Photochemistry of iron(iii)-lactic acid complex in aqueous solutions

Photochemistry of a 1: 1 Fe^III-lactic acid complex, [Fe(Lact)]⁺, in aqueous solutions was studied by stationary photolysis, nanosecond laser flash photolysis (355 nm, 6 ns), and femtosecond pump-probe spectroscopy (400 nm, 200 fs). The quantum yield of photolysis of [Fe(Lact)]⁺ upon excitation at 355 nm is 0.4 and 0.22 in the deoxygenated and air-saturated solutions, respectively. Weak transient absorption in the range 500–750 nm was observed in the nanosecond experiments. It was assigned to a [Fe^II...-O-CH(Me)-COO^·]⁺ radical complex. The spectral properties of the ligand-to-metal charge transfer excited state and the characteristic time of formation of the radical complex (1.5 ps) were determined in the femtosecond spectroscopy experiments. A reaction mechanism was proposed, which involves inner-sphere electron transfer in the excited complex with the formation of a radical complex [Fe^II...-O-CH(Me)-COO^·]⁺ and its subsequent transformation to the end product of the photochemical reaction.     

The Photochemistry of [FeIIIN3(cyclam‐ac)]PF6 at 266 nm

The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe^IIIN₃(cyclam‐ac)]PF₆ ([ 1 ]PF₆), where cyclam‐ac represents the 1,4,8,11‐tetraazacyclotetradecane‐1‐acetate ligand, has been shown to be wavelength‐dependent, leading either to the rare high‐valent iron(V) nitrido complex [Fe^VN(cyclam‐ac)]PF₆ ([ 3 ]PF₆) after cleavage of the azide N_α? N_β bond, or to a photoreduced Fe^II species after Fe? N_azide bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step‐scan and rapid‐scan time‐resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron‐containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe? N_azide bond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [ 1 ]PF₆, the solvent‐stabilized complex cation [Fe^II(CH₃CN)(cyclam‐ac)]⁺ ( 2 red ) together with the azide radical (N₃^.) is formed with a relative yield of 80 %, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step‐scan experiments with a higher time resolution reveal the formation of azide anions (N₃^?) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 red to form [Fe^IIN₃(cyclam‐ac)] ( 1 red ) as a secondary product of the photochemical decomposition of 1 . Molecular oxygen was further used to quench 1 red and 2 red to form what seems to be the elusive complex [Fe(O₂)(cyclam‐ac)]⁺ ( 6 ).     

Accelerated Oxygen Atom Transfer and C−H Bond Oxygenation by Remote Redox Changes in Fe3Mn‐Iodosobenzene Adducts

We report the synthesis, characterization, and reactivity of [LFe₃(PhPz)₃OMn(^sPhIO)][OTf]_x ( 3 : x =2; 4 : x =3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X‐ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, ⁵⁷Fe Mössbauer and X‐ray absorption spectroscopy provided unique insights into how changes in oxidation state ( Fe^III ₂ Fe^IIMn^II vs. Fe^III ₃ Mn^II ) influence oxygen atom transfer in tetranuclear Fe₃Mn clusters. In particular, a one‐electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude.     

A study of Roman glass by reflectance and electron paramagnetic resonance spectroscopies

Reflectance and electron paramagnetic resonance (EPR) spectroscopies were used to study 25 fragments of Roman glass. Colour coordinates were used for an unbiased classification of the glasses in colour groups, which accounted for the presence of blue, blue-green, green, yellow-green, yellow and purple samples. Reflectance spectra were recorded in the 250–2500 nm wavelength range and showed absorption bands characteristic of Fe^II, Fe^III and Mn^III ions; furthermore, Co^II and Cu^II bands were observed in the spectra of the blue glasses. A decrease of the absorbance ratio of Fe^II to Fe^III ions was observed moving from blue-green to green and yellow-green glasses; however, yellow fragments still proved to be reduced glasses. EPR spectra displayed the characteristic patterns of Fe^III and Mn^II ions, with g-values in the 2–5 interval and spectral features depending on the relative content of the two elements. The characteristic pattern of the V^IV ion (g ≈ 2) and signals due to the formation of iron-sulphur complexes (g ≈ 6) appeared in the spectrum of a dark yellow glass, recorded at 77 K.     

Photochemische Eigenschaften der Systeme von Chloroeisen(III)-Komplexen in einigen nicht-wäßrigen Lösungsmitteln

Photochemical properties of Fe^III Chloro Complex Systems in Some Non-aqueous Solvents The results on the relationship between the composition of Fe^III chloro complexes in acetone, methanol, their mixtures resp., and their photochemical properties are presented. It has been found that the presence of Fe^II ions in irradiated systems is observed till a certain time. This time as well as the values of Phi; Fe^II depend on the ratio of [Fe^III]:[Cl^?] and the composition of the solvent mixture. Photochemical properties of some Fe^III chloro complexes are discussed.     

Heat-induced changes in microstructure and magnetic properties of Co0.75Ni0.75[Fe(CN)6] · XH2O

Polycrystalline Co_0.75Ni_0.75[Fe(CN)₆]?·?XH₂O was prepared by coprecipitation. The coprecipitated powder was annealed in vacuum at 80°C, 100°C, and 130°C. Variation of microstructural and magnetic properties with different annealed temperatures was studied by Fourier-transform infrared, X-ray diffraction, and magnetization measurements. The differences in magnetic phase transition temperature, coercivity, remanence, and effective magnetization were studied in detail. The magnetic contribution mainly results from Fe^III–CN–Co^II/Ni^II and Fe^III–NC–Co^II/Ni^II because Fe^II–CN–Co^III/Ni^II carries no net spin. After annealing at 130°C, the microstructures Fe^III–CN–Co^II/Ni^II and Fe^III–NC–Co^II/Ni^II convert to Fe^II–CN–Co^III/Ni^II. Differences in magnetic properties may be attributed to heat-induced microstructural changes.     

Mixed benzohydroxamato-acetylacetonato metal complexes: spectral,thermal and photochemical behaviour

Summary Bis(acetylacetonato)VO^II,–Co^II,–Ni^II,–Cu^II,–Zn^II, –UO ₂ ^II and tris(acetylacetonato)Fe^III react with benzohydroxamic acid to yield the corresponding mixed ligand complexes as a result of displacement of one acetylacetone molecule. Intermolecular association may be the reason for six-coordination geometry around the metal ions. A t.g.a. study of the complexes shows, in most cases, initial loss of alcohol and water molecules associated with the complexes; subsequent decomposition steps are characterised by very sharp weight loss. The photochemical stability of the complexes has been studied. Intraligand excitation causes a decomposition in the case of Fe^III and VO^II-complexes but no detectable effect for Co^II, Ni^II, Cu^II, Zn^II, or UO ₂ ^II -complexes.     

An unusual electrochemically induced demetalation of iron-substituted Keggin-type undecatungstozincate ZnW11FeIII(H2O)O7−39

Firstly reported for Fe-containing transition metal substituted polyoxometalates was an unusual Fe-centered demetalation process induced by the reduction of ZnW₁₁Fe^III to ZnW₁₁Fe^II, which resulted in a new couple of Fe-relating redox waves at positive potentials.     

Iron(II)-8-mercaptoquinoline,iron(II)-5-chloro-8-mercaptoquino-line and iron(II)-5-bromo-8-mercaptoquinoline complexing in the iron(III) hexacyanoferrate(II) gelatin-immobilized matrix systems

Novel complexing processes in the Fe^II-8-mercaptoquinoline, Fe^II-5-chloro-8-mercaptoquinoline and Fe^II-5-bromo-8-mercaptoquinoline systems, not used previously in coordination chemistry, namely complexing as an iron(III)hexacyanoferrate(II) gelatin-immobilized matrix (GIM) in contact with an aqueous solution of the corresponding ligand, have been observed and analysed. Incorporation of these ligands into the inner coordination sphere is preceded by the decomposition of the immobilized compound KFe[Fe(CN)₆] to form hydroxides or oxohydroxides of Fe^II and Fe^III under the action of OH^- ions. It has been shown that Fe^IIFeIII redox process and the formation of FeB₃ chelates (B^- is a singly deprotonated form of the corresponding ligand) take place during complexing under such conditions.     

Probing the Activity of Iron Peroxo Porphyrin Intermediates in the Reaction Layer during the Electrochemical Reductive Activation of O2

Herein we report the first example of using scanning electrochemical microscopy (SECM) to quantitatively analyze O₂ reductive activation in organic media catalyzed by three different Fe porphyrins. For each porphyrin, SECM can provide in one single experiment the redox potential of various intermediates, the association constant of Fe^II with O₂, and the pK_a of the Fe^III(OOH^?)/ Fe^III(OO^2?) couple. The results obtained can contribute to a further understanding of the parameters controlling the catalytic efficiency of the Fe porphyrin towards O₂ activation and reduction.     

Complexes of 1-isonicotinoyl-4-benzoyl-3-thiosemicarbazide with manganese(II), iron(III), chromium(III), cobalt(II), nickel(II), copper(II) and zinc(II)

1-Isonicotinoyl-4-benzoyl-3-thiosemicarbazide (IBtsc) and its Cr^III, Mn^II, Fe^III, Co^II, Ni^II, Cu^II and Zn^II complexes have been prepared and characterized by elemental analyses, magnetic susceptibility measurements, u.v.–vis., i.r., n.m.r. and FAB mass spectral data. The room temperature e.s.r. spectra of the Cr^III, Fe^III and Cu^II complexes yield values, characteristic of octahedral, tetrahedral and square-planar complexes, respectively. The Mössbauer spectra of [Fe(IBtsc-H)Cl₂] at room temperature and at 78 K suggest the presence of high-spin Fe^III. The Ni^II, Cr^III and Cu^II complexes show semiconducting behaviour in the solid state, but the Zn^II complex is an insulator at room temperature. IBtsc and its soluble complexes have been screened against several bacteria, fungi and tumour cell lines.     

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

Non‐heme (L)Fe^III and (L)Fe^III‐O‐Fe^III(L) complexes (L=1,1‐di(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) underwent reduction under irradiation to the Fe^II state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and DFT studies support a mechanism in which irradiation results in charge‐transfer excitation of a Fe^III?μ‐O?Fe^III complex to generate [(L)Fe^IV=O]²⁺ (observed transiently during irradiation in acetonitrile), and an equivalent of (L)Fe^II. Under aerobic conditions, irradiation accelerates reoxidation from the Fe^II to the Fe^III state with O₂, thus closing the cycle of methanol oxidation to methanal.     

Synthesis and reactivity of metal-containing monomers

Acid and neutral Co^II, Cu^II, Ni^II, Zn^II, Fe^II, and Fe^III maleates, fumarates, and itaconates were obtained and characterized. The methods for their synthesis were optimized, and the valence state and coordination of metals were studied. Co^II and Fe^II hydrogen maleates, Co^II maleate, and Co^II fumarate were examined by X-ray diffraction analysis. The ligands based on unsaturated dicarboxylic acids can be mono-, bi-, and tetradentate, which results in the formation of acid salts, chain and three-dimensional coordination polymers, whose double bond is not involved in the coordination. The strong antiferromagnetic exchange (μ_elf=1.41 and 0.34 μ_B at 290 and 80 K, respectively) was detected in Cu^II itaconate. Based on the data of Mössbauer spectroscopy, the partial reduction of Fe^III to Fe^II during the synthesis of Fe^III maleate was shown to occur: δFe=0.43 and 1.27 mm s^?1, ΔE _Q=0.57 and 3.13 mm s^?1 and Γ=0.37 and 0.28 mm s^?1 atT=298 K for Fe^III and Fe^II, respectively.     

Activation of nitrite ion by biomimetic iron(III) complexes

The rate of reaction of NO ₂ ⁻ ion with various Fe^III porphyrins in the presence of PPh₃ is shown to depend on the redox potential of the Fe^III center. There is a linear relationship between the ease of reduction of the Fe^III to Fe^II and the kinetics for the formation of the Fe^II porphyrin nitrosyl adduct, with concomitant oxidation of PPh₃ to PPh₃O. Cyclic voltammograms show reversible one-electron reductions that can be ascribed to the Fe^III/Fe^II couple ranging from E_1/2 = −343 to −145 mV (versus Ag/AgCl). The order of increasing half-wave reduction potentials for the Fe^III/Fe^II porphyrin redox centers studied is octaethylporphyrin > etioporphyrin I > deuteroporphyrin IX dimethyl ester > protoporphyrin IX dimethyl ester > α,β,γ,δ-tetraphenylporphyrin. This sequence of redox potentials complements the pseudo first-order kinetics ( to m s ⁻¹) for the oxidation of PPh₃ and subsequent Fe^II porphyrin nitrosyl adduct formation. The rates of reaction of biomimetic Fe^III porphyrins with NO ₂ ⁻ ion demonstrate how metal center redox properties are influenced by the surrounding ligand. In this paper we have elucidated a possible mechanistic control for the rate of this reaction.     

Electron Transfer in the Cs⊂{Mn4Fe4} Cubic Switch: A Soluble Molecular Model of the MnFe Prussian-Blue Analogues

A mixed-valence {Mn^II₃Mn^IIIFe^II₂Fe^III₂} cyanide-bridged molecular cube hosting a caesium cation, Cs⊂{Mn₄Fe₄}, was synthesized and structurally characterized by X-ray diffraction. Cyclic-voltammetry measurements show that its electronic state can be switched between five different redox states, which results in a remarkable electrochromic effect. Magnetic measurements on fresh samples point to the occurrence of a spin-state change near room temperature, which could be ascribed to a metal-to-metal electron transfer converting the {Fe^II−CN−Mn^III} pair into a {Fe^III−CN−Mn^II} pair. This feature was only previously observed in the polymeric MnFe Prussian-blue analogues (PBAs). Moreover, this novel switchable molecule proved to be soluble and stable in organic solvents, paving the way for its integration into advanced materials.     

Iron(III)‐Complexes Engaged in the Biochemical Processes in Seawater. II. Voltammetry of Fe(III)‐Malate Complexes in Model Aqueous Solution

Characteristics of iron(III) complexes with malic acid in 0.55 mol L^?1 NaCl were investigated by voltammetric techniques. Three iron(III)‐malate redox processes were detected in the pH range from 4.5 to 11: first one at ?0.11 V, second at ?0.35 V and third at ?0.60 V. First process was reversible, so stability constants of iron(III) and iron(II) complexes were calculated: log K₁(Fe^III(mal))=12.66±0.33, log β₂(Fe^III(mal)₂)=15.21±0.25, log K₁(Fe^II(mal))=2.25±0.36, and log β₂(Fe^II(mal)₂)=3.18±0.32. In the case of second and third reduction process, conditional cumulative stability constants of the involved complexes were determined using the competition method: log β(Fe(mal)₂(OH)_x)=15.28±0.10 and log β(Fe(mal)₂(OH)_y)=27.20±0.09.     

Light-Induced Bidirectional Metal-to-Metal Charge Transfer in a Linear Fe2Co Complex

It is a challenge to reversibly switch both magnetism and polarity using light irradiation. Herein we report a linear Fe₂Co complex, whereby interconversion between Fe^III_LS(μ-CN)Co^II_HS(μ-NC)Fe^III_LS (LS=low-spin, HS=high-spin) and Fe^III_LS(μ-CN)Co^III_LS(μ-NC)Fe^II_LS linkages could be achieved upon heating and cooling, or alternating laser irradiation at 808 and 532 nm. The electron spin arrangement and charge distribution were simultaneously tuned accompanying bidirectional metal-to-metal charge transfer, providing switchable polarity and magnetism in the complex.     

A Structural and Functional Model for the Tris-Histidine Motif in Cysteine Dioxygenase

The iron(II) complexes [Fe(L)(MeCN)₃](SO₃CF₃)₂ (L are two derivatives of tris(2-pyridyl)-based ligands) have been synthesized as models for cysteine dioxygenase (CDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and the Fe−N_py bond lengths [1.953(4)–1.972(4) Å] are similar to those in the Cys-bound Fe^II-CDO; Fe−N_His: 1.893–2.199 Å. The iron(II) centers of the model complexes exhibit relatively high Fe^III/II redox potentials (E_1/2=0.988–1.380 V vs. ferrocene/ferrocenium electrode, Fc/Fc⁺), within the range for O₂ activation and typical for the corresponding nonheme iron enzymes. The reaction of in situ generated [Fe(L)(MeCN)(SPh)]⁺ with excess O₂ in acetonitrile (MeCN) yields selectively the doubly oxygenated phenylsulfinic acid product. Isotopic labeling studies using ¹⁸O₂ confirm the incorporation of both oxygen atoms of O₂ into the product. Kinetic and preliminary DFT studies reveal the involvement of an Fe^III peroxido intermediate with a rhombic S= Fe^III center (687–696 nm; g≈2.46–2.48, 2.13–2.15, 1.92–1.94), similar to the spectroscopic signature of the low-spin Cys-bound Fe^IIICDO (650 nm, g≈2.47, 2.29, 1.90). The proposed Fe^III peroxido intermediates have been trapped, and the O−O stretching frequencies are in the expected range (approximately 920 and 820 cm⁻¹ for the alkyl- and hydroperoxido species, respectively). The model complexes have a structure similar to that of the enzyme and structural aspects as well as the reactivity are discussed.     

Removal of chromate from aqueous solution by reduction with nanoscale Fe–Al layered double hydroxide

Nanoscale layered double hydroxides of Fe^II and Al^III (Fe–Al LDH) have been applied for removal of chromate (Cr^VI) from aqueous solution. Given the reaction stoichiometry, Cr^VI was completely reduced to Cr^III and coprecipitated with Fe^III and Al^III oxyhydroxides. The extent of Cr^VI removal decreased with increasing initial pH and decreasing molar ratio of Cr^VI/structural Fe^II in the LDH. The chromate reduction rate at different initial concentrations of Cr^VI was well described by the pseudo-second-order model with reaction rate constant ranging from 197.4 to 13.53 (mmol min)^?1. Initial pH and substitution of various amounts of Fe^III in the LDH structure had little effect on the reaction rate. Backtransformation of Cr^III to Cr^VI by birnessite Mn oxide (δ-MnO₂) after 40 days of reaction was less than 1% of the initial Cr (as Cr^III solid), indicating high stability of the final reaction products and high efficiency of nanoscale Fe–Al LDHs for removal of chromate from aqueous solution.     

Synthesis,characterization and magnetic properties of mixed-valence iron complexes with 2-pyridyl oximes

Abstract

Two new mixed-valence iron complexes with 2-pyridyl oximes, [Fe(mpko)₃Fe(H₂O)₂(NO₃)](NO₃)·2H₂O (1) (mpko^? = methyl(2-pyridyl)ketone oximate) and [{Fe(dpko)₃}₂Fe](ClO₄)·4H₂O (2) (dpko^? = bis(2-pyridyl)ketone oximate), have been prepared by reaction of Fe^III with mpkoH in methanol (1) and Fe^II with dpkoH in methanol/water (2). Dinuclear Fe^II(low-spin)Fe^III(high-spin) and trinuclear Fe^II(low-spin)Fe^III(high-spin)Fe^II(low-spin) cations are present in the crystal structure of 1 and 2, respectively. Intermolecular hydrogen bonds in 1 lead to weak antiferromagnetic interactions between pairs of neighboring Fe^III centers, which allows observation of single-ion zero-field splitting effects.