Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

Electric field effect of positive and negative charges of substituents on electronic structure and reactivity of oxoiron(IV) porphyrin π-cation radical complex

Electric field effect by the positive and negative changes near the active site is an important factor for controlling the reactivity of metalloenzymes. Previously, we reported that the positive charge of the N-methyl-2-pyridinium cation increases the reactivity of oxoiron(IV) porphyrin π-cation radical complex (Compound I), due to the attractive Coulomb interaction with electrons in Compound I. To further investigate the electric field effect, we study here the effect of the negative charge of the sulfonate group on the electronic structure and reactivity using Compound I of meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrin (TMPS-I). Although Compound I has been known as a very unstable complex, TMPS-I is very stable in 0.1 M acetate buffer at pH = 6. The half-life of TMPS-I is estimated to be 6.9 × 10³ s, which is the longest in Compound I previously reported. The redox potential of TMPS-I is estimated to be 0.76 V vs SCE in phosphate buffer, pH = 10. Kinetic analysis with stopped-flow technique indicates TMPS-I is less reactive than Compounds I reported previously. However, ¹H NMR and EPR spectra of TMPS-I are very close to those of Compounds I reported previously. The DFT calculations show that the orbital energy of Compound I is drastically altered by the positive and negative charges on the meso-phenyl group, suggesting the electric field effect. The difference of the reactivity of Compound I can be rationalized with the change of the orbital energy caused by the intramolecular electric field effect of the positive and negative charges.

Energy Landscape for the Electrocatalytic Oxidation of Water by a Single-Site Oxomanganese(V) Porphyrin

A cationic manganese porphyrin, Mn^III-TDMImP, is an efficient, homogeneous, single-site water oxidation electrocatalyst at neutral pH. The measured turnover frequency for oxygen production is 32 s^-1. Mechanistic analyses indicate that Mn^V(O)(OH₂), the protonated form of the corresponding trans-Mn^V(O)₂ species, is generated from the Mn^III(OH₂)₂ precursor in a 2-e^- two-proton process and is responsible for O-O bond formation with a H₂O molecule. Chloride ion is a competitive substrate with H₂O for the Mn^V(O)(OH₂) oxidant, forming hypochlorous acid with a rate constant that is 3 orders of magnitude larger than that of water oxidation. The data allow the construction of an experimental energy landscape for this water oxidation catalysis process.

Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

Activation and reduction of O₂ and H₂O₂ by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

Mechanistic insights into the selectivity of norcarane oxidation by oxoMn(V) porphyrin complexes

OxoMn(V) porphyrin complexes perform competitive hydroxylation, desaturation, and radical rearrangement reactions using diagnostic substrate norcarane. Initial C-H cleavage proceeds through the two hydrogen abstraction steps from the two adjacent carbon on the norcarane, then the selective reaction is performed to generate various products. Using density functional theory calculations, we show that the hydroxylation and desaturation reactions are triggered by a rate-determining H-abstraction step, whereas the rate-determining step for the radical rearrangement is located at the rebound step ( TS2 ). We find that the  endo- 2  reaction is favorable over other reactions, which is consistent with the experimental result. Furthermore, the competitive pathways for norcarane oxidation depend on the non-covalent interaction between norcarane and porphyrin-ring, and orbital energy gaps between donor and acceptor orbitals because of stable or unstable acceptor orbital. The stereo- and regio-selectivities of norcarane oxidation are hardly sensitive to the zero-point energy and thermal free energy corrections.

Synthesis, characterization, and reactivity of oxoiron(IV) porphyrin π-cation radical complexes bearing cationic N-methyl-2-pyridinium group

Electronic charge near the active site is an important factor for controlling the reactivity of metalloenzymes. Here, to investigate the effect of the cationic charge near the heme in heme proteins, we synthesized new iron porphyrin complexes (1 and 2) having cationic 3-methyl-N-methyl-2-pyrdinium group and N-methyl-2-pyridinium group at one of the four meso-positions, respectively. The N-methyl-2-pyridinium groups could be introduced by Stille coupling used palladium catalysts. Oxoiron(IV) porphyrin π-cation radical complexes (Compound I) of 1 (1-CompI) and 2 (2-CompI) are soluble in most organic solvents, allowing direct comparison of their electronic structure and reactivity with Compound I of tetramesitylporphyrin (3-CompI) and tetrakis-(2,6-dichlorophenyl)porphyrin (4-CompI) under the same conditions. Spectroscopic data for 1-CompI are close to those for 3-CompI, but the redox potential for 1-CompI is close to that of 4-CompI. Kinetic analysis of the epoxidation reactions shows that 1-CompI and 2-CompI are (~250-fold) more reactive than 3-CompI, and comparable to 4-CompI. DFT calculations allow to propose that the positive shift of the redox potential and the enhanced reactivity of 1-CompI and 2-CompI is induced by the intramolecular electric field effect of N-methyl-2-pyridinium cation, not by the electron-withdrawing effect.

A Kinetic Study on the Efficient Formation of High-Valent Mn(TPPS)-oxo Complexes by Various Oxidants

New, more efficient methods of wastewater treatment, which will limit the harmful effects of textile dyes on the natural environment, are still being sought. Significant research work suggests that catalysts based on transition metal complexes can be used in efficient and environmentally friendly processes. In this context, a number of compounds containing manganese have been investigated. A suitable catalyst should have the capacity to activate a selected oxidant or group of oxidants, in order to be used in industrial oxidation reactions. In the present study we investigated the ability of MnIII(TPPS), where TPPS = 5,10,15,20-tetrakis(4-sulphonatophenyl)-21H,23H-porphyrine, to activate five different oxidants, namely hydrogen peroxide, peracetic acid, sodium hypochlorite, potassium peroxomonosulfate and sodium perborate, via the formation of high valent Mn(TPPS)-oxo complexes. Kinetic and spectroscopic data showed that the oxidation process is highly pH dependent and is strongly accelerated by the presence of carbonate in the reaction mixture for three of the five oxidizing agents. The highest efficiency for the oxidation of MnIII(TPPS) to high-valent Mn(TPPS)-oxo complexes, was found for peracetic acid at pH ≈ 11 in 0.5 M carbonate solution, which is at least an order of magnitude higher than the rate constants found for the other tested oxidants under similar conditions.

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide. These species are composed of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates. These complexes have been studied to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this Review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. This Review is organized by structural type of metal-oxygen intermediate and broken into subsections based on the metal (manganese and iron) and porphyrinoid ligand (porphyrin, corrole, and corrolazine).

Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine

A reactive hydroxoferric porphyrazine complex, [(PyPz)Fe^III(OH) (OH₂)]⁴⁺ (1, PyPz = tetramethyl-2,3-pyridino porphyrazine), has been prepared via one-electron oxidation of the corresponding ferrous species [(PyPz)Fe^II(OH₂)₂]⁴⁺ (2). Electrochemical analysis revealed a pH-dependent and remarkably high Fe^III-OH/Fe^II-OH₂ reduction potential of 680 mV vs Ag/AgCl at pH 5.2. Nernstian behavior from pH 2 to pH 8 indicates a one-proton, one-electron interconversion throughout that range. The O-H bond dissociation energy of the Fe^II-OH₂ complex was estimated to be 84 kcal mol^-1. Accordingly, 1 reacts rapidly with a panel of substrates via C-H hydrogen atom transfer (HAT), reducing 1 to [(PyPz)Fe^II(OH₂)₂]⁴⁺ (2). The second-order rate constant for the reaction of [(PyPz)Fe^III(OH) (OH₂)]⁴⁺ with xanthene was 2.22 × 10³ M^-1 s^-1, 5-6 orders of magnitude faster than other reported Fe^III-OH complexes and faster than many ferryl complexes.

Immobilized Lignin Peroxidase-Like Metalloporphyrins as Reusable Catalysts in Oxidative Bleaching of Industrial Dyes

Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.

Spectroscopic and kinetic evidence for redox cycling, catalase and degradation activities of Mn(III)(TPPS) in a basic aqueous peroxide medium

Mn(III)(TPPS) was found to react rapidly with hydrogen peroxide in basic aqueous solution to form intermediate (TPPS)Mn(V)[double bond, length as m-dash]O and (TPPS)Mn(IV)[double bond, length as m-dash]O species which, in the presence of excess H2O2, are reduced fully back to Mn(III)(TPPS) with clear evidence for redox cycling of Mn(III)(TPPS). The system shows very strong catalase and degradation activities.

A comprehensive evaluation of catalase-like activity of different classes of redox-active therapeutics

Because of the increased insight into the biological role of hydrogen peroxide (H2O2) under physiological and pathological conditions and the role it presumably plays in the action of natural and synthetic redox-active drugs, there is a need to accurately define the type and magnitude of reactions that may occur with this intriguing and key species of redoxome. Historically, and frequently incorrectly, the impact of catalase-like activity has been assigned to play a major role in the action of many redox-active drugs, mostly SOD mimics and peroxynitrite scavengers, and in particular MnTBAP(3-) and Mn salen derivatives. The advantage of one redox-active compound over another has often been assigned to the differences in catalase-like activity. Our studies provide substantial evidence that Mn(III) N-alkylpyridylporphyrins couple with H2O2 in actions other than catalase-related. Herein we have assessed the catalase-like activities of different classes of compounds: Mn porphyrins (MnPs), Fe porphyrins (FePs), Mn(III) salen (EUK-8), and Mn(II) cyclic polyamines (SOD-active M40403 and SOD-inactive M40404). Nitroxide (tempol), nitrone (NXY-059), ebselen, and MnCl2, which have not been reported as catalase mimics, were used as negative controls, while catalase enzyme was a positive control. The dismutation of H2O2 to O2 and H2O was followed via measuring oxygen evolved with a Clark oxygen electrode at 25°C. The catalase enzyme was found to have kcat(H2O2)=1.5×10(6)M(-1) s(-1). The yield of dismutation, i.e., the maximal amount of O2 evolved, was assessed also. The magnitude of the yield reflects an interplay between the kcat(H2O2) and the stability of compounds toward H2O2-driven oxidative degradation, and is thus an accurate measure of the efficacy of a catalyst. The kcat(H2O2) values for 12 cationic Mn(III) N-substituted (alkyl and alkoxyalkyl) pyridylporphyrin-based SOD mimics and Mn(III) N,N'-dialkylimidazolium porphyrin, MnTDE-2-ImP(5+), ranged from 23 to 88M(-1) s(-1). The analogous Fe(III) N-alkylpyridylporphyrins showed ~10-fold higher activity than the corresponding MnPs, but the values of kcat(H2O2) are still ~4 orders of magnitude lower than that of the enzyme. While the kcat(H2O2) values for Fe ethyl and n-octyl analogs were 803.5 and 368.4M(-1) s(-1), respectively, the FePs are more prone to H2O2-driven oxidative degradation, therefore allowing for similar yields in H2O2 dismutation as analogous MnPs. The kcat(H2O2) values are dependent on the electron deficiency of the metal site as it controls the peroxide binding in the first step of the dismutation process. SOD-like activities depend on electron deficiency of the metal site also, as it controls the first step of O2(●-) dismutation. In turn, the kcat(O2(●-)) parallels the kcat(H2O2). Therefore, the electron-rich anionic non-SOD mimic MnTBAP(3-) has essentially very low catalase-like activity, kcat(H2O2)=5.8M(-1) s(-1). The catalase-like activities of Mn(III) and Fe(III) porphyrins are at most, 0.0004 and 0.05% of the enzyme activity, respectively. The kcat(H2O2) values of 8.2 and 6.5M(-1) s(-1) were determined for electron-rich Mn(II) cyclic polyamine-based compounds, M40403 and M40404, respectively. The EUK-8, with modest SOD-like activity, has only slightly higher kcat(H2O2)=13.5M(-1) s(-1). The biological relevance of kcat(H2O2) of MnTE-2-PyP(5+), MnTDE-2-ImP(5+), MnTBAP(3-), FeTE-2-PyP(5+), M40403, M40404, and Mn salen was evaluated in wild-type and peroxidase/catalase-deficient E. coli.

A bioinspired polymer-bound Mn-porphyrin as an artificial active center of catalase

The complex comprising a cationic Mn-porphyrin and carboxymethyl poly(1-vinylimidazole) (CM-PVIm) was prepared as an artificial active center of catalase. Interestingly, the catalase activity of the complex depends on the chain length of the polymer and the chemical structure of Mn-porphyrin. This study is one step forward in the development of a new class of water-soluble catalase mimics.

Rational design of superoxide dismutase (SOD) mimics: the evaluation of the therapeutic potential of new cationic Mn porphyrins with linear and cyclic substituents

Our goal herein has been to gain further insight into the parameters which control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP(5+), MnTnHexOE-2-PyP(5+), MnTE-2-PyPhP(5+), and MnTPhE-2-PyP(5+)) that bear the same positive charge and same number of carbon atoms at meso positions of porphyrin core were explored. The carbon atoms of their meso substituents are organized to form either linear or cyclic structures of vastly different redox properties, bulkiness, and lipophilicities. These Mn porphyrins were compared to frequently studied compounds, MnTE-2-PyP(5+), MnTE-3-PyP(5+), and MnTBAP(3-). All Mn(III) porphyrins (MnPs) have metal-centered reduction potential, E1/2 for Mn(III)P/Mn(II)P redox couple, ranging from -194 to +340 mV versus NHE, log kcat(O2(•-)) from 3.16 to 7.92, and log kred(ONOO(-)) from 5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol and water, log POW, was in the range -1.67 to -7.67. The therapeutic potential of MnPs was assessed via: (i) in vitro ability to prevent spontaneous lipid peroxidation in rat brain homogenate as assessed by malondialdehyde levels; (ii) in vivo O2(•-) specific assay to measure the efficacy in protecting the aerobic growth of SOD-deficient Saccharomyces cerevisiae; and (iii) aqueous solution chemistry to measure the reactivity toward major in vivo endogenous antioxidant, ascorbate. Under the conditions of lipid peroxidation assay, the transport across the cellular membranes, and in turn shape and size of molecule, played no significant role. Those MnPs of E1/2 ∼ +300 mV were the most efficacious, significantly inhibiting lipid peroxidation in 0.5-10 μM range. At up to 200 μM, MnTBAP(3-) (E1/2 = -194 mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP(5+) with 129 mV more positive E1/2 (-65 mV vs NHE) was fully efficacious at 50 μM. The E1/2 of Mn(III)P/Mn(II)P redox couple is proportional to the log kcat(O2(•-)), i.e., the SOD-like activity of MnPs. It is further proportional to kred(ONOO(-)) and the ability of MnPs to prevent lipid peroxidation. In turn, the inhibition of lipid peroxidation by MnPs is also proportional to their SOD-like activity. In an in vivo S. cerevisiae assay, however, while E1/2 predominates, lipophilicity significantly affects the efficacy of MnPs. MnPs of similar log POW and E1/2, that have linear alkyl or alkoxyalkyl pyridyl substituents, distribute more easily within a cell and in turn provide higher protection to S. cerevisiae in comparison to MnP with bulky cyclic substituents. The bell-shape curve, with MnTE-2-PyP(5+) exhibiting the highest ability to catalyze ascorbate oxidation, has been established and discussed. Our data support the notion that the SOD-like activity of MnPs parallels their therapeutic potential, though species other than O2(•-), such as peroxynitrite, H2O2, lipid reactive species, and cellular reductants, may be involved in their mode(s) of action(s).

Acid-base and electrochemical properties of manganese meso(ortho- and meta-N-ethylpyridyl)porphyrins: voltammetric and chronocoulometric study of protolytic and redox equilibria

Growing interest in redox-active compounds as therapeutics for oxidative stress-related diseases led to the design of metalloporphyrins as some of the most potent functional SOD-mimics. Herein we report a detailed electrochemical study of the protolytic and redox equilibria of manganese ortho and meta substituted N-ethylpyridyl porphyrins (MnPs), MnTE-2-PyP(5+) and MnTE-3-PyP(5+), in aqueous solutions. The electrochemical parameters of redox processes for all experimentally available species have been determined, as well as their diffusion coefficients and estimated sizes of aqueous cavities. The results indicate that possible changes of the intracellular acidity cannot affect the antioxidant activity of MnPs in vivo, since no change in the E(Mn(III)P/Mn(II)P) values was observed below pH 10. Furthermore, the results confirm that both of these MnPs can be efficient redox scavengers of peroxynitrite (ONOO(-)), another major damaging species in vivo. This can occur by either single-electron reduction or two-electron reduction of ONOO(-), involving either the Mn(IV)P/Mn(III)P redox couple or Mn(IV)P/Mn(II)P redox couple. In addition to kred(ONOO(-)) reported previously, the thermodynamic parameters calculated herein imply a strong and identical driving force for the reaction of both ortho and meta isomeric MnPs with ONOO(-). An enlargement of both Mn(III)P complexes upon an increase of the solution pH was also observed and attributed to the reduction of positive charge on the central ion caused by deprotonation of the axial water molecules. This expansion of aqueous cavities suggests the formation of a solvent cage and the increased lipophilicity of Mn(III)P complexes caused by increased electron density on the Mn ion.

Efficient water oxidation catalyzed by homogeneous cationic cobalt porphyrins with critical roles for the buffer base

A series of cationic cobalt porphyrins was found to catalyze electrochemical water oxidation to O2 efficiently at room temperature in neutral aqueous solution. Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin, with a highly electron-deficient meso-dimethylimidazolium porphyrin, was the most effective catalyst. The O2 formation rate was 170 nmol · cm(-2) · min(-1) (k(obs) = 1.4 × 10(3) s(-1)) with a Faradaic efficiency near 90%. Mechanistic investigations indicate the generation of a Co(IV)-O porphyrin cation radical as the reactive oxidant, which has accumulated two oxidizing equivalents above the Co(III) resting state of the catalyst. The buffer base in solution was shown to play several critical roles during the catalysis by facilitating both redox-coupled proton transfer processes leading to the reactive oxidant and subsequent O-O bond formation. More basic buffer anions led to lower catalytic onset potentials, extending below 1 V. This homogeneous cobalt-porphyrin system was shown to be robust under active catalytic conditions, showing negligible decomposition over hours of operation. Added EDTA or ion exchange resin caused no catalyst poisoning, indicating that cobalt ions were not released from the porphyrin macrocycle during catalysis. Likewise, surface analysis by energy dispersive X-ray spectroscopy of the working electrodes showed no deposition of heterogeneous cobalt films. Taken together, the results indicate that Co-5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrin is an efficient, homogeneous, single-site water oxidation catalyst.

Hydrogen peroxide as oxidant in bio-mimetic catalysis by manganese porphyrin: Theoretical DFT studies

The aim of this study is to elucidate the geometry and electronic structure of various adducts that may be formed between manganese(III) (Mn(III)) porphyrin and hydrogen peroxide. Hydrogen peroxide may interact with Mn(III) porphyrin either as H2O2 or, after dissociation, as OOH–. In the former, it may decompose into two hydroxo groups, which acquire OH– character or an oxo group (=O) and a water molecule. Therefore, the following systems are considered: MnP(H2O2)+, MnP(H2O2)(OH), MnP(OH)3, [Formula: see text], MnPO+, MnPO(OH), MnP(OOH), MnP(OOH)(OH)–, and the possible transformations between them are taken into account. The reported studies are performed within the Density Functional Theory (DFT) method with the GGA-BP functional. The geometry and electronic structures of the structures found along the studied reaction pathways are discussed in terms of interatomic distances, valence angles, Mulliken charges, and spin densities. It was found that different active oxygen species may be formed in the reaction between Mn(III) porphyrin and hydrogen peroxide. As manganese is a transition metal, numerous possible spin states for each of the studied structures are found, where the relative energies of different multiplicities depend strongly on the ligands present in the complex. In view of the catalytic properties, all oxygen-containing ligands are negatively charged, which results in their behaviour as nucleophiles towards hydrocarbons. Finally, the analysis of charge and spin populations on different parts of the studied systems indicate the porphyrin ligand as active in charge transfer processes.

Water-soluble ionic benzoporphyrins

Novel ionic water-soluble tetrabenzoporphyrins have been successfully synthesized via a cascade reaction based on the Heck reaction. The UV-Vis spectra of these porphyrins displayed red-shifted and broadened Soret bands, and significantly enhanced Q bands. These porphyrins are highly water soluble.

Electron- and hydride-transfer reactivity of an isolable manganese(V)-oxo complex

The electron-transfer and hydride-transfer properties of an isolated manganese(V)−oxo complex, (TBP8Cz)Mn(V)(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)−oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*) to give the direct formation of [(TBP8Cz)Mn(III)(OH)]− ([2-OH]−), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)−oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn(IV) complex, was not observed during the reduction of 1. However, a Mn(IV) complex was successfully generated via one-electron oxidation of the Mn(III) precursor complex 2 to give [(TBP8Cz)Mn(IV)]+ (3). Complex 3 exhibits a characteristic absorption band at λ(max) = 722 nm and an EPR spectrum at 15 K with g(max)′ = 4.68, g(mid)′ = 3.28, and g(min)′ = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 Mn(IV)S = 3/2 ground state. Although electron transfer from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron transfer (k(et)) versus acid concentration was observed, providing insight into the critical involvement of H+ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD+ analogues and [2-OH]−. The logarithms of the observed second-order rate constants of hydride transfer (k(H)) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.

Manganese porphyrins catalyze selective C-H bond halogenations

We report a manganese porphyrin mediated aliphatic C-H bond chlorination using sodium hypochlorite as the chlorine source. In the presence of catalytic amounts of phase transfer catalyst and manganese porphyrin Mn(TPP)Cl 1, reaction of sodium hypochlorite with different unactivated alkanes afforded alkyl chlorides as the major products with only trace amounts of oxygenation products. Substrates with strong C-H bonds, such as neopentane (BDE =∼100 kcal/mol) can be also chlorinated with moderate yield. Chlorination of a diagnostic substrate, norcarane, afforded rearranged products indicating a long-lived carbon radical intermediate. Moreover, regioselective chlorination was achieved by using a hindered catalyst, Mn(TMP)Cl, 2. Chlorination of trans-decalin with 2 provided 95% selectivity for methylene-chlorinated products as well as a preference for the C2 position. This novel chlorination system was also applied to complex substrates. With 5α-cholestane as the substrate, we observed chlorination only at the C2 and C3 positions in a net 55% yield, corresponding to the least sterically hindered methylene positions in the A-ring. Similarly, chlorination of sclareolide afforded the equatorial C2 chloride in a 42% isolated yield. Regarding the mechanism, reaction of sodium hypochlorite with the Mn(III) porphyrin is expected to afford a reactive Mn(V)═O complex that abstracts a hydrogen atom from the substrate, resulting in a free alkyl radical and a Mn(IV)-OH complex. We suggest that this carbon radical then reacts with a Mn(IV)-OCl species, providing the alkyl chloride and regenerating the reactive Mn(V)═O complex. The regioselectivity and the preference for CH(2) groups can be attributed to nonbonded interactions between the alkyl groups on the substrates and the aryl groups of the manganese porphyrin. The results are indicative of a bent [Mn(v)═O---H---C] geometry due to the C-H approach to the Mn(v)═O (dπ-pπ)* frontier orbital.

cis-Dihydroxylation of alkenes with oxone catalyzed by iron complexes of a macrocyclic tetraaza ligand and reaction mechanism by ESI-MS spectrometry and DFT calculations

[Fe(III)(L-N(4)Me(2))Cl(2)](+) (1, L-N(4)Me(2) = N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) is an active catalyst for cis-dihydroxylation of various types of alkenes with oxone at room temperature using limiting amounts of alkene substrates. In the presence of 0.7 or 3.5 mol % of 1, reactions of electron-rich alkenes, including cyclooctene, styrenes, and linear alkenes, with oxone (2 equiv) for 5 min resulted in up to >99% substrate conversion and afforded cis-diol products in up to 67% yield, with cis-diol/epoxide molar ratio of up to 16.8:1. For electron-deficient alkenes including α,β-unsaturated esters and α,β-unsaturated ketones, their reactions with oxone (2 equiv) catalyzed by 1 (3.5 mol %) for 5 min afforded cis-diols in up to 99% yield with up to >99% substrate conversion. A large-scale cis-dihydroxylation of methyl cinnamate (9.7 g) with oxone (1 equiv) afforded the cis-diol product (8.4 g) in 84% yield with 85% substrate conversion. After catalysis, the L-N(4)Me(2) ligand released due to demetalation can be reused to react with newly added Fe(ClO(4))(2)·4H(2)O to generate an iron catalyst in situ, which could be used to restart the catalytic alkene cis-dihydroxylation. Mechanistic studies by ESI-MS, isotope labeling studies, and DFT calculations on the 1-catalyzed cis-dihydroxylation of dimethyl fumarate with oxone reveal possible involvement of cis-HO-Fe(V)═O and/or cis-O═Fe(V)═O species in the reaction; the cis-dihydroxylation reactions involving cis-HO-Fe(V)═O and cis-O═Fe(V)═O species both proceed by a concerted but highly asynchronous mechanism, with that involving cis-HO-Fe(V)═O being more favorable due to a smaller activation barrier.

Manganese(V)-oxo corroles in hydride-transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to manganese(V)-oxo corroles proceeds via proton-coupled electron transfer, followed by rapid electron transfer. The redox potentials (E(red)) of manganese(V)-oxo corroles exhibit a good correlation with their reactivity in hydride-transfer reactions.

A highly reactive p450 model compound I

The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe(III)-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin pi-radical cation. This intermediate was subsequently transformed into the well-characterized OFe(IV)-4-TMPyP. Global analysis afforded a second-order rate constant k(1) = (1.59 +/- 0.06) x 10(7) M(-1) s(-1) for the formation of 1 followed by a first-order decay with k(2) = 8.8 +/- 0.1 s(-1). (1)H and (13)C NMR determined 9-xanthydrol to be the major product (approximately 90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% (18)OH(2) indicated 21% (18)O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d(2) revealed a modest kinetic isotope effect, k(H)/k(D) = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k(3) = (3.6 +/- 0.3) x 10(6) M(-1) s(-1). Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Brønsted-Evans-Polanyi relationship for similar substrates determined the H-OFe(IV)-4-TMPyP bond dissociation energy to be approximately 100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.

High-valent manganese(v)-oxo porphyrin complexes in hydride transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to trans-dioxomanganese(v) porphyrin complexes proceeds via proton-coupled electron transfer, followed by rapid electron transfer.

Electrochemically driven generation of manganese(IV,V)-oxo multiporphyrin arrays and their redox properties with manganese(III) species in Langmuir-Blodgett films

High-valency manganese (IV,V)-oxo porphyrins have been electrochemically generated and in situ spectrally characterized in multiporphyrin arrays, which were formed by an interfacial coordination reaction of Na2PdCl4 with manganese (III) tetrapyridylporphyrin (MnTPyP). Multilayers of the Pd-MnTPyP multiporphyrin arrays were obtained by the Langmuir-Blodgett (LB) method. The redox behaviors of manganese in the multiporphyrin arrays were pH-dependent. Spectroelectrochemical experiments revealed a reversible redox process between Pd-Mn(III)TPyP and its Mn(IV)-oxo species, but an irreversible process between Pd-Mn(III)TPyP and its Mn(V)-oxo species. The Pd-Mn(IV)TPyP multiporphyrin arrays could be spontaneously reduced to their Mn(III) complex, while the Pd-Mn(V)TPyP arrays were rather stable in basic solutions (pH > 10.5). However, when the Pd-Mn(V)TPyP multiporphyrin arrays were washed by or immersed in water, they were immediately reduced to their Mn(III) complex. Because these well-organized multiporphyrin arrays are of high thermal and chemical stability, they are potential molecular materials in the studies of natural and artificial catalytic processes as well as redox-based molecular switches.

A rational basis for the axial ligand effect in C-H oxidation by [MnO(porphyrin)(X)]+ (X = H2O, OH-, O2-) from a DFT study

Oxyl radical character in the MnO group of the title system is shown from a density functional theory study to be essential for efficient C-H cleavage, which is a key step in C-H oxidation. Since oxyl species have elongated Mn-O bonds relative to the more usual oxo species of type MnO, the normal expectation would be that high trans-influence ligands X should facilitate oxyl character by elongating the Mn-O bond and thus enhance both oxyl character and reactivity. Contrary to this expectation, but in line with the experimental data (Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc. 2007, 129, 12416), we find that reactivity increases along the series X = O(2-) < OH(-) < H2O for the following reasons. The ground-state singlet (S) is unreactive for all X, and only the higher-energy triplet (T) and quintet (Q) states have the oxyl character needed for reactivity, but the higher trans-influence X ligands are also shown to increase the S/T and S/Q gaps, thus making attainment of the needed T and Q states harder. The latter effect is dominant, and high trans-influence X ligands thus disfavor reaction. The higher reactivity in the presence of acid noted by Groves and co-workers is thus rationalized by the preference for having X = H2O over OH(-) or O(2-).