Axial ligand and spin-state influence on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Iron porphyrins-catalysed oxidation of α-alkyl substituted mono and dimethoxylated benzyl alcohols

The mechanisms of oxidation of a series of a-alkyl substituted mono and dimethoxylated benzyl alcohols catalysed by mesotetrakis(4-N-methylpyridynium)porphyrin iron (III) chloride (FeTMPyPCl) and meso-tetrakis(4-sulfonatophenyl)porphyrin iron (III) chloride (FeTSPPCl) in aqueous solution with KHSO₅ as oxygen atom donor and by meso-tetrakis(pentafluorophenyl)-porphyrin iron (III) chloride (FeTPFPPCl) in dichloromethane employing iodosylbenzene as oxidant have been investigated. In the highly polar aqueous medium an electron transfer mechanism is operating. With FeTMPyPCl, which is a much more efficient catalyst than FeTSPPCl due to the presence of stronger electron withdrawing substituents, formation of side-chain oxidation products accompanies generation of nuclear oxidation products. In the low polar solvent dichloromethane, two competing mechanism have been suggested: hydrogen atom transfer and formation of a complex between the active species iron-oxo porphyrin radical cation and the substrate.      

Formation of iron(III) meso-chloro-isoporphyrin as a reactive chlorinating agent from oxoiron(IV) porphyrin π-cation radical

Iron(III) isoporphyrin, a tautomer of porphyrin with a saturated meso carbon, is one of the isoelectronic forms of oxoiron(IV) porphyrin π-cation radical, which is known as an important reactive intermediate of various heme enzymes. The isoporphyrin has been believed to be incapable of catalyzing oxygenation and oxidation reactions. Here, we report that an oxoiron(IV) porphyrin π-cation radical can be converted to iron(III) meso-chloro-isoporphyrin in the presence of trifluoroacetic acid and chloride ion. More importantly, this study shows the first evidence that iron(III) meso-chloro-isoporphyrin is an excellent reactive agent for chlorinating aromatic compounds and olefins. The results of this study suggest that the mechanism involves electrophilic chlorination of substrate with iron(III) meso-chloro-isoporphyrin.     

Self-assembled monolayer of an iron(III) porphyrin disulphide derivative on gold

A novel iron(III) porphyrin disulphide derivative have been successfully immobilised on gold surfaces by self-assembly. The redox response of the modified electrodes was compared with the obtained for a similar iron porphyrin in solution, confirming the immobilisation of the metalloporphyrin. The gravimetric data obtained by electrochemical quartz crystal microbalance (EQCM) during adsorption allowed an estimation of the electrode coverage, providing further evidence for the formation of the porphyrin SAM. The modified electrodes were also measured by conventional and imaging ellipsometry. The electrocatalytic activity of the two modified electrodes was tested for the reduction of the molecular oxygen.     

Reactivity of Nickel(II) Porphyrins in oCVD Processes—Polymerisation,Intramolecular Cyclisation and Chlorination

Oxidative chemical vapour deposition of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) with iron(III) chloride as oxidant yielded a conjugated poly(metalloporphyrin) as a highly coloured thin film, which is potentially useful for optoelectronic applications. This study clarified the reactive sites of the porphyrin monomer NiDPP by HRMS, UV/Vis/NIR spectroscopy, cyclic voltammetry and EPR spectroscopy in combination with quantum chemical calculations. Unsubstituted meso positions are essential for successful polymerisation, as demonstrated by varying the porphyrin meso substituent pattern from di- to tri- and tetraphenyl substitution. DFT calculations support the proposed radical oxidative coupling mechanism and explain the regioselectivity of the C−C coupling processes. Depositing the conjugated polymer on glass slides and on thermoplastic transparent polyethylene naphthalate demonstrated the suitability of the porphyrin material for flexible optoelectronic devices.     

Synthesis and chemistry of an iron(III) tetraphenylporphyrin with a covalently-attached phenolate tail

We report the synthesis of a phenolate-tailed iron(III) tetraphenylporphyrin, 1. The phenolate ligand is covalently-attached to the porphyrin ring and is coordinated to the iron(III) center. This phenolate ligand increases the rate of oxygen atom transfer to the metal center.     

Effects of ion-carrier substituents on the potentiometric-response characteristics in anion-selective membrane electrodes based on iron porphyrins.

The potentiometric response characteristics with respect to salicylate anion of several membrane electrodes based on iron(III) tetraphenylporphyrin chloride (FeTPPCl) and derivatives with electrophilic and nucleophilic substituents, incorporated into plasticized polyvinylchloride (PVC) membranes were investigated. Complexes tetraphenyl porphyrin iron(III) chloride (FeTPPCl; A), tetrakis (4-methoxyphenyl) porphyrin iron(III) chloride (Fe(TOCH3PP)Cl; B), tetrakis (2,6-dichlorophenyl) porphyrin iron(III) chloride (Fe(TDClPP)Cl; C), tetrakis (4-nitrophenyl) porphyrin iron(III) chloride (Fe(TNO2PP)Cl; D), and tetrakis (pentafluorophenyl) porphyrin iron(III) chloride (Fe(TPFPP)Cl; E) were used as anion carriers in the membrane electrodes. The sensitivity, working range, detection limit, response mechanism, and selectivity of the membrane sensor toward interference shows a considerable dependence on the type of carrier substituent and the pH value of the sample solution. Potentiometric investigations in solutions of various pH show that the carrier complex containing fluoro substituents (E), which have very strong electron-accepting properties and a high ability to form hydrogen bonds, is capable of serving as a positively charged ionophore. Some other ionophores are capable of serving as both charged and neutral carriers under different conditions. The electrodes prepared in this work show super-Nernstian slopes with respect to salicylate concentration, which tend to a Nernstian response (slope near to -59 mV decade-1) upon an increase of the pH of the test solution. The results of UV/Vis absorption spectroscopy are used for interpretation of the formation of an oxene complex between salicylate and iron porphyrins.     

A Highly Oxidized Cobalt Porphyrin Dimer: Spin Coupling and Stabilization of the Four‐Electron Oxidation Product

A highly oxidized cobalt porphyrin dimer is reported. Each cobalt(II) ion and porphyrin ring underwent 1e oxidation with iodine as the oxidant to give a 4e‐oxidized cobalt(III) porphyrin π‐cation radical dimer. The bridging ethylene group allows for substantial conjugation of the porphyrin macrocycles, thus leading to a strong antiferromagnetic coupling between the π‐cation radicals and to stabilization of the singlet state. X‐ray crystallography clearly showed that the complex may be considered as a real supramolecule rather than two cobalt(III) porphyrin π‐cation radicals that interact through space.     

Factors affecting the catalytic epoxidation of olefins by iron porphyrin complexes and H2O2 in protic solvents

The catalytic epoxidation of cyclohexene by iron(III) porphyrin complexes and H2O2 has been investigated in alcohol solvents to understand factors affecting the catalyst activity in protic solvents. The yields of cyclohexene oxide and the Fe(III/II) reduction potentials of iron porphyrin complexes were significantly affected by the protic solvents, and there was a close correlation between the product yields and the reduction potentials of the iron porphyrin catalysts. The role of alcohol solvents was proposed to control the electronic nature of iron porphyrin complexes that determines the catalyst activity in the epoxidation of olefins by H2O2. We have also demonstrated that an electron-deficient iron porphyrin complex can catalyze the epoxidation of olefins by H2O2 under conditions of limiting substrate with high conversion efficiency in a solvent mixture of CH3OH and CH2Cl2.     

Intersystem crossing versus electron transfer in porphyrin-based donor-bridge-acceptor systems: influence of a paramagnetic species

We have investigated how the spin state of an acceptor influences the photophysical processes in a donor-bridge-acceptor (D-B-A) system. The system of choice has zinc porphyrin as the electron donor and high- or low-spin iron(III) porphyrin as the acceptor. The spin state of the acceptor porphyrin is switched simply by coordinating imidazole ligands to the metal center. The D-A center-center distance is 26 A, and the bridging chromophore varies from pi-conjugated to a sigma-bonded system. The presence of a high-spin iron(III) porphyrin in such systems has previously been shown to significantly enhance intersystem crossing in the remote zinc porphyrin donor, whereas no significant electron transfer to the iron porphyrin acceptor was observed, even though the thermodynamics would allow for photoinduced electron transfer. Here, we demonstrate that by switching the acceptor to a low-spin state, the dominating photophysical process is drastically changed; the low-spin system shows long-range electron transfer on the picosecond time-scale, and intersystem crossing occurs at its "normal" rate.     

Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).     

Immobilization of heme complex by radiation polymerization

Transparent films containing an iron-porphyrin complex were synthesized by ⁶⁰Co γ-ray irradiation of aqueous solutions of ironporphyrin complex and a water soluble monomer such as 2-hydroxyethylmethacrylate or 1-vinyl-2-pyrrolidone. The iron(II)porphyrin complex was immobilized in the film by covalent bonding without any denaturation, under anaerobic condition or by protection of the ironporphyrin complex with carbon monoxide. After the irradiation of iron(III)porphyrin, the central iron ion was reduced spontaneously to the ferrous state. The films containing the iron(II)porphyrin adsorbed quantitatively carbon monoxide gas.     

Efficient and Bio‐inspired Conversion of Cellulose to Formic Acid Catalyzed by Metalloporphyrins in Alkaline Solution

A bio‐inspired approach for efficient conversion of cellulose to formic acid (FA ) was developed in an aqueous alkaline medium. Metalloporphyrins mimicking cytochrome P450 exhibit efficiently and selectively catalytic performance in catalytic conversion of cellulose. High yield of FA about 63.7% was obtained by using sulfonated iron(III ) porphyrin as the catalyst and O₂ as the oxidant. Iron(III )‐peroxo species, TSPPFe^IIIOO ⁻, was involved to cleave the C‐C bonds of gluconic acid to FA in this catalytic system. This approach used relatively high concentration of cellulose and ppm concentration of catalyst. This work may provide a bio‐inspired route to efficient conversion of cellulose to FA .     

Aerobic catalytic photooxidation of olefins by an electron-deficient Pacman bisiron(III) mu-oxo porphyrin

The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) mu-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) mu-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions.     

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO^? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl^? or CF₃SO₃^?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Chemoselective aerobic oxidation of 4-allylanisol by Fe(III) porphyrins in an aqueous system

The allyl moiety of 4-allyl-anisol was oxidized in the presence of a Fe(III) porphyrin derivative to the corresponding α,β-unsaturated aldehyde in an initial oxidation step with perfect chemoselectivity. Molecular oxygen was employed as the sole environmental innocuous oxidant. The reaction was performed in an aqueous buffer/CH₂Cl₂ mixture using the detergent Tween 80 to homogenize the system.     

Spectroscopic and Mechanistic Studies on Oxidation Reactions Catalyzed by the Functional Model SR Complex for Cytochrome P450: Influence of Oxidant,Substrate, and Solvent

Kinetic and mechanistic studies on the formation of an oxoiron(IV) porphyrin cation radical bearing a thiolate group as proximal ligand are reported. The SR complex, a functional enzyme mimic of P450, was oxidized in peroxo‐shunt reactions under different experimental conditions with variation of solvent, temperature, and identity and excess of oxidant in the presence of different organic substrates. Through the application of a low‐temperature rapid‐scan stopped‐flow technique, the reactive intermediates in the SR catalytic cycle, such as the initially formed SR acylperoxoiron(III) complex and the SR high‐valent iron(IV) porphyrin cation radical complex [( SR ^.+)Fe^IV?O], were successfully identified and kinetically characterized. The oxidation of the SR complex under catalytic conditions provided direct spectroscopic information on the reactivity of [( SR ^.+)Fe^IV?O] towards the oxidation of selected organic substrates. Because the catalytically active species is a synthetic oxoiron(IV) porphyrin cation radical bearing a thiolate proximal group, the effect of the strong electron donor ligand on the formation and reactivity/stability of the SR high‐valent iron species is addressed and discussed in the light of the reactivity pattern observed in substrate oxygenation reactions catalyzed by native P450 enzyme systems.     

Effects of a nonplanar porphyrin rings on the spin-spin interactions in low-spin ferric porphyrin radical cationst

Low-spin ferric porphyrin radical cations formed by the oxidation of chloro(meso-tetraalkylporphyrinato)iron(III) followed by the addition of bulky 2-methylimidazole show antiferromagnetic coupling, which is interpreted in terms of the interaction between porphyrin a2u and iron d(xy), orbitals caused by the S4 ruffling of the porphyrin core.     

Molecular assembling using axial phenolate on an iron N-confused porphyrin complex

The reaction of sodium phenolate and Fe(HCTPPH)Br assembles a dimeric iron(III) complex with a channel-like solid state packing; the intramolecular iron-to-iron distance of 8.864 A is the longest among the available N-confused porphyrin dimeric complexes.     

Electronic structure analysis of iron(III)-porphyrin complexes by X-ray absorption spectra at the C, N and Fe K-edges.

X-ray absorption near edge structure (XANES) measurements at the C, N, and Fe K absorption edges were performed for iron(III)-tetraphenylporphyrin (FeTPP), iron(III)-tetrakis(p-carboxyphenyl)porphyrin (FeTCPP), and iron(III)-tetrakis(p-sulfonatophenyl)porphyrin (FeTSPP). The spectral shapes differ in the Fe K XANES, but not in C and N K XANES among FeTPP, FeTCPP, and FeTSPP. Crosschecks of XANES data for C, N, and Fe K absorption edges in combination with discrete variational (DV)-Xalpha molecular orbital (MO) calculations indicate that each p-electron-withdrawing group on four meso-phenyl substitutes in an Fe(III)-porphyrin complex brings about a unique electron state through the complex because of the electron-withdrawal strength, itself. Consequently, they affect the positive charge of the center Fe(III) ion.