Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2-bis(2,2′-bipyridyl-6-yl)ethane(H2O)2]2+: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate

The iron(II) triflate complex ( 1 ) of 1,2-bis(2,2′-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30 % H₂O₂ to an acetonitrile solution of 1 yielded 2 , a green compound with λ_max=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔE_Q) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm⁻¹ that downshifted to 832, 540 and 441 cm⁻¹ when 1 was treated with H₂¹⁸O₂. All the spectroscopic data support the initial formation of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0 °C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C−H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C−H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, k_H/k_D, of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.     

Catalytically Active Iron(IV)oxo Species Based on a Bis(pyridinyl)phenanthrolinylmethane

Starting from the mononuclear iron(II) complex [Fe(MeCPy₂Phen)(MeCN)₂]²⁺, a non-heme Fe(IV)oxo complex [Fe^IV(MeCPy₂Phen)O]²⁺ was synthesized via oxidation with meta-chloroperoxybenzoic acid (mCPBA). The Fe(IV)oxo complex was characterized using UV/Vis spectroscopy, Mößbauer spectroscopy and CSI mass spectrometry. The ability of this species to oxidize C−H bonds was tested with cyclohexane and adamantane as model substrates. For cyclohexane, an alcohol-to-ketone ratio (A/K) of 6.1 and efficiencies up to 55 were obtained. In case of adamantane, the ratio of tertiary over secondary products (3°/2°) is 38. In combination, this indicates the iron(IV)oxo complex being the catalytically active species.     

Visible‐Light‐Induced Metal‐Free Allylic Oxidation Utilizing a Coupled Photocatalytic System of g‐C3N4 and N‐Hydroxy Compounds

Graphitic carbon nitride (g‐C₃N₄) and N‐hydroxy compounds can function as a non‐metal photocatalytic system to activate O₂ for the selective allylic oxidation under mild conditions, avoiding the employment of any metal derivative or organic oxidizing agents. Interestingly, the novel photocatalytic system affords a remarkably high selectivity towards the formation of aldehydes, especially in the oxidation of toluene. By combining the unique nature of g‐C₃N₄ (surface basicity, semiconductor features, high stability) and the remarkable catalytic oxidation reactivity of nitroxyl radicals, this photocatalytic system opens up a mild and efficent access for C H bond activation.     

Iron and Single Electron Transfer: All is in the Ligand

This account describes some advances we have made in the field of iron catalysis. Two types of reactivity have been uncovered. Based on the use of an iron(II) precatalyst in the presence of NaBH₄, the first one consists in a SET which can be useful for the reductive dehalogenation of iodide and bromide derivatives. Switching to the non-innocent bis-iminopyridine ligands promotes a previously undescribed Csp²−H activation reaction leading to biaryl derivatives. First clues into the intricate nature of the mechanism were obtained and suggested that the redox-active bis-iminopyridine ligand acts as an electron reservoir. The resulting buildup of electron density triggers the C−H bond breaking. All these findings are discussed in light of the existing literature and perspectives are given.     

Insights into Activation of Cobalt Pre-Catalysts for C(sp2)−H Functionalization

The activation of readily prepared, air-stable cobalt(II) bis(carboxylate) pre-catalysts for the functionalization of C(sp²)−H bonds has been systematically studied. With the pyridine bis(phosphine) chelate, ^iPrPNP, treatment of 1 - (O₂C^tBu)₂ with either B₂Pin₂ or HBPin generated cobalt boryl products. With the former, reduction to (^iPrPNP)Co^IBPin was observed while with the latter, oxidation to the cobalt(III) dihydride boryl, trans-(^iPrPNP)Co(H)₂BPin occurred. The catalytically inactive cobalt complex, Co[PinB(O₂C^tBu)₂]₂, accompanied formation of the cobalt-boryl products in both cases. These results demonstrate that the pre-catalyst activation from cobalt(II) bis(carboxylates), although effective and utilizes an air-stable precursor, is less efficient than activation of cobalt(I) alkyl or cobalt(III) dihydride boryl complexes, which are quantitatively converted to the catalytically relevant cobalt(I) boryl. Related cobalt(III) dihydride silyl and cobalt(I) silyl complexes were also synthesized from treatment of trans-(^iPrPNP)Co(H)₂BPin and (^iPrPNP)CoPh with HSi(OEt)₃, respectively. No catalytic silylation of arenes was observed with either complex likely due to the kinetic preference for reversible C−H reductive elimination rather than product- forming C−Si bond formation from cobalt(III). Syntheses of the cobalt(II) bis(carboxylate) and cobalt(I) alkyl of ^iPrPONOP, a pincer where the methylene spacers have been replaced by oxygen atoms, were unsuccessful due to deleterious P−O bond cleavage of the pincer. Despite their structural similarity, the rich catalytic chemistry of ^iPrPNP was not translated to ^iPrPONOP due to the inability to access stable cobalt precursors as a result of ligand decomposition via P−O bond cleavage.     

Mechanism‐Based Inhibitor of DNA Cytosine‐5 Methyltransferase by a SNAr Reaction with an Oligodeoxyribonucleotide Containing a 2‐Amino‐4‐Halopyridine‐C‐Nucleoside

In chromatin, 5‐methylcytosine (mC), which represents the fifth nucleobase in genomic DNA, plays a role as an inducer of epigenetic changes. Tumor cells exhibit aberrant DNA methylation patterns, and inhibition of human DNA cytosine‐5 methyltransferase (DNMT), which is responsible for generating mC in CpG sequences, is an effective strategy to treat various cancers. Here, we describe the design, synthesis, and evaluation of the properties of 2‐amino‐4‐halopyridine‐C‐nucleosides (d^XP) and oligodeoxyribonucleotides (ODNs) containing d^XP as a novel mechanism‐based inhibitor of DNMTs. The designed ODN containing ^XPpG forms a complex with DNMTs by covalent bonding through a nucleophilic aromatic substitution (S_NAr) reaction, and its cell proliferation activity is investigated. This study suggests that d^XP in a CpG sequence of DNA could serve as a potential nucleic acid drug lead in cancer chemotherapy and a useful chemical probe for studies of epigenetics. Our molecular design using a S_NAr reaction would be useful for DNMTs and other protein–DNA interactions.     

Advanced oxidation of raw and biotreated textile industry wastewater with O3, H2O2 /UV‐C and their sequential application

The advanced chemical oxidation of raw and biologically pretreated textile wastewater by (1) ozonation, (2) H₂O₂ /UV − C oxidation and (3) sequential application of ozonation followed by H₂O₂ /UV − C oxidation was investigated at the natural pH values (8 and 11) of the textile effluents for 1 h. Analysis of the reduction in the pollution load was followed by total environmental parameters such as TOC, COD, UV–VIS absorption kinetics and the biodegradability factor, f_B. The successive treatment combination, where a preliminary ozonation step was carried out prior to H₂O₂ /UV − C oxidation without changing the total treatment time, enhanced the COD and TOC removal efficiency of the H₂O₂ /UV − C oxidation by a factor of 13 and 4, respectively, for the raw wastewater. In the case of biotreated textile effluent, a preliminary ozonation step increased COD removal of the H₂O₂ /UV − C treatment system from 15% to 62%, and TOC removal from 0% to 34%. However, the sequential process did not appear to be more effective than applying a single ozonation step in terms of TOC abatement rates. Enhancement of the biodegradability factor (f_B) was more pronounced for the biologically pretreated wastewater with an almost two‐fold increase for the optimized Advanced Oxidation Technologies (AOTs). For H₂O₂ /UV − C oxidation of raw textile wastewater, apparent zero order COD removal rate constants (k_app), and the second order OH· formation rates (r_i) have been calculated. © 2001 Society of Chemical Industry     

Novel Sulfonated CNN Pincer Ligands for Facile C−H Activation at a Pt(II) Center

Two novel sulfonated CNN-pincer ligands 1b and 1c and the corresponding chloro and aqua complexes K[^CNNLPtCl] and ^CNNLPt(H₂O), 3b – 3c and 2b – 2c , were prepared and fully characterized including single crystal X-ray diffraction. Along with the previously described complexes 2a and 3a , the derivatives of a CNN pincer ligand 1a , these complexes form a family of structurally similar compounds where the pincer core rigidity increases in the series 2a (3a) < 2b (3b) < 2c ( 3c ), as deduced from their XRD data. The increased ligand rigidity affects the aqua ligand dissociation energy of the ^CNNLPt(H₂O) complexes, as it follows from DFT calculations and as is reflected in the increased reactivity of the aqua complexes 2a , 2b and 2c in processes that involve aqua ligand loss. Among these processes the formation of the presumed dinuclear complexes ^CNNL₂Pt₂ and, importantly, catalytic C−D bond cleavage in C₆D₆ were studied in 2,2,2-trifluoroethanol solutions. The C−D bond cleavage reactivity was quantified as the rate of H/D exchange between C₆D₆ and CF₃CH₂OH at 80 °C.     

CuII Pyrazolyl-Benzimidazole Dinuclear Complexes with Remarkable Antioxidant Activity

Three dinuclear coordination complexes generated from 1-n-butyl-2-((5-methyl-1H-pyrazole-3-yl)methyl)-1H-benzimidazole ( L ), have been synthesized and characterized spectroscopically and structurally by single crystal X-ray diffraction analysis. Reaction with iron(II) chloride and then copper(II) nitrate led to a co-crystal containing 78 % of [Cu(NO₃)(μ-Cl)( L’ )]₂ ( C₁ ) and 22 % of [Cu(NO₃)(μ-NO₃)( L’ )]₂ ( C₂ ), where L was oxidized to a new ligand L^’ . A mechanism is provided. Reaction with copper chloride led to the dinuclear complex [Cu(Cl)(μ-Cl)( L) ]₂ ( C₃ ). The presence of N−H⋅⋅⋅O and C−H⋅⋅⋅O intermolecular interactions in the crystal structure of C₁ and C₂ , and C−H⋅⋅⋅N and C−H⋅⋅⋅Cl hydrogen bonding in the crystal structure of C₃ led to supramolecular structures that were confirmed by Hirshfeld surface analysis. The ligands and their complexes were tested for free radical scavenging activity and ferric reducing antioxidant power. The complex C₁ / C₂ shows remarkable antioxidant activities as compared to the ligand L and reference compounds.     

Important role of hydrogen peroxide for storing of iron ions in human L-subunit ferritin

The tetranuclear iron(III) compound where two μ-oxo bridged iron(III) complexes are bridged by four carboxylato groups of H(dpal), was isolated from the reaction mixture of Fe(dpal)Cl₂ and hydrogen peroxide, implying that hydrogen peroxide contributes to the formation of an oxo-bridge diiron species via O–O bond cleavage (H(dpal)=N,N-bis(2-picolylmethyl)-β-alanine). This suggests that hydrogen peroxide released in H-subunit of human herritin plays an important role for storing of iron atoms in L-subunit of human ferritin.     

Chemical looping combustion characteristics and kinetic behaviour of Sr-doped perovskite-type CaFeO3 oxygen carriers: Theoretical and experimental investigations

Oxygen carriers (OCs) with typical perovskite structures have attracted attention for use in chemical looping combustion (CLC) owing to their unique tunable structures and excellent performance. Thus, a further improvement in the reactivity and a deep understanding of the kinetic behaviour in CLC are highly desirable for such perovskite OCs. In this study, a series of Sr-doped perovskite-structured CaFeO₃ OCs (denoted as Sr_xCa_1−xFeO₃) were synthesized. The CLC characteristics, kinetic behaviour, and doping mechanism were systematically investigated via experiments and density functional theory (DFT) calculations. The activation energies of Sr_xCa_1−xFeO₃ OCs with various Sr contents were found to be in the range of 36.6–40.1 kJ/mol and lower than that of CaFeO₃ (62.7 kJ/mol), indicating that the Sr doping enhanced the reactivity of CaFeO₃. Among the OCs, Sr_0.4Ca_0.6FeO₃, which had the lowest activation energy and the fastest release of lattice oxygen, was regarded as the optimum OC. DFT calculations indicated that the reaction energy barrier of Sr_xCa_1−xFeO₃ (0.73–1.06 eV) was lower than that of CaFeO₃ (2.18 eV). This suggests that Sr doping and the regulation of the reaction pathways are essential drivers for enhancing the reactivity of Sr_xCa_1−xFeO₃, which affects the release of lattice oxygen and the morphological properties of OC particles.     

Impact of Fe Content in Laboratory-Produced Soot Aerosol on its Composition,Structure, and Thermo-Chemical Properties

Soot aerosol, which is a major pollutant in the atmosphere of urban areas, often contains not only carbonaceous matter but also inorganic material. These species, for example, iron compounds, originated from impurities in fuel or lubricating oil, additives or engine wear may change the physico-chemical characteristics of soot and hence its environmental impact. We studied the change of composition, structure, and oxidation reactivity of laboratory-produced soot aerosol with varying iron content. Soot types of various iron contents were generated in a propane/air diffusion flame by adjusting the doping amount of iron pentacarbonyl Fe(CO)₅ to the flame. Scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) was combined with cluster analysis (CA) to separate individual particles into definable groups of similar chemical composition representing the particle types in dependence of the iron content in soot. Raman microspectroscopy (RM) and infrared spectroscopy were applied for the characterization of the graphitic soot structure, hydrocarbons, and iron species. For the analysis of soot reactivity, temperature-programmed oxidation (TPO) was used. It is demonstrated that iron is most dominantly present in the form of amorphous Fe (III) oxide crystallizing to hematite α-Fe₂O₃ upon thermal treatment. Iron contaminations do not change the soot microstructure crucially, but Fe(CO)₅ doping of the flame impacts hydrocarbon composition. Soot oxidation reactivity strongly depends on the iron content, as the temperature of maximum carbon (di)oxide emission T _max follows an exponential decay with increasing iron content in soot. Based on the results of the thermo-chemical characterization of laboratory-produced internally mixed iron-containing soot, we can conclude that iron-containing combustion aerosol samples cannot be characterized unambiguously by current thermo-optical analysis protocols.

Copyright 2012 American Association for Aerosol Research     

Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.     

Oxidative Degradation of Thiaproline Derivatives in Aqueous Solutions Induced by •OH Radicals

The participation of neighboring amino and carboxyl groups in thiazolidyne-4-carboxylic acid (thiaproline) and its two derivatives, thiazolidyne-2-carboxylic acid and thiazolidyne-2,4-dicarboxylic acid, is demonstrated during ^.OH-radical-induced oxidation in aqueous solutions. The reaction of ^.OH radicals with these compounds does not lead to the expected one-electron oxidized sulfur-centered radical cations and subsequently to the intermolecularly (S∴S)-bonded dimeric radical cations (even at very high concentration of thiaprolines (>50 mM )), but rather to the elimination of CO₂ with the parallel formation of αN radicals. The latter radicals formed in thiaproline and thiazolidyne-2,4-dicarboxylic acid subsequently undergo β-fragmentation of the C S bond, leading to the thiyl radicals (RS^.). The αN and RS^. radicals were probed and quantified by one-electron reduction of p-nitroacetophenone (PNAP) and by one-electron oxidation of the monoanion of ascorbic acid (AH⁻), respectively. The calculated stabilization enthalpies of the αN radicals show that the loss of CO₂ is thermodynamically a favorable process. The Gibbs free energies of β-fragmentation processes in thiaproline and thiazolidyne-2,4-dicarboxylic acid are equal to −3.7 and −10.2 kJ mol⁻¹, respectively, showing that at room temperature both are exergonic. A general ^.OH-radical-induced oxidation mechanism of thiaproline derivatives in aqueous solutions is proposed.     

One‐Pot Directing Group Formation/C−H Bond Functionalization via Copper(I) and Ruthenium(II) Catalysis

A copper(I)‐catalyzed oxidation leading to the formation of 2‐pyridyl ketone directing groups has been efficiently coupled with a ruthenium(II)‐catalyzed C(sp²)−H bond arylation transformation without performing any intermediate work‐up. This sustainable approach was further extended to sequential three‐step transition metal‐catalyzed transformations in which up to four new bonds (C–O, C–C, C–H and O–H) are formed in a one‐pot fashion. This is the first example in which two different transition metal catalysts are sequentially employed for the directing group formation and the C−H bond functionalization, respectively.

     

Metalloporphyrins in Oxidative Catalysis. Oxygen Transfer Reactions of Oxochromium Porphyrins

The oxidation of chloro-5-10,15,20-tetramesitylporphyrinotoiron(III) with peroxyacids affords a reactive oxoiron(IV)-porphyrin cation radical species 2. The characterization of 2 and its oxochromium analogs 3, 4 and 5 are reviewed. The nature of reactive oxochromium species derived from chromyl reagents is also reviewed. The oxidation of triphenylphosphine by CrOTPP (11), CrOTTP (13) and CrOTMP (14) is described. Variations in the rate constants indicate that steric factors affect the rate of oxygen atom transfer. Activation parameters for the oxidation of triphenylphosphine by 14 are ΔH^≠ = 6.96 kcal/mol and ΔS^≠ = −39 eu. The oxidation of t-butylphenylcarbinol (18) by CrOTPP gave predominantly benzaldehyde via carbon—carbon bond cleavage while the chromium(III) porphyrin-catalyzed oxidation of 18 by iodosylbenzene afforded t-butylphenylketone.     

Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics,electron distributions,and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processes

We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX₂]⁰ (P = porphyrin; X = F, Cl, PF₆) and (b) high-valent manganese-oxo species. For [(P)Mn(PF₆)₂]⁰, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO₄)₂]⁰ (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl₂]⁰ and [(P)MnF₂]⁰ have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for Mn^IV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the Mn^IV = O bond is significantly weaker than the Fe^IV = O bond in an analogous molecule. Thus, the optimized metal—oxygen bond distances are 1.654 and 1.674 Å for (P)Fe^IV(O)(Py) and (P)Mn^IV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that Mn^IV = O stretching frequencies are over 10% lower than Fe^IV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF₆)]⁰, [(P)Mn(O)(Py)]⁺, and [(P)Mn(O)(F)]⁰, the ground states clearly correspond to a (d_xy)² Mn(V) configuration and the short Mn–O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn–O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species.