Synthesis, Characterization, and Spectroscopic Studies of 2,6-Dimethylpyridyl-Linked Cu(I)-CNC Complexes: The Electronic Influence of Aryl Wingtips on Copper Centers.

Six new Cu(I) complexes containing pincer ligands of the type 2,6-bis(3-alkyl/arylimidazol-2-ylidene) methylpyridine I(R/R'Ar) CN̂C, where R = trifluoroethyl (TFE) and R' = 4-CF3, 4-NO2, 4-CN, 4-H, and 4-CH3, have been synthesized. These complexes, namely, [Cu(I(TFE)CN̂C)]PF6, 1-TFE; [Cu(ICF3Ar CN̂C]PF6, 2-CF3; [Cu(INO2Ar CN̂C)]PF6, 3-NO2; [Cu(ICNAr CN̂C]PF6, 4-CN; [Cu(IHAr CN̂C)]2(PF6)2, 5-H; and [Cu(ICH3Ar CN̂C)]2(PF6)2, 6-CH3, were fully characterized by 1H, 13C, and HMBC NMR spectroscopy, elemental analysis, electrochemical studies, and single-crystal X-ray crystallography. The crystallographic data revealed different structures and copper nuclearities for the complexes bearing aryl wingtips with electron-withdrawing (2-CF3, 3-NO2, and 4-CN) and electron-donating (5-H and 6-CH3) substituents. The solution-phase conductivity measurements in acetonitrile revealed a mix-electrolyte behavior for these complexes, supporting the presence of both mono- and binuclear forms of each complex. The fast monomer-dimer equilibrium of the Cu-CNC complexes at room temperature is reflected in their simple 1H NMR spectra in acetonitrile. However, both mono- and binuclear forms were identifiable in 1H diffusion-ordered spectroscopy (DOSY) at low temperatures. The dynamic behavior of these complexes in solution was further examined by variable-temperature 1H NMR (VT 1H NMR) experiments, and the relevant thermodynamic parameters were determined. The process was also probed by one-dimensional rotating-frame Overhauser enhancement spectroscopy (1D ROESY) experiments to elucidate the coexisting species in solution. The 2,6-dimethylpyridyl-linked Cu-CNC complexes also presented a quasi-reversible Cu(II)/Cu(I) couple in cyclic voltammetry studies, wherein a clear influence of the aryl wingtips on the E1/2 values was observed. Furthermore, the percent buried volumes (% Vbur) of the complexes were calculated, showing a similar steric hindrance around copper in all complexes. These findings support the importance of electronic effects, induced by the aryl wingtips, on the preferred coordination geometry, copper nuclearity, and redox properties of the Cu-CNC complexes.

pH Changes That Induce an Axial Ligand Effect on Nonheme Iron(IV) Oxo Complexes with an Appended Aminopropyl Functionality.

Nonheme iron enzymes often utilize a high-valent iron(IV) oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV) oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity toward olefin epoxidation upon changes in the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen-atom-transfer (OAT) reaction with substrates. In particular, these changes have occurred because of protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis, and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that replacement of the pendant amine with a neutral or anionic ligand dramatically lowers the rate-determining barriers of cyclooctene epoxidation. The computational work further establishes that the change in OAT is due to electrostatic interactions of the pendant amine cation that favorably affect the barrier heights.

An elusive thermal [2 + 2] cycloaddition driven by visible light photocatalysis: tapping into strain to access C2-symmetric tricyclic rings.

A mild and operationally simple methodology is reported for the synthesis of cyclobutane rings imbedded within a C2-symmetric tricyclic framework. The method uses visible light and an iridium-based photocatalyst to drive the oft-stated "forbidden" thermal [2 + 2] cycloaddition of cycloheptenes and analogs. Importantly, it generates cyclobutane with four new stereocenters with excellent stereoselectivity, and perfect regioselectivity. The reaction is propelled forward when the photocatalyst absorbs a visible light photon, which transfers this energy to the cycloheptene. Key to success is, upon excitation to the triplet via sensitization from the photocatalyst, the double bond isomerizes to give the transient, highly strained, trans-cycloheptene. The trans-cycloheptene undergoes a strain relieving thermal, intermolecular [π2s + π2a] cycloaddition with another cis-cycloheptene. X-ray analysis reveals that the major product is the head-to-head, C2-symmetric all trans-cyclobutane. Additionally, a dramatic display structural complexity enhancement is observed with the use of chiral cycloheptenols possessing one stereocenter, which results in the formation of cyclobutanes with six contiguous stereocenters with good to excellent diastereocontrol, and can be used to isolate single stereoisomers of stereochemically complex cyclobutanes in good yield.

Correction: An elusive thermal [2 + 2] cycloaddition driven by visible light photocatalysis: tapping into strain to access C2-symmetric tricyclic rings.

Correction for 'An elusive thermal [2 + 2] cycloaddition driven by visible light photocatalysis: tapping into strain to access C2-symmetric tricyclic rings' by Kamaljeet Singh et al., Org. Biomol. Chem., 2018, DOI: 10.1039/c8ob01273c.

Cu(I) Complexes of Pincer Pyridine-Based N-Heterocyclic Carbenes with Small Wingtip Substituents: Synthesis and Structural and Spectroscopic Studies.

Six new Cu(I) complexes with pincer N-heterocyclic carbene (NHC) ligands of the type 2,6-bis(3-alkylimidazol-2-ylidene)pyridine, I(R)CNC, and 2,6-bis(3-alkylimidazol-2-ylidene)methylpyridine, I(R)C^N^C, where R = Me, Et, and iPr have been synthesized using Cu precursors and bis(imidazolium) salts. All of these compounds, namely, [Cu2(IMeCNC)2](PF6)2, 1; [Cu2(IEtCNC)2](PF6)2, 2; [Cu2(IiPrCNC)2](PF6)2, 3; [Cu(IMeC^N^C)](PF6), 4; [Cu(IEtC^N^C)](PF6), 5; and [Cu(IiPrC^N^C)](PF6), 6, have been characterized by 1H and 13C NMR spectroscopies, elemental analysis, solution conductivity, and electrochemical studies. Single crystal X-ray structures were obtained for all complexes except 1. The crystallographic data reveal a binuclear structure containing two Cu atoms at a close distance, 2.622-2.811 Å for all the complexes except 5, which shows a unique mononuclear structure. Spatial syn arrangement of ethyl groups and extensive π-π stacking in the solid state accounts for the mononuclear structure of complex 5. A pseudolinear coordination geometry about metal centers consisting of two Cu-carbene bonds, as well as weak Cu-pyridine interactions, exist among all the complexes independent of their ligand. Solution-state conductivity data reveal a dominant 1:2 electrolyte behavior for 1-3 but 1:1 electrolyte for 4-6, consistent with the sustainable binuclear structure in solutions of Cu(I)-I(R)CNC complexes. Cyclic voltammetry and differential pulse voltammetry studies reveal an irreversible and two quasi-reversible peaks for the one-electron oxidation of solvent-bound and solvent-free binuclear and mononuclear Cu-NHC species in complexes 1-3. In contrast, the reversible Cu(II)/Cu(I) couples of 4-6 at potentials close to that of complexes with tripodal polydentate NHC scaffolds indicate the electronic and structural flexibility of I(R)C^N^C ligands to accommodate both Cu(I) and Cu(II) ions.

Quantum mechanics/molecular mechanics study on the oxygen binding and substrate hydroxylation step in AlkB repair enzymes.

AlkB repair enzymes are important nonheme iron enzymes that catalyse the demethylation of alkylated DNA bases in humans, which is a vital reaction in the body that heals externally damaged DNA bases. Its mechanism is currently controversial and in order to resolve the catalytic mechanism of these enzymes, a quantum mechanics/molecular mechanics (QM/MM) study was performed on the demethylation of the N(1) -methyladenine fragment by AlkB repair enzymes. Firstly, the initial modelling identified the oxygen binding site of the enzyme. Secondly, the oxygen activation mechanism was investigated and a novel pathway was found, whereby the catalytically active iron(IV)-oxo intermediate in the catalytic cycle undergoes an initial isomerisation assisted by an Arg residue in the substrate binding pocket, which then brings the oxo group in close contact with the methyl group of the alkylated DNA base. This enables a subsequent rate-determining hydrogen-atom abstraction on competitive σ- and π-pathways on a quintet spin-state surface. These findings give evidence of different locations of the oxygen and substrate binding channels in the enzyme and the origin of the separation of the oxygen-bound intermediates in the catalytic cycle from substrate. Our studies are compared with small model complexes and the effect of protein and environment on the kinetics and mechanism is explained.

Pycup--a bifunctional, cage-like ligand for (64)Cu radiolabeling.

In developing targeted probes for positron emission tomography (PET) based on (64)Cu, stable complexation of the radiometal is key, and a flexible handle for bioconjugation is highly advantageous. Here, we present the synthesis and characterization of the chelator pycup and four derivatives. Pycup is a cross-bridged cyclam derivative with a pyridyl donor atom integrated into the cross-bridge resulting in a pentadentate ligand. The pycup platform provides kinetic inertness toward (64)Cu dechelation and offers versatile bioconjugation chemistry. We varied the number and type of additional donor atoms by alkylation of the remaining two secondary amines, providing three model ligands, pycup2A, pycup1A1Bn, and pycup2Bn, in 3-4 synthetic steps from cyclam. All model copper complexes displayed very slow decomplexation in 5 M HCl and 90 °C (t1/2: 1.5 h for pycup1A1Bn, 2.7 h for pycup2A, 20.3 h for pycup2Bn). The single crystal crystal X-ray structure of the [Cu(pycup2Bn)](2+) complex showed that the copper was coordinated in a trigonal, bipyramidal manner. The corresponding radiochemical complexes were at least 94% stable in rat plasma after 24 h. Biodistribution studies conducted in Balb/c mice at 2 h postinjection of (64)Cu labeled pycup2A revealed low residual activity in kidney, liver, and blood pool with predominantly renal clearance observed. Pycup2A was readily conjugated to a fibrin-targeted peptide and labeled with (64)Cu for successful PET imaging of arterial thrombosis in a rat model, demonstrating the utility of our new chelator in vivo.

Does hydrogen-bonding donation to manganese(IV)-oxo and iron(IV)-oxo oxidants affect the oxygen-atom transfer ability? A computational study.

Iron(IV)-oxo intermediates are involved in oxidations catalyzed by heme and nonheme iron enzymes, including the cytochromes P450. At the distal site of the heme in P450 Compound I (Fe(IV) -oxo bound to porphyrin radical), the oxo group is involved in several hydrogen-bonding interactions with the protein, but their role in catalysis is currently unknown. In this work, we investigate the effects of hydrogen bonding on the reactivity of high-valent metal-oxo moiety in a nonheme iron biomimetic model complex with trigonal bipyramidal symmetry that has three hydrogen-bond donors directed toward a metal(IV)-oxo group. We show these interactions lower the oxidative power of the oxidant in reactions with dehydroanthracene and cyclohexadiene dramatically as they decrease the strength of the OH bond (BDEOH ) in the resulting metal(III)-hydroxo complex. Furthermore, the distal hydrogen-bonding effects cause stereochemical repulsions with the approaching substrate and force a sideways attack rather than a more favorable attack from the top. The calculations, therefore, give important new insights into distal hydrogen bonding, and show that in biomimetic, and, by extension, enzymatic systems, the hydrogen bond may be important for proton-relay mechanisms involved in the formation of the metal-oxo intermediates, but the enzyme pays the price for this by reduced hydrogen atom abstraction ability of the intermediate. Indeed, in nonheme iron enzymes, where no proton relay takes place, there generally is no donating hydrogen bond to the iron(IV)-oxo moiety.

Rationalization of the barrier height for p-Z-styrene epoxidation by iron(IV)-oxo porphyrin cation radicals with variable axial ligands.

A versatile class of heme monoxygenases involved in many vital functions for human health are the cytochromes P450, which react via a high-valent iron(IV) oxo heme cation radical species called Compound I. One of the key reactions catalyzed by these enzymes is C═C epoxidation of substrates. We report here a systematic study into the intrinsic chemical properties of substrate and oxidant that affect reactivity patterns. To this end, we investigated the effect of styrene and para-substituted styrene epoxidation by Compound I models with either an anionic (chloride) or neutral (acetonitrile) axial ligand. We show, for the first time, that the activation enthalpy of the reaction is determined by the ionization potential of the substrate, the electron affinity of the oxidant, and the strength of the newly formed C-O bond (approximated by the bond dissociation energy, BDE(OH)). We have set up a new valence bond model that enables us to generalize substrate epoxidation reactions by iron(IV)-oxo porphyrin cation-radical oxidants and make predictions of rate constants and reactivities. We show here that electron-withdrawing substituents lead to early transition states, whereas electron-donating groups on the olefin substrate give late transition states. This affects the barrier heights in such a way that electron-withdrawing substituents correlate the barrier height with BDE(OH), while the electron affinity of the oxidant is proportional to the barrier height for substrates with electron-donating substituents.

Synthesis and ligand non-innocence of thiolate-ligated (N4S) Iron(II) and nickel(II) bis(imino)pyridine complexes.

The known iron(II) complex [Fe(II)(LN3S)(OTf)] (1) was used as starting material to prepare the new biomimetic (N4S(thiolate)) iron(II) complexes [Fe(II)(LN3S)(py)](OTf) (2) and [Fe(II)(LN3S)(DMAP)](OTf) (3), where LN3S is a tetradentate bis(imino)pyridine (BIP) derivative with a covalently tethered phenylthiolate donor. These complexes were characterized by X-ray crystallography, ultraviolet-visible (UV-vis) spectroscopic analysis, (1)H nuclear magnetic resonance (NMR), and Mössbauer spectroscopy, as well as electrochemistry. A nickel(II) analogue, [Ni(II)(LN3S)](BF4) (5), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed 1-3 and 5 undergo a single reduction process with E(1/2) between -0.9 V to -1.2 V versus Fc(+)/Fc. Treatment of 3 with 0.5% Na/Hg amalgam gave the monoreduced complex [Fe(LN3S)(DMAP)](0) (4), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron paramagnetic resonance (EPR) spectroscopy (g = [2.155, 2.057, 2.038]), and Mössbauer (δ = 0.33 mm s(-1); ΔE(Q) = 2.04 mm s(-1)) spectroscopy. Computational methods (DFT) were employed to model complexes 3-5. The combined experimental and computational studies show that 1-3 are 5-coordinate, high-spin (S = 2) Fe(II) complexes, whereas 4 is best described as a 5-coordinate, intermediate-spin (S = 1) Fe(II) complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (S(total) = 1/2) ground state. Complexes 2 and 3 are shown to react with O2 to give S-oxygenated products, as previously reported for 1. In contrast, the monoreduced 4 appears to react with O2 to give a mixture of sulfur oxygenates and iron oxygenates. The nickel(II) complex 5 does not react with O2, and even when the monoreduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O2.

Innovative Technique for Posterior Fixation of Vertically Unstable Pelvic Ring Fracture: A Case Report.

An obese 57-year-old woman with known hypertension and diabetes mellitus sustained multiple injuries during an accident, which caused anterior-posterior fracture-dislocation of the pelvic ring. Due to the drawbacks of conventional stabilizing methods for anterior-posterior fracture-dislocations of the hip in this setting, such as the inability to visualize anatomical landmarks fluoroscopically for the iliosacral screw technique and the compromised L5 pedicle preventing lumbopelvic fixation, the patient underwent an innovative Hula Hoop technique described here. Using the Hula Hoop technique, a technique that has rarely been studied in humans, we avoided an invasive open procedure, decreased anesthesia time, reduced the size and number of incisions, and minimized bleeding. After three months of routine physiotherapy and occupational therapy, the patient was able to walk with a walker and an ankle-foot orthosis.

A Brief Overview on Crack Patterns, Repair and Strengthening of Historical Masonry Structures.

Given that a significant fraction of buildings and architectural heritage in Europe's historical centers are masonry structures, the selection of proper diagnosis, technological surveys, non-destructive testing, and interpretations of crack and decay patterns is paramount for a risk assessment of possible damage. Identifying the possible crack patterns, discontinuities, and associated brittle failure mechanisms within unreinforced masonry under seismic and gravity actions allows for reliable retrofitting interventions. Traditional and modern materials and strengthening techniques create a wide range of compatible, removable, and sustainable conservation strategies. Steel/timber tie-rods are mainly used to support the horizontal thrust of arches, vaults, and roofs and are particularly suitable for better connecting structural elements, e.g., masonry walls and floors. Composite reinforcing systems using carbon, glass fibers, and thin mortar layers can improve tensile resistance, ultimate strength, and displacement capacity to avoid brittle shear failures. This study overviews masonry structural diagnostics and compares traditional and advanced strengthening techniques of masonry walls, arches, vaults, and columns. Several research results in automatic surface crack detection for unreinforced masonry (URM) walls are presented considering crack detection based on machine learning and deep learning algorithms. In addition, the kinematic and static principles of Limit Analysis within the rigid no-tension model framework are presented. The manuscript sets a practical perspective, providing an inclusive list of papers describing the essential latest research in this field; thus, this paper is useful for researchers and practitioners in masonry structures.

Valence tautomerism in a high-valent manganese-oxo porphyrinoid complex induced by a Lewis acid.

Addition of the Lewis acid Zn(2+) to (TBP(8)Cz)Mn(V)(O) induces valence tautomerization, resulting in the formation of [(TBP(8)Cz(+•))Mn(IV)(O)-Zn(2+)]. This new species was characterized by UV-vis, EPR, the Evans method, and (1)H NMR and supported by DFT calculations. Removal of Zn(2+) quantitatively restores the starting material. Electron-transfer and hydrogen-atom-transfer reactions are strongly influenced by the presence of Zn(2+).

Regioselectivity of aliphatic versus aromatic hydroxylation by a nonheme iron(II)-superoxo complex.

Many enzymes in nature utilize molecular oxygen on an iron center for the catalysis of substrate hydroxylation. In recent years, great progress has been made in understanding the function and properties of iron(IV)-oxo complexes; however, little is known about the reactivity of iron(II)-superoxo intermediates in substrate activation. It has been proposed recently that iron(II)-superoxo intermediates take part as hydrogen abstraction species in the catalytic cycles of nonheme iron enzymes. To gain insight into oxygen atom transfer reactions by the nonheme iron(II)-superoxo species, we performed a density functional theory study on the aliphatic and aromatic hydroxylation reactions using a biomimetic model complex. The calculations show that nonheme iron(II)-superoxo complexes can be considered as effective oxidants in hydrogen atom abstraction reactions, for which we find a low barrier of 14.7 kcal mol(-1) on the sextet spin state surface. On the other hand, electrophilic reactions, such as aromatic hydroxylation, encounter much higher (>20 kcal mol(-1)) barrier heights and therefore are unlikely to proceed. A thermodynamic analysis puts our barrier heights into a larger context of previous studies using nonheme iron(IV)-oxo oxidants and predicts the activity of enzymatic iron(II)-superoxo intermediates.

Manganese substituted Compound I of cytochrome P450 biomimetics: a comparative reactivity study of Mn(V)-oxo versus Mn(IV)-oxo species.

Manganese-oxo porphyrins have been well studied as biomimetic models of cytochromes P450 and are known to be able to catalyze substrate hydroxylation reactions. Recent experimental studies [J.Y. Lee, Y.-M. Lee, H. Kotani, W. Nam, S. Fukuzumi, Chem. Commun. (2009) 704] showed that Mn(V)-oxo porphyrins react rapidly with 10-methyl-9,10-dihydroacridine (AcrH(2)) via a proton-coupled-electron-transfer followed by an electron transfer. In this work, we present a computational study on the reactivity patterns of Mn(V)-oxo and Mn(IV)-oxo with respect to AcrH(2). This study shows that although both oxidants are capable of hydroxylating AcrH(2), the Mn(V)-oxo species is the more active oxidant. We have generalized these observations with thermodynamic cycles that explain the reaction mechanisms and electron transfer processes. For the Mn(V)-oxo mechanism the reactions proceed with a fast spin state crossing from the ground state singlet to the triplet spin state prior to a hydrogen atom transfer followed by another electron transfer. The present results are fully consistent with previous studies on iron-oxo porphyrins and manganese-oxo porphyrins and shows that the interplay of low lying singlet and triplet spin state surfaces influences the reaction mechanisms and kinetics.

Computational studies of DNA base repair mechanisms by nonheme iron dioxygenases: selective epoxidation and hydroxylation pathways.

DNA base repair mechanisms of alkylated DNA bases is an important reaction in chemical biology and particularly in the human body. It is typically catalyzed by an α-ketoglutarate-dependent nonheme iron dioxygenase named the AlkB repair enzyme. In this work we report a detailed computational study into the structure and reactivity of AlkB repair enzymes with alkylated DNA bases. In particular, we investigate the aliphatic hydroxylation and C[double bond, length as m-dash]C epoxidation mechanisms of alkylated DNA bases by a high-valent iron(iv)-oxo intermediate. Our computational studies use quantum mechanics/molecular mechanics methods on full enzymatic structures as well as cluster models on active site systems. The work shows that the iron(iv)-oxo species is rapidly formed after dioxygen binding to an iron(ii) center and passes a bicyclic ring structure as intermediate. Subsequent cluster models explore the mechanism of substrate hydroxylation and epoxidation of alkylated DNA bases. The work shows low energy barriers for substrate activation and consequently energetically feasible pathways are predicted. Overall, the work shows that a high-valent iron(iv)-oxo species can efficiently dealkylate alkylated DNA bases and return them into their original form.

Origin of the correlation of the rate constant of substrate hydroxylation by nonheme iron(IV)-oxo complexes with the bond-dissociation energy of the C-H bond of the substrate.

Mononuclear nonheme iron containing systems are versatile and vital oxidants of substrate hydroxylation reactions in many biosystems, whereby the rate constant of hydroxylation correlates with the strength of the C-H bond that is broken in the process. The thermodynamic reason behind these correlations, however, has never been established. In this work results of a series of density functional theory calculations of substrate hydroxylation by a mononuclear nonheme iron(IV)-oxo oxidant with a 2 His/1 Asp structural motif analogous to alpha-ketoglutarate dependent dioxygenases are presented. The calculations show that these oxidants are very efficient and able to hydroxylate strong C-H bonds, whereby the hydrogen abstraction barriers correlate linearly with the strength of the C-H bond of the substrate that is broken. These trends have been rationalized using a valence bond (VB) curve-crossing diagram, which explains the correlation using electron transfer mechanisms in the hydrogen abstraction processes. We also rationalized the subsequent reaction step for radical rebound and show that the barrier is proportional to the electron affinity of the iron(III)-hydroxo intermediate complex. It is shown that nonheme iron(IV)-hydroxo complexes have a larger electron affinity than heme iron(IV)-hydroxo complexes and therefore also experience larger radical rebound barriers, which may have implications for product distributions and rearrangement reactions. Thus, detailed comparisons between heme and nonheme iron(IV)-oxo oxidants reveal the fundamental differences in monoxygenation capabilities of these important classes of oxidants in biosystems and synthetic analogues for the first time and enable us to make predictions of experimental processes.

Carbon dioxide: a waste product in the catalytic cycle of alpha-ketoglutarate dependent halogenases prevents the formation of hydroxylated by-products.

We present the first density functional theory study on alpha-ketoglutarate dependent halogenases and focus on the mechanism starting from the iron(IV)-oxo species. The studies show that the high-valent iron(IV)-oxo species reacts with substrates via an initial and rate determining hydrogen abstraction that is characterized by a large kinetic isotope effect (KIE) of 26.7 leading to a radical intermediate. This KIE value is in good agreement with experimental data. The reaction proceeds via two-state reactivity patterns on competing quintet and septet spin state surfaces with close lying hydrogen abstraction barriers. However, the septet spin radical intermediate gives very high barriers for hydroxylation and chlorination whereas the barriers on the quintet spin state surface are much lower. The calculations give extra information regarding the nature of the intermediates and a prediction of a new low-energy mechanism starting from the radical intermediate, whereby a waste product from an earlier step in the catalytic cycle (CO(2)) is recycled and takes the hydroxyl radical away to form bicarbonate via an OH trapping mechanism. As a consequence, this mechanism prevents the occurrence of hydroxylated byproduct and gives a rationale for the sole observance of halogenated products. By contrast, a direct halogenation reaction cannot compete with hydroxylation due to higher reaction barriers. Our findings support experimental work in the field and give a rationale for the lack of hydroxylation products in alpha-ketoglutarate dependent halogenases.

Activation of hydrocarbon C-H bonds by iodosylbenzene: how does it compare with iron(IV)-oxo oxidants?

Combined experimental and theoretical studies on the reactivity of iodosylbenzene (PhIO) show that PhIO is capable of activating weak C-H bonds of hydrocarbons via a hydrogen abstraction mechanism.

Structural characterization and remarkable axial ligand effect on the nucleophilic reactivity of a nonheme manganese(III)-peroxo complex.

The dark side of the Mn: A manganese(III) complex bearing a 13-membered macrocyclic ligand (1, see picture) binds a peroxo ligand in a side-on eta(2) fashion. The reactivity of 1 is influenced by the introduction of anionic ligands trans to the peroxo group. Electronic and structural changes upon trans-ligand binding explain the increased nucleophilicity of the resulting complexes 1-X.