ni²⁺ (15); have been prepared and investigated by Mössbauer and IR spectroscopy. The X-ray crystal structure for the 1·2CH₃OH complex indicates that it crystallizes in the monoclinic crystal system (P2₁/n) and has a structure typical of μ₃-O-bridged trinuclear iron(III) compounds. Coordination compounds 1, 4, 7, 8 can be used as regulators of the biochemical composition of cyanobacterium Spirulina platensis biomass. The supplementation of these compounds, in concentrations exceeding 5-10 mg/l, increases the content of iron, amino acids, peptides and carbohydrates in Spirulina.     

X-ray and Fe Mössbauer study on a Fe(III) complex of 2-acetyl-1,3-indandione

Single-crystal X-ray structure of an iron(III) complex of 2-acetyl-1,3-indandione is resolved which reveals that a racemic mixture, composed of Δ-fac and Λ-fac stereoisomers, is formed. Both species have octahedral geometry, but slightly distinguishable. Detailed Mössbauer data indicate a high-spin electronic structure of the Fe(III) centres Fe1 and Fe2 with spin-spin magnetic relaxation process. Two different approaches for computer processing of the experimental Mössbauer spectra are considered.     

The Fe(III) and Ga(III) coordination chemistry of 3-(1-hydroxymethylidene) and 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione: novel tetramic acid degradation products of homoserine lactone bacterial quorum sensing molecules

Bacteria use small diffusible molecules to exchange information in a process called quorum sensing (QS). An important class of quorum sensing molecules used by Gram-negative bacteria is the family of N-acylhomoserine lactones (HSL). It was recently discovered that a degradation product of the QS molecule 3-oxo-C₁₂-homoserine lactone, the tetramic acid 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione, is a potent antibacterial agent, thus implying roles for QS outside of simply communication. Because these tetramic acids also appear to bind iron with appreciable affinity it was suggested that metal binding might contribute to their biological activity. Here, using a variety of spectroscopic tools, we describe the coordination chemistry of both the methylidene and decylidene tetramic acid derivatives with Fe(III) and Ga(III) and discuss the potential biological significance of such metal binding.     

Synthesis, structure and Mössbauer characterization of polymeric iron(II) complexes with bidentate thiourea ligands

Complexes of FeCl₂ with the known bis(3-methyl-2-thione-imidazolyl)methane (L¹) and the new bis(3-tert-butyl-2-thione-imidazolyl)methane (L²) are reported. For both [L¹FeCl₂]_n (3) and [L²FeCl₂]_n (4) X-ray crystallography reveals that 1D-polymeric chain structures are present in the solid state, with the two mercaptoimidazolyl units of L¹ and L² coordinating to different metal ions. Complexes 3 and 4 are further characterized by Mössbauer spectroscopy and SQUID magnetometry. NMR spectroscopy suggests that the complexes largely dissociate in polar solvents. X-ray structures of L² and its precursor bis(imidazolium) salt are also reported.     

Evidence for polynuclear iron(III) clusters in the root nodule bacterium, Rhizobium leguminosarum bv. viciae WSM710

Cells of the root nodule bacterium Rhizobium leguminosarum bv. viciae WSM710 were cultured in a medium containing 20 M ⁵⁷Fe. Mössbauer spectra of the cells at 5.5 and 3.7 K indicated that the major form of iron present in the cells was in the form of polynuclear iron(III) clusters. At 5.5 K the spectral component associated with these clusters was in the form of a superposition of a broad feature (large magnetic hyperfine field distribution) and a doublet. On lowering the temperature of the cells to 3.7 K, the spectral component was transformed into resolved magnetic hyperfine field splitting which yielded a magnetic hyperfine field of 42.4 T when fitted with broad Lorentzian peaks. These spectral characteristics are typical of the hydrated iron(III) phosphate cores of several bacterioferritins. A small fraction (11%) of the Mössbauer spectral area of the cells was in the form of a doublet which yielded parameters ( = 1.35 mm/s; E_Q = 3.15 mm/s) indicative of iron(II). The parameters are very similar to those of a spectral component previously observed in several other microbes (R. Böhnke and B.F. Matzanke (1995) BioMetals 8, 223-230) and which has been associated with a 2.2 kDa oligomeric iron(II) carbohydrate phosphate     

Synthesis and characterization of three mononuclear Fe(III) complexes containing bipodal and tripodal ligands: X-ray molecular structure of the dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate

In this work, we present the synthesis and characterization of three mononuclear iron(III) complexes: dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate (1), trichloro[N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III) (2) and trichloro[bis-(2-pyridylmethyl)amine]iron(III) (3). The complexes were characterized by cyclic voltammetry, conductivimetry, elemental analyses, and by electronic, infrared and Mössbauer spectroscopies. Complex 1 was also characterized by X-ray structural analysis, which showed an iron center coordinated to one amide, one tertiary amine, two pyridine groups and two chloride ions. While for 1 the X-ray molecular structure and the infrared spectrum confirm the coordination of the amide group by the oxygen atom, the infrared spectrum of 2 indicates that the ester group present in the ligand is not coordinated, resulting in a N₃Cl₃ donor set, similar to the one present in 3. However, in 3 there is a secondary amine while in 2 a tertiary amine exists. These structural differences result in distinguishable variations in the Lewis acidity of the iron center, which could be evaluated by the analysis of the redox potential of the complexes, as well as by Mössbauer parameters. Thus, the Lewis acidity decreases in the following order: 1 > 2 > 3. It is important to notice that 1 has the amide group coordinated to the iron center, a feature present in metalloenzymes as lipoxygenase and isopenicillin N synthase, and in a small number of mononuclear iron(III) complexes.     

Three Co(III)-Fe(II) complexes based on hexacyanoferrates: Syntheses, spectroscopic and structural characterizations

Red or orange crystals of [Co(NH₃)₆]₂Cl₂[Fe(CN)₆] · 4H₂O (1), [Co(en)₃]₂Cl₂[Fe(CN)₆] · 2H₂O (2) and [Co(en)₃]₄[Fe(CN)₆]₃ · 21.6H₂O (3) were isolated from the aqueous systems Co³⁺-L_N-[Fe(CN)₆]⁴⁻ (L_N = NH₃, en = 1,2-diaminoethane). In all isolated samples the combination of Mössbauer (δ values were from the range −0.07 to −0.08 mm/s) and IR spectra (ν(CN) stretching vibrations in the range 2015-2047 cm⁻¹) confirms the presence of low spin Fe(II) in [Fe(CN)₆]⁴⁻ anions. X-ray structure analyses corroborate the ionic character of all studied compounds. These contain diamagnetic [Co(NH₃)₆]³⁺ (1) or [Co(en)₃]³⁺ (2 and 3) complex cations and diamagnetic [Fe(CN)₆]⁴⁻ complex anions. In compounds 1 and 2 chloride anions are present, too. All three compounds contain water of crystallization, in compound 3 as many as 21.6 molecules per formula unit.     

Complexation of iron(III) by catecholate-type polyphenols

We report here a complete physico-chemical study of the chelation of iron(III) by catechin (L¹), an abundant polyphenol in green tea. Using a fruitful combination of electrospray mass spectrometry, absorption spectrophotometry and potentiometry, we have characterized three ferric complexes of catechin (L¹Fe, and (L¹)₃Fe) as well as a ternary complex L¹FeNTA when an exogenous ligand (nitrilotriacetic acid) is added to the medium. Thanks to this study, we discuss the influence of an exogenous tetradentate ligand in the ferric recognition processes by catecholate-type polyphenols.     

A new example of a tetranuclear iron(III) cluster containing the [Fe4O2] core: preparation, X-ray crystal structure, magnetochemistry and Mössbauer study of [Fe4O2(O2CMe)6(N3)2(phen)2]

Two synthetic procedures have been employed that allow access to the new tetranuclear cluster [Fe₄O₂(O₂CMe)₆(N₃)₂(phen)₂] (1), where phen is 1,10-phenanthroline. Complex 1 · 3MeCN displays an unusual structural asymmetry (observed for the second time) in its [Fe₄O₂]⁸⁺ core that can be considered as a hybrid of the bent (butterfly) and planar dispositions of four metal ions seen previously in such compounds with transition metals. Complex 1 has been characterized by variable-temperature magnetic susceptibility studies, and by IR and variable-temperature ⁵⁷Fe Mössbauer spectroscopies. Magnetochemical data reveal a diamagnetic ground state (S=0) with antiferromagnetic body-body and body-wingtip interactions between the iron(III) ions of the butterfly core (J_bb=−11 cm⁻¹, J_wb=−70 cm⁻¹). Magnetochemical and Mössbauer studies on 1 show that its structural asymmetry has practically no influence on these properties compared with the more symmetric types.     

The kinetics of the complex formation between iron(III)-ethylenediaminetetraacetate and hydrogen peroxide in aqueous solution

The kinetics of the formation of the purple complex [Fe^III(EDTA)O₂]³⁻, between Fe^III-EDTA and hydrogen peroxide was studied as a function of pH (8.22-11.44) and temperature (10-40 °C) in aqueous solutions using a stopped-flow method. The reaction was first-order with respect to both reactants. The observed second-order rate constants decrease with an increase in pH and appear to be related to deprotonation of Fe^III-EDTA ([Fe(EDTA)H₂O]⁻ ⇔ Fe(EDTA)OH]²⁻ + H⁺). The rate law for the formation of the complex was found to be d[Fe^IIIEDTAO₂]³⁻/dt=[(k₄[H⁺]/([H⁺] + K₁)][Fe^III-EDTA][H₂O₂], where k₄=8.15±0.05×10⁴ M⁻¹ s⁻¹ and pK₁=7.3. The steps involved in the formation of [Fe(EDTA)O₂]³⁻ are briefly discussed.     

N-Alkylation of tripodal iron(III) imidazolate complexes: Reactivity and structure

The N-alkylation of iron(III) complexes of the tripodal imidazolate complexes derived from the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 2-imidazolecarboxaldehyde (2ImH), 4-imidazolecarboxaldehyde (4ImH) or 4-methyl-5-imidazolecarboxaldehyde (5-Me4ImH) was investigated. While each complex possesses three nucleophilic imidazolate nitrogen atoms, only the complex derived from 2-imidazolecarboxaldehyde, Fetren(2Im)₃, was completely alkylated under the ambient conditions used in this work. Using methyl iodide as the alkylating agent, a correlation between spin state of the product and degree of methylation was observed. Low spin iron complexes were more nucleophilic than high spin systems. The structure reactivity relationship was exploited in the reaction of Fetren(2Im)₃ with methyl iodide and allyl iodide to give [Fetren(N-Me2Im)₃]²⁺ and [Fetren(N-allyl2Im)₃]²⁺. The products are iron(II) due to reduction of the iron(III) by iodide ion which builds up in the reaction mixture as the alkylation reaction proceeds. These complexes were characterized by a number of methods including EA, IR, ES-MS, Mössbauer spectroscopy, magnetic susceptibility and X-ray diffraction.     

Functional model for catecholase-like activity: Synthesis, structure, spectra, and catalytic activity of iron(III) complexes with substituted-salicylaldimine ligands

Four new mononuclear iron(III) complexes with the substituted-salicylaldimine ligands, [Fe(L¹)(TCC)] (1), [Fe(L²)(TBC)] (2), [Fe(L³)(TBC)] (3) and [Fe(L⁴)(TCC)](CH₃CN) (4) (HL¹ = N′-(5-OH-salicylaldimine)-diethylenetriamine, HL² = (N′-(5-Cl-salicylaldimine)-diethylenetriamine, HL³ N′-(5-Br-salicyl-aldimine)-dipropylenetriamine, HL⁴ = (N′-3,5-Br-salicylaldimine)-dipropylenetriamine, H₂TCC = tetrachlorocatechol, and H₂TBC = tetrabromocatechol), were prepared and characterized by XRD, EPR, and Mössbauer spectroscopy. The coordination sphere of the Fe(III) in complexes 1-4 is a distorted octahedral with N₃O₃ donors set which constructed by the Schiff-base ligands and the catecholate substrates of TBC or TCC. The in situ prepared Fe(III) complexes [Fe(L¹)Cl₂], [Fe(L²)Cl₂], [Fe(L³)(Cl₂)], and [Fe(L⁴)Cl₂] in absence of TBC or TCC show a high catecholase-like activity for the oxidation of 3,5-DTBC to the corresponding quinone 3,5-DTBQ.     

Influence of Cr(III) and Cr(VI) on the interaction between sparfloxacin and calf thymus DNA

Cr(III) and Cr(VI) have different binding capacity with sparfloxacin, and have different combination modes with calf thymus DNA. Selecting these two different metal ions, the influence of them on the binding constants between sparfloxacin (SPFX) and calf thymus DNA, as well as the related mechanism has been studied by using absorption and fluorescence spectroscopy. The result shows that Cr(III) has weaker binding capacity to SPFX in the SPFX-Cr(III) binary system, but influences the binding between SPFX and DNA obviously in SPFX-DNA-Cr(III) ternary system. However, although Cr(VI) has a stronger binding capacity to SPFX, it has no effect on the binding between SPFX and DNA. Referring to the different modes of Cr(III) and Cr(VI) binding to DNA, the mechanism of the influence of metal ions on the binding between SPFX and DNA has been proposed. SPFX can directly bind to DNA by chelating DNA base sites. If a metal ion at certain concentration binds mainly to DNA bases, it can decrease the binding constants between SPFX and DNA through competing with SPFX. While if a metal ion at certain concentration mainly binds to phosphate groups of DNA, it can increase the binding constants by building a bridge between SPFX and DNA. If a metal ion at certain concentrations binds neither to bases nor phosphate groups in DNA, it will have no effect on the binding constant between SPFX and DNA. Our result supports Palumbo's conclusion that the binding between SPFX and the phosphata groups is the precondition for the combination between SPFX and DNA, which is stabilized through stacking interactions between the condensed rings of SPFX and DNA bases.     

Disproportionation of hydroxylamine by water-soluble iron(III) porphyrinate compounds

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-(N-ethylpyridinium-2yl)-porphyrinate ([Fe^III(TEPyP)]⁵⁺), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([Fe^III(TPPS)]³⁻), and microperoxidase 11 ([Fe^III(MP11)]) were studied for different [Fe^III(Porph)]/[HA] ratios, under anaerobic conditions at neutral pH. Efficient catalytic processes leading to the disproportionation of HA by these iron(III) porphyrinates were evidenced for the first time. As a common feature, only N₂ and N₂O were found as gaseous, nitrogen-containing oxidation products, while NH₃ was the unique reduced species detected. Different N₂/N₂O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [Fe^III(TEPyP)]⁵⁺ and [Fe^III(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [Fe^III(TPPS)]³⁻ led to the well characterized soluble intermediate, [Fe^II(TPPS)NO]⁴⁻. Free-radical formation was only evidenced for [Fe^III(TEPyP)]⁵⁺, as a consequence of a metal centered reduction. We discuss the catalytic pathways of HA disproportionation on the basis of the distribution of gaseous products, free radicals formation, the nature of porphyrinic intermediates, the Fe^II/Fe^III redox potential, the coordinating capabilities of each complex, and the kinetic analysis. The absence of revealed either that no HAO-like activity was operative under our reaction conditions, or that , if formed, was consumed in the reaction milieu.     

Bonding of the iron Lewis acid-THF adduct CpFe(CO)2 · THF to silica gel and its implication for cyclopropanation reactions

The adsorption of the iron Lewis acid-THF adduct CpFe(CO)₂ · THF (1) onto the silica gel has been observed to dramatically alter the cis:trans ratio for cyclopropanation reactions versus the homogeneous catalyzed reactions. To better understand this dramatic change in selectivity, we investigated the nature of bonding of 1 on silica with a number of analytical techniques. X-ray photoelectron spectroscopy showed the presence of a new peak at 687.7 eV for the silica-supported catalyst, which indicated possible fluorination by the anion. Further experiments using solid state NMR showed that a new boron species was also generated by the adsorption onto the silica gel. Mössbauer spectroscopy showed that adsorption of the iron Lewis acid-THF adduct onto the silica gel did not change the oxidation state to the iron; however, diffuse reflectance infrared spectroscopy showed the loss of surface hydroxyl groups and a shift in one of the C-O absorptions to higher wave numbers. The combined data suggest fluorination of the silica surface by the anion. This theory was tested by adsorption of the iron Lewis acid-THF adduct onto a polytrimethyl hydrosilylsilicate resin and sodium perchlorate treated silica. Analysis showed that both fluorination and physical adsorption of the catalysts occur, although fluorination was found to predominate for binding.     

Isolation of a singly oxo-bridged Fe(III) dinuclear complex: Synthesis, crystal structure, spectroscopic and magnetic studies

The μ-oxo dinuclear complex {Fe₂O(tptz)₂[N(CN)₂]₂(NO₃)₂} (1) (where tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine) has been synthesised and characterised by elemental analysis, FT-IR, UV-vis, cyclic voltammetry, Mössbauer spectroscopy, and variable-temperature magnetic susceptibility measurements and single crystal X-ray diffraction. The iron centres have a pentagonal-bipyramidal geometry. The dimeric neutral complex exhibits typical Fe-μ-O bond lengths of 1.763(1) Å and a bridge angle of 180.00°. The Fe?Fe separation is 3.526(3) Å. The Mössbauer spectrum at room temperature consists of one quadrupole doublet with an isomer shift of 0.41 mm/s and a quadrupole splitting of 1.12 mm/s. Variable-temperature magnetic susceptibility measurements have been measured in the temperature range 300-2 K, revealing an intramolecular antiferromagnetic coupling (J = −211.6 cm⁻¹).     

Synthesis and structure of iron (III) and iron (II) complexes in S4P2 environment created by diethyldithiocarbamate and 1,2-bis(diphenylphosphino)ethane chelation: Investigation of the electronic structure of the complexes by Mössbauer and magnetic studies

Iron (II) and iron (III) complexes, [Fe^II(DEDTC)₂(dppe)] · CH₂Cl₂ (1), [Fe^II(ETXANT)₂(dppe)] (2) (DEDTC = diethyldithiocarbamate, ETXANT = ethyl xanthate, dppe = 1,2-bis (diphenylphosphino) ethane), and [Fe^III(DEDTC)₂(dppe)] [Fe^IIICl₄] (3) have been synthesized and characterized. Since 3 contains two magnetic centers, an anion metathesis reaction has been conducted to replace the tetrahedral FeCl₄⁻ by a non-magnetic BPh₄⁻ ion producing [Fe^III(DEDTC)₂(dppe)]BPh₄ (4) for the sake of unequivocal understanding of the magnetic behavior of the cation of 3. With the similar end in view, the well-known FeCl₄⁻ ion, the counter anion of 3, is trapped as PPh₄[Fe^IIICl₄] (5) and its magnetic property from 298 to 2 K has been studied. Besides the spectroscopic (IR, UV-Vis, NMR, EPR, Mass and XPS) characterization of the appropriate compounds, especially 2, others viz. 1, 3 and 4 have been structurally characterized by X-ray crystallography. While Fe^II complexes, 1 and 2, are diamagnetic, the Fe^III systems, namely the cations of 3, and 4 behave as low-spin (S = 1/2) paramagnetic species from 298 to 50 K. Below 50 K 3 shows gradual increase of χ_MT up to 2 K suggesting ferromagnetic behavior while 4 exhibits gradual decrease of magnetic moment from 60 to 2 K, indicating the occurrence of weak antiferromagnetic interaction. These conclusions are supported by the Mössbauer studies of 3 and 4. The Mössbauer pattern of 1 exhibits a doublet site for diamagnetic (2-400 K) Fe^II. The compounds 1, 2 and 4 encompass interesting cyclic voltammetric responses involving Fe^II, Fe^III and Fe^IV.     

The first iron(III) complexes with cyclin-dependent kinase inhibitors: Magnetic, spectroscopic (IR, ES+ MS, NMR, Fe Mössbauer), theoretical, and biological activity studies

The first Fe^III complexes 1-6 with cyclin-dependent kinase (CDK) inhibitors of the type [Fe(L_n)Cl₃]·nH₂O (n = 0 for 1, 1 for 2, 2 for 3-6; L₁-L₆ = C2- and phenyl-substituted CDK inhibitors derived from 6-benzylamino-9-isopropylpurine), have been synthesized and characterized by elemental analysis, IR, ⁵⁷Fe Mössbauer, ¹H and ¹³C NMR, and ES+ mass spectroscopies, conductivity and magnetic susceptibility measurements, and thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The study revealed that the compounds are mononuclear, tetrahedral high-spin (S = 5/2) Fe^III complexes with an admixture of an S = 3/2 spin state originating probably from five-coordinated Fe^III ions either connecting with a bidentate coordination mode of the CDK inhibitor ligand or relating to the possibility that one crystal water molecule enters the coordination sphere of the central atom in a portion of molecules of the appropriate complex. Nearly spin-only value of the effective magnetic moment (5.82 μ_eff/μ_B) was determined for compound 1 due to absence of crystal water molecule(s) in the structure of the complex. Based on NMR data and DFT calculations, we assume that the appropriate organic ligand is coordinated to the Fe^III ion through the N7 atom of a purine moiety. The cytotoxicity of the complexes was tested in vitro against selected human cancer cell lines (G-361, HOS, K-562 and MCF-7) along with the ability to inhibit the CDK2/cyclinE kinase. The best cytotoxicity (IC₅₀: 4-23 μM) and inhibition activity (IC₅₀: 0.02-0.09 μM) results have been achieved in the case of complexes 2-4, and complexes 3, 4 and 6, respectively. In addition, the X-ray structure of 2-chloro-6-benzylamino-9-isopropylpurine, i.e. a precursor for the preparation of L₁, L₄ and L₅, is also described.