Hexacoordinate Iron Research Articles

Magnetic field modulation of high spin hexa-coordinated iron sites to enhance catalytic activity

The external alternating magnetic field (AMF) strategy is the most promising research topic to solve the bottleneck of catalyst activity enhancement. However, the mechanism between the arrangement of the spin electrons under the AMF and the catalytic activity enhancement for water splitting is still unclear. Herein, we synthesize Fe3O4@CNTs heterostructure as a research model and resolve the mechanism by system theoretical analysis and in-situ Raman characterization. The multiple orbital interactions of σ(dz2, pz, s) and π(dxz-px, dyz-py) promote the hexa-coordinated Feoct for both the reductive HER and oxidative OER processes. The AMF excites the transition from low to high spin configurations of the Feoct sites, which accelerates charge transfer of unpaired d electrons and optimizes adsorption and desorption interactions to intermediates during the reaction processes, resulting in a significant enhancement of the electrocatalytic activity (HER: 32 mV at 10 mA cm−2, OER: 179 mV at 100 mA cm−2).

Impact of counter anions on structural and magnetic properties of iron(III) meso-tetraphenylporphyrin tetrahydrofurane solvates

Intensively researched iron meso-tetraphenylporphyrin (FeTPP) complexes are central to the functionalities of many biological systems while at the same time they possess intriguing magnetic properties. Here we report on the synthesis and structural characterization of FeTPP complexes with axial tetrahydrofuran (THF) ligand and different non-coordinating counter anions. Structurally, the most interesting feature of these complexes is the stretching of the distance between the iron(III) and the oxygen atom of the THF ligand for different counter anions. This parameter affects the magnetic anisotropy of FeTPP as studied here with magnetic susceptibility, χm, and X-band electron paramagnetic resonance (EPR). Both magnetic probes are consistent with the iron(III) in its high-field state S = 5/2. Simulations of temperature dependencies of χm and EPR spectra show that the zero-field splitting magnetic anisotropy parameter D decreases with the tetragonal elongation of the hexa-coordinated iron(III) geometry, which is qualitatively discussed within the ligand field theory.

Electrochemical Proton Reductions Catalyzed by the Simpler Hexacoordinate Iron Compounds as Functional Mimics of the Active Site in [FeFe] Hydrogenase

Ligand-substitution reaction of [(μ-S2C4N2H2)Fe2(CO)6] (1) generated a normal mono-substituted diiron dithiolate derivative [(μ-S2C4N2H2)Fe2(CO)5(PMe3)] (1P) and a simpler hexacoordinate mononuclear compound [(μ-S2C4N2H2)Fe([CO)2(PMe3)2] (2) via distinct reactivity pathways under identical conditions. 1P and 2 could act as the electrochemically functional mimics of the [2Fe] sub-cluster and the distal Fe moiety of the active site of [FeFe] hydrogenase. The electrochemical investigations showed that 2 catalyzed the production of hydrogen from weak acid (acetic acid, HOAc) via two catalytic processes with an initial metal-orbital based reduction. In contrast, the electrocatalytic reaction of 2 with stronger acid (trifluoroacetic acid, TFA) occurred via an initial ligand protonation, and proceeded through different pathways that involved distinct oxidation states of the catalyst. The most striking result obtained in this study was that the hydrogen formation by 2 from TFA occurred at a relatively low overpotential as small as −0.28 V. It could be rationalized by the exclusive employment of iron(II) and iron(I) redox levels in the catalytic cycle, which was consistent with the enzymatic process. The observations might shed light on some aspects of ways by which the model compounds catalyzed the reduction of protons.

A mononuclear iron carbonyl complex [Fe(μ-bdt)(CO)2(PTA)2] with bulky phosphine ligands: a model for the [FeFe] hydrogenase enzyme active site with an inverted redox potential.

A mononuclear hexa-coordinated iron carbonyl complex [Fe(μ-bdt)(CO)2(PTA)2] 1 (bdt = 1,2-benzenedithiolate; PTA = 1,3,5-triaza-7-phosphaadamantane) with two bulky phosphine ligands in the trans position was synthesized and characterized by X-ray structural analysis coulometry data, FTIR, electrochemistry and electronic structure calculations. The complex undergoes a facilitated two-electron reduction 1/12- and shows an inverted one-electron reduction for 1/1- at higher potentials. Electrochemical investigations of 1 are compared to the closely related [Fe(bdt)(CO)2(PMe3)2] compound. A mechanistic suggestion for the hydrogen evolution reaction upon proton reduction from acid media is derived. The stability of 1 in both weak and strong acids is monitored by cyclic voltammetry.

Syntheses, crystal structure, spectroscopic, redox and magnetic properties of oxo- and carboxylato-bridged polynuclear iron(III) complexes with phenolate- and pyridine-substituted benzimidazole ligands

Using the same trinuclear [Fe3O]+7 core having oxo and carboxylate bridges, two different types of polynuclear iron(III) complexes have been synthesized. The bidentate ligands, (2-hydroxyphenyl)benzimidazole (HL1) and (2-pyridyl)benzimidazole (L2), produce the dinuclear and tetranuclear complexes [Fe2(L1)4(C6H5COO)]NO3·3H2O (1) and [Fe4O2(C6H5COO)7(L2)2]NO3·H2O·CH3CN (2·CH3CN), respectively. Complex 1 crystallizes in the monoclinic space group P2(1)/c, while 2·CH3CN crystallizes in the monoclinic space group C2/c. In 1, the two hexa-coordinated iron(III) centers are bridged by the two phenolate oxygens of the ligand HL1. The tetranuclear entity 2 consists of a [Fe4(μ3-O)2]8+ unit comprising four FeIII centers with a “butterfly” arrangement. Each pair of iron(III) centers occupy the “body” or “hinge” and “wing-tip” sites, respectively. The Fe2IIIFe2III complex 2 undergoes two stepwise one electron reductions at E1/2=−0.625V and −0.11V, while 1 displays an irreversible reduction wave at EP·C=−0.3V. Variable-temperature magnetic susceptibility measurements have been carried out for 1 and 2 in the temperature range 2–300K. Complex 1 exhibits a very weak Fe⋯Fe exchange interaction, J=−0.24(1)cm−1 (H=−J(S1·S2)). In complex 2, moderate antiferromagnetic exchange interactions occur among the four high-spin FeIII centers. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to the prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) is evaluated as −73(3) cm−1, using the spin Hamiltonion model H=−J1(S1·S2+S2·S3+S3·S4+S4·S1)−J2(S2·S4).

Substrate Hydroxylation by the Oxido–Iron Intermediate in Aromatic Amino Acid Hydroxylases: A DFT Mechanistic Study

AbstractSubstrate hydroxylation by an FeIV=O cluster model of the active center in aromatic amino acid hydroxylases (AAHs) has been investigated by means of DFT calculations. Whereas benzene was used as a model for the aromatic amino acid substrate, the water‐free FeIV=O cluster model has been used in previous studies of enzyme activation and formation of the hydroxylating intermediate. This cluster model also has the pterin cofactor placed in the first coordination sphere of the iron atom and differs substantially from models used in previous computational studies of AAH‐catalyzed hydroxylations. The formation of the (Fe)O–C(benzene) bond is associated with a free‐energy barrier (12.6 kcal mol–1) that is slightly lower than that calculated previously for the formation of the FeIV=O species. The subsequent steps, the NIH shift and the tautomerization leading to the phenol product are both associated with lower energy barriers. The substrate hydroxylation is followed by protonation of the oxidized iron‐bound cofactor to give the pterin‐4a‐carbinolamine product. The latter is subsequently dissociated upon rebinding of water molecules to produce the hexacoordinate iron complex of the enzyme resting state. Consequently, no product is released before the oxidation of both the substrate and the cofactor has been completed. Finally, the current study completes the catalytic cycle and regenerates the catalyst, with a barrier energy comparable to that of the (Fe)O–C(benzene) bond formation.

Correlation between the SOD-like activity of hexacoordinate iron(II) complexes and their Fe3+/Fe2+ redox potentials

Abstract Hexacoordinate, high-spin iron(II) complexes with different isoindoline-derivative ligands have been prepared and characterized. The Fe3+/Fe2+ redox transition is reversible in each case, but the E1/2 values vary in a ~ 400 mV range depending on the ligand. SOD-like activity of the complexes was determined by indirect methods with cytochrome c, and nitroblue tetrazolium indicator at pH 7.6. The measured activities correlate with the redox potentials for the Fe3+/Fe2+ couples. The results indicate that the superoxide dismutation takes place via an inner-sphere mechanism at the iron site.

Synthesis, characterization, and formation constant of hexacoordinate iron(III) complexes

Twelve iron(III) complexes [FeIII(LX)2]ClO4, where (LX)− is the deprotonated form of a series of asymmetric ligands containing pyridine and substituted phenol moieties (XPh(OH)–CH=N–(CH2) n –Py that X = H, 5-Br, 5-NO2, 5-OMe, 4-OMe, 3-OMe, and n = 1, 2), were synthesized and characterized by 1H NMR, IR, UV-Vis spectroscopy, mass spectrometry, and elemental analysis. Formation constants were measured using UV-Vis spectrophotometric titration at constant ionic strength (0.10 M NaClO4) at 25 (±0.1)°C. The trend of the complex formation of Fe(III) ion with a given tridentate ligand decreases as follows:

Microperoxidase 8 adsorbed on a roughened silver electrode as a monomeric high-spin penta-coordinated species: characterization by SERR spectroscopy and electrochemistry.

Microperoxidase 8 (MP8), a heme octapeptide obtained by hydrolytic digestion of cytochrome c, was adsorbed at the surface of a roughened silver electrode in order to provide a new supported biomimetic system for hemoproteins. A combination of two techniques was used to study its redox and coordination properties: electrochemistry and surface-enhanced resonance Raman (SERR) spectroscopy. This allowed us to show that MP8 could be adsorbed as a monolayer at the surface of the roughened silver electrode, where it could undergo a reversible electron transfer. Under those conditions, a redox potential of -0.4 V vs. SCE (-0.16 V vs. NHE) was measured for MP8, which was almost identical to that reported for N-acetyl-MP8 in aqueous solution. In addition, whereas MP8 appeared to aggregate in solution, and led to a mixture of high-spin penta-coordinated (5cHS) and low-spin hexa-coordinated (6cLS) iron(III) or iron(II) species, it was recovered almost exclusively as a monomeric high-spin penta-coordinated species at the surface of the electrode, both in the reduced and in the oxidized states. This then allowed a free coordination site on the iron, on the distal face of MP8 accessible to ligands. Accordingly, experiments performed in the presence of potassium cyanide demonstrated that MP8 adsorbed on a silver electrode could be ligated by a sixth CN(-) ligand. Thus there is the possibility of binding several kinds of ligands such as O(2) or H(2)O(2), which will open the way to biocatalysis of oxidation reactions at the surface of an electrode, or ligands such as drugs which will lead to the design of new biosensors for molecules of biological interest.

Surface structure effects on direct reduction of iron oxides by Shewanella oneidensis

The atomic and electronic structure of mineral surfaces affects many environmentally important processes such as adsorption phenomena. They are however rarely considered relevant to dissimilatory bacterial reduction of iron and manganese minerals. In this regard, surface area and thermodynamics are more commonly considered. Here we take a first step towards understanding the nature of the influence of mineral surface structure upon the rate of electron transfer from Shewanella oneidensis strain MR-1 outer membrane proteins to the mineral surface and the subsequent effect upon cell “activity.” Cell accumulation has been used as a proxy for cell activity at three iron oxide single crystal faces; hematite (001), magnetite (111) and magnetite (100). Clear differences in cell accumulation at, and release from the surfaces are observed, with significantly more cells accumulating at hematite (001) compared to either magnetite face whilst relatively more cells are released into the overlying aqueous phase from the two magnetite faces than hematite. Modeling of the electron transfer process to the different mineral surfaces from a decaheme (protoporphyrin rings containing a central hexacoordinate iron atom), outer membrane-bound cytochrome of S. oneidensis has been accomplished by employing both Marcus and ab initio density functional theories. The resultant model of electron transfer to the three oxide faces predicts that over the entire range of expected electron transfer distances the highest electron transfer rates occur at the hematite (001) surface, mirroring the observed cell accumulation data. Electron transfer rates to either of the two magnetite surfaces are slower, with magnetite (111) slower than hematite (001) by approximately two orders of magnitude. A lack of knowledge regarding the structural details of the heme-mineral interface, especially in regards to atomic distances and relative orientations of hemes and surface iron atoms and the conformation of the protein envelope, precludes a more thorough analysis. However, the results of the modeling concur with the empirical observation that mineral surface structure has a clear influence on mineral surface-associated cell activity. Thus surface structure effects must be accounted for in future studies of cell-mineral interactions.

Iron coordination chemistry with tetra-, penta- and hexadentate bispidine-type ligands

Described is the synthesis of tetra-, penta- and hexadentate bispidine ligands with two tertiary amine and two, three or four additional donors (pyridine, phenolate or alcoholate; bispidine=3,7-diazabicyclo[3.3.1]nonanone, coordinating substituents at positions 2,4; 2,4,7; 2,3,4; 2,3,4,7) and of their hexa-coordinate iron(II) complexes. Crystal structural analyses reveal that all complexes are six-coordinate, with one or two co-ligands, and all structures with the tetradentate bispidine ligand are asymmetrical with respect to the two tertiary amine donors, with short FeN1 and long FeN2 bonds (N1: position 3, N2: position 7). This is the same structural type as found for the Jahn–Teller labile copper(II) compounds, the manganese(II) and chromium(III) complexes but different from copper(I), zinc(II) and some cobalt(II) complexes with MN1≥MN2. Additional donors at N2 modify the structures, but do not lead to a change to the other structural type; additional donors at N1 lead to structures with MN1∼MN2. Solution studies (NMR, UV–Vis, electrochemistry, magnetism) indicate that the co-ligands may be substituted by solvent, with the donors trans to N2 being more labile than those trans to N1, but the over-all structural properties in solution are similar to those in the solid state. The complexes are stable towards oxidation, all except one have high spin electronic configuration. The oxidation potentials strongly depend on the two co-ligands.

Synthetic models of the reduced active site of superoxide reductase.

We report the synthesis, structural and spectroscopic characterization, and magnetic and electrochemical studies of a series of iron(II) complexes of the pyridyl-appended diazacyclooctane ligand L(8)py(2), including several that model the square-pyramidal [Fe(II)(N(his))(4)(S(cys))] structure of the reduced active site of the non-heme iron enzyme superoxide reductase. Combination of L(8)py(2) with FeCl(2) provides [L(8)py(2)FeCl(2)] (1), which contains a trigonal-prismatic hexacoordinate iron(II) center, whereas a parallel reaction using [Fe(H(2)O)(6)](BF(4))(2) provides [L(8)py(2)Fe(FBF(3))]BF(4) (2), a novel BF(4)(-)-ligated square-pyramidal iron(II) complex. Substitution of the BF(4)(-) ligand in 2 with formate or acetate ions affords distorted pentacoordinate [L(8)py(2)Fe(O(2)CH)]BF(4) (3) and [L(8)py(2)Fe(O(2)CCH(3))]BF(4) (4), respectively. Models of the superoxide reductase active site are prepared upon reaction of 2 with sodium salts of aromatic and aliphatic thiolates. These model complexes include [L(8)py(2)Fe(SC(6)H(4)-p-CH(3))]BF(4) (5), [L(8)py(2)Fe(SC(6)H(4)-m-CH(3))]BF(4) (6), and [L(8)py(2)Fe(SC(6)H(11))]BF(4) (7). X-ray crystallographic studies confirm that the iron(II)-thiolate complexes model the square-pyramidal geometry and N(4)S donor set of the reduced active site of superoxide reductase. The iron(II)-thiolate complexes are high spin (S = 2), and their solutions are yellow in color because of multiple charge-transfer transitions that occur between 300 and 425 nm. The ambient temperature cyclic voltammograms of the iron(II)-thiolate complexes contain irreversible oxidation waves with anodic peak potentials that correlate with the relative electron donating abilities of the thiolate ligands. This electrochemical irreversibility is attributed to the bimolecular generation of disulfides from the electrochemically generated iron(III)-thiolate species.

Synthesis, Crystal Structure and Magnetic Properties of Novel Complex [μ-(NC)-Fe(CN)3(NO)-μ-(CN)-Cu(ept)]n·4nH2O (ept = N-(2-Aminoethyl)propane-1,3-diamine)

The novel complex [μ-(NC)-Fe(CN)3(NO)-μ-(CN)-Cu(ept)]n·4nH2O (ept = N-(2-aminoethyl)- propane-1,3-diamine) was obtained by the reaction of Cu(ClO4)2·6H2O with N-(2-amino- ethyl)propane-1,3-diamine and Na2[Fe(CN)5NO]·2H2O in water. This compound was characterized by IR, UV-VIS and EPR spectroscopies and magnetic measurement. Single-crystal X-ray structure analysis revealed that the title complex has a one-dimensional polymeric structure containing hexacoordinate iron(II) with five cyanide ligands (two of them, in trans position, bridging) and one nitrosyl group, and pentacoordinate copper(II) with N-(2-aminoethyl)propane-1,3-diamine and two sites occupied by bridging cyanide ligands. Magnetic investigation revealed a very weak antiferromagnetic interaction between the copper atoms (superexchange interaction parameter J = -1.7(1) cm-1; H = -JSiSi+1) within the chain through the diamagnetic [Fe(CN)5NO]2- ions.

Spin crossover in mononuclear and binuclear iron(III) complexes with pentadentate Schiff-base ligands

A series of mononuclear hexacoordinate iron(III) complexes, [Fe( 5 L )(py)]BPh 4, and binuclear hexacoordinate iron(III) complexes, [( 5 L )Fe(μ 2-bpy)Fe( 5 L )](BPh 4) 2, has been prepared and their magnetic properties were investigated; the pentadentate ligands were derivatives of 5 L =saldptn=N,N′-bis(2-hydroxybenzyliden)-1,7-diamino-4-azaheptane. Temperature variation of the effective magnetic moment for them shows that a spin transition from the low-spin to the high-spin state occurs. The magnetic data were fitted to an Ising-like model appropriate for the mono- and binuclear systems.

Dinuclear and Mononuclear Iron(II)−Thiolate Complexes with Mixed CO/CN- Ligands: Synthetic Advances for Iron Sites of [Fe]-Only Hydrogenases

The pentacoordinate, 16-electron FeII complex [PPN][Fe(CO)2(CN)(S,NH-C6H4)] (1), stabilized by strong S, N π-donation of chelating [S,NH-C6H4]2- ligand, was prepared by the reaction of 2-aminophenyl disulfide and [PPN][Fe(CO)4(CN)]. Protonation of complex 1 by electrophiles (2-mercaptopyrimidine and 2-aminophenylthiol) yielded hexacoordinate iron(II)−thiolate cyanocarbonyl complexes [PPN][Fe(CO)(CN)(S-C4H3N2)2] (5) and [PPN][Fe(CO)2(CN)(S-C6H4NH2)(S,NH2-C6H4)] (3), respectively. The IR spectrum of complex 5 in the aprotic solvent CH3CN displayed a weak ν(CN) band at 2090 cm-1 and a strong ν(CO) band at 1945 cm-1. Chemical oxidation of complex 5 in CH3CN at − 20 °C with [Cp2Fe][PF6] displayed absorption bands at 2096 and 1962 cm-1 which were assigned to the ν(CN) and ν(CO) vibrational frequencies respectively of the thermally unstable neutral FeIII(CO)(CN)(S-C4H3N2)2. Complex 5 was reobtained upon addition of [PPN][BH4] to FeIII(CO)(CN)(S-C4H3N2)2 in CH3CN at −20 °C. The first dinuclear Fe(II)−thiolate cya...

Artificial peroxidase-like hemoproteins based on antibodies constructed from a specifically designed ortho-carboxy substituted tetraarylporphyrin hapten and exhibiting a high affinity for iron-porphyrins

In order to get catalytic antibodies modelling peroxidases BALB/c mice have been immunized with iron(III)α,α,α,β- mesotetrakis-orthocarboxyphenyl-porphyrin (Fe(ToCPP))-KLH conjugates. Monoclonal antibodies have been produced by the hybridoma technology. Three antibodies, 2 IgG, and 1 IgG 2a, were found to bind both Fe(ToCPP) and the free base ToCPPH 2 with similar binding constants. None of those antibodies was found to bind tetraphenylporphyrin. Those results suggest that the recognition of Fe(ToCPP) by the antibodies was mainly due to the binding of the carboxylate groups to some amino acid residues of the protein. True K d values of 2.9 × 10 −9 M and 5.5 × 10 −9 M have been determined for the two IgG 1-Fe(ToCPP) complexes. Those values are the best ones ever reported for iron-porphyrin-antibody complexes. UV-vis. studies have shown that the two IgG 1-Fe(ToCPP) complexes were highspin hexacoordinate iron(III) complexes, with no amino acid residue binding the iron, whereas the IgG 2α-Fe(ToCPP) complex was a low-spin hexacoordinate iron(III) complex with two strong ligands binding the iron atom. Both IgG 1 -Fe(ToCPP) complexes were found to catalyze the oxidation of 2,2′-azinobis (3ethylbenzothiazoline-6-sulfonic acid (ABTS) 5-fold more efficiently than Fe(ToCPP) alone whereas the binding of IgG 2a to this iron-porphyrin had no effect on its catalytic activity. k cat values of 100 min −1 and 63 min −1 and k cat/ K m. values of 105 M −1 s −1 and 119 M −1 s −1 have been found respectively for the two IgG 1-Fe(ToCPP) complexes.

The Raman Spectral Study on the Solution Structure of Iron(III)-edta Complexes

Abstract The Raman spectra of iron(III) complexes with ethylenediaminetetraacetate were observed for the purpose of elucidating the structure of the complexes in solution. In a Raman spectral study on the solid complexes, the spectra in the 300–600 cm−1 region were found to be diagnostic to coordination geometry. Namely, the coordination number can be determined on the basis of the wavenumber difference of the two major bands observed in the 450–600 cm−1 region; the wavenumber differences observed for heptacoordinate complexes lie between 32 and 42 cm−1, while that for the hexacoordinate complex is 62 cm−1. Complexes containing a sexidentate edta exhibit a single intense band around 470 cm−1 in the 450–500 cm−1 region, while complexes containing a quinquedentate edta exhibit two intense bands in this region. These Raman spectral criteria were then applied for the determination of the solution structure. The Raman spectral features observed for solutions of Fe(III)-edta complexes indicate that the any alkali metal salts and acid salts of FeIII(edta)− give sexidentate heptacoordinate complex ions in water. The dimeric complex which is formed in an alkaline solution was characterized as being a dinuclear complex consisting of two quinquedentate hexacoordinate iron(III) moieties with a Fe–O–Fe bridging unit.

The preparation and characterisation of some complexes of iron(II) with amino acids

The following complexes of iron(II) with the amino acids glycine, alanine, phenylglycine, phenylalanine, leucine, serine, aspartic acid, glutamic acid, glutamine, tryptophan, histidine, methionine, S-methylcysteine, cystine, and glycylglycine have been isolated: Fe(Gly) 2, Fe(Ala) 2, Fe(Phegly) 2, Fe(Phe) 2·2H 20, Fe(Leu) 2·2H 20, Fe(Ser) 2, Fe(Asp)·2H 20, Fe(Glu)·2H 20, Fe(Gln) 2, Fe(Trp) 2, Fe(His) 2·H 20 and 2H 20, Fe(Met) 2, Fe(MeCys) 2, Fe(CysCys) and Fe(GlyGly) 2. Their magnetic behaviour, reflectance spectra, and Mössbauer parameters are consistent with high spin, hexacoordinate iron(II), and imply extended structures involving carboxylate bridges.

The phenomenon of ‘dioxygen protection’ during autoxidation of a lacunar macrobicyclic iron(II) reversible dioxygen carrier. An outer-sphere autoxidation mechanism

The autoxidation kinetics of an iron(II) complex which is a dioxygen carrier and which contains a sizeable protected cavity around one axial co-ordination site of the metal ion display a maximum in rate at ca. 1 Torr of dioxygen pressure at 0 °C and this unusual behaviour is interpreted as evidence for an outer-sphere oxidation mechanism involving hexaco-ordinated iron(II) species.

Kinetics of the fast charge and spin relaxation of coordination compounds studied by the Mossbauer Emission Spectroscopy

Abstract