π-Lewis Base Activation of Carbonyls and Hexafluorobenzene.

We report hitherto elusive side-on η2-bonded palladium(0) carbonyl (anthraquinone, benzaldehyde) and arene (benzene, hexa-fluorobenzene) palladium(0) complexes and present the catalytic hydrodefluorination of hexafluorobenzene by cyclohexene. The comparison with respective cyclohexene, pyridine and tetrahydrofuran complexes reveals that the experimental ligand binding strengths follow the order THF < C6H6 < C6F6 < cyclohexene < pyridine < benzaldehyde < anthraquinone. To understand this surprising order, the complexes' electronic structures were elucidated by nuclear magnetic resonance (NMR), single crystal X-Ray diffraction (sc-XRD), ultraviolet/visible (UV/Vis) electronic absorption, infrared (IR) vibrational, Pd L3-edge X-ray absorption (XAS), and X-ray photoelectron (XP) spectroscopic techniques, complemented by Density Functional Theory (DFT) calculations including energy decomposition (EDA-NOCV) and effective oxidation state (EOS) analyses.

Reinvestigation of the mechanism of dioxygen activation at a Mn(cyclam) center.

This study deals with the unprecedented reactivity of a [(cyclam)Mn(OTf)] (3-cis; OTf = CFSO) with O, which, depending on the presence or absence of a hydrogen atom donor like 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H), selectively generates di-μ-oxo Mn(III)Mn(IV) (1) or Mn (2) complexes, respectively. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction, X-ray absorption spectroscopy, and electron paramagnetic resonance. Oxygenation reactions carried out with labeled O and Resonance Raman spectroscopy unambiguously show that the oxygen atoms present in the MnMn dimer originate from O.

A bis-Phenolate Carbene-Supported bis-μ-Oxo Iron(IV/IV) Complex with a [Fe(μ-O)Fe] Diamond Core Derived from Dioxygen Activation.

The diiron(II) complex, [(OCO)Fe(MeCN)] (, MeCN = acetonitrile), supported by the bis-phenolate carbene pincer ligand, 1,3-bis(3,5-di--butyl-2-hydroxyphenyl)benzimidazolin-2-ylidene (OCO), was synthesized and characterized by single-crystal X-ray diffraction, H nuclear magnetic resonance, infrared (IR) vibrational, ultraviolet/visible/near-infrared (UV/vis/NIR) electronic absorption, Fe Mössbauer, X-band electron paramagnetic resonance (EPR) and SQUID magnetization measurements. Complex activates dioxygen to yield the diferric, μ-oxo-bridged complex [(OCO)Fe(py)(μ-O)Fe(O(C═O)O)(py)] () that was isolated and fully characterized. In , one of the iron-carbene bonds was oxidized to give a urea motif, resulting in an O(C═O)O binding site, while the other Fe(OCO) unit remained unchanged.

A Bioinspired Nonheme Fe-(O)-Cu Complex with an = 1 Ground State.

Cytochrome oxidase (CcO) is a heme copper oxidase (HCO) that catalyzes the natural reduction of oxygen to water. A profound understanding of some of the elementary steps leading to the intricate 4e/4H reduction of O is presently lacking. A total spin = 1 Fe-(O)-Cu () intermediate is proposed to reduce the overpotentials associated with the reductive O-O bond rupture by allowing electron transfer from a tyrosine moiety without the necessity of any spin-surface crossing.

Origin of Unprecedented Formation and Reactivity of Fe═O Species via Oxygen Activation: Role of Noncovalent Interactions and Magnetic Coupling.

Emulating the capabilities of the soluble methane monooxygenase (sMMO) enzymes, which effortlessly activate oxygen at diiron(II) centers to form a reactive diiron(IV) intermediate Q, which then performs the challenging oxidation of methane to methanol, poses a significant challenge. Very recently, one of us reported the mononuclear complex [(cyclam)Fe(CHCN)] (), which performed a rare bimolecular activation of the molecule of O to generate two molecules of Fe═O without the requirement of external proton or electron sources, similar to sMMO. In the present study, we employed the density functional theory (DFT) calculations to investigate this unique mechanism of O activation.

Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates.

The reaction of Li[(TAML)Co]·3HO (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf) and Y(OTf)) or triflic acid affords a blue species , which is converted reversibly to a green species upon cooling to 193 K. The electronic structures of and have been determined by combining advanced spectroscopic techniques (X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD)) with theoretical studies. Complex is best represented as an = 1/2 [(Sol)(TAML)Co---OH(LA)] species (LA = Lewis/Brønsted acid and Sol = solvent), where an = 1 Co(III) center is antiferromagnetically coupled to = 1/2 TAML, which represents a one-electron oxidized TAML ligand.

Long-Term Stability of Light-Induced Ti Defects in TiO Nanotubes for Amplified Photoelectrochemical Water Splitting.

This study shows that the simple approach of keeping anodic TiO nanotubes at 70 °C in ethanol for 1 h results in improved photoelectrochemical water splitting activity due to initiation of crystallization in the material amplified by the light-induced formation of a Ti -V states under UV 365 nm illumination. For the first time, the light-induced Ti -V states are generated when oxygen is present in the reaction solution and are stable when in contact with air (oxygen) for a long time (two months). We confirmed here that the amorphous or nearly amorphous structure of titania supports the survival of Ti species in contact with oxygen.

Trapping of a phenoxyl radical at a non-haem high-spin iron(II) centre.

The activation of dioxygen at haem and non-haem metal centres, and subsequent functionalization of unactivated C‒H bonds, has been a focal point of much research. In iron-mediated oxidation reactions, O binding at an iron(II) centre is often accompanied by an oxidation of the iron centre. Here we demonstrate dioxygen activation by sodium tetraphenylborate and protons in the presence of an iron(II) complex to form a reactive radical species, whereby the iron oxidation state remains unaltered in the presence of a highly oxidizing phenoxyl radical and O.

A high-spin alkylperoxo-iron(iii) complex with -anionic ligands: implications for the superoxide reductase mechanism.

The NO macrocycle of the 12-TMCO ligand stabilizes a high spin ( = 5/2) [Fe(12-TMCO)(OOBu)Cl] (3-Cl) species in the reaction of [Fe(12-TMCO)(OTf)] (1-(OTf)) with -butylhydroperoxide (BuOOH) in the presence of tetraethylammonium chloride (NEtCl) in acetonitrile at -20 °C. In the absence of NEtCl the oxo-iron(iv) complex 2 [Fe(12-TMCO)(O)(CHCN)] is formed, which can be further converted to 3-Cl by adding NEtCl and BuOOH. The role of the -chloride ligand in the stabilization of the Fe-OOBu moiety can be extended to other anions including the thiolate ligand relevant to the enzyme superoxide reductase (SOR).

Reactivities of iron(IV)-oxido compounds with pentadentate bispidine ligands.

The FeO complexes of bispidines (3,7-diazabicyclo[3.3.1]nonane derivatives) are known to be highly reactive oxidants - with the tetradentate bispidine, the so far most reactive ferryl complex has been reported and two isomeric pentadentate ligands also lead to very reactive high-valent oxidants.

A New Thiolate-Bound Dimanganese Cluster as a Structural and Functional Model for Class Ib Ribonucleotide Reductases.

In class Ib ribonucleotide reductases (RNRs) a dimanganese(II) cluster activates superoxide (O ⋅ ) rather than dioxygen (O ), to access a high valent Mn -O -Mn species, responsible for the oxidation of tyrosine to tyrosyl radical. In a biomimetic approach, we report the synthesis of a thiolate-bound dimanganese complex [Mn (BPMT)(OAc) ](ClO) (BPMT=(2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-methylthiophenolate) (1) and its reaction with O ⋅ to form a [(BPMT)MnO Mn] complex 2. Resonance Raman investigation revealed the presence of an O-O bond in 2, while EPR analysis displayed a 16-line S =1/2 signal at g=2 typically associated with a Mn Mn core, as detected in class Ib RNRs.

Spectroscopic Properties of a Biologically Relevant [Fe (μ-O) ] Diamond Core Motif with a Short Iron-Iron Distance.

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.

Evidence of Sulfur Non-Innocence in [Co (dithiacyclam)] -Mediated Catalytic Oxygen Reduction Reactions.

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)] (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S N -cyclam)] (2; S N -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere.

The influence of secondary interactions on the [Ni(O)] mediated aldehyde oxidation reactions.

A rate enhancement of one to two orders of magnitude can be obtained in the aldehyde deformylation reactions by replacing the -N(CH) groups of [Ni(O)(Me[12]aneN)] (Me[12]aneN = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and [Ni(O)(Me[13]aneN)] (Me[13]aneN = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane) complexes by -NH in [Ni(O)([12]aneN)] (2; [12]aneN = 1,4,7,10-tetraazacyclododecane) and [Ni(O)([13]aneN)] (4; [13]aneN = 1,4,7,10-tetraazacyclotridecane). Based on detailed spectroscopic, reaction-kinetics and theoretical investigations, the higher reactivities of 2 and 4 are attributed to the changes in the secondary-sphere interactions between the [Ni(O)] and [12]aneN or [13]aneN moieties, which open up an alternative electrophilic pathway for the aldehyde oxidation reaction. Identification of primary kinetic isotope effects on the reactivity and stability of 2 when the -NH groups of the [12]aneN ligand are deuterated may also suggest the presence of secondary interaction between the -NH groups of [12]aneN and [Ni(O)] moieties, although, such interactions are not obvious in the DFT calculated optimized structure at the employed level of theory.

Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center.

A mononuclear nonheme cobalt(ii) complex, [(TMGtren)Co(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMGtren)Co(O)]-(Sc(OTf)) (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf). Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant.

Spectroscopic Characterization of a Reactive [Cu (μ-OH) ] Intermediate in Cu/TEMPO Catalyzed Aerobic Alcohol Oxidation Reaction.

Cu /TEMPO (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl) catalyst systems are versatile catalysts for aerobic alcohol oxidation reactions to selectively yield aldehydes. However, several aspects of the mechanism are yet unresolved, mainly because of the lack of identification of any reactive intermediates. Herein, we report the synthesis and characterization of a dinuclear [L1 Cu ] complex 1, which in presence of TEMPO can couple the catalytic 4 H /4 e reduction of O to water to the oxidation of benzylic and aliphatic alcohols.

Ligand-Constraint-Induced Peroxide Activation for Electrophilic Reactivity.

μ-1,2-peroxo-bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron-rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms.

Transition metal-mediated O-O bond formation and activation in chemistry and biology.

Oxygen evolution and reduction reactions are fundamental processes in biological energy conversion schemes, which represent an attractive method for artificial energy conversion for a world still largely depending on fossil fuels. A range of metalloenzymes achieve these challenging tasks in biology by activating water and dioxygen using cheap and abundant transition metals, such as iron, copper, and manganese. High-valent metal-oxo/oxyl, metal-superoxo, and/or metal-(hydro)peroxo species are common reactive intermediates that are found in the O-O bond formation and activation reactions.

A bioinspired oxoiron(IV) motif supported on a NS macrocyclic ligand.

A mononuclear oxoiron(iv) complex 1-trans bearing two equatorial sulfur ligations is synthesized and characterized as an active-site model of the elusive sulfur-ligated FeIV[double bond, length as m-dash]O intermediates in non-heme iron oxygenases. The introduction of sulfur ligands weakens the Fe[double bond, length as m-dash]O bond and enhances the oxidative reactivity of the FeIV[double bond, length as m-dash]O unit with a diminished deuterium kinetic isotope effect, thereby providing a compelling rationale for nature's use of the cis-thiolate ligated oxoiron(iv) motif in key metabolic transformations.

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C-H Bond Oxidation Reactions.

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 Fe =O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral Fe =O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand.

Iron and manganese oxo complexes, oxo wall and beyond.

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [FeO(porphyrinato)] cofactors bearing π-radical porphyrinato ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms.

Catalytic dioxygen reduction mediated by a tetranuclear cobalt complex supported on a stannoxane core.

The synthesis, spectroscopic characterization (infrared, electron paramagnetic resonance and X-ray absorption spectroscopies) and density functional theoretical calculations of a tetranuclear cobalt complex CoL1 involving a nonheme ligand system, L1, supported on a stannoxane core are reported. CoL1, similar to the previously reported hexanuclear cobalt complex CoL2, shows a unique ability to catalyze dioxygen (O) reduction, where product selectivity can be changed from a preferential 4e/4H dioxygen-reduction (to water) to a 2e/2H process (to hydrogen peroxide) only by increasing the temperature from -50 to 30 °C. Detailed mechanistic insights were obtained on the basis of kinetic studies on the overall catalytic reaction as well as by low-temperature spectroscopic (UV-Vis, resonance Raman and X-ray absorption spectroscopies) trapping of the end-on μ-1,2-peroxodicobalt(iii) intermediate 1.

Stoichiometric Formation of an Oxoiron(IV) Complex by a Soluble Methane Monooxygenase Type Activation of O at an Iron(II)-Cyclam Center.

In soluble methane monooxygenase enzymes (MMO), dioxygen (O) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate that performs the challenging oxidation of methane to methanol. An analogous mechanism of O activation at mono- or dinuclear iron centers is rare in the synthetic chemistry. Herein, we report a mononuclear non-heme iron(II)-cyclam complex, -, that activates O to form the corresponding iron(IV)-oxo complex, -, via a mechanism reminiscent of the O activation process in MMO.

Omomyc Reveals New Mechanisms To Inhibit the MYC Oncogene.

The MYC oncogene is upregulated in human cancers by translocation, amplification, and mutation of cellular pathways that regulate Myc. Myc/Max heterodimers bind to E box sequences in the promoter regions of genes and activate transcription. The MYC inhibitor Omomyc can reduce the ability of MYC to bind specific box sequences in promoters of MYC target genes by binding directly to E box sequences as demonstrated by romatin mmunorecipitation (CHIP).

Heme and Nonheme High-Valent Iron and Manganese Oxo Cores in Biological and Abiological Oxidation Reactions.

Utilization of O as an abundant and environmentally benign oxidant is of great interest in the design of bioinspired synthetic catalytic oxidation systems. Metalloenzymes activate O by employing earth-abundant metals and exhibit diverse reactivities in oxidation reactions, including epoxidation of olefins, functionalization of alkane C-H bonds, arene hydroxylation, and -dihydroxylation of arenes. Metal-oxo species are proposed as reactive intermediates in these reactions.