Iron Porphyrin Research Articles

Second Coordination Sphere Effect Shifts CO2 to CO Reduction by Iron Porphyrin from Fe0 to FeI.

Iron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO2 ) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO2 from the well-admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV-Visible spectroscopies, we underpinned a two-electron activation step of CO2 starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)-COOH species. The addition of another electron and a proton to the latter species converged to the cleavage of a C-O bond with the loss of water molecule resulting in an Fe(II)-CO species. DFT analyses of these postulated intermediates are in good agreement with our collected spectroscopic data, allowing us to propose an alternative pathway in the catalytic CO2 reduction with iron porphyrin catalyst. Such a remarkable shift opens new lines of research in the design of molecular catalysts to reach low overpotentials in performing multi-electronic CO2 reduction catalysis.

Second Coordination Sphere Effect Shifts CO2 to CO Reduction by Iron Porphyrin from Fe0 to FeI

AbstractIron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO2) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO2 from the well‐admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV‐Visible spectroscopies, we underpinned a two‐electron activation step of CO2 starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)−COOH species. The addition of another electron and a proton to the latter species converged to the cleavage of a C−O bond with the loss of water molecule resulting in an Fe(II)−CO species. DFT analyses of these postulated intermediates are in good agreement with our collected spectroscopic data, allowing us to propose an alternative pathway in the catalytic CO2 reduction with iron porphyrin catalyst. Such a remarkable shift opens new lines of research in the design of molecular catalysts to reach low overpotentials in performing multi‐electronic CO2 reduction catalysis.

Peroxymonosulfate activation by trace iron(III) porphyrin for facile degradation of organic pollutants via nonradical oxidation

Nonradical species with great resistance to interference have shown great advantages in complex wastewater treatment. Herein, a novel system constructed by biodegradable tetrakis-(4-carboxyphenyl)-porphyrinatoiron(III) (FeIII-TCPP) and peroxymonosulfate (PMS) was proposed for facile decontamination. Nonradical pathway is observed in FeIII-TCPP/PMS, where 1O2 and high-valent iron-oxo species play dominant roles. The genres and valence of high-valent iron-oxo species, including iron(IV)-oxo porphyrin radical-cationic species [OFeIV-TCPP•+] and iron(IV)-hydroxide species [FeIV-TCPP(OH)], are ascertained, along with their generation mechanism. The axial ligand on the iron axial site affects the ground spin state of FeIII-TCPP, further influencing the thermodynamic reaction pathway of active species. With trace catalyst in micromoles, FeIII-TCPP exhibits high efficiency by degrading bisphenol S (BPS) completely within 5 min, while Co2+/PMS can only achieve a maximum of 26.2% under identical condition. Beneficial from nonradical pathways, FeIII-TCPP/PMS demonstrates a wide pH range of 3–10 and exhibits minimal sensitivity to interference of concomitant materials. BPS is primarily eliminated through β-scission and hydroxylation. Specifically, 1O2 electrophilically attacks the C–S bond of BPS, while high-valent iron-oxo species interacts with BPS through an oxygen-bound mechanism. This study provides novel insights into efficient activation of PMS by iron porphyrin, enabling the removal of refractory pollutants through nonradical pathway.

Remote Enantioselective Epoxidation Reactions Catalyzed by Chiral Iron Porphyrin Complexes with a Hydrogen-Bonding Site

AbstractIron porphyrin complexes, which were linked via a para-phenylethynyl group to a chiral scaffold with a lactam binding site, were probed as catalysts in the enantioselective epoxidation of 4-(ω-alkenyl)-quinolones. It was found that the 3-butenyl group in the substrate accounts for the highest enantioselectivity (up to 44% ee) and the absolute configuration of an oxirane product was elucidated by electron diffraction. A two-point hydrogen bond of the substrate to the catalyst is likely responsible for enantioface differentiation at a remote position. The study shows chirality transfer to be possible via four nonstereogenic carbon atoms between the binding site of the substrate and its reactive C=C double bond.

Iron Dioxygen Adduct Formed during Electrochemical Oxygen Reduction by Iron Porphyrins Shows Catalytic Heme Dioxygenase Reactivity.

Heme dioxygenases oxidize the indole ring of tryptophan to kynurenine which is the first step in the biosynthesis of several important biomolecules like NAD, xanthurenic acid, and picolinic acid. A ferrous heme dioxygen adduct (or FeIII-O2•-) is the oxidant, and both the atoms of O2 are inserted in the product and its catalytic function has been difficult to emulate as it is complicated by competing rapid reactions like auto-oxidation and/or formation of the μ-oxo dimer. In situ resonance Raman spectroscopy technique, SERRS-RDE, is used to probe the species accumulated during electrochemical ORR catalyzed by site-isolated imidazole-bound iron porphyrin installed on a self-assembled monolayer covered electrode. These in situ SERRS-RDE data using labeled O2 show that indeed a FeIII-O2•- species accumulate on the electrode during ORR between -0.05 and -0.30 V versus Ag/AgCl (satd. KCl) and is reduced by proton coupled electron transfer to a FeIII-OOH species which, on the other hand, builds up on the electrode between -0.20 and -0.40 V versus Ag/AgCl (satd. KCl). This FeIII-OOH species then gives way to a FeIV═O species, which accumulates at -0.50 V versus Ag/AgCl (satd. KCl). When 2,3-dimethylindole is present in the solution and the applied potential is held in the range where FeIII-O2•- species accumulate, it gets oxidized to N-(2-acetylphenyl)acetamide retaining both the oxygens from O2 mimicking the reaction of heme dioxygenases. Turnover numbers more than 104 are recorded, establishing this imidazole-bound ferrous porphyrin as a functional model of heme dioxygenases.

Optimization and Enhancement of the Peroxidase-like Activity of Hemin in Aqueous Solutions of Sodium Dodecylsulfate.

Iron porphyrins play several important roles in present-day living systems and probably already existed in very early life forms. Hemin (= ferric protoporphyrin IX = ferric heme b), for example, is the prosthetic group at the active site of heme peroxidases, catalyzing the oxidation of a number of different types of reducing substrates after hemin is first oxidized by hydrogen peroxide as the oxidizing substrate of the enzyme. The active site of heme peroxidases consists of a hydrophobic pocket in which hemin is embedded noncovalently and kept in place through coordination of the iron atom to a proximal histidine side chain of the protein. It is this partially hydrophobic local environment of the enzyme which determines the efficiency with which the sequential reactions of the oxidizing and reducing substrates proceed at the active site. Free hemin, which has been separated from the protein moiety of heme peroxidases, is known to aggregate in an aqueous solution and exhibits low catalytic activity. Based on previous reports on the use of surfactant micelles to solubilize free hemin in a nonaggregated state, the peroxidase-like activity of hemin in the presence of sodium dodecyl sulfate (SDS) at concentrations below and above the critical concentration for SDS micelle formation (critical micellization concentration (cmc)) was systematically investigated. In most experiments, 3,3',5,5'-tetramethylbenzidine (TMB) was applied as a reducing substrate at pH = 7.2. The presence of SDS clearly had a positive effect on the reaction in terms of initial reaction rate and reaction yield, even at concentrations below the cmc. The highest activity correlated with the cmc value, as demonstrated for reactions at three different HEPES concentrations. The 4-(2-hydroxyethyl)-1-piperazineethanesulfonate salt (HEPES) served as a pH buffer substance and also had an accelerating effect on the reaction. At the cmc, the addition of l-histidine (l-His) resulted in a further concentration-dependent increase in the peroxidase-like activity of hemin until a maximal effect was reached at an optimal l-His concentration, probably corresponding to an ideal mono-l-His ligation to hemin. Some of the results obtained can be understood on the basis of molecular dynamics simulations, which indicated the existence of intermolecular interactions between hemin and HEPES and between hemin and SDS. Preliminary experiments with SDS/dodecanol vesicles at pH = 7.2 showed that in the presence of the vesicles, hemin exhibited similar peroxidase-like activity as in the case of SDS micelles. This supports the hypothesis that micelle- or vesicle-associated ferric or ferrous iron porphyrins may have played a role as primitive catalysts in membranous prebiotic compartment systems before cellular life emerged.

Enhanced interfacial water dissociation on a hydrated iron porphyrin single-atom catalyst in graphene

Single Atom Catalysis (SAC) is an expanding field of heterogeneous catalysis in which single metallic atoms embedded in different materials catalyze a chemical reaction, but these new catalytic materials still lack fundamental understanding when used in electrochemical environments. Recent characterizations of non-noble metals like Fe deposited on N-doped graphitic materials have evidenced two types of Fe-N4 fourfold coordination, either of pyridine type or of porphyrin type. Here, we study these defects embedded in a graphene sheet and immersed in an explicit aqueous medium at the quantum level. While the Fe-pyridine SAC model is clear cut and widely studied, it is not the case for the Fe-porphyrin SAC that remains ill-defined, because of the necessary embedding of odd-membered rings in graphene. We first propose an atomistic model for the Fe-porphyrin SAC. Using spin-polarized ab initio molecular dynamics, we show that both Fe SACs spontaneously adsorb two interfacial water molecules from the solvent on opposite sides. Interestingly, we unveil a different catalytic reactivity of the two hydrated SAC motives: while the Fe-porphyrin defect eventually dissociates an adsorbed water molecule under a moderate external electric field, the Fe-pyridine defect does not convey water dissociation.

Polymeric membrane with nanohybrids of Cu nanocomposites and metalloporphyrin-based nanosheets for enzyme-like catalytic degradation of Congo Red

Metalloporphyrin-based nanozymes are effective peroxidase-like catalysts for dye degradation; however, their reusability is a challenge due to the nanoscale dimensions. Membrane technology has the advantages of high efficiency, low energy consumption, and easy operation. Here, we integrate poly (vinylidene fluoride) (PVDF) membrane with nanozyme for effective decolorization of Congo Red (CR). A nanozyme (Cu@Cu-FeTCPP) is prepared by growth of copper hydroxide-copper oxide (Cu(OH)2-CuO) nanocomposites onto the metal organic framework (MOF) nanosheet consisting of tetrakis-(4-carboxyphenyl)-porphyrin iron ligands and cupric ion nodes. The functional membrane is achieved by loading Cu@Cu-FeTCPP nanozyme on the PVDF membrane surface (Cu@Cu-FeTCPP/PVDF). The as-prepared membrane shows catalytic activity for the degradation of CR. Under flow through conditions, the catalytic degradation in tangential flow plays a major role in overall decolorization of CR, far exceeding the contribution of adsorption. The membrane can rapidly catalyze the dye degradation even at different operating pressures (0.1–0.7 kPa) and feed solutions with higher CR concentrations (eg. 60 mg·L-1). Compared with the static batch catalysis and the free nanozyme catalysis, the membrane exhibits higher decolorization efficiency and outstanding reusability in the continuous flow process owing to the promoted mass transfer. After 40 min of continuous flow reaction in a feed solution containing CR (40 mg·L-1) and H2O2 (50 mM), the decolorization efficiency approaches 90% and remains above 85% after five catalytic cycles. The membrane with enzyme-mimicking ability has application prospect in wastewater treatment.

Proton Activation in the Presence of a Weak Acid Facilitated by Second Coordination Sphere Effects in Iron Porphyrins**

AbstractThe catalytic activity of two iron‐based porphyrin complexes containing pyridine‐functionalized second coordination spheres, referred to as Py2XPFe and CuPy2XPFe, have been investigated for the hydrogen evolution reaction (HER) and compared with the unsubstituted analog TMPFe in acetonitrile. The CuPy2XPFe incorporates a second metal center within the pyridine residues and represents a heterodinuclear system, while the structurally analogous Py2XPFe lacks an additional metal in the second coordination sphere. Both the Py2XFe and CuPy2XPFe complexes are observed to activate the weak acid, acetic acid (AcOH) at the FeII/I couple rather than at the more energy intensive FeI/0 couple observed for the unfunctionalized TMPFe complex at low acid concentrations. The ability of the monometallic Py2XPFe to activate the weak proton source at the FeII/I couple manifests as an ECEC(E)′ type electrochemical mechanism, rather than an EECC(E)′ type mechanism as observed for TMPFe. The CuPy2XPFe displays improved reactivity compared with the Py2XPFe and TMPFe under the same substrate conditions, however the mechanistic nuances into bimetallic CuPy2XPFe system are currently unclear. The concentration of AcOH was incrementally increased and the rate constants of the initial protonation step i. e. formation of a hydridic species (k1,app), as well as of the protonation of the hydridic species to produce dihydrogen (kglobal) were calculated using the Foot‐of‐the‐wave and plateau current rate equation methodologies, respectively. The kinetic analysis indicates that the activation of protons through the ECEC(E)′ type mechanism for the Py2XPFe results in a system in which k1,app< kglobal. As both the monometallic Py2XPFe and bimetallic CuPy2XPFe activate the AcOH at less cathodic potentials than TMPFe under identical conditions, the overpotentials (ηTOF/2) for these complexes are thus dramatically lower. The reactivity difference of the Py2XPFe when compared to non‐substituted iron porphyrin analogs is due to the hanging group's influence, which is believed to either act as hydrogen bond promoters for the active site or aids to stabilize a catalytic intermediate species during catalysis.

Bioinspired construction of histidine-doped porphyrin covalent organic framework nanozyme with enhanced peroxidase-like activity for sensitive uric acid detection

Imitating catalytic sites of natural enzymes is a promising approach to accurately improve the catalytic performances of nanozymes. Herein, inspired by the structure of natural horseradish peroxidase (HRP), three-component condensation strategy is unitized to design histidine (His) functionalized iron porphyrin covalent organic frameworks (Fe-COF-H3) nanozyme with peroxidase-like (POD-like) activity. The introduced His could form favorable microenvironment around the iron porphyrin structure, enhancing the catalytic capacity of COF nanozyme. Taking advantage of its excellent POD-like activity, Fe-COF-H3 was utilized as a carrier to immobilize urate oxidase (UOx) for the construction of enzyme-nanozyme cascade system (UOx@Fe-COF-H3). The UOx@Fe-COF-H3 was successfully applied for the colorimetric detection of uric acid (UA) in biological samples. This work not only offers a new insight to develop high-efficiency COF nanozymes, but also expands the application of COF nanozyme for the construction of enzyme-nanozyme cascade system.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction.

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

A Porphyrin Iron(III) π-Dication Species and its Relevance in Catalyst Design for the Umpolung of Nucleophiles.

Isoporphyrins have recently been identified as remarkable species capable of turning the nucleophile attached to the porphyrin ring into an electrophile, thereby providing umpolung of reactivity (Inorg. Chem. 2022, 61, 8105-8111). They are generated by nucleophilic attack on an iron(III) π-dication, a class of species that has received scant attention. Here, we explore the effect of the porphyrin meso-substituent and report a iron(III) π-dication bearing the meso-tetraphenylporphyrin (TPP) ligand. We provide an extensive study of the species by UV/Vis absorption, 2 H NMR, EPR, applied field Mössbauer, and resonance Raman spectroscopy. We further explore the system's highly dynamic and tunable properties and address the nature of the axial ligands as well as the conformation of the porphyrin ring. The insights presented are essential for the rational design of catalysts for the umpolung of nucleophiles. Such catalytic avenues could for example provide a novel method for electrophilic chlorinations. We further examine the importance of electronic tuning of the porphyrin by nature of the meso-substituent as a factor in catalyst design.

A Porphyrin Iron(III) π‐Dication Species and its Relevance in Catalyst Design for the Umpolung of Nucleophiles

AbstractIsoporphyrins have recently been identified as remarkable species capable of turning the nucleophile attached to the porphyrin ring into an electrophile, thereby providing umpolung of reactivity (Inorg. Chem. 2022, 61, 8105–8111). They are generated by nucleophilic attack on an iron(III) π‐dication, a class of species that has received scant attention. Here, we explore the effect of the porphyrin meso‐substituent and report a iron(III) π‐dication bearing the meso‐tetraphenylporphyrin (TPP) ligand. We provide an extensive study of the species by UV/Vis absorption, 2H NMR, EPR, applied field Mössbauer, and resonance Raman spectroscopy. We further explore the system's highly dynamic and tunable properties and address the nature of the axial ligands as well as the conformation of the porphyrin ring. The insights presented are essential for the rational design of catalysts for the umpolung of nucleophiles. Such catalytic avenues could for example provide a novel method for electrophilic chlorinations. We further examine the importance of electronic tuning of the porphyrin by nature of the meso‐substituent as a factor in catalyst design.

Effect of magnetic field modification on oxidative stability of myoglobin in sarcoplasm systems

This study aimed to investigate the effect of magnetic fields (0, 3, 6, 12 mT) on the oxidation characteristics of myoglobin (Mb) in the sarcoplasmic protein (SP) system and to understander the underlying mechanism. The metmyoglobin content, Soret band of heme iron porphyrin, protein conformation and molecular weight distribution were measured in different Mb and SP samples. The results showed that the primary oxidation site of hydroxyl radical on Mb was likely to be the porphyrin ring structure and the side chain group of protein rather than the central iron atoms, what’s more, 12 mT magnetic field treatment had an inhibitory effect on the oxidative damage induced by hydroxyl radical.

Fe-doped two-dimensional graphite carbon nitride: An effective nanozymetic catalyst for electrochemical detection of cell-released H2O2

In this work, a simple one-step pyrolysis process was designed to synthesize Fe-doped graphite carbon nitride (Fe–C3N4) nanosheets with FeCl3 as Fe source. The resulted samples were characterized by various characterization techniques, and the results showed that Fe species might form Fe–N bonds with N atoms of C3N4, which was similar to the iron (II) porphyrin active center of horseradish peroxidase. Then Fe–C3N4 nanosheet was further mixed with carbon nanotube (CNT) to obtain Fe–C3N4@CNT nanocomposite, which was further modified on the self-made flexible printed electrode (PE) to construct the flexible working electrode. Fe–C3N4 nanosheets show good catalytic activity for H2O2 and the introduction of CNT can effectively improve the electron transfer efficiency at the modified electrode interface. By combining with wireless portable electrochemical workstation, the sensor shows a wider detection range (10.0 μmol L−1–20.0 mmol L−1) and a lower limit of detection (0.38 μmol L−1, S/N = 3) for the H2O2 analysis. Furthermore, the sensor can be applied to the real time and in-situ analysis of H2O2 secreted from live cells. In word, this self-supported sensing electrode shows great potential applications in monitoring the pathological and physiological dynamics of cancer cells.

Selective Electrocatalytic Reduction of NO to NH3 by Iron Porphyrins at Physiologically Relevant Potentials

Selective Electrocatalytic Reduction of NO to NH₃ by Iron Porphyrins at Physiologically Relevant Potentials

Iron(III) porphyrin complexes as metalloradical catalysts.

Iron(III) porphyrin complexes as metalloradical catalysts.

Selective Electrocatalytic Reduction of Nitrous Oxide to Dinitrogen with an Iron Porphyrin Complex

Selective Electrocatalytic Reduction of Nitrous Oxide to Dinitrogen with an Iron Porphyrin Complex

Controlled Potential Electrolysis: Transition from Fast to Slow Regimes in Homogeneous Molecular Catalysis. Application to the Electroreduction of CO2 Catalyzed by Iron Porphyrin

AbstractMolecular catalysis of electrochemical reactions is a field of intense activity because of the current interest in electrifying chemical transformations, including both electrosynthesis of organic molecules and production of fuels via small molecule activation. Controlled potential electrolysis (CPE) is often coupled with in situ in operando spectroscopic methods with the aim to gather mechanistic information regarding the catalytic species involved. Herein, considering a simple mechanism for a homogeneous molecular catalysis of an electrochemical reaction, we establish the concentration profile of the catalyst in the electrolysis cell enabling to envision the information that can be obtained from the coupling of this CPE with a spectroscopic probe in the cell compartment. We show how the characteristic parameters of the system (catalytic rate constant, cell dimensions and stirring rate) affect the response with particular emphasis on the transition between two limiting cases, namely a ‘fast’ catalysis regime where catalysis only takes place in a small layer adjacent to the electrode surface and a ‘slow’ catalysis regime where catalysis takes place in the bulk of the solution. These formal concepts are then illustrated with an experimental example, the electroreduction of CO2 in dimethylformamide homogeneously catalyzed by iron tetraphenylporphyrin and followed by UV‐vis spectroscopy.

Iron porphyrin for selective electrochemical reduction of CO2: Synthesis, characterization and effect of electrolyte

The electrochemical CO2reductionreaction to form valued by-products is a challenging and a sustainable process. In general, CO2 reduction to carbon neutral fuels requires selective and well-designed catalysts. Herein, we report perfluorinated iron porphyrin (FeFPor) for CO2-to-CO electroreduction in different bicarbonate electrolytes, byproducts distribution in lithium, sodium, potassium and cesium were analyzed. The immobilized FeFPor on the carbon microporous layer of the carbon cloth exhibited a high selectivity for carbon monoxide production over a wide range of potentials from −0.5 to −1 V vs. RHE in all bicarbonate solution with the NaHCO3 being the best catalytic medium. The highest conversion efficiency to CO production reached 97% at −0.5 V vs. RHE in 0.5 M NaHCO3 in near neutral aqueous solution (pH = 7.2). The CO electrosynthesis dropped when using lithium, potassium and cesium bicarbonates, which was due to the CO2 concentration and reactions at the inner electrode-solution reactions.