H2O2 Oxidations Catalyzed by an Iron(III) Corrolazine: Avoiding High-Valent Iron–Oxido Species?

Axial Ligation Impedes Proton-Coupled Electron-Transfer Reactivity of a Synthetic Compound-I Analogue

The nature of the axial ligand in high-valent iron-oxo heme enzyme intermediates and related synthetic catalysts is a critical structural element for controlling proton-coupled electron-transfer (PCET) reactivity of these species. Herein, we describe the generation and characterization of three new 6-coordinate, iron(IV)-oxo porphyrinoid-π-cation-radical complexes and report their PCET reactivity together with a previously published 5-coordinate analogue, FeIV(O)(TBP8Cz+•) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3-) (2) (Cho, K. A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer. J. Am. Chem. Soc. 2012, 134, 7392-7399). The new complexes FeIV(O)(TBP8Cz+•)(L) (L = 1-methyl imidazole (1-MeIm) (4a), 4-dimethylaminopyridine (DMAP) (4b), cyanide (CN-)(4c)) can be generated from either oxidation of the ferric precursors or by addition of L to the Compound-I (Cpd-I) analogue at low temperatures. These complexes were characterized by UV-vis, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies, and cryospray ionization mass spectrometry (CSI-MS). Kinetic studies using 4-OMe-TEMPOH as a test substrate indicate that coordination of a sixth axial ligand dramatically lowers the PCET reactivity of the Cpd-I analogue (rates up to 7000 times slower). Extensive density functional theory (DFT) calculations together with the experimental data show that the trend in reactivity with the axial ligands does not correlate with the thermodynamic driving force for these reactions or the calculated strengths of the O-H bonds being formed in the FeIV(O-H) products, pointing to non-Bell-Evans-Polanyi behavior. However, the PCET reactivity does follow a trend with the bracketed reduction potential of Cpd-I analogues and calculated electron affinities. The combined data suggest a concerted mechanism (a concerted proton electron transfer (CPET)) and an asynchronous movement of the electron/proton pair in the transition state.

Catalytic oxidative desulfurization of model fuel utilizing functionalized HMS catalysts: characterization, catalytic activity and mechanistic studies

In this study, a novel Fe(ii) porphyrin conjugated amino modified HMS catalyst was synthesized and characterized by BET, XRD, TEM, SEM, and UV-vis techniques.

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).

Highly selective and efficient oxidation of sulfide to sulfoxide catalyzed by platinum porphyrins

Two platinum porphyrins, meso-tetramesitylporphyrinatoplatinum and meso-tetrakis(pentaflourophenyl) porphyrinatoplatinum, are explored for catalytic application in the selective oxidation of sulfide to sulfoxide by iodosylbenzene. The obtained overall turnover number of 90,000 in the oxidation of thioanisole in the presence of meso-tetrakis(pentaflourophenyl) porphyrinatoplatinum indicates the pronounced catalytic activity of the platinum porphyrins. Perfect selectivity toward sulfoxide or sulfone also was achieved via stoichiometric control of reactants.

Visible light catalyzed methylsulfoxidation of (het)aryl diazonium salts using DMSO

The visible light catalyzed methylsulfoxidation of (het)aryl diazonium salts using DMSO is illustrated.

Quantum mechanical/molecular mechanical calculated reactivity networks reveal how cytochrome P450cam and Its T252A mutant select their oxidation pathways

Quantum mechanical/molecular mechanical calculations address the longstanding-question of a "second oxidant" in P450 enzymes wherein the proton-shuttle, which leads to formation of the "primary-oxidant" Compound I (Cpd I), was severed by mutating the crucial residue (in P450cam: Threonine-252-to-Alanine, hence T252A). Investigating the oxidant candidates Cpd I, ferric hydroperoxide, and ferric hydrogen peroxide (Fe(III)(O2H2)), and their reactions, generates reactivity networks which enable us to rule out a "second oxidant" and at the same time identify an additional coupling pathway that is responsible for the epoxidation of 5-methylenylcamphor by the T252A mutant. In this "second-coupling pathway", the reaction starts with the Fe(III)(O2H2) intermediate, which transforms to Cpd I via a O-O homolysis/H-abstraction mechanism. The persistence of Fe(III)(O2H2) and its oxidative reactivity are shown to be determined by interplay of substrate and protein. The substrate 5-methylenylcamphor prevents H2O2 release, while the protein controls the Fe(III)(O2H2) conversion to Cpd I by nailing-through hydrogen-bonding interactions-the conformation of the HO(•) radical produced during O-O homolysis. This conformation prevents HO(•) attack on the porphyrin's meso position, as in heme oxygenase, and prefers H-abstraction from Fe(IV)OH thereby generating H2O + Cpd I. Cpd I then performs substrate oxidations. Camphor cannot prevent H2O2 release and hence the T252A mutant does not oxidize camphor. This "second pathway" transpires also during H2O2 shunting of the cycle of wild-type P450cam, where the additional hydrogen-bonding with Thr252 prevents H2O2 release, and contributes to a successful Cpd I formation. The present results lead to a revised catalytic cycle of Cytochrome P450cam.

Selective sulfoxidation with hydrogen peroxide catalysed by a titanium catalyst

A cyclopentadienyl–silsesquioxane titanium complex efficiently catalyses the selective oxidation of sulfides to sulfoxides (85–98% yield in 5 min at ambient temperature) and sulfones with near-stochiometric amount of aqueous H₂O₂ in methanol.

A mononuclear non-heme high-spin iron(III)-hydroperoxo complex as an active oxidant in sulfoxidation reactions

We report the first direct experimental evidence showing that a high-spin iron(III)-hydroperoxo complex bearing an N-methylated cyclam ligand can oxidize thioanisoles. DFT calculations showed that the reaction pathway involves heterolytic O-O bond cleavage and that the choice of the heterolytic pathway versus the homolytic pathway is dependent on the spin state and the number of electrons in the d(xz) orbital of the Fe(III)-OOH species.

Generation of a high-valent iron imido corrolazine complex and NR group transfer reactivity

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe(III)(TBP8Cz)] (TBP8Cz = octakis(4-tert-butylphenyl)corrolazinato) with the commercially available chloramine-T (Na(+)TsNCl(-)) leads to oxidative N-tosyl transfer to afford [Fe(IV)(TBP8Cz(+•))(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV-vis, Mössbauer (δ = -0.05 mm s(-1), ΔE(Q) = 2.94 mm s(-1)), and EPR (X-band (15 K), g = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (S(total) = 1/2 ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe(IV)(TBP8Cz(+•))(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph3P═NTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M(-1) s(-1). These data indicate that [Fe(IV)(TBP8Cz(+•))(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe(III)(TBP8Cz) + chloramine-T + PPh3, [Fe(IV)(TBP8Cz)(NPPh3)] and [Fe(III)(TBP8Cz)(OPPh3)], which were definitively characterized by X-ray crystallography. The sequential production of Ph3P═NTs, Ph3P═NH, and Ph3P═O was observed by (31)P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe(III) complex was then rationally synthesized and structurally characterized from Fe(III)(TBP8Cz) and OPPh3, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (S = 3/2 and low spin (S = 1/2) Fe(III) corrolazines, as well as a formally Fe(IV) corrolazine which may also be described by its valence tautomer Fe(III)(Cz(+•)).

Understanding the oxidative relationships of the metal oxo, hydroxo, and hydroperoxide intermediates with manganese(IV) complexes having bridged cyclams: correlation of the physicochemical properties with reactivity

Multiple transition metal functional groups including metaloxo, hydroxo, and hydroperoxide groups play significant roles in various biological and chemical oxidations such as electron transfer, oxygen transfer, and hydrogen abstraction. Further studies that clarify their oxidative relationships and the relationship between their reactivity and their physicochemical properties will expand our ability to predict the reactivity of the intermediate in different oxidative events. As a result researchers will be able to provide rational explanations of poorly understood oxidative phenomena and design selective oxidation catalysts. This Account summarizes results from recent studies of oxidative relationships among manganese(IV) molecules that include pairs of hydroxo/oxo ligands. Changes in the protonation state may simultaneously affect the net charge, the redox potential, the metal-oxygen bond order (M-O vs M═O), and the reactivity of the metal ion. In the manganese(IV) model system, [Mn(IV)(Me(2)EBC)(OH)(2)](PF(6))(2), the Mn(IV)-OH and Mn(IV)═O moieties have similar hydrogen abstraction capabilities, but Mn(IV)═O abstracts hydrogen at a more than 40-fold faster rate than the corresponding Mn(IV)-OH. However, after the first hydrogen abstraction, the reduction product, Mn(III)-OH(2) from the Mn(IV)-OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the Mn(III)-OH from the Mn(IV)═O moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the Mn(IV)═O moiety proceeds by concerted oxygen atom transfer, but the reaction with the Mn(IV)-OH functional group proceeds by electron transfer. In addition, the manganese(IV) species with a Mn(IV)-OH group has a higher redox potential and demonstrates much more facile electron transfer than the one that has the Mn(IV)═O group. Furthermore, an increase in the net charge of the Mn(IV)-OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted electron transfer does not enhance hydrogen abstraction remarkably. The Mn(IV)-OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding Mn(IV)-OH and Mn(IV)═O moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the Mn(IV)-OH and Mn(IV)═O moieties, we have clarified the correlation between the physicochemical properties of these active intermediates, including net charge, redox potential, and metal-oxygen bond order, and their reactivities. The reactivity differences between the metal oxo and hydroxo moieties on these manganese(IV) functional groups after the first hydrogen abstraction have provided clues for understanding their occurrence and functions in metalloenzymes. The P450 enzymes require an iron(IV) oxo form rather than an iron(IV) hydroxo form to perform substrate hydroxylation. However, the lipoxygenases use an iron(III) hydroxo group to dioxygenate unsaturated fatty acids rather than an iron(III) oxo species, a moiety that could facilitate hydroxylation reactions. These distinctly different physicochemical properties and reactivities of the metal oxo and hydroxo moieties could provide clues to understand these elusive oxidation phenomena and provide the foundation for the rational design of novel oxidation catalysts.

Formation and characterization of five- and six-coordinate iron(III) corrolazine complexes

Electronic structures of five- and six-coordinate iron(III) corrolazine complexes are determined by means of 1H NMR, 13C NMR, EPR, and Mössbauer spectroscopy as well as SQUID magnetometry. A series of five-coordinate complexes, [FeIII(TBP8Cz)(L)]* where the axial ligands(L) are cyanide(CN-), imidazole(HIm), 1-methylimidazole(1-MeIm), 4-(N,N-dimethylamino)pyridine(DMAP), pyridine(Py), 4-cyanopyridine(4-CNPy), and tert-butylisocyanide(tBuNC), are obtained by the addition of 1 to 2 equiv. of the ligands to the dichloromethane solutions of FeIII(TBP8Cz) at 298 K: TBP8Cz is a trianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)corrolazine. These complexes commonly show the S = 3/2 at 298 K. By contrast, formation of the six-coordinate complexes depends on the nature of the axial ligands. While the addition of 3 equiv. of CN- has completely converted FeIII(TBP8Cz) to (Bu4N)2[FeIII(TBP8Cz)(CN)2] at 298 K, the conversion to the bis-adduct is only attained below ca. 200 K in the case of HIm, 1-MeIm, and DMAP even in the presence of 50 equiv. of the ligands. If the axial ligand is Py, 4-CNPy, or tBuNC, the formation of [FeIII(TBP8Cz)(L)2] is confirmed only at an extremely low temperature (15 K). Close inspection of the 1H NMR and EPR spectra has revealed that all the bis-adducts adopt the (dxy)2(dxz, dyz)3 ground state. While FeIII(TBP8Cz) forms paramagnetic bis- and mono-adduct in toluene solution at 298 K in the presence of excess amount of CN- and tBuNC, respectively, the corresponding porphyrazine complex, [FeIII(TBP8Pz)]Cl , forms diamagnetic bis-CN and bis-tBuNC under the same conditions: TBP8Pz is a dianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)-porphyrazine. Thus, the iron(III) ion of porphyrazine complex is more easily reduced than that of the corresponding corrolazine complex.

Oxidation of Benzylic Azides Catalysed by Iron(III) Chloride with Hydrogen Peroxide as Oxidant

A simple and general method for the oxidation of benzylic azides to aldehydes using the inexpensive and environmentally benign iron (III) chloride as catalyst with 30% aqueous H2O2 as oxidant is described. Broad substrate scope and high tolerance against several functional groups make the process synthetically useful.

Preparation, size control, surface deposition, and catalytic reactivity of hydrophobic corrolazine nanoparticles in an aqueous environment

Nanoparticles, each consisting of one of the three molecular corrolazine (Cz) compounds, H(3)(TBP(8)Cz), Mn(III)(TBP(8)Cz), and Fe(III)(TBP(8)Cz) (TBP(8)Cz = octakis(4-tert-butylphenyl)corrolazinato), were prepared via a facile mixed-solvent technique. The corrolazine nanoparticles (MCz-NPs) were formed in H(2)O/THF (10:1) in the presence of a small amount of a polyethylene glycol derivative (TEG-ME) added as a stabilizer. This technique allows highly hydrophobic Czs to be "dissolved" in an aqueous environment as nanoparticles, which remain in solution for several months without visible precipitation. The MCz-NPs were characterized by UV-visible spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) imaging, and shown to be spherical particles from 100-600 nm in diameter with low polydispersity indices (PDI = 0.003-0.261). Particle size is strongly dependent on Cz concentration. The H(3)Cz-NPs were adsorbed on to a modified self-assembled monolayer (SAM) surface and imaged by atomic force microscopy (AFM). Adsorption resulted in disassembly of the larger H(3)Cz-NPs to smaller H(3)Cz-NPs, whereby the resulting particle size can be controlled by the surface energy of the monolayer. The Fe(III)Cz-NPs were shown to be competent catalysts for the oxidation of cyclohexene with either PFIB or H(2)O(2) as external oxidant. The reactivity and product selectivity seen for Fe(III)Cz-NPs differs dramatically from that seen for the molecular species in organic solvents, suggesting that both the nanoparticle structure and the aqueous conditions may contribute to significant changes in the mechanism of action of the Fe(III)Cz catalyst.

Highly efficient catalase activity of metallocorroles

The iron(iii) complex of a bipolar and amphiphilic corrole, which binds strongly to proteins and undergoes protein-mediated cellular uptake, catalyzes the decomposition of hydrogen peroxide faster (k(cat) = 6400 M(-1) s(-1)) and more efficiently (turnover frequency >120 s(-1)) than previously reported synthetic compounds with catalase-like activity.

Iron-catalyzed ligand-free three-component coupling reactions of aldehydes, terminal alkynes, and amines

A tri-umph in many respects: The iron-catalyzed ligand-free, one-pot three-component coupling reactions of aldehydes, terminal alkynes, and amines in the presence of 4 A molecular sieves yields the corresponding propargylamines in good to excellent yields, displays a broad substrate scope, and is economical and environmentally friendly (see scheme).

Formation of stable and metastable porphyrin- and corrole-iron(IV) complexes and isomerizations to iron(III) macrocycle radical cations

Oxidations of three porphyrin-iron(III) complexes (1) with ferric perchlorate, Fe(ClO(4))(3), in acetonitrile solutions at -40 degrees C gave metastable porphyrin-iron(IV) diperchlorate complexes (2) that isomerized to known iron(III) diperchlorate porphyrin radical cations (3) when the solutions were warmed to room temperature. The 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) systems were studied by UV-visible spectroscopy. Low temperature NMR spectroscopy and effective magnetic moment measurements were possible with the TPP and TMP iron(IV) complexes. Reactions of two corrole systems, 5,10,15-tris(pentafluorophenyl)corrole (TPFC) and 5,15-bis(pentafluorophenyl)-10-p-methoxyphenylcorrole (BPFMC), also were studied. The corrole-iron(IV) chlorides reacted with silver salts to give corrole-iron(IV) complexes. The corrole-iron(IV) nitrate complexes were stable at room temperature. (TPFC)-iron(IV) toslyate, (TPFC)-iron(IV) chlorate, and (BPFMC)-iron(IV) chlorate were metastable and rearranged to their electronic isomers iron(III) corrole radical cations at room temperature. (TPFC)-iron(III) perchlorate corrole radical cation was the only product observed from reaction of the corrole-iron(IV) chloride with silver perchlorate. For the metastable iron(IV) species, the rates of isomerizations to the iron(III) macrocycle radical cation electronic isomers in dilute acetonitrile solutions were relatively insensitive to electron demands of the macrocyclic ligand but reflected the binding strength of the ligand to iron. Kinetic studies at varying temperatures and concentrations indicated that the mechanisms of the isomerization reactions are complex, involving mixed order reactivity.

Novel iron(III) porphyrazine complex. Complex speciation and reactions with NO and H2O2

The complex [iron(III) (octaphenylsulfonato)porphyrazine] (5-), Fe (III)(Pz), was synthesized. The p K a values of the axially coordinated water molecules were determined spectrophotometrically and found to be p K a 1 = 7.50 +/- 0.02 and p K a 2 = 11.16 +/- 0.06. The water exchange reaction studied by (17)O NMR as a function of the pH was fast at pH = 1, k ex = (9.8 +/- 0.6) x 10 (6) s (-1) at 25 degrees C, and too fast to be measured at pH = 10, whereas at pH = 13, no water exchange reaction occurred. The equilibrium between mono- and diaqua Fe (III)(Pz) complexes was studied at acidic pH as a function of the temperature and pressure. Complex-formation equilibria with different nucleophiles (Br (-) and pyrazole) were studied in order to distinguish between a five- (in the case of Br (-)) or six-coordinate (in the case of pyrazole) iron(III) center. The kinetics of the reaction of Fe (III)(Pz) with NO was studied as a model ligand substitution reaction at various pH values. The mechanism observed is analogous to the one observed for iron(III) porphyrins and follows an I d mechanism. The product is (Pz)Fe (II)NO (+), and subsequent reductive nitrosylation usually takes place when other nucleophiles like OH (-) or buffer ions are present in solution. Fe (III)(Pz) also activates hydrogen peroxide. Kinetic data for the direct reaction of hydrogen peroxide with the complex clearly indicate the occurrence of more than one reaction step. Kinetic data for the catalytic decomposition of the dye Orange II by H 2O 2 in the presence of Fe (III)(Pz) imply that a catalytic oxidation cycle is initiated. The peroxide molecule first coordinates to the iron(III) center to produce the active catalytic species, which immediately oxidizes the substrate. The influence of the catalyst, oxidant, and substrate concentrations on the reaction rate was studied in detail as a function of the pH. The rate increases with increasing catalyst and peroxide concentrations but decreases with increasing substrate concentration. At low pH, the oxidation of the substrate is not complete because of catalyst decomposition. The observed kinetic traces at pH = 10 and 12 for the catalytic cycle could be simulated on the basis of the obtained kinetic data and the proposed reaction cycle. The experimental results are in good agreement with the simulated ones.

Corrolazines: new frontiers in high-valent metalloporphyrinoid stability and reactivity

The activation of dioxygen and its analogues, such as hydrogen peroxide, by metalloporphyrins leads to the generation of high-valent metal-oxo species. This process is critically important to heme-catalyzed reactions, such as for cytochrome P450, and synthetic porphyrin-catalyzed oxidations. We have synthesized a new ring-contracted porphyrinoid system called a corrolazine that is designed to stabilize high oxidation states, including high-valent metal-oxo species. The corrolazine ligand stabilizes manganese(V) terminal oxo and terminal imido complexes for isolation, both of which are only transiently observed with normal porphyrin macrocycles. Examination of both oxygen atom transfer and hydrogen atom abstraction reactions for the Mn(V)-oxo complex has led to a number of mechanistic insights regarding these transformations. The activation of H 2O 2 to give the Mn(V)-oxo complex exhibits some dramatic and unexpected axial ligand effects that call into question the normal role of axial ligands in O-O bond cleavage pathways.