Selective Ortho‐Hydroxylation–Defluorination of 2‐Fluorophenolates with a Bis(μ‐oxo)dicopper(III) Species

The bis(μ‐oxo)dicopper(III) species [Cu^III₂(μ‐O)₂(m‐XYL^MeAN)]²⁺ ( 1 ) promotes the electrophilic ortho‐hydroxylation–defluorination of 2‐fluorophenolates to give the corresponding catechols, a reaction that is not accomplishable with a (η²:η²‐O₂)dicopper(II) complex. Isotopic labeling studies show that the incoming oxygen atom originates from the bis(μ‐oxo) unit. Ortho‐hydroxylation–defluorination occurs selectively in intramolecular competition with other ortho‐substituents such as chlorine or bromine.     

Mononuclear Manganese–Peroxo and Bis(μ‐oxo)dimanganese Complexes Bearing a Common N‐Methylated Macrocyclic Ligand

Mononuclear Mn^III–peroxo and dinuclear bis(μ‐oxo)Mn^III₂ complexes that bear a common macrocyclic ligand were synthesized by controlling the concentration of the starting Mn^II complex in the reaction of H₂O₂ (i.e., a Mn^III–peroxo complex at a low concentration (≤1 mM ) and a bis(μ‐oxo)Mn^III₂ complex at a high concentration (≥30 mM )). These intermediates were successfully characterized by various physicochemical methods such as UV–visible spectroscopy, ESI‐MS, resonance Raman, and X‐ray analysis. The structural and spectroscopic characterization combined with density functional theory (DFT) calculations demonstrated unambiguously that the peroxo ligand is bound in a side‐on fashion in the Mn^III–peroxo complex and the Mn₂O₂ diamond core is in the bis(μ‐oxo)Mn^III₂ complex. The reactivity of these intermediates was investigated in electrophilic and nucleophilic reactions, in which only the Mn^III–peroxo complex showed a nucleophilic reactivity in the deformylation of aldehydes.     

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO^? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl^? or CF₃SO₃^?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Hydroxylation of Aromatics with the Help of a Non‐Haem FeOOH: A Mechanistic Study under Single‐Turnover and Catalytic Conditions

Ferric–hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low‐spin Fe^III(OOH) complexes characterized to date, [(L₅²)Fe(OOH)]²⁺ is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single‐turnover conditions. In this report we show that [(L₅²)Fe(OOH)]²⁺ decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first‐order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first‐order rate constants. Combining the kinetic data obtained at different temperatures and under different single‐turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5‐D₃]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by Fe^III(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged Fe^IV(O) and OH ^. species from the Fe^III(OOH) complex have been obtained for the first time. After homolytic O? O cleavage, a caged pair of oxidants [Fe^IVO+HO ^. ] is generated that act in unison to hydroxylate the aromatic ring: HO ^. attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by Fe^IVO to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re‐aromatization step. Spin‐trapping experiments in the presence of 5,5‐dimethyl‐1‐pyrroline N‐oxide and GC‐MS analyses of the intermediate products further support the proposed mechanism.     

A Bis(μ‐oxido)dinickel(III) Complex with a Triplet Ground State

A bis(μ‐oxido)dinickel(III) complex was synthesized and characterized by single crystal X‐ray diffraction, resonance Raman, and ESI‐mass measurements. Magnetic susceptibility measurements by SQUID and EPR spectroscopy reveal that the complex has a triplet ground state, which is unprecedented for high‐valent metal (M) complexes with [M₂(μ‐O)₂] diamond core. DFT studies indicate ferromagnetic coupling of the nickel(III) centers. The complex exhibits hydrogen abstraction reactivity and oxygenation reactivity toward external substrates.     

Synthetic and Structural Studies on Copper 1 H‐[1,10]‐Phenanthrolin‐2‐one Coordination Complexes: Isolation of a Novel Intermediate During 1,10‐Phenanthroline Hydroxylation

The synthesis and crystal structure elucidation of a novel dinuclear heteroleptic copper(II) complex has led to an alternative mechanism in the formation of covalent hydrates. During further studies on the synthesis and properties of [Cu₂(ophen)₂] ( 1 ), a dinuclear complex of copper(I) with 1 H‐[1,10]‐phenanthrolin‐2‐one (Hophen), two intermediates/alternative products 2 and 3 were isolated. The dinuclear, antiferromagnetic complex [Cu₂(ophen)₂(phen)₂] ? (NO₃)₂ ? 9H₂O ( 3 , phen=1,10‐phenanthroline) contains two five‐coordinate copper(II) ions, both with trigonal‐bipyramidal coordination, which are bridged together through deprotonated hydroxyl groups with a Cu? Cu non‐bonding distance of 3.100 Å. Complex [Cu(phen)₂(H₂O)] ? (NO₃)₂ ( 2 ) is a polymorph of a previously reported material. The occurrence of 2 and 3 has led us to propose a variation to the Gillard mechanism for the formation of covalent hydrates in bidentate N‐heterocycles in which the attacking nucleophile may be the deprotonated form of 2 , [Cu(phen)₂(OH)]^?, rather than free OH^?.     

Theoretical Studies on the Asymmetric Baeyer–Villiger Oxidation Reaction of 4‐Phenylcyclohexanone with m‐Chloroperoxobenzoic Acid Catalyzed by Chiral Scandium(III)–N,N′‐Dioxide Complexes

The mechanism and enantioselectivity of the asymmetric Baeyer–Villiger oxidation reaction between 4‐phenylcyclohexanone and m‐chloroperoxobenzoic acid ( m ‐CPBA ) catalyzed by Sc^III–N,N′‐dioxide complexes were investigated theoretically. The calculations indicated that the first step, corresponding to the addition of m ‐CPBA to the carbonyl group of 4‐phenylcyclohexanone, is the rate‐determining step (RDS) for all the pathways studied. The activation barrier of the RDS for the uncatalyzed reaction was predicted to be 189.8 kJ mol^?1. The combination of an Sc^III–N,N′‐dioxide complex and the m ‐CBA molecule can construct a bifunctional catalyst in which the Lewis acidic Sc^III center activates the carbonyl group of 4‐phenylcyclohexanone while m ‐CBA transfers a proton, which lowers the activation barrier of the addition step (RDS) to 86.7 kJ mol^?1. The repulsion between the m‐chlorophenyl group of m ‐CPBA and the 2,4,6‐iPr₃C₆H₂ group of the N,N′‐dioxide ligand, as well as the steric hindrance between the phenyl group of 4‐phenylcyclohexanone and the amino acid skeleton of the N,N′‐dioxide ligand, play important roles in the control of the enantioselectivity.     

Mechanistic Insights on the Iodine(III)‐Mediated α‐Oxidation of Ketones

The synthesis of α‐substituted carbonyl compounds is of great importance due to their ubiquity in both natural and man‐made biologically active compounds. The field of hypervalent iodine chemistry has been a great contributor to access these molecules. For example, the α‐oxidation of carbonyl compounds has been one of the most investigated iodine(III)‐mediated stereoselective transformations. Yet, it is also the transformation that has met the most challenge in terms of achieving high stereoselectivities. The different mechanistic pathways of the iodine(III)‐mediated α‐tosyloxylation of ketones have been investigated. The calculations suggest an unprecedented iodine(III)‐promoted enolization process. Indications that iodonium intermediates could serve as proficient Lewis acids are reported. This concept could have broad impact and foster new developments in the field of hypervalent iodine chemistry.     

The Copper(II)‐Catalyzed Oxidation of Glutathione

The kinetics and mechanisms of the copper(II)‐catalyzed GSH (glutathione) oxidation are examined in the light of its biological importance and in the use of blood and/or saliva samples for GSH monitoring. The rates of the free thiol consumption were measured spectrophotometrically by reaction with DTNB (5,5′‐dithiobis‐(2‐nitrobenzoic acid)), showing that GSH is not auto‐oxidized by oxygen in the absence of a catalyst. In the presence of Cu²⁺, reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper–glutathione complex by the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG (glutathione disulfide) catalyzed by copper(II). When the initial concentrations of GSH are at least threefold in excess of Cu²⁺, the rate law is deduced to be ?d[thiol]/dt=k[copper–glutathione complex][O₂]^0.5[H₂O₂]^?0.5. The 0.5^th reaction order with respect to O₂ reveals a pre‐equilibrium prior to the rate‐determining step of the GSSG formation. In contrast to [Cu²⁺] and [O₂], the rate of the reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of the competing formation of an inactive form of the copper–glutathione complex (binding to glutamic and/or glycine moieties).     

The Striking Stabilization of Ni0(η6‐Arene) Complexes by an Ylide‐Like Silylene Ligand

The right mix does the trick : Elusive {Ni⁰(η⁶‐arene)} moieties can be dramatically stabilized by the N‐heterocyclic silylene ligand 1 , which has a zwitterionic mesomeric structure. The σ, π‐acid–base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2 , which enables arene exchange studies at a Ni⁰ center. Addition of B(C₆F₅)₃ to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6‐iPr₂C₆H₃).

     

Synthesis,Characterization, and Reactivity of Cobalt(III)–Oxygen Complexes Bearing a Macrocyclic N‐Tetramethylated Cyclam Ligand

Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [Co^III(15‐TMC)(O₂)]⁺, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [Co^III(15‐TMC)(OOH)]²⁺. The cobalt(III)–hydroperoxo complex was further converted to [Co^III(15‐TMC‐CH₂‐O)]²⁺ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and ¹⁸O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a Co^IV–oxo (or Co^III–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.     

Formation and High Reactivity of the anti‐Dioxo Form of High‐Spin μ‐Oxodioxodiiron(IV) as the Active Species That Cleaves Strong C−H Bonds

Recently, it was shown that μ‐oxo‐μ‐peroxodiiron(III) is converted to high‐spin μ‐oxodioxodiiron(IV) through O?O bond scission. Herein, the formation and high reactivity of the anti‐dioxo form of high‐spin μ‐oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic‐absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ‐oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene<ethylbenzene≈cumene<trans‐β‐methylstyrene. The rate constants increased proportionally to the substrate concentration at low substrate concentration. At high substrate concentration, however, the rate constants converge to the same value regardless of the kind of substrate. This is explained by a two‐step mechanism in which anti‐μ‐oxodioxodiiron(IV) is formed by syn‐to‐anti transformation of the syn‐dioxo form and reacts with substrates as the oxidant. The anti‐dioxo form is 620 times more reactive in the C?H bond cleavage of ethylbenzene than the most reactive diiron system reported so far. The KIE for the reaction with toluene/[D₈]toluene is 95 at ?30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H₂O₂.     

Dissociation Kinetics of Open‐Chain and Macrocyclic Gadolinium(III)‐Aminopolycarboxylate Complexes Related to Magnetic Resonance Imaging: Catalytic Effect of Endogenous Ligands

The kinetics of the metal exchange reactions between open‐chain Gd(DTPA)^2? and Gd(DTPA‐BMA), macrocyclic Gd(DOTA)^? and Gd(HP‐DO3A) complexes, and Cu²⁺ ions were investigated in the presence of endogenous citrate, phosphate, carbonate and histidinate ligands in the pH range 6–8 in NaCl (0.15 M ) at 25 °C. The rates of the exchange reactions of Gd(DTPA)^2? and Gd(DTPA‐BMA) are independent of the Cu²⁺ concentration in the presence of citrate and the reactions occur via the dissociation of Gd³⁺ complexes catalyzed by the citrate ions. The HCO₃^?/CO₃^2? and H₂PO₄^? ions also catalyze the dissociation of complexes. The rates of the dissociation of Gd(DTPA‐BMA), catalyzed by the endogenous ligands, are about two orders of magnitude higher than those of the Gd(DTPA)^2?. In fact near to physiological conditions the bicarbonate and carbonate ions show the largest catalytic effect, that significantly increase the dissociation rate of Gd(DTPA‐BMA) and make the higher pH values (when the carbonate ion concentration is higher) a risk‐factor for the dissociation of complexes in body fluids. The exchange reactions of Gd(DOTA)^? and Gd(HP‐DO3A) with Cu²⁺ occur through the proton assisted dissociation of complexes in the pH range 3.5–5 and the endogenous ligands do not affect the dissociation rates of complexes. More insights into the interaction scheme between Gd(DTPA‐BMA) and Gd(DTPA)^2? and endogenous ligands have been obtained by acquiring the ¹³C NMR spectra of the corresponding diamagnetic Y(III)‐complexes, indicating the increase of the rates of the intramolecular rearrangements in the presence of carbonate and citrate ions. The herein reported results may have implications in the understanding of the etiology of nephrogenic systemic fibrosis, a rare disease that has been associated to the administration of Gd‐containing agents to patients with impaired renal function.     

Metal‐Ion Induced In Situ Ligand Oxidation for Self‐Assembled Clusters: from Bis(5‐(2‐pyridine‐2‐yl)‐1,2,4‐triazole‐3‐yl)methane to Alcohol or Ketone

Hydrothermal reactions of metal nitrates and ligand bis(5‐(pyridine‐2‐yl)‐1,2,4‐triazol‐3‐yl)methane (H₂L¹) gave three cluster compounds, {Cr₂}, {Zn₁₂} and {Fe₈}. Notably, methylene group of H₂L¹ was in situ oxidized either to hydroxymethylated (L²‐O)³⁻ in the metallo‐ring {Zn₁₂} or to a rigid carbonylated (L³=O)²⁻ in the screw‐type {Fe₈}. In light of comparative experimental results, NO₃⁻ was deduced to be of a catalytic role in the ligand oxidation. Metal ion could be regarded as an “induced” tool for clusters generation in self‐assembly process.     

Reactivity and Operational Stability of N‐Tailed TAMLs through Kinetic Studies of the Catalyzed Oxidation of Orange II by H2O2: Synthesis and X‐ray Structure of an N‐Phenyl TAML

The catalytic activity of the N‐tailed (“biuret”) TAML (tetraamido macrocyclic ligand) activators [Fe{4‐XC₆H₃‐1,2‐( N COCMe₂ N CO)₂NR}Cl]^2? ( 3 ; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me ( a ); NO₂, Me ( b ); H, Ph ( c )] in the oxidative bleaching of Orange II dye by H₂O₂ in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4‐XC₆H₃‐1,2‐( N COCMe₂ N CO)₂CMe₂}OH₂]^? ( 1 ) and the more aggressive analogue [Fe(Me₂C{CON(1,2‐C₆H₃‐4‐X) N CO}₂)OH₂]^? ( 2 ). Catalysis by 3 of the reaction between H₂O₂ and Orange II (S) occurs according to the rate law found generally for TAML activators (v=k_Ik_II[Fe^III][S][H₂O₂]/(k_I[H₂O₂]+k_II[S]) and the rate constants k_I and k_II at pH 7 both decrease within the series 3 b > 3 a > 3 c . The pH dependency of k_I and k_II was investigated for 3 a . As with all TAML activators studied to‐date, bell‐shaped profiles were found for both rate constants. For k_I, the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a . For k_II, the broad bell pH profile exhibits a maximum at pH about 10.5. The condition k_I?k_II holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants k_i for suicidal inactivation of the active forms of complexes 3 a – c were calculated using the general formula ln([S₀]/[S_∞])=(k_II/k_i)[Fe^III]; here [Fe^III], [S₀], and [S_∞] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X‐ray characterization of 3 c are also described.