Iron Porphyrin Complexes Research Articles

Influence of the Fe(II) Spin State on Iron-Ligand Bonds in Heme Model Iron-Porphyrin Complexes with 4, 5 and 6 Ligands.

In this work heme models with four [Fe(II)(P)], five [Fe(II)(P)Im], [Fe(II)(P)O2] and six ligands [Fe(II)(P)(Im)O2], where P=porphyrin, with different spin states (ms=5, 3 and 1) of the iron atom were investigated using relativistic-corrected quantum chemistry methods (PW6B95-D3-DKH/jorge-TZP-DKH). Dependence of the iron-ligand bond properties on (i) spin state and (ii) number of ligands were analyzed using natural bond orbital analysis, electron density topology, electrostatic potential and electron localization function. It is shown that reversible binding of O2 is possible in case of formation of semicoordination bond between Fe(II) and imidazole. Binding of the fifth and sixth ligand from the energetic and orbital points of view is more favorable for the triplet Fe(II) state. At the same time for the six-coordinated complex [Fe(II)(P)(Im)O2] interconversion of Fe(II) electrons of valent 3d orbital from quintet to triplet and vice versa is possible under thermal fluctuations (energy barriers less than 2 kcal/mol).

Alkali Metal Ion Tuning of a Crown Ether‐Appended Porphyrin for Electrocatalytic CO2 Reduction

Iron‐porphyrin systems are among the best catalysts to reduce selectively CO2 to CO. Electronic modification of the porphyrin skeleton by substituents can enhance the catalytic efficiency of the complexes. In this work, we modified the iron porphyrin complex with crown‐ether auxiliaries and tested the effect of complexation with alkali metals on their electrocatalytic properties for CO2 reduction. The presence of Na+ or K+ resulted in a positive shift of the onset potential by 210 mV while not compromising the catalytic current. This effect was maintained after the immobilization at the vulcanized carbon support, and the catalyst showed the Faradaic efficiency above 90% for CO2 to CO reduction in aqueous solution of NaHCO3 or KHCO3 in a wide range of overpotentials. Hence, host‐guest chemistry at the periphery of the CO2 reduction catalysts can tune the electronic properties of the catalysts and thereby increase the catalytic efficiency.

Mechanistic insight into electrocatalytic CO2 reduction to formate by the iron(I) porphyrin complex: A DFT study

Electrocatalytic reduction of CO2 into value-added chemicals has been considered as a promising pathway to mitigate the energy crisis and global warming. Iron porphyrins have been extensively studied for electrocatalytic CO2 reduction reaction (CO2RR), known for their ability to promote CO2-to-CO conversion. However, the mechanism of CO2-to-HCOO− conversion by Fe porphyrin remains unclear. Here, by means of density functional theory (DFT) calculations, we investigated the detailed mechanism of a novel Fe porphyrin catalyst for CO2 reduction to HCOO− in its Fe(I) state. Our results demonstrated that the reduction of CO2 to HCOO− proceeds via the C-protonation of an FeII-OCO•− complex, rather than through the hydrolysis of an FeIII-COOH complex or CO2 insertion in an Fe−H bond. Furthermore, the FeIII-COOH complex is found to be a unstable intermediate. Protonation of its hydroxyl group, accompanied by C−OH bond cleavage to produce CO, is both thermodynamically and kinetically unfeasible. Instead, the FeIII-COOH complex undergoes a coordination switch followed by a conformational change to form the active FeII-OCO•− complex, which promotes the production of HCOO−. Moreover, the single-electron reduction of FeIII-COOH gives FeII-COOH, leading to formation of CO rather than HCOO−. The insights gained from this study may contribute to design of electrocatalysts for selective CO2 reduction to formate.

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex.

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex

AbstractIn the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3‐arylmethyl‐substituted 2‐quinolones and 2‐pyridones underwent an enantioselective amination reaction (20 examples; 93–99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two‐point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Can Iron-Porphyrins Behave as Single-Molecule Magnets?

The study of magnetic properties, especially the magnetic anisotropy of iron-porphyrin complexes employing multiconfigurational methods, is quite challenging due to many strongly correlated electrons in nearly degenerate orbitals. However, a prerequisite for observing the magnetic anisotropy and slow magnetization relaxation, the zero-field splitting parameter, D, was experimentally observed decades ago for halide-based axially ligated penta-coordinate Fe(III)-porphyrins. In these complexes, the signs of D were reported mostly as positive; in a few cases, inconclusive signs of the D parameter were also mentioned. However, no ab initio calculations have been reported to shed light on this. Deciphering the electronic structure of these penta-coordinated complexes employing the complete active space self-consistent field method and N-electron valence second-order perturbation theory, we confirm the positive D values. However, a negative D value is highly desired to observe the single-molecule magnet properties without an external magnetic field, which we observed in the Fe(II)-porphyrin complexes with axial imidazole ligands instead of halide ligands. The detailed analysis of the multireference wave functions unravels the role of axial ligands in determining the sign and magnitude of the D parameters.

Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction.

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Proximity-based defensive mutualism between Streptomyces and Mesorhizobium by sharing and sequestering iron.

Microorganisms living in soil maintain intricate interactions among themselves, forming the soil microbiota that influences the rhizosphere microbiome and plant growth. However, the mechanisms underlying the soil microbial interactions remain unclear. Streptomyces and Mesorhizobium are commonly found in soil and serve as plant growth-promoting rhizobacteria (PGPR). Here, we identified an unprecedented interaction between the colonies of red-soil-derived Streptomyces sp. FXJ1.4098 and Mesorhizobium sp. BAC0120 and referred to it as "proximity-based defensive mutualism (PBDM)." We found that metabolite-mediated iron competition and sharing between the two microorganisms were responsible for PBDM. Streptomyces sp. FXJ1.4098 produced a highly diffusible siderophore, desferrioxamine, which made iron unavailable to co-cultured Mesorhizobium sp. BAC0120, thereby inhibiting its growth. Streptomyces sp. FXJ1.4098 also released poorly diffusible iron-porphyrin complexes, which could be utilized by Mesorhizobium sp. BAC0120, thereby restoring the growth of nearby Mesorhizobium sp. BAC0120. Furthermore, in ternary interactions, the PBDM strategy contributed to the protection of Mesorhizobium sp. BAC0120 close to Streptomyces sp. FXJ1.4098 from other microbial competitors, resulting in the coexistence of these two PGPR. A scale-up pairwise interaction screening suggested that the PBDM strategy may be common between Mesorhizobium and red-soil-derived Streptomyces. These results demonstrate the key role of iron in complex microbial interactions and provide novel insights into the coexistence of PGPR in soil.

Metal-ligand delocalization of iron and cobalt porphyrin complexes in aqueous solutions probed by soft X-ray absorption spectroscopy.

Metal-ligand delocalization of metal porphyrin complexes in aqueous solutions was investigated by analyzing the electronic structure of both the metal and ligand sides using soft X-ray absorption spectroscopy (XAS) at the metal L2,3-edges and nitrogen K-edge, respectively. In the N K-edge XAS spectra of the ligands, the energies of the CN π* peaks of cobalt protoporphyrin IX (CoPPIX) are higher than iron protoporphyrin IX (FePPIX). The energy difference between the two lowest peaks in the XAS spectrum of CoPPIX is also larger than that of FePPIX. Nitrogen K-edge inner-shell calculations of metalloporphyrins with different central metals indicate that the energy differences between these peaks reflect the electronic configurations and spin multiplicities of metalloporphyrins. We also investigated the hydration structure of CoPPIX in aqueous solution by analyzing the electronic structure of the ligand and revealed that CoPPIX maintains its five-coordination geometry in aqueous solution. The present study shows high performance of N K-edge XAS of ligands for studying the coordination structures of metalloporphyrins in solutions rather than the metal L2,3-edges of central metals.

Intramolecular electrostatic effects on reduction of CO2 to CO by iron porphyrin complexes: Insights from DFT calculations

Iron porphyrin (Por) complexes were found to be highly efficient molecular catalysts for electrocatalytic CO2 reduction and the catalytic performance can be modulated by the modification of ligands. Herein, density functional theory calculations were employed to investigate how the presence of second-sphere anionic carboxylate groups influences the CO2-to-CO conversion by Fe porphyrins. The kinetics and thermodynamics of individual chemical steps in the reaction cycle were systematically studied. Computations indicated that the catalytic active species is the ferrous complex with the two extra electrons being stored in the π system of porphyrin and the rate-determining step is the CO2 binding. The synchronization of the Fe-to-CO2 and ligand-to-Fe electron transfers leads to the formation of FeIII(CO22−)(Por•−). The experimentally observed activity trend could be rationalized by the Brønsted-Evans-Polanyi principle. The electrostatic repulsive between the negatively charged hanging group and the CO2 moiety destabilizes the CO2 adduct and thereby raises the binding energy. Then, the two-step protonation occurs easily to give Fe(II)-CO. The further one-electron reduction triggers the dissociation of CO to regenerate of the formal Fe(I) porphyrin catalyst. The insights gained from this study may be helpful in designing efficient electrocatalysts for CO2 reduction.

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(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

Distortion effect on the ligation of imidazoles to water-insoluble iron(III) porphyrin

Axial ligands control the various chemical reactions triggered by heme proteins. Thus, studies on the reaction of iron porphyrin with ligands are important for understanding their functions. Here, a spectroscopic investigation was performed to clarify the distortion effect of the porphyrin ring on the ligation of imidazoles (imidazole (Im), 1-methyl- and 2-methylimidazole (1-MeIm and 2-MeIm)) to iron(III) porphyrin. The overall stability constant ([Formula: see text] was found to have increased, while [Formula: see text] decreased with increasing distortion of the porphyrin core. However, the [Formula: see text] of the tetraphenyl-substituted complex, which is considered the least distorted, was not minimal, while the [Formula: see text] was not maximal. These results suggest that the stability of bis-ligand iron(III) porphyrin complexes depends on the increase in the saddle-type distortion of the porphyrin, the type of distortion, and the steric hindrance at the surrounding substituents. Furthermore, the ligation of 1-MeIm to iron(III) porphyrin hydroxo complexes with the porphyrin ring distortion was investigated. The ligation of 1-MeIm to the iron(III) dodecaphenylporphyrin hydroxo complex (DPPFeOH) suggests that a reaction mechanism different from the general reaction mechanism may occur simultaneously, i.e., a simultaneous reaction. In addition, the [Formula: see text] of DPPFeOH was found to be approximately 15 times smaller than that of the chloride complex. The relationship between the distortion and stability of bis-ligands iron(III) porphyrin complexes obtained in this study is expected to assist in the elucidation of the model reaction of heme proteins.

Catalytic Degradation of Triphenylmethane Dyes with an Iron Porphyrin Complex as a Cytochrome P450 Model.

The organic dyes used in printing and dyeing wastewater have complex components, diverse structures and strong chemical stability, which make them not suitable for treatment and difficult to degrade in the environment. Porphyrins are macromolecules with 18 π electrons formed by four pyrrole molecules connected with a methylene bridge that has a stable structure. Porphyrin combines with iron to form an active intermediate with a structure similar to the cytochrome P450 enzyme, so they are widely used in the biomimetic field. In the current study, 5,10,15,20-tetra (4-carboxyphenyl) porphine ferric chloride (III) (Fe(III)TCPP) was used as a catalyst and iodosobenzene was used as an oxidant to explore the catalytic degradation of triphenylmethane dyes, such as rhodamine B (RhB) and malachite green (MG). The results of UV-Vis spectral analysis have shown that the conversion rate of the rhodamine B was over 90% when the amount of Fe(III)TCPP was 0.027 mM and the amount of iodosobenzene was eight equivalents. When the catalyst was 0.00681 mM and the amount of the oxidant was five equivalents, the conversion rate of the malachite green reached over 95%. This work provides a feasible method for the degradation of triphenylmethane dyes.

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry.

The iron-based porphyrin complex containing a bispyridine-based hanging unit termed Py2XPFe was previously used as an effective catalyst for the reduction of protons to molecular hydrogen in solution. Here, the molecular compound was immobilized on a modified gold electrode surface and investigated by spectroelectrochemical methods under catalytic conditions. Immobilization of the Py2XPFe was facilitated using a pyridine-based amine linker molecule grafted to the gold electrode by electrochemical amine oxidation. The linker molecule denoted in this report as Pyr-1 allows for effective coordination of the iron porphyrin compound to the modified gold surface through axial coordination of the pyridine component to the Fe center. Resonance Raman spectroelectrochemistry was performed on the immobilized catalyst in pH 7 buffer at increasing cathodic potentials. This facilitates the electrochemical hydrogen evolution reaction (HER) while concurrently allowing for the observation of the v4, v3, and v2 porphyrin marker bands, which are sensitive to oxidation and spin state changes at the metal center. The observed changes in these bands at decreasing potential indicate that the immobilized Py2XPFe exists in the formal high-spin FeIII state before being reduced to the low-spin FeII state resulting from axial interaction with the linker moiety. This FeII state likely acts as the precatalyst for the HER reaction. Surfaced enhanced Raman spectroelectrochemistry was also conducted on the system as the gold electrode provides a sufficient surface enhancement effect so as to observe the bonding nature of the pyridine substituents within the second coordination sphere. As the potential is lowered cathodically, the pyridine ring breathing modes at 999 cm-1 are shown to increase in intensity due to protonation, which reach an intensity saturated limit whereat HER is conducted. This suggests that in pH 7 buffer, the increase in cathodic potentials facilitates protonation of the pyridine-based second coordination sphere. The extent to which protonation occurs can be viewed as a function of decreasing potential due to an increase in proton flux at the immobilized catalyst which, at the required onset potential for catalysis, aids in the reduction of protons to molecular hydrogen.

Probing the spin states of tetra‐coordinated iron(II) porphyrins by their vibrational and pre K‐edge x‐ray absorption spectra

AbstractTetra‐coordinated iron(II) porphyrins are observed in triplet (intermediate spin, IS) ground state with low‐lying quintet (high spin, HS) and singlet (low spin, LS) excited states. It is well known in the literature that the bond distances are dependent on the spin state such that the distances are larger for HS compared to the LS/IS states. This implies the existence of strong magneto‐structural correlations. Herein, the possibility to obtain the spin‐state information for a series of tetra‐coordinated square planar iron(II) porphyrin complexes have been explored by exploiting the magneto‐structural correlations obtained from first principle calculations. The spin‐state dependent nature of the chemical bonds and spin polarization on the N‐atoms play a crucial role in the vibrational dynamics of the central Fe‐atom. Theoretical nuclear resonance vibrational spectroscopy (NRVS) is adopted to investigate the vibrational dynamics and the Fe‐atom associated spectral peaks and their spin‐dependent features. We find that NRVS studies along with iron pre‐K‐edge x‐ray absorption analysis convincingly provide the information on the spin state, which could be utilized to identify the spin states without adopting any magnetic probes.

Iron-Complex-Based Supramolecular Framework Catalyst for Visible-Light-Driven CO2 Reduction.

Molecule-based heterogeneous photocatalysts without noble metals are one of the most attractive systems for visible-light-driven CO2 reduction. However, reports on this class of photocatalysts are still limited, and their activities are quite low compared to those containing noble metals. Herein, we report an iron-complex-based heterogeneous photocatalyst for CO2 reduction with high activity. The key to our success is the use of a supramolecular framework composed of iron porphyrin complexes bearing pyrene moieties at meso positions. The catalyst exhibited high activity for CO2 reduction under visible-light irradiation (29100 μmol g-1 h-1 for CO production, selectivity 99.9%), which is the highest among relevant systems. The performance of this catalyst is also excellent in terms of apparent quantum yield for CO production (0.298% at 400 nm) and stability (up to 96 h). This study provides a facile strategy to create a highly active, selective, and stable photocatalyst for CO2 reduction without utilizing noble metals.

Methodic Approach of Atomic-Force Microscopy (AFM) to Study Morphological Changes of Cells and Model Systems

For the first time AFM (atomic-force microscopy) was used to record significant changes in the geometric parameters of the image of erythrocytes in vitro under conditions of glycolytic starvation (ATP (Adenosine triphosphate) deficiency). The difference in the action of antioxidants, phenosan K, and Ihfan-10 on erythrocytes that we detected with AFM seems to be mainly due to their difference in hydrophobicity. We used the AFM method to research the self-organization of the components of the active center of P450 (Porphyrin-450) metalloenzymes that are part of a class of hemoproteins with functions of affinity to molecular oxygen O2. Stable supramolecular nanostructures in the form of triangular prisms based on the iron porphyrin complex with amino acids due to self-assembly involving intermolecular hydrogen bonds were received. A possible scheme for the formation of such structures is proposed.

Silver nanostructure-modified graphite electrode for in-operando SERRS investigation of iron porphyrins during high-potential electrocatalysis.

In-operando spectroscopic observation of the intermediates formed during various electrocatalytic oxidation and reduction reactions is crucial to propose the mechanism of the corresponding reaction. Surface-enhanced resonance Raman spectroscopy coupled to rotating disk electrochemistry (SERRS-RDE), developed about a decade ago, proved to be an excellent spectroscopic tool to investigate the mechanism of heterogeneous oxygen reduction reaction (ORR) catalyzed by synthetic iron porphyrin complexes under steady-state conditions in water. The information about the formation of the intermediates accumulated during the course of the reaction at the electrode interface helped to develop better ORR catalysts with second sphere residues in the porphyrin rings. To date, the application of this SERRS-RDE setup is limited to ORR only because the thiol self-assembled monolayer (SAM)-modified Ag electrode, used as the working electrode in these experiments, suffers from stability issues at more cathodic and anodic potential, where H2O oxidation, CO2 reduction, and H+ reduction reactions occur. The current investigation shows the development of a second-generation SERRS-RDE setup consisting of an Ag nanostructure (AgNS)-modified graphite electrode as the working electrode. These electrodes show higher stability (compared to the conventional thiol SAM-modified Ag electrode) upon exposure to very high cathodic and anodic potential with a good signal-to-noise ratio in the Raman spectra. The behavior of this modified electrode toward ORR is found to be the same as the SAM-modified Ag electrode, and the same ORR intermediates are observed during electrochemical ORR. At higher cathodic potential, the signatures of Fe(0) porphyrin, an important intermediate in H+ and CO2 reduction reactions, was observed at the electrode-water interface.