Binuclear Iron Research Articles

Theoretical Study on Methane Oxidation Catalyzed by Fe/ZSM-5: The Significant Role of Water on Binuclear Iron Active Sites

In the present study, density functional theory (DFT) calculations were performed to investigate the reaction mechanism of methane oxidation catalyzed by ZSM-5-supported binuclear iron species. A variety of binuclear iron sites such as [Fe(μ-O)Fe]2+, [Fe(μ-O)2Fe]2+, [Fe(μ-O)(μ-OH)Fe]+, and [HOFe(μ-O)FeOH]2+ were considered. The conversion of methane to methanol is decomposed into two processes: C–H activation and methanol formation. It is found that the anhydrous [Fe(μ-O)Fe]2+ sites exhibit the lowest reactivity for methane oxidation due to high energy barriers for both C–H activation and methanol formation steps, while the [Fe(μ-O)(μ-OH)Fe]+ and [HOFe(μ-O)FeOH]2+ sites are found to exhibit higher reactivity for methane oxidation in comparison with the anhydrous sites. This high reactivity is mainly attributed to the presence of the terminal or bridged hydroxyls, which can either provide a significant coordination effect or act as an oxidant for methane oxidation. Moreover, we find that on both [Fe(μ-O)Fe...

A comparative DFT study of oxygen reduction reaction on mononuclear and binuclear cobalt and iron phthalocyanines

The oxygen reduction reaction (ORR) catalyzed by mononuclear and planar binuclear cobalt (CoPc) and iron phthalocyanine (FePc) catalysts is investigated in detail by density functional theory (DFT) methods. The calculation results indicate that the ORR activity of Fe-based Pcs is much higher than that of Co-based Pcs, which is due to the fact that the former could catalyze 4e- ORRs, while the latter could catalyze only 2e- ORRs from O2 to H2O2. The original high activities of Fe-based Pcs could be attributed to their high energy level of the highest occupied molecular orbital (HOMO), which could lead to the stronger adsorption energy between catalysts and ORR species. Nevertheless, the HOMO of Co-based Pcs is the ring orbital, not the 3d Co orbital, thereby inhibiting the electron transfer from metal to adsorbates. Furthermore, compared with mononuclear FePc, the planar binuclear FePc has more stable structure in acidic medium and more suitable adsorption energy of ORR species, making it a promising non-precious electrocatalyst for ORR.

The binuclear form of dinitrosyl iron complexes with thiol-containing ligands in animal tissues

It has been established that treatment of mice with sodium nitrite, S-nitrosoglutathione and the water-soluble nitroglycerine derivative isosorbide dinitrate (ISDN) as NO donors initiates in vivo synthesis of significant amounts of EPR-silent binuclear dinitrosyl iron complexes (B-DNIC) with thiol-containing ligands in the liver and other tissues of experimental mice. This effect is especially apparent if NO donors are administered to mice simultaneously with the Fe2+-citrate complex. Similar results were obtained in experiments on isolated liver and other mouse tissues treated with gaseous NО in vitro and during stimulation of endogenous NO synthesis in the presence of inducible NO synthase. B-DNIC appeared in mouse tissues after in vitro treatment of tissue samples with an aqueous solution of diethyldithiocarbamate (DETC), which resulted in the transfer of iron-mononitrosyl fragments from B-DNIC to the thiocarbonyl group of DETC and the formation of EPR-detectable mononitrosyl iron complexes (MNIC) with DETC. EPR-Active MNIC with N-methyl-d-glucamine dithiocarbamate (MGD) were synthesized in a similar way. MNIC-MGD were also formed in the reaction of water-soluble MGD-Fe2+ complexes with sodium nitrite, S-nitrosoglutathione and ISDN.

Synthesis, crystal structure and magnetic property of a binuclear iron(III) complex with phenol–pyrazole based ligands

ABSTRACTThe compound [Fe2(Hmpp)4(OMe)2]⋅3MeOH (1) (Hmpp = 2-(4-methyl-1H-pyrazol-3-yl)phenol) was characterized by elementary analysis, IR, X-ray crystallography, and magnetic measurements. The X-ray crystallography results reveal that the molecule 1 consists of a binuclear iron(III) natural complex and three methanol molecules. In the complex, two Fe3+ ions are bridged by two μ2-OMe. Magnetic measurements indicate that the two Fe3+ ions are antiferromagnetically coupled.

Structure/function correlations over binuclear non-heme iron active sites.

Binuclear non-heme iron enzymes activate O2 to perform diverse chemistries. Three different structural mechanisms of O2 binding to a coupled binuclear iron site have been identified utilizing variable-temperature, variable-field magnetic circular dichroism spectroscopy (VTVH MCD). For the μ-OH-bridged Fe(II)2 site in hemerythrin, O2 binds terminally to a five-coordinate Fe(II) center as hydroperoxide with the proton deriving from the μ-OH bridge and the second electron transferring through the resulting μ-oxo superexchange pathway from the second coordinatively saturated Fe(II) center in a proton-coupled electron transfer process. For carboxylate-only-bridged Fe(II)2 sites, O2 binding as a bridged peroxide requires both Fe(II) centers to be coordinatively unsaturated and has good frontier orbital overlap with the two orthogonal O2 π* orbitals to form peroxo-bridged Fe(III)2 intermediates. Alternatively, carboxylate-only-bridged Fe(II)2 sites with only a single open coordination position on an Fe(II) enable the one-electron formation of Fe(III)-O2 (-) or Fe(III)-NO(-) species. Finally, for the peroxo-bridged Fe(III)2 intermediates, further activation is necessary for their reactivities in one-electron reduction and electrophilic aromatic substitution, and a strategy consistent with existing spectral data is discussed.

Protein effects in non-heme iron enzyme catalysis: insights from multiscale models.

Many non-heme iron enzymes have similar sets of ligands but still catalyze widely different reactions. A key question is, therefore, the role of the protein in controlling reactivity and selectivity. Examples from multiscale simulations, primarily QM/MM, of both mono- and binuclear non-heme iron enzymes are used to analyze the stability of these models and what they reveal about the protein effects. Consistent results from QM/MM modeling are the importance of the hydrogen bond network to control reactivity and electrostatic stabilization of electron transfer from second-sphere residues. The long-range electrostatic effects on reaction barriers are small for many systems. In the systems where large electrostatic effects have been reported, these lead to higher barriers. There is thus no evidence of any significant long-range electrostatic effects contributing to the catalytic efficiency of non-heme iron enzymes. However, the correct evaluation of electrostatic contributions is challenging, and the correlation between calculated residue contributions and the effects of mutation experiments is not very strong. The largest benefits of QM/MM models are thus the improved active-site geometries, rather than the calculation of accurate energies. Reported differences in mechanistic predictions between QM and QM/MM models can be explained by differences in hydrogen bonding patterns in and around the active site. Correctly constructed cluster models can give results with similar accuracy as those from multiscale models, but the latter reduces the risk of drawing the wrong mechanistic conclusions based on incorrect geometries and are preferable for all types of modeling, even when using very large QM parts.

Mono- and binuclear non-heme iron chemistry from a theoretical perspective.

In this minireview, we provide an account of the current state-of-the-art developments in the area of mono- and binuclear non-heme enzymes (NHFe and NHFe2) and the smaller NHFe(2) synthetic models, mostly from a theoretical and computational perspective. The sheer complexity, and at the same time the beauty, of the NHFe(2) world represents a challenge for experimental as well as theoretical methods. We emphasize that the concerted progress on both theoretical and experimental side is a conditio sine qua non for future understanding, exploration and utilization of the NHFe(2) systems. After briefly discussing the current challenges and advances in the computational methodology, we review the recent spectroscopic and computational studies of NHFe(2) enzymatic and inorganic systems and highlight the correlations between various experimental data (spectroscopic, kinetic, thermodynamic, electrochemical) and computations. Throughout, we attempt to keep in mind the most fascinating and attractive phenomenon in the NHFe(2) chemistry, which is the fact that despite the strong oxidative power of many reactive intermediates, the NHFe(2) enzymes perform catalysis with high selectivity. We conclude with our personal viewpoint and hope that further developments in quantum chemistry and especially in the field of multireference wave function methods are needed to have a solid theoretical basis for the NHFe(2) studies, mostly by providing benchmarking and calibration of the computationally efficient and easy-to-use DFT methods.

Redox reactions of binuclear tetranitrosyl iron complexes with bridging N-C-S ligands

Reduction of neutral binuclear tetranitrosyl iron complexes of “μ-N-C-S” structural type of [Fe2(SR)2(NO)4] composition with R being functional azaheterocyclic thiolyls has been studied by cyclic voltammetry in aprotic solvents in a wide range of potential scan rates. Redox potentials of these reactions have been determined. The consecutive transfer of two electrons was shown to be typical for all the complexes. For some complexes the products of the first electron addition are unstable, while for the others the products of reduction by both the first and the second electrons are stable. The calculations of geometric and electronic structures of monoanions, dianions and intermediates forming upon the reduction of the complexes were performed by DFT methods. Using quantum-chemical modeling, electrochemical reduction of the complexes has been studied; their energy and theoretical redox potentials have been calculated.

Tartrate Resistant Acid Phosphatase Assisted Degradation of Single-Wall Carbon Nanotubes (SWCNTs).

In this study, we explored the capability of tartrate resistant acid phosphatase (TRAcP), a bone resorption marker, to degrade carboxylated single-walled carbon nanotubes (C-SWCNTs). Optical observations and Raman and high-resolution transmission electron microscopy studies show that the enzyme contributes to the degradation of C-SWCNTs, although the degradation is not complete. Molecular modeling implemented to investigate the binding sites for carboxylated and pristine SWCNTs to TRAcP elucidate the varying proximity of SWCNTs to the binuclear iron active site and the active site residues of TRAcP, which is clearly dependent upon the degree of carboxylation introduced into the SWCNT model. The modeling results presented provide justification to the propensity of TRAcP to degrade the C-SWCNTs alluding to the possibility of C-SWCNTs to be used as a potential degradable biomaterial for use in therapeutic applications of mineralized tissue related conditions.

Synthesis and characterization of a novel binuclear iron phthalocyanine/reduced graphene oxide nanocomposite for non-precious electrocatalyst for oxygen reduction

Binuclear iron phthalocyanine/reduced graphene oxide (bi-FePc/RGO) nanocomposite with good electrocatalytic activity for ORR in alkaline medium was prepared in one step. High angle annular dark field image scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy element mapping results show bi-FePc was uniformly distributed on RGO. An obvious cathodic peak located at about −0.23 V (vs. SCE) in CV and an onset potential of −0.004 V (vs. SCE) in LSV indicate the as-prepared bi-FePc/RGO nanocomposite possesses high activity which is closed to Pt/C for ORR. The ORR on bi-FePc/RGO nanocomposite follows four-electron transfer pathway in alkaline medium. Compared with Pt/C, there is only a slight decrease (about 0.02 V vs. SCE) for bi-FePc/RGO nanocomposite when the methanol exists. The excellent activity and methanol tolerance in alkaline solutions proves that bi-FePc/RGO nanocomposite could be considered as a promising cathode catalyst for alkaline fuel cells.

Bridging hydrogen atoms versus iron–iron multiple bonding in binuclear borole iron carbonyls

The geometries and energetics of the binuclear unsubstituted borole iron carbonyls (C4H4BH)2Fe2(CO)n (n=5, 4, 3, 2, 1) have been studied by density functional theory for comparison with the previously studied related substituted borole iron carbonyls (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] having different substituents on the boron atoms. The lowest energy (C4H4BH)2Fe2(CO)n (n=5, 4, 3) structures have terminal borole ligands related to those in the (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] systems as well the isoelectronic (η5-C5H5)2Mn2(CO)n systems. However, the lowest energy structure of the dicarbonyl (C4H4BH)2Fe2(CO)2 is an unusual quintet spin state structure with one of the borole ligands bridging the central Fe2 unit by forming a three-center two-electron BHFe bond to one iron atom as well as functioning as a pentahapto ligand to the other iron atom. For the (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] systems the tricarbonyls (n=3) having formal FeFe triple bonds of lengths ∼2.2Å analogous to the experimentally known (η5-Me5C5)2Mn2(μ-CO)3 structure appear to be favorable structures. An analogous (C4H4BH)2Fe2(μ-CO)3 structure is found for the unsubstituted borole ligand. However, this structure appears to be disfavored relative to disproportionation into (C4H4BH)2Fe2(CO)n+1+(C4H4BH)2Fe2(CO)n−1.

Nitric oxide donation by the binuclear tetranitrosyl iron complexes in the presence of erythrocytes

The kinetic regularities of nitric oxide donation in the presence of erythrocytes by representatives of a new class of synthetic nitric oxide donors, binuclear tetranitrosyl iron complexes (B-TNIC) [Fe2(SR)2(NO)4] with thiol-containing ligands, where R is pyrimidin-2-yl, 1-methylimidazol-2-yl, benzothiazol-2-yl, and penicillamine residue, were established. The NO-donating ability of B-TNIC was estimated from the apparent rate constants of the first order for the formation of intraerythrocyte methemoglobin with the variation of the initial concentration of the complex. During the standard experimental time (15—17 min), three of the four complexes released in the solution no more than a quarter of available NO groups. Their NO-donating ability turned out to be variable increasing with an increase in the initial concentration of the complex. At the same time, the complex bearing penicillamine residues as ligands donated almost all NO groups within approximately 1 min. Features of NO donation by the B-TNIC in the presence of erythrocytes are related to the formation in the system of an additional equilibrium pool of the membrane-associated complex, which is characterized by a reduced rate of hydrolytic dissociation with NO releasing to the solution. The NO-donating ability of the B-TNIC in the presence of erythrocytes is determined by the ratio of volumes of the free and membrane-associated pools of the complex.

Binuclear phospholyl iron carbonyls: The limited role of the phosphorus atom in metal complexation

The structures and thermodynamics of the binuclear phospholyl iron carbonyls (C4H4P)2Fe2(CO)n (n=6, 5, 4, 3, 2) have been investigated using density functional theory. The low-energy (C4H4P)2Fe2(CO)n (n=4, 3, 2) structures are found to have direct iron–iron bonds and terminal five-electron donor pentahapto η5-C4H4P rings with the phosphorus lone pairs not involved in the bonding to the iron atoms. They are thus analogous to the corresponding cyclopentadienyliron carbonyl derivatives. However, they differ from the binuclear phospholyl manganese carbonyls (C4H4P)2Mn2(CO)n (n=5, 4) for which structures with bridging seven-electron donor η5, η1-C4H4P phospholyl rings are the lowest energy structures by substantial margins. Partially bonded phospholyl rings, as well as direct Fe–Fe bonds, are found in the carbonyl-rich (C4H4P)2Fe2(CO)n (n=6, 5) species. The pentacarbonyl (C4H4P)2Fe2(CO)5 does not appear to be a viable species since it is disfavored relative to CO loss and to disproportionation into (C4H4P)2Fe2(CO)6+(C4H4P)2Fe2(CO)4.

The hapticity of the acenaphthylene ligand in its mononuclear, binuclear, and trinuclear iron carbonyl complexes

The structures and energetics of the acenaphthylene iron carbonyl complexes (C12H8)Fe(CO)n (n = 4, 3, 2), (C12H8)Fe2(CO)n (n = 8, 7, 6, 5, 4), and (C12H8)Fe3(CO)n (n = 9, 8) have been examined using density functional theory.

Communication—High-Performance and Non-Precious Bifunctional Oxygen Electrocatalysis with Binuclear Ball-Type Phthalocyanine Based Complexes for Zinc-Air Batteries

In this study, the electrocatalytic activities and zinc-air battery performances of newly synthesized 4,4′-{(diphenylmethylene) bis(4,1-phenylene)bis(oxy)}-bridged binuclear ball-type iron(II), cobalt(II) and zinc(II) phthalocyanine, M2Pc2 complexes were investigated. Electrochemical measurements indicated that Fe2Pc2 complex is a good alternate to Pt/C catalyst.

Structure and activity of the anticaking agent iron(III) meso-tartrate.

Iron(III) meso-tartrate, a metal-organic complex, is a new anticaking agent for sodium chloride. A molecular structure in solution is proposed, based on a combination of experimental and molecular modelling results. We show that the active complex is a binuclear iron(iii) complex with two bridging meso-tartrate ligands. The iron atoms are antiferromagnetically coupled, resulting in a reduced paramagnetic nature of the solution. In solution, a water molecule coordinates to each iron atom as a sixth ligand, resulting in an octahedral symmetry around each iron atom. When the water molecule is removed, a flat and charged site is exposed, matching the charge distribution of the {100} sodium chloride crystal surface. This charge distribution is also found in the iron(iii) citrate complex, another anticaking agent. This gives a possible adsorption geometry on the crystal surface, which in turn explains the anticaking activity of the iron(III) meso-tartrate complex.

Electrochemistry of Simple Organometallic Models of Iron-Iron Hydrogenases in Organic Solvent and Water.

Synthetic models of the active site of iron-iron hydrogenases are currently the subjects of numerous studies aimed at developing H2-production catalysts based on cheap and abundant materials. In this context, the present report offers an electrochemist's view of the catalysis of proton reduction by simple binuclear iron(I) thiolate complexes. Although these complexes probably do not follow a biocatalytic pathway, we analyze and discuss the interplay between the reduction potential and basicity and how these antagonist properties impact the mechanisms of proton-coupled electron transfer to the metal centers. This question is central to any consideration of the activity at the molecular level of hydrogenases and related enzymes. In a second part, special attention is paid to iron thiolate complexes holding rigid and unsaturated bridging ligands. The complexes that enjoy mild reduction potentials and stabilized reduced forms are promising iron-based catalysts for the photodriven evolution of H2 in organic solvents and, more importantly, in water.

CD/MCD/VTVH-MCD Studies of Escherichia coli Bacterioferritin Support a Binuclear Iron Cofactor Site.

Ferritins and bacterioferritins (Bfrs) utilize a binuclear non-heme iron binding site to catalyze oxidation of Fe(II), leading to formation of an iron mineral core within a protein shell. Unlike ferritins, in which the diiron site binds Fe(II) as a substrate, which then autoxidizes and migrates to the mineral core, the diiron site in Bfr has a 2-His/4-carboxylate ligand set that is commonly found in diiron cofactor enzymes. Bfrs could, therefore, utilize the diiron site as a cofactor rather than for substrate iron binding. In this study, we applied circular dichroism (CD), magnetic CD (MCD), and variable-temperature, variable-field MCD (VTVH-MCD) spectroscopies to define the geometric and electronic structures of the biferrous active site in Escherichia coli Bfr. For these studies, we used an engineered M52L variant, which is known to eliminate binding of a heme cofactor but to have very minor effects on either iron oxidation or mineral core formation. We also examined an H46A/D50A/M52L Bfr variant, which additionally disrupts a previously observed mononuclear non-heme iron binding site inside the protein shell. The spectral analyses define a binuclear and an additional mononuclear ferrous site. The biferrous site shows two different five-coordinate centers. After O2 oxidation and re-reduction, only the mononuclear ferrous signal is eliminated. The retention of the biferrous but not the mononuclear ferrous site upon O2 cycling supports a mechanism in which the binuclear site acts as a cofactor for the O2 reaction, while the mononuclear site binds the substrate Fe(II) that, after its oxidation to Fe(III), migrates to the mineral core.

The antitumor activity of the S-nitrosoglutathione and dinitrosyl iron complex with glutathione: Comparative studies

The antitumor activity of the binuclear form of dinitrosyl iron complexes with glutathione against Lewis lung carcinoma was found earlier with intraperitoneal administration of the complexes. This activity was also observed when this preparation was injected subcutaneously. The complex inhibited the tumor growth by 43% upon subcutaneous injection at a daily dose of 100 µM/kg (as calculated per one iron atom in the binuclear dinitrosyl iron complex) for 10 or 15 days. The effect was observed during the first 2 weeks after tumor transplantation. After this, the tumors began to grow at a rate that was equal to or even higher than that for the control animals. The mean survival time for the treated mice exceeded the control values by 30%. Binuclear dinitrosyl iron complexes were also effective against Ca-755 adenocarcinoma with intraperitoneal administration. In this case, however, the mean survival time for the treated animals only increased by 7%. It was also shown that S-nitrosoglutathione inhibited the growth of Lewis lung carcinoma and Ca-755 adenocarcinoma by 70 and 90%, respectively. However, in contrast to binuclear dinitrosyl iron complexes, the antitumor effect of S-nitrosoglutathione decreased with an increase in the daily dose of the compound from 200 to 400 µM/kg. The initial antitumor effect of binuclear dinitrosyl iron complexes and S-nitrosoglutathione is suggested to be due to NO that is released from both compounds. The subsequent suppression of the effect is caused by the activation of antinitrosative and antioxidant defense systems in tumors.

166 - Thionitrosyl Binuclear Iron Complexes - a New Class of Agents for Anti-Aging Therapy

166 - Thionitrosyl Binuclear Iron Complexes - a New Class of Agents for Anti-Aging Therapy