Narrow Results

Density functional theory (DFT) calculations have been carried out for a variety of iron and manganese porphyrin complexes with metal oxidation states of +3 and above. In general, DFT gives very good descriptions of molecular structure and electron distributions, but appears less reliable in predictions of the relative energetics of different spin states of transition metal compounds. For instance, the fact that our calculations favor a quartet state for the five-coordinate complex, (P)FeCl, over a sextet state may be regarded as a shortcoming of currently available exchange-correlation functionals. This problem is corrected at the considerably more demanding spin-restricted open-shell coupled-cluster level of theory. For both +3 and +4 metal oxidation states, the manganese 3d subshells appear to be significantly more spatially contracted than the 3d subshells of analogous iron compounds. This appears to be a key factor underlying the anomalously low MnO stretching frequencies of Mn(IV)-oxo porphyrins, compared to the FeO stretching frequencies of Fe(IV)-oxo porphryins. Similarly, a dramatic ruffling-induced redistribution of unpaired spin density in the case of an A_2u-type iron(IV)-oxo porphyrin cation radical, recently reported by Deeth, has been ascribed to a metal(d_xy)–porphyrin(a_2u) orbital interaction that becomes symmetry-allowed on ruffling of the porphyrin ring. If the axial ligand in a compound I species is strongly basic such as imidazolate (e.g. horseradish peroxidase) or mercaptide (chloroperoxidase and cytochrome P450), it too can be strongly noninnocent and carry a significant unpaired electron spin population. Similarly, theoretical modeling of a synthetic 'perferryl' complex, reported by Morishima and co-workers, has revealed a noninnocent axial methoxide ligand.

The reaction of the 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrinatoiron(III) cation, [Fe^III(TMPyP)]⁵⁺ (1; prepared as the pentaperchlorate), with hydrogen peroxide (H₂O₂) in aqueous solution was studied by spectrophotometric methods. The porphyrin 1 was rapidly oxidized with an excess of H₂O₂ under aerobic conditions to the oxo-iron(IV) porphyrin, [Fe^IVO(TMPyP)]⁴⁺ (2), having UV-visible absorption maxima at 436 and 450 nm. Computer deconvolution of the time-resolved absorption spectra revealed that the absorption spectra < 0.5 s were not reproduced well by a simple combination of the two spectra of 1 and 2, indicating that transient species were formed at the initial stage. Addition of sodium nitrite (NaNO₂), which is thought to be a one-electron donor, to the stoichiometric reaction mixture of 1 and H₂O₂ increased the initial formation rate of 2. The presence of uric acid (UA) caused a significant delay in the formation of 2. The delay of the appearance of 2 was directly proportional to the starting concentration, [UA]₀, but other kinetic profiles were not changed significantly. Based on these observations and the kinetic analysis, we confirmed the involvement of the oxo-iron(IV) porphyrin radical cation, [Fe^IVO(TMPyP^•)]⁵⁺ (3), as a compulsory intermediate in the rate-determining step of the overall reaction, 1 + H₂O₂→ 2, with a rate constant k = 4.4 × 10⁴ M⁻¹.s⁻¹. The rate constants for the reaction between 2 and NaNO₂, and between 2 and UA were estimated to be 9.0 M⁻¹s⁻¹ and 5.45 × 10⁶M⁻¹.s⁻¹, respectively.

Data from the last few years have revealed a novel biological role of the tetrahydrobiopterin (H₄B) cofactor, in one-electron transfers to the heme of the active site of NO-synthases (NOSs) with intermediate formation of a H₄B-derived radical. These electron transfers play a key role in the catalytic cycles of the two steps catalyzed by NOS, the N^ω-hydroxylation of L-arginine, and the three-electron oxidation of N^ω-hydroxyarginine to L-citrulline and NO. Recent experiments performed between various tetrahydropterins and iron porphyrins have shown that the one-electron transfer from tetrahydropterins, such as the natural cofactors H₄B and tetrahydrofolate or the synthetic 6,7-dimethyltetrahydropterin (diMeH₄P), to Fe(III) porphyrins of sufficiently high redox potentials (> about -100 mV versus NHE for the Fe(III)/Fe(II) couple) is a very general reaction that occurs with formation of a tetrahydropterin-derived radical. Reaction of diMeH₄P with a stable porphyrin Fe(II)-O₂ complex leads to a diMeH₄P-derived radical and a transient Fe(III)-OOH complex, mimicking the reaction between H₄B and heme Fe(II)-O₂ in the NOS catalytic cycle. Tetrahydropterins such as diMeH₄P also reduce hemeproteins Fe(III) of sufficiently high redox potentials, such as cytochromes c and b₅ or metmyoglobin, to the corresponding hemeproteins Fe(II).

This paper reports a UV-Vis spectroscopic study on the interactions of chloroquine with anionic, cationic and neutral tetra-arylporphyrins and their iron(III) complexes in aqueous buffer and in methanolic solution. This study reveals that in water at pH 6.4 cooperative ion-pair and π-π interactions lead to complex formation between the anionic porphyrin species and chloroquine. In methanolic solution no π-π complexation is observed. The neutral and cationic porphyrins and metalloporphyrins do not form complexes with chloroquine. On the basis of these results, the iron(III) porphyrins have been used as catalysts for the oxidation of chloroquine by iodosylbenzene. The main oxidation product in all systems is monodesethylchloroquine. The anionic iron(III) porphyrins are the most active catalysts; in aqueous solution they are selective for oxidative deethylation whereas in methanol they also give five other oxidation products. The cationic and neutral iron(III) porphyrins are poor or inactive catalysts for chloroquine oxidation by iodosylbenzene. The effect of iron(III) porphyrin/chloroquine interactions on the yields and selectivity of the oxidation is discussed.

Five iron tetraphenylporphyrins including the (TPP)FeCl and its derivatives which contain electron-withdrawing or electron-donating substituents on the meso-phenyl rings of the porphyrin macrocycle were examined as catalysts for the electoreduction of dioxygen in 1.0 M HClO₄. The investigated compounds are represented as (TpRPP)FeCl, where TpRPP is the dianion of the para-substituted tetraphenylporphyrin and R = NEt₂, OMe, H, Br or CF₃. Cyclic voltammetry and linear sweep voltammetry at a rotating disk electrode (RDE) or a rotating ring disk electrode (RRDE) were utilized to examine the catalytic activity of the compounds. The number of electrons transferred and the percentage of H₂O₂ produced as the product of O₂ electroreduction were calculated. The investigated iron porphyrins containing electron-withdrawing substituents (R = Br and CF₃) were shown to be more efficient catalysts for O₂ reduction than the compounds having electron-donating groups (R = NEt₂ and OMe) on the four phenyl rings of the porphyrin macrocycle.

Two iron porphyrins, (TPP)FeCl and (OEP)FeCl, where TPP and OEP are the dianions of tetraphenylporphyrin and octaethylporphyrin, respectively, were utilized as catalysts for the electroreductive dechlorination of α-hexachlorocyclohexane (α-HCH) which was monitored by electrochemistry, in situ UV-visible spectroelectrochemistry and controlled potential electrolysis in N,N′-dimethylformamide. GC-MS analysis of the α-HCH degradation products revealed the stepwise formation of pentachlorocyclohexene and tetrachlorocyclohexadiene as intermediates, prior to generation of the final dechlorination products which consisted of an isomeric mixture of trichlorobenzenes. Based on identification of the intermediates and final products in the reaction, an overall dechlorination mechanism of α-hexachlorocyclohexane is proposed.

Iron–porphyrins ($i.e.,$ hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple-electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.

The modeling of iron porphyrins is essential in the study of heme proteins. These complexes readily undergo autooxidation and dimerization; however, this dimerization process can be inhibited by introducing phenyl groups. Herein, the dimerization of highly substituted iron(III) porphyrins is investigated. The stability of water-insoluble iron(III) hydroxo complexes was monitored via thin-layer chromatography and was found to increase with the number of phenyl groups. Dodecaphenyl-substituted complexes formed hydroxo complexes, tetraphenyl and hexaphenyl complexes formed $\mu$-oxo dimers, and octaphenyl and decaphenyl complexes existed as hydroxo complex-$\mu$-oxo dimer mixtures. Crystals of the dodecaphenyl-substituted iron(III) hydroxo complexes were successfully isolated. The decaphenyl-substituted complex was unique in that the respective crystals of both the hydroxo complexes and $\mu$-oxo dimer could be isolated from the chloride complexes. Furthermore, the synthesis and proton equilibria of the new porphyrin iron(III) water-soluble complexes were investigated. Under neutral to basic conditions, at equilibrium, dodecaphenyl-substituted complexes exclusively formed hydroxo complexes, whereas octaphenyl, hexaphenyl, and tetraphenyl complexes exclusively formed $\mu$-oxo dimers, and decaphenyl complexes existed as hydroxo complex-$\mu$-oxo dimer mixtures. Additionally, the order of the dimerization reaction was examined using the initial-rate and half-life methods, which confirmed that the dimerization is a second-order reaction that follows a mechanism wherein two hydroxo complexes form a dihydroxo-bridged intermediate. This result was supported by the dimerization of water-soluble iron(III) hydroxo complexes. The new hydroxo complexes with non-planar porphyrin cores synthesized in this study are expected to have a significant impact on heme protein reaction modeling.

Iron–porphyrins (i.e., hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple–electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.